Pretreatment processing usually takes place in the prescreening of feed­stock, where unsuitable materials are screened out. Typical pretreatment operations include drying, devolatilization, size reduction and particle size classification, torrefaction, pelletizing, and mixing.

1.1.1 Chemical and Biochemical Reactions

Highly oxygenated and high molecular-weighted biomass organic materials need to be converted to lighter molecular-weighted and simpler chemicals via a variety of targeted and accompanying chemical reactions, depending upon the desired outcomes. Such reactions include complete combustion, partial oxidation, pyrolysis, gasification, reformation, hydrocracking, lique­faction, hydrolysis, enzymatic hydrolysis, fermentation, and more. The reac­tions listed here are not single chemical reactions and each is rather a class of chemical reactions. For example, gasification includes steam gasification, hydrogasification, carbon dioxide gasification, partial oxidation, and a com­bination of these. Conventional process technologies are more applicable to first-generation biofuels, whereas more advanced process technologies are being developed for second-generation biofuels.