JP7723562B2 - Hydrogen production equipment including solid oxide fuel cell systems - Google Patents

Description

The present invention relates to a hydrogen production device including a solid oxide fuel cell system equipped with a cell stack that generates electricity through an electrochemical reaction (fuel cell reaction) between a reformed fuel gas obtained by reforming a fuel gas and an oxidant gas.

When hydrogen is burned or subjected to an electrochemical reaction (fuel cell reaction) to generate electricity (including hydrogen power generation in fuel cell vehicles), no carbon dioxide (hereinafter also referred to as "CO2") is emitted during the combustion or electrochemical reaction. However, the most common hydrogen production technologies are those that combine the conversion of natural gas into a hydrogen-rich gas using the steam reforming method with gas separation and purification using methods such as the pressure swing method, or those that produce hydrogen as a by-product in the steelmaking process. However, producing hydrogen using these methods results in the emission of carbon dioxide during the production stage.

For this reason, in recent years, the value of more environmentally friendly hydrogen (hereinafter also referred to as "H2") has been increasing, with attention also being paid to the amount of carbon dioxide emitted during the production stage, and low-CO2-emission hydrogen, which emits less carbon dioxide during production, is now recognized as having greater environmental value than hydrogen produced by conventional methods.

As a result, attention is being paid to a method of producing hydrogen using water electrolysis hydrogen generators that use electricity (natural energy-generated electricity) generated by natural energy power generation systems (such as solar power generation systems and wind power generation systems) that utilize natural energy such as sunlight and wind power. Known water electrolysis hydrogen generation systems include alkaline and solid polymer types, and both are commercially available. Anion exchange membrane water electrolysis and solid oxide steam electrolysis are also being developed as methods of producing hydrogen through electrolysis.

Due to the high equipment and electricity costs involved with producing hydrogen through water electrolysis, it cannot compete in terms of hydrogen production costs with mainstream methods such as reforming natural gas or producing hydrogen as a by-product from steelmaking. As a result, it is only used in limited applications, such as some industrial uses where there is stable local demand for hydrogen, and primarily for technology demonstration purposes. In the long term, it is expected that technological developments in water electrolysis hydrogen generation equipment will reduce equipment costs, but in the short to medium term, it will be desirable to produce hydrogen cheaply using expensive water electrolysis hydrogen generation equipment.

On the other hand, a well-known high-efficiency power generation technology for natural gas and other fuels is the solid oxide fuel cell system (hereinafter also referred to as "SOFC system"), which houses a solid oxide cell stack in a container, using a solid electrolyte as a membrane that conducts oxide ions. In this solid oxide fuel cell system, the cell stack is composed of multiple stacked fuel cells, with a fuel electrode (anode) on one side of the solid electrolyte in each fuel cell, and an oxygen electrode (cathode) on the other side. The operating temperature of the cell stack in this solid oxide fuel cell system is high, approximately 700 to 900°C. At such high temperatures, electricity is generated by an electrochemical reaction between the hydrogen, carbon monoxide, and hydrocarbons in the fuel gas (reformed fuel gas) and the oxygen in the air.

Such SOFC systems use hydrocarbon fuel gases such as natural gas or biofuel gas (a fuel gas composed of a mixture of methane and carbon dioxide) as the raw fuel gas. This SOFC system includes a reformer for, for example, steam reforming the hydrocarbon fuel gas, a fuel gas supply means for supplying fuel gas (raw fuel gas) to the reformer, an air supply means for supplying air as an oxidant gas, and a cell stack having a fuel electrode (anode) and an oxygen electrode (cathode). Reformed fuel gas from the reformer is supplied to the fuel electrode of the cell stack, and air from the air supply means is supplied to the oxygen electrode. Electricity is generated by an electrochemical reaction between the reformed fuel gas and the oxidant gas in the cell stack.

In this SOFC system, the cell stack and reformer are housed in a high-temperature space, and a high-temperature state is maintained by burning anode off-gas from the fuel electrode (anode) of the cell stack. For example, a combustor is housed in this high-temperature space, and anode off-gas and cathode off-gas from the cell stack are supplied to the combustor, and the high-temperature space is maintained at a high temperature using the combustion heat in this combustor (see, for example, Patent Document 1).

In such SOFC systems, anode off-gas and cathode off-gas from the cell stack are sent to a combustor and combusted in this combustor, resulting in combustion exhaust gas containing nitrogen and oxygen in addition to water and carbon dioxide. To capture carbon dioxide from such combustion exhaust gas, it is necessary to dissolve the carbon dioxide in an alkaline absorption solution and then apply heat to release the carbon dioxide (regenerating the absorption solution), which requires thermal energy for this release.

Furthermore, a method has been proposed that takes advantage of the high temperature operation of SOFC systems as a way to completely oxidize anode off-gas from the cell stack of an SOFC system to water and carbon dioxide (see, for example, Patent Document 2). This method uses an oxygen ion conductor or a mixed conductor that conducts oxygen ions and electrons, and, without nitrogen being mixed into the anode off-gas from the cell stack, the difference in oxygen activity between the anode off-gas and the cathode off-gas causes the oxygen in the cathode off-gas to react with the remaining hydrogen and carbon monoxide in the anode off-gas, resulting in almost complete oxidation.

This method using a selectively permeable membrane takes advantage of the high-temperature operation of the SOFC system, and because it is driven by the difference in oxygen activity between the anode off-gas and cathode off-gas, it has the advantage of not causing excessive reaction. However, the technological maturity of selectively permeable membranes is still low, which results in problems such as high costs and low reliability.

Japanese Patent Application Laid-Open No. 2008-21596 Japanese Patent Application Laid-Open No. 2000-3719

When producing hydrogen, using electricity generated by a natural energy power generation device (e.g., a solar power generation device) that utilizes natural energy as the main power source for a water electrolysis hydrogen generation device can reduce the amount of carbon dioxide emitted during hydrogen production, but there is a problem in that the electricity generated by the natural energy power generation device fluctuates depending on natural conditions.

For example, when using a solar power generation system, the amount of sunlight received that contributes to power generation is small during nighttime hours or during rainy or cloudy hours, resulting in an operating state where the power output of the solar power generation system is low or almost nonexistent. Furthermore, when using a wind power generation system, the wind force that contributes to power generation is weak during times of weak winds, resulting in an operating state where the power output of the wind power generation system is low or almost nonexistent.

When the natural energy power generation device is operating with low power output, the utilization rate of the hydrogen production equipment is low, making it difficult to produce hydrogen efficiently, resulting in the problem of higher hydrogen production costs.

It is also possible to use commercial electricity from a grid (commercial power supply) when the power output of a natural energy power generation device (for example, a solar power generation device) is low, but in such cases, carbon dioxide is emitted when commercial electricity is generated, making it difficult to sufficiently reduce carbon dioxide emissions per unit of hydrogen produced.

For this reason, when using a natural energy power generation device, it is possible to consider using a secondary battery as an auxiliary means to suppress fluctuations in power output. However, with a solar power generation device, for example, there are long periods of time when power output is low (at least half a day at night, and even longer periods on rainy days), so a large-capacity secondary battery is required to effectively suppress fluctuations in power output. While this is not a problem for the amount of power needed for a short period of time, such as a few tens of minutes, when trying to store the power needed for a long period of time, such as several hours, it is not easy to achieve economic viability when considering factors such as the cost of the secondary battery.

The object of the present invention is to provide a hydrogen production device that can assist in the supply of power to a water electrolysis hydrogen production device and economically produce hydrogen while minimizing carbon dioxide emissions during hydrogen production.

A hydrogen production apparatus according to a first aspect of the present invention includes a water electrolysis hydrogen generation apparatus that electrolyzes water to generate hydrogen and oxygen, a solid oxide fuel cell system that generates electricity using a hydrocarbon fuel gas as a raw fuel gas, and a power supply controller that controls drive power for operating the water electrolysis hydrogen generation apparatus,
The solid oxide fuel cell system includes a reformer that reforms a raw fuel gas to generate a reformed fuel gas, a cell stack that generates electricity by an electrochemical reaction between the reformed fuel gas from the reformer and an oxidant gas, and a combustor that combusts an anode off-gas from an anode of the cell stack,
The oxygen produced in the water electrolysis hydrogen generation apparatus is supplied to the combustor through an oxygen supply line, and the battery-generated power generated in the cell stack is supplied to the water electrolysis hydrogen generation apparatus via the power supply controller.

Furthermore, in the hydrogen production device described in claim 2 of the present invention, the rated power generation output of the solid oxide fuel cell system is 30% or less of the rated power consumption of the water electrolysis hydrogen production device.

Furthermore, in the hydrogen production device described in claim 3 of the present invention, natural energy-generated power from a natural energy power generation device that generates power using natural energy, commercial power supply from a grid power source, and the cell-generated power from the solid oxide fuel cell system are supplied to the power supply controller, and the power supply controller preferentially supplies the natural energy-generated power and the cell-generated power to the water electrolysis hydrogen production device.

Furthermore, in the hydrogen production device described in claim 4 of the present invention, the solid oxide fuel cell system further includes a condenser that condenses moisture contained in the combustion exhaust gas from the combustor, a gas-liquid separator that separates the condensed water condensed in the condenser, and an exhaust gas recovery and purification line that recovers and purifies the combustion exhaust gas from which the condensed water has been separated by the gas-liquid separator, and carbon dioxide is recovered and purified through the exhaust gas recovery and purification line.

The hydrogen production apparatus described in claim 1 of the present invention includes a water electrolysis hydrogen generator that electrolyzes water to produce hydrogen and oxygen, a solid oxide fuel cell system that generates electricity using a hydrocarbon fuel gas as raw fuel gas, and a power supply controller that controls the driving power supplied to the water electrolysis hydrogen generator. The oxygen generated by the water electrolysis hydrogen generator is supplied to the combustor of the solid oxide fuel cell system via an oxygen supply line, so that the oxygen generated by the water electrolysis hydrogen generator can be supplied to this combustor and effectively used to combust anode off-gas. Furthermore, the cell-generated power generated by the cell stack is supplied to the water electrolysis hydrogen generator via the power supply controller, so that the power generated by this cell stack can be used as part of the driving power for the water electrolysis hydrogen generator, thereby increasing the driving power of the water electrolysis hydrogen generator and improving its capacity utilization rate.

Furthermore, according to the hydrogen production device described in claim 2 of the present invention, the rated power output of the solid oxide fuel cell system is 30% or less of the rated power consumption of the water electrolysis hydrogen generation device, so the water electrolysis hydrogen generation device can be operated with an effective facility utilization rate.

Furthermore, according to the hydrogen production device described in claim 3 of the present invention, natural energy-generated power from a natural energy power generation device that generates power using natural energy, commercial power supply from a grid power source, and battery-generated power from a solid oxide fuel cell system are supplied to a power supply controller, and the power supply controller preferentially supplies the natural energy-generated power and battery-generated power to the water electrolysis hydrogen production device, thereby reducing consumption of commercial power supply from the grid power source and reducing carbon dioxide emissions during hydrogen production. Note that natural energy power generation devices include solar power generation devices that use sunlight and wind power generation devices that use wind power.

Furthermore, according to the hydrogen production device described in claim 4 of the present invention, the combustion exhaust gas from the combustor of the solid oxide fuel cell system is sent to a condenser and a gas-liquid separator, the moisture in the combustion exhaust gas is condensed in the condenser, and this condensed water is separated in the gas-liquid separator, after which it is recovered and purified through the exhaust gas recovery and purification line, thereby reducing carbon dioxide emissions from this solid oxide fuel cell system.

In a solid oxide fuel cell system using this hydrogen production device, hydrocarbon fuel gas is used as the raw fuel gas. This fuel gas is reformed in a reformer to produce reformed fuel gas. This reformed fuel gas is then sent to the fuel electrode (anode) of the cell stack to generate electricity through an electrochemical reaction. The anode off-gas from the cell stack is then sent to the combustor, and oxygen generated in the water electrolysis hydrogen generator is also sent to the combustor, where it is burned using this oxygen. In such an SOFC system, the combustion exhaust gas from the combustor becomes carbon dioxide containing moisture. However, the moisture is condensed in a condenser and then separated in a gas-liquid separator, leaving only the remaining carbon dioxide to flow through the exhaust gas recovery and purification line. Thus, by recovering and purifying the combustion exhaust gas (carbon dioxide) flowing through the exhaust gas recovery and purification line, carbon dioxide emissions from the solid oxide fuel cell system can be reduced.

1 is a block diagram showing a simplified embodiment of a hydrogen production device according to the present invention; FIG. 2 is a diagram showing a simplified embodiment of a solid oxide fuel cell system used in the hydrogen production device of FIG. 1 . FIG. 1 is a diagram showing a typical example of the relationship between the time of day of a solar power generation device and its relative output. FIG. 10 is a diagram showing the relationship between the ratio of the output power of the SOFC system to the input power of the water electrolysis hydrogen generator and the improvement effect index per SOFC power generation output. FIG. 10 is a diagram for explaining the difference in capacity factor before and after adding an SOFC system in a water electrolysis hydrogen generation apparatus, and the improvement effect index per SOFC power generation output when an SOFC system is added.

Hereinafter, a hydrogen production device including a solid oxide fuel cell system according to the present invention will be described with reference to the accompanying drawings.

First, one embodiment of a hydrogen production device will be described with reference to Figure 1. In Figure 1, the illustrated hydrogen production device 1 includes a water electrolysis hydrogen production device 2 that produces hydrogen by electrolyzing water, a solar power generation device 4 that generates electricity using sunlight (constituting a natural energy power generation device that generates electricity using natural energy), and a solid oxide fuel cell system 6 that generates electricity using hydrocarbon fuel gas as raw fuel gas.

The water electrolysis hydrogen generator 2 may be of any known type that electrolyzes water to produce hydrogen. The hydrogen produced by the electrolysis of water is the target product, which is stored in a hydrogen tank 10 or the like via a hydrogen supply line 8, or is supplied to downstream hydrogen-using equipment (not shown) for consumption. The produced hydrogen can also be used as fuel gas for fuel cell vehicles, industrial or residential fuel cells, etc.

Furthermore, oxygen is produced as a by-product during water electrolysis. As described below, this by-product (oxygen) is supplied to the solid oxide fuel cell system 6 through the oxygen supply line 12 at a pressure of, for example, approximately 200 kPa and consumed. By using this oxygen in the solid oxide fuel cell system 6, the by-product (oxygen) produced during water electrolysis can be effectively utilized. Furthermore, a portion of this by-product (oxygen) (excess oxygen) may be stored in, for example, an oxygen tank 16 via the oxygen storage line 12.

The power used for water electrolysis in the water electrolysis hydrogen generator 2 is supplied from a power supply controller 18. This power supply controller 18 is supplied with solar-generated power (natural energy-generated power) from the solar power generation device 4, battery-generated power from the solid oxide fuel cell system 6, and commercial power from a grid power supply 21 (e.g., a 200V commercial power supply). The power supply controller 18 selects one or more of the solar-generated power, battery-generated power, and commercial power to operate the water electrolysis hydrogen generator 2, and supplies the required power.

In this embodiment, the power supply controller 18 controls the use of solar-generated power from the solar power generation device 4 and battery-generated power from the solid oxide fuel cell system 6, and controls the use of commercially available power when there is a significant power shortage even after using the solar-generated power and battery-generated power. By controlling in this manner, solar-generated power and battery-generated power are used preferentially, and as will be understood from the description below, hydrogen production costs can be reduced and carbon dioxide emissions can also be kept low.

As can be seen from the above description, in order to efficiently produce hydrogen and minimize carbon dioxide emissions, the hydrogen production device 1 can be operated using solar-generated power from the solar power generation device 4 and cell-generated power from the solid oxide fuel cell system 6. In this case, even when solar-generated power from the solar power generation device 4 is not available, the water electrolysis hydrogen generation device 2 can be operated using cell-generated power from the solid oxide fuel cell system 6, thereby generating oxygen, a by-product of water electrolysis, and supplying this oxygen to the solid oxide fuel cell system 6. In this case, any surplus power can be addressed by reducing the power generation output of the solid oxide fuel cell system 6.

Next, referring primarily to Figure 2, the solid oxide fuel cell system 6 (SOFC system) used in this hydrogen production device 1 will be described. In Figure 2, the solid oxide fuel cell system 6 (SOFC system) shown consumes hydrocarbon fuel gas (e.g., city gas, LP gas, biogas, etc.) as raw fuel gas to generate electricity, and includes a reformer 20 for reforming the fuel gas, and a solid oxide cell stack 22 that generates electricity through an electrochemical reaction (fuel cell reaction) between the reformed fuel gas reformed in the reformer 20 and air as an oxidant gas.

The cell stack 22 is constructed by stacking multiple solid oxide fuel cells via interconnector plates to generate electricity through electrochemical reactions. Although not shown, it includes a solid electrolyte 24 that conducts oxygen ions, a fuel electrode 26 (anode) provided on one side of the solid electrolyte 24, and an oxygen electrode 28 (cathode) provided on the other side of the solid electrolyte 24. The solid electrolyte 24 is made of, for example, zirconia doped with yttria.

The fuel electrode 26 side of this cell stack 22 is connected to the reformer 20 via a reformed fuel gas supply line 30. In this configuration, the reformer 20 is configured as an integrated unit with a vaporizer 32 for vaporizing the reforming water. The vaporizer 32 may also be configured separately from the reformer 20, and the water vapor vaporized by the vaporizer 32 may be supplied to the reformer 20 via a water vapor supply line (not shown).

This vaporizer 32 is connected to a water supply source (not shown) (e.g., consisting of a water tank or water recovery tank) via a water supply line 34, and reforming water from the water supply source is supplied to the vaporizer 32 via the water supply line 34. The reformer 20 contains a reforming catalyst, such as alumina supported ruthenium, which steam reforms the raw fuel gas supplied via the fuel gas supply line 36 with the steam vaporized in the vaporizer 32.

This fuel gas supply line 36 is equipped with a fuel supply pump 38 (fuel gas supply means), a fuel flow meter 40 (flow rate sensor), a desulfurizer 42, and a shutoff solenoid valve 44, arranged in this order from the vaporizer 32 upstream. The desulfurizer 42 removes sulfur components contained in the raw fuel gas (sulfur components in the odorant), and the shutoff solenoid valve 44 closes to shut off the fuel gas supply line 36 when the supply of raw fuel gas is stopped.

Furthermore, the fuel supply pump 38 pressurizes the raw fuel gas flowing through the fuel gas supply line 36 and supplies it to the vaporizer 32, the fuel flow meter 40 measures the flow rate of the raw fuel gas flowing through the fuel gas supply line 36, and the system control unit (not shown) of the SOFC system 6 compares the set flow rate value of the raw fuel gas with the flow rate value measured by the fuel flow meter 40, and if this measured flow rate value is smaller (or larger) than the set flow rate value, increases (or decreases) the rotation speed of the fuel supply pump 38, thereby adjusting the supply flow rate of the raw fuel gas to the set flow rate value of the SOFC system.

A water supply pump 46 (water supply means) is also provided in the water supply line 34, and this water supply pump 46 supplies reforming water from a water supply source (not shown) to the vaporizer 32 through the water supply line 34. A system control unit (not shown) of the SOFC system 6 controls the rotation speed of this water supply pump 46, and increases (or decreases) the rotation speed of the water supply pump 40 when the supply flow rate of the reforming water is less (or more) than the set flow rate value, thereby adjusting the supply flow rate of the reforming water to the set flow rate value.

Furthermore, the oxygen electrode 28 side of the cell stack 22 is connected to an air blower 50 (air supply means) via an air supply line 48. The air blower 50 supplies air as an oxidant gas to the oxygen electrode 28 side of the cell stack 22 through the air supply line 48. An air flow meter 52 that measures the air flow rate is disposed on this air supply line 48, and a system control unit (not shown) of the SOFC system 6 compares the set air flow rate value with the flow rate value measured by the air flow meter 52, and increases (or decreases) the rotation speed of the air blower 50 when the measured flow rate value is smaller (or larger) than the set flow rate value, thereby adjusting the air supply flow rate to the set flow rate value of the SOFC system.

This SOFC system 6 is equipped with a combustor 54 that combusts anode offgas from the fuel electrode 26 (anode) of the cell stack 22. More specifically, the exhaust side of the fuel electrode 26 of the cell stack 22 is connected to the combustor 54 via an anode offgas feed line 56, and the anode offgas from the cell stack 22 is fed to the combustor 54 via this anode offgas feed line 56. An oxygen feed line 12 from the water electrolysis hydrogen generator 12 is connected to the combustor 54, and an electromagnetic shut-off valve 58 and a mass flow controller 60 (oxygen flow rate control means) are provided on the oxygen feed line 12. The mass flow controller 60 controls the flow rate of oxygen flowing through the oxygen feed line 12 as described below, and feeds it downstream to the combustor 54. When the oxygen supply is stopped, the electromagnetic shut-off valve 58 closes to shut off the oxygen feed line 12.

A first heat exchanger 62 is also provided to allow heat exchange between the cathode offgas discharged from the oxygen electrode 28 (cathode) of the cell stack 22 and the air flowing through the air supply line 48. A cathode offgas discharge line 64 is provided on the discharge side of the oxygen electrode 28 of the cell stack 22, and a first heat exchanger 62 is disposed in this cathode offgas discharge line 64. In this first heat exchanger 62, heat is exchanged between the cathode offgas flowing through the cathode offgas discharge line 64 and the air flowing through the air supply line 48, and the air heated by this heat exchange is supplied to the oxygen electrode 28 side of the cell stack 22.

Furthermore, a second heat exchanger 66 is provided to allow heat exchange between the combustion exhaust gas discharged from the combustor 54 and the air flowing through the air supply line 48. A combustion exhaust gas discharge line 68 is provided on the exhaust side of the combustor 54, and a second heat exchanger 66 is disposed in this combustion exhaust gas discharge line 68. In this second heat exchanger 66, heat exchange occurs between the combustion exhaust gas flowing through the combustion exhaust gas discharge line 68 and the air flowing through the air supply line 48. With this configuration, the air from the air blower 50 is heated by heat exchange with the cathode off-gas in the first heat exchanger 62, and is further heated by heat exchange with the combustion exhaust gas in the second heat exchanger 66. This air, heated in two stages, is supplied to the oxygen electrode 28 side of the cell stack 22.

In this SOFC system 6, the reformer 20 and vaporizer 32 are disposed in contact with or close to the combustor 54, and the reformer 20 and vaporizer 32 are heated and maintained at a predetermined temperature by the heat of combustion generated by the combustion of anode off-gas in the combustor 54. The cell stack 22, combustor 54, reformer 20, vaporizer 32, first heat exchanger 62, and second heat exchanger 66 are housed in a high-temperature space 82 surrounded by thermal insulation (not shown), and the combustion heat from the combustor 54 maintains the high-temperature state within this high-temperature space 82.

To minimize carbon dioxide emissions, the SOFC system 6 is further configured as follows: A third heat exchanger 70 and a gas-liquid separator 72 are disposed downstream in this order in the combustion exhaust gas discharge line 68. The third heat exchanger 70 functions as a condenser for condensing moisture in the combustion exhaust gas. For example, the third heat exchanger 70 may be a heat exchanger used to store the thermal energy of the combustion exhaust gas as hot water in a hot water storage tank (not shown) of a hot water storage device. The third heat exchanger 70 exchanges heat between water from the hot water storage tank (not shown) of the hot water storage device and the combustion exhaust gas flowing through the combustion exhaust gas discharge line 68. This heat exchange cools the combustion exhaust gas from the combustor 54 and condenses the moisture contained therein, while the hot water heated by the heat exchange is stored in the hot water storage tank. The condenser does not have to be a third heat exchanger 70 that exchanges heat with water from a hot water storage device; it can also be one that exchanges heat with cooling water, or with air, as long as it cools the combustion exhaust gas by heat exchange and condenses the moisture contained therein.

Meanwhile, the combustion exhaust gas cooled by heat exchange flows further downstream through the combustion exhaust gas discharge line 68, where the condensed water condensed in the third heat exchanger 70 is separated by the gas-liquid separator 72. The gas-liquid separator 72 is composed of, for example, a drain separator. The moisture contained in the combustion exhaust gas is cooled and condensed by heat exchange in the third heat exchanger 70, and the condensed water is separated by this gas-liquid separator 72. The combustion exhaust gas from which the moisture has been separated flows further downstream. The condensed water separated by the drain separator may be recovered, for example, in a water recovery tank (not shown) and reused as reforming water.

A three-way valve 74 is disposed downstream of the exhaust gas recovery line 68. One discharge side of the three-way valve 74 is connected to an exhaust gas recovery and purification line 76 (specifically, a carbon dioxide recovery and purification line), and the other discharge side is connected to an exhaust gas treatment line 78. When this three-way valve 74 is in a first switching state, it connects the combustion exhaust gas discharge line 68 to the exhaust gas recovery and purification line 76, and the combustion exhaust gas from the combustion exhaust gas discharge line 68 flows into the exhaust gas recovery and purification line 76 to be recovered and purified. When it is in a second switching state, it connects the combustion exhaust gas discharge line 68 to the exhaust gas treatment line 78, and the combustion exhaust gas from the combustion exhaust gas discharge line 68 flows into the exhaust gas treatment line 78 to be treated as required.

In the solid oxide fuel cell system 6, the operating state is stable during rated operation, so the anode off-gas from the cell stack 22 can be completely combusted in the combustor 54 using oxygen from the water electrolysis hydrogen generator 2. In such a case, the three-way valve 74 is held in the first switching state, and carbon dioxide as combustion exhaust gas is recovered and purified through the exhaust gas recovery and purification line 76. On the other hand, during startup operation or low-output operation, it is difficult to completely combust the anode off-gas from the cell stack 22 using oxygen in the combustor 54. In such a case, the three-way valve 74 is held in the second switching state, and the combustion exhaust gas flows through the exhaust gas treatment line 78, where it is treated as required and then released, for example, into the atmosphere.

The combustion exhaust gas (CO2) recovered through this exhaust gas recovery and purification line 76 can be used to produce synthetic fuels (jet fuel, liquid fuels such as methanol, and gas fuels such as methane and propane), and can also be used to synthesize chemical products such as olefins and urethanes.

This hydrogen production device 1 can reduce carbon dioxide emissions during hydrogen production, and this SOFC system 6 can also reduce carbon dioxide emissions during power generation operation. This will be explained below. Power generation is performed using hydrocarbon fuel gas (e.g., natural gas, biogas (mainly methane or CO2), etc.) as the raw fuel gas for the SOFC system 6. When the raw fuel gas is completely oxidized without being mixed with the air supplied to the cell stack 22, the products are carbon dioxide (CO2) and water (H2O). When these products are cooled and the moisture is removed, the carbon dioxide remains in the combustion exhaust gas and can be separated. The separated carbon dioxide can be recovered and purified by supplying it to the exhaust gas recovery and purification line 76.

More specifically, when the operating conditions of the SOFC system 6 are such that the fuel utilization rate is, for example, 80%, oxygen from the oxygen electrode 28 side of the cell stack 22 is supplied to the anode 26 side through the electrolyte 24 so that 80% of the raw fuel gas undergoes an electrochemical reaction. As a result, the gas composition at the exhaust outlet of the anode 26 is such that the reformed fuel gas is partially oxidized with pure oxygen to, for example, 80%, and contains hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and water (H2O). Because the typical operating temperature of the cell stack 22 in the SOFC system 6 is 700°C or higher and a large amount of water is present, the methane content is extremely low at less than 0.1%.

To completely oxidize this gas into CO2 and H2O, the anode off-gas from the cell stack 22 is sent to the combustor 54, and oxygen generated as a by-product in the water electrolysis hydrogen generator 2 is also sent to the combustor 54, where the anode off-gas is combusted with oxygen. When the fuel utilization rate of the cell stack 22 is 80%, the amount of oxygen required to burn 20% of the raw fuel gas is sent from the water electrolysis hydrogen generator 2 to the combustor 54 of the SOFC system 6 via the oxygen supply line 12. By controlling the amount of oxygen supplied from the water electrolysis hydrogen generator 2 in this manner, the anode off-gas from the cell stack 22 can be completely oxidized into CO2 and H2O.

When oxygen (so-called pure oxygen) from the water electrolysis hydrogen generator 2 is supplied to the combustor 54 in the high-temperature space 82 of the SOFC system 6, the rate of oxidation degradation of the cell stack 22 due to contact with pure oxygen or high-concentration oxygen is greater than the rate of oxidation degradation due to contact with air. Therefore, it is desirable to limit the contact of pure oxygen to the combustor 54 itself and the supply pipe to it (oxygen supply line 12). Furthermore, the combustion conditions for the anode off-gas in the combustor 54 are desirably stoichiometric (stoichiometric) combustion, which results in a post-combustion composition consisting only of H2O and CO2. This allows the post-combustion combustion exhaust gas to be cooled to separate the condensed water, and the remaining CO2 to be recovered via the exhaust gas recovery line 76.

The above-described hydrogen production system 1, which combines a water electrolysis hydrogen generator 2 with a natural energy power generation device (e.g., a solar power generation device 4) and an SOFC system 6, is particularly effective in reducing hydrogen production costs. The power generated by a natural energy power generation device (e.g., a solar power generation device 4) fluctuates greatly, with periods of low output often lasting for several hours or more. However, by combining this natural energy power generation device with an SOFC system 6, the battery-generated power from the SOFC system 6 can be supplied to the water electrolysis hydrogen generator 2, thereby increasing the capacity utilization rate of the water electrolysis hydrogen generator 2. This increase in capacity utilization rate has a significant effect, especially when the capacity utilization rate of the water electrolysis hydrogen generator 6 is low.

Next, we will explain the effect of improving the capacity factor (defined here as the capacity factor relative to rated operation over calendar hours) of the water electrolysis hydrogen generation equipment using battery-generated power from the SOFC system.

This section explains the case where the goal is to reduce CO2 emissions during hydrogen production. If the power source for the water electrolysis hydrogen generator is, for example, a solar power generation system, fluctuations in the power generation output will determine the capacity factor of the water electrolysis hydrogen generator (defined here as (calendar time) x (utilization factor for rated operation)). This makes it possible to calculate the fluctuating portion of the hydrogen production cost (yen/Nm3). The relationship between the capacity factor and the portion of the hydrogen production cost (yen/Nm3) corresponding to the capacity factor is proportional to 1/u, where u is the capacity factor. Therefore, for example, if the capacity factor increases from 0.7 to 0.8, the effect will be significantly greater than if the capacity factor increases from 0.3 to 0.4.

The solar power generation system 4 has large fluctuations in the output of solar power generation electricity, and the power output of this solar power generation system 4 may be low, resulting in a low capacity utilization rate for the water electrolysis hydrogen generation system. However, by using the SOFC system 6 as part of the power source for the water electrolysis hydrogen generation system 2 and supplying battery-generated power from this SOFC system 6, particularly when the capacity utilization rate is low, it is possible to achieve results with a relatively small increase in power generation capacity, and the economic viability of hydrogen production is likely to be improved despite an increase in the equipment costs associated with the SOFC system 6.

Here, we will estimate the economic improvement effect when a water electrolysis hydrogen generation system is combined with a solar power generation system and an SOFC system. Figure 4 shows a typical example of the power generation output of a solar power generation system by time of day. In Figure 4, the solid line A shows the relative output on a sunny day, the dashed line B shows the relative output on a cloudy day, and the dashed line C shows the relative output on a rainy day. Figure 4 is a simplified representation of a typical example, with the maximum power generation output on a sunny day set to 1.0, and the relative output to this maximum power generation output is shown. For example, the New Energy and Industrial Technology Development Organization and others have published detailed databases by region and by sunshine, making it possible to carry out detailed calculations.

In this calculation, we assumed four sunny days, four cloudy days, and two rainy days over a 10-day period. We calculated the capacity factor (utilization rate per calendar hour) of the water electrolysis hydrogen generator when the power output of the solar power generator was connected to the water electrolysis hydrogen generator. While the power output of the solar power generator fluctuates within an hour, these small fluctuations can be absorbed by a small-capacity power storage function in either the solar power generator or the water electrolysis hydrogen generator. Therefore, we calculated the capacity factor as fluctuating in hourly increments, as shown in Figure 4. In this calculation, the maximum power output of the solar power generator (the power output when sunlight is strongest on a sunny day) was set to 1.0, and the rated input power (i.e., the device capacity) to the water electrolysis hydrogen generator was set to 0.6, 0.4, and 0.2. We also assumed that the water electrolysis hydrogen generator can produce hydrogen proportional to the partial input. That is, if the amount of hydrogen produced when "1" amount of power is supplied to the water electrolysis hydrogen generation device is "1," then the amount of hydrogen produced when "0.5" amount of power is supplied was calculated to be "0.5." The capacity factor of the water electrolysis hydrogen generation device over a 10-day period was then calculated for a hydrogen production device that did not incorporate an SOFC system. After that, the capacity factor of the water electrolysis hydrogen generation device over a 10-day period was calculated for a hydrogen production device that incorporated an SOFC system.

Figure 5 shows the calculated capacity utilization rate of the water electrolysis hydrogen generation equipment before and after adding the SOFC system. As shown in these calculation results, adding an SOFC system increases the capacity utilization rate of the water electrolysis hydrogen generation equipment, improving its capacity utilization rate.

Furthermore, the effect of this improvement in capacity factor was calculated by taking the capacity factor as u, (1/u) as the capital cost improvement index, and the difference in (1/u) between the presence and absence of the SOFC system as the capital cost improvement index for adding the SOFC system. This capital cost improvement index for adding the SOFC system was then divided by the power output of the SOFC system to calculate the improvement effect index per power output of the SOFC system. The results of this calculation are shown in Figure 5, and the trend of the improvement effect index is shown in Figure 4.

Figure 5 shows calculations for the rated power consumption of the water electrolysis hydrogen generator set to 0.2, 0.4, and 0.6, relative to the power output of the solar power generation system (1.0) in Figure 3. This calculation shows that even when the relationship between the rated input power of the water electrolysis hydrogen generator and the maximum output power of the solar power generation system changes (varying between 0.2 and 0.6), the improvement in the equipment cost of the water electrolysis hydrogen generator per SOFC system does not change significantly, and actually decreases as the proportion of SOFC systems introduced increases. It is clear from past performance and technical characteristics (increasing the power output requires increasing the number of fuel cells) that SOFC systems generally tend to have limited economies of scale in terms of cost and efficiency. While this must be taken into account in the details, the calculation results in Figures 4 and 5 show that in a hydrogen production system combining a solar power generation system and an SOFC system, the power output of the SOFC system relative to the rated power input of the water electrolysis hydrogen generator should be at most 0.3, and preferably 0.2 or less.

Although one embodiment of the hydrogen production device according to the present invention has been described above, the present invention is not limited to this embodiment, and various changes and modifications can be made without departing from the scope of the present invention.

For example, in the hydrogen production device described above, a water electrolysis hydrogen generation device is combined with a solar power generation device and an SOFC system as a natural energy power generation device, but instead of this combination, the same effects as described above can be obtained by combining a wind power generation device and an SOFC system as a natural energy power generation device.

Furthermore, for example, the hydrogen production device described above employs both the technical feature of consuming the by-product (oxygen) of the water electrolysis hydrogen generation device in the SOFC system and the technical feature of recovering the combustion exhaust gas (carbon dioxide) emitted from the SOFC system, but it is also possible to employ only one of these technical features.

1 Hydrogen production device 2 Water electrolysis hydrogen generation device 4 Solar power generation device 6 Solid oxide fuel cell system (SOFC system)
12 oxygen supply line 18 power supply controller 20 reformer 22 cell stack 54 combustor 60 mass flow controller 70 third heat exchanger 72 gas-liquid separator 74 three-way valve 76 exhaust gas recovery and purification line 78 exhaust gas treatment line

Claims (4)

The system comprises a water electrolysis hydrogen generator that electrolyzes water to generate hydrogen and oxygen, a solid oxide fuel cell system that generates electricity using a hydrocarbon fuel gas as a raw fuel gas, and a power supply controller that controls the drive power for operating the water electrolysis hydrogen generator,
The solid oxide fuel cell system includes a reformer that reforms a raw fuel gas to generate a reformed fuel gas, a cell stack that generates electricity by an electrochemical reaction between the reformed fuel gas from the reformer and an oxidant gas, and a combustor that combusts an anode off-gas from an anode of the cell stack,
a hydrogen production apparatus, characterized in that oxygen produced in the water electrolysis hydrogen generation apparatus is supplied to the combustor through an oxygen supply line, and battery-generated power generated in the cell stack is supplied to the water electrolysis hydrogen generation apparatus via the power supply controller. The hydrogen production device described in claim 1, characterized in that the rated power generation output of the solid oxide fuel cell system is 30% or less of the rated power consumption of the water electrolysis hydrogen generation device. The hydrogen production device described in claim 1 or 2, characterized in that the power supply controller is supplied with natural energy-generated power from a natural energy power generation device that generates power using natural energy, commercial power supply from a grid power source, and the cell-generated power from the solid oxide fuel cell system, and the power supply controller preferentially supplies the natural energy-generated power and the cell-generated power to the water electrolysis hydrogen production device. The hydrogen production device described in any one of claims 1 to 3, characterized in that the solid oxide fuel cell system further includes a condenser that condenses moisture contained in the combustion exhaust gas from the combustor, a gas-liquid separator that separates the condensed water condensed in the condenser, and an exhaust gas recovery and purification line that recovers and purifies the combustion exhaust gas from which the condensed water has been separated by the gas-liquid separator, and carbon dioxide is recovered and purified through the exhaust gas recovery and purification line.