U.S. patent application number 12/130448 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 | 20090297993 12/130448 |
Document ID | / |
Family ID | 41377654 |
Filed Date | 2009-12-03 |
United States Patent
Application |
20090297993 |
Kind Code |
A1 |
Fan; Zhen ; et al. |
December 3, 2009 |
Method of and System For Generating Power By Oxyfuel Combustion
Abstract
An oxyfuel combustion system for generating power that includes
a furnace for combusting carbonaceous fuel and substantially pure
oxygen to produce exhaust gas including mainly carbon dioxide and
water. An exhaust gas channel system discharges the exhaust gas
from the furnace. The exhaust gas channel system has an upstream
channel, an outlet channel and a gas recycling channel. The
upstream channel recycles a recycling portion of the exhaust gas
through the recycling channel to the furnace, and conveys an end
portion of the exhaust gas through the outlet channel for final
processing. The upstream channel is divided between a first divider
piece and a connecting piece into a first exhaust gas channel
portion and a second exhaust gas channel portion. A gas-gas heat
exchanger arranged in the first exhaust gas channel portion
transfers heat from exhaust gas in the first exhaust gas channel
portion to gas in the gas recycling channel. A first economizer
arranged in the second exhaust gas channel portion transfers heat
from exhaust gas in the second exhaust gas channel portion to a
flow of feedwater in a feedwater line, and a second economizer
arranged in the exhaust gas channel system downstream of the
connecting piece transfers heat from gas in the exhaust gas channel
system to the flow of feedwater in the feedwater line.
Inventors: |
Fan; Zhen; (Parsippany,
NJ) ; Eriksson; Timo; (Varkaus, FI) ; 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: |
41377654 |
Appl. No.: |
12/130448 |
Filed: |
May 30, 2008 |
Current U.S.
Class: |
431/4 ; 60/320;
60/39.511 |
Current CPC
Class: |
F23J 15/006 20130101;
F23J 15/06 20130101; Y02E 20/30 20130101; Y02E 20/34 20130101; F22B
35/002 20130101; Y02E 20/363 20130101; F27D 17/004 20130101; Y02E
20/344 20130101; F23C 9/00 20130101; F27D 17/008 20130101; F22B
35/001 20130101; Y02P 10/122 20151101; F23L 7/007 20130101; F22B
37/00 20130101 |
Class at
Publication: |
431/4 ;
60/39.511; 60/320 |
International
Class: |
F23J 7/00 20060101
F23J007/00 |
Claims
1. A method of generating power by oxyfuel combustion, the 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 an
exhaust gas stream from the furnace; (d) dividing the exhaust gas
stream in a final divider piece into a recycling portion and an end
portion; (e) recycling the recycling portion through a gas
recycling channel to the furnace; and (f) conveying the end portion
through an outlet channel to final processing, wherein the method
also comprises the steps of: (g) dividing the exhaust gas stream in
a first divider piece, arranged upstream of the final divider
piece, into a first exhaust gas stream and a second exhaust gas
stream; (h) transferring heat from the first exhaust gas stream to
gas in the gas recycling channel by a gas-gas heat exchanger to
form a cooled first exhaust gas stream; (i) transferring heat from
the second exhaust gas stream to a flow of feedwater in a feedwater
line by a first economizer to form a cooled second exhaust gas
stream; (j) combining the cooled first exhaust gas stream and the
cooled second exhaust gas stream in a connecting piece arranged
upstream of the final divider piece to form a combined exhaust gas
stream; and (k) conveying at least a portion of the combined
exhaust gas stream through a second economizer arranged to transfer
heat from the combined exhaust gas stream to the flow of feedwater
in the feedwater line.
2. The method according to claim 1, wherein the second economizer
is arranged in the exhaust gas channel upstream of the final
divider piece.
3. The method of claim 1, further comprising a step of transferring
heat from the end portion of the exhaust gas to the stream of
substantially pure oxygen.
4. The method of claim 3, further comprising a step of mixing the
stream of substantially pure oxygen and the recycling portion as a
combined oxidant gas in a mixer, and feeding the combined oxidant
gas into the furnace.
5. The method according to claim 3, wherein the second economizer
is arranged in the recycling channel to transfer heat from the
recycling portion of the exhaust gas.
6. The method according to claim 1, wherein the first economizer is
arranged in the feedwater line immediately downstream of the second
economizer.
7. The method according to claim 1, further comprising a step of
controlling the division ratio of the first exhaust gas stream and
the second exhaust gas stream.
8. The method according to claim 2, wherein, in the first operating
mode, the temperature of the exhaust gas decreases in the second
economizer by at least 30.degree. C.
9. The method according to claim 1, further comprising performing
the first operating mode alternatingly with a second operating
mode, in which the recycling portion is minimized, and wherein the
oxidant gas comprises a stream of air introduced into the gas
recycling line upstream of the gas-gas heat exchanger.
10. The method according to claim 9, wherein, in the second
operation mode, the temperature of the exhaust gas changes in the
second economizer by less than 10.degree. C.
11. A system for generating power by oxyfuel combustion, the system
comprising: a furnace for combusting carbonaceous fuel; an oxygen
channel for feeding substantially pure oxygen from an oxygen supply
into the furnace for combusting the fuel with the oxygen to produce
exhaust gas comprising mainly carbon dioxide and water; an exhaust
gas channel system connected to the furnace for discharging the
exhaust gas from the furnace, wherein the exhaust gas channel
system comprises an upstream channel, an outlet channel and a gas
recycling channel, wherein the upstream channel is connected by a
final divider piece to the gas recycling channel and the outlet
channel, for recycling a recycling portion of the exhaust gas
through the recycling channel to the furnace, and for conveying an
end portion of the exhaust gas through the outlet channel for final
processing, wherein the upstream channel is divided between a first
divider piece and a connecting piece into a first exhaust gas
channel portion and a second exhaust gas channel portion; a gas-gas
heat exchanger arranged in the first exhaust gas channel portion to
transfer heat from exhaust gas in the first exhaust gas channel
portion to gas in the gas recycling channel; a first economizer
arranged in the second exhaust gas channel portion to transfer heat
from exhaust gas in the second exhaust gas channel portion to a
flow of feedwater in a feedwater line; and a second economizer
arranged in the exhaust gas channel system downstream of the
connecting piece to transfer heat from gas in the exhaust gas
channel system to the flow of feedwater in the feedwater line.
12. The system according to claim 11, wherein the second economizer
is arranged in the upstream channel.
13. The system according to claim 11, further comprising an oxygen
heater arranged in the oxygen channel, connected to a gas cooler
arranged in the outlet channel for heating the substantially pure
oxygen by heat obtained from the end portion of the exhaust
gas.
14. The system according to claim 13, further comprising a mixer to
mix the substantially pure oxygen and the recycling portion as a
combined oxidant gas and a channel for feeding the combined oxidant
gas into the furnace.
15. The system according to claim 13, wherein the second economizer
is arranged in the gas recycling channel.
16. The system according to claim 11, wherein the first economizer
is arranged in the feedwater line immediately downstream of the
second economizer.
17. The system according to claim 1 1, further comprising a damper
in one of the first exhaust gas channel portion and the second
exhaust gas channel portion for controlling the division ratio of
the exhaust gas.
18. The system according to claim 11, further comprising: a damper
arranged in the gas recycling channel for controlling the recycling
portion; and an air intake arranged in the gas recycling channel
for introducing a stream of air as the oxidant gas, 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.
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 dual-firing or flexi-burn combustion
system, i.e., to 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 therefrom and,
possibly, the exhaust gas is purified in order to reduce inert
gases originating 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, in which 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 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 making any changes in the
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 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] A conventional boiler based on combusting carbonaceous fuel
by air usually comprises a set of heat transfer surfaces, such as
evaporators, superheaters, reheaters, economizers, and an
air-heater, arranged sequentially in the furnace and an exhaust gas
channel upstream of an electrostatic precipitator (ESP) or a fabric
filter. It is also known to arrange superheaters, reheaters and
economizers in parallel exhaust gas channels portions, or a low
pressure economizer parallel to an air heater.
[0008] U.S. Pat. No. 6,202,574 shows an oxyfuel combusting boiler
having, in the exhaust gas channel, downstream of superheaters,
reheaters and economizers, a further sequence set of the flue gas
coolers, comprising a recycled flue gas heater, a pure oxygen
heater and a feedwater heater. German patent publication DE 103 56
701 A1 shows an oxyfuel combustion boiler system comprising an
oxygen heater and a recycled flue gas heater arranged in series or
in parallel in the exhaust gas channel.
[0009] PCT patent publication WO 2006/131283 shows a dual firing
oxyfuel combustion boiler having, downstream of an air heater, a
series of heat exchangers, which are in an oxyfuel combustion mode
connected to the feedwater supply line so as to compensate for heat
energy, which is in an oxyfuel combustion mode used for the air
separation or CO.sub.2 liquefaction. This system is quite
complicated due to the valves and controllers required for
controlling the feedwater flow in the oxyfuel combustion mode.
[0010] 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
[0011] An object of the present invention is to provide a new
method of and a system for oxyfuel combustion.
[0012] According to an aspect of the present invention, a method of
generating power by oxyfuel combustion is provided, 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 an exhaust gas stream from the furnace,
dividing the exhaust gas stream in a final divider piece to a
recycling portion and an end portion, recycling the recycling
portion through a gas recycling channel to the furnace, and
conveying the end portion through an outlet channel to final
processing, wherein the method also comprises the steps of dividing
the exhaust gas stream in a first divider piece arranged upstream
of the final divider piece to a first exhaust gas stream and a
second exhaust gas stream, transferring heat from the first exhaust
gas stream to gas in the gas recycling channel by a gas-gas heat
exchanger to form a cooled first exhaust gas stream, transferring
heat from the second exhaust gas stream to a flow of feedwater in a
feedwater line by a first economizer to form a cooled second
exhaust gas stream, combining the cooled first exhaust gas stream
and the cooled second exhaust gas stream in a connecting piece
arranged upstream of the final divider piece to form a combined
exhaust gas stream, and conveying at least a portion of the
combined exhaust gas stream through a second economizer arranged to
transfer heat from the combined exhaust gas stream to the flow of
feedwater in the feedwater line.
[0013] 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 substantially pure oxygen from an oxygen supply
into the furnace for combusting the fuel with the oxygen to produce
exhaust gas, comprising mainly carbon dioxide and water, an exhaust
gas channel system connected to the furnace for discharging the
exhaust gas from the furnace, wherein the exhaust gas channel
system comprises an upstream channel, an outlet channel and a gas
recycling channel, wherein the upstream channel is connected by a
final divider piece to the gas recycling channel and the outlet
channel, for recycling a first portion, a so-called recycling
portion, of the exhaust gas through the recycling channel to the
furnace, and for conveying a second portion, a so-called end
portion, of the exhaust gas through the outlet channel for final
processing, wherein the system further comprises dividing the
upstream channel between a first divider piece and a connecting
piece into a first exhaust gas channel portion and a second exhaust
gas channel portion, a gas-gas heat exchanger arranged in the first
exhaust gas channel portion to transfer heat from exhaust gas in
the first exhaust gas channel portion to gas in the gas recycling
channel, a first economizer arranged in the second exhaust gas
channel portion to transfer heat from exhaust gas in the second
exhaust gas channel portion to a flow of feedwater in a feedwater
line, and a second economizer arranged in the exhaust gas channel
system downstream of the connecting piece to transfer heat from gas
in the exhaust gas channel system to the flow of feedwater in the
feedwater line.
[0014] The power generating system according to the present
invention preferably comprises an oxygen heater arranged to the
oxygen channel, which oxygen heater is advantageously connected to
a gas cooler arranged in the outlet channel, so as to heat the
substantially pure oxygen by heat obtained from the end portion of
the exhaust gas. This oxygen heating system may consist of a
gas-gas heat exchanger, in which heat is transferred directly from
the end portion of the exhaust gas to the stream of substantially
pure oxygen, but, advantageously, it is based on circulating a heat
transfer medium, typically water, by a pump in a circulating pipe
system between a separately-arranged gas cooler and an oxygen
heater. When using the present invention, the feeding rate of the
relatively pure oxygen is advantageously 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%.
[0015] The recycling portion of the exhaust gas 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 recycling portion of the exhaust 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 combined oxidant gas is formed to be fed via a
channel into the furnace. An advantage resulting from heating the
oxygen stream prior to mixing with recycling gas is the avoiding of
moisture or acid gas condensation from the recycling gas on an
O.sub.2 injector pipe, which may result if the temperature of the
O.sub.2 stream is too low. Generally, the mixing of a heated pure
oxygen stream with a heated recycling portion of the exhaust gas
makes it possible to efficiently control the temperature, flow rate
and oxygen content of the combined oxidant gas.
[0016] The gas recycling 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 a mixed gas, which may be fed to the furnace separately,
for example, as primary and secondary oxidant gas. 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.
[0017] When the present invention is used for an oxyfuel combusting
boiler retrofitted from an air-firing boiler, the flow rate of the
recycling portion of the exhaust gas is advantageously adjusted so
as to maintain the desired gas velocity in the furnace, in which
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 be maintained at about its original level to avoid,
e.g., corrosion or material strength problems of the furnace
walls.
[0018] Due to the high heat capacity of the exhaust gas generated
in the oxyfuel combustion 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 oxyfuel combustion than in air-firing combustion. Thus, when
changing an air-firing steam generation process to oxyfuel
combustion, the fuel feeding rate is advantageously increased by at
least 10%, whereby the original furnace temperature or heat flux
can still be maintained. As a result of the increased firing, an
increased amount of heat is available, e.g., for steam generation
and for heating of the oxidant gas.
[0019] In a conventional air-firing boiler, a large portion of
steam extracted from steam turbines is used for preheating
feedwater. In an oxyfuel combustion boiler, advantageously, at
least a portion of steam extracted from the steam turbines is used
for driving compressors in an air separation unit (ASU) or in a
carbon dioxide purification and compression unit (CCU), and,
correspondingly, an increased amount of the preheating of feedwater
is performed in economizers arranged in the exhaust gas channel.
Because of this arrangement, and due to increased steam generation
based on the increased firing mentioned above, there is in oxyfuel
combustion a need for an especially efficient system of
economizers.
[0020] The first economizer is advantageously arranged in the
feedwater line immediately downstream of the second economizer. By
such an arrangement, the first and second economizers are in a
direct feedwater flow connection, i.e., that the same stream of
feedwater always flows through both of the economizers, and there
are no branch pipes with control valves, for controlling the flow
of feedwater between the two economizers. In this way, the
economizers according to the present invention provide a simple
system which can be adjusted for optimal heating of the feedwater.
The adjustment is preferably performed by controlling a damper in
one of the first exhaust gas channel portion and the second exhaust
gas channel portion, so as to vary the division ratio of the
exhaust gas between the two exhaust gas channel portions.
[0021] According to a preferred embodiment of the present
invention, the second economizer is arranged in the upstream
channel, i.e., to the exhaust gas channel upstream of the final
divider piece. Usually, the upstream channel comprises a dust
separator, such as an electrostatic precipitator (ESP) or a fabric
filter. The second economizer is advantageously arranged upstream
of the dust separator, whereby the temperature of the exhaust gas
can be adjusted to be suitable for the operating range of the dust
separator.
[0022] Typically, a majority of the exhaust gas, for example, about
80%, flows through the first exhaust gas channel portion, and a
smaller portion, for example, about 20%, flows through the second
exhaust gas channel portion. Thus, when the first exhaust gas
stream is cooled in the gas-gas heat exchanger, for example, from
about 310.degree. C. to about 210.degree. C., and the second
exhaust gas stream is cooled in the first economizer, for example,
to about 170.degree. C., the combined exhaust gas will have
downstream of the connecting piece a temperature of about
200.degree. C. Thereby, the combined exhaust gas stream is
advantageously cooled in a second economizer arranged in the
upstream channel from about 200.degree. C., for example, to about
150.degree. C. This arrangement of the economizers provides the
possibility of simultaneously heating both the recycling gas, in
the gas-gas exchanger, and the feedwater, in the economizers, to
their optimal temperatures, without risk of acid condensation on
the economizers or the downstream dust separator.
[0023] According to another preferred embodiment of the present
invention, which is especially advantageous when the outlet channel
comprises an exhaust gas cooler connected to an oxygen heater, the
second economizer is arranged in the gas recycling channel instead
of the upstream channel. Thereby, the second economizer transfers
heat only from the recycling portion of the exhaust gas to the
feedwater. This arrangement provides the advantage that the
temperature of the exhaust gas remains relatively high, typically,
about 200.degree. C., when the exhaust gas enters the exhaust gas
cooler, and the oxygen stream can correspondingly be heated by the
oxygen heater to a relatively high temperature. Naturally, it is
also possible to have a split second economizer, a part of which is
located in the upstream channel and another part in the gas
recycling channel.
[0024] According to an especially advantageous embodiment of the
present invention, the system comprises an air intake arranged in
the gas recycling channel for introducing a stream of air as the
oxidant gas and a damper arranged in the gas recycling channel for
controlling the recycling portion. The purpose of the air intake is
to render possible a second operating mode, an air-firing mode,
which can be used alternatingly with the first operating mode. In
the second operating mode, the recycling portion is minimized and
air is used as the oxidant gas, instead of substantially pure
oxygen or a combined stream of oxygen and recycling portion of the
exhaust gas. The air intake is advantageously 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.
[0025] In the second operating mode, the combustion system is
decoupled from the oxygen supply, and the exhaust gas comprises
nitrogen, carbon dioxide and water, as its main components. Because
of the large portion of nitrogen in the exhaust gas, the system is
also decoupled from the carbon dioxide purification and compression
unit (CCU), 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 a system for and a method of dual-firing oxyfuel
combustion, which can be easily switched from oxyfuel combustion to
air-firing combustion, and back, without making any modifications
in the construction, even on-line, without stopping the power
generation during the change.
[0026] Because, in the second operating mode, the oxygen supply is
not in use and the carbon dioxide of exhaust gas is not purified
and sequestered, the auxiliary power consumption of these processes
is minimized, and the system provides a higher total efficiency
than in oxyfuel combustion, at the cost of releasing carbon dioxide
to the environment. The air-firing operating mode is advantageously
used when the demand from power is especially high, for example,
during the summer or the day-time. Alternatively, the air-firing
mode may be temporarily used, e.g., based on varying economic
conditions, or when the oxygen supply, carbon dioxide purification
and compression unit or carbon dioxide storage system, are, for
some reason, not available.
[0027] When using the first operation mode, the initial temperature
of the cold gas in the gas-gas heat exchanger, i.e., the
temperature of the recycling gas, is relatively high and, thus, the
exhaust gas cools in the gas-gas heat exchanger only by about
100.degree. C., typically, to about 200.degree. C. Therefore, the
exhaust gas carriers downstream of the gas-gas heat exchanger steal
a large amount of heat energy, a considerable portion of which is
advantageously utilized to heat the feedwater in the second
economizer. Thus, the second economizer is arranged so that the
exhaust gas cools therein, in the first operating mode, preferably,
by at least about 30.degree. C., even more preferably, by at least
about 40.degree. C. Typically, the flue gas cools in the second
economizer, in the first operating mode, from a temperature between
about 170.degree. C. and about 220.degree. C. to a temperature
between about 120.degree. C. and about 170.degree. C., i.e., so as
to stay above the acid gas dew point. In order to obtain the
desired exhaust gas temperature, the first and second economizers
are preferably LP economizers arranged upstream of a de-aerator.
When a low pressure aerator is used, the first and second
economizers can also be arranged upstream of the de-aerator.
[0028] In the second operating mode, the stream of recycling
exhaust gas is advantageously replaced by an about as large a
stream of air which, however, is at a much lower temperature than
is the recycling exhaust gas. Therefore, the exhaust gas cools,
then, in the gas-gas heat exchanger to a much lower temperature,
typically, to about 120.degree. C. In these conditions, the
temperature of the exhaust gas downstream of the gas-gas heat
exchanger is, typically, already close to that of feedwater
entering the second heat exchanger, and there is very little, if
any, heat transfer in the second economizer. Advantageously, the
temperature of the exhaust gas changes, in the second operating
mode, in the second economizer by less than about 10.degree. C.
[0029] As described above, in the oxyfuel combustion mode, a large
portion of steam extracted from steam turbines is used for driving
compressors in the ASU or the CCU. In the air-firing mode, when the
ASU and CCU are not in use, this steam is saved for preheating the
feedwater, and the need for feedwater preheating in the economizers
is greatly reduced. As described above, the present arrangement
decreases, in the air-firing mode, automatically, the heat transfer
duty of the economizers. The preheating of the feedwater is, in the
air-firing mode, also advantageously decreased by using the damper
in one of the first and second exhaust gas channel portions, so as
to decrease the share of exhaust gas flowing through the second
exhaust gas channel portion.
[0030] 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
[0031] FIG. 1 is a schematic diagram of an oxyfuel-combusting power
plant in accordance with a preferred embodiment of the present
invention.
[0032] FIG. 2 is a schematic diagram of an oxyfuel-combusting power
plant in accordance with another preferred embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0033] 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 system 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.
[0034] The exhaust gas channel system 20 comprises an upstream
channel portion 54, a recycling channel 28 and an outlet channel
58, whereby, the exhaust gas stream is divided in a final divider
piece 26 into a recycling portion, conveyed through the recycling
gas channel 28 back to the furnace 14, and an end portion, which is
conveyed through the outlet channel 58 for final processing.
[0035] The oxidant gas is preferably a mixture of substantially
pure oxygen, produced form an air stream 22 in the air separation
unit (ASU) 24, and at least a portion of the recycling portion of
the exhaust gas. Another portion of the recycling portion, not
shown in FIG. 1, may be conducted, for example, as sealing or
conveying gas for the boiler 12. The exhaust gas recycling channel
28 advantageously comprises means, such as a fan 30 and a damper
32, for controlling the exhaust gas recycling rate. 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 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.
[0036] As is conventional, the furnace 14 usually comprises
evaporation surfaces, not shown in FIG. 1, and the upstream channel
portion 54 of the exhaust gas channel system 20 further comprises
heat exchanger surfaces 34, for example, superheaters, reheaters
and HP economizers. For the sake of simplicity, FIG. 1 only shows
one such heat exchanger surface 34, but, in practice, the upstream
portion of the exhaust gas channel system usually comprises
multiple superheating, reheating and HP economizer surfaces for
recovering heat from the exhaust gas.
[0037] In the upstream portion 54 of the exhaust gas channel system
20 are also arranged, downstream of the steam generating heat
exchange surfaces 34, a gas-gas heat exchanger 36, for example, a
regenerative heat exchanger, for transferring heat from the exhaust
gas directly to the recycling portion of the exhaust gas, and a
first economizer 38, for transferring heat to feedwater flowing in
a feedwater line 40. According to the present invention, the
gas-gas heat exchanger 36 is advantageously arranged in a first
exhaust gas channel portion 42 and the first economizer 38 is
arranged in a second exhaust gas channel portion 44, which channel
portions are connected in parallel between an initial divider piece
46 and a connecting piece 48. One of the first exhaust gas channel
portion 42 and the second exhaust gas channel portion 44
advantageously comprises a damper 50 for adjusting the division of
the exhaust gas to the parallel channel portions.
[0038] Downstream of the connecting piece 48 is advantageously
connected to a second economizer 52, for transferring heat from the
combined stream of exhaust gas to the feedwater flowing in the
feedwater line 40. By using such a combination of economizers, it
is possible to simultaneously heat both the gas in the recycling
channel 28, by the gas-gas heat exchanger 36, and the feedwater, by
the economizers 38, 52, to their optimal temperatures without risk
of acid condensation on the economizers.
[0039] The upstream portion 54 of the exhaust gas channel system 20
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
56.
[0040] In accordance with the main object of oxyfuel combustion,
i.e., to recover carbon dioxide from the exhaust gas, the outlet
channel 58 is equipped with means, schematically represented by a
carbon dioxide processing unit 60, for cooling, cleaning and
compressing carbon dioxide. The unit 60 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 62, and other possible impurities, from the carbon dioxide.
A stream of carbon dioxide 64 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. FIG. 1 separately shows a
condensing gas cooler 66, located upstream of the carbon dioxide
processing unit 60, for initially removing water from the exhaust
gas.
[0041] In order to transfer energy from the end portion of the
exhaust gas to the stream of substantially pure oxygen, the outlet
channel 58 is preferably equipped with a gas cooler 68, which is
connected by a liquid heat transfer medium circulation to an oxygen
heater 70 arranged in an oxygen channel 72 downstream of the oxygen
supply 24. The heat transfer medium, usually water, is preferably
circulated by a pump 74 in tubing 76 extending between the gas
cooler 68 and the oxygen heater 70, which are usually, in practice,
located at distant portions of the power plant 10.
[0042] The oxygen channel 72 may be connected directly to the
furnace 14, but, according to a preferred embodiment of the present
invention, the oxygen channel 72 and the exhaust gas recycling
channel 28 are both connected to a mixer 78, 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.
[0043] According to a preferred embodiment of the present
invention, the system also comprises an air intake 80 for feeding
air to the furnace 14. The air stream is preferably introduced into
the gas recycling channel 28 upstream of the gas-gas heat exchanger
36, whereby it is possible to transfer heat directly from the
exhaust gas to the stream of air. The purpose of air intake 80 is
to enable switching from oxyfuel combustion to air-firing
combustion. Thus, when introducing air to the gas recycling channel
28, the oxygen supply is stopped and the recycling of exhaust gas
is minimized, preferably, totally stopped, by the damper 32. 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 82.
[0044] An air stream flowing, in the air-firing mode, in the gas
recycling channel 28 may advantageously already be preheated
upstream of the gas-gas heat exchanger 36 by a gas heater 86. The
gas heater 86, which is advantageously arranged in the gas
recycling channel 28 downstream of the fan 30, may be connected to
the gas cooler 68 by a side loop of the tubing 76, which is then,
in the air firing mode, connected to transfer heat obtained from
the end portion of the exhaust gas to the gas heater 86 instead of
the oxygen heater 70. The gas heater 86 may, alternatively, be a
conventional steam coil heater arranged in the gas recycling
channel 28, which is preferably used only in the air-firing
mode.
[0045] FIG. 2 shows a schematic diagram of a power plant 10' in
accordance with another preferred embodiment of the present
invention. Elements of the power plant 10' corresponding to similar
elements in power plant 10 shown in FIG. 1 are shown with the same
reference numbers as those in FIG. 1.
[0046] The power plant 10' differs from the power plant 10 shown in
FIG. 1 mainly in that the second economizer 52' is arranged in the
gas recycling channel 28 instead of the upstream portion 54 of the
exhaust gas channel system 20. Thus, the end portion of the exhaust
gas remains at a higher temperature, and the oxygen stream can be
heated to a higher temperature by the oxygen heater 84 than by the
heater 70 of the embodiment shown in FIG. 1. The oxygen heater 84
is, here, shown as a direct gas-gas heater, but it may
alternatively comprises an oxygen heating system based on
circulating a heat transfer medium between a separate exhaust gas
cooler and oxygen heater, as shown in FIG. 1. It is also possible
that oxygen is heated in two successive heaters, for example, first
in a heating system of the type shown in FIG. 1, and then, in a
direct gas-gas heat exchanger, as shown in FIG. 2.
[0047] According to a preferred embodiment of the present
invention, the system comprises an air intake 80, as in the system
shown in FIG. 1, for feeding, in an air-firing mode, air as the
oxidant to the furnace 14. As shown in FIG. 2, though, the air
stream may be preheated by a conventional steam coil heater 86'
arranged in the gas recycling channel 28. If, however, the system
comprises a separate exhaust gas cooler 68 connected to an oxygen
heater 70 in the oxygen channel 72, by using tubing for circulating
a fluid transfer medium, as shown in FIG. 1, the tubing may also
contain a side loop, to be used in the air-firing mode for heating
air by a gas heater arranged in the gas recycling channel 28.
[0048] 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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