Review of heterogeneous catalysts for sub- and supercritical water gasification of biomass and wastes

Nowadays, substantial efforts are devoted to decrease our dependence on fossil fuels. This change will heavily rely on development of new and improved catalytic processes. Over the past two decades, catalytic hydrogen production from wet biomass and organic compounds in sub- and supercritical water (SCW) has gained significant attention. In this process, catalysts are employed to enhance the gas formation rate at moderate temperatures. Catalysts can be also utilized to shift the product distribution toward a more desirable compound (e.g. hydrogen). The effectiveness of various types of heterogeneous catalysts, mainly containing nickel and ruthenium, have been demonstrated for hydrothermal gasification of organic compounds. Catalyst formulation along with operating conditions such as temperature and feed concentration can significantly affect the conversion and selectivity of the process. This paper reviews the major findings of hydrothermal gasification over the past two decades with the aid of heterogeneous catalysts in terms of activity, hydrogen selectivity and stability. Commercially available and laboratory-prepared catalysts including supported and skeletal metal catalysts, activated carbon, oxides, metal wires and other innovative catalysts are considered. Results of supercritical water gasification (SCWG) of various feedstocks reported in the literature are compared and possible mechanisms and rates of deactivation of heterogeneous catalysts are discussed.