fuel cell system

Abstract

To provide a fuel cell system capable of heating an off-gas path before the supply of reformed gas during starting operation.SOLUTION: A solid oxide type fuel cell system is equipped with an evaporator that evaporates water to generate steam, a reformer that generates reformed gas reforming raw material gas by using the steam, a front stage side cell stack that has a first fuel electrode to which the reformed gas is supplied, and generates power by using the reformed gas, and a rear stage side cell stack that has a second fuel electrode to which a first anode off gas is supplied from the first fuel electrode through a first anode off gas path, and generates power by using the first anode off gas. As starting operation from a cold state, water is supplied to the evaporator to generate steam, and the steam is supplied to the reformer without supplying raw material gas, thereby performing a first circulation operation sequentially circulating the steam in the front stage side cell stack, the first anode off gas path, and the rear stage side cell stack in this order.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] Various types of fuel cells have been developed as an efficient and environmentally friendly power source. In particular, solid oxide fuel cells (SOFCs) have a high power generation efficiency of over 50%, and are therefore used in fuel cell systems with a wide range of outputs, from household to industrial use.

[0003] Meanwhile, aiming for a power generation efficiency of over 60%, technological developments are also underway to improve fuel utilization rates, such as by increasing the number of cell stack stages and recirculating anode off-gas (sometimes abbreviated as "off-gas" in this application). For example, Patent Document 1 discloses an example of a multi-stage fuel cell system in which solid oxide fuel cell cell stacks are arranged in multiple stages and off-gas discharged from a preceding cell stack is supplied to a succeeding cell stack. Here, for convenience, the path of off-gas from a preceding cell stack to a succeeding cell stack will be referred to as the "off-gas path."

[0004] The off-gas path of the multi-stage fuel cell system disclosed in Patent Document 1 and the like is configured to extract the off-gas from the upstream side during power generation operation to the outside of the power generation module, which is an insulated region, so that it can be recycled as fuel for the downstream side, and water vapor is removed from the off-gas in the non-insulated region. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-138573 Summary of the Invention [Problem to be solved by the invention]

[0006] In fuel cell systems that use source gases such as city gas, the source gas is reformed using steam to produce reformed gas, which is then used in the power generation reaction. The operating temperature of a high-temperature SOFC cell stack during power generation is 500-700°C. Therefore, when transitioning the power generation module from a cold state to a warm state where power can be generated, a startup operation is performed to supply reducing reformed gas to protect the cell stack while raising the temperature of each part within the power generation module to an appropriate temperature for power generation. In a multi-stage fuel cell system, as part of this startup operation, reformed gas may be generated and supplied to a cell stack in a front stage, and the reformed gas that has passed through the front stage may also be supplied to a cell stack in a rear stage via an off-gas path.

[0007] However, if there are low-temperature points in the reformed gas path and water vapor condenses at those points, there is a risk of carbon deposition (coking) occurring due to a change in the equilibrium composition of the reformed gas. Carbon deposition not only leads to blockage of the gas flow path but can also degrade the catalytic activity of the electrode material. Therefore, in order to prevent carbon deposition, it is necessary to operate the system during startup so as not to change the equilibrium composition of the reformed gas. As mentioned above, since the off-gas path is located in a non-adiabatic region, it is difficult for the temperature to rise during startup. Therefore, it is important to preheat the off-gas path before starting the supply of reformed gas.

[0008] SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a fuel cell system that can warm up an off-gas path prior to supplying reformed gas during startup operation. [Means for solving the problem]

[0009] The fuel cell system of the present invention is a solid oxide fuel cell system including an evaporator that evaporates water to generate water vapor, a reformer that uses the water vapor to generate reformed gas by reforming a feed gas, an upstream cell stack having a first fuel electrode to which the reformed gas is supplied and that generates power using the reformed gas, and a downstream cell stack having a second fuel electrode to which the first anode off-gas is supplied from the first fuel electrode via a first anode off-gas path and that generates power using the first anode off-gas, and is configured to perform a first flow operation as a startup operation from a cold state, in which water is supplied to the evaporator to generate water vapor and the water vapor is supplied to the reformer without supplying feed gas, thereby flowing the water vapor through the upstream cell stack, the first anode off-gas path, and the downstream cell stack in that order. This configuration allows the anode off-gas path to be warmed prior to the supply of reformed gas during startup.

[0010] More specifically, the above configuration may be configured to monitor the temperature of the first anode off-gas passage during the first circulation operation, and start supplying the raw material gas to the reformer when the temperature satisfies a predetermined reference condition. With this configuration, it is possible to supply the raw material gas after the first anode off-gas passage has been appropriately warmed.

[0011] More specifically, the above configuration may be configured to monitor the temperature of the steam flowing through the first anode off-gas passage during the first circulation operation, and start supplying the raw material gas to the reformer when the temperature satisfies a predetermined reference condition. With this configuration, it is possible to supply the raw material gas after the first anode off-gas passage has been appropriately warmed.

[0012] More specifically, the above configuration may be configured such that the supply flow rate of water to the evaporator after the start of supply of the raw material gas is adjusted so that the molar ratio S / C is 2 to 15, where C is a set value for the molar flow rate of carbon contained in the raw material gas supplied to the reformer and S is a set value for the molar flow rate of water vapor supplied to the reformer, and the supply flow rate of water to the evaporator during the first circulation operation is adjusted so that water vapor having the molar ratio S / C of 2 to 15 is supplied to the reformer, assuming that the raw material gas is supplied to the reformer at the set value C. Furthermore, the supply flow rate of water to the evaporator during the first circulation operation is preferably set to a value greater than the supply flow rate of water to the evaporator after the start of supply of the raw material gas.

[0013] More specifically, the above configuration may include an off-gas burner that burns the second anode off-gas discharged from the second fuel electrode, and the evaporator evaporates water by heat exchange with the combustion exhaust gas produced by burning the second anode off-gas, and the startup operation may involve generating an alternative gas to replace the combustion exhaust gas without using the second anode off-gas, and performing a second circulation operation to circulate the alternative gas through the evaporator.

[0014] Furthermore, more specifically, the above configuration may include a heat exchanger that exchanges heat between the combustion exhaust gas and air supplied to the air electrode of at least one of the front-stage cell stack and the rear-stage cell stack, and the second circulation operation may be an operation of circulating the alternative gas through the heat exchanger as well.

[0015] Furthermore, more specifically, the above configuration may be configured to include a startup burner that combusts the raw material gas, and the second circulation operation may be configured to circulate the alternative gas produced by burning the raw material gas with the startup burner to the evaporator. [Effects of the Invention]

[0016] According to the fuel cell system of the present invention, it is possible to warm up the off-gas passage prior to supplying the reformed gas during startup operation. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a schematic configuration diagram of a fuel cell system according to an embodiment of the present invention; [Figure 2] 10 is a flowchart relating to an operation in a temperature increase mode. DETAILED DESCRIPTION OF THE INVENTION

[0018] An embodiment of the present invention will be described below with reference to the accompanying drawings. Fig. 1 is a schematic diagram of a fuel cell system 1 according to this embodiment. In Fig. 1, with regard to fluid paths, paths related to fuel gas (gas paths) are indicated by solid arrows, paths related to reforming water (water paths) are indicated by dashed arrows, paths related to air (air paths) are indicated by dotted arrows, and other paths are indicated by dashed arrows. Furthermore, the outline arrows in Fig. 1 indicate fluids passing through each path.

[0019] 1, the fuel cell system 1 includes a reformer 11, an evaporator 12, a front-stage cell stack 13, a water recovery device 14, a rear-stage cell stack 15, an off-gas burner 16a, and a startup burner 16b. In this way, the fuel cell system 1 is a multi-stage fuel cell system including the front-stage cell stack 13 and the rear-stage cell stack 15.

[0020] These cell stacks 13 and 15 are both high-temperature solid oxide fuel cells (SOFCs), and their operating temperature during power generation is approximately 500 to 700° C. A solid oxide fuel cell generates power 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).

[0021] For ease of explanation, in this embodiment, the anode and cathode of the front-stage cell stack 13 are referred to as the first anode 13a and the first cathode 13b. On the other hand, the anode and cathode of the rear-stage cell stack 15 are referred to as the second anode 15a and the second cathode 15b. In this embodiment, as an example, only one front-stage cell stack 13 and one rear-stage cell stack 15 are provided, but a plurality of either or both of these may be provided in parallel.

[0022] Most of the elements of the fuel cell system 1, except for the water recovery device 14, are located within the heat insulating area TA shown in Figure 1. The heat insulating area TA is covered with heat insulating material, etc., and consideration is given to minimizing heat release to the outside. This gives the fuel cell system 1 an advantageous configuration in terms of thermal self-sustainability.

[0023] The fuel cell system 1 is provided with a raw material gas receiving section Pg that receives raw material gas (city gas containing methane as an example in this embodiment) and an air receiving section Pa that receives air. The raw material gas and air are basically supplied from outside the fuel cell system 1.

[0024] The raw material gas receiving section Pg is connected to the reformer 11 via a first gas path Lg1, and a first blower B1 is provided midway along the first gas path Lg1. At least a portion of the first gas path Lg1 downstream of the first blower B1 is arranged to pass through the evaporator 12. A sixth gas path Lg6 branches off from a branch point α located between the first blower B1 and the evaporator 12 on the first gas path Lg1 and is connected to a startup burner 16b.

[0025] The reformer 11 is connected to the first fuel electrode 13a via a second gas path Lg2. The first 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, and a cooling air path 14c, and is connected to the second fuel electrode 15a via a fourth gas path Lg4.

[0026] 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 the water in the water tank 14a can be continuously supplied to the evaporator 12 as reforming water Wa. This allows the fuel cell system 1 to achieve water independence. The water pump W1 can adjust the amount of reforming water Wa supplied from the water tank 14a to the evaporator 12 (water path Lw).

[0027] The second fuel electrode 15a is connected to the off-gas burner 16a via a fifth gas path Lg5. A first air path La1 extends from the air receiving section Pa, and a second blower B2 is provided midway along the first air path La1. At a branch point β located downstream of the second blower B2, the first air path La1 branches into a second air path La2 and a third air path La3. The second air path La2 is connected to the first air electrode 13b, and the third air path La3 is connected to the startup burner 16b.

[0028] As will be described in detail later, the startup burner 16b serves to combust the raw material gas supplied during startup operation to generate an alternative gas Gx that replaces the combustion exhaust gas Gf. A combustion exhaust gas path Lx extends from the off-gas burner 16a so as to pass through the evaporator 12. A path for the alternative gas Gx extending from the startup burner 16b is connected to a connection point γ on the combustion exhaust gas path Lx upstream of the evaporator 12.

[0029] The first air electrode 13b is connected to the second air electrode 15b via a fourth air path La4, and the second air electrode 15b is connected to the off-gas burner 16a via a fifth air path La5.

[0030] Furthermore, a first heat exchanger H1 and a second heat exchanger H2 are provided in the fuel cell system 1. The first heat exchanger H1 is a self-reheating heat exchanger that allows heat exchange to occur between a fluid passing through a predetermined position in the third gas path Lg3 (a position between the front-side cell stack 13 and the water recovery device 14) and a fluid passing through a predetermined position in the fourth gas path Lg4 (a position between the water recovery device 14 and the rear-side cell stack 15).

[0031] The second heat exchanger H2 is an air preheater that allows heat exchange to occur between a fluid passing through a predetermined position in the combustion exhaust gas path Lx (a position between the evaporator 12 and the connection point γ) and a fluid passing through a predetermined position in the first air path La1 (a position between the second blower B2 and the branch point β).

[0032] In the fuel cell system 1, the branch point α is provided with a device (such as a flow rate adjusting valve) that can adjust the flow rate of the raw material gas Ga at each branch destination (the latter portion of the first gas path Lg1 and the sixth gas path Lg6). This makes it possible to change the setting of the state of the branch point α (the flow rate of the raw material gas Ga at each branch destination). Furthermore, the branch point β is provided with a device (such as a flow rate adjusting valve) that can adjust the flow rate of the air Aa at each branch destination (the second air path La2 and the third air path La3). This makes it possible to change the setting of the state of the branch point β (the flow rate of the air Aa at each branch destination). In addition, the rotation speeds of the first blower B1 and the second blower B2 may be increased or decreased when adjusting the flow rates of the raw material gas Ga and / or the air Aa.

[0033] Focusing on the operating mode of the fuel cell system 1, the fuel cell system 1 transitions from a cold state during shutdown to a temperature rise mode in which a predetermined startup operation is performed, and then to a normal power generation mode (a state in which each cell stack 13, 15 generates power and supplies it to the outside). Below, the operation of the fuel cell system 1 in the power generation mode and the temperature rise mode will be explained in order.

[0034] First, we will explain the operation in the power generation mode of the fuel cell system 1. In this power generation mode, the state of the branch point α is set so that all of the raw material gas Ga passing through the branch point α flows into the first gas path Lg1 (i.e., flows toward the reformer 11). Furthermore, in this power generation mode, the state of the branch point β is set so that all of the air Aa passing through the branch point β flows into the second air path La2 (i.e., flows toward the first air electrode 13b).

[0035] The raw material gas Ga supplied from the raw material gas receiving section Pg into the first gas path Lg1 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 reforming 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 inflowing reforming water Wa by heat exchange to generate steam (superheated steam), and mixes this with the raw material gas Ga flowing through the first gas path Lg1. The mixed gas of raw material gas Ga and steam generated in this way flows into the reformer 11.

[0036] The supply flow rate of the reforming water Wa to the evaporator 12 is adjusted by the water pump W1 so that the reformed gas is appropriately generated in the reformer 11. More specifically, the supply flow rate is adjusted so that the molar ratio S / C falls within a range of 2 to 3, where C is the set value for the molar flow rate of carbon contained in the raw material gas Ga supplied to the reformer 11 via the first gas path Lg1, and S is the set value for the molar flow rate of water vapor supplied to the reformer 11. The supply flow rate of the raw material gas Ga is also adjusted according to the power generation amount of each of the cell stacks 13, 15.

[0037] The reformer 11 reforms the raw material gas Ga using steam received from the evaporator 12 to generate a reformed gas Gb, which is then sent to the downstream side. The reformer 11 has a catalyst for steam reforming, and reacts methane contained in the raw material gas Ga with steam to generate a reformed gas Gb containing carbon monoxide and hydrogen. Although steam reforming is an endothermic reaction, the reformer 11 is able to stably generate the reformed gas Gb thanks to the heat supply from the off-gas burner 16a. The reformed gas Gb sent from the reformer 11 flows into the first fuel electrode 13a via the second gas path Lg2.

[0038] Meanwhile, in parallel with the supply of the raw material gas Ga and the reforming water Wa, 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 to the downstream side by the action of the second blower B2. After being heated by heat exchange in the second heat exchanger H2, this air Aa proceeds to the second air path La2.

[0039] The air Aa flowing through the second air path La2 flows into the first air electrode 13b. The upstream cell stack 13 generates electricity using the reformed gas Gb that has flowed into the first fuel electrode 13a and the air Aa that has flowed into the first fuel electrode 13b, and discharges a first off-gas Gc (anode off-gas) from the first fuel electrode 13a. The power generated by this power generation is supplied to the outside via a power supply line (not shown). The first off-gas Gc contains fuel components such as hydrogen that have not reacted at the first fuel electrode 13a. The remaining air Ab, which contains oxygen and other components that have not reacted at the first air electrode 13b, is discharged from the first air electrode 13b to the second air electrode 15b via the fourth air line La.

[0040] The first off-gas Gc discharged from the first fuel electrode 13a passes through the third gas path Lg3 and the first heat exchanger H1 before flowing into the water recovery device 14. The first off-gas Gc undergoes heat exchange as it passes through the first heat exchanger H1. The first heat exchanger H1 heats the regenerated off-gas Gd, which will be described later, by exchanging heat with the first off-gas Gc. Due to the heat exchange in the first heat exchanger H1, the temperature of the regenerated off-gas Gd increases and the temperature of the first off-gas Gc decreases.

[0041] In the water recovery device 14, the condenser 14b cools the first off-gas Gc to a temperature equal to or lower than the dew point temperature by exchanging heat with cooling air Ad supplied from the outside to a cooling air path 14c, thereby condensing the water vapor contained in the first off-gas Gc. Due to the heat exchange in the condenser 14b, the temperature of the cooling air Ad increases and the temperature of the first off-gas Gc decreases.

[0042] The first off-gas Gc that has passed through the condenser 14b is separated into water and steam in the water tank 14a, and the condensed water is collected in the water tank 14a, while the remaining uncondensed portion flows into the fourth gas line Lg4 as regenerated off-gas Gd. The regenerated off-gas Gd corresponds to the first off-gas Gc that has been subjected to the water-steam separation process. The water recovery device 14 is located outside the insulating area TA so that water vapor can be efficiently removed from the first off-gas Gc.

[0043] The flow path for the cooling air Ad in the condenser 14b may be either a sealed type or an open type. In the former case, a plate-type, shell-and-plate-type, or shell-and-tube-type heat exchanger may be used, and a blower may be used to send the cooling air Ad to the heat transfer surface. In the latter case, an air-cooled radiator may be used, and a fan may be used to ventilate the cooling air Ad over the surface of the heat exchange core through which the first off-gas Gc flows. Note that a radiator has the advantage of reducing the power consumption of the fuel cell system 1's accessories, since it has low pressure loss when supplying the cooling air Ad and allows for easy adjustment of the air flow rate.

[0044] The condensed water collected in the water tank 14a is supplied to the evaporator 12 via the water path Lw as described above, and is reused as reforming water Wa. Note that by removing water vapor from the first off-gas Gc to generate the regenerated off-gas Gd, it becomes possible to efficiently utilize the unreacted fuel components in the first off-gas Gc in the downstream cell stack 15.

[0045] The regenerated off-gas Gd that flows into the fourth gas line Lg4 is heated by heat exchange in the first heat exchanger H1 and then flows into the second fuel electrode 15a. In parallel with this, air Ab that passes through the aforementioned fourth air path La4 flows into the second air electrode 15b. The downstream cell stack 15 generates power using the regenerated off-gas Gd that flows into the second fuel electrode 15a and the air Ab that flows into the second air electrode 15b, and discharges the second off-gas Ge (anode off-gas) from the second fuel electrode 15a. The power generated by this power generation is supplied to the outside via a power supply line (not shown).

[0046] The second off-gas Ge contains fuel components that did not react at the second fuel electrode 15a. The remaining air Ac, which contains oxygen and other components that did not react at the second air electrode 15b, is discharged from the second air electrode 15b. The second off-gas Ge flows into the off-gas burner 16a via the fifth gas path Lg5, and the air Ac discharged from the second air electrode 15b flows into the off-gas burner 16a via the fifth air path La5.

[0047] The off-gas burner 16a burns a gas mixture of air Ac and second off-gas Ge discharged from the downstream cell stack 15 to generate heat, and discharges the combustion exhaust gas Gf generated by the combustion into the combustion exhaust gas path Lx. The off-gas burner 16a is disposed near the reformer 11 so that the heat generated from the off-gas burner 16a is efficiently transferred to the reformer 11 by radiative heat transfer and convective heat transfer.

[0048] The combustion exhaust gas Gf passing through the combustion exhaust gas path Lx exchanges heat with the air Aa when passing through the second heat exchanger H2. Due to the heat exchange in the second heat exchanger H2, the temperature of the air Aa increases and the temperature of the combustion exhaust gas Gf decreases.

[0049] After passing through the second heat exchanger H2, the combustion exhaust gas Gf exchanges heat with the reforming water Wa when passing through the evaporator 12, and is then discharged to the outside of the fuel cell system 1. The evaporator 12 heats the reforming water Wa through heat exchange with the combustion exhaust gas Gf, thereby generating steam as described above.

[0050] Next, the operation of the fuel cell system 1 in the temperature rise mode will be described with reference to the flowchart shown in Fig. 4. In the initial state of this temperature rise mode, the state of the branch point α is set so that all of the raw material gas Ga passing through the branch point α flows into the sixth gas path Lg6 (i.e., so that it flows toward the startup burner 16b). Furthermore, in the initial state of the temperature rise mode, the state of the branch point β is set so that all of the air Aa passing through the branch point β flows into the third air path La3 (i.e., so that it flows toward the startup burner 16b).

[0051] In the temperature increase mode, a series of startup operations, steps S1 to S8 shown in FIG. 2, are executed. During the startup operation, the supply of cooling air Ad to the condenser 14b is stopped. First, the fuel cell system 1 activates the startup burner 16b (step S1). The startup burner 16b receives the raw material gas Ga from the raw material gas receiving section Pg via the sixth gas path Lg6 and the air Aa from the air receiving section Pa via the third air path La3, and combusts the raw material gas Ga using the air Aa. The high-temperature gas generated by the combustion in the startup burner 16b is discharged to the combustion exhaust gas path Lx as a high-temperature substitute gas Gx that replaces the combustion exhaust gas Gf.

[0052] Since the alternative gas Gx passes through the combustion exhaust gas path Lx, the temperature of the evaporator 12 provided in this path gradually increases. Furthermore, since the alternative gas Gx also passes through the second heat exchanger H2 provided in the combustion exhaust gas path Lx, it can also be used for heat exchange in the second heat exchanger H2. As described above, in this embodiment, a startup operation (corresponding to the second circulation operation according to the present invention) is performed in which the alternative gas Gx is generated without using the second off-gas Ge and this alternative gas Gx is circulated through the evaporator 12 and the second heat exchanger H2. The amount of the alternative gas Gx produced in the startup burner 16b is adjusted to gradually decrease as the temperature of each part of the fuel cell system 1 increases.

[0053] Furthermore, the fuel cell system 1 starts supplying air Aa from the air receiving section Pa to the first air electrode 13b (step S2). That is, the fuel cell system 1 sets the state of the branch point β so that at least a portion of the air Aa passing through the branch point β flows into the second air path La2 (i.e., so that it flows toward the first air electrode 13b). This air Aa is heated using the heat of the alternative gas Gx as it passes through the second heat exchanger H2 provided in the first air path La1, and is then supplied to the front-stage cell stack 13 via the second air path La2 and further to the rear-stage cell stack 15 via the fourth air path La4. As the heated air Aa is supplied, the temperatures of the cell stacks 13, 15 gradually rise.

[0054] While increasing the temperatures of the evaporator 12 and each of the cell stacks 13, 15 in this manner, the fuel cell system 1 monitors the timing when the temperatures of the evaporator 12 and each of the cell stacks 13, 15 reach a specified temperature (step S3). The specified temperature is set assuming the temperature when the evaporator 12 and each of the cell stacks 13, 15 are appropriately warmed, and may be set to different temperatures for the evaporator 12 and each of the cell stacks 13, 15.

[0055] When these temperatures reach the specified temperatures (Yes in step S3), the fuel cell system 1 starts supplying the reforming water Wa to the water path Lw at a first set flow rate V1 (step S4). This first set flow rate V1 will be described in detail later.

[0056] The reforming water Wa supplied to the water path Lw is heated in the evaporator 12 to become water vapor (superheated steam), which flows into the reformer 11 and then flows sequentially through the second gas path Lg2, the front-stage cell stack 13, the third gas path Lg3, the water recovery device 14, and the fourth gas path Lg4 into the rear-stage cell stack 15. As a result, the water vapor path, including the first off-gas path (the series of paths from the third gas path Lg3 to the fourth gas path Lg4), is gradually heated, and the temperature of the condenser 14b provided in the first off-gas path also rises. At this point, since the raw material gas Ga has not yet been supplied to the reformer 11, no reformed gas is generated, but since water vapor is a reducing gas, each of the cell stacks 13 and 15 is protected by the flow of water vapor.

[0057] While increasing the temperature of the first off-gas path in this manner, the fuel cell system 1 monitors the timing when the temperature of the first off-gas path reaches a predetermined temperature (step S5). The predetermined temperature is set based on the temperature when the first off-gas path is adequately warmed. The temperature of the first off-gas path is preferably monitored for the condenser 14b, which has a large heat transfer surface and is prone to water condensation due to heat dissipation. For example, this is achieved by measuring the heat transfer surface temperature on the low-temperature side (the cooling air flow path side). Furthermore, if it is difficult to attach a temperature sensor to the condenser 14b, this may be achieved by measuring the outer wall temperature of the off-gas piping near the condenser 14b. When the temperature of the first off-gas path reaches the predetermined temperature (Yes in step S5), the fuel cell system 1 starts supplying the raw material gas Ga from the raw material gas receiving unit Pg to the reformer 11 (step S6). That is, the fuel cell system 1 sets the state of the branch point α so that at least a portion of the raw material gas Ga passing through the branch point α flows into the first gas path Lg1 (i.e., flows toward the reformer 11).

[0058] This starts the generation of reformed gas in the reformer 11, and the reformed gas flows into the rear-side cell stack 15 via the second gas path Lg2, the front-side cell stack 13, the third gas path Lg3, the water recovery device 14, and the fourth gas path Lg4 in this order. As a result, a reformed gas that is more reducing than water vapor is supplied to each of the cell stacks 13, 15, thereby protecting each of the cell stacks 13, 15. Note that, at the stage where the operation of step S6 is performed, the amount of air Aa supplied from the air receiving part Pa to the first air electrode 13b may be increased.

[0059] Furthermore, the fuel cell system 1 changes the supply amount of the reforming water Wa to the water path Lw to a second set flow rate V2 (step S7). This second set flow rate V2 will be described in detail later. Thereafter, the fuel cell system 1 monitors the timing at which a predetermined power generation start condition is satisfied (step S8).

[0060] This power generation start condition is a condition under which each cell stack 13, 15 can generate power stably, and is set, for example, to the temperature of each part of the fuel cell system 1 exceeding a predetermined temperature. When the power generation start condition is satisfied (Yes in step S8), the operating mode of the fuel cell system 1 shifts from the temperature increase mode to the power generation mode, and the operation of the power generation mode already described is executed.

[0061] The startup operation described above can raise the temperature of each part in the power generation module provided in the fuel cell system 1 to an appropriate temperature for power generation, and can transition the power generation module from a cold state to a warm state in which power can be generated. Furthermore, by supplying the raw material gas Ga to the raw material gas receiving part Pg (step S6), it is possible to supply a reformed gas with strong reducing properties and protect each of the cell stacks 13, 15.

[0062] Prior to the operation of step S6, the fuel cell system 1 performs the operation of step S4 of starting the supply of reforming water Wa to the water path Lw, thereby circulating water vapor. That is, in this embodiment, the reforming water Wa is supplied to the evaporator 12 to generate water vapor, and the water vapor is supplied to the reformer 11 without supplying raw material gas, thereby performing a startup operation (corresponding to a first circulation operation according to the present invention) in which the water vapor is circulated through the front-stage cell stack 13, the third gas path Lg3, the water recovery device 14, the fourth gas path Lg4, and the rear-stage cell stack 15 in this order.

[0063] This prevents carbon deposition (coking) in the reformed gas path. In other words, if the operation of step S4 were omitted, there is a risk that the reformed gas would flow through a low-temperature area in the reformed gas path. This would cause water vapor to condense at that location, potentially causing a change in the equilibrium composition of the reformed gas, resulting in carbon deposition. In particular, the area near the water recovery device 14, located outside the thermal insulation area TA, is less likely to heat up during startup, making it more susceptible to carbon deposition. Carbon deposition not only clogs the gas flow path, but can also degrade the catalytic activity of the electrode material. However, in this embodiment, the operation of step S4 is performed, and water vapor, rather than reformed gas, is circulated. This preheats the reformed gas path, including the area near the water recovery device 14, thereby avoiding a change in the equilibrium composition during reformed gas supply and preventing carbon deposition.

[0064] The first set flow rate V1 described above corresponds to the supply flow rate of the reforming water Wa to the evaporator 12 before the supply of the raw material gas Ga starts (step S6). On the other hand, the second set flow rate V2 described above corresponds to the supply flow rate of the reforming water Wa to the evaporator 12 after the supply of the raw material gas Ga starts (step S6).

[0065] The second set flow rate V2 is set so that the molar ratio S / C is in the range of 2 to 15, where C is the set value of the molar flow rate of carbon contained in the raw material gas Ga supplied to the reformer 11, and S is the set value of the molar flow rate of water vapor supplied to the reformer 11. In this way, the second set flow rate V2 is set so that the molar ratio S / C is equivalent to that in the power generation mode described above. This makes it possible to match the composition of the reformed gas to the composition in the power generation mode. Note that when the reforming water Wa is supplied at the second set flow rate V2, the supply flow rate of the raw material gas Ga (amount related to the set value C) is set so that it is equivalent to the supply flow rate at the minimum output in the power generation mode (for example, 40% of the rated current).

[0066] On the other hand, assuming that the raw material gas Ga is supplied to the reformer 11 at the set value C (that is, the raw material gas Ga is supplied so that the molar flow rate of carbon becomes the set value C), the first set flow rate V1 is set so that the molar ratio S / C described above falls within a range of 2 to 15. That is, before and after the start of the supply of the raw material gas Ga, the first set flow rate V1 and the second set flow rate V2 are set within the same range of the molar ratio S / C.

[0067] When the second set flow rate V2 is set under the condition that the molar ratio S / C is relatively large (for example, 8 or more), the first set flow rate V1 can be set to the same value as the second set flow rate V2. Under this condition, the flow rate of water vapor at the minimum output in the power generation mode is sufficiently large, so that the time required for heating the first off-gas path by the water vapor is short.

[0068] Conversely, when the second set flow rate V2 is set under conditions where the molar ratio S / C is relatively small (for example, less than 8), it is preferable to set the first set flow rate V1 to a value larger than the second set flow rate V2 (for example, about twice as large). Under these conditions, the flow rate of steam at the minimum output in the power generation mode is relatively small, so by generating steam in excess of this flow rate, it is possible to shorten the time required to heat the first off-gas path with the steam.

[0069] If the first set flow rate V1 is set so that the molar ratio S / C is less than 2, it will take a long time (for example, more than one hour) to heat up the first off-gas passage, which could result in a long time before power generation becomes possible. On the other hand, if the first set flow rate V1 is set so that the molar ratio S / C is greater than 15, the time required to heat up the first off-gas passage will be significantly reduced, but this will require the installation of over-specified equipment (such as a water pump that increases the maximum flow rate of the reforming water or an evaporator that can heat the reforming water more strongly) to increase the amount of steam generated. Furthermore, the alternative gas flow rate of the startup burner 16b will also need to be increased, which will require the startup burner 16b to be larger and will result in unnecessary consumption of raw material gas. Taking these factors into consideration, the first set flow rate V1 is set so that steam having a molar ratio S / C in the range of 2 to 15 is supplied to the reformer 11.

[0070] In this embodiment, the timing to start supplying the raw material gas Ga to the reformer 11 is determined based on the temperature of the first anode off-gas path when the first circulation operation is performed, but it may also be determined based on the temperature of the steam circulating through the first anode off-gas path. By monitoring the temperature of the steam, condensation of the steam in the first anode off-gas path can be avoided, thereby preventing local carbon deposition.

[0071] In addition, in this embodiment, the raw material gas and air are supplied to the startup burner 16b in such a manner that the raw material gas is supplied from the raw material gas receiving section Pg via branch point α, and the air is supplied from the air receiving section Pa via branch point β, but other methods may also be used. For example, both or either the raw material gas and the air may be supplied directly to the startup burner 16b from outside the fuel cell system 1 via a receiving section and a blower provided separately from the raw material gas receiving section Pg or the air receiving section Pa, without passing through these sections.

[0072] Although the 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 are illustrative in all respects and should be considered not to be limiting. The technical scope of the present invention is defined by the claims, not by the description of the above embodiments, and should be understood to include all modifications that fall within the meaning and scope of the claims. [Industrial Applicability]

[0073] The present invention can be used in a fuel cell system using a solid oxide fuel cell. [Explanation of symbols]

[0074] 1. Fuel cell system 11 Reformer 12 Evaporator 13 Front-stage cell stack 13a 1st fuel electrode 13b First air electrode 14 Water recovery device 14a Water tank 14b Condenser 14c Cooling air path 15 Rear-stage cell stack 15a 2nd fuel electrode 15b Second air electrode 16a Offgas burner 16b Start-up Burner Aa, Ab, Ac air Ad Cooling air B1 No. 1 blower B2 Second blower Ga source gas Gb reformed gas Gc No. 1 off-gas Gd Regenerated Offgas Ge No. 2 Offgas Gf combustion exhaust gas H1 1st heat exchanger H2 2nd heat exchanger La1 First air path La2 Second air path La3 Third air path La4 4th air path La5 5th air path Lg1 First gas path Lg2 Second gas path Lg3 Third gas path Lg4 4th gas path Lg5 5th gas route Lg6 6th gas route Lx Combustion exhaust gas route Lw water path Pa Air receiving section Pg Raw material gas receiving section TA Insulation Area W1 Water Pump

Claims

1. A solid oxide fuel cell system, comprising: an evaporator that evaporates water to generate water vapor; a reformer for generating a reformed gas by reforming a raw material gas using the water vapor; a front-stage cell stack having a first fuel electrode to which the reformed gas is supplied and which generates power using the reformed gas; a downstream cell stack having a second fuel electrode to which a first anode off-gas is supplied from the first fuel electrode via a first anode off-gas path, the downstream cell stack generating power using the first anode off-gas; When starting from a cold state, a first flow operation in which water is supplied to the evaporator to generate steam, and the steam is supplied to the reformer without supplying raw material gas, thereby causing the steam to flow through the upstream cell stack, the first anode off-gas path, and the downstream cell stack in that order.

2. monitor the temperature of the first anode off-gas path during the first circulation operation; 2. The fuel cell system according to claim 1, wherein the supply of raw material gas to the reformer is started when the temperature satisfies a predetermined reference condition.

3. monitor a temperature of the water vapor flowing through the first anode off-gas path during the execution of the first flow operation; 2. The fuel cell system according to claim 1, wherein the supply of raw material gas to the reformer is started when the temperature satisfies a predetermined reference condition.

4. a supply flow rate of water to the evaporator after the start of supply of the raw material gas is adjusted so that a molar ratio S / C falls within a range of 2 to 15, where C is a set value of a molar flow rate of carbon contained in the raw material gas supplied to the reformer, and S is a set value of a molar flow rate of water vapor supplied to the reformer; The fuel cell system according to claim 2 or 3, characterized in that the supply flow rate of water to the evaporator during the execution of the first circulation operation is adjusted so that, assuming that the raw material gas is supplied to the reformer at the set value C, water vapor having a molar ratio S / C within a range of 2 to 15 is supplied to the reformer.

5. 5. The fuel cell system according to claim 4, wherein the flow rate of water supplied to the evaporator during the first circulation operation is set to a value greater than the flow rate of water supplied to the evaporator after the supply of the raw material gas starts.

6. an off-gas burner that combusts a second anode off-gas discharged from the second fuel electrode; the evaporator evaporates water by heat exchange with combustion exhaust gas produced by burning the second anode off-gas, As the startup operation, 6. The fuel cell system according to claim 1, wherein a second circulation operation is performed in which an alternative gas that replaces the combustion exhaust gas is generated without using the second anode off gas, and the alternative gas is circulated through the evaporator.

7. a heat exchanger that exchanges heat between the combustion exhaust gas and air supplied to an air electrode of at least one of the front-stage cell stack and the rear-stage cell stack; 7. The fuel cell system according to claim 6, wherein the second circulation operation is an operation of circulating the alternative gas also through the heat exchanger.

8. a startup burner that burns the raw material gas; 8. The fuel cell system according to claim 6, wherein the second flow operation comprises burning the raw material gas with the startup burner and flowing the substitute gas generated through the evaporator.

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