Solid oxide fuel cell system

Abstract

To provide a solid oxide fuel cell (SOFC) system that can have a function of collecting carbon dioxide by relatively small-scaled design change.SOLUTION: An SOFC system includes a modifier (4) that modifies a source fuel gas, a cell stack (6) that generates power by an electrochemical reaction between a modified fuel gas and an oxidizing agent gas, a combustion chamber (44) that burns an anode off-gas from the cell stack (6), and an oxygen supply source that supplies oxygen to the combustion chamber (44). The controller calculates the necessary oxygen calculation amount required to completely oxidize the anode off-gas without leaving excess oxygen, supplies oxygen in the oxygen calculation amount that is smaller than this necessary oxygen calculation amount to the combustion chamber (44), varies the fuel use rate by controlling to correct the power generation current of the cell stack (6) on the basis of a detection signal from an oxygen sensor (82), and completely oxidizes the anode off-gas without leaving excess oxygen without changing the oxygen supply quantity.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a solid oxide fuel cell system equipped with a cell stack that generates electricity through an electrochemical reaction (fuel cell reaction) between a reformed fuel gas obtained by reforming a fuel gas and an oxidant gas. [Background technology]

[0002] A solid oxide fuel cell system (hereinafter also referred to as "SOFC 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 housing. In this solid oxide fuel cell system, the cell stack is composed of a plurality of stacked fuel cells, and each fuel cell has a fuel electrode (anode) on one side of the solid electrolyte and an oxygen electrode (cathode) on the other side. The operating temperature of the cell stack in this solid oxide fuel cell system is high, approximately 700 to 900°C, and at such high temperatures, hydrogen, carbon monoxide, and hydrocarbons in the fuel gas (reformed fuel gas) undergo an electrochemical reaction with oxygen in the air, thereby generating electricity.

[0003] In such SOFC systems, hydrocarbon fuel gas such as natural gas or biofuel gas (a fuel gas composed of a mixture of methane and carbon dioxide) is used as the raw fuel gas. This SOFC system includes a reformer for, for example, steam reforming the hydrocarbon fuel gas, a fuel gas supply means for supplying fuel gas (raw fuel gas) to the reformer, an air supply means for supplying air as an oxidant gas, and a cell stack having a fuel electrode (anode) and an oxygen electrode (cathode), where reformed fuel gas from the reformer is supplied to the fuel electrode of the cell stack, and air from the air supply means is supplied to the oxygen electrode, and power is generated by an electrochemical reaction between the reformed fuel gas and the oxidant gas in the cell stack (see, for example, Patent Document 1).

[0004] In this SOFC system, the cell stack and reformer are housed in a high-temperature space, and a high-temperature state is maintained by combusting anode offgas from the fuel electrode (anode) of the cell stack. For example, a combustion zone is provided above the cell stack, and a reformer is disposed above this combustion zone. Anode offgas from the fuel electrode (anode) side of the cell stack and cathode offgas from the oxygen electrode (cathode) side are sent to the combustion zone, where oxygen in the cathode offgas combusts the fuel gas remaining in the anode offgas. This combustion heat is used to heat the reformer and maintain the high-temperature space at a high temperature.

[0005] In such SOFC systems, instead of providing a combustion region above the cell stack, a system equipped with a combustor has also been proposed (see, for example, Patent Document 2). In this SOFC system, anode offgas from the fuel electrode (anode) side of the cell stack is fed to the combustor through an anode offgas feed passage, and cathode offgas from the oxygen electrode (cathode) side of the cell stack is fed to the combustor through a cathode offgas feed passage, where the oxygen in the cathode offgas combusts the fuel gas remaining in the anode offgas.

[0006] In these SOFC systems, anode off-gas from the cell stack is combusted using cathode off-gas, and the resulting combustion exhaust gas contains nitrogen and oxygen in addition to water and carbon dioxide. To recover carbon dioxide from such combustion exhaust gas, it is necessary to dissolve the carbon dioxide in an alkaline absorption solution and then apply heat to cause the carbon dioxide to desorb (regenerate the absorption solution). This desorption requires thermal energy, and therefore it is not practical to recover carbon dioxide from combustion exhaust gas in such SOFC systems.

[0007] Therefore, a method has been proposed for recovering carbon dioxide contained in the combustion exhaust gas emitted from an SOFC system, taking advantage of the fact that the SOFC system operates at high temperatures (see, for example, Patent Document 3). In this method, an oxygen ion conductor or a mixed conductor of oxygen ions and electrons is used, and the difference in oxygen activity between the anode off-gas and the cathode off-gas causes the oxygen in the cathode off-gas to react with the remaining hydrogen and carbon monoxide in the anode off-gas, resulting in almost complete oxidation, without nitrogen being mixed into the anode off-gas from the cell stack. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-285340 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-21596 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-3719 Summary of the Invention [Problem to be solved by the invention]

[0009] The method using the above-mentioned permselective membrane can take advantage of the high-temperature operation of the SOFC system, and is characterized by the fact that the process does not proceed excessively because it is driven by the difference in oxygen activity between the anode off-gas and the cathode off-gas. However, the technological maturity of the permselective membrane is still low, which results in problems such as high costs and low reliability.

[0010] An object of the present invention is to provide a solid oxide fuel cell system that can be equipped with a carbon dioxide capture function with relatively small design changes. [Means for solving the problem]

[0011] The solid oxide fuel cell system according to claim 1 of the present invention comprises a reformer that reforms a hydrocarbon-based raw fuel gas to produce a reformed fuel gas, a fuel gas supply means for supplying the raw fuel gas to the reformer, a water supply means for supplying reforming water to the reformer, an oxidant gas supply means for supplying an oxidant gas, a cell stack that generates electricity by an electrochemical reaction between the reformed fuel gas from the reformer and the oxidant gas from the oxidant gas supply means, a combustor that combusts anode off-gas from an anode of the cell stack, an oxygen supply source that supplies oxygen to the combustor, an oxygen supply control means for controlling the amount of oxygen supplied from the oxygen supply source to the combustor, a power generation current setting means that sets a power generation current of the cell stack, and a controller that controls the fuel gas supply means, the oxidant gas supply means, the water supply means, the oxygen supply control means, and the power generation current setting means, The combustion exhaust gas from the combustor is configured to be sent to an exhaust gas recovery and purification line through a combustion exhaust gas discharge line, and a condenser for condensing moisture contained in the combustion exhaust gas and a gas-liquid separator for separating the condensed water condensed in the condenser are disposed in the combustion exhaust gas discharge line, an oxygen concentration detection means for detecting an oxygen concentration in the combustion exhaust gas is further provided in association with the combustor; the controller includes a required oxygen amount calculation means for calculating a calculated amount of oxygen required to completely oxidize the anode off-gas from the cell stack without excess oxygen, a calculated oxygen supply amount setting means for setting a calculated oxygen supply amount that is smaller than the calculated required oxygen amount calculated by the required oxygen amount calculation means, and a generated current correction means for increasing or decreasing a generated current of the cell stack, the power generation current setting means sets a power generation current based on the supply amount of raw fuel gas supplied from the fuel gas supply means, and the power generation current correction means corrects and controls the power generation current setting means based on the oxygen concentration detection signal of the oxygen concentration detection means so as to vary the fuel utilization rate of the cell stack; This allows the anode off-gas fed to the combustor to be completely oxidized without excess oxygen.

[0013] Furthermore, the claims of the present invention 2 In the solid oxide fuel cell system described above, the controller further includes an upper limit utilization rate determination means for determining whether an upper limit fuel utilization rate has been reached due to fluctuations in the fuel utilization rate based on increases or decreases in the power generation current, and when the upper limit utilization rate determination means determines that the upper limit fuel utilization rate has been reached, the controller prohibits the power generation current setting means from making an increase correction to the power generation current based on the oxygen concentration detection signal of the oxygen concentration detection means.

[0014] Furthermore, the claims of the present invention 3In the solid oxide fuel cell system described above, the controller further includes a lower limit utilization rate determination means for determining whether a lower limit fuel utilization rate has been reached due to fluctuations in the fuel utilization rate based on increases or decreases in the power generation current, and when the lower limit utilization rate determination means determines that the lower limit fuel utilization rate has been reached, the controller prohibits the power generation current setting means from making a reduction correction to the power generation current based on the oxygen concentration detection signal of the oxygen concentration detection means.

[0015] Furthermore, the claims of the present invention 4 In the solid oxide fuel cell system described above, an exhaust gas treatment line is branched off from the combustion exhaust gas discharge line at a connection point with the exhaust gas recovery and purification line, and a flow path switching means is disposed at the branch point of the exhaust gas recovery and purification line and the exhaust gas treatment line, and when the upper limit utilization rate determination means of the controller determines that the upper limit fuel utilization rate has been reached or when the lower limit utilization rate determination means determines that the fuel utilization rate has dropped to the lower limit fuel utilization rate, the flow path switching means is switched from a first connection state connecting the combustion exhaust gas discharge line and the exhaust gas recovery and purification line to a second connection state connecting the combustion exhaust gas discharge line and the exhaust gas treatment line.

[0016] Furthermore, the claims of the present invention 5 The solid oxide fuel cell system described in is characterized in that a buffer tank for storing anode off-gas is provided in an anode off-gas supply line that supplies anode off-gas from the anode of the cell stack to the combustor.

[0017] Furthermore, the claims of the present invention 6 In the solid oxide fuel cell system described in the above, the oxygen supplied to the combustor is oxygen generated by electrolysis of water in a water electrolysis hydrogen generation device that electrolyzes water to generate hydrogen and oxygen. [Effects of the Invention]

[0018] According to the solid oxide fuel cell system of claim 1 of the present invention, the cell stack generates electricity through an electrochemical reaction between reformed fuel gas, which is obtained by reforming raw fuel gas from a fuel gas supply means in a reformer, and oxidant gas from an oxidant gas supply means, and anode off-gas from the cell stack is supplied to a combustor, to which oxygen from an oxygen supply source is supplied and the anode off-gas is combusted in the combustor. At this time, the controller corrects and controls the power generation current setting means based on an oxygen concentration detection signal from the oxygen concentration detection means, so that the fuel utilization rate of the cell stack varies, which changes the composition of the anode off-gas supplied to the combustor, and as a result, the anode off-gas supplied to the combustor can be completely oxidized without excess oxygen, and the combustion exhaust gas from the combustor can be made to consist of water and carbon dioxide.

[0019] In addition, a condenser and a gas-liquid separator are provided in the combustion exhaust gas discharge line through which the combustion exhaust gas with this composition (water and carbon dioxide) flows from the combustor. Therefore, the moisture in the combustion exhaust gas is condensed in the condenser, and this condensed water is separated in the gas-liquid separator. In this way, the moisture-removed combustion exhaust gas, i.e., carbon dioxide, can be recovered and purified through the exhaust gas recovery and purification line.

[0020] Furthermore, The required oxygen amount calculation means calculates the calculated amount of oxygen required to completely oxidize the anode off-gas from the cell stack without any excess oxygen, the calculated oxygen supply amount setting means sets a calculated oxygen supply amount that is less than the calculated required oxygen amount, and the oxygen supply control means supplies this calculated oxygen supply amount to the combustor. The generated current correction means corrects and controls the generated current setting means based on the oxygen concentration detection signal from the oxygen concentration detection means, so that the anode off-gas from the cell stack can be completely oxidized without any excess oxygen without changing the amount of oxygen supplied through the oxygen supply control means.

[0021] When the oxygen concentration detection means detects a shortage of oxygen, it means that the anode off-gas cannot be completely oxidized in the combustor due to a shortage of oxygen.In this case, the power generation current correction means corrects and controls the power generation current setting means so that the fuel utilization rate of the cell stack increases.By controlling in this way, the fuel gas components (hydrogen and carbon monoxide) in the anode off-gas are reduced, and as a result, the anode off-gas can be completely oxidized without excess oxygen without changing the amount of oxygen supplied through the oxygen supply control means.

[0022] Furthermore, when the oxygen concentration detection means detects an excess of oxygen, it means that complete oxidation has occurred in the combustor and there is an excess of oxygen. In this case, the power generation current correction means corrects and controls the power generation current setting means so that the fuel utilization rate of the cell stack decreases. By controlling in this manner, the fuel gas components (hydrogen and carbon monoxide) in the anode off-gas increase, and as a result, the anode off-gas can be completely oxidized without an excess of oxygen without changing the amount of oxygen supplied through the oxygen supply control means.

[0023] Furthermore, the claims of the present invention 2 According to the solid oxide fuel cell system described in the above, the controller includes an upper limit utilization rate determination means for determining whether the upper limit fuel utilization rate has been reached, and when the upper limit utilization rate determination means determines that the upper limit utilization rate has been reached, the controller prohibits the power generation current setting means from increasing the power generation current based on the oxygen concentration detection signal from the oxygen concentration detection means, thereby suppressing an increase in the fuel utilization rate and preventing deterioration of the cell stack due to an excessively high fuel utilization rate. Power generation operation at an excessively high fuel utilization rate leads to deterioration of the metallic nickel component used on the oxygen electrode (anode) side of the cell stack.

[0024] Furthermore, the claims of the present invention 3According to the solid oxide fuel cell system described in the above, the controller includes a lower limit utilization rate determination means for determining whether the lower limit fuel utilization rate has been reached. When the lower limit utilization rate determination means determines that the lower limit utilization rate has been reached, the controller prohibits the power generation current setting means from reducing and correcting the power generation current based on the oxygen concentration detection signal from the oxygen concentration detection means. This suppresses a decrease in the fuel utilization rate and prevents overheating due to combustion of anode off-gas in the combustor.

[0025] Furthermore, the claims of the present invention 4 According to the solid oxide combustion cell system described above, an exhaust gas treatment line is branched off from the exhaust gas discharge line at the connection point with the exhaust gas recovery and purification line, and a flow path switching means is disposed at this branch connection point. Therefore, when the upper limit utilization rate determination means determines that the upper limit fuel utilization rate has been reached (or the lower limit utilization rate determination means determines that the fuel utilization rate has dropped to the lower limit fuel utilization rate), the flow path switching means is switched to a second connection state that connects the combustion exhaust gas discharge line and the exhaust gas treatment line, thereby preventing combustion exhaust gas containing incompletely oxidized hydrogen and carbon monoxide (or combustion exhaust gas containing excessively supplied oxygen) from flowing into the exhaust gas recovery and purification line.

[0026] Furthermore, the claims of the present invention 5 According to the solid oxide fuel cell system described in the above, a buffer tank is provided in the anode off-gas supply line through which the anode off-gas of the cell stack flows, thereby making it possible to mitigate fluctuations in the flow rate and composition of the anode off-gas supplied to the combustor.

[0027] Furthermore, the claims of the present invention 6 In the solid oxide fuel cell system described in the above, oxygen generated by electrolysis of water in a water electrolysis hydrogen generator is used as the oxygen supplied to the combustor, so that by-products of water electrolysis can be effectively utilized. [Brief explanation of the drawings]

[0028] [Figure 1]1 is a simplified overall view showing a first embodiment of a solid oxide fuel cell system according to the present invention; [Figure 2] FIG. 2 is a simplified block diagram showing a control system of the solid oxide fuel cell system of FIG. 1. [Figure 3] 3 is a flowchart showing the flow of control by a control system of the solid oxide fuel cell system of FIG. 2. [Figure 4] FIG. 3 is a block diagram showing a simplified control system of a second embodiment of a solid oxide fuel cell system according to the present invention. [Figure 5] 5 is a flowchart showing the flow of control during rated operation by the control system of the solid oxide fuel cell system of FIG. 4. [Figure 6] 5 is a flowchart showing the flow of control during partial load operation by the control system of the solid oxide fuel cell system of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0029] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A solid oxide fuel cell system according to the present invention will now be described with reference to the accompanying drawings. First, a first embodiment of the solid oxide fuel cell system will be described with reference to FIG.

[0030] In FIG. 1, the illustrated solid oxide fuel cell system 2 (SOFC system) consumes hydrocarbon fuel gas (e.g., city gas, LP gas, biogas, etc.) as raw fuel gas to generate electricity, and is equipped with a reformer 4 for reforming the fuel gas, and a solid oxide cell stack 6 that generates electricity by an electrochemical reaction (fuel cell reaction) between the reformed fuel gas reformed in the reformer 4 and air as an oxidant gas.

[0031] The cell stack 6 is constructed by stacking multiple solid oxide fuel cell cells via interconnector plates for generating electricity through electrochemical reactions, and is equipped with a solid electrolyte 8 that conducts oxygen ions, a fuel electrode 10 (anode) provided on one side of the solid electrolyte 8, and an oxygen electrode 12 (cathode) provided on the other side of the solid electrolyte 8, and the solid electrolyte 8 is made of, for example, zirconia doped with yttria.

[0032] The fuel electrode 10 side of this cell stack 6 is connected to the reformer 4 via a reformed fuel gas supply line 14, and in this embodiment, the reformer 4 is configured as an integrated unit with a vaporizer 16 for vaporizing the reforming water. Note that the vaporizer 16 may be configured separately from the reformer 4, and the water vapor vaporized in the vaporizer 16 may be supplied to the reformer 4 via a water vapor supply line (not shown).

[0033] The vaporizer 16 is connected to a water supply source (not shown) (for example, composed of a water tank or a water recovery tank) via a water supply line 18, and reforming water from the water supply source is supplied to the vaporizer 16 through the water supply line 18. The reformer 4 contains a reforming catalyst, for example, alumina supported with ruthenium, and the raw fuel gas supplied via the fuel gas supply line 20 is steam reformed by the reforming catalyst with the steam vaporized in the vaporizer 16.

[0034] Arranged in this fuel gas supply line 20, in this order from the vaporizer 16 toward the upstream side, are a fuel supply pump 22 (constituting fuel gas supply means), a fuel flow meter 24 (fuel gas flow rate sensor), a desulfurizer 26, and a shutoff electromagnetic valve 28. The desulfurizer 26 removes sulfur components contained in the raw fuel gas (sulfur components in the odorant), and the shutoff electromagnetic valve 28 closes to shut off the fuel gas supply line 20 when the supply of raw fuel gas is stopped.

[0035] Furthermore, the fuel supply pump 22 (fuel gas supply means) increases the pressure of the raw fuel gas flowing through the fuel gas supply line 20 and supplies it to the vaporizer 16, the fuel flow meter 24 measures the flow rate of the raw fuel gas flowing through the fuel gas supply line 20, and the controller 32 (see Figure 2) of the SOFC system 2 compares the set flow rate value of the raw fuel gas with the measured flow rate value of the fuel flow meter 24, and controls the fuel supply pump 22 to increase (or decrease) its rotation speed when this measured flow rate value is smaller (or larger) than the set flow rate value, thereby adjusting the supply flow rate of the raw fuel gas to the set flow rate value of the SOFC system 2.

[0036] In addition, a water pump 34 (constituting a water supply means) and a water flow meter 36 (water flow sensor) are arranged in the water supply line 18, and this water pump 34 supplies reforming water from a water supply source (not shown) to the vaporizer 16 through the water supply line 18, and the water flow meter 36 measures the supply flow rate of the reforming water flowing through this water supply line 18.

[0037] The controller 32 (FIG. 2) of the SOFC system 6 compares the set flow rate value of the reforming water with the flow rate value measured by the water flow meter 36, and controls the rotation speed of the water pump 34 to increase (or decrease) when the measured flow rate value is smaller (or larger) than the set flow rate value, thereby adjusting the supply flow rate of the reforming water to the set flow rate value of the SOFC system 2.

[0038] Furthermore, an air supply line 38 is connected to the oxygen electrode 12 side of this cell stack 6, and an air blower 40 (constituting oxidant gas supply means) and an air flow meter 42 (air flow sensor) are arranged on this air supply line 38. The air blower 40 supplies air as an oxidant gas to the oxygen electrode 12 side of the cell stack 6 through the air supply line 38, and the air flow meter 42 measures the supply flow rate of air flowing through the air supply line 38.

[0039] The controller 32 (FIG. 2) of the SOFC system 2 compares the set flow rate value of the air with the flow rate value measured by the air flow meter 42, and controls the rotation speed of the air blower 40 to increase (or decrease) when the measured flow rate value is smaller (or larger) than the set flow rate value, thereby adjusting the air supply flow rate to the set flow rate value of the SOFC system 2.

[0040] This SOFC system 2 is provided with a combustor 44 for combusting anode offgas from the fuel electrode 10 (anode) of the cell stack 6. More specifically, the discharge side of the fuel electrode 10 of the cell stack 6 is connected to the combustor 44 via an anode offgas feed line 46, and the anode offgas from the cell stack 6 is fed to the combustor 44 through this anode offgas feed line 46.

[0041] An oxygen supply line 48 that supplies oxygen from an oxygen supply source (not shown) is connected to the combustor 44, and an electromagnetic cutoff valve 50 and a mass flow controller 52 (constituting oxygen flow rate control means) are disposed on the oxygen supply line 48. The mass flow controller 52 controls the flow rate of oxygen flowing through the oxygen supply line 48 to supply it downstream to the combustor 44, and the electromagnetic cutoff valve 50 closes to shut off the oxygen supply line 48 when the supply of oxygen is stopped.

[0042] In this embodiment, oxygen is supplied through an oxygen supply line 48 at a predetermined pressure (for example, a pressure of about 200 kPa), and the controller 32 (FIG. 2) of the SOFC system 2 compares the set flow rate value of oxygen with the measured flow rate value of the mass flow controller 52, and controls the opening of the mass flow controller 52 to be larger (or smaller) when the measured flow rate value is smaller (or larger) than the set flow rate value, thereby adjusting the oxygen supply flow rate to the set flow rate value of the SOFC system 2.

[0043] The oxygen supply source (not shown) may be an oxygen tank filled with oxygen, or may be a water electrolysis hydrogen generator that electrolyzes water to produce hydrogen and oxygen. In this case, the hydrogen produced by the water electrolysis hydrogen generator is the target product, and oxygen produced as a by-product at this time is delivered to the combustor 44 via the oxygen delivery line 48, thereby enabling effective use of the oxygen by-product of the water electrolysis hydrogen generator.

[0044] The hydrogen produced by the water electrolysis hydrogen generator is stored in a hydrogen tank (not shown) or is supplied to downstream hydrogen-using facilities for consumption. The hydrogen produced can also be used as fuel gas for fuel cell vehicles, industrial or residential fuel cells, etc.

[0045] In this SOFC system 2, a first heat exchanger 54 is provided to perform heat exchange between the cathode offgas discharged from the oxygen electrode 12 (cathode) of the cell stack 6 and the air flowing in the air supply line 38. A cathode offgas discharge line 56 is provided on the discharge side of the oxygen electrode 12 of the cell stack 6, and a first heat exchanger 54 is disposed in this cathode offgas discharge line 56. In this first heat exchanger 54, heat is exchanged between the cathode offgas flowing in the cathode offgas discharge line 56 and the air flowing in the air supply line 38, and the air heated by this heat exchange is supplied to the oxygen electrode 12 side of the cell stack 6.

[0046] Furthermore, a second heat exchanger 58 is provided so that heat exchange occurs between the combustion exhaust gas discharged from the combustor 44 and the air flowing in the air supply line 38. A combustion exhaust gas discharge line 60 is provided on the discharge side of the combustor 44, and the second heat exchanger 58 is disposed in this combustion exhaust gas discharge line 60, and in this second heat exchanger 58, heat exchange occurs between the combustion exhaust gas flowing in the combustion exhaust gas discharge line 60 and the air flowing in the air supply line 38.

[0047] With this configuration, the air from the air blower 40 is heated by heat exchange with the cathode off-gas in the first heat exchanger 54, and is further heated by heat exchange with the combustion exhaust gas in the second heat exchanger 58, and the air heated in this two-stage manner is supplied to the oxygen electrode 12 side of the cell stack 6.

[0048] In this SOFC system 2, the reformer 4 and the vaporizer 16 are disposed in contact with or in close proximity to the combustor 44, and the reformer 4 and the vaporizer 16 are heated and maintained at a predetermined temperature by the combustion heat generated by the combustion of the anode off-gas in this combustor 44. Furthermore, the cell stack 6, the combustor 44, the reformer 4, the vaporizer 16, the first heat exchanger 54, and the second heat exchanger 58 are housed in a high-temperature space 62 surrounded by a heat insulating material (not shown), and the combustion heat from the combustor 44 maintains the interior of this high-temperature space 62 at a high temperature.

[0049] To minimize carbon dioxide emissions, the SOFC system 2 is further configured as follows: A third heat exchanger 62 and a gas-liquid separator 64 are arranged in this order downstream in the combustion exhaust gas discharge line 60. The third heat exchanger 62 functions as a condenser for condensing moisture in the combustion exhaust gas and may be, for example, a heat exchanger used to store the thermal energy of the combustion exhaust gas as hot water in a hot water storage tank (not shown) of a hot water storage device. In this third heat exchanger 62, heat is exchanged between water (cooled water) from the hot water storage tank (not shown) of the hot water storage device and the combustion exhaust gas flowing through the combustion exhaust gas discharge line 60. This heat exchange cools the combustion exhaust gas from the combustor 44 and condenses the moisture contained therein, while the hot water heated by the heat exchange is stored in the hot water storage tank.

[0050] Furthermore, the condenser does not have to be the third heat exchanger 62 that exchanges heat with water from such a hot water storage device, but may be one that exchanges heat with, for example, cooling water, or one that exchanges heat with air, as long as it cools the combustion exhaust gas by heat exchange and condenses the moisture contained therein.

[0051] Meanwhile, the combustion exhaust gas cooled by heat exchange flows further downstream through the combustion exhaust gas discharge line 60, and the condensed water condensed in the third heat exchanger 62 is separated by a gas-liquid separator 64. The gas-liquid separator 64 is configured, for example, by a drain separator, and the moisture contained in the combustion exhaust gas is cooled and condensed by heat exchange in the third heat exchanger 62, the condensed water is separated by this gas-liquid separator 64, and the combustion exhaust gas from which the moisture has been separated flows further downstream. The condensed water separated by the drain separator may be recovered, for example, in a water recovery tank (not shown) and reused as reforming water.

[0052] An exhaust gas recovery and purification line 66 (specifically, a carbon dioxide recovery and purification line) and an exhaust gas treatment line 68 are branched and connected downstream of the combustion exhaust gas discharge line 60, and a three-way switching valve 70 (constituting flow path switching means) is disposed at this branch connection. The exhaust gas recovery and purification line 66 is connected to one discharge side of the three-way switching valve 70, and the exhaust gas treatment line 68 is connected to the other discharge side.

[0053] When this three-way switching valve 70 (flow path switching means) is in a first switching state, it connects the combustion exhaust gas discharge line 60 with the exhaust gas recovery and purification line 66, and the combustion exhaust gas from the combustion exhaust gas discharge line 60 flows into the exhaust gas recovery and purification line 66 to be recovered and purified, and when it is in a second switching state, it connects the combustion exhaust gas discharge line 60 with the exhaust gas treatment line 68, and the combustion exhaust gas from the combustion exhaust gas discharge line 60 flows into the exhaust gas treatment line 68 to be treated as required.

[0054] The flow path switching means for switching the flow of combustion exhaust gas may be configured with two on-off valves, i.e., first and second on-off valves (not shown), instead of the three-way switching valve 70. In this case, for example, a first on-off valve is provided in the exhaust gas recovery / purification line 66, and a second on-off valve is provided in the exhaust gas treatment line 68, and when the combustion exhaust gas from the combustion exhaust gas discharge line 60 is caused to flow into the exhaust gas recovery / purification line 66, the first on-off valve is switched to an open state and the second on-off valve is switched to a closed state, and when the combustion exhaust gas is caused to flow into the exhaust gas treatment line 68, the first on-off valve is switched to a closed state and the second on-off valve is switched to an open state.

[0055] As will be explained later, when the anode off-gas from the cell stack 6 can be completely combusted without excess oxygen in the combustor 44 as will be explained later, the three-way selector valve 70 is held in the first switching state, and carbon dioxide as the combustion exhaust gas is recovered and purified through the exhaust gas recovery and purification line 66. On the other hand, when it is difficult to completely oxidize this anode off-gas in the combustor 44 as will be explained later, or when there is a possibility that the combustion exhaust gas contains excess oxygen, the three-way selector valve 70 is switched to the second switching state, and the combustion exhaust gas flows through the exhaust gas treatment line 68, is treated as required, and is then discharged, for example, into the atmosphere.

[0056] The combustion exhaust gas (CO2) recovered through this exhaust gas recovery and purification line 66 can be used to produce synthetic fuels (jet fuel, liquid fuels such as methanol, gas fuels such as methane and propane), and can also be used to synthesize chemical products such as olefins and urethanes.

[0057] In this SOFC system 2, anode off-gas from the fuel electrode 10 side of the cell stack 6 is completely combusted (completely oxidized) in the combustor 44, and then recovered and purified as carbon dioxide (CO2). Referring to FIG. 1 and FIG. 2, this embodiment further includes an oxygen concentration detection means for detecting the oxygen concentration in association with the combustor 44. A sensor using a zirconia solid electrolyte, i.e., an oxygen sensor 82 or an A / F ratio sensor (air-fuel ratio sensor), can be suitably used as this oxygen concentration detection means, which is disposed at the outlet of the combustor 44 or the inlet of the combustion exhaust gas discharge line 60. The discharge side of the combustor 44 is an installation location that provides an advantageous temperature environment for the oxygen sensor 82 (or A / F sensor) to operate.

[0058] In the detection sections of oxygen sensor 82 and the A / F sensor, platinum electrodes are provided on both sides of a zirconia solid electrolyte. The A / F sensor is further structured such that a diffusion-controlling layer made of porous ceramic (e.g., alumina) that limits the diffusion of gas molecules is provided on the exhaust-side electrode, and an A / F sensor with such a structure is characterized by a faster response speed than oxygen sensor 82, which measures only electromotive force.

[0059] In this embodiment, a generated current sensor 84 is provided in association with the cell stack 6 as a generated current measuring device for measuring the generated current. Cell stack Based on this generated current, it is possible to know the load state of the cell stack 6, that is, whether the partial load power generation state is the rated power generation state.

[0060] In this embodiment, the operation of the SOFC system 2 is controlled by a control system shown in Fig. 2. Explaining further with reference to Fig. 2, the SOFC system 2 includes a controller 32 for controlling the operation of the entire system, and this controller 32 is composed of a microcontroller or the like in which a CPU, memory, timer, etc. are implemented in a single integrated circuit, and measurement signals from the fuel flow meter 24, air flow meter 42, water flow meter 36, oxygen sensor 82, generated current sensor 84, etc. are sent to the controller 32, and based on these detection signals (measurement signals), the controller 32 controls the fuel supply pump 22, air blower 40, water pump 34, mass flow controller 52, and three-way switching valve 70 as described below.

[0061] The controller 32 of this embodiment includes a power generation current setting means 92, a fuel utilization rate setting means 94, a rated current determination means 96, a fuel supply amount calculation means 98, an air supply amount calculation means 100, a water supply amount calculation means 102, and an operation control means 104. The power generation current setting means 92 sets the power generation current of the cell stack 6, and the power generation output of the cell stack 6 is set based on the set power generation current. The fuel utilization rate setting means 94 sets the fuel utilization rate in the cell stack 6, i.e., the proportion of fuel gas consumed for power generation in the cell stack 6. Furthermore, based on the power generation current set by the power generation current setting means 98 and the fuel utilization rate set by the fuel utilization rate setting means 94, the fuel supply amount calculation means 98 calculates the supply amount of raw fuel gas to be supplied to the anode 10 side of the cell stack 6, the air supply amount calculation means 100 calculates the supply amount of air to be supplied to the cathode 12 side of the cell stack 6, and the water supply amount calculation means 102 calculates the supply amount of reforming water to be supplied to the vaporizer 16.

[0062] In addition, the operation control means 104 controls the fuel supply pump 22 so as to achieve the supply amount calculated by the fuel supply amount calculation means 98, controls the air blower so as to achieve the supply amount calculated by the air supply amount calculation means 100, and controls the water pump 34 so as to achieve the supply amount calculated by the water supply amount calculation means 102.

[0063] The controller 32 further includes a required oxygen amount calculation means 106 and a calculated oxygen supply amount setting means 108, and the operation control means 104 includes a power generation current correction means 110. The required oxygen amount calculation means 106 calculates a calculated required oxygen amount necessary for complete combustion (complete oxidation) of the anode off-gas from the fuel electrode 10 side of the cell stack 6 in the combustor 44 without excess oxygen, and the calculated oxygen supply amount setting means 108 sets a calculated oxygen supply amount less than this calculated required oxygen amount, which is set to a value that is, for example, about 2 to 5% smaller than the calculated required oxygen amount calculated by the required oxygen amount calculation means 106, and the operation control means 104 controls the mass flow controller 52 so that the oxygen supplied to the combustor 44 is this calculated oxygen supply amount.

[0064] When this calculated oxygen supply amount is supplied to the combustor 44, there is a slight shortage of oxygen to completely combust the anode off-gas. Therefore, the generated current correcting means 110 corrects the generated current setting means 92 based on the detection signal (i.e., the oxygen concentration detection signal) from the oxygen sensor 82, and thus the generated current from the fuel supply pump 22 is corrected. Raw fuel gas By correcting the power generation current of the cell stack 6 without changing the amount of oxygen supplied, it is possible to supply the combustor 44 with the amount of oxygen necessary to completely combust the anode off-gas without any excess oxygen, thereby completely combusting the anode off-gas into carbon dioxide and water.

[0065] To explain this more specifically, when the fuel utilization rate under the operating conditions of the SOFC system 6 is, for example, 80%, oxygen in the air from the oxygen electrode 10 side of the cell stack 6 is supplied to the anode 10 side through the electrolyte 8 so that 80% of the raw fuel gas undergoes an electrochemical reaction. As a result, the gas composition at the exhaust outlet of the anode 10 is such that the reformed fuel gas is partially oxidized, for example, to 80% with pure oxygen, and contains hydrogen (H), carbon monoxide (CO), carbon dioxide (CO), and water (H0). Because the general operating temperature of the cell stack 6 in the SOFC system 6 is 700°C or higher and a large amount of water is present, the methane content is extremely low at less than 0.1%.

[0066] In order to completely oxidize this gas into CO2 and HO, the anode off-gas from the cell stack 6 is fed to a combustor 44, and oxygen supplied from an oxygen supply source (not shown) (e.g., an oxygen tank, a water electrolysis hydrogen generator) is fed to the combustor 44, where the anode off-gas is oxidized by the oxygen in the combustor 44. If the fuel utilization rate of the cell stack 6 is, for example, 80%, oxygen in an amount less than the amount of oxygen required to combust 20% of the raw fuel gas is fed from the oxygen supply source (not shown) through an oxygen feed line 48 to the combustor 44 of the SOFC system 6, and the amount of power generation current of the cell stack 6 is increased or decreased, thereby completely oxidizing the anode off-gas from the cell stack 22 into CO2 and HO. For example, when the oxygen concentration detection means (oxygen sensor 82) detects an oxygen shortage, this means that the anode off-gas cannot be completely oxidized in the combustor 44 and there is a shortage of oxygen. In this case, the amount of oxygen consumed in the combustor 44 must be reduced. Therefore, the power generation current correction means 110 corrects and controls the power generation current setting means 92 so as to increase the fuel utilization rate in the cell stack 6. If the fuel utilization rate is increased without changing the supply amount of raw fuel gas supplied from the fuel supply pump 22, the amount of raw fuel gas (reformed fuel gas) consumed in the cell stack 6 increases, which reduces the fuel gas components (hydrogen and carbon monoxide) in the anode off-gas. Therefore, without changing the amount of oxygen supplied through the mass flow controller 52, the amount of fuel gas components in the anode off-gas decreases, reducing the amount of oxygen consumed in the combustor 44 and resolving the oxygen shortage state. As a result, the anode off-gas can be completely oxidized. Furthermore, when the oxygen concentration detection means (oxygen sensor 82) detects excess oxygen, this means that there is excess oxygen even after the anode off-gas is completely oxidized in the combustor 44. In this case, it is necessary to increase the oxygen consumption in the combustor 44. Therefore, the power generation current correction means 110 corrects and controls the power generation current setting means 92 so that the fuel utilization rate in the cell stack 6 decreases. If the fuel utilization rate is reduced without changing the supply amount of raw fuel gas supplied from the fuel supply pump 22, the consumption amount of raw fuel gas (reformed fuel gas) in the cell stack 6 decreases, and as a result, the fuel gas components (hydrogen and carbon monoxide) in the anode off-gas increase. Therefore, without changing the amount of oxygen supplied through the mass flow controller 52, the fuel gas components in the anode off-gas increase, increasing the oxygen consumption in the combustor 44 and resolving the excess oxygen state. As a result, the anode off-gas can be completely oxidized.

[0067] When oxygen (so-called pure oxygen) from an oxygen supply source (not shown) is supplied to the combustor 44 in the high-temperature space 62 of the SOFC system 6, the rate of oxidation degradation of the cell stack 6 due to contact with pure oxygen or high-concentration oxygen is greater than the rate of oxidation degradation due to contact with air. Therefore, it is desirable that the pure oxygen only come into contact with the combustor 44 itself and the supply pipe (oxygen supply line 48) to it. Furthermore, the combustion conditions for the anode off-gas in the combustor 54 are desirably stoichiometric combustion, in which the post-combustion composition is only H2O and CO2. For this reason, it is desirable to supply oxygen from the oxygen supply source in an amount that allows the anode off-gas to be completely combusted without excess oxygen. This makes it possible to cool the combustion exhaust gas after combustion, separate the condensed water, and recover the remaining CO2 through the exhaust gas recovery line 76.

[0068] Next, the operation of the above-described SOFC system 2 will be described with reference to Figures 1 to 3. When this SOFC system 2 is operated, a start-up operation is performed (step S1) mainly with reference to Figures 2 and 3. During this start-up operation, the operating temperature of the cell stack 6 is low and operation is unstable, making it difficult to completely combust (completely oxidize) the anode off-gas from the cell stack 6 in the combustor 44 without excess oxygen. Therefore, the three-way switching valve 70 (flow path switching valve) is maintained in the second switching state (step S2).

[0069] In this startup operation, the fuel supply amount calculation means 98 calculates the amount of raw fuel gas to be supplied during startup operation, the air supply amount calculation means 100 calculates the amount of air to be supplied during startup operation, the water supply amount calculation means 102 calculates the amount of reforming water to be supplied during startup operation, and the required oxygen amount calculation means 106 calculates the amount of oxygen required to combust the anode off-gas during startup operation. These calculated supply amounts are set (step S3), and the operation control means 104 controls the fuel supply pump 22, the air blower 40, the water pump 34, and the mass flow controller 52 to supply these supply amounts, and the startup operation is performed in this manner.

[0070] During this startup operation, the three-way switching valve 70 is held in the second switching state, and the anode off-gas from the cell stack 6 is burned with oxygen (supplied through the oxygen feed line 48) in the combustor 44, and then flows into the exhaust gas treatment line 68 through the combustion exhaust gas discharge line 60 and the three-way switching valve 70. During this startup operation, when the operating temperature of the cell stack 6 rises and it becomes possible to generate electricity, the power generation current setting means 92 sets the power generation current for the startup operation, and the power generation output is set based on this power generation current setting, and is output at this set power generation current (step S4).

[0071] Then, when the power generation current (power generation output) of the cell stack 6 increases to the rated current (rated voltage) and the rated current determination means 96 determines that the rated current has been reached, the process proceeds from step S5 to step S6, and rated operation of the SOFC system 2 is performed. In this rated operation, the operating temperature of the cell stack 6 increases and the operating state stabilizes, making it possible to completely combust the anode off-gas from the cell stack 6 in the combustor 44 without excess oxygen, as follows, and therefore the three-way switching valve 70 (flow path switching valve) is switched from the second switching state to the first switching state (step S7).

[0072] In this rated operation, the power generation current setting means 92 sets the power generation current of the cell stack 6 to the rated current (step S8), the fuel supply amount calculation means 98 calculates the supply amount of raw fuel gas during rated operation, the air supply amount calculation means 100 calculates the supply amount of air during rated operation, and the water supply amount calculation means 102 calculates the supply amount of reforming water during rated operation; these calculated supply amounts are set (step S9); the operation control means 104 controls the fuel supply pump 22, the air blower 40, and the water pump 34 to supply these supply amounts; and in this manner, rated operation is performed.

[0073] In this rated operation, a calculated oxygen supply amount is set for the amount of oxygen supplied through the oxygen supply line 48 (step S10). That is, the required oxygen amount calculation means 106 calculates the calculated required oxygen amount necessary for complete combustion (complete oxidation) of the anode off-gas from the cell stack 6 as described above, the calculated oxygen supply amount setting means 108 sets a calculated oxygen supply amount that is less than this calculated required oxygen amount, and the controller 32 controls the mass flow controller 52 to supply oxygen of the calculated oxygen supply amount to the combustor 44. Then, for the combustion (oxidation) of the anode off-gas in the combustor 44, the power generation current of the cell stack 6 is corrected and controlled based on the detection signal (measurement signal) from the oxygen sensor 82.

[0074] Specifically, a detection signal from the oxygen sensor 82 is sent to the controller 32 (step 11), and the generated current correction means 110 determines whether there is an oxygen surplus or oxygen deficiency state based on this detection signal and corrects the generated current of the cell stack 6. When there is an oxygen surplus state due to the oxidation of the combustion exhaust gas in the combustor 44, there is a risk that the supplied oxygen will flow to the anode 10 side of the cell stack 6. Therefore, the process proceeds from step S12 to step S13, and the generated current correction means 110 reduces the generated current of the cell stack 6 to perform a downward correction of the fuel utilization rate. When the fuel utilization rate decreases due to this downward correction, the proportion of fuel gas (hydrogen and carbon monoxide) remaining in the anode off-gas from the cell stack 6 increases, which increases the amount of oxygen consumed in the combustor 44 and resolves the oxygen surplus state.

[0075] Furthermore, when there is an oxygen deficiency state where the combustion exhaust gas from the combustor 44 does not contain oxygen, the anode off-gas cannot be completely combusted (completely oxidized) in the combustor 44, so the process moves from step S12 to step S14 and then to step S15, where the power generation current correction means 110 increases the power generation current of the cell stack 6 to perform an increasing correction on the fuel utilization rate. When the fuel utilization rate increases as a result of this increasing correction, the proportion of fuel gas (hydrogen and carbon monoxide) remaining in the anode off-gas from the cell stack 6 decreases, which reduces the amount of oxygen consumed in the combustor 44 and resolves the oxygen deficiency state.

[0076] By correcting and controlling the power generation current of the cell stack 6 in this way, the anode off-gas can be completely combusted (completely oxidized) without excess oxygen in the combustor 44, and the anode off-gas becomes composed of water and carbon dioxide. This type of corrective control of the power generation current of the cell stack 6, which uses the detection signal of the oxygen sensor 82, is performed continuously throughout the rated operation of the SOFC system 2.

[0077] During this rated operation, the three-way switching valve 70 is held in the first switching state, and the combustion exhaust gas from the combustor 44 is cooled in the third heat exchanger 62 (condenser) to condense moisture, and after the condensed water is removed in the gas-liquid separator 64, it flows through this three-way switching valve 70 to the exhaust gas recovery / purification line 66. The combustion exhaust gas flowing through this exhaust gas recovery / purification line 66 has had moisture removed and has become carbon dioxide, so the carbon dioxide can be recovered and purified through this exhaust gas recovery / purification line 66.

[0078] When this rated operation is to be terminated, the process proceeds to step S16 and then step S17, where the SOFC system 2 is shut down. During this shut down operation, the three-way switching valve 70 is switched to the second switching state (step S18), and the combustion exhaust gas from the combustor 44 flows through the combustion exhaust gas discharge line 60 and the three-way switching valve 70 to the exhaust gas treatment line 68.

[0079] In the above-described embodiment, when the SOFC system 2 is in rated operation, the detection signal from the oxygen sensor 82 is used to correct and control the power generation current of the cell stack 6, but by configuring it as in the following second embodiment, it is possible to perform correction and control of the power generation current using the oxygen sensor 82 even during partial load operation after rated operation, and to recover and purify the combustion exhaust gas (carbon dioxide).In the second embodiment, components that are substantially the same as those in the first embodiment are given the same reference numerals, and their description will be omitted.

[0080] 4 showing the control system of the SOFC system of the second embodiment, this controller 32A, like the first embodiment, includes a power generation current setting means 92A, a fuel utilization rate setting means 94A, a rated current determination means 96A, a fuel supply amount calculation means 98A, an air supply amount calculation means 100A, a water supply amount calculation means 102A, a required oxygen amount calculation means 106A, an oxygen calculation supply amount setting means 108A, and an operation control means 104A. In addition to a power generation current correction means 110A, the operation control means 104A further includes an upper limit utilization rate determination means 122, a lower limit utilization rate determination means 124, and a correction prohibition signal generation means 126. A memory means 128 is registered with a detection signal from the oxygen sensor 82 to determine the power generation current of the cell stack 6, in other words, the upper limit fuel utilization rate and the lower limit fuel utilization rate when correcting and controlling the fuel utilization rate.

[0081] Upper limit utilization rate determination means 122 determines whether the fuel utilization rate in cell stack 6 has reached the upper limit fuel utilization rate by correcting the power generation current of cell stack 6, lower limit utilization rate determination means 124 determines whether the fuel utilization rate to cell stack 6 has decreased to the lower limit fuel utilization rate by correcting the power generation current of cell stack 6, and correction prohibition signal generation means 124 generates a correction prohibition signal based on the upper limit fuel utilization rate determination by upper limit utilization rate determination means 122 and the lower limit fuel utilization rate determination by lower limit utilization rate determination means 124. The other configurations of the SOFC system of this second embodiment are substantially the same as those of the first embodiment described above.

[0082] 5, rated operation of this SOFC system will be described. When the cell stack 6 reaches the rated current (rated output under rated load conditions) (i.e., the rated current determination means 96A determines that the rated current has been reached), the three-way switching valve 70 (flow path switching valve) is switched to the first switching state (step S21). The power generation current setting means 92A sets the power generation current of the cell stack to the rated current (step S22). The fuel supply amount calculation means 98A calculates the amount of raw fuel gas to be supplied during rated operation, the air supply amount calculation means 100A calculates the amount of air to be supplied during rated operation, and the water supply amount calculation means 102A calculates the amount of reforming water to be supplied during rated operation. These calculated supply amounts are set (step S23), and the operation control means 104A controls the fuel supply pump 22, the air blower 40, and the water pump 34 to supply these supply amounts.

[0083] During this rated operation, the oxygen sensor 82 (oxygen concentration detection means) detects the oxygen concentration of the combustion exhaust gas flowing from the combustor through the combustion exhaust gas discharge line 60 (step S24), and the detection signal (oxygen concentration detection signal) of this oxygen sensor 82 is used to perform correction control of the power generation current of the cell stack in the same manner as described above.

[0084] That is, required oxygen amount calculation means 106A calculates the calculated amount of oxygen required to completely combust the anode off-gas from the cell stack, calculated oxygen supply amount setting means 108A sets a calculated oxygen supply amount that is less than this calculated required oxygen amount, and operation control means 104A controls the mass flow controller 52 (oxygen flow rate control means) so that this calculated oxygen supply amount of oxygen is supplied to the combustor. In this oxygen supply state, there is a tendency for oxygen to be insufficient for the combustion of the anode off-gas in the combustor, so correction control of the power generation current of the cell stack is performed based on the detection signal from oxygen sensor 82.

[0085] The generated current correction means 110A determines whether there is an oxygen surplus or oxygen deficiency state based on the detection signal of this oxygen sensor 82. If the combustion exhaust gas is in an oxygen surplus state, the process proceeds from step S25 to step S26, where the generated current correction means 110A corrects and decreases the generated current of the cell stack without changing the amount of raw fuel gas supplied from the fuel supply pump 22. In this way, the fuel utilization rate of the cell stack is corrected and decreased, and the amount of fuel gas (hydrogen and carbon monoxide) contained in the anode off-gas increases without changing the amount of oxygen supplied through the mass flow controller 52 (oxygen supply control means), thereby eliminating the oxygen surplus.

[0086] At this time, when the fuel utilization rate drops from the upper limit fuel utilization rate described later, it is assumed that the anode off-gas can be completely combusted in the combustor, and the process proceeds from step S26 to step S27 and then to step S28, where the three-way switching valve 70 is switched to the first switching state, and the combustion exhaust gas from the combustor flows into the exhaust gas recovery and purification line 66.

[0087] Furthermore, if this correction for the decrease in the fuel utilization rate causes the fuel utilization rate to fall to the lower limit, the process proceeds from step S29 to step S30, where the lower limit utilization rate determination means 124 determines that the fuel utilization rate has fallen to the lower limit, and based on this determination result, the correction prohibition signal generation means 126 generates a correction prohibition signal, which prohibits further decreases in the fuel utilization rate, and the cell stack is operated at this lower limit fuel utilization rate, thereby preventing overheating due to combustion of anode off-gas in the combustor. At this time, an insufficient supply of oxygen occurs in the combustor, and some hydrogen and carbon monoxide remain in the combustion exhaust gas from the combustor. Therefore, the three-way switching valve 70 is switched to the second switching state, and the combustion exhaust gas from the combustor flows to the exhaust gas treatment line 68 (step S3).

[0088] Furthermore, if the combustion exhaust gas is in an oxygen-deficient state, the process proceeds from step S25 to step S32 and then to step S33, where the power generation current correction means 110A increases the power generation current of the cell stack without changing the amount of raw fuel gas supplied from the fuel supply pump 22. This increases the fuel utilization rate of the cell stack, and the amount of fuel gas (hydrogen and carbon monoxide) contained in the anode off-gas decreases without changing the amount of oxygen supplied via the mass flow controller 52 (oxygen supply control means), thereby resolving the oxygen deficiency. At this time, if the fuel utilization rate increases from the lower limit fuel utilization rate (described later), it is assumed that the anode off-gas can be completely combusted in the combustor. Therefore, the process proceeds from step S33 to step S34 and then to step S35, where the three-way switching valve 70 is switched to the first switching state, and the combustion exhaust gas from the combustor flows into the exhaust gas recovery and purification line 66.

[0089] Furthermore, if this increase correction of the fuel utilization rate causes the fuel utilization rate to rise to the upper limit fuel utilization rate, the process proceeds from step S36 to step S37, where the upper limit utilization rate determination means 122 determines that the fuel utilization rate has risen to the upper limit fuel utilization rate, and based on this determination result, the correction prohibition signal generation means 126 generates a correction prohibition signal, and based on this correction prohibition signal, further increases in the fuel utilization rate are prohibited, and the cell stack is operated at this upper limit fuel utilization rate, thereby preventing deterioration of the cell stack due to an excessively high fuel utilization rate. At this time, an excess of oxygen occurs in the combustor, and the combustion exhaust gas from the combustor contains oxygen, so the three-way switching valve 70 is switched to the second switching state, and the combustion exhaust gas from the combustor flows to the exhaust gas treatment line 68 (step S38).

[0090] When the generated current drops from such rated operation (rated load operation) to partial load operation, the process proceeds to step S39 to step S40, and partial load operation is performed. This partial load operation is performed in the same manner as the rated load operation described above.

[0091] 6, in this partial load operation, the three-way switching valve 70 (flow path switching valve) is switched to the first switching state (step S41). Also, the power generation current setting means 92A sets the power generation current of the cell stack to be the partial load power generation current (step S42), further, the fuel supply amount calculation means 98A calculates the supply amount of raw fuel gas during partial load operation, the air supply amount calculation means 100A calculates the supply amount of air during partial load operation, and the water supply amount calculation means 102A calculates the supply amount of reforming water during partial load operation, and these calculated supply amounts are set (step S43), and the operation control means 104A controls the fuel supply pump 22, the air blower 40, and the water pump 34 to supply these supply amounts.

[0092] Even during this partial operation, the oxygen sensor 82 (oxygen concentration detection means) detects the oxygen concentration of the combustion exhaust gas flowing from the combustor through the combustion exhaust gas discharge line 60 (step S44), and the detection signal (oxygen concentration detection signal) of this oxygen sensor 82 is used to perform correction control of the power generation current of the cell stack in the same manner as described above.

[0093] That is, even in this partial load operation, the required oxygen amount calculation means 106A calculates the calculated amount of oxygen required to completely combust the anode off-gas from the cell stack, the calculated oxygen supply amount setting means 108A sets a calculated oxygen supply amount that is less than this calculated required oxygen amount, and the operation control means 104A controls the mass flow controller 52 (oxygen flow rate control means) so that this calculated oxygen supply amount of oxygen is supplied to the combustor, and in this oxygen supply state, correction control of the power generation current of the cell stack is performed based on the detection signal from the oxygen sensor 82.

[0094] The generated current correction means 110A determines whether there is an oxygen surplus or oxygen deficiency state based on the detection signal of this oxygen sensor 82. If the combustion exhaust gas is in an oxygen surplus state, the process proceeds from step S45 to step S46, where the generated current correction means 110A corrects and decreases the generated current of the cell stack without changing the amount of raw fuel gas supplied from the fuel supply pump 22. In this way, the fuel utilization rate of the cell stack is corrected and decreased, and the amount of fuel gas (hydrogen and carbon monoxide) contained in the anode off-gas increases without changing the amount of oxygen supplied through the mass flow controller 52 (oxygen supply control means).

[0095] At this time, when the fuel utilization rate is reduced from the upper limit fuel utilization rate, which will be described later, the process proceeds from step S46 to step S47 and then to step S48, where the three-way switching valve 70 is switched to the first switching state.

[0096] Furthermore, if the fuel utilization rate falls to the lower limit fuel utilization rate as a result of this correction for the decrease in the fuel utilization rate, the process proceeds from step S49 to step S50, and based on the result of the judgment made by the lower limit utilization rate judgment means 124 that the fuel utilization rate has fallen to the lower limit fuel utilization rate, the correction prohibition signal generation means 126 generates a correction prohibition signal, further decrease in the fuel utilization rate is prohibited, and the cell stack is operated at this lower limit fuel utilization rate. At this time, the three-way switching valve 70 is switched to the second switching state, and the combustion exhaust gas from the combustor flows to the exhaust gas treatment line 68 (step S51).

[0097] Furthermore, if the combustion exhaust gas is in an oxygen-deficient state, the process proceeds from step S45 to step S52 and then to step S53, where the power generation current correction means 110A increases the power generation current of the cell stack without changing the amount of raw fuel gas supplied from the fuel supply pump 22. This increases the fuel utilization rate of the cell stack, and the amount of fuel gas (hydrogen and carbon monoxide) contained in the anode off-gas decreases without changing the amount of oxygen supplied through the mass flow controller 52 (oxygen supply control means), thereby resolving the oxygen deficiency. At this time, the process proceeds from step S53 to step S54 and then to step S55, where the three-way switching valve 70 is switched to the first switching state, and the combustion exhaust gas from the combustor flows into the exhaust gas recovery and purification line 66.

[0098] Furthermore, if this increase correction of the fuel utilization rate causes the fuel utilization rate to rise to the upper limit fuel utilization rate, the process proceeds from step S56 to step S57, where the upper limit utilization rate determination means 122 determines that the fuel utilization rate has risen to the upper limit fuel utilization rate, and based on this determination result, the correction prohibition signal generation means 126 generates a correction prohibition signal, which prohibits further increases in the fuel utilization rate, and the cell stack operates at this upper limit fuel utilization rate. At this time, the three-way switching valve 70 is switched to the second switching state, and the combustion exhaust gas from the combustor flows to the exhaust gas treatment line 68 (step S58).

[0099] When the power generation output drops from such rated operation (rated load operation) to partial load operation, the process proceeds from step S39 to step S40, and partial load operation is performed. Then, when this partial load operation ends and the operation returns to normal operation, the process returns to step S21 via steps S59 and S60. Furthermore, when operation is to be terminated after partial load operation, the process proceeds from step S59 to step S60, and operation is terminated (the process proceeds to step S17 in the flowchart shown in FIG. 3, and termination operation is performed).

[0100] Although the embodiment of the solid oxide fuel cell system (SOFC system) according to the present invention has been described above, the present invention is not limited to such an embodiment, and various changes and modifications are possible without departing from the scope of the present invention.

[0101] In the above-described embodiment, the required oxygen amount calculation means 106 (106A) calculates the required oxygen amount, the calculated oxygen supply amount setting means 108 (108A) sets the calculated oxygen supply amount based on this required oxygen amount, and when this calculated oxygen supply amount is being supplied, the generated current correction means 110 (110A) corrects and controls the generated current of the cell stack 6 based on the detection signal of the oxygen sensor 82 (oxygen concentration detection means). However, instead of this configuration, the required oxygen amount calculated by the required oxygen amount calculation means 106 (106A) may be used, and when this required oxygen amount is being supplied, the generated current correction means 110 (110A) corrects and controls the generated current of the cell stack 6 based on the detection signal of the oxygen sensor 82 (oxygen concentration detection means).

[0102] In addition, a buffer tank (not shown) may be provided in the anode off-gas supply line 46 that supplies the anode off-gas from the fuel electrode 10 side of the cell stack 6 to the combustor 44. By configuring in this manner, it is possible to mitigate fluctuations in the flow rate and composition of the anode off-gas supplied to the combustor 44. [Explanation of symbols]

[0103] 2. Solid oxide fuel cell system 4 Reformer 6 Cell stack 22 Fuel supply pump (fuel gas supply means) 32,32A controller 34 Water pump (water supply means) 40 Air blower (oxidant gas supply means) 44 Combustor 46 Combustion exhaust gas supply line 52 Mass flow controller (oxygen flow rate control means) 62 Third heat exchanger (condenser) 64 Gas-liquid separation equipment 66 Exhaust gas recovery and purification line 70 Three-way switching valve 82 Oxygen sensor (oxygen concentration detection means) 106, 106A Required oxygen amount calculation means 108, 108A Oxygen calculation supply amount setting means 110,110A power generation current correction means 122 Upper limit utilization rate determination means 124 Lower limit utilization rate determination means 126 Correction prohibition signal generation means

Claims

1. the system comprises a reformer that reforms a hydrocarbon-based raw fuel gas to produce a reformed fuel gas, fuel gas supply means for supplying the raw fuel gas to the reformer, water supply means for supplying reforming water to the reformer, oxidant gas supply means for supplying oxidant gas, a cell stack that generates electricity by an electrochemical reaction between the reformed fuel gas from the reformer and the oxidant gas from the oxidant gas supply means, a combustor that combusts anode off-gas from an anode of the cell stack, an oxygen supply source that supplies oxygen to the combustor, oxygen supply control means for controlling the amount of oxygen supplied from the oxygen supply source to the combustor, power generation current setting means for setting a power generation current of the cell stack, and a controller for controlling the fuel gas supply means, the oxidant gas supply means, the water supply means, the oxygen supply control means, and the power generation current setting means, The combustion exhaust gas from the combustor is configured to be sent to an exhaust gas recovery and purification line through a combustion exhaust gas discharge line, and a condenser for condensing moisture contained in the combustion exhaust gas and a gas-liquid separator for separating the condensed water condensed in the condenser are disposed in the combustion exhaust gas discharge line, In association with the combustor, oxygen concentration detection means is further provided for detecting the oxygen concentration in the combustion exhaust gas, and the controller includes required oxygen amount calculation means for calculating a calculated amount of oxygen required to completely oxidize the anode off-gas from the cell stack without excess oxygen, calculated oxygen supply amount setting means for setting a calculated oxygen supply amount that is smaller than the calculated required oxygen amount calculated by the required oxygen amount calculation means, and generated current correction means for increasing or decreasing the generated current of the cell stack, a power generation current setting means for setting a power generation current based on the amount of raw fuel gas supplied from the fuel gas supply means; and a power generation current correction means for correcting and controlling the power generation current setting means based on an oxygen concentration detection signal from the oxygen concentration detection means so as to vary the fuel utilization rate of the cell stack, thereby completely oxidizing the anode off-gas fed to the combustor without excess oxygen.

2. The solid oxide fuel cell system of claim 1, characterized in that the controller further includes an upper limit utilization rate determination means for determining whether an upper limit fuel utilization rate has been reached due to fluctuations in the fuel utilization rate based on increases or decreases in the power generation current, and when the upper limit utilization rate determination means determines that the upper limit fuel utilization rate has been reached, the controller prohibits the power generation current setting means from increasing the power generation current based on the oxygen concentration detection signal of the oxygen concentration detection means.

3. The solid oxide fuel cell system of claim 2, characterized in that the controller further includes a lower limit utilization rate determination means for determining whether a lower limit fuel utilization rate has been reached due to fluctuations in fuel utilization rate based on increases or decreases in the power generation current, and when the lower limit utilization rate determination means determines that the lower limit fuel utilization rate has been reached, the controller prohibits the power generation current setting means from making a decrease correction to the power generation current based on the oxygen concentration detection signal of the oxygen concentration detection means.

4. A solid oxide fuel cell system as described in claim 3, characterized in that an exhaust gas treatment line branches off from the combustion exhaust gas discharge line at a connection point with the exhaust gas recovery and purification line, and a flow path switching means is arranged at the branch point of the exhaust gas recovery and purification line and the exhaust gas treatment line, and when the upper limit utilization rate determination means of the controller determines that the upper limit fuel utilization rate has been reached or when the lower limit utilization rate determination means determines that the fuel utilization rate has dropped to the lower limit fuel utilization rate, the flow path switching means switches from a first connection state connecting the combustion exhaust gas discharge line and the exhaust gas recovery and purification line to a second connection state connecting the combustion exhaust gas discharge line and the exhaust gas treatment line.

5. 5. The solid oxide fuel cell system according to claim 1, wherein an anode off-gas supply line that supplies anode off-gas from the anode of the cell stack to the combustor is provided with a buffer tank that stores the anode off-gas.

6. 6. The solid oxide fuel cell system according to claim 1, wherein the oxygen supplied to the combustor is oxygen produced by electrolysis of water in a water electrolysis hydrogen production device that electrolyzes water to produce hydrogen and oxygen.

Images