Effects of Microwave Heating Paths on Pores and Cracks in Bituminous Coal

Abstract

To follow the effect of the microwave heating path on the structure of coal, eight 50 mm diameter, 30 mm long dry coal cores from the same coal seam with similar pore structure characteristics were microwave-treated using 4 pathways. The T 2 spectrum, pore-volume, temperature, mass, and visual changes of coal samples were analyzed before and after microwave heating. The microwave heating path affected the macropores and microcracks and the crack development mode. When the same microwave energy was applied, microwave heating on the coal was mainly manifested by the opening of closed pores, before the pyrolysis temperature of the coal was reached. Increasing the energy density caused the water vapor to move from constant pressure expansion to constant volume expansion. This resulted in an exponential growth of the mesopore and macropore volumes. Meanwhile, the micropore volume increased due to the collapse of pore structures. As a result, high-power microwave heating could accelerate the vaporization rate of water. The rapid expansion of water vapor volume brought about a microwave heating effect similar to the “steam explosion”. The resulting local tensile stress enabled the cracks to develop, expand, and connect to others continuously. Thus, it formed a complex crack network leading to the outside of the coal sample.

Effect of moisture on microwave ignition of bituminous coal

“…Because the pores and fractures were relatively independent (permeability less than 12 mD), the steam was confined to the pore space. The thermal expansion of steam in this process and the subsequent increase in pressure difference [ 37 ] led to a significant self accelerating heating effect [ 38 ]. Under the excitation of microwave, the temperature of steam continued to rise, and finally induced the ignition of surrounding volatiles.…”

Experimental Research on the Influence of Microwave Radiation on Coal Permeability and Microstructure

“…Among the methods, NMR is the most widely used method. Yao et al, 31 Hong et al, 32,33 and Li et al 34 analyzed the pore structure of coal before and after microwave by NMR. Hong et al studied the pore structure of coal before and after microwave by MIP.…”

“…Among the methods, NMR is the most widely used method. Yao et al, Hong et al, , and Li et al . analyzed the pore structure of coal before and after microwave by NMR.…”