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Current research status at home and abroad and their advantages and disadvantages: Biogas purification technologies include washing method, solvent method, membrane separation method, pressure swing adsorption method, and cryogenic method. China’s biogas purification technology started relatively late, with the water washing method and pressure swing adsorption method being commonly used. With the progress made in membrane separation technologies in developed countries in Europe and America in recent years, China has also begun to develop in this area. Since the cryogenic method is used in large, medium, and small biogas projects, it involves high investment costs and extremely high energy consumption, which makes it unsuitable for widespread adoption; therefore, no further details will be provided on it. 1. Water washing method (CH4 recovery rate: 80-90%) Principle: CO2 and H2S are removed by taking advantage of the fact that acidic oxides have a higher solubility in water than CH4. Water can be regenerated and reused through methods such as pressure reduction and air lifting ； If the conditions permit, fresh water can be used entirely, eliminating the need for water reclamation. The washing process is actually also a physical absorption process, suitable for situations with high CO2 partial pressure. Advantage: Simple process ； Technological maturity ； Water, as an absorbent, is pollution-free and easily available. Disadvantage: The CH4 loss rate is higher than that of other technologies ； The energy utilization efficiency is lower than that of other technologies ； High water consumption makes it unsuitable for water-scarce areas ； Microorganisms that grow easily cause tower blockage ； After purification, drying treatment is required. Development trend: The wash process has reached a mature stage, and it is mainly controlled by companies such as Mamberg in Sweden, DMT in the Netherlands, and Flotech in New Zealand. It is widely used in the field of biogas purification abroad. According to statistics from the International Energy Agency (IEA) in 2014, both the number of units using the washing method and the amount of biogas purified by this method accounted for more than one-third of the total, yet their market share is on the decline. 2 Solvent method (CH4 recovery rate: 98.5%) Principle: The solvent method can be divided into two types: chemical solvent method and physical solvent method. The former has high energy consumption and is less commonly used ； The latter removes CO2 and H2S by taking advantage of the different solubilities of acidic gases and CH4 in the solvent. Common physical solvents include propylene carbonate (PC method), polyethylene glycol dimethyl ether (NHD method), and the formulated solvent Selexol, among others ； Advantage: High purity of the product gas ； High methane recovery rate ； The solvent circulation volume is low, resulting in low power consumption. Disadvantage: The solvent regeneration process requires a large amount of thermal energy, resulting in high energy consumption ； The biogas production process does not generate steam as a by-product; it requires steam to be produced separately, which consumes methane and reduces the overall yield of biological natural gas ； Chemical solvents have an impact on the environment, and corresponding control measures must be taken; in areas with strict environmental regulations, the use of solvents is restricted. Trend: In the overseas biogas purification market share, the solvent method accounts for over 1/3. The entire process flow is adapted from the corresponding chemical processes, with solvents such as Selexol and Genosorb being used. The domestic solvent-based decarburization process is well-established in the chemical industry, and improved methods such as the MDEA method, PC method, and NHD method developed by institutions like Nanhua Design Institute and Sichuan Chemical Industry Design Institute can be utilized as references. Applying ionic solutions to biogas purification is a new topic that has attracted significant international attention. 3 Membrane separation (CH4 recovery rate: 95%) Principle: The membrane separation method takes advantage of the different adsorption capacities of various gas components on the membrane surface, as well as their differing dissolution and diffusion rates. Driven by the pressure difference across the membrane, most components such as CO2 and a small amount of CH4 pass through the membrane wall into the permeate side and are separated there, while most of the CH4 remains on the high-pressure side and is output as the main component of biogas. Advantages: The device occupies little space, is easy to operate, and has low energy consumption. Disadvantages: Membrane elements are expensive, prone to damage, and plasticization and aging reduce their lifespan; maintenance costs are high ； Membrane elements are sensitive to oil and dust, and strict requirements exist for oil and dust removal in the initial stage ； The CH4 recovery rate (≥95%) is low. Applications and development: According to statistics from the International Energy Agency (IEA), in Europe and the United States, the number of membrane separation units used for biogas purification as well as their processing capacity have seen significant increases since 2010. Based on these trends, it is estimated that by 2016, membrane separation units will account for around 10% of the market share in this area. 4. Pressure swing adsorption: Pressure swing adsorption is recognized as the gas purification technique with the lowest operating costs; however, its low methane recovery rate prevents it from being used on a large scale in biogas purification. This problem can be solved by developing adsorbents specifically for biogas, thereby improving the CH4/CO2 separation coefficient and increasing the CO2 adsorption capacity, which lays a technical foundation for the widespread use of pressure swing adsorption technology in the field of biogas purification. 5 Honghu Technology’s pressure swing adsorption technology for biogas purification achieves a CH4 recovery rate of 99%; the CH4 content in the exhaust gas is below 1%, while the purity of the output gas ranges from 97% to 99% (adjustable). It combines the advantages of traditional pressure swing adsorption, solves the problem of low recovery rates, and at the same time reduces the size of the installation, lowers the total investment cost, and decreases operating expenses. The operating cost of producing SNG from biogas is 0.25 yuan per standard cubic meter of biogas.