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A dust explosion is the rapid combustion of fine particles suspended in the air, often but not always in 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 oxygen.

Dust explosions are a frequent hazard in underground coal mines, in grain elevators, and other industrial environments. Dust explosions 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.

Terminology

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 inside the workplace. Historically, fatalities from dust explosions have largely been the result of secondary dust explosions.

Conditions required

There are four necessary conditions for a dust explosion:

• A combustible dust
• The dust is suspended in the air at a sufficiently high concentration
• There is an oxidant (typically atmospheric oxygen)
• There is an ignition source

If the rapid combustion occurs in a confined space, enormous overpressures can build up, causing major structural damage and flying shrapnel. The sudden release of energy from a "detonation" can produce a shock wave, 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". In some traditional analyses of dust explosions, confinement is considered to be a fifth requirement; this is not an essential condition, but can greatly aggravate the physical damage that results.

Sources of dust

Many materials which are commonly known to oxidize can generate a dust explosion, such as coal, sawdust, and magnesium. However, many otherwise mundane organic materials can also be dispersed into a dangerous dust cloud, such as grain, flour, sugar, powdered milk, cocoa, coffee, and pollen. Many powdered metals (such as aluminum, magnesium, and titanium) can form explosive suspensions in air.

The dust can arise from activities such as transporting grain, and grain silos have often exploded. 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 14 workers at the Washburn A Mill, and another 4 in adjacent buildings. A similar problem occurs in sawmills and other places dedicated to woodworking. Thermobaric weapons, depending upon their fuel, are also a potential and intentional source of dust.

Although not strictly a dust, paper particles emitted during processing - especially rolling, unrolling, calendaring/slitting, and sheet-cutting - are another known 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.

To support combustion, the dust must also 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.

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

Sources of ignition

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

• electrostatic discharge
• friction
• arcing from machinery or other equipment;
• hot surfaces, including e.g. overheated bearings
• fire

However it is often difficult to determine the exact source of ignition post-explosion. When a source cannot be found, it will often be cited as static electricity. Static charges can occur by friction at the surfaces of particles as they move against one another, and build up to levels leading to a sudden discharge to earth.

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 combustible concentration at or below 25% of the LEL is considered safe. Similarly, if the fuel/air ratio increases above the upper explosive limit (UEL), there is insufficient oxidant to permit combustion to continue at the necessary rate.

Mechanism

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 1g/cm 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 50µm 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 burning (such as wood), can produce a powerful explosion when finely divided, and can be ignited by even a small spark.

• Demonstration of an open-air dust explosion
• Experimental setup
• 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 skin burns in unprotected persons. In a tightly enclosed space, the sudden depletion of oxygen can cause asphyxiation.

Protection and mitigation

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, a methane explosion can initiate a coal dust explosion, which can then engulf an entire working pit. Incombustible stone dust may be spread along mine roadways, or suspended from trays in the roof, so as to dilute the coal dust raised ahead of the combustion zone by a shockwave, to the point where it cannot burn. Mines may also be sprayed with water to inhibit ignition.

Some industries exclude oxygen from dust-raising processes, a precaution known as "inerting". Typically this uses nitrogen, carbon dioxide, or argon, which are incombustible gases and inhibit combustion. The same method is also used in large storage tanks where flammable vapors can accumulate. However, use of oxygen-free gases brings a risk of asphyxiation of the workers. Workers who need illumination in enclosed spaces where a dust explosion is a high risk often use lamps designed for divers, as they have no risk of producing an open spark due to their waterproof design.

Good housekeeping practices, such as eliminating build-up of combustible dust deposits that could be disturbed and lead to a secondary explosion, also help mitigate the problem.

Best engineering control measures which can be found in the National Fire Protection Association (NFPA) Combustible Dust Standards include:

• Wetting
• Oxidant concentration reduction
• Deflagration venting
• Deflagration pressure containment
• Deflagration suppression
• Deflagration venting through a dust retention and flame-arresting device

Accidents

• May 2, 1878: A grain dust explosion in the Washburn "A" Mill in Minneapolis, Minnesota kills 22 people, destroying the largest grain mill in the world and leveling five other mills, effectively reducing the milling capacity of the city by one-third to one-half. The disaster prompts mills throughout the country to install better ventilation systems to prevent dust build-up.
• September 19, 1921: The Mount Mulligan mine disaster occurred in Mount Mulligan, Far North Queensland, Australia. A series of coal dust explosions in the local coal mine, audible as much as 30 kilometres (19 mi) away, rocked the close-knit township.
• April 26, 1942: A coal and gas explosion at the Benxihu Colliery, kills 1,549 Chinese miners, 34% of the miners working that day, making it the worst coal mining accident in history.
• 1977: Westwego, Louisiana grain elevator explosion
• March 15, 1987: Harbin Linen Textile Plant, People's Republic of China, kills 58 and 177 were injured
• January 29, 2003: The West Pharmaceutical Services plant in Kinston, North Carolina, suffers a rubber dust explosion.
• February 7, 2008: Combustible sugar dust fires and explosions at the Imperial Sugar facility in Port Wentworth, Georgia kill fourteen.
• June 27, 2015: Formosa Fun Coast explosion: about 500 people injured when Holi-like colored powder causes a dust explosion at an outdoor music and color festival in Formosa Fun Coast, New Taipei, Taiwan.

See also

• Air to fuel ratio
• Coal dust
• Combustion
• Explosion vent
• Flour bomb

References

2. http://www.mnopedia.org/event/1878-washburn-mill-explosion
3. ^ !StaubEx_0805_e.pdf
4. NFPA 69 8.3.1

• John Barton (ed.): Dust Explosion Prevention and Protection (A Practical Guide) Institution of Chemical Engineers, Rugby 2002, ISBN 0-85295-410-7.
• Rolf K. Eckhoff: Dust Explosions in the Process Industries, 2nd ed. Butterworth-Heinemann, Oxford 1997, ISBN 0-7506-3270-4.

External links

For stories about incidents in France and the USA see

• Combustible dust explosion investigation products from the Chemical Safety Board
• Combustible Dust Policy Institute-ATEX
• OSHA case studies of dust explosions

For information on how to protect a process plants and grain handling facilities from the risk of dust hazard explosions, see

• Dust Explosions - The Dangers, and The Precautions Needed To Avoid Them
• Hazard Monitoring Equipment - Selection, Installation and Maintenance
• Seminars for Combustible Dust Safety
• FAQs: Solutions for Minimizing Dust Explosion Risks

• Chemical processes
• Explosions
• Hazards
• Industrial fires and explosions