Methane-air Mixture Explosion Research Articles

Fundamentals of forecasting explosive gas accumulation formation in working areas of coal mines

This article presents a methodological approach to solving the problem of predicting local methane accumulations in order to prevent occupational injuries resulting from explosions of methane-air mixtures. On the basis of the analysis of accidents that occurred in coal mines and industrial injuries of miners (mine workers) it is established that the leading role belongs to explosions of methane-air mixtures. The method/methodology considered in the article is based on a comprehensive analysis, first of all, of both mining and geological and other influencing factors and conditions of operating mines of the Pechersk coal basin, as well as aerogas-dynamic processes of mass transfer of air flows in mine workings. The assumption about influence of mining equipment on the process of formation of methane accumulation sites is made. The algorithm of consecutive actions of managerial engineering and technical personnel to reduce industrial injuries is proposed. The developed algorithm implies the use of applied software for modelling aerogas-dynamic processes in the mining excavation operating region. FlowVision and ANSYS Fluent application software packages were selected to determine the methane accumulation locations based on the analysis of existing application programmes.

Effect of Co3O4 powder on the explosion characteristics and CO products of methane-air mixture

Rapid removing toxic CO for methane-air mixture explosions is essential in reducing casualties, but relevant research is rare. This paper proposed a novel approach for efficient CO removal and explosion suppression for methane-air mixture explosions using Co3O4 powder. The effect of Co3O4 powder on the explosion characteristics and basic parameters to quantify the CO removal performance have been studied, mainly including maximum explosion pressure (Pm), combustion time (tc), flame propagation velocity, and average CO removal rate (Rave.). Results indicated that explosion intensity and CO concentration decreased significantly with Co3O4 powder, due to increased heat absorption capacity and the active site of CO catalytic oxidation. However, the average CO removal rate decreased as the Co3O4 powder usage and CH4 partial pressure increased, which could reduce the dispersion degree of the Co3O4 dust cloud. The CO removal was achieved through the CH4 catalytic combustion and CO catalytic oxidation, which could reduce the CO generation and oxidize the generated CO to harmless CO2, respectively. This work provides a novel routine to lessen the explosion hazards and has promising applications in emergency rescue.

Effects of transverse concentration gradients on the vented explosion of methane-air mixtures

Experiments were performed in a 90-L vessel with a side vent to investigate the effects of transverse concentration gradients on the vented explosion of non-uniform methane-air mixture. Controlled by a mass flow meter, methane was injected uniformly into the vessel through 18 nozzles that are evenly distributed at the top to configure a methane-air mixture with an average methane volume fraction of 10.9%. Various transverse concentration gradients were obtained by altering the free diffusion time of the methane in the vessel. The deflagration flame was recorded through a high-speed camera at a frequency of 1000 Hz, and the internal overpressure at different locations was captured through three piezoelectric sensors. Experimental results reveal that larger concentration gradient resulted in lower flame propagation rate, longer time for the burst of the vent cover, and stronger pressure oscillations. Concentration gradient affects the typical flame structure, especially the formation and development of the tulip flame. Under the present experimental conditions, Helmholtz oscillations occurred in all tests. Concentration gradient has a negative correlation with the frequency of Helmholtz oscillations but a positive correlation with the duration of oscillations. In conclusion, the variation of concentration gradient obviously affects flame propagation and pressure evolution during vented explosions, which deserves more attention in the design of explosion protection.

Дегазация разрабатываемых пластов угля направленной трассы скважинами

The technology for conducting degassing work in the Russian mines is determined by the experience of half a century of using methods and means to reduce the methane abundance of excavation areas based on drilling wells along the developed and adjacent coal seams into mined-out spaces. The parameters of degassing and methane emission sources are legalized by the industry regulations, scientifically substantiated, and published in the scientific and practical works. The mines of the Kuznetsk coal basin and the Vorkuta deposit, mainly with high productivity of coal mining in the conditions of developing a suite of methane-bearing coal seams using complex mechanized faces were selected as objects of study: named after S.M. Kirov, named after V.D. Yalevsky (formerly «Kotinskaya»), Boldyrevsky coal seams (index 24) and seam 52. Gas hazard of the coal mines is determined by the volume of methane emissions, explosions of methane-air mixtures, loss of people life and is caused by the joint influence of natural, mining, organizational and subjective factors. The main indicator of gas hazard when developing a suite of methane-bearing coal seams is the absolute methane abundance of the excavation area. The method was used to predict the intensity of methane release from the exposed surface of a mined coal seam into the bottomhole space of the longwall and adjacent coal seams. Methane release from the exposed surface of a contiguous coal seam, subject to unloading from the rock pressure, was determined by analogy with the mined seam. The release of methane from the broken coal was established experimentally. When the working face unloads adjacent coal seams, the intensity of methane release from them into the working face is summed up.

Effects of ignition position on the explosion of methane-air mixtures with concentration gradients

To investigate the effects of ignition position on the explosion of non-uniform methane-air mixtures, experiments were carried out in a closed pipe of 1 m length and 0.3m × 0.3 m cross-section. Methane is uniformly charged at the top of the pipe and then diffused freely, creating a concentration gradient. The concentration gradient of the methane-air mixture depends on the diffusion time. The effects of the three ignition positions – top (IP1), center (IP2) and bottom (IP3) - on the flame propagation at different ignition delay time (tig) were investigated. The ignition position and tig are the key factors influencing the success of ignition. For IP2 and IP3, the flame propagation velocity increases with the increase of tig, but the flame propagation velocity at IP1 does not change significantly with an increase in tig. When tig ≤ 25min, the temperature (Tmax) and maximum overpressure (Pmax) in the pipe at IP1 are significantly higher than those at IP2 and IP3. When the methane-air mixtures are nearly uniform, the Tmax and Pmax at different ignition locations are almost identical. The maximum pressure rise rate, (dP/dt)max, was affected by the flame pipe collision and different flame shapes caused by the ignition position.

Experimental study on gas explosions of methane-air mixtures in a full-scale residence building

The field tests were carried out in a full-scale residence building with volume of 105 m3 to reveal the hazards and mechanism of methane-air mixture explosions. The real layout and variations of gas concentration, initial state and ignition position are considered in the explosion tests. The pressure–time histories and flame development of each test were recorded and analyzed. The concentration distribution was also measured by using the fixed concentration sensors and mobile robots. It is found that when the methane diffuses freely in the resident buildings, concentration stratification can be observed in the vertical direction and the concentration of region with height of 0.64 ∼ 2.38 m is within the explosive limit in the 9.5 vol% case. In the real scenario, the maximum overpressure of gas explosions in residence buildings is less than 10 kPa due to the effective venting of doors and windows. The concentration of 9.5 vol%, uniform gas distribution and ignition position in room 2 can bring about more intensive gas explosions and in turn result in higher overpressure loads and stronger external flame propagation. What is more, the acoustic oscillation is significantly restrained by the disturbance on the acoustic field caused by the effective venting, gas stratification and obstacle distribution in residence buildings.

Effect of flammable gases produced from spontaneous smoldering combustion of coal on methane explosion in coal mines

Methane released from underground coal mines forms a highly explosive mixture when combined with air, which can be aggravated by the flammable gases produced from spontaneous smoldering combustion of coal in the mines. In this work, the flammable gases from coal smoldering combustion under conditions close to those in a coal mine are experimentally characterized by a self-designed quartz cylinder. The flammable gases mainly consist of CO at the initial stage of the smoldering combustion while they are composed of CO, H2 and CH4 at temperatures above 300 °C. Then, their effect on explosion of methane-air mixtures is investigated via experiments and simulations. The explosion characteristics and flame propagation of the typical in-mine methane-air mixtures enriched by the smoldering combustion products are studied using a 20 L spherical vessel reactor and a high-speed camera. Among all the methane-air mixtures tested, the addition of the smoldering combustion products to fuel-lean methane-air mixtures most remarkably enhance methane explosion. In the simulations, a sensitivity analysis is conducted to identify the dominant elementary reactions during the explosion process. As the share of the smoldering combustion products added to the methane-air mixture increases, the dominant elementary reaction for methane consumption is found to eventually shift to enhance methane explosion by generating a high concentration of OH and O. The results of this work not only improve the understanding of methane explosion but also benefit the development of new methane explosion suppression technology for coal-mining industry.

Gas explosion suppression performance of modified gel-type dry waters

In this paper, the gas explosion (methane-air mixture explosion) suppression properties of the gel and modified dry water powders were investigated using a transient flame propagation system. The results revealed that the moisturising ability of dry water powders containing gel or ammonium dihydrogen phosphate (the chemical formula is NH4H2PO4, and its abbreviation is MAP) has been improved. The optimal explosion suppression performance was achieved when the concentration of MAP in the aqueous solvent was 10% and the dry water concentration was 520 g/m3. In a 10 vol% gas explosion, the gellan gum gel dry water, calcium alginate gel dry water, 10 mass% MAP–modified dry water, 10 mass% MAP–modified gellan gum dry water, and 10 mass% MAP–modified calcium alginate dry water decreased the maximum flame propagation velocity by 17.50%, 20.63%, 25.62%, 26.76%, and 28.44%, respectively; decreased the maximum flame temperature (as measured at a point in the lower part of the pipeline) by 37.84%, 42.47%, 45.55%, 46.31%, and 53.71%, respectively; and decreased the maximum explosion venting pressure by 6.43%, 9.16%, 13.77%, 16.50%, 21.08%, and 36.70%, respectively. According to thermogravimetric analysis, the mass losses of the MAP-modified dry waters during pyrolysis were greater than those of the unmodified dry waters. Studies have revealed that MAP-modified calcium alginate dry water can effectively suppress explosions and has a stable particle structure.

Effects of heat transfer mechanism on methane-air mixture explosion in 20 L spherical device

This study developed a model of methane one-step combustion mechanism based on the CFD code GASFLOW-MPI to understand the influence mechanism and degree of influence of heat transfer mechanism on methane explosion. Moreover, this study proposed the addition of heat transfer mathematical models using GASFLOW-MPI, including thermal radiation and convection heat transfer. Further, the effects of thermal radiation and convective heat transfer on the shock wave during a methane explosion in a 20 L spherical explosion tank were studied through numerical simulation and the results were compared with the experimental data. The numerical simulation results were found to reasonably predict the peak pressure value and pressure decay process of a methane explosion. In addition, through comparisons of the effects of adiabatic and numerical simulations considering heat loss, the peak pressure of gas explosion calculated via adiabatic simulation were found to be overestimated. Moreover, during the methane-air mixed explosion experiment in the 20 L spherical device, the heat loss was observed to be primarily caused by heat radiation, accounting for more than 76.01% of the total heat loss, followed by convection heat transfer, accounting for 23.99% of the total heat loss. The research results showed that heat loss significantly influenced the methane explosion process. Additionally, for the methane-air mixture explosion experiment in a 20 L spherical device, thermal radiation was the most critical factor that resulted in heat loss in the methane explosion process. Therefore, the influence of thermal radiation and convective heat transfer mechanism on methane explosion must be considered in the numerical simulation of a methane explosion.

Effects of mesh aluminium alloy and aluminium velvet on the explosion of H2/air, CH4/air and C2H2/air mixtures

MAA (mesh aluminium alloy) is one of the most widely used explosion suppression materials in military and civilian applications. To systematically research the effect of MAA on the explosion reactions of flammable gases, we investigated the effect of MAA and AV (aluminium velvet) on the explosion of hydrogen-air, methane-air and acetylene-air mixtures. The results indicated that MAA and AV suppress the methane-air mixture explosion but significantly promote the explosions of the hydrogen-air mixture and the acetylene-air mixture. MAA and AV have the dual effect of promoting and suppressing an explosion. In addition, with an increase in filling density, the promotion of MAA and AV first strengthens and then weakens. The results of this study show that the properties of flammable gas, not the shapes of the explosion suppression materials, determine whether the dominant effect of explosion suppression material is promotion or suppression. Use of explosion suppression materials is not suitable for all flammable gases, especially highly reactive chemical fuels. Applying explosion suppression materials blindly may greatly increase safety risk.

Gas explosions of methane-air mixtures in a large-scale tube

12 batches of premixed methane-air mixture explosion tests were conducted in a 30 m long testing tube with section dimensions of 0.8 m × 0.8 m to explore the loading characteristics of gas explosion inside the tube. The pressure-time histories and flame travel distance of each test were recorded and analyzed. A 3D numerical model was developed and numerical simulations were carried out by using the open-source Computation Fluid Dynamic (CFD) toolbox OpenFOAM. Based on testing and numerical results, the characteristics of gas explosion loading inside the large-scale tube were revealed. It is found that the existence of vents can reduce the peak pressure by 13%–91% and increase the flame travelling distance inside the tube. The peak pressure increases with the rising concentration when the gas concentration range is 7.5%–9.5% and decreases with the rising concentration when the gas concentration ranges 9.5%–11.5%. End venting can reduce the peak pressure along the tube by 17%–69%. With the decrease of vent number, the gas explosion inside the tube is enhanced and the side vents closed to the ignition can reduce the gas explosion loads inside the tube effectively.

Coupling effects of venting and inerting on explosions in interconnected vessels

The coupling effects of venting and CO2inerting on stoichiometric methane-air mixture explosions were investigated in an isolated vessel and interconnected vessels. The results indicate that venting mitigates the explosion intensity, especially for small vessels. For vessels connected by pipes, a venting design following EN 14994 (2007) and NFPA 68 (2013) could not meet the venting requirements. For an isolated big vessel and interconnected vessels, increasing the CO2 volume fraction (Φ) from 0 to 15.0 vol% decreased the maximum explosion overpressure (Pmax) and maximum rate of overpressure rise ((dP/dt)max) and delayed tmax. For closed interconnected vessels, Pmax varied approximately linearly with Φ. For both isolated vessel and interconnected vessels, the coupling effects of venting and CO2 inerting on methane-air explosion were more efficient than those of individual mitigative method (that is, venting alone or CO2 inerting alone).

Effects of concentration and initial turbulence on the vented explosion characteristics of methane-air mixtures

The testing of vented explosion of methane-air mixtures was conducted in a custom-designed 4.5-m3 steel chamber. Methane concentrations in the mixed gas varied between 7 and 13 vol% and the static hysteresis time was set as 0, 1, and 5 min to characterize strong, medium and weak turbulence levels. The effects of concentration and initial turbulence on pressure characteristics and flame development were investigated. It is found that the quasi-static pressure in the chamber during vented explosion can be divided into three stages: pressure release, Helmholtz oscillation, and acoustic oscillation. The Helmholtz oscillation at a frequency of about 20–40 Hz is produced after the peak pressure P1 caused by the pressure release at all concentrations; however, the acoustic oscillation at a frequency of about 300 Hz only occurs after Helmholtz oscillation when the concentration is near the optimum. In the early stage of internal flame development, the cellular structure is generated due to hydrodynamic instability and diffusive-thermal instability, and the flame front is distorted owing to the instability caused by venting. Initial turbulence distorts the flame front, significantly increases the peak pressure, and stimulates the generation of acoustic oscillation at the upper and lower limits of concentration. Under the influence of initial high turbulence, the internal flame propagation distance has a power function relationship with time, and the turbulence acceleration factor and distance also show a power function relationship.

Application of flame arrester in mitigation of explosion and flame deflagration of ventilation air methane

Fugitive methane emissions from underground coal mines are considered a potent greenhouse gas source with a large global warming potential. Application of ventilation air methane thermal oxidiser technology reduces this risk to a large extent. However, it imposes the risk of accidental fires and explosions in the mine site which may claim many lives and cause massive property damage. As such, the necessity of employing safety barriers such as flame arrester seems crucial to protect the mine against the accidental flames and explosion propagation originated from the thermal oxidiser.The aim of this work was to evaluate the effectiveness of an inline flame arrester for the mitigation of methane explosions in a large scale detonation tube. The results corresponding to this study indicated that the flame arrester was capable to effectively mitigate the explosion flames of 5%, 7.5% and 9.5% methane air mixtures (for a 10 m fuel filled section). The flame arrester reduced the pressure wave velocity by 88% and 40% for the explosions of methane-air mixtures of 7.5% and 9.5% methane, respectively. The flame propagation velocities for the 7.5% and 9.5% methane concentrations reduced approximately 32% and 44%, respectively. A significant pressure build-up was observed upstream of the flame arrester when compared with a duct without flame arrester. In addition, the build-up pressure was relatively higher for larger methane concentrations (e.g. 9.5%). These observations highlights that the maximum design pressure of an industrial process plant must be carefully considered when implementing an inline flame arrester due to the possibility of a sudden pressure increase cause by an explosion.

Chemical Control of Combustion, Explosion, and Detonation of Gases

The identification of the chain origin of gas combustion at atmospheric and elevated pressure has opened up new aspects of the theory and wide-ranging possibilities for effectively managing the processes of combustion, explosion, and detonation by controlling the branching and loss rates of active intermediate particles, such as atoms and radicals, by using inhibitors and promoters. In this paper, we briefly describe methods for preventing fires and explosions of methane-air mixtures, including those in coal mines, ignition and explosion of hydrogen-air mixtures, and the transition of combustion to detonation in the current ramjet engine model. The results of interdepartmental tests and experimental data illustrating the destruction of a stationary detonation wave by the small impurities of the lower hydrocarbons are given.

Assessing safety conditions in underground excavations after a methane-air mixture explosion

A technique for evaluation of shock wave impulse after a methane-air mixture explosion is elaborated. The numerical model developed in previous studies has been verified in the laboratory by using laser initiation of explosives and measuring the pressure impulses of explosion products on a ballistic pendulum. To evaluate the mechanical impulse the functional correlations between its magnitude, the swing angle, and the pendulum characteristics have been derived analytically. The reliability of experimental results is ensured by calibrating the sensor that measures the pendulum swing angle and estimating the impulse measurement errors caused by specifics of angle measurements by a digital voltmeter, pendulum axis friction, and the pauses between measurements. Testing the developed technique to evaluate the shock wave impact showed satisfactory consistency of experimental and theoretical results with the momentum deviation below 9%, which confirms model applicability and correct reproducibility of the shock wave propagation process.

Numerical simulation on structure effects for linked cylindrical and spherical vessels

This paper presents a simulation study on the methane–air mixture explosions through using the eddy-dissipation concept (EDC) model in FLUENT. The aims are to investigate the structure effects of methane–air mixture explosions in a spherical vessel, cylindrical vessel and different systems of cylindrical vessels connected with pipe. Meanwhile, in order to study the characteristics of methane-air mixture explosions in the linked vessels, changes of flame temperature and airflow velocity in the linked vessels are simulated and analyzed. The results suggest that the effect of structural changes of a single vessel on the gas explosion intensity is clear, and the explosion intensity of a spherical vessel is greater than that of a cylindrical vessel. The simulation results of different structural forms of a cylindrical vessel connected with pipelines show that the time to reach the peak value of explosion pressure is the shortest in the linked vessels, and the explosion pressure rising rate is highest at the vessel’s center. For the linked vessels, after ignition, the airflow ahead of the flame propagates to the secondary vessel, and the maximum airflow velocity of every monitoring point in the linked vessels increases. The detonation occurs when the flame propagates to the secondary vessel, which leads to a severe secondary explosion in the secondary vessel. The studies can provide an important reference for the safe design of industrial vessels.

Experimental and numerical study on connecting pipe and vessel size effects on methane–air explosions in interconnected vessels

The size effects on a methane–air mixture explosion in the interconnected vessels were investigated in this article. The vessels were interconnected by pipes of various lengths or diameters. Varied pipe lengths were analyzed by experiment. The results indicate that the maximum explosion pressure and the maximum rate of pressure rise in the primary and secondary vessels increase with pipe length. To investigate the effects of pipe diameter and volume ratio on methane–air mixtures’ explosion in the interconnected vessels, a computational fluid dynamics model was implemented. The model was validated by comparison with experimental results. A fair agreement was observed between the simulation results and experimental data. The simulation results indicate that an increase in the pipe diameter will reduce the danger of explosion. The maximum explosion pressure in both vessels increases when the volume ratio increases. When the primary vessel is larger than the secondary vessel, the maximum rate of pressure rise in the primary vessel decreases with volume ratio. However, the maximum rate of pressure rise in the secondary vessel increases. The maximum rate of pressure rise changes inconspicuously while the secondary vessel is larger than the primary one. Hence, the cubic-root law is not applicable to an explosion in the interconnected vessels. These conclusions can support the safe design of chemical equipment.

Confined explosion of methane-air mixtures under turbulence

In this study, a large-scale investigation of completely confined methane-air explosions was conducted in a 1-m3 spherical explosion chamber. The effects of turbulence and explosive powders on explosion parameters such as the deflagration index, maximum explosion pressure and burning velocity were examined. Theoretical calculations were conducted and are presented alongside the experimental data. The study suggests that the presence of turbulence increases the maximum explosion pressure. The values of the deflagration indices and burning velocities were found to be increased by the turbulence. The presence of an explosive powder provides similar effects to turbulence, and the values of the maximum explosion pressure, deflagration index and burning velocities increased with increases of the mass of the explosive powders. The magnitude of the turbulence generated in the explosion chamber was determined theoretically by employing Damköhler’s correlation.

Numerical simulation of size effects of gas explosions in spherical vessels

To study the law of sizes on gas explosions, numerical simulations of methane–air mixture explosions in spherical vessels were performed. The law of sizes on gas explosions is studied using FLUENT simulations with the [Formula: see text] two-equation turbulent model, the eddy-dissipation-concept model, thermal dissipation at a wall boundary, the P1 model, and the SIMPLE algorithm. The experimental results suggest that under an adiabatic condition without energy loss, the maximum explosion pressures in different spherical vessels are all 0.82 MPa, and the effect on the explosion intensity in spherical vessels is small. Under the condition of heat dissipation at the wall boundary, the maximum explosion pressure increases with volume of the spherical vessel. However, the explosion intensity in this condition is lower than that in adiabatic condition. Also, the size effect is not obvious. The size effect on the explosion intensity is significant under the combined effects of heat dissipation at the wall boundary and thermal radiation, where the maximum explosion pressure increases with volume of spherical vessels. On the contrary, the maximum pressure rising rate decreases with the volume of the spherical vessels; this rule coincides with the “cube” law. The studies on the size effects of methane–air mixture explosions in a spherical vessel provide an important reference for establishing a model system that can be used to test and design industrial vessels.