It is generally appreciated that, if suspended in air, coal dust in a mine or wheat chaff in a grain elevator can explode violently.
In fact, many finely divided materials can ignite when in suspension, including polymer powders, powdered
metals such as Al, toners in copy machines, and sugar dust.
Serious dust explosion incidents occur all too regularly, sometimes causing massive damage and serious injury
or loss of life. Some dust ignitions are traceable to electrostatic sparks, though the conditions needed
to achieve electrostatic ignition of airborne powders and dusts are stringent. The dust must be suspended in air and the density
of the cloud must fall within a certain range. Further, the spark must occur at the right time and must be
sufficiently energetic. Achieving all these conditions in the setting of a lecture demonstration is very challenging. Nevertheless,
it is rather easy and quite instructive to demonstrate dust ignition itself. The diagram below shows a very simple setup using
a candle as the ignition source and wheat flour as the fuel.
To perform this demonstration reliably and safely, use a cardboard tube such as a Quaker Oats container, never metal, plastic, or glass! The container must have a volume no larger than ~1 liter. A plastic straw or small glass tube connected to a 2' length of rubber tubing is inserted through a hole in the side near the bottom of the container. The candle is mounted securely inside and a small amount of wheat flour is piled around its base. Use just enough wheat flour so that when you blow through the tube, a dense cloud of wheat dust forms. After lighting the candle and positioning yourself away from the top of the container, blow a puff of air into the tube. Ignition almost always occurs unless the air flow blows out the candle or if too much or too little flour is used. You need to experiment to achieve ignition reliably. For safety, it is very important to keep the area above the cardboard tube clear. Wear goggles! Placing a paper lid loosely on top of the container increases the power of the ignition but remember that this lid can fly off the top rapidly. Also, the flame, though very brief in duration, can singe your hair. Stay well clear! Make sure there are no matches or flammable liquids present.
WARNING: This demonstration is somewhat messy. After each ignition, a dirty mixture of unburnt flour and blackened particles ash settles around the base of the tube. You should never attempt it on the dining room table! The video shows ignition of plain wheat flour placed inside a ~1 liter cardboard container with a lid: CLICK HERE to view it. Watch this video closely several times. Characteristic of many dust deflagrations, a flame briefly flares up after the initial explosion has occurred. This happens because the concussion of the initial explosion kicks up additional flour from the bottom of the container into the air where it ignites and burns. This scenario is actually typical in coal mine explosions, where a methane explosion occurs first and then coal dust is kicked up off the mine floor, suspended in the air, and then ignited. The secondary deflagration is often far more devastating and wide-spread in its effect within the mine than is the initial methane explosion. Therefore, it is vitally important to prevent the build-up of coal dust on equipment and on the floor of a mine.
Bartknecht, W., Dust Explosions: course, prevention, protection, Springer-Verlag, 1989.
Britton, L.G., Avoiding Static Ignition Hazards in Chemical Operations, Center for Chemical Process Safety (AIChE), 1999.
Jones, T.B. and King, J.L., Powder Handling and Electrostatics, Lewis Publishers, 1991.