Recent advances in tar conversion to H2-rich syngas during biomass and wastes gasification: Leveraging in-situ approach

A major roadblock in biomass waste (BMW) and municipal solid waste (MSW) gasification is the formation of tar, an unavoidable by-product that reduces energy conversion efficiency and contaminates downstream systems. Tar can be addressed through in-situ or ex-situ methods. While ex-situ approaches are widely studied for their efficiency and ease of control, in-situ methods offer advantages such as improved mass and heat transfer, energy self-sufficiency, and lower infrastructure costs. Converting tar into high-value fuels and chemicals through feedstock selection, optimized conditions, and catalytic upgrading is an effective strategy, with catalysts playing a key role in enhancing product selectivity. This review summarizes recent progress in BMW/MSW tar upgrading through feedstock selection and reaction optimization, with a novel focus on the performance of monometallic (Ni, Fe) and bimetallic (Ni–Fe, Ni–Co) catalysts for converting tar into value-added fuels. Data from multiple studies are compiled, analyzed, and presented in figures and tables. It is noted that temperature plays a crucial role in the gasification process by promoting tar cracking reactions, with different gasification agents having specific critical temperatures typically ranging from 700 to 900 °C. Beyond this range, the reverse water-gas shift reaction (WGSR) may dominate, reducing H₂ production. Thermal catalytic tar upgrading with monometallic catalysts shows high upgrading efficiency but suffers from low stability and poor resistance to carbon deposition after a single use in lifetime tests. In contrast, bimetallic catalysts improve stability by approximately 3.5 times compared to monometallic catalysts and achieve over 95 % tar conversion into valuable products. Despite these advances, sintering and coke deposition remain significant challenges, highlighting the need for improved catalyst modification strategies to enhance both stability and activity.