There are four methods to achieve biogas purification, namely absorption method, pressure swing adsorption method, low-temperature condensation method, and membrane separation method.
The absorption purification method utilizes the physical and chemical absorption characteristics of organic amine solutions (primary amine, secondary amine, tertiary amine, steric hindrance amine, etc.) and carbon dioxide to achieve decarbonization and purification of methane. It involves an absorption reaction with carbon dioxide in biogas under pressure and room temperature conditions in the absorption tower, and a reverse desorption reaction of the absorbed rich liquid under pressure and heating conditions in the regeneration tower, releasing high-purity carbon dioxide gas, At the same time, the regeneration of the rich liquid has the ability to reabsorb carbon dioxide, thereby achieving continuous decarbonization and purification of methane from biogas in the absorption tower, and enabling continuous absorption and regeneration of the decarbonized liquid.
变压吸附提纯法是利用吸附剂（如分子筛等）对二氧化碳的选择性吸附特点, 即在吸附剂上二氧化碳相对其他气态组分有较高的分离系数, 来达到对沼气中二氧化碳进行脱除的目的。在吸附过程中，原料气在加压条件下其中的二氧化碳被吸附在吸附塔内，甲烷等其他弱吸附性气体作为净化气排出，当吸附饱和后将吸附柱减压甚抽成真空使被吸附的二氧化碳释放出来。为了保证对气体的连续处理要求，变压吸附法少需要两个吸附塔, 也可是三塔、四塔或更多。
The pressure swing adsorption purification method utilizes the selective adsorption characteristics of adsorbents (such as molecular sieves) for carbon dioxide, which means that carbon dioxide has a higher separation coefficient compared to other gaseous components on the adsorbent, in order to achieve the purpose of removing carbon dioxide from biogas. During the adsorption process, carbon dioxide in the feed gas is adsorbed into the adsorption tower under pressure conditions, and other weakly adsorbed gases such as methane are discharged as purified gas. When the adsorption is saturated, the adsorption column is depressurized or even evacuated to release the adsorbed carbon dioxide. In order to ensure continuous gas processing requirements, the pressure swing adsorption method requires at least two adsorption towers, which can be three, four, or more.
The low-temperature condensation purification method utilizes the high liquefaction temperature of carbon dioxide to liquefy the carbon dioxide in biogas through low-temperature action, and the methane component is discharged as non condensable gas to purify the product gas. In order to reduce operational energy consumption, regenerative technology is usually used to recover the remaining cooling capacity.
The membrane separation purification method is achieved by utilizing the different permeability effects of different gas components through the membrane under pressure driving. Generally, carbon dioxide has a fast permeation rate, and as a fast gas, it is discharged through the permeable gas, while methane has a slow permeation rate. As a slow gas, it is obtained as a purified product gas in the form of permeable residual gas. In engineering, multi-stage membrane separation technology is often used to increase the concentration of methane gas.
At present, the main methods used in biogas purification engineering are absorption and pressure swing adsorption. The low-temperature condensation method and membrane separation method are still rarely applied due to their technological maturity and economic feasibility.
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