In general, the ease with which a cloud of combustible powder/
dust ignites, and the violence with which it explodes increases with
decreasing particle size. If the particles in an organic powder/dust
are so small that devolatilization no longer controls the explosion
rate, further particle size reduction will not increase the overall
combustion rate further. For many metals such as Mg, Al and Si, the
limiting particle is considerably smaller than for most organic
materials and coal. This is because metal particles do not produce
a homogeneous combustible gas phase by devolatilization/pyrolysis
as organic materials do, but tend to burn as discrete entities.
Fig. 2 shows how the combustion rate of clouds of aluminium dust
in air increases systematically with the specific surface area of the
dust. A specific surface area of 6.5 m2/g corresponds to mono-sized
spheres of diameter 0.34 mm.