# Pore Structure of Coals by Mercury Intrusion, N\({}_{2}\) Adsorption and NMR: A Comparative Study
## 2 Experiments and Methods  
### Coal Basic Information and Experiments  
The 16 coal samples were collected from different mining areas in three coalbearing basins of Junggar, Ordos, and Qinshui. The samples range from low-metamorphic lignite to subbituminous coal and bituminous coal, as well as high-metamorphic anthracite. The petrographic and proximate analysis results are shown in Table 1. The MIP, N2 GA, and NMR experiments were conducted. The mercury intrusion porosimetry (MIP) tests were conducted on the 16 selected coal samples using 9420 automatic mercury intrusion porosimeter by Micromeritics as per our previous research [4]. The coal sample were dried to constant weight at 105 \({}^{\circ}\)C. The amount of intruded and ejected mercury volume in the range of 0.004-206 MPa were determined. The low temperature nitrogen absorption/desorption experiment (N2 GA) were also carried out. The experiments were completed with the experiment device of SASAP-2020 automatic surface analyzer by Micromeritics. The procedure is as follows: Firstly, the coal samples were broken and the coal powder of 60-80 mesh was screened. After the device was degassed, the samples were put in the unit. At the liquid nitrogen temperature (77k), the adsorption amount in the experiment is completely determined by the relative pressure P/P0, where P is the actual pressure of nitrogen, and P0 is the saturated vapor pressure of nitrogen at 77 K. According to the results of low-temperature nitrogen adsorption experiment, the pore specific surface area can be calculated by BET equation when P/P0 is between 0.05 and 0.35. Moreover, the pore volume and pore diameter distribution of coal can be analyzed by BJH equation when P/P0 is greater than or equal to 0.4 [1]. The low field nuclear magnetic resonance (NMR) experiments were also carried out on 16 coal samples. The pretreatment process is as follows: first, cylindrical cores approximately 25 mm in diameter and 50 mm in length were drilled on fresh coals. Then, coal pillar was dried to constant weight at 105 \({}^{\circ}\)C and placed in a vacuum pump for 48 h. Following that, the samples were injected  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Sample** & **Coal** & \multirow{2}{*}{\ \begin{tabular}{c} **R\({}_{0.000}\)/\%** \\ **Basin** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Coal Composition (Vol.’s)** \\ **Vitix** \\ \end{tabular}  } & \multicolumn{4}{c}{**Proximate Analysis (wt\({}^{\circ}\)s)**} & \multirow{2}{*}{**Porosity/\%**. Permeability/mD**} \\ \cline{4-11}  & & & & **Viti  
into distilled water and then saturated for 72 h under the pressure of 8 MPa. When the coal sample reached the fully saturated water state, the experimental pretreatment was completed. The NMR experiment consists of three steps: 1) The coal sample under saturated water were set in Rec Core 2500 low field nuclear magnetic resonance of 2.38 MHz to obtain the inverse T2 relaxation time spectrum. The experiment parameters were set as follows: 0.240 ms of echo time interval (T\({}_{\text{E}}\)), 6 s of wait time (T\({}_{\text{W}}\)), 8000 of echo number (N\({}_{\text{ECH}}\)), 64 of scanning frequency (N\({}_{\text{S}}\)), and the experimental temperature 25 \({}^{\circ}\)C; 2) To detect the best centrifugal force and make the samples reach the ideal state of bound water, the centrifugal rotational rates were set to 3000, 4000, 5000, 6000, 7000 r/min, respectively (corresponding to the centrifugal force of 0.69, 0.92, 1.15, 1.38, 1.61 MPa) within the centrifugal time of 1 h; 3) the coal samples after centrifugation were subjected to NMR experiments under the same test environment. Furthermore, NMR signal calibration experiment is also required. The NMR signal calibration experiments were conducted to determine the NMR nuclear magnetic signal corresponding to the amount of water content in the experiments.