Fresh coal samples (as large as possible) collected from the Fushun West Open Pit Mine were sealed on site and transported to the laboratory. When preparing the experimental samples, the oxide layer on the surface was stripped first, and the samples were crushed to 80-120 mesh (particle size is 0.124-0.178 mm) for experiment followed.
(1) Coal quality analysis. Moisture and volatile content were measured by industrial analyzer (5E-MAG6700, China). The element components of samples were determined by test using element analyzer (Vario EL Ⅲ, Germany).
(2) Analysis of specific surface area and pore size. The automatic physical and chemical adsorption analyzer (Autosorb-iq-c, American) was used to test the specific surface area and pore size distribution characteristics.
(3) Temperature-programmed experiment at high temperature. The coal samples were crushed and sieved into 5 different particle sizes including 0~0.9mm, 0.9~3mm, 3~5mm, 5~7mm and 7~10mm, and 1 kg each was taken to form a mixed particle size sample. The high-temperature-programmed experimental test was carried out in the spontaneous combustion experimental system of (XKGW-1, Xi’an University of Science and Technology, China). The heating rate was 1℃/min, the air flow was set at 120 mL/min, and the temperature rising range was 30-600℃. The gas was pumped every 15℃ increasing, and sent to gas chromatograph (SP-3430, American) for analyzing gas component.
(4) Thermal analysis experiment. Differential scanning calorimetry (DSC, TA-SDTQ600, American) experiment was done by using thermal analyzer to study heat flow curve and the exothermic characteristics of the sample.
(5) Free radical concentration test. The electron spin resonance spectrometer (MS5000, Germany) was used to test the free radical concentration at different characteristic temperature points.
Test results and discussion
Coal quality analysis. The coal seam in the West Open-pit Mine was formed at Jurassic Cretaceous period, and is a kind of long-flame coal. The coal quality analysis results is shown in Table 1, where Mad is moisture content on an air-dry basis (ad), Aad is ash content, Vad is volatile matter content, FCad is fixed carbon content, and C, H, O, N, S are carbon, hydrogen, oxygen and nitrogen, sulfur element respectively.
Table 1. Coal analysis result (%)
It can be seen that the Mad content is only 2.94%, less than 5%. The Vad content is much higher than some conventional data, which is very helpful for spontaneous combustion. The sulfur content is only 0.33%, which is good for use.
Analysis of specific surface area and pore size. As shown in Table 2, the specific surface area of coal sample is up to 39.3788 m2/g, increasing the chance of a reaction in contact with oxygen. It can be seen that the macropores (pore size >50 nm) and mesopores (2-50nm) of the coal sample is more than 95%, and it is very helpful for oxygen adsorption beneficial for low temperature oxidation. The apparent characteristics is very helpful to explain spontaneous combustion is easy happening.
Table 2. Specific surface area and pore size distribution
||Specific surface area（m2/g）
Variation of oxygen concentration. The law of oxygen consumption is one of the most important macro parameters to reflect the severity of spontaneous combustion. The variation curves of oxygen concentration and consumption rate with temperature are shown in Figure 2. In the low-temperature stage, the oxidation reaction of coal samples was relatively gentle, and there is only a little active functional groups reacting with oxygen. A large number of active functional groups had not been activated, and the consumption of oxygen was low.
The oxygen concentration decreased sharply and oxygen consumption rate increased rapidly after 150 ℃. At 270 ℃, the oxygen concentration reached the point of 6.986 %. At this time, the active functional groups in coal molecules were gradually activated, resulting in chemical reactions of coal samples, and the reaction rate was accelerating. A large number of active functional groups participated in the reaction and consumed a lot of oxygen. When reaching 330 ℃, the oxygen concentration began to rise significantly, and rose to 16.04 % at 390 ℃. At this temperature, a large number of functional groups inside the mineral molecules began to decompose. The sample has just reached the ignition point, and the heat generated by combustion is low. However, the decomposition of functional groups requires a lot of energy, resulting in a slight decrease in the combustion effect of the sample and the oxygen concentration rose slightly. After 390 ℃, the oxygen concentration began to decline sharply, until the end of the experiment, it fell to 6-7 %. The oxygen consumption rate is in the opposite trend to the oxygen concentration.
The law of indicator gas. Experiments show that the coal will release carbon oxide and hydrocarbons (olefins, alkanes) in the process of the oxidation and spontaneous combustion. The composition and concentration of gas have a corresponding relationship with temperature. In this experiment, the concentrations of CO, CO2, CH4, C2H4, C2H6 and C3H8 generated and their variation laws were measured.
(1) Carbon oxide. The gas of CO and CO2 were detected from the beginning in the low temperature oxidation stage ( before 200 ℃ ), the gas precipitation changed slowly, as shown in Figure 3. When the coal temperature was lower than 180 ℃, the CO concentration increased slowly, and a large number of water oxygen complexes were generated by the reaction of coal with water and oxygen. With the increase of temperature, some water oxygen complexes were converted into CO and CO2. After 200 ℃, the gas concentration increased rapidly. The CO concentration reached the first max point at 350 ℃. At this stage, the coal sample reacted strongly with oxygen, resulting in an increase in oxygen consumption rate, a rapid decline in oxygen concentration, and a large amount of CO was generated. The gas concentration dropped a little because of decreasing of the content of functional groups such as carboxyl and aliphatic hydrocarbons. As the oxidation reaction of the sample gradually intensified, the C=C double bonds and some oxygen-containing heterocycles in the coal molecule were cracked, the production of CO increased again reaching a peak of 60760 ppm at 555 ℃, then began to drop until the end of the experiment.
In the low temperature oxidation stage, a large amount of CO2 gas was adsorbed between the sample macromolecules due to van der Waals force. In the process of heating, CO2 gas would gradually desorb, and the gas concentration increased rapidly in the rapid heating stage. In the high temperature stage, the growth rate slowed down slightly, and then increased rapidly. The CO2 concentration was up to 168704 ppm at 465 ℃, and then fluctuated slightly because of the oxidation of aromatic hydrocarbons and aliphatic hydrocarbons, and some CO2 gas was produced again. Then all kinds of functional groups were exhausted, and the gas concentration decreased rapidly.
(2) Hydrocarbon. The variations of CH4, C2H6, C3H8 and C2H4 concentrations with coal temperature were generally similar, as shown in Figure 4. The concentration of these four gases were lower before 300 ℃, but gradually increased with temperature. Since CH4 was easy to desorb from coal, it was detected at lower temperature. The gas of C2H6, C3H8 and C2H4 appeared at 120 ℃, 195 ℃ and 135 ℃, respectively, and their concentration were relatively low, and increased slowly with the increase of coal temperature.
The concentrations of CH4, C2H6, C3H8 and C2H4 increased sharply from 340 ℃, 330 ℃, 350 ℃ and 345 ℃, and reached the peak value at 480 ℃, 430 ℃, 435 ℃ and 435 ℃ respectively. In addition, a certain amount of humic acid contained in coal also produced hydrocarbon gases by thermal decomposition reaction. After that, the decomposition of active functional groups ended, and the concentration of CO was in a high rising state, which inhibited the precipitation of hydrocarbon gases. The concentration of CH4 dropped after 480 ℃, but it was still at a high level until the end of combustion. The concentration of C2H6, C3H8, and C2H4 gas decreased sharply after 435 ℃, 480 ℃, and 480 ℃, respectively, and remained at a low level after 530 ℃ as it was in the low temperature oxidation stage.
Characteristic temperature. Characteristic temperature is a macroscopic representation of the influence of external environment on the reaction process in the micro-process of coal sample oxidation. Using the growth rate analysis method of index gas CO, as shown in Figure 3. The characteristic temperature points of high temperature oxidation of coal samples were obtained, such as critical temperature ( T1 ), crack temperature ( T2 ), active temperature ( T3 ), speed-up temperature ( T4 ) and ignition temperature ( T5 ), as shown in Table 3.
Table 3. Characteristic temperatures (℃)
The critical temperature stage is the stage of water evaporation and desorption, corresponding to the stage before the critical temperature point. Crack – active – speedup temperature stage is the stage of oxygen inhalation and weight gain, which corresponds to process between the critical temperature and the speedup temperature. The speedup – ignition temperature stage is the stage from the speed-up temperature to the ignition temperature, which indicates that the coal samples undergo thermal decomposition and start violent oxidative cracking. The combustion stage is the stage from ignition temperature to burnout. The critical temperature, crack temperature and active temperature occurred in the low temperature oxidation stage. The CO concentration of the sample increased slowly before the active temperature T3, and a large number of CO and CO2 precipitated after the active temperature, and the corresponding gas concentration increased sharply.
Exothermic properties. The oxidation spontaneous combustion is a continuous exothermic process, and the exothermic characteristics determine the spontaneous combustion properties. The DSC curve of the sample is shown in Figure 5, and the change of heat release with temperature is shown in Figure 6. According to the DSC results of coal samples, it can be seen that the heat flow values decreases at the initial stage ( the first stage ), this is due to the evaporation of water absorbing heat, while the released heat is generated by the physical adsorption of coal. The heat release is relatively small, so this stage is in the endothermic process. In the rapid heating stage ( the second stage ), the heat flow value increased with temperature, reaching the first peak of heat flow at about 320 ℃. At this time, the active functional groups, such as aliphatic hydrocarbons and oxygen-containing functional groups, reacted with oxygen first, producing a large amount of gas and releasing a large amount of heat. In the high temperature combustion stage ( the third stage ), the temperature was at about 490 ℃, and the heat flow value began to decrease after reaching the second peak. A large number of small molecule active agent structures and combustible materials reacted with oxygen, and the concentration of carbon oxides increased continuously. Finally, the exothermic peak and the peak of CO and CO2 gas concentrations of DSC curve appeared. The heat release intensity of the sample was very weak, and the heat release increased sharply. After the burnout temperature ( 600 ℃ ) ( the fourth stage ), the heat flow fall rapidly. At this time, the heat release intensity became weak, but the heat release still increased slowly, and finally reached the peak value of 4714 J/g.
Variation of free radicals. There were a certain amount of free radicals in coal formation, and a large number of free radicals will be generated in the process of spontaneous combustion. The concentration of free radicals has a great effect on spontaneous combustion. As shown in Figure 7. the concentration of free radicals at different characteristic temperature are increasing linearly with temperature.
The temperature increasing will promote production of the free radicals. The free radicals concentration at the active temperature is about 50% higher than which at the critical temperature point. At this time, some structural basic units, bridge bonds, and chemical bonds began to decompose and break, resulting in a large number of free radicals, and whose concentration at the ignition temperature is about twice as high as which at the critical temperature. The activity of free radicals will promote the coal-oxidation reaction and heating until combustion, and meanwhile it will also promote the precipitation of the indicator gas.