MW-based pretreatment approach utilizes both thermal and non-thermal effects gen­erated by an extensive intermolecular collision as a result of realignment of polar molecules with MW oscillations. Compared to conventional heating, electromag­netic field generated by MW has the ability to directly interact with the material to produce heat, thereby accelerating chemical, physical, and biological processes. The advantages of employing MW rather than the conventional heating include reduction of process energy requirements, selective processing and capability for instantaneous starting and ceasing of the process. This also offers enormous benefits such as en­ergy efficiency due to rapid and selective heating and the possibility for developing a compact process.

When MW is applied to pretreatment of lignocellulosic biomass, the unique fea­ture of selectively heating the more polar part will result in an improved disruption of the recalcitrant (treatment-resistant) structures of lignocellulose. With the nonthermal effects, electromagnetic field enhances the destruction of crystalline structures and changes the super molecular structure of lignocellulosic material thereby improving its reactivity.

MW pretreatment is also an energy-efficient and environmentally benign tech­nology that aids in the transport of chemicals into the substrates. The project team from the US Department of Energy in partnerships with research institutes including the Oak Ridge National Laboratory [10] has showed that by opening the cellular microstructures of wood, for example, MW pretreatment could permit pumping chemicals for easy access of even large sections (10 cm longx 10 cm diameter) of hardwoods. The project team has demonstrated that, for both hardwood and soft­wood chips, MW pretreatment can decrease both H-factor and chemicals required to pulp hardwoods and softwoods by greater than 40 % with acceptable quality. The steam pressure generated inside the wood breaks the pit membranes and vessel cell walls, thereby enhancing the woods permeability to chemicals and process liquors.

Other than the lignocellulosic biomass, the use of MW for pretreatment of samples for a more efficient oil extraction and pretreatment of FFAs for biodiesel conversion has also been proposed [11, 12].