Effects of steam treatment on the internal moisture and physicochemical structure of coal and their implications for coalbed methane recovery

Based on the advantages of steam in applications of coal gasification, drying of low-rank coals, and heavy oil recovery, steam is considered to have great potential in the recovery of coalbed methane (CBM). This study revealed the effects of steam on CBM recovery by characterizing the changes in the surface morphology, internal moisture, pore-fissure structure and molecular structure of coal. The results demonstrated that steam treatment at 100 degrees C (saturated steam) caused the expansion and extension of the cracks of lignite and induced new cracks, which were conducive to CBM diffusion and migration. However, saturated steam resulted in only partial moisture removal from the seepage pores of bitumite and anthracite, which might be beneficial for alleviating the "water blocking" effect. As the steam temperature rose above 200 degrees C (superheated steam), over 96% of the moisture in water-saturated bitumite and anthracite was removed, including irreducible water, and the resulting enlargement of coal pores and the enhancement in pore-fissure connectivity helped provide more channels for CBM flow. However, after steam treatment at 200-400 degrees C, the decrease in the amount of hydroxyl groups, shortening of aliphatic side chains and enhancement in the aromaticity of the coals would contribute to CBM adsorption.