Kinetics of the major coal seam in typical coal mining areas 1 of Southern Junggar coalfield, Xinjiang, China

: In the present study, the authors collected coal samples from the Wudong (WD), 10 Dahuangshan (DHS), and Sikeshu (SKS) coal mines in the Southern Junggar coalfield. The 11 collected coal was ground into particle sizes of 0.25–0.38 mm, 0.15–0.18 mm, 0.109–0.12 mm, 12 0.08–0.096 mm, and <0.075 mm. The experimental data was acquired using diverse methods, such 13 as in situ infrared spectroscopic analysis, temperature-programmed oxidation, and 14 thermogravimetric analysis. The results show that the number of oxygen-containing functional 15 groups increased with the decline in particle size, indicating that a smaller particle size may 16 facilitate oxidation reactions and spontaneous coal combustion. On the basis of the analysis of the 17 elements of coal samples, it can be concluded that the proportion of oxygen elements of coal 18 samples in three mining areas is: WD ＞ SKS ＞ SHS. The oxygen consumption rate of the DHS and 19 WD coal samples increased exponentially when the temperature increased; the rate of SKS coal 20 samples initially rose significantly and then decreased. The gas generation rates for the different 21 gases indicated that temperatures of 90 ℃ or 130 ℃ could accelerate the oxidation reaction. The 22 pollutants produced by the oxidation of the SKS coal samples were higher than those by DHS and 23 WD. For all particle diameters, the T 3 for the SKS samples is smaller than that for WD and DHS, a higher exothermic heat, the coal sample will more readily ignite.

groups increased with the decline in particle size, indicating that a smaller particle size may 16 facilitate oxidation reactions and spontaneous coal combustion. On the basis of the analysis of the 17 elements of coal samples, it can be concluded that the proportion of oxygen elements of coal 18 samples in three mining areas is: WD＞SKS＞SHS. The oxygen consumption rate of the DHS and 19 WD coal samples increased exponentially when the temperature increased; the rate of SKS coal 20 samples initially rose significantly and then decreased. The gas generation rates for the different 21 gases indicated that temperatures of 90 ℃ or 130 ℃ could accelerate the oxidation reaction. The 22 pollutants produced by the oxidation of the SKS coal samples were higher than those by DHS and 23 WD. For all particle diameters, the T 3 for the SKS samples is smaller than that for WD and DHS, 24 indicating that the SKS coal sample will combust more readily. With the decrease in particle size, 25 the activation energy showed an increase in low-temperature oxidation stage. While the activation 26 energy of the WD and SKS samples decreased in high temperature stage, that of the DHS 27 increased before decreasing. A lower of activation energy means that the coal will have a higher 28 risk of spontaneous combustion. Based on the differential scanning calorimetry (DSC) curve, SKS 29 coal sample has the highest exothermic heat, the DHS has the second, and WD has the least. With

Introduction 34
Coal is prone to spontaneous combustion in underground mining (Cheng et al., 2017). Coal 35 oxidation at low temperatures is the heat source liable for the self-heating and spontaneous 36 combustion of coal. Self-heating of coal begins when adequate oxygen from the air is sufficient to 37 support the reaction between coal and oxygen. The heat produced by low-temperature oxidation of 38 coal is not sufficiently dissipated either by conduction or convection, hence an increase in 39 temperature within the coal mass arises (Onifade et al., 2020(Onifade et al., , 2019. Coal spontaneous combustion 40 (CSC) causes huge economic losses and casualties, with the toxic and harmful gases produced 41 during coal combustion not only polluting the working environment, but also causing great 42 damage to the ecological environment (Kong et al., 2017). Some scholars (Niu et  conditions, moisture content, and particle size) to prevent fires from CSC and control the pollution 45 from the burning coal. 46 Oxidative combustion of coal is a complex physico-chemical process affected by many 47 Mountain and the southern margin of the Junggar Basin, approximately 40 km southwest of Wusu 124 City. It is part of the Southern Junggar coalfield and is the main coal producer in the 125 Wusu-Kuytun-Dushanzi golden region. The Wudong Coal Mine (WD) is located near the southern 126 margin of the Junggar Basin and in the middle of the Southern Junggar coalfield. The thickness of 127 its coal seam varies significantly, tending to be thick in the east and thin toward the west, and the 128 seams are steeply inclined. The horizontal stratification and fully mechanised top coal caving 129 methods have been adopted for mining WD. Presently, the primary coal seam is Group B, and the 130 The coal samples used in this study were collected from the WD, DHS, and SKS mines in the 137 Southern Junggar coalfield. The collected raw coal was crushed and screened to produce samples 138 with various particle sizes: 0.25 to 0.38 mm, 0.15 to 0.18 mm, 0.109 to 0.12 mm, 0.08 to 0.096 139 mm, and <0.075 mm. Table 1 presents an analysis of the properties of the collected coal samples. 140 were as follows: RES 4.0 cm -1 , SCANS 120, and wavelength in the range of 400 to 4,000 cm -1 . 147

Temperature-Programmed Oxidation (TPO) Analysis 148
The intact coal samples were crushed, and five coal samples with particle sizes of 0.25-0.38 149 mm, 0.15-0.18 mm, 0.109-0.12 mm, 0.08-0.096 mm, and <0.075 mm were mixed. Then, the 150 mixture was tested on a BPG-907A experiment table for the temperature-programmed oxidation. 151 The heating rate was 0.3 C/min, the temperature range was 30 C to 170 C, and the airflow rate 152 was 120 mL/min. With every 10 C increase, the exit gas was drawn to analyse its composition 153 (especially O 2 , CO, CO 2 , and C n H m ) and concentration using a gas chromatograph. The test was 154 terminated when the temperature of the coal sample was higher than or equal to the temperature of 155 the furnace chamber (i.e. critical temperature). 156

TGA Analysis 157
A STA 7300 thermal analyser (made by Hitachi, Japan) was used. The experimental 158 conditions were as follows: The heating rate was 10 C/min, the atmosphere was air, the airflow 159 rate was 200 mL/min, and the temperature increased from the ambient temperature to 1000 C. Thermogravimetric curves were generated using the TGA data (the particle size of coal 163 sample from SKS was 0.15-0.18 mm) (as shown in Fig. 2). 164 165 Fig. 2. TG and DTG curves of coal sample As shown in Fig. 2, T 1 to T 5 denote the characteristic temperature of the coal at different 167 stages. Specifically, T 1 denotes the inflection temperature at which a coal sample gains weight 168 after water loss and oxygen uptake; T 2 denotes the initial temperature of pyrolysis; T 3 denotes the 169 ignition temperature; T 4 denotes the temperature at which the combustion rate is maximised, and 170 T 5 denotes the burn-off temperature. 171 2.3.2 Calculating the non-isothermal gasification kinetics parameters 172 The activation energy was calculated using the Coats-Redfern integral method. The 173 combustion reaction between coal and oxygen is viewed as a first-order reaction. According to the 174 Arrhenius, the reaction rate of coal combustion can be calculated: 175 (1) 176 where k denotes the reaction rate constant, A denotes the frequency factor, E denotes the activation 177 energy (kJ/mol), T denotes the reaction temperature (K), and R denotes the gas constant (R = 8.314 178 In the coal-oxygen reaction process, the mass conversion rate is as follows: 180 where α denotes the mass conversion rate in the combustion process of coal sample, m 0 denotes 182 the mass of the coal sample when the thermogravimetric experiment began, m t denotes the mass of 183 coal sample t after the beginning of the experiment, and m ∞ denotes the mass of the coal sample at 184 the end. The reaction rate is calculated as follows: 185 where k denotes the chemical reaction rate, and t denotes time. 187 The following equations were obtained after the integral operation and the Coats-Redfern 188 approximate function were calculated: 189 For the general reaction temperature province and activation energy (E), E/RT is greater than 192 or equal to 1, and 1-2RT/E is approximately 1. Therefore, Equation (4) can be rewritten as 193 Equation (6)  Coal has a complex macromolecular structure that is mainly composed of aromatic 201 compounds with poor condensation, aliphatic hydrocarbons, oxygen-containing functional groups, 202 and other active groups. Infrared spectroscopy was used to study the basic chemical characteristics 203 of the coal samples. Table 2 lists the functional groups detected through the in-situ infrared 204 analysis of our coal samples. 205   As shown in the figures above, the functional groups display consistent trends for the 243 absorbance and peak areas. As the particle size decreases, the absorbance and peak areas of 244 aliphatic hydrocarbons vary as follows: 1) for the coal samples from WD, the absorbance 245 increases first and then decreases; the peak areas decrease first, then increase before finally 246 decreasing again; 2) for the coal samples from DHS, the absorbance and peak areas do not vary 247 significantly; 3) for the coal samples from SKS, both the absorbance and peak areas increase (as 248 shown in Fig. 4 and Fig. 5).The absorbance and peak areas for the aliphatic hydrocarbons in the 249 coal samples are ranked as WD > DHS > SKS for particle size ranges of 0.25-0.38 mm, 0.15-0.18 250 mm, and 0.109-0.12 mm, WD > SKS > DHS for 0.08-0.096 mm, and DHS > WD > SKS for 251 those <0.075 mm. 252 As the particle size decreases, the absorbance and peak area of the aromatic hydrocarbons 253 vary as follows: 1) for the coal samples from WD, the absorbance and peak area increase first and 254 then decrease; 2) for those from DHS, the absorbance decreases first and then increases, and the 255 peak area increases first, then decreases, and finally increases; 3) for the samples from SKS, the 256 absorbance increases first, then decreases, and finally increases, and the peak area increases (as 257 shown in Fig. 6 and Fig. 7). The absorbance of the aromatic hydrocarbons in coal samples are 258 ranked as follows: DHS > WD > SKS for particle size ranges of 0.25-0.38 mm and <0.075 mm, 259 WD > DHS > SKS for 0.15-0.18 mm, 0.109-0.12 mm, and 0.08-0.096 mm; WD > DHS > SKS 260 for all particle size ranges overall. 261 As the particle size decreases, the absorbance and peak area of oxygen-containing functional 262 group vary as follows: 1) for the samples from WD, the absorbance decreases, whereas for those 263 from DHS and SKS, the absorbance increases; 2) for the samples from WD and DHS, the peak 264 area does not vary significantly, whereas for those from SKS, the peak area increases first, then decreases, and finally increases (as shown in Fig. 8 and Fig. 9). The absorbance of the 266 oxygen-containing functional group in the coal samples are ranked as follows: DHS > WD > SKS 267 for the particle size range of 0.25-0.38 mm, DHS > WD > SKS for 0.15-0.18 mm, DHS > SKS > 268 WD for 0.109-0.12 mm, and SKS > DHS > WD for 0.08-0.096 mm and <0.075 mm. 269 As mentioned in previous studies, the content of oxygen-containing functional groups is an 270 index to measure if the coal is prone to oxidation. The experimental data reveal that for coal 271 samples from SKS, the smaller the particle size is, the more prone the coal samples are to 272 oxidation, and the particle size does not affect the oxidation propensity of the samples from WD Oxygen consumption rate is an index to measure the intensity of the coal-oxygen reaction. 291 For the samples from WD and DHS, the oxygen consumption rate positively correlated with 292 temperature, and for those from SKS, the oxygen consumption rate increased first and then 293 decreased with the rise in temperature (as shown in Fig. 10). 294 As shown in Fig. 11, the coal samples are ranked as follows by the yield of CO and CO 2 : 295 SKS > DHS > WD. In addition to the CO 2 generation rate of SKS coal sample higher than the CO 296 generation rate, the CO 2 output rate of the other two mines are lower than the CO generation rate. 297 This indicates that coal combustion in SKS produces the most severe environmental pollution, 298 followed by DHS, and then WD. 299 The emission concentration of CO is positively correlated with temperature for the samples 300 from WD and DHS, and for those from SKS, the emission concentration is high initially, then 301 follows the same trend as that of the samples from WD and DHS after temperature reaches 80 C 302 (as shown in Fig. 12). For the coal samples from WD and DHS, the emission concentration of CO 303 increases slowly when the temperature is below 90 C, then displays a marked increase over 90 C, 304 indicating a speed-up in the oxidation reaction. After temperature is above 130 C, the emission 305 concentration of CO increases sharply, indicating a further speed-up in the oxidation reaction. For the coal samples from SKS, the emission concentration of CO is quite high initially, indicating that 307 the oxidation reaction is quite fast initially, and then slows down until the degree of oxidation is 308 the same as that of the samples from WD and SKS. For the samples from the three coal mines, the 309 concentration of oxygen basically shares the same trend with the rise in temperature (decrease first, 310 and then increase when temperature reaches 130 C). The concentration of oxygen increases first 311 with a sharp decrease for the samples from WD and DHS; for SKS, the concentration of oxygen 312 continues to increase, but at a lower rate than that of the samples from WD and DHS. As shown in Figures 13 -17, the characteristic temperatures T 2 , T 3 , T 4 , and T 5 of the coal 325 samples from SKS are all lower than those from WD and DHS, whereas the characteristic 326 temperature T 1 of the samples from SKS is higher than that from WD and DHS. As the particle 327 size decreased, the T 1 decreased for the samples from WD, decreased first and then increased for 328 DHS samples, and increased first and then decreased for SKS samples. 329 As the particle size decreases, the T 2 for the WD samples varies slightly, decreases first and 330 then varies slightly for DHS, and decreases first and then increases for SKS. As the particle size 331 decreases, the T 3 for the WD samples decreases first, then increases, and finally decreases. The T 3 332 of the samples from DHS increases first, then decreases, and finally increases, and the T 3 for SKS 333 decreases first and then increases. Because T 3 denotes the ignition temperature of coal, the lower 334 the ignition temperature is, the more prone the coal is to spontaneous combustion. Therefore, the 335 coal from SKS is the most prone to spontaneous combustion, followed by DHS, and then WD. 336 As the particle size decreases, the T 4 of the samples from WD and DHS tend to decrease, and 337 the T 4 of those from SKS increases first and then decreases. For the T 5 , the samples from WD 338 show an increase first, then a decrease, and finally an increase. The T 5 of the coal samples from 339 DHS varies irregularly, and the T 5 of the SKS samples decreased before increasing. Fig. 18. Activation energy of coal sample at the T 1 -T 2 stage with different particle sizes 343 344 Fig. 19. Activation energy of coal sample at the T 3 -T 5 stage with different particle sizes 345

Analysis of Activation Energy 341 342
As shown in Fig. 18 and Fig. 19, during the low-temperature oxidation stage, the activation 346 energy of the samples from DHS and SKS increases first and then decreases overall, from WD, 347 decreases first and then increases, and from DHS, is higher than that from WD and SKS. During 348 the high-temperature oxidation combustion stage, the activation energy for the DHS and SKS 349 samples increases first and then decreases, and the activation energy for WD decreases first and 350 then increases. Based on the proximate analysis, the coal samples are ranked as follows by volatile 351 components: SKS > DHS > WD. For the SKS samples, the volatile content is the highest, and the 352 T 3 and T 5 are lower than those for WD and DHS. Therefore, a higher volatile content indicates that 353 the samples are more prone to ignition, and the coal samples burned out more readily. 354 Volatile content is not the only factor that affects the characteristic temperatures (e.g. element 355 content), nor does it only impact the characteristic temperatures. The activation energy of the coal 356 tends to decline with higher volatile components, so the coal particles will be more likely to ignite. 357 In addition, the precipitation of volatiles changes the morphological structure of the coke, making 358 the residual coke porous, resulting in a larger specific surface area and air contact area, thus 359 making the residual coke more prone to oxidation and combustion. As a result, the minimum 360 energy required for coal combustion (i.e. activation energy) is lower (Table 1). 361

Analysis of exothermic heat in the oxidation process of coal 362
DSC was performed to measure the quantity of heat required for maintaining a 363 zero-temperature difference per unit time between a sample and a reference object to reflect how 364 the enthalpy of the sample varies with temperature. DSC curves were generated from the 365 thermogravimetric data using the Origin software. Specifically, the integral operation was 366 conducted on the X axis to determine the exothermic heat at different stages of spontaneous 367 combustion. In the graph, below 0 on the Y axis is the heat absorbed by spontaneous combustion, 368 and above 0 of the Y axis is the exothermic heat of spontaneous combustion. According to the 369 characteristic temperatures determined by the thermogravimetric experiment, the coal-oxygen 370 reaction process is divided into three stages: 1) oxygen absorption and weight-gaining stage 371 (T 1 -T 2 ), 2) dehydration stage (T 2 -T 3 ), and 3) combustion stage (T 3 -T 5 ). In this study, only the 372 exothermic heat at the combustion stage was calculated. Figures 12 to 14 respectively show the 373 DSC curves of coal samples from WD, DHS, and SKS. Figure 15 shows the exothermic heat of 374 the coal samples with different particle sizes from the three mines at the combustion stage.  As shown in Fig. 20 to 22, the DSC curves of the samples from the three coal mines reflect 384 their exothermic heat. Among the samples from WD, exothermic heat was only recorded for 385 particle sizes of <0.075 for oxidation reactions, indicating low exothermic heat generated by 386 oxidation reactions. Among the coal samples from DHS, the DSC values for particle sizes of 0.15-387 0.18 mm, 0.08-0.096 mm, and <0.075 mm are greater than 0 (i.e. exothermic heat is generated), 388 indicating average exothermic heat generated by oxidation reactions. For both WD and DHS, 389 smaller particle sizes are beneficial to the generation of exothermic heat. For the SKS samples, all 390 particle sizes record exothermic heat, indicating high exothermic heat generated by oxidation 391 reactions. Additionally, the samples with a particle size of <0.075 mm are different from those 392 with a particle size of 0.25-0.38 mm in terms of peak height and peak width. As shown in the 393 above figures, however, both particle sizes generate high exothermic heat during the oxidation 394 reaction. 395 Fig. 23 demonstrates that the coal samples from WD generate the lowest exothermic heat, 396 followed by DHS and SKS. Additionally, the exothermic heat generated by the SKS samples with 397 different particle sizes varies irregularly but is overall higher than that from WD and DHS. 398 Therefore, the coal samples from SKS are the most prone to spontaneous combustion.

Conclusions 400
First, elemental analysis shows that, overall, the content of oxygen-containing functional 401 groups tends to increase with the decrease in particle size; therefore, the smaller the particle size is, 402 the more prone the coal samples are to oxidation and spontaneous combustion. The proportions of 403 oxygen in the samples is ranked as WD > SKS > DHS. In summary, the coal samples from SKS 404 are the most prone to oxidation, followed by the WD samples, and the samples from DHS are least 405 prone to oxidation. 406 Second, for coal samples from DHS and WD, the oxygen consumption rate continues to 407 increase exponentially with the rise in temperature; for coal samples from SKS, oxygen 408 consumption rate increases exponentially initially, and then decreases. The different gases 409 emission show that the oxidation reaction speeds up at the temperatures of 90 C and 130 C. 410 These gases show that coal samples from SKS are more prone to spontaneous combustion than 411 those from DHS and WD. Furthermore, during this reaction, the samples from SKS produce more 412 pollutants than those from DHS and WD. 413 Third, for coal samples from WD and DHS, the T 3 declines as the particle size decreases, 414 indicating that the smaller the particle size is, the lower the ignition temperature of coal sample is. 415 For the SKS samples, the T 3 rises as the particle size decreases, but the T 3 of all sample particle 416 sizes is lower than that of the samples from WD and DHS, indicating that coal samples from SKS 417 are more prone to combustion. 418 The reaction activation energy of experimental coal samples overall tends to increase at the 419 stage of low-temperature oxidation. At the stage of high-temperature oxidation, as the particle size 420 decreases, the activation energy of the samples from WD and SKS decreases, whereas that of the 421 DHS samples increases first and then decreases. Overall, at the stage of high-temperature 422 oxidation combustion, lower activation energy implies that coal is more prone to ignition, causing 423 a higher risk of spontaneous combustion. 424 Volatile content is also a factor that affects the spontaneous combustibility of the coal 425 samples. Higher volatile contents cause the samples to be more prone to ignition and burn out 426 sooner. The amount of exothermic heat reveals that in the SKS samples, higher exothermic heat 427 indicates that the samples will be more prone to spontaneous combustion. 428 In practice, the fire areas in SKS generated by coal mining are still on fire, but not yet 429 harnessed. This also indicates that coal of SKS is very prone to spontaneous combustion. 430