There are four methods for biogas purification, namely absorption method, pressure swing adsorption method, low-temperature condensation method and membrane separation method.
1、 Absorption purification
The absorption and purification method is realized by using the physicochemical absorption characteristics of organic amine solution and carbon dioxide, that is, it reacts with carbon dioxide in biogas under pressure and normal temperature in the absorption tower to decarbonize and purify methane, and the absorbed rich solution reacts reversely under decompression and heating conditions in the regeneration tower to release high-purity carbon dioxide gas, At the same time, the rich liquid is regenerated and has the ability to reabsorb carbon dioxide, so as to realize the continuous decarburization and purification of methane in the absorption tower, and make the decarburized liquid carry out continuous absorption and regeneration cycle.
2、 PSA purification
PSA purification method uses the selective adsorption characteristics of adsorbents (such as molecular sieves) for carbon dioxide, that is, carbon dioxide has a higher separation coefficient than other gaseous components on the adsorbent to achieve the purpose of removing carbon dioxide from biogas. During the adsorption process, the carbon dioxide in the feed gas is adsorbed in the adsorption tower under pressurized 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 vacuumized to release the adsorbed carbon dioxide. In order to ensure the continuous treatment of gas, PSA requires at least two adsorption towers, or three towers, four towers or more.
3、 Low temperature condensation purification
Low temperature condensation purification method uses the characteristics of high liquefaction temperature of carbon dioxide to liquefy carbon dioxide in biogas through low temperature, and methane component is discharged as non condensable gas to purify product gas. In order to reduce operation energy consumption, regenerative technology is usually used to recover the remaining cooling capacity.
4、 Membrane separation and purification
The membrane separation and purification method is realized by using the different permeability of different gas components through the membrane under pressure. Generally, the penetration rate of carbon dioxide is fast, which is discharged as fast gas through gas, and the penetration rate of methane is slow. As slow gas, purified product gas is obtained in the form of residual gas. In engineering, multistage membrane separation process is often used to improve the concentration of methane.
Biogas purification project mainly adopts absorption, pressure swing adsorption and membrane separation methods. Low temperature condensation method is rarely used due to technical maturity and economy.