Thermal fracturing of anthracite under low-energy microwave irradiation: An experimental study

Abstract

Microwave-assisted coalbed methane (CBM) development is a very promising technology for engineering applications. The mechanism of the macro and micro thermal fracture of coal must be further discussed because of the thermal inhomogeneity of microwave caused by the heterogeneity and structural complexity of coal. In this study, the temperature, weight and P-wave velocity of anthracite were tested under low-energy microwave irradiation. The characteristics of fracture initiation, extension, and connectivity in coal after microwave irradiation were extracted from macroscopic and microscopic observations. Then the mechanisms of microwave thermal fracturing in anthracite were analyzed. The results showed that the temperature of anthracite under low-energy microwave irradiation varies asynchronously. The temperature distributions with high and low temperature partitions exhibit obvious bedding effects. The coal mass loss varies in stages with microwave irradiation time, first increasing slowly and then increasing dramatically above 500 °C. The P-wave velocity presents a decreasing trend with the increase of input microwave energy. Coal generates a complex fracture network at the micro and macro scales, with fractures expanding outwards in a divergent pattern. The nonuniformity of the temperature distribution promotes fracture development. It is related to the asynchronous heating caused by the mineral composition, water content, pore fracture structure, and electromagnetic field in coal. It is of great significance to improve the thermal fracturing efficiency by using the nonuniformity of microwave heat in the application of microwave-assisted CBM development.