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A dust explosion is the rapid combustion of fine particles suspended in the air within an enclosed location. Dust explosions can occur where any dispersed powdered combustible material is present in high-enough concentrations in the atmosphere or other oxidizing gaseous medium, such as pure oxygen. In cases when fuel plays the role of a combustible material, the explosion is known as a fuel-air explosion.

Dust explosions are a frequent hazard in coal mines, grain elevators, and other industrial environments. They are also commonly used by special effects artists, filmmakers, and pyrotechnicians, given their spectacular appearance and ability to be safely contained under certain carefully controlled conditions.

Thermobaric weapons utilize this principle by rapidly saturating an area with an easily combustible material and then igniting it to produce explosive force. These weapons are the most powerful non-nuclear weapons in the world.[1]

Terminology

If rapid combustion occurs in a confined space, enormous overpressures can build up, causing major structural damage and flying debris. The sudden release of energy from a "detonation" can produce a shockwave, either in open air or in a confined space. If the spread of flame is at subsonic speed, the phenomenon is sometimes called a "deflagration", although looser usage calls both phenomena "explosions".

Dust explosions may be classified as being either "primary" or "secondary" in nature. Primary dust explosions may occur inside process equipment or similar enclosures, and are generally controlled by pressure relief through purpose-built ducting to the external atmosphere. Secondary dust explosions are the result of dust accumulation inside a building being disturbed and ignited by the primary explosion, resulting in a much more dangerous uncontrolled explosion that can affect the entire structure. Historically, fatalities from dust explosions have largely been the result of secondary dust explosions.[2]

Conditions required

Diagram showing the five requirements for a dust explosion

There are five necessary conditions for a dust explosion:[3]

  • A combustible dust
  • The dust is dispersed in the air at a sufficiently high concentration
  • There is an oxidant (typically atmospheric oxygen)
  • There is an ignition source
  • The area is confined—a building can be an enclosure

Sources of dust

Diagram showing the five requirements for a dust explosion

There are five necessary conditions for a dust explosion:[3]

  • A combustible dust
  • The dust is dispersed in the air at a sufficiently high concentration
  • There is an oxidant (typically atmospheric oxygen)
  • There is an ignition source
  • The area is confined—a building can be an enclosure

pressure relief through purpose-built ducting to the external atmosphere. Secondary dust explosions are the result of dust accumulation inside a building being disturbed and ignited by the primary explosion, resulting in a much more dangerous uncontrolled explosion that can affect the entire structure. Historically, fatalities from dust explosions have largely been the result of secondary dust explosions.[2]

There are five necessary conditions for a dust explosion:[3]

  • A combustible dust
  • The dust is dispersed in the air at a sufficiently high concentration
  • There is an oxidant (typically atmospheric oxygen)
  • There is an ignition source
  • The area is confined—a building can be an enclosure

Sources of dust

coal and sawdust. In addition, many otherwise mundane organic materials can also be dispersed into a dangerous dust cloud, such as grain, flour, starch, sugar, powdered milk, cocoa, coffee, and pollen. Powdered metals (such as aluminum, magnesium, and titanium) can form explosive suspensions in air, if finely divided.

Explosive dust can arise from activities such as transporting grain, and grain silos have often been demolished violently. Mining of coal leads to coal dust, and flour mills likewise have large amounts of flour dust as a result of milling. A gigantic explosion of flour dust destroyed a mill in Minnesota on May 2, 1878, killing 18 workers at the Washburn A Mill and another four in adjacent buildings.[4] A similar problem occurs in sawmills and other places dedicated to woodworking.

Since the advent of industrial production–scale metal powder–based additive manufacturing (AM) in the 2010s, there is growing need for more information and experience with preventing dust explosions and fires from the traces of excess metal powder sometimes left over after laser sintering or other fusion methods.[5] For example, in machining operations downstream of the AM build, excess powder liberated from porosities in the support structures can be exposed to sparks from the cutting interface.[5] Efforts are underway not only to build this knowledgebase within the industry but also to share it with local fire departments, who do periodic fire-safety inspections of businesses in their districts and who can expect to answer alarms at shops or plants where AM is now part of the production mix.[5]

Although not strictly a dust, paper particles emitted during processing - especially rolling, unrolling, calendaring/slitting, and sheet-cutting - are also known to pose an explosion hazard. Enclosed paper mill areas subject to such dangers commonly maintain very high air humidities to reduce the chance of airborne paper dust explosions.

In special effects pyrotechnics, lycopodium powder[2] and non-dairy creamer[6] are two common means of producing safe, controlled fire effects.

To support rapid combustion, the dust must consist of very small particles with a high surface area to volume ratio, thereby making the collective or combined surface area of all the particles very large in comparison t

Explosive dust can arise from activities such as transporting grain, and grain silos have often been demolished violently. Mining of coal leads to coal dust, and flour mills likewise have large amounts of flour dust as a result of milling. A gigantic explosion of flour dust destroyed a mill in Minnesota on May 2, 1878, killing 18 workers at the Washburn A Mill and another four in adjacent buildings.[4] A similar problem occurs in sawmills and other places dedicated to woodworking.

Since the advent of industrial production–scale metal powder–based additive manufacturing (AM) in the 2010s, there is growing need for more information and experience with preventing dust explosions and fires from the traces of excess metal powder sometimes left over after laser sintering or other fusion methods.[5] For example, in machining operations downstream of the AM build, excess powder liberated from porosities in the support structures can be exposed to sparks from the cutting interface.[5] Efforts are underway not only to build this knowledgebase within the industry but also to share it with local fire departments, who do periodic fire-safety inspections of businesses in their districts and who can expect to answer alarms at shops or plants where AM is now part of the production mix.[5]

Although not strictly a dust, paper particles emitted during processing - especially rolling, unrolling, calendaring/slitting, and sheet-cutting - are also known to pose an explosion hazard. Enclosed paper mill areas subject to such dangers commonly maintain very high air humidities to reduce the chance of airborne paper dust explosions.

In special effects pyrotechnics, lycopodium powder[2] and non-dairy creamer[6] are two common means of producing safe, controlled fire effects.

To support rapid combustion, the dust must consist of very small particles with a high surface area to volume ratio, thereby making the collective or combined surface area of all the particles very large in comparison to a dust of larger particles. Dust is defined as powders 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 total surface area of all the particles.

Below a certain value, the lower explosive limit (LEL), there is insufficient dust to support the combustion at the rate required for an explosion.[7] A combustible concentration at or below 25% of the LEL is considered safe.[8] Similarly, if the fuel to air ratio increases above the upper explosive limit (UEL), there is insufficient oxidant to permit combustion to continue at the necessary rate.

Determining the minimum explosive concentration or maximum explosive concentration of dusts in air is difficult, and consulting different sources can lead to quite different results. Typical explosive ranges in air are from few dozens grams/m3 for the minimum limit, to few kg/m3 for the maximum

Determining the minimum explosive concentration or maximum explosive concentration of dusts in air is difficult, and consulting different sources can lead to quite different results. Typical explosive ranges in air are from few dozens grams/m3 for the minimum limit, to few kg/m3 for the maximum limit. For example, the LEL for sawdust has been determined to be between 40 and 50 grams/m3.[9] It depends on many factors including the type of material used.

Typically, normal atmospheric oxygen can be sufficient to support a dust explosion if the other necessary conditions are also present. High-oxygen or pure oxygen environments are considered to be especially hazardous, as are strong oxidizing gases such as chlorine and fluorine. Also, particulate suspensions of compounds with a high oxidative potential, such as peroxides, chlorates, nitrates, perchlorates, and dichromates, can increase risk of an explosion if combustible materials are also present.

Sources of ignition

Dusts have a very large surface area compared to their mass. Since burning can only occur at the surface of a solid or liquid, where it can react with oxygen, this causes dusts to be much more flammable than bulk materials. For example, a 1 kilogram (2.2 lb) sphere of a combustible material with a density of 1 g/cm3 would be about 12.4 centimetres (4.9 in) in diameter, and have a surface area of 0.048 square m

Dusts have a very large surface area compared to their mass. Since burning can only occur at the surface of a solid or liquid, where it can react with oxygen, this causes dusts to be much more flammable than bulk materials. For example, a 1 kilogram (2.2 lb) sphere of a combustible material with a density of 1 g/cm3 would be about 12.4 centimetres (4.9 in) in diameter, and have a surface area of 0.048 square metres (0.52 sq ft). However, if it were broken up into spherical dust particles 50 µm in diameter (about the size of flour particles) it would have a surface area of 120 square metres (1,300 sq ft). This greatly-increased surface area allows the material to burn much faster, and the extremely small mass of each particle allows them to catch on fire with much less energy than the bulk material, as there is no heat loss to conduction within the material.

When this mixture of fuel and air is ignited, especially in a confined space such as a warehouse or silo, a significant increase in pressure is created, often more than sufficient to demolish the structure. Even materials that are traditionally thought of as nonflammable (such as aluminum), or slow burni

When this mixture of fuel and air is ignited, especially in a confined space such as a warehouse or silo, a significant increase in pressure is created, often more than sufficient to demolish the structure. Even materials that are traditionally thought of as nonflammable (such as aluminum), or slow burning (such as wood), can produce a powerful explosion when finely divided, and can be ignited by even a small spark.

Experimental setup

  • Finely-ground flour is dispersed

    Finely-ground flour is dispersed

  • Cloud of flour is ignited

  • Fireball spreads rapidly

  • Intense radiant heat has nothing to ignite here

  • Fireball and superheated gases rise

  • Aftermath of explosion, with unburned flour on the ground

  • Effects

    A dust explosion can cause major damage to structures, equipment, and personnel from violent overpressure or shockwave effects. Flying objects and debris can cause further damage. Intense radiant heat from a fireball can ignite the surroundings, or cause severe radiant heat from a fireball can ignite the surroundings, or cause severe skin burns in unprotected persons. In a tightly enclosed space, the sudden depletion of oxygen can cause asphyxiation. Where the dust is carbon based (such as in a coal mine), incomplete combustion may cause large amounts of carbon monoxide (the miners' after-damp) to be created. This can cause more deaths than the original explosion as well as hindering rescue attempts.[10][11]

    Protection and mitigation

    Protecting process plant and grain handling facilities from the risk of dust hazard explosions: