Method for preparing methanol based on cow dung and its application

Abstract

The application provides a method for preparing methanol based on cow dung and application thereof, and relates to the technical field of solid waste treatment, and comprises the following steps: modifying cow dung after mixing with a composite modifier to obtain modified cow dung; the composite modifier comprises one or more of phosphate compounds, alkaline compounds and inorganic fillers; the modified cow dung is mixed with a gasification catalyst, and catalytic gasification is carried out under a gasification agent atmosphere to obtain crude synthesis gas; the crude synthesis gas is purified, and then a water gas shift reaction is carried out under the action of a catalyst to adjust the hydrogen-carbon ratio; the synthesis gas after the hydrogen-carbon ratio is adjusted is subjected to a catalytic synthesis reaction under the action of a synthesis catalyst to obtain methanol. The application solves the problems of cow dung resource waste, poor adaptability of the biomass methanol preparation process, low gasification efficiency, easy deactivation of the catalyst and insufficient methanol yield in the prior art, and realizes high-value utilization of cow dung and efficient preparation of clean methanol.

Description

Technical Field

[0001] This invention relates to the field of solid waste treatment technology, and in particular to a method for preparing methanol based on cow dung and its application. Background Technology

[0002] Methanol, as an important clean fuel and chemical feedstock, has wide applications in energy substitution and chemical synthesis. Traditional methanol production mainly relies on fossil fuels such as coal and natural gas, which not only faces the pressure of resource depletion but also generates large amounts of greenhouse gases and pollutants.

[0003] Biomass energy, as a renewable energy source, boasts advantages such as abundant resources and environmental friendliness, making its conversion into methanol a research hotspot in recent years. Cow dung is a unique biomass resource in the Qinghai-Tibet Plateau and surrounding areas, with a massive annual production. Currently, most cow dung is directly discarded or simply incinerated, resulting in resource waste.

[0004] The existing biomass methanol production technology has the following defects: (1) Insufficient pretreatment of biomass raw materials makes it difficult to destroy the fiber structure, resulting in low subsequent gasification efficiency; (2) The purity of the synthesis gas (H2+CO) produced during the gasification process is low, and the impurities such as tar, sulfides, and nitrogen oxides contained therein will poison the synthesis catalyst and reduce the methanol yield; (3) The H2 / CO ratio in the synthesis gas produced by biomass gasification is generally low, which is difficult to meet the reaction requirements of methanol synthesis for a high hydrogen-to-carbon ratio, and the hydrogen-to-carbon ratio needs to be adjusted by water-gas shift.

[0005] Due to the characteristics of cow dung (high fiber content and ash containing minerals such as potassium and calcium), existing biomass-to-methanol technologies are not directly suitable, resulting in problems such as low gasification efficiency, rapid catalyst deactivation, and insufficient methanol yield. Therefore, developing a methanol production method that is adapted to the characteristics of cow dung raw materials, has high efficiency, and produces high-purity products has significant resource utilization value and environmental significance. Summary of the Invention

[0006] One of the objectives of this invention is to provide a method for preparing methanol based on cow dung, so as to at least solve one of the technical problems existing in the prior art.

[0007] The second objective of this invention is to provide a method for preparing methanol based on cow dung and its application in the preparation of clean liquid fuels.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, the present invention provides a method for preparing methanol based on cow dung, comprising the following steps: (a) Modified cow manure is obtained by mixing cow manure with a composite modifier and then subjecting the mixture to modification treatment; the composite modifier includes one or more of phosphate compounds, alkaline compounds and inorganic fillers. (b) The modified cow dung is mixed with a gasification catalyst and catalytically gasified in a gasification atmosphere to obtain crude syngas; (c) The crude syngas is purified and then subjected to a water-gas shift reaction under the action of a catalyst to adjust the hydrogen-to-carbon ratio; (d) The syngas with the adjusted hydrogen-to-carbon ratio is subjected to a catalytic synthesis reaction under the action of a synthesis catalyst to obtain methanol.

[0009] Furthermore, in step (a), the cow dung is screened and dried before the modification treatment; Preferably, the moisture content of the cow manure after screening and drying is ≤10%, and the particle size is 20-40 mesh; the temperature of the screening and drying process is 60-80℃.

[0010] Furthermore, in step (a), the mass ratio of the cow dung to the composite modifier is 100:5-8; Preferably, the temperature of the modification treatment is 120-150℃, the pressure of the modification treatment is 0.3-0.5MPa, and the time of the modification treatment is 2-3h; Preferably, the mass ratio of the phosphate compound, the basic compound, and the inorganic filler is 2-4:1-3:1; Preferably, the phosphate compound includes one or more of potassium dihydrogen phosphate, sodium dihydrogen phosphate, and ammonium dihydrogen phosphate; the alkaline compound includes one or two of calcium hydroxide and magnesium hydroxide; and the inorganic filler is one or more of silica, diatomaceous earth, bentonite, kaolin, and zeolite powder.

[0011] Furthermore, in step (b), the gasification catalyst includes a supported composite catalyst; the supported composite catalyst uses γ-Al2O3 as a support and is loaded with nickel components, CeO2 and La2O3; Preferably, the nickel component accounts for 5-8 wt% of the total mass of the supported composite catalyst, the CeO2 accounts for 2-3 wt% of the total mass of the supported composite catalyst, and the La2O3 accounts for 1-2 wt% of the total mass of the supported composite catalyst. Furthermore, in step (b), the mass ratio of the modified cow dung to the gasification catalyst is 100:3-5; the mass ratio of the gasification agent to the modified cow dung is 1.5-2.0. Preferably, the vaporizing agent comprises one or both of water vapor and oxygen; the volume ratio of the water vapor to the oxygen is 1.5-3:1; Preferably, the temperature of the catalytic gasification is 850-950℃ and the pressure is 0.8-1.2MPa.

[0012] Furthermore, in step (c), the purification process includes sequential solid-gas separation, tar removal, and acid gas absorption; wherein the tar removal uses a ceramic membrane with a filtration accuracy of ≤0.5μm, and the acid gas absorption process uses a methanol washing process. Preferably, in step (c), the catalyst used includes a Cu-Zn-Al-O composite oxide catalyst; Preferably, in step (c), the conversion reaction temperature is 200-250°C and the pressure is 1.0-1.5 MPa; Preferably, in step (c), the molar ratio of H2 to CO in the syngas is adjusted to 2.0-2.1 by the water-gas shift reaction.

[0013] Further, in step (d), the synthesis catalyst comprises a ZnO-Cr2O3-Al2O3-based catalyst and optional auxiliary agents; the auxiliary agents comprise ZrO2; and the content of the auxiliary agents in the synthesis catalyst is 0.5-1 wt%.

[0014] Preferably, the catalytic synthesis reaction is carried out at a temperature of 220-260°C, a pressure of 5.0-8.0 MPa, and a space velocity of 5000-8000 h⁻¹. -1 .

[0015] Furthermore, step (d) also includes compressing the unreacted gas after the synthesis reaction and returning it to the synthesis reactor for recycling, with a recycling ratio of 3-5:1.

[0016] Furthermore, after step (d), the method further includes multi-tower distillation purification of the obtained methanol, wherein the multi-tower distillation purification includes pre-distillation column treatment, pressurized distillation column treatment and atmospheric pressure distillation column treatment; Preferably, the top temperature of the pre-distillation column is 65-70°C; Preferably, the top temperature of the pressurized distillation column is 120-130°C; Preferably, the top temperature of the atmospheric distillation column is 64-65°C.

[0017] Secondly, the present invention provides an application of a method for preparing methanol based on cow dung in the preparation of clean liquid fuels.

[0018] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a method for preparing methanol from cow dung. By pretreating cow dung with a composite modifier, its high-fiber, recalcitrant physical properties are effectively improved, enhancing the thermochemical reactivity of the raw material and solving the problem of low efficiency in direct cow dung gasification. Then, catalytic gasification generates syngas, promoting the efficient conversion of carbon elements into syngas precursors and improving the yield and quality of crude syngas. Next, purification and water-gas shift processes are combined to adjust the hydrogen-to-carbon ratio. After removing impurities such as dust and tar through purification, the hydrogen-to-carbon ratio is further adjusted via water-gas shift reaction, overcoming the technical obstacle of insufficient hydrogen in biomass-derived syngas to meet subsequent synthesis requirements. Finally, the adjusted syngas is efficiently converted to methanol under the action of a synthesis catalyst. These steps are sequentially linked, from initial raw material activation and gasification upgrading to gas composition control and final synthesis into methanol, realizing methanol production from plateau-specific biomass and improving the efficiency and added value of cow dung resource utilization. Detailed Implementation

[0019] Unless otherwise defined herein, the scientific and technical terms used in conjunction with this invention shall have the meanings commonly understood by one of ordinary skill in the art. The meaning and scope of terms shall be clear; however, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or foreign definitions. In this application, unless otherwise stated, the use of "or" means "and / or". Furthermore, the use of the term "comprising" and other forms is non-limiting.

[0020] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The first aspect of this invention provides a method for preparing methanol based on cow dung, comprising the following steps: (a) Modified cow manure is obtained by mixing cow manure with a composite modifier and then modifying the mixture; the composite modifier includes phosphate compounds, alkaline compounds and inorganic fillers. (b) The modified cow dung is mixed with a gasification catalyst and catalytically gasified in a gasification atmosphere to obtain crude syngas; (c) The crude syngas is purified and then subjected to a water-gas shift reaction under the action of a catalyst to adjust the hydrogen-to-carbon ratio; (d) The syngas with the adjusted hydrogen-to-carbon ratio is subjected to a catalytic synthesis reaction under the action of a synthesis catalyst to obtain methanol.

[0022] This invention effectively solves the problems of waste of cow manure resources, poor adaptability of biomass to methanol process, low gasification efficiency, easy deactivation of catalyst, and insufficient methanol yield in the prior art, and realizes the high-value utilization of cow manure and the efficient preparation of clean methanol.

[0023] This invention addresses the characteristics of cow dung by designing a composite modifier consisting of phosphate compounds, alkaline compounds, and inorganic fillers. High-temperature and high-pressure pretreatment disrupts the fiber structure of the cow dung while optimizing ash properties, thus solving the problem of low gasification activity. A Ni-CeO2-La2O3 / γ-Al2O3 composite gasification catalyst is constructed to synergistically promote cow dung gasification and tar cracking, improving syngas purity. An integrated process of "pretreatment-catalytic gasification-purification and proportioning-methanol synthesis-distillation" is employed to achieve efficient conversion of cow dung into high-purity methanol, demonstrating strong process adaptability.

[0024] This invention achieves the following through pretreatment with a composite modifier: mild hydrolysis of cellulose and hemicellulose, resulting in a loose structure, increased porosity, and improved reactivity; formation of a high-temperature stable phase from potassium, phosphate, and silicon components, increasing the ash melting point, eliminating the risk of slagging, and converting it into a tar cracking co-catalyst; introduction of highly dispersed calcium hydroxide for in-situ desulfurization, acid neutralization, increased ash melting point, and catalytic tar conversion; and combined with Ni-CeO2-La2O3 / γ-Al2O3 catalytic gasification, achieving deep tar cracking, efficient carbon conversion, and a significant improvement in syngas quality. Ultimately, it eliminates the adverse effects of high fiber, high potassium, and high calcium content in cow dung, achieving efficient raw material matching.

[0025] This invention overcomes the compatibility bottleneck of existing biomass-to-methanol technology with cow manure. Through the synergistic design of composite modification and a dedicated catalyst, the yield of cow manure to methanol is increased to 180-200 kg / ton, far exceeding the existing technology level. It innovatively employs a partial conversion reaction to precisely adjust the H2 / CO ratio of the syngas, combined with a three-stage purification process, avoiding catalyst poisoning and extending the catalyst lifespan to over 8000 hours. This solves the key problems of rapid catalyst deactivation and high cost in biomass-to-methanol production. It achieves a combination of waste resource utilization and clean energy production, with low energy loss and significant environmental benefits, providing a new path for the high-value utilization of cow manure and possessing important practical value.

[0026] Furthermore, this invention utilizes a coupled process of catalytic gasification, high-temperature ceramic membrane dust and tar removal, low-temperature methanol washing for deep desulfurization and decarbonization, and precise water-gas conversion ratio adjustment, which eliminates the need for separate H2 / CO / CO2 separation devices and allows for the direct acquisition of syngas components that meet the requirements of methanol synthesis.

[0027] In some preferred embodiments, in step (a), the cow dung is screened and dried prior to the modification treatment; Preferably, the moisture content of the cow manure after screening and drying is ≤10%, and the particle size is 20-40 mesh; the temperature of the screening and drying process is 60-80℃, for example, 60℃, 65℃, 70℃, 75℃, 80℃, etc.

[0028] In some preferred embodiments, in step (a), the mass ratio of the cow dung to the composite modifier is 100:5-8, for example, it can be 100:5, 100:6, 100:7, 100:8, etc. Preferably, the temperature of the modification treatment is 120-150℃, for example, 120℃, 130℃, 140℃, 150℃, etc.; the pressure of the modification treatment is 0.3-0.5MPa, for example, 0.3MPa, 0.4MPa, 0.5MPa, etc.; and the time of the modification treatment is 2-3h, for example, 2h, 2.5h, 3h. Preferably, the mass ratio of the phosphate compound, the basic compound, and the inorganic filler is 2-4:1-3:1; wherein "2-4" can be, for example, 2, 3, and 4; wherein "1-3" can be, for example, 1, 2, and 3; preferably, the phosphate compound includes one or more of potassium dihydrogen phosphate, sodium dihydrogen phosphate, and ammonium dihydrogen phosphate; the basic compound includes one or two of calcium hydroxide and magnesium hydroxide; and the inorganic filler is one or more of silica, diatomaceous earth, bentonite, kaolin, and zeolite powder. The composite modifier is preferably a combination of potassium dihydrogen phosphate, calcium hydroxide, and silica.

[0029] Specifically, this invention addresses the issues of high fiber content and unique ash characteristics in cow dung by employing a composite modifier pretreatment process. Potassium dihydrogen phosphate catalyzes the degradation of cellulose and hemicellulose, calcium hydroxide neutralizes the acidity of the raw materials and lowers the ash melting point, and silica optimizes the pore structure of the raw materials. The synergistic effect of these three factors fully destroys the structure of cow dung fibers, significantly enhances the gasification reaction activity, and improves gasification efficiency.

[0030] Further explanation: (1) Regarding potassium dihydrogen phosphate: Potassium dihydrogen phosphate can provide phosphate ions to catalyze the hydrolysis of glycosidic bonds in cellulose and hemicellulose in cow dung. At the same time, it introduces potassium ions as a co-catalyst for the gasification reaction. Its acidity is weak, so it will not excessively destroy the carbon structure of the raw materials, and it has strong thermal stability. Potassium dihydrogen phosphate adopts a weakly acidic system, which only catalyzes the hydrolysis of glycosidic bonds in cellulose and hemicellulose in cow dung, destroys the dense lignocellulose structure, and improves the porosity and reactivity of the raw materials. At the same time, it avoids excessive oxidation and degradation of the carbon skeleton of the raw materials, and retains the complete and loose porous carbon structure, so that the gas-solid contact is more sufficient, the carbon conversion is more complete, and the fly ash loss is lower during the subsequent gasification process, thereby improving the syngas yield and the final methanol yield.

[0031] (2) Calcium hydroxide can neutralize the organic acids produced by the decomposition of lignin in cow manure, reduce the acidity of the raw material, and regulate the melting point of ash to prevent slagging in the gasifier; at the same time, calcium ions can catalyze the cracking of tar and are stable at high temperatures. Magnesium hydroxide has both alkaline neutralization and flame retardant effects, and can inhibit the local overheating and carbonization of cow manure during the modification process; the decomposition products MgO and H2O, H2O can participate in the hydrolysis reaction of fibers, and synergistically promote the modification of raw materials. Adding calcium hydroxide can significantly increase the melting temperature of ash, so that the ash can maintain the solid particle form under gasification conditions, avoid coking, slagging and bed loss, ensure the stable operation of the fluidized bed, and facilitate solid slag discharge. Calcium hydroxide is weakly alkaline and only loosens fibers without eroding carbon. It can gently break the hydrogen bond connection between cellulose and hemicellulose in cow manure, promote the loosening and opening of the fiber structure, and increase the specific surface area and gasification reaction activity of the raw material; its effect is only on loosening the physical structure of lignocellulose, without oxidizing or degrading the carbon skeleton, and can retain the complete porous carbon structure, avoiding excessive decomposition of raw materials and carbon loss.

[0032] (3) Silica can serve as an inert framework to support cow dung particles and prevent raw material agglomeration during the modification process; it forms a porous SiO2 framework during the high-temperature gasification stage, improving the permeability of the raw materials and the specific surface area of ​​the reaction; at the same time, it forms low-melting-point silicates with calcium and potassium ions in the ash, reducing slagging. Other types also meet the characteristics of inertness, high specific surface area, and good thermal stability.

[0033] It is understood that in this invention, the composite modifier functions as follows: it disrupts the dense structure of cellulose and hemicellulose in cow manure, lowers the pyrolysis temperature, and increases the gasification reaction activity; it adjusts the acidity and alkalinity of the raw materials, neutralizes the organic acids produced by pyrolysis, and reduces the risk of equipment corrosion; it increases the ash melting temperature, preventing slagging and coking during fluidized bed gasification; it provides a porous structure and dispersion framework, preventing raw material particle agglomeration and enhancing gas-solid contact efficiency; and it introduces catalytic components such as potassium and calcium to promote tar cracking in situ and increase syngas yield.

[0034] In some preferred embodiments, in step (b), the gasification catalyst includes a supported composite catalyst; the supported composite catalyst uses γ-Al2O3 as a support and is loaded with nickel components, CeO2 and La2O3.

[0035] Specifically, the present invention employs a supported Ni-CeO2-La2O3 / γ-Al2O3 composite gasification catalyst. The addition of CeO2 and La2O3 can inhibit the sintering of Ni particles and improve the stability of the catalyst, while promoting tar cracking and CO generation, thereby increasing the CO content and reducing the tar content in the crude syngas.

[0036] Further explanation reveals that the supported composite catalyst, using γ-Al₂O₃ as a support, possesses a high specific surface area and good structural stability, enabling highly dispersed active components. The nickel component, as the main active component, efficiently catalyzes the catalytic cracking of tar components in cow dung pyrolysis products, while simultaneously promoting the carbon-water vapor gasification reaction and increasing the syngas (H₂+CO) yield. CeO₂ exhibits excellent oxygen storage and release capabilities, regulating the redox atmosphere in the reaction zone, inhibiting carbon accumulation on the catalyst surface, and suppressing high-temperature sintering of the nickel active component. La₂O₃ enhances the interaction between the metal and the support, further improving the catalyst's thermal stability and resistance to deactivation. The synergistic effect of these components results in a catalyst exhibiting high activity, high stability, and high tar cracking efficiency in cow dung catalytic gasification.

[0037] Preferably, the nickel component accounts for 5-8 wt% of the total mass of the supported composite catalyst, based on Ni element, for example, 5 wt%, 6 wt%, 7 wt%, 8 wt%, etc.; CeO2 accounts for 2-3 wt% of the total mass of the supported composite catalyst, for example, 2 wt%, 2.5 wt%, 3 wt%, etc.; and La2O3 accounts for 1-2 wt% of the total mass of the supported composite catalyst, for example, 1 wt%, 1.5 wt%, 2 wt%, etc. In some preferred embodiments, in step (b), the mass ratio of the modified cow dung to the gasification catalyst is 100:3-5, for example, it can be 100:3, 100:4, 100:5, etc.; the mass ratio of the gasification agent to the modified cow dung is 1.5-2.0, for example, it can be 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, etc. Preferably, the vaporizing agent includes one or both of water vapor and oxygen; the volume ratio of the water vapor to the oxygen is 1.5-3:1, for example, it can be 1.5:1, 2:1, 3:1, etc. Preferably, the temperature of the catalytic gasification is 850-950℃, for example, 850℃, 900℃, 950℃, etc., and the pressure is 0.8-1.2 MPa, for example, 0.8 MPa, 0.9 MPa, 1.0 MPa, 1.1 MPa, 1.2 MPa, etc.

[0038] Specifically, this invention employs a medium-pressure circulating fluidized bed gasifier, with the gasification temperature controlled at 850-950℃ and the operating pressure at 0.8-1.2 MPa. This bed type provides uniform temperature distribution and sufficient gas-solid contact, with a solid material residence time of 20-60 min and a gas residence time of 1.0-3.0 s. This ensures both complete pyrolysis of cow dung and complete carbon conversion, while also enabling deep cracking of tar into CO and H2 under the action of a nickel-based composite catalyst, effectively improving syngas yield and quality. Simultaneously, the medium-pressure operation reduces subsequent syngas compression energy consumption and provides better pressure matching with the methanol synthesis section.

[0039] In some preferred embodiments, step (c) includes sequential solid-gas separation (preferably cyclone separation), tar removal, and acid gas absorption; wherein the tar removal uses a ceramic membrane with a filtration accuracy of ≤0.5μm, and the acid gas absorption uses a methanol washing process.

[0040] Specifically, this invention achieves deep removal of impurities such as dust, tar, and sulfides through a three-stage purification process of "cyclone separation + ceramic filtration + low-temperature methanol washing," avoiding poisoning of the synthesis catalyst. Combined with a partial conversion reaction, it precisely adjusts the H2 / CO ratio to the optimal value, solving the key problem of imbalance in the syngas ratio of biomass gasification, eliminating the need for additional H2 supplementation, and reducing process costs.

[0041] Preferably, in step (c), the catalyst used includes a Cu-Zn-Al-O composite oxide catalyst.

[0042] Specifically, Cu-Zn-Al-O composite oxide catalysts have advantages such as high activity at low temperatures, controllable conversion depth, strong resistance to impurities in biomass gas sources, good stability, and high selectivity. They can adjust the H2 / CO molar ratio of cow dung gasification syngas to the optimal range of 2.0-2.1 for methanol synthesis under mild conditions of 200-250℃ and 1.0-1.5MPa.

[0043] Preferably, in step (c), the conversion reaction temperature is 200-250℃, for example, it can be 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, etc., and the pressure is 1.0-1.5MPa, for example, it can be 1.0 MPa, 1.1 MPa, 1.2 MPa, 1.3 MPa, 1.4 MPa, 1.5 MPa, etc.; Preferably, in step (c), the molar ratio of H2 to CO in the syngas is adjusted to 2.0-2.1 by the water-gas shift reaction.

[0044] In some preferred embodiments, in step (d), the synthesis catalyst comprises a ZnO-Cr2O3-Al2O3-based catalyst and optional auxiliary agents; the auxiliary agents comprise ZrO2; and the content of the auxiliary agents in the synthesis catalyst is 0.5-1 wt%, for example, it can be 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, etc.

[0045] Specifically, the addition of ZrO2 as an additive to the synthesis catalyst of this invention can improve the catalyst's resistance to carbon deposition and thermal stability, and extend the catalyst's service life to more than 8000 hours; unreacted syngas is recycled to improve the raw material conversion rate, and the yield of cow dung to methanol can reach 180-200 kg / ton (based on dry cow dung), which is much higher than the existing biomass to methanol technology (120-150 kg / ton).

[0046] Furthermore, this synthesis catalyst exhibits excellent high-temperature resistance, operating stably at 220-260℃ and adapting to fluctuations in syngas operating conditions. ZrO2 significantly improves the catalyst's resistance to carbon deposition, inhibits surface carbon deposition, and extends its service life. It also demonstrates stronger tolerance to trace impurities such as sulfur, tar, and dust in biomass syngas, making it less susceptible to poisoning and deactivation. The catalyst has a stable structure and high mechanical strength, making it suitable for long-term continuous operation. It exhibits high methanol selectivity and produces fewer byproducts, which is beneficial for obtaining high-purity methanol products.

[0047] Preferably, the temperature of the catalytic synthesis reaction is 220-260℃, for example, 220℃, 230℃, 240℃, 250℃, 260℃, etc.; the pressure is 5.0-8.0 MPa, for example, 5.0 MPa, 6.0 MPa, 7.0 MPa, 8.0 MPa, etc.; and the space velocity is 5000-8000 h⁻¹. -1 .

[0048] In some preferred embodiments, step (d) further includes compressing the unreacted gas after the synthesis reaction and returning it to the synthesis reactor for recycling, with a recycling ratio of 3-5:1, such as 3:1, 4:1, 5:1, etc.

[0049] In some preferred embodiments, after step (d), the method further includes multi-tower distillation purification of the obtained methanol, the multi-tower distillation purification including pre-distillation column treatment, pressurized distillation column treatment and atmospheric distillation column treatment.

[0050] To further explain, this invention employs a three-tower distillation process: a pre-distillation tower, a pressurized distillation tower, and an atmospheric distillation tower. The pre-distillation tower removes light components and non-condensable gases, preventing impurity accumulation; the pressurized distillation tower increases the relative volatility of methanol, water, and ethanol under pressurized conditions, achieving efficient purification; and the atmospheric distillation tower further removes heavy components, tar, and higher alcohol impurities, ensuring product purity.

[0051] The advantages of pressurized distillation are: increasing the boiling point of methanol, allowing methanol vapor at the top of the column to be directly condensed through circulating cooling water, significantly reducing condensation energy consumption; increasing separation efficiency through pressurization, making it easier to achieve efficient separation of methanol from ethanol and water; achieving thermal integration of pressurized and atmospheric distillation columns, resulting in high energy utilization; and ensuring stable system operation and strong resistance to impurity interference, making it particularly suitable for the complex component conditions of cow dung-based crude methanol.

[0052] Preferably, the top temperature of the pre-distillation column is 65-70°C, for example, it can be 65°C, 66°C, 67°C, 68°C, 69°C, 70°C, etc. Preferably, the top temperature of the pressurized distillation column is 120-130°C, for example, 120°C, 125°C, 130°C, etc. Preferably, the top temperature of the atmospheric distillation column is 64-65°C, for example, 64°C, 64.5°C, 65°C, etc.

[0053] In a preferred embodiment of the present invention, the method for preparing methanol based on cow dung includes the following steps: 1. Raw material pretreatment (1) Screening and drying: Select fresh cow dung, remove impurities such as stones and weeds, dry it at 60-80℃ until the moisture content is ≤10%, and crush it to a particle size of 20-40 mesh to obtain cow dung powder; (2) Catalytic modification: The dry cow dung powder and the composite modifier are mixed evenly at a mass ratio of 100:5-8, and kept at 120-150℃ and 0.3-0.5MPa for 2-3 hours. After cooling to room temperature, the modified cow dung is obtained. The composite modifier is composed of potassium dihydrogen phosphate, calcium hydroxide and silicon dioxide at a mass ratio of 3:2:1.

[0054] 2. Catalytic gasification (1) The modified cow dung and the gasification catalyst are mixed evenly at a mass ratio of 100:3-5 and fed into a medium-pressure circulating fluidized bed gasifier; the gasification catalyst is a supported composite catalyst, with γ-Al2O3 as the support, and loaded with 5-8% Ni, 2-3% CeO2 and 1-2% La2O3 by mass fraction, and prepared by impregnation-calcination method; (2) A gasifying agent is introduced into the gasifier, and the gasification temperature is controlled at 850-950℃, the pressure is controlled at 0.8-1.2MPa, the mass ratio of gasifying agent to raw material is controlled at 1.5-2.0, and the gasification reaction time is controlled at 30-40min to obtain crude syngas; the gasifying agent is a mixture of water vapor and oxygen with a volume ratio of 2:1.

[0055] 3. Syngas purification and proportioning (1) The crude syngas is sequentially passed through a cyclone separator (to remove dust), a ceramic filter membrane (with a filtration accuracy of 0.1 μm to remove tar mist), and a low-temperature methanol scrubbing tower (to absorb sulfides and CO2) to obtain purified syngas; the tar content in the purified syngas is ≤5 mg / m³. 3 Sulfide content ≤ 0.1 ppm, CO2 content ≤ 0.5 vol% (2) The purified syngas is fed into the shift reactor. Under the action of the shift catalyst, the reaction temperature is controlled at 200-250℃ and the pressure at 1.0-1.5MPa to carry out a partial shift reaction: CO + H2O CO2 + H2, adjusting the H2 / CO ratio in the syngas to 2.0-2.1 (the H2 / CO ratio is adjusted by controlling the reaction temperature, reaction pressure, space velocity, and water vapor / CO molar ratio); the conversion catalyst is a Cu-Zn-Al-O composite oxide catalyst.

[0056] 4. Methanol Synthesis (1) The syngas after adjustment is preheated to 220-260℃ and fed into a fixed-bed synthesis reactor. Under the action of the synthesis catalyst, the reaction pressure is controlled at 5.0-8.0 MPa and the space velocity at 5000-8000 h⁻¹. -1 The methanol synthesis reaction proceeds as follows: CO + 2H₂ CH3OH, CO2 + 3H2 CH3OH + H2O; the synthesis catalyst is a ZnO-Cr2O3-Al2O3 composite catalyst, and 0.5-1% ZrO2 by mass is added as an auxiliary agent; (2) The reaction products are cooled and condensed (temperature 0-10℃) and separated into gas and liquid to obtain crude methanol and unreacted synthesis gas; the unreacted synthesis gas is compressed and returned to the synthesis reactor for recycling, with a recycling ratio of 3-5:1.

[0057] 5. Refining of crude methanol Crude methanol is fed into a distillation column and a three-column distillation process is adopted: a pre-distillation column (top temperature 65-70℃, removing light components), a pressurized distillation column (top temperature 120-130℃, purifying methanol), and an atmospheric distillation column (top temperature 64-65℃, removing heavy components) to obtain a refined methanol product with a purity ≥99.9%.

[0058] A second aspect of this invention provides an application of a method for preparing methanol based on cow dung in the production of clean liquid fuels. This invention achieves high-value utilization of cow dung, converting waste into clean methanol and reducing environmental pollution; the integrated process design results in low energy loss, high product yield and purity, and significant economic and environmental benefits.

[0059] The present invention will be further illustrated below by way of examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0060] Example 1 This embodiment provides a method for preparing methanol based on cow dung, the specific process of which is as follows: Step 1: Raw material pretreatment Fresh cow dung was selected, impurities were removed, and it was dried at 70℃ to a moisture content of 8%, then pulverized to 30 mesh to obtain cow dung powder. 100 kg of cow dung powder was mixed evenly with 6 kg of composite modifier (3 kg of potassium dihydrogen phosphate, 2 kg of calcium hydroxide, and 1 kg of silicon dioxide), and kept at 130℃ and 0.4 MPa for 2.5 h. After cooling, modified cow dung was obtained.

[0061] Step 2, Catalytic gasification Preparation of gasification catalyst: Select catalysts with a specific surface area of ​​150–200 m² 2 Using γ-Al2O3 particles as a support, an equal-volume impregnation method was employed. A mixed solution of nickel nitrate hexahydrate (Ni(NO3)2·6H2O), cerium nitrate hexahydrate (Ce(NO3)3·6H2O), and lanthanum nitrate hexahydrate (La(NO3)3·6H2O) was used for the first impregnation. The catalyst was then loaded with 6% Ni, 2.5% CeO2, and 1.5% La2O3, and calcined at 550℃ for 4 hours to obtain a supported composite catalyst. 100 kg of modified cow dung was mixed with 4 kg of the gasification catalyst and fed into a circulating fluidized bed gasifier. A water-steam-oxygen mixed gasifying agent with a volume ratio of 2:1 was introduced. The gasification temperature was controlled at 900℃, the pressure at 1.0 MPa, and the mass ratio of gasifying agent to raw material at 1.8. The reaction was carried out for 35 minutes to obtain crude syngas.

[0062] Step 3: Syngas purification and proportioning The crude syngas is subjected to dust removal by a cyclone separator, tar mist removal by a ceramic filter membrane, and sulfide and CO2 removal by a low-temperature methanol scrubbing tower to obtain purified syngas (tar content 3 mg / m³). 3 The purified syngas (containing 0.08 ppm sulfide and 0.3 vol%) is fed into a shift reactor. Under the action of a Cu-Zn-Al-O catalyst, the temperature is controlled at 220℃ and the pressure at 1.2 MPa to carry out a partial shift reaction, adjusting the H2 / CO ratio to 2.05.

[0063] The preparation process of the Cu-Zn-Al-O catalyst is as follows: (1) Preparation of ingredients and mixed salt solution To prepare 100g of Cu-Zn-Al-O catalyst, 162g of copper nitrate, 78g of zinc nitrate, and 37g of aluminum nitrate were weighed. The above metal salts and 0.5g of PEG-6000 were added to 500mL of deionized water and placed in a 60℃ water bath. The mixture was stirred at 400r / min until completely dissolved, yielding a mixed salt solution with a total metal ion concentration of 0.6mol / L.

[0064] Prepare a 12% sodium carbonate precipitant solution for later use, and preheat the solution temperature to 60℃.

[0065] (2) Coprecipitation reaction A parallel-flow feeding method was employed to ensure simultaneous precipitation of metal ions and avoid component segregation. The mixed salt solution and sodium carbonate solution were simultaneously and uniformly added dropwise to a stirred reactor at a rate of 12 mL / min. During the reaction, the reaction temperature was maintained at 65℃, the stirring rate at 500 rpm, and the pH value of the system was monitored in real time. The pH was stabilized at 7.5 by adjusting the feeding rate. After the addition was complete, the reaction was stirred for another 45 minutes to ensure complete precipitation, forming a blue-green basic copper-zinc carbonate-aluminum hydroxide coprecipitate precursor.

[0066] (3) Aging treatment After the precipitation reaction is complete, stop stirring, maintain the reactor temperature at 65℃, and allow it to stand for 3 hours for aging. The purpose of aging is to allow the precursor particles to undergo crystal reconstruction, reduce lattice defects, and form precipitated particles with a larger specific surface area and a more stable structure, laying the foundation for subsequent calcination to form highly active composite oxides.

[0067] (4) Solid-liquid separation and washing Vacuum filtration (0.07 MPa) was used to separate the precipitated filter cake from the mother liquor. The filter cake was repeatedly washed with 65°C deionized water, with each wash using three times the mass of the filter cake, until the washing liquid was undetectable (detected by ion chromatography) to avoid residual sodium ions poisoning the active sites of the catalyst.

[0068] (5) Drying treatment The washed filter cake was spread evenly on a tray and placed in an oven. The heating program was set to increase the temperature from room temperature to 110°C at a rate of 5°C / h and hold for 10h to completely remove free water and water of crystallization from the filter cake, thus obtaining a blue-green catalyst precursor dry powder.

[0069] (6) Segmented roasting activation The precursor powder is transferred to a muffle furnace and calcined using a segmented heating process to avoid rapid high-temperature decomposition that could damage the structure. First stage: room temperature → 200℃, heating rate 10℃ / min, hold for 1 hour to remove residual organic dispersants and a small amount of carbonate decomposition products.

[0070] Second stage: 200℃→380℃, heating rate 5℃ / min, holding for 2.5h, the precursor undergoes a decomposition reaction to generate Cu-Zn-Al-O composite oxide: After calcination, the mixture was naturally cooled to room temperature to obtain a black Cu-Zn-Al-O composite oxide catalyst powder with a specific surface area of ​​90-110 m². 2 / g.

[0071] (7) Molding and Reduction Activation Molding: The catalyst powder was mixed evenly with 4% guar gum powder, and an appropriate amount of deionized water was added to knead it into a dough-like consistency. The resulting strip-shaped catalyst with a diameter of 3 mm was prepared by extrusion molding machine and dried at 110℃ for 5 hours.

[0072] Reduction and activation: The CuO in the catalyst needs to be reduced before use. The reduction conditions are as follows: Reducing gas: H2-N2 mixture (H2 volume fraction 8%); Temperature: 230℃, Pressure: 0.2MPa, Space velocity: 1500h -1 ; Reduction time: 5 hours, until the content in the exhaust gas stabilizes.

[0073] Step 4: Methanol Synthesis The adjusted synthesis gas was preheated to 240°C and fed into a fixed-bed synthesis reactor. Under the action of a ZnO-Cr2O3-Al2O3-ZrO2 catalyst (with 0.8% ZrO2 added as an additive), the pressure was controlled at 6.5 MPa and the space velocity at 6000 h⁻¹. -1 The reaction products are cooled and condensed, and then separated into gas and liquid. The unreacted syngas is recycled (recycle ratio 4:1) to obtain crude methanol.

[0074] The preparation method of 100g ZnO-Cr2O3-Al2O3-ZrO2 catalyst containing 0.8% ZrO2 is as follows: (1) Preparation of mixed salt solution Calculate the amount of each metal salt precursor according to the formula ratio: To prepare 100g of catalyst containing 0.8% ZrO2, weigh 2.1g of zirconium oxychloride (calculated according to the conversion relationship between oxides and salts), 155g of zinc nitrate, 82g of chromium nitrate, and 18g of aluminum nitrate.

[0075] The above metal salts were added to 500 mL of deionized water and placed in a 50°C water bath. The mixture was stirred at 300 r / min until completely dissolved, resulting in a mixed salt solution with a total metal ion concentration of 0.8 mol / L.

[0076] Prepare a 15% ammonium bicarbonate precipitant solution and preheat it to 50°C for later use.

[0077] (2) Coprecipitation reaction A parallel-flow feeding method was employed to ensure simultaneous precipitation of all components and avoid stratification: the mixed salt solution and ammonium bicarbonate solution were simultaneously and uniformly added dropwise to a stirred reactor at a rate of 10 mL / min. During the reaction, the following conditions were maintained: temperature 50-55℃, stirring speed 400 r / min, and system pH 7.0-7.5 (controlled by adjusting the feeding rate; excessively high pH can lead to premature precipitation of Al(OH)3, while excessively low pH can cause premature precipitation of Zn). 2+ (Incomplete precipitation). After the addition is complete, continue stirring for 60 minutes to allow the precipitate particles to grow fully and form a uniform basic carbonate-hydroxide coprecipitate precursor.

[0078] (3) Aging treatment After the precipitation reaction is complete, stirring is stopped, and the reactor temperature is maintained at 55°C for 4 hours of static aging. This aging process allows the precursor particles to undergo crystal remodeling, reducing lattice defects and promoting Zr... 4+ Uniform doping into the ZnO / Cr2O3 lattice lays the foundation for the formation of a stable composite structure during subsequent calcination.

[0079] (4) Solid-liquid separation and washing Plate and frame filter press (pressure 0.3-0.5 MPa) was used to separate the precipitated filter cake from the mother liquor. The filter cake was repeatedly washed with 50℃ deionized water, with each washing solution being 4 times the mass of the filter cake, until no white precipitate was detected in the washing solution (using AgNO3 solution, no white precipitate was found) and (using Nessler's reagent, no yellow precipitate was found), to avoid poisoning of the active centers by impurity ions.

[0080] (5) Drying treatment The washed filter cake was spread evenly on a tray and placed in an oven. A stepped heating drying program was set: room temperature → 80℃, heating rate 5℃ / h, holding for 4h; 80℃ → 120℃, heating rate 10℃ / h, holding for 10h; after drying, a loose blue-green precursor powder was obtained with a moisture content ≤1%.

[0081] (6) Segmented roasting activation Calcination is the core step in forming the active phase of the composite oxide. A segmented heating process is used to avoid rapid high-temperature decomposition that damages the pore structure: the precursor dry powder is transferred into a muffle furnace, and the temperature is programmed. First stage: room temperature → 250℃, heating rate 10℃ / min, hold for 1h, to remove residual bicarbonate and water of crystallization; Second stage: 250℃ → 450℃, heating rate 5℃ / min, hold for 2h, to decompose the precursor into metal oxides; Third stage: 450℃ → 600℃, heating rate 5℃ / min, hold for 3h, to promote the formation of solid solution structure between ZrO2 and ZnO-Cr2O3-Al2O3.

[0082] After calcination, the mixture was naturally cooled to room temperature to obtain a dark gray ZnO-Cr2O3-Al2O3-ZrO2 composite catalyst powder with a specific surface area of ​​60-80 m². 2 / g, average pore size 3-6nm.

[0083] (7) Forming and screening Mix the catalyst powder with 3% graphite powder evenly, add an appropriate amount of deionized water and knead into a dough-like consistency.

[0084] Catalyst strips with a diameter of 4 mm were prepared by extrusion molding and dried at 120℃ for 6 h. Catalyst particles with a length of 5-10 mm were obtained by sieving, with a compressive strength ≥20 N / mm, which meets the packing requirements of a fixed-bed reactor.

[0085] Step 5: Refining crude methanol A three-tower distillation process was adopted, with the top temperature of the pre-distillation tower at 68℃, the top temperature of the pressurized distillation tower at 125℃, and the top temperature of the atmospheric distillation tower at 64.5℃, to obtain refined methanol product.

[0086] Example 2 This embodiment provides a method for preparing methanol based on cow dung, which differs from Embodiment 1 in that: In step 1, the amount of composite modifier added is 5 kg (the mass ratio of potassium dihydrogen phosphate, calcium hydroxide, and silicon dioxide is still 3:2:1). In step 2, the amount of gasification catalyst added is 3 kg, and the gasification temperature is 850℃; In step 4, the synthesis pressure is 5.0 MPa.

[0087] Example 3 This embodiment provides a method for preparing methanol based on cow dung, which differs from Embodiment 1 in that: In step 1, the amount of composite modifier added is 8 kg (the mass ratio of potassium dihydrogen phosphate, calcium hydroxide, and silicon dioxide is still 3:2:1). In step 2, the amount of gasification catalyst added is 5 kg, and the gasification temperature is 950℃; In step 4, the synthesis pressure is 8.0 MPa.

[0088] Example 4 This embodiment provides a method for preparing methanol based on cow dung, which differs from Embodiment 1 in that: In step 1, the mass ratio of potassium dihydrogen phosphate, calcium hydroxide, and silicon dioxide in the composite modifier remains 2:3:1.

[0089] Example 5 This embodiment provides a method for preparing methanol based on cow dung, which differs from Embodiment 1 in that: In step 1, the mass ratio of potassium dihydrogen phosphate, calcium hydroxide, and silicon dioxide in the composite modifier remains 4:1:1.

[0090] Example 6 This embodiment provides a method for preparing methanol based on cow dung, which differs from Embodiment 1 in that: in step 4, no auxiliary agent ZrO2 is added.

[0091] Example 7 This embodiment provides a method for preparing methanol based on cow dung, which differs from Embodiment 1 in that: In step 1, the amount of composite modifier added is 4 kg (the mass ratio of potassium dihydrogen phosphate, calcium hydroxide, and silicon dioxide is still 3:2:1). In step 2, the amount of gasification catalyst added is 2 kg.

[0092] Example 8 This embodiment provides a method for preparing methanol based on cow dung, which differs from Embodiment 1 in that: In step 1, the amount of composite modifier added is 9 kg (the mass ratio of potassium dihydrogen phosphate, calcium hydroxide, and silicon dioxide is still 3:2:1). In step 2, the amount of gasification catalyst added is 6 kg.

[0093] Example 9 This embodiment provides a method for preparing methanol based on cow dung. The difference from Embodiment 1 is that in step 1, the composite modifier is a combination of calcium hydroxide and silicon dioxide (the mass ratio of calcium hydroxide to silicon dioxide is 1:1).

[0094] Example 10 This embodiment provides a method for preparing methanol based on cow dung. The difference from Embodiment 1 is that in step 2, the gasification catalyst is a single Ni / γ-Al2O3 catalyst, and the rest of the process is the same as in Embodiment 1.

[0095] Comparative Example 1 This comparative example provides a method for preparing methanol based on cow dung. The difference between this method and Example 1 is that the catalytic modification treatment in step 1 is not performed, while the rest of the process is the same as in Example 1.

[0096] Comparative Example 2 This comparative example provides a method for preparing methanol based on cow dung, which differs from Example 1 in that the synthesis gas is not partially converted and adjusted (H2 / CO=1.1).

[0097] Test case Test methods: The tar content in crude syngas was determined by quantitative analysis using gas chromatography-mass spectrometry (GC-MS) in accordance with the "Determination of Tar and Dust Content in Biomass Gas" (GB / T 40508-2021); the purity of refined methanol was determined by gas chromatography; and the yield of cow dung to methanol was obtained by calculation.

[0098] The test results are shown in Table 1.

[0099] Table 1

[0100] The data in Table 1 show that Examples 1-3 had tar content of 22-26 mg / m³. 3 The three indicators of methanol purity (99.91-99.95%) and methanol yield (183-198 kg / ton) are generally superior, demonstrating the synergistic effect of the optimized parameter combination on process performance. Although Examples 4-5 used boundary values ​​(such as a modifier component ratio of 2:3:1 or 4:1:1), the yield and purity remained at a high level (yield 180-182 kg / ton, purity ≥99.89%). The parameters of Examples 6-10 deviated from the preferred conditions of the present invention, and their performance decreased. In Example 10, the catalyst activity decreased by 50% after 3000 hours of use.

[0101] The performance of Comparative Example 1 (without modification) and Comparative Example 2 (without hydrogen-carbon ratio adjustment) decreased significantly, and the gasification efficiency of Comparative Example 1 decreased by 18%, and the catalyst life was shortened to 4500 h. This further proves the important role of the composite modification + synergistic catalysis + precise ratio adjustment of the present invention in achieving the efficient conversion of cow dung into high-purity methanol.

[0102] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing methanol based on cow dung, characterized in that, Includes the following steps: (a) Modified cow manure is obtained by mixing cow manure with a composite modifier and then subjecting the mixture to modification. The composite modifier includes one or more of phosphate compounds, basic compounds, and inorganic fillers; (b) The modified cow dung is mixed with a gasification catalyst and catalytically gasified in a gasification atmosphere to obtain crude syngas; (c) The crude syngas is purified and then subjected to a water-gas shift reaction under the action of a catalyst to adjust the hydrogen-to-carbon ratio; (d) The syngas with the adjusted hydrogen-to-carbon ratio is subjected to a catalytic synthesis reaction under the action of a synthesis catalyst to obtain methanol.

2. The method for preparing methanol based on cow dung according to claim 1, characterized in that, In step (a), the cow dung is screened and dried before modification treatment; Preferably, the moisture content of the cow manure after screening and drying is ≤10%, and the particle size is 20-40 mesh; the temperature of the screening and drying process is 60-80℃.

3. The method for preparing methanol based on cow dung according to claim 1, characterized in that, In step (a), the mass ratio of the cow dung to the composite modifier is 100:5-8; Preferably, the temperature of the modification treatment is 120-150℃, the pressure of the modification treatment is 0.3-0.5MPa, and the time of the modification treatment is 2-3h; Preferably, the mass ratio of the phosphate compound, the basic compound, and the inorganic filler is 2-4:1-3:1; Preferably, the phosphate compound includes one or more of potassium dihydrogen phosphate, sodium dihydrogen phosphate, and ammonium dihydrogen phosphate; the alkaline compound includes one or two of calcium hydroxide and magnesium hydroxide; and the inorganic filler is one or more of silica, diatomaceous earth, bentonite, kaolin, and zeolite powder.

4. The method for preparing methanol based on cow dung according to claim 1, characterized in that, In step (b), the gasification catalyst includes a supported composite catalyst; the supported composite catalyst uses γ-Al2O3 as a support and is loaded with nickel, CeO2 and La2O3; Preferably, the nickel component accounts for 5-8 wt% of the total mass of the supported composite catalyst, the CeO2 accounts for 2-3 wt% of the total mass of the supported composite catalyst, and the La2O3 accounts for 1-2 wt% of the total mass of the supported composite catalyst.

5. The method for preparing methanol based on cow dung according to claim 1, characterized in that, In step (b), the mass ratio of the modified cow dung to the gasification catalyst is 100:3-5; the mass ratio of the gasification agent to the modified cow dung is 1.5-2.

0. Preferably, the vaporizing agent comprises one or both of water vapor and oxygen; the volume ratio of the water vapor to the oxygen is 1.5-3:1; Preferably, the temperature of the catalytic gasification is 850-950℃ and the pressure is 0.8-1.2MPa.

6. The method for preparing methanol based on cow dung according to claim 1, characterized in that, In step (c), the purification process includes solid-gas separation, tar removal, and acid gas absorption treatment performed sequentially; wherein, the tar removal uses a ceramic membrane with a filtration accuracy of ≤0.5μm, and the acid gas absorption treatment uses a methanol washing process; Preferably, in step (c), the catalyst used includes a Cu-Zn-Al-O composite oxide catalyst; Preferably, in step (c), the conversion reaction temperature is 200-250°C and the pressure is 1.0-1.5 MPa; Preferably, in step (c), the molar ratio of H2 to CO in the syngas is adjusted to 2.0-2.1 by the water-gas shift reaction.

7. The method for preparing methanol based on cow dung according to claim 1, characterized in that, In step (d), the synthesis catalyst comprises a ZnO-Cr2O3-Al2O3-based catalyst and optional auxiliary agents; the auxiliary agents comprise ZrO2; and the content of the auxiliary agents in the synthesis catalyst is 0.5-1 wt%. Preferably, the catalytic synthesis reaction is carried out at a temperature of 220-260°C, a pressure of 5.0-8.0 MPa, and a space velocity of 5000-8000 h⁻¹. -1 .

8. The method for preparing methanol based on cow dung according to claim 1, characterized in that, Step (d) also includes compressing the unreacted gas after the synthesis reaction and returning it to the synthesis reactor for recycling, with a recycling ratio of 3-5:

1.

9. The method for preparing methanol based on cow dung according to claim 1, characterized in that, After step (d), the method further includes multi-tower distillation purification of the obtained methanol, wherein the multi-tower distillation purification includes pre-distillation column treatment, pressurized distillation column treatment and atmospheric distillation column treatment; Preferably, the top temperature of the pre-distillation column is 65-70°C; Preferably, the top temperature of the pressurized distillation column is 120-130°C; Preferably, the top temperature of the atmospheric distillation column is 64-65°C.

10. The application of the method for preparing methanol based on cow dung according to any one of claims 1-9 in the preparation of clean liquid fuels.

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