Experimental study on the evolutional trend of pore structures and fractal dimension of low-rank coal rich clay subjected to a coupled thermo-hydro-mechanical-chemical environment

In order to study the thermo-hydro-mechanical-chemical (THMC) behaviors of low-rank coal rich clay (LRCRC), a triaxial testing machine with THMC effects of high pressure and high temperature was built. Then, mercury injection tests, fractal dimension analysis and infrared spectroscopy analysis were carried out on LRCRC specimens subjected to a coupled THMC environment. The results showed that: 1) as the temperature increased, the sub-micro pores gradually transformed into other types of pores. Moreover, the formation rate of new pores during pyrolysis was slower than the expansion rate of existing pores; 2) the overall meso-physical parameters and fractal dimension of specimen decreased first and then increased with the increase in temperature; 3) at a given temperature, for sub-micropores (0–10 nm), the reduction rate of microscopic physical parameters and pore size distribution ratio of specimen under superheated steam was less than that under nitrogen atmosphere. For the other pores (>10 nm), the growth rate of microscopic physical parameters and pore size distribution ratio of specimen under superheated steam was greater than that under nitrogen atmosphere. Compared with nitrogen, water vapor was more favorable for pyrolysis of specimen; 4) at a given temperature, the structural parameters (Car/(Car + CO), Car/Hal) of pyrolysis residues of specimen under superheated steam atmosphere were larger than those under nitrogen atmosphere, indicating that the amount of decomposition of aliphatic hydrocarbons and oxygen-containing functional groups was greater than that under nitrogen atmosphere. The research results in this paper can provide useful theoretical guidance and technical support for mining of low-rank clay rich coal.