Mark...
It's been a long time since I learned anything about combustion. So my 'theory' might be shaky now. Any 'Smokeys' on the forum will no doubt jog my grey cells!
That little demo was fine, but planer chippings and shavings will always ignite like that if thrown into a column of naked flame. But they aren't likely to explode if subjected to sparks. For one thing they don't float in the air so readily. If a spark or naked flame drops into a pile of shavings, they will probably smoulder, and eventually, as the heat builds up, they will ignite. They can also combust spontaneously, if they are left in a pile, and allowed to heat up for long enough.
An explosion is 'a rapid combustion, caused by heat or shock', and the main concern with airborne explosions, is electrical sparking (heat) setting light to fine dust. The fine particles are so close to one another, and just a small spark can set them going. The combustion is genuinely rapid and has an 'explosive' effect. It even occurs in flour mills, and has accounted for more than one mill being destroyed. This is why a good extraction system separates the dust from the larger solids. It's also a good case for having some kind of fan to exhaust dust laden air out of the shop; through a filter of course.
It's rather like plastic explosive. Plastic explosive is a liquid explosive mixed into a paste with inert filler. You can hammer the stuff; even set fire to it, but it won't explode unless it's subjected to a 'shock', either by a primer exploding, or by an electrical impulse setting off a detonator. Back in the day, we used it to start fires, when we were set loose in the Scottish Highlands, on so-called survival training. Where ordinary people would get plastic explosive, I dunno! (hammer)
HTH.
John
