Process method for preparing natural gas by coupling CO2 catalytic hydrogenation

Abstract

The invention relates to a process method for preparing natural gas by coupling CO2 catalytic hydrogenation, which comprises the following steps: feeding purified CO2 gas and purified hydrogen into a pre-methanation unit for reaction to obtain pre-methane gas, and then feeding the pre-methane gas into a deep methanation unit for reaction to obtain deep methanation gas; the deep methane gas enters a separation tank after being subjected to multi-stage heat exchange cooling, and methane gas at the top of the separation tank enters a decarburization unit to obtain methane-rich gas; the purified CO2 gas is divided into n + m strands, and the n + m strands are respectively introduced into corresponding pre-methanation reactors in n stages of pre-methanation reactor assemblies and deep methanation reactors in m stages of deep methanation reaction assemblies; all the purified hydrogen is introduced into the first-stage pre-methanation reactor assembly or is divided into n strands to be respectively introduced into the corresponding n-stage pre-methanation reactor assemblies. The method is simple in process, easy to start and stop, mild in reaction condition, high in CO2 one-way conversion rate, low in energy consumption, low in equipment investment and operation cost, and suitable for large-scale production.

Description

Technical Field

[0001] The present invention relates to the field of carbon neutrality and carbon reduction technology, and specifically to a process for producing natural gas by coupling CO2 catalytic hydrogenation. Background Art

[0002] According to the International Energy Agency's forecast, as countries attach importance to carbon emission reduction technologies, it is expected that by 2040, the amount of electricity generated by renewable energy will account for more than 60% of the total electricity. However, green electricity mainly comes from solar energy and wind energy, and the output is unstable. It is difficult and costly to develop large-scale energy conversion technologies that can adjust seasonal output changes. Using "green electricity" to electrolyze water to produce hydrogen, and then using green hydrogen and CO2 methanation to generate methane is an effective technology for converting electrical energy into chemical energy. This method can realize the recycling of CO2 and the storage of green hydrogen. The generated methane can also be stored and transported using existing natural gas pipelines and infrastructure, and is expected to become a large-scale, long-term energy storage technology in the new energy era.

[0003] At present, my country's wind power, solar energy and other renewable energy sources are developing rapidly, but renewable energy generation is intermittent and volatile, which makes it difficult to integrate renewable electricity into the power grid on a large scale. At the same time, the hydrogen produced by water electrolysis is also volatile. Therefore, in order to achieve stable production of natural gas, this requires that the production equipment of natural gas must meet the situation of wide load fluctuations.

[0004] Because the methanation reaction is a highly exothermic reaction, under adiabatic conditions, the temperature rises by 60°C for every 1% mol of CO2 that is converted. In addition, various side reactions will occur at high temperatures, which also have a certain impact on the activity of the catalyst. Therefore, the reaction temperature must be controlled.

[0005] Patent CN201910541748.5 discloses a biogas preparation device and method that combines biogas decarbonization and purification technology with carbon dioxide hydrogenation and methanation. The present invention prepares biogas by hydrogenation and methanation of carbon dioxide in biogas while purifying the biogas in biogas. However, the produced hydrogen enters the gas mixer through the gas inlet of the gas mixer, and is fully mixed with the obtained carbon dioxide gas in the gas mixer and reacts after being heated.

[0006] Patent CN202322737119.4 discloses a production system for the Sabatier reaction of carbon dioxide hydrogenation and methanation, in which the carbon dioxide main pipe is connected to the shell of the primary methanation inlet tower heat exchanger, and a circulating gas compressor is provided in the process.

[0007] Patent CN202111407955.5 discloses a coupling system and method for producing methane from carbon dioxide and LNG from natural gas, which involves large-scale circulation of CO2.

[0008] Patent CN201610037828.3 discloses a method for producing natural gas by staged methanation of carbon dioxide. The reactants of the reaction control effect and the products of the previous reaction (CH4 and H2O) are used as temperature control measures to avoid the influence of adding water vapor to control the temperature on the equilibrium of the methanation reaction in the traditional methanation process. However, a traditional adiabatic methanation reactor is used. The volume content of CH4 in the product gas (dry basis) is 95%, the molar concentration of H2 is less than 2%, the molar concentration of CO2 is less than 2%, and the CO2 content is relatively high.

[0009] Although many studies have been conducted on various links of CO2 catalytic hydrogenation comprehensive utilization, the hydrogen preparation, CO2 capture, and CO2 hydrogenation to natural gas involved in natural gas production have been studied or improved separately, without systematic analysis and optimization based on the reaction characteristics of each process. Therefore, in order to realize the hydrogenation utilization of CO2 and hydrogen and green hydrogen in industrial exhaust gas, reduce carbon emissions, reduce the production cost and comprehensive energy consumption of natural gas, and increase the supply of natural gas, it is urgent to develop a process technology for coupling CO2 catalytic hydrogenation to natural gas. Summary of the invention

[0010] The purpose of the present invention is to solve the above technical problems and to provide a process for coupling CO2 catalytic hydrogenation to produce natural gas, which has a simple process, is easy to start and stop, has mild reaction conditions, has a high CO2 single-pass conversion rate, has low energy consumption, has low equipment investment and operating costs, and is suitable for large-scale production.

[0011] The technical solution includes sending the purified CO2 gas and the purified hydrogen into a pre-methanation unit for reaction to obtain pre-methane gas, and then sending them into a deep methanation unit for reaction to obtain deep methanation gas; the deep methane gas enters a separation tank after multi-stage heat exchange and cooling, and the methane gas at the top of the separation tank enters a decarbonization methanation tower to obtain methane-rich gas;

[0012] The pre-methanation unit comprises n-stage pre-methanation reactor assemblies connected in series, wherein the pre-methanation reactor assembly comprises a pre-methanation reactor, a steam superheater and a boiler water preheater connected in series; the deep methanation unit comprises m-stage deep methanation reaction assemblies connected in series, wherein the deep methanation reaction assembly comprises an inlet heat exchanger, a deep methanation reactor with a steam drum, an outlet heat exchanger and a separator connected in series;

[0013] The purified CO2 gas is divided into n+m streams, which are respectively introduced into the corresponding pre-methanation reactor in the n-stage pre-methanation reactor assembly and the deep methanation reactor in the m-stage deep methanation reaction assembly; the purified hydrogen is all introduced into the first-stage pre-methanation reactor assembly or is divided into n streams and respectively introduced into the corresponding n-stage pre-methanation reactor assembly;

[0014] The pre-methanation reactor is an adiabatic methanation reactor or a chilled methanation reactor.

[0015] The quenched methanation reactor comprises a shell, a main air inlet is arranged at the top of the shell, an air outlet is arranged at the bottom, 2 to 4 catalyst beds are arranged vertically in the shell, a distributor is arranged between two adjacent catalyst beds, and a secondary air inlet is arranged on the side wall of the shell above each distributor; the purified CO2 gas entering the quenched methanation reactor is divided into multiple streams again, which are respectively introduced into the main air inlet and the secondary air inlet, and the purified hydrogen and / or the reaction gas from the previous stage pre-methanation reactor assembly are introduced into the main air inlet.

[0016] When n is greater than or equal to 2, the volume of the purified CO2 gas introduced into the premethanation reactor in the next-stage premethanation reactor assembly is greater than or equal to the volume of the purified CO2 gas introduced into the premethanation reactor in the previous-stage premethanation reactor assembly.

[0017] When n is equal to 2, the volume ratio of the first purified CO2 gas to the second purified CO2 gas is 0.1:1 to 2:1.

[0018] When n is equal to 3, the volume fraction ratio of the first stream of purified CO2 gas, the second stream of purified CO2 gas, and the third stream of purified CO2 gas is 0.05:0.05:1.0 to 0.08:0.1:1.

[0019] The gas temperature at the outlet of each catalyst bed layer is controlled to be 450-600°C by adjusting the amount of purified CO2 gas entering each secondary air inlet of the quenched methanation reactor.

[0020] When the purified hydrogen is divided into n streams and enters each pre-methanation reactor assembly, when n is equal to 2, when the purified hydrogen is divided into 2 streams and enters the pre-methanation reactor assembly, the hydrogen-carbon molar ratio at the first-stage inlet is adjusted to 8:1-6:1, and the remainder enters the second-stage pre-methanation reactor; when n is equal to 3, when the purified hydrogen is divided into 3 streams and enters the pre-methanation reactor assembly, the hydrogen-carbon molar ratio at the first-stage inlet is adjusted to 8:1-6:1, the hydrogen-carbon molar ratio at the second-stage inlet is adjusted to 7:1-5:1, and the remainder enters the tertiary pre-methanation reactor.

[0021] The operating temperature of the pre-methanation reactor is 250-600°C, the operating pressure is 2-9.5 MPa, and the space velocity is 5000-20000 h -1 .

[0022] The operating temperature of the deep methanation reactor is 245-520°C, the operating pressure is 2-9.5 MPa, and the space velocity is 4000-18000 h -1 .

[0023] The deep methanation reactor is a shell-and-tube methanation reactor, which produces steam at a pressure level of 4.0 to 5.5 MPaG. The steam produced is superheated by the steam superheater at the outlet of the pre-methanation unit to obtain high-quality superheated steam and send it out of the boundary area, or directly produces medium-pressure saturated steam and sends it out of the boundary area.

[0024] The decarbonization unit is a dry decarbonization tower, a wet decarbonization tower, or a complete methanation decarbonization tower.

[0025] In view of the problems existing in the background technology, the inventor has made the following improvements:

[0026] 1) Use split feed: The CO2 gas is divided into n+m shares according to the pre-methanation reactor assembly and the deep methanation reactor, and the purified CO2 gas is fed into each pre-methanation reactor and the deep methanation reactor to adjust the gas ratio and carbon concentration at the inlet, so as to achieve the purpose of controllable bed temperature and high single-pass carbon conversion rate. The gas flow rate of each CO2 gas is controlled according to the outlet temperature and the thermodynamic equilibrium conversion rate to achieve reasonable distribution. It can also be flexibly applied to the working conditions of green hydrogen H2 flow fluctuations (scenarios with volatility of green hydrogen sources), and flexibly synthesize green methane without the risk of bed temperature fluctuations due to flow fluctuations.

[0027] 2) The pre-methanation reactor adopts a chilled methanation reactor with multiple catalyst beds arranged in sections. In addition to the main air inlet (for introducing hydrogen or the reaction gas of the previous stage), an auxiliary air inlet is also arranged above the distributor of each catalyst bed. The CO2 gas entering the pre-methanation reactor is also divided into shares and enters the main air inlet and the auxiliary air inlet respectively, which is used to adjust the outlet temperature of each catalyst bed layer to achieve the purpose of controllable bed temperature. At the same time, when applied to green hydrogen operating conditions fluctuating, the layered control method is adopted to facilitate the adjustment of the inlet gas composition, so as to achieve the purpose of one-way passage of the raw gas and controllable outlet temperature.

[0028] 3) The process of the present invention is simple, the equipment investment and operation cost are low, there is no CO2 gas circulation in the whole process, and there is no need to set up a circulating gas compressor. The comprehensive energy consumption is low, the CO2 single-pass conversion rate is high, and high-quality superheated steam can be produced. It is suitable for single-series devices with a scale of more than one billion cubic meters. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a process flow chart of the present invention.

[0030] Figure 2 Schematic diagram of the structure of the quenched methanation reactor.

[0031] Among them, ST101 filter, ST102 CO2 purification unit, ST201 purifier, C102 first booster, C101 second booster; E101 heat exchanger, R101 first-level pre-methanation reactor, E1011 first-level steam superheater, E1012 first-level boiler water preheater, R102 second-level pre-methanation reactor, E2021 second-level steam superheater, E1022 second-level boiler water preheater, E201 first-level import heat exchanger, R201 first-level deep methanation reactor, E2011 first-level outlet heat exchanger, E2012 first-level outlet heat exchanger, S201 first-level separator, E202 second-level import heat exchanger, R202 second-level deep methanation reactor, S202 second-level separator, E2021 second-level outlet heat exchanger, E2022 second-level outlet heat exchanger, S202 second-level separator, R301 decarbonization reactor, S301 separator.

[0032] Among them, there are a shell 1, a main air inlet 2, an auxiliary air inlet 3, an air outlet 4, a catalyst bed 5, and a distributor 6. DETAILED DESCRIPTION

[0033] The present invention is further explained below in conjunction with the accompanying drawings:

[0034] See also Figure 1 The hydrogen-containing gas is purified by the purifier ST201, and the purified hydrogen is pressurized by the second booster C101 and then heat-exchanged by the heat exchanger E101 to obtain purified hydrogen; the hydrogen-containing gas is derived from hydrogen produced by water decomposition, industrial hydrogen-rich gas, such as by-product hydrogen of a propane dehydrogenation unit;

[0035] The carbon-containing gas passes through the filter ST101 and the CO2 purification unit ST102 in sequence, and then is pressurized by the second booster C101 to obtain purified CO2 gas; the carbon-containing gas comes from air, biomass combustion gas, low-temperature methanol-washed exhaust gas or industrial carbon-containing exhaust gas.

[0036] The purified hydrogen is sent to the pre-methanation unit to react with the purified CO2 gas to obtain pre-methane gas, and the pre-methane gas is continuously sent to the deep methanation unit to further react with the purified CO2 gas to obtain deep methanation gas. The deep methanation gas enters the decarbonization unit (decarbonization reactor R301) after further cooling to obtain methane-rich gas, and then enters the S301 separator after condensation. The condensate at the bottom is sent out of the boundary area in the form of process condensate, and a methane-rich gas product is obtained at the top of the separator, which is sent out of the boundary area for the production of SNG / CNG, or produced after deep cooling. LNG.

[0037] The premethanation unit includes n-stage (n≥1) premethanation reactor assemblies connected in series, and the premethanation reactor assembly includes a premethanation reactor, a steam superheater and a boiler water preheater connected in series. The reaction gas exiting the premethanation reactor passes through the steam superheater in sequence to produce steam as a by-product, and then passes through the boiler water preheater for further heat exchange and cooling. In this embodiment, it includes two stages of premethanation reactor assemblies connected in series, and the first-stage premethanation reactor assemblies are respectively a first-stage premethanation reactor R101, a first-stage steam superheater E1011 and a first-stage boiler water preheater E1012, and the second-stage premethanation reactor assemblies are respectively a second-stage premethanation reactor R201, a second-stage steam superheater E2011 and a second-stage boiler water preheater E2012.

[0038] The deep methanation unit comprises m-stage (m≥1) deep methanation reaction components connected in series, and the deep methanation reaction components comprise an inlet heat exchanger connected in series, a deep methanation reactor with a steam drum, two outlet heat exchangers and a separator. After heat exchange in the inlet heat exchanger, the reactor is sent to the deep methanation reactor with a steam drum for further reaction. The reaction gas from the deep methanation reactor is first heat exchanged and cooled in two outlet heat exchangers connected in series and then sent to the separator for gas-liquid separation. In this embodiment, two-stage deep methanation reaction components are included. The first-stage deep methanation reaction component includes a first-stage inlet heat exchanger E201, a first-stage deep methanation reactor R201 with a first deep steam drum S201, two first-stage outlet heat exchangers E2011 and E2012 connected in series, and a first-stage separator S201; the second-stage deep methanation reaction group includes a second-stage inlet heat exchanger E202, a second-stage deep methanation reactor R202 with a first deep steam drum S201, two second-stage outlet heat exchangers E2021 and E2022 connected in series, and a second-stage separator S202.

[0039] When a multi-stage pre-methanation reactor assembly and a deep methanation reaction assembly are connected in series, the number of open stages can be flexibly adjusted according to changes in the upstream gas volume. If the gas volume is small, the number of stages of the pre-methanation reactor assembly and the deep methanation reaction assembly is reduced through valve control, otherwise it is increased.

[0040] In this embodiment, the purified CO2 gas is divided into m+n=4 streams, which correspond to the first pre-methanation reactor R101 and the second pre-methanation reactor R102 in the pre-methanation reactor assembly, and the first deep methanation reactor R201 and the second deep methanation reactor R202 in the deep methanation reaction assembly, while the purified hydrogen is divided into two streams and enters the first pre-methanation reactor R101 and the second pre-methanation reactor R102 respectively.

[0041] Among them, the total ratio of the purified hydrogen and purified CO2 gases introduced is controlled to be 3.95-4.15. When n is equal to 2, when the purified CO2 enters the pre-methanation reactor assembly in two streams, the volume ratio of the first purified CO2 gas entering the first-level pre-methanation reactor R101 and the second purified CO2 gas entering the second-level pre-methanation reactor R102 is 0.1:1-2:1.

[0042] When the purified hydrogen is divided into two streams and enters the pre-methanation reactor assembly, the hydrogen-carbon molar ratio at the inlet of the first pre-methanation reactor R101 is adjusted to 8:1-6:1, and the remainder enters the second pre-methanation reactor R102.

[0043] The operating temperature of the pre-methanation reactor is 250-600°C, the operating pressure is 2-9.5MPa, and the space velocity is 5000-20000h -1 The deep methanation reactor has an operating temperature of 245-520°C, an operating pressure of 2-9.5 MPa, and a space velocity of 4000-18000 h -1 .

[0044] The pre-methanation reactor may be an adiabatic methanation reactor or a chilled methanation reactor.

[0045] See also Figure 2 The chilled methanation reactor in this embodiment comprises a shell 1, a main air inlet 2 is provided at the top of the shell, an air outlet 4 is provided at the bottom, 2 to 4 catalyst beds 5 are vertically arranged in the shell 1, a distributor 6 is provided between two adjacent catalyst beds 5, and a secondary air inlet 3 is provided on the side wall of the shell above each distributor 6;

[0046] The purified CO2 gas entering the quenched methanation reactor is divided into multiple streams again, one of which is introduced into the main air inlet 2 together with the purified hydrogen (or the reaction gas from the previous stage), and the remaining streams are introduced into multiple auxiliary air inlets 3 as quenching gases to control the outlet temperature of each bed layer at 450-600°C, so as to achieve the purpose of controllable reaction temperature. The specific ratio of each purified CO2 gas is controlled according to the outlet gas temperature of the bed layer. When the outlet gas temperature of a certain bed layer is too high, the flow rate of the introduced CO2 gas is appropriately reduced.

[0047] The deep methanation reactor is a shell-and-tube methanation reactor, which can generate steam with a grade of 4.0-5.5MPaG through a steam drum. After the generated steam is superheated in a steam superheater, high-quality medium-pressure steam can be obtained and sent out of the boundary area.

[0048] There is no CO2 gas circulation in the entire process of the method in this embodiment, and there is no need to set up a circulating gas compressor. The CO2 single-pass conversion rate can reach more than 99.5%, which can adapt to H2 flow fluctuations (green hydrogen scenario) and flexibly synthesize green methane. Ultimately, the effective coupling of CO2 and hydrogen to produce natural gas reaction is achieved, and the produced natural gas meets the category of "GBT 33445-2023 Coal-to-Synthetic Natural Gas".

[0049] The hot spot temperature of the first-level deep methanation reactor R201 is about 514°C, and the hot spot is located near 1 / 3 of the bed inlet. The hot spot temperature of the second-level deep methanation reactor R202 is about 507°C, and the hot spot is located near 1 / 3 of the bed inlet. After continuous operation for 20,000 hours, the hot spot temperature of the first-level deep methanation reactor R201 is about 518°C, and the hot spot is located near 3 / 4 of the bed inlet. The hot spot temperature of the second-level deep methanation reactor R202 is about 517°C, and the hot spot is located near 3 / 4 of the bed inlet. The life of the methanation catalyst can reach more than 2 years.

[0050] In another embodiment: the pre-methanation unit includes n-stage (n=3) pre-methanation reactor assemblies connected in series, and the deep methanation unit includes m-stage (m=3) deep methanation reaction assemblies connected in series. When n is equal to 3, the volume fraction ratio of the first purified CO2 gas entering the first-stage pre-methanation reactor, the second purified CO2 gas entering the second-stage pre-methanation reactor, and the third purified CO2 gas entering the tertiary pre-methanation reactor is 0.05:0.05:1.0~0.08:0.1:1; when the purified hydrogen is divided into three streams and enters the pre-methanation reactor assembly, the hydrogen-carbon molar ratio at the inlet of the first-stage pre-methanation reactor is adjusted to 8:1~6:1, and the hydrogen-carbon molar ratio at the inlet of the second-stage pre-methanation reactor is adjusted to 7:1~5:1, and the rest enters the tertiary pre-methanation reactor.

[0051] There is no CO2 gas circulation in the entire process of the method in this embodiment, and there is no need to set up a circulating gas compressor. The CO2 single-pass conversion rate can reach more than 99.5%, which can adapt to H2 flow fluctuations (green hydrogen scenario) and flexibly synthesize green methane. Ultimately, the effective coupling of CO2 and hydrogen to produce natural gas reaction is achieved, and the produced natural gas meets the category of "GBT 33445-2023 Coal-to-Synthetic Natural Gas".

[0052] The hot spot temperature of the first-stage deep methanation reactor R201 is about 508°C, and the hot spot is located near the 1 / 4 of the bed inlet. The hot spot temperature of the second-stage deep methanation reactor R202 is about 503°C, and the hot spot is located near the 1 / 4 of the bed inlet. The hot spot temperature of the third-stage deep methanation reactor R203 is about 507°C, and the hot spot is located near the 1 / 4 of the bed inlet. After continuous operation for 25,000 hours, the hot spot temperature of the first-stage deep methanation reactor R201 is about 512°C, and the hot spot is located near the 3 / 4 of the bed inlet. The hot spot temperature of the second-stage deep methanation reactor R202 is about 513°C, and the hot spot is located near the 3 / 4 of the bed inlet. The hot spot temperature of the third-stage deep methanation reactor R203 is about 514°C, and the hot spot is located near the 3 / 4 of the bed inlet. The catalyst life of the methanation catalyst can reach more than 2 years.

Claims

1. A process for coupling CO2 catalytic hydrogenation to produce natural gas, comprising sending purified CO2 gas and purified hydrogen into a pre-methanation unit to react to obtain pre-methane gas, and then sending them into a deep methanation unit to react to obtain deep methanation gas; the deep methanation gas enters a separation tank after multi-stage heat exchange and cooling, and the methane gas at the top of the separation tank enters a decarbonization unit to obtain methane-rich gas; characterized in that The pre-methanation unit comprises n-stage pre-methanation reactor assemblies connected in series, wherein the pre-methanation reactor assembly comprises a pre-methanation reactor, a steam superheater and a boiler water preheater connected in series; the deep methanation unit comprises m-stage deep methanation reaction assemblies connected in series, wherein the deep methanation reaction assembly comprises an inlet heat exchanger, a deep methanation reactor with a steam drum, an outlet heat exchanger and a separator connected in series; The purified CO2 gas is divided into n+m streams, which are respectively introduced into the corresponding premethanation reactor in the n-stage premethanation reactor assembly and the deep methanation reactor in the m-stage deep methanation reaction assembly; the purified hydrogen is all introduced into the first-stage premethanation reactor assembly or is divided into n streams and respectively introduced into the corresponding n-stage premethanation reactor assembly.

2. The process for producing natural gas by coupled CO2 catalytic hydrogenation as claimed in claim 1, characterized in that: The pre-methanation reactor is an adiabatic methanation reactor or a chilled methanation reactor.

3. The process for producing natural gas by coupled CO2 catalytic hydrogenation as claimed in claim 2, characterized in that: The quenched methanation reactor comprises a shell, a main air inlet is arranged at the top of the shell, an air outlet is arranged at the bottom, 2 to 4 catalyst beds are arranged vertically in the shell, a distributor is arranged between two adjacent catalyst beds, and a secondary air inlet is arranged on the side wall of the shell above each distributor; the purified CO2 gas entering the quenched methanation reactor is divided into multiple streams again, which are respectively introduced into the main air inlet and the secondary air inlet, and the purified hydrogen and / or the reaction gas from the previous stage pre-methanation reactor assembly are introduced into the main air inlet.

4. The process for producing natural gas by coupled CO2 catalytic hydrogenation according to any one of claims 1 to 3, characterized in that: When n is greater than or equal to 2, the volume of the purified CO2 gas introduced into the premethanation reactor in the next-stage premethanation reactor assembly is greater than or equal to the volume of the purified CO2 gas introduced into the premethanation reactor in the previous-stage premethanation reactor assembly.

5. The process for producing natural gas by coupled CO2 catalytic hydrogenation as claimed in claim 4, characterized in that: When n is equal to 2, the volume ratio of the first purified CO2 gas to the second purified CO2 gas is 0.1:1 to 0.9:

1.

6. The process for producing natural gas by coupled CO2 catalytic hydrogenation as claimed in claim 4, characterized in that: When n is equal to 3, the volume fraction ratio of the first stream of purified CO2 gas, the second stream of purified CO2 gas, and the third stream of purified CO2 gas is 0.05:0.05:1.0 to 0.3:0.6:

1.

7. The process for producing natural gas by coupled CO2 catalytic hydrogenation as claimed in claim 3, characterized in that: The gas temperature at the outlet of each catalyst bed layer is controlled to be 450-600°C by adjusting the amount of purified CO2 gas entering each secondary air inlet of the quenched methanation reactor.

8. The process for producing natural gas by coupled CO2 catalytic hydrogenation according to any one of claims 1 to 3, characterized in that: The purified hydrogen is divided into n streams and enters each pre-methanation reactor assembly. When n is 2, when the purified hydrogen is divided into 2 streams and enters the pre-methanation reactor assembly, the hydrogen-carbon molar ratio at the first-stage inlet is adjusted to 8:1-6:1, and the remainder enters the second-stage pre-methanation reactor; when n is 3, when the purified hydrogen is divided into 3 streams and enters the pre-methanation reactor assembly, the hydrogen-carbon molar ratio at the first-stage inlet is adjusted to 8:1-6:1, the hydrogen-carbon molar ratio at the second-stage inlet is adjusted to 7:1-5:1, and the remainder enters the tertiary pre-methanation reactor.

9. The process for producing natural gas by coupled CO2 catalytic hydrogenation according to any one of claims 1 to 3, characterized in that: The operating temperature of the pre-methanation reactor is 250-600°C, the operating pressure is 2-9.5MPa, and the space velocity is 5000-20000h -1 .

10. The process for producing natural gas by coupled CO2 catalytic hydrogenation according to any one of claims 1 to 3, characterized in that: The operating temperature of the deep methanation reactor is 230-520°C, the operating pressure is 2-9.5 MPa, and the space velocity is 4000-18000 h -1 .

11. The process for producing natural gas by coupled CO2 catalytic hydrogenation as claimed in claim 10, characterized in that: The deep methanation reactor is a shell-and-tube methanation reactor, which produces steam at a pressure level of 4.0 to 5.5 MPaG. The steam produced is superheated by the steam superheater at the outlet of the pre-methanation unit to obtain high-quality superheated steam and send it out of the boundary area, or directly produces medium-pressure saturated steam and sends it out of the boundary area.

12. The process for producing natural gas by coupled CO2 catalytic hydrogenation according to claim 1, characterized in that: The decarbonization unit is a dry decarbonization tower, a wet decarbonization tower, or a complete methanation decarbonization tower.

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