Depending on how heat is generated, gasification technology can be classified as either directly — or indirectly-heated gasification. For directly-heated gasification, pyrolysis and gasification reactions are conducted in a single vessel, with heat arising from feedstock combustion with oxygen. The syngas generated from this method has low heating values (4-6 MJ/m3 or ~100-140 Btu/ft3). For indirectly — heated gasification, the heat-generating process (combustion of char) is separated from the pyrolysis and gasification reactions, which generates high heating value syngas (12-18 MJ/m3 or ~300-400 Btu/ft3). Low heating value syngas is usually used to generate steam or electricity via a boiler or gas turbine, while high heat­ing value syngas can also be used as a feedstock for subsequent conversion to fuels and chemicals [22]. According to the flow direction of the feedstock material and oxidant, gasifiers can basically be classified into five types (Table 2, Fig. 3).

Although a portion of the feedstocks are converted to heat during gasifica­tion, conversion efficiencies of biomass to syngas are relatively high: e. g. 50-75% on weight basis [22]. This gasification efficiency is mainly due to the utilization of lignin and other organic substances, which cannot be used directly in acid or enzymatic hydrolyzing processes.