Graham Brodie

Simultaneous Heat and Moisture Diffusion During Microwave Heating of Moist Wood

The influence of simultaneous heat and moisture diffusion on moisture movement in porous materials, such as cotton, has been known since the late 1940s. Until now this theory has not been applied to the problem of microwave heating in moist materials.

Allometric relationships for estimating biomass in grey box (Eucalyptus microcarpa)

Summary A procedure is described for obtaining allometric relationships between stem diameter and aboveground biomass for grey box. Eucalyptus microcarpa. a commonly occurring tree in the Victorian and New South Wales Riverina. While usually having a single trunk, a significant proportion of grey box individuals have multiple stems from near the ground. This is an artifact of enhanced epicormic growth when juvenile, often resulting from stock grazing disturbance. The procedure treats each stem formed from below 30 cm above ground as a discrete tree that shares a proportion of the butt and other elements common to all stems. Significant allometric relationships, using both the commonly-applied log-transformed method, and a recently proposed additive error method, were developed between stem diameter at breast height and at 30 cm above ground, tree height and width, and the dry weights of each aboveground component. These relationships were similar to those previously obtained for single-stemmed trees in southern Australia.

Microwave Pretreatment of Backsawn Messmate Boards to Improve Drying Quality. Part 1

The findings of an investigation into the microwave (MW) modification and conventional kiln drying of backsawn/flatsawn messmate stringybark (Eucalyptus obliqua LHerit) are summarized. The project was conducted in two parts. Part 1, the subject of this article, investigates whether low-intensity microwave modification results in more or less check formation than occurs during conventional drying of eucalypts. Part 2 of the investigation will determine the acceleration in kiln drying that can be achieved through microwave modification of the wood structure and in turn improving permeability. Significant improvement in microwave modified samples compared to controls in relation to check (internal/honeycombing and surface) formation during drying was observed, with Schedule 1 (70 kWh/m3) having 55% of samples without checks compared to the control samples at 31%. The study also found that there is a significant reduction in surface checking (number and depth) in MW modified boards compared to the controls. The control samples exhibited the highest proportion of surface checking, with 65% possessing one or more surface checks, of which 82% extended more than one quarter the width of the sample. This was in marked contrast to Schedule 2 (75 kWh/m3), which resulted in 43% having one or more surface checks, of which 25% extended more than one quarter the width of the sample. The control samples also returned the highest rate of internal/honeycombing checking with 28% of samples affected, while samples pretreated according to MW Schedule 2 yielded the lowest rate, with 11% affected.

The Influence of Load Geometry on Temperature Distribution During Microwave Heating

Uneven temperature distributions during microwave heating in multi-mode ovens are commonly reported. The influence of uneven field distribution can be partially overcome by using turntables, mode stirrers, and well designed launchers. The next most important influence over temperature distributions is the geometry of the work load. New analytical studies of microwave heating in rectangular, cylindrical, and spherical coordinate systems demonstrate that object geometry and consistency profoundly affect temperature/moisture distributions during microwave heating. These analyses show that the highest temperatures in rectangular blocks usually occur in the corners, just below the surface. For small-diameter cylinders with low dielectric loss, the resulting temperature distribution resembles a dumbbell with two temperature peaks along the longitudinal axis; however, as the loss factor or diameter of the cylinder increases, the temperature profile becomes an annulus of high temperature below the upper and lower circular surfaces. Heating in small-diameter spheres is focused in the center, but like the cylinder, there is a transformation to subsurface heating as the diameter or loss factor of the material increases. Heating time also affects temperature distribution, with the peak in temperature moving towards the core of most objects as heating time increases.

Microwave Technologies as Part of an Integrated Weed Management Strategy: A Review

Interest in controlling weed plants using radio frequency or microwave energy has been growing in recent years because of the growing concerns about herbicide resistance and chemical residues in the environment. This paper reviews the prospects of using microwave energy to manage weeds. Microwave energy effectively kills weed plants and their seeds; however, most studies have focused on applying the microwave energy over a sizable area, which requires about ten times the energy that is embodied in conventional chemical treatments to achieve effective weed control. A closer analysis of the microwave heating phenomenon suggests that thermal runaway can reduce microwave weed treatment time by at least one order of magnitude. If thermal runaway can be induced in weed plants, the energy costs associated with microwave weed management would be comparable with chemical weed control.

Biochar produced from biosolids using a single-mode microwave: Characterisation and its potential for phosphorus removal

The amount of biosolids increases every year, and social and environmental concerns are also rising due to heavy metals and pathogen contamination. Even though biosolids are considered as a waste material, they could be used as a precursor in several applications, especially in agriculture due to the presence of essential nutrients. Microwave assisted pyrolysis (MWAP) is a promising technology to safely manage biosolids, while producing value-added products, such as biochar, that can be used to improve soil fertility. This study examined the impact of pyrolysis temperature between 300xa0°C and 800xa0°C on the chemical and physical properties of biochar obtained from biosolids via MWAP. Preliminary phosphorus adsorption tests were carried out with the biochar produced from biosolids. This research demonstrated that pyrolysis temperature affects biochar specific surface area, ash and volatiles content, but does not impact heavily on the pH, chemical composition and crystalline phases of the resultant biochar. Biochar yield decreases as the pyrolysis temperature increases. Phosphorus adsorption capacity of biochar was approximately around 15xa0mg/g of biochar. Biochar resulting from MWAP is a potential candidate for land application with an important role in water and nutrient retention, due to the high surface area.

Design of a Microwave Chamber for the Purpose of Drying of Wood Components for Furniture

A specialized multiple-magnetron, microwave chamber was designed and built to facilitate research into the drying of bent timber components for furniture manufacture. This article reports on the design of this chamber and the findings from experimental drying of wood components. The microwave chamber provides three power levels (2, 4, and 6 kW) by engaging appropriate numbers of standard 1 kW microwave magnetrons. Preliminary experiments revealed that good drying quality was achieved when the microwave power level was set to 2 kW and cycled such that microwave power was applied to the chamber for 30 s, followed by a 90 s relaxation time during which no microwave power was applied and internal heat and moisture diffusion could alleviate hot spots inside the wood samples. The use of microwave drying reduced the drying time from weeks or months to hours. Straight components that started with a low moisture content and all bent components, irrespective of their initial moisture content, were dried with no evidence of drying degrade or collapse. However, the straight components that started with a high moisture content showed severe surface collapse. This may have been because these samples were taken from poor-quality material that was considered unsuitable for bending.

Microwave Soil Heating for Controlling Ryegrass Seed Germination

Several studies have evaluated the effectiveness of microwave soil heating to control a range of weed species. Ryegrass species are problematic weeds of cropping systems across many of the major crop production regions of Australia. This study determined the effect of irradiation time, soil moisture, and distance into the soil from the irradiated surface on the efficacy of microwave soil heating in reducing the germination of both perennial (Lolium perenne) and annual (Lolium rigidum) ryegrass seeds. The study used a horn antenna to apply the microwave energy to the soil. Temperature increased linearly with heating time in dry sand, although the rate of temperature increase depended on distance from the irradiated surface. The rate of temperature increase was highest at 2 cm depth (˜5.6°C min-1) and lowest at 10 cm depth (˜2.6°C min-1). The germination of seeds in the dry sand was unaffected until the microwave treatment time reached 12 min. Even after 12 min of heating in dry sand, only seeds that were close to the irradiated surface were affected, with the germination percentage of seeds on the surface dropping to 2% of the control treatments germination response. There was no effect on seeds that were 10 cm from the irradiated surface. The rate of temperature increase in wet sand was more complex. The temperature in the first 2 cm of sand rose rapidly (˜18.3°C min-1 at 2 cm) during the first 4 min of heating and then stabilized between 80°C and 90°C. As the heating time continued beyond 4 min, the temperature at other locations farther from the irradiated surface also rose and stabilized between 80°C and 90°C; however, the heating rate was slower than near the surface (˜7.8°C min-1 at 10 cm). Heating in the wet sand reduced seed germination at all depths to less than 2.5% of the control treatments germination response after 8 min of microwave treatment. Seeds that were within 5 cm of the irradiated surface of the wet sand were all killed (0% germination) after only 4 min of microwave heating.

Microwave modification of sugar cane to enhance juice extraction during milling.

Abstract Sugar extraction from cane requires shredding and crushing, both of which are energy intensive activities. Cane shredders account for almost 30% of the total power requirements for the juice extraction train in a sugar mill with four mills. Shredder hammers also wear quickly during the crushing season and need to be regularly maintained or replaced. Microwave pre-treatment of other plant based materials has resulted in significant reductions in total processing energy. This paper briefly reviews the underlying structure of sugar cane and how microwave pre-treatment may interact with sugar cane. Microwave treatment reduced the strength of sugar cane samples to 20 % of its untreated value. This strength reduction makes it easier to crush the cane and leads to a 320 % increase in juice yield compared with untreated cane when cane samples were crushed in a press. There was also a 68 % increase in Brix %, a 58 % increase in total dissolved solids, a 58 % reduction in diffusion time, a 39 % increase in Pol %, and a 7 % increase in juice purity compared with the control samples after 60 minutes of diffusion in distilled water.

Microwave Heating in Moist Materials

The definition of “microwaves” is somewhat arbitrary; however microwaves are usually considered to be electromagnetic waves in the frequency range from 300 MHz to 300 GHz. Before World War II, there is little evidence of work on radio frequency or microwave heating; however Kassner (1937b) mentions industrial applications of microwave energy in two of his patents on spark-gap microwave generators (Kassner, 1937a, 1937b, 1938). Unfortunately early studies in radio frequency heating concluded that microwave heating of food stuffs would be most unlikely because the calculated electric field strength required to heat biological materials would approach the breakdown voltage of air (Shaw & Galvin, 1949). A fortuitous discover by Spencer that microwave energy could heat food (Murray, 1958) lead to a series of patents (Spencer, 1947, 1949, 1952) and the development of microwave cooking equipment. The major advantages of microwave heating are its short start-up, precise control and volumetric heating (Ayappa et al., 1991); however microwave heating suffers from: uneven temperature distributions (Van Remmen et al., 1996; Brodie, 2008); unstable temperatures (Vriezinga, 1996, 1998, 1999; Vriezinga et al., 2002); and rapid moisture movement (Brodie, 2007b). This chapter will explore the theory and practice of microwave heating in moist materials.