JP2003217603A - Fuel cell cogeneration system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To recover thermal energy from offgas, and to sufficiently burn unburnt components in the offgas before discharging. <P>SOLUTION: This fuel cell cogeneration system for supplying power and hot water is provided with an offgas combustor 5 to burn the offgas from an anode electrode 41 of a fuel cell stack 4, a first heat exchanger 6 installed downstream a stack cooling part 42 to recover combustion heat of the offgas as warm water, and a passage 25 for feeding the offgas residual after combustion in the offgas combustor 5 to a reformer combustion burner 2. Residual offgas fed from the offgas combustor 5 is burnt in the reformer combustion burner 2 before it is discharged. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell cogeneration system for supplying electric power and hot water in combination, in which an anode off gas of a fuel cell is burned by an off gas combustor and an off gas combustion heat is burned by a first heat exchanger. While recovering heat as hot water, the off-gas combusted by the off-gas combustor is supplied to the reformer combustion burner through the passage, and the remaining off-gas combustor supplied by the reformer combustion burner remains. The present invention relates to a fuel cell cogeneration system that burns off gas and discharges it.

[0002]

2. Description of the Related Art A conventional fuel reformer for a fuel cell (Japanese Patent Application Laid-Open No. Hei 7 (1999) -77242)
-192742) is a fuel cell F as shown in FIG.
The catalytic combustor S was disposed in the off-gas exhaust system E, and the off-gas was burned and exhausted.

A conventional fuel cell plant (Japanese Patent Laid-Open No. 5-20
5759), as shown in FIG. 6, the residual oxygen component in the exhaust gas discharged from the combustion section B of the reformer K is consumed in the catalytic combustor S as a purge gas containing a low concentration or containing no oxygen component. Was supplied to V.

The conventional fuel cell hot water supply cogeneration system (Japanese Unexamined Patent Publication No. 2001-176527) has an oxygen electrode of the fuel cell F by an off-gas burner S arranged at a downstream portion of the fuel cell F as shown in FIG. And the exhaust gas of the hydrogen electrode was burned and discharged.

A conventional polymer electrolyte fuel cell power generator (Japanese Patent Laid-Open No. 2001-93550) has an exhaust gas containing combustible substances such as residual hydrogen and unreformed fuel from a fuel electrode FP as shown in FIG. Is sent to the exhaust gas burner S,
Here, a part of the exhaust gas of the oxidizer electrode OP is mixed and burned, and the exhaust gas is passed through the steam heater VH, the fuel-mixed gas preheater PH, the water evaporator WV, and the water condenser WG in this order. there were.

[0006]

The above-mentioned conventional fuel reformer for a fuel cell is one in which the off gas is combusted and exhausted by the catalytic combustor S arranged in the off gas discharge system E of the fuel cell F. Since the off gas is processed only by the off gas burner, there is a problem that unreformed reformed gas is discharged together with the exhaust gas due to the structure of the off gas burner.

In the above conventional fuel cell plant, the residual oxygen component in the exhaust gas discharged from the combustion section B of the reformer K is consumed in the catalytic combustor S as a purge gas which has a low concentration or contains no oxygen component. Since it is supplied to the containment vessel V, the residual oxygen component is merely consumed in the catalytic combustor, and there is a problem that thermal energy cannot be recovered from the residual component. .

Further, in the above conventional fuel cell hot water supply cogeneration system, the off gas burner S disposed in the downstream portion of the fuel cell F burns and discharges the exhaust gas of the oxygen electrode and the hydrogen electrode of the fuel cell F. However, there is a problem that thermal energy cannot be recovered from the exhaust gas.

In the conventional polymer electrolyte fuel cell power generator, the exhaust gas containing combustible substances such as residual hydrogen and unreformed fuel from the fuel electrode FP is exhaust gas burner S.
To a steam heater VH, a fuel mixed gas preheater PH, a water evaporator WV, and a water condenser WG in that order. Since it is passed, thermal energy is recovered from the exhaust gas, but there is a problem that it is eventually released.

Therefore, in the fuel cell cogeneration system for supplying electric power and hot water together, the inventor of the present invention transfers the offgas combustion heat associated with the combustion of the anode offgas of the fuel cell in the offgas combustor to the downstream side of the stack cooling section. While recovering heat as hot water by the installed first heat exchanger, the off gas remaining after being burned by the off gas combustor is supplied to the reformer combustion burner through the passage, and the reformer is supplied. Focusing on the technical idea of the present invention of combusting and exhausting the remaining off-gas supplied by the combustion burner, as a result of further research and development, it is possible to recover thermal energy from the off-gas and The present invention has been achieved to achieve the purpose of exhausting the unburned components therein after they have been sufficiently burned.

[0011]

A fuel cell cogeneration system according to the present invention (a first invention according to claim 1) is a fuel cell cogeneration system for supplying electric power and hot water in combination. An off-gas combustor for burning off-gas, a first heat exchanger installed on the downstream side of the stack cooling section for recovering the heat of off-gas combustion as hot water, and the remaining off-gas combustor for combustion A passage is provided for supplying offgas to the reformer combustion burner, and the remaining offgas supplied from the offgas combustor is burned and exhausted by the reformer combustion burner.

In the fuel cell cogeneration system of the present invention (the second invention according to claim 2), in the first invention, an offgas air control valve for controlling the amount of offgas air supplied to the offgas combustor for burning the offgas. It is equipped with.

The fuel cell cogeneration system of the present invention (the third invention according to claim 3) is the hot water in which heat is recovered by off-gas combustion heat by the first heat exchanger in the first invention or the second invention. Is supplied to the battery stack via the heat exchanger of the hot water storage tank.

In the fuel cell cogeneration system of the present invention (the fourth invention according to claim 4), the off-gas combustor and the reformer combustion burner are connected to each other in any one of the first to third inventions. A first directional control valve that switches the supply direction so as to enable the supply or exhaust of the off gas left after being combusted by the off gas combustor to the reformer combustion burner is disposed in the passage. It is a thing.

The fuel cell cogeneration system of the present invention (the fifth invention according to claim 5) is the fuel cell cogeneration system according to the first invention, wherein the reformed gas from the reformer is supplied in accordance with the operating state of the system and the demand for hot water. A second direction switching valve for switching the supply direction is provided to switch the supply destination.

The fuel cell cogeneration system of the present invention (sixth invention according to claim 6) is the fuel cell cogeneration system according to the fifth invention, wherein the modification is supplied through the second directional control valve during warm-up operation. A third directional control valve for supplying a quality gas to the off-gas combustor is provided, and the combustion heat of the off-gas combustor is used to raise the temperature of the cooling water in the stack cooling section.

The fuel cell cogeneration system of the present invention (the seventh invention according to claim 7) is the fuel cell cogeneration system according to the sixth invention, wherein the reformed gas is supplied to the off-gas combustor when the demand for hot water supply in a steady state is high. To heat the cooling water in the stack cooling section by using the combustion heat of the off-gas combustor.

The fuel cell cogeneration system of the present invention (the eighth invention according to claim 8) is the fuel cell cogeneration system according to the fifth invention, wherein the reformed gas is fed to the reformer combustion burner when the power demand in the steady state is high. Is to be supplied to.

The fuel cell cogeneration system of the present invention (the ninth invention according to claim 9) is the fuel cell cogeneration system according to the sixth invention, wherein the first to third directional control valves switch three-way connection relationships. The reformer, the reformer combustion burner, and the off-gas combustor are controlled by a three-way valve.

[0020]

The fuel cell cogeneration system according to the first aspect of the present invention, which has the above-described structure, is a fuel cell cogeneration system that supplies electric power and hot water in combination, and is used for combusting the anode offgas of a fuel cell in an offgas combustor. The accompanying off-gas combustion heat is recovered as hot water by the first heat exchanger installed on the downstream side of the stack cooling unit, and the off-gas remaining after being combusted by the off-gas combustor is modified through the passage. The remaining off-gas supplied by the reformer combustion burner is combusted and exhausted by supplying the reformer combustion burner to the quality device combustion burner, thereby enabling thermal energy recovery from the off-gas and unburned gas in the off-gas. The effect is that the components are sufficiently burned and then discharged.

The fuel cell cogeneration system of the second invention having the above-mentioned constitution is the same as the first invention,
The off-gas air control valve controls the amount of off-gas air supplied to the off-gas combustor that burns the off-gas, so that the amount of heat recovery can be controlled.

In the fuel cell cogeneration system of the third invention having the above-mentioned structure, in the first invention or the second invention, the hot water recovered by the off-gas combustion heat by the first heat exchanger is stored in the hot water storage tank. Since it is supplied to the battery stack via the heat exchanger, it has the effects of increasing the hot water supply capacity and raising the temperature of the cooling water in the stack cooling section of the battery stack.

In the fuel cell cogeneration system of the fourth invention having the above-mentioned configuration, in the fuel cell cogeneration system of any one of the first invention to the third invention, the fuel cell cogeneration system is provided in the passage connecting the offgas combustor and the reformer combustion burner. The first direction switching valve provided switches the supply direction so as to enable the supply or exhaust of the off gas left after being combusted by the off gas combustor to the reformer combustion burner. When the temperature of the furnace combustion burner is abnormal, there is an effect that it is possible to stop the supply of the off gas and exhaust the off gas.

The fuel cell cogeneration system of the fifth invention having the above-mentioned structure is the same as that of the first invention.
By switching the supply direction according to the operating state of the system and the demand for hot water by the second directional control valve,
It is possible to switch the supply destination for supplying the reformed gas from the reformer.

A fuel cell cogeneration system according to a sixth aspect of the present invention having the above structure is the same as the fifth aspect of the invention.
During the warm-up operation, the reformed gas supplied through the second directional switching valve by the third directional switching valve is supplied to the offgas combustor, so the combustion heat of the offgas combustor is used. As a result, it is possible to raise the temperature of the cooling water in the stack cooling unit.

The fuel cell cogeneration system of the seventh invention having the above-mentioned constitution is the same as the sixth invention,
When the demand for hot water supply at regular times is high, the reformed gas is supplied to the off-gas combustor and the combustion heat of the off-gas combustor is used to raise the temperature of the cooling water in the stack cooling section. This has the effect of effectively raising the temperature of water.

The fuel cell cogeneration system according to the eighth aspect of the present invention having the above structure is the same as the fifth aspect,
Since the reformed gas is supplied to the reformer combustion burner when the power demand in the constant time is high, it is possible to meet the high power demand in the steady state.

The fuel cell cogeneration system of the ninth invention having the above-mentioned structure is the same as that of the sixth invention,
The three-way valve constituting the first to third directional control valves controls switching of the three-way communication relationship to establish the communication relationship between the reformer, the reformer combustion burner, and the off-gas combustor. Since the control is performed, it is possible to effectively recover the thermal energy from the off-gas and to exhaust the unburned components in the off-gas after they are sufficiently burned.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION The embodiments of the present invention will be described below.
This will be described with reference to the drawings.

(Embodiment) The fuel cell cogeneration system according to the present embodiment is the anode electrode of the fuel cell stack 4 in the fuel cell cogeneration system for supplying both electric power and hot water as shown in FIGS. 1 to 4. An off-gas combustor 5 for combusting the off-gas of 41, a first heat exchanger 6 installed downstream of the stack cooling unit 42 for recovering the off-gas combustion heat as hot water, and the off-gas combustor 5 2 for supplying the off-gas left by combustion by the reformer combustion burner 2
5, the reformer combustion burner 2 is configured to burn and exhaust the remaining off-gas supplied from the off-gas combustor 5.

The off-gas combustor 5 in this embodiment
Is constituted by a catalytic combustor as an example.

An off-gas air control valve 51 is provided in a secondary air passage 50 for supplying secondary air to the off-gas combustor 5 for burning the off-gas, and is supplied to the off-gas combustor 5 to control the heat recovery amount. The off-gas air amount as the secondary air is controlled.

The first heat exchanger 6 communicates with the heat exchanger 62 of the hot water storage tank 60 via a pipe, and the heat exchanger 6
2 communicates with the stack cooling unit 42 of the fuel cell stack 4 via a pipe, and the hot water recovered by the heat of combustion of the offgas from the cell stack 4 exchanges the heat in the hot water storage tank 60. It is configured to be supplied to the stack cooling unit 42 of the battery stack 4 via the container 62 to raise the temperature of the stack cooling water.

The first directional control valve is disposed in the passage 25 that connects the offgas combustor 5 and the reformer combustion burner 2, and reforms the offgas left after combustion by the offgas combustor 5. It is constituted by a three-way valve 10 which switches the supply direction so as to allow supply or exhaust to the combustion furnace burner 2.

The second directional control valve is arranged in the passage 14 that connects the reformer 1 and the battery stack 4, and the second directional control valve from the reformer 1 depends on the operating state of the system and the demand for hot water. It comprises a three-way valve 8 for switching the supply direction in order to switch the supply destination for supplying the reformed gas.

The third directional control valve is arranged in the passage 90 which connects the passage 14 and the passage 25, and through the three-way valve 8 as the second directional control valve during the warm-up operation. The three-way valve 9 that supplies the supplied reformed gas to the off-gas combustor 5 or the reformer combustion burner 2 is used to generate the cooling water of the stack cooling unit by using the combustion heat of the off-gas combustor. The temperature is raised.

The shut-off valve 3 is disposed in a passage 91 between the passage 90 and the anode of the battery stack 4, and the combustion burner 2 or the off-gas combustor 5 is passed through the passage 90 for the off-gas of the anode 41. It controls the opening and closing of the supply to.

The controller 50 is connected to the off-gas air control valve 51, the shutoff valve 3, the three-way valve 8, the three-way valve 9 and the three-way valve 10, and controls the opening / closing, the flow rate, and the supply direction. In addition to recovering the heat of the energy of the off-gas and utilizing it effectively, it outputs a control signal for exhausting the unburned components in the off-gas after sufficient combustion.

That is, when the demand for hot water supply in a steady state is high, the shut valve 3 is opened so that the reformed gas is supplied to the off-gas combustor 5, and the three-way valve 9 is controlled in the a direction. The heat of combustion of the off-gas combustor 5 is used to exchange heat with the cooling water by the first heat exchanger 6 to raise the temperature of the cooling water in the stack cooling section.

When the electric power demand in the steady state is high, the reformed gas is supplied to the three-way valve 8, the three-way valve 9 and the three-way valve 10.
It is supplied to the reformer combustion burner 2 via.

Details of the method of operating the system in this embodiment will be described with reference to the flowcharts of FIGS. 3 and 4. As shown in FIG. 3, when the system is activated, the reformer 1 starts preparing for reforming. At this time, the shut valve 3 is closed, and the connecting direction of the 3-way valve 8 is a, and the connecting direction of the 3-way valve 9 is b.

When the reforming reaction is started, the reformed gas becomes rich in hydrogen. At this stage, the connecting direction of the three-way valve 9 is switched to a, and at the same time, secondary air is supplied to the off-gas combustor 5 to burn the reformed gas. The temperature of the cooling water is raised by the reformed gas until the stack cooling water can generate power.

When the preparation for power generation is completed, the shut valve 3 is opened and the communication direction of the three-way valve 8 is switched to b, and the mode is switched.

In the hot water supply (main) mode in which hot water (hot water supply) is desired, the connection direction of the three-way valve 9 is set to a, and secondary air is supplied to the off-gas combustor 5 to increase the power generation start output, It is determined whether or not the utilization rate is a certain percentage (%) or more, and if it is more than a certain percentage (%), the off-gas combustor 5 is used.
Stops the supply of secondary air to the.

In the power generation mode in which electricity is desired,
The temperature of the stack cooling water is compared with the set temperature, and when it is higher than the set temperature, the connection direction of the three-way valve 9 is set to b, the supply of secondary air to the off-gas combustor 5 is stopped, and the power generation start output is set. It is increased and it is determined whether or not the hydrogen utilization rate is a certain rate (%) or more. If it is more than a certain rate (%), the supply of secondary air to the off-gas combustor 5 is stopped.

When the temperature of the stack cooling water is lower than the set temperature, the connection direction of the three-way valve 9 is set to a, and the same control as in the hot water supply mode is performed.

Even in the hot water supply mode and the power generation main mode, when the supply rate of secondary air to the off-gas combustor 5 is stopped when the hydrogen utilization rate is equal to or higher than a constant rate (%), the steady operation shown in FIG. 4 is performed. , Mode switching is performed.

In a hot water supply (main) mode in which hot water (hot water supply) is desired, off gas is controlled to be supplied to the off gas combustor 5, and the off gas is supplied to the off gas combustor 5 to cause the three-way valve 9 to operate. The connection direction is set to a, and it is determined whether or not the hydrogen utilization rate is equal to or higher than a certain rate (%). When the hydrogen utilization rate is equal to or more than the certain rate (%), the supply of secondary air to the offgas combustor 5 is stopped. When the ratio is less than a certain ratio (%), secondary air is supplied to the off-gas combustor 5.

In the power generation mode in which electricity is desired,
The temperature of the stack cooling water is compared with a set temperature, and when the temperature is higher than the set temperature, the connection direction of the three-way valve 9 is set to b in order to supply the off gas to the combustion burner 2, and 2 to the off gas combustor 5 Stop the supply of the next air.

When the temperature of the stack cooling water is lower than the set temperature, the off-gas is supplied to the off-gas combustor 5, the connection direction of the three-way valve 9 is set to a, and the hydrogen utilization rate is a constant rate (%). Whether or not it is above is determined, and when it is above a certain ratio (%), the supply of secondary air to the off-gas combustor 5 is stopped. When the ratio is less than a certain ratio (%), secondary air is supplied to the off-gas combustor 5.

When the temperature of the stack cooling water is lower than the set temperature, the connection direction of the three-way valve 9 is set to a and the same control as in the hot water supply mode is performed.

In the fuel cell cogeneration system of the present embodiment, the off gas combustion heat generated by the combustion of the off gas in the anode 41 of the fuel cell in the off gas combustor 5 is installed on the downstream side of the stack cooling section 42. The heat is recovered as hot water by the heat exchange by the heat exchanger 6 of No. 1, and the off gas remaining after being burned by the off gas combustor 5 is supplied to the reformer combustion burner 2 through the passage 25 to change the temperature. Burner burner 2
Since the remaining off-gas is burned and exhausted by the above method, it is possible to recover thermal energy from the off-gas, and at the same time, the unburned components in the off-gas are sufficiently burned and then discharged.

By controlling both the reformer burner 2 and the offgas burner 5 to burn off gas generated from the fuel cell stack (FC) 4 in the system of the present embodiment, the unburned gas in the offgas is controlled. It has the advantage of exhausting after burning the fuel components sufficiently.

When heat recovery is required, the off gas burner 5 is mixed with air, and H2 is burned in the off gas burner to recover the heat. It has the advantage that it can be controlled by adjusting the amount.

Since the residual H2 and residual methane are further burned by the reforming burner 2 and exhausted, there is an advantage that the unburned components in the off gas are sufficiently burned and then discharged.

When the heat recovery is not performed, the off-gas burner 5 is bypassed and the reforming burner 2 controls the entire process. It is controlled so as to process H2 or to release all when abnormal.

As a method of treating off-gas in the FC generator system of this embodiment, the off-gas burner 5 and the reformer burner 2 are connected in series so that the heat recovery amount can be adjusted and the residual methane can be treated. Therefore, in the off-gas combustor 5, in catalytic combustion, H2 is burned to recover heat, and the amount of mixed air is adjusted to control the amount of heat recovered.
Further, the remaining off-gas in the reforming burner 2 is
It is burned and exhausted.

The system of the present embodiment is also provided with a circuit that does not flow to the off-gas combustor 5 at all, and makes it possible to burn the entire amount with the reforming burner 2.

The system of the present embodiment also includes a circuit for discharging the off gas without supplying the off gas to the reforming burner 2 when the temperature of the reforming burner 2 is abnormal. It has an advantage of avoiding promoting the temperature abnormality of 2.

Furthermore, the system of the present embodiment enables the temperature of the cooling water of the fuel cell stack 4 to be quickly and efficiently warmed by the waste heat of the off gas in the anode 41 of the fuel cell when the system is started. Have advantages.

As described above, the system of this embodiment is
When the conventional off-gas is treated only by the off-gas burner, the problem that unreformed reformed gas is discharged together with the exhaust gas due to the structure of the off-gas burner is avoided.

Further, when the off-gas is processed only by the reforming burner 2, the off-gas amount changes when the load fluctuates, and an amount that cannot be processed is generated in some cases, and the reforming burner becomes abnormal at high temperature. This is to avoid such a situation.

The above-described embodiments and examples are given as examples for the purpose of explanation, and the present invention is not limited to them. The present invention can be understood from the claims, the detailed description of the invention and the description of the drawings. Modifications and additions can be made without departing from the technical idea of the present invention that can be recognized by those skilled in the art.

In the above embodiments and examples,
Although an example of using a heat exchanger has been described as an example, the present invention is not limited thereto, and a catalyst combustor for combusting the anode offgas of the fuel cell and the combustor by the offgas combustor are used. A modified example of the fuel cell cogeneration system including a passage for supplying the remaining off-gas to the reformer burner, and burning and discharging the remaining off-gas supplied from the catalytic combustor by the reformer combustion burner. Can be adopted.

In the present modification, it is possible to reliably burn the part that has not burned in the catalytic combustor with the reformer burner (flame combustion), and the catalytic combustor has a wide range of combustible air-fuel ratios, and in particular air-fuel ratio control Since it is possible to perform combustion only by flowing the off gas without the need of performing the above, it has advantages such as excellent controllability.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing a fuel cell cogeneration system according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a flow of off-gas in the system of the present embodiment.

FIG. 3 is a chart showing a control procedure when the system of the present embodiment is activated.

FIG. 4 is a chart diagram showing a control procedure during steady operation of the system of the present embodiment.

FIG. 5 is an overall block diagram showing a conventional fuel reformer for a fuel cell.

FIG. 6 is a block circuit diagram showing a conventional fuel cell plant.

FIG. 7 is a block circuit diagram showing a conventional fuel cell hot water supply cogeneration system.

FIG. 8 is a block circuit diagram showing a conventional polymer electrolyte fuel cell power generator.

[Explanation of symbols]

2 reformer combustion burner 4 Fuel cell stack 5 Off-gas combustor 6 First heat exchanger 8, 9, 10 3-way switching valve 25 passages 41 Anode 42 Stack cooling unit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H01M 8/06 H01M 8/06 G (72) Inventor Kazuhiro Nagata 2-1-1 Asahi-cho, Kariya city, Aichi prefecture In Seiki Co., Ltd. (72) Inventor Koichiro Hara 1 Toyota-cho, Toyota-shi, Aichi F-term in Toyota Motor Corporation (reference) 3L025 AC01 AC07 4G040 EA03 EA06 EB12 EB42 EB43 EB44 5H027 AA06 BA01 BA09 BA16 BA17 DD06 MM08 MM13

Claims (9)

[Claims] 1. A fuel cell cogeneration system for supplying electric power and hot water in combination, an off-gas combustor for burning anode off-gas of a fuel cell, and a stack cooling unit for recovering off-gas combustion heat as hot water. A first heat exchanger installed on the downstream side of the exhaust gas, and a passage for supplying the off gas remaining after being combusted by the off gas combustor to the reformer combustion burner, and the off gas combustor by the reformer combustion burner. A fuel cell cogeneration system, characterized in that the remaining off-gas supplied from the unit is combusted and exhausted. 2. The fuel cell cogeneration system according to claim 1, further comprising an off-gas air control valve that controls an amount of off-gas air supplied to the off-gas combustor that combusts the off-gas. 3. The structure according to claim 1, wherein the hot water recovered by the heat of offgas combustion by the first heat exchanger is supplied to the battery stack via the heat exchanger of the hot water storage tank. Fuel cell cogeneration system characterized by being 4. The reforming of off-gas remaining after being burned by the off-gas combustor in the passage connecting the off-gas combustor and the reformer combustion burner according to any one of claims 1 to 3. Fuel cell cogeneration system, characterized in that a first directional control valve for switching the supply direction is provided so as to enable supply or exhaust to the burner combustion burner. 5. The second directional control valve according to claim 1, wherein the second direction switching valve switches a supply direction to switch a supply destination of the reformed gas supplied from the reformer according to an operating state of the system and a demand for hot water. Fuel cell cogeneration system characterized by 6. The third directional control valve for supplying the reformed gas supplied via the second directional control valve to the off-gas combustor during warm-up operation according to claim 5, A fuel cell cogeneration system characterized by using the combustion heat of an off-gas combustor to raise the temperature of cooling water in a stack cooling section. 7. The stack cooling unit according to claim 6, wherein the reformed gas is supplied to the off-gas combustor and the heat of combustion of the off-gas combustor is used when the demand for hot water supply at a constant time is high. A fuel cell cogeneration system characterized by raising the temperature of cooling water. 8. The fuel cell cogeneration system according to claim 5, wherein the reformed gas is supplied to the reformer combustion burner when the power demand in the constant time is high. 9. The method according to claim 6, wherein the first to third directional control valves control switching of three-way communication relationships to connect the reformer, the reformer combustion burner, and the off-gas combustor. A fuel cell cogeneration system comprising a three-way valve for controlling communication.