Three-dimensional numerical simulation on the thermal response of oil shale subjected to microwave heating

Abstract

Microwave heating technology has received significant attention as an alternative to conventional heating methods for oil shale upgrading. To investigate the thermal response of oil shale such as temperature distribution, product migration, pressure generation, and thermal stress under microwave irradiation, numerical simulations were conducted based on a fully coupled three-dimensional electromagnetic-thermal-chemical-hydraulic-mechanical model by COMSOL Multiphysics software. The influence of microwave frequency and power on the temperature distribution of oil shale sample is investigated. The results reveal that the temperature distribution of oil shale sample is not uniform under microwave heating due to the nonuniform distribution of electric field. Microwave heating is most efficient at a frequency of 2.45 GHz because of the size and shape of rectangular waveguide. The temperature rise of oil shale is characterized by “slow-fast”, because the dielectric property of oil shale is temperature-dependent. Once the reaction temperature is over 500 °C, the heating rate increases significantly, and this phenomenon can be more obvious under high microwave power. Under 800 W, kerogen decomposition starts at 120 s in the sample center, the highest temperature region. The migration of products is dominated first by diffusion and then by convection, because the production of oil and gas forms a significant pressure gradient within oil shale over time. Compared with electrical heating, microwave heating accelerates the generation of pressure gradient and thermal stress, enlarging the porosity and enhancing the permeability of oil shale. In addition, the stepwise microwave heating mode is proposed to avoid excessive temperature and proved to be feasible during the oil shale retorting process. The outcomes show great potential of microwave heating in replacing conventional electrical heating for oil shale upgrading.