Method for geological storage of carbon dioxide in coal seams and bioconversion of methane therefrom

Abstract

The present application relates to the technical field of carbon dioxide capture, utilization, storage and biological conversion, and particularly relates to a method for carbon dioxide geological storage in coal seams and biological conversion of methane. The method comprises the following steps: S100: selecting a target storage coal seam suitable for conversion of methane according to geological conditions; S200: regulating environmental influencing factors of the target storage coal seam to make it suitable for conversion of methane from CO2; S300: deploying a CO2 injection device and a CH4 extraction device to inject CO2 into the target storage coal seam; S400: storing CO2 in the target storage coal seam; S500: planting methanogenic bacteria in the target storage coal seam through the CO2 injection device to biologically convert CO2 into methane; and S600: extracting methane after long-term conversion. The present application uses abandoned coal mine systems and exhausted coal seams in goaf to construct a physical trap environment for CO2 geological storage, which saves drilling and completion costs, ensures the safety of CO2 geological storage, and makes the exhausted coal seams have economic utilization value again.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon dioxide capture, utilization, sequestration and biological conversion, in particular to a method for carbon dioxide geological sequestration in coal seams and biological conversion of methane. BACKGROUND

[0002] In the aspect of environmental protection, with the rapid change of the earth's climate, research has found that the excessive emission of carbon dioxide is closely related to the world's major environmental problems, and has caused great threat to the world's environmental problems, so achieving energy saving and emission reduction and double carbon peak has become the current development trend. In the aspect of development, as a world coal production and coal consumption country, the overexploitation of coal in China has led to the exhaustion of coal resources in many coal-producing areas, forming underground cavities. The negative effects of underground cavities on buildings will affect the reuse of the overall mined-out area surface, and even more, the surface subsidence of the mined-out area will cause farmland destruction, soil erosion, affect economic development and threaten the safety of residents. The China Natural Gas Development Report (2022) shows that the domestic natural gas production in the first half of 2022 was 112 billion cubic meters, and the natural gas import volume was 741 billion cubic meters. Among them, 312 billion cubic meters of pipeline gas were imported; 428 billion cubic meters of LNG were imported. Promote the low-carbon transformation of the oil and gas industry, promote the integration development of natural gas and new energy; based on the advantages of the industry development, carry out research and demonstration of key technologies such as carbon dioxide capture, utilization and sequestration (CCUS). The problems and current situation of environmental protection, development, energy and policy have given birth to the fusion and innovation technology of carbon dioxide geological sequestration and biological conversion.

[0003] The technical bottleneck restricting the development of carbon dioxide geological storage and biological conversion is mainly in the selection of environmental coal seams and microbial conversion. In the selection of environmental coal seams, the intraspecific and interspecific relationships of different bacteria are a major focus of research. It is generally believed that methanogens and sulfate-reducing bacteria form a major symbiotic and competitive relationship in the environment, and temperature, bacterial immobilization, initial number ratio of bacterial population, pH, sulfate concentration, acetic acid concentration, inhibitor addition, coalification degree of coal seam, coal particle size, etc. in the external environment have important influence. In microbial conversion, functional microorganisms include methanogens and other bacteria in the environment. Methanogens are the main functional microorganisms for converting CO2 into CH4, including six orders of methanococcales, methanomicrobiales, and methanobacteriales, involving more than 10 families and more than 30 genera. Methanogens can convert H2 and CO2, CH3COOH or methyl compounds (such as CH3OH, CH3NH2, etc.) into CH4 under the catalytic action of biological enzymes. Among them, hydrogenotrophic methanomicrobiales and metabolically diverse methanococcales are in a dominant position. Research has found that the rate of carbon dioxide conversion to methane is limited by the process of methanation of intermediate products. The main functional substance in this process is coenzyme F430 in the methyl coenzyme M reductase, and the active substance is the transition metal Ni located in the center of coenzyme F430. Through the change of the valence state of the transition metal Ni, electron transfer is realized, and CH4 is produced by microbial reduction. This process involves the widely concerned breaking of the C-S bond in organic compounds catalyzed by transition metal complexes and the enzyme molecular modification research to improve the catalytic performance of transition metal complexes. Therefore, it is very urgent to develop a method for carbon dioxide geological storage in coal seams and biological conversion of methane. SUMMARY

[0004] To solve the above problems, the present application provides a method for carbon dioxide geological storage in coal seams and biological conversion of methane, which controls variables to obtain the contribution of temperature, bacterial immobilization, initial number ratio of bacterial population, pH, sulfate concentration, inhibitor addition, coalification degree of coal seam, and coal particle size to improving the conversion performance of methanogens, and plans a method for selecting environmental coal seams. According to the catalytic activity of transition metals, a transition metal with higher catalytic activity is selected to replace the Ni-based coenzyme F430 that catalyzes the methanation reaction, in order to modify the functional enzyme. At the same time, an external environment that is conducive to the survival, reproduction and activity of methanogens is provided, and the biological conversion of CO2 is further accelerated and increased in production.

[0005] The present application adopts the following technical scheme: a method for carbon dioxide geological storage in coal seams and biological conversion of methane, comprising:

[0006] S100: According to the geological conditions, select a coal seam suitable for converting methane for storage;

[0007] S200: Regulate the environmental impact factors of the target coal seam for sealing, which is suitable for CO2 converting methane;

[0008] S300: Deploy CO2 injection device and CH4 extraction device to inject CO2 into the target coal seam for sealing;

[0009] S400: Seal CO2 in the target coal seam for sealing;

[0010] S500: Methanogenic bacteria are planted in the target coal seam for sealing through the CO2 injection device for biological conversion into methane;

[0011] S600: After long-term conversion, methane extraction is carried out.

[0012] In the step S100, the selected target coal seam for sealing is a depleted coal seam.

[0013] In the step S200, the environmental impact factors include: acetic acid concentration, coal seam temperature, coal seam filler, methanogenic bacteria quantity, coal seam PH value, sulfate concentration and COD to sulfate concentration ratio, inhibitor, coalification degree of coal seam, coal particle size and coal H / C atomic ratio.

[0014] The Michaelis constant value of the acetic acid concentration in the depleted coal seam is about 0.7 mmol / L, and the depleted coal seam contains a large number of acetate-producing bacteria;

[0015] The temperature in the depleted coal seam is in the medium temperature range of 38℃;

[0016] The depleted coal seam is added with granular sludge filler suitable for microbial growth and reproduction;

[0017] The number of naturally surviving methanogenic bacteria in the depleted coal seam is significantly higher than that of sulfate-reducing bacteria;

[0018] The PH value of the depleted coal seam is in the neutral high range of 6.8-7.5;

[0019] The sulfate concentration in the depleted coal seam is controlled below 1000 mg / L, and the ratio of COD to sulfate concentration is best maintained above 10:1 and at least controlled above 5:1;

[0020] About 20 mmol / L of Na2MoO4 inhibitor is added to the depleted coal seam;

[0021] The coal type of the depleted coal seam is high metamorphic anthracite;

[0022] The residual coal particle size of the depleted coal seam is in the range of 10-40 μm, and the coal H / C atomic ratio is about 0.5%.

[0023] The step S300 includes:

[0024] S301: The CO2 injection device is connected to the target coal seam through the waste air intake shaft, and the CH4 extraction device is connected to the target coal seam through the waste air return shaft,

[0025] S302: The CH4 extraction device is used to extract free CH4 in the upper fracture zone of the target coal seam, the water drainage system in the target coal seam is used to drain water, and the water injection device is used to inject water into the upper part of the target coal seam.

[0026] S303: The CO2 injection device is used to inject CO2 into the target coal seam, the displacement effect of CO2 on CH4 in coal pores is utilized to release CH4 adsorbed between coal pores, and the CH4 extraction device is used to extract CH4, part of CO2 is adsorbed in coal pores, and CO2 adsorption storage is achieved.

[0027] In the step S303, a proper amount of H2 is mixed in the injected CO2 at a CH ratio of 1:3 or a CH ratio of 1:4 to provide sufficient reaction substrates for methanogens.

[0028] The step S400 includes closing all air shafts connected to the target coal seam to achieve physical closure, using the roadway in the target coal seam to realize interconnection between regions, and ensuring that CO2 can be injected into all regions.

[0029] In the step S500, the methanogens are modified before being planted in the target coal seam to improve the catalytic ability of functional enzymes in the methanation reaction.

[0030] The methanogens are modified by changing the active transition metal Ni in the coenzyme F430 of the methylenetetrahydromethanopterin reductase in the methanogens to transition metal Ru.

[0031] The catalytic activity of the enzyme can be improved and the synthesis rate of CH4 can be increased by replacing the transition metal of coenzyme F430 active center. Therefore, the Gaussian 16 program is used to reasonably determine the transition metal with high catalytic activity, and the quantitative calculation is verified by related functions and methods. Then, the methanogens with high activity are screened and enriched, and under the guidance of the quantitative calculation results, the active transition metal Ni in the coenzyme F430 of the methanogen methyl coenzyme M reductase is mutated into active transition metals Fe, Cu, Ru, Rh, Pd and Pt, respectively, to explore the efficiency of the enzyme containing different transition metals in catalyzing CO2 and other substrates to generate CH4. The results show that the Ru element has higher low-temperature catalytic activity and CH4 selectivity, but its durability is poor, and it can form Ru(Co)x compounds with CO at high temperature, which can sublimate and reduce the activity of the catalyst. In addition, the cost of noble metal is high, and it cannot be industrialized at present. Therefore, the patent proposes the optimal modification scheme by comprehensively considering the conversion efficiency, engineering practice and economic benefit, that is, Ru element doping is carried out on the Ni-based coenzyme F430.

[0032] Compared with the prior art, the present application has the following beneficial effects:

[0033] 1. The method for geological storage of carbon dioxide in exhausted coal seams provided by the present patent provides a higher acetic acid concentration for the environment of exhausted coal seams, uses a medium temperature close to 38 DEG C, supplements fillers, ensures that MPB has an absolute initial advantage, maintains a high alkaline pH value to control the concentration of sulfides, reduces the concentration of sulfate in the environment, increases the number of hydrogen-producing bacteria to maintain a high H / C atomic ratio of the coal seam, selectively adds metal salt inhibitors to maintain a high biological activity and number of MPB and improve the competitiveness of MPB in the bacterial community, thereby improving the biological conversion efficiency of CO2. In addition, the selection of vitrinite coal seams with low coalification degree and small coal particle size can provide a favorable external environment for improving the gas production and gas production rate of methanogens.

[0034] 2. The method provided by the present patent uses the abandoned coal mine system and the exhausted coal seam in the goaf to construct a physical trap environment for CO2 geological storage, which saves the drilling and completion cost, ensures the safety of CO2 geological storage, and makes the exhausted coal seam have economic utilization value again.

[0035] 3. The method provided by the present patent can effectively utilize the methane produced in the exhausted coal seam and avoid resource waste and the influence of methane gas leakage on the environment as much as possible.

[0036] 4. Because it is difficult to achieve complete production of coal in reality, a large amount of coal powder, coal slime and other forms of coal resources will be left in the exhausted coal seam after mining. These coal resources can be used as natural filler materials to provide attachment sites for microorganisms, and can be converted into CH4 by microorganisms under suitable conditions, thereby increasing the production of CH4 and avoiding waste of resources.

[0037] 5. The water accumulation injection method proposed in the method can inject the underground water drained during mining into the geological environment above the exhausted coal seam to increase the fluid pressure of the stratum above the target coal seam, thereby balancing the fluid pressure of the exhausted coal seam increased due to the injection of CO2, reducing the pressure difference between the upper and lower strata, and reducing the risk of rupture of the trap environment. In addition, the injection of the underground water drained during mining into the geological environment above the exhausted coal seam can reduce the leakage amount of CO2 by using the dissolution of water to CO2. Therefore, the method for geological storage of carbon dioxide in the exhausted coal seam of the patent can improve the safety of the structure, and avoid the problem of possible CO2 leakage, thereby improving the safety of CO2 storage.

[0038] 6. The method for improving the catalytic ability of functional enzymes in the methanation reaction proposed in the patent can modify the functional enzymes through software simulation. The initial energy barrier of the catalytic methanation reaction before and after modification characterizes the catalytic ability of the functional enzymes, reduces the cost investment of experiments, and more intuitively obtains relevant data by using calculation means, thereby guiding the modification of methanogenic bacteria by using biological technology, and effectively reducing the blindness of biological researchers in this aspect. At the same time, planting the modified methanogenic bacteria in the exhausted coal seam can greatly improve the speed and efficiency of the biological conversion of CO2 to CH4. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 Schematic diagram of the carbon dioxide coal seam storage method;

[0040] Figure 2 Flowchart of the carbon dioxide geological storage and biological conversion method;

[0041] Figure 3 Flowchart of the biological conversion acceleration and yield increase method;

[0042] In the figure: 1-methanogenic bacteria, 2-target storage coal seam, 3-fracture zone, 4-water injection layer, 5-abandoned air intake well, 6-CO2 injection device, 7-water injection device, 8-sieve (orifice plate), 9-abandoned air return well, 10-CH4 extraction device. DETAILED DESCRIPTION

[0043] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments; based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0044] The present application adopts the following technical solutions: a method for carbon dioxide geological storage in coal seams and biological conversion, comprising the following steps:

[0045] S100: According to the geological conditions, the selected target storage coal seam 2 is a depleted coal seam. The depth of the depleted coal seam is 1000m or more, which can ensure that the CO2 pressure injected into the depleted coal seam is in a high range, and other conditions are controlled to exist in a supercritical state; the goaf of the depleted coal seam has a good state of the reserved coal pillar and the pressure bearing device between the roof and the floor during mining, and the whole has a relatively stable structure; the depleted coal seam should be in a high-gas area, and the coal seam is rich in various types of methanogenic bacteria.

[0046] S200: Regulate the environmental impact factors of the target storage coal seam 2, which is suitable for CO2 conversion of methane. In the depleted coal seam, the sulfate-reducing bacteria are the main competitive species of the methanogenic bacteria 1, and by regulating the external environmental factors, the methanogenic bacteria 1 has a competitive advantage. (Hereinafter, the methanogenic bacteria is referred to as MPB; the sulfate-reducing bacteria is referred to as SRB).

[0047] In terms of acetic acid concentration, when the acetic acid concentration is low, SRB has absolute advantage; but when the acetic acid concentration is high, MPB is the dominant species. At the same time, since 68% of CH4 in nature comes from acetic acid nutrition type methanogenic bacteria 1, the number of acetic acid bacteria in the environment can be increased to convert CO2 into acetic acid and then into CH4.

[0048] In terms of temperature, the use of a medium temperature range close to 38℃ makes MPB always at the most suitable growth temperature. The use of electron flow consumption to characterize the activity of the two, it is found that the electron flow consumption at 38℃ is the smallest, which indicates that the competitiveness of SRB is the worst, that is, 38℃ is more conducive to the growth and reproduction of MPB.

[0049] In terms of immobilization, the design of the reactor will directly affect the growth of SRB and MPB. Since MPB has stronger adhesion ability to the filler than SRB, it will cause the competitive disadvantage of SRB. Studies have shown that compared with the bacterial flora in water, the number of MPB attached to the biofilm increases by more than 200 times, while the number of SRB increases by only 30 times, so it is helpful to the stable progress of the methanogenic reaction to add suitable microbial growth fillers on the basis of existing coal powder and coal slime.

[0050] In terms of initial quantity, if one of the two bacterial populations has a significant advantage in the initial system, the competitive advantage will be difficult to eliminate, so the number of MPB should be ensured to have an absolute advantage before CO2 storage, and be planted if necessary.

[0051] In terms of PH, the PH value affects the state of sulfide in water, and indirectly affects the activity of SRB through the toxicity of sulfide in different states. When the PH value is neutral and slightly high, the competition of MPB is dominant, and ensuring a high PH value is beneficial to enhancing the competitiveness of MPB.

[0052] In terms of sulfate concentration, the growth of MPB is better than that of SRB under lower sulfate concentration, so the coal seam with lower sulfate concentration should be selected when selecting depleted coal seams.

[0053] In terms of inhibitors, the use of Na2MoO4 can inhibit the growth and reproduction of SRB while activating MPB, thereby improving the competitive ability of MPB to substrate in anaerobic treatment.

[0054] In terms of coalification degree and particle size of coal seam, biological gas production simulation experiments of coal samples with different coal ranks and different particle sizes show that within a certain range, biological gas production and gas production rate are inversely proportional to coalification degree and particle size; in addition, coal samples with high H / C atomic ratio have more potential for biological methane production, so a proper amount of H2 should be mixed when injecting CO2 to provide sufficient substrate for biological conversion.

[0055] S300: deploy CO2 injection device 6 and CH4 extraction device 10 to inject CO2 into the target storage coal seam 2.

[0056] The specific process is as follows:

[0057] S301: Before CO2 storage, use CH4 extraction device 10 to extract free CH4 in the upper fracture zone 3 of the depleted coal seam; use the water drainage system to drain the accumulated water in the depleted coal seam and reinject it into the upper part of the target coal seam. Make full use of abandoned wellways and devices in the depleted coal seam; deploy CO2 storage device, CO2 injection device 6, CH4 extraction device 10, and CH4 collection device on the ground; deploy filling device and supporting device at the lower part of the ground; the CO2 storage device is connected with the extraction device; the CO2 injection device 6 is connected with the depleted coal seam through the abandoned air inlet well 5; the CH4 extraction device 10 is connected with the depleted coal seam through the abandoned air return well 9.

[0058] S302: Because a large amount of free methane from the depleted coal seam will be gathered in the fracture zone 3, the initial extraction of methane can be carried out by using the CH4 extraction device 10, and the CH4 extraction device 10 arranged in the fracture zone 3 should be provided with a screen (orifice plate) 8 with a suitable aperture. In addition, the accumulated water is extracted by using the drainage system of the depleted coal seam, and the extracted accumulated water is injected back into the geological environment above the depleted coal seam through a pipeline to increase the formation fluid pressure above the depleted coal seam, while reducing the leakage amount of CO2 by increasing the dissolution of leaked CO2 in water.

[0059] S303: CO2 is injected into the depleted coal seam by using the abandoned air inlet well of the depleted coal seam, and an appropriate amount of H2 is mixed in the injected CO2 to provide sufficient reaction substrate for methanogenic bacteria, while all air inlets are closed to achieve physical closure during geological storage of CO2, and the roadway in the depleted coal seam is used to realize interconnection between regions to ensure that CO2 can be injected into all regions to achieve full utilization of space. Under the action of microorganisms for a long time, most of the CO2 undergoes methanation reaction to produce CH4, and a small part of the CO2 is adsorbed into the coal seam and finally permanently fixed in the form of carbonate.

[0060] S400: All air inlets connected to the target storage coal seam are closed to achieve physical closure, and the roadway in the target storage coal seam is used to realize interconnection between regions to ensure that CO2 can be injected into all regions.

[0061] S500: Methanogenic bacteria are planted in the target storage coal seam by using a CO2 injection device to carry out biological conversion to produce methane.

[0062] The methanogenic bacteria are modified before being planted in the target storage coal seam to improve the catalytic ability of functional enzymes in the methanation reaction.

[0063] In the process of biological conversion of CO2 to produce CH4, the methyl coenzyme M reductase in the methanogenic bacteria catalyzes the last step in the CH4 synthesis pathway, which is also the rate-limiting step in the CH4 synthesis pathway. The active center of MCR has a necessary coenzyme F430, and the nickel in the active center of the coenzyme F430 of the active methyl coenzyme M reductase is in a +1 valence state, which realizes electron transfer through the change of the valence state of the Ni atom, and then generates CH4 under the reduction of microorganisms. This process involves the widely concerned breaking of C-S bond in organic matter catalyzed by transition metal complexes, as well as the modification of enzyme molecules to affect the catalytic performance of transition metal complexes.

[0064] The catalytic activity of the enzyme can be improved by replacing the transition metal in the coenzyme F430 active center, thereby increasing the synthesis rate of CH4. The Gaussian16 program is used to reasonably determine the transition metal with high catalytic activity, and the related functions and methods are used for quantitative calculation. First, the Gaussian16 program is used to construct the structural model of the functional enzyme and the reactant. The model includes shearing and simplifying the coenzyme F430 and the reactant, and fixing the surrounding protein environment, so as to reduce the workload and time of subsequent quantitative calculation. It is worth noting that the solvent environment used in this model is not a conventional organic or inorganic solution, but a CPCM implicit solvent model with a dielectric constant of 4.

[0065] Secondly, the Gaussian16 program is used to search for the structure optimization of the reactant, the transition state of the reaction, the frequency calculation and the intrinsic reaction coordinate test, the structure optimization of the initial state and the final state of the reaction, the energy correction of the system and the calculation of the reaction barrier under the non-restrictive DFT theory level using B3LYP exchange and correlation function and LANL2TZ basis set of transition metal and 6-311G* basis set and 6-31G* basis set of non-metallic atoms. Among them, we use the reaction barrier obtained by quantitative calculation to characterize the catalytic performance of coenzyme F430. Within a certain range, the lower the reaction barrier, the better the catalytic performance of coenzyme F430, and vice versa, the higher the reaction barrier, the worse the catalytic performance of coenzyme F430.

[0066] Furthermore, from the four aspects of structure, dynamics, energy and AIM theory, the quantitative data are analyzed by measuring bond length and bond angle, drawing Arrhenius equation graph and Eyring equation graph, reaction barrier graph and topological graph of electron density scalar field, respectively.

[0067] Compared with Ni-based methanation catalyst, Ru-based methane catalyst has higher low-temperature catalytic activity and CH4 selectivity. Secondly, Ru can be used as an electronic assistant to adjust the redox performance of the catalyst, so Ru is used as an ideal replacement transition metal for exploration. In addition, according to the catalytic activity order of different transition metal catalysts in the CO2 reduction reaction, seven transition metals of Ru, Rh, Fe, Cu, Co, Pt and Pd are used in the exploration process.

[0068] The active methanogen enrichment culture was screened out, and under the guidance of quantitative calculation results, the active transition metal Ni in coenzyme F430 of methanogen methyl coenzyme M reductase was respectively mutated to active transition metals Ru, Rh, Fe, Cu, Co, Pt, Pd by using genetic engineering technology and induced gene mutation technology, and the efficiency of enzyme catalysis of CO2 and other substrates to generate CH4 was explored. The results show that Ru element has higher low-temperature catalytic activity and CH4 selectivity, but its durability is poor, it can form Ru(Co)x compound with CO, which is easy to sublimate at high temperature, causing the reduction of catalyst activity, and the cost of noble metal is higher, which cannot be industrialized at present. Therefore, the optimal modification scheme is proposed by comprehensively considering the conversion efficiency, engineering practice and economic benefit, that is, Ru element is doped in Ni-based coenzyme F430. Considering the long-term and systematic nature of CH4 biological conversion in engineering practice, the real environment has not been practiced yet, but the test results show that the modified methanogen has higher conversion efficiency and stability than the natural methanogen, the environmental adaptability is improved, the ability to resist adverse external factors is enhanced, and the interspecific competition ability is significantly improved.

[0069] S600: After long-term conversion, methane extraction is carried out.

Claims

1. A method of geological sequestration of carbon dioxide in coal seams and its bioconversion to methane, characterized by, The method comprises the following steps: S100: selecting a target coal seam suitable for converting methane according to geological conditions; S200: regulating environmental factors of the target coal seam suitable for converting methane, the environmental factors comprising: the target coal seam has a Michaelis constant value of acetic acid concentration of 0.7 mmol / L and contains a large number of acetic acid-producing bacteria; the temperature in the target coal seam is in a medium temperature range of 38 DEG C; granular sludge fillers suitable for the growth and reproduction of microorganisms are added to the target coal seam; methanogens in the target coal seam have a significant advantage over sulfate-reducing bacteria in quantity; the pH value of the target coal seam is in a neutral high range of 6.8-7.5; the sulfate concentration in the target coal seam is controlled below 1000 mg / L, and the ratio of COD to sulfate concentration is maintained above 10:1; 20 mmol / L of Na2MoO4 inhibitor is added to the target coal seam; the target coal seam is high-rank anthracite; the residual coal particle size in the target coal seam is in a range of 10-40 um, and the atomic ratio of H / C of the coal is 0.5%; S300: deploying a CO2 injection device (6) and a CH4 extraction device (10) to inject CO2 into the target coal seam; S400: sealing CO2 in the target coal seam; S500: planting methanogens in the target coal seam through the CO2 injection device (6) to biologically convert CO2 into methane; the methanogens are modified before being planted in the target coal seam to improve the catalytic ability of functional enzymes in the methanation reaction; the methanogens are modified by using genetic engineering technology and induced gene mutation technology to mutate the active transition metal Ni in the coenzyme F430 of the methylenetetrahydromethanopterin reductase in the methanogens into the active transition metal Ru; S600: after long-term conversion, methane is extracted.

2. The method of carbon dioxide coal seam geological storage and its biological conversion methane according to claim 1, characterized in that, In the step S100, the target coal seam is a depleted coal seam.

3. The method of carbon dioxide coal seam geological storage and its biological conversion methane according to claim 1, characterized in that, The step S300 comprises: S301: the CO2 injection device (6) is communicated with the target coal seam (2) through the abandoned air inlet well (5), and the CH4 extraction device (10) is communicated with the target coal seam (2) through the abandoned air return well (9); S302: using the CH4 extraction device (10) to extract free CH4 in the upper fractured zone (3) of the target coal seam (2), using the water drainage system in the target coal seam to drain water and using the water return device (7) to return the water to the upper part (4) of the target coal seam; S303: using the CO2 injection device (6) to inject CO2 into the target coal seam (2), using the displacement effect of CO2 on CH4 in the coal pores to desorb CH4 adsorbed between the coal pores, and using the CH4 extraction device (10) to extract the CH4, so that part of the CO2 is adsorbed between the coal pores to achieve the adsorption sealing of CO2.

4. The method of carbon dioxide coal seam geological storage and its biological conversion methane according to claim 3, characterized in that, In the step S303, a proper amount of H2 is mixed in the injected CO2 at a CH ratio of 1:3 or 1:4 to provide sufficient reaction substrates for the methanogens.

5. The method of carbon dioxide coal seam geological sequestration and its biological conversion methane of claim 1, wherein, The step S400 comprises: closing all air shafts connected with the target sealed coal seam, realizing interconnection of each region by using the roadway in the target sealed coal seam, and ensuring that CO2 can be injected into all regions.

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