Methane-air Explosion Research Articles

A study of pressure characteristics of methane explosion in a 20 m buried tunnel and influence on structural behaviour of concrete elements

With increasing use of natural gas in urban metropolitan areas, utility tunnels that contain utility and gas lines have become critical infrastructure. A leak within a gas pipe may cause methane-air explosions in the tunnel, leading to structural damage and casualties. Thus, it is necessary to investigate the response of structural members against explosion loads in the tunnel. Few studies in the open literature have studied the effects of a methane-air explosion in a full-scale concrete tunnel. This study presents two 9.5% methane-air explosions in a tunnel with a dimension of 20000 mm × 1800 mm × 600 mm. The pressure characteristics are summarized and compared with existing pressure–time curve of the methane-air explosion in typical vented containers. Similarities of pressure characteristics between the current study and previous studies avaliable from the literature are identified. Apart from explosion pressure characteristics, concrete structural specimens with a dimension of 1800 mm × 400 mm × 90 mm are also investigated. Geopolymer concrete, ultra high performance concrete (UHPC) and conventional concrete with compressive strength of approximately 70 MPa, 150 MPa and 30 MPa, respectively, were used to manufacture the testing specimens subjected to the methane-air explosions in the tunnel. In this study, the cracks were observed on all specimens. Due to large size of the tunnel and gas leakage during blast, the pressure distribution in the tunnel is not as uniform as observed in methane-air explosion chamber from the previous literatures. The conventional concrete specimen positioned between two pressure sensors is selected to validate the numerical model in the present study. The calibrated numerical model is then used to study the structural responses of concrete specimen subjected to the captured pressure.

Combined effects of obstacle and fine water mist on gas explosion characteristics

Abstract Combined effects of obstacles and fine water mist on a methane-air explosion of a semi-closed pipe were investigated experimentally. In this study, the diameter of the water mist, the location, and the number of obstacles was considered. The results demonstrated that 5 µm water mist present a significant suppression affected while 45 µm shows a slight promotion effected on a gas explosion of the condition without obstacles. In the presence of an obstacle, however, the inhibitory effect of 5 µm water veils of mist dropped significantly during flame propagation, and the effect of 45 µm water veils of mist changed from the enhancement of inhibition, and its inhibitory effect was significant. The inhibitory effect of 45 µm water veils of mist on gas explosion weakened firstly and then enhanced with the increasing distance between obstacle location from the ignition location as well as in several obstacles.

Characteristics of methane-air explosions in large-scale tunnels with different structures

To investigate the characteristics of methane-air explosions in large-scale tunnels with different structures, the CFD code for gas explosion modelling of FLACS was validated and used to model the explosions in tunnelling faces, coalfaces and crosscuts, where explosions mostly occurred in coalmines. The maximum overpressures were obtained as about 150–1400 kPa but varied for places with different cross-sections. The confinement degree and combustible mixture volume compete for controlling the overpressures, the space congestion degree influences the competition. For the tunnelling faces and crosscuts with low congestions, the increase of confinement degree caused by reducing cross-sections takes the dominant effect, which makes the overpressure in 3 m × 2 m cross-section tunnels higher. However, the 6 m × 4 m coalface produces the largest overpressure as the highly congested coalface strengthens the effect of increasing the mixture volume on pressure rise. Since the pressure reflection results from space constraints, a higher confinement degree causes a greater reflection. In crosscuts with both ends closed, the delayed combustion of the unburnt rich mixture after the seal’s failure generates a high pressure than assumed. Because of containing the same volume combustible mixtures in the crosscut modelling with the same cross-section, little difference exists in the overpressure regardless of sealing conditions. The flame is more inclined to propagate towards free spaces, and the flame of a larger volume combustible mixture fills a longer tunnel more comprehensively. In straight tunnelling faces, the flame length can be calculated as the product of the equivalent side length of the cross-section and its corresponding simulated constant.

Thermal Runaway Pressures of Iron Phosphate Lithium-Ion Cells as a Function of Free Space Within Sealed Enclosures

Mining vehicle manufacturers are developing lithium-ion (Li-ion) battery electric vehicles as an alternative to diesel-powered vehicles. In gassy underground mines, explosion-proof (XP) enclosures are commonly used to enclose electrical ignition sources to prevent propagation of an internal methane (CH4) air explosion to a surrounding explosive atmosphere. Li-ion batteries can create pressurized explosions within sealed enclosures due to thermal runaway (TR). Researchers at the National Institute for Occupational Safety and Health (NIOSH) measured TR pressures of lithium iron phosphate (LFP) cells as a function of free space within sealed enclosures and observed an inverse power relationship. A well-confined cell produced 294 bar (4260 psia) of pressure during a TR, far exceeding minimum pressure containment specifications for conventional XP enclosures. Results indicate that adding enough free space surrounding LFP cells can reduce TR pressures to levels below that expected for CH4-air ignitions. Measured TR temperatures were below the minimum autoignition temperature of CH4-air.

Effect of concentration and ignition position on vented methane–air explosions

Experiments were conducted in a 1 m3 vessel with a top vent to investigate the effect of methane concentration and ignition position on pressure buildup and flame behavior. Three pressure peaks (p1, p2, and Pext) and two types of pressure oscillations (Helmholtz and acoustic oscillations) were observed. The rupture of vent cover results in p1 that is insensitive to methane concentration and ignition position. Owing to the interaction between acoustic wave and the flame, p2 forms in the central and top ignition explosions when the methane–air mixture is near–stoichiometric. When the methane–air mixture is centrally ignited, p2 first increases and then decreases with an increase in the methane concentration. The external explosion-induced Pext is observed only in the bottom ignition explosions with an amplitude of several kilopascals. Under the current experimental conditions, flame–acoustic interaction leads to the most serious explosions in central ignition tests. Methane concentration and ignition position have little effect on the frequency of Helmholtz and acoustic oscillations; however, the Helmholtz oscillation lasts longer and first decreases and then increases as the methane concentration increases for top ignition cases. The ignition position significantly affects the Taylor instability of the flame front resulting from the Helmholtz oscillation.

Investigation on methane-air explosion overpressure in bifurcated tubes according to methane concentrations and bifurcation angles

ABSTRACT To understand the maximum overpressure propagation characteristics of methane-air explosion in a bifurcated tube, the distribution and mutation coefficient of maximum overpressure caused by methane-air explosions were investigated with various methane concentrations (5%, 7%, 9%, 11%, and 13%) and various bifurcation angles (30°, 45°, 60°, and 90°). The results show that: in terms of the overall trend, the increase in tube bifurcation angle is positively correlated with the maximum overpressure value of methane-air explosion in the main and branch tube sections, and the increase in these values was larger for the former. The methane concentration also significantly affects the explosion maximum overpressure in the main and branch tube sections. The closer the methane concentration is to the stoichiometric concentration, the greater the explosion maximum overpressure. The attenuation amplitude of the explosion maximum overpressure before and after the bifurcation point is affected by both the bifurcation angle and the methane concentration. The greater the bifurcation angle of the tube, the closer the methane concentration is to the stoichiometric concentration, the smaller the attenuation amplitude of the explosion overpressure in the main tube section before and after the bifurcation point, and the greater the attenuation amplitude of the explosion overpressure in the branch tube section before and after the bifurcation point. The research results can provide guidance for the design and implementation of explosion prevention and suppression measures in coal mines.

Experimental and numerical study on the effect of low vent burst pressure on vented methane-air deflagrations

This paper experimentally investigated the effect of low vent burst pressure on the overpressure buildup and flame evolution during the vented methane-air deflagrations in a 1m3 rectangular vessel. CFD software FLACS was assessed in simulation of vented methane-air explosions against the experiment. Experimental results demonstrate that four overpressure peaks of Popen, Pout, Pext and Pacc from the vent failure, flame venting, external explosion and acoustically enhanced combustion, respectively, were observed except for the free venting case, in which only two overpressure peaks of Pout and Pext were monitored. The flame behaviors corresponding to different overpressure peaks were monitored especially at Pacc, where an abrupt increase in flame luminosity was observed on the bottom of the vessel due to the combustion of pockets of flammable mixture. Popen and Pacc increase with the vent burst pressure while Pout and Pext are nearly independent of the vent burst pressure. Double flame accelerations were observed owing to the vent rupture and flame venting. The phenomena of Helmholtz oscillation and the bulk motion of flame bubble always appeared after the vent rupture. The predicted overpressure agrees reasonably well with the experimental data. The discrepancy of the predicted and measured maximum peak overpressures reduces with increasing vent burst pressure. The predicted maximum peak overpressure and external flame length are slightly larger than those in experiment, which would facilitate the safe design in explosion venting and offer a better guidance in risk assessment. The simulation results also show that two overpressure peaks were recorded resulting from the pressure wave propagating above the vessel and the external explosion, respectively.

Investigation of methane-air explosions and its destruction at longwall coalface in underground coalmines

ABSTRACT Methane-air explosions are the most fatal accidents in underground coalmines. To understand its destruction, a worst-case full-scale methane-air explosion at a longwall coalface was simulated and analyzed based on accident investigations. It was found the flame covers a short length with an expansion ratio of about 6.67 at the longwall coalface, and crosscuts reduce the ratio to 3.67. Bifurcation reduces the peak overpressure by 25%–32% and the peak velocity of blast waves by 18.5%–23.5%. Corners have a significant effect on mitigating the peak overpressure but have a negligible impact on the blast velocity. In a long straight tunnel, the peak overpressure and blast velocity decay slowly and linearly by just 18.5% and 17.3%. A high-speed blast wave with debris and toxic and anoxic airflows at a velocity higher than 100 m/s were obtained from the accident investigations and in most of the simulation domain, it is the biggest challenge to evacuation and the most destructive force at the far-field disaster areas of methane-air explosions in underground coalmines. Thus, crosscuts, corners and blast velocity retarding methods should be more adopted to mitigate explosions and improve the rescue and evacuation safety.

Steel/basalt rebar reinforced Ultra-High Performance Concrete components against methane-air explosion loads

Ultra-High Performance Concrete (UHPC) is a relatively new construction material, which has been investigated over the past few decades. Despite its exceptional mechanical strength, UHPC still requires passive steel reinforcement to maximise its bending capacity and the overall material cost will be high. The basalt fibre rebar has a higher mechanical strength than steel rebar with lower cost. In addition, it also has better alkali resistance and good cost-effectiveness. The basalt fibre rebar is therefore considered as a potential alternative reinforcement in the structural member. In this study, a recently developed UHPC formula was adopted, the conventional steel rebar and basalt fibre rebar were used as reinforcement. The developed components were tested against static flexural and methane-air explosion loads. In the four-point flexural tests, the basalt fibre rebar reinforced specimen (400 mm × 100 mm × 100 mm) performed more ductile structural behaviour with higher flexural strength. Two large scale methane-air explosion tests were conducted in buried utility tunnels with different length (i.e., 12000 mm × 1800 mm × 600 mm and 20000 mm × 1800 mm × 600 mm). The experimental test in shorter tunnel yielded lower explosion pressure [1] with marginal structural response. The test in longer tunnel achieved a higher explosion pressure on concrete elements. The C30 and UHPC specimens (1800 mm × 400 mm × 90 mm) with steel/basalt fibre rebar reinforcement were tested. The pressure and deflection data revealed that basalt fibre rebar reinforced UHPC component had a more ductile structural behaviour against accidental gas explosion.

A comparative and experimental study on performances of Natural gas-Air Combustion, Helium, and Air as Propellants in a Gas Gun

ABSTRACT In this study, the performance of a single-stage gas gun with a natural gas-air mixture as the propellant gas is investigated by measuring the explosion pressure and the projectile velocity. In this work, the combined effect of elevated initial pressure, mixing, equivalence ratio, and ignition energy on the explosion pressure and projectile velocity is studied. To assess the performance of the natural gas-air combustion, a series of tests with helium and air as the propellant gas are carried out to measure the projectile velocity. The fuel used in this study consisted of 84.2% methane, 9.5% ethane, and 3.7% propane. The helium gas was 99.999% pure. The volume fraction of fuel in the natural gas-air mixture varied between 4.98% and 17.3%, and the flammability limits of the natural gas were between 4.41% and 14.47%. The equivalence ratio Φ was used to control the fuel concentration in the mixture. The equivalence ratio in this study was 0.5, 0.75, 1, 1.5, and 2. The experimental results indicated that despite the low percentage of methane and high percentage of higher hydrocarbons in natural gas, the maximum pressure still occurs at the equivalence ratio 1, the finding which had already been proved to be the case for pure methane. However, the maximum combustion pressure is reduced compared to other fuels with higher methane and lower hydrocarbon content. Also, the study showed that the maximum combustion pressure increased with rising the ignition energy also when the mixer was used to blend the natural gas-air mixture. The experimental results showed that at low pressures, the projectile velocity was higher using helium compared to the same pressures produced by the combustion of methane-air mixture. However, at higher pressures, the projectile velocity was much higher for the pressures produced by combustion due to its lower molecular weight and the heat produced in the combustion process. The price analysis also indicated that for the same output pressure, the cost of a test with a methane-air explosion is 78 times lower than that of a test with helium.

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).

Experimental Study on Explosion of Premixed Methane-air with Different Porosity and Distance from Ignition Position

ABSTRACT In order to suppress the overpressure and flame damage caused by the explosion of premixed methane-air, a self-built duct with a 10 × 10 × 100 cm was used to conduct an experimental study on the explosive characteristics and propagation rules of 9.5% methane under the effect of different porous materials at a fixed position. The methods of suppressing methane-air explosion in confined space are optimized. By analyzing the effect of porosity and distance of the porous material on the explosion, the overpressure and flame propagation characteristics after the methane-air explosion and under the influence of the dual factors are studied. It provides data support to explain the mechanism of blocking and suppressing premixed methane-air explosion when porous materials with different porosity and different positions. Studies have shown that the different porosity of porous materials and the position from the explosion source have significant and different effects on the propagation of overpressure and flame. The 10 PPI porous material has a strong promoting effect on the overpressure and flame propagation after the explosion, it accelerates the overpressure rising rate and increases the overpressure peak, and enhances the turbulence intensity of the explosion flame and accelerates the flame propagation rate. The 20 PPI porous material failed to suppress the explosion during the acceleration phase, but had certain suppression on the explosion flame in other phases. The effect of 30 PPI porous material to suppress explosion overpressure and flame depends on where it is placed. Both 20 PPI and 30 PPI porous materials can quench the flame and block the propagation of high-temperature carbonaceous products. This research results have theoretical value and guiding significance for the prevention and emergency treatment of methane explosion accidents.

Numerical simulation of the inhibition process of methane-air explosion under cold aerosol

In order to study the methane-air explosion suppression problem, a certain size experimental tube was used as the physical model, and Gambit was used for modelling. The ANSYS 15.0 software was used to simulate the methane-air explosion suppression process under the action of cold aerosol, and the simulation results were compared with the results of the experimental studies in the open literature. It can be seen from the numerical simulation results that the concentration of cold aerosol had a significant effect on the methane-air explosion process. When the concentration of cold aerosol was low, the methane-air would explode. The pressure-time curve in the experimental tube showed four stages: the explosion induction period, the pressure rise phase, the explosion pressure peak value phase and the explosion pressure decay phase. With the increase of the aerosol concentration, the explosion induction period was significantly prolonged, and the maximum explosion pressure was slightly decreased, and the time to reach the maximum explosion pressure was also significantly prolonged. When the cold aerosol concentration reached a certain threshold, the methane-air no longer exploded. Compared with the experimental results, the simulated values of the explosion induction period, the maximum explosion pressure value, the maximum explosion pressure time and the final pressure value under different cold aerosol concentrations matched well with the experimental measurements. Therefore, numerical simulation can provide a reference for the study of methane-air explosion suppression performance under cold aerosol.

Investigation on the overpressure of methane-air mixture gas explosions in straight large-scale tunnels

To investigate the overpressure of methane-air explosions in straight large-scale tunnels, the computational fluid dynamics (CFD) code of Flame Accelerator Simulator (FLACS) was used and validated against experiments conducted at three different scales, and the effects of the volume concentration of methane in air, the blockage ratio (BR), the tunnel length, and the cross-section were studied. When analysed using the GaussAmp mathematical model, the maximum peak overpressure appears at a volume concentration of 10.30 % of methane in air. Blockage ratios (BR) of 0.15 and 0.3 resulted in the combustion of methane-air mixtures with the volume concentration of 6.5 % and 14.0 % of methane in air, producing a fatal overpressure of 21 kPa. When the BR increases up to 0.75, both the lean and rich mixtures cause a peak overpressure of over 60 kPa. Combustion of the same methane-air mixture produces the same overpressure, which decays approximately linearly at the same slope owing to a smooth wall roughness before travelling near the outlet, independent of the specific tunnel length. A method to characterise the cross-sections was proposed, and the maximum peak overpressure of different lengths of methane-air mixtures in different cross-sectional tunnels was found, presenting various regimes from a hump shape to a wave-like uplift and bowl shape. The cross-section parameters determine the degree of confinement and further control the maximum peak overpressure in the modelled tunnels. An exponential asymptotic model can be used to conveniently obtain the maximum peak overpressure. These phenomena indicate that approximately square-shaped cross-sections should be selected to avoid an extremely high overpressure in large-scale tunnels with the potentially significant accumulation of methane-air mixtures.

Experimental Research on The Coupling Effect of Hole Plate Size and Initial Pressure on The Pressure of Methane-Air Explosion

In order to study the effects of different orifice sizes and initial pressures on the explosion process of methane-air mixture, the explosion tests of methane-air mixture were carried out under different orifice sizes (16mm-160mm) and initial pressures (0kPa-50kPa) using PTB pipeline experimental device. The experimental results show that with the decrease of orifice size and the increase of initial pressure, the maximum explosion pressure of methane-air increases. The maximum explosion pressure is 5 times of the minimum explosion pressure in the limit state. The coupling effect of orifice size and initial pressure on explosion pressure is greater than effect of single factor. Finally, the influence surface is drawn to describe the relationship between the size of orifice plate and the initial pressure and the maximum explosion pressure of methane-air mixture.

Thermal and kinetics mechanism of explosion mitigation of methane-air mixture by N2/CO2 in a closed compartment

The natural gas is extensively used in energy and chemical industries. The natural gas explosion accidents frequently occur in civil and industrial building closed space which are significantly different from gas explosions in a large length-width ratio tube. In current research, the effectiveness of the usage of N2, CO2 and mixture of both suppressants in preventing and controlling gas explosion in a closed compartment is discussed. The thermal and reaction kinetics mechanisms involving inert gas suppression are discovered based upon the physical and chemical properties of both gases. The measurements of maximum explosion pressure, pressure rising rate and velocity of flame propagation are performed. The phenomena of fire core floatation and the “mushroom-shaped” flame in downward flame front are observed under the suppression of CO2 or mixture of N2/CO2. The inhibiting effect of CO2 on CH4-air explosion is significantly better than that of N2. The properties of physical heat absorption and chemical elementary reaction of both gases are analyzed to account for the difference of the explosion suppression effect. The findings of current research can provide basic data and theoretical guidance for inerting treatment or fire protection design of methane-air explosion in civil and industrial closed spaces.

Effects of Propane on the Flammability Limits and Chemical Kinetics of Methane–Air Explosions

ABSTRACT This work presents a comprehensive account of how adding C3H8 impacts the possibility of explosion for CH4–air mixtures. A series of experiments was carried out to determine the upper and lower flammability limits (UFL and LFL, respectively), and the explosive risk value for different mixtures of CH4 and C3H8. The critical oxygen concentration, the concentrations of CH4 and N2 at the point where the LFL and UFL merge, and the CH4 explosive triangle were obtained for nitrogen-diluted mixtures. In addition, we used a detailed mechanism (GRI-Mech3.0) in the CHEMKIN software package to acquire the impacts of adding C3H8 on the chemical kinetics of the explosion of CH4. The flammable limits of CH4 decrease linearly with the addition of C3H8 while the flammable range increases slightly. The explosive risk value for CH4 in air increases greatly upon adding C3H8 following a parabolic relationship. Nitrogen dilution noticeably decreases the UFL and slightly increases the LFL, ultimately resulting in these points merging. The concentration of CH4 and nitrogen at this merging point varies linearly when the C3H8 concentration is varied between 0% and 2.0%. The addition of C3H8 also expands and shifts leftwards and downwards CH4’s explosive triangle. Numerical analysis reveals that the maximum temperature and pressure during the explosion, and the formation rates of CO and NOx, noticeably increase after adding C3H8. Sensitivity coefficients of key dominant reactions show that overall more •H, •O, and •OH are consumed when C3H8 is added. Furthermore, the time between ignition and maximum pressure (the burning time) is evidently shortened by the addition of C3H8, which also shortens the explosion duration.

Experimental study on the explosion and flame emission behaviors of methane-ethylene-air mixtures

To investigate the effects of ethylene on methane-air explosions on a macro-to-micro level, the corresponding explosion and flame emission spectral behaviors were experimentally studied in a 20-L spherical chamber. The maximum explosion pressure, maximum pressure rising rate, and explosion duration were tested. The emission spectra of the key intermediates OH*, H* and CH2O* were obtained synchronously by using a photomultiplier-monochromator method. The detailed mechanism of GRI-Mech 3.0 was employed to analyze the kinetic reactions in the explosion process. A comprehensive analysis was performed by correlating the explosion parameters and emission spectral data. The results indicated that the added C2H4 would enhance the explosion of the fuel-lean methane-air mixture and attenuate the explosion of the fuel-rich mixture. It first promotes and then weakens the explosion of the stoichiometric methane-air mixture on the basis of the added volume fraction. The behavior of free radicals is well in accordance with the explosion parameters. The added C2H4 promotes the accumulation of the intermediates. Moreover, the variation in the production rate for the dominant reactions to generate the intermediates, caused by the addition of C2H4, directly results in a change in the average rising rate and maximum emission intensity of the intermediates.

Effect of a single orifice plate on methane-air explosion in a constant volume vessel: Position and blockage ratio dependence

In this study, the explosion behaviors of methane-air mixtures were investigated experimentally in a cylinder vessel (210 mm inner diameter and 210 mm height) filled with a single orifice plate at atmospheric pressure (1 atm) and room temperature (298 K). The equivalence ratios of the test mixtures ranged from 0.7 to 1.3. The blockage ratios of the orifice plates were from 0.49 to 0.94, and the distance between the orifice plates and the ignition point (S) were 25 mm, 50 mm and 75 mm. The results show that the maximum explosion pressure in the presence of an orifice plate. However, the effects of the orifice plate on the maximum rate of pressure rise are various. At the fuel-lean side, the maximum pressure rise rate is increased significantly (larger than that of the stoichiometric condition). As the distance between the orifice plate and ignitor increases, the maximum pressure rise rate increases. In the case of the fuel-rich conditions, the maximum pressure rise rate is the smallest forS = 25 mm, followed by the no plate condition, S = 50 mm andS = 75 mm. The mechanisms are left for further study.

The influence of dust originating from carbon black nanopowders on the explosion characteristics of lean methane/air mixtures within a turbulent environment

Industrial requirements present some unique challenges that can be met only by the application of engineered nanomaterials (ENMs). The completion of risk assessments based on the knowledge of the change in the explosion severity of turbulent gas/air mixtures caused by the accidental dispersion of nanopowders is vital before integrating such materials into existing systems. In this work, known amounts of selected carbon black nanopowders were mixed with methane near the lower-flammability-limit (LFL) to form hybrid mixtures of variable dust concentration. Mixtures were ignited in a 23 L cylindrical combustion vessel that allowed the control of isotropic turbulence through specially designed fans. The particle size distribution, the explosion pressure history and the flame speed derived from high speed Schlieren cine photographs, were measured. The influence of dispersed amounts of nanopowders on explosion severity was investigated by comparing the results with those obtained for pure methane-air explosions. Results indicated that the release of a relatively low mass of nanopowder into methane-air mixtures resulted in a more severe explosion than that of a higher amount. Also, despite the very low content of volatiles in the selected nanopowders, a hybrid mixture can be ignitable at an equivalence ratio well below the LFL of the gas. However this ignitability was shown to be related to the agglomeration state of powder in the dispersion phase, indicating that as the mean particle size decreases, a hybrid mixture with an extremely low content of flammable gas could be ignitable. From a risk assessment point of view, these results may have great significance for specific industrial processes aiming to utilise ENMs.