Research and Application of CO₂ Fire Prevention Mechanism and Key Technologies in Mines: A Review

Abstract

Spontaneous coal combustion adversely affects coal mine safety and restricts safe, efficient, and green coal mining. Inert gas fire prevention and extinguishing technology is a widely used fire prevention and extinguishing method in coal mines. CO₂ is often used as the primary raw material for inert gas fire prevention and extinguishing, owing to its good inerting and cooling characteristics. However, the lack of data on the physical and chemical properties and fire extinguishing mechanism of CO₂ by field personnel has limited the efficient application of CO₂ in coal mine fire prevention and extinguishing. To explore the practical application effects of CO₂ fire prevention and extinguishing technology on coal mine sites, this paper summarised and analysed the research and development status of CO₂ fire prevention and extinguishing technology and expounded the physical and chemical properties, phase characteristics, and fire prevention and extinguishing mechanisms of CO₂. The CO₂ pipeline, CO₂ pipeline intelligent monitoring and control system, CO₂ inerting mechanism, and comprehensive gas fire prevention and extinguishing technology are summarised and discussed. This study provides a systematic theoretical basis for the field application of CO₂ fire prevention and extinguishing technology.

1. Introduction

Coal remains one of the most important fossil fuels used in power engineering, metallurgy, the chemical industry, and other fields. Coal accounts for 68.8% and 57.7% of primary energy production and consumption structures, respectively. Coal is expected to remain the primary energy source in China for a long time [1,2]. The spontaneous combustion of coal seams is China’s main coal seam mining. Spontaneous combustion fires account for 85–90% of mine fires [3,4]. Spontaneous coal combustion affects various countries, including Germany, the United States, Australia, South Africa, Poland, the Czech Republic, India, Pakistan, and Indonesia. This problem has long plagued the global coal industry. In Germany, the Ruhr industrial area suffers from approximately 10 mine fires caused by the annual spontaneous coal combustion. In India, spontaneous coal combustion fires account for 75% of all coalfield fires, and the fire situation in the Jharia Coalfield is particularly severe [5,6]. Spontaneous coal combustion disasters are prone to occur during coal mining, transportation, storage, and other production processes, resulting in a large waste of resources and a serious threat to the safety of workers [7,8]. Complete fire prevention and extinguishing technology systems have been developed [9]. Common prevention and control measures for spontaneous coal combustion mainly include grouting [10], pressure injection of inert gas [11], spraying inhibitor [12], and gel injection [13]. The key to inert gas fire prevention and extinguishing technology is controlling the oxygen level in the fire area [14,15]. Compared with other fire prevention and extinguishing technologies, this technology has the advantages of fast fire extinguishing speed, no pollution, and fast production recovery; therefore, it is a comprehensive fire prevention and extinguishing measure used in mines [16].

Inert gas is used in mine fire prevention and extinguishing because of its good performance, and the technical methods for fire prevention and extinguishing are relatively mature [17]. As an inert gas fire-extinguishing material, CO₂ has been widely used in various fire prevention and control [18]. In the prevention and control of mine fires, CO₂ can reach fire source locations that are difficult to reach using solid fire extinguishing materials such as mud or colloids. Simultaneously, CO₂ does not react with coal to produce flammable and explosive gases such as water at high temperatures. Compared with other fire prevention and extinguishing technologies, CO₂ has significant advantages in terms of cooling effect, asphyxiation, environmental protection, economy, and rapid production recovery [19]. Given the advantages of CO₂ fire prevention measures, CO₂ technology has been used in coal seam fire prevention and control in some mining areas worldwide and has achieved positive results.

This review summarises the history of CO₂ fire prevention and extinguishing technology development through a literature review and discusses the CO₂ fire prevention and extinguishing mechanisms, process parameters, prevention and control technology equipment, and on-site practical application effects. Based on the available data and existing problems, the development trends of CO₂ transmission pipelines, actual cooling effect monitoring, CO₂ inerting mechanisms, and composite gas fire prevention and extinguishing technology are summarised to provide a reference for on-site fire prevention and extinguishing work and theoretical research on fire prevention and extinguishing technology.

2. History of CO₂ Fire Prevention Technology Development

2.1. Sources of CO₂

By 2023, global CO₂ emissions will increase to 37.4 billion tons (1.1%), most originating from fossil fuel burning during industrial processes and transportation [20]. Thermal power plants and coal chemical enterprises are the primary fossil-fuel combustion sources. Attempts have been made to capture and purify CO₂ in the exhaust gas produced by combustion to provide high-purity CO₂. The comprehensive utilisation of CO₂ has a positive effect on the unification of environmental, economic, and social benefits [21]. The carbon cycle of a coal mine is illustrated in Figure 1.

2.2. History of CO₂ Fire Extinguishing Technology Development

CO₂ has a long history and wide application in fire prevention and extin0guishing. In the early 19th century, researchers began exploring the use of compressed gas for fire extinguishing. In 1816, British Captain Manby invented a barrel fire extinguisher, laying the foundation for developing fire extinguishers. Meanwhile, the French doctor Gallier innovated a portable chemical fire extinguisher. In 1905, the Russian Professor Laurent invented a foam-extinguishing agent, and, in 1909, Davidson in New York used CO₂ to squeeze carbon tetrachloride out and extinguish the fire. Since then, various small CO₂ fire extinguishers have been developed. For example, in 1929, the United States government promulgated the first CO₂ fire extinguishing system standard, marking the entry of CO₂ fire extinguishing agents for large-scale use. Later, Japan developed its first CO₂ fire extinguishing system in 1933. The application of CO₂ fire extinguishing systems in China began in the 1950s and was mainly used in the shipbuilding industry by the end of the 1970s. Currently, CO₂ gas fire-extinguishing technology is used in many fields. The history of CO₂ fire extinguishing technology development is shown in Figure 2.

2.3. Coal Mine CO₂ Fire Prevention Technology

CO₂ fire prevention and extinguishing technology is one of the main methods for mine inert gas fire prevention and extinguishing. Inert gases used in mine fire prevention and extinguishing include CO₂, N₂, and gases produced by combustion (such as furnace smoke) [23]. In 1851, the first use of inert gas to extinguish coal mine fires was furnace smoke to extinguish a large area of fire in the Clackmanan coal mine in Scotland. In the first half of the 20th century, inert gas fire prevention and extinguishing technology gradually became common in the United States, Poland, and other countries as the main measures for mine fire prevention and extinguishing [24]. In 1949, the Doubravave coal mine in the Czech Republic first used N₂ to extinguish fires [25]. In the following 10 years, N₂ was used to prevent and control mine fires in countries such as West Germany, the former Soviet Union, France, and Poland, and satisfactory fire prevention and extinguishing effects were achieved [26,27]. In the 1960s, Qi, an academician at the Chinese Academy of Engineering, attempted to inject furnace smoke into underground fire areas to control fires. This began research on inert gas fire prevention and extinguishing technology in China [28]. After 1980, China’s Chongqing Tianfu Mining Bureau and Liaoning Fushun Mining Bureau attempted to inject N₂ to prevent spontaneous coal seam combustion [29,30] and successfully applied it to the Fushun Longfeng Mine and Yanzhou Xinglongzhuang Mine and achieved a good fire extinguishing effect. Since 2000, N₂ injection fire prevention and extinguishing technology has gradually matured and is widely used in mines with spontaneous combustion risks worldwide. In addition, Poland developed a GAG system device in 1970, which has been applied to many coal mine fires and tests in Australia, the United States, and New Zealand, among other countries. Górnicze Aparaty Gaśnicze (GAG) uses military jet engines to produce high-humidity inert gases (a mixture of mainly CO₂, N₂, and water vapour) to reduce the oxygen concentration in the fire zone, thereby preventing and controlling mine fires [31,32]. The Queensland Mine Rescue Service (QMRS) and Commonwealth Scientific and Industrial Research Organisation (CSIRO) jointly developed a GAG device and applied it to mines, such as the Loveridge mine in West Virginia, the Pinnacle mine in West Virginia, the Loveridge mine in West Virginia, and the Pike River mine in New Zealand, and successfully extinguished mine fires [33,34,35]. In the late 1970s, China developed a fuel-inert gas fire-extinguishing technology that uses kerosene, removes oxygen by combustion, and produces inert gases, such as CO₂ and N_2, to extinguish the fire. Simultaneously, five specifications of DQ and YZD products were studied, DQ series has DQ-150, DQ-500 and DQ-1000 types, YZD series includes YZD-20/700 and YZD-20/5000 types, among which DQ series is mainly used in roadways and other disaster areas with small resistance. YZD series is mainly used in pressure injection fire prevention and extinguishing in goaf and other areas, field practices were carried out in the Xujiagou and Datong Dadougou coal mines in Henan Province, and a good fire extinguishing effect was achieved [36,37].

CO₂ mainly originates from industrial waste liquids and waste gas. It has the advantages of remarkable cooling and fire extinguishing effects, wide use range, short fire extinguishing cycle, safety and environmental protection, and low equipment purchase and maintenance cost [38]. Therefore, this fire extinguishing technology and equipment has good economic, social, and environmental benefits and long-term development prospects. In the 1890s, the Sandwell Park and Abram coal mines in the United Kingdom used CO₂ to extinguish underground fires [39]. The Westfield, Smith, and Brumaugh Bureaus of Mines used CO₂ to extinguish the fire that broke out at the Valier mine in the United States in 1949 and successfully extinguished the fire by injecting CO₂ gas and liquid CO₂ into the fire area [40]. In 1957, CO₂ in the form of dry ice in the Penokee mine successfully extinguished a mine fire. In 1959, a fire broke out at the Koehler mine in the United States, and liquid CO₂ and dry ice were used to extinguish and successfully control the fire [41]. As a large coal-producing country, China began using CO₂ gas fire prevention technology after 2000. In 2003, the Yanzhou Nantun Coal Mine transported CO₂ gas to the working face through an underground pipeline to conduct fire extinguishing work. In 2003, Tianzhu Coal Mine successfully injected CO₂ into 3214 working faces to extinguish the fire. Subsequently, working faces with the hidden dangers of spontaneous combustion gradually began to use CO₂ as an inert gas for fire prevention and control in the goaf [42]. With the in-depth study and many practical applications of inert gas fire prevention and extinguishing technology, the production equipment and processes of CO₂ fire prevention and extinguishing technology have made significant breakthroughs, laying a solid foundation for developing CO₂ fire prevention and extinguishing technology. At the beginning of the 21st century, with the development of liquid CO₂ transportation technology, large-flow pipeline transportation of liquid CO₂ began to appear. Recently, attempts have been made to prevent and extinguish mine CO₂ fires. The Qingshuiying, Yangchangwan, Dongrong No.2, and Huashan coal mines have achieved good results in CO₂ prevention and control of the spontaneous combustion of coal seams and goaves.

3. Physicochemical Properties of CO₂ and Fire Prevention Mechanism

3.1. Physicochemical Properties and Phase Characteristics of CO₂

3.1.1. Physical and Chemical Properties of CO₂

In nature, CO₂ accounts for 0.03–0.04% of atmospheric volume [43] and is primarily produced by the combustion of carbon-containing substances and animal metabolism. CO₂ is a colourless, slightly sour, chemically stable, inert gas at room temperature and atmospheric pressure. It does not react with other substances but can react with strong reducing agents at extremely high temperatures. In addition, from a combustion perspective, CO₂ itself cannot burn and does not support combustion; therefore, CO₂ is widely used as a fire-extinguishing agent in various fields [44]. The physical parameters of CO₂ are listed in

Table 1. CO₂ physical property parameters. Reprinted with permission from Ref. [45]. Copyright 2023, China University of Geosciences.

Parameter Name	Parameter Value
Relative molecular mass	44.01
Density	1.97 kg/m3
Relative density	1.53
Melting point (0.52 MPa)	−56.6 °C
Boiling point (0.1 MPa)	−78.5 °C
Heat of gasification	573.6 kJ/kg
Heat of melting	198.9 kJ/kg
Heat of Sublimation	151.6 kJ/kg
Specific volume (21.1 °C, l atm)	0.5457 m3/kg
Vapor pressure (20 °C)	7.17 MPa

.

3.1.2. Phase Characteristics of CO₂

CO₂ has three characteristic points: critical point (31.3 °C, 7.38 MPa), triple point (−56.6 °C, 0.52 MPa), and freezing point (−78.5 °C, 0.10 MPa). The three feature points divide CO₂ into four forms: gas, liquid, solid, and supercritical, at different pressures and temperatures, as shown in Figure 3. The line between the triple and critical points is the CO₂ saturation line. The three states of CO₂—liquid, gas, and solid—are in equilibrium at the triple point, whereas the gas and liquid properties at the critical point are very similar. The interface between the two phases disappears and becomes a phase [46]. This indicates that the critical point is gas–liquid two-phase mixed CO₂ is at a critical point. When in a supercritical state, CO₂ has the characteristics of both liquid and gaseous phases; that is, its density is close to that of a liquid, but it has the viscosity, diffusivity, and fluidity of a gas, which is often used in long-distance CO₂ pipeline transportation. When the pressure reaches a certain value and the temperature exceeds −56.6 °C, CO₂ transforms from gas to liquid. When the temperature is lower than −56.6 °C and the pressure is higher than 0.52 MPa, CO₂ will transform from liquid to solid. When the pressure is below 0.52 MPa and the temperature reaches a certain value, solid CO₂ is directly sublimated to the gaseous state [47]. There are three forms of CO₂ pipeline transport: gaseous, liquid, and supercritical [48].

3.2. Mechanism Underlying CO₂ Fire Prevention

Researchers have also investigated the mechanism by which CO₂ inhibits spontaneous coal combustion. The adsorption characteristics of CO₂, the effects of CO₂ on the gas and heat production of coal oxidation, the effects of CO₂ on the functional groups in coal, and the effect of inert gases on the free-radical reaction of coal have been studied [50,51]. Jin [52] used the Monte Carlo method to establish a coal structure model to study the competitive adsorption capacity of different concentrations of flue gas in coal and indicated that the order of coal adsorption gas selection was CO₂ > O₂ > N₂. Zheng [53] obtained the following order of gas adsorptive selectivity on a coal surface using gas adsorption analysis: CO₂ > CH₄ > O₂ > N₂. At room temperature, the CO₂ adsorbed on the coal surface hindered the spontaneous combustion of the coal. Abunowara [54] studied the experimental results of CO₂ adsorption using adsorption isotherms, thermodynamics, and kinetics models and concluded that the adsorption of CO₂ on the coal substratum is favourable and autogenic, and the adsorbed CO₂ molecules aggregated more on the coal substratum. Day [55] obtained the adsorption heat range based on the Clausius–Clapeyron equation by comparing and analysing the relationship between the adsorption amount and degree of metamorphism from the CO₂ adsorption amount. Zhai [56] used a liquid CO₂ cooling device to study the cooling behaviour of high-temperature loose coal with different particle sizes under the same liquid flow rate. The results showed that the smaller the particle size, the better the cooling effect of the coal body. The cooling rates of the measuring points near and above the liquid CO₂ outlet were higher than those of the lower coal body. It was concluded that a dry ice ball below the liquid CO₂ outlet hindered the cooling effect of the low-temperature liquid (gas) on the lower coal body. Ma [57] analysed the influences of different volume fractions of CO₂ on the combustion characteristics of coal using TG-DSC. The experiments showed that a high volume of CO₂ reduced the pre-exponential factor, the apparent activation energy of coal and the exothermal intensity, and inhibited its oxidation and combustion. Ma [58] also studied the differences in the functional groups of coal oxidised under CO₂/O₂ and N₂/O₂ atmospheres using FTIR. The authors reported a significant difference in the changes in the functional groups, such as -OH, C=O, and -COO-, in the two atmospheres. N₂ and CO₂ differ in the microscopic aspects of the inhibition of spontaneous coal combustion. Miao [59] measured the ESR in situ spectrum, spectral line width, free radical concentration, and other free-radical parameters while transforming a coal gas atmosphere at different temperatures. The results showed that the higher the CO₂ injection temperature is, the higher the peak height is, the paramagnetic centre of coal in a CO₂ atmosphere is lower than that in a dry air atmosphere, and the free radical level first decreases and then increases. After the atmosphere changed, the ESR spectral line width of the coal body suddenly decreased and stabilised at approximately 0.45. The inhibition effect of CO₂ injection at 70 °C on free radicals is the best.

As a mine fire prevention and extinguishing material, the main principle of CO₂ is reflected in three aspects: asphyxiation and oxygen isolation, inerting and explosion suppression, and cooling to extinguish fire [60,61]. The mechanisms of CO₂ fire prevention and extinguishment are illustrated in Figure 4.

(1)

Isolation effect of asphyxia

The spontaneous combustion of coal seams results from the combination of coal and oxygen; without oxygen, this reaction cannot occur [63]. It is generally believed that when the O₂ concentration in the goaf is 5–15%, the coal can undergo oxidation and spontaneous combustion, while at below 5%, the coal is deflagration [64,65]. Therefore, injecting CO₂ into a fire area or goaf with a high-temperature fire point can rapidly dilute the oxygen concentration in the goaf. Simultaneously, as the density of CO₂ is higher than that of air, a positive pressure is formed in the goaf, and air entry is reduced. Moreover, coal has a strong adsorption effect on CO₂, making it easy for CO₂ to cover the surface of the coal combustion point to replace O₂ and reduce the O₂ concentration [66]. Finally, the O₂ concentration in the goaf is lower than the critical O₂ concentration for spontaneous coal combustion, which prevents the oxidation and spontaneous combustion of coal, resulting in fire extinguishing owing to hypoxia.

(2)

Inerting explosion suppression effect

CO₂ injection can inhibit the production of oxidation products, such as CO, to a certain extent. Owing to the inerting effect of CO₂ and the elimination of free radicals, the explosion risk of a mixture of CO₂ with air and combustible gas is significantly reduced. The inert and explosion suppression effects of CO₂ were better than those of the other inert gases. For example, the critical O₂ concentration for the explosion of an air–methane mixture containing additional N₂ was 12%, and the critical O₂ concentration for the extinction of an open flame in the fire zone was 9.5%. For an air–methane mixture containing CO₂, the critical O₂ concentration of the explosion can reach 14.6%, and the critical O₂ concentration of flame extinction in the fire zone is 11.5% [67,68].

(3)

Cooling effect

The oxidation rate of coal is closely related to the O₂ content and temperature. Spontaneous coal combustion often involves three stages: incubation, self-heating, and combustion [69]. After liquid CO₂ is injected into the goaf, it instantaneously becomes gaseous, and the volume expands rapidly, absorbing a large amount of heat. Therefore, it has an evident cooling effect on the goaf [70]. Combined with the strong adsorption capacity of coal for CO₂, the adsorption heat is transferred to the CO₂ gas during the adsorption process, curbing the combustion chain reaction. The cooling effect of liquid CO₂ plays a significant role in fire prevention and extinguishing.

3.3. Analysis of CO₂ Fire Prevention Performance

There are N₂ and CO₂ fire prevention technologies in the use of coal mine inert gas fire prevention technology at home and abroad; however, CO₂ has unique advantages over N₂ [71,72]. Their fire prevention and extinguishing performances were compared based on the following four aspects:

(1)

Comparison adsorption comparison

Under the same temperature and pressure conditions, the adsorption capacities of coal for CO₂ and N₂ were 48 and 8 L/kg, respectively; i.e., the adsorption capacity of CO₂ in coal was six times that of N₂ [73]. The ability of coal to adsorb CO₂ is stronger than that for N₂, and CO₂ is preferentially adsorbed. The adsorption sites on the surface of coal molecules are quickly occupied by CO₂, indicating that their affinity for CO₂ is stronger [74,75].

(2)

Comparison of critical oxygen concentration

When N₂ was used for flame retardancy and explosion resistance, the oxygen concentration in the fire zone decreased to 9.5% and 11.5%, respectively. When CO₂ is selected for flame retardancy and explosion resistance, the oxygen concentration in the fire zone must be reduced by 12.0% and 14.6%, respectively, to play a role [76]. The oxygen concentration of CO₂ is higher than that of N₂; therefore, using CO₂ to prevent and extinguish fires is more effective than that of N₂.

(3)

Comparison of inert coverage in fire zones

The fire source points in the goaf were mostly residual coal and generally located in the middle and lower parts of the goaf. The density of N₂ is lower than that of air; therefore, the injected N₂ easily diffuses outwards and cannot achieve a good inerting effect. When CO₂ is injected into a fire area, CO₂ can stay in the fire area longer. In other words, under the same conditions, CO₂ can achieve an explosion–suppression effect earlier and wider than N₂ [77].

(4)

Comparison of separation purity

Regardless of whether membrane separation or adsorption separation technology is used to produce N₂, it is impossible to separate all the O₂. As the liquefaction boiling point of O₂ is −182.96 °C and the liquefaction boiling point of N₂ is −196 °C, some O₂ is retained in liquid N₂ during liquefaction. When the liquid CO₂ is produced in the chemical plant, the boiling point of CO₂ liquefaction is −56.6 °C, which is much different from the boiling point of O₂ liquefaction; hence, it will not be mixed with O₂ during the liquefaction process. Therefore, the purity remains high, close to 100% [78,79]. This shows that, in terms of liquid purity, CO₂ has a high purity and is conducive to the inerting of fire gas. A comparison of the fire prevention and extinguishing performance parameters of CO₂ and N₂ is presented in

Table 2. Comparison of fire prevention performance parameters of CO₂ and N₂. Reprinted with permission from Ref. [80]. Copyright 2017, Elsevier BV. Reprinted with permission from Ref. [81]. Copyright 2021, American Chemical Society.

Contrast Parameters	CO2	N2	Differences between CO2 and N2
Liquid purity (%)	Close to 100	Close to 97	All conform to the coal mine safety regulations
Relative density	1.53	0.967	The submergence effect of CO2 is good.
Adsorption capacity of coal (L/kg)	48	8	The adsorption capacity of CO2 is 6 times that of N2.
Maximum oxygen concentration in inert gas (%)	0	2~3	Separation of liquid CO2 with high purity
Critical oxygen concentration for explosion prevention (%)	14.6	11.5	The critical oxygen concentration of CO2 is high
Critical oxygen concentration for extinguishing an open flame (%)	12	9.5	The critical oxygen concentration of the extinguished flame of CO2 is high.
Gas volume expansion	600	700	The volume expansion of CO2 gas is slightly small
Application scenario	Pole tilt face fire	Fire on the upper stratified face	The application scenarios are different and the fire prevention and extinguishing gas is selected according to the conditions of the field working face.

.

Previous studies have also compared the fire prevention and extinguishing performances of CO₂ and N₂ [82,83]. Cui [84] studied the adsorption characteristics of three single-component gases using coal of different metamorphic grades. With an increase in the metamorphic degree, the adsorption capacities of N₂, methane, and CO₂ in the same coal body increased under the same experimental conditions. It was concluded that coal could adsorb CO₂ and preferentially adsorb CO₂; that is, CO₂ can better inhibit spontaneous coal combustion and stably adsorb onto the coal body, and when the pressure increases, the adsorption capacity is enhanced. Si [85] studied the competitive adsorption behaviour of binary mixed-component gases (CO₂/O₂, CO₂/N₂, and O₂/N₂) in different proportions from the aspects of adsorption capacity, competitive adsorption heat, and interaction energy. The results showed that in the competitive component gas, CO₂ had the strongest adsorption effect on the entire system; that is, coal preferentially adsorbed CO₂, and thus, CO₂ could better inhibit spontaneous coal combustion. Pang [86] studied the flame propagation characteristics in a 12 L cylindrical explosive tank and further explained the dust explosion mechanism of low-density polyethylene (LDPE) with different inert gases using thermogravimetric analysis and related theories. It concluded that the LOC of CO₂ is typically higher than that of N₂; therefore, the inert effect of CO₂ is better than that of N₂. Wang [87] inerted coal samples from the Xinglongzhuang Coal Mine with CO₂ and N₂ gas for 12 h and used a temperature-programmed experimental system to compare and analyse the variation characteristics of the CO production rate and O₂ consumption rate of coal samples during the temperature-programmed experiment. The authors concluded that the amount of CO produced by the coal sample adsorbed with CO₂ gas was small after desorption was completed, and CO₂ had a better effect in inhibiting the spontaneous combustion of coal samples than N₂. Ma [88] analysed the change in gas concentration in a fire area after adopting a combined inert gas fire prevention and extinguishing technology through local fire control measures of the tail roadway with N₂ injection first and liquid CO₂ injection later. The authors found that CO₂ had a better diffusivity and dilution effect than N₂, and the dilution effects of O₂, C₂H₄, and C₂H₆ were more evident, indicating that CO₂ can better reduce the oxidation reactivity of coal. Liu [89] conducted oxidation experiments on coal samples in two inert gas environments (N₂-O₂ and CO₂ -O₂) and modelled the mass flow rates. The oxidation reaction constant in the CO₂-O₂ environment was only 40–85% of that in the N₂-O₂ environment, indicating that CO₂ had a stronger inhibitory effect on coal spontaneous combustion.

3.4. Studies on the Influencing Factors of CO₂ Fire Prevention Performance

3.4.1. Effect of CO₂ Concentration on Coal Oxidation Rate

To study the influence of CO₂ on the low-temperature oxidation reaction of coal, some scholars have used a temperature-programmed experimental device to investigate the spontaneous combustion characteristics of coal samples under different CO₂ concentrations and analysed the influence of spontaneous combustion characteristic parameters on the change in CO₂ concentration during the experiment. The results showed that the effects of different concentrations of CO₂ on the inhibition of the oxidation and spontaneous combustion of coal differed. To determine the inhibitory effect of CO₂ on the exothermic performance of coal oxidation at low temperatures, Wang [90] used differential scanning calorimetry to observe heat flow at different CO₂ concentrations and heating rates. CO₂ exhibited different degrees of inhibition at all stages of coal oxidation and heating. The Flynn–Wall–Ozawa method was used to determine whether the inhibitory effect of the CO₂ concentration on the exothermic stage of the low-temperature oxidation of coal was better than that on the endothermic stage. Zhang [91] used a C80 microcalorimeter to measure and calculate the constant-temperature heat release and activation energy of coal samples. In the high-temperature combustion stage, the critical, maximum peak, ignition, and burnout temperatures of the coal samples were analysed using a synchronous thermal analyser. The authors showed that increasing the concentration of the inert gas could gradually reduce the oxidation combustion rate and improve the inhibition of spontaneous coal combustion. Li [92] measured the inhibitory effects of different CO₂ concentrations on spontaneous coal combustion oxidation using temperature-programmed experiments. The authors concluded that CO₂ with a concentration of more than 30% had a good inhibitory effect on the oxidation and spontaneous combustion processes of coal, and the higher the CO₂ concentration, the stronger the ability to inhibit the oxidation and spontaneous combustion of coal. Liu [93] studied the effect of different concentrations of CO₂ on the low-temperature oxidation (below 160 °C) of coal using temperature-programmed experiments. The results showed that at 160 °C, CO₂ had an inert effect on the low-temperature oxidation of the coal. CO₂ with a volume fraction of >31% exhibited a good inerting effect, and the inerting effect of CO₂ with a volume fraction of >50% was more pronounced. At temperatures below 100 °C, the inerting effect of CO₂ at different concentrations was not significant.

3.4.2. Effect of CO₂ Injection Position on Coal Oxidation Rate

The coal oxidation rate at different CO₂ injection positions has been investigated using a combination of field tests and numerical simulations. Wang et al. [94] designed and conducted a simulation experiment on the quenching effect of low-temperature gaseous CO₂ on spontaneous coal combustion and tested its migration and cooling characteristics in high-temperature-scattered coal. The authors concluded that the size of the cooling range and diffusion rate of the cooling area were inversely proportional to the distance from the centre axis and proportional to the injection time. The injection time played a leading role in the cooling effect of high-temperature coal, followed by the injection position. Gaseous CO₂ rapidly diffused into the loose coal, filled the vicinity of the release port within 5 s, and filled the container within 30 s. The average diffusion rate of CO₂ in loose coal was 0.051 m/s. Hao [95] numerically simulated the effects of injection flow rate, temperature, and injection position of liquid CO₂ on the gas concentration field, temperature field, and width of the goaf. It was concluded that with the injection of a low-temperature CO₂ position in the goaf moving deep, the width of the oxidation zone in the goaf first slowly increased, rapidly decreased, and finally tended to stabilise. The temperature at the high-temperature point in the oxidation zone of the goaf rapidly decreased, slowly increased, and finally linearly increased. Based on the measured data, Wang et al. [96] studied the distribution of the multicomponent gas concentration field, diffusion, and spontaneous combustion temperature field in the process of CO₂ injection using numerical simulations.

The optimal range of the CO₂ injection position in the goaf from the working face and a reasonable injection flow rate were obtained; the maximum width of the stable oxidation zone in the goaf was simulated. Li [97] conducted a closed O₂ consumption experiment on coal samples in the Jiudaoling Mine and applied the experimental O₂ consumption parameters to a numerical simulation of CO₂ injection fire prevention and extinguishing in the goaf. The position of the CO₂ injection and the width of the O₂ concentration zone in the goaf at different positions were obtained. It was concluded that with a deeper CO₂ injection, the width of the spontaneous combustion oxidation zone first decreased and then increased. Bai [98] studied and analysed the effects of different injection positions, injection flow rates and different temperatures of CO₂ on fire prevention and extinguishing. Based on the numerical simulation, Design Expert was used to design the experiment, and a quadratic regression response surface model of the maximum width of the oxidation zone in the goaf was constructed. The maximum width of the oxidation zone in the goaf under different conditions was predicted and analysed. The authors showed that the maximum width of the oxidation zone decreased with an increase in the flow rate of the CO₂ injection and increased with an increase in the CO₂ injection temperature. Song [99] used a physical simulation model to study the inerting effect and migration law of CO₂ in a large inclined goaf and analysed and compared the difference between O₂ reduction and inert gas migration. The authors showed that with an increase in CO₂ injection, the range and width of the oxidation zone decreased, and the ends of the loose and oxidation zones moved closer to the working face.

4. CO₂ Fire Prevention Technology

At present, CO₂ fire prevention and extinguishing technologies adopt three types of process systems: ground pressure injection [100], downhole pressure injection [101], and dry ice phase-change systems [102]. Ground pressure injection includes two processes: ground liquid CO₂ pressure with direct injection and ground liquid CO₂ gasification pressure injection. Downhole pressure injection has two processes: underground mobile liquid CO₂ pressure maintaining direct injection and underground mobile liquid CO₂ gasification pressure injection. The gasification perfusion method solves the problem of condensation blockage in the long-distance pipeline transportation of liquid CO₂; however, the cooling, fire prevention, and extinguishing effects of liquid CO₂ after gasification are greatly weakened [103,104].

4.1. Surface Injection

Ground pressure injection includes two processes: ground liquid CO₂, direct injection of ground liquid CO₂, and gasification pressure injection. The ground liquid CO₂ pressure-keeping direct injection system uses long-distance pipelines to directly transport liquid CO₂ in a large tank car to the vicinity of the fire prevention and extinguishing area, which then injects the liquid CO₂ into the target area through pre-embedded pipelines or boreholes [105]. The system is primarily composed of surface drilling, liquid CO₂ storage tank cars, pressurisation device, flow control valves, pressure meter (P), temperature meter (T), flow meter (Q), system control cabinets, high-pressure stainless-steel pipes, and other equipment [106]. Through this process, the temperature of liquid CO₂ is low when injected into the target area, and the rapid phase change at the outlet position can reduce the ambient temperature to below −15 °C. Typically, CO₂ injected into the downhole is in a three-phase mixed state [107]. The perfusion system is simple and can perform large-flow perfusion, which has a significant effect on the management of fire areas.

The ground liquid CO₂ gasification pressure injection vaporises the liquid CO₂ in the tank car or stored through the ground vaporisation device and then transports the gas phase CO₂ to the underground target area through the original fire prevention pipeline to achieve the purpose of inerting and heat replacement in the fire area. The temperature of CO₂ after gasification is generally ~0 ± 5 °C, mainly in the form of gas [108].

The system is mainly composed of a ground tank car, gasification device, gas storage tank, liquid–gas conversion unit of electric control cabinet-autothermal type, liquid–gas conversion unit-strong heat type, flow control valve, gas pressure control unit, safety valve, P, T, Q, system control cabinet, transmission pipeline, and so on [109]. A schematic of the ground pressure injection system is shown in Figure 5. In Figure 5, for the pressurisation device (a), the schematic is the direct injection diagram of the ground liquid CO₂ pressure holding; for the gasification device (b), the schematic is the ground liquid CO₂ gasification pressure injection diagram.

4.2. Downhole Injection

Downhole pressure injection includes downhole mobile liquid CO₂ pressure-maintaining direct injection and downhole mobile liquid CO₂ gasification pressure injection. The underground mobile liquid CO₂ pressure-keeping direct injection subpacks the liquid CO₂ in the large tank car into a small mobile storage tank that is easy to move and transport, transporting the storage tank to a location close to the underground fire prevention and extinguishing sites. Liquid CO₂ is directly injected into fire prevention and extinguishing areas through pipelines or boreholes. This system includes a production process system and an auxiliary system. The production process system comprises a liquid storage tank and liquid-phase pipeline, while the auxiliary system comprises a booster pump, safety valve, pressure meter (P), temperature meter (T), flow meter (Q), and control valve. However, due to its complex operation, governance efficiency is low [111].

Downhole mobile liquid CO₂ gasification pressure injection uses a liquid CO₂ storage tank train to transport liquid CO₂ to the vicinity of the underground fire area, vaporise liquid CO₂ through the gasification device, and then inject liquid CO₂ into the fire area through the gas phase pipeline. They can cover and inert the fire area, extinguish the fire source, and quickly cool it [112,113]. This system included a production process system and an auxiliary system. The production process system comprises a liquid storage tank and gas-phase pipeline, and the auxiliary system comprises a gasification device, safety valve, temperature meter (T), pressure meter (P), control valve, and gas-phase pipeline [114]. A process diagram of the downhole pressure injection system is shown in Figure 6. In Figure 6, for the booster pump (a), the schematic diagram shows the downhole mobile liquid CO₂ pressure-holding direct injection, and for the gasification device (b), the schematic diagram shows the downhole mobile liquid CO₂ gasification pressure injection.

The Xi’an University of Science and Technology [115] developed China’s first mine-used liquid CO₂ booster pump, driven by electric power. The problem of limited long-distance transportation of liquid CO₂ in coal mines is solved by low-temperature pressurisation and long-distance, stable, direct, continuous, and rapid transportation of liquid CO₂ in coal mines is realised. The pump is mainly composed of a motor, pulley mechanism, cylinder block assembly, piston assembly, sealing assembly, inlet and outlet valve, etc. Its working principle is that the rotation motion of the motor is transformed into a reciprocating motion, and the output power of the motor is transmitted to the cylinder block assembly. The motor is successfully converted by belt pulley, eccentric wheel, connecting rod, and crosshead. Under this action, the liquid CO₂ in the storage tank can be pressurized by the conveying pump to achieve long-distance high-pressure transportation. The designed mine mobile liquid CO₂ storage tank comprises a liquid storage tank, self-pressurised control system, liquid pipeline, gas pipeline, safety device, and control valve. The liquid CO₂ tank is directly transported to the underground coal mine, and CO₂ is poured into the dangerous area of spontaneous combustion, which plays a role in fire prevention and extinguishing and provides important equipment support for mine fire prevention and extinguishing in China. The mine mobile liquid CO₂ storage tank and liquid CO₂ pump are shown in Figure 7.

4.3. Dry Ice Phase-Change Fire Prevention

Liquid CO₂ is unstable and, hence, can be easily gasified. Liquid CO₂ has high pressure and temperature requirements for the storage tank, whereas dry ice is stable and easy to transport. Therefore, some scholars have proposed placing dry ice in the goaf and using the characteristics of dry ice sublimation to prevent and extinguish fires [117]. The dry ice phase-change fire prevention and extinguishing system involves injecting dry ice into the goaf through a gas pipeline through a dry ice phase-change generator to promote the rapid phase-change of dry ice into low-temperature CO₂ gas through water flow heat exchange, which can achieve the effect of cooling, covering oxygen insulation, and inerting the spontaneous combustion of residual coal [118].

The dry ice phase-change fire prevention and extinguishing system mainly includes dry ice phase-change generators, inlet and outlet water pipelines, gas-injection pipeline safety valves, pressure meter (P), temperature meter (T), flow meter (Q), and control valves. The dry ice phase-change generator is mainly composed of tanks, heat exchange copper tubes, water heating pipelines, and monitoring instruments [119]. A schematic of the dry ice phase-change fire-extinguishing device is shown in Figure 8.

The technological parameters, advantages, and disadvantages of the CO₂ fire prevention process are presented in

Table 3. Comparison of process parameters of CO₂ fire extinguishing technology. Reprinted with permission from Ref. [120]. Copyright 2016, China Coal mine Safety professional committee.

Fire of Extinguishing and Preventing System	Ground Liquid CO2 Pressure Maintaining Direct Injection	Ground Liquid CO2 Gasification Pressure Injection	Downhole Mobile Liquid CO2 Pressure Injection	Downhole Mobile Liquid CO2 Gasification Pressure Injection	Dry Ice Phase-Change Fire Prevention
Device	Liquid CO2 storage tank car, gasification device, pressurization device, flow control valve, pressure meter (P), temperature meter (T), flow meter (Q), etc.	Gasification device, gas storage tank, gas pressure control unit, safety valve, pressure meter (P), temperature meter (T), flow meter (Q), control valve, etc.	Liquid storage tank, booster pump, safety valve, pressure meter (P), temperature meter (T), flow meter (Q), control valve, etc.	Liquid storage tank, gasification device, safety valve, pressure meter (P), temperature meter (T), control valve, etc	Dry ice phase-change generator, injection pipe safety valve, pressure meter (P), temperature meter (T), flow meter (Q), control valve, etc.
Pipeline	High-pressure stainless-steel pipe, sealing, pressure control and other conditions are required	High-pressure hose or steel pipe	High-pressure hose or steel pipe	High-pressure hose or steel pipe	High-pressure hose or steel pipe
Ease of operation	Very demanding	High demand	High demand	High demand	High demand
Pressure injection capacity	Continuous injection, up to 60 t/h	It can reach 5–10 t/h	It can reach 5–10 t/h	It can reach 5–10 t/h	It can reach 5–10 t/h
Cooling effect	The outlet temperature is −25 to 10 °C, and the dry ice formed can reach −60 °C	Outlet temperature −2 to 5 °C	Outlet temperature −1 to 0 °C	Outlet temperature −5 to 5 °C	Outlet temperature −5 to 0 °C
Line clogging	The possibility of a pressure-holding section is low	The ground may partially appear	The ground may partially appear	It may appear at the end of the pipe	It may appear at the end of the pipe
Scope of application	Prevent coal spontaneous combustion in key underground areas; Control and direct extinguishing of large fire areas	Large area underground fire prevention	Small fire area control and emergency response	Small fire area control and emergency response	Large area underground fire prevention

.

5. Field Application and Effect

CO₂ concentration can be used as a key parameter to reflect the degree of risk to the goaf, corner of the working face, closed wall, and other areas. At the same time, it can be used as an important index to investigate the effects of fire prevention and extinguishing to effectively protect the safety of underground workers. In China, the ‘Coal Mine Fire Prevention and Extinguishing Rules’ issued by the National Safety Supervision Bureau stipulate that ‘When CO₂ fire prevention is adopted, the oxygen concentration in the return corner of the working face must be monitored. After the spontaneous combustion fire in the goaf of the working face is closed (or other closed areas where spontaneous combustion fire occurs), CO₂ is continuously injected into the closed area, and the concentration of carbon dioxide in the inlet and return air flow of the coal mining face must be monitored. When the carbon dioxide concentration in the inlet air flow exceeds 0.5% or the carbon dioxide concentration in the return air flow exceeds 1.5%, the perfusion must be stopped, the personnel must be evacuated, measures must be taken, treatment must be carried out, and the oxygen concentration in the closed area must not be greater than 5.0% [121].’ During fire extinguishing, parameters, such as gas and temperature in the fire zone should be continuously monitored, the fire extinguishing effect should be investigated, and fire extinguishing measures should be improved until the fire zone reaches the extinguishing standard. At the same time, the ‘Coal Mine Safety Regulations’ issued by the Ministry of Emergency Management stipulate that, ‘When the fire area has the following two most important conditions, it is the fire area extinguished, that is, the air temperature in the fire area is reduced to below 30 °C or the same as the daily temperature in the pre-fire area; the oxygen concentration in the air of the fire area was reduced to less than 5.0% [122].’ In addition, the safety awareness and professional skills of field workers are important cornerstones to ensure the smooth progress of production and prevent accidents. Therefore, it is necessary to strengthen the training of coal mine field operators on emergency rescue plans, disaster avoidance routes, self-rescue and mutual rescue, and safety and risk avoidance knowledge. Underground workers must also be proficient in the use of self-rescue devices and emergency shelters. Tang et al. [19] injected composite inert gas into the goaf of the 5306 working face in the Tangkou Coal Mine. At the same time, after injecting CO₂ composite inert gas into the goaf, the CO concentration in the return air flow and return air angle of the working face decreased rapidly from 14.9 ppm and 4821 ppm to 0.1 ppm and 21 ppm, respectively. During the gas injection process, the CO₂ concentration in the return air does not exceed 15%, which ensures the safety of the working face personnel. Cao et al. [61] conducted a fire prevention and extinguishing experiment through an intelligent long-distance pressure-maintaining CO₂ conveying system using the Huoji soil well of Daliuta Coal Mine in the Shendong Mining Area as an industrial test site. The practice has proved that the concentration of CO decreased from 790 ppm to 41 ppm, indicating that liquid CO₂ has a significant effect on fire prevention. In order to explore the intelligent locking performance of CO₂ long-distance pressure-holding transportation, four observation points were set up in the working face in the field application. The CO₂ sensor at the observation point will detect when the CO₂ concentration is higher than 0.8%, and the CO₂ pressure-keeping transportation system is intelligently locked, and the ground control terminal of the system also issues an early warning in time. This effectively prevents the problem of CO₂ concentration exceeding the limit during transportation and prevents the occurrence of hypoxic asphyxiation accidents. Si et al. [85] established a physical model of goaf by combining a numerical simulation with a field test, and analysed the safety of CO₂ injection volume to the fire area, O₂ concentration distribution, and CO₂ fire prevention and extinguishing technology. The relationship between the effect of CO₂ injection and safety production is balanced. The results show that the distribution of the O₂ flow field in goaf changes with the change in the CO₂ injection rate. The O₂ between the working face and the injection port is diluted, with the O₂ at the air inlet being significantly reduced. Therefore, before using CO₂ fire extinguishing technology, the amount of CO₂ injection should be calculated according to the actual situation of the mine, and the gas concentration should be observed at all times during the injection process to ensure the smooth progress of CO₂ fire extinguishing technology and the safety of personnel.

The distribution range of the spontaneous combustion area in the 110,205 working face of the Qingshuiying Coal Mine was large, and the high-temperature area exhibited a dynamic distribution. To completely resolve the problem of spontaneous combustion in the working face, Zhou [123] implemented the direct injection of liquid CO₂ in a closed goaf to comprehensively cool and inert the coal in the goaf. Long-distance transportation of liquid CO₂ was adopted, and liquid CO₂ was directly poured into the closed goaf through a long-distance pipeline using a ground tanker. During this perfusion, the perfusion of liquid CO₂ is divided into four pipelines and the daily circulation of the casing in the borehole is injected; the total amount of liquid CO₂ was 548.88 t.

After 18 d of sealing, the measured CO concentration in each observation hole was 0–50 ppm, the CO₂ concentration was approximately 99%, while the O₂ concentration was below 5%. After all the indicators met the conditions for unsealing, the working face was unsealed. After ventilation was restored, owing to the large density of CO₂ and the large dip angle of the working face, the emission intensity and time were increased to ensure safety. Figure 9 shows the CO concentration change diagram of the 110,205 working face at the Qingshuiying Coal Mine.

After the unsealing of the 110,205 fully mechanized mining face, the CO concentration in the working face, return air flow, and upper corner was 0–3, 0–2, and 1–6 ppm, respectively. The highest part of the working face support was 14 ppm, and the temperature detection in the working face and the borehole was maintained at 18–22 °C. By analysing the gas composition, concentration data, and temperature variation law of the collected observation points, we found that, owing to the better adsorption of CO₂ and coal, CO₂ can better occupy coal fissures and discharge O₂. After ventilation was restored, the contact area between the coal and O₂ was also reduced, which had a good wrapping effect on the coal body and inhibited the reignition of the coal seam. Simultaneously, the temperature of the coal body was significantly reduced, and the spontaneous combustion period of the coal was prolonged. From the opening of the working face to the completion of withdrawal, there was no high concentration of CO in the observation hole, and the CO concentration in the upper corner was maintained at approximately 5 ppm.

Jing et al. [124] adopted a closed direct injection liquid CO₂ anti-fire technology on the working face of the Yangchangwan Coal Mine to prevent spontaneous combustion in the goaf of working face II020205. The liquid CO₂ entered a stainless-steel pipeline laid underground through vertical drilling. Liquid CO₂ was injected into the goaf of working face II020205 through the return air lane. Figure 10 shows the CO₂ injection pipeline on the working face of Site II020205.

The CO concentrations at different positions on the initial working face fluctuated constantly with time. The CO gas concentration data for the II020205 working face of the Yangchangwan Coal Mine are shown in Figure 11.

After injecting liquid CO₂ into the goaf, the concentration of each gas varied significantly at different locations. When the pressure injection amount reached a certain degree, the CO₂ concentration in the return air roadway of the II020205 working face reached more than 9%, the O₂ concentration dropped below 3%, the CO gas content rapidly decreased, and finally dropped to 0%, which played a significant role in inerting. With the continuous diffusion of CO₂ in the goaf, the CO concentration gradually stabilised at all positions of the working face and tended to 10 ppm in the upper corner of the working face. Combined with the distribution law of CO₂ in the goaf, the concentration of CO₂ gradually increased after it was injected into the goaf. With the diffusion of CO₂ in the goaf and continuous adsorption of CO₂ by the residual coal, the concentration of CO₂ gradually decreased. The combination of residual coal and CO₂ reduced the contact area between O₂ and residual coal, thus blocking the oxidation of residual coal and decreasing the concentration of CO. This indicates that the pressure injection of liquid CO₂ in the goaf effectively inhibited the spontaneous combustion of residual coal in the goaf.

6. Shortcomings and Prospects of CO₂ Fire Prevention Technology

6.1. Shortcomings

CO₂ fire prevention and extinguishing technologies are widely used to treat underground fire areas in coal mines. However, during transportation, CO₂ undergoes three processes: supercritical, gas–liquid two-phase coexistence, and gas phase. CO₂ gasification leads to a significant increase in pressure and impact on the pipeline. Owing to the large pressure difference and the throttling effect, the temperature drops sharply and even drops below the CO₂ three-phase point. A temperature that is too low damages the pipeline, and CO₂ has a strong Joule–Thomson effect, resulting in a sharp decrease in temperature near the leakage port and a large amount of dry ice, resulting in low-temperature brittleness of the pipeline and pipeline bursting. When CO₂ contains water during transportation, it partially dissolves and forms carbonic acid, which increases pipeline corrosion. When CO₂ is injected into the downhole, the concentration and distribution of CO₂ change dynamically owing to downhole gas flow, temperature change, and pressure fluctuation, among other factors. Therefore, it is difficult to monitor inert coverage. At the same time, owing to the consumption and leakage of CO₂, the inert effect decreases with injection time, which makes monitoring the inert duration challenging. Due to the influence of these factors, it was difficult to investigate the actual cooling effect at the site. Considering these problems, the following prospects are proposed for future work on CO₂ fire prevention and extinguishing technology to promote the development and application of CO₂ fire prevention and extinguishing technology in mine goaves.

6.2. Prospects

(1)

Studying material and crack propagation of CO₂ transport pipeline

The corrosion of pipelines, temperature and pressure changes, and other problems in CO₂ transportation may cause pipeline rupture; therefore, it is necessary to study the corrosion resistance, low-temperature resistance, and high-pressure resistance of pipeline materials. In the design of CO₂ pipelines, the resistance to ductile fracture propagation is an important factor that must be considered. Therefore, the effects of different phases of CO₂ on pipe performance, pipe performance indicators, and crack propagation control must be studied to improve the reliability and safety of pipelines and provide a more reliable guarantee for CO₂ transportation.

(2)

Intelligent monitoring and control system for CO₂ transmission pipelines

The operational environment of underground pipelines is complex; therefore, predicting the leakage point during the transportation process is difficult. To ensure that CO₂ reaches the designated area, it is essential to monitor and locate the leakage point in the pipeline in real time. Simultaneously, owing to the blockage caused by phase change, the temperature and pressure control requirements for CO₂ in the transportation process are extremely high. After a long period of injection into the inerting area, the ageing of the inerting effect was determined by the temperature and concentration of various index gases. Therefore, it is necessary to develop a multifunctional and comprehensive CO₂ intelligent monitoring and control system to monitor the temperature, flow rate, and conveying pressure of CO₂ during transportation in real time and make appropriate adjustments to ensure the safety of transportation technology.

(3)

Multifactor coupling analysis of the inert mechanism

Combined with the temperature, pressure, and other conditions of the coal seam, investigating the inerting mechanism under multi-factor coupling further explores the diffusion and adsorption characteristics of CO₂ in coal bodies, especially for deep coal seams, and studies on the distribution and diffusion law of CO₂ under complex geological conditions will ensure effective coverage of high-temperature fire points, through numerical simulation and experimental verification, establishing a CO₂ inerting effect prediction model to adjust the optimisation of field application parameters.

(4)

Microscopic mechanism of CO₂ inerting

Further exploration of the inhibitory effect of CO₂ on the coal–oxygen recombination reaction, especially by analysing the change in the active group caused by the interaction between CO₂ and the coal matrix, will elucidate the mechanism by which CO₂ reduces the rate of coal–oxygen recombination, thereby prolonging the latency of spontaneous coal combustion. An in-depth study of coal microstructure characteristics, especially pore structure and surface functional groups, on CO₂ adsorption performance can elucidate the interaction mechanism between coal and CO₂ molecules and improve the CO₂ adsorption efficiency and inerting effect to provide theoretical support.

(5)

Comprehensive fire prevention technology

By exploring the combination of CO₂ and other fire extinguishing technologies to form an integrated fire prevention system through the synergistic effect to improve the fire prevention effect, one can fully use their respective advantages and complementary effects to improve fire efficiency and adaptability and better respond to various fire scenes and to improve the fire extinguishing effect.

7. Conclusions

The CO₂ anti-fire technology in the containment of fire has a significant effect. However, there remain several gaps in our understanding of field personnel, which limits the wide application of CO₂ fire prevention technology. Based on the available data and existing challenges with CO₂ anti-fire technology, the future development trend of CO₂ anti-fire technology prospects, and the inerting mechanism of CO₂ anti-fire technology, the evolution of the coal molecular microstructure and the influence of different-phase CO₂ mechanisms are further explored. Simultaneously, strengthening the joint application research and field test verification with other anti-fire technologies will enhance the effectiveness of CO₂ anti-fire technology for the prevention and control of spontaneous coal combustion disasters.