Current Status and Expected Future Trends in Dust Explosion Research - Research Essay Example
bstract In spite of extensive research and development for more than 100 years to prevent and mitigate dust explosions in the process industries, this hazard continues to threaten industries that manufacture, use and/or handle powders and dusts of combustible materials - Current Status and Expected Future Trends in Dust Explosion Research introduction. Lack of methods for predicting real dust cloud structures and ? ame propagation processes has been a major obstacle to prediction of course and consequences of dust explosions in practice. However, work at developing comprehensive numerical simulation models for solving these problems is now on its way.
This requires detailed experimental and theoretical studies of the physics and chemistry of dust cloud generation and combustion. The present paper discusses how this kind of work will promote the development of means for prevention and mitigation of dust explosions in practice. However, progress in other areas will also be discussed, e. g. ignition prevention. The importance of using inherently safe process design, building on knowledge in powder science and technology, and of systematic education/training of personnel, is also emphasized. q 2005 Elsevier Ltd. All rights reserved.
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Keywords: Dust explosions; Dust explosion prevention; Dust explosion mitigation; Dust explosion research 1. Introduction The dust explosion hazard continues to represent a constant threat to process industries that manufacture, use and/or handle powders and dusts of combustible materials. However, substantial advances have been made through extensive research and development world-wide for more than 100 years. Table 1 gives an overview of the most important methods currently used for preventing and mitigating dust explosions in the process industries. In dust explosion prevention and mitigation, as in many other
challenges encountered by the process industries, there is an inevitable con? ict between the short-term needs of the users of knowledge and technology, and the long-term strive for the ‘perfect’ solution. Industry will always need practicable solutions that can be implemented more or less immediately. It cannot wait for the ideal solutions that may become available in some distant future. However, industrial pragmatism must not, on the other hand, block the constant strive for better solutions based on improved basic understanding of the phenomena involved.
* Tel. : C55 582858; fax: C55 589440. E-mail address: rolf. [email protected] uib. no In the present paper an attempt will be made at illustrating how research on relevant fundamental phenomena can promote further development of the practical means for preventing and mitigating dust explosions in industry listed in Table 1. The paper is essentially based on the comprehensive review of the state-of-the art in dust explosion research given in Chapter 9 of Eckhoff (2003), comprising about 600 references to works published from 1990 to 2003.
In the present condensed summary, only a limited selection of these references is included. 2. The role of fundamental knowledge in assessing and controlling dust explosion hazards in practice. Over the last 20 years there has been a gradual shift in approach in dust explosion prevention and mitigation, from simple dogmatic design methods, towards more sophisticated ones opening up for increased ? exibility and tailoring. However, fundamental knowledge is essential for proper understanding of the practical aspects. In recent years the appreciation of the bene?
ts that can be harvested from cross-fertilization between fundamental research and applied research and development has been increasing. Advanced numerical models will play an increasingly important role in solving 0950-4230/$ – see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10. 1016/j. jlp. 2005. 06. 012 226 R. K. Eckhoff / Journal of Loss Prevention in the Process Industries 18 (2005) 225–237 Table 1 Means of preventing and mitigating dust explosions. A schematic overview Prevention Preventing ignition sources Smouldering combustion in dust, dust ? res Other types of open ? ames (e. g.
hot work) Hot surfaces (electrically, thermically or mechanically heated) Heat from mechanical impact (metal sparks and hot-spots) Electric sparks and arcs. Electrostatic discharges Preventing explosive dust cloud Inerting of dust cloud by N2, CO2 and rare gases Intrinsic inerting of dust cloud by combustion gases Inerting of dust cloud by adding inert dust Keeping dust conc. outside explosive range Explosion-pressure resistant construction Explosion isolation (sectioning) Explosion venting Automatic explosion suppression Partial inerting of dust cloud by inert gas Good housekeeping (dust removal/cleaning) Mitigation
practical design problems. The development of such models requires detailed experimental and theoretical studies of the relevant physical and chemical aspects. Table 2 summarizes some fundamental research topics that are essential for further development of preventive and mitigatory methods, indicated in Table 1. However, before addressing the speci? c methods, research on two common basic problems, must be addressed, viz. dust cloud generation, and ? ame propagation in dust clouds. 3. Generation of explosive dust clouds in the process industries 3. 1.
A historical perspective Nearly 130 years ago professor Weber, one of the pioneers of dust explosion research, stressed the importance of accounting for dust cohesion and dust dispersibility. In his excellent paper on the ignitability and explosibility of wheat ? our Weber (1878) Table 2 Fundamental aspects addressed in dust explosion research Dust cloud formation processes Inter-particle forces in dust deposits (cohesion) Entrainment of particles from dust deposits by shock waves passing across the deposit surface Entrainment of particles from dust deposits by turbulent gas ?
ows Dust cloud ignition processes Ignition of single particles and clouds Ignition by smouldering combustion in dust layers/deposits Ignition by hot surfaces Ignition by ? ying burning metal particles Flame propagation processes in dust clouds Microscopic aspects (Single-particle ignition) and combustion in hot oxidizer gas Laminar and turbulent ? ames in dust clouds Blast waves generated by burning dust clouds Blast wave properties as a function of properties of burning dust clouds Effects of blast waves on humans and mechanical structures
Mechanisms of heat transfer (conduction, convection, radiation) Ability of blast waves from dust explosions to transform dust layers into explosive dust clouds (coupled to ? rst column of table) Ignition by electric sparks and arcs Transport of dust particles in turbulent gas ? ows Ignition by electrostatic discharges Limit conditions for ? ame propagation in dust clouds (particle properties, dust conc. , oxygen conc. , geometry). Measurement and characterization of turbulence in dust clouds Measurement and characterization of spatial distribution of particles in dust clouds
Ignition by hot gas jets Ignition by shock waves Ignition by hot-spots from focused light beams In? uences on dust cloud ignition sensitivity of cloud properties (composition, size, shape of particles, dust concentration, composition, turbulence, temperature and pressure of gas phase) Acceleration of ? ames in dust clouds by turbulence mechanisms Detonation phenomena in dust clouds R. K. Eckhoff / Journal of Loss Prevention in the Process Industries 18 (2005) 225–237 227 emphasized that the cohesion of the ? our, which is caused by inter-particle
adhesion, has a strong in? uence on the ability of the ? our to disperse into explosive dust clouds. Weber suggested that two large dust explosion ? disasters, one in Szczecin (Stettin) and one in Munchen, were mainly due to the high dispersibility of the ? ours. He also demonstrated, using simple but convincing laboratory experiments, that the dispersibility, or dustability, of wheat ? our increased as its moisture content decreased. A global de? nition of dust dispersibility is given in Eckhoff (2003), Chapter 3. 3. 2.
The ‘state’ of a dust cloud The characterization of the ‘state’ of a dust cloud is far more complicated than characterizing the ‘state’ of a premixed quiescent gas mixture. For a quiescent gas the thermodynamic state is completely de? ned by the chemical composition, the pressure and the temperature. For a dust cloud, however, the stable state of equilibrium will be complete separation, with all the particles settled out at the bottom of the system. Therefore, in the context of dust explosions, the relevant ‘state’ will always be dynamic.
In various industrial environments, as well as in experiments with dust clouds (disregarding sophisticated ‘microgravity’ experiments), gravity and other inertia forces act on the dust particles, giving rise to a complex dynamic picture. In the ideal static dust cloud, all the particles would be located in ? xed positions, either ordered or at random. The closest approximation to the ideal dust cloud that can be encountered in practice is probably a cloud in which the particles are settling in quiescent gas under the in? uence of gravity alone. 3. 3.
Generation of primary dust clouds inside process equipment In order for an explosive dust cloud to be formed from a layer/deposit, the layer/deposit must be exposed to a process that disperses the particles in the air to the extent that the dust concentration drops into the explosive range. Most often such dispersion generating explosive clouds takes place intentionally inside process equipment, e. g. by handling and transportation in various process equipment (in mills, dryers, mixers, bucket elevators and other conveyors, silos, ? lters, cyclones, connecting ducts etc. ).
It seems reasonable to expect that comprehensive numerical models will be developed for predicting the dust cloud structures (spatial distributions of effective particle size, dust concentration, turbulence and global ? ow) that will be generated in various practical situations in industry. Knowing this initial cloud structure is essential because it has a major impact both on the ignition sensitivity of the cloud and the course of development of the primary explosion. Therefore, adequate information about the initial dust cloud structure is also essential for realistic modelling
of dust explosions in process plant. However, the development of adequate numerical models is only in its infancy, and little information of practical use has been traced. The works of Hauert, Vogl, and Radandt (1994); Kosinski et al. (2001) constitute valuable initial contributions. However, more experimental and theoretical work is needed in this area. In order to guide fundamental research in this area in the direction of maximum practical relevance, further information about dust cloud structures that are typical in industrial operation is required.
This not only means the cloud structures in normal plant operation, but perhaps even more importantly the structures existing during abnormal transient phases, including plant start-up and close-down. The occurrence of dust explosions may sometimes seem more likely during such periods than under normal steady-state conditions. 3. 4. Generation of secondary dust clouds inside and/or outside process equipment by blast waves from primary dust explosion The blast wave from a primary dust explosion can generate secondary explosive clouds ahead of the ? ame by entraining dust deposits and layers there.
Lebecki, Sliz, Dyduch, and Wolanski (1990) investigated such processes experimentally in a 100 m long gallery of cross-section 3 m2. Kauffman, Sichel, and Wolanski (1992) and Austin et al. (1993) summarized their extensive research on entrainment of dust layers in long tubes by the blast wave heading a dust explosion propagating along the tube. Boiko and Poplavski (1996) studied the effect of the dust concentration in a dust cloud behind a shock wave, on the acceleration of the cloud. Data from this kind of work are essential in the development of comprehensive dust explosion codes.
Klemens, Kosinski, and Oleszczak (2002a,b) presented a mathematical model for simulating the process of entrainment of dust particles from a dust layer, by the gas ? ow behind a shock or a rarefaction wave passing across the layer. Fedorov and Gosteev (2002) and Fedorov and Fedorova (2002) presented mathematical models describing the initial stage of the entrainment of single dust particles from a dust layer by a gas ? ow passing across the layer, and numerical simulations of the entrainment of dust particles from a near-wall dust layer by a shock wave propagating across the layer. 3. 5.
Dust dispersibility tests Various test methods have been proposed for evaluating the ease with which dust clouds can be produced from deposits and layers of powders/dusts (Breum, 1999; ? Dahmann & Mocklinghoff, 2000; Eckhoff, 2003, Chapter 7; Jong, Hoffmann, & Finkers, 1999; Tamanini & Ural, 1992). 228 R. K. Eckhoff / Journal of Loss Prevention in the Process Industries 18 (2005) 225–237 4. Flame propagation in dust clouds 4. 1. General An important difference between dust clouds and premixed gases is that in dust clouds inertial forces can produce fuel concentration gradients (displacement of particles in relation to gas phase).
Furthermore, thermal radiation may contribute signi? cantly to the heat transfer from the ? ame to the unburnt cloud, depending on the type of particle material (e. g. light metals). More work is needed to explore the role of thermal radiation in the development and course of dust explosions. Lee, Zhang, and Knystautas (1992) showed that theoretical equilibrium properties of dust cloud combustion (constant-pressure adiabatic ? ame temperatures, and maximum constant-volume explosion pressures) calculated by standard computer codes are in good agreement with experimental data obtained by various workers.
Wolanski (1990) reviewed the problems involved in determining ? ame structures, laminar or quasi-laminar burning velocities, lower ? ammability limits, and conditions required for ? ame acceleration and transition to detonation in dust clouds. The in? uences of added inert particles were also considered. Much research work has been done on various aspects of combustion of liquid sprays and mists (Eckhoff, 1991), which is in part also relevant even in the context of dust explosions. 4. 2. Laminar ?
ames in dust clouds It has often been assumed that the laminar burning velocity of a given dust cloud is a basic combustion property of the cloud, which is closely related also to the burning velocities at various levels of turbulence, and hence to the ? ame propagation through that type of cloud at large. Numerous experimental studies have been performed of quasi-laminar upwards ? ame propagation in vertical tubes with the top closed and the bottom open. Often a ‘true laminar’ burning velocity was calculated by multiplying the observed upwards ?
ame speed by the ratio of the cross-section of the tube to the approximately constant observed surface area of the hemispherical/parabolic ? ame front. However, this method contains a logical inconsistency, in that the observed constant shape of the ? ame front is incompatible with assuming a constant laminar burning velocity perpendicularly to any point of the ? ame surface. Valuable work on laminar dust ? ames in stabilized burners was carried out in the past, e. g. by Smoot and co-workers (see Eckhoff, 2003, p. 273). An excellent recent contribution to improved understanding of the nature of laminar dust ?
ames was given by Dahoe, Hanjalic, and Scarlett (2002). They used a burner apparatus to produce stable cornstarch ? ames in air, and the laminar burning velocity was measured via laser Doppler anemometry (LDA). It was found that the laminar burning velocity varied with ? ame shape, and this was accounted for by introducing the ‘Markstein length’ of a dust/air ? ame. This parameter is speci? c for any given dust cloud. It has a magnitude of the order of the laminar ? ame thickness of that speci? c dust cloud, and serves as a measure of the sensitivity of the laminar burning velocity to changes in the ?
ame shape. Dahoe et al. emphasized that neither the theoretical derivation nor the experimental determination of the Markstein length is trivial, and that much remains to be learnt about its precise dependence on the chemical and physical properties of the speci? c combustible mixture being investigated. This work also suggests that time seems ripe for reconsidering earlier work to determine laminar burning velocities of dust clouds in vertical tube experiments, because buoyancy and ? ame stretch probably contributed signi? cantly to formation of the upwards moving ?
ame front. The conclusion seems to be that a satisfactory method for experimental determination of laminar burning velocities of dust clouds remains to be developed. 4. 3. Turbulent ? ame propagation in dust clouds Access to adequate sub-models of ? ame propagation in turbulent dust clouds is essential when developing comprehensive numerical codes for dust explosion propagation. Understanding ? ame acceleration, due to ? ame distortion and turbulence produced by the propagating explosion itself, is central for understanding both dust and gas explosions in practice.
Extensive experimental research programmes have been conducted to resolve basic ? ame acceleration mechanisms in gas explosions in obstructed geometries. Central contributors are Moen, Lee, Hjertager, Fuhre, and Eckhoff (1982); Hjertager, Fuhre, and Bjoerkhaug (1988); Bakke and Wingerden (1992). ` The fundamental studies of Rzal-Rebiere and Veys` siere (1994) provide signi? cant insight in possible differences between turbulent combustion of premixed gases and dust clouds. They investigated the interaction of a laminar maize starch/air ? ame with an obstacle, viz. a sphere, a disk or an annulus.
With the annulus, ? amequenching phenomena were observed, which were attributed to centrifugal separation of dust particles and air in the turbulent eddies. This is a very important observation, indicating that the burning rate of a dust cloud may not necessarily respond to turbulence in the same way as the burning rate of a premixed gas. Further work towards improved understanding of the relation between the dynamic state of a dust cloud and its combustion rate is needed. By employing the experimental facilities used in previous extensive gas explosion experiments, e. g.
in the experiments by Moen et al. (1982), and repeating these experiments using dust clouds instead of premixed gas, valuable insight could R. K. Eckhoff / Journal of Loss Prevention in the Process Industries 18 (2005) 225–237 229 be gained. Systematic comparison of results would yield an overview of similarities and discrepancies, which would help to focus basic research efforts on important areas where dust cloud combustion may differ signi? cantly from combustion of premixed gases. Signi? cant differences between combustion of premixed gases and dust clouds also exist on the microscopic scale.
For example, the basic microscopic turbulence mechanisms that promote the combustion process must be identi? ed. The results of Mitgau (1996) and Mitgau, Wagner, and Klemens (1997) indicate that more ef? cient replacement of gaseous reaction products by fresh air round each particle may be a strong basic turbulent combustion enhancement mechanism. 4. 4. Comprehensive mathematical models of turbulent ? ame propagation in dust clouds ? Kjaldman (1992), being one of the pioneers in this ? eld, summarized his early work on applying computational ? uid dynamics (CFD) to turbulent dust explosion propagation.
Subsequent contributions were made by Bielert and Sichel (2001); Korobeinikov, Semenov, Klemens, Wolanski, and Kosinski (2002); Kosinski, Klemens, and Wolanski (2002); Rose, Roth, and Frolov ? ? (1999); Smirnov, Nikitin, and Legros (2000); Worsdorfer, Sippel, Fuisting, and Kneer (2001); Zhong, Teodorczyk, Deng, and Dang (2002). In developing a comprehensive numerical code for dust explosion simulation, corresponding existing codes for gas explosion simulation constitute a logical starting point. The comprehensive FLACS code originally developed by Hjertager et al.
(1988) is currently being used as a basis for developing the corresponding dust explosion code DESC. Arntzen, Salvesen, Nordhaug, Storvik, and Hansen (2003); Hansen, Skjold, and Arntzen (2004), and Wingerden, Arntzen, and Kosinski (2001) presented dust explosion simulations using preliminary versions of the DESC code. Very recently Skjold, Arntzen, Hansen, Storvik, and Eckhoff (2004) presented results from simulation of dust explosions in a large silo of 236 m3, performed in Norway about 20 years ago, using an improved version of the same code. Skjold et al.
(2004a) presented further DESC simulations of explosions in other process equipment. It seems clear that this type of comprehensive numerical computer simulation codes will become the future tool for predicting of the course of dust explosion scenarios encountered in the process industries, including process units interconnected by ducts and conveyor lines. However, development of, and con? dence in, any such comprehensive computer code have to be built on extensive validation against full-scale dust explosion experiments, covering a wide range of dusts, initial dust clouds states, and geometrical con?
gurations. 5. Preventing explosive dust clouds 5. 1. Inherently safe process design Most often one tries to ? ght the dust explosion hazard by adding preventive and mitigatory measures to an existing process. However, the technical measures adopted are often expensive, and safety procedures may fail. Inherent safety is an alternative approach implying that the process itself be designed in such a way that no explosion hazard exists. Kletz (1999), the ‘father’ of the inherently-safe-process design concept, outlined its basic philosophy and recommended the use of it whenever feasible.
In the context of preventing and mitigating dust explosions inherently safe process design could include use of production, treatment, transportation and storage operations where dust cloud generation is kept at a minimum. One example is use of mass ? ow silos and hoppers instead of the frequently used funnel ? ow types. Eckhoff (1997) emphasized the importance of knowing powder science and technology when striving for inherently safe process design in industries having a dust explosion hazard.
Amyotte and Khan (2002) proposed a framework for directing the concept of inherently safe process design speci? cally towards reducing the dust explosion hazard in industry. Hopefully, such initiatives will promote further work in this important area. 5. 2. Inerting by adding inert gas Inert dust clouds can be generated by mixing the air with an inert gas such as nitrogen or carbon dioxide to a level at which the dust cloud can no longer propagate a selfsustained ? ame. Some further insight has been gained over the last few years.
For example, in contrast to what had been found earlier for coal dust, Wilen (1998) measured an increase of the limiting oxygen concentration (LOC) for inerting bio-mass dust clouds, with increasing initial pressure in the range 5–18 bar. Schwenzfeuer, Glor, and Gitzi (2001) found that LOC for ignition of dust clouds by electrostatic discharges, or metal sparks from mechanical impact, were signi? cantly higher than the conservative limit determined in standard tests, using a very strong pyrotechnical ignition source. Whilst a reduction of the oxygen content in the atmosphere can prevent dust explosions, it may introduce a suffocation risk.
However, research has shown that adding a few vol. % of CO2 to the gas mixture reduces the critical oxygen threshold for suffocation considerably. An inert gas mixture utilizing this effect was described by Dansk Fire Eater A/S (1992). 5. 3. Keeping the dust concentration below LEL In principle, keeping the concentration of dust in the cloud below the minimum explosive limit (LEL), is a means of maintaining dust clouds non-explosive. 230 R. K. Eckhoff / Journal of Loss Prevention in the Process Industries 18 (2005) 225–237 However, the method has limited applicability in practice.
Mittal (1993) discussed various mathematical models for calculating minimum explosive concentrations of dust clouds. 6. Preventing ignition sources 6. 1. Smouldering layers, deposits and nests A practical question that has been asked is whether metal particle sparks from single accidental impacts can initiate combustion in dust layers/deposits. Hesby (2000) found that the number of sparks from single accidental impacts of steel objects is all too low to be able to cause ignition of the layers of organic dusts studied. Krause and Hensel (1996) presented a numerical method by which non-steady temperature ?
elds in dust deposits can be computed. This enables numerical analysis of a number of practical cases of self-heating/self ignition that cannot be analyzed using the classical thermal explosion theory of Frank-Kamenetzki. Krause and Schmidt (2001) studied experimentally critical thermal conditions that may lead to initiation of smouldering processes, or to further development of such processes, once initiated. Gummer and Lunn (2003) found that smouldering nests were poor ignition sources for most dust clouds, whereas, ? aming nests caused ignition more readily.
More work is needed to clarify both the conditions under which smouldering or ? aming nests of various materials are generated in industrial plant, and the circumstances under which such nests will ignite explosive clouds of various dusts. 6. 2. Hot surfaces In the past, the minimum hot-surface temperature for ignition of a dust cloud has often been regarded as if it were a universal constant for a given cloud. Consequently, results from small-scale laboratory tests were often applied directly in design of large-scale industrial plant. However, minimum hot-surface ignition temperatures of dust clouds vary signi?
cantly with scale, as well as with the geometry of the hot surface in relation to the dust cloud. There is a need for both a more differentiated basic understanding and a more differentiated testing approach. Development of numerical models for dynamic simulation of hot-surface ignition processes encountered in practice is foreseen. 6. 3. Electric/electrostatic discharges between two metal electrodes Electric and electrostatic discharges between two metal electrodes can be generated in a number of ways, e. g. in switches, by failures in electric circuits, and by discharge of static electricity.
The parameters in? uencing the minimum energy required for igniting a dust cloud by an electric spark include voltage and current characteristics across the spark gap, spark gap geometry and electrode material, as well as all the dust cloud parameters. The latter include particle material and particle size/shape distributions, dust moisture content, dust concentration, and the dynamic state of the dust cloud with respect to the spark gap. Minimum ignition energies (MIE) of clouds of a given dust material decreases strongly with the ? neness of the dust. Eckhoff (1995) discussed the in?
uence of dust ? neness on MIE of ferroalloys dusts. In the past dust ? neness was often speci? ed just as a mass percentage ? ner than an arbitrary size, e. g. 74 or 63 mm, without any speci? cation of the distribution of particle sizes below these limits. This complicates the analysis of published experimental data, and more systematic research is needed to clarify the exact in? uence of particle size. In the case of metal alloys the most hazardous components may sometimes accumulate in the ? ne tail of the particle size distribution (e. g. Mg in MgFeSi), and special investigations are required.
Lorenz and Schiebler (2001) presented the results from a comprehensive, detailed experimental and theoretical investigation of the energy transfer processes taking place during an electrostatic spark discharge. The temperature and pressure development in the spark channel during its formation and subsequent expansion were investigated. This also included cooling of the channel by thermal radiation. The dependence of the ability of a given discharged electrical energy to ignite a dust cloud on these basic physical spark characteristics was emphasized.
Recently Randeberg and Eckhoff (2004) investigated an alternative method for measuring MIEs of explosive dust clouds, which may be in better accordance with accidental electrostatic spark ignition in industrial plant. In the conventional method a special electronic system is employed for optimal synchronization of the dust cloud and the spark discharge. In the work of Randeberg and Eckhoff the transient dust cloud itself was used to initiate spark breakdown between a pair of electrodes pre-set at a high voltage somewhat below the breakdown voltage in dust-free
air. Using this method, the MIEs of three dusts were determined. The results were of the same order, although somewhat higher than those obtained using the conventional method. Up to now the lower spark energy limit for apparatuses commonly used for determining MIEs of dust clouds has been 1–3 mJ. However, recently Randeberg, Olsen, and Eckhoff (2005) presented a new test method that permits MIE determination for dust clouds, using synchronized sparks, down to the order of 0. 03 mJ. 6. 4.
Electrostatic one-electrode discharges With regard to the even more complex one-electrode electrostatic discharge types (corona, brush, propagating brush, etc. ), valuable experimental insight has been gained during the last years. The issue of whether brush discharges R. K. Eckhoff / Journal of Loss Prevention in the Process Industries 18 (2005) 225–237 231 can ignite dust clouds was revisited experimentally by Larsen, Hagen, Wingerden, and Eckhoff (2001), who were in fact able to ignite clouds of sulphur dust in oxygenenriched air by true brush discharges.
However, ignition in air only was never observed. Because of the very low MIE of clouds of sulphur dust in air, this indicates that ignition of even the most sensitive dust clouds by brush discharges in air, is unlikely. 6. 5. Glowing/burning particles Ignition of dust clouds by small burning metal particles (impact sparks, metal sparks) generated by mechanical impact is a complex process, and comprehensive, practically useful theories do not seem to be within sight. Such theories must comprise several complex sub-processes. The ?
rst is the generation and initial heating of the metal particle by the impact. The second is the ignition of the ? ying hot particle and the subsequent burning process. The third is the heat transfer to the dust cloud, which ultimately determines whether ignition occurs or not. 6. 6. Electrical equipment The present situation internationally concerning standards for apparatuses for use in areas containing combustible dust is confusing, as discussed by Eckhoff (2004a). Whereas, the International Electrotechnical Commission (IEC) has decided to base its development on
European Union ‘Atex’ philosophy, the European Union Atex 94/9/ EC Directive does not distinguish adequately between combustible dusts and combustible gases/vapours. In particular the Directive does not point out the vast differences between the ways in which explosive gas clouds on the one hand, and explosive dust clouds on the other, are generated and sustained in industrial practice. This has resulted in undue alignment of a series of new IEC standards for electrical apparatuses for combustible dusts with established standards for gases/vapours.
The current European Union Atex 1999/92/EC Directive also lacks the required distinction between gases and dusts, which gives rise to problems with area classi? cation. 6. 7. Other ignition sources Proust (2002) determined experimentally the minimum laser beam power required for igniting dust clouds by the heat absorbed by a solid target heated by the laser beam. The variable parameters included the laser beam diameter, the duration of the irradiation, the target material (combustible/ non-combustible), and the type of dust.
Initiation of dust explosions by shock waves has been studied by several workers, including Wolanski (1990); Klemens, Klammer, and Wolanski (1998). 7. Protective/mitigatory measures 7. 1. Full con? nement The applicability of the concept is limited because of high equipment costs. However, the method is being used in some special cases, e. g. when the powder/dust is highly toxic, and completely reliable con? nement is absolutely necessary. Current experimental m