# Pore Structure of Coals by Mercury Intrusion, N\({}_{2}\) Adsorption and NMR: A Comparative Study
## 3 Results
### Low Temperature Nitrogen Absorption/Desorption Experiment  
BET calculation results show that the values of the specific surface area are between 0.116-5.030 m\({}^{2}\)/g, with an average of about 1.5 m\({}^{2}\)/g (data shown in Table 2). Among the samples of different coal ranks, the pore specific surface area of low rank coals in southern Junggar basin and high rank coals in northern Qinshui is relatively low, with the average value below 1.0 m\({}^{2}\)/g. While in the eastern Ordos basin, the specific surface area is generally higher, with average value of 2.5 m\({}^{2}\)/g or more. BJH pore volumes are ranged from \(0.47\times 10^{-3}\) to \(13.46\times 10^{-3}\) mL/g, which vary greatly between different coal samples. However, overall, the pore volume of the coals in northern Qinshui is the smallest, which is mainly determined by the characteristics of low porosity and pore size in high rank coals. The PSD characteristics show that the content of pores in the eastern Ordos basin is the highest, ranging from 83.7% to 92.4%, with an average content of 90.0%, while the average pore diameter is the lowest of 4.82 nm and the specific surface area is the highest, which is favorable for CBM storage. The pore type can be classified into three categories based on IUPAC classification (Figure 2): type I, type II, and type III represent closed pore, open pore, and pore with thin neck bottle, respectively. In the figure of the relationship between relative pressure and adsorption capacity, there are great differences in adsorption capacity because of the difference in porosities and PSD. In order to compare the types of adsorption loop, the adsorption capacity is normalized and the maximum adsorption capacity is set at 1 mL/g STP. The characteristics of the adsorption loop are shown in Figure 3. The normalized treatment does not change the type of adsorption loop, which is helpful to compare the pore structure between different coal samples. Figure 1: Type of mercury intrusion curve of coal sample (**(a)** type I, **(b)** type II, **(c)** Type III, **(d)** Type IV ). Figure 2: Typical low temperature N\({}_{2}\) adsorption loop type based on International Union of Applied Chemistry (IUPAC) classification ((**a**) type I, (**b**) type II, (**c**) type III). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample No.**} & **BET SSA/(m\({}^{2}\)/g)** & **BJH PV/(10\({}^{-3}\) mL/g)** & \multirow{2}{*}{**D\({}_{p}\)/nm**} & \multicolumn{3}{c}{**PV Percentage/\%**} & **Ad/De. Curve Type** \\ \cline{5-8}  & & & & **<10 nm** & **10 – 100 nm** & **>100 nm** \\ \hline
**TB** & **1.879** & **4.90** & **7.19** & 25.0 & 59.5 & 15.5 & II \\ BS & 1.858 & 6.95 & 7.09 & 26.5 & 55.1 & 18.4 & I \\ SW & 0.559 & 2.28 & 9.02 & 23.3 & 59.1 & 17.6 & I \\ TA & 2.190 & 5.06 & 2.80 & 64.4 & 28.0 & 7.6 & II \\ FL & 0.116 & 0.47 & 7.69 & 17.5 & 61.0 & 21.5 & I \\ BD & 2.585 & 7.07 & 5.61 & 42.7 & 49.0 & 8.3 & III \\ SL & 5.030 & 13.46 & 4.07 & 54.7 & 36.0 & 9.3 & III \\ WYJ & 4.083 & 7.53 & 3.12 & 59.6 & 31.9 & 8.5 & III \\ TL & 1.654 & 5.24 & 5.28 & 43.9 & 47.4 & 8.7 & II \\ QLT & 0.477 & 2.55 & 8.49 & 22.9 & 60.8 & 16.3 & II \\ DEP & 0.375 & 1.60 & 7.60 & 23.2 & 59.7 & 17.1 & I \\ HS & 0.314 & 1.13 & 7.18 & 25.2 & 58.7 & 16.1 & I \\ DP & 0.816 & 2.00 & 4.91 & 27.1 & 51.5 & 21.4 & II \\ CYH & 0.303 & 1.97 & 13.00 & 17.4 & 69.5 & 13.1 & II \\ WK & 0.246 & 0.87 & 15.55 & 14.1 & 68.8 & 17.1 & II \\ SK & 0.928 & 1.84 & 2.76 & 48.7 & 36.9 & 14.4 & II \\ \hline \hline \end{tabular} Note: SSA- specific surface area; PV-pore volume; D\({}_{p}\)-The average pore diameter; Ad/De.-Adsorption/desorption.  
\end{table} 
Table 2: Experiment porosity test results by nitrogen adsorption (N\({}_{2}\)GA). The pore morphology of the coal samples BS, SW, FL, DEP, and HS is mostly the type I, which is airtight at one end, including cylinder, parallel plate, and pointed split pores. When liquid nitrogen condenses or evaporates in this kind of pores, there is the same kind of the gas-liquid interface hemispherical meniscus.