U.S. patent application number 11/960958 was filed with the patent office on 2009-06-25 for method of controlling a process of generating power by oxyfuel combustion.
This patent application is currently assigned to Foster Wheeler Energy Corporation. Invention is credited to Zhen Fan, Horst Hack, Archibald Robertson, Andrew Seltzer.
Application Number | 20090158978 11/960958 |
Document ID | / |
Family ID | 40787095 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090158978 |
Kind Code |
A1 |
Seltzer; Andrew ; et
al. |
June 25, 2009 |
METHOD OF CONTROLLING A PROCESS OF GENERATING POWER BY OXYFUEL
COMBUSTION
Abstract
A method of controlling a process of generating power in a power
plant with a boiler by combusting carbonaceous fuel with
substantially pure oxygen. At full load conditions, the method
includes introducing a first carbonaceous fuel feed stream into a
furnace, introducing a first substantially pure oxygen feed stream
into the furnace for combusting the first carbonaceous fuel feed
stream with the oxygen, and recirculating a portion of the exhaust
gas discharged from the furnace at a first recirculation flow rate
to the furnace, to form, together with the first substantially pure
oxygen feed stream, a first inlet gas stream having a predetermined
average oxygen content, thereby discharging exhaust gas from the
furnace at a first discharge flow rate. In second load conditions,
corresponding to at most 90% load, the method includes introducing
a second carbonaceous fuel feed stream into the furnace,
introducing a second substantially pure oxygen feed stream into the
furnace for combusting the second carbonaceous fuel feed stream
with the oxygen, and recirculating a portion of the exhaust gas
discharged from the furnace at a second recirculation flow rate to
the furnace, to form, together with the second substantially pure
oxygen feed stream, a second inlet gas stream, so as to discharge
exhaust gas from the furnace at a second discharge flow rate, and
controlling the second recirculation flow rate to be from the first
recirculation flow rate to a value providing the second discharge
flow rate to be substantially as high as the first discharge flow
rate.
Inventors: |
Seltzer; Andrew;
(Livingston, NJ) ; Fan; Zhen; (Parsippany, NJ)
; Hack; Horst; (Hampton, NJ) ; Robertson;
Archibald; (Whitehouse Station, NJ) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
Foster Wheeler Energy
Corporation
Clinton
NJ
|
Family ID: |
40787095 |
Appl. No.: |
11/960958 |
Filed: |
December 20, 2007 |
Current U.S.
Class: |
110/345 |
Current CPC
Class: |
F23L 2900/07006
20130101; F23L 2900/07005 20130101; Y02E 20/344 20130101; F23L
2900/07007 20130101; F23C 9/003 20130101; Y02E 20/322 20130101;
F23C 2202/30 20130101; Y02E 20/32 20130101; Y02E 20/34 20130101;
F23C 2202/50 20130101; F23L 7/007 20130101 |
Class at
Publication: |
110/345 |
International
Class: |
F23C 9/00 20060101
F23C009/00 |
Claims
1. A method of controlling a process of generating power in a power
plant with a boiler by combusting carbonaceous fuel with
substantially pure oxygen, the method comprising, at full load
conditions, the steps of: (a1) introducing a first carbonaceous
fuel feed stream into a furnace; (b1) introducing a first
substantially pure oxygen feed stream into the furnace for
combusting the first carbonaceous fuel feed stream with the oxygen;
(c1) discharging exhaust gas via an exhaust gas channel from the
furnace; (d1) recovering heat from the exhaust gas by heat exchange
surfaces arranged in the exhaust gas channel; and (e1)
recirculating a portion of the exhaust gas via an exhaust gas
recirculating channel, at a first recirculation flow rate to the
furnace, to form, together with the first substantially pure oxygen
feed stream, a first inlet gas stream having a predetermined
average oxygen content, thereby discharging exhaust gas from the
furnace at a first discharge flow rate, and, in second load
conditions, corresponding to at most 90% of the full load, the
steps of: (a2) introducing a second carbonaceous fuel feed stream
into the furnace; (b2) introducing a second substantially pure
oxygen feed stream into the furnace for combusting the second
carbonaceous fuel feed stream with the oxygen; (c2) discharging
exhaust gas via the exhaust gas channel from the furnace; (d2)
recovering heat from the exhaust gas by the heat exchange surfaces
arranged in the exhaust gas channel; and (e2) recirculating a
portion of the exhaust gas via the exhaust gas recirculating
channel at a second recirculation flow rate to the furnace, to
form, together with the second substantially pure oxygen feed
stream, a second inlet gas stream, so as to discharge exhaust gas
from the furnace at a second discharge flow rate, and controlling
the second recirculation flow rate to be from the first
recirculation flow rate to a value providing the second discharge
flow rate to be substantially as high as the first discharge flow
rate.
2. The method according to claim 1, wherein the second load
conditions corresponds to at most 80% of the full load.
3. The method according to claim 2, wherein the second load
conditions correspond to at most 70% of the full load.
4. The method according to claim 3, wherein the average oxygen
content of the first inlet gas stream is, by volume, from about 20%
to about 25%, and the average oxygen content of the second inlet
gas stream is 0.70 to 0.78 times the average oxygen content of the
first inlet gas stream.
5. The method according to claim 3, wherein the average oxygen
content of the first inlet gas stream is, by volume, from about 20%
to about 25%, and the average oxygen content of the second inlet
gas stream is 0.72 to 0.75 times the average oxygen content of the
first inlet gas stream.
6. The method according to claim 3, wherein the average oxygen
content of the first inlet gas stream is, by volume, from about 40%
to about 60%, and the average oxygen content of the second inlet
gas stream is 0.73 to 0.82 times the average oxygen content of the
first inlet gas stream.
7. The method according to claim 3, wherein the average oxygen
content of the first inlet gas stream is, by volume, from about 40%
to about 60%, and the average oxygen content of the second inlet
gas stream is 0.77 to 0.81 times the average oxygen content of the
first inlet gas stream.
8. The method according to claim 1, further comprising a step of
measuring the discharge flow rate.
9. The method according to claim 1, further comprising a step of
measuring the recirculation flow rate.
10. The method according to claim 1, further comprising controlling
the second recirculation flow rate by a fan.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of controlling a
process of generating power by oxyfuel combustion. More
particularly, the present invention relates to controlling oxyfuel
combustion in different load conditions.
[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 at least 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, as when combusting the fuel with
air.
[0005] The feeding rate of oxygen into a combustion system is
regularly controlled together with the feeding rate of fuel, so
that almost complete combustion of the fuel is obtained. In
conventional air-firing at full load, typically, a relatively low
level, say 3%, of excess oxygen in the flue gas is sufficient to
keep the CO level of the flue gas sufficiently low, but at low
loads, a higher level of excess air is needed to maximize steam
superheating and to complete combustion. The increased excess air
at low loads leads to reduced boiler efficiency due to increased
thermal stack losses.
[0006] In conventional combustion with air, harmful effects caused
by too high of combustion temperatures, such as increased NO.sub.X
emissions or corrosion, or material strength problems of the
furnace walls, are often prevented by recirculating a portion of
the flue gas back to the furnace. Thus, the oxygen content of the
inlet gas is reduced from the about 21% of air to a lower value,
and the combustion temperature is thereby lowered.
[0007] One of the advantages of oxyfuel combustion is the
possibility to increase thermal efficiency of the process by using
high combustion temperatures. However, combustion with nearly pure
oxygen as the inlet gas would provide very high combustion
temperatures. Therefore, in order to avoid harmful effects of too
high of combustion temperatures, especially when repowering
air-fired boilers to oxyfuel combustion, flue gas recirculation is
advantageously used to lower the average oxygen content of the
inlet gas.
[0008] U.S. Pat. No. 6,935,251 discloses a method of combusting
fuel with an oxidant stream comprising an oxygen-rich gas stream
mixed with recirculated flue gas. According to this method, the
rate of the flue gas recirculation is adjusted, so that the
resulting mass flow rate of the flue gas is less than the
corresponding mass flow rate of the flue gas generated by using air
as the oxidant stream. By using such a reduced flue gas mass flow,
the size of the flue gas channel and the pollution control
equipment therein can be minimized. U.S. Pat. No. 6,418,865
suggests repowering an air-combustion boiler to oxycombustion by
adjusting the exhaust gas recirculation rate so as to maintain the
heat transfer at the original specification.
[0009] One of the requirements of a power generation process is its
applicability for use in different power demand conditions with
high efficiency. According to conventional practice, reduced steam
outputs are achieved by operating the boiler with reduced fuel and
air feeding rates. Japanese patent publication No. 2007-147162
discloses a combustion control method of an oxygen burning boiler,
wherein oxygen is supplied in an amount corresponding to the boiler
load, and the exhaust gas recirculation rate is controlled so as to
obtain the required absorption of heat for producing steam.
[0010] For oxyfuel combustion, especially when the flue gas mass
flow is less than that in combustion with air, the operation at low
loads may lead to a distorted flow pattern of the flue gas,
increasing the risk of operational problems, for example, due to
excessive fouling or dust accumulation in low-velocity regions of
the exhaust gas channel. Thus, there is a need for an improved
method of controlling oxyfuel combustion in different load
conditions.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is to provide a method of
controlling a process of generating power by oxyfuel combustion at
different load conditions.
[0012] According to an aspect of the present invention, a method of
controlling a process of generating power in a power plant with a
boiler by combusting carbonaceous fuel with substantially pure
oxygen is provided, the method comprising, at full load conditions,
the steps of (a1) introducing a first carbonaceous fuel feed stream
into a furnace, (b1) introducing a first substantially pure oxygen
feed stream into the furnace for combusting the first carbonaceous
fuel feed stream with the oxygen, (c1) discharging exhaust gas via
an exhaust gas channel from the furnace, (d1) recovering heat from
the exhaust gas by heat exchange surfaces arranged in the exhaust
gas channel, and (e1) recirculating a portion of the exhaust gas
via an exhaust gas recirculating channel at a first recirculation
flow rate to the furnace, to form, together with the first
substantially pure oxygen feed stream, a first inlet gas stream
having a predetermined average oxygen content, thereby discharging
exhaust gas from the furnace at a first discharge flow rate, and in
second load conditions, corresponding to 90% or less of the full
load, the steps of (a2) introducing a second carbonaceous fuel feed
stream into the furnace, (b2) introducing a second substantially
pure oxygen feed stream into the furnace for combusting the second
carbonaceous fuel feed stream with the oxygen, (c2) discharging
exhaust gas via the exhaust gas channel from the furnace, (d2)
recovering heat from the exhaust gas by the heat exchange surfaces
arranged in the exhaust gas channel, and (e2) recirculating a
portion of the exhaust gas via the exhaust gas recirculating
channel at a second recirculation flow rate to the furnace, to form
together with the second substantially pure oxygen feed stream a
second inlet gas stream, so as to discharge exhaust gas from the
furnace at a second discharge flow rate, and controlling the second
recirculation flow rate to be from the first recirculation flow
rate to a value providing the second discharge flow rate to be
substantially as high as the first discharge flow rate.
[0013] Any reference to a gas flow rate, throughout this
description, can be considered to mean a volume flow rate, unless
otherwise stated. By a "substantially pure oxygen feed stream" is
meant an oxygen-rich stream, usually, having a purity of at least
95%, from an oxygen supply, such as a cryogenic air separator. The
substantially pure oxygen feed stream is at all loads, as usual,
such that substantially all of the fuel feed stream will be
combusted with the oxygen, which means that the exhaust gas stream
comprises a small amount, for example, 3%, of residual oxygen. The
process also regularly comprises conventional measures for cleaning
the exhaust gas from impurities, such as sulfur dioxide. The
portion of the exhaust gas, which is not recirculated to the
furnace, may be discharged from the system by condensing water and
recovering carbon dioxide for sequestration or further use.
[0014] According to the present invention, the second fuel feeding
rate corresponds to reduced load conditions, i.e., 90% or less of
the full load. The second load conditions may preferably be 80% or
less of the full load, even more preferably, 70% or less of the
full load. According to the present method, the exhaust gas
recirculation rate is, at reduced load conditions, adjusted so that
the flow rate of gas discharged from the furnace remains at a
sufficiently high range. By having a high exhaust gas flow rate at
all load conditions, the designed flow pattern of the exhaust gas,
and the distribution of heat transfer in different heat transfer
surfaces of the boiler, can be maintained. In practice, the exhaust
gas flow rate may be fixed to a predetermined value or range, which
depends on the conditions in question. The selected value or range
is naturally such that it guarantees problem-free operation in all
load conditions. Thus, the exhaust gas flow is sufficient to
prevent, for example, unwanted excessive dust accumulation in
low-velocity regions.
[0015] According to a conventional method, where the exhaust gas
flow rate decreases at low load conditions, the distribution of
heat absorbed in different heat transfer surfaces can be distorted,
because of the varying relative amount of heat transferred with the
exhaust gas. Thereby, for example, the amount of superheating of
steam or the preheating of the feedwater in the exhaust gas channel
may, at low load conditions, become too low. According to the
present invention, the distribution of heat transfer in different
heat transfer surfaces can be maintained even at low load
conditions. Because the sufficient flow rate of gas discharged from
the furnace is, according to the present invention, achieved by
recirculating exhaust gas, not by discharging extra exhaust gas to
the environment, the problem of reduced thermal efficiency due to
thermal stack losses, is avoided.
[0016] According to the present invention, the flue gas
recirculation is increased at low load conditions, so as to at
least partially compensate for the decreased production of
combustion gas. This method of controlling the flue gas
recirculation clearly differs from the conventional method used in
combustion with air, in which flue gas recirculation is used to
avoid too high of temperatures in the furnace. At low load
conditions, when the temperature in the furnace already decreases
due to a reduced fuel feed rate, the need for conventional flue gas
recirculation is minimized.
[0017] The prevention of harmful decreasing of the exhaust gas flow
rate at low load conditions by increasing or at least maintaining
the recirculating gas flow rate is especially beneficial in oxyfuel
combustion, where the equipment for high exhaust gas recirculation
is usually readily available, and the gas flow to be compensated
for consists mainly of the decreased CO.sub.2 production. This is
in clear contrast with air-fired combustion, where exhaust gas
recirculation is normally low or missing, and the change of the
exhaust gas at low loads includes, in addition to a reduced flow of
carbon dioxide, also, as a larger component, a decreased flow of
nitrogen. Thus, the application of the present invention in
air-fired combustion would bring about high costs, due to the
arrangements needed for high additional exhaust gas recirculation
at low loads.
[0018] When using the present invention, the recirculated gas flow
rate may be increased at low loads by the same amount, in moles, as
the substantially pure oxygen feed stream is decreased. Thereby,
the volume flow rate of the exhaust gas remains constant.
Alternatively, the recirculated gas flow rate may be increased at
low loads by a smaller amount or, at least, the recirculated gas
flow rate shall advantageously be maintained at a constant level.
In all of these alternatives, the share of exhaust gas recirculated
to the furnace is, at low loads, increased from that at full load.
Thereby, the average oxygen content of the inlet gas is, at low
loads, decreased.
[0019] When retrofitting an air-fired boiler for oxyfuel
combustion, it is usually required to maintain, as much as
possible, of the original combustion system, and, therefore, it is
advantageous to at least partially keep the original furnace, flue
gas channel and heat transfer surfaces. Thus, in order to obtain an
average oxygen content of the inlet gas, which is close to that of
air, the oxyfuel combustion process of a retrofitted boiler
advantageously uses a high exhaust gas recirculation rate. Thereby,
the fuel can be combusted by almost maintaining the original
temperatures and gas flow rates. A similar construction is also
advantageously used in dual-firing boilers, i.e., in boilers, which
can be used both for combustion with air and for oxyfuel
combustion. To obtain average oxygen contents of the inlet gas of
20% to 25%, typically, exhaust gas recirculation rates of about 81%
to 75%, respectively, are required, the exact values depending on
the level of impurities and residual oxygen in the flue gas.
[0020] For oxycombustion boilers in which the designed oxygen
content of the inlet gas is relatively low, say 20% to 25%, it is
especially advantageous to increase the recirculated gas flow rate
at low loads, so that the flow rate of the exhaust gas remains
substantially constant. The reason for this is that, for such a low
oxygen boiler, even the maintaining of the exhaust gas flow rate
increases the recirculated gas flow rate only by a relatively small
amount. Alternatively, the exhaust gas flow rate can be allowed to
slightly decrease, which means that the flow rate of the
recirculated gas is increased even less.
[0021] For example, the maintaining of the exhaust gas flow rate,
when changing the load from 100% to 70% in a boiler having, at full
load, an inlet gas oxygen content of 25%, is obtained by increasing
the flow rate of the recirculated exhaust gas by about 10%. Because
the flow rate of the exhaust gas is constant, the share of exhaust
gas recirculated back to the furnace changes as the flow rate of
the recirculated gas, i.e., by about 10%, typically, from 75% to
82%. The oxygen content of the inlet gas is thereby decreased from
25% to about 18%, i.e., it is multiplied by 0.72. Correspondingly,
for a boiler with a 20% full load inlet gas oxygen content, the
exhaust gas flow rate can be maintained, when reducing the load to
70%, by increasing the recirculation gas flow rate by about 7%,
whereby the oxygen content of the inlet gas decreases from 20% to
about 15%, i.e., it is multiplied by 0.75.
[0022] According to a preferred embodiment of the present
invention, the exhaust gas recirculation of an oxycombustion boiler
having, at full load, an oxygen content of the inlet gas of 20% to
25%, is, at 70% load, increased, such that the average oxygen
content of the inlet gas stream is reduced to a value, which is
preferably 0.70 to 0.78 times, even more preferably, 0.72 to 0.75
times, the average oxygen content of the inlet gas stream at full
load.
[0023] A new boiler, which is especially designed for oxyfuel
combustion, is usually intended for combustion of fuel at a
relatively high temperature with an inlet gas having an average
oxygen content, which is clearly higher than that of air. The
furnace and flue gas channel of such a boiler are advantageously
clearly smaller in size than those of a corresponding air-fired
combustion system. A new oxyfuel combustion boiler thus differs
considerably from an oxyfuel combustion boiler, which is
retrofitted from an air-fired boiler or from a combustion system
intended for oxyfuel combustion and air-fired combustion, for
example, in the arrangement of heat transfer surfaces. The present
invention can, however, advantageously be used both in new and in
retrofitted oxyfuel combustion boilers.
[0024] The average full load oxygen content of the inlet gas of a
new oxyfuel combustion boiler may advantageously be, for example,
about 40% to about 60%. These oxygen contents are typically
obtained by recirculating about 58% to about 35%, respectively, of
the exhaust gas back to the furnace. Then, for example, at 70%
load, the flow rate of the exhaust gas can be maintained at its
original value by increasing the exhaust gas recirculation to a
value of about 71% to about 55%, respectively. These increased
recirculation rates result in the average oxygen content of the
inlet gas decreasing to a value of about 29% to about 43%,
respectively, i.e., the oxygen contents are multiplied by about
0.72. A drawback of this controlling procedure is that the flow
rate of the recirculated gas is thereby considerably increased at
low loads. For example, the maintaining of the exhaust gas flow
rate when changing the load from 100% to 70% in a boiler having, at
full load, an inlet gas oxygen content of 60% would require an
increase of about 55% of the recirculation gas flow rate at full
load. Readiness for such a high increase would mean having a
largely oversized recirculation channel and a correspondingly
oversized fan for the operation at low loads. In order to minimize
additional costs due to such devices, it is advantageous to use
somewhat lowered gas recirculation, at low loads, in high oxygen
boilers.
[0025] If, alternatively, for an oxycombustion boiler designed for
60% full load inlet gas oxygen content, the flow rate of the
recirculation gas is maintained at 70% load at the same value as at
full load, the flow rate of the exhaust gas is typically decreased
by almost 20%. This is obtained by increasing the share of the
exhaust gas recirculated back to the furnace from about 35% to
about 44%, whereby the oxygen content of the inlet gas is reduced
from 60% to about 52%, i.e., it is multiplied by 0.87. In this
case, the recirculation devices designed on the basis of the gas
streams at full load can be used, but the decreased flow rate of
the exhaust gas may cause problems.
[0026] In cases when such a decreased exhaust gas flow rate already
causes problems of dust accumulation or distorted heat transfer,
the recirculated gas flow rate is advantageously, at low loads,
controlled to be between the values of the examples described
above. For example, if the recirculated gas flow rate of an
oxycombustion boiler designed for 60% full load inlet gas oxygen
content is, at 70% load, increased by 20% of that at full load,
whereby the inlet gas oxygen content is decreased to about 48%,
i.e., it is multiplied by 0.80, the flow rate of the exhaust gas
decreases only by about 12% of that obtained at full load.
[0027] According to a preferred embodiment of the present
invention, the exhaust gas recirculation of an oxycombustion boiler
having, at full load, an oxygen content of the inlet gas of 40% to
60%, is, at 70% load, increased, such that the average oxygen
content of the inlet gas stream is reduced to a value, which is
preferably 0.73 to 0.82 times, even more preferably, 0.77 to 0.81
times, the average oxygen content of the inlet gas stream at full
load. It has been surprisingly found that these ranges provide cost
efficient and problem-free operation of the boiler.
[0028] The exhaust gas discharge rate is, in practice, adjusted at
full load so as to obtain a desired average oxygen of the inlet
gas. The process of controlling the exhaust gas flow, at low loads,
in accordance with the present invention, can be based on
controlling the exhaust gas recirculation rate directly on the
basis of the load, or on the basis of a measured fuel feeding rate.
Alternatively, the controlling of the exhaust gas recirculation can
be based on a measured flow rate of the exhaust gas discharged from
the furnace. The exhaust gas recirculating rate can be controlled
by adjusting the speed of a fan used for recirculating the exhaust
gas, either directly to a set value, based on a desired
recirculation rate, or by comparing a measured recirculated gas
flow rate with the desired recirculation rate.
[0029] The present invention can, advantageously, be used in
different types of power generating boilers, especially, in
circulating fluidized bed (CFB) boilers and pulverized coal (PC)
boilers. The fuel is advantageously solid fuel, especially, coal,
biofuel or refuse-derived fuel. The substantially pure oxygen is
typically obtained from an oxygen supply, such as a cryogenic or a
membrane-based air separation unit.
[0030] The substantially pure oxygen and the recirculated exhaust
gas may be fed to the furnace as separate streams, or they may be
mixed to form an inlet stream to be fed to the furnace. The inlet
gas may consist of a single gas flow, or it may consist of several
flows, such as fluidization gas and secondary gas, in a CFB boiler,
or primary, secondary and upper furnace inlet gas in a PC boiler.
It is also possible that gas streams with different compositions
are introduced into different portions of the furnace.
[0031] The brief description given above, 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
[0032] FIG. 1 is a schematic diagram of an oxyfuel-combusting power
plant suitable for application of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0033] FIG. 1 shows a schematic diagram of a boiler plant 10 with 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
introducing oxygen-containing inlet 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 inlet gas. The details
and type of some elements of the boiler 12, such as the fuel
feeding means 16 and inlet gas feeding means 18, naturally depend
on the type of the boiler. Details such as, for example, burners,
coal mills, means for separately feeding primary and secondary
inlet gas, are, however, not important for the purposes of the
present invention, and they are thus not shown in FIG. 1.
[0034] The oxygen-containing inlet 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 recirculated via an exhaust gas recirculating channel 26. The
exhaust gas recirculating channel 26 advantageously comprises
means, such as a fan 28, for controlling the exhaust gas
recirculating rate. The recirculating rate of the inlet gas is, in
retrofitted boilers, advantageously adjusted such that the average
02 content of the inlet gas is, at full load, close to that of air,
preferably from 20% to 25%. A new oxyfuel boiler may advantageously
be designed for a much higher oxygen content of the inlet gas, and
the recirculating rate of the exhaust gas is, correspondingly, much
lower. In some applications of the present invention, it is also
possible to introduce the streams of recirculated exhaust gas and
substantially pure oxygen separately into the furnace 14, for
example, in different portions of the furnace.
[0035] The walls of the furnace 14 are preferably formed as a
tube-wall construction, which forms evaporating heat transfer
surfaces 30, for converting preheated feedwater to steam. An
upstream portion of the exhaust gas channel 20 comprises a
superheating heat transfer surface 34 for recovering heat from the
exhaust gas to superheat the steam. For the sake of simplicity,
FIG. 1 shows only one superheating surface, but, in practice, the
upstream portion 32 of the exhaust gas channel usually comprises
multiple superheating and reheating surfaces.
[0036] The downstream portion of the flue gas channel 20
advantageously comprises one or more economizers 38, 42 for
preheating feedwater to be fed to the evaporating heat transfer
surfaces, and a gas-gas heater 40 for heating the inlet gas. The
exhaust gas channel 20 also usually comprises different units for
cleaning the exhaust gas from particles and gaseous pollutants, but
they are not shown in FIG. 1.
[0037] Downstream of the branch point for the exhaust gas
recirculating channel 26 is advantageously arranged means for
producing liquid carbon dioxide, typically, at a pressure of about
110 bar, so that it can be transported for further use or to be
stored to a suitable place. Therefore, FIG. 1 shows compressors 44,
46 for pressurizing the exhaust gas, and an economizer 48 arranged
between the compressors for inter-stage cooling. The carbon dioxide
liquefying system usually comprises, in practice, more than two
compression stages, usually, at least four stages, in order to
increase the efficiency of the system. In FIG. 1, the economizer 42
is shown as a condensing cooler, whereby water is removed from the
exhaust gas. The carbon dioxide sequestration system also usually
comprises means for completely drying all water from the exhaust
gas, and means for separating oxygen, and possible other
impurities, from the carbon dioxide, which are, however, not shown
in FIG. 1.
[0038] When generating power by the boiler 10, the load level,
i.e., the amount of fuel fed by the fuel feeding means 16 and the
amount of oxygen provided by the air separation unit 24 are
controlled by main control device 50, on the basis of the
prevailing power demand. Then, according to the present invention,
the exhaust gas recirculating rate is adjusted by the fan 28, so
that the flow of the exhaust gas remains at a predetermined range.
The speed of the fan 28 can be adjusted on the basis of, for
example, the prevailing load, or a measured fuel feeding rate. The
adjustment can, alternatively, be made on the basis of recirculated
gas flow and/or the exhaust gas flow, as measured by suitable means
54, 52.
[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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