A system and method for co-production of graphene and natural gas

Abstract

The application provides a system and method for co-production of graphene and natural gas, which comprises a biomass gasification device, a purification and separation device, a methane cracking device and a methanation device. The devices are connected with each other, so that internal energy cascade utilization and material directional conversion can be realized. The first mixed gas separated by the purification and separation device is used for hydrogen production by methane cracking and co-production of high value-added carbon material graphene, and the second mixed gas is used for methanation reaction with cracking gas to produce natural gas. The results show that the purity of the target product graphene is 95%-98%, and the purity of the target product methane is 94%-98%. The system can realize value-added utilization and directional conversion of carbon resources. Compared with the prior art, by using the system, through multi-stage waste heat recovery and energy cascade utilization, the comprehensive thermal efficiency of the system can be further improved to 82%-88%.

Description

Technical Field

[0001] This application relates to the field of energy utilization technology, and in particular to a system and method for co-producing graphene and natural gas. Background Technology

[0002] Natural gas, as a clean and efficient fossil energy source, has wide applications in residential gas supply, industrial fuel, and power generation. With the deepening of my country's "dual carbon" goals, the technological route for producing biomass-synthetic natural gas from renewable biomass resources has received increasing attention.

[0003] In existing biomass gasification and co-production natural gas systems, the hydrogen-to-carbon ratio (H2 / CO molar ratio) of the syngas produced by biomass gasification is typically only 1.0–2.0, far lower than the theoretical hydrogen-to-carbon ratio (approximately 3.0–4.0) required for methanation. To meet the feed requirements for methanation, existing technologies generally employ two hydrogen replenishment schemes: one is hydrogen replenishment via water-gas shift reaction, and the other is hydrogen replenishment through water electrolysis.

[0004] However, the utilization pathways of CH4 and CO2 in existing systems are relatively simple, and the economic value of carbon resource conversion products is low. Summary of the Invention

[0005] This application provides a system and method for co-producing graphene and natural gas to solve the technical problems of the relatively simple utilization pathways of CH4 and CO2 and the low economic value of carbon resource conversion products in existing systems.

[0006] The specific details of the invention are as follows: In a first aspect, this application provides a system for co-producing graphene and natural gas, the system comprising: A biomass gasification device is used to cause biomass raw materials to undergo a gasification reaction under the action of water vapor and oxygen, generating gaseous products containing carbon monoxide, methane, hydrogen and carbon dioxide. A purification and separation device, connected to the biomass gasification device, is used to receive the gaseous products and perform purification and separation treatment to obtain a first mixed gas rich in methane and carbon dioxide, and a second mixed gas rich in hydrogen and carbon monoxide. A methane cracking unit, connected to the purification and separation unit, is used to receive the first mixed gas and carry out a catalytic cracking reaction to obtain the target product graphene and cracked gas containing hydrogen and carbon monoxide. The methanation unit is connected to the purification and separation unit and the methane cracking unit. It is used to receive the second mixed gas and the cracked gas, and to carry out the methanation reaction to generate the target product methane. The heat released by the methanation reaction is used to generate water vapor through heat exchange. The water vapor is used as a gasification agent and fed back to the biomass gasification unit to supply the biomass raw materials for gasification reaction.

[0007] Optionally, the system further includes: The first heat exchange device is connected to the biomass gasification device and the methane cracking device, and is used to receive the cracked gas and the biomass raw material, and perform a first-stage heat exchange treatment to preheat the biomass raw material. The second heat exchange device is connected to the biomass gasification device, the first heat exchange device, and the methanation device. It is used to receive the pyrolysis gas discharged from the first heat exchange device and perform secondary heat exchange treatment with oxygen. The oxygen after heat exchange is used as a gasification agent and is introduced into the biomass gasification device to participate in the reaction. The pyrolysis gas after heat exchange is used as a reaction feedstock and is introduced into the methanation device to participate in the reaction.

[0008] Optionally, the system further includes: The third heat exchange device is connected to the biomass gasification device, the methane cracking device, the purification and separation device, and the methanation device. It is used to receive the gaseous products, the first mixed gas, and part of the methane discharged from the methanation device, and to perform a first-stage heat exchange treatment. The first mixed gas and part of the methane after heat exchange and heating are fed into the methane cracking device as reaction raw materials for catalytic cracking reaction. The fourth heat exchange device is connected to the third heat exchange device, the methanation device and the purification and separation device. It is used to receive the gaseous products discharged from the third heat exchange device and the second mixed gas, and to perform secondary heat exchange treatment. The second mixed gas after heat exchange and heating is used as a reaction raw material and fed into the methanation device for methanation reaction.

[0009] Optionally, the methanation unit is equipped with a steam generation unit to recover the heat released during the reaction of the methanation unit, convert the demineralized water into saturated steam, and use it as a gasifying agent to be fed back to the biomass gasification unit for the biomass feedstock to undergo a gasification reaction.

[0010] Optionally, the purification and separation device includes: a desulfurization unit, a dechlorination unit, and a pressure swing adsorption unit; The desulfurization unit is connected to the biomass gasification device and is used to receive the gaseous products and perform desulfurization treatment. The dechlorination unit is connected to the desulfurization unit and the pressure swing adsorption unit, and is used to receive the gaseous products discharged from the desulfurization unit and perform dechlorination treatment, and send the dechlorinated gaseous products into the pressure swing adsorption unit. The pressure swing adsorption unit is connected to the methanation device and the methane cracking device, and is used to receive the gaseous products discharged from the dechlorination unit and perform separation processing to obtain the first mixed gas and the second mixed gas; the first mixed gas is used as a reaction feedstock and fed into the methane cracking device for catalytic cracking reaction; the second mixed gas is used as a reaction feedstock and fed into the methanation device for methanation reaction.

[0011] Secondly, this application provides a method for co-producing graphene and natural gas, the method being applicable to the aforementioned system, the method comprising the following steps: Biomass feedstock, water vapor, and oxygen are gasified in a biomass gasification device to produce gaseous products containing carbon monoxide, methane, hydrogen, and carbon dioxide. The gaseous product is purified and separated in a purification and separation device to obtain a first mixed gas rich in methane and carbon dioxide, and a second mixed gas rich in hydrogen and carbon monoxide. The first mixed gas is subjected to catalytic cracking reaction in a methane cracking unit to obtain the target product graphene and cracked gas containing hydrogen and carbon monoxide. The second mixed gas and the pyrolysis gas are subjected to a methanation reaction in a methanation device to generate the target product methane. The heat released by the methanation reaction is used to generate water vapor through heat exchange. The water vapor is fed back to the biomass gasification device as a gasifying agent to supply the biomass feedstock for gasification reaction.

[0012] Optionally, a molten medium is used in the catalytic cracking reaction, and the molten medium is selected from a molten alloy, wherein the molten alloy is Bi. 43 In 37 Sn 20 Doped with metallic nickel and metallic copper; The amount of nickel doped is 0.4wt% to 0.6wt% and the amount of copper doped is 0.1wt% to 0.4wt% based on the mass of the molten alloy.

[0013] Optionally, the method further includes: The biomass feedstock and the pyrolysis gas are subjected to primary heat exchange treatment in a first heat exchange device to obtain preheated biomass feedstock and primary pyrolysis gas. The oxygen and the primary pyrolysis gas are subjected to secondary heat exchange treatment in a second heat exchange device to obtain preheated oxygen and secondary pyrolysis gas. The preheated biomass raw material and preheated oxygen are used as reaction raw materials in the biomass gasification device for gasification reaction. The secondary cracked gas is used as a reaction feedstock in the methanation unit for methanation reaction.

[0014] Optionally, the method further includes: The gaseous product, the first mixed gas, and a portion of the methane discharged from the methanation unit are subjected to a first-stage heat exchange process in a third heat exchanger to obtain a first-stage gaseous product, preheated methane, and a preheated first mixed gas. The primary gaseous product and the second mixed gas are subjected to secondary heat exchange treatment in the fourth heat exchange device to obtain the secondary gaseous product and the preheated second mixed gas. The preheated first mixed gas and the preheated portion of methane are used as reactants in the methane cracking unit for catalytic cracking reaction. The preheated second mixed gas is used as a reaction raw material in the methanation device for methanation reaction.

[0015] Optionally, at least one of the following conditions must be met: (1) The temperature of the gasification reaction is 800℃~900℃ and the pressure is 0.2MPa~1.2MPa; (2) The temperature of the catalytic cracking reaction is 850℃~950℃, and the pressure is 0.10MPa~0.15MPa; (3) The temperature of the methanation reaction is 300℃~350℃; (4) Based on the volume of the gaseous products, the gaseous products include 18% to 25% carbon monoxide, 25% to 33% hydrogen, 15% to 24% carbon dioxide, and 8% to 18% methane; (5) The mass ratio of water vapor to biomass raw material is 0.5 to 1.5, and the mass of oxygen is 0.15 to 0.25 times the theoretical mass of oxygen required for complete combustion of biomass raw material; (6) By volume ratio, the first mixed gas comprises 50% to 65% carbon dioxide, 30% to 45% methane, 3% to 10% carbon monoxide, and less than 4% hydrogen. (7) By volume ratio, the second mixed gas comprises 45% to 55% hydrogen, 30% to 40% carbon monoxide, 3% to 6% carbon dioxide, and 1% to 3% methane; (8) The purity of the target product graphene is 95%~98%; the purity of the target product methane is 94%~98%.

[0016] Compared with the prior art, this application has the following advantages: The system for co-producing graphene and natural gas provided in this application connects a biomass gasification unit and a purification and separation unit to separate the gaseous products into a first mixed gas rich in methane and carbon dioxide, and a second mixed gas rich in hydrogen and carbon monoxide. These are then sent to a methane cracking unit and a methanation unit, respectively. The first mixed gas is used for methane cracking to produce hydrogen and co-producing high-value-added carbon material graphene. The second mixed gas is mixed with the cracked gas and then subjected to a methanation reaction, achieving component-based utilization according to quality. Simultaneously, a portion of the methane generated by the system itself is used for catalytic cracking to produce hydrogen, providing the necessary hydrogen for the methanation unit, eliminating the need for external water electrolysis or water-gas shift reaction. Furthermore, by feeding a first mixed gas into the methane cracking unit, both methane catalytic cracking and carbon gasification reactions occur simultaneously. The carbon dioxide in the first mixed gas preferentially reacts with the amorphous carbon produced from methane cracking to convert it into CO, thereby removing the amorphous carbon and obtaining high-quality graphene. The target product, graphene, has a purity of 95%–98%, and the target product, methane, has a purity of 94%–98%, achieving value-added utilization and targeted conversion of carbon resources. Compared with existing technologies, the system using this application can further improve the overall thermal efficiency to 82%–88%. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A schematic diagram of the structure of a system for co-producing graphene and natural gas provided in an embodiment of this application is shown.

[0019] Figure label: 1-Biomass gasification unit, 11-Biomass feeding unit, 12-Biomass gasification reaction unit, 2-Methane cracking unit, 21-Gas-solid separation unit, 22-Graphene storage unit, 23-Methane cracking reaction unit, 3-Methanation unit, 31-Gas-liquid separation unit, 32-Methanation reaction unit, 4-First heat exchanger, 5-Second heat exchanger, 6-Third heat exchanger, 7-Fourth heat exchanger, 8-Purification and separation unit, 81-Desulfurization unit, 82-Dechlorination unit, 83-Pressure swing adsorption unit, 9-First connecting pipeline, 10-Second connecting pipeline, 13-Third connecting pipeline, 14-Fourth connecting pipeline, 15-Mixing device, 16-First fluid conveying device, 17-Second fluid conveying device, 18-Third fluid conveying device, 19-Fourth fluid conveying device. Detailed Implementation

[0020] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its application or use. Based on the embodiments of this application, any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the protection scope of this application. Furthermore, all other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this application.

[0021] Specific experimental steps or conditions are not specified in the embodiments; they can be performed according to the conventional experimental steps or conditions described in the prior art. Reagents and other instruments used, unless otherwise specified, are all commercially available conventional reagent products. Furthermore, the accompanying drawings are merely illustrative diagrams of the embodiments of this application and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore, repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0022] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of this application specification.

[0023] Furthermore, the technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0024] Biomass gasification coupled with methanation is a key technology that uses thermochemical methods to convert biomass such as agricultural and forestry waste, livestock and poultry manure, and industrial organic waste into syngas rich in CO, CO2, H2, and CH4, and then synthesizes alternative natural gas through methanation. This approach not only enables the efficient resource utilization of biomass but also significantly reduces infrastructure upgrade costs by leveraging existing natural gas pipeline networks and end-use facilities. It is widely recognized as one of the most promising biomass energy utilization technologies for industrialization.

[0025] To meet the requirements of methanation feed (the methanation reaction is CO + 3H2) CH4 + H2O, CO2 + 4H2 According to the requirements of CH4+2H2O, existing technologies generally adopt the following two hydrogen replenishment schemes: one is hydrogen replenishment by water-gas shift reaction, which uses water-gas shift reaction (CO+H2O→CO2+H2) to react some CO with water vapor to generate H2, thereby increasing the hydrogen-carbon ratio; the other is hydrogen replenishment by electrolysis of water, which uses wind and solar power generation units to electrolyze water to produce green hydrogen, providing an external hydrogen source for the methanation unit.

[0026] The existing biomass gasification and co-production natural gas systems still have the following technical problems and shortcomings in practical applications: First, when using water-gas shift reaction for hydrogen supplementation, a large amount of highly reactive CO components are consumed and converted into less reactive CO2, which not only reduces the quality of syngas but also consumes a large amount of steam, increasing system energy consumption and equipment investment. Second, the scheme relying on external water electrolysis for hydrogen supplementation is limited by factors such as uneven distribution of green electricity resources, high hydrogen production costs, and poor power supply stability, resulting in poor economic efficiency and difficulty in achieving large-scale and stable hydrogen supplementation supply, leading to a significantly higher end-user cost of biomass-synthesized natural gas than fossil natural gas. Third, the existing biomass gasification to methane system has low heat recovery efficiency and high energy consumption, resulting in a large amount of waste heat and affecting overall energy utilization efficiency. Fourth, the utilization pathways of CH4 and CO2 in the existing system are relatively simple, and the economic value of carbon resource conversion products is low. Although related technologies have incorporated methane cracking for hydrogen production, the CO2 methanation reaction requires a higher hydrogen-to-carbon ratio (4:1), 33% higher than CO hydrogenation (3:1), implying a larger hydrogen production load. Furthermore, this technology is primarily used in single-source carbon sequestration power generation scenarios with high-purity gas sources, making it difficult to integrate with the multi-component syngas from biomass gasification in the methane synthesis stage. Therefore, there is an urgent need to develop a process that eliminates the need for water-gas shift reactions, does not rely on external water electrolysis for hydrogen production, can adapt to diverse syngas components, precisely control the hydrogen-to-carbon ratio during the synthesis stage, and simultaneously improves system heat recovery efficiency and the economic value of carbon resource conversion.

[0027] To address the aforementioned problems, this application provides a system for the co-production of graphene and natural gas. Figure 1 A schematic diagram of the system for co-producing graphene and natural gas provided in an embodiment of this application is shown; as follows: Figure 1 As shown, the system includes: Biomass gasification device 1 is used to cause biomass raw materials to undergo a gasification reaction under the action of water vapor and oxygen, generating gaseous products containing carbon monoxide, methane, hydrogen and carbon dioxide. The purification and separation device 8 is connected to the biomass gasification device 1 and is used to receive gaseous products and perform purification and separation treatment to obtain a first mixed gas rich in methane and carbon dioxide, and a second mixed gas rich in hydrogen and carbon monoxide. The methane cracking unit 2 is connected to the purification and separation unit 8. It is used to receive the first mixed gas and carry out a catalytic cracking reaction to obtain the target product graphene and cracked gas containing hydrogen and carbon monoxide. The methanation unit 3 is connected to the purification and separation unit 8 and the methane cracking unit 2. It is used to receive the second mixed gas and the cracked gas, and to carry out the methanation reaction to generate the target product methane. The heat released by the methanation reaction is used to generate water vapor through heat exchange. The water vapor is used as a gasification agent and is fed back to the biomass gasification unit 1 to supply the biomass raw materials for gasification reaction.

[0028] It should be noted that the biomass gasification device 1 includes a biomass feeding unit 11 and a biomass gasification reaction unit 12, which are connected. The biomass feeding unit 11 can be a screw feeder, through which biomass raw materials are fed into the biomass gasification reaction unit 12 for gasification reaction. The biomass gasification reaction unit 12 can be a fluidized bed gasifier.

[0029] It should be noted that oxygen is introduced into the biomass gasification device 1 through the first connecting pipe 9.

[0030] It should be noted that the methane cracking device 2 includes a gas-solid separation unit 21, a graphene storage unit 22, and a methane cracking reaction unit 23. The methane cracking reaction unit 23 is connected to the gas-solid separation unit 21. The methane cracking reaction unit 23 is used to receive the first mixed gas and part of the methane discharged from the methanation device 3, and to carry out a catalytic cracking reaction. The gas-solid separation unit 21 receives the cracking products and performs gas-solid separation to obtain the target product graphene and cracked gas containing hydrogen and carbon monoxide. The gas-solid separation unit 21 is connected to the graphene storage unit 22 and the methanation device 3. The graphene storage unit 22 receives and stores the target product graphene, and the cracked gas is introduced into the methanation device 3 to carry out the methanation reaction.

[0031] It should also be noted that a heating device is provided around the methane cracking reaction unit 23. The heating device is used to heat the molten medium to catalyze the cracking reaction. For example, the heating device can be an electromagnetic induction coil.

[0032] It should be noted that the methanation unit 3 includes a gas-liquid separation unit 31 and a methanation reaction unit 32. The methanation reaction unit 32 is connected to the gas-liquid separation unit 31. The methanation reaction unit 32 is used to receive the second mixed gas and the cracked gas and carry out the methanation reaction. The gas-liquid separation unit 31 is used to receive the methanation reaction products and carry out gas-liquid separation to generate the target product methane. Among them, after gas-liquid separation, 30% to 60% is transported out and merged into the pipeline network or liquefied and stored as the target product natural gas, and 40% to 70% of the methane is returned to the methane cracking unit 2 through the second connecting pipeline 10 to replenish the raw materials for the catalytic cracking reaction and form a material cycle.

[0033] It should also be noted that the system further includes a mixing device 15, which is connected to the methanation device 3, the purification and separation device 8, and the methane cracking device 2. The mixing device 15 is used to receive the second mixed gas from the purification and separation device 8 and the cracked gas discharged from the methane cracking device 2 and mix them to form a third mixed gas (which, by gas volume, contains 70%~80% hydrogen, 10%~20% carbon monoxide, and 2%~5% methane). The hydrogen-to-carbon ratio in the mixing device 15 is adjusted to 3.0~3.4:1, and then the third mixed gas is finally fed into the methanation device 3 through the second fluid conveying device 17 for methanation reaction.

[0034] It should also be noted that the purification and separation device 8 performs purification and separation treatment to obtain a first mixed gas rich in methane and carbon dioxide, and a second mixed gas rich in hydrogen and carbon monoxide. The purification and separation device 8 is connected to the fourth fluid conveying device 19 and the third fluid conveying device 18 respectively. The first mixed gas is fed into the methane cracking device 2 through the fourth fluid conveying device 19 and the third connecting pipe 13, and the second mixed gas is fed into the methanation device 3 through the third fluid conveying device 18 and the fourth connecting pipe 14.

[0035] In this embodiment, the biomass gasification device 1 is connected to the purification and separation device 8 to separate the gaseous products into a first mixed gas rich in methane and carbon dioxide, and a second mixed gas rich in hydrogen and carbon monoxide. These are then sent to the methane cracking device 2 and the methanation device 3, respectively. The first mixed gas is used for methane cracking to produce hydrogen and co-produce high-value-added carbon material graphene. The second mixed gas is mixed with the cracked gas and then undergoes a methanation reaction, achieving component-based separation and utilization. Simultaneously, some of the methane generated by the system itself is used for catalytic cracking to produce hydrogen, providing the necessary hydrogen for the methanation device 3, eliminating the need for external water electrolysis or water-gas conversion reactions. Furthermore, by feeding a first mixed gas into the methane cracking unit 2, both methane catalytic cracking and carbon gasification reactions occur simultaneously in the unit. The carbon dioxide in the first mixed gas preferentially reacts with the amorphous carbon produced from methane cracking to convert it into CO, thereby removing the amorphous carbon and obtaining high-quality graphene. The target product, graphene, has a purity of 95%–98%, and the target product, methane, has a purity of 94%–98%, achieving value-added utilization and targeted conversion of carbon resources. Compared with existing technologies, the system using this application can further improve the overall thermal efficiency to 82%–88%.

[0036] Optionally, in some implementations, the system further includes: The first heat exchange device 4 is connected to the biomass gasification device 1 and the methane cracking device 2. It is used to receive cracked gas and biomass raw materials and perform primary heat exchange treatment to preheat the biomass raw materials. The second heat exchanger 5 is connected to the biomass gasification unit 1, the first heat exchanger 4 and the methanation unit 3. It is used to receive the cracked gas discharged from the first heat exchanger 4 and perform secondary heat exchange treatment with oxygen. The oxygen after heat exchange is used as a gasification agent and is fed into the biomass gasification unit 1 to participate in the reaction. The cracked gas after heat exchange is used as a reaction feedstock and is fed into the methanation unit 3 to participate in the reaction.

[0037] It should be noted that the first heat exchange device 4 can be a shell-and-tube heat exchanger, and the second heat exchange device 5 can be a counter-flow heat exchanger.

[0038] In this embodiment, by setting up a two-stage heat exchange device, the high-temperature cracked gas generated by the methane cracking unit 2 is used to sequentially preheat the biomass feedstock and the gasifying agent oxygen, thereby realizing the cascade recovery and utilization of the waste heat of the cracked gas. This not only reduces the energy input for biomass gasification and oxygen heating, but also eliminates the need for an external preheating heat source, simplifies the process flow, and reduces equipment investment. In other words, the two-stage heat exchange device can realize the cascade utilization of heat within the system, significantly reducing external energy consumption.

[0039] Optionally, in some implementations, the system further includes: The third heat exchange device 6 is connected to the biomass gasification device 1, the methane cracking device 2, the purification and separation device 8 and the methanation device 3. It is used to receive gaseous products, the first mixed gas and part of the methane discharged from the methanation device 3, and to perform first-stage heat exchange treatment. The first mixed gas and part of the methane after heat exchange and heating are used as reaction raw materials and fed into the methane cracking device 2 for catalytic cracking reaction. The fourth heat exchange device 7 is connected to the third heat exchange device 6, the methanation device 3 and the purification and separation device 8. It is used to receive the gaseous products discharged from the third heat exchange device 6 and the second mixed gas, and to perform secondary heat exchange treatment. The second mixed gas after heat exchange and heating is used as a reaction raw material and fed into the methanation device 3 for methanation reaction.

[0040] It should be noted that the third heat exchange device 6 and the fourth heat exchange device 7 can be shell-and-tube countercurrent heat exchangers.

[0041] It should be noted that some of the methane discharged from the methanation unit 3 is fed into the methane cracking unit 2 through the second connecting pipe 10.

[0042] In this embodiment, a two-stage heat exchange structure, consisting of a third heat exchange device 6 and a fourth heat exchange device 7, is used to preheat the feedstock of the methane cracking unit 2 and the methanation unit 3 in stages, with the gaseous products generated by the biomass gasification unit 1 as the concentrated heat source. This fully recovers the system's waste heat, eliminating the need for additional raw material heating equipment, significantly reducing system energy consumption and equipment investment. At the same time, it precisely matches the feed temperature requirements of the methane cracking and methanation reactions, improving reaction conversion efficiency and the overall system's coupled operational stability.

[0043] Optionally, in some embodiments, the methanation unit 3 is provided with a steam generation unit to recover the heat released during the reaction of the methanation unit 3, convert the demineralized water into saturated steam, and use it as a gasifying agent to be fed back to the biomass gasification unit 1 for the biomass feedstock to undergo a gasification reaction.

[0044] It should be noted that the steam generation unit is located in the reactor shell jacket of the methanation reaction unit 32.

[0045] It should be noted that the steam generation unit is connected to the first fluid conveying device 16, which is used to send demineralized water into the steam generation unit.

[0046] In this embodiment, the methanation unit 3 has a built-in steam generation unit. On the one hand, it removes the strong exothermic reaction of methanation in a timely manner through demineralized water, controls the reaction temperature range, protects catalytic activity, and avoids thermal runaway. On the other hand, it recovers the waste heat of the reaction to produce saturated steam, which is directly fed back to the biomass gasification unit 1 as a gasification agent to participate in the gasification reaction, realizing the cascade utilization of heat and steam self-sufficiency, eliminating the need for an external steam boiler, reducing system energy consumption and equipment investment, and improving the energy and carbon resource utilization efficiency of the entire process.

[0047] Optionally, in some embodiments, the purification and separation device 8 includes: a desulfurization unit 81, a dechlorination unit 82, and a pressure swing adsorption unit 83; Among them, the desulfurization unit 81 is connected to the biomass gasification device 1 and is used to receive gaseous products and perform desulfurization treatment. The dechlorination unit 82 is connected to the desulfurization unit 81 and the pressure swing adsorption unit 83. It is used to receive the gaseous products discharged from the desulfurization unit 81 and perform dechlorination treatment, and send the dechlorinated gaseous products into the pressure swing adsorption unit 83. The pressure swing adsorption unit 83 is connected to the methanation unit 3 and the methane cracking unit 2. It is used to receive the gaseous products discharged from the dechlorination unit and perform separation treatment to obtain a first mixed gas and a second mixed gas. The first mixed gas is used as a reaction feedstock and fed into the methane cracking unit 2 for catalytic cracking reaction. The second mixed gas is used as a reaction feedstock and fed into the methanation unit 3 for methanation reaction.

[0048] It should be noted that the desulfurization unit 81 can be a desulfurization tower, the dechlorination unit 82 can be a dechlorination tower, and the pressure swing adsorption unit 83 can be a pressure swing adsorption tower.

[0049] It should be noted that the air inlet of the desulfurization unit 81 is connected to the biomass gasification device 1 to receive the gaseous products produced by biomass gasification. The desulfurization unit 81 is filled with dry or wet desulfurization adsorbent to remove sulfur-containing harmful impurities such as hydrogen sulfide from the gaseous products, thereby completing the desulfurization and purification. The outlet of the desulfurization unit 81 is connected to the inlet of the dechlorination unit 82. The dechlorination unit 82 is filled with a special dechlorination adsorbent to further remove chlorine-containing corrosive impurities such as hydrogen chloride from the gaseous products after desulfurization, remove trace amounts of chlorine components, and avoid poisoning and failure of the subsequent adsorption and catalytic media and corrosion and salt formation of the equipment. The outlet of the dechlorination unit 82 is connected to the pressure swing adsorption unit 83. The pressure swing adsorption unit 83 is equipped with multiple pressure swing adsorption towers and programmable valve groups. It utilizes the difference in adsorption capacity of the adsorbent for different gas components under different pressures to selectively adsorb and desorb the purified gaseous products. After separation, a first mixed gas rich in methane and carbon dioxide and a second mixed gas rich in hydrogen and carbon monoxide are obtained.

[0050] The pressure swing adsorption unit 83 delivers the first mixed gas to the methane cracking unit 2 as the reaction feedstock for catalytic cracking of the molten medium; at the same time, it delivers the second mixed gas to the methanation unit 3 as the feedstock gas for the methanation reaction, thereby achieving precise distribution and fractional utilization of the purified gas components.

[0051] In this embodiment, the desulfurization unit 81, dechlorination unit 82, and pressure swing adsorption unit 83 are arranged in series to remove harmful impurities such as sulfides and chlorides from the syngas step by step, effectively preventing poisoning and deactivation of subsequent adsorbents and various catalysts, equipment corrosion, and pipeline blockage. Then, the pressure swing adsorption unit 83 achieves precise separation of gaseous products, and the two mixed gases obtained from the separation are respectively supplied to the methane cracking unit 2 and the methanation unit 3. The material matching is reasonable, the process coupling is strong, and the system operation stability, catalyst life and comprehensive utilization value of hydrocarbon resources are significantly improved.

[0052] This application also provides a method for co-producing graphene and natural gas, applicable to the above-mentioned system, the method comprising the following steps: Step S1: The biomass raw material, water vapor and oxygen are gasified in a biomass gasification device to generate gaseous products containing carbon monoxide, methane, hydrogen and carbon dioxide. Step S2: The gaseous product is purified and separated in a purification and separation device to obtain a first mixed gas rich in methane and carbon dioxide, and a second mixed gas rich in hydrogen and carbon monoxide. Step S3: The first mixed gas is subjected to catalytic cracking reaction in a methane cracking unit to obtain the target product graphene and cracked gas containing hydrogen and carbon monoxide. Step S4: The second mixed gas and the cracked gas are subjected to a methanation reaction in a methanation unit to generate the target product methane. The heat released by the methanation reaction is used to generate water vapor through heat exchange. The water vapor is used as a gasification agent and fed back to the biomass gasification unit to supply the biomass feedstock for gasification reaction.

[0053] It should be noted that biomass raw materials are derived from at least one of agricultural waste and industrial waste; agricultural waste includes rice husks, straw, waste wood, and livestock and poultry manure, while industrial waste includes papermaking waste, sugar refining or traditional Chinese medicine waste. The particle size range of biomass raw materials is 1mm to 30mm.

[0054] It should be noted that, under the protection of inert nitrogen gas, the biomass feedstock and the cracked gas discharged from the methane cracking unit undergo countercurrent indirect heat exchange in the first heat exchange unit, raising the temperature of the biomass feedstock to 300℃~400℃. The above process completes the deep dehydration and mild pyrolysis activation of the biomass feedstock, and the generated trace amount of pyrolysis gas is introduced into the biomass gasification unit for recycling.

[0055] In the embodiments of this application, the method for co-producing graphene and natural gas can make full use of CH4 and CO2 to solve the problem of relatively single utilization path, and can also achieve co-production of graphene and natural gas, with high economic value of carbon resource conversion products.

[0056] Optionally, in some embodiments, a molten medium is used in the catalytic cracking reaction. The molten medium is selected from a molten alloy, and the molten alloy is Bi. 43 In 37 Sn 20 Doped with metallic nickel and metallic copper; Among them, based on the mass of the molten alloy, the doping amount of metallic nickel is 0.4wt%~0.6wt%, and the doping amount of metallic copper is 0.1wt%~0.4wt%.

[0057] For example, metallic nickel and metallic copper are active components, wherein the doping amount of metallic nickel is one of 0.4wt%, 0.5wt%, 0.6wt%, or any two of them.

[0058] The copper doping concentration is within the range of 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, or any combination thereof. The system utilizes the synergistic reaction of CH4 and CO2 generated internally in a methane cracking unit to simultaneously achieve hydrogen self-sufficiency and the co-production of high-value-added carbon nanomaterials (graphene).

[0059] It should be noted that Bi 43 In 37 Sn 20 It is a low-melting-point alloy with a melting point range of 70℃ to 100℃.

[0060] It should be noted that the catalytic cracking reaction in the methane cracking unit uses a molten medium, and the methane conversion rate is 93%~96%. The high-quality graphene obtained is collected and used in downstream industries such as batteries.

[0061] In this embodiment, the process of methane catalytic cracking is CH4→C+2H2. In the catalytic cracking reaction, since a molten medium is used, carbon is generated on the surface of the bubbles, avoiding covering the active centers. Methane is introduced into the melt in the form of bubbles and catalytically cracks at the gas-liquid interface. Carbon nucleates and grows on the bubble wall and does not deposit inside the melt. Through the rise and rupture of the bubbles, the carbon is directly carried away from the liquid phase.

[0062] The reason carbon is directly carried away from the liquid phase is that the density of solid carbon is much smaller than that of molten metal. Buoyancy causes carbon particles to float rapidly to the surface of the melt and be carried out of the reactor with the gas. This avoids carbon products coating active sites or clogging reaction channels, allowing the catalyst to maintain its activity for a long time. In addition, the molten medium has a large heat capacity, fast thermal conductivity, and uniform temperature, which can effectively inhibit deep cracking of methane and reduce the formation of coking.

[0063] In summary, this application utilizes the characteristics of the molten medium to achieve continuous carbon removal through buoyancy and gas entrainment, enabling the carbon generated from the catalytic cracking of methane to quickly detach from the catalyst surface, thus preventing catalyst deactivation due to carbon buildup from the source.

[0064] Optionally, in some implementations, the method further includes: The biomass feedstock and pyrolysis gas are subjected to primary heat exchange treatment in the first heat exchange device to obtain preheated biomass feedstock and primary pyrolysis gas. Oxygen and primary pyrolysis gas are subjected to secondary heat exchange in a second heat exchange device to obtain preheated oxygen and secondary pyrolysis gas. Preheated biomass feedstock and preheated oxygen are used as reaction feedstocks in a biomass gasification device for gasification reaction. Secondary cracked gas is used as a reaction feedstock in a methanation unit for methanation reaction.

[0065] It should be noted that the temperature of the preheated biomass feedstock is 300℃~400℃, and the temperature of the primary pyrolysis gas drops to 450℃~550℃.

[0066] The temperature range of oxygen after preheating is 200℃~250℃, and the temperature of the secondary cracked gas drops to 250℃~300℃.

[0067] In this embodiment, waste heat from methane cracking gas is recovered through two-stage gradient heat exchange, while preheating of biomass feedstock and oxygen as gasification agent are completed simultaneously, achieving full recovery of waste heat in stages and reducing the demand for external heat sources.

[0068] Optionally, in some implementations, the method further includes: The gaseous product, the first mixed gas, and a portion of the methane discharged from the methanation unit are subjected to a first-stage heat exchange process in a third heat exchange unit to obtain the first-stage gaseous product, the preheated portion of methane, and the preheated first mixed gas. The primary gaseous product and the second mixed gas are subjected to a secondary heat exchange process in the fourth heat exchange device to obtain the secondary gaseous product and the preheated second mixed gas. The preheated first mixed gas and a portion of the preheated methane are used as reactants in a methane cracking unit for catalytic cracking reaction. The preheated second mixed gas is used as a reaction feedstock in a methanation unit for methanation reaction.

[0069] It should be noted that the third mixed gas in the mixing unit comes from the preheated second mixed gas and the secondary cracked gas.

[0070] It should be noted that the temperature of the gaseous products is 800℃~900℃, the temperature of the preheated first mixed gas and the preheated methane is 400℃~500℃, and the temperature of the primary gaseous products drops to 450℃~500℃.

[0071] The temperature of the preheated second mixed gas is 250℃~300℃, and the temperature of the secondary gaseous products drops to 40℃~60℃.

[0072] In this embodiment, the gaseous products of biomass gasification are used as the waste heat source. The waste heat is recovered in stages through the third and fourth heat exchange devices, realizing the internal reuse of the system's waste heat. This simplifies the process, saves energy, and reduces equipment investment.

[0073] Optionally, in some embodiments, the temperature of the gasification reaction is 800°C to 900°C, and the pressure is 0.2 MPa to 1.2 MPa.

[0074] For example, the temperature of the gasification reaction is within the range of one or both of 800°C, 830°C, 850°C, 880°C, and 900°C, and the pressure is within the range of one or both of 0.2 MPa, 0.5 MPa, 0.8 MPa, 1.0 MPa, and 1.2 MPa.

[0075] In this embodiment, the above-mentioned gasification reaction conditions take into account the gasification reaction conversion rate, gaseous product quality, system energy consumption, and equipment cost, and are suitable for the entire process of subsequent purification and separation, methane cracking, and methanation, ensuring the continuous, stable, and efficient operation of the entire process.

[0076] Optionally, in some embodiments, the temperature of the catalytic cracking reaction is 850°C to 950°C, and the pressure is 0.10 MPa to 0.15 MPa.

[0077] For example, the temperature of the catalytic cracking reaction is a range of one or both of 850°C, 880°C, 900°C, 920°C, and 950°C, and the pressure is a range of one or both of 0.10 MPa, 0.12 MPa, and 0.15 MPa.

[0078] In this embodiment, the reaction temperature meets the activation energy required for the breaking of the CH bond in methane, ensuring efficient methane cracking, increasing hydrogen yield, and simultaneously accommodating the stable liquid phase operating range of the molten medium, maintaining the gas-liquid interface reaction environment. The pressure range facilitates stable reactor feed, optimizes feed gas residence time, and improves methane cracking conversion rate.

[0079] Optionally, in some embodiments, the temperature of the methanation reaction is 300°C to 350°C.

[0080] For example, the temperature of the methanation reaction is a range of 300°C, 320°C, 350°C, or any two of these values.

[0081] In the embodiments of this application, the methanation reaction is a strongly exothermic and reversible reaction. The above-mentioned temperature is moderate, which can ensure that the reaction proceeds efficiently in the forward direction, while avoiding the reverse shift of the equilibrium and the decrease in methane yield.

[0082] Optionally, in some embodiments, the gaseous products include 18% to 25% carbon monoxide, 25% to 33% hydrogen, 15% to 24% carbon dioxide, and 8% to 18% methane, based on the volume of the gaseous products.

[0083] For example, the gaseous products include carbon monoxide in the range of 18%, 20%, 22%, 25%, hydrogen in the range of 25%, 28%, 30%, 33%, carbon dioxide in the range of 15%, 18%, 20%, 22%, 24%, and methane in the range of 8%, 10%, 12%, 15%, 18%.

[0084] In this embodiment, the partial pressures of the components in the gaseous product are significantly different. After separation by the pressure swing adsorption unit, the first mixed gas and the second mixed gas can be stably separated, which respectively meet the feed requirements of methane cracking and methanation reaction.

[0085] Optionally, in some embodiments, the mass ratio of water vapor to biomass feedstock is 0.5 to 1.5, and the mass of oxygen is 0.15 to 0.25 times the theoretical mass of oxygen required for complete combustion of biomass feedstock.

[0086] For example, the mass ratio of water vapor to biomass feedstock is one or both of the following values: 0.5, 0.8, 1.0, 1.2, 1.5, and the mass of oxygen is one or both of the following values: 0.15 times, 0.18 times, 0.20 times, 0.22 times, 0.25 times the theoretical mass of oxygen required for complete combustion of biomass feedstock.

[0087] Optionally, in some embodiments, the first mixed gas comprises, by volume, 50% to 65% carbon dioxide, 30% to 45% methane, 3% to 10% carbon monoxide, and less than 4% hydrogen.

[0088] For example, the first mixed gas includes 50%, 52%, 55%, 58%, 60%, 63%, 65% carbon dioxide, 30%, 35%, 40%, 45% methane, 3%, 5%, 8%, 10% carbon monoxide, and less than 4% hydrogen.

[0089] In this embodiment, methane and carbon dioxide are the main components, and the feed is adapted to catalytic cracking in a molten medium. The carbon dioxide preferentially reacts with the amorphous carbon from the cracking of methane, reducing low-value by-product carbon and improving the purity of graphene. Meanwhile, the generated carbon monoxide can be used as a raw material for the methanation unit, thereby improving the comprehensive utilization rate of carbon resources.

[0090] Optionally, in some embodiments, the second mixed gas comprises, by volume, 45% to 55% hydrogen, 30% to 40% carbon monoxide, 3% to 6% carbon dioxide, and 1% to 3% methane.

[0091] For example, the second gas mixture includes hydrogen in the range of 45%, 48%, 50%, 52%, 55%, carbon monoxide in the range of 30%, 33%, 36%, 40%, carbon dioxide in the range of 3%, 4%, 5%, 6%, and methane in the range of 1%, 2%, 3%.

[0092] In this embodiment, the hydrogen-carbon composition ratio is adapted to the methanation reaction, eliminating the need for additional hydrogen replenishment or water-gas conversion, thus reducing process energy consumption and equipment investment.

[0093] Optionally, in some embodiments, the purity of the target product graphene is 95% to 98%; the purity of the target product methane is 94% to 98%.

[0094] For example, the purity of the target product graphene is a range of 95%, 96%, 97%, 98%, or any two of these values; the purity of the target product methane is a range of 94%, 95%, 96%, 97%, 98%, or any two of these values.

[0095] In this embodiment, the graphene of the above purity possesses excellent electrical and thermal conductivity and structural mechanical properties, meeting the requirements for high-value industrial applications and adapting to large-scale continuous production. The above methane can directly meet the requirements for pipeline grid connection, industrial fuel, and resource recycling, without the need for additional complex refining and purification equipment, simplifying the process, reducing investment, and adapting to internal methane reflux within the system.

[0096] In traditional biomass gasification processes for producing natural gas using water electrolysis or water-gas conversion for hydrogen supplementation, the overall system thermal efficiency is approximately 60%–75%. Compared to existing technologies, the system using this application, through multi-stage waste heat recovery and energy cascade utilization, can further improve the overall system thermal efficiency to 82%–88%.

[0097] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0098] For the sake of simplicity, the method embodiments are described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to this application. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and components involved are not necessarily essential to this application.

[0099] The present application has been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of the present application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present application. Therefore, the content of this specification should not be construed as a limitation of the present application.

Claims

1. A system for co-producing graphene and natural gas, characterized in that, The system includes: A biomass gasification device is used to cause biomass raw materials to undergo a gasification reaction under the action of water vapor and oxygen, generating gaseous products containing carbon monoxide, methane, hydrogen and carbon dioxide. A purification and separation device, connected to the biomass gasification device, is used to receive the gaseous products and perform purification and separation treatment to obtain a first mixed gas rich in methane and carbon dioxide, and a second mixed gas rich in hydrogen and carbon monoxide. A methane cracking unit, connected to the purification and separation unit, is used to receive the first mixed gas and carry out a catalytic cracking reaction to obtain the target product graphene and cracked gas containing hydrogen and carbon monoxide. The methanation unit is connected to the purification and separation unit and the methane cracking unit. It is used to receive the second mixed gas and the cracked gas, and to carry out the methanation reaction to generate the target product methane. The heat released by the methanation reaction is used to generate water vapor through heat exchange. The water vapor is used as a gasification agent and fed back to the biomass gasification unit to supply the biomass raw materials for gasification reaction.

2. The system according to claim 1, characterized in that, The system also includes: The first heat exchange device is connected to the biomass gasification device and the methane cracking device, and is used to receive the cracked gas and the biomass raw material, and perform a first-stage heat exchange treatment to preheat the biomass raw material. The second heat exchange device is connected to the biomass gasification device, the first heat exchange device, and the methanation device. It is used to receive the pyrolysis gas discharged from the first heat exchange device and perform secondary heat exchange treatment with oxygen. The oxygen after heat exchange is used as a gasification agent and is introduced into the biomass gasification device to participate in the reaction. The pyrolysis gas after heat exchange is used as a reaction feedstock and is introduced into the methanation device to participate in the reaction.

3. The system according to claim 1, characterized in that, The system also includes: The third heat exchange device is connected to the biomass gasification device, the methane cracking device, the purification and separation device, and the methanation device. It is used to receive the gaseous products, the first mixed gas, and part of the methane discharged from the methanation device, and to perform a first-stage heat exchange treatment. The first mixed gas and part of the methane after heat exchange and heating are used as reaction raw materials and fed into the methane cracking device for catalytic cracking reaction. The fourth heat exchange device is connected to the third heat exchange device, the methanation device and the purification and separation device. It is used to receive the gaseous products discharged from the third heat exchange device and the second mixed gas, and to perform secondary heat exchange treatment. The second mixed gas after heat exchange and heating is used as a reaction raw material and fed into the methanation device for methanation reaction.

4. The system according to claim 1, characterized in that, The methanation unit is equipped with a steam generation unit, which is used to recover the heat released during the reaction of the methanation unit, convert the demineralized water into saturated steam, and use it as a gasifying agent to be fed back to the biomass gasification unit for the biomass feedstock to undergo a gasification reaction.

5. The system according to claim 1, characterized in that, The purification and separation device includes: a desulfurization unit, a dechlorination unit, and a pressure swing adsorption unit; The desulfurization unit is connected to the biomass gasification device and is used to receive the gaseous products and perform desulfurization treatment. The dechlorination unit is connected to the desulfurization unit and the pressure swing adsorption unit, and is used to receive the gaseous products discharged from the desulfurization unit and perform dechlorination treatment, and send the dechlorinated gaseous products into the pressure swing adsorption unit. The pressure swing adsorption unit is connected to the methanation device and the methane cracking device, and is used to receive the gaseous products discharged from the dechlorination unit and perform separation processing to obtain the first mixed gas and the second mixed gas; the first mixed gas is used as a reaction feedstock and fed into the methane cracking device for catalytic cracking reaction; the second mixed gas is used as a reaction feedstock and fed into the methanation device for methanation reaction.

6. A method for co-producing graphene and natural gas, characterized in that, The method is applicable to the system described in any one of claims 1-5, and the method includes the following steps: Biomass feedstock, water vapor, and oxygen are gasified in a biomass gasification device to produce gaseous products containing carbon monoxide, methane, hydrogen, and carbon dioxide. The gaseous product is purified and separated in a purification and separation device to obtain a first mixed gas rich in methane and carbon dioxide, and a second mixed gas rich in hydrogen and carbon monoxide. The first mixed gas is subjected to catalytic cracking reaction in a methane cracking unit to obtain the target product graphene and cracked gas containing hydrogen and carbon monoxide. The second mixed gas and the pyrolysis gas are subjected to a methanation reaction in a methanation unit to obtain the target product methane. The heat released by the methanation reaction is used to generate water vapor through heat exchange. The water vapor is fed back to the biomass gasification unit as a gasifying agent to supply the biomass feedstock for gasification reaction.

7. The method according to claim 6, characterized in that, The catalytic cracking reaction uses a molten medium, which is selected from a molten alloy, and the molten alloy is Bi. 43 In 37 Sn 20 Doped with metallic nickel and metallic copper; The amount of nickel doped is 0.4wt% to 0.6wt% and the amount of copper doped is 0.1wt% to 0.4wt% based on the mass of the molten alloy.

8. The method according to claim 6, characterized in that, The method further includes: The biomass feedstock and the pyrolysis gas are subjected to primary heat exchange treatment in a first heat exchange device to obtain preheated biomass feedstock and primary pyrolysis gas. The oxygen and the primary pyrolysis gas are subjected to secondary heat exchange treatment in a second heat exchange device to obtain preheated oxygen and secondary pyrolysis gas. The preheated biomass raw material and preheated oxygen are used as reaction raw materials in the biomass gasification device for gasification reaction. The secondary cracked gas is used as a reaction feedstock in the methanation unit for methanation reaction.

9. The method according to claim 6, characterized in that, The method further includes: The gaseous product, the first mixed gas, and a portion of the methane discharged from the methanation unit are subjected to a first-stage heat exchange process in a third heat exchanger to obtain a first-stage gaseous product, preheated methane, and a preheated first mixed gas. The primary gaseous product and the second mixed gas are subjected to secondary heat exchange treatment in the fourth heat exchange device to obtain the secondary gaseous product and the preheated second mixed gas. The preheated first mixed gas and the preheated portion of methane are used as reactants in the methane cracking unit for catalytic cracking reaction. The preheated second mixed gas is used as a reaction raw material in the methanation device for methanation reaction.

10. The method according to any one of claims 6 to 9, characterized in that, At least one of the following conditions must be met: (1) The temperature of the gasification reaction is 800℃~900℃ and the pressure is 0.2MPa~1.2MPa; (2) The temperature of the catalytic cracking reaction is 850℃~950℃, and the pressure is 0.10MPa~0.15MPa; (3) The temperature of the methanation reaction is 300℃~350℃; (4) Based on the volume of the gaseous products, the gaseous products include 18% to 25% carbon monoxide, 25% to 33% hydrogen, 15% to 24% carbon dioxide, and 8% to 18% methane; (5) The mass ratio of water vapor to biomass raw material is 0.5 to 1.5, and the mass of oxygen is 0.15 to 0.25 times the theoretical mass of oxygen required for complete combustion of biomass raw material; (6) By volume ratio, the first mixed gas comprises 50% to 65% carbon dioxide, 30% to 45% methane, 3% to 10% carbon monoxide, and less than 4% hydrogen. (7) By volume ratio, the second mixed gas comprises 45% to 55% hydrogen, 30% to 40% carbon monoxide, 3% to 6% carbon dioxide, and 1% to 3% methane; (8) The purity of the target product graphene is 95%~98%; the purity of the target product methane is 94%~98%.

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