At a guess...

John in NM
And it is just that, the danger is likely minimal due to the type of coal - anthracite. It is the hardest and most dense type of coal, and from what I read, the hardest to ignite and the least dusty.

I had exactly the same thought you did however, and I would probably be pretty careful about such a system, since vacuums move dust in air and that is the textbook means of generating static followed by a spark.

I bet if you substituted normal bituminous coal, you'd get a boom. I remember visiting a portland cement plant once, and the tour was fascinating - they cooked the various minerals to make the cement using air entrained coal dust - it burned like a gas, though I've long since forgotten the temperature (eg like acetylene or propane?). They must have done a lot of work to prevent sparking, even though all the ducting was stainless it is a huge risk - just like with pressurized gases.

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