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Explosions of vapour/dust hybrid mixtures: A particular class
Abstract
The aim of this work is to demonstrate the particular aspects of the explosion of hybrid mixtures with respect to either vapour or dust explosions. This work is focused on pharmaceutical products from excipients to active drug substances and their associated solvents. Experiments with an explosion sphere has been used to determined the influence of dusts and vapours concentrations on the severity of explosions (maximum pressure Pmax and maximum rate of pressure rise (dP/dt)max). Results clearly show that the explosion behaviour of such hybrid mixtures reveals significant differences with respect to either vapour or dust explosions; especially a promotion effect on the combustion kinetics and on the rate of pressure rise for poor mixtures. The influences of the introduction of small amount of vapour on the minimum explosible dust concentration (MEC) and of dust addition on the lower explosion limit (LEL) are noteworthy.Graphical abstractThis work shows the peculiar aspects of hybrid mixtures explosion as regard to vapours or dusts explosions. Results clearly demonstrate a promotion effect on the combustion kinetics and on the rate of pressure rise for poor mixtures. The influences of the introduction of small amount of vapours on explosion limits are also noteworthy.
... However, gas and dust explosion tests may be essential to understand better the phenomenon itself and its fundamental steps (Cloney et al. 2017). Particularly promising is the study of hybrid mixtures explosion, which could potentially represent and simulate complex phenomena involving two combustible phases (Abbas et al. 2022b;Dufaud et al. 2009;Guo et al. 2020;. In this work, hybrid mixtures were exploited to assess the role of pyrolysis during the rapid combustion of an organic powder. ...
... This energy was chosen as it is both sufficiently high to ignite the pure dust (MIE cellulose and char) and sufficiently low to limit overdriving effect (Taveau et al. 2017). As no standard procedure exists (yet) to determine the explosion severity of hybrid mixtures (Spitzer et al. 2020), the procedure used for powders (EN 14034) has been adapted to such mixtures (Dufaud et al. 2009). The sphere was partially vacuumed to pressures as low as 50 mbar (punctually 30 mbar). ...
... Representation of the maximum rate of pressure rise of magnesium stearate/ethanol hybrid mixtures(Dufaud et al. 2009) Explosion regime chart for nicotinic acid/methane hybrid mixtures(Garcia- Agreda et al. 2011); MEC and LFL correspond to the Minimal Explosion Concentration and the Lower Flammable Limit, respectively. ...
On a misty night on the 14th of December 1785, a "spontaneous inflammation", as described later by Count Morozzo Della Rocca, occurred in a bakery in Turin, Italy. From that episode, considered the first-ever documented accident caused by a dust explosion, the scientific community has built a profound knowledge of this phenomenon. However, comprehending the numerous physical and chemical mechanisms involved is still incomplete.
This work aims to identify the processes that have a primary role in organic dust explosions, study them and propose experimental tools to characterise them better. Choosing operating conditions coherent with those encountered during a dust explosion will be crucial.
... De plus, considérer que le domaine d'inflammabilité d'un tel mélange est simplement borné par la concentration minimale explosive de la poussière et la limite inférieure d'explosivité du gaz est manifestement une erreur. En effet, un mélange de gaz, dont la concentration est inférieure à la LIE et de poudre, dont la teneur est inférieure à la CME, peut générer des atmosphères explosibles [47,48]. Figure 3.8 -Représentation normalisée de la limite inférieure d'inflammabilité de mélanges hybrides composés de poussières combustibles et de gaz inflammables [3] En pratique, la relation linéaire de Le Chatelier a été jugée valide par Cashdollar [34] pour le mélange de poussière de charbon et de méthane. ...
... d'hexane. De telles évolutions ont été relevées par plusieurs auteurs [48]. La figure 4.13 confirme cette évolution de la C max en fonction de solvants [11]. ...
... En 2006, des chercheurs japonais et français[1] ont également étudié ce type de mélange hybride en travaillant sur le polyuréthane en présence de vapeurs de cyclopentane. Il ressort de cette étude que plus la proportion de cyclopentane est importante dans l'atmosphère, plus le polyuréthane devient inflammable et explosif jusqu'à atteindre un palier pour une concentration de 5300 ppm de cyclopentane.Plus récemment, des études ont été faites au sein du LRGP[47,48], notamment dans le cadre de la thèse de M. Traoré[11], sur l'influence du mélange des poussières pharmaceutiques (stearate de magnésium, niacine et antibiotique) et de solvants, sur la pression maximale et la vitesse maximale de montée en pression atteintes au cours de leur explosion. Au début de son travail, il a étudié les explosions d'un même antibiotique en présence de différents solvants (acétone, éther diisopropylique, éthanol, toluène). ...
La sensibilité et la sévérité d'explosion des différents mélanges gaz/vapeur-poussière ont été étudiées grâce à des dispositifs standards (sphère de 20 L, tube de Hartmann). Les spécificités des explosions de mélanges hybrides gaz/poussière ont été mises en évidence. En fait, même pour des concentrations de gaz inférieures à la limite inférieure d'explosivité (LIE), la probabilité d'inflammation et la gravité d'explosion peuvent être considérablement augmentées, ce qui permettra notamment de conduire à de grands changements dans la détermination des zones ATEX. Il a été, par exemple, démontré que ces mélanges peuvent être explosifs même lorsque la concentration en poudre et la concentration en vapeur sont respectivement en dessous de la concentration minimale explosive et de la LIE. En outre, des effets de synergie ont été observés et la vitesse de montée en pression de mélanges hybrides peut être supérieure à celles des gaz purs. Les origines de ces spécificités ne doivent pas être recherchées dans la modification d'un paramètre unique, mais peuvent probablement être attribuées aux effets combinés sur l'hydrodynamique (propagation de la flamme), le transfert thermique et la cinétique de combustion. Des expériences ont été menées afin de souligner l'importance de chaque contribution. Basé sur des schémas cinétiques classiques à coeur rétrécissant prenant en compte des diverses contraintes lors d'une réaction non-catalytique de gaz/solide et sur des modèles de combustion homogène pour les gaz, un modèle a été développé pour représenter l'évolution temporelle de la pression d'explosion pour ces mélanges
... However, considering that the concentration's flammable domain is simply framed by the MEC of the dust and the LEL of the gas is a mistake. In fact, mixtures containing concentrations of dust and gas below the LEL and MEC are still sensitive to flammability (Dufaud et al., 2008;Dufaud et al., 2009). Furthermore, the Le Chaterlier's mixing rule, as described by Equation 1.16, is often inappropriate for predicting the concentration explosive limits of dust-gas mixtures. ...
... This can be done with a contour-graph while efficiently transmitting the information. This has been done by Traore (2007) and Dufaud et al. (2009), but it is still necessary to consider the position of the experimental points. This is important because it allows to visualize empty areas where more experimental data might be necessary to confidently assert the behaviour of the severity parameters in terms of the concentration values. ...
... This kind of behaviour seems to be exhibited by telithromycine/acetone, telithromycine/toluene and niacin/diisopropylether mixtures whose severity maps can be observed in Figures 1.13 to 1.15. Data for these figures comes from the thesis work of Traore (2007), some of which was published by Dufaud et al. (2009). The emphasis given before to the word "seems", arises from the fact that the data was originally presented, only as a contour plot without the presence of the experimental points over it. ...
Predicting the flame propagation during a dust/gas hybrid mixture explosion in complex geometries is a challenge that mobilizes numerous resources. One approach consists on experimentally determining the inherent characteristics of dust-air mixtures, like the laminar flame speed, and using them as input for Computational Fluid Dynamics (CFD) simulation programs. Nevertheless, the experimental characterization of the burning rates of turbulent dust clouds in air still delicate due to the variability of the properties of powders (particle size distribution, moisture…), the physical impossibility to generate a quiescent dust cloud and the impact of powder on the flame radiation among others. The ultimate goal of this work was to develop an approach to assess fundamental flame propagation properties, from closed vessel experiments and pressure-time evolution curves, but specially from the analysis of flame velocity as a function of its stretching and of the hydrodynamic instabilities. In a first step, the turbulence of the initial dust cloud has been studied. The impact of the pyrolysis phase on organic dusts explosion has also been highlighted both experimentally and by means of model for flash pyrolysis. Furthermore, the explosive behaviour of gas-dusts hybrid mixtures composed of pyrolysis gases and organic dusts has been analysed. Finally, the turbulence/combustion interactions during flame propagation have been studied in order to extract the “pseudo” laminar flame velocity of dusts clouds or hybrid mixtures
... Denkevits [7] investigated the explosibility of hybrid hydrogen/graphite dust mixtures. Dufaud et al. [8][9] presented the results of hybrid explosions involving pharmaceutical dust and their associated solvents. Garcia-Agreda et al. [10] characterised the explosibility of nicotinic acid dust as a function of methane concentration. ...
... It can be seen from Fig. 4 that the maximum explosion pressure P ex of lycopodium dust increases with the increase of equivalent ratio of methane and ethylene, indicating that the addition of methane or ethylene to the lycopodium dust cloud increases the maximum explosion pressure P ex significantly, and the maximum explosion pressure P ex of lycopodium dust is slightly dependent on the flammable gas content. The increase in P ex can be attributed to the increase of reactant amount taken by the additional flammable gas [4][5][6][7][8]. From Fig. 4 it can also be found that for hybrid mixtures with low gas concentrations (0b Ф b0.4), the increase in P ex taken by the additional methane and ethylene with the same equivalent ratio is almost equivalent, whereas for hybrid mixtures with higher gas concentrations (Ф N0.4), it reveals that the increase in P ex taken by the additional ethylene is obviously higher than that taken by the additional methane with the same equivalent ratio. ...
... It can be seen from Fig. 6 and Fig. 7 that the presence of flammable gas in a dust cloud increases the (dP/dt) ex significantly. The increase in (dP/dt) ex can be attributed to the increase of combustion kinetic induced by the additional flammable gas [4][5][6][7][8][9][10][11]. Similar to P ex , the increase in (dP/dt) ex taken by the additional methane and ethylene with the same equivalent ratio Ф at low values (0b Ф b0.4 for lycopodium dust, 0b Ф b 0.2 for polyethylene dust) is almost equivalent. ...
A standard 20 L spherical chamber was used to study the explosibility of hybrid mixtures systematically. Four different hybrid mixtures, which were composed of two different flammable gases (methane and ethylene) and two different flammable organic dusts (lycopodium and polyethylene), were selected. The maximum explosion pressure Pex and the maximum explosion rate of pressure rise (dP/dt)ex of the four different hybrid mixtures were measured under initial high turbulence conditions over a wide range of composition concentrations (volume concentration y for gas and mass concentration c for dust). Explosion behaviours of the four different hybrid mixtures were analysed and compared with each other. Experimental results have shown that adding different flammable gases to the same dust cloud can all increase the explosion severity of the dust. However, the increase is almost equivalent when different flammable gases with the same equivalent ratio at low values are added. Only if the equivalent ratio is sufficiently higher can a higher increase in the explosion severity be induced by the more explosible gas. Adding the same flammable gas to different dust clouds clearly increases the explosion severity of all these dusts, but the increased ratios of Pmax and (dP/dt)max are higher for the less explosible dusts, indicating that the influence of flammable gas on the explosion severity is more pronounced for less explosible dusts.
... The characterization of dust explosions also involves the measurement of variables like the maximum overpressure (P max ), the maximum rate of pressure rise dP/dt max , and minimum explosion concentration (MEC) under standardized procedures (Cashdollar and Zlochower, 2007;Sun et al., 2006). Experimental setups use to perform measurements of the P max and dP/dt max parameters are the 20 L Siwek sphere and the 1 m 3 vessel (Bind et al., 2011;Dufaud et al., 2009). The K St index is then deduced from a commonly used relationship, the cube-root law (Skjold, 2003). ...
... The standardized equipment used to perform the dust explosion severities tests was the 20 L sphere designed by Siwek (Dufaud et al., 2009). This experimental setup consists of a hollow sphere made of stainless steel, a dust storage container connected with the chamber through a dust outlet electro valve, a pair of electrodes holding two pyrotechnic igniters of 5 kJ at the sphere center (Cashdollar, 1994), and two piezoelectric pressure sensors to record the explo-sion overpressure development. ...
Due to its industrial applications, the assessment of the explosibility parameters of aluminum powder is essential. However, they strongly depend on the Particle Size Distribution (PSD), which impacts both the rate-limiting step of the combustion mechanism and the radiative transfer. This article describes the influence of the test procedure on the explosion severity of six aluminum samples, having primary particle diameters ranging between 0.04 and 125 μm. The PSD measured in situ after dispersion in a 20 L sphere differs from those obtained before dispersion, particularly for nanoparticles for which the presence of agglomerates is evidenced. The explosion severity parameters were measured at different turbulence levels by changing the nozzle geometry and ignition delay time. The impact of the injection procedure varies from micron-sized aluminum dust to nanoparticles due to their low inertia. Moreover, if an alternative nozzle could be more appropriate at a lower turbulence level, the rebound nozzle is always the most conservative option for standard test conditions. Finally, the mean particle surface area was identified as an appropriate indicator of the micro/nano scale's explosivity performance. Results suggest that below 3 m2/g, the combustion would be diffusion-limited and kinetic-limited otherwise.
... According to Worsfold et al. (2012) the LFL is reduced by the addition of a small amount of dust. An example of that is given by Dufaud et al. (2009) who indicated that the LFL of toluene is 8 %, although, it has been shown to explode at a concentration of 4 % after addition of combustible dust. Also, methane, due to its unique oxidation characteristics, has been found to be less ignitable in methane-air mixtures compared to other hydrocarbons. ...
... On the other hand, the maximum rate of pressure rise parameter has presented to be influenced by the flammable gas concentration, even above the MEC, and this effect is diminished as the dust concentration increases above the MEC. At the leaner dust concentrations the presence of the flammable gas has the maximum effect on the explosion parameters as the controlling variable of the combustion kinetics is led by the gas(Dufaud et al., 2009). ...
This work investigates the explosion and dispersion potential of engineered nanoparticles (ENP). The European Union (EU) sponsored this investigation, firstly to predict or estimate risks posed by the use of engineered nanomaterials (ENM), and secondly to implement procedures for the purpose of risk mitigation. These include establishing exposure control limits and controlling and monitoring exposure, including the accidental explosive or massive release of ENP into the environment.
To this end, the release of ENP originating from specific nanopowders was simulated in a 31 m3 airtight chamber of controllable environment. Their loss and dispersion characteristics were studied under ventilated and unventilated conditions. The explosion characteristics of specific ENP in lean hybrid blends of nanoparticles with methane and air, were studied in a 23 L cylindrical combustion vessel providing the adjustment of isotropic turbulence induced by specially designed fans. The influence of ENP on the explosion severity was evaluated by comparing the results obtained for pure methane explosions. Via a 6-jet Collison nebuliser (CN) combined with a considerably modified preparatory process of the tested nanopowders suspensions in water, ENP of Titanium and Silica Dioxide (TiO2 and SiO2) were injected continuously into the dispersion chamber. A specially designed dust injector was used for the introduction of two types of carbon black (CB) nanopowders (Corax N550 and Printex XE2) into the combustion vessel. An arrangement of particulate instrumentation was applied for tracking the evolution of particle number concentration (PNC) and particle size distribution (PSD) at points near to the source within the dispersion chamber. In addition, PSD measurements were conducted in the dust clouds generated for the explosion tests. Using a Log10-normal modal fitting program, the characteristics of groups present within the PSDs, were mathematically described. An indoor aerosol model for the study of the differential effect of coagulation and deposition on the changes of PNC with time in the dispersion chamber, was applied. Finally, the explosion severity was characterised by measurements of the explosion pressure history and of flame speed derived from high speed Schlieren cine photographs.
Results indicated that by reducing the ventilation rate the leftover PNC of ultrafine particles (diameter ≤ 100 nm) was gradually increased at the end of the evacuation process. In parallel, at the high ventilation rates the spatial ventilation efficiency was shown to be optimal close to the inlet diffuser. However, by decreasing the ventilation rate, ventilation efficiency was shown to be independent of the location in the chamber. The study of particle interactions under unventilated conditions indicated that different growth rates due to agglomeration were induced on the two types of dispersed ENP. For fine particles (diameter > 100 nm) of both materials, the model indicated that their losses were dominated by deposition at high PNC, whereas for ultrafine particles, heterogeneous coagulation was the main removal mechanism. However, the model indicated stronger deposition at low PNC, and weaker homogeneous coagulation at high PNC, for ultrafine and fine SiO2 particles respectively, compared to their TiO2 counterparts. The explosion tests indicated that the addition of variable concentrations of ENP in methane resulted in higher burning rates and flame acceleration than those demonstrated by the respective particle-free methane air mixtures. In addition, the mixture of the highest fraction in ultrafine ENP yielded the most severe explosion, while this mixture was of the lowest dust concentration. Finally, hybrid mixtures with methane below its lower flammability limit (LFL) were shown to be ignitable. Furthermore, the level of this extension below LFL was shown to be dependent to the material as different extensions were performed by Corax N550 and Printex XE2 hybrid mixtures.
The investigation recommends that in order to design efficient ventilation systems for nanotechnology workplace, only specific ventilation rates and arrangements of inlet/outlet diffusers, should be considered. Exposure to accidentally released ENP is expected to be different for different materials and strongly related to their emission profile. Finally, the generation of a dust cloud from a minor amount of a nanopowder combined with a low amount of a flammable gas and an electrostatic spark may result in a severe explosion of higher impacts for human health and installations than those induced by the explosion of a higher dust concentration hybrid mixture. Also this work demonstrated that as the mean particle size in the dust cloud decreases, a hybrid mixture of an extremely low content of gas could become ignitable. The latter could be applicable not only in the field relating to the risk assessment of ENP but also in generic technological applications involving airborne nanoparticles (e.g. soot particles) suspended in flammable gases (e.g. automotive applications that use natural gas).
... Bu kaza sonucunda birçok ölüm ve yaralanma meydana gelmiştir. Dufaud ve diğ.[8] hibrit karışımların patlamasının buhar veya toz patlamalarıyla benzer özelliklerini incelemişlerdir. Costin[9] yaptığı çalışmada kapalı bir alanda havada yayılan sıvılaştırılmış petrol gazından patlayıcı bir atmosferin patlamasını sayısal modelleme kullanarak incelemiştir.Patlamaların önlenmesi ve patlamadan korunma çalışma ortamları için önemli iki kavramdır. ...
... Unlike unitary gas explosions, which have been widely studied in the past decades, only minimal research has been focused on solid-gas hybrid mixtures, and data concerning hybrid mixtures can rarely be found. In a hybrid mixture, the admixture gas may be present below its LFL, of which battery gaseous emission is 7.9 %, and the particle content may be below its MEC p , but their combination can become explosible [41]. ...
When the concentration of gas is below its lower flammable limit and the content of the particles is below its minimum explosible concentration, it can be explosible. Thermal abuse tests for 50 Ah lithium-ion batteries were conducted under an inert atmosphere. The battery failure venting emissions were collected and their composition was identified previously. It still needs further research for the flammability characteristics and ignition conditions for hybrid mixture emissions venting from a large format lithium-ion battery thermal failure. In this study, the likelihood, severity, and kinetics of the batteries' hybrid mixture emissions flammability have been researched. To analyze battery emission flammability characteristics, five indexes have been investigated; they are ignition temperature (T ig), ignition time (τ ig), maximum explosion pressure (P max), pressure rise maximum rate ((dP/dτ) max), and size-normalized maximum rate pressure rise (K St). Based on quantitative analysis for the hybrid mixture flammability characteristics, it indicates that particle size and gas content can impact hybrid mixture emission flammability characteristics. As for the mixture with finer particles, the optimum combustion condition of the particle and gas concentration pairs were 0.3 and 0.7. The ignition temperature for the optimum combustion conditions was 495°C. The pressure rising velocity was 23 Mpa/s during ignition. High solvent content and low particle concentration are the conditions for optimum combustion. This indicates that the gas admixture solvent plays a major role in the combustion kinetics of the mixture emission. This research offers a fundamental perspective and provides preventive and protective thermal safety designs that are more suitable for large format commercial battery thermal systems.
... Thus, stringent safety precautions should be taken throughout the production process. Moreover, the explosion of dust particles is influenced by several factors, such as particle size (Azam and Mishra, 2019;Gao et al., 2013), dust concentration (Li et al., 2020a), moisture content (Chang et al., 2022;Niu et al., 2020), ignition energy (Janès et al., 2014;Kuai et al., 2011), oxygen content (Kim et al., 2016;Zhang et al., 2018a), inert dust content (Addai et al., 2016b), dust dispersion (Chen et al., 2014;Mishra and Azam, 2018;Serrano et al., 2021), and the presence of inflammable gases (Dufaud et al., 2008(Dufaud et al., , 2009Pilão et al., 2006). In general, the sensitivity and severity of dust explosions increase with dust concentration, oxygen concentration, inflammable gas concentration, and particle size, whereas the degree of dust explosion increases with a decrease in the contents of water and inert materials (Kuracina et al., 2021;Zhang et al., 2018b). ...
This study investigated the effects of different concentrations of ethylene and propene gas on the explosion characteristics of polyethylene (PE) dust. After the addition of ethylene and propene gases, the maximum explosion pressure of PE dust increased from 6.7 barg to 9.1 and 7.6 barg by 1.4 and 1.1 times, respectively. Furthermore, the lower explosion limit (LEL) decreased; the addition of even a small amount of these inflammable gases was sufficient to produce a hazardous gas/dust hybrid mixture with low LEL. PE dust tests in a 2 vol. % propene environment revealed that an initial explosion occurred in the hybrid gas mixture, and the heat was transferred to the remainder of the incompletely mixed gas/dust mixture, thereby leading to a two-stage and more complete reaction. In addition, 2 vol. % propene tests also exhibited changes in the maximum pressure rise rate increase, resulting in strong St-2 level explosions because the propene reached its LEL of 2 vol. %. Therefore, propene underwent a complete combustion, resulting in a higher rate of pressure, and shortened time to maximum rate of pressure rise (TMRPR). Thus, the use of volatile organic compounds in dust manufacturing processes considerably increases the risk of explosion incidents.
... Полученные результаты характерны не только для смесей метана с угольной пылью. Так в работе [18] изучалась способность к взрыву паропылевых смесей, образующихся в фармацевтической промышленности. В [19] измерялись максимальное давление и его рост при сгорании взвеси графитовой пыли в водородовоздушной смеси, находящейся в 20-ти литровой бомбе постоянного объема. ...
The spontaneous combustion of the coal microparticles of fractions 1–20 μm and 20–32 μm in an air atmosphere and the inflammation of the coal microparticles of fraction 20–32 μm in a methane–air mixture at temperatures of 700–1100 K were investigated with the use of a rapid compression machine. A contactless measurement of the temperature of the coal particles ignited spontaneously in a gas has shown that this temperature can reach 2500 ± 200 K and substantially exceeds the temperature of the gas at the end of its compression stroke. It was established that the coal microparticles burning in a stoichiometric methane–air mixture are local hotbeds of fire in this mixture at a temperature as high as 1400 K and that the gas is ignited in the neighborhood of these hotbeds. The limiting temperatures of ignition of the coal microparticles in an air atmosphere free of methane and in a methane–air mixture were determined. The measured times of delay in the ignition of the methane by the coal microparticles in a hybrid methane–air mixture agree with the delay times of ignition of a pure methane–air mixture under the same conditions to within the experimental error.
... Engler is the first scholar at home and abroad to study the explosion characteristics of fuel-air mixture. He studied and proposed the gas-dust dust explosion characteristics [6] [7]. Tan Yumei et al. studied and proposed the explosion characteristics of propylene oxide vapor-aluminum powder-air mixture [8]. ...
In order to improve the feasibility and accuracy of predicting the explosion power of fuel air mixture, a method of BP neural network prediction is proposed by combining factor analysis method with BP neural network. Using factor analysis method, the original data of 9 fuel air mixture explosion power factors were processed by dimensionality reduction data, 2 common factors were obtained, and 2 common factors were substituted for 9 fuel air mixtures as input layer parameters of BP neural network. A prediction model of coal and gas outburst with the combination of factor analysis method and BP Neural network is established to predict the explosion power of fuel air mixture. The prediction model of the explosion power of the fuel air mixture is selected to verify the improved BP neural network prediction models, and the final results are as follows: The relative error range of four prediction samples is 0.16%-7.58%, all less than 10%. The improved BP neural network prediction method is used to solve the problem of the traditional BP neural network because of the excessive number of input layer parameters, low data processing efficiency, slow iteration rate and low precision, which provides a new research way for predicting the explosion power of fuel air mixture.
... In contrast several other studies (Denkevits, 2007;Dufaud et al., 2009;Khalil, 2013;Denkevits and Hoess, 2015;Addai, 2016) advocate that the worst case scenario occurs at different concentrations of dust and gas in the hybrid mixture. ...
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.
... Pour des richesses inférieures à l'unité, l'addition de faibles quantités de combustible gazeux accroît donc la sévérité des explosions. Dans quelques cas particuliers, la sévérité de l'explosion peut être plus importante pour un mélange hybride que pour le gaz ou la vapeur seule (Dufaud et al., 2009). C'est le cas pour le mélange hexane -tourteaux (figure 3b -tableau 1) contrairement au mélange méthane -amidon. ...
... Furthermore, the influence of dust in hybrid mixtures with a gas below its LFL has been discussed by Dufaud et al. (2009), while several indications that explosions can occur even at these low equivalence ratio (φ) values have been reported by Amyotte et al. (2007), Cashdollar (1996) and Cashdollar (2000). Attempts to ignite hybrid mixtures composed of propane and dispersed amounts of a CB nanopowder below the LFL of gas, have been conducted by Kosinski et al. (2013), however they reported that this ignition was not possible due to the negligible amount of volatiles in the tested nanopowder. ...
... Denkevits (2007) investigated the explosibility of hybrid hydrogen/graphite dust mixtures. Dufaud et al. (2009Dufaud et al. ( , 2011 presented the results of hybrid explosions involving pharmaceutical dust and their associated solvents. Garcia-Agreda et al. (2011) characterised the explosibility of nicotinic acid dust as a function of methane concentration. ...
Vented hybrid mixture explosions were conducted in a 20-L chamber with different venting diameters and static activation pressures. Simultaneously, the maximum explosion pressure and the maximum rate of pressure rise of hybrid mixtures were also determined. It was found that the addition of methane to lycopodium dust led to an increase in both the maximum explosion pressure and the maximum rate of pressure rise and a decrease in the optimum dust concentration. Both the maximum explosion pressure and the maximum rate of pressure rise of hybrid mixtures were higher than those of lycopodium dust, but lower than those of methane. Similarly, the addition of methane to lycopodium dust led to an increase in the maximum reduced pressure, and the maximum reduced pressure increased with increase of the methane concentration. This effect was more pronounced for small vents and high static activation pressures. The maximum reduced pressure of the hybrid mixture was higher than that of lycopodium dust, but lower than that of methane. This was consistent with the relationship of the maximum explosion pressure between the three different systems. However, the increase in the maximum reduced pressure of lycopodium dust taken by the additional methane was obviously higher than that in the maximum explosion pressure, indicating that the influence of methane on the maximum reduced pressure of lycopodium dust was more significant. Adding methane to lycopodium dust increased the longest vented flame length and decreased the duration time of the external flame. Similar to the maximum reduced pressure, the longest flame length of the hybrid mixture was longer than that of lycopodium dust, but shorter than that of methane. However, it is the converse for the duration time of the external flame.
... Q4 Since Engler's first experiments in 1885 (Engler, 1885), several researches have been carried out through the world with the aims of preventing the occurrence or mitigating the consequence of a hybrid mixture explosions. Some of these researches on hybrid mixtures explosions can be found in the literature (Agreda, 2011;Addai et al., 2015a,b,c;Dufaud, 2009;Amyotte, 2010). The main findings of these studies could be summarized non-exhaustively by the assertions that both the ignition sensitivity and severity of the dust can be strongly increased by the addition of a few percent of combustible gases or vapors, even with contents lower than the lower explosion limit (LEL). ...
Explosion hazards involving mixtures of different states of aggregation continue to occur in facilities where dusts, gases or solvents are handled or processed. In order to prevent or mitigate the risk associated with these mixtures, more knowledge of the explosion behavior of hybrid mixtures is required. An experimental investigation of the minimum ignition temperature (MIT) and the lower explosion limit/minimum explosion concentration (LEL/MEC) of three component hybrid mixtures of combustible dusts, gases and vapors in air were performed in a modified Godbert-Greenwald (GG) furnace. The study comprised three dusts, two gases and two solvents vapors. For the dusts alone, the experimental protocol was in accordance with the European standard EN 50281-2-1, whereas for the gases, solvent vapors and mixtures of these with dusts, this protocol had to be modified slightly. It was found that the MITs of the gases and solvent vapors decreased significantly when a combustible dust at a concentration below the MEC of the dust alone, was added. A significant decrease in the MIT was also found if the gas/vapor atmosphere was mixed with a dispersed combustible dust that could not be ignited alone at the given temperature in the GG furnace. For example, the MIT of methane decrease from 600 °C to 585 °C when a small amount of starch was added. This hybrid MIT further decreased to 490 °C when a non-explosible third component, ethanol, was added. It was also found that a hybrid cloud of a combustible dust at a concentration less than the MEC and a combustible gas at a concentration less than the LEL could be ignited in the GG furnace. For example, the MEC of starch decreased from about 145 g/m³ to 40 g/m³ when methane at a concentration below the LEL was added. This explosible concentration of starch– methane mixture further decreased to 25 g/m³ when a small fraction of a third component, hexane, was added.
... Such risk is not limited to the integrity of the plant or equipment but also includes health concerns for operators when dust forms unconfined clouds in closed systems. A number of factors influence the dust explosion hazard, including particle size; dust concentration; oxidizer concentration; ignition temperature; turbulence of the dust cloud; maximum rate of pressure rise; admixed inert dust concentration; presence of flammable gases [32][33][34][35][36] . Among several factors to be taken into account in determining the explosion risk, the dust concentration and the variation of pressure and turbulence could be decisive [37][38][39] . ...
In the framework of the on-going research on dust explosion, both a physical and chemical theory and numerical models extensively validated are yet to be found. In order to develop control tools essential to continuously measure a set of key parameters for dust explosiveness, the authors performed stainless steel dust mobilization experiments inside STARDUST-U facility using a controlled air inlet whose features are comparable to the ones typical of several industrial scenarios. Dust particles velocity and concentration within the cloud were measured with non-invasive diagnostics involving imaging techniques and a custom software. The STARDUST-U facility showed capability to provide useful data for validation of numerical models. Furthermore, a custom software allowed to determine the relative dustiness, defined as a non-dimensional parameter proportional to dust concentration and not dependent on dust mass and vessel volume. This study is a first step towards a complete integration of the air inlet modeling and dust tracking software, in order to determine dustiness inside the cloud. The authors believe that the imaging techniques presented could represent a valuable tool for industry in order to perform continuous monitoring of vessels with the aim of controlling and mitigating dust explosion risks.
... Hybrid mixtures of combustible dust and gas-phase fuel pose a serious risk of accidental explosions in many industrial settings (Eckhoff, 2003). Examples of such hybrid mixtures include pulverized coal and methane in coal mines (Liu et al., 2007), pharmaceutical dust and solvent vapors (Dufaud et al., 2009), organic dust with fermentation gases (Pilão et al., 2006), paint pigments and solvents (Dufaud et al., 2007), plastic powders and solvents (Amyotte et al., 2010), or even metal particles with hydrogen, natural gas or other vapors in various industries (Denkevitz, 2007, Denkevitz and Hoess, 2015. ...
The propagation of constant-pressure flames through hybrid aluminum-methane-oxidizer mixtures in transparent latex balloons is investigated using high-speed cameras that record both the dust dispersal and the subsequent spherical flame propagation processes. For mixtures without excess oxygen in the post-methane flame zone, the aluminum acts only as an inert diluent at low concentrations, resulting in a decline of the flame speed with aluminum concentration. Past a critical concentration around 100 g/m³, the flame speed stops decreasing and remains constant with increasing aluminum concentration, a behavior which is attributed to the formation of a high-temperature aluminum flame front that thermally couples to the methane flame. When there is excess oxygen to react with the aluminum, the flame speed monotonically increases with aluminum concentration before reaching a plateau at aluminum concentrations above 150 g/m³. The difference in behavior at low concentrations in mixtures with and without excess oxygen is explained by the ability of aluminum particles reacting with free oxygen to ignite and burn in the diffusion-limited combustion mode. The igniting particles are able to form high-temperature micro-diffusion flames which attain high combustion rates even at the relatively low bulk gas temperatures typical for methane flames.
... The main conclusions of previous studies [11][12][13][14][15][16][17][18][19][20] on the explosion properties of hybrid mixtures could be summarized non-exhaustively by the following assertions, that the ignition sensitivity of the powder can be strongly increased by the addition of a few percent of combustible gases or vapours, even with contents lower than the LEL. It has notably been shown that hybrid mixtures can also be explosible when the concentrations of the dust and the gas are both below their respective MEC and LEL, respectively. ...
Investigation of the ignition behavior (minimum ignition temperature [MIT]) of dusts, gases, or solvents and their mixtures has been undertaken by performing a series of tests in the modified Godbert–Greenwald furnace. Four flammable gases (methane, propane, ethanol, and isopropanol) as well as three combustible dusts (starch, lycopodium, and toner) were used as materials. For the dusts alone test, the experimental protocol was in accordance with the European standard EN 50281-2-1, whereas for the gases, solvent vapors, and mixtures of these with the dusts, this protocol had to be modified slightly. The experimental results demonstrate the significant decrease of the MIT of either gas, solvent, or dust and an increase in the explosion likelihood when a small amount dust, which is either below the minimum explosion concentration or not ignitable itself at the given temperature, are admixed with gas or solvent and vice versa. For example, the MIT of methane decrease from 600°C to 585°C when a small amount of starch was added. This hybrid MIT further decreased to 490°C when a third component (ethanol) was added. The same effect was noticed when a small amount of gas was added to a dust; for example, toner with a MIT of 460°C decreases to 450°C when a small amount of ethanol was added, which further decreased to 430°C when a third component (propane) was added. The result also confirm that an explosion is possible for a process or a system where hybrid mixtures are generated even if the temperature is below the MIT of a single substance and, hence, the MIT of hybrid mixtures cannot be predicted by simply overlapping the effects of the single dust, gas, or solvent.
... As a result of this, since Engler's experiments in 1885 (Engler, 1885), several researches have been carried out through the world with the aims of preventing the occurrence or mitigating the consequence of a hybrid mixture explosion. Some of these researches on hybrid mixtures explosions can be found in the literature (Agreda, 2011, Addai et al., 2015a, 2015b, 2015c, Dufaud, 2009, Amyotte, 2010. The main findings of these studies could be summarized nonexhaustively by the assertions that both the ignition sensitivity and severity of the dust can be strongly increased by the addition of a few percent of combustible gases or vapors, even with contents lower than the lower explosion limit (LEL). ...
Explosion hazards involving mixtures of different states of aggregation continue to occur in facilities where dusts, gases or solvents are handled or processed. In order to prevent or mitigate the risk associated with these mixtures, more knowledge of the explosion behavior of hybrid mixtures is required. An experimental investigation of the minimum ignition temperature (MIT) and the lower explosion limit/minimum explosion concentration (LEL/MEC) of three component hybrid mixtures of a combustible dusts, gases and vapors in air were performed in a modified Godbert- Greenwald (GG) furnace. The study comprised three dusts as well as two gases and two solvent vapors. For the dusts alone, the experimental protocol was in accordance with the European standard EN 50281-2-1, whereas for the gases, solvent vapors and mixtures of these with dusts, this protocol had to be modified slightly. It was found that the MITs of the gases and solvent vapors decreased significantly when a combustible dust at a concentration below the MEC of the dust alone, was added. A significant decrease of the MIT was also found if the gas/vapor atmosphere was mixed with a dispersed combustible dust that could not be ignited alone at the given temperature in the GG furnace. For example, the MIT of methane decrease from 600°C to 585°C when a small amount of starch was added. This hybrid MIT further decreased to 490°C when a non-explosible third component, ethanol, was added. It was also found that a hybrid cloud of a combustible dust at a concentration less than the MEC and a combustible gas at a concentration less than the LEL could be ignited in the GG furnace. For example, the MEC of starch decreased from about 145 g/m3 to 40 g/m3 when methane at a concentration below the LEL was added. This explosible concentration of starch- methane mixture further decreased to 25 g/m3 when a small fraction of a third component, hexane, was added.
... However, these kinds of mixtures are usually encountered where gases, solvents and dusts are either handle or process. Some recent studies on the explosion sensitivity and severities of hybrid mixture explosion include; Dufaud et al., (2009), Amyotte et al., (2010), Garcia-Agreda et al., (2011), etc.). The main conclusions of the previous studies could non-exhaustively be summarized by the following assertions that the ignition sensitivity of the powder can be strongly increased by the addition of a few percent of combustible gas or vapour, even with contents lower than their LEL. ...
The present paper reports on the experimental and theoretical investigations on the lower explosion limits of single dusts, gases as well of two phase mixtures such as gas/dust, vapor/dust, spray/vapor and vapor/gas. The materials used were corn starch, lycopodium, toner and high density polyethylene as dusts, methane and hydrogen as combustible (perfect) gases and acetone and isopropanol as sprays or vapours (real gases). The experiments were performed in the standardized 20-lters spherical explosion chamber where modifications were done to allow input of spray, solvent and gas. The test protocol was according to EN 14034 with an electrical ignition source. The experimental results demonstrate a significant enhancement in explosion likelihood by solvent, gas or spray admixture with dust and vice versa. They also confirm that a hybrid mixture explosion is possible even when the concentrations of both components are lower than their minimum explosion concentration (MEC) respectively lower explosivity limit (LEL). For example, the MEC of starch decreases from 150 g/m3 decrease to 20, 30, 125g/m3 and 125g/m3 when small amounts of isopropanol spray, acetone vapor, methane gas and hydrogen gas respectively were added. These concentrations were all below the LEL of the individual substance. Comparisons have been done between the lower explosible limit of the experimental data and classical models such as those developed by Bartknecht, Le Chatelier, MKOPSC and our newly proposed models. With the exception of the Le Chatelier and MKOPSC model, the other models were in agreement with the experimental result for safety point of view.
... The ignition sensitivity of combustible dust can be strongly increased by an addition of a few percentages of combustible gases or vapors, even with contents lower than the LEL. It has notably been shown that hybrid mixtures can also be explosible when both the concentrations of the dust and the vapor are below their respective explosible limits (Addai et al., 2015a,b,c,d,e;Pilão et al., 2006;Liu et al., 2007;Dufaud et al., 2009). It has been noticed that the minimum explosion energy (MIE) of dust clouds could decrease as soon as a few percentages of combustible gases or solvents were added (Khalili et al., 2012). ...
Mixtures of suspended combustible dust and flammable gas are usually encountered in various processes and systems where substances of different states of aggregate are handled. Knowing the lowest amount of energy needed to ignite such mixtures are critical to identify possibilities of accidental hazards in industry. Investigation of the minimum ignition energy (MIE) of a hybrid mixture of two flammable gases (methane and propane) and eight combustible dusts (wheat flour, starch, protein, polyethylene, peat, dextrin, wood coal and brown coal) were carried out in the modified Hartmann apparatus. The determination of the MIE of the dusts alone was in accordance with the European standard EN 50281. In the case of hybrid mixtures testing, this protocol had to be slightly modified, as hybrid mixtures are not included in the standard mentioned. The device used is limited to a lowest ignition energy of 4 mJ. Thus, the MIE of pure gases could not be as tested directly, as their values are all below that energy. The MIE values as well as the lower explosible limits (LEL) for gases were taken from the literature. To determine the MIE of hybrid mixtures at different concentrations of gas below the respective LEL were added to the pressurized air that used to generate the dust cloud in the MIE apparatus. The experimental results demonstrated a significant decrease of the MIE of the dusts and an increase in the likelihood of explosion when a small amount of gas that was below its LEL was mixed with the dust. For example, the MIE of polypropylene was observed to decrease from 116 mJ to 5 mJ when only 1 vol % of propane (below its LEL) was added. Moreover, an empirical model to predict the MIE of hybrid mixtures was presented and further compared with the experimental results were done.
... A number of studies have been conducted by different authors on the explosion characteristics of hybrid mixtures [6][7][8][9][10][11][12][13] with the aim of either preventing the occurrence or mitigating the consequence of hybrid mixture explosions. But, there are still open questions regarding the phenomena involved, as well as the sensitivity of hybrid explosions. ...
Experimental investigations of the lower explosion limits (LEL) of three-component hybrid mixtures of six combustible dusts, three gases, and four solvents were performed in the modified Godbert-Greenwald furnace. The test protocol was in accordance with European standard EN 50281-2-1 which is originally used to determine the minimum ignition temperature of dusts. Modification was done on the equipment to test for the explosion limits for dusts, gases, solvents, and hybrid mixtures. In order to prove the validity of our experimental procedure, the LEL for pure gases were initially tested and the results were compared with values found in literature obtained from the standard procedure which show very good agreement. The experimental results demonstrated a significant decrease of the explosion limits of gas, solvent, or dust and an increase in the likelihood of explosion when a small amount of dust was mixed with gas or solvent and vice versa. For example, the minimum explosible concentration (MEC) of high density polyethylene (HDPE) of 174 g/m3 decreased to 130 g/m3 upon addition of methane the concentration of which itself was below the LEL. The MEC of HDPE further decreased to 65 g/m3 when a nonexplosible concentration of hexane was added. © 2016 American Institute of Chemical Engineers Process Saf Prog, 2016
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/vapour 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.
Ammonia is a promising energy carrier for realizing a carbon-neutral society. In particular, solid particle cloud–ammonia co-combustion is considered as an efficient and feasible method to reduce CO2 emissions from the thermal power generation sector by using particle-fuel such as solid waste, biomass, and pulverized coal particles in combustors. However, the fundamental turbulent flame propagation mechanism of solid particle cloud–ammonia co-combustion remains unknow. Therefore, the present study intends to investigate and validate the general turbulent flame propagation mechanism of solid particle cloud–ammonia co-combustion. To achieve this aim, silica particle cloud–ammonia–oxygen–nitrogen mixing combustion, silica particle cloud–acetylene–air mixing combustion, and PMMA particle cloud–ammonia–oxygen–nitrogen co-combustion experiments were conducted. The results showed that the turbulent flame propagation velocity of silica particle cloud–gas-fuel–oxidizer mixing combustion is lower than that of pure gas-fuel–oxidizer combustion. However, the comparison of the turbulent flame propagation velocity of PMMA particle cloud–ammonia co-combustion and that of pure ammonia combustion, showed that whether the flame propagation of the co-combustion was higher than that of the pure ammonia combustion was dependent on the equivalence ratio of the ammonia-oxidizer. Therefore, the consistency of the results between the current study of PMMA particle cloud–ammonia co-combustion and the previous study for coal particle cloud–ammonia co-combustion indicates the turbulent flame propagation mechanism of solid particle cloud–ammonia co-combustion is dominated by the negative effect of the heat sink by unburned particles and the local equivalence ratio increment effect in the preheat zone of the flame front by the addition of the volatile matter, and that the positive effect of radiation from soot particles has little effect on the turbulent flame propagation of co-combustion for small-scale flames. Further, the influence of the heterogeneous combustion of char particles on the turbulent flame propagation of solid particle cloud–ammonia co-combustion is minor because of its slow combustion process. Based on the validated turbulent flame propagation mechanism of co-combustion, new numerical simulation models for solid particle cloud–ammonia co-combustion can be developed in the future.
To clarify the relationship of explosion severity between the flammable gas, dust, and their mixtures, along with developing models for predicting the explosion severity of hybrid mixtures, four different kinds of hybrid mixtures with wide range of concentrations were selected in this study. The maximum explosion pressure as well as the maximum rate of pressure rise for these four hybrid mixtures were measured in a standard 20 L spherical chamber. It was obtained that addition of flammable gas to dust cloud increases both the maximum explosion pressure and the maximum rate of pressure rise. However, the increase in the maximum rate of pressure rise was more pronounced than that of the maximum explosion pressure, indicating that the influence of the flammable gas addition on the maximum rate of pressure rise of dust was more effective. Both the maximum explosion pressure and the maximum rate of pressure rise of the hybrid mixtures increased with an increase in the flammable gas concentration. At any flammable gas concentration, both the maximum explosion pressure and the maximum rate of pressure rise of the hybrid mixtures were smaller than those of the pure flammable gas but greater than those of the pure dust. That is to say, the explosion severity of hybrid mixtures was less than that of the flammable gas, but greater than that of dust. The maximum explosion pressure of the hybrid mixture increased linearly with an increase in the flammable gas equivalent ratio Ф. In addition, the maximum rate of pressure rise of the hybrid mixture increased following a second-order function by increasing the flammable gas equivalent ratio Ф. Building on this latest finding, two models were developed for predicting the maximum explosion pressure and the maximum rate of pressure rise of the hybrid mixtures with satisfactory predictive performances.
Although the minimum ignition temperature is an important safety characteristic and of practical relevance in industrial processes, actually only standard operation procedures are available for pure substances and single-phase values. Nevertheless, combinations of substances or mixtures are used in industrial processes and up to now it is not possible to provide a standardised minimum ignition temperature and in consequence to design a process safely with regard to the substances used.
In order to get minimum ignition temperatures for frequently used hybrid mixtures, first, the minimum ignition temperatures and ignition frequencies were determined in the modified Godbert-Greenwald furnace for two single phase solids and a liquid substance. Second, minimum ignition temperatures and ignition frequencies were determined for several combinations as hybrid mixture of dust and liquid.
In parallel to the determination of ignition temperatures a new camera and computer system to differentiate ignition from non-ignition is developed. First results are promising that such a system could be much less operator depended.
By a high number of repetitions to classify regions of ignition the base is laid to decide about a new procedure for a hybrid standard and updating existing ones, too. This is one of the necessary aims to be reached in the Nex-Hys project.
A noticeable decrease of minimum ignition temperatures below the MIT of the pure solids was observed for the one hybrid mixture tested, yet. Furthermore more widely dispersed area of ignition is shown. In accordance to previously findings, the results demonstrate a strong relationship between likelihood of explosion and amount of added solvent. In consequence the hybrid mixture is characterized by a lower minimum ignition temperature than the single dust.
Emerging technologies and processes, such as nano and green technologies, sustainable processes, and additive manufacturing processes, have led to the development and handling of “novel” materials, which pose significant safety issues as they exhibit properties different from those of commonly used materials. The main differences are related to the particle size (from micro to nano), shape (non spherical), density and structure (porous particles), and composition (metal alloys and hybrid mixtures). Data on flammability and explosion behavior, as well as standard procedures and equipment derived for spherical dusts with micro size, cannot be directly applied to these emerging materials.
In this chapter, the main literature findings on the flammability and explosion behavior of hybrid mixtures, non spherical dusts, nano materials, and additive manufacturing dusts are discussed. The physical-chemical properties of the particles have an impact on relevant issues, such as the interaction with turbulence, the combustion reaction mechanism, and the dust propensity to dispersion/sedimentation (dustiness). The full characterization of the link between the particle properties and these issues is an essential preliminary step in the study of the flammability and explosion behavior of emerging materials.
The main objective of this work is to perform an exhaustive analysis of the explosion severity of wheat starch/pyrolysis gases hybrid mixtures in the 20-L Sphere Test and to determine how severe hybrid mixture explosions can be when compared to one phase explosions. Several experimental tests were performed at different concentrations of both phases and CFD simulations were run alongside to complement the experimental findings. The data were analysed using severity maps and adapted the stoichiometric parameters proposed in the literature. It was found that the explosion regimes proposed in the literature are not applicable to all types of mixtures given that, for wheat starch/pyrolysis gases, each regime could not be uniquely-characterized. In contrast, analysing the data using parameters that consider the chemical reactions is a superior methodology as it provides a comparative basis for assessing the explosion severity of different types of combustible materials, including hybrid mixtures.
The minimum explosible concentration (MEC) and flame propagation behaviors of propane/L-leucine powder hybrid mixtures were experimentally investigated. The results of the experiment demonstrated that the MEC values decreased as the propane concentration increased. Additionally, the findings of the analysis proved that the flame propagation limitation in dust explosions increased with the increase in propane concentration. The results indicated two things. First, the flame speed of stoichiometric propane-air/L-leucine powder hybrid mixtures in the initial period of propagation was almost constant notwithstanding an increase in the L-leucine concentration. Second, the dimensionless flame speed was close to unity because the speed of the hybrid mixture was strongly affected by the propane–air premixed flame speed, although the flame speed increased with the increase in propane concentration. Further, it was found that the separate propagating flames of dust/gas hybrid mixtures had a considerable difference in flame speeds were observed.
This study comprises an assessment of the behaviour of wheat starch/pyrolysis gases hybrid mixtures under different operating conditions of Ignition Delay Time (tv) and nozzle geometry in the context of the 20-L Sphere Test. The behaviour of the system was determined through the study of the interaction between the chemical reactions of the phases and the turbulence levels produced by the different operating configurations. For these purposes, CFD simulations based on a coupled Eulerian-Lagrangian formulation were developed considering the two main steps of the test: i) Dispersion of the powder, and ii) Pyrolysis/gas-oxidation reactions. The CFD model included a few important improvements to other computational methodologies proposed in previous studies, such as the incorporation of more rigorous kinetic mechanisms for the devolatilization reactions of the starch particles and the combustion reactions of the pyrolysis gases. In parallel, experimental tests were performed in order to validate the CFD model and characterize the powder samples. In particular, the composition of the pyrolysis gases emitted by the particles was determined by a modified setup of the Godbert-Greenwald furnace, and the Particle Size Distribution (PSD) was measured through laser diffraction techniques. The performance of the CFD model was satisfactory in terms of predicting parameters related to the hydrodynamic or reaction-kinetic behaviour of the system, such as the pressure evolution during dispersion and the (dP/dt)m (average deviations from experimental values of ~4.0% and 11.9%, respectively). Furthermore, the results suggest that the combined effect of the dust-phase and gas-phase reactions have a distinctive dissipative effect on the turbulence levels and that higher turbulence decay levels during the explosion step of the test are related to higher explosion severities.
Aluminum powder was always chosen as an additive to improve the explosive performance. In this work, experiments were performed to investigate the lower flammability limit (LFL) of volatile liquid fuel-aluminum powder mixtures using a 20 L closed spherical stainless steel vessel at a temperature of 20 °C (293 K) and 40 °C (313 K). The volatile liquid fuels tested in the work were diethyl ether (DEE), epoxypropane (PO), n-pentane and n-hexane. DEE, PO and n-pentane are in the liquid phase at room temperature and can easily transition to the gas phase at 40 °C (313 K). Through a series of experiments carried out, it was found that the change in phase would affect the interaction between the components. Aluminum powder always has an inhibitory effect on the flammability of the mixtures when it is mixed with gas-phase fuels. The inhibition effect was most obvious when the aluminum powder concentration reached 200 g/m³. While the interaction between aluminum powder and liquid-phase volatile fuels was promotion and was influenced by the component proportion and the type of the volatile fuels.
The post-explosion gaseous environment in a coal mine influences the occurrence of secondary explosions. The flammable and explosion risk of post-explosion gases (CH4/air and CH4/coal dust/air mixtures) was evaluated. Explosion tests were carried out in a 20-L spherical explosion device under conditions with different air-fuel ratios. The composition and volume fraction of post-explosion gases were collected and analyzed. CO and H2 are two main possible flammable gas products with increasing initial fuel-air ratio. Unexpectedly, H2 was generated after a CH4/air explosion when the initial φ(CH4) ≥ 13%, in which condition the concentration of O2 was far from sufficient to oxidize CH4. The volume fraction of flammable gas in residual gases of CH4/coal dust/air mixture explosions was higher than that of CH4/air mixtures. It was found that the secondary explosion risk increased significantly when coal dust was involved in a methane explosion. The lower explosion limit (LEL) of post-explosion flammable gases decreased with the increasing mass concentration of coal dust. The flammable component of such post-explosion gases affected the secondary explosion.
The aim of the work presented here is a comparison of hybrid mixture explosion parameters obtained in the explosion chambers used in European Standard EN 14034 and explore the influence of the explosion volume and the ignition source on the explosion parameters of the hybrid mixtures.
Explosion chambers of the two volumes, 20 l and 1 m³, specified in the Standard, were used to carry out standard procedures according to EN 14034 to determine hybrid mixture explosion parameters such as maximum overpressure and maximum rate of pressure rise. Three flammable dusts widely accepted as standards were chosen – Pittsburgh seam bituminous coal dust, Lycopodium Clavatum spores and Niacin. Two flammable gases (methane and hydrogen) were used. Methane and hydrogen are used for standard testing of flammable gas mixtures explosion parameters in explosion chambers. The explosion parameters of various mixtures of flammable dusts, flammable gases and air were measured. Standard ignition sources for dust dispersion, two 5 kJ chemical igniters, were used in both chambers. Explosion parameters were also measured using the standard permanent spark described in EN 15967 as an ignition source for a comparison of the effect of different ignition energies on explosion parameters.
The results show a significant increase of normalised maximum rates of pressure rise in a 20 l chamber compared with a 1 m³ chamber caused by higher turbulence levels in the smaller chamber. It was also shown that the permanent spark could be used for easily ignitable dusts and, in some cases, can produce even higher rates of pressure rise than chemical igniters.
Metallic dusts with low values for maximum pressure Pmax and volume-normalized maximum rate of pressure rise KSt as determined in bench-scale apparatus have been labeled as “marginally explosible.” These dusts pose unique challenges because they may be shown to be explosible using standardized bench-scale apparatus, yet they may not be found to be explosible using larger standardized test chambers. Furthermore, some metallic powders produce higher KSt values with increasing test scale.
Results and preliminary analysis stemming from a comprehensive experimental campaign involving selected iron and aluminum powders of varying specific surface area are presented.
Fine-, medium-, and coarse-particle size iron powder with Siwek 20-L KSt values less than 45 bar⋅m/s were also found to be explosible in the Fike 1-m3 vessel. The coarse iron was only found to be non-explosible with the sub-75 μm fraction removed. The fine, medium, and coarse iron powders produced lower maximum rates of pressure rise across all concentrations in the 20-L chamber compared to the 1-m3 vessel. Fine-, medium-, and coarse-particle aluminum powders were also tested. The coarse aluminum powder could be exploded under the corresponding experimental conditions of 20-L chamber but not under the corresponding experimental conditions of 1-m3 chamber. The volume-normalized maximum rate of pressure rise for the finest aluminum powder was found to be overdriven in the 20-L vessel using an ignition energy of 10 kJ compared to tests performed with the same ignition energy in the 1-m3 vessel. An ignition energy of 5 kJ was found to be more appropriate for testing fine aluminum in the 20-L sphere. In contrast, the medium-particle size aluminum produced a KSt value substantially higher in the 1-m3 vessel compared to the 20-L chamber.
The aim of this work is to study the effects of a multi-source ignition in a wide energy range on a dust explosion. More particularly, we focus on the pressure history inside a partitioned vessel. The initiation is introduced in the form of internal energy and a calculation methodology, particularly interesting in the field of the risk assessment is used to simulate the transmission of the explosion from one compartment to another adjacent one by the means of the hot flow through the shared orifice and finally to generalise this methodology to a complex multi-partitioned structure.
The basic characteristics of the model have been developed for the ignition and the combustion of propulsive powders and adapted to dust suspensions with appropriate parameters linked to simplified kinetics. A simple representation of the combustion phenomena based on energy transfers and the action of specific molecular species is presented. The model allows the study of the influence of various parameters such as the dust concentration, the different ignition energies and their locations, the size of the inner openings or the venting areas. The theoretical results have been compared with many data available in the literature and indicate correct preliminary trends.
Flammable gas/solid hybrid mixture explosions are not well understood because of the interaction of the thermal transfer process, the combustion kinetics mechanisms and the interactions between turbulence and combustion. The main objective on this work is to study the explosion severity and flame burning velocities of carbon black nanoparticles/methane to better understand the influence of added nanopowders in gas explosions. Tests have been performed in a flame propagation tube and in the standard 20 L explosion sphere. The influence of carbon black particles on the explosions severity and in the front flame propagation has been appreciated by comparing the results obtained for pure gas mixtures. It appeared that the carbon black nanoparticles insertion increases around 10% the explosion severity for lean methane mixtures. Therefore, it seems that nanoparticles has an impact on the severity of the explosion even for quiescent systems, contrary to systems involving micro-sized powders that requires a dispersion at high turbulence levels. The increment on the maximum rate of pressure rise is higher for powders with lower elementary particle diameter, which is notably due to the fragmentation phenomena. A flame propagation numerical model associated to a gas/carbon black mixture has been developed to examine the influence of carbon blacks on the flame propagation. The results of the numerical model suggest that the radiative heat contribution promotes the flame acceleration. This result is consistent with the experimental increase on the explosion severity for some hybrid mixtures
The laminar flame speed is an essential input for Computational Fluid Dynamics simulation programs aiming to predict the effects of explosions. In this study, an approach to assess fundamental flame propagation properties from the analysis of the flame velocity as a function of its stretching and hydrodynamic instabilities was developed. A numerical tool was developed to analyze videos of propagating flames in order to estimate their unstretched burning velocities.
Markstein’s theory, developed for gases and assuming a linear relation between the flame stretch and its speed, was then extended to dust clouds and hybrid mixtures of starch and methane. At first, the approach was validated with pure methane and was extended to pure starch and hybrid mixtures of both compounds.
Finally, it appears that hybrid mixtures, especially when the gas concentration is greater than the lower explosive limit, can present a synergetic effect enabling faster flame propagation with regard to pure gas flames. Indeed, the stretching of a gas flame is strongly influenced by the addition of dusts. Nevertheless, for lower gas concentrations and larger dust concentrations called ‘dust-driven regime’, the presence of powders tends to limit the flame velocity to that of the less reactive compound, i.e. the dust.
During incomplete combustions or nano-size carbon blacks generation, atmospheres of carbonaceous nanopowders and combustible gases are encountered. These hybrid mixtures exhibit specific explosive behaviors, which can notably be caused by the modification of the initial turbulence level or by changes in oxidation reactions. In order to either support or reject such assertions, various nanoparticles/methane mixtures were tested, some with carbonaceous nanopowders, some with inert nanopowders (alumina). The aim of this work is then to compare the influences of alumina and carbon black nanoparticles insertion on the explosion severity and on the flame velocity of methane. Tests were performed in a 20 L explosion sphere and in a 1 m vertical flame propagation tube. An estimation of the unstretched flame velocity is obtained assuming a linear relationship between the burning velocity and Karlovitz stretch factor. It appears that the use of carbon black nanoparticles increases the explosion overpressure for lean methane mixtures by approximately 10%. Similar behaviors have been observed for hybrid mixtures involving alumina particles for fuel lean conditions. For alumina, non-significant changes are observed for fuel rich mixtures. Moreover, a considerable diminution of the explosion severity for fuel rich mixtures when carbon black nanoparticles are dispersed into the reaction vessel. Regarding the flame propagation test for stoichiometric methane concentration, higher unstretched burning velocities were obtained for carbon black hybrid mixtures compared to alumina mixtures. These results suggest soot or carbonaceous nanopowders not only impact the oxidation kinetics, but also the flame stretching and heat transfer.
Hybrid mixtures of combustible dust and flammable gas represent an enhanced industry hazard due to increased explosion severity over that for the constituent fuels at their given concentrations. The current investigation extends the understanding of hybrid explosion dynamics by identifying and evaluating new explosion regimes on the dust/gas concentration plane. This work builds on previous studies that identified five regimes: gas-driven explosion, dual-fuel explosion, dust-driven explosion, synergic/synergistic explosion, and no explosion. For low ignition energy (e.g., spark ignition) the gas-driven and dual-fuel regimes are extended to include: gas-only explosion, two-stage explosion, single-stage explosion, and dust-only explosion. For high ignition energy (e.g., 10-kJ ignitors) the hybrid behaviour in the dust-driven regime depends on the dust reaction mechanism. For heterogeneous combustion, addition of flammable gas has a minor impact on explosion parameters. For homogeneously reacting dust, two new regimes are proposed: isolated particle combustion close to the dust flammability limit and group combustion further away. It is hypothesized that flammable gas addition has a larger impact in the isolated regime, as the gas acts to bridge individual diffusion flames during flame propagation and explosion. Ongoing research is investigating this hypothesis and reviewing the use of Computational Fluid Dynamics to close the gaps in understanding for hybrid explosion systems.
This work aims to study the influence of low concentrations of carbon black nanoparticles in gas mixtures on the front flame velocity. Due to their low settling velocity, nanoparticles offer the opportunity to study the hybrid mixture explosion at low turbulence levels of dispersion. They can also be used as particles to model the presence of soot. The flame velocity of carbon black nanoparticles/methane/air mixtures was measured in a vertical 1 m long tube with a square crosssection connected to a gas mixing system. Dust clouds are generated by a pulse of methane/air mixture at 5 barg from the bottom of the tube, where the mixture is also ignited. A high-speed video camera is used to record the flame propagation. An estimation of the laminar burning velocity is obtained using the method proposed by Andrews and Bradley. Although this method may not be precise for laminar flame velocity estimations, it offers a first approximation for hybrid systems explosions. The influence of the initial turbulence was also studied by varying the ignition delay. The influence of low concentrations of carbon black nanoparticles on the front flame velocity has been appreciated by comparing the results obtained for gaseous mixtures explosions at different turbulence levels. The burning velocity of gaseous mixture seems to increase when the initial turbulence of the system is augmented. However, when the initial turbulence is significant, the front flame velocity seems to decrease, suggesting that the flame kernel can be strongly destabilized by turbulent vortices. Moreover, it appears that the flame burning velocity can slightly decrease when carbon black nanoparticles concentration is increased. The unstretched burning velocity is decreased by 43% when 20 mg of carbon black nanoparticles are added to the system. This trend could be explained by the enhancement of the heat radiation transfer of the system. The results are then compared to the explosions trends in a 20 L spherical vessel.
Explosion hazards involving mixtures of different states of aggregation continue to occur in
facilities where dusts, gases or solvents are handled or processed. In order to prevent or mitigate
the risk associated with these mixtures, more knowledge of the explosion behavior of hybrid
mixtures is required. The aim of this study is to undertake an extensive investigation on the
explosion phenomenon of hybrid mixtures to obtain insight into the driving mechanisms and the
explosion features affecting the course of hybrid mixture explosions. This was accomplished by
performing an extensive experimental and theoretical investigation on the various explosion
parameters such as: minimum ignition temperature, minimum ignition energy, limiting oxygen
concentration, lower explosion limits and explosion severity. Mixtures of twenty combustible
dusts ranging from food substances, metals, plastics, natural products, fuels and artificial
materials; three gases; and six solvents were used to carry out this study. Three different standard
equipments: the 20-liter sphere (for testing lower explosion limits, limiting oxygen concentration
and explosion severity), the modified Hartmann apparatus (for testing minimum ignition energy)
and the modified Godbert–Greenwald (GG) furnace (for testing minimum ignition temperature)
were used. The test protocols were in accordance with the European standard procedures for
dust testing for each parameter. However, modifications were made on each equipment in order
to test the explosion properties of gases, solvents, and hybrid mixtures. The experimental results
demonstrated a significant decrease of the minimum ignition temperature, minimum ignition
energy and limiting oxygen concentration of gas or solvent and increase in the likelihood of
explosion when a small amount of dust, which was either below the minimum explosion
concentration or not ignitable by itself, was mixed with gas or solvent and vice versa. For example,
methane with minimum ignition temperature of 600 °C decreased to 530 °C when 30 g/m3 of
toner dust, which is 50 % below its minimum explosible concentration was, added. A similar
explosion behavior was observed for minimum ignition energy and limiting oxygen concentration.
Furthermore, it was generally observed that the addition of a non-explosible concentration of
flammable gas or spray to a dust-air mixture increases the maximum explosion pressure to some
extent and significantly increases the maximum rate of pressure rise of the dust mixture, even
though the added concentrations of gases or vapor are below its lower explosion limit. Finally, it
could be said that, one cannot rely on the explosion properties of a single substance to ensure
full protection of an equipment or a process if substances with different states of aggregate are
present.
To evaluate the hazard of combined gaseous epoxypropane-aluminum dust explosions, a 5 L spherical exploding device was used to measure the lower limit of explosion densities of aluminum dust in aluminum dust-air mixtures and gaseous epoxypropane-aluminum dust-air hybrid mixtures, and gaseous epoxypropane in gaseous epoxypropane-air mixtures. The results show that the existence of gas epoxypropane can reduce the lower limit of explosion density of the hybrid mixtures, and enlarge the rate of pressure rise of the hybrid mixtures; at low aluminum dust concentration, the maximum overpressures rise due to the existence of gaseous epoxypropane, and then fall with increasing aluminum dust concentration.
Ideally, explosion risk is identified and prevented at an early stage of process design, forming a key part of the inherently safer design (ISD) approach. However, in practice, explosion risk often cannot be eliminated completely. Reliable preventive and protective systems must therefore be applied, as part of the now widely applied barrier or independent layer of protection approach (LOPA) to process design. Doing this requires a comprehensive knowledge of available systems. However, such a comprehensive overview of the various systems does not yet exist. This paper provides such an overview. The commercially available explosion prevention and mitigative systems applicable to gas, dust, mist and hybrid (gas-aerosol) explosions are discussed, including basic principles and proper application for single and combined systems, for confined installations. Limitations of the applicability as well as limitations of the research-base of the various systems are also discussed.
Explosibility of polyurethane dusts produced in the recycling process of refrigerator and the ways to prevent the dust explosion were studied. In recent years, cyclopentane is often used as the foaming agent and this produces explosive atmosphere in the shredding process. The minimum explosive concentration of polyurethane dust, influence of coexisting cyclopentane gas on the explosibility, effect of relative humidity on the minimum explosive concentration of polyurethane dusts, the minimum ignition energy, influence of cyclopentane mixture on the explosion severity, etc. were investigated.The minimum explosive dust concentration decreased with the increase of cyclopentane concentration and increased with the increase of relative humidity. The minimum ignition energy was about 11mJ. The ignition energy decreased with the increase of the cyclopentane gas concentration. The cyclopentane gas concentration up to about 5300ppm did not influence too much on the explosion index (Kst) and maximum explosion pressure. From these, it would be a good way to increase the relative humidity and to regulate the cyclopentane concentration in the shredding process to prevent the dust explosion hazard.
The dust explosibility characteristics of Lycopodium, Cornstarch, Pittsburgh Bituminous Coal and Calcium Sterate were measured in a 1 m3 spherical explosion chamber at ambient pressures, ambient temperatures and at three different turbulence levels. Explosibility parameters measured were the maximum pressure, maximum pressure rate of rise and the initial pressure rate of rise. Propane explosibility and Hybrid explosibility (Cornstarch in the presence of propane) were also measured. For the hybrid system, the lower limits of explosibility and the effect of low propane concentrations on the explosibility of optimum Cornstarch concentrations were investigated. Finally, a relationship between maximum pressure rate of rise and initial pressure rate of rise was sought in order to lay a foundation for explosion suppression and explosion venting engineering methodologies.
This paper reports US Bureau of Mines (USBM) research on the explosibility of coal dusts. The purpose of this work is to improve safety in mining and other industries that process or use coal. Most of the tests were conducted in the USBM 20 litre laboratory explosibility chamber. The laboratory data show relatively good agreement with those from full-scale experimental mine tests. The parameters measured included minimum explosible concentrations, maximum explosion pressures, maximum rates of pressure rise, minimum oxygen concentrations, and amounts of limestone rock dust required to inert the coals. The effects of coal volatility and particle size were evaluated, and particle size was determined to be at least as important as volatility in determining the explosion hazard. For all coals tested, the finest sizes were the most hazardous. The coal dust explosibility data are compared to those of other hydrocarbons, such as polyethylene dust and methane gas, in an attempt to understand better the basics of coal combustion.
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.
The explosion characteristics of coal dust/air and methane/coal dust/air mixtures have been determined experimentally. All tests were conducted at initial pressures of nominally 1.0 bar in a 26 / spherical explosion bomb. Run-of-mine coal from the Prince, Lingan and Phalen seams of the Cape Breton Development Corporation was used. Two size fractions of each coal were tested at dust concentrations ranging from the lean flammability limit to 1.0 kg m−3. The explosion parameters measured for each test were the maximum explosion pressure, Pmax, and the maximum rate of pressure rise, (dP/dt)max. Methane addition to the coal dust/air mixtures was found to increase both Pmax and (dP/dt)max, the effect being most significant at low dust concentrations. A reduction in mass mean diameter of the coal or an increase in the parent coal volatile content was found to have a similar effect on Pmax and (dP/dt)max. These observations are consistent with a description of coal dust flame propagation by gas-phase combustion of devolatilization products.
Diss. Nr. 6498 techn. Wiss. ETH Zürich.
Flammability and explosion propagation of methane/coal dust hybrid mixtures
- Torrent
J.G. Torrent, J.C. Fuchs, Flammability and explosion propagation of methane/coal dust hybrid mixtures, 23rd Int. Conf. Of the Safe Mining Research Institute: Washington DC, 1989.
Dusts explosion experiments: measurements of explosion indices of graphite dust in hydrogen-containing atmospheres
- Denkevits
A. Denkevits, Dusts explosion experiments: measurements of explosion indices of graphite dust in hydrogen-containing atmospheres, Wissenschaftliche Berichte FZKA 7114, Karlsruhe, 2005.