JP2012218947A - Fuel reforming system, and method for controlling fuel reforming system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel reforming system and a method for controlling the fuel reforming system, properly controlling the amount of steam.SOLUTION: The fuel reforming system includes: a reformer including an evaporation part in which reforming water is evaporated and a reforming part in which fuel gas is generated by the steam reforming reaction of raw fuel with reforming water evaporated in the evaporation part; and an S/C control part which controls the ratio S/C of the number of moles of steam to the number of moles of carbon in the raw fuel in the reformer, wherein the S/C control part changes a target value of the S/C in the same direction as at least one of directions increasing and decreasing the amount of fuel gas generation required for the reformer.

Description

  The present invention relates to a fuel reforming system and a control method for the fuel reforming system.

  Hydrogen consumed in a fuel cell or the like is generated, for example, in a fuel reforming system. In a fuel reforming system, hydrogen is generated by a steam reforming reaction between a raw fuel such as hydrocarbon and reformed water in a reformer. Since the steam reforming reaction is a chemical reaction process, if the amount of reforming water supplied to the reformer becomes unstable, the amount of generated hydrogen becomes unstable. Therefore, Patent Document 1 discloses a technique for deriving a steam carbon ratio control instruction value based on at least one of the reforming fuel supply amount, the temperature of the carbon monoxide reduction unit, and the temperature of water vapor.

JP 2006-273619 A

  However, in the technique of Patent Document 1, an excess or deficiency of the amount of water vapor may occur due to a water vapor generation response delay when the generated power is transient.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a fuel reforming system and a fuel reforming system control method capable of appropriately controlling the amount of steam supplied to the reformer. And

  A fuel reforming system according to the present invention includes a reforming unit that vaporizes reformed water, and a reforming unit that generates fuel gas by a steam reforming reaction between raw fuel and reformed water vaporized in the vaporizing unit. A S / C control unit that controls a ratio S / C of the number of moles of water vapor to the number of moles of carbon in the raw fuel in the reformer, and the S / C control unit includes the S / C control unit, The target value of the S / C is changed in the same direction as at least one of the increasing / decreasing directions of the fuel gas generation amount required for the reformer. In the fuel reforming system according to the present invention, it is possible to suppress an excessive amount of water vapor and / or a shortage of water vapor. Therefore, the amount of water vapor can be properly controlled.

  The S / C control unit is configured to increase or decrease the amount of fuel gas generation required for the reformer when the amount of change in the amount of fuel gas generation required for the reformer exceeds a predetermined value. The S / C may be changed in the same direction. In this case, the S / C is controlled when the required fluctuation amount of the fuel gas generation amount is large.

  The pressure sensor further detects a gas pressure in the reformer, and the S / C control unit has a fluctuation amount of a fuel gas generation amount required for the reformer being equal to or less than the predetermined value and the pressure sensor When the fluctuation amount of the gas pressure detected by the engine exceeds a threshold value, at least one of the reforming water flow rate supplied to the reformer and the raw fuel flow rate supplied to the reformer is periodically It may be varied. In this case, the bumping frequency of the reformed water can be suppressed. The S / C control unit may stop the periodic fluctuation when the fluctuation amount of the gas pressure detected by the pressure sensor becomes equal to or less than the threshold value.

  A pressure sensor for detecting a gas pressure in the reformer; and a pressure reducing means for reducing the pressure in the reformer, wherein the pressure reducing means generates fuel gas required for the reformer. When the amount of change in the amount is equal to or less than the predetermined amount and the amount of change in the gas pressure detected by the pressure sensor exceeds a threshold value, the pressure in the reformer may be periodically changed. In this case, the bumping frequency of the reformed water can be suppressed. The pressure reducing means may stop the periodic fluctuation when the fluctuation amount of the gas pressure detected by the pressure sensor becomes equal to or less than the threshold value.

  The variation amount of the gas pressure may be a standard deviation of the gas pressure detected by the pressure sensor. When the S / C is changed, the S / C control unit sets the target value of the S / C to the original value after a predetermined time has elapsed or when the S / C change amount reaches a predetermined amount. You may return. A fuel cell that generates power using the fuel gas may be further provided, and a fuel gas generation amount required for the reformer may be correlated with a power generation amount required for the fuel cell. The fuel cell may be a solid oxide fuel cell. You may further provide the combustion chamber which gives the heat | fever obtained by burning the off gas of the said fuel cell to the said vaporization part.

  A control method for a fuel reforming system according to the present invention includes: a vaporization unit that vaporizes reformed water; a reforming unit that generates fuel gas by a steam reforming reaction between raw fuel and reformed water vaporized in the vaporization unit; The number of moles of carbon in the raw fuel in the reformer in the same direction as at least one of the increasing and decreasing directions of the amount of fuel gas generation required for the reformer It includes an S / C control step for changing a target value of the ratio S / C of the number of moles of water vapor relative to the water vapor. In the control method of the fuel reforming system according to the present invention, it is possible to suppress an excessive amount of water vapor and / or a shortage of water vapor. Therefore, the amount of water vapor can be properly controlled.

  ADVANTAGE OF THE INVENTION According to this invention, the control method of the fuel reforming system and fuel reforming system which can control appropriately the amount of water vapor | steam supplied to a reformer can be provided.

1 is a block diagram illustrating an overall configuration of a fuel cell system including a fuel reforming system according to Embodiment 1. FIG. It is a fragmentary perspective view containing the cross section of a fuel cell. It is a figure which shows the relationship between an anode pressure and a reforming water flow rate, and is the graph which plotted the relationship between an anode pressure and a reforming water flow rate. It is a figure for demonstrating the experimental result of the change of S / C ratio when generated electric power changes. It is a figure for demonstrating an example of the flowchart performed when controlling a S / C ratio when the request | required generated electric power requested | required of a fuel cell fluctuates. It is a figure for demonstrating the relationship between the request | requirement electric power of a general electrical load, and the request | requirement generated electric power with respect to a fuel cell. It is a figure which shows transition of the anode pressure at the time of making the reforming water supply amount increase gradually in the state which kept the combustion heat quantity in a combustion chamber constant. It is a figure which shows the boiling curve of liquid water. It is a figure for demonstrating the shift according to the pressure of the boiling curve. It is a figure which shows an example of the flowchart showing bump boiling suppression control.

  Hereinafter, the best mode for carrying out the present invention will be described.

  FIG. 1 is a block diagram illustrating an overall configuration of a fuel cell system 100 including a fuel reforming system according to a first embodiment. As shown in FIG. 1, the fuel cell system 100 includes a grid interconnection device 110, a control unit 120, and a fuel cell device 130.

  The grid interconnection device 110 is a device that switches electrical connection and disconnection between the fuel cell device 130 and the general electric load 200. The general electric load 200 is supplied with power from the system power supply and / or the fuel cell device 130. The system power source is a household power source or the like supplied to each home, and may be a business power source or the like supplied to a store, a factory, or the like. The general electric load 200 is an electric load in each home, store, factory, or the like. When the fuel cell device 130 and the general electric load 200 are connected, the grid interconnection device 110 converts the power generated by the fuel cell device 130 into alternating current power and supplies it to the general electric load 200 with a built-in inverter. To do.

  The control unit 120 is a device that controls the fuel cell device 130, and includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. The control unit 120 obtains the required generated power P_req for the fuel cell device 130 from the required power of the general electric load 200, and obtains the generated power command value P_fc required for the fuel cell device 130. The required generated power P_req is larger than the required power of the general electric load 200 because the power consumption of each auxiliary machine of the fuel cell device 130 and the inverter loss of the grid interconnection device 110 are considered in the required generated power P_req. Become. The control unit 120 controls each unit of the fuel cell device 130 based on the generated power instruction value P_fc. Details will be described later.

  The fuel cell apparatus 130 includes a raw fuel supply unit 10, a reformed water supply unit 20, an oxidant gas supply unit 30, a reformer 40, a combustion chamber 50, a fuel cell 60, a heat exchanger 70, a condensed water tank 80, an exhaust gas. A suction pump 90 and a pressure sensor 91 are provided.

  The raw fuel supply unit 10 includes a raw fuel pump 11 for supplying raw fuel such as hydrocarbons to the reformer 40, a desulfurizer 12 for removing sulfur components in the raw fuel, and the like. As an example of the hydrocarbon, city gas can be used. The reforming water supply unit 20 supplies the reforming water tank 21 for storing the reforming water necessary for the steam reforming reaction in the reformer 40 and the necessary reforming water from the condensed water tank 80 to the reforming water tank 21. And a flow rate regulator 23 for supplying the reforming water stored in the reforming water tank 21 to the reformer 40. The oxidant gas supply unit 30 includes an air pump for supplying an oxidant gas such as air to the cathode 61 of the fuel cell 60. The reformer 40 includes a vaporization unit 41 for vaporizing the reformed water and a reforming unit 42 for generating fuel gas by a steam reforming reaction. The fuel cell 60 has a structure in which an electrolyte 63 is sandwiched between a cathode 61 and an anode 62. In FIG. 1, both the oxidant gas supply unit 30 and the exhaust suction pump 90 are shown, but only one of them may be provided. For example, by providing only the oxidant gas supply unit 30, it is possible to realize a oxidant gas pressure feeding system. In addition, by providing only the exhaust suction pump 90, an oxidant gas suction system can be realized.

  FIG. 2 is a partial perspective view including a cross section of the fuel cell 60. As shown in FIG. 2, the fuel cell 60 has a flat plate-like overall shape. A plurality of fuel gas passages 32 penetrating along the axial direction (longitudinal direction) are formed in the conductive support 31 having gas permeability. A fuel electrode 33, a solid electrolyte 34, and an oxygen electrode 35 are laminated in this order on one plane on the outer peripheral surface of the conductive support 31. On the other plane facing the oxygen electrode 35, an interconnector 37 is provided via a bonding layer 36, and a P-type semiconductor layer 38 for reducing contact resistance is provided thereon. The fuel electrode 33 functions as the anode 62 in FIG. 1, the oxygen electrode 35 functions as the cathode 61 in FIG. 1, and the solid electrolyte 34 functions as the electrolyte 63 in FIG. The fuel cell 60 may have a stack structure in which a plurality of single cells shown in FIG. 2 are stacked.

By supplying the reformed gas containing hydrogen to the fuel gas passage 32, hydrogen is supplied to the fuel electrode 33. On the other hand, oxygen is supplied to the oxygen electrode 35 by supplying an oxidant gas containing oxygen around the fuel cell 60. As a result, the following electrode reactions occur at the oxygen electrode 35 and the fuel electrode 33 to generate power. The power generation reaction is performed at 600 ° C. to 1000 ° C., for example.
Oxygen electrode: 1 / 2O ₂ + 2e ⁻ → O ²⁻ (solid electrolyte)
Fuel electrode: O ²⁻ (solid electrolyte) + H ₂ → H ₂ O + 2e ⁻

The material of the oxygen electrode 35 has oxidation resistance and is porous so that gaseous oxygen can reach the interface with the solid electrolyte 34. The solid electrolyte 34 has a function of moving oxygen ions O ²⁻ from the oxygen electrode 35 to the fuel electrode 33. The solid electrolyte 34 is composed of an oxygen ion conductive oxide. Further, since the solid electrolyte 34 physically separates the fuel gas and the oxidant gas, the solid electrolyte 34 is stable and dense in the oxidizing / reducing atmosphere. The fuel electrode 33 is made of a material that is stable in a reducing atmosphere and has an affinity for hydrogen. The interconnector 37 is provided to electrically connect the fuel cells 60 in series, and is dense to physically separate the fuel gas and the oxidant gas.

For example, the oxygen electrode 35 is formed of a lanthanum cobaltite-based perovskite complex oxide having high conductivity of both electrons and ions. The solid electrolyte 34 is formed of ZrO ₂ (YSZ) containing Y ₂ O ₃ having high ion conductivity. The fuel electrode 33 is formed of a mixture of Ni having high electronic conductivity and ZrO ₂ (YSZ) containing Y ₂ O ₃ . The interconnector 37 is made of LaCrO ₃ or the like having a high electronic conductivity and solid-dissolved alkaline earth oxide. These materials are preferably close in thermal expansion coefficient.

  Next, an outline of the operation of the fuel cell device 130 during power generation will be described with reference to FIG. The raw fuel pump 11 supplies a required amount of raw fuel gas to the reformer 40 in accordance with instructions from the control unit 120. In this case, the sulfur component in the raw fuel gas is removed by the desulfurizer 12.

  The flow controller 23 supplies a required amount of reforming water to the reformer 40 in accordance with instructions from the control unit 120. The reforming water supplied to the reformer 40 is evaporated by the heat received from the combustion chamber 50 in the vaporization unit 41 and supplied to the reforming unit 42. In the reforming unit 42, a steam reforming reaction occurs between the steam and the raw fuel gas using the heat received from the combustion chamber 50. Thereby, fuel gas containing hydrogen is generated. The fuel gas generated in the reformer 40 is supplied to the anode 62 of the fuel cell 60. The oxidant gas supply unit 30 supplies a required amount of oxidant gas to the cathode 61 of the fuel cell 60 in accordance with instructions from the control unit 120. Thereby, power generation is performed in the fuel cell 60.

  The oxidant off-gas discharged from the cathode 61 and the fuel off-gas discharged from the anode 62 flow into the combustion chamber 50. In the combustion chamber 50, combustible components of the fuel off-gas are burned by oxygen in the oxidant off-gas. The heat obtained by the combustion is given to the reformer 40 and the fuel cell 60. As described above, in the fuel cell device 130, combustible components such as hydrogen and carbon monoxide contained in the fuel off-gas can be burned in the combustion chamber 50.

  The heat exchanger 70 exchanges heat between the exhaust gas discharged from the combustion chamber 50 and tap water flowing in the heat exchanger 70. Condensed water obtained from the exhaust gas by heat exchange is stored in a condensed water tank 80. The tap water heated by heat exchange is stored in a hot water tank as hot water. The pump 22 supplies the condensed water stored in the condensed water tank 80 to the reformed water tank 21 in accordance with an instruction from the control unit 120. The exhaust suction pump 90 exhausts exhaust gas in accordance with instructions from the control unit 120. The pressure sensor 91 detects a gas pressure in the reformer 40 (hereinafter also referred to as an anode pressure), and transmits the detected anode pressure to the control unit 120. The pressure sensor 91 may be provided at any location on the pipe from the flow controller 23 to the anode 62. The control unit 120 calculates the flow rate of reforming water supplied to the reforming unit 42 based on the anode pressure received from the pressure sensor 91.

  Here, the relationship between the anode pressure and the reforming water flow rate will be described. FIG. 3 is a graph showing the relationship between the anode pressure and the reforming water flow rate, and is a graph plotting the relationship between the anode pressure and the reforming water flow rate. In FIG. 3, a white area is an area where white plots are overlapped and is a transient power generation area. A region drawn with a black plot is a steady power generation region. In FIG. 3, the horizontal axis represents the anode pressure, and the vertical axis represents the reforming water flow rate. Referring to FIG. 3, the reforming water flow rate has a linear relationship with the anode pressure. Therefore, the flow rate of reforming water supplied to the reforming unit 42 can be estimated based on the anode pressure. In this case, expensive equipment such as a steam flow meter may not be used. Thereby, cost can be suppressed.

  Next, the S / C ratio in the reforming unit 42 will be described. The S / C ratio is the ratio of the number of moles of water vapor to the number of moles of carbon in the raw fuel supplied to the reforming unit 42. Therefore, when the S / C ratio is high, the steam supplied to the reforming unit 42 is larger than the raw fuel, and when the S / C ratio is low, the steam supplied to the reforming unit 42 is Less than raw fuel.

  When the S / C ratio is too small, the carbon in the raw fuel reacts with the reforming catalyst of the reforming section 42 and the constituent component of the anode 62 (for example, nickel). When the reforming catalyst reacts with carbon, the function of the reforming catalyst as a catalyst decreases. Thereby, the reforming efficiency of the reforming unit 42 is reduced. Moreover, when carbon and the structural component of the anode 62 react, for example, NiC will be produced | generated and the anode 62 will expand. In this case, the fuel cell 60 may be cracked and the fuel gas may leak. Thereby, the power generation performance of the fuel cell 60 decreases.

  On the other hand, when the S / C ratio becomes excessive, excess water vapor blocks the hydrogen path of the anode 62. In this case, hydrogen becomes insufficient and oxygen becomes excessive at the three-phase interface of the anode 62. Thereby, an oxide is produced | generated when the structural component (for example, nickel) of the anode 62 reacts with oxygen, and the electronic conductivity function of the anode 62 falls. Further, the anode 62 expands as the oxide is generated. As the oxide is reduced by hydrogen, the anode 62 contracts. By repeating this expansion and contraction, the fuel cell 60 may be cracked and the fuel gas may leak. Thereby, the power generation performance of the fuel cell 60 decreases.

  From the above, the S / C ratio is preferably controlled within an appropriate range. By the way, in the fuel cell device 130 described above, steam obtained by vaporizing the reforming water in the reforming water tank 21 in the vaporizing section 41 is used as the steam used for the steam reforming reaction. In order to generate the desired amount of fuel gas in the reformer 40, the amount of water vapor corresponding to the amount of fuel gas required for the reformer 40 is required, and the amount of heat required to generate this amount of water vapor is required. It is. In order to generate the required amount of water vapor, a plurality of steps as described above are required. Therefore, the response delay during the water vapor generation is larger than that of the raw fuel supply and the oxidant gas supply. From the above, when the amount of fuel gas generation required for the reformer 40 increases, the amount of water vapor is insufficient and the S / C ratio tends to be excessively low, which is required for the reformer 40. When the amount of fuel gas generated decreases, the amount of water vapor tends to be excessive and the S / C ratio tends to be excessive. The fuel gas generation amount required for the reformer 40 correlates with the required generated power required for the fuel cell 60. That is, the amount of fuel gas generation required for the reformer 40 is determined by the required generated power required for the fuel cell 60.

  FIG. 4 is a diagram for explaining an experimental result of a change in the S / C ratio when the generated power changes. In FIG. 4, the horizontal axis indicates the elapsed time, and the vertical axis indicates the generated power of the fuel cell 60 and the S / C ratio in the reforming unit 42. In the figure, an S / C control target value, an excessive S / C sign determination value, and an under S / C sign determination value are shown. The excessive S / C sign determination value is a threshold value for determining whether or not the S / C ratio is excessive. The excessively small S / C sign determination value is a threshold value for determining whether or not the S / C ratio is excessively small. S / C in the figure is an estimate of the control value of the raw fuel pump 11 and the detection value of the pressure sensor 91.

  Referring to FIG. 4, when the generated power increases rapidly, the S / C ratio becomes lower than the under S / C sign determination value. When the rate of change in the generated power decreases, the S / C ratio becomes a value within the range between the excessive S / C sign determination value and the under S / C sign determination value. When the generated power is rapidly reduced, the S / C ratio exceeds the excessive S / C sign determination value. As described above, also in the experimental results, it was confirmed that the S / C ratio becomes excessive when the required generated power increases, and the S / C ratio becomes excessive when the required generated power decreases.

  Therefore, in this embodiment, the S / C control target value (target value) is changed in the same direction as at least one of the increasing / decreasing directions of the required generated power required for the fuel cell 60. Thereby, the amount of water vapor can be controlled appropriately. For example, when the required generated power required for the fuel cell 60 increases, the S / C control target value is increased. Thereby, the response delay of the amount of water vapor | steam is suppressed, and it can suppress that water vapor | steam runs short. When the required generated power required for the fuel cell device 130 decreases, the target value for S / C control is decreased. Thereby, the response delay of the amount of water vapor is suppressed, and it is possible to suppress the water vapor from becoming excessive.

  Note that the S / C control target value may be increased or decreased in accordance with both the increase and decrease directions of the required generated power required for the fuel cell device 130. In this case, the amount of water vapor can be appropriately controlled regardless of which required power generation varies in the increase / decrease direction. The S / C ratio can be controlled by controlling at least one of the reforming water supply amount by the flow rate regulator 23 and the raw fuel supply amount by the raw fuel pump 11. Therefore, in this embodiment, the control unit 120, the raw fuel pump 11 and the flow rate regulator 23 function as an S / C control unit. However, it is preferable to control the S / C ratio by controlling the amount of reforming water supplied by the flow rate regulator 23 from the viewpoint of suppressing excess / deficiency of water vapor.

  FIG. 5A is a diagram for explaining an example of a flowchart executed when controlling the S / C ratio when the required generated power required for the fuel cell 60 fluctuates. The flowchart in FIG. 5A is periodically executed, for example, every second. First, referring to FIG. The control unit 120 obtains the required generated power P_req for the fuel cell 60 from the required power of the general electric load 200 (step S1).

  Next, the control unit 120 determines whether or not the fluctuation value | ΔP_req | of the required generated power P_req is larger than the determination value ΔP_req_ref (step S2). The fluctuation value | ΔP_req | is an absolute value of a difference between the required generated power P_req at the time of execution of the current flowchart and the required generated power P_req at the time of execution of the previous flowchart. The determination value ΔP_req_ref is a determination threshold value for determining whether or not the fluctuation in the required generated power P_req is large, and is about 10 W / s, for example. By executing step S2, it is possible to determine whether or not the required generated power fluctuates greatly. In other words, unnecessary S / C control when the required generated power is small can be avoided.

  When it determines with "Yes" in step S2, the control part 120 determines whether (DELTA) P_req is a positive value (step S3). By executing step S3, it is possible to determine whether the required generated power is increasing or decreasing. When it determines with "Yes" in step S3, the control part 120 sets the target value sbyc of S / C ratio to the target value sbyc_up (step S4). The target value sbyc_up is a value (for example, 3.0) that is larger than the normal target value sbyc_std (for example, 2.5) when the fluctuation in the required generated power is small. By executing step S4, the S / C target value can be varied in accordance with the variation direction of the required generated power P_req.

  Next, the control unit 120 determines whether or not the estimated S / C ratio sbyc_p obtained based on the detection result of the pressure sensor 91 is smaller than the excessive S / C sign determination value sbyc_lwr (step S5). The under S / C sign determination value sbyc_lwr is a value smaller than the normal target value sbyc_std, for example, 2.0. By executing step S5, it can be determined whether or not the current S / C ratio is within an appropriate range. When it determines with "Yes" in step S5, the control part 120 sets the generated electric power command value P_fc to the value which deducted generation | occurrence | production suppression electric power (DELTA) P_fc from the request | required generated electric power P_req (step S6). By executing step S6, the power generated by the fuel cell 60 can be temporarily reduced. Thereby, water vapor deficiency can be suppressed.

  Next, the control unit 120 obtains the raw fuel flow rate Qf based on the generated power command value P_fc, and controls the raw fuel pump 11 so that the raw fuel flow rate supplied to the reformer 40 becomes the raw fuel flow rate Qf. (Step S7). By executing step S7, it is possible to realize a raw fuel flow rate for controlling the S / C ratio within an appropriate range. FIG. 5B is a diagram for explaining the relationship between the generated power command value P_fc and the raw fuel flow rate Qf. As shown in FIG. 5B, the raw fuel flow rate Qf increases as the generated power command value P_fc increases. However, when the generated power instruction value P_fc is small, the raw fuel flow rate Qf is constant. Further, the raw fuel flow rate Qf increases as the generated power command value P_fc increases, but the increase rate of the raw fuel flow rate Qf decreases as the generated power command value P_fc increases.

Next, the control unit 120 obtains the oxidant gas flow rate Qa according to the following formula (1), and controls the oxidant gas supply unit 30 so that the oxidant gas flow rate supplied to the cathode 61 becomes the oxidant gas flow rate Qa. (Step S8). By executing step S8, the oxidant gas flow rate can be controlled to a flow rate according to the required generated power P_req. In the following formula (1), “λ” is an excess air ratio (for example, 1.8), and “stc” is a theoretical mixing ratio.
Qa = Qf × λ × stc / 0.21 (1)

  Next, the control unit 120 obtains the reforming water flow rate Qw supplied to the reformer 40 by multiplying the raw fuel flow rate Qf by the target value sbyc of the S / C ratio, and is supplied to the reformer 40. The flow rate regulator 23 is controlled so that the reformed water flow rate becomes the reformed water flow rate Qw (step S9). By executing step S9, the amount of water vapor can be controlled appropriately. Thereafter, the control unit 120 ends the execution of the flowchart.

  When it determines with "No" in step S3, the control part 120 sets the target value sbyc of S / C ratio to the target value sbyc_dn (step S10). The target value sbyc_dn is a value (for example, 1.0) that is smaller than the normal target value sbyc_std (for example, 2.5) when the variation in the required generated power P_req is small. By executing step S10, the S / C target value can be varied in accordance with the variation direction of the required generated power P_req.

  Next, the control unit 120 sets the generated power command value P_fc to the requested generated power P_req (step S11). Next, the control part 120 performs step S7-step S9. In addition, when it determines with "No" in step S5, the control part 120 performs step S11 and step S7-step S9.

  When it is determined as “No” in step S <b> 2, the controller 120 determines that the estimated S / C ratio sbyc_p obtained from the detection result of the pressure sensor 91 is an excessive S / C sign determination value sbyc_lwr and an excessive S / C sign determination value. It is determined whether it is between sbyc_upr (step S12). By executing step S12, it can be determined whether or not the S / C ratio in the reforming unit 42 is within an appropriate range. When it determines with "No" in step S12, the control part 120 performs step S3.

  When it determines with "Yes" in step S12, the control part 120 sets the target value sbyc of S / C ratio to the normal target value sbyc_std (step S13). Next, the control unit 120 sets the generated power command value P_fc to the requested generated power P_req (step S14). Thereafter, the control unit 120 executes Steps S7 to S9.

  According to the flowchart of FIG. 5A, the target value of the S / C ratio can be increased or decreased in accordance with the increase / decrease direction of the required generated power required for the fuel cell device 130. Thereby, the response delay of the amount of steam supplied to the reformer 40 can be suppressed. As a result, the amount of water vapor can be controlled appropriately.

  FIG. 6 is a diagram for explaining the relationship between the required power of the general electric load 200 and the required generated power P_req for the fuel cell 60. In FIG. 6, the horizontal axis indicates elapsed time, and the vertical axis indicates power. Considering the auxiliary machine power consumption in the fuel cell device 130 and the loss in the inverter of the grid interconnection device 110, the required generated power P_req for the fuel cell 60 is larger than the required power for the general electric load 200.

  Referring to FIG. 6, as the required power of general electric load 200 increases, required generated power P_req for fuel cell 60 also increases. However, when the maximum point of inverter efficiency in the grid interconnection device 110 is set to the output rated point of the fuel cell 60, the required power of the general electric load 200 is larger and the required power and required power of the general electric load 200 are larger. The requested generated power P_req is set so that the difference from the generated power P_req is small.

  According to the flowchart of FIG. 5A, when the required generated power P_req for the fuel cell 60 increases, the generated power command value P_fc is set to be smaller by the power generation suppression power ΔP_fc, so the generated power command value P_fc Is smaller than the required generated power P_req.

  In addition, when the control unit 120 changes the S / C target value in accordance with the fluctuation of the required generated power P_req in step S4 or step S10, the control unit 120 is after a predetermined time has elapsed or when the S / C change amount reaches the predetermined amount. Alternatively, the S / C target value may be restored. Whether or not the S / C change amount has reached a predetermined amount can be determined based on the detection result of the pressure sensor 91. In addition, the control unit 120 may correct the predetermined time such that the predetermined time becomes longer as the fluctuation range of the required generated power P_req is larger. Further, the control unit 120 may determine the predetermined period based on a learning value based on a deviation between a correction control result corresponding to the fluctuation range of the required generated power P_req and a control expected value.

  In addition, when the control unit 120 determines in step S12 that the S / C ratio in the reforming unit 42 is not within the proper range and changes the S / C target value, the S / C ratio continues for a predetermined time or more. When returning to the proper range, the S / C target value may be returned to the original value. In this case, the control unit 120 may exclude short-time fluctuations in the detection result of the pressure sensor 91 using a low-pass filter. This is because erroneous control due to instantaneous fluctuations such as bumping of reformed water can be avoided.

(About bumping)
Next, vaporization (boiling) of reformed water will be described. As described above, the reformed water is vaporized in the vaporization unit 41. When the vaporization state of the reformed water shifts to bumping, the anode pressure may fluctuate rapidly. In this case, it becomes difficult to detect the reforming water flow rate using the pressure sensor 91. That is, it becomes difficult to detect the S / C ratio using the pressure sensor 91. Therefore, it is preferable to avoid bumping of the reformed water as much as possible.

  FIG. 7 is a graph showing the transition of the anode pressure when the reforming water supply amount is gradually increased while the combustion heat amount in the combustion chamber 50 is kept constant. In FIG. 7, the horizontal axis indicates the elapsed time, and the vertical axis indicates the S / C ratio, the anode pressure, the anode pressure fluctuation index, and the combustion part heat quantity. In addition, the value of S / C ratio is shown on the right vertical axis. The anode pressure fluctuation index is a standard deviation (the number of samples n = 50 in FIG. 7) of the difference between the anode pressure measured last time and the anode pressure measured this time. The amount of heat in the combustion section is the heat of combustion in the combustion chamber 50.

  As shown in FIG. 7, when the S / C ratio is small, the anode pressure fluctuation index is also small. When the S / C ratio is larger than a predetermined value, the anode pressure fluctuation index becomes equal to or greater than the predetermined value. When the C ratio becomes larger than a predetermined value, the anode pressure fluctuation index rapidly decreases. Therefore, it can be seen that bump boiling occurs when the S / C ratio is in the predetermined value range (the reforming water supply amount is in the predetermined range). On the other hand, depending on the S / C ratio, it can be seen that bumping can be avoided.

Here, the boiling curve of liquid water will be described. FIG. 8 is a diagram showing a boiling curve of liquid water, and is an experimental result showing a relationship between the temperature Tw of the heat transfer surface in the liquid water and the heat flux Q. In FIG. 8, the horizontal axis represents the logarithmic value (superheat degree log ΔT _sat ) of the difference ΔT _sat = T _w −T _sat between the heat transfer surface T _w and the saturation temperature T _sat, and the vertical axis represents the heat flux Q. The heat flux Q is the amount of heat that the liquid water takes away as water vapor.

Referring to FIG. 8, when the temperature of the heat transfer surface is low, boiling does not occur, and liquid water is dominated by heat transfer due to natural retention. When the heat transfer surface temperature gradually rises and ΔT _sat increases, boiling starts at point P2. Thereafter, the heat flux Q rises rapidly and rises almost linearly between the points P2 to P3. When the point P3 is exceeded, the rise of the heat flux Q slows down and reaches a critical heat flux point (CHF). In FIG. 8, the critical heat flux point is represented by point P4.

  Between the points P2 and P4, steam bubbles are generated intermittently from a fixed point. A specific point where the vapor bubbles are generated is referred to as a foaming point. The number of foaming points increases or decreases depending on the heat flux. The feature of boiling between the points P2 to P4 is the generation of vapor bubbles from the foaming point. Boiling between the points P2 to P4 is referred to as nucleate boiling. Around the point P3, the number of foaming points increases and the bubble generation cycle is shortened, and the violently generated bubbles merge with each other to partially cover the heat transfer surface.

When ΔT _sat is further increased, giant bubbles are generated, and these giant bubbles are further heated and ruptured, increasing the pressure of the system. This state is called bumping (or burnout). In this bumping state, the state of the liquid water instantaneously moves between points P4 to P5 to P6 to P7. If bumping occurs, the amount of water vapor produced will vary. In this case, it becomes difficult to control the S / C ratio using the pressure sensor 91.

  In addition, when a heat-transfer surface is 100 degreeC vicinity, the lifetime of a water droplet becomes the shortest. On the other hand, if the heat transfer surface further rises and bumping occurs, the life of the water droplets becomes longer. This is presumably because the process of generating, bursting, and vaporizing giant bubbles is performed during bumping. Thus, it becomes difficult to control the S / C ratio using the pressure sensor 91 due to bumping. Therefore, in order to control the S / C ratio more accurately using the pressure sensor 91, it is preferable not to bump the reforming water.

  Therefore, in the present embodiment, the frequency of bumping is suppressed by periodically changing at least one of the reforming water flow rate supplied to the reformer 40 and the raw fuel flow rate supplied to the reformer 40. To do. In addition, the boiling curve shown in FIG. 8 shifts according to the pressure to which liquid water is exposed. Specifically, as shown in FIG. 9, the boiling curve shifts to the low temperature side when the pressure to which the liquid water is exposed increases, and shifts to the high temperature side when the pressure to which the liquid water is exposed decreases. From such a result, the bumping frequency can be suppressed by periodically changing the pressure in the reformer 40. Accordingly, by periodically varying at least one of the reforming water flow rate supplied to the reformer 40, the raw fuel flow rate supplied to the reformer 40, and the pressure in the reformer 40, the bumping frequency Can be suppressed.

  FIG. 10 is a diagram illustrating an example of a flowchart representing bump boiling suppression control. The flowchart of FIG. 10 is executed when the S / C ratio is not changed from the target value. For example, in step S2 of FIG. 5A, it is determined that the fluctuation value | ΔP_req | If not, it is executed. Further, the flowchart of FIG. 10 is periodically executed, for example, every second.

  Referring to FIG. 10, control unit 120 reads fuel gas pressure p_and from pressure sensor 91 (step S21). Next, the control unit 120 calculates the anode pressure fluctuation σ_p_and (step S22). The anode pressure fluctuation σ_p_and is, for example, the standard deviation of 50 p_and samples. Next, the control unit 120 determines whether or not the anode pressure fluctuation σ_p_and exceeds a bump boiling determination value σ_ref (for example, about 0.15 kPa) (step S23). By executing step S23, it can be determined whether or not bumping of reformed water has occurred.

  When it determines with "Yes" in step S23, the control part 120 determines whether the control object correction coefficient k is over 1.0 (step S24). When it is determined as “Yes” in step S24, the control unit 120 substitutes the decrease side correction coefficient k_l for the control target correction coefficient k because the reforming water flow rate increase control is being performed (step S25). The reduction-side correction coefficient k_l is a value less than “1.0”, for example “0.8”.

Next, the control unit 120 obtains the reforming water flow rate Qw according to the following formula (2), and controls the flow rate regulator 23 so that the reforming water flow rate supplied to the reformer 40 becomes the reforming water flow rate Qw. (Step S26). In the following formula (2), Qw_base is a reforming water flow rate for controlling the S / C ratio to a target value. Thereafter, the control unit 120 ends the execution of the flowchart.
Qw = k × Qw_base (2)

  When it is determined “No” in step S24, the control unit 120 substitutes the increase-side correction coefficient k_h for the control target correction coefficient k because the reforming water flow rate reduction control is being performed. The increase side correction coefficient k_h is a value exceeding “1.0”, for example, “1.2”. Next, the control part 120 performs step S26, and complete | finishes the execution of a flowchart.

  Since the flowchart of FIG. 10 is periodically executed, the reforming water flow rate can be periodically increased or decreased. Thereby, bumping frequency can be suppressed. In addition, since the process is repeated until the anode pressure does not fluctuate, bumping can be avoided. In addition, what is necessary is just to replace Qw_base in step S26 with a raw fuel flow volume, when increasing / decreasing the raw fuel flow supplied to the reformer 40 periodically. When the anode pressure is periodically increased or decreased, Qw_base in step S26 may be replaced with the suction amount by the exhaust suction pump 90.

  In addition, in order to further suppress the bumping frequency, the cycle when the reforming water flow rate supplied to the reformer 40, the raw fuel flow rate supplied to the reformer 40, and the pressure in the reformer 40 are varied. The fluctuation range may be changed. Further, hysteresis may be set for the bump boiling determination value σ_ref. For example, until it is determined as “Yes” in Step S23, the bump boiling determination value σ_ref is set to a high value (for example, 0.15 kPa) to suppress unnecessary control. In Step S23, it is determined as “Yes” once. Thereafter, the bumping determination value σ_ref may be set lower (for example, 0.10 kPa) to avoid bumping more reliably.

  The above-described embodiments can be applied to any other type of fuel cell such as a solid polymer type, a solid oxide type, and a carbonated molten salt type. However, since the solid oxide fuel cell has an internal reforming function, the structure of the reforming part is simple. Therefore, when a solid oxide fuel cell is used, there is a tendency that the bump pressure is not suppressed by using the subsequent volume of the reforming unit. Therefore, the structure according to the above embodiment is particularly effective for the solid oxide fuel cell.

DESCRIPTION OF SYMBOLS 10 Raw fuel supply part 11 Raw fuel pump 12 Desulfurizer 20 Reformed water supply part 21 Reformed water tank 22 Pump 23 Flow regulator 30 Oxidant gas supply part 40 Reformer 41 Vaporization part 42 Reforming part 50 Combustion chamber 60 Fuel cell 61 Cathode 62 Anode 63 Electrolyte 70 Heat exchanger 80 Condensed water tank 90 Exhaust suction pump 91 Pressure sensor 100 Fuel cell system 110 System interconnection device 120 Control unit 130 Fuel cell device

Claims (12)

A reformer comprising: a vaporization unit that vaporizes reformed water; and a reforming unit that generates fuel gas by a steam reforming reaction between raw fuel and the reformed water vaporized in the vaporization unit;
An S / C control unit that controls a ratio S / C of the number of moles of water vapor to the number of moles of carbon in the raw fuel in the reformer,
The S / C control unit changes the target value of the S / C in the same direction as at least one of the increasing / decreasing directions of the fuel gas generation amount required for the reformer. .   The S / C control unit is configured to increase or decrease the amount of fuel gas generation required for the reformer when the amount of change in the amount of fuel gas generation required for the reformer exceeds a predetermined value. The fuel reforming system according to claim 1, wherein the S / C is changed in the same direction. A pressure sensor for detecting a gas pressure in the reformer;
The S / C control unit is configured such that a fluctuation amount of the fuel gas generation amount required for the reformer is not more than the predetermined value and a fluctuation amount of the gas pressure detected by the pressure sensor exceeds a threshold value. 3. The fuel reforming according to claim 2, wherein at least one of a flow rate of reforming water supplied to the reformer and a raw fuel flow rate supplied to the reformer is periodically changed. system.   4. The fuel according to claim 3, wherein the S / C control unit stops the periodic fluctuation when the fluctuation amount of the gas pressure detected by the pressure sensor becomes equal to or less than the threshold value. 5. Reforming system. A pressure sensor for detecting a gas pressure in the reformer;
Pressure reducing means for reducing the pressure in the reformer, and
The pressure reducing means is configured such that when the fluctuation amount of the fuel gas generation amount required for the reformer is equal to or less than the predetermined amount and the fluctuation amount of the gas pressure detected by the pressure sensor exceeds a threshold value, The fuel reforming system according to claim 2, wherein the pressure in the reformer is periodically changed.   6. The fuel reforming according to claim 5, wherein the pressure reducing means stops the periodic fluctuation when the fluctuation amount of the gas pressure detected by the pressure sensor becomes equal to or less than the threshold value. system.   The fuel reforming system according to any one of claims 3 to 6, wherein the fluctuation amount of the gas pressure is a standard deviation of the gas pressure detected by the pressure sensor.   When the S / C is changed, the S / C control unit sets the target value of the S / C to the original value after a predetermined time has elapsed or when the S / C change amount reaches a predetermined amount. It returns, The fuel reforming system as described in any one of Claims 1-7 characterized by the above-mentioned. A fuel cell for generating electricity using the fuel gas;
The fuel reforming system according to any one of claims 1 to 8, wherein a fuel gas generation amount required for the reformer correlates with a power generation amount required for the fuel cell.   The fuel reforming system according to claim 9, wherein the fuel cell is a solid oxide fuel cell.   11. The fuel reforming system according to claim 9, further comprising a combustion chamber that supplies heat obtained by burning off-gas of the fuel cell to the vaporization unit. A fuel reforming system comprising a reformer comprising: a vaporizing unit that vaporizes reformed water; and a reforming unit that generates fuel gas by a steam reforming reaction between raw fuel and the reformed water vaporized in the vaporizing unit. In
The target value of the ratio S / C of the number of moles of water vapor relative to the number of moles of carbon in the raw fuel in the reformer in the same direction as at least one of the increasing / decreasing directions of the amount of fuel gas required for the reformer A control method for a fuel reforming system, comprising an S / C control step for changing the fuel efficiency.

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