JP2007200771A - Reforming catalyst temperature control system and control method for fuel cell power generator - Google Patents

Abstract

The present invention provides a reforming catalyst temperature control system for a fuel cell power generation apparatus and a control method therefor, which has a wide operation control range that can avoid power generation stoppage as much as possible and continuously obtain a more stable power generation amount.
SOLUTION: Temperature setting means for determining a reformer set temperature corresponding to a set current of a fuel cell; a hydrogen utilization rate and an empty capacity determined corresponding to a temperature difference between the measured temperature and the set temperature of the reformer. A flow rate calculating means for calculating a raw fuel supply flow rate and a combustion air supply flow rate so as to achieve an fuel ratio, and a raw fuel supply means for supplying raw fuel to the reformer or a combustion air supply means for supplying combustion air to the reformer. When at least one of them reaches the supply limit, there is provided set current control means for performing control for reducing the set current.
[Selection] Figure 1

Description

  A reformer that has a burner for heating the reforming catalyst and reforms the raw fuel with the reforming catalyst to generate a reformed gas rich in hydrogen, and electrically connects the hydrogen and the oxidizing gas contained in the reformed gas More particularly, the present invention relates to a fuel cell that generates electricity through chemical reaction, and a fuel cell power generator that heats the reforming catalyst by burning a fuel off-gas discharged from the fuel cell. Regarding the method.

  A fuel cell is a device that directly converts chemical energy of fuel into electrical energy without passing through mechanical energy or thermal energy, and can achieve high energy efficiency. As a well-known form of a fuel cell, a pair of electrodes are arranged with an electrolyte layer in between, a fuel gas containing hydrogen is supplied to one electrode (anode side), and oxygen is supplied to the other electrode (cathode side). The electromotive force is obtained by utilizing an electrochemical reaction that occurs between the two electrodes. Below, an equation representing an electrochemical reaction occurring in the fuel cell is shown. Formula (1) represents the reaction on the anode side, Formula (2) represents the reaction on the cathode side, and the reaction represented by Formula (3) proceeds in the entire fuel cell.

H ₂ → 2H ++ 2e ⁻ (1)
1 / 2O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O (2)
H ₂ + 1 / 2O ₂ → H ₂ O (3)
Fuel cell power generators are classified according to the type of electrolyte used. Among these fuel cells, solid polymer fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, etc. Due to the nature, it is possible to use oxidizing gas or carbon dioxide containing carbon dioxide. Therefore, in these fuel cells, normally, air is used as an oxidizing gas, and a gas containing hydrogen generated by steam reforming a hydrocarbon-based raw fuel such as natural gas by a fuel reformer is used as a fuel gas.

  For this reason, a fuel cell system including such a fuel cell is provided with a reformer and a carbon monoxide converter, and the reformer and the carbon monoxide converter reform the raw fuel to supply fuel gas. Is generated.

Formula (4) shows, for example, the reforming reaction of methane in the reformer.
CH ₄ + H ₂ O → CO + 3H ₂ (+206.14 kJ / mol: endothermic reaction) (4)
Since the reforming reaction of methane is an endothermic reaction as shown in the equation (4), the granular reforming catalyst is converted to 600 by using the combustion exhaust gas obtained by burning the fuel off-gas from the fuel cell after adding water vapor to the methane. By maintaining at ˜700 ° C., a reformed gas rich in hydrogen is produced.

  This reformed gas leaving the reformer is fed to a carbon monoxide converter to reduce the carbon monoxide in the reformed gas, where the carbon monoxide is reduced to less than 1% and is in phosphoric acid form. In the case of a fuel cell (PAFC), this gas can be introduced into the fuel cell to generate electricity.

  On the other hand, the polymer electrolyte fuel cell (PEFC) has a low operating temperature of 60 to 80 ° C., so if carbon monoxide is present in the reformed gas, it becomes a catalyst poison and deteriorates its performance. In order to further reduce the carbon monoxide concentration, the reformed gas is supplied to a carbon monoxide remover controlled at a predetermined temperature, where the carbon monoxide concentration is reduced to 10 ppm or less.

  FIG. 4 is a schematic configuration diagram of a conventional polymer electrolyte fuel cell power generator using this type of fuel reformer. In FIG. 4, the raw fuel 12 from which the sulfur content has been removed by the desulfurizer 1 is mixed with the reforming water supplied by the reforming water supply pump 8, and then supplied to the steam generator 2. After being vaporized, it is supplied to the reformer 3 and reformed into a hydrogen-rich reformed gas by the steam reforming reaction shown in the formula (4). Thereafter, the carbon concentration is supplied to the carbon monoxide converter 4 to increase the hydrogen concentration by the carbon monoxide conversion reaction, and is further supplied to the carbon monoxide remover 5 together with a certain amount of air (not shown) to selectively oxidize the carbon monoxide. As a result, the carbon monoxide concentration is reduced to 10 ppm or less, and then supplied to the fuel cell 6.

  In the fuel cell 6, the hydrogen utilization rate (defined by the amount of hydrogen consumed by the fuel cell / the amount of supplied hydrogen) is normally controlled to 75 to 85%, that is, 75 to 85% of hydrogen is consumed in the fuel cell. After being consumed, the fuel off-gas 14 discharged from the fuel cell returns to the reformer 3 and is mixed with the combustion air 13 and burned in the burner 3a. This combustion heat is supplied to the steam reforming reaction (endothermic reaction) shown in the formula (4). Further, the heat of the combustion exhaust gas 9 in the burner 3a is used for evaporation of the reforming water in the steam generator 2.

  In FIG. 4, part number 7 is a reforming water tank, 10 is a raw fuel gas blower, 11 is a combustion air blower, 15 is a heating fuel described later, and 21 is a reforming water supply path.

  By the way, in the fuel cell power generator shown in FIG. 4, in the apparatus of Patent Document 1, the temperature control system of the reformer is configured as follows. A reforming raw material temperature detector for detecting the temperature of the reforming raw material flowing into the catalyst layer, and a plurality of catalyst layer temperature detectors for detecting the temperatures of the catalyst layers arranged at a plurality of locations in the flow direction of the reforming raw material of the catalyst layer. The average temperature calculator that calculates the average catalyst temperature by averaging the temperatures detected by these catalyst layer temperature detectors, and the deviation from the average catalyst temperature output from the average temperature calculator and the average catalyst reference temperature A correction temperature calculator for calculating a correction temperature to be set, and a temperature setter for setting a correction set temperature obtained by correcting the set temperature corresponding to the load by the load command with the correction temperature from the correction temperature calculator. The temperature of the catalyst layer is controlled from the deviation between the corrected preset temperature and the reforming raw material temperature detected by the reforming raw material temperature detector. In addition, a temperature detector is provided at the catalyst layer outlet (the highest temperature part) of the reformer, a desired control temperature is set for the catalyst layer outlet temperature, and the measured temperature (actual temperature) at the catalyst layer outlet is It is described that the configuration may be such that the set temperature is controlled. In FIG. 5, the target state is a state where the difference between the corrected set temperature calculated by the above-described control and the actual temperature is zero.

  When the actual temperature of the reformer is higher than the set temperature, the flow rate of combustion air to the reformer burner is increased in proportion to the difference to cool the reformer (a control in FIG. 5). ). Conversely, when the actual temperature of the reformer is lower than the set temperature, the supply amount of the heating fuel 15 is increased in proportion to the difference, and the required combustion air to the reformer burner. The amount is also increased (h control in FIG. 5).

  The temperature control system for the fuel cell power generator shown in FIG. 5 has the following problems. For example, the heating fuel 15 supplying means and the increase / decrease control means are required, which complicates the system. Furthermore, if the difference between the actual temperature of the reformer and the set temperature is too large, it will exceed the upper limit of the amount of combustion air or heating fuel supplied to the reformer burner, whether it is too high or too low. As a result, there is a problem that the operation of the fuel cell power generation apparatus has to be stopped due to an abnormal reformer temperature.

  As described above, Patent Document 2 is known as a reforming catalyst temperature control system for a fuel cell power generator related to reformer temperature control in which the heating fuel 15 is not supplied. Patent Document 2 includes the temperature control system described in the prior art section of the specification and FIGS. 6 and 7, the solution section of the abstract of Patent Document 2, and the temperatures described in FIGS. Two types of control systems are disclosed.

  The former temperature control system of Patent Document 2 is related to the temperature control system of the present invention, which will be described later, and the graph disclosed in FIG. 7 of Patent Document 2 indicates that “the temperature measured by the temperature sensor becomes the target value. Thus, it is a graph showing temperature control output characteristics of a temperature control system that controls the flow rate of combustion air and the flow rate of raw fuel (city gas). FIG. 6 is an explanatory diagram showing the same temperature control output characteristics as those in FIG. The increase in the raw fuel flow rate in FIG. 6 results in a decrease in the hydrogen utilization rate when the battery output is constant, and thus has substantially the same effect as the increase in the heating fuel flow rate in FIG. The detailed description of FIG. 6 will be described later in connection with the present invention.

On the other hand, the latter temperature control system of Patent Document 2 is the following temperature control system if the description of the abstract is cited. “When the temperature of the reformer is measured with a temperature sensor and the measured temperature is higher than the target temperature, the DC current value is increased by the DC current value control means, and the amount of reformed gas consumed by the fuel cell body is increased. The amount of off-gas extracted from the fuel cell main body and supplied to the reformer is decreased, the amount of gas burned in the reformer is decreased, the temperature of the reformer is lowered, and the target temperature is approached. When the reformer temperature is lower than the target temperature, the direct current value control means reduces the direct current value to reduce the amount of reformed gas consumed by the fuel cell body, and the reformer is taken out from the fuel cell body and reformed. The amount of off-gas supplied to the reformer is increased, the amount of gas combusted in the reformer is increased, the reformer temperature is increased, and the target temperature is approached. "
That is, the temperature control system increases or decreases the power generation amount (load factor) corresponding to the temperature of the reformer, thereby decreasing or increasing the amount of off-gas, and the temperature measured by the temperature sensor becomes the target value. It is the temperature control system which controls to become (refer FIG. 7).
Japanese Patent Laid-Open No. 8-27385 JP 2002-289226 A

  By the way, neither of the former and the latter temperature control systems disclosed in Patent Document 1 and Patent Document 2 are described regarding the operation method when the combustion air flow rate or raw fuel flow rate reaches the supply limit. It is considered that when the limit is reached, it is assumed that the operation of the fuel cell power generator is stopped. Moreover, although the latter temperature control system of patent document 2 performs the temperature control of the reformer by increasing or decreasing the power generation amount (load factor), from the consumer side, as much as possible during normal operation. It is desirable that the power generation amount (load factor) is constant and a stable power generation amount can be obtained continuously.

  The present invention has been made in view of the above-described problems of the prior art. The object of the present invention is to avoid the stoppage of power generation as much as possible in the temperature control of the reformer, and to maintain a more stable power generation amount. It is an object of the present invention to provide a reforming catalyst temperature control system and a control method thereof for a fuel cell power generator having a wide operation control range.

  The above-mentioned subject is achieved by the following. That is, a reformer that has a burner that heats the reforming catalyst and reforms the raw fuel with the reforming catalyst to generate a reformed gas rich in hydrogen, hydrogen and oxidizing gas contained in the reformed gas, In a reforming catalyst temperature control system of a fuel cell power generation apparatus that heats the reforming catalyst by burning fuel off-gas discharged from the fuel cell and generating electricity by electrochemically reacting the fuel cell, the fuel cell A temperature setting means for determining a set temperature of the reformer corresponding to the set current of the gas, and a hydrogen utilization rate and an air-fuel ratio determined corresponding to a temperature difference between the measured temperature of the reformer and the set temperature. At least of a flow rate calculating means for calculating the raw fuel supply flow rate and the combustion air supply flow rate, and a raw fuel supply means for supplying raw fuel to the reformer or a combustion air supply means for supplying combustion air to the reformer. One is at supply limit If it is, it is characterized in that a set current control means for controlling to reduce the set current (claim 1).

  The amount of decrease in the set current may be calculated and controlled according to equation (5).

Amount of decrease in set current = coefficient x absolute value of temperature difference + constant (5)
2. The control method for a reforming catalyst temperature control system according to claim 1, wherein when the set current of the fuel cell is reduced, the amount of reduction is reduced so that the measured temperature approaches the set temperature. The combustion air supply means and the set current control means are controlled (claim 2).

  Further, in the control method of the reforming catalyst temperature control system according to claim 2, when the measured temperature is higher than the set temperature, the flow rate of combustion air supplied to the burner is increased, and the flow rate of combustion air reaches the supply limit. When it reaches, the set current of the fuel cell is reduced in a state where the air-fuel ratio is kept constant (claim 3).

  Also, in the control method of the reforming catalyst temperature control system according to claim 2, when the measured temperature is lower than a set temperature, the raw fuel supply flow rate to the reformer is increased and the air-fuel ratio becomes constant. When the flow rate of the combustion air supplied to the burner is increased, and when at least one of the raw fuel supply flow rate or the combustion air flow rate reaches the supply limit, the fuel cell is set with the air-fuel ratio maintained constant. The current is reduced (claim 4). At this time, the ratio of the fuel off-gas flow rate to the raw fuel flow rate increases.

  According to the present invention, it is possible to provide a reforming catalyst temperature control system for a fuel cell power generation apparatus with a wide operation control range that avoids power generation stoppage as much as possible and continuously obtains a more stable power generation amount and a control method therefor. In particular, according to the invention of claim 3, when the ambient environment of the reformer is hot, it is possible to avoid power generation stop as much as possible and continuously obtain a more stable power generation amount. According to the invention of claim 4, when the ambient environment of the reformer is cold, biogas with a varying methane concentration is used as the raw fuel, and when this methane concentration is reduced, and the original pressure of the raw fuel In the case of a temporary decrease, it is possible to avoid power generation stop as much as possible and continuously obtain a more stable power generation amount.

Next, an embodiment of the present invention will be described with reference to FIG. 1, FIG. 2, and FIG.
FIG. 1 is a block diagram of a reforming catalyst temperature control system according to an embodiment of the present invention. The reforming catalyst temperature control system calculates the raw fuel flow rate and the combustion air flow rate at regular intervals. The raw fuel supply flow rate output to the raw fuel supply unit and the combustion air flow rate output to the combustion air supply unit are calculated using the set current and the reforming catalyst temperature measured by the reforming catalyst temperature sensor as inputs. The temperature setting means determines the set temperature according to the set current as shown in Equation (6).
Setting temperature = (− 20 × setting current / maximum setting current) +700 (6)
Next, the temperature difference calculating means subtracts the reforming catalyst temperature measured by the reforming catalyst temperature sensor from the set temperature to calculate the temperature difference. Next, the flow rate calculation means calculates the raw fuel flow rate and the combustion air flow rate in the next cycle of the control calculation according to the procedure shown in FIG. The supply limit detection means compares the raw fuel flow rate and the combustion air flow rate calculated by the flow rate calculation means with the respective flow rate upper limit values of the raw fuel gas blower 10 and the combustion air blower 11, and at least one exceeds the flow rate upper limit value. In this case, the set current is changed by the set current control means, and recalculation is performed with a new set current value. When the flow rate upper limit value is not exceeded, the raw fuel supply means and the combustion air supply means operate at the raw fuel flow rate and the combustion air flow rate calculated by the flow rate calculation means, and the fuel cell current control means is a condition of the set current. To control the fuel cell current. The operation continuity determining unit determines whether the temperature of the reforming catalyst temperature sensor is an abnormal value by comparing with a predetermined value. When the temperature is abnormal, the operation stop control means stops the operation of the reformer.

  FIG. 2 is a more detailed block diagram of a part of the reforming catalyst temperature control system according to the embodiment of the present invention. When the temperature difference is 0 or less, the hydrogen utilization rate is calculated by the equation (7).

Hydrogen utilization rate = 0.5 × temperature difference + 80 (7)
If the temperature difference is greater than 0, the hydrogen utilization rate does not change the current value.

  The fuel cell hydrogen consumption flow rate calculation means calculates a hydrogen consumption flow rate corresponding to the set current. The raw fuel flow rate calculation means calculates the raw fuel flow rate from the hydrogen consumption flow rate and the hydrogen utilization rate.

  When the temperature difference is 0 or less, the current air-fuel ratio is not changed. When the temperature difference is larger than 0, the following processing is performed. First, the air-fuel ratio of the next cycle is calculated by equation (8).

Estimated air-fuel ratio = 0.025 × temperature difference +1.5 (8)
If the calculated air / fuel ratio is equal to or lower than the upper limit of the air / fuel ratio, the air / fuel ratio is changed to the calculated air / fuel ratio. If the calculated air / fuel ratio is larger than the upper limit of the air / fuel ratio, the current air / fuel ratio is not changed. The combustion air flow rate calculation means calculates a combustion air flow rate from the raw fuel flow rate and the air-fuel ratio. In this way, the flow rate calculation means calculates the raw fuel flow rate and the combustion air flow rate.

  FIG. 3 is an explanatory diagram of a means for controlling the hydrogen utilization rate and the air-fuel ratio according to the embodiment of the present invention. FIG. 6 is an output characteristic diagram of the reforming catalyst temperature control system serving as the base of FIG. 3, and is an explanatory diagram showing the operation method substantially the same as FIG. 7 of Patent Document 2 as described above. .

  3 and FIG. 6 are common to the a control and b control shown on the left and right of the target state, and FIG. 3 is different from FIG. 6 in that c control and d control shown on the left and right are further added.

  First, each control in FIG. 3 will be described. The target state and a control are the same as in FIG. That is, when the actual temperature of the reformer is higher than the set temperature, the set current is kept constant, and the reformer burner is proportional to the temperature difference obtained by subtracting the set temperature from the actual temperature of the reformer. The reformer is cooled by increasing the flow rate of combustion air to (a control). In this case, the set current, the raw fuel flow rate, and the fuel off-gas amount are constant, that is, the hydrogen utilization rate is constant.

  When the actual temperature of the reformer is lower than the set temperature, the set current is kept constant, and the hydrogen utilization rate is reduced in proportion to the temperature difference obtained by subtracting the set temperature from the actual temperature of the reformer ( b control). In order to reduce the hydrogen utilization rate, the raw fuel flow rate to the reformer is substantially increased. This increases the amount of fuel off-gas and acts to raise the temperature of the reformer. At this time, the flow rate of the combustion air supplied to the burner also increases so that the air-fuel ratio in the burner becomes constant. In the a control, b control, c control, and d control shown in FIG. 3, the reforming water flow rate is controlled at a constant ratio with respect to the raw fuel flow rate.

  Next, FIG. 3 relating to the operation method of the present invention based on FIG. 6 will be described. As described above, in FIG. 3, the a control and b control shown on the left and right of the target state are the same as those in FIG. In FIG. 3, when the temperature difference is further increased to a positive value and the combustion air flow rate to the reformer burner reaches the upper limit, the air-fuel ratio is thereafter controlled to be constant to lower the set current (power generation amount) (c control). In this case, as the power generation amount decreases, the raw fuel flow rate (corresponding reforming water amount) and the fuel off-gas amount decrease. Further, since the air-fuel ratio is constant, the combustion air flow rate is also reduced. As a result, the fuel cell can be continuously operated while the set current is lowered while acting in the direction of lowering the temperature of the reformer.

  On the other hand, even if the temperature difference becomes further negative and the hydrogen utilization rate is lowered, the actual temperature of the reformer cannot approach the set temperature, and eventually the raw fuel flow rate or the combustion air to the reformer burner When any of the flow rates reaches the upper limit, the air-fuel ratio is thereafter controlled to be constant and the set current (power generation amount) is lowered (d control). Then, the ratio of the fuel off gas flow rate to the raw fuel flow rate increases. That is, while the amount of heat absorbed by the reforming reaction is reduced, the amount of heat supplied by the combustion of the fuel off-gas is increased, so that the fuel cell continues with the set current lowered while acting to increase the temperature of the reformer. Driving is possible.

Block diagram of a reforming catalyst temperature control system according to an embodiment of the present invention 1 is a block diagram of a part of a reforming catalyst temperature control system according to an embodiment of the present invention. Explanatory drawing of the means to control the hydrogen utilization rate and air-fuel ratio according to the embodiment of the present invention Schematic configuration diagram of a conventional polymer electrolyte fuel cell power generator Output characteristic diagram of reforming catalyst temperature control system of Patent Document 1 Output characteristic diagram of reforming catalyst temperature control system substantially the same as FIG. 7 of Patent Document 2 Output characteristic diagram of reforming catalyst temperature control system described in Patent Document 2

Explanation of symbols

  1: desulfurizer, 2: steam generator, 3: reformer, 3a: burner, 4: carbon monoxide converter, 5: carbon monoxide remover, 6: fuel cell, 7: water tank for reforming, 8 : Reforming water supply pump, 10: raw fuel gas blower, 11: combustion air blower, 12: raw fuel, 13: combustion air, 14: fuel off gas, 16: temperature sensor.

Claims (4)

A reformer that has a burner for heating the reforming catalyst and reforms the raw fuel with the reforming catalyst to generate a reformed gas rich in hydrogen, and electrically connects the hydrogen and the oxidizing gas contained in the reformed gas In a reforming catalyst temperature control system for a fuel cell that generates electricity by chemically reacting, and a fuel cell power generator that heats the reforming catalyst by burning a fuel off-gas discharged from the fuel cell,
Temperature setting means for determining a set temperature of the reformer corresponding to the set current of the fuel cell; a hydrogen utilization factor and an empty rate determined corresponding to a temperature difference between the measured temperature of the reformer and the set temperature; A flow rate calculating means for calculating a raw fuel supply flow rate and a combustion air supply flow rate so as to achieve an fuel ratio, and a raw fuel supply means for supplying raw fuel to the reformer or a combustion air supply for supplying combustion air to the reformer A reforming catalyst temperature control system comprising: a set current control unit that performs control to reduce a set current when at least one of the units reaches a supply limit.   When reducing the set current of the fuel cell, the raw fuel supply means, the combustion air supply means, and the set current control means are controlled so that the measured temperature approaches the set temperature. The control method of the reforming catalyst temperature control system according to claim 1.   When the measured temperature is higher than the set temperature, the flow rate of combustion air supplied to the burner is increased, and when the flow rate of combustion air reaches the supply limit, the fuel cell is set with the air-fuel ratio maintained constant. The method for controlling a reforming catalyst temperature control system according to claim 2, wherein the current is reduced.   When the measured temperature is lower than the set temperature, the raw fuel supply flow rate to the reformer is increased, the combustion air flow rate to be supplied to the burner is increased so that the air-fuel ratio becomes constant, and the raw fuel supply flow rate is further increased. 3. The reforming catalyst temperature according to claim 2, wherein when at least one of the combustion air flow rates reaches a supply limit, the set current of the fuel cell is reduced while maintaining the air-fuel ratio constant. Control system control method.

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