JP3897149B2 - Solid oxide fuel cell and Stirling engine combined system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is relatively small in size and high in dispersion scale, which realizes a combined cycle with a solid oxide fuel cell and a Stirling engine suitable for use of waste heat. The present invention relates to a solid oxide fuel cell / Stirling engine combined system for efficient energy conversion.
[0002]
[Prior art]
Conventionally, as an energy conversion system for distributed cogeneration, a gas engine, a diesel engine, a gas turbine engine, and the like have been used and developed as a power generation engine, and in particular, a highly efficient solid oxide fuel cell and In the combination of the above, research and development are progressing mainly for the combination with the micro gas turbine. In the combined system of the solid oxide fuel cell and the micro gas turbine, it is possible to increase the pressure of the solid oxide fuel cell unit, and the load factor of the solid oxide fuel cell can be improved. Furthermore, the unused energy of the solid oxide fuel cell can be burned completely in the micro gas turbine combustor, and a power generation efficiency of about 65% may be realized with a system of about 100 kW.
[0003]
On the other hand, in the kW class that is expected to spread on a household scale, which is the subject of the present invention, a small gas engine with an overall efficiency of 85% and a power generation efficiency of about 20%, and a polymer electrolyte fuel cell (PEFC) with a power generation efficiency of 30% However, the realization of a system with higher power generation efficiency is desired. In this class, the scale of the solid oxide fuel cell can be reduced, but since it is not consistent with the size of the combined micro gas turbine, it is difficult to combine with the conventional micro gas turbine. A Stirling engine combine system is considered. As described in Japanese Utility Model Laid-Open No. 4-119297, an afterburner is provided on the outlet side of the fuel cell, and the combined system is configured by further heating the high-temperature gas discharged from the fuel cell by the afterburner and supplying it to the Stirling engine. To improve efficiency.
[0004]
[Problems to be solved by the invention]
However, solid electrolyte fuel cells have many advantages, such as extremely high efficiency and the fact that oxygen, which is an oxidant, passes through the solid oxide film, so that hydrocarbon fuels such as methane can be used. Although it is known as a certain energy conversion device, since the operating temperature is high and the energy of exhaust gas is large, it is effective to combine it with other energy conversion devices for further efficiency improvement. Currently, the most researched form is a combination with a small gas turbine, and a feasibility study of several hundred kW class has already begun.
[0005]
On the other hand, the power generation capacity required on a household scale is from 1 kW to several kW class. In this class, the efficiency of the bottoming turbine is reduced, so the combined benefits with the gas turbine are less advantageous and difficult to realize. .
In addition, since the demand on the household scale is extremely fluctuating in load, a power generation system that can be applied to the demand is required to have a rapid start-up and high power generation efficiency with respect to a partial load.
[0006]
In the present invention, high-efficiency power generation efficiency can be obtained by combining well-known technologies such as a solid oxide fuel cell and a Stirling engine in small-scale distributed use, and the solid oxide fuel cell and the combustor are integrated. In order to minimize the heat capacity by making the structure and enable rapid start-up, and to improve power generation efficiency at partial load, the flow rate of fuel and air supplied by the load signal and the working gas pressure of the Stirling engine are controlled. The purpose is to operate under optimal operating conditions.
Japanese Utility Model Application No. 4-119297 describes a combined system of a solid oxide fuel cell and a Stirling engine. Since this system uses a closed system for underwater vehicles, the oxidizer is generally used as air. The home system has the following problems.
First, due to fuel and oxidant issues, the system must supply pure hydrogen and pure oxygen.
Next, a branch pipe is installed in the oxidant (oxygen) and fuel (hydrogen) pipes supplied to the solid oxide fuel cell, and this branch pipe is connected to the afterburner unit, and the high-temperature gas discharged from the fuel cell is further removed. It is configured to increase the temperature. However, since the details of the start-up and partial load operation methods of this system are not described in the text, the details are unknown, but when the afterburner is operated as a start-up burner at the start-up, the battery unit becomes the battery reaction start temperature. The heat generated by combustion is not directly supplied to the battery side, but is heated by heat exchange between the exhaust gas and the supplied fuel / oxidizer, and the oxygen and hydrogen supplied to the battery unit are not sufficiently mixed In that case, it will be circulated without burning in the afterburner section, and the combustion temperature will not rise sufficiently, so it will take a long time to rise to the operating temperature, or supply oxidant and fuel more than necessary As a result, the supply power increases. Also, when air is used as the oxidant in the combined system of Japanese Utility Model Publication No. Hei 4-119297, the concentration of oxidant (oxygen) required for battery operation is limited to 20% of the air, so the air flow rate is the fuel. In order to warm up to the battery operating temperature (800-900 ° C), the system must be further connected to the air supply side. It will be difficult not to provide not only exhaust gas (about 850 ° C) but also a preheating structure by heat exchange using high-temperature gas from an afterburner.
In addition, the fuel and oxidant piping from the fuel cell fuel and oxidant supply piping to the afterburner is branched to feed back pressure fluctuations accompanying combustion in the afterburner to the upstream fuel cell piping, and the fuel cell The battery reaction of the battery fluctuates, and problems such as fluctuations in electrical output are likely to occur. Furthermore, when fuel is burned using air as an oxidant in an afterburner, it is necessary to generate a high temperature combustion gas region of a certain level or more in order to achieve stable combustion. There is a problem that undesirable components are discharged.
In the conventional example, since partial load operation is not described, it is unclear, but in order to operate partial load with high efficiency by combining the fuel cell and SE, it is necessary to control the operation state considering the characteristics of both. Needed.
[0007]
[Means for Solving the Problems]
When the afterburner is installed separately as in the conventional example, the temperature of the combustion gas is raised to a high temperature in a short time during start-up, and the start-up time is shortened, or the fuel and oxidant supply piping is connected between the fuel cell and the afterburner. In order to solve the problems such as the mutual interference of fluctuations caused by branching at NOx and the generation of NOx by ensuring combustion stability in the afterburner, unreacted oxidation discharged from the cell at the outlet of the solid oxide fuel cell itself The combustor combusts the agent and fuel, and supplies heat directly to the upstream side, and the gas burned from the combustor passes through the Stirling engine heater, the air preheater, and the reforming steam generator in order. It is characterized in that it is guided to one gas flow path.
The solid oxide fuel cell / Stirling engine combined system according to the present invention is a combined system of a solid oxide fuel cell and a Stirling engine. When controlling the output of the system according to the power load, The temperature and the temperature of the Stirling engine heater are controlled to be constant.
The solid oxide fuel cell / Stirling engine combined system of the present invention is provided with a flow rate / pressure control device, and the flow rate / pressure control device controls the supply amount of air and fuel and the operating pressure of the Stirling engine. The temperature of the electrolyte fuel cell and the temperature of the Stirling engine heater are controlled to be constant.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows an outline of the overall configuration of a solid oxide fuel cell / Stirling engine combined system. This system includes a blower 1, an air preheater 2, a solid electrolyte fuel cell 3, and a solid oxide fuel cell. Integrated combustor 4, Stirling engine heater 5, reforming steam generator 6, fuel flow control valve 7, pre-reformer 8, Stirling engine 9, pressure control tank 10, flow rate / pressure control device 11, The system includes a Stirling engine cooler 12, a Stirling engine regenerator 13, a generator 14, and an inverter 15, and supplies electricity to an electric power load 16.
[0009]
A signal is input from the electric load 16 to the flow rate / pressure control device 11, and a control signal is sent to the blower 1 and the fuel flow rate control valve 7 according to the electric load state, and the working chamber of the Stirling engine 9 and the pressure control tank are sent. 10 is configured to send a control signal to control valves 20, 20 provided on a pipe connecting to 10.
[0010]
In this system, air is compressed by a blower 1 and sent, and heat is supplied from an exhaust gas of the system by an air preheater 2 to be heated and supplied to a cathode (air electrode) 17 of the solid oxide fuel cell 3. Only oxygen passes through the anode (fuel electrode) 18 side by the solid oxide film 19 and reacts with the fuel to generate electric power. The heat generated in the solid oxide film 19 is supplied to the exhaust gas, and the exhaust gas is mixed with the fuel that has not reacted on the anode 18 side in the combustor 4 integrated at the outlet of the solid oxide fuel cell 3 and burned. Then, heat is supplied to the heater 5 for Stirling engine.
The exhaust gas heat-exchanged by the Stirling engine heater 5 is used as a heat source for generating steam by the reforming steam generator 6 after the air is heated by the air preheater 2 and discharged to the atmosphere.
Thus, by arranging the combustor 4 at the outlet of the solid oxide fuel cell 3 and integrating the solid oxide fuel cell 3 and the combustor 4, the time constant at the time of start-up or sudden heat load is obtained. In addition to reducing the heat capacity to be reduced, the formation of a single gas flow path through the solid oxide fuel cell 3, the Stirling engine 9, the air preheater 2, and the reforming steam generator 6 enables startup and rapid load. A quick and smooth response can be realized at the time of fluctuation. In addition, the reforming steam generator 6 can be placed downstream of the air preheater 2 to recover the surplus exhaust gas heat to the limit to maintain high power generation efficiency.
[0011]
In this system, the flow rate of the fuel is adjusted by the flow rate control valve 7, sent to the system, and mixed with the steam generated by the reforming steam generator 6. The fuel mixed with the water vapor is heated by the air preheater 2, partly reformed to hydrogen by the pre-reformer 8, and supplied to the anode 18 of the solid oxide fuel cell 3.
The hydrogen supplied to the anode 18 passes through the solid oxide film 19 of the solid oxide fuel cell 3 and reacts with the oxygen separated from the air on the cathode 17 side, thereby generating power and generating heat of the solid oxide fuel cell 3. Is done. The fuel consumed here is limited to about 80% due to the protection of the solid oxide fuel cell.
The fuel that has not been consumed by the solid oxide fuel cell 3 is mixed with air from which some oxygen has been removed on the cathode 17 side, burned by the solid oxide fuel cell-integrated combustor 4, and heated to a Stirling engine. Supplied to the heater 5.
[0012]
In this system, the operation of the Stirling engine 9 is general, and the Stirling engine heater 5 supplies heat and operates.
In the Stirling engine regenerator 13, when the working medium passes from the Stirling engine heater 5 through the Stirling engine regenerator 13 and enters the Stirling engine cooler 12, heat is transferred from the working medium to the Stirling engine regenerator 13. When the working medium is stored and stored, and the working medium flows, the working medium cooled by the Stirling engine cooler 12 is heated again to prevent heat loss during the reciprocating motion of the working medium. ing.
[0013]
Since the electricity generated by the solid oxide fuel cell 3 is direct current and the generator 14 driven by the Stirling engine 9 is unrectified alternating current, power is supplied to the power load 16 via the inverter 15.
[0014]
The operating conditions of this system are: methane for fuel, air for oxidizer, atmospheric pressure, 70% hydrogen utilization, 3: 1 steam carbon ratio, 1.5 to 3.5 mass flow ratio of air to fuel, number of units Is the result of evaluating the performance of this system by system analysis assuming that the Stirling engine's illustrated efficiency is changed from 0.8 to 1.5, the Stirling engine regenerator's temperature efficiency is changed to 0.7, 0.8, and 0.9. These are shown in FIG. 2 and FIG. FIG. 2 shows the power generation efficiency, and FIG. 3 shows the dependence of the efficiency on the excess air ratio.
The efficiency of the system is somewhat affected by the number of units, but it depends strongly on the mass flow ratio of air and fuel and the temperature efficiency of the regenerator for the Stirling engine. In particular, the former should be operated with the mass flow ratio as the control target. This indicates that high power generation efficiency can be maintained even during partial loads. For example, if the mass flow ratio of air and fuel is fixed at 3, the efficiency is 40% or more at a temperature efficiency of 0.7, the efficiency is 50% or more at a temperature efficiency of 0.8, and the efficiency close to 60% is achieved at a temperature efficiency of 90%. .
Even when the fuel is hydrogen, an efficiency of about 50% at a temperature efficiency of 0.7, an efficiency of about 55% at a temperature efficiency of 0.8, and an efficiency of about 60% at a temperature efficiency of 90% can be realized.
[0015]
In general, at the time of partial load, the supply amount of fuel and air is decreased to decrease the output, but this causes the cell temperature of the solid oxide fuel cell to decrease and the electric resistance to increase. It is known that the power generation efficiency is lowered, and accordingly, the working gas temperature is lowered and the thermal efficiency is lowered due to a decrease in the Stirling engine heat input amount.
In this system, as shown in [0014], in order to minimize the decrease in power generation efficiency at such partial load, the supply amount of air and fuel to the system is controlled from the load condition of the power load 16. The temperature of the solid oxide fuel cell 3 is prevented and the operating pressure of the Stirling engine 9 is controlled to obtain a working gas pressure condition that provides a high efficiency with respect to the heat input. .
That is, the pressure control device 11 adjusts the blower 1 and the fuel flow rate control valve 7 to control the mass flow rate ratio, thereby keeping the cell temperature of the solid oxide fuel cell 3 constant and the rotational speed of the Stirling engine 9. The amount of heat sent from the Stirling engine heater 5 to the Stirling engine 9 side is controlled by adjusting the valve 20 of the pressure control tank 10 according to the load signal, and the temperature of the Stirling engine heater 5 is kept constant. Hold. For this reason, high efficiency can be maintained even at a partial load.
[0016]
【The invention's effect】
As is apparent from the above description, according to the present invention, even in a small-scale distributed power supply of the household scale, high-efficiency power generation is possible and energy can be used effectively for the following reasons.
(1) By making the outlet of the solid oxide fuel cell a combustor and integrating the solid oxide fuel cell and the combustor, the heat capacity that becomes a time constant at the time of start-up or sudden heat load can be reduced. By providing a single gas flow path through the solid oxide fuel cell, the Stirling engine, the air preheater, and the reforming steam generator, it is possible to realize a quick and smooth response during start-up and sudden load fluctuations.
(2) Further, a reforming steam generator is placed downstream of the exhaust gas air preheater, and the excess exhaust gas heat amount can be recovered to the limit to maintain high power generation efficiency.
(3) By controlling the supply amount of air and fuel to the system from the state of electric power load to prevent the temperature drop of the solid oxide fuel cell, and controlling the pressure of the Stirling engine, the heat input is highly efficient. Is obtained, and the operation can be performed under the condition of high power generation efficiency as a whole. That is, by controlling the blower, the fuel flow rate control valve, and the pressure control tank valve by the pressure control circuit, the temperature of the solid oxide fuel cell and the temperature of the Stirling engine heater are controlled to be constant. High efficiency can be maintained even at partial loads.
[Brief description of the drawings]
FIG. 1 is a diagram showing an outline of the overall configuration of a solid oxide fuel cell / Stirling engine combined system according to an embodiment of the present invention;
FIG. 2 is a graph showing power generation efficiency of a solid oxide fuel cell / Stirling engine combined system according to an embodiment of the present invention.
FIG. 3 is a graph showing the excess air dependency of the efficiency of the solid oxide fuel cell / Stirling engine combined system according to the embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Blower 2 Air preheater 3 Solid oxide fuel cell 4 Solid oxide fuel cell integrated combustor 5 Stirling engine heater 6 Reforming steam generator 7 Fuel flow control valve 8 Pre-reformer 9 Stirling engine 10 Pressure Control tank 11 Flow rate / pressure control device 12 Stirling engine cooler 13 Stirling engine regenerator 14 Generator 15 Inverter 16 Power load 17 Cathode (air electrode)
18 Anode (fuel electrode)
19 Solid oxide film 20 Control valve

Claims (1)

The fuel is mixed with the steam generated by the reforming steam generator, the fuel mixed with the steam is heated by the air preheater and supplied to the anode of the solid oxide fuel cell, and the air is heated by the air preheater. In the combined system of the solid oxide fuel cell supplied to the cathode of the solid oxide fuel cell and the Stirling engine, the solid oxide fuel cell outlet itself is connected to the unburned oxidant discharged from the cell. A combustor capable of combusting fuel and transferring heat generated by the combustion directly to the solid oxide fuel cell, and the gas combusted by the combustor into a Stirling engine heater, the air preheater, and the reformer formed to direct steam generator to a single gas flow path through the order, the supply amount of air and fuel supplied to the solid oxide fuel cell and star Solid oxide, wherein the controller controls so that the temperature of the heater for temperature and Stirling engine of the solid oxide fuel cell constant by providing the flow rate and pressure control device for controlling the pressure of the working gas ring engine Fuel cell and Stirling engine combined system.

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