Method for preparing low-carbon methanol by coupling biochar with carbon capture

Abstract

The invention discloses a method for preparing low-carbon methanol by coupling biochar with carbon capture. The method comprises a route for preparing methanol from biochar and a route for preparing methanol from carbon capture, carrying out thermal cracking on the biomass to obtain biochar, cracking gas and a cracking solution; in the route of preparing methanol from biochar, biochar is ground and then is subjected to gasification reaction with oxygen, obtained crude synthesis gas is subjected to primary purification treatment, then H2S and CO2 in the synthesis gas are removed, and synthesis reaction is performed to obtain crude methanol; in the carbon capture methanol preparation route, pyrolysis gas and pyrolysis liquid are subjected to oxygen-enriched combustion, generated heat is used for supplying energy to thermal cracking, tail gas generated in the oxygen-enriched combustion process is purified, and purified waste gas is obtained; then capturing CO2 in the purified waste gas through a carbon dioxide capturing device, mixing the CO2 with removed CO2, and supplementing green hydrogen to carry out synthetic reaction to obtain crude methanol; and rectifying the crude methanol obtained by the two routes to obtain the low-carbon methanol.

Description

Technical Field

[0001] This invention relates to the field of low-carbon methanol synthesis technology, and more specifically, to a method for producing low-carbon methanol using biochar coupled with carbon capture. Background Technology

[0002] With the increasing depletion of traditional fossil fuels and the escalation of environmental problems, biomass, as a renewable and environmentally friendly energy resource, has attracted widespread attention for its efficient conversion and utilization. Methanol is an important basic chemical raw material and clean fuel. Developing technologies for producing methanol from biomass is of great significance for alleviating dependence on fossil fuels and promoting the development of green chemical industry.

[0003] Existing biomass-to-methanol technologies suffer from problems such as low conversion efficiency, complex processes, and poor energy consumption control. For example, the connection between biomass pyrolysis and gasification is not smooth, resulting in limited raw material utilization; resources such as carbon dioxide in the exhaust gas are not fully recovered and utilized, causing waste and environmental burden; and the energy supply system is not stable enough, affecting the continuous operation of the overall process.

[0004] Therefore, there is an urgent need for an optimized biomass-to-methanol process to achieve efficient conversion, resource recycling, and stable energy supply. Summary of the Invention

[0005] The purpose of this invention is to overcome the aforementioned deficiencies in existing technologies and provide a method for producing low-carbon methanol using biochar coupled with carbon capture. Based on parallel biochar-to-methanol and carbon capture-to-methanol routes, different methanol production routes are selected for pyrolysis products, maximizing resource utilization and effectively controlling costs. This forms a flexible, green methanol production system, realizing the resource utilization of waste, reducing equipment maintenance costs, and generating clean, green methanol, thus achieving both economic and ecological benefits. Simultaneously, it fully utilizes CO2 separated from syngas, combining it with CO2 and hydrogen from the tail gas generated by the oxygen-enriched combustion of cracked gas and cracking liquid to produce methanol, achieving the recycling of carbon resources. This combines resource recycling and low-carbon emission reduction benefits, providing a more efficient and green technical solution for biomass-to-methanol production.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: A method for producing low-carbon methanol from biochar coupled with carbon capture, the method comprising a biochar-to-methanol route and a carbon capture-to-methanol route configured in parallel; specifically including the following steps: S1. The biomass is thermally pyrolyzed to obtain biochar, pyrolysis gas and pyrolysis liquid; wherein the biochar is processed using a biochar-to-methanol route, and the pyrolysis gas and the pyrolysis liquid are processed using a carbon capture-to-methanol route. S2. In the biochar-to-methanol route, the biochar is ground and then gasified with oxygen. The resulting crude syngas is purified once to remove ash and carbon black impurities. Then, the crude syngas after the first purification is purified a second time to remove H2S and CO2. The purified syngas is then used for synthesis reaction under the condition of hydrogen-to-carbon ratio of 2 to 2.2 to obtain crude methanol. S3. In the carbon capture methanol production route, the cracked gas and the cracked liquid are subjected to oxygen-enriched combustion. The heat generated is used to power the thermal cracking in step S1. The tail gas generated during the oxygen-enriched combustion process is purified to obtain clean waste gas. Subsequently, CO2 in the clean waste gas is captured by a carbon dioxide capture device. The captured CO2 is then mixed with the CO2 removed in step S2 through a desorption device. Green hydrogen is added to carry out the synthesis reaction under the condition of a hydrogen-to-carbon ratio of 3 to 3.5 to obtain crude methanol. S4. Distill the crude methanol obtained in step S2 and the crude methanol obtained in step S3 to obtain low-carbon methanol.

[0007] Optionally, step S2 further includes adjusting the ratio between hydrogen and carbon components using a water-gas shift reaction or an electrolysis-based green hydrogen supplementation method to form syngas with a hydrogen-to-carbon ratio of 2-2.2. When using the water-gas shift reaction, it specifically includes: adjusting the ratio between hydrogen and carbon components in the crude syngas after primary purification through a water-gas shift reaction, wherein the pressure of the water-gas shift reaction is 2-3 MPa and the temperature is 180-350°C, to form syngas with a hydrogen-to-carbon ratio of 2-2.2, which is then subjected to secondary purification and a synthesis reaction to obtain crude methanol. When using the electrolysis-based green hydrogen supplementation method, it specifically includes: adjusting the ratio between hydrogen and carbon components in the purified syngas obtained after secondary purification through an electrolysis-based green hydrogen supplementation method to form syngas with a hydrogen-to-carbon ratio of 2-2.2, which is then subjected to a synthesis reaction to obtain crude methanol.

[0008] Optionally, step S1 includes: crushing the biomass into particles of 2-6 mm and heating and drying it until the moisture content is 10-20%, and then thermally pyrolyzing the treated biomass under anaerobic conditions at a temperature of 350-650℃ and a heating rate of 10-100℃ / min to obtain biochar, pyrolysis gas and pyrolysis liquid.

[0009] Optionally, in step S1, the source of the biomass includes one or more agricultural and forestry wastes selected from rice husks, sawdust, rice straw, and corn stalks.

[0010] Optionally, in step S2, the gasification reaction of the biochar after grinding with oxygen specifically includes: grinding the biochar to obtain biochar powder with a particle size <100μm, and then gasifying the biochar powder with oxygen to obtain crude syngas.

[0011] Optionally, in step S2, the gasification reaction of the biochar after grinding with oxygen specifically includes: grinding the biochar to obtain biochar powder with a particle size <100μm, mixing the biochar powder and water at a mass-volume ratio of 1:1 to obtain a biochar slurry, and then gasifying the biochar slurry with oxygen to obtain crude syngas.

[0012] Optionally, in step S2, the grinding is ball milling; the rotation speed of the ball mill is 200~400 r / min, and the ball milling time is 20~60 min.

[0013] Optionally, in step S2, the pressure of the gasification reaction is 1.0~9.0 MPa, and the temperature of the gasification reaction is 800~1600℃; the crude synthesis gas includes two or more of CO, CO2, H2 and CH4; in step S2, the temperature of the primary purification treatment is 400℃; in step S2, the secondary purification treatment adopts a low-temperature methanol washing method; in step S2, the pressure of the synthesis reaction is 2.5~5 MPa, the temperature is 200~300℃, the space velocity is maintained at 3000~10000 ml / (g cat·h), and the methanol selectivity is >80%.

[0014] Optionally, in step S3, the collecting agent used in the carbon dioxide capture device includes at least one of ethanolamine (MEA), phase change solvent (PCS), and dimethyl carbonate (DMC); in step S3, the oxygen-enriched combustion temperature is 600~1300℃; in step S3, the purification treatment temperature is 30~400℃; in step S3, the capture temperature is 30~100℃, and the capture pressure is atmospheric pressure; in step S3, the synthesis reaction pressure is 3~10MPa, the temperature is 200~300℃, the space velocity is maintained at 3000~10000 ml / (g cat·h), and the methanol selectivity is >90%.

[0015] Optionally, the method uses off-grid power or grid power for energy supply; when using off-grid power, the unit methanol carbon intensity is less than 15g CO2eq / MJ; when using grid power, the unit methanol carbon intensity is less than 50g CO2eq / MJ.

[0016] Implementing the embodiments of the present invention will have the following beneficial effects: (1) Based on the parallel biochar methanol production route and carbon capture methanol production route, the present invention selects different methanol production routes for pyrolysis products, which has high flexibility and scalability, realizes the maximum utilization of resources and effective cost control, improves production efficiency and solves the energy consumption and emission problems in traditional processes.

[0017] (2) Achieving the dual goals of carbon emission reduction and carbon utilization: The biochar used in this invention comes from the pyrolysis or gasification of biomass and is a negative carbon material. In the process, since the production of methanol from biochar will produce excess CO2, this invention makes full use of the CO2 separated from the synthesis gas, and the CO2 and hydrogen produced from the tail gas of the oxygen-enriched combustion of the cracked gas and cracked liquid to produce methanol, thereby realizing the recycling of carbon resources and converting them into high-value-added chemicals. It has both resource recycling and low-carbon emission reduction benefits, providing a more efficient and green technical solution for the production of methanol from biomass, and realizing the integration of carbon capture-carbon conversion-carbon sequestration.

[0018] (3) Promote the green and low-carbon transformation of methanol: When coupled with renewable hydrogen (green hydrogen), the carbon emissions of methanol throughout its life cycle can be significantly reduced, approaching or even achieving negative emissions of methanol.

[0019] (4) Improved energy utilization efficiency: By-product gases (such as CO, H2, CH4) generated during biochar preparation can be used as energy supplements for biomass pyrolysis, which helps to achieve energy recovery and overall system efficiency optimization.

[0020] (5) Economic and industrial synergy benefits: Compared with traditional CCS, CCU products (methanol) have market value and can improve the economics of carbon capture and utilization. Methanol is an important chemical raw material and energy carrier, which can be coupled with industries such as coal chemical, renewable energy, and transportation fuel to promote the development of green industrial chains. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the process for producing low-carbon methanol using biochar coupled with carbon capture, as described in this invention. Detailed Implementation

[0022] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0023] This invention discloses a method for producing low-carbon methanol using biochar coupled with carbon capture. The method includes a biochar-to-methanol route and a carbon capture-to-methanol route set up in parallel; specifically, it includes the following steps: S1. The biomass is thermally pyrolyzed to obtain biochar, pyrolysis gas and pyrolysis liquid; wherein the biochar is processed using the biochar-to-methanol route, and the pyrolysis gas and pyrolysis liquid are processed using the carbon capture-to-methanol route.

[0024] In one specific embodiment, step S1 includes: crushing the biomass into particles of 2-6 mm and heating and drying it until the moisture content is 10-20%; then, under anaerobic conditions, thermally decomposing the treated biomass at a temperature of 350-650°C (preferably 380-450°C) and a heating rate of 10-100°C / min to obtain biochar, pyrolysis gas and pyrolysis liquid.

[0025] In one specific embodiment, in step S1, the source of biomass includes one or more agricultural and forestry wastes selected from rice husks, sawdust, rice straw, and corn straw.

[0026] S2. In the biochar-to-methanol route, the biochar is ground and then gasified with oxygen. The resulting crude syngas is purified to remove ash and carbon black impurities. Then, the crude syngas after the first purification is purified a second time to remove H2S and CO2. The purified syngas is then used for synthesis under the condition of a hydrogen-to-carbon ratio of 2 to 2.2. The reaction formula is shown in (I), and crude methanol is obtained.

[0027] CO + 2H₂ → CH₃OH (I) In one specific embodiment, step S2, where the biochar is ground and then gasified with oxygen, specifically includes: grinding the biochar to obtain biochar powder with a particle size <100μm, and then gasifying the biochar powder with oxygen to obtain crude syngas.

[0028] In one specific embodiment, step S2, the gasification reaction of biochar after grinding with oxygen specifically includes: grinding biochar to obtain biochar powder with a particle size <100μm, mixing biochar powder and water at a mass-volume ratio of 1:1 to obtain biochar slurry, and then gasifying the biochar slurry with oxygen to obtain crude syngas.

[0029] In one specific embodiment, in step S2, the grinding is ball milling; the ball milling speed is 200~400 r / min, and the ball milling time is 20~60 min.

[0030] In one specific embodiment, in step S2, the pressure of the gasification reaction is 1.0~9.0 MPa, and the temperature of the gasification reaction is 800~1600℃; the crude syngas includes two or more of CO, CO2, H2 and CH4.

[0031] In one specific embodiment, in step S2, the temperature of the first purification process is 400°C.

[0032] In one specific embodiment, step S2 further includes adjusting the ratio between hydrogen and carbon components by using a water-gas shift reaction or by using water electrolysis to supplement green hydrogen, so as to form syngas that meets the requirement of a hydrogen-to-carbon ratio of 2 to 2.2.

[0033] In one specific embodiment, when the water-gas shift reaction method is used, it specifically includes: adjusting the ratio between hydrogen and carbon components of the crude synthesis gas after primary purification through a water-gas shift reaction to form synthesis gas that meets the requirement of a hydrogen-carbon ratio of 2 to 2.2, and then sequentially undergoing secondary purification and synthesis reaction to obtain crude methanol.

[0034] In one specific embodiment, the water-gas shift reaction formula is shown in (II), the pressure of the water-gas shift reaction is 2~3MPa, the temperature of the water-gas shift reaction is 180~350℃, and the catalyst used is a Ni-based catalyst.

[0035] CO + H₂O → CO₂ + H₂ (II) In one specific embodiment, when the green hydrogen supplementation method using water electrolysis is adopted, it specifically includes: adjusting the ratio between hydrogen and carbon components of the purified synthesis gas obtained after secondary purification treatment by water electrolysis to form a synthesis gas that meets the requirement of a hydrogen-carbon ratio of 2 to 2.2, and then obtaining crude methanol through a synthesis reaction.

[0036] In one specific embodiment, in step S2, the secondary purification process employs a low-temperature methanol washing method.

[0037] In one specific embodiment, in step S2, the synthesis reaction pressure is 2.5~5MPa, the temperature is 200~300℃, the space velocity is maintained at 3000~10000 ml / (g cat·h), and the methanol selectivity is >80%.

[0038] In one specific embodiment, in step S2, the catalyst used in the synthesis reaction is a Cu-Zn-Al based catalyst.

[0039] In the S3 carbon capture methanol production route, the cracked gas and cracked liquid are subjected to oxygen-enriched combustion. The heat generated is used to power the thermal cracking in step S1. The tail gas generated during the oxygen-enriched combustion process is purified to remove impurities such as fly ash and water, resulting in clean exhaust gas. Subsequently, CO2 in the clean exhaust gas is captured by a carbon dioxide capture device. The captured CO2 is then mixed with the CO2 removed in step S2 through a desorption device, and green hydrogen is added to carry out the synthesis reaction under the condition of a hydrogen-to-carbon ratio of 3 to 3.5. The reaction formula is shown in (III), and crude methanol is obtained.

[0040] CO2 + 3H2 → CH3OH + H2O (III) In one specific embodiment, in step S3, the capturing agent used in the carbon dioxide capture device includes at least one of ethanolamine (MEA), phase change solvent (PCS), and dimethyl carbonate (DMC).

[0041] In one specific embodiment, in step S3, the temperature of oxygen-enriched combustion is 600~1300℃.

[0042] In one specific embodiment, in step S3, the purification process is carried out at a temperature of 30~400℃.

[0043] In one specific embodiment, in step S3, the temperature of the collection is 30~100℃, and the pressure of the collection is atmospheric pressure.

[0044] In one specific embodiment, in step S3, the synthesis reaction pressure is 3~10MPa, the temperature is 200~300℃, the space velocity is maintained at 3000~10000 ml / (g cat·h), and the methanol selectivity is >90%.

[0045] In one specific embodiment, in step S3, the catalyst used in the synthesis reaction is a Cu-Zn-Al based catalyst.

[0046] S4. Distill the crude methanol obtained in step S2 and the crude methanol obtained in step S3 to obtain low-carbon methanol.

[0047] In one specific embodiment, the method uses either off-grid power or grid power for energy supply; when using off-grid power, the unit methanol carbon intensity is less than 15 gCO2eq / MJ; when using grid power, the unit methanol carbon intensity is less than 50 gCO2eq / MJ.

[0048] The following are specific embodiments. Example 1 The method for producing low-carbon methanol using biochar coupled with carbon capture in this embodiment includes a biochar-to-methanol route and a carbon capture-to-methanol route set up in parallel; specifically, it includes the following steps: S1. Corn stalks are crushed into particles of 2-6 mm and dried until the moisture content is 15%. The treated corn stalks are then fed into a continuous pyrolysis furnace and thermally pyrolyzed at 400°C under anaerobic conditions at a heating rate of 25°C / min to obtain biochar, pyrolysis gas, and pyrolysis liquid. The biochar is processed using the biochar-to-methanol route, while the pyrolysis gas and pyrolysis liquid are processed using the carbon capture-to-methanol route.

[0049] In the S2 biochar-to-methanol route, biochar is ball-milled at 300 r / min for 35 min. The resulting biochar powder (<100 μm) is obtained by sieving. This powder is then gasified with oxygen at a pressure of 4.2 MPa and a temperature of 1400 °C to obtain crude syngas. This crude syngas contains two or more of CO, CO2, H2, and CH4. After cooling and purification at 400 °C to remove ash and carbon black impurities, a secondary purification process is performed using methanol as the removal agent and low-temperature methanol washing technology to remove H2S and CO2, yielding purified syngas. Electrolysis of water is used to supplement green hydrogen to adjust the hydrogen-to-carbon ratio in the purified syngas, forming a syngas with a hydrogen-to-carbon ratio of 2.05. The syngas is then subjected to a synthesis reaction, as shown in (I). The reaction pressure is 2.8 MPa, the temperature is 230 °C, and the space velocity is maintained at 8000 ml / (g). The catalyst used was a Cu-Zn-Al based catalyst, which yielded crude methanol.

[0050] CO + 2H₂ → CH₃OH (I) In the S3 carbon capture methanol production route, the cracked gas and cracked liquid are subjected to oxygen-enriched combustion at 900℃. The heat generated is used to power the thermal cracking in step S1. The tail gas generated during the oxygen-enriched combustion process is purified at 40℃ to remove impurities such as fly ash and water, resulting in clean waste gas with CO2 as the main component. Subsequently, CO2 in the clean waste gas is recovered by carbon dioxide capture. The capture agent used in the carbon dioxide capture device is ethanolamine, the operating temperature is 30℃, and the pressure is atmospheric pressure. The captured CO2 is mixed with the CO2 removed in step S2 through a desorption device, and then green hydrogen is added to carry out the synthesis reaction under the condition of hydrogen-carbon ratio of 3.05. The synthesis reaction pressure is 4MPa, the temperature is 247℃, and the space velocity is maintained at 8500 ml / (g cat·h). The catalyst used in the synthesis reaction is a Cu-Zn-Al based catalyst, and the reaction formula is shown in (Ⅲ), to obtain crude methanol.

[0051] CO2 + 3H2 → CH3OH + H2O (III) S4. Distill the crude methanol obtained in step S2 and the crude methanol obtained in step S3 to obtain low-carbon methanol.

[0052] The method in this embodiment uses green electricity, with a carbon emission intensity of 12.81 g CO2eq / MJ.

[0053] Example 2 The method for producing low-carbon methanol using biochar coupled with carbon capture in this embodiment includes a biochar-to-methanol route and a carbon capture-to-methanol route set up in parallel; specifically, it includes the following steps: S1. Corn stalks are crushed into particles of 2-6 mm and dried until the moisture content is 15%. The treated corn stalks are then fed into a continuous pyrolysis furnace and thermally pyrolyzed at 450°C under anaerobic conditions at a heating rate of 30°C / min to obtain biochar, pyrolysis gas, and pyrolysis liquid. The biochar is processed using the biochar-to-methanol route, while the pyrolysis gas and pyrolysis liquid are processed using the carbon capture-to-methanol route.

[0054] In the S2 biochar-to-methanol route, biochar is ball-milled at a speed of 300 r / min for 20 min. Biochar powder with a particle size <100 μm is obtained by sieving. The biochar powder is then mixed with water at a mass-to-volume ratio of 1:1 to prepare a biochar slurry. The biochar slurry is then gasified with oxygen at a pressure of 4 MPa and a temperature of 1300 °C to obtain crude syngas. The crude syngas contains two or more of CO, CO2, H2, and CH4. After cooling and purification at 400 °C to remove ash and carbon black impurities, the purified crude syngas undergoes a water-gas shift reaction at a pressure of 2 MPa and a temperature of 250 °C. A Ni-based catalyst is used, and the reaction formula is shown in (II) to form syngas with a hydrogen-to-carbon ratio of 2.05. Subsequently, methanol was used as the removal agent, and the synthesis gas was subjected to secondary purification treatment using low-temperature methanol washing technology to remove H2S and CO2. The purified synthesis gas was then used for the synthesis reaction, as shown in (Ⅰ). The synthesis reaction pressure was 2.5 MPa, the temperature was 240 °C, and the space velocity was maintained at 8000 ml / (g cat·h). The catalyst used was a Cu-Zn-Al based catalyst, and crude methanol was obtained.

[0055] CO + 2H₂ → CH₃OH (I) CO + H₂O → CO₂ + H₂ (II) In the S3 carbon capture methanol production route, the cracked gas and cracked liquid are subjected to oxygen-enriched combustion at 900℃. The heat generated is used to power the thermal cracking in step S1. The tail gas generated during the oxygen-enriched combustion process is purified at 40℃ to remove impurities such as fly ash and water, resulting in clean waste gas with CO2 as the main component. Subsequently, CO2 in the clean waste gas is recovered by carbon dioxide capture. The capture agent used in the carbon dioxide capture device is ethanolamine, the operating temperature is 30℃, and the pressure is atmospheric pressure. The captured CO2 is mixed with the CO2 removed in step S2 through a desorption device, and then green hydrogen is added to carry out the synthesis reaction under the condition of hydrogen-carbon ratio of 3.1. The synthesis reaction pressure is 4.5MPa, the temperature is 240℃, and the space velocity is maintained at 8500 ml / (g cat·h). The catalyst used in the synthesis reaction is a Cu-Zn-Al based catalyst, and the reaction formula is shown in (Ⅲ), to obtain crude methanol.

[0056] CO2 + 3H2 → CH3OH + H2O (III) S4. Distill the crude methanol obtained in step S2 and the crude methanol obtained in step S3 to obtain low-carbon methanol.

[0057] The method in this embodiment uses grid electricity, and its carbon emission intensity is 45.07 gCO2eq / MJ.

[0058] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for producing low-carbon methanol using biochar coupled with carbon capture, characterized in that, The method includes a parallel biochar-to-methanol route and a carbon capture-to-methanol route; specifically, it includes the following steps: S1. The biomass is thermally pyrolyzed to obtain biochar, pyrolysis gas and pyrolysis liquid; wherein the biochar is processed using a biochar-to-methanol route, and the pyrolysis gas and the pyrolysis liquid are processed using a carbon capture-to-methanol route. S2. In the biochar-to-methanol route, the biochar is ground and then gasified with oxygen. The resulting crude syngas is purified once to remove ash and carbon black impurities. Then, the crude syngas after the first purification is purified a second time to remove H2S and CO2. The purified syngas is then used for synthesis reaction under the condition of hydrogen-to-carbon ratio of 2 to 2.2 to obtain crude methanol. S3. In the carbon capture methanol production route, the cracked gas and the cracked liquid are subjected to oxygen-enriched combustion. The heat generated is used to power the thermal cracking in step S1. The tail gas generated during the oxygen-enriched combustion process is purified to obtain clean waste gas. Subsequently, CO2 in the clean waste gas is captured by a carbon dioxide capture device. The captured CO2 is then mixed with the CO2 removed in step S2 through a desorption device. Green hydrogen is added to carry out the synthesis reaction under the condition of a hydrogen-to-carbon ratio of 3 to 3.5 to obtain crude methanol. S4. Distill the crude methanol obtained in step S2 and the crude methanol obtained in step S3 to obtain low-carbon methanol.

2. The method for producing low-carbon methanol using biochar coupled with carbon capture according to claim 1, characterized in that, Step S2 also includes adjusting the ratio between hydrogen and carbon components by using a water-gas shift reaction or by using water electrolysis to supplement green hydrogen, so as to form syngas that meets the requirement of a hydrogen-to-carbon ratio of 2 to 2.

2. When the water-gas shift reaction method is used, the specific steps include: adjusting the ratio between hydrogen and carbon components by passing the crude syngas after primary purification through a water-gas shift reaction. The pressure of the water-gas shift reaction is 2-3 MPa, and the temperature of the water-gas shift reaction is 180-350°C, forming syngas that meets the requirement of a hydrogen-to-carbon ratio of 2-2.

2. Subsequently, the syngas undergoes secondary purification and synthesis reaction to obtain crude methanol. When the green hydrogen supplementation method using water electrolysis is adopted, the specific steps include: adjusting the ratio between hydrogen and carbon components of the purified synthesis gas obtained after secondary purification treatment by water electrolysis to form a synthesis gas that meets the requirement of a hydrogen-to-carbon ratio of 2 to 2.2, and then obtaining crude methanol through a synthesis reaction.

3. The method for producing low-carbon methanol via biochar coupled with carbon capture according to claim 1, characterized in that, Step S1 includes: crushing the biomass into particles of 2-6 mm and heating and drying it until the moisture content is 10-20%. Then, under anaerobic conditions, the treated biomass is thermally pyrolyzed at a temperature of 350-650℃ and a heating rate of 10-100℃ / min to obtain biochar, pyrolysis gas and pyrolysis liquid.

4. The method for producing low-carbon methanol via biochar coupled with carbon capture according to claim 1, characterized in that, In step S1, the source of the biomass includes one or more agricultural and forestry wastes selected from rice husks, sawdust, rice straw, and corn stalks.

5. The method for producing low-carbon methanol via biochar coupled with carbon capture according to claim 1, characterized in that, In step S2, the gasification reaction of the biochar after grinding with oxygen specifically includes: grinding the biochar to obtain biochar powder with a particle size of <100μm, and then gasifying the biochar powder with oxygen to obtain crude syngas.

6. The method for producing low-carbon methanol via biochar coupled with carbon capture according to claim 1, characterized in that, In step S2, the gasification reaction of the biochar after grinding with oxygen specifically includes: grinding the biochar to obtain biochar powder with a particle size of <100μm, mixing the biochar powder and water at a mass-volume ratio of 1:1 to obtain a biochar slurry, and then gasifying the biochar slurry with oxygen to obtain crude syngas.

7. The method for producing low-carbon methanol via biochar coupled with carbon capture according to claim 1, characterized in that, In step S2, the grinding is ball milling; the rotation speed of the ball mill is 200~400 r / min, and the ball milling time is 20~60 min.

8. The method for producing low-carbon methanol via biochar coupled with carbon capture according to claim 1, characterized in that, In step S2, the pressure of the gasification reaction is 1.0~9.0 MPa, and the temperature of the gasification reaction is 800~1600℃; the crude syngas includes two or more of CO, CO2, H2 and CH4. In step S2, the temperature of the first purification process is 400°C; In step S2, the secondary purification process employs a low-temperature methanol washing method; In step S2, the synthesis reaction is carried out at a pressure of 2.5-5 MPa, a temperature of 200-300°C, a space velocity of 3000-10000 ml / (g cat·h), and a methanol selectivity of >80%.

9. The method for producing low-carbon methanol via biochar coupled with carbon capture according to claim 1, characterized in that, In step S3, the capturing agent used in the carbon dioxide capture device includes at least one of ethanolamine MEA, phase change solvent PCS, and dimethyl carbonate DMC; In step S3, the temperature for oxygen-enriched combustion is 600~1300℃; In step S3, the purification process is carried out at a temperature of 30~400℃; In step S3, the temperature of the collection is 30~100℃, and the pressure of the collection is atmospheric pressure; In step S3, the pressure of the synthesis reaction is 3~10MPa, the temperature is 200~300℃, the space velocity is maintained at 3000~10000ml / (g cat·h), and the methanol selectivity is >90%.

10. The method for producing low-carbon methanol via biochar coupled with carbon capture according to claim 1, characterized in that, The method uses either off-grid or grid power for energy supply.

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