U.S. patent application number 14/207700 was filed with the patent office on 2014-09-18 for integrated power generation using molten carbonate fuel cells.
This patent application is currently assigned to EXXONMOBIL RESEARCH AND ENGINEERING COMPANY. The applicant listed for this patent is Timothy Andrew Barckholtz, Paul J. Berlowitz, Frank H. Hershkowitz, Anita S. Lee. Invention is credited to Timothy Andrew Barckholtz, Paul J. Berlowitz, Frank H. Hershkowitz, Anita S. Lee.
Application Number | 20140260310 14/207700 |
Document ID | / |
Family ID | 50397350 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140260310 |
Kind Code |
A1 |
Berlowitz; Paul J. ; et
al. |
September 18, 2014 |
Integrated Power Generation Using Molten Carbonate Fuel Cells
Abstract
In various aspects, systems and methods are provided for
integrated operation of molten carbonate fuel cells with turbines
for power generation. Instead of selecting the operating conditions
of a fuel cell to improve or maximize the electrical efficiency of
the fuel cell, an excess of reformable fuel can be passed into the
anode of the fuel cell to increase the chemical energy output of
the fuel cell. The increased chemical energy output can be used for
additional power generation, such as by providing fuel for a
hydrogen turbine.
Inventors: |
Berlowitz; Paul J.; (Glen
Gardner, NJ) ; Barckholtz; Timothy Andrew;
(Whitehouse Station, NJ) ; Hershkowitz; Frank H.;
(Basking Ridge, NJ) ; Lee; Anita S.; (Jersey City,
NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Berlowitz; Paul J.
Barckholtz; Timothy Andrew
Hershkowitz; Frank H.
Lee; Anita S. |
Glen Gardner
Whitehouse Station
Basking Ridge
Jersey City |
NJ
NJ
NJ
NJ |
US
US
US
US |
|
|
Assignee: |
EXXONMOBIL RESEARCH AND ENGINEERING
COMPANY
Annandale
NJ
|
Family ID: |
50397350 |
Appl. No.: |
14/207700 |
Filed: |
March 13, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61788628 |
Mar 15, 2013 |
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61787587 |
Mar 15, 2013 |
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61787697 |
Mar 15, 2013 |
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61787879 |
Mar 15, 2013 |
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61884376 |
Sep 30, 2013 |
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61884545 |
Sep 30, 2013 |
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61884565 |
Sep 30, 2013 |
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61884586 |
Sep 30, 2013 |
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61884605 |
Sep 30, 2013 |
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61884635 |
Sep 30, 2013 |
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61889757 |
Oct 11, 2013 |
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Current U.S.
Class: |
60/780 |
Current CPC
Class: |
H01M 8/04 20130101; H01M
8/0618 20130101; H01M 2300/0051 20130101; Y02P 10/122 20151101;
C21B 15/00 20130101; H01M 2250/10 20130101; C01B 3/50 20130101;
C10G 2/32 20130101; H01M 8/04111 20130101; H01M 8/04156 20130101;
H01M 8/04761 20130101; H01M 8/0612 20130101; Y02E 20/16 20130101;
C01B 2203/00 20130101; Y02P 70/50 20151101; C01B 2203/061 20130101;
Y02P 20/10 20151101; H01M 2250/405 20130101; C01B 3/16 20130101;
C01B 2203/84 20130101; C10K 3/04 20130101; H01M 8/141 20130101;
Y02E 50/10 20130101; C01B 2203/046 20130101; C07C 1/0485 20130101;
F02C 6/18 20130101; C04B 7/367 20130101; H01M 2250/407 20130101;
C01B 3/48 20130101; C01B 2203/0495 20130101; C04B 2290/20 20130101;
Y02E 20/14 20130101; Y02P 30/00 20151101; Y02P 30/20 20151101; C01B
2203/86 20130101; C01C 1/04 20130101; C10G 2/332 20130101; C01B
2203/0405 20130101; C07C 29/152 20130101; H01M 8/0687 20130101;
C10G 2/34 20130101; H01M 8/145 20130101; C01B 2203/067 20130101;
Y02T 10/12 20130101; H01M 8/04805 20130101; C01B 2203/02 20130101;
C01B 2203/066 20130101; C01B 2203/068 20130101; H01M 8/04843
20130101; C01B 3/34 20130101; C01B 2203/1241 20130101; C01B
2203/0415 20130101; C01B 2203/1247 20130101; C07C 29/1518 20130101;
H01M 8/06 20130101; C01B 2203/148 20130101; C01B 2203/0205
20130101; H01M 8/0637 20130101; C01B 2203/0475 20130101; H01M 8/14
20130101; H01M 2008/147 20130101; H01M 8/0625 20130101; H01M
8/04097 20130101; H01M 8/0662 20130101; H01M 8/0668 20130101; C01B
2203/0233 20130101; C01B 2203/0283 20130101; Y02E 20/18 20130101;
C21B 2300/02 20130101; H01M 8/0693 20130101; H01M 8/04014 20130101;
C25B 3/02 20130101; H01M 8/0631 20130101; C01B 2203/1205 20130101;
Y02B 90/10 20130101; Y02P 20/129 20151101; C01B 2203/0227 20130101;
Y02E 60/50 20130101; F02C 3/22 20130101; C01B 2203/04 20130101;
C01B 2203/062 20130101 |
Class at
Publication: |
60/780 |
International
Class: |
F02C 3/22 20060101
F02C003/22 |
Claims
1. A method for producing electricity, the method comprising:
introducing a fuel stream comprising a reformable fuel into an
anode of a molten carbonate fuel cell, an internal reforming
element associated with the anode, or a combination thereof;
introducing a cathode inlet stream comprising CO.sub.2 and O.sub.2
into a cathode of the molten carbonate fuel cell; generating
electricity within the molten carbonate fuel cell, the molten
carbonate fuel cell being operated at a fuel utilization of about
60% or less; generating an anode exhaust comprising H.sub.2, CO,
and CO.sub.2; separating, from at least a portion of the anode
exhaust, a first H.sub.2-rich gas stream comprising at least about
80 vol % H.sub.2; and combusting at least a portion of the first
H.sub.2-rich gas stream to produce electricity.
2. The method of claim 1, further comprising performing a water gas
shift process on the anode exhaust, the at least a portion of the
anode exhaust, or a combination thereof.
3. The method of claim 1, further comprising separating CO.sub.2
from the anode exhaust, the at least a portion of the anode
exhaust, or a combination thereof.
4. The method of claim 1, further comprising separating H.sub.2O
from the anode exhaust, the at least a portion of the anode
exhaust, or a combination thereof.
5. The method of claim 1, wherein the separating step comprises:
performing a water gas shift process on the anode exhaust or at
least a portion of the anode exhaust to form a shifted anode
exhaust portion; and separating H.sub.2O and CO.sub.2 from the
shifted anode exhaust portion to form the first H.sub.2-rich gas
stream.
6. The method of claim 1, wherein the first H.sub.2-rich gas stream
comprises at least about 90 vol % H.sub.2.
7. The method of claim 1, wherein combusting step comprises
generating steam from heat generated by the combustion, and
producing electricity from at least a portion of the generated
steam.
8. The method of claim 1, wherein the combusting step comprises
combusting the at least a portion of the first H.sub.2-rich gas
stream in a turbine.
9. The method of claim 1, wherein the cathode inlet stream
comprises exhaust from combustion of a carbon-containing fuel in a
combustion turbine.
10. The method of claim 9, wherein the carbon-containing fuel
comprises at least 5 vol % of inert gases.
11. The method of claim 9, wherein the carbon-containing fuel
comprises at least about 10 vol % CO.sub.2.
12. The method of claim 9, wherein the carbon-containing fuel
comprises at least about 10 vol % N.sub.2.
13. The method of claim 1, wherein the anode exhaust has a ratio of
H.sub.2:CO of at least about 3.0:1.
14. The method of claim 1, further comprising forming a second
H.sub.2-containing stream from the anode exhaust, the at least a
portion of the anode exhaust, the first H.sub.2-rich gas stream, or
a combination thereof; and recycling at least a portion of the
second H.sub.2-containing stream to the combustion turbine.
15. The method of claim 1, wherein at least about 90 vol % of the
reformable fuel is methane.
16. The method of claim 1, wherein the molten carbonate fuel cell
is operated at a thermal ratio of about 0.25 to about 1.0.
17. The method of claim 1, wherein an amount of the reformable fuel
introduced into the anode, the internal reforming element
associated with the anode, or the combination thereof, is at least
about 75% greater than an amount of hydrogen reacted in the molten
carbonate fuel cell to generate electricity.
18. The method of claim 1, wherein a ratio of net moles of syngas
in the anode exhaust to moles of CO.sub.2 in a cathode exhaust is
at least about 2.0:1.
19. The method of claim 1, wherein a fuel utilization in the anode
is about 50% or less and a CO.sub.2 utilization in the cathode is
at least about 60%.
20. The method of claim 1, wherein the molten carbonate fuel cell
is operated to generate electrical power at a current density of at
least about 150 mA/cm.sup.2 and at least about 40 mW/cm.sup.2 of
waste heat, the method further comprising performing an effective
amount of an endothermic reaction to maintain a temperature
differential between an anode inlet and an anode outlet of about
100.degree. C. or less.
21. The method of embodiment 20, wherein performing the endothermic
reaction consumes at least about 40% of the waste heat.
22. The method of claim 1, wherein an electrical efficiency for the
molten carbonate fuel cell is between about 10% and about 40% and a
total fuel cell efficiency for the molten carbonate fuel cell is at
least about 55%.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Ser. Nos.
61/787,587, 61/787,697, 61/787,879, and 61/788,628, all filed on
Mar. 15, 2013, each of which is incorporated by reference herein in
its entirety. This application also claims the benefit of U.S. Ser.
Nos. 61/884,376, 61/884,545, 61/884,565, 61/884,586, 61/884,605,
and 61/884,635, all filed on Sep. 30, 2013, each of which is
incorporated by reference herein in its entirety. This application
further claims the benefit of U.S. Ser. No. 61/889,757, filed on
Oct. 11, 2013, which is incorporated by reference herein in its
entirety.
[0002] This application is related to 25 other co-pending U.S.
applications, filed on even date herewith, and identified by the
following Attorney Docket numbers and titles: 2013EM104-US2
entitled "Integrated Power Generation and Carbon Capture using Fuel
Cells"; 2013EM104-US3 entitled "Integrated Power Generation and
Carbon Capture using Fuel Cells"; 2013EM107-US2 entitled
"Integrated Power Generation and Carbon Capture using Fuel Cells";
2013EM107-US3 entitled "Integrated Power Generation and Carbon
Capture using Fuel Cells"; 2013EM108-US2 entitled "Integrated Power
Generation and Carbon Capture using Fuel Cells"; 2013EM108-US3
entitled "Integrated Power Generation and Carbon Capture using Fuel
Cells"; 2013EM109-US2 entitled "Integrated Power Generation and
Carbon Capture using Fuel Cells"; 2013EM109-US3 entitled
"Integrated Power Generation and Carbon Capture using Fuel Cells";
2013EM272-US2 entitled "Integrated Power Generation and Chemical
Production using Fuel Cells"; 2013EM273-US2 entitled "Integrated
Power Generation and Chemical Production using Fuel Cells at a
Reduced Electrical Efficiency"; 2013EM274-US2 entitled "Integrated
Power Generation and Chemical Production using Fuel Cells";
2013EM277-US2 entitled "Integrated Power Generation and Chemical
Production using Fuel Cells"; 2013EM278-US2 entitled "Integrated
Carbon Capture and Chemical Production using Fuel Cells";
2013EM279-US2 entitled "Integrated Power Generation and Chemical
Production using Fuel Cells"; 2013EM285-US2 entitled "Integrated
Operation of Molten Carbonate Fuel Cells"; 2014EM047-US entitled
"Mitigation of NOx in Integrated Power Production"; 2014EM049-US
entitled "Integrated of Molten Carbonate Fuel Cells in
Fischer-Tropsch Synthesis"; 2014EM050-US entitled "Integrated of
Molten Carbonate Fuel Cells in Fischer-Tropsch Synthesis";
2014EM051-US entitled "Integrated of Molten Carbonate Fuel Cells in
Fischer-Tropsch Synthesis"; 2014EM052-US entitled "Integrated of
Molten Carbonate Fuel Cells in Methanol Synthesis"; 2014EM053-US
entitled "Integrated of Molten Carbonate Fuel Cells in a Refinery
Setting"; 2014EM054-US entitled "Integrated of Molten Carbonate
Fuel Cells for Synthesis of Nitrogen Compounds"; 2014EM055-US
entitled "Integrated of Molten Carbonate Fuel Cells with
Fermentation Processes"; 2014EM056-US entitled "Integrated of
Molten Carbonate Fuel Cells in Iron and Steel Processing"; and
2014EM057-US entitled "Integrated of Molten Carbonate Fuel Cells in
Cement Processing". Each of these co-pending U.S. applications is
hereby incorporated by reference herein in its entirety.
FIELD OF THE INVENTION
[0003] In various aspects, the invention is related to power
production processes integrated with use of molten carbonate fuel
cells.
BACKGROUND OF THE INVENTION
[0004] Molten carbonate fuel cells utilize hydrogen and/or other
fuels to generate electricity. The hydrogen may be provided by
reforming methane or other reformable fuels in a steam reformer
that is upstream of the fuel cell or within the fuel cell.
Reformable fuels can encompass hydrocarbonaceous materials that can
be reacted with steam and/or oxygen at elevated temperature and/or
pressure to produce a gaseous product that comprises hydrogen.
Alternatively or additionally, fuel can be reformed in the anode
cell in a molten carbonate fuel cell, which can be operated to
create conditions that are suitable for reforming fuels in the
anode. Alternately or additionally, the reforming can occur both
externally and internally to the fuel cell.
[0005] Traditionally, molten carbonate fuel cells are operated to
maximize electricity production per unit of fuel input, which may
be referred to as the fuel cell's electrical efficiency. This
maximization can be based on the fuel cell alone or in conjunction
with another power generation system. In order to achieve increased
electrical production and to manage the heat generation, fuel
utilization within a fuel cell is typically maintained at 70% to
75%.
[0006] U.S. Published Patent Application 2011/0111315 describes a
system and process for operating fuel cell systems with substantial
hydrogen content in the anode inlet stream. The technology in the
'315 publication is concerned with providing enough fuel in the
anode inlet so that sufficient fuel remains for the oxidation
reaction as the fuel approaches the anode exit. To ensure adequate
fuel, the '315 publication provides fuel with a high concentration
of H.sub.2. The H.sub.2 not utilized in the oxidation reaction is
recycled to the anode for use in the next pass. On a single pass
basis, the H.sub.2 utilization may range from 10% to 30%. The '315
reference does not describe significant reforming within the anode,
instead relying primarily on external reforming.
[0007] U.S. Published Patent Application 2005/0123810 describes a
system and method for co-production of hydrogen and electrical
energy. The co-production system comprises a fuel cell and a
separation unit, which is configured to receive the anode exhaust
stream and separate hydrogen. A portion of the anode exhaust is
also recycled to the anode inlet. The operating ranges given in the
'810 publication appear to be based on a solid oxide fuel cell.
Molten carbonate fuel cells are described as an alternative.
[0008] U.S. Published Patent Application 2003/0008183 describes a
system and method for co-production of hydrogen and electrical
power. A fuel cell is mentioned as a general type of chemical
converter for converting a hydrocarbon-type fuel to hydrogen. The
fuel cell system also includes an external reformer and a high
temperature fuel cell. An embodiment of the fuel cell system is
described that has an electrical efficiency of about 45% and a
chemical production rate of about 25% resulting in a system
coproduction efficiency of about 70%. The '183 publication does not
appear to describe the electrical efficiency of the fuel cell in
isolation from the system.
[0009] U.S. Pat. No. 5,084,362 describes a system for integrating a
fuel cell with a gasification system so that coal gas can be used
as a fuel source for the anode of the fuel cell. Hydrogen generated
by the fuel cell is used as an input for a gasifier that is used to
generate methane from a coal gas (or other coal) input. The methane
from the gasifier is then used as at least part of the input fuel
to the fuel cell. Thus, at least a portion of the hydrogen
generated by the fuel cell is indirectly recycled to the fuel cell
anode inlet in the form of the methane generated by the
gasifier.
[0010] An article in the Journal of Fuel Cell Science and
Technology (G. Manzolini et. al., J. Fuel Cell Sci. and Tech., Vol.
9, February 2012) describes a power generation system that combines
a combustion power generator with molten carbonate fuel cells.
Various arrangements of fuel cells and operating parameters are
described. The combustion output from the combustion generator is
used in part as the input for the cathode of the fuel cell. One
goal of the simulations in the Manzolini article is to use the MCFC
to separate CO.sub.2 from the power generator's exhaust. The
simulation described in the Manzolini article establishes a maximum
outlet temperature of 660.degree. C. and notes that the inlet
temperature must be sufficiently cooler to account for the
temperature increase across the fuel cell. The electrical
efficiency (i.e. electricity generated/fuel input) for the MCFC
fuel cell in a base model case is 50%. The electrical efficiency in
a test model case, which is optimized for CO.sub.2 sequestration,
is also 50%.
[0011] An article by Desideri et al. (Intl. J. of Hydrogen Energy,
Vol. 37, 2012) describes a method for modeling the performance of a
power generation system using a fuel cell for CO.sub.2 separation.
Recirculation of anode exhaust to the anode inlet and the cathode
exhaust to the cathode inlet are used to improve the performance of
the fuel cell. The model parameters describe an MCFC electrical
efficiency of 50.3%.
SUMMARY OF THE INVENTION
[0012] In an aspect, a method for producing electricity is
provided. The method includes introducing a fuel stream comprising
a reformable fuel into the anode of a molten carbonate fuel cell,
an internal reforming element associated with the anode, or a
combination thereof; introducing a cathode inlet stream comprising
CO.sub.2 and O.sub.2 into the cathode of the fuel cell; generating
electricity within the molten carbonate fuel cell, the molten
carbonate fuel cell being operated at a fuel utilization of about
60% or less; generating an anode exhaust comprising H.sub.2, CO,
and CO.sub.2; separating, from at least a portion of the anode
exhaust, a gas stream comprising at least about 80 vol % H.sub.2;
and combusting at least a portion of the separated gas stream to
produce electricity.
BRIEF DESCRIPTION OF THE FIGURES
[0013] FIG. 1 schematically shows an example of a configuration for
molten carbonate fuel cells and associated reforming and separation
stages.
[0014] FIG. 2 schematically shows another example of a
configuration for molten carbonate fuel cells and associated
reforming and separation stages.
[0015] FIG. 3 schematically shows an example of the operation of a
molten carbonate fuel cell.
[0016] FIG. 4 schematically shows an example of a combined cycle
system for generating electricity based on combustion of a
carbon-based fuel.
[0017] FIG. 5 schematically shows an example of a combined cycle
system for generating electricity based on combustion of a
carbon-based fuel.
[0018] FIG. 6 schematically shows an example of a configuration for
generating electricity.
[0019] FIG. 7 shows results of simulations of a system for
generating electricity.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Overview
[0020] In various aspects, the operation of molten carbonate fuel
cells can be integrated with a variety of chemical and/or materials
production processes, including but not limited to providing a
chemical output that can be used to generate further electrical
power. For example, at least a portion of the anode exhaust can be
used as a fuel for a turbine powered by combustion of hydrogen.
Using an MCFC to provide hydrogen for a hydrogen turbine can allow
for enhanced power generation, while optionally but advantageously
reducing or minimizing the amount of greenhouse gases generated
and/or exhausted to the environment.
Integration with a Hydrogen Turbine
[0021] In some aspects, an objective in producing low-carbon power
can be to increase or maximize the total power output while
maintaining high CO.sub.2 capture efficiency and/or while
efficiently utilizing existing systems. In a conventional system, a
gas turbine can be connected to a MCFC system such that the gas
turbine produces an exhaust stream comprising CO.sub.2 that serves
as a component of the cathode inlet providing heat and CO.sub.2 to
the cathode. For this configuration, as is known in the literature,
CO.sub.2 can be captured by conventional means and the MCFC system
can be operated at relatively high fuel utilizations (typically
above 70% to about 80%, or about 75%) to maintain heat balance
within the MCFC under ordinary operating conditions.
[0022] The efficient utilization of the MCFC can be improved by
lowering the fuel utilization to process excess fuel, for example
methane, and produce excess syngas as an exhaust. This
exhaust/effluent can undergo various separations to yield a syngas
stream that can be useful for a variety of chemical and industrial
purposes. However, where syngas is not useful as a feed stock,
and/or for cases where electrical power generation can be a primary
goal, generation of a syngas stream may not provide additional
low-carbon power.
[0023] In various aspects, systems and methods are provided for
producing an increased or maximized amount of electrical power from
a fixed MCFC system while optionally but preferably providing for
consistent high carbon capture. In some aspects, such a system can
be provided by combining the use of a conventional gas turbine as
the CO.sub.2 source for the MCFC cathode, a low fuel utilization
for production of high amounts of syngas, and a combination of
separation and/or conversion systems that can allow for increased
production of hydrogen derived from the MCFC anode exhaust. This
hydrogen stream derived from the anode exhaust can then be
introduced into a second hydrogen turbine where additional power
can be generated with reduced or minimized emissions of CO.sub.2.
Because the second turbine can be powered by the
hydrogen-containing stream derived from the anode exhaust, the
amount of additional CO.sub.2 generated in order to power the
second turbine can be limited to, for example, carbon oxide(s) and
carbon fuel residual components in the hydrogen-containing stream.
Additionally or alternatively, the hydrogen from the anode exhaust
can be used to generate electricity in other manners, such as by
combusting the hydrogen to raise steam, which can then be used to
generate electricity. Further additionally or alternately, a
portion of the hydrogen derived from the anode exhaust can be used
as an input for the first (conventional) turbine. This can be
beneficial, for example, if the carbon-containing fuel for the
first turbine has an elevated content of inerts, such as CO.sub.2
and/or N.sub.2.
[0024] In addition to use within a refinery, hydrogen can more
generally be useful for a wide variety of products and processes,
as it produces only water vapor on combustion. However, most
conventional approaches to making hydrogen can require large
emissions of carbon. For example, production of hydrogen from steam
reforming of methane can produce CO.sub.2 (from the carbon in the
methane) and waste heat. Production of hydrogen from electrolysis
can require electricity, which is typically generated for the
electrical grid based on combustion of a mixture of fossil fuels.
These production systems can all typically result in effluent
exhausts comprising CO.sub.2. Applications such as fuel cell
vehicles can require low-temperature fuel cells which utilize high
purity hydrogen. While the vehicle does not produce much carbon
emissions, the production of the hydrogen for the vehicle can be
inefficient, not easily adapted to smaller-scale, and can produce
significant carbon emissions.
[0025] In some additional or alternative aspects, the systems and
methods herein can facilitate separation of CO.sub.2 efficiently
from the process as an integral part of the separation and hydrogen
purification steps. The CO.sub.2 can then be captured and/or used
for other useful processes. This can occur at high overall system
efficiency, as compared to conventional means of producing net
hydrogen production/export, especially at small scale and under
variable load.
[0026] The use of a MCFC system for hydrogen production for use in
subsequent processes that may generate electrical power and/or heat
can allow for low-emission energy production at high efficiency and
with reduced or minimized carbon emissions. The MCFC system can
dynamically respond to varying needs for hydrogen by adjusting the
ratio of chemical energy production to electrical energy production
and can be suitable for uses where loads and demands may not be
constant--varying from heightened electrical production with little
or no excess hydrogen to heightened hydrogen production.
Additionally, the integrated system can be scaled over a wider
range of applications with high efficiency, as compared with larger
scale systems such as methane steam reformers. This could allow,
for example, for co-production of hydrogen for other uses, such as
a fuel cell vehicle system, and for variable electrical power or
simply to vary electrical power output.
[0027] For example, in some operating configurations, the base gas
turbine (such as a turbine powered by combustion of a
carbon-containing fuel) can be run at constant high-efficiency
conditions with the MCFC system run under variable fuel
utilizations to yield different electrical and hydrogen production
values, which can control the electrical output from the entire
system. The amount of hydrogen and electricity can be varied
depending on overall demand while maintaining overall high system
efficiency. Hydrogen for export into boilers and/or other combined
heat and power systems can allow for the constant production of
electricity, e.g., in stand-alone generation, along with a variable
amount of carbon-free heat via the production of hydrogen with
subsequent combustion in heater/boiler and/or similar systems. For
example, an installation could produce primarily electricity in the
summer for air conditioning systems while switching to a mix of
primarily chemical energy in the winter for heating operations.
During high electrical demand, increased hydrogen can be sent to
the hydrogen turbine for maximized electrical production. Other
applications can include systems designed to provide on-site
hydrogen such as in laboratories and other technical and
manufacturing facilities, where some hydrogen can be needed along
with a need for electrical energy.
[0028] In aspects involving hydrogen production and/or electrical
power generation, the anode inlet can be fed by fresh methane,
another suitable hydrocarbon fuel, and/or the combination of fresh
fuel and recycled CO and/or H.sub.2 from the various processes. The
anode outlet stream comprising H.sub.2 and/or CO can provide the
components to produce hydrogen. This is typically done through some
combination of reaction, separation, and purification steps. An
example can be a first stage employing water-gas shift to shift as
much as possible (nearly all) of the CO to H.sub.2 by the reaction
H.sub.2O+CO.dbd.H.sub.2+CO.sub.2, followed by a second (and
potentially subsequent) stage(s) that can remove H.sub.2O and/or
CO.sub.2 from the H.sub.2, and can provide a suitable purity
product. Such stages can include PSA, cryogenic separation,
membranes, and/or other known separation methods, either
individually or in combination. The off-gasses from these steps can
be recycled and/or can be used to provide heat for inlet streams.
The separated CO.sub.2 can be used as a recycle stream and/or can
be captured and optionally used for other processes. The cathode
inlet can be composed of recycled CO.sub.2 from the overall process
and/or of CO.sub.2 produced by the combustion of fresh (or
recycled) fuel used to provide heat to the inlet streams. In some
preferred aspects, the cathode inlet can include at least a portion
of a combustion exhaust from a first conventional combustion
turbine. The cathode effluent can typically be exhausted to the
atmosphere, optionally but preferably after heat recovery to, for
example, provide heat for other process streams and/or in combined
cycle electrical production, though the cathode effluent could
optionally but less preferably be sent for further treatment, if
desired.
[0029] MCFC systems integrated into carbon-free heat and power
applications can be used over a range of operating conditions
comprising high (e.g., from about 60% to about 70%) fuel
utilization with low hydrogen make to low fuel utilizations (e.g.,
from about 20% to about 60%) with increased hydrogen production.
Examples of low fuel utilizations can include at least about 20%,
such as at least about 30%, and/or about 60% or less, such as about
50% or less. The exact operational range of an individual
application can vary widely both by application and over time. The
ability to adapt to this operational range can be a desirable
advantage. The number of separation stages and the purity achieved
can depend on the ultimate application. Simple production of
hydrogen for low-emissions heat can be tolerant to modest
impurities in the hydrogen, as even a few percent CO.sub.2 and/or
CO in emitted streams could still result in very low overall carbon
emissions. High purification applications such as fuel cell
vehicles and/or hydrogen for laboratories can require multiple
steps (e.g. cryogenic separation, followed by PSA) to achieve
purity specifications.
[0030] In other configurations, the MCFC can be operated under
lower fuel utilizations with the excess anode outlet fuel used to
provide heat and/or power. In both cases, an advantage can be that
the "base load" power output of the fixed gas turbine and MCFC
systems can remain approximately constant, while a small
sub-section of the overall process, the hydrogen turbine, can be
used for variable load. The combination of variable fuel
utilization and variable hydrogen turbine feed can allow the
overall plant to meet a large variety of needs for heat,
electricity, and/or hydrogen demand, while operating the major
subsystems of the plant at fairly consistent operating
conditions.
[0031] FIG. 6 shows an example flow sheet of an integrated power
generation MCFC process that can produce low CO.sub.2 emissions
power from a convenient carbon-containing fuel, such as natural gas
and/or methane. In this scheme a natural gas fired turbine 640 can
combust oxidant (air) 601 and methane/natural gas 602 to generate
power and exhaust gas stream 603. Exhaust 603 can be fed into a
cathode of MCFC 630. The anode of MCFC 630 can be fed additional
fuel (methane/natural gas) 605 and steam 606. Through an
electrochemical reaction, the MCFC 630 can generate power, can
produce a CO.sub.2 depleted cathode exhaust 604, and can produce an
anode exhaust 607 that contains H.sub.2/CO.sub.2/CO. Heat can be
recovered from cathode exhaust 604, and then cathode exhaust 604
can be emitted to the atmosphere and/or subject to further
treatment, if desired. Anode exhaust 607 can be shifted in a water
gas shift reactor 660 to increase the H.sub.2 concentration. The
shift reactor effluent can undergo separations 670 to remove water
608, recover CO.sub.2 609, and form a separated stream 610
containing H.sub.2. CO.sub.2-containing stream 609 can be
compressed to pipeline conditions and can then be sold for use,
used for a different process, and/or sequestered. The separated
stream 610 can be combined with oxidant (air) 611 and sent to a
hydrogen turbine 650 to generate additional power. The exhaust 612
from the hydrogen turbine 650 can be mostly water and N.sub.2 and
can be emitted to the atmosphere and/or subject to further
treatment, if desired.
[0032] As an example, simulations were performed using a
configuration similar to the system shown in FIG. 6. Comparative
simulations were also performed for systems where a hydrogen
turbine was not included. In the comparative simulations, fuel
(comprising hydrogen) from the anode exhaust was instead recycled
to the combustion zone for the conventional turbine. It is noted
that the size of the conventional turbine was held constant in the
simulations, so recycle of fuel from the anode exhaust resulted in
a reduction in the amount of fresh natural gas delivered to the
conventional turbine.
[0033] The results from the simulations are shown in FIG. 7. The
results in FIG. 7 appeared to show that, at conventional fuel
utilizations, such as a fuel utilization of .about.75%, use of a
second hydrogen turbine may not be as beneficial. At a fuel
utilization of .about.75%, the results appeared to show that using
the second hydrogen turbine can reduce the overall efficiency of
generation of electrical power while also reducing the net power
produced.
[0034] For a fuel utilization of about 50%, the simulation results
in FIG. 7 appeared to show the benefits of operating with a second
hydrogen turbine. The overall efficiency of the system with the
second hydrogen turbine still appeared to be lower, as the overall
efficiency was about 50% versus the .about.55% that was simulated
for the comparative example. However, the simulations appeared to
show that operating at about 50% fuel utilization resulted in a
larger amount of power produced (about 624 MW) than any of the
comparative examples, while also appearing to have lower emissions
of CO.sub.2 per MWhr (about 144 lbs/MWhr) relative to any of the
comparative examples. The simulations at a fuel utilization of
about 30% appeared to show even greater benefits of producing large
hydrogen volumes by operating at lower fuel utilization. The
simulations appeared to show significantly increased power
production (about 790 MW) while also significantly reducing the
amount of CO.sub.2 emissions (about 113 lbs/MWhr). The combination
of increased power and reduced CO.sub.2 emissions appeared to be
achieved in the simulations in part due to the increased amount of
CO.sub.2 captured (about 1.92 Mtons/yr at about 50% fuel
utilization, about 2.56 Mtons/year at about 30% fuel utilization).
As a result, the simulations appeared to show that use of a
hydrogen turbine, in combination with a fuel utilization of about
60% or less, such as about 50% or less, can provide unexpectedly
low CO.sub.2 emissions per unit of energy generated while providing
increased electrical energy production.
Additional Fuel Cell Operation Strategies
[0035] As an addition, complement, and/or alternative to the fuel
cell operating strategies described herein, a molten carbonate fuel
cell can be operated so that the amount of reforming can be
selected relative to the amount of oxidation in order to achieve a
desired thermal ratio for the fuel cell. As used herein, the
"thermal ratio" is defined as the heat produced by exothermic
reactions in a fuel cell assembly divided by the endothermic heat
demand of reforming reactions occurring within the fuel cell
assembly. Expressed mathematically, the thermal ratio
(TH)=Q.sub.EX/Q.sub.EN, where Q.sub.EX is the sum of heat produced
by exothermic reactions and Q.sub.EN is the sum of heat consumed by
the endothermic reactions occurring within the fuel cell. Note that
the heat produced by the exothermic reactions corresponds to any
heat due to reforming reactions, water gas shift reactions, and the
electrochemical reactions in the cell. The heat generated by the
electrochemical reactions can be calculated based on the ideal
electrochemical potential of the fuel cell reaction across the
electrolyte minus the actual output voltage of the fuel cell. For
example, the ideal electrochemical potential of the reaction in a
MCFC is believed to be about 1.04V based on the net reaction that
occurs in the cell. During operation of the MCFC, the cell will
typically have an output voltage less than 1.04 V due to various
losses. For example, a common output/operating voltage can be about
0.7 V. The heat generated is equal to the electrochemical potential
of the cell (i.e. .about.1.04V) minus the operating voltage. For
example, the heat produced by the electrochemical reactions in the
cell is .about.0.34 V when the output voltage of .about.0.7V. Thus,
in this scenario, the electrochemical reactions would produce
.about.0.7 V of electricity and .about.0.34 V of heat energy. In
such an example, the .about.0.7 V of electrical energy is not
included as part of Q.sub.EX. In other words, heat energy is not
electrical energy.
[0036] In various aspects, a thermal ratio can be determined for
any convenient fuel cell structure, such as a fuel cell stack, an
individual fuel cell within a fuel cell stack, a fuel cell stack
with an integrated reforming stage, a fuel cell stack with an
integrated endothermic reaction stage, or a combination thereof.
The thermal ratio may also be calculated for different units within
a fuel cell stack, such as an assembly of fuel cells or fuel cell
stacks. For example, the thermal ratio may be calculated for a
single anode within a single fuel cell, an anode section within a
fuel cell stack, or an anode section within a fuel cell stack along
with integrated reforming stages and/or integrated endothermic
reaction stage elements in sufficiently close proximity to the
anode section to be integrated from a heat integration standpoint.
As used herein, "an anode section" comprises anodes within a fuel
cell stack that share a common inlet or outlet manifold.
[0037] In various aspects of the invention, the operation of the
fuel cells can be characterized based on a thermal ratio. Where
fuel cells are operated to have a desired thermal ratio, a molten
carbonate fuel cell can be operated to have a thermal ratio of
about 1.5 or less, for example about 1.3 or less, or about 1.15 or
less, or about 1.0 or less, or about 0.95 or less, or about 0.90 or
less, or about 0.85 or less, or about 0.80 or less, or about 0.75
or less. Additionally or alternately, the thermal ratio can be at
least about 0.25, or at least about 0.35, or at least about 0.45,
or at least about 0.50. Additionally or alternately, in some
aspects the fuel cell can be operated to have a temperature rise
between anode input and anode output of about 40.degree. C. or
less, such as about 20.degree. C. or less, or about 10.degree. C.
or less. Further additionally or alternately, the fuel cell can be
operated to have an anode outlet temperature that is from about
10.degree. C. lower to about 10.degree. C. higher than the
temperature of the anode inlet. Still further additionally or
alternately, the fuel cell can be operated to have an anode inlet
temperature that is greater than the anode outlet temperature, such
as at least about 5.degree. C. greater, or at least about
10.degree. C. greater, or at least about 20.degree. C. greater, or
at least about 25.degree. C. greater. Yet still further
additionally or alternately, the fuel cell can be operated to have
an anode inlet temperature that is greater than the anode outlet
temperature by about 100.degree. C. or less, such as by about
80.degree. C. or less, or about 60.degree. C. or less, or about
50.degree. C. or less, or about 40.degree. C. or less, or about
30.degree. C. or less, or about 20.degree. C. or less.
[0038] As an addition, complement, and/or alternative to the fuel
cell operating strategies described herein, a molten carbonate fuel
cell (such as a fuel cell assembly) can be operated with increased
production of syngas (or hydrogen) while also reducing or
minimizing the amount of CO.sub.2 exiting the fuel cell in the
cathode exhaust stream. Syngas can be a valuable input for a
variety of processes. In addition to having fuel value, syngas can
be used as a raw material for forming other higher value products,
such as by using syngas as an input for Fischer-Tropsch synthesis
and/or methanol synthesis processes. One option for making syngas
can be to reform a hydrocarbon or hydrocarbon-like fuel, such as
methane or natural gas. For many types of industrial processes, a
syngas having a ratio of H.sub.2 to CO of close to 2:1 (or even
lower) can often be desirable. A water gas shift reaction can be
used to reduce the H.sub.2 to CO ratio in a syngas if additional
CO.sub.2 is available, such as is produced in the anodes.
[0039] One way of characterizing the overall benefit provided by
integrating syngas generation with use of molten carbonate fuel
cells can be based on a ratio of the net amount of syngas that
exits the fuel cells in the anode exhaust relative to the amount of
CO.sub.2 that exits the fuel cells in the cathode exhaust. This
characterization measures the effectiveness of producing power with
low emissions and high efficiency (both electrical and chemical).
In this description, the net amount of syngas in an anode exhaust
is defined as the combined number of moles of H.sub.2 and number of
moles of CO present in the anode exhaust, offset by the amount of
H.sub.2 and CO present in the anode inlet. Because the ratio is
based on the net amount of syngas in the anode exhaust, simply
passing excess H.sub.2 into the anode does not change the value of
the ratio. However, H.sub.2 and/or CO generated due to reforming in
the anode and/or in an internal reforming stage associated with the
anode can lead to higher values of the ratio. Hydrogen oxidized in
the anode can lower the ratio. It is noted that the water gas shift
reaction can exchange H.sub.2 for CO, so the combined moles of
H.sub.2 and CO represents the total potential syngas in the anode
exhaust, regardless of the eventual desired ratio of H.sub.2 to CO
in a syngas. The syngas content of the anode exhaust (H.sub.2+CO)
can then be compared with the CO.sub.2 content of the cathode
exhaust. This can provide a type of efficiency value that can also
account for the amount of carbon capture. This can equivalently be
expressed as an equation as
Ratio of net syngas in anode exhaust to cathode CO.sub.2=net moles
of (H.sub.2+CO).sub.ANODE/moles of (CO.sub.2).sub.CATHODE
[0040] In various aspects, the ratio of net moles of syngas in the
anode exhaust to the moles of CO.sub.2 in the cathode exhaust can
be at least about 2.0, such as at least about 3.0, or at least
about 4.0, or at least about 5.0. In some aspects, the ratio of net
syngas in the anode exhaust to the amount of CO.sub.2 in the
cathode exhaust can be still higher, such as at least about 10.0,
or at least about 15.0, or at least about 20.0. Ratio values of
about 40.0 or less, such as about 30.0 or less, or about 20.0 or
less, can additionally or alternately be achieved. In aspects where
the amount of CO.sub.2 at the cathode inlet is about 6.0 volume %
or less, such as about 5.0 volume % or less, ratio values of at
least about 1.5 may be sufficient/realistic. Such molar ratio
values of net syngas in the anode exhaust to the amount of CO.sub.2
in the cathode exhaust can be greater than the values for
conventionally operated fuel cells.
[0041] As an addition, complement, and/or alternative to the fuel
cell operating strategies described herein, a molten carbonate fuel
cell (such as a fuel cell assembly) can be operated at a reduced
fuel utilization value, such as a fuel utilization of about 50% or
less, while also having a high CO.sub.2 utilization value, such as
at least about 60%. In this type of configuration, the molten
carbonate fuel cell can be effective for carbon capture, as the
CO.sub.2 utilization can advantageously be sufficiently high.
Rather than attempting to maximize electrical efficiency, in this
type of configuration the total efficiency of the fuel cell can be
improved or increased based on the combined electrical and chemical
efficiency. The chemical efficiency can be based on withdrawal of a
hydrogen and/or syngas stream from the anode exhaust as an output
for use in other processes. Even though the electrical efficiency
may be reduced relative to some conventional configurations, making
use of the chemical energy output in the anode exhaust can allow
for a desirable total efficiency for the fuel cell.
[0042] In various aspects, the fuel utilization in the fuel cell
anode can be about 50% or less, such as about 40% or less, or about
30% or less, or about 25% or less, or about 20% or less. In various
aspects, in order to generate at least some electric power, the
fuel utilization in the fuel cell can be at least about 5%, such as
at least about 10%, or at least about 15%, or at least about 20%,
or at least about 25%, or at least about 30%. Additionally or
alternatively, the CO.sub.2 utilization can be at least about 60%,
such as at least about 65%, or at least about 70%, or at least
about 75%.
[0043] As an addition, complement, and/or alternative to the fuel
cell operating strategies described herein, a molten carbonate fuel
cell can be operated at conditions that increase or maximize syngas
production, possibly at the detriment of electricity production and
electrical efficiency. Instead of selecting the operating
conditions of a fuel cell to improve or maximize the electrical
efficiency of the fuel cell, operating conditions, possibly
including an amount of reformable fuel passed into the anode, can
be established to increase the chemical energy output of the fuel
cell. These operating conditions can result in a lower electrical
efficiency of the fuel cell. Despite the reduced electrical
efficiency, optionally, but preferably, the operating conditions
can lead to an increase in the total efficiency of the fuel cell,
which is based on the combined electrical efficiency and chemical
efficiency of the fuel cell. By increasing the ratio of reformable
fuel introduced into the anode to the fuel that is actually
electrochemically oxidized at the anode, the chemical energy
content in the anode output can be increased.
[0044] In some aspects, the reformable hydrogen content of
reformable fuel in the input stream delivered to the anode and/or
to a reforming stage associated with the anode can be at least
about 50% greater than the net amount of hydrogen reacted at the
anode, such as at least about 75% greater or at least about 100%
greater. Additionally or alternately, the reformable hydrogen
content of fuel in the input stream delivered to the anode and/or
to a reforming stage associated with the anode can be at least
about 50% greater than the net amount of hydrogen reacted at the
anode, such as at least about 75% greater or at least about 100%
greater. In various aspects, a ratio of the reformable hydrogen
content of the reformable fuel in the fuel stream relative to an
amount of hydrogen reacted in the anode can be at least about
1.5:1, or at least about 2.0:1, or at least about 2.5:1, or at
least about 3.0:1. Additionally or alternately, the ratio of
reformable hydrogen content of the reformable fuel in the fuel
stream relative to the amount of hydrogen reacted in the anode can
be about 20:1 or less, such as about 15:1 or less or about 10:1 or
less. In one aspect, it is contemplated that less than 100% of the
reformable hydrogen content in the anode inlet stream can be
converted to hydrogen. For example, at least about 80% of the
reformable hydrogen content in an anode inlet stream can be
converted to hydrogen in the anode and/or in an associated
reforming stage(s), such as at least about 85%, or at least about
90%. Additionally or alternately, the amount of reformable fuel
delivered to the anode can be characterized based on the Lower
Heating Value (LHV) of the reformable fuel relative to the LHV of
the hydrogen oxidized in the anode. This can be referred to as a
reformable fuel surplus ratio. In various aspects, the reformable
fuel surplus ratio can be at least about 2.0, such as at least
about 2.5, or at least about 3.0, or at least about 4.0.
Additionally or alternately, the reformable fuel surplus ratio can
be about 25.0 or less, such as about 20.0 or less, or about 15.0 or
less, or about 10.0 or less.
[0045] As an addition, complement, and/or alternative to the fuel
cell operating strategies described herein, a molten carbonate fuel
cell (such as a fuel cell assembly) can also be operated at
conditions that can improve or optimize the combined electrical
efficiency and chemical efficiency of the fuel cell. Instead of
selecting conventional conditions for maximizing the electrical
efficiency of a fuel cell, the operating conditions can allow for
output of excess synthesis gas and/or hydrogen in the anode exhaust
of the fuel cell. The synthesis gas and/or hydrogen can then be
used in a variety of applications, including chemical synthesis
processes and collection of hydrogen for use as a "clean" fuel. In
aspects of the invention, electrical efficiency can be reduced to
achieve a high overall efficiency, which includes a chemical
efficiency based on the chemical energy value of syngas and/or
hydrogen produced relative to the energy value of the fuel input
for the fuel cell.
[0046] In some aspects, the operation of the fuel cells can be
characterized based on electrical efficiency. Where fuel cells are
operated to have a low electrical efficiency (EE), a molten
carbonate fuel cell can be operated to have an electrical
efficiency of about 40% or less, for example, about 35% EE or less,
about 30% EE or less, about 25% EE or less, or about 20% EE or
less, about 15% EE or less, or about 10% EE or less. Additionally
or alternately, the EE can be at least about 5%, or at least about
10%, or at least about 15%, or at least about 20%. Further
additionally or alternately, the operation of the fuel cells can be
characterized based on total fuel cell efficiency (TFCE), such as a
combined electrical efficiency and chemical efficiency of the fuel
cell(s). Where fuel cells are operated to have a high total fuel
cell efficiency, a molten carbonate fuel cell can be operated to
have a TFCE (and/or combined electrical efficiency and chemical
efficiency) of about 55% or more, for example, about 60% or more,
or about 65% or more, or about 70% or more, or about 75% or more,
or about 80% or more, or about 85% or more. It is noted that for a
total fuel cell efficiency and/or combined electrical efficiency
and chemical efficiency, any additional electricity generated from
use of excess heat generated by the fuel cell can be excluded from
the efficiency calculation.
[0047] In various aspects of the invention, the operation of the
fuel cells can be characterized based on a desired electrical
efficiency of about 40% or less and a desired total fuel cell
efficiency of about 55% or more. Where fuel cells are operated to
have a desired electrical efficiency and a desired total fuel cell
efficiency, a molten carbonate fuel cell can be operated to have an
electrical efficiency of about 40% or less with a TFCE of about 55%
or more, for example, about 35% EE or less with about a TFCE of 60%
or more, about 30% EE or less with about a TFCE of about 65% or
more, about 25% EE or less with about a 70% TFCE or more, or about
20% EE or less with about a TFCE of 75% or more, about 15% EE or
less with about a TFCE of 80% or more, or about 10% EE or less with
about a TFCE of about 85% or more.
[0048] As an addition, complement, and/or alternative to the fuel
cell operating strategies described herein, a molten carbonate fuel
cell (such as a fuel cell assembly) can be operated at conditions
that can provide increased power density. The power density of a
fuel cell corresponds to the actual operating voltage V.sub.A
multiplied by the current density I. For a molten carbonate fuel
cell operating at a voltage V.sub.A, the fuel cell also can tend to
generate waste heat, the waste heat defined as (V.sub.0-V.sub.A)*I
based on the differential between V.sub.A and the ideal voltage
V.sub.0 for a fuel cell providing current density I. A portion of
this waste heat can be consumed by reforming of a reformable fuel
within the anode of the fuel cell. The remaining portion of this
waste heat can be absorbed by the surrounding fuel cell structures
and gas flows, resulting in a temperature differential across the
fuel cell. Under conventional operating conditions, the power
density of a fuel cell can be limited based on the amount of waste
heat that the fuel cell can tolerate without compromising the
integrity of the fuel cell.
[0049] In various aspects, the amount of waste heat that a fuel
cell can tolerate can be increased by performing an effective
amount of an endothermic reaction within the fuel cell. One example
of an endothermic reaction includes steam reforming of a reformable
fuel within a fuel cell anode and/or in an associated reforming
stage, such as an integrated reforming stage in a fuel cell stack.
By providing additional reformable fuel to the anode of the fuel
cell (or to an integrated/associated reforming stage), additional
reforming can be performed so that additional waste heat can be
consumed. This can reduce the amount of temperature differential
across the fuel cell, thus allowing the fuel cell to operate under
an operating condition with an increased amount of waste heat. The
loss of electrical efficiency can be offset by the creation of an
additional product stream, such as syngas and/or H.sub.2, that can
be used for various purposes including additional electricity
generation further expanding the power range of the system.
[0050] In various aspects, the amount of waste heat generated by a
fuel cell, (V.sub.0-V.sub.A)*I as defined above, can be at least
about 30 mW/cm.sup.2, such as at least about 40 mW/cm.sup.2, or at
least about 50 mW/cm.sup.2, or at least about 60 mW/cm.sup.2, or at
least about 70 mW/cm.sup.2, or at least about 80 mW/cm.sup.2, or at
least about 100 mW/cm.sup.2, or at least about 120 mW/cm.sup.2, or
at least about 140 mW/cm.sup.2, or at least about 160 mW/cm.sup.2,
or at least about 180 mW/cm.sup.2. Additionally or alternately, the
amount of waste heat generated by a fuel cell can be less than
about 250 mW/cm.sup.2, such as less than about 200 mW/cm.sup.2, or
less than about 180 mW/cm.sup.2, or less than about 165
mW/cm.sup.2, or less than about 150 mW/cm.sup.2.
[0051] Although the amount of waste heat being generated can be
relatively high, such waste heat may not necessarily represent
operating a fuel cell with poor efficiency. Instead, the waste heat
can be generated due to operating a fuel cell at an increased power
density. Part of improving the power density of a fuel cell can
include operating the fuel cell at a sufficiently high current
density. In various aspects, the current density generated by the
fuel cell can be at least about 150 mA/cm.sup.2, such as at least
about 160 mA/cm.sup.2, or at least about 170 mA/cm.sup.2, or at
least about 180 mA/cm.sup.2, or at least about 190 mA/cm.sup.2, or
at least about 200 mA/cm.sup.2, or at least about 225 mA/cm.sup.2,
or at least about 250 mA/cm.sup.2. Additionally or alternately, the
current density generated by the fuel cell can be about 500
mA/cm.sup.2 or less, such as 450 mA/cm.sup.2, or less, or 400
mA/cm.sup.2, or less or 350 mA/cm.sup.2, or less or 300 mA/cm.sup.2
or less.
[0052] In various aspects, to allow a fuel cell to be operated with
increased power generation and increased generation of waste heat,
an effective amount of an endothermic reaction (such as a reforming
reaction) can be performed. Alternatively, other endothermic
reactions unrelated to anode operations can be used to utilize the
waste heat by interspersing "plates" or stages into the fuel cell
array that are in thermal communication but not fluid
communication. The effective amount of the endothermic reaction can
be performed in an associated reforming stage, an integrated
reforming stage, an integrated stack element for performing an
endothermic reaction, or a combination thereof. The effective
amount of the endothermic reaction can correspond to an amount
sufficient to reduce the temperature rise from the fuel cell inlet
to the fuel cell outlet to about 100.degree. C. or less, such as
about 90.degree. C. or less, or about 80.degree. C. or less, or
about 70.degree. C. or less, or about 60.degree. C. or less, or
about 50.degree. C. or less, or about 40.degree. C. or less, or
about 30.degree. C. or less. Additionally or alternately, the
effective amount of the endothermic reaction can correspond to an
amount sufficient to cause a temperature decrease from the fuel
cell inlet to the fuel cell outlet of about 100.degree. C. or less,
such as about 90.degree. C. or less, or about 80.degree. C. or
less, or about 70.degree. C. or less, or about 60.degree. C. or
less, or about 50.degree. C. or less, or about 40.degree. C. or
less, or about 30.degree. C. or less, or about 20.degree. C. or
less, or about 10.degree. C. or less. A temperature decrease from
the fuel cell inlet to the fuel cell outlet can occur when the
effective amount of the endothermic reaction exceeds the waste heat
generated. Additionally or alternately, this can correspond to
having the endothermic reaction(s) (such as a combination of
reforming and another endothermic reaction) consume at least about
40% of the waste heat generated by the fuel cell, such as consuming
at least about 50% of the waste heat, or at least about 60% of the
waste heat, or at least about 75% of the waste heat. Further
additionally or alternately, the endothermic reaction(s) can
consume about 95% of the waste heat or less, such as about 90% of
the waste heat or less, or about 85% of the waste heat or less.
[0053] As an addition, complement, and/or alternative to the fuel
cell operating strategies described herein, a molten carbonate fuel
cell (such as a fuel cell assembly) can be operated at conditions
corresponding to a decreased operating voltage and a low fuel
utilization. In various aspects, the fuel cell can be operated at a
voltage V.sub.A of less than about 0.7 Volts, for example less than
about 0.68 V, less than about 0.67 V, less than about 0.66 V, or
about 0.65 V or less. Additionally or alternatively, the fuel cell
can be operated at a voltage V.sub.A of at least about 0.60, for
example at least about 0.61, at least about 0.62, or at least about
0.63. In so doing, energy that would otherwise leave the fuel cell
as electrical energy at high voltage can remain within the cell as
heat as the voltage is lowered. This additional heat can allow for
increased endothermic reactions to occur, for example increasing
the CH.sub.4 conversion to syngas.
DEFINITIONS
[0054] Syngas: In this description, syngas is defined as mixture of
H.sub.2 and CO in any ratio. Optionally, H.sub.2O and/or CO.sub.2
may be present in the syngas. Optionally, inert compounds (such as
nitrogen) and residual reformable fuel compounds may be present in
the syngas. If components other than H.sub.2 and CO are present in
the syngas, the combined volume percentage of H.sub.2 and CO in the
syngas can be at least 25 vol % relative to the total volume of the
syngas, such as at least 40 vol %, or at least 50 vol %, or at
least 60 vol %. Additionally or alternately, the combined volume
percentage of H.sub.2 and CO in the syngas can be 100 vol % or
less, such as 95 vol % or less or 90 vol % or less.
[0055] Reformable fuel: A reformable fuel is defined as a fuel that
contains carbon-hydrogen bonds that can be reformed to generate
H.sub.2. Hydrocarbons are examples of reformable fuels, as are
other hydrocarbonaceous compounds such as alcohols. Although CO and
H.sub.2O can participate in a water gas shift reaction to form
hydrogen, CO is not considered a reformable fuel under this
definition.
[0056] Reformable hydrogen content: The reformable hydrogen content
of a fuel is defined as the number of H.sub.2 molecules that can be
derived from a fuel by reforming the fuel and then driving the
water gas shift reaction to completion to maximize H.sub.2
production. It is noted that H.sub.2 by definition has a reformable
hydrogen content of 1, although H.sub.2 itself is not defined as a
reformable fuel herein. Similarly, CO has a reformable hydrogen
content of 1. Although CO is not strictly reformable, driving the
water gas shift reaction to completion will result in exchange of a
CO for an H.sub.2. As examples of reformable hydrogen content for
reformable fuels, the reformable hydrogen content of methane is 4
H.sub.2 molecules while the reformable hydrogen content of ethane
is 7 H.sub.2 molecules. More generally, if a fuel has the
composition CxHyOz, then the reformable hydrogen content of the
fuel at 100% reforming and water-gas shift is n(H.sub.2 max
reforming)=2x+y/2-z. Based on this definition, fuel utilization
within a cell can then be expressed as n(H.sub.2 ox)/n(H.sub.2 max
reforming) Of course, the reformable hydrogen content of a mixture
of components can be determined based on the reformable hydrogen
content of the individual components. The reformable hydrogen
content of compounds that contain other heteroatoms, such as
oxygen, sulfur or nitrogen, can also be calculated in a similar
manner.
[0057] Oxidation Reaction: In this discussion, the oxidation
reaction within the anode of a fuel cell is defined as the reaction
corresponding to oxidation of H.sub.2 by reaction with
CO.sub.3.sup.2- to form H.sub.2O and CO.sub.2. It is noted that the
reforming reaction within the anode, where a compound containing a
carbon-hydrogen bond is converted into H.sub.2 and CO or CO.sub.2,
is excluded from this definition of the oxidation reaction in the
anode. The water-gas shift reaction is similarly outside of this
definition of the oxidation reaction. It is further noted that
references to a combustion reaction are defined as references to
reactions where H.sub.2 or a compound containing carbon-hydrogen
bond(s) are reacted with O.sub.2 to form H.sub.2O and carbon oxides
in a non-electrochemical burner, such as the combustion zone of a
combustion-powered generator.
[0058] Aspects of the invention can adjust anode fuel parameters to
achieve a desired operating range for the fuel cell. Anode fuel
parameters can be characterized directly, and/or in relation to
other fuel cell processes in the form of one or more ratios. For
example, the anode fuel parameters can be controlled to achieve one
or more ratios including a fuel utilization, a fuel cell heating
value utilization, a fuel surplus ratio, a reformable fuel surplus
ratio, a reformable hydrogen content fuel ratio, and combinations
thereof.
[0059] Fuel utilization: Fuel utilization is an option for
characterizing operation of the anode based on the amount of
oxidized fuel relative to the reformable hydrogen content of an
input stream can be used to define a fuel utilization for a fuel
cell. In this discussion, "fuel utilization" is defined as the
ratio of the amount of hydrogen oxidized in the anode for
production of electricity (as described above) versus the
reformable hydrogen content of the anode input (including any
associated reforming stages). Reformable hydrogen content has been
defined above as the number of H.sub.2 molecules that can be
derived from a fuel by reforming the fuel and then driving the
water gas shift reaction to completion to maximize H.sub.2
production. For example, each methane introduced into an anode and
exposed to steam reforming conditions results in generation of the
equivalent of 4 H.sub.2 molecules at max production. (Depending on
the reforming and/or anode conditions, the reforming product can
correspond to a non-water gas shifted product, where one or more of
the H.sub.2 molecules is present instead in the form of a CO
molecule.) Thus, methane is defined as having a reformable hydrogen
content of 4 H.sub.2 molecules. As another example, under this
definition ethane has a reformable hydrogen content of 7 H.sub.2
molecules.
[0060] The utilization of fuel in the anode can also be
characterized by defining a heating value utilization based on a
ratio of the Lower Heating Value of hydrogen oxidized in the anode
due to the fuel cell anode reaction relative to the Lower Heating
Value of all fuel delivered to the anode and/or a reforming stage
associated with the anode. The "fuel cell heating value
utilization" as used herein can be computed using the flow rates
and Lower Heating Value (LHV) of the fuel components entering and
leaving the fuel cell anode. As such, fuel cell heating value
utilization can be computed as
(LHV(anode_in)-LHV(anode_out))/LHV(anode_in), where LHV(anode_in)
and LHV(anode_out) refer to the LHV of the fuel components (such as
H.sub.2, CH.sub.4, and/or CO) in the anode inlet and outlet streams
or flows, respectively. In this definition, the LHV of a stream or
flow may be computed as a sum of values for each fuel component in
the input and/or output stream. The contribution of each fuel
component to the sum can correspond to the fuel component's flow
rate (e.g., mol/hr) multiplied by the fuel component's LHV (e.g.,
joules/mol).
[0061] Lower Heating Value: The lower heating value is defined as
the enthalpy of combustion of a fuel component to vapor phase,
fully oxidized products (i.e., vapor phase CO.sub.2 and H.sub.2O
product). For example, any CO.sub.2 present in an anode input
stream does not contribute to the fuel content of the anode input,
since CO.sub.2 is already fully oxidized. For this definition, the
amount of oxidation occurring in the anode due to the anode fuel
cell reaction is defined as oxidation of H.sub.2 in the anode as
part of the electrochemical reaction in the anode, as defined
above.
[0062] It is noted that, for the special case where the only fuel
in the anode input flow is H.sub.2, the only reaction involving a
fuel component that can take place in the anode represents the
conversion of H.sub.2 into H.sub.2O. In this special case, the fuel
utilization simplifies to (H.sub.2-rate-in minus
H.sub.2-rate-out)/H.sub.2-rate-in. In such a case, H.sub.2 would be
the only fuel component, and so the H.sub.2 LHV would cancel out of
the equation. In the more general case, the anode feed may contain,
for example, CH.sub.4, H.sub.2, and CO in various amounts. Because
these species can typically be present in different amounts in the
anode outlet, the summation as described above can be needed to
determine the fuel utilization.
[0063] Alternatively or in addition to fuel utilization, the
utilization for other reactants in the fuel cell can be
characterized. For example, the operation of a fuel cell can
additionally or alternately be characterized with regard to
"CO.sub.2 utilization" and/or "oxidant" utilization. The values for
CO.sub.2 utilization and/or oxidant utilization can be specified in
a similar manner.
[0064] Fuel surplus ratio: Still another way to characterize the
reactions in a molten carbonate fuel cell is by defining a
utilization based on a ratio of the Lower Heating Value of all fuel
delivered to the anode and/or a reforming stage associated with the
anode relative to the Lower Heating Value of hydrogen oxidized in
the anode due to the fuel cell anode reaction. This quantity will
be referred to as a fuel surplus ratio. As such the fuel surplus
ratio can be computed as
(LHV(anode_in)/(LHV(anode_in)-LHV(anode_out)) where LHV(anode_in)
and LHV(anode_out) refer to the LHV of the fuel components (such as
H.sub.2, CH.sub.4, and/or CO) in the anode inlet and outlet streams
or flows, respectively. In various aspects of the invention, a
molten carbonate fuel cell can be operated to have a fuel surplus
ratio of at least about 1.0, such as at least about 1.5, or at
least about 2.0, or at least about 2.5, or at least about 3.0, or
at least about 4.0. Additionally or alternately, the fuel surplus
ratio can be about 25.0 or less.
[0065] It is noted that not all of the reformable fuel in the input
stream for the anode may be reformed. Preferably, at least about
90% of the reformable fuel in the input stream to the anode (and/or
into an associated reforming stage) can be reformed prior to
exiting the anode, such as at least about 95% or at least about
98%. In some alternative aspects, the amount of reformable fuel
that is reformed can be from about 75% to about 90%, such as at
least about 80%.
[0066] The above definition for fuel surplus ratio provides a
method for characterizing the amount of reforming occurring within
the anode and/or reforming stage(s) associated with a fuel cell
relative to the amount of fuel consumed in the fuel cell anode for
generation of electric power.
[0067] Optionally, the fuel surplus ratio can be modified to
account for situations where fuel is recycled from the anode output
to the anode input. When fuel (such as H.sub.2, CO, and/or
unreformed or partially reformed hydrocarbons) is recycled from
anode output to anode input, such recycled fuel components do not
represent a surplus amount of reformable or reformed fuel that can
be used for other purposes. Instead, such recycled fuel components
merely indicate a desire to reduce fuel utilization in a fuel
cell.
[0068] Reformable fuel surplus ratio: Calculating a reformable fuel
surplus ratio is one option to account for such recycled fuel
components is to narrow the definition of surplus fuel, so that
only the LHV of reformable fuels is included in the input stream to
the anode. As used herein the "reformable fuel surplus ratio" is
defined as the Lower Heating Value of reformable fuel delivered to
the anode and/or a reforming stage associated with the anode
relative to the Lower Heating Value of hydrogen oxidized in the
anode due to the fuel cell anode reaction. Under the definition for
reformable fuel surplus ratio, the LHV of any H.sub.2 or CO in the
anode input is excluded. Such an LHV of reformable fuel can still
be measured by characterizing the actual composition entering a
fuel cell anode, so no distinction between recycled components and
fresh components needs to be made. Although some non-reformed or
partially reformed fuel may also be recycled, in most aspects the
majority of the fuel recycled to the anode can correspond to
reformed products such as H.sub.2 or CO. Expressed mathematically,
the reformable fuel surplus ratio
(R.sub.RFS)=LHV.sub.RF/LHV.sub.OH, where LHV.sub.RF is the Lower
Heating Value (LHV) of the reformable fuel and LHV.sub.OH is the
Lower Heating Value (LHV) of the hydrogen oxidized in the anode.
The LHV of the hydrogen oxidized in the anode may be calculated by
subtracting the LHV of the anode outlet stream from the LHV of the
anode inlet stream (e.g., LHV(anode_in)-LHV(anode_out)). In various
aspects of the invention, a molten carbonate fuel cell can be
operated to have a reformable fuel surplus ratio of at least about
0.25, such as at least about 0.5, or at least about 1.0, or at
least about 1.5, or at least about 2.0, or at least about 2.5, or
at least about 3.0, or at least about 4.0. Additionally or
alternately, the reformable fuel surplus ratio can be about 25.0 or
less. It is noted that this narrower definition based on the amount
of reformable fuel delivered to the anode relative to the amount of
oxidation in the anode can distinguish between two types of fuel
cell operation methods that have low fuel utilization. Some fuel
cells achieve low fuel utilization by recycling a substantial
portion of the anode output back to the anode input. This recycle
can allow any hydrogen in the anode input to be used again as an
input to the anode. This can reduce the amount of reforming, as
even though the fuel utilization is low for a single pass through
the fuel cell, at least a portion of the unused fuel is recycled
for use in a later pass. Thus, fuel cells with a wide variety of
fuel utilization values may have the same ratio of reformable fuel
delivered to the anode reforming stage(s) versus hydrogen oxidized
in the anode reaction. In order to change the ratio of reformable
fuel delivered to the anode reforming stages relative to the amount
of oxidation in the anode, either an anode feed with a native
content of non-reformable fuel needs to be identified, or unused
fuel in the anode output needs to be withdrawn for other uses, or
both.
[0069] Reformable hydrogen surplus ratio: Still another option for
characterizing the operation of a fuel cell is based on a
"reformable hydrogen surplus ratio." The reformable fuel surplus
ratio defined above is defined based on the lower heating value of
reformable fuel components. The reformable hydrogen surplus ratio
is defined as the reformable hydrogen content of reformable fuel
delivered to the anode and/or a reforming stage associated with the
anode relative to the hydrogen reacted in the anode due to the fuel
cell anode reaction. As such, the "reformable hydrogen surplus
ratio" can be computed as
(RFC(reformable_anode_in)/(RFC(reformable_anode_in)-RFC(anode_out)),
where RFC(reformable_anode_in) refers to the reformable hydrogen
content of reformable fuels in the anode inlet streams or flows,
while RFC (anode_out) refers to the reformable hydrogen content of
the fuel components (such as H.sub.2, CH.sub.4, and/or CO) in the
anode inlet and outlet streams or flows. The RFC can be expressed
in moles/s, moles/hr, or similar. An example of a method for
operating a fuel cell with a large ratio of reformable fuel
delivered to the anode reforming stage(s) versus amount of
oxidation in the anode can be a method where excess reforming is
performed in order to balance the generation and consumption of
heat in the fuel cell. Reforming a reformable fuel to form H.sub.2
and CO is an endothermic process. This endothermic reaction can be
countered by the generation of electrical current in the fuel cell,
which can also produce excess heat corresponding (roughly) to the
difference between the amount of heat generated by the anode
oxidation reaction and the carbonate formation reaction and the
energy that exits the fuel cell in the form of electric current.
The excess heat per mole of hydrogen involved in the anode
oxidation reaction/carbonate formation reaction can be greater than
the heat absorbed to generate a mole of hydrogen by reforming. As a
result, a fuel cell operated under conventional conditions can
exhibit a temperature increase from inlet to outlet. Instead of
this type of conventional operation, the amount of fuel reformed in
the reforming stages associated with the anode can be increased.
For example, additional fuel can be reformed so that the heat
generated by the exothermic fuel cell reactions can be (roughly)
balanced by the heat consumed in reforming, or even the heat
consumed by reforming can be beyond the excess heat generated by
the fuel oxidation, resulting in a temperature drop across the fuel
cell. This can result in a substantial excess of hydrogen relative
to the amount needed for electrical power generation. As one
example, a feed to the anode inlet of a fuel cell or an associated
reforming stage can be substantially composed of reformable fuel,
such as a substantially pure methane feed. During conventional
operation for electric power generation using such a fuel, a molten
carbonate fuel cell can be operated with a fuel utilization of
about 75%. This means that about 75% (or 3/4) of the fuel content
delivered to the anode is used to form hydrogen that is then
reacted in the anode with carbonate ions to form H.sub.2O and
CO.sub.2. In conventional operation, the remaining about 25% of the
fuel content can be reformed to H.sub.2 within the fuel cell (or
can pass through the fuel cell unreacted for any CO or H.sub.2 in
the fuel), and then combusted outside of the fuel cell to form
H.sub.2O and CO.sub.2 to provide heat for the cathode inlet to the
fuel cell. The reformable hydrogen surplus ratio in this situation
can be 4/(4-1)=4/3.
[0070] Electrical efficiency: As used herein, the term "electrical
efficiency" ("EE") is defined as the electrochemical power produced
by the fuel cell divided by the rate of Lower Heating Value ("LHV")
of fuel input to the fuel cell. The fuel inputs to the fuel cell
includes both fuel delivered to the anode as well as any fuel used
to maintain the temperature of the fuel cell, such as fuel
delivered to a burner associated with a fuel cell. In this
description, the power produced by the fuel may be described in
terms of LHV(el) fuel rate.
[0071] Electrochemical power: As used herein, the term
"electrochemical power" or LHV(el) is the power generated by the
circuit connecting the cathode to the anode in the fuel cell and
the transfer of carbonate ions across the fuel cell's electrolyte.
Electrochemical power excludes power produced or consumed by
equipment upstream or downstream from the fuel cell. For example,
electricity produced from heat in a fuel cell exhaust stream is not
considered part of the electrochemical power. Similarly, power
generated by a gas turbine or other equipment upstream of the fuel
cell is not part of the electrochemical power generated. The
"electrochemical power" does not take electrical power consumed
during operation of the fuel cell into account, or any loss
incurred by conversion of the direct current to alternating
current. In other words, electrical power used to supply the fuel
cell operation or otherwise operate the fuel cell is not subtracted
from the direct current power produced by the fuel cell. As used
herein, the power density is the current density multiplied by
voltage. As used herein, the total fuel cell power is the power
density multiplied by the fuel cell area.
[0072] Fuel inputs: As used herein, the term "anode fuel input,"
designated as LHV(anode_in), is the amount of fuel within the anode
inlet stream. The term "fuel input", designated as LHV(in), is the
total amount of fuel delivered to the fuel cell, including both the
amount of fuel within the anode inlet stream and the amount of fuel
used to maintain the temperature of the fuel cell. The fuel may
include both reformable and nonreformable fuels, based on the
definition of a reformable fuel provided herein. Fuel input is not
the same as fuel utilization.
[0073] Total fuel cell efficiency: As used herein, the term "total
fuel cell efficiency" ("TFCE") is defined as: the electrochemical
power generated by the fuel cell, plus the rate of LHV of syngas
produced by the fuel cell, divided by the rate of LHV of fuel input
to the anode. In other words, TFCE=(LHV(el)+LHV(sg
net))/LHV(anode_in), where LHV(anode_in) refers to rate at which
the LHV of the fuel components (such as H.sub.2, CH.sub.4, and/or
CO) delivered to the anode and LHV(sg net) refers to a rate at
which syngas (H.sub.2, CO) is produced in the anode, which is the
difference between syngas input to the anode and syngas output from
the anode. LHV(el) describes the electrochemical power generation
of the fuel cell. The total fuel cell efficiency excludes heat
generated by the fuel cell that is put to beneficial use outside of
the fuel cell. In operation, heat generated by the fuel cell may be
put to beneficial use by downstream equipment. For example, the
heat may be used to generate additional electricity or to heat
water. These uses, when they occur apart from the fuel cell, are
not part of the total fuel cell efficiency, as the term is used in
this application. The total fuel cell efficiency is for the fuel
cell operation only, and does not include power production, or
consumption, upstream, or downstream, of the fuel cell.
[0074] Chemical efficiency: As used herein, the term "chemical
efficiency", is defined as the lower heating value of H.sub.2 and
CO in the anode exhaust of the fuel cell, or LHV(sg out), divided
by the fuel input, or LHV(in).
[0075] Neither the electrical efficiency nor the total system
efficiency takes the efficiency of upstream or downstream processes
into consideration. For example, it may be advantageous to use
turbine exhaust as a source of CO.sub.2 for the fuel cell cathode.
In this arrangement, the efficiency of the turbine is not
considered as part of the electrical efficiency or the total fuel
cell efficiency calculation. Similarly, outputs from the fuel cell
may be recycled as inputs to the fuel cell. A recycle loop is not
considered when calculating electrical efficiency or the total fuel
cell efficiency in single pass mode.
[0076] Syngas produced: As used herein, the term "syngas produced"
is the difference between syngas input to the anode and syngas
output from the anode. Syngas may be used as an input, or fuel, for
the anode, at least in part. For example, a system may include an
anode recycle loop that returns syngas from the anode exhaust to
the anode inlet where it is supplemented with natural gas or other
suitable fuel. Syngas produced LHV (sg net)=(LHV(sg out)-LHV(sg
in)), where LHV(sg in) and LHV(sg out) refer to the LHV of the
syngas in the anode inlet and syngas in the anode outlet streams or
flows, respectively. It is noted that at least a portion of the
syngas produced by the reforming reactions within an anode can
typically be utilized in the anode to produce electricity. The
hydrogen utilized to produce electricity is not included in the
definition of "syngas produced" because it does not exit the anode.
As used herein, the term "syngas ratio" is the LHV of the net
syngas produced divided by the LHV of the fuel input to the anode
or LHV (sg net)/LHV(anode in). Molar flow rates of syngas and fuel
can be used instead of LHV to express a molar-based syngas ratio
and a molar-based syngas produced.
[0077] Steam to carbon ratio (S/C): As used herein, the steam to
carbon ratio (S/C) is the molar ratio of steam in a flow to
reformable carbon in the flow. Carbon in the form of CO and
CO.sub.2 are not included as reformable carbon in this definition.
The steam to carbon ratio can be measured and/or controlled at
different points in the system. For example, the composition of an
anode inlet stream can be manipulated to achieve a S/C that is
suitable for reforming in the anode. The S/C can be given as the
molar flow rate of H.sub.2O divided by the product of the molar
flow rate of fuel multiplied by the number of carbon atoms in the
fuel, e.g. one for methane. Thus,
S/C=f.sub.H20/(f.sub.CH4.times.#C), where f.sub.H20 is the molar
flow rate of water, where f.sub.CH4 is the molar flow rate of
methane (or other fuel) and #C is the number of carbons in the
fuel.
[0078] EGR ratio: Aspects of the invention can use a turbine in
partnership with a fuel cell. The combined fuel cell and turbine
system may include exhaust gas recycle ("EGR"). In an EGR system,
at least a portion of the exhaust gas generated by the turbine can
be sent to a heat recovery generator. Another portion of the
exhaust gas can be sent to the fuel cell. The EGR ratio describes
the amount of exhaust gas routed to the fuel cell versus the total
exhaust gas routed to either the fuel cell or heat recovery
generator. As used herein, the "EGR ratio" is the flow rate for the
fuel cell bound portion of the exhaust gas divided by the combined
flow rate for the fuel cell bound portion and the recovery bound
portion, which is sent to the heat recovery generator.
[0079] In various aspects of the invention, a molten carbonate fuel
cell (MCFC) can be used to facilitate separation of CO.sub.2 from a
CO.sub.2-containing stream while also generating additional
electrical power. The CO.sub.2 separation can be further enhanced
by taking advantage of synergies with the combustion-based power
generator that can provide at least a portion of the input feed to
the cathode portion of the fuel cell.
[0080] Fuel Cell and Fuel Cell Components: In this discussion, a
fuel cell can correspond to a single cell, with an anode and a
cathode separated by an electrolyte. The anode and cathode can
receive input gas flows to facilitate the respective anode and
cathode reactions for transporting charge across the electrolyte
and generating electricity. A fuel cell stack can represent a
plurality of cells in an integrated unit. Although a fuel cell
stack can include multiple fuel cells, the fuel cells can typically
be connected in parallel and can function (approximately) as if
they collectively represented a single fuel cell of a larger size.
When an input flow is delivered to the anode or cathode of a fuel
cell stack, the fuel stack can include flow channels for dividing
the input flow between each of the cells in the stack and flow
channels for combining the output flows from the individual cells.
In this discussion, a fuel cell array can be used to refer to a
plurality of fuel cells (such as a plurality of fuel cell stacks)
that are arranged in series, in parallel, or in any other
convenient manner (e.g., in a combination of series and parallel).
A fuel cell array can include one or more stages of fuel cells
and/or fuel cell stacks, where the anode/cathode output from a
first stage may serve as the anode/cathode input for a second
stage. It is noted that the anodes in a fuel cell array do not have
to be connected in the same way as the cathodes in the array. For
convenience, the input to the first anode stage of a fuel cell
array may be referred to as the anode input for the array, and the
input to the first cathode stage of the fuel cell array may be
referred to as the cathode input to the array. Similarly, the
output from the final anode/cathode stage may be referred to as the
anode/cathode output from the array.
[0081] It should be understood that reference to use of a fuel cell
herein typically denotes a "fuel cell stack" composed of individual
fuel cells, and more generally refers to use of one or more fuel
cell stacks in fluid communication. Individual fuel cell elements
(plates) can typically be "stacked" together in a rectangular array
called a "fuel cell stack". This fuel cell stack can typically take
a feed stream and distribute reactants among all of the individual
fuel cell elements and can then collect the products from each of
these elements. When viewed as a unit, the fuel cell stack in
operation can be taken as a whole even though composed of many
(often tens or hundreds) of individual fuel cell elements. These
individual fuel cell elements can typically have similar voltages
(as the reactant and product concentrations are similar), and the
total power output can result from the summation of all of the
electrical currents in all of the cell elements, when the elements
are electrically connected in series. Stacks can also be arranged
in a series arrangement to produce high voltages. A parallel
arrangement can boost the current. If a sufficiently large volume
fuel cell stack is available to process a given exhaust flow, the
systems and methods described herein can be used with a single
molten carbonate fuel cell stack. In other aspects of the
invention, a plurality of fuel cell stacks may be desirable or
needed for a variety of reasons.
[0082] For the purposes of this invention, unless otherwise
specified, the term "fuel cell" should be understood to also refer
to and/or is defined as including a reference to a fuel cell stack
composed of set of one or more individual fuel cell elements for
which there is a single input and output, as that is the manner in
which fuel cells are typically employed in practice. Similarly, the
term fuel cells (plural), unless otherwise specified, should be
understood to also refer to and/or is defined as including a
plurality of separate fuel cell stacks. In other words, all
references within this document, unless specifically noted, can
refer interchangeably to the operation of a fuel cell stack as a
"fuel cell". For example, the volume of exhaust generated by a
commercial scale combustion generator may be too large for
processing by a fuel cell (i.e., a single stack) of conventional
size. In order to process the full exhaust, a plurality of fuel
cells (i.e., two or more separate fuel cells or fuel cell stacks)
can be arranged in parallel, so that each fuel cell can process
(roughly) an equal portion of the combustion exhaust. Although
multiple fuel cells can be used, each fuel cell can typically be
operated in a generally similar manner, given its (roughly) equal
portion of the combustion exhaust.
[0083] "Internal reforming" and "external reforming": A fuel cell
or fuel cell stack may include one or more internal reforming
sections. As used herein, the term "internal reforming" refers to
fuel reforming occurring within the body of a fuel cell, a fuel
cell stack, or otherwise within a fuel cell assembly. External
reforming, which is often used in conjunction with a fuel cell,
occurs in a separate piece of equipment that is located outside of
the fuel cell stack. In other words, the body of the external
reformer is not in direct physical contact with the body of a fuel
cell or fuel cell stack. In a typical set up, the output from the
external reformer can be fed to the anode inlet of a fuel cell.
Unless otherwise noted specifically, the reforming described within
this application is internal reforming.
[0084] Internal reforming may occur within a fuel cell anode.
Internal reforming can additionally or alternately occur within an
internal reforming element integrated within a fuel cell assembly.
The integrated reforming element may be located between fuel cell
elements within a fuel cell stack. In other words, one of the trays
in the stack can be a reforming section instead of a fuel cell
element. In one aspect, the flow arrangement within a fuel cell
stack directs fuel to the internal reforming elements and then into
the anode portion of the fuel cells. Thus, from a flow perspective,
the internal reforming elements and fuel cell elements can be
arranged in series within the fuel cell stack. As used herein, the
term "anode reforming" is fuel reforming that occurs within an
anode. As used herein, the term "internal reforming" is reforming
that occurs within an integrated reforming element and not in an
anode section.
[0085] In some aspects, a reforming stage that is internal to a
fuel cell assembly can be considered to be associated with the
anode(s) in the fuel cell assembly. In some alternative aspects,
for a reforming stage in a fuel cell stack that can be associated
with an anode (such as associated with multiple anodes), a flow
path can be available so that the output flow from the reforming
stage is passed into at least one anode. This can correspond to
having an initial section of a fuel cell plate not in contact with
the electrolyte and instead can serve just as a reforming catalyst.
Another option for an associated reforming stage can be to have a
separate integrated reforming stage as one of the elements in a
fuel cell stack, where the output from the integrated reforming
stage can be returned to the input side of one or more of the fuel
cells in the fuel cell stack.
[0086] From a heat integration standpoint, a characteristic height
in a fuel cell stack can be the height of an individual fuel cell
stack element. It is noted that the separate reforming stage and/or
a separate endothermic reaction stage could have a different height
in the stack than a fuel cell. In such a scenario, the height of a
fuel cell element can be used as the characteristic height. In some
aspects, an integrated endothermic reaction stage can be defined as
a stage that is heat integrated with one or more fuel cells, so
that the integrated endothermic reaction stage can use the heat
from the fuel cells as a heat source for the endothermic reaction.
Such an integrated endothermic reaction stage can be defined as
being positioned less than 5 times the height of a stack element
from any fuel cells providing heat to the integrated stage. For
example, an integrated endothermic reaction stage (such as a
reforming stage) can be positioned less than 5 times the height of
a stack element from any fuel cells that are heat integrated, such
as less than 3 times the height of a stack element. In this
discussion, an integrated reforming stage and/or integrated
endothermic reaction stage that represent an adjacent stack element
to a fuel cell element can be defined as being about one stack
element height or less away from the adjacent fuel cell
element.
[0087] In some aspects, a separate reforming stage that is heat
integrated with a fuel cell element can correspond to a reforming
stage associated with the fuel cell element. In such aspects, an
integrated fuel cell element can provide at least a portion of the
heat to the associated reforming stage, and the associated
reforming stage can provide at least a portion of the reforming
stage output to the integrated fuel cell as a fuel stream. In other
aspects, a separate reforming stage can be integrated with a fuel
cell for heat transfer without being associated with the fuel cell.
In this type of situation, the separate reforming stage can receive
heat from the fuel cell, but the decision can be made not to use
the output of the reforming stage as an input to the fuel cell.
Instead, the decision can be made to use the output of such a
reforming stage for another purpose, such as directly adding the
output to the anode exhaust stream, and/or for forming a separate
output stream from the fuel cell assembly.
[0088] More generally, a separate stack element in a fuel cell
stack can be used to perform any convenient type of endothermic
reaction that can take advantage of the waste heat provided by
integrated fuel cell stack elements. Instead of plates suitable for
performing a reforming reaction on a hydrocarbon fuel stream, a
separate stack element can have plates suitable for catalyzing
another type of endothermic reaction. A manifold or other
arrangement of inlet conduits in the fuel cell stack can be used to
provide an appropriate input flow to each stack element. A similar
manifold or other arrangement of outlet conduits can additionally
or alternately be used to withdraw the output flows from each stack
element. Optionally, the output flows from a endothermic reaction
stage in a stack can be withdrawn from the fuel cell stack without
having the output flow pass through a fuel cell anode. In such an
optional aspect, the products of the exothermic reaction can
therefore exit from the fuel cell stack without passing through a
fuel cell anode. Examples of other types of endothermic reactions
that can be performed in stack elements in a fuel cell stack can
include, without limitation, ethanol dehydration to form ethylene
and ethane cracking.
[0089] Recycle: As defined herein, recycle of a portion of a fuel
cell output (such as an anode exhaust or a stream separated or
withdrawn from an anode exhaust) to a fuel cell inlet can
correspond to a direct or indirect recycle stream. A direct recycle
of a stream to a fuel cell inlet is defined as recycle of the
stream without passing through an intermediate process, while an
indirect recycle involves recycle after passing a stream through
one or more intermediate processes. For example, if the anode
exhaust is passed through a CO.sub.2 separation stage prior to
recycle, this is considered an indirect recycle of the anode
exhaust. If a portion of the anode exhaust, such as an H.sub.2
stream withdrawn from the anode exhaust, is passed into a gasifier
for converting coal into a fuel suitable for introduction into the
fuel cell, then that is also considered an indirect recycle.
Anode Inputs and Outputs
[0090] In various aspects of the invention, the MCFC array can be
fed by a fuel received at the anode inlet that comprises, for
example, both hydrogen and a hydrocarbon such as methane (or
alternatively a hydrocarbonaceous or hydrocarbon-like compound that
may contain heteroatoms different from C and H). Most of the
methane (or other hydrocarbonaceous or hydrocarbon-like compound)
fed to the anode can typically be fresh methane. In this
description, a fresh fuel such as fresh methane refers to a fuel
that is not recycled from another fuel cell process. For example,
methane recycled from the anode outlet stream back to the anode
inlet may not be considered "fresh" methane, and can instead be
described as reclaimed methane. The fuel source used can be shared
with other components, such as a turbine that uses a portion of the
fuel source to provide a CO.sub.2-containing stream for the cathode
input. The fuel source input can include water in a proportion to
the fuel appropriate for reforming the hydrocarbon (or
hydrocarbon-like) compound in the reforming section that generates
hydrogen. For example, if methane is the fuel input for reforming
to generate H.sub.2, the molar ratio of water to fuel can be from
about one to one to about ten to one, such as at least about two to
one. A ratio of four to one or greater is typical for external
reforming, but lower values can be typical for internal reforming.
To the degree that H.sub.2 is a portion of the fuel source, in some
optional aspects no additional water may be needed in the fuel, as
the oxidation of H.sub.2 at the anode can tend to produce H.sub.2O
that can be used for reforming the fuel. The fuel source can also
optionally contain components incidental to the fuel source (e.g.,
a natural gas feed can contain some content of CO.sub.2 as an
additional component). For example, a natural gas feed can contain
CO.sub.2, N.sub.2, and/or other inert (noble) gases as additional
components. Optionally, in some aspects the fuel source may also
contain CO, such as CO from a recycled portion of the anode
exhaust. An additional or alternate potential source for CO in the
fuel into a fuel cell assembly can be CO generated by steam
reforming of a hydrocarbon fuel performed on the fuel prior to
entering the fuel cell assembly.
[0091] More generally, a variety of types of fuel streams may be
suitable for use as an input stream for the anode of a molten
carbonate fuel cell. Some fuel streams can correspond to streams
containing hydrocarbons and/or hydrocarbon-like compounds that may
also include heteroatoms different from C and H. In this
discussion, unless otherwise specified, a reference to a fuel
stream containing hydrocarbons for an MCFC anode is defined to
include fuel streams containing such hydrocarbon-like compounds.
Examples of hydrocarbon (including hydrocarbon-like) fuel streams
include natural gas, streams containing C1-C4 carbon compounds
(such as methane or ethane), and streams containing heavier C5+
hydrocarbons (including hydrocarbon-like compounds), as well as
combinations thereof. Still other additional or alternate examples
of potential fuel streams for use in an anode input can include
biogas-type streams, such as methane produced from natural
(biological) decomposition of organic material.
[0092] In some aspects, a molten carbonate fuel cell can be used to
process an input fuel stream, such as a natural gas and/or
hydrocarbon stream, with a low energy content due to the presence
of diluent compounds. For example, some sources of methane and/or
natural gas are sources that can include substantial amounts of
either CO.sub.2 or other inert molecules, such as nitrogen, argon,
or helium. Due to the presence of elevated amounts of CO.sub.2
and/or inerts, the energy content of a fuel stream based on the
source can be reduced. Using a low energy content fuel for a
combustion reaction (such as for powering a combustion-powered
turbine) can pose difficulties. However, a molten carbonate fuel
cell can generate power based on a low energy content fuel source
with a reduced or minimal impact on the efficiency of the fuel
cell. The presence of additional gas volume can require additional
heat for raising the temperature of the fuel to the temperature for
reforming and/or the anode reaction. Additionally, due to the
equilibrium nature of the water gas shift reaction within a fuel
cell anode, the presence of additional CO.sub.2 can have an impact
on the relative amounts of H.sub.2 and CO present in the anode
output. However, the inert compounds otherwise can have only a
minimal direct impact on the reforming and anode reactions. The
amount of CO.sub.2 and/or inert compounds in a fuel stream for a
molten carbonate fuel cell, when present, can be at least about 1
vol %, such as at least about 2 vol %, or at least about 5 vol %,
or at least about 10 vol %, or at least about 15 vol %, or at least
about 20 vol %, or at least about 25 vol %, or at least about 30
vol %, or at least about 35 vol %, or at least about 40 vol %, or
at least about 45 vol %, or at least about 50 vol %, or at least
about 75 vol %. Additionally or alternately, the amount of CO.sub.2
and/or inert compounds in a fuel stream for a molten carbonate fuel
cell can be about 90 vol % or less, such as about 75 vol % or less,
or about 60 vol % or less, or about 50 vol % or less, or about 40
vol % or less, or about 35 vol % or less.
[0093] Yet other examples of potential sources for an anode input
stream can correspond to refinery and/or other industrial process
output streams. For example, coking is a common process in many
refineries for converting heavier compounds to lower boiling
ranges. Coking typically produces an off-gas containing a variety
of compounds that are gases at room temperature, including CO and
various C1-C4 hydrocarbons. This off-gas can be used as at least a
portion of an anode input stream. Other refinery off-gas streams
can additionally or alternately be suitable for inclusion in an
anode input stream, such as light ends (C1-C4) generated during
cracking or other refinery processes. Still other suitable refinery
streams can additionally or alternately include refinery streams
containing CO or CO.sub.2 that also contain H.sub.2 and/or
reformable fuel compounds.
[0094] Still other potential sources for an anode input can
additionally or alternately include streams with increased water
content. For example, an ethanol output stream from an ethanol
plant (or another type of fermentation process) can include a
substantial portion of H.sub.2O prior to final distillation. Such
H.sub.2O can typically cause only minimal impact on the operation
of a fuel cell. Thus, a fermentation mixture of alcohol (or other
fermentation product) and water can be used as at least a portion
of an anode input stream.
[0095] Biogas, or digester gas, is another additional or alternate
potential source for an anode input. Biogas may primarily comprise
methane and CO.sub.2 and is typically produced by the breakdown or
digestion of organic matter. Anaerobic bacteria may be used to
digest the organic matter and produce the biogas. Impurities, such
as sulfur-containing compounds, may be removed from the biogas
prior to use as an anode input.
[0096] The output stream from an MCFC anode can include H.sub.2O,
CO.sub.2, CO, and H.sub.2. Optionally, the anode output stream
could also have unreacted fuel (such as H.sub.2 or CH.sub.4) or
inert compounds in the feed as additional output components.
Instead of using this output stream as a fuel source to provide
heat for a reforming reaction or as a combustion fuel for heating
the cell, one or more separations can be performed on the anode
output stream to separate the CO.sub.2 from the components with
potential value as inputs to another process, such as H.sub.2 or
CO. The H.sub.2 and/or CO can be used as a syngas for chemical
synthesis, as a source of hydrogen for chemical reaction, and/or as
a fuel with reduced greenhouse gas emissions.
[0097] In various aspects, the composition of the output stream
from the anode can be impacted by several factors. Factors that can
influence the anode output composition can include the composition
of the input stream to the anode, the amount of current generated
by the fuel cell, and/or the temperature at the exit of the anode.
The temperature of at the anode exit can be relevant due to the
equilibrium nature of the water gas shift reaction. In a typical
anode, at least one of the plates forming the wall of the anode can
be suitable for catalyzing the water gas shift reaction. As a
result, if a) the composition of the anode input stream is known,
b) the extent of reforming of reformable fuel in the anode input
stream is known, and c) the amount of carbonate transported from
the cathode to anode (corresponding to the amount of electrical
current generated) is known, the composition of the anode output
can be determined based on the equilibrium constant for the water
gas shift reaction.
K.sub.eq=[CO.sub.2][H.sub.2]/[CO][H.sub.2O]
[0098] In the above equation, K.sub.eq is the equilibrium constant
for the reaction at a given temperature and pressure, and [X] is
the partial pressure of component X. Based on the water gas shift
reaction, it can be noted that an increased CO.sub.2 concentration
in the anode input can tend to result in additional CO formation
(at the expense of H.sub.2) while an increased H.sub.2O
concentration can tend to result in additional H.sub.2 formation
(at the expense of CO).
[0099] To determine the composition at the anode output, the
composition of the anode input can be used as a starting point.
This composition can then be modified to reflect the extent of
reforming of any reformable fuels that can occur within the anode.
Such reforming can reduce the hydrocarbon content of the anode
input in exchange for increased hydrogen and CO.sub.2. Next, based
on the amount of electrical current generated, the amount of
H.sub.2 in the anode input can be reduced in exchange for
additional H.sub.2O and CO.sub.2. This composition can then be
adjusted based on the equilibrium constant for the water gas shift
reaction to determine the exit concentrations for H.sub.2, CO,
CO.sub.2, and H.sub.2O.
[0100] Table 1 shows the anode exhaust composition at different
fuel utilizations for a typical type of fuel. The anode exhaust
composition can reflect the combined result of the anode reforming
reaction, water gas shift reaction, and the anode oxidation
reaction. The output composition values in Table 1 were calculated
by assuming an anode input composition with an about 2 to 1 ratio
of steam (H.sub.2O) to carbon (reformable fuel). The reformable
fuel was assumed to be methane, which was assumed to be 100%
reformed to hydrogen. The initial CO.sub.2 and H.sub.2
concentrations in the anode input were assumed to be negligible,
while the input N.sub.2 concentration was about 0.5%. The fuel
utilization U.sub.f (as defined herein) was allowed to vary from
about 35% to about 70% as shown in the table. The exit temperature
for the fuel cell anode was assumed to be about 650.degree. C. for
purposes of determining the correct value for the equilibrium
constant.
TABLE-US-00001 TABLE 1 Anode Exhaust Composition Uf % 35% 40% 45%
50% 55% 60% 65% 70% Anode Exhaust Composition H.sub.2O %, wet 32.5%
34.1% 35.5% 36.7% 37.8% 38.9% 39.8% 40.5% CO.sub.2 %, wet 26.7%
29.4% 32.0% 34.5% 36.9% 39.3% 41.5% 43.8% H.sub.2 %, wet 29.4%
26.0% 22.9% 20.0% 17.3% 14.8% 12.5% 10.4% CO %, wet 10.8% 10.0%
9.2% 8.4% 7.5% 6.7% 5.8% 4.9% N.sub.2 %, wet 0.5% 0.5% 0.5% 0.4%
0.4% 0.4% 0.4% 0.4% CO.sub.2 %, dry 39.6% 44.6% 49.6% 54.5% 59.4%
64.2% 69.0% 73.7% H.sub.2 %, dry 43.6% 39.4% 35.4% 31.5% 27.8%
24.2% 20.7% 17.5% CO %, dry 16.1% 15.2% 14.3% 13.2% 12.1% 10.9%
9.7% 8.2% N.sub.2 %, dry 0.7% 0.7% 0.7% 0.7% 0.7% 0.7% 0.7% 0.7%
H.sub.2/CO 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.1 (H.sub.2--CO.sub.2)/
0.07 -0.09 -0.22 -0.34 -0.44 -0.53 -0.61 -0.69 (CO + CO.sub.2)
[0101] Table 1 shows anode output compositions for a particular set
of conditions and anode input composition. More generally, in
various aspects the anode output can include about 10 vol % to
about 50 vol % H.sub.2O. The amount of H.sub.2O can vary greatly,
as H.sub.2O in the anode can be produced by the anode oxidation
reaction. If an excess of H.sub.2O beyond what is needed for
reforming is introduced into the anode, the excess H.sub.2O can
typically pass through largely unreacted, with the exception of
H.sub.2O consumed (or generated) due to fuel reforming and the
water gas shift reaction. The CO.sub.2 concentration in the anode
output can also vary widely, such as from about 20 vol % to about
50 vol % CO.sub.2. The amount of CO.sub.2 can be influenced by both
the amount of electrical current generated as well as the amount of
CO.sub.2 in the anode input flow. The amount of H.sub.2 in the
anode output can additionally or alternately be from about 10 vol %
H.sub.2 to about 50 vol % H.sub.2, depending on the fuel
utilization in the anode. At the anode output, the amount of CO can
be from about 5 vol % to about 20 vol %. It is noted that the
amount of CO relative to the amount of H.sub.2 in the anode output
for a given fuel cell can be determined in part by the equilibrium
constant for the water gas shift reaction at the temperature and
pressure present in the fuel cell. The anode output can further
additionally or alternately include 5 vol % or less of various
other components, such as N.sub.2, CH.sub.4 (or other unreacted
carbon-containing fuels), and/or other components.
[0102] Optionally, one or more water gas shift reaction stages can
be included after the anode output to convert CO and H.sub.2O in
the anode output into CO.sub.2 and H.sub.2, if desired. The amount
of H.sub.2 present in the anode output can be increased, for
example, by using a water gas shift reactor at lower temperature to
convert H.sub.2O and CO present in the anode output into H.sub.2
and CO.sub.2. Alternatively, the temperature can be raised and the
water-gas shift reaction can be reversed, producing more CO and
H.sub.2O from H.sub.2 and CO.sub.2. Water is an expected output of
the reaction occurring at the anode, so the anode output can
typically have an excess of H.sub.2O relative to the amount of CO
present in the anode output. Alternatively, H.sub.2O can be added
to the stream after the anode exit but before the water gas shift
reaction. CO can be present in the anode output due to incomplete
carbon conversion during reforming and/or due to the equilibrium
balancing reactions between H.sub.2O, CO, H.sub.2, and CO.sub.2
(i.e., the water-gas shift equilibrium) under either reforming
conditions or the conditions present during the anode reaction. A
water gas shift reactor can be operated under conditions to drive
the equilibrium further in the direction of forming CO.sub.2 and
H.sub.2 at the expense of CO and H.sub.2O. Higher temperatures can
tend to favor the formation of CO and H.sub.2O. Thus, one option
for operating the water gas shift reactor can be to expose the
anode output stream to a suitable catalyst, such as a catalyst
including iron oxide, zinc oxide, copper on zinc oxide, or the
like, at a suitable temperature, e.g., between about 190.degree. C.
to about 210.degree. C. Optionally, the water-gas shift reactor can
include two stages for reducing the CO concentration in an anode
output stream, with a first higher temperature stage operated at a
temperature from at least about 300.degree. C. to about 375.degree.
C. and a second lower temperature stage operated at a temperature
of about 225.degree. C. or less, such as from about 180.degree. C.
to about 210.degree. C. In addition to increasing the amount of
H.sub.2 present in the anode output, the water-gas shift reaction
can additionally or alternately increase the amount of CO.sub.2 at
the expense of CO. This can exchange difficult-to-remove carbon
monoxide (CO) for carbon dioxide, which can be more readily removed
by condensation (e.g., cryogenic removal), chemical reaction (such
as amine removal), and/or other CO.sub.2 removal methods.
Additionally or alternately, it may be desirable to increase the CO
content present in the anode exhaust in order to achieve a desired
ratio of H.sub.2 to CO.
[0103] After passing through the optional water gas shift reaction
stage, the anode output can be passed through one or more
separation stages for removal of water and/or CO.sub.2 from the
anode output stream. For example, one or more CO.sub.2 output
streams can be formed by performing CO.sub.2 separation on the
anode output using one or more methods individually or in
combination. Such methods can be used to generate CO.sub.2 output
stream(s) having a CO.sub.2 content of 90 vol % or greater, such as
at least 95% vol % CO.sub.2, or at least 98 vol % CO.sub.2. Such
methods can recover about at least about 70% of the CO.sub.2
content of the anode output, such as at least about 80% of the
CO.sub.2 content of the anode output, or at least about 90%.
Alternatively, in some aspects it may be desirable to recover only
a portion of the CO.sub.2 within an anode output stream, with the
recovered portion of CO.sub.2 being about 33% to about 90% of the
CO.sub.2 in the anode output, such as at least about 40%, or at
least about 50%. For example, it may be desirable to retain some
CO.sub.2 in the anode output flow so that a desired composition can
be achieved in a subsequent water gas shift stage. Suitable
separation methods may comprise use of a physical solvent (e.g.,
Selexol.TM. or Rectisol.TM.); amines or other bases (e.g., MEA or
MDEA); refrigeration (e.g., cryogenic separation); pressure swing
adsorption; vacuum swing adsorption; and combinations thereof. A
cryogenic CO.sub.2 separator can be an example of a suitable
separator. As the anode output is cooled, the majority of the water
in the anode output can be separated out as a condensed (liquid)
phase. Further cooling and/or pressurizing of the water-depleted
anode output flow can then separate high purity CO.sub.2, as the
other remaining components in the anode output flow (such as
H.sub.2, N.sub.2, CH.sub.4) do not tend to readily form condensed
phases. A cryogenic CO.sub.2 separator can recover between about
33% and about 90% of the CO.sub.2 present in a flow, depending on
the operating conditions.
[0104] Removal of water from the anode exhaust to form one or more
water output streams can also be beneficial, whether prior to,
during, or after performing CO.sub.2 separation. The amount of
water in the anode output can vary depending on operating
conditions selected. For example, the steam-to-carbon ratio
established at the anode inlet can affect the water content in the
anode exhaust, with high steam-to-carbon ratios typically resulting
in a large amount of water that can pass through the anode
unreacted and/or reacted only due to the water gas shift
equilibrium in the anode. Depending on the aspect, the water
content in the anode exhaust can correspond to up to about 30% or
more of the volume in the anode exhaust. Additionally or
alternately, the water content can be about 80% or less of the
volume of the anode exhaust. While such water can be removed by
compression and/or cooling with resulting condensation, the removal
of this water can require extra compressor power and/or heat
exchange surface area and excessive cooling water. One beneficial
way to remove a portion of this excess water can be based on use of
an adsorbent bed that can capture the humidity from the moist anode
effluent and can then be `regenerated` using dry anode feed gas, in
order to provide additional water for the anode feed. HVAC-style
(heating, ventilation, and air conditioning) adsorption wheels
design can be applicable, because anode exhaust and inlet can be
similar in pressure, and minor leakage from one stream to the other
can have minimal impact on the overall process. In embodiments
where CO.sub.2 removal is performed using a cryogenic process,
removal of water prior to or during CO.sub.2 removal may be
desirable, including removal by triethyleneglycol (TEG) system
and/or desiccants. By contrast, if an amine wash is used for
CO.sub.2 removal, water can be removed from the anode exhaust
downstream from the CO.sub.2 removal stage.
[0105] Alternately or in addition to a CO.sub.2 output stream
and/or a water output stream, the anode output can be used to form
one or more product streams containing a desired chemical or fuel
product. Such a product stream or streams can correspond to a
syngas stream, a hydrogen stream, or both syngas product and
hydrogen product streams. For example, a hydrogen product stream
containing at least about 70 vol % H.sub.2, such as at least about
90 vol % H.sub.2 or at least about 95 vol % H.sub.2, can be formed.
Additionally or alternately, a syngas stream containing at least
about 70 vol % of H.sub.2 and CO combined, such as at least about
90 vol % of H.sub.2 and CO can be formed. The one or more product
streams can have a gas volume corresponding to at least about 75%
of the combined H.sub.2 and CO gas volumes in the anode output,
such as at least about 85% or at least about 90% of the combined
H.sub.2 and CO gas volumes. It is noted that the relative amounts
of H.sub.2 and CO in the products streams may differ from the
H.sub.2 to CO ratio in the anode output based on use of water gas
shift reaction stages to convert between the products.
[0106] In some aspects, it can be desirable to remove or separate a
portion of the H.sub.2 present in the anode output. For example, in
some aspects the H.sub.2 to CO ratio in the anode exhaust can be at
least about 3.0:1. By contrast, processes that make use of syngas,
such as Fischer-Tropsch synthesis, may consume H.sub.2 and CO in a
different ratio, such as a ratio that is closer to 2:1. One
alternative can be to use a water gas shift reaction to modify the
content of the anode output to have an H.sub.2 to CO ratio closer
to a desired syngas composition. Another alternative can be to use
a membrane separation to remove a portion of the H.sub.2 present in
the anode output to achieve a desired ratio of H.sub.2 and CO, or
still alternately to use a combination of membrane separation and
water gas shift reactions. One advantage of using a membrane
separation to remove only a portion of the H.sub.2 in the anode
output can be that the desired separation can be performed under
relatively mild conditions. Since one goal can be to produce a
retentate that still has a substantial H.sub.2 content, a permeate
of high purity hydrogen can be generated by membrane separation
without requiring severe conditions. For example, rather than
having a pressure on the permeate side of the membrane of about 100
kPaa or less (such as ambient pressure), the permeate side can be
at an elevated pressure relative to ambient while still having
sufficient driving force to perform the membrane separation.
Additionally or alternately, a sweep gas such as methane can be
used to provide a driving force for the membrane separation. This
can reduce the purity of the H.sub.2 permeate stream, but may be
advantageous, depending on the desired use for the permeate
stream.
[0107] In various aspects of the invention, at least a portion of
the anode exhaust stream (preferably after separation of CO.sub.2
and/or H.sub.2O) can be used as a feed for a process external to
the fuel cell and associated reforming stages. In various aspects,
the anode exhaust can have a ratio of H.sub.2 to CO of about 1.5:1
to about 10:1, such as at least about 3.0:1, or at least about
4.0:1, or at least about 5.0:1. A syngas stream can be generated or
withdrawn from the anode exhaust. The anode exhaust gas, optionally
after separation of CO.sub.2 and/or H.sub.2O, and optionally after
performing a water gas shift reaction and/or a membrane separation
to remove excess hydrogen, can correspond to a stream containing
substantial portions of H.sub.2 and/or CO. For a stream with a
relatively low content of CO, such as a stream where the ratio of
H.sub.2 to CO is at least about 3:1, the anode exhaust can be
suitable for use as an H.sub.2 feed. Examples of processes that
could benefit from an H.sub.2 feed can include, but are not limited
to, refinery processes, an ammonia synthesis plant, or a turbine in
a (different) power generation system, or combinations thereof.
Depending on the application, still lower CO.sub.2 contents can be
desirable. For a stream with an H.sub.2-to-CO ratio of less than
about 2.2 to 1 and greater than about 1.9 to 1, the stream can be
suitable for use as a syngas feed. Examples of processes that could
benefit from a syngas feed can include, but are not limited to, a
gas-to-liquids plant (such as a plant using a Fischer-Tropsch
process with a non-shifting catalyst) and/or a methanol synthesis
plant. The amount of the anode exhaust used as a feed for an
external process can be any convenient amount. Optionally, when a
portion of the anode exhaust is used as a feed for an external
process, a second portion of the anode exhaust can be recycled to
the anode input and/or recycled to the combustion zone for a
combustion-powered generator.
[0108] The input streams useful for different types of
Fischer-Tropsch synthesis processes can provide an example of the
different types of product streams that may be desirable to
generate from the anode output. For a Fischer-Tropsch synthesis
reaction system that uses a shifting catalyst, such as an
iron-based catalyst, the desired input stream to the reaction
system can include CO.sub.2 in addition to H.sub.2 and CO. If a
sufficient amount of CO.sub.2 is not present in the input stream, a
Fischer-Tropsch catalyst with water gas shift activity can consume
CO in order to generate additional CO.sub.2, resulting in a syngas
that can be deficient in CO. For integration of such a
Fischer-Tropsch process with an MCFC fuel cell, the separation
stages for the anode output can be operated to retain a desired
amount of CO.sub.2 (and optionally H.sub.2O) in the syngas product.
By contrast, for a Fischer-Tropsch catalyst based on a non-shifting
catalyst, any CO.sub.2 present in a product stream could serve as
an inert component in the Fischer-Tropsch reaction system.
[0109] In an aspect where the membrane is swept with a sweep gas
such as a methane sweep gas, the methane sweep gas can correspond
to a methane stream used as the anode fuel or in a different low
pressure process, such as a boiler, furnace, gas turbine, or other
fuel-consuming device. In such an aspect, low levels of CO.sub.2
permeation across the membrane can have minimal consequence. Such
CO.sub.2 that may permeate across the membrane can have a minimal
impact on the reactions within the anode, and such CO.sub.2 can
remain contained in the anode product. Therefore, the CO.sub.2 (if
any) lost across the membrane due to permeation does not need to be
transferred again across the MCFC electrolyte. This can
significantly reduce the separation selectivity requirement for the
hydrogen permeation membrane. This can allow, for example, use of a
higher-permeability membrane having a lower selectivity, which can
enable use of a lower pressure and/or reduced membrane surface
area. In such an aspect of the invention, the volume of the sweep
gas can be a large multiple of the volume of hydrogen in the anode
exhaust, which can allow the effective hydrogen concentration on
the permeate side to be maintained close to zero. The hydrogen thus
separated can be incorporated into the turbine-fed methane where it
can enhance the turbine combustion characteristics, as described
above.
[0110] It is noted that excess H.sub.2 produced in the anode can
represent a fuel where the greenhouse gases have already been
separated. Any CO.sub.2 in the anode output can be readily
separated from the anode output, such as by using an amine wash, a
cryogenic CO.sub.2 separator, and/or a pressure or vacuum swing
absorption process. Several of the components of the anode output
(H.sub.2, CO, CH.sub.4) are not easily removed, while CO.sub.2 and
H.sub.2O can usually be readily removed. Depending on the
embodiment, at least about 90 vol % of the CO.sub.2 in the anode
output can be separated out to form a relatively high purity
CO.sub.2 output stream. Thus, any CO.sub.2 generated in the anode
can be efficiently separated out to form a high purity CO.sub.2
output stream. After separation, the remaining portion of the anode
output can correspond primarily to components with chemical and/or
fuel value, as well as reduced amounts of CO.sub.2 and/or H.sub.2O.
Since a substantial portion of the CO.sub.2 generated by the
original fuel (prior to reforming) can have been separated out, the
amount of CO.sub.2 generated by subsequent burning of the remaining
portion of the anode output can be reduced. In particular, to the
degree that the fuel in the remaining portion of the anode output
is H.sub.2, no additional greenhouse gases can typically be formed
by burning of this fuel.
[0111] The anode exhaust can be subjected to a variety of gas
processing options, including water-gas shift and separation of the
components from each other. Two general anode processing schemes
are shown in FIGS. 1 and 2.
[0112] FIG. 1 schematically shows an example of a reaction system
for operating a fuel cell array of molten carbonate fuel cells in
conjunction with a chemical synthesis process. In FIG. 1, a fuel
stream 105 is provided to a reforming stage (or stages) 110
associated with the anode 127 of a fuel cell 120, such as a fuel
cell that is part of a fuel cell stack in a fuel cell array. The
reforming stage 110 associated with fuel cell 120 can be internal
to a fuel cell assembly. In some optional aspects, an external
reforming stage (not shown) can also be used to reform a portion of
the reformable fuel in an input stream prior to passing the input
stream into a fuel cell assembly. Fuel stream 105 can preferably
include a reformable fuel, such as methane, other hydrocarbons,
and/or other hydrocarbon-like compounds such as organic compounds
containing carbon-hydrogen bonds. Fuel stream 105 can also
optionally contain H.sub.2 and/or CO, such as H.sub.2 and/or CO
provided by optional anode recycle stream 185. It is noted that
anode recycle stream 185 is optional, and that in many aspects no
recycle stream is provided from the anode exhaust 125 back to anode
127, either directly or indirectly via combination with fuel stream
105 or reformed fuel stream 115. After reforming, the reformed fuel
stream 115 can be passed into anode 127 of fuel cell 120. A
CO.sub.2 and O.sub.2-containing stream 119 can also be passed into
cathode 129. A flow of carbonate ions 122, CO.sub.3.sup.2-, from
the cathode portion 129 of the fuel cell can provide the remaining
reactant needed for the anode fuel cell reactions. Based on the
reactions in the anode 127, the resulting anode exhaust 125 can
include H.sub.2O, CO.sub.2, one or more components corresponding to
incompletely reacted fuel (H.sub.2, CO, CH.sub.4, or other
components corresponding to a reformable fuel), and optionally one
or more additional nonreactive components, such as N.sub.2 and/or
other contaminants that are part of fuel stream 105. The anode
exhaust 125 can then be passed into one or more separation stages.
For example, a CO.sub.2 removal stage 140 can correspond to a
cryogenic CO.sub.2 removal system, an amine wash stage for removal
of acid gases such as CO.sub.2, or another suitable type of
CO.sub.2 separation stage for separating a CO.sub.2 output stream
143 from the anode exhaust. Optionally, the anode exhaust can first
be passed through a water gas shift reactor 130 to convert any CO
present in the anode exhaust (along with some H.sub.2O) into
CO.sub.2 and H.sub.2 in an optionally water gas shifted anode
exhaust 135. Depending on the nature of the CO.sub.2 removal stage,
a water condensation or removal stage 150 may be desirable to
remove a water output stream 153 from the anode exhaust. Though
shown in FIG. 1 after the CO.sub.2 separation stage 140, it may
optionally be located before the CO.sub.2 separation stage 140
instead. Additionally, an optional membrane separation stage 160
for separation of H.sub.2 can be used to generate a high purity
permeate stream 163 of H.sub.2. The resulting retentate stream 166
can then be used as an input to a chemical synthesis process.
Stream 166 could additionally or alternately be shifted in a second
water-gas shift reactor 131 to adjust the H.sub.2, CO, and CO.sub.2
content to a different ratio, producing an output stream 168 for
further use in a chemical synthesis process. In FIG. 1, anode
recycle stream 185 is shown as being withdrawn from the retentate
stream 166, but the anode recycle stream 185 could additionally or
alternately be withdrawn from other convenient locations in or
between the various separation stages. The separation stages and
shift reactor(s) could additionally or alternately be configured in
different orders, and/or in a parallel configuration. Finally, a
stream with a reduced content of CO.sub.2 139 can be generated as
an output from cathode 129. For the sake of simplicity, various
stages of compression and heat addition/removal that might be
useful in the process, as well as steam addition or removal, are
not shown.
[0113] As noted above, the various types of separations performed
on the anode exhaust can be performed in any convenient order. FIG.
2 shows an example of an alternative order for performing
separations on an anode exhaust. In FIG. 2, anode exhaust 125 can
be initially passed into separation stage 260 for removing a
portion 263 of the hydrogen content from the anode exhaust 125.
This can allow, for example, reduction of the H.sub.2 content of
the anode exhaust to provide a retentate 266 with a ratio of
H.sub.2 to CO closer to 2:1. The ratio of H.sub.2 to CO can then be
further adjusted to achieve a desired value in a water gas shift
stage 230. The water gas shifted output 235 can then pass through
CO.sub.2 separation stage 240 and water removal stage 250 to
produce an output stream 275 suitable for use as an input to a
desired chemical synthesis process. Optionally, output stream 275
could be exposed to an additional water gas shift stage (not
shown). A portion of output stream 275 can optionally be recycled
(not shown) to the anode input. Of course, still other combinations
and sequencing of separation stages can be used to generate a
stream based on the anode output that has a desired composition.
For the sake of simplicity, various stages of compression and heat
addition/removal that might be useful in the process, as well as
steam addition or removal, are not shown.
Cathode Inputs and Outputs
[0114] Conventionally, a molten carbonate fuel cell can be operated
based on drawing a desired load while consuming some portion of the
fuel in the fuel stream delivered to the anode. The voltage of the
fuel cell can then be determined by the load, fuel input to the
anode, air and CO.sub.2 provided to the cathode, and the internal
resistances of the fuel cell. The CO.sub.2 to the cathode can be
conventionally provided in part by using the anode exhaust as at
least a part of the cathode input stream. By contrast, the present
invention can use separate/different sources for the anode input
and cathode input. By removing any direct link between the
composition of the anode input flow and the cathode input flow,
additional options become available for operating the fuel cell,
such as to generate excess synthesis gas, to improve capture of
carbon dioxide, and/or to improve the total efficiency (electrical
plus chemical power) of the fuel cell, among others.
[0115] In a molten carbonate fuel cell, the transport of carbonate
ions across the electrolyte in the fuel cell can provide a method
for transporting CO.sub.2 from a first flow path to a second flow
path, where the transport method can allow transport from a lower
concentration (the cathode) to a higher concentration (the anode),
which can thus facilitate capture of CO.sub.2. Part of the
selectivity of the fuel cell for CO.sub.2 separation can be based
on the electrochemical reactions allowing the cell to generate
electrical power. For nonreactive species (such as N.sub.2) that
effectively do not participate in the electrochemical reactions
within the fuel cell, there can be an insignificant amount of
reaction and transport from cathode to anode. By contrast, the
potential (voltage) difference between the cathode and anode can
provide a strong driving force for transport of carbonate ions
across the fuel cell. As a result, the transport of carbonate ions
in the molten carbonate fuel cell can allow CO.sub.2 to be
transported from the cathode (lower CO.sub.2 concentration) to the
anode (higher CO.sub.2 concentration) with relatively high
selectivity. However, a challenge in using molten carbonate fuel
cells for carbon dioxide removal can be that the fuel cells have
limited ability to remove carbon dioxide from relatively dilute
cathode feeds. The voltage and/or power generated by a carbonate
fuel cell can start to drop rapidly as the CO.sub.2 concentration
falls below about 2.0 vol %. As the CO.sub.2 concentration drops
further, e.g., to below about 1.0 vol %, at some point the voltage
across the fuel cell can become low enough that little or no
further transport of carbonate may occur and the fuel cell ceases
to function. Thus, at least some CO.sub.2 is likely to be present
in the exhaust gas from the cathode stage of a fuel cell under
commercially viable operating conditions.
[0116] The amount of carbon dioxide delivered to the fuel cell
cathode(s) can be determined based on the CO.sub.2 content of a
source for the cathode inlet. One example of a suitable
CO.sub.2-containing stream for use as a cathode input flow can be
an output or exhaust flow from a combustion source. Examples of
combustion sources include, but are not limited to, sources based
on combustion of natural gas, combustion of coal, and/or combustion
of other hydrocarbon-type fuels (including biologically derived
fuels). Additional or alternate sources can include other types of
boilers, fired heaters, furnaces, and/or other types of devices
that burn carbon-containing fuels in order to heat another
substance (such as water or air). To a first approximation, the
CO.sub.2 content of the output flow from a combustion source can be
a minor portion of the flow. Even for a higher CO.sub.2 content
exhaust flow, such as the output from a coal-fired combustion
source, the CO.sub.2 content from most commercial coal-fired power
plants can be about 15 vol % or less. More generally, the CO.sub.2
content of an output or exhaust flow from a combustion source can
be at least about 1.5 vol %, or at least about 1.6 vol %, or at
least about 1.7 vol %, or at least about 1.8 vol %, or at least
about 1.9 vol %, or at least greater 2 vol %, or at least about 4
vol %, or at least about 5 vol %, or at least about 6 vol %, or at
least about 8 vol %. Additionally or alternately, the CO.sub.2
content of an output or exhaust flow from a combustion source can
be about 20 vol % or less, such as about 15 vol % or less, or about
12 vol % or less, or about 10 vol % or less, or about 9 vol % or
less, or about 8 vol % or less, or about 7 vol % or less, or about
6.5 vol % or less, or about 6 vol % or less, or about 5.5 vol % or
less, or about 5 vol % or less, or about 4.5 vol % or less. The
concentrations given above are on a dry basis. It is noted that the
lower CO.sub.2 content values can be present in the exhaust from
some natural gas or methane combustion sources, such as generators
that are part of a power generation system that may or may not
include an exhaust gas recycle loop.
[0117] Other potential sources for a cathode input stream can
additionally or alternately include sources of bio-produced
CO.sub.2. This can include, for example, CO.sub.2 generated during
processing of bio-derived compounds, such as CO.sub.2 generated
during ethanol production. An additional or alternate example can
include CO.sub.2 generated by combustion of a bio-produced fuel,
such as combustion of lignocellulose. Still other additional or
alternate potential CO.sub.2 sources can correspond to output or
exhaust streams from various industrial processes, such as
CO.sub.2-containing streams generated by plants for manufacture of
steel, cement, and/or paper.
[0118] Yet another additional or alternate potential source of
CO.sub.2 can be CO.sub.2-containing streams from a fuel cell. The
CO.sub.2-containing stream from a fuel cell can correspond to a
cathode output stream from a different fuel cell, an anode output
stream from a different fuel cell, a recycle stream from the
cathode output to the cathode input of a fuel cell, and/or a
recycle stream from an anode output to a cathode input of a fuel
cell. For example, an MCFC operated in standalone mode under
conventional conditions can generate a cathode exhaust with a
CO.sub.2 concentration of at least about 5 vol %. Such a
CO.sub.2-containing cathode exhaust could be used as a cathode
input for an MCFC operated according to an aspect of the invention.
More generally, other types of fuel cells that generate a CO.sub.2
output from the cathode exhaust can additionally or alternately be
used, as well as other types of CO.sub.2-containing streams not
generated by a "combustion" reaction and/or by a combustion-powered
generator. Optionally but preferably, a CO.sub.2-containing stream
from another fuel cell can be from another molten carbonate fuel
cell. For example, for molten carbonate fuel cells connected in
series with respect to the cathodes, the output from the cathode
for a first molten carbonate fuel cell can be used as the input to
the cathode for a second molten carbonate fuel cell.
[0119] For various types of CO.sub.2-containing streams from
sources other than combustion sources, the CO.sub.2 content of the
stream can vary widely. The CO.sub.2 content of an input stream to
a cathode can contain at least about 2 vol % of CO.sub.2, such as
at least about 4 vol %, or at least about 5 vol %, or at least
about 6 vol %, or at least about 8 vol %. Additionally or
alternately, the CO.sub.2 content of an input stream to a cathode
can be about 30 vol % or less, such as about 25 vol % or less, or
about 20 vol % or less, or about 15 vol % or less, or about 10 vol
% or less, or about 8 vol % or less, or about 6 vol % or less, or
about 4 vol % or less. For some still higher CO.sub.2 content
streams, the CO.sub.2 content can be greater than about 30 vol %,
such as a stream substantially composed of CO.sub.2 with only
incidental amounts of other compounds. As an example, a gas-fired
turbine without exhaust gas recycle can produce an exhaust stream
with a CO.sub.2 content of approximately 4.2 vol %. With EGR, a
gas-fired turbine can produce an exhaust stream with a CO.sub.2
content of about 6-8 vol %. Stoichiometric combustion of methane
can produce an exhaust stream with a CO.sub.2 content of about 11
vol %. Combustion of coal can produce an exhaust stream with a
CO.sub.2 content of about 15-20 vol %. Fired heaters using refinery
off-gas can produce an exhaust stream with a CO.sub.2 content of
about 12-15 vol %. A gas turbine operated on a low BTU gas without
any EGR can produce an exhaust stream with a CO.sub.2 content of
.about.12 vol %.
[0120] In addition to CO.sub.2, a cathode input stream must include
O.sub.2 to provide the components necessary for the cathode
reaction. Some cathode input streams can be based on having air as
a component. For example, a combustion exhaust stream can be formed
by combusting a hydrocarbon fuel in the presence of air. Such a
combustion exhaust stream, or another type of cathode input stream
having an oxygen content based on inclusion of air, can have an
oxygen content of about 20 vol % or less, such as about 15 vol % or
less, or about 10 vol % or less. Additionally or alternately, the
oxygen content of the cathode input stream can be at least about 4
vol %, such as at least about 6 vol %, or at least about 8 vol %.
More generally, a cathode input stream can have a suitable content
of oxygen for performing the cathode reaction. In some aspects,
this can correspond to an oxygen content of about 5 vol % to about
15 vol %, such as from about 7 vol % to about 9 vol %. For many
types of cathode input streams, the combined amount of CO.sub.2 and
O.sub.2 can correspond to less than about 21 vol % of the input
stream, such as less than about 15 vol % of the stream or less than
about 10 vol % of the stream. An air stream containing oxygen can
be combined with a CO.sub.2 source that has low oxygen content. For
example, the exhaust stream generated by burning coal may include a
low oxygen content that can be mixed with air to form a cathode
inlet stream.
[0121] In addition to CO.sub.2 and O.sub.2, a cathode input stream
can also be composed of inert/non-reactive species such as N.sub.2,
H.sub.2O, and other typical oxidant (air) components. For example,
for a cathode input derived from an exhaust from a combustion
reaction, if air is used as part of the oxidant source for the
combustion reaction, the exhaust gas can include typical components
of air such as N.sub.2, H.sub.2O, and other compounds in minor
amounts that are present in air. Depending on the nature of the
fuel source for the combustion reaction, additional species present
after combustion based on the fuel source may include one or more
of H.sub.2O, oxides of nitrogen (NOx) and/or sulfur (SOx), and
other compounds either present in the fuel and/or that are partial
or complete combustion products of compounds present in the fuel,
such as CO. These species may be present in amounts that do not
poison the cathode catalyst surfaces though they may reduce the
overall cathode activity. Such reductions in performance may be
acceptable, or species that interact with the cathode catalyst may
be reduced to acceptable levels by known pollutant removal
technologies.
[0122] The amount of O.sub.2 present in a cathode input stream
(such as an input cathode stream based on a combustion exhaust) can
advantageously be sufficient to provide the oxygen needed for the
cathode reaction in the fuel cell. Thus, the volume percentage of
O.sub.2 can advantageously be at least 0.5 times the amount of
CO.sub.2 in the exhaust. Optionally, as necessary, additional air
can be added to the cathode input to provide sufficient oxidant for
the cathode reaction. When some form of air is used as the oxidant,
the amount of N.sub.2 in the cathode exhaust can be at least about
78 vol %, e.g., at least about 88 vol %, and/or about 95 vol % or
less. In some aspects, the cathode input stream can additionally or
alternately contain compounds that are generally viewed as
contaminants, such as H.sub.2S or NH.sub.3. In other aspects, the
cathode input stream can be cleaned to reduce or minimize the
content of such contaminants.
[0123] In addition to the reaction to form carbonate ions for
transport across the electrolyte, the conditions in the cathode can
also be suitable for conversion of nitrogen oxides into nitrate
and/or nitrate ions. Hereinafter, only nitrate ions will be
referred to for convenience. The resulting nitrate ions can also be
transported across the electrolyte for reaction in the anode. NOx
concentrations in a cathode input stream can typically be on the
order of ppm, so this nitrate transport reaction can have a minimal
impact on the amount of carbonate transported across the
electrolyte. However, this method of NOx removal can be beneficial
for cathode input streams based on combustion exhausts from gas
turbines, as this can provide a mechanism for reducing NOx
emissions. The conditions in the cathode can additionally or
alternately be suitable for conversion of unburned hydrocarbons (in
combination with O.sub.2 in the cathode input stream) to typical
combustion products, such as CO.sub.2 and H.sub.2O.
[0124] A suitable temperature for operation of an MCFC can be
between about 450.degree. C. and about 750.degree. C., such as at
least about 500.degree. C., e.g., with an inlet temperature of
about 550.degree. C. and an outlet temperature of about 625.degree.
C. Prior to entering the cathode, heat can be added to or removed
from the combustion exhaust, if desired, e.g., to provide heat for
other processes, such as reforming the fuel input for the anode.
For example, if the source for the cathode input stream is a
combustion exhaust stream, the combustion exhaust stream may have a
temperature greater than a desired temperature for the cathode
inlet. In such an aspect, heat can be removed from the combustion
exhaust prior to use as the cathode input stream. Alternatively,
the combustion exhaust could be at very low temperature, for
example after a wet gas scrubber on a coal-fired boiler, in which
case the combustion exhaust can be below about 100.degree. C.
Alternatively, the combustion exhaust could be from the exhaust of
a gas turbine operated in combined cycle mode, in which the gas can
be cooled by raising steam to run a steam turbine for additional
power generation. In this case, the gas can be below about
50.degree. C. Heat can be added to a combustion exhaust that is
cooler than desired.
Fuel Cell Arrangement
[0125] In various aspects, a configuration option for a fuel cell
(such as a fuel cell array containing multiple fuel cell stacks)
can be to divide the CO.sub.2-containing stream between a plurality
of fuel cells. Some types of sources for CO.sub.2-containing
streams can generate large volumetric flow rates relative to the
capacity of an individual fuel cell. For example, the
CO.sub.2-containing output stream from an industrial combustion
source can typically correspond to a large flow volume relative to
desirable operating conditions for a single MCFC of reasonable
size. Instead of processing the entire flow in a single MCFC, the
flow can be divided amongst a plurality of MCFC units, usually at
least some of which can be in parallel, so that the flow rate in
each unit can be within a desired flow range.
[0126] A second configuration option can be to utilize fuel cells
in series to successively remove CO.sub.2 from a flow stream.
Regardless of the number of initial fuel cells to which a
CO.sub.2-containing stream can be distributed to in parallel, each
initial fuel cell can be followed by one or more additional cells
in series to further remove additional CO.sub.2. If the desired
amount of CO.sub.2 in the cathode output is sufficiently low,
attempting to remove CO.sub.2 from a cathode input stream down to
the desired level in a single fuel cell or fuel cell stage could
lead to a low and/or unpredictable voltage output for the fuel
cell. Rather than attempting to remove CO.sub.2 to the desired
level in a single fuel cell or fuel cell stage, CO.sub.2 can be
removed in successive cells until a desired level can be achieved.
For example, each cell in a series of fuel cells can be used to
remove some percentage (e.g., about 50%) of the CO.sub.2 present in
a fuel stream. In such an example, if three fuel cells are used in
series, the CO.sub.2 concentration can be reduced (e.g., to about
15% or less of the original amount present, which can correspond to
reducing the CO.sub.2 concentration from about 6% to about 1% or
less over the course of three fuel cells in series).
[0127] In another configuration, the operating conditions can be
selected in early fuel stages in series to provide a desired output
voltage while the array of stages can be selected to achieve a
desired level of carbon separation. As an example, an array of fuel
cells can be used with three fuel cells in series. The first two
fuel cells in series can be used to remove CO.sub.2 while
maintaining a desired output voltage. The final fuel cell can then
be operated to remove CO.sub.2 to a desired concentration but at a
lower voltage.
[0128] In still another configuration, there can be separate
connectivity for the anodes and cathodes in a fuel cell array. For
example, if the fuel cell array includes fuel cathodes connected in
series, the corresponding anodes can be connected in any convenient
manner, not necessarily matching up with the same arrangement as
their corresponding cathodes, for example. This can include, for
instance, connecting the anodes in parallel, so that each anode
receives the same type of fuel feed, and/or connecting the anodes
in a reverse series, so that the highest fuel concentration in the
anodes can correspond to those cathodes having the lowest CO.sub.2
concentration.
[0129] In yet another configuration, the amount of fuel delivered
to one or more anode stages and/or the amount of CO.sub.2 delivered
to one or more cathode stages can be controlled in order to improve
the performance of the fuel cell array. For example, a fuel cell
array can have a plurality of cathode stages connected in series.
In an array that includes three cathode stages in series, this can
mean that the output from a first cathode stage can correspond to
the input for a second cathode stage, and the output from the
second cathode stage can correspond to the input for a third
cathode stage. In this type of configuration, the CO.sub.2
concentration can decrease with each successive cathode stage. To
compensate for this reduced CO.sub.2 concentration, additional
hydrogen and/or methane can be delivered to the anode stages
corresponding to the later cathode stages. The additional hydrogen
and/or methane in the anodes corresponding to the later cathode
stages can at least partially offset the loss of voltage and/or
current caused by the reduced CO.sub.2 concentration, which can
increase the voltage and thus net power produced by the fuel cell.
In another example, the cathodes in a fuel cell array can be
connected partially in series and partially in parallel. In this
type of example, instead of passing the entire combustion output
into the cathodes in the first cathode stage, at least a portion of
the combustion exhaust can be passed into a later cathode stage.
This can provide an increased CO.sub.2 content in a later cathode
stage. Still other options for using variable feeds to either anode
stages or cathode stages can be used if desired.
[0130] The cathode of a fuel cell can correspond to a plurality of
cathodes from an array of fuel cells, as previously described. In
some aspects, a fuel cell array can be operated to improve or
maximize the amount of carbon transferred from the cathode to the
anode. In such aspects, for the cathode output from the final
cathode(s) in an array sequence (typically at least including a
series arrangement, or else the final cathode(s) and the initial
cathode(s) would be the same), the output composition can include
about 2.0 vol % or less of CO.sub.2 (e.g., about 1.5 vol % or less
or about 1.2 vol % or less) and/or at least about 0.5 vol % of
CO.sub.2, or at least about 1.0 vol %, or at least about 1.2 vol %
or at least about 1.5 vol %. Due to this limitation, the net
efficiency of CO.sub.2 removal when using molten carbonate fuel
cells can be dependent on the amount of CO.sub.2 in the cathode
input. For cathode input streams with CO.sub.2 contents of greater
than about 6 vol %, such as at least about 8%, the limitation on
the amount of CO.sub.2 that can be removed is not severe. However,
for a combustion reaction using natural gas as a fuel and with
excess air, as is typically found in a gas turbine, the amount of
CO.sub.2 in the combustion exhaust may only correspond to a
CO.sub.2 concentration at the cathode input of less than about 5
vol %. Use of exhaust gas recycle can allow the amount of CO.sub.2
at the cathode input to be increased to at least about 5 vol %,
e.g., at least about 6 vol %. If EGR is increased when using
natural gas as a fuel to produce a CO.sub.2 concentration beyond
about 6 vol %, then the flammability in the combustor can be
decreased and the gas turbine may become unstable. However, when
H.sub.2 is added to the fuel, the flammability window can be
significantly increased, allowing the amount of exhaust gas recycle
to be increased further, so that concentrations of CO.sub.2 at the
cathode input of at least about 7.5 vol % or at least about 8 vol %
can be achieved. As an example, based on a removal limit of about
1.5 vol % at the cathode exhaust, increasing the CO.sub.2 content
at the cathode input from about 5.5 vol % to about 7.5 vol % can
correspond to a .about.10% increase in the amount of CO.sub.2 that
can be captured using a fuel cell and transported to the anode loop
for eventual CO.sub.2 separation. The amount of O.sub.2 in the
cathode output can additionally or alternately be reduced,
typically in an amount proportional to the amount of CO.sub.2
removed, which can result in small corresponding increases in the
amount(s) of the other (non-cathode-reactive) species at the
cathode exit.
[0131] In other aspects, a fuel cell array can be operated to
improve or maximize the energy output of the fuel cell, such as the
total energy output, the electric energy output, the syngas
chemical energy output, or a combination thereof. For example,
molten carbonate fuel cells can be operated with an excess of
reformable fuel in a variety of situations, such as for generation
of a syngas stream for use in chemical synthesis plant and/or for
generation of a high purity hydrogen stream. The syngas stream
and/or hydrogen stream can be used as a syngas source, a hydrogen
source, as a clean fuel source, and/or for any other convenient
application. In such aspects, the amount of CO.sub.2 in the cathode
exhaust can be related to the amount of CO.sub.2 in the cathode
input stream and the CO.sub.2 utilization at the desired operating
conditions for improving or maximizing the fuel cell energy
output.
[0132] Additionally or alternately, depending on the operating
conditions, an MCFC can lower the CO.sub.2 content of a cathode
exhaust stream to about 5.0 vol % or less, e.g., about 4.0 vol % or
less, or about 2.0 vol % or less, or about 1.5 vol % or less, or
about 1.2 vol % or less. Additionally or alternately, the CO.sub.2
content of the cathode exhaust stream can be at least about 0.9 vol
%, such as at least about 1.0 vol %, or at least about 1.2 vol %,
or at least about 1.5 vol %.
Molten Carbonate Fuel Cell Operation
[0133] In some aspects, a fuel cell may be operated in a single
pass or once-through mode. In single pass mode, reformed products
in the anode exhaust are not returned to the anode inlet. Thus,
recycling syngas, hydrogen, or some other product from the anode
output directly to the anode inlet is not done in single pass
operation. More generally, in single pass operation, reformed
products in the anode exhaust are also not returned indirectly to
the anode inlet, such as by using reformed products to process a
fuel stream subsequently introduced into the anode inlet.
Optionally, CO.sub.2 from the anode outlet can be recycled to the
cathode inlet during operation of an MCFC in single pass mode. More
generally, in some alternative aspects, recycling from the anode
outlet to the cathode inlet may occur for an MCFC operating in
single pass mode. Heat from the anode exhaust or output may
additionally or alternately be recycled in a single pass mode. For
example, the anode output flow may pass through a heat exchanger
that cools the anode output and warms another stream, such as an
input stream for the anode and/or the cathode. Recycling heat from
anode to the fuel cell is consistent with use in single pass or
once-through operation. Optionally but not preferably, constituents
of the anode output may be burned to provide heat to the fuel cell
during single pass mode.
[0134] FIG. 3 shows a schematic example of the operation of an MCFC
for generation of electrical power. In FIG. 3, the anode portion of
the fuel cell can receive fuel and steam (H.sub.2O) as inputs, with
outputs of water, CO.sub.2, and optionally excess H.sub.2, CH.sub.4
(or other hydrocarbons), and/or CO. The cathode portion of the fuel
cell can receive CO.sub.2 and some oxidant (e.g., air/O.sub.2) as
inputs, with an output corresponding to a reduced amount of
CO.sub.2 in O.sub.2-depleted oxidant (air). Within the fuel cell,
CO.sub.3.sup.2- ions formed in the cathode side can be transported
across the electrolyte to provide the carbonate ions needed for the
reactions occurring at the anode.
[0135] Several reactions can occur within a molten carbonate fuel
cell such as the example fuel cell shown in FIG. 3. The reforming
reactions can be optional, and can be reduced or eliminated if
sufficient H.sub.2 is provided directly to the anode. The following
reactions are based on CH.sub.4, but similar reactions can occur
when other fuels are used in the fuel cell.
<anode reforming>CH.sub.4+H.sub.2O=>3H.sub.2+CO (1)
<water gas shift>CO+H.sub.2O=>H.sub.2+CO.sub.2 (2)
<reforming and water gas shift
combined>CH.sub.4+2H.sub.2O=>4H.sub.2+CO.sub.2 (3)
<anode H.sub.2
oxidation>H.sub.2+CO.sub.3.sup.2-=>H.sub.2O+CO.sub.2+2e.sup.-
(4)
<cathode>1/2O.sub.2+CO.sub.2+2e.sup.-=>CO.sub.3.sup.2-
(5)
[0136] Reaction (1) represents the basic hydrocarbon reforming
reaction to generate H.sub.2 for use in the anode of the fuel cell.
The CO formed in reaction (1) can be converted to H.sub.2 by the
water-gas shift reaction (2). The combination of reactions (1) and
(2) is shown as reaction (3). Reactions (1) and (2) can occur
external to the fuel cell, and/or the reforming can be performed
internal to the anode.
[0137] Reactions (4) and (5), at the anode and cathode
respectively, represent the reactions that can result in electrical
power generation within the fuel cell. Reaction (4) combines
H.sub.2, either present in the feed or optionally generated by
reactions (1) and/or (2), with carbonate ions to form H.sub.2O,
CO.sub.2, and electrons to the circuit. Reaction (5) combines
O.sub.2, CO.sub.2, and electrons from the circuit to form carbonate
ions. The carbonate ions generated by reaction (5) can be
transported across the electrolyte of the fuel cell to provide the
carbonate ions needed for reaction (4). In combination with the
transport of carbonate ions across the electrolyte, a closed
current loop can then be formed by providing an electrical
connection between the anode and cathode.
[0138] In various embodiments, a goal of operating the fuel cell
can be to improve the total efficiency of the fuel cell and/or the
total efficiency of the fuel cell plus an integrated chemical
synthesis process. This is typically in contrast to conventional
operation of a fuel cell, where the goal can be to operate the fuel
cell with high electrical efficiency for using the fuel provided to
the cell for generation of electrical power. As defined above,
total fuel cell efficiency may be determined by dividing the
electric output of the fuel cell plus the lower heating value of
the fuel cell outputs by the lower heating value of the input
components for the fuel cell. In other words, TFCE=(LHV(el)+LHV(sg
out))/LHV(in), where LHV(in) and LHV(sg out) refer to the LHV of
the fuel components (such as H.sub.2, CH.sub.4, and/or CO)
delivered to the fuel cell and syngas (H.sub.2, CO and/or CO.sub.2)
in the anode outlet streams or flows, respectively. This can
provide a measure of the electric energy plus chemical energy
generated by the fuel cell and/or the integrated chemical process.
It is noted that under this definition of total efficiency, heat
energy used within the fuel cell and/or used within the integrated
fuel cell/chemical synthesis system can contribute to total
efficiency. However, any excess heat exchanged or otherwise
withdrawn from the fuel cell or integrated fuel cell/chemical
synthesis system is excluded from the definition. Thus, if excess
heat from the fuel cell is used, for example, to generate steam for
electricity generation by a steam turbine, such excess heat is
excluded from the definition of total efficiency.
[0139] Several operational parameters may be manipulated to operate
a fuel cell with excess reformable fuel. Some parameters can be
similar to those currently recommended for fuel cell operation. In
some aspects, the cathode conditions and temperature inputs to the
fuel cell can be similar to those recommended in the literature.
For example, the desired electrical efficiency and the desired
total fuel cell efficiency may be achieved at a range of fuel cell
operating temperatures typical for molten carbonate fuel cells. In
typical operation, the temperature can increase across the fuel
cell.
[0140] In other aspects, the operational parameters of the fuel
cell can deviate from typical conditions so that the fuel cell is
operated to allow a temperature decrease from the anode inlet to
the anode outlet and/or from the cathode inlet to the cathode
outlet. For example, the reforming reaction to convert a
hydrocarbon into H.sub.2 and CO is an endothermic reaction. If a
sufficient amount of reforming is performed in a fuel cell anode
relative to the amount of oxidation of hydrogen to generate
electrical current, the net heat balance in the fuel cell can be
endothermic. This can cause a temperature drop between the inlets
and outlets of a fuel cell. During endothermic operation, the
temperature drop in the fuel cell can be controlled so that the
electrolyte in the fuel cell remains in a molten state.
[0141] Parameters that can be manipulated in a way so as to differ
from those currently recommended can include the amount of fuel
provided to the anode, the composition of the fuel provided to the
anode, and/or the separation and capture of syngas in the anode
output without significant recycling of syngas from the anode
exhaust to either the anode input or the cathode input. In some
aspects, no recycle of syngas or hydrogen from the anode exhaust to
either the anode input or the cathode input can be allowed to
occur, either directly or indirectly. In additional or alternative
aspects, a limited amount of recycle can occur. In such aspects,
the amount of recycle from the anode exhaust to the anode input
and/or the cathode input can be less than about 10 vol % of the
anode exhaust, such as less than about 5 vol %, or less than about
1 vol %.
[0142] Additionally or alternately, a goal of operating a fuel cell
can be to separate CO.sub.2 from the output stream of a combustion
reaction or another process that produces a CO.sub.2 output stream,
in addition to allowing generation of electric power. In such
aspects, the combustion reaction(s) can be used to power one or
more generators or turbines, which can provide a majority of the
power generated by the combined generator/fuel cell system. Rather
than operating the fuel cell to optimize power generation by the
fuel cell, the system can instead be operated to improve the
capture of carbon dioxide from the combustion-powered generator
while reducing or minimizing the number of fuels cells required for
capturing the carbon dioxide. Selecting an appropriate
configuration for the input and output flows of the fuel cell, as
well as selecting appropriate operating conditions for the fuel
cell, can allow for a desirable combination of total efficiency and
carbon capture.
[0143] In some embodiments, the fuel cells in a fuel cell array can
be arranged so that only a single stage of fuel cells (such as fuel
cell stacks) can be present. In this type of embodiment, the anode
fuel utilization for the single stage can represent the anode fuel
utilization for the array. Another option can be that a fuel cell
array can contain multiple stages of anodes and multiple stages of
cathodes, with each anode stage having a fuel utilization within
the same range, such as each anode stage having a fuel utilization
within 10% of a specified value, for example within 5% of a
specified value. Still another option can be that each anode stage
can have a fuel utilization equal to a specified value or lower
than the specified value by less than an amount, such as having
each anode stage be not greater than a specified value by 10% or
less, for example, by 5% or less. As an illustrative example, a
fuel cell array with a plurality of anode stages can have each
anode stage be within about 10% of 50% fuel utilization, which
would correspond to each anode stage having a fuel utilization
between about 40% and about 60%. As another example, a fuel cell
array with a plurality of stages can have each anode stage be not
greater than 60% anode fuel utilization with the maximum deviation
being about 5% less, which would correspond to each anode stage
having a fuel utilization between about 55% to about 60%. In still
another example, one or more stages of fuel cells in a fuel cell
array can be operated at a fuel utilization from about 30% to about
50%, such as operating a plurality of fuel cell stages in the array
at a fuel utilization from about 30% to about 50%. More generally,
any of the above types of ranges can be paired with any of the
anode fuel utilization values specified herein.
[0144] Still another additional or alternate option can include
specifying a fuel utilization for less than all of the anode
stages. For example, in some aspects of the invention fuel
cells/stacks can be arranged at least partially in one or more
series arrangements such that anode fuel utilization can be
specified for the first anode stage in a series, the second anode
stage in a series, the final anode stage in a series, or any other
convenient anode stage in a series. As used herein, the "first"
stage in a series corresponds to the stage (or set of stages, if
the arrangement contains parallel stages as well) to which input is
directly fed from the fuel source(s), with later ("second,"
"third," "final," etc.) stages representing the stages to which the
output from one or more previous stages is fed, instead of directly
from the respective fuel source(s). In situations where both output
from previous stages and input directly from the fuel source(s) are
co-fed into a stage, there can be a "first" (set of) stage(s) and a
"last" (set of) stage(s), but other stages ("second," "third,"
etc.) can be more tricky among which to establish an order (e.g.,
in such cases, ordinal order can be determined by concentration
levels of one or more components in the composite input feed
composition, such as CO.sub.2 for instance, from highest
concentration "first" to lowest concentration "last" with
approximately similar compositional distinctions representing the
same ordinal level.)
[0145] Yet another additional or alternate option can be to specify
the anode fuel utilization corresponding to a particular cathode
stage (again, where fuel cells/stacks can be arranged at least
partially in one or more series arrangements). As noted above,
based on the direction of the flows within the anodes and cathodes,
the first cathode stage may not correspond to (be across the same
fuel cell membrane from) the first anode stage. Thus, in some
aspects of the invention, the anode fuel utilization can be
specified for the first cathode stage in a series, the second
cathode stage in a series, the final cathode stage in a series, or
any other convenient cathode stage in a series.
[0146] Yet still another additional or alternate option can be to
specify an overall average of fuel utilization over all fuel cells
in a fuel cell array. In various aspects, the overall average of
fuel utilization for a fuel cell array can be about 65% or less,
for example, about 60% or less, about 55% or less, about 50% or
less, or about 45% or less (additionally or alternately, the
overall average fuel utilization for a fuel cell array can be at
least about 25%, for example at least about 30%, at least about
35%, or at least about 40%). Such an average fuel utilization need
not necessarily constrain the fuel utilization in any single stage,
so long as the array of fuel cells meets the desired fuel
utilization.
Applications for CO.sub.2 Output after Capture
[0147] In various aspects of the invention, the systems and methods
described above can allow for production of carbon dioxide as a
pressurized fluid. For example, the CO.sub.2 generated from a
cryogenic separation stage can initially correspond to a
pressurized CO.sub.2 liquid with a purity of at least about 90%,
e.g., at least about 95%, at least about 97%, at least about 98%,
or at least about 99%. This pressurized CO.sub.2 stream can be
used, e.g., for injection into wells in order to further enhance
oil or gas recovery such as in secondary oil recovery. When done in
proximity to a facility that encompasses a gas turbine, the overall
system may benefit from additional synergies in use of
electrical/mechanical power and/or through heat integration with
the overall system.
[0148] Alternatively, for systems dedicated to an enhanced oil
recovery (EOR) application (i.e., not comingled in a pipeline
system with tight compositional standards), the CO.sub.2 separation
requirements may be substantially relaxed. The EOR application can
be sensitive to the presence of O.sub.2, so O.sub.2 can be absent,
in some embodiments, from a CO.sub.2 stream intended for use in
EOR. However, the EOR application can tend to have a low
sensitivity to dissolved CO, H.sub.2, and/or CH.sub.4. Also,
pipelines that transport the CO.sub.2 can be sensitive to these
impurities. Those dissolved gases can typically have only subtle
impacts on the solubilizing ability of CO.sub.2 used for EOR.
Injecting gases such as CO, H.sub.2, and/or CH.sub.4 as EOR gases
can result in some loss of fuel value recovery, but such gases can
be otherwise compatible with EOR applications.
[0149] Additionally or alternately, a potential use for CO.sub.2 as
a pressurized liquid can be as a nutrient in biological processes
such as algae growth/harvesting. The use of MCFCs for CO.sub.2
separation can ensure that most biologically significant pollutants
could be reduced to acceptably low levels, resulting in a
CO.sub.2-containing stream having only minor amounts of other
"contaminant" gases (such as CO, H.sub.2, N.sub.2, and the like,
and combinations thereof) that are unlikely to substantially
negatively affect the growth of photosynthetic organisms. This can
be in stark contrast to the output streams generated by most
industrial sources, which can often contain potentially highly
toxic material such as heavy metals.
[0150] In this type of aspect of the invention, the CO.sub.2 stream
generated by separation of CO.sub.2 in the anode loop can be used
to produce biofuels and/or chemicals, as well as precursors
thereof. Further additionally or alternately, CO.sub.2 may be
produced as a dense fluid, allowing for much easier pumping and
transport across distances, e.g., to large fields of photosynthetic
organisms. Conventional emission sources can emit hot gas
containing modest amounts of CO.sub.2 (e.g., about 4-15%) mixed
with other gases and pollutants. These materials would normally
need to be pumped as a dilute gas to an algae pond or biofuel
"farm". By contrast, the MCFC system according to the invention can
produce a concentrated CO.sub.2 stream (.about.60-70% by volume on
a dry basis) that can be concentrated further to 95%+ (for example
96%+, 97%+, 98%+, or 99%+) and easily liquefied. This stream can
then be transported easily and efficiently over long distances at
relatively low cost and effectively distributed over a wide area.
In these embodiments, residual heat from the combustion source/MCFC
may be integrated into the overall system as well.
[0151] An alternative embodiment may apply where the CO.sub.2
source/MCFC and biological/chemical production sites are
co-located. In that case, only minimal compression may be necessary
(i.e., to provide enough CO.sub.2 pressure to use in the biological
production, e.g., from about 15 psig to about 150 psig). Several
novel arrangements can be possible in such a case. Secondary
reforming may optionally be applied to the anode exhaust to reduce
CH.sub.4 content, and water-gas shift may optionally additionally
or alternately be present to drive any remaining CO into CO.sub.2
and H.sub.2.
[0152] The components from an anode output stream and/or cathode
output stream can be used for a variety of purposes. One option can
be to use the anode output as a source of hydrogen, as described
above. For an MCFC integrated with or co-located with a refinery,
the hydrogen can be used as a hydrogen source for various refinery
processes, such as hydroprocessing. Another option can be to
additionally or alternately use hydrogen as a fuel source where the
CO.sub.2 from combustion has already been "captured." Such hydrogen
can be used in a refinery or other industrial setting as a fuel for
a boiler, furnace, and/or fired heater, and/or the hydrogen can be
used as a feed for an electric power generator, such as a turbine.
Hydrogen from an MCFC fuel cell can further additionally or
alternately be used as an input stream for other types of fuel
cells that require hydrogen as an input, possibly including
vehicles powered by fuel cells. Still another option can be to
additionally or alternately use syngas generated as an output from
an MCFC fuel cell as a fermentation input.
[0153] Another option can be to additionally or alternately use
syngas generated from the anode output. Of course, syngas can be
used as a fuel, although a syngas based fuel can still lead to some
CO.sub.2 production when burned as fuel. In other aspects, a syngas
output stream can be used as an input for a chemical synthesis
process. One option can be to additionally or alternately use
syngas for a Fischer-Tropsch type process, and/or another process
where larger hydrocarbon molecules are formed from the syngas
input. Another option can be to additionally or alternately use
syngas to form an intermediate product such as methanol. Methanol
could be used as the final product, but in other aspects methanol
generated from syngas can be used to generate larger compounds,
such as gasoline, olefins, aromatics, and/or other products. It is
noted that a small amount of CO.sub.2 can be acceptable in the
syngas feed to a methanol synthesis process, and/or to a
Fischer-Tropsch process utilizing a shifting catalyst.
Hydroformylation is an additional or alternate example of still
another synthesis process that can make use of a syngas input.
[0154] It is noted that one variation on use of an MCFC to generate
syngas can be to use MCFC fuel cells as part of a system for
processing methane and/or natural gas withdrawn by an offshore oil
platform or other production system that is a considerable distance
from its ultimate market. Instead of attempting to transport the
gas phase output from a well, or attempting to store the gas phase
product for an extended period, the gas phase output from a well
can be used as the input to an MCFC fuel cell array. This can lead
to a variety of benefits. First, the electric power generated by
the fuel cell array can be used as a power source for the platform.
Additionally, the syngas output from the fuel cell array can be
used as an input for a Fischer-Tropsch process at the production
site. This can allow for formation of liquid hydrocarbon products
more easily transported by pipeline, ship, or railcar from the
production site to, for example, an on-shore facility or a larger
terminal.
[0155] Still other integration options can additionally or
alternately include using the cathode output as a source of higher
purity, heated nitrogen. The cathode input can often include a
large portion of air, which means a substantial portion of nitrogen
can be included in the cathode input. The fuel cell can transport
CO.sub.2 and O.sub.2 from the cathode across the electrolyte to the
anode, and the cathode outlet can have lower concentrations of
CO.sub.2 and O.sub.2, and thus a higher concentration of N.sub.2
than found in air. With subsequent removal of the residual O.sub.2
and CO.sub.2, this nitrogen output can be used as an input for
production of ammonia or other nitrogen-containing chemicals, such
as urea, ammonium nitrate, and/or nitric acid. It is noted that
urea synthesis could additionally or alternately use CO.sub.2
separate from the anode output as an input feed.
Integration Example: Applications for Integration with Combustion
Turbines
[0156] In some aspects of the invention, a combustion source for
generating power and exhausting a CO.sub.2-containing exhaust can
be integrated with the operation of molten carbonate fuel cells. An
example of a suitable combustion source is a gas turbine.
Preferably, the gas turbine can combust natural gas, methane gas,
or another hydrocarbon gas in a combined cycle mode integrated with
steam generation and heat recovery for additional efficiency.
Modern natural gas combined cycle efficiencies are about 60% for
the largest and newest designs. The resulting CO.sub.2-containing
exhaust gas stream can be produced at an elevated temperature
compatible with the MCFC operation, such as 300.degree.
C.-700.degree. C. and preferably 500.degree. C.-650.degree. C. The
gas source can optionally but preferably be cleaned of contaminants
such as sulfur that can poison the MCFC before entering the
turbine. Alternatively, the gas source can be a coal-fired
generator, wherein the exhaust gas would typically be cleaned
post-combustion due to the greater level of contaminants in the
exhaust gas. In such an alternative, some heat exchange to/from the
gas may be necessary to enable clean-up at lower temperatures. In
additional or alternate embodiments, the source of the
CO.sub.2-containing exhaust gas can be the output from a boiler,
combustor, or other heat source that burns carbon-rich fuels. In
other additional or alternate embodiments, the source of the
CO.sub.2-containing exhaust gas can be bio-produced CO.sub.2 in
combination with other sources.
[0157] For integration with a combustion source, some alternative
configurations for processing of a fuel cell anode can be
desirable. For example, an alternative configuration can be to
recycle at least a portion of the exhaust from a fuel cell anode to
the input of a fuel cell anode. The output stream from an MCFC
anode can include H.sub.2O, CO.sub.2, optionally CO, and optionally
but typically unreacted fuel (such as H.sub.2 or CH.sub.4) as the
primary output components. Instead of using this output stream as
an external fuel stream and/or an input stream for integration with
another process, one or more separations can be performed on the
anode output stream in order to separate the CO.sub.2 from the
components with potential fuel value, such as H.sub.2 or CO. The
components with fuel value can then be recycled to the input of an
anode.
[0158] This type of configuration can provide one or more benefits.
First, CO.sub.2 can be separated from the anode output, such as by
using a cryogenic CO.sub.2 separator. Several of the components of
the anode output (H.sub.2, CO, CH.sub.4) are not easily condensable
components, while CO.sub.2 and H.sub.2O can be separated
individually as condensed phases. Depending on the embodiment, at
least about 90 vol % of the CO.sub.2 in the anode output can be
separated to form a relatively high purity CO.sub.2 output stream.
Alternatively, in some aspects less CO.sub.2 can be removed from
the anode output, so that about 50 vol % to about 90 vol % of the
CO.sub.2 in the anode output can be separated out, such as about 80
vol % or less or about 70 vol % or less. After separation, the
remaining portion of the anode output can correspond primarily to
components with fuel value, as well as reduced amounts of CO.sub.2
and/or H.sub.2O. This portion of the anode output after separation
can be recycled for use as part of the anode input, along with
additional fuel. In this type of configuration, even though the
fuel utilization in a single pass through the MCFC(s) may be low,
the unused fuel can be advantageously recycled for another pass
through the anode. As a result, the single-pass fuel utilization
can be at a reduced level, while avoiding loss (exhaust) of
unburned fuel to the environment.
[0159] Additionally or alternatively to recycling a portion of the
anode exhaust to the anode input, another configuration option can
be to use a portion of the anode exhaust as an input for a
combustion reaction for a turbine or other combustion device, such
as a boiler, furnace, and/or fired heater. The relative amounts of
anode exhaust recycled to the anode input and/or as an input to the
combustion device can be any convenient or desirable amount. If the
anode exhaust is recycled to only one of the anode input and the
combustion device, the amount of recycle can be any convenient
amount, such as up to 100% of the portion of the anode exhaust
remaining after any separation to remove CO.sub.2 and/or H.sub.2O.
When a portion of the anode exhaust is recycled to both the anode
input and the combustion device, the total recycled amount by
definition can be 100% or less of the remaining portion of anode
exhaust. Otherwise, any convenient split of the anode exhaust can
be used. In various embodiments of the invention, the amount of
recycle to the anode input can be at least about 10% of the anode
exhaust remaining after separations, for example at least about
25%, at least about 40%, at least about 50%, at least about 60%, at
least about 75%, or at least about 90%. Additionally or alternately
in those embodiments, the amount of recycle to the anode input can
be about 90% or less of the anode exhaust remaining after
separations, for example about 75% or less, about 60% or less,
about 50% or less, about 40% or less, about 25% or less, or about
10% or less. Further additionally or alternately, in various
embodiments of the invention, the amount of recycle to the
combustion device can be at least about 10% of the anode exhaust
remaining after separations, for example at least about 25%, at
least about 40%, at least about 50%, at least about 60%, at least
about 75%, or at least about 90%. Additionally or alternately in
those embodiments, the amount of recycle to the combustion device
can be about 90% or less of the anode exhaust remaining after
separations, for example about 75% or less, about 60% or less,
about 50% or less, about 40% or less, about 25% or less, or about
10% or less.
[0160] In still other alternative aspects of the invention, the
fuel for a combustion device can additionally or alternately be a
fuel with an elevated quantity of components that are inert and/or
otherwise act as a diluent in the fuel. CO.sub.2 and N.sub.2 are
examples of components in a natural gas feed that can be relatively
inert during a combustion reaction. When the amount of inert
components in a fuel feed reaches a sufficient level, the
performance of a turbine or other combustion source can be
impacted. The impact can be due in part to the ability of the inert
components to absorb heat, which can tend to quench the combustion
reaction. Examples of fuel feeds with a sufficient level of inert
components can include fuel feeds containing at least about 20 vol
% CO.sub.2, or fuel feeds containing at least about 40 vol %
N.sub.2, or fuel feeds containing combinations of CO.sub.2 and
N.sub.2 that have sufficient inert heat capacity to provide similar
quenching ability. (It is noted that CO.sub.2 has a greater heat
capacity than N.sub.2, and therefore lower concentrations of
CO.sub.2 can have a similar impact as higher concentrations of
N.sub.2. CO.sub.2 can also participate in the combustion reactions
more readily than N.sub.2, and in doing so remove H.sub.2 from the
combustion. This consumption of H.sub.2 can have a large impact on
the combustion of the fuel, by reducing the flame speed and
narrowing the flammability range of the air and fuel mixture.) More
generally, for a fuel feed containing inert components that impact
the flammability of the fuel feed, the inert components in the fuel
feed can be at least about 20 vol %, such as at least about 40 vol
%, or at least about 50 vol %, or at least about 60 vol %.
Preferably, the amount of inert components in the fuel feed can be
about 80 vol % or less.
[0161] When a sufficient amount of inert components are present in
a fuel feed, the resulting fuel feed can be outside of the
flammability window for the fuel components of the feed. In this
type of situation, addition of H.sub.2 from a recycled portion of
the anode exhaust to the combustion zone for the generator can
expand the flammability window for the combination of fuel feed and
H.sub.2, which can allow, for example, a fuel feed containing at
least about 20 vol % CO.sub.2 or at least about 40% N.sub.2 (or
other combinations of CO.sub.2 and N.sub.2) to be successfully
combusted.
[0162] Relative to a total volume of fuel feed and H.sub.2
delivered to a combustion zone, the amount of H.sub.2 for expanding
the flammability window can be at least about 5 vol % of the total
volume of fuel feed plus H.sub.2, such as at least about 10 vol %,
and/or about 25 vol % or less. Another option for characterizing
the amount of H.sub.2 to add to expand the flammability window can
be based on the amount of fuel components present in the fuel feed
before H.sub.2 addition. Fuel components can correspond to methane,
natural gas, other hydrocarbons, and/or other components
conventionally viewed as fuel for a combustion-powered turbine or
other generator. The amount of H.sub.2 added to the fuel feed can
correspond to at least about one third of the volume of fuel
components (1:3 ratio of H.sub.2:fuel component) in the fuel feed,
such as at least about half of the volume of the fuel components
(1:2 ratio). Additionally or alternately, the amount of H.sub.2
added to the fuel feed can be roughly equal to the volume of fuel
components in the fuel feed (1:1 ratio) or less. For example, for a
feed containing about 30 vol % CH.sub.4, about 10% N.sub.2, and
about 60% CO.sub.2, a sufficient amount of anode exhaust can be
added to the fuel feed to achieve about a 1:2 ratio of H.sub.2 to
CH.sub.4. For an idealized anode exhaust that contained only
H.sub.2, addition of H.sub.2 to achieve a 1:2 ratio would result in
a feed containing about 26 vol % CH.sub.4, 13 vol % H.sub.2, 9 vol
% N.sub.2, and 52 vol % CO.sub.2.
Exhaust Gas Recycle
[0163] Aside from providing exhaust gas to a fuel cell array for
capture and eventual separation of the CO.sub.2, an additional or
alternate potential use for exhaust gas can include recycle back to
the combustion reaction to increase the CO.sub.2 content. When
hydrogen is available for addition to the combustion reaction, such
as hydrogen from the anode exhaust of the fuel cell array, further
benefits can be gained from using recycled exhaust gas to increase
the CO.sub.2 content within the combustion reaction.
[0164] In various aspects of the invention, the exhaust gas recycle
loop of a power generation system can receive a first portion of
the exhaust gas from combustion, while the fuel cell array can
receive a second portion. The amount of exhaust gas from combustion
recycled to the combustion zone of the power generation system can
be any convenient amount, such as at least about 15% (by volume),
for example at least about 25%, at least about 35%, at least about
45%, or at least about 50%. Additionally or alternately, the amount
of combustion exhaust gas recirculated to the combustion zone can
be about 65% (by volume) or less, e.g., about 60% or less, about
55% or less, about 50% or less, or about 45% or less.
[0165] In one or more aspects of the invention, a mixture of an
oxidant (such as air and/or oxygen-enriched air) and fuel can be
combusted and (simultaneously) mixed with a stream of recycled
exhaust gas. The stream of recycled exhaust gas, which can
generally include products of combustion such as CO.sub.2, can be
used as a diluent to control, adjust, or otherwise moderate the
temperature of combustion and of the exhaust that can enter the
succeeding expander. As a result of using oxygen-enriched air, the
recycled exhaust gas can have an increased CO.sub.2 content,
thereby allowing the expander to operate at even higher expansion
ratios for the same inlet and discharge temperatures, thereby
enabling significantly increased power production.
[0166] A gas turbine system can represent one example of a power
generation system where recycled exhaust gas can be used to enhance
the performance of the system. The gas turbine system can have a
first/main compressor coupled to an expander via a shaft. The shaft
can be any mechanical, electrical, or other power coupling, thereby
allowing a portion of the mechanical energy generated by the
expander to drive the main compressor. The gas turbine system can
also include a combustion chamber configured to combust a mixture
of a fuel and an oxidant. In various aspects of the invention, the
fuel can include any suitable hydrocarbon gas/liquid, such as
syngas, natural gas, methane, ethane, propane, butane, naphtha
diesel, kerosene, aviation fuel, coal derived fuel, bio-fuel,
oxygenated hydrocarbon feedstock, or any combinations thereof. The
oxidant can, in some embodiments, be derived from a second or inlet
compressor fluidly coupled to the combustion chamber and adapted to
compress a feed oxidant. In one or more embodiments of the
invention, the feed oxidant can include atmospheric air and/or
enriched air. When the oxidant includes enriched air alone or a
mixture of atmospheric air and enriched air, the enriched air can
be compressed by the inlet compressor (in the mixture, either
before or after being mixed with the atmospheric air). The enriched
air and/or the air-enriched air mixture can have an overall oxygen
concentration of at least about 25 volume %, e.g., at least about
30 volume %, at least about 35 volume %, at least about 40 volume
%, at least about 45 volume %, or at least about 50 volume %.
Additionally or alternately, the enriched air and/or the
air-enriched air mixture can have an overall oxygen concentration
of about 80 volume % or less, such as about 70 volume % or
less.
[0167] The enriched air can be derived from any one or more of
several sources. For example, the enriched air can be derived from
such separation technologies as membrane separation, pressure swing
adsorption, temperature swing adsorption, nitrogen plant-byproduct
streams, and/or combinations thereof. The enriched air can
additionally or alternately be derived from an air separation unit
(ASU), such as a cryogenic ASU, for producing nitrogen for pressure
maintenance or other purposes. In certain embodiments of the
invention, the reject stream from such an ASU can be rich in
oxygen, having an overall oxygen content from about 50 volume % to
about 70 volume %, can be used as at least a portion of the
enriched air and subsequently diluted, if needed, with unprocessed
atmospheric air to obtain the desired oxygen concentration.
[0168] In addition to the fuel and oxidant, the combustion chamber
can optionally also receive a compressed recycle exhaust gas, such
as an exhaust gas recirculation primarily having CO.sub.2 and
nitrogen components. The compressed recycle exhaust gas can be
derived from the main compressor, for instance, and adapted to help
facilitate combustion of the oxidant and fuel, e.g., by moderating
the temperature of the combustion products. As can be appreciated,
recirculating the exhaust gas can serve to increase CO.sub.2
concentration.
[0169] An exhaust gas directed to the inlet of the expander can be
generated as a product of combustion reaction. The exhaust gas can
have a heightened CO.sub.2 content based, at least in part, on the
introduction of recycled exhaust gas into the combustion reaction.
As the exhaust gas expands through the expander, it can generate
mechanical power to drive the main compressor, to drive an
electrical generator, and/or to power other facilities.
[0170] The power generation system can, in many embodiments, also
include an exhaust gas recirculation (EGR) system. In one or more
aspects of the invention, the EGR system can include a heat
recovery steam generator (HRSG) and/or another similar device
fluidly coupled to a steam gas turbine. In at least one embodiment,
the combination of the HRSG and the steam gas turbine can be
characterized as a power-producing closed Rankine cycle. In
combination with the gas turbine system, the HRSG and the steam gas
turbine can form part of a combined-cycle power generating plant,
such as a natural gas combined-cycle (NGCC) plant. The gaseous
exhaust can be introduced to the HRSG in order to generate steam
and a cooled exhaust gas. The HRSG can include various units for
separating and/or condensing water out of the exhaust stream,
transferring heat to form steam, and/or modifying the pressure of
streams to a desired level. In certain embodiments, the steam can
be sent to the steam gas turbine to generate additional electrical
power.
[0171] After passing through the HRSG and optional removal of at
least some H.sub.2O, the CO.sub.2-containing exhaust stream can, in
some embodiments, be recycled for use as an input to the combustion
reaction. As noted above, the exhaust stream can be compressed (or
decompressed) to match the desired reaction pressure within the
vessel for the combustion reaction.
Example of Integrated System
[0172] FIG. 4 schematically shows an example of an integrated
system including introduction of both CO.sub.2-containing recycled
exhaust gas and H.sub.2 or CO from the fuel cell anode exhaust into
the combustion reaction for powering a turbine. In FIG. 4, the
turbine can include a compressor 402, a shaft 404, an expander 406,
and a combustion zone 415. An oxygen source 411 (such as air and/or
oxygen-enriched air) can be combined with recycled exhaust gas 498
and compressed in compressor 402 prior to entering combustion zone
415. A fuel 412, such as CH.sub.4, and optionally a stream
containing H.sub.2 or CO 187 can be delivered to the combustion
zone. The fuel and oxidant can be reacted in zone 415 and
optionally but preferably passed through expander 406 to generate
electric power. The exhaust gas from expander 106 can be used to
form two streams, e.g., a CO.sub.2-containing stream 422 (that can
be used as an input feed for fuel cell array 425) and another
CO.sub.2-containing stream 492 (that can be used as the input for a
heat recovery and steam generator system 490, which can, for
example, enable additional electricity to be generated using steam
turbines 494). After passing through heat recovery system 490,
including optional removal of a portion of H.sub.2O from the
CO.sub.2-containing stream, the output stream 498 can be recycled
for compression in compressor 402 or a second compressor that is
not shown. The proportion of the exhaust from expander 406 used for
CO.sub.2-containing stream 492 can be determined based on the
desired amount of CO.sub.2 for addition to combustion zone 415.
[0173] As used herein, the EGR ratio is the flow rate for the fuel
cell bound portion of the exhaust gas divided by the combined flow
rate for the fuel cell bound portion and the recovery bound
portion, which is sent to the heat recovery generator. For example,
the EGR ratio for flows shown in FIG. 4 is the flow rate of stream
422 divided by the combined flow rate of streams 422 and 492.
[0174] The CO.sub.2-containing stream 422 can be passed into a
cathode portion (not shown) of a molten carbonate fuel cell array
425. Based on the reactions within fuel cell array 425, CO.sub.2
can be separated from stream 422 and transported to the anode
portion (not shown) of the fuel cell array 425. This can result in
a cathode output stream 424 depleted in CO.sub.2. The cathode
output stream 424 can then be passed into a heat recovery (and
optional steam generator) system 450 for generation of heat
exchange and/or additional generation of electricity using steam
turbines 454 (which may optionally be the same as the
aforementioned steam turbines 494). After passing through heat
recovery and steam generator system 450, the resulting flue gas
stream 456 can be exhausted to the environment and/or passed
through another type of carbon capture technology, such as an amine
scrubber.
[0175] After transport of CO.sub.2 from the cathode side to the
anode side of fuel cell array 425, the anode output 435 can
optionally be passed into a water gas shift reactor 470. Water gas
shift reactor 470 can be used to generate additional H.sub.2 and
CO.sub.2 at the expense of CO (and H.sub.2O) present in the anode
output 435. The output from the optional water gas shift reactor
470 can then be passed into one or more separation stages 440, such
as a cold box or a cryogenic separator. This can allow for
separation of an H.sub.2O stream 447 and CO.sub.2 stream 449 from
the remaining portion of the anode output. The remaining portion of
the anode output 485 can include unreacted H.sub.2 generated by
reforming but not consumed in fuel cell array 425. A first portion
445 of the H.sub.2-containing stream 485 can be recycled to the
input for the anode(s) in fuel cell array 425. A second portion 487
of stream 485 can be used as an input for combustion zone 415. A
third portion 465 can be used as is for another purpose and/or
treated for subsequent further use. Although FIG. 4 and the
description herein schematically details up to three portions, it
is contemplated that only one of these three portions can be
exploited, only two can be exploited, or all three can be exploited
according to the invention.
[0176] In FIG. 4, the exhaust for the exhaust gas recycle loop is
provided by a first heat recovery and steam generator system 490,
while a second heat recovery and steam generator system 450 can be
used to capture excess heat from the cathode output of the fuel
cell array 425. FIG. 5 shows an alternative embodiment where the
exhaust gas recycle loop is provided by the same heat recovery
steam generator used for processing the fuel cell array output. In
FIG. 5, recycled exhaust gas 598 is provided by heat recovery and
steam generator system 550 as a portion of the flue gas stream 556.
This can eliminate the separate heat recovery and steam generator
system associated with the turbine.
[0177] In various embodiments of the invention, the process can be
approached as starting with a combustion reaction for powering a
turbine, an internal combustion engine, or another system where
heat and/or pressure generated by a combustion reaction can be
converted into another form of power. The fuel for the combustion
reaction can comprise or be hydrogen, a hydrocarbon, and/or any
other compound containing carbon that can be oxidized (combusted)
to release energy. Except for when the fuel contains only hydrogen,
the composition of the exhaust gas from the combustion reaction can
have a range of CO.sub.2 contents, depending on the nature of the
reaction (e.g., from at least about 2 vol % to about 25 vol % or
less). Thus, in certain embodiments where the fuel is carbonaceous,
the CO.sub.2 content of the exhaust gas can be at least about 2 vol
%, for example at least about 4 vol %, at least about 5 vol %, at
least about 6 vol %, at least about 8 vol %, or at least about 10
vol %. Additionally or alternately in such carbonaceous fuel
embodiments, the CO.sub.2 content can be about 25 vol % or less,
for example about 20 vol % or less, about 15 vol % or less, about
10 vol % or less, about 7 vol % or less, or about 5 vol % or less.
Exhaust gases with lower relative CO.sub.2 contents (for
carbonaceous fuels) can correspond to exhaust gases from combustion
reactions on fuels such as natural gas with lean (excess air)
combustion. Higher relative CO.sub.2 content exhaust gases (for
carbonaceous fuels) can correspond to optimized natural gas
combustion reactions, such as those with exhaust gas recycle,
and/or combustion of fuels such as coal.
[0178] In some aspects of the invention, the fuel for the
combustion reaction can contain at least about 90 volume % of
compounds containing five carbons or less, e.g., at least about 95
volume %. In such aspects, the CO.sub.2 content of the exhaust gas
can be at least about 4 vol %, for example at least about 5 vol %,
at least about 6 vol %, at least about 7 vol %, or at least about
7.5 vol %. Additionally or alternately, the CO.sub.2 content of the
exhaust gas can be about 13 vol % or less, e.g., about 12 vol % or
less, about 10 vol % or less, about 9 vol % or less, about 8 vol %
or less, about 7 vol % or less, or about 6 vol % or less. The
CO.sub.2 content of the exhaust gas can represent a range of values
depending on the configuration of the combustion-powered generator.
Recycle of an exhaust gas can be beneficial for achieving a
CO.sub.2 content of at least about 6 vol %, while addition of
hydrogen to the combustion reaction can allow for further increases
in CO.sub.2 content to achieve a CO.sub.2 content of at least about
7.5 vol %.
Alternative Configuration--High Severity NOx Turbine
[0179] Gas turbines can be limited in their operation by several
factors. One typical limitation can be that the maximum temperature
in the combustion zone can be controlled below certain limits to
achieve sufficiently low concentrations of nitrogen oxides (NOx) in
order to satisfy regulatory emission limits. Regulatory emission
limits can require a combustion exhaust to have a NOx content of
about 20 vppm or less, and possible 10 vppm or less, when the
combustion exhaust is allowed to exit to the environment.
[0180] NOx formation in natural gas-fired combustion turbines can
be a function of temperature and residence time. Reactions that
result in formation of NOx can be of reduced and/or minimal
importance below a flame temperature of about 1500.degree. F., but
NOx production can increase rapidly as the temperature increases
beyond this point. In a gas turbine, initial combustion products
can be mixed with extra air to cool the mixture to a temperature
around 1200.degree. F., and temperature can be limited by the
metallurgy of the expander blades. Early gas turbines typically
executed the combustion in diffusion flames that had stoichiometric
zones with temperatures well above 1500.degree. F., resulting in
higher NOx concentrations. More recently, the current generation of
`Dry Low Nox` (DLN) burners can use special pre-mixed burners to
burn natural gas at cooler lean (less fuel than stoichiometric)
conditions. For example, more of the dilution air can be mixed in
to the initial flame, and less can be mixed in later to bring the
temperature down to the .about.1200.degree. F. turbine-expander
inlet temperature. The disadvantages for DLN burners can include
poor performance at turndown, higher maintenance, narrow ranges of
operation, and poor fuel flexibility. The latter can be a concern,
as DLN burners can be more difficult to apply to fuels of varying
quality (or difficult to apply at all to liquid fuels). For low BTU
fuels, such as fuels containing a high content of CO.sub.2, DLN
burners are typically not used and instead diffusion burners can be
used. In addition, gas turbine efficiency can be increased by using
a higher turbine-expander inlet temperature. However, because there
can be a limited amount of dilution air, and this amount can
decrease with increased turbine-expander inlet temperature, the DLN
burner can become less effective at maintaining low NOx as the
efficiency of the gas turbine improves.
[0181] In various aspects of the invention, a system integrating a
gas turbine with a fuel cell for carbon capture can allow use of
higher combustion zone temperatures while reducing and/or
minimizing additional NOx emissions, as well as enabling DLN-like
NOx savings via use of turbine fuels that are not presently
compatible with DLN burners. In such aspects, the turbine can be
run at higher power (i.e., higher temperature) resulting in higher
NOx emissions, but also higher power output and potentially higher
efficiency. In some aspects of the invention, the amount of NOx in
the combustion exhaust can be at least about 20 vppm, such as at
least about 30 vppm, or at least about 40 vppm. Additionally or
alternately, the amount of NOx in the combustion exhaust can be
about 1000 vppm or less, such as about 500 vppm or less, or about
250 vppm or less, or about 150 vppm or less, or about 100 vppm or
less. In order to reduce the NOx levels to levels required by
regulation, the resulting NOx can be equilibrated via thermal NOx
destruction (reduction of NOx levels to equilibrium levels in the
exhaust stream) through one of several mechanisms, such as simple
thermal destruction in the gas phase; catalyzed destruction from
the nickel cathode catalyst in the fuel cell array; and/or assisted
thermal destruction prior to the fuel cell by injection of small
amounts of ammonia, urea, or other reductant. This can be assisted
by introduction of hydrogen derived from the anode exhaust. Further
reduction of NOx in the cathode of the fuel cell can be achieved
via electrochemical destruction wherein the NOx can react at the
cathode surface and can be destroyed. This can result in some
nitrogen transport across the membrane electrolyte to the anode,
where it may form ammonia or other reduced nitrogen compounds. With
respect to NOx reduction methods involving an MCFC, the expected
NOx reduction from a fuel cell/fuel cell array can be about 80% or
less of the NOx in the input to the fuel cell cathode, such as
about 70% or less, and/or at least about 5%. It is noted that
sulfidic corrosion can also limit temperatures and affect turbine
blade metallurgy in conventional systems. However, the sulfur
restrictions of the MCFC system can typically require reduced fuel
sulfur levels that reduce or minimize concerns related to sulfidic
corrosion. Operating the MCFC array at low fuel utilization can
further mitigate such concerns, such as in aspects where a portion
of the fuel for the combustion reaction corresponds to hydrogen
from the anode exhaust.
ADDITIONAL EMBODIMENTS
Embodiment 1
[0182] A method for producing electricity, the method comprising:
introducing a fuel stream comprising a reformable fuel into an
anode of a molten carbonate fuel cell, an internal reforming
element associated with the anode, or a combination thereof;
introducing a cathode inlet stream comprising CO.sub.2 and O.sub.2
into a cathode of the molten carbonate fuel cell; generating
electricity within the molten carbonate fuel cell, the molten
carbonate fuel cell being operated at a fuel utilization of about
60% or less; generating an anode exhaust comprising H.sub.2, CO,
and CO.sub.2; separating, from at least a portion of the anode
exhaust, a first H.sub.2-rich gas stream comprising at least about
80 vol % (e.g., at least about 90%) H.sub.2; and combusting at
least a portion of the first H.sub.2-rich gas stream to produce
electricity.
Embodiment 2
[0183] The method of embodiment 1, further comprising (i)
performing a water gas shift process on the anode exhaust, the at
least a portion of the anode exhaust, or a combination thereof;
(ii) separating CO.sub.2 and/or H.sub.2O from the anode exhaust,
the at least a portion of the anode exhaust, or a combination
thereof; or (iii) both (i) and (ii).
Embodiment 3
[0184] The method of embodiment 1 or 2, wherein the separating step
comprises: performing a water gas shift process on the anode
exhaust or at least a portion of the anode exhaust to form a
shifted anode exhaust portion; and separating H.sub.2O and CO.sub.2
from the shifted anode exhaust portion to form the first
H.sub.2-rich gas stream.
Embodiment 4
[0185] The method of any of the above embodiments, wherein
combusting step comprises generating steam from heat generated by
the combustion, and producing electricity from at least a portion
of the generated steam.
Embodiment 5
[0186] The method of any of the above embodiments, wherein the
combusting step comprises combusting the at least a portion of the
first H.sub.2-rich gas stream in a turbine.
Embodiment 6
[0187] The method of any of the above embodiments, wherein the
cathode inlet stream comprises exhaust from combustion of a
carbon-containing fuel in a combustion turbine.
Embodiment 7
[0188] The method of embodiment 6, wherein the carbon-containing
fuel comprises one or more of: at least 5 vol % of inert gases; at
least about 10 vol % CO.sub.2; and at least about 10 vol %
N.sub.2.
Embodiment 8
[0189] The method of any of the above embodiments, wherein the
anode exhaust has a ratio of H.sub.2:CO of at least about 2.5:1
(e.g., at least about 3.0:1, at least about 4.0:1, or at least
about 5.0:1).
Embodiment 9
[0190] The method of any of the above embodiments, further
comprising forming a second H.sub.2-containing stream from the
anode exhaust, the at least a portion of the anode exhaust, the
first H.sub.2-rich gas stream, or a combination thereof; and
recycling at least a portion of the second H.sub.2-containing
stream to the combustion turbine.
Embodiment 10
[0191] The method of any of the above embodiments, wherein at least
about 90 vol % of the reformable fuel is methane.
Embodiment 11
[0192] The method of any of the above embodiments, wherein the
molten carbonate fuel cell is operated at a thermal ratio from
about 0.25 to about 1.5 (e.g., from about 0.25 to about 1.3, from
about 0.25 to about 1.15, from about 0.25 to about 1.0, from about
0.25 to about 0.85, from about 0.25 to about 0.8, or from about
0.25 to about 0.75).
Embodiment 12
[0193] The method of any of the above embodiments, wherein an
amount of the reformable fuel introduced into the anode, the
internal reforming element associated with the anode, or the
combination thereof, is at least about 50% greater (e.g., at least
about 75% greater or at least about 100% greater) than an amount of
hydrogen reacted in the molten carbonate fuel cell to generate
electricity.
Embodiment 13
[0194] The method of any of the above embodiments, wherein a ratio
of net moles of syngas in the anode exhaust to moles of CO.sub.2 in
a cathode exhaust is at least about 2.0:1 (e.g., at least about
3.0, at least about 4.0, at least about 5.0, at least about 10.0,
or at least about 20.0), and optionally about 40.0 or less (e.g.,
about 30.0 or less or about 20.0 or less).
Embodiment 14
[0195] The method of any of the above embodiments, wherein a fuel
utilization in the anode is about 50% or less (e.g., about 30% or
less, about 25% or less, or about 20% or less) and a CO.sub.2
utilization in the cathode is at least about 60% (e.g., at least
about 65%, at least about 70%, or at least about 75%).
Embodiment 15
[0196] The method of any of the above embodiments, wherein the
molten carbonate fuel cell is operated to generate electrical power
at a current density of at least about 150 mA/cm.sup.2 and at least
about 40 mW/cm.sup.2 (e.g., at least about 50 mW/cm.sup.2, at least
about 60 mW/cm.sup.2, at least about 80 mW/cm.sup.2, or at least
100 mW/cm.sup.2) of waste heat, the method further comprising
performing an effective amount of an endothermic reaction to
maintain a temperature differential between an anode inlet and an
anode outlet of about 100.degree. C. or less (e.g., about
80.degree. C. or less or about 60.degree. C. or less), and
optionally wherein performing the endothermic reaction consumes at
least about 40% (e.g., at least about 50%, at least about 60%, or
at least about 75%) of the waste heat.
Embodiment 16
[0197] The method of any of the above embodiments, wherein an
electrical efficiency for the molten carbonate fuel cell is between
about 10% and about 40% (e.g., between about 10% and about 35%,
between about 10% and about 30%, between about 10% and about 25%,
between about 10% and about 20%) and a total fuel cell efficiency
for the fuel cell of at least about 50% (e.g., at least about 55%,
at least about 60%, at least about 65%, at least about 70%, at
least about 75%, or at least about 80%).
[0198] Although the present invention has been described in terms
of specific embodiments, it is not necessarily so limited. Suitable
alterations/modifications for operation under specific conditions
should be apparent to those skilled in the art. It is therefore
intended that the following claims be interpreted as covering all
such alterations/modifications that fall within the true
spirit/scope of the invention.
* * * * *