Combustible dust

A model for theoretical investigation of turbulent mixing and combustion of polydispersed mixtures in confined volumes is developed. The numerical model and the created software make it possible to determine the characteristics of polydispersed mixtures turbulent combustion and ignition. The model is validated against with experiments on dust combustion in confined volumes under different initial turbulization of the mixture. 

Curstedt T, Hafinan M, Robertson B, et al. 1983. Rabbit lung after long-term exposure to low nickel dust combustion. I. Effects on phospholipid concentration and surfactant activity. Environ Res 30 89-94. 

Dusts. Most combustible solids can produce combustible dusts. Combustible dusts are particularly hazardous they have a very high surface area to volume ratio. When finely divided as powders or dusts, solids bum quite differently from the original material in the bulk. Dust and fiber deposits can spread fire across a room or along a ledge or roof beam very quickly. Accumulations of dust can smoulder slowly for long periods, giving little indication that combustion has started until the fire suddenly flares up, possibly when no one suspects a problem. 

Removing impurities. Lubricants keep engine components clean by preventing the formation of deposits and suspended impurities (dust, combustion residue, engine wear particles) that then build up on the oil filter. 

Settled dust can also decompose, following the same chemical reaction. However, the reaction rate will be much slower because the dust and the oxidizing agent are not well mixed. The reaction progresses as a so-called dust combustion. 

Safety technical numbers as well as the corresponding experimental methods for their determination are required in order to enable an assessment of the risk related to handling powders and dust. In the evaluation the different reaction mechanisms, namely dust combustion and dust explosion, have to be accounted for. 

The characterization of a dust combustion risk, which means the assessment of hazards related to handling substances in the presence of settled dust, is commonly based on the following safety technical numbers ... 

Define combustible dusts. Combustible dusts are fine particles, fibers, chips, chunks, or fiakes that could be an explosion hazard when they re suspended in air under certain conditions. Many products can be fuel for a dust explosion when they re used in powdered form. Here are some examples ... 

Gasoline engine equipment such as carburetors, injectors, intake manifolds, valve systems and combustion chambers, are subject to fouling by the fuel itself, the gases recycled from the crankcase, or even dust and particulates arriving with poorly filtered air. Three types of problems then result ... 

The element is commercially obtained from the dusts of smelters processing zinc ores, as well as recovered from combustion by-products of certain coals. A large reserve of the elements for future uses in insured in coal sources. 

Dust Filter. The cloth or bag dust filter is the oldest and often the most reHable of the many methods for removing dusts from an air stream. Among their advantages are high (often 99+%) collection efficiency, moderate pressure drop and power consumption, recovery of the dust in a dry and often reusable form, and no water to saturate the exhaust gases as when a wet scmbber is used. There are also numerous disadvantages maintenance for bag replacement can be expensive as well as a sometimes unpleasant task these filters are suitable only for low to moderate temperature use they cannot be used where Hquid condensation may occur they may be hazardous with combustible and explosive dusts and they are bulky, requiring considerable installation space. 

Thermal drying has been studied in conjunction with a rail shipment of ca 1200 km from North Dakota to Illinois. Oil was appHed at 6-8 L/t to suppress dust loss, and cracks around the doors in the base of the cat were sealed to prevent ignition. Stable shipment and stockpiling were then possible (31). Thermal drying may be carried out to further reduce the moisture content as requited for briquetting or for mote efficient pulverizing and combustion. 

Maleic Anhydride. The ACGIH threshold limit value in air for maleic anhydride is 0.25 ppm and the OSHA permissible exposure level (PEL) is also 0.25 ppm (181). Maleic anhydride is a corrosive irritant to eyes, skin, and mucous membranes. Pulmonary edema (collection of fluid in the lungs) can result from airborne exposure. Skin contact should be avoided by the use of mbber gloves. Dust respirators should be used when maleic anhydride dust is present. Maleic anhydride is combustible when exposed to heat or flame and can react vigorously on contact with oxidizers. The material reacts exothermically with water or steam. Violent decompositions of maleic anhydride can be catalyzed at high temperature by strong bases (sodium hydroxide, potassium hydroxide, calcium hydroxide, alkaU metals, and amines). Precaution should be taken during the manufacture and use of maleic anhydride to minimize the presence of basic materials. 

The common types of dryers are rotary, hearth, flash (spray), and fluidized beds (10). Hot gases are used invariably to remove moisture. The gas flow can be either cocurrent or countercurrent to the flow of soHds, the former tends to be more efficient. In the hearths, the gas flow is countercurrent as the soHds are raked down from one hearth to the next below. Flash dryers are very rapid because the soHds are exposed only briefly to the hot gases. Fluidized-bed dryers, which use hot gases to suspend the soHds, are rapid and efficient, but require elaborate dust coHection systems. These are preferred when fine soHds are involved, and are used commonly for drying fine coal. Indirect-fired dryers are used when the soHds are heat sensitive or combustible. 

Each plant or laboratory should adopt definite rules and procedures for electrical iastahations and work. All iastahations should be ia accordance with the National Electrical Code (NEC) for the type of ha2ard, eg. Class I flammable gas or vapor Class II organic, metallic, or conductive dusts and Class III combustible fibers and the degree of process containment, eg. Division 1 open and Division 2 closed (67). Regardless of the flammabiUty of the materials ia the iastaHed operations, changes ia procedure involving use of such materials often occur, sometimes without concurrent alteration of the electrical iastaHation. 

Potassium forms corrosive potassium hydroxide and Hberates explosive hydrogen gas upon reaction with water and moisture. Airborne potassium dusts or potassium combustion products attack mucous membranes and skin causing bums and skin cauterization. Inhalation and skin contact must be avoided. Safety goggles, full face shields, respirators, leather gloves, fire-resistant clothing, and a leather apron are considered minimum safety equipment. 

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