A photovoltaic and biomass coupled system for preparing bio-natural gas, a preparation method and an optimization coupling method

Abstract

This invention discloses a photovoltaic and biomass coupled biogas production system, preparation method, and optimized coupling method, belonging to the field of carbon dioxide methanation technology. This invention addresses the biomass-to-biogas process under fluctuating photovoltaic conditions by coupling direct methanation and decarbonization of biogas. The biogas flow rate entering the methanation process is adjusted according to the hydrogen production. Excess biogas enters a carbon separation unit for CO2 storage. Controlling the circulation rate of the biogas methanation reactor ensures a stable space velocity and meets conversion requirements. Simultaneously, the oxygen produced during hydrogen production is used for oxygen-enriched combustion of fermentation biogas residue, overcoming the poor combustion performance caused by the low calorific value and high moisture content of the residue. Furthermore, the heat released from biogas methanation is comprehensively utilized to produce steam, meeting the steam consumption requirements of the electrolytic hydrogen production and decarbonization processes, thus improving the energy efficiency of the coupled system.

Description

Technical Field

[0001] This invention relates to the field of renewable energy utilization, and in particular to a photovoltaic and biomass coupling preparation system, preparation method and optimized coupling method. Background Technology

[0002] In the current low-carbon context, methane has become an option for low-carbon dispatch in new power systems, serving as a natural gas energy source. The carbon dioxide methanation reaction is an important pathway for the resource utilization of carbon dioxide, producing methane while reducing carbon dioxide emissions, which can then be directly used in existing natural gas energy networks.

[0003] According to publicly available literature, biogas can be produced through anaerobic fermentation using biomass as raw material, and then converted into biomethane through methanation with hydrogen. However, when hydrogen levels fluctuate, the conventional method involves biogas storage. This involves calculating the required biogas volume based on hydrogen flow rate, controlling the biogas flow to meet hydrogen consumption, and supplying excess or insufficient biogas through storage tanks. However, when the scale is large, biogas storage itself poses safety risks.

[0004] A search of patent literature databases yielded no clear solution. Summary of the Invention

[0005] The technical problem to be solved by this invention is to improve the production conversion efficiency in the process of large-scale industrial safe production of biogas. The purpose of this invention is to provide a technical solution that can be applied to industrial production scenarios. By optimizing the design of the biogas conversion system and preparation method, it can adapt to the fluctuation of hydrogen production, avoid the problem of combustible gas storage, and promote the overall high capacity of the entire production system.

[0006] The technical solution adopted in this invention is: a photovoltaic and biomass coupled biogas production system, comprising: a solid oxide electrolysis cell, in which photovoltaic power is used to electrolyze water vapor to produce hydrogen and oxygen-enriched air; an anaerobic fermentation tank, in which biomass undergoes anaerobic fermentation to produce biogas and biogas residue; a steam boiler, which uses biogas residue as fuel to produce steam; a gas distributor, which regulates the flow of biogas or CO2 between a CO2 absorption tower and a biogas methanation reactor; a CO2 absorption tower, in which countercurrent operation is adopted, an organic amine absorbent absorbs CO2 from biogas to form a rich liquid, producing a gas rich in methane; a CO2 desorption tower, in which countercurrent contact is adopted, separating CO2 from the CO2-absorbed rich liquid, and the decarbonized organic amine absorbent is returned to the CO2 absorption tower as feed liquid; a biogas methanation reactor, in which hydrogen is drawn in and reacts with the decarbonized CO2 in biogas to produce biogas; and a natural gas pipeline network, in which the methane-rich gas and biogas are coupled and then enter the natural gas pipeline network.

[0007] Using a high-temperature solid oxide electrolysis cell to produce hydrogen can effectively couple heat with biogas methanation, improving the utilization efficiency of the exothermic heat from methanation and further enhancing overall energy efficiency.

[0008] Electrolysis of water to produce hydrogen requires a large amount of electricity, and the current costs of electricity and hydrogen storage remain very high. Using photovoltaic power generation to provide the necessary electricity is an effective solution. The downstream biogas conversion process needs to quickly adapt to the fluctuations in photovoltaic hydrogen production. By coupling production through integrated systems, the overall energy utilization efficiency can be effectively improved.

[0009] The biogas conversion process involves biomass fermentation to obtain biogas, which, after purification, undergoes a methanation reaction with hydrogen obtained from electrolysis. After dehydration, biogas is obtained. However, biogas production from biomass can only utilize the easily digestible organic matter; other components such as cellulose and lignin are difficult to convert. The resulting fermentation residue, due to its low calorific value and high water content, is typically only used for composting under conventional methods, making it difficult to utilize its heat. This invention, however, allows for the comprehensive utilization of the fermentation residue to produce steam, thereby improving overall energy efficiency. The solid oxide electrolyzer uses high-temperature electrolysis, which can improve hydrogen production capacity.

[0010] By coupling direct methanation and decarbonization of biogas, the flow rate of biogas entering the methanation process is adjusted according to the amount of hydrogen produced. Excess biogas enters a carbon separation unit, and the resulting methane and biomethane from methanation are output together to the natural gas pipeline network. The separated CO2 is stored to supplement the required CO2 when hydrogen production is high. On the other hand, the oxygen produced in the hydrogen production process is used for oxygen-enriched combustion of the fermentation biogas residue, overcoming the poor combustion performance caused by the low calorific value and high moisture content of the biogas residue. At the same time, the heat released from biogas methanation is comprehensively utilized to produce steam, meeting the steam consumption of the electrolytic hydrogen production and decarbonization processes, thereby improving the system's energy efficiency.

[0011] In this scheme, high-temperature and high-pressure electrolysis is further used in the solid oxide electrolysis cell, and the oxygen-enriched air released is fed into a steam boiler for oxygen-enriched combustion of biogas residue; the steam produced by the steam boiler is used to enter the solid oxide electrolysis cell or the CO2 desorption tower; the CO2 storage tank transfers the CO2 released from the CO2 desorption tower and distributes it to the biogas methanation reactor via a gas distributor to participate in the CO2 methanation reaction; the biogas methanation reactor produces biogas through a heat recovery and circulation device; part of the gas in the heat recovery and circulation device recovers heat and is circulated back to the biogas methanation reactor to regulate the flow rate of the circulating gas.

[0012] The produced oxygen is used for oxygen-enriched combustion of biogas residue, and the steam produced by the steam boiler is used in a solid oxide electrolysis cell to release CO2 from the biogas, which then participates in the CO2 methanation reaction, further improving overall utilization efficiency. High-temperature and high-pressure electrolysis further enhances hydrogen production capacity. Energy utilization efficiency is further improved through heat recovery and recycling devices.

[0013] Another technical solution adopted in this invention is: a method for preparing biogas by coupling photovoltaic and biomass, comprising the following steps: S1: using an anaerobic fermentation tank to obtain biogas and biogas residue produced by anaerobic fermentation of biomass; S2: using a steam boiler, with biogas residue as fuel, and using oxygen-enriched air released from a solid oxide electrolysis cell for combustion to produce water vapor; S3: using photovoltaic power, introducing water vapor produced by the steam boiler into the solid oxide electrolysis cell, and electrolyzing it to produce hydrogen and oxygen-enriched air, with the hydrogen entering the biogas methanation reactor; S4: using a biogas purification mechanism consisting of a CO2 absorption tower and a CO2 desorption tower, producing methane after purification, and CO2 after water separation; S5: adjusting the flow distribution of biogas in the CO2 absorption tower and the biogas methanation reactor through a gas distributor to regulate the flow fluctuation of CO2 entering the biogas methanation reactor; S6: the methane and biogas are coupled and then enter the natural gas pipeline network.

[0014] By coupling the direct separation of methane from biogas and the decarbonization of biogas, the comprehensive utilization of water vapor, the regulation of hydrogen production, and the simultaneous output of the separated methane and the biomethane obtained after methanation to the natural gas pipeline network, the system's output is increased. On the other hand, the oxygen generated during hydrogen production is used for oxygen-enriched combustion of the fermentation residue, overcoming the poor combustion performance caused by the low calorific value and high water content of the residue. Simultaneously, the heat released from biogas methanation is comprehensively utilized to generate steam, meeting the steam consumption requirements of the electrolytic hydrogen production and decarbonization processes, thereby improving the system's energy conversion efficiency.

[0015] In this scheme, the outlet of the biogas methanation reactor is further connected to a heat recovery and circulation device to produce biogas. After recovering heat, part of the gas from the heat recovery and circulation device is recycled back to the biogas methanation reactor to regulate the flow distribution of the circulating gas and stabilize hydrogen fluctuations.

[0016] Furthermore, the heat recovery and circulation device has a flow ratio of 1:1 to 10:1 for the circulating gas to the output biogas.

[0017] Furthermore, in the biogas methanation reactor, the space velocity within the reactor remains stable during hydrogen fluctuations, with a stable space velocity range of 1000~1000 h⁻¹. -1 The CO2 conversion rate is not less than 98%, and the reaction temperature is 250~450℃.

[0018] Furthermore, the steam boiler uses oxygen-enriched air as a combustion aid, and the steam produced has the following parameters: pressure not less than 0.3 MPa and superheat temperature not less than 600℃.

[0019] Steam boilers can also use oxygen-enriched air as a combustion aid, producing steam with the following parameters: pressure not less than 1 MPa and temperature not less than 200℃.

[0020] Furthermore, the gas distributor maintains the H2 / CO2 molar ratio in the biogas methanation reactor at 4:1 to 4.1:1.

[0021] Another technical solution adopted in this invention is: an optimized coupling method for the production of biogas by coupling photovoltaic and biomass, comprising the following steps: T1: constructing a coupled production system for biogas based on an anaerobic fermentation tank, a solid oxide electrolysis tank, and a biogas methanation reactor; T2: considering the production parameters of the coupled production system, and taking the lowest energy consumption cost for biogas production within a cycle as the optimization objective, establishing a coupled production model; T3: using the mass ratio of hydrogen produced by the solid oxide electrolysis tank, biogas produced by the anaerobic fermentation tank, and CO2 released by the CO2 desorption tower as constraints, optimizing the solution to obtain the optimal production state with the lowest production energy consumption cost.

[0022] This technical solution aims to minimize the energy consumption cost of biogas production within a given cycle. It considers the coupled production processes of anaerobic digesters, solid oxide electrolyzers, and biogas methanation reactors, and establishes a corresponding equipment coupling production model. Under the actual conditions of meeting large-scale production arrangements, it optimizes the production ratio of hydrogen from the solid oxide electrolyzer, biogas from the anaerobic digester, and CO2 from the CO2 desorption tower at each time period, thereby optimizing the coupling configuration, reducing energy consumption costs, and improving the overall system operating efficiency.

[0023] In this scheme, the optimization objective of minimizing the energy consumption cost of biogas production within the cycle is further defined as follows.

[0024]

[0025] In the formula, This refers to the input electrical power of the solid oxide electrolytic cell. The mass of CO2 consumed per unit of the biogas methanation reactor. The electricity price for hydrogen production during time period t; The unit cost of CO2 desorption used to produce biogas during time period t; Indicates the optimization cycle; Indicates the unit preparation time.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows.

[0027] This invention uses photovoltaic power generation for electrolytic hydrogen production, and the generated oxygen is used for oxygen-enriched combustion of biogas residue, overcoming the problem of poor combustion performance caused by the low calorific value and high water content of biogas residue, thus improving the overall energy utilization efficiency; the obtained steam can meet the needs of biogas decarbonization, electrolytic hydrogen production steam raw materials, etc., realizing the coupling of photovoltaic and biomass to produce biogas.

[0028] This invention adapts to hydrogen flow fluctuations caused by fluctuating photovoltaic power by coupling biogas decarbonization and biogas methanation. When the biogas load changes, it only affects the CO2 load of the absorbent in the decarbonization process, and only affects the steam energy consumption after entering the desorption unit, thus fully adapting to fluctuations. The biogas flow rate entering the methanation unit always follows the hydrogen production change. By changing the flow rate of the circulating gas, the space velocity in the reactor is kept stable, ensuring the CO2 conversion rate, thereby adapting to hydrogen flow fluctuations. Attached Figure Description

[0029] Figure 1 This is a flowchart of the coupling preparation process of the present invention. Detailed Implementation

[0030] The technical solution of the present invention will be further described in detail below through embodiments.

[0031] Example 1.

[0032] like Figure 1 As shown, this embodiment provides a photovoltaic and biomass coupled biogas production system, comprising: a solid oxide electrolysis cell, employing high-temperature and pressurized electrolysis to electrolyze water vapor into hydrogen and oxygen-enriched air using photovoltaic power; an anaerobic fermentation tank, where biomass undergoes anaerobic fermentation to produce biogas and biogas residue; oxygen-enriched air released from the solid oxide electrolysis cell entering a steam boiler; a steam boiler, using biogas residue as fuel, producing steam through oxygen-enriched combustion; the steam produced by the steam boiler being used to enter the solid oxide electrolysis cell and also to enter a CO2 desorption tower; a gas distributor, regulating the flow of biogas or CO2 between the CO2 absorption tower and the biogas methanation reactor; and a CO2 absorption tower, employing countercurrent operation within the tower, where organic amine absorbent absorbs... CO2 from biogas is collected to form a rich liquid, producing a gas rich in methane. A CO2 desorption tower, with counter-current contact, separates CO2 from the CO2-absorbed rich liquid. The decarbonized organic amine absorbent is returned to the CO2 absorption tower as feed. The CO2 released from the CO2 desorption tower is stored and then distributed to the biogas methanation reactor via a gas distributor to participate in the CO2 methanation reaction. The biogas methanation reactor draws in hydrogen to react with the decarbonized CO2 from the biogas, producing biomethane through a heat recovery and circulation device. A portion of the gas from the heat recovery and circulation device recovers heat and is recycled back to the biogas methanation reactor, regulating the flow rate of the circulating gas. Finally, the methane-rich gas couples with the biomethane and enters the natural gas pipeline network.

[0033] Example 2.

[0034] like Figure 1 As shown in the figure, this embodiment provides a method for preparing biogas by coupling photovoltaic and biomass, the process of which includes:

[0035] Hydrogen production process by electrolysis. The electricity generated by photovoltaic power generation is directly used to produce hydrogen in a solid oxide electrolyzer, without using energy storage equipment to store excess electricity generated by fluctuating photovoltaic power; water vapor enters the solid oxide electrolyzer, hydrogen is obtained at the cathode, and after dehydration, it enters the methanation reactor; the oxygen produced at the anode is purged away by air to obtain oxygen-enriched air.

[0036] Biomass fermentation process. Anaerobic fermentation is used to produce biogas. The biomass used is conventional kitchen waste, lignocellulose and other biomass. After preliminary dehydration, the fermentation residue is fed into a steam boiler for combustion.

[0037] Oxygen-enriched combustion process. Fermented biogas residue is used for oxygen-enriched combustion, and the oxygen-enriched air is obtained from the anode of the electrolytic hydrogen production process; the heat generated by combustion is used to produce steam, which is used as steam feedstock for solid oxide electrolytic hydrogen production and heating energy consumption for the biogas decarbonization process.

[0038] Biogas decarbonization process. The organic amine method is used to absorb CO2 from biogas, and the decarbonized biogas is output to the pipeline network as biomethane; the absorbent is regenerated by steam heating, and the steam used comes from the oxygen-enriched combustion process of biogas residue, and the desorbed CO2 is stored in a storage tank.

[0039] Biogas methanation process. Hydrogen and biogas are mixed and then fed into the methanation reactor. After preheating, the mixture enters the catalyst bed and undergoes methanation to produce biogas. The gas leaving the reactor recovers the heat released to produce steam, and a portion of the gas is returned to the inlet of the methanation reactor as recycle gas to mix with fresh feed gas, ensuring a stable space velocity within the reactor.

[0040] Example 3.

[0041] like Figure 1 As shown in the figure, this embodiment provides a method for preparing biogas by coupling photovoltaic and biomass, the steps of which include:

[0042] The system simultaneously includes a biogas CO2 absorption tower, a CO2 desorption tower, and a biogas methanation reactor. A gas distributor regulates the flow distribution of biogas between the CO2 absorption tower and the biogas methanation reactor to accommodate fluctuations in photovoltaic power generation and hydrogen flow.

[0043] The CO2 produced by biogas decarbonization is stored, and when the hydrogen flow rate is too high, it is sent to the biogas methanation unit to meet the biogas methanation reaction requirements.

[0044] The oxygen-enriched air produced by the hydrogen production in the solid oxide electrolysis cell is used for the oxygen-enriched combustion of fermented biogas residue, which promotes the combustion performance of the fermented biogas residue. The steam produced by the steam boiler is used in the solid oxide electrolysis cell and decarbonization device.

[0045] The outlet of the biogas methanation reactor is connected to a heat recovery and circulation device. After recovering heat from part of the gas, it is circulated back to the biogas methanation reactor. The flow rate of the circulating gas is adjusted to ensure that the space velocity in the reactor remains stable when hydrogen fluctuates.

[0046] like Figure 1 The heat recovery and circulation device shown has a flow ratio of 1:1 to 10:1 for the circulating gas to the output biogas.

[0047] like Figure 1 The biogas methanation reactor shown maintains a stable space velocity, with a stable space velocity range of 1000~1000 h⁻¹. -1 The CO2 conversion rate is not less than 98%, and the reaction temperature is 250~450℃.

[0048] like Figure 1 The steam boiler shown uses oxygen-enriched air as a combustion aid and produces two types of steam: one with a pressure of not less than 0.3 MPa and a superheat temperature of not less than 600℃; and the other with a pressure of not less than 1 MPa and a temperature of not less than 200℃.

[0049] like Figure 1 The gas distributor shown maintains the H2 / CO2 molar ratio in the biogas methanation reactor at 4:1 to 4.1:1, and can also control the gas distribution between biogas and CO2 in the storage tank.

[0050] Example 4.

[0051] like Figure 1 As shown in the figure, this embodiment provides a method for preparing biogas by coupling photovoltaic and biomass, the steps of which include:

[0052] The electricity generated by the photovoltaic panel enters the solid oxide electrolytic cell, through which high-temperature atmospheric pressure steam at 600℃ is introduced, and the temperature of the electrolytic cell is 650℃. The cathode gas leaves the solid oxide electrolytic cell and, after cooling and dehydration, yields hydrogen. The anode is purged with air, and the oxygen produced enters the air to obtain oxygen-enriched air.

[0053] Biomass enters the anaerobic fermentation tank to produce biogas, which is then sent to the CO2 absorption tower or biogas methanation reactor through a gas distributor. Meanwhile, the unconverted biogas residue is dewatered and then burned in a steam boiler.

[0054] The steam boiler uses oxygen-enriched air and biogas residue to generate steam. The superheated steam at 0.3 MPa and 600°C enters the solid oxide electrolysis cell, and the steam at 1 MPa and 200°C enters the CO2 desorption tower. The production rate of different steam is adjusted according to the steam demand of the electrolysis cell and the desorption tower.

[0055] The gas distributor adjusts the amount of biogas entering the biogas methanation reactor based on the actual hydrogen production, ensuring a molar ratio of hydrogen to CO2 of 4.05:1. The biogas, hydrogen, and recycle gas are mixed before entering the biogas methanation reactor, maintaining a catalyst bed space velocity of 2000 h⁻¹. -1 The average temperature is 300℃, and the CO2 conversion rate is 98%. The gas leaving the reactor is immediately subjected to heat recovery to generate steam at 150℃ and 0.3 MPa. The steam enters the boiler for superheating. Part of the heat-recovered reaction gas is recycled, and part is output as biogas. The molar ratio of recycled gas to output gas is controlled between 1:1 and 10:1 according to the total amount of hydrogen. When the raw material hydrogen is too low, the recycling ratio is always maintained at 10:1.

[0056] Biogas entering the CO2 absorption tower undergoes CO2 absorption using MEA (Mechanical Absorption Array), with an absorption temperature of 50℃ and a CO2 removal rate of 98%. The lean gas, mainly composed of methane, enters the natural gas pipeline along with the biogas. The rich absorption liquid enters the desorption tower, where steam from the boiler is used for desorption at a temperature of 140℃. The desorbed CO2 is then sent to a storage tank.

[0057] The gas distributor calculates based on the amount of hydrogen produced. When the CO2 in all the biogas cannot meet the H2 / CO2 molar ratio, the gas in the CO2 storage tank is sent to the biogas methanation reactor.

[0058] Example 5.

[0059] like Figure 1 As shown in the figure, this embodiment provides an optimized coupling method for the production of biogas by coupling photovoltaic and biomass, which includes the following steps.

[0060] A coupled preparation system for biogas was constructed based on the coupling of an anaerobic fermentation tank, a solid oxide electrolysis tank, and a biogas methanation reactor.

[0061] Considering the preparation parameters of the coupled preparation system, and with the goal of minimizing the energy consumption cost of preparing biogas within a cycle, a coupled preparation model is established.

[0062] Among them, the optimization objective algorithm that minimizes the energy consumption cost of biogas production within the cycle is:

[0063]

[0064] In the formula, This refers to the input electrical power of the solid oxide electrolytic cell. The mass of CO2 consumed per unit of the biogas methanation reactor. The electricity price for hydrogen production during time period t; The unit cost of CO2 desorption used to produce biogas during time period t; Indicates the optimization cycle; Indicates the unit preparation time.

[0065] Using the mass ratio of hydrogen produced by the solid oxide electrolysis cell, biogas produced by the anaerobic fermentation cell, and CO2 released by the CO2 desorption tower as constraints, the optimal preparation state with the lowest preparation energy consumption cost is obtained by optimization.

[0066] As can be seen from the above embodiments, the technical solution of the photovoltaic and biomass coupling system for producing biogas, the preparation method, and the optimized coupling method provided by this invention address the biomass-to-biogas process under fluctuating photovoltaic conditions. This invention couples direct methanation and decarbonization of biogas, adjusting the biogas flow rate into methanation based on hydrogen production. Excess biogas enters a carbon separation device for CO2 storage. Controlling the circulation rate of the biogas methanation reactor ensures a stable space velocity and meets conversion requirements. Simultaneously, the oxygen generated during hydrogen production is used for oxygen-enriched combustion of fermentation residue, overcoming the poor combustion performance caused by the low calorific value and high moisture content of the residue. Furthermore, the heat released from biogas methanation is comprehensively utilized to produce steam, meeting the steam consumption requirements of the electrolytic hydrogen production and decarbonization processes, thus improving the energy efficiency of the coupling system.

Claims

1. A system for the production of bio-SNG by coupling photovoltaics with biomass, characterized in that it comprises: include: In a solid oxide electrolytic cell, photovoltaic power is used to electrolyze water vapor into hydrogen and oxygen-enriched air; In the anaerobic fermentation tank, biomass is anaerobic fermented to produce biogas and biogas residue; The steam boiler uses biogas residue as fuel to produce steam; high-temperature and pressurized electrolysis is used in the solid oxide electrolysis cell, and the oxygen-enriched air desorbed is introduced into the steam boiler to participate in the oxygen-enriched combustion of biogas residue; the steam produced by the steam boiler is introduced into the solid oxide electrolysis cell and then into the CO2 desorption tower. Gas distributor one adjusts the flow rate of biogas in the CO2 absorption tower and biogas methanation reactor according to the amount of hydrogen produced. In the CO2 absorption tower, countercurrent operation is adopted inside the tower. The organic amine absorbent absorbs CO2 in the biogas to form a rich liquid and produces gas rich in methane. In the CO2 desorption tower, the tower is in countercurrent contact, separating CO2 from the rich liquid that has absorbed CO2, and the decarbonized organic amine absorbent is returned to the CO2 absorption tower as feed liquid. The CO2 storage tank transfers the CO2 released from the CO2 desorption tower to the biogas methanation reactor via the second gas distributor, where it participates in the CO2 methanation reaction. The biogas methanation reactor draws in hydrogen and reacts it with CO2 from biogas to produce biogas. Natural gas pipelines are formed by coupling methane-rich gases with biogas before they enter the natural gas pipeline network.

2. The photovoltaic and biomass coupled biogas production system according to claim 1, characterized in that: The biogas methanation reactor produces biogas through a heat recovery and circulation device; part of the gas in the heat recovery and circulation device recovers heat and is then recycled back to the biogas methanation reactor, regulating the flow rate of the circulating gas.

3. A method for preparing bio-natural gas by coupling photovoltaic and biomass, using the system for preparing bio-natural gas by coupling photovoltaic and biomass according to claim 1 or 2, characterized in that, include: S1: Utilize anaerobic fermentation tanks to obtain biogas and biogas residue produced by anaerobic fermentation of biomass; S2: Using a steam boiler, biogas residue is used as fuel, and oxygen-enriched air desorbed from a solid oxide electrolysis cell is used for combustion to produce steam. S3: Using photovoltaic power, steam produced by a steam boiler is introduced into a solid oxide electrolysis cell, and hydrogen and oxygen-enriched air are electrolyzed. The hydrogen enters the biogas methanation reactor. S4: A biogas purification mechanism consisting of a CO2 absorption tower and a CO2 desorption tower, which produces methane after purification and CO2 after desorption. S5: The flow distribution of biogas in the CO2 absorption tower and biogas methanation reactor is adjusted by gas distributor one to adapt to the fluctuations in photovoltaic power generation and hydrogen flow; when the hydrogen flow is too large, CO2 in the CO2 storage tank is sent to the biogas methanation device through gas distributor two to meet the biogas methanation reaction. S6: Methane and biogas are coupled together and then enter the natural gas pipeline network.

4. The method for preparing biogas by coupling photovoltaic and biomass according to claim 3, characterized in that: The biogas methanation reactor outlet is connected to a heat recovery and circulation device to produce biogas. After recovering heat, part of the gas from the heat recovery and circulation device is recycled back to the biogas methanation reactor to regulate the flow distribution of the circulating gas and stabilize hydrogen fluctuations.

5. The method for preparing biogas by coupling photovoltaic and biomass according to claim 4, characterized in that: The heat recovery and circulation device has a flow ratio of 1:1 to 10:1 for the circulating gas and the output biogas.

6. The method for preparing biogas by photovoltaic coupling according to claim 4 or 5, characterized in that: The biogas methanation reactor maintains a stable space velocity during hydrogen fluctuations, achieves a CO2 conversion rate of no less than 98%, and operates at a reaction temperature of 250~450℃.

7. The method for preparing biogas by coupling photovoltaic and biomass according to claim 6, characterized in that: The steam boiler uses oxygen-enriched air as a combustion aid and produces steam with a pressure of not less than 0.3 MPa and a superheat temperature of not less than 600℃; or a pressure of not less than 1 MPa and a temperature of not less than 200℃.

8. The method for preparing biogas by coupling photovoltaic and biomass according to claim 6, characterized in that: Maintain the molar ratio of H2 / CO2 in the biogas methanation reactor at 4:1 to 4.1:

1.

9. An optimized coupling method for the production of biogas by photovoltaic and biomass coupling, employing the photovoltaic and biomass coupling system for the production of biogas as described in claim 1 or 2, characterized in that, include: T1: Construct a coupled preparation system for biogas based on an anaerobic fermentation tank, a solid oxide electrolysis tank, and a biogas methanation reactor; T2: Considering the preparation parameters of the coupled preparation system, and with the goal of minimizing the energy consumption cost of preparing biogas within the cycle, a coupled preparation model is established. T3: Using the mass ratio of hydrogen produced by the solid oxide electrolysis cell, biogas produced by the anaerobic fermentation cell, and CO2 released by the CO2 desorption tower as constraints, the optimal preparation state with the lowest preparation energy consumption cost is obtained by optimization.

10. The optimized coupling method for photovoltaic-biomass coupled production of bio- natural gas according to claim 9, characterized in that, The optimization objective for minimizing the energy consumption cost of biogas production within the cycle is: In the formula, This refers to the input electrical power of the solid oxide electrolytic cell. The mass of CO2 consumed per unit of the biogas methanation reactor. The electricity price for hydrogen production during time period t; The unit cost of CO2 production for biogas preparation during time period t; To optimize the cycle; Preparation time is expressed in units.

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