Monday, December 24, 2007

Dust Explosions

Dust Explosion in a Coal Mine



There are two kinds of sparks. One kind is the electrical spark, where a potential charge bridges the gap between two conductors. Electrical sparks span the range from that little flash you see when you touch the door knob after walking across the carpet when the air is dry, to the lightning flash caused by clouds playing with themselves.

The other kind of spark is basically a small particle burning in air. Those are the ones you see coming off of the campfire, or when you strike flint. One of the fun things to do with the old Gilbert Chemistry set is to take some of the iron filings and shake a few into the flame of the alcohol lamp. Or you can make a paste of some iron filings, potassium nitrate, charcoal, and a starch binder. Put it on a stick, let it dry and you have sparklers. Alternately, you can just drive to the nearest store that has a 50 foot sign that says !!!!FIREWORKS!!!! and just buy a few.

People don’t usually think of iron as something that burns, because most of the iron we deal with every day won’t. Very few solids will actually burn in ordinary air (air under pressure, pure oxygen, chlorine, fluorine, etc. are another matter). Wood, for example, emits combustible gases when heated, so wood burns by what is a complex, multistage process. Charcoal is similar; it has to be very hot, and then the carbon combines with oxygen to make CO, and the CO is what burns in a charcoal flame (which you mostly don’t want, since you’d rather have hot charcoal embers). Moreover, and this is very important, charcoal is porous, so it has a large surface area.

One of the most important features of physical geometry is the cube-square law. This is the recognition that, as things get larger, they have proportionally more volume to surface area (or any other two dimensional feature such as cross section). That’s because, as things get larger, their surface area changes as the square of linear dimension, while the volume increases as a cube, so volume goes up faster. Conversely, as things get smaller, surface effects begin to get more and more important. Chemical reactions between two different substances can only occur where the substances touch each other; for a solid and a gas, that is at the surface of the solid.

So oxidation of a bulk solid takes place at its surface, and, for big objects, that tends to be pretty slow. So iron slowly oxidizes on its surface to form rust. Because rust is porous and crumbly (that being because iron oxide is much bigger than the original iron), rust is progressive and will slowly rust the whole piece of iron. Aluminum, on the other hand, forms a tough oxide film on its surface, and that protects the rest of the aluminum from oxidation.

Chop a metal bar into fine enough pieces, however, and the dynamic changes. Iron filings will oxidize much more rapidly than if they were in an iron bar. Moreover, the oxidation rate increases with temperature. Heat the filings enough and the oxidation will take off and the whole filing will rapidly burn: a spark. You can also burn ordinary steel wool, incidentally.

Chop your particles even further and they will get to the size where they can be suspended in air. Get the right mix of combustible particles and air, and the burning can coordinate itself into a self-propagating flame front. Confine the particles in an enclosure and you can get a dust explosion.

I’ve been talking about iron particles, but any combustible material will do, and the most common types of dust explosions are in coal mines and in granaries and flour mills. The coal mine part seems pretty obvious, but people don’t usually think of flour as an explosive or even as combustible. Strictly speaking flour isn’t explosive; it’s just that, under the right conditions, it can burn rapidly enough in a confined space to cause what is called a “pressure burst” explosion. In other words, under the right conditions, a grain elevator can become a pipe bomb.



Grain elevators used to protect against dust buildup by rapid ventilation. That, however, sometimes caused local air quality problems, violations of the ambient air quality standards for particulate matter. So ventilation was reduced, and air filters were added, and sometimes the air filters weren’t maintained as properly as they should, so bang.

Actually, the dust filters themselves were sometimes a problem, since they concentrated the dust in a small space, and sometimes the filter itself would have a dust explosion. Sometimes, because the filter contained a lot of previously collected dust, a minor filter explosion would push the contained dust into a larger volume, and then a larger, secondary explosion would result.

That’s a specific example of the more general problem in dust explosions: secondary explosions. Often the amount of dust in the air was fairly small in comparison to the amount of settled dust in a container, or in a coal mine, or whatever. The first, small explosion would shake the area enough to push a lot more dust into suspension, and then that dust would ignite and cause the much larger pressure burst. In some ways, the geometry of the situation resembles that in a CVCC engine, where a small ignition chamber shoots a flame front into a larger combustion chamber. Only in the case of a flour mill or coal mine, the primary ignition also mixes the fuel with in the larger chamber.

There’s a standard educational lab demonstration that uses a pipe or small container, a funnel attached to an air hose, some flour and a candle to demonstrate dust explosions. A more dramatic demonstration of the secondary explosion effect would be to put a pan of flour on top of the first pipe, and have the pan shoot the flour into the air in a larger volume, like a shipping container perhaps. However, that might be a little rough on the shipping container.

Besides, I don’t want to give anyone ideas. The day that bags of flour are viewed as the tools of terrorists is the day that I’m packing my bags for New Zealand.

4 comments:

JP Stormcrow said...

The day that bags of flour are viewed as the tools of terrorists is the day that I’m packing my bags for New Zealand.

They use bags of pulverized sheep dung for that purpose over there.

James Killus said...

Okay, now that's a terror weapon.

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