Microwave heating of coal slime based on multi-physics field simulations: Regulating waveguide port size and sample radius to improve microwave utilization efficiency

Microwave pyrolysis, a promising green-processing technology with high efficiency and low consumption, is particularly applicable in the pyrolysis of solid waste fuels such as coal slime. The microwave thermal effect plays an imperative role in the pyrolysis process. Low microwave utilization efficiency in single-mode microwave cavities is caused by the irregular field intensity distribution, and since of the high manufacturing costs of the equipment, it is unclear the relationship between waveguide port size, microwave field intensity distribution, and the temperature of sample. In this study, the effects of different waveguide port sizes and sample radius on the heating characteristics of coal slime were investigated using COMSOL Multiphysics software to construct a coupled multiphysics finite-element model. Moreover, the effects of the radius of the coal slime on the microwave utilization efficiency. The results demonstrated that only two types of wave distribution were found within the cavity, α and β, under the size of the waveguide port. The field intensity distribution in the β-type cavity was uniform, which prompted the sample temperature from 176 °C to 657 °C and microwave utilization efficiency from 10.16 % to 33.87 %. The maximum microwave utilization efficiency was increased from 33.87 % to 67.22 % by adjusting the radius of the sample at the optimal port size. The efficiency of the microwaves was maximized when the width-to-height ratio of the waveguide is 70:65 and the sample radius is 30–35 mm, and the sample temperature was maintained in a more desirable range. From the findings of this study, both the waveguide port size and sample radius are essential influencing factors in determining the sample temperature and microwave energy utilization efficiency during the heating of coal slime.