Page 291 - Dust Explosions in the Process Industries

260 Dust Explosions in the Process lndustries

Howard and Essenhigh (1965, 1966, 1967), discussing the results of their extensive
research on coal particle combustion, first indicated that ignition of a bituminous coal
particle generally occurs on the solid surface of the particle rather than in the volatile
pyrolysis products. However, in their final conclusion (Howard and Essenhigh, 1966),
they differentiated among various mechanisms on the basis of particle size. The classi-
cal view, of ignition taking place in the volatiles, still seemed valid for particle diame-
ters larger than 65 pm. Smaller particles would, however, not be able to generate a
sufficiently concentrated envelope of volatiles to prevent oxygen from diffusing to the
solid carbon surface. For particle diameters smaller than 15 pm, the ignition reaction is
more or less entirely heterogeneous oxidation at the particle surface.
The essential point in Howard and Essenhigh’s argument is the assumption that, for
particles of diameters smaller than 100 pm, the total devolatilization time is independ-
ent of particle size. This impliesthat the average flow of volatiles per unit of particle sur-
face area increases with the particle size. For very small particles, the volatile flux is
insufficient to maintain a volatile flame envelope round the particle.
In a more recent investigation of the devolatilizationprocess by Johnson, Murdoch,
and Williams (1988), Howard and Essenhigh’s assumptionof negligible influence of par-
ticle size on devolatilizationrates (or total devolatilization times) was maintained for the
range of particle sizes typical of most pulverized fuels and explosible dusts. These work-
ers studied the devolatilizationof monolayers of coal particles in an inert atmosphere,
at heating rates from 100 to 1500 Ws. The results also indicated that, for 10-1000 pm
diameterparticles of bituminous coal resting on an electrically heated filament,the heat-
ing rate had little influence on the devolatilizationyield, which rather was determined
by the peak temperature. The maximum rate of devolatilization and maximum hydro-
carbon yield occurred at peak temperatures between 700 and 1000K.
Froelich et al. (1987) studied the combustion in air at 1400 K of single 80-100 pm
diameter coal particles containing 30% volatile matter. They used the experimentally
determined relationshipbetween particle temperature (two-colorpyrometer) and time in
a furnace of known temperature to calculate the rate of gasification of the solid carbon
of a coal particle.After about 5 ms in the furnace, the particle temperaturereached a sharp
peak of 2200 K, which was attributed to the devolatilization and ignition of the volatiles.
A second, less-sharp temperature rise, which started at about 10 ms and terminated at
about 60 ms, had a peak value of about 1800K and was associated with the gasification
of the solid carbon.
In their theoretical analysis, Froelich et al. assumed that

The particle was a perfect and homogeneous sphere.
The temperature of the particle was uniform.
Either the diameter or the density of the particle remained constant (devolatilization
or combustion of solid carbon).
The furnace and the particle were black and gray bodies, respectively.
The particle was in permanent thermal equilibrium with the gas and walls of the fur-
nace.

The following equation was proposed: