System and method for treating off-gas discharged from fuel cells

Abstract

To provide a system and a method for processing an off-gas discharged from a fuel cell capable of converting an off-gas that is discharged from a fuel cell and contains carbon dioxide, hydrogen, and water at a specific ratio to a gas containing methane.SOLUTION: A processing method of an off-gas discharged from a fuel cell includes removing water from an off-gas that is discharged from a fuel cell and contains carbon dioxide of 10 vol.% or more and 40 vol.% or less, hydrogen of 5 vol.% or more and 20 vol.% or less, and water of 40 vol.% or more and 80 vol.% or less to obtain a CO2 rich gas of a low water content, adding hydrogen to the obtained CO2 rich gas of a low water content to obtain a mixed gas, and reacting the mixed gas in the presence of a CO2 methanation catalyst to obtain a methane rich gas.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing off-gas discharged from a fuel cell. of Processing Reason Stem and process More specifically, the present invention relates to a method for converting a specific proportion of carbon dioxide, hydrogen, and water from an off-gas discharged from a fuel cell into a gas containing methane. of Processing Reason Stem and process It concerns the method. [Background technology]

[0002] There is a known technology to produce methane by reducing carbon dioxide, a greenhouse gas, with hydrogen. However, comprehensive consideration is required from the standpoints of power supply, equipment costs, operating costs, carbon dioxide capture efficiency, thermal efficiency, etc., in order to capture carbon dioxide and utilize the methane obtained by the methanation reaction.

[0003] For example, Patent Document 1 discloses a fuel cell power generation system in which fuel and an oxidant are supplied to generate power in a fuel cell, the fuel cell comprising a fuel electrode to which the fuel or a reformed gas produced by reforming the fuel is supplied, and an electrolyte that allows anions to pass through, and a methane generation means to which a fuel electrode exhaust gas discharged from the fuel electrode is supplied and which reacts carbon monoxide and / or carbon dioxide in the fuel electrode exhaust gas with hydrogen to convert it into methane.

[0004] Patent Document 2 discloses a fuel cell power generation system that includes a fuel cell that generates electricity by chemically reacting hydrogen contained in a fossil fuel with oxygen in the air, carbon dioxide separation means that separates carbon dioxide contained in exhaust gas discharged from the fuel cell, hydrocarbon production means that produces hydrocarbons by reacting the carbon dioxide separated by the carbon dioxide separation means with hydrogen, and hydrogen supply means that supplies hydrogen to the hydrocarbon production means, and that supplies the hydrocarbons produced by the hydrocarbon production means to the fuel cell.

[0005] Patent Document 3 discloses a reversible system for generating electricity using a solid oxide fuel cell (SOFC), including a fuel cell (SOFC) including at least one solid oxide individual electrochemical cell, each formed of a cathode, an anode, and an electrolyte inserted between the cathode and the anode, a gas-liquid phase separator connected to the outlet of the fuel cell, a methanation reactor suitable for carrying out a methanation reaction, the inlet of which is connected to the outlet of the phase separator, the outlet of which is connected to the inlet of the fuel cell, and a mixture resulting from the methanation reactor is introduced into the fuel cell, and a reversible hydrogen storage tank suitable for storing hydrogen, the outlet of which is connected to the inlet of the methanation reactor.

[0006] Patent Document 4 discloses an operation system for a distributed power source provided for the purpose of stable operation of a grid power, characterized in having at least hydrogen obtained from surplus energy of a distributed power source (e.g., a fuel cell) that is a power source other than grid power and has restrictions on load fluctuations using city gas as fuel, a hydrocarbon fuel production means for producing hydrocarbon fuels such as methane, ethane, propane, butane, etc. from hydrogen obtained from the surplus energy and carbon dioxide obtained from within and / or outside the operation system, a hydrocarbon fuel injection means for injecting the hydrocarbon fuel produced by the hydrocarbon fuel production means into a city gas pipeline system, and a carbon dioxide separation and capture device for separating and capturing carbon dioxide generated during operation of the distributed power source that has restrictions on load fluctuations using city gas as fuel.

[0007] Patent Document 5 discloses a fuel cell combined power generation system comprising: a fuel cell to which a fuel gas mainly composed of methane and an oxidizing gas are supplied; a gas turbine device equipped with a combustor to which exhaust fuel gas discharged from the fuel cell is supplied; a generator driven by the gas turbine device; a methanation device to which a carbon-containing gas and a hydrogen-containing gas are supplied and which converts at least a portion of the gas into methane; a hydrogen supply system to supply hydrogen to the fuel cell; and a control unit to supply methane-containing gas produced in the methanation device to the combustor when hydrogen is supplied from the hydrogen supply system to the fuel cell.

[0008] Patent Document 6 discloses an SOFC fuel recycling system characterized by comprising a SOFC (solid oxide fuel cell) module that uses hydrogen and carbon monoxide as fuel and generates electricity and heat by electrochemically combining these fuels with an oxidant, a first reaction tank in the fuel exhaust gas passage of the SOFC module that produces hydrogen and carbon monoxide from carbon dioxide and water vapor contained in the exhaust, a reflux path that returns the hydrogen and carbon monoxide produced in the first reaction tank to the SOFC, and a second reaction tank in the middle of the reflux path that produces methanol and / or methane from hydrogen and unreacted carbon dioxide.

[0009] Patent Document 7 discloses a fuel cell power production system comprising a flue gas generator configured to generate flue gas containing carbon dioxide and oxygen, a fuel supply, and a fuel cell assembly including a cathode section configured to receive the flue gas generated by the flue gas generator and output a cathode exhaust, an anode section configured to receive fuel from the fuel supply and output an anode exhaust containing hydrogen and carbon dioxide, a methanator configured to receive the anode exhaust, convert at least a portion of the hydrogen in the anode exhaust to methane, and discharge a methanized anode exhaust, a cooler assembly configured to cool the methanized anode exhaust to a predetermined temperature to liquefy the carbon dioxide in the methanized anode exhaust, and a gas separation assembly configured to receive the cooled methanized anode exhaust and separate the liquefied carbon dioxide from residual fuel gas. US Patent No. 5,399,633 teaches that in some embodiments, the concentration of carbon dioxide in the anode exhaust gas is in the range of 60-75 mol % (dry basis), including all ranges and values ​​therebetween; the anode exhaust before methanation comprises 20-25 mol % hydrogen and 65-75 mol % carbon dioxide on a dry basis; and the methanated anode exhaust comprises 5-10 mol % hydrogen and 75-85 mol % carbon dioxide on a dry basis.

[0010] Furthermore, for example, Patent Document 8 discloses a methane production apparatus that produces methane using hydrogen and carbon dioxide as raw material gases, and is characterized in that the methane production apparatus includes a plurality of reactors containing a catalyst that promotes a methanation reaction, which is a reaction from the raw material gas to the methane, a plurality of communication passages that connect two adjacent reactors among the plurality of reactors and send a product gas generated in a previous stage reactor to a subsequent stage reactor, a raw material gas inlet section that introduces the raw material gas into the previous stage reactor among the plurality of reactors, a cooling section in the communication passage that cools the product gas generated in the previous stage reactor to a temperature equal to or higher than the temperature at which the methanation reaction starts and lower than the temperature at which the methanation reaction stops, and a water removal section that removes water from the product gas cooled by the cooling section, located between the cooling section and the subsequent stage reactor in at least any of the communication passages.

[0011] Patent Document 9 discloses a methane production apparatus for producing methane from carbon dioxide and hydrogen, comprising: a first reactor that contains a catalyst having a methane conversion performance and causes a methanation reaction using a raw material gas containing carbon dioxide and hydrogen supplied from a supply source; a second reactor that is disposed downstream of the first reactor and causes a methanation reaction using a reaction mixed gas containing methane produced in the first reactor; and a reaction mixed gas boosting unit that is disposed on a gas flow path between the first reactor and the second reactor and makes the pressure of the reaction mixed gas supplied to the second reactor higher than the pressure of the raw material gas supplied to the first reactor. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] JP 2008-204783 A [Patent Document 2] JP 2019-8908 A [Patent Document 3] Special Publication No. 2018-519617 (Patent No. 6615220) [Patent Document 4] JP 2009-77457 A [Patent Document 5] JP 2007-152313 A [Patent Document 6] JP 2003-109638 A [Patent Document 7] Special Publication No. 2019-515442 [Patent Document 8] JP 2015-107942 A [Patent Document 9] JP 2019-142807 A Summary of the Invention [Problem to be solved by the invention]

[0013] The object of the present invention is to provide an off-gas discharged from a fuel cell, which contains a specific ratio of carbon dioxide, hydrogen and water, and can be converted into a gas containing methane. of Processing Reason Stem and process The objective of the present invention is to provide a method. [Means for solving the problem]

[0014] As a result of investigations aimed at solving the above problems, the present invention has been completed, including the following aspects.

[0015] [1] A fuel cell that discharges off-gas containing 10% by volume or more and 40% by volume or less of carbon dioxide, 5% by volume or more and 20% by volume or less of hydrogen, and 40% by volume or more and 80% by volume or less of water; A CO2 methanation reactor for converting carbon dioxide to methane; a line for transporting gas from the hydrogen source to the CO2 methanation reactor; and A line for transporting gas from the fuel cell to a CO2 methanation reactor. 、 The temperature of the off-gas at the exhaust port of the fuel cell is between 115℃ and 185℃. An off-gas treatment system.

[0016] 〔2 The fuel cell is a solid oxide fuel cell. 〕 The processing system described herein. [ 3 〕 The line for transporting the gas to the CO2 methanation reactor further includes a moisture remover for removing water from the gas, a compressor or compressor for increasing the pressure of the gas, and / or a heat exchanger or heater for adjusting the temperature of the gas. [1] Or〔2〕 The processing system according to claim 1 . [ 4 a CO selective oxidation reactor for converting carbon monoxide to carbon dioxide; a line for transporting gas from the oxygen source to the CO selective oxidation reactor; and Further comprising a line for transporting gas from the CO methanation reactor to the CO selective oxidation reactor. 3 ] The processing system according to any one of the preceding claims. [5] A fuel cell that discharges off-gas containing 10% by volume or more and 40% by volume or less of carbon dioxide, 5% by volume or more and 20% by volume or less of hydrogen, and 40% by volume or more and 80% by volume or less of water; CO for converting carbon dioxide to methane 2 Methanation reactor, Hydrogen source to CO 2 a line for transporting gas to a methanation reactor; CO from fuel cells 2 a line for transporting gas to a methanation reactor; A CO selective oxidation reactor for converting carbon monoxide to carbon dioxide; a line for transporting gas from the oxygen source to the CO selective oxidation reactor; and CO 2 A system for treating off-gas emitted from a fuel cell, comprising a line for transporting gas from a methanation reactor to a CO selective oxidation reactor. [6] A water remover for removing water from the gas is further provided in the line for transporting the gas from the CO2 methanation reactor to the CO selective oxidation reactor. [4] or [5] The processing system according to claim 1 . [7] Further comprising a line for discharging the gas generated in the CO selective oxidation reaction device; A water remover for removing water from the gas is further provided in the line for discharging the gas produced in the CO selective oxidation reaction device. 4 The processing system according to any one of the above items (1) to (6). [8] A fuel cell that discharges off-gas containing 10% by volume or more and 40% by volume or less of carbon dioxide, 5% by volume or more and 20% by volume or less of hydrogen, and 40% by volume or more and 80% by volume or less of water; CO for converting carbon dioxide to methane 2 Methanation reactor, Hydrogen source to CO 2 a line for transporting gas to a methanation reactor; CO from fuel cells 2 a line for transporting gas to the methanation reactor; and The system includes a line for discharging the gas generated in the CO selective oxidation reaction device, The system for treating off-gas discharged from a fuel cell further comprises a moisture remover for removing water from the gas, the moisture remover being disposed in a line for discharging the gas produced in the CO selective oxidation reaction device.

[0017] [ 9 〕 A low-water-content CO2-rich gas is obtained by removing water from an off-gas discharged from a fuel cell, the off-gas containing 10% by volume to 40% by volume, 5% by volume to 20% by volume, and 40% by volume to 80% by volume of water; Hydrogen is added to the obtained low-moisture CO2-rich gas to obtain a mixed gas, The mixed gas is reacted in the presence of a CO2 methanation catalyst to obtain a methane-rich gas. 、 Water is removed from methane-rich gas to obtain low-water content methane-rich gas. Including fruit , When the off-gas discharged from the fuel cell does not satisfy at least one of the following: carbon dioxide content is 10% by volume or more and 40% by volume or less, hydrogen content is 5% by volume or more and 20% by volume or less, and water content is 40% by volume or more and 80% by volume or less, the off-gas is directly discharged into the atmosphere. Method for treating off-gas discharged from fuel cells 。 Effect of the Invention

[0018] The present invention process System and process According to the method, off-gas containing a specific ratio of carbon dioxide, hydrogen, and water discharged from a fuel cell can be converted to a gas containing methane with high efficiency. According to the present invention, the emission of carbon dioxide discharged from a fuel cell into the atmosphere can be reduced, and the generated methane can be used as a fuel, a raw material for city gas, etc. The off-gas having the composition ratio specified by the present invention contains almost or completely no substances that poison the catalyst, so that the deterioration of the catalyst in the reactor of the CO2 methanation reaction device or the CO selective oxidation reaction device is prevented, and long-term operation is possible without replacing the catalyst. In addition, by using the off-gas having the composition ratio specified by the present invention, it is easy to make the methane-rich gas obtained by the CO2 methanation reaction or by the CO2 methanation reaction and the CO selective oxidation reaction meet the specifications of the gas grid. Table 1 shows an example of the main specifications of the gas grid.

[0019] [Table 1] [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram illustrating an example of a processing system according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention will be described in more detail below by showing embodiments of the present invention. Note that these are merely examples for the purpose of explanation, and the technical scope of the present invention is not limited by these embodiments in any way.

[0022] The treatment system of the present invention includes a fuel cell that discharges off-gas, a CO2 methanation reactor, and a line for transporting gas to the CO2 methanation reactor.

[0023] The treatment method of the present invention includes removing water from the off-gas discharged from a fuel cell to obtain a low-moisture CO2-rich gas, adding hydrogen to the obtained low-moisture CO2-rich gas to obtain a mixed gas, and reacting the mixed gas in the presence of a CO2 methanation catalyst to obtain a methane-rich gas.

[0024] The off-gas discharged from the fuel cell used in the present invention contains carbon dioxide at 10% by volume or more and 40% by volume or less, preferably 28% by volume or more and 33% by volume or less, hydrogen at 5% by volume or more and 20% by volume or less, preferably 8% by volume or more and 18.3% by volume or less, and water at 40% by volume or more and 80% by volume or less, preferably 41% by volume or more and 62% by volume or less. The off-gas may contain ammonia, carbon monoxide, or nitrogen. The off-gas preferably has a temperature at the outlet of the fuel cell of 115°C or more and 185°C or less. The composition ratio, temperature, flow rate, etc. of the off-gas may be measured inline using a sensor s1 or the like, or the composition ratio of the off-gas may be measured offline using gas chromatography or the like. When the temperature of the off-gas is extremely low, an extra amount of heat is required for temperature adjustment. When the amount of heat for temperature adjustment is insufficient, it is difficult to adjust the temperature to a temperature suitable for the CO2 methanation reaction.

[0025] When the off-gas discharged from the fuel cell does not satisfy at least one of the following: carbon dioxide 10% to 40% by volume, preferably 28% to 33% by volume; hydrogen 5% to 20% by volume, preferably 8% to 18.3% by volume; and water 40% to 80% by volume, preferably 41% to 62% by volume; the off-gas can be discharged directly into the atmosphere. When the fuel cell is operated at 30% or less of its rated load, the amount of off-gas discharged from the fuel cell is small, the temperature of the off-gas is low, or the amount of carbon dioxide or hydrogen is small. In this case, the off-gas can be discharged directly into the atmosphere. The sensors s1 and s2 and the valves V1 and V2 can be connected by a control system, and the opening of the valve V1 or V2 can be automatically controlled according to the detected gas composition ratio.

[0026] The fuel cell that discharges the off-gas as described above is not particularly limited in type, but is preferably a solid oxide fuel cell (SOFC: Solid Oxide Fuel Cell). Examples of fuel for the fuel cell include hydrocarbons such as methane, ethane, propane, and butane, and alcohols such as methanol and ethanol. Among these, gases mainly containing methane, such as city gas, are preferred.

[0027] The off-gas discharged from the fuel cell can be supplied to a CO2 methanation reactor via a gas transport line. During the transport of the off-gas, it can be cooled in a cooler 9 and the drain water 21 can be removed in a drain separator 8 to produce a low-moisture CO2-rich gas.

[0028] The hydrogen source is not particularly limited as long as it is a source of gas mainly containing hydrogen. The hydrogen-containing gas may be obtained by electrolysis of water, by reforming methanol or methane gas produced from fossil fuels such as petroleum or natural gas or biomass, or by-product in steel mills, chemical plants, etc. Hydrogen may be produced from surplus electricity generated by power generation using renewable energy. Hydrogen from the hydrogen source can be supplied to the CO2 methanation reactor via a gas transport line. During the transport of the gas mainly containing hydrogen, it can be cooled by a cooler and drain water can be removed by a drain separator. The hydrogen source may be a hydrogen storage alloy tank. When a hydrogen storage alloy tank is used, the alloy stored at a low temperature can be heated to obtain hydrogen at a pressure corresponding to the temperature, which has the advantage that a compressor can be omitted.

[0029] The gas mainly containing hydrogen and the low moisture CO2 rich gas may be mixed in a line mixer, mixer, etc. during transportation, or may be mixed when they reach a reactor in the CO2 methanation reaction device. Furthermore, during gas transportation, the gas can be pressurized or compressed or its temperature adjusted in a pressurizer or compressor, or a heat exchanger or heater to a pressure or temperature suitable for the CO2 methanation reaction. The ratio of the supply amount of the gas mainly containing hydrogen to the supply amount of the low moisture CO2 rich gas can be changed by measuring the composition ratio, flow rate, temperature, etc. with sensors s3, s2, etc., and controlling the opening of valves V3, V2, etc. with the control system 10. The supply amount of the gas mainly containing hydrogen can be calculated, for example, by the following formula based on the amounts of carbon monoxide, carbon dioxide, and hydrogen contained in the low moisture CO2 rich gas. (H2 flow rate supplied by gas mainly containing hydrogen)= (CO flow rate in low moisture CO2 rich gas) x 3 + (CO2 flow rate in low moisture CO2 rich gas) x 4 -(H2 flow rate in low moisture CO2 rich gas)

[0030] The CO2 methanation reactor is equipped with a reactor having an inlet for a raw gas (gas containing hydrogen and carbon dioxide) and an outlet for a reaction product (gas containing mainly methane and, secondarily, carbon monoxide). The reactor can be of a tube type or a tank type. The reactor usually has a device for adjusting the temperature inside the reactor. The device for adjusting the temperature usually includes a jacket or a heat transfer tube for heat exchange with the reaction gas, a control device for controlling the amount of heat exchanged, and a measuring device for measuring the temperature inside the reactor. The heat transfer medium that can be passed through the jacket or the heat transfer tube has a temperature of preferably 20 to 60°C. The temperature of the heat transfer medium can be adjusted, for example, by a separately provided cooler (cooling tower).

[0031] The CO2 methanation reaction is a chemical reaction that produces methane from carbon dioxide and hydrogen (CO2 + 4H2 → CH4 + 2H2O). The temperature in the CO2 methanation reaction is usually 200 to 500°C, preferably 220 to 350°C. The pressure during the CO2 methanation reaction is not particularly limited, but is preferably normal pressure (about 0.1 MPa) or higher and less than 1 MPa. In the CO2 methanation reaction, the chemical reactions that produce carbon monoxide (CO2+ H2→ CO + H2O) and methanol (CO2+ 3H2→ CH3OH + H2O) can occur simultaneously.

[0032] A methanation catalyst can be used for the CO2 methanation reaction. Examples of the methanation catalyst include Ni-based catalysts, platinum group metal-based catalysts, and other precious metal-based catalysts. A specific example of the methanation catalyst is nickel aluminate (NiAl x O y ), Ru / NiAl x O y , Ru / Al2O3, Ru / TiO2, Ni / TiO2, Ru-Ni / TiO2, etc. As the methanation catalyst, those in the shape of powder, granules, pellets, flat plate, corrugated plate, corrugated, honeycomb, etc. can be appropriately used. The methanation catalyst may be loaded or packed in the inner cavity of the reactor, or may be attached to the inner wall of the inner cavity of the reactor.

[0033] The CO2 methanation reaction may be performed by connecting only one CO2 methanation reactor, or by connecting multiple CO2 methanation reactors in series. The CO2 methanation reaction may also be performed by connecting multiple CO2 methanation reactors in parallel. It is preferable to remove water from the gas discharged from the CO2 methanation reactor as drain water using a moisture remover or the like, and supply the gas adjusted to a low water content to the next CO2 methanation reactor or the CO selective oxidation reactor. When the gas discharged from the CO2 methanation reactor contains little water, the gas may be passed through a bypass line (not shown) of the moisture remover to reduce energy consumption. In an embodiment in which multiple CO2 methanation reactors are connected in parallel, it is easy to deal with the case where the flow rate of the off-gas discharged from the fuel cell fluctuates.

[0034] The gas produced by the CO2 methanation reaction may contain carbon monoxide. The carbon monoxide concentration is not particularly limited, but may be 100 ppm or more. Since carbon monoxide is toxic to the human body, it is preferable to treat the gas so that the carbon monoxide concentration is 30 ppm or less. Examples of treatment methods for reducing the carbon monoxide concentration include converting carbon monoxide to methane by a CO methanation reaction, converting carbon monoxide to carbon dioxide by a CO selective oxidation reaction, and adsorbing or absorbing carbon dioxide using an adsorbent or absorbent.

[0035] In one embodiment of the present invention, oxygen is added to the carbon monoxide in the gas produced by the CO2 methanation reaction in the presence of a CO selective oxidation catalyst to cause an oxidation reaction (2CO + O2 → 2CO2). The upper limit of the temperature in the CO selective oxidation reaction (PROX reaction) is usually 200°C, preferably 150°C, more preferably 120°C, and even more preferably 100°C, and the lower limit is usually 20°C, preferably 35°C, more preferably 50°C, and even more preferably 80°C. The higher the temperature in the CO selective oxidation reaction, the more likely the methane oxidation reaction will occur. It is preferable to add oxygen so that the molar ratio of oxygen (O2) to carbon monoxide (CO) is 2 to 6. The pressure during the CO selective oxidation reaction is not particularly limited, but is preferably normal pressure (about 0.1 MPa) or more and less than 1 MPa. The gas produced by the CO2 methanation reaction may also contain hydrogen and water, some of which is chemically converted to water by the oxygen used in the PROX reaction.

[0036] The CO selective oxidation reaction apparatus is equipped with a reactor having an inlet for the gas produced in the CO2 methanation reaction, an inlet for the gas containing oxygen, and an outlet for the reaction product (mainly a gas containing methane). The reactor can be of a tube type or a tank type. The reactor usually has a device for adjusting the temperature inside the reactor. The temperature adjustment device usually includes a jacket or a heat transfer tube for heat exchange with the reaction gas, a control device for controlling the amount of heat exchanged, and a measuring device for measuring the temperature inside the reactor. In order to adjust the temperature, it is preferable to have a device (such as a heat exchanger) for cooling the gas produced in the CO2 methanation reaction apparatus on the line that supplies the gas produced in the CO2 methanation reaction apparatus to the CO selective oxidation reaction apparatus. This cooling can remove water (H2O) and further adjust the temperature to a temperature suitable for the CO selective oxidation reaction.

[0037] Examples of CO selective oxidation catalysts used in the CO selective oxidation reaction include oxide catalysts, platinum group metal (Pt, Ru, Rh, Pd, etc.) catalysts, other precious metal catalysts, photocatalysts, etc. Specific examples of CO selective oxidation catalysts include Ru / Al2O3, Ru / C, Rh porphyrin / C, Co x -Fe2O, Co3O4, Cu / CeO2-ZrO2, Ni / CeO2-ZrO2, Co / CeO2-ZrO2, Fe / CeO2-ZrO2, Pt / Al2O3, CuMn2O4, CuZnO, Pt / SiO2, Pd / Al2O3, Pt / SnO2, Pd / CeO2, Pt / TiO2, PdCl2-CuCl2 / C, Au / TiO2, Au / Fe2O3, etc. As the CO selective oxidation catalyst, powder, granules, pellets, flat plate, corrugated plate, corrugated, honeycomb, etc. can be appropriately used. The CO selective oxidation catalyst may be a catalyst in which an element having an oxidation function, such as Pt, Ru, Rh, or Pd, is supported on a support such as C, Al2O3, SiO2, TiO2, or ZrO2. The CO selective oxidation catalyst may be loaded or packed into the bore of the reactor, or may be attached to the inner wall of the bore of the reactor.

[0038] The CO selective oxidation reaction is preferably carried out so that the carbon monoxide concentration in the resulting methane-rich gas is 30 ppm or less. The space velocity (GHSV) in the reactor of the CO selective oxidation reaction apparatus is preferably 4000 to 150000 h -1 , more preferably 5000 to 10000h -1 It is.

[0039] The treatment system of the present invention further includes a line for discharging the gas generated in the CO selective oxidation reaction device, and preferably includes a moisture remover for removing water from the gas in the middle of the line for discharging the gas generated in the CO selective oxidation reaction device. The CO selective oxidation reaction can reduce carbon monoxide from a gas containing mainly methane and secondary carbon monoxide, and as a result, a methane-rich gas is obtained. The methane-rich gas obtained by the present invention may be used as it is as a fuel for a fuel cell. The methane-rich gas obtained by the present invention can easily meet the specifications of a gas grid by appropriately performing water removal, calorific value adjustment (addition of gas such as propane or ethane), pressure adjustment, etc., and therefore can be supplied to a gas grid. The pressure of the gas in the gas grid is usually 5 to 7 MPa. The means for removing water is not particularly limited, and can be performed, for example, by an adsorbent, a desiccant, or a moisture remover.

[0040] The methane-rich gas obtained by the present invention can be supplied as fuel to a gas turbine, which can generate electricity. Since the combustion exhaust gas from the gas turbine usually contains carbon dioxide, it can be used as a CO2 source in the method for producing the methane-rich gas.

[0041] (Example) Using the treatment system of the present invention shown in Figure 1, the SOFC off-gas having the composition ratio shown in Table 2 was subjected to a CO2 methanation reaction at a H2 / CO2 molar ratio of 4, and then the resulting gas was subjected to a CO selective oxidation reaction to obtain a methane-rich gas. The gas temperature, pressure, composition ratio, and gas amount were measured using sensors s1, s4, s5, s6, and s7. The results are shown in Table 2. It can be seen that the gas discharged from the moisture remover 41 satisfies the specifications shown in Table 1 and can be supplied to the gas grid.

[0042] [Table 2] [Explanation of symbols]

[0043] 3: CO selective oxidation reactor 16: Heat transfer medium inlet 17: Heat transfer medium outlet 7: Gas mixer 10: Control system s1, s2, s3, s4, s5, s6, s7: sensors V1, V2, V3: Flow rate adjustment valve 19: First heat transfer medium inlet 20: First heat transfer medium outlet 29: Second heat transfer medium inlet 30: Second heat transfer medium outlet 11: First CO2 methanation reactor 21: Second CO2 methanation reactor 12,22,32: Heat exchanger 9,15,25,35: Cooler 8, 18, 28, 38: Drain separator (moisture remover) 21:Drain water 40: Compressor 41: Moisture remover

Claims

1. a fuel cell which discharges off-gas containing 10% by volume or more and 40% by volume or less of carbon dioxide, 5% by volume or more and 20% by volume or less of hydrogen, and 40% by volume or more and 80% by volume or less of water; CO for converting carbon dioxide to methane ２ Methanation reactor, Hydrogen source to CO ２ a line for transporting gas to the methanation reactor; and CO from fuel cells ２ a line for transporting the gas discharged from the fuel cell to a methanation reactor; An off-gas treatment system for treating off-gas whose temperature at the exhaust port of a fuel cell is 115°C or higher and 185°C or lower.

2. 10. The treatment system of claim 1, wherein the fuel cell is a solid oxide fuel cell.

3. CO ２ 3. The treatment system according to claim 1 or 2, further comprising a moisture remover for removing water from the gas, a compressor or a compressor for increasing the pressure of the gas, and / or a heat exchanger or a heater for adjusting the temperature of the gas, in a line for transporting the gas to the methanation reaction apparatus.

4. a CO selective oxidation reactor for converting carbon monoxide to carbon dioxide; a line for transporting gas from the oxygen source to the CO selective oxidation reactor; and CO ２ 4. The treatment system according to claim 1, further comprising a line for transporting gas from the methanation reactor to the CO selective oxidation reactor.

5. A fuel cell that discharges off-gas containing 10% by volume or more and 40% by volume or less of carbon dioxide, 5% by volume or more and 20% by volume or less of hydrogen, and 40% by volume or more and 80% by volume or less of water. a CO2 methanation reactor for converting carbon dioxide to methane; a line for transporting gas from a hydrogen source to the CO2 methanation reactor; a line for transporting gas from the fuel cell to a CO2 methanation reactor; a CO selective oxidation reactor for converting carbon monoxide to carbon dioxide; a line for transporting gas from the oxygen source to the CO selective oxidation reactor; and CO ２ A system for treating off-gas emitted from a fuel cell, comprising a line for transporting gas from a methanation reactor to a CO oxidation reactor.

6. CO ２ 6. The treatment system according to claim 4, further comprising a moisture remover for removing water from the gas, the moisture remover being disposed in a line for transporting the gas from the methanation reactor to the CO selective oxidation reactor.

7. Further comprising a line for discharging the gas generated in the CO selective oxidation reaction device; 7. The treatment system according to claim 4, further comprising a moisture remover for removing water from the gas, the moisture remover being disposed in a line for discharging the gas produced in the CO selective oxidation reaction device.

8. A fuel cell that discharges off-gas containing 10% by volume or more and 40% by volume or less of carbon dioxide, 5% by volume or more and 20% by volume or less of hydrogen, and 40% by volume or more and 80% by volume or less of water. a CO2 methanation reactor for converting carbon dioxide to methane; a line for transporting gas from a hydrogen source to the CO2 methanation reactor; a line for transporting gas from the fuel cell to a CO2 methanation reactor; and A line is provided for discharging the gas generated in the CO selective oxidation reaction device, The system for treating off-gas discharged from a fuel cell further comprises a moisture remover for removing water from the gas, the moisture remover being provided in a line for discharging the gas produced in the CO selective oxidation reaction device.

9. The low-water-content CO is produced by removing water from off-gas discharged from a fuel cell, the off-gas containing 10% by volume to 40% by volume, 5% by volume to 20% by volume, and 40% by volume to 80% by volume. ２ Obtain rich gas, The obtained low water content CO ２ Add hydrogen to the rich gas to obtain a mixed gas; The mixed gas is ２ reacting in the presence of a methanation catalyst to obtain a methane-rich gas; removing water from the methane-rich gas to obtain a low water content methane-rich gas; A method for treating off-gas discharged from a fuel cell, comprising discharging the off-gas directly into the atmosphere when the off-gas does not satisfy at least one of 10% by volume or more and 40% by volume or less of carbon dioxide, 5% by volume or more and 20% by volume or less of hydrogen, and 40% by volume or more and 80% by volume or less of water.

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