oa South African Journal of Chemical Engineering - On the sphericity of coal and char particles
Most solid particles of practical interest are irregular in shape. In fixed-bed and fluidized-bed reactors, shape of particles can influence bed permeability, gas flow distribution, diffusion of mass and heat and reaction rates. The most popular empirical factor to describe non-spherical shapes of particles is sphericity. In this study sphericity of Highveld coal particles in the range 4.7-106 mm and chars formed by devolatilization of this coal in the range 3.4-45 mm was determined using 3-D laser scanning technique. It was found that the relative variation in sphericity of the coal particles does not exceed 15% and, therefore, it can be possible to describe Highveld coal particles using an average value of sphericity calculated for all size classes. This average value is 0.793 (standard deviation 0.034) and it is very close to the sphericity of particles in the +37.5-53.0 mm size class (øS = 0.798). In contrast to this, the sphericity of devolatalized coal particles were observed to decrease slightly with increasing particle diameter, assuming an average value of 0.749 (standard deviation 0.042).
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