The pyrolysis of biomass feedstocks may be endothermic or exothermic, de­pending on the temperature of the reactants. For most biomass containing highly oxygenated hemicellulosics and cellulosics as the major components, pyrolysis is endothermic at temperatures below about 400 to 450°C and exo­thermic at higher temperatures. Once the necessary temperature has been reached in a properly designed system, little or no external heat is needed to sustain the process. The principal exothermic reactions that occur in the gas and solid phases during biomass pyrolysis are shown in Table 8.2. These reactions include the reduction of the carbon oxides to methane and methanol, the water gas shift reaction, and carbonization of celluloses. Substantial quanti­ties of hydrogen are required for reduction of carbon oxides to methane and methanol, but hydrogen is not required for the water gas shift, which produces hydrogen, and the char formation reactions listed in Table 8.2. The pyrolysis temperature should be high enough to generate the requisite hydrogen for reduction of the carbon oxides. The water formed on biomass pyrolysis and the vaporization of the physically contained moisture in the fresh feed can participate in the water gas shift reaction. Interestingly, the exothermicity of cellulose carbonization is quite high per monomeric unit (C6Hi0O5). Since the char yields on conventional biomass pyrolysis range up to about 35 wt %, and the fixed carbon contents of the chars are high, char formation would be expected to be a dominant driving force for biomass pyrolysis at the lower temperatures at which autogenous pyrolysis begins, but which generate less hydrogen. At these temperatures, pyrolysis is generally reaction — rate controlled, and at higher temperatures, the process becomes mass-transfer controlled. Energy to initiate biomass pyrolysis can be supplied by an external heat source or a portion of the pyrolysis products such as char and low — energy gas.