JP2024106827A - Electrochemical cell system and method for operating the electrochemical cell system - Google Patents

Abstract

[Problem] To facilitate recovery of carbon dioxide from exhaust fuel gas. [Solution] The SOFC power generation system includes a topping SOFC 11a, a bottoming SOFC 11b to which exhaust fuel gas discharged from the topping SOFC 11a is supplied, a detection unit 32 that detects the state quantity of the exhaust fuel gas discharged from the bottoming SOFC 11b, and a control device that controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b based on the result detected by the detection unit 32. The control device controls the current command value so that the fuel utilization rate of the SOFC power generation system is within a specified range, the carbon dioxide concentration in the exhaust fuel gas discharged from the bottoming SOFC 11b is equal to or greater than a first threshold, and the amount of power generation required for the SOFC power generation system is satisfied. [Selected Figure] Figure 7

Description

The present disclosure relates to an electrochemical cell system and a method for operating an electrochemical cell system.

Fuel cells, which generate electricity by chemically reacting a fuel gas with an oxidizing gas, have characteristics such as excellent power generation efficiency and environmental friendliness. Among them, solid oxide fuel cells (SOFCs) use ceramics such as yttria-stabilized zirconia as an electrolyte, and generate electricity by reacting the fuel gas in a high-temperature atmosphere of approximately 700°C to 1000°C, such as hydrogen, city gas, natural gas, petroleum, methanol, gasification gas produced by a gasification facility from a carbon-containing raw material, and biogas made from biomass.
In addition, electrolysis cells, which produce hydrogen and oxygen by electrochemically decomposing water, are a hydrogen production method that does not emit carbon dioxide and has excellent environmental characteristics. Among them, solid oxide electrolysis cells (SOECs) use ceramics such as yttria-stabilized zirconia as an electrolyte and can produce hydrogen more efficiently than other electrolysis cells because they use high-temperature steam as a raw material. In addition, for the purpose of decarbonization, co-electrolysis is also possible, using carbon dioxide (CO2) as a raw material and electrolytic hydrogen as a reducing agent to directly produce carbon monoxide (CO).
Furthermore, some solid oxide electrochemical cells can be used as reversible solid oxide electrochemical cells (RSOCs), which have a power generation function as a fuel cell and a function of producing hydrogen and oxygen through a reverse reaction by supplying electric power and high-temperature steam from the outside.

A cascade-type power generation system in which multiple SOFCs are connected in series is known as a power generation system that uses SOFCs (for example, see Patent Document 1). In a cascade-type power generation system, fuel gas and air are supplied to an SOFC in a first stage to generate power, and fuel gas (exhaust fuel gas) discharged from the SOFC in the first stage is supplied to an SOFC in a second stage to generate power.

JP 2004-199979 A

A power generation system using an SOFC is known in which exhaust fuel gas discharged from the SOFC is mixed with air and reused in an external reformer, a combustor such as a micro gas turbine, etc., thereby improving thermal efficiency. In such a power generation system, the exhaust fuel gas introduced into the external reformer or the combustor is mixed with air, so that the carbon dioxide concentration contained in the gas discharged from the power generation system is reduced.
On the other hand, a power generation system using an SOFC may be provided with a carbon dioxide capture device that captures carbon dioxide from the exhaust gas. In the carbon dioxide capture device, the carbon dioxide capture efficiency decreases when the concentration of carbon dioxide in the gas is low.
In addition, by increasing the fuel utilization rate of the power generation system, the carbon dioxide concentration in the exhaust gas can be increased; however, when the composition of the fuel gas supplied to the SOFC fluctuates or when the amount of power generation is changed to follow the power demand, it can be difficult to control the fuel utilization rate.

The present disclosure has been made in consideration of these circumstances, and aims to provide an electrochemical cell system and an operating method for an electrochemical cell system that can facilitate the recovery of carbon dioxide from exhaust gas.

In order to solve the above problems, the electrochemical cell system and the operating method of the electrochemical cell system of the present disclosure employ the following measures.
An electrochemical cell system according to one embodiment of the present disclosure is an electrochemical cell system having a plurality of fuel cells that generate electricity by reacting a fuel gas supplied to an anode and an oxidizing gas supplied to an cathode, and includes a first fuel cell and a second fuel cell to which exhaust fuel gas discharged from the first fuel cell is supplied, a detection unit that detects the state quantity of the exhaust fuel gas discharged from the second fuel cell, and a control unit that controls current command values of the first fuel cell and the second fuel cell based on the results detected by the detection unit, wherein the control unit controls the current command value so that the fuel utilization rate of the electrochemical cell system is within a specified range, the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell is equal to or greater than a first threshold value, and the amount of electricity generated required for the electrochemical cell system is satisfied.

An operating method of an electrochemical cell system according to one aspect of the present disclosure is a method for operating an electrochemical cell system having a plurality of fuel cells that generate electricity by reacting a fuel gas supplied to an anode with an oxidizing gas supplied to an cathode, the electrochemical cell system comprising a first fuel cell and a second fuel cell to which exhaust fuel gas discharged from the first fuel cell is supplied, a detection unit that detects the state quantity of the exhaust fuel gas discharged from the second fuel cell, and a control unit that controls the current command value of the first fuel cell and the second fuel cell based on the result detected by the detection unit, and the control unit controls the current command value so that the fuel utilization rate of the electrochemical cell system is within a specified range, the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell is equal to or greater than a first threshold, and the amount of electricity generated required for the electrochemical cell system is satisfied.

This disclosure makes it easier to recover carbon dioxide from exhaust gas.

1 illustrates an embodiment of an electrochemical cell stack according to an embodiment of the present disclosure. 1 illustrates an embodiment of an electrochemical cell module according to an embodiment of the present disclosure. 1 illustrates one embodiment of a cross-section of an electrochemical cell cartridge according to an embodiment of the present disclosure. 1 is a schematic configuration diagram of an SOFC power generation system according to an embodiment of the present disclosure. 4 is a map showing the correspondence between the density of exhaust fuel gas and the fuel utilization rate, the carbon dioxide concentration, and the hydrogen concentration. 4 is a flowchart illustrating a process performed by a control device according to an embodiment of the present disclosure. FIG. 2 is a diagram illustrating a process performed by a control device according to an embodiment of the present disclosure. 1 is a graph showing the relationship between exhaust fuel gas density and current values of a bottoming SOFC and a topping SOFC in an SOFC power generation system according to an embodiment of the present disclosure.

Below, an embodiment of an electrochemical cell system and an operating method of an electrochemical cell system according to the present disclosure will be described with reference to the drawings.

In the following, for the sake of convenience, the positional relationship of each component described using the expressions "upper" and "lower" with respect to the paper surface indicates the vertically upper side and vertically lower side, respectively. Also, in this embodiment, for components that can obtain similar effects in the vertical and horizontal directions, the vertical direction on the paper surface is not necessarily limited to the vertical direction, but may correspond to, for example, a horizontal direction perpendicular to the vertical direction.
In the following, a cylindrical solid oxide fuel cell (SOFC) is used as an example of a cell stack of solid oxide electrochemical cells, but this is not necessarily the case and a flat cell stack may be used. The electrochemical cell is formed on a substrate, but an electrode (fuel electrode or air electrode) may be formed thick and serve as the substrate.

First, referring to FIG. 1, a cylindrical cell stack using a substrate tube will be described as an example according to this embodiment. When a substrate tube is not used, for example, the fuel electrode may be formed thick to serve as the substrate tube, and the use of the substrate tube is not limited. In addition, the substrate tube in this embodiment is described as having a cylindrical shape, but the substrate tube may be cylindrical and is not necessarily limited to a circular cross section, and may be, for example, elliptical. A cell stack such as a flat tubular cylinder in which the peripheral side of the cylinder is crushed vertically may also be used. Here, FIG. 1 shows one aspect of a cell stack according to the embodiment. The cell stack 101 includes, as an example, a cylindrical substrate tube 103, a plurality of electrochemical unit cells 105 formed on the outer peripheral surface of the substrate tube 103, and an interconnector 107 formed between adjacent electrochemical unit cells 105. The electrochemical unit cell 105 is formed by stacking a fuel electrode 109, a solid electrolyte membrane 111, and an air electrode 113. In addition, the cell stack 101 includes a lead film 115 electrically connected via an interconnector 107 to the air electrode 113 of the electrochemical single cell 105 formed at the end of the electrochemical single cell 105 at the axially outermost end of the base tube 103 among the multiple electrochemical single cells 105 formed on the outer peripheral surface of the base tube 103, and a lead film 115 electrically connected to the fuel electrode 109 of the electrochemical single cell 105 formed at the other end of the electrochemical single cell 105 at the outermost end.

The base tube 103 is made of a porous material and is mainly composed of, for example, CaO-stabilized _ZrO2 (CSZ), a mixture of CSZ and nickel oxide (NiO) (CSZ+NiO), _{Y2O3 -} stabilized _ZrO2 ( _YSZ ), or _MgAl2O4 . The base tube 103 supports the electrochemical unit cell 105, the interconnector 107, and the lead film 115, and diffuses the fuel gas supplied to the inner peripheral surface of the base tube 103 through the pores of the base tube 103 to the fuel electrode 109 formed on the outer peripheral surface of the base tube 103.

The fuel electrode 109 is made of a composite oxide of Ni and a zirconia-based electrolyte material, and Ni/YSZ is used, for example. The thickness of the fuel electrode 109 is 50 μm to 250 μm, and the fuel electrode 109 may be formed by screen printing a slurry. In this case, the Ni component of the fuel electrode 109 has a catalytic effect on the fuel gas. This catalytic effect is to react with the fuel gas supplied through the substrate tube 103, for example, a mixed gas of methane (CH ₄ ) and water vapor, and reform it into hydrogen (H ₂ ) and carbon monoxide (CO). The fuel electrode 109 also electrochemically reacts the hydrogen (H ₂ ) and carbon monoxide (CO) obtained by reforming with oxygen ions (O ^2- ) supplied through the solid electrolyte membrane 111 near the interface with the solid electrolyte membrane 111 to generate water (H ₂ O) and carbon dioxide (CO ₂ ). At this time, the electrochemical unit cell 105 generates electricity by the electrons released from the oxygen ions.
Fuel gases that can be supplied to the fuel electrode 109 of a solid oxide electrochemical cell and used include hydrocarbon gases such as hydrogen ( _H2 ), carbon monoxide (CO), and methane ( _CH4 ), city gas, and natural gas, as well as gasified gases produced by gasification equipment from carbon-containing raw materials such as petroleum, methanol, and coal, and biogas made from biomass.

For the solid electrolyte membrane 111, YSZ is mainly used, which has airtightness that makes it difficult for gas to pass through and high oxygen ion conductivity at high temperatures. This solid electrolyte membrane 111 moves oxygen ions (O ^2- ) generated at the air electrode to the fuel electrode. The thickness of the solid electrolyte membrane 111 located on the surface of the fuel electrode 109 is 10 μm to 100 μm, and the solid electrolyte membrane 111 may be formed by screen printing a slurry.

The air electrode 113 is made of, for example, _LaSrMnO3 -based oxide or _LaCoO3 -based oxide, and a slurry is applied to the air electrode 113 by screen printing or using a dispenser. This air electrode 113 dissociates oxygen in an oxidizing gas such as air supplied near the interface with the solid electrolyte membrane 111, and combines with electrons supplied from the outside to generate oxygen ions ( ^O2- ).
The air electrode 113 may have a two-layer structure. In this case, the air electrode layer (air electrode intermediate layer) on the solid electrolyte membrane 111 side is made of a material that exhibits high ionic conductivity and has excellent catalytic activity. The air electrode layer (air electrode conductive layer) on the air electrode intermediate layer may be made of a perovskite oxide represented by Sm-doped ceria or Sr and Ca-doped _LaMnO3 . This can further improve the power generation performance.
An oxidizing gas is a gas that contains approximately 15% to 30% oxygen. Air is a typical example, but other gases such as a mixture of combustion exhaust gas and air, or a mixture of oxygen and air can also be used.

The interconnector 107 is made of a conductive _perovskite oxide represented by M1 _{- xLxTiO3} (M is an alkaline earth metal element, L is a lanthanoid element) such as _SrTiO3 , and is screen-printed with a slurry. The interconnector 107 is a dense film so that the fuel gas and the oxidizing gas do not mix. The interconnector 107 also has stable durability and electrical conductivity under both an oxidizing atmosphere and a reducing atmosphere. The interconnector 107 electrically connects the air electrode 113 of one electrochemical unit cell 105 to the fuel electrode 109 of the other electrochemical unit cell 105 in adjacent electrochemical unit cells 105, thereby connecting the adjacent electrochemical unit cells 105 in series.

The lead film 115 is made of a composite material of Ni and a zirconia-based electrolyte material, such as Ni/YSZ, or an M1 _{- xLxTiO3} (M is an alkaline earth metal element, and L is a lanthanoid element) such as _SrTiO3 , since it is required to have electronic conductivity and a thermal expansion coefficient close to that of the other materials constituting the cell stack 101. This lead film 115 leads out the DC power generated by the multiple electrochemical _single cells 105 connected in series by the interconnector 107 to the vicinity of the end of the cell stack 101.

The substrate tube 103 on which the fuel electrode 109, the solid electrolyte membrane 111 and the interconnector 107 slurry films are formed is co-sintered in air at a sintering temperature of, for example, 1350° C. to 1450° C.
Next, the base tube 103 on which the slurry film of the air electrode 113 is formed is sintered in the atmosphere. The sintering temperature is, for example, 1100° C. to 1250° C. The sintering temperature here is set to a lower temperature than the co-sintering temperature after the base tube 103 to the interconnector 107 are formed.

Next, the electrochemical cell cartridge and electrochemical cell module according to this embodiment will be described with reference to Figures 2 and 3. Here, Figure 2 shows one embodiment of a solid oxide fuel cell (SOFC) module according to this embodiment. Also, Figure 3 shows a cross-sectional view of one embodiment of a solid oxide fuel cell (SOFC) cartridge according to this embodiment.

As shown in FIG. 2, the SOFC module (electrochemical cell module) 201 includes, for example, a plurality of SOFC cartridges (electrochemical cell cartridges) 203 and a module container 205 that houses the plurality of SOFC cartridges 203. Although FIG. 2 illustrates a cylindrical SOFC cell stack 101, this is not necessarily the case, and a flat cell stack may be used, for example. The SOFC module 201 also includes a fuel gas supply pipe 207, a plurality of fuel gas supply branch pipes 207a, a fuel gas exhaust pipe 209, and a plurality of fuel gas exhaust branch pipes 209a. The SOFC module 201 also includes an oxidizing gas supply pipe (not shown), an oxidizing gas supply branch pipe (not shown), an oxidizing gas exhaust pipe (not shown), and a plurality of oxidizing gas exhaust branch pipes (not shown).

The fuel gas supply pipe 207 is provided inside the module container 205 and is connected to a fuel gas supply unit that supplies fuel gas with a predetermined gas composition and a predetermined flow rate corresponding to the power generation amount of the SOFC module 201, and is also connected to multiple fuel gas supply branch pipes 207a. This fuel gas supply pipe 207 branches and guides the fuel gas at a predetermined flow rate supplied from the above-mentioned fuel gas supply unit to multiple fuel gas supply branch pipes 207a. In addition, the fuel gas supply branch pipe 207a is connected to the fuel gas supply pipe 207 and is connected to multiple SOFC cartridges 203. This fuel gas supply branch pipe 207a guides the fuel gas supplied from the fuel gas supply pipe 207 to the multiple SOFC cartridges 203 at an approximately equal flow rate, and approximately uniforms the power generation performance of the multiple SOFC cartridges 203.

The fuel gas discharge branch pipe 209a is connected to the multiple SOFC cartridges 203 and is also connected to the fuel gas discharge pipe 209. This fuel gas discharge branch pipe 209a guides the exhaust fuel gas discharged from the SOFC cartridges 203 to the fuel gas discharge pipe 209. In addition, the fuel gas discharge pipe 209 is connected to the multiple fuel gas discharge branch pipes 209a, and a portion of the fuel gas discharge pipe 209 is disposed outside the module container 205. This fuel gas discharge pipe 209 guides the exhaust fuel gas discharged from the fuel gas discharge branch pipe 209a at a substantially uniform flow rate to the outside of the module container 205.

The module container 205 is operated with an internal pressure of atmospheric pressure to approximately 3 MPa and an internal temperature of atmospheric temperature to approximately 550°C, so a material that is pressure resistant and resistant to corrosion by oxidizing agents such as oxygen contained in oxidizing gases is used. For example, stainless steel materials such as SUS304 are suitable.

In this embodiment, a configuration in which multiple SOFC cartridges 203 are grouped together and stored in the module container 205 is described, but this is not limited thereto, and for example, the SOFC cartridges 203 can also be stored in the module container 205 without being grouped together.

3, the SOFC cartridge 203 includes a plurality of cell stacks 101, a power generation chamber 215, a fuel gas supply header 217, a fuel gas exhaust header 219, an oxidizing gas (air) supply header 221, and an oxidizing gas exhaust header 223. The SOFC cartridge 203 also includes an upper tube plate 225a, a lower tube plate 225b, an upper insulator 227a, and a lower insulator 227b. In this embodiment, the SOFC cartridge 203 has a structure in which the fuel gas supply header 217, the fuel gas discharge header 219, the oxidizing gas supply header 221, and the oxidizing gas discharge header 223 are arranged as shown in FIG. 3, so that the fuel gas and the oxidizing gas flow in opposite directions inside and outside the cell stack 101. However, this is not necessarily required. For example, the fuel gas and the oxidizing gas may flow in parallel inside and outside the cell stack 101, or the oxidizing gas may flow in a direction perpendicular to the longitudinal direction of the cell stack 101.

The power generation chamber 215 is an area formed between the upper insulator 227a and the lower insulator 227b. This power generation chamber 215 is an area in which the electrochemical single cells 105 of the cell stack 101 are arranged, and generates electricity by electrochemically reacting fuel gas with oxidizing gas. The temperature near the center of the cell stack 101 in the longitudinal direction of the power generation chamber 215 may be monitored by a temperature measurement unit (such as a temperature sensor or thermocouple), and during steady-state operation of the SOFC module 201, a high-temperature atmosphere of approximately 700°C to 1000°C is created.

The fuel gas supply header 217 is an area surrounded by the upper casing 229a and upper tube plate 225a of the SOFC cartridge 203, and is connected to the fuel gas supply branch pipe 207a by the fuel gas supply hole 231a provided at the top of the upper casing 229a. The multiple cell stacks 101 are joined to the upper tube plate 225a by a seal member 237a, and the fuel gas supply header 217 guides the fuel gas supplied from the fuel gas supply branch pipe 207a through the fuel gas supply hole 231a into the inside of the base tube 103 of the multiple cell stacks 101 at a substantially uniform flow rate, thereby substantially equalizing the power generation performance of the multiple cell stacks 101.

The fuel gas discharge header 219 is an area surrounded by the lower casing 229b and the lower tube plate 225b of the SOFC cartridge 203, and is connected to the fuel gas discharge branch pipe 209a (not shown) by the fuel gas discharge hole 231b provided in the lower casing 229b. In addition, the multiple cell stacks 101 are joined to the lower tube plate 225b by a seal member 237b, and the fuel gas discharge header 219 collects the exhaust fuel gas that passes through the inside of the base tube 103 of the multiple cell stacks 101 and is supplied to the fuel gas discharge header 219, and directs it to the fuel gas discharge branch pipe 209a via the fuel gas discharge hole 231b.

The oxidizing gas of a predetermined gas composition and a predetermined flow rate corresponding to the power generation amount of the SOFC module 201 is branched to the oxidizing gas supply branch pipe and supplied to the multiple SOFC cartridges 203. The oxidizing gas supply header 221 is an area surrounded by the lower casing 229b, the lower tube plate 225b, and the lower heat insulator 227b of the SOFC cartridge 203, and is connected to an oxidizing gas supply branch pipe (not shown) by an oxidizing gas supply hole 233a provided on the side of the lower casing 229b. This oxidizing gas supply header 221 guides a predetermined flow rate of oxidizing gas supplied from the oxidizing gas supply branch pipe (not shown) through the oxidizing gas supply hole 233a to the power generation chamber 215 through the oxidizing gas supply gap 235a described later.

The oxidizing gas discharge header 223 is an area surrounded by the upper casing 229a, upper tube plate 225a, and upper insulation 227a of the SOFC cartridge 203, and is connected to an oxidizing gas discharge branch pipe (not shown) via an oxidizing gas discharge hole 233b provided on the side of the upper casing 229a. This oxidizing gas discharge header 223 guides the exhaust oxidizing gas supplied to the oxidizing gas discharge header 223 from the power generation chamber 215 through an oxidizing gas discharge gap 235b (described later) to the oxidizing gas discharge branch pipe (not shown) via the oxidizing gas discharge hole 233b.

The upper tube plate 225a is fixed to the side plate of the upper casing 229a between the top plate of the upper casing 229a and the upper insulator 227a so that the upper tube plate 225a, the top plate of the upper casing 229a, and the upper insulator 227a are approximately parallel. The upper tube plate 225a also has a number of holes corresponding to the number of cell stacks 101 provided in the SOFC cartridge 203, into which the cell stacks 101 are inserted. The upper tube plate 225a airtightly supports one end of the multiple cell stacks 101 via either or both of the sealing member 237a and the adhesive member, and isolates the fuel gas supply header 217 from the oxidizing gas exhaust header 223.

The upper insulation 227a is disposed at the lower end of the upper casing 229a so that the upper insulation 227a, the top plate of the upper casing 229a, and the upper tube plate 225a are approximately parallel to each other, and is fixed to the side plate of the upper casing 229a. The upper insulation 227a also has a number of holes corresponding to the number of cell stacks 101 provided in the SOFC cartridge 203. The diameter of the holes is set to be larger than the outer diameter of the cell stacks 101. The upper insulation 227a has an oxidizing gas exhaust gap 235b formed between the inner surface of the hole and the outer surface of the cell stack 101 inserted into the upper insulation 227a.

The upper insulation 227a separates the power generation chamber 215 from the oxidizing gas discharge header 223, and prevents the atmosphere around the upper tube sheet 225a from becoming hot, which can lead to a decrease in strength and an increase in corrosion caused by the oxidizing agent contained in the oxidizing gas. In addition, a metal material with high temperature resistance, such as a Ni-based alloy, may be used to prevent the upper tube sheet 225a and the like from being exposed to high temperatures in the power generation chamber 215 and thermally deforming the upper tube sheet 225a and the like due to the temperature difference. The upper insulation 227a also guides the exhaust oxidizing gas that has passed through the power generation chamber 215 and been exposed to high temperatures through the oxidizing gas discharge gap 235b to the oxidizing gas discharge header 223.

According to this embodiment, the structure of the SOFC cartridge 203 described above allows the fuel gas and the exhaust oxidizing gas to flow in opposite directions inside and outside the cell stack 101. As a result, the exhaust oxidizing gas exchanges heat with the fuel gas supplied to the power generation chamber 215 through the inside of the base tube 103, and is cooled to a temperature at which the upper tube plate 225a, etc., made of a metal material, does not deform, such as buckling, and is supplied to the oxidizing gas discharge header 223. In addition, the fuel gas is heated by heat exchange with the exhaust oxidizing gas discharged from the power generation chamber 215 and is supplied to the power generation chamber 215. As a result, fuel gas preheated to a temperature suitable for power generation can be supplied to the power generation chamber 215 without using a heater or the like.

The lower tube plate 225b is fixed to the side plate of the lower casing 229b between the bottom plate of the lower casing 229b and the lower insulator 227b so that the lower tube plate 225b, the bottom plate of the lower casing 229b, and the lower insulator 227b are approximately parallel to each other. The lower tube plate 225b also has a number of holes corresponding to the number of cell stacks 101 provided in the SOFC cartridge 203, into which the cell stacks 101 are inserted. The lower tube plate 225b airtightly supports the other ends of the multiple cell stacks 101 via either or both of the sealing member 237b and the adhesive member, and isolates the fuel gas discharge header 219 from the oxidizing gas supply header 221.

The lower insulator 227b is disposed at the upper end of the lower casing 229b so that the lower insulator 227b, the bottom plate of the lower casing 229b, and the lower tube plate 225b are approximately parallel to each other, and is fixed to the side plate of the lower casing 229b. The lower insulator 227b also has a number of holes corresponding to the number of cell stacks 101 provided in the SOFC cartridge 203. The diameter of the holes is set to be larger than the outer diameter of the cell stacks 101. The lower insulator 227b has an oxidizing gas supply gap 235a formed between the inner surface of the hole and the outer surface of the cell stack 101 inserted into the lower insulator 227b.

The lower insulator 227b separates the power generation chamber 215 from the oxidizing gas supply header 221, and prevents the atmosphere around the lower tube sheet 225b from becoming hot, which would result in a decrease in strength and an increase in corrosion caused by the oxidizing agent contained in the oxidizing gas. The lower tube sheet 225b is made of a metal material that is resistant to high temperatures, such as Inconel, and prevents the lower tube sheet 225b from being exposed to high temperatures and being thermally deformed by the large temperature difference within the lower tube sheet 225b. The lower insulator 227b also guides the oxidizing gas supplied to the oxidizing gas supply header 221 through the oxidizing gas supply gap 235a to the power generation chamber 215.

According to this embodiment, the structure of the SOFC cartridge 203 described above allows the exhaust fuel gas and the oxidizing gas to flow in opposite directions inside and outside the cell stack 101. As a result, the exhaust fuel gas that passes through the inside of the base tube 103 and the power generation chamber 215 exchanges heat with the oxidizing gas supplied to the power generation chamber 215, and is cooled to a temperature at which the lower tube plate 225b made of a metal material and the like do not deform, such as buckling, and is supplied to the fuel gas discharge header 219. In addition, the oxidizing gas is heated by heat exchange with the exhaust fuel gas and is supplied to the power generation chamber 215. As a result, it is possible to supply the oxidizing gas heated to a temperature required for power generation to the power generation chamber 215 without using a heater or the like.

The DC power generated in the power generation chamber 215 is conducted to the vicinity of the end of the cell stack 101 by the lead film 115 made of Ni/YSZ or the like provided on the multiple electrochemical single cells 105, and then collected by the current collector (not shown) of the SOFC cartridge 203 via a current collector plate (not shown) and taken out to the outside of each SOFC cartridge 203. The DC power conducted out to the outside of the SOFC cartridge 203 by the current collector is connected to the generated power of each SOFC cartridge 203 in a predetermined number of series and parallel connections, conducted out to the outside of the SOFC module 201, and converted to a predetermined AC power by a power conversion device (such as an inverter) such as a power conditioner (not shown) and supplied to the power supply destination (for example, a load facility or a power system).

Next, the SOFC power generation system according to this embodiment will be described.
As shown in Fig. 4, the SOFC power generation system (electrochemical cell system) 10 includes a plurality of SOFCs 11. Specifically, the SOFC power generation system 10 has a plurality of SOFCs 11 arranged in cascade (series) on the upstream side and downstream side. In the following description, the SOFC 11 arranged on the upstream side (upstream side) is referred to as a topping SOFC (first fuel cell) 11a, and the SOFC arranged on the downstream side (downstream side) is referred to as a bottoming SOFC (second fuel cell) 11b. In addition, when it is not necessary to distinguish between the topping SOFC 11a and the bottoming SOFC 11b, they will simply be referred to as SOFC 11.
In addition, in the following description and drawings, the topping SOFC 11a may be referred to as a topping module (TM), and the bottoming SOFC 11b may be referred to as a bottoming module (BM).

The SOFC 11 generates electricity by reacting fuel gas as a reducing agent and air as an oxidizing agent (oxidizing gas) at a predetermined operating temperature. As described above, the SOFC 11 is composed of an SOFC module, and contains an assembly of multiple cell stacks 101 provided in a module container 205 of the SOFC module. The cell stack 101 includes an anode 109, an cathode 113, and a solid electrolyte membrane 111 (see FIGS. 1 to 3 ).
The SOFC 11 generates electricity by supplying air to the air electrode 113 and fuel gas to the fuel electrode 109, and converts the electricity into a predetermined AC power by a power conversion device (not shown).

The fuel gas supplied to the SOFC11 is a combustible gas, and examples of such gas include gas produced by vaporizing liquefied natural gas (LNG), natural gas, city gas, hydrogen (H2), carbon monoxide (CO), hydrocarbon gas such as methane (CH4), gas produced by gasification equipment for carbonaceous raw materials (oil, coal, etc.), and biogas made from biomass. Fuel gas refers to fuel gas whose heating value has been adjusted to a substantially constant value in advance.

An oxidizing gas supply line 12 that supplies oxidizing gas (air) is connected to the upstream end (air inlet) of the air electrode 113 of the SOFC 11. The oxidizing gas supply line 12 is provided with an air blower 12a for circulating air. In addition, a first heat exchanger 30 is provided downstream of the air blower 12a in the oxidizing gas supply line 12. The first heat exchanger 30 exchanges heat between the air flowing through the oxidizing gas supply line 12 and the fuel gas discharged from a third fuel gas line 23 described later, heating the air and cooling the fuel gas. The oxidizing gas supply line 12 branches downstream of the first heat exchanger 30 into a topping side supply line 12b that supplies air to the topping SOFC 11a and a bottoming side supply line 12c that supplies air to the bottoming SOFC 11b.

The downstream end (air outlet) of the air electrode 113 of the SOFC 11 is connected to an exhaust oxidizing gas exhaust line 13 that exhausts the exhaust air used in the reaction. The exhaust oxidizing gas exhaust line 13 has a topping side exhaust line 13a that exhausts the exhaust air from the topping SOFC 11a, and a bottoming side exhaust line 13b that exhausts the exhaust air from the bottoming SOFC 11b. The downstream end of the topping side exhaust line 13a and the downstream end of the bottoming side exhaust line 13b merge. The line after the topping side exhaust line 13a and the bottoming side exhaust line 13b merge is the exhaust oxidizing gas exhaust line 13.

The downstream end of a first fuel gas line 20 that supplies fuel gas is connected to the upstream end (fuel gas inlet) of the fuel electrode 109a of the topping SOFC 11a. A fuel gas supply unit is connected to the upstream end of the first fuel gas line. In addition, a fuel gas control valve 20a is provided in the first fuel gas line to adjust the flow rate of fuel gas supplied to the fuel electrode 109.

The downstream end (fuel gas outlet) of the fuel electrode 109a of the topping SOFC 11a is connected to the upstream end of the second fuel gas line 21 that discharges the fuel gas after being used in the reaction. The downstream end of the second fuel gas line 21 is connected to the upstream end (fuel gas inlet) of the fuel electrode 109b of the bottoming SOFC 11b. The second fuel gas line 21 supplies the fuel gas discharged from the topping SOFC 11a to the fuel electrode 109 of the bottoming SOFC 11b. A recirculation line 22 branches off from the second fuel gas line 21 to recirculate the fuel gas discharged from the downstream end (fuel gas outlet) of the fuel electrode 109a of the topping SOFC 11a to the upstream end (fuel gas inlet) of the fuel electrode 109a of the topping SOFC 11a. The downstream end of the recirculation line 22 is connected to the downstream side of the fuel gas control valve 20a of the first fuel gas line 20. In addition, the second fuel gas line 21 is provided with a fuel gas blower 21a for circulating fuel gas through the second fuel gas line 21 and the recirculation line 22. The fuel gas blower 21a is provided on the second fuel gas line 21 upstream of the branch point of the recirculation line 22.

The downstream end (fuel gas outlet) of the fuel electrode 109b of the bottoming SOFC 11b is connected to the upstream end of the third fuel gas line 23, which discharges the fuel gas used in the reaction. The third fuel gas line 23 guides the fuel gas discharged from the bottoming SOFC 11b (hereinafter, the fuel gas discharged from the bottoming SOFC 11b is referred to as "exhaust fuel gas") to the carbon dioxide capture device 33. The third fuel gas line 23 is provided with a first heat exchanger 30, a second heat exchanger 31, and a detection unit 32 in this order from the upstream side. The detection unit 32 detects the state quantity (composition, etc.) of the exhaust fuel gas flowing. The detection unit 32 may be a density meter, and the composition of the exhaust fuel gas may be obtained from the relationship between the composition and density of the exhaust fuel gas prepared in advance. By using a density meter as the detection unit 32, rapid measurement is possible and the cost of the detection unit 32 can be reduced.

As described above, the first heat exchanger 30 exchanges heat between the air flowing through the oxidizing gas supply line 12 and the exhaust fuel gas discharged from the bottoming SOFC 11b, thereby heating the air and cooling the exhaust fuel gas. At this time, the moisture in the exhaust fuel gas condenses, and the condensed moisture (drain) is discharged from the first heat exchanger 30 via the first drain line 24. In this way, the first heat exchanger 30 cools the exhaust fuel gas and removes the moisture in the exhaust fuel gas.

The second heat exchanger 31 cools the exhaust fuel gas by exchanging heat between the exhaust fuel gas discharged from the first heat exchanger 30 and the refrigerant supplied by the refrigerant line 25. At this time, the moisture in the exhaust fuel gas condenses, and the condensed moisture (drain) is discharged from the second heat exchanger 31 by the second drain line 26. In this way, the second heat exchanger 31 cools the exhaust fuel gas and removes the moisture in the exhaust fuel gas.

The detection unit 32 detects the composition of the exhaust fuel gas discharged from the second heat exchanger 31. The detection unit 32 is provided downstream of the first heat exchanger 30 and the second heat exchanger 31, and therefore detects the composition of the exhaust fuel gas from which moisture has been removed by the first heat exchanger 30 and the second heat exchanger 31. The detection unit 32 transmits the detected information to the control device 40.

As described above, the SOFC power generation system 10 according to the present embodiment includes a topping SOFC 11a and a bottoming SOFC 11b, and the bottoming SOFC 11b is disposed downstream of the topping SOFC 11a. The bottoming SOFC 11b is supplied with fuel gas discharged from the topping SOFC 11a. That is, the fuel gas used in the topping SOFC 11a and reduced in fuel components (hydrocarbon, hydrogen, carbon monoxide) is supplied to the bottoming SOFC 11b for reuse. In addition, the fuel gas containing steam is recirculated to the topping SOFC 11a via a recirculation line 22 branched from a system (second fuel gas line 21) from the topping SOFC 11a to the bottoming SOFC 11b, and the molar ratio of steam to carbon in the fuel gas (S/C: steam carbon ratio) required for internal reforming of the fuel gas in the topping SOFC 11a is ensured. The bottoming SOFC 11b is supplied with fuel gas containing water vapor discharged from the topping SOFC 11a, so that the S/C required for internal reforming is ensured. By using a cascade connection in which the fuel gas discharged from the SOFC on the front side is reused in the SOFC on the rear side, the S/C of the topping SOFC 11a and the bottoming SOFC 11b is maintained at an appropriate value, while improving the fuel utilization rate of the entire SOFC power generation system 10, and the fuel gas is efficiently used for power generation, thereby improving the power generation system efficiency. Here, the fuel utilization rate is calculated by dividing the amount of fuel components consumed in the SOFC power generation reaction by the amount of fuel components supplied to the SOFC. The amount of fuel components consumed in the power generation reaction can be calculated from the current command. The amount of fuel components is not limited to hydrocarbons and includes hydrogen and the like that can be supplied to the SOFC fuel cell.
In this embodiment, the ratio between the output of the topping SOFC 11a and the output of the bottoming SOFC 11b (hereinafter referred to as "output ratio") is about 10:1. That is, the output of the topping SOFC 11a is about 10 times the output of the bottoming SOFC 11b. The output ratio is set for the purpose of improving the tracking ability to the target fuel utilization rate. Specifically, when the fuel flow rate input to the system is matched with the current value of the topping SOFC 11a, the current value of the bottoming SOFC 11b needs to be corrected to adjust the fuel utilization rate in the system. Furthermore, when tracking the power demand, the current value of the topping SOFC 11a also needs to be corrected by the amount of the output fluctuation due to the current value correction of the bottoming SOFC 11b. The larger the output ratio, the smaller the correction amount of the current value of the topping SOFC 11a and the correction to the fuel flow rate supplied to the system are, so that it is possible to finely adjust the system fuel utilization rate by correcting the current command value of the bottoming SOFC 11b.

The SOFC power generation system 10 includes a control device (control unit) 40. The control device 40 receives the detection results of the detection unit 32. The control device 40 also operates each valve provided in the SOFC power generation system 10. The control device 40 also controls the topping SOFC 11a and the bottoming SOFC 11b.

The control device (Controller) includes, for example, a CPU (Central Processing Unit: processor), a main memory, a secondary storage (secondary storage: memory), etc. Furthermore, the control device may include a communication unit for transmitting and receiving information to and from other devices.
The main storage device is composed of writable memory such as cache memory and RAM (Random Access Memory), and is used as a working area for reading execution programs of the CPU and writing processing data by the execution programs.
The secondary storage device is a non-transitory computer readable storage medium, such as a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM, or a semiconductor memory.
As an example, a series of processes for realizing various functions is stored in a secondary storage device in the form of a program, and the CPU reads this program into the main storage device and executes information processing and arithmetic processing to realize various functions. Note that the program may be pre-installed in the secondary storage device, provided in a state stored in a computer-readable storage medium, or distributed via wired or wireless communication means. Examples of computer-readable storage media include magnetic disks, magneto-optical disks, CD-ROMs, DVD-ROMs, and semiconductor memories.

The control device 40 has a current command value control unit that controls the power generation amount (current command value) of the topping SOFC 11a and the bottoming SOFC 11b based on the detection result of the detection unit 32. The current command value control unit determines a current command value based on the detection result of the detection unit 32, and sets the current values of the topping SOFC 11a and the bottoming SOFC 11b to be the determined current command value.
A current command value is set according to the target load (electricity demand), and various controls corresponding to the output current value (control of the fuel gas supply amount, the pressure difference between the fuel electrode system and the air electrode system, the SOFC supply air temperature, the fuel gas recirculation flow rate, etc.) are performed.

The current command value control unit controls the current command value so that the fuel utilization rate of the entire SOFC power generation system 10 (topping SOFC 11a and bottoming SOFC 11b) is within a specified range, the concentration of carbon dioxide contained in the exhaust fuel gas discharged from the bottoming SOFC 11b is equal to or greater than a first threshold value, and the amount of power generation required of the SOFC power generation system 10 is satisfied.
Moreover, the current command value control unit controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b so that the carbon dioxide concentration in the exhaust fuel gas becomes equal to or lower than a second threshold value.

Specifically, the current command value control unit controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b so that the fuel utilization rate of the entire SOFC power generation system 10 is in the range of 90% or more and 94% or less. The fuel utilization rate (Uf) is calculated based on the composition of the fuel gas at a point upstream of the point where the recirculation line 22 of the first fuel gas line 20 joins (point P0 in FIG. 4) and the composition of the fuel gas at a point upstream of the first heat exchanger 30 of the third fuel gas line 23 (point P1 in FIG. 4).

In addition, the current command value control unit detects the concentration of carbon dioxide contained in the exhaust fuel gas discharged from the bottoming SOFC 11b and after moisture has been removed in the first heat exchanger 30 and the second heat exchanger 31 using the detection unit 32, and controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b so that the carbon dioxide concentration is 80 volume% or more.
In addition, the current command value control unit detects the hydrogen concentration contained in the exhaust fuel gas discharged from the bottoming SOFC 11b and after moisture has been removed in the first heat exchanger 30 and the second heat exchanger 31 using the detection unit 32, and controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b so that the hydrogen concentration is in the range of 14 volume % or more and 17 volume % or less.

The current command value control unit corrects the current command value of the topping SOFC11a based on the output ratio between the topping SOFC11a and the bottoming SOFC11b, and sets the current of the topping SOFC11a with the corrected current command value. Specifically, when the output ratio between the topping SOFC11a and the bottoming SOFC11b is about n:1, the current command value control unit sets the correction amount of the current command value of the topping SOFC11a to 1/n of the correction amount of the current command value of the bottoming SOFC11b. For example, when the output ratio between the topping SOFC11a and the bottoming SOFC11b is 10:1, the correction amount of the current command value of the topping SOFC11a is set to 1/10 of the correction amount of the current command value of the bottoming SOFC11b. The bottoming SOFC11b has a larger current correction width than the topping SOFC11a, resulting in better controllability.

The control device 40 may have a main memory device or a secondary memory device that stores a map that specifies the correspondence between the density of the exhaust fuel gas and the composition of the exhaust fuel gas (the carbon dioxide concentration and hydrogen concentration in the exhaust fuel gas). In this case, the control device 40 has an estimation unit that estimates the carbon dioxide concentration and hydrogen concentration in the exhaust fuel gas based on the map from the density of the exhaust fuel gas measured by the detection unit 32.

Specifically, the control device 40 may store a map shown in Fig. 5 that defines the correspondence between the density of the exhaust fuel gas and the carbon dioxide concentration, hydrogen concentration, and fuel utilization rate in the exhaust fuel gas. In Fig. 5, the horizontal axis indicates the density of the exhaust fuel gas, and the vertical axis indicates the carbon dioxide concentration, hydrogen concentration, and fuel utilization rate. That is, the map in Fig. 5 defines estimated values of the carbon dioxide concentration, hydrogen concentration, and fuel utilization rate in the exhaust fuel gas for each density value (between about ^1.34 kg/Nm3 and ^about 1.67 kg/Nm3). Note that the carbon dioxide concentration and hydrogen concentration in Fig. 5 are concentrations in the exhaust fuel gas after moisture is removed by the first heat exchanger 30 and the second heat exchanger 31.

The map shown in FIG. 5 shows that as the density of the exhaust fuel gas increases, the carbon dioxide concentration in the exhaust fuel gas also increases. In addition, as the density of the exhaust fuel gas increases, the fuel utilization rate also increases. In addition, as the density of the exhaust fuel gas increases, the hydrogen concentration in the exhaust fuel gas decreases. The estimation unit estimates the carbon dioxide concentration from the density of the exhaust fuel gas measured by the detection unit 32 based on the map in FIG. 5.

In this embodiment, when fuel gas mainly composed of hydrocarbons is supplied, the density of the exhaust fuel gas whose carbon dioxide concentration is estimated to be about 80% (first threshold value) is set as the lower limit value X1. Also, the density of the exhaust fuel gas whose hydrogen concentration is estimated to be about 14% is set as the upper limit value X2. When the density of the exhaust fuel gas becomes X1, the estimation unit estimates that the carbon dioxide concentration in the exhaust fuel gas is about 80% (first threshold value) and the hydrogen concentration is about 17%. Also, when the density of the exhaust fuel gas becomes X2, the estimation unit estimates that the carbon dioxide concentration in the exhaust fuel gas is about 82% (second threshold value) and the hydrogen concentration is about 14%.
The lower limit value X1 of the density of the exhaust fuel gas may be a density corresponding to a lower limit value of the carbon dioxide concentration at which carbon dioxide can be efficiently captured from the exhaust fuel gas (which depends on the type and specifications of the carbon dioxide capture device 33). Also, the upper limit value X2 of the density of the exhaust fuel gas may be a density corresponding to the upper limit values of the carbon dioxide concentration and the hydrogen concentration at which damage to the cell stack 101, etc. can be suppressed in the topping SOFC 11a and the bottoming SOFC 11b.

Next, the process performed by the control device 40 will be described with reference to the flowchart of FIG.
When the process starts, the control device 40 receives a power demand (required power generation amount) D for the SOFC power generation system 10. The power demand D is input from outside the control device 40 (step S1). Next, the control device 40 sets the fuel flow rate and the current values of the topping SOFC 11a (topping module (TM)) and the bottoming SOFC 11b (bottoming module (BM)) so that the sending end output of the SOFC power generation system 10 satisfies the power demand D (step S2).

Next, the control device determines whether the density of the exhaust fuel gas is equal to or less than X1 (step S3). In step S3, the control device may estimate the carbon dioxide concentration in the exhaust fuel gas from the output of the detection unit 32, and determine whether the estimated carbon dioxide concentration is equal to or less than a predetermined value.
If it is determined in step S3 that the density of the exhaust fuel gas is equal to or lower than X1, the process proceeds to step S4. In step S4, the current set value is increased by ΔI so that the output of the bottoming SOFC 11b increases by ΔP. In other words, in step S4, the current set value of the bottoming SOFC 11b set in step S2 is corrected. Also, in step S4, the current set value of the bottoming SOFC 11b is corrected so that the fuel utilization rate falls within a specified range.

Next, proceeding to step S5, in order to maintain a sending end output that satisfies the power demand D for the SOFC power generation system 10, the output of the topping SOFC 11a is reduced by the amount of the increase in the output of the bottoming SOFC 11b. Specifically, the current set value is reduced by ΔI/n so that the output of the topping SOFC 11a is reduced by ΔP. The correction value of the current set value is set to a value based on the output ratio between the topping SOFC 11a and the bottoming SOFC 11b. In this way, in step S5, the current set value of the topping SOFC 11a set in step S2 is corrected.

After completing step S5, the control device 40 proceeds to step S10. In step S10, the fuel flow rate is changed to a value corresponding to the changed current setting value. Specifically, the control device 40 controls the fuel gas control valve 20a to adjust the flow rate of the fuel gas flowing through the first fuel gas line 20. After completing step S10, the control device 40 returns to step S3.

If it is determined in step S3 that the density of the exhaust fuel gas is greater than X1, the process proceeds to step S6. In step S6, it is determined whether the density of the exhaust fuel gas is equal to or greater than X2. Note that in step S6, the carbon dioxide concentration and/or hydrogen concentration in the exhaust fuel gas may be estimated from the output of the detection unit 32, and it may be determined whether the estimated carbon dioxide concentration and/or hydrogen concentration is equal to or greater than a predetermined value.
If it is determined in step S6 that the density of the exhaust fuel gas is equal to or greater than X2, the process proceeds to step S7. In step S7, the current set value is reduced by ΔI' so that the output of the bottoming SOFC 11b is reduced by ΔP'. In other words, in step S4, the current set value of the bottoming SOFC 11b set in step S2 is corrected. Also, in step S7, the current set value of the bottoming SOFC 11b is corrected so that the fuel utilization rate falls within a specified range.

Next, proceed to step S8, and in order to maintain a sending end output that satisfies the power demand D for the SOFC power generation system 10, the output of the topping SOFC 11a is increased by the amount of the reduction in the output of the bottoming SOFC 11b. Specifically, the current set value is increased by ΔI'/n so that the output of the topping SOFC 11a increases by ΔP'. The correction value of the current set value is set to a value based on the output ratio between the topping SOFC 11a and the bottoming SOFC 11b. In this way, in step S8, the current set value of the topping SOFC 11a set in step S2 is corrected.

After completing step S8, the control device 40 proceeds to step S10. In step S10, the fuel flow rate is changed to a value corresponding to the changed current setting value. Specifically, the control device 40 controls the fuel gas control valve 20a to adjust the flow rate of the fuel gas flowing through the first fuel gas line 20. After completing step S10, the control device 40 returns to step S3.

If it is determined in step S6 that the density of the exhaust fuel gas is less than X2, the process proceeds to step S9. In step S9, operation continues with the current output (current setting value) of the topping SOFC 11a and bottoming SOFC 11b. When step S9 is completed, the process returns to step S1.

Next, the process performed by the control device 40 will be described with reference to FIG.
First, the control device 40 receives a power demand (required power generation amount) for the SOFC power generation system 10 (step S11). The control device 40 sets a current set value for the topping SOFC 11a and a current set value for the bottoming SOFC 11b based on the power demand (steps S12 and S13).

Meanwhile, the detection unit 32 measures the composition (hereinafter, for example, density) of the exhaust fuel gas discharged from the bottoming SOFC 11b and from which moisture has been removed by the first heat exchanger 30 and the second heat exchanger 31. The detection unit 32 transmits the measurement results to the control device 40. The control device 40 estimates the hydrogen concentration and carbon dioxide concentration in the exhaust fuel gas using the map of FIG. 5 based on the measurement results received from the detection unit 32 (step S14). The control device 40 determines whether or not the current values of the topping SOFC 11a and the bottoming SOFC 11b set in steps S12 and S13 need to be corrected based on the carbon dioxide concentration (step S15).

If correction is necessary, the output (current setting value) of the bottoming SOFC11b is corrected. Specifically, a correction value ΔI for the current setting value of the bottoming SOFC11b is determined (step S17) to increase the output of the bottoming SOFC11b by ΔP (step S16). Next, the correction value ΔI determined in step S17 is added to the current setting value set in step S12 to determine the corrected current setting value of the bottoming SOFC11b. The corrected current setting value of the bottoming SOFC11b is transmitted to the PCS37 (Power Conditioning System) of the bottoming SOFC11b, and the current value of the bottoming SOFC11b is controlled to become the corrected current setting value (step S18).

When the output of the bottoming SOFC 11b is corrected in steps S16 to S18, the output of the topping SOFC 11a is reduced by the amount of the increase in the output of the bottoming SOFC 11b in order to maintain the output that satisfies the power demand D for the SOFC power generation system 10. Specifically, a correction value ΔI/n for the current set value of the topping SOFC 11a is determined (step S20) to reduce the output of the topping SOFC 11a by ΔP (step S19). Next, the correction value ΔI/n determined in step S20 is added to the current set value set in step S13 to determine the corrected current set value of the topping SOFC 11a. The corrected current set value of the topping SOFC 11a is transmitted to the PCS 36 of the topping SOFC 11a, and the current value of the topping SOFC 11a is controlled to be the corrected current set value (step S21). In addition, the flow rate of the fuel gas flowing through the first fuel gas line 20 is set based on the corrected current setting value of the topping SOFC 11a (step S22), and the flow rate of the fuel gas flowing through the first fuel gas line 20 is adjusted by adjusting the opening of the fuel gas control valve 20a.

Next, the relationship between the change in exhaust fuel gas density and the current value of the bottoming SOFC11b and the topping SOFC11a will be explained using Figure 8. In Figure 8, the horizontal axis indicates the flow of time, and the vertical axis indicates (a) the exhaust fuel gas density, (b) the carbon dioxide concentration, (c) the current value of the bottoming SOFC11b, and (d) the current value of the topping SOFC11a.

8, when the density of the exhaust fuel gas measured by the detector 32 decreases, the estimated carbon dioxide concentration in the exhaust fuel gas also decreases (T1). Accordingly, the current setting value of the bottoming SOFC 11b increases, and the current setting value of the topping SOFC 11a decreases. Furthermore, when the decrease in the density of the exhaust fuel gas measured by the detector 32 stops, the decrease in the estimated carbon dioxide concentration in the exhaust fuel gas also stops (T2). Accordingly, the increase in the current setting value of the bottoming SOFC 11b and the decrease in the current setting value of the topping SOFC 11a also stop.
When the density of the exhaust fuel gas measured by the detector 32 starts to increase, the estimated carbon dioxide concentration in the exhaust fuel gas also increases (T3). Accordingly, the current setting value of the bottoming SOFC 11b decreases, and the current setting value of the topping SOFC 11a increases. Furthermore, when the increase in the density of the exhaust fuel gas measured by the detector 32 stops, the estimated carbon dioxide concentration in the exhaust fuel gas also stops increasing (T4). Accordingly, the decrease in the current setting value of the bottoming SOFC 11b and the increase in the current setting value of the topping SOFC 11a also stop.

On the other hand, when the density of the exhaust fuel gas measured by the detector 32 increases, the estimated carbon dioxide concentration in the exhaust fuel gas also decreases (T5). Accordingly, the current setting value of the bottoming SOFC 11b decreases, and the current setting value of the topping SOFC 11a increases. Furthermore, when the increase in the density of the exhaust fuel gas measured by the detector 32 stops, the estimated carbon dioxide concentration in the exhaust fuel gas also stops increasing (T6). Accordingly, the decrease in the current setting value of the bottoming SOFC 11b and the increase in the current setting value of the topping SOFC 11a also stop.
When the density of the exhaust fuel gas measured by the detector 32 starts to decrease, the estimated carbon dioxide concentration in the exhaust fuel gas also decreases (T7). Accordingly, the current setting value of the bottoming SOFC 11b increases, and the current setting value of the topping SOFC 11a decreases. Furthermore, when the decrease in the density of the exhaust fuel gas measured by the detector 32 stops, the decrease in the estimated carbon dioxide concentration in the exhaust fuel gas also stops (T8). Accordingly, the increase in the current setting value of the bottoming SOFC 11b and the decrease in the current setting value of the topping SOFC 11a also stop.
Although the behavior of the hydrogen concentration is not shown in Figure 8, the estimated hydrogen concentration in the exhaust fuel gas increases as the density of the exhaust fuel gas measured by the detection unit 32 decreases, and the hydrogen concentration increases as the density of the exhaust fuel gas measured by the detection unit 32 increases.

According to this embodiment, the following advantageous effects are obtained.
In this embodiment, the control device 40 controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b so that the fuel utilization rate, which is the utilization rate of the fuel gas in the topping SOFC 11a and the bottoming SOFC 11b, is within a specified range and the carbon dioxide concentration in the exhaust fuel gas discharged from the bottoming SOFC 11b is equal to or higher than a first threshold value (specifically, about 80%). This makes it possible to increase the carbon dioxide concentration in the exhaust fuel gas discharged from the bottoming SOFC 11b. This makes it easier to recover carbon dioxide from the exhaust fuel gas.

In particular, it has been difficult to stably control the fuel utilization rate when the composition of the fuel gas being supplied fluctuates frequently or in a transient state when the power generation output is changed to follow the power demand, but in this embodiment, it is possible to stably control the fuel utilization rate.

In addition, in this embodiment, the control device 40 controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b so as to satisfy the amount of power generation required for the SOFC power generation system 10. This makes it possible to satisfy the amount of power generation required for the SOFC power generation system 10.

In addition, in this embodiment, the control device 40 controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b so that the carbon dioxide concentration of the exhaust fuel gas detected by the detection unit 32 is equal to or lower than a second threshold value (specifically, approximately 82%). This makes it possible to prevent the carbon dioxide concentration in the exhaust fuel gas from increasing excessively (in other words, the fuel gas components in the exhaust fuel gas from being excessively reduced). Therefore, damage to the bottoming SOFC 11b caused by an excessive reduction in the fuel gas components in the bottoming SOFC 11b can be prevented.

In addition, the density of the exhaust fuel gas is relatively easy to detect, and continuous detection is also relatively easy. In this embodiment, the density of the exhaust fuel gas is detected, and the control device 40 controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b based on the carbon dioxide concentration estimated by the estimation unit from the density of the exhaust fuel gas. This allows the control device 40 to relatively easily control the current command values of the topping SOFC 11a and the bottoming SOFC 11b, even when the composition of the fuel gas frequently changes.

In addition, in this embodiment, the control device 40 corrects the current command values of the topping SOFC 11a and the bottoming SOFC 11b based on the output ratio between the topping SOFC 11a and the bottoming SOFC 11b. This makes it possible to more appropriately satisfy the required power generation amount for the SOFC power generation system 10.

In addition, in this embodiment, the control device 40 controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b so that the fuel utilization rate is in the range of 90% or more and 94% or less. This allows the topping SOFC 11a and the bottoming SOFC 11b to consume fuel gas components preferably, so that the carbon dioxide concentration in the exhaust fuel gas discharged from the bottoming SOFC 11b can be more preferably increased. This makes it easier to recover carbon dioxide from the exhaust fuel gas.

In addition, in this embodiment, the detection unit 32 detects the density of the exhaust fuel gas from which moisture has been removed by the first heat exchanger 30 and the second heat exchanger 31. This makes it possible to detect the density of the exhaust fuel gas more accurately.

In this embodiment, the control device 40 controls the current command values of the topping SOFC 11a and the bottoming SOFC 11b so that the hydrogen concentration in the exhaust fuel gas is in the range of 14% or more and 17% or less. This allows the topping SOFC 11a and the bottoming SOFC 11b to preferably consume hydrogen, which is a fuel gas component, and therefore the carbon dioxide concentration in the gas discharged from the bottoming SOFC 11b can be more preferably increased. This makes it easier to recover carbon dioxide from the exhaust fuel gas.
Moreover, an excessive decrease in the hydrogen concentration in the exhaust fuel gas can be suppressed, and therefore, in the bottoming SOFC 11b, damage to the cell stack 101 and the like caused by an excessive decrease in the hydrogen concentration can be suppressed.

In this way, in this embodiment, the bottoming SOFC 11b does not focus only on the power generation output, but controls the current value to increase the carbon dioxide concentration in the exhaust fuel within a range that meets the power demand. This makes it possible to suitably increase the carbon dioxide concentration in the exhaust fuel gas discharged from the bottoming SOFC 11b in a cascade-type SOFC power generation system 10 in which multiple SOFCs 11 are connected in series.

Note that this disclosure is not limited to the above-described embodiments, and can be modified as appropriate without departing from the spirit and scope of the present disclosure.

For example, the topping SOFC 11a may have multiple SOFCs connected in parallel.

The SOFCs 11 may also be connected in series in three or more stages. In that case, the composition of the exhaust fuel gas discharged from the SOFC 11 located at the most downstream position is detected, and the current command values of all the SOFCs 11 are controlled based on the detected composition.

In the above embodiment, the detection unit 32 is provided to measure the density of the exhaust fuel gas discharged from the bottoming SOFC 11b, and the carbon dioxide concentration in the exhaust fuel gas is estimated based on the detected density. However, the present disclosure is not limited to this. For example, a carbon dioxide concentration meter that measures the carbon dioxide concentration in the exhaust fuel gas discharged from the bottoming SOFC 11b may be provided instead of the detection unit 32. Also, a hydrogen concentration meter that measures the hydrogen concentration in the exhaust fuel gas may be provided instead of (or in addition to) the carbon dioxide concentration meter.
By measuring the carbon dioxide concentration in the exhaust fuel gas in this manner, the carbon dioxide concentration in the exhaust fuel gas can be grasped more accurately. This makes it possible to adjust the carbon dioxide concentration in the exhaust fuel gas more accurately. Therefore, the carbon dioxide concentration in the exhaust fuel gas can be more suitably increased, making it easier to recover carbon dioxide from the exhaust fuel gas.

In addition, in the above embodiment, the power generation output of the bottoming SOFC11b is not emphasized, so it is desirable to use a highly durable cell that is capable of generating electricity at low temperatures in the bottoming SOFC11b.

In addition, the high concentration of carbon dioxide discharged from the bottoming SOFC11b may be mixed into the fuel gas supplied to the SOFC11 to prevent spontaneous ignition.

The electrochemical cell system and the method of operating the electrochemical cell system described in the above-described embodiment can be understood, for example, as follows.

The electrochemical cell system according to the first aspect of the present disclosure is an electrochemical cell system (10) having a plurality of fuel cells (11) that generate electricity by reacting a fuel gas supplied to a fuel electrode with an oxidizing gas supplied to an air electrode, and includes a first fuel cell (11a), a second fuel cell (11b) to which exhaust fuel gas discharged from the first fuel cell (11a) is supplied, a detection unit (32) that detects the state quantity of the exhaust fuel gas discharged from the second fuel cell (11b), and a control unit (40) that controls the current command value of the first fuel cell (11a) and the second fuel cell (11b) based on the result detected by the detection unit (32). The control unit (40) controls the current command value so that the fuel utilization rate of the electrochemical cell system is within a specified range, the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell (11b) is equal to or greater than a first threshold, and the amount of electricity generated required for the electrochemical cell system (10) is satisfied.

In the above configuration, the control unit controls the current command values of the first fuel cell and the second fuel cell so that the fuel utilization rate of the electrochemical cell system is within a specified range and the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell is equal to or higher than a first threshold value. This makes it possible to increase the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell, thereby making it easier to recover carbon dioxide from the exhaust fuel gas.
In the above configuration, the control unit controls the current command values of the first fuel cell and the second fuel cell so as to satisfy the amount of power generation required for the electrochemical cell system, thereby making it possible to satisfy the required amount of power generation for the entire electrochemical cell system.
Examples of the state quantities of the exhaust fuel gas include the density of the exhaust fuel gas, the carbon dioxide concentration in the exhaust fuel gas, and the hydrogen concentration in the exhaust fuel gas.

In the electrochemical cell system according to the second aspect of the present disclosure, in the first aspect described above, the control unit (40) controls the current command values of the first fuel cell (11a) and the second fuel cell (11b) so that the carbon dioxide concentration in the exhaust fuel gas detected by the detection unit (32) is equal to or lower than a second threshold value that is higher than the first threshold value.

In the above configuration, the control unit controls the current command values of the first fuel cell and the second fuel cell so that the carbon dioxide concentration of the exhaust fuel gas detected by the detection unit is equal to or lower than the second threshold value. This makes it possible to prevent the carbon dioxide concentration in the exhaust fuel gas from increasing excessively (in other words, the fuel gas components in the exhaust fuel gas from being excessively reduced). Therefore, damage to the second fuel cell caused by an excessive reduction in the fuel gas components in the second fuel cell can be prevented.

In the electrochemical cell system according to the third aspect of the present disclosure, in the first or second aspect described above, the detection unit (32) detects the density of the exhaust fuel gas discharged from the second fuel cell (11b), and the control unit (40) has an estimation unit that estimates the carbon dioxide concentration in the exhaust fuel gas based on the density of the exhaust fuel gas, and controls the current command values of the first fuel cell (11a) and the second fuel cell (11b) based on the carbon dioxide concentration estimated by the estimation unit.

The density of the exhaust fuel gas is relatively easy to detect. In the above configuration, the density of the exhaust fuel gas is detected, and the control unit controls the current command values of the first fuel cell and the second fuel cell based on the carbon dioxide concentration estimated by the estimation unit from the density of the exhaust fuel gas. This allows the control unit to relatively easily control the current command values of the first fuel cell and the second fuel cell.

In the electrochemical cell system according to the fourth aspect of the present disclosure, in any one of the first to third aspects, the control unit (40) corrects the current command value of the first fuel cell (11a) based on the ratio between the output of the first fuel cell (11a) and the output of the second fuel cell (11b).

In the above configuration, the control unit corrects the current command value for the first fuel cell based on the output ratio between the first fuel cell and the second fuel cell. This allows the current command value for the first fuel cell to be set to a value based on the output ratio. Therefore, the required power generation amount for the entire electrochemical cell system can be more appropriately met.

In the electrochemical cell system according to the fifth aspect of the present disclosure, in any one of the first to fourth aspects, the control unit (40) controls the current command values of the first fuel cell (11a) and the second fuel cell (11b) so that the fuel utilization rate is in the range of 90% or more and 94% or less.

In the above configuration, the control unit controls the current command values of the first fuel cell and the second fuel cell so that the fuel utilization rate is in the range of 90% or more and 94% or less. This allows the first fuel cell and the second fuel cell to consume fuel gas components in an optimal manner, and therefore the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell can be increased more optimally. This makes it easier to recover carbon dioxide from the exhaust fuel gas.

The electrochemical cell system according to the sixth aspect of the present disclosure is any one of the first to fifth aspects described above, and includes a gas-liquid separation section (30, 31) that removes moisture from the exhaust fuel gas discharged from the second fuel cell (11b), and the detection section (32) detects the composition of the exhaust fuel gas from which moisture has been removed by the gas-liquid separation section (30, 31).

In the above configuration, the detection unit detects the composition of the exhaust fuel gas from which moisture has been removed by the gas-liquid separation unit. This makes it possible to detect the composition of the exhaust fuel gas more accurately.

The electrochemical cell system according to the seventh aspect of the present disclosure is the sixth aspect, in which the control unit (40) controls the current command values of the first fuel cell (11a) and the second fuel cell (11b) so that the carbon dioxide concentration in the exhaust fuel gas is 80% or more.

In the above configuration, the control unit controls the current command values of the first fuel cell and the second fuel cell so that the carbon dioxide concentration in the exhaust fuel gas is 80% or more. This makes it possible to more suitably increase the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell, thereby making it easier to recover carbon dioxide from the exhaust fuel gas.
Furthermore, carbon dioxide is relatively easy to control among the components contained in the exhaust fuel gas, so the control unit can perform control more appropriately.

The electrochemical cell system according to the eighth aspect of the present disclosure is the sixth or seventh aspect, in which the control unit (40) controls the current command values of the first fuel cell (11a) and the second fuel cell (11b) so that the hydrogen concentration in the exhaust fuel gas is in the range of 14% or more and 17% or less.

In the above configuration, the control unit controls the current command values of the first fuel cell and the second fuel cell so that the hydrogen concentration in the exhaust fuel gas is in the range of 14% or more and 17% or less. This allows the first fuel cell and the second fuel cell to preferably consume hydrogen, which is a fuel gas component, and therefore the carbon dioxide concentration in the gas exhausted from the second fuel cell can be more preferably increased. This makes it easier to recover carbon dioxide from the exhaust fuel gas.
Moreover, an excessive decrease in the hydrogen concentration in the exhaust fuel gas can be suppressed, and therefore damage to the second fuel cell caused by an excessive decrease in the hydrogen concentration can be suppressed.
Furthermore, among the components contained in the exhaust fuel gas, hydrogen is relatively easy to control, so the control unit can perform control more appropriately.

The method for operating an electrochemical cell system according to the first aspect of the present disclosure is a method for operating an electrochemical cell system (10) having a plurality of fuel cells that generate electricity by reacting a fuel gas supplied to an anode with an oxidizing gas supplied to an cathode, the electrochemical cell system (10) comprising a first fuel cell (11a) and a second fuel cell (11b) to which exhaust fuel gas discharged from the first fuel cell (11a) is supplied, a detection unit (32) that detects the state quantity of the exhaust fuel gas discharged from the second fuel cell (11b), and a control unit (40) that controls the current command values of the first fuel cell (11a) and the second fuel cell (11b) based on the result detected by the detection unit (32), and a control step in which the control unit (40) controls the current command value so that the fuel utilization rate of the electrochemical cell system is within a specified range, the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell (11b) is equal to or greater than a first threshold, and the amount of electricity required for the electrochemical cell system (10) is satisfied.

10: SOFC power generation system 12: Oxidizing gas supply line 12a: Air blower 12b: Topping side supply line 12c: Bottoming side supply line 13: Exhaust oxidizing gas discharge line 13a: Topping side discharge line 13b: Bottoming side discharge line 20: First fuel gas line 20a: Fuel gas control valve 21: Second fuel gas line 21a: Fuel gas blower 22: Recirculation line 23: Third fuel gas line 24: First drain line 25: Refrigerant line 26: Second drain line 30: First heat exchanger 31: Second heat exchanger 32: Detection unit 33: Carbon dioxide capture device 40: Control device 101: Cell stack 103: Base tube 105: Electrochemical single cell 107: Interconnector 109: Anode 111: Solid electrolyte membrane 113 : Air electrode 115 : Lead film 201 : SOFC module 203 : SOFC cartridge 205 : Module container 207 : Fuel gas supply pipe 207a : Fuel gas supply branch pipe 209 : Fuel gas exhaust pipe 209a : Fuel gas exhaust branch pipe 215 : Power generation chamber 217 : Fuel gas supply header 219 : Fuel gas exhaust header 221 : Oxidizing gas supply header 223 : Oxidizing gas exhaust header 225a : Upper tube plate 225b : Lower tube plate 227a : Upper heat insulator 227b : Lower heat insulator 229a : Upper casing 229b : Lower casing 231a : Fuel gas supply hole 231b : Fuel gas exhaust hole 233a : Oxidizing gas supply hole 233b : Oxidizing gas exhaust hole 235a : Oxidizing gas supply gap 235b : Oxidizing gas exhaust gap 237a : Sealing member 237b : Sealing member

Claims (9)

An electrochemical cell system having a plurality of fuel cells that generate electricity by reacting a fuel gas supplied to an anode with an oxidizing gas supplied to an cathode,
a first fuel cell; and a second fuel cell to which exhaust fuel gas discharged from the first fuel cell is supplied.
a detection unit for detecting a state quantity of exhaust fuel gas discharged from the second fuel cell;
a control unit that controls current command values for the first fuel cell and the second fuel cell based on the result detected by the detection unit,
The control unit controls the current command value so that the fuel utilization rate of the electrochemical cell system is within a specified range, the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell is equal to or greater than a first threshold value, and the amount of power generation required for the electrochemical cell system is satisfied. The electrochemical cell system according to claim 1, wherein the control unit controls the current command values of the first fuel cell and the second fuel cell so that the carbon dioxide concentration in the exhaust fuel gas detected by the detection unit is equal to or lower than a second threshold value that is higher than the first threshold value. the detection unit detects a density of exhaust fuel gas discharged from the second fuel cell,
2. The electrochemical cell system according to claim 1, wherein the control unit has an estimation unit that estimates a carbon dioxide concentration in the exhaust fuel gas based on a density of the exhaust fuel gas, and controls current command values of the first fuel cell and the second fuel cell based on the carbon dioxide concentration estimated by the estimation unit. The electrochemical cell system according to claim 1, wherein the control unit corrects the current command value of the first fuel cell based on the ratio between the output of the first fuel cell and the output of the second fuel cell. The electrochemical cell system according to claim 1, wherein the control unit controls the current command values of the first fuel cell and the second fuel cell so that the fuel utilization rate is in the range of 90% or more and 94% or less. a gas-liquid separator for removing moisture from the exhaust fuel gas discharged from the second fuel cell;
2. The electrochemical cell system according to claim 1, wherein the detection section detects the composition of the exhaust fuel gas from which moisture has been removed in the gas-liquid separation section. The electrochemical cell system according to claim 6, wherein the control unit controls the current command values of the first fuel cell and the second fuel cell so that the carbon dioxide concentration in the exhaust fuel gas is 80% or more. The electrochemical cell system according to claim 6, wherein the control unit controls the current command values of the first fuel cell and the second fuel cell so that the hydrogen concentration in the exhaust fuel gas is in the range of 14% or more and 17% or less. A method for operating an electrochemical cell system having a plurality of fuel cells that generate electricity by reacting a fuel gas supplied to an anode with an oxidizing gas supplied to an cathode, comprising the steps of:
The electrochemical cell system comprises:
a first fuel cell; and a second fuel cell to which exhaust fuel gas discharged from the first fuel cell is supplied.
a detection unit for detecting a state quantity of exhaust fuel gas discharged from the second fuel cell;
a control unit that controls current command values for the first fuel cell and the second fuel cell based on the result detected by the detection unit,
An operating method for an electrochemical cell system, comprising a control step in which the control unit controls a current command value so that the fuel utilization rate of the electrochemical cell system is within a specified range, the carbon dioxide concentration in the exhaust fuel gas discharged from the second fuel cell is equal to or greater than a first threshold value, and the amount of power generation required for the electrochemical cell system is satisfied.