Performance evaluation of a novel two-stage waste wood gasifier: Influence of steam-to-biomass ratio and in-situ steam generation on tar formation

The shift from high-quality woodchips to low-quality feedstocks increasingly shapes biomass gasification. Small- to medium-scale plants face challenges from heterogeneous feedstock and tar formation, with a research gap in producing energetically useable product gas without extensive cleaning. This study investigates gasification of different waste wood fractions in a two-stage process using a novel bubbling fluidised bed reactor without bed material. Two steam generation strategies were applied: steam-saturated air and direct water injection. Combining secondary air with steam-saturated primary air improved temperature control, reduced slagging, and ensured stable thermal stratification across feed rates. While raising the steam-to-biomass ratio did not significantly enhance efficiency, it strongly reduced tar: PAH16 and BTX decreased by up to 85 % and 65 %. Gas composition showed slight increases in H 2 and CH 4 , a decrease in CO, and stable values for heating value, gas yield, and conversion efficiency. The product gas exhibited an LHV of 3.7-4.2 MJNm −3 , gas yields of 1.9-2.8 Nm 3 kg −1 , and cold gas efficiencies of 52.5-57.8 %, comparable to values reported for high-quality woodchips. Total tar concentrations remained low (0.92-2.8 gNm −3 ), demonstrating that autothermal operation with controlled secondary air and steam-saturated primary air provides both efficient energy recovery and effective tar reduction, even for low-quality feedstocks. • Novel two-stage bubbling fluidised bed enabled waste wood gasification without additional bed material. • Combined secondary air and steam-saturated air improved stability and inhibited slag formation. • Steam addition reduced PAH16 and BTX by up to 85% and 65%. • High gas yields (1.9–2.8 Nm 3 kg -1 ) achieved cold gas efficiencies up to 57.8%. • Low tar contents without gas cleaning: PAH16 120–403 mgNm -3 , total 0.9–2.8 gNm -3 .