Low-temperature gasification of biomass and lignite: Consideration of key thermochemical phenomena, rearrangement of reactions, and reactor configuration

This paper discusses gasification of solid fuels, such as biomass and lignite, at temperatures well below 1000 C, which potentially realizes a loss of chemical energy (LCE) smaller than 10% but encounters difficulty in fast and/or complete solid-to-gas conversion in conventional reactor systems. First, key thermochemical and catalytic phenomena are extracted from complex reactions involved in the gasification. These are interactions between intermediates (i.e., volatiles and char), catalysis of inherent and extraneous metallic species, and very fast steam gasification of nascent char. Second, some ways to control the key phenomena are proposed conceptually together with those to rearrange homogeneous/heterogeneous reactions in series/parallel. Third, implementation of the proposed concepts is discussed assuming different types of gasifiers consisting of a single-fluidized bed, dual-fluidized bed, triple-bed circulating fluidized bed, and/or fixed (moving) bed. The triple-fluidized bed can attain gasification with a LCE as small as 10% by introducing enhancement and/or elimination of the key phenomena and another way to recuperate heat from gas turbine and/or fuel cells (i.e., power generators in gasification combined cycles) into chemical energy of fuel gas. A particular type of fixed-bed gasifier is proposed, which is separated from a pyrolyzer to realize not only control of the key phenomena but also temporal/spatial rearrangement of exothermic and endothermic reactions. This type of gasifier can make a LCE smaller than 4%. Even a conventional single-fluidized bed provides simple and effective gasification, when tar-free/reactive char is used as the fuel instead of parent the one and contributes to a novel integrated gasification fuel cell combined cycles with a theoretical electrical efficiency over 80%.