Experimental study on inert products,moisture, and particle size effect on the minimum ignition energy of combustible dusts

Handling combustible dusts not only continues to pose a risk to industry but can also affect the safety of society. Explosion risk could be avoided or mitigated trying to guarantee inherent safety throughout the product life chain. One way to reduce the risks when dealing with combustible dust is to increase the Minimum Ignition Energy (MIE) in order to decrease combustible dust ignition sensitivity. To achieve this decrease, the inertization technique, also known as moderation, will be used. It consists of adding inert powders or humidity to the combustible dust. As sometimes end-users also must deal with the handling of flammable dusts, this study aims to find the most optimal inert for toner waste from printers and Holi powder (organic coloured dust from Indian parties), taking Lycopodium as a reference. Calcium carbonate, sodium bicarbonate and gypsum are proposed as inert materials. In addition, with the aim of giving a second use to biomass boiler waste or boiler slagging, this waste will be analyzed as inert, as well as how humidity affects the combustible dusts. Then, sodium bicarbonate will be tested at different granulometries to evaluate the effect of particle size on moderation process. The tests were carried out in the modified Hartmann apparatus or MIKE 3.0. Mechanisms such as decomposition of inert dust have been analyzed by thermogravimetric analysis (TGA)). The results show that gypsum and moisture are the best performing inert followed by calcium carbonate. Boiler slagging and solid bicarbonate contribute to a decrease in the MIE in some of the tests. The reasons for this deviation are discussed in the presented article. When sodium bicarbonate is analyzed at different particle sizes, it is found that the optimum particle size does not match the particle size of the combustible dust. According to the tests, there is an optimum point for which the inert powder provides better results.     

Effect of particle size and dust layer size on ignition characteristics of PMMA dust layer on hot surface

Deposition of combustible dust on a hot surface is a hidden danger of fire. In this work, polymethylmethacrylate (PMMA) dust was selected to analyse the influence of dust layer diameter, dust particle size and dust layer thickness on the ignition characteristics of PMMA dust layer. Critical heating temperatures and ignition time had been measured. The STA-GC/MS-FTIR analysis was used to determine that the main products of PMMA pyrolysis were MMA, CO, CO₂, and C₂H₄, of which CO and C₂H₄ were transported to the ambient to cause gas phase combustion on the surface of the dust layer. For 10 mm thick dust layer, the critical heating temperatures of 5 μm PMMA, 100 nm PMMA, and 30 μm PMMA were 300 °C, 330 °C, and 320 °C. As the thickness of the dust layer increased, the gas transport path became longer, the critical heating temperature and ignition time increased. The characteristic particle size (D [3,2]) was utilized to represent the true particle size, and the ignition time increased with the increase of the characteristic particle size. The increase in the diameter of the dust layer had a slight effect on the temperature history and ignition time of the dust layer.     

Safe handling of combustible powders during transportation,charging, discharging and storage

The knowledge of the ignition behavior of dust–air mixtures due to electrical sparks (MIE, Minimum Ignition Energy) and hot surfaces (MIT, Minimum Ignition Temperature) is important for risk assessments in chemical production plants. The ignition behavior determines the extent and hence the cost of preventive protection measures.This paper describes the use of the minimum ignition energy and minimum ignition temperature as very important safety indexes in practice.     

When solids meet solids: A glimpse into dust mixture explosions

Mixing an inert solid or a less flammable compound with a combustible dust can be regarded as a direct application of the inherent safety principle of moderation. An experimental investigation was carried out to determine the evolution of the ignition sensitivity and the explosion severity of such various mixtures as a function of their compositions. It demonstrates that the introduction of small amounts of highly combustible powders (such as sulphur or nicotinic acid) to a less flammable dust (such as microcrystalline cellulose or carbon black) can strongly influence the ignition sensitivity as well as the explosion severity.It has notably been shown that the ignition sensitivity of solid/solid mixtures significantly rises up when only 10–5%wt. of highly flammable dust is introduced. Simple models can often be applied to estimate the minimum ignition energy, minimum ignition temperature and minimum explosive concentration of such mixtures. Concerning the dust explosivity, three cases have been studied: mixtures of combustibles dusts without reaction, dusts with reactions between the powders, combustible dusts with inert solid. If the evolution of the maximum explosion pressure can be estimated by using thermodynamic calculations, the maximum rate of pressure rise is more difficult to predict with simple models, and both combustion kinetics and hydrodynamics of the dust clouds should be taken into account. These results were also extended to flammable dust/solid inertant mixture. They clearly show that the concentration of solid inertant at which the ignition is not observed anymore could reach 95%wt. As a consequence, the common recommendation of solid inertant introduction up to 50–80%wt. to prevent dust explosion/ignition should be reconsidered.     

Minimum ignition temperatures and explosion characteristics of micron-sized aluminium powder

To forestall, control, and mitigate the detrimental effects of aluminium dust, a 20-L near-spherical dust explosion experimental system and an HY16429 type dust-cloud ignition temperature test device were employed to explore the explosion characteristics of micron-sized aluminium powder under different ignition energies, dust particle sizes, and dust cloud concentration (C_dust) values; the minimum ignition temperature (MIT) values of aluminium powder under different dust particle sizes and C_dust were also examined. Flame images at different times were photographed by a high-speed camera. Results revealed that under similar dust-cloud concentrations and with dust particle size increasing from 42.89 to 141.70 μm, the MIT of aluminium powder increased. Under various C_dust values, the MIT of aluminium dust clouds attained peak value when concentrations enhanced. Furthermore, the increase of ignition energy contributed to the increase of the explosion pressure (P_ex) and the rate of explosion pressure rise [(dP/dt)_ex]. When dust particle size was augmented gradually, the P_ex and (dP/dt)_ex attenuated. Decreasing particle size lowered both the most violent explosion concentration and explosive limits.     

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.     

The effect of admixed material on the minimum explosible concentration of oil shale

The minimum explosible concentration (MEC) in the air atmosphere at the boundary between an explosion and no explosion in a dust cloud, has been investigated for several particle sizes of oil shale and for mixtures of oil shale and inert powder of different particle size. Limestone, stone dust and coarse particle size of oil shale were used as inert materials. Measurements were made in a standard small vertical tube apparatus. The results obtained indicated that the minimum explosible concentration is dependent on the particle size, i.e., values of MEC decrease with a decrease in the size of the particles. Below 70 μm, values of MEC become almost constant. Admixture of limestone as low as 5% to oil shale is sufficient to reduce the MEC values significantly.     

Minimum ignition energy of mixtures of combustible dusts

Most industrial powder processes handle mixtures of various flammable powders. Consequently, hazard evaluation leads to a reduction of the disaster damage that arises from dust explosions. Determining the minimum ignition energy (MIE) of flammable mixtures is critical for identifying possibility of accidental hazard in industry. The aim of this work is to measure the critical ignition energy of different kinds of pure dusts with various particle sizes as well as mixtures thereof.The results show that even the addition of a modest amount of a highly flammable powder to a less combustible powder has a significant impact on the MIE. The MIE varies considerably when the fraction of the highly flammable powder exceeds 20%. For dust mixtures consisting of combustible dusts, the relationship between the ignition energy of the mixture and the minimum ignition energy of the components follows the so-called harmonic model based upon the volume fraction of the pure dusts in the mixture. This correlation provides results which show satisfactory agreement with the experimental values.     

Inhibiting effects by Fe2O3 on combustion and explosion characteristics of ABS resin

The global increase in the use of, and reliance on, plastics has prompted the demand for acrylonitrile-butadiene-styrene (ABS) resin in various fields. With this increased requirement, numerous failures have occurred in the ABS process. Those incidents, resulting from electrostatic discharge, powder accumulation, heat accumulation, construction sparks, and plant fires, have caused dust fire and explosions.In this study, the ABS resin was gleaned from the site and tested for its explosion parameters, including minimum ignition temperature of dust cloud (MIT_C), minimum ignition energy (MIE), and minimum explosion concentration (MEC). To improve loss prevention in the manufacturing process, ferric oxide (Fe₂O₃) as an inert additive was added in the ABS powder. According to the MIE test, Fe₂O₃ has an apparent inhibiting effect on dust explosion for the ABS dust. With the proportion of Fe₂O₃ increased from 25 to 50 mass% in ABS, the MIE increased from 67 to 540 mJ. The explosion tests via 20-L apparatus indicated that Fe₂O₃ mixed with ABS could not increase the MEC significantly. However, the explosion pressure dropped by increasing in the ratio of Fe₂O₃ in ABS. This inerting strategy of ABS was deemed to substantially lessen the probability and severity of fire and explosion.     

Effect of superfine KHCO3 and ABC powder on ignition sensitivity of PMMA dust layer

In order to evaluate the flame-retardant capacity of KHCO₃ and ABC on the ignition of PMMA dust layer accumulation on hot surfaces, the ignition time and critical heating temperature of PMMA/KHCO₃ and PMMA/ABC dust layer were experimentally investigated. The thermal stability of the mixed dust, the condensed phase products and gas phase products of the mixed dust combustion were analyzed to reveal the flame-retardant mechanism. The ignition time of 30 μm PMMA was obviously longer than that of 5 μm PMMA, and the critical heating temperature was close to that of 5 μm PMMA. KHCO₃ and ABC could greatly extend the ignition time of the PMMA dust layer and increase the critical heating temperature of the dust layer. ABC was more effective than KHCO₃. The decomposition of KHCO₃ and ABC absorbed the heat and inhibits the pyrolysis of PMMA. The HPO₃ and P₂O₅ generated by the decomposition of ABC would cover the surface of PMMA aggregates or particles and act as a physical barrier. The main light combustible gas produced by PMMA pyrolysis were CO and C₂H₄. The CO₂ generated during the decomposition of KHCO₃ could dilute the combustible gas in the ambient to inhibit the combustion of PMMA.     

Minimum ignition energy of LDPE dust/ethylene hybrid mixture

An experimental device for evaluating the minimum ignition energy (MIE) of LDPE dust/ethylene hybrid mixture was built with the innovative mixing mode. The MIE of the hybrid mixture that contained ethylene below its lower explosive limit (LEL) was studied. The result indicated that adding a small amount of ethylene significantly reduced the MIE of the original dust cloud. All the MIEs with five different particle sizes were found to show similar trends of exponential attenuation with the increase of ethylene concentration; such attenuating effect grew as the dust particle size rose. When ethylene concentration increased and approached to its LEL, the reaction mechanism dominated by combustible dust turned into one dominated by combustible gas. The MIE decreased first and then increased with the dust mass and increased with the dust particle size. A multifactor mathematical correlation model of the MIE with the dust particle size and ethylene concentration was developed.     

Study of dust cloud behaviour in the modified Hartmann tube using the image subtraction method (ISM)

A dispersion of fine particles in the air is needed for a dust explosion to occur since an explosion is the fast combustion of particles in the air. When particles are poorly dispersed, agglomerated, or their concentration is low, the combustion velocity decreases, and deflagration would not occur. The combustion rate is strictly related to dust concentration. Therefore, the maximum explosion pressure rise occurs at dust concentration close to stoichiometric. Conversely, Minimum Explosion Concentration (MEC) is the lower limit at which self-sustained combustion and a pressure rise are possible. Dust explosion tests are designed to reproduce the dispersion and generation of dust clouds in industrial ambiences by using dispersion devices activated by pressurised air pulses. The resulting dust cloud, which has a marked transient character, is considered representative of real clouds by current standards. Over time, several studies have been carried out to optimise these devices (e.g. to reduce the inhomogeneity of the cloud in the 20 L sphere). The Minimum Ignition Energy (MIE) of dust is measured using the Mike3 modified Hartmann tube, where the ignition attempt is made 60–180 ms after dust dispersion regardless of dust characteristics.This work investigates the dust clouds’ actual behaviour inside the modified Hartmann tube before ignition using high-velocity video movies and a new image post-treatment method called Image Subtraction Method (ISM). Movies are recorded with high-speed cameras at a framerate of 2000 fps and elaborated with an on-purpose developed LabVIEW® code. Concentration (mass per volume) and dispersion pressure are varied to evaluate their effect on dust clouds. Maise starch, iron powder and silica powder are chosen to investigate the effect of particle density and size on the cloud structure and turbulence. This approach will help to investigate the structure of the dust cloud, the shape and size of the particle lumps and the change in dust concentration over time. In addition, information on the actual concentration and cloud turbulence at the ignition location and delay time were obtained, which may help identify the local turbulence scale and widen the characterisation of the cloud generated in the Hartmann tube.     

固体惰性介质对煤粉爆炸压力的影响研究

通过对固体惰性介质在减轻煤粉爆炸作用的实验研究,给出影响固体惰化剂作用效果的主要影响因素。实验分别选用来自加拿大和中国的3种煤粉和石灰石,对每种实验样品的成分、粒度都进行分析。用20L球形容器进行实验,测定煤粉中加入不同含量的石灰石后煤粉爆炸的Pmax和(dp/dt)max值。结果表明,石灰石能够起到减轻煤粉爆炸影响的作用,并且随着煤粉粒度的减小,要达到相同的抑爆效果需要的石灰石的用量将加大。     

A numerical study of the auto-ignition temperatures of CH4–Air,C3H8–Air,CH4–C3H8–Air and CH4–CO2–Air mixtures

The auto-ignition temperature (AIT) is an important parameter in the process industries. In order to ensure a safe working environment in process industries, it is important to predict the AIT of combustible gases or vapors. In this study, the AITs of natural gas mixtures (CH₄–Air, C₃H₈–Air, CH₄–C₃H₈–Air and CH₄–CO₂–Air) are calculated based on a detailed kinetic model. To create a more practical model, different ignition criteria and convective heat transfer coefficients are investigated and compared against one another, resulting in the temperature criterion and a convective heat transfer coefficient of h = 50 W/(m² K). The results showed that the AITs of CH₄–Air and C₃H₈–Air decrease with an increase of equivalence ratios. While the propane ratio increasing, the AIT of CH₄–C₃H₈–Air decreasing. Reaction path analysis of natural gas mixtures (CH₄–C₃H₈) was also carried out to explain this phenomenon, yielding results showing that C₃H₈ is the main reaction during the ignition induction period. In addition the AIT of CH₄ increases slowly in positive correlation with CO_2, which plays a role of an inert gas. Comparing the results with literature work revealed a deviation of about 10%. Thus, it can be reasonably concluded that the AIT of a low hydrocarbons mixtures such as natural gas can be reliably predicted with detailed kinetic model.     

Study of the spontaneous ignition of stoichiometric tetrafluoroethylene–air mixtures at elevated pressures

The Ignition Temperature (IT) of stoichiometric tetrafluoroethylene–air mixtures on hot walls was determined in a 3-dm³-reactor. Tests at elevated pressure conditions were performed, namely at 5, 15 and 25 bar(a), showing a decrease of the IT with the initial pressure. Furthermore, the measured ignition temperatures of stoichiometric tetrafluoroethylene–air mixtures were lower than the ignition temperatures required for the decomposition pure tetrafluoroethylene (Minimum Ignition Temperature of Decomposition, MITD) reported in previous works.Equations from the Semenov thermal explosion theory on spontaneous ignition were used to identify approximate combustion kinetics of tetrafluoroethylene from the experimental results. The determined kinetics was used for the prediction of the IT of stoichiometric tetrafluoroethylene-air by simplified calculation methods. A very good agreement with the experimental results was observed.     

Does the dust explosion risk increase when moving from μm-particle powders to powders of nm-particles?

Based on experience with powders of particle sizes down to the 1–0.1 μm range one might expect that dust clouds from combustible nm-particle powders would exhibit extreme ignition sensitivities (very low MIEs) and extreme explosion rates (very high K_St-values). However, there are two basic physical reasons why this may not be the case. Firstly, complete transformation of bulk powders consisting of nm-particles into dust clouds consisting of well-dispersed primary particles is extremely difficult to accomplish, due to very strong inter-particle cohesion forces. Secondly, should perfect dispersion nevertheless be achieved, the extremely fast coagulation process in clouds of explosive mass concentrations would transform the primary nm-particles into much larger agglomerates within fractions of a second. Furthermore, for organic dusts and coal the basic mechanism of flame propagation in dust clouds suggests that increased cloud explosion rates would not be expected as the particle size decreases into the <1 μm range. An overall conclusion is that dust clouds consisting of nm primary particles are not expected to exhibit more severe K_St-values than clouds of μm primary particles, in agreement with recent experimental evidence. In the case of the ignition sensitivity recently published evidence indicates that MIEs of clouds in air of some metal powders are significantly lower for nm particles than for μm particles. A possible reason for this is indicated in the paper.     

Experimental investigation of the influence of inert solids on ignition sensitivity of organic powders

This work presents the results of the experimental characterization of the ignition sensitivity of solid inertant/combustible powders mixtures. Three inert solids (alumina, Kieselguhr, aerosil) and eleven organic powders have been considered and the following parameters have been determined: (1) the minimum ignition energy, (2) the minimum ignition temperature in cloud and (3) the minimum ignition temperature in 5 mm layer. The effects of the addition of inert solids are described and a simple model is proposed to represent the experimental results.Generally, increasing inert solid content in a powder leads to a higher minimum ignition energy as well as a higher minimum ignition temperatures in cloud and in layer. In some cases, the flammability is influenced above a threshold concentration value, which can be quite high (up to 85 wt.%). Indeed, the proposed model shows a zone below the minimum ignition concentration (MIC), which does not enable an efficient or safe inerting: either the admixed inert solid does not provide a sufficient effect, or it can even facilitate the ignition of the dust by notably improving its dispersability.The influence of key parameters such as the thermal conductivity or optical properties on the efficiency of the inerting by admixed solid need to be further assessed in a future work in view to better understand the mechanisms involved and to extend the scope to other types of oxidizable materials.     

Minimum ignition energies of pure magnesium powders due to electrostatic discharges and nitrogen's effect

This paper experimentally investigated the relation between the minimum ignition energy (MIE) of magnesium powders as well as the effect of inert nitrogen (N₂) on the MIE. The modified Hartmann vertical-tube apparatus and four kinds of different-sized pure magnesium powders (median particle size, D50; 28.1 μm–89.8 μm) were used in this study. The MIE of the most sensitive magnesium powder was 4 mJ, which was affected by the powder particle size (D50; 28.1 μm). The MIE of magnesium powder increased with an increase in the N₂ concentration for the inerting technique. The magnesium dust explosion with an electrostatic discharge of 1000 mJ was suppressed completely at an N₂ concentration range of more than 98%. The experimental data presented in this paper will be useful for preventing magnesium dust explosions generated from electrostatic discharges.     

The effect of admixed material on the flaming and smouldering combustion of dust layers

The rate of propagation of flame and the rate of propagation of smouldering in still air, have been investigated for several particle sizes, mixtures of coarse and fine dusts of the same material, and mixtures containing partially coarse dusts of limestone. Wood sawdust and grass dust were used as the industrial materials. Measurements were made by using a mold of acceptable dimensions. The results obtained indicated that the flaming and smouldering rates are dependent on the particle size and the depth of the layer and the values of the smouldering rates were found to be about 20% of the values obtained with flaming. Also, an admixture of fine dust of 50% of the same material to coarse dust for a 0.5 cm layer of both materials is sufficient to increase the values of the flaming rate by 61% for wood sawdust and the smouldering rate by 88% and 52% for wood sawdust and grass dust, respectively. The admixture of limestone as low as 10% was sufficient to produce zero propagation for both types of burning.     

敏感条件对粉尘云最小点火能的影响规律分析

为使粉尘云最小点火能实验测量更准确,从多个方面分析影响最小点火能的测量因素,并根据粉尘云状态、粉尘颗粒固有性质、点火电路等几个方面对影响粉尘云最小点火能的因素,即敏感条件进行了分类。在实验测量中,具体归纳为:粉尘浓度、粉尘湿度、粉尘粒度及其分布、粉尘挥发份含量、粉尘温度(环境温度)、粉尘云的湍流度、粉尘分散质量、粉尘云初始压力、环境氧浓度、电极材料、电极直径和电极末端曲率、电极间距、电火花持续时间、点火延迟时间、电火花能量密度、火花触发电路、可燃气体影响、实验次数等18个影响因素。重点分析了敏感条件对最小点火能的影响规律,从粉尘云点火机理和过程出发,着重分析一些敏感条件对最小点火能影响的内在原因和实质。