Dust particles flammability

The flammability and explosivity of high-sulfur petroleum coke dust (particle size <75 pm) were examined. Air-dried powder was non-explosive but fire-prone above 400°C. A 5 mm layer became incandescent at 420-470° and a dust cloud ignited at 520-660°C [1]. The fire and explosion hazards of petroleum coke or anthracite, when used in the manufacture of furnace electrodes, may be reduced by heat treatment [2],... 

As might be expected, particle size is an important factor in assessing the flammability of dusts. It appears that above some size, dust particles are relatively ineffective as contributors to dust explosions. Figure 20.4 shows this effect for several dusts. In this figure only the mean particle size is given. However, it is expected that particle size distribution would also be a complicating factor in assessing the flammability of dusts. 

The mechanism of dust explosions Because dusts can behave in a similar manner to gases, they possess similar explosive characteristics. The mechanism of propagation is the same as for gases except that it is the actual particles of solid matter that are heated to ignition, not the molecules of the substance. Dusts exhibit upper and lower flammable levels, ignition temperature and other combustion characteristics which are dependent on the size of the dust particles. Normally only the lower flammable level of a dust is relevant since, long before the upper level is reached, the dust concentration becomes an environmental hazard. Combustion characteristics of several dusts are shown in Table 25.2. 

In the typical fluidized-bed dryer, the drying gas is hot air. Air is pulled over heating coils and through the dryer by an induced-draft fan. A particular hazard here is the possibility of dust explosions. Fine particles, still in a fairly dense phase, are surroimded by warm or hot air. The hazard is more severe if flammable solvents are being removed. A synergistic effect may exist between dusts and flammable vapors. Even at vapor concentrations below the LEE, the ignition energy of the mixture may be considerably lower than that of the dust alone. This emphasizes the need for the precautions listed for other dryers and in the section on dust explosions (Chapter 2). 

Sealant Manufacturing. Most sealants use mineral-based fillers which may contain small amounts of crystalline siHca. If crystalline siHca is present, dust control is important to prevent inhalation of these particles. Crystalline siHca is a known cause of siHcosis, a debiHtating disease of the lung. Another common safety concern in sealant manufacturing is the use of flammable materials. Not all sealants use flammable ingredients, but for those that do, proper inerting and grounding are needed to prevent potential explosions. 

Vei y small solid fuel particles such as sawdust, agricultural grains, or coal dust can sustain flames when they are suspended in air. In fact, very serious fires have occurred in grain storage towers and coal mines because of the flammability of suspended dusts. The combustion of the individual particles follows the usual pattern of solid particle burning— devolatization and char burning. The combustion of the whole cloud of particles is similar to spray combustion and its characteristics depend on the nature of the fuel, size of the particles, and the number of particles in a given volume. 

Increasing the surface area of a combustible solid enhances the ease of ignition. Solid particles less than about 10 pm in diameter settle slowly in air and comprise float dust . Such particles behave, in some ways, similarly to gas and, if the solid is combustible, a flammable dust—air mixture containing a distribution of particle sizes can form within certain limits. 

Dust explosion This explosion results from the rapid combustion of fine solid particles. Many solid materials (including common metals such as iron and aluminum) become flammable when reduced to a fine powder. 

The limits of flammability of dusts have been investigated (30G, 34G, 35G). Using various metal powders, Boyle and Llewellyn (15G) have found the minimum ignition energy to be proportional to particle size. Browning, Tyler, and Krall (10) report a variation of the lean limit as a function of particle size particles of about 60-micron diameter give the leanest combustible mixtures. 

CA 47, 8342 (1953) (The lower the content of the volatile matter, the lower was the flammability of coal dust and below 27.27% content the coal was no longer flammable. Addn of 40% pptdCa carbonate, 45%marble, or 50% silica or alumina diminished flammability The smaller was the particle size of nonflammable addns, the greater was the reduction of flammability) S)K.Matsumoto, JMin-inglnstJapan 68, 260-82 (1952) 70, 463-66... 

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