# Pore Structure of Coals by Mercury Intrusion, N\({}_{2}\) Adsorption and NMR: A Comparative Study
## 3 Results
### The Porosity and Pore Size Distribution by Nuclear Magnetic Resonance (NMR)
#### 3.3.3 Pore Size Distribution by Nuclear Magnetic Resonance (NMR)  
The NMR technology characterizing the pore structure of coals needs to calculate the T\({}_{2}\) cutoff value (T\({}_{2\text{C}}\)) to perform the T\({}_{2\text{C}}\) and size conversion. As shown in Figure 7, the cumulative amplitude curve of NMR signal is respectively applied to the T2 spectrum before and after centrifugation, and the line parallel to X-axis is made from the maximum cumulative amplitude curve after centrifugation until it intersects the cumulative amplitude curve of saturated water. From the intersection point, the perpendicular line is introduced to the X-axis and then the value of T\({}_{2\text{C}}\) can be obtained. As shown in Table 4, the T\({}_{2\text{C}}\) value of coal samples ranges from 0.70 to 37.25 ms, with an average of 10.0 ms. The pore size of the coals can be calculated based on Equation (5) and the results of the corresponding pore volume are shown in Table 4. According to the PSD characteristics of coal samples in Figure 8, the experimental coal samples are mainly developed with micropores and transition pores, accounting for  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample No.**} & \multirow{2}{*}{**NMR Porosity/\%**} & \multirow{2}{*}{**T\({}_{2\text{C}}\)/ms**} & \multicolumn{5}{c}{**Pore Volume/\%**} \\ \cline{3-6}  & & & **Micropores** & **Small Pores** & **Mesopores** & **Macropores** \\ \hline
**TB** & **18.66** & 7.76 & 45.20 & 35.18 & 14.49 & 5.13 \\ BS & 20.43 & 11.60 & 38.10 & 42.82 & 18.59 & 0.49 \\ SW & 7.74 & 1.04 & 3.96 & 60.37 & 12.61 & 23.06 \\ TA & 3.06 & 2.77 & 27.16 & 51.44 & 10.78 & 10.62 \\ FL & 4.69 & 0.70 & 2.08 & 60.54 & 6.03 & 31.34 \\ BD & 4.24 & 2.28 & 22.44 & 63.51 & 5.31 & 8.74 \\ SL & 7.04 & 4.45 & 28.47 & 47.68 & 19.01 & 4.84 \\ WJY & 2.18 & 1.13 & 3.36 & 55.73 & 16.81 & 24.10 \\ TL & 2.87 & 21.65 & 45.59 & 29.34 & 22.73 & 2.34 \\ QLT & 0.91 & 12.40 & 51.34 & 7.82 & 37.91 & 2.93 \\ DEP & 1.03 & 0.71 & 2.08 & 67.62 & 16.51 & 13.78 \\ HS & 4.35 & 15.20 & 47.52 & 12.92 & 31.85 & 7.70 \\ DP & 3.23 & 13.80 & 64.13 & 8.29 & 19.88 & 7.70 \\ CYH & 2.19 & 25.64 & 78.01 & 4.03 & 13.82 & 4.14 \\ WK & 3.66 & 2.27 & 6.98 & 79.81 & 9.56 & 3.66 \\ SK & 5.57 & 37.25 & 78.78 & 2.44 & 13.91 & 4.88 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Analysis of pore characteristics of experimental coal samples by Nuclear magnetic resonance (NMR).  
Figure 6: Nuclear magnetic porosity versus gas porosity.  
59.09%-86.79% of the total pore volume, with an average of 73.42%, which is basically similar to the results of N2 GA experiment. Moreover, the proportion of micropores gradually rises with the increase of coal rank, from 23.30% of low-rank coals to 25.55% of medium-rank coals and finally to 55.08% of high-rank coals.  
Figure 8: Nuclear Magnetic Resonance (NMR) pore size distribution characteristics.  
Figure 7: Nuclear Magnetic Resonance (NMR) T\({}_{2}\) distribution of experimental coal samples and calculation of T\({}_{2C}\).