Validation of a new formula for predicting the lower flammability limit of hybrid mixtures

A correlation of the lower flammability limit for hybrid mixtures was recently proposed by us. The experimental conditions including ignition energy and turbulence which play a primary role in a gas or dust explosion were at fixed values. The sensitivity of such experimental conditions to the accuracy of the proposed formula was not thoroughly discussed in the previous work. Therefore, this work studied the effect of varying the ignition energy and turbulence intensity to the formula proposed in our previous paper. For ignition energy effect, results from methane/niacin mixture demonstrated that the MEC and LFL will not be affected by changing ignition energy. There is no distinguishable difference among gas explosion index (K_G) and dust explosion index (K_St) derived from tests with every ignition energy (2.5 kJ, 5 kJ and 10 kJ) in a 36 L vessel. The proposed formula is independent of ignition energy. For turbulence effect, the proposed formula can have a good prediction of the explosion and non-explosion zone if the ignition delay time is within a certain range. The formula prediction is good as the ignition delay time increases up to 100 ms in this work. Propane/niacin and propane/cornstarch mixtures are also tested to validate the proposed formula. It has been confirmed that the proposed formula predicts the explosion and non-explosion zone boundary of such mixtures.     

A review on hybrid mixture explosions: Safety parameters,explosion regimes and criteria,flame characteristics

The hybrid mixture of combustible dusts and flammable gases/vapours widely exist in various industries, including mining, petrochemical, metallurgical, textile and pharmaceutical. It may pose a higher explosion risk than gas/vapor or dust/mist explosions since the hybrid explosions can still be initiated even though both the gas and the dust concentration are lower than their lower explosion limit (LEL) values. Understanding the explosion threat of hybrid mixtures not only contributes to the inherent safety and sustainability of industrial process design, but promotes the efficiency of loss prevention and mitigation. To date, however, there is no test standard with reliable explosion criteria available to determine the safety parameters of all types of hybrid mixture explosions, nor the flame propagation and quenching mechanism or theoretical explanation behind these parameters. This review presents a state-of-the-art overview of the comprehensive understanding of hybrid mixture explosions mainly in an experimental study level; thereby, the main limitations and challenges to be faced are explored. The discussed main contents include the experimental measurement for the safety parameters of hybrid mixtures (i.e., explosion sensitivity and severity parameters) via typical test apparatuses, explosion regime and criterion of hybrid mixtures, the detailed flame propagation/quenching characteristics behind the explosion severities/sensitivities of hybrid mixtures. This work aims to summarize the essential basics of experimental studies, and to provide the perspectives based on the current research gaps to understand the explosion hazards of hybrid mixtures in-depth.     

Effects of ignition energy on fire and explosion characteristics of dilute hybrid fuel in ventilation air methane

Deflagration explosions of coal dust clouds and flammable gases are a major safety concern in coal mining industry. Accidental fire and explosion caused by coal dust cloud can impose substantial losses and damages to people and properties in underground coal mines. Hybrid mixtures of methane and coal dust have the potential to reduce the minimum activation energy of a combustion reaction. In this study the Minimum Explosion Concentration (MEC), Over Pressure Rise (OPR), deflagration index for gas and dust hybrid mixtures (K_st) and explosive region of hybrid fuel mixtures present in Ventilation Air Methane (VAM) were investigated. Experiments were carried out according to the ASTM E1226-12 guideline utilising a 20 L spherical shape apparatus specifically designed for this purpose.Resultsobtained from this study have shown that the presence of methane significantly affects explosion characteristics of coal dust clouds. Dilute concentrations of methane, 0.75–1.25%, resulted in coal dust clouds OPR increasing from 0.3 bar to 2.2 bar and boosting the K_st value from 10 bar m s⁻¹ to 25 bar m s⁻¹. The explosion characteristics were also affected by the ignitors’ energy; for instance, for a coal dust cloud concentration of 50 g m⁻³ the OPR recorded was 0.09 bar when a 1 kJ chemical ignitor was used, while, 0.75 bar (OPR) was recorded when a 10 kJ chemical ignitor was used.For the first time, new explosion regions were identified for diluted methane-coal dust cloud mixtures when using 1, 5 and 10 kJ ignitors. Finally, the Le-Chatelier mixing rule was modified to predict the lower explosion limit of methane-coal dust cloud hybrid mixtures considering the energy of the ignitors.     

Experimental study on the minimum explosion concentration of anthracite dust: The roles of O2 mole fraction,inert gas and CH4 addition

The explosion characteristics of anthracite coal dust with/without small amount of CH₄ (1.14 vol %) were investigated by using a 20 L spherical explosion apparatus with an emphasis on the roles of oxygen mole fraction and inert gas. Two methods based on overpressure and combustion duration time were used to determine the minimum explosion concentration (MEC) or the lower explosion limit (LEL) of the pure anthracite coal dust and the hybrid coal-methane mixtures, respectively. The experiment results showed that increasing oxygen mole fraction increases the explosion risk of coal dust: with increasing oxygen mole fraction, the explosion pressure (P_ex) and the rate of explosion pressure rise ((dp/dt)_ex)) increase, while MEC decreases. The explosion risk of anthracite dust was found to be lower after replacing N₂ with CO₂, suggesting that CO₂ has a better inhibition effect on explosion mainly due to its higher specific heat. However, the addition of 1.14% CH₄ moderates the inhibition effect of CO₂ and the promotion effect of O₂ on anthracite dust explosion for some extent, increasing explosion severity and reducing the MEC of anthracite dust. For hybrid anthracite/CH₄ mixture explosions, Barknecht's curve was found to be more accurate and conservative than Chatelier's line, but neither are sufficient from the safety considerations. The experimental results provide a certain help for the explosion prevention and suppression in carbonaceous dust industries.     

Explosibility of cork dust in methane/air mixtures

Explosibility studies of hybrid methane/air/cork dust mixtures were carried out in a near-spherical 22.7 L explosibility test chamber, using 2500 J pyrotechnic ignitors. The suspension dust burned as methane/air/dust clouds and the uniformity of the cork dust dispersion inside the chamber was evaluated through optical dust probes and during the explosion the pressure and the temperature evolution inside the reactor were measured. Tested dust particles had mass median diameter of 71.3 μm and the covered dust cloud concentration was up to 550 g/m³. Measured explosions parameters included minimum explosion concentration, maximum explosion pressures and maximum rate of pressure rise. The cork dust explosion behavior in hybrid methane/air mixtures was studied for atmospheres with 1.98 and 3.5% (v/v) of methane. The effect of methane content on the explosions characteristic parameters was evaluated. The conclusion is that the risk and explosion danger rises with the increase of methane concentration characterized by the reduction of the minimum dust explosion concentration, as methane content increases in the atmosphere. The maximum explosion pressure is not very much sensitive to the methane content and only for the system with 3.5% (v/v) of methane it was observed an increase of maximum rate of pressure rise, when compared with the value obtained for the air/dust system.     

Influences of coal dust components on the explosibility of hybrid mixtures of methane and coal dust

In order to study the influences of coal dust components on the explosibility of hybrid mixture of methane and coal dust, four kinds of coal dust with different components were selected in this study. Using the standard 20 L sphere, the maximum explosion pressure, explosion index and lower explosion limits of methane/coal dust mixtures were measured. The results show that the addition of methane to different kinds of coal dust can all clearly increase their maximum explosion pressure and explosion index and decrease their minimum explosion concentration. However, the increase in the maximum explosion pressure and explosion index is more significant for coal dust with lower volatile content, while the decrease in the minimum explosion concentration is more significant for coal dust with higher volatile content. It is concluded that the influence of methane on the explosion severity is more pronounced for coal dust with lower volatile content, but on ignition sensitivity it is more pronounced for coal dust with higher volatile content. Bartknecht model for predicting the lower explosion limits of methane/coal dust mixture has better applicability than Le Chatelier model and Jiang model. Especially, it is more suitable for hybrid mixtures of methane and high volatile coal dust.     

Lower explosion limit of hybrid mixtures of burnable gas and dust

Hybrid mixtures – mixtures of burnable dusts and burnable gases – pose special problems to industries, as their combined Lower Explosion Limit (LEL) can lie below the LEL of the single substances. Different mathematical relations have been proposed by various authors in literature to predict the Lower Explosion Limit of hybrid mixtures (LEL_hybrid). The aim of this work is to prove the validity or limitations of these formulas for various combinations of dusts and gases. The experiments were executed in a standard 20 L vessel apparatus used for dust explosion testing. Permanent spark with an ignition energy of 10 J was used as ignition source. The results obtained so far show that, there are some combinations of dust and gas where the proposed mathematical formulas to predict the lower explosible limits of hybrid mixtures are not safe enough.     

Explosion of lycopodium-nicotinic acid–methane complex hybrid mixtures

With the terms “complex hybrid mixtures”, we mean mixtures made of two or more combustible dusts mixed with flammable gas or vapors in air (or another comburent).In this work, the flammability and explosion behavior of selected complex hybrid mixtures was studied. In particular, we investigated mixtures of nicotinic acid, lycopodium and methane. We performed explosion tests in the 20-L explosion vessel at different overall (nicotinic plus lycopodium) dust concentrations, nicotinic acid/lycopodium ratios, and methane concentrations.An exceptional behavior (in terms of unexpected values of rate of pressure rise and pressure) was found for the complex hybrid mixtures containing lycopodium and nicotinic acid in equal amounts. This mixture was found to be much more reactive than all the other dust mixtures, whatever the dust concentration and the methane content.     

Experimental investigation on the lower flammability limits of diethyl ether/ n-pentane/epoxypropane-air mixtures

Diethyl ether (DEE), epoxypropane (PO) and n-pentane have excellent ignition and combustion performance; hence, they have a wide variety of applications in industry and advanced aviation propulsion systems. As these fuels are flammable at normal temperature and pressure, their explosive characteristics need to be explored. In this study, the lower flammability limits (LFLs) of vapor mixtures of DEE/PO/n-pentane in air were measured in 20 L, closed, stainless steel spherical vessels. Experimental results were obtained at ambient atmospheric pressure and an initial temperature of 40 °C. The experimental results show that the LFLs of DEE-air, n-pentane -air, and PO-air are 1.81 vol%, 1.41 vol% and 2.44 vol%, respectively. The LFLs of binary/ternary fuel mixtures under different compositions were tested, and the experimental results are compared with the classical Le Chatelier's formula. The results show that, for the binary fuels (i.e., DEE/PO, DEE/n-pentane, PO/n-pentane)-air mixtures, the maximum difference of the LFLs between Le Chatelier's formula and the experimental results is 6.10%. For the ternary fuels (i.e., DEE/PO/n-pentane)-air mixtures, the maximum difference of the LFLs between the two results is 6.33%. Due to the adiabatic flame temperature of each single fuel mixture being close, the Le Chatelier's formula is applicable for an estimation of the LFL for DEE/PO/n-pentane-air mixtures.     

Explosibility of hydrogen–graphite dust hybrid mixtures

To evaluate the hazard of combined hydrogen/dust explosions under severe accident conditions in International Thermonuclear Experimental Reactor (ITER), standard method of 20-L-sphere was used to measure the explosion indices of 4-μm fine graphite dust in lean hydrogen/air mixtures. The mixtures were ignited by a weak electric spark. The tested fuel concentrations were 8–18 vol% H₂ and 25–250 g/m³ dust. If the hydrogen content is higher than 10 vol%, the dust constituent can be induced to explode by the hydrogen explosion initiated by a weak electric spark. Depending on the fuel component concentrations, the explosions proceed in either one or two stages. In two-stage explosions occurring at low hydrogen and dust concentrations, the mixture ignition initiates first a fast hydrogen explosion followed by a slower phase of the dust explosion. With increasing dust concentration, the dust explodes faster and can overlap the hydrogen-explosion stage. At higher hydrogen concentrations, the hybrid mixtures explode in one stage, with hydrogen and dust reacting at the same time scale. Maximum overpressures of hybrid explosions are higher than those observed with hydrogen alone; maximum rates of pressure rise are lower in two-phase explosions and, generally, higher in one-stage explosions, than those characteristic of the corresponding H₂/air mixtures.     

Explosion behavior of hydrogen–methane/air mixtures

The effects of enriching natural gas with hydrogen on local flame extinction, combustion instabilities and power output have been widely studied for both stationary and mobile systems. On the contrary, the issues of explosion safety for hydrogen–methane mixtures are still under investigation.In this work, experimental tests were performed in a 5 L closed cylindrical vessel for explosions of hydrogen–methane mixtures in stoichiometric air. Different compositions of hydrogen–methane were tested (from pure methane to pure hydrogen) at varying initial pressures (1, 3 and 6 bar).Results have allowed the quantification of the combined effects of both mixture composition (i.e., hydrogen content in the fuel) and initial pressure on maximum pressure, maximum rate of pressure rise and burning velocity. The measured burning velocities were also correlated by means of a Le Chatelier’s Rule-like formula. Good predictions have been obtained (at any initial pressure), except for mixtures with hydrogen molar content in the fuel higher than 50%.     

Experimental measurement and numerical analysis of binary hydrocarbon mixture flammability limits

The flammability limits of binary hydrocarbon mixtures in air were measured in a combustion apparatus using an innovative method developed for this apparatus. The experimental results were obtained at standard conditions (room temperature and ambient atmospheric pressure) with upward flame propagation. The experimentally determined flammability limits for pure hydrocarbons (methane and ethylene) were compared with existing data reported in the literature. Le Chatelier's Law was fit to all experimental data to obtain LFLs and UFLs for various two-component combinations of saturated and unsaturated hydrocarbons (methane, ethylene, acetylene, propane, propylene, and n-butane). A modification of this law was used if experimental observations showed large deviations from Le Chatelier's predictions. Also, experimentally measured flammability limit data of the binary hydrocarbon mixtures were analytically related to the stoichiometric concentrations.     

Methane/coal dust/air explosions and their suppression by solid particle suppressing agents in a large-scale experimental tube

Methane/coal dust/air explosions under strong ignition conditions have been studied in a 199 mm inner diameter and 30.8 m long horizontal tube. A fuel gas/air manifold assembly was used to introduce methane and air into the experimental tube, and an array of 44 equally spaced dust dispersion units was used to disperse coal dust particles into the tube. The methane/coal dust/air mixture was ignited by a 7 m long epoxypropane mist cloud explosion. A deflagration-to-detonation transition (DDT) was observed, and a self-sustained detonation wave characterized by the existence of a transverse wave was propagated in the methane/coal dust/air mixtures.The suppressing effects on methane/coal dust/air mixture explosions of three solid particle suppressing agents have been studied. Coal dust and the suppressing agent were injected into the experimental tube by the dust dispersion units. The length of the suppression was 14 m. The suppression agents examined in this study comprised ABC powder, SiO₂ powder, and rock dust powder (CaCO₃). Methane/coal dust/air explosions can be efficiently suppressed by the suppression agents characterized by the rapid decrease in overpressure and propagating velocity of the explosion waves.     

Analysis on research trends with dust explosions by bibliometric approach

Highly destructive combustible dust explosions, which is prone to cause secondary explosion, has been a concern in industrial processes. To understand the current development and status of research on dust explosions, 1276 publications related to dust explosions from 1998 to 2021 were indexed through the Web of Science Core Collection database. CiteSpace and VOSviewer were used to visualize and analyze the collected literature information. The number of articles related to dust explosions has increased from 12 in 1998 to 191 in 2021. China, the United States, and Canada are the major contributors in this field. Dalhousie University, Beijing Institute of Technology, and Dalian University of Technology are at the core of dust explosion research. Wei Gao, Paul Amyotte, and Chi-Min Shu are the most prolific researchers. Journal of Loss Prevention in the Process Industries, Powder Technology, and Process Safety and Environmental Protection are the major sources of publications related to dust explosions. The research topic of dust explosions mainly evolves into four aspects: explosion characteristics and influencing factors, research media, explosion suppression, and numerical simulation. New research hotspots have appeared related to gas–dust hybrid mixtures, nanomaterials, and powder suppressants. The results can help researchers in the dust explosion field to quickly determine the research frontier and the overall situation.     

On the explosion and flammability behavior of mixtures of combustible dusts

In the work presented in this paper, the explosion and flammability behavior of combustible dust mixtures was studied. Lycopodium, Nicotinic acid and Ascorbic acid were used as sample dusts.In the case of mixtures of two dusts, the minimum explosive concentration is reproduced well by a Le Chatelier's rule-like formula, whereas the minimum ignition energy is a linear combination of the ignition energies of the pure dusts.An unexpected behavior has been found in relation to the explosion behavior and the reactivity. When mixing Lycopodium and Nicotinic acid or Ascorbic acid, the rate of pressure rise of the mixture is much higher than the rate of pressure rise obtained by linearly averaging the values of the pure dusts (according to their weight proportions), thus suggesting that strong synergistic effects arise; but it is comparable to that of the most reactive dust in the mixture.The observed behavior seems to be linked to the presence of minerals in the Lycopodium particles which catalyze oxidation reactions of Nicotinic acid and Ascorbic acid, as suggested by TG analysis.In the case of mixtures of three dusts, a similar behavior is observed when the concentration of Lycopodium is twice that of the other two dusts.     

Large-scale dust explosions in vessel-pipe systems

This study investigates dust explosions in vessel-pipe systems to develop a better understanding of dust flame propagation between interconnected vessels and implications for the proper application of explosion isolation systems. Cornstarch dust explosions were conducted in a large-scale setup consisting of a vented 8-m³ vessel and an attached pipe with a diameter of 0.4 m and a length of 9.8 m. The ignition location and effective dust reactivity were varied between experiments. The experimental results are compared against previous experiments with initially quiescent propane-air mixtures, demonstrating a significantly higher reactivity of the dust explosions due to elevated initial turbulence, leading to higher peak pressures and faster flame propagation. In addition, a physics-based model developed previously to predict gas explosion dynamics in vessel-pipe systems was extended for dust combustion. The model successfully predicts the pressure transients and flame progress recorded in the experiments and captures the effects of ignition location and effective dust reactivity.     

Influence of different ignition delay times on the pressure rise rate in hybrid mixture explosions in the 20-L sphere

For the development of a standardized method for measuring the explosion safety characteristics of combustible hybrid dust/vapor mixtures, the influence of the ignition delay time needs to be investigated. The ignition delay time, defined as the time between the injection of dust and the activation of the ignition source, is related to the turbulence of the mixture and thus to the pressure rise rate. The ignition source for pure vapors, however, has to be activated in a quiescent atmosphere according to the standards. Nevertheless, when measuring the explosion safety characteristics of hybrid mixtures, it is important that the dust be in suspension around the igniter. Like pure dust/air mixtures, hybrid dust/vapor/air mixtures need to be ignited in a turbulent atmosphere to keep the dust in suspension.This work will therefore investigate the influence of ignition delay times on the severity of hybrid explosions. It was generally found that at shorter ignition delay times, (dp/dt)_ex increased due to higher turbulence and decreases as the dust sinks to the bottom of the 20 L-sphere. This effect is more pronounced for hybrid mixtures with higher vapor content compared to dust content.     

EVA树脂粉尘/甲烷/空气杂混物爆炸特性

在20 L爆炸实验装置中,开展了3种不同中值粒径的EVA树脂粉尘/甲烷/空气所组成的杂混物爆炸特性研究,探究了甲烷浓度对粉尘爆炸下限、最大爆炸压力的影响。结果表明,尽管添加的甲烷气体浓度低于爆炸下限,仍使得粉尘爆炸下限得以降低,粒径较大的EVA III粉尘,当甲烷体积分数为1%时,爆炸下限降低约25%;粒径较小的EVA I粉尘,当混入甲烷体积分数为4%时,爆炸下限则降低80%;甲烷体积分数每增加1%,可燃粉尘最大爆炸压力上升约10%,但对于粒径较小的EVA I粉尘,当甲烷体积分数为4%时,最大爆炸压力的上升呈现突变趋势,上升近50%。     

Dust explosion risk moderation for flocculent dusts

The research presented in this paper is focused on dust explosions of coarse and fine flocculent (or fibrous) samples of wood and polyethylene. Hybrid mixtures of fibrous polyethylene and admixed ethylene were also studied. Experimentation was conducted by following standardized test procedures and using standardized apparatus for determination of maximum explosion pressure, size-normalized maximum rate of pressure rise, minimum explosible concentration, minimum ignition energy, and minimum ignition temperature. A general trend was observed of enhanced explosion likelihood and consequence severity with a decrease in material diameter, as well as enhanced consequence severity with admixture of a flammable gas to the combustion atmosphere. The same phenomena are well-established for dusts composed of spherical particles; this highlights the importance of inherently safer design and the principle of moderation in avoiding the generation of fine sizes of flocculent dusts and hybrid mixtures of such materials with flammable gases.In addition to presenting experimental findings, the paper describes phenomenological modelling efforts for the flocculent polyethylene using four geometric equivalence models: radial equivalence, volumetric equivalence, surface area equivalence, and specific surface area equivalence. The surface area equivalence model was found to yield the best estimates of maximum rate of pressure rise for the flocculent polyethylene samples investigated experimentally.     

Theoretical evaluation of lower explosion limit of hybrid mixtures

Lower explosion limits of hybrid fuel mixtures are usually determined through time consuming and expensive experiments. Although, mathematical expressions like Le-Chatelier's Law and Bartknecht curve have been used by many researchers to predict the LEL of hybrid mixtures, significant deviations remain unexplained. This research work, presents a more sophisticated and general approach for the determination of LEL of hybrid mixtures.Assuming that the combustion kinetics of pure species are independent and unchanged by the presence of other combustible species, complete conversion of the reactants and no heat losses, a simple mathematical model has been derived from the enthalpy balance of the whole system. For the experimental validation of the modelled values, modified version of 20L sphere has been employed, following the European standard (EN 14034-3: 2011) as experimental protocol. Hybrid mixtures of three dusts with two gases were selected for the scope of this publication. By analyzing the modelled as well as the experimental values, it can be concluded that the LEL values of the individual components in the hybrid mixture set the upper and lower limit for the LEL of the hybrid mixture provided the total amount of fuel in the system is considered as the concentration of the hybrid mixture. Moreover, the amount of dust or gas required to render the hybrid mixture flammable mainly depends on the energy contribution upon combustion of the individual species to raise the temperature of the whole system from ambient to the flame temperature.Le-Chatelier's Law and Bartknecht curve are empirical relations, which might hold true for a first-order approximation of LEL of hybrid mixtures, but do not represent the most conservative values of LEL reported in literature. This implies that there is a non-zero probability of occurrence of an explosible mixture in the non-explosible concentrations ranges defined by these relations. Considering these arguments, the authors suggest to employ the model presented in this paper – which presents reasonably conservative values of LEL of hybrid mixtures – for theoretical calculation of LEL of hybrid mixtures, when no precise experimental data is available.