Experimental study on mineral variation in coal under microwave irradiation and its influence on coal microstructure

Abstract

Microwave technology is regarded as a promising new method for coalbed methane production. An investigation was carried out using a self-developed controllable microwave device to study the mineral variation in coal under microwave irradiation and its influence on coal microstructure. The mineral phase change, mass loss, P-wave velocity, and microstructure evolution of coal under different microwave conditions were studied using an energy dispersion spectrometer, ultrasonic detector, and scanning electron microscope. The results show that the mass loss of coal increases from slightly to significantly with microwave irradiation time, while with the increase of microwave power, it shows a trend of increasing steadily to slightly decrease. The mineral variation process includes water-soluble minerals migrating with water evaporation, sulfide minerals releasing sulfur-containing gas and the fuming of fine-grained volatile substances. Mineral variation is one of the main reasons for the damage of coal microstructure. In this process, the coal body is damaged by water vapor pressure, sulfur-containing gas and pyrolysis gas release and the fume of fine-grained materials, resulting in new pores and weak structural planes. Besides, increasing the microwave power to the power threshold and then prolonging the microwave time has significantly affected microstructure damage.