Gasification and Synthesis

Gasification involves the conversion of biomass into a combustible, non­condensable gas mixture by partial oxidation of biomass at high temperature (800-1,300 °C). The resulting gaseous mixtures consist mostly of carbon monoxide (CO), hydrogen (H2) and traces of methane (CH4). Unlike bio-oil, incondensable biogas cannot be stored easily, safely or economically. Therefore, this low heating value gas fuel must be used immediately. Different gasification technologies exist using a broad range of gasifiers, namely: fixed bed gasifiers; including updraft, downdraft and crossdraft gasifiers, fluidised bed gasifiers; including bubbling fluidised bed (BFB), circulating fluidised bed and entrained flow gasifiers, multi-bed gasifiers; including indirect heating, pyrolysis/char, cyclone and plasma gasifiers (Bridgwater 1995; Siedlecki et al. 2011; Gabra et al. 2001; Rutberg et al. 2011). Gasification conditions in various types of gasifiers have been thoroughly reviewed by Pfeifer etal. (2011).

The gasification products of lignocellulose can be applied to various channels of bioenergy production, such as thermal heat generation (steam, hot water), electrical power production by a steam or gas turbine/engine and as synthesis gas (or syngas), which can be used for the production of liquid fuels (biodiesel), hydrogen, methane, mixed alcohols and other chemicals. From raw syngas (the main product of gasification and a by-product of pyrolysis) a series of liquid hydrocarbons can be produced, such as bio-synthetic natural gas (SNG), bio­hydrogen, biomethanol, ethanol, dimethylether and Fischer-Tropsch fuels, which are synthesised via different catalytic processes (Swain et al. 2011; Zinoviev et al. 2010). Fischer-Tropsch (FT) fuels are mainly aliphatic straight chain and branched hydrocarbons and primary alcohols. The product distribution obtained from FT fuels include light hydrocarbon methane (CH4), ethylene (C2H4), ethane (C2H5), LPG (C3-C4), propane (C3), butane (C4), gasoline (C5-Ci2), diesel fuel (Ci3-C22) and wax (C23-C33) (Naik et al. 2010).

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