Sequential pyrolysis and potassium-catalyzed steam-oxygen gasification of woody biomass in a continuous two-stage reactor

A recently proposed type of gasification (Type IV) has a potential to produce syngas from biomass or lignite at chemical energy recovery over 96% on an LHV basis with H₂O/C and O₂/C molar ratios as small as 0.5 and 0.2, respectively. This type of gasification converts biomass or lignite in a sequence of pyrolysis, gas-phase partial oxidation of volatiles, and simultaneous steam gasification of char and steam reforming of the volatiles under catalysis of potassium. The present authors experimentally simulated a Type IV gasification of woody biomass loaded with 1.5 wt-% K in a laboratory-scale continuous reactor system. The pyrolysis at 550°C in a screw conveyer reactor and the subsequent steam gasification/reforming at 700-720°C in the bed of K-loaded char demonstrated steady-state carbon conversion over 99%, residual tar concentration in the syngas well below 100 mg/m³_N-dry, and CH₄ concentration below 3 vol-%-dry simultaneously. The potassium retained by the char catalyzed the steam reforming/gasification, consuming externally added steam and in situ formed one from the pyrolysis and partial oxidation at consumption more than 0.7 mol H₂O/mol C. The composition of syngas reached near-equilibrium among H₂, steam, CO, and CO₂ at 700-720°C. This was caused by fast reactions within the bed of the K-loaded char; steam reforming/gasification forming CO₂/H₂ and reverse water-gas shift reaction.