**10. Summary and future directions**

Haswell had summarized available information related to the fundamentals of microwave

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 heating-

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

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‐

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

chemistry in sample preparations and other applications [25].

472 Biofuels - Status and Perspective

cooling process), among many other possible advantages.

heating, and the possibility for developing a compact process.

losic material thereby improving its reactivity.

liquors.

This research focuses on the feasibility of converting oil palm biomass wastes to bio-oil under sub- and supercritical water. The effects of reaction temperature and pressure on the efficiency of the liquefaction of EFB, MCF and PKS were investigated. Based on the obtained results, the optimum liquefaction condition for EFB, PMF and PKS could be obtained at supercritical conditions of water. EFB gave the highest bio-oil yield at 390°C and 25 MPa, whereas for PMF and PKS, the highest bio-oil yields were at 450°C and 30 MPa. Lignocellulosic contents of the biomass affect their optimum liquefaction condition and the yield of bio-oil. Temperature and pressure have also different impacts on the bio-oil yield. Compared to pyrolysis, the proposed hydrothermal liquefaction method using sub- and supercritical fluid utilizes the water originally present in the feedstock, thus avoiding the energy-intensive drying procedures.

Future directions will cover evaluation of global kinetics of the liquefaction rate based on the reaction equilibrium data collected and use of various catalysts to enhance liquefaction rates. The major contribution expected from this research will be a new technology as applied to the liquefaction of biomass using sub- and supercritical fluid.
