Standard Radar Frequency Letter-Band Nomenclature (IEEE Standard 521-1984)
Microwave frequencies occupy portions of the electromagnetic spectrum between 300 MHz to 300 GHz. The full range of microwave frequencies is subdivided into various bands (Table 1). Because microwaves are also used in the communication, navigation and defence industries, their use in thermal heating is restricted to a small subset of the available frequency bands. In Australia, the commonly used frequencies include 434 ± 1 MHz, 922 ± 4 MHz, 2450 ± 50 MHz and 5800 ± 75 MHz . These frequencies have been set aside for Industrial, Scientific and Medical (ISM) applications. All these frequencies interact to some degree with moist materials.
|L band||1 to 2||30.0 to 15.0|
|S band||2 to 4||15 to 7.5|
|C band||4 to 8||7.5 to 3.8|
|X band||8 to 12||3.8 to 2.5|
|Ku band||12 to 18||2.5 to 1.7|
|K band||18 to 27||1.7 to 1.1|
|Ka band||27 to 40||1.1 to 0.75|
|V band||40 to 75||0.75 to 0.40|
|W band||75 to 110||0.40 to 0.27|
The major advantages of microwave heating are its short startup, precise control, and volumetric heating . In industry, microwave heating is used for drying [2-5], oil extraction from tar sands, cross-linking of polymers, metal casting , medical applications , pest control , enhancing seed germination , and solvent free chemistry . Microwave heating has been applied to various agricultural and forestry problems and products since the 1960’s . Studies have been undertaken to use microwave energy: to improve crop handling, storage and preservation; to provide pest and weed control for agricultural production, for food preservation and quarantine purposes; and for preconditioning of products for better quality and more energy efficient processing. This chapter is concerned with microwave heating applications in the agricultural and forestry industries for purposes other than human food processing and consists of a review, update, and discussion of some potential applications that may be of interest to the microwave power and agricultural industries.
2. A brief review of microwave heating in moist materials
Most agricultural and forest products are a heterogeneous mixture of various organic molecules and water, arranged in various geometries. There are some important features of microwave heating that will determine the final temperature and moisture distribution during microwave processing.
Any realistic study of microwave heating in moist materials must account for simultaneous heat and moisture diffusion through the material. The coupling between heat and moisture is well known but not very well understood . Henry  first proposed the theory for simultaneous diffusion of heat and moisture in a textile package. Crank  later presented a more thorough development of Henry’s work. Since then, this theory has been rewritten and used by many authors [11, 14-17]. Microwave heating can be described by a combined heat and moisture diffusion equation that includes a volumetric heating term associated with the dissipation of microwave energy in the material :
This can be expressed in a simpler form if ( = pMv + nT:
The constants of association, p and n, are calculated to satisfy:
Essentially, the combined heat and moisture diffusion coefficient (() has two independent values, implying that heating and moisture movement occurs in two independent waves. The slower wave of the coupled heat and moisture system is always slower than either the isothermal diffusion constant for moisture or the constant vapour concentration diffusion constant for heat diffusion, whichever is less, but never by more than one half [12, 13]. The faster wave is always many times faster than either of these independent diffusion constants.
Considerable evidence exists in literature for rapid heating and drying during microwave processing [5, 18]; therefore it is reasonable to assume that the faster diffusion wave dominates microwave heating in moist materials whereas the slower wave dominates conventional heating. A slow heat and moisture diffusion wave should also exist during microwave heating; however observing this slow wave during microwave heating experiments may be difficult and no evidence of its influence on microwave heating has been seen in literature so far.
The fast heat and moisture diffusion wave has a profound effect on biological materials during microwave heating. In particular, very rapid heat and moisture diffusion during microwave heating yields: faster heating compared to conventional heating; and localized steam explosions which may rupture plant and animal cells [18, 19].
Other important phenomenaassociated with microwave heating include: non-uniform heat and moisture distribution due to the geometry of the microwave applicator  and the geometry of the heated material ; and phenomenon such as thermal runaway which manifest itself as localised “hot spots” and very rapid rises in temperature . The volumetric heating term (q) in equation (2) is strongly influenced by the geometry of the heated material. Ayappa et al.  demonstrate that the equation for electromagnetic power distribution generated in a slab of thickness (W) can be described by:
From this it can be deduced that the temperature/moisture profiles in thick slabs and rectangular blocks usually result in subsurface heating where the maximum temperature is slightly below the material surface .
The microwave’s electric field distribution in the radial dimension of a cylinder can be described by :
The temperature/moisture profiles in small-diameter cylinders, such as a plant stem, usually exhibit pronounced core heating [23, 24]. On the other hand, temperature profiles in large cylinders exhibit subsurface heating, with the peak temperature occurring slightly below the surface .
Microwave heating in spheres is similar to that in cylinders. The microwave’s electric field distribution in the radial dimension of a cylinder can be described by :
This analysis can be used, in conjunction with experimental data, to better understand how microwave heating affects agricultural and forestry products.
3. Crop drying
Many studies have investigated the application of microwave energy to speed up crop and wood drying [25, 26]. Higgins and Spooner  investigated alfalfa, which was microwave-dried for 7, 8, 9 or 10 min in a microwave oven, compared with field and convective oven-dried alfalfa. They found no differences in crude protein,
Adu and Otten  studied the kinetics of microwave drying of white beans. They found that microwave drying was a falling rate process. When constant power was absorbed, seed temperature increased rapidly to a maximum value during the initial stages of drying and began to decrease gradually during the latter stages of drying. To maintain a constant drying temperature, the microwave power had to be increased progressively as the moisture content of the beans decreased due to drying.
This is linked to reductions in the dielectric properties of the beans as moisture is removed , which reduces the interactions between the microwave fields and the beans. The gradual decrease in seed temperature, when the drying rate decreases, is opposite to what is observed during conventional hot air drying. This may be caused by a progressively increasing heat of desorption during the drying process , which is a common phenomenon in hygroscopic solids. Thus, the microwave heating characteristics observed for white beans may apply to other hygroscopic solids, such as soils, wood, and fodder chaff.
Microwave drying is fast, as may be expected from the coupling of heat and moisture transport described earlier. The drying curve (Figures 1 and 3) exhibits a short relatively slow drying period, followed by a much faster almost linear relationship between applied microwave energy and moisture loss. This is followed by a more conventional falling rate drying period (Figure 1); however prolonged microwave treatment at high power leads to a phenomenon known as “thermal runaway”, which causes charring (Figure 2). Microwave treatment profoundly affects the germination performance of grains, with any reasonable application of microwave power totally inhibiting grain germination (Table 2). The microwave drying curve can be described by:
Problems with thermal runaway during microwave drying can be overcome by using cyclic drying instead of continuous microwave heating . In this technique, microwave energy is applied for a short time to induce rapid heating and moisture movement and then the product is allowed to equilibrate during a period with no microwave heating. This technique has been successfully applied to timber drying  (Figure 3). The resulting drying curve is still described by equation (10).
|Microwave Power (%)||Microwave treatment time (minutes)|
Walde, et al.  studied the effect of microwave drying on the grinding properties of wheat. The microwave dried samples were crisp and consumed less energy for grinding compared to the control samples. The Bond’s work index for the bulk sample was 2.26 kWh kg-1 compared to 2.41 kWh kg-1 for the control samples of equal moisture content. These studies indicated that microwave drying of wheat before grinding helps reduce power consumption in wheat milling. The microwave drying did not change the total protein content, but there were some functional changes in the protein, which was evident from gluten measurements.
Studies have also been carried out on the dry milling characteristics of maize grains, which were dried previously from different initial moisture contents (MCi) in a domestic microwave oven . The MCi ranged from 9.6% to 32.5% on a dry sample basis. Drying was also carried out in a convective dryer at temperatures of 65 – 90 °C. The drying rate curve showed a typical case of moisture loss by diffusion from grains. The dried samples were ground in a hammer mill and the Bond’s work index was found to decrease with increasing duration of microwave drying. There was no difference in protein and starch content between the different treatments. Viscosity measurements were made with 10% suspensions of the flour in water which were heated to 80 – 90 °C and allowed to cooled. Viscosity decreased with increasing microwave drying of the grains. The colour analysis showed that flour of the microwave-dried samples was brighter than the control and convective dried samples. Based on these and other studies, microwave drying of agricultural and forestry commodities appear to be a viable alternative to conventional methods, especially when rapid drying and high throughputs of moist material are desirable.
Dried timber, nuts and fruits are commonly treated by chemical fumigation to control field and storage pests before being shipped to domestic and international markets. Because chemical fumigants such as methyl bromide are no longer available , there is a heightened interest in developing non–chemical pest control. An important key to developing successful thermal treatments is to balance the need for complete insect mortality with minimal impact on the product quality. A common difficulty in using conventional hot–air disinfestation is the slow heating rate, non–uniform temperature distribution, and possible heat damage to heat–sensitive commodities . A more promising approach is to heat the commodity rapidly using radio frequency (RF) or microwave dielectric heating to control insects [35, 36].
Interest in controlling insects, using electromagnetic energy, dates back nearly 70 years. Headlee [37, 38], cites one earlier report of experiments determining lethal exposures for several insect species to 12 MHz electric fields and the body temperatures produced in honey bees due to dielectric heating. Nelson  has shown that microwaves can kill insects in grain; however one of the challenges for microwave insect control is to differentially heat the insects in preference to their surrounds. Nelson  shows that differential heating depends on microwave frequency. It appears that using a 2.45 GHz microwave system, which is the frequency used in domestic microwave ovens, heats the bulk material, which then transfers heat to the insects; however lower frequencies heat the insects without raising the temperature of the surrounding material beyond 50°C .
Nzokou et al  investigated the use of kiln and microwave heat treatments for the sanitisation of emerald ash borer (
In spite of this, with the high costs and level of energy needed to thoroughly heat logs to the desired 65 °C using conventional heating, microwave heating is still a very attractive solution for rapid heat sterilisation of infested wood materials . The problem of ensuring appropriate temperature distribution inside treated materials can be easily overcome by using appropriate microwave applicators rather than a multi-mode cavity . Several options are available including conveyer belt feeds through a long choke tunnel into a purpose built applicator , or projecting a very intense but short duration microwave field pulse into the material, using an antenna. Plaza
It has been shown earlier that microwave heating in moist materials, such as the body of an insect, induces a very fast moving wave of heat and water vapour . The intensity of this wave is directly linked to the intensity of the microwave fields , therefore using very intense microwave fields may rupture the internal organs of insects, due to local steam explosions.
The interaction of electromagnetic energy with matter is determined by the dielectric properties of the material. The permittivity of a material can be expressed as a complex quantity, the real part (κ’) of which is associated with the capability of the material for storing energy in the electric field of the electromagnetic wave, and the imaginary part (κ”) is associated with the conversion of electromagnetic energy to heat inside the material . This is the phenomenon commonly referred to as dielectric heating. The dielectric properties also determine the reflectivity of a material.
The power dissipated per unit volume in a nonmagnetic, uniform material exposed to radio frequency (RF) or microwave fields can be expressed as:
Therefore in a system composed of two or more materials, there will be preferential heating in favour of the material with the least reflectivity and higher dielectric loss factor. The rate of temperature increase also depends on the density and thermal capacity of the heated material [35, 36]:
Termites are a good example of insects that infest economically important products. For example, in the United States, the annual cost of treating damage caused by the Formosan termite (
Liquid water exhibits dielectric relaxation at around 22 GHz  (Figure 4). There are higher dielectric relaxations in water at about 280 GHz , 4.5 THz and 15.4 THz . The dielectric properties of grains, soil and wood also depend on their moisture content  (Figure 5).
Dry wood-in-service is in hydro-thermodynamic equilibrium with its surroundings. This condition is known as the equilibrium moisture content . Depending on the atmospheric conditions, equilibrium moisture content is usually about 12% moisture on a dry wood weight for weight basis. When termites invade wood, they often import moisture into the structure to maintain a suitable microclimate for their foraging activities. The maximum moisture content that wood can attain before free water begins to form is known as fibre saturation. This occurs at about 25 - 30 % moisture content , depending on the wood species. Fibre saturation refers to the state when all the cells are free of water and only bound water is found within the cell walls. Usually termites do not increase the moisture content beyond fibre saturation. The dielectric properties of termites (modelled as water) and wood at fibre saturation are significantly different from each other (Figure 6).
Treatment of termite infestations using microwave energy, at 2.45 GHz, has been available for some time [50, 51]. This technique does not directly heat the termites, but heats the surrounding wood to more than 55°C , which then causes termite mortality. Unfortunately, the combination of high reflectivity and low dielectric losses for water in the lower microwave frequency band (2.45 GHz) means that there is virtually no differential heating between the termites and wood that is at fibre saturation; however significant differential heating should occur once the frequency increases above 20 GHz (Figure 7). Research in the field of ultra-high frequencies (>20 GHz) indicates that these frequencies may selectively heat insect pests in favour of the materials they infest [52, 53]. Therefore research into ultra-high frequency microwave based insect control should yield some valuable insights over the coming decades .
Devine et al.  conducted a trial in which microwave radiation, coupled with steam heat, was used to treat organic waste (1,136 kg of culled turkey carcasses), designed to simulate a small-scale poultry mortality event. They inoculated the turkey carcasses with
5. Effect of microwave heating on seeds and plants
In 2006, the cost of weed management and loss of production to Australian agricultural industries was estimated to be about $4 billion annually . Depletion of the weed seed bank is critically important to overcoming infestations of various weed species . Mechanical and chemical controls are the most common methods of weed management in cropping systems [58, 59]. The success of these methods usually depends on destroying the highest number of plants during their early growth stages  before they interfere with crop production and subsequently set further seed. These strategies must be employed continually to deplete the weed seed bank.
Interest in the effects of high frequency electromagnetic waves on biological materials dates back to the late 19th century, while interest in the effect of high frequency waves on plant material began in the 1920’s . In many cases, short exposure of seeds to radio frequency and microwave radiation resulted in increased germination and vigour of the emerging seedlings [61, 62]; however, long exposure usually resulted in seed death .
Barker and Craker  investigated the use of microwave heating in soils of varying moisture content (10-280 g water/kg of dry soil) to kill ‘Ogle’ Oats (
Several patents dealing with microwave treatment of weeds and their seeds have been registered [67-69]; however none of these systems appear to have been commercially developed. This may be due to concerns about the energy requirements to manage weed seeds in the soil using microwave energy. In a theoretical argument based on the dielectric and density properties of seeds and soils, Nelson  demonstrated that using microwaves to selectively heat seeds in the soil “
Microwaves can kill a range of weed seeds in the soil [63, 64, 72], however fewer studies have considered the efficacy of using microwave energy to manage already emerged weed plants. Davis et al.  considered the effect of microwave energy on bean (
Brodie et al.  studied the effect of microwave treatment on Marshmallow (
Horn antennas (Figure 8), like the design used in the experimental prototype are very popular for microwave communication systems . The vertical plane of the horn antenna is usually referred to as the E-plane, because of the orientation of the electrical field (or E-field) in the antenna’s aperture. The horizontal plane is referred to as the H-plane, because of the orientation of the magnetic field (or H-field) of the microwave energy. The H-plane electric field distribution in the aperture of a horn antenna, fed from a wave-guide propagating in the TE10 mode, is approximated by:
In the case of a cylindrical object, such as a plant stem, the microwave’s electric field distribution created by a horn antenna  can be described by:
The resulting temperature distribution can be described by  :
Three horn applicators, with varying aperture dimension (180mm by 90 mm; 130 mm by 43 mm; and 86 mm by 20 mm), were developed and tested during various experiments [59, 73, 76, 77]. Aperture size of the horn applicator profoundly affects the treatment time needed to kill plants, with the smaller aperture needing much less time to provide a lethal dose; however the total energy density needed to kill the plants (microwave output power density multiplied by treatment time) was the same irrespective of the horn aperture size. The resulting lethal dose, which was sufficient to kill all the test species, was 350 J cm-2 for each plant.
Because energy rather than treatment time is the key factor in plant mortality, two options for using microwave energy to manage weeds become evident; either a prolonged exposure to very diffuse microwave fields or a strategic application of an intensely focused microwave pulse is sufficient to kill plants. Bigu-Del-Blanco
Based on energy calculations for plants and seeds on the surface of sandy soil [72, 76], the energy needed to kill dry seeds is an order of magnitude higher than the energy needed to kill already emerged plants. The microwave energy dose needed to kill a paddy melon or fleabane plant was approximately 350 J cm-2 (or 35 GJ ha-1) . This is an order of magnitude higher than the embodied energy (2.2 – 3.0 GJ ha-1) associated with chemical weed management [79-82]; however the real microwave energy requirements on a large scale will depend on the plant density and spatial distribution. Therefore the microwave energy requirements may be greatly reduced if appropriate techniques, such as weed seeker systems , are employed to only turn on the microwave unit when the system encounters a weed in the field. The growing problems of herbicide resistance  also warrants ongoing research and development of microwave weed control technologies. Other strategies may also reduce this energy requirement even further.
It has been well documented that the dielectric properties of most materials are temperature and moisture dependent [22, 85-88]. Ulaby and El-Rayes [89, 90] studied the dielectric properties of plant materials at microwave frequencies. Plants with high moisture content have higher dielectric constants and will therefore interact more with the microwave fields, rendering them more susceptible to microwave damage.
Equation (15) was used in an iterative calculation, where the new dielectric properties for plant based materials were recalculated after every second of microwave heating, based on the changes in temperature and moisture content of the plant during that interval of the microwave heating progresses. This results in non-linear heating responses and sudden jumps in temperature when there is no change in the applied microwave field strength (Figure 9). The sudden jump in temperature for the 15 mm diameter stem is the result of “thermal runaway”. The onset of thermal runaway is also dependent on the microwave field intensity (Figure 10) and the heat transfer properties of the heated material (Figure 11). Under the influence of simultaneous heat and moisture diffusion during microwave heating, the effective thermal conductivity of a microwave heated material can be many times the normal value for the plant tissue  (see equation (3)).
In most cases, thermal runaway is a problem during microwave heating. It usually leads to undesirable charring of the microwave heated material (see Figure 2) ; however it has been very effectively used in some applications such as the development of a microwave drill [92, 93] and preconditioning of wood for further preservative treatment and drying [94, 95].
Vriezinga has concluded that thermal runaway in moist materials, and water in general, is caused by: the specific characteristic of the dielectric properties of water, which decrease with increasing temperature  (Figure 4); and resonance of the electromagnetic waves within the irradiated medium due to changes in the dielectric properties of the material during heating [87, 88]. Resonance will only occur when the object’s dimensions are some multiple of the wave length of the microwave fields inside the object.That is why thermal runaway only becomes evident in the 15 mm diameter stem (Figure 9), while the smaller stems are too small to allow internal field resonance. Internal steam pressure, induced by thermal runaway, may cause stem rupture, if sufficient microwave field intensity can be focused onto the plants (Figure 10).
Moriwaki et al  studied the dehydrochlorination of polyvinyl chloride (PVC) by microwave irradiation using an optical fibre thermo-sensor to investigate the relationship between temperature and microwave absorption onto PVC. Their observations were that: at the beginning of microwave irradiation, the temperature rose in direct proportion to the strength of the incident microwave power and irradiation time; after exceeding a critical condition, the temperature rose quickly (thermal runaway); higher incident microwave power led to thermal runaway starting earlier in the heating process; and higher pre-heating temperatures also led to a faster onset of thermal runaway conditions. These findings are consistent with the modelling displayed in Figures 9, 10, and 11.
Total treatment time, and therefore total applied microwave energy, could be significantly reduced, if thermal runaway can be induced in weed plants during microwave treatment. For example, extrapolating the data presented in Figure 9, it takes approximately 200 to 250 seconds for the 10 mm diameter stem to reach 40 °C; however the 15 mm diameter stem reaches 40 °C in 25 seconds under the same applied microwave power. Therefore the energy required to achieve this temperature rise in the 15 mm diameter stem is only 10 % of the energy needed to heat the 10 mm diameter stem.
Based on existing data, phenomena such as thermal runaway, and the nonlinear temperature/ microwave field strength relationships, it is difficult to discuss “scale up” from small laboratory studies and modelling exercises such as have been used here; however if thermal runaway can be induced in plant tissues, treatment time, and the associated treatment energy, may be drastically reduced; resulting in comparable energy needs to those associated with conventional chemical weed control. This scenario can only be explored by further research into the microwave heating of living plants and plant based materials.
In weed control, microwave radiation is not affected by wind, which extends the application periods compared with conventional herbicide spraying. Energy can also be focused onto individual plants without affecting adjacent plants . This would be very useful for in-crop or spot weed control activities. Microwave energy can also kill the roots and seeds that are buried to a depth of several centimetres in the soil [73, 97].
6. Microwave treatment of animal fodder
Hay is an important feed source for ruminant animals so every effort should be made to improve its feed conversion efficiency and reduce the risk of importing weed seeds as hay is transported from one location to another. Similarly, cereal grains are the base of most horse rations, because they are a valuable source of digestible energy; however their use is always associated with some risk.
The major concern when feeding cereal grains to horses is the risk of incomplete starch digestion in the small intestine, which enables significant amounts of starch to pass through to the caecum and colon. When starch is able to reach these organs it rapidly ferments producing an accumulation of acidic products, which place the horse at risk of developing serious and potentially fatal illnesses such as laminitis, colic and ulcers .
When 25 kg bags of lucerne fodder, treated in an experimental 6 kW, 2.45 GHz, microwave heating chamber  were subjected to a similar
The efficiency of chaff and fodder treatment using microwave energy depends on the applied microwave energy and the frequency at which the microwave system operates. Absorbed energy, calculated by measuring the combination of sensible (temperature rise) and latent heat (moisture loss) in treated samples, is much higher at 2.45 GHz than at 922 MHz (Figure 12). It is also evident that efficiency (i.e. the ratio of absorbed energy to applied microwave energy) decreases as the applied microwave energy increases (Figure 13). This is attributable to the increasing transparency of the fodder material to microwave energy as it dries during microwave treatment. The dielectric properties, and therefore the microwave heating effect, reduce as the moisture content of plant materials decrease (Figure 5). Some of these problems of material transparency during microwave treatment can be overcome by compressing the fodder, which increases its ability to absorb microwave energy (Figure 14).
Brodie et al.  treated 25 kg samples of lucerne chaff in an experimental 6 kW microwave chamber . The temperature in the air space at the top of the lucerne bags rose to 100 °C in ~12 min and fluctuated above 100 °C for the remainder of the treatment time (Figure 15). The maximum temperature in the air space was 115 °C. The maximum temperature in the lucerne (99.5 °C) was measured by the probe facing the microwave magnetrons whereas the maximum temperature measured by the probe in the front of the bag, facing the door of the microwave chamber, was only 94 °C.
The temperature in the lucerne increased steadily at a rate of ~2 °C/min of microwave heating time for the first 20–25 min of heating. At this stage there was a sudden increase in heating rate (~6 °C/min) until the temperature stabilised at ~98 °C (Figure 15). The sudden jump in the heating rate after some time of steady heating may be evidence of thermal runaway (Figure 9). Observation of the treated chaff showed no signs of charring; however the chaff was dry and crisp . The onset of thermal runaway dramatically increases the heating efficiency. In this example, the heating rate during thermal runaway is three times higher than during the normal heating phase. Provided charring can be avoided, inducing thermal runaway in the treated chaff may drastically improve treatment efficiency. The onset of thermal runaway is usually quicker when the microwave field intensity is higher (Figure 10) and the thermal conductivity of the material is increased. In the case of fodder chaff, thermal conductivity is proportional to density, which may partially explain why increasing the material density significantly increases the heating rate of the chaff (Figure 14). This needs further exploration.
7. Microwave assisted extraction
During microwave assisted extraction (MAE), plant materials such as wood, seeds and leaves are suspended in solvents and the mixture is exposed to microwave heating instead of conventional heating. Enhanced rates of plant oil extraction have been observed for a range of plant materials. Chen and Spiro  examined the extraction of the essential oils of peppermint and rosemary from hexane and ethanol mixtures and found that yields were more than one third greater in the microwave assisted extractions. Saoud
Although less well described in the literature, an alternative approach for utilizing microwave heating of plant based materials has been to treat the materials with microwave energy prior to conventional extraction processes . Microwave preconditioning of sugar cane prior to juice diffusion studies led to significant decreases in colour and significant increases in juice yield, Brix %, purity and Pol % . Microwave treatment significantly reduced the compression strength of the sugar cane samples , especially while the cane was still hot from the microwave treatment. This treatment option reduced the compressive strength of the cane to about 18 % of its original strength, implying that much less energy would be required to crush the cane for juice extraction.
Controlled application of microwave heating to green timber [18, 41, 94, 112-114] results in local steam explosions and can directly manipulate both permeability and density with potentially less strength loss than is caused by conventional steam conditioning. This technique does not attempt to dry the wood using microwave energy. Rather, it is used to modify the wood structure to facilitate faster drying in more conventional systems. This technology has the potential to: relieve internal log stresses in susceptible species; substantially accelerate drying; improve preservative treatment and resin uptake; and produce new wood-based products for commercial applications. Application of microwave processing technology has the potential to streamline production and to facilitate conveyor belt automation in the timber industry.
8. Microwave assisted pyrolysis and bio-fuel extraction
Three different thermo-chemical conversion processes are possible, depending on the availability of oxygen during the process: combustion (complete oxidation), gasification (partial oxidation) and pyrolysis (thermo-chemical degradation without oxygen). Among these, combustion is the most common option for recovering energy. Combustion is also associated with the generation of carbon oxides, sulphur, nitrogen, chlorine products (dioxins and furans), volatile organic compounds, polycyclic aromatic hydrocarbons, and dust ; however gasification and pyrolysis offer greater efficiencies in energy production, recovery of other compounds and less pollution.
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 . Wood, crops, agricultural and forestry residues, and sewage sludges  can be subjected to pyrolysis processes to recover valuable chemicals and energy.
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 . 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 . 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 . In microwave heating, the lower temperatures in the microwave cavity can also be useful for condensing the final pyrolysis vapours on the cavity walls.
Microwave assisted pyrolysis yields more gas and less carbonaceous (char) residue, which demonstrate the efficiency of microwave energy . 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 heating at lower temperatures .
Bio-fuel extraction is facilitated when microwave energy is used to thermally degrade various organic polymers to facilitate extraction of sugars for fermentation . 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; and Makombu (
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 and opportunities within the primary industries.
= combined temperature and moisture vapour parameter
Em= Microwave energy (J)
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
r= radial distance form the centre of a cylinder or sphere (m)
x= linear distance across the aperture of a horn antenna (m)
z= linear distance from the surface of a slab (m)
Γ= internal reflection coefficient
β= imaginary part of the complex wave number
δ= phase shift of microwave fields at the surface of a material
εο= electrical permittivity of free space
γ= combined diffusivity for simultaneous heat and moisture transfer
κ' = relative dielectric constant of the material
κ" = dielectric loss factor of the material
λ= wave length inside a material (m)
ρ= composite material density (kg m-3)
σ= constant of association relating moisture vapor concentration to moisture content in a solid
τ= transmission coefficient for incoming microwave
ω= angular frequency (rad s-1)