A dust explosion can be deleterious if they occur. It is this factor that makes it important to study this phenomenon. A dust explosion can be defined as volatile combustion of dust, which is often suspended in air within an enclosure resulting to destructive effects due to thermal radiation, overpressure, and projectiles. It is not often easy to avoid such occurrences. For instance, processes that handle combustible materials are important for various purposes, but the danger of a dust explosion is apparent (Barton, p. 1). It is, therefore, vital to understand the causes of dust explosion, the conditions under which they occur, their mechanism, protective measures against them, and solutions to dust explosions.
Causes
There are several sources of dust explosions. These include combustible solid materials such as coal, flour, wood, sawdust, starch, rubber, pollen, sugar, plastics, pharmaceutical and some metals (Barton, p. 1). Powdered metals such as aluminium, magnesium, and titanium are likely to explode if they form suspension in the air.
The dust may come from activities like grain transportation; certainly grain silos experience dust explosion. Moreover, activities like coal mining generate coal dust, and saw mills and some carpentry activities result to saw dust. Milling in flourmills also generates huge amounts of dust. Other potential causes of dust explosion include thermobaric weapons, whose risk depends on their fuel. Even with these sources, there are certain conditions that must be satisfied for a dust explosion to occur.
Conditions
There are basically five conditions that are necessary for a dust explosion to occur. These include a dust that is combustible, the dust should be suspended in air and at appropriate concentration, an oxidant, a source of ignition, and it should be in confinement. All these conditions must be available simultaneously for deflagration or dust explosion to occur. Note, however, that deflagration is possible even without confinement. Deflagration refers to the process that results to the explosion.
Deflagration is the “propagation of a combustion zone at a velocity that is less than the speed of sound in the un-reacted medium (Cholin p. 5). This process normally results in huge rise in pressure if the explosion occurs in a confinement. Only the four conditions mentioned above that are necessary for its occurrence. The combustible material must be in form of solid particles that are adequately small for the process to continue. Furthermore, it must be suspended in the oxidizing medium, which is often the air, and in adequately high concentration. Just as the explosion is caused, an igniter must be applied.
Although the definition of combustible particulate solid is not available in “model building codes”, (Cholin, p. 18) it is necessary to understand the characteristics of combustible solid particle required for dust explosion and deflagration. In general, dusts are known as solid particles with the size of about 420 microns or smaller (Cholin, p.20). However, fibres or flakes larger than this value still pose the threat of a dust explosion and deflagration. In fact, 420 micron is basically larger than the particle of ordinary table salt. The rate of the dust combustion is dependent on the sizes of these particles among other factors. The smaller the sizes of these particles the more they are combustible or hazardous. Rod-shaped or flat flake particles give more surface are for the burning process hence accelerating it. It is worth to note that seldom are all the dust particles uniform. Therefore, it is the average size of these particles that is considered in determining suspended dust is sufficient enough to promulgate a flame front. Agglomeration of the particles, which normally occurs due to humidity, may be more hazardous (Cholin, p. 26).
Suspension of the particles enables them to spread into larger surface area for faster rate of combustion to occur. The small the particulate solids the more easily they are suspended. The suspension, further, enables a concentration gradient to develop.
A minimum concentration of dust is necessary for propagation of flame front to be necessary. It is dependent on the chemical composition and the size of the particulate solid, and is given as the minimum explosible concentration (MEC) or lower explosive limit (LEL); typically, it is in the range of between 10 gm-3 to 100gm-3. Below this limit, it is unlikely for a dust explosion to occur; and similarly, above this limit the condition is considered sufficient for combustion to occur. Increase in concentration of the dust increases the violence of explosion. However, at a certain point of optimum concentration, further concentration yields no difference in the explosion violence. Barton asserts that this concentration is often above the amount that is “theoretically” required to react with available oxygen” (p. 2). At the upper explosion limit, it is not possible to propagate, and this limit is usually not very distinctively defined.
Ignition becomes possible when adequate energy is supplied to the combustible particulate matter suspended in consideration of the unit volume and time. Ignition starts when temperature, which is described as energy per unit mass (Cholin, P. 46), reaches a certain pint known as the auto-ignition.
Mechanism
The stoichiometry of the concentration of the combustible particulate solid is vital is vital in understanding the nature of combustion of various dusts. Different combustible solid particles exhibit different temperature of combustion, and they will either elevate or suppress this temperature. Adequate energy, may come from either electrical or thermal sources, is necessary to initiate a combustion. As a result of the small volume to surface area ratio, the flame is likely to propagate very rapidly as will its flame front. Furthermore, pressure builds up due to the thermal effect of the gas. In a confined enclosure, the pressure amount to a point known as overpressure making the explosion more violent.
Protection and solutions
It is vital to take precautionary measures to mitigate the occurrence of dust explosions. Various industry or sources of dust explosion need various alternatives measures for prevention of this occurrence. In the mining of coal, for instance, it is helpful to spread dust along the mine roadways as well as hanged in trays from the roof. This helps to dilute the coal dust to a point combustion is impossible. Moreover, coal mines can be sprayed with water to prevent ignition. Some mitigation measures involve inerting, where air is excluded out of the processing process.
Other protection measures include the following: reducing the concentration of oxidant, venting and suppression of the deflagration, and the containment of the deflagration pressure.
In the event that a dust explosion occurs even after implementing and observing various protective measures meant to prevent its occurrence, it is also vital to take measures to mitigate it effect. Measure should be put in place to limit destruction of properties and injuries death to people occupying, for instance, a building. These include measures such enhanced integrity of buildings, provision of emergency routes among others.
References
Barton, John, Dust explosion prevention and protection: a practical guide, Institution of Chemical Engineers, Great Britain, 2002.
Cholin, John M. Explosions 101, University of Maryland, Oakland New Jersey, 2007.
Gagnon, M Design of fire alarm systems, 2nd edition.