Wet Cleaning Technologies for Biomass-Derived Syngas: A Review

Biomass gasification and pyrolysis are thermochemical conversion technologies with significant potential for the future development of a sustainable energy system worldwide, which can help to replace several fuels and chemicals currently provided by fossil fuels. Gasification and pyrolysis generate solid, liquid, and gaseous products. The gas is a mixture of gases composed mainly of CO, CO2, H-2, CH4, N-2, and small concentrations of impurities that are relevant and can have significant drawbacks in its subsequent use as fuel. Among those impurities are included tars, particulate matter, nitrogen-based compounds (NH3, HCN), sulfur-based compounds (H2S, COS), hydrogen halides (HCl, HF), and trace metals (Na, K). Thus, cleaning syngas is an essential process to get a gas with suitable characteristics before use in further applications such as internal combustion engines, gas turbines, hydrogen production, and other chemical applications. In this context, this work presents a review of the main wet cleaning systems that have been developed for syngas cleaning and conditioning. For wet cleaning, wet scrubbers are generally used, and this is a relatively mature and highly effective process. The main drawbacks are the generation of a residual liquid stream that is difficult to treat and eliminate, and the decrease in energy efficiency, by cooling the syngas. Therefore, this review critically examines the significant challenges in syngas wet cleanup and discusses its major recent advances.