U.S. patent application number 10/421783 was filed with the patent office on 2004-01-22 for warm-up device for catalytic reactor.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Abe, Mitsutaka.
Application Number | 20040011703 10/421783 |
Document ID | / |
Family ID | 29545685 |
Filed Date | 2004-01-22 |
United States Patent
Application |
20040011703 |
Kind Code |
A1 |
Abe, Mitsutaka |
January 22, 2004 |
Warm-up device for catalytic reactor
Abstract
A fuel cell power plant is provided with a reformer (18)
producing a reformate gas and a carbon monoxide removal device (1)
removing carbon monoxide from the reformate gas as a result of
catalytic reactions. When the carbon monoxide removal device (1) is
operating at a low temperature, a heat exchanger (10) supplies
gaseous heat transmitting medium to a heat exchange portion (5) of
the carbon monoxide removal device (1) and warms the catalyst using
heat exchange operations with the catalyst in the carbon monoxide
removal device (1). The pressure regulation valve (12) regulates
the pressure of the gaseous heat transmitting medium and maintains
the temperature of the catalyst between the catalyst activation
temperature and the predetermined upper limiting temperature of the
catalyst in order to employ latent heat resulting from the heat of
condensation of the steam for heating the catalyst.
Inventors: |
Abe, Mitsutaka;
(Yokosuka-shi, JP) |
Correspondence
Address: |
McDERMOTT, WILL & EMERY
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
NISSAN MOTOR CO., LTD.
|
Family ID: |
29545685 |
Appl. No.: |
10/421783 |
Filed: |
April 24, 2003 |
Current U.S.
Class: |
208/62 |
Current CPC
Class: |
H01M 8/0612 20130101;
H01M 8/0662 20130101; H01M 8/2457 20160201; H01M 8/04014 20130101;
Y02E 60/50 20130101; H01M 8/04302 20160201; H01M 8/241 20130101;
H01M 8/04225 20160201 |
Class at
Publication: |
208/62 |
International
Class: |
C10G 057/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 4, 2002 |
JP |
2002-162790 |
Claims
What is claimed is:
1. A warm-up device for a catalytic reactor performing a
predetermined chemical reaction mediated by a catalyst activated at
a predetermined activation temperature, the warm-up device
comprising: a vapor supply mechanism supplying gaseous heat
transmitting medium; a heat exchange portion performing heat
exchange operations between the catalyst and the gaseous heat
transmitting medium; a pressure control valve regulating a pressure
of the gaseous heat transmitting medium in the heat exchange
portion; and a passage recirculating the heat transmitting medium
condensed in the heat exchange portion.
2. The warm-up device as defined in claim 1, wherein the
recirculation passage is provided with a tank storing the heat
transmitting medium.
3. The warm-up device as defined in claim 1, wherein the heat
transmitting medium comprises an aqueous solution of ethylene
glycol.
4. The warm-up device as defined in claim 1, wherein the vapor
supply mechanism has a function of supplying the heat transmitting
medium in a liquid state to the heat exchange portion 5 so as to
absorb a heat of the catalyst in the heat exchange portion, and the
warm-up device further comprises a radiator which radiates heat
from the heat transmitting medium.
5. The warm-up device as defined in claim 1, wherein the catalytic
reactor comprises a carbon monoxide removal device adapted to be
used in a fuel cell power plant for producing a hydrogen-rich gas
by removing carbon monoxide from a reformate gas, the fuel cell
power plant comprising a fuel cell stack which generates electric
power using the hydrogen-rich gas, and a reformer producing the
reformate gas by reforming a hydrocarbon fuel.
6. The warm-up device as defined in claim 5, wherein the reformer
comprises a device producing the reformate gas by steam reforming
operations on the hydrocarbon fuel, the heat transmitting medium
comprises water, the vapor supply mechanism has a function of
supplying water in a liquid state to the heat exchange portion, and
the warm-up device further comprises a valve supplying the reformer
with steam produced by evaporation of the water in the heat
exchange portion by a heat of the catalyst.
7. The warm-up device as defined in claim 6, wherein the vapor
supply mechanism comprises a pre-heater preheating water, and the
warm-up device further comprises a sensor detecting a power
generation load on the fuel cell stack, and a controller
functioning to calculate a heat amount required for generating a
required amount of steam which is to be supplied to the reformer
based on the power generation load, calculate a reaction heat
amount produced in the carbon monoxide removal device when the
required amount of steam is supplied to the reformer and a
corresponding amount of the reformate gas is supplied to the carbon
monoxide removal device, and control the pre-heater according to a
difference between the required heat amount and the produced heat
amount.
8. The warm-up device as defined in claim 1, wherein the vapor
supply mechanism comprises a heater heating the heat transmitting
medium and a pump supplying the heat transmitting medium to the
heater.
9. The warm-up device as defined in claim 1, wherein the warm-up
device further comprises a sensor detecting a temperature of the
catalyst and a controller functioning to reduce an opening of the
pressure control valve when the temperature of the catalyst is
lower than the activation temperature in order to maintain a
condensation temperature of the heat transmitting medium to a value
between the catalyst activation temperature and a predetermined
upper limiting temperature for the catalyst.
10. The warm-up device as defined in claim 9, wherein the warm-up
device further comprises a sensor detecting an oxygen concentration
of an atmosphere around the catalyst and the controller further
functions to perform a determination as to whether the atmosphere
around the catalyst is an oxidizing environment or a reducing
environment based on the oxygen concentration, and to set the upper
limiting temperature for the catalyst to different values according
to a result of the determination.
Description
FIELD OF THE INVENTION
[0001] This invention relates to warming up of a catalytic reactor
such as a carbon monoxide removal device provided in a fuel cell
power plant.
BACKGROUND OF THE INVENTION
[0002] A fuel cell power plant using reformate gas is provided with
a reformer which generates the reformate gas from a hydrocarbon
fuel. However carbon monoxide (CO) is contained in the reformate
gas. Since CO causes deterioration of the fuel cell, CO contained
in the reformate gas must be removed before the reformate gas is
supplied to the fuel cell stack. Preferential oxidation is known as
a means of removing CO from the reformate gas. The preferential
oxidation is a reaction under the presence of a catalyst.
[0003] In order to promote preferential oxidation, the temperature
of the catalyst has to be maintained at a predetermined activation
range. When the power plant is started under cold conditions, it is
therefore necessary to perform warming up of the catalyst until the
temperature of the catalyst rises to the activation temperature
range.
[0004] When the fuel cell power plant is used as a source of
driving power for a vehicle, it is preferred that the required
warm-up period is shortened. JP2000-028590 published by the
Japanese Patent Office in 2000 discloses a method of combusting a
gaseous mixture of air and fuel to generate high-temperature
combustion gas and heating the catalyst from the outside of the CO
removal device using the combustion gas.
SUMMARY OF THE INVENTION
[0005] In this prior-art technique, it is necessary to increase the
temperature of the combustion gas in order to shorten the time
period required for warming up of the catalyst, by applying a large
amount of heat to the catalyst. However when the temperature of the
combustion gas increases, the problem arises that a part of the
catalyst in proximity to the combustion gas starts to deteriorate
before the temperature of the whole catalyst increases. On the
other hand, when the temperature of the combustion gas is reduced,
the amount of heat applied to the catalyst by the combustion gas
decreases and as a result the time period required for warming up
of the power plant lengthens.
[0006] It is therefore an object of this invention to advance the
activation of the catalyst of a catalytic reactor while preventing
deterioration of the catalyst.
[0007] In order to achieve the above object, this invention
provides a warm-up device for a catalytic reactor performing a
predetermined chemical reaction mediated by a catalyst activated at
a predetermined activation temperature. The warm-up device
comprises a vapor supply mechanism supplying gaseous heat
transmitting medium, a heat exchange portion performing heat
exchange operations between the catalyst and the gaseous heat
transmitting medium, a pressure control valve regulating a pressure
of the gaseous heat transmitting medium in the heat exchange
portion, and a passage recirculating the heat transmitting medium
condensed in the heat exchange portion.
[0008] The details as well as other features and advantages of this
invention are set forth in the remainder of the specification and
are shown in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic diagram of a fuel cell power plant
according to this invention.
[0010] FIG. 2 is similar to FIG. 1, but showing a situation where
the power plant is warmed up.
[0011] FIG. 3 is a flowchart showing a start-up control routine
executed by a controller according to this invention.
[0012] FIG. 4 is a schematic diagram of a fuel cell power plant
according to a second embodiment of this invention.
[0013] FIG. 5 is similar to FIG. 4, but showing a situation where
the power plant is warmed up.
[0014] FIG. 6 is a flowchart showing a start-up control routine
executed by a controller according to the second embodiment of this
invention.
[0015] FIG. 7 is a schematic diagram of a fuel cell power plant
according to a third embodiment of this invention.
[0016] FIG. 8 is a flowchart showing a pre-heat control routine
executed by a controller according to the third embodiment of this
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] Referring to FIG. 1 of the drawings, a fuel cell power plant
for driving a vehicle is provided with a fuel cell stack 2
comprising a plurality of polymer electrolyte fuel cells
(PEFC).
[0018] The fuel cell stack 2 is provided with an anode 2A and a
cathode 2B. A hydrogen-rich gas is supplied to the anode 2A through
a three-way valve 14 from a carbon monoxide (CO) removal device 1.
Air is supplied to the cathode 2B from an air supply pipe 24
through a valve. The three-way valve 14 has the function to switch
over between a section supplying hydrogen-rich gas from the CO
removal device 1 to the anode 2A of the fuel cell stack 2 and a
section supplying the hydrogen-rich gas to a combustor 3 through a
bypass passage 26.
[0019] The fuel cell stack 2 generates electric power as a result
of electrochemical reactions between hydrogen (H2) in the
hydrogen-rich gas supplied to the anode 2A and oxygen (O2) in air
supplied to the cathode 2B. Power generation operations result in
the discharge of anode effluent from the anode 2A to an
anode-effluent passage 27 and cathode effluent from the cathode 2B
to a cathode effluent passage 28. A certain amount of hydrogen is
remains in the anode effluent and a certain amount of oxygen
remains in the cathode effluent. The respective discharges are
supplied to the combustor 3 through the anode-effluent passage 27
and the cathode-effluent passage 28. High-temperature combustion
gas is produced as a result of combustion of the discharges in the
combustor 3. The bypass passage 26 is joined to the anode-effluent
passage 27.
[0020] High-temperature combustion gas produced in the combustor 3
is supplied to the heat exchanger 10 through a three-way valve 15.
The three-way valve 15 has the function to switch over a section
supplies the high-temperature combustion gas to the heat exchanger
10 and a section which discharges the high-temperature combustion
gas into the atmosphere.
[0021] The heat exchanger 10 heats and vaporizes a heat
transmitting medium using heat exchange operations between the
high-temperature combustion gas and the heat transmitting medium
which is supplied from a pump 7.
[0022] An aqueous solution of ethylene glycol may be used as the
heat transmitting medium.
[0023] The gaseous heat transmitting medium produced in the heat
exchanger 10 is transferred to a heat exchange portion 5 in the CO
removal device 1 through a vapor supply passage 30 and performs
heat exchange operations with the preferential oxidation catalyst
in the CO removal device 1. Thereafter the heat transmitting medium
is condensed in a pressure regulating valve 12 and recirculated to
a tank 9 through a radiator 8. The pressure regulating valve 12
regulates the vapor pressure of the heat transmitting medium by
adjusting the cross-sectional area of the passage for the heat
transmitting medium. Consequently it is possible to maintain the
gaseous heat transmitting medium in the heat exchange portion 5 to
a preferred temperature range. At the same time, a part of the
gaseous heat transmitting medium is condensed and re-enters liquid
phase.
[0024] A thermostat 6 is provided in the outlet of the tank 9. A
bypass passage 25 is connected to the thermostat 6. The bypass
passage 25 introduces liquid heat transmitting medium discharged
from the pressure regulating valve 12 directly into the pump 7 by
bypassing the tank 9 and the radiator 8. The thermostat 6 controls
the temperature of the heat transmitting medium supplied to the
pump 7 to a predetermined temperature as a result of selectively
connecting the bypass passage 25 or the tank 9 to a suction port of
the pump 7. The predetermined temperature of the thermostat 6 is 80
degrees centigrade.
[0025] Methanol as a hydrocarbon fuel is supplied together with
steam and air from the air supply pipe 24 to a reformer 18. The
reformer 18 produces reformate gas by performing known autothermal
reforming (ATR) operations comprising a combination of steam
reforming and partial oxidation reforming. These operations are
performed with respect to the gaseous mixture of methanol, air and
steam. The reformate gas comprises a concentration of 1.5% of
carbon monoxide.
[0026] The reformate gas is transferred to the CO removal device 1
through a reformate gas supply pipe 4. Air from the air supply pipe
25 is also supplied to the reformate gas supply pipe 4 through a
flow rate control valve 21. A catalyst supporting
platinum--ruthenium on aluminum, abbreviated as Pt--Ru/Al2O3, is
stored as a preferential oxidation catalyst in the CO removal
device 1. Carbon monoxide is removed from the reformate gas using
the reaction below with respect to the gaseous mixture of air and
reformate gas flowing into the reformate gas supply pipe 4.
2CO+O.sub.2.fwdarw.2CO.sub.2
[0027] The CO removal device performing promoting this reaction is
generally termed a preferential oxidation reactor (PROX
reactor).
[0028] The concentration of carbon monoxide is reduced by the CO
removal device 1 to a level of 40 parts per million (ppm) which
does not cause poisoning the fuel cell. The reformate gas after
removal of the carbon monoxide is termed a hydrogen-rich gas in the
following description. The hydrogen-rich gas is supplied to the
anode 2A of the fuel cell stack 2 through the three-way valve
14.
[0029] The fuel cell power plant is provided with a controller 50
for controlling the switching operation of the three-way valves 14,
15, the openings of the flow rate control valve 21 and the pressure
regulating valve 12 and the operation of the pump 7. The controller
50 comprises a microcomputer provided with a central processing
unit (CPU), a read-only memory (ROM), a random access memory (RAM)
and an input/output interface (I/O interface). The controller 21
may comprise a plurality of microcomputers.
[0030] In order to perform this control, the fuel cell power plant
is provided with a level sensor 11 detecting the surface level of
the aqueous solution of ethylene glycol in the heat exchanger 10, a
temperature sensor 13 detecting the catalyst temperature in the CO
removal device 1, an oxygen sensor 16 detecting the concentration
of oxygen in the reformate gas and a pressure sensor 17 detecting
the vapor pressure of the heat transmitting medium in the CO
removal device 1. The data detected by these sensors are input as
signals to the controller 50.
[0031] The temperature of the catalyst must be restricted to a
predetermined temperature range in order to ensure suitable
operation of the CO removal device 1. The lower limit for the
temperature range is the catalyst activation temperature of 120
degrees centigrade. The upper limit for the temperature range is a
temperature of 200 degrees centigrade representing the temperature
limit at which reverse shift reactions do not occur in the CO
removal device 1. A reverse shift reaction is described below.
CO.sub.2+H.sub.2.fwdarw.CO+H.sub.2O
[0032] Since it is preferred to ensure rapid increases in the
catalyst temperature when performing start-up of the fuel cell
power plant, the upper limit of the temperature range is set to an
upper limiting temperature at which the catalyst does not
deteriorate. The upper limiting temperature in a reducing gaseous
environment is set to 350 degrees centigrade at which the catalyst
begins to undergo sintering. The upper limiting temperature in an
oxidizing gaseous environment is set to 120 degrees centigrade at
which the catalyst begins to undergo oxidation deterioration.
[0033] During normal operation, the reformate gas entering the CO
removal device 1 comprises a reducing gaseous environment
containing large amount of hydrogen. During start-up of the power
plant, either reducing or oxidizing gaseous environments are
possible. During steady-state operation of the fuel cell power
plant, exothermic preferential oxidation reactions are performed in
the CO removal device 1. Thus during steady-state operation of the
fuel cell power plant, the heat exchange portion 5 has the function
of reducing the high temperature of the catalyst in the CO removal
device 1.
[0034] In the steady-state operation of the power plant as shown in
FIG. 1, the three-way valve 14 is maintained to the section in
which hydrogen-rich gas is supplied to the anode 2A of the fuel
cell stack 2 and the three-way valve 15 is maintained to the
section discharging the combustion gas into the atmosphere. The
three-way valves 14 and 15 in the figures close the connecting
passage at the black triangle and open the connecting passage at
the white triangle. The connecting passages connected to the two
white triangles are connected to one another.
[0035] In the steady state operation of the power plant, heating of
the heat transmitting medium is not performed by the heat exchanger
10 and the heat transmitting medium absorbs excess heat from the
catalyst at the heat exchange portion 5 in the CO removal device 1
and radiates it in the radiator 8. The thermostat 6 regulates the
temperature of the heat transmitting medium at the suction port of
the pump 7 at the constant temperature of 80 degrees
centigrade.
[0036] Next referring to FIG. 2, start-up operations of the fuel
cell power plant under cold conditions will be described. At this
time, the three-way valve 14 is maintained at the section
connecting the bypass passage 26 to the CO removal device 1 and the
three-way valve 15 is maintained to the section supplying
combustion gas from the combustor 3 to the heat exchanger 10.
[0037] As a result, since the reformer 18 and the CO removal device
1 are not activated during start-up, a gaseous mixture of
unreformed fuel is directly supplied from the reformer 18 to the
combustor 3 via the CO removal device 1 and the bypass passage
26.
[0038] Furthermore since the power generation operations have not
been commenced by the fuel cell stack 2, air in the air supply pipe
24 is supplied without modification from the cathode-effluent
passage 28 to the combustor 3. High-temperature combustion gas is
produced in the combustor 3 as a result of combustion of this
gaseous mixture. The combustion gas is supplied to the heat
exchanger 10 via the three-way valve 15 and vaporizes the heat
transmitting medium in the heat exchanger 10.
[0039] The gaseous heat transmitting medium heats the preferential
oxidation catalyst in the heat exchange portion 5 of the CO removal
device 1. Thereafter in response to temperature conditions, it
condenses in the pressure regulating valve 12 and is recycled to
the suction port of the pump or to the tank 9 through the radiator
8. The pressure regulating valve 12 regulates the condensation
temperature and the vaporization temperature in the heat exchange
portion 5 as described above.
[0040] Referring to FIG. 3, a start-up control routine performed by
the controller 50 when starting up the fuel cell power plant will
be described. This routine is performed only once when start-up of
the fuel cell power plant is commanded.
[0041] Firstly in a step S1, the controller 50 reads a temperature
TCO of the CO removal device 1 detected by the temperature sensor
13.
[0042] Then in a step S2, it is determined whether or not the
temperature TCO of the CO removal device 1 has reached a
temperature of 120 degrees centigrade which is the catalyst
activation temperature described above.
[0043] When the temperature TCO of the CO removal device 1 has
reached a temperature of 120 degrees C., the controller 50
immediately terminates the routine.
[0044] When the temperature TCO of the CO removal device 1 is less
than a temperature of 120 degrees centigrade, in a step S3, the
controller 50 operates the three-way valve 14 to connect the CO
removal device 1 to the combustor 3 through the bypass passage 26
so as to supply an unreformed gaseous mixture from the CO removal
device 1 to the combustor 3. Further, the controller 50 operates
the three-way valve 15 to supply the combustion gas from the
combustor 3 to the heat exchanger 10 in order to heat the heat
transmitting medium in the heat exchanger 10.
[0045] Then in a step S4, the controller 50 reads the liquid level
H of the heat transmitting medium in the heat exchanger 10 detected
by the level sensor 11.
[0046] In a step S5, the controller 50 determines whether or not
the level H of the heat transmitting medium in the heat exchanger
10 has reached a predetermined level. The predetermined level
represents a value at which it is determined whether or not a
sufficient amount of heat transmitting medium is stored in the heat
exchanger 10 in order to vaporize the heat transmitting medium.
[0047] When the level H of heat transmitting medium is less than
the predetermined level, the controller 50 operates the pump 7 in a
step S6 and transfers heat transmitting medium in the tank 9 to the
heat exchanger 10 so that the level H reaches the predetermined
level.
[0048] After the process in the step S6 is completed, the
controller 50 performs the process in a step S7. In the step S5,
when the level H of heat transmitting medium has reached the
predetermined level, the controller 50 skips the step S6 and
performs the process in the step S7.
[0049] In the step S7, the controller 50 reads the oxygen
concentration of reformate gas detected by the oxygen sensor 16.
The oxygen concentration in the unreformed gaseous mixture is
detected immediately after start-up when the reformer 18 is not
operating.
[0050] Then in a step S8, the controller 50 uses the oxygen
concentration in order to determine whether or not the gaseous
environment of the catalyst in the CO removal device 1 is reducing
or oxidizing. When the gaseous environment is oxidizing, the target
temperature of the heat exchange portion 5 is set to a temperature
of 110 degrees centigrade which is lower than the upper limiting
temperature for an oxidizing gaseous environment.
[0051] A target pressure of gaseous heat transmitting medium in the
heat exchange portion 5 is set in order to achieve the target
temperature of 110 degrees centigrade. When an aqueous solution of
ethylene glycol is used as the heat transmitting medium, the target
pressure corresponding to the target temperature of 110 degrees
centigrade is atmospheric pressure plus 0.01 megapascals.
[0052] Herein, the target temperature of 110 degrees centigrade for
the oxidizing gaseous environment is less than the catalyst
activation temperature of 120 degrees centigrade, but once the
reformer 18 has started reforming reaction, the resultant reformate
gas has a higher temperature than the target temperature and
rapidly raises the catalyst temperature in the CO removal device 1
to the activation temperature. The reformate gas also provides a
reducing gaseous environment in the CO removal device 1.
[0053] When the catalyst environment is reducing, the target
temperature of the heat exchange portion 5 is set to 130 degrees
centigrade which is higher than the target temperature in oxidizing
environment. The target pressure for the vaporized heat
transmitting fluid in the heat exchange portion 5 is set to achieve
the target temperature of 130 degree centigrade. When using an
aqueous solution of ethylene glycol as the heat transmitting fluid,
the target pressure corresponding to the temperature of 130 degrees
centigrade is atmospheric pressure plus 0.1 megapascals.
[0054] The target temperature for the reducing gaseous environment
is set so that the condensation temperature of the heat
transmitting medium is positioned between the upper limiting
temperature at which catalyst deterioration does not occur and the
catalyst activation temperature.
[0055] Then in a step S9, the controller 50 reads the pressure of
the gaseous heat transmitting medium in the heat exchange portion 5
detected by the pressure sensor 17.
[0056] In a step S10, the controller 50 feedback controls the
opening of the pressure regulating valve 12 in response to the
deviation of the detected pressure from the target pressure. Since
the pressure in the heat exchange portion 5 is maintained to the
target pressure, the process in the step S10 allows the gaseous
heat transmitting medium supplied from the steam supply passage 30
to the heat exchange portion 5 to be condensed in the heat exchange
portion 5. Thus large amounts of condensation heat are generated
and effectively heat the catalyst. However the temperature of the
heat exchange portion 5 does not increase to a high temperature
resulting in catalyst deterioration.
[0057] After the process in the step S10, the controller 50 repeats
the process from the step S1. When the temperature TCO of the CO
removal device 1 reaches a temperature of 120 degrees centigrade,
the routine is terminated.
[0058] After the temperature TCO of the CO removal device 1 has
reached a temperature of 120 degrees centigrade, the controller 50
executes steady-state operation as shown in FIG. 1. That is to say,
the three-way valve 15 is switched so that the combustion gas from
the combustor 3 is discharged into the atmosphere. The pressure
regulating valve 12 is placed in a fully open position and the
discharge amount of the pump 7 is controlled to a predetermined
amount adapted for steady-state operation. The flow rate control
valve 21 is opened and air is supplied to enable preferential
oxidation reaction in the CO removal device 1. The three-way valve
14 is switched so that hydrogen rich gas is supplied from the CO
removal device 1 to the anode 2A.
[0059] During steady-state operation, as described above, the heat
transmitting medium which is maintained to a temperature of 80
degrees centigrade by the thermostat 6 absorbs the excess heat from
the catalyst in the heat exchange portion 5 of the CO removal
device 1. This heat is radiated from the radiator 8. In this
manner, it is possible to heat and cool the catalyst in the CO
removal device 1 using the same heat transmitting medium and the
same heat exchange portion 5.
[0060] In this fuel cell power plant, since the heat of combustion
of the anode effluent combusted in the combustor 3 is used as the
source of heat for heating the heat transmitting medium, a
preferred energy-efficient solution is obtained.
[0061] Referring to FIGS. 4-6, a second embodiment of this
invention will be described.
[0062] Referring to FIG. 4, a fuel cell power plant according to
this embodiment differs from the power plant according to the first
embodiment in that pure water is used as the heat transmitting
medium instead of the aqueous solution of ethylene glycol, and the
supply of steam to the reformer 18 is also performed using the
water in the tank 9.
[0063] For this purpose, a three-way valve 19 is provided
downstream of the pressure regulation valve 12. The three-way valve
19 selectively leads the flow of water or steam from the pressure
regulation valve 12 to the radiator 8 or the reformer 18 in
response to the signals from the controller 50. The bypass passage
25 and the thermostat 6 in the first embodiment are not provided in
this embodiment.
[0064] As shown in FIG. 5, during start-up of the power plant under
cold conditions, the controller 50 heat the water from the tank 9
in the heat exchanger 10 and supplies the resulting steam to the
heat exchange portion 5 of the CO removal device 1. The controller
50 controls the opening of the pressure regulation valve 12 so that
the condensation temperature for the steam coincides with the
target temperature set in proximity to the catalyst activation
temperature. The condensed water is recycled from the pressure
regulation valve 12 through the radiator 8 and the three-way valve
19 to the tank 9.
[0065] When the reformer 18 reaches a state in which reforming
reactions are enabled, the controller 50 places the pressure
regulation valve 12 in a fully open position, switches the
three-way valve 19 and supplies steam discharged from the pressure
regulation valve 12 to the reformer 18. Thereafter the reformer 18
produces reformate gas by reforming the gaseous fuel making use of
air supplied from the air supply pipe 24 and steam discharged from
the heat exchange portion 5 in the CO removal device 1.
[0066] During steady-state operations as shown in FIG. 1, the water
level in the heat exchange portion 5 is detected using the level
sensor 20. The controller 50 controls the discharge amount of the
pump 7 so that the water level is maintained to a target water
level. The switching of the three-way valves 14 and 15 is the same
as that described with reference to the first embodiment.
[0067] Referring to FIG. 6, a start-up control routine executed by
the controller 50 during start-up of the fuel cell power plant will
be described. This routine is a replacement of the start-up control
routine of FIG. 3 of the first embodiment.
[0068] The process in the step S1 and S2 is the same as that of the
routine of FIG. 3 according to the first embodiment.
[0069] In a step S21, the controller 50 switches the three-way
valves 14 and 15 in the same manner as the step S3 of the first
embodiment, but in addition, the three-way valve 19 is switched so
that the water flowing out of the pressure regulating valve 12
flows into the radiator 18.
[0070] Then in a step S22, the controller 50 detects the water
level H in the heat exchanger 10 detected by the level sensor
11.
[0071] In a step S23, the controller 50 determines whether or not
the level H has reached a predetermined level. The predetermined
level represents a value at which it is determined whether or not a
sufficient amount of water is stored in the heat exchanger 10 in
order to produce steam.
[0072] When the water level H is less than the predetermined level,
the controller 50 operates the pump 7 in a step S24 and supplies
water in the tank 9 to the heat exchanger 10 so that the water
level H reaches the predetermined level.
[0073] After the process in the step S24, the controller 50
performs the process in the step S7.
[0074] In the step S23, when the water level H has reached the
predetermined level, the controller 50 skips the step S24 and
performs the process of the step S7.
[0075] In the step S7, the controller 50 reads the oxygen
concentration of reformate gas detected by the oxygen sensor 16 in
the same manner as the first embodiment.
[0076] Then in a step S25, the controller 50 uses the oxygen
concentration in order to determine whether or not the gaseous
environment of the catalyst in the CO removal device 1 is a
reducing environment or an oxidizing environment. When the gaseous
environment is oxidizing, the target temperature of the heat
exchange portion 5 is set to a temperature of 110 degrees
centigrade which is lower than the upper limiting temperature for
an oxidizing gaseous environment. The pressure of steam in the heat
exchange portion 5 is set in order to reach the target temperature
of 110 degrees centigrade. The target pressure corresponding to
this target temperature when water is used as the heat transmitting
medium is atmospheric pressure plus 0.04 megapascals.
[0077] When the gaseous environment of the catalyst is reducing,
the target temperature of the heat exchange portion 5 is set to 130
degrees centigrade which is higher than that for an oxidizing
atmosphere. The steam pressure of the heat exchange portion 5 is
then regulated to achieve this target temperature. The target
pressure corresponding to this target temperature when water is
used as the heat transmitting medium is atmospheric pressure plus
0.17 megapascals.
[0078] The target pressure for the reducing gaseous environment is
set so that the condensation temperature of the steam is a value
between the upper limiting temperature at which catalyst
deterioration does not occur and the catalyst activation
temperature.
[0079] The process in the step S9 and the step S10 is the same as
that of the routine of FIG. 3 according to the first
embodiment.
[0080] In the step S2, after the temperature TCO of the CO removal
device 1 reaches a temperature of 120 degrees centigrade the
controller 50 executes the steady-state operation as shown in FIG.
4. In the same manner as the first embodiment, in addition to
operating the flow rate control valve 21, the pressure regulation
valve 12 and the three-way valves 14, 15, the three-way valve 19 is
switched so that steam flowing out of the pressure regulation valve
12 is supplied to the reformer 18. Furthermore the operation of the
pump 7 is controlled so that the water level in the heat exchange
portion 5 detected by the level sensor 20 is maintained to the
target water level.
[0081] According to this embodiment, during steady-state operation,
it is possible to use steam generated in the heat exchange portion
5 of the CO removal device 1 for steam reforming operations of fuel
in the reformer 18. Furthermore during start-up under cold
conditions, operating the pressure regulation valve 12 makes it
possible to perform efficient warming up of the catalyst in the CO
removal device 1 using the heat of condensation from steam in the
heat exchange portion 5.
[0082] Referring to FIGS. 7 and 8, a third embodiment of this
invention will be described.
[0083] Referring to FIG. 7, a fuel cell power plant according to
this embodiment corresponds to the fuel cell power plant according
to the second embodiment with the addition of a temperature sensor
22, 23 and a load sensor 29.
[0084] The temperature sensor 22 detects a temperature of water
supplied to the heat exchanger 10 from the pump 7. The temperature
sensor 23 detects a temperature of steam supplied to the CO removal
device 1 from the heat exchanger 10. The load sensor 29 detects a
power generation load exerted on the fuel cell stack 2 from the
electrical apparatus driven by the fuel cell stack 2. The
temperature and the power generation load detected by the sensors
are input as respective signals into the controller 50.
[0085] The control routine performed during start-up of the fuel
cell power plant according to this embodiment is the same as that
in the second embodiment. This embodiment differs from the second
embodiment with respect to the preheating of water in the heat
exchanger 10 depending on operating conditions during steady-state
operation.
[0086] After the catalyst in the CO removal device 1 reaches an
activation temperature as a result of executing the start-up
control routine in FIG. 6, steam produced in the heat exchange
portion 5 of the CO removal device 1 is supplied to the reformer
18. In other words, the heat exchange portion 5 must produce the
amount of steam required for the reformer 18. However when the heat
release resulting from preferential oxidation reactions in the CO
removal device 1 is low, it is sometimes the case that the required
amount of steam can not be supplied to the reformer 18. For
example, there is the possibility that this type of problem will
arise during transient operating conditions such as when the power
generation load on the fuel cell stack 2 increases or when shifting
from start-up operation to steady-state operation.
[0087] In this embodiment, pre-heating of the water in the heat
exchanger 10 is performed so that the heat exchange portion 5
generates the amount of steam required by the reformer 1 during the
above situations.
[0088] Referring to FIG. 8, a pre-heating control routine performed
by the controller 50 for the above purpose will be described. This
routine is executed at intervals of a hundred milliseconds during
steady-state operation of the fuel cell power plant, that is to
say, after the start-up control routine is completed.
[0089] Firstly in a step S31, the controller 50 uses the power
generation load on the fuel cell stack 2 detected by the load
sensor 29 in order to calculate a heat amount Q1 to produce the
amount of steam required by the reformer 18. The amount of hydrogen
required by the fuel cell stack 2 is determined corresponding to
the power generation load of the fuel cell stack 2. As a result,
the reformate gas supply amount of the reformer 18 is determined
and the amount of steam required for reforming operation can be
determined. The heat amount Q1 is the amount of heat added to water
by the heat exchange portion 5 in order to produce the required
amount of steam.
[0090] Then in a step S32, the controller 50 calculates the amount
of heat Q2 produced by the supplied amount of reformate gas causing
preferential oxidation reactions mediated by the catalyst in the CO
removal device 1.
[0091] Then in a step S33, the controller 50 calculates the
shortfall Q3 by subtracting the reaction heat amount Q2 from the
consumed heat amount Q1.
[0092] In a step S34, the controller 50 calculates the temperature
increase factor .DELTA.T=Tb-Ta due to the heat exchanger 10 based
on the water temperature Ta detected by the temperature sensor 22
and the water temperature Tb detected by the temperature sensor 23.
However on the first occasion that this routine is performed, when
start-up control is completed, the three-way valve 15 is switched
so that combustion gas produced by the combustor 3 is discharged
into the atmosphere. Consequently the temperature increase factor
.DELTA.T in the heat exchanger 10 is zero.
[0093] Then in a step S35, the controller 50 calculates the
discharge amount F of the pump 7. This is calculated based on the
steam amount required by the reformer 1.
[0094] In a step S36, the pre-heat amount Q4 is calculated based on
Equation (1) below. The pre-heat amount Q4 on the first occasion
the routine is performed is zero.
Q4=.DELTA.T F Cp (1)
[0095] where, Cp=the specific heat of water.
[0096] In a step S37, the controller 50 compares the shortfall Q3
of the heat amount with the pre-heat amount Q4. When the shortfall
Q3 is greater than the pre-heat amount Q4, it means that the amount
of steam produced by the heat exchange portion 5 using the heat of
catalytic reactions in the CO removal device 1 will not satisfy the
amount of steam required by the reformer 18. In this case, in a
step S38, the controller 50 operates the three-way valve 15 so that
the supply amount of combustion gas to the heat exchanger 10 from
the combustor 3 increases. On the first occasion on which the
routine is performed, since the pre-heat amount Q4 is zero, if the
shortfall Q3 is a positive value, the supply of combustion gas to
the heat exchanger 10 from the combustor 3 in the step S38 is
resumed.
[0097] On the other hand, when the shortfall Q3 is not greater than
the pre-heat amount Q4, it means that the amount of steam produced
by the heat exchange portion 5 using the heat of catalytic
reactions in the CO removal device 1 will satisfy the amount of
steam required by the reformer 18. In this case, in a step S39, the
controller 50 operates the three-way valve 15 so that the supply
amount of combustion gas to the heat exchanger 10 from the
combustor 3 decreases.
[0098] The reason that the supply amount of combustion gas to the
heat exchanger 10 from the combustor 3 in the step S39 is reduced
even when the shortfall Q3 is equal to the pre-heat amount Q4 in
the step S38 is as follows.
[0099] During an increase in load, the reaction amount in the CO
removal device 1 is increased and the catalyst temperature is
increases as a result of the increase in the supply amount of
reformate gas. Thus it is thought to be due to the fact that the
shortfall Q3 in the amount of heat tends to decrease.
[0100] Even after completion of start-up control, when the heat
exchange portion 5 of the CO removal device 1 can not supply the
amount of steam required by the reformer 18 during a transient
operation state, it is still possible to supplement the shortfall
Q3 in the amount of heat by pre-heating the water supplied to the
heat exchange portion 5. Thus it is possible to increase the
transient response characteristics of the fuel cell stack 2.
[0101] In this embodiment, the shortfall Q3 in the amount of heat
was calculated on the basis of the amount of heat produced by
catalytic reactions in the CO removal device 1 and the amount of
heat Q1 consumed in producing steam required by the reformer 18.
However it is possible to determine the amount of pre-heating by
comparing the amount of steam produced by the heat exchange portion
5 as a result of catalyst reactions in the CO removal device 1 with
the amount of steam required by the reformer 18. In this case, the
shortfall in the steam amount can be calculated and the amount of
heat required to produce an amount of steam corresponding to the
shortfall can be calculated as the shortfall Q3.
[0102] In this embodiment, a valve which can control the flow rate
of combustion gas supplied to the heat exchanger 10 is used in the
three-way valve. When determining the opening of the three-way
valve 15 in the step S38 or step S39, it is preferable to first
calculate the combustion gas pressure upstream of the three-way
valve 15 from the combustion gas temperature in the combustor 3 and
a composition of gas supplied to the combustor 3. The target
opening of the three-way valve 15 is then determined based on the
combustion gas pressure, the combustion gas temperature, the
temperature of water supplied to the heat exchanger 10, and the
difference between the shortfall Q3 and the pre-heat amount Q4. On
the basis of this determination, it is possible to vary the
pre-heat amount Q4 in a highly accurate manner with respect to
fluctuations in the shortfall Q3 in the amount of heat.
[0103] The contents of Tokugan 2002-162790, with a filing date of
Jun. 4, 2002 in Japan, are hereby incorporated by reference.
[0104] Although the invention has been described above by reference
to certain embodiments of the invention, the invention is not
limited to the embodiments described above. Modifications and
variations of the embodiments described above will occur to those
skilled in the art, in light of the above teachings.
[0105] For example, in each of the above embodiments, although the
embodiments applied the invention to the warming up of a carbon
monoxide removal device, it is possible to apply the invention to
the warming up of all types of catalytic reactor requiring warming
up.
[0106] The embodiments of this invention in which an exclusive
property or privilege is claimed are defined as follows.
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