Study on the Thermal Effects and Characteristics of Free Radical Evolution in Coal Oxidation at Different Moisture Content

Abstract

To investigate the influence of moisture on the exothermic properties of coal oxidation, this study investigated the variation in thermal effect and radical parameters in the oxidation process of coal under different moisture content. This was achieved through thermogravimetric–differential thermal experiments and electron spin resonance experiments. Additionally, the study analyzed the impact of free radicals on the characteristics of the thermal effect of coal at different oxidation stages using the product–moment correlation method. The results indicate that the moisture content of 8% is a critical point for promoting or inhibiting the oxidation reaction of coal. Below this threshold, it promotes the oxidation reaction, while above it, it plays an inhibitory role. The most significant promotional effect was observed at 8% moisture content, which increased both the weight loss and heat release by 8.61% and 1567.04 J/g, respectively, while also significantly enhancing free radical species and complexity. Conversely, when the moisture content of coal reached 20%, there was a notable inhibition effect, with a reduced weight loss and exothermic capacity by 4.94% and 2705.03 J/g, respectively, along with decreased free radical species and complexity. The free radical species and heat effect parameters in coal showed a strong correlation at all stages of low-temperature oxidation, thus indicating that g-factor can characterize the coal oxidation process to some extent.

1. Introduction

Coal, as a significant global energy source, is also the primary energy source in China [1,2,3]. With the continuous mining of coal resources, deep coal resources have become the main focus, accounting for more than 60% of the proven reserves [4]. However, mining coal from deeper levels presents challenges, such as increased water influx, resulting in changes in the moisture content of coal and the formation of coal with varying moisture content [5,6]. It has been observed that moisture can impact the coal spontaneous combustion (CSC), exerting a complex effect on the process [7,8,9,10]. CSC is a complex non-linear gradual physicochemical change process involving physical adsorption, chemical adsorption, and chemical reaction between coal and oxygen. In this series of reactions, moisture plays a key role, and its content variation has different effects on various stages of CSC [11,12].

From a macroscopic perspective, previous studies have extensively utilized thermal analyses, such as thermogravimetric analysis (TG), differential scanning calorimetry (DSC), and adiabatic oxidation tests, among others, to examine changes in macroscopic thermal characteristic parameters during the processes of CSC [13,14,15,16,17,18]. For instance, Li et al. [19] and Xu et al. [15] conducted TG experiments to investigate the spontaneous combustion characteristics of coal samples with varying moisture content. Their findings revealed that moisture not only decreased the activation energy of coal but also heightened its propensity for spontaneous combustion, particularly in low-rank coals. Additionally, Li et al. [17] discovered through TG-DSC experiments that moisture hinders coal spontaneous combustion by physically isolating oxygen and evaporating water during the slow oxidation stage. Conversely, it catalyzes CSC through chemical adsorption during the accelerated oxidation stage. They also noted that an appropriate moisture content enhances the coal’s oxygen adsorption capacity through hydrogen bonding formation, while excessively high or low moisture content is detrimental to this process. Saffari et al. [20] and Beamish et al. [21] further emphasized the dual role of moisture content in either accelerating or inhibiting coal spontaneous combustion, especially in slowing down spontaneous combustion at high moisture levels. The collective findings of these studies indicate that moisture plays a significant role in influencing the CSC. Furthermore, it is evident that the impact of water on these properties is closely intertwined with its effect on the microscopic reactions occurring within coal.

From a microscopic perspective, moisture has a significant impact on the generation and reaction pathways of free radicals and peroxy complexes by altering the microstructure of coal. This includes changes in the type and quantity of functional groups, as well as alterations in pore structure, which ultimately lead to variations in macroscopic characteristic parameters [22,23]. Studies conducted by Qu et al. [24] and Niu et al. [25] have validated that prolonged exposure to moisture results in dilution of the coal skeleton. Additionally, water molecules can create more active sites through adsorption during the initial stages of oxidation, thereby promoting the generation and accumulation of free radicals and facilitating low-temperature oxidation. On the other hand, Song et al. [26] and Huang et al. [27] investigated the microscopic morphology, free radical parameters, and thermal characteristics parameters of water-soaked coals using SEM, ESR, and FTIR techniques. Their findings visually demonstrated how moisture content affects physical oxygen uptake, intersection temperature, critical temperature, highlighting its role in accelerating the oxidative spontaneous combustion process of coal. Therefore, it can be stated that there is a close correlation between the thermal effects of moisture on microstructural changes and free radical reactions.

In light of the strong connection between microstructural changes and macroscopic characteristic parameters, scholars have utilized mathematical methods to establish correlation models between the two [28,29,30]. Zhang et al. [31] utilized the grey correlation method to investigate the changes in key microstructures during different phases of CSC. Jiang et al. [32] examined the correlation between the concentration of free radicals and the macro-exothermic correlation of coal low-temperature oxidation. Li et al. [33] explored the variation in gas concentration, radical active sites, and functional groups during coal oxidation, and established the inter-transformation relationship between macroscopic gas concentration and microscopic radicals and functional groups. Additionally, Gao et al. [34] and Duan et al. [35] further validated the significant correlation between free radicals and functional groups while emphasizing the crucial role of free radicals in CSC interaction mechanisms. Furthermore, Sikandar Azam et al. [36] revealed a positive correlation between free radical parameters and coalification degree parameters through Pearson’s correlation coefficient analysis, offering a valuable tool for understanding coal dust properties and free radical behavior from a new perspective. These studies demonstrate that the method of correlation analysis has proven to be feasible and effective in exploring the interaction between thermal effects and microscopic properties.

In summary, scholars have conducted numerous experimental studies and theoretical analyses on the impact of moisture on the influence of moisture on the characteristic parameter changes in the macroscopic thermal effect of CSC, as well as the microscopic characteristics of CSC during the process. Due to the complexity of coal molecules, the mechanism of how moisture affects the low-temperature oxidation of coal is not clear. Therefore, further study is needed to understand the law governing how moisture affects the oxidative–exothermic characteristics of CSC. In addition, few studies have combined the thermal effect and the law of free radicals change in low-temperature oxidation to study the oxidative exothermic process with different moisture content. This paper aims to analyze this correlation by studying both macroscopic thermal effects and microscopic free radical parameter changes in order to determine how free radicals influence the thermal effect of coal oxidation using a method that correlates the product–moment.

2. Materials and Methods

2.1. Experimental Coal Samples

Bituminous coal from Liangshuijing Coal Mine was selected. When sampling, fresh coal samples should be selected underground, sealed, stored, and transported back to the laboratory. After removing the external oxide layer, a crusher and coal screen were used to screen out coal with the particle size of 0.10–0.15 mm as test samples, and to keep them dry and sealed. According to the national standard of the People’s Republic of China GB/T 212-2008 “Industrial Analysis of Coal”, the coal samples were analyzed using a 5E-MAG6700 proximate analyzer. Additionally, according to GB/T 31391-2015 “Elemental Analysis of Coal”, the coal samples were analyzed by using a Vario EL Ultimate analyzer. The results are shown in

Table 1. Proximate analysis and ultimate analysis.

Proximate Analysis (%)				Ultimate Analysis (%)
Moisture (Mad 1)	Ash (Aad)	Volatile (Vad)	Fixed Carbon (FCad)	Cad	Had	Nad	Oad
4.87	10.82	27.49	56.82	75.16	4.34	0.75	19.58

¹ ad-air dry basis.

.

Coal samples with different moisture content were prepared by using the water immersion method [27]. Firstly, the coal samples were dried in a vacuum drying oven for 24 h to eliminate the original moisture in the coal. Subsequently, they were divided into 6 groups labeled as a, b, c, d, e, and f, and placed into sealed bags (20 g in each group). Group a served as the control group, while the remaining 5 groups of coal samples were intended to have a moisture content of 4%, 8%, 12%, 16%, and 20%, respectively. The moisture content ($M_c$) of the coal samples was calculated using the following Formula (1):

$$M_c={\displaystyle \frac{m_w}{m_w+m_c}}\times 100\%$$

(1)

where $m_w$ represents the moisture mass and $m_c$ represents the mass of the coal sample. Following calculations based on Formula (1), distilled water was uniformly injected into each sealed bag containing the coal samples using a syringe with graduated scale. The bags were then immediately sealed after removing the air and stored at a low temperature away from light for 7 days to ensure thorough impregnation of moisture into the pores and fissures within the coal body. After calculating the average value three times according to Formula (1), it was determined that the actual moisture content of the coal samples was measured at 4.14%, 8.08%, 12.05%, 16.16%, and 20.13%, respectively.

2.2. Experimental Conditions and Methods

2.2.1. Thermogravimetric Analysis and Differential Scanning Calorimetry (TG-DSC) Experiment

To investigate the relationship between heat loss and heat release parameters during coal oxidation and the moisture content of coal samples, TG-DSC experiments were conducted on coal samples with varying moisture content. The TGA simultaneous thermal analyzer was utilized to measure the heat loss of the coal samples, while the DSC model DSC200F3 was employed to carry out DSC experiments on the coal samples. Experimental conditions were as follows: heating rate: 5 °C/min, temperature range: 30~700 °C, sample size: 10 mg, air flow rate: 100 mL/min, and atmosphere: air.

2.2.2. Electron Spin Resonance (ESR) Experiment

In order to investigate the relationship between moisture content and free radical parameters during the oxidation process, ESR signals of coal samples with different moisture content were tested using a Bruker EMX-8 spectrometer from Germany. The ESR spectra were obtained at different temperatures by placing 10 mg of the sample in a quartz tube with an outer diameter of 2 mm. The quartz thin-walled tube containing the coal sample was then placed in a resonance chamber for experimental testing. Experimental conditions were as follows: heating rate: 5 °C/min, temperature range: 30~300 °C, sample size: 10 mg, air flow rate: 100 mL/min, atmosphere: air, time constant: 5.12 ms, scanning time: 20 s, working frequency: 9.87 GHz(X-band), and modulation frequency: 100 kHz.

We used the indirect calibration method to calculate the free radical concentration of coal samples, and the ESR spectrum of the coal sample under the same conditions as the standard sample DPPH (1,1-diphenyl-2-picrylhydrazyl, g-factor value is 2.0036) with a known concentration was measured. The free radical concentration of the coal sample can be calculated by using the following Equation (2):

$${\displaystyle \frac{N_g}{N_s}}={\displaystyle \frac{A_g}{A_s}}$$

(2)

where $N_g$ represents the free radical concentration of the test coal sample, $N_s$ represents the free radical concentration of the standard sample, $A_g$ represents the integrated area of the test coal sample, and $A_s$ represents the integrated area of the standard sample.

2.2.3. Product–Moment Correlation Coefficient

In order to investigate the correlation between the macroscopic heat effect and microscopic free radical parameters of coals with different moisture content, we utilized the product–moment correlation coefficient to calculate the correlation between two variables and determine the degree of correlation. This method offers wide applicability and clear statistical significance, allowing for an intuitive understanding and analysis of the strength of the correlation between two variables as well as clarification of their positivity or negativity. Therefore, we employed the product–moment correlation method to calculate the correlation between weight loss, heat release, and free radical parameters, respectively.

Given the data, X: {x₁, x₂, …, x_n} and Y: {y₁, y₂, …, y_n}, where x and y are calculated samples, the calculation formula of correlation coefficient r is shown in the following Formula (3):

$$r={\displaystyle \frac{n{\sum }_{i=1}^n\left(X_i-E\left(X\right)\right)\left(Y_i-\mathrm{E}\left(\mathrm{Y}\right)\right)}{\sqrt{{\sum }_{i=1}^n{\left(X_i-E\left(X\right)\right)}^2{\sum }_{i=1}^n{\left(Y_i-\mathrm{E}\left(\mathrm{Y}\right)\right)}^2}}}$$

(3)

where E(X) and E(Y) are the average values of the given data X and Y, respectively, and the calculation formula is shown in the following Formula (4):

$$E\left(X\right)={\displaystyle \frac{{\sum }_{i=1}^n{X}_i}{n}}, E\left(Y\right)={\displaystyle \frac{{\sum }_{i=1}^n{Y}_i}{n}}$$

(4)

To ensure the comprehensiveness of the analysis, the correlation coefficients between weight loss, heat release and free radical concentration, g-factor, and line width are calculated, respectively. The correlation coefficients $r_{pq}$ between each pair of data will be obtained by calculating p, which is the number of X’s involved in the calculation, where q is the number of Y’s involved in the calculation, so p = 5 and q = 5. The calculated $r_{pq}$ is listed in the correlation coefficient matrix R shown in the following Formula (5):

$$R=\left[\begin{array}{ccc}{r}_{11}& \cdots & r_{1q}\\ ⋮& \ddots & ⋮\\ r_{p1}& \cdots & r_{pq}\end{array}\right]$$

(5)

The calculation results of $r_{pq}$ fall within the value range of [−1, 1], where 1 indicates complete positive correlation, −1 indicates complete negative correlation, and 0 indicates no linear correlation. Based on the value range of the calculation results of $r_{pq}$, the correlation between the thermal effect and the free radical parameters in the oxidation process of coals with different moisture content can be analyzed.

3. Results and Discussion

3.1. Analysis of the Oxidation Characteristics Parameters of Coal at Different Moisture Content

3.1.1. Analysis of Thermal Weight Loss Parameters

Figure 1 illustrates the TG/DTG curves of coal with different moisture content. It is evident from Figure 1 that the overall trend in the TG/DTG curves of coal samples with different moisture content remains consistent. Based on the changes in the TG/DTG curves and the spontaneous combustion reaction process of coal, several critical temperature points are defined: T₁ represents the temperature at which moisture in coal begins to evaporate and reaches the maximum weight loss rate, T₂ signifies the temperature at which the thermogravimetric curve reaches its lowest point due to the complete evaporation of water in coal, and T₃ indicates the temperature at which the mass of coal samples reaches a maximum due to oxygen absorption. Furthermore, T₄ denotes the ignition point temperature where a rapid decrease in the TG/DTG curve occurs as coal sample starts to burn, T₅ represents the temperature at which the maximum weight loss rate is achieved by the coal sample, and, finally, T₆ signifies the burnout temperature where the mass of the coal sample remains relatively unchanged after burning.

Table 2. Characteristic temperature point of the thermal weightlessness of coal samples.

Moisture Content/%	T1/°C	T2/°C	T3/°C	T4/°C	T5/°C	T6/°C
0	63.51	178.08	284.28	407.11	467.44	551.875
4	61.13	173.03	280.21	403.37	466.89	550.87
8	60.03	172.18	278.38	400.81	467.60	550.14
12	57.38	179.56	278.29	401.73	468.86	551.28
16	59.07	177.73	277.91	402.54	466.77	552.44
20	59.89	180.41	273.26	406.84	468.28	551.99

presents characteristic temperature points of thermal weight loss for different moisture content of coal samples. A comprehensive analysis reveals that moisture content has varying influences on each characteristic temperature. With increasing moisture content, it is observed that T₁ initially decreases before increasing again. The value of T1 for coals with different moisture content is lower than that for raw coal, showing a maximum decrease of 9.65%. This can be attributed to wetting heat promotion dominating before reaching T₁ when moisture content is low, while the wetting heat promotion is dominant. With the increase in moisture content, T₂ initially decreases and then increases. The T₂ of low moisture content coal is lower than that of raw coal, while the T₂ of high moisture content coal is higher than that of raw coal. This indicates that a small amount of water increases the water-containing complex in coal, promotes the generation of free radicals in the early stage, and accelerates the coal–oxygen complex reaction. Excessive water will block the pore structure of coal, hinder oxygen absorption, and delay T₂ due to the large heat absorption of water evaporation. As for T₃ and T₄, they show an upward trend with increasing moisture content, with a maximum increase of 4.24 °C and 7.6 °C, respectively. This is because after water evaporation, swelling occurs, which produces a large number of pore spaces, providing more reaction sites. More oxygen is then needed to react with these sites making it difficult for the active structure in coal to undergo the oxidative decomposition reaction in time, thus moving the characteristic temperature to the high-temperature region. The changing trend of T₅ and T₆ with increasing moisture content is not obvious as their overall change range is very small, indicating that moisture content has a significant influence on coal before the ignition temperature during the low-temperature oxidation process.

According to the characteristic temperature point, the thermal weight loss process can be divided into four stages: from the initial temperature to T₂ is the stage of water loss weight loss, from T₂ to T₃ is the stage of oxygen absorption weight gain, from T₃ to T₆ is the stage of combustion, and after T₆ is the stage of complete combustion. From Figure 1, it can be observed that coal with different moisture content exhibits varying degrees of oxidation weight loss. The weight loss of coal samples with a moisture content of 8% or less significantly increases, with a total weight loss of 8.61% compared to the original coal at an 8% moisture content. Conversely, a moisture content above 8% results in a decrease in coal weight loss, with a total weight loss of 4.94% compared to the original coal at a 20% moisture content. This is attributed to the evaporation of water and the desorption of gases leading to weight loss, as well as the occurrence of coal–oxygen complex reactions due to the combination of some active structures with oxygen. The impact of moisture on weight loss is influenced by various factors, such as water evaporation and swelling effects. Additionally, weight loss is also affected by the rate of coal–oxygen composite reactions caused by moisture, leading to secondary effects, such as adsorption and desorption oxygen capacity, gas product desorption, etc. Overall, this results in different weight loss impacts, with 8% moisture content as the dividing point.

3.1.2. Analysis of Thermal Release Parameters

Figure 2 illustrates the DSC experimental curves of coal samples with different moisture content, showing the variation in heat flow (HF) with temperature. The DSC curves can be differentiated to obtain the DDSC curve, i.e., the derivative of HF (dHF), in order to analyze the rate of change in HF.

According to the characteristics of the DSC and DDSC curves and the process of coal oxidation, three characteristic temperature points were selected, denoted as T_D1, T_D2, and T_D3, respectively. Three reaction stages were divided based on the peaks of curves and the characteristic temperature points: before T_D1 is the moisture evaporation stage (Stage 1), between T_D1 and T_D2 is the oxidative decomposition stage (Stage 2), between T_D2 and T_D3 is the combustion stage (Stage 3), and after T_D3 is the burnout stage (Stage 4). As shown in Figure 2, with the increase in moisture content, the heat flow of coal samples exhibits a trend of first increasing and then decreasing. The peak point at a moisture content of 8% shows an increase in coal sample HF of 6.71%, while at a moisture content of 20%, it decreases by 16.91%. Different moisture content coals exhibit a short endothermic stage in the early reaction period due to slow coal–oxygen complex reactions during this phase. Heat is absorbed from water evaporation and gas desorption at a rate greater than that released from chemical reactions, resulting in overall endothermic behavior. As the coal temperature gradually rises, chemical oxygen absorption becomes dominant as its rate accelerates. The release of heat from oxygen absorption exceeds that from water evaporation, leading to overall exothermic behavior. With accelerated chemical oxygen absorption and reactions within coal, side chains begin to break along with bridge bonds; active groups consume oxygen to produce unstable intermediate products until reaching ignition temperature when combustion begins vigorously with an exothermic rate far exceeding endothermic rates, resulting in rapid increases in HF rates until they reach their maximum levels. The overall changes in DSC curves for coals with different moisture content show three distinct peaks as temperatures rise, and these peaks demonstrate significant differences among coal samples with different moisture content, indicating that moisture content has a certain influence on coal’s thermal effects.

The DSC curves of coal with different moisture content were integrated, respectively, for the first three stages, obtaining the total heat release and stage heat release, as shown in Figure 3. The heat release in Stage 1 is between −70.45 and −126.26 J/g; this is because the evaporation of moisture and gas desorption in coal during Stage 1 both absorb heat, resulting in a heat absorption rate greater than the heat release rate of coal oxidation reactions. With an increase in moisture content, the heat absorption capacity of Stage 1 increases continuously. The percentage increase in heat absorption at 4%, 8%, 12%, 16%, and 20% moisture content is 6.3%, 11.06%,33.07%,26.83%, and 41%, respectively; this is because more moisture increases the heat absorption capacity. Stage 2 and Stage 3 are both exothermic stages, with Stage 2 releasing between 2146.92 and 2861.13 J/g, accounting for 21.10%~23.33% of the total heat release. The heat release increases first and then decreases with the increase in moisture content. The heat release of Stage 3 is 3.60 to 3.77 times that of Stage 2, and it also increases first and then decreases with the increase in moisture content. This indicates that lower moisture content can increase the heat release of coal, while higher moisture content will decrease the heat release of coal.

3.2. Analysis of Free Radical Parameters of Coal at Different Moisture Content

3.2.1. Analysis of ESR Spectrogram

Figure 4 shows the ESR spectrogram of coal with different moisture content during the heating process. It can be observed from Figure 4 that coal with different moisture content exhibits stable and smooth ESR signals, and these signals can be consistently detected throughout the heating process, indicating the presence of relatively stable free radicals in coal. The ESR spectra of coal samples generally consist of a mixture of Gaussian and Lorentzian line shapes. In Figure 4, as the temperature increases, the absorption peak intensity of the ESR spectra gradually increases while the line width decreases, making the overall ESR spectra closer to narrow Lorentzian lines. This suggests that free radicals in coal are sensitive to temperature changes.

Moreover, with an increase in moisture content, there is a trend for both an initial increase followed by a decrease in absorption peak intensity of the ESR spectral lines, as well as a decrease followed by an increase in line width. Overall, this indicates that moisture has a certain influence on free radicals in coal.

3.2.2. Analysis of Free Radical Concentration

The free radical concentration (N_g) represents the sum of the concentrations of all the free radicals in the sample, with a higher N_g indicating a greater presence of free radicals. Figure 5 shows the variation in N_g during the oxidation process for coal samples with different moisture content. The order of initial N_g in coal with different moisture content is 8% > 4% > 0% > 12% > 16% > 20%. The coal sample with an 8% moisture content exhibited the highest increase in N_g, which was elevated by 2.806 × 10¹⁵ spin/mm³ compared to the original coal.

As temperature increases, N_g initially decreases, then significantly rises before decreasing again. This is because changes in free radical concentration are primarily related to the rates of generation and consumption of free radicals. When the generation rate exceeds the consumption rate, N_g increases; conversely, it decreases. Before reaching critical temperature, there is weak reactivity between coal and oxygen. Evaporation of moisture from coal leads to an increase in the consumption rate of free radicals, resulting in a decrease in N_g. After exceeding the critical temperature, reaction rates accelerate and functional groups within coal continue to activate, leading to an increase in the generation rate of free radicals and, subsequently, an increase in N_g. When temperature exceeds the activation temperature, further acceleration occurs in coal–oxygen reactions, causing an intensification of the chain reactions of free radicals and leading to an increase followed by a decrease in their consumption rate. Furthermore, there are differences in variations within different moisture content as to how much N_g changes over time during the oxidation process, as shown by Figure 5. The overall trend indicates that as the moisture content rises, it initially causes larger fluctuations but eventually decreases, suggesting that moisture content has an influence on the changes seen within N_g during the coal oxidation process. The swelling effect caused by water can create new pores within coal, leading to fractures of bonds within its structure, thus generating more free radicals. Simultaneously due to increased hydrogen ions and hydroxide ions brought about by moisture content, this accelerates complex reactions between oxygen and coal, further increasing the differences found between the generation and consumption rates within coal of different moisture content. When the moisture content is low, the swelling effect promotes an increase in N_g without being rapidly consumed. However, when the moisture content is high, it not only promotes an increase in N_g but also accelerates the consumption of free radicals, resulting in an overall lower initial N_g.

3.2.3. Analysis of the G-Factor

The g-factor reflects the strength of the interaction between free radical electron spin and an applied magnetic field. The value of the g-factor depends on the electron structure of the free radical and its chemical environment, representing the types of free radical present. Changes in the g-factor indicate variations in the types of free radicals in a sample. The value of the g-factor can be obtained by measuring the position of resonance absorption peaks. The variation in g-factor values for different moisture content coals during oxidation is shown in Figure 6. The initial value of g-factor t is between 2.00142 and 2.00166, which is less than 2.0023, and it is important to note that the g-factor of free electrons is known to be 2.0023, which indicates that the free radical centered on carbon atoms is the main one in coal samples [37]. This is because the presence of water can cause the side chains in the molecular structure of coal to break, leading to the generation of more free radicals with carbon atoms at their center. According to Figure 5, it can be seen that the g-factor in coal with different moisture content shows a fluctuating downward trend as the temperature increases. This indicates that the types of free radicals in coal constantly change to aromatic radicals with the increase in temperature. The g-factor of raw coal is higher than that of other water-bearing coal samples before 180 °C, which is because the adsorption of water increases the aliphatic structure, as well as the reaction between carbon free radicals and oxygen, promoting the free radical chain reactions [38]. This suggests that moisture can promote the conversion of hydroxyl, carboxyl, carbonyl, and other heteroatomic functional groups in coal to aromatic hydrocarbon free radicals through free radical reaction at the early stage of reaction, thus reducing the complexity of free radical species in coal.

At the initial stage, with the increase in moisture content at the same temperature, the variation in the g-factor is relatively scattered. After 180 °C, with the increase in moisture content, the overall trend of the g-factor shows an initial increase followed by a decrease. This is because before 180 °C, the moisture in coal is constantly evaporating, due to factors, such as water evaporation, the steam pressure effect, and water wetting heat, and the generation and consumption rate of free radicals in coal with different moisture content are affected by complex factors. After 180 °C, the stage of water evaporation basically ends. As mentioned earlier, it can be seen that in coal samples with different moisture content, carbon-centered free radicals are dominant. Therefore, moisture does not directly influence the types of free radicals. At the same time, due to continuous temperature rise, heating causes coal molecules to continuously break and decompose side chains and functional groups into active sites for free radical reactions which release heat energy. The coal–oxygen reaction causes the continuous “carbonization” of coal molecules; “carbonization” means that carbon-based free radicals dominate in coal, leading to a decreasing trend in g-factor as temperature rises.

3.2.4. Analysis of Line Width

The width of the absorption peak (ΔH) can be directly obtained by measuring the peak width of the ESR resonance absorption curve, which describes the interaction between particles and reflects the distribution and strength of spin states of free radical electrons. ΔH is closely related to two types of interactions: spin–lattice relaxation and spin–spin relaxation. A narrower ΔH indicates a longer spin–spin relaxation time for electrons within the substance, as well as more intense electron exchange interactions between free radicals. The variation in ΔH during the oxidation process of coal with different moisture content is shown in Figure 7. The ΔH of coal samples with different moisture content shows a decreasing trend during the heating process; this is because with the increase in temperature, the degree of the condensation reaction of the radical fragments produced at the beginning of the coal oxidation process increases, the degree of condensation of the aryl ring increases, macromolecular radicals are generated, and the spin–lattice hesitation time of the electrons becomes longer. Additionally, as the coal oxidation reaction proceeds, coal is gradually dehydrogenated, the interaction between unpaired electrons in coal is enhanced, the relaxation time is prolonged, and the ΔH becomes narrower [39]. As shown in Figure 7, moisture content has an impact on ΔH; with the increase in the moisture content, ΔH as a whole shows a trend of increasing at first and then decreasing, indicating that water generates more active free radicals to participate in the reaction through the initial heat accumulation of wetting heat and vapor pressure effect. When the moisture content is 8%, ΔH reaches its lowest point relative to raw coal, decreasing by 14.31%. As analyzed earlier, this is because at 8% moisture content, N_g is the highest; therefore, increased N_g and enhanced spin–spin interaction resulting in a narrower ΔH value, and this moisture content can promote the free radical chain reaction in coal.

3.3. Analysis of the Correlation between Free Radicals and Thermal Effects

From the previous analysis, it is evident that moisture has a significant impact on the changes in free radicals and thermal effect parameters in coal during low-temperature oxidation. This influence varies at different stages of the process. Additionally, there appears to be a correlation between the change in free radicals and the thermal effect parameters of coal samples. To investigate this correlation further, we calculated the correlation coefficients between free radical Ng, g-factor, ΔH, and the weight loss (ML) and heat release (HR) of coal samples with different moisture content using the product–moment correlation method. The resulting correlation coefficients are listed in a matrix to clarify the relationship between each parameter in Figure 8.

The correlation coefficient values between ML and the Ng, g-factor, and ΔH of coal samples with different moisture content within stage 1 were from 0.698 to 0.918, from 0.948 to 0.995, and from 0.92 to 0.983, respectively. This indicates that the change in ML was positively correlated with the decrease in Ng, g-factor, and ΔH within stage 1. This correlation can be attributed to the fact that stage 1 primarily involves moisture evaporation, leading to a continuous decrease in coal weight as well as changes in the pore structure and properties due to the evaporation of moisture. These changes make it easier for free radicals in coal to approach each other, increasing their interactions and promoting free radical reactions in coal, resulting in a decrease in the Ng, g-factor, and ΔH. Within stage 1, HR is negatively correlated with g-factor and ΔH with correlation coefficients from −0.838 to −0.951 and from −0.878 to −0.952, respectively, indicating a positive correlation with heat uptake within this stage. Heat absorption is required for moisture evaporation within stage 1, which promotes free radical reactions in coal, leading to reductions in the g-factor and ΔH. There is no obvious correlation between HR and Ng within stage 1, suggesting no direct relationship between the concentration of free radicals in coal and its exothermic capacity during low-temperature oxidation process. Overall, during the low-temperature oxidation process of coal, due to the continuous occurrence of the free radical chain reaction, the concentration of free radicals increases due to the breakage of the side chain of coal molecules after being heated and the conversion of the functional groups, and at the same time the free radical chain reaction decreases the concentration of free radicals continuously. The free radical consumption and generation lead to the continuous change in free radical concentration, which advances the coal oxidation reaction process.

In stage 2, the correlation between Ng and the ML and HR of coal samples with different water content did not show significant correlation. Similarly, the correlation between free radical concentration and thermal effect parameters was poor, consistent with the reasons described in stage 1. The correlation coefficients of ML with g-factor and ΔH ranged from −0.455 to −0.948 and from −0.705 to −0.946, respectively, both showing a negative correlation due to oxygen absorption and oxidative decomposition occurring mainly in stage 2. The weak covalent bonds of the coal structure are broken to form free radicals and active sites after being heated, the more free radicals involved in the reaction, the stronger their interactions become, resulting in lower g-factor and ΔH values. The adsorption of oxygen by coal molecules through these free radicals facilitates oxygen adsorption, increasing the ML of coal samples. Similarly, HR showed a negative correlation with g-factor and ΔH ranging from −0.706 to −0.987 and from −0.852 to −0.935, respectively, because HR shows an increasing trend with temperature due to a free radical chain reaction releasing heat that promotes the oxidative decomposition of coal producing more reactive structures and free radicals. The analysis indicates that both g-factor and ΔH can characterize the weight loss reaction process as well as the heat release reaction process for coal samples in stage 1 and stage 2. Overall, the g-factor consistently maintains a strong correlation with ML and HR in both stages, suggesting its ability to characterize the oxidation reaction process of coal during the moisture evaporation stage as well as during the oxidative exothermic stage.

4. Conclusions

[1]

A critical moisture content has been identified in the process of coal oxidation, which can either enhance or suppress the exothermic reaction. A moisture content of 8% is the threshold. Below this threshold, the exothermic reaction of coal oxidation is promoted and, conversely, the reaction is inhibited. At 8% moisture content, the exothermic reaction is notable, causing an increase in weight loss and heat release by 8.61% and 14.65%, respectively. At 20% moisture content, the suppression is significant, leading to a reduction in weight loss and heat release by 4.94% and 10.64%, respectively.

[2]

Moisture content plays a dual role in the free radical chain reaction of coal oxidation. When the moisture content of coal is less than 8%, the chain reaction of free radicals in coal is enhanced, and vice versa. At a moisture content of 8%, there is a significant enhancement in the coal, leading to an increase in both the concentration and species variety of free radicals, as well as an increase in complexity. The concentration of free radicals increases by 2.806 × 10¹⁵ spin/mm³ higher compared to that in raw coal. When the moisture content reaches 20%, it has the strongest weakening effect on the free radical chain reaction, resulting in a significant decrease in both the concentration and variety of free radicals, as well as complexity.

[3]

The change in free radicals is the essential reason for the macroscopic thermal effect characteristics. The variation in free radical species during the coal oxidation process shows a strong correlation with the characteristic parameters of heat effects both in the stage of moisture evaporation and in the stage of exothermic oxidation. Therefore, to some extent, the species of free radicals can characterize the progress of the coal oxidation reaction. These findings have important implications for developing new types of spontaneous combustion retardants for water-soaked coal as well as for researching prevention technologies for coal spontaneous combustion.