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A dust explosion is the explosive combustion
Combustion

Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering....
 of a dust suspended in air in an enclosed location, which results in harmful effects of overpressure, thermal radiation, and ensuing projectiles.

e are five necessary conditions for a dust
Dust

Dust is a general name for minute solid particles with diameters less than 20 Thou . Particles in the Earth's atmosphere arise from various sources such as soil dust lifted up by wind, volcanic eruptions, and pollution....
 explosion:
  1. A combustible dust;
  2. The dust is suspended in the air at a proper concentration
    Concentration

    In chemistry, concentration is the measure of how much of a given chemical substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent....
    ;
  3. There is an oxidant (typically atmospheric oxygen);
  4. The dust is confined;
  5. There is an ignition source.


If any of these five conditions is missing there can be no dust explosion or deflagration
Deflagration

Deflagration is a technical term describing subsonic combustion that usually propagates through thermal conductivity . Most "fire" found in daily life, from flames to explosions, is technically deflagration....
, with the exception of #4 if there is no confinement a deflagration can still occur.

materials which are commonly known to combust can generate a dust explosion, such as coal
Coal

Coal is a readily combustion black or brownish-black sedimentary rock. The harder forms, such as anthracite, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure....
, sawdust
Sawdust

File:Saw dust .jpgSawdust is composed of fine particles of wood. This material is produced from cutting with a saw, hence its name. It has a variety of practical uses, including serving as a mulch, or as an alternative to clay cat litter, or as a fuel, or for the manufacture of particleboard....
, and magnesium
Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, atomic weight 24.3050 and common oxidation number +2.Magnesium, an alkaline earth metal, is the ninth most abundance of the chemical elements in the universe by mass....
.






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Encyclopedia


A dust explosion is the explosive combustion
Combustion

Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering....
 of a dust suspended in air in an enclosed location, which results in harmful effects of overpressure, thermal radiation, and ensuing projectiles.

Conditions for dust explosion

There are five necessary conditions for a dust
Dust

Dust is a general name for minute solid particles with diameters less than 20 Thou . Particles in the Earth's atmosphere arise from various sources such as soil dust lifted up by wind, volcanic eruptions, and pollution....
 explosion:
  1. A combustible dust;
  2. The dust is suspended in the air at a proper concentration
    Concentration

    In chemistry, concentration is the measure of how much of a given chemical substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent....
    ;
  3. There is an oxidant (typically atmospheric oxygen);
  4. The dust is confined;
  5. There is an ignition source.


If any of these five conditions is missing there can be no dust explosion or deflagration
Deflagration

Deflagration is a technical term describing subsonic combustion that usually propagates through thermal conductivity . Most "fire" found in daily life, from flames to explosions, is technically deflagration....
, with the exception of #4 if there is no confinement a deflagration can still occur.

Sources of dust

Many materials which are commonly known to combust can generate a dust explosion, such as coal
Coal

Coal is a readily combustion black or brownish-black sedimentary rock. The harder forms, such as anthracite, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure....
, sawdust
Sawdust

File:Saw dust .jpgSawdust is composed of fine particles of wood. This material is produced from cutting with a saw, hence its name. It has a variety of practical uses, including serving as a mulch, or as an alternative to clay cat litter, or as a fuel, or for the manufacture of particleboard....
, and magnesium
Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, atomic weight 24.3050 and common oxidation number +2.Magnesium, an alkaline earth metal, is the ninth most abundance of the chemical elements in the universe by mass....
. However, many otherwise mundane materials can also lead to a dangerous dust cloud such as grain
GRAIN

GRAIN is an international non-governmental organization based in Barcelona, Spain, which works toward sustainable agriculture. It was formed upon the realization that the genetic diversity of the world's food crops are being drastically eliminated....
, flour
Flour

Flour is a powder made of cereal grains. It is the main ingredient of bread, which is a staple food for many civilizations, making the availability of adequate supplies of flour a major economic and political issue at various times throughout history....
, sugar
Sugar

Sugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many other sources....
, powdered milk
Powdered milk

Powdered milk is a manufactured dairy product made by evaporating milk to Drying . One purpose of drying milk is to preserve it; milk powder has a far longer shelf life than liquid milk and does not need to be refrigeration, due to its low moisture content....
 and pollen
Pollen

Pollen is a fine to coarse powder consisting of Gametophyte , which produce the male gametes of spermatophyta. A hard coat covering the pollen grain protects the sperm cells during the process of their movement between the stamens of the flower to the pistil of the next flower....
. Many powdered metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
s (like aluminium
Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
 and titanium
Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the ?space age metal?, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver colour....
), can form explosive suspensions in air.

The dust can arise from activities such as transporting grain and indeed grain silos do regularly have explosions. Mining of coal leads to coal dust
Coal dust

Coal dust is a fine Powder form of coal, which is created by the crushing, grinding, or pulverizing of coal. Because of the brittle nature of coal, coal dust can be created during mining, transportation, or by mechanically handling coal....
 and flour mills likewise have large amounts of flour dust as a result of milling. A similar problem occurs in saw mills and other places dedicated to carpentry. Thermobaric weapon
Thermobaric weapon

Thermobaric weapons distinguish themselves from conventional explosive weapons by using atmospheric oxygen, instead of carrying an oxidizer in their explosives....
s, depending upon their fuel, are also a potential and intentional source of dust.

The dust must also consist of very small particles, where the surface area is very large, and so will support combustion
Combustion

Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering....
. Dust
Dust

Dust is a general name for minute solid particles with diameters less than 20 Thou . Particles in the Earth's atmosphere arise from various sources such as soil dust lifted up by wind, volcanic eruptions, and pollution....
 is defined as powders
Powder (substance)

A powder is a dry, bulk solid composed of a large number of very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms powder and granular are sometimes used to distinguish separate classes of material....
 with particles less than about 500 micrometres in diameter, but finer dust will present a much greater hazard than coarse particles by virtue of the larger surface area.

Sources of ignition

There are many sources of ignition and a naked flame need not be one: over one-half of the dust explosions in Germany in 2005 were from non-flame sources. Common sources of ignition include:
  • electrostatic discharge
    Electrostatic discharge

    Electrostatic discharge is the sudden and momentary electric current that flows between two objects at different electrical potentials caused by direct contact or induced by an electrostatic field....
    ;
  • friction
    Friction

    File:Friction alt.svgFriction is the force resisting the relative lateral motion of solid surfaces, fluid layers, or material elements in contact....
    ;
  • sparks from machinery;
  • sparks from electrical equipment;
  • hot surfaces;
  • fire.
However it is always very hard to determine the exact source of the ignition, post explosion. So when a source cannot be found, then there is a tendency to cite the cause as being static electricity.

Concentration

Below a certain value, the lower explosive limit (LEL), there is simply insufficient dust to support the combustion at the rate required for an explosion. A figure 20% lower than the LEL is considered safe. Similarly, if the fuel/air ratio increases above the upper explosive limit there is insufficient oxidant to permit combustion to continue at the necessary rate.

Mechanism of dust explosions

Different dusts will have different combustion temperatures and dust of various types will either suppress or elevate this temperature in relation to the stoichiometric concentration of the dusts. It is necessary that sufficient energy, generally either thermal or electrical, be applied to trigger combustion. Due to the small volume in relation to the large surface area, combustion can then proceed very rapidly and the flame front can also travel quickly. For example, 1 kg of powder, 120 micrometres in diameter will have a surface area of 50 sq/m or 540 sq/ft. Due to the thermal expansion of the gas, the pressure increases. In an enclosed space this leads to the condition called overpressure
Overpressure

Overpressure can mean:* In geology: the pressure regime in a Stratigraphy that exhibits higher-than-hydrostatic pressure in its porosity structure....
.

Protection and Mitigation from dust explosions

Much research has been carried out in Europe and elsewhere to understand how to control these dangers, but explosions still occur. The alternatives for making processes and plants safer depend on the industry. In the coal mining industry, stone dust is spread along mine roadways, or suspended from trays in the roof, so as to dilute the coal dust raised ahead of the combustion
Combustion

Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering....
 zone by the shock wave
Shock wave

A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field....
, to the point where it cannot burn. Mines may also be sprayed with water to inhibit ignition. Some industries exclude air from the process, known as inerting. Typically this uses nitrogen or carbon dioxide, and if this is done properly nothing can burn.

Other best engineering control measures which can be found in the National Fire Protection Association
National Fire Protection Association

The National Fire Protection Association is a United States organization charged with creating and maintaining minimum standards and requirements for fire prevention and suppression activities, training, and equipment, as well as other life-safety codes and standards....
 (NFPA) Combustible Dust Standards include:
  • Oxidant Concentration Reduction
  • Deflagration venting
  • Deflagration pressure containment
  • Deflagration suppression
  • Deflagration venting through a dust retention and flame-arresting device


See also

  • Air to fuel ratio
  • Flour bomb
    Flour bomb

    There are two types of flour bomb:* The relatively innocuous use of flour in a fragile container, thrown at a person or object to produce an inconvenient stain....


External links

For stories about incidents in France and the USA see
  • various completed investigations


Preventing and Mitigating Combustible Dust Explosions