Biogas Purification and Upgrading Technologies

Abstract

The fact that most countries do not promote the use of biogas as energy vector via tax incentives entails the need for an optimization of biogas upgrading technologies in order to support a cost-competitive utilization of this renewable energy source. Nowadays, the contaminants present in biogas such as CO2, H2S, H2O, N2, O2, siloxanes, and halocarbons are removed through the implementation of costly and environmentally unfriendly upgrading processes. Conventional biogas upgrading is based on physical/chemical technologies leading to CH4 purities of 88–98% and removal efficiencies of higher than 99% for H2S, halocarbons, and siloxanes. Unfortunately, their high energy and chemical demands limit the environmental and economic sustainability of these conventional biogas upgrading technologies. In this sense, biological processes have emerged in the past decade as an economic and environmentally friendly alternative to conventional biogas upgrading technologies. Thus, biotechnologies such as microalgae-based CO2 fixation, H2-assisted litoautotrophic CO2 bioconversion to CH4, enzymatic CO2 dissolution or fermentative CO2 reduction have been consistently shown to result in CO2 removals of 80–100% with CH4 purities of 88–100%, while allowing the valorization of CO2 into bioproducts of commercial interest (therefore preventing its release to the atmosphere). Similarly, H2S removals > 99% are consistently achieved in aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors, and digesters under microaerobic conditions. In addition, recent investigations have shown the potential biodegradability of siloxanes and halocarbons under both aerobic and anaerobic conditions. This chapter constitutes a state of the art comparison of physical/chemical and biological technologies for the removal of CO2, H2S, halocarbons, and siloxanes from biogas.