Effect of Metals on Supercritical Water Gasification of Cellulose and Lignin

Abstract

We gasified cellulose and lignin in supercritical water, using quartz reactors, and quantified the catalytic effect of metals by adding them to these reactors in different forms. We used nickel, iron, copper, zinc, and zirconium wires, ruthenium powder, and Raney nickel slurry. The presence of metals was more likely to increase gas yields to a measurable extent when the catalyst surface area/biomass weight ratio was at least 15 mm2/mg (5.0 wt % biomass loading in our experiments). Nickel and copper typically provided higher gas yields at 5.0 wt % loading, and nickel provided the highest H2 yields at 1.0 wt % loading. SCWG at 500 °C with nickel at 240 mm2/mg generated 16 mmol/g of H2 from cellulose. CH4 yields were not strongly influenced by the presence of metals. With nickel wires in the reactor, gas with about 40−50% of the energy content in the original biomass was produced from cellulose and lignin. All of the metals tested except copper produced H2 from water when exposed to SCWG conditions with no biomass. It is important that this background H2 formation be accounted for when interpreting results from SCWG experiments in the presence of metals. Exposure of nickel wires to supercritical water did not reduce their activity for H2 production.