Mini-Review on Char Catalysts for Tar Reforming during Biomass Gasification: The Importance of Char Structure

With the depletion of fossil fuel reserves and the increasing concern about greenhouse gas emission, the need to develop technologies for using renewable and sustainable energy resources is becoming urgent. Solar, winds, geothermal, and hydro as well as nuclear energy are all considered as clean energy sources to (partially) substitute fossil fuels in energy generation. Biomass, however, remains the only renewable energy source consisting of hydrocarbon compounds, which is a necessity for making basic building blocks for chemical industries. The development of feasible gasification technologies to produce syngas (primarily H₂ and CO) is considerably inhibited by the "tar issues". The tar content in the product gas has to be extremely low in order to effectively use the gas downstream, such as for synthesizing liquid fuels and generating electricity. Although there are a wide range of options for removing, cracking, and catalyzing the tar, this review mainly discusses the catalytic reforming approach using char as catalysts, of which the importance of the carbon structure of chars will be underpinned because the char structure including matrix carbon structure and functional groups on the char surface not only affects the reactivity of nonmetal active sites but also largely dominates the role/fate of internally existing and externally added metal species.