Fuel cell system

Abstract

To provide a fuel cell system that can suppress condensation of combustion exhaust gas discharged into the atmosphere.SOLUTION: A solid oxide type fuel cell system that is equipped with an off-gas burner that burns anode off-gas discharged from a fuel electrode of a cell stack, and uses combustion exhaust gas as heat, and the combustion exhaust gas after heat utilization is mixed with heated air and discharged outside the system.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a fuel cell system using a solid oxide fuel cell.

Background Art

[0002] Conventionally, various types of fuel cells have been developed as power sources with good power generation efficiency and environmental friendliness. In particular, since a solid oxide fuel cell (SOFC) can obtain a high power generation efficiency of 50% or more, it is used in fuel cell systems with a wide output range from household to industrial use.

[0003] In an externally reformed fuel cell system using a hydrocarbon fuel such as city gas as a primary fuel, for example, as disclosed in Patent Document 1, the primary fuel is reformed in a reformer, and the reformed gas thus obtained is used for power generation by a cell stack. In addition, in the system, an off-gas burner for burning the off-gas discharged from the fuel electrode of the cell stack is generally provided so that heat is supplied from the off-gas burner to the reformer or the like.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The combustion exhaust gas emitted from the off-gas burner mentioned above contains more water vapor than the atmosphere because it is produced by burning hydrogen that was not used in the power generation reaction, resulting in a higher dew point temperature. Therefore, when the combustion exhaust gas cools down as it is discharged into the atmosphere, condensation is likely to occur, and as a result, for example, the condensed water can corrode the exhaust piping. In addition, if the humidity of the atmosphere is high, the exhaust air may become supersaturated, causing white smoke to be generated in the exhaust stack. This white smoke can also obstruct visibility, so there are concerns about adverse effects on the surrounding area.

[0006] In view of the above problems, the present invention aims to provide a fuel cell system that can suppress condensation of combustion exhaust gas discharged into the atmosphere. [Means for solving the problem]

[0007] The fuel cell system according to the present invention is a solid oxide type fuel cell system that includes an off-gas burner for burning anode off-gas discharged from the fuel electrode of a cell stack, and utilizes the combustion exhaust gas for thermal purposes. The system comprises an air preheater that preheats the air supplied to the air electrode of the cell stack using the combustion exhaust gas, and an evaporator that heats the reformed water using the combustion exhaust gas to generate steam used for steam reforming, wherein the air preheater and the evaporator The combustion exhaust gas after heat utilization , generated using the waste heat within the system The system is configured to mix the exhaust gas with heated air and discharge it outside the system. This configuration makes it possible to suppress condensation of the combustion exhaust gas discharged into the atmosphere.

[0008] More specifically, the above configuration is: The off-gas burner comprises a condenser that cools and condenses the water vapor contained in the anode off-gas, and a water tank that recovers the condensed water separated from the cooled anode off-gas as the reformed water, and the off-gas burner burns the anode off-gas after the water separation. The structure is also good. .Ma More specifically, the above configuration is as follows: The condenser exchanges heat between the water vapor contained in the anode off-gas and the cooling air, and the cooling air that has passed through the condenser is used as the heated air. It can also be used as a composition.

[0009] More specifically, the above configuration is: The temperature of the heated air is equal to or greater than the temperature of the combustion exhaust gas after heat utilization, and the mixing ratio of the combustion exhaust gas to the heated air is within the range of 1:2 to 1:30. It can also be used as a composition. [Effects of the Invention]

[0010] According to the fuel cell system of the present invention, it is possible to suppress condensation of combustion exhaust gas discharged into the atmosphere. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram of the fuel cell system according to this embodiment. [Modes for carrying out the invention]

[0012] One embodiment of the present invention will be described below with reference to the drawings. Figure 1 is a schematic diagram of the fuel cell system 1 according to this embodiment. In Figure 1, the fluid pathways are shown as follows: solid arrows indicate pathways related to fuel gas (gas pathways), dashed arrows indicate pathways related to reformed water (water pathways), dotted arrows indicate pathways related to air (air pathways), and dashed-dotted arrows indicate other pathways. The white arrows shown in Figure 1 indicate the fluid passing through each pathway.

[0013] As shown in Figure 1, the fuel cell system 1 includes a reformer 11, an evaporator 12, a cell stack 13, a water recovery device 14, an air preheater 15, an off-gas burner 16, and a mixing unit 17 (a space for mixing the combustion exhaust gas Gf and cooling air Ad, which will be described later).

[0014] The cell stack 13 is a solid oxide fuel cell (SOFC), and its operating temperature is, for example, around 700°C. A solid oxide fuel cell can generate electricity by generating an electrochemical reaction when a fuel gas containing hydrogen is supplied to the fuel electrode (anode) and air containing oxygen is supplied to the air electrode (cathode). In this embodiment, as an example, only one cell stack 13 is provided, but multiple cell stacks 13 may be provided in parallel.

[0015] Most of the elements of the fuel cell system 1, excluding the water recovery device 14 and the mixing unit 17, are located within the thermal insulation region TA shown in Figure 1. The thermal insulation region TA is covered with thermal insulation material, and care has been taken to minimize heat release to the outside. As a result, the fuel cell system 1 has an advantageous configuration in terms of thermal self-sufficiency.

[0016] The fuel cell system 1 is provided with a raw material gas receiving section Pg for receiving a raw material gas (in this embodiment, for example, city gas containing methane), and an air receiving section Pa for receiving air. The raw material gas and air are basically supplied from outside the fuel cell system 1.

[0017] The raw material gas receiving section Pg is connected to a reformer 11 via a first gas path Lg1, and a first blower B1 is provided in the middle of the first gas path Lg1. At least a part of the downstream side of the first blower B1 in the first gas path Lg1 is arranged to pass through an evaporator 12.

[0018] The reformer 11 is connected to a fuel electrode 13a of a cell stack 13 via a second gas path Lg2. The fuel electrode 13a is connected to a water recovery device 14 via a third gas path Lg3. The water recovery device 14 includes a water tank 14a, a condenser 14b, a cooling air path 14c, and an inlet 14d for cooling air (outside air), and is connected to an off-gas burner 16 via a fourth gas path Lg4. The cooling air path 14c extends from the inlet 14d and is connected to a mixing section 17 via the condenser 14b. An exhaust pipe Ly for exhausting gas to the outside extends from the mixing section 17. An exhaust cylinder for exhausting gas from a high position is arranged on the downstream side of the exhaust pipe Ly.

[0019] The water tank 14a is connected to the evaporator 12 via a water path Lw. A water pump W1 is provided in the water path Lw, and it is possible to continuously supply the water in the water tank 14a to the evaporator 12 as reformed water Wa. Thereby, the fuel cell system 1 can achieve water self-sufficiency. The water pump W1 can adjust the supply amount of the reformed water Wa from the water tank 14a to the evaporator 12.

[0020] A first air path La1 extends from the air intake section Pa, and a second blower B2 is provided in the middle of the first air path La1. The first air path La1 is connected to the air electrode 13b of the cell stack 13. The air electrode 13b is connected to the off-gas burner 16 via a second air path La2. The off-gas burner 16 is connected to the mixing section 17 via a combustion exhaust gas path Lx. An evaporator 12 is provided in the middle of the combustion exhaust gas path Lx.

[0021] The air preheater 15 is configured to perform heat exchange between a fluid passing through a predetermined position (a position between the off-gas burner 16 and the evaporator 12) of the combustion exhaust gas path Lx and a fluid passing through a predetermined position (a position between the second blower B2 and the cell stack 13) of the first air path La1. Thereby, it is possible to preheat the air Aa using the heat of the combustion exhaust gas Gf.

[0022] Next, an outline of the operation of the fuel cell system 1 will be described. The raw material gas Ga supplied into the first gas path Lg1 from the raw material gas intake section Pg is sent to the downstream side by the action of the first blower B1 and flows into the evaporator 12. In parallel with the supply of the raw material gas Ga, the reformed water Wa supplied from the water tank 14a into the water path Lw by the water pump W1 flows into the evaporator 12. The evaporator 12 heats the incoming reformed water Wa by heat exchange to generate water vapor (superheated steam), and mixes this with the raw material gas Ga flowing through the first gas path Lg1. The mixed gas of the raw material gas Ga and water vapor thus generated flows into the reformer 11.

[0023] The reformer 11 reforms the raw material gas Ga using the water vapor received from the evaporator 12, generates a reformed gas Gb, and sends it to the downstream side. The reformer 11 has a catalyst for steam reforming, reacts methane contained in the raw material gas Ga with water vapor, and generates a reformed gas Gb containing carbon monoxide and hydrogen. Although steam reforming is an endothermic reaction, the reformer 11 can stably generate the reformed gas Gb by the heat supply from the off-gas burner 16. The reformed gas Gb sent out from the reformer 11 flows into the fuel electrode 13a via the second gas path Lg^{2}.

[0024] Meanwhile, in parallel with the supply of the raw material gas Ga and reformed water Wa mentioned above, air Aa is supplied from the air receiving section Pa into the first air path La1. The air Aa in the first air path La1 is sent downstream by the action of the second blower B2. This air Aa is heated by heat exchange in the air preheater 15 and then flows into the air electrode 13b.

[0025] The cell stack 13 generates electricity using the reformed gas Gb flowing into the fuel electrode 13a and the air Aa flowing into the air electrode 13b, and also discharges off-gas Gc (anode off-gas) from the fuel electrode 13a. The electricity generated is supplied to the outside via a power supply line (not shown). The off-gas Gc contains fuel components such as hydrogen that were unreacted in the fuel electrode 13a. From the air electrode 13b, the remaining air Ab, which contains unreacted oxygen and other components, is discharged to the off-gas burner 16 via the fourth air line La.

[0026] The off-gas Gc discharged from the fuel electrode 13a flows through the third gas path Lg3 and into the water recovery device 14. In the water recovery device 14, the condenser 14b cools the off-gas Gc to below the dew point temperature by exchanging heat with the cooling air Ad supplied from the inlet 14d to the cooling air path 14c, thereby condensing the water vapor contained in the off-gas Gc. Due to the heat exchange in the condenser 14b, the temperature of the cooling air Ad rises and the temperature of the off-gas Gc falls. The heated cooling air Ad (heated air) then flows into the mixing unit 17.

[0027] The off-gas Gc that has passed through the condenser 14b is separated into gas and water in the water tank 14a. The condensed water is recovered in the water tank 14a, while the remaining portion that did not condense flows into the fourth gas path Lg4 as residual off-gas Gd. The residual off-gas Gd corresponds to the off-gas Gc that has undergone gas and water separation. The water recovery device 14 is located outside the adiabatic region TA to efficiently remove water vapor from the off-gas Gc.

[0028] The cooling air passage for the condenser 14b may be either closed or open. In the former case, a heat exchanger such as a plate type, shell-and-plate type, or shell-and-tube type can be used, and cooling air is supplied to the heat transfer surface by a blower. In the latter case, an air-cooled radiator can be used, and cooling air is ventilated by a fan over the surface of the heat exchange core through which the off-gas Gc flows. The radiator has the advantage of reducing the auxiliary power consumption of the fuel cell system 1 because it has low pressure loss when supplying cooling air and the airflow rate can be easily adjusted.

[0029] The condensed water collected in the water tank 14a is supplied to the evaporator 12 via the water path Lw, as described above, and reused as reformed water Wa. Furthermore, by removing water vapor from the off-gas Gc to generate residual off-gas Gd, it becomes possible to efficiently burn the unreacted fuel components in the off-gas Gc in the off-gas burner 16.

[0030] The remaining off-gas Gd that flows into the fourth gas path Lg4 flows into the off-gas burner 16. The off-gas burner 16 burns a mixture of air Ab discharged from the air electrode 13b and the remaining off-gas Gd to generate heat, and discharges the combustion exhaust gas Gf produced by the combustion into the combustion exhaust gas path Lx. The off-gas burner 16 is located near the reformer 11, and the heat generated from the off-gas burner 16 is efficiently transferred to the reformer 11 by radiant heat transfer and convective heat transfer.

[0031] As the combustion exhaust gas Gf passes through the combustion exhaust gas path Lx, it exchanges heat with air Aa as it passes through the air preheater 15. Due to the heat exchange in the air preheater 15, the temperature of the air Aa rises and the temperature of the combustion exhaust gas Gf decreases. After passing through the air preheater 15, the combustion exhaust gas Gf exchanges heat with the reformed water Wa as it passes through the evaporator 12 before flowing into the mixing section 17. The evaporator 12 heats the reformed water Wa through heat exchange with the combustion exhaust gas Gf, generating steam as described above.

[0032] As already explained, cooling air Ad and combustion exhaust gas Gf flow into the mixing unit 17. The mixing unit 17 mixes the combustion exhaust gas Gf with the cooling air Ad to produce a mixed gas, which is then discharged to the outside (into the atmosphere) of the fuel cell system 1 via the exhaust pipe Ly. This suppresses condensation of the combustion exhaust gas Gf discharged into the atmosphere, thereby minimizing corrosion of the exhaust pipe Ly and the generation of white smoke.

[0033] In other words, the combustion exhaust gas Gf contains hydrogen that was not used in the power generation reaction, and therefore contains more water vapor than the atmosphere, resulting in a higher dew point temperature. Furthermore, since the combustion exhaust gas Gf is used for heat in the air preheater 15 and evaporator 12, the temperature of the combustion exhaust gas Gf after heat utilization is lower than before heat utilization, making it more prone to condensation. If such combustion exhaust gas Gf after heat utilization is discharged into the atmosphere alone, its temperature will drop further, causing condensation, and this condensed water may corrode the exhaust piping Ly. However, in this embodiment, the dew point temperature is lowered by mixing it with cooling air Ad, making it less likely for the combustion exhaust gas Gf to condense. As a result, corrosion of the exhaust piping Ly is suppressed, and even if the humidity of the atmosphere is high, the exhaust air is less likely to become supersaturated, thus suppressing the generation of white smoke in the exhaust stack.

[0034] Furthermore, the cooling air Ad is heated by heat exchange in the condenser 14b (heat recovery from off-gas Gc), and the temperature of the heated cooling air Ad (cooling air Ad at the stage of flowing into the mixing section 17) is higher than the temperature of the combustion exhaust gas Gf (combustion exhaust gas Gf at the stage of flowing into the mixing section 17) after heat utilization. Therefore, when the combustion exhaust gas Gf and the cooling air Ad are mixed in the mixing section 17, a situation in which the temperature of the combustion exhaust gas Gf drops and condensation is promoted is avoided. From the viewpoint of making the combustion exhaust gas Gf less prone to condensation, it is preferable that the temperature of the cooling air Ad mixed with the combustion exhaust gas Gf be as high as possible.

[0035] The specific configuration of the mixing section 17 should preferably be determined considering the type of condenser 14b. For example, if the condenser 14b is a plate heat exchanger, a mixing chamber is connected to the end of the combustion exhaust gas path Lx, and cooling air is introduced into the chamber by a blower in the cooling air path 14c to counteract the low-pressure combustion exhaust gas Gf flowing towards the exhaust stack. Alternatively, if the condenser 14b is a radiator, the airflow from the fan after passing through the heat exchange core is taken into the mixing chamber, and the combustion exhaust gas Gf is released from the combustion exhaust gas path Lx to counteract the low-pressure cooling air flowing into the chamber towards the exhaust stack.

[0036] Furthermore, the heating of the cooling air Ad can be performed while maintaining the thermal independence of the fuel cell system 1 by using the waste heat within the fuel cell system 1 (in this embodiment, the heat of the off-gas Gc). However, as a heating method for generating heated air according to the present invention, in addition to the method using waste heat within the fuel cell system, it is also possible to employ, for example, a method in which a heater is driven by electricity generated and heated air is generated using the heat of the heater.

[0037] Furthermore, regarding the mixing ratio of combustion exhaust gas Gf and cooling air Ad in the mixing section 17, if the ratio of cooling air Ad to combustion exhaust gas Gf is less than 2, the partial pressure of water vapor in these mixed gases remains relatively high, which may increase the likelihood of water vapor condensation when the temperature drops. On the other hand, if the ratio of cooling air Ad to combustion exhaust gas Gf exceeds 30, the partial pressure of water vapor in the mixed gases becomes sufficiently low, but the power consumption of the blower and fan and the amount of heat released to the outside increase excessively in order to increase the flow rate of cooling air Ad, which may reduce the power generation efficiency of the fuel cell system 1. Considering these circumstances, it is desirable to set the mixing ratio within the range of 1:2 to 1:30.

[0038] As described above, the fuel cell system 1 according to this embodiment is a solid oxide type fuel cell system that utilizes the combustion exhaust gas Gf discharged from the off-gas burner 16 for thermal energy, and mixes the combustion exhaust gas Gf after thermal energy utilization with heated air and discharges it outside the system. Therefore, it is possible to suppress condensation of the combustion exhaust gas Gf discharged into the atmosphere. The fuel cell system 1 includes a condenser 14b that condenses the water vapor contained in the off-gas Gc by heat exchange with cooling air Ad, and a water tank 14a that separates the off-gas Gc into gas and water and recovers the condensed water. The off-gas burner 16 burns the remaining off-gas Gd (off-gas Gc after gas and water separation), and the cooling air Ad that has passed through the condenser 14b is used as the heated air.

[0039] Furthermore, the fuel cell system according to the present invention can also be configured as a multi-stage fuel cell system having multiple cell stacks. When the fuel cell system 1 of this embodiment is configured as a multi-stage fuel cell system, for example, the cell stack 13 can be configured as a front-stage cell stack, and a rear-stage cell stack can be provided. In this case, an example configuration can be adopted in which residual off-gas Gd flows into the fuel electrode of the rear-stage cell stack, the off-gas discharged from the fuel electrode flows into the off-gas burner 16, and air Ab flows into the air electrode of the rear-stage cell stack, and the air discharged from the air electrode flows into the off-gas burner 16.

[0040] Although embodiments of the present invention have been described above, the configuration of the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the invention. In other words, the above embodiments should be considered to be illustrative in all respects and not restrictive. The technical scope of the present invention is indicated not by the above description of embodiments, but by the claims, and should be understood to include all modifications that fall within the meaning and scope equivalent to the claims. [Industrial applicability]

[0041] This invention is applicable to fuel cell systems using solid oxide fuel cells. [Explanation of symbols]

[0042] 1. Fuel cell system 11. Modifier 12 Evaporator 13-cell stack 13a Fuel electrode 13b Air electrode 14. Water recovery device 14a Water Tank 14b Condenser 14c Cooling air path 14d Cooling air inlet 15 Air preheater 16 Off-gas burner 17 Mixing section Aa, Ab Air Ad Cooling air B1 First Blower B2 Second Blower Ga raw material gas Gb reformed gas Gc Off-gas Gd Residual Off-gas Gf combustion exhaust gas La1 First air path La2 Second air path Lg1 First Gas Path Lg2 Second Gas Path Lg3 Third Gas Path Lg4 4th gas pathway Lx Combustion Exhaust Gas Path Ly exhaust piping Lw Water Path Pa air intake section Pg raw material gas receiving section TA Insulation Area W1 Water Pump Wa (modified water)

Claims

1. A solid oxide fuel cell system that includes an off-gas burner for burning anode off-gas discharged from the fuel electrode of a cell stack, and utilizes the combustion exhaust gas for thermal purposes, An air preheater that preheats the air supplied to the air electrode of the cell stack by the combustion exhaust gas, The system includes an evaporator that heats the reformed water with the combustion exhaust gas to generate steam for use in steam reforming, A fuel cell system characterized by mixing the combustion exhaust gas, after heat utilization in the air preheater and the evaporator, with heated air generated using the waste heat within the system, and discharging the mixture outside the system.

2. A condenser for cooling and condensing the water vapor contained in the anode off gas, The system includes a water tank for recovering the condensed water separated from the anode off-gas after cooling as the reformed water, The fuel cell system according to claim 1, characterized in that the off-gas burner burns the anode off-gas after gas-water separation.

3. The condenser exchanges heat between the water vapor contained in the anode off gas and the cooling air, The fuel cell system according to claim 2, characterized in that the cooling air that has passed through the condenser is used as the heated air.

4. The temperature of the heated air is equal to or greater than the temperature of the combustion exhaust gas after heat utilization. A fuel cell system according to any one of claims 1 to 3, characterized in that the mixing ratio of the combustion exhaust gas and the heated air is within the range of 1:2 to 1:30.

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