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Microwave heating of waste results in chemical breakdown that can lead to conversion of mixed waste materials to fuel. Heating waste mixtures with microwave energy rather than incineration results in faster breakdown and can therefore be more efficient. Here we address heating of small volumes of mixed food waste materials with widely differing and temperature-dependent electrical properties. Uniform heating is accomplished with mode mixing within a loaded cavity and by spatial power combining of solid-state power amplifiers (SSPAs). We present a heating comparison of two circuit-combined and spatially-combined 2.45 GHz 70-W 65% efficient GaN SSPAs with controlled relative phase. The heating efficacy is shown to improve by volumetric combining inside the waste loading. The temperature changes in several locations and for several common waste materials and mixtures are investigated and compared to FEM electromagnetic simulations, as well as FDTD multi-physics simulations that incorporate thermal dependence of material properties. The approach is scalable in volume and power, demonstrated by a simulation comparison of the 1.4 L small cavity to a 5.2 L volume.INDEX TERMS Microwave heating, power amplifiers, efficiency, power combining, multi-physics modelling.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

This paper presents an efficiency study for scalable microwave waste management. When waste with carbon content is subjected to volume power densities on the order of 0.25W/cm3 at GHz frequencies, it converts to solid coke fuel with oil and gas bi-products that can further be processed for fuel, leaving no trace. For an efficient process, a well-controlled uniform RF field should be maintained in a non-uniform and time-variable material. We are developing a 2.45-GHz active microwave cavity with solid-state (GaN) spatially power combined sources for lower volumes, Fig.1. In the energy balance calculations, the input energy into the system consists of the waste chemical energy and the DC electrical energy used to obtain the RF power with an efficiency that can reach 70% for kW power levels. The efficiency of RF power conversion to heat in the waste mass is calculated from full-wave simulations for typical waste mixtures and ranges from 10 to 90% depending on the material and cavity filling. The output energy estimates are collected from various pyrolysis process descriptions, e.g. [1], with the total energy being that of the solid fuel (35MJ/kg) and oil caloric values, e.g. 40MJ/kg for plastics and about 10-15MJ/kg for nonplastics [2]. A byproduct is flue gas which can be converted to Syngas [3]. The total worse-case carbon footprint balance (0.3-3) calculations will be presented.     Fig. 1. Block diagram of active microwave cavity for waste to fuel conversion.   References D. Czajczyńska, “Potential of pyrolysis processes in the waste management sector,” Thermal Science and Engineering Progress, vol. 3, p. 171. Sept., 2017. J.A. Onwudili, “Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: effects of temperature and residence time,” Journal of Analytical and Applied Pyrolysis, vol. 86 p. 293–303. Nov., 2009. S. Chunshan, "Tri-reforming of methane: a novel concept for synthesis of industrially useful synthesis gas with desired H2/CO ratios using CO2 in flue gas of power plants without CO2 separation." Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem 49, no. 1 (2004): 128.

This paper presents simulation and measurement results of a 2–4 GHz octave bandwidth interference suppression circuit. The circuit accomplishes the function of a tunable frequency notch through an interferometer architecture. The relative delay in the interferometer paths is varied with GaN monolithic microwave integrated circuit tunable delay lines. The delay is adjusted by varying the drain voltage of cold‐FET connected high electron mobility transistors acting as varactors. Two types of periodically‐loaded delay lines are compared: a uniform and a tapered design. A simple theoretical study, relating the delays and amplitudes in the interferometer circuit branches, is developed to inform the design. Two interference suppression hybrid circuits are implemented, and measurements demonstrate a 25–40 dB notch across the 2.24–4 GHz range for the uniform delay line, and 2.32–4.13 GHz for the tapered design. The return loss for both designs remains below 10 dB. Measurements with two tones spaced 0.5 and 1 GHz for varying tone power are performed to quantify suppression. The circuit can handle an input power of 37 dBm and maintains performance with two simultaneous 25 dBm tones spaced 0.5 GHz apart. Linearity is characterised with 10 MHz two‐tone measurements, and the circuit demonstrates a 3rd‐order intercept input power larger than 30 dBm for control biases above −12 V.