Study on the Inhibitory Effect and Mechanism of Modified Ultrafine ABC Powder on CH₄/Coal Dust Coexistence Explosions

Abstract

This study investigated the inhibitory effect and mechanism of modified ultrafine ABC powder on the explosion of a methane (CH₄)/coal dust mixed system. Through experiments, it was found that the addition of ABC powder significantly weakened the deflagration characteristics of the CH₄/coal dust mixture system. During decomposition, heat was absorbed to generate ammonia and phosphoric acid. Inert gases such as CO₂ and water vapor produced during decomposition could dilute the oxygen concentration. Phosphate ions produced during the decomposition of ammonium phosphate would bind with free radicals during combustion, reducing their reactivity. The explosion reaction was suppressed through a dual mechanism of physical cooling and chemical consumption of free radicals. The experimental results showed that the weight loss rate of modified ABC powder was 49% at 800 °C, while the weight loss rate of unmodified ABC powder was 78%. The modified ABC powder had better thermal stability and could absorb more heat at high temperatures, further suppressing explosive reactions. This study provides a new modification scheme for explosion suppressants for coal mine safety, which has important theoretical and practical application value.

1. Introduction

Coal is the most common fossil fuel in the world, which can be used in various fields such as power generation, heating, industrial production, and transportation [1,2,3,4]. Due to insufficient ventilation capacity, mines are also prone to CH₄ accumulation. Once exposed to high-temperature heat sources, CH₄/coal powder coexistence explosion accidents are likely to occur [5,6]. Therefore, suppressing the explosion of CH₄/coal dust mixture system through inert substances can effectively prevent or reduce the damage caused by the explosion, which is an essential link in coal mining engineering sites [7,8].

The explosion of CH₄/coal powder in the coal mining field can cause great harm; the explosion characteristics and suppression laws of the mixed system have been studied [9,10,11]. Liu et al. [12] studied various types of detonation inhibitors, primarily to assess their inhibitory efficacy against CH₄ (mashgas)/coal dust mixtures. They found that the detonation wave velocity and overpressure without the addition of explosion suppressants were 1560 m/s and 6.0 MPa, respectively. However, after the addition of explosion suppressants, the parameters decreased to 650 m/s and 0.5 MPa, respectively. Yu et al. [13] grafted phosphorus-containing substances (ammonium polyphosphate) onto fly ash microspheres through acid-base modification, co-condensation, and amidation, resulting in significantly prolonged explosion time, reduced pressure, and (dP/dt)_max in the prepared composite powder. Chen et al. [14,15,16] discussed in depth the effects of APP, Al(OH)₃, and fly ash, which are common materials, on the explosion characteristics of CH₄/coal powder. The inhibitory effects of APP were better than those of Al (OH)₃ and fly ash, and APP had the best inhibitory effect. Zhao et al. [17,18] further studied the inhibition effect of two-phase inhibitors (N₂/APP, CO₂/fly ash) on CH₄/coal powder explosion flames. The addition of APP can prevent the reignition of CH₄/coal powder explosion flames after passing through the N₂ layer. The amount of CO₂ and fly ash added is proportional to the inhibition effect of the explosion system. Jiang et al. [19] conducted a water mist interference test on methane coal powder using dimethyl phosphate and phytic acid and found that the addition of additives significantly enhanced its heat absorption performance, effectively suppressing the temperature and spread rate during CH₄/coal combustion. Research has shown that the main direction of current explosion suppressant research is focused on the development of new explosion suppressants and composite explosion suppressants [20,21,22,23,24]. Li et al. [25] conducted a comparative study on the explosion suppression effects of ABC powder and potassium bicarbonate/zeolite composite explosion suppressant (PBZ) on hydrogen-doped natural gas (HCNG). The comparison found that for HCNG explosion with 20% hydrogen added, the optimal concentrations of ABC powder and PBZ composite powder suppressants were 0.12 g/L and 0.1 g/L, respectively. Zhang et al. [26] successfully prepared ABC powder (NH₄H₂PO₄)/RM composite powder using the anti-solvent method. The synergistic inhibitory effect between NH₄H₂PO₄ and RM resulted in comparable inhibitory performance compared to pure NH₄H₂PO₄ or red mud powder. Han et al. [27] mixed nickel salt, sodium hypophosphite, and Hβ-Al₂O₃ carrier to generate NiBP active component through sodium borohydride reduction reaction. Then, by adjusting the boron phosphorus ratio to optimize the structure, NiBP is uniformly dispersed on the surface of the carrier, forming a composite explosion suppression material that combines free radical capture and thermal barrier functions. Liang et al. [28] calcined the slag of the biomass power plant at high temperature and modified it by the sol–gel method. Through mechanochemical technology, ultrafine KHCO₃ particles and modified slag were ball milled in proportion to form KHCO₃/modified slag composite explosion suppression powder. Li et al. [29] synthesized Ni-MOF using terephthalic acid and hexahydrate nickel chloride as raw materials; subsequently, piperazine pyrophosphate (PAPP) was introduced as a functional component in the same system and prepared by co-solvent thermal reaction of the PAPP@Ni-MOF compound material. However, due to the complexity of production processes and cost factors, most new and composite explosion suppressants are difficult to achieve in large-scale industrial applications. Among the existing explosion suppressants, ABC powder has become one of the most widely used due to its excellent fire extinguishing and explosion suppression performance. However, ABC dry powder has significant moisture absorption and agglomeration problems after ultrafine refinement, which not only reduces its spraying performance but also has adverse effects on its explosion suppression effect. Therefore, in the preparation of ABC-type ultrafine dry powder explosion suppressants, surface modification techniques must be used to improve their performance, including inhibiting the agglomeration of ultrafine powders, enhancing moisture resistance, and improving flowability, in order to ensure their effectiveness in practical applications.

This study selected typical ultrafine ABC-type dry powder explosion suppressants as experimental objects and focused on conducting modification experiments from two aspects: hydrophobicity and flowability. In the experiment, the silane coupling agent KH550 and talc powder were used to modify ABC powder. By conducting experiments on the detonation characteristics of CH₄/coal powder, the optimal explosion conditions were determined, and the influence of modified ABC powder on the explosion characteristics of CH₄/coal powder was systematically studied.

2. Experimental Materials and Methods

2.1. Material Preparation

The coal used in the experiment is brown coal. The coal was ground by a crusher, and 89.1% of the coal powder particles had a particle size below 75 µm. The results of industrial analysis and elemental analysis are shown in

Table 1. Industrial and elemental analysis.

Sample	Industrial Analysis (wt%, ad)				Elemental Analysis (wt%, ad)
	Moisture	Ash Content	Volatile Matter	Fixed Carbon	C	H	N	O	S
Lignite	12.28	7.93	39.55	42.24	56.24	4.47	1.22	37.47	0.60

. ABC powder and silane coupling agent KH550 were purchased from Shanghai McLean Biochemical Technology Co., Ltd., Shanghai, China. The ABC powder was sieved through a 3000 mesh sieve, and 90% of the particles had a diameter less than 10 µm. Talc powder purchased from Shandong Yousuo Chemical Technology Co., Ltd. in Linyi, China.

2.2. Experimental Methods

2.2.1. Preparation of Modified Samples

In this experiment, the airflow grinder is used as the grinding equipment, and the equipment manufacturer is China Shandong Weifang essence Powder Technology Co., Ltd., Weifang, China. The SHR high-speed heating mixer is used as a modification equipment to perform surface modification treatment on ABC powder. The equipment manufacturer is Jiangsu Bell Machinery Co., Ltd. in Wuxi, China. During the modification process, hydrophobic agents (silane coupling agent KH550) and flow aids (talc powder) were selected as modifiers. The specific experimental steps are as follows:

Sample number 1: Take 200 g of ABC powder raw material, place it in an air flow crusher, and grind it at a speed of 400 m/s for 40 min. The ground sample was dried in a vacuum drying oven for 24 h and labeled as sample 1.

Sample number 2: Take 1000 g of ABC powder and place it in a high-speed heating mixer. After the temperature rises to 70 °C, atomize and spray 3%, 4%, and 5% (mass ratio) of silane coupling agent KH550 (after alcoholysis with anhydrous ethanol) into the mixer, and stir continuously for 1 h. Subsequently, 200 g of modified ammonium dihydrogen phosphate was placed in a jet mill and ground at a speed of 400 m/s for 40 min. The ground samples were dried in a vacuum drying oven for 24 h and labeled as samples 2-1, 2-2, and 2-3, respectively.

Sample number 3: Mix ABC powder and talc powder in mass ratios of 3%, 4%, and 5%, with a total mass of 200 g, and place them in an air flow crusher to grind at a speed of 400 m/s for 40 min. The ground samples were dried in a vacuum drying oven for 24 h and labeled as samples 3-1, 3-2, and 3-3, respectively.

All modified powder samples were subjected to hydrophobicity and flowability tests to evaluate their modification effect. Hydrophobicity is measured by contact angle, moisture absorption rate, and moisture content, while flowability is characterized by a flowability index tester. Finally, the optimal modification scheme will be selected based on the test results.

2.2.2. Experimental Procedure

The lignite was ground by a crusher before the experiment, and 89.1% of the coal powder particle size was below 75 µm. In the process of mining coal resources, the coexistence of low-concentration CH₄ (less than 5% by volume) and coal dust generally exists in underground operation points. Once a high-temperature heat source is encountered, a CH₄/coal coexistence explosion will occur. As shown in Figure 1 and Figure 2, the 20 L spherical explosive tank experimental device [30,31,32] and flame propagation experimental device [33,34] were used to study the effect of low concentration CH₄ on CH₄/coal dust co-explosion. Two sets of equipment were purchased from Jilin Hongyuan Scientific Instrument Co., Ltd. in China. The suppression agent is ABC powder with a D50 of 24 µm.

According to the volume of the device, the required amount of dust and gas is converted. A certain amount of coal powder is placed in the powder storage bin of the explosive device, and the interior of the device is evacuated. Air and methane are introduced into the gas storage tank according to the ratio. Click the start button, and the methane/air mixture blows the coal powder into the explosive device to form an explosive condition. After 60 ms, the electrode will ignite the mixed system. The specific operation steps can refer to the standards (ISO6184-1 [35] and GB/T 16428-1996 [36]). The flame propagation device is a transparent tube with a diameter of 70 mm and a length of 600 mm. Based on the volume conversion of gas and dust, the gas mixture is used to lift the dust and ignite it with a delay of 30 ms. The data are recorded by a camera, and the specific operation method refers to the standard (GB/T 3836.12-2019/ISO/IEC 80079-20-2:2016 [37,38]).

3. Results and Discussion

3.1. Characterization Parameters of Modified ABC Powder

3.1.1. Hydrophobicity and Fluidity

The contact angle is an important indicator for characterizing the hydrophobicity of powders, and measuring the contact angle of powders can further our understanding of the interactions at the solid–liquid interface. Contact angle measurements below 90° indicate hydrophilicity, while measurements above 90° indicate hydrophobicity. The contact angle of sample 1 is 41.35°, while the contact angles of samples 2-1, 2-2, and 2-3 are 78.56, 90.29, and 80.34, respectively. Referring to GA578-2005, the moisture absorption and moisture content before and after modification were tested, and the parameters of sample 1 were 1.87% and 0.29%, respectively. The parameters of samples 2-1, 2-2, and 2-3 are 1.66%, 1.59%, and 1.60%, respectively, with moisture contents of 0.21%, 0.17%, and 0.19%, respectively. Therefore, 4% silane coupling agent KH550 is selected as the hydrophobic agent.

The flowability was measured using the HY-100A powder compaction density meter. The equipment is produced by Haoyu Technology Co., Ltd. in Dandong, China. The flowability of ultrafine powder explosion suppressants has a significant impact on their release in the explosion space. The better the flowability of the explosion suppressant, the higher the spraying efficiency during use. At the same time, after release, it can better exert its total submergence effect in the explosion space, increasing the explosion suppression efficiency. The flowability of sample 1 is 0.78, while the flowabilities of samples 3-1, 3-2, and 3-3 are 0.66, 0.58, and 0.53, respectively. The smaller the liquidity index, the lower the liquidity. Therefore, 5% talc powder is selected as the flow aid.

3.1.2. Scanning Electron Microscopy Image Analysis

The ABC powder explosion suppressant before and after modification is shown in Figure 3. Although the explosion suppressant before modification was sieved through a 3000 μm sieve, the agglomeration phenomenon was very obvious, and the surface was dull and dull. The modified ABC powder (MABC) particles have a clear and distinct structure and are evenly distributed. The observed phenomenon can be attributed to the significant reduction in surface energy of particulate matter following the application of hydrophobic agents and flow-enhancing additives. This surface modification effectively mitigates the steric hindrance effect between adjacent particles, thereby substantially reducing the propensity for powder agglomeration. KH550 is chemically grafted onto the ABC surface through Si-O bonds, and talc powder forms a steric hindrance layer through physical adsorption, jointly reducing van der Waals forces between particles and enhancing the overall flow characteristics of the powder system.

3.2. Explosion Overpressure Characteristics of CH₄/Coal Powder Mixture System

3.2.1. Explosion Overpressure Characteristics of CH₄/Coal Powder Mixed System

Figure 4 shows the effect of CH₄ concentration on the P_max and (dP/dt)_max of CH₄/coal powder coexistence. The trend of the explosion pressure process in repeated experiments is similar, with an error of no more than 5% between the P_max and the (dP/dt)_max. The coexistence of single coal dust and mixed systems has the same development process, which first reaches the P_max at an extremely fast speed and then slowly decreases. As the concentration of CH₄ gas in the environment gradually increases, the P_max at which an explosion occurs when mixed with coal dust also continues to rise. The reason behind this phenomenon is that CH₄ gas has a smaller minimum ignition energy (MIE) compared to coal dust. After ignition, methane gas is first ignited and releases enormous energy. The liberation of this thermal energy plays a pivotal role in facilitating the pyrolysis of coal powder, thereby significantly augmenting the evolution of combustible volatile matter. These volatiles, predominantly comprising light hydrocarbons and other flammable gases, serve as essential contributors to the intensification of coal dust explosions. Their rapid ignition and subsequent combustion, characterized by high reaction rates and substantial heat release, are fundamental to the development of violent explosive events. This process establishes a self-sustaining cycle where the initial energy release promotes further pyrolysis, leading to increased volatile production and, consequently, more intense combustion reactions. Therefore, the ignition of CH₄ initiates a self-propagating chain reaction with dual effects: the thermal energy effect is manifested as the high heat flux released by methane combustion, which triggers the pyrolysis of coal dust particles. The volatile matter does not increase significantly, and the turbulent flow field enhances the mixing efficiency of volatile matter and oxygen; coal dust triggers group synchronous pyrolysis through radiation absorption, expanding the explosion range. At the level of reaction kinetics, the free radicals generated by methane cracking initiate a chain reaction, and the thermal feedback effect accelerates energy release. The two are coupled through positive feedback, with pyrolysis providing fuel to sustain the reaction and releasing heat to further accelerate pyrolysis. The enhanced energy transfer from CH₄ combustion to pulverized-air systems is shown in two measurable parameters: first, it results in an increase in the P_max of the CH₄/pulverized coal mixture due to more complete combustion and greater energy release; and, second, it results in a gradual increase in the (dP/dt)_max as the concentration of CH₄ increases, reflecting the system’s accelerated combustion dynamics and faster energy release characteristics. This is because the (dP/dt) _max reflects the speed of the system oxidation reaction, and the combustible gas CH₄ is easier to ignite than solid coal powder particles. The thermal energy liberated from CH4 combustion is transferred to coal dust particles through both conductive heat transfer and radiative mechanisms, thereby accelerating their thermal decomposition and subsequent reaction kinetics. This energy transfer process significantly reduces the induction period required for the system to attain its peak pressure, consequently leading to a marked (dP/dt)_max. Experimental observations demonstrate that at low CH₄ concentrations (with volume fractions below 5%), the flame propagation velocity and explosive intensity of CH₄/coal dust hybrid mixtures exhibit a positive correlation with increasing CH₄ concentration. This concentration-dependent behavior is attributed to the enhanced thermal feedback mechanism between gas-phase combustion and solid-phase pyrolysis. Based on these findings, a CH₄/coal dust mixture containing 4.5% CH₄ (volume fraction) was selected as the optimal concentration for investigating the suppression efficiency of explosion suppressant powders, as this concentration represents the threshold where the hybrid mixture demonstrates significant explosive characteristics while maintaining experimental safety margins.

3.2.2. The Inhibition Law of ABC or MABC on Explosion Overpressure Characteristics of Mixed Systems

Figure 5 and Figure 6 show in detail the effects of ABC powder and MABC on the evolution of explosion pressure over time in the CH₄/coal powder mixture system. At the beginning of the explosion, the pressure within the equipment remains relatively low, and the corresponding curve exhibits a sharp upward trend followed by a gradual decline. The P_max, (dP/dt) _max, and pressure peak arrival time (t_Pmax) are selected as indicators to evaluate the characteristics of the explosion. Research has shown that the P_max of CH₄ at a concentration of 4.5% is generally above 0.6 MPa. As shown in Figure 5b, the P_max of adding 4.5% CH₄ to coal powder is 0.581 MPa. This is because after adding coal powder to CH₄, the coal powder is easily decomposed by heat to produce combustible gases such as CO and CH₄. However, the amount of oxygen in the explosion tank is limited, and the increase in combustible substances inevitably leads to the fixed oxygen content in the explosion chamber being unable to support the complete reaction of combustible substances. The thermal decomposition of ABC powder requires the consumption of oxygen, which competes with CH₄/coal powder combustion to form oxygen. The addition of ABC forces oxygen to be divided into two parts: one for fuel combustion and the other for ABC decomposition. Therefore, as the proportion of ABC powder added increases, the maximum explosive pressure remains almost unchanged. After adding modified ABC powder, there was a slight decrease in the maximum explosion pressure. After adding 75% MABC, the pressure began to rise between 70 and 100 ms and entered a rapid rise phase at 850 ms. Finally, at 1000 ms, the pressure continued to rise but did not reach its maximum value. MABC can improve the oxygen utilization efficiency per unit mass of powder through structural adjustment, which has a more significant explosion suppression effect than ABC powder. However, the decrease is still limited by the total oxygen content.

However, after adding pre-modified ABC powder to CH₄/coal dust, the maximum explosion pressure did not decrease significantly. This phenomenon occurs as the explosion chamber constitutes an enclosed system, wherein the thermal decomposition of ABC powder generates substantial gaseous byproducts, leading to a consequent rise in internal pressure within the vessel. As the ABC powder added increases, the Pmax remains almost unchanged. Because the oxygen content in the sealed space of the explosion tank is constant. Although the amount of ABC explosion suppressant added has increased, the fixed oxygen content in the explosion tank can only support partial pyrolysis of ABC powder and cannot support sufficient thermal decomposition of ABC explosion suppressant. Consequently, with an increasing proportion of ABC powder, the P_max remains relatively stable. After adding modified ABC powder, there was a slight decrease in the P_max. After adding 75% MABC, the pressure began to rise between 70 and 100 ms and entered a rapid rise phase at 850 ms. Finally, at 1000 ms, the pressure continued to rise but did not reach its maximum value.

After adding ABC powder before and after modification, the (dP/dt)_max value demonstrates a consistent downward trend. When 50% ABC powder or MABC was added, the (dP/dt)_max decreased by 38.9% and 50%, respectively. This phenomenon is also accompanied by an extension of the t_Pmax value. This change reveals a key characteristic of the explosion process: the rate of oxidation reaction in the system has slowed down. Owing to the distinctive physical characteristics of ABC powder, it exhibits an exceptional capacity to absorb external heat, while its pyrolysis byproducts create a thermal insulation layer on the coal powder surface. This protective barrier not only reduces the temperature within the gas system but also inhibits the conduction of heat into the internal structure of the coal powder particles. With the enhancement of this insulation effect, the oxidation reaction inside the system is limited. Due to the fact that the rate of oxidation reaction depends on the chemical reaction itself, and the energy required for the reaction comes from the collision and radiation heat of reactant molecules. Therefore, when the oxidation reaction rate of the system slows down, it means that the energy required for the reaction decreases, resulting in a corresponding reduction in the time taken by the system to reach its pressure peak. Through systematic analysis of experimental data, the addition of 25%wt, 50%wt, and 75%wt MABC resulted in a decrease of 22.87%, 50%, and 64.3% in (dP/dt)_max, respectively. After adding ABC powder, the levels decreased by 16.22%, 38.92%, and 44.30%, respectively. It can be concluded that modified ABC powder exhibits a significant explosion suppression effect as an explosion inhibitor, and its inhibitory ability is significantly better than that of unmodified ABC powder. This phenomenon may be attributed to a significant reduction in the agglomeration of ABC powder after modification treatment, resulting in a significant improvement in particle dispersion and enabling it to be more evenly distributed in the explosion space. The modified ABC powder has a larger specific surface area and higher surface activity, which enables more effective contact with combustible substances in explosive systems, thereby suppressing the progress of explosive reactions through a dual mechanism of physical cooling and chemical free radical consumption. In addition, the modification treatment also enhances the thermal stability of ABC powder, enabling it to maintain high explosion suppression performance in high-temperature environments. These results indicate that surface modification treatment significantly improves the explosion suppression efficiency of ABC powder, providing an important theoretical basis and practical guidance for its application in the field of industrial safety.

3.3. The Propagation Law of Explosive Flames

3.3.1. The Influence of CH₄ Concentration on the Propagation Mode of Detonation Flames

In the Hartmann flame propagation experiment, first, a flame propagation experiment was conducted on a single coal powder without adding CH₄ gas, as shown in Figure 7a. When a single coal powder exploded, the coal powder was ignited 18 ms after ignition, and a weak pale yellow flame appeared near the ignition electrode and gradually spread upwards into an orange flame. At 378 ms, the flame reached its highest point, with a height of nearly 580 mm. Before 100 ms, the flame was relatively weak, which may be due to incomplete coal powder pyrolysis at this stage, which fails to produce enough combustible gases. As the flame continues to climb, its length begins to show a significant increase trend, and its propagation speed also becomes faster accordingly. This is because during combustion, pulverized coal particles are decomposed by heat, releasing a significant quantity of combustible gases and volatile compounds. Under the action of thermal expansion, these gases and substances form a strong buoyancy force, which not only pushes the sinking particles but also prompts the small particles that have not yet fully burned to move upward. This upward movement brings heat and further ascent of the flame, making it increasingly bright.

Subsequently, CH₄ with volume fractions of 1.5%, 2.5%, 3.5%, and 4.5% was added to the coal powder for CH₄/coal powder flame propagation experiments. As illustrated in Figure 7 and Figure 8, the ignition time of the CH₄/coal dust hybrid explosion occurs earlier than that of a coal powder explosion alone. This is because the MIE of CH₄ is lower than that of coal powder. After ignition, CH₄ burns first and then quickly triggers the combustible components in the coal dust, forming a mixed system coexisting flame. As the CH₄ concentration increases, the flame brightness intensifies continuously, there is an increase in flame fullness, and there is an advancement in ignition time. The transmission height has basically reached 600 mm. In repeated experiments, the flame shape may not be exactly the same, but the error in flame propagation speed and height does not exceed 5%.

3.3.2. The Inhibitory Effect of ABC Powder on Flame Propagation Before and After Modification

In Figure 9 and Figure 10, the flame propagation images with the addition of ABC powder and MABC are shown. Figure 9a–d shows that a distinct yellow flame appears at the electrode at the moment of ignition. With the passage of time, the flame becomes clear, while the flame state becomes fuller. The explosion flame not only propagates towards the pipe wall but also in two directions, up and down. The flame spreads to the bottom of the pipeline in a very short time, and the flame front shape is evenly distributed when propagating upwards. The flame spread within the CH₄/coal powder distribution range. Progressive expansion of the gaseous phase, emerging as the dominant factor in the explosive reaction, propels the flame front to the quartz tube’s upper boundary. The presence of inert particulate matter curtails flame propagation distance, attenuates combustion intensity, and induces flame segmentation with observable discontinuity phenomena. As shown in Figure 10, the modified ABC powder exhibits significantly better performance in suppressing methane/coal dust mixed explosions than unmodified ABC powder. The experimental results showed that under the same addition conditions, the flame treated with modified ABC powder exhibited a significant darkening phenomenon, with a significant decrease in flame propagation height and an increase in flame fracture area. ABC powder needs to be added with 80% to completely suppress flames, while MABC only needs 60% to achieve the same effect, saving 25% of MABC usage. Compared with ABC powder, MABC has higher efficiency in suppressing methane/coal dust mixed explosions and can significantly reduce the required explosion suppression dose.

The flame propagation images of the explosion suppressant/CH₄/coal powder mixture were processed and analyzed. The CH₄/coal dust co-existing explosion flame propagation height and velocity were found. The results are shown in Figure 11. At any time during the propagation process, the height of the CH₄/coal dust co-explosion flame was lower than that of the CH₄/coal dust mixed explosion flame without the addition of explosion suppressants after adding modified ABC powder. The peak flame height parameter was inversely proportional to the concentration of ABC powder. After adding 20% ABC powder and MABC, the time for flame propagation to 600 mm was reduced by 71 ms and 75 ms, respectively. The increase in the amount of addition continuously reduced the peak flame propagation height, and the addition of 80% ABC powder and 60% MABC reduced the peak propagation height to below 150 mm. The experimental data based on Figure 8 show that the maximum flame propagation speed (V_max) of the CH₄/coal dust mixture system reached 17.75 mm/ms. After adding 20% ABC powder and MABC, the V_max of the explosion in the system was significantly reduced by 56% and 78%, respectively. The V_max of the explosion showed a clear decreasing trend, and significant flame velocity pulsation propagation phenomena were observed during the experimental process. The mechanism of this phenomenon is mainly attributed to the following factors: turbulence induced by high-pressure jet, thermal expansion process in the flow field, and inherent instability characteristics of gas–solid two-phase flames. Values of 20%, 40%, 60%, and 80% MABC, respectively, were added to V_max of 7.07 mm/ms, 3.1 mm/ms, 1.64 mm/ms, and 0.75 mm/ms. Values of 20%, 40%, 60%, and 80% ABC powder were added, respectively, and the V_max was 14.75 mm/ms, 7.4 mm/ms, 4.31 mm/ms, and 2.38 mm/ms. The value of the maximum speed abandoned the initial data of the flame, which was caused by fluctuations resulting in excessively large values.

By comparing and analyzing the experimental results with the previous two sections, it can be concluded that the modified ABC powder exhibits superior explosion suppression performance. This enhancement effect mainly stems from the following mechanism: firstly, surface modification treatment effectively reduces the surface polarity of ABC powder, thereby significantly improving its dispersion characteristics and alleviating powder agglomeration phenomenon; Secondly, the modified ABC powder has a larger specific surface area, which significantly increases its contact interface with the gas/coal dust mixture system; Finally, the augmented specific surface area of the powder markedly amplifies its thermal absorption capacity and cooling efficacy, thereby achieving better explosion suppression performance.

3.4. Thermal Decomposition Analysis of ABC Powder Before and After Modification

Figure 12 shows the TG-DSC curves of ABC powder and MABC. ABC powder and MABC had similar weight loss curves between 35 °C and 180 °C; the weight loss curves of ABC powder and MABC exhibited similar characteristics, indicating that the mass change in the two explosion suppressants was relatively small at this stage, with only a small amount of water evaporation occurring. When the temperature rose from 180 °C to 600 °C, the mass of ABC powder decreased by about 38%, which is consistent with the theoretical calculation value of the chemical equation NH₄H₂PO₄ → 2NH₃ + P₂O₅ + H₂O, indicating that ABC powder completely decomposes within this temperature range. The corresponding DSC curve showed a significant endothermic peak at 210–220 °C, further confirming the decomposition process of ABC powder. The weight loss of ABC powder in the temperature range of 600–800 °C is mainly attributed to the crystal transformation of P₂O₅ and its accompanying evaporation and sublimation phenomena. The DSC curve shows a relatively gentle endothermic peak between 620 and 700 °C.

In contrast, the weight loss rate of MABC was only 30% at 180–380 °C, and reached 49% when the temperature rose to 800 °C. The DSC curve had two distinct endothermic peaks: the first endothermic peak appeared at 210–220 °C, mainly corresponding to the evaporation of surfactants and partial decomposition of ABC powder; the second endothermic peak appeared at 300–350 °C, which is related to the melting process of P₂O₅ (the melting point of P₂O₅ is about 340 °C). According to the comprehensive analysis of the TG-DSC curve, within the temperature range of 180–800 °C, ABC powder underwent complete decomposition accompanied by the evaporation and sublimation of P₂O₅, with a total weight loss rate of 78%; the MABC underwent evaporation of surfactants, partial decomposition of ABC powder, and melting of P₂O₅, with a total weight loss rate of 49%. This result indicated that MABC had excellent thermal stability and could maintain high structural integrity in high-temperature environments. MABC retained 51% of its mass at 800 °C, while the temperature in high-temperature underground areas (such as goaf) could reach 300–500 °C, indicating its ability to maintain structural stability in practical environments. When a fire or explosion occurs simultaneously, the temperature can reach thousands of degrees, and good thermal stability can allow MABC to absorb more heat. In addition, the modified ABC powder exhibited stronger endothermic ability in explosive reaction systems, which provided an important thermodynamic basis for its high efficiency in explosion suppression applications.

3.5. Inhibition Mechanism of ABC Powder on Explosion of Mixed System

The combustion process in the coexistence system of methane and coal dust is a complex multiphase reaction involving the interaction of gas-phase combustion, solid-phase combustion, and pyrolysis oxidation reaction. The preferential combustion of methane provides energy support for the pyrolysis and combustion of coal dust, and the pyrolysis and combustion of coal dust further intensify the combustion process of methane, forming a mutually reinforcing cyclic reaction mechanism. This process not only leads to the continuous release and combustion of volatile substances but also releases a large amount of heat through solid-phase combustion reactions, ultimately determining the accumulation of pressure inside the container and the intensity of combustion.

ABC powder produces a large amount of solid product phosphorus pentoxide during the endothermic decomposition process, which is dispersed in the explosion space. Phosphorus pentoxide can act as a coating to isolate oxygen and hinder the surface combustion and volatilization analysis of oil shale particles. Combustible volatile gases generate highly active free radicals OH· and H· during explosive combustion, which play a crucial role in combustion reactions. The explosive reaction mechanism is primarily governed by the thermal energy released through radical recombination processes, particularly the exothermic reactions OH·+ H·→H₂O and H·+ O₂→H₂O. These chain-terminating steps represent the major heat-producing pathways in explosive combustion. When ABC dry powder is added to the explosion reaction, it will first undergo a decomposition reaction in the combustion flame: NH₄H₂PO₄ → NH₃ + H₃PO₄ + H₂O, followed by H₃PO₄ continuing to decompose: H₃PO₄ → HPO₃ + H₂O. And HPO₃ can bind with the reactive free radical H· in the combustion reaction: HPO₃ + H·→ PO₂·+ H₂O. The intermediate product PO₂· formed by this reaction can bind with the reactive free radical OH · in the explosion reaction: PO₂· + OH·→ P₂O₅ + H₂O, ultimately generating the final stable products P₂O₅ and H₂O. The chemical inhibition process is achieved through the gradual reduction in OH· and H· radical concentrations, which decreases the reaction’s exothermic rate and disrupts the chain propagation cycle. This radical scavenging mechanism effectively suppresses the explosive reaction through chemical means.

4. Conclusions

This study investigated the inhibitory effect and mechanism of modified ultrafine ABC powder on the explosion of the methane/coal dust mixture system through systematic experiments and theoretical analysis. The main conclusions are as follows:

(1)

Methane significantly exacerbates the intensity of coal dust explosions, and its concentration is positively correlated with the Pmax and (dP/dt) of the mixed system. The heat released by methane combustion accelerates the pyrolysis of coal powder, forming a self-sustaining chain reaction that leads to a significant increase in explosion intensity.

(2)

Modified ABC powder was treated with a hydrophobic agent (KH550) and a flow aid (talc powder) to improve particle dispersion and flowability, making it more evenly distributed in the explosion space. Compared to unmodified powder, its explosion suppression efficiency is significantly improved: adding 60–80% modified ABC can completely suppress flame propagation, and the amount of explosion suppression agent used is reduced by about 20%.

(3)

ABC powder undergoes endothermic decomposition to produce P₂O₅, which covers the surface of coal powder to form an insulation layer and reduce the system temperature. The decomposition products HPO₃ and PO₂ consume key free radicals in the explosive chain reaction, interrupting the transmission of the combustion chain. Modified powders maintain higher activity at high temperatures due to enhanced thermal stability.