Study on the thermal kinetics of anthracite oxidation induced by water and associated pyrite

Pyrite and water in coal have considerable inuence on coal spontaneous combustion and threaten the safety of mine production gravely. To reveal the inuence mechanism of water and associated pyrite on oxidation kinetics of coal–oxygen composite reaction, the pyrite of 0%, 1%, 2%, 4%, 6% and the moisture of 1%, 5%, 10%, 15% and 20% were mixed with the coal samples to obtain 25 coal samples. Thermogravimetric analysis technology was conducted to explore the changes of mass and characteristic temperatures of coal samples treated with water and associated pyrite during the low– temperature oxidation, and kinetic analysis of the oxidation process was discussed based on multiple heating rates(5 °C/min, 10 °C/min and15 °C/min).The results show that water and associated pyrite had a great inuence on coal in oxygen absorption and weight gain stage ( T 3 ~ T 5 ), and there was a proportion range with the largest synergistic oxidation contribution. The apparent activation energy of the coal sample appeared changes, but the mechanism model did not, indicating that water and pyrite could affect the oxidation process of the coal sample externally. When water and associated pyrite exhibit synergistic interaction, there have a range that water was 10~15% and associate pyrite was 2~4% had the largest promotion and contribution to anthracite oxidation. The results have important scientic value and practical guiding signicance for the further study on prediction, prevention and control of high sulfur anthracite spontaneous combustion.


Introduction
Coal utilities are currently the major source of electricity in China, and in foreseeable future, coal will continue to be main energy and important industrial raw materials all over the word, and still plays an important role in economic development (Xu et al.2018; Xie et al. 2020). At present, low-sulfur coal is widely exploited and utilized; so, coal of high sulfur may have a signi cant role to play in future. Sulfur in coal can be divided into organic sulfur and inorganic sulfur; meanwhile, the main form of inorganic sulfur is pyrite (FeS 2 ) ). In actually anthracite mining process, the content of pyrite increased under the in uence of mining depth. At the same time, it was affected by the in ltration of surface and groundwater in the mining process. The air leakage caused by water exploration and release provided enough oxygen for anthracite, which lead to the spontaneous combustion of high-sulfur anthracite (Huang 2013). Burning high-sulfur coal would produce carbon dioxide and sulfur dioxide, they were the main sources of greenhouse gases and acid rain, respectively (Gu et al. 2017;Liang et al. 2019). In order to ensure production safety, reduce energy loss and whilst minimising the impact on the environment, we need to study the in uence of water and associated pyrite on coal oxidation process.
Sulfur is promising storage medium, which can be used for thermal energy storage at high temperature with a lower cost, and has good charge-discharge performance at high temperature (Jin et al. 2020; Nithyanandam et al. 2018). As for the in uence of pyrite on CSC, studies have found that pyrite has a great in uence on low temperature oxidation in humid environment (Arisoy and Beamish 2015; Arisoy et al. 2017). In the past decades, numerous researches have been carried out in learning the effect of pyrite on low-temperature coal oxidation. According to the Fenton reaction, FeS 2 in coal reacts with oxygen and water, the resulting H 2 SO 4 and Fe(OH) 3 colloids make a difference to CSC (Arisoy et al. 2017;Doulati et al. 2008;Li 2007;Yu et al. 2013). At the same time, the reaction gives off heat, which accumulated continuously and further increased the macroscopic calori c value of coal, promoted coal sample oxidation, and nally caused CSC. In recent years, Deng et al. (2015), Wang et al. (2020) found that pyrite has a signi cant effect on the spontaneous combustion of coal. The addition of 2.0 mass% FeS 2 exerted a prominent promoting effect on gas coal oxidation, the 5~7%FeS 2 had a great impact on anthracite. Yang et al. (2019) found that temperatures corresponding to the maximum weight loss and peak temperatures of coal samples were lowered with higher pyrite content, also calculated the apparent activation energy (E a ) using Flynn-Wall-Ozawa (FWO) method, it was conclude that coal with higher pyrite content was more prone to spontaneous combustion. Zhang et al. (2020a) have found the external moisture content has a signi cant impact on coal quality variation in the process of oxidation. The increase of moisture content would make the TG and DSC curve of CSC showing hysteresis, and inhibit the coal oxidation, which is not conducive to CSC (Jia et al. 2020). In addition, water-immersed coal can affect coal initial heat release temperature and heat release in the low temperature oxidation process, compared with raw coal, it was more likely to spontaneously ignite bituminous coal immersed for 30~90 days using Arrhenius Formal, and found that the coal sample released more heat at the low temperature stage, with lower activation energy and easier oxidation than raw coal. At the same time, the structure of coal sample will change to some extent after water immersion.
Domestic and foreign scholars have made some progress in the study of CSC which affected by water and pyrite, and accumulated a good research foundation. However, hardly study on oxidation kinetics effect of water and pyrite on CSC, such as the thermogravimetry, temperature and kinetics in the process of low temperature oxidation. Therefore, on the basis of previous studies, this paper investigated the in uence of water and associated pyrite on anthracite mass and characteristic temperature in low temperature oxidation through TG-DTG experiments. In addition, based on the synergistic effect of water and pyrite, the kinetic characteristic parameters of coal oxidation process were calculated and the most probable function was determined. Meanwhile, the mechanism of its effect on anthracite oxidation reaction was revealed, the optimal in uence ratio range was also determined; which provided a key support for further inhibiting the oxidation process of pyrite bearing coal from the perspective of thermal reaction.

Sample preparation
Taking the anthracite of Baijiao Coal Mine (Sichuan Province, China) as the research object, the fresh and unoxidized coal samples collected from the site were transported to the laboratory in sealed packaging of multi-layer plastic and nylon bags. Meanwhile, the fresh coal samples crushed and ground to less than 200 mesh, and each coal sample was ca. 100g.
In order to accurately study the effect of external moisture on coal oxidation at low temperature, the coal sample was dehydrated. The speci c method was to dry the coal samples in vacuum at 30℃ for 48hours.Due to the lower internal moisture of Baijiao anthracite, the in uence of internal moisture was not considered. So, different moisture was added to the coal samples after many calculations and experiments. Mixture well and leave in a cool, airtight place for a week until moisture was absorbed. Finally, coal samples with external moisture content of 1%, 5%, 10%, 15% and 20% were obtained. Pyrite was crushed to 0.075mm and added to the samples in order to gain anthracite samples with pyrite content of 0%, 1%, 2%, 4% and 6%.

Elemental analysis and proximate analysis
Samples underwent technical and element analysis via a 5E-MAG6700 (Kaiyuan, Changsha, China) proximate analyser and Vario ELIII (Elementar, German) organic element analyser before experiments.
The moisture (M ad ), ash (A ad ), volatility (V ad ), xed carbon (FC ad ), and element content were presented in Table 1.

TG-DTG experiment
The TG-DTG experiments were carried out from 30 to 400 °C, at heating rates of 5, 10 and 15 ℃/min in air and N 2 atmosphere of 50 mL/min, and the experimental sample volume was 5mg. The TG-DTG curves of coal samples with different water and pyrite were obtained, which showed the different characteristics of coal oxidation at low temperature.

Kinetic methods
In the study of coal oxidation kinetics, the rate of coal weight loss (gain) can be expressed by differential method and integral methods. G(α) represents the integral of the reaction mechanism function, as shown below. f (α) represents the differential form of the reaction mechanism function: According to Arrhenius formula, the relationship between reaction constant and temperature (absolute temperature) is: The experiment adopts non-isothermal temperature rise, so T = T 0 +β.The simultaneous Eqs. Where, R is the universal gas constant, 8.314 J/(mol·K); β is the heating rate, K/min; E a is the apparent activation energy; α is the conversion rate of coal at time, t; A is the pre-exponential factor.
(1) Achar differentiation Integrate both sides of Eq. (1) and take the logarithm to get: (2) Group-Redfern integral method In this paper, n = 1. Eq. (4) and Eq. (6) were tted linearly respectively to calculate the apparent activation energy and the pre-exponential factor.

Characterization of coal samples
Through proximate and elemental analysis of raw coal (YM), and 25 experimental coal samples, indexes of each element, moisture and ash were obtained, as shown in Table 1. In raw coal, the content of C was higher, while H was lower; it indicated that Baijiao anthracite had a high degree of metamorphism. Meanwhile, the lower content of O in raw coal, which was only 1.51%, suggested the oxygen condition of spontaneous combustion of raw coal was relatively low ).
With the increase of moisture content, the content of O and H increased accordingly, and it was greatest when water content was 1~5%. Hence, the increased of moisture content will lead to the increase of oxygen-containing functional groups. According to Table 1, it could be concluded that the increase of pyrite content would increase sulfur in coal, which may enhance the environmental impact of sulfur oxide (SO X ) release. Conversely, the impact on other elements was small. The V ad showed a trend of decrease after the rst increase, this was because the water in coal and pyrite would have a synergistic reaction, with the increased of water, the production such as CO 2 will increase. The variation was strongest at about 10% moisture. After that, the water lm and vapor pressure (Arisoy et al. 2017;Yu et al.2013) would be produced due to excessive water, and the changing effect will be weakened.

Characteristics temperatures
The TG and derivative-TG (DTG) curves during coal oxidation process were presented in Figs. 1-3. It should be illustrated that this paper took the heating rate of 10℃/min as an example to research the in uence of water and associated pyrite on TG-DTG curve. As shown in Fig. 1 The characteristic temperatures of all samples were presented in Fig. 2, the temperatures of the maximum evaporation of water and desorption of gases of 25 coal samples ranged from 90 ℃ to 110 ℃. Under the different moisture content and with the increase of pyrite, T 1 was reduced, although the change was nonlinear. When the moisture content increased, the critical temperature displayed a trend of increasing rst and then decreasing. This was because the increase of water content leads to the increase of water evaporation in the early stage of low-temperature oxidation and the longer evaporation time. However, it was not the higher the moisture content, the higher the critical temperature. There was a critical moisture content in this experiment, which was 10~15%.
With the increase of pyrite which in different water content, the variation trend of T 2 was different. When pyrite content was less than 6%, with the increase of moisture content, the T 2 rst fall and then rose and then decreased, reached the lowest value at 5%, and reached the highest temperature when the water content was 10~15%. According to Fig. 2, the dry cracking temperature of 25 coal samples were about 105~126 ℃, and the increase of pyrite content would raise the temperature to a certain extent.
Started from T 3 , the oxidation of coal became intense, which was the starting of oxygen absorption and weight gain stage of coal sample. The variation of T 3 was shown in Fig. 3a, under the 20% of moisture content, the temperature of coal samples was lower when pyrite content was 2~4%. Meanwhile, with the increase of moisture content, the trend of T 3 was different, among which the coal samples with water content of 10% and 15% had lower temperature. The change of temperature was affected by many factors, on one hand, the products Fe(OH) 3  factors, the temperature of maximum oxidization mass gain was not the same.
T 5 was the temperature represented the maximum value of sample mass ratio, which the number of active groups and the adsorption amount of oxygen in the coal reached the maximum. Notably, as presented in Fig. 3b, when moisture content was greater than 1%, T 5 was rst decreased, then increased and last decreased with the increase of pyrite content, and it was smaller when the pyrite content was 2~4%. When pyrite content less than 6%, T 5 was rst rose and then fall and nally rose with the raise of moisture content, and was lower at 10~15%. The reason for this was the reaction between water and pyrite could give off heat, when water content was 10~15%, it could fully react with the 2~4%pyrite. Under the in uence of heat release, the active groups that were originally not involved in coal oxygen reaction at a lower temperature also began to participate in the reaction and reached the ignition temperature in early. T 5 was within the range of 368~388℃, especially the temperature of coal samples W1P0, W5P6 and W20P6 were relatively low (Fig. 2).

Variation of coal mass
In order to further research the impact of water and associated pyrite on coal low temperature oxidation, the mass changes in different stages on the TG curve were observed, date in Table 2 shown the mass change of two stages (Deng et al.2018a). Note: △M 1 : The loss ratio of water evaporation and desorption stage; △M 2 : The loss ratio of oxygen absorption and weight gain stage

Mass change during evaporation and desorption stage
As shown in Table 2, under the different moisture content, as pyrite increased, the mass changes in the water evaporation and desorption stage was rst increased and then decreased and nally increased, and the mass was the largest when the water content was 10~15%; meanwhile, the maximum change was achieved when pyrite was 2~4% (Fig. 4a). After T 1 , larger active group in coal began to react with oxygen and generate CO and CO 2 , at this time, the reaction rate of pyrite with O 2 was also accelerated, which made the gas generation and consumption rate achieve balance. Finally, the mass changes of coal became stability. The variation of coal mass affect by many factors, such as the evaporation of coal water, the production of water-oxygen complex and the generation of pyrite with oxygen.
Observing the mass changes of 25 coal samples in water evaporation and gas desorption stage, discovered the mass changes of coal samples ranged from 0.12 to 0.26%, among which the mass changes of coal sample W15P2 was the largest, followed by W10P4, 0.25%,and then mass variation of W10P2, W10P6 and W15P4 were 0.24%.Therefore, when pyrite content was 2~4% and water was 10~15%, it had the strongest ability to promote the oxidation of coal in the water evaporation and desorption stage.

Mass change during oxygen absorption and weight gain stage
Under different moisture content, with the rise of pyrite content, the sample mass generally showed a trend of increasing rst and then decreasing and last increasing, and the weight gain was the largest when water was 10~15%.At the same time, the weight gain was the largest when the pyrite content was 2~4% (Fig. 4b). Coal oxidation is a complex chemical interplay and temperature is the most crucial factor affecting the reaction process. After T 3 , coal samples entered into a violent oxidation stage, and the oxygen uptake increased continuously. In the process of oxidation, 2~4% pyrite reacted with 10~15% water in the coal, eventually advancing the oxidation process and lowering the temperature. Therefore, the mass variation of 2~4% pyrite and 10~15% moisture coal samples will also be large at this stage. Meanwhile, 15% moisture was the critical water content in this paper. When the water content was lower than 15%, due to the swelling effect of water on coal, the pores inside the coal and water-oxygen complex generated in the initial stage of coal oxidation increased, which accelerated the oxidation reaction of coal samples (Liu et al. 2016).On the contrary, when the water was too large, the formation of liquid lm prevented the coal oxidation, at the end, the variation of coal samples gradually decreased.
The mass change of 25 coal samples in oxygen absorption and weight gain stage was ranged from 2.14~2.49%. The larger mass change of coal samples was: W15P2 > W10P4 > W15P4, W10P2, W10P6.
The results showed that the synergistic effect of pyrite 2~4% and moisture 10~15% was the strongest in promoting the oxidation of coal at the oxygen absorption and weight gain stage.

Kinetic parameters
The coal oxidation reaction was the sum of complex primitive reactions. Activation energy was a very important kinetic parameter in coal oxidation, which can re ect the coal reaction di culty to a certain extent. A great deal of work has been done (Qi et al. 2015;Deng et al. 2018b) in the study of oxidation reaction kinetics to obtain different methods for solving E a , A and deducing the most probable mechanism function.In this paper, Achar differential method and Coastal-Redfern integral method were used for kinetic analysis and calculation of coal oxidation reaction, and Bagchi method was used to infer the reaction mechanism function. The experimental data and the 18 solid reaction mechanism functions in the Table 3 were substituted into Eq. (5)  Due to technical limitations, Table 3 is provided in the Supplementary Files section. Table 4 summarized the differential and integral results of sample W1P0 under 18 mechanism functions at 10 ℃/min. Table 4 Differential and integral kinetic parameters of W1P0 under different mechanism functions sample critical point of the oxidation activity. On the whole, it could be conclude that when the coal sample was synergistically affected by water and pyrite, E a decreased as a whole, and the oxidation activity increased. There were critical content points of water and pyrite, which were 15% and 2%, respectively. In addition, it suggested that the chemical reaction of water and pyrite on coal active groups brings changes, which was manifested as the changes of apparent activation energy in the oxidation process. In this stage, coal molecules needed less energy to complete the coal oxygen adsorption reaction.
Due to technical limitations, Table 5 is provided in the Supplemtneary Files section.
Based on the analysis of sample probable mechanism, the mechanism function of different water and pyrite coal samples did not change during this stage. It implied that although water and associated pyrite can promote the process of coal oxidation, the reaction mechanism was not changed; water and pyrite can affect the degree of coal oxidation, but it does not fundamentally change the oxidation mechanism.

Reaction mechanism
Low temperature oxidation of coal sample is a complex chemical reaction. When coal contains different water and associated pyrite, the coal characteristics will change correspondingly, such as the increase of oxygen-containing functional group and volatile component. All of these will affect coal oxidation, and nally present the changes of characteristic temperatures, coal weight, and so on, in the oxidation process. It can be found from the study that, as shown in Fig. 7, water and pyrite can react chemically and give off heat in coal. Under the comprehensive action of reaction heat and moisture wetting heat, the speed of coal water evaporation was accelerated, made the critical temperature reduce and coal sample began to release heat and gases. At the initial stage of oxidation, the water in the coal sample continued to evaporate, and the oxidation reaction was relatively gentle and the heat generation and gases was less. Meanwhile, under the in uence of pyrite, T 2 had a certain increase, coal samples from T 3 began a strong coal reaction. Under the dual action of water and pyrite reaction product Fe(OH) 3 colloid to ll the pore structure of coal samples, blocked coal oxygen reaction, water and oxygen complex andH 2 SO 4 acid solution to promote coal oxygen reaction, T 5 was in advance. At the same time, when water was 10~15% and pyrite was 2~4%, the oxidation of anthracite required the least activation energy and the strongest oxidation reaction.

Conclusion
Coal spontaneous combustion has always been a di cult problem all over the word. Water and associated pyrite is of signi cance for coal mining, transport and utilizing. The in uence of water and associated on coal oxidation had investigated through TG-DTG experiment and dynamic calculation. It was found that water and associated pyrite could not change the oxidation reaction mechanism of coal, but could accelerate or delay coal oxidation process, and it had a great in uence on the oxygen absorption and weight gain stage (126~388 ℃). Coal oxidation process affected by water and associated pyrite was consistent with the n = 4 Avrami-Erofeev equation. It was also observed that the synergistic effect of water 10~15% and pyrite content 2~4% had the strongest effect on coal oxidation. In summary, the content of water and pyrite will affect the spontaneous ignition time of anthracite coal seam, we should pay close attention to the content of water and associated pyrite during mining.