U.S. patent application number 11/690577 was filed with the patent office on 2007-09-27 for method for improving gasification efficiency through the use of waste heat.
Invention is credited to John Paul Gaus, Philip D. Leveson.
Application Number | 20070220810 11/690577 |
Document ID | / |
Family ID | 38531856 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070220810 |
Kind Code |
A1 |
Leveson; Philip D. ; et
al. |
September 27, 2007 |
METHOD FOR IMPROVING GASIFICATION EFFICIENCY THROUGH THE USE OF
WASTE HEAT
Abstract
A method for recycling the waste heat generated from an external
process, which is fuelled by syngas, into a gasification process to
enhance the energy density of the syngas produced as well as the
overall gasification efficiency of the system. A method is provided
for utilizing the waste heat contained in a stream exiting in the
syngas fueled process to vaporize water and produce steam. The
steam is then upgraded by first exchanging energy with the hot
syngas exiting the gasifier and then within the gasifier itself to
a temperature where significant steam gasification of the biomass
occurs. The process within the gasifier is driven by introducing a
small amount of air into the gasifier such that some biomass is
directly combusted to provide the heat required by the process.
Inventors: |
Leveson; Philip D.; (Hannawa
Falls, NY) ; Gaus; John Paul; (Watertown,
NY) |
Correspondence
Address: |
POWELL GOLDSTEIN LLP
ONE ATLANTIC CENTER
FOURTEENTH FLOOR 1201 WEST PEACHTREE STREET NW
ATLANTA
GA
30309-3488
US
|
Family ID: |
38531856 |
Appl. No.: |
11/690577 |
Filed: |
March 23, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60785519 |
Mar 24, 2006 |
|
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|
60785520 |
Mar 24, 2006 |
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Current U.S.
Class: |
48/197FM |
Current CPC
Class: |
C10J 2300/1884 20130101;
C10J 2300/1892 20130101; C10J 2300/0973 20130101; Y02P 20/129
20151101; Y02P 20/145 20151101; C10J 2300/0916 20130101; C10J
2300/1861 20130101; C10J 2300/0956 20130101; Y02E 20/18 20130101;
C10J 3/00 20130101; C10J 2300/1671 20130101; C10J 2300/1838
20130101 |
Class at
Publication: |
048/197.0FM |
International
Class: |
C10L 3/00 20060101
C10L003/00 |
Claims
1) A method for improving the efficiency of electricity production
by a gasifier and generator combination, comprising: a) utilizing
the waste heat produced by the generator to produce low quality
steam; b) exchanging heat from the gasifier outlet stream to
increase the temperature and quality of the steam; c) introducing
the steam along with some air into a suitable gasifier device; and,
d) superheating the steam through oxidation of biomass to the point
where at least partial reaction between the steam and biomass
occurs.
2) The method of claim 1, wherein a portion of waste heat produced
by the generator is utilized in any biomass preparation
operations.
3) The method of claim 1, wherein where a portion of the
electricity produced by the generator is utilized in any biomass
preparation operation.
4) The method of claim 1, wherein a portion of waste heat produced
by the generator is utilized to preheat the oxygen containing
stream prior to being fed into the gasifier.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of co-pending U.S.
Provisional Patent Application Nos. 60/785,519, filed Mar. 24,
2006, entitled METHOD TO IMPROVE GASIFICATION EFFICIENCY THROUGH
THE USE OF WASTE HEAT, and 60/785,520, filed Mar. 24, 2006,
entitled GASIFICATION SYSTEMS, and commonly assigned to the
assignee of the present application, the disclosures of which are
incorporated by reference in their entirety herein.
FIELD OF THE INVENTION
[0002] The present invention relates to gasifier equipment and to
the process of gasification of carbon containing solids into
combustible gases. The improvement may be used to enrich the
calorific density and hydrogen content of the produced syngas while
simultaneously improving the thermal efficiency of the gasification
process.
BACKGROUND OF THE ART
[0003] Gasification processes convert carbon-containing solids of
liquids into combustible gases that ideally contain all the energy
originally present in the feed. In reality this is not easily
achieved, although with good thermal management it is possible to
operate with energy efficiencies in excess of 90%. The technique
yields a combustible gas, which is typically rich in carbon
monoxide, hydrogen and methane from a carbon containing solid.
Gaseous fuels have many advantages over solid fuels. They are
typically cleaner burning reducing particulate carbon, hydrocarbon
and carbon monoxide emissions. It is also much easier to remove
sulphur, halogen and nitrogen containing volatile compounds from
the syngas through scrubbing and adsorption techniques prior to
combustion rather than cleaning the flue gases or the solid fuels.
It is becoming increasingly attractive to consider maximizing the
hydrogen concentration in the produced syngas through the water gas
shift reaction and then sequestering the carbon dioxide for this
gas stream. This technique is being adopted by some integrated
gasification combined cycle ("IGCC") coal gasification plants as a
method to reduce carbon oxide emissions.
[0004] Almost all carbon containing solids are suitable fuels for
gasification systems. Examples include, but are not limited to,
coals, lignites, plant matter and plant matter derived products,
animal wastes, oil and oil derived products. Increasing interest is
being expressed towards the use of biomass and animal wastes as
these offer a potentially viable route to creating fuel streams
which do not contribute to the addition of carbon dioxide to the
atmosphere. The mechanism is that the plant picks up carbon dioxide
during the growing season and a similar quantity of carbon dioxide
is released during combustion resulting in a near net zero addition
of carbon dioxide to the atmosphere. This technique offers a large
scale route were a significant fraction of the energy requirements
of the planet can be readily produced in an environmentally
friendly manner as well as being eligible for carbon based tax
credits as presently offered by various governments.
[0005] One disadvantage of gasification is that the gas stream
produced has a relatively weak energy density. For an air blown
system the energy content per unit volume is around a fifth to a
seventh that contained in natural gas and around one twentieth that
of liquefied petroleum gas ("LPG"). This low energy density
detracts from the economics of compressing the gas and transporting
through pipelines to anywhere other than over short to moderate
distances. Thus the gaseous fuel produced from gasification is
typically used on or near the production facility.
[0006] The use of biomass as a feedstock for gasification systems
is becoming increasingly economically as well as environmentally
attractive. Potential local uses for the syngas may include,
running generators to produce electrical power, using the fuel to
offset natural gas in heating applications or to convert the syngas
into a liquid fuel, and other uses. The conversion to a liquid fuel
can be readily accomplished by the catalytic reduction of carbon
monoxide by hydrogen to produce methanol, ethanol or synthetic
middle distillates. In this case the fuel can be readily
transported to be the market place.
[0007] A typical biomass has an energy density around 18 kJ/g on a
dry basis. On a wet basis this value can be substantially less and
can even be less than zero, indicating that the fuel is not capable
of burning in a sustainable manner while liberating energy. On a
dry basis biomass has a calorific value around half that of coal.
The low energy density, its low packing density and difficulty in
handling make the economics of transporting biomass large distances
unfeasible. Thus the utilization of biomass for small to medium
scale distributed energy producing processes has some synergy. The
biomass for such a process would be sourced locally and probably
within a twenty mile radius. Power may be generated and used to
reverse feed already saturated power delivery lines. In such a
system local communities would utilize locally grown biomass and
potentially make use of some volume of waste currently being land
filled to generate their own power or convert the material into
fuels. In effect a community could become power and fuel self
sufficient while producing no greenhouse gas emissions.
[0008] Biomass is a very broad term and includes all solids derived
from plant matter, animal wastes as well as organic municipal
waste. Suitable biomasses include, but are not limited to, sawdust,
wood, straw, alfalfa seed straw, barley straw, bean straw, corn
cobs, corn stalks, cotton gin trash, rice hulls, paper, municipal
solid waste, barks and animal wastes. It is interesting that almost
all biomass has the same ratio of carbon to hydrogen to oxygen,
which is summarized as CH.sub.1.4O.sub.0.6.
[0009] The stoichiometric gasification equation is shown below:
CH.sub.1.4O.sub.0.6+0.2O.sub.2.fwdarw.CO+0.7H.sub.2 (1)
[0010] Performing an energy balance across the system reveals that
the products contain more energy than the reactants, hence some of
the biomass is burnt to offset this imbalance. Hence a more
realistic gasification process may be represented as:
CH.sub.1.4O.sub.0.6+0.4O.sub.2.fwdarw.0.7CO+0.6H.sub.2+0.3CO.sub.2+0.1H.s-
ub.2O (2)
[0011] If air is used as the oxidant the process becomes
CH.sub.1.4O.sub.0.6+0.4O.sub.2+1.6N.sub.2.fwdarw.0.7CO+0.6H.sub.2+0.3CO.s-
ub.2+0.1H.sub.2O+1.6N.sub.2 (3)
[0012] As can be seen in equation (3) the nitrogen associated with
the oxygen acts to dilute the energy containing components by
approximately one half. The reaction may also be facilitated
through utilizing steam as the oxidant. This process is expressed
as: CH.sub.1.4O.sub.0.6+0.4H.sub.2O.fwdarw.CO+1.1H.sub.2 (4)
[0013] The syngas produced through equation 4 has a much higher
energy density than air derived syngas. The total energy content of
the syngas is also about twice than the air derived product,
however, the process is strongly endothermic and requires a
substantial external energy input. The energy can be transferred
into the process through heat transfer mechanisms, this may include
externally heating the gasifier, through the use of heating
elements within the gasifier or through passing hot inert solids
into the gasification bed. Either of these techniques greatly
complicates the overall design of the gasifier. A second technique
utilizes a large excess of superheated steam, such that the
sensible heat contained in the steam is used to provide the energy
for the process. However, this involves the construction of a large
steam generator, thus increasing the capital expenditure of the
process and generates the need for an external fuel input.
SUMMARY OF THE INVENTION
[0014] The present invention comprises, in one exemplary
embodiment, a method which allows the waste heat generated from an
external process, which is fueled by syngas, to be recycled into a
gasification process to enhance the energy density of the syngas
produced as well as the overall gasification efficiency of the
system. The invention also relates to a method of utilizing the
waste heat contained in a stream exiting in the syngas-fueled
process to vaporize water and produce steam. The steam is then
upgraded by first exchanging energy with the hot syngas exiting the
gasifier and then within the gasifier itself to a temperature where
significant steam gasification of the biomass occurs. The process
within the gasifier is driven by introducing a small amount of air
into the gasifier such that some biomass is directly combusted to
provide the heat required by the endothermic processes. By the
exchange of heat in this manner the volume of oxygen required by
the process is vastly reduced and hence the volume of associated
nitrogen diluent introduced is also minimized. This manner of
operation significantly reduces the cost of the ancillary equipment
as no external steam or oxygen generator is required. The method
maximizes the energy content of the produced gas and under certain
circumstances allows gasification efficiencies greater than 100% to
be achieved. For the purposes of the present disclosure, the
gasification efficiency is defined as the energy content of the
produced gas divided by the energy content in the original biomass.
The improvement becomes much increased if amounts of steam much
higher than required by stoichiometry are utilized. Particularly
favorable results are achieved with steam ratios in the range of
1:10 times that of stoichiometry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Exemplary embodiments of the present invention are
illustrated in the drawings in which like reference characters
designate the same or similar parts throughout the figures of
which:
[0016] FIG. 1 is a schematic flow diagram illustrating how waste
energy from a generator powered by an internal combustion engine
can be effectively recycled back to the gasification process to
improve the quality of the syngas produced and improve the thermal
efficiency of the gasification process, and
[0017] FIG. 2 is a schematic flow diagram illustrating how waste
energy from a generator powered by an internal combustion turbine
can be effectively recycled back to the gasification process to
improve the quality of the syngas produced and improve the thermal
efficiency of the gasification process.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0018] Gasification systems often make use of air as the oxidant in
the process. The disadvantage of the use of air is that the
associated nitrogen acts to dilute the syngas produced and results
in the production of a syngas with a low energy density. A weak
syngas can still be readily utilized but results in larger
downstream equipment, higher blower costs and higher de-rates of
downstream electrical generation equipment. The nitrogen can be
removed from the system by utilizing an air separation unit to
enrich the air. The gas produced in this case has a much higher
energy density, approaching twice that obtained from an air blown
system, but the capital and operational costs of an air separation
unit is high. A third technique is to utilize steam as the oxidant.
Utilizing steam results in a syngas which has a high calorific
value and is high in hydrogen and so exhibits good flame velocity
attributes. However, the gasification reactions which involve steam
are highly endothermic such that external energy must be supplied
to the system, either through external heating techniques, the
introduction of hot inert material into the gasification bed or
through the use of large volumes of excess steam such that the
steam contains appreciable quantities of sensible heat. This
results in a process which requires some form of external energy
input and as such requires utilizing a fuel.
[0019] In the present invention, the external process that is
consuming the syngas produced in the gasifier is thermally
integrated with the gasification process itself. By operating in
this regime waste heat from the process can be efficiently and
conveniently used to enhance the gasification process to produce a
syngas with a higher energy density, a higher in hydrogen
concentration and in a thermally more efficient manner as compared
to an air blown system. The exemplary embodiment of the method
described hereinbelow utilizes a gasifier operating with and
without the energy recycle and discusses how the process becomes
integrated within a continuous process to convert biomass into
electrical power using an internal combustion engine generator and
a turbine powered generator.
EXAMPLES
[0020] The invention will be further described in connection with
the following examples, which are set forth for purposes of
illustration only. Parts and percentages appearing in such examples
are by weight unless otherwise stipulated.
Example 1
[0021] A 15 cm, down-draft stratified gasifier with an integral tar
cracking and hydrocarbon reforming lower chamber was used to
convert biomass into syngas. In the upper zone the biomass
undergoes the decomposition process commencing with devolatization
followed by flaming pyrolysis and finally char gasification. In the
lower zone a small amount of air is introduced into the syngas such
that a small fraction is further oxidized. The heat liberated by
this oxidation allows higher order hydrocarbons and tars to be
broken down into carbon monoxide and hydrogen. The result of the
thermal treatment is that a syngas which is essentially free of
tars and higher order hydrocarbons is produced. The air flow to the
gasifier was adjusted such that the maximum bed temperature was
850.degree. C. The syngas produced exiting the system was cooled to
40.degree. C. such that any condensable matter is liquefied. The
syngas was filtered using a 5 micron polyester filter, passed
through a blower and was then used to power a 4 KW YAMAHA.TM.
TRIFUEL.TM. generator. The air entering the system was preheated in
a plate heat exchanger using the hot syngas exiting the gasifier,
in a counter current arrangement. A gas chromatograph was used to
analyze the composition of the gas exiting the gasifier system. A
typical analysis of the syngas produced is shown below in Table 1.
TABLE-US-00001 TABLE 1 Gas composition produced by an air blown
system Gas % by volume Range H.sub.2 17.6 +/-2% CO 21.0 +/-2%
CO.sub.2 9.9 +/-1% CH.sub.4 >0.5 +/-.1% C.sub.2H.sub.4 0 --
C.sub.2H.sub.6 0 -- N.sub.2 51.0 +/-3.sup.
[0022] In a subsequent experiment the oxidant was adjusted to
contain a mixture of air and steam. The steam flowrate used
represented the volume of steam that could be raised using half of
the waste heat that is available to be captured from the generator.
A similar experiment was conducted, the oxidants again being
preheated by the syngas exiting the system and the gas analysis was
found to be as shown in Table 2 below. TABLE-US-00002 TABLE 2 Gas
composition produced by an air-steam blown system: Gas % by Volume
H.sub.2 27.5 CO 26 CO.sub.2 8 CH.sub.4 >0.5 C.sub.2H.sub.4 0
C.sub.2H.sub.6 0 N.sub.2 38
[0023] Table 2 clearly demonstrates the improvements in the syngas
energy density and hydrogen content that are achieved by recycling
the waste heat from an external device into the gasification
system.
Example 2
[0024] FIG. 1 illustrates an exemplary, nonlimiting embodiment of a
continuous process for recycling waste heat from an electricity
generator powered by an internal combustion engine into a
gasification system. The result is to both enrich the quality of
the gas being produced there and improve the overall thermal
efficiency of the gasifier. Biomass 10 is fed via stream 12 into a
gasification 14. In the gasifier 14, volatile matter and a good
fraction of the fixed carbon is converted into gaseous components.
The ash, non-volatiles and any unconverted fixed carbon exit via
the ash outlet into a collector 16. The hot syngas stream 18 exits
the gasifier 14 and is partially cooled in a booster heat exchanger
20. A number of heat exchangers are suitable for this operation,
including, but not limited to, shell and tube, plate duct, welded
plate and diffusion bonded plate heat exchangers. It may be
advantageous to orientate the exchanger 20 such that the gas stream
flows in a vertical plane to minimize any ash deposits occurring
there to minimize fouling effects. The heat exchanger 20 is used to
transfer energy from the hot syngas exiting the gasifier and
preheat the oxidants entering the gasifier 20. The partially cooled
syngas exits the heat exchanger via stream 22 and then may undergo
some treatment in a syngas clean up module 24. Typically, this will
involve further cooling of the syngas to allow the separation and
collection of condensables followed by some method of particulate
removal. Cyclones, spray system, wash columns and filters are all
suitable for this operation. If required or desired, volatile
compounds containing sulphur, halogens or nitrogen can be removed
at this stage using scrubbing and/or adsorbent-based techniques.
Carbon dioxide may also be sequestered at this stage. The cooled
and cleaned gas then enters a generator 26 via stream 28. A number
of different generators are suitable to utilize syngas. Some, such
as the Jenbacher range, use a compressor to increase the energy
density per unit volume of the syngas. The two output streams 30,
32 from the generator 28 include the electrical power 30 and the
waste heat contained in the cooling system and the sensible heat
contained in the exhaust gases. In FIG. 1 the waste heat and
sensible heat have been combined to form a combined waste heat
stream 32. Some generators already have a waste capture system in
place to provide combined heat and power solutions, again the
Jenbacher range is an example of such a unit. The combined waste
heat 32 flows into a steam generator heat exchanger 34 where the
energy is used to provide the latent heat of vaporization to water
36 and convert liquid water into steam which exits the unit via
stream 40. If desired some of the steam generated can be diverted
as a stream 41 to ancillary equipment or processes (e.g., back to
the generator 26). The remaining steam is mixed with the air or
oxygen from a source 42 prior to entering the booster heat
exchanger 20 via stream 44. The stream is superheated to a
temperature close to the temperature of the syngas exiting the
gasifier 20 and to a temperature above where gasification processes
are initiated. The superheated stream 46 exits the heat exchanger
20 and then enters the gasifier 14.
[0025] FIG. 2 illustrates one exemplary embodiment of a continuous
process for recycling waste heat from a generator powered by some
form of internal combustion turbine. The system is similar to that
described above with respect to the exemplary embodiment shown in
FIG. 1, the difference being that only the exhaust stream 50 from a
turbine 52 is utilized to convert liquid water to steam.
[0026] Although only a few exemplary embodiments of this invention
have been described in detail above, those skilled in the art will
readily appreciate that many modifications are possible in the
exemplary embodiments without materially departing from the novel
teachings and advantages of this invention. Accordingly, all such
modifications are intended to be included within the scope of this
invention as defined in the following claims.
[0027] Therefore, it is intended that the invention not be limited
to the particular embodiments disclosed as the best mode
contemplated for carrying out this invention, but that the
invention will include all embodiments falling within the scope and
spirit of the appended claims. It should further be noted that any
patents, applications and publications referred to herein are
incorporated by reference in their entirety.
* * * * *