Dust explosions combustion process

NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, 1997 edition. 

A confined explosion occurs when there is a rapid combustion of a fuel and an oxidizer inside an enclosure (e.g., building, vessel, or duct), developing sufficient pressure to cause the enclosure to rupture. Examples of confined explosions include gas or dust explosions inside buildings, storage tanks, or process equipment. 

General References Crowl and Louvar, Chemical Process Safety Fundamentals with Applications, 2d ed., Prentice-Hall, Upper Saddle River, N.J., 2002, Chaps. 6 and 7. Crowl, Understanding Explosions, American Institute of Chemical Engineers, New York, 2003. Eckoff, Dust Explosions in the Process Industries, 2d ed., Butterworth-Heinemann, now Elsevier, Amsterdam, 1997. Kinney and Graham, Explosive Shocks in Air, 2d ed., Springer-Verlag, New York, 1985. Lewis and von Elbe, Combustion, Flames and Explosions of Gases, 3d ed., Academic Press, New York, 1987. Mannan, Lees Loss Prevention in the Process Industries, 3d ed., Elsevier, Amsterdam, 2005, Chap. 16 Fire, Chap. 17 Explosion. 

Long-term investigations of the processes of turbulent combustion in dust explosions have contributed to great progress achieved in this branch of science [1]. 

In dust explosions the combustion process is very rapid. The flame speed is high compared with that in gas deflagrations. Detonations normally do not occur in dust explosions in industrial plants. 

Most organic solids, metals, and some combustible inorganic salts can form explosive dust clouds. In order to have a dust explosion, certain elements are necessary (1) Particles of dust of suitable size, (2) A sufficient source of ignition energy, (3) A concentration of dust within explosive limits. If an explosive dust in air that meets the above criteria occurs in a process, an explosion should be considered inevitable. The process designer of inherently safer plants must take into account the possibility of dust explosions and design accordingly. 

In dust explosions the combustion process is very rapid. The flame speed is high compared to that in gas deflagrations. However, detonations normally do not occur in dust explosions in industrial plants. In a serious industrial dust explosion, two steps often occur. First, a primary explosion occurs in part of a plant, causing an air disturbance. Second, the air disturbance disperses dust and causes a secondary explosion. The secondary explosion is often more destructive than the primary explosion. There is a great deal of literature on dust explosions, which is available to the process designer. See, for example, Lees (1980) for a bibliography on dust explosions. 

Eckhoff, Dust Explosions in the Process Industries, Butterworth-Heinemann, London 1991. Health, Safety and Loss Prevention in the Oil, Chemical and Process Industries, Butterworth-Heinemann, Singapore, 1993. NFPA 69, Standard on Explosion Prevention Systems, 1992. VDI-Beport 975, Safe Handling of Combustible Dust, VDI-Verlag GmbH, Diisseldorf, 1992. VDI-Guideline 2263, Dust Fires and Dust Explosions, Beuth Verlag, Berhn, 1992. 

NFPA 654 (2006) Standard for the prevention of fire and dust explosions from the manufacturing, processing and handling of combustible particulate solids. (National Fire Protection Association). 

Statistics clearly show that fire/explosion hazards could exist in any plant/equipment that handles or processes a combustible dust. The consequences of a dust explosion can range from disruption of production to loss of plant and injury or fatality of personnel. 

Dust must be airborne If a layer of dust on a surface is ignited, the burning process will be relatively slow, releasing heat over a long period of time. However, if a sufficient concentration of finely divided dust particles are suspended in air and ignited, because of the availability of oxygen around each particle, the combustion process will be very rapid giving rise to a dust explosion. 

Dust must have particle size distribution capable of propagating flame The sensitivity of a dust cloud to ignition and the resulting explosion violence (severity) increase with a decrease in particle size. This is because the combustion process involves chemical reaction at the solid-oxidant interface. Therefore, as the dust particles get finer the total surface area that is available for oxidation will increase. It should be noted that, in practice, very often dust clouds are made up of particles with sizes ranging from fine to coarse. As the fines become airborne more readily, they play a more prominent role in the initial ignition and explosion propagation. 

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