JP6956665B2 - Method of methaneization of carbon dioxide in combustion exhaust gas and methane production equipment - Google Patents

Description

The present invention relates to a methaneization method for obtaining methane by separating carbon dioxide contained in combustion exhaust gas and reacting it with hydrogen.

Methane is produced by the following equation using carbon dioxide, for example. This reaction itself has been known for a long time.
CO ₂ + 4H ₂ aCH ₄ + 2H ₂ O (g) ΔH = -165.0 kJ / mol

Patent Document 1 discloses a methanation reactor that methanizes a gas containing _{CO and H 2.} In the methanation reactor, a Cu—Zn-based low-temperature shift catalyst is arranged on the upstream side, and a methanation catalyst is arranged on the downstream side. In this methanation reactor, the _{gas containing CO and H 2} is methanized. In the low temperature shift reactor on the upstream side, the CO shift reaction of the following equation proceeds.
CO + H ₂ OaCO ₂ + H ₂

It is considered that most of the carbon monoxide is converted to carbon dioxide by reacting with water vapor by the low temperature shift catalyst, and the methaneization reaction of carbon dioxide is proceeding on the methanation catalyst. Methanation catalysts have long been used to remove carbon monoxide and carbon dioxide from hydrogen for ammonia synthesis. As the methanation catalyst, it is known that a catalyst supporting Ni or Ru exhibits high activity (Non-Patent Documents 1 and 2).

Patent Document 2 discloses a catalyst for a methaneization reaction for producing methane by hydrogenating carbon dioxide, carbon monoxide and carbon dioxide, or a mixed gas containing these as main components. When the total amount of the metallization reaction catalyst based on the metal in the elemental state is 100%, one or more rare earth elements M selected from Sm, Ce, La and Y are 2.4 to 18. 75 atomic%, Zr occupies 22.5-69 atomic%. However, the ratio of Zr: M (atomic ratio) = 3 to 11.5. Further, it has a chemical composition in which Ni, which is an iron group element responsible for catalytic activity, accounts for 25 to 70 atomic%. Therefore, the catalyst for the methanation reaction is formed by supporting Ni in a metallic state on an oxide having a tetragonal zirconia structure that is stabilized by incorporating a part of Ni into the crystal structure together with a rare earth element. This catalyst shows high activity even at a low temperature of 250 ° C. or lower, and is said to be suitable for producing methane from a mixed gas composed of carbon dioxide, carbon monoxide and hydrogen obtained by gasifying biomass or the like. ..

When methane production by a methanation reaction is carried out on a large scale, the issue is what to look for as a raw material for carbon dioxide. Large-scale carbon dioxide sources include thermal power plants that use coal or biomass as fuel, and industrial processes such as steel and cement manufacturing. As a method of separating carbon dioxide from the combustion exhaust gas of carbon-containing fuels such as coal and biomass, a chemical absorption method that chemically separates carbon dioxide into an absorption liquid using a solvent such as amine, and physically carbon dioxide under high pressure. Known methods include a physical absorption method in which an absorption solution absorbs and separates the substance, a solid absorption method in which an amine or the like is supported on a solid instead of a solvent and used as an absorbent, and a film separation method using a film that selectively permeates carbon dioxide. There is.

Among these, the chemical absorption method has already been technically established and put into practical use. In the chemical absorption method, monoethanolamine, methyldiethanolamine, piperazine, 2-amino-2-methylpropanol and the like are used as amines, and one or two or more of these are combined to form an aqueous solution as an absorption solution. In the separation of carbon dioxide from the combustion exhaust gas by the chemical absorption method, the combustion exhaust gas is brought into contact with the absorption liquid at a temperature of about 60 ° C. from room temperature in the absorption tower, and the carbon dioxide in the combustion exhaust gas is absorbed by the absorption liquid. Next, the absorption liquid is guided to the regeneration tower and heated to about 100 ° C. to 150 ° C. to release carbon dioxide from the absorption liquid that has absorbed carbon dioxide. The absorption liquid that has released carbon dioxide exchanges heat with the absorption liquid that has absorbed carbon dioxide as needed, is further cooled, and then is used again as the absorption liquid.

The chemical absorption method and the solid absorption method require a relatively large amount of heat in the separation step after absorbing carbon dioxide. The chemical absorption method requires heat of _{90 to 150 kJ (2 to} 3.5 MJ / kg-CO 2) per 1 mol of carbon dioxide. However, since the methanation reaction is accompanied by heat generation of 165 kJ per 1 mol of carbon dioxide, the heat generation of the methanation reaction can cover the above-mentioned required heat.

Pat. A metanation process is disclosed that involves the conversion of carbon dioxide produced or obtained, or more particularly carbon dioxide gas, to methane at a metanation plant.

The thermal energy generated as waste heat from the conversion of carbon dioxide to methane in the metanation plant is at least partially extracted into at least one material stream and / or thermal energy stream, and this at least one material stream and / Alternatively, the thermal energy stream flows into at least one medium flowing into the combustion chamber of the steam generator of the power station on the burner side, into the water / steam circuit of the power station, upstream of the metanational plant from a process engineering point of view. Connected carbon dioxide exhaust gas treatment or carbon dioxide treatment, more specifically to the power station flue gas treatment plant and / or to one or more operating stages of ancillary industrial plants, at least partially. The metanational process characterized by the above is disclosed.

Patent Document 4 describes a generator that converts the energy obtained by burning fossil fuel into electric power, a dehydrogenation reactor that generates hydrogen from a hydrogenated aromatic compound by a dehydrogenation reaction, and the generator by a methaneation reaction. An energy supply system including a metanation reaction device that produces methane from carbon dioxide in the exhaust gas generated by combustion in the above and hydrogen generated from the dehydrogenation reaction device is disclosed. The generator is characterized by burning methane produced by the metanation reactor together with the fossil fuel. Further, Patent Document 4 describes that the amount of heat generated by the metanation reaction in the metanation reactor may _{be supplied to the regeneration unit of the CO 2} separation apparatus and used for regeneration of monoethanolamine.

The methanation reaction is generally carried out in a temperature range of about 250 ° C. to 700 ° C., whereas the amine absorption liquid in the chemical absorption method is regenerated up to about 150 ° C.

Carbon-containing fuels such as coal and biomass usually contain sulfur. This sulfur content changes to sulfur oxides (sulfur dioxide and sulfur trioxide) with combustion. In the chemical absorption method and the solid absorption method, sulfur oxides are also absorbed by the absorbent or the absorbent, but a part of them is released when carbon dioxide is released from the absorbent or the absorbent. Therefore, the separated carbon dioxide contains a trace amount of sulfur oxides.

In Non-Patent Document 3, in the separation of carbon dioxide from the exhaust gas of a coal-fired power plant, carbon dioxide was separated from the exhaust gas after desulfurization of the exhaust gas to reduce the sulfur dioxide concentration to 10 ppm by a chemical absorption method using amine. In this case, the concentration of sulfur dioxide contained in the separated carbon dioxide is estimated to be 34 to 135 ppm.

As described above, a catalyst containing Ni or Ru as an active component is used as the methanation catalyst, but these have a problem that they are very susceptible to poisoning by sulfur. Catalysts containing Ni or Ru as active ingredients are widely used not only for methanation but also for steam reforming, which is the reverse reaction of methanation. Sulfur poisoning is also a serious problem in the steam reforming reaction, and therefore various hydrocarbon desulfurization methods are being studied.

For example, Patent Document 5 uses a higher-order desulfurization agent obtained by hydrogen-reducing a copper oxide-zinc oxide-aluminum oxide mixture prepared by a co-precipitation method using a copper compound, a zinc compound and an aluminum compound as raw materials. , It has been shown that the sulfur content in the hydrocarbon can be 5 ppb or less.

In Patent Document 6, a mixture containing a copper compound and a zinc compound and an aqueous solution of an alkaline substance are mixed to generate a precipitate, and the obtained precipitate is calcined to obtain a copper oxide-zinc oxide mixture molded product. A desulfurizing agent is disclosed, which comprises impregnating this molded product with iron and / or nickel, further calcining it, and hydrogen-reducing the obtained oxide calcined product.

It is said that when these desulfurizing agents are used, hydrocarbons, which are raw materials for steam reforming, are highly desulfurized, so that deterioration of the steam reforming catalyst containing Ni or Ru as an active ingredient can be suppressed.

The sulfur component contained in the hydrocarbon is hydrogen sulfide and an organic sulfur compound such as thiol and sulfide, and sulfur dioxide is not usually contained. Carbon dioxide may be separated from methane fermentation gas or gasification gas of coal or biomass, but the sulfur compounds contained in such gas are also hydrogen sulfide, COS and organic sulfur compounds, and sulfur dioxide is usually Not included.

As a method for removing sulfur dioxide from the gas to be treated, there is a wet desulfurization method by the lime gypsum method used in coal-fired power plants, etc., but the removal rate of sulfur dioxide is generally limited to about 95% (non-). Since Patent Document 3), it is not sufficient to protect the methanation catalyst.

A method for purifying carbon dioxide is also known. Patent Document 7 is a method for purifying a carbon dioxide gas stream, in which the carbon dioxide gas stream to be treated is selected from the group consisting of a desiccant, a zeolite or an ion-exchanged zeolite, and activated carbon. Disclosed are methods that include passing through at least one adsorbent bed containing at least two adsorbent layers. According to this method, water, sulfur species and other impurities are said to be removed from carbon dioxide.

Patent Document 8 describes a step of treating a carbon dioxide-containing supply flow such as combustion exhaust gas to remove SOx and NOx with activated carbon, a step of treating at an atmospheric temperature or lower to produce a product flow and an exhaust flow, and a pressure. A method of producing a high-purity carbon dioxide stream is disclosed by a series of steps including a step of treating the exhaust stream by swing adsorption or physical or chemical absorption to produce a product stream that is recirculated to the supply stream. ..

Although these methods can obtain high-purity carbon dioxide, there is a problem that the purification cost is very high. The carbon dioxide-based gas released from the amine absorber contains water vapor, but the water vapor does not strongly inhibit the methanation reaction, but rather has the effect of suppressing carbon precipitation, so it is a large cost. There is no need to remove it with water vapor.

Japanese Unexamined Patent Publication No. 60-235893 JP-A-2009-34650 Special Table 2016-531973 Japanese Unexamined Patent Publication No. 2015-51901 Japanese Unexamined Patent Publication No. 1-259088 Japanese Unexamined Patent Publication No. 11-61154 Special Table 2009-504383 Gazette Special Table 2012-503543

Edited by Chemical Engineering Association, Chemical Process Collection, 1970, p.153. Catalysis Society, Catalysis Handbook, 2008, p.535. Lee, Keener and Yang, Journal of Air & Waste Management Association, Vol. 59, 2009, p.725-732.

In view of the above problems, the problem to be solved by the present invention is that carbon dioxide contained in the combustion exhaust gas is contained in the combustion exhaust gas and methaneized when methane is obtained by separating carbon dioxide and reacting with hydrogen. It is an object of the present invention to provide a technique capable of suppressing the concentration of a sulfur compound which is a catalyst poison of a catalyst.

The characteristic composition of the methanation method according to the present invention is
It is a methanation method that obtains methane by a methanation reaction using a catalyst using hydrogen and carbon dioxide contained in the combustion exhaust gas of carbon-containing fuel as raw materials.
a) A step of bringing the combustion exhaust gas into contact with a carbon dioxide absorbent to absorb carbon dioxide in the combustion exhaust gas.
b) A step of heating the carbon dioxide absorbing material that has absorbed carbon dioxide to extract a first gas containing carbon dioxide as a main component, and
c) The first hydrogen, which is a first amount of hydrogen, is added to the first gas to obtain a second gas, and the second gas is passed through a desulfurizer filled with a desulfurizing agent to allow sulfur in the second gas. The process of removing the compound and
d) A point including a step of adding a second amount of hydrogen, the second hydrogen, to the third gas that has undergone the step of removing the sulfur compound, and converting it into methane by a methanation reaction through a methanation catalyst. It is in.

According to this characteristic configuration, even if the sulfur compound is contained in the carbon dioxide-based gas (first gas) separated from the combustion exhaust gas, the sulfur compound is treated so as to be removed by the desulfurizing agent. NS. Therefore, according to the above configuration, carbon dioxide having a reduced sulfur compound or carbon dioxide containing substantially no sulfur compound can be obtained. Therefore, when carbon dioxide and hydrogen are reacted to carry out a methanation reaction, sulfur poisoning of the methanation catalyst is suppressed.

Further, at the time of desulfurization, a first amount of first hydrogen is added to a gas containing carbon dioxide as a main component (first gas). Then, after that, a second amount of hydrogen is additionally added to the third gas in which the second gas composed of the first gas and the first hydrogen is desulfurized, and the methanation reaction is carried out. Therefore, by limiting the amount of the first hydrogen added at the time of desulfurization to the first amount, the second gas (gas containing the first gas and the first hydrogen) supplied to the desulfurization agent at the time of desulfurization The amount can be suppressed, the contact time between the sulfur compound containing carbon dioxide, sulfur dioxide and the like, and the second gas such as hydrogen and the desulfurizing agent can be secured, and the deterioration of the desulfurization performance can be suppressed.

In addition, compared to the case where a large amount of hydrogen is added to carbon dioxide at once, the reaction between carbon dioxide and hydrogen is achieved by adding hydrogen stepwise to the gas containing carbon dioxide as the main component (first to third gases). It is possible to suppress an excessive rise in temperature due to the heat of reaction caused by carbon dioxide. Thereby, the desulfurization reaction can be safely carried out.

Further characteristic configurations of the methanation method according to the present invention are:
The first amount is less than the total amount of hydrogen required to complete the methanation reaction that produces methane by reacting the first gas with hydrogen.

According to this characteristic configuration, the first amount of the first hydrogen is less than the total amount of hydrogen required to react the first gas with hydrogen to complete the methanation reaction. Therefore, compared to the case where a large amount of hydrogen is added to carbon dioxide at one time, the contact time between the sulfur compound containing carbon dioxide, sulfur dioxide, etc., and the second gas such as hydrogen, and the desulfurizing agent is secured, and the desulfurization performance is improved. The decrease can be suppressed and the desulfurization reaction can be safely carried out.

Further characteristic configurations of the methanation method according to the present invention are:
The second amount is at least the amount obtained by subtracting the first amount from the total hydrogen amount.

According to this characteristic configuration, the second amount of dihydrogen is greater than or equal to the total amount of hydrogen required to complete the methanation reaction minus the first amount. By adding such a second amount of secondary hydrogen to carbon dioxide that has undergone the desulfurization step, methane can be efficiently produced by carbon dioxide and hydrogen.

Further characteristic configurations of the methanation method according to the present invention are:
The point is that the volume% of the first hydrogen with respect to the second gas is 0.1% by volume or more and 5% by volume or less.

According to this characteristic configuration, by setting the first amount of the first hydrogen in the above range, it is possible to efficiently perform desulfurization and suppress an excessive rise in temperature due to the heat of reaction due to the reaction between carbon dioxide and hydrogen. Can be done.
When the volume% of the first hydrogen in the second gas is 0.1% by volume or more, the reaction rate of the desulfurization reaction can be increased, and the completion of the desulfurization reaction can be almost achieved. On the contrary, when the volume% of the first hydrogen in the second gas is less than 0.1% by volume, the reaction rate of the desulfurization reaction may be slow and the completion of the desulfurization reaction may not be achieved.
Further, when the volume% of the first hydrogen in the second gas is 5% by volume or less, it is possible to suppress an excessive rise in temperature due to the heat of reaction due to the progress of the methanation reaction. On the contrary, when the volume% of the first hydrogen in the second gas exceeds 5% by volume, the temperature may rise excessively due to the heat of reaction due to the progress of the methanation reaction.

Further characteristic configurations of the methanation method according to the present invention are:
The volume% of the first hydrogen with respect to the second gas is 10 times or more the volume% of the sulfur compound with respect to the second gas.

According to this characteristic configuration, the volume of the first hydrogen with respect to the second gas is 10 times or more the volume of the sulfur compound with respect to the second gas. As a result, the reaction rate of the desulfurization reaction can be increased, and the completion of the desulfurization reaction can be almost achieved.

Further characteristic configurations of the methanation method according to the present invention are:
In the step b), the reaction heat from the methanation reaction in the step d) is used to heat the carbon dioxide absorbent to take out the first gas.

According to this characteristic configuration, the energy required for the separation of carbon dioxide can be covered by the heat of reaction of the methanation reaction. Therefore, since energy input is substantially unnecessary for the desulfurization reaction, efficient methanation and economically excellent methanation are possible according to the methanation method of the present invention.

Further characteristic configurations of the methanation method according to the present invention are:
The point is that the carbon dioxide absorbent is an aqueous solution of an amine compound.

According to this characteristic configuration, carbon dioxide can be separated from combustion exhaust gas at low cost by using an amine compound, and economically excellent methanation is possible. In the separation of carbon dioxide from the combustion exhaust gas by the chemical absorption method using an amine compound solution, the separated carbon dioxide may contain a trace amount of sulfur compound. However, according to the above-mentioned characteristic composition, since the sulfur compound contained in carbon dioxide is desulfurized with a desulfurizing agent while adding a first amount of primary hydrogen, carbon dioxide can be inexpensively used by using an inexpensive amine compound. Desulfurization can also be achieved while separating.

Further characteristic configurations of the methanation method according to the present invention are:
The point is that the desulfurizing agent contains at least one component metal or oxide composed of iron, nickel, cobalt and copper and zinc oxide.

The desulfurizing agent having this characteristic structure is an inexpensive desulfurizing agent having a large amount of sulfur adsorbed. Therefore, the replacement cost of the desulfurizing agent can be suppressed, and methanation can be carried out by an economically excellent method.

Further characteristic configurations of the methanation method according to the present invention are:
The first hydrogen and the second hydrogen are obtained by electrolyzing water using surplus electric power from a power generation facility.

According to this feature configuration, surplus electricity from the power generator is converted to hydrogen, and this hydrogen and carbon dioxide separated from the combustion exhaust gas are used for methane conversion, so methane is injected into the existing city gas pipeline and used. can. Therefore, it is not necessary to newly install a gas storage facility. Further, since it is not necessary to go through the process of conversion from hydrogen to electric power other than the conversion from surplus electric power to hydrogen, it is possible to suppress the decrease in the amount of available electric power due to the decrease in conversion efficiency.

The characteristic configuration of the methane production facility according to the present invention is
A methane production facility that uses hydrogen and carbon dioxide contained in the combustion exhaust gas of carbon-containing fuel as raw materials to obtain methane through a methaneization reaction using a catalyst.
An absorption tower that brings the combustion exhaust gas into contact with the carbon dioxide absorber to absorb the carbon dioxide in the combustion exhaust gas.
A regeneration tower that heats the carbon dioxide absorbent material that has absorbed carbon dioxide in the absorption tower to take out a first gas containing carbon dioxide as a main component, and a regeneration tower.
A desulfurizer that removes sulfur compounds in the second gas by treating the second gas containing the first gas from the regeneration tower and the first hydrogen, which is the first amount of hydrogen, with a desulfurizing agent. ,
It is provided with a methanation reactor in which a second amount of hydrogen, second hydrogen, is added to the third gas that has passed through the desulfurization device and converted into methane by a methanation reaction that has passed through a methanation catalyst.

According to this characteristic configuration, even if the sulfur compound is contained in the carbon dioxide-based gas (first gas) separated from the combustion exhaust gas, the sulfur compound is treated so as to be removed by the desulfurizing agent. NS. Therefore, according to the above configuration, carbon dioxide having a reduced sulfur compound or carbon dioxide containing substantially no sulfur compound can be obtained. Therefore, when carbon dioxide and hydrogen are reacted to carry out a methanation reaction, sulfur poisoning of the methanation catalyst is suppressed.

Further, at the time of desulfurization, a first amount of first hydrogen is added to a gas containing carbon dioxide as a main component (first gas). Then, after that, a second amount of hydrogen is additionally added to the third gas in which the second gas composed of the first gas and the first hydrogen is desulfurized, and the methanation reaction is carried out. Therefore, by limiting the amount of the first hydrogen added at the time of desulfurization to the first amount, the second gas (gas containing the first gas and the first hydrogen) supplied to the desulfurization agent at the time of desulfurization The amount can be suppressed, the contact time between the sulfur compound containing carbon dioxide, sulfur dioxide and the like, and the second gas such as hydrogen and the desulfurizing agent can be secured, and the deterioration of the desulfurization performance can be suppressed.

In addition, compared to the case where a large amount of hydrogen is added to carbon dioxide at once, the reaction between carbon dioxide and hydrogen is achieved by adding hydrogen stepwise to the gas containing carbon dioxide as the main component (first to third gases). It is possible to suppress an excessive rise in temperature due to the heat of reaction caused by carbon dioxide. Thereby, the desulfurization reaction can be safely carried out.

Further characteristic configurations of the methane production facility according to the present invention are:
The first hydrogen and the second hydrogen are obtained by electrolyzing water using surplus electric power from a power generation facility.

According to this feature configuration, surplus electricity from the power generator is converted to hydrogen, and this hydrogen and carbon dioxide separated from the combustion exhaust gas are used for methane conversion, so methane is injected into the existing city gas pipeline and used. can. Therefore, it is not necessary to newly install a gas storage facility. Further, since it is not necessary to go through the process of conversion from hydrogen to electric power other than the conversion from surplus electric power to hydrogen, it is possible to suppress the decrease in the amount of available electric power due to the decrease in conversion efficiency.

This is an example showing the methane production flow in the methane production facility of the present invention.

[Embodiment]
Hereinafter, embodiments of a method for methaneizing carbon dioxide in the methane production facility according to the present invention will be described with reference to the drawings.

In the following embodiment, a methaneization method for producing methane by removing sulfur dioxide in a gas containing carbon dioxide as a main component separated from combustion exhaust gas is described. However, the sulfur component contained in the gas containing carbon dioxide as a main component separated from the combustion exhaust gas is not limited to sulfur dioxide, and examples thereof include sulfur trioxide. Therefore, the methanation method according to the following embodiment can be applied even when the gas containing carbon dioxide as a main component separated from the combustion exhaust gas contains sulfur compounds such as sulfur dioxide and sulfur trioxide.

As shown in FIG. 1, the methane production facility 100 includes a carbon dioxide separation facility 1 that separates carbon dioxide from the combustion exhaust gas 101 of a carbon-containing fuel, a carbon dioxide pretreatment facility 2 that is a raw material for methaneization, and hydrogen and dioxide. It is equipped with a methane conversion facility 3 that obtains methane from a mixed gas of carbon.

The carbon dioxide separation facility 1 treats the combustion exhaust gas 101 of the carbon-containing fuel to obtain the combustion exhaust gas 102 from which carbon dioxide has been removed and the gas 201 (first gas) containing carbon dioxide as a main component.
The combustion exhaust gas 101 of the carbon-containing fuel may be the combustion exhaust gas of fossil fuels such as coal, petroleum coke, and heavy oil, and may be biomass, more specifically, forestry waste such as waste wood and waste, and rice straw. It may be the combustion exhaust gas of agricultural waste such as. These generally contain about 100 ppm to 1% sulfur.

Since coal, petroleum coke, etc. contain a large amount of sulfur in the fuel, their combustion exhaust gas usually contains high-concentration sulfur oxides (sulfur compounds such as sulfur dioxide and sulfur trioxide), which is the carbon dioxide separation facility 1. Performance may deteriorate in a short time. In the case of such combustion exhaust gas, it is desirable to reduce the sulfur oxide concentration in the combustion exhaust gas to preferably 100 ppm or less, more preferably 10 ppm or less (volume basis) by a known desulfurization process such as the lime gypsum method. ..

The combustion exhaust gas 101 of the carbon-containing fuel that has entered the carbon dioxide separation facility 1 comes into contact with the amine solution (carbon dioxide absorber) in the absorption tower 11, and the carbon dioxide in the combustion exhaust gas is absorbed by the amine solution. The combustion exhaust gas 102 from which carbon dioxide has been removed is extracted from the upper part of the absorption tower 11. The amine solution that has absorbed carbon dioxide is sent to the regeneration tower 14 by the pump 12. At this time, the amine solution that has absorbed carbon dioxide is heat-exchanged with the high-temperature amine solution returned from the regeneration tower 14 by the heat exchanger 13, and after the temperature is raised, it is sent to the regeneration tower 14.

A reboiler 15 is provided in the lower part of the regeneration tower 14, and the amine solution is heated to release carbon dioxide. Steam (G in FIG. 1) obtained in the methanation facility 3 described later is used as the heat source of the reboiler 15, and the steam condensed water is recovered. This is appropriately refined and used as a raw material for steam in the methanation facility 3.

The carbon dioxide-releasing amine solution extracted from the lower part of the regeneration tower 14 is replaced by a flow control valve if the pressure inside the regeneration tower 14 is kept sufficiently higher than the pressure inside the absorption tower 11. , It is sent to the upper part of the absorption tower 11 via the heat exchanger 13 and the cooler 17.

The released carbon dioxide is cooled by the cooler 18, a part of the accompanying water is condensed and separated by the drum 19, and then the carbon dioxide-based gas 201 (first gas) separated from the combustion exhaust gas is used as the main component. ). The gas 201 contains a part of sulfur dioxide contained in the combustion exhaust gas 101 of the carbon-containing fuel. The concentration depends on the sulfur dioxide concentration in the combustion exhaust gas 101 of the carbon-containing fuel, but is usually about 0.1 ppm to 100 ppm.

If a solution that absorbs a large amount of carbon dioxide per unit amount of the solution and requires as little heat as possible to desorb carbon dioxide is used as the amine solution, the energy required for separating carbon dioxide is small, resulting in a small amount of energy. The efficiency of methanation also increases. If the regeneration temperature is 200 ° C. or lower, more preferably 150 ° C. or lower, and the amount of heat of regeneration per 1 kg of carbon dioxide is 3.5 MJ or less, the calorific value of the methanation reaction, which will be described later, is used to determine the total amount of heat of regeneration. It is suitable because it can be covered. As such a solution, for example, an aqueous solution of monoethanolamine can be used.

The pretreatment facility 2 for the methanation raw material adds a first amount of hydrogen 203 (first hydrogen) to the carbon dioxide-based gas 201 separated from the combustion exhaust gas through a valve 21 to obtain the first amount of hydrogen 203 (first hydrogen). After making the gas 205 (second gas) containing carbon dioxide as a main component to which hydrogen 203 is added, the gas 205 is treated by the desulfurizer 23. The desulfurizer 23 removes sulfur dioxide contained in the carbon dioxide-based gas 201 separated from the combustion exhaust gas 101. The desulfurizer 23 is filled with zinc oxide and a desulfurizing agent composed of Fe, Co, Ni, and Cu in the state of metal or oxide, and sulfur dioxide is fixed on the desulfurizing agent by the following reaction.
SO ₂ + 3H ₂ → H ₂ S + 2H ₂ O
H ₂ S + Zn O → Zn S + H _{2 O}

From the desulfurizer 23, a gas 206 (third gas) containing desulfurized carbon dioxide as a main component is generated. A second amount of hydrogen 204 (second hydrogen) is added to the gas 206. The total hydrogen content of the first amount of hydrogen 203 and the second amount of hydrogen 204 is the total hydrogen required to almost complete the methanation reaction of the carbon dioxide-based gas 201 separated from the combustion exhaust gas. The amount is preferable. Alternatively, the total hydrogen content of the first amount of hydrogen 203 and the second amount of hydrogen 204 may be greater than or equal to the total hydrogen content required to substantially complete the methanation reaction.

As described above, the carbon dioxide-based gas 201 separated from the combustion exhaust gas contains sulfur dioxide. Usually, the lime gypsum method is used for desulfurization of sulfur dioxide-containing gas. The lime gypsum method is carried out in an oxidizing atmosphere and no hydrogen is added. However, in the present embodiment, as described above, attention is newly paid to the addition of the first amount of hydrogen 203 to the gas 201 containing carbon dioxide as the main component separated from the combustion exhaust gas, and the sulfur dioxide in the desulfurizer 23 is used. Sulfur is reduced to hydrogen sulfide and reacted with ZnO to fix it as ZnS.

Since the desulfurization reaction does not proceed sufficiently at a temperature that is too low, it is preferable that the temperature at which the gas 205 before entering the desulfurization apparatus 23 is preheated and brought into contact with the desulfurization agent is about 150 to 350 ° C.

If the hydrogen concentration in the carbon dioxide-based gas 205 to which the first amount of hydrogen 203 is added is too small, the desulfurization reaction cannot be completed and the reaction rate becomes slow to provide sufficient desulfurization performance. I can't get it. Therefore, the hydrogen concentration in the gas 205 is 10 times or more the sulfur dioxide concentration, and is preferably 0.1% by volume or more from the viewpoint of obtaining a sufficient desulfurization reaction rate. The gas 205 contains carbon dioxide, sulfur dioxide, a first amount of hydrogen added, and the like. And here, the hydrogen concentration is the volume of hydrogen with respect to the volume of the gas 205. Similarly, the sulfur dioxide concentration is the volume of sulfur dioxide relative to the volume of gas 205.

On the other hand, if the hydrogen concentration in the gas 205 is too high, the methanation reaction tends to proceed. If the hydrogen concentration is 5% by volume or less, methanation will proceed, and even if all the hydrogen is consumed in the methanation reaction, the temperature rise will be only about 50 ° C. Therefore, the desulfurization reaction should be carried out safely. Can be done. Further, when hydrogen is added in a larger amount than necessary for the desulfurization reaction, the volume of the gas 205 increases and the contact time with the desulfurization agent decreases, so that there is a concern that the desulfurization performance may deteriorate. Therefore, the hydrogen concentration to be added is preferably 5% by volume or less so that the volume of the gas 205 does not increase too much.

By limiting the hydrogen concentration in the gas 205 as described above, as a result, the reaction rate of methanation is reduced, and even if methanation progresses, the methanation reaction ends when hydrogen is exhausted. .. As a result, damage to the desulfurizing agent and equipment due to an increase in temperature due to the heat of reaction due to excessive progress of methanation can be suppressed.

For example, when the sulfur dioxide concentration in the gas 201 is 10 ppm (the balance is carbon dioxide), 2% by volume of hydrogen 203 is added through the valve 21 in terms of the volume ratio to carbon dioxide, and 1.96% by volume of hydrogen is added. As a gas 205 composed of 9.8 ppm sulfur dioxide and carbon dioxide (remaining), this is treated by the desulfurizer 23. Then, hydrogen 204 having a volume ratio of 398% with respect to carbon dioxide is added through the valve 22 to obtain a pretreated methanation raw material 207. Here, the molar ratio of carbon dioxide to hydrogen was set to 1: 4, but if there is a restriction on the volume concentration of hydrogen or carbon dioxide remaining in the product, for example, the ratio of hydrogen is slightly increased and the residual carbon dioxide remains. It is possible to reduce the volume concentration of hydrogen, or to reduce the volume concentration of residual hydrogen by slightly lowering the volume ratio of hydrogen. The molar ratio of carbon dioxide to hydrogen is usually about 1: 3.9 to 4.1.

The hydrogen 202 used as a raw material here needs to be substantially free of sulfur compounds (sulfur dioxide, hydrogen sulfide, etc.). Hydrogen obtained by electrolysis of water usually satisfies this condition, but hydrogen obtained by other methods may require desulfurization treatment in advance.

In the methanation facility 3, methane is obtained from a mixed gas of hydrogen and carbon dioxide by a methanation reaction. Since the methanation reaction is accompanied by a large amount of heat, heat is removed by using a plurality of reactors and using a heat exchanger between the reactors. In particular, the first-stage reactor, which has a large calorific value, is diluted by recycling a part of the gas after the reaction to suppress the temperature rise.

In FIG. 1, the reactors are set to three stages (methaneization reactors 31, 33, 35), and a part of the outlet gas of the first stage is returned to the inlet of the first stage methanation reactor 31 by the recycling pump 37. Shows the case. Further, heat exchangers 32, 34, 36 are provided at the outlets of the reactors 31, 33, 35. Water is supplied to the low temperature side of the heat exchangers 32, 34, and 36, and steam (G in FIG. 1) is taken out. The removed steam is used as a regenerative heat source for the carbon dioxide separation facility 1. The high-temperature reaction gas from the reactors 31, 33, 35 is supplied to the high-temperature side of the heat exchangers 32, 34, 36, and the cooled reaction gas is taken out.

The methanation reactors 31, 33, and 35 are filled with a catalyst (methaneation catalyst) that is active in methanation. Specifically, a catalyst in which at least one metal selected from Ni, Ru, and Rh is supported on a heat-resistant inorganic oxide carrier such as alumina is preferably used. Since these catalysts exhibit sufficient methanation activity at about 250 ° C. to 300 ° C., the raw material gas should be supplied to each reactor at 250 ° C. to 300 ° C. The inlet temperature of the methanation reactors 31, 33, 35 of each stage is adjusted by adjusting the amount of water supplied to the low temperature side of the heat exchangers 32, 34, 36, or by adjusting the heat exchangers 32, 34, 36. This is done by providing a bypass on the high temperature side and adjusting the flow rate.

In the methanization facility 3 of FIG. 1, the adiabatic methanization reactors 31, 33, 35 and the heat exchangers 32, 34, 36 are combined in multiple stages, but the heat exchange function is incorporated inside the reactor. A heat exchange type reactor may be used, and if necessary, an adiabatic type methanation reactor and a reactor and an independent heat exchanger and a heat exchange type reactor may be combined to form a methanization facility. ..

According to the above configuration, since energy efficiency is high and the life of the methaneization catalyst can be maintained for a long time, methane can be produced from the combustion exhaust gas of carbon-containing fuel and hydrogen as raw materials by an economical method. ..

More specifically, even if a sulfur compound such as sulfur dioxide is contained in the carbon dioxide-based gas (first gas) separated from the combustion exhaust gas, the sulfur compound is removed by the desulfurizing agent. Will be processed. Therefore, according to the above configuration, carbon dioxide having a reduced sulfur compound or carbon dioxide containing substantially no sulfur compound can be obtained. Therefore, when carbon dioxide and hydrogen are reacted to carry out a methanation reaction, sulfur poisoning of the methanation catalyst is suppressed.

Further, at the time of desulfurization, a first amount of first hydrogen is added to a gas containing carbon dioxide as a main component (first gas). Then, after that, a second amount of hydrogen is additionally added to the third gas in which the second gas composed of the first gas and the first hydrogen is desulfurized, and the methanation reaction is carried out. Therefore, by limiting the amount of the first hydrogen added at the time of desulfurization to the first amount, the second gas (gas containing the first gas and the first hydrogen) supplied to the desulfurization agent at the time of desulfurization The amount can be suppressed, the contact time between the sulfur compound containing carbon dioxide, sulfur dioxide and the like, and the second gas such as hydrogen and the desulfurizing agent can be secured, and the deterioration of the desulfurization performance can be suppressed.

In addition, compared to the case where a large amount of hydrogen is added to carbon dioxide at once, the reaction between carbon dioxide and hydrogen is achieved by adding hydrogen stepwise to the gas containing carbon dioxide as the main component (first to third gases). It is possible to suppress an excessive rise in temperature due to the heat of reaction caused by carbon dioxide. Thereby, the desulfurization reaction can be safely carried out.

[Other Embodiments]
The configurations disclosed in the above-described embodiment (including other embodiments, the same shall apply hereinafter) can be applied in combination with the configurations disclosed in the other embodiments as long as there is no contradiction. Moreover, the embodiment disclosed in the present specification is an example, and the embodiment of the present invention is not limited to this, and can be appropriately modified without departing from the object of the present invention.

(1) In the above embodiment, there is no limitation on the raw material of hydrogen. As a raw material for hydrogen used in the above embodiment, surplus electricity of various power generation systems such as solar power generation, wind power generation, and fuel cell systems installed in each household can be used. In this case, hydrogen can be obtained by electrolyzing water using surplus electric power.

In recent years, renewable energies such as solar power generation and wind power generation have become widespread from the viewpoint of global warming countermeasures. Since these renewable energies do not use fossil fuels during power generation, they can be said to contribute to the prevention of global warming. On the other hand, since the amount of power generated by solar power generation and wind power generation depends on the weather, there is a concern that a large amount of surplus power will be generated when the introduction proceeds on a large scale.

The use of storage batteries and pumped storage power generation is envisioned as a measure against surplus electricity, but both have problems such as cost, and are effective in stabilizing supply and demand on a time axis such as diurnal fluctuations and weekly fluctuations in electricity demand. However, there is a problem that the cost is considerably high as a measure against surplus power on a longer time axis.

Using such surplus electricity, water is electrolyzed to obtain hydrogen, and this hydrogen is used to desulfurize carbon dioxide and convert carbon dioxide to methane, thereby injecting methane into the existing city gas pipeline. Can be used. Therefore, it is not necessary to newly install a gas storage facility. For example, when hydrogen is generated from raw materials, it is necessary to store it in a tank on the spot because there is currently no large-scale transportation infrastructure for transporting hydrogen. However, as described above, by using hydrogen for the conversion of methane, it is not necessary to newly install a hydrogen storage facility.

Further, it is conceivable to convert the surplus electric power into hydrogen, and when necessary, reconvert the hydrogen into electric power by a power generation facility using a fuel cell or an internal combustion engine. However, there is a problem that the energy loss during power generation is relatively large. That is, since the conversion efficiency cannot be maintained high in the process of converting surplus electric power to hydrogen, the process of reconverting hydrogen to electric power, and the like, only a part of the original electric power can be reused as electric power. However, by generating hydrogen using surplus electricity and converting it to methane using the generated hydrogen, the methane produced by the conventional combustion equipment for natural gas-based city gas can be used as it is. Therefore, there is almost no additional cost in the transportation infrastructure and the equipment used, and there is a great advantage that it can be widely used.

(2) In the above embodiment, a plurality of stages of methanation reactors 31, 33, 35 are used. However, the number of methanation reactors is not limited to this, and may be one, two, or four or more.

(3) In the above embodiment, when the gas containing carbon dioxide as a main component separated from the combustion exhaust gas contains sulfur compounds such as sulfur dioxide and sulfur trioxide, the sulfur compounds are removed from the carbon dioxide and methaneized. The method of methanation was explained. However, the above methanation method does not exclude the application to hydrogen sulfide and the like.

1: Carbon dioxide separation equipment 2: Pretreatment equipment 3: Methaneization equipment 11: Absorption tower 14: Regeneration tower 23: Desulfurizers 31, 33, 35: Methanization reactors 32, 34, 36: Heat exchanger 100: Methane Manufacturing equipment 101: Combustion exhaust gas 102: Combustion exhaust gas 201: Gas 202: Hydrogen 205: Gas 207: Methaneization raw material

Claims (11)

It is a methanation method that obtains methane by a methanation reaction using a catalyst using hydrogen and carbon dioxide contained in the combustion exhaust gas of carbon-containing fuel as raw materials.
a) A step of bringing the combustion exhaust gas into contact with a carbon dioxide absorbent to absorb carbon dioxide in the combustion exhaust gas.
b) A step of heating the carbon dioxide absorbing material that has absorbed carbon dioxide to extract a first gas containing carbon dioxide as a main component, and
c) The first hydrogen, which is a first amount of hydrogen, is added to the first gas to obtain a second gas, and the second gas is passed through a desulfurizer filled with a desulfurizing agent to allow sulfur in the second gas. The process of removing the compound and
d) Methane including a step of adding a second amount of hydrogen, the second hydrogen, to the third gas that has undergone the step of removing the sulfur compound, and converting the second hydrogen into methane by a methanation reaction through a methanation catalyst. Method of conversion. The methaneization method according to claim 1, wherein the first amount is less than the total amount of hydrogen required to complete the methanation reaction for producing methane by reacting the first gas with hydrogen. The methanation method according to claim 2, wherein the second amount is equal to or more than an amount obtained by subtracting the first amount from the total hydrogen amount. The methanation method according to any one of claims 1 to 3, wherein the volume% of the first hydrogen with respect to the second gas is 0.1% by volume or more and 5% by volume or less. The methanation method according to any one of claims 1 to 4, wherein the volume% of the first hydrogen with respect to the second gas is 10 times or more the volume% of the sulfur compound with respect to the second gas. The first gas is taken out by heating the carbon dioxide absorbent by utilizing the heat of reaction from the methanation reaction in the step d) in the step b). The methanation method described in. The methanation method according to any one of claims 1 to 6, wherein the carbon dioxide absorbent is an aqueous solution of an amine compound. The methanation method according to any one of claims 1 to 7, wherein the desulfurizing agent contains at least one component metal or oxide composed of iron, nickel, cobalt and copper and zinc oxide. The methanation method according to any one of claims 1 to 8, wherein the first hydrogen and the second hydrogen are obtained by electrolyzing water using surplus electric power from a power generation facility. A methane production facility that uses hydrogen and carbon dioxide contained in the combustion exhaust gas of carbon-containing fuel as raw materials to obtain methane through a methaneization reaction using a catalyst.
An absorption tower that brings the combustion exhaust gas into contact with the carbon dioxide absorber to absorb the carbon dioxide in the combustion exhaust gas.
A regeneration tower that heats the carbon dioxide absorbent material that has absorbed carbon dioxide in the absorption tower to take out a first gas containing carbon dioxide as a main component, and a regeneration tower.
A desulfurizer that removes sulfur compounds in the second gas by treating the second gas containing the first gas from the regeneration tower and the first hydrogen, which is the first amount of hydrogen, with a desulfurizing agent. ,
A methane production facility including a methanation reactor in which a second amount of hydrogen, second hydrogen, is added to the third gas that has passed through the desulfurizer and converted into methane by a methanation reaction that has passed through a methanation catalyst. The methane production facility according to claim 10, wherein the first hydrogen and the second hydrogen are obtained by electrolyzing water using surplus electric power from a power generation facility.

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