Combusting particle, material

Combustible, dusty material, with particle size less than approximately 200 mesh, has the potential to explode if a sufficient concentration in air is present along with an ignition source. 

The data shown in Fig. 20.4 are for tests in which dusts with narrow size distributions were used. Hertzberg and Cashdollar (1987) pointed out that with broad size distributions, strong size dependencies tend to be blurred, hut the trends should remain the same. With small mean particle sizes, all flammable material is volatilized from the particles, and the combustion process is homogeneous. As the particle size increases, only the surface material is volatilized so that more dust is required overall to sustain combustion. Finally, above some upper limit of particle size, the volatilization rate is such that a minimum concentration of combustible volatile material is not achieved. 

Combustion particles are of complex chemistry, carrying most of the trace elements, toxins or carcinogens generated from the combustion process. Combustion of different types of fuels results in emissions of various trace elements which are present in the fuel material. In most cases there is not just one specific element that is related to the combustion of a particular fuel, but a source profile of elements [2]. For example, motor vehicle emissions contain Br, Ba, Zn, Fe and Pb (in countries where leaded petrol is used) and coal combustion results in the emission of Se, As, Cr, Co, Cu and Al. For comparison, the crustal elements include Mg, Ca, Al, K, Sc, Fe and Mn. Since most of the trace elements are nonvolatile, associated with ultrafine particles and less prone to chemical transformations, they often remain in the air for prolonged periods of time in the form in which they were emitted. 

The major processes for creating atmospheric fine particles (diameter < 2.5 pm) are combustion and gas-to-particle conversion (GPC). Whereas combustion particles are emitted directly to the atmosphere (primary aerosol), gas-to-particle conversion refers to the chemistry that leads to particulate matter by converting volatile gases into condensable substances under atmospheric conditions. Gas-to-particle conversion leads to an increase in the mass of preexisting particles and under some circumstances may lead to the creation of new particles. Particulate material produced by GPC is referred to as secondary aerosol. 

Particles suspended in air after incomplete combustion of materials, soil carbon... 

In a more all encompassing classification, Doubleday (1999) produced a morphologically based description for all combustion derived particles collected in the eastern Canadian Arctic (including those from waste incineration, aircraft emission and domestic burning), where the fly-ash particles considered in this chapter formed classes, However, this was only used as a classification tool and not for fuel-type apportionment of the fly-ash particles. When applied to sediment material from the same area the results were expressed in terms of percentages of total particles or as percentages of total combustion particles. 

Spontaneous combustion may develop due to the heating of coal particles and slow combustion. Toxic material may be released with the oxidation of compounds present in the waste material. Runoffs, with minimal iron pyrites content, from the tip surface, promote hazards and the acid compounds may leach into domestic water supplies. All these effects may be partly or wholly overcome with proper management of the planning and design of waste heaps and good operating practices. Moreover, encouragement of the commercial use of waste material is beneficial. 

There is a wide range of suitable means of connecting conductors and we shaii look at these in a moment. Whatever method is used to connect iive conductors, the connection must be contained in an enciosed compartment such as an accessory for example, a switch or socket box or a junction box. Aitemativeiy, an equipment enclosure may be used for example, a motor enclosure or an enclosure partly formed by non-combustible building material (lET Regulation 526.5). This is because faulty joints and terminations in live conductors can attain very high temperatures due to the effects of resistive heating. They might also emit arcs, sparks or hot particles with the consequent risk of fire or other harmful thermal effects to adjacent materials. 

Other properties of dusts that affect the likelihood of ignition and the severity of combustion include particle size, concentration, oxygen presence, presence of impurities, moisture content, and air turbulence. Small particles ignite easier than large particles. Fine dusts often have high rates of pressure rise during combustion. Similar to flammable gases and vapors, certain concentrations of dusts in air are combustible. Particle size affects the concentration required for combustion. If a concentration is too low, combustion is not likely. The presence of inert material in a dust can reduce its... 

Smoke Smoke is carbon or soot particles, generally less than 0.1 pm in size. Smoke results from incomplete combustion of materials containing carbon. 

Table 5.9 Selected material properties for combusting particle.

For cases where the industrial process involves high temperatures and pressures, as in pressurized fluidized-bed combustion, it becomes possible to select the pressure in the cold model in such a way as to maintain the gas density, and hence the particle material, the same as in the industrial unit. This is illustrated in the second cold model of Table 13.3, where, for the chosen example, it results in a scale factor of approximately one-half. The first cold model of Table 13.3 operates with the same gas as the industrial unit under conditions of ambient pressure as well as ambient temperature. The fact that the model size is only marginally less than the original does not necessarily impose a severe limitation, as the effect of bed internals... 

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