# Pore Structure of Coals by Mercury Intrusion, N\({}_{2}\) Adsorption and NMR: A Comparative Study
## 4 Discussion
### The Pore Size Distribution from Different Techniques  
In order to compare the difference of PSD from three technologies, the porosity characteristics of coal were compared, combined with their individual test range of the pore size. As shown in Figure 10, the results of three experiments show that the PSD and porosity from NMR experiment are the largest, with the characterization range between 1 and 10000 nm, which basically covers the micropore to macropore segments required for the experiment. In the measurement range of MIP experiment, the characteristic PSD is basically consistent with NMR results, but there is peak dislocation. In addition to this, the results based on MIP are obvious or slightly higher than that from NMR when the pore size is less than 10 nm, which is mainly caused by two reasons: Firstly, excessive pressure during mercury injection could lead to the fracturing of some smaller micropores [11]. Secondly, large pores connected by throat can only be measured in the MIP experiment, resulting in the overestimation of pore size [12]. The variation of pore size from N\({}_{2}\)GA based on BJH method is similar to that of NMR, however N\({}_{2}\)GA value is obviously smaller and the curve compliance degree is not high. In general, the smaller the total pore volume is, the closer the characterization result is to the NMR result and the more accurate the pore characterization range is in the range of 10-100 nm. The characterization results of the pores with diameter less than 10 nm is significantly smaller, while that with diameter larger than 100 nm is larger \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample No.**} & \multicolumn{2}{c}{**Mercury Intrusion Porosimetry (MIP)**} & \multicolumn{2}{c}{**Nitrogen Adsorption (N\({}_{2}\)GA)**} & \multicolumn{2}{c}{**Nuclear Magnetic Resonance (NMR)**} \\ \cline{2-7}  & **Apparent** & **Pore** & **\(\theta_{\text{MNP}}\)/\%** & **Pore** & **\(\theta_{\text{N2 GA}}\)/\%** & **Pore Volume** & **\(\theta_{\text{NMR}}\)/\%** \\  & **Density/g/cm\({}^{3}\)** & **Volume/mL/g** & **Volume/mL/g** & **Volume/mL/g** & **/mL/g** & **/mL/g** & **0\({}_{\text{NMR}}\)/\%** \\ \hline TB & 1.28 & 0.0505 & 6.46 & 0.0049 & 0.63 &.1458 & 18.66 \\ BS & 1.34 & 0.1004 & 13.46 & 0.0069 & 0.93 &.1525 & 20.43 \\ SW & 1.06 & 0.0492 & 5.21 & 0.0023 & 0.24 & 0.0730 & 7.74 \\ TA & 1.57 & 0.0139 & 2.18 & 0.0051 & 0.79 & 0.0195 & 3.06 \\ FL & 1.26 & 0.0263 & 3.31 & 0.0005 & 0.06 & 0.0372 & 4.69 \\ BD & 1.96 & 0.0150 & 2.94 & 0.0071 & 1.39 & 0.0216 & 4.24 \\ SL & 1.41 & 0.0349 & 4.92 & 0.0135 & 1.90 & 0.0499 & 7.04 \\ WJY & 1.72 & 0.0119 & 2.05 & 0.0075 & 1.30 & 0.0127 & 2.18 \\ TL & 1.24 & 0.0123 & 1.53 & 0.0052 & 0.65 & 0.0231 & 2.87 \\ QLT & 1.31 & 0.0033 & 0.43 & 0.0025 & 0.33 & 0.0069 & 0.91 \\ DEP & 1.34 & 0.0075 & 1.00 & 0.0016 & 0.21 & 0.0077 & 1.03 \\ HS & 1.22 & 0.0184 & 2.25 & 0.0011 & 0.14 & 0.0357 & 4.35 \\ DP & 1.31 & 0.0087 & 1.14 & 0.0020 & 0.26 & 0.0247 & 3.23 \\ CYH & 1.42 & 0.0034 & 0.48 & 0.0020 & 0.28 & 0.0154 & 2.19 \\ WK & 1.59 & 0.0209 & 3.33 & 0.0009 & 0.14 & 0.0230 & 3.66 \\ SK & 1.28 & 0.0091 & 1.17 & 0.0018 & 0.24 & 0.0435 & 5.57 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Determination of different porosities and pore volume characterization of the porosity results. The possible reason is that the pore diameter of the adsorption layer is less than 10 nm, and the pore diameter calculated by the Kelvin Equation has a greater influence on the radius of the backfill layer, resulting in the smaller pore volume calculated by BJH method [13; 14]. However, when the pores are larger than 100 nm in diameter, the relative pressure is close to 1, indicating that the condensation and evaporation rate is close to liquid nitrogen molecules, and the measuring range cut-off. However, at this time a layer of liquid nitrogen molecule film will be adsorbed on the wall of some large and medium-sized pores. This part of volume will also be added in the calculation using BJH method, resulting in the overestimation of pore volume.