Recent advances in microwave-assisted thermal upgrading of low-rank coal

Microwave heating technology possesses the unique advantages of rapid and uniform heating, selective heating, flexible operation, safety and environmental protection, which offers a new way to achieve the clean and high-efficient utilization of low-rank coal. Based on the upgrading technology of microwave drying and pyrolysis of low-rank coal, the dielectric response characteristics of low-rank coal towards microwave were analyzed, and the factors influencing the microwave drying upgradation process of low-rank coal were summarized. Furthermore, the effect rules of microwave drying upgradation on the quality and physicochemical properties of low-rank coal were expounded, the impact of various process conditions on the microwave pyrolysis upgrading of low-rank coal are emphatically reviewed. Additionally, the characteristics of the introduction of microwave absorbers and catalysts on enhancing the microwave pyrolysis upgrading of low-rank coal are analyzed in detail. The results show that the moisture and mineral content of low-rank coal are the key factors of affecting its dielectric loss ability, and the rapid dielectric response of internal moisture towards microwave is beneficial to achieve the rapid drying and quality improvement of low-rank coal under microwave irradiation. Microwave drying can effectively improve the fixed carbon content and combustion calorific value of low- rank coal，and the internal moisture can be preferentially heated for rapid migration with microwave radiation, which is conducive to promoting the development of pore structure and the enhancement of grindability of low- rank coal. The oxygen-containing functional groups with polarity are easy to be decomposed under microwave induction, which is beneficial for inhibiting the spontaneous combustion of low-rank coal. Compared with conventional pyrolysis, microwave-assisted pyrolysis upgrading of low-rank coal shows numerous unique advantages involving fast heating rate, high pyrolysis efficiency, abundant light tar components, and high syngas production. Microwave power, radiation time, final pyrolysis temperature and reaction atmosphere are the main factors that affect the microwave pyrolysis process of low-rank coal. Target-oriented optimization of pyrolysis process parameters is the core for achieving the clean and high-efficient utilization of low-rank coal. The introduction of microwave absorbers and catalysts to fully strengthen the microwave pyrolysis process of low-rank coal is an effective way to increase the utilization rate of microwave energy, enhance the pyrolysis efficiency of coal, directional control of product distribution and improve the quality of pyrolysis products. In addition, microwave-assisted co-pyrolysis of low-rank coal and hydrogen-rich materials such as biomass, oil shale and waste plastics can enhance the production of light tar components and increase the yield the effective gas, which is the crucial development direction to realize the clean and high-efficient utilization of low-rank coal