A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading

The lack of tax incentives for biomethane use requires the optimization of both biogas production and upgrading in order to allow the full exploitation of this renewable energy source. The large number of biomethane contaminants present in biogas (CO₂, H₂S, H₂O, N₂, O₂, methyl siloxanes, halocarbons) has resulted in complex sequences of upgrading processes based on conventional physical/chemical technologies capable of providing CH₄ purities of 88–98 % and H₂S, halocarbons and methyl siloxane removals >99 %. Unfortunately, the high consumption of energy and chemicals limits nowadays the environmental and economic sustainability of conventional biogas upgrading technologies. In this context, biotechnologies can offer a low cost and environmentally friendly alternative to physical/chemical biogas upgrading. Thus, biotechnologies such as H₂-based chemoautrophic CO₂ bioconversion to CH₄, microalgae-based CO₂ fixation, enzymatic CO₂ dissolution, fermentative CO₂ reduction and digestion with in situ CO₂ desorption have consistently shown CO₂ removals of 80–100 % and CH₄ purities of 88–100 %, while allowing the conversion of CO₂ into valuable bio-products and even a simultaneous H₂S removal. Likewise, H₂S removals >99 % are typically reported in aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors and digesters under microaerophilic conditions. Even, methyl siloxanes and halocarbons are potentially subject to aerobic and anaerobic biodegradation. However, despite these promising results, most biotechnologies still require further optimization and scale-up in order to compete with their physical/chemical counterparts. This review critically presents and discusses the state of the art of biogas upgrading technologies with special emphasis on biotechnologies for CO₂, H₂S, siloxane and halocarbon removal.