JP7409971B2 - Solid oxide fuel cell system - Google Patents

Description

The present invention relates to a solid oxide fuel cell system equipped with a cell stack that generates power through an electrochemical reaction between reformed fuel gas, which is obtained by reforming fuel gas, and oxidant gas.

A solid oxide fuel cell system is known in which a solid oxide cell stack using a solid electrolyte as a membrane that conducts oxide ions is housed in a storage container. In this solid oxide fuel cell system, a cell stack is configured by stacking a plurality of fuel cells, and a fuel electrode for oxidizing fuel gas is provided on one side of a solid electrolyte in each fuel cell, An oxygen electrode for reducing oxygen in the air (oxidant gas) is provided on the other side. The operating temperature of the fuel cells in this solid oxide fuel cell system is as high as approximately 700 to 900°C, and at such high temperatures, hydrogen, carbon monoxide, and hydrocarbons in the fuel gas (reformed fuel gas) Electricity is generated through an electrochemical reaction with oxygen in the air.

Such a solid oxide fuel cell system includes a reformer for steam reforming fuel gas (raw fuel gas), and electricity for reformed fuel gas and oxidant gas reformed in the reformer. A cell stack that generates electricity through a chemical reaction , an air supply means for supplying air as an oxidant gas to the oxygen electrode (air electrode) side of the cell stack, and supplying fuel gas (raw fuel gas) to the reformer. A system has been proposed in which the cell stack and the reformer are housed in a high-temperature space where the cell stack and the reformer are kept at a high temperature (see, for example, Patent Document 1).

In this solid oxide fuel cell system, a combustion zone (combustion space) is provided above the cell stack, and a reformer is disposed above the combustion zone. Then, reformed fuel gas from the reformer is fed to the fuel electrode side of the cell stack, air from the air supply means is fed to the air electrode side of the cell stack, and electricity is generated through an electrochemical reaction in this cell stack. will be held. Fuel off-gas from the fuel electrode side of the cell stack (i.e., anode off-gas) and air off-gas from the air electrode side (i.e., cathode off-gas) are fed to the combustion zone and burned, and this combustion heat is used to generate a high-temperature space. is maintained at a high temperature, and the reformer etc. are heated.

A solid oxide fuel cell system has also been proposed that includes a dedicated combustor instead of providing a combustion zone above the cell stack (see, for example, Patent Document 2). In this solid oxide fuel cell system, fuel off-gas (anode off-gas) from the fuel electrode side of the cell stack is sent to the combustor through the fuel off-gas supply channel, and air off-gas from the air electrode side of the cell stack is (Cathode off-gas) is sent to the combustor through the air off-gas supply flow path, and in this combustor, the fuel off-gas is combusted by the air off-gas, and this combustion heat is used to maintain the high-temperature space in a high-temperature state. The reformer etc. are heated.

In recent years, among such solid oxide fuel cell systems, systems with improved power generation efficiency have been proposed (for example, see Patent Document 3). In this solid oxide fuel cell system, fuel off-gas (anode off-gas) from the cell stack is extracted from the high-temperature space and sent to the heat exchanger for exhaust heat recovery, where it is stored in hot water. It is cooled by heat exchange with water from the device, and this cooling condenses water contained in the fuel off-gas, and the condensed water is separated in a gas-liquid separator. Then, a part (about 10 to 40%) of the fuel off-gas (anode off-gas) from which water has been removed is returned to the decompression area (fuel gas supply system) upstream of the fuel gas supply pump, and this returned fuel off-gas is mixed with fuel gas (for example, city gas) from a fuel gas supply source and supplied to the reformer. On the other hand, the remaining fuel off-gas (not returned to the fuel gas supply system) is sent to the combustor in the high-temperature space, where it is combusted by air off-gas (cathode off-gas).

By returning a portion of the fuel off-gas to the fuel gas supply system in this manner, the power generation efficiency of the fuel cell system can be increased. In general, the fuel utilization rate of a cell stack is normally around 82-83%, but by returning fuel off-gas, this fuel utilization rate can be increased to around 87-90%, resulting in a gain of around 5-7 points. It becomes possible to make improvements, and thereby it becomes possible to increase power generation efficiency.

JP2005-285340A Japanese Patent Application Publication No. 2008-21596 Japanese Patent Application Publication No. 2018-198116

In solid oxide fuel cell systems, such as those used for home cogeneration, where the power generation output (generated power) is small, on the order of several hundred watts, there is heat radiation loss from the high-temperature housing that houses the cell stack. In a power generation state where the power generation output of the cell stack is lower than the rated power generation state (i.e., partial load power generation state), the ratio of heat radiation loss to the power generation output increases, making it difficult to operate this fuel cell system under high power generation efficiency conditions. . In such a case, the operating condition of the fuel cell system is to lower the fuel utilization rate.

In the above-mentioned solid oxide fuel cell system that aims for high power generation efficiency, when the fuel utilization rate becomes low during partial load power generation, the fuel off-gas (anode off-gas) from the cell stack is sent to the combustor. Carbon may be deposited in the off-gas supply channel and the recycle channel branching from the fuel off-gas supply channel, and when this carbon deposition occurs, the fuel off-gas supply channel and/or the recycling channel may be blocked. This can lead to such problems. In particular, when carbon deposits occur in the recycle flow path, the flow resistance of the recycle flow path increases, and the flow rate of fuel off-gas recycled to the fuel gas supply system through the recycle flow path decreases, increasing the fuel cell system's power generation efficiency. It becomes difficult to drive.

An object of the present invention is to provide a solid oxide fuel cell system that can suppress carbon deposition in a recycling channel that returns part of the fuel off-gas from the fuel electrode side of the cell stack to the fuel gas supply system. be.

A solid oxide fuel cell system according to claim 1 of the present invention includes a reformer for steam reforming fuel gas, and a reformed fuel gas and an oxidant gas reformed in the reformer. a cell stack that generates electricity through an electrochemical reaction ; an oxidant gas supply means for supplying the oxidant gas to the cell stack; and a fuel gas supply means for supplying the fuel gas to the reformer. , a water supply means for supplying reforming water used for steam reforming, a combustor provided in association with the reformer, and a steam condenser for condensing water vapor contained in the fuel off-gas from the cell stack. a recycling channel for returning a portion of the fuel off-gas to the upstream side of the fuel gas supply means; and a recycle channel for controlling the operation of the fuel gas supply means, the oxidant gas supply means, and the water supply means. a controller, the reformed fuel gas from the reformer is fed to the fuel electrode side of the cell stack, and the oxidant gas from the oxidant gas supply means is fed to the oxygen electrode side of the cell stack. The water vapor contained in the fuel off-gas supplied from the fuel electrode side of the cell stack is cooled and condensed in the water vapor condensing section, and a part of the fuel off-gas from which the water vapor has been removed is recycled. A solid oxide fuel cell system in which the fuel gas is returned to the upstream side of the fuel gas supply means through a flow path, the remainder of which is sent to the combustor, and is burned by the oxidant off-gas from the oxygen electrode side of the cell stack. And,
The controller includes a first target S/C calculating means for calculating a first target S/C, which is an S/C condition for preventing carbon deposition in the recycling channel, and a system operating S/C. and a control means for controlling the operation of the water supply means, and the first target S/C calculation means is a proportion of the fuel off-gas returned through the recycling flow path. The first target S/C is calculated using the recycling rate, and the operating S/C setting means sets the first target S/C to be equal to or higher than the first target S/C calculated by the first target S/C calculating means. The operating S/C is set, and the control means controls the operation of the water supply means so that the operating S/C is set.

The solid oxide fuel cell system according to claim 2 of the present invention further includes a first temperature detection means for detecting the fuel electrode outlet temperature of the cell stack, and the first target S of the controller. The /C calculation means is configured to calculate the first value based on the recycle rate, the fuel utilization rate determined by the fuel gas supply flow rate and the generated current or generated output of the cell stack, and the fuel electrode outlet temperature of the cell stack. It is characterized by calculating the target S/C.

Further, in the solid oxide fuel cell system according to claim 3 of the present invention, the first target S/C calculated by the first target S/C calculation means is a carbon deposition test using a boudoir reaction. Carbon precipitation value K,
K=PCO ₂ /(PCO) ² (partial pressure: MPa)
A safe carbon deposition value Ks is calculated by adding a safety factor to the carbon deposition value K, and the gas composition is balanced using the fuel utilization rate, the recycling rate, and the fuel electrode outlet temperature of the cell stack as variables. Regarding the safe carbon deposition value Ks in the gas composition calculated and equilibrium calculated,
Ks=PCO ₂ /(PCO) ² (partial pressure: MPa)
The S/C value is calculated based on the fuel utilization rate, the recycling rate, and the fuel electrode outlet temperature of the cell stack using the safe carbon deposition value Ks. It is characterized by

Further, in the solid oxide fuel cell system according to claim 4 of the present invention, the controller sets a second target S/C that is an S/C condition for preventing carbon deposition in the reformer. The target S/C determination selection means further includes a second target S/C calculation means for calculating and a target S/C determination selection means for determining and selecting a target S/C of the system, and the target S/C determination selection means is configured to determine the first target S/C. C and the second target S/C, the operating S/C setting means determines that the operating S/C is greater than or equal to the first target S/C when the first target S/C is selected. The operating S/C is set such that when the second target S/C is selected, the operating S/C is set to be equal to or higher than the second target S/C , and the control means , the operation of the water supply means is controlled so that the S/C is in operation.

Furthermore, the solid oxide fuel cell system according to claim 5 of the present invention further includes a second temperature detection means for detecting the reformer inlet temperature of the reformer, and the second temperature detection means for detecting the reformer inlet temperature of the reformer. The C calculation means calculates the second target S/C based on the reformer inlet temperature.

According to the solid oxide fuel cell system according to claim 1 of the present invention, electricity is generated by an electrochemical reaction between the reformer for steam reforming the fuel gas, the reformed fuel gas, and the oxidizing gas. A cell stack, an oxidant gas supply means for supplying oxidant gas to the cell stack, a fuel gas supply means for supplying fuel gas to a reformer, and a water supply means for supplying reforming water. , a water vapor condensing section for condensing water vapor contained in the fuel off-gas from the cell stack, a recycling channel for returning a portion of the fuel off-gas to the upstream side of the fuel gas supply means, a fuel gas supply means, and an oxidizing gas. It is equipped with a supply means and a controller for controlling the operation of the water supply means. Then, the controller sets a first target S/C calculation means for calculating a first target S/C, which is an S/C condition for preventing carbon deposition in the recycling channel, and an operating S/C of the system. the first target S/C calculation means calculates the first target S/C using a recycle rate that is the proportion of fuel off-gas returned through the recycling flow path, The /C setting means sets the operating S/C to be equal to or higher than the first target S/C calculated by the first target S/C calculating means, so the water supply means sets the operating S/C to be equal to or higher than the first target S/C calculated by the first target S/C calculating means. The reforming water is supplied so as to suppress carbon deposition in the recycle flow path that returns the fuel off-gas to the fuel gas supply system, and prevent the recycle flow path from being blocked by precipitated carbon. Therefore, a part of the fuel off-gas (anode off-gas) from the fuel electrode side of the cell stack stably flows through the recycling channel to the fuel gas supply system, allowing long-term operation with high power generation efficiency, and reducing carbon deposition. It is possible to avoid deterioration in system reliability due to

Further, according to the solid oxide fuel cell system according to claim 2 of the present invention, the first target S/C calculation means of the controller is based on the fuel utilization rate, the recycling rate, and the fuel electrode outlet temperature of the cell stack. Since the first target S/C is calculated using the same method, it is possible to calculate the first target S/C at which carbon precipitation does not occur in the recycle channel.

Further, according to the solid oxide fuel cell system according to claim 3 of the present invention, the first target S/C calculated by the first target S/C calculation means is a carbon deposition test using a boudoir reaction. Carbon precipitation value K,
K=PCO ₂ /(PCO) ² (partial pressure: MPa)
Calculate the safe carbon deposition value Ks by adding a safety factor to this, and then perform an equilibrium calculation of the gas composition using the fuel utilization rate, recycling rate, and fuel electrode outlet temperature of the cell stack as variables. Regarding the safe carbon precipitation value Ks in
Ks=PCO ₂ /(PCO) ² (partial pressure: MPa)
Since this is the S/C value calculated as the S/C for , it is possible to calculate the first target S/C at which carbon precipitation does not occur in the recycle channel as required.

Further, according to the solid oxide fuel cell system according to claim 4 of the present invention, the controller sets the second target S/C, which is the S/C condition for preventing carbon deposition in the reformer. The target S/C determination selection means further includes a second target S/C calculation means for calculating, and a target S/C determination selection means for determining and selecting the target S/C of the system, and the target S/C determination selection means is configured to determine the first target S/C and the target S/C. The operating S/C setting means determines and selects the larger one of the second target S/Cs, and the operating S/C setting means sets the operating S/C so that it is equal to or greater than the first target S/C when the first target S/C is selected. is set, and when the second target S/C is selected, the operating S/C is set to be equal to or higher than the second target S/C , thereby suppressing carbon deposition in the reformer and recycle flow path. be able to.

Furthermore, according to the solid oxide fuel cell system according to claim 5 of the present invention, the second target S/C calculation means calculates the second target S/C based on the reformer inlet temperature. , it is possible to calculate a second target S/C in which carbon precipitation does not occur in the reformer.

1 is an overall diagram schematically showing a first embodiment of a solid oxide fuel cell system according to the present invention. FIG. 3 is a diagram showing the relationship between the cell stack's power generation current and fuel utilization rate. FIG. 3 is a diagram showing the relationship between cell stack power generation current and recycling rate. 2 is a block diagram schematically showing a control system of the solid oxide fuel cell system of FIG. 1. FIG. FIG. 2 is an overall diagram schematically showing a second embodiment of a solid oxide fuel cell system according to the present invention. The figure which shows the relationship between the inlet temperature of a reformer and S/C. 6 is a block diagram schematically showing a control system of the solid oxide fuel cell system of FIG. 5. FIG. 8 is a flowchart showing the flow of setting the operating S/C by the control system of FIG. 7; FIG. FIG. 3 is a block diagram schematically showing a control system of a third embodiment of a solid oxide fuel cell system according to the present invention. 10 is a flowchart showing the flow of setting the operating S/C by the control system of FIG. 9.

EMBODIMENT OF THE INVENTION Hereinafter, various embodiments of the solid oxide fuel cell system according to the present invention will be described with reference to the accompanying drawings. First, a first embodiment of a solid oxide fuel cell system according to the present invention will be described with reference to FIGS. 1 to 4.

In FIG. 1, the illustrated solid oxide fuel cell system 2 generates electricity using fuel gas such as city gas or LP gas as raw fuel gas, and reformes the fuel gas (raw fuel gas). a solid oxide type cell that generates electricity (so-called power generation reaction) through an electrochemical reaction between the reformed fuel gas reformed in the reformer 4 and air as an oxidant gas. It is equipped with a stack 6.

The cell stack 6 is composed of a plurality of solid oxide fuel cells stacked together via a current collecting member to generate electricity through a power generation reaction, and includes a solid material that conducts oxygen ions (not shown). It includes an electrolyte, a fuel electrode provided on one side of the solid electrolyte, and an oxygen electrode (air electrode) provided on the other side of the solid electrolyte, and zirconia doped with yttria, for example, is used as the solid electrolyte.

The fuel electrode side 8 of this cell stack 6 is connected to the reformer 4 via a reformed fuel gas supply channel 10, and in this form, the reformer 4 is a vaporizer for vaporizing reforming water. 12 as a reforming unit. The vaporizer 12 is configured separately from the reformer 4, and is configured to feed the steam vaporized in the vaporizer 12 to the reformer 4 via a water vapor feed channel (not shown). You can also do this.

The carburetor 12 is connected to a fuel gas supply source (not shown) (for example, composed of a buried pipe, a storage tank, etc.) for supplying fuel gas via a fuel gas supply flow path 14, and Fuel gas (raw fuel gas) from a supply source is supplied to the vaporizer 12 through the fuel gas supply channel 14 . Note that this fuel gas supply channel 14 may be connected to the reformer 4 so that fuel gas from the fuel gas supply source is directly supplied to the reformer 4.

Further, reforming water is supplied to this vaporizer 12 through a water supply channel 18 . A reforming catalyst is housed in the reformer 4. For example, ruthenium supported on alumina is used as the reforming catalyst, and this reforming catalyst vaporizes the fuel gas supplied through the fuel gas supply channel 14. The vaporized water vapor is used for steam reforming in the vessel 12.

This fuel gas supply channel 14 includes, in order from the carburetor 12 toward the upstream side, a desulfurizer 20, a first throttle member 22, a fuel gas supply pump 24 (constituting fuel gas supply means), and a second throttle member 26. , a zero governor 28 as a pressure regulating member, a fuel flow sensor 30, and a cutoff valve 32 are provided. The desulfurizer 20 removes sulfur components contained in the fuel gas (sulfur components in the odorant), and the fuel gas supply pump 24 boosts the pressure of the fuel gas flowing through the fuel gas supply channel 14 and supplies it to the vaporizer 12. The fuel gas supply flow rate is adjusted by controlling the rotation speed of the fuel gas supply pump 24. When the rotation speed of the fuel gas supply pump 24 is increased (or decreased), the fuel gas supply flow path 14 The supply flow rate of fuel gas supplied through the fuel gas increases (or decreases).

Further, the zero governor 28 adjusts the fuel gas supplied from the fuel gas supply source (not shown) through the fuel gas supply flow path 14 to a predetermined pressure (i.e., atmospheric pressure), and the fuel gas flow sensor 30 adjusts the fuel gas The flow rate of the fuel gas supplied through the supply passage 14 is measured, and when the cutoff valve 32 is in the closed state, the cutoff valve 32 shuts off the fuel gas supply passage 14 and stops the supply of fuel gas. Further, the first and second throttle members 22 and 26 located on both sides (i.e., the downstream side and the upstream side) of the fuel gas pump 24 are provided to stabilize the flow rate of the fuel gas flowing through the fuel gas supply channel 14. , the first throttle member 22 is composed of, for example, a capillary tube, and the second throttle member 26 is composed of, for example, a throttle member having a small orifice. Moreover, the fuel gas flow rate sensor 30 can be configured, for example, from a thermal type flow rate sensor. Note that when the fuel gas can be stably supplied, the first throttle member 22 may be omitted, and the second throttle member 26 may be replaced with, for example, a buffer tank. .

A water supply pump 34 (constituting water supply means) is disposed in the water supply channel 18, and the water supply pump 34 supplies water for reforming to the vaporizer 12 through the water supply channel 18 as described later. be done. By controlling the rotation speed of this water supply pump 34, the supply flow rate of reforming water is adjusted, and when the rotation speed of the water supply pump 34 is increased (or decreased), reforming water is supplied through the water supply flow path 18. As the supply flow rate of quality water increases (or decreases), the steam carbon ratio (also referred to as "S/C") is adjusted as described below by controlling the operation of the water supply pump 34.

The steam carbon ratio (S/C) is the ratio of the number of moles of reforming water supplied to the reformer 4 to the number of moles of carbon in the fuel gas (raw fuel gas) supplied to the reformer 4. Yes, this S/C, the supply flow rate of raw fuel gas and the supply flow rate of reforming water are as follows.
S/C = (22.4X (supply flow rate of reforming water)/18)/(1.2X (supply flow rate of fuel gas)) ... (1)
In this equation (1), "22.4" is the volume of gas in the standard state, "18" is the molecular weight of water, and "1.2" is the supply of fuel gas. This is a coefficient for calculating the number of moles of carbon from the flow rate (flow rate per unit time). It is a typical value when the fuel gas is based on natural gas, and it depends on the composition of the fuel gas (raw fuel gas). do.

The oxygen electrode (air electrode) side 36 of the cell stack 6 is connected to an air blower 40 as an air supply means via an air supply channel 38. The air blower 40 supplies air (oxidant gas) to the oxygen electrode side 36 (air electrode side) of the cell stack 6 through the air supply channel 38. The air supply flow rate is adjusted by controlling the rotation speed of the air blower 40, and when the rotation speed of the air blower 40 is increased (or decreased), the air (oxidant gas ) supply flow rate increases (or decreases).

The fuel off-gas (that is, anode off-gas) discharged from the fuel electrode side 8 of the cell stack 6 is fed to the steam condensing section 46 through the fuel off-gas outlet flow path 44, and is contained in the fuel off-gas in the steam condensing section 46. The water vapor present in the fuel off-gas is condensed and removed, and the fuel off-gas from which the water vapor has been removed is fed to the combustor 50 through the fuel off-gas feed passage 48 . Further, the air off-gas (oxidizing agent off-gas) (i.e. cathode off-gas) discharged from the oxygen electrode side 36 of the cell stack 6 is sent to the combustor 50 through the air off-gas supply passage 52 (oxidizing agent off-gas supply passage). In this combustor 50, fuel off-gas (containing fuel gas) from the fuel electrode side 8 of the cell stack 6 and air off-gas (containing oxygen) from the oxygen electrode side 36 (air electrode side) are ) are burned.

The carburetor 12 and the reformer 4 are arranged in contact with or close to the combustor 50, and the combustion heat of this fuel off-gas is used to heat the carburetor 12, the reformer 4, etc. A high temperature space 64, which will be described later, is maintained at a high temperature. Combustion exhaust gas from the combustor 50 is exhausted to the atmosphere through an exhaust gas exhaust flow path 54.

Furthermore, a recycling passage 56 is provided branching off from the fuel off-gas supply passage 48 , and the downstream side of this recycling passage 56 is provided with the fuel gas supply passage 14 , specifically, the fuel gas supply pump 24 . It is connected to a part between the part and the part where the second aperture member 26 is provided. Since the recycling channel 56 is provided in this way, a part of the fuel off-gas (anode off-gas) flowing through the fuel off-gas supply channel 48 is transferred to the fuel gas supply system through the recycling channel 56, specifically, to the fuel gas supply channel 48. The fuel gas is returned to the upstream side of the fuel gas supply pump 24 in the supply flow path 14 , and the remainder is fed to the combustor 50 through the fuel off-gas feed flow path 48 .

In this embodiment, a reforming unit (reformer 4 and vaporizer 12), cell stack 6, and combustor 50 are housed in a high temperature housing 62. The high-temperature housing 62 is made of metal (for example, stainless steel), and its inner surface is covered with a heat insulating member (not shown). 4 and vaporizer 12), the cell stack 6, and the combustor 50 are maintained at a high temperature within this high temperature space 64.

In this solid oxide fuel cell system 2, a part of the fuel off-gas outlet channel 44 is led out of the high temperature housing 62 (high temperature space 64) and connected to the steam condensing section 46, and the fuel from the steam condensing section 46 is A portion of the off-gas feed passage 48 is introduced from outside the high temperature housing 62 (high temperature space 64) into the inside thereof and is connected to the combustor 50. In this embodiment, the steam condensing unit 46 includes a first heat exchanger 66 for exhaust heat recovery, a gas-liquid separator 68 for separating condensed water from fuel off-gas, and a gas-liquid separator 68 for removing impurities contained in the condensed water. and a water recovery container 72 for storing condensed water. The gas-liquid separator 68 is composed of, for example, a drain separator, and the first heat exchanger 66 is used in combination with, for example, a hot water storage device 74 for a co-generation system that stores the heat of fuel off-gas as hot water. .

The hot water storage device 74 includes a hot water storage tank 76 for storing hot water, a circulation passage 78 that circulates through the first heat exchanger 66, and a circulation pump 80 disposed in the circulation passage 78. When used in combination with this hot water storage device 74, the water in the hot water storage tank 76 is circulated through the circulation flow path 66 by the circulation pump 80, and the water flowing through the circulation flow path 78 and the fuel off-gas derived flow are combined with the first heat exchanger 66. Heat exchange is performed with the fuel off-gas flowing through the passage 44, and hot water heated by the heat exchange is stored in the hot water storage tank 76. Moreover, the fuel off-gas is cooled by this heat exchange and the water vapor contained therein is condensed, and the fuel off-gas and condensed water (condensed water) flow to the gas-liquid separator 68 on the downstream side.

The gas-liquid separator 68 separates condensed water and fuel off-gas, and the separated condensed water flows to the lower ion exchanger 70, where impurities such as ionic components are removed by the ion exchange resin built into this. The water is then stored in the water recovery tank 72. The water (condensed water) recovered in the water recovery tank 72 is used as reforming water, and is fed to the vaporizer 12 through the water supply channel 18 by the action of the water supply pump 34.

Further, a second heat exchanger 82 for air preheating is provided in relation to the air supply channel 38 (oxidant gas supply channel), and this second heat exchanger 82 connects the air supply channel 38 Heat exchange is performed between the flowing air (oxidizing gas) and the combustion exhaust gas discharged through the exhaust gas exhaust flow path 54, and the air (oxidizing gas) heated by this heat exchange is used as oxygen in the cell stack 6. It is fed to the pole side 36.

Furthermore, a third heat exchanger 84 for residual heat of the fuel off-gas is arranged in relation to the fuel off-gas supply passage 48 , and this third heat exchanger 84 is configured to handle the fuel off-gas flowing through the fuel off-gas supply passage 48 . Heat exchange is performed between the fuel off-gas flowing through the fuel off-gas outlet passage 44 , and the fuel off-gas heated by this heat exchange is sent to the combustor 50 . Note that the second and third heat exchangers 82, 84 are housed in the high temperature housing 62 (high temperature space 64) and are maintained at a high temperature.

This solid oxide fuel cell system 2 is configured so that the power generation output (generated current) of the cell stack 6 varies depending on the size of the power load. For example, when the power load increases (or decreases), Following this, the power generation output (power generation current) of the cell stack 6 is also configured to increase (or decrease). When the solid oxide fuel cell system 2 is controlled in this way, the relationship between the generated current of the cell stack 6 and the fuel utilization rate in the cell stack 6 is, for example, as shown in FIG. As the power generation current increases, the fuel utilization rate also increases; for example, when the generated current is 10A, the fuel utilization rate is around 55%, but when the generated current increases to the rated value of 30A, the fuel utilization rate increases to around 82%. Note that this fuel utilization rate is the amount of fuel gas consumed in the power generation reaction in the cell stack 6 relative to the amount of fuel gas supplied to the cell stack 6, that is, (fuel utilization rate = amount of fuel gas consumed) /fuel gas supply amount).

Furthermore, in the solid oxide fuel cell system 2 equipped with the recycling flow path 56, the relationship between the generated current of the cell stack 6 and the recycling rate is, for example, the relationship shown in FIG. 3, and as the generated current increases, the recycling rate increases. For example, when the generated current is 10 A, the recycle rate is around 0.33, but when the generated current is 30 A, the recycle rate decreases to about 0.3. Note that this recycle rate is the amount of recycle feed that is returned to the fuel gas supply system through the recycle flow path 56 relative to the feed amount of fuel off-gas that is fed from the fuel electrode side 8 of the cell stack 6, that is, (recycle rate = This is the ratio of recycle feed amount/fuel off-gas feed amount).

This recycling rate is evaluated by, for example, extracting a small amount of fuel gas at the outlet of the fuel gas supply pump 24, determining the gas composition of the extracted fuel gas by gas chromatography, and calculating the fuel utilization rate (fuel gas flow rate and power generation current). ) can be calculated to determine the recycling rate. By determining the recycle rate in advance by changing the conditions of the generated current, the data shown in FIG. 3 (that is, the relationship between the generated current of the cell stack 6 and the recycle rate) can be obtained. In Figure 3, the recycling rate is high where the generated current is low, but this is due to the component configuration around the fuel gas supply pump 24 (second throttle member 26, etc.), and this component configuration reduces the generated current. The recycling rate also changes when the amount is low. Although it is difficult to constantly measure the recycling rate in this solid oxide fuel cell system 2, by comparing the duty ratio of the fuel gas supply pump and the flow rate detected by the fuel gas flow rate sensor 30, it is possible to determine whether the system is in its original operating state. It is possible to understand whether this is the case.

The relationship between the power generation output of the cell stack 6 and the fuel utilization rate has been described with reference to FIG. The relationship between power generation output and recycling rate is similar to that shown in Figure 3. Therefore, the generated current of the cell stack 6 in FIGS. 2 and 3 can be replaced with the generated output of the cell stack 6.

In such a solid oxide fuel cell system 2, in a partial load power generation state where the fuel utilization rate is low, there is a risk that carbon precipitation will occur in the recycle channel 56 branching from the fuel off-gas supply channel. In order to suppress carbon precipitation, the S/C is set as follows to control the supply amount of reforming water.

Here, a method of calculating S/C to prevent carbon precipitation from occurring in the recycle channel 56 will be described. In this solid oxide fuel cell system 2, when the fuel off-gas recycling rate is high, the fuel utilization rate of the fuel gas is low, and the fuel electrode outlet temperature of the cell stack 6 is high, the fuel off-gas flows through the recycling path 56. (The temperature is lowered by flowing through the steam condensing section 46), which makes carbon precipitation more likely to occur. The mechanism of this carbon precipitation is a reaction known as the boudoir reaction (CO disproportionation reaction).
2CO→C+CO ₂ ...(a)
It can be expressed as In the equilibrium of this reaction formula (a), CO can stably exist at a high concentration at high temperatures, but when the temperature drops at that composition, the reaction shifts to the right and carbon begins to precipitate.

In this solid oxide fuel cell system 2, the fuel off-gas decreases from the fuel electrode outlet temperature of the cell stack 6 (normally, for example, around 700°C) to around 100°C, so that the equilibrium calculation of reaction equation (a) This results in carbon precipitation, and the configuration of this solid oxide fuel cell system 2 becomes impossible.

Therefore, the inventors of the present invention need only investigate the carbon precipitation when the composition of the fuel off gas (obtained from the anode off) passes through a temperature range of 550 to 600°C through a carbon precipitation test. It was confirmed that there is no need to consider the temperature

<Carbon deposition test method>
This carbon deposition test was not conducted using the composition of the fuel off gas (anode off gas), but by reforming natural gas-based city gas (13A) to have an S/C of 1.0 to 2.0. The typical reforming outlet temperature in the reformer 4 was set to 650°C, and the temperature of the alumina and nickel catalyst as the reforming catalyst was set in the temperature range of 550 to 600°C. The test was conducted by maintaining the temperature state for 5 hours or more and examining carbon precipitation.

Since the gas composition at the reforming catalyst outlet of the reformer 4 is an equilibrium composition based on the ability of the reforming catalyst and the gas flow rate, the conditions under which carbon deposition was observed in this test were summarized. Then, according to reaction formula (a), from the equilibrium composition at the reforming outlet stage of the reformer 4, the carbon precipitation value K under the conditions where carbon starts to precipitate,
K=PCO ₂ /(PCO) ² ...(2)
I asked for The partial pressure in this equation (2) is MPa. The smaller this carbon precipitation value K is, the more easily carbon precipitation occurs. By using this equation (2), the carbon precipitation value K at which carbon precipitation actually starts to occur can be determined in this test.

<Fuel off-gas conditions/system operating conditions>
The composition of the fuel off-gas (anode off-gas) in the cell stack 6 of this solid oxide fuel cell system 2 is defined by S/C, fuel utilization rate, recycling rate, and temperature of the fuel off-gas (specifically, the cell Four parameters were extracted (fuel electrode outlet temperature of stack 6). Once these four parameters are determined, the equilibrium composition of the fuel off-gas will be determined. Since the fuel electrode side of the cell stack 6 has a large amount of nickel and is at a high temperature, it can be assumed to have an equilibrium composition at its outlet temperature. ).

Therefore, using three variables: fuel utilization rate, recycling rate, and fuel electrode outlet temperature of the cell stack 6, the gas composition of the fuel off-gas is calculated in equilibrium, and using the above equation (2),

K'= _PCO2 /(PCO) ² ...(3)
Find the S/C that satisfies. At this time, the value K' may be the value K obtained using equation (2) in the carbon precipitation test method described above, but the safe carbon precipitation value (Ks=S× K) may also be used. In this way, the S/C at which carbon precipitation does not occur in the recycling channel 56 can be determined.

<Specific carbon deposition test>
When the above-mentioned carbon precipitation test method was performed using a catalytic reaction experimental device to determine the S/C at which no oxygen precipitation occurred in the recycle channel 56, the following results were obtained. The test using this catalytic reaction experimental device was not conducted using the composition of the fuel off-gas (anode off-gas), but using natural gas-based city gas (13A) modified to have an S/C of 1.0 to 2.0. The exit temperature of the reforming catalyst was controlled at 650°C, the reformed gas was cooled while passing through α-alumina installed on the downstream side, and held in the temperature range of 550 to 600°C for 5 hours to prevent carbon precipitation. I checked to see if it existed. In this test, a carbon deposition test was conducted using four types of reformed gases with S/C of 1.0, 1.2, 1.5, and 2.0. As a result of this test, no carbon was precipitated in the reformed gas with an S/C of 1.5, but a trace amount of carbon was precipitated in the reformed gas with an S/C of 1.2.

A similar carbon deposition test using this catalytic reaction experimental apparatus was conducted using a porous body containing metallic nickel as a main component instead of α-alumina as a reforming catalyst. In the same manner as described above, four types of reformed gases having S/C of 1.0, 1.2, 1.5, and 2.0 were used. As a result of this test, the amount of carbon precipitated increased significantly, but with the reformed gas with S/C of 1.2, carbon precipitated, but with the reformed gas with S/C of 1.5, carbon precipitated. There was no precipitation, and the presence or absence of carbon precipitation was a common result.

From this test result, the carbon precipitation value K was calculated using the above equation (2) from the equilibrium composition of the reformed gas with an S/C of 1.5 and a reforming outlet temperature of 650°C. The value K was 38 (K=38). Catalysis literature “Catalysis Science and
Technology (Edited by John R.
Anderson and Michel Bouda, Volume 5, 1984, the equilibrium constant of the above equation (2) is 107 at 600°C and 450 at 550°C. If the carbon precipitation value K is lower than these values, the balance will be on the carbon precipitation side. However, from the results of this test, carbon deposition can be avoided if the gas composition corresponds to K=38.

Here, the gas composition is calculated in equilibrium using three variables: the fuel utilization rate, the recycling rate, and the fuel electrode outlet temperature of the cell stack 6, and the carbon deposition value K in the above equation (2) is the value in the above equation (3). Find the S/C to become K'. Multiplying by the safety factor S, for example, when the safe carbon deposition value Ks is set to 100 (Ks=100), the results are as follows, and the results of this calculation are summarized as shown in Table 1.

That is, the function for determining the steam carbon ratio S/C is:
S/C=f (fuel utilization rate, recycling rate, fuel electrode outlet temperature)...(4)
When S/C is calculated using this equation (4), the results shown in Table 1 are obtained.

上述したことから理解されるように、リサイクル流路５６を通してのリサイクル率、燃料ガスの燃料利用率及びセルスタック６の燃料極出口温度の３つのパラメータの値が判れば、これらの値に基づいて、リサイクル流路５６での炭素析出が生じないＳ／Ｃを求めることができる。

As can be understood from the above, if the values of the three parameters, ie, the recycling rate through the recycling channel 56, the fuel utilization rate of the fuel gas, and the fuel electrode outlet temperature of the cell stack 6, are known, then based on these values, , it is possible to determine the S/C at which carbon precipitation does not occur in the recycle channel 56.

The results of Table 1 can be expressed as an approximate expression, and in this approximate expression, S/C can be expressed by the following formula.

In this embodiment, the S/C at which carbon precipitation does not occur is determined using this approximate formula, but it can also be determined using the above Table 1 instead of using this approximate formula.

Referring to FIGS. 1 and 4, this solid oxide fuel cell system 2 is configured as follows in order to realize the above-described S/C calculation method. A temperature sensor 92 (constituting a first temperature detection means) is provided to detect the fuel electrode outlet temperature of the cell stack 6 . Although it is desirable to directly measure this fuel electrode outlet temperature, if measurement is difficult, the temperature at the fuel electrode side outlet may be estimated from another temperature representative of the cell stack 6. By measuring in advance the correlation between different temperatures and operating conditions of the cell stack 6 and the fuel electrode outlet temperature, it is possible to estimate the fuel electrode outlet temperature of the cell stack 6 with a reasonable degree of accuracy, and such temperature can be It may also be used as the fuel electrode outlet temperature.

This solid oxide fuel cell system 2 includes a controller 94 for controlling the system, and a detection signal from a temperature sensor 92 is sent to the controller 94. The controller 94 is composed of a microprocessor or the like, and includes a fuel utilization rate calculation means 96, a recycle rate calculation means 98, a target S/C calculation means 100, an operating S/C setting means 102, and a control means 104.

The fuel utilization rate calculation means 96 calculates the fuel utilization rate as described later, and the recycle rate calculation means 98 calculates the recycle rate as described later, and the target S/C calculation means 100 (first target S/C (functioning as a calculation means) calculates the target S/C (which becomes the first target S/C) as described later, and the operating S/C setting means 102 sets the operating S/C as described later. , the control means 104 controls the operation of the water supply pump 34 to achieve the set operating S/C.

The controller 94 further includes a memory means 106, which stores a generated current-fuel utilization rate map such as the one shown in FIG. 2, and a generated current-recycle rate map such as the one shown in FIG. and target S/C calculation data (that is, approximate equation data of the above equation (5)) are registered.

This solid oxide fuel cell system 2 is configured so that the generated current (power generation output) of the cell stack 6 fluctuates in accordance with the fluctuations in the power load. For example, as shown in FIG. Following this, the power generation output (power generation output) of the cell stack 6 is configured to vary within a range of, for example, 4 to 30A (400 to 3000W).

The solid oxide fuel cell system 2 is controlled, for example, as follows. When the power generation output (power generation output) of the cell stack 6 is determined in accordance with the electric power load, the fuel utilization rate calculation means 96 calculates the generated current-fuel utilization rate map registered in the memory means 106 (for example, see FIG. 2). Based on this, a fuel utilization rate corresponding to the generated current is calculated, and the system is operated at this fuel utilization rate. That is, the control means 104 controls the operation of the fuel gas supply pump 24, air blower 40, and water supply pump 34 so that this fuel utilization rate is achieved, and the cell stack 6 is operated to generate electricity so that this fuel utilization rate is achieved. .

Then, in order to prevent carbon precipitation from occurring in the recycle channel 56, the operating S/C is set as follows. That is, the recycle rate calculation means 98 calculates the recycle rate corresponding to the generated current based on the generated current-recycle rate map registered in the memory means 106 (for example, see FIG. 3). Next, the target S/C calculation means 100 (first target S/C calculation means) reads out the target S/C calculation data registered in the memory means 106 and calculates the fuel utilization calculated by the fuel utilization rate calculation means 96. The target S/C is calculated using the recycle rate calculated by the recycle rate calculation means 98, and the fuel electrode outlet temperature of the cell stack 6 detected by the temperature sensor 92 (first temperature detection means).

When the target S/C is determined in this way, the operating S/C setting means 102 sets this target S/C to be the operating S/C of the system, and the control means 104 sets this operating S/C to be the operating S/C of the system. /C, thereby supplying reforming water in an amount that suppresses carbon deposition in the recycle flow path 56, thereby preventing troubles such as clogging of the recycle flow path. can.

In addition, in this first embodiment, the target S/C calculated by the target S/C calculation means 100 is set as the operating S/C, but by setting a value larger than this target S/C, Carbon precipitation in the recycle channel 56 can be suppressed, and the operating S/C may be set to a value that is, for example, about 1.1 to 1.3 times the target S/C.

Next, a second embodiment of the solid oxide fuel cell system according to the present invention will be described with reference to FIGS. 5 to 8. In this second embodiment, the operating S/C is configured to suppress carbon deposition in the recycle flow path and carbon deposition in the reformer. Incidentally, in the following embodiments, the same reference numerals are given to the elements that are substantially the same as those in the above-described first embodiment, and the description thereof will be omitted.

5 and 7, in the solid oxide fuel cell system 2A of the second embodiment, a temperature sensor that detects the fuel electrode outlet temperature of the cell stack 6, that is, a first temperature sensor 92 (first temperature detection In addition to the temperature sensor 102 (constituting the second temperature detecting means), a temperature sensor for detecting the inlet temperature of the reformer 4, that is, a second temperature sensor 102 (constituting the second temperature detecting means) is provided. , 112 are sent to the controller 94A.

In addition to the fuel utilization rate calculation means 96, the recycle rate calculation means 98, the first target S/C calculation means 100, the operating S/C setting means 102A and the control means 104, the controller 94A also includes a second target S/C It includes a calculation means 114 and a target S/C judgment selection means 116. The second target S/C calculation means 114 calculates the second target S/C, which is an S/C condition under which carbon precipitation does not occur in the reformer 4, as described below, and the target S/C determination selection means 116 determines and selects the larger one of the first target S/C and the second target S/C, and the operating S/C setting means 102A selects the first target S/C when the first target S/C is large. /C is set as the working S/C, and when the second target S/C is large, this second target S/C is set as the working S/C.

Furthermore, in addition to the generated current-fuel utilization rate map, generated current-recycle rate map, and first target S/C calculation data, a reformer inlet temperature-S/C map is registered in the memory means 106A. The other configurations of this second embodiment are substantially the same as those of the first embodiment described above.

In the solid oxide fuel cell system 2A of the second embodiment, the operating S/C is set as follows to prevent carbon precipitation from occurring in both the recycle channel 56 and the reformer 4. Ru. Referring primarily to FIGS. 7 and 8, first, the first target S/C is calculated in the same manner as described above. That is, the fuel utilization rate calculation means 96 calculates the fuel utilization rate corresponding to the generated current of the cell stack 6 based on the generated current-fuel utilization rate map (for example, see FIG. 2) (step S1), and calculates the recycle rate. The means 98 calculates the recycle rate corresponding to the generated current based on the generated current-recycle rate map (for example, see FIG. 3) registered in the memory means 106 (step S2). Then, the first target S/C calculation means 100 reads out the target S/C calculation data registered in the memory means 106, and reads out the above fuel utilization rate, the above recycle rate, and the first temperature sensor 92 (first temperature detection means). A first target S/C is calculated using the detected temperature (fuel electrode outlet temperature of the cell stack 6) (step S3).

Next, a second target S/C is calculated. That is, the second target S/C calculation means 114 calculates the inlet temperature of the reformer 4 (i.e., the detection by the second temperature sensor 112) based on the reformer inlet temperature-S/C map (see FIG. 6, for example). (step S4), and this S/C becomes the second target S/C. If the supply amount of reforming water is controlled to achieve the second target S/C, carbon precipitation in the reformer 4 will be suppressed.

Once the first target S/C and the second target S/C are determined in this way, the target S/C determination and selection means 116 determines the first target S/C and the second target S/C. The larger one is compared and selected (step S5). When the second target S/C is equal to or higher than the first target S/C, the process advances from step S6 to step S7, and the operating S/C setting means 102A sets the second target S/C as the operating S/C. Then, the control means 104 controls the operation of the water supply pump 34 to achieve this operating S/C (second target S/C) (step S8), thereby suppressing carbon deposition in the reformer 4. amount of reforming water is supplied. Note that the amount of reforming water supplied at this time is greater than the amount at which carbon precipitation occurs in the recycle channel 56, so that carbon precipitation does not occur in the recycle channel 56.

Further, when the first target S/C is larger than the second target S/C, the process advances from step S6 to step S9, and the operating S/C setting means 102A sets the first target S/C as the operating S/C. , the control means 104 controls the operation of the water supply pump 34 to achieve this operating S/C (first target S/C) (step S10), thereby reducing the amount of carbon precipitation in the recycling channel 56. of reforming water will be supplied. Note that the amount of reforming water supplied at this time is greater than the amount at which carbon precipitation occurs in the reformer 4, so that carbon precipitation does not occur in the reformer 4.

In addition, in this second embodiment, when the first target S/C calculated by the first target S/C calculation means 100 is selected, this first target S/C is set as the operating S/C. However, by setting a value larger than the first target S/C, carbon precipitation in the recycle flow path 56 can be suppressed, and the first target S/C, for example 1.1 to 1, can be set as the operating S/C. It may be set to a value of about 1.3 times. Further, when the second target S/C calculated by the second target S/C calculation means 114 is selected, this second target S/C is set as the operating S/C; By setting a value larger than /C, carbon precipitation in the reformer 4 can be suppressed, and the operating S/C can be set to a value of, for example, about 1.1 to 1.3 times the second target S/C. You may also set it.

Next, a third embodiment of the solid oxide fuel cell system according to the present invention will be described with reference to FIGS. 9 and 10. In this third embodiment, the second target S/C is configured to be corrected based on the reformer inlet temperature and the cell stack temperature.

In this third embodiment, a temperature sensor that detects the temperature of the cell stack 6, that is, a first temperature sensor 92 (constituting the first temperature detection means), and a temperature sensor that detects the inlet temperature of the reformer 4, that is, In addition to the second temperature sensor 102 (constituting the second temperature detecting means), a temperature sensor for detecting the temperature of the cell stack 6, that is, a third temperature sensor (constituting the third temperature detecting means) is provided. Detection signals from the first to third temperature sensors 92, 112, 122 are sent to the controller 94B.

The controller 94B also includes a fuel utilization rate calculation means 96, a recycle rate calculation means 98, a first target S/C calculation means 100, a second target S/C calculation means 114, a target S/C determination selection means 116, and an operation S/C calculation means 114. In addition to the /C setting means 102B and the control means 104, it further includes a first correction value calculation means 124 and a second correction value calculation means 126. The first correction value calculation means 124 calculates a first correction value as described below based on the inlet temperature of the reformer 4, and the second correction value calculation means calculates a first correction value as described below based on the temperature of the cell stack 6. The second target S/C calculation means 114B calculates the second target S/C based on the reference S/C data, the first correction value, and the second correction value. In the embodiment, the second target S/C is obtained by adding these values (second target S/C = basic S/C + first correction value + second correction value), and the specific configuration thereof is as follows. is substantially the same as the configuration disclosed in Japanese Patent Application Laid-Open No. 2013-187118, so please refer to this publication.

Furthermore, in addition to the generated current-fuel utilization rate map, generated current-recycle rate map, and first target S/C calculation data, the memory means 106B stores a reference S/C used when calculating the second target S/C. C data, a first correction map (i.e., a map showing the relationship between the reformer inlet temperature and the first correction value), a second correction map (i.e., a map showing the relationship between the cell stack temperature and the second correction value) and second target S/C calculation data are registered. The other configurations of this third embodiment are substantially the same as those of the second embodiment described above.

In this third embodiment, the operating S/C is set in the following manner, substantially similar to the second embodiment described above. First, the first target S/C is calculated in the same manner as described above. That is, steps S21 to S23 are executed in the same manner as steps 1 to S3 in the second embodiment.

Next, a second target S/C is calculated. In this third embodiment, the first correction value calculation means 124, based on the first correction map (map showing the relationship between the reformer inlet temperature and the first correction value) registered in the memory means 106B, A first correction value corresponding to the inlet temperature of the reformer 4 (that is, the temperature detected by the second temperature sensor 112) is calculated (step S24). Further, the second correction value calculation means 126 calculates the temperature of the cell stack 6 (i.e., the A second correction value corresponding to the temperature detected by the third temperature sensor 122 is calculated (step S25).

Then, the second target S/C calculation means 114B uses the second target S/C calculation data, the reference S/C data, the first correction value and the second correction value calculated by the first correction value calculation means 124. A second target S/C is calculated by adding the second correction value calculated by the calculation means 126 (step S26).

When the first target S/C and the second target S/C are determined in this way, the target S/C determination and selection means 116 determines the first target S/C and the second target S/C. The larger one is compared with the target S/C and selected (step S27). When the second target S/C is equal to or higher than the first target S/C, the process advances from step S28 to step S29, and the operating S/C setting means 102B sets the second target S/C as the operating S/C, The control means 104 controls the operation of the water supply pump 34 to achieve this operating S/C (second target S/C) (step S30).

Further, when the first target S/C is larger than the second target S/C, the process advances from step S28 to step S31, and the operating S/C setting means 102A sets the first target S/C as the operating S/C. , the control means 104 controls the operation of the water supply pump 34 to achieve this operating S/C (first target S/C) (step S32).

Although various embodiments of the solid oxide fuel cell system according to the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes can be made without departing from the scope of the present invention. It is possible.

2,2A solid oxide fuel cell system 4 reformer 6 cell stack 8 fuel electrode side 12 vaporizer 24 fuel gas supply pump (fuel gas supply means)
34 Water supply pump (water supply means)
36 Air electrode side (oxygen electrode side)
40 Air blower (air supply means)
46 Steam condensing section 48 Fuel off-gas supply passage 50 Combustor 56 Recycle passage 62 High temperature housing 92 First temperature sensor (first temperature detection means)
94, 94A, 94B Controller 96 Fuel utilization rate calculation means 98 Recycle rate calculation means 100 Target S/C calculation means (first target S/C calculation means)
102, 102A, 102B Operating S/C setting means 104 Control means 112 Second temperature sensor (second temperature detection means)
114 Second S/C calculation means 116 Target S/C judgment selection means

Claims (5)

a reformer for steam reforming fuel gas; a cell stack for generating electricity through an electrochemical reaction of the reformed fuel gas and oxidizing gas reformed in the reformer; An oxidant gas supply means for supplying the cell stack, a fuel gas supply means for supplying the fuel gas to the reformer, and a water supply means for supplying reforming water used for steam reforming. , a combustor provided in association with the reformer, a steam condensing section that condenses water vapor contained in the fuel off-gas from the cell stack, and a part of the fuel off-gas on the upstream side of the fuel gas supply means. and a controller for controlling the operation of the fuel gas supply means, the oxidant gas supply means, and the water supply means, the reformed fuel gas from the reformer is returned to the The oxidizing gas from the oxidizing gas supply means is fed to the fuel electrode side of the cell stack, and the oxidizing gas from the oxidizing gas supply means is fed to the fuel off gas from the fuel electrode side of the cell stack. The water vapor contained therein is cooled and condensed in the water vapor condensing section, a part of the fuel off-gas from which the water vapor has been removed is returned to the upstream side of the fuel gas supply means through the recycling flow path, and the remainder is A solid oxide fuel cell system that is fed to the combustor and combusted by oxidant off-gas from the oxygen electrode side of the cell stack,
The controller includes a first target S/C calculating means for calculating a first target S/C, which is an S/C condition for preventing carbon deposition in the recycling channel, and a system operating S/C. and a control means for controlling the operation of the water supply means, and the first target S/C calculation means is a proportion of the fuel off-gas returned through the recycling flow path. The first target S/C is calculated using the recycling rate, and the operating S/C setting means sets the first target S/C to be equal to or higher than the first target S/C calculated by the first target S/C calculating means. A solid oxide fuel cell system, wherein the operating S/C is set, and the control means controls the operation of the water supply means so that the operating S/C is set. The controller further includes a first temperature detection means for detecting a fuel electrode outlet temperature of the cell stack, and the first target S/C calculation means of the controller detects the recycle rate, the supply flow rate of the fuel gas, and the cell stack. The solid oxidizer according to claim 1, wherein the first target S/C is calculated based on the fuel utilization rate determined by the generated current or generated output and the fuel electrode outlet temperature of the cell stack. Physical fuel cell system. The first target S/C calculated by the first target S/C calculation means is a carbon deposition value K obtained by a carbon deposition experiment using a boudoir reaction,
K=PCO ₂ /(PCO) ² (partial pressure: MPa)
Calculate the safe carbon deposition value K' by adding a safety factor to the carbon deposition value K, and further calculate the gas composition using the fuel utilization rate, the recycling rate, and the fuel electrode outlet temperature of the cell stack as variables. Regarding the safe carbon deposition value K' in the equilibrium calculated gas composition,
K'= PCO ₂ / (PCO) ² (partial pressure: MPa)
Calculate the S/C to achieve the following, and use the safe carbon deposition value K' to calculate the S/C value based on the fuel utilization rate, the recycling rate, and the fuel electrode outlet temperature of the cell stack. The solid oxide fuel cell system according to claim 2, characterized in that: The controller includes a second target S/C calculating means for calculating a second target S/C, which is an S/C condition for preventing carbon precipitation in the reformer, and a second target S/C calculating means for determining a target S/C of the system. The target S/C determination selection means determines and selects the larger of the first target S/C and the second target S/C, and The S/C setting means sets the operating S/C so that when the first target S/C is selected, the operating S/C is equal to or higher than the first target S/C, and the second target S/C is When selected, the operating S/C is set to be equal to or higher than the second target S/C , and the control means controls the operation of the water supply means so that the operating S/C is achieved. The solid oxide fuel cell system according to claim 1 or 2, characterized in that: The reformer further includes a second temperature detection means for detecting a reformer inlet temperature, and the second target S/C calculation means calculates the second target S/C based on the reformer inlet temperature. 5. The solid oxide fuel cell system according to claim 4, wherein C is calculated.

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