Gasification of in-Forest Biomass Residues

Abstract

Described is a laboratory-scale continuous-feed supercritical water gasification (SCWG) system. The system is operated using real-world Ponderosa Pine sawmill residues at high biomass loadings, short mean residence times (2-5 sec), and 27.7 MPa pressures. Each run with the SCWG system typically processed several 100 g of biomass/water slurry mixture. We evaluated the effect of operating temperatures (from 700K to 900K) and biomass feedstock loadings (5% to 15% by weight in water) on solids conversion and gaseous product composition. Biomass-to-gasified product conversion efficiencies ranged from 89% to 99%, by mass. Gaseous products were primarily composed of CO2, H2, CH4, and CO, generally in that order of prevalence. The highest hydrogen yield, 43% mole percent, was achieved at 900k with a 5% biomass loading. In general, low biomass loadings corresponded to higher H2:CO2 ratios, but never did we observe stoichiometries that could be explained purely by steam reforming or steam reforming plus water gas shift chemistries. Methanation & Hydrogenation chemistry also occurred, but the mole fraction of CH4 never exceeded 10%. We hypothesize that the real-world biomass samples used here intrinsically include gas-bubbles in the slurry, enabling partial or complete oxidation to occur along with the more conventional SCWG chemistries. As a result, the observed syngas composition was shown to depend more on biomass loading than on processing temperature. In-situ Raman testing was also evaluated as a possible means of monitoring SCWG real time. Biomass (lignin, cellulose, and hemicellulose) were all detected along with variations in concentration. Additionally effluent composition was verified to not contain intermediary compounds.