Abstract
In agriculture and industry, it is often necessary to heat surface layers of material like soil, timber, concrete, and so on, with microwave (MW) energy. Traditional MW irradiators (antennas) cannot restrain their heating to the surface, with the energy penetrating deeply into the material. Slow-wave comb applicators can provide the required energy distribution in the surface layer. Theoretical analyses of the comb applicators used for heating were carried out and on this basis, three comb applicators were designed and made for soil treatment: two applicators Comb 1 and Comb 2 for frequency 2.45 GHz and Comb 3 for frequency 0.922 GHz. An experimental study of applicators was carried out using two MW plants: 30 kW (2.45 GHz) and 60 kW (0.922 GHz) for heating soil with moisture content in the range from 32% to 173% and density 460 to 1290 kg m−3. The study showed that comb applicators provide the following advantages: reduction in energy dissipation in material depth and release of the significant part of applied MW energy in layers close to the applicator surface. Comb applicators can provide the required soil top layer treatment (sterilization) with reasonable efficiency and can be recommended for practical use in shallow soil treatment for weed seed and pathogen control in agricultural applications. Comb applicators can also be used for effective heating and MW treatment of the surface layers of wood, concrete, bricks, plastics, and other dielectric materials.
Highlights
It has been demonstrated that microwave soil heating can deactivate weed seeds [1,2] and some soil borne organisms [3,4] in soil; microwave technology is being developed as a potential weed control strategy [5,6,7]
Almost all the energy was absorbed by 200 mm along the applicator under any soil moisture content regime
The maximum energy release took place at 100–120 mm from the beginning of the applicator. Both Comb 1 and Comb 2 give similar energy distributions in the soil along the applicator length, across the applicator width and in soil depth with moisture contents (MC) ranging from 32% to 174%
Summary
It has been demonstrated that microwave soil heating can deactivate weed seeds [1,2] and some soil borne organisms [3,4] in soil; microwave technology is being developed as a potential weed control strategy [5,6,7]. Especially in zero till cropping systems, the soil seed bank is within 2 cm of the soil surface [8,9]; to minimise energy requirements, it is necessary to only heat the surface layers with microwaves (MW). Traditional MW irradiators (antennas) cannot restrict their heating effect to only the surface layers and energy penetrates deeply into the material where the MW fields decay exponentially, according to Maxwell’s equations describing propagation in a semi-infinite lossy medium [10], as energy is naturally absorbed by the material. The resulting energy wastage due to MW field transmission beyond the required heating depth (for example, if the heating depth in soil for killing weed seeds is only 10–20 mm), is very significant. Development of special MW irradiators for surface treatment, to increase process efficiency, is required.
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