Supercritical Water Gasification of Biomass.

Abstract

We performed Supercritical Water Gasification (SCWG) for cellulose and lignin for the first time in the absence of catalytic effects from metallic walls, by using quartz reactors. We quantified the catalytic effect of metals by adding metal wires to the reactors. We also performed the first systematic study of the effect of variables on gas yields. We varied time (2.5 to 75 minutes), temperature (365-725C), water density (0.00-0.18 g/cm3), and biomass loading (1.0 wt %-33.3 wt %). In the absence of metals, high temperatures and water densities provide the highest gas yields. Up to 3.3 mmol/g of H2 were obtained from cellulose (at 0.18 g/cm3) and up to 7.5 mmol/g (at 725C) from lignin. Up to 2.6 mmol/g of CH4 were obtained from cellulose (at 600C), and up to 9.0 mmol/g from lignin (at 725C). The highest energetic content (LHV gas/LHV biomass) was 20.0 % from cellulose (at 600C), and 37.4 % from lignin (at 725C). The presence of metals increases gas yields to a significant extent if the catalyst surface area/biomass weight ratio is at least 15.4 mm2/mg (5.0 wt % biomass loading). Nickel and copper provide high yields at 5.0 wt % loading, and nickel provides the highest yields at 1.0 wt % loading. Nickel at 240 mm2/mg provides 23.5 mmol/g of H2 from cellulose and 21.1 mmol/g of H2 from lignin, which is close to equilibrium yields. CH4 yields are not strongly influenced by the presence of metals. With nickel, it is possible to generate a gas with almost 50 % energetic content from cellulose, and 45 % from lignin. The non-catalytic results were used to fit the first kinetic model describing gas formation in SCWG. The proposed 11 reactions and the concept of a generic intermediate led to a model that can successfully fit the base case experimental data for cellulose and lignin. We verified that the model can predict yields at different biomass loadings and water densities. Its equilibrium predictions agree with thermodynamic calculations, and the rate constants obtained for the water-gas shift are in the range reported by previous authors.