JP7116651B2 - Solid oxide fuel cell system - Google Patents

Description

The present invention relates to a solid oxide fuel cell system having a cell stack that generates power by oxidation and reduction of a reformed fuel gas obtained by reforming a raw fuel and an oxidant gas.

2. Description of the Related Art Conventionally, solid oxide fuel cell systems in which a solid oxide cell stack using a solid electrolyte as a membrane that conducts oxide ions is housed in a container have been known. In this solid oxide fuel cell system, the 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 the 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 this fuel cell is relatively high, about 700 to 1000°C. Electricity is generated by causing an electrochemical reaction.

Typical small fuel cell systems for domestic use include a solid oxide fuel cell system (so-called SOFC) and a polymer electrolyte fuel cell system (so-called PEFC). In this polymer electrolyte fuel cell system (PEFC), the balance of the heat recovered from the fuel cell system (in other words, hot water stored in the hot water tank) is monitored with respect to the domestic hot water demand, and the fuel cell system Operation control is performed to stop the operation or restrict the power generation output. In addition, the solid oxide fuel cell system (SOFC) has high power generation efficiency and a small ratio of heat to power output. done. This type of operation control is also not suitable for frequent starting and stopping due to the high power generation temperature (so-called operating temperature) of the cell stack.

This solid oxide fuel cell system basically operates continuously day and night, and with regard to heat utilization, a hot water storage tank is used. keeps In the current solid oxide fuel cell system, high power generation efficiency can be obtained even at a power generation output scale of about 700 W. The efficiency is about 52% (AC sending end, lower heating value standard). If the power generation efficiency is high, the installer will be able to obtain economic benefits even if it is installed in a house with little heat demand, and the target market can be expected to expand. being promoted. In the future, if the power generation is further improved, the installer will be able to obtain economic benefits and environmental friendliness with a simple heat utilization form that does not use heat or does not require a hot water storage tank, so it is important to further improve power generation efficiency. It has become.

A solid oxide fuel cell system suitable for home use includes a reformer for steam reforming the raw fuel gas, and oxidation of the reformed fuel gas reformed by the reformer and the oxidant gas. and a cell stack for generating power by reduction, an air supply means for supplying air as an oxidant gas to the oxygen electrode (air electrode) side of the cell stack, and a fuel for supplying raw fuel gas to the reformer A gas supply means is provided, and a cell stack and a reformer are housed in a high-temperature space in which a high temperature is maintained (see, for example, Patent Document 1).

In this solid oxide fuel cell system, a combustion zone is provided above the cell stack, and a reformer is arranged above this combustion zone. The reformed fuel gas from the reformer is supplied to the fuel electrode side of the cell stack, the air from the air supply means is supplied to the air electrode side of the cell stack, and the electrochemical reaction in the cell stack generates electricity. is done. Fuel off-gas from the fuel electrode side of the cell stack (that is, anode off-gas) and air off-gas from the air electrode side (that is, cathode off-gas) are fed to the combustion zone and burned, and the combustion heat is used to create a high-temperature space. is maintained at a high temperature, and the reformer and the like are heated.

As a solid oxide fuel cell system, instead of providing a combustion zone on the upper side of the cell stack, there has also been proposed a system provided with a dedicated combustor (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 supplied to the combustor through the fuel off-gas supply channel, and air off-gas from the air electrode side of the cell stack is supplied to the combustor. (Cathode off-gas) is fed to the combustor through the air off-gas feeding channel, where the fuel off-gas is combusted by the air off-gas, and the combustion heat is used to keep the high-temperature space at a high temperature, The reformer and the like are heated.

The power generation efficiency of such a solid oxide fuel cell system is approximately proportional to the product of the average generated voltage of the fuel cell and the fuel utilization rate. Hydrogen (H ₂ ) in the pole-side gas is consumed to produce water (H ₂ O). In general, the fuel electrode of a solid oxide fuel cell uses a cermet (a mixture of metallic nickel and ceramics) in which metallic nickel particles are present together with ceramic particles. As the hydrogen to water ratio (H ₂ /H ₂ O) increases, the metallic nickel particles will begin to oxidize. When the metal nickel particles on the fuel electrode side of the fuel cell are oxidized, their electrical resistance increases, so that no power is generated in the vicinity of the fuel electrode side exit of the fuel cell. The area where nickel particles are oxidized will expand, and the area where power generation will be difficult will expand.

On the other hand, the fuel electrode thus oxidized is reduced under other conditions during power generation (partial load operation state, start state, stop state, etc.) and returns to the nickel state, and the nickel particles are repeatedly oxidized and reduced. However, if the number of repetitions increases, there is a risk that the material on the fuel electrode side of the fuel cell will undergo dimensional changes. If the material on the fuel electrode side undergoes a dimensional change, cracks will occur in the solid electrolyte that is joined to the fuel electrode. For this reason, it is necessary to increase the fuel utilization rate of the solid oxide fuel cell system in order to increase the power generation efficiency of the system, but if it is too high, the cell stack may be damaged.

In addition, in terms of reaction equilibrium, in the operating temperature range of the solid oxide fuel cell (cell stack), if hydrogen (H ₂ ):water (H ₂ 0) is less than 5:95, hydrogen (H ₂ ) is known to oxidize. In fact, since water vapor is generated by power generation at the electrolyte/anode interface of the fuel cell, the H ₂ /H ₂ 0 value tends to be lower at the electrolyte/anode interface than in the bulk (gas flow path). . In addition, the distribution state of the fuel gas to each fuel cell and the distribution state of the fuel gas within the fuel cell are also added, and the upper limit of the fuel utilization rate that can actually be used is hydrogen (H ₂ ):water (H ₂ 0) is limited to much lower fuel utilization than 5:95.

As a method for improving the fuel utilization rate of a solid oxide fuel cell system, part of the fuel offgas (that is, anode offgas) used for power generation in the cell stack is returned to the supply side of the cell stack, and this returned fuel is It is well known to mix the off-gas with the fuel gas and feed it back to the cell stack. In this case, it is known that the fuel utilization rate of the fuel cell system can be further increased by removing carbon dioxide (CO2) and water (H2O) contained in the fuel off-gas (see, for example, Patent Document 3). ).

JP-A-2005-285340 Japanese Unexamined Patent Application Publication No. 2008-21596 Japanese Patent No. 2981571

However, the fuel cell system of Patent Document 3 is configured assuming a power generation output of several tens of KW or more. (Anode off-gas) is returned to the fuel gas supply side, and how to grasp the recycling rate of the fuel off-gas. Throughout this specification, the recycling rate is the ratio of the fuel off-gas flowing out of the fuel electrode side of the cell stack that is returned to the fuel gas supply side. is returned to the fuel supply side, and the remaining 70% is distributed so as to flow downstream, the recycling rate is expressed as 30%.

This recycling rate is determined by the pressure loss characteristics of the flow path such as the recycling flow path, the flow path from the vaporizer/reformer to the cell stack, and the flow path from the cell stack to the atmosphere, and the fuel gas supply means (e.g., fuel gas pump). ), and the higher the recycling rate, the higher the fuel utilization rate can be set. However, if the fuel utilization rate of the cell stack is set high while the recycling rate is low, the fuel utilization rate becomes too high, which may lead to a shortened life of the cell stack.

Small fuel cell systems used in household cogeneration systems have a small power output per unit, but when used in many households, the number of units installed increases, so maintenance-free is one of the strengths of the product. is considered important for As of 2016, among domestic fuel cell cogeneration systems, polymer electrolyte fuel cell systems (PEFC) and solid oxide fuel cell systems (SOFC) have been sold domestically as maintenance-free specifications for about 10 years. During long-term operation over 10 years, scale deposition of trace impurities contained in the reforming water in the vaporizer and cracking of the catalyst in the reformer occur, and these causes do not lead to blockage. It often happens that the pressure loss increases from the initial state. If the pressure loss increases due to such factors, the recycling rate of the fuel off-gas will decrease from the initial level. In addition, in the solid oxide fuel cell system, the recycling rate is also affected by state variables such as cell stack temperature and air flow rate and operating conditions. For this reason, in a solid oxide fuel cell system in which the fuel off-gas is returned to the upstream side of the fuel gas supply means, it is desirable to recycle the fuel off-gas while maintaining an appropriate fuel utilization rate. For this reason, it is desired to realize a method for easily grasping the recycling rate of fuel off-gas.

An object of the present invention is to provide a solid oxide fuel cell system that can grasp fluctuations in the recycling rate of fuel off-gas due to fluctuations in pressure loss and set a fuel utilization rate that matches this recycling rate. be.

A solid oxide fuel cell system according to claim 1 of the present invention comprises: a reformer for steam reforming raw fuel gas; a vaporizer for generating steam used for steam reforming; a cell stack for generating electricity by oxidation and reduction of the reformed fuel gas reformed in the reformer and oxygen in the air; fuel gas supply means for supplying the raw fuel gas to the reformer; and the cell stack an air supply means for supplying air to the cell stack, a fuel flow rate measuring means for measuring the flow rate of the raw fuel gas, a cooler for cooling the fuel off-gas after the power generation reaction in the cell stack, and the cooling a combustor for combusting the fuel off-gas after being cooled by the combustor and the air off-gas after the power generation reaction of the cell stack; A solid oxidizer comprising: a recycle channel for leading to an upstream side of a supply means; a recycle valve disposed in the recycle channel; and a controller for controlling operations of the fuel gas supply means and the air supply means. A physical fuel cell system,
The controller includes fuel utilization rate setting means for setting a fuel utilization rate and fuel utilization rate correcting means for correcting the fuel utilization rate. and the driving duty ratio of the fuel gas supply means in the closed state or the small opening state of the recycle valve.

Further, in the solid oxide fuel cell system according to claim 2 of the present invention, a reformer for steam reforming raw fuel gas, a vaporizer for generating steam used for steam reforming, a cell stack that generates electricity by oxidation and reduction of the reformed fuel gas reformed by the reformer and oxygen in the air; a fuel gas supply means for supplying the raw fuel gas to the reformer; air supply means for supplying air to the cell stack; fuel flow rate measurement means for measuring the flow rate of the raw fuel gas; a combustor, a recycling passage for guiding part of the fuel off-gas to the upstream side of the fuel gas supply means, a cooler for cooling the fuel off-gas flowing through the recycling passage, and the recycling passage A solid oxide fuel cell system comprising a recycle valve disposed in and a controller for controlling the operation of the fuel gas supply means and the air supply means,
The controller includes fuel utilization rate setting means for setting a fuel utilization rate and fuel utilization rate correcting means for correcting the fuel utilization rate. and the driving duty ratio of the fuel gas supply means in the closed state or the small opening state of the recycle valve.

Further, in the solid oxide fuel cell system according to claim 3 of the present invention, the fuel utilization rate correcting means corrects the driving duty ratio difference of the fuel gas supply means before and after opening and closing of the recycle valve or before and after the degree of opening of the recycle valve. is greater than the reference duty ratio difference value, it is determined that the fuel off-gas recycling rate is fluctuating on the increasing side, and the fuel utilization rate is corrected to increase. When the driving duty ratio difference of the fuel gas supply means is smaller than the reference duty ratio difference value, it is determined that the fuel off-gas recycling rate is fluctuating toward the decreasing side, and correction is made so that the fuel utilization rate becomes smaller. and

Further, in the solid oxide fuel cell system according to claim 4 of the present invention, the recycle valve is composed of an on-off valve having a valve body for opening and closing the recycle flow path, and the on-off valve has the A bypass passage is provided to bypass the valve body, and part of the fuel off-gas flowing in the recycling passage flows upstream of the fuel gas supply means through the bypass passage.

Further, in the solid oxide fuel cell system according to claim 5 of the present invention, the recycle valve is composed of an on-off valve having a valve body for opening and closing the recycle flow path, and the A through hole is provided in the valve body, and part of the fuel off-gas flowing in the recycling passage flows through the through hole of the valve body to the upstream side of the fuel gas supply means.

Further, in the solid oxide fuel cell system according to claim 6 of the present invention, the recycle valve is composed of a proportional valve provided with a valve body for controlling opening and closing of the recycle flow path, and the valve body A part of the fuel off-gas flowing through the recycle flow path flows upstream of the fuel gas supply means through the proportional valve when the opening is small.

Further, in the solid oxide fuel cell system according to claim 7 of the present invention, temperature detection means is provided in relation to the fuel gas supply means, and the temperature detected by the temperature detection means exceeds a predetermined set temperature. Further, the controller permits control of opening or large opening of the recycle valve based on a detection signal from the temperature detection means.

According to the solid oxide fuel cell system according to claims 1 and 2 of the present invention, fuel gas supply means for supplying fuel gas to the fuel electrode side of the cell stack and oxidant to the air electrode side of the cell stack are provided. A controller for controlling the operation of the air supply means for supplying air as a The fuel utilization rate correcting means determines the drive duty ratio of the fuel gas supply means when the recycle valve is open (or when the degree of opening is large) and the fuel gas when the recycle valve is closed (or when the degree of opening is small). Since the fuel utilization rate is corrected based on the drive duty ratio of the supply means, the fuel utilization rate can be corrected to match fluctuations in the fuel off-gas recycling rate, and operation can be performed at a high fuel utilization rate suitable for the system.

Further, according to the solid oxide fuel cell system according to claim 3 of the present invention, the fuel utilization rate correcting means determines the drive duty ratio of the fuel gas supply means before and after opening and closing (or before and after the degree of opening) of the recycle valve. When the difference is greater than the reference duty ratio difference value, the fuel utilization rate is corrected to increase, and the driving duty ratio difference of the fuel gas supply means before and after opening and closing the recycle valve (or before and after the degree of opening) is the reference duty ratio difference value. Since the fuel utilization rate is corrected to be smaller when it is smaller, the fuel utilization rate is corrected to match the fluctuation of the fuel off-gas recycling rate, and operation can be performed at a high fuel utilization rate suitable for the system.

Further, according to the solid oxide fuel cell system according to claim 4 of the present invention, the recycle valve is composed of an on-off valve having a valve body for opening and closing the recycle flow path, and the valve body is bypassed. Since the bypass flow path is provided, a part of the fuel off-gas flows to the upstream side of the fuel gas supply means through the bypass flow path even when the valve body is in the closed state, and the fuel off-gas is converted into the raw fuel gas. It can be mixed and fed to the reformer.

Further, according to the solid oxide fuel cell system according to claim 5 of the present invention, the recycle valve is composed of an on-off valve having a valve body for opening and closing the recycle flow path, and the valve of the on-off valve Since the body is provided with a through-hole, part of the fuel off-gas flows through the through-hole of the valve body to the upstream side of the fuel gas supply means even when the valve body is closed, and this fuel off-gas is converted into the raw fuel gas. It can be mixed and fed to the reformer.

According to the solid oxide fuel cell system of claim 6 of the present invention, the recycle valve is composed of a proportional valve having a valve body for controlling opening and closing of the recycle flow path. Part of the fuel off-gas flows through the proportional valve to the upstream side of the fuel gas supply means even when the opening of the body is small, and this fuel off-gas can be mixed with the raw fuel gas and supplied to the reformer. can.

Furthermore, according to the solid oxide fuel cell system according to claim 7 of the present invention, the temperature detection means is arranged in relation to the fuel gas supply means, and the temperature detected by the temperature detection means exceeds the predetermined set temperature. If it exceeds, the controller permits the control of opening the recycle valve (or large opening state). As a result, the fuel off-gas does not flow to the upstream side of the fuel gas supply means through the recycle flow path, and as a result, dew condensation in the fuel gas supply means can be suppressed.

1 is a simplified diagram showing the entirety of a first embodiment of a solid oxide fuel cell system according to the present invention; FIG. FIG. 2 is a block diagram schematically showing a control system of the solid oxide fuel cell system of FIG. 1; FIG. 3 is a flow chart showing the flow of control by the control system in FIG. 2; FIG. A test fuel cell system for determining in advance the relationship between the duty ratio of the fuel gas supply means, the fuel gas flow rate, the air flow rate, and the recycling rate. FIG. 5 is a flow chart showing a flow for obtaining a predetermined relationship in advance using the test fuel cell system shown in FIG. 4; FIG. FIG. 2 is a simplified diagram showing the entirety of a second embodiment of a solid oxide fuel cell system according to the present invention;

Various embodiments of the solid oxide girder fuel cell system according to the present invention will now be described with reference to the accompanying drawings. First, a solid oxide fuel cell system of a first embodiment will be described with reference to FIGS. 1 to 3. FIG.

In FIG. 1, the illustrated solid oxide fuel cell system 2 consumes raw fuel gas (for example, city gas, LP gas, etc.) to generate power. and a solid oxide cell stack 6 that generates electricity by oxidation and reduction of the fuel gas reformed in the reformer 4 and air as an oxidant.

The cell stack 6 is configured by stacking a plurality of solid oxide fuel cells for generating power through a fuel cell reaction via current collecting members, and conducts oxygen ions (not shown). It comprises a solid electrolyte, a fuel electrode provided on one side of the solid electrolyte, and an 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 through a reformed fuel gas supply channel 10. In this embodiment, the reformer 4 is a vaporizer for vaporizing reforming water. 12 and integrally configured. The vaporizer 12 is configured separately from the reformer 4 so that the vapor vaporized by the vaporizer 12 is fed to the reformer 4 through a vapor feed passage (not shown). can be

The vaporizer 12 is connected to a fuel gas supply source (not shown) for supplying raw fuel gas through a fuel gas supply passage 14 (consisting of, for example, an embedded pipe, a storage tank, etc.). Raw fuel gas from a gas supply source is supplied to the vaporizer 12 through a fuel gas supply passage 14 . The fuel gas supply passage 14 may be connected to the reformer 4 to directly supply the raw fuel gas from the fuel gas supply source to the reformer 4 .

The vaporizer 12 is also connected to a water supply source (not shown) (consisting of, for example, a water tank, a water recovery tank, etc.) through a water supply channel 18, and reforming from the water supply source is performed. Service water is supplied to vaporizer 12 through water supply channel 18 . A reforming catalyst is housed in the reformer 4. For example, a reforming catalyst in which ruthenium is supported on alumina is used, and the fuel gas supplied through the fuel gas supply passage 14 is vaporized by this reforming catalyst. Steam reforming is performed with the steam vaporized in the vessel 12 .

In the fuel gas supply passage 14, from the carburetor 12 toward the upstream side, a desulfurizer 20, a first throttle member 22, a fuel gas pump 24 (constituting fuel gas supply means), a second throttle member 26, and a pressure regulator are arranged in this order. A zero governor 28, a fuel flow rate sensor 30 (constituting fuel flow rate measuring means) and a shutoff valve 32 are provided as members. The desulfurizer 20 removes the sulfur component contained in the raw fuel gas (the sulfur component in the odorant), and the fuel gas pump 24 pressurizes the raw fuel gas flowing through the fuel gas supply passage 14 and supplies it to the vaporizer 12. supply.

Further, the zero governor 28 adjusts the raw fuel gas supplied from the fuel gas supply source (not shown) through the fuel gas supply passage 14 to a predetermined pressure (that is, atmospheric pressure), and the fuel gas flow rate sensor 30 The flow rate of the raw fuel gas supplied through the gas supply passage 14 is measured, and when the cutoff valve 32 is closed, it shuts off the fuel gas supply passage 14 to stop the supply of fuel gas. Further, the first and second throttle members 22 and 26 located on both sides of the fuel gas pump 24 are provided to stabilize the flow rate of the raw fuel gas flowing through the fuel gas supply passage 14. The first throttle member 22 is, for example, Constructed from a capillary tube, the second throttle member 26 is constructed from, for example, a throttle member having a small orifice. Further, the fuel gas flow sensor 30 can be composed of, for example, a thermal flow sensor, and by using this thermal flow sensor, the flow rate of the raw fuel gas can be instantaneously measured.

Further, a water pump 34 is provided in the water supply channel 18, and the water pump 34 supplies reforming water from a water supply source (not shown) through the water supply channel 18 to the vaporizer 12. . When the raw fuel gas can be stably supplied, the first throttle member 22 may be omitted, or the second throttle member 26 may be replaced by a buffer tank. .

The air electrode side 36 of the cell stack 6 is connected through an air supply passage 38 to an air blower 40 as air supply means. ) are provided. The air blower 40 supplies air (oxidant) through the air supply channel 38 to the air electrode side 36 of the cell stack 6 , and the air flow rate sensor 42 measures the flow rate of the air flowing through the air supply channel 38 .

In this embodiment, the rotation control of the fuel gas pump 24 (and the water pump 34 and the air blower 40) is controlled by the duty ratio of the drive current (so-called drive duty ratio). (and water pump 34, air blower 40) increases, the supply flow rate of raw fuel gas (and reforming water, air) increases, while the drive duty ratio decreases, fuel gas pump 24 (and water pump 34) , the rotation speed of the air blower 40) becomes small, and the supply flow rate of the raw fuel gas (and reforming water and air) is small. be.

Fuel off-gas (that is, anode off-gas) discharged from the fuel electrode side 8 of the cell stack 6 is supplied to the combustor 46 through the fuel off-gas supply passage 44, as will be described in detail later. Air off-gas discharged from the cathode side 36 (that is, cathode off-gas) is fed through an air off-gas feed channel 48 to a combustor 46 where it is fed from the fuel electrode side 8 of the cell stack 6. The fuel off-gas (containing fuel gas) and the air off-gas (containing oxygen) from the air electrode side 36 are combusted, and the combustion heat of this fuel off-gas is used to drive the vaporizer 12 and reformer 4. heated. Combustion exhaust gas from the combustor 46 is discharged to the atmosphere through an exhaust gas discharge passage 50 .

In this embodiment, a first heat exchanger 52 is disposed in association with the fuel offgas feed channel 44 , the first heat exchanger 52 being upstream of the fuel offgas feed channel 44 from the cell stack 6 . fuel offgas flowing through a portion (a portion inside the high temperature housing 52 to be described later) and a fuel flowing through a downstream portion of the fuel offgas delivery channel 44 (a portion located inside the high temperature housing 54 again after being introduced outside the high temperature housing 54) It exchanges heat with the off-gas.

A second heat exchanger 56 is also disposed in association with the air supply passage 38 and the exhaust gas discharge passage 50 , and the second heat exchanger 56 is adapted for the combustion exhaust discharged through the exhaust gas discharge passage 50 . Heat is exchanged between the gas and the air flowing through the air supply channel 38 .

In this embodiment, reformer 4 , cell stack 6 , vaporizer 12 , combustor 38 , first heat exchanger 52 and second heat exchanger 56 are housed in high temperature housing 54 . The high-temperature housing 54 is made of metal (for example, stainless steel), the inner surface of which is covered with a heat insulating member (not shown), and defines a high-temperature space 58 inside thereof, the reformer 4 and the cells The stack 6 , evaporator 12 , combustor 30 , first heat exchanger 52 and second heat exchanger 56 are kept hot in this hot space 58 .

Further, in this solid oxide fuel cell system 2, a part of the fuel off-gas supply channel 44 is outside the high temperature housing 54 in order to temporarily cool the fuel off-gas from the fuel electrode side 8 of the cell stack 6. After being led out, it is again introduced into the high-temperature housing 54 and led to the combustor 46 . Further, in relation to this, a third heat exchanger 60 is further arranged in the middle portion of the fuel offgas supply channel 44 (specifically, the portion located outside the high temperature housing 54). This third heat exchanger 60 is used, for example, in combination with a circulation flow path 62 of a hot water storage device (not shown) for a co-generation system for storing the heat of the fuel off-gas as hot water, and supplying the fuel gas off-gas. Heat is exchanged between the fuel off-gas flowing through the flow path 44 and water from the hot water storage device, and the third heat exchanger 60 functions as a cooler for cooling the fuel off-gas.

A drain separator 64 is disposed downstream of the third heat exchanger 60, and water vapor contained in the fuel off-gas cooled by heat exchange by the third heat exchanger 60 is condensed into water. is separated from the fuel off-gas at the drain separator 64 and drained to the outside, and the fuel off-gas from which moisture has been removed is fed to the combustor 46 through the fuel off-gas feed channel 44 . Instead of discharging the condensed water to the outside, the condensed water may be recovered in a water recovery tank (not shown), and the recovered condensed water may be used as reforming water.

This solid oxide fuel cell system is further configured to return part of the fuel off-gas to the fuel gas supply channel 14 for recycling. More specifically, the fuel off-gas supply channel 44 (specifically, the portion exposed to the outside of the high temperature housing 54 and downstream of the drain separator 64) and the fuel gas supply channel 14 (specifically, is provided with a recycle flow path 66 so as to connect the portion between the fuel gas pump 24 and the second throttle member 26 , and a recycle valve 68 is disposed in the recycle flow path 66 .

With this configuration, when the recycle valve 68 is opened, the fuel offgas flowing through the fuel offgas supply passage 44 (in this form, the water is removed after being cooled in the third heat exchanger 60). A part of the fuel off-gas (after the fuel off-gas) is returned to the fuel gas supply channel 14 through the recycle channel 66, and this returned fuel off-gas is mixed with the raw fuel gas and sent to the carburetor 12 through the desulfurizer 20. be. Also, when the recycle valve 68 is closed, the recycle flow path 66 is closed and the fuel off-gas is not recycled to the fuel gas supply flow path 14 .

In this embodiment, since the downstream side of the recycle flow path 66 is connected between the fuel gas pump 24 and the second throttle member 26 disposed downstream of the zero governor 28, the fuel gas pump 24 operates to recycle the raw fuel. When the gas is supplied, the connection portion of the fuel gas supply channel 14 with the recycle channel 66 becomes somewhat negative pressure, and this negative pressure is used to recycle the fuel off-gas through the recycle channel 66. .

In this embodiment, as a desulfurizing agent used in the desulfurizer 20 , an ultra-high-order desulfurizing agent having a high desulfurizing action is used. system desulfurization agents are used. In connection with the use of such a desulfurizing agent, it is preferable to configure as follows.

The recycle valve 68 is composed of, for example, an on-off valve having a valve element (not shown) for opening and closing the recycle flow path 66. Bypassing the valve element of the on-off valve ) may be provided. When such an on-off valve is used, even when the recycle valve 68 (in this case, on-off valve) is in a closed state, a small amount of the fuel off-gas flowing through the recycle flow path 66 passes through the bypass flow path to the fuel gas supply flow path 14 ( Specifically, the fuel off-gas containing hydrogen is mixed with the raw fuel gas and fed to the desulfurizing agent in the desulfurizer 20 through the fuel gas supply passage 14. A desulfurizing agent can be used under hydrogen usage conditions.

The recycle valve 68 is composed of, for example, an on-off valve having a valve body (not shown) for opening and closing the recycle flow path 66, and a through hole (not shown) is provided in the valve body of the on-off valve. You may do so. When such an on-off valve is used, even when the recycle valve 68 (in this case, on-off valve) is in a closed state, a small amount of the fuel off-gas flowing through the recycle passage 66 passes through the through-hole of the valve body to the fuel gas supply passage 14. (Upstream side of the fuel gas pump 24), even with this configuration, the fuel off-gas containing hydrogen can be mixed with the raw fuel gas.

The recycle valve 68 is composed of, for example, a proportional valve having a valve element (not shown) for controlling the opening and closing of the recycle flow path 66. may be controlled to open and close. When such a proportional valve is used, even when the recycle valve 68 is in the small opening state, part of the fuel off-gas flowing in the recycle passage 66 passes through the proportional valve to the fuel gas supply passage (upstream side of the fuel gas pump 24). ), and even with this configuration, the fuel off-gas containing hydrogen gas can be mixed with the raw fuel gas.

Next, the power generation operation of this solid oxide fuel cell system 2 will be described. During power generation operation, raw fuel gas from a fuel gas supply source (not shown) is supplied through the fuel gas supply passage 14 by the fuel gas pump 24, and the raw fuel gas thus supplied is supplied in the manner described above. The fuel off-gas to be recycled through the recycle passage 66 is mixed, the mixed fuel gas is fed to the fuel gas pump 24, and the mixed fuel gas pressurized by the fuel gas pump 24 is sent through the fuel gas supply passage 14 to the desulfurizer 20. sent to

In the desulfurizer 20 , the sulfur component contained in the mixed fuel gas is removed by the desulfurizing agent. Since the mixed fuel gas to be desulfurized contains hydrogen, the desulfurizing agent stably exerts an excellent desulfurization action and can remove sulfur components contained in the mixed fuel gas as desired. can.

The raw fuel gas (mixed fuel gas) desulfurized by the desulfurizer 20 is supplied to the vaporizer 12 through the fuel gas supply passage 14 . The vaporizer 12 is also supplied with reformed water through a water supply passage 18, vaporized by the vaporizer 12 into steam, and the generated steam and raw fuel gas (mixed fuel gas) are mixed. It is sent to the reformer 4 .

In the reformer 4 , the raw fuel gas (mixed fuel gas) is steam-reformed by steam, and the steam-reformed raw fuel gas passes through the reformed fuel gas feed passage 10 to the fuel electrode side 8 of the cell stack 6 . sent to Air is supplied to the air electrode side 36 of the cell stack 6 through an air supply channel 38 .

In the cell stack 6, power is generated by oxidation and reduction of the reformed fuel gas flowing on the fuel electrode side 8 and the air (oxygen in the air) flowing on the air electrode side 36, and the DC power obtained by the power generation is Although not shown, the AC power is converted into AC power through a power conditioner and supplied to a household demand end.

Fuel off-gas (which contains fuel gas) from the anode side 8 of the cell stack 6 is delivered to the combustor 46 through the fuel off-gas delivery passage 44, and air off-gas from the cathode side 36 is delivered to the air off-gas delivery. The combustion exhaust gas from the combustor 46 is discharged to the atmosphere through an exhaust gas discharge channel 50.

During this power generation operation, in the first heat exchanger 52, the fuel offgas flowing from the cell stack 6 through the upstream portion of the fuel offgas feed passage 44 to its intermediate portion (the portion located outside the high temperature housing 54) and this Heat exchange is performed between the intermediate portion and the fuel off-gas flowing to the combustor 66 through the downstream portion of the fuel off-gas supply channel 44 , and the fuel off-gas heated by this heat exchange is supplied to the combustor 46 . .

In the second heat exchanger 56, heat is exchanged between the combustion exhaust gas flowing through the exhaust gas discharge passage 50 and the air flowing through the air supply passage 38, and the air heated by this heat exchange is converted into a cell. It is fed to cathode side 36 of stack 6 .

Furthermore, in the third heat exchanger 60, heat is exchanged between the fuel off-gas flowing through the fuel off-gas supply passage 44 and the water flowing through, for example, the circulation passage 62 of the hot water storage device. The warm water is stored in a hot water storage tank (not shown) of the hot water storage device.

Further, when the recycle valve 68 is opened during such power generation operation, part of the fuel off-gas flowing through the fuel off-gas supply passage 44 is recycled to the fuel gas supply passage 14 through the recycle passage 66. The resulting fuel off-gas is mixed with the raw fuel gas flowing through the fuel gas supply passage 14 . Further, when the recycle valve 68 is closed, a small amount of the fuel off-gas is recycled through the recycle passage 66 and a bypass passage (not shown) of the recycle valve 68, and this small amount of fuel off-gas flows through the fuel gas supply passage 14. It is mixed with the flowing raw fuel gas.

In this solid oxide fuel cell system, the fuel utilization rate is corrected by the control system shown in FIG. 2 together with FIG. 1, the illustrated solid oxide fuel cell system 2 includes a controller 72 for controlling the operation of the system, and is controlled by this controller 72 as described later.

More specifically, detection signals from the air flow rate sensor 42 (air flow rate measuring means) and the fuel gas flow rate sensor 30 (fuel flow rate measuring means) are sent to the controller 72, which controls the fuel gas pump 24, air blower 40 and Recycle valve 68 is controlled as described below.

The illustrated controller 72 is composed of, for example, a microprocessor, and includes operation control means 74, fuel utilization rate setting means 76, flow rate stability determination means 78, fuel utilization rate drop setting means 80, duty ratio difference calculation means 82, fuel utilization rate It includes determination means 84 , fuel utilization correction means 86 and memory means 88 . The operation control means 74 controls the operation of the fuel gas pump 24, the air blower 40, the recycle valve 68, etc. as will be described later, and the fuel utilization rate setting means 76 sets the fuel utilization rate of the system. The fuel utilization rate here refers to the ratio of the raw fuel gas supply flow rate consumed for power generation in the cell stack 6, and in this solid oxide fuel cell system 2, for example, a fuel utilization rate of 80% means that the supply 80% of the raw fuel gas is consumed for power generation, and the remaining 20% is combusted in the combustor 46, and this combustion heat keeps the inside of the high temperature housing 54 in a high temperature state.

Further, the flow rate stability determination means 78 determines whether the flow rate of the raw fuel gas is stable based on the flow rate detected by the fuel gas flow rate sensor 30, and the fuel utilization rate drop setting means 80 determines the fuel utilization rate as described later. For example, lower by 0.5 points. For example, in the rated power generation state of the cell stack 6, the raw fuel gas consumption is constant, so the supply flow rate of the raw fuel gas is increased to lower the fuel utilization rate.

Furthermore, the duty ratio difference calculation means 82 calculates the drive duty ratio difference of the drive current of the fuel gas pump 24 as described later, and the fuel utilization rate determination means 84 calculates the drive duty ratio of the fuel pump 24 before and after the recycle valve 68 is opened and closed. Based on (in this embodiment, the drive duty ratio difference), it is determined whether the set fuel utilization rate is appropriate. The utilization rate is corrected as described later. A reference map is registered in advance in the memory means 88 as a criterion for judging whether the set fuel utilization rate is appropriate.

Here, the reference map registered in the memory means 88 will be explained. In order to create this reference map, the drive duty ratio of the fuel gas pump, the fuel gas flow rate, the air flow rate, and the recycle rate were calculated using the test fuel cell system (in other words, the fuel cell system for acquiring various data) shown in FIG. , and the drive duty ratio difference is determined in advance, and a reference map is created using this relationship. The basic configuration of the test fuel cell system 2P shown in FIG. 4 is the same as that of the solid oxide fuel cell system described above. The same reference numerals are given to the same members as in the fuel cell system, and the description thereof is omitted.

In this test fuel cell system 2P, a branched flow path 72 is provided in the fuel gas supply flow path 14 (specifically, a portion between the first throttle member 22 and the desulfurizer 20 ). An on-off valve 74 is provided at 72 . Further, the fuel gas supply channel 14 (specifically, the portion between the first throttle member 22 and the desulfurizer 20 , which is downstream of the branch channel 72 as shown in FIG. A needle valve 76 is provided at the upstream portion of the flow path 72 . Other configurations of this test fuel cell system 2P are the same as those of the solid oxide fuel cell system described above.

In such a solid oxide fuel cell system, the recycling rate of the fuel off-gas is mainly: b) the flow resistance of the fuel off-gas flow path from the fuel electrode side outlet of the cell stack 6 to the discharge port of the exhaust gas discharge flow path 50; and c d) the negative pressure generated upstream of the fuel gas pump 24; Among these, if maintenance-free for about 10 years, the change is large: The pressure loss in the flow path (in particular, the vaporizer 12 and the reformer 4) and b) the pressure loss in the fuel offgas flow path from the fuel electrode side outlet of the cell stack 6 to the exhaust port of the exhaust gas discharge flow path 50. be.

Therefore, in the initial state of the test fuel cell system 2P, the needle valve 76 is provided between the fuel gas pump 24 and the desulfurizer 20 , and the mixed fuel gas (raw fuel gas and fuel off-gas) flowing through the fuel gas supply passage 14 is A branch flow path 72 is provided for extracting a mixed gas of (1), and an on-off valve 74 is provided in this branch flow path 72 .

The needle valve 76 can change the flow path resistance of the fuel gas supply flow path 14 under various conditions. In other words, the drive duty ratio of the drive current of the fuel gas pump 24, the supply flow rate of the raw fuel gas, the supply flow rate of air, the recycling rate of the fuel off-gas, and the opening and closing of the on-off valve 68 (when the pressure loss of the flow resistance fluctuates) The relationship between the front and rear driving duty ratio differences can be obtained as follows, and these relationships are registered in the memory means 88 (see FIG. 2) of the controller 72 as a reference map.

These relationships can be obtained by performing the procedure shown in FIG. 5, for example. Referring to FIG. 5 together with FIG. 4, in order to create this reference map, the cell stack 6 is operated to generate power, for example, at rated power (step S31). In this rated power generation operation, the recycle valve 68 is opened (step S32), part of the fuel off-gas is returned to the fuel gas supply channel 14 through the recycle channel 66 and recycled, and then the needle valve 76 is opened and closed. Then, the channel resistance condition of the fuel gas supply channel 14 is set (step S33).

In such an operating state, when the rated power generation state of the cell stack 6 stabilizes (in other words, the power generation output of the cell stack 6 stabilizes), the process moves from step S34 to step S35 to acquire relevant data for the reference map. is done. First, control is performed to lower the fuel utilization rate of the system by 2 to 5 points, for example, 3 points. This control is performed by increasing the drive duty ratio of the fuel gas pump 24 to increase the supply flow rate of the raw fuel gas.

In this way, the power generation operation is performed with a reduced fuel utilization rate, and the supply flow rate of the raw fuel gas (that is, the flow rate measured by the fuel gas flow sensor 30) and the supply flow rate of air (that is, the flow rate measured by the air flow sensor 42) is stabilized, the process proceeds from step S36 to step S37, and various information when the recycle valve 68 is open, specifically, the drive duty ratio of the fuel gas pump 24, the supply flow rate of the raw fuel gas, the supply flow rate of air, and the recycling rate is acquired, and the acquired various information is recorded.

This recycling rate is obtained by extracting a part of the mixed fuel gas flowing through the fuel gas supply channel 14 through the branch channel 72 with the on-off valve 74 opened, and analyzing the components of the extracted mixed fuel gas with a gas chromatograph. Then, the recycling rate of the fuel off-gas can be calculated based on the result of this component analysis and the set fuel utilization rate.

After that, the recycling valve 68 is closed (step S38), and the recycling of the fuel off-gas through the recycling passage 66 is finished. When the supply flow rate of the raw fuel gas (that is, the flow rate measured by the fuel gas flow sensor 30) and the supply flow rate of air (that is, the flow rate measured by the air flow sensor 42) are stabilized after the recycling of the fuel off-gas is completed in this way, , the process advances from step S39 to step S40 to obtain various information when the recycle valve 68 is closed, specifically the drive duty ratio of the fuel gas pump 24, the supply flow rate of the raw fuel gas, and the supply flow rate of air. Various types of acquired information are recorded.

After that, the drive duty ratio difference of the fuel gas pump 24 before and after the recycle valve 68 is opened and closed (difference value between the drive duty ratio when the recycle valve 68 is open and the drive duty ratio when the recycle valve 68 is closed) is calculated (step S41). ), and this difference value is also recorded in relation to the fuel gas supply flow rate before and after the recycle valve 68 is opened and closed. Note that the ratio of the drive duty ratios before and after the opening and closing of the recycling valve 68 (for example, the ratio of the drive duty ratio in the open state to the drive duty ratio in the closed state) is calculated instead of calculating the difference in the drive duty ratio before and after opening and closing the recycle valve 68. You may make it calculate.

The work for creating such a reference map is performed until the setting conditions are completed, and the work is repeated after returning to step S32. It is desirable that such measurement be performed over a plurality of conditions over a range of air flow rates, including the initial air flow rate of the fuel cell system and a flow rate that takes into consideration an increase in air flow rate due to deterioration of the cell stack 6 over time.

In this way, various types of information about the required conditions (that is, the drive duty ratio of the fuel gas pump 24, the raw fuel gas supply flow rate, the air supply flow rate, the recycling rate, and the drive duty ratio difference before and after opening and closing the recycling valve 68) are acquired. is completed, the process proceeds from step S42 to step S43, a reference map is created based on the acquired various information, and this created reference map is registered in the memory means 88 of the controller 72. FIG.

Next, referring to FIG. 3 together with FIGS. 1 and 2, the fuel utilization rate is corrected as follows during the power generation operation of the solid oxide fuel cell system 2 described above. For example, during rated power generation of the cell stack 6, first, it is checked whether the recycle valve 68 is kept open (step S1). , the recycle valve 68 is switched to the open state (step S2), and then the process proceeds to step S3.

In step S3, it is determined whether or not the output of the cell stack 6 is stable at the rated power generation output, and if it is stable at the rated power generation output, the process proceeds to step S4, where related data relating to correction of the fuel utilization rate is acquired. done. First, under the same conditions as when the above reference map is created, control is performed to lower the fuel utilization rate of the system by 2 to 5 points, for example, 3 points (step S4). This control is performed by the fuel utilization rate drop setting means 80 (see FIG. 2), and when the fuel utilization rate is set to 77%, for example, when the power generation operation is performed at a fuel utilization rate of, for example, 80%. is set, the drive duty ratio of the fuel gas pump 24 is increased and the supply flow rate of the raw fuel gas is increased.

In the power generation operation with the fuel utilization rate lowered in this way, the supply flow rate of the raw fuel gas (that is, the flow rate measured by the fuel gas flow sensor 30) and the supply flow rate of air (that is, the flow rate measured by the air flow sensor 42) are stable. Then, the process proceeds from step S5 to step S6. That is, when the flow rate stability determination means 78 (see FIG. 2) determines that the supply flow rate of the raw fuel gas and the air supply flow rate have stabilized, various information when the recycle valve 68 is open, specifically the fuel gas pump 24 The drive duty ratio, raw fuel gas supply flow rate, and air supply flow rate are acquired, and the acquired various information is recorded (step S6).

After that, the recycling valve 68 is closed (step S7), thereby completing the recycling of the fuel off-gas through the recycling passage 66. FIG. In this way, the recycling of the fuel off-gas is completed, and the supply flow rate of the raw fuel gas (that is, the flow rate measured by the fuel gas flow sensor 30) and the supply flow rate of air (that is, the flow rate measured by the air flow sensor 42) are stabilized. That is, when the flow rate stability determination means 78 (see FIG. 2) determines that the supply flow rate of the raw fuel gas and the supply flow rate of air are stable, the process proceeds from step S8 to step S9, and various operations are performed when the recycle valve 68 is closed. Information, specifically, the drive duty ratio of the fuel gas pump 24, the raw fuel gas supply flow rate, and the air supply flow rate are acquired, and the acquired various information is recorded.

After that, the drive duty ratio difference of the fuel gas pump 24 before and after opening and closing the recycle valve 68 (difference value between the drive duty ratio when the recycle valve 68 is open and the drive duty ratio when the recycle valve 68 is closed) is calculated (step S41). ), this calculated drive duty ratio difference value is also stored.

Then, it is determined whether the fuel utilization rate of the cell stack 6 is properly set based on the change in the information before and after opening and closing the recycle valve 68. If it is not properly set, the fuel utilization rate is corrected as follows. is done like That is, the fuel utilization rate determining means 84 (see FIG. 2) compares the drive duty ratio difference before and after opening and closing the recycle valve 68 with the reference duty ratio difference value (read out from the reference map registered in the memory means 88). .

As the reference duty ratio difference value, the corresponding drive duty ratio difference value is read from the reference map registered in the memory means 88, and the read drive duty ratio difference value is used as the reference duty ratio difference value. A comparison is made with the drive duty ratio difference before and after opening and closing. At this time, the drive duty ratio difference value of the fuel gas supply flow rate in the reference map corresponding to the fuel gas supply flow rate before and after the recycle valve 68 is opened and closed is read. Fluctuations in the ratio difference are used to capture fluctuations in the recycling rate.

When the drive duty ratio difference before and after opening and closing the recycle valve 68 is smaller than the reference duty ratio difference value, it is determined that the fuel utilization rate in the cell stack 6 is high. If the driving duty ratio difference of the fuel gas pump 24 is smaller than the reference duty ratio difference value, the amount of fuel off-gas recycled through the recycling passage 66 is small, and the fuel off-gas recycling rate fluctuates on the decreasing side (that is, The recycling rate is smaller than the set value), and in such a case, the fuel utilization rate determining means 84 determines that the "fuel utilization rate is high", and proceeds from step S11 to step S12 to reduce the fuel utilization rate. The rate correcting means 86 corrects the fuel utilization rate to lower it, for example, by 0.5 points in this embodiment, and the system is operated to generate power with the corrected fuel utilization rate.

Further, the fuel utilization rate determination means 84 compares the drive duty ratio difference before and after the recycle valve 68 is opened and closed with this reference duty ratio difference value (read out from the reference map registered in the memory means 88), and determines the drive duty ratio. When the ratio difference is larger than the reference duty ratio difference value, it is determined that the fuel utilization rate in the cell stack 6 is low. When the driving duty ratio difference of the fuel gas pump 24 is larger than the reference duty ratio difference value, the amount of fuel off-gas recycled through the recycling passage 66 is large, and the fuel off-gas recycling rate fluctuates on the increasing side (that is, The recycling rate is higher than the set value), and in such a case, the fuel utilization rate determining means 84 determines that the "fuel utilization rate is small" and moves from step S11 to step S13 to reduce the fuel utilization rate. A rate correction means 86 corrects so as to increase the fuel utilization rate.

At this time, when the fuel utilization rate in the cell stack 6 has reached the upper limit fuel utilization rate (for example, set to around 85%), it is not preferable for the system to further increase the fuel utilization rate. Move to S14. On the other hand, when the fuel utilization rate has not reached the upper limit fuel utilization rate, the process proceeds from step S14 to step S15, and the fuel utilization rate correcting means 86 corrects the fuel utilization rate so as to increase it, for example, 0.5 in this embodiment. Correction is made to raise the points, and the system is operated to generate power with the corrected fuel utilization rate.

In the above-described embodiment, the drive duty ratio difference and the reference duty ratio difference value are compared in consideration of changes in the supply flow rate of the raw fuel gas (flow rate measured by the fuel gas flow rate sensor 30) before and after the recycle valve 68 is opened and closed. However, in addition to the change in the supply flow rate of the raw fuel gas before and after opening and closing the recycling valve 68, the change in the supply flow rate of air before and after opening and closing the recycling valve 68 is taken into consideration, and the drive duty ratio difference and the reference duty ratio are calculated. It is also possible to compare with the ratio difference value.

In the above-described embodiment, the drive duty ratio difference of the fuel gas pump 24 before and after opening and closing the recycle valve 68 is compared with the reference duty ratio difference value. and a second reference duty ratio difference value (a value larger than the first reference duty ratio difference value), and when it is determined whether the fuel off-gas recycling rate fluctuates on the decreasing side, the first reference duty ratio difference value and the second reference duty ratio difference value may be used when it is determined that the fuel off-gas recycling rate is increasing.

In the solid oxide fuel cell system 2 of the embodiment shown in FIGS. 1 to 3, the fuel off-gas from the cell stack 6 is flowed outside the high temperature housing 54 and cooled by the third heat exchanger 60 outside the high temperature housing 54. Then, the cooled fuel off-gas is introduced again into the high temperature housing 54 and supplied to the combustor 46, and a part of this cooled fuel off-gas is returned to the upstream side of the fuel gas pump 24 through the recycling passage 66. The same can be applied to solid oxide fuel cell systems of the following forms.

A second embodiment of the solid oxide fuel cell system will be described with reference to FIG. In the solid oxide fuel cell system of the second embodiment, members that are substantially the same as those of the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

6, in the illustrated solid oxide fuel cell system 2A, the fuel offgas feed passage 44A for guiding the fuel offgas from the cell stack 6 to the combustor 46 is located inside the high temperature housing 54 of the fuel electrode of the cell stack 6. It connects the discharge side and the inlet side of the combustor 46 . A recycling passage 66A that returns part of the fuel off-gas to the upstream side of the fuel gas pump 24 has one end connected to the fuel off-gas supply passage 44A inside the high temperature housing 54 and the other end outside the high temperature housing 54. and is connected to a predetermined portion of the fuel gas supply passage 14 (a portion between the fuel gas pump 24 and the second throttle member 26). A third heat exchanger 60, a drain separator 64 and a recycle valve 68 are arranged in this order on the other end side of the recycle passage 66A (a portion extending outside the high temperature housing 54). Other configurations of the solid oxide fuel cell system 2A of the 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 fuel off-gas from the fuel electrode side of the cell stack 6 is supplied to the combustor 46 through the fuel off-gas supply passage 44A. The fuel off-gas thus produced is combusted by the air off-gas supplied from the air electrode side of the cell stack 6 through the air off-gas supply passage 48 . Further, when the recycle valve 68 is open, part of the fuel off-gas flowing through the fuel off-gas supply channel 44A passes through the recycle channel 66A to the fuel gas supply channel 14 (specifically, the upstream portion of the fuel gas pump 24). ), mixed with the raw fuel gas flowing through the fuel gas supply passage 14, and supplied toward the desulfurizer 20 downstream.

The fuel off-gas flowing through the recycle flow path 66A is cooled by heat exchange in the third heat exchanger 60, water in the fuel off-gas is condensed by this cooling, and the condensed water is separated from the fuel off-gas by the drain separator 64. Afterwards, the fuel off-gas from which moisture has been removed is returned to the fuel gas supply channel 14 .

Also in this solid oxide fuel cell system 2A, the fuel utilization rate of the cell stack 6 can be corrected in the same manner as in the solid oxide fuel cell system of the first embodiment. It is possible to achieve effects similar to those of the form.

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 changes and modifications can be made without departing from the scope of the present invention. It is possible.

For example, in the solid oxide fuel cell system described above, in order to suppress the occurrence of dew condensation in the fuel gas pump 24 (fuel gas supply means), the system may be configured as follows. That is, a temperature detection means (for example, a temperature detection sensor) is provided in relation to the fuel gas pump 24 (fuel gas supply means), and when the temperature detected by this temperature detection means exceeds a predetermined set temperature (for example, 30° C.), The controller 72 may generate an operation permission signal based on the detection signal from the temperature detection means, and the recycle valve 68 may be allowed to be open or open greatly based on the operation permission signal. . In this configuration, when the temperature of the fuel gas pump 24 is low, the fuel off-gas is not recycled to the upstream side of the fuel gas pump 24 through the recycle flow path 66, so that water vapor remaining in the fuel off-gas is discharged into the fuel gas pump. Condensation at 24 is prevented.

Further, in the above-described embodiment, an on-off valve is used as the recycle valve 68, and the fuel utilization rate of the cell stack 6 is corrected based on the driving duty ratio of the fuel gas pump 24 when the on-off valve is in the open state and the closed state. However, a proportional valve may be used as the recycle valve 68. In this case, the fuel in the cell stack 6 is determined based on the duty ratio of the fuel gas pump when the proportional valve is in a large opening state or a small opening state. Utilization rate will be corrected.

2, 2A solid oxide fuel cell system 4 reformer 6 cell stack 12 vaporizer 14 fuel gas supply channel 24 fuel gas pump 30 fuel gas flow rate sensor (fuel gas flow force measuring means)
44, 44A fuel offgas supply channel 46 combustor 54 high temperature housing 66, 66A recycle channel 68 recycle valve 72 controller 82 duty ratio difference calculation means 88 fuel utilization rate correction means

Claims (7)

A reformer for steam reforming the raw fuel gas, a vaporizer for generating steam used for steam reforming, and a reformed fuel gas reformed by the reformer and oxygen in the air. A cell stack that generates electricity by oxidation and reduction, a fuel gas supply means for supplying raw fuel gas to the reformer, an air supply means for supplying air to the cell stack, and a flow rate of the raw fuel gas. a cooler for cooling the fuel off-gas after the power generation reaction in the cell stack; and the fuel off-gas after being cooled by the cooler and the power generation reaction of the cell stack. a combustor for combusting the post-air off-gas; a recycle flow path for guiding a portion of the fuel off-gas after being cooled by the cooler to the upstream side of the fuel gas supply means; and the recycle flow. A solid oxide fuel cell system comprising: a recycle valve disposed in a channel; and a controller for controlling the operation of the fuel gas supply means and the air supply means,
The controller includes fuel utilization rate setting means for setting a fuel utilization rate and fuel utilization rate correcting means for correcting the fuel utilization rate. and correcting the fuel utilization rate based on the drive duty ratio of the fuel gas supply means in the closed state or the small opening state of the recycle valve. Oxide fuel cell system. A reformer for steam reforming the raw fuel gas, a vaporizer for generating steam used for steam reforming, and a reformed fuel gas reformed by the reformer and oxygen in the air. A cell stack that generates electricity by oxidation and reduction, a fuel gas supply means for supplying raw fuel gas to the reformer, an air supply means for supplying air to the cell stack, and a flow rate of the raw fuel gas. a combustor for burning the fuel off-gas and air off-gas after the power generation reaction in the cell stack, and a part of the fuel off-gas on the upstream side of the fuel gas supply means a recycle passage for guiding to the recycle passage, a cooler for cooling the fuel off-gas flowing through the recycle passage, a recycle valve disposed in the recycle passage, the fuel gas supply means and the air supply means A controller for controlling the operation of a solid oxide fuel cell system comprising:
The controller includes fuel utilization rate setting means for setting a fuel utilization rate and fuel utilization rate correcting means for correcting the fuel utilization rate. and correcting the fuel utilization rate based on the drive duty ratio of the fuel gas supply means in the closed state or the small opening state of the recycle valve. Oxide fuel cell system. The fuel utilization rate correcting means increases the recycling rate of the fuel off-gas when the drive duty ratio difference of the fuel gas supply means before and after opening and closing of the recycle valve or before and after the degree of opening is greater than a reference duty ratio difference value. If the difference in drive duty ratio of the fuel gas supply means before and after opening and closing of the recycle valve or before and after the degree of opening of the recycle valve is smaller than the reference duty ratio difference value 3. The solid oxide fuel cell system according to claim 1, wherein it is determined that the recycling rate of the fuel off-gas is fluctuating to the decreasing side, and the correction is made so that the fuel utilization rate becomes smaller. The recycling valve is composed of an on-off valve having a valve body for opening and closing the recycling flow path, and a bypass flow path is provided by bypassing the valve body of the on-off valve. 4. The solid oxide fuel cell system according to any one of claims 1 to 3, wherein part of the flowing fuel off-gas flows upstream of said fuel gas supply means through said bypass passage. The recycling valve is composed of an on-off valve having a valve body for opening and closing the recycling flow path. 4. The solid oxide fuel cell system according to any one of claims 1 to 3, wherein a part of the fuel gas flows through the through-hole of the valve body to the upstream side of the fuel gas supply means. The recycle valve is composed of a proportional valve having a valve body for controlling the opening and closing of the recycle flow path. 4. The solid oxide fuel cell system according to claim 1, wherein the fuel gas flows upstream of said fuel gas supply means through a proportional valve. Temperature detection means is provided in relation to the fuel gas supply means, and when the temperature detected by the temperature detection means exceeds a predetermined set temperature, the controller controls the recycle valve according to a detection signal from the temperature detection means. 4. The solid oxide fuel cell system according to any one of claims 1 to 3, characterized in that it permits control to set the open state or the large opening state.

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