Dust explosion


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.

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.

Conditions required

There are five necessary conditions for a dust explosion:
Below a certain value, the lower explosive limit, there is 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 to air ratio increases above the upper explosive limit, 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 limit. For example, the LEL for sawdust has been determined to be between 40 and 50 grams/m3. It depends on many factors including the type of material used.

Oxidant

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.

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 sphere of a combustible material with a density of 1 g/cm3 would be about in diameter, and have a surface area of. However, if it were broken up into spherical dust particles 50 µm in diameter it would have a surface area of. 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, or slow burning, can produce a powerful explosion when finely divided, and can be ignited by even a small spark.

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. Where the dust is carbon based, incomplete combustion may cause large amounts of carbon monoxide to be created. This can cause more deaths than the original explosion as well as hindering rescue attempts.

Protection and mitigation

Much research has been carried out in Europe and elsewhere to understand how to control these dangers, but dust 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 mine pit. As a precaution, incombustible stone dust may be spread along mine roadways, or stored in trays hanging from the roof, to dilute the coal dust stirred up 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 which can displace oxygen. 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 underwater divers, as they have no risk of producing an open spark due to their sealed 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 Combustible Dust Standards include:
Dust clouds are a common source of explosions, causing an estimated 2,000 explosions annually in Europe. The table lists notable incidents worldwide.
EventDateLocationCountrySource materialFatalitiesInjuriesNotes
Washburn "A" Mill explosionMinneapolis, MinnesotaUSAgrain dust22Destroyed the largest grain mill in the world and leveled five other mills, effectively reducing the milling capacity of Minneapolis by one-third to one-half. Prompted mills throughout the country to install better ventilation systems to prevent dust build-up.
Milwaukee Works explosionMilwaukee, WisconsinUSAFeed grinding plant34The blast was felt for miles around and completely leveled the plant owned by the company.
Douglas Starch Works explosionCedar Rapids, IowaUSAcorn starch4330The blast was felt for miles around and completely leveled the plant owned by the company.
Port Colborne explosionPort ColborneCanadagrain1016Blast also destroyed the steamer Quebec, which was near the grain elevator
Large terminal grain elevator in Kansas CityKansas City, MissouriUSA1410Originated in basement of elevator, during a cleanup period, and travelled up through the elevator shaft
Mount Mulligan mine disasterMount Mulligan, QueenslandAustraliacoal dust75The series of coal dust explosions within a mine rocked the close-knit township and was audible as far as away.
Benxihu Colliery explosionBenxi, LiaoningManchukuo coal dust and gas1,54934% of the miners working that day were killed. This is the world's worst-ever coal-mining accident.
Westwego grain elevator explosionWestwego, LouisianaUSAgrain dust3613
Galveston grain elevator explosionGalveston, TexasUSAgrain dust20
Bird's Custard factory explosionBanburyUKcorn starch9
Metz malt factory explosionMetzFrancebarley dust121
Harbin textile factory explosionHarbinChinaflax dust58177
Blaye grain explosionBlayeFrancegrain dust111Explosion in a grain storage facility at the Société d’Exploitation Maritime Blayaise killed 11 people in nearby offices and injured one.
West Pharmaceutical Services explosionKinston, North CarolinaUSApolyethylene dust638
Imperial Sugar explosionPort Wentworth, GeorgiaUSAsugar dust1442
2014 Kunshan explosionKunshanChinametal powder146114
Formosa Fun Coast explosionNew TaipeiTaiwancolored starch powder15498Explosion when Holi-like colored powder was released at an outdoor music and color festival at the Formosa Fun Coast.
Bosley wood flour mill explosionBosley, CheshireUKwood flour44