## 4 Discussion
### Analysis of Specific Surface Area Changes of Mixed Coal before and after Adsorption  
Figure 9 shows the percentage difference of macro and micro pores' specific surface area (after adsorption/before adsorption). From the test results, it was found that the macro pore specific surface area ratio showed a decreasing trend before reaching the mass ratio of 3:3. With the increase of the mass proportion of tectonic coal, the specific surface areas of medium and macro pores before and after adsorption showed a decreasing trend first, followed by an increasing trend. When the qualities of tectonic coal and virgin coal were approximately equal, the trend reached its minimum. Because the mixed coal was dominated by virgin coal, the adsorption capacity of the mixture was dominated by that of the virgin coal. The swelling induced by gas adsorption in the pores of coal was not obvious. When the gas could not be completely desorbed from the coal, the resistance of desorbed gas was measured using a physical adsorption instrument to test the pore specific surface area. The specific surface area of macro pores showed a declining trend. When the qualities of tectonic coal and virgin coal were approximately equal, the proportion of the specific surface area of macro pores reached its minimum. With the increase in quality of the tectonic coal, the pore structure became more developed. Micro pores connected to each other to form small holes by the adsorption of gas, which caused the specific surface area of macro pores in the mixed coal to continuously increase. With the increase in quality of the tectonic coal, the pore volume gradually reached equilibrium, which also caused the specific surface area to increase slowly. It can be seen from Figure 9 that the specific surface area changing trend is contrary to that of Figure 3 with the increase in quality of the tectonic coal. When the qualities of tectonic coal and virgin coal were approximately equal, the trend reached a minimum and then declined. By fitting the experimental data, the variation of the pore specific surface area and micro pore specific surface area in the total coal sample before and after adsorption was obtained according to the following relationship:  
\[\theta=\sum_{i=i}^{2}\theta_{i}=\sum_{i=1}^{2}\frac{B_{oi}}{1+B_{1i}e^{-B_{2}x_ {i}}} \tag{5}\]  
where \(\theta\) is the difference between the proportions of specific surface area of coal before and after adsorption; \(\theta_{i}\) (\(i=1\), \(2\)) is the difference between the proportions of specific surface area of micro poresor macro pores before and after adsorption; and \(x_{i}\) (\(i\) = 1, 2) is the proportion of structural coal to total coal sample. \(B_{0i}\), \(B_{1i}\), \(B_{2i}\) are fitting constants. The Relationship between the Specific Surface Area before and after Adsorption and Swelling Deformation and Strain  
The adsorption amount of mixed coal varied under different pressures and different contents of tectonic coal. The swelling also varied. Therefore, there must be some connection between the gas adsorption capacity and the coal swelling deformation. The strain of coal after adsorption can be expressed as follows according to the results of previous studies [51]. \[\varepsilon_{m}=K_{m}\times W_{m} \tag{6}\]  
where \(\varepsilon_{m}\) is the volume strain after adsorption. \(W_{m}\) is the amount of gas adsorbed by a unit mass coal under a certain temperature, pressure, and mass ratio of tectonic coal to the mixed coal, mL\(\cdot\)g\({}^{-1}\). \(K_{m}\) is the expansion strain coefficient, g\(\cdot\)mL\({}^{-1}\). \(m\) is the abbreviation of mixture, denoting the mixed coal sample. The relationship between the gas adsorption and the amount of tectonic coal shown in Figure 9 is expressed by Equation (1).