The recalcitrant nature of biomass makes pretreatment an essential step to facilitate the conversion of biomass to renewable fuels and bio-based products. Of all the existing pretreatment technologies, microwave (MW) has been considered one of the most promising methods. As a rapid and effective heating source with both thermal and nonthermal effects, MW can directly interact with the material, thereby accelerating chemical, physical, and biologic reactions. MW irradiation has been successfully used in the pretreatment of various types of biomass including agricultural residues, woody biomass, grass, energy plants, and industrial residuals. This chapter reviews the progress on the use of MW irradiation for the pretreatment of biomass, addressing the following most commonly preferred MW-assisted biomass pretreatments: (1) MW/water alone, (2) MW/alkali, (3) MW/acid, (4) MW/ionic liquid, (5) MW/salt, and (6) other combined MW-assisted pretreatment.
