JPS63239777A - Operation method for fuel cell power generating plant - Google Patents

Abstract

PURPOSE:To improve the power generation efficiency in a partial load operation by using a multispindle type expansion turbine, driving an air compressor and a generator with separate expansion turbines, and controlling the expansion turbine to drive the generator at a constant rotation frequency. CONSTITUTION:A multispindle expansion turbine is used, an air compressor 36 and a generator 38 are driven by separate turbines, the rotation frequency of the turbine 37 to drive the air compressor 36 is varied to make the flow of the air 27 variable, and the turbine 75 to drive the generator 38 is operated at a constant rotation frequency. Since the energy of the high temperature and high pressure fuel cell exhaust gas can be collected by the driving force as an electric output, and the feeding amount of the air 27 in a partial load operation can be reduced, the motive force for the air compressor can be reduced. Consequently, the power generation efficiency of the power generating plant in a partial load operation can be maintained at a high rate.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method of controlling a fuel cell power plant, and more particularly to a method of operating a fuel cell power plant suitable for achieving high power generation efficiency even at partial load.

[Conventional technology]

In conventional fuel cell power generation plants,
As disclosed in Publication No. 31, an attempt was made to improve the power generation efficiency at the rated value by supplying unreacted fuel from the fuel cell anode to the combustion section of the reformer and recovering heat. However, with regard to controlling the plant in consideration of power generation efficiency at partial load, characteristics at partial load have not been studied.

[Problem that the invention seeks to solve]

In other words, the conventional technology does not take into consideration the improvement of power generation efficiency under one partial load, and in order to improve power generation efficiency during rated load operation, an expansion turbine and a compressor are installed to configure a pressurized power generation plant. Therefore, there are operating constraints on the expansion turbine and air compressor, i.e., it is necessary to supply air at a constant pressure to the fuel cell at a constant rotation speed, so the gas flow rate and temperature that pass through the expansion turbine and compressor must be Since it is necessary to maintain a constant value, and auxiliary fuel is supplied to the expansion turbine, the power generation efficiency of the fuel cell itself improves, but the problem is that the power generation efficiency of the plant as a whole decreases at partial load. there were.

Furthermore, since an auxiliary fuel system is required, there is a problem in that the method and apparatus for controlling the amount of auxiliary fuel and air at one partial load are complicated.

An object of the present invention is to provide a method of operating a pressurized fuel cell power plant that can maintain high partial load power generation efficiency.

[Means for solving problems]

The above purpose uses a multi-shaft expansion turbine, drives an air compressor and a generator with separate expansion turbines, changes the rotational speed of the air compressor to vary the air supply amount, and outputs a fuel cell load signal or This is achieved by supplying fuel by selecting a high value of the fuel cell reaction temperature signal, the fuel cell oxidant gas supply temperature, or the expansion turbine inlet gas temperature signal.

[Effect]

Using a multi-shaft expansion turbine, the air compressor and generator are driven by separate turbines, and the rotational speed of the turbine that drives the air compressor is varied to vary the air flow rate, while the turbine that drives the generator is kept constant. By increasing the rotational speed, the energy contained in the high-temperature, high-pressure fuel cell exhaust gas can be recovered as electric power, and the amount of air supplied during partial load can be reduced, reducing the power required for air compression. Compared to an operation method in which auxiliary fuel is supplied and the operating conditions of the expansion turbine and air compressor are kept constant, the flow rate of auxiliary fuel to heat up the excess air during two partial loads is reduced. The power generation efficiency of the power generation plant at partial load can be maintained at a high level.

As the amount of heat supplied to the expansion turbine changes, the rotational speed changes, and the air flow rate and pressure balance with the shaft power.However, the fuel cell load signal or fuel cell reaction temperature signal,
Alternatively, by selecting the high value of the fuel cell oxidant gas temperature signal and controlling the fuel supplied to the fuel cell, it is possible to maintain optimal operating conditions in response to changes in the air supply amount and changes in the fuel cell operating conditions. The power generation plant of the load power generation plant can be maintained at a high level.

By controlling the fuel cell oxidant gas supply temperature with the fuel supplied to the fuel cell in accordance with the load factor, an auxiliary combustor is installed and at least a portion of the gas is supplied as auxiliary fuel by bypassing the fuel cell. Compared to the conventional case, it is possible to supply excess fuel to the fuel cell and increase the hydrogen equivalent partial pressure in the fuel cell anode, which increases the voltage of the fuel cell and increases the power generation efficiency at partial load of the fuel cell power plant. improves.

According to this embodiment, an auxiliary fuel system is not required, the system configuration is simplified, and the operating method, control method, and device can be simplified.

〔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/cd and is sent to the reformer 4.
supplied to In the reformer 4, fuel such as natural gas is passed to the waste heat recovery boiler 41 of the expansion turbine 75 in the reformer reaction section 5.
A reforming reaction is caused with the steam 3 generated by the above, and the fuel is reformed into a gas containing hydrogen and carbon oxide as main components.

A reaction gas 7 containing reformed hydrogen and carbon oxide as main components is supplied to an anode 9 of a fuel cell 8 at about 600°C.

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

In this example, carbonates such as lithium carbonate and potassium carbonate are used as the electrolyte, and the temperature ranges from about 550°C to molten state.
A molten carbonate operating at a temperature of 700°C is used.

7 The reaction gas 7 supplied to the node 9 is a mixed gas of air and carbon dioxide gas (oxidant gas) 3 supplied to the cathode 11.
Reacts with 0. At the cathode 11, the oxidant gas receives electrons and becomes carbonate ions, which enter the electrolyte. At the anode 9, hydrogen and carbonate ions in the electrolyte react to generate carbon dioxide gas and water, and release electrons. As a result, electrons move from the anode to the cathode and a current is generated.

The seven-node exhaust gas 12 of the fuel cell 8 contains carbon dioxide gas and water produced by the reaction between hydrogen and carbon oxide in the reaction gas 7 and carbonate ions.

The anode exhaust gas 12 of the fuel cell 8 is passed through the gas/gas heat exchanger 1
3 to exchange heat and cool. Further, the anode exhaust gas 12 is cooled by a gas cooler, and water generated in the anode exhaust gas 12 is separated by a steam/water separator 15.

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

The steam reforming reaction in which the fuel 1 is reacted with the steam 3 to reform into hydrogen and carbon oxide is an endothermic reaction.

It is necessary to apply heat from the outside. In this embodiment, the anode exhaust gas 12 of the fuel cell 8 is supplied to the reformer combustion section 68,
The reaction heat is supplied by burning unreacted fuel such as hydrogen and carbon oxide contained in the gas. The reaction of the reformer 4 depends on the ratio of fuel 1 and steam 3 supplied to the reformer reaction section 5, and
In order to keep the reaction temperature constant, the flow rate of steam 3 is controlled in proportion to the fuel flow rate 1, and the combustion temperature in the reformer combustion section 6 is controlled by controlling the excess air ratio.

The oxidant gas 3o, which is a mixed gas of air and carbon dioxide, is supplied to the cathode 11 of the fuel cell 8, and the air is pressurized by an air compressor 36 to 6 to 10 kg/aJ during rated load operation. Supplied as. On the other hand, carbon dioxide gas is supplied as the reformer combustion section exhaust gas 21. The exhaust gas 21 is produced at a rate of 6 to 10 kg/c during rated load operation by pressurizing the anode exhaust gas 20 serving as fuel with the booster compressor 18 and pressurizing the air 27 with the compressor 36.
Pressurized to ot.

It is known that the higher the reaction pressure, reaction temperature, and fuel concentration in the reaction gas, the higher the power generation efficiency of the fuel cell 8, which is the ratio of the amount of heat that can be extracted as electrical output than the amount of heat possessed by the reaction gas.

Regarding the reaction gas temperature, although the power generation efficiency of a fuel cell increases at high pressure, it is generally set to 10 kg/a+f or less due to legal and regulatory restrictions.

Of the heat contained in the gas reacted in the fuel cell, the heat that cannot be extracted as electrical output is converted into heat by resistance such as polarization contact resistance, so the fuel cell needs to be cooled.

Fuel cells that use molten carbonate as an electrolyte have a high reaction temperature of about 550°C to 700°C, so when cooling with water, there is a large temperature difference between the gas being cooled and the water being cooled, causing problems such as thermal stress. be.

The fuel cell has an anode 9 and a cathode 11 of the fuel cell 8.
It is cooled by flowing a large amount of gas into it.

The fuel cell cathode exhaust gas 32 is passed through the first stage expansion turbine 3
7 performs work to drive the compressor 36, and the second stage expansion turbine 75 drives the generator 38 to generate electrical output and recover heat.

A fuel cell power generation plant has a reformer 4, a fuel cell 8, and an expansion turbine 37.75 that balance each other to form an effective heat recovery system, resulting in an overall plant power generation efficiency of 55 to 60% during rated load operation. can be achieved.

The power generation efficiency of the fuel cell 8 during partial load operation is improved because the current flowing through the cell is reduced, the resistance is reduced, and the voltage is increased.

The power generation efficiency of a fuel cell power plant is as follows: In the conventional technology shown in Figure 2, a heat recovery system using an expansion turbine 37 is used to improve power generation efficiency during rated load operation, so the power generation efficiency at partial load is descend. The expansion turbine 37 recovers heat from the high-temperature, high-pressure fuel cell cathode exhaust gas 32 to drive the compressor 36, and the excess heat is used to generate the generator 38.
Since the generator drives the generator and generates electrical output, it is necessary to keep the frequency of the generated power constant, so it is operated at a constant rotation speed in the entire load range. Therefore.

The expansion turbine 37 that drives the generator 38 also operates at a constant rotation speed. In order to maintain the fuel cell operating pressure constant even during partial load operation, it is necessary to operate the compressor to maintain a constant discharge air amount, according to the compressor characteristic curve shown in FIG.

The air flow rate required by the fuel cell 8 during partial load operation is:
From the point of view of cooling the fuel cell 8.

The battery voltage increases and the amount of heat required to cool the battery decreases. Part load characteristics of the fuel cell 8 are shown in FIGS. 4 and 5. FIG. 4 shows the ratio when the heat input to the fuel cell 8 during rated load operation is set as 100.

The amount of heat 80 supplied to the fuel cell 8 decreases excessively compared to the decrease in load because the voltage of the battery increases during partial load operation and the power generation efficiency improves. The cooling heat amount 82 of the battery is excessively reduced compared to the load factor because power is generated with high power generation efficiency using a small amount of fuel supplied. FIG. 5 is a redrawing of FIG. 4 with the heat input to the fuel cell 8 at each load being 100, and it can be seen that the cooling heat jft82 of the cell decreases relatively as the load decreases.

Cooling of the fuel cell is performed by gas passing through the anode 9 and cathode 11, but the amount of cooling heat 83 due to the anode gas decreases in proportion to the fuel heat input 80, so as shown in FIG. The ratio to 80 remains constant. The air supplied to the cathode 11 for cooling the fuel cell 8 needs to be supplied in accordance with the decrease in fuel supplied to the fuel cell 8 and the decrease in cooling heat amount due to an increase in cell voltage.

In the prior art shown in FIG. 2, an auxiliary combustor 71 is installed at the inlet of the expansion turbine 37, and during 1 part load operation, the auxiliary fuel 45 is combusted with surplus air 44 in the fuel cell 8 and the reformer combustion section 6. Since the amount of work in the expansion turbine 37 is kept constant by supplying the high temperature gas to the expansion turbine 3°7, the power generation efficiency of the fuel cell power generation plant at partial load decreases.

According to the embodiment shown in FIG. 1, the expansion turbine 37 that drives the air compressor 36 and the expansion turbine 75 that drives the generator 38 are separate shafts, and the expansion turbine 75 that drives the generator 38 is controlled to a constant rotation speed. By doing so, effective heat recovery can be achieved during rated load operation, and the amount of air supplied during partial load operation can be reduced, so that the amount of auxiliary fuel can be reduced and the power generation efficiency of the power generation plant can be improved.

FIG. 3 shows an example of the flow rate and pressure characteristics of the air compressor 37. Compressor 36 of first stage expansion turbine 37
During partial load operation, as the inlet heat amount of the expansion turbine 37 decreases, as shown in the characteristic curve 88, the rotation speed and pressure ratio decrease, and the flow rate decreases. Therefore, the amount of surplus air at partial load is reduced, and the amount of auxiliary fuel can be reduced.
The power generation efficiency during partial load operation is improved compared to the conventional example.

Furthermore, in this embodiment, the reaction temperature of the fuel cell is controlled by supplying fuel by selecting the high value of the fuel cell load signal 62 and the fuel cell reaction temperature signal (separator metal temperature signal 63). It becomes possible to operate the plant under load in response to changes in the operating state such as changes in the flow rate of the machine discharge air 27 and a relative decrease in the amount of heat dissipated from the fuel cell due to an increase in the voltage of the fuel cell.

In the prior art shown in FIG. 2, this is performed using the recycled flow rate of the cathode output log gas 33, and when the battery heat dissipation amount decreases as the battery voltage increases during one partial load, the recycled flow rate 33 increases and the recycle compressor 34 There is a problem that the power of

Further, according to the present invention, the fuel cell anode 9 is supplied in relatively excess amount by the amount of fuel necessary to maintain the reaction temperature of the fuel cell compared to the fuel flow rate necessary for generating electricity with the fuel cell. That's what happened.

Unreacted fuel in the gas at the outlet of the anode 9 increases, and the cell voltage also increases compared to the prior art.

The improved value of power generation efficiency during partial load operation according to this embodiment will be explained based on a No. 2 5,000 KW class or higher power plant.

In the conventional example, when the fuel cell is loaded at 50%, fuel 1 corresponding to 46% of the rated fuel flow rate is supplied, and fuel corresponding to 16% of the rated fuel flow rate is supplied as the auxiliary fuel 45. The expansion turbine 37 is operating at rated load, and the electrical output at the rated time is 56% for the fuel cell and 5% for the expansion turbine.
When the fuel cell is loaded at 50%, the fuel cell is 28% and the expansion turbine is 5%, resulting in a total of 33%. Therefore, the power generation efficiency is 61%
This will be approximately 53%, a decrease of 13%.

In this embodiment, 55% of the rated fuel is supplied to the fuel cell 8 when the fuel cell is operated at 50% load. In the fuel cell 8, since the fuel concentration in the anode increases, a fuel cell output of 50% of the rated value is obtained with a calorific value of 45% of the rated value. surplus 10
% of the heat amount is supplied to the expansion turbine 37, so the air amount of the expansion turbine is reduced to about 60%, and the electrical output is also reduced to about 60%.
It becomes 0%. Therefore, with 55% human heat of the rated fuel, it is possible to obtain an output of 31% of the rated fuel ratio, and the power generation efficiency is approximately 56%.
This is an improvement of about 6% compared to the conventional technology.

FIG. 6 shows a second embodiment of the present invention. In Example 2, the fuel cell reaction temperature is determined by the cathode inlet/outlet log gas temperature signal 6.
It is given as 7.68. The present invention is characterized in that the reaction temperature of the fuel cell is controlled by the fuel flow rate 1, and does not specify a temperature detection method.

FIG. 7 shows a third embodiment of the present invention. The third embodiment is characterized in that the temperature of the oxidizing gas 21 at the exit of the reformer is controlled. If the oxidant gas temperature is kept constant, the battery reaction temperature will decrease by the amount of decrease in the amount of cooling heat accompanying the increase in battery voltage. For example, 0.85 V/cell during rated load operation,
Under the condition of 80% fuel utilization rate, at 50% fuel cell output,
Example 1.2 by supplying a temperature of about 50' CIa converting agent gas
The same effect can be obtained.

FIG. 8 shows an example of Embodiment 4 of the present invention. In this embodiment, the expansion turbine inlet gas temperature 32 is controlled.

By controlling the fuel flow rate by selecting the high value of the fuel cell load signal, fuel cell reaction temperature signal, or oxidizer supply temperature signal, operation corresponding to changes in air flow rate and fuel cell heat balance can be performed. It can be carried out.

During partial load operation, the fuel supplied to the fuel cell reduces cell heat loss due to an increase in cell voltage due to a decrease in electrical resistance, and prevents overcooling of the fuel cell due to relatively excessive air flow. The fuel gas concentration at the fuel cell increases and the cell voltage increases, improving plant efficiency.

The bus pass system for the auxiliary fuel system and auxiliary air system can be deleted, simplifying the system configuration.

Since the load on the plant can be controlled by controlling the flow rate of fuel supplied to the fuel cell, operational control becomes simple.

No. 9 @ shows the deviation between the power generation efficiency of the plant according to the present invention and the power generation efficiency of a conventional plant in relation to the fuel cell load factor.

Power generation efficiency improves to 85 by reducing the amount of auxiliary fuel, increases to 86 by increasing the fuel gas concentration in the fuel cell anode, but increases to 87 by decreasing the operating pressure of the fuel cell.
The improvement value of plant power generation efficiency is 8.
It will be like 4. In the figure, 65 is a fuel flow control valve, 66 is a master control device, 70 is an auxiliary fuel control valve, and 72 is an excess air control valve.

〔Effect of the invention〕

According to the present invention, by using a multi-shaft expansion turbine, driving the air compressor and the generator with separate expansion turbines, and controlling the expansion turbine that drives the generator to a constant rotation speed,
As the compressor-driven expansion turbine heat input decreases, the air flow rate decreases, resulting in less auxiliary fuel and improved plant efficiency during part-load operation.

[Brief explanation of drawings]

1 and 6 are system diagrams of one embodiment of the present invention, and FIG.
Figure 8 is a system diagram of Embodiment 2, Figure 8 is a system diagram of Embodiment 3, Figure 2 is a conventional system diagram, Figure 3 is a compressor characteristic curve diagram, Figures 4 and 5 are fuel cell FIG. 9, a partial load characteristic diagram of a power generation plant, is a diagram showing the effects of the present invention. 4...Reformer, 5...Reformer reaction section, 6...Reformer combustion frame base 3 times 4v [with] C scale

Claims (1)

[Scope of Claims] 1. A fuel cell power generation plant comprising a reformer, a fuel cell, an expansion turbine driven by the exhaust gas of the fuel cell, an air compressor driven by the expansion turbine, and a generator, comprising: By using a turbine, the expansion turbine that drives the air compressor and the expansion turbine that drives the generator are installed separately, and the expansion turbine that drives the generator is controlled to a constant rotation speed, thereby reducing compressor discharge. A method of operating a fuel cell power generation plant, characterized in that the air flow rate is made variable and the amount of heat from the exhaust gas of the fuel cell is recovered as electrical output. 2. The fuel cell power generation according to claim 1, wherein a high value of a load signal of the fuel cell and a reaction temperature signal of the fuel cell is selected to control the flow rate of fuel supplied to the fuel cell. How to operate the plant. 3. A method of operating a fuel cell power generation plant according to claim 1 or 2, characterized in that the oxidant gas supply temperature of the fuel cell is used instead of the reaction temperature signal of the fuel cell. 4. The method of operating a fuel cell power plant according to claim 1 or 2, characterized in that the inlet or outlet gas temperature of the expansion turbine is used instead of the reaction temperature signal of the fuel cell.