MW technology relies on the use of electromagnetic waves to generate heat by the oscillation of molecules upon MW absorption. The electromagnetic spectrum for MWs is in between infrared radiation and radiofrequencies of 30 GHz to 300 MHz, respectively, corresponding to wavelengths of 1 cm to 1 m. Domestic and indus­trial MW systems are required to operate at either 12.2 cm (2.45 GHz) or 33.3 cm (900 MHz) in order not to interfere with the wavelength ranges being utilized for RADAR transmissions and telecommunications [5].

In MW-assisted heating, unlike the conventional methods, the heat is generated within the material, thus rapid heating occurs. As a result of this rapid heating, many MW-assisted organic reactions are accelerated, incomparable with those obtained using the conventional methods. Thus, higher yields and selectivity of target com­pounds can be obtained at shorter reaction times. In addition, many reactions not

possible using the conventional heating methods, had been reported to occur under MW heating. Some very useful information on the fundamentals of MW-enhanced chemistry, its sample preparation, and applications are well presented in the book edited by Kingston and Haswell [6]. The advantages of MW heating are briefly summarized in Table 6.1.

Other than the above-mentioned advantages of rapid, internal, and selective heat­ing, MW non-thermal effects on reaction likely occur, obtaining dramatic increase in the yield even at milder conditions. The MW non-thermal effect is defined as the system response to electromagnetic energy not attributed to temperature variation [7]. Although doubts are cast on the true existence of non-thermal effects, some evi­dences had been reported and postulates had also been made by several researchers. These were summarized in a review article published by de la Hoz et al. [8] com­paring them with the thermal effects. The review of Jacob et al. [9] on thermal and non-thermal interaction of MWs with materials attributed some interesting results on specific MW effects. Evidences on reaction rate enhancement due to some reasons other than the thermal effects such as “hotspots” or localized heating, molecular agitation, improved transport properties were discussed. They suggested that due to the interaction of MW with the materials, heating cannot be simply treated as that similar to the conventional methods as there are a lot of possible mechanisms of activation of materials that might possibly occur.

The above-mentioned thermal and non-thermal effects of MW irradiation offer enormous benefits to the pretreatment of biomass for synthesis of biofuels in­cluding energy efficiency, development of a compact process, rapid heating, and instant on-off process (instant heating-cooling process), among many other possible advantages.