Page 307 - Dust Explosions in the Process Industries

276 Dust Explosions in the Process Industries

fraction of volatiles assumed to take part in the combustion of Pittsburgh seam bitumi-
nous coal dust decreases with increasing dust concentration.
In a subsequentpaper, Hertzberg et al. (1987) formulated a three-stage model for coal
dust flame propagation:

1. Heating and devolatilizationof dust particles.
2. Mixing of emitted volatiles with air in the space between the particles.
3. Gas phase combustion of premixed volatile/air.

Each stage is characterized by a time constant. For small particles and low dust con-
centrations,the combustion process is controlled by stage 3, whereas for large particles
and high dust concentrations,stage 1controls the combustion rate. When discussingthe
influence of particle size on devolatilizationin coal dust flames, Hertzberg et al. (1987)
suggested that, for particles smaller than 50-100 pm diameter, devolatilization is com-
plete and not rate limiting for the combustion reaction; that is, p in Figure 4.11 is equal
to unity. On the basis of measurement of pyrolysis rates of single particles and micro-
scopic studies of particle morphology, they concluded that the pyrolysis wave preced-
ing a coal dust flame is nonisothermal, with a velocity proportional to the net absorbed
heat flux intensity and inversely proportional to the overall enthalpy change of the com-
bustion reaction.
In view of Hertzberg et al.’s suggestion of a limiting particle diameter of 50-100 pm,
it is interesting to consider the influence of particle size on maximum explosion pres-
sure and maximum rate of pressure rise of lignite dust in air in a 1 m3vessel, as meas-
ured by Scholl(l981).As shown in Figure 4.12, there was no further systematicincrease
of the two parameters with decreasing particle size below 60-80 pm diameter, in accor-
dance with what would be expected on the basis of the hypothesis of Hertzberg et al.

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