U.S. patent application number 12/130474 was filed with the patent office on 2009-12-03 for method of and system for generating power by oxyfuel combustion.
This patent application is currently assigned to FOSTER WHEELER ENERGIA OY. Invention is credited to Timo Eriksson, Zhen Fan, Ossi Sippu.
Application Number | 20090293782 12/130474 |
Document ID | / |
Family ID | 41378193 |
Filed Date | 2009-12-03 |
United States Patent
Application |
20090293782 |
Kind Code |
A1 |
Eriksson; Timo ; et
al. |
December 3, 2009 |
METHOD OF AND SYSTEM FOR GENERATING POWER BY OXYFUEL COMBUSTION
Abstract
A method of generating power by oxyfuel combustion. Carbonaceous
fuel and oxidant gas are fed into a furnace. In a first operating
mode, the oxidant gas includes a stream of substantially pure
oxygen conveyed from an oxygen supply for combusting the fuel with
the oxygen to produce exhaust gas including mainly carbon dioxide
and water. The exhaust gas is discharged from the furnace and is
divided into a recycling portion and an end portion. The recycling
portion is recycled to the furnace. Heat is transferred from the
end portion to the stream of substantially pure oxygen by
circulating a liquid heat transfer medium in a passage between an
exhaust gas cooler and an oxygen heater.
Inventors: |
Eriksson; Timo; (Varkaus,
FI) ; Fan; Zhen; (Parsippany, NJ) ; Sippu;
Ossi; (Varkaus, FI) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
1290 Avenue of the Americas
NEW YORK
NY
10104-3800
US
|
Assignee: |
FOSTER WHEELER ENERGIA OY
Espoo
FI
|
Family ID: |
41378193 |
Appl. No.: |
12/130474 |
Filed: |
May 30, 2008 |
Current U.S.
Class: |
110/205 ;
110/233; 60/39.511; 60/39.52 |
Current CPC
Class: |
F22B 35/002 20130101;
F23J 2215/50 20130101; F23C 2202/30 20130101; F23L 7/007 20130101;
Y02E 20/32 20130101; Y02E 20/322 20130101; Y02E 20/348 20130101;
F23C 9/08 20130101; Y02E 20/34 20130101; Y02E 20/344 20130101; F23L
2900/07001 20130101; F23L 2900/07007 20130101; F23C 9/003 20130101;
F23L 15/045 20130101; F23L 2900/07006 20130101 |
Class at
Publication: |
110/205 ;
110/233; 60/39.511; 60/39.52 |
International
Class: |
F23C 9/08 20060101
F23C009/08; F23C 9/00 20060101 F23C009/00; F23L 7/00 20060101
F23L007/00 |
Claims
1. A method of generating power by oxyfuel combustion, said method
comprising the steps of: (a) feeding carbonaceous fuel into a
furnace; (b) feeding oxidant gas into the furnace, wherein, in a
first operating mode, the oxidant gas comprises a stream of
substantially pure oxygen conveyed from an oxygen supply for
combusting the fuel with the oxygen to produce exhaust gas
comprising mainly carbon dioxide and water; (c) discharging the
exhaust gas from the furnace; (d) dividing the exhaust gas into a
recycling portion and an end portion; (e) recycling the recycling
portion to the furnace; and (f) transferring heat from the end
portion to the stream of substantially pure oxygen by circulating a
liquid heat transfer medium in a passage between an exhaust gas
cooler and an oxygen heater.
2. The method according to claim 1, further comprising the steps
of: (g) transferring heat in a gas-gas heat exchanger from the
exhaust gas to the recycling portion to produce a stream of heated
recycling gas; (h) mixing the stream of substantially pure oxygen
with the stream of heated recycling gas so as to form a stream of
mixed gas; and (i) feeding the stream of mixed gas as the oxidant
gas into the furnace.
3. The method according to claim 1, wherein the liquid heat
transfer medium is water.
4. The method according to claim 1, wherein the exhaust gas cooler
is of a corrosion resistant type.
5. The method according to claim 1, wherein the first operating
mode is performed alternatingly with a second operating mode, in
which the recycling portion is minimized, and the oxidant gas
comprises a stream of air.
6. The method of claim 2, wherein the first operating mode is
performed alternatingly with a second operating mode, in which the
recycling portion is minimized, and the oxidant gas comprises a
stream of air introduced into the gas recycling line, so as to
transfer heat in the gas-gas heat exchanger from the exhaust gas to
the air stream.
7. The method according to claim 5, wherein the second operating
mode comprises a step of transferring heat from the end portion of
the stream of air by circulating a liquid heat transfer medium in a
passage between the exhaust gas cooler and an air heater.
8. The method according to claim 6, wherein the second operating
mode comprises a step of transferring heat from the end portion of
the stream of air by circulating a liquid heat transfer medium in a
passage between the exhaust gas cooler and an air heater.
9. The method according to claim 7, wherein the second operating
mode comprises further steps of dividing a parallel stream from the
exhaust gas stream and transferring heat from the parallel stream
into the stream of air by circulating a liquid heat transfer medium
between a second exhaust gas cooler and the air heater.
10. A system for generating power by oxyfuel combustion, said
system comprising: a furnace for combusting carbonaceous fuel; an
oxygen channel for feeding the substantially pure oxygen from an
oxygen supply to the furnace for combusting the fuel with the
oxygen to produce exhaust gas comprising mainly carbon dioxide and
water; an exhaust gas channel connected to the furnace for
discharging the exhaust gas from the furnace; branch piping for
dividing the exhaust gas into a recycling portion and an end
portion; a gas recycling channel for feeding the recycling portion
of the exhaust gas to the furnace; and an exhaust gas cooler
arranged in the exhaust gas channel downstream of the branch piping
and an oxygen heater arranged in the oxygen channel connected by a
passage for transferring heat from the end portion to the stream of
substantially pure oxygen by circulating a liquid heat transfer
medium in the passage.
11. The system according to claim 10, further comprising: a gas-gas
heat exchanger for transferring heat from the exhaust gas to the
recycling portion; a mixer arranged to connect the gas recycling
channel downstream of the gas-gas heat exchanger and the oxygen
channel downstream of the oxygen heater for mixing the recycling
portion with the stream of substantially pure oxygen so as to form
a stream of mixed gas; and a channel for feeding the stream of
mixed gas as the oxidant gas into the furnace.
12. The system according to claim 10, wherein the exhaust gas
cooler is of a corrosion resistant type.
13 The system according to claim 10, further comprising: a flow
controller arranged in the recycling gas channel for controlling
the recycling portion; and an air intake for introducing a stream
of air as the oxidant gas instead of substantially pure oxygen.
14. The system according to claim 11, further comprising: a flow
controller arranged in the recycling gas channel for controlling
the recycling portion; and an air intake for introducing a stream
of air as the oxidant gas, instead of substantially pure oxygen
into the gas recycling line, wherein the air intake is arranged
upstream of the gas-gas heat exchanger so as to transfer heat from
the exhaust gas to the air stream in the gas-gas heat
exchanger.
15. The system according to claim 13, further comprising an air
heater connected by a passage to the exhaust gas cooler for
transferring heat from the end portion to the stream of air by
circulating a liquid heat transfer medium in the passage between
the exhaust gas cooler and the air heater.
16. The system according to claim 14, further comprising an air
heater connected by a passage to the exhaust gas cooler for
transferring heat from the end portion to the stream of air by
circulating a liquid heat transfer medium in the passage between
the exhaust gas cooler and the air heater.
17. The system according to claim 15, further comprising: second
branch piping for dividing from the exhaust gas stream a parallel
stream in a parallel channel option; and a second exhaust gas
cooler arranged in the parallel channel portion for transferring
heat from the parallel stream into the stream of air by circulating
a liquid heat transfer medium in a passage between the second
exhaust gas cooler and the air heater.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of and a system
for generating power by oxyfuel combustion. The invention
especially relates to a flexi-burn or dual-firing combustion
system, i.e., a system which can be easily switched between the
modes of oxyfuel combustion and combustion with air.
[0003] 2. Description of the Related Art
[0004] Oxyfuel combustion is one of the methods suggested for
removing CO.sub.2 from the combustion 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, without having to separate it from a gas stream having
nitrogen as its main component, such 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. The produced exhaust gas is then ready for
sequestration of CO.sub.2 when water is removed from the oxidant,
fuel and air-leakage. This purification is typically done by
CO.sub.2 condensation at a low temperature under high pressure.
CO.sub.2 can be separated from the exhaust gas, for example, by
cooling it to a relatively low temperature, while compressing it to
a pressure greater than one hundred ten bar.
[0006] In order to avoid very high combustion temperatures
resulting from combustion with pure oxygen, it is advantageous to
use an oxyfuel combustion boiler, 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 gas of, for
example, about 20 to about 28%. Such oxyfuel combustion boilers can
advantageously be built by modifying existing air-firing boilers.
Due to many uncertainties related to oxyfuel combustion with the
capture and storage of carbon dioxide, there is also a need for
dual firing boilers, i.e., boilers which can be changed from
oxyfuel combustion to air-firing, and back, as easily as possible,
preferably, without any changes in the actual construction. With
such a dual firing boiler, it is possible to have maximum power
output, by using air-firing combustion, during a high load demand,
such as during the summer or the day-time, and applying 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 separation unit or CO.sub.2 sequestration unit is
out of order.
[0007] U.S. Pat. No. 6,202,574 suggests a combustion system for
firing fossil fuel with substantially pure oxygen to produce
exhaust gas having carbon dioxide and water as its two largest
constituents. A portion of the exhaust gas is recycled to the
combustion chamber and the rest of the exhaust gas is compressed
and stripped to produce carbon dioxide in a liquid phase. The
recycled exhaust gas and the substantially pure oxygen stream are
preheated by the exhaust gas in respective gas-gas heat
exchangers.
[0008] German patent publication DE 103 56 703 A1 shows an oxyfuel
combustion boiler system comprising a feedwater heater and a
gas-gas heat exchanger acting as an oxygen heater arranged to cool
the exhaust gas downstream of a recycle gas take-off.
[0009] In order to more economically generate power when minimizing
carbon dioxide emissions, there is a need for an improved method of
and a system for oxyfuel combustion, especially, by using a dual
firing combustion system.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to provide a new
method of and a system for oxyfuel combustion.
[0011] According to one aspect, the present invention provides a
method of generating power by oxyfuel combustion, the method
comprising the steps of feeding carbonaceous fuel into a furnace,
feeding oxidant gas into the furnace, wherein, in a first operating
mode, the oxidant gas comprises a stream of substantially pure
oxygen conveyed from an oxygen supply for combusting the fuel with
the oxygen to produce exhaust gas comprising mainly carbon dioxide
and water, discharging the exhaust gas from the furnace, dividing
the exhaust gas into a recycling portion and an end portion,
recycling the recycling portion of the furnace, and transferring
heat from the end portion to the stream of substantially pure
oxygen by circulating a liquid heat transfer medium, such as water,
in a passage between an exhaust gas cooler and an oxygen
heater.
[0012] According to another aspect, the present invention provides
a system for generating power by oxyfuel combustion, the system
comprising a furnace for combusting carbonaceous fuel, an oxygen
channel for feeding the substantially pure oxygen from an oxygen
supply to the furnace for combusting the fuel with the oxygen to
produce exhaust gas comprising mainly carbon dioxide and water, an
exhaust gas channel connected to the furnace for discharging the
exhaust gas from the furnace, branch piping for dividing the
exhaust gas into a recycling portion and an end portion, a gas
recycling channel for feeding the recycling portion of the exhaust
gas to the furnace, an exhaust gas cooler arranged in the exhaust
gas channel downstream of the branch piping, and an oxygen heater
arranged in the oxygen channel connected by a passage for
transferring heat from the end portion to the stream of
substantially pure oxygen by a circulating liquid heat transfer
medium in the passage.
[0013] The process of transferring heat from the exhaust gas to the
oxidant gas improves the efficiency of the boiler and the process
as a whole. The present invention differs from the prior art
solution shown in U.S. Pat. No. 6,202,574, for example, in that the
pure oxygen stream is heated by low grade heat obtained from the
end portion of the exhaust gas, which is to be discharged from the
system, and not from the exhaust gas upstream of the take off point
of the recycled gas. Thereby, the thermal efficiency of the process
is improved. While the exhaust gas cooler is at the low temperature
portion of the exhaust gas channel, it may cool below the acid
condensation temperature during the heat transfer. Therefore, the
exhaust gas cooler is advantageously of a corrosion resistant type,
such as a plastic gas cooler.
[0014] The recycling portion of the exhaust gas comprises, often in
oxyfuel combustion, a majority, typically, about 65 to about 80%,
of the exhaust gas discharged from the furnace, whereby the end
portion of the exhaust gas is about or less than a third of the
exhaust gas stream. On the other hand, the end portion is always
naturally about as large as the oxygen stream. Therefore, the
arranging of the exhaust gas cooler downstream of the point
dividing the recycling portion provides another advantage in that
the gas flows of the end portion of the exhaust gas and the oxygen
stream are as large, whereby it is relatively easy to obtain an
energy balance between the two flows at advantageous temperature
levels. The end portion of the exhaust gas is, in practice,
virtually free of dust, while being located downstream of an
electron dust separator (ESP) or a bag house. This provides
conditions for arranging an effective and a compact heat exchanger.
It also increases the safety of the system, while there is very
little of igniting or explosive dust present.
[0015] According to the present invention, heat is transferred form
the exhaust gas to the oxygen flow by means of a liquid transfer
medium, instead of transferring the heat directly in a gas-gas heat
exchanger. This feature provides the advantage that a leak in the
exhaust gas cooler, which may occur especially when the heat
exchanger is used below the acid dew point temperature, may only
cause a leak of water, and not a leak of explosive oxygen, into the
exhaust gas channel.
[0016] Another advantage of transferring heat from the exhaust gas
to the oxygen by means of a liquid heat transfer medium is that, in
practice, it usually provides a relatively simple construction,
although it appears to make the system more complicated. The reason
for this is that the oxygen supply, typically, an air separation
unit (ASU), is usually located at a portion of the power plant
other than a final exhaust gas processing system, usually
comprising units for cleaning, capturing and storage of carbon
dioxide. Thus, the heat transferring distance may be quite long,
and it is easier to transfer the heat such long distances in
relatively small water tubes by making an additional excursion to
much larger channels carrying hot exhaust gas or explosive oxygen
gas.
[0017] According to a preferred embodiment of the present
invention, the method further comprises steps of transferring heat
in a gas-gas heat exchanger from the exhaust gas to the recycling
portion of the exhaust gas to produce a stream of heated recycling
gas. The recycling gas stream and the stream of substantially pure
oxygen may be conducted to the furnace separately, but, according
to a preferred embodiment of the present invention, the stream of
substantially pure oxygen is mixed with the stream of heated
recycling gas in a mixer arranged to connect the gas recycling
channel downstream of the gas-gas heat exchanger and the oxygen
channel downstream of the oxygen heater. Thus, a stream of mixed
gas is formed to be fed as the oxidant gas via a channel into the
furnace.
[0018] The feeding rate of the relatively pure oxygen is 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%. By mixing the pure oxygen stream, heated by
the transfer heat medium as described above, with recycling gas
heated in a gas-gas heat exchanger, it is possible to efficiently
control the temperature, flow rate and oxygen content of the mixed
gas to be used as the oxidant gas in the furnace.
[0019] The recycling gas channel and the oxygen channel may
advantageously be divided in multiple parallel lines, which are
separately connected in multiple mixers, so as to form multiple
streams of mixed gas, which may be used, for example, as primary
and secondary oxidant gas in the furnace. By separately controlling
the gas flows in the parallel recycling gas lines and oxygen lines,
it is possible to separately control the flows and oxygen contents
of the oxidant gas streams.
[0020] According to a preferred embodiment of the present
invention, the gas feeding rate to a boiler retrofitted from
air-firing to oxycombustion is adjusted so as to maintain the
original gas velocity in the furnace, whereby the oxygen content of
the oxidant gas is advantageously adjusted to be near to that of
air, typically, from about 18% to about 28%. The furnace
temperature or heat flux of the retrofitted boiler shall
advantageously also be maintained at about its original level to
avoid, e.g., corrosion or material strength problems of the furnace
walls.
[0021] Due to the high heat capacity of the exhaust gas generated
in the oxycombustion process, having carbon dioxide as its main
component, when compared to that of conventional exhaust gas,
having nitrogen as its main component, the same volume flow of
exhaust gas at the same temperature carries more heat in the case
of oxycombustion, than in air-firing combustion. Thus, when
changing an air-firing steam generating process to oxycombustion,
the fuel feeding rate can be increased by at least 10%, and still
maintain the original furnace temperature or heat flux. Thereby, an
increased amount of heat is available, e.g., for steam generation
and for heating of the oxidant gas.
[0022] According to an especially advantageous embodiment of the
present invention, the first operating mode is performed
alternatingly with a second operating mode, a so-called an
air-firing mode, wherein the recycling portion is minimized, for
example, by a damper and the oxidant gas comprises a stream of air.
Advantageously, the system comprises an air intake for introducing
air into the gas recycling line upstream of the gas-gas heat
exchanger. Thus, the gas-gas heat exchanger is used in the second
operating mode to transfer heat from the exhaust gas to the air
stream.
[0023] In the second operating mode, the combustion system is
advantageously decoupled from the oxygen supply, and the exhaust
gas comprises nitrogen, carbon dioxide and water, as its main
components. Therefore, the system is also decoupled from the carbon
dioxide capturing and storage units, and the exhaust gas is
released to the environment through a stack. One of the main ideas
of the present invention is that it provides an oxyfuel combustion
method which can be easily switched to air-firing combustion, and
back, without any modifications in the actual construction, even
on-line, without stopping the power generation during the
change.
[0024] In the second operating mode, the oxygen supply is not in
use and, because the content of the exhaust gas is that in
conventional combustion, the carbon dioxide of the exhaust gas is
not purified and sequestered. Thus, the auxiliary power consumption
of these processes is minimized, and the system provides a higher
total efficiency than that in oxygen-combustion, at the cost of
releasing carbon dioxide to the environment. The air-firing
operating mode is advantageously used when the demand for power is
especially high, for example, in the summer or during the day-time.
Alternatively, the air-firing mode may be temporarily used, e.g.,
based on varying economical conditions, or when the oxygen supply
or carbon dioxide capturing and storage systems are, for some
reason, not available.
[0025] The method advantageously comprises, in the second operating
mode, a step of transferring heat from the exhaust gas to the
stream of air by circulating a liquid heat transfer medium in a
passage between an exhaust gas cooler and an air heater. Because
the exhaust gas flow rate is, in the second operating mode,
typically much higher than the flow rate of the end portion of the
exhaust gas in the first operating mode, advantageously, at least
one other channel with an exhaust gas cooler may be formed parallel
to the exhaust gas channel portion comprising the exhaust gas
cooler used in the oxycombustion mode. Thereby, the heat transfer
medium is circulated in the air-firing mode between the air heater
and at least two parallel exhaust gas coolers. Thus, the air
stream, which has a clearly higher flow rate than does the oxygen
stream in the oxycombustion mode, can be efficiently heated by the
exhaust gas.
[0026] 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 drawing.
BRIEF DESCRIPTION OF THE DRAWING
[0027] FIG. 1 is a schematic diagram of an oxy-fuel combusting
power plant in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] FIG. 1 shows a schematic diagram of a power plant 10 in
accordance with a preferred embodiment of the present invention.
The power plant 10 comprises a boiler 12, which may be, for
example, a pulverized coal (PC) boiler or a circulating fluidized
bed (CFB) boiler. The furnace 14 of the boiler comprises
conventional fuel feeding means 16, means for feeding oxidant gas
18 into the furnace, 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 type 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 boiler. Such details, for
example, as burners, coal mills, means for separately feeding
primary and secondary inlet gas, are, however, not important for
the present invention, and are, thus, not shown in FIG. 1.
[0029] The oxidant gas is preferably a mixture of substantially
pure oxygen, produced from an air stream 22 in an air separation
unit (ASU) 24, and a portion of the exhaust gas, which is recycled
via an exhaust gas recycling channel 26. The exhaust gas recycling
channel 26 advantageously comprises a flow controller, such as a
controllable fan 28 and/or a damper 30, for controlling the exhaust
gas recycling rate. The recycling rate of the exhaust gas is
advantageously adjusted such that the average O.sub.2 content of
the oxidant gas is close to that of air, preferably, from about 18%
to about 28%. In some applications of the present invention, 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, into, for example, different portions
of the furnace 14.
[0030] As is conventional, the furnace 14 usually comprises
evaporation surfaces, not shown in FIG. 1, and the exhaust gas
channel 20 further comprises heat exchanger surfaces 32, 34, for
example, superheaters and economizers. For the sake of simplicity,
FIG. 1 shows only two heat exchanger surfaces 32, 34, but, in
practice, the exhaust gas channel 20 usually comprises multiple
superheating, reheating and economizer surfaces for recovering heat
from the exhaust gas. Between the steam generating heat exchange
surfaces 32 and 34, there is arranged a gas-gas heat exchanger 36,
usually, a regenerative heat exchanger, for transferring heat from
the exhaust gas directly to the recycling portion of the exhaust
gas.
[0031] The exhaust gas channel 20 usually comprises conventional
units for cleaning the exhaust gas from particles and gaseous
pollutants, which are, in FIG. 1, schematically represented only by
a dust separator 38. The dust separator 38 and possible other gas
cleaning units are advantageously arranged upstream of the branch
point 40 of the exhaust gas recycling channel 26. At the branch
point, the exhaust gas stream is divided into a recycling portion,
conveyed through the recycling gas channel 26 back to the furnace
14 and an end portion, which is conveyed through the end portion 42
of the exhaust gas channel 20 for final processing.
[0032] In accordance with the main object of oxyfuel combustion,
i.e., to recover carbon dioxide from the exhaust gas, the end
portion 42 of the exhaust gas channel 20 is equipped with a device,
schematically represented by a carbon dioxide capturing unit 44,
for cooling, cleaning and compressing carbon dioxide. The unit 44
usually comprises a dryer for completely drying all water from the
exhaust gas, and a separator for separating a stream of
non-condensable gas, such as oxygen 50, and possibly other
impurities, from the carbon dioxide. A stream of carbon dioxide 46
is typically captured in a liquid or supercritical state, at a
pressure of, for example, about one hundred ten bar, so that it can
be transported for further use or to be stored in a suitable place.
FIG. 1 shows, separately, a condensing gas cooler 48, located
upstream of the carbon dioxide capturing unit 44, for initially
removing water from the exhaust gas.
[0033] In order to transfer energy from the end portion of the
exhaust gas to the stream of substantially pure oxygen, the end
portion 42 of the exhaust gas channel 20 is, according to the
present invention, equipped with a gas cooler 52, which is
connected by a liquid heat transfer medium circulation to an oxygen
heater 54 arranged in an oxygen channel 56 downstream of the oxygen
supply 24. The heat transfer medium, usually water, is, preferably,
circulated by a pump 58 in a tubing 60 extending between the gas
cooler 52 and the oxygen heater 54, which are usually, in practice,
located at distant portions of the power plant 10.
[0034] The oxygen channel 56 may be connected directly to the
furnace 14, but, according to a preferred embodiment of the present
invention, the oxygen channel 56 and the exhaust gas recycling
channel 26 are both connected to a mixer 62, and a stream of mixed
gas is lead as the oxidant gas to the furnace through the oxidant
gas feeding means 18. This system makes it possible to separately
control the temperature, flow rate and oxygen content of the
oxidant gas.
[0035] According a preferred embodiment of the present invention,
the system also comprises an air intake 64 for feeding air to the
furnace. The air stream is, preferably, introduced into the exhaust
gas recycling channel 26 upstream of the gas-gas heat exchanger 36,
whereby it is possible to transfer heat directly form the exhaust
gas to the stream of air. The purpose of the air intake 64 is to
enable switching from oxyfuel combustion to air-firing combustion.
Thus, when introducing air to the gas recycling line, the oxygen
supply is stopped, and the recycling of exhaust gas is minimized,
preferably, totally stopped, by the damper 30. The exhaust gas
comprises, in the air-firing mode, carbon dioxide and water mixed
with a large amount of nitrogen, whereby, it is not possible to
easily capture the carbon dioxide from the exhaust gas, which is,
thus, in this case, released to the environment through a stack
66.
[0036] While the air stream obtained from the environment is,
typically, at a much lower temperature than is the recycling
exhaust gas, the heating of air in the gas-gas heat exchanger 36 is
usually not sufficient, but more heat is advantageously transferred
to the air stream by an air heater 68. The air heater 68 is also
advantageous in order to increase the inlet temperature of the air
in the gas-gas heat exchanger 36, so as to avoid flue gas
condensation and related problems on the heat exchanger surfaces.
The air heater 68 is preferably connected to the exhaust gas cooler
52, which is also used in the oxycombustion mode, by means of heat
transfer medium circulation. Thus, the tubing 60 used for water
circulation in the oxycombustion is in the air-firing combustion
mode connected to a side tubing 70, a so-called air heating arm,
including a circulation pump 72.
[0037] By opening a valve 74 in the air heating arm, and closing a
valve 76 in the main tubing, in the so-called oxygen heating arm,
the heat transfer medium can be switched to circulate through the
air heater 68 instead of the oxygen heater 54. If the water
circulation pump is arranged in the common portion of the water
tubing 60, it is, in the above-described case, sufficient to have
only one circulation pump. Because the flow rate of air in the
air-firing mode is much higher than that of the oxygen in the
oxycombustion mode, and the flow rate of exhaust gas in the
air-firing mode is much larger than that of the end portion of the
exhaust gas in the oxycombustion mode, the circulation rate of the
heat transfer medium is advantageously lower in the oxycombustion
mode than that in the air-firing mode.
[0038] According to an alternative embodiment of the present
invention, parallel to the end portion of the exhaust gas channel
42, is arranged a parallel channel portion 78. The portion of the
flue gas flowing through the parallel channel portion 78 can be
varied by a damper 80 to be, for example, between about 0% and
about 75%. The parallel channel portion comprises another exhaust
gas cooler 82, which is in the air-firing mode connected in
parallel to the exhaust gas cooler 52. Thus, heat is transferred in
the oxycombustion mode from the exhaust gas cooler 52 to the oxygen
heater 54, and, in the air-firing combustion mode, from two exhaust
gas coolers 52, 82 to the air heater 68. By properly adjusting the
heat transfer rates, it is possible to obtain required inlet gas
temperatures and sufficient cooling of the exhaust gas in both
operating modes.
[0039] 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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