Method for producing liquid biological methane
By optimizing the inlet and discharge system of fermentation tanks, precise temperature control and multi-stage purification technology, combined with wet + dry desulfurization and low-temperature liquefaction processes, the problems of low efficiency, imperfect pollutant treatment and resource waste in traditional biogas fermentation processes have been solved, efficient biogas purification and liquid biomethane production have been achieved, and the sustainable development of clean energy has been promoted.
Patent Information
- Application Number
- CN202510558636.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-30
- Publication Date
- 2025-07-25
AI Technical Summary
The traditional biogas fermentation process has problems such as low fermentation efficiency, imperfect pollutant treatment, low utilization rate of by-products and difficult to scale, resulting in low gas production rate, equipment corrosion, degradation of combustion performance and waste of resources.
The optimized fermentation tank inlet and discharge system, precise temperature control, wet + dry desulfurization, CO2 low-temperature separation and multi-stage compression and refrigeration cycle technology are adopted, and the efficient biogas fermentation, multi-stage purification and low-temperature liquefaction technology are combined to achieve efficient purification and liquefaction of biogas.
It significantly improves the gas production rate and methane concentration of biogas, reduces the content of hydrogen sulfide and carbon dioxide, achieves efficient purification and energy-saving liquefaction, recovers high-purity CO2 and organic fertilizers, and builds a full-chain resource system to support the efficient, low-carbon and large-scale development of biomass energy.
Smart Images
Figure CN120366393A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of producing liquid biomethane, and particularly to a method for producing liquid biomethane. Background Art
[0002] Traditional biogas fermentation processes have encountered a series of complex and urgent challenges in practice, which have seriously hindered the further improvement of their production efficiency and economic benefits, and restricted their wide application in the field of renewable energy. The primary problem lies in the low fermentation efficiency. The uneven distribution of raw materials makes it difficult for microorganisms to comprehensively contact and effectively degrade all raw materials, thus seriously affecting the efficiency of biodegradation and gas production. At the same time, the low fermentation temperature is also an important reason for the low fermentation efficiency. The temperature difference further inhibits the activity of microorganisms, resulting in a significant decrease in the gas production rate and a methane content far lower than the ideal level. Secondly, the imperfect treatment of pollutants is also a major problem faced by traditional biogas fermentation processes. Biogas often contains high concentrations of hydrogen sulfide and carbon dioxide. These two gases not only reduce the calorific value and combustion efficiency of biogas, thus affecting its quality as a clean energy source, but also hydrogen sulfide has strong corrosiveness, which can damage equipment and pose safety hazards. Although traditional desulfurization and decarbonization technologies can reduce the content of these two gases to a certain extent, the effect is not ideal and it is difficult to meet the requirements of high-quality clean energy. In order to purify methane in biogas, methods such as multi-stage fine CO2 removal, such as amine solution deacidification, are often used, which have high regeneration energy consumption and high equipment costs, restricting the large-scale application of the process and making it difficult for traditional biogas fermentation processes to be promoted on a larger scale. Finally, the low utilization rate of by-products is also one of the problems that need to be solved urgently in traditional biogas fermentation processes. Biogas residues and biogas slurry, as by-products of the fermentation process, contain rich nutrients, but in traditional processes, they are often used in a rough manner, such as directly as farm fertilizers, etc. This low-value utilization method greatly limits the economic value of biogas residues and biogas slurry. At the same time, carbon dioxide in biogas is often directly discharged into the atmosphere without being effectively recovered and utilized, which not only causes waste of resources but also has a negative impact on the environment.
[0003] Traditional biogas processes (such as anaerobic digesters) have multiple technical bottlenecks: First, the fermentation efficiency is low. Limited by uneven raw material distribution and insufficient temperature control, the gas production rate is low (the methane content is usually 40% - 60%) and it is difficult to operate stably. Second, pollutant treatment is imperfect. High-concentration H2S (1000 - 3000 ppm) and CO2 (>30%) are not effectively removed, resulting in equipment corrosion and decreased combustion performance. Third, the utilization rate of by-products is low. Biogas residues and biogas slurry are mostly used as low-value fertilizers or landfilled, and CO2 is directly emitted or inefficiently utilized, failing to form a resource closed-loop. Fourth, it is difficult to scale up. The equipment is complex, the energy consumption is redundant, and the investment and operation and maintenance costs are high, making it difficult to meet commercial demands. Therefore, the traditional biogas process can no longer meet the development needs of modern biomass energy in terms of efficiency, environmental protection, and economy. Summary of the Invention
[0004] Based on the technical problems existing in the background art, the present invention proposes a method for producing liquid bio-methane.
[0005] A method for producing liquid bio-methane proposed by the present invention includes raw material fermentation, biogas purification, biogas dehydration, and cryogenic distillation.
[0006] Preferably, the raw material fermentation includes pretreating the raw materials, heating, and anaerobic fermentation to obtain biogas and sand-containing materials.
[0007] More preferably, the raw materials are organic solid wastes, and the organic solid wastes are selected from one or more of livestock and poultry manure and agricultural straws.
[0008] More preferably, the mass ratio of the livestock and poultry manure to the agricultural straw is 1:(1 - 2).
[0009] More preferably, the pretreatment includes one or more of impurity removal and crushing.
[0010] More preferably, the sand-containing materials include filtrate and filter residue.
[0011] More preferably, the sand-containing materials are transported to a solid-liquid separation tank, and the filtered biogas slurry is sent to a primary dehydrator. The solid content rate in the biogas slurry is reduced to 5% by means of screw pressing, and then it is transported to a plate and frame filter press for secondary dehydration to reduce the solid content rate of the biogas slurry to less than 1%. The filter residue is transported to a solid fertilizer processing unit by a belt conveyor. The filtrate from the plate and frame filter press is collected by a biogas slurry storage tank and divided into two paths. One part enters a flocculation sedimentation tank to remove heavy metal elements in the biogas slurry and kill pests and diseases, and the obtained clear liquid is transported to a liquid fertilizer processing unit. The other path is heated and then sent to a mixing tank to adjust the solid content rate of the raw materials and provide a suitable fermentation temperature.
[0012] More preferably, the heating temperature is 50 - 60°C.
[0013] More preferably, the anaerobic fermentation includes controlling the temperature of the fermentation tank to be maintained at 50 °C and fermenting for 25 days.
[0014] More preferably, the biogas includes methane, CO2, and H2S.
[0015] Preferably, the biogas purification includes subjecting the biogas to wet desulfurization and dry desulfurization to obtain purified biogas.
[0016] More preferably, the desulfurizing agent for the wet desulfurization is an iron ion chelating agent.
[0017] More preferably, the iron ion chelating agent is a compound with a cyclic structure formed by the coordination bonding of a central iron ion and a polydentate ligand.
[0018] More preferably, the polydentate ligand is selected from one or more of amino carboxylic acids and mercapto compounds.
[0019] More preferably, the molar ratio of the iron ion to the polydentate ligand is 1:(2 - 3), and the coordination number is 6.
[0020] More preferably, the desulfurizing agent for the dry desulfurization is a Fe2O3-ZnO mixture, where the molar ratio of Fe2O3 to ZnO is (1 - 2):(1 - 2).
[0021] More preferably, the biogas purification includes transporting the biogas to a desulfurization absorption tower. The rich desulfurized liquid flowing out from the bottom of the tower enters the oxidation regeneration tank to complete the regeneration step of the desulfurizing agent. Oxygen is continuously introduced into the bottom of the oxidation tank under the action of a blower. During this period, the chelated iron ions are reduced to obtain lean desulfurized liquid, and elemental sulfur precipitates are generated, namely sulfur slurry. The sulfur slurry precipitated at the bottom of the oxidation tank is sent to a sulfur slurry plate and frame filter press for solid-liquid separation to obtain filtrate and sulfur cake; the separated filtrate is returned to the oxidation tank, and the regenerated qualified lean desulfurized liquid is transported to the desulfurization absorption tower by a lean liquid pump to participate in the absorption of H2S gas, realizing the circulation of the desulfurized liquid in the system. During this period, the obtained sulfur cake is sent to a sulfur purification unit to complete the finishing process. The desulfurized biogas is transported from the top of the desulfurization absorption tower to a biogas condenser to condense out most of the water, and then enters a gas-liquid separator to separate the condensed water. The dehydrated biogas enters a dry desulfurization tower to further reduce the hydrogen sulfide content in the biogas to below 4 ppm to obtain purified biogas.
[0022] Preferably, the biogas dehydration includes first introducing the purified biogas into a biogas cold drying unit, cooling and dehydrating it, then entering a first-stage booster to boost the pressure to 0.5 - 1 Mpa, and then passing through a second-stage booster to boost the pressure to 4 - 5 Mpa. The boosted biogas enters the dehydration unit to obtain dehydrated biogas.
[0023] More preferably, the dehydration unit includes an adsorption tower, a hot blow tower, and a cold blow tower, with a three-tower circulation process.
[0024] More preferably, the packing material of the adsorption tower is 4A molecular sieve.
[0025] The packing material of the adsorption tower is 4A molecular sieve to ensure that the water molecule concentration in the coalbed methane is effectively reduced to below 1 ppm.
[0026] More preferably, the dried gas after dehydration treatment flows from the bottom of the adsorption tower to the liquefaction unit for subsequent liquefaction and separation process.
[0027] Preferably, the cryogenic rectification includes feeding the dehydrated biogas into the shallow cooling section of the large cold box for heat exchange and cooling to -65°C, then entering the high-pressure rectification tower. Gas methane is obtained at the top of the high-pressure rectification tower. The gas methane is depressurized to 3 - 4 Mpa through a throttle valve and enters the reflux absorption tower; gas methane is obtained at the top of the reflux absorption tower and continues to enter the deep cooling section of the large cold box for temperature reduction and is cooled to -162°C to complete the liquefaction process, obtaining liquid biomethane.
[0028] Preferably, after the cryogenic rectification, it further includes depressurizing and loading the obtained liquid biomethane; specifically, the liquid biomethane is depressurized to 0.1 - 0.5 Mpa through a throttle valve and sent to the LNG loading skid to complete the external transportation process.
[0029] More preferably, gas CO2 is obtained at the bottom of the high-pressure rectification tower. The gas CO2 returns to the shallow cooling section of the cold box for temperature reduction and is cooled to about -55°C to complete the liquefaction process, obtaining liquid CO2; the liquid CO2 passes through a throttle valve, and the pressure is reduced to 0.3 MPa and enters the liquid CO2 storage tank.
[0030] More preferably, the liquid CO2 obtained at the bottom of the reflux absorption tower converges with the biogas cooled to -65°C from the dehydration unit and enters the high-pressure rectification tower to complete the reflux process.
[0031] More preferably, the cooling capacity of the large cold box is mainly provided by the refrigerant compression refrigeration cycle.
[0032] More preferably, the refrigerant in the refrigerant compressor includes nitrogen, methane, propane, and isobutane, and the molar ratio is (0.05 - 0.2):0.2:0.35:0.35.
[0033] More preferably, the refrigerant compressor includes a low-pressure stage refrigerant compressor and a high-pressure stage refrigerant compressor.
[0034] More preferably, the refrigeration process includes passing the refrigerant successively through a low-pressure stage refrigerant compressor and a high-pressure stage refrigerant compressor, and then separating it through a refrigerant gas-liquid separator to obtain gaseous refrigerant and liquid refrigerant. The gaseous refrigerant and the liquid refrigerant are respectively flowed into the shallow cooling section area of the large cold box and precooled to -50°C. Subsequently, the liquid refrigerant is throttled through a J-T valve and enters a medium-temperature refrigerant separator. Here, it converges with the throttled gaseous refrigerant that has been reheated in the lower section of the cold box, and then enters the upper part of the cold box again. After being reheated to 37-38°C, it flows back to the inlet of the mixed refrigerant compressor. On the other hand, the precooled gaseous refrigerant continues to penetrate into the deep cooling section of the large cold box for deep cooling treatment to -162°C. After the throttling effect of the gaseous refrigerant, the temperature further decreases, and it enters a low-temperature refrigerant separator. Then, this part of the gaseous refrigerant flows back to the lower part of the cold box for reheating, and enters the upper part of the cold box again together with the throttled liquid refrigerant to complete the reheating process. At the same time, a refrigerant recovery tank is provided for the storage and preparation of the mixed refrigerant and to facilitate the replenishment of the losses of various refrigerants during the operation of the device. At the same time, the recovery device can be used to recover the refrigerant when the device stops to reduce the refrigerant emissions and save the refrigerant cost.
[0035] The beneficial effects of the present invention are as follows:
[0036] Liquefied Biomethane (LBM) is a clean energy source that removes impurities (such as hydrogen sulfide H2S, carbon dioxide CO2, etc.) in biogas (mainly composed of methane CH4) to an extremely low concentration and concentrates its methane concentration to more than 99.5% through deep purification and cryogenic liquefaction technologies. Its production process combines high-efficiency biogas fermentation, multi-stage purification, and cryogenic liquefaction technologies. The final product is liquid methane at -162°C, which has a high energy density and meets the natural gas standard, and can be widely used in transportation fuels, industrial raw materials, and distributed energy supply.
[0037] The process for producing liquid biomethane proposed in the present invention has the following advantages: 1. The feeding and discharging system of the fermentation tank is optimized, and the fermentation environment inside the fermentation tank is improved. This improvement ensures that the fermentation raw materials can enter the fermentation tank evenly and stably, and at the same time effectively controls the material flow during the fermentation process, avoiding problems such as low fermentation efficiency and unstable product quality caused by material accumulation or uneven distribution. 2. During the high-temperature fermentation stage of biogas, by precisely controlling the temperature of the fermentation broth within the optimal range of 50 - 60 °C, not only an ideal growth environment is provided for microorganisms, but also the decomposition rate of organic matter is greatly promoted. Under this suitable temperature condition, the metabolic activities of microorganisms are significantly enhanced, so that organic matter can be more effectively converted into biogas, significantly increasing the biogas production rate and shortening the retention time. 3. The wet desulfurization technology uses a liquid phase rich in catalysts to oxidize hydrogen sulfide in biogas into elemental sulfur, thereby effectively removing hydrogen sulfide from biogas. This technology has the advantages of large gas treatment capacity and high desulfurization efficiency, and is especially suitable for the treatment of biogas with high sulfur content. After wet desulfurization, dry desulfurization is used to further remove the residual hydrogen sulfide in biogas to ensure that the sulfur content of biogas is less than 10 ppm. 4. The low-temperature separation technology is used to separate carbon dioxide from biogas. By adopting a combination of a high-pressure rectification tower and a reflux absorption tower, not only the energy consumption is reduced, but also high-content CO2 can be effectively removed to less than 50 ppm, meeting the natural gas specification standards. At the same time, the obtained low-temperature liquid CO2 can be used for other purposes in industrial production, such as chemical raw materials, food preservation, and energy storage, etc., and can also be used as one of the important measures for greenhouse gas emission reduction. 5. The multi-stage compression refrigeration cycle technology is adopted. First, the raw gas methane is multi-stage compressed to 5 Mpa to meet the production conditions of liquid biomethane. At the same time, the methane-nitrogen-propane-isobutane mixed refrigerant is multi-stage compressed, which not only provides cooling capacity for the low-temperature rectification separation of CO2, but also reduces the liquefaction energy consumption of CO2 and biomethane.
[0038] By optimizing the feeding and discharging system of the fermentation tank and precisely controlling the temperature (50 - 60 °C) to improve the gas production efficiency, and combining wet + dry desulfurization, low-temperature separation of CO2 and multi-stage compression refrigeration cycle technology, the present invention realizes efficient purification and energy-saving liquefaction. At the same time, high-purity CO2 (industrial raw material / carbon sequestration) and high-value organic fertilizers (biogas residue / liquid fertilizer) are recovered, constructing a "raw material - energy - by-product" whole-chain resource utilization system, providing key technical support for the efficient, low-carbon and large-scale development of biomass energy.
[0039] The present invention combines technologies such as high-temperature anaerobic digestion of biogas, wet desulfurization and dry desulfurization, low-temperature separation technology of CO2, and multistage compression refrigeration cycle technology, etc., to achieve large-scale and efficient conversion of biogas into liquid biomethane. This innovative technological process not only significantly improves the utilization efficiency and economic benefits of biogas, but also opens up a new path and injects strong impetus for the sustainable development of clean energy. Brief Description of the Drawings
[0040] Figure 1 It is a flowchart of the method for producing liquid biomethane proposed by the present invention;
[0041] 1 - Homogenization tank, 2 - Crusher, 3 - Mixing tank, 4 - Fermentation tank, 5 - Solid-liquid separation tank, 6 - Primary dehydrator, 7 - Plate and frame filter press, 8 - Biogas slurry storage tank, 9 - Flocculation sedimentation tank, 10 - Liquid fertilizer processing unit, 11 - Solid fertilizer processing unit, 12 - Biogas holder, 13 - Desulfurization absorption tower, 14 - Oxidation tank, 15 - Sulfur slurry plate and frame filter press, 16 - Sulfur purification unit, 17 - Biogas condenser, 18 - Gas-liquid separator, 19 - Dry desulfurization tower, 20 - Biogas cold drying unit, 21 - Primary booster, 22 - Secondary booster, 23 - Dehydration unit, 24 - Shallow cooling section of large cold box, 25 - Deep cooling section of large cold box, 26 - High-pressure rectification tower, 27 - Reflux absorption tower, 28 - Liquid CO2 storage tank, 29 - LNG loading skid, 30 - Low-pressure refrigerant compressor, 31 - High-pressure refrigerant compressor, 32 - Refrigerant gas-liquid separator, 33 - Refrigerant recovery tank. Detailed Embodiments
[0042] The technical solutions of the present invention are described in detail through specific embodiments.
[0043] The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels without special instructions.
[0044] Embodiment
[0045] Raw material fermentation: Livestock and poultry manure is directly unloaded into the receiving tank of the homogenization tank 1 by a transport vehicle. A stirrer is installed on the upper part of the homogenization tank 1 to ensure that the manure in the homogenization tank 1 does not deposit and caking. A sand removal screw is provided at the bottom to remove the sand and gravel in the manure. The pretreated manure is then transported to the mixing tank 3. Wheat straw and corn straw are sent to the crusher 2 by a conveyor belt. A two-stage crushing process is adopted to keep the particle size of the materials at the outlet of the crusher 2 within the range of 3-5 cm. The crushed straw enters the mixing tank 3 by a belt conveyor. The straw and manure are fully mixed in the mixing tank 3 at a ratio of 1.5:1. At the same time, a heating system is set to make the temperature of the mixed straw manure 55°C and the solid content of the raw material above 15%. Subsequently, the straw manure enters the top of the fermentation tank 4 through vacuum transportation for anaerobic fermentation steps. A high-temperature anaerobic fermentation process is adopted to control the temperature of the fermentation tank 4 to be maintained at 50°C. According to the test, the optimal residence time for raw material fermentation is 25 days. The biogas components produced by fermentation mainly include methane, CO2 and H2S, which are collected and transported to the biogas holder 12 by a fan. The fermented substrate is pressed into the sand removal tank by gravity. The supernatant is then pumped back into the fermentation tank 4 by the fermentation tank circulation pump, and the sand-containing material is transported to the solid-liquid separation tank 5. The filtered biogas slurry is sent to the first-stage dehydrator 6, and the solid content in the biogas slurry is reduced to 5% by the spiral pressing method. Subsequently, it is transported into the plate and frame filter press 7 for secondary dehydration to reduce the solid content of the biogas slurry to less than 1%, obtaining filter residue and filtrate. The filter residue is transported to the solid fertilizer processing unit 11 by a belt conveyor; the filtrate is collected by the biogas slurry storage tank 8 and is divided into two paths. One part enters the flocculation sedimentation tank 9 to remove heavy metal elements in the biogas slurry and kill pests and diseases, and the obtained clear liquid is transported to the liquid fertilizer processing unit 10. The other path is heated and sent to the mixing tank to adjust the solid content of the raw material and provide a suitable fermentation temperature.
[0046] Biogas purification: The biogas produced after fermentation needs to be further refined and purified, using wet + dry desulfurization technology. It is transported from the gas holder to the desulfurization absorption tower 13. The wet desulfurization technology mainly uses an iron ion chelating agent as a catalyst. The iron ion chelating agent is a compound with a cyclic structure formed by the coordination bond of the central iron ion and amino carboxylic acid. The molar ratio of iron ion to amino carboxylic acid is 1:2, and it forms a bond with the adjacent Fe atom, and the coordination number is 6. The following overall reaction occurs in the solution: H2S + 2Fe 3+ →2H + + S + 2Fe 2+ ; The system adopts a unique chelating agent, so that the divalent iron and trivalent iron in the water-mixed solution can maintain a stable ionic state within a wide pH value range (2-9), preventing the formation of iron hydroxide or iron sulfide precipitation. The rich desulfurization liquid flowing out from the bottom of the tower and absorbing hydrogen sulfide enters the oxidation tank 14 to complete the regeneration step of the desulfurization liquid. Oxygen is continuously introduced into the bottom of the oxidation tank 14 under the action of a blower. During this period, the chelated iron ions are reduced to obtain sulfur slurry and poor desulfurization liquid. The following chemical reaction occurs: 1 / 2O2(g) + H2O + 2Fe2+ → 2OH - + 2Fe 3+ 。The sulfur slurry precipitated at the bottom of the oxidation tank 14 is sent to the sulfur slurry plate and frame filter press 15 for solid-liquid separation to obtain filtrate and sulfur cake. The separated filtrate is returned to the oxidation tank 14, and the regenerated qualified lean desulfurized liquid is transported by the lean liquid pump to the desulfurization absorption tower 13 to participate in the absorption of H2S gas, realizing the circulation of the desulfurized liquid in the system. The sulfur cake is sent to the sulfur purification unit 16 to complete the finishing process. The desulfurized biogas is transported from the top of the desulfurization absorption tower 13 to the biogas condenser 17 to condense out most of the water, and then enters the gas-liquid separator 18 to separate the condensed water. The dehydrated biogas enters the dry desulfurization tower 19. The desulfurizer for dry desulfurization is a mixture of Fe2O3-ZnO, and the molar ratio of Fe2O3 to ZnO is 1:1, further reducing the hydrogen sulfide content in the biogas to less than 4 ppm.
[0047] Dehydration and drying: The pressure of the purified biogas is about 20 kPa. First, it enters the biogas cold drying unit 20, is cooled and dehydrated, then enters the first-stage booster 21 to be boosted to 0.8 Mpa, and then passes through the second-stage booster 22 to be boosted to 5 Mpa. The boosted biogas enters the dehydration unit 23, adopting a three-tower circulation process. This process simultaneously performs three processes: adsorption, hot blow, and cold blow. The adsorption tower selects 4A molecular sieve as the filler to ensure that the water molecule concentration in the coalbed methane is effectively reduced to below 1 ppm. The dried gas after dehydration treatment flows from the bottom of the drying tower to the liquefaction unit for subsequent liquefaction and separation processes.
[0048] Cryogenic rectification: The raw gas of biogas (40 °C, 5 Mpa) from the dehydration unit 23 enters the shallow cooling section 24 of the large cold box to be heat-exchanged and cooled to -65 °C, and then enters the high-pressure rectification tower 26. Through the cryogenic rectification process of biogas, high-purity CO2 with a purity of 99.9% is obtained at the bottom of the high-pressure rectification tower 26, returns to the shallow cooling section of the cold box for cooling, and is cooled to about -55 °C to complete the liquefaction process. The liquid CO2 passes through a throttle valve, and the pressure is reduced to 0.3 MPa and enters the liquid CO2 storage tank 28. The product gas at the top of the high-pressure rectification tower 26 has an outlet temperature of -74 °C and contains 7% CO2. The pressure is reduced to 4 Mpa through a throttle valve and enters the reflux absorption tower 27. The product gas at the top of the reflux absorption tower 27 has an outlet temperature of -87 °C, and the CO2 concentration in the biomethane is only 10 ppm, meeting the national standard specifications. The purified biomethane continues to enter the deep cooling section 25 of the large cold box for cooling, and is cooled to about -162 °C to complete the liquefaction process. Then, the pressure is reduced to 0.2 Mpa through a throttle valve and sent to the LNG loading skid 29 to complete the external transportation process. The bottom of the reflux absorption tower 27 obtains a liquid phase composition with 8% CO2, which meets the biogas cooled to -65 °C from the dehydration unit 23 and enters the high-pressure rectification tower 26 to complete the reflux process.
[0049] The cooling capacity of the cold box is mainly provided by the refrigeration cycle of the refrigerant compressor. The main components of the refrigerant are a mixed refrigerant of nitrogen-methane-propane-isobutane. The molar ratio of nitrogen, methane, propane, and isobutane is 0.1:0.2:0.35:0.35. This process adopts two-stage compression. The mixed refrigerant first passes through the low-pressure stage refrigerant compressor 30 (boosted to 1.4 Mpa), the high-pressure stage refrigerant compressor 31 (boosted to 3.1 Mpa), and then is separated by the refrigerant gas-liquid separator 32. The gas-liquid two-phase mixed refrigerant from the refrigerant gas-liquid separator 32 flows into the shallow cooling section 24 area of the large cold box respectively and is precooled to -50°C. Subsequently, the liquid-phase refrigerant is throttled through the J-T valve and enters the medium-temperature refrigerant separator. Here, it converges with the throttled gas-phase refrigerant that has been reheated in the lower section of the cold box, and then enters the upper part of the cold box again. After reheating to 37-38°C, it returns to the inlet of the mixed refrigerant compressor. On the other hand, the precooled gas-phase refrigerant continues to enter the deep cooling section 25 of the large cold box for deep cooling treatment to -162°C. After the throttling effect of the gas-phase refrigerant, the temperature further decreases, and it enters the low-temperature refrigerant separator. Then, this part of the gas-phase refrigerant returns to the lower part of the cold box for reheating and enters the upper part of the cold box again together with the throttled liquid-phase refrigerant to complete the reheating process. At the same time, a refrigerant recovery tank 33 is set up for the storage and preparation of the mixed refrigerant and to facilitate the replenishment of the losses of various refrigerants during the operation of the device. At the same time, the recovery device can be used to recover the refrigerant when the device stops to reduce the refrigerant emission and save the refrigerant cost.
[0050] As described above, it is only the preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution of the present invention and its inventive concept, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.
Claims
1. A method for producing liquid biomethane, characterized in that, It includes raw material fermentation, biogas purification, biogas dehydration, and cryogenic rectification.
2. The method according to claim 1, characterized in that The raw material fermentation mentioned above includes pre-treating the raw materials, heating them, and anaerobically fermenting them to obtain biogas and sand-containing materials.
3. The method according to claim 2, characterized in that, The raw materials are organic solid wastes, and the organic solid wastes are selected from one or more of livestock and poultry manure and agricultural straws; the pre-treatment includes one or more of impurity removal and crushing; the sand-containing materials include filtrate and filter residue; the heating temperature is 50 - 60 °C.
4. The method according to claim 1, wherein The biogas purification mentioned above includes subjecting the biogas to wet desulfurization and dry desulfurization to obtain purified biogas.
5. The method according to claim 4, characterized in that The desulfurizer for the wet desulfurization is an iron ion chelating agent; the desulfurizer for the dry desulfurization is a Fe2O3-ZnO mixture, where the molar ratio of Fe2O3 to ZnO is (1 - 2):(1 - 2).
6. The method according to claim 1, wherein The biogas dehydration mentioned above includes first sending the purified biogas into a biogas cold dryer unit, cooling and dehydrating it, then sending it into a first-stage booster to boost the pressure to 0.5 - 1 Mpa, and then boosting the pressure to 4 - 5 Mpa through a second-stage booster. The boosted biogas enters the dehydration unit to obtain dehydrated biogas.
7. The method according to claim 6, wherein The dehydration unit includes an adsorption tower, a hot blow tower, and a cold blow tower. The packing of the adsorption tower is 4A molecular sieve.
8. The method according to claim 1, characterized in that, The cryogenic rectification mentioned above includes sending the dehydrated biogas into the shallow cooling section of a large cold box to exchange heat and cool it to -65 °C, then sending it into a high-pressure rectification tower. Gas methane is obtained at the top of the high-pressure rectification tower. The gas methane is depressurized to 3 - 4 Mpa through a throttle valve and enters a reflux absorption tower; gas methane is obtained at the top of the reflux absorption tower and continues to enter the deep cooling section of the large cold box for cooling and is cooled to -162 °C to complete the liquefaction process to obtain liquid biomethane.
9. The method according to claim 8, wherein The cooling capacity of the large cold box is provided by the refrigerant compression refrigeration cycle; the refrigerant compressor includes a low-pressure stage refrigerant compressor and a high-pressure stage refrigerant compressor.
10. The method according to claim 9, wherein The refrigerant in the refrigerant compressor includes nitrogen, methane, propane, and isobutane.