Ignition Of Dust Clouds Research Articles

Effect of large particle mixing on cloud ignition and explosion of fine rice husk

Large particles mixed with fine dust are commonly found in the processing and transportation of grain dust. The mixture of dust is exposed to the risk of dust explosions when suspended in the environment. Using a 20 L spherical explosion device and the dust cloud minimum ignition energy (MIE) experimental platform, this study examines the effects of crushed brown rice mixture on the ignition and explosion of fine rice husk dust. The discovery of large granules of crushed brown rice being lifted up to form flow "obstacles" in the mixed dust cloud has a similar inhibitory effect to inert materials, causing the MIE of the dust cloud to increase from 40 mJ to 150 mJ. However, 5 % of crushed brown rice will lower the 10 mJ dust cloud's ignition energy when the concentration of dust clouds is 500 g/m3. Through testing the dispersibility of mixed dust clouds, it was found that fine dust particles adsorbed on the surface of large particles will peel off after being lifted, improving the dispersibility of the mixed dust cloud and forming a system of mixed dust clouds with finer effective particle size, thereby promoting ignition. Furthermore, due to the improvement in the dispersibility of mixed dust clouds by large particles, at mixing ratios below 30 %, large particles will also significantly increase the maximum rate of pressure rise (dP/dt)max, increasing the likelihood of dust ignition and the consequences of an explosion. Therefore, these findings should be taken into consideration during the process of grain processing and transportation.

A numerical model for the prediction of the minimum ignition temperature of dust clouds

This paper presents a numerical model for the prediction of the minimum ignition temperature (MIT) of dust clouds. First, a physical model is developed for the dust cloud ignition in the Godbert-Greenwald furnace. A numerical approach is then applied for the MIT prediction based on the physical model. The model considers heat transfer between the air and dust particles, the dust particle reaction kinetics, and the residence times of dust clouds in the furnace. In general, for the 13 dusts studied, the calculated MIT data are in agreement with the experimental values. There is also great accordance between the experimental and numerical MIT variation trends against particle size. Two different ignition modes are discovered. The first one consists in ignition near the furnace wall for bigger particles characterized by rather short residence times. In the second mode, the ignition starts from the center of the furnace by self-heating of the dust cloud for smaller particles with longer residence times. For magnesium, as dust concentration increases, the lowest ignition temperature of the dust cloud IT(conc) decreases first, then transits to increase at a certain point. The transition happens at different dust concentrations for different particle sizes. Moreover, the MIT of the magnesium dust cloud generally increases as particle size increases, but the increasing trend stagnates within a certain medium particle size range.

Investigation of niacin and aluminum dust cloud ignition characteristics in an explosion hazard testing device using high-speed imaging

Experimental investigations of dust explosions in standard industrial testing equipment such as the MIKE3 minimum ignition energy (MIE) testing device are promising ways of producing fundamental insights into dust cloud ignitability. However, advanced experimental methods must be developed to characterize the dust cloud ignition and combustion behavior of various types of dust. In this work, high-speed broadband and species-specific chemiluminescence imaging diagnostics are implemented to explore similarities and differences between organic and metal powder ignition kernel development. A selected set of 600-mg niacin (ϕ = 2.9) and aluminum (ϕ = 1.6) powder samples were dispersed in the air and ignited using a high-voltage spark inside a standard MIKE3 device. The resulting broadband flame emission was recorded at 4 kHz using a high-speed camera for the first 10 ms after the spark. For the niacin sample, species-specific chemiluminescence emissions from excited-state hydroxyl (OH*) and methylidyne (CH*) radicals were also recorded at 4 kHz and 1 kHz, respectively. The flame kernel in each image frame was detected using an intensity thresholding algorithm and tracked throughout the video sequence, yielding quantitative size, position, and velocity measurements of the evolving flame kernel during the first 10 ms after the ignition spark. For the niacin sample, a continuous and non-uniform reaction zone composed of burning particle clusters and excited-state radicals was observed. The niacin flame kernels grew from 5 mm to 17 mm with an initial velocity of 5 m/s. Conversely, an intensely bright reaction zone and discrete burning particles near the flame kernel boundary were observed in the aluminum sample. The aluminum flame kernels grew from 7 mm to 10 mm with an initial velocity of 3 m/s. The niacin and aluminum flame kernels traveled 7–11 mm away from the central spark gap and slowed down to 1 m/s by the end of the 10-ms period. This time-resolved imaging study, when coupled with previously reported three-dimensional particle and flow field data prior to the arrival of the ignition spark, sets the foundation for an improved multi-physics understanding of the initiation of dust cloud ignition and explosion processes.

A theoretical model for the prediction of the minimum ignition energy of dust clouds

In this study, a physical model of the dust cloud ignition process is developed for both cylindrical coordinates with a straight-line shaped ignition source and spherical coordinates with a point shaped ignition source. Using this model, a numerical algorithm for the calculation of the minimum ignition energy (MIE) is established and validated. This algorithm can evaluate MIEs of dusts and their mixtures with different dust concentrations and particle sizes. Although the average calculated cylindrical MIE (MIEcylindrical) of the studied dusts only amounts to 63.9% of the average experimental MIE value due to reasons including high idealization of the numerical model and possible energy losses in the experimental tests, the algorithm with cylindrical coordinates correctly predicts the experimental MIE variation trends against particle diameter and dust concentration. There is a power function relationship between the MIE and particle diameter of the type MIE ∝ dpk with k being approximately 2 for cylindrical coordinates and 3 for spherical coordinates. Moreover, as dust concentration increases MIE(conc) first drops because of the decreasing average distance between particles and, at fuel-lean concentrations the increasing dust cloud combustion heat; however, after the dust concentration rises beyond a certain value, MIE(conc) starts to increase as a result of the increasingly significant heat sink effect from the particles and, at fuel-rich concentrations the no longer increasing dust cloud combustion heat.

Testing of dust clouds for the electrostatic-spark ignition hazard in industry. Need for a modified approach?

Current standard test methods for electric-spark minimum ignition energies (MIEs) of dust clouds in air require that a series inductance of at least 1–2 mH be included in the electric-spark discharge circuit. The reason is to prolong the spark discharge duration and thus minimize the spark energy required for ignition. However, when assessing the minimum electrostatic energy ½CU2 for dust cloud ignition by accidental electrostatic-spark discharges, current testing standards require that the series inductance of at least 1–2 mH be removed from the spark discharge circuit. No other changes of apparatus and test procedure are required. The present paper questions whether this simple approach is always adequate. The reason is that in practice in industry accidental electrostatic-spark discharge circuits may contain large ohmic resistances due to corrosion, poor electrical grounding connections, poorly electrically conducting construction materials etc. The result is increased spark discharge durations and reduced mechanical disturbance of the dust cloud by the blast wave emitted by the spark. Therefore, testing for minimum ½CU2 for ignition by accidental electrostatic spark discharges may not only require removal of the series inductance of 1–2 mH from the standard MIE spark discharge circuit. Additional tests may be needed with one or more quite large series resistances Rs inserted into the spark discharge circuit. The present paper proposes a modified standard test procedure for measurement of the minimum electrostatic-spark ignition energy of dust clouds that accounts for these effects.

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 (Cdust) values; the minimum ignition temperature (MIT) values of aluminium powder under different dust particle sizes and Cdust 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 Cdust 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 (Pex) and the rate of explosion pressure rise [(dP/dt)ex]. When dust particle size was augmented gradually, the Pex and (dP/dt)ex attenuated. Decreasing particle size lowered both the most violent explosion concentration and explosive limits.

Ignition and explosion characteristics of micron-scale aluminum–silicon alloy powder

Aluminum–silicon (Al–Si) alloys are used in numerous applications, particularly in the automotive and electronics industries. However, because of the production of dust in the course of alloy production, the associated processes are prone to dust explosion, which can result in considerable damages. To examine the ignition and explosion characteristics of micron-scale Al–Si alloy powder, we varied the ignition energy (Ei), particle size, and dust concentration in our experiments. The effects of particle size and dust concentration on minimum ignition temperature (MIT), and Ei, particle size, and dust concentration on the explosions were studied by testing ignition and explosion parameters in a dust cloud ignition temperature testing device and a nearly spherical 20-L explosion vessel, respectively. The MIT of the Al–Si alloy powder considerably increased with increase in particle size. However, when the particle size reached a certain specific degree, the dust cloud could not be ignited. The MIT of the Al–Si alloy powder initially decreased and then increased with increase in dust concentration. In addition, a specific concentration resulted in the Al–Si alloy powder attaining the MIT, which had the highest explosion hazard. The results of the explosion test indicated that the explosion pressure (Pex) and the rate of explosion pressure rise [(dP/dt)ex] of the sample increased with increase in Ei. Similarly, the lowest Ei increased with increase in particle size; Pex of the sample alleviated as particle size increased, and Pex and (dP/dt)ex of the Al–Si alloy powder first increased and then decreased in the mass concentration range 100–900 g/m3. For four samples, the most severe explosions corresponding to the highest Pex were observed at various concentrations: 600, 700, and 800 g/m3 at an Ei of 5 kJ and 500, 600, 700, and 800 g/m3 at 9 kJ. This result indicated that the attenuation of particle size requires a reduction in the Ei, and at the same time was able to effect the decrease of the most severe explosion concentration and the minimum explosible concentrations. Thermal analysis demonstrated the accelerated oxidation stage of Al–Si alloy powder, which started at 800 °C. In addition, compared with aluminum powder of similar particle size, the continuous oxidation stage started earlier and the oxidation rate was higher. Finally, comparing Al alloy with Al powder, pure Al powder demonstrated a higher Pex in an extensive range of mass concentration, indicating its higher explosion risk.

Origin and development of the Godbert-Greenwald furnace for measuring minimum ignition temperatures of dust clouds

Hot surfaces that can ignite explosible dust clouds can occur in various process situations in industry, e.g. inside furnaces, burners and dryers of various kinds. Hot surfaces can also be generated accidentally e.g. by frictional overheating of bearings and other mechanical parts, and electrically. To avoid accidental ignition of dust clouds by hot surfaces, it is essential to know the minimum temperature of any hot surface, at which a given explosible dust cloud that makes contact with that surface, will ignite. Currently the Godbert-Greenwald (G-G) furnace is the most commonly used apparatus for experimental laboratory-scale assessment of this minimum temperature for various dusts.The G-G furnace was developed by Godbert and Greenwald in 1935. Two pioneering “forerunners” of this apparatus were described by M. J. Taffanel and A. Durr (1911) in France, and by R.V. Wheeler (1913) in UK. However, neither the two “forerunners” nor the G-G furnace itself were originally intended for assessing maximum permissible temperatures of hot surfaces in industrial plant for preventing accidental hot-surface ignition of dust clouds there. This new use of the G-G furnace was not introduced in UK and the USA until in the 1940ies.The G-G furnace still appears to be the most widely used apparatus for assessing minimum hot-surface temperatures (MITs) for ignition of explosible dust clouds in air in industrial plant. However, not least in view of the history of the furnace, it is clear that the MITs that it produces do not apply to all possible hot-surface ignition scenarios that can be foreseen in industry. In most countries this problem is overcome by introducing a rather generous safety margin between the measured MIT for a given dust and the maximum permissible hot-surface temperature in industrial plants that produce and/or handle this dust.

Characterization of a spark discharge for dust cloud ignition

Spark discharges are widely used to ignite flammable gases, liquids, or dust. For a better understanding of the interaction between the spark discharge and the ignited media (gas, liquid, or dust), it is necessary to measure some key parameters of the spark, especially the space‐time variation of its temperature. Determination of temperature gradients would allow a more precise and realistic simulation of the ignition process. In fact, electrons and particles in the discharge zone get their energy with increasing temperature before interacting with particles of the media to ignite the flame. In this study, optical emission spectroscopy of the spark discharge between two tungsten electrodes was performed. Assuming excitation balance between the WI lines, a Boltzmann plot after an Abel inversion gives the excitation temperature and its space‐time variation. For a 100‐μs time discharge, at 80‐μs delay, we measured 7,000 K at the centre of the column zone, 4,100 K at the centre of the cathode zone, and 3,600 K at the centre of the anode zone. Assuming a singly ionized tungsten plasma and excitation equilibrium, we used also the Saha–Boltzmann equation to calculate the plasma composition. The electron density at the column zone was about 3 × 1017 cm−3, which is two orders of magnitude higher than in the rest of the spark.

Dust ignition characteristics of different coal ranks, biomass and solid waste

A vertical furnace into which a portion of a dust/air mixture is blown, was used to study dust cloud ignition. A dust/air cloud was formed in the working chamber space, with an average dust concentration of 4.8·10-2kg/m3. Ignition studies were conducted for one separated particle fraction of 63–90 μm. The purpose of the paper is to determine the parameters characterising the ignition (minimum ignition temperature, i.e. critical temperature of the furnace, where ignition takes place, and ignition time) and to describe the mechanism of ignition of coals with various rank (from brown coal then bituminous coal to anthracite) under conditions of particle interaction (cloud ignition). In addition, for comparison purposes, tests of ignition of various types of biomass, sewage sludge, segregated municipal waste and sewage sludge were carried out under the same experimental conditions. The moment of flame appearance (ignition) was recorded using two optical detectors. Ignition delay time was determined at different furnace temperatures ranging from the ignition temperature to 1100 °C. The tests performed show that the values of minimum ignition temperatures and the ignition delay times for the analysed coals depend to a large extent on the coal rank. The ignition temperature and ignition time decreased when the volatile matter content in the coal increased and the calorific value of the fuel decreased, i.e. when the rank of coal decreased. For the remaining category of the fuels tested, it was observed that, unlike for coals, the ignition temperature and the ignition delay time increases with the increase of volatile matter content of fuel and the decrease of the calorific value of fuel. Based on the research, it was concluded that under experimental conditions, the ignition of dust clouds of fuels with a volatile matter content below 65–70% daf (anthracites, bituminous coals and some brown coal) occurred according to the heterogeneous mechanism. However, for the content of volatile matters in the fuel above this value, volatile matter are ignited in the gaseous environment (homogeneous ignition). On the basis of the experimentally determined ignition characteristics, an attempt is presented in the paper to interpret the test results based on a simplified model of a heterogeneous ignition of a dust cloud and a model of homogeneous ignition of volatile matters in the vicinity of a fuel particle.

Explosibility of metallic waste dusts

Abstract Among the industrial sectors that are affected by dust explosions, the metal working industry is one of the most frequently involved. Metal dusts are often the by-products of mechanical workings. Dust producing machines are widely distributed, small in size and are generally located in workplaces. Abatement plants are also often located in the working area. The companies that are involved in these explosions are often small, and thus often have limited resources. These factors generally lead to difficulties in managing the risk of explosions. This paper has the aim of investigating the flammability of waste dust produced by metal workings, and to define the dust properties that are more likely to lead to an explosion. For this purpose, a simple and fast flammability test has been used as a cheap way of characterise the flammability of the samples. The test has been called the Speedy Esplosibility Test (SET), and it is similar to the procedure suggested in the new ISO/IEC standard ( ISO/IEC 2016 ) that came into force recently. SET is composed of 4 different tests, derived from standard procedures, each of which represents a different ignition mechanism: • High voltage continuous arc ignition and glowing wire ignition in a Hartman 1.2 l tube (based on UNI EN 13821:2004 ); • Dust cloud ignition in a G-G furnace and dust layer ignition on a hot plate (based on UNI EN 50281:1999 ). The SET results are compared with the standard flammability classification obtained according to ISO, 2016ISO/IEC 80079:2016, with the standard KSt measurement in the 20-L Siwek Sphere, and with tests in the 20-L sphere with 2 × 1 kJ igniters, respectively according to the UNI EN 14034: 2011 part 2 and part 3 . Moreover, the morphology and chemical nature of the dusts have also been determined and their effects on dust explosibility are discussed.

Experimental study on the minimum ignition temperature of coal dust clouds in oxy-fuel combustion atmospheres

BAM furnace apparatus tests were conducted to investigate the minimum ignition temperature of coal dusts (MITC) in O2/CO2 atmospheres with an O2 mole fraction from 20 to 50%. Three coal dusts: Indonesian Sebuku coal, Pittsburgh No.8 coal and South African coal were tested. Experimental results showed that the dust explosion risk increases significantly with increasing O2 mole fraction by reducing the minimum ignition temperature for the three tested coal dust clouds dramatically (even by 100°C). Compared with conventional combustion, the inhibiting effect of CO2 was found to be comparatively large in dust clouds, particularly for the coal dusts with high volatile content. The retardation effect of the moisture content on the ignition of dust clouds was also found to be pronounced. In addition, a modified steady-state mathematical model based on heterogeneous reaction was proposed to interpret the observed experimental phenomena and to estimate the ignition mechanism of coal dust clouds under minimum ignition temperature conditions. The analysis revealed that heterogeneous ignition dominates the ignition mechanism for sub-/bituminous coal dusts under minimum ignition temperature conditions, but the decrease of coal maturity facilitates homogeneous ignition. These results improve our understanding of the ignition behaviour and the explosion risk of coal dust clouds in oxy-fuel combustion atmospheres.

Ignition behaviour and flame stability of different ranks coals in oxy fuel atmosphere

The aim of the research has been to investigate the impact of a type of atmosphere onto the ignition of fuel and the stability of dust–gas flames. Three black coals and two brown coals were applied in the research. It covered performing their physicochemical analyses and determining the induction time of ignition of single particles and dust–air cloud as well as parameters of dust flames and their stability. The research program referred to various aspects of the ignition of fuel, including: effects of its temperature, size of grains, type of atmosphere (air, O2/CO2), O2 share (5–30%) in the O2/CO2 atmosphere. The conducted research highlighted that the ignition of fuel is connected with a degree of its carbonisation, temperature and the type of atmosphere at which it takes place. The ignition of black coals is more difficult (hindered) than the one of brown coals. For its enhancement it is required to apply (among others): higher oxygen concentrations, higher dust concentrations, fined dust milling or higher ambient temperatures. A type of gas (N2, CO2) with which oxygen was mixed had a fundamental impact onto the characteristics of ignition. For all the examined coals, the induction times of ignition of dust cloud measured in the air atmosphere were shorter than the ones measured in the oxy atmosphere. The presented research demonstrated that – for the stability of flames and the parameters of fuel – the specific thermal capacity and density of main gases making up a mixture with oxygen are of fundamental importance.

Suitability of ignition source “exploding wire” for determination of dust explosion characteristics in the 20-L-sphere

Several safety characteristics of dusts are determined in the 20-L-sphere (also known as SIWEK Chamber) according to international standards. Dust cloud ignition is carried out using pyrotechnical igniters. Due to various disadvantages of such igniters the need for alternative ignition sources arises again and again. An alternative could be an ignition source which is known as “exploding wire” or “fuse wire”. The paper presents test results of a comparative study between both ignition sources for the determination of the safety characteristics “Maximum Explosion Pressure” and “Maximum Rate of Explosion Pressure Rise” of five selected dusts in the 20-L-sphere. In addition to that the ignition mechanisms of both ignition sources were analyzed by high speed camera recordings and the ignition energy was determined with electric and calorimetric recordings. The paper shows results of measurements of the ignition energy of both ignition sources as well as sequences of the flame propagation.

Tests on Suitability of the Ignition Source “exploding Wire” for the Determination of Explosion Characteristics of Combustible Dusts in the 20-l-sphere

According to international standards several safety characteristics of dusts are determined in the 20-L-sphere (also known as SIWEK Chamber). Dust cloud ignition is carried out using pyrotechnical igniters. Due to various disadvantages of such igniters the need for alternative ignition sources arises again and again. An alternative ignition source could be the so called “exploding wire” or “fuse wire. The paper presents test results of a comparative study between both ignition sources for the determination of the safety characteristics Maximum Explosion Pressure and Maximum Explosion Pressure Rise of five selected dusts in the 20-L-sphere. In addition to that the ignition mechanisms of both ignition sources were analysed by high speed camera recordings and the ignition energy was determined with electric and calorimetric recordings.

Ignition: A Century of Research and an Assessment of Our Current Status

Ignition has been systematically, scientifically studied for about a century now. But the development of knowledge in this area has been uneven, and while an advanced understanding has been reached in some areas, knowledge is scant in others. A review of the literature of the field indicates that the following are some of the notable areas where engineering calculation methods are poor or lacking: hot-surface ignition of liquids; ignition of liquid sprays or aerosols; ignition of solids from hot bodies (hot solids or liquids); ignition of solids by small flames; glowing ignition of solids, especially of lignocellulosic materials; almost all aspects of the ignition of dust clouds; and electrical fires. It is also noted that, despite a century's research, there have been very few theories developed in the ignition field that have engineering utility. The vast majority of theoretical development efforts have produced unwieldy computational models that would require a plethora of generally unavailable input d...

Initiation of dust explosions by electric spark discharges triggered by the explosive dust cloud itself

An investigation of ignition of dust clouds by the use of electric spark discharges triggered by the explosive dust cloud itself has been conducted. This method of triggering capacitive sparks probably represents a realistic mechanism for initiating accidental dust explosions in industrial practice. Unlike the conventional method for determining the minimum ignition energy (MIE) in the laboratory, the delay between dust dispersion and spark discharge is not a degree of freedom. In stead, the transient dust cloud itself is used to initiate spark breakdown between electrodes set at a high voltage lower than breakdown in pure air. In the present study, different kinds of dusts were tested as ‘spark triggers’, and they exhibited quite different abilities to trigger breakdown. Large particles were found to initiate breakdown at lower voltages than smaller ones. In general, conductive particles were not found to initiate breakdown at lower voltages than dielectric ones when using the same dust concentration. Minimum ignition energies (MIE) of three dusts ( Lycopodium clavatum, sulphur and maize starch) were determined using the authors' method of study. The MIEs were somewhat higher than those obtained using conventional methods, but relatively close to the values obtained through conventional methods.

A study on the static electrification of powders during pneumatic transportation and the ignition of dust cloud

Large amounts of powders are being handled in industrial activities, and these are facing static electrification that might lead to dust explosion or fire. Therefore, it is important to know about the static electrification in detail in the powder handling processes to prevent the hazards. The authors investigated the static electrification characteristics such as the relation between the powder charge and dust concentration, transportation velocity, transportation distance, etc., using pneumatic transportation system. Also, the influences of spark duration time, ignition energy and feeding ratio of the ignition energy to initiate the dust explosion were investigated because there is a potential risk of dust explosion due to the static electrification. From the results of the experiments, the information on the static electrification and the ignition of the dust cloud was obtained. This would contribute to maintain safe operation of the powder handling, especially pneumatic transportation, by regulating the transport conditions.

Ignition of dust clouds by brush discharges in oxygen enriched atmospheres

Abstract Brush discharge ignitions of sulphur dust in oxygen-enriched atmospheres have been established. From a total of approximately 300 trials, brush discharges were able to start explosions of sulphur dust in three trials. In these trials the atmospheres contained 55 vol%, 60 vol% and 70 vol% oxygen, respectively. The work is motivated by the fact that no brush discharge ignition of dust–air mixtures has been observed in laboratory trials, despite an equivalent energy of the discharge above the minimum ignition energy of some dusts. By adding oxygen to the atmosphere in which the brush discharge is generated, the dust–air mixture will be more prone to ignition. If the critical oxygen concentration to establish ignition of very sensitive dusts such as sulphur is considerably higher than the oxygen concentration in air, it may be stated that a brush discharge cannot ignite dust–air mixtures at atmospheric conditions. High-speed video recordings show that all three ignitions occurred at the top edge of an evolving dust cloud. The research is still ongoing.

Static electricity hazards of flexible intermediate bulk container (FIBC)

AbstractFlexible intermediate bulk containers are being widely used. Much concern has been expressed regarding the electrostatic discharge hazards around flammable vapor/air mixtures and some very sensitive dust clouds. Methods were developed to analyze and test powders with FIBC materials to predict full scale electrostatic charging characteristics. Comparison with full scale FIBC operations shows very good correlation. Operations using FIBCs, including grounded versions are not recommended in flammable vapor environments. When dusty powders with low dust cloud ignition energies are used for FIBC systems, engineering analysis and testing is required to assure ESD safety.