Fuel gas production method

Abstract

To provide a method for obtaining fuel gas reduced in a hydrogen concentration by a simple and economically advantageous method without requiring reaction in an extremely high-pressure condition, and installation of an expensive decarbonation facility when fuel gas having methane as the main component is manufactured by subjecting hydrogen and carbon oxide to methanizing reaction in the presence of a methanizing catalyst.SOLUTION: A method for manufacturing fuel gas includes steps of: a methanizing step of reacting hydrogen with carbon oxide in the presence of a methanizing catalyst so as to obtain fuel gas containing 2% or more and 20% or less of hydrogen on the basis of volume after dehydration; and a dehydrogenation reaction step of adding alcohol to the fuel gas obtained in the methanizing step, and reacting the alcohol with the hydrogen in the fuel gas in the presence of a dehydrogenation catalyst so as to obtain fuel gas containing paraffin hydrocarbon. The alcohol is selected from alcohol with a carbon number of 2 or more and 4 or less, and the dehydrogenation catalyst has an activity on dehydration reaction of alcohol and hydrogenation reaction of olefin.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a fuel gas containing methane as a main component and having a high heating value by subjecting hydrogen and carbon oxides to a methanation reaction in the presence of a methanation catalyst, and more particularly to a method for producing a fuel gas having a high hydrocarbon concentration and a reduced hydrogen concentration. [Background technology]

[0002] In recent years, from the perspective of global warming countermeasures, attention has been focused on carbon-neutral fuels, which do not substantially increase the carbon dioxide concentration in the atmosphere even when burned.

[0003] Methane can be obtained by capturing carbon dioxide from exhaust gases generated in industrial processes and thermal power plants, and reacting it with hydrogen obtained by electrolysis using electricity generated by renewable energy sources such as solar power and wind power. Methane obtained by this method can be considered a carbon-neutral fuel that does not contribute to global warming, as no additional carbon dioxide is produced when it is burned.

[0004] The methanation reaction (formula 1) in which carbon dioxide and hydrogen are reacted to produce methane is known. CO2+4H2→ CH4+2H2O (Formula 1)

[0005] Patent Document 1 discloses a method for methanating a gas containing CO and H2, characterized in that a methanation reactor is used in which a Cu-Zn-based low-temperature shift catalyst is arranged on the upstream side and a methanation catalyst is arranged on the downstream side when methanating a gas containing CO and H2. Since the CO shift reaction (formula 2) proceeds in the upstream low-temperature shift reactor, most of the carbon monoxide contained in the raw material gas reacts with water vapor and is converted to carbon dioxide, and it is considered that the methanation reaction of carbon dioxide proceeds on the downstream methanation catalyst. CO+H2O → CO2+H2 (Formula 2)

[0006] Methanation reactions have long been used to remove carbon monoxide and carbon dioxide from hydrogen for ammonia synthesis, and catalysts supporting Ni, Ru, etc. are known to exhibit high activity (Non-Patent Documents 1 and 2).

[0007] Methanation, in which carbon oxides (carbon monoxide and carbon dioxide) are reacted with hydrogen to obtain methane, is an industrially established technology (e.g., Non-Patent Document 3). However, there are still challenges to be overcome in obtaining a fuel gas of a quality that can be used as a raw material for city gas.

[0008] Natural gas is generally used as a raw material for city gas, and is composed mainly of methane, with small amounts of ethane, propane, and butane. Natural gas does not normally contain hydrogen or carbon monoxide, and carbon dioxide is removed during the natural gas refining process. In particular, in the case of city gas produced from liquefied natural gas, hydrogen, carbon monoxide, and carbon dioxide are almost completely removed during the liquefaction refining process, so there is virtually no hydrogen, carbon monoxide, or carbon dioxide in the gas.

[0009] The concentration of hydrocarbons other than methane (ethane, propane, and butane) contained in natural gas varies depending on the natural gas's place of origin and refining method, so when producing city gas, propane or butane is usually added to adjust the calorific value to within a certain range, and then an odorant is added to ensure safety before the gas is sent to consumers through city gas pipelines.

[0010] When hydrogen, carbon monoxide and carbon dioxide are contained in city gas, they can cause the following problems:

[0011] First, carbon monoxide is highly toxic, and if the gas leaks, there is a risk of poisoning. The permissible concentration is set at 200 ppm, and from a safety standpoint, it is desirable to keep the concentration in fuel gas below this level. Even taking into account dilution with air, it must be kept below 1000 ppm.

[0012] Secondly, carbon dioxide is not only non-flammable but also acts to suppress combustion. Therefore, if it is mixed into fuel gas in high concentrations, it not only reduces the efficiency of gas transport in the ducts due to the reduction in the calorific value of the fuel gas, but also may cause a decrease in the efficiency of the combustion appliance.

[0013] Finally, although hydrogen is a fuel gas, its calorific value per unit volume is only about one-third that of methane, the main component of city gas. Therefore, when hydrogen is mixed into fuel gas whose main component is methane, the calorific value per unit volume decreases. Furthermore, hydrogen is known to have a large impact on combustion equipment due to its fast combustion speed.

[0014] As described above, when hydrogen, carbon monoxide and carbon dioxide are mixed into city gas, they have various effects at each stage of gas supply and consumption, so the quality standards for gas accepted into city gas pipeline networks generally set restrictions on the concentrations of hydrogen, carbon monoxide and carbon dioxide.

[0015] In pipeline networks where fuel filling stations for natural gas vehicles exist, there are known examples where the upper limit of hydrogen concentration is set at 2% by volume (Non-Patent Document 4).Other known examples include a hydrogen concentration of 4% or less by volume, a carbon dioxide concentration of 0.5% or less by volume, and a carbon monoxide concentration of 0.05% or less by volume (Non-Patent Document 5), as well as a total concentration of methane and ethane of 93% or more by volume, and a total concentration of components other than hydrocarbons of 4% or less by volume (Non-Patent Document 6).

[0016] The contamination of city gas with hydrogen, carbon monoxide, and carbon dioxide poses problems other than city gas quality. As mentioned above, city gas is supplied after being adjusted to a certain calorific value range by adding propane or butane so that it can be used stably in combustion appliances. Therefore, if hydrogen or carbon dioxide is present in high concentrations, it is necessary to mix in large amounts of propane and butane, which are necessary to adjust the calorific value. This not only increases the cost of gas production, but also impairs the carbon neutrality of the produced gas if fossil fuel-derived propane and butane are used to adjust the calorific value.

[0017] The methanation reaction of carbon dioxide (Equation 1) is an equilibrium reaction, and under normal industrial operating conditions, carbon dioxide and hydrogen cannot be completely converted to methane. When a mixed gas with a stoichiometric ratio (hydrogen:carbon dioxide = 4:1) is reacted at normal pressure (0.1 MPa), the equilibrium conversion rate of carbon dioxide to methane is 95.0% when the reaction temperature is 300°C, 97.5% when the reaction temperature is 250°C, and 98.9% when the reaction temperature is 200°C.

[0018] Thus, at normal pressure, only fuel gas containing a large amount of hydrogen can be obtained. Because the methanation reaction is an exothermic reaction, the lower the temperature, the higher the equilibrium conversion rate. However, in the case of a catalytic reaction, the lower the temperature, the lower the catalytic activity. For this reason, there is a lower limit to the reaction temperature, and in the case of a normal methanation catalyst, a temperature of 250°C or higher is required to obtain a practical reaction rate (Non-Patent Document 4, Patent Documents 2 and 3).

[0019] The methanation reaction of carbon dioxide (Equation 1) is a reaction in which the amount of substance (molar number) decreases, so the higher the pressure, the higher the equilibrium conversion rate. Comparing at 250°C, the equilibrium conversion rate of carbon dioxide to methane (when reacted at a stoichiometric ratio of hydrogen:carbon dioxide = 4:1) is 97.5% at normal pressure (0.1 MPa), but improves to 98.9% at 0.7 MPa and 99.5% at 5 MPa. However, even when a mixed gas of hydrogen:carbon dioxide = 4:1 is reacted at 250°C and 5 MPa, the equilibrium composition (composition by volume after dehydration excluding the water produced) is 97.45% methane, 2.04% hydrogen, and 0.51% carbon dioxide. Therefore, in order to obtain a fuel gas with less than 2% hydrogen (by volume), the reaction must be carried out under high-pressure conditions exceeding 5 MPa.

[0020] There are several known methods for reducing the hydrogen concentration in fuel gas obtained by a methanation reaction. For example, if the methanation reaction is carried out under conditions where the amount of hydrogen is less than the stoichiometric ratio, and the excess carbon dioxide is removed by a decarbonation treatment, both the hydrogen content and the carbon dioxide content can be reduced (Patent Document 4). There is an example where this method has been adopted in the methanation of coke oven gas (Non-Patent Document 3). However, the cost of decarbonation equipment is generally high, and a large amount of energy is consumed in operation, which reduces its economic efficiency.

[0021] As another method, a method is also known in which the methanation reaction is carried out in multiple stages, the gas produced in the intermediate stages is cooled, and the water is condensed and separated (Non-Patent Document 4, Patent Documents 2 and 3). By removing the produced water, the methanation reaction can be allowed to proceed further to the production side, and the conversion rate of carbon dioxide to methane can be improved. However, this method requires heat exchange equipment, which increases the equipment costs, and there is also the problem that if too much produced water is removed, the composition will enter the region where carbon deposition occurs in equilibrium, making it difficult to control the water separation process.

[0022] Furthermore, in both of the methods of carrying out the methanation reaction under conditions in which the amount of hydrogen is less than the stoichiometric ratio and then removing excess carbon dioxide by a decarbonation treatment, and carrying out the methanation reaction in multiple stages and cooling the gas produced in the intermediate stages to condense and separate the water, the conditions are favorable for the production of carbon monoxide in equilibrium, and therefore there is a problem that the carbon monoxide concentration in the produced fuel gas is high. [Prior art documents] [Patent documents]

[0023] [Patent Document 1] Japanese Patent Application Publication No. 60-235893 [Patent Document 2] JP 2015-124217 A [Patent Document 3] JP 2018-135283 A [Patent Document 4] JP 2019-26595 A [Non-patent literature]

[0024] [Non-Patent Document 1] Chemical Process Journal, edited by the Society of Chemical Engineers, 1970, p.153 [Non-Patent Document 2] Catalysis Society of Japan, Catalyst Handbook, 2008, p.535 [Non-Patent Document 3] Kawagoe, Matsuda, Matsushima and Uematsu, Hitachi Review, Vol. 68, No. 10, 1986, p. 73 [Non-Patent Document 4] E. I. Koytsoumpa and S. Karellas, Renewable and Sustainable Energy Reviews, vol. 94, 2018, p. 536 [Non-Patent Document 5] Biogas Purchasing Guidelines, Osaka Gas Co., Ltd., 2008 [Non-Patent Document 6] Biogas Purchasing Guidelines, Tokyo Gas Co., Ltd., 2008 Summary of the Invention [Problem to be solved by the invention]

[0025] In view of the above problems, an object of the present invention is to provide a method for producing a fuel gas containing methane as a main component by subjecting hydrogen and carbon oxides to a methanation reaction in the presence of a methanation catalyst, which method provides a fuel gas having a reduced hydrogen concentration in a simple and economical manner without requiring a reaction under extremely high pressure conditions or the installation of expensive decarbonation equipment. [Means for solving the problem]

[0026] A characteristic configuration of the fuel gas production method according to the present invention includes a methanation step of reacting hydrogen and carbon oxides in the presence of a methanation catalyst to obtain a fuel gas containing 2% to 20% hydrogen on a volumetric basis after dehydration, and a dehydration / hydrogenation reaction step of adding an alcohol to the fuel gas obtained in the methanation step and then reacting the alcohol with hydrogen in the fuel gas in the presence of a dehydration / hydrogenation catalyst to obtain a fuel gas containing paraffinic hydrocarbons having a carbon number of 2 to 4, wherein the alcohol is one or more alcohols selected from alcohols having a carbon number of 2 to 4, and the dehydration / hydrogenation catalyst has activity in the dehydration reaction of the alcohol and the hydrogenation reaction of olefins.

[0027] According to this characteristic configuration, hydrogen contained in the fuel gas mainly composed of methane, which contains a relatively high concentration of hydrogen obtained by performing a methanation reaction while avoiding extremely high pressure conditions, is converted into paraffin hydrocarbons and water vapor by reaction with alcohol, so that a fuel gas with a reduced hydrogen concentration can be obtained. Furthermore, since the calorific value per unit volume of paraffin hydrocarbons having a carbon number of 2 to 4 is higher than that of methane, when adjusting the calorific value, the amount of propane, butane, etc. added for adjusting the calorific value can be reduced. Furthermore, when using alcohol derived from biomass obtained by fermenting sugar obtained from sugarcane, corn, etc. as the alcohol, the carbon neutrality of the fuel gas is not impaired.

[0028] A further characteristic feature of the fuel gas production method according to the present invention is that the ratio of hydrogen to carbon oxides supplied to the methanation step is a value in which the value of {(hydrogen)+(carbon monoxide)} / {(carbon monoxide)+(carbon dioxide)} calculated on a substance amount (molar) basis is 4.04 or more and 4.08 or less, and the gas supplied to the dehydration hydrogenation reaction step has a molar ratio of alcohol having a carbon number of 2 to 4 to hydrogen in the fuel gas obtained in the methanation step of 0.45 or more and 0.9 or less.

[0029] Although it is possible to reduce hydrogen by reacting with alcohol, it is not possible to reduce carbon dioxide. Therefore, in this characteristic configuration, the methanation reaction is performed under conditions where the amount of hydrogen supplied to the methanation catalyst is in excess, thereby sufficiently reducing the carbon dioxide concentration in the fuel gas obtained by the methanation step, and in the subsequent dehydration / hydrogenation reaction step, the excess hydrogen is treated by reaction with alcohol. This produces a fuel gas in which the concentration of carbon dioxide is sufficiently reduced in addition to that of hydrogen.

[0030] For example, when only carbon dioxide is contained as carbon oxide, if the methanation reaction is carried out under conditions of excess hydrogen, such that the molar ratio of hydrogen to carbon dioxide (hydrogen / carbon dioxide) supplied to the methanation catalyst is approximately 4.04 or more and 4.08 or less, the carbon dioxide concentration in the fuel gas obtained by the methanation process can be sufficiently reduced.

[0031] In other words, by carrying out the methanation reaction under conditions of excess hydrogen by adjusting the ratio of hydrogen to carbon supplied to the methanation step, i.e., {(hydrogen) + (carbon monoxide)} / {(carbon monoxide) + (carbon dioxide)}, calculated on a substance amount basis, to 4.04 or more and 4.08 or less, the carbon dioxide concentration in the fuel gas obtained by the methanation step can be sufficiently reduced.

[0032] A further characteristic feature of the method for producing a fuel gas according to the present invention is that the dehydration hydrogenation catalyst includes a solid acid catalyst and at least one of palladium and platinum, and the at least one of palladium and platinum is supported on the solid acid catalyst.

[0033] According to this configuration, the reaction between hydrogen and alcohol is carried out at a sufficient rate while the steam reforming reaction that produces hydrogen and carbon dioxide is suppressed, so that a fuel gas with a reduced hydrogen concentration can be obtained effectively. [Brief description of the drawings]

[0034] [Figure 1] FIG. 1 is a block flow diagram showing a fuel gas production method of the present invention. [Diagram 2] FIG. 1 shows the effect of temperature and pressure on the equilibrium conversion of the methanation reaction of carbon dioxide. [Diagram 3] FIG. 3 is an enlarged view of the region of FIG. 2 where the equilibrium conversion is 80 to 100%. [Figure 4] This is an example of the configuration of a reactor for a methanation reaction (a fixed-bed adiabatic multi-stage reactor). [Diagram 5] FIG. 2 is a diagram showing the conversion rate of carbon dioxide to methane and the gas temperature in each reaction stage when a methanation reaction is carried out using a fixed-bed adiabatic multi-stage reactor. [Figure 6] This is an example of the configuration of a reactor for a methanation reaction (a fixed-bed adiabatic multistage reactor having a recycle line that returns the first-stage outlet gas to the first-stage inlet). [Figure 7] This is an example of the structure of a methanation reaction reactor (heat exchange type reactor). [Figure 8] This is an example of the reactor configuration for a methanation reaction (the first stage is a fixed-bed adiabatic reactor with a recycle line, and the second stage is a heat exchange type reactor). [Figure 9] FIG. 1 is a diagram showing the effect of the inlet hydrogen / carbon dioxide ratio on the hydrogen and carbon dioxide concentrations in the equilibrium composition (after dehydration) of the carbon dioxide methanation reaction (reaction pressure 0.7 MPa). [Figure 10] FIG. 1 is a diagram showing the effect of the inlet hydrogen / carbon dioxide ratio on the hydrogen and carbon dioxide concentrations in the equilibrium composition (after dehydration) of the carbon dioxide methanation reaction (reaction pressure 5 MPa). [Figure 11]FIG. 2 is a diagram showing the effect of the inlet hydrogen / carbon dioxide ratio on the hydrogen and carbon dioxide concentrations in the equilibrium composition (after dehydration) of the fuel gas obtained by the fuel gas production method of the present invention (ethanol / hydrogen molar ratio of 0.9). [Figure 12] FIG. 2 is a diagram showing the effect of the inlet hydrogen / carbon dioxide ratio on the hydrogen and carbon dioxide concentrations in the equilibrium composition (after dehydration) of the fuel gas obtained by the fuel gas production method of the present invention (ethanol / hydrogen molar ratio of 0.5). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] [Embodiment] Hereinafter, an embodiment of the fuel gas production method according to the present invention will be described. Fig. 1 is a block flow diagram showing the fuel gas production method according to the present invention. The fuel gas production method according to this embodiment has a methanation step (methanation reactor 10) and a dehydration / hydrogenation reaction step (dehydration / hydrogenation reactor 20).

[0036] In the methanation step, a mixed gas 1 of hydrogen and carbon oxides is brought into contact with a methanation catalyst packed in a methanation reactor 10 to carry out a methanation reaction. This methanation reaction produces a fuel gas 2 that is mainly composed of methane and contains 2% to 20% hydrogen by volume after dehydration.

[0037] In the dehydration hydrogenation reaction step, first, alcohol 3 is added to the fuel gas 2 obtained in the methanation step through a flow rate control valve 30. Then, the fuel gas 2 to which alcohol 3 has been added is fed into a dehydration hydrogenation reactor 20, and the fuel gas 2 to which alcohol 3 has been added is brought into contact with a dehydration hydrogenation catalyst packed in the dehydration hydrogenation reactor 20. Here, the dehydration hydrogenation catalyst is a catalyst active in the dehydration reaction of alcohol and the hydrogenation reaction of olefin. In the dehydration hydrogenation reaction step, paraffin hydrocarbons are produced by the reaction of alcohol with hydrogen. Since the hydrogen contained in the fuel gas 2 is consumed by this reaction, a fuel gas 4 with a reduced hydrogen concentration is obtained. The obtained fuel gas 4 contains methane as a main component and paraffin hydrocarbons derived from the alcohol 3. The alcohol 3 is one or more alcohols selected from alcohols having a carbon number of 2 to 4, and the carbon number of the paraffin hydrocarbons contained in the fuel gas 4 is the same as that of the alcohol 3 used.

[0038] [Methanation process conditions] The hydrogen and carbon oxides used in the mixed gas 1 in the methanation step may be produced by any method as long as they have a purity and properties that do not interfere with the methanation reaction in the presence of a methanation catalyst. The hydrogen may be electrolytic hydrogen obtained by electrolyzing water, for example. The carbon oxides are carbon monoxide or carbon dioxide, or a mixture thereof. The carbon dioxide may be carbon dioxide recovered from combustion exhaust gas by a known carbon dioxide recovery method such as an amine absorption method, or may be carbon dioxide contained in biogas obtained by methane fermentation of organic matter. Biogas is usually obtained as a mixed gas of methane and carbon dioxide, and carbon dioxide may be separated from this mixed gas and used in the methanation reaction, or methane may be used in the methanation reaction without being separated.

[0039] When the hydrogen and carbon oxides contain sulfur components, halogen compounds, siloxane compounds, heavy hydrocarbons, and the like, these may cause deterioration of the methanation reaction catalyst, and therefore it is preferable to remove these, as necessary, before subjecting the mixture to the methanation reaction.

[0040] As the methanation catalyst used in the methanation step, a known methanation catalyst containing Ni, Ru or the like can be used.

[0041] There are no particular restrictions on the type of methanation reactor 10 used in the methanation step, so long as the hydrogen concentration in the fuel gas 2 after the methanation reaction is 2% or more and 20% or less on a volumetric basis after dehydration. However, the methanation reaction is an equilibrium reaction that generates a relatively large amount of heat, and the equilibrium conversion rate varies greatly depending on the temperature and pressure, as shown in Figures 2 and 3, and is higher at lower temperatures and higher pressures. On the other hand, there are problems in that it is difficult to ensure the activity of the methanation catalyst at low temperatures, and that equipment costs are high at higher pressures.

[0042] In addition, since the methanation reaction generates a relatively large amount of heat, the gas temperature rises as the reaction proceeds, and the equilibrium conversion rate decreases accordingly, which cannot be ignored. For these reasons, when a fixed-bed adiabatic reactor, which is most commonly used in chemical processes, is used as the methanation reactor 10, it is usually difficult to obtain a desired conversion rate in one stage, so it is preferable to connect the reactors in multiple stages and introduce the outlet gas, whose temperature has risen due to the heat of reaction, into the next-stage reactor after cooling it.

[0043] FIG. 4 shows an example of the configuration of the methanation reactor 10 when it is provided as a fixed-bed adiabatic multi-stage reactor. In this reactor configuration, five sets of reactors (11a-15a) in which the methanation reaction proceeds and heat exchangers (11b-15b) are provided. In the configuration of the methanation reactor 10 shown in FIG. 4, when a raw material gas having a stoichiometric ratio of hydrogen:carbon dioxide = 4:1 (molar ratio) is reacted at an inlet temperature of 250°C at each stage and a reaction pressure of 0.7 MPa, the temperatures at the inlet and outlet of each stage and the conversion rate of carbon dioxide to methane are as shown in FIG. 5. At the outlet of the fourth stage (14a), the conversion rate of carbon dioxide to methane is 94.3%, and at this stage, the hydrogen concentration in the gas (based on the volume after dehydration with moisture removed; the same applies below) is 18.3%, which can be reduced to 20% or less. At the outlet of the fifth stage (15a), the conversion rate of carbon dioxide to methane is 97.7%, and the hydrogen concentration in the gas has dropped to 8.3%, making it possible to reduce it to 10%. Therefore, when methanation is performed by a simple adiabatic multi-stage reaction, under the above-mentioned temperature and pressure conditions, four stages are required to reduce hydrogen to 20% or less by volume, and five stages are required to reduce it to 10% or less. In addition, the large heat generated by methanation causes the outlet temperature of the first stage to reach approximately 700°C, which is problematic in terms of the durability of the catalyst.

[0044] FIG. 6 shows an example of the configuration in which the methanation reactor 10 is provided as a fixed-bed adiabatic multi-stage reactor having a recycle line. In this reactor configuration, three sets of reactors (11a-13a) in which the methanation reaction proceeds and heat exchangers (11b-13b) are provided, and the first set of these is provided with a recycle line 11c for returning the product after heat exchange to the reactor 11a. When this reactor configuration is used, the durability of the catalyst is improved because the temperature rise is suppressed by the dilution effect. In addition, the lowering of the reactor outlet temperature has the effect of increasing the equilibrium conversion rate, so that the number of reactor stages can be reduced in some cases compared to a simple multi-stage reactor configuration.

[0045] FIG. 7 shows an example of the configuration when the methanation reactor 10 is provided as a heat exchange type reactor. In this reactor configuration, the reaction can be carried out while maintaining the catalyst layer temperature at a predetermined temperature, which makes it possible to obtain a high conversion rate in a single-stage reactor. However, the heat exchange type reactor has a complex structure, and the cost of equipment and maintenance may be high. In addition, if the balance between the heat generated by the reaction and the heat removed by heat exchange is lost, localized high temperature areas may occur, causing the catalyst to deteriorate in a short period of time.

[0046] Figure 8 shows an example of a methanation reactor 10 configured by combining a fixed-bed adiabatic reactor having a recycle line and a heat exchange type reactor. In this reactor configuration, the set of reactors (12a, 13a) and heat exchangers (12b, 13b) from the second stage onwards in the example of Figure 6 is replaced with one heat exchange type reactor. In this reactor configuration, it is possible to reduce the number of reactor stages and suppress equipment costs while ensuring the durability of the catalyst.

[0047] [Example of Methanation Process] In the following, it is assumed that the methanation reactor 10 has the reactor configuration shown in FIG. 8, a sufficient amount of methanation catalyst is used, and an equilibrium composition for the methanation reaction and the CO shift reaction is obtained under specified conditions.

[0048] When the molar ratio of hydrogen to carbon dioxide (hydrogen / carbon dioxide molar ratio) at the inlet is in the range of 3.8 to 4.2 and the second-stage reaction temperature is 250°C, the concentrations of hydrogen and carbon dioxide contained in the gas after the methanation reaction are as shown in FIG. 9 when the reaction pressure is 0.7 MPa. When the hydrogen / carbon dioxide molar ratio is 4.0, the hydrogen concentration is 4.4% and the carbon dioxide concentration is 1.1%. When the hydrogen / carbon dioxide molar ratio is reduced, the hydrogen concentration decreases, but even when the molar ratio is reduced to 3.8, the hydrogen concentration does not become 2% or less and the carbon dioxide concentration increases to 5.6%. In other words, when the reaction pressure is 0.7 MPa, it is not possible to obtain a fuel gas with a sufficiently reduced hydrogen concentration and carbon dioxide concentration.

[0049] When the reaction pressure is 5 MPa, the hydrogen and carbon dioxide contained in the gas after the methanation reaction are as shown in FIG. 10. When the hydrogen / carbon dioxide molar ratio is 4.0, the hydrogen concentration is 2.04% and the carbon dioxide concentration is 0.51%. Under these conditions, a fuel gas can be obtained that generally meets the quality standard of a hydrogen concentration of 2% or less and a carbon dioxide concentration of 0.5% or less (this standard is the strictest among the generally known standards). However, under these conditions, even a slight decrease in the hydrogen / carbon dioxide molar ratio increases the carbon dioxide concentration, and conversely, a slight increase in the hydrogen / carbon dioxide molar ratio increases the hydrogen concentration. Thus, even a slight change in the hydrogen / carbon dioxide molar ratio greatly affects the purity of the resulting fuel gas.

[0050] In other words, when obtaining fuel gas mainly composed of methane through the methanation reaction between hydrogen and carbon dioxide only, in order to obtain fuel gas that satisfies the quality standards of a hydrogen concentration of 2% or less and a carbon dioxide concentration of 0.5% or less, a high-pressure reaction facility with an operating pressure sufficiently higher than 5 MPa is required.

[0051] [Conditions for the dehydration / hydrogenation reaction step] The alcohol 3 used in the dehydration hydrogenation reaction step is one or more alcohols selected from alcohols having from 2 to 4 carbon atoms. That is, the alcohol 3 is one type of alcohol selected from the group consisting of ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and 2-methyl-2-butanol, or a mixture of multiple alcohols selected from this group.

[0052] The alcohol 3 used in the dehydration hydrogenation reaction step may be produced by any method as long as it has a purity and properties that do not interfere with the reaction (described in detail later) that proceeds in the dehydration hydrogenation reaction step. For example, alcohol produced by a fermentation method using sugarcane, corn, or the like as a raw material can be suitably used. In the method for producing fuel gas according to this embodiment, the alcohol 3 may contain a normal amount of water. However, if the water content is too high, evaporation heat is required to vaporize the water, which may reduce the efficiency of fuel gas production. In consideration of this, the water content of the alcohol 3 is preferably 50 mass % or less.

[0053] The reactions in the dehydration / hydrogenation reactor 20 proceed as follows: That is, in the dehydration / hydrogenation reactor 20, the dehydration reaction of alcohol (Equation 3) and the hydrogenation reaction of olefins (Equation 4) proceed. C n H 2n+1 OH → C n H 2n +H2O (Eq. 3) C n H 2n + H2 → C n H 2n+2 (Formula 4) Here, n is 2, 3, or 4.

[0054] Here, paraffin hydrocarbons (C n H 2n+2 ) contains the paraffinic hydrocarbons corresponding to the alcohol 3 used. For example, if the alcohol 3 contains ethanol (C2H5OH, n=2), the paraffinic hydrocarbons contain ethane (C2H6).

[0055] If the amount of alcohol 3 added is small, the reduction in the hydrogen concentration in the fuel gas 4 will be insufficient. On the other hand, when the amount of alcohol 3 added is large, the hydrogen concentration becomes extremely low, so that the olefin hydrogenation reaction (formula 4) does not proceed sufficiently, and olefins may remain in the fuel gas 4. In addition, since the olefin concentration during the reaction is high, the olefins may polymerize on the catalyst, causing problems with deterioration of the catalyst due to carbon deposition. Therefore, the amount of alcohol 3 added is preferably set so that the molar ratio (alcohol) / (hydrogen) of alcohol 3 to hydrogen contained in fuel gas 2 obtained in the methanation step is 0.45 or more and 0.90 or less.

[0056] The dehydration / hydrogenation catalyst used in the dehydration / hydrogenation reaction step is preferably active in the dehydration reaction of alcohols (formula 3) and the hydrogenation reaction of olefins (formula 4), but is substantially inactive in the steam reforming reaction of paraffin hydrocarbons (specifically, methane, ethane, propane, and butane). If a catalyst active in the steam reforming reaction of paraffin hydrocarbons is used, even if hydrogen is reduced in the hydrogenation reaction of olefins, new hydrogen is generated by the steam reforming reaction of paraffin hydrocarbons, so that the hydrogen concentration in the fuel gas may not be reduced. An example of a catalyst that exhibits such reaction selectivity is a catalyst in which at least one of palladium and platinum is supported on a solid acid catalyst.

[0057] The reaction temperature when converting alcohol to paraffin hydrocarbons by reaction with hydrogen using a dehydration hydrogenation catalyst is preferably 200°C or higher and 400°C or lower. Since dehydration hydrogenation catalysts generally exhibit good activity at temperatures of 200°C or higher, the reaction between alcohol 3 and hydrogen (Equations 3 and 4) is likely to proceed when the reaction temperature is 200°C or higher. Furthermore, the steam reforming reaction of paraffin hydrocarbons is likely to be suppressed when the reaction temperature is 400°C or lower. The reaction temperature is more preferably 250°C or higher and 350°C or lower.

[0058] The type of the dehydration / hydrogenation reactor 20 used in the dehydration / hydrogenation reaction step is not particularly limited, and may be, for example, a fixed-bed adiabatic reactor, a fixed-bed adiabatic reactor having a recycle line, a heat exchange type reactor, or the like.

[0059] [Example of the dehydration hydrogenation reaction process] In the following, an example will be described in which the reactor configuration shown in Fig. 8 is adopted as the methanation reactor 10, and a dehydration / hydrogenation reactor 20 (not shown) configured as a fixed-bed adiabatic reactor is provided downstream of the methanation reactor 10. In this example, ethanol is used as the alcohol 3. In the methanation reactor 10, the methanation reaction and the CO shift reaction reach equilibrium, and in the dehydration / hydrogenation reactor 20, the dehydration reaction of ethanol and the hydrogenation reaction of ethylene reach equilibrium.

[0060] In the following example, the reaction temperature of the heat exchange reactor 12d in the methanation reactor 10 was set to 250°C. The inlet temperature of the dehydration hydrogenation reactor 20 was set to 250°C so that the reaction proceeded under adiabatic conditions. The reaction pressure was set to 0.7 MPa in both the methanation reactor 10 and the dehydration hydrogenation reactor 20. The concentrations of hydrogen and carbon dioxide in the fuel gas 4 obtained in this case are shown in Figures 11 and 12 relative to the hydrogen / carbon dioxide molar ratio of the mixed gas 1 supplied to the methanation reactor 10.

[0061] FIG. 11 shows the relationship between the composition of the mixed gas 1 and the concentrations of hydrogen and carbon dioxide in the fuel gas 4 when the molar ratio of the amount of ethanol added to the hydrogen contained in the fuel gas 2 is 0.9. When the hydrogen / carbon dioxide molar ratio of the mixed gas 1 is 4.0, the hydrogen concentration is 0.44% and the carbon dioxide concentration is 1.09%. When only the methanation process is performed under similar conditions, the hydrogen concentration is 4.4% and the carbon dioxide concentration is 1.1% (FIG. 9). This shows that the hydrogen concentration of the fuel gas 2 obtained by the methanation process is significantly reduced by the dehydration reaction process. In addition, when the hydrogen / carbon dioxide molar ratio is 4.04, the hydrogen concentration is 0.55%, the carbon dioxide concentration is 0.43%, the methane concentration is 94.1%, and the ethane concentration is 4.9%, and the concentration of the hydrocarbon (methane + ethane) component exceeds 99%. When only the methanation step was carried out, the hydrogen concentration was 5.5% and the carbon dioxide concentration was 0.43%, so it can be seen that even in this case, the hydrogen concentration was significantly reduced by the dehydration hydrogenation reaction step.

[0062] Although the methanation step and the dehydration / hydrogenation step (dehydration and hydrogenation) are both performed at the same inlet temperature (250°C), hydrogen remains in the methanation step and hydrogen is reduced in the dehydration / hydrogenation step for the following reasons. The methanation reaction of carbon dioxide is an exothermic reaction that generates 41 kJ per mole of hydrogen. On the other hand, the hydrogenation reaction of ethylene is a reaction that generates a slightly large amount of heat (136 kJ exothermic per mole of ethylene), and even when combined with the dehydration reaction of ethanol (endothermic heat of 45 kJ per mole of ethanol), it is an exothermic reaction of about 90 kJ per mole of hydrogen. Therefore, the equilibrium between the production of ethylene by dehydration of ethanol and the production of ethane by the subsequent hydrogenation of ethylene is significantly biased toward the production of ethane. For this reason, the ethanolysis reaction proceeds sufficiently even under temperature conditions that leave hydrogen in the methanation step.

[0063] This explanation also applies to other alcohols that may be included in alcohol 3. For example, when alcohol 3 includes 1-propanol, the endothermic heat in the dehydration reaction of 1-propanol is 33 kJ per mole of 1-propanol, and the heat generated in the hydrogenation reaction of propylene is 125 kJ per mole of propylene, so that the part of the dehydration hydrogenation reaction process involving 1-propanol is an exothermic reaction of 92 kJ per mole of hydrogen. For other alcohols that may be included in alcohol 3, the entire dehydration hydrogenation reaction process is an exothermic reaction of about 65 to 95 kJ per mole of hydrogen. Therefore, similar to the explanation for the case where ethanol is used above, it can be seen that the reaction in the dehydration hydrogenation reaction process also proceeds sufficiently for other alcohols that may be included in alcohol 3.

[0064] In the dehydration hydrogenation reaction step, the carbon dioxide cannot be reduced. If the hydrogen / carbon dioxide molar ratio used in the methanation reaction is slightly higher than 4.0 and the methanation step is performed under conditions of a slight excess of hydrogen, the carbon dioxide concentration in the fuel gas 2 after the methanation step can be reduced. Here, the hydrogen concentration in the fuel gas 2 increases due to the excess addition of hydrogen, but this hydrogen can be reduced in the dehydration hydrogenation reaction step. If the hydrogen / carbon dioxide molar ratio of the mixed gas 1 used in the methanation step is 4.04, the hydrogen concentration in the resulting fuel gas is 0.55% and the carbon dioxide concentration is 0.43%. In other words, the quality standards of a hydrogen concentration of 2% or less and a carbon dioxide concentration of 0.5% or less, which cannot be achieved even under high-pressure reaction conditions of 5 MPa by the methanation step alone, can be achieved by a reaction at a low pressure of 0.7 MPa by using the method according to this embodiment.

[0065] If the hydrogen / carbon dioxide molar ratio of the mixed gas 1 to be subjected to the methanation step is further increased, the carbon dioxide concentration in the resulting fuel gas can be further reduced. For example, if the hydrogen / carbon dioxide molar ratio of the mixed gas 1 to be subjected to the methanation step is 4.2, the hydrogen concentration in the resulting fuel gas 4 will be 1.7%, and the carbon dioxide concentration will be 0.1% or less. In addition, the methane concentration will be 83.3%, and the ethane concentration will be 15.0%, so that the carbon dioxide concentration in 1 m3 of fuel gas will be 1.7%, and the carbon dioxide concentration in 1 m3 of fuel gas will be 1.7%. 3The calorific value per unit is 44 MJ, making the gas almost identical in properties to general natural gas-based city gas.

[0066] On the other hand, in addition to the fact that a larger amount of alcohol 3 is required to be added, when the reaction (dehydration and hydrogenation) of the dehydration hydrogenation reaction step is performed under adiabatic conditions, the outlet temperature of the reaction of the dehydration hydrogenation reaction step increases, and the olefin concentration increases, which may cause deterioration of the catalyst due to carbon deposition, and an increase in the hydrogen and carbon dioxide concentrations due to the simultaneous occurrence of a steam reforming reaction. From this perspective, it is preferable that the hydrogen / carbon dioxide molar ratio of the mixed gas 1 to be subjected to the methanation step does not exceed 4.08. If the hydrogen / carbon dioxide molar ratio is in the range of 4.04 to 4.08, it is easy to achieve the quality standard of a hydrogen concentration of 2% or less and a carbon dioxide concentration of 0.5% or less. In addition, when at least one of carbon monoxide and carbon dioxide is contained as carbon oxides, if the ratio of hydrogen to carbon oxides supplied to the methanation step satisfies the value of {(hydrogen)+(carbon monoxide)} / {(carbon monoxide)+(carbon dioxide)} calculated on a substance amount basis of 4.04 to 4.08, the above quality standard is easy to achieve.

[0067] FIG. 12 shows the relationship between the composition of the mixed gas 1 and the concentrations of hydrogen and carbon dioxide in the fuel gas 4 when the molar ratio of the amount of ethanol added to the hydrogen contained in the fuel gas 2 is 0.5. In the example of FIG. 12, the amount of ethanol added is smaller than that in the example of FIG. 11, so that the hydrogen concentration in the resulting fuel gas 4 is higher than that in FIG. 11, but it can be understood that the hydrogen concentration in the fuel gas 4 is reduced compared to FIG. 9. If the hydrogen / carbon dioxide molar ratio of the mixed gas 1 is in the range of 4.04 to 4.08, all of the quality standards of hydrogen concentration 4% or less, carbon dioxide concentration 0.5% or less, and components other than hydrocarbons 4% or less can be achieved. This condition cannot be realized in a methanation reaction at 0.7 MPa, no matter how the hydrogen / carbon dioxide molar ratio of the mixed gas to be subjected to the methanation reaction is set, so it is clear that the method of the present invention is useful for producing high-quality fuel gas without using high-pressure reaction equipment. Even in this case, similarly to the above, when at least one of carbon monoxide and carbon dioxide is contained as the carbon oxides, if the ratio of hydrogen to carbon oxides supplied to the methanation step is a ratio that satisfies the value of {(hydrogen)+(carbon monoxide)} / {(carbon monoxide)+(carbon dioxide)}, calculated on an amount of substance basis, of 4.04 or more and 4.08 or less, the above quality standard is easily achieved.

[0068] Other embodiments Finally, other embodiments of the method for producing fuel gas according to the present invention will be described. Note that the configurations disclosed in the following embodiments can be combined with the configurations disclosed in other embodiments as long as no contradiction occurs.

[0069] In the above embodiment, the methanation reactor 10 used in the methanation step is particularly described as a combination of a fixed-bed adiabatic reactor having a recycle line and a heat exchange reactor, as shown in Fig. 8. However, the reactor used in the methanation step according to the present invention is not particularly limited as long as it can produce a fuel gas containing 2% by volume or more and 10% by volume or less of hydrogen. Examples of such a reactor include a fixed-bed adiabatic multi-stage reactor and a single-stage or multi-stage heat exchange reactor.

[0070] In the above embodiment, the pressure in the methanation step is 0.7 MPa. However, the pressure in the methanation step according to the present invention is not particularly limited as long as a fuel gas containing 2% by volume or more and 10% by volume or less of hydrogen is obtained. However, if the pressure is 0.5 MPa or more, the hydrogen and carbon dioxide contents in the fuel gas obtained in the methanation step are reduced, and therefore the hydrogen and carbon dioxide contents in the final fuel gas product are also likely to be reduced. In addition, since the cost of the equipment used in the methanation step is easily reduced, the pressure is preferably 3 MPa or less, and more preferably 1 MPa or less.

[0071] In the above embodiment, the dehydration / hydrogenation reactor 20 used in the dehydration / hydrogenation reaction step is particularly described as a fixed-bed adiabatic reactor. However, the reactor used in the dehydration / hydrogenation reaction step according to the present invention is not particularly limited as long as the reactions represented by the formulas 3 and 4 proceed. Examples of such a reactor include a fixed-bed adiabatic reactor having a recycle line and a heat exchange reactor.

[0072] In the above embodiment, the pressure in the dehydration hydrogenation reaction step is 0.7 MPa. However, the pressure in the dehydration hydrogenation reaction step according to the present invention is not particularly limited as long as the reactions represented by formulas 3 and 4 proceed. However, if the pressure is 0.5 MPa or more, the contents of hydrogen and carbon dioxide in the resulting fuel gas can be easily reduced without using a large amount of the dehydration hydrogenation catalyst. In addition, the pressure is preferably 3 MPa or less, and more preferably 1 MPa or less, because it is easy to reduce the cost of the equipment used in the dehydration hydrogenation reaction step.

[0073] In the above embodiment, the pressure in the methanation step and the dehydration hydrogenation reaction step are the same (0.7 MPa). However, the pressure in the methanation step and the dehydration hydrogenation reaction step according to the present invention may be the same or different.

[0074] The present invention does not exclude the use of conventionally known means for improving methane conversion, and may be configured, for example, to cool the reaction gas and condense and separate a portion of the water vapor in the middle of a multi-stage reaction. Even in this case, the use of the method of the present invention makes it possible to reduce the concentrations of hydrogen and carbon dioxide in the fuel gas without excessively removing water vapor. EXAMPLES

[0075] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0076] Example 1 A silica support (Fuji Silysia Chemical, Cariact G-6) was impregnated with an aqueous solution of tungstosilicic acid (SiO2·12WO3·26H2O) and palladium nitrate, evaporated to dryness in a water bath, dried at 120°C, and calcined in air at 350°C for 4 hours to prepare a catalyst (Pd / SiW / silica catalyst) in which 20 parts by mass of tungstosilicic acid and 2 parts by mass of palladium are supported on 80 parts by mass of silica support. In this catalyst, the tungstosilicic acid supported on the silica support forms a solid acid, and palladium is supported on this.

[0077] The Pd / SiW / silica catalyst was tableted and classified into 1.0 to 2.5 mm tablets. 6 mL of the tablets were packed into a SUS reaction tube (inner diameter 16 mm). The tube was heated in an electric furnace while passing a gas of 2% hydrogen and the balance nitrogen until the temperature reached 225°C. The tube was held for 2 hours to reduce the catalyst, and the temperature was then further raised and held at 275°C.

[0078] Ethanol was added to 400 mL per minute (volume at standard conditions of 0°C and 101.325 kPa) of fuel gas consisting of 32.6% methane, 65.2% steam, 2.0% hydrogen, and 0.14% carbon dioxide, so that the molar ratio of ethanol to hydrogen in the fuel gas was 0.46. This fuel gas with added ethanol was passed through the catalyst under a pressure of 0.7 MPa (absolute pressure).

[0079] The composition of 32.6% methane, 65.2% water vapor, 2.0% hydrogen, and 0.14% carbon dioxide corresponds to the equilibrium composition when a gas with a (hydrogen) / (carbon dioxide) value of 4.04, calculated on a mass of substance basis, is subjected to a methanation reaction at a temperature of 250°C and a pressure of 0.7 MPa (absolute pressure), and simulates the fuel gas obtained in the methanation process, which contains approximately 6% hydrogen on a volumetric basis after dehydration.

[0080] After the reaction, the fuel gas was cooled to separate water, and the concentrations of methane, ethane, hydrogen, carbon monoxide, and carbon dioxide were quantified using a gas chromatograph.

[0081] The concentrations of each component in the fuel gas after the reaction (by volume after dehydration) were: hydrogen 3.1%, carbon dioxide 0.44%, carbon monoxide 0.09%, ethane 2.6%, and the remainder methane.

[0082] Example 2 The same test as in Example 1 was carried out, except that the amount of ethanol added was changed so that the molar ratio of ethanol to hydrogen in the fuel gas was 0.79.

[0083] The concentrations of each component in the fuel gas after the reaction (based on the volume after dehydration) were: hydrogen 1.42%, carbon dioxide 0.45%, carbon monoxide 0.08%, ethane 4.5%, and the remainder methane.

[0084] Comparative Example 1 The same test as in Example 1 was carried out, except that ethanol was not added.

[0085] The concentration of each component in the fuel gas after the reaction (volume basis after dehydration) was 5.7% hydrogen, 0.39% carbon dioxide, and the remainder methane. No carbon monoxide was detected.

[0086] [Evaluation of Examples 1 and 2 and Comparative Example 1] As shown in Comparative Example 1, when ethanol is not added to the fuel gas containing hydrogen, there is substantially no reaction even when the fuel gas is passed through the Pd / SiW / silica catalyst. On the other hand, as shown in Example 1, when ethanol is added to the fuel gas containing hydrogen so that the molar ratio of ethanol to hydrogen is 0.46 and then the fuel gas is passed through the Pd / SiW / silica catalyst, hydrogen reacts quantitatively with ethanol to produce water vapor and ethane, and the fuel gas is cooled and the water is separated to remove the water vapor, so that the hydrogen concentration in the fuel gas is reduced and ethane is produced. Under the conditions of Example 1, the hydrogen concentration is reduced to 3.1%, so that the standard of a hydrogen concentration of 4% can be met. As shown in Example 2, when ethanol is added to the fuel gas containing hydrogen so that the molar ratio of ethanol to hydrogen is 0.79 and then the fuel gas is passed through the Pd / SiW / silica catalyst, the hydrogen concentration is reduced to 1.42%, so that even a standard of a hydrogen concentration of 2% can be met.

[0087] The carbon dioxide concentrations in the fuel gas after the reaction were 0.44%, 0.45%, and 0.39% in Examples 1 and 2 and Comparative Example 1, respectively. In Examples 1 and 2, 0.08 to 0.09% of carbon monoxide was detected in the fuel gas after the reaction. This is thought to be because the steam reforming reaction of ethanol proceeded slightly, and a part of the ethanol was converted to carbon monoxide, carbon dioxide, and hydrogen. Nevertheless, most of the reacted ethanol was converted to ethane, and the carbon dioxide concentration was suppressed to 0.5% or less and the carbon monoxide concentration to 0.1% or less, so it can be said that there is no major problem in using the generated fuel gas as city gas. [Industrial Applicability]

[0088] The present invention can be used, for example, as a method for producing fuel gas to be supplied as city gas. [Explanation of symbols]

[0089] 1: Mixed gas 2: Fuel gas 3: Alcohol 4: Fuel gas 10: Methanation reactor 11a to 15a: Reactor 11b~15b: Heat exchanger 11c: Recycling Line 12d: Heat exchange reactor 20: Dehydration and hydrogenation reactor 30: Flow control valve

Claims

1. a methanation step of reacting hydrogen with carbon oxides in the presence of a methanation catalyst to obtain a fuel gas containing 2% to 20% hydrogen by volume after dehydration; a dehydration / hydrogenation reaction step of adding alcohol to the fuel gas obtained in the methanation step, and then reacting the alcohol with hydrogen in the fuel gas in the presence of a dehydration / hydrogenation catalyst to obtain a fuel gas containing paraffin hydrocarbons having a carbon number of 2 to 4, The alcohol is one or more alcohols selected from alcohols having 2 to 4 carbon atoms, The dehydration / hydrogenation catalyst has activity in the dehydration reaction of alcohol and the hydrogenation reaction of olefin.

2. a ratio of hydrogen to carbon oxides supplied to the methanation step is such that the value of {(hydrogen)+(carbon monoxide)} / {(carbon monoxide)+(carbon dioxide)} calculated on a substance amount (molar) basis is 4.04 or more and 4.08 or less; 2. The method for producing a fuel gas according to claim 1, wherein the gas supplied to the dehydration hydrogenation reaction step has a molar ratio of alcohol having a carbon number of 2 to 4 to hydrogen in the fuel gas obtained in the methanation step of 0.45 to 0.

9.

3. the dehydration hydrogenation catalyst comprises a solid acid catalyst and at least one of palladium and platinum; 3. The method for producing a fuel gas according to claim 1, wherein the at least one of palladium and platinum is supported on the solid acid catalyst.

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