U.S. patent application number 12/107198 was filed with the patent office on 2009-10-22 for oxyfuel combusting boiler system and a method of generating power by using the boiler system.
This patent application is currently assigned to FOSTER WHEELER ENERGY CORPORATION. Invention is credited to Timo Eriksson, Zhen Fan, Horst Hack, Archibald Robertson, Andrew Seltzer, Ossi Sippu.
Application Number | 20090260585 12/107198 |
Document ID | / |
Family ID | 41200054 |
Filed Date | 2009-10-22 |
United States Patent
Application |
20090260585 |
Kind Code |
A1 |
Hack; Horst ; et
al. |
October 22, 2009 |
Oxyfuel Combusting Boiler System and a Method of Generating Power
By Using the Boiler System
Abstract
Carbonaceous fuel is combusted with an oxidant gas in a furnace
of a boiler system to generate power. Oxidant gas is fed into the
furnace for combusting the fuel to produce exhaust gas, the exhaust
gas is discharged from the furnace via an exhaust gas channel, a
stream of feedwater is conveyed from a final economizer arranged in
the exhaust gas channel to evaporating and superheating heat
exchange surfaces arranged in the furnace and in the exhaust gas
channel for converting the feedwater to superheated steam, the
superheated steam is converted in a high-pressure steam turbine for
generating power, a first portion of steam is extracted from the
high-pressure steam turbine for preheating the feedwater, a second
portion of steam is conveyed from the high-pressure steam turbine
to reheating heat exchange surfaces arranged in the exhaust gas
channel for generating reheated steam, and the reheated steam is
expanded in an intermediate pressure steam turbine for generating
power. The oxidant gas can be a mixture of substantially pure
oxygen and recycled exhaust gas, and the ratio of the first and
second portions of steam can be controlled to obtain a desired flue
gas temperature in the exhaust gas channel downstream of the final
economizer.
Inventors: |
Hack; Horst; (Hampton,
NJ) ; Seltzer; Andrew; (Livingston, NJ) ; Fan;
Zhen; (Parsippany, NJ) ; Robertson; Archibald;
(Whitehouse Station, NJ) ; Eriksson; Timo;
(Varkaus, FI) ; Sippu; Ossi; (Varkaus,
FI) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
FOSTER WHEELER ENERGY
CORPORATION
Clinton
NJ
|
Family ID: |
41200054 |
Appl. No.: |
12/107198 |
Filed: |
April 22, 2008 |
Current U.S.
Class: |
122/7R ;
60/649 |
Current CPC
Class: |
Y02E 20/344 20130101;
F01K 7/38 20130101; Y02E 20/34 20130101; Y02E 20/12 20130101; F01K
7/24 20130101; F01K 13/00 20130101; F22B 1/02 20130101 |
Class at
Publication: |
122/7.R ;
60/649 |
International
Class: |
F22B 1/18 20060101
F22B001/18; F01K 25/06 20060101 F01K025/06 |
Claims
1. A method of generating power by combusting carbonaceous fuel
with an oxidant gas in a furnace of a boiler system, the method
comprising the steps of: (a) feeding carbonaceous fuel into the
furnace at a fuel feeding rate; (b) feeding oxidant gas into the
furnace for combusting the fuel to produce exhaust gas; (c)
discharging the exhaust gas from the furnace via an exhaust gas
channel; (d) conveying a stream of feedwater at a feedwater
conveying rate from a final economizer arranged in the exhaust gas
channel to evaporating and superheating heat exchange surfaces
arranged in the furnace and in the exhaust gas channel for
converting the feedwater to superheated steam; (e) expanding the
superheated steam in a high-pressure steam turbine for generating
power; (f) extracting a first portion of steam from the
high-pressure steam turbine for preheating the feedwater; (g)
conveying a second portion of steam from the high-pressure steam
turbine to reheating heat exchange surfaces arranged in the exhaust
gas channel for generating reheated steam; and (h) expanding the
reheated steam in an intermediate pressure steam turbine for
generating power, wherein, in first operating conditions, the
oxidant gas is a mixture of substantially pure oxygen and recycled
exhaust gas and the ratio of the first and second portions of steam
is controlled so as to obtain a desired flue gas temperature in the
exhaust gas channel downstream of the final economizer.
2. The method according to claim 1, wherein the fuel feeding rate
and the feedwater conveying rate are adjusted so as to obtain a
desired furnace temperature.
3. The method according to claim 2, wherein, in second operating
conditions, the oxidant gas is air and, when operating the
combustion system at full load at the first and second operating
conditions, the fuel feeding rate is, in the first operating
conditions, higher than that in the second operating
conditions.
4. The method according to claim 3, wherein, when operating the
combustion system at full load of the first and second operating
conditions, the first portion of steam in the first operating
conditions is less than that in the second operating conditions,
and the second portion of steam is, in the first operating
conditions, greater than that in the second operating
conditions.
5. The method according to claim 3, wherein the system comprises a
gas-gas heat exchanger and heat is transferred in the gas-gas heat
exchanger in the first and second operating conditions from the
exhaust gas in the exhaust gas channel to at least a portion of the
oxidant gas.
6. The method according to claim 1, wherein the controlling
comprises measuring the exhaust gas temperature.
7. The method according to claim 3, wherein the feedwater conveying
rate is greater in the first operating conditions than that in the
second operating conditions.
8. The method according to claim 1, wherein the method comprises,
in the first operating conditions, a further step of pressurizing a
portion of the exhaust gas in multiple exhaust gas compressors so
as to produce liquid or supercritical carbon dioxide.
9. The method according to claim 1, wherein the method comprises,
in the first operating conditions, a further step of extracting a
portion of steam from the intermediate-pressure steam turbine for
driving a compressor.
10. The method according to claim 9, wherein, in the first
operating conditions, the oxygen is mixed with the recycled exhaust
gas so as to produce oxidant gas having an average oxygen content,
by volume, from about 18% to about 28%.
11. A boiler system for generating power by combusting carbonaceous
fuel in a furnace of the boiler system, the boiler system
comprising: means for feeding carbonaceous fuel into the furnace;
means for feeding substantially pure oxygen and recycled exhaust
gas as an oxidant gas into the furnace for combusting the fuel to
produce exhaust gas; an exhaust gas channel for discharging the
exhaust gas from the furnace; means for conveying a stream of
feedwater from a final economizer arranged in the exhaust gas
channel to evaporating and superheating heat exchange surfaces
arranged in the furnace and in the exhaust gas channel for
converting the feedwater to be superheated steam; a high-pressure
steam turbine for expanding the superheated steam for generating
power; means for extracting a first portion of steam from the
high-pressure steam turbine for preheating the feedwater; means for
conveying a second portion of steam from the high-pressure steam
turbine to reheating heat exchange surfaces arranged in the exhaust
gas channel for generating reheated steam; an intermediate-pressure
steam turbine for expanding the reheated steam for generating
power; and means for controlling the ratio of the first and second
portions of steam so as to obtain a desired flue gas temperature in
the exhaust gas channel downstream of the final economizer.
12. The boiler system according to claim 11, wherein the boiler
system comprises means for feeding air as an oxidant gas into the
furnace for combusting the fuel to produce exhaust gas.
13. The boiler system according to claim 11, wherein the boiler
system comprises a gas-gas heater exchanger for transferring heat
from the exhaust gas in the exhaust gas channel to at least a
portion of the oxidant gas.
14. The boiler system according to claim 11, wherein the means for
controlling comprise means for measuring the exhaust gas
temperature.
15. The boiler system according to claim 11, wherein the boiler
system comprises multiple exhaust gas compressors for pressurizing
a portion of the exhaust gas so as to produce liquid or
supercritical carbon dioxide.
16. The boiler system according to claim 11, wherein the boiler
system comprises means for extracting a portion of steam from the
intermediate-pressure steam turbine for driving a compressor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an oxyfuel combusting
boiler system and a method of generating power by using the boiler
system. The invention relates especially to a dual-firing boiler
system, i.e., a boiler system which can be operated by using either
air or a mixture of substantially pure oxygen and recycled exhaust
gas as the oxidant gas, i.e., as the oxygen carrier gas.
[0003] 2. Description of the Related Art
[0004] Oxyfuel combustion is one of the methods suggested for
removing CO.sub.2 from the exhaust gases of a power generating
boiler, such as a pulverized coal (PC) boiler or a circulating
fluidized bed (CFB) boiler. Oxyfuel combustion is based on
combusting carbonaceous fuel with substantially pure oxygen,
typically, of about 95% purity, so as to have carbon dioxide and
water as the main components of the exhaust gas discharged from the
boiler. Thereby, the carbon dioxide can be captured relatively
easily from the exhaust gas, without having to separate it from a
gas stream having nitrogen as its main component, as when
combusting the fuel with air.
[0005] Generating power by oxyfuel combustion is more complicated
than conventional combustion by air, because of the need of an
oxygen supply, for example, a cryogenic or membrane based air
separation unit (ASU), where oxygen is separated from other
components of air, mainly, nitrogen. The produced exhaust gas is
then ready for sequestration of CO.sub.2 when water is removed
therefrom and, possibly, the exhaust gas is purified in order to
reduce inert gases originating from the oxidant, fuel or
air-leakage. This purification is typically done by CO.sub.2
condensation at a low temperature and/or a high pressure. CO.sub.2
can be separated from the exhaust gas, for example, by cooling to a
relatively low temperature, while compressing it to a pressure
greater than 110 bar. Both the production of oxygen and the
compression and purification of carbon dioxide increase the total
production costs of the power generation process, for example, by
decreasing the net power produced in the process.
[0006] Combustion using oxygen differs from combustion using air,
mainly by having a higher combustion temperature and a smaller
combustion volume. Because oxyfuel combustion is still a developing
technology, it is considered to be advantageous to design so-called
first generation oxyfuel combustion boilers, where the combustion
conditions are arranged to be close to those of air-firing
combustion. This can be done by recycling exhaust gas back to the
furnace, so as to provide an average O.sub.2 content of the oxidant
of, for example, 20-28%. Such first-generation oxyfuel combustion
boilers can advantageously be built by modifying existing
air-firing boilers. Due to many uncertainties related to oxyfuel
combustion with capture and storage of carbon dioxide, there is
also a need for dual-firing boilers, i.e., boilers which can be
changed from air-firing to oxyfuel combustion and back, as easily
as possible, and preferably, without any changes in the actual
construction. With such a dual-firing boiler, it is also possible
to have a maximum power output, by using air-firing combustion,
during high load demand, such as in the summer or during the
daytime, and to apply oxyfuel combustion with CO.sub.2 removal in
other conditions. Also, it is possible to use a dual-firing boiler
in an air-firing mode, for example, when the air separation unit or
CO.sub.2 sequestration unit is out of order.
[0007] U.S. Pat. No. 6,418,865 discloses a boiler for combusting
fuel with oxygen-enriched air, which boiler can be made by
retrofitting an air-firing boiler, wherein flue gas is
re-circulated to the furnace so as to have a flame temperature and
total mass flow approximately the same as that for combustion with
air.
[0008] Patent publication number WO 2006/131283 discloses a
retrofitted dual firing boiler, where fresh air exiting an air
heater is either conveyed directly, in the air-firing mode, to the
combustion chamber, or it is, in the oxyfuel combustion mode,
cooled by feedwater of the boiler, compressed by utilizing steam
extracted from a high pressure steam turbine and conveyed to an air
separator unit for producing oxygen. The net power generated in the
CO.sub.2 capturing oxyfuel combustion mode of the process disclosed
in WO 2006/131283 is considerably reduced from that of the
air-firing mode.
[0009] In order to more economically generate power by an oxyfuel
combusting boiler system, there is a need for an improved method
and boiler system for minimizing the loss of produced power,
especially, in a dual-firing boiler.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide an oxyfuel
combusting boiler system and a method of using the boiler system,
so as to minimize the loss of produced power.
[0011] In one aspect, the present invention provides a method of
generating power by combusting carbonaceous fuel with an oxidant
gas in a furnace of a boiler system, the method comprising the
steps of feeding carbonaceous fuel into the furnace at a fuel
feeding rate, feeding oxidant gas into the furnace for combusting
the fuel to produce exhaust gas, discharging the exhaust gas from
the furnace via an exhaust gas channel, conveying a stream of
feedwater at a feedwater conveying rate from a final economizer
arranged in the exhaust gas channel to evaporating and superheating
heat exchange surfaces arranged in the furnace and in the exhaust
gas channel, for converting the feedwater to superheated steam,
expanding the superheated steam in a high-pressure steam turbine
for generating power, extracting a first portion of steam from the
high-pressure steam turbine for preheating the feedwater, conveying
a second portion of steam from the high-pressure steam turbine to
reheating heat exchange surfaces arranged in the exhaust gas
channel for generating reheated steam, and expanding the reheated
steam in an intermediate-pressure steam turbine for generating
power, wherein, in first operating conditions, the oxidant gas is a
mixture of substantially pure oxygen and recycled exhaust gas, and
the ratio of the first and second portions of steam is controlled
so as to obtain a desired flue gas temperature in the exhaust gas
channel downstream of the final economizer.
[0012] In another aspect, the present invention provides a boiler
system for generating power by combusting carbonaceous fuel in a
furnace of the boiler system, the boiler system comprising means
for feeding carbonaceous fuel into the furnace, means for feeding
substantially pure oxygen and recycled exhaust gas as an oxidant
gas into the furnace for combusting the fuel to produce exhaust
gas, an exhaust gas channel for discharging the exhaust gas from
the furnace, means for conveying a stream of feedwater from a final
economizer arranged in the exhaust gas channel to evaporating and
superheating heat exchange surfaces arranged in the furnace and in
the exhaust gas channel, for converting the feedwater to
superheated steam, a high-pressure steam turbine for expanding the
superheated steam for generating power, means for extracting a
first portion of steam from the high-pressure steam turbine for
preheating the feedwater, means for conveying a second portion of
steam from the high-pressure steam turbine to reheating heat
exchange surfaces arranged in the exhaust gas channel for
generating reheated steam, an intermediate-pressure steam turbine
for expanding the reheated steam for generating power, and means
for controlling the ratio of the first and second portions of
steam, so as to obtain a desired flue gas temperature in the
exhaust gas channel downstream of the final economizer.
[0013] The decreasing amount of steam extracted from the
high-pressure steam turbine for preheating the feedwater naturally
lowers the temperature of the feedwater entering a final economizer
in the exhaust gas channel. Thus, the decreasing of this steam
extraction increases the temperature difference between the
feedwater and the exhaust gas in the final economizer. Thereby, the
decreasing of the steam extraction indirectly increases the rate of
heat exchange taking place in the final economizer.
Correspondingly, the increasing of the amount of steam conveyed
from the high-pressure steam turbine to the reheating heat exchange
surfaces increases the heat exchange rate taking place at the
reheating surfaces. In some cases, it may be useful to increase the
heat transfer area of the reheating surfaces in order to obtain the
desired, increased heat transfer rate. Both of the above-described
measures enhances the cooling of the exhaust gas in the exhaust gas
channel, and together, they provide an especially efficient method
for controlling the temperature of the exhaust gas.
[0014] When using the present invention, the fuel feeding rate and
the feedwater conveying rate are advantageously adjusted so as to
obtain a desired furnace temperature. This, together with the
above-discussed method for controlling the temperature of the
exhaust gas, provides an efficient method of adjusting the
temperature profile of an oxyfuel combustion boiler retrofitted
from an air-firing boiler, to be nearly the same as that of the
air-firing combustion, and to avoid, e.g., corrosion or material
strength problems of the boiler walls. According to an advantageous
embodiment of the present invention, the fuel feeding rate at full
load is when modifying an air-firing boiler for oxyfuel combustion,
increased by 20% and, correspondingly, the feedwater conveying rate
is at the same time increased by 10%. Thus, as a consequence of the
method of the present invention, due to the higher firing rate,
more energy can be released from the fuel when using oxyfuel
combustion, and thereby, the net power loss, caused by the
oxycombustion process as a whole, is minimized.
[0015] According to an especially advantageous embodiment of the
present invention, the oxyfuel combustion boiler is a dual-firing
boiler, i.e., an oxyfuel combusting boiler, which can, in special
operating conditions, for example, when the oxygen supply is not
operational, be used for combustion with air. When comparing the
combustion, at full load, in normal operating conditions, i.e., in
the so-called first operation conditions, with a mixture of oxygen
and recycled exhaust gas as the oxidant, to that in the so-called
second operation conditions, by using air as the oxidant, the fuel
feeding rate in the first operating conditions is advantageously
higher than that in the second operating conditions. The fuel
feeding rate in oxyfuel combustion is preferably at least 10%
higher, even more preferably, at least 15% higher, than that in the
air-firing combustion. Due to the higher fuel feeding rate, the
total firing rate of the boiler is increased, and the loss of
produced power is minimized.
[0016] The use of an increased fuel feeing rate in the oxyfuel
combustion, while still maintaining the furnace temperature, is
advantageously partly based on the increased heat exchange in the
evaporation surfaces, due to decreased temperature, and possibly,
also increased flow rate, of the feedwater. As discussed above, the
feedwater temperature can advantageously be lowered, especially
before the final economizer, but to some extent, also after the
final economizer, in oxyfuel combustion, by decreasing the
extraction of steam for preheating the feedwater from that in
air-firing combustion.
[0017] The furnace temperature is naturally, also to a large
extent, determined by the exhaust gas cycling rate, which affects
both the rate of feeding relatively cold inlet gas to the furnace
and the rate of convective heat flow, by the exhaust gas, from the
furnace. The exhaust gas recycling rate may, in the oxyfuel
combustion mode, advantageously be determined such that the average
oxygen content, by volume, of the oxidant gas is at a desired
level, typically, from about 18% to about 28%. The exhaust gas
recycling rate in the oxyfuel combustion mode may alternatively be
determined so as to maintain a desired gas flow velocity, usually,
the same as that in air-firing combustion, in the furnace.
[0018] The increased convective heat flow from the furnace is
partially based on the fact that the mass and heat capacity of the
exhaust gas of oxyfuel combustion, having carbon dioxide as its
main component, are larger than those of the exhaust gas of
air-firing combustion, having nitrogen as its main component. The
high heat flow brings about that the exhaust gas carries an
increased amount of heat to the exhaust gas channel, where the heat
is advantageously recovered by an increased heat exchange rate in
the reheating surfaces and the final economizer, as discussed
above.
[0019] According to a preferred embodiment of the present
invention, the system comprises a gas-gas heat exchanger, where
heat is transferred from the exhaust gas in the exhaust gas channel
to at least a portion of the oxidant gas. Thus, the same gas-gas
heat exchanger is advantageously used in air-firing combustion to
transfer heat from the exhaust gas to the combustion air, and in
oxyfuel combustion to transfer heat from the exhaust gas to at
least a portion of the oxidant gas.
[0020] As is common in oxyfuel combustion, the substantially pure
oxygen is advantageously produced in an air separation unit (ASU),
for example, a cryogenic or membrane based air separation unit.
Correspondingly, a portion of the exhaust gas is advantageously
cooled and pressurized in multiple exhaust gas compressors, so as
to sequestrate liquid or supercritical carbon dioxide. Due to this
auxiliary equipment, the net power produced by an oxyfuel
combusting boiler tends to be considerably less than that of a
corresponding air-firing boiler. According to an advantageous
embodiment of the present invention, at least a portion of the
exhaust compressors is directly driven by mechanical energy of
auxiliary steam turbines using steam extracted from the steam
turbine system. This steam is advantageously generated by firing
more and saved from reducing the extraction of steam used for
feedwater heating. Thus, the need for auxiliary power for the
compression of carbon dioxide is minimized. Correspondingly, in a
case in which the oxygen supply comprises a cryogenic air
separation unit having compressors for pressurizing air, one or
more of these compressors can also be driven directly by the
auxiliary steam turbines, so as to further decrease the need for
auxiliary power.
[0021] According to the present invention, the substantially pure
oxygen and recycled exhaust gas can be fed to the boiler as
separate streams, or as a mixture of the two streams. It is also
possible to feed to the boiler multiple streams, which can be
identical mixture streams, or streams having different temperatures
or compositions. The multiple streams can naturally have different
purposes in the furnace, such as primary, secondary and overfire
gas streams of a PC boiler, or streams of fluidizing gas and
secondary gas of a CFB boiler.
[0022] The feeding rate of oxygen is always, in practice,
determined on the basis of the fuel feeding rate, so as to provide
sufficiently complete combustion of the fuel. Usually, the oxygen
feeding rate is controlled by monitoring the content of residual
oxygen in the exhaust gas, which should stay at a suitable level,
typically, about 3%.
[0023] An advantage of an oxycombustion power generation process in
accordance with the present invention is that it can be taken to
use relatively easily, by retrofitting an conventional air-firing
boiler, such as a PC boiler or a CFB boiler. Advantageously, the
modification mainly comprises the implementation of an oxygen
supply, such as a cryogenic air separation unit, equipment for
carbon dioxide sequestration, means for extensive exhaust gas
recycling and means for controlling the ratio of the steam flows
from the high-pressure steam turbine to the feedwater preheaters
and reheater surfaces. In some cases, the modification may also
require the use of updated steam turbines and a steam condenser, as
well as increased heat exchange surfaces in the upstream portion of
the exhaust gas channel. When controlling the temperatures in the
boiler, as discussed above, the same combustion system can be used
in oxyfuel combustion and in air-firing combustion, thus enabling
the use of the system as a dual-firing steam generator.
[0024] The above brief description, as well as other 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, embodiment of the present invention, taken in
conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
[0025] FIG. 1 is a schematic diagram of an oxy-fuel combusting
power plant in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0026] FIG. 1 shows a schematic diagram of an oxycombustion boiler
system 10 in accordance with a preferred embodiment of the present
invention. The boiler system 10 comprises a boiler 12, which may
be, for example, a pulverized coal (PC) boiler or a circulating
fluidized bed (CFB) boiler. The boiler 12 comprises conventional
fuel feeding means 16, such as a fuel supply pipe means for
introducing oxidant gas into the furnace 14 of the boiler, such as
a gas supply line 18, and an exhaust gas channel 20 for discharging
exhaust gas produced by combusting the fuel with the oxygen of the
oxidant gas. The details and types of some elements of the boiler
12, such as the fuel feeding means 16 and oxidant gas feeding means
18, depend, naturally, on the type of the boiler. Such details, for
example, burners, coal mills, means for separately feeding primary
and secondary oxidant gas, are, however, not important for the
present invention, and they are thus not shown in FIG. 1.
[0027] The oxycombustion boiler system 10 is advantageously
retrofitted from an existing air-firing boiler, mainly by adding
equipment 24 for purifying and sequestering carbon dioxide from the
exhaust gas, and an oxygen supply 26, such as a cryogenic or
membrane-based air separation unit (ASU), for producing
substantially pure oxygen from an air stream 28. Because combustion
with pure oxygen tends to create combustion temperatures which are
too high for the construction of an air-firing boiler, the boiler
system 10 is preferably designed so as to maintain the temperature
profiles in the furnace and the exhaust gas channel to be close to
those of an original air firing boiler. Most preferably, the boiler
system 10 is designed as a dual-firing boiler, i.e., a boiler which
can be easily switched between oxyfuel combustions and air-firing
combustion. At the same time, the system is designed so as to have
the loss of produced net power in the oxycombustion mode be as low
as possible.
[0028] According to the present invention, the oxidant gas,
introduced from gas supply line 18 into the furnace 14, is in
normal operating conditions, so-called first operating conditions,
and includes a mixture of substantially pure oxygen and a portion
of cooled exhaust gas, which is recycled via an exhaust gas
recycling channel 30. The exhaust gas recycling channel 30
advantageously comprises means, such as a fan (not shown in FIG.
1), for controlling the exhaust gas recycling rate. The recycling
rate of the exhaust gas is advantageously adjusted such that the
average oxygen content of the oxidant gas is close to that of air,
preferably, from 18% to 28%. In some applications of the present
invention, it is also possible to introduce the streams of recycled
exhaust gas and substantially pure oxygen, or different oxidant gas
compositions, separately, into the furnace 14, for example, in
different portions of the furnace.
[0029] The walls of the furnace 14 are preferably formed as a
tube-wall construction, which forms an evaporating heat transfer
surface 32, for converting preheated feedwater to steam. The high
temperature portions of the boiler 12, especially, the upstream end
of the exhaust gas channel 20, comprise superheating heat transfer
surfaces 34 for recovering heat from the exhaust gas to produce
superheated steam to be conveyed to the inlet of a high-pressure
steam turbine 36 for generating power in a generator 38. Expanded
steam in line 42 is conveyed from the outlet side of the
high-pressure steam turbine 36 to reheating heat transfer surfaces
40 for recovering further heat from the exhaust gas. For some
cases, primary superheating and reheating surfaces may be located
in the exhaust gas channel 20 and additional finishing superheating
and reheating surfaces, for example in the furnace 14.
[0030] Another portion of steam from the high-pressure turbine 36
may be converted through line 42 to a feedwater heater 44. Reheated
steam is conveyed from the reheating heat exchange surfaces 40 the
feedwater heater 44 to the inlet of an intermediate-pressure steam
turbine 46 for generating power. The intermediate-pressure steam
turbine 46 may comprise a line 48 for extracting steam from the
steam turbine 46 for other purposes, advantageously, for generating
mechanical power in an auxiliary steam turbine for driving
compressors in the air separation unit 26 or carbon dioxide
purification and sequestration unit 24. In practice, the steam
turbine system also usually comprises at least a low-pressure steam
turbine, which is, however, not shown in FIG. 1. There may also be
more feedwater heaters than the single feedwater heater 44 shown in
FIG. 1.
[0031] The steam cycle of the boiler 12 comprises, in a
conventional manner, a condenser 50 downstream of the
intermediate-pressure steam turbine 46. The condensed steam, i.e.,
feedwater of the next steam cycle, is conducted from the condenser
50 for preheating in an economizer system typically comprising at
least a first economizer 52 and a final economizer 54, to be again
converted to steam in the evaporation surfaces 32. Additional
feedwater heating may be performed in the feedwater heater 40 by
steam extracted from the high-pressure steam turbine 36.
[0032] According to the present invention, the exhaust gas
temperature is controlled in oxyfuel combustion by adjusting the
rate of extracting intermediate-pressure steam from the
high-pressure steam turbine 36 to the feedwater preheater 44, by
means 56, such as a regulation valve, arranged in the steam line
42. When this rate is decreased, the temperature of the feedwater
entering the final economizer 54 is lowered, and the rate of heat
exchange taking place in the final economizer 54 increases. At the
same time, a higher portion of steam remains to be conveyed to the
reheating heat transfer surfaces 40, which increases the rate of
heat exchange taking place in the final economizer 54. At the same
time, a higher portion of steam remains to be conveyed to the
reheating heat transfer surfaces 40, which increases the rate of
heat exchange taking place at the reheating heat exchange surfaces
40. Thus, both of these effects increase the cooling of the exhaust
gas, and, thereby, they can be used to efficiently control the
exhaust gas temperature. The controlling of the exhaust gas
temperature may advantageously be based on measuring the
temperature of the exhaust gas downstream of the final economizer
54 by a thermometer 58.
[0033] According to the present invention, the loss of produced net
power is minimized by arranging the conditions such that more fuel
can be fired while still maintaining the temperatures in the
furnace 14 and in the exhaust gas channel 20. The temperature in
the furnace 14 can be maintained by adjusting the exhaust gas
recycling rate to a suitable level and by controlling the
temperature and flow rate of the feedwater. When the exhaust gas
recycling rate is adjusted such that the volume flow of gas in the
furnace 14 remains at a desired level, the temperature in the
furnace 14 can typically still be adjusted to its desired level by
the measures discussed above. Due to the increased mass flow and
high heat capacity of the exhaust gas, consisting mainly of carbon
dioxide, the convective heat, carried by the exhaust gas, is
increased, even if the temperature in the furnace 14 is unchanged.
This additional heat can then be recovered by decreasing the
extraction of steam for feedwater preheating, by the means 56, and
increasing the reheating rate, as discussed above, as well as by
increasing feedwater flow due to increased main steam
generation.
[0034] A recuperative or regenerative gas-gas heat exchanger 60 is
advantageously arranged in the exhaust gas channel, downstream of
the final economizer 54. A gas-gas heat exchanger 60 can be of a
recuperative or regenerative type, for transferring heat from the
exhaust gas to the oxidant gas of the boiler 12. The exhaust gas
channel 20 also usually comprises different units for cleaning the
exhaust gas from particles and gaseous pollutants, but because they
are not important for the present invention, such units are not
shown in FIG. 1.
[0035] In accordance with the main object of oxyfuel combustion,
i.e., to recover carbon dioxide from the exhaust gas, the end
portion of the exhaust gas channel 20 is equipped with means 24,
such as a separator, to produce liquid or supercritical carbon
dioxide, typically, at a pressure of about 110 bar, so that it can
be transported for further use or to be stored in a suitable place.
The carbon dioxide purification and sequestration system also
usually comprises means for completely drying all water from the
exhaust gas, and means for separating oxygen, and other possible
impurities, from the carbon dioxide, which are, however, not shown
in FIG. 1. Such means for drying and means for separating are
individually known in the art.
[0036] The water content of the recycled exhaust gas is
advantageously lowered before the exhaust gas is recycled to the
furnace 14. Therefore, the exhaust gas recycling line 30 is
advantageously branched off from the exhaust gas channel 20
downstream of the first economizer 52, which functions as a
condensing cooler of the exhaust gas. Thereby, the water content of
the recycled gas is reduced, also causing a reduction of the water
content in the furnace 14 and in the exhaust gas discharged from
the furnace 14. Because the O.sub.2 content of the exhaust gas has
to be maintained at a suitable level, at about 3% by volume, in
order to guarantee sufficiently complete combustion of the fuel,
the reducing of the water content reduces the O.sub.2/CO.sub.2
ratio in the exhaust gas. Alternatively, the condensing cooler can
be downstream of the branch point of the recycled exhaust gas.
[0037] Because of the efficient methods for controlling the
temperatures in the furnace 14 and in the exhaust gas channel 20,
as described above, the oxyfuel combustion system shown in FIG. 1
can be constructed relatively easily by retrofitting an existing
air-firing boiler. For the same reasons, the boiler system can also
be used as a dual firing boiler, which can be switched between
oxyfuel combustion and air-firing combustion, without any physical
modification of the system. This is achieved by arranging means 62,
such as an air inlet supply line, for introducing fresh air as the
oxidant gas, to replace the mixture of oxygen and recycled exhaust
gas, and a stack 64 for releasing the exhaust gas to the
environment. Advantageously, air inlet 62 is arranged in the
recycling gas channel 30, in such a way that the gas-gas heater 60
can be used alternatively as an air heater. In an air-firing
combustion mode, the temperature profiles in the furnace 14 and in
the exhaust channel 20 can be adjusted to their desired values by
adjusting the fuel feeding rate and steam reheating rate to
suitable values, by using the principles discussed above.
[0038] 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.
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