**Nomenclature**

68 The Development and Application of Microwave Heating

recover valuable chemicals and energy.

heating at lower temperatures [115].

**9. Conclusion** 

Most studies of pyrolysis behaviour have considered lingo-cellulosic materials, which comprise of a mixture of hemicellulose, cellulose, lignin and minor amounts of other organic compounds. While cellulose and hemicelluloses form mainly volatile products during pyrolysis due to the thermal cleavage of the sugar units, lignin mainly forms char since it is not readily cleaved into lower molecular weight fragments [115]. Wood, crops, agricultural and forestry residues, and sewage sludges [116] can be subjected to pyrolysis processes to

Conventional heating transfers heat from the surface towards the centre of the material by convection, conduction and radiation; however microwave heating is a direct conversion of electromagnetic energy into thermal energy within the volume of the material [20]. In microwave heating, the material is at higher temperature than its surroundings, unlike conventional heating where it is necessary for the surrounding atmosphere to reach the desired operating temperature before heating the material [115]. Consequently, microwave heating favours pyrolysis reactions involving the solid material, while conventional heating improves the reactions that take place in surroundings, such as homogeneous reactions in the gas-phase [115]. In microwave heating, the lower temperatures in the microwave cavity

Microwave assisted pyrolysis yields more gas and less carbonaceous (char) residue, which demonstrate the efficiency of microwave energy [115]. The conversion rates in microwave assisted pyrolysis are always higher than those observed in conventional heating at any temperature. The differences between microwave heating and conventional heating seems to be reduced with temperature increase, which points to the higher efficiency of microwave

Bio-fuel extraction is facilitated when microwave energy is used to thermally degrade various organic polymers to facilitate extraction of sugars for fermentation [117]. These sugars can then be fermented and distilled to create fuel alcohols. Woody plant materials are commonly subjected to microwave assisted bio-fuel extraction; however other materials such as discharge from food processing industries, agriculture and fisheries can also be processed using these techniques. Other materials that have been subjected to microwave assisted bio-fuel extraction include: soybean residue; barley malt feed; tea residues; stones from Japanese apricots; corn pericarp, which is a by-product from corn starch production;

Microwave and radio frequency heating have many potential applications in the agricultural and forestry industries. This chapter has discussed a few of these, but there are many more that have not been included. The purpose of this chapter was to encourage practitioners within the microwave engineering and agricultural and forestry industries to explore the many possibilities of applying microwave heating to address many problems

can also be useful for condensing the final pyrolysis vapours on the cavity walls.

and Makombu (*Laminaria japonica*), which is a kind of brown sea algae.

and opportunities within the primary industries.

= combined temperature and moisture vapour parameter *av*= air space fraction in the material *b*= Microwave drying constant to be determined experimentally *C*= thermal capacity of the composite material (J kg-1 °C-1) *Da*= vapor diffusion coefficient of water vapor in air (m2 s-1) *E*= electric field associated with the microwave (V m-1) *Eo*= magnitude of the electric field external to the work load (V m-1) Em= Microwave energy (J) *f*= complex wave number of the form *f* = + *j h*= convective heat transfer at the surface of a heated object (W m-1 K-1) *io*(*x*) = modified spherical Bessel function of the first kind of order zero *Io*(*x)*= modified Bessel function of the first kind of order zero *jo*(*x*) = spherical Bessel function of the first kind of order zero *Jo*(*x*) = Bessel function of the first kind of order zero *i1*(*x*) = modified spherical Bessel function of the first kind of order one *I1*(*x)*= modified Bessel function of the first kind of order one *k*= thermal conductivity of the composite material (W m-1 °C-1) *L*= latent heat of vaporization of water (J kg-1) *Mv*= moisture vapor concentration in the pores of the material (kg m-3) MC= Moisture content (kg kg-1 dry matter) MCi= Initial moisture content (kg kg-1 dry matter) MCf= Final moisture content (kg kg-1 dry matter) n = constant of association relating water vapor concentration to internal temperature of a solid p = constant of association relating internal temperature of a solid to water vapor concentration *q*= volumetric heat generated by microwave fields (W m-3) r= radial distance form the centre of a cylinder or sphere (m) *ro*= external radius of the cylinder or sphere (m) *t*= heating time (s) *T*= temperature (°C) *W*= thickness of the slab (m) x= linear distance across the aperture of a horn antenna (m) z= linear distance from the surface of a slab (m) = internal reflection coefficient = real part of the complex wave number *f* = imaginary part of the complex wave number *f* = phase shift of microwave fields at the surface of a material = electrical permittivity of free space = combined diffusivity for simultaneous heat and moisture transfer ' = relative dielectric constant of the material

```
" = dielectric loss factor of the material 
= wave length inside a material (m) 
= composite material density (kg m-3) 
s= density of the solid material (kg m-3) 
= constant of association relating moisture vapor concentration to moisture content in a 
solid 
= transmission coefficient for incoming microwave 
v= tortuosity factor 
= angular frequency (rad s-1)
```