Research on the Flame-Retardant Performance of Antioxidant Gel Foam in Preventing Spontaneous Coal Combustion

Abstract

Antioxidant gel foams are promising materials for coal mine fire prevention due to their unique physicochemical properties. To address the limitations of conventional suppression methods under high-temperature conditions, this study investigates a newly developed antioxidant gel foam and its mechanism in inhibiting coal spontaneous combustion. A novel antioxidant gel foam was formulated by incorporating TBHQ and modified montmorillonite into a sodium alginate-based gel system. This formulation enhances the thermal stability, water retention, and free radical scavenging capacity of the gel. This study uniquely combines multi-scale experimental methods to evaluate the performance of this material in coal fire suppression. Multi-scale experiments, including FTIR, leakage air testing, programmed temperature rise, and small-scale fire extinction, were conducted to evaluate its performance. Experimental results indicate that the antioxidant gel foam exhibits excellent thermal stability in the temperature range of 200–500 °C. Its relatively high decomposition temperature enables it to effectively resist structural damage in high-temperature environments. During thermal decomposition, the gel releases only a small amount of gas, while maintaining the integrity of its internal micro-porous structure. This characteristic significantly delays the kinetics of coal oxidation reactions. Further research revealed that the spontaneous combustion ignition temperature of coal samples treated with the gel was significantly higher, and the oxygen consumption rate during spontaneous combustion was significantly reduced, indicating that the gel not only effectively suppressed the acceleration of the combustion reaction but also significantly reduced the release of harmful gases such as HCl. Scanning electron microscope analysis confirmed that the gel maintained a good physical structure under high temperatures, forming an effective oxygen barrier, which further enhanced the suppression of coal spontaneous combustion. These findings provide important theoretical and practical guidance for the application of antioxidant gel foams in coal mine fire prevention and control, confirming that this material has great potential in coal mine fire safety, offering a new technological approach to improve coal mine safety.

1. Introduction

Coal spontaneous combustion not only causes significant resource and economic losses but also generates large amounts of toxic gases, seriously endangering the health and safety of underground workers. With the increase in mining depth, the gas content in coal seams gradually rises, further exacerbating the risks of spontaneous combustion fires and gas explosions. To address these issues, there is an urgent need to develop more effective fire suppression materials to enhance coal mine safety and reduce the hazards caused by coal spontaneous combustion. Although various methods for suppressing coal spontaneous combustion, such as water injection and grouting, have been proposed, these methods are limited in effectiveness under high-temperature conditions and are prone to failure. Furthermore, some existing suppression methods pose environmental risks, raising considerable concern. Therefore, the development of efficient and environmentally friendly materials for coal spontaneous combustion suppression is of great practical significance. However, existing materials generally exhibit low stability under high-temperature conditions, making it difficult to suppress coal spontaneous combustion over long periods. Hence, researching materials that retain excellent thermal stability under high-temperature environments has become a key issue.

In recent years, antioxidant gel foams have gained widespread attention in the field of coal spontaneous combustion suppression due to their excellent thermal insulation properties and chemical stability. Tang et al. [1] used orthogonal experiments to determine that a foam prepared with 2% SDBS, 0.5% guar gum, and 1% MgCl₂ was more effective in suppressing coal spontaneous combustion and fire spreading. Fire-retardant performance was examined from three angles: free radicals, functional groups, and CO generation rate. The results showed that at low temperatures, the foam primarily relied on physical suppression methods, while at high temperatures, it mainly relied on chemical suppression methods. Wang et al. [2] analyzed the strengthening effect of fly ash on three-phase foams through the perspective of interfacial properties, particle adsorption effects, and rheological properties, finding that fly ash particles could reduce the surface tension of the liquid film, effectively prevent water drainage, and increase foam strength, thereby slowing down the foam coalescence and rupture rate. Huang et al. [3] formulated a gel material with sodium silicate as the main component, sodium bicarbonate as a coagulant, and sodium polyacrylate as a polymer additive. Fire extinguishing experiments confirmed that this material had significant advantages, including reducing the fire scene temperature, lowering the concentrations of O₂ and CO, and increasing the concentration of CO₂, effectively suppressing the spread of fire. Wang et al. [4] prepared a novel gel by mixing konjac glucomannan and fly ash, which effectively suppressed temperature rise, heat accumulation, and CO generation in the coal oxidation combustion process, reducing the likelihood of coal spontaneous combustion. Fire extinguishing experiments showed that this gel, when applied to the surface of burning coal, significantly reduced the fire temperature and CO emissions, stably extinguishing the fire. Fan et al. [5] introduced a polymer plasticizer into conventional sodium silicate gel to obtain a new plasticized gel. Infrared spectroscopy analysis showed that this gel effectively suppressed the oxidation of methylene groups in the coal body. Field tests revealed that this plastic gel effectively inhibited oxidation in the coal particles, significantly lowering the coal temperature and ensuring the safe evacuation of workers. Zhang et al. [6] studied the optimal formulation of gel foam components and found that the gel foam treatment of coal samples showed better suppression of spontaneous combustion than untreated coal and MgCl₂-treated coal. However, these studies mainly focused on the thermal decomposition behavior of antioxidant gel foams, with limited systematic analysis of their specific mechanism of action and long-term stability in suppressing coal spontaneous combustion.

In summary, although previous studies have explored the thermal stability of antioxidant gel foams and their effects on suppressing coal spontaneous combustion, there is limited research on the chemical reaction processes under high temperatures and the specific mechanisms by which these gels inhibit coal spontaneous combustion. To fill this gap, this study investigates the cooling effect of antioxidant gel foam on coal spontaneous combustion and its ability to suppress coal pile reignition through Fourier infrared spectroscopy, leakage air experiments, programmed temperature experiments, and small-scale coal pile fire extinguishing experiments. The results of this study will provide important theoretical support and technical guidance for improving coal mine safety, with broad application value.

Three types of gel foams were used in this study: original gel foam, sodium silicate gel foam, and antioxidant gel foam. Among them, the antioxidant gel foam was synthesized using a compound system consisting of sodium alginate as the gelling agent, calcium chloride as the crosslinker, TBHQ (tert-butylhydroquinone) as the antioxidant, and modified montmorillonite as the thermal stabilizer. The antioxidant gel foam exhibits excellent thermal stability and high viscosity, which help maintain foam integrity at high temperatures. Its antioxidants scavenge free radicals, while montmorillonite enhances water retention and forms a dense physical barrier, jointly suppressing coal oxidation and spontaneous combustion.

2. Experimental Section

2.1. Fourier Transform Infrared Spectroscopy (FTIR) Experiment

The experiment was conducted using a Fourier-transform infrared spectrometer (FTIR, Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA) to measure the samples in the wavelength range of 400–4000 cm⁻¹, with a resolution of 2 cm⁻¹ and a scanning frequency of 64 scans per second. First, we crushed and sieved the coking coal, selecting coal samples with particle sizes ranging from 40 to 60 mesh. The samples were vacuum-dried for 10 h and then divided into four portions (each weighing 1000 g). One portion was kept as a control sample of raw coal, while the other three portions were mixed with 100 g of original gel foam, sodium silicate gel foam, and antioxidant gel foam each and then dried at a constant temperature of 100 °C for 24 h. For the infrared spectroscopy test, the KBr pellet method was used. The dried KBr was mixed with 10 mg of the coal sample in a fixed proportion, thoroughly ground, and pressed into a pellet (pressure 10 MPa, lasting 5 min). Each sample group was measured 11 times, and the effect of the gel foam on the functional groups in the coal was analyzed by comparing the infrared spectra.

2.2. Leakage Air Test

The experimental setup for testing the sealing ability of antioxidant gel foam primarily consists of an air supply system, pressure valve, pressure regulator valve, flow regulator valve, pressure relief device, vacuum pressure gauge, and coal sample chamber. The experimental apparatus is a steel coal sample chamber, with connections for gas pipes at both the top and bottom of the chamber, as shown in Figure 1.

The experiment used coking coal blocks, crushed and sieved to 40–80 mesh, as the blank control coal sample. The experiment was conducted in three stages: In the first stage, 1000 g of the mixed-particle-size coal sample was placed in the coal sample chamber, the chamber was evacuated for 60 min, and the pressure reading was recorded. In the second stage, 200 g of antioxidant gel foam was spread over the 1000 g coal sample, the chamber was evacuated for 60 min, and the pressure was recorded. In the third and fourth stages, 200 g of sodium silicate gel foam and original gel foam were covered, and the same steps were repeated.

2.3. Programmed Temperature Experiment

The flame-retardant performance of antioxidant gel foam was investigated using a coal spontaneous combustion programmed temperature experiment setup. The coal samples were crushed into five particle sizes: 0–1 mm, 1–3 mm, 3–5 mm, 5–7 mm, and 7–10 mm. These were mixed in proportion to form 1000 g samples, resulting in four groups: the raw coal sample group, original gel foam treatment group, sodium silicate gel foam treatment group, and antioxidant gel foam treatment group. After pretreatment, the coal samples were placed at room temperature for 12 h to stabilize in the mass before being loaded into the sample chamber. Before the experiment, the gas line and thermocouple were connected, and dry air was passed at a flow rate of 120 mL/min. The initial temperature of the programmed temperature furnace was set to 30 °C, and once achieved, the temperature was raised at a rate of 0.8 °C/min. Gas samples were collected every 10 °C, and the concentrations of gases such as CO and C₂H₄ were analyzed using a chromatograph. The experiment continued until the temperature reached 170 °C. The experimental principle is shown in Figure 2.

2.4. Small-Scale Coal Pile Fire Extinguishing Experiment

To investigate the fire extinguishing ability of antioxidant gel foam, a small-scale fire extinguishing experiment furnace was designed to simulate a fire scenario. Four groups of coal samples, each weighing 2000 g, were prepared. After 30 min of ignition, the original gel foam, sodium silicate gel foam, and antioxidant gel foam were injected into the samples, evenly covering the surface of the coal samples (with a coal to gel foam mass ratio of 10%). A thermocouple was inserted into the center of the coal pile, and the temperature change was recorded every minute, with the maximum temperature reaching 1300 °C (accuracy ± 1 °C). Throughout the experiment, the coverage of the foam and the dynamic temperature changes were recorded by a camera. The experimental setup included a camera, foam container, and temperature data acquisition system, as shown in Figure 3.

3. Results and Discussion

3.1. Analysis of the Effect of Gel Foam on Functional Groups in Coal

Coal molecules possess a variety of active functional groups, and these groups exhibit unique reaction characteristics when exposed to specific wavelengths. By utilizing a Fourier-transform infrared spectrometer, the functional groups in the coal samples treated with three types of gel foam, as well as the original coal sample, absorb energy due to vibrations at different frequencies. The absorption frequencies differ significantly due to the inherent characteristics of each type of functional group. As a result, various absorption peaks appear in different wavelength regions of the infrared spectrum. By comparing the differences in the intensity of characteristic functional group absorption peaks between the gel foam-treated coal samples and the original coal sample, the impact of the three gel foams on the coal’s functional groups is determined, further explaining the flame-retardant mechanism of the foam.

Various functional groups in coal undergo changes in quantity as temperature increases during low-temperature oxidation. By observing the content of specific functional groups in coal during the low-temperature oxidation process, the degree of coal oxidation can be assessed [7]. Therefore, by observing the functional group content at different wavelengths in the original coal and antioxidant gel foam-treated coal samples, the ability of antioxidant gel foam to suppress coal oxidation can be evaluated. The infrared spectra of the original coal and the three gel foam-treated coal samples are shown in Figure 4.

As shown in Figure 4, during the heating process, the overall absorbance curves of the three gel foam-treated coal samples exhibit a lower trend compared to the original coal. This phenomenon indicates that the number of active functional groups in the coal samples decreased after treatment with the gel foams. Among the treatments, the antioxidant gel foam exhibited the smallest absorption peak intensity, followed by the original gel foam and sodium silicate gel foam. In the wavelength range of 4000–400 cm⁻¹, the absorption peak trends of functional groups in the four sample groups were similar, but the intensity and area showed significant differences. This is primarily reflected in four regions: the hydroxyl vibration wavelength (3153–3700 cm⁻¹), aliphatic C-H vibration wavelength (2767–2988 cm⁻¹), carboxyl vibration wavelength (1800–1975 cm⁻¹), and carbonyl vibration wavelength (1500–1710 cm⁻¹), as detailed in

Table 1. Characteristic infrared absorption peaks of major functional groups in coal samples.

No.	Wavelength (cm−1)	Functional Group	Functional Group Assignment
1	3700–3153	-OH	—OH Hydroxyl Stretching Vibration Region
2	2920–2767	-CH	C—H Stretching Vibration (alkyl, methylene, methyl groups)
3	1975–1800	-COOH	—COOH Carboxyl Group
4	1710–1500	C=O	Carbonyl Stretching Vibration Region

.

The intensity of the absorption peaks for the sample groups is shown in

Table 2. Absorption peak intensities of functional groups in raw coal and coal samples treated with three types of gel foam.

Functional Group	Original Coal	Original Gel Foam	Sodium Silicate Gel Foam	Antioxidant Gel Foam
Hydroxyl	0.261	0.374	0.873	0.327
Aliphatic	0.898	0.405	0.896	0.328
Carboxyl	0.152	0.026	0.078	0.019
Carbonyl	1.835	1.450	1.320	1.052

. Compared to the original coal, after treatment with the original gel foam, sodium silicate gel foam, and antioxidant gel foam, the hydroxyl absorption peak intensity increased. Specifically, the original gel foam increased the absorption peak intensity by 0.113, the sodium silicate gel foam increased it by 0.612, and the antioxidant gel foam increased it by 0.066. This is attributed to the introduction of the gel foam, which increased the crystalline water content in the coal, thereby releasing more free hydroxyl groups. However, the increase in free hydroxyl groups was minimal in the antioxidant gel foam-treated coal, indicating that antioxidant gel foam can quickly consume the free hydroxyl groups in the coal sample during oxidation, hindering the chain reaction process of coal oxidation [8]. This is because the antioxidant gel foam can absorb moisture from both the coal and the air in various ways. Furthermore, the modified montmorillonite particles in the antioxidant components have high water absorption and retention properties, which also indicates that the antioxidant gel foam has good thermal stability.

The reduction in the aliphatic C-H absorption peak intensity can be attributed to the interaction between the aliphatic hydrocarbon functional groups and active oxygen or hydroxyl radicals. For the aliphatic C-H group, the absorption peak intensity decreased by 0.493 for the original gel foam, 0.002 for the sodium silicate gel foam, and 0.57 for the antioxidant gel foam. This decrease was primarily due to the effective elimination of free radicals in the coal by the antioxidant components, which reduced the number of methyl and methylene functional groups and effectively blocked their interaction with other groups. After treatment with antioxidant gel foam, the decrease in the aliphatic hydrocarbon absorption peak intensity in the coal sample was smaller than in the original coal, indicating that antioxidant gel foam can reduce the consumption of aliphatic hydrocarbons during coal combustion and heating, thus effectively suppressing coal oxidation [9]. This is due to the antioxidant gel foam’s ability to eliminate free radicals in the coal, slowing down the oxidation reaction during coal combustion.

The reduction in the carboxyl absorption peak suggests that the increase in the carboxyl content is suppressed after adding gel foam. The absorption peak intensity for carboxyl groups in the antioxidant gel foam-treated coal is reduced by 0.133 compared to that of the original coal. This is because antioxidants can consume active groups such as -COO- in the coal, blocking the chain cycle in the coal-oxygen composite reaction and reducing the formation of -COOH compounds [10]. The H ions in TBHQ react with -OH to form water, halting the chemical reactions in the coal combustion oxidation process. Additionally, montmorillonite can exchange metal ions, and these ions further form compounds that effectively block the reaction between coal and oxygen. Furthermore, TBHQ can react with Fe⁺, releasing H⁺ and improving antioxidant capacity. FeS accelerates the oxidation process during coal combustion, while montmorillonite can replace Fe²⁺ with Na⁺, Ca⁺, and other ions. Therefore, the antioxidant gel foam suppresses spontaneous coal combustion by eliminating free radicals and slowing the supply of free radicals required for coal combustion through these two mechanisms.

The reduction in the carbonyl absorption peak is the most significant for the antioxidant gel foam-treated coal, with the absorption peak intensity decreasing by 0.783 compared to the original coal. This indicates that less C=O is generated during the coal’s heating and oxidation. This is because the antioxidant gel foam has a significant physical moisture-retaining effect, effectively hindering the progress of oxidation reactions. At the same time, the antioxidant gel foam accelerates the decomposition of aldehydes, aromatic acids, and other functional groups, thereby reducing the overall carbonyl content.

Similar trends in the suppression of aliphatic and oxygen-containing functional groups have also been reported in coal oxidation studies involving antioxidant additives, such as those by Qiao et al. (2019) [9] and Iglesias et al. (1998) [11]. These results confirm the effectiveness of free radical scavenging in altering coal’s reactive structure.

In summary, although the lack of pre-drying FTIR spectra limits the direct tracking of functional group evolution, the observed post-treatment differences combined with known antioxidant pathways strongly support the proposed inhibition mechanism of the gel foam.

3.2. Analysis of the Leakage Air Blocking Ability of Antioxidant Gel Foam

Antioxidant gel foam contains surfactant components, and during the initial formation phase, it exhibits low viscosity, enhancing its flowability. Compared to water injection and grouting methods, which struggle to cover higher locations in mined-out areas, antioxidant gel foam fully utilizes the advantages of its foam structure. Its overall density significantly increases, making it an ideal carrier for transporting other substances. This allows it to penetrate the mined-out area in a three-dimensional manner, effectively acting on the coal body over a large area, as shown in Figure 5. The antioxidant gel foam fills the fractures in the mined-out area, blocking the airflow pathways and effectively sealing air leaks. This prevents contact between oxygen and residual coal in the mined-out area, creating a physical barrier.

Figure 6 shows the pressure variation curves over time in the coal sample chamber before and after injecting the antioxidant gel foam, original gel foam, and sodium silicate gel foam. From the graph, it can be observed that the pressure curve can be divided into two stages: the negative pressure increase stage and the pressure stabilization stage. At the beginning of the experiment, by using sealing materials and a vacuum pump, a negative pressure state is created inside the coal sample chamber. When the negative pressure inside the chamber stabilizes, the negative pressure value is reached and the time taken to stabilize the pressure varies. When only coal is placed in the coal sample chamber, after about 5 min of vacuum pump operation, the vacuum gauge reading remains unchanged, stabilizing at −26.4 kPa. When original gel foam and sodium silicate gel foam are injected, the pressure in the coal sample chamber stabilizes after about 18 min of vacuum pump operation, with the maximum negative pressure values reaching −70.8 kPa and −59.062 kPa, respectively. When antioxidant gel foam is injected, the pressure stabilizes after about 19 min of vacuum pump operation, with the maximum negative pressure value reaching −86.772 kPa.

As shown in Figure 6, after the injection of antioxidant gel foam, the time required for the vacuum chamber to stabilize the pressure is extended, and the negative pressure increases. This is because, after the addition of antioxidant gel foam, a portion of the foam covers the surface of the coal body, while another portion flows into the coal fissures under the action of the vacuum pump, which takes more time, thus extending the time to stabilize the negative pressure. During the seepage process, the gelling agent and crosslinking agent interact to form a stable weak gel structure, effectively blocking air leakage cracks and enhancing the overall integrity of the coal body. This results in a good filling and sealing effect, preventing the coal from coming into contact with air.

Comparing the negative pressures formed by the three types of gel foam under the vacuum pump, it is found that when antioxidant gel foam is applied to the coal body, the measured system’s negative pressure on the vacuum gauge is higher, indicating that antioxidant gel foam has a stronger ability to fill the gaps between coal particles and provides a better sealing effect. Therefore, the addition of antioxidant gel foam significantly improves the leakage resistance of crushed raw coal. As the concentration of antioxidant gel foam increases, the leakage resistance of the coal also increases correspondingly.

The foaming agent components in the antioxidant gel foam can wet the coal samples and wrap them around the surface of the coal [2]. After mixing with the crushed coal, it adsorbs and fills the leakage channels in the coal particle gaps, improving the leakage resistance of the coal samples. At the same time, the gel components of the antioxidant gel foam possess a three-dimensional framework, which enhances the strength and viscosity of the antioxidant gel [6]. The modified montmorillonite in the antioxidant components strengthens the intermolecular interactions, improving the adsorption of large amounts of water within the gel, forming a high-strength liquid film that increases the foam’s resistance to pressure and vibration. Additionally, due to the repulsive forces between the charged particles, the aggregation of modified montmorillonite is reduced, leading to a broader distribution of the modified montmorillonite. This enhances the foam’s stability, increases its flow resistance, and improves the air leakage blocking ability of the antioxidant gel foam [13].

3.3. Analysis of the Flame-Retardant Performance of Antioxidant Gel Foam

Figure 7 shows the CO concentration change curves during the heating process for the original coal and the three gel foam-treated coal samples. As seen from the figure, the CO concentration change trends during the heating process are generally the same for the different samples, but there are differences in the CO concentrations released by each sample at the same temperature. The concentration of CO released during coal oxidation reflects the extent of the oxidation process—a lower CO concentration indicates a lower degree of oxidation.

From the figure, it can be seen that among the four samples, the original coal sample releases the highest concentration of carbon monoxide during the heating process. The sample treated with sodium silicate gel foam releases a slightly lower concentration of CO compared to the original coal sample, followed by the sample treated with original gel foam. The antioxidant gel foam-treated sample exhibits the lowest level of gas emission.

Before 90 °C, there is no significant difference in the CO concentrations released by the four samples during the heating process. However, once the temperature exceeds 90 °C, the differences in gas concentrations become more apparent.

Among the samples treated with original gel foam and sodium silicate gel foam, although the onset temperature for CO release does not shift significantly, the concentration of released CO is reduced to some extent, partially inhibiting the coal’s oxidation. In contrast, the antioxidant gel foam-treated sample shows a 20–30 °C delay in reaching equivalent gas concentrations compared to raw coal, indicating a stronger suppression of low-temperature oxidation.

This delay in CO and C₂H₄ release aligns with the findings of Liu et al. (2022) [14], who reported that thermal stabilizers effectively postponed the onset of coal spontaneous combustion and the associated gas emissions. Such comparative results validate the inhibition mechanism attributed to the antioxidant components.

This phenomenon is mainly due to the gel foam’s ability to bond with the coal body to some extent. Once the foam covers the surface of the coal particles, the weak gel formed by the gelling agent creates a protective layer that prevents air from coming into contact with the coal, thus hindering the coal oxidation during the heating process.

Figure 8 shows the release of C₂H₄ during the heating process for the original coal and the three gel foam-treated coal samples. From the figure, it can be seen that the C₂H₄ concentration change followed a similar trend to that of CO. At the same temperature, the sample that was not treated with foam released the highest concentration of C₂H₄, followed by the samples treated with sodium silicate gel foam and original gel foam, while the sample treated with antioxidant gel foam released the least C₂H₄. The appearance of C₂H₄ gas indicates the initiation of accelerated oxidation in the coal. Unlike CO, the original coal sample, original gel foam-treated sample, sodium silicate gel foam-treated sample, and antioxidant gel foam-treated sample all have different initial temperatures for C₂H₄ release, which occur at 130 °C, 150 °C, 150 °C, and 160 °C, respectively.

From the experimental results, it can be concluded that the gel foam treatments delay the coal’s oxidation and spontaneous combustion process to some extent, with the sample treated with antioxidant gel foam showing the most significant effect.

3.4. Analysis of Fire Extinguishing and Cooling Performance Results

(1)

Comparison of Fire Extinguishing Effectiveness:

Using the coal pile fire extinguishing experiment setup, the remaining coal and temperature variations at different times were compared between the three types of gel foam and the sample without gel foam after ignition. The effectiveness of antioxidant gel foam, sodium silicate gel foam, and original gel foam in suppressing coal spontaneous combustion was compared.

Based on the coverage of the three gel foams at different times over the high-temperature fire source and surrounding areas, the following observations were made:

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After 30 min of coal burning, gel foam was applied, and 1 min after application, the gel foam surface remained relatively intact, with good coverage of the high-temperature area.

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Over time, the coverage area of the three gel foams varied. When sodium silicate gel foam was injected into the burning coal pile, the foam’s liquid film thinned quickly due to the loss of water owing to the high temperature, causing the foam to collapse rapidly, and the foam remained for a short time. After 1 h of coverage, the gel foam exhibited the most significant reduction, with its coverage area shrinking by about half and its thickness visibly diminished. After 1.5 h of foam coverage, and after 2 h of coal burning, the foam was almost entirely gone.

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When the original gel foam was injected into the burning coal pile, the foam broke down more slowly than the sodium silicate gel foam, resulting in a longer coverage time. After 1 h of coverage, more than half of the coverage area remained, and the foam thickness was less reduced. After 1.5 h of foam coverage, some foam remained at the edge of the coal pile burning area.

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When antioxidant gel foam was injected into the burning coal pile, the foam broke down the slowest and covered the coal surface for the longest time. After 1 h of coverage, about one-third of the coverage area remained, and the foam thickness was reduced the least. After 1.5 h of foam coverage, the most foam remained, with foam present at both the center and edges of the coal pile, and the foam thickness was greater than that of the original gel foam.

The high temperature caused the water in the sodium silicate gel foam and original gel foam to vaporize quickly as it seeped into the fire source point, leading to rapid foam breakdown and a short duration of action. In contrast, the antioxidant gel foam had better thermal stability, with slower water vaporization as it seeped into the fire source point, maintaining overall stability. The antioxidant gel foam fully covered the fire zone, blocking contact between the high-temperature fire source and oxygen. Even when the gel foam dried and cracked during the fire extinguishing process, the antioxidant agents within it could still reduce the content of methyl, methylene, hydroxyl, and carboxyl groups in the coal’s free radical chain reactions, increase the stable ether bond content, and suppress spontaneous coal combustion.

(2)

Temperature Field Variation

As shown in Figure 9, by analyzing the data collected by the temperature measurement system before and after the injection of the non-gel foam, original gel foam, sodium silicate gel foam, and antioxidant gel foam, the temperature change curves for raw coal combustion and the temperature changes during the extinguishing of coal pile fires using the three types of gel foam were obtained.

From Figure 9, it can be observed that the first stage of combustion was from 0 to 30 min. After igniting the coal pile in the iron box, the coal burned intensely, causing the temperature to rise to around 600 °C. The second stage began when the three gel foams were injected at 30 min. The phase in which the three gel foams were used to extinguish the coal combustion was from 30 to 70 min. After applying the same amount (300 g) of gel foam for fire suppression, the temperatures of the three gel foam-treated coal samples quickly decreased.

The coal sample treated with sodium silicate gel foam exhibited a temperature reduction of approximately 258 °C, after which the cooling effect plateaued and the combustion source remained active. Similarly, the sample treated with original gel foam cooled to around 210 °C but did not achieve complete extinguishment. In contrast, the sample treated with antioxidant gel foam demonstrated a markedly superior cooling effect, with the temperature continuing to decline past the ignition threshold during the transition from the second to the third stage. This enhanced performance can be attributed to the rapid and uniform surface coverage provided by the foam, which promptly suppressed open flames and isolated the coal from oxygen. Additionally, upon contact with the high-temperature coal surface, the water content within the foam vaporized, absorbing substantial latent heat and accelerating the reduction in coal surface temperature.

The third stage, from 70 to 120 min, showed that the temperatures of the coal samples treated with sodium silicate gel foam and original gel foam rebounded and then stabilized. However, the temperature of the coal sample treated with antioxidant gel foam continued to decrease, reaching around 16.5 °C, and the fire source was completely extinguished. This is because, in the antioxidant agent, montmorillonite was evenly distributed in the foam, enhancing the foam’s strength and reducing the likelihood of foam dehydration. Similar cooling behavior and delayed reignition effects have been observed in prior studies involving composite gel fire suppressants (Zhang et al., 2020 [10]; Fan et al., 2020 [5]), supporting the conclusion that gel-based thermal insulation and water retention are key to long-term suppression.

To further enhance understanding of the gel’s thermochemical properties, future studies could employ TGA, DSC, and cone calorimetry, as demonstrated by Huang et al. (2021) [15] in their evaluation of coal inhibition materials. These methods enable detailed analysis of decomposition stages and heat release behavior, offering valuable insights into fire suppressant mechanisms under high-temperature conditions.

After the foam ruptured, the montmorillonite was able to cover the coal, and the montmorillonite, having lost moisture and lowered the temperature, could again absorb water molecules, continuing to wet the coal and further cool it down. Before rupture, the modified montmorillonite in the antioxidant gel foam was able to effectively limit the contraction, expansion, and coalescence of the liquid film surface. After the antioxidant gel foam dried and cracked due to moisture loss, the modified montmorillonite was still able to block the coal fissures. By absorbing the moisture released during coal combustion, the montmorillonite would have wet the coal surface, effectively preventing coal oxidation and spontaneous combustion.

4. Conclusions

In this study, the effects of antioxidant gel foam on leakage air in mined-out areas, coal active functional groups, coal spontaneous combustion suppression, and heat accumulation were systematically investigated using Fourier Transform Infrared Spectroscopy (FTIR), leakage air tests, programmed temperature experiments, and small-scale coal pile fire extinguishing experiments. The results confirm its excellent fire suppression performance and reveal its mechanisms in inhibiting coal oxidation and spontaneous combustion at the microscopic level. The main conclusions are as follows:

• FTIR Experiments: The antioxidant gel foam exhibited the best suppression effect on the functional group content during low-temperature coal oxidation. Its antioxidants effectively eliminated free radicals and active oxygen in the coal oxidation system, reduced the formation of -COOH compounds, and significantly inhibited the free radical reaction process of coal spontaneous combustion.
• Leakage Air Test: The leakage blocking ability of antioxidant gel foam was superior to that of the original gel foam and sodium silicate gel foam. Under the same conditions, the negative pressure of the vacuum gauge for the sample treated with antioxidant gel foam reached −86.772 kPa, indicating its effective seepage into the coal particle gaps, isolating oxygen, and blocking the leakage air channels.
• Programmed Temperature Experiment: The antioxidant gel foam demonstrated the most significant suppression effect on coal spontaneous combustion. At 170 °C, the CO release rate for the coal sample treated with antioxidant gel foam was only 30.0 mmol/cm³·s, much lower than that of the original gel foam (84.1 mmol/cm³·s) and sodium silicate gel foam (52.8 mmol/cm³·s). At 100 °C, the suppression rate reached 80%, significantly delaying the coal oxidation process.
• Coal Pile Fire Extinguishing Experiment: The antioxidant gel foam exhibited excellent fire extinguishing and cooling performance. It remained on the coal surface for an extended period and quickly extinguished the fire. After 40 min of treatment, the cooling effect of antioxidant gel foam was significantly better than the other two types of foam, and within 40–90 min, the temperature continued to drop to 16.5 °C, completely extinguishing the fire, while the other foam-treated groups showed temperature rebounds.

In summary, the antioxidant gel foam shows excellent thermal stability and flame-retardant performance under high-temperature conditions. It effectively suppresses coal spontaneous combustion reactions, blocks leakage air channels, and rapidly extinguishes fire sources.

In future work, we plan to conduct thermochemical analyses, including TGA, DSC, and cone calorimetry, to better understand the gel’s heat release and decomposition behaviors. Additionally, aging tests under high-humidity environments will be carried out to evaluate long-term performance. In particular, temperature field variation measurements of the antioxidant gel foam after exposure to humid environments are recommended to assess thermal diffusion and retention capacity under moisture stress conditions. Field-scale applications of the gel system in real mine environments will also be explored.

These findings provide not only theoretical guidance but also practical support for advancing coal mine fire prevention strategies. The antioxidant gel foam offers a promising and sustainable solution to improve coal mine safety.