JP4896601B2 - Operation control method of solid oxide fuel cell - Google Patents

Description

  The present invention relates to a method for controlling the operation of a solid oxide fuel cell. More specifically, the present invention relates to a solid oxide fuel cell stack or a horizontally striped solid oxide fuel cell bundle, which has a fuel utilization rate that is safe and stable as a whole. The present invention relates to a method of controlling to be appropriate or maximum.

  Solid oxide fuel cells (hereinafter referred to as “SOFC” where appropriate) include a flat plate type, a cylindrical type, an integral laminated type, a horizontal stripe type, and other various types, but these are the same in principle. The unit cell or cell includes an anode and a cathode arranged with a solid oxide electrolyte in between, and is constituted by a three-layer structure of anode / solid oxide electrolyte / cathode. The SOFC includes (a) a self-supporting membrane type in which the structure is maintained by the electrolyte membrane itself, (b) a support membrane type in which the electrolyte membrane is supported by a thick anode, and (c) an insulating porous support substrate. A form in which a battery is arranged on top is also considered.

  FIGS. 1A to 1C are views (cross-sectional views) for explaining an example of the cells. FIG. 1A shows a self-supporting membrane cell 1 in which an anode 2 is disposed on the lower surface of the electrolyte membrane 3 and a cathode 4 is disposed on the upper surface of the electrolyte membrane 3. FIG. 1 (b) shows a support membrane type cell 1 in which an electrolyte membrane 3 is disposed on an anode 2 and a cathode 4 is disposed on the electrolyte membrane 3. FIG. 1C shows a cell 1 in which an anode 2, an electrolyte 3 and a cathode 4 are sequentially arranged on a porous support substrate 5.

As the constituent material of the anode, a material mainly composed of Ni, a material composed of a mixture of Ni and yttria stabilized zirconia, and the like are used. As the solid oxide electrolyte, yttria stabilized zirconia, scandia stabilized zirconia, yttria doped ceria, Gadria-doped ceria or the like is used, Sr-doped LaMnO ₃ or the like is used as the constituent material of the cathode, and ZrO ₂ or the like in which Ni and a rare earth element oxide are dissolved as the constituent material of the porous support base. . The porous support substrate is made porous by mixing and sintering graphite powder or the like in the constituent material at the time of production.

  By the way, since only one cell can obtain a voltage of about 0.8 V at most, in order to obtain practical power, cells and cells are alternately stacked and stacked via an interconnector. FIG. 2 is a diagram schematically showing an example of the mode, in which two cells are arranged via the interconnectors 6 and 7, and the mode in which fuel and air cross flow is shown. That is, interconnectors and cells are alternately stacked for the purpose of distributing and supplying and discharging air and fuel to and from the cathode and anode at the same time as connecting adjacent cells electrically in series. In addition, the AA line cross section in FIG. 2 is corresponded in sectional drawing of Fig.1 (a)-(c).

  FIGS. 3A to 3B are diagrams for explaining an example of the lamination. FIG. 3A is a diagram showing the positional relationship between each cell 1 and the fuel flow path and air flow path of the interconnectors 6 and 7, and shows a mode in which the fuel flow path and the air flow path flow in parallel. FIG. 3B is a perspective view showing the SOFC stack that is stacked and configured as shown in FIG. 3A, and illustration of interconnectors and the like is omitted. FIG. 3B shows a case where the number of cells is 17, but the number is set as appropriate. During operation of the SOFC, electric power can be obtained by flowing fuel on the anode side of the cell, flowing oxidant gas on the cathode side, and connecting an external load between both electrodes. In the present specification, the oxidant gas will be described using air as an example, but the same applies when oxygen-enriched air, oxygen, or the like is used.

FIG. 3C is a diagram showing one cell constituting the stack shown in FIGS. 3A to 3B and showing oxygen, electron flow, etc. in the air. ) More enlarged. As shown in FIG. 3C, oxygen in the air flowing on the cathode side becomes oxide ions (O ²⁻ ) at the cathode 4 and reaches the anode 2 through the electrolyte 3. Here, it reacts with the fuel flowing on the anode side to emit electrons, and generates reaction products such as electricity and water or carbon dioxide. Used air at the cathode is discharged as cathode offgas, and used fuel at the anode is discharged as anode offgas containing unused fuel and reaction products such as water vapor and carbon dioxide.

  By the way, in SOFC, hydrogen and carbon monoxide are fuels, but methane among hydrocarbons is steam-reformed by the catalytic action of a metal that is a component of the anode, such as nickel, to become hydrogen and carbon monoxide. . For this reason, in SOFC, hydrogen, carbon monoxide, methane, or a fuel composed of two or more thereof may be introduced directly into the anode. However, if the fuel contains hydrocarbons with 2 or more carbon atoms other than methane, carbon will be generated and deposited in the SOFC piping, especially the fuel inlet tube and anode to the anode, which will inhibit the electrochemical reaction. Battery performance will deteriorate.

Therefore, if the raw fuel comprising a hydrocarbon number C ₂ or more carbon atoms, pre reformed hydrogen is changed to the pre-reforming gas containing carbon monoxide and methane steam reforming process or a partial oxidation process. Instead of the preliminary reforming, methane may be reformed and changed to hydrogen or carbon monoxide to be a reformed gas. When the raw fuel is reformed by the steam reforming method, the methane conversion steam (mole) ratio (S / C ratio) is 2 or more (more than twice the amount of steam required for complete steam reforming), preferably 3 or more. Is done.

  Here, in this specification, the fuel before the pre-reformation or reforming is called “raw fuel”, and the raw fuel is pre-reformed or reformed by the steam reforming method or the partial oxidation method. Pre-reformed fuel (hydrogen, carbon monoxide, methane, or a fuel containing two or more thereof) and reformed fuel (hydrogen, carbon monoxide, or both) to be introduced into the SOFC anode Is simply called “fuel”.

  By the way, in the SOFC, it is necessary to control the flow rate of the raw fuel in order to supply an appropriate amount of fuel to the anode, but the control can be generally performed as follows. One factor that determines SOFC efficiency is fuel utilization. The fuel utilization rate is a ratio of the amount of fuel actually contributing to power generation to the amount of fuel introduced into the anode, and is a ratio indicating how much of the fuel introduced into the anode is used for power generation. Therefore, the higher the fuel utilization rate, the higher the power generation efficiency. In general, the device is designed to increase the fuel utilization rate as much as possible, and is operated at the highest fuel utilization rate.

However, there is a theoretical and practical upper limit on the fuel utilization rate. FIG. 4 is a diagram for explaining the fact, and shows the change in the cell voltage near the fuel outlet in the SOFC cell when the current density is fixed at, for example, 0.2 A / cm ² and the fuel utilization rate is increased. . As shown in FIG. 4, the voltage gradually decreases as the fuel utilization rate in the SOFC cell increases, and when the fuel utilization rate exceeds about 90%, the cell voltage rapidly drops.

  The phenomenon in which the cell voltage decreases in this way means an increase in the oxygen partial pressure on the anode side. When the oxygen partial pressure increases to a certain value or more, the catalytic metal, for example, Ni in the anode is oxidized. Then, it changes to NiO, and the anode is damaged by the lattice expansion caused by the oxidation of Ni, and the safety is impaired. This is called “fuel depletion”, which not only impairs power generation but also causes cell deterioration.

  Since the fuel supplied to the anode of the SOFC cell is consumed sequentially toward the outlet, fuel depletion may occur in a single cell, a SOFC stack in which a plurality of cells are arranged, or a plurality of SOFC stacks in which a plurality of cells are arranged. In the SOFC stack formed by arranging the above, it can usually occur at the fuel outlet side of the anode. In addition, in actual SOFC cells and SOFC stacks, slight fuel leaks and gas diffusion inside the electrodes are rate-limiting (dominant), and as a result, the fuel utilization rate is practically limited to about 85%. It becomes. These points are the same in the case of a cell stack in a horizontal stripe type SOFC bundle described later, and the following points are the same in a horizontal stripe type SOFC bundle described later.

  Therefore, as shown in FIG. 5, a cell potential at which the anode is not oxidized, for example, a lower limit potential of about 0.6 V, that is, a limit voltage is set. Then, while keeping the cell voltage below the limit voltage (that is, while applying a limiter so that the cell voltage does not fall below the limit voltage), the fuel utilization rate is set to a predetermined value, that is, the fuel utilization rate corresponding to the limit voltage, for example, A control method is adopted in which the safety range is set to 80%, that is, a range in which a margin of safety is added to the limit fuel utilization rate.

  As a mode of controlling the fuel utilization rate to a predetermined value as described above, it is conceivable to control the supply amount of the raw fuel so that the cell voltage does not fall below the limit voltage. For example, as shown in FIG. 5, the predetermined fuel utilization rate is set to 80%, the current value is monitored (that is, the current value is measured and monitored), and a predetermined amount of raw fuel calculated from the measured current value is supplied. In accordance with this, the amount of steam for preliminary reforming or reforming and the amount of air for power generation are also controlled so that the fuel utilization rate is always about 80%.

  6-7 is a figure explaining the example of the aspect of the operation control. FIG. 6 shows, for example, the case of a single SOFC stack as shown in FIG. 3, for example, with a fuel utilization of 80% (limit voltage, that is, the fuel utilization corresponding to the cell voltage of 0.6 V in the example of FIG. 5). Set to. During power generation, the current value from the SOFC stack is monitored over time, and control is performed to supply a predetermined raw fuel amount calculated from the measured current value so that the fuel utilization rate is always 80% or less. In accordance with this, the amount of pre-reforming or reforming steam and the amount of air to the cathode are also controlled. These controls are performed by a central processing unit (CPU) with a storage device and an input / output device.

  FIG. 7 shows an SOFC stack in which a plurality of SOFC stacks are arranged. In FIG. 6, description of the raw fuel reformer and the like shown in FIG. 6 is omitted. The fuel supplied to each SOFC stack is controlled to a predetermined amount by a mass flow controller (MFC) or the like according to the load and supplied to the anode of each cell of each stack, and the air amount corresponding to this is supplied to each cell of each stack. Supplied to the cathode. In this case, if the scales of the SOFC stacks are the same, the amount of fuel branched from the common fuel conduit and supplied to the SOFC stacks is the same.

  In such control, for example, when city gas is used as raw fuel, its composition (= component of raw fuel, amount ratio of each component) has some degree of freedom, but is known in advance, and correspondingly Since the pre-reformed gas or reformed gas, that is, the composition of the fuel (= hydrogen, carbon monoxide, methane in the case of the pre-reformed gas, methane, and their ratio) is known, the raw fuel The amount of increase / decrease can be controlled uniquely or almost uniquely.

  Speaking from the side of the fuel supplied to the anode, the composition of the pre-reformed gas or the reformed gas in the fuel is uniquely or almost uniquely controlled by controlling the increase / decrease amount of the raw fuel. However, the calculation of the fuel utilization rate is obtained uniformly from only the current value for each cell and the ratio to the total amount of fuel supplied (that is, the estimated fuel utilization rate). Due to the time lag of the fuel supply system, the actual fuel utilization rate does not necessarily match the estimated fuel utilization rate.

  In addition, as an example, in a stack in which a plurality of flat-plate SOFC cells are stacked, for example, the temperature of the center cell (outside of the stacked cells, the center cell, not the upper or lower cell) The fuel supplied to each cell may also be distributed due to the higher temperature and further reaction. For this reason, the average fuel usage rate of the entire stack and the fuel usage rate in the local cell in the central part do not coincide with each other.

  The present inventors have measured the oxygen partial pressure in a local cell in a SOFC stack having a plurality of cells, a SOFC stack having a plurality of SOFC stacks, etc., and detected the fuel utilization rate. A method for preventing fuel depletion in the local cells of the SOFC stack, maximizing their efficiency and operating stably and a system therefor have been developed [Application for Japanese Patent Application No. 2004-333079 (Application) Date = November 15, 2004), Japanese Patent Application No. 2005-059759 (application date = March 3, 2005, hereinafter referred to as “759 application”)].

Application for Japanese Patent Application No. 2004-330779 Application for Japanese Patent Application No. 2005-059759

  Of these two applications, the invention according to the 759 application corresponds to the direct prior art to the present invention. There are several aspects of the invention according to the 759 application. Basically, for the local cell in the SOFC stack where fuel is most likely to be exhausted, (a) the fuel supply to the anode and the fuel outlet of the anode The oxygen sensor is arranged in the part to measure the partial pressure of oxygen, and (b) the amount of generated current in the local cell and (c) the amount of water vapor introduced for reforming are measured, whereby the fuel utilization rate of the local cell is determined. It estimates and controls the fuel supply amount to the entire SOFC stack so that fuel depletion does not occur in the local cell.

  According to this, when operating the SOFC stack, it is possible to prevent fuel depletion in the local cell and to control the SOFC stack safely and stably so that the fuel utilization rate is appropriate or maximized. As a result, damage to the anode caused by oxidation of the catalytic metal, for example, Ni, in the anode of the local cell constituting the SOFC stack can be prevented, and the SOFC can be operated safely and stably.

  Thus, although the invention according to the 759 application is very useful in practice, it is necessary to arrange an oxygen sensor for the control, and a zirconia oxygen sensor is used as a general oxygen sensor. There are several types of zirconia type oxygen sensors. Broadly speaking, when the oxygen concentration is high, a limiting current type oxygen sensor (that is, a single-chamber type) is used, and when the oxygen concentration is low, a full density cell type is used. Although an oxygen sensor (that is, a two-chamber type) is used, it is necessary to use a battery-type oxygen sensor (two-chamber type) for measuring the oxygen concentration of a gas having a low oxygen partial pressure, such as fuel used for SOFC.

  However, in the case of a battery-type oxygen sensor (two-chamber type), the structure is complicated (see FIG. 12 to be described later), and oxygen (air) serving as a reference needs to be supplied separately.

  In the present invention, further improvements are made on the premise of the invention according to the 759 application, and a SOFC stack formed by arranging a plurality of SOFC stacks arranged with a plurality of cells and a SOFC stack arranged with a plurality of cells. Using local cell stack of local cell and horizontal stripe type SOFC bundle as oxygen sensor, providing a method to control SOFC stack or horizontal stripe type SOFC bundle so that the fuel utilization rate is appropriate or maximum as a whole safely and stably. It is intended to do.

The present invention (1) is an operation control method for a solid oxide fuel cell stack having a plurality of cells. And
(A) Utilizing a local cell that is most likely to cause fuel depletion among the cells constituting the stack, and measuring the oxygen partial pressure based on the electromotive force and temperature on the fuel supply side and the fuel outlet side,
(B) Estimating the fuel utilization of the local cell by measuring the amount of current generated by the local cell and the amount of water vapor introduced for reforming the raw fuel,
(C) The fuel supply amount to the whole solid oxide fuel cell stack is controlled so that fuel exhaustion does not occur in the local cell.

In the present invention (1), as an aspect of measuring the oxygen partial pressure in (a) above, a sensor lead wire for measuring electromotive force is arranged on the fuel supply side anode and the electrolyte surface, and the temperature is measured for the local cell. A thermocouple for measurement is arranged to constitute oxygen sensor A, and a sensor lead wire for measuring electromotive force is arranged on the anode and electrolyte surface on the fuel outlet side, and a thermocouple for measuring temperature is arranged to provide oxygen. This can be done by configuring the sensor B.
The present invention (1) is applicable to a solid oxide fuel cell stack of a flat plate type, a cylindrical type, an integrally laminated type, and other various types.

The present invention (2) is an operation control method for a solid oxide fuel cell stack in which a plurality of solid oxide fuel cell stacks each having a plurality of flat cells are arranged. And
(A) Utilizing a local cell that is most likely to be depleted of fuel in each of the plurality of stacks, and measuring an oxygen partial pressure based on the electromotive force and temperature on the fuel supply side and the fuel outlet side ,
(B) Estimating the fuel utilization of the local cell by measuring the amount of current generated by the local cell and the amount of water vapor introduced for reforming the raw fuel,
(C) The fuel supply amount to the whole solid oxide fuel cell stack is controlled so that fuel exhaustion does not occur in the local cell.

  In the present invention (2), as a mode of measuring the oxygen partial pressure in (a), a sensor lead wire for measuring electromotive force is disposed on the fuel supply side anode and the electrolyte surface for the local cell, and the temperature A thermocouple for measurement is arranged to constitute oxygen sensor A, and a sensor lead wire for measuring electromotive force is arranged on the anode and electrolyte surface on the fuel outlet side, and a thermocouple for measuring temperature is arranged to provide oxygen. This can be done by configuring the sensor B.

The present invention (3) is an operation control method for a solid oxide fuel cell bundle composed of a plurality of horizontally-striped solid oxide fuel cell stacks. And
(A) using the local cell stack that is most likely to cause fuel depletion among the plurality of cell stacks, and measuring the oxygen partial pressure based on the electromotive force and temperature on the fuel supply side and the fuel outlet side;
(B) Estimating the fuel utilization rate of the local cell stack by measuring the amount of current generated by the local cell stack and the amount of water vapor introduced for reforming the raw fuel,
(C) The fuel supply amount to the whole solid oxide fuel cell bundle is controlled so that fuel depletion does not occur in the local cell stack.

The present invention (4) is a method for controlling the operation of a solid oxide fuel cell bundle comprising a plurality of solid oxide fuel cell bundles composed of a plurality of horizontal stripe solid oxide fuel cell stacks. is there. And
(A) The oxygen partial pressure is measured on the basis of the electromotive force and temperature on the fuel supply side and the fuel outlet side using the local cell stack that is most likely to cause fuel depletion among the cell stacks in the plurality of bundles. With
(B) Estimating the fuel utilization rate of the local cell stack by measuring the amount of current generated by the local cell stack and the amount of water vapor introduced for reforming the raw fuel,
(C) The fuel supply amount to the whole solid oxide fuel cell bundle is controlled so that fuel depletion does not occur in the local cell stack.

  In the present invention (3) to (4), as an aspect for measuring the oxygen partial pressure in (a), the measurement of the oxygen partial pressure in (a) is performed on the fuel cell side anode and electrolyte for the local cell stack. A sensor lead wire for measuring electromotive force is arranged on the surface, a thermocouple for measuring temperature is arranged to constitute an oxygen sensor A, and a sensor for measuring electromotive force is formed on the anode and the electrolyte surface on the fuel outlet side. The oxygen sensor B can be configured by arranging a lead wire and a thermocouple for temperature measurement.

  According to the present invention, when operating the SOFC stack and the horizontal stripe type SOFC bundle, fuel depletion is prevented in the local cell and the local cell stack, respectively, and the SOFC stack and the horizontal stripe type SOFC bundle can be safely and stably operated. The fuel utilization rate can be controlled to be appropriate or maximum. This prevents damage to the anode caused by oxidation of the catalytic metal in the local cell stack of the SOFC stack or the local cell stack of the horizontal stripe SOFC bundle, for example, Ni, and the SOFC stack or horizontal stripe type. The SOFC bundle can be operated safely and stably.

  Speaking of the present invention in relation to the invention related to the 759 application, the control of the invention related to the 759 application requires a separate oxygen sensor, whereas in the present invention, the SOFC stack is configured. The local cell or the local cell stack itself constituting the horizontal stripe SOFC bundle has a function as an oxygen sensor, so that it is not necessary to separately prepare an oxygen sensor having a complicated structure. Further, when an oxygen sensor is separately provided, optional parts necessary for using the oxygen sensor, such as wiring and supply of reference oxygen (air), become unnecessary.

In the SOFC stack or the horizontal stripe type SOFC bundle targeted in the present invention, a fuel obtained by pre-reforming or reforming a hydrocarbon-based raw fuel is supplied to the anode of the SOFC. For these preliminary reforming or reforming, a steam reforming method or a partial oxidation method may be applied, but a steam reforming method is preferably applied. In SOFC, methane is also used as fuel in addition to hydrogen and carbon monoxide. Therefore, in the present invention, a reformed gas having a pre-reformed level obtained by reforming hydrocarbons of C ₂ or higher is used as a fuel. It is also possible to use a reformed gas at a reforming level obtained by reforming methane as fuel.

FIG. 8 shows the average fuel utilization rate in the entire SOFC stack in which a plurality of cells are stacked as shown in FIGS. FIG. 5 is a graph plotting average cell voltage and voltage of a cell in which fuel depletion occurs locally. As an example, the change in cell voltage near the fuel outlet in the SOFC cell when the current density is fixed at 0.2 A / cm ² and the fuel utilization rate is increased is shown.

  As shown in FIG. 8, the average cell voltage of the SOFC stack as a whole gradually decreases as the fuel utilization rate increases. For example, when the fuel utilization rate is 80%, the average cell voltage exceeds 0.65V. On the other hand, the cell voltage of the local cell (that is, the cell at the center of the stacked SOFC stack) rapidly decreases as the fuel utilization rate increases. For example, when the fuel utilization rate is 80%, the voltage of the local cell has dropped to 0.5 V, and fuel depletion has occurred.

  This fuel depletion is caused by uneven fuel distribution. For example, in the central cell of the stacked SOFC stack, the temperature becomes higher, so that the molar flow rate of the fuel flowing through the region relatively decreases due to the increase in gas viscosity and the decrease in density. In this case, the fuel utilization rate becomes high, but this means that sufficient fuel does not reach the center cell, that is, the local cell, particularly on the fuel outlet side. The voltage drops.

FIG. 9 is a diagram for explaining the situation and fact, and FIG. 9 (a) corresponds to FIG. 3 (c). As shown in FIG. 9A, oxygen in the air is supplied from the cathode 4 side through the electrolyte 3 to the anode 2 side in the form of oxygen ions (O ²⁻ ), and reacts with hydrogen in the fuel on the anode side to react with water vapor. Is generated. Then, hydrogen in the fuel is consumed sequentially from the upstream side of the fuel to the downstream side, and the concentration of water vapor increases. As described above, in the higher temperature local cell, the increase in viscosity and density of these gases occur. Due to the decrease, the molar flow rate of the fuel relatively decreases, and as the fuel flows downstream, the hydrogen concentration decreases, the oxygen concentration increases relatively, and the electromotive force of the local cell decreases. In particular, when the total current amount exceeds the supplied fuel amount, the voltage of the local cell is remarkably lowered, and so-called “fuel depletion” occurs, in which current cannot be obtained.

  As described above, even if the average fuel utilization rate of the entire SOFC stack is a predetermined value, the fuel utilization rate is high in the local cell, fuel is exhausted, and there is a risk of damaging the local cell. This can also occur in a local cell in a SOFC stack formed by arranging a plurality of SOFC stacks.

FIG. 9B is a diagram in which the portion shown in FIG. 9A is written as a battery circuit diagram for explanation. In FIG. 9B, E _{1 to} E ₆ are electromotive forces at the respective locations of the cell, and I _{1 to} I ₆ are current values corresponding thereto. As shown in FIG. 9B, the current flows in the opposite direction to the flow of oxygen ions (O ²⁻ ). Actually, since the electromotive force E decreases as it goes downstream of the fuel flow, the current values are not uniform, and I ₁ > I ₂ > I ₃ > I ₄ > I ₅ > I ₆ . In the lower part of FIG. 9B, the case where the fuel is hydrogen is shown as an example, but the same applies when the fuel is carbon monoxide or a mixed gas of both.

  Here, oxygen in the air supplied to the cathode side enters into the fuel flowing on the anode side in the form of water vapor, so that the amount of water vapor in the fuel flowing on the anode side increases accordingly. Become. Therefore, as shown in the lower part of FIG. 9B, if the fuel is hydrogen, the water vapor concentration in the fuel at the anode side inlet is 0% (hydrogen concentration is 100%), and the fuel utilization rate is 80%. The water vapor concentration at the anode side most downstream is 80% (conversely, the hydrogen concentration is 20%). Since the S / C ratio is usually 2 or more, surplus steam is contained during steam reforming of the raw fuel, and the steam is turned on by that amount.

  By the way, when the number of cells stacked as an SOFC stack is small, the effect of fuel depletion in such a local cell becomes prominent so that it can be read by an average cell voltage signal. However, when the number of cells is large, the effect is slowed down due to the large number of cells, that is, the number of parameters, and does not appear clearly. Therefore, even if the entire cell potential or the average cell potential is monitored, a local drop in cell potential cannot be read. For this reason, even if it is recognized that fuel exhaustion has not occurred in a normal operation control method, in fact, fuel exhaustion may occur in the local cell, and the local cell may be damaged.

  The present invention relates to a SOFC stack in which a plurality of cells are arranged, a SOFC stack in which a plurality of SOFC stacks in which a plurality of cells are arranged are arranged, a local cell as described above in a horizontally striped SOFC bundle, and a local cell. This prevents fuel depletion in the stack and controls the local cell and the local cell stack not to be damaged.

<Basic configuration and operation of the present invention>
The basic configuration and operation of the present invention are as follows (1) to (5). Here, the SOFC stack will be described as an example, but the same applies to a horizontally striped SOFC bundle.
(1) Of the cells that make up the SOFC stack, the most likely fuel depletion, that is, the local cell, occurs between the anode and the electrolyte at two locations on the fuel inlet side and the anode fuel outlet side. A sensor lead wire for power measurement is arranged, and thermocouples (= thermopile) for temperature measurement are arranged at the two locations. And
(2) During operation of the SOFC stack, the electromotive force (that is, open electromotive force) and temperature are measured by the two sensor lead wires and the thermocouple, respectively.
(3) Based on the electromotive force signal (electromotive force value) and temperature measured in (2) above, the oxygen partial pressure of the fuel flowing through the fuel flow path on the fuel supply side and the fuel flowing through the fuel flow path on the fuel outlet side The oxygen partial pressure is measured, and the fuel utilization rate in the local cell is detected.
(4) The fuel usage rate detected in (3) above is compared with the set value (set fuel usage rate).
(5) In the comparison of (4) above, if the detected fuel utilization rate is lower than the set value, it is evidence that fuel is dead in the local cell, so the total amount of fuel supplied to the SOFC stack is increased. . This prevents fuel depletion in the local cell.

  Each cell constituting the SOFC stack is supplied with a fuel having the same composition from a common fuel supply pipe via a fuel distributor to each cell. Of the electromotive force measurement units at two locations of the local cell, the sensor lead wires for measuring the electromotive force on the fuel inlet side are arranged on the anode surface and the electrolyte on the fuel inlet side of the cell to measure the electromotive force on the fuel outlet side. The sensor lead wire is disposed on the anode surface on the fuel outlet side of the cell and the electrolyte. At the same time, a thermocouple for temperature measurement is disposed at the position where each sensor lead wire is disposed. In this case, the tips of both thermocouples may be arranged at or near the location where each sensor lead wire is arranged, for example, on the electrolyte surface on the air flow path side.

  FIGS. 10-11 is a figure which shows the aspect which has arrange | positioned the sensor lead wire and thermocouple for electromotive force measurement in the local cell of the SOFC stack comprised by laminating | stacking a several flat type cell. First, FIG. 10 is a perspective view of FIG. 10, and FIG. 10B is a cross-sectional view taken along line XX in FIG. 10A. The fuel distribution unit and the used fuel discharge unit shown in FIG. 10B are omitted in FIG.

  As shown in FIG. 10, sensor leads and thermocouples are arranged on the fuel inlet side and the fuel outlet side of a local cell, that is, a cell that is likely to run out of fuel among the cells constituting the SOFC stack, respectively. Thereby, the function of an oxygen sensor is given to the local cell, and the local cell is used as an oxygen sensor.

  Here, for example, in the SOFC stack as shown in FIG. 10, the cell at the center of the stack becomes higher during the operation, and therefore the cell that is likely to run out of fuel is usually the cell at the center of the stack. However, SOFC stacks are usually housed and installed in insulated containers, and spent fuel and spent fuel from each cell are burned in the insulated container and then discharged. It may be a cell that is above the center of the stack, or it may be a cell that is below the center of the stack, such as when used fuel and exhaust of exhaust gas from the used fuel are provided at the bottom of the insulated container. There is also a possibility.

<Identification of local cell and local cell stack>
Therefore, in the present invention, in the SOFC stack, which cell is the cell that is most likely to be depleted of fuel is determined in advance by a preliminary test or the like, and this local cell has a function as an oxygen sensor. In the SOFC stack, the number of cells that are hotter and likely to run out of fuel is not limited to one, and there may be a plurality of cells. In this case, one of them may be appropriately selected as an oxygen sensor. Provide functionality. The same applies to the horizontal stripe cell stack in the horizontal stripe SOFC bundle described later.

<Aspects of oxygen partial pressure measurement using local cell>
FIG. 11 is a diagram for taking out a local cell in which an electromotive force measurement sensor lead wire and a temperature measurement thermocouple are arranged as shown in FIG. 10 and more specifically explaining their arrangement mode. In FIG. 11, the front side is the fuel inlet side, and the opposite side is the fuel outlet side, that is, the used fuel outlet side. As shown in FIG. 11, sensor lead wires are disposed on the lower surface of the anode 2 on the fuel inlet side and the upper surface of the electrolyte 3, and a thermocouple is disposed. In addition, a sensor lead wire is disposed on the lower surface of the anode 2 on the fuel outlet side and the upper surface of the electrolyte 3, and a thermocouple is disposed.

  In FIG. 11, 8 is a planar electrode disposed on the upper surface of the electrolyte 3, and the tip of the sensor lead wire is connected to this. The tip of the sensor lead wire may be connected to the cathode 4 instead of the upper surface of the electrolyte 3.

  The tip of the sensor lead wire arranged on the lower surface of the anode 2 is connected to the position of the lower surface of the anode 2 opposite to the position of the planar electrode 8, but the same as the planar electrode 8 at the position of the lower surface of the anode 2. A planar electrode may be arranged and connected to this.

  Further, the tip of each thermocouple on the fuel inlet side and the fuel outlet side may be disposed at or near the position where each sensor lead wire is disposed, but in FIG. 11, a planar shape disposed on the upper surface of the electrolyte 3 on the air flow path side. The case where it arrange | positions to an electrode is shown. Pt is used as a constituent material for the sensor lead wire, thermocouple lead, and planar electrode, and other materials are selected as appropriate.

  Here, as shown in FIGS. 10 to 11, the oxygen sensor on the fuel inlet side is the oxygen sensor A and the oxygen sensor on the fuel outlet side is the oxygen sensor B for the local cell that functions as the oxygen sensor. That is, the oxygen sensor A and the oxygen sensor B are each configured by arranging “an electromotive force measurement sensor lead wire and a temperature measurement thermocouple” in the local cell. Then, when the SOFC stack is operated, the oxygen sensor A and the oxygen sensor B measure the partial pressure of oxygen in the fuel flowing through these portions, that is, the fuel inlet side and the fuel outlet side.

<Principle and function of oxygen sensor A and oxygen sensor B using local cells>
In the present invention, the partial pressure of oxygen is measured using a local cell. The local cell that functions as an oxygen sensor plays an important role in the present invention, and this enables measurement of a very small amount of oxygen in the fuel flowing on the anode side. In the present invention, the oxygen partial pressure in the fuel flowing through the local cell at the fuel inlet side of the anode and the fuel outlet side of the anode in the SOFC stack in the SOFC stack is measured by the oxygen sensor A and oxygen sensor B, respectively. And based on the measured value, the fuel utilization rate in the said local cell is detected, and it uses for operation control of a SOFC stack or a horizontal stripe type SOFC bundle.

<Principle and function of conventional oxygen sensor>
Here, a conventional oxygen sensor will be described as an example of the oxygen sensor A and the oxygen sensor B using local cells in the present invention. There are several types of oxygen sensors. Among them, the complete concentration cell type oxygen sensor (two-chamber type) is a zirconia stabilized with yttrium (Y ₂ O ₃ ) or calcia (CaO) doped zirconia as an electrolyte. And the like, and the oxygen partial pressure difference between the electrodes sandwiching the stabilized zirconia is changed to electric power for measurement. This oxygen sensor applies the so-called SOFC principle and supplies air to the cathode side for use as a reference (ie, reference gas or reference gas), so that even a very small amount of oxygen in the gas can be used with high accuracy. It can be measured accurately.

  FIG. 12 is a diagram for explaining the principle, structure, and function of the oxygen sensor. As shown in FIG. 12, stabilized zirconia is disposed between porous ceramic layers as an electrode protective layer, and a cathode is provided on one side and an anode on the other side of both porous ceramic layers. Pt electrodes are preferably used as both electrode materials. Then, air is circulated on the cathode side, and a measurement gas is circulated on the anode side. Thus, by measuring the electromotive force E between both electrodes in an open circuit state, it is possible to know the partial pressure of oxygen in the gas to be measured.

The voltage of the oxygen sensor, that is, the electromotive force E is expressed by the following Nernst equation (Y) as in the case of SOFC. In formula (Y), Po ₂ (c) is the oxygen partial pressure on the cathode side (air side), Po ₂ (a) is the oxygen partial pressure on the anode side (the gas to be measured with low oxygen concentration), and F is the Faraday constant. , R is a gas constant, and T is an oxygen sensor temperature (K). Po ₂ (c) is 0.20948 when air is used as a reference.

Since the oxygen concentration in the air (oxygen = 20.948 vol%) is constant, the oxygen concentration in the gas (gas) on the other side where the oxygen concentration is low can be measured. In the oxygen sensor, the oxygen partial pressure of the gas to be measured having a low oxygen concentration is identified by calculating from the sensor electromotive force E using the equation (Y). Also shows the formula (Z) in case of obtaining by common logarithm of Po ₂ (a).

  In the present invention, the oxygen sensor A and the oxygen sensor B using the local cell measure a very small partial pressure of oxygen in the fuel flowing through the fuel flow path of the local cell using the principle of such an oxygen sensor. Is.

<Specific Embodiment of Oxygen Partial Pressure Measurement by Oxygen Sensor A and Oxygen Sensor B in Local Cell>
In the present invention, the oxygen partial pressure of the fuel flowing through the fuel flow path at the fuel inlet and the oxygen in the fuel flowing through the fuel flow path at the fuel outlet based on the electromotive force signals and the measured temperatures from the oxygen sensors A and B The partial pressure is measured, and the fuel utilization rate in the local cell is detected based on the measured partial pressure.

Steam reforming of, for example, C _{1 to} C ₄ saturated hydrocarbons, which are components of hydrocarbon raw fuel, is represented by the following reaction formulas (1) to (4). The CO and H ₂ in the production system in these reaction formulas are produced through a so-called “elementary process of reaction”. The same applies to C _{2 to} C ₄ unsaturated hydrocarbons.

In this way, the gas composition when the hydrocarbon-based raw fuel is completely reformed by the steam reforming reaction is a mixed gas of hydrogen, carbon monoxide, four components of steam (surplus steam) and carbon dioxide. . Of these, carbon dioxide is a reaction component of carbon monoxide and water vapor. As described above, the reformed gas is a system containing CO, H ₂ , H ₂ O and CO _2. Strictly speaking, the constituent atoms of each gas are mixed in the state of radicals or ions and dissociated and reacted with each other. And oxygen is involved in the reaction with CO and H ₂ .

Looking at this for H ₂ and CO, as shown in the following reaction formulas (5) and (6), ½ mol (1 / 2O ₂ ) of oxygen reacts to become H ₂ O and CO ₂ , respectively. .

Here, the equilibrium constant K is indicated by the partial pressure P of each gas. In addition, although the equilibrium constant of gas reaction is described as Kp, it is set to K by the relationship with the code | symbol used below. For example, the reaction of equation (5) is described by placing the left side of equation (5), ie, the partial pressure product of the original gas, in the denominator, and placing the right side, ie, the partial pressure product of the generating gas, in the numerator. Thus, for example, assuming that the equilibrium constant in the equation (5) at a temperature of 750 ° C. is K ₁ and the equilibrium constant in the equation (6) is K ₂ , K ₁ and K ₂ can be calculated from the following equations (7) and (8), respectively. ₁ = 6.0 × 10 ⁹ and K ₂ = 7.6 × 10 ⁹ , and the equilibrium constants K ₁ and K ₂ are both very large values.

Equations (5) and (6) are equilibrium equations that must be satisfied independently. However, in reformed gas, since Equations (5) and (6) usually hold simultaneously, the shift of Equation (9) below The reaction formula holds. This is naturally derived by subtracting (6) from (5). For this reason, a mixed gas containing CO, H ₂ O, H ₂ and CO ₂ is introduced into the anode of the SOFC. Strictly speaking, in addition to these components, the above formulas (5) and (6) As shown by the equation, oxygen is also included.

As described above, the gas composition when completely reformed by the steam reforming reaction of the hydrocarbon-based raw fuel is composed of four types of hydrogen and carbon monoxide as the original system, and steam and carbon dioxide as the production system. It becomes a mixed gas of components. Here, as is clear from the equations (7) (K ₁ = 6.0 × 10 ⁹ ) and (8) (K ₂ = 7.6 × 10 ⁹ ), the equations (5) and (6) In addition, oxygen is present on the order of a partial pressure of about 10 ⁻²⁰ , though very little. In the present invention, the partial pressure of such a trace amount of oxygen is measured through a sensor lead wire and a thermocouple arranged in the local cell.

Here, in the above formula Y or Z, the electromotive force measured by the oxygen sensor A is E, the temperature measured by the thermocouple is T, and Po is used when the oxidizing gas flowing to the cathode is air. ₂ (c) is 0.20948.
That is, the partial pressure of oxygen at the fuel inlet side (fuel supply side to the anode) of the local cell is measured by a thermocouple by substituting the voltage measured via the sensor lead wire as E in the above formula Y or Z. Is substituted as T, and since “Po ₂ (c)” in the above formula Y or Z is the oxygen partial pressure on the cathode side, 0.20948 is substituted when the oxidizing gas is air. can get.

Similarly, in the above formula Y or Z, the electromotive force measured by the oxygen sensor B is E, the temperature measured by the thermocouple is T, and when the oxidizing gas flowing to the cathode is air, Po ₂ ( c) is 0.20948, but the oxygen partial pressure is reduced by the amount of oxygen consumption. However, the air utilization rate is usually about 30%, and the decrease in oxygen partial pressure can be almost ignored.
In other words, the oxygen partial pressure at the fuel outlet side (fuel discharge side from the anode) of the local cell is measured with a thermocouple by substituting the voltage measured via the sensor lead wire as E in the above formula Y or Z. Is substituted as T, and since “Po ₂ (c)” in formula Y or Z is the oxygen partial pressure on the cathode side, 0.20948 is substituted when the oxidant gas is air. It is obtained by.

Here, as described above, the decrease in the oxygen partial pressure in the air on the fuel discharge side is almost negligible, but since the air utilization rate can usually be predicted to be about 30% in advance, the oxygen partial pressure in the air on the fuel discharge side "Po ₂ (c)" may be obtained by substituting and correcting.

<Specific mode of fuel utilization determination by oxygen partial pressure in fuel>
The present invention basically presupposes a case where the fuel composition is clear. However, if the fuel composition is clear, for example, fuel using city gas as a raw fuel, if the flow rate of the fuel is known, Since the amount of supplied fuel can be calculated, the fuel utilization rate can be calculated immediately from the amount of generated current. The same applies to the fuel utilization rate in individual cells that do not cause fuel depletion as well as the entire SOFC stack.

  However, even if the fuel composition is clear, that is, the fuel composition is known, the fuel flow to the individual cells in the SOFC stack is not uniform, and there is a cell that may cause fuel depletion. Cannot calculate the fuel utilization rate in the local cell. Therefore, in the present invention, the fuel utilization rate of the local cells constituting the SOFC stack is calculated by the following method even when the fuel flow rate in each cell is unknown.

As described above, when H ₂ and CO in the fuel produced by the steam reforming reaction of the hydrocarbon-based raw fuel are observed, each of ½ mol of oxygen (Equation (5) and Equation (6)) 1 / 2O ₂ ) reacts to become H ₂ O and CO ₂ .

Here, the initial number of moles of the C component and H component in the hydrocarbon, which is the raw fuel of the fuel introduced into the anode, and the added water vapor: H ₂ O are as follows.
C: c mol, H: a mol, H ₂ O: B mol

For example, when the raw fuel is composed of 1 mole of CH ₄ and 1 mole of C ₂ H ₆ , it is three times the stoichiometric amount of steam required for steam reforming [methane converted steam (mole) ratio (S / C When the ratio) = 3] is added, the result is as follows.
c = 1 + 2 = 4 mol, a = 4 + 6 = 10 mol, B = (1 + 2) × 3 = 9 mol

Here, after the raw fuel is completely reformed, the gas composition on the fuel inlet side (fuel supply unit) to the anode of the cell is set as follows.
H ₂ : p mol, CO: q mol, H ₂ O: r mol, CO ₂ : s mol

In consideration of mass balance with respect to H, C, and O, the following equations (10) to (12) are obtained.

When these equations (10) to (12) are modified with respect to r and s, the following equations (13) to (14) are obtained, respectively.

The balanced equations K ₁ and K _{2 in the} equations (5) and (6) are as shown in the following equations (15) and (16), respectively.

When r and s obtained by modifying Expressions (10) to (12) are substituted into Expressions (15) and (16) and rearranged, the following Expressions (17) and (18) are obtained.

Then, the oxygen partial pressure at the fuel inlet side (fuel supply side to the anode) of the local cell is Po ₂ (1), and the oxygen partial pressure at the fuel outlet side (anode fuel outlet side) is Po ₂ (2). In order to simplify the calculation, t ₁ (1), t ₂ (1), t ₁ (2), and t ₂ (2) are defined as in the following formulas (19) to (22). In equations (19) to (22), the oxygen partial pressure: Po ₂ (1) is measured by the oxygen sensor A, and the oxygen partial pressure: Po ₂ (2) is a value measured by the oxygen sensor B. Since K ₁ and K ₂ are constants, the variables in the equations (19) to (22) are only Po ₂ (1) and Po ₂ (2).

Here, J represents the amount of generated current, that is, the amount of oxygen ions (O ²⁻ ) supplied to the anode. H ₂ : The amount of oxygen ions reacting with 1 mol is 1 mol [1/2 of O ²⁻ , see formula (5)]. CO: The amount of oxygen ions reacting with 1 mol is 1 mol [= O ²⁻ 1/2, see equation (6)], so defining the effective fuel amount Q as the amount of fuel that reacts with 1 mol of oxygen ions, Q = a / 2 + 2c = p + q. Accordingly, the effective fuel amount Q is represented by the following equation (23).

Then, when the equation (23) is modified with respect to c, the following equation (24) is obtained.

Here, for simplification, M (1) and M (2) are further defined as in the following formulas (25) and (26).

When the equation (24) is solved for the effective fuel amount Q, the following equation (27) is obtained.

Here, since the fuel usage rate is Uf = J / Q, the fuel usage rate Uf is expressed by the following equation (28).

Thus, if the fuel composition is known, even if the fuel flow rate is unknown, the amount of steam B supplied for steam reforming, the fuel inlet side (fuel supply side to the anode) and the fuel outlet side of the target local cell From the oxygen partial pressure of the fuel (on the fuel outlet side of the anode), the fuel utilization rate Uf in the local cell can be calculated. That is,
(1) The oxygen partial pressures on the fuel inlet side and the fuel outlet side of the target local cell are Po ₂ (1) measured by the oxygen sensor A and Po ₂ (2) measured by the oxygen sensor B, respectively. . Since this is a variable at t ₁ (1), t ₂ (1), t ₁ (2), t ₂ (2) in the equation (28) as defined by the equations (19) to (22), ( It is substituted into the equation (28) via the equations (19) to (22).
(2) Po ₂ (1) and Po ₂ (2) are expressed as M (1) and M (2) in the formula (28) through the formulas (19) to (22) and (25) to (26). Is assigned as
(3) The amount of oxygen ions (O ²⁻ ) J supplied to the cathode corresponds to the amount of generated current and is obtained by measuring the amount of generated current, and is substituted into equation (28).
(4) Since the water vapor amount B is the amount of water vapor that is steam reformed from the raw fuel as fuel, it is substituted as the B amount in equation (28).

  In the present invention, for the local cell of the SOFC stack, (a) an electromotive force measuring sensor lead wire and a thermocouple are arranged on the fuel supply side of the anode and the fuel outlet side of the anode, respectively. The sensor B is used to measure the partial pressure of oxygen in the fuel flowing through the respective locations, and (b) the amount of current generated in the local cell and (c) the amount of water vapor introduced for raw fuel reforming. Then, based on these measured values, the fuel utilization rate Uf in the local cell is calculated and estimated as described in (1) to (4) above, and the entire SOFC stack is prevented so that fuel depletion does not occur in the local cell. The amount of fuel supplied to the can be controlled.

  The generated current amount of the local cell can be obtained by dividing the generated current amount measured for the entire stack by the number of cells constituting the stack. Here, the generated current amount measured as the whole stack is the current amount measured by the current monitor (A) in FIG.

  FIG. 13 is a diagram for explaining an example in which the operation control method of the present invention is applied to an SOFC stack. During power generation, the current value and voltage value from the SOFC stack are monitored over time, and the oxygen partial pressure in the fuel on the fuel supply side to the anode of the local cell and the fuel outlet side of the anode by the oxygen sensor A and oxygen sensor B Monitor the partial pressure of oxygen in the fuel. Then, control is performed so that a predetermined raw fuel amount calculated from the measured oxygen partial pressure value and the generated current amount is supplied so that the fuel utilization rate of the local cell is always 80% or less, and accordingly, It also controls the amount of steam for raw fuel reforming and the amount of air to the cathode. These controls can be performed by a central processing unit (CPU) with a storage device and an input / output device.

  FIG. 14 is a diagram illustrating control modes after the start of operation in the mode example of FIG. The initial fuel quantity is introduced at the start of operation. Note that the initial set fuel amount is set in advance as a fuel amount necessary for steady operation in the SOFC stack to which the present operation control system is applied. Thereafter, during operation, the generated current value and the total current potential value are monitored as normal output control. When the average battery potential as a whole drops below the limit voltage, that is, when “average battery potential <limit voltage”, control is performed to increase the fuel flow rate. Conversely, if the average battery potential is equal to or higher than the limit voltage, that is, “average battery potential> limit voltage”, the SOFC stack as a whole is normal, and the operation is continued as it is.

  While performing normal operation control in this way, the oxygen sensor A and oxygen sensor B measure the oxygen partial pressure of the fuel flowing on the fuel supply side to the anode of the local cell and the fuel flowing on the fuel outlet side of the anode. At the same time, the amount of generated current of the local cell and the amount of water vapor introduced for reforming are measured, and the fuel utilization rate Uf of the local cell is calculated based on the measured values. If the fuel utilization rate Uf is larger than the set fuel utilization rate, that is, “fuel utilization rate Uf> set fuel utilization rate”, fuel depletion occurs in the local cell or is beginning to occur. Increase the fuel supply to the entire SOFC stack, as shown as “Fuel Flow Increase”.

  On the other hand, if the fuel usage rate Uf of the local cell is smaller than the set fuel usage rate, that is, if “fuel usage rate Uf <set fuel usage rate”, there is no fuel depletion in the local cell, so the operation is continued as it is. However, if the difference of “fuel utilization rate Uf <set fuel utilization rate” is large at that time, control is performed so as to decrease the fuel flow rate according to the difference. As described above, when the fuel utilization rate Uf of the local cell is smaller than the set fuel utilization rate, the fuel supply amount to the entire SOFC stack may be reduced as necessary depending on the degree.

  By performing such control at regular intervals or at predetermined intervals, it is possible to prevent fuel depletion in a local cell where fuel depletion is predicted to occur, and to control the operation of the SOFC stack as a whole without causing performance degradation. be able to. The set fuel utilization rate is set to a cell potential at which the anode is not oxidized, for example, a lower limit potential of about 0.6 V, for example, as shown in FIG. 5 described above, for each cell (including local cells) constituting the stack. Correspondingly, the fuel utilization rate is set in advance.

<Operation control mode of SOFC stack with multiple SOFC stacks>
The present invention is similarly applied to operation control of a SOFC stack formed by arranging a plurality of SOFC stacks having a plurality of flat plate cells. FIG. 15 is a diagram for explaining a mode of operation control of a SOFC stack in which three SOFC stacks are arranged. In the case of a SOFC stack in which a plurality of SOFC stacks are arranged, it is also used by being accommodated in a heat insulating container or the like. Further, even in the stack disposed in the central portion, the local cell in the central portion is expected to have a higher temperature and fuel depletion is expected to occur.

  Therefore, in the SOFC stack in which a plurality of SOFC stacks having a plurality of flat plate cells are arranged, in the same manner as described above, an oxygen sensor using a local cell that is expected to cause the most fuel depletion. A and oxygen sensor B are constructed and arranged. Then, in the same manner as described above, the fuel utilization rate in the local cell is estimated, the fuel supply amount to the entire SOFC stack is controlled so that fuel exhaustion does not occur, and fuel exhaustion in the local cell is prevented, The entire SOFC stack can be controlled without degrading performance. The present invention is not limited to three, and is similarly applied to SOFC stacks in which a plurality of SOFC stacks such as two, four, and five are arranged.

  Although the description has been given mainly of the flat plate type SOFC as described above, the present invention is similarly applied to a cylindrical type, a horizontal stripe type, an integrally laminated type, and other various types of SOFCs.

<Operation control mode of SOFC stack with multiple cylindrical SOFC cells>
The present invention is also applied to operation control of a SOFC stack in which a plurality of cylindrical SOFC cells are arranged. FIG. 16 is a diagram for explaining the mode, and shows the main points. Even in the case of an SOFC stack in which a plurality of cylindrical SOFC cells are arranged, for example, a cell arranged in the central part of the stack is likely to be hotter than a peripheral cell, that is, a cell away from the central part, Fuel depletion is expected.

  Therefore, as shown in FIG. 16 (a), in the SOFC stack in which a plurality of cylindrical SOFC cells are arranged, the local cell that is expected to cause the most fuel exhaustion is used in the same manner as described above. Oxygen sensor A and oxygen sensor B are configured. This type of SOFC stack is also generally used by accommodating a plurality of cylindrical SOFC cells in a cylindrical, cubic, rectangular or other insulating container. In Fig.16 (a), the row | line | column of the arrange | positioned center part is shown, and description of the heat insulation container is abbreviate | omitted. FIG. 16 (b) is a diagram showing the flow direction of fuel and air by taking out one cylindrical SOFC cell.

  In the case of a cylindrical SOFC cell as well, the oxygen sensor A is configured by arranging a sensor lead wire for electromotive force measurement on the fuel supply side anode and the electrolyte surface and a thermocouple for temperature measurement. Then, the sensor lead wire for electromotive force measurement is arranged on the anode and the electrolyte surface on the fuel outlet side, and the thermocouple for temperature measurement is arranged to constitute the oxygen sensor B. Then, in the same manner as described above, the fuel utilization rate in the local cell is estimated, the fuel supply amount to the entire SOFC stack is controlled so that fuel exhaustion does not occur, and fuel exhaustion in the local cell is prevented, Operation control is performed so that the performance of the SOFC stack as a whole does not deteriorate.

<Mode of Horizontally Striped SOFC Bundle Applying the Present Invention>
The present invention is also applied to operation control of a horizontal stripe type SOFC bundle. The basic structure of the horizontal stripe SOFC bundle is that a plurality of SOFC cells are arranged in a horizontal stripe pattern on the surface of a porous support base having a fuel flow channel on the inside, and adjacent cells are electrically connected in series. The basic unit itself constitutes a stack. The structure of each cell is the type shown in FIG. 1C in the form shown in FIG.

  Here, there are various cross-sectional shapes of the porous support substrate, such as a circular shape, an elliptical shape, a polygonal shape (rectangle, rectangle, etc.), and the number of fuel flow passages inside the porous support substrate is not limited to one, but includes a plurality. There are various aspects. The surface of the porous support substrate has a shape corresponding to the outer surface of the cross-sectional shape, and a plurality of SOFC cells are arranged on the surface in the form of horizontal stripes along the fuel flow direction of the fuel flow path. For this reason, for example, when the cross-sectional shape of the porous support substrate is circular, a cylindrical cell stack is formed, and when the cross-sectional shape is rectangular, a rectangular cell stack is formed.

  In the present specification, a unit having such a basic structure is referred to as “horizontal stripe type SOFC cell stack”, “horizontal stripe type cell stack”, “SOFC cell stack”, or simply “cell stack”. A horizontal stripe SOFC bundle is formed by integrating a plurality of horizontal stripe cell stacks.

  Hereinafter, the operation control of the horizontal stripe type SOFC bundle according to the present invention will be described by taking a horizontal stripe type SOFC bundle composed of a rectangular parallelepiped (cross section rectangular or flat) cell stack as an example. The same applies to the bundle.

<Operation control mode of horizontal stripe SOFC bundle (Part 1)>
FIG. 17 is a diagram for explaining the arrangement relationship of each member and the like for an example of a horizontally striped SOFC bundle. 17 (b) is a plan view, FIG. 17 (a) is a left side view thereof (a view of FIG. 17 (b) viewed from the left), and FIG. 17 (c) is a right side view thereof (FIG. 17 (b)). Figure viewed from the right side]. FIG. 18 is a view showing a fuel flow state in the horizontal stripe SOFC bundle as shown in FIG.

  As shown in FIG. 17, a plurality of horizontally-striped SOFC cell stacks 10 are arranged vertically with the fuel supply pipe 19 as the center. That is, a plurality of horizontally-striped SOFC cell stacks 10 are arranged such that the positions of the fuel introduction side openings of the respective fuel flow paths are aligned at one end and the used fuel discharge side opening at the other end is directed to the opposite side. Each SOFC cell stack 10 is fixed, and each cell stack group composed of a plurality of horizontally-striped SOFC cell stacks 10 is arranged on both the upper and lower sides of the manifold 18 around the manifold 18 for distributing fuel. A fuel supply pipe 19 is disposed on one of the left and right ends of the manifold 18 and the other is closed.

  In each SOFC cell stack 10, a plurality of cells are arranged in a horizontal stripe shape, and adjacent cells are electrically connected in series by an interconnector. Each SOFC cell stack 10 has a plurality of cells 11 arranged in a horizontal stripe pattern as shown in FIGS. 17 (a) and 17 (c), but each SOFC cell stack 10 in FIG. 17 (b). The description of the cell 11 is omitted. The SOFC cell stacks 10 forming the cell stack group are arranged at equal intervals or substantially equal intervals in parallel to the plane, and are fixed to the manifold 18. Although FIG. 17 shows a case where the total number of SOFC cell stacks 10 is 40 (20 each in the vertical direction as shown in FIG. 17B), the number is appropriately set.

  In FIG. 18, the fuel supplied from the fuel supply pipe 19 is introduced into the fuel flow path 16 of each cell stack 10 while flowing through the space in the manifold 18. The introduced fuel contributes to power generation while flowing through the fuel flow path 16, and used fuel including unused fuel is discharged from the fuel flow path 16 of each cell stack 10. In FIG. 18, the flow direction of the fuel is indicated by an arrow, a is a fuel introduction port of the fuel channel 16, and b is a used fuel outlet port of the fuel channel 16.

  FIG. 19 is a perspective view of the horizontal stripe SOFC bundle of the example shown in FIGS. In addition, there are various horizontal stripe SOFC bundles such as a mode in which a SOFC cell stack group, that is, a plurality of horizontal stripe SOFC cell stacks are arranged only on one side from the manifold 18, all of which are horizontal stripe SOFC bundles. .

  As an example of the arrangement mode of the horizontal stripe type SOFC bundle, an air distribution mechanism is arranged in parallel to the lower surface thereof. FIG. 20 is a diagram showing an example of the mode of the distribution mechanism, and is shown as a perspective view corresponding to FIG. The air distribution mechanism 30 includes an air supply pipe 31 and an air distribution container 32, and also corresponds to the entire area of the lower surface of the SOFC bundle as shown in FIG. 19, and a plurality of air discharge holes that distribute air toward the surface. 33 is provided.

  FIG. 21 is a diagram showing an aspect in which the horizontally striped SOFC bundle 20 and the air distribution mechanism 30 as shown in FIGS. As shown in FIG. 21, the horizontal striped SOFC bundle 20 is disposed on the upper part of the air distribution mechanism 30 and is normally used by being housed in a heat insulating container 34. Used fuel including unused fuel is discharged from the left and right end faces of each cell stack constituting the bundle, and burned by the used air in the off-gas combustion zone of the end face. The combustion exhaust gas is discharged from a discharge pipe (not shown) provided in the heat insulating container 34.

  FIG. 22 is a view for explaining the flow direction of air or the like during operation of the unit including the SOFC bundle 20 and the air distribution mechanism 30. As shown in FIG. 22, the air supplied from the air supply pipe 31 and discharged from the air discharge hole 33 circulates as shown by the arrows in FIG. That is, the air flows toward the left and right ends of each cell stack 10 after contributing to power generation while passing between the cell stacks 10 arranged in plane parallel with the SOFC bundle. Further, spent fuel including unused fuel is discharged from the left and right end surfaces of each cell stack 10.

  In such a horizontal stripe type SOFC bundle, of the cell stack group, for example, the cell stack arranged at the center is likely to be hotter than the cell stacks on both sides, and fuel depletion is expected to occur. In FIG. 17, this is a cell stack shown as a local cell stack in FIG. Therefore, the local cell stack has a function as an oxygen sensor.

  That is, in the horizontally striped SOFC bundle, as shown in FIG. 19, the oxygen sensor A and the oxygen sensor B are arranged in the same manner as described above with respect to the local cell stack that is most likely to cause fuel depletion. Note that the number of cell stacks at which the temperature is higher and fuel exhaustion is likely to occur is not limited to one, and there may be a plurality of cell stacks. In this case, the oxygen sensor A, The oxygen sensor B is configured.

  FIG. 23 is a plan view of the local cell stack taken out, and FIG. 24 is a perspective view thereof. As shown in FIGS. 23 to 24, with respect to the local cell stack, the sensor lead wire for electromotive force measurement is arranged on the anode and the electrolyte surface on the fuel supply side, and the thermocouple for temperature measurement is arranged to provide the oxygen sensor A. In addition, an oxygen sensor B is configured by arranging a sensor lead wire for electromotive force measurement on the anode and the electrolyte surface on the fuel outlet side, and a thermocouple for temperature measurement.

  FIG. 25 is an enlarged view of a cross section taken along line AA in FIG. As shown in FIG. 25, sensor lead wires are disposed on the upper surface of the electrolyte 13 on the fuel supply side and the lower surface of the anode 12, respectively, and thermocouples are disposed.

  In FIG. 25, reference numeral 18 denotes a planar electrode disposed on the upper surface of the electrolyte 13, and the tip of the sensor lead wire is connected to this. Note that the tip of the sensor lead wire may be connected to the cathode 14 instead of the upper surface of the electrolyte.

  The tip of the sensor lead arranged on the lower surface of the anode 12 is connected to the position of the lower surface of the anode 12 opposite to the position of the planar electrode 18, but the same as the planar electrode 18 at the position of the lower surface of the anode 2. A planar electrode may be arranged and connected to this. The sensor lead wire can be taken out from the lower surface of the anode 12 as appropriate, for example, from the lateral side of the cell stack in the longitudinal direction as shown in FIG.

  Further, the tip of the thermocouple lead may be disposed at or near the location where each sensor lead is disposed. In FIGS. 23 to 25, the side portion of the planar electrode 18 disposed on the upper surface of the electrolyte 13 on the air flow path side. The case where it has arrange | positioned is shown. Pt is used as a constituent material for the sensor lead wire, thermocouple lead, and planar electrode, and other materials are selected as appropriate.

  The above points regarding the oxygen sensor A are the same for the oxygen sensor B disposed on the fuel discharge side. Then, during the operation and operation of the horizontally striped SOFC bundle, the oxygen sensor A and the oxygen sensor B measure the partial pressure of oxygen in the fuel flowing through both of them, that is, the fuel inlet side and the fuel outlet side. Then, in the same manner as described above, the fuel utilization rate in the local cell stack is estimated, and the fuel supply amount to the entire SOFC bundle is controlled so that the fuel does not run out. Therefore, it is possible to control the operation of the SOFC bundle as a whole without causing performance degradation.

Here, Po ₂ (c) of the air flowing to the cathode is 0.20948, but the oxygen partial pressure is reduced by the amount of oxygen consumption. However, the air utilization rate is usually about 30% as in the case of the SOFC stack described above, and the decrease in the oxygen partial pressure is almost negligible. In the case of the horizontal stripe SOFC bundle illustrated in FIGS. 17 to 19, the air distribution mechanism 30 is arranged as shown in FIG. 21, and therefore, the decrease in the oxygen partial pressure on the cathode side is caused on the fuel supply side and the fuel discharge side. Since it does not substantially change, it is not necessary to correct the oxygen partial pressure in the air on the fuel discharge side when substituting for “Po ₂ (c)” in the above-mentioned formula Y or Z.

<Operation control mode of horizontal stripe SOFC bundle (Part 2)>
The present invention is also applied to a horizontal stripe SOFC bundle in which a plurality of horizontal stripe SOFC bundles are arranged. FIG. 26 is a diagram for explaining the mode. As shown in FIG. 26, horizontal stripe type SOFC bundles 20 as shown in FIGS. That is, the horizontal stripe SOFC bundle 20 and the air distribution mechanism 30 are alternately arranged and accommodated in the heat insulating container 34. Used fuel including unused fuel is discharged from the left and right end faces of each cell stack constituting each bundle, and burned with used air in an off-gas combustion zone on each end face.

The combustion occurs in the region on the used fuel discharge end side on the left and right of each cell stack, and the entire surface of each cell stack 10, that is, the entire area from both end surface sides to the fuel supply side, that is, the manifold side is heated by the radiant heat, The upper and lower cell stacks 10 are also heated uniformly.
However, if a specific portion of the bundles arranged above and below, for example, a central bundle or a combustion gas discharge port is provided at the upper part of the heat insulating container 34, the upper bundle is heated to a higher temperature. In addition, among the cell stack groups constituting the bundle, the cell stack at the center is likely to be hotter than the cell stacks on both sides, and it is expected that fuel depletion will occur.

  Therefore, in the same manner as in the above-described <Operation control mode of horizontal stripe SOFC bundle (part 1)>, oxygen sensor A and oxygen sensor B are utilized using the local cell stack that is expected to cause the most likely fuel depletion. Configure. Thus, in the same manner as described above, the fuel utilization rate in the local cell stack is estimated at the time of operation, and the fuel supply amount to a plurality of SOFC bundles is controlled so that fuel exhaustion does not occur. It is possible to prevent fuel depletion in the local cell stack and to control the operation of the entire horizontal striped SOFC bundle in which a plurality of horizontal striped SOFC bundles are arranged without causing performance degradation.

The figure explaining the example of an aspect of a SOFC cell Diagram explaining an example of SOFC stack mode The figure explaining the example of a mode of cell lamination in a SOFC stack A diagram for explaining the relationship between fuel utilization and cell voltage in SOFC Diagram explaining the operation control method of SOFC using fuel utilization rate The figure explaining the example of the mode of normal operation control of SOFC stack The figure explaining the example of the mode of normal operation control of the SOFC stack which consists of a plurality of SOFC stacks A plot of the average cell voltage and the voltage of the cell where fuel depletion occurred locally A diagram to explain "fuel depletion" in a local cell, voltage and current status, etc. The figure explaining the sensor lead wire for electromotive force measurement with respect to a local cell, and the arrangement | positioning aspect of a thermocouple The figure explaining the arrangement mode of the sensor lead wire and thermocouple for electromotive force measurement to a local cell in more detail Diagram explaining the principle, structure and function of a conventional oxygen sensor The figure explaining the example of an aspect which applies the operation control method of this invention to a SOFC stack The figure explaining the control aspect after the time of the driving | operation start in FIG. The figure explaining the aspect which applies this invention to the operation control of the SOFC stack which has arrange | positioned two or more of the SOFC stacks provided with several cells. The figure explaining the aspect which applies this invention to the operation control of the SOFC stack which has arrange | positioned several cylindrical SOFC cells. The figure explaining the arrangement relation etc. of each member of a horizontal stripe type SOFC bundle (an example) Diagram showing fuel flow in a horizontal stripe SOFC bundle The figure which showed the horizontal stripe type | mold SOFC bundle of the example shown to FIGS. 17-18 as a perspective view The figure which shows the example of the aspect of the air distribution mechanism with respect to a horizontal stripe type | mold SOFC bundle The figure which shows the aspect which accommodated the horizontal stripe type | mold SOFC bundle and the air distribution mechanism in the heat insulation container The figure explaining the distribution direction of air etc. at the time of operation of a unit containing a SOFC bundle and an air distribution mechanism Figure showing a top view of the local cell stack. Figure showing a perspective view of the local cell stack. The figure which expanded and showed the AA line cross section in FIG. The figure explaining the aspect which applies this invention with respect to the horizontal stripe type | mold SOFC bundle which has arrange | positioned the plurality of horizontal stripe type SOFC bundles

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 SOFC cell 2,12 Anode 3,13 Electrolyte 4,14 Cathode 5,15 Porous support base | substrate 6,7 Interconnector V1-V3 Flow control valve 8 Planar electrode arrange | positioned on the electrolyte 3 upper surface 10 Horizontally striped SOFC cell stack 11 A plurality of cells in the horizontal stripe type SOFC cell stack 10 16 Fuel flow path of the cell stack 10 a Fuel inlet port of the fuel flow path 16 b Fuel used outlet port of the fuel flow path 18 Each horizontal stripe type SOFC cell stack 10 is fixed , Fuel distribution manifold 19 fuel supply pipe 20 horizontal stripe SOFC bundle 30 air distribution mechanism 31 air supply pipe 32 air distribution container 33 multiple air discharge holes 34 heat insulation container

Claims (8)

An operation control method for a solid oxide fuel cell stack having a plurality of cells,
(A) Utilizing a local cell that is most likely to cause fuel depletion among the cells constituting the stack, and measuring the oxygen partial pressure based on the electromotive force and temperature on the fuel supply side and the fuel outlet side,
(B) Estimating the fuel utilization of the local cell by measuring the amount of current generated by the local cell and the amount of water vapor introduced for reforming the raw fuel,
(C) A method for controlling the operation of a solid oxide fuel cell stack, characterized in that the amount of fuel supplied to the entire solid oxide fuel cell stack is controlled so that fuel exhaustion does not occur in a local cell.   In the measurement of the oxygen partial pressure in (a), for the local cell, a sensor lead wire for electromotive force measurement is arranged on the anode and electrolyte surface on the fuel supply side, and a thermocouple for temperature measurement is arranged for oxygen. This is done by configuring sensor A and arranging sensor leads for measuring electromotive force on the anode and electrolyte surface on the fuel outlet side, and configuring oxygen sensor B by arranging thermocouples for temperature measurement. The operation control method for a solid oxide fuel cell stack according to claim 1.   2. The solid oxide fuel cell stack according to claim 1, wherein the solid oxide fuel cell stack including the plurality of cells is a flat oxide, cylindrical or monolithic solid oxide fuel cell stack. A fuel cell stack operation control method. An operation control method for a solid oxide fuel cell stack in which a plurality of solid oxide fuel cell stacks having a plurality of flat plate cells are arranged,
(A) Utilizing a local cell that is most likely to be depleted of fuel in each of the plurality of stacks, and measuring an oxygen partial pressure based on the electromotive force and temperature on the fuel supply side and the fuel outlet side ,
(B) Estimating the fuel utilization of the local cell by measuring the amount of current generated by the local cell and the amount of water vapor introduced for reforming the raw fuel,
(C) A method for controlling the operation of a solid oxide fuel cell stack, characterized in that the amount of fuel supplied to the entire solid oxide fuel cell stack is controlled so that fuel exhaustion does not occur in a local cell.   In the measurement of the oxygen partial pressure in (a), for the local cell, a sensor lead wire for electromotive force measurement is arranged on the anode and electrolyte surface on the fuel supply side, and a thermocouple for temperature measurement is arranged for oxygen. This is done by configuring sensor A and arranging sensor leads for measuring electromotive force on the anode and electrolyte surface on the fuel outlet side, and configuring oxygen sensor B by arranging thermocouples for temperature measurement. The operation control method for a solid oxide fuel cell stack according to claim 4. An operation control method for a solid oxide fuel cell bundle composed of a plurality of horizontal stripe solid oxide fuel cell stacks,
(A) using the local cell stack that is most likely to cause fuel depletion among the plurality of cell stacks, and measuring the oxygen partial pressure based on the electromotive force and temperature on the fuel supply side and the fuel outlet side;
(B) Estimating the fuel utilization rate of the local cell stack by measuring the amount of current generated by the local cell stack and the amount of water vapor introduced for reforming the raw fuel,
(C) A method for controlling the operation of a horizontally-striped solid oxide fuel cell bundle, wherein the amount of fuel supplied to the entire solid oxide fuel cell bundle is controlled so that fuel exhaustion does not occur in the local cell stack. An operation control method for a solid oxide fuel cell bundle in which a plurality of solid oxide fuel cell bundles composed of a plurality of horizontal stripe solid oxide fuel cell stacks are arranged,
(A) The oxygen partial pressure is measured on the basis of the electromotive force and temperature on the fuel supply side and the fuel outlet side using the local cell stack that is most likely to cause fuel depletion among the cell stacks in the plurality of bundles. With
(B) Estimating the fuel utilization rate of the local cell stack by measuring the amount of current generated by the local cell stack and the amount of water vapor introduced for reforming the raw fuel,
(C) A method for controlling the operation of a horizontally-striped solid oxide fuel cell bundle, wherein the amount of fuel supplied to the entire solid oxide fuel cell bundle is controlled so that fuel exhaustion does not occur in the local cell stack. In the measurement of the oxygen partial pressure of (a), for the local cell stack, a sensor lead wire for electromotive force measurement is arranged on the anode and electrolyte surface on the fuel supply side, and a thermocouple for temperature measurement is arranged. The oxygen sensor A is configured, and a sensor lead wire for electromotive force measurement is arranged on the anode and the electrolyte surface on the fuel outlet side, and a thermocouple for temperature measurement is arranged to constitute the oxygen sensor B. The operation control method of a horizontal stripe type solid oxide fuel cell bundle according to claim 6 or 7,

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