Understanding Dust Explosions - the Role of Powder Science and

Transcription

1 Understanding Dust Explosions - the Role of Powder Science and Technology Rolf K. Eckhoff Professor emeritus, University of Bergen, Dept. of Physics and Technology, Bergen, Norway. Scientific/technical adviser,, Malmö, Sweden.

2 What is a dust explosion?

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4 Controlled, vented wheat grain dust explosion experiment in a 500 m 3 silo cell in Norway in 1980

5 Destructive dust explosion experiment (maize starch) in a 500 m 3 silo in Norway in 1982

6 Objective of present paper.. to show that adequate understanding of dust explosion phenomena in the process industries requires knowledge of central topics of powder science and technology

7 Basic powder-technology-related topics that are essential for understanding dust explosions Particle and powder characterization Production of fine particles by crushing, grinding and abrasion Powder mechanics (mechanical strength of powder deposits) Particle segregation in powder beds Dust cloud generation/powder dispersion/powder fluidization

8 Powder-technology-related properties of dust clouds influencing i their ignition iti sensitivity and burning rate (explosion violence) Distribution of primary particle sizes in dust/powder Particle shape characteristics Degree of agglomeration of dust particles, i.e. real `particle' size distribution in dust cloud Dust concentration distribution in cloud Degree of turbulence of cloud.

9 In some special situations such as in air jet mills, explosive dust clouds are generated in situ, i.e. the dust particles become suspended d in the air as they are produced

10 In most cases, explosive dust clouds are generated by re-entrainment and re-dispersion of powders and dusts that have been produced at an earlier stage and allowed to accumulate as layers or heaps

11 Re-suspension/re-dispersion of powder/dust occurs intentionally during pneumatic transport in pipes, by filling of silos and hoppers, in fluidized beds, in spray dryers and other types of dryers, in mixers and blenders and in screening machinery

12 Unintentional re-dispersion may be due to uncontrolled flow ( flooding ) in funnel- flow silos and hoppers, bursting of sacks and bags containing powder, or by sudden blasts of air generated by primary dust explosions elsewhere in the plant

13 Factors influencing the ignitability and explosibility of dust clouds in air (I) Chemical composition of the dust, including its moisture content Distributions of particle sizes and shapes in the dust, determining the specific surface area of the dust in the fully dispersed state

14 Materials that can give dust explosions Natural organic materials (grain, wood, linen, sugar, etc.) Synthetic organic materials (plastics, organic pigments, pesticides, pharmaceuticals etc.). Coal and peat Metals (aluminium, magnesium, titanium, zinc, iron, etc.)

15 Heats of combustion (oxidation) of various substances per mole O 2 consumed p 2

16 Influence of specific surface area of aluminium i powder of maximum rate of rise of explosion pressure during dust explosions in air in a standard 1 m3 closed bomb. From Bartknecht (1978)

17 Influence of mean particle diameter on minimum explosive dust concentration for three different dusts in the 20-litres USBM closed explosion bomb. From Hertzberg and Cashdollar (1987)

18 Minimum electric spark ignition energy (MIE) of clouds in air of three different powders, as functions of particle size. From Bartknecht (1987) Theoretical line for poly-ethylene from Kalkert and Schecker (1979)

19 Influence of chemistry (starch or protein) and specific surface area of natural organic materials on maximum rate of pressure rise in closed 1.2 liter Hartmann bomb. From Eckhoff (1977/1978)

20 Influence of chlorine in dust material molecule on maximum explosion pressure and maximum rate of pressure rise in 1 m 3 standard ISO vessel, for various particle sizes. From Bartknecht (1978)

21 Factors influencing the ignitability and explosibility of dust clouds in air (II) Degree of dispersion (or agglomeration) of dust particles, i.e. the effective specific surface area available to the combustion process in the dust cloud in the actual industrial i situation ti

22 Illustration of a perfectly dispersed dust cloud consisting of primary particles only, and a cloud consisting of agglomerates. From Eckhoff (2003)

23 One can define a theoretical dispersibility D max of a particular material as the mass that could ideally be dispersed into primary particles per unit of work, W, performed: D max = 1/W min But no realistic dispersion process can be onehundred per cent efficient. This may be accounted for by incorporating an efficiency factor, K: D real = K/ W min, 0 < K < 1

24 Inter-particle forces in powders include: van der Waals' forces Electrostatic forces Forces due to liquid bridges and capillary under-pressure

25 In the dispersion of cohesive powders composed of very small particles (<5-10 µm), inter-particle forces play a major role, and inter-particle bonds cannot be broken unless the particle agglomerates are exposed to very large shear forces.

26 This means that complete dispersion into primary particles is only possible in high-velocity flow fields,, or if the particles are exposed to high-velocity impacts

27 Cross section of nozzle for dispersing agglomerates of cohesive dust particles. From Yamamoto and Suganuma (1984)

28 Effective particle size distributions of an airborne talc dust after dispersal at different flow velocities through orifices. Rw is the percentage by weight of the effective particles that are larger than the size x. From Yamamoto and Suganuma (1984)

29 Max. rates of pressure rise versus dust concentration in dust explosions in a 1.2-litre closed bomb with clouds in air of maize starch containing different fractions of agglomerates. Eckhoff and Mathisen (1977/1978)

30 Factors influencing the ignitability and explosibility of dust clouds in air (III) Dust concentration

31 Range of explosive dust concentrations for maize starch, compared with typical range of concentrations relevant to industrial hygiene, and with a typical density of dust deposits/layers. From Eckhoff (2003)

32 Influence of dust concentration on explosion rate and ignition sensitivity of dust cloud

33 Influence of average dust concentration in a maize starch explosion in an experimental silo cell of volume 236 m 3, height 22 m and length-to-diameter 6, on the maximum explosion pressure generated in the silo. 5.7 m 2 vent opening in silo roof. Ignition close to silo bottom. From Eckhoff (2003)

34 Influence of average dust concentration on the minimum electric spark ignition energy (MIE) of clouds of an anti oxidant in air, in the standard 1 m3 closed vessel. From Bartknecht (1979)

35 Factors influencing the ignitability and explosibility of dust clouds in air (IV) Distribution ib ti of initial iti turbulence in the actual cloud Possibility of generation of explosion-induced induced turbulence in the still unburnt part of the cloud (location of ignition source important parameter)

36 Influence of initial turbulence on minimum electric spark ignition energy (MIE) of a dust cloud. Experiments with various dusts in a 20-litres closed explosion bomb. From Glarner (1984)

37 Influence of initial turbulence on explosion rate of a dust cloud. Experiments with 420 g/m 3 of lycopodium in air in a 1.2-litre closed explosion bomb. Bars: ±1 std. dev. From Eckhoff (1977)

38 Time of arrival of bituminous coal dust/air flame as a function of distance from ignition point at closed end of gallery of length 260 m and diameter 3.2 m. Pressure at closed end as a function of time. Nominal average dust concentration 500 g/m3 (From Fischer, 1957)

39 The role of powder science and technology in dust explosion prevention and control in practice

40 Inherently safe process design Avoidance of undesired particle segregation

41 Some segregation mechanisms Percolation of small particles through a bed of flarger ones by e.g. vibration Air current segregation Segregation due to differences in density, shape shape and surface properties of particles

42 Illustration of migration of a fraction of small particles through a bed of larger particles (percolation). From Eckhoff (1976a)

43 Inherently safe process design Example: Application of powder mechanics to silo design to avoid undesired segregation and dust cloud formation

44 Illustration ti of uncontrolled flow, with flooding, of bulk material in funnel flow causing generation of explosive dust clouds

45 Illustration of generation of smouldering nests in stagnant zones in funnel flow silos

46 Smooth controlled flow and re-mixing of segregated bulk material in mass flow silo during discharge of stored bulk material

47 Casting of MgFeSi alloys Old Odcasting method: Large solidification flakes Substantial segregation of Mg during solidification Material with high Mg content most brittle and created most fine dust Therefore: More very fine dust then necessary was produced More Mg in fine dust than average content Because MIE decreases with decreasing particle size and increasing Mg content, too much fine dust of very low MIE was generated

48 Casting of MgFeSi alloys New casting method: Small solidification flakes Minimal segregation, i.e. less fine dust and less Mg in finest dust Therefore: A better main product Considerably reduced dust explosion hazard

49 EXPLOSION PREVENTION PREVENTING EXPLOSIVE DUST CLOUDS Process design to prevent undesired generation of dust clouds and particle size segregation ( Inherent safety ) PREVENTING EXPLOSION MITIGATION IGNITION SOURCES Smouldering combustion in dust, dust fires Explosion-pressure resistant construction Intrinsic inerting of dust cloud by combustion gases Inerting of dust cloud by adding inert dust Other types of open flames (e.g. hot work) Hot surfaces (electrically or mechanically heated) Explosion isolation (sectioning) Explosion venting Keeping dust conc. outside explosive Heat from mechanical impact Automatic explosion suppression range (metal sparks and hot-spots) Inerting of dust clouds by N 2, CO 2 Electric sparks and Partial inerting of dust cloud and rare gases arcs and electrostatic discharges by inert gas Good housekeeping (dust removal/cleaning)

50 Preventing explosive dust clouds Inerting of dust cloud by adding inert dust (mixing/segregation, dispersion) Keeping dust conc. outside explosive range

51 Explosion mitigation

52 Explosion isolation (sectioning) i.e. closure of connections between process units in time to prevent passage of propagating dust flame. Depends on flame propagation speed, which in turn, for a given chemistry, depends on - particle size - degree of dust dispersion - dust concentration - dust cloud turbulence

53 Explosion isolation Illustration of an explosion isolation system at a transfer point between two horizontal conveyors, based on strong reinforced concrete walls and an explosion proof rotary lock that is interlocked with pressure sensors detecting any abnormal pressure rise on either side. From Bühler, Switzerland

54 Explosion venting

55 Explosion venting The maximum explosion pressure in a vented dust explosion, Pred, is a result of two competing processes, viz.: Burning of the dust cloud within the enclosure, which produces heat and hence increases the pressure Flow of unburnt, burning and burnt dust cloud out of the enclosure through the vent, which relieves the pressure

56 Typical vent panel that opens in a controlled way at a specified internal over-pressure

57 Explosion venting Influence of initial dust cloud turbulence on maximum pressure in vented maize starch explosions in a 64 m 3 chamber. Dust concentration 250 g/m 3. Vent size 5.6 m 2. From Tamanini (1989)

58 Explosion venting Results from vented maize starch and wheat grain dust explosions in a 500 m 3 silo cell. Comparison with P red /vent area correlations used in various countries. From Eckhoff (2003)

59 Explosion venting Results from vented maize starch explosions in a 20 m 3 silo cell, demonstration the marked influence of the mode of dust cloud generation on the maximum explosion pressure P red in the vented silo. From Eckhoff (2003)

60 Explosion venting Influence of location of the ignition point in a 236 m 3 slim silo cell on the maximum explosion pressure P red in the vented silo. From Eckhoff (1990)

61 Explosion venting: Fig 113 Quenching tube

62 Explosion venting: Fig 114 Quenching-tube

63 Explosion venting: Quenching tube

64 Explosion suppression

65 Fig 118 Explosion pressure sensor for suppression systems

66 Fig 119 Explosion suppression system

67 Fig 120 Large (45 l) explosion suppressor for powder suppressant

68 Explosion suppression Extinguishing agent permanently pressurized Large-diameter discharge orifice Very fast opening of valve for immediate release of extinguishing agent by means of an explosive charge

69 Explosion suppression Illustration of the mass of suppressant required and delivered as functions of time, for obtaining reliable, critical and failed suppression. From Moore (1987)

70 Explosion suppression Illustration of how failed suppression can result from too late of suppressant injection, too low injection rate, and too small quantity of suppressant injected. From Moore (1987)

71 Secondary dust explosions

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73 How thick does a dust layer have to be to produce hazardous volumes of explosive dust cloud?

74 Potential dust explosion hazard of even very thin layers of combustible dust

75 We must understand: Entrainment of dust particles from layers by air flows Movement of dust particles suspended in air flows

76 Concluding statement Hopefully this lecture has contributed to the appreciation of knowledge of powder science and technology as an essential premise for a genuine understanding of dust explosions