U.S. patent application number 12/238768 was filed with the patent office on 2010-04-01 for method of combusting sulfur-containing fuel.
This patent application is currently assigned to FOSTER WHEELER ENERGY CORPORATION. Invention is credited to Horst Hack.
Application Number | 20100077947 12/238768 |
Document ID | / |
Family ID | 41445554 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100077947 |
Kind Code |
A1 |
Hack; Horst |
April 1, 2010 |
METHOD OF COMBUSTING SULFUR-CONTAINING FUEL
Abstract
A method of combusting sulfur-containing fuel in a circulating
fluidized bed boiler includes the steps of (a) feeding
sulfur-containing fuel into a furnace of the circulating fluidized
bed boiler, (b) combusting the fuel with oxidant gas consisting
essentially of pure oxygen and circulated exhaust gas, so as to
form exhaust gas having carbon dioxide and water as its main
components, and (c) feeding calcium carbonate containing material
into the furnace so as to capture sulfur dioxide into calcium
sulfate in the furnace. The temperature in the furnace is
maintained above 870.degree. C.
Inventors: |
Hack; Horst; (Hampton,
NJ) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
1290 Avenue of the Americas
NEW YORK
NY
10104-3800
US
|
Assignee: |
FOSTER WHEELER ENERGY
CORPORATION
Clinton
NJ
|
Family ID: |
41445554 |
Appl. No.: |
12/238768 |
Filed: |
September 26, 2008 |
Current U.S.
Class: |
110/345 ;
110/216; 110/234 |
Current CPC
Class: |
Y02E 20/322 20130101;
F23J 7/00 20130101; F23C 9/003 20130101; Y02E 20/32 20130101; F23L
2900/07001 20130101; Y02E 20/34 20130101; F23L 7/007 20130101; F23C
10/04 20130101; F23J 2215/20 20130101; Y02E 20/344 20130101 |
Class at
Publication: |
110/345 ;
110/234; 110/216 |
International
Class: |
F23C 9/00 20060101
F23C009/00; F23C 10/00 20060101 F23C010/00; F23J 15/02 20060101
F23J015/02; F23L 7/00 20060101 F23L007/00; F23N 3/00 20060101
F23N003/00 |
Claims
1. A method of combusting sulfur-containing fuel in a circulating
fluidized bed boiler, the method comprising the steps of: (a)
feeding sulfur-containing fuel into a furnace of the circulating
fluidized bed boiler; (b) combusting the fuel with oxidant gas
consisting essentially of pure oxygen and circulated exhaust gas,
so as to form exhaust gas having carbon dioxide and water as its
main components; and (c) feeding calcium carbonate containing
material into the furnace so as to capture sulfur dioxide into
calcium sulfate in the furnace, wherein the temperature in the
furnace is maintained above 870.degree. C.
2. A method according to claim 1, wherein the temperature in the
furnace is maintained above 900.degree. C.
3. A method according to claim 1, wherein the temperature in the
furnace is maintained above 930.degree. C.
4. A method according to claim 3, wherein the circulating fluidized
bed boiler generates steam at a temperature of at least about
650.degree. C.
5. A method according to claim 4, wherein the circulating fluidized
bed boiler generates steam at a temperature of at least about
700.degree. C.
6. A method according to claim 1, wherein the sulfur-containing
fuel is bituminous coal.
7. A method according to claim 1, wherein the oxygen content of the
oxidant gas is at least about 21%.
8. A method according to claim 5, wherein the oxygen content of the
oxidant gas is from about 21% to about 28%.
9. A method according to claim 5, wherein the oxygen content of the
oxidant gas is at least about 28%.
10. A method according to claim 7, wherein the oxygen content of
the oxidant gas is from about 28% to about 40%.
11. A method according to claim 1, wherein the average carbon
dioxide content in the furnace is about 70%.
12. A method according to claim 1, wherein the average carbon
dioxide content in the furnace is from about 50% to about 70%.
13. A method according to claim 1, wherein the average carbon
dioxide content in the furnace is from about 70% to about 80%.
14. A method according to claim 2, wherein the sulfur-containing
fuel is bituminous coal.
15. A method according to claim 3, wherein the sulfur-containing
fuel is bituminous coal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of efficiently
combusting sulfur-containing fuel in a circulating fluidized bed
boiler by using oxyfuel combustion, i.e., the fuel is combusted by
using pure oxygen or a mixture of a substantially pure oxidant and
circulated fluidizing gas as the oxidant gas. The invention
especially relates to oxyfuel combustion, in which the fuel is
combusted with an oxidant gas having an oxygen content of about the
same as that or higher than that of air.
[0003] 2. Description of the Related Art
[0004] Combustion of sulfur-containing fuels, such as coal,
generates sulfur dioxide, SO.sub.2, which is an acid rain creating
pollutant, if released to the atmosphere. In order to minimize
pollution of the environment, today's combustion systems often
comprise special equipment, such as an exhaust gas scrubber,
arranged in the exhaust gas channel to remove sulfur dioxide from
the exhaust gas. Such special equipment is usually expensive to
build and to operate.
[0005] When combusting fuel in a circulating fluidized bed boiler,
most of released sulfur dioxide is usually already captured in the
furnace, by feeding calcium carbonate (CaCO.sub.3) containing
material, typically, limestone, into the furnace. The calcium
carbonate, CaCO.sub.3, is calcined in the furnace to calcium oxide,
CaO, which captures sulfur dioxide into calcium sulfate, according
to the reactions:
CaCO.sub.3->CaO+CO.sub.2 (i)
CaO+SO.sub.2+1/2O.sub.2->CaSO.sub.4. (ii)
The formed calcium sulfate can then be removed from the furnace
together with the ashes. Thus, a combustion process using a
circulating fluidized bed boiler does not need additional sulfur
capturing equipment in the exhaust gas channel, or the efficiency
of such equipment can be relatively low.
[0006] The reactivity of calcium carbonate to capture sulfur
dioxide in a conventional circulating fluidized bed boiler
increases with temperature, reaching an optimum value, typically,
around 850.degree. C. The optimum temperature may vary depending on
the type of the fuel and the sorbent, and other parameters of the
process. The efficiency of the capturing of sulfur dioxide drops
quite rapidly at temperatures above the optimum. Many explanations
are proposed for the reduced capturing at higher temperatures,
including sintering of pores of the CaO particles, decomposition of
CaSO.sub.4 with CO, instability of the intermediate product,
CaSO.sub.3, and the depletion of oxide in the region of the
capturing process. Thus, the process of combusting
sulfur-containing fuel, especially, bituminous coal, in a fluidized
bed boiler, has, in practice, a desired operating temperature,
typically, near to about 850.degree. C. The combustion temperature
has an effect to the highest obtainable steam temperature and,
thereby, the thermal efficiency of the boiler.
[0007] H. Liu, et al. have suggested in an article published in
Fuel (2000) 945-953 that, when using oxyfuel combustion in a
fluidized bed, the calcium carbonate based sulfur capture is
replaced at temperatures below 850.degree. C. by direct sulfation
from CaCO.sub.3 to CaSO.sub.4. This reaction enables a higher
sulfation degree of the calcium, due to the porosity of the
CaCO.sub.3 particles generated therein by the counter-diffusion of
the CO.sub.2.
[0008] At low partial pressures of carbon dioxide, CO.sub.2,
prevailing in combustion processes using air as the oxidant gas,
the equilibrium of the reaction (i) is, in the typical temperatures
of fluidized bed combustion, always on the right hand side, i.e.,
CaCO.sub.3 calcines to CaO. On the other hand, at typical partial
pressures of CO.sub.2 prevailing in a circulating fluidized bed
boiler when using oxyfuel combustion, for example, at about 0.7
atm, the CaCO.sub.3 does not calcine to CaO below a calcination
temperature of about 870.degree. C., and SO.sub.2 is captured by
direct sulfation. However, above the calcination temperature,
CaCO.sub.3 calcines to CaO, and direct sulfation should not be
important.
[0009] Due to the above-mentioned reasons, there is a need for an
improved combustion method of a circulating fluidized bed boiler,
in order to improve the thermal efficiency of the boiler, while
still effectively capturing sulfur dioxide in the furnace.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide a method of
efficiently combusting sulfur-containing fuel in a circulating
fluidized bed boiler.
[0011] According to an aspect of the present invention, a method of
combusting sulfur-containing fuel in a circulating fluidized bed
boiler is provided, the method comprising the steps of feeding
sulfur-containing fuel into a furnace of the circulating fluidized
bed boiler, combusting the fuel with oxidant gas consisting
essentially of pure oxygen and circulated exhaust gas, so as to
form exhaust gas having carbon dioxide and water as its main
components, and feeding calcium carbonate containing material into
the furnace so as to capture sulfur dioxide into calcium sulfate in
the furnace, wherein the temperature in the furnace is maintained
above 870.degree. C.
[0012] According to other embodiments of the present invention, the
temperature in the furnace is preferably maintained above
900.degree. C., even more preferably, above 930.degree. C.
[0013] As is well known to a person skilled in the art of fluidized
bed combustion, the temperature in the furnace can be maintained at
a desired level by controlling the fuel feeding rate or the cooling
rate of the furnace. The heat absorption rate can advantageously be
controlled by using a controllable heat exchange chamber arranged
in the hot loop of the circulating fluidized bed boiler. The oxygen
feeding rate is usually controlled so that substantially complete
combustion of the fuel is obtained, and a desired excess oxygen
level, typically, about 3%, remains in the exhaust gas.
[0014] The present invention is based on a series of surprising
results obtained by the inventor, when adding calcium carbonate to
a fluidized bed combustion reactor maintained at different
temperatures, when combusting bituminous coal by using a mixture of
oxygen and recirculated exhaust gas as the oxidant gas. The results
will be discussed in more detail below, in the section entitled
DETAILED DESCRIPTION OF THE INVENTION.
[0015] It appears that the conditions giving rise to a decreasing
sulfur capture at high temperatures in air-firing fluidized bed
boilers are changed in oxyfuel combustion. As a result of the
change, the temperature range of decreasing efficiency appears to
be removed, or at least shifted to a higher temperature. An
improved sulfur capture in the furnace in oxyfuel combustion can be
expected, at any temperature, on the basis of the extended time of
SO.sub.2 presence in the furnace, due to the recirculation of the
exhaust gas. In addition to that, the contents of the gas in the
furnace are, in oxyfuel combustion, clearly different than those in
air combustion. Thus, the enhanced sulfur capture at high
temperatures may be related to the higher content of CO.sub.2 or
O.sub.2 in the furnace.
[0016] Similar enhanced sulfur capture at high temperatures has
been observed earlier in air-firing fluidized bed boilers when used
at elevated pressures of 10 to 20 atm. In that case, the improved
sulfur capture at high temperatures is probably related to changing
of the sulfur dioxide capturing process to direct sulfation of
CaCO.sub.3, as described above. In the present case, when enhanced
sulfur capture was observed at ambient pressure, and at
temperatures above 870.degree. C., the sulfur capturing process
remains to be via the route including calcination of CaCO.sub.3 to
CaO, and direct sulfation is not important.
[0017] The efficiency of calcium carbonate based sulfur capture in
a circulating fluidized bed boiler at even higher temperatures,
say, above 1000.degree. C., is not yet known. Anyhow, the present
observations encourage testing of the sulfur capture up to
1000.degree. C. and above, in order to find the full usable
operation range for oxyfuel combustion of sulfur-containing fuels
in circulating fluidized bed boilers, in order to obtain high steam
temperatures. However, it is clear that a fluidized bed boiler
cannot be operated above the softening temperature of the bed,
which depends on the fuel, being, for example, about 1100.degree.
C.
[0018] The use of combustion temperatures higher than those used in
conventional air-firing fluidized bed combustion allows a
circulating fluidized bed boiler to generate steam at higher
temperatures, while still achieving a high level of in-furnace
sulfur dioxide capture. Thus, the present invention helps to
facilitate improved steam cycle efficiency in supercritical and
ultra-supercritical circulating fluidized bed boiler applications.
The boiler is then preferably used to generate steam at a
temperature of at least about 650.degree. C., even more preferably,
at least about 700.degree. C.
[0019] Preferably, the oxygen content of the oxidant gas used in
the process is about the same as that, or somewhat above that of
air, i.e., about or above 21%. According to a preferred embodiment
of the present invention, the oxygen content of the oxidant gas is
about 28%, typically, from about 26% to about 30%. In some cases,
the oxygen content of the oxidant gas may be above about 28%, for
example, from about 28% to about 40%. In some other cases, the
oxygen content of the oxidant gas may be from about 21% to about
28%, for example, about 24%.
[0020] In oxyfuel combustion, when the oxidant gas is a mixture of
substantially pure oxygen and recycled exhaust gas, the main
components of the exhaust gas are carbon dioxide, water and, for
example, about 3% of excess oxygen. The oxygen content of the
oxidant gas is thus determined by the recycling rate of the exhaust
gas. Thus, for example, an oxygen content of 28% is obtained by
recycling approximately 70% of the exhaust gas.
[0021] The average carbon dioxide content in the combustion reactor
may alternatively be considered as the key parameter for
advantageous oxyfuel combustion of sulfur-containing fuels in
fluidized bed boilers. The carbon dioxide content in the furnace
depends on the exhaust gas circulation rate and water content of
the circulated exhaust gas. According to the present experiments,
the average carbon dioxide content in the combustion reactor is
advantageously about 70%, typically, from about 68% to about 72%.
In other advantageous cases, the average carbon dioxide content in
the furnace may be even higher, for example, from about 70% to
about 80%, especially, when recycling dry and relatively cold
exhaust gas. In still other advantageous cases, the average carbon
dioxide content in the combustion reactor is lower than 70%, for
example, from about 50% to about 70%.
[0022] The above brief description, as well as further objects,
features, and advantages of the present invention will be more
fully appreciated by reference to the following detailed
description of the currently preferred, but nonetheless
illustrative, embodiments of the present invention, taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic diagram of a typical circulating
fluidized bed combustion reactor used for oxyfuel combustion.
[0024] FIG. 2 is a schematic diagram of experimentally observed
calcium utilization in oxygen and air combustion of bituminous coal
as a function of average temperature in the combustion reactor.
DETAILED DESCRIPTION OF THE INVENTION
[0025] FIG. 1 shows a schematic diagram of a typical power plant 10
used for oxyfuel combustion of sulfur-containing fuel, such as
bituminous coal. The power plant 10 comprises a circulating
fluidized bed (CFB) boiler 12 with a furnace 14 comprising
conventional means for feeding fuel 16 and calcium carbonate,
CaCO.sub.3, containing sorbent 18 for capturing sulfur dioxide,
SO.sub.2, generated when combusting the fuel with an oxidant gas,
to calcium sulfate, CaSO.sub.4. The oxidant gas is fed to the
furnace as primary oxidant gas 20, fed through a grid at the bottom
of the furnace, and secondary oxidant gas 22, fed to the furnace at
a higher level. Oxidant gas may, in some cases, be fed to the
furnace at even more locations and levels.
[0026] Exhaust gas produced by combusting the fuel with the oxidant
gas in a fluidized bed formed in the furnace 14 is led from the
furnace through a particle separator 24 to an exhaust gas channel
26. The temperature in the furnace 14 may be controlled to a
desired value, to be discussed later, by controlling the fuel feed
rate and by a heat exchanger 28 advantageously connected to a
return leg arranged for leading separated particles from the
particle separator 24 to the lower portion of the furnace 14. The
temperature and, advantageously, temperature distribution in the
furnace can be monitored by temperature measuring means 30, such as
thermocouples, arranged at one or more suitable locations in the
furnace.
[0027] An exhaust gas recycling channel 32 is branched off from the
exhaust gas channel 26, so as to recycle a portion of the exhaust
gas back to the furnace 14. The exhaust gas recycling channel 32
advantageously comprises means, such as a fan 34 and a damper 36,
for controlling the exhaust gas recycling rate. The rest of the
exhaust gas is conveyed through an end portion 38 of the exhaust
gas channel 26 for final processing.
[0028] As is conventional, the furnace 14 usually comprises
evaporation surfaces, not shown in FIG. 1, and the exhaust gas
channel 26 further comprises heat exchanger surfaces 40, for
example, superheaters, reheaters and economizers, for generating
steam by the boiler 12. For the sake of simplicity, FIG. 1 only
shows one such heat exchanger surface 40, but, in practice, the
exhaust gas channel system usually comprises multiple superheating,
reheating and economizer surfaces for recovering heat from the
exhaust gas.
[0029] A gas-gas heat exchanger 42, for example, a regenerative
heat exchanger, is advantageously also arranged in the exhaust gas
channel 26, downstream of the steam generating heat exchange
surfaces 40, for transferring heat from the exhaust gas to the
recycling portion of the exhaust gas. The exhaust gas channel 26
also usually comprises conventional units for cleaning particles
and gaseous pollutants from the exhaust gas, which units are
schematically represented in FIG. 1 only by a dust separator 44.
Between the gas-gas heat exchanger 42 and the dust separator 44,
there is shown a further heat exchanger 40', a low-pressure
economizer, which may alternatively be arranged downstream of the
dust separator or in the exhaust gas recycling channel 32.
[0030] In accordance with the main object of oxyfuel combustion,
i.e., to recover carbon dioxide from the exhaust gas, having carbon
dioxide as its largest component, the end portion 38 of the exhaust
gas channel 26 is equipped with exhaust gas processing means,
schematically represented by a carbon dioxide processing unit 46,
for cooling, cleaning and compressing carbon dioxide. The carbon
dioxide processing unit 46 usually comprises a dryer for completely
drying all water from the exhaust gas, and a separator for
separating a stream of non-condensable gas 48, such as oxygen, and
other possible impurities, from the carbon dioxide. A stream of
carbon dioxide 50 is typically captured in a liquid or
supercritical state, at a pressure of, for example, about one
hundred ten bars, so that it can be transported to further use or
to be stored in a suitable place.
[0031] FIG. 1 separately shows a condensing gas cooler 52, located
upstream of the carbon dioxide processing unit 46, for initially
removing water from the exhaust gas. The condensing gas cooler may
be arranged either in the end portion 38 of the exhaust gas channel
26, as in FIG. 1, or upstream of the branch point of the recycling
exhaust gas channel 32. In the latter case, the water content in
the recycling exhaust gas and in the furnace 14 is minimized.
[0032] The oxidant gas is preferably produced by mixing a stream of
substantially pure oxygen 54, produced from an air stream 56 in an
air separation unit (ASU) 58, and at least a portion of the
recycling portion of the exhaust gas in a mixing unit 60. The air
separation unit produces a gas stream consisting essentially of
pure oxygen, containing, typically, at least about 95% oxygen. The
recycling rate of the exhaust gas is advantageously adjusted such
that the resulting gas flow rate in the furnace 14 obtains a
desired value, whereby the average O.sub.2 content of the oxidant
gas is, typically, close to that of air, preferably, from about 21%
to about 28%. In some other cases, the oxygen content of the
oxidant gas may be higher, for example, from about 28% to about
40%. It is also possible to introduce the streams of recycled
exhaust gas and substantially pure oxygen separately, or multiple
streams with different O.sub.2 contents, produced in separate
mixing units, at, for example, different heights, into the furnace
14.
[0033] FIG. 2 shows a diagram of experimentally observed calcium
utilization, in percents, in otherwise similar oxyfuel combustion
and air combustion processes of bituminous coal as a function of
average temperature in the combustion reactor. The calcium
utilization is defined as:
Calcium utilization=RET/(Ca/S),
where RET is sulfur retention [%], and Ca/S is calcium to fuel
sulfur feed ratio [mol/mol]. Experimental points of oxyfuel
combustion are shown as black squares, whereas points measured in
air combustion are shown as hatched triangles.
[0034] As can be seen in FIG. 2, the calcium utilization decreases
with temperature in air combustion, from about 50% at 800.degree.
C. to about 30% at 900.degree. C. In oxyfuel combustion, the
calcium utilization does not show any clear decrease in the
temperature range extending up to 930.degree. C. To the contrary,
the calcium utilization seems to increase, from slightly above 50%
to near to 60%, when increasing the temperature from 800.degree. C.
to 900.degree. C.
[0035] The measurements suggest, surprisingly, that, when using
oxyfuel combustion, sulfur-containing fuels, especially, bituminous
coals, can be advantageously combusted in a circulating fluidized
bed boiler at a temperature, which is preferably higher than
870.degree. C., more preferably, higher than 900.degree. C. and,
even more preferably, higher than 930.degree. C., while still
obtaining efficient sulfur capture in the furnace. Such a
combustion process renders operation possible to generate steam at
a higher temperature than conventionally, preferably, at a
temperature of at least 650.degree. C., even more preferably, at
least about 700.degree. C., and to thereby obtain an improved
thermal efficiency of the boiler.
[0036] While the invention has been described herein by way of
examples in connection with what are, at present, considered to be
the most preferred embodiments, it is to be understood that the
invention is not limited to the disclosed embodiments, but is
intended to cover various combinations or modifications of its
features, and several other applications included within the scope
of the invention as defined in the appended claims.
* * * * *