A Study on the Flame and Pressure Characteristics of Ultrafine Calcium Carbonate (CaCO₃) Powder in Suppressing Gas Explosions

Abstract

This study investigates the suppression effect of ultrafine CaCO₃ powder on gas explosions through a series of experiments conducted in a medium-scale explosion tube (9.6 m in length) with varying concentrations of ultrafine CaCO₃. The gas explosion suppression concentration was established at 9.5%. The results indicate that, at concentrations of 125 g/m³ and 300 g/m³, ultrafine CaCO₃ powder significantly reduced the flame propagation speed of the gas explosion. Among the concentrations that did not fully suppress the gas explosion, 20 g/m³ effectively mitigated the flame propagation speed and overpressure throughout the entire process. The concentration of 75 g/m³ demonstrated a suppression–promotion–suppression–promotion pattern regarding the flame propagation rate, with significant fluctuations observed throughout the process. While the maximum suppression rate reached 60.9%, the maximum promotion rate was 131.12%. At 10 g/m³, the flame propagation rate initially increased before decreasing, with the most effective suppression observed within the first 6.6 m, where flame propagation suppression increased rapidly from 58.13% to 74.82%, and the peak explosion overpressure was reduced by up to 62.94%. These findings contribute valuable insights for the development of effective gas explosion suppression strategies in mining environments.

1. Introduction

Coal mine gas, primarily composed of methane, poses a significant explosion risk under specific conditions, such as when its concentration reaches the explosive limit, an ignition source is present, and sufficient oxygen is available. The resulting shockwave and high-temperature flame not only directly damage underground infrastructure but may also trigger secondary disasters, including coal dust explosions and tunnel collapses, thereby amplifying the incident’s impact [1,2]. For example, on 12 October 2020, a gas explosion at the Zuoyuan Fusheng Coal Mine of the Shanxi Lu’an Group resulted in four fatalities, one injury, and direct economic losses of CNY 11.33 million [3]. Incomplete statistics indicate that, from 2011 to 2022, 112 gas explosion accidents occurred in Chinese coal mines, leading to 923 deaths [4]. Consequently, to mitigate gas explosion accidents, it is imperative to thoroughly investigate effective gas explosion suppression technologies capable of inhibiting the propagation of initial explosion flames and pressure. This research aims to enhance coal mine safety, safeguard workers’ lives and property, and improve the overall production safety.

The current gas explosion suppression technologies include inert gas suppression [5], water-based suppression [6], powder-based suppression [7], and porous material suppression techniques [8]. Among these, powder-based suppression presents significant advantages for underground coal mine applications, particularly in mitigating secondary explosions. Both domestic and international researchers have conducted extensive studies on powder-based explosion suppression technologies [9,10]. Research has demonstrated that powders such as NH₄H₂PO₄, Mg(OH)₂, NaHCO₃, calcium carbonate (CaCO₃), SiO₂, and Al(OH)₃ exhibit suppressive effects on gas explosions [11]. For instance, Wang et al. investigated the suppressive effects of Al(OH)₃ and Mg(OH)₂ powders on CH₄/air explosions in a 20 L confined container, obtaining explosion parameters including the maximum explosion pressure, upper explosive limit, and lower explosive limit at varying gas and powder concentrations [12]. Dounia et al. utilized theoretical analysis and numerical simulation methods to examine the suppression effects of NaHCO₃ powders with different particle sizes on CH₄/air flames. Their study revealed that the maximum particle size capable of decomposing and effectively participating in chemical reactions within the flame front depends on the flame speed [13]. Jia examined the impact of the KHCO₃ powder particle size and powder spraying pressure on methane explosions in a network of 8100 mm × 5500 mm pipes, concluding that both the particle size and spraying pressure significantly influenced the suppression effect [14]. Luo Zhenmin et al. conducted a 20 L explosion ball experiment to investigate the effects of NH₄H₂PO₄ powder after endothermic decomposition on methane explosions. Their findings indicated that the thermal decomposition products of NH₄H₂PO₄, such as ammonia gas and phosphorus pentoxide, exhibited a weakening effect on the methane explosion process [15]. Liu et al. studied the inhibition of gas explosions using nano-SiO₂ powder under the condition of obstacles in a 20 L near-spherical vessel. It was found that the concentration of 100 mg/L nano-SiO₂ powder could reduce the explosion limit range of methane to 6.95–13.75%, and the maximum overpressure of an alkane air body with a methane concentration of 7% could be reduced to 0.179 MPa [16].

Extensive research has shown that the suppression efficacy of various powder-based suppressants is influenced by both external factors (such as the powder spraying pressure and ignition delay time) and the intrinsic properties of the suppressants (including the powder type, particle size, and concentration). For a given type of powder suppressant, a smaller average particle size typically results in enhanced suppression effectiveness [17,18]. In recent years, researchers worldwide have applied ultrafine powders, developed in firefighting technologies, to mitigate gas explosions in confined spaces [19]. Ding Chao et al. examined the suppression of gas explosions using ultrafine NH₄H₂PO₄ powder with a median particle size of 8 μm in a 100 L explosion container. Their research indicates that, when the suppression device’s trigger time is fixed, if the NH₄H₂PO₄ powder amount falls below a critical value, the powder generally enhances and intensifies flame propagation [20]. Dounia et al. derived an analytical solution for the critical particle size, demonstrating that, above this size, minimal or no suppression effects are observed. However, when particles are sufficiently small, they decompose in the flame to produce gaseous suppressants, exerting effective chemical suppression [21]. Li et al. investigated the suppression performance of ordinary ABC dry powder, ultrafine ABC dry powder, and CO₂ composite suppressants on methane/air premixed gas in a confined chamber. Their findings revealed that ultrafine ABC dry powder exhibited superior explosion reduction compared to ordinary ABC dry powder, and the combination of CO₂ and ultrafine ABC dry powder demonstrated a synergistic effect in mitigating methane/air explosions [22]. Xie et al. conducted experimental studies on the synergistic suppression of gas explosions using hydrophobic nanosilica (SiO₂) and nanosized CaCO₃ in an open-space explosion experimental setup with a total length of 1000 mm and an internal diameter of 10 mm. Their results indicated optimal gas explosion suppression when SiO₂ and CaCO₃ were mixed in a 1:1 ratio [23]. Wen Hu et al. employed a 20 L explosion ball experiment to examine the suppression effect of ultrafine Al(OH)₃ powder on the methane explosion pressure [24].

While existing research on single powder-based suppressants primarily focuses on compounds such as ammonium dihydrogen phosphate (NH₄H₂PO₄) and sodium bicarbonate (NaHCO₃), studies on CaCO₃ powder are relatively scarce. Despite this lack of research, CaCO₃ powder has certain advantages in terms of resource richness, thermal stability, cost, and environmental friendliness. Compared with common explosion suppression powders such as NaHCO₃ and NH₃H₂PO₄, CaCO₃ has a higher decomposition temperature and can maintain stability at higher temperatures, which helps to prolong the duration of the explosion suppression effect [25,26]. Therefore, in this study, we selected CaCO₃ powder as the suppressant material and examined its effectiveness in mitigating the gas explosion pressure and flame characteristics at different concentrations within a medium-scale explosion tube. The aim was to provide theoretical insights for the advancement of powder-based explosion suppression technologies in coal mine applications.

2. Mechanism of CaCO₃ Powder in Suppressing Gas Explosions

Methane explosions encompass complex chain reactions and the generation and propagation of free radicals. Key free radicals, including H− and OH−, are essential in maintaining the chain reaction during methane explosions. These highly reactive species can initiate and transfer energy, thus accelerating the explosion process. CaCO₃ powder possesses the ability to absorb the H− and OH− free radicals produced during methane explosions, effectively interrupting the chain reaction mechanism.

The thermal decomposition of CaCO₃ powder at high temperatures yields CaO and CO₂. During this process, CaCO₃ powder absorbs thermal energy from the methane explosion, consequently reducing the reaction rate and inhibiting the formation and propagation of free radicals. This mechanism plays a crucial role in suppressing methane explosions. Additionally, the CaO and CO₂ produced from CaCO₃ decomposition chemically interact with H− and OH− free radicals generated during the methane explosion. This interaction consumes these free radicals in the methane explosion chain reaction, thereby providing effective chemical suppression. The primary reactions involved in this process can be represented by the following equation [27,28,29]:

$$\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3\to \mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$$

(R1)

The heterogeneous reactions involving CaCO₃ in the chemical reactions of the methane explosion can be expressed as

$$\mathrm{C}\mathrm{a}\mathrm{O}+2\mathrm{O}\mathrm{H}-+2\mathrm{H}-\to {\mathrm{H}}_2\mathrm{O}+\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2$$

(R2)

Regarding secondary reactions, forward (or reverse) reactions in conditions of limited (or excessive) liquid water can be expressed as

$$\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2\rightleftharpoons \mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(R3)

3. Experimental Equipment and Method

3.1. Experimental System

Figure 1 depicts the experimental setup. The apparatus consists of several essential components: a mid-scale pipeline, a premixed gas configuration system, an ignition system, a dust spraying system, the Donghua data acquisition system, and a synchronized control acquisition system. The mid-scale pipeline is a horizontally positioned square steel pipe measuring 20 cm × 20 cm × 960 cm, with a thickness of 0.1 cm. The pipeline’s front end functions as the ignition point, while the middle section comprises multiple shock tube segments, and the rear end is sealed with a steel blind flange. The premixed gas configuration system includes a gas storage tank, an air compressor, a vacuum pump, a methane cylinder, and a configuration cabinet. The Donghua data acquisition system, model DH8302, from the Donghua Corporation features a maximum sampling rate of 1 MHz per channel and primarily utilizes pressure and flame sensors. The pressure sensors are ICP piezoelectric sensors with a maximum range of 6.9 MPa and a resonant frequency exceeding 500 kHz. The flame sensors are custom-designed optoelectronic signal converters that transform flame signals into electrical signals, with an AD card recording the voltage divider signal of the resistance. The configuration cabinet regulates the connection of the premixed gas, a mixture of methane and air, through flexible hoses.

The premixed gas configuration adheres to Dalton’s Law of Partial Pressures. Initially, methane at a 9.5% concentration (based on its proportion in the total volume of gas in the storage tank) is introduced into a prepared gas storage tank. Subsequently, air is added until the predetermined required amount is achieved. After sealing all valves, the mixture is allowed to settle and undergoes complete premixing. The dust dispersion system consists of an air compressor with a flow rate of 40.41 m³/min and flexible hoses connecting a portable air compressor, a dust chamber, and the shock tube. Previous studies have shown that, when the particle size is 6.5 μm, the particles are too small to disperse effectively and tend to remain suspended near the injection port [30]. The ignition system employs a custom-developed pulsed high-voltage discharge device, activated by the TTL platform of a multi-channel synchronized controller. This electric spark ignites the premixed gas within the shock tube.

Prior to dust injection, the air inside the pipeline was evacuated using a vacuum pump until the pressure gauge indicated −100 kPa (gauge pressure). Methane and air were then introduced into the pipeline using the partial pressure method. Gas injection was stopped once the pressure gauge reached −3 kPa. The ignition delay time was set to 600 ms by the control system, with 100 ms allocated for dust injection and 500 ms for dust dispersion. For dispersion, the controller pressurized the air compressor to 3 kPa, which was then directed into the dust container to propel the ultrafine CaCO₃ powder into the pipeline, forming a dust cloud. The high-pressure airflow facilitated the mixing of the CaCO₃ dust cloud with the methane/air mixture in the pipeline. Preliminary experiments confirmed that this injection timing and pressure configuration allowed the pressure gauge to return to 0 kPa before ignition. To ensure the reliability of the results, each experimental condition was replicated three times. The most representative data set was then selected for analysis using a test method based on the experimental outcomes. The experiments showed that, under identical experimental conditions, CaCO₃ dust clouds without methane did not result in explosions. Additionally, the used ultrafine CaCO₃ powder was collected for centralized disposal. When large quantities of ultrafine CaCO₃ powder are used, the resulting CaO generated after the explosion poses a potential alkalization risk to the environment; therefore, the residual powder must be properly collected and treated.

3.2. Experimental Method

A methane/air premixed gas with an optimal concentration of 9.5% was used in this investigation as the experimental medium [31]. To suppress methane explosions, CaCO₃ powder with a particle size range of 4.5 µm to 6 µm was utilized as an inert suppressant agent. We examined the suppression effects of ultrafine CaCO₃ of 99% purity on the methane explosion pressure and flame propagation characteristics at various concentrations. The experiment was conducted under eight distinct conditions, as outlined in

Table 1. Experimental conditions.

Condition Number	Ultrafine CaCO3 Concentration	Condition Number	Ultrafine CaCO3 Concentration
1	0 g/m3	2	5 g/m3
3	10 g/m3	4	20 g/m3
5	25 g/m3	6	75 g/m3
7	125 g/m3	8	300 gm3

. The concentration settings of ultrafine CaCO₃ were determined based on the volume of the pipeline and the mass of the CaCO₃ powder. Flame sensors and pressure sensors were positioned at identical axial locations at distances of 200 cm, 455 cm, 600 cm, 660 cm, and 835 cm from the ignition end of the pipeline, designated from left to right as F1–F4 and P1–P5, respectively. A dust injection point was established 370 cm from the ignition end, situated 85 cm from the P2 or F2 monitoring point. Each injection of ultrafine CaCO₃ employed a pressure of 3 kPa. Figure 2 depicts the pipeline model.

4. Experimental Results

4.1. Flame Propagation Velocity

The inhibition of the methane explosion’s flame propagation speed by ultrafine CaCO₃ powder is illustrated in Figure 3, while Figure 4 depicts the flame propagation rate. Figure 3 and Figure 4 demonstrate that, at measurement point F1, the flame propagation speeds in conditions 2 and 4–8 are lower than in condition 1. The most significant decrease occurs in condition 7, with a 20.8% reduction. This phenomenon is attributed to the initial stage of a methane explosion, where the precursor compression wave’s propagation speed exceeds that of the flame. The resulting disturbance lifts dust from pipe walls, propelling it towards the injection point via the reflected wave, thus reducing the initial flame propagation speed. Condition 3 exhibits a slight 0.7% increase in the flame propagation speed, which falls within the permissible fluctuation range and does not contribute to enhanced flame propagation. At test point F2, the flame propagation speeds in conditions 2, 3, 5, and 6 surpass that of condition 1, with increases of 21.9%, 7.5%, 14.0%, and 16.2%, respectively. This is due to the formation of a “dust cloud wake zone” behind the injection point, where the particle concentration is slightly lower than in front but higher than at F3. In this region, ultrafine calcium carbonate enhances mixing through turbulence without reaching complete combustion suppression, forming an “optimal acceleration window”. As the flame propagates past the injection point, high-temperature combustion products heat the dust cloud, causing some calcium carbonate to undergo endothermic decomposition via reaction (R1). The released CO₂ further dilutes oxygen, although endothermic decomposition may locally reduce the temperature. However, near the injection point, the combustion-enhancing effect dominated by turbulence mixing prevails. In condition 4, flame propagation speed increases by 0.12 m/s compared to F1, yet it remains 10.8% lower than the pure methane explosion speed. This is because ultrafine particles adsorb reactive free radicals on their surfaces, interrupting the chain reaction through a heterogeneous reaction (R2). In conditions 7 and 8, no light signals are detected, indicating complete flame extinction due to the flame retardant, reducing the flame propagation speed to 0 m/s.

At test point F3, the flame propagation velocities for working conditions 2 to 6 exhibited decreases of 19.8%, 58.15%, 33.6%, 52.2%, and 60.9%, respectively. This reduction can be attributed to the increasing distance from the dust injection point, where the dust cloud gradually dilutes due to diffusion and sedimentation, potentially lowering the concentration below the suppression threshold. At this point, the heat absorption and dilution effects of CaCO₃ become predominant. CO₂, an inert gas released by the thermal decomposition of CaCO₃, dilutes the oxygen concentration in the deflagration reaction system. Additionally, the thermal capacity of the generated CaO absorbs reaction heat, leading to a gradual reduction in the methane deflagration intensity. Furthermore, the surfaces of ultrafine dust particles adsorb key free radicals, such as H− and OH−, involved in the combustion chain reaction. These radicals disrupt chain propagation through reaction (R2). Although the flame propagation velocity at F3 for working condition 4 increased by 5.14 m/s compared to F2, it remained suppressed relative to the flame propagation velocity of pure methane. This phenomenon occurs because the pressure wave generated by the initial explosion reaches this point and, when combined with the reflected wave from the pipeline end, forms a local high-pressure zone. The high pressure compresses the unburned gas, increasing its density and temperature, which drives the flame velocity into the “super-driven” mode.

At test point F4, the flame propagation velocities for working conditions 2 to 5 were suppressed, decreasing to 26.0%, 74.8%, 57.2%, and 58.6%, respectively. This suppression was primarily due to dust diffusion and suppression mechanisms, with the thermal boundary layer effect also playing a significant role. The combined influence of wall heat loss and dust heat absorption reduced the unburned gas temperature below the ignition threshold, causing the flame propagation to enter a “decay zone” with an exponential velocity decrease. Conversely, the flame propagation velocity for working condition 6 increased sharply to 131.1% of that for pure methane. This increase may be attributed to the heightened presence of liquid water within the pipeline, promoting the reaction of ultrafine CaCO₃ as per reaction (R3). This process releases substantial heat, with the exothermic reaction outweighing the endothermic heat absorption of CaCO₃, creating a localized high-temperature state and intensifying the methane explosion’s flame propagation velocity. Furthermore, the shockwave accelerates methane fuel particles, forming a high-energy “particle jet” that penetrates the flame front and disturbs the unburned region, triggering a local detonation-to-deflagration transition (DDT). During the DDT, the flow field’s turbulence intensity grows geometrically, and the heat release rate increases exponentially. Consequently, the convective heat transfer coefficient between the reaction medium and the confined wall experiences a significant multiplicative increase, resulting in a sharp rise in the flame propagation velocity.

4.2. Explosion Pressure Peak

Figure 5 illustrates the pressure intensity curves for the first to third shockwave peaks at measurement points P2 and P3 for working conditions 1 to 6. The data reveal that, regardless of the wave type (first compression wave, second reflected wave, or third shockwave), the explosion pressure at the P2 measurement point is highest when the dust concentration is 10 g/m³ and lowest when the ultrafine CaCO₃ concentration is 25 g/m³. For example, at an ultrafine CaCO₃ concentration of 10 g/m³, the peak pressure of the third shockwave exceeds that of the pure methane explosion by 3 kPa. Conversely, at a concentration of 25 g/m³, the peak pressure of the third shockwave is 8 kPa lower than the pressure from a pure methane explosion, representing a reduction of approximately 19.5%. At ultrafine CaCO₃ concentrations of 5 g/m³, 20 g/m³, and 75 g/m³, the peak pressures for the first to third shockwaves deviate by ±1 kPa from those generated by pure methane explosions. Several factors contribute to these observations. At 5 g/m³, calcium carbonate enhances mixing through turbulence and partially absorbs heat, resulting in an energy release rate comparable to the original explosion level, thus causing only slight changes in the pressure peak. At 20 g/m³, thermal absorption, free radical quenching, and turbulence promotion reach a dynamic equilibrium. The energy release rate is partially suppressed but remains relatively high, leading to pressure wave attenuation and generation rates similar to those in pure methane explosions. At 75 g/m³, the high dust concentration increases acoustic impedance, shortening the pressure oscillation cycle and accelerating energy dissipation. Although the flame is significantly suppressed, the rapid compression of the unburned region during the early stages of the explosion can still generate pressure pulses similar to those of the original explosion through “inertial overpressure”.

Similarly, at the P3 measurement point, when the ultrafine CaCO₃ concentration is 5 g/m³, 20 g/m³, or 75 g/m³, the peak pressures for the first to third shockwaves are all lower compared to those generated by the pure methane explosion, indicating that the CaCO₃ powder effectively mitigates the gas explosion. For example, at a concentration of 75 g/m³, the peak pressure intensity of the shockwaves generated by the methane explosion decreases by 3 kPa to 5 kPa. At an ultrafine CaCO₃ concentration of 25 g/m³, the peak pressure intensities of the first compression wave and the third shockwave decrease by 10 kPa, while the second reflected wave’s peak pressure intensity decreases by 8 kPa. The maximum reduction in the peak pressure intensity can reach 29.4%, demonstrating the most significant suppression effect on the methane explosion. However, at a CaCO₃ concentration of 10 g/m³, the pressure intensity at P3 was slightly higher than that of the pure methane explosion, suggesting that this concentration exhibits a less pronounced suppression effect compared to higher concentrations.

At a concentration of 10 g/m³ ultrafine CaCO₃, the overall pressures at P2 and P3 exceed that of pure methane. However, the peak pressure intensity of the P3 shockwave is lower than that of P2, both at the initial compression wave (I) and the subsequent reflected wave (II). This suggests that 10 g/m³ ultrafine CaCO₃ exhibits a moderate inhibitory effect on pure methane explosions. This phenomenon results from the formation of incident and reflected waves of turbulent shockwaves within the pipeline, which diminish dust dispersion. At this concentration, the ultrafine CaCO₃ initially acted as a catalyst, promoting the oxidation reaction of methane and resulting in a higher third shockwave pressure at P3 compared to P2. However, this catalytic effect was transient. The CO₂ released via reaction (R1) rapidly transformed the CaCO₃ into an effective suppressant, causing the pressure at P3 to reach a peak before gradually decreasing until it was extinguished. As illustrated in Figure 3, the flame propagation velocity at this concentration exhibited a trend of initially increasing and then decreasing. Notably, the initial flame propagation velocity under this condition was the highest among all eight experimental conditions, reaching 16.76 m/s. Due to the temporary catalytic effect, the flame velocity further increased to 17.02 m/s during propagation. However, as the flame advanced toward P3, it was increasingly inhibited by the inert dust, resulting in a gradual decline in the propagation speed until extinction. Additionally, due to the influence of pressure fluctuations generated between the ignition point and sensor F1, the first three shockwaves were subject to considerable interference from uncontrollable factors. Consequently, during the explosion suppression process, the pressure observed may occasionally exceed that of the pure methane explosion, explaining why the peak pressures at this concentration were, in some cases, higher than those of the pure methane case.

As shown in Figure 6, the experimental results demonstrate that ultrafine CaCO₃ at different concentrations has a significant suppressive effect on the maximum explosion pressure generated by pure methane explosions. The 3D surface plot clearly illustrates that, under otherwise identical conditions, ultrafine calcium carbonate effectively reduces the peak overpressure of methane explosions. Moreover, it can be observed that the P3 position serves as a turning point for the 10 g/m³ dust concentration, while P4 represents the inflection point for the 20 g/m³ concentration. In terms of overall trends, the maximum explosion pressure resulting from pure methane explosions increases linearly along the pipeline. A similar linear trend is observed for the 5 g/m³ ultrafine CaCO₃ concentration, although its peak overpressure remains consistently lower than that of pure methane. In contrast, dust concentrations of 10 g/m³, 20 g/m³, and 75 g/m³ exhibit a trend of initially increasing and then decreasing the maximum overpressure. Concentrations of 25 g/m³, 125 g/m³, and 300 g/m³ show a trend of a rapid increase followed by a slight decrease and then a minor rebound.

In cases where the methane explosion flame was not fully suppressed, the 2D plot in Figure 6 shows that, at measurement point P1, the maximum explosion pressure (P_max) was the lowest at a CaCO₃ concentration of 25 g/m³, with a value of only 12.29 kPa—approximately 0.43 times that of the pure methane explosion. Conversely, the highest P_max at this point was observed at 10 g/m³, reaching 32.87 kPa, or 1.15 times that of pure methane. At point P2, the lowest P_max occurred under the 75 g/m³ condition (64.32 kPa), while the highest was under 25 g/m³ (85.79 kPa), corresponding to 0.89 and 1.19 times the P_max of pure methane, respectively. Similarly, at P3, the highest P_max (131.8 kPa) was observed at 5 g/m³ and the lowest (99.74 kPa) at 75 g/m³, yielding suppression rates of 9.96% and 31.86%, respectively. At P4, the maximum overpressure was 178.93 kPa at 5 g/m³, while the minimum was 111.78 kPa at 10 g/m³, with suppression rates of 4.11% and 40.10%, respectively. At P5, the highest P_max (218.46 kPa) again occurred at 5 g/m³, while the lowest value (95.69 kPa) was recorded at 10 g/m³. The latter achieved the highest suppression rate of 62.94%.

Figure 6 shows that, under conditions where the methane explosion flame was completely suppressed, the maximum overpressure generated throughout the entire pipeline at a CaCO₃ concentration of 300 g/m³ was consistently lower than that at 125 g/m³. Both concentrations effectively suppressed the explosion pressure over the entire process, with suppression rates ranging from 22.46% to 68.03% for 300 g/m³ and from 41.25% to 76.03% for 125 g/m³. However, instances were observed where the flame was extinguished while a portion of the pressure persisted. This phenomenon can be attributed to two main factors: the partial agglomeration of the powder and the relatively high dust concentration at 125 g/m³, leading to the formation of a dense dust cloud that obstructed both flame propagation and the transmission of some pressure waves. Due to the small particle size, the CaCO₃ powder tended to accumulate near the injection port after being introduced into the pipeline. As a result, flame extinction was detected at sensor F2, indicating the effective suppression of both the flame and a significant portion of the pressure. Nonetheless, owing to the excessive overpressure generated by the methane explosion, some pressure waves were still able to penetrate the dense dust cloud and propagate to the end of the pipeline, where they were reflected and continued to oscillate within the pipe. A similar mechanism was observed for the 300 g/m³ concentration condition.

As illustrated in Figure 7, it can also be observed that, from ignition to the flame reaching sensor F1, the flame sustained combustion for 379.83 ms under the 125 g/m³ ultrafine CaCO₃ dust condition. By the time the flame arrived at F1, the first compression wave had already reached P5. Comparing this with P1, it is evident that, within 3 ms of the flame reaching F1, the maximum explosion overpressure was recorded at P1. A similar pattern was observed under the 300 g/m³ condition. Furthermore, flame propagation relies on the surrounding thermal energy to sustain combustion. Prior to reaching F1, the heat generated by methane combustion was gradually transmitted to the unburned region ahead via the advancing shockwave. However, upon encountering the dust cloud, the shockwave’s progression was impeded, and the heat energy was diluted. Given that a high dust concentration requires a higher temperature to initiate powder reactions, the diluted heat failed to provide sufficient energy for the CaCO₃ dust cloud to react. Consequently, the pressure at P1 rapidly transitioned from overpressure to underpressure. The shockwave passing through P1 repeatedly compensated for this underpressure, thereby influencing the subsequent pressure wave behavior. Although P1 experienced a transition to negative pressure, methane combustion continued—albeit at a reduced rate—which, in turn, sustained the oscillatory behavior of the pressure waves. Flame extinction was primarily caused by the significant dilution of oxygen due to the dense dust cloud, leading to an oxygen deficiency. Nevertheless, the residual temperature and pressure generated by methane combustion persisted. This was mainly because the dust cloud formed a densely woven high-concentration region that interfered with the propagation of pressure waves, resulting in residual pressure waves. These residual waves were subsequently counteracted by the negative pressures at P2 and P5 and further dissipated through collisions and friction with the pipeline walls, ultimately leading to energy loss and the complete attenuation of the pressure waves.

4.3. Discussion

The flame and pressure suppression effects of ultrafine CaCO₃ powder on methane explosions can be evaluated by assessing the degree of suppression at individual measurement points or by analyzing them from a spatial perspective based on the distance from the ignition end along the horizontal axis.

Analysis of methane explosion flame characteristics: The flame propagation characteristics during methane explosions can be categorized into two cases. In the first case, the complete suppression of flame propagation was achieved. At concentrations of 125 g/m³ and 300 g/m³, the flame velocity dropped to 0 m/s within 85 cm upstream of the dust injection point, fully demonstrating the suppressive advantage of high dust concentrations. In the second case, the suppression was incomplete. At the same measurement point, 200 cm from the ignition end, the best suppression was observed with 25 g/m³ of ultrafine CaCO₃ powder, achieving a suppression rate of 18.87%. The 10 g/m³ concentration showed slightly weaker performance: after briefly acting as a catalytic promoter, it rapidly transitioned into an inhibitor and continued to exert a suppression effect throughout the subsequent stages. At 455 cm, the 20 g/m³ concentration achieved the best suppression effect (10.83%), whereas the 5 g/m³ concentration promoted the explosion by 21.86%. At 600 cm, the 75 g/m³ concentration exhibited the highest suppression rate (60.90%), while the 5 g/m³ concentration showed the lowest (19.76%). At 660 cm, the 10 g/m³ concentration demonstrated the most effective suppression, with the rate increasing dramatically from 58.13% to 74.82%. Conversely, the 75 g/m³ concentration exhibited a significant promoting effect (131.12%). Horizontally, the 5 g/m³ and 25 g/m³ concentrations displayed a suppression–promotion–suppression pattern, the 10 g/m³ dust showed a promotion–suppression trend, the 20 g/m³ concentration maintained consistent suppression, and the 75 g/m³ concentration exhibited a suppression–promotion–suppression–promotion pattern throughout the test.

Analysis of methane explosion pressure characteristics: Due to the influence of uncontrollable factors during the propagation of the initial three pressure waves (from the ignition point to F1), it is possible for the peak overpressure in the presence of inert dust to exceed that of pure methane explosions. Regarding the peak explosion intensity, under conditions where flame suppression was incomplete, the most effective pressure suppression at 200 cm from the ignition point was observed with a 25 g/m³ CaCO₃ concentration, achieving a suppression rate of 57%. The suppression trend followed a pattern of rapid increase—slight decrease—slight increase. Similarly, at 455 cm, the 75 g/m³ concentration exhibited superior suppression compared to other concentrations, with a suppression rate of 19% and a trend of an initial increase followed by a decrease. At 600 cm, the best suppression was again achieved with 75 g/m³, yielding a suppression rate of 31.86%. At 660 cm, 10 g/m³ showed the most effective suppression, following a rise–fall pattern with a suppression rate of 40.10%. At 835 cm, 10 g/m³ also performed the best, achieving the highest suppression rate among all conditions with incomplete flame suppression, reaching 62.94%. Under conditions of complete flame suppression, dust concentrations of 125 g/m³ and 300 g/m³ were both effective in suppressing the maximum overpressure generated by methane explosions. The corresponding suppression rates ranged from 22.46% to 68.03% for 125 g/m³ and from 41.25% to 76.03% for 300 g/m³. Although some pressure waves persisted temporarily after the flame was extinguished, the peak overpressure had already formed at the moment of flame extinction. The intensity of the residual pressure waves exhibited a gradually decreasing trend, consistent with the process of energy dissipation, eventually fading out due to energy loss.

5. Conclusions

In this research, the inhibitory effect of varying concentrations of ultrafine CaCO₃ powder on the methane explosion flame propagation velocity and explosion pressure within a medium-scale gas explosion pipeline was examined.

(1)

When the concentration of CaCO₃ powder is 125 g/m³ and 300 g/m³, the suppression effect on methane explosions is the most significant, with complete flame quenching observed. However, the explosion-induced pressure is not immediately eliminated; it gradually dissipates after repeated collisions of the shockwaves with the duct walls and mutual cancellation between positive and negative pressure zones due to energy loss. When the dust concentration is 20 g/m³, the suppression effect is optimal among the incomplete suppression conditions, with continuous flame suppression observed throughout the entire flame propagation process. Additionally, the first to third pressure waves at test points P2 and P3 are reduced, and the maximum explosion overpressure significantly decreases. When the concentration is 25 g/m³, the powder accelerates flame propagation at F2 (downstream of the injection point), and the resulting maximum overpressure is the highest among all test conditions. Therefore, this concentration is not recommended for industrial applications.

(2)

Although increasing the concentration of ultrafine CaCO₃ powder can reduce the propagation rate of methane explosion flames to some extent, if the flame propagation is not completely interrupted, both excessively low and high concentrations may participate in chain reactions and temporarily accelerate methane combustion, thereby releasing more heat. As a result, ultrafine CaCO₃ may act as a transient catalyst during the explosion process, accelerating flame propagation and further promoting the transmission of the flame and pressure. Therefore, in practical applications, it is recommended that an appropriate concentration of ultrafine CaCO₃ is adopted as a suppressant for methane explosions.

(3)

The distance between the powder injection point and the test point significantly influences the effectiveness of inert powders in suppressing gas explosions, particularly regarding flame propagation. When the distance between the ignition end and the powder injection point is 3.7 m, and the distance from the injection point to the observation point is 1.7 m, selecting an ultrafine CaCO₃ powder concentration of 125 g/m³ can completely suppress the flame and significantly reduce the maximum explosion overpressure. Within 4.55 m from the ignition end, using a concentration of 20 g/m³ is effective in suppressing both the flame and pressure, with a maximum flame suppression rate of 15.58%. Similarly, within 6 m, a concentration of 10 g/m³ can achieve approximately 60% flame suppression, although the initial flame acceleration effect of 75 g/m³ is 2.3 times that of 10 g/m³. Within 6.6 m, 10 g/m³ is preferable for the suppression of the flame speed, with suppression increasing from 58.13% to 74.82%, while 20 g/m³ is optimal when considering both flame and pressure suppression.