The use of renewable sources of energy such as biomass is an alternative that has the potential to reduce our reliance on imported oil while addressing environmental concerns. Among the technologies that convert biomass into fuels, Supercritical Water Gasification (SCWG) has been suggested to process wet feedstocks because of the ability of water to dissolve organic components of plant materials at supercritical conditions, promoting formation of gases such as H2 and CH4, and limiting the amount of tar and char formed as residues.
Metal catalysts can improve SCWG efficiency in a variety of ways. Some catalysts promote the cleavage of C-C bonds, which is crucial to achieve higher gasification yields. Others promote reactions in the gas-phase changing product composition towards H2 production, such as water-gas shift. This research project aims to determine which metal catalysts are more effective to conduct SCWG, and the conditions where catalytic effects are maximized.
We gasified cellulose and lignin as model compounds for biomass in supercritical water, and avoided catalytic effects from the reactor walls by using quartz reactors. We ran experiments in the absence of catalysts and in the presence of nickel, iron or copper wires to evaluate their effects on product yields. In order to determine how experimental conditions affected catalysis, we varied temperature from 400 to 725 °C, biomass loading from 5.0 to 33.3 wt %, and water density from 0.08 g/ml to 0.18 g/ml.
The results indicate that high temperatures, low loadings and high water densities promoted catalytic activity. Nickel increases yields of the gas products (H2 in particular) at all conditions studied, while iron and copper increase yields only at specific conditions. When CH4 and energetic yields are also taken into account, iron is more effective at high temperatures and copper at low biomass loadings.