Methanation reactor apparatus utilizing endothermic reactions to remove heat of reaction and endothermic material regeneration process

Abstract

[Problem] There is a need for a device that synthesizes methane from hydrogen and carbon dioxide gases, but removes reaction heat and further reduces the progress and temperature of the reaction to improve the conversion rate of methane. [Solution] The present invention uses the powders of chemical heat accumulators magnesium hydroxide and magnesium carbonate as part of the fluid medium of a multi-stage fluidized bed to absorb and store the heat generated during the methanation reaction in the temperature range in which the methanation reaction proceeds, and is able to use the carbon dioxide gas generated from the magnesium carbonate as a raw material gas for the methanation reaction. When the magnesium oxide produced by heat absorption is discharged and regenerated in an external regeneration device, the stored heat is recovered, and the regenerated powder is reintroduced at the highest stage of the multi-stage fluidized bed at a temperature lower than the internal temperature of the device, so that the progress and temperature of the reaction can be reduced as the reaction gas rises through the device, and the conversion rate of the generated methane is improved.

Description

TECHNICAL FIELD

[0001] In recent years, in order to prevent an increase in the concentration of carbon dioxide, a gas that causes global warming, in the atmosphere or to reduce the concentration of carbon dioxide in the atmosphere, carbon capture and storage (CCS), a technology for separating and recovering carbon dioxide, and carbon capture utilization and separation (CCUS) technology, including its use, are being developed.

[0002] On the other hand, in terms of promoting the use of renewable energy, in order to make fluctuating energy, such as solar and wind energy, into a stable energy supply source according to demand, hydrogen is generated by electrolysis using these energies (green hydrogen). Also, hydrogen conversion processes for producing, storing, and utilizing such hydrogen are being developed and demonstrated.

[0003] In addition, a methanation (= methanation reaction) process that converts carbon dioxide captured by the above-described CCS technology and hydrogen from such renewable energy into methane is being developed, which is safer, more compatible with current infrastructure, and easier to use.

[0004] The present invention relates to a CCUS technology that combines a CCS technology that captures carbon dioxide and a process that converts hydrogen into methane. BACKGROUND

[0005] Currently, the mainstream of the technology for absorbing, separating, and recovering carbon dioxide from the atmosphere or flue gas is an absorption method using an amine-based absorbent. There are many types of amine absorption liquids available for use, but they all absorb carbon dioxide in flue gas at room temperature to 50°C, and then heat it to 100-150°C to remove carbon dioxide and recover carbon dioxide. In addition, as described in Non-Patent Literature 3, there is also a method of impregnating solid particles with an amine to absorb carbon dioxide.

[0006] On the other hand, according to Non-Patent Literature 1, about 120 years ago, Sabatier discovered a method of synthesizing methane from hydrogen and carbon dioxide in the presence of a Ni-based catalyst. However, to this day, it has not been realized as an actual mass production process. The main reason for this is that this reaction occurs at temperatures below about 600°C, and it is an equilibrium reaction that is accompanied by a large amount of heat generation. When the temperature of the reaction gas increases, methane decomposition (steam reforming reaction) that is the reverse of the methanation reaction begins to dominate, thereby reducing the conversion and yield. As a result, the methane gas produced by this reaction contains a high concentration of unreacted hydrogen gas, which is highly explosive, making it difficult to safely and easily use the methane.

[0007] Nowadays, the main industrial use of this technology, which utilizes hydrogen conversion into methane, is limited to the removal of carbon dioxide, which is a reaction inhibitor in the ammonia synthesis process, and a small amount of the gas described in Non-Patent Literature 2.

[0008] In this case, since the reaction heat generated by the methanation of a small amount of carbon dioxide is diluted by a large amount of surrounding gas, the influence of heat generation is minimal, and does not become a hindrance. However, in the reaction of hydrogen and high-concentration carbon dioxide (concentration close to 100%) in the absence of such a dilution gas, the influence of heat generation is greater, and the heat is not sufficiently removed from the catalyst surface, which is the reaction field. As described above, in addition to the reduction of the methane conversion rate due to the temperature increase, the catalyst itself is deteriorated due to the heat. In addition, since the generated heat is not easily removed, this is one of the reasons why the process of generating methane from high-concentration carbon dioxide and hydrogen has not been put into practical use to date.

[0009] However, in recent years, due to the increasing necessity of global warming countermeasures, according to Non-Patent Literature 6, a catalyst with high activity at low temperatures has been developed; and according to Patent Literature 1 and Non-Patent Literatures 4, 5, a reactor designed to remove the generated reaction heat or control the reaction amount has been developed and demonstrated.

[0010] Further, in Non-Patent Literature 1, carbon dioxide obtained by recovering carbon dioxide in flue gas by using an amine-based absorption method is used as carbon dioxide, which is a raw material of the present methanation method. That is, a process of converting green hydrogen into methane using carbon dioxide in flue gas is being investigated and developed.

[0011] [Related Art Literature]

[0012] [Patent Literature]

[0013] [Patent Literature 1]: The following method is described in Japanese Patent Application No. 2016-524796 (PCT / EP2014 / 064625) and Japanese Patent Application No. 2020-63206. Carbon dioxide obtained from flue gas by using an absorption method (such as the above-described amine-based absorption method) and hydrogen are used to generate methane. In this method, the heat obtained by removing heat from the methanation reactor is used for preheating boiler combustion air.

[0014] [Patent Literature 2]: Japanese Patent Application No. 2020-71387 proposes a moving bed and a multi-stage fluidized bed equipped with a swing mechanism that prevents drift from occurring and enables uniform contact between gas and solids by providing horizontal swing motion to fine powders that are particularly difficult to fluidize.

[0015] [Non-Patent Literature]

[0016] [Non-patent literature 1] Paul Sabatier proposed in 1913 in "Catalysis in Organic Chemistry" that carbon dioxide reacts with hydrogen to produce methane in the presence of a Ni catalyst.

[0017] [Non-patent literature 2] According to Kuriyama Tsuyoshi, Chemistry and Education 66(11), P529 (2018), the methanation reaction is described for removing trace amounts of carbon dioxide in the ammonia industry production.

[0018] [Non-patent literature 3]

[0019] According to https: / / www.nedo.go.jp / news / press / AA5_101330.html on July 13, 2020, NEDO and Kawasaki Heavy Industries, Ltd. and the Earth Innovation Institute (cooperation, introduced "Carbon dioxide separation and recovery technology using solid absorption method", which uses amine impregnated particles. They are developing a methane production process and working to reduce the cost of carbon dioxide separation and recovery technology.

[0020] [Non-patent literature 4]

[0021] In "Review on methanation-From fundamentals to current projects", pages 276-296, Fuel 2016, 166 (Stefan Ronsch et al.), a review of the methanation process technology and its problems is summarized, respectively.

[0022] [Non-patent literature 5]

[0023] According to https: / / www.nedo.go.jp / news / press / AA5_101217.html in the New Energy and Industrial Technology Development Organization (NEDO) released on October 16, 2019, INPEX Corporation and Hitachi Shipbuilding Co., Ltd. jointly announced that the development of the "Methane synthesis test facility using carbon dioxide" has been completed, and began to try to prepare for full-scale production.

[0024] [Non-patent literature 6]

[0025] In "Toyama University Hydrogen Isotope Science Research Center Research Report", 36, 39-44, (2016), Yab et al. reported that TiO2 particles carrying nanoscale Ru metal catalysts are active at low temperatures of 150-200°C.

[0026] [Non-Patent Literature 7]

[0027] In "Journal of the Japan Society of Ceramics, 71, P61, (1963)", Hamano Kenya reported that the dehydration reaction of magnesium hydroxide is about 2% up to about 290°C; and when it exceeds 330°C, it starts to rapidly dehydrate, and will be completed at 430°C.

[0028] [Non-Patent Literature 8]

[0029] In "Journal of the Chemical Society of Japan, p.57-64, (1979)", Sawada Yuuhei et al. reported the measurement results of the thermal decomposition process of basic magnesium carbonate.

[0030] [Non-Patent Literature 9]

[0031] In "Chemical Engineering 31(6), P538 (1967)", Maeda Taro, Yamakawa Norio reported that the heat transfer coefficient of the fluidized bed is large.

[0032] [Non-Patent Literature 10]

[0033] In "Examples and problems of chemical heat storage technology": Japan Society of Automotive Engineers No. 14-14 Symposium, Takayuki Kobayashi presented on February 13, 2015, at http: / / www.energy.gr.jp / wp-content / uploads / 2018 / 02 / Ch HeatStorage.pdf Figure 5 Various chemical heat storage agents and their operating temperatures are shown in the above. Among them, magnesium hydroxide and magnesium carbonate are indicated as chemical heat storage agents capable of operating in the range of 300-500°C. SUMMARY

[0034] [Problems to be solved by the invention]

[0035] The absorption temperature of carbon dioxide in the absorption method using an amine-based absorbent depends on the type of amine, and is said to be about 20-60°C, which is lower than the normal flue gas temperature (about 140°C). Therefore, in order to absorb carbon dioxide in the flue gas, it is necessary to immediately reduce the flue gas temperature.

[0036] Reducing the flue gas temperature reduces the "chimney effective height" prescribed in the Air Pollution Control Law. It is considered necessary to raise the temperature again to about 140°C, or to enhance the exhaust fan and increase the exhaust air velocity. In addition, it is known that after the desulfurization process, the amine-based absorbent reacts with a small amount of SOx in the flue gas and gradually deteriorates, so periodic refining and replenishment of the absorbent are required.

[0037] Therefore, there is a need for a new method capable of absorbing and fixing carbon dioxide without lowering the temperature of flue gas and without deteriorating the absorbent.

[0038] On the other hand, Reaction Formula 1 shows a methanation reaction for synthesizing methane from carbon dioxide and hydrogen. As described above, this reaction is an equilibrium reaction and an exothermic reaction.

[0039] Here, the Gibbs free energy change G value of each component involved in the present reaction is used to give the Gibbs free energy change AG value of the methanation reaction by Equation 1. When the value of Equation 1 is negative, the reaction proceeds; and when the value of Equation 1 is positive, the reaction proceeds in the opposite direction, i.e., the reverse reaction. Therefore, AG was calculated for Reaction Formula 1 at 0-1000°C, and the results are shown in Table 1. Figure 1 Generally, the AG value and the enthalpy change AH value of many compounds at each temperature can be referenced from various thermodynamic databases, such as the Handbook of Chemistry. Here, these values were calculated by using ASPEN PLUS (used as a program simulator having these databases). From Figure 1 It is known that the AG value of the methanation reaction is 0 at around 580°C and is negative at lower temperatures, so the methanation reaction proceeds; but at higher than 580°C, the AG value turns positive, so the reverse reaction (steam reforming reaction of methane) begins to dominate.

[0040] Further, when the enthalpy of each component is calculated, the reaction in which methane is generated in Reaction Formula 1 is an exothermic reaction, but the steam reforming reaction as the reverse reaction is an endothermic reaction.

[0041] Generally, the reaction rate of an endothermic reaction is easily controlled by giving heat. However, in the case of an exothermic reaction and an Arrhenius-type reaction, when the heat removal ability of the reaction heat is insufficient, the reaction temperature increases, and the reaction rate also increases with the increase in the reaction temperature, as a result of which the reaction can runaway.

[0042] However, in the case of this methanation reaction, when the temperature increases, the reverse reaction begins to dominate; so, although this does not lead to a reaction runaway, the reaction yield is still reduced. Therefore, it is important to have sufficient heat removal ability to suppress the increase in the reaction temperature.

[0043] [Reaction Formula 1]

[0044]

[0045] [Equation 1]

[0046]

[0047] Further, because this reaction occurs on the surface of the catalyst, the temperature of the catalyst tends to increase due to the heat generated by the reaction. When the heat is not removed from the catalyst soon, the temperature of the catalyst increases; as described above, as a result, not only is the conversion rate reduced, but also the life of the catalyst is shortened.

[0048] Therefore, the problem to be solved by the present methanation reaction is to remove the reaction heat quickly from the reaction field containing the catalyst.

[0049] Next, the methanation reaction is carried out at less than 580°C. The dehydration reaction of magnesium hydroxide in Reaction Formula 2 is an endothermic reaction; because Figure 1 The ΔG value in Reaction Formula 2 is negative at more than about 270°C, so this means that this dehydration reaction proceeds. In fact, this is almost consistent with the report in Non-Patent Literature 7 that "the dehydration reaction of magnesium hydroxide occurs at 300°C or more".

[0050] Next, from Figure 1 It can be seen that the decarboxylation reaction of magnesium carbonate in Reaction Formula 3 also proceeds because the ΔG value is negative at more than 410°C. In fact, according to Non-Patent Literature 8, the decarboxylation reaction depends on the concentration of carbon dioxide in the atmosphere, but what is consistent with the report is that when the carbon dioxide partial pressure is less than 5 mol%, the entire decarboxylation occurs at 400°C or more.

[0051] From these dehydration and decarboxylation reactions, magnesium oxide is generated, but at Figure 1 In Reaction Formula 2, the calculation line of the dehydration reaction of magnesium hydroxide is in the positive range at less than 270°C. This means that the reverse reaction (hydration) proceeds at less than this temperature. That is, as shown in Reaction Formula 4, magnesium oxide is converted to magnesium hydroxide by reaction with water. Also, in the carbonation reaction of magnesium hydroxide in Reaction Formula 5, Figure 1 The calculation line in Reaction Formula 5 is in the negative region in a wide temperature range, which means that carbonation proceeds even at less than 600°C. Therefore, carbonation is desired even at a flue gas temperature of 140°C.

[0052] [Reaction Formula 2]

[0053] Mg(OH)2→ MgO + H2O

[0054] [Reaction Formula 3]

[0055] MgCO3→ MgO + CO2

[0056] [Reaction Formula 4]

[0057] MgO + H2O → Mg(OH)2

[0058] [Reaction Formula 5]

[0059] Mg(OH)2+ CO2→ MgCO3+ H2O

[0060] Now, the equilibrium constant K at an arbitrary temperature T can be obtained from ΔG by Equation 2, where R is a gas constant. In addition, from Reaction 1, Equation 3 gives a relationship between the equilibrium constant K and the concentration of each component. That is, by the ΔG value at an arbitrary temperature T, the K value can be obtained from Equation 2; then, by the K value, the equilibrium composition of each component at an arbitrary temperature can be calculated from Equation 3. The equilibrium composition according to Equation 3 is determined by trial and error. The methanation reaction will be performed at each temperature to approach this equilibrium composition.

[0061] Figure 2 is a graph showing the change in the carbon dioxide equilibrium conversion according to the temperature change determined in this way. In this graph, the solid line indicates the case where the reaction gas is not diluted with steam; the broken line indicates the change in the equilibrium conversion when 1 mol of steam is added to the reaction gas. As will be described below, this corresponds to the case where the magnesium hydroxide absorbs the reaction heat and dehydrates to generate 1 mol of water vapor.

[0062] At a temperature of around 580°C, the conversion of carbon dioxide to methane is 50%. Also, at around 350°C, the equilibrium conversion is about 87%, shown by the broken line; then, the unreacted carbon dioxide is reduced to about 13%.

[0063] [Equation 2]

[0064]

[0065] [Equation 3]

[0066]

[0067] In addition, from Figure 2 It can be seen that it is effective to perform the reaction at a lower temperature in order to obtain a high methane conversion. That is, in this reaction, the lower the reaction temperature, the higher the conversion of the reaction. Therefore, in order to increase the conversion at the outlet of the reactor, it is effective to lower the reaction temperature as the reaction proceeds.

[0068] However, as described above, when the reaction, this reaction generates heat, so if the heat removal capacity is not sufficient, this reaction heat raises the temperature and in turn reduces the conversion. On the other hand, according to the reaction kinetics, the lower the reaction temperature, the lower the reaction rate. Therefore, the higher the reaction temperature is better. This is the dilemma and the problem to be solved for this methanation reaction.

[0069] Therefore, at the initial stage of the reaction, the reaction temperature is increased to proceed the reaction; but after that, the reaction is carried out while gradually lowering the reaction temperature; and at the outlet of the reaction device where the temperature starts to lower, the reaction rate should be increased. There is a need for a reaction device that can increase the methane conversion rate without lowering the reaction rate even in the low temperature region by increasing the catalyst concentration or using a higher-activity catalyst.

[0070] Here, the review of the past research and development progress is as follows.

[0071] The methanation reaction in Patent Document 1 and Non-Patent Documents 4 and 5 is carried out in the presence of a Ni-based catalyst in the temperature range of 300 to 400°C.

[0072] Further, a process of obtaining heat by this heat removal inside or outside the process has been proposed.

[0073] As for the catalyst, the catalyst is shaped into pellets or honeycomb, they are filled into a reaction tube, the reaction gas is passed through this reaction tube, and an indirect cooling method of removing the heat generated inside the reaction tube using external cooling of the reaction tube is used.

[0074] The reaction is carried out in the reactor at 300 to 400°C, and according to Figure 2 , the equilibrium conversion rates at the reactor outlet temperatures of 350°C and 300°C are about 87% and 92%, respectively.

[0075] In order to make the actual conversion rate close to the equilibrium conversion rate, it is necessary to ensure sufficient reaction time. Assuming that the conversion rate in the methanation reactor proceeds to 90%, the gas components generated from the raw material gas (1 kmol of carbon dioxide and 4 kmol of hydrogen) are 0.90 kmol of methane and 1.80 kmol of water vapor.

[0076] And, the components of the unreacted gas are 0.10 kmol of carbon dioxide and 0.40 kmol of hydrogen. When converted to concentration, the concentration of unreacted hydrogen is 12.5 mol%; it is dangerous and undesirable to use methane containing such a high concentration of extremely explosive hydrogen as a general-purpose fuel.

[0077] In recent years, according to Non-Patent Document 6, a Ru-supported catalyst with higher activity has been developed even at 150 to 200°C in order to further increase the conversion rate. Therefore, the reaction is first carried out at 300 to 400°C, and then the generated gas is cooled to 150°C; if the reaction is further carried out in the presence of this catalyst, the conversion rate will be able to approach 0.98 (i.e., the equilibrium conversion rate).

[0078] When the conversion rate is 0.98, the concentration of unreacted hydrogen will be reduced to 2.6 mol%. That is, the danger of generated methane can be greatly reduced. In summary, increasing the conversion rate of methane in the methanation reaction and reducing the concentration of unreacted hydrogen in the product gas is not only important in terms of reaction yield, but also important in terms of safety. For this reason, after the reaction at 300-400°C, it is necessary to increase the reactor to perform further reaction at 150-300°C to increase the equilibrium conversion rate to nearly 98%.

[0079] Before entering the specific structure of the apparatus, the elements of the apparatus should be organized. For this reason, it is assumed that the methanation reaction is performed by connecting reactors in a plurality of different temperature ranges, in which the temperature is gradually reduced from 500°C to 200°C, 100°C at a time.

[0080] That is, in the first stage, a conventional catalyst is used to perform the reaction in the temperature range of 400-500°C, and then the reaction gas is cooled to react in the range of 300-400°C before entering the second stage.

[0081] In addition, in the third stage, it is also cooled to the range of 300-200°C. At this temperature, the reaction rate is significantly reduced, so it is assumed that the high-activity Ru catalyst described in Non-Patent Literature 6 is used for this reaction in this temperature range.

[0082] In this way, by reducing the reaction temperature as the reaction proceeds and using a higher-activity catalyst in the lower temperature region, it is possible to increase the conversion rate without significantly reducing the methanation reaction rate; then, it is possible to reduce the concentration of remaining unreacted hydrogen; and it is possible to safely use the generated methane.

[0083] However, in this process, it is necessary to connect multiple reactors and install coolers to reduce the gas temperature between them, which means that this process will become complex.

[0084] A compact reactor with high heat removal performance and capable of reducing the reaction temperature as the reaction progresses is needed. If such a reactor is achieved, it should be possible to utilize such a compact reactor to increase the final conversion rate of the methanation reaction and reduce the concentration of unreacted hydrogen.

[0085] [Means for solving the problem]

[0086] In the case of a three-stage reactor as described above, a cooler is required between each reactor to reduce the temperature of the reaction gas, resulting in a complex process. Therefore, a compact process that does not require a cooler is needed.

[0087] According to Non-Patent Literature 9, a fluidized bed is a reactor having excellent heat transfer performance. In the present invention, in order to solve the problem of heat removal, a fluidized bed having excellent heat transfer performance is used, and a reaction gas supplied from the bottom of the reactor is used as a fluidizing gas.

[0088] Figure 3 The concept of a primary fluidized bed methanation reactor using an inorganic powder as a fluidizing medium and a reaction gas as a fluidizing gas is shown. The device configuration is the same as that of a normal fluidized bed device. That is, there is a dispersion plate 4 at the bottom of the fluidized bed reactor 1, and a powder bed 2 of the fluidizing medium is located thereon. The powder is sulfided by a reaction gas 3 flowing through the dispersion plate 4. A catalyst is placed in this sulfided powder in the form of particles or honeycomb, which is not shown in the figure.

[0089] A heat transfer pipe 5 for removing heat is also immersed in this fluidized bed, and the fluidized powder is cooled by flowing a cooling water or a hot medium oil into the pipe 5 to absorb and remove the heat generated in this fluidized bed. The mixing property of the fluidized bed is complete mixing, and its high heat transfer performance makes the temperature in the bed almost uniformly distributed. Therefore, the temperature in this reactor does not decrease as the reaction proceeds. Therefore, in the case of this primary fluidized bed, when the reaction temperature is 300-400°C, the concentration of residual hydrogen gas in the produced methane exceeds 10%, as described above.

[0090] Next, by lowering the reaction temperature of this fluidized bed to 200-300°C, it is possible to increase the conversion rate and decrease the concentration of residual hydrogen gas; however, this means that the lower the temperature, the lower the reaction rate. Therefore, in order to increase the reaction rate, it is necessary to take corresponding measures, such as using a high-activity catalyst, increasing the amount of catalyst, or increasing the residence time of the reaction gas in this reactor, i.e., increasing the size of the reactor.

[0091] From the above study, as a preferred reactor, in order to increase the methane conversion rate at the outlet of the reactor, a plurality of fluidized bed reactors having different internal temperatures are connected in series, as described above. For example, it is possible to further increase the conversion rate by supplying a gas to a reactor at 300-400°C, and then passing the outlet gas from this reactor to the next reactor at 150-300°C.

[0092] Therefore, in the present invention, a multi-stage fluidized bed is applied, in which a plurality of such fluidized bed reactors can be integrated into one reactor. Its concept is shown as follows.

[0093] The powder, which is the fluidizing medium used in the multi-stage fluidized bed, is cooled to the temperature of the fluidizing medium in the uppermost stage of the multi-stage fluidized bed or lower than this temperature using a powder cooler provided outside the multi-stage fluidized bed; then, the cooled powder is continuously fed into the uppermost stage of this multi-stage fluidized bed reactor. And, as the powder flows down into the multi-stage fluidized bed, its temperature is raised by absorbing the heat inside the reactor. The powder, which has become higher in temperature by absorbing heat, is continuously extracted. Then, the powder is cooled by the above-mentioned external cooler and circulated into the uppermost stage of the multi-stage fluidized bed. By this, a temperature distribution is formed in which the temperature of the lowermost stage is the highest, the temperature of the uppermost stage is the lowest, and the temperature of each stage between the uppermost stage and the lowermost stage gradually decreases from the lower to the upper.

[0094] The concept and configuration of this reactor are similar to a multi-stage distillation column, in which the fluidizing medium is cooled from the outside and circulated to form a temperature distribution as described above. In a distillation column, the liquid is heated at the bottom of the column by a reboiler to evaporate, and the vapor coming out from the top of the column is cooled and condensed by an external condenser, and a part of the condensate is refluxed into the column. By refluxing the cooled condensate to the top of the column, a stepwise temperature distribution is formed in which the temperature at the top is low and the temperature at the bottom is high. The liquid, which is cooled and condensed by refluxing, is similar to the powder, which is cooled by the external cooler of the multi-stage fluidized bed of the present invention, to be charged into the uppermost stage to cool the inside of the reactor.

[0095] Figure 4 is a schematic diagram of a two-stage fluidized bed to explain this concept more specifically. In order to lower the temperature of the upper stage of the reactor, the powder, which is cooled by the external cooler 11 to a temperature lower than that of the upper stage of the reactor, is continuously fed into the upper stage of the fluidized bed reactor 1 through the powder feeding pipe 13. That is, in contrast to lowering the temperature of the upper stage of this two-stage fluidized bed by a cooling heat pipe, the internal temperature is lowered to a lower temperature by feeding in the cooled powder. As a result, the temperature of the upper stage of the fluidized bed 1 is lower than that of the lower stage of the fluidized bed 7. The powder fed into the upper stage absorbs the reaction heat of the internal gas and the sensible heat of the gas temperature drop, raising the temperature, and then enters the lower stage of the fluidized bed 7.

[0096] In the lower stage of the fluidized bed, the lower-temperature powder flowing down from the upper stage absorbs the reaction heat and the sensible heat of the gas in this region, and then raises its own temperature, thereby suppressing the temperature rise of the fluidized bed 7.

[0097] As a result, the temperature of the upper stage of the fluidized bed becomes lower than that of the lower stage of the fluidized bed, and on the other hand, the reaction gas, which is higher in temperature in the lower stage of the fluidized bed, proceeds with the reaction, and then the reaction gas flows up into the upper stage, which is lower in temperature than the lower stage. By lowering the gas temperature, the equilibrium conversion rate is further improved; and by using a higher-activity catalyst at a low temperature, the reaction is further proceeded. Ultimately, it is possible to increase the methane conversion rate and reduce the residual hydrogen concentration.

[0098] Since powder is continuously fed from the upper stage, it is necessary to continuously extract the amount of powder heated to a higher temperature corresponding to the molar fed from the discharge section 9 of the fluidized bed 7 in the lower stage bottom. The extracted high-temperature powder is fed to the upper portion of the moving bed 12 in the external heat exchanger 11, cooled from the inside, extracted from the bottom, and fed again to the methanation by the powder feeding pipe 13. That is, it is fed to the fluidized bed 1 upper stage of the reactor. The configuration of the reaction gas 3, the dispersion plate 4, and the heat removal pipe 5 is the same as in the Figure 3

[0099] The above is the meaning and role of using a multi-stage fluidized bed and circulating the fluidizing medium between it and the external powder cooler. The above is a simplified two-stage fluidized bed for explaining the concept of the present invention. In the case of applying this method to the methanation reaction, the temperature varies greatly between about 600°C to about 150°C, so a multi-stage fluidized bed having three or more stages is preferred.

[0100] Next, a more detailed description of a multi-stage fluidized bed having such a temperature distribution will be given below. Here, it is assumed that the temperature of the uppermost stage of the multi-stage fluidized bed is in the range of 140-200°C, the temperature of the lowermost stage is in the range of 300-500°C, and the temperature of each intermediate stage is sequentially decreased from the lowest to the highest. In each stage, catalyst particles or catalyst molded bodies are arranged, and carbon dioxide and hydrogen, which are raw materials for the reaction, are supplied from the bottom of this multi-stage fluidized bed, serving as fluidizing gas. The powder in the fluidized bed is fluidized with this gas stream, and then the gas is brought into contact with the catalyst in the fluidized bed to synthesize methane. Since the temperature range at the reactor outlet uppermost stage is 140-200°C, the equilibrium conversion of methane increases as the reaction temperature decreases; a lower concentration of unreacted residual hydrogen and a higher concentration of methane can be obtained compared to the methane obtained from a conventional single methanation reactor with a reaction temperature of 300-500°C.

[0101] In this multi-stage fluidized bed, the descending powder and the ascending gas are brought into direct countercurrent contact and exchange heat, similar to a tray distillation column. As a result, a multi-stage fluidized bed with the lowest temperature at the uppermost stage and the highest temperature at the lowermost stage is achieved without using a powder heater or a gas cooler between the fluidized bed stages of different temperatures. This configuration greatly simplifies the equipment, and is one of the prominent features of the present invention.

[0102] In addition, hot spots can often occur in packed bed reactors filled with catalysts that initiate exothermic reactions. Since the fluidized bed has high heat transfer performance, the temperature distribution is uniform, and it is expected that the longer life of the catalyst will be extended compared to conventional packed bed reactors.

[0103] ​Next, powders used as fluidization media for this multi-stage fluidized bed are desired to be safe inorganic compounds, which are inexpensive, stable and have no toxicity or environmental load. That is, for example, sand, silicon dioxide, alumina, magnesium oxide and calcium oxide are such candidates.

[0104] Here, the heat load of each stage of this multi-stage fluidized bed will be described with a multi-stage fluidized bed having a three-stage configuration as an example.

[0105] As an illustration of the device concept of the present application, a three-stage multi-stage reactor is assumed, which temperature is changed by 100°C each time from 200°C to 500°C. At the bottom stage of the 500°C reactor, a reaction gas is injected, and the reaction gas flows into the next upper stage of 400°C.

[0106] Figure 2 The left bar of the three vertical bars shows the equilibrium conversion and the relative reaction amount to reach this conversion in the 400-500°C temperature range in this multi-stage fluidized bed. Next, this gas rises and enters the upper stage of the fluidized bed in the 300-400°C temperature range, and indicates the conversion increment and the relatively increased reaction amount as the reaction proceeds further. In addition, it also shows the reaction amount increase in the 200-300°C range. The reaction amount in the 400-500°C range is the reaction from zero conversion, that is, this means that the heat generated is relatively large at the bottom side of this reactor. From here, the value of the equilibrium conversion is read, and the heat generated in each stage is roughly estimated. Figure 2

[0107] From Figure 2 the broken line, the equilibrium conversion at 450°C is 80%. Assuming that the actual reaction rate reached is 80% of this equilibrium conversion, the reaction amount in this stage will be 64% of the total reaction. This indicates that 64% of the heat of reaction of the methanation reaction occurs in this temperature region.

[0108] On the contrary, according to Figure 2 , the equilibrium conversion in the 350°C region in the next stage is 92%. Similarly, when the actual conversion is 80%, the methane conversion is 74%. This means that the reaction proceeds by another 10% from 64% in the 450°C region to 74% in the 350°C region. That is, the amount of heat removal required for the lowest stage is larger than that required in the second stage because it starts from zero conversion of the reaction amount, and is about 6.4 times larger than the reaction amount of the second stage. Therefore, a large amount of heat needs to be removed from the lowest fluidized bed, and the heat removal capacity needs to be enhanced by increasing the heat transfer area, such as installing a large number of heat transfer tubes.

[0109] ​To solve this problem, the present invention uses inorganic powders that have been described as fluidizing media, and further mixes powders of substances that absorb heat in the range of 300 to 600°C to decompose into powders. Thus, one designs a multi-stage fluidized bed in which a part or all of the heat released by the methanation reaction generated in the temperature range of 300°C or higher is absorbed by the decomposition reaction of these compounds.

[0110] As a substance that absorbs heat, decomposes in the range of 300 to 600°C, and can be regenerated after decomposition, magnesium hydroxide powder is a chemical heat storage agent shown in Non-Patent Literature 10, that is, it dehydrates and absorbs heat at 300°C or higher according to Reaction Formula 2; and magnesium carbonate powder decarbonates at 400°C or higher according to Reaction Formula 3. By using these substances, a part or all of the heat of the methanation reaction occurring in the temperature range of 300°C or higher in this multi-stage fluidized bed reactor can be absorbed by these endothermic reactions of these chemical heat storage agents. Therefore, in the present invention, these substances are used to enhance the heat removal capacity in this temperature range.

[0111] That is, the lowest stage of the multi-stage fluidized bed generates a large amount of reaction heat, and needs to enhance its heat removal capacity. By using an endothermic agent that performs an endothermic reaction in this temperature range, and mixing it with the fluidizing medium flowing from the uppermost stage to the lowermost stage, the heat removal load in this temperature range can be greatly reduced.

[0112] The endothermic reaction of these chemical heat storage agents, that is, the dehydration reaction of magnesium hydroxide generates magnesium oxide, and the decarbonation reaction of magnesium carbonate also generates magnesium oxide.

[0113] As shown in Reaction Formula 4, this magnesium oxide can be regenerated to magnesium hydroxide by a hydration reaction with water. In addition, as shown in Reaction Formula 5, this magnesium hydroxide can be regenerated to magnesium carbonate by a reaction with carbon dioxide. That is, it is possible to regenerate this magnesium oxide to magnesium hydroxide, and then to magnesium carbonate, and to repeatedly recycle them.

[0114] Since both of these regeneration treatment reactions are exothermic reactions, this means that in these regeneration processes, the reaction heat absorbed in the methanation reactor is recovered and utilized.

[0115] Here, when this carbonation reaction is performed by absorbing carbon dioxide in a plant flue gas, this means that the carbon dioxide is fixed as magnesium carbonate. In addition, when this magnesium carbonate is used as a supply source of carbon dioxide, one of the raw gas for this methanation, this means that the carbon dioxide in this magnesium carbonate is converted to methane. That is, it is possible to generate methane from carbon dioxide in the flue gas, which is one of the CCUS technologies.

[0116] And, at this time, when "green hydrogen" is used as hydrogen for synthesizing methane, it means that a fuel easy for us to use is synthesized from renewable energy and greenhouse gases, or green hydrogen is converted into more usable methane with global warming gases.

[0117] Another feature is that the apparatus for such a methanation reaction is a multi-stage fluidized bed, so it is possible to select and use different optimum catalysts for each temperature range of each stage.

[0118] In addition to the catalysts that are often used at 300-500°C, a catalyst having high activity at low temperature has been developed. Therefore, in the low temperature range of the multi-stage fluidized bed, a catalyst having high activity even in the low temperature range of 150-200°C for low temperature can be used. In the high temperature range of this fluidized bed, a conventional methanation reaction catalyst such as a Ni catalyst can be used; and in the medium temperature range between these catalysts, it is possible to select any one of these catalysts or use a mixture of catalysts.

[0119] That is, in a reactor using a multi-stage fluidized bed in which the use temperature decreases as the reaction proceeds, by selecting and using different optimum catalysts for each temperature range, even in the case where the reaction rate is not slowed down at the reactor discharge side where the reaction temperature decreases and the reaction rate decreases, it is possible to synthesize high-concentration methane.

[0120] In such a reactor, in order to prevent these catalysts with the fluidizing medium from moving downward to the lower stage of the multi-stage fluidized bed, the catalyst particles should be larger than the hole diameter of the downcomer, or the catalyst should be shaped into a honeycomb shape to be held in each stage.

[0121] Next, because the chemical heat storage agent mixed in the fluidizing medium is in powder form, fine powder having a high specific surface area is preferably used in order to increase the reaction rate. In this case, it is generally considered difficult to fluidize fine powder, so it is necessary to provide a means for fluidizing fine powder.

[0122] [Effects of the Invention]

[0123] The above is a conceptual description of the present invention, and in the following, the feasibility of the concept of removing reaction heat by absorbing heat through the endothermic reaction of a chemical heat storage agent, which is the concept of the present invention, will be confirmed by heat calculation.

[0124] Table 1 shows the exothermic value and the endothermic value of each reaction under standard conditions. In addition, the reaction start temperature of each reaction is shown in the rightmost column.

[0125] First, the bottom row of this table shows the heat of combustion when 4 kmol of hydrogen is combusted, and its value is 968 MJ / kmol. From the second row from the bottom, the value of the heat of combustion of methane is shown to be 803 MJ / kmol. The difference between these two values is 165 MJ / kmol, which corresponds to 17% of the heat of combustion of hydrogen.

[0126] Next, from the third row from the bottom, the heat value generated when 4 kmol of hydrogen is reacted with 1 kmol of carbon dioxide to produce 1 kmol of methane is shown to be 165 MJ / kmol and is equal to the difference between the heat of combustion of hydrogen and the heat of combustion of methane. In other words, this difference means that, when 4 kmol of hydrogen is converted to methane, a portion of the energy it possesses is released to the outside as reaction heat.

[0127] This calculation shows that, from an energy quantitative point of view, it is more advantageous to directly combust hydrogen than to use methane converted from hydrogen. However, hydrogen needs to be stored under high pressure, and it is a substance that has a high risk of hydrogen explosion once it leaks, and in order to use it safely, it must be equipped with strict safety equipment.

[0128] On the other hand, because methane is a fuel used for household gas stoves and the like, it is preferable to convert it to methane and use it safely because it is possible to use it safely using existing infrastructure. However, by converting to methane, the energy is reduced by 17%, so a method is desired to recover and efficiently use this 17% of heat as much as possible.

[0129] In the present invention, by utilizing the dehydration reaction of magnesium hydroxide and the decarbonation reaction of magnesium carbonate, which is endothermic above 300°C, 17% of this heat loss can be absorbed and stored in the generated magnesium oxide. If this stored heat is efficiently used in other processes, then this 17% of energy will not be lost.

[0130] First, the dehydration reaction of magnesium hydroxide proceeds above 300°C, and its endothermic amount is 81 MJ / kmol; second, the endothermic amount of the decarbonation reaction of magnesium carbonate is 118 MJ / kmol, and the sum of these is 199 MJ / kmol. That is, because it is larger than the heat released by the methanation reaction (165 MJ / kmol), it is possible to absorb and store all of the reaction heat of the methanation reaction by adjusting the amounts of these two reactions.

[0131] Although the above study was performed using enthalpy under standard conditions, actual calculations should be performed using the enthalpy values at each reaction temperature. The reactions and heat calculations at each reaction temperature are described below.

[0132] [Table 1]

[0133]

[0134] Table 2 shows the reaction amount and the reaction heat of the methanation reaction in the temperature range of 300-400°C and 400-500°C, and the corresponding endothermic reactions and their endothermic amounts. Hereinafter, in the two temperature ranges, it is shown that all the exothermic values due to the methanation reaction can be offset by these endothermic reactions.

[0135] First, a heat calculation is performed with an intermediate temperature of 450°C to calculate the reaction heat in the temperature range of 400-500°C. The reaction heat of the methanation at 450°C is 183 MJ / kmol, which is higher than the value at the standard state of 25°C. Second, because the equilibrium conversion rate at 450°C is 0.72 (from Figure 2 ), assuming that the reaction reaches this equilibrium conversion rate, the heat released is 132 MJ / kmol. On the other hand, the decarbonation heat of 1 kmol of magnesium carbonate at 450°C is 114 MJ / kmol. Therefore, 18 MJ / kmol of the methanation reaction heat is excessive and cannot be absorbed.

[0136] Third, when performing a heat calculation for the temperature range of 300-400°C, the calculation is performed with an intermediate value of 350°C as the temperature. According to Figure 2 , the equilibrium conversion rate at 350°C is 0.90, so assuming that the reaction conversion rate proceeds from 0.72 to 0.90, the increment of the conversion rate is 0.18, and at this time, the reaction amount of carbon dioxide is 0.18 kmol. Also, because the reaction heat of the methanation at 350°C is 180 MJ / kmol, the reaction heat in this temperature range is 32 MJ / kmol. Adding this value to the residual heat of 18 MJ / kmol in the range of 400-500°C, the amount of heat to be removed is 50 MJ / kmol. On the other hand, because the heat of the dehydration reaction of 1 kmol of magnesium hydroxide at 350°C is 76 MJ / kmol, this amount of heat of 50 MJ / kmol can be absorbed by the dehydration reaction of 0.66 kmol of magnesium hydroxide. Therefore, the entire methanation reaction heat above 300°C can be absorbed and offset by the decarbonation of 1 kmol of magnesium carbonate and the dehydration of 0.66 kmol of magnesium hydroxide. That is, it is possible to suppress the temperature rise in the reactor without using a heat transfer pipe for heat removal.

[0137] [Table 2]

[0138]

[0139] The above assumes that 18 MJ / kmol heat at 400-500°C is brought into the 300-400°C temperature range. In addition, another way is that the decarbonation reaction of 1.16 kmol of magnesium carbonate absorbs all 132 MJ of heat; next, 32 MJ of heat can be absorbed by the dehydration reaction of 0.42 kmol of magnesium hydroxide, and then, 1 kmol of methanation absorbs all the reaction heat. In this case, without bringing the heat in the 400-500°C region into the 300-400°C region, all of the reaction heat is absorbed.

[0140] The above means that, without using a heat removal method using a heat transfer pipe, the heat removed from the 300-500°C high-temperature section, in which a large amount of heat is generated by the reaction, can be absorbed by using an appropriate amount of these chemical heat accumulators in the methanation reactor. In addition, this indicates that the heat generated by the exothermic reaction can be stored in magnesium oxide.

[0141] These two endothermic reactions generate 1.66 kmol of magnesium oxide in the former case and 1.58 kmol of magnesium oxide in the latter case. By hydration with water, the magnesium oxide can be regenerated into magnesium hydroxide; then, by reacting this magnesium hydroxide with carbon dioxide, it can be regenerated and used as magnesium carbonate. Also, because these regeneration reactions are all exothermic reactions, it is possible to recover and use the heat absorbed in the methanation reactor during this regeneration process.

[0142] In other words, by converting hydrogen into methane and using it, 17% of the energy of hydrogen is reduced, and will drop to 83%. However, by absorbing this heat loss in the heat accumulator and using it during regeneration, part or all of this 17% of energy loss can be effectively utilized, rather than being lost.

[0143] As described above, the methanation reactor of the present application has the following features.

[0144] 1. A fluidized bed, rather than a conventional packed bed, having high heat transfer performance for the reactor, a reaction gas is used as a fluidizing gas, and an inorganic powder is used as a fluidizing medium.

[0145] 2. The powder cooled by the external cooler as the fluidizing medium is fed into the top of the multi-stage fluidized bed reactor and can flow downward and absorb the reaction heat inside the reactor, and then the powder raised to a high temperature is extracted from the bottom. By circulating the fluidizing medium between the external cooler and the multi-stage fluidized bed reactor, by forming a multi-stage fluidized bed having a temperature distribution in which the temperature inside the reactor decreases from the bottom to the top, the conversion rate of methane at the outlet of the reactor can be improved, in which the temperature of the reaction gas flowing upward in the reactor gradually decreases as the reaction proceeds.

[0146] 3. In the temperature range above 300℃ (where the heat of reaction is large), inorganic powder of chemical heat storage agent can be used as a fluidizing medium to enhance the heat removal capacity in this temperature range by mixing and using magnesium hydroxide dehydrated above 300℃ and magnesium carbonate decarbonated above 400℃.

[0147] 4. Because the magnesium oxide obtained from these dehydration and decarbonation processes is a chemical heat storage agent, the heat absorbed in the methanation reaction can be recovered and used as heat in the regeneration process, where the carbonization reaction occurs after the hydration reaction. That is, it is possible to minimize the energy loss generated when converting hydrogen energy into methane.

[0148] Here, according to the rough calculations in Table 2, when the inorganic material powder flows downward in a multi-stage fluidized bed, the powder receives the heat of reaction from the rising gas; on the other hand, when the gas rises in this multi-stage fluidized bed, it comes into contact with the powder to provide its heat of reaction, i.e., they exchange heat with each other. However, in the above thermal calculations, the heat exchange between the sensible heat of the powder-side temperature increase and the sensible heat of the gas-side temperature decrease is not considered. To verify the robustness of the present invention, in addition to the heat exchange of reaction, it is necessary to take into account the sensible heat of the powder temperature increase and the sensible heat of the gas temperature decrease before performing the calculations.

[0149] Of course, in calculations that include these sensible heats, it is necessary to consider not only the components involved in the reaction, but also the sensible heats of the endothermic agents and gaseous components not involved in the reaction.

[0150] Therefore, for the following three cases AC, the compositional changes of each component within each temperature range, the reaction amount and heat of reaction at this time, the temperature change and sensible heat during the temperature rise of the downward flowing powder, and the temperature drop of the rising reactive gas and the sensible heat during the temperature drop should be considered. Then, taking the above into account, by calculating the changes in the composition of each component and the heat transfer between the components, it is verified whether the exothermic and endothermic processes can be balanced as expected.

[0151] A. When carbon dioxide is supplied from cylinders, etc.;

[0152] B. When flue gas is used directly as a source of carbon dioxide;

[0153] C. When magnesium carbonate powder is used as a source of carbon dioxide.

[0154] The calculation results for these three scenarios are described below. Note that this thermal calculation does not consider the difference in reaction rates between the gaseous side and the chemical heat storage agent, nor does it consider heat losses, such as heat dissipation from the equipment.

[0155] The thermal calculation results are described below. In Table 3-6, the right side shows the compositional changes and heat release (carbon dioxide, hydrogen, methane, water vapor) on the gas side (i.e., the heat-generating side); the left side shows the same situation on the heat-absorbing side, the compositional changes of the inorganic powders (magnesium oxide, magnesium hydroxide, and magnesium carbonate) and the values ​​of the heat absorption. In these calculations, magnesium oxide is used as a powder not involved in the reaction, but sand or silica, etc., can be used as heat-absorbing materials, as mentioned above.

[0156] Table 3 shows the composition changes in the simplest case where carbon dioxide is supplied by a cylinder or tank.

[0157] Here, the heat generated by the methanation reaction in the temperature range above 300°C is absorbed by the heat of the dehydration reaction of magnesium hydroxide. That is, magnesium carbonate is not used. In this calculation, the reactor consists of two stages: an upper stage (140-300°C) and a lower stage (300-400°C). The small figures in Table 3 show the composition of the gas and inorganic powder after each reaction stage.

[0158] In this thermal calculation, the temperature range of the lower stage of the reactor is 300-400℃ when the heat of reaction and sensible heat are calculated separately; however, in the heat of reaction calculation, the value at 350℃ is used as the median of this range to calculate the enthalpy change. Because the temperature change in this range is 100℃ when the temperature rises or falls, each sensible heat is obtained from each component and enthalpy value at 300-400℃.

[0159] The upper stage temperature range of the reactor is 140-300℃, but the representative temperature used to calculate the heat of reaction is assumed to be 200℃, and the temperature variation within this range is 140-300℃. Therefore, the temperature variation is set to 160℃.

[0160] The 1 kmol carbon dioxide and 4 kmol hydrogen required for the methanation reaction are supplied to the reactor from the bottom, as shown in the lower right of Table 3. When these reactant gases are supplied directly at room temperature (25°C), the temperature inside the reactor drops. Therefore, the reactor temperature is raised to 400°C by exchanging heat with a 400°C high-temperature powder pre-extracted from the reactor. That is, the hydrogen is preheated before being fed into the reactor.

[0161] The calculation results under these conditions are as follows. On the powder side, a mixture of 2.08 kmol magnesium hydroxide powder (used as an endothermic material) and 4.01 kmol magnesium oxide powder is fed from the top of the reactor at 140°C. The powder that has fully reacted during the downward flow is extracted from the bottom of the reactor, and after the feed gas is preheated as described above, the powder is sent to the regeneration process.

[0162] Within a temperature range of 140-300℃, powder fed from the top of the reactor does not undergo dehydration, meaning its composition remains unchanged, and the powder absorbs sensible heat from the gas to raise its temperature.

[0163] In the next stage, the temperature range is 300-400℃. It is assumed that 80% of the 2.08 kmol magnesium hydroxide fed in is dehydrated, and the remaining 20% ​​(0.416 kmol) of magnesium hydroxide does not react. (This is merely an assumption for trial calculations, and the actual dehydration rate determined experimentally is expected.) Therefore, the 0.416 kmol of magnesium hydroxide extracted from the bottom of the reactor is 5.674 kmol of magnesium oxide, which is the sum of the 4.01 kmol fed in and the 1.66 kmol generated by the dehydration reaction. The amount of water vapor generated in this dehydration process is 1.66 kmol, and this water vapor rises along with the reaction gases.

[0164] Next, the calculation results for the gas side will be explained.

[0165] 1.0 kmol of carbon dioxide and 4.0 kmol of hydrogen enter the lower stage of the reactor at a temperature range of 300-400°C, acting as reactants at 400°C. They exchange heat with the powder flowing downwards from the upper stage, then the reactants reach 300°C and flow into the upper layer. At this point, according to... Figure 2 The equilibrium conversion rate of the methanation reaction is 0.89. However, since the actual conversion rate of this reaction will not reach the equilibrium conversion rate during the actual residence time in the fluidized bed, this equilibrium conversion rate is assumed to be 85% of the equilibrium rate. As a result, the gaseous components after the reaction are: methane = 0.77 kmol, carbon dioxide = 0.23 kmol, water = 1.54 kmol, and hydrogen = 0.92 kmol. These components are shown in the lower part of the thermally generated side on the right-hand side of Table 3.

[0166] Next, within a temperature range of 140-300℃, it is assumed that the methane conversion rate increases from 0.85 to 0.98. Therefore, the gas composition at the reactor outlet is: methane = 0.98 kmol, carbon dioxide = 0.02 kmol, water = 3.62 kmol, and hydrogen = 0.08 kmol.

[0167] Magnesium oxide powder, which is not involved in the reaction, acts as an endothermic agent, absorbing sensible heat from the reaction gases as it flows downward into the reactor.

[0168] Here, considering the temperature of the powder after regeneration, the temperature of the powder fed into the top of the reactor is assumed to be 140°C.

[0169] If the powder temperature is lower or higher than this temperature, the temperature can be adjusted by decreasing or increasing the amount of magnesium oxide powder to be fed in.

[0170] [Table 3]

[0171]

[0172] Next, Table 4 shows the trial results of heat transfer obtained from the temperature and enthalpy changes of each component based on the compositional changes in Table 3. The values ​​in brackets [] in Table 4 indicate heat in MJ (megajoules). The heat transfer in the reactor from the bottom of the reactor is described below.

[0173] In countercurrent contact processes, such as in moving bed reactors, hydrogen directly exchanges heat with a mixture of 6.09 kmol of magnesium oxide and magnesium hydroxide powder extracted from the bottom of the reactor at 400°C, receiving 60 MJ of heat to preheat it to 400°C before feeding it back to the bottom of the reactor. Conversely, after the temperature is reduced from 400°C to 288°C through this heat exchange, the mixed powder is fed into a regeneration process. (In reality, a temperature difference of at least several degrees Celsius is required between the powder on the heating side and the gas on the heated side, but this temperature difference is assumed to be zero here, as it is an approximation.)

[0174] The reaction gas, which enters the lower stage of the reactor at 400°C, reacts to form methane with a conversion rate of 77%, and then releases 144 MJ of heat.

[0175] This heat is provided to the mixed powder at approximately 300°C flowing downwards from the upper stage of the reactor. Additionally, by providing 19 MJ of its own sensible heat to the powder during the temperature drop (which the powder absorbs), the temperature of the reactant gas itself drops to 300°C while the gas flows upwards within the reactor to enter the 300°C region at the upper stage of the reactor.

[0176] On the other hand, by absorbing and exchanging the 37 MJ sensible heat of the reactant gas as its temperature decreases, as well as the heat of methanation at the lower stage of the reactor, the temperature of the 300°C mixed powder entering the lower stage of the reactor from the upper stage is increased. That is, at this point, 1.66 kmol of magnesium hydroxide absorbs 127 MJ of dehydration heat from the reactant gas, then converts to magnesium oxide through dehydration, and is further heated to 400°C.

[0177] In summary, the heat supplied to the powder from the reactant gases in the lower stage of the reactor is shown on the right side of the table.

[0178] [Heat supplied to the powder from the gas] = Heat of reaction released by the gas + Sensible heat during the temperature drop of the gas = 144 + 19 = 163 MJ.

[0179] The heat received by the powder from the gas is shown on the left side of Table 4.

[0180] [Heat received by the powder from the gas] = Heat of reaction absorbed + Sensible heat of the powder used for heating = 127 + 37 = 164 MJ.

[0181] At this point, the two values ​​are almost equal, and (the heat released + the sensible heat during the gas temperature drop) and (the heat absorbed + the sensible heat during the powder temperature rise) cancel each other out. Therefore, under these reaction conditions, no heat transfer tubes are needed for heat removal to maintain a constant temperature, and no temperature changes occur in the next stage of the reactor.

[0182] [Table 4]

[0183]

[0184] Next, the heat transfer in the upper stage of the reactor will be described in the same manner.

[0185] Similarly, the inorganic powder flowing downwards into the reactor and the rising reaction gas exchange their heat through countercurrent contact.

[0186] The 300°C reaction gas enters the upper stage of the reactor from the lower stage, where the carbon dioxide undergoes a further reaction up to 98%, releasing 31MJ of heat, which is then provided to the downward-flowing inorganic powders of magnesium oxide and magnesium hydroxide. The temperature of the reaction gas then drops to 140°C.

[0187] On the other hand, the mixed powder fed into the upper part of the reactor at 140°C receives 62 MJ (which is the sum of 31 MJ of heat from the methanation reaction and 31 MJ of sensible heat released from the reaction gas as its temperature drops from 300°C to 140°C), and then, by receiving this heat, the powder temperature rises to 300°C. That is,

[0188] [Heat provided by the gas to the powder] = Heat released + Sensible heat due to temperature drop of the gas = 31 + 31 = 62 MJ.

[0189] [Heat received by the powder from the gas] = Heat used to heat the powder = 62 MJ.

[0190] The two heats become equal. Similarly, it is noted that the heat generated is offset by the heat absorbed, the temperature in the upper stage of the reactor does not change, and a constant temperature is maintained.

[0191] The above calculations do not include heat removal from heat removal heat transfer tubes inserted into the fluidized bed.

[0192] In fact, instead of relying solely on chemical heat storage materials for heat removal, it is also possible to combine heat removal with heat transfer tubes to control the internal temperature, thereby increasing the energy of responses to various disturbances in the process. When using heat transfer tubes to remove heat, the amount of inorganic powder supplied corresponding to the heat removed by the heat transfer tubes can be reduced for adjustment.

[0193] Incidentally, in Tables 3 and 4, to simplify thermal calculations, the reactor interior is described as having a two-stage structure, namely, an upper stage and a lower stage.

[0194] However, it is not necessary to be limited to two-level configurations; of course, more configuration levels are allowed.

[0195] Next, Table 5 shows the relationship between compositional changes and heat transfer when the flue gas is assumed to be directly supplied to the reactor as a source of carbon dioxide, similar to Tables 3 and 4. However, the compositional changes and heat transfer are summarized in one table. That is, the values ​​in the sub-tables indicated by solid lines in each stage of this table indicate the composition of each component in each reaction zone, similar to Table 3. Furthermore, the small values ​​indicated by dashed lines in the table indicate the amount of heat of reaction and sensible heat, similar to Table 4.

[0196] [Table 5]

[0197]

[0198] The flue gas temperature used here is 140°C. The composition is shown at the bottom of Table 5 and is assumed to be: carbon dioxide = 1 kmol, water vapor = 2.5 kmol, nitrogen = 5.0 kmol, and residual oxygen = 0.25 kmol.

[0199] To avoid introducing residual oxygen from the flue gas into the methanation reactor, hydrogen, with a stoichiometric amount of the remaining oxygen, is used for pre-combustion into water vapor. The gas composition after this hydrogen-oxygen combustion becomes: carbon dioxide = 1 kmol, water vapor = 3.0 kmol, and nitrogen = 5.0 kmol.

[0200] At this point, the heat of combustion caused by the combustion of hydrogen and oxygen is 123 MJ, and 74 MJ of this heat is consumed to raise the flue gas temperature from 140°C to 400°C.

[0201] Furthermore, the 4 kmol of hydrogen supplied to the methanation reaction receives 44 MJ of heat from this combustion heat and is also heated to 400°C. After deducing these figures, the remaining residual heat is 5 MJ, but this heat is carried from the bottom of the reactor along with the flue gas and hydrogen to the next stage of the reactor. In reality, this 5 MJ of heat only raises the temperature of the supplied gas by a few degrees Celsius from 400°C.

[0202] On the gas side, in the lower stage of the reactor, the reactant gases react to form: methane = 0.8 kmol, carbon dioxide = 0.2 kmol, water vapor = 4.60 kmol, hydrogen = 0.8 kmol, and nitrogen = 5.0 kmol. On the other hand, 80% of the 2.34 kmol magnesium hydroxide in the powder undergoes a dehydration reaction, releasing 1.87 kmol of water vapor. By adding 8.38 kmol and 1.87 kmol, the amount of magnesium oxide is 10.25 kmol. Similarly, 0.468 kmol of unreacted magnesium hydroxide remains.

[0203] Based on the above compositional changes, the heat transfer in the lower stage of the reactor is calculated by adding the heat of reaction of the reactant gas (144 MJ), the sensible heat of gas cooling (52 MJ), and the heat introduced by the reactant gas (5 MJ). That is, the total heat is 144 + 52 + 5 = 201 MJ, and this heat is provided to the powder flowing downward from the upper stage of the reactor.

[0204] Here, the heat absorbed during the dehydration of 1.87 kmol magnesium hydroxide is 143 MJ, and the heat required to heat the mixed powder to 300-400 °C is 58 MJ. The total is 143 + 58 = 201 MJ, where the heat release is also offset by heat absorption. In summary, in the lower stage of the reactor,

[0205] [Heat provided by the gas to the powder] = (Heat of gas reaction + Sensible heat of gas temperature drop + Waste heat of oxygen-hydrogen combustion) = 144 + 52 + 5 = 201 MJ.

[0206] [Heat received by the powder from the gas] = (sensible heat of the mixed powder) = 143 + 58 = 201 MJ.

[0207] That is, the released heat is offset by the amount of heat absorbed.

[0208] The same applies to the upstream stage of the reactor; each composition change is shown in the table, and the heat transfer based on this composition change is as follows:

[0209] [Heat provided by the gas to the powder] = (Heat of gas reaction + Sensible heat of gas temperature drop) = 31 + 67 = 98 MJ.

[0210] [Heat received by the powder from the gas] = (sensible heat of the mixed powder upon heating) = 98 MJ.

[0211] Therefore, the absorbed heat offsets the released heat. As shown above, it is possible to synthesize methane from hydrogen by using carbon dioxide-containing flue gas, in which residual oxygen has been previously removed, as the feedstock gas for the methanation reaction, based on thermal calculations.

[0212] In the process of amine absorption of carbon dioxide gas by directly supplying flue gas, a small amount of SOx can degrade the amine absorbent liquid when the flue gas is directly supplied to the amine absorption process. Since this invention does not use amine-based absorbent liquids, this concern is avoided. There is a concern that the catalyst used in the methanation reaction might be poisoned by the small amount of SOx in the flue gas. However, in this invention, the amounts of magnesium hydroxide and magnesium oxide (which are also desulfurizing agents) are much larger than the catalyst. Therefore, SOx is converted to MgSO4 and is considered harmless.

[0213] As a third case, Table 6 assumes that the carbon dioxide released from the magnesium carbonate decarbonation reaction is used as a source of carbon dioxide supply. Similarly, this is evident from the results of the thermal calculations. Based on the compositional changes of each inorganic powder component and thermal calculations, the reaction conditions under which the heat generated and absorbed at each stage can offset each other are shown.

[0214] Here, the magnesium carbonate obtained by the carbonation of magnesium hydroxide is often basic magnesium carbonate. According to thermal calculations, the chemical composition of basic magnesium carbonate can be considered as a mixture of magnesium hydroxide and magnesium carbonate, so this trial calculation treats it as a mixture of magnesium hydroxide and magnesium carbonate.

[0215] The main difference between Tables 3 and 5 is that, in this thermal calculation, the reactor interior consists of three stages (upper, middle, and lower).

[0216] The temperature at the bottom of the reactor is in the range of 400-500℃, at which the decarbonation reaction takes place.

[0217] In this scenario, the heat from the methanation reaction alone is insufficient to achieve temperatures above 400°C. Here, we assume that the insufficient heat is compensated by oxy-hydrogen combustion with air, and that this combustion heat is used to raise the temperature to 500°C. That is, oxy-hydrogen combustion occurs between 25°C air (0.14 kmol O2, 0.56 kmol N2) and 0.28 kmol hydrogen, and the combustion heat at this point can raise the gas temperature to 500°C. Of course, if the temperature of the hydrogen source gas can be increased by other preheating methods, this oxy-hydrogen combustion is unnecessary.

[0218] [Table 6]

[0219]

[0220] The calorific value of this oxygen-hydrogen combustion is 69 MJ. The heat required to raise the air temperature from 25°C to 500°C is 13 MJ, and the heat required to raise the temperature of the 4 kmol of hydrogen used for methanation is 56 MJ. Furthermore, when the gas heated to 500°C by this combustion heat is carried into the 400-500°C region at the bottom of the reactor, the excess heat is 55 MJ. Again, this excess heat effectively raises the temperature of the feed gas above 500°C. When this oxygen-hydrogen combustion occurs in the lowest stage of the reactor, the heat generated by the hydrogen combustion simultaneously carries out a decarbonation reaction (endothermic reaction), thus preventing excessive temperature rise.

[0221] Next, within this 400-500℃ range, the equilibrium conversion rate of the methanation reaction is approximately 70%. Assume the actual reaction rate to methane is 70%, or 49%, with a calorific value of 91 MJ. The gas composition at this point is as shown in the table below.

[0222] Furthermore, the heat generated when the reactant gas cools from 500°C to 400°C (sensible heat of cooling) is 19 MJ. Therefore, in the lower stage of the reactor,

[0223] [Heat supplied from gas to powder] = Heat carried in by gas + Heat of reaction + Sensible heat of cooling = 55 + 91 + 19 = 165 MJ.

[0224] On the other hand, the input amounts of the mixed powder used as the heat-absorbing material are: magnesium oxide = 4.74 kmol, magnesium hydroxide = 0.45 kmol and magnesium carbonate = 1.0 kmol.

[0225] In this lowest stage, the temperature is higher than in the middle stage, so all the magnesium hydroxide has been dehydrated and converted into magnesium oxide, with the following amounts: magnesium oxide = 5.19 and magnesium carbonate = 1.0 kmol. The heat required to raise the temperature of this mixed powder to 400-500°C is 51 MJ, and the heat required to completely decarbonate 1 kmol of magnesium carbonate is 114 MJ.

[0226] [Heat received by the powder from the gas] = heat of endothermic reaction + sensible heat of powder heating up = 114 + 51 = 165 MJ.

[0227] Furthermore, the heat generated is offset by the heat absorbed. (Similarly, the reaction rate of magnesium carbonate can, of course, be 50% or 80%. For example, when the reaction rate is 50%, the amount of magnesium carbonate to be fed is doubled; and the amount of heat absorbed can be adjusted by reducing the amount of magnesium oxide corresponding to the sensible heat of magnesium carbonate as an unreacted component.)

[0228] Within this 400-500℃ range, the carbon dioxide released from magnesium carbonate through decarbonation becomes the raw material gas for the methanation reaction. Therefore, when reacting with hydrogen, this gas flows upward in the reactor along with other gases.

[0229] Next, the composition of the reactor stage, gas side and powder side, is shown in Table 6. Based on these compositional changes,

[0230] On the gas side,

[0231] Heat of methanation reaction = 54 MJ;

[0232] The sensible heat of gas cooling is 17 MJ.

[0233] And their sum is 71 MJ.

[0234] [Heat supplied to the powder by the gas] = Heat of the exothermic reaction + Sensible heat of the gas cooling = 54 + 17 = 71 MJ.

[0235] On the powder side,

[0236] The heat of dehydration reaction of 0.45 kmol magnesium hydroxide is 34 MJ;

[0237] The heat generated by the powder heating up is 37 MJ;

[0238] And their sum is 71 MJ.

[0239] [Heat received by the powder from the gas] = heat of endothermic reaction + sensible heat of the powder = 34 + 37 = 71 MJ.

[0240] Similarly, the heat generated is offset by the heat absorbed. 0.45 kmol of water vapor is released and rises in the reactor along with the reactant gases.

[0241] Finally, similarly, in the top stage of the reactor,

[0242] The heat of methanation is 31 MJ, the sensible heat of gas cooling is 27 MJ, and their sum is...

[0243] [Heat provided by the gas to the powder] = Exothermic reaction heat + Sensible heat of cooling = 31 + 27 = 58 MJ.

[0244] Furthermore, this value is equal to the heat required to raise the temperature of the powder from 140°C to 300°C, which is 58 MJ.

[0245] [Heat received by the powder from the gas] = Sensible heat of the powder = 58 MJ.

[0246] Therefore, even under these conditions, the heat generated can be offset by the heat absorbed by each stage in the reactor, and the temperature in the reactor can be stabilized.

[0247] In the case shown in Table 6, magnesium carbonate powder is used after being mixed with inorganic powder as a fluidizing medium in a methanation reactor. This magnesium carbonate can be obtained by reacting magnesium oxide with carbon dioxide in flue gas. This means that methane is generated using carbon dioxide gas as a raw material; once converted to magnesium carbonate, it is then decarbonated in the reactor to obtain the carbon dioxide gas. That is, this is one type of CCUS technology.

[0248] As described above, in Tables 3 to 6, under three different scenarios with varying carbon dioxide supply sources used as raw materials, the heat release from the methanation reactor can be completely offset by inputting a chemically endothermic material with adjusted composition and quantity, even without the presence of heat transfer tubes for heat removal. That is, it demonstrates that the temperature in each stage of the multi-stage fluidized bed can be controlled to a constant level even without the use of heat transfer tubes for heat removal.

[0249] Of course, we can assume situations beyond these three, where, in any case, the amount and proportion of heat absorption can be adjusted by varying the quantity and ratio of each of the magnesium oxide, magnesium hydroxide, and magnesium carbonate powders. In the absence of heat transfer tubes for heat removal, the temperature in each temperature zone of this type of reactor remains constant.

[0250] Currently, the above calculations do not consider the reaction rate of the powder, the methanation reaction rate, and the residence time (=reaction time) for each case. These will be taken into account below, and it will be shown how to match the absorbed heat and generated heat in each fluidized bed.

[0251] Generally, gaseous reactions are very fast, with the combustion rate of gases being a typical example. Conversely, because the reaction rate of powders is a solid-state reaction rate and depends on their specific surface area, the reaction rate is much slower if the particles are in block form, such as charcoal. In fact, according to Non-Patent Literature 7, the dehydration reaction rate of magnesium hydroxide crystalline powder is about 50 minutes (=3000 seconds) to achieve a dehydration rate of about 90% at around 350°C, which is much slower than that of gases.

[0252] On the other hand, regarding the residence time of powder and gas in a fluidized bed, when the powder and gas react in equal molar amounts, the specific volume of the gas is much larger than that of the powder. The gas flow rate is very fast compared to the powder flow rate; that is, the residence time of the gas becomes very short compared to that of the powder.

[0253] As mentioned above, powders have a slow reaction rate but a long residence time; while gases have a fast reaction rate but a short residence time.

[0254] The heat absorption and heat generation in each stage are proportional to the respective amounts of powder and gas reacting, and the amount of reaction is proportional to the product of the reaction rate and the residence time. Therefore, the heat balance in each stage should be discussed based on the value of the product of (equal amounts of reaction).

[0255] Although a rough estimate has been made, the results for the reaction amounts of powder and gas in the fluidized bed will be shown below, taking into account the residence time.

[0256] Equation 4 is for the fluidized bed volume V of the first stage in a multi-stage fluidized bed. FB Fluidized bed cross-sectional area S and fluidized bed height H FB The relationship between them. Because the fluidized bed contains heat transfer tubes and catalyst for heat removal, when their occupied volumes are V... 管 and V cat Equation 5 gives the actual volume V of the fluidizing medium occupied by the powder and gas. fpg Here, when this V fpg When the porosity is ε, in this V fpg The volume V occupied by the powder fp The left-hand side of Equation 6 represents the volume V occupied by the fluidizing gas. fg The right-hand side of equation 6 represents this.

[0257] Here, the powder flows at a molar flow rate Q. p The volume V occupied by powder continuously flowing into the fluidized bed fp The average dwell time τ p Then at a rate Q p Leaving. Here, because the powder flowing into this fluidized bed contains the molar flow rate Q of substances involved in the reaction and substances not involved in the reaction. Rp and Q ip Therefore, the molar flow rate Q of the powder p It is the sum of these two molar flow rates (left side of Equation 7). On the other hand, the gas side also includes the reacting gaseous components and the unreacted gaseous components. Molar flow rate Q g It is the sum of the two mentioned above, and is expressed on the right side of Equation 7.

[0258] Here, when magnesium-based compounds are used as chemical heat storage agents, the reaction involves powder Q. Rp It is either magnesium hydroxide or magnesium carbonate. And as a substance not involved in the reaction, Q... ip It is magnesium oxide produced by a thermal absorption reaction.

[0259] The gas Q involved in the reaction rp This refers to carbon dioxide and hydrogen; and the gas Q not involved in the reaction. igIt refers to nitrogen gas contained in the input gas, and methane or water vapor used as reaction products.

[0260] Regarding Equation 8, the left side shows the reaction formula for the powder; and the right side shows the reaction formula for the methanation reaction of the gas. The stoichiometric coefficients of each component are shown in each reaction formula below. Since a dehydration reaction is shown here, after the gas-side reaction, 1 kmol of water vapor generated by the dehydration reaction should be added to the molar flow rate, and this molar flow rate of water vapor is shown in parentheses in Equation 8 as (1). In the case of decarbonation reaction, all the carbon dioxide generated is consumed as a raw material for methanation.

[0261] [Equation 4]

[0262]

[0263] [Equation 5]

[0264] V fpg =(V FB -V 管 -V cat )

[0265] [Equation 6]

[0266] V fp =V fpg (1-ε), V fg =V fpg ε

[0267] [Equation 7]

[0268] Q p =Q ip +Q rp Q g =Q ig +Q rg

[0269] [Equation 8]

[0270] Mg(OH)₂→MgO+H₂O↑, CO z +4H2→CH4+2H2O(+H2O)

[0271] 1→1+(1), [1+4=5]→[1+2=3](+1)

[0272] By multiplying this molar flow rate by the specific volumes v of the powder and gas respectively... p and v g To determine the respective volumetric flow rates U of the powder and gas flowing through this fluidized bed. p and U gHere, when the molar volume ratio changes before and after the reaction, the average values ​​before and after the reaction are used, and these average values ​​are v0 and v1, respectively. p(av) and v g(av) Therefore, the volumetric flow rate of the powder is U on the left side of Equation 9. p(av) And the gas volumetric flow rate U g(av) On the right side of Equation 9, by using these volumetric flow rates, the occupied volume in the fluidized bed is divided by the volumetric flow rate to obtain Equation 10, which is used to calculate the time through the fluidized bed, i.e., the residence time τ. p and τ g Substituting equations 6 and 9 into equation 10, we obtain equations 11 and 12. Therefore, we obtain the relationship between the residence times of powder and gas in the fluidized bed.

[0273] Here, compared to the number of moles of powder reacting, according to Equation 8, the number of moles of gas before the reaction is 5 mol, and the number of moles after the reaction is 3 mol. Therefore, when the average value in this calculation is 4 mol, since Equation 13 gives the ratio of the molar flow rates of powder and gas, Equation 14 gives τ. p With τ g The ratio of .

[0274] [Equation 9]

[0275] U p(av) =Q p ×v p(av) U g(av) =Q g ×v g(av)

[0276] [Equation 10]

[0277]

[0278] [Equation 11]

[0279]

[0280] [Equation 12]

[0281]

[0282] [Equation 13]

[0283] 4Q rp ≈Q rg (av)

[0284] [Equation 14]

[0285]

[0286] Here, when comparing the molar standard volume ratio of magnesium oxide as an inorganic powder with that of a gas, the molar volume ratio v of magnesium oxide is determined based on the true density value of magnesium oxide. p The molar volume ratio of magnesium hydroxide to magnesium carbonate is approximately 0.011 [L / mol]. The molar volumes of magnesium hydroxide and magnesium carbonate are 0.025 [L / mol] and 0.028 [L / mol], respectively. On the other hand, in an ideal gaseous state at 400°C, the molar volume of the gas is approximately 50 [L / mol], thus the ratio is approximately 4500-1800 times. Therefore, in the following calculations, the average value is taken as 3000 times for the following checks, as shown in Equation 13.

[0287] Generally, the porosity ε of particles with good flowability is assumed to be approximately 0.3, and the value of (1-ε) / ε is approximately 2.3, as shown on the right side of Equation 16. Therefore, these values ​​are substituted into Equation 14. Then, the ratio τ of the average residence time of powder and gas in the fluidized bed is used. p / τ g The value is approximately 28,000, as shown in Equation 17. That is, in a methanation reactor, this means that the residence time of the gas (=pass time) is much shorter than the residence time of the powder passing through the fluidized bed.

[0288] [Equation 15]

[0289]

[0290] [Equation 16]

[0291]

[0292] [Equation 17]

[0293]

[0294] In a fluidized bed at 350°C, when the residence time of magnesium hydroxide powder is 3000 seconds, the dehydration rate reaches approximately 90% during this period. However, the residence time of the gas in this fluidized bed is only about 0.11 seconds. When the amount of gaseous reaction (equal to heat generation) within this 0.11 seconds is equal to 90% of the heat absorbed by the dehydration reaction of the magnesium hydroxide present therein, the heat absorption and heat generation are equal. That is, the reaction temperature neither rises nor falls and remains constant.

[0295] However, despite the fast reaction rate of the gas, there is still concern that a residence time of 0.11 seconds may be too short. Therefore, the following shows how to address the mismatch between the reaction rate and residence time (when the residence time of the gas is too short, or when the amount of gas reacting (heat released) is greater than the amount of endothermic reaction).

[0296] When the residence time of the gas is short and the amount of reaction on the gas side is less than expected, the heat released is reduced; however, on the other hand, the endothermic reaction of the powder will not occur unless heat is provided; as a result, the amount of reaction of the powder is equal to the amount of reaction of the gas.

[0297] Conversely, if the amount of gas reacting is greater than the amount of powder reacting, then the amount of heat absorbed will be insufficient relative to the amount of heat generated, thus the temperature will rise. However, as Figure 2 As shown, the increase in temperature in the reaction field leads to a decrease in the equilibrium conversion rate, and the reverse reaction (steam reforming, which is an endothermic reaction) corresponding to this decrease in conversion rate occurs; as a result, the release of heat from the gas is suppressed to reach equilibrium.

[0298] Based on the above considerations, when the powder supply rate Q p and gas supply rate Q g When constant, the amount of gas reacted in each stage of a fluidized bed is related to the gas reaction rate and residence time τ. g The product is given; furthermore, the reaction rate can be adjusted by the amount and activity of the catalyst in the fluidized bed. On the other hand, according to Equation 4-12, the gas residence time τ g (Time spent in the fluidized bed) and fluidized bed height H FB It is proportional to the porosity ε of the fluidized bed.

[0299] Given the residence time τ required to obtain the desired gas reaction rate in each fluidized bed. g The fluidized bed height is H g At the same time, and similarly, the residence time τ required to obtain the desired amount of powder reaction is also given. p The fluidized bed height is H p Additionally, when the condition is H p >H g (That is, this means that when the gas reaction rate is faster), the amount of powder reacting will not exceed the amount of gas reacting, as described above. Because the results of these two quantities are equal, the fluidized bed height can be set to the height H that gives the desired amount of gas reacting. p That is, H FB =H p .

[0300] On the other hand, if H p <H g Then the height of the fluidized bed must be H. g In order to obtain the desired amount of gaseous reaction, i.e., H FB =H g However, at this point, the supply amount Q of the inorganic powder (2.08 kmol magnesium hydroxide and 4.01 kmol magnesium oxide in the example in Table 3) is... pThe supply quantity Q of gases (1.0 kmol carbon dioxide and 4.0 kmol hydrogen) g It shouldn't change. This is because the heat absorbed by the powder and the heat absorbed by the gas are determined by Q. p and Q g The definition is independent of the amount of powder present in the fluidized bed (retention rate). In other words, at this point, H... FB Become more than H p High (despite the powder's residence time τ) p (If it takes longer than required), the heat absorbed by the powder will not increase more than the heat generated by the gas, so this is not a problem.

[0301] This can be achieved by adjusting each stage of the downwell ( Figure 5 The height of the fluidized bed can be adjusted by adjusting the height of the fluidized bed (21). Of course, adjusting the supply of reactive gas to balance the heat generated by disturbances during the operation of the device is also effective.

[0302] Next, it is possible to improve H FB To obtain the desired gas reaction rate, in the examples in Tables 3 to 6, the temperature difference between each stage is set to 100°C to simplify calculations. Considering thermal strain in actual equipment, this temperature difference is preferably smaller. Therefore, for example, by dividing the fluidized bed with a 100°C temperature difference into four stages and setting the temperature difference between each stage to 25°C, the thermal strain of the equipment can be significantly reduced. Furthermore, increasing the number of stages is preferable because the flow properties of the powder in a multi-stage fluidized bed become closer to plug flow. This knowledge should be reflected in the device design.

[0303] Here, the reaction rate of the powder depends on its specific surface area. Therefore, a larger particle size will reduce the reaction rate. Therefore, it is preferable to use fine powder or porous particles obtained by granulating fine powder. However, fluidized beds using fine powder are prone to drift and are generally considered difficult to fluidize. In order to enable fluidization of fine powder and homogenization of the powder, it is preferable to use a multi-stage fluidized bed utilizing the oscillating mechanism of Patent Document 2.

[0304] In a fluidized bed utilizing fine powder, the uniform fluidization facilitates the expansion of the bed height, which in turn increases the porosity within the bed, leading to a longer gas residence time. Conversely, the reduced volume occupied by the powder results in a shorter powder residence time. Therefore, care must be taken not to shorten the residence time excessively.

[0305] Next, attention should be paid to the property changes accompanying the powder reaction. That is, when the reaction proceeds to 100%, all the substances are transformed into different other substances. In other words, the true specific gravity of the particles changes; as a result, changes occur in particle properties, such as particle size. Therefore, when the dehydration and decarbonation reactions and regeneration of powders are repeatedly carried out, it is conceivable that these property changes will affect operating conditions, such as powder handling. To suppress the effect of this property change on the powder flow characteristics, powder handling, such as encapsulating the powder with an inactive material (such as silica), can be considered. The simplest approach might be to limit the reaction rate to 50% or 30% without such handling.

[0306] The example in Table 6 is a trial calculation when the reaction rate of 1 kmol of magnesium carbonate is as high as 100%. If this is set to 50%, the heat removed in the 400-500°C range will be halved and insufficient. In this case, the amount of magnesium carbonate powder to be added needs to be doubled to 2 kmol, while maintaining the amount of magnesium carbonate powder reacting at 1 kmol. At this point, regarding sensible heat, the sensible heat per mole of magnesium carbonate between 140°C and 500°C is approximately 2.3 times the molar sensible heat of magnesium oxide. Therefore, because the amount of magnesium carbonate to be added has increased by 1 kmol, the input amount of 2.3 kmol of magnesium oxide corresponding to this sensible heat should be reduced from 4.74 kmol to 2.44 kmol.

[0307] The above calculations are only on-machine calculations, and the actual temperature inside the reactor can vary due to various factors. To handle these variations, it is preferable to control the generated heat by adjusting the supply of the reactant gas, as this provides a faster response to device conditions. Furthermore, when heat transfer tubes are installed for the purpose of partial heat removal, the amount of heat removed through the heat transfer tubes can be adjusted.

[0308] As described above, one of the main features of this invention is the use of inorganic powders as a fluidizing medium and chemical heat storage agents for removing the heat of the methanation reaction. These chemical heat storage agents are magnesium hydroxide, magnesium carbonate, and magnesium oxide, which are endothermic at temperatures above 300°C. Because these powders do not change unless they come into contact with moisture or carbon dioxide, they can be stably stored for long periods (heat storage) at atmospheric pressure.

[0309] Therefore, if the magnesium oxide powder extracted and cooled by the methanation reactor is fed into the carbon dioxide-containing flue gas emitted from the plant and stored in silos, the magnesium oxide can be used to generate the heat required when these plants need heat. When these plants require heat, the required amount of magnesium oxide is removed, hydrated during the regeneration process, and further carbonized, and then the stored heat can be recovered and used during regeneration.

[0310] Generally, the reason why waste heat utilization rates in factories don't increase is that the temperature, quantity, and time of the released waste heat don't match the temperature, quantity, and time requirements of the processes utilizing it. However, at the peak of heat utilization in a factory, extracting and regenerating these powders allows the heat generated to be recovered and used, for example, to preheat boiler water or boiler combustion air. That is, it can compensate for a portion of the factory's heat usage.

[0311] This means that the 17% energy lost when converting hydrogen to methane is not lost and can be used effectively elsewhere.

[0312] The features of the methanation reactor utilizing the multi-stage fluidized bed of this invention are summarized.

[0313] A. It improves the heat removal performance of the reaction heat generated in the reactor, inhibits the rise in catalyst surface temperature, and helps to extend catalyst lifetime.

[0314] B. By progressively reducing the temperature in the multi-stage fluidized bed from the bottom to the top of the reactor, and by selecting and using catalysts that function effectively within each temperature range, high methane conversion can be achieved even at the reactor outlet at low temperatures without reducing the reaction rate. This also allows for a reduction in residual hydrogen concentration.

[0315] C. To address the significant exothermic reaction in multi-stage fluidized beds operating at temperatures above 300°C, chemical heat storage agents that decompose and absorb heat, such as magnesium hydroxide and magnesium carbonate powder, are mixed with inorganic powders used as the fluidizing medium. The mixed powder is then fed into a methanation reactor, where heat removal is achieved within this temperature range. These endothermic reactions greatly enhance the heat removal capacity within this temperature range.

[0316] D. The magnesium oxide obtained from the dehydration reaction of magnesium hydroxide and the decarbonation reaction of magnesium carbonate can be regenerated and reused, and the heat generated during this regeneration process can be recovered and reused. By using this recovered heat, energy loss can be reduced when converting hydrogen to methane.

[0317] Magnesium oxide, magnesium hydroxide, and magnesium carbonate are chemical heat storage agents, and they can be stably stored and transported at atmospheric pressure as long as moisture and carbon dioxide are isolated. Then, carbon dioxide in the factory flue gas (i.e., the carbon dioxide source) reacts to form magnesium carbonate during the magnesium oxide regeneration process. Therefore, by fixing the carbon dioxide in the flue gas into magnesium carbonate and feeding it to a methanation reactor, this carbon dioxide can be converted into methane for reuse.

[0318] F. Because magnesium carbonate, which reacts with carbon dioxide, can be easily stored and transported at atmospheric pressure, even if the operating time and processing capacity of the carbon dioxide regeneration source are not matched with the methanation reactor, and even if the plants serving as carbon dioxide sources are small in scale and geographically dispersed, after transporting the magnesium carbonate obtained from the flue gas of these plants to the methanation reactor and storing all the collected magnesium carbonate, the carbon dioxide in the magnesium carbonate can be converted into methane by removing the required amount of magnesium carbonate from storage. That is, effective operation can be performed independently and separately without mutual interference. Attached Figure Description

[0319] [ Figure 1 The relationship between the Gibbs free energy change ΔG and the temperature of the various reactions involved in this process.

[0320] [ Figure 2 Regarding the methanation reaction, the relationship between the conversion rate of carbon dioxide to methane and temperature is obtained from the equilibrium constant K calculated from ΔG. In this graph, the vertical bars represent the conversion rate in each stage of a multi-stage system.

[0321] [ Figure 3 A primary fluidized bed equipped with heat transfer tubes for heat removal.

[0322] [ Figure 4 A secondary fluidized bed equipped with heat transfer tubes for heat removal, wherein powder circulates between the reactor and an external cooler.

[0323] [ Figure 5 A conceptual diagram of a multi-stage fluidized bed equipped with a oscillating mechanism and heat transfer tubes for heat removal.

[0324] [ Figure 6 An example of the arrangement of a honeycomb catalyst, heat transfer tubes for heat removal, and a oscillating perforated plate in one stage of a multi-stage fluidized bed.

[0325] [ Figure 7 The flowchart of the regeneration process shows the process of regenerating part or all of magnesium oxide into magnesium hydroxide on the left and the process of regenerating part or all of magnesium hydroxide into magnesium carbonate on the right.

[0326] [ Figure 8 A moving bed equipped with a oscillating mechanism is used when magnesium oxide is regenerated into magnesium hydroxide or when magnesium hydroxide is regenerated into magnesium carbonate.

[0327] [ Figure 9 The flow of chemical heat storage agent and heat when the chemical heat storage agent circulates between the methanation process and the chemical heat storage agent regeneration process.

[0328] [ Figure 10The diagram shows a plant that generates hydrogen from renewable energy, an adjacent methanation unit, a conveyor line for magnesium carbonate powder containing carbon dioxide to fix carbon dioxide from multiple plant flue gases, carbon dioxide supplied to the methanation reaction, and a concept diagram of magnesium oxide from the methanation plant being recycled back to the plant for regeneration. Detailed Implementation

[0329] The possibilities and characteristics of a multi-stage fluidized bed methanation reactor utilizing a chemical regenerative agent have been described above. The equipment for achieving this functionality will be described below.

[0330] The dehydration of magnesium hydroxide and the hydration of magnesium oxide, as well as the decarbonation of magnesium carbonate and the carbonation of magnesium hydroxide or magnesium oxide, are chemical heat storage agents added to the fluidizing medium. These reactions occur at the solid surface. Therefore, to increase the reaction rate and the residence time of the gas, fine powders with a high specific surface area are preferably used. However, fine powders are generally considered difficult to self-fluidize. Therefore, it is necessary to design a method to achieve uniform fluidization of fine powders.

[0331] To fluidize the fine powder, a vibrating fluidized bed is used, in which the entire fluidized bed vibrates up and down. Of course, this method can also be applied to this methanation reactor, but for this system, because the vibration energy is proportional to the mass and amplitude of the equipment, the larger the equipment, the greater the energy required for vibration.

[0332] On the other hand, in a multi-stage fluidized bed using the oscillation mechanism of Patent Document 2, the powder inside is gently agitated without the need for a moving device.

[0333] Therefore, the mass to be moved is much smaller than that of a vibrating fluidized bed system in which the entire device vibrates.

[0334] It is said that the preferred vibration frequency for a vibrating fluidized bed should be 5-50 Hz, but in the case of an oscillating mechanism, this value is as small as about 0.01-1 Hz. Since vibration energy is proportional to the square of the vibration frequency, it can be seen that the energy required for this oscillating motion is much smaller than that required for a vibrating fluidized bed.

[0335] As described above, from the perspective of energy requirements, in the methanation reactor of the present invention, preferably, a multi-stage fluidized bed equipped with a oscillating mechanism is used so as to fluidize fine powder and homogenize the powder.

[0336] Next, Figure 5 This diagram illustrates a concept for a methanation reactor using a multi-stage fluidized bed. In this diagram, the fluidized bed has a four-stage structure, but is not limited to four stages; more than four stages can be used. In this diagram, inorganic powder at 140°C is fed to the top stage, the bottom temperature is 400°C or 500°C, and the intermediate temperatures vary within each stage.

[0337] Furthermore, in this figure, it is assumed that the fine powder is used as a powder, and the aforementioned oscillating mechanism is used to assist in the uniform fluidization of the fine powder.

[0338] For this purpose, a motor 16 for driving the swing mechanism, a drive shaft 17 for transmitting rotational motion to the swing plate, and a perforated swing plate 18 are shown.

[0339] When the oscillating mechanism is used, in addition to the average packing density of powder in the bed, the powder entering the dispersion plate from the supply port 15 is distributed on the entire surface of the dispersion plate and is smoothly leveled by utilizing the oscillating motion of the oscillating perforated plate 18.

[0340] Similarly, here we assume the use of a large-particle-size catalyst 19, which is then rolled onto a dispersion plate by the movement of a oscillating plate. The reason for the large particle size of the catalyst particles 19 in this figure is that, in the absence of fluidization by a fluidizing gas, they remain in each stage of the fluidized bed. Of course, when using molded bodies, such as honeycomb structures, it is not necessary to roll the catalyst.

[0341] Figure 6 A diagram showing the layout of the catalyst, heat transfer tubes for heat removal, and porous oscillating plates within the bed is presented, in this case only one stage of a multi-stage fluidized bed.

[0342] The powder enters the lower fluidized bed from the upper fluidized bed through the downcomer 20.

[0343] Here, the honeycomb catalyst 19 is fixed below the heat transfer tube 5 for heat removal. When the reactant gas passes through the honeycomb catalyst, it reacts on the catalyst surface. Below the honeycomb catalyst, in the powder bed, there is an oscillating porous plate 18; the horizontal oscillating motion averages the packing density of the powder in the horizontal direction, prevents drift, and ensures uniform contact between the gas, powder, and catalyst.

[0344] After a predetermined residence time in the fluidized bed, the powder escapes from the weir 21 of the downcomer, enters the downcomer 20, and moves to the lower fluidized bed.

[0345] Magnesium hydroxide and magnesium carbonate in inorganic powders absorb the heat of reaction and are converted into magnesium oxide in a methanation reactor. In their regeneration process, the magnesium oxide-containing inorganic powders react with water vapor to convert the magnesium oxide into magnesium hydroxide; then the magnesium hydroxide reacts with carbon dioxide to form magnesium carbonate.

[0346] Figure 7 This is a block diagram of the magnesium oxide regeneration process. The left side of the diagram shows the block flow of magnesium oxide undergoing a hydration reaction after cooling to form magnesium hydroxide. The right side of the diagram shows the block flow of partially or completely carbonizing magnesium hydroxide to form magnesium carbonate.

[0347] Magnesium oxide powder extracted from methanation reactor 28 is cooled in external powder cooler 11 and then returned to methanation reactor 28 as magnesium hydroxide in hydration reaction unit 30. Additionally, when a portion of the magnesium hydroxide is to be prepared into magnesium carbonate, this portion is prepared into magnesium carbonate in carbonation reaction unit 31.

[0348] Here, the recycling ratio of the powder to be processed is not necessarily 100%.

[0349] exist Figures 3 to 6 In the three trials, the ratio of magnesium hydroxide to magnesium carbonate in the powder was approximately 25-35% of the amount of inorganic powder added, and the remainder was inorganic powder (such as magnesium oxide) that had not participated in methanation. In other words, magnesium oxide regeneration can be performed by regenerating only the amount of magnesium hydroxide and magnesium carbonate required to offset the heat released in the methanation reactor.

[0350] Therefore, in this block flow diagram, a portion of magnesium oxide from the external powder cooler 11 and a portion of magnesium hydroxide powder from the hydration reactor 30 are extracted and mixed with the treated powder to generate methane, and then the mixed powder is conveyed to the chemical reactor 28.

[0351] Figure 8 This is a conceptual diagram of a scenario where a moving bed is used as a regeneration reactor for chemical regenerators.

[0352] Once the powder extracted from the methanation reactor is cooled by an external powder cooler, the powder is then fed into the upper part 10 of the regeneration reactor.

[0353] Preferably, the inorganic powder is regenerated using a moving bed that enables efficient countercurrent contact between the powder and the gas.

[0354] When using a moving bed for regeneration, drift is prone to occur when fine powder is used or when the device size is increased even without using fine powder. Therefore, it is preferable to use a moving bed with a swing mechanism to suppress drift and enable uniform regeneration of the inorganic heat storage agent.

[0355] exist Figure 8 In this moving bed, the oscillating perforated plate 18 and the heat transfer tubes 5 for heat removal are alternately installed along the height direction. The rotational motion of the drive motor 16 is transmitted to the perforated oscillating plate via the drive shaft 17 to generate oscillating motion. Therefore, the powder bed is leveled along the horizontal direction to prevent drift and to ensure uniform contact between the gas and the powder.

[0356] In the hydration reaction of magnesium oxide, since wet steam is preferably used for water vapor (i.e., reaction gas), the wet steam is dispersed from the gas distributor 27 at the bottom and supplied to the moving bed to form a plug flow.

[0357] After regeneration, the powder is extracted from the bottom of the reactor and conveyed to the carbonization step via conveyor pipe 13, or transferred to the methanation reactor via a storage tank.

[0358] In the carbonization reaction of magnesium hydroxide, carbon dioxide or flue gas (also a reaction gas) is supplied from the lower air distributor 27 to be uniformly dispersed in this moving bed.

[0359] Finally, the product gases exiting the methanation reactor are unreacted carbon dioxide, hydrogen, and methane vapor at 140°C. Before storing this product gas in storage tanks, it is... Figure 5 The gas cooler 25 shown cools and condenses the contained vapors, thereby separating and removing them from the generated methane gas.

[0360] The small amount of unreacted carbon dioxide contained can be removed by methods such as chemical absorption using an aqueous solution of calcium hydroxide to convert it into calcium carbonate, and then methane containing about 1% hydrogen can be obtained.

[0361] Industrial applicability

[0362] This invention relates to a reactor for synthesizing methane from hydrogen and carbon dioxide. Carbon dioxide is a greenhouse gas, and its primary source is boilers burning fossil fuels such as coal, oil, and natural gas. Hydrogen generated from renewable energy sources is preferred if it can cover the total energy from these fuels. However, hydrogen is highly explosive and must be stored at considerably high pressures. Using hydrogen as a general fuel carries significant risks. Therefore, if this hydrogen could be converted into methane and used, its safety and ease of use would be greatly improved. Thus, a highly efficient methanation reactor is desired.

[0363] As mentioned above, methane is an excellent all-purpose fuel; however, on the other hand, by converting hydrogen into methane, the thermal energy is reduced by about 17%.

[0364] If the heat generated in the methanation reactor is released into the environment during heat removal, 17% of the thermal energy of the hydrogen will be lost. If this heat can be stored and then effectively utilized, all the thermal energy can be effectively used. For this purpose, it is preferable to use inorganic powders containing magnesium hydroxide and magnesium carbonate (i.e., chemical heat storage agents) as the fluidizing medium and as a method for removing the heat of reaction in a multi-stage fluidized bed used in the methanation reactor.

[0365] Furthermore, when the carbon dioxide released from the decarbonation of this magnesium carbonate can be used as a raw material for methanation, this technology will become a CCUS technology that fixes carbon dioxide into magnesium carbonate and uses carbon dioxide more efficiently.

[0366] In concrete terms, this CCUS technology can be used to fix carbon dioxide into magnesium carbonate from flue gas from power plant boilers, cement industries, steel industries, and other sources of large-scale carbon dioxide production. This magnesium carbonate is then fed to a hydrogen production facility, where it can be used to synthesize methane in a methanation reactor. As a result, a carbon cycle can be achieved by using this methane or supplying it to other sources as fuel.

[0367] Currently, the mainstream CCS technology for capturing carbon dioxide utilizes amine-based absorbents, and naturally, this carbon dioxide can also be used as a feedstock for methanation. However, the carbon dioxide must be pressurized for storage.

[0368] On the other hand, magnesium carbonate is considered advantageous because it can be stored stably for a long time under atmospheric pressure and is easy to transport.

[0369] This technology allows magnesium carbonate powder to be stored in storage tanks and easily retrieved and transported. This technology can be applied not only to the large-scale carbon dioxide generation sources mentioned above, but also to small-scale carbon dioxide generation sources that are separate and dispersed from each other.

[0370] Therefore, carbon dioxide from these small, distributed carbon dioxide sources can also be converted into methane by converting most of the carbon dioxide produced by these small-scale plants into magnesium carbonate and then feeding it into methanation reactors for aggregation and storage.

[0371] This means that the technology is not limited to boilers in large-scale power plants, but rather the carbon dioxide in the flue gas from most of the small, geographically dispersed plants is fixed into magnesium carbonate, which is then transported to facilities with methanation reactors and converted into methane.

[0372] While capturing and converting carbon dioxide from large-scale sources is undoubtedly important, it is ultimately also necessary to capture and convert it from smaller, geographically dispersed sources. The method of this invention is also useful in such cases.

[0373] exist Figure 9 The present invention outlines the heat and mass flow.

[0374] The heat of combustion of hydrogen, initially 968 MJ / kmol, is converted to methane in methanation reactor 28, releasing 165 MJ / kmol of heat of combustion through thermal generation, reducing the heat to 803 MJ / kmol. In this figure, the internal temperature of the methanation reactor is approximately 500°C at the bottom and 200°C at the top; however, the fluidized bed temperature gradually changes between the bottom and top. By reducing the reaction temperature in this manner as the gaseous reaction proceeds, the conversion rate to methane at the reactor outlet can be increased.

[0375] By absorbing the heat generated in this reaction, magnesium hydroxide dehydrates to magnesium oxide, and magnesium carbonate decarbonates to magnesium oxide. That is, the resulting magnesium oxide absorbs 165 MJ / kmol of heat and stores it.

[0376] Next, the magnesium oxide powder is transferred to the regeneration facility 34 via a circulating loop 36 of the chemical heat storage agent, such as by powder conveying. Here, because the 165 MJ / kmol of reaction heat absorbed in the methanation reactor is released during magnesium oxide regeneration, it can be recovered and reused. This means that the heat generated in the methanation reactor is carried to the regeneration facility by hot steam 34 and recovered and used in the regeneration facility.

[0377] In this regeneration facility, magnesium oxide is regenerated into magnesium hydroxide and magnesium carbonate, which are then transported again via loop 36 to the methanation reactor for reuse. Thus, the magnesium-based chemical regenerative agent acts as a medium for transferring heat from the methanation reactor 28 to the regeneration unit 34, and as a transport medium for transferring carbon dioxide from the regeneration unit 34 to the methanation reactor 28. The magnesium-based chemical regenerative agent circulates between these two facilities. Here, when powder agglomerates or breaks up in the methanation reactor and regeneration unit, a pulverizing or granulation process should be added to the loop to facilitate powder handling.

[0378] Figure 10 The flow between the methanation reactor and the regeneration reactor is shown. Figure 9 The concept of heat and material flow. Powder containing carbon dioxide fixed as magnesium carbonate, collected from multiple plants 34, is transported to a methanation facility 28 that stores hydrogen produced from renewable energy sources. The magnesium carbonate and hydrogen supplied by a hydrogen generation facility 35 are used to generate methane. The methane is then stored in a methane gas container 29.

[0379] Magnesium oxide powder extracted from methanation reactor 28 is cooled to room temperature by external powder cooler 11 and then stored in storage tank 32, such as a silo. This magnesium oxide powder is returned to carbon dioxide source plant 34, hydrated to magnesium hydroxide, and then regenerated into magnesium carbonate by absorbing carbon dioxide from the plant flue gas. Once the magnesium hydroxide and magnesium carbonate are stored in storage tank 33, they are transported to methanation facility 28. In other words, carbon dioxide in the flue gas from a distant plant can be converted into methane through a process that converts it into magnesium carbonate. On the other hand, the heat energy stored in each plant 34 when hydrogen is converted into methane can be used to generate heat during the regeneration into magnesium carbonate.

[0380] That is, according to this viewpoint, the energy lost during the conversion of hydrogen into methane can be recovered and used in other plants.

[0381] On the other hand, each plant provides carbon dioxide in the form of magnesium carbonate in exchange for that amount of heat. Furthermore, even if the amount and timing of heat generation differ from demand, the energy exchange between the methanation reaction facility and the carbon dioxide generation source can be operated independently of each other via heat storage agents, storage tanks, and transportation. This is also an advantage of the present invention.

[0382] according to Figure 1 The calculations suggest that even at around 140°C, the carbonization reaction of magnesium hydroxide can proceed fully.

[0383] On the other hand, in amine absorption methods, the flue gas temperature needs to be cooled to 20-60°C for carbon dioxide absorption; however, this cooling is not required in this invention. That is, without cooling, the carbon dioxide in the flue gas is directly converted into magnesium carbonate. Therefore, another advantage is that it does not affect the effective stacking height.

[0384] Currently, as a measure to combat global warming, there are strong calls to reduce carbon dioxide emissions and promote the use of renewable energy.

[0385] According to the present invention, in the cases shown in Table 5, carbon dioxide extracted after burning fuel in the boiler is used directly; or in the cases shown in Table 6, carbon dioxide is fixed into magnesium carbonate and used. In either case, green hydrogen obtained from renewable energy sources can be used to convert carbon dioxide into fuel methane, which is then reused as a synthetic fuel. In other words, this is a CCUS technology that achieves a complete carbon cycle and contributes to a dramatic reduction in carbon dioxide emissions as a countermeasure against global warming and a reduction in the use of fossil resources.

[0386] [Label Description]

[0387] 1. Fluidized bed or moving bed device

[0388] 2. Fluidized bed or moving bed of powder

[0389] 3. Gas Flow

[0390] 4 Gas distribution plate

[0391] 5. Heat transfer tubes for cooling

[0392] 6. Cooling medium (water, oil, etc.)

[0393] 7. Fluidized bed lower stage

[0394] 8. Connecting pipe for reaction gases

[0395] 9. Powder emission section

[0396] 10 Powder conveyor line from the bottom of the reactor

[0397] 11 External Powder Cooler

[0398] 12 mobile beds

[0399] 13 Powder conveying (supply) line to the top of the reactor

[0400] 14 Multistage fluidized beds

[0401] 15 powder supply units for multi-stage fluidized beds

[0402] 16 Drive motor for the swing mechanism

[0403] 17 Drive shaft for the swing mechanism

[0404] 18. Oscillating perforated plate

[0405] 19. Catalyst particles, honeycomb catalysts

[0406] 20 Downcomer

[0407] 21. Overflow weir of downcomer

[0408] 22. Divider plate at the bottom of the downcomer

[0409] 23 Raw material gas supply port

[0410] 24. Gas outlet

[0411] 25 Generative Gas Cooler

[0412] 26. Gas generated after cooling

[0413] 27. Gas diffuser (gas distributor)

[0414] 28 Methanation reactor

[0415] 29. Methane Generator

[0416] 30 Hydration Reactor

[0417] 31 Carbonization reactor

[0418] 32 Magnesium Oxide Storage Tank

[0419] 33. Powder storage tank after regeneration treatment

[0420] 34. Plant (hydration reaction facility + carbon dioxide source)

[0421] 35 Hydrogen generation device

[0422] 36. Circulation loop of chemical heat storage agent

[0423] 37 Absorbed reaction heat flow

Claims

1. A method for reacting carbon dioxide with hydrogen to produce methane in the presence of a catalyst, using a multi-stage fluidized bed reactor, wherein, The top stage has the lowest temperature, and the temperature of each stage decreases sequentially from bottom to top. When a multi-stage fluidized bed is applied to a reactor, the powder used as the fluidizing medium is cooled by an external powder cooler to the temperature of the fluidizing medium in the top stage of the multi-stage fluidized bed or a lower temperature. It is then continuously fed to the top stage of the multi-stage fluidized bed and cooled internally as it flows downwards. The powder itself absorbs heat from the reactor interior to raise its temperature, and powder that has absorbed heat and reached a high temperature is continuously extracted from the bottom of the multi-stage fluidized bed. The extracted powder is then cooled by the external powder cooler and returned to the multi-stage fluidized bed. The fluidized bed is circulated at the top; wherein the multi-stage fluidized bed reactor forms a temperature distribution in the vertical direction; the temperature of the uppermost stage is 100-300°C, and the temperature of the lowermost stage is 300-600°C; wherein, by supplying carbon dioxide and hydrogen as fluidizing gases from the bottom of the multi-stage fluidized bed, catalyst particles or catalyst compacts are arranged in each stage of the multi-stage fluidized bed having this temperature distribution; the powder in each fluidized bed is fluidized and further contacted with the catalyst in the fluidized bed for methanation; then methane is synthesized by reaction; inorganic powder containing magnesium hydroxide and magnesium carbonate is used as the fluidizing medium.

2. The method according to claim 1, wherein, The catalyst for the methanation reaction is a Ni-based catalyst.

3. The method according to claim 2, wherein, Methane is synthesized by using flue gas containing carbon dioxide, which is a feedstock gas for the methanation reaction, as a source of carbon dioxide gas, together with hydrogen.

4. The method according to claim 1, wherein, To improve the reaction rate of the chemical heat storage agent powder mixed in the fluidized medium, fine powder with a high specific surface area is used; wherein, the multi-stage fluidized bed is provided in conjunction with a oscillating mechanism to fluidize and homogenize the powder.

5. The method according to claim 1, wherein, The temperature of the uppermost level is 140-200℃, and the temperature of the lowermost level is 400-500℃.

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