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.