JP2008269887A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately estimate a degree of conversion without increasing cost independent of a catalyst state in a reforming part in a fuel cell system. <P>SOLUTION: The fuel cell system is equipped with a fuel cell generating electric power with supplied fuel gas and oxidant gas; a reforming part producing fuel gas including hydrogen by reforming fuel for reforming supplied with a fuel pump; and a combustion part to which only anode offgas from a fuel electrode of the fuel cell is supplied and burning anode offgas with air for combustion, and heating the reforming part with combustion gas in steady operation. The fuel cell system detects ion current correlating with fuel amount for reforming in the anode offgas supplied to the combustion part and burned (step 108), and a degree of conversion representing a ratio converted to hydrogen out of fuel for reforming supplied to the reforming part is estimated based on ion current being detected with an ion current detecting means (step 110). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fuel cell system.

  As one type of fuel cell system, one shown in Patent Document 1 is known. As shown in FIG. 11 of Patent Document 1, in the fuel cell system, the reformer catalyst packed bed temperature measuring temperature sensor 6 for measuring the temperature of the reformed catalyst packed bed of the reformer 8, and the modified sensor. A reformer having a predetermined reformer catalyst packed bed temperature detected by the reformer catalyst packed bed temperature measuring temperature sensor 6 in response to a temperature detection signal from the temperature sensor 6 for measuring the catalyst packed bed temperature. The degradation state of the reformer 8 is diagnosed by collating it with collation data on the relationship between the catalyst packed bed temperature and the methane conversion rate of the reformer 8 (step S4). Then, a period until reaching the lower limit value of the methane conversion rate or a period until reaching the upper limit value of the deterioration amount of the reforming catalyst is calculated (step S5). Then, the reforming catalyst replacement timing is determined based on the period until reaching the lower limit value of the methane conversion rate thus obtained, or the period until reaching the upper limit value of the deterioration amount of the reforming catalyst (step) S6).

As another format, one shown in Patent Document 2 is known. As shown in FIG. 1 of Patent Document 2, in the fuel cell system, the hydrocarbon sensor 22 detects the concentration of hydrocarbons contained in the product gas G, and the control unit 24 calculates the conversion rate of the raw material. Since at least one of the fan 10 and the pump 21 can be controlled by the control unit 24 so that the calculated conversion rate is 99% or less, hydrocarbons of 1% or more of the raw material fuel can be left in the offgas or the like. it can. A flame detector for detecting a flame based on the ion current of the flame is also provided.
JP 2000-268840 A Japanese Patent Laid-Open No. 2002-075412

  In the fuel cell system described in Patent Document 1 described above, the conversion rate is calculated based on the catalyst temperature of the reformer, and the deterioration state of the reformer 8 is diagnosed. However, when the catalyst is deteriorated or there is a tolerance of the catalyst, an error may occur in the calculated conversion rate. In the fuel cell system described in Patent Document 2 described above, the hydrocarbon sensor 22 detects the concentration of hydrocarbons contained in the product gas G, and the control unit 24 calculates the conversion rate of the raw material. However, since the hydrocarbon sensor is generally an expensive part, there is a problem that the cost of the fuel cell system is increased.

  The present invention has been made to solve the above-described problems. In a fuel cell system, the conversion rate can be accurately estimated regardless of the state of the catalyst in the reforming unit without incurring an increase in cost. With the goal.

  In order to solve the above problems, the structural feature of the invention according to claim 1 is that a fuel cell for generating electric power by using a fuel gas and an oxidant gas respectively supplied to a fuel electrode and an oxidant electrode, and a fuel supply for reforming A reforming section for generating a fuel gas containing hydrogen by reforming the reforming fuel supplied by the means, and during the steady operation, only the anode offgas from the fuel electrode of the fuel cell is supplied and the anode offgas A fuel cell system comprising: a combustion section that burns with a combustion oxidant gas and heats the reforming section with the combustion gas; an ion current detection means for detecting an ion current of a flame in the combustion section; A conversion rate estimating means for estimating a conversion rate indicating a ratio of the reformed fuel supplied to the reforming unit that has been reformed to hydrogen based on the ion current detected by the current detecting means; It is that it was e.

  The invention according to claim 2 is characterized in that the reforming fuel supply means is controlled by the reforming fuel supply means so that the conversion rate estimated by the conversion rate estimation means becomes the target conversion rate. At least one of a reforming fuel supply amount control means for controlling the supply amount of the fuel for combustion and a combustion oxidant gas supply amount control means for controlling the supply amount of the combustion oxidant gas supplied to the combustion section It is to be prepared.

  The structural feature of the invention according to claim 3 is represented by the ratio of the heat quantity of hydrogen consumed in the fuel cell to the heat quantity of the reforming fuel supplied to the reforming section in claim 1 or claim 2. The optimum conversion rate with the highest process efficiency is set as the target conversion rate.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the conversion rate estimating means estimates the conversion rate in consideration of the output power of the fuel cell. is there.

  In the invention according to claim 1 configured as described above, during the steady operation, the reforming fuel supplied to the reforming unit is partly reformed to hydrogen and the rest is reformed without being reformed. The fuel is discharged as is. Fuel gas containing these hydrogen and unreformed reforming fuel is supplied to the fuel cell, and hydrogen is consumed by power generation. The anode off-gas from the fuel cell contains unreformed reforming fuel and unburned hydrogen and other combustible gases. This anode gas is supplied to the combustion section and burned. In the combustion in the combustion section, no additional cooking is performed in which a combustible gas such as reforming fuel is separately added to the combustion section for combustion. This is so-called no additional cooking. In the case of no additional cooking, the amount of reforming fuel in the anode off-gas burned in the combustion section is the same as the amount of reforming fuel that has not been reformed in the reforming section. The ionic current value when hydrogen is burned is significantly smaller than the ionic current value when the reforming fuel (hydrocarbon such as natural gas) is burned. Therefore, the ion current value is hardly affected by the amount of combustion heat of hydrogen in the anode off gas. Therefore, the conversion rate of the unreformed reforming fuel amount can be detected by the ion current value. The conversion rate (%) is (reforming fuel amount reformed in reforming unit / reforming fuel supply amount to reforming unit) × 100, that is, 100− (unreformed in reforming unit) This is because the amount of reforming fuel / the amount of reforming fuel supplied to the reforming section) × 100.

  From the above, the conversion rate estimation means indicates the ratio of the reformed fuel supplied to the reforming unit that has been reformed to hydrogen based on the ion current detected by the ion current detection means. Can be estimated. As a result, the conversion rate can be accurately estimated by detecting the ionic current by the ionic current detecting means which is generally cheaper than the hydrocarbon sensor and using the detection result. Therefore, the conversion rate can be calculated accurately without incurring an increase in cost regardless of the state of the catalyst in the reforming section.

  In the invention according to claim 2 configured as described above, in the invention according to claim 1, the reforming fuel supply amount control means causes the conversion rate estimated by the conversion rate estimation means to be the target conversion rate. In addition, the supply amount of reforming fuel is controlled by controlling the reforming fuel supply means. Alternatively, the combustion oxidant gas supply amount control unit controls the supply amount of the combustion oxidant gas so that the conversion rate estimated by the conversion rate estimation unit becomes the target conversion rate. As a result, the conversion rate can be accurately and accurately controlled to the target conversion rate. Therefore, by stabilizing the conversion rate, the hydrogen utilization rate can also be stably controlled, and as a result, the stability of the fuel cell system can be improved. In addition, even if there is a variation in the flow rate of the reforming fuel supply means, the conversion rate and hydrogen utilization rate can be stably controlled in a balanced manner, so an inexpensive reforming fuel supply means with a relatively large flow rate variation can be used. The cost of the fuel cell system can be reduced.

  In the invention according to claim 3 configured as described above, in the invention according to claim 1 or 2, the amount of heat of hydrogen consumed in the fuel cell relative to the amount of heat of the reforming fuel supplied to the reforming section. Since the optimal conversion rate with the largest process efficiency represented by the ratio is the target conversion rate, high process efficiency operation can be performed by controlling the conversion rate to the target conversion rate.

  In the invention according to claim 4 configured as described above, in the invention according to any one of claims 1 to 3, the conversion rate estimation means calculates the conversion rate in consideration of the output power of the fuel cell. Since the estimation is performed, more optimal operation can be performed according to the output power of the fuel cell.

  Hereinafter, an embodiment of a fuel cell system according to the present invention will be described. FIG. 1 is a schematic diagram showing an outline of this fuel cell system. The fuel cell system includes a fuel cell 10 and a reformer 20 that generates a reformed gas (fuel gas) containing hydrogen gas necessary for the fuel cell 10.

  The fuel cell 10 includes a fuel electrode 11, an air electrode 12 that is an oxidant electrode, and an electrolyte 13 interposed between the electrodes 11 and 12, and supplies the reformed gas supplied to the fuel electrode 11 and the air electrode 12. Electric power is generated using air (cathode air), which is the oxidant gas. Note that air-enriched gas may be supplied instead of air.

  The reformer 20 steam reforms the reforming fuel and supplies a hydrogen-rich reformed gas to the fuel cell 10. The reformer 20 includes a reforming unit 21, a cooling unit 22, a carbon monoxide shift reaction unit (hereinafter referred to as “carbon reforming unit”). , A CO shift unit) 23, a carbon monoxide selective oxidation reaction unit (hereinafter referred to as a CO selective oxidation unit) 24, a combustion unit 25, and an evaporation unit 26. Examples of the reforming fuel include natural gas, gas fuel for reforming such as LPG, and liquid fuel for reforming such as kerosene, gasoline, and methanol. In the present embodiment, description will be made on natural gas.

  The reforming unit 21 generates and derives a reformed gas from a mixed gas that is a reforming raw material in which reforming water is mixed with the reforming fuel. The reforming portion 21 is formed in a bottomed cylindrical shape, and includes an annular folded channel 21a extending along the axis in the annular cylindrical portion.

  The return channel 21 a of the reforming unit 21 is filled with a catalyst 21 b (for example, a Ru or Ni-based catalyst) and introduced from the reforming fuel introduced from the cooling unit 22 and the steam supply pipe 51. The gas mixture with the steam reacts and is reformed by the catalyst 21b to generate hydrogen gas and carbon monoxide gas (so-called steam reforming reaction). At the same time, a so-called carbon monoxide shift reaction occurs in which carbon monoxide generated in the steam reforming reaction reacts with steam to transform into hydrogen gas and carbon dioxide. These generated gases (so-called reformed gas) are led to a cooling unit (heat exchange unit) 22. The steam reforming reaction is an endothermic reaction, and the carbon monoxide shift reaction is an exothermic reaction. The activation temperature range of the catalyst 21b of the reforming unit 21 is 400 ° C to 800 ° C. If the temperature of the reforming unit 21 is within the activation temperature range of the catalyst 21b, the higher the temperature, the more hydrogen is generated.

  Further, a temperature sensor 21 c that measures the temperature in the reforming unit 21, for example, the temperature near the wall between the combustion unit 25 (T 1) is provided in the reforming unit 21. The detection result of the temperature sensor 21 c is transmitted to the control device 30.

  The cooling unit 22 is a heat exchanger (heat exchange unit) in which heat exchange is performed between the reformed gas derived from the reforming unit 21 and a mixed gas of reforming fuel and reformed water (steam). The temperature of the reformed gas having a high temperature is lowered by the mixed gas having a low temperature and led to the CO shift unit 23, and the temperature of the mixed gas is raised by the reformed gas and led to the reforming unit 21. Yes.

  Specifically, a reforming fuel supply pipe 41 connected to a fuel supply source (not shown) (for example, a city gas pipe) is connected to the cooling unit 22. The reforming fuel supply pipe 41 is provided with a fuel pump 42, a desulfurizer 46, and a reforming fuel valve 43 in order from the upstream. The reforming fuel valve 43 opens and closes the reforming fuel supply pipe 41. The fuel pump 42 is reforming fuel supply means for supplying reforming fuel and adjusting the supply amount. The desulfurizer 46 removes sulfur (for example, sulfur compounds) in the fuel. The fuel that is supplied to the reforming unit 21 and reformed is called reforming fuel, and the fuel that is directly supplied to the combustion unit 25 and combusted is called combustion fuel.

  A steam supply pipe 51 connected to the evaporation section 26 is connected between the reforming fuel valve 43 of the reforming fuel supply pipe 41 and the cooling section 22. The steam supplied from the evaporation unit 26 is mixed with the reforming fuel, and the mixed gas is supplied to the reforming unit 21 through the cooling unit 22.

  The evaporation unit 26 evaporates the reformed water to generate water vapor, and supplies the water vapor to the reforming unit 21 via the cooling unit 22. The evaporator 26 is formed in a cylindrical shape so as to cover and contact the outer peripheral wall of the combustion gas passage 27.

  A water supply pipe 52 connected to a reforming water tank (not shown) is connected to a lower portion (for example, a lower portion of the side wall surface and a bottom surface) of the evaporation section 26. A water vapor supply pipe 51 is connected to the upper part (for example, the upper part of the side wall surface) of the evaporation unit 26. The reformed water introduced from the reformed water tank is heated by the heat from the combustion gas and the heat from the CO selective oxidation unit 24 in the course of flowing through the evaporation unit 26 to become water vapor and the water vapor supply pipe 51. And, it is led out to the reforming unit 21 through the cooling unit 22. The water supply pipe 52 is provided with a reforming water pump 53 and a reforming water valve 54 in order from the upstream. The reforming water pump 53 supplies reforming water to the evaporation unit 26 and adjusts the reforming water supply amount. The reforming water valve 54 opens and closes the water supply pipe 52.

  The evaporation unit 26 is provided with a temperature sensor 26 a that detects the temperature of water vapor in the evaporation unit 26. The detection result of the temperature sensor 26 a is transmitted to the control device 30. If the temperature of the water vapor can be detected, for example, the temperature sensor 26 a may be provided in the vicinity of the inlet of the cooling unit 22 or in the water vapor supply pipe 51 between the evaporation unit 26 and the cooling unit 22. This steam temperature TS is the temperature of the reforming water supplied to the reforming unit 21.

  The CO shift unit 23 is a unit that reduces carbon monoxide in the reformed gas supplied from the reforming unit 21 through the cooling unit 22, that is, a carbon monoxide reducing unit. The CO shift unit 23 includes a folded channel 23a extending along the vertical direction. The return channel 23a is filled with a catalyst 23b (for example, a Cu—Zn-based catalyst). In the CO shift unit 23, a so-called carbon monoxide shift reaction in which carbon monoxide and water vapor contained in the reformed gas introduced from the cooling unit 22 react with the catalyst 23b to be converted into hydrogen gas and carbon dioxide gas. Has occurred. This carbon monoxide shift reaction is an exothermic reaction.

  In the CO shift unit 23, a temperature sensor 23c that measures the temperature in the CO shift unit 23 is provided. The detection result of the temperature sensor 23 c is transmitted to the control device 30.

  The CO selective oxidation unit 24 further reduces the carbon monoxide in the reformed gas supplied from the CO shift unit 23 and supplies it to the fuel cell 10, that is, a carbon monoxide reduction unit. The CO selective oxidation unit 24 is formed in a cylindrical shape, and is provided so as to cover the outer peripheral wall of the evaporation unit 26. The CO selective oxidation unit 24 is filled with a catalyst 24a (for example, a Ru or Pt catalyst).

  In the CO selective oxidation unit 24, a temperature sensor 24b for measuring the temperature in the CO selective oxidation unit 24 is provided. The detection result of the temperature sensor 24 b is transmitted to the control device 30.

  A connecting pipe 89 connected to the CO shift section 23 and a reformed gas supply pipe 71 connected to the fuel electrode 11 of the fuel cell 10 are connected to the lower and upper side walls of the CO selective oxidation section 24, respectively. ing. An oxidation air supply pipe 61 is connected to the connection pipe 89. As a result, the reformed gas from the CO shift unit 23 and the oxidizing air from the atmosphere are introduced into the CO selective oxidation unit 24. The oxidizing air supply pipe 61 is provided with an oxidizing air pump 62 and an oxidizing air valve 63 in order from the upstream. The oxidizing air pump 62 supplies oxidizing air and adjusts the supply amount. The oxidation air valve 63 opens and closes the oxidation air supply pipe 61.

  Therefore, carbon monoxide in the reformed gas introduced into the CO selective oxidation unit 24 reacts (oxidizes) with oxygen in the oxidizing air to become carbon dioxide. This reaction is an exothermic reaction and is promoted by the catalyst 24a. Thereby, the reformed gas is derived by further reducing the carbon monoxide concentration (10 ppm or less) by the oxidation reaction, and is supplied to the fuel electrode 11 of the fuel cell 10.

  A CO selective oxidation unit 24 is connected to the inlet of the fuel electrode 11 of the fuel cell 10 via a reformed gas supply pipe 71, and the combustion unit 25 is connected to the outlet of the fuel electrode 11 via an offgas supply pipe 72. Is connected. The bypass pipe 73 bypasses the fuel cell 10 and directly connects the reformed gas supply pipe 71 and the offgas supply pipe 72. The reformed gas supply pipe 71 is provided with a first reformed gas valve 74 between the branch point of the bypass pipe 73 and the fuel cell 10. The off gas supply pipe 72 is provided with an off gas valve 75 between the junction with the bypass pipe 73 and the fuel cell 10. A second reformed gas valve 76 is provided in the bypass pipe 73.

  A cathode air supply pipe 67 is connected to the inlet of the air electrode 12 of the fuel cell 10, and an exhaust pipe 82 is connected to the outlet of the air electrode 12. Air is supplied to the air electrode 12, and off-gas is exhausted. The cathode air supply pipe 67 is provided with a cathode air pump 68 and a cathode air valve 69 in order from the upstream. The cathode air pump 68 supplies cathode air and adjusts the supply amount. The cathode air valve 69 opens and closes the cathode air supply pipe 67.

  The combustion part 25 generates combustion gas for heating the reforming part 21 and supplying heat necessary for the steam reforming reaction. The lower end part is inserted into the inner peripheral wall of the reforming part 21. It is arranged with a space. As shown in FIG. 2, the combustion section 25 includes a main body 25a, a cylindrical combustion cylinder 25b that stands on the main body 25a and communicates with the main body 25a, and an ignition electrode (not shown). The combustion unit 25 is ignited according to a command from the control device 30.

  The main body 25a is supplied with a combustion fuel, a combustible gas such as an anode off-gas and a reformed gas, and an oxidant gas such as combustion air for burning (oxidizing) the combustible gas. The base end of the combustion cylinder 25b is erected from the inner peripheral edge of the injection port 25a1 of the main body 25a. The combustion cylinder 25b communicates with the main body 25a, and a mixed gas of combustible gas and combustion air derived from the main body 25a is introduced into the combustion cylinder 25b through the proximal end opening 25b1 of the combustion cylinder 25b.

  Specifically, as shown in FIG. 1, the other end of an off-gas supply pipe 72 having one end connected to the outlet of the fuel electrode 11 is connected to the main body 25a, and the fuel cell system (the reformer 20) is connected. The reformed gas from the reformer 20 is supplied through the reformed gas supply pipe 71, the bypass pipe 73, and the off-gas supply pipe 72 during the start-up operation of the fuel cell system (reformer 20) during steady operation. The anode off-gas discharged from the battery 10 (reformed gas containing unreformed reforming fuel in the reforming unit 21 and unused hydrogen in the fuel electrode 11) is supplied.

  Further, a combustion air supply pipe 64 is connected to the main body 25a so that combustion air that is a combustion oxidant gas for burning combustion fuel, reformed gas, or anode off-gas is supplied. It has become. The combustion air supply pipe 64 is provided with a combustion air pump (combustion oxidant gas supply means) 65 and a combustion air valve 66. The combustion air pump 65 sucks combustion air from the atmosphere and discharges it to the combustion unit 25, and adjusts the amount of combustion air supplied to the combustion unit 25 in accordance with a command from the control device 30. The combustion air valve 66 opens and closes the combustion air supply pipe 64 in accordance with a command from the control device 30.

  Furthermore, between the desulfurizer 46 and the reforming fuel valve 43 of the reforming fuel supply pipe 41, one end is connected between the fuel cell 10 of the reforming gas supply pipe 71 and the first reformed gas valve 74. The other end of the combustion fuel supply pipe 44 is connected. A combustion fuel valve 45 is provided in the combustion fuel supply pipe 44. The combustion fuel valve 45 opens and closes the combustion fuel supply pipe 44. One end is connected between the fuel cell 10 of the offgas supply pipe 72 and the offgas valve 75 and the other end is connected between the combustion air valve 66 of the combustion air supply pipe 64 and the combustion unit 25. A combustion fuel supply pipe 47 is provided. A combustion fuel valve 48 is provided in the combustion fuel supply pipe 47. The combustion fuel valve 48 opens and closes the combustion fuel supply pipe 47.

  From the time when the system is started to the time when the supply of the reforming fuel to the reforming unit 21 is started, the reforming fuel valve 43, the first reformed gas valve 74, and the offgas valve 75 are closed, and the combustion fuel. The valve 45 and the combustion fuel valve 48 are opened, and the fuel pump 42 is driven. As a result, the combustion fuel is directly supplied to the combustion section 25 through the combustion fuel supply pipe 44, the fuel cell 10 and the combustion fuel supply pipe 47 without passing through the reforming section 21.

  In addition, the reforming fuel valve 43 and the second reformed gas valve 76 are opened from the start of supply of the reforming fuel to the reforming unit 21 to the start of steady operation (power generation), and the combustion fuel valve 45 is opened. And the combustion fuel valve 48 is closed, and the fuel pump 42 is driven. The first reformed gas valve 74 and the offgas valve 75 remain closed. Thus, in order to avoid supplying reformed gas having a high carbon monoxide concentration from the CO selective oxidation unit 24 to the fuel cell 10, the reformed gas from the CO selective oxidation unit 24 is supplied to the fuel cell 10 in the combustion unit 25. Supplied directly without going through.

  During steady operation (power generation), the reforming fuel valve 43, the first reformed gas valve 74, and the offgas valve 75 are opened, and the combustion fuel valve 45, the combustion fuel valve 48, and the second reformed gas valve 76 are opened. Is closed and the fuel pump 42 is driven. As a result, the combustion portion 25 contains the anode off-gas from the fuel electrode 11 of the fuel cell 10 (hydrogen that has been supplied to the fuel electrode 11 of the fuel cell 10 and discharged without being consumed, or unreformed reforming fuel). Only the reformed gas is supplied. At this time, no additional cooking is performed in the combustor 25 in which a combustible gas such as reforming fuel is separately added and burned. This is so-called no additional cooking.

  As described above, in the combustion unit 25, the combustion fuel, reformed gas, or anode off gas (these are combustible gases) supplied to the combustion unit 25 are combusted by the combustion air supplied to the combustion unit 25. Hot combustion gas is generated. This combustion gas is formed between the reforming section 21 and the heat insulating section 28 and between the heat insulating section 28 and the evaporation section 26 and disposed so as to heat the reforming section 21 and the evaporation section 26. It flows through the flow path 27, passes through the exhaust pipe 81, and is discharged to the outside as combustion exhaust gas. The combustion gas heats the reforming catalyst 21a of the reforming unit 21 so as to be in the activation temperature range, and heats the evaporation unit 22 to generate water vapor.

  Further, as shown in FIG. 2, the combustion unit 25 is a physical quantity detection unit that detects an ionic current that is a physical quantity correlated with the amount of reforming fuel in the anode off-gas that flows into the combustion unit 25 and burns. An ion current detector 90 is provided. The ion current detector 90 detects the ion current flowing through the flame 25c by installing an electrode 91 (frame rod) on the immediate side of the flame 25c by utilizing the fact that the flame has a function of rectifying the current. Is. The ion current detection device 90 includes an electrode 91, a power source 92 that applies a voltage between the electrode 91 and the combustion cylinder 25 b (or the main body 25 a), and a current detection array that is arranged between the electrode 91 and the power source 92. The resistor 93 and a current detection circuit 94 that detects the current by detecting the voltage across the current detection resistor 93 and transmits the detection result to the control device 30.

  The fuel cell system also includes a control device 30. The control device 30 includes the temperature sensors 21c, 23c, 24b, and 26a, the pumps 42, 53, 62, 65, and 68, and the valves 43 and 45. , 48, 54, 63, 66, 69, 74, 75, 76, the combustion part 25 (ignition electrode) and the current detection circuit 94 are connected (see FIG. 3). The control device 30 includes a microcomputer (not shown), and the microcomputer includes an input / output interface, a CPU, a RAM, and a ROM (all not shown) connected through a bus. Based on the temperature from the temperature sensors 21c, 23c, 24b, and 26a, and the ion current value from the current detection circuit 94, the CPU selects each pump 42, 53, 62, 65, 68, and each valve 43, 45, 48, 54. , 63, 66, 69, 74, 75, 76 and the combustion section 25 are controlled to operate the fuel cell system. The RAM temporarily stores variables necessary for executing the program, and the ROM stores the program.

  A storage device 31 is connected to the control device 30. As shown in FIG. 4, the storage device 31 stores characteristics indicating the correlation between the conversion rate and the ion current value for each required supply amount of reforming fuel corresponding to the required power generation amount of the fuel cell 10. It is. The required power generation amount of the fuel cell 10 is the power consumption (total power consumption) of the power load (home appliance) connected to the fuel cell 10. In FIG. 4, the horizontal axis represents the conversion rate (%), and the vertical axis represents the ionic current value (mA). The conversion rate is the ratio of the reformed fuel supplied to the reforming unit 21 that has been reformed to hydrogen among the reformed fuel supplied to the reforming unit 21.

  This characteristic is created based on data measured using an actual machine. Specifically, a simulated gas simulating a reformed gas (that is, an anode off gas) corresponding to a specified supply amount and a specified conversion rate of reforming fuel is burned to obtain a conversion rate-ion current value correlation. For example, the simulated gas corresponding to the conversion rates of 80%, 85%, and 90% is burned at the specified supply amount An of the reforming fuel, and the characteristic curve fa (n) of FIG. 4 is created. In FIG. 4, the air ratio λ is 1.5.

  The air ratio λ is expressed by (actual combustion air input amount / ideal combustion air input amount). In order to change the air ratio λ, the actual input amount (supply amount) of combustion air may be changed.

  For example, when reforming fuel is supplied at a specified supply amount An, a simulated gas with a specified conversion rate equivalent to 80% is supplied, and the supply amount of combustion air is changed to measure the ion current value. Thus, the characteristic curve f11 shown in FIG. 5 is created. Similarly, characteristic curves f12, f13, and f14 when the specified conversion rate is set to 85%, 90%, and 95% are created. In FIG. 5, the horizontal axis represents the air ratio λ, and the vertical axis represents the ion current value (mA). FIG. 5 shows a case where the reforming fuel is supplied at a certain specified supply amount An.

  As is apparent from FIG. 5, the conversion rate is constant even when the air ratio λ changes. This is because if the air ratio λ is changed, the amount of combustion air supplied will change, but the effect it has on the flammability of the reforming fuel, ie methane, is within the practical air ratio λ range (to ensure low emissions). This is because the air ratio λ range is 1.2 to 2.0).

  Further, the ion current value increases as the conversion rate decreases. This is because, if the conversion rate in the reforming section 21 is low, the unreformed reforming fuel increases and the reforming fuel is supplied to the combustion section 25. This is because the ionic current value increases due to an increase in the fuel for use. Further, during the steady operation, since the hydrocarbon-based combustible gas other than the anode off-gas is not separately supplied to the combustion unit 25, unreformed reforming fuel is burned in the combustion unit 25 in the reforming unit 21. Thus, an ion current value corresponding to the amount of the reforming fuel is generated. Therefore, the amount of reformed fuel supplied to the reforming unit 21 and the amount of unreformed reformable fuel that can be calculated by the conversion rate are correlated with the ionic current value. As a result, the conversion rate and the ionic current value are also correlated. There is a relationship.

  Next, when the correlation between the conversion rate and the ionic current value shown in FIG. 5 created in this way is shown in a graph, it is shown as fa (n) in FIG. This fa (n) indicates the correlation between the conversion rate and the ionic current value when the reforming fuel is supplied at the supply amount An. Further, when the reforming fuel is supplied at An + 1 larger than the supply amount An, it is indicated by fa (n + 1) obtained by translating fa (n) upward by a predetermined amount. When the reforming fuel is supplied at An + 2, which is greater than the supply amount An + 1, it is indicated by fa (n + 2) obtained by translating fa (n + 1) by a predetermined amount. Further, when the reforming fuel is supplied at An-1 which is smaller than the supply amount An, it is indicated by fa (n-1) obtained by translating fa (n) downward by a predetermined amount. When the reforming fuel is supplied at An-2 that is less than the supply amount An-1, it is indicated by fa (n-2) obtained by translating fa (n-1) downward by a predetermined amount.

  Thus, FIG. 4 shows characteristics indicating the correlation between the conversion rate and the ionic current value for each required supply amount of reforming fuel corresponding to the required power generation amount of the fuel cell 10, fa (n),. It is shown. If the conversion rate is the same, the absolute amount of unreformed reforming fuel in the reforming unit 21 increases as the required supply amount of reforming fuel, that is, the supply amount of reforming fuel, increases. Therefore, the characteristic fa moves upward. Further, as the supply amount of reforming fuel decreases, the absolute amount of unreformed reforming fuel in the reforming unit 21 decreases, so the characteristic fa moves downward.

  Further, as shown in FIG. 6, the storage device 31 is a process represented by the conversion rate and the ratio of the heat quantity of hydrogen consumed in the fuel cell 10 to the heat quantity of the reforming fuel supplied to the reforming unit 21. A characteristic indicating a correlation with efficiency is stored. In FIG. 6, the horizontal axis represents the conversion rate (%), and the vertical axis represents the process efficiency (%).

  This characteristic is created based on data measured using an actual machine. Specifically, the process efficiency is measured by changing the conversion rate while keeping the supply amount of the reforming fuel constant. The conversion rate and the hydrogen utilization rate are determined based on the heat balance of the reformer 20 with respect to the sweep current value (power generation amount) in order to avoid reheating. Actually, the combustion gas composition (conversion rate) is combusted under specified conditions, and the hydrogen utilization rate (hydrogen amount in off-gas) is varied so that the actual reformed gas conversion rate becomes the specified conversion rate. In the above evaluation, the process efficiency value is measured in a state where the combustion gas conversion rate condition and the actual reformed gas conversion rate result are the same and the heat balance is achieved.

  For example, when the reforming fuel is supplied at a certain supply amount An, the characteristic curve fb (n) is created by changing (converting) the conversion rate and measuring (or calculating) the process efficiency. Similarly, the characteristic curves fb (n + 1), fb (n + 2), fb (n-1), fb (n) when the supply amount of reforming fuel is An + 1, An + 2, An-1, and An-2. -2). The characteristics fb (n-2), fb (n-1), fb (n), fb (n + 1), and fb (n + 2) measured in this way are shown in FIG. The supply amount An + 1 is larger than An, and the supply amount An + 2 is larger than An + 1. The supply amount An-1 is smaller than An, and the supply amount An-2 is smaller than An-1.

  As shown in FIG. 6, the conversion rate-process efficiency characteristics revealed that the process efficiency has a maximum value Ya when the conversion rate is the optimum conversion rate Xa.

  The optimum conversion rate Xa having the highest process efficiency may be stored as a numerical value for each reforming fuel supply amount, that is, for each reforming fuel required supply amount corresponding to the required power generation amount of the fuel cell 10. .

  The operation of the fuel cell system described above will be described. When a start switch (not shown) is turned on at time t0, control device 30 starts the start-up operation of the fuel cell system. That is, the fuel cell system enters the warm-up mode. The control device 30 opens the combustion air valve 66 and drives the combustion air pump 65 to supply the combustion air to the combustion unit 25. Further, the control device 30 energizes the ignition electrode of the combustion unit 25. Further, the control device 30 closes the reforming fuel valve 43, the first reformed gas valve 74, and the offgas valve 75, opens the combustion fuel valves 45 and 48, drives the fuel pump 42, and supplies the combustion fuel. The fuel is supplied directly to the combustion unit 25 via the fuel cell 10. As a result, the combustion fuel is burned in the combustion section 25, and the reforming section 21 and the evaporation section 26 are heated by the combustion gas.

  When activated, the control device 30 supplies a predetermined amount of water to the evaporation unit 26 (if the system is in negative pressure, the water is supplied after the negative pressure is eliminated), and the supply of water is temporarily stopped. To do. Thereafter, the control device 30 determines that water vapor has been generated when the temperature sensor 26a becomes equal to or higher than a predetermined value (for example, 100 ° C.). The control device 30 starts supplying water at a predetermined flow rate to the evaporation unit 26 after confirming the generation of water vapor.

  Thereafter, the control device 30 opens the reforming fuel valve 43, closes the combustion fuel valves 45 and 48, the first reformed gas valve 74 and the offgas valve 75, drives the fuel pump 42, and reforms the fuel. Is supplied to the reforming unit 21. Further, the control device 30 opens the oxidation air valve 64 to drive the oxidation air blower 63 and supplies the oxidation air to the CO selective oxidation unit 24 by a predetermined flow rate (predetermined supply amount). Thus, the reformed fuel and steam mixed gas are supplied to the reforming unit 21, and the reforming unit 21 generates the reformed gas by causing the steam reforming reaction and the carbon monoxide shift reaction described above. The reformed gas derived from the reforming unit 21 is reduced in carbon monoxide gas by the CO shift unit 23 and the CO selective oxidation unit 24 and is derived from the CO selective oxidation unit 24 without passing through the fuel cell 10. The fuel is directly supplied to the combustion unit 25 through the bypass pipe 73 and burned.

  During the generation of the reformed gas, the control device 30 uses the temperature sensor 24b (and / or the temperature sensor 23c) to control the temperature of the catalyst 24a of the CO selective oxidation unit 24 (and / or the catalyst 23b of the CO shift unit 23). If the detected temperature is equal to or higher than the predetermined temperature, the first reformed gas valve 74 and the offgas valve 75 are opened assuming that the carbon monoxide concentration in the reformed gas is equal to or lower than the predetermined low concentration. The second reformed gas valve 76 is closed to connect the CO selective oxidation unit 24 to the inlet of the fuel electrode 11 of the fuel cell 10 and to the outlet of the fuel electrode 11 to the combustion unit 25. Thereby, the start-up operation for warming up the fuel cell system is completed, and then the steady operation is started. That is, the fuel cell system enters the power generation mode.

  In the power generation mode, the control device 30 includes a reforming fuel, a combustion fuel, a combustion air, an oxidation air, a cathode air, and a desired output power (current / power consumed by the load device). The reformed water is supplied. The control device 30 calculates the supply amount of the reforming fuel so as to obtain a desired output power, drives the fuel pump 42 so as to obtain the supply amount, and calculates the calculated reforming fuel supply amount and S / C ( Based on the steam carbon ratio), the supply amount of the reforming water is calculated, and the reforming water pump 53 is driven so as to obtain the supply amount. Further, the control device 30 calculates the supply amount of the combustion air based on the supply amount of the reforming fuel, and drives the combustion air blower 66 so as to obtain the supply amount. In addition, the control device 30 calculates the supply amount of the oxidation air so that the carbon monoxide is less than or equal to a predetermined amount, and drives the oxidation air blower 63 so as to obtain the supply amount. Then, the control device 30 calculates a supply amount of cathode air sufficient to react with the reformed gas supplied from the reformer 20, and drives the cathode air blower 68 so as to obtain the supply amount. . When the stop switch is pushed, the fuel cell system performs a stop operation and stops.

  Further, derivation (estimation) of the conversion rate in the reforming unit 21 and control of the fuel pump 42 based on the derived conversion rate will be described with reference to FIGS. 7 and 8. When starting the steady operation, the control device 30 executes the program shown in FIG. 7 every predetermined short time.

  In step 102, the control device 30 determines a reforming fuel supply amount (required fuel supply amount) based on the required power generation amount for the fuel cell 10, and in step 104 drives the fuel pump 42 with the determined supply amount. To do. In step 106, the control device 30 selects a conversion rate-ion current value characteristic according to the reforming fuel supply amount determined in step 102. For example, assume that fa (n) is selected.

  Further, the control device 30 detects the ion current value by the ion current detection device 90 (step 108), and derives (estimates) the conversion rate from the detected ion current value and the characteristics selected in step 106 (step 110). ).

  Then, the control device 30 adjusts the supply amount of the fuel pump 42 so that the derived conversion rate becomes the target conversion rate. Specifically, in step 112, the control device 30 reads (acquires) the target conversion rate from the storage device 31, and compares the acquired target conversion rate with the conversion rate derived in step 110. The target conversion rate is preferably the optimum conversion rate Xa with the highest process efficiency.

  As shown in FIG. 8, when the conversion rate is smaller than the target conversion rate, the supply amount of reforming fuel is increased in increments of a predetermined amount (step 114). When the supply amount of reforming fuel increases, the endothermic amount necessary for reforming increases, but the amount of unreformed reforming fuel flowing into the combustion section 25 increases, so the amount of combustion heat in the combustion section 25 Is larger than the necessary amount of heat absorption, so that the combustion becomes excessive and the conversion rate increases. Further, as shown in FIG. 8, when the detected ion current value is larger than the target ion current value Ia corresponding to the target conversion rate, the supply amount of reforming fuel is increased by a predetermined amount in order to increase the conversion rate. Let

  If the conversion rate is equal to the target conversion rate, the supply amount of the reforming fuel is maintained as it is (step 116).

  If the conversion rate is larger than the target conversion rate, the supply amount of the reforming fuel is decreased in increments of a predetermined amount (step 118). When the supply amount of reforming fuel decreases, the endothermic amount necessary for reforming decreases, but the amount of unreformed reforming fuel flowing into the combustion section 25 decreases, so the amount of combustion heat in the combustion section 25 Is less than the necessary amount of heat absorption, so that the combustion becomes insufficient and the conversion rate decreases. Further, as shown in FIG. 8, when the detected ion current value is smaller than the target ion current value Ia corresponding to the target conversion rate, the supply amount of reforming fuel is decreased by a predetermined amount in order to lower the conversion rate. You may make it make it.

  In addition, the process of step 108 mentioned above is an ionic current detection means for detecting an ionic current correlated with the amount of reforming fuel in the anode off-gas that flows into the combustion section 25 and burns. Conversion rate indicating the ratio of the reformed fuel supplied to the reforming unit 21 to hydrogen based on the ion current detected by the ion current detecting means in the processing of steps 106 to 110 described above. Is a conversion rate estimation means for estimating Supplying the fuel for reforming by controlling the fuel pump 42 as the reforming fuel supply means so that the conversion rate estimated by the conversion rate estimating means in the processing of steps 112 to 118 described above becomes the target conversion rate. A reforming fuel supply amount control means for controlling the amount.

  As is apparent from the above description, in this embodiment, during the steady operation, the reforming fuel supplied to the reforming unit 21 is partly reformed to hydrogen and the rest is reformed without being reformed. It is discharged as quality fuel. Fuel gas containing these hydrogen and unreformed reforming fuel is supplied to the fuel cell 10, of which hydrogen is consumed by power generation. The anode off-gas from the fuel cell 10 contains unreformed reforming fuel and unburned combustible gas such as hydrogen. This anode gas is supplied to the combustion section 25 and burned. In the combustion in the combustion unit 25, no additional cooking is performed in which a combustible gas such as a reforming fuel is separately added to the combustion unit 25 for combustion. This is so-called no additional cooking. In the case of no additional cooking, the amount of reforming fuel in the anode off-gas burned by the combustion unit 25 is the same as the amount of reforming fuel that has not been reformed by the reforming unit 21. The ionic current value when hydrogen is burned is significantly smaller than the ionic current value when the reforming fuel (hydrocarbon such as natural gas) is burned. Therefore, the ion current value is hardly affected by the amount of combustion heat of hydrogen in the anode off gas. Therefore, the conversion rate of the unreformed reforming fuel amount can be detected by the ion current value. The conversion rate (%) is (reforming fuel amount reformed in reforming unit / reforming fuel supply amount to reforming unit) × 100, that is, 100− (unreformed in reforming unit) This is because the amount of reforming fuel / the amount of reforming fuel supplied to the reforming section) × 100.

  From the above, the conversion rate estimation means (steps 106 to 110) uses hydrogen in the reforming fuel supplied to the reforming unit 21 based on the ion current detected by the ion current detection means (step 108). It is possible to estimate the conversion rate indicating the ratio of those modified. Thus, the conversion rate can be accurately estimated by detecting the ionic current by the ionic current detection device 90, which is an ionic current detection means generally cheaper than the hydrocarbon sensor, and using the detection result. Therefore, regardless of the state of the catalyst in the reforming unit 21, the conversion rate can be accurately calculated without incurring an increase in cost.

  Further, the reforming fuel supply amount control means (steps 112 to 118) controls the fuel pump 42, which is the reforming fuel supply means, so that the conversion rate estimated by the conversion rate estimation means becomes the target conversion rate. Thus, the supply amount of the reforming fuel is controlled. As a result, the conversion rate can be accurately and accurately controlled to the target conversion rate. Therefore, by stabilizing the conversion rate, the hydrogen utilization rate can also be stably controlled, and as a result, the stability of the fuel cell system can be improved. In addition, even if there is a variation in the flow rate of the reforming fuel supply means, the conversion rate and hydrogen utilization rate can be stably controlled in a balanced manner, so an inexpensive reforming fuel supply means with a relatively large flow rate variation can be used. The cost of the fuel cell system can be reduced.

  Instead of such reforming fuel supply amount control means, or together with the reforming fuel supply amount control means, the combustion oxidation is performed so that the conversion rate estimated by the conversion rate estimation means becomes the target conversion rate. The supply amount of the combustion air that is the combustion oxidant gas may be controlled by controlling the combustion air pump 65 that is the agent gas supply means. In this case, a program corresponding to the flowchart shown in FIG. 7 may be executed. However, when the conversion rate is smaller than the target conversion rate, the supply amount is decreased, and when the conversion rate is larger than the target conversion rate, the supply amount is increased. This is because when the amount of combustion oxidant gas supplied decreases, the amount of cooling heat generated by the combustion oxidant gas decreases, so the temperature of the reforming section 21 rises and the conversion rate increases. On the other hand, when the reforming fuel supply amount increases, the anode off-gas input to the combustion unit 25 also increases due to the lack of renewal. As a result, the amount of combustion heat in the combustion unit 25 increases, and the temperature of the reforming unit 21 increases. Increases and the conversion rate increases.

  As a result, the conversion rate can be accurately and accurately controlled to the target conversion rate. Therefore, by stabilizing the conversion rate, the hydrogen utilization rate can also be stably controlled, and as a result, the stability of the fuel cell system can be improved. In addition, even if there is a variation in the flow rate of the combustion oxidant gas supply means, the conversion rate and hydrogen utilization rate can be stably controlled in a balanced manner, so an inexpensive combustion oxidant gas supply means with a relatively large flow rate variation should be used. Thus, the cost of the fuel cell system can be reduced.

  Further, since the optimum conversion rate with the highest process efficiency represented by the ratio of the heat amount of hydrogen consumed by the fuel cell 10 to the heat amount of the reforming fuel supplied to the reforming unit 21 is set as the target conversion rate, High process efficiency operation can be achieved by controlling the rate to the target conversion rate.

  Further, the conversion rate estimating means (steps 106 to 110) converts the conversion rate-ion current according to the required power generation amount (output power; the reforming fuel supply amount determined in step 102) of the fuel cell 10 in step 106. By selecting the value characteristic, the conversion rate is estimated in consideration of the output power of the fuel cell 10, so that more optimal operation can be performed according to the output power of the fuel cell 10.

  In the above-described embodiment, a blower may be used instead of the pump in the pump that supplies gas.

It is a schematic diagram showing an outline of one embodiment of a fuel cell system according to the present invention. It is a schematic diagram which shows the cross section of the combustion part shown in FIG. 1, and an ion current detection apparatus. It is a block diagram which shows the fuel cell system shown in FIG. It is a map which shows the characteristic curve of the conversion rate and ion current value for every required fuel supply amount memorize | stored in the memory | storage device shown in FIG. It is a figure which shows the relationship between a combustion state and an ionic current value for every conversion rate. It is a map which shows the characteristic curve of the conversion rate and process efficiency for every required fuel supply amount. 4 is a flowchart of a control program executed by the control device shown in FIG. 3. It is a figure which shows each relationship of the fuel flow for a reforming and an ionic current value, the fuel flow for a reforming, a conversion rate, and the fuel flow for a reforming, and a hydrogen utilization factor.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Fuel cell, 11 ... Fuel electrode, 12 ... Air electrode, 20 ... Reformer, 21 ... Reformer, 21c ... Temperature sensor, 22 ... Cooling part (heat exchange part), 23 ... Carbon monoxide shift reaction part (CO shift part), 24 ... carbon monoxide selective oxidation reaction part (CO selective oxidation part), 25 ... combustion part, 25a ... main body, 25b ... combustion cylinder, 26 ... evaporation part, 27 ... combustion gas flow path, 28 ... Heat insulation part, 30 ... control device (ion current detection means (step 108), conversion rate estimation means (steps 106-110), reforming fuel supply amount control means (steps 112-118)), 41 ... fuel supply pipe, 42 ... Fuel pump (reforming fuel supply means), 43 ... Reforming fuel valve, 44, 47 ... Combustion fuel supply pipe, 45, 48 ... Combustion fuel valve, 46 ... Desulfurizer, 51 ... Steam supply pipe 52 ... Water supply pipe, 53 ... Reformed water pump 54 ... reforming water valve, 61 ... oxidation air supply pipe, 62 ... oxidation air pump, 63 ... oxidation air valve, 64 ... combustion air supply pipe, 65 ... combustion air pump (combustion oxidant gas supply) Means), 66 ... Combustion air valve, 67 ... Cathode air supply pipe, 68 ... Cathode air pump, 69 ... Cathode air valve, 71 ... Reformed gas supply pipe, 72 ... Off-gas supply pipe, 73 ... Bypass pipe , 74 ... first reformed gas valve, 75 ... off-gas valve, 76 ... second reformed gas valve, 81, 82 ... exhaust pipe, 89 ... connecting pipe, 90 ... ion current detector (ion current detector).

Claims (4)

A fuel cell that generates electric power using the fuel gas and the oxidant gas respectively supplied to the fuel electrode and the oxidant electrode;
A reforming section for generating the fuel gas containing hydrogen by reforming the reforming fuel supplied by the reforming fuel supply means;
A combustion section for supplying only the anode off-gas from the fuel electrode of the fuel cell during steady operation, burning the anode off-gas with a combustion oxidant gas, and heating the reforming section with the combustion gas; Fuel cell system,
Ionic current detection means for detecting the ionic current of the flame of the combustion section;
Conversion rate estimating means for estimating a conversion rate indicating a ratio of the reformed fuel supplied to the reforming unit that has been reformed to hydrogen based on the ion current detected by the ion current detecting means. A fuel cell system comprising:   2. The reforming according to claim 1, wherein the reforming fuel supply unit is controlled to control a supply amount of the reforming fuel so that the conversion rate estimated by the conversion rate estimating unit becomes a target conversion rate. A fuel further comprising at least one of a combustion fuel supply amount control means and a combustion oxidant gas supply amount control means for controlling a supply amount of the combustion oxidant gas supplied to the combustion section Battery system.   3. The optimum conversion rate with the largest process efficiency represented by the ratio of the amount of heat of hydrogen consumed in the fuel cell to the amount of heat of the reforming fuel supplied to the reforming unit according to claim 1 or 2. A fuel cell system having the target conversion rate.   4. The fuel cell system according to claim 1, wherein the conversion rate estimation unit estimates the conversion rate in consideration of output power of the fuel cell. 5.

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