Study on Permeability and Flow Characteristics of Composite Thermosensitive Hydrogel and Its Fire Prevention and Extinguishment Performance

Abstract

This study focuses on the issues of poor fluidity, low penetration into residual coal, and suboptimal inhibition of coal spontaneous combustion associated with traditional coal mine gel fire retardants. The permeability and flow characteristics of a sodium alginate-based composite thermosensitive hydrogel, as well as its fire prevention and extinguishment performance, were investigated. The findings suggest that the thermosensitive hydrogel behaves as a pseudoplastic fluid at 40 °C and a yield-pseudoplastic fluid at 65 °C, exhibiting shear-thinning behavior with increasing shear rate. In a 1000 m pipeline transportation system, the hydrogel’s shear rate was 108.838 s^–1, and the pipeline pressure drop was 1.608 MPa, confirming its suitability for pipeline transport. The mass retention ratio of the hydrogel in coal samples was 23.94%, nearly twice that of water, indicating better retention in coal samples. The rate of hydroxyl consumption in coal samples subjected to the hydrogel treatment decreased by approximately 40 times. Upon heating, the CO generation rate of the hydrogel-treated coal remained lower than that of the untreated coal, while its activation energy was higher. Additionally, the hydrogel effectively reduced coal temperature rise and provided a sustained cooling effect on the oxidation process, significantly enhancing fire prevention and extinguishment efficiency.

1. Introduction

According to the International Energy Agency (IEA) report titled China’s Coal Demand, 2023 and 2035, China’s coal demand reached 350 million tons of standard coal in 2023. However, despite the potential reduction in coal consumption due to the industrialization of new energy sources as part of China’s energy transformation and upgrading, coal remains a widely used traditional energy source in China. In recent years, China’s annual coal production has surpassed 3.5 billion metric tons. With the depletion of shallow coal resources, coal mining is progressively moving to greater depths. Mines exceeding 1000 m are expected to emerge as the primary focus for future coal extraction. The growing depth of mining operations has resulted in a higher frequency of various accidents, with coal spontaneous combustion (CSC) being a significant potential hazard that impacts the safety of coal mine production. As of October 2021, among the 4113 operating coal mines in China, 2392 mines (58.2%) were identified as having the potential for CSC. The Coal Mine Fire Prevention and Extinguishing Regulations explicitly state that mines exploiting easily combustible coal seams must develop specialized fire prevention and extinguishing designs and implement comprehensive preventive measures. Therefore, efficient and environmentally friendly technologies for mitigating CSC continue to be a focal point of current research.

At present, technologies for preventing and controlling CSC primarily consist of grouting, spraying fire retardants, injecting inert gases, injecting foamed materials, and injecting gels. − Grouting utilizes the covering and encapsulating properties of the grout to achieve oxygen isolation. However, due to the poor water retention of the grout, it is prone to drying and cracking, thereby losing its fire-retardant effectiveness. Chemical fire retardants work by using chemical reagents to interrupt coal-oxygen free radical reactions, suppress the oxidation of active functional groups, and capture free radicals, thereby achieving the desired inhibitory effect. However, these retardants are costly and can cause environmental pollution. Inert gases are not suitable for long-term retention on the surface of coal bodies, which weakens their oxygen-isolating effect. Foamed materials have poor stability and a relatively short duration of effectiveness. These issues limit the effective application of the aforementioned technologies. In contrast, gel materials, due to their high heat capacity and good adhesion, have attracted significant attention in coal mine fire prevention and control, and have emerged as an effective method for preventing CSC.

Commonly used fire-extinguishing gel materials encompass inorganic silica gels, organic polymer gels, and composite gels. , However, the current application of gel materials also faces issues such as high costs, complex preparation conditions, and poor fluidity. To address these challenges, researchers have conducted extensive studies. Ma et al. developed a mine fire-resistant gel using sludge, polyacrylamide, water glass, and baking soda as primary raw materials, which demonstrated significant economic and environmental benefits. Ren et al. modified fly ash through acid–base combined activation and prepared a hydrogel with excellent flame-retardant properties using bentonite and reticular polyacrylamide as raw materials. Hu et al. combined calcium carbide slag with triethylenetetramine to prepare a hydrogel and fixed CO₂ through mineralization reactions to achieve long-term prevention of CSC. Ma et al. employed an acid–base combined approach to activate fly ash and created an active colloid featuring a three-dimensional network structure, in which fly ash solid particles were embedded as inert components within the framework. This not only encapsulated more water but also enhanced the colloid’s strength and its water retention capability. All the above studies utilized inorganic solid waste materials to prepare hydrogels, which are cost-effective and align with China’s carbon peak and carbon neutrality strategy. Yan et al. designed a novel sodium alginate/nonionic polysaccharide/inorganic borate cross-linker/calcium carbonate (SA/P/B/Ca) gel with a dual-gel network structure, offering enhanced stability. Huang et al. synthesized a double-network hydrogel by solution polymerization of hyaluronic acid, poly­(vinyl alcohol), and the nanomaterial palygorskite, which exhibited superior stability and water retention. Zhou et al. introduced divalent metal ions Fe³⁺ and Al³⁺ into the chain structure of sodium carboxymethyl cellulose solution through ionic cross-linking, thereby improving the hydrogel’s mechanical strength, high-temperature stability, and water retention capacity. In addition, some researchers have leveraged the swelling and adhesive characteristics of hydrogel materials for sealing and preventing leaks, thereby decreasing the exposure of coal to air, which helps to slow down the oxidation process and prevent spontaneous combustion. Xu et al. incorporated acrylamide into the interlayer of layered organic nanomontmorillonite and introduced sodium lignosulfonate as a carbon source to prepare an expansive nanohydrogel, enhancing its stability and swelling performance at high temperatures. Li et al. prepared a hydrogel using polyacrylamide, polydopamine, sodium lignosulfonate, and carbon nanofibers. The interaction between polyacrylamide and polydopamine created a cross-linked polymer structure, enhancing the material’s ability to adhere to coal surfaces and effectively isolate oxygen. Shi et al. incorporated tannic acid into the hydrogel to boost the polymer system’s adhesive properties, allowing the hydrogel to firmly bond with coal surfaces and thereby enhance material effectiveness and sealing performance.

The aforementioned studies have all enhanced the performance of gel materials in various ways and advanced the evolution of gel materials for mine fire control. However, research on the flow characteristics of gel materials is still limited. In the context of coal mine fire control, gel materials need to have good fluidity to be transported through pipelines to the fire source and penetrate deep into the coal seams for fire blocking. On the other hand, they must also effectively adhere to the coal surface to achieve cooling, fire-retardant, and oxygen-isolating fire-extinguishing functions. Therefore, research on the flow characteristics of gel materials is essential. Thermosensitive hydrogels possess both fluidity and adhesion and are functional hydrogels that can undergo sol–gel phase transitions controlled by temperature. Specifically, they maintain good fluidity at low temperatures and solidify on the coal surface at high temperatures to achieve fire prevention and extinguishing objectives. Wang et al. prepared a gas-filled thermosensitive hydrogel using the hydrophilic and hydrophobic groups within hydroxypropyl methylcellulose, the expansion properties of expandable microspheres, and chitosan, which exhibited good fluidity at a viscosity of 50 mPa·s. Our team previously developed an AB-type thermosensitive hydrogel, where the A component includes poly­(vinyl alcohol), methyl cellulose, sodium carboxymethyl cellulose, and guar gum. This gel has strong fluidity at low temperatures, facilitating transportation through pipelines. As the temperature rises, it gradually undergoes phase transition to form a high-viscosity solid material, with a viscosity approximately 11 times that at room temperature. Although the aforementioned studies have analyzed the fluid characteristics of thermosensitive hydrogels, they lack theoretical design for the pipelines used to transport hydrogels. In light of this, this paper further investigates the permeability and flow characteristics of a sodium alginate-based composite thermosensitive hydrogel. We have designed and calculated the relevant parameters for pipeline transportation and compared and analyzed the fire-retardant and fire-extinguishing performance of hydrogels and water on CSC. This study offers theoretical insights and practical foundations for utilizing composite thermosensitive hydrogel materials in the context of coal mine fire control.

2. Experimental Section

2.1. Materials

The substances needed to formulate hydrogels encompass methyl cellulose (MC, analytical grade, Aladdin), hydroxypropyl methylcellulose (HPMC, analytical grade, Aladdin), sodium alginate (SA, food grade, sourced from Qingdao Mingyue Seaweed Group Co., Ltd.), and guar gum (GG, food grade, from Qingzhou Rongmeier Biotechnology Co., Ltd.).

2.2. Preparation of Thermosensitive Hydrogel

Initially, precise quantities of MC and HPMC were measured and individually dissolved in deionized water. The solutions were stirred at 60 °C using an electric stirrer at a speed of 300 r/min for 30 min. The two solutions were then sealed and left undisturbed for 12 h to eliminate foam, resulting in MC and HPMC solutions with mass fractions of 5% each. Subsequently, appropriate amounts of SA and GG were weighed and dissolved in deionized water, agitated for 30 min in a constant-temperature water bath maintained at 25 °C, to obtain SA and GG solutions with mass fractions of 2% each. The SA solution was then physically blended with the GG solution at a mass ratio of 9:1 to produce a modified SA solution. Finally, the MC solution, HPMC solution, modified SA solution, and deionized water were mixed together and stirred magnetically at 25 °C for 30 min until the solution was homogeneous. The mixture was left undisturbed for 2 h to allow the hydrogel to fully cross-link, yielding a thermosensitive hydrogel with a composition of 1.25 wt % MC, 1.25 wt % HPMC, and 0.2 wt % modified SA. Figure illustrates the synthesis of the hydrogel.

1.

Illustration of Hydrogel Synthesis.

2.3. Rheological Property Tests

The thermosensitive hydrogel prepared in this study exhibits a phase transition temperature of 60 °C. The rheological characteristics of the hydrogel were assessed at 40 and 65 °C, corresponding to states before and after the phase transition, utilizing a rotational rheometer. The shear rate was adjusted from 0.1 to 1000 s^–1 to determine the hydrogel’s viscosity and shear stress.

2.4. Permeability Tests

The permeability of the thermosensitive hydrogel through coal samples was assessed using a permeability testing apparatus, as shown in Figure . A plexiglass tube, featuring an outer diameter of 50 mm, an inner diameter of 46 mm, and a length of 300 mm, was vertically mounted. Coal lumps with particle sizes between 10 and 15 mm were packed into the tube to mimic abandoned coal in a goaf. A volume of 200 g of hydrogel or deionized water was injected into the top of the tube. The duration it took for the solution to travel from the top to the bottom of the tube was measured, and the mass of the solution in a beaker was continuously monitored using an electronic balance until the reading stabilized.

2.

Flowchart of Hydrogel Permeability Test.

2.5. Fire-Retardant and Extinguishing Performance Tests

2.5.1. Surface Functional Group Analysis

Twenty-five grams of coal sample, which had been sifted through a 200-mesh sieve, were mixed uniformly with either 25 g of water or 25 g of thermosensitive hydrogel. The mixtures were subsequently put into a vacuum drying oven at 40 °C for 72 h to obtain the untreated coal sample and the hydrogel-treated coal sample at ambient temperature. Following the same procedure, coal samples were also prepared at 70 °C to obtain the untreated and hydrogel-treated coal samples at 70 °C. After drying, all four groups of coal samples were ground and passed through a 200-mesh sieve again. The infrared spectra of the samples were examined with a Fourier-transform infrared spectrometer (Bruker, VERTEX80 V, Germany). The spectral scan covered a range from 4000 to 350 cm^–1, with a resolution set at 0.4 cm^–1 and 64 scans performed.

2.5.2. Fire-Retardant Performance Experiment

Coal samples with a particle size ranging from 100 to 200 mesh were dried in a vacuum drying oven at 30 °C for 48 h and then kept for subsequent use. The experimental group consisted of coal samples obtained by uniformly mixing 10 g of dried coal that had not been treated with hydrogel and 5 g of thermosensitive hydrogel, while the control group consisted of 10 g of dried coal that had not been treated with hydrogel. The experimental setup for the fire-retardant performance test is shown in Figure . The coal samples were positioned in a designated container, with an airflow rate maintained at 80 mL/min, a heating rate of 1 °C/min, and a temperature range spanning from 30 to 300 °C. Samples were taken every time the coal sample temperature rose by 10 °C and were analyzed for gas components using a gas chromatograph (Beijing Dongxi Analytical Instruments Co., Ltd., GC-4000A).

3.

Schematic Diagram of Inhibition Test Apparatus.

2.5.3. Cooling Performance Test

The cooling performance test setup is depicted in Figure . A ceramic crucible, featuring an inner diameter of 80 mm and a height of 120 mm, was positioned on an intelligent temperature-controlled heating plate (Bangyuan-BY1515). Inside the crucible, experimental coal lumps were placed, with a 10 mm ventilation hole at the bottom. K-type thermocouples were positioned to monitor the temperature of the coal lumps, with the temperature data being collected via a data acquisition module (Jing Simai Ke Hua Technology Co., Ltd., M2101). The cooling performance test consisted of two groups:

• (1)50 mm Coal Layer Cooling Test: Coal lumps sized 10–15 mm were chosen and arranged into a 50 mm thick layer inside the crucible, with a thermocouple positioned atop the coal layer. The temperature-controlled heating plate was initially set to 300 °C to heat the coal layer in the crucible until the temperature reading of the thermocouple above the coal layer reached 80 °C. The heating plate was then turned off, and the crucible was removed. Next, 100 g of thermosensitive hydrogel was added to the crucible, and the duration for the temperature above the coal layer to decrease to 30 °C was measured. Continuous monitoring was conducted, and when the temperature above the coal layer dropped to 25 °C, the crucible was placed back on the heating plate set at 300 °C, and the time it took for the coal layer temperature to reach 100 °C was recorded. The same procedure was followed for the cooling test using 100 g of water for comparison.
• (2)100 mm Coal Layer Cooling Test: Following the procedure in (1), a 100 mm thick coal layer was arranged inside the crucible, with thermocouples positioned both on top of and within the middle of the coal layer. When the temperature reading of the thermocouple in the middle of the coal layer reached 80 °C, 200 g of hydrogel was poured into the crucible, and the readings of the two thermocouples were recorded. The same procedure was followed for the cooling test using 200 g of water for comparison.

4.

Schematic Diagram of Cooling Test Apparatus.

2.5.4. Fire-Extinguishing Performance Evaluation

The effectiveness of the thermosensitive hydrogel in extinguishing fires was assessed using a combustion test setup, depicted in Figure . The combustion chamber is a metal cylinder measuring 160 mm in diameter and 270 mm in height. A ventilation opening, 30 mm in diameter, is situated on the side 50 mm above the base, with a metal mesh positioned 70 mm above the base. For the experiment, coal lumps sized between 10 and 15 mm were chosen. The coal lumps were ignited using an alcohol block and then placed on the metal screen inside the combustion furnace. Additional coal lumps were added until the combustion furnace was filled. K-type thermocouples were positioned at the base and the upper surface of the coal lumps. When the coal lumps reached a certain temperature (approximately 900 °C) and began to burn, 1500 g of hydrogel was injected into the combustion furnace from the top, and the temperature changes of the coal body were recorded.

5.

Schematic Diagram of Combustion Test Apparatus.

3. Results and Discussion

3.1. Rheological Properties

The viscosity of the thermosensitive hydrogel at different temperatures as a function of shear rate is shown in Figure . As depicted in Figure , the hydrogel’s viscosity diminishes as the shear rate rises at both 40 and 65 °C, demonstrating the shear-thinning behavior typical of pseudoplastic fluids.

6.

Hydrogel Viscosity vs Shear Rate Curve.

At 40 °C the thermosensitive hydrogel has not yet undergone a phase transition and remains in a sol state with relatively low viscosity. When subjected to external shear forces, the van der Waals forces within the sol gradually weaken, which results in a gradual reduction in the apparent viscosity of the hydrogel solution. At 65 °C the hydrogel undergoes a phase transition from a liquid state to a gel state, resulting in a sharp increase in viscosity. When subjected to external shear forces, the viscosity begins to decrease. Under low shear rate conditions (<2 s^–1), the hydrogel’s viscosity rapidly diminishes as the shear rate increases. Under medium and high shear rate conditions (2∼1000 s^–1), the rate of viscosity decrease becomes more gradual. This is due to the alteration of the molecular chain network structure of the hydrogel phase under the influence of external shear forces, which reduces the degree of cross-linking. The large hydrogel molecules gradually align in the direction of shear, reducing flow resistance and causing varying degrees of reduction in the hydrogel’s apparent viscosity. This shear-thinning property can be leveraged in coal mine fire prevention and extinguishing applications by increasing the shear rate during pumping to lower the hydrogel’s viscosity, thereby reducing its flow resistance.

Figure illustrates the correlation between shear stress and shear rate of the hydrogel at 40 and 65 °C. As the shear rate increases, the shear stress shows a clear upward trend under both temperature conditions. At the same shear rate, the shear stress of the hydrogel at 40 °C is significantly higher than that at 65 °C.

7.

Hydrogel Shear Stress vs Shear Rate Curve.

Based on the trends observed in the curves, the Herschel-Bulkley (H–B) rheological eq (eq ) was employed for curve fitting, with the fitting outcomes presented in Figure .

$$\tau ={\tau }_0+K·{\gamma }^n$$ 1

8.

Fitted Curves of Thermosensitive Hydrogel under Different Temperature Conditions.

In the equation, τ represents the shear stress (Pa), τ₀ is the yield stress (Pa), K is the consistency coefficient (N·s ⁿ /m²), γ represents the shear rate (s^–1), and n is the flow behavior index, with 0 < n < 1.

The fitting curves in Figure have correlation coefficients R ² exceeding 0.99, indicating an excellent fit. This suggests that the H–B rheological equation can effectively describe the fluid characteristics of the thermosensitive hydrogel. In Figure a, the hydrogel at 40 °C does not exhibit a yield stress and behaves as a pseudoplastic fluid within non-Newtonian fluids; whereas at 65 °C (Figure b), the hydrogel shows a yield stress value of 1.37732 Pa, exhibiting characteristics of a yield-pseudoplastic fluid. Based on the hydrogel’s rheological properties, the corresponding pipeline transportation flow characteristic parameters can be further calculated.

In coal mine operations, the hydrogel is primarily conveyed to the underground fire prevention and extinguishing areas through grouting pipelines. During transportation, the friction between the hydrogel solution and the pipeline wall cannot be ignored. To streamline the problem and aid in theoretical analysis, this study employs the following assumptions: (1) The hydrogel solution is treated as an incompressible, homogeneous, and isotropic fluid. (2) Upon entering a circular pipe, the hydrogel solution is presumed to have a uniform velocity distribution at the inlet interface. (3) After the solution passes through the inlet section, it is assumed that the flow enters a steady-state flow condition. The pipeline transportation flow characteristic parameters are calculated according to eqs to .

For the hydrogel solution at 40 °C, which is a pseudoplastic fluid, the flow pressure drop in the conveying pipeline is calculated using eq :

$$\mathrm{\Delta }P=\frac{2\rho U^2\mathrm{fL}}{D}$$ 2

In the equation, ΔP represents the pressure drop in the pipeline, measured in MPa; ρ indicates the density of the fluid, measured in kg/m³; U is the velocity of the fluid, measured in m/s; f is the Fanning friction factor, which is given by f = 16/Re, where Re is the modified Reynolds number calculated using eq ; L is the length of the pipeline, measured in meters; D is the inner diameter of the pipeline, measured in meters.

$$\mathit{Re}=\frac{\rho U^{2-n}{D}^n}{K^{\prime }{8}^{n-1}}$$ 3

In the equation, n denotes the flow behavior index, while K′ represents the modified consistency coefficient, which adheres to eq .

$$K^{\prime }=K{\left(\frac{1+3n}{4n}\right)}^n$$ 4

The shear rateγ­(in units of s^–1) experienced by the thermosensitive hydrogel on the pipeline wall is given by

$$\gamma =\frac{8U}{D}·\frac{1+3n}{4n}$$ 5

The hydrogel solution was set to flow at a rate of 30 m³/h through a 1000 m long DN100 steel pipe with an inner diameter of 95 mm. Based on the values obtained from the constitutive equation fitting and relevant calculations, it was determined that in a 1000 m long pipeline, the shear rate of the hydrogel was 108.838 s^–1, and the pipeline pressure drop was 1.608 MPa. The relatively low pressure drop indicates that under the current flow rate, the hydrogel solution has a high flow efficiency and low energy loss in the pipeline, making it suitable for pipeline transportation.

3.2. Permeability Characteristics

Permeability is a key property of thermosensitive hydrogel fluids for combating fires. If the permeability is too high, the hydrogel solution cannot effectively retain on the coal surface and its fractures, thereby failing to effectively isolate oxygen. Conversely, if the permeability is too low, the slow permeation rate will prevent the hydrogel from penetrating into the coal fractures and spontaneous combustion zones in a timely and sufficient manner, also failing to achieve an effective fire control function. The formula for calculating the permeability of the hydrogel solution is given in eq :

$$K=\frac{m_1}{m_0}\times \mathrm{100\%}$$ 6

In the equation, K represents the permeability of the solution, in units of %; m ₁ is the mass of the solution that flows into the beaker, in units of g; m ₀ is the total mass of the solution injected into the coal sample, in units of g.

Figure shows the experimental results of the permeability of thermosensitive hydrogel and deionized water.

9.

Curve of hydrogel permeability versus time.

Due to the low viscosity of water, it quickly flows through the entire coal sample and into the beaker after being poured onto the coal sample’s top. In contrast, the thermosensitive hydrogel does not begin to flow out from the bottom of the coal sample until 340 s. This demonstrates that water permeates through the coal sample too rapidly, while the hydrogel moves through the coal sample at a much slower rate. After 184 s, the weight of water in the beaker stabilizes at 175.18 g, corresponding to a permeability of 87.59% for water, with 12.07% of the water mass retained within the coal sample. In comparison, the weight of the hydrogel in the beaker stabilizes after 3900 s, at which point the permeability of the hydrogel is 76.07%, with 23.94% of the hydrogel mass retained in the coal sample. These results demonstrate that although the permeability of the thermosensitive hydrogel is lower than that of water, it can effectively penetrate and be retained in the coal sample, achieving the goal of effectively encapsulating the coal and isolating it from oxygen. This characteristic grants the thermosensitive hydrogel considerable value for application in coal mine fire control.

3.3. Fire Prevention and Extinguishing Performance

3.3.1. Influence of Thermosensitive Hydrogel on Coal Functional Groups

The infrared spectra for the four samples are depicted in Figure . The stretching vibration of the hydroxyl group (−OH) occurs within the range of 3700 to 3200 cm^–1. the stretching vibrations of the methyl group (−CH₃) and methylene group (−CH₂) in aliphatic hydrocarbons correspond to the regions near 2950 and 2850 cm^–1, respectively, while the deformation vibration of −CH₃ is in the range of 1437∼1419 cm^–1. The infrared spectra indicate that the peak positions of the samples at room temperature and 70 °C do not show significant changes, but there are differences in the intensities of the absorption peaks. This indicates that incorporating hydrogel at both temperatures does not change the types of functional groups in the coal samples but only influences their concentrations.

10.

Infrared spectra of coal samples before and after hydrogel treatment at room temperature and 70 °C.

The infrared spectra were deconvoluted and fitted using the second derivative method via the OMNIC software. The functional groups in the main wavenumber regions that exhibited significant differences between the hydrogel-treated coal samples and the original coal samples were compared, as shown in Figure . Based on the characteristic changes of the main functional groups, the influence of the hydrogel on the spontaneous combustion tendency of coal can be analyzed.

11.

Infrared spectra fitting of the four samples.

By examining the positions and areas of the deconvoluted peaks in the infrared spectra, the actual areas of the functional groups can be ascertained. This allows for the quantification of the contents of aliphatic hydrocarbons and hydroxyl groups in the coal samples, as shown in Table .

1. Fitted Peak Area Values of the Main Functional Groups in the Four Coal Samples.

samples	aiphatic hydrocarbons	hydroxyl groups
room-temperature raw coal sample	512.091	620.229
70 °C raw coal sample	480.528	497.099
room-temperature hydrogel-treated coal sample	476.130	541.448
70 °C hydrogel-treated coal sample	469.902	538.526

During the initial oxidation phase of coal at low temperatures, aliphatic hydrocarbons are the primary functional groups that release heat during coal oxidation. This is attributed to the high reactivity of the aliphatic hydrocarbon side chains, which are involved throughout the oxidation of coal. As indicated in Table , the content of aliphatic hydrocarbons in all samples diminishes as the temperature rises. For the raw coal sample, the content of aliphatic hydrocarbons at 70 °C is 6.16% lower than that at room temperature. In contrast, for the hydrogel-treated coal sample, the content of aliphatic hydrocarbons at 70 °C is only 1.31% lower than that at room temperature. The lower consumption rate of the functional group in the hydrogel-treated coal sample indicates that the addition of hydrogel can reduce the reactivity of aliphatic hydrocarbons and inhibit their low-temperature oxidation reactions.

The hydroxyl content of all coal samples decreased with increasing temperature. However, the rate of hydroxyl group consumption in the hydrogel-treated coal samples was lower than in the untreated coal samples, indicating that the hydrogel reduced the oxidative reactivity of the coal samples to some extent and played a role in delaying the oxidation process of coal.

3.3.2. Inhibitory Effect of Thermosensitive Hydrogel on CO Generation

During the oxidation and temperature rise process of coal self-ignition, characteristic gases such as CO and C₂H₄ are produced. Among them, CO is released in large quantities and can be detected over a wide temperature range. It can continuously reflect the oxidation and temperature rise process of coal and is a primary indicator of coal self-ignition. The variations in CO gas for raw coal samples and thermosensitive hydrogel-treated coal samples are depicted in Figure .

12.

Variation curves of CO concentration with temperature before and after hydrogel treatment.

As shown in Figure , CO is present throughout the oxidation and temperature rise process, and its generation increases gradually with temperature. When the temperature is below 100 °C, both samples produce low concentrations of CO, with the hydrogel-treated coal sample releasing slightly less CO than the raw coal sample. After exceeding 100 °C, the CO generation from the untreated coal sample increases exponentially, while for the hydrogel-treated coal sample, the CO generation only begins to increase rapidly after reaching 150 °C. At 220 °C, the CO concentration in the untreated coal sample is 8593 ppm, whereas the hydrogel-treated coal sample produces a CO concentration of 6279 ppm. Starting from 230 °C, the CO generation rate slows down for both samples, and the hydrogel-treated coal sample consistently generates CO at a lower rate than the raw coal sample. It can be observed that throughout the entire programmed temperature rise process, the hydrogel-treated coal sample produces less CO gas than the raw coal sample, suggesting that the hydrogel exerts a certain inhibitory influence on the coal oxidation process. The inhibition efficiency E of the hydrogel can be determined based on CO generation, using the following equation:

$$E=\frac{X-Y}{X}\times 100\%$$ 7

where E is the inhibition efficiency (%), X is the CO concentration (ppm) produced by the raw coal sample at temperature T, and Y is the CO concentration (ppm) produced by the hydrogel-treated coal sample at the same temperature T.

The results of the inhibition efficiency of the thermosensitive hydrogel are shown in Figure . A higher inhibition efficiency indicates a better inhibitory effect. As depicted in the figure, the inhibition efficiency of the hydrogel remains at 100% when the temperature is below 90 °C. Starting from 90 °C, CO begins to be generated in the hydrogel-treated coal samples, and the inhibition efficiency fluctuates between 30 and 60%. The highest inhibition efficiency of 58.14% is achieved at 130 °C. After 220 °C, the inhibition efficiency of the hydrogel decreases and stabilizes around 25%. Overall, the hydrogel demonstrates the most effective inhibition on coal samples below 200 °C, with an inhibition efficiency consistently higher than 30%.

13.

Inhibition efficiency of the hydrogel.

3.3.3. Effect of Thermosensitive Hydrogel on Apparent Activation Energy

The apparent activation energy influences the rate of coal oxidation reactions. Based on the Arrhenius equation, the oxidation reaction rate of coal at any given temperature can be calculated:

$$v(T_i)=v({\mathrm{O}}_2)=\frac{v(\mathrm{C}\mathrm{O})}{m}=\frac{v(\mathrm{C}{\mathrm{O}}_2)}{g}=A{c}_{{\mathrm{O}}_2}^{n_{\mathrm{r}}}\exp (-\frac{E}{R{T}_i})$$ 8

where v represents the reaction rate of the coal sample (mol·m^–3·s^–1), T_i denotes the thermodynamic temperature (K), A is the pre-exponential factor (s^–1), n _r is the reaction order, R is the universal gas constant (8.314 J/(mol·K)), and E is the apparent activation energy (J/mol).

The airflow in the coal sample container moves axially from the inlet to the outlet. The container is heated uniformly, resulting in a consistent temperature distribution. The CO generation rate at any axial position dx can be calculated using the following equation:

$$S·v(\mathit{CO})\mathrm{d}x={\mathit{kv}}_{\mathrm{g}}\mathrm{d}c$$ 9

where S represents the cross-sectional area of the coal sample container (m²), v­(CO) is the rate of CO generation (mol·m^–3·s^–1), k is the conversion factor for units (22.4 × 10⁹ mol/m⁴), and v _g denotes the gas flow velocity (m/s).

During the experiment, if the air supply is maintained at a constant rate, the oxygen concentration at the inlet remains unchanged. Substituting eq into eq yields

$$\mathit{AS}{\mathrm{mc}}_{{\mathrm{O}}_2}^{n_{\mathrm{r}}}\exp (-\frac{E}{R{T}_i})\mathrm{d}x={\mathit{kv}}_{\mathrm{g}}\mathrm{d}c$$ 10

Integrating the above equation gives

$${\int }_0^1\frac{\mathit{ASL}{\mathrm{mc}}_{{\mathrm{O}}_2}^{n_{\mathrm{r}}}\exp (-\frac{E}{R{T}_i})\mathrm{d}x={\int }_0^C{\mathit{kv}}_{\mathrm{g}}}{d}c$$ 11

Taking the logarithm of both sides results in

$$\ln C_{\mathrm{co}}=-\frac{E}{R{T}_i}+\ln \left(\frac{\mathit{ASL}{\mathrm{mc}}_{{\mathrm{O}}_2}^{n_{\mathrm{r}}}}{{\mathit{kv}}_{\mathrm{g}}}\right)$$ 12

From eq , it can be seen that there is a linear relationship between ln C _CO and 1/T_i . The apparent activation energy E can be determined by plotting ln C _CO against 1/T_i using the experimentally obtained CO concentrations, fitting the curve, and determining the slope −E/R.

Figure presents the curves of ln C _CO versus 1/T_i for untreated coal and hydrogel-modified coal samples within the temperature range from 90 to 300 °C. All curves in the figure exhibit correlation coefficients exceeding 0.9, suggesting that the fitted curves accurately reflect the activation energy of the coal samples across various stages.

14.

Curves of ln C _CO versus 1/T_i for untreated coal samples and hydrogel-modified coal samples in the temperature range of 90∼300 °C.

As depicted in Figure , the coal oxidation process during the temperature increase can be categorized into three distinct phases: 90∼120, 130∼210, and 220∼300 °C. The apparent activation energies for each stage are presented in Table .

2. Apparent Activation Energies (kJ/mol) of Coal Samples and Hydrogel-treated Coal Samples.

temperature range (°C)	raw coal sample	hydrogel-treated coal sample
90∼120	3.65	3.67
130∼210	6.96	8.06
220∼300	2.50	2.50

According to Table , within the temperature interval from 130 to 210 °C, the activation energy for the hydrogel-modified coal sample exceeds that of the untreated coal sample by 1.10 kJ/mol. This suggests a comparatively diminished reactivity of the hydrogel-modified coal, indicating a lower propensity for spontaneous combustion in the hydrogel-treated coal.

In the temperature range from 90 to 120 °C, the oxidation reactions of both coal samples are relatively mild. When the temperature is between 220 and 300 °C, most of the hydrogel has decomposed, and its inhibitory effect on the coal sample is limited. Therefore, the difference in activation energy between the two coal samples is not significant in these two stages.

3.3.4. Cooling Performance of Thermosensitive Hydrogel

The cooling performance of thermosensitive hydrogel on a 50 mm thick coal layer is shown in Figure .

15.

Comparison of cooling effects of water and thermosensitive hydrogel on a 50 mm thick coal layer.

In Figure , the moment at 0 s is when the heat source is immediately removed after the temperature above the coal layer reaches 80 °C, and cooling is initiated using either water or hydrogel. As depicted in Figure , when water is employed for cooling, the temperature at the coal layer’s surface immediately jumps to 94.9 °C upon contact with water. This phenomenon occurs because water rapidly evaporates upon contact, causing a temporary spike in the coal layer’s surface temperature before it starts to decline.The duration needed for the coal layer’s temperature to drop from 80 to 30 °C is 1268 s. When hydrogel is used for cooling, the surface temperature of the coal layer drops sharply upon contact with the hydrogel. This is because the hydrogel undergoes a rapid phase change upon contact with the hot coal layer, filling the surface pores of the coal and making it difficult for the subsequently poured hydrogel to infiltrate. Consequently, a substantial quantity of hydrogel interacts with the coal layer’s surface, resulting in a swift reduction in surface temperature. Following this, the temperature at the coal layer’s surface oscillates around 50 °C and progressively declines to 30 °C. The duration for the temperature to fall from 80 to 30 °C is 1596 s, which is 328 s longer than when water is used. This indicates that the hydrogel has a high cooling efficiency for high-temperature coal bodies during this stage, although the cooling rate is slower in the low-temperature range (below approximately 50 °C).

When the temperature above the coal layer drops to 25 °C, the heat source is added again (as shown in Figure ). The cooling time for the hydrogel-treated coal sample is longer, and the time for adding the heat source is delayed. After the heat source is added again, the temperature of the coal layer cooled by water and hydrogel both go through three phases: an initial increase, a subsequent decline, and a final increase. For the coal layer treated with hydrogel, each phase lasts longer compared to the coal layer cooled by water. The time needed for the hydrogel-treated coal layer to rise from 25 to 100 °C is 1234 s, which is 87% longer than that for water.

When heating the coal layer that has been cooled by water, the initial stage consists of the vaporization of water attached to the coal layer, which leads to an increase in surface temperature. This stage lasts for 111 s. The second stage occurs after the water vapor evaporates and takes away some heat, causing the coal layer temperature to drop from 85.6 to 63.4 °C. This stage lasts for 261 s. The third stage features another increase in the temperature at the coal layer’s surface, with a heating rate comparable to that observed during the initial heating process. This indicates that the water adhering to the coal layer has completely evaporated, and the heat source is now directly acting on the coal layer itself.

When heating the coal layer cooled by hydrogel, the first stage involves the phase change of the hydrogel adhering to the coal block, releasing water vapor that moves upward and causes the surface temperature to rise. However, since the latent heat of the hydrogel is higher than that of water and a greater proportion of hydrogel remains on the coal block surface, the time needed for the surface temperature to increase is 225 s longer than that for the water-cooled coal layer. The second stage involves a temperature drop from 82.6 to 75 °C, lasting for 339 s. This stage is still longer than that for the water-cooled coal layer, but the temperature decrease is smaller. This is because the hydrogel has better water retention, and the limited amount of released water vapor takes away less heat. The third stage involves the simultaneous heating of the coal block and the hydrogel adhering to its surface, but at a slower rate. Overall, since more hydrogel remains in the coal layer after infiltration compared to water, it can prolong the heating rate of the coal body and thus inhibit CSC.

After the cooling experiment, the surfaces of the coal layers treated with water and hydrogel are shown in Figure . When water is used for cooling, almost no water remains on the surface of the coal layer. In contrast, some hydrogel remains in the gaps of the coal layer. This occurs because the high-temperature phase change of the hydrogel inhibits further infiltration, leading to visible hydrogel residue on the coal layer’s surface that continues to provide cooling effects.

16.

Morphology of the coal layer surface after cooling with water and hydrogel.

The cooling experiment results of thermosensitive hydrogel on a 100 mm thick coal layer are shown in Figure .

17.

Cooling effect of water and hydrogel on a 100 mm thick coal layer.

As shown in Figure , when the depth of the coal layer increases, the cooling effects of water and hydrogel on the coal layer differ in the initial stage. At the instant of pouring the hydrogel, the temperature decrease in the coal layer remains notably greater compared to the water solution. Throughout the entire process, both the surface temperature and the temperature at the midpoint of the hydrogel-treated coal layer are generally lower than those of the water-treated coal layer. Due to its good fluidity and low viscosity, water uniformly cools the entire coal layer without significant differences based on location. In the first stage of cooling with water, the water solution flows through the entire coal body, causing a general downward trend in both the surface and middle temperatures of the coal layer. At the end of this stage, the surface temperature of the coal layer drops to 39.8 °C, and the middle temperature drops to 51.2 °C. However, the movement of water vapor still causes some temperature fluctuations in the coal layer. In the second stage, the water vapor remaining in the coal layer gaps moves upward from the bottom, causing a temporary increase in the middle and surface temperatures of the coal layer. The surface temperature rises to 48.8 °C, and the middle temperature rises to 54 °C. Compared to a single-layer coal layer, the increased thickness of the coal layer and the increased amount of water solution poured in result in more water retained in the coal layer, making the effect of water vapor on the coal temperature more pronounced. In the third stage, both the middle and surface temperatures of the coal layer decrease.

The cooling impact of hydrogel on the coal layer varies between the surface and the interior of the layer. On the surface, the effect of hydrogel is consistent with the cooling effect observed in the single-layer coal layer, with an initial temperature drop followed by a temperature rebound.For the middle of the coal layer, the increased thickness results in a longer duration for the hydrogel to traverse the coal layer, and the middle section is more influenced by the lower coal layer. When the top section of the coal layer rapidly cools due to the large amount of hydrogel passing through, the water vapor generated from the phase change of the hydrogel in the middle and lower sections of the coal layer causes the middle temperature to continue rising for a period of time. This continues until the coal layer temperature drops to the same level as the hydrogel temperature, after which both decrease to room temperature together. For the double-layer coal layer, the surface and middle temperatures of the coal layer treated with hydrogel are mostly lower than those treated with water at most times. Therefore, it can be concluded that the cooling effect of hydrogel on thicker coal layers is superior to that of water. This is because hydrogel has a higher viscosity and a greater retention ratio than water, allowing the same mass of hydrogel to more effectively act on the coal layer, resulting in a more pronounced cooling effect.

3.3.5. Performance Analysis of Thermosensitive Hydrogel for Fire Suppression

The fire-extinguishing performance of the hydrogel solution is depicted in Figure , while the temperature change curve for the coal layer is illustrated in Figure .

18.

Combustion state of coal pile and fire-extinguishing effect of hydrogel.

19.

Variation curve of temperature with time during hydrogel fire-extinguishing process.

When the coal block after fire-extinguishing (c) in Figure is cut vertically, it can be observed that the hydrogel fills the gaps between coal blocks and adheres them together, effectively isolating the coal from oxygen, thereby accomplishing the fire suppression. As shown in Figure , when the hydrogel comes into contact with the hot surface of the coal, it undergoes a phase transition, absorbing heat and releasing some water in the process, which aids in cooling the coal. After the injection of hydrogel, the coal layer cools rapidly, with the upper temperature decreasing from 394.4 to 81.4 °C and the lower temperature decreasing from 937.7 to 98.2 °C. After 1200 s, the top temperature of the coal layer decreases to 27.4 °C, while the bottom temperature falls to 62.3 °C, with no upward trend in temperature, indicating that the hydrogel has an excellent cooling and fire-extinguishing effect.

4. Conclusions

• (1)The thermosensitive hydrogel exhibits pseudoplastic fluid characteristics at 40 °C and yield-pseudoplastic fluid characteristics at 65 °C. With increasing shear rate, the viscosity of the hydrogel decreases, showing a ″shear-thinning″ phenomenon. The shear rate of the hydrogel in a 1000 m pipeline is 108.838 s^–1, and the pipeline pressure drop is 1.608 MPa, indicating that the hydrogel has low resistance and energy loss during flow, rendering it appropriate for pipeline conveyance.
• (2)The permeability of the hydrogel is 76.06%, lower than that of water (87.93%), but the mass retention ratio on the coal surface is 23.94%, nearly twice that of water. This indicates that the hydrogel not only has good permeability but also bonds effectively with the coal surface, effectively isolating oxygen and inhibiting coal-oxygen contact.
• (3)In hydrogel-treated coal samples, the functional groups exhibit a lower consumption rate relative to those in untreated coal samples. The reactivity of aliphatic hydrocarbons is diminished, and the consumption of hydroxyl groups is curtailed. As the temperature increases, the hydrogel decelerates the oxidation process of the coal samples. The CO generation rate is lower than that observed in untreated coal samples. By augmenting the apparent activation energy of the coal samples, the hydrogel effectively mitigates the low-temperature oxidation process of coal.
• (4)The hydrogel’s high viscosity and superior water retention capabilities offer a prolonged cooling effect for high-temperature coal bodies. Moreover, the hydrogel remains in the coal body in greater quantities than water solutions. After reheating, the time required for the temperature to increase in the hydrogel-treated coal layer is 87% longer compared to the water-treated coal layer, resulting in a slower heating rate. This sustained cooling effect effectively enhances the fire prevention and suppression capabilities of the hydrogel for coal bodies, highlighting its benefits in coal mine fire control and extinguishing.

In summary, the sodium alginate-based composite thermosensitive hydrogel developed in this study demonstrates excellent fluidity, permeability, and cooling fire-extinguishing performance. These characteristics significantly enhance the fire-extinguishing effectiveness of the hydrogel. In practical applications, the hydrogel is transported through pipelines to the abandoned coal in the mined-out area. Its shear-thinning behavior and low flow resistance ensure that the hydrogel can smoothly reach the high-temperature coal seam surface without causing blockages in the pipelines. The moderate permeability not only guarantees that the hydrogel can slowly penetrate into the coal that requires cooling, achieving complete cooling effects, but also allows the hydrogel to penetrate deeply into the coal seam to effectively retard deep heat sources. In addition, as a thermosensitive hydrogel, it can undergo phase transitions upon heat absorption, sealing the pores of the coal layer and thus isolating coal from contact with oxygen, reducing the potential for oxidation. The cost of this hydrogel is also relatively low, with the raw materials having good biocompatibility and biodegradability. The synergistic effect of these characteristics makes the sodium alginate-based composite thermosensitive hydrogel promising as a new type of environmentally friendly and safe mine fire prevention and extinguishing material.

The data that support the findings of this study have been deposited in the Mendeley Data Repository and can be accessed at DOI: 10.17632/hkk3mfmfgg.1­(https://data.mendeley.com/preview/hkk3mfmfgg?a=992ff439-7dea-4807-af69-ef150181a45a).

The initial draft of the manuscript was composed by Y.Q.. Z.W., W.Z., and L.Z. provided valuable suggestions for the revision of the draft. S.J. determined the research direction, and Y.Z. performed material preparation and data collection.

The authors declare no competing financial interest.