2.1 Pore structure change rule
Based on the low-field MRI results, Figure 1(a) shows that in the range of peaks on the left, the T2 value corresponding to the peak value of the dry coal sample was basically stable. In the range of the right peak, compared with the raw coal, the width of the peak of the coal sample that was dried for 8 h increased significantly, showing an increase to the left and right sides, and the intensity of the peak also increased significantly. This result indicated that during the drying process of the saturated coal, the content of micropores to mesopores in the coal decreased somewhat, but the decrease in the amplitude was not significant. The number of larger pores and cracks corresponding to the second peak value and the size of the pore diameter increased significantly—that is, the expansion and generation of larger pores and cracks in the drying process was the main cause of the change of the pore structure.
Figure 1(b) shows that new pores with sizes of 6550–10680 nm were generated in coal sample 1-2 and 63900–28350 pores or cracks were generated in coal sample 1-3 after 8 h of drying. Similarly, the total integral area of the pores, namely the pore volume, did not increase significantly in the range below 1000 nm, whereas the total amount of larger pores increased significantly, and large numbers of new pores and fissures appeared.
In Figure 2(a), the change of the peak on the left is basically similar to that of the other drying degrees. The width and the peak value of the peak increased, but the increase was not significant, indicating that the pore size, content, and total amount of relatively small pores did not change significantly during the drying process. The distribution of the peak on the right side showed that after 24 h of drying, the peak extended significantly to the left and right sides, the peak width increased significantly, and the peak signal increased significantly, indicating that large pores and cracks occurred in the coal after 24 h of drying.
Figure 2(b) shows the comparison of the pore structures of the coal sample and raw coal after drying for 24 h. Except for the pores below 1000 nm, which had changes like the other drying time changes, there were no significant changes. For coal sample 2-1, the pore sizes in the range of 7700–20470 nm were the newly generated large pores or cracks after drying for 24 h. Correspondingly, the size range for coal sample 2-2 was 4730–20470 nm, and that for coal sample 2-3 was 6550–9070 nm.
Figure 3(a) shows the pore scanning results for the coal before and after drying for 48 h. In the range of the left peak, the variation trend was basically consistent with those for drying for 8 h and 24 h. In the range of the left peak, the width and the peak value of the peak increased slightly, but the increase was not significant. Within the range of the right peak, however, taking coal sample 3-3 as an example, the left boundary of the right peak extended significantly to the left and to the right. The increase of the right peak indicated that more and larger pores and cracks were generated or expanded. The significant increase of the left peak indicated that the pores within the range of the original right peak also increased.
The comparison between the coal samples that were dried for 48 h and raw coal is shown in Figure 3(b). For coal sample 3-1, larger pores or cracks with pore sizes of 7700–17400 nm were generated due to 48 h of drying. For the other coal samples, although there was no significant increase in the larger pore size, the total amount of pores corresponding to the original pore size increased significantly, which also indicated that drying resulted in a significant increase in the large pores in the coal.
Table 1 shows the porosity change and the porosity increase percentage of the coal during the drying process. Compared with raw coal, the porosity of the coal increased after drying, and the longer the drying time was, the greater the porosity increase proportion of the coal was.
Table 1
Porosity changes of raw coal and saturated dry coal
Raw coal
|
Porosity (%)
|
Saturated dry coal
|
Porosity (%)
|
Percentage increase in porosity (%)
|
Average porosity increases (%)
|
1-1-Rc
|
20.1
|
1-1-ad-8h
|
22.0
|
9.7
|
9.4
|
1-2-Rc
|
20.6
|
1-2-ad-8h
|
22.7
|
10.3
|
1-3-Rc
|
21.0
|
1-3-ad-8h
|
22.7
|
8.2
|
2-1-Rc
|
19.1
|
2-1-ad-24h
|
22.3
|
16.4
|
16.5
|
2-2-Rc
|
18.1
|
2-2-ad-24h
|
21.3
|
17.7
|
2-3-Rc
|
20.0
|
2-3-ad-24h
|
23.1
|
15.4
|
3-1-Rc
|
20.2
|
3-1-ad-48h
|
24.8
|
22.4
|
20.1
|
3-2-Rc
|
22.5
|
3-2-ad-48h
|
26.8
|
19.1
|
3-3-Rc
|
21.4
|
3-3-ad-48h
|
25.4
|
18.9
|
Table 2 shows the percentage of the pore volume in different pore size ranges during the drying process. As illustrated in Figures 4–6, the proportion of micropores in the coal samples used in this experiment was very low, and the total amount of micropores in the 18 tested coal samples decreased to 45% of the original coal on average, except that the proportion increased only slightly after the drying of samples 1-3 and 3-2. Two samples with low relative errors in the same group were averaged, and the proportion of micropores in the coal samples decreased to 53%, 61%, and 61% of that in raw coal after drying for 8 h, 24 h, and 48 h, respectively. Except for samples 1-3 and 3-2, the average results of the other 16 groups of coal samples showed that the content of mesopores was reduced to 49% after drying. However, large pores (100–1000 nm) and large pores or cracks (>1000 nm) were not detected in the raw coal, except for sample 1-3. After drying, the existence of large pores or cracks could be detected in the individual coal samples of each group after drying for 8 h. When the drying time reached 24 h, the occurrence of macropores could be detected in all of the dried coal samples, and the proportion of macropores increased gradually with the drying time. The proportions of macropores in the coal after drying for 8 h, 24 h, and 48 h were 1.2%, 1.8%, and 7.5% on average. Although the proportion of macropores in the pore system of the coal increased with the drying time, the proportion of large pores was not large. At the same time, large pores and cracks larger than 1000 nm were detected in all of the coal samples after drying, and the proportions of large pores and cracks in the coal samples after drying for 8 h, 24 h, and 48 h reached 83%, 87%, and 73% on average, respectively.
Table 2
Pore volume ratios for different pore sizes of raw coal and saturated coal after drying
Raw coal
|
<10 nm
|
10–100 nm
|
100–1000 nm
|
>1000 nm
|
Saturated dry coal
|
<10 nm
|
10–100 nm
|
100–1000 nm
|
>1000 nm
|
Pore volume ratio (%)
|
Pore volume ratio (%)
|
1-1-Rc
|
4.4
|
41.8
|
0.0
|
53.8
|
1-1-ad-8h
|
2.3
|
20.5
|
0.0
|
77.2
|
1-2-Rc
|
4.1
|
39.4
|
0.0
|
56.5
|
1-2-ad-8h
|
1.7
|
15.0
|
0.0
|
83.4
|
1-3-Rc
|
0.7
|
6.4
|
3.9
|
89.0
|
1-3-ad-8h
|
0.9
|
8.3
|
7.9
|
82.9
|
2-1-Rc
|
3.1
|
32.3
|
0.0
|
64.6
|
2-1-ad-24h
|
1.1
|
10.8
|
1.1
|
87.1
|
2-2-Rc
|
4.6
|
33.3
|
0.0
|
62.1
|
2-2-ad-24h
|
1.0
|
6.8
|
2.6
|
89.5
|
2-3-Rc
|
3.8
|
31.2
|
0.0
|
65.0
|
2-3-ad-24h
|
1.6
|
11.7
|
1.8
|
84.9
|
3-1-Rc
|
3.9
|
44.1
|
0.0
|
52.0
|
3-1-ad-48h
|
1.1
|
10.8
|
4.9
|
83.2
|
3-2-Rc
|
2.3
|
24.6
|
0.0
|
73.2
|
3-2-ad-48h
|
2.1
|
18.7
|
10.8
|
68.4
|
3-3-Rc
|
5.1
|
56.3
|
0.0
|
38.6
|
3-3-ad-48h
|
2.5
|
22.6
|
6.8
|
68.0
|
Combined with the significant drying shrinkage of the coal and the drying and cracking phenomenon of the shallow surface, we found that the volume of high water-bearing coal in the drying process had significant drying shrinkage, and many large pores and cracks larger than 1000 nm, accounting for 73–87% of the total, appeared in the shallow surface. The generation of deep cracks in the block coal was much lower than that in the shallow surface. Therefore, the deep layer of the coal was accompanied by significant drying shrinkage and significant reductions in the micropore, pore, and mesopore content. In summary, the porosity and the large pores and cracks of coal increased significantly during the drying process, resulting in an increase of large channels for oxygen flow and storage in the coal as well as in enhanced connectivity.
2.2 Changes in oxygen consumption rate and exothermic intensity
According to the temperature, oxygen concentration, carbon monoxide, and carbon dioxide concentration of coal samples during the temperature-programmed experiment, the oxygen consumption rates for different coal samples in the low-temperature oxidation stage were calculated as shown in Figure 7.
When the temperature of the coal sample was lower than 70°C, the oxygen consumption rate of the raw coal was higher than that of the coal sample dried for 48 h, which in turn was higher than that of the other dry coal samples soaked in water. The oxygen consumption rates of the coal samples dried for 8 h and 24 h were lower than those for the raw coal and the coal samples dried for 48 h, but the difference between the rates was small.
When the temperature of the coal sample was in the range of 70–90°C, the oxygen consumption rate of the raw coal was the highest, followed by that of the coal samples dried for 48 h, then by that the coal samples dried for 8 h, and the lowest oxygen consumption rate was that for the coal samples dried for 24 h after soaking in water. When the temperature of the coal sample was in the range of 90–100°C, the order of the oxygen consumption rates was raw coal > coal sample dried for 8 h > coal sample dried for 48 h > coal sample dried for 24 h. When the coal temperature was within the range of 100–120°C, the order of the oxygen consumption rates was raw coal > coal sample dried for 8 h > coal sample dried for 24 h > coal sample dried for 48 h. When the coal temperature was in the range of 120–140°C, the order of the oxygen consumption rates was raw coal > coal sample dried for 24 h > coal sample dried for 8 h > coal sample dried for 48 h. When the temperature of the coal was in the range of 140–170°C, the order of the oxygen consumption rates was coal sample dried for 24 h > raw coal > coal sample dried for 8 h > coal sample dried for 48 h.
For the temperature-programmed condition, the spontaneous combustion risk for the coal samples could be directly determined using the oxygen consumption rate and the heat release intensity of the coal samples, and the coal samples with a high oxygen consumption rate and heat release intensity had a high spontaneous combustion risk. At different temperature stages, the sequence of spontaneous combustion risks for different coal samples was different, and there were significant differences in the spontaneous combustion risks due to different temperatures and different treatment methods for the coal samples. When the temperature of the coal sample was below 90°C, the spontaneous combustion risk for the coal sample dried for 48 h was close to that of the raw coal but higher than those of the coal samples dried for 8 h and 24 h. When the temperature of the coal sample reached 90–120°C, the raw coal had the highest spontaneous combustion risk, followed by the samples that were dried for 8 h, 24 h, and 48 h, which showed the highest spontaneous combustion risk of the raw coal. The longer the drying time after soaking in water was, the higher the spontaneous combustion risk of the coal sample was. When the temperature of the coal sample reached the range of 120–140°C, the spontaneous combustion risk of the raw coal was close to that of the coal sample dried for 24 h and higher than that of the coal sample dried for 8 h, and the lowest was that of the coal sample dried for 48 h. When the temperature of the coal sample was higher than 140°C, the spontaneous combustion risk of the coal sample dried for 24 h was the highest, followed by those for the raw coal, and then the coal sample dried for 8 h after soaking in water, and the lowest risk was that for the coal sample dried for 48 h.
Coal oxidizes with oxygen, releasing a large amount of heat. According to the experimental results, as shown in Figure 8, different treatment processes had a significant impact on the upper limit of the heat intensity released by coal oxidation. The change rule of the upper limit of the heat intensity with temperature was basically consistent with the change rule of the oxygen consumption rate with temperature. When the coal temperature was lower than 70°C, the upper limit of the heat release intensity of the coal sample was lower, but the upper limit of the heat release intensity of the raw coal sample was the highest in general, followed by the coal sample that was soaked and dried for 48 h. When the temperature of the coal sample was higher than 90°C, the upper limit of the heat release intensity of the coal sample began to increase significantly. In the range of 100–120°C, the upper limit of the heat release intensity of the raw coal was the highest, followed by that for the sample dried for 8 h after soaking in water, and then by that for the sample dried for 24 h, and the lowest was that for the coal sample dried for 48 h. When the temperature of the coal sample was above 160°C, the upper limit of the heat release intensity of the coal sample dried for 24 h after soaking in water was the highest, higher than that of the raw coal. The coal sample dried for 8 h was next to the raw coal, and the coal sample dried for 48 h after soaking in water had the lowest upper limit.
The upper limit of the exothermic strength and the change of the oxygen consumption rate with the temperature fully indicated that the oxidation capacity and the exothermic strength of the coal decreased and increased in a certain temperature range after a certain length of time of air drying at room temperature. Generally, when the temperature of the coal sample reached approximately 120°C, the oxidation capacity, and the heat release intensity of the coal sample after soaking and drying for 24 h gradually became close to those of the raw coal. When the temperature of the coal sample was above 140°C, the oxidation capacity, and the heat release intensity of the coal sample after soaking and drying for 24 h gradually became higher than those of the raw coal, and the difference between the coal sample and the raw coal after soaking and drying for 8 h began to narrow. This indicated that the spontaneous combustion risk of the coal samples dried for 24 h after soaking in water gradually increased when the coal temperature was higher than 120°C. When the temperature of the coal samples was higher than 140°C, the spontaneous combustion risk was significantly higher than that of raw coal, and the difference between the spontaneous combustion risk for the coal samples dried for 8 h and the raw coal gradually decreased. The effects of the drying after soaking on the spontaneous combustion risk of the coal were complex, and there were both promoting and inhibiting effects. The two effects were significantly affected by the degree of drying and the temperatures of the coal samples. The oxidation capacity, heat release intensity, and spontaneous combustion risk of the coal samples dried for 24 h at the high-temperature stage were significantly higher than those of the raw coal and the other coal samples with different drying treatments or without drying treatment after soaking.