Rodriguez Oyarzun, Jose Miguel
(2019)
Scalable microwave applicators for high-temperature processing.
PhD thesis, University of Nottingham.
Abstract
There have been numerous attempts to develop microwave-based processes for applications including waste treatment, metallurgy and advanced material synthesis. At a laboratory scale improvement in energy efficiency and reduction in the environmental impact for many of these applications has been demonstrated to be potentially significant. Unfortunately, very few if any applications of high-temperature microwave processes, with products reaching more than 250 °C, have ever reached a commercial scale. A multiplicity of factors have hindered such developments including a lack of understanding of microwave-material interactions and their variation during real processing conditions and also lack of consideration of material transport techniques for moving the products through the microwave heating systems and their robustness and scalability to full scale. A clear gap in the development of high-temperature microwave applications has been highlighted, with no or very few, clear scale-up strategies or hardware solutions able to meet the requirements around robustness for continuous operation and ultimately commercial implementation. In this work, three concepts have been developed which have enabled a range of scalable options for microwave applicators that can both underpin proof of concept and full-scale processes, crucially at high temperature. An elliptical cylindrical cavity applicator, a hybrid toroidal fluidised bed, and a metallic conveyor belt system are presented. All of these concepts have been developed to the prototype stage, and the design and performance have been studied by numerical simulation and validated through experimental measurements and cross-platform simulation. The simulation framework was also validated against a number of different processing systems with several test materials evidencing suitability for the design and evaluation of microwave heating systems for high-temperature application.
For liquids and fine powders, and especially when high power densities within the product are required, an elliptical cylindrical cavity provides a stable and practical replacement to more traditional single-mode cavities. Simulated and experimental results showed good agreement with a -10 dB bandwidth of 9 MHz and 8 MHz and quality factors of 198 and 233 respectively. The treatment of waste oils and plastics, as well as the purification of minerals like quartz powders, could be potentially suitable applications for this class of applicator. For particulates from 1 to 20 mm, the hybrid fluidised bed system demonstrates significant potential for use in high-temperature processes, especially for medium to high microwave loss products. Only a 1 MHz difference between experimental and measurement -10 dB bandwidth and practically the same resonant frequency was observed. Potential applications are considered to be the pyrolysis of biomasses, coals and various carbonaceous materials and the reduction of iron ores. The system overcomes particulate size limitations found in conventional fluidised beds as well as reduce microwave field homogeneity issues by thorough mixing of the product without integrated moving parts. For larger particles, greater than 10 mm in size, the metal conveyor belt system provides a suitable candidate for high-throughput processes in which the major limitation is the lack of robustness and microwave compatibility of conveying technology. Similar resonant levels of -32 dB, and 69 MHz and 76 MHz -10 dB bandwidth for the experimental and simulation respectively were produced, with a similar heating behaviour. Envisaged potential applications include coke-making from low ranking coals and the direct reduction of iron.
Further work includes the finalisation of commissioning for experimental use in a variety of applications. The performance of test work that validates the power handling capabilities and robustness of the different systems under a variety of conditions and materials. The implementation of continuous processing strategies in the developed systems to study their engineering performance and commercial prospects.
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