Design of biogas upgrading processes based on ionic liquids

Abstract

Biogas stands out as an alternative to traditional sources of energy since it presents a high methane content, and it is mainly produced by anaerobic digestions of organic wastes. Typical biogas streams do not only consist of biomethane but also carbon dioxide, water, ammonia, hydrogen sulfide or siloxanes, depending on the source of the organic waste. Therefore, it is important to remove all these contaminants to obtain a high-quality stream in a process known as biogas upgrading. Currently, there is not a predominant technology, all of them presenting advantages and drawbacks to be solved. In this work, we test a biogas upgrading process based on CO2 chemical absorption by ionic liquids (ILs). The complete process was evaluated involving absorber and stripping columns in a wide range of operating temperatures and pressures to reach biomethane of 97% purity from an industrial biogas stream. Best specific energy consumptions are found at 50 °C in the absorber and 95 °C in the stripper at atmospheric pressure. Increasing the operating pressure of the absorber to 6 bar reduces the energy consumption from 0.8 kWh/Nm3 to 0.2 kWh/Nm3. This is mainly because of the reduction in IL flow (almost a half) and that the thermal energy needed is provided by the exothermic reaction and no external requirements are needed. IL-based proposal for biogas upgrading was found able to efficiently retain CO2 but also other main impurities (H2S, H2O, siloxanes) producing biomethane with quality standards. Results from IL-based process reveal savings in operating cost and nearly the same equipment investment costs than available technologies (PSA, water and amine scrubbing, and membranes) for biogas upgrading process.