Drying characteristics of oil shale under microwave heating based on a fully coupled three-dimensional electromagnetic-thermal-multiphase transport model

Microwave heating (MWH) is a promising drying method for increasing the utilization efficiency of oil shale, but its drying mechanism is not understood completely. In this study, a fully coupled three-dimensional electromagnetic-thermal-multiphase transport model was developed to investigate the drying characteristics of oil shale under MWH. The model was improved by a mesh convergence test, and the laboratory experiments were conducted to validate the model. The obtained temperature profiles showed that microwave irradiation resulted in a high heating rate, but led to an uneven temperature distribution within oil shale. To avoid excessive temperature, we proposed intermittent microwave heating (IMH) and demonstrated its feasibility. Under IMH, moisture evaporation occurred in a very short time owing to the volumetric heating advantage of MWH. A high gas pressure gradient was found in the oil shale sample, which improved multiphase transportation from the interior of the sample to the exterior. By calculating the convective and diffusive fluxes of liquid water and vapor, the main factors controlling drying efficiency of oil shale could be determined. Furthermore, a higher intermittency led to a higher moisture loss under IMH as long as the drying temperature did not exceed the upper limit. Since this model is well-established for oil shale drying under MWH, it can be applied to other porous rocks such as coal, sandstone, and shale, which undergo significant evaporation and other phase changes under MWH.