Study of the Effect of Alkali Metal Ions (Li⁺, Na⁺, K⁺) in Inhibiting the Spontaneous Combustion of Coal

Abstract

The essence of coal spontaneous combustion lies in the existence of a large number of chemically active functional groups in the coal molecule, such as aldehyde group (-CHO) and methoxy group (-OCH₃) in the side chain structure of coal molecule, which can be easily oxidized, thus triggering the spontaneous combustion process. Retardant is a more widely used technology to prevent the spontaneous combustion of coal, but the research on the microscopic level of the mechanism of coal spontaneous combustion retardation has been weak for many years, so deepening the exploration in this field is crucial for the optimization of the retardation strategy. The inhibition effect of Li⁺, Na⁺, and K⁺ inhibitors was investigated through the programmed warming experiments, and the results showed that the carbon monoxide production and oxygen consumption of coal samples inhibited by Li⁺, Na⁺, and K⁺ inhibitors were reduced to different degrees compared with that of the original coal, which proved that it had an inhibitory effect on the spontaneous combustion of coal. In order to deeply investigate the interaction between the molecular structure properties of coal and alkali metal ions, the complexes formed by three typical alkali metal ions-Li⁺, Na⁺, and K⁺-with specific reactive groups (-CHO and -OCH₃) in coal were investigated with the help of the quantum chemical calculation software Gaussian 16W, and the following conclusions were made after analyzing the complexes: on the one hand, the complexes formed by Li⁺, Na⁺, and K⁺ with the reactive groups in coal can occupy the sites where the reactive groups bind with oxygen, reduce the chance of coal oxygen contact and inhibit its oxidation process; on the other hand, the coordinating action of alkali metal ions increases the maximum energy barrier that needs to be overcome for the reaction of the originally active groups, resulting in the coal molecules in the process of oxidation reaction, increasing the difficulty of the reaction, thus effectively curbing the tendency of spontaneous combustion of coal.

1. Introduction

As a major component of global energy consumption, the safe, efficient and clean utilization of coal has always been a hot spot of research in the energy field [1]. The phenomenon of spontaneous coal combustion is a major hazard in coal mine safety, and its occurrence is closely related to the oxidation reaction of reactive groups in the coal molecule [2]. Spontaneous coal combustion not only leads to the loss of coal resources, but may also cause serious safety accidents such as fires and explosions, posing a great threat to the life safety of miners and environmental quality [3]. Therefore, in-depth study of the mechanism of spontaneous coal combustion and exploration of effective preventive and control measures are of great significance to ensure the safe production of coal mines [4,5,6,7].

The essence of coal spontaneous combustion is the presence of a large number of chemically active functional groups in the coal molecule, such as aldehyde (-CHO), methoxy (-OCH₃), etc. These functional groups are prone to oxidative reactions and release heat when they come into contact with oxygen, and if the heat is not emitted in a timely manner, spontaneous combustion may be triggered [8,9]. In order to prevent spontaneous combustion of coal, retardants are widely used as an effective technical means [10,11,12]. Retardants prevent spontaneous combustion of coal by changing the chemical properties or physical state of coal and inhibiting the reaction of reactive groups in coal with oxygen [13,14,15]. Although the macroscopic effects of retardants have been investigated, there is still a lack of in-depth studies on their microscopic mechanism of action, especially the detailed process of the interaction between alkali metal ions in the retardant and the reactive groups in coal.

In the previous research, Wang Shuangming et al. [16] conducted an in-depth reflection on the status of the main energy source of coal and green mining in China, and put forward a new idea of green mining of coal. Deng Jun et al. [17] used quantum chemistry to explore the behavioral characteristics of reactive groups within coal in the oxidation process, and especially pointed out that hydroxyl (-OH) has a decisive role in the oxidation of side-chain reactive groups, which deepened the understanding of the mechanism of spontaneous combustion of coal. He Shuaiyin et al. [18] investigated the inhibitory effect of metal ions on the activity of oxygen-containing functional groups in coal by using quantum chemistry, which provided a theoretical basis for the molecular design of the retardant. Qi et al. [19] analyzed the charge distribution, structural parameters, and molecular orbitals of the aldehyde group by using a quantum mechanical method, and determined that the C-H bond in the structure of the aldehyde group is the active site for the existence of the aldehyde group. Zhu et al. [20] constructed a coal model that contains different positions of the hydroxyl groups in the coal model in order to deeply investigate the influence of the specific position of hydroxyl groups in coal on its reaction properties. It was shown that the H atoms in the hydroxyl groups acted as key active sites in the reaction. During the intermediate stage of coal auto-ignition, the carboxyl radicals undergo decarbonylation and decarboxylation reactions, and the rate of these reactions is gradually enhanced with the increase in temperature [19,20]. Alkali metal ions themselves are widely available elements in nature, and their compounds (e.g., LiCl, NaCl, KCl) have high solubility in water, and are easily degraded in the environment without producing persistent pollution. In addition, these compounds are applied in coal mines in relatively small quantities, and do not produce toxic by-products, thus having a low impact on the environment. Future studies could further assess their environmental impacts in long-term use, especially the potential impacts on groundwater, soil, and ecosystems [10].

In this study, the interactions of alkali metal ions Li⁺, Na⁺, and K⁺ with aldehyde and methoxy groups in coal and their inhibition effects on spontaneous combustion of coal were investigated by quantum chemical calculations and programmed warming experiments. The aim of this research is to provide a deeper understanding of the microscopic mechanisms of coal spontaneous combustion and to optimize the use of alkali metal ions as inhibitors [21].

2. Materials and Methods

2.1. Programmed Warming Experiment

2.1.1. Preparation of Samples

Coal samples of 18–30 mesh, LiCl, KCl, and NaCl were weighed with an accurate and reliable electronic balance, and the alkali metal salts were prepared according to the concentrations of 5%, 10%, 15%, and 20%. The coal samples and the inhibitor solution were put into clean glass containers prepared in advance and, at the same time, one coal sample was added with an equal amount of pure water and mixed well as the control group, and the prepared samples were placed in a room temperature environment for 12 h, after which the moistened coal samples were put into a vacuum drying oven together, and dried for 24 h in an oxygen-free environment at a drying temperature of 40 °C, and after the drying was completed, the dried coal samples were taken out for the program warming experiment. After completion of drying, the dried coal samples were taken out to carry out the program warming experiment. Each experimental condition (different concentrations of LiCl, NaCl, and KCl blocking agents) was repeated three times, and the results were in good agreement, with the error of the data controlled within 5%. In this study, experiments and calculations have been carried out mainly for lignite coals, but the mechanism of inhibition of spontaneous combustion of coal by alkali metal ions is universal. Coals of different origins and deterioration degrees differ in molecular structure, especially the content and distribution of reactive groups (e.g., -CHO, -OCH₃) may be different. However, alkali metal ions reduce their contact with oxygen by forming complexes with the reactive groups, and this mechanism is applicable in different types of coals. Future studies can further validate the applicability of this inhibitor in different coal types (e.g., bituminous coal, anthracite) to ensure its wide application prospects.

2.1.2. Program Heating Experiment

Put the coal sample into the coal sample tank, cover the tank tightly, and make sure the tank is well sealed to prevent gas leakage. Connect the inlet and outlet pipes, and put the temperature sensor into the coal sample tank. The gas flow meter was set at 10 mL/min, the starting temperature was set at 30 °C, the termination temperature was set at 180 °C, and the heating time was set at 75 min, i.e., the heating rate was set at 2 °C/min. The temperature and the production of the indicator gas were recorded and monitored by the data acquisition system in the computer. The programmed warming system is shown in Figure 1.

3. Results and Discussion

3.1. Indicator Gas and Oxygen Consumption Analysis

3.1.1. Analysis of Indicator Gases

In this experiment, we studied the changes in CO production during the oxidation process with the heating time after the lignite was treated with three different kinds of inhibitors with different mass concentrations of LiCl, NaCl, and KCl, and evaluated the effect of the types and concentrations of inhibitors on slowing down or inhibiting the oxidation of spontaneous combustion of the coal, and the results were obtained as shown in Figure 2 and Figure 3.

As can be seen from Figure 2, the original coal sample started to increase slowly at 70 °C, and it was only at 90 °C that the concentration of CO changed slightly significantly for the first time, after which, the generation of CO showed an exponential increase with the increase in temperature, and the concentration of CO was already more than 800 ppm at 180 °C. The overall of coal samples treated with the blocking agent were lower than that of the original coal in the generation of CO, and as can be seen in the figure, the blocking agent effect of KCl was the most effective compared to NaCl and LiCl.

As can be seen from Figure 3, the CO concentration gradually decreased with the increase in the inhibitor concentration. The highest value of CO concentration in the coal sample with LiCl inhibitor is about 700 ppm, the highest value of CO concentration in the coal sample with NaCl inhibitor is about 650 ppm, and the highest value of CO concentration in the coal sample with KCl inhibitor is about 600 ppm, which all have the effect of inhibiting the generation of CO.

Through the above analysis, it can be seen that with the increase in temperature, the active groups in coal and oxygen reacted to generate CO gas and release heat to make the temperature of the coal body increase, the concentration of CO increased slowly when the temperature was low, and increased rapidly when the temperature reached a certain degree, and the oxidization process of the coal samples treated with the inhibitor was inhibited to varying degrees. Firstly, because of the formation of liquid film on the surface of the coal body, the retardant can isolate the coal from oxygen, which is still effective even after drying; secondly, the metal ions (Li⁺, Na⁺, K⁺) in the retardant and the active groups in the coal form a stable complex, which reduces the contact between coal and oxygen and inhibits spontaneous combustion.

3.1.2. Oxygen Consumption Analysis

The tendency of spontaneous combustion of coal can be observed based on the measured oxygen concentration. In this experiment, the oxygen consumption of coal samples treated with different concentrations of KCl, NaCl, and LiCl retardants at different temperatures was investigated to obtain the results shown in Figure 4 and Figure 5.

As can be seen from Figure 4, the oxygen volume fraction of the resist-treated coal samples was slightly lower than that of the original coal samples at room temperature. This is because the resist formed a protective film on the coal surface, reducing the direct contact between coal and oxygen. With the increase in temperature, the oxidation reaction of coal increased and consumed more oxygen. The volume fraction of oxygen starts to decrease when the raw coal is at 70~80 °C; at this time, the degree of decrease of oxygen volume fraction is very slow; when the temperature reaches 100 °C, the volume fraction of oxygen decreases obviously, and the oxidation reaction of the coal samples is intensified. And the slow decrease in coal samples treated with inhibitor started only at 80~90 °C, which was due to the fact that the inhibitor reduced the contact between coal and oxygen by forming a protective film and complexing with the active groups in coal, thus inhibiting the oxidation process of coal. When the temperature reached 170 °C, the volume fractions of different concentrations of oxygen were around 16%, 16.5%, 17%, and 17.5%, respectively. In addition, the oxygen consumption of coal samples treated with KCl inhibitor was the smallest, followed by NaCl, and LiCl was the most.

As can be seen from Figure 5, with the increase in temperature, the volume fraction of oxygen in the coal sample tank decreased slowly at first, and then decreased significantly, and the volume fraction of oxygen in the coal samples treated with the inhibitor was lower than that of the original coal samples. And with the increase in inhibitor concentration, the volume fraction of oxygen in the coal sample tank increased to a certain extent, indicating that the inhibitor concentration has a promotional effect on slowing down the oxidation process of coal.

3.1.3. Calculation of Apparent Activation Energy

In order to further explore the inhibition of CSC by different ratios of inhibitors, the apparent activation energy of coal samples was calculated using the Arrhenius formula. The calculation formula is as follows:

$$\mathrm{f}\left(\mathrm{c}\right)=\mathrm{l}\mathrm{n}{\mathrm{c}}_0-\mathrm{l}\mathrm{n}\mathrm{c}$$

$$\mathrm{l}\mathrm{n}(\mathrm{f}(\mathrm{c})/{\mathrm{T}}^2)=\mathrm{l}\mathrm{n}(\mathrm{A}\mathrm{R}/\mathrm{w}{\mathrm{E}}_{\mathrm{a}})-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}\mathrm{T}}}$$

where C₀ is the initial oxygen molar concentration, mol/cm³; C is the oxygen molar concentration at the moment of T, mol/cm³; T is the thermodynamic temperature, K; A is the pre-finger factor; w is the rate of warming (°C/min); Ea is the apparent activation energy (kJ/mol); and R is the gas constant 8.314 J/(mol-K). Using origin software (https://www.originlab.com/), it was plotted to establish a linear relationship, in which the activation energy can be sought according to the slope of the straight line. After calculation, it can be obtained to the activation energy of the original coal is 59.31 kJ/mol. As can be seen from Figure 6, the activation energy of the coal samples treated with 10% KCl, NaCl, and LiCl solutions increased by 8.52, 6.89, and 6.05 kJ/mol, respectively, compared with the original coal samples.

3.1.4. Retardation Rate

The amount of CO released from the raw and inhibited coal samples can be used to calculate the inhibition rate; the higher the inhibition rate, the greater the inhibition effect, in order to quantitatively evaluate the inhibition effect of the inhibitor at different inhibition temperatures. The formula for calculating the blocking rate is as follows.

$$\mathrm{r}(\mathrm{T})={\displaystyle \frac{{\mathrm{w}}_1\left(\mathrm{T}\right)-{\mathrm{w}}_2\left(\mathrm{T}\right)}{{\mathrm{w}}_1\left(\mathrm{T}\right)}}\times 100\mathrm{\%}$$

where r(T) is the inhibition rate when the coal temperature is T, %; w₁ is the amount of CO released from the original coal sample at T temperature, ppm; and w₂ is the amount of CO released from the coal sample after inhibition at T temperature, ppm.

As shown in Figure 7, the variation curves of inhibition rate with temperature at different inhibition temperatures were comparatively investigated, and the effect of inhibition concentration on the inhibition effect of coal at different temperature stages was studied. The inhibition rate gradually increased in the temperature range of 70–130 °C at different inhibition concentrations. The inhibition rate started to decrease rapidly around 130 °C. The number of active groups in the oxygen reaction of coal started to rise after 70 °C, at which time the inhibition rate had a tendency to increase steadily. The inhibition rate of different coal samples gradually decreased after 130 °C, which was mainly due to the large amount of volatilization of water in coal caused by the increase in coal temperature, and the inhibition effect gradually weakened until disappeared.

3.2. Geometric Configuration of the Simplified Structure of the Coal Molecule

The simplified molecular structure of coal calculated on the 6-311G(d, p)/LanL2DZ basis set using the Gaussian16W package and the B3LYP method in density-functional theory (DFT) is shown in Figure 8.

The information of the optimized geometries in terms of electron energy, number of imaginary frequencies, polarizability, bond lengths between atoms, bond angles, and charge distributions were obtained by the calculation of Gaussian 16 software. In the optimized geometrical configurations of benzaldehyde and anisole, the electronic energies are −345.573442 Hartree and −346.771316 Hartree, respectively, and neither of them has imaginary frequency, which indicates that the molecular structure at this time is a stable structure with the lowest energy in the ground state; the bond lengths of C-H bond as well as the C=O double bond in the aldehyde group are around 1.1 nm and 1.2 nm, respectively, and the bond lengths of the methoxy group are around 1.36 nm and 1.08 nm, respectively; the charge distribution is shown in

Table 1. Electrostatic charge layout of simplified structure of coal molecule.

Benzaldehyde			Anisole
Atomic Number	Atomic Species	Atomic Charge	Atomic Number	Atomic Species	Atomic Charge
1	C	−0.09771	1	C	−0.16107
2	C	−0.08583	2	C	0.15881
3	C	−0.09033	3	C	−0.13709
4	C	−0.11735	4	C	−0.11082
5	C	−0.10332	5	C	−0.13218
6	C	−0.11838	6	C	−0.10821
7	H	0.12082	7	H	0.11764
8	H	0.11898	8	H	0.11823
9	H	0.12786	9	O	−0.26099
10	H	0.12116	10	H	0.12254
11	H	0.12033	11	H	0.11826
12	C	0.19701	12	H	0.11760
13	O	−0.26437	13	C	−0.15821
14	H	0.07112	14	H	0.11393
			15	H	0.10078
			16	H	0.10078

.

The active groups in coal are represented by aldehyde group and methoxy group, which are able to complex with metal ions. The O atoms in aldehyde group and methoxy group have more negative charges, and can provide electrons to metal ions, while the empty orbitals of metal ions can accept lone pair of electrons from aldehyde group and methoxy group, which interact with each other to form ligand bonds.

According to the theory of frontier orbitals, frontier orbitals (HOMO and LUMO) are the key factors determining the chemical reactivity of molecules. The HOMO and LUMO orbitals of the simplified structure of the optimized coal molecule are shown in Figure 9.

The HOMO orbitals of benzaldehyde and anisole have higher electron cloud densities in the vicinity of the O atom in the reactive group -CHO and the adjacent C atom and the O atom in -OCH₃ and the C atom on the benzene ring, respectively, relative to the other positions, which are able to promote the rapidity of the chemical reaction and can, through the formation of a stable chemical bond, to slow down the rate of oxidation reaction of coal with oxygen.

3.2.1. Geometrical Configuration of Complexes

In the structure of the coal molecule, the O atoms in the aldehyde group and the O atoms in the methoxy group of its reactive group have more negative charges, and will show stronger electrical properties, and in the process of interacting with the metal ions (Li⁺, Na⁺, K⁺) to form a complex, it is easy to provide the electrons to the metal ions to form a coordination bond, which is generated between the metal ions and the O atoms in the reactive group. The complex configurations are shown in Figure 10 and Figure 11.

As can be seen from Figure 10 and Figure 11, the simplified structures of the coal molecules—benzaldehyde and anisole with Li⁺, Na⁺, and K⁺, respectively—form ligand bonds that are all located between the aldehyde group O and the methoxy O atoms and the metal ions, and a folded structure is formed at this position. In these complexes, the bond lengths of the C-H bonds in the benzene ring are all around 1.08 nm, while the bond lengths of the C-C bonds between the C atoms on the benzene ring and the C atoms on the aldehyde group are around 1.48 nm, and the bond lengths of the C-H bonds in the aldehyde group, as well as the C=O double bonds are around 1.1 nm and 1.2 nm, respectively; while the bond lengths of the C-O bonds between the C atoms on the benzene ring and the O atoms on the methoxy group are around 1.39 nm, and the bond lengths of the C-O bond in methoxy and the C-H bond in methoxy are around 1.36 nm and 1.08 nm, respectively. The small changes in bond lengths before and after coordination and the absence of imaginary frequencies in the optimized geometrical configurations indicate that the calculated geometrical configurations are located at the saddle point position on the potential energy surface, suggesting a good structural stability. The detailed description of the coordination structure is shown by the bond lengths and bond angles of the geometrical configurations, as shown in

Table 2. Coordination structure bond length.

Ligand and Central Atom	Atomic	Key Length/nm
-CHO and Li+	O (13)-Li+ (15)	1.79189
-CHO and Na+	O (13)-Na+ (15)	2.13588
-CHO and K+	O (13)-K+ (15)	2.49810
-OCH3 and Li+	O (9)-Li+ (17)	1.96442
-OCH3 and Na+	O (9)-Na+ (17)	2.49489
-OCH3 and K+	O (9)-K+ (17)	2.96479

and

Table 3. Coordination structure bond angle.

Ligand and Central Atom	Atomic	Key Angle/°
-CHO and Li+	C (12)-O (13)-Li+ (15)	87.60935
-CHO and Na+	C (12)-O (13)-Na+ (15)	147.85904
-CHO and K+	C (12)-O (13)-K+ (15)	99.48480
-OCH3 and Li+	C (2)-O (9)-Li+ (17)	113.77708
-OCH3 and Na+	C (2)-O (9)-Na+ (17)	115.05559
-OCH3 and K+	C (2)-O (9)-K+ (17)	114.62615

.

3.2.2. Molecular Frontier Orbitals and Stability Analysis

The HOMO orbital diagrams of the complexes formed by the -CHO functional group and -OCH₃ functional group with Li⁺, Na⁺ and K⁺, as calculated by the B3LYP method and the 6-311G(d,p)/LanL2DZ group, are shown in Figure 12 and Figure 13.

The electron cloud densities of the highest occupied orbitals in benzaldehyde and anisole molecules are composed of C atoms and aldehyde groups on the benzene ring and O atoms on the methoxy group. The electron cloud densities of the highest occupied orbitals of the complexes formed by -CHO functional group, -OCH₃ functional group and Li⁺, Na⁺, and K⁺ in Figure 12 and Figure 13 are on the C atoms on the benzene ring and on the O atoms on the aldehyde group and methoxy group, and a very small fraction of the electrons are present on the H atoms, with a slight decrease in the charge densities of the O atoms on the aldehyde group and the methoxy group, which suggests that the intervention of metal ions, to a varying degrees makes the components of the oxygen atom orbitals in the highest occupied orbitals of the molecule decrease which, in turn, reduces the reactivity of the reactive groups and enhances the antioxidant capacity of the coal.

Kinetic stability concerns the ability of a substance to keep its structure and properties unchanged under environmental changes, and is closely related to the electronic structural properties, especially the energy level difference E of frontier orbitals and the energy level of HOMO. Larger values of E indicate that HOMO electrons are less likely to migrate to LUMO, reducing the tendency for chemical reactions and thus improving kinetic stability. The higher the absolute value of HOMO energy, the stronger the molecular control of electrons, enhancing the intrinsic stability of the molecule. The frontier orbital properties of the reactive groups and complexes, such as energy level differences and HOMO energies, are listed in

Table 4. Molecular frontier orbital energy and energy level difference in -CHO, -OCH₃ and Li⁺, Na⁺, K⁺.

Structure Name	EHOMO/eV	ELUMO/eV	ΔE/eV
-CHO active group	−6.943620	−1.712075	5.231545
-CHO and Li+ complexes	−2.814183	−1.699162	1.115021
-CHO complex with Na+	−2.890001	−2.057976	0.832025
-CHO with K+ complex	−2.577066	−1.611542	0.965524
-OCH3 active group	−5.854907	0.109031	5.963938
-OCH3 and Li+ complexes	−2.539168	−0.659273	1.879895
-OCH3 and Na+ complexes	−2.793568	−1.164018	1.62955
-OCH3 and K+ complexes	−2.452604	−1.212991	1.239613

, revealing their critical impact on molecular stability.

As can be seen from Table 4, the HOMO orbital energy and LUMO orbital energy of the reactive group -CHO in coal before the formation of the complexes are −6.94362 eV and −1.712075 eV, respectively, and the energy level difference between the two is 5.231545 eV; the HOMO orbital energy and LUMO orbital energy of the reactive group -OCH₃ in coal are −2.814183 eV and −1.699162 eV, respectively; the energy level difference between the two is 5.963938 eV. 1.699162 eV; and the energy level difference between them is 5.963938 eV. The HOMO orbital energy and energy level difference can partly reflect the stability of the molecule, and the chemical stability of the molecule can be reflected by the HOMO orbital energy and energy level difference for the molecules of the same class with sufficient similarity when the characteristics of other reactants are not considered, but the reaction energy barrier of the molecule is the key factor in determining the stability of the molecule in concrete terms; therefore, the activation energies required for the reaction of the complexes were investigated in subsequent studies.

3.2.3. Analysis of Electrostatic Charge Layout

There is a close relationship between the atomic charge distribution in a molecule and the interactions between nucleophilic groups with a greater electron cloud density and electrophilic groups with a greater affinity for electrons and atoms. On the basis of the natural bonding orbital theory, the electronic structure and chemical bonding properties in molecules are analyzed by partially delocalizing the molecular orbitals. The active part of a nucleophilic reaction is usually characterized by a strong electronegativity and a tendency to accept electrons given from other atoms or groups of atoms, while the active part of an electrophilic reaction is usually characterized by atoms or groups of atoms that have a greater attraction for electrons and tend to give electrons. These active parts can provide a basis for how fast or slow the reaction proceeds and how it is carried out. The atomic charge distribution of the reactive groups -CHO and -OCH₃ in coal to form complexes with metal ions (Li⁺, Na⁺, K⁺) is shown in

Table 5. Static charge layout of -CHO and Li⁺, Na⁺, and K⁺.

Atomic Species and Serial Number	Atomic Charge/e	Atomic Species and Serial Number	Atomic Charge/e	Atomic Species and Serial Number	Atomic Charge/e
C (1)	−0.03930	C (1)	−0.06555	C (1)	−0.05810
C (2)	−0.10891	C (2)	−0.09929	C (2)	−0.08889
C (3)	−0.09567	C (3)	−0.05677	C (3)	−0.08210
C (4)	−0.14645	C (4)	−0.13812	C (4)	−0.15115
C (5)	−0.02334	C (5)	−0.06281	C (5)	−0.03738
C (6)	−0.13938	C (6)	−0.13696	C (6)	−0.13908
H (7)	0.11812	H (7)	0.11861	H (7)	0.11527
H (8)	0.11404	H (8)	0.11458	H (8)	0.11141
H (9)	0.11887	H (9)	0.11631	H (9)	0.11035
H (10)	0.11896	H (10)	0.11855	H (10)	0.11507
H (11)	0.11267	H (11)	0.11557	H (11)	0.11035
C (12)	0.24561	C (12)	0.26908	C (12)	0.24040
O (13)	−0.27817	O (13)	−0.34111	O (13)	−0.28830
H (14)	0.08283	H (14)	0.07415	H (14)	0.06900
Li (15)	0.42012	Na (15)	0.47377	K (15)	0.47313

and

Table 6. Static charge layout of -OCH₃ and metal ion complexes.

Atomic Species and Serial Number	Atomic Charge/e	Atomic Species and Serial Number	Atomic Charge/e	Atomic Species and Serial Number	Atomic Charge/e
C (1)	−0.15354	C (1)	−0.15507	C (1)	−0.15626
C (2)	0.15495	C (2)	0.15548	C (2)	0.15585
C (3)	−0.12970	C (3)	−0.13643	C (3)	−0.13886
C (4)	−0.10638	C (4)	−0.10666	C (4)	−0.10709
C (5)	−0.12451	C (5)	−0.12706	C (5)	−0.12807
C (6)	−0.10545	C (6)	−0.10522	C (6)	−0.10519
H (7)	0.12115	H (7)	0.12011	H (7)	0.11993
H (8)	0.12143	H (8)	0.12055	H (8)	0.12034
O (9)	−0.29901	O (9)	−0.28278	O (9)	−0.27710
H (10)	0.12021	H (10)	0.12126	H (10)	0.12027
H (11)	0.12274	H (11)	0.12117	H (11)	0.12083
H (12)	0.12120	H (12)	0.12012	H (12)	0.11993
C (13)	−0.15298	C (13)	−0.15535	C (13)	−0.15636
H (14)	0.11653	H (14)	0.11532	H (14)	0.11466
H (15)	0.10925	H (15)	0.10728	H (15)	0.10649
H (16)	0.10925	H (16)	0.10426	H (16)	0.10285
Li (17)	0.47487	Na (17)	0.48301	K (17)	0.48779

.

Theoretically, the valence state of Li⁺, Na⁺ and K⁺ in the compound is +1, but the positive charge number of the three ions in the electrostatic charge layout table above is less than 1, and the actual situation deviates from the theoretical data. This is due to the metal ion in the coal in the process of chemical reaction of the active group, the metal ion accepts from the ligand to provide the negatively charged electrons, and the positively charged electrons in the metal ion to neutralize each other, so that the actual valence of the metal ion is less than the theoretical valence, thus forming a coordination bond. In this case, the metal ions accept the negative electrons provided by the O atoms in the -CHO and -OCH₃ functional groups to form a coordination bond, thus reducing the contact between the active groups and the oxygen in the air, and further inhibiting the spontaneous combustion of coal.

3.2.4. Transition State Analysis

The positive charge of the complex is mainly concentrated in the metal ions and the C atoms in the -CHO and -OCH₃ active groups, while the negative charge is on the O atoms in the -CHO and -OCH₃ active groups. When the active groups in the coal react with the oxygen in the air, it is known that the O atoms are the first to attack the C atoms in the -CHO and -OCH₃ active groups and, therefore, the transition state of the molecular structure is analyzed. Initial guesses were made, and the TS search in Gaussian software was used to optimize the structure of the guessed transition states of the complexes, while the IRC calculations were performed to obtain the reaction paths and energy emergence of the reactions, as shown in Figure 14 and Figure 15.

As can be seen from Figure 14a, the energy required for the benzaldehyde molecule to change from the lower energy normal state to the higher energy transition state where the chemical reaction can easily occur is 0.801108313 kJ/mol, and the energy of the reactant is −420.553262 Hartree, and the energy at the highest point in the process is −420.7490185730 Hartree, and the reaction The minimum energy required for the reaction to occur is −0.1957566 Hartree; from Figure 14b, it can be seen that the energy required for the complex formed by the benzaldehyde molecule and Li⁺ to change from the lower energy normal state to the higher energy transition state in which the chemical reaction can easily take place is 36.274396343 kJ/mol, and the energy of the reactant is −428.094796 Hartree, the energy of the highest point in the process is −428.2793012450 Hartree, and the minimum energy required for the reaction to be able to take place is −0.1957566 Hartree; as can be seen from Figure 14c, the energy required for the complex formed by the benzaldehyde molecule and Na⁺ to change from the lower energy normal state to the higher energy transition state where the chemical reaction can easily take place is 40.71111343 kJ/mol, and the energy of the reactant is −428.094796 Hartree. The energy required is 40.711365319 kJ/mol, the energy of the reactants is −582.852713 Hartree, the energy of the highest point in the process is −420.9192438330 Hartree, and the minimum energy required for the reaction to be able to take place is 161.933469167 Hartree; from Figure 14d, it can be seen that the energy required for the complex formed by the benzaldehyde molecule and K⁺ to change from the lower energy normal state to the higher energy transition state where the chemical reaction can easily take place is 133.721892486 kJ/mol, and the energy of the reactant is −1020.463115 Hartree, and the energy of the highest point in the process is −448.8493185280 Hartree, and the minimum energy required for the reaction to be able to occur is 571.613796472 Hartree.

As can be seen from Figure 15a, the energy required for the anisole molecule to change from the lower energy normal state to the higher energy transition state where the chemical reaction can easily take place is 32.7132810395 kJ/mol, and the energy of the reactant is −421.751136 Hartree, and the energy of the highest point in the reaction process is −421.8280050970 Hartree. The minimum energy required for the reaction to be able to occur is −0.076869097 Hartree; from Figure 15b, the energy required for the complex formed by the benzaldehyde molecule and Li⁺ to change from the lower energy normal state to the higher energy transition state in which the chemical reaction can easily take place is 0.1488684755 kJ/mol, and the energy of the reactants is −429.255568 Hartree, the energy of the highest point in the process is −429.5154261950 Hartree, and the minimum energy required for the reaction to take place is −0.259858195 Hartree; it can be seen from Figure 15c that the complex formed by the benzaldehyde molecule and Na⁺ is transformed from the lower-energy normal state into the transition state of the higher-energy chemical reaction that can easily take place. The energy required for the transition state in which the chemical reaction occurs is 4.590308678 kJ/mol, the energy of the reactants is −584.036673 Hartree, the energy of the highest point in the process is −422.1665186550 Hartree, and the minimum energy required for the reaction to be able to occur is 161.870154345 Hartree; from Figure 15d, the energy required for the complex formed by the benzaldehyde molecule with K⁺ to change from the lower energy normal state to the higher energy transition state where the chemical reaction can easily take place is 2.808266306 kJ/mol, and the energy of the reactant is −1021.645257 Hartree, and the energy of the highest point in the process is −450.0955951200 Hartree and the minimum energy required for this reaction to be able to occur is 571.54966188 Hartree.

Combining the results of these analyses, it can be concluded that when metal ions (Li⁺, Na⁺ and K⁺) react chemically with the reactive groups -CHO and -OCH₃ in coal to form complexes, the energy required for the reaction is higher compared to the energy required for the reaction of the reactive groups -CHO and -OCH₃, and the reaction proceeds at a slower rate, and the reaction is more difficult to carry out. Therefore, when the coal is oxidized, the inhibitors with metal ions (Li⁺, Na⁺ and K⁺) can better stop the oxidation reaction process of the coal, and the inhibition effect of the inhibitor with the addition of K⁺ is much better than that of the inhibitor with the addition of Li⁺ and Na⁺. Moreover, since the energy required for the reaction between Li⁺ and the complexes of reactive groups -CHO and -OCH₃ is very small compared with that required for the reaction of the reactive groups themselves, both of them have the same ability to react, and thus the inhibition effect of the inhibitor with the addition of Li⁺ is not obvious. The results obtained in this study were compared with previous literature data, showing that the inhibition effect of alkali metal ions on coal spontaneous combustion is consistent with the findings of other researchers, such as Deng Jun et al. (2020) [17] and He Shuaiyin et al. [18] (2023).

4. Conclusions

In this study, the formation of complexes between alkali metal ions Li⁺, Na⁺, and K⁺ and the reactive groups -CHO and -OCH₃ in coal was investigated using quantum chemical calculations, and the effect of inhibition of spontaneous combustion of coal by the inhibitor containing Li⁺, Na⁺, and K⁺ was verified by programmed warming experiments as well as calculations of inhibition rates and activation energies, and the following conclusions were drawn:

(1)

The results of the programmed temperature rise experiments show that the inhibition effect of different concentrations of the retardants on spontaneous combustion of coal is different, and the inhibition effect will be enhanced with the increase in the concentration of the retardants, and the inhibition effect tends to level off when the concentration of the retardants is more than 15%, and the inhibition effect of KCl is the best under the same concentration.

(2)

Metal ions form complexes with active groups, reduce the chemical activity of active groups, enhance the antioxidant capacity of coal, and effectively prevent spontaneous combustion of coal. Through Gaussian software simulation, the geometrical configuration, electronic structure and energy changes in the complexes were analyzed, revealing that the coordination bond formed between the metal ions and oxygen atoms reduces the contact between the reactive groups and oxygen, and delays the oxidation reaction. The analysis of frontier orbitals and electrostatic charge layouts further confirmed that the addition of metal ions reduced the activity of the reactive groups and improved the stability of the complexes. Transition state analysis showed that the addition of metal ions raised the reaction energy threshold, making the coal oxidation reaction more difficult. Therefore, its application value includes that it can effectively reduce the risk of spontaneous coal combustion and reduce the occurrence of fire and explosion accidents in coal mines; inhibiting spontaneous coal combustion reduces the loss of coal resources and improves the utilization rate of the resources; reduces the emission of hazardous gases (e.g., CO) produced by spontaneous combustion of coal and reduces the pollution to the environment; and reduces the loss of accidents and resource losses, and reduces the economic losses of the coal mining enterprises.

In conclusion, the addition of alkali metal ions (Li⁺, Na⁺, and K⁺) significantly increased the thermal resistance of coal by approximately 10–15 °C, depending on the concentration of the inhibitor. The inhibitors also altered the ash properties, reducing the formation of volatile compounds and improving the overall stability of the coal during combustion. These changes are expected to enhance the efficiency of boiler operations by reducing the risk of spontaneous combustion and improving the combustion process.