Experimental Study on Inhibition Characteristics of Imidazolium-Ionic-Liquid-Loaded Sepiolite Composite Inhibitor

Abstract

In response to the prevalent issues of short inhibition cycles and poor environmental compatibility in traditional inhibitors, this study prepared a new sepiolite-based composite inhibitor by loading imidazolium ionic liquid onto sepiolite. Through TG-DTG analysis, cone calorimeter experiments, and FTIR spectroscopy, we comparatively investigated the combustion characteristics of the composite inhibitor and its effects on the oxidation properties, inhibition performance, and active functional groups of coal samples. The results demonstrate that appropriate loading optimizes the thermal stability of sepiolite. Compared with conventional inhibitors, the composite exhibited the minimum weight loss rate at characteristic temperatures and achieved greater delays in critical temperature points of coal samples. The composite inhibitor delayed ignition time by 27–44 s compared to conventionally inhibited coal. The 3% [BMIM][BF₄]/sepiolite formulation showed CO emission peak intensity 3.02 times that of raw coal within 0–200 s, while reducing CO₂ production rate by 10.56% compared to MgCl₂-treated samples at 1000 s. The P_PFI exhibited maximum enhancement. Post-inhibition analysis revealed a 22–51% reduction in peak areas of active functional groups, indicating that the sepiolite-based composite achieves inhibition through synergistic physical and chemical interactions. Ultimately, a sepiolite-based composite inhibitor with environmental benignity was developed, whose inhibition performance is significantly enhanced compared to the traditional inhibitor MgCl₂. This research provides theoretical foundations for developing advanced inhibitor materials in coal mine applications.

1. Introduction

With the continuous increase in mining depths and the significant intensification of coal extraction operations, secondary disasters induced by coal spontaneous combustion have manifested diversified characteristics, including but not limited to chain reactions triggered by coal seam self-ignition, explosion risks from high-concentration dust clouds, and gas deflagration accidents caused by abnormal gas accumulation [1,2,3,4]. These compound mine hazards have resulted in substantial socioeconomic losses and posed critical threats to personnel safety [5].

Coal spontaneous combustion poses a critical hazard to mining safety, and the development of high-efficiency environmentally benign composite inhibitors has emerged as a pivotal research priority in this field [6,7]. Current conventional inhibitors face prevalent limitations including short inhibition duration and poor environmental compatibility. Specific manifestations include inorganic salts exhibiting proneness to deliquescence and leaching; gel-based formulations potentially releasing harmful gaseous byproducts; and halide-based inhibitors undergoing decomposition with the subsequent emission of corrosive ions, thereby inducing metallic corrosion and groundwater contamination [8,9]. Recent years have witnessed multidimensional explorations in the development of composite inhibition systems, yielding diversified multilayered technological pathways. Tang [10] systematically investigated the synergistic effects between MgCl₂ and phosphate, revealing that, at a Cl/P ratio of 2:1, hydroxyl content was reduced by 84.5%, the CO generation rate decreased by 71.3%, and the thermogravimetric analysis demonstrated a 62.8% reduction in exothermic peak area. Li et al. [11] synthesized MgAlZn-LDHs via hydrothermal synthesis, with temperature-programmed experiments confirming a 38 °C elevation in critical self-heating temperature, a 27.4 kJ·mol⁻¹ increase in activation energy, and an 82.3% reduction in free radical concentration. Although the composite inhibitors developed in current studies demonstrate significantly enhanced inhibition efficacy compared to conventional formulations, they still face persistent challenges including intricate preparation processes and suboptimal environmental compatibility.

Ionic liquids (ILs), as emerging chemical inhibitors, are characterized by their environmental benignity and chemical stability. The interactions between ILs and coal matrices facilitate both the dissolution of active functional groups and the suppression of oxidative reaction pathways underlying spontaneous combustion. Furthermore, the inhibition efficacy exhibits significant variation across different IL formulations [12,13]. As one of the most extensively utilized categories of ILs, imidazolium-based ILs demonstrate marked cost-effectiveness while retaining inhibitory characteristics. Bai et al. [14] focused on analyzing the inhibition mechanism of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) on coal, concluding that the dissolution of aliphatic components in coal by imidazolium-based ILs accelerates coalification, which suppresses the oxidative active sites of coal. Lin et al. [15] conducted research on wettability by compounding 1-butyl-3-methylimidazolium tetrafluorochlorate ([BMIM][Cl]) with α-olefin sulfonate at a molar ratio of 3:2 to prepare a novel composite inhibitor solution. Contact angle measurements revealed that the surface contact angle of treated coal samples sharply decreased from 78° for raw coal to 29.6°, representing a reduction of 62.05%. Despite the demonstrated advantages of ILs in molecular tunability and interfacial modulation capabilities, single-component ILs still face challenges including thermal instability at elevated temperatures and prohibitively high costs.

The natural layered chain silicate mineral sepiolite, characterized by abundant availability and low cost, exhibits unique advantages in active site adsorption due to its distinctive fibrous porous structure and exceptional thermal stability [16]. Based on the analysis of material properties and functional compatibility, sepiolite (SEP) demonstrates significant physical inhibition potential. Owing to its abundant resource reserves and notable cost-effectiveness advantages, this mineral has achieved industrial-scale applications in flame-retardant material systems across multiple sectors. However, its functional characteristics and mechanisms of action in mine fire prevention scenarios remain systematically undocumented in the literature to date [17,18]. Its layered silicate structure and elevated surface adsorption capacity enable the synergistic co-loading of moisture and chemical agents [19,20]. The superior thermodynamic stability ensures structural integrity maintenance in high-temperature environments, while the inherent viscous properties confer effective sealing capability for coal matrix pores [21,22,23].

The SEP/ILs composite system exhibits breakthrough potential in environmental friendliness: the former, as a natural mineral, enables green mining and low-cost application, while the latter avoids heavy metal pollution through molecular structure regulation [24,25]. Through thermogravimetric–differential thermogravimetric analysis (TG-DTG), cone calorimeter tests, and Fourier-transform infrared spectroscopy (FTIR) experiments conducted on the sepiolite-based composite inhibitor, the inhibition performance and mechanisms of the composite inhibitor were systematically investigated, providing a theoretical basis for the development of novel composite inhibitors.

2. Experimental

2.1. Material Preparation

2.1.1. Preparation of Sepiolite-Based Composite Inhibitor

The purified SEP (Xinglei Sepiolite Co., Ltd., Neixiang County, China) with a purity of 99%. [BMIM][BF₄] and [BMIM][Cl] (Macklin Biochemical Technology Co., Ltd., Shanghai, China), both with a purity of 99%.

IL-modified SEP was prepared via the liquid-phase loading method. First, 4 g of SEP was dispersed in distilled water and mechanically stirred at 500 r/min for 24 h at room temperature to fully activate the SEP surface. Based on previous studies, an IL loading ratio of 2–5% was adopted [26]. The IL aqueous solution was quantitatively added to the SEP dispersion system according to the proportions specified in

Table 1. Formulation ratios of sepiolite-based composites with different ionic liquid components.

Composite Inhibitor Sample		Sample ID	Sepiolite, g	Ionic Liquid, g	Water, g
Sepiolite	0%	SEP	20	0	200
[BMIM][BF4]-tc	2%	SEP + 2% [BF4]−	20	4	196
	3%	SEP + 3% [BF4]−	20	6	194
	4%	SEP + 4% [BF4]−	20	8	192
	5%	SEP + 5% [BF4]−	20	10	190
[BMIM][CL]-tc	2%	SEP + 2% Cl−	20	4	196
	3%	SEP + 3% Cl−	20	6	194
	4%	SEP + 4% Cl−	20	8	192
	5%	SEP + 5% Cl−	20	10	190

. The mixture was continuously stirred at 800 r/min for 24 h to facilitate the binding of ILs to the SEP surface. After the reaction, the mixture was centrifuged at 10,500 r/min for 5 min to achieve solid–liquid separation. The collected solid material underwent five washing cycles to remove unbound components. Finally, the washed solid was dried in a vacuum drying oven at 60 °C for 24 h, yielding sepiolite-based composite inhibitors with varying loading concentrations.

2.1.2. Preparation of Inhibited Coal Samples

Fresh coal samples collected from the underground coal face of a specific mine were transported to the laboratory where they were crushed and sieved to 60–80 mesh using a rapid pulverizer for industrial analysis. The industrial analysis results are presented in

Table 2. Proximate analysis results of coal samples.

Moisture, %	Volatile Matter, %	Ash, %	Fixed Carbon, %
0.39	35.89	16.54	47.18

.

The inhibitor was compounded with coal samples at a mass ratio of 1:4 [27]. Specifically, 20 g of the inhibitor was thoroughly mixed with 80 g of pulverized and sieved (60–80 mesh) coal samples to prepare inhibitor-modified coal samples. Untreated raw coal was set as the blank control group. All samples were subjected to vacuum drying at 50 °C for 24 h, followed by cooling to constant weight in a desiccator.

2.2. Experimental Procedure

The experimental flowchart is shown in Figure 1.

2.2.1. TG-DTG Experiment

The TG-DTG experiment was conducted using a NETZSCH STA449F5 simultaneous thermal analyzer (NETZSCH-Gerätebau GmbH, Selb, Germany). Each sample was tested twice to obtain valid data, and the final result was taken as the average of the two measurements to minimize the impact of ambient temperature and equipment-related issues on the experimental outcomes. The sample size was 5 mg and the temperature range was from 30 to 800 °C. The heating rate was set at 10.0 °C/min [28,29]. During the experiment, standard air was continuously introduced at a rate of 100.0 mL/min. After the test was completed, the experimental data were preserved.

2.2.2. FTIR Experiment

The test was conducted using a ALPHA Fourier Transform Infrared Spectrometer (Bruker Optik GmbH, Ettlingen, Germany), with the instrument parameters set to a resolution of 4 cm⁻¹, a scan accumulation of 32 times, and a wavenumber range of 4000–400 cm⁻¹.

2.2.3. Cone Calorimeter Experiment

Cone calorimeter tests on inhibited coal samples were conducted using a TTech-KZG06 cone calorimeter (Ttech Instrument Technology Co., Ltd., Suzhou, China). Each sample was tested three times to obtain valid data, and the final result was taken as the average of the three measurements. This approach aims to mitigate the impacts of ambient temperature, equipment-related issues, and insufficient combustion caused by excessive moisture in the coal samples on the experimental outcomes. The experiments were performed in an environmental chamber under constant temperature and humidity conditions. The heat radiation intensity was calibrated to 45 kW·m⁻² using a blackbody furnace, corresponding to a surface temperature of 765 °C for the conical heater. Prior to testing, the instrument was preheated for 12 h, followed by heat flux calibration to ensure radiation flux stability. A 50 g preprocessed coal sample was weighed and loaded into an aluminum-foil-wrapped sample holder (dimensions: 100 mm × 100 mm × 10 mm). The holder was then positioned on the sample stage beneath the conical heater. Three consecutive tests were performed, and the average value of the data was calculated.

3. Analysis and Discussion

3.1. Combustion Characteristics Analysis of Sepiolite-Based Composite Inhibitor

The TG curves in Figure 2 elucidate the thermal stability evolution of [BMIM][BF₄]⁻ modified SEP. In the 100–300 °C range, modified samples exhibited significantly lower mass loss than SEP. Specifically, SEP + 4% [BF₄]⁻ and SEP + 5% [BF₄]⁻ showed mass losses of only 3.8% and 4.0%, respectively, indicating that ILs effectively suppressed the desorption of adsorbed water by occupying interlayer domains. In the high-temperature regime (300–1000 °C), loaded samples displayed accelerated mass loss due to the thermal decomposition of organic components: SEP + 5% [BF₄]⁻ exhibited a 16.1 percentage point mass loss in the 300–400 °C interval, with drastic mass changes attributed to chain degradation reactions of overloaded ILs. Notably, low-loading samples such as SEP + 2% [BF₄]⁻ surpassed the pristine sample in residual mass at the terminal temperature (76.3% vs. 71.5%), demonstrating that moderate loading enhances high-temperature stability. The DTG curves further corroborate differences in loading mechanisms. All modified samples exhibited new decomposition peaks in the 365–389 °C range, corresponding to the breakdown of the [BMIM][BF₄] cationic framework. SEP + 5% [BF₄]⁻ showed a sharp peak at 369 °C with a decomposition rate of −3.1% /min, 1.7–2.3 times faster than low-loading samples, resulting from physically adsorbed IL layers on clay surfaces rather than chemical bonding. This observation correlates with the abrupt mass loss of SEP + 5% [BF₄]⁻ in the 300–400 °C range, suggesting that ILs exceeding 4% loading fail to fully integrate into the SEP layered structure, existing partially as surface-physiosorbed species. Comprehensive analysis identifies SEP + 3% [BF₄]⁻ as the optimal modified formulation, exhibiting superior thermal stability (82.1% residue at terminal temperature) and the broadest DTG peak profile, thereby providing a strategic basis for subsequent applications.

The TG curves in Figure 3 elucidate the regulatory effects of IL modification on the thermal stability of SEP. After IL treatment, the modified samples exhibited significantly reduced mass loss in the 100–300 °C range. Specifically, SEP + 3% Cl⁻ and SEP + 5% Cl⁻ showed mass losses of only 4.8% and 3.2%, respectively, indicating that ILs effectively suppressed the thermal desorption of water molecules by occupying interlayer domains within the clay mineral structure. In the high-temperature regime (200–900 °C), the residual mass progressively decreased with increasing IL loading, attributed to the stepwise thermal decomposition of loaded [BMIM][Cl] above 300 °C, with decomposition spanning 280–450 °C. Data confirm that moderate loading (3–5% Cl⁻) enhances thermal stability while avoiding secondary decomposition effects caused by excessive organic components. The incorporation of ILs markedly altered the thermal response characteristics, as evidenced by systematic shifts in DTG peak positions: modified samples exhibited new decomposition peaks in the 435–450 °C range (SEP + 2% Cl⁻: 450 °C; SEP + 3% Cl⁻: 435 °C; SEP + 4% Cl⁻: 445 °C; SEP + 5% Cl⁻: 449 °C), corresponding to the thermal degradation of the imidazolium ring in [BMIM][Cl]. The temperature variations correlate with interfacial interaction strength between ILs and SEP. Quantitative DTG peak analysis revealed a 61% reduction in peak intensity for SEP + 5% Cl⁻ within 100–200 °C compared to pristine SEP, confirming that ILs effectively inhibit zeolitic water release by occupying interlayer spaces. Based on experimental results, two optimized formulations SEP + 3% [BF₄]⁻ and SEP + 5% Cl⁻ were selected as the primary systems for subsequent investigations.

3.2. Analysis of the Effect of Sepiolite-Based Composite Inhibitor on Coal Oxidation Characteristics

Based on the optimized experimental results, the experimental design focused on demonstrating the advantages of the novel composite inhibitor over traditional MgCl₂ inhibitors, with detailed formulation parameters for each system provided in

Table 3. Formulation parameters of coal samples treated with different inhibitors.

Inhibitor Name	Sample ID	Solute Component, g	Water, g	Coal, g
Raw Coal	0	0	0	80
20%-MgCl2	1	4	16
10%-SEP	2	2	18
5%-[BMIM][BF4]	3	1	19
5%-[BMIM][Cl]	4	1	19
10%-SEP + 3% [BF4]−	5	2	18
10%-SEP + 5% [Cl]−	6	2	18

.

Figure 4 presents the thermogravimetric response characteristics of inhibited coal samples and untreated raw coal samples as a function of temperature. A comparative analysis of the curve morphology demonstrates that both exhibit analogous evolutionary patterns in TG-DTG profiles.

Based on characteristic temperature parameters, the coal oxidation process can be divided into four distinct stages: dehydration and weight loss stage (T₁–T₂); oxygen absorption and weight gain stage (T₂–T₃); pyrolysis activation stage (T₃–T₄); and combustion reaction stage (T₄–T₆). These characteristic temperature parameters serve as kinetic indicators for coal–oxygen complex reactions, quantitatively characterizing the spontaneous combustion propensity of coal matrices.

Table 4. Characteristic temperatures and mass loss of coal samples.

Inhibitor Name	Coal Sample ID	T1, °C	T2, °C	T3, °C	T5, °C	T6, °C	Mass Loss, %
Raw Coal	0	35.5	121.5	310.3	540.4	628.3	85.9
20%-MgCl2	1	58.9	130.9	314.9	497.9	577.9	84.8
10%-SEP	2	70.3	122.6	316.4	534.8	642.3	85.5
5%-[BMIM][BF4]	3	62.8	222.3	315.7	538.9	640.8	85.7
5%-[BMIM][Cl]	4	83.7	148.6	302.8	547.6	743.6	88.7
10%-SEP + 3% [BF4]−	5	74.5	155.5	312.5	554.5	650.3	84.8
10%-SEP + 5% [Cl]−	6	70.4	144.4	310.4	550.4	649.8	85.2

, compiled based on the data from Figure 3, comparatively presents the differential distribution of characteristic temperature parameters among various coal samples in the thermogravimetric analysis (TGA).

The data in Table 4 reveal that, with the exception of Samples 1^# and 4^#, the characteristic temperature parameters (T₁–T₆) of inhibitor-treated coal samples exhibit positive shifts accompanied by reduced mass loss rates compared to raw coal. These results confirm the efficacy of the inhibition system in suppressing chain reactions during coal oxidation. Notably, the T₃ parameter does not display significant regularity, which is closely related to the reaction pathway characteristics of the pyrolysis stage governed by the activation energy threshold.

Experimental data demonstrate that all inhibitor-treated samples exhibit significantly elevated T₁ parameters compared to raw coal. As this temperature parameter negatively correlates with coal spontaneous combustion propensity, the results confirm the inhibitory effects of SEP-based materials, ILs, and MgCl₂ on coal self-ignition. A comparative analysis reveals that SEP-treated Samples 2^#, 5^#, and 6^# show over 20% greater T1 enhancement than inhibited coal Sample 1^# (MgCl₂-treated), evidencing the superior inhibition efficacy of SEP-based composite inhibitors in elevating the critical temperature for coal spontaneous combustion. All inhibited coal samples display substantially higher T₂ parameters than untreated raw coal (Sample 0^#). Notably, samples functionalized with 3% [BMIM][BF₄] increase the desiccation temperature threshold beyond 100 °C. Mechanistic studies indicate that [BMIM][BF₄] effectively disrupts the chain processes of coal–oxygen complex reactions via the catalytic cleavage of oxygen-containing functional groups in coal matrices, a mechanism validated across all IL-containing samples (Samples 3^#–6^#). Intriguingly, SEP-inhibited coal exhibits relatively lower T₂ parameters compared to MgCl₂-treated counterparts, attributable to the incomplete suppression of oxidation activity through the physical adsorption of oxygen-containing groups by SEP. This highlights the fundamental differences between SEP physical adsorption mechanisms and IL chemical modification effects.

Comparative data show systematic increases in T₄–T₆ parameters for SEP-treated coal versus MgCl₂-treated groups, with ignition temperature offsets reaching 15.2–23.8 °C. Mechanistic analysis reveals that thermally stable mineral phases in situ generated by SEP under heating conditions synergistically retard coal combustion kinetics through combined physical barrier effects and chemisorption. This phenomenon confirms that the SEP flame-retardant effect predominantly operates from the coal pyrolysis to combustion stages, with sustained efficacy spanning the accelerated chain reaction period through to the complete burnout phase, further substantiating the performance advantages of sepiolite-based inhibitors in high-temperature regimes.

A comparative analysis of composite inhibitors with two distinct additives reveals that, with the exception of the T₄ parameter in Sample 6^#, the sepiolite-based composite inhibition system significantly elevates characteristic temperature thresholds and reduces mass loss magnitudes during coal oxidation. Through the systematic evaluation of synergistic inhibition performance based on characteristic temperature offsets and mass loss rates, the formulation of Sample 5^# (SEP loaded with 3% [BMIM][BF₄]) was identified as achieving the optimal balance between inhibition efficacy and cost-effectiveness, demonstrating the highest potential for engineering applications.

3.3. Cone Calorimeter Experiments on the Influence of Composite Inhibitors on Coal Combustion Characteristics

3.3.1. Ignition Time Analysis

The ignition time threshold (ITT), characterizing the time interval from spark application to sustained flame formation under standardized heat flux conditions, exhibits an inverse correlation with material flammability. A comparative analysis of ignition characteristics between raw coal and inhibited coal samples, as illustrated in Figure 5, demonstrates the significant prolongation of ignition delay time in treated samples, confirming the effective suppression of initial ignition processes by inhibitors. Post-ignition combustion phenomena for each coal sample are depicted in Figure 6. As can be seen from Figure 6, the raw coal exhibits a vigorous flame state at the moment of ignition, while the coal samples treated with the composite inhibitor show a weak flame state.

As shown in Figure 5 and Figure 6, compared to MgCl₂-treated coal samples, the three SEP-inhibited coal samples exhibit prolonged ignition delay times ranging from 8 to 44 s, indicating superior inhibition efficacy of SEP over traditional inhibitors during the initial stages of coal spontaneous combustion. This phenomenon originates from SEP’s unique layered structure, which provides a high specific surface area capable of adsorbing several times its own mass in moisture. During the initial inhibition phase, moisture evaporation effectively delays the initiation of combustion chain reactions. Experimental data reveal that Samples 5^# and 6^# demonstrate enhanced flame-retardant performance compared to samples treated solely with SEP or ILs, confirming a synergistic enhancement effect between SEP and ILs. This synergism arises from their structural complementarity at the molecular level, which significantly improves free radical capture efficiency through interfacial coupling interactions.

3.3.2. Analysis of Heat Release Characteristics

HRR and THR, as critical parameters for evaluating coal combustion processes, reveal differences in thermal decomposition behavior through their numerical characteristics. HRR magnitude reflects combustion intensity, where higher HRR values indicate the greater flammability and enhanced combustion performance of coal samples. As evidenced by the experimental data in

Table 5. Parameters of heat release rate and total heat release for different coal samples.

Inhibitor Name	Coal Sample ID	Time to Peak Heat Release Rate, s	Peak Heat Release Rate, (kW·m−2)	Total Heat Release, (MJ·m−2)
Raw Coal	0	71	88.17	48.17
20%-MgCl2	1	179	83.11	36.50
10%-SEP	2	238	82.45	35.94
5%-[BMIM][BF4]	3	196	59.05	33.60
5%-[BMIM][Cl]	4	223	53.03	35.96
10%-SEP + 3% [BF4]−	5	276	59.21	32.74
10%-SEP + 5% [Cl]−	6	199	68.84	33.68

and the dynamic variation curves in Figure 7, distinct differences in heat release parameters are observed among modified coal samples.

Experimental data from Table 5 and Figure 7 demonstrate that the HRR curve of raw coal 0^# exhibits distinct bimodal characteristics. The dynamic pyrolysis behavior manifests as follows: the initial combustion-formed char layer undergoes structural collapse under sustained thermal radiation, resulting in the exposure of unburned coal beneath and triggering a secondary heat release process. The peak HRR of this sample during the combustion acceleration phase occurs at 71 s, reaching a quantified value of 88.17 kW·m⁻². Notably, all three inhibition systems—MgCl₂, ILs, and sepiolite-based composites—effectively regulate coal combustion kinetics by suppressing free radical chain reactions, significantly delaying the emergence of HRR peaks.

As evidenced by Table 5, the peak time of Inhibited Coal Sample 5^# is delayed to 276 s, representing the maximum delay compared to raw coal. This confirms that the synergistic effects of multi-component inhibition systems enhance combustion suppression efficacy. The peak heat release rate (PHRR) of the six inhibited coal samples exhibits a reduction range of 5.0–39.8% relative to the raw coal baseline value of 88.17 kW·m⁻². Among these, Inhibited Coal Sample 4# demonstrates optimal flame-retardant performance, with its PHRR reduced to 53.03 kW·m⁻² (a 39.8% decrease). Comparative studies reveal that the PHRR of Inhibited Coal Sample 5^# in the sepiolite-based composite system is 14% lower than that of Sample 6^#. This discrepancy stems from improved free radical quenching efficiency due to differences in surface active site density, confirming that the structural optimization of the carrier significantly influences combustion intensity modulation.

Figure 8 illustrates THR variation curves for different coal samples. Thermodynamic analysis from Figure 8 and Table 5 demonstrates that Inhibited Coal Sample 5^# exhibits significant heat release suppression efficacy at t > 100 s, with its THR reduced by 32.03% compared to raw coal 0^#. The final stabilized THR value of 32.74 MJ·m⁻² represents the lowest level among all experimental groups. This sample maintains a 14.8% greater heat release suppression differential than the other five inhibition systems, confirming that its layered structure-loaded system achieves the profound modulation of combustion energy release through dual mechanisms of physical barrier effects and chemical bonding interactions.

3.3.3. Analysis of CO and CO₂ Gas Emissions

The dynamic emission characteristics of characteristic toxic gaseous products (CO and CO₂) during coal combustion provide critical insights into free radical oxidation pathway variations through monitoring their concentration evolution, offering pivotal decision-making support for the design of mine fire prevention and control systems. As shown in Figure 9 and Figure 10, the experimentally derived toxic gas generation kinetics—specifically the CO generation rate and CO₂ yield during the combustion of modified coal samples—reveal differential regulation mechanisms of inhibition systems on incomplete oxidation reaction pathways.

As shown in Figure 9, raw coal 0^# exhibits significant fluctuations in CO generation rate during the 0–100 s interval, with its peak corresponding to intense volatile matter release. From 100 to 1000 s, the CO generation rate transitions into a dynamic equilibrium state. This phenomenon originates physically from heat accumulation effects caused by excessive coal packing density, which induces restricted oxidation reactions of underlying fuels under oxygen-deficient pyrolysis conditions, forming diffusion-limited combustion kinetics. All six inhibited coal samples significantly enhance CO generation intensity. Notably, Inhibited Coal Sample 5^# demonstrates a characteristic CO emission peak during the 0–200 s interval, with an instantaneous intensity reaching 3.02 times the baseline value of raw coal. This confirms that the composite inhibition system alters coal–oxygen reaction pathways via selective chemisorption, preferentially directing combustion toward incomplete oxidation channels favoring CO production. Furthermore, Sample 5^# maintains a higher CO generation rate than raw coal throughout the 200–1000 s phase, revealing the porous carrier structure’s persistent interference with free radical chain reactions.

The CO₂ yield curves for different coal samples are shown in Figure 10. Raw coal 0# exhibits a continuous increase in CO₂ yield during the 0–300 s interval, followed by stabilization between 300 and 1000 s. In contrast, all inhibited coal samples demonstrate significantly lower CO₂ yields than raw coal during the initial 0–300 s phase, indicating the effective suppression of combustion by various inhibitors. Notably, Inhibited Coal Sample 2^# shows the lowest CO₂ yield of between 200 and 300 s of 0.87 kg·kg⁻¹, highlighting the primary inhibition capability of SEP in this critical timeframe. Between 300 and 550 s, certain inhibited samples (2^#, 3^#, 4^#, and 6^#) exhibit CO₂ yields exceeding raw coal levels. This phenomenon arises from the deactivation of single-component inhibition systems post 400 s, leading to partial combustion recovery. Specifically, Sample 6# fails to achieve anticipated synergistic effects due to insufficient interfacial compatibility between SEP and [BMIM][Cl]. A comparative analysis reveals that both Samples 1^# and 5^# surpass raw coal in CO₂ yield after 550 s. However, by the terminal 1000 s, Sample 5^# achieves a CO₂ yield of 2.64 kg·kg⁻¹, representing a 13.2% reduction compared to Sample 1^#. These results conclusively demonstrate that the composite inhibitor system of SEP loaded with 3% [BMIM][BF₄] delivers optimal combustion inhibition performance.

3.3.4. Fire Risk Analysis of Coal Combustion Characteristics

The Fire Performance Index (PFI) and Fire Growth Index (FGI) jointly characterize coal combustion propensity. An increase in PFI values indicates a reduction in fire risk levels, while a decrease in FGI values reflects diminished potential hazards through the delayed attainment of peak heat release rate (HRR). Based on combustion characteristic parameters, this study systematically determined fire risk assessment metrics for both raw coal and inhibited coal samples.

The PFI is calculated by Equation (1) as follows: [30]

$$P_{PFI}=t_{IIT}/P_{PHRR}$$

(1)

where P_PFI: Fire Performance Index, m²·s·kW⁻¹; t_IIT: ignition time, s; P_PHRR: peak heat release rate, kW·m⁻².

The FGI is calculated by Equation (2) as follows: [31]

$$F_{FGI}=P_{PHRR}/t_P$$

(2)

where F_FGI: Fire Growth Index, kW·(m²·s)⁻¹; t_P: time to reach peak heat release rate, s; P_PHRR: peak heat release rate, kW·m⁻².

Based on the data from Figure 5 and

Table 6. Fire Performance Index and Fire Growth Index of treated coal samples.

Inhibitor Name	Coal Sample ID	Time to Ignition, s	Fire Performance Index, (m2·s·kW−1)	Fire Growth Index, (kW·m−2·s−1)
Raw Coal	0	31	0.35	1.24
20%-MgCl2	1	115	1.38	0.46
10%-SEP	2	123	1.49	0.35
5%-[BMIM][BF4]	3	115	1.94	0.31
5%-[BMIM][Cl]	4	106	1.99	0.24
10%-SEP + 3% [BF4]−	5	159	2.68	0.21
10%-SEP + 5% [Cl]−	6	142	2.06	0.34

, the P_PFI and F_GFI of modified coal samples were derived through formula calculations, with detailed results provided in Table 4. A comparative analysis of the tabulated data reveals that all inhibitor-treated coal samples exhibit increased P_PFI values compared to raw coal 0^#. The specific PPFI enhancement magnitudes for Samples 1^#-6^# are as follows: 1.03, 1.14, 1.59, 1.64, 2.33, and 1.71, respectively. These results highlight that Inhibited Coal Sample 5^# achieves the greatest P_PFI increase, thereby effectively reducing the fire risk level of coal. Concurrently, Sample 5^# exhibits the lowest FGFI value of 0.21 kW·(m2·s)⁻¹, confirming that coal samples treated with the SEP-loaded 3% [BMIM][BF₄] composite inhibitor demonstrate significant advantages in fire risk control.

3.4. Quantitative Analysis of FTIR Spectra for Coal Samples

To investigate the effects of the sepiolite-based composite inhibitor on coal functional groups, the SEP-loaded 3% [BMIM][BF₄] composite inhibitor system was designated as the experimental group based on prior optimization results, with untreated raw coal serving as the control group.

To investigate the effects of the sepiolite-based composite inhibitor on coal functional groups, this study established correspondence relationships between major characteristic functional groups and spectral absorption peaks (

Table 7. Assignment table of major characteristic functional groups and spectral absorption peaks in coal.

Vibration Modes	Wavenumber Range, cm−1	Functional Groups	Spectral Band Assignment
Oxygen-Containing Functional Groups	3590–3697	-OH	Free Non-Associated Hydroxyl Groups
	3200–3500		Intermolecular Hydrogen-Bonded Association
	1715–1790	C=O	C=O Stretching Vibration
Aliphatic Hydrocarbons	2950–2975	-CH3	Asymmetric CH3 Vibration
	2850–2860		Symmetric CH2 Stretching Vibration
	2915–2940	-CH2-	Asymmetric CH2 Stretching Vibration
Aromatic Hydrocarbons	1490–1620	C=C	Conjugated C=C In-Plane Ring Vibration

) based on existing infrared spectral knowledge and relevant coal chemistry, considering the influence of specific functional groups on coal oxidation characteristics. In Table 7, -OH groups are validated as the most reactive moieties during spontaneous combustion, capable of both spontaneous reactions with oxygen and generation via the chemical adsorption of oxygen; C=O groups, distinct from -OH, serve as critical transitional moieties in coal self-ignition as oxidation products of aliphatic hydrocarbons; aliphatic C-H groups (including methyl and methylene) exhibit reduced content through reactions with atmospheric oxygen, forming various oxygen-containing functional groups that intensify spontaneous combustion; and C=C aromatic rings demonstrate high reactivity, participating in reactions during initial coal oxidation stages. Additionally, aromatic rings are generated during coal self-ignition processes [32,33,34]. The reduction in these functional groups quantitatively reflects the microscopic inhibition performance of the inhibitor.

Given the complex chemical composition of coal and the convoluted nature of its FTIR spectra, three characteristic spectral regions were selected for analysis, 1500–1800 cm⁻¹, 2800–3000 cm⁻¹, and 3200–3700 cm⁻¹, corresponding to distinct coal functional groups, as specified in Table 6. A peak deconvolution analysis of coal sample FTIR spectra was performed in Origin software, yielding the following conclusions.

As shown in Figure 11 and Figure 14, the sepiolite-based composite inhibitor exhibits inhibitory effects on both C=O and C=C functional groups. The raw coal displays a distinct minor peak at 1731 cm⁻¹, which, according to Table 6, corresponds to carbonyl group vibrational modes. However, this peak disappears in treated coal samples, conclusively demonstrating the composite inhibitor’s capability to reduce C=O content. [BMIM][BF₄] inhibition mechanism: The IL component effectively suppresses carbonyl group formation by blocking the generation of oxidative intermediates (e.g., ketones, carboxylic acids) during coal oxidation [35]. Mg²⁺ ions in SEP undergo complexation reactions with carboxylic acid groups in coal, forming stable COO-Mg structures. This interaction reduces the reactivity of C=O bonds in carboxyl groups [36]. Figure 12 displays the FTIR spectra of coal samples in the 2600–3000 cm⁻¹ region, predominantly attributed to aliphatic hydrocarbons. Compared to methylene groups, the composite inhibitor demonstrates more pronounced suppression effects on methyl groups. A quantitative analysis of peak areas reveals a 51% reduction in methyl group peak area and a 22% decrease in methylene group peak area for treated coal samples compared to raw coal. The [BMIM]⁺ cations, adsorbed onto SEP, are transported into coal pore structures through physical adsorption and electrostatic interactions. This process disrupts C-H bonds in aliphatic hydrocarbons, effectively inhibiting their oxidation into alkyl radicals [37]. Figure 13 reflects the effect of the composite inhibitor on hydroxyl (-OH) groups in oxygen-containing functional groups of coal. The processed infrared spectra exhibit substantial peak reduction compared to raw coal, particularly the disappearance of peaks at 3326 cm⁻¹ and 3367 cm⁻¹ after treatment. This phenomenon arises from the strong electronegativity of fluorine atoms in [BF₄]⁻, which form hydrogen bonds with hydroxyl groups in coal, leading to hydroxyl bond cleavage or passivation. Through a comprehensive analysis of functional group suppression, it can be seen that the composite inhibitor significantly reduces peak areas of key functional groups in coal, including C=O, -OH, and -CH₃, effectively decreasing the proportion of surface-active groups and thereby achieving flame-retardant effects.

Figure 14. Changes in functional group peak areas of raw coal and optimally formulated flame-retardant coal samples.

Figure 14. Changes in functional group peak areas of raw coal and optimally formulated flame-retardant coal samples.

3.5. Mechanism of Sepiolite-Based Composite Inhibitor in Suppressing Coal Spontaneous Combustion

The chemical structure of coal is fundamentally a network system composed of condensed aromatic nuclei formed by varying numbers of benzene rings, hydrogenated aromatic rings, and heterocyclic structures [38]. The core trigger for coal spontaneous combustion originates from oxidation interactions between surface-active groups on coal and oxygen molecules. These interfacial reactions initiate self-accelerating exothermic effects, ultimately leading to a sustained energy-releasing chain process [39,40]. The schematic diagram of coal inhibition by sepiolite-based composite inhibitor is shown in Figure 15.

1.

Physical Inhibition Mechanism: During the initial stage of coal spontaneous combustion, SEP adsorbs and retains substantial moisture on its surface due to its high specific surface area and structural characteristics. This moisture absorbs heat during temperature elevation, effectively reducing thermal accumulation on the coal surface. Additionally, SEP contains structural water within its framework, which desorbs at elevated temperatures, further lowering the coal matrix temperature. Physical inhibition suppresses the coal self-ignition reaction rate by altering the reaction conditions.

2.

Chemical Inhibition Mechanism: In the early stage of coal spontaneous combustion, [BMIM][BF₄] infiltrates coal pores through SEP-mediated loading, effectively neutralizing surface-active groups to mitigate self-ignition. Concurrently, Mg²⁺ ions in SEP chemically interact with carboxylic acid groups in coal via complexation reactions, forming stable COO-Mg structures that reduce the reactivity of C=O bonds in carboxyl groups. Chemical inhibition suppresses the coal self-ignition reaction rate by decreasing the population of surface-active groups and interrupting subsequent chain reactions.

4. Conclusions

1.

SEP achieves thermal performance optimization through the precise loading of functional components at specific ratios. Compared to SEP, the modified materials effectively suppress secondary thermal decomposition while maintaining thermal stability: quantitative DTG curve analysis reveals 53% and 61% reductions in peak intensities within the 100–200 °C low-temperature weight loss region for SEP + 3% Cl⁻ and SEP + 5% Cl⁻, respectively, confirming the significant mitigation of organic component volatilization. The SEP + 2% [BF₄]⁻ system exhibits optimal comprehensive performance, demonstrating an 82.1% residue rate at 800 °C and the narrowest DTG peak half-width, indicative of the highly uniform dispersion of loaded components within the matrix, thereby avoiding stepwise thermal decomposition caused by localized agglomeration. Based on orthogonal experimental analysis, two optimized formulations of sepiolite-based composites SEP + 3% [BF₄]⁻ and SEP + 5% Cl⁻ were selected as the primary systems for subsequent investigations.

2.

TG-DTG analysis confirms the significant inhibitory effect of the composite inhibitor on coal spontaneous combustion. Inhibited Coal Sample 5^# (SEP loaded with 3% [BMIM][BF₄]) exhibits optimal inhibition performance, achieving an 84.8% mass retention rate and the maximum delay in characteristic temperatures. Cone calorimeter tests further reveal the synergistic flame-retardant mechanism. Compared to MgCl₂-inhibited coal, the composite system delays ignition time by 8–44 s and significantly postpones the peak heat release rate. Critical parameters indicate that Sample 5^# shows a 3.02-fold increase in CO emission peak intensity within the 0–200 s interval compared to raw coal, while reducing the CO₂ production rate by 13.2% (to 2.64 kg·kg⁻¹) at 1000 s relative to MgCl₂-treated samples. The PPFI increases maximally to 2.33 m²·s·kW⁻¹, and the F_GFI reaches the lowest value (0.21 kW·m⁻²·s⁻¹), conclusively validating the superior synergistic flame-retardant performance of the SEP-loaded 3% [BMIM][BF₄] composite inhibitor.

3.

The quantitative FTIR analysis of coal samples before and after treatment with the sepiolite-based composite inhibitor reveals post-inhibition reductions of 22–51% in peak areas for -OH, -CH₃, and -CH₂- groups, demonstrating the composite inhibitor’s pronounced suppression of coal spontaneous combustion at the molecular level. The sepiolite-based composite inhibitor operates through dual inhibition mechanisms: Physical inhibition: SEP’s high surface area and structural water adsorption mitigate heat accumulation. Chemical inhibition: [BMIM][BF₄] disrupts radical chain reactions, while Mg²⁺ ions form stable complexes with oxygen-containing functional groups, reducing their oxidative activity. This integrated physical–chemical mechanism effectively interrupts the self-accelerating exothermic processes underlying coal spontaneous combustion, providing a robust theoretical foundation for developing advanced fire prevention technologies in mining engineering.