Study on the Reaction Path of -CH₃ and -CHO Functional Groups during Coal Spontaneous Combustion: Quantum Chemistry and Experimental Research

Abstract

Coal spontaneous combustion (CSC) is a disaster that seriously threatens safe production in coal mines. Revealing the mechanism of CSC can provide a theoretical basis for its prevention and control. Compared with experimental research is limited by the complexity of coal molecular structure, the quantum chemical calculation method can simplify the complex molecular structure and realize the exploration of the mechanism of CSC from the micro level. In this study, toluene and phenylacetaldehyde were used as model compounds, and the quantum chemical calculation method was adopted. The reaction processes of the methyl and aldehyde groups with oxygen were investigated with the aid of the Gaussian 09 software, using the B3LYP functional and the 6-311 + G(d,p) basis set and including the D3 dispersion correction. On this basis, the generation mechanisms of CO and CO₂, two important indicator gases in the process of CSC, were explored. The calculation results show that the Gibbs free energy changes and enthalpy changes in the two reaction systems are both of negative values. Accordingly, it is judged that the reactions belong to spontaneous exothermic reactions. In the reaction processes, the activation energy of CO is less than that of CO₂, indicating that CO is formed more easily in the above-two reaction processes. In addition, the variations in concentrations of important oxidation products (CO and CO₂) and main active functional groups (such as methyl, carboxyl and carbonyl) with temperature were revealed through a low-temperature oxidation experiment. The experimental results verify the accuracy of the above quantum chemical reaction path. Moreover, it is also found that the generation mechanisms of CO and CO₂ in coal samples with different metamorphic degrees are different. To be specific, for low-rank coal (HYH), CO and CO₂ mainly come from the oxidation of alkyl side chains; for high-rank coal (CQ), CO is produced by the oxidation of alkyl side chains, and CO₂ is attributed to the inherent oxygen-containing structure.

1. Introduction

Coal spontaneous combustion (CSC) affects the safe production and sustainable development of coal mines. CSC, a rather complex physical and chemical process [1,2,3], has not been clearly described at the chemical level; therefore, revealing the mechanism of CSC can provide a theoretical basis for its prevention and control. Coal has a complex molecular structure. Its structural unit is composed of regular and chemically stable aromatic nuclei and irregular and chemically active oxygen-containing functional groups and alkyl side chains. The content of oxygen-containing functional groups and alkyl side chains decreases with the increase in coalification degree. Existing studies show that oxygen-containing functional groups and alkyl side chains tend to participate in the low-temperature oxidation process and play a crucial role in CSC. Wang [4] found that with the progress of oxidation, the content of methyl and methylene in coal plunges, while the content of oxygen-containing functional groups gradually increases. Therefore, it is considered that the initial stage of CSC mainly witnesses the reaction of active groups such as methyl and methylene with oxygen, followed by the decomposition and con version of unstable intermediates into stable solid products such as carboxyl and carbonyl. Mathews [5] found that the initial oxidation of coal takes place at the benzyl position where peroxy radicals are formed and then transform into quinones and carboxyls. Zhou [6] disclosed that in the process of low-temperature oxidation, the alkyl structure decreases while the oxygen-containing functional groups increase with the rise in temperature; based on the above finding, Zhang [7] considered that highly reactive methyl, methylene and methylene react with oxygen in the air to form oxygen-containing functional groups, and the oxidation of these active groups serves as the initial heat source of CSC.

In short, oxygen-containing functional groups and alkyl side chains in coal are essential active groups promoting the process of CSC. However, experimental research fails to reveal the mechanism of reaction between active groups and oxygen due to the complexity of coal molecular structure. In addition, the mechanisms of generation of indicator gases CO and CO₂ during CSC are controversial. Considering the above reasons, in recent years, people have started to adopt the quantum chemical calculation method to study the CSC-related free-radical mechanism. This method, which features precise and fast calculation, can simplify the complex molecular structure of coal and support the exploration of the coal-oxygen free-radical reaction process from the micro level. Based on the calculation of the density functional theory (DFT) and the frontier orbital theory, Xin et al. [8] analyzed the electron transfer of active center in coal, established 13 free-radical reactions and their reaction sequence relations in the process of CSC, and revealed the oxidation kinetics of coal. They reached the conclusion that these 13 free-radical reactions are a chain cycle with low activation energy initiated by oxygen, in which the primary structure of coal is constantly transformed into carbon free radicals and releases gaseous products. By using the frontier orbital method in quantum chemistry, Qi et al. and Zhu et al. [9,10] studied the active sites in the aldehyde group model compounds, speculated the possible attack position of oxygen free radicals, and calculated the activation energy in the reaction process. Nevertheless, the existing quantum chemical theory concerning the mechanism of CSC has not considered the weak interaction between coal molecules. Therefore, the calculation results in this study are corrected through dispersion correction [11,12,13,14]. In fact, since coals with different structures have different types and contents of active groups, they correspond to varying generation mechanisms of CO and CO₂ during CSC. At present, the mechanism is mostly explored through relevant experiments, while it is rarely investigated from a micro level using the quantum chemistry theory, which requires further research. Moreover, different indicator gases are used in different countries and applied to different-rank coals. In view of the above reasons, in this study, ph-CH₃ and ph-CH₂CHO were used as the model compounds, and their complete radical reaction paths with oxygen (O₂) were theoretically calculated using the DFT with the aid of the Gaussian 09 software [15,16,17]. In addition, the thermal energy, enthalpy change and Gibbs free energy of reactants, intermediates, transition states and products in the reaction process were obtained through the introduction of D3 correction [18,19]. Finally, the energy change in the reaction and the feasibility of the reaction path were determined.

2. Methods and Experiments

2.1. Establishment of Molecular Model and Calculate Content

As can be known from the above introduction, oxygen-containing functional groups and alkyl side chains are the primary active groups that have a considerable impact on CSC. However, according to previous studies [20,21,22,23], the reactivity of active groups in coal is less affected by the number of benzene rings. Therefore, the reaction process between these active groups and oxygen can be studied by reasonably constructing active group model compounds (toluene and phenylacetaldehyde) and conducting calculation in the light of the quantum chemical theory. In this study, with the aid of Gaussian 09 software (Gaussian 09, Revision A.02, Gaussian Inc., Wallingford, CT, USA, 2016) [24,25,26], the reaction processes of methyl, aldehyde and oxygen radicals in the model compounds were simulated by selecting the B3LYP functional and 6-311 + G(d,p) basis set [27,28]. Moreover, the structure of reactants and products was optimized through Opt freq, and the energy of the reaction system was corrected through D3. Furthermore, according to the frontier orbital theory, the electron gain/loss and transfer ability of reactant molecules were analyzed to identify the active sites of reactants. Meanwhile, the transition states involved in the reaction were found by the method of opt = TS, which was then verified by the intrinsic reaction coordinates (IRC). Finally, the reaction paths of methyl, aldehyde and oxygen radicals were obtained according to the final theory of reactants, products and transition states.

2.2. Experimental

2.2.1. Preparation of Coal Samples

Two different-rank coals, i.e., Chaoqiang anthracite (CQ) and Hunyuan lignite (HYH), were selected for as the experimental coal samples.

Table 1. Results of proximate and ultimate analyses on coal samples.

Coal Sample	Proximate (w/%)				Ultimate (wdaf/%)
	Mad	Aad	Vdaf	FCd	Odaf	Cdaf	Hdaf	Ndaf
CQ	2.57	53.52	22.76	35.90	5.87	79.38	2.91	1.32
HYH	6.59	15.58	37.14	53.07	15.66	74.49	4.36	1.31

shows the results of proximate analysis and ultimate analysis of the above two kinds of coal samples. After being transported to the lab, the coal samples were taken out of the sealed bag, dried in vacuum at 50 °C for 12 h, and then cooled to room temperature. Subsequently, 30 g of each kind of dried coal sample was ground to 0.18–0.25 mm and put into a sealed bag for storage.

2.2.2. Fourier-Transform Infrared Spectroscopy (FTIR) Experiment

The temperatures of the two kinds of coal samples were raised from 30 °C to 180 °C at a heating rate of 1 °C/min. When the temperature of the tubular furnace reached 60 °C, 90 °C, 120 °C, 150 °C and 180 °C [29], the quartz tube was quickly drawn out for sampling. The experiment results were analyzed by a Nicolet-iS10 Fourier-transform infrared spectrometer. Before the test, 2 mg of raw coal and 200 mg of dry KBr were fully ground and mixed, and then the mixtures were pressed under 10 MPa for 2 min with a tablet press pressure to construct uniform thin sheets for FTIR measurement. The wave number range was 4000–400 cm⁻¹. The experimental process is given in Figure 1.

2.2.3. Low-Temperature Oxidation Experiment

In the experiment, the concentration of CO, an important gas product during low-temperature oxidation of raw coal and inhibited coal, was detected through an FUL9790 gas chromatograph, and the external standard method was used for quantitative analysis. From 30 °C, the concentrations of CO₂ and CO released from the sample during low-temperature oxidation were monitored at an interval of 10 °C in real time. Finally, the variations law of gas-product concentrations with temperature were obtained.

3. Free-Radical Reaction Processes of ph-CH₃ and ph-CH₂CHO with O₂

3.1. Reaction of ph-CH₃ with O₂

In the calculation, the reaction system of ph-CH₃ with O₂ is optimized to obtain stable structures of the reactants, transition states, intermediates and final products.

The reaction path of ph-CH₃ with O₂ is as shown in Figure 2 [30,31,32] (R is the reactant; TS1-TS5 is the transition state; IM1-IM5 is the intermediate; P and P1 are the products).

3.2. Reaction of ph-CH₂CHO with O₂

Similarly, the reaction path of ph-CH₂CHO with O₂ is obtained based on the results of quantum chemistry calculation by using the same method as above [33,34,35].

The structural models of reactants (r1), transition states (ts2 and ts4), intermediates (im1-im5) and products (p and p1) in the reaction process after structural optimization are exhibited in Figure 3.

4. Results and Discussion

4.1. Frontier Orbital Theory

According to the frontier orbital theory, the electrons on HOMO are the most active and changeable among molecules because they have the highest energy and undergo the least binding, while LUMO has the lowest energy in all unoccupied orbitals and is the most likely to accept electrons. Thus, these two orbitals determine the gain, loss and transfer ability of electrons. When O₂ reacts with coal molecules, it can be regarded as a nucleophile that attacks the highly reactive groups in coal. The electron density distributions on LUMO and HOMO of toluene and phenylacetaldehyde are displayed in Figure 4.

4.2. Reaction of ph-CH₃ with O₂

4.2.1. Analysis of Reaction Mechanism and Energy

As can be seen from Figure 2, in the reaction process, the O atom in the oxygen molecule first attacks the H atom in the methyl group to generate peroxide radicals. The -O-O bond in the peroxide radicals breaks and combines with the H atom to generate the hydroxyl group, which then connects with the C atom. The O atom attracts the H atom in the hydroxyl group, resulting in the breakage of -O-H and the generation of hydroxyl radicals, which are attracted to each other by the C atom and connected with C. Oxygen electron donation in the -C-O bond leads to the breakage of -C-H to form the transition state structure (TS4) of alternating C-O single and double bonds. Next, the carbonyl group and water are rapidly generated, which fall off to produce CO. As listed in

Table 2. Thermodynamic parameters of ph-CH₃ with O₂ reaction.

Stagnation Point	Corrected E (kJ/mol)	Enthalpy H (kJ/mol)	Gibbs Free Energy G (kJ/mol)	Enthalpy Change (kJ/mol)	Gibbs Free Energy Change G (kJ/mol)	Activation Energy Ea (kJ/mol)
R	−1,107,488.4	−1,107,451.38	−1,107,596.72	0	0
IM1	−1,107,934.7	−1,107,654.65	−1,107,770.27	−203.27	−173.55	105.02
TS1	−1,107,829.7	−1,107,452.17	−1,107,596.87	−0.79	−0.15
IM2	−1,107,514.7	−1,107,481.18	−1,107,603.45	−29.8	−6.73	131.27
TS2	−1,107,383.4	−1,107,906.13	−1,108,017.46	−454.75	−420.74
IM3	−1,107,934.7	−1,107,906.13	−1,108,017.45	−454.75	−420.73	157.53
TS3	−1,107,777.2	−1,107,762.36	−1,107,871.98	−310.98	−275.26
IM4	−1,108,197.3	−1,107,732.20	−1,107,844.49	−280.82	−247.77	105.02
TS4	−1,108,092.3	−1,107,940.63	−1,108,069.98	−489.25	−473.26
P + CO + H2O	−1,108,118.5	−1,107,910.61	−1,108,075.84	−459.23	−479.12
IM5	−1,302,353	−1,301,871.87	−1,302,003.05	0	0	236.39
TS5	−1,302,116.7	−1,301,871.87	−1,302,003.07	−0.021	−0.015
P1 + CO2	−1,302,589.3	−1,302,461.24	−1,302,594.85	−589.34	−591.79

E is the energy (E = electronic energy (EE) + zero-point energy); H is the enthalpy; G is the Gibbs free energy; ΔH is the enthalpy change, which reflects the absorbed or released heat in the reaction; ΔG is the Gibbs free energy change, which reflects whether the reaction is spontaneous (1 hartree = 2625.5 kJ/mol) [36].

, in this process, the energy of the system decreases by 626 kJ/mol; the is −459.23 kJ/mol; the G is −479.12 kJ/mol; and the energy barriers to be overcome are 105 kJ/mol, 131 kJ/mol, 157 kJ/mol and 105 kJ/mol, respectively. The aldehyde group (TS3) generated in this process continues to react with O₂ to form carboxyl radicals. The bond between the carbon atom in the carboxyl radical and the carbon atom connected to the benzene ring is constantly stretched until it breaks to produce CO_2. In this process, the energy of the system decreases by 391.1 kJ/mol, the is −589.34 kJ/mol, and the G is −591.79 kJ/mol. Since the changes in Gibbs free energy are negative, it can be concluded that these two reactions are spontaneous. Therefore, the total heat released in the free-radical reaction process of the methyl group with oxygen is 1048.57 kJ/mol. For the reaction of ph-CH₃ with O₂, the energy of the reaction system is significantly reduced, indicating that CO is generated more easily than CO₂. However, more heat is generated during CO₂ generation, which brings a more significant thermal effect to the system. As a result, the temperature of coal rises sharply after CO₂ generation. As shown in Table 2, the activation energy is over 100 kJ/mol, which means that the reaction occurs in the middle and late stage of low-temperature oxidation. At the same time, the reaction absorbs and stores massive heat for the subsequent CSC stage.

4.2.2. Analysis of Transition States and Activation Energy

The frequencies of transition states (TS1, TS2, TS3, TS4 and TS5) produced in the reaction were calculated. The calculation results show that each of them has only one imaginary frequency (the uniqueness of imaginary frequency can verify the correctness of transitional states), i.e., −1294.58 icm⁻¹, −1664 icm⁻¹, −1814.87 icm⁻¹, −1446 icm⁻¹ and −784 icm⁻¹, respectively. The changes in potential energy along the intrinsic reaction coordinate in the reaction process are illustrated in Figure 5 and Figure 6, from which it can be observed that the transition states have the highest energy. The frequency of the transition states in the reaction process is calculated, and the results suggest that these transition states have only one imaginary frequency. Afterwards, the intrinsic reaction coordinates of the transition states are calculated, and the results indicate that the transition states correspond to the highest energy. The above calculations verify the feasibility of the reaction paths.

As can be seen in Table 2 and Figure 5, IM4 first needs to overcome 105.02 kJ/mol of energy to generate the aldehyde group and hydroxyl radicals in the process of CO generation. Then, the -C-C- bond between the benzene ring and the aldehyde group breaks to generate CO, water and benzene. As listed in Figure 2, due to the O attraction of IM4, the C-C bond is shortened and the angle gradually increases to 147.23°, preparing to form the C=O structure. The bond between the single broken H free radical and the adjacent O free radical is gradually shortened, and the angle eventually becomes 105°, forming water molecules. In addition, it can be observed from Table 2 and Figure 6 that in IM5, the hydroxyl radical and the oxygen of -C-O in the carboxyl radical attract each other. They need to overcome 236.29 kJ/mol of energy to form a transition state (TS5) of alternating double and single O-O bonds. The instability of the transition state leads to the rapid breakage of the O-O bond to form the hydroxyl group and the carboxyl radical. Then, the -C-C-bond between the benzene ring and the carboxyl radical breaks to release CO₂. When the O of the hydroxyl radical in IM5 gradually moves closer to the C-O bond, the O-O bond is shortened to a length of 0.121 nm, while the C-C and C-O bonds rarely change, forming the TS5 structure. During the gradual generation of CO₂, the angle of the O=C=O bond gradually increases and finally approaches 180°.

4.3. Reaction of ph-CH₂CHO with O₂

4.3.1. Analysis on Reaction Mechanism and Energy

The reaction path of the aldehyde group indicates that the oxygen atom in the oxygen molecule first attacks the aldehyde group in ph-CH₂CHO. As a result, the -C-H bond in the aldehyde group is broken, while -C≡O and the hydroxyl radical are formed. As oxidation proceeds, the -C-H- bond of methylene in the phenylacetaldehyde is broken, and the transition state ts2 is formed. The hydrogen combines with the oxygen atom quickly to generate the hydroxyl radical, which then combines with the -C-H bond to promote the breakage of the -C-C- bond and release CO. As listed in

Table 3. Thermodynamic parameters of ph-CH₂CHO with O₂ reaction.

Stagnation Point	Corrected E (kJ/mol)	Enthalpy H (kJ/mol)	Gibbs Free Energy G (kJ/mol)	Enthalpy Change (kJ/mol)	Gibbs Free Energy Change G (kJ/mol)	Activation Energy Ea (kJ/mol)
r1	−1,405,085.25	−1,405,018.3	−1,405,174.1	0	0
im1	−1,405,521.21	−1,405,323.1	−1,405,449.9	−304.8	−275.8	13.51
ts2	−1,405,507.70	−1,405,323.1	−1,405,449.9	−304.8	−275.8
im2	−1,405,291.92	−1,405,204.7	−1,405,350.7	−186.4	−176.6
p + CO + H2O	−1,405,564.02	−1,405,518.6	−1,405,680.2	−500.3	−506.1
im4	−1,206,760.51	−1,206,291.2	−1,206,411.4	0	0	26.46
ts4	−1,206,734.05	−1,206,410.9	−1,206,554.9	−119.7	−143.5
im5	−1,206,890.46	−1,206,410.8	−1,206,554.9	−119.6	−143.5
p1 + CO2	−1,405,640.19	−1,405,611.6	−1,405,757.9	−593.3	−583.8

E is the energy (E = electronic energy (EE) + zero-point energy); H is the enthalpy; G is the Gibbs free energy; ΔH is the enthalpy change, which reflects the absorbed or released heat in the reaction; ΔG is the Gibbs free energy change, which reflects whether the reaction is spontaneous (1 hartree = 2625.5 kJ/mol) [36].

, in this process, the energy of the system decreases by 478.77 kJ/mol, the is −500.3 kJ/mol, the G is −506.10 kJ/mol, and 13.51 kJ/mol of energy barrier needs to be overcome. After removing the hydroxyl radical, the generated im2 continues to be oxidized to form alternating single and double C-O bonds, resulting in the breakage of the -C-C- bond and the release of CO₂. The generated unstable benzyl radical (im5) will combine with the hydroxyl radical in the reaction process to generate phenyl ethanol (p). During the process, the energy of the system decreases by 129.95 kJ/mol, 119.6 kJ/mol of heat is released, and the Gibbs free energy changes to −143.5 kJ/mol. The results in Table 3 show that the activation energy is less than 50 kJ/mol. Such activation energy means the reactions can completely take place in the early stage of CSC, indicating that the aldehyde functional group is prone to react with oxygen. For the aldehyde, the heat mainly comes from the process of CO generation. CO generation has a great influence on the CSC process, while the effect of CO₂ generation is very limited.

4.3.2. Analysis of Transition States and Activation Energy

The aldehyde group has a relatively simple reaction mechanism. It has only two transition states, ts2 and ts4, each of which has only one imaginary frequency, namely, −1279.23 icm⁻¹ and −133.2 icm⁻¹, respectively. The activation energy and the changes in potential energy along the intrinsic reaction coordinate in the transition states are presented in Figure 7 and Figure 8, respectively.

As can be seen from Table 3 and Figure 7, as the oxidation proceeds continuously, methylene C-H in ph-CH₂CHO breaks to generate the hydrogen radical, and then it gradually approaches the oxygen atom to form the ts2 structure. In this process, 16.51 kJ/mol of energy needs to be overcome. The ts2 rapidly generates the hydroxyl radical and quickly combines with -C-H· in ph-CH₂CHO, so that the C-C bond in -C-C≡O is broken to release CO. Meanwhile, the hydroxyl radical attracts the hydrogen atom to generate water. According to Figure 3, from im1 to ts2, the adjacent C-C bond, C=O bond and O-O bond between the two hydroxyl radicals on the benzene ring do not change much, while the C-C bond connected between C=O increases to a length of 0.14 nm, ready to remove CO. In addition, the angle between H-O-H in im1 and p shrinks from 107° to 105°, during which water molecules are formed. It can be seen from Table 3 and Figure 8 that after removing the hydroxyl radical, im2 continues to oxidize the carboxyl radical. It needs to overcome 26.46 kJ/mol of energy to break the -C-C- bond to form extremely unstable ts4. As the angle of O=C=O in CO₂ gradually changes from 167° to 180°, CO₂ is released finally. The transition state has only one imaginary frequency. Moreover, the change in the reaction potential energy also verifies the rationality of the reaction path. Similarly, in the process of aldehyde group reaction, the activation energy of CO is lower than that of CO₂. CO is first released in the low-temperature oxidation process of coal, and the release rate is higher than that of CO₂.

4.4. Changes in CO and CO₂ Concentrations during Low-Temperature Oxidation

Figure 9 and Figure 10 show the change curves of CO and CO₂ concentrations of CQ and HYH coal samples during low-temperature oxidation (30–180 °C). The concentrations of CO and CO₂ are exponentially related to temperature with the rise in temperature. As illustrated in Figure 9, for the HYH coal sample, the initial generation temperature of CO is 60 °C and the inflection point temperature is 100 °C; for the CQ coal sample, the two temperatures are 80 °C and 120 °C, respectively, both being lower than those of the CQ coal sample. Moreover, the CO concentration of the HYH coal sample is much higher than that of the CQ coal sample. When the temperature reaches the inflection point, the CO concentrations of both samples surge, which indicates that the coal oxygen reaction is developing to the deep oxidation stage. As presented in Figure 8, the CO₂ release curve resembles the CO release curve, and for both samples, the initial generation temperatures and inflection point temperature of CO₂ are basically the same as those of CO. The above results are not completely consistent with the previous conclusions on the activation energies of CO and CO₂ calculated by the quantum chemistry method for two reasons: first, CO₂ is adsorbed on coal more easily [37,38]. With the rise in temperature, this part of adsorbed CO₂ is gradually desorbed from coal and released. Second, the active groups in coal are complex, and the types and numbers of active groups in different-rank coal samples are also quite different. In addition to methyl and aldehyde groups, common active groups in coal also include methylene, carboxyl and hydroxyl groups. The CO and CO₂ production mechanisms of these active groups are different from those of methyl and aldehyde groups. These groups are a focus of research in our follow-up study. Moreover, the above quantum chemical calculation results show that the activation energies of the methyl group to produce CO and CO₂ are 105 kJ/mol and 236 kJ/mol, and those of the aldehyde group to produce CO and CO₂ are 13 kJ/mol and 26 kJ/mol, respectively.

4.5. Changes in Active Groups during Low-Temperature Oxidation

FTIR is widely used to study the change in microstructure during coal oxidation. By analyzing the FTIR spectra, the changes in functional groups in coal can be clearly mastered, based on which the change in microstructure during coal oxidation can be grasped. The chemical structures of different-rank coal samples differ, and the intensities of absorption peaks of main active groups in the coal structure are also different. Considering such a fact, the FTIR spectra of CQ and HYH coal samples at different temperatures were comparatively analyzed to obtain the changes in their main active functional groups during low-temperature oxidation.

In the hope of acquiring the variations in concentrations of active functional groups in coal, the functional groups in the aliphatic vibration range 3000–2800 cm⁻¹ and the oxygen-containing functional group in the vibration range 1800–1500 cm⁻¹ were fitted by PeakFit software. Accordingly, the stretching and shrinking vibration peak areas of the main functional groups in the two ranges were obtained. Since the aromatic content in coal barely changes in the low-temperature oxidation stage, the relative intensities of different functional groups are defined as the ratios of absorption peak intensities of functional groups to that of aromatic C=C, so as to realize quantitative analysis of the FTIR spectra. In this way, the absorption peak intensities of functional groups were treated as standard [39,40,41].

A great difference can be found between the FTIR spectra of HYH and CQ coal samples. The peak intensities of hydroxyl, alkyl and carbonyl in the HYH coal sample are significantly higher than those in the CQ coal sample. With the rise in temperature, the concentrations of active functional groups fluctuate. Since the main active groups involved in the previous quantum chemical calculation are C-H, C=O and COOH, this paper will focus on the relationship of the three active groups with temperature and CO and CO₂ concentrations.

The relative absorption peak intensities of CQ and HYH coal samples are compared in Figure 11. According to the low-temperature oxidation experiment, HYH produces CO and CO₂ at 60 °C. The analysis of FTIR spectra shows that when the temperature rises to 60 °C, the CH₃ concentration declines mildly, and the concentrations of C=O and COOH increase slightly. At this time, the generation of CO and CO₂ can be deduced as follows: alkyl side chains are oxidized to form aldehyde groups, which further generate carboxyl groups and CO, and carboxyl groups break to produce CO₂. The reaction equation is: Coal-CH₃Coal-C=OCoal-COOHCO, CO₂.

The results of peak fitting based on the FTIR spectra are shown in Figure 12, and variations of CH₃, C=O, COOH functional groups in HYH and CQ coal samples are illustrated in Figure 13, Figure 14 and Figure 15.

At 120 °C, the concentrations of CO and CO₂ produced by the HYH coal sample soar. However, the analysis of FTIR spectra demonstrates that the concentration of CH₃ first jumps and then plunges, and the concentrations of C=O and COOH decrease slightly. This phenomenon can be explained as follows: at 120 °C, the stable chemical bonds of coal molecules begin to break and form alkyl side chains; the highly reactive alkyl side chains produced can react with oxygen rapidly, resulting in a sharp decrease in the concentration of alkyl side chains. Meanwhile, alkyl is oxidized to generate C=O, COOH, CO and CO₂. At this time, the amounts of C=O and COOH generated are smaller than the amount of them consumed, leading to a slight decrease in the concentrations of carbonyl and carboxyl groups.

According to the low-temperature oxidation experiment, CQ produces CO and CO₂ at 80 °C. The analysis of FTIR spectra indicates that when the temperature rises to 80 °C, the concentration of CH₃ increases first and then decreases, as does the concentration of C=O, but the concentration of COOH drops continuously. The reason for this phenomenon is that as CQ coal sample contains little CH₃, the amount of C=O produced by CH₃ oxidation is not high. At this time, CO mainly comes from oxidation and inherent C=O in coal, whereas CO₂ is mainly attributed to the inherent oxygen-containing structure COOH in coal.

At 150 °C, the concentrations of CO and CO₂ produced by the CQ coal sample rise sharply. The analysis of FTIR spectra shows that the concentration of CH₃ soars, the concentration of C=O grows slowly, and the concentration of COOH decline slightly. This indicates that at 150 °C, a large number of alkyl side chains are formed and further oxidized to produce C=O. Part of C=O becomes CO, and the rest is oxidized to COOH and finally forms CO₂. Compared with the low-rank HYH coal sample, C=O in the high-rank CQ coal sample is difficult to oxidize to form COOH at this stage.

5. Conclusions

(1) From the point of view of Gibbs free energy, the Gibbs free energy values of both methyl and aldehyde groups experience negative changes, indicating that the reaction is moving toward the CSC reaction. This is also a sign of rationality of the reaction path. From the perspective of enthalpy change, the enthalpy changes in both methyl and aldehyde groups are negative throughout the reaction, indicating that their reactions are exothermic. This finding is consistent with the actual situation of low-temperature oxidation of coal. With respect to the reaction energy, the energy released by the methyl group during the reaction is 1084 kJ/mol, and the energy released by the aldehyde group is 593 kJ/mol; the former releases much more heat than the latter (almost twice as much). In terms of the activation energy, the aldehyde group is more likely to react with oxygen to form CO and CO₂. For different reactive groups, a methyl group releases less energy than CO₂ in generating CO, while the opposite is true for aldehyde.

(2) Through quantum chemistry calculation, the reaction paths of ph-CH₃ and ph-CH₂CHO oxidation at low temperatures were obtained, respectively.

The reaction path of ph-CH₃ oxidation in coal at low temperatures is:

$${\mathrm{ph}\text{-}\mathrm{CH}}_3\to \mathrm{IM}1 \dots \mathrm{TS}2\to {\mathrm{ph}\text{-}\mathrm{HC}\text{-}\left(\mathrm{OH}\right)}_2\to \{\begin{array}{c}\mathrm{ph}\text{-}\mathrm{COO}\to {\mathrm{ph}\text{-}\mathrm{OH}+\mathrm{CO}}_2\\ \mathrm{ph}\text{-}\mathrm{CHO}\to {\mathrm{P}+\mathrm{H}}_2\mathrm{O}+\mathrm{CO}\end{array}$$

The reaction path of ph-CH₂CHO oxidation in coal at low temperatures is:

$${\mathrm{ph}\text{-}\mathrm{CH}}_2\text{-}\mathrm{CHO}\to \mathrm{im}1, \mathrm{ts}2\to \mathrm{ph}\text{-}\mathrm{CH}\text{-}\mathrm{C}\equiv \mathrm{O}\to \{\begin{array}{c}{\mathrm{ph}\text{-}\mathrm{CHO}+\mathrm{CO}+\mathrm{H}}_2\mathrm{O}\\ {\mathrm{ph}\text{-}\mathrm{CH}}_2{\mathrm{OH}+\mathrm{CO}}_2\end{array}$$

(3) The results of the low-temperature oxidation experiment on two different-rank coal samples reveal the following low-temperature oxidation path of active functional groups in coal: alkyl side chains are oxidized to form aldehyde groups, which further generate carboxyl groups and CO, and carboxyl groups break to produce CO₂. This conclusion verifies the accuracy of methyl and aldehyde reaction paths obtained above. The experimental results also show that different-rank coal samples correspond to varying mechanisms of CO and CO₂ generation. Specifically, for low-rank coal (HYH), the CO and CO₂ produced in the spontaneous combustion mainly comes from the oxidation of alkyl side chains. For high-rank coal (CQ), the CO produced mainly results from the oxidation of alkyl side chains, while the CO₂ produced is attributed to the inherent oxygen-containing structure in coal.