JPH088108B2 - Fuel cell power plant - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a fuel cell power plant, and more particularly to a fuel cell power plant having high power generation efficiency even under partial load.

[Conventional technology]

A conventional fuel cell power plant, as described in Japanese Patent Publication No. 58-56231, supplies unreacted fuel from the anode of the fuel cell to the reformer combustion section, recovers heat, etc. Was trying to improve the power generation efficiency.

However, regarding the control of the plant in consideration of the power generation efficiency at the partial load, no special consideration was given to the characteristics at the partial load had not been examined.

[Problems to be solved by the invention] The prior art does not consider generation efficiency improvement at partial load, and installs an expansion turbine and a compressor to improve power generation efficiency at rated operation. It constituted a pressurized power plant.

However, in the related art, the operational constraint of the expansion turbine and the compressor, that is, it is necessary to supply the air having a constant pressure to the fuel cell at a constant rotation speed, so that the flow rate of the gas passing through the expansion turbine and the compressor and It was necessary to keep the temperature almost constant. Further, since the auxiliary fuel is supplied to the expansion turbine, there is a problem that the power generation efficiency at the partial load is reduced and the flow control of the auxiliary fuel and the auxiliary air at the partial load becomes complicated.

An object of the present invention is to provide a pressurized fuel cell power plant operated at a high pressure of 1 kg / cm ² or more, and a simple and simple fuel cell power plant capable of maintaining high power generation efficiency under partial load. It is in.

[Means for solving problems]

The fuel cell power plant of the present invention includes a reformer for reforming fuel, a compressor for compressing air, an anode to which the fuel reformed by the reformer is supplied, and air compressed by the compressor. A fuel cell that has a cathode supplied through a reformer and that generates electricity by the reaction of fuel and air, and an expansion turbine that is driven by exhaust gas discharged from the cathode side and that is connected to a compressor by a single shaft. , With. Then, a system for guiding the exhaust gas discharged from the anode side to the reformer,
Adjust the fuel flow rate so that the inlet temperature of the expansion turbine and the system that guides air to the outlet side of the reformer by bypassing the reformer are approximately constant even when the fuel cell is under partial load. And a control device.

That is, the flow rate of the fuel is controlled so that the operating conditions of the expansion turbine are substantially constant over the entire operating load range including the partial load state. Then, the flow rate of air is controlled so that the gas temperature in the combustion section of the reformer becomes substantially constant. Therefore, the excess air can be used as cooling air for the gas discharged from the combustion section of the reformer.

[Action]

A system that guides the exhaust gas discharged from the anode side to the reformer, a system that bypasses the reformer to the outlet side of the reformer, and a system that expands the expansion turbine even if the fuel cell is in a partial load state. By having a control device that adjusts the flow rate of fuel so that the inlet temperature is substantially constant, expansion is possible in all operating load zones without installing an auxiliary combustor that was formed on the inlet side of the expansion turbine. The operating conditions of the turbine can be kept substantially constant.

Further, by controlling the gas temperature at the inlet side of the expansion turbine with the fuel supplied to the fuel cell, more fuel is supplied to the fuel cell compared to the case where the fuel cell is bypassed and the auxiliary fuel is supplied to the auxiliary combustor. Can supply the fuel of
The hydrogen partial pressure at the anode can be increased. Therefore,
The voltage of the fuel cell can be increased, and the power generation efficiency of the fuel cell power plant is improved.

Further, the flow rate of air supplied to the combustion section of the reformer is controlled so that the gas temperature of the combustion section of the reformer becomes constant.
By using the surplus air as cooling air for the gas emitted from the combustion section of the reformer, through the fuel cell, even when the unreacted fuel supplied to the combustion section of the reformer increases, The operation corresponding to this becomes possible.

By mixing and cooling the surplus air with the gas discharged from the combustion section of the reformer and supplying it to the fuel cell, the temperature of the gas supplied from the combustion section of the reformer to the fuel cell can be lowered, thus reducing the wall thickness. .

The fuel supplied to the fuel cell power plant is supplied to the expansion turbine through the reformer, the fuel cell, and the combustion section of the reformer. Further, air is supplied to the expansion turbine via the compressor, the combustion section of the reformer, and the fuel cell. This eliminates the need for a fuel and air flow distribution system and their control devices, and simplifies the system configuration.

〔Example〕

 An embodiment of the present invention will be described below with reference to FIG.

The fuel 1 is pressurized to about 6 to 10 kg / cm ² and supplied to the reformer 4. In the reformer reaction part 5 without the reformer 4, the fuel such as natural gas and the steam 3 generated by the exhaust heat recovery boiler 41 that guides the exhaust gas of the expansion turbine 37 are reformed to generate fuel. Reform into a gas containing hydrogen and carbon monoxide as main components.

The reformed reaction gas 7 containing hydrogen and carbon monoxide as main components is supplied to the anode 9 of the fuel cell 8 at a temperature of about 600 ° C.

The fuel cell 8 is composed of a stack of fuel cells. Each fuel cell includes a positive electrode, a negative electrode, and an electrolyte 10 arranged between these two electrodes, a gas passage (the positive electrode and the positive electrode gas passage are referred to as cathode 11) provided on the non-electrolyte side of the positive electrode, and a non-electrolyte of the negative electrode. Gas passage provided on the side (the negative electrode and the negative electrode gas passage are referred to as an anode).

In this embodiment, a carbonate such as lithium carbonate or potassium carbonate is used as the electrolyte, and it is in a melting state at about 550 ° C to 7 ° C.
It uses a fused carbonate which operates at a temperature of 00 ° C.

The reaction gas 7 supplied to the anode 9 reacts with the mixed gas 30 of air and carbon dioxide gas supplied to the cathode 11. At the cathode 11, the mixed gas receives electrons and becomes carbonate ions, which enter the electrolyte. At the anode 9, hydrogen reacts with carbonate ions in the electrolyte to generate carbon dioxide gas and water,
Emits electrons. As a result, electrons move from the anode to the cathode, and a current is generated.

The anode exhaust gas 12 of the fuel cell 8 contains hydrogen gas and carbon monoxide in the reaction gas 7, carbon dioxide gas generated by the reaction of carbonate ions in the electrolyte 10, water, and unreacted reaction gas.

The fuel cell 8 and the anode exhaust gas 12 are heat-exchanged and cooled in the gas / gas heat exchange 13. Further, the water cooled by the gas cooler is separated by the steam separator 15 into water produced in the anode exhaust gas 12.

The anode exhaust gas 17 from which water has been separated is compressed by the compressor 18 and supplied to the reformer combustion section 6.

The reforming reaction for reacting the fuel 1 with the steam 3 to reform it into hydrogen and carbon monoxide is an endothermic reaction, and it is necessary to give heat from the outside. In the present embodiment, to the reformer combustion section 6,
The fuel cell 8 and the anode exhaust gas 12 are supplied, and unreacted fuel such as hydrogen and carbon monoxide contained in the gas is burned to supply reaction heat. The reaction of the reformer 4 controls the flow rate of the steam 3 in proportion to the fuel flow rate 1 so that the ratio of the fuel 1 supplied to the reformer reaction section 5 to the steam 3 and the reaction temperature are constant. The combustion temperature of the reformer combustion section 6 is controlled by controlling the excess air ratio.

The mixed gas 30 of air and carbon dioxide gas supplied to the fuel cell 8 and the cathode 11 is 6 to 6 by the air compressor 36.
It is pressurized to 10 kg / cm ² and supplied as compressed air 27. On the other hand, carbon dioxide gas is supplied as the exhaust gas 21 of the reformer combustion section. The exhaust gas 21 is pressurized to 6 to 10 kg / cm ² by pressurizing the anode exhaust gas 20 as fuel with the boost compressor 18 and the combustion air 29 with the compressor 36.

It is known that in the fuel cell 8, the higher the reaction pressure and reaction temperature of the anode 9 and the fuel concentration in the reaction gas, the higher the power generation efficiency, which is the ratio that can be taken out as an electrical output from the amount of heat of the reaction gas.

Regarding the reaction gas temperature, the temperature at which the molten carbonate that is the electrolyte can maintain the molten state is approximately 550 ° C to 700 ° C.
Limited to ℃.

Regarding the reaction gas pressure, the higher the power generation efficiency of a fuel cell, the higher it becomes.
It is generally less than kg / cm ² .

Of the heat quantity of the gas reacted in the fuel cell, the heat quantity that cannot be taken out as an electric output is exchanged for heat due to resistance such as polarization and contact resistance, so that the fuel cell needs to be cooled.

Fuel cells that use molten carbonate as an electrolyte have a high reaction temperature of approximately 550 ° C to 700 ° C, so when cooling with water,
When cooled, the temperature difference between the gas and the cooled water is large, and there are problems such as thermal stress. In general, the fuel cell is cooled by flowing a large amount of gas to the fuel cell 8, the anode 9 and the cathode 11. In this embodiment, the fuel cell is cooled by the gas passing through the fuel cell 8 and the cathode 11.
The reaction temperature of the fuel cell 8 is controlled by controlling the cathode inlet gas temperature 52 by recirculating the cathode outlet gas 33. In addition, the above recirculation system is
This prevents the temperature of 52 from becoming too low and the molten carbonate, which is the electrolyte of the fuel cell 8, to solidify or generate excessive thermal stress.

In a fuel cell power plant that operates at high temperature, the power of the compressor for cooling is large, the gas pressure is increased, the volume flow rate of the gas passing through the compressor is reduced, and the power generation efficiency of the entire plant is increased. Will be needed.

The fuel cell cathode exhaust gas 32 performs work in the expansion turbine 37, drives the compressor 36, and generates an electric output in the generator 38 to recover heat.

In the fuel cell power plant, the reformer 4, the fuel cell 8, and the expansion turbine 37 are mutually balanced to form an effective heat recovery system, so that a total plant power generation efficiency of about 60% at the rated load operation can be achieved. .

Regarding the power generation efficiency of the fuel cell 8 at the time of partial load operation, the current flowing through the cell decreases and the voltage increases, so the power generation efficiency improves, but the power generation efficiency as a fuel cell power plant is
Since the heat recovery system using the expansion turbine 37 is used to improve the total power generation efficiency during the rated load operation, the total power generation efficiency during the partial load operation is reduced. This is because the expansion turbine 37 recovers heat from the high-pressure, high-temperature fuel cell cathode exhaust gas 32, drives the compressor 36, and drives the generator 38 with an excess amount of heat to generate an electric output. Since the generator needs to keep the frequency of the generated power constant, it is necessary to operate at a constant rotation speed in all load bands, and the expansion turbine 37 that drives the generator 38 must also operate at a constant rotation speed. become. The compressor 36 needs to supply the air having almost the same pressure as the rated load operation required by the fuel cell 8 even during the partial load operation. However, in order to reduce the power of the compressor 36, the expansion turbine 37 Driving the compressor 36, the compressor 36
Since it is necessary to supply air with a constant pressure at a constant rotation speed, the discharge air amount 37 of the compressor 36 is almost the same as that at the rated load operation from the relationship between the pressure ratio and the flow rate of the compressor 36 even at the partial load operation. Will be the same. If the power generator 38 is not driven by the expansion turbine 37, the rotation speed can be varied to reduce the load on the expansion turbine 37 and the compressor 36, but the power generation efficiency will be reduced because power is not generated. It is also possible to control the rotational speed of the turbine that drives the compressor 36 and the turbine that drives the generator 38 to reduce the load on the gas turbine, but the system configuration becomes complicated, and it is currently in practical use. Since it is a different type of gas turbine from the existing large-capacity gas turbine, it needs to be newly developed for fuel cell power plants.

From the above, it is necessary that the expansion turbine 37 and the compressor 36 perform substantially the same operation as the rated load operation even during the partial load operation of the fuel cell power plant.

On the other hand, from the viewpoint of cooling the fuel cell 8, the air flow rate required for the fuel cell 8 during the partial load operation is such that the cell voltage rises and the amount of heat required for cooling the cell decreases. 3 and 4 show the partial load characteristics of the fuel cell 8.
Figure 3 shows the heat input to the fuel cell 8 during rated load operation.
Indicates the ratio. Heat quantity supplied to the fuel cell 8 80
In the case of partial load operation, the battery voltage rises and the power generation efficiency improves, so that the load drops excessively compared to the load drop. The cooling heat quantity 82 of the battery is less supplied fuel,
Since the power is generated with high power generation efficiency, it decreases excessively compared to the load factor. Figure 4 shows the heat input to the fuel cell 8 at each load.
Although FIG. 3 is rewritten as 0, it can be seen that the cooling heat quantity 82 of the battery relatively decreases as the load decreases. The fuel cell 8 is cooled by the gas that passes through the anode 9 and the cathode 11, but the cooling heat amount 83 by the anode gas decreases in proportion to the fuel heat input 80. Therefore, as shown in FIG. The ratio to is constant. The air supplied to the cathode 11 for cooling the fuel cell 8 needs to be supplied in accordance with the decrease in the fuel supplied to the fuel cell 8 and the decrease in the cooling heat amount due to the increase in the cell voltage.

It is necessary for the expansion turbine 37 and the compressor 36 to maintain substantially the rated operating conditions, and the surplus air is supplied to the expansion turbine 37 by raising the temperature of the auxiliary fuel and supplying it to the expansion turbine 37, so the power generation efficiency of the fuel cell power generation plant decreases.

In the prior art shown in FIG. 2, an auxiliary combustor 43 is installed at the inlet of the expansion turbine 37, and excess air 44 in the fuel cell 8 and the reformer combustor 6 during partial load operation is combusted with the auxiliary fuel 45, By supplying the expansion turbine 37 as high-temperature gas, the expansion turbine 37 is maintained in substantially the same operating condition as the rated operation.

In the embodiment of the present invention shown in FIG. 1, the load signal 62 of the fuel cell is obtained during both the rated operation and the partial load operation.
By selecting the high value of the expansion turbine inlet gas temperature signal 59 and controlling the fuel flow rate adjusting valves 65 and 68, the temperature of the expansion turbine inlet gas 32 can be controlled in the full load range, so the flow rate and pressure of the compressor 36 can be controlled. Compressor discharge air pressure from ratio characteristics
27 is almost constant. To the fuel cell anode 9, the amount of fuel required to maintain the temperature of the expansion turbine inlet gas 32 is higher than that required to generate power in the fuel cell 8,
As compared with the prior art, the fuel is supplied in a relatively excessive amount, and unreacted fuel in the gas at the outlet of the anode 9 increases. Although the fuel concentration of the anode inlet gas 7 is the same, since the fuel is excessively supplied, the unreacted fuel increases at the anode outlet 12 and the fuel concentration at the anode 9 increases, so that the voltage in the fuel cell increases. And the efficiency of power generation increases.

The unreacted fuel from the anode outlet 12 is supplied to the reformer combustion section 6, and in the reforming combustion section 6, the air flow rate is controlled by the air control valve 73 so that the combustion temperature becomes constant. The rest,
By controlling the pressure at the inlet of the bypass valve 74 to be constant by the air bypass valve 74, the combustion section 6 is bypassed and supplied to the combustion section exhaust gas cooler 69 to cool the combustion section exhaust gas.
The fuel to the reformer combustion section 6 is relatively excessive during partial load operation, but the reaction conditions of the reformer reaction section 5 can be maintained by controlling the air flow rate so that the combustion temperature becomes constant. . However, since the fuel to the combustion section 6 is excessively opposed to each other, the exhaust gas temperature of the combustion section rises. In the present invention, the temperature rise of the exhaust gas from the reformer outlet is prevented by mixing and cooling the reaction gas for the combustion cell 8 into the cooler 69 that is integral with the reformer combustion section.

In the conventional example shown in FIG. 2, the flow rate of the air supplied to the fuel cell cathode 11 is reduced more than the air flow rate required for the reaction in accordance with the cooling heat amount of the fuel cell 8, so that the voltage of the fuel cell 8 is lowered. However, in the present invention, in the entire load range,
Since the entire amount of air discharged from the compressor 36 is supplied to the fuel cell cathode 11, it is possible to prevent a voltage drop due to a decrease in oxygen concentration in the reaction gas supplied to the cathode.

According to the present embodiment, conventionally, an auxiliary combustor is installed at the inlet of the expansion turbine 36 and the auxiliary fuel is supplied to keep the operating condition of the expansion turbine 37 substantially the same as the rated operation. This is performed by selecting the high value of the load signal and the load signal of the expansion turbine inlet gas temperature to control the flow rate of the fuel supplied to the fuel cell. Therefore, the auxiliary combustor 71, the auxiliary fuel system 45, and the air bypass shown in FIG. Since the system 44 is unnecessary and the operation of the expansion turbine 37 and the compressor 36 can be controlled by using the normal fuel system in the reformer combustion section 6, the system configuration can be simplified and the control can be simplified.

The improvement value of the power generation efficiency during partial load operation according to the present embodiment will be described based on a power plant of 25,000 KW class or higher.

In the conventional example, when the fuel cell is 50% loaded, the rated combustion flow rate is 46%.
Fuel corresponding to% is supplied as fuel 1. Also,
Fuel equivalent to 16% of the rated value is supplied as auxiliary fuel 45. Since the expansion turbine is operating at rated load,
Power plant output is 46% fuel cell, 8% expansion turbine
Of 54%. On the other hand, since the fuel heat input is 62% of the rated value, the power generation efficiency drops to 87%.

In this embodiment, similarly, 60% of the rated fuel at 50% load is supplied to the fuel cell 8. In the fuel cell 8, the hydrogen equivalent concentration (carbon monoxide 1 m
ol is converted to 1 mol of hydrogen and 1 mol of methane is converted to 4 mol of hydrogen)
Rises and the voltage rises, so with the amount of heat of 44% of the rated time,
Obtain the fuel cell electric output of 50% of the rated value. The surplus 16% of heat is supplied to the expansion turbine 37 by 16%, so the expansion turbine 37 can maintain the rated load operation, and the plant efficiency is
It only drops to about 90%. Therefore, the plant efficiency at 50% load is improved by about 4% relative value.

FIG. 5 shows a second embodiment of the present invention. In this embodiment, since the expansion turbine inlet gas temperature 32 is about 700 ° C., which is a relatively high temperature, the temperature of the expansion turbine outlet exhaust gas 39 at about 400 ° C. is detected and controlled. Different from Since the expansion turbine 39 is operated at a substantially rated operation, the relationship between the inlet gas and the outlet gas of the expansion turbine 37 is constant, so that the temperature of the expansion turbine inlet gas 32 can be controlled also in this embodiment.

FIG. 6 shows a third embodiment of the present invention. In this embodiment, the high value of the fuel cell load signal 62 and the expansion turbine load signal are selected to control the fuel regulating valve. In the present embodiment, the work of the expansion turbine 37 and the compressor 36 is different depending on the atmospheric temperature and the atmospheric pressure. Therefore, the characteristics of the expansion turbine load signal for the atmospheric temperature and the atmospheric pressure are given to the master controller 66. Every time, it is necessary to change the set value.

FIG. 7 shows a fourth embodiment of the present invention. In this embodiment, since the temperature of the gas at the inlet of the expansion turbine 37 is controlled by the fuel of the fuel 1 supplied to the fuel cell, there is a time delay corresponding to the volume of the pipes and equipment compared to the case where the auxiliary combustor 71 is used. However, in this embodiment, the transient response characteristic of the load change is dealt with by the auxiliary fuel adjusting valve 72 for the transient change, and by switching the auxiliary fuel 45 and the fuel 1 while the load is settling. And the thermal efficiency of the partial load are both improved.

FIG. 8 shows the deviation between the plant power generation efficiency according to the present invention and the plant power generation efficiency according to the prior art in relation to the fuel cell load factor.

According to the present invention, during partial load operation, excess fuel is supplied to the fuel cell and the fuel concentration increases, so the power generation efficiency improves as shown at 85. On the other hand, as the fuel flow rate increases, the heat loss due to the cooling of the anode outlet gas increases, and the power generation efficiency decreases as indicated by 87. Further, since the fuel in the fuel cell can be burned and the high-temperature gas can be supplied to the cathode of the fuel cell, the power of the cathode circulation fan can be reduced, and the power generation efficiency increases as shown in 86. The power generation efficiency at the power transmission end is [(85) + (86) − (87) = (84)]. In addition, in a trial calculation example of power generation efficiency at the transmission end assuming a scale of 25 MW or more, when the fuel cell 50% load operation, 3.7% due to voltage increase, 0.4% due to heat loss, 0.6% due to reduction of auxiliary machine power, At the power transmission end, 3.9% (relative value) power generation efficiency is improved.

Further, the load response characteristic can be improved by the system configuration in which the present invention and the conventional auxiliary fuel system are combined.

FIG. 9 shows the load change characteristics of each system. In a fuel cell power plant, the electric facilities of the fuel cell and the orthogonal switch respond instantaneously, so the response characteristics of the reformer and the time delay of fuel supply to the fuel cell are the limiting factors when the load changes. For example, when the load is changed from 50% to 100%, the conventional example shows a load change characteristic of 89. There is a delay corresponding to the volume between the reformer and the anode of the fuel cell, and then the fuel flow rate increases in accordance with the load characteristics of the reformer, and the load rises. In the present invention, since the fuel is in excess of the fuel required in the fuel cell, the load can be rapidly increased by utilizing the excess fuel. However, since the load of the expansion turbine can be increased only through the fuel cell, an extra time delay occurs compared to the conventional example. (Shown at 90.) By the combination of the present invention and the conventional example, the load of the expansion turbine can be increased by the auxiliary fuel supply, so that the load change characteristic is improved as shown at 91.

〔The invention's effect〕

According to the present invention, the operating conditions of the expansion turbine can be kept substantially constant. Even under partial load, the fuel gas concentration in the fuel cell anode increases and the cell voltage can be increased, so plant efficiency can be improved.

Further, the bypass system of the auxiliary fuel system and the auxiliary air system can be deleted, so that the system configuration can be simplified.

[Brief description of drawings]

FIG. 1 is a first embodiment of the present invention, FIG. 5 is a second embodiment, FIG. 6 is a third embodiment, and FIG. 7 is a system diagram of a fourth embodiment. The figure is a conventional system diagram, and FIGS. 3 and 4 are partial load characteristics of a fuel cell power plant, FIG.
FIG. 9 and FIG. 9 respectively show the effects of the present invention. 1 ... Fuel, 3 ... Steam, 4 ... Reformer, 5 ... Reformer reaction part, 6 ... Reformer combustion part, 7 ... Reformed gas, 8 ...
Fuel cell, 9 ... Fuel cell anode, 11 ... Fuel cell cathode, 12 ... Anode outlet gas, 20 ... Anode outlet gas, 21 ... Reformer combustion part outlet gas, 26 ... Air, 32 ...
… Expansion turbine inlet gas, 36 …… Compressor, 37 Expansion turbine, 38 …… Generator, 39 …… Expansion turbine exhaust gas, 54 ……
Battery temperature control valve, 56 ... Expansion turbine inlet thermometer, 59 ...
… Expansion turbine inlet temperature signal, 61 …… Expansion turbine load signal, 62 …… Fuel cell load signal, 65 …… Fuel control valve, 68
…… Fuel control valve, 73 …… Combustion temperature control valve, 74 …… Bypass air flow control valve.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kenshi Yokosuka Kenshi Yokosuka 4-6 Kanda Surugadai, Chiyoda-ku, Tokyo Inside Hitachi, Ltd. (72) Inventor Hiroshi Hattori 3-2-1 Sachimachi, Hitachi City, Ibaraki Prefecture Hitachi Inside Engineering Co., Ltd. (72) Inventor Naruhisa Sugita 502 Kintatecho, Tsuchiura City, Ibaraki Prefecture Inside the Institute of Mechanical Research, Hiritsu Manufacturing Co., Ltd. (56) Reference JP-A-57-212778 (JP, A) JP-A-58 -133780 (JP, A)

Claims (1)

[Claims] 1. A reformer for reforming fuel, a compressor for compressing air, an anode to which fuel reformed by the reformer is supplied, and air compressed by the compressor are the reformer. A fuel cell that has a cathode supplied through a quality device and that generates electricity by the reaction of the fuel and air, and an expansion that is driven by exhaust gas discharged from the cathode side and that is connected to the compressor by one shaft. In a fuel cell power plant having a turbine, a system that guides the exhaust gas discharged from the anode side to the reformer, and a system that guides the air to the outlet side of the reformer by bypassing the reformer. And a control device for adjusting the flow rate of the fuel so that the inlet temperature of the expansion turbine is substantially constant even when the fuel cell is in a partial load state. .