Evolutions of Pore and Crack Structure of Coal under Hot Steam Heating

Hot steam injection in coal seams can raise the molecular kinetic energy of methane and promote gas desorption, which provides a new idea to solve the difficulty in gas extraction in most coal seams in China. To explore the law of pore and crack expansion in coal under the action of hot steam, bituminous coal and anthracite samples were obtained from Jining Coal Mine (JN) and Linhua Coal Mine (LH), respectively. With the aid of a self-built experimental platform for stream-treated coal, a contrastive study was conducted before and after heat treatment. In terms of macrocracks, photos of cracks on the coal surface before and after hot steam treatment were captured by a high-definition camera; then, the cracks in the photos were extracted, processed, and analyzed. In terms of micropores, evolutions of micropores and cracks in coal before and after heat treatment were studied on the basis of mercury intrusion. The following results were obtained: After hot steam treatment at 140 °C for 180 min, the areas of macrocracks on the surface of the JN bituminous coal sample and the LH anthracite sample jump by 13.05 and 3.76 times, respectively, and their porosities reach 1.075% and 0.081%, respectively. The JN coal surface has more developed cracks than the LH coal surface after hot steam treatment. The cumulative volumes of mercury intrusion for JN coal and LH coal rise by 11.4% and 3.6%, respectively. The volumes of micropores and minipores in coal decrease, while those of mesopores and macropores increase. Hot steam promotes the development of pores and cracks in coal to a certain extent, and its pore expansion effect is more remarkable for JN coal than for LH coal. The mechanism of the action of hot steam on coal is concluded as follows: Uneven expansion force occurs in coal during heating due to the anisotropy of coal, the different expansion coefficients of different components in coal, and the temperature gradients. Constrained by the mutual deformation of matrix, when strength limit is exceeded, pores and cracks of coal develop and expand from weak media, so that micropores and minipores are transformed into mesopores and macropores.