AJW said:
Dust clouds I think have been known to actually explode under the right
conditions. Coal I suspect in a fine enough dispersion might be a possibility.
Coal is pretty dirty carbon, so it's reasonable to assume it'd be somewhat
conductive. If that's so, I can imagine a discharge propagating through it.
After all, there are lots of sharp edges, and that leads to high field
gradients. When one has a bit of a corona discharge it effectively reduces the
air gap, and cascading could happen.
Or not.
Coal dust has been known to explode. I would suspect that if a spark occurs
and the dust was dense enough, there would be explosive combustion. Flame
stability is a major concern with coal fired steam generators using fine
coal (as dust in an air suspension) - if the flame goes out and fuel is
still being blown in there is a chance of a hot spot igniting the coal or
sudden explosive ignition when the flame returns - bye bye boiler.
It has also happened with flour as well due to static or other sparks. Any
combustable material in a fine enough suspension can explode.