Characteristics of pores and methane adsorption of low-rank coal in China

摘要

The pores and methane adsorption characteristics of low-rank coals are different from those of high-rank coals. In part, the differences are due to maceral composition. For this paper, mercury intrusion porosimetry and isothermal adsorption experiments were carried out on dozens of coal samples with R-o,R-max between 0.24 and 0.65%. The results show that the pores between 10 and 100 nm in size are the most important in low-rank coal. It is found that pore volumes, pore-specific surface areas, and porosity first decrease and then increase with increasing R-o,R-max. The minimum values are reached when R-o,R-max = 0.5%. There are two reasons for this. One is that the porosity is reduced because sediments are compacted and gradually lose moisture when R-o,R-max < 0.5%. The other is that the hydrocarbon generation results in the production of many small-diameter gas pores and the bitumens do not block the pores when R-o,R-max is between 0.5 and 0.65%. Coal rank is not the dominant factor affecting methane adsorption capacity in low-rank coal. No obvious correlation exists between Langmuir volume (V-L) and R-o,R-max. With regard to coal rank, the effect of maceral composition on the size of pores and the capacity for methane adsorption is greater in low-rank coals than in high-rank coals. Significantly, micropore volumes and V-L, display a parallel "three-step-style" change with increasing maceral content for vitrinite/huminite and inertinite. For vitrinite/huminite, both micropore volume and V-L, first decrease, then increase, and finally decrease with increasing vitrinite/huminite content. For ineitinite, the relationship is the opposite. It is concluded that the increase of a single maceral constituent leads to a regular distribution of micropore volumes and the three-step-style change in micropore volume results in a similar three-step-style change in methane adsorption capacity. (C) 2015 Elsevier B.V. All rights reserved.