Solar thermochemical conversion of biomass

The purpose of this paper is first to briefly describe the usual routes of biomass thermochemical conversion and then to discuss the possibility of using concentrated solar energy to provide the necessary heat for the processes. Gasification, fast and slow pyrolysis are more particularly described. They can be carried out for the preparation of a vast range of possible products that can be used as energy carriers and/or as a source of chemical commodities. The gasification processes are intended for the preparation of gas mixtures (CO, H-2, etc.) for chemical synthesis, heat or electricity generation. The fast pyrolysis formerly carried out for gas production (CO, Fl,, light hydrocarbons, etc.) is now mainly studied with the objective to produce liquids (bio-oils). Slow pyrolysis is in use for a long time for the preparation of solids (charcoal). The nature and quality of the products depend mainly on the experimental conditions of the process (temperature, heating rates, residence times, etc.). The possibility of a solar entry in the gasification and pyrolysis processes is then discussed. The technical and scientific benefits, as well as the difficulties, are underlined, showing the necessity to design new types of specific reactors. From a fundamental point of view the advantages are also underlined of using a concentrated radiation as a laboratory tool for studying the very fast primary steps of biomass thermal decomposition as well as the possible existence of intermediate short life time species that are still not well known. (C) 1998 Elsevier Science Ltd. All rights reserved.