A critical review of biochar versus hydrochar and their application for H2S removal from biogas

Biogas contains significant quantities of undesirable and toxic compounds, such as hydrogen sulfide (H2S), posing severe concerns when used in energy production-related applications. Therefore, biogas needs to be upgraded by removing H2S to increase their bioenergy application attractiveness and lower negative environmental impacts. Commercially available biogas upgradation processes can be expensive for small and medium-scale biogas production plants, such as wastewater treatment facilities via anaerobic digestion process. In addition, an all-inclusive review detailing a comparison of biochar and hydrochar for H2S removal is currently unavailable. Therefore, the current study aimed to critically and systematically review the application of biochar/hydrochar for H2S removal from biogas. To achieve this, the first part of the review critically discussed the production technologies and properties of biochar vs. hydrochar. In addition, exisiting technologies for H2S removal and adsorption mechanisms, namely physical adsorption, reactive adsorption, and chemisorption, responsible for H2S removal with char materials were discussed. Also, the factors, including feedstock type, activation strategies, reaction temperature, moisture content, and other process parameters that could influence the adsorption behaviour are critically summarised. Finally, synergy and trade-offs between char and biogas production sectors and the techno-economic feasibility of using char for the adsorption of H2S are presented. Biochar's excellent structural properties coupled with alkaline pH and high metal content, facilitate physisorption and chemisorption as pathways for H2S removal. In the case of hydrochar, H2S removal occurs mainly via chemisorption, which can be attributed to well-preserved surface functional groups. Challenges of using biochar/hydrochar as commercial adsorbents for H2S removal from biogas stream were highlighted and perspectives for future research were provided.