Numerical investigation of microwave-assisted pyrolysis of a tar-rich coal seam: an efficient and safe strategy with adjustable power heating

Microwave heating has the advantages of no physical contact, high energy utilization rate, and rapid controllability. Firstly, a multi-physics field coupling model of electromagnetism, heat transfer, fluid flow and chemical reaction was established to conduct numerical simulation of microwave pyrolysis of a tar-rich coal seam, and the pyrolysis characteristics were explored under different continuous heating powers. In order to further enhance the thermal decomposition efficiency and reduce energy consumption, on-off heating and stepped heating schemes were subsequently designed. The results showed that although increasing heating power can accelerate coal conversion and promote tar production within the same period of time, excessively high temperature will form local high-temperature zones, which will affect long-term stable operation of the microwave heating system. The on-off heating mode not only ensures safety, but also reduces energy consumption costs. The stepped heating scheme significantly improves seepage and transport properties of the substance, greatly enhances pyrolysis efficiency and ensures output of products, but the energy consumption rises compared to the on-off heating. The four-stage stepwise heating strategy can complete pyrolysis within 404 days, and the output of tar reaches over 3800 kg within 450 days. Taking into account high efficiency, safety, stability and economy comprehensively, different heating methods can be selected according to actual demands and priorities. The research results can enhance the understanding and knowledge of coal layer pyrolysis using microwaves and break through the scale limitation of traditional laboratory experiments, which is providing guidance for the schemes and feasibility of microwaving large-scale coal seams.