Review on biogas upgrading technologies for producing biomethane

Biogas can be upgraded to biomethane, giving more utilization possibilities, such as utilization as autogas, or distant utilization by using the existing natural gas grid. Upgrading of biogas has gained increased attention due to rising oil and natural gas prices and increasing targets for renewable fuel quotes in many countries. This paper analyzes the possibility of the substitution of natural gas by biogas according to the characteristics and components of biogas and natural gas, and reviewed the biogas upgrading technologies including water scrubbing, organic physical scrubbing, chemical scrubbing, pressure swing adsorption (PSA), membrane separation, cryogenic upgrading, and the in situ methane enrichment processes. Among these upgrading technologies, water scrubbing and PSA are the main two processes from the point of view of commercialization, because of their comprehensive advantages in technical and economic aspects. Water scrubbing is a very simple process. In a scrubber, CO2 and H2S dissolve into the water, while CH4 does not because of their difference in solubility. PSA is the second most employed technique. In PSA processes, biogas is compressed to a vessel where it is put in contact with an adsorbent that will selectively retain CO2. In a similar manner to water scrubbing, it is possible to use chemicals to absorb CO2. However, chemical scrubbing may have higher energetic penalties since the CO2 removal in biogas is a bulk removal process. Membrane separation technology is an excellent technology with good prospects for separating CO2 and CH4. Owing to its superiority in upgrading cost and simplifying upgrading equipment, in situ methane enrichment technology is relatively simple, and there is no need for much auxiliary equipment. Therefore, it has a potential for a lower upgrading cost compared to other techniques, especially when it is used in a biogas plant with high concentration solids as materials. Furthermore, research and application of new materials on biogas upgrading technologies has become an important direction of research and development. MOFs used in the PSA process, and the application of ionic liquids on chemical scrubbing process, as well as research on composite membrane separation process have all reflected this trend.