Reactions between char and gasifying agents are usually the controlling step and of a core role for the overall biomass gasification process due to the relatively low reaction rate. Char reactivity will be greatly affected via interacting with volatiles, such as steam, hydrocarbons, tarry compounds, and other light gas species. By taking the updraft/downdraft moving bed and fluidized bed gasifier as examples, this review discussed the effects of char generation and evolution in various reactor environments on its following reaction behaviors. The characteristics of biomass and subsequent char gasification are further examined in terms of feedstock types and their inherent inorganics. Then, the effects of operation conditions and gasifying reagents on biomass gasification are outlined mainly from the point of char production and the subsequent volatiles−char interaction. Finally, some directions and suggestions considering char conversion and utilization are addressed for the design and betterment of the biomass gasification process.

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