Supercritical water gasification of biomass: an experimental study of model compounds and potential biomass feeds

Gasification of biomass in supercritical water is a complex process. In supercritical water ideally the biomass structure and the larger molecules are broken down into smaller, gaseous components under the influence of radicals. However, the biomass is normally fed to the system at low temperature and, hence, should traverse a large temperature range before the supercritical conditions are met. During this warming-up (mostly ionic-) reactions may occur leading to the formation of intermediate compounds which are difficult to gasify, but lead to e.g. carbon formation that can cause blockage of process equipment. In case of uncatalyzed gasification the required temperatures are often above 650 °C and thus far above the supercritical temperature for water (374 °C). Together with the required high operating pressures (above 221 bar), this leads to high demands for the materials of construction and, hence, high costs. Reducing the operating temperature to below 600 °C is, next to maximizing gasification efficiency and minimizing carbon formation, an important target for catalyst development. In this thesis, attractive and relevant biomass feed streams have been used; like (crude) glycerine, a by-product from the biodiesel production, algae slurries and aqueous phases produced during the fast pyrolysis of lignocellulosic biomass as well as the aqueous phase produced during upgrading (by hydrogenation) of the primary pyrolysis oil. Results for these raw materials are presented for both the uncatalyzed gasificaion as well as for gasification in the presence of existing (commercial-) and newly developed (in this project) catalysts. For the model compounds chosen, glycerol and ethylene glycol, significant progress is made in the area of catalyst development and deduction of the prevailing reaction mechanisms. Additionally, a more fundamental study into the effect of the molecular structure of n-alcohols and n-carboxylic acids on the gasification performance is performed.