**9. Coupling with microwave irradiation**

Various catalytic processes for production of biofuels from palm oil and oil palm biomass were also summarized by Chew and Bhatia [21]. This includes reviews on catalytic processes of palm oil to produce biodiesel, and cracking to produce high-grade biofuels. This review also discusses biomass gasification to produce hydrogen and syngas, and its conversion to liquid

**Figure 7.** Typical GC-MS chromatograms of bio-oil obtained from EFB, PMF and PKS at the optimum condition (T =

390°C, P=25 MPa).

16 Book Title

**(EFB)**

**(PMF)**

**(PKS)**

**7.3 Results of GC-MS analyses of the obtained bio-oils** 

470 Biofuels - Status and Perspective

Microwave technology utilizes electromagnetic waves for heating as a result of friction produced by oscillation of molecules as they realign with microwave upon absorption. Microwaves operate in between infrared radiation and radiofrequencies of 30 GHz to 300 MHz, corresponding to wavelengths of 1 cm to 1 m. Strict government regulations only allow domestic and microwave apparatus to operate at either 122.2 cm (2.45 GHz) or 33.3 cm (900 MHz) to avoid interference with RADAR transmissions and telecommunications [24].

Using microwave, rapid heating occurs as a result of heat being generated within the material, unlike the conventional method where heating of material is performed by conduction from heat sources outside of the vessel. Many microwave-assisted reactions are accelerated due to this rapid internal heating, resulting to tremendous increase in reaction rates compared to the conventional methods. Thus, even at shorter reaction times, higher yields and selectivity of target compounds are expected. Some other reactions not possible with the conventional heating methods had also been reported to proceed by microwave irradiation. Kingston and Haswell had summarized available information related to the fundamentals of microwave chemistry in sample preparations and other applications [25].

Microwave non-thermal effects on reaction have also been reported to occur, thus even at milder conditions, dramatic increase in the yield can be obtained. Hoz et al. reported in a review of related topics some evidences and postulates on the existence of this phenomenon, but doubts still remain for many researchers [26]. Jacob et al. has reported some interesting results on specific microwave effects on thermal and non-thermal interaction of microwaves with materials [27]. Some of the topics discussed include "hot spots" or localized heating, molecular agitation, improved transport properties of materials with microwaves, and some evidences on reaction rate enhancements as result of these improved properties of materials. A lot of mechanisms of activation of materials are thought to be possible with microwave interaction; thus, it is difficult to simply treat microwave heating as similar to that of the conventional.

The above-mentioned thermal and non-thermal effects of microwave irradiation offer enor‐ mous benefits to the treatment of biomass for synthesis of biofuels including energy efficiency, development of a compact process, rapid heating and instant on-off process (instant heatingcooling process), among many other possible advantages.

Microwave-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 microwave oscillations. Compared to conventional heating, electromagnetic field generated by microwave has the ability to directly interact with the material to produce heat, thereby accelerating chemical, physical and biological processes. The advantages of employing microwave rather than the conventional heating include reduction of process energy require‐ ments, 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 microwave is applied to the treatment of lignocellulosic biomass, the unique feature of selectively heating the more polar part will result in an improved disruption of the recalcitrant structures of lignocellulose. With the non-thermal effects, electromagnetic field enhances the destruction of crystalline structures and changes the super molecular structure of lignocellu‐ losic material thereby improving its reactivity.

Microwave 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 [28] has showed that by opening the cellular microstructures of wood, for example, microwave pretreatment could permit pumping chemicals for easy access of even large sections (10 cm long x 10 cm diameter) of hardwoods. The project team has demonstrated that, for both hardwood and softwood chips, microwave pretreatment can decrease both Hfactor 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 microwave for pretreatment of samples for a more efficient oil extraction and pretreatment of free fatty acids for biodiesel conversion has also been proposed.
