Porosity changes in bituminous and anthracite coal with ultrasonic treatment

Abstract Improving pore structures is key for enhancing coalbed methane (CBM) recovery. The variations in pores of Chinese bituminous and anthracite coal were analysed by using low-pressure N 2 gas adsorption and CO 2 gas adsorption during 20 kHz and 50 kHz ultrasonic treatment. Although the shape of the physisorption isotherm and hysteresis loop, that indicate the pore type of coal, were not changed after ultrasonic treatment, the pore size distribution (PSD) of the treated coal differed from that of the raw coal, especially for bituminous coal with tissue pores, intercrystal pores, and gas pore clusters. An obvious increase in bituminous coal pores was noted, although only a slight change was noted in anthracite pores except for macropores during ultrasonic treatment. The 50 kHz ultrasonic treatment contributed to pore development more than 20 kHz, and created sub-micropores, mesopores, and macropores in bituminous coal fractures, after rapid growth in the treatment process. In the 20 kHz ultrasonic field, these ranges of pores represent continuous and monotonous growth. The range of micropores in bituminous coal increased rapidly before 36 h of ultrasonic treatment, followed by a slow increase. Throughout the ultrasonic treatment process, the largest growth of micropores, sub-micropores, mesopores, and macropores in the bituminous coal were about 148.5%, 418.4%, 228.3%, and 43.6%, respectively, with the higher frequency of ultrasonic treatment; for the lower frequency of the ultrasonic field, the increment values were 115.0%, 179.9%, 103.9%, and 76.4%, respectively. The cumulative pore volume of anthracite showed a 61.9% increase during the 50 kHz ultrasonic treatment after macropore fracturing.