A chemical reaction
that is the bane of grain farmers everywhere.
Essentially, dust explosions are very much like other low level explosions, in that they consist of some combustible material combustiing at an exceedingly fast rate, quite like smokeless powder in rifle rounds. The reason rifles don't explode is that the chamber in which the cartridge resides is designed to withstand the high pressures caused by the combustion of the powder. Grain silos are not. When the finely powdered grains (i.e. flour) are suspended in the air, they are much more likely to combust, being almostly completely surrounded by abundant oxygen. The combustion reaction is tremendously fast, and pressure within the silo rapidly increases beyond the silo's ability to contain it. Therefore, BOOM goes the silo.
When I was in high school, my Chemistry teacher demonstrated dust explosions for us using a 1 quart paint can modified by the addition of a long copper blowpipe soldered to the side of the can, near the bottom. Into the can he placed a small amount of flour, and a lit candle. He then loosely fitted to can lid, and blew a puff of air in through the blowpipe. This atomized the flour cum explosive, and the lid was blown off the can with a bright flash and small *bang*.
This was among the neatest applications of practical chemistry we'd ever seen, and the next time the teacher was out of the room, we decided to replicate the feat. Being teen-aged males, we of course we not content to use flour, and quickly decided the powdered magnesium would be a suitable substitute.
Needless to say, we all ended up in the principal's office, having blown the can apart, stuck the lid 2 inches dep into the ceiling, and left an approximately 7' diameter burned circle in the ceiling tiles. Isn't chemistry grand?