Decoupling thermal and non-thermal effects of the microwaves for lignocellulosic biomass pretreatment

In order to develop an efficient biorefinery concept based on the sustainable use of biomass, pretreatments are a major issue as they consume a considerable amount of energy, among others. Competitive and sustainable bioconversion technologies are required and microwave (MW) treatment has emerged as an interesting option during the recent years. Nevertheless, MW action mode on lignocellulosic biomass is not yet fully understood. With some exceptions, experimental investigations dealing with MW pretreatment do not address this important aspect nor energy and power level effects on biomass conversion. To study the action mode of MW on lignocellulosic biomass and its energy absorption behavior, MW assisted pretreatments of three types of grass biomass of industrial interest were performed at several solid to liquid (S/L) ratios, i.e. 1/21 or 1/9, with different solvents (water, ethanol, acid water or alkaline water) and different MW power densities (2.38 or 4.76 W/g). Incident, reflected and absorbed MW power levels and the reaction temperatures were monitored on-line. Thermal and non-thermal MW effects were studied by comparing experiments performed with a glass reactor and an in-house-made jacketed glass reactor designed to avoid the heating of sample. Under the tested conditions, only thermal MW effects have been detected. In an attempt to optimize the bio-solubilization of biomass components, microwave pretreatment with increased pressure and incident power density (7.5 W/g) was also tested. The cell wall was significantly impacted with a solubilization of 33% of hemicelluloses, which opens the way to industrial applications. To our knowledge, this is the first reported experimental study that addresses the non-thermal MW effects on biomass by decoupling heating effect from irradiation.