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Q & A: Starting fires

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Most recent answer: 10/22/2007
Q:
how do fires start?
- Anonymous
A:
Fires start in a variety of different ways. The ingredients of fires are fuel, oxygen, and something to get the burning reaction going. This last item is needed because while the oxidation reaction that is burning releases energy, it needs a little activation energy to get it going. Once going, the energy released by the burning fuel is enough to supply the activation energy needed to ignite other fuel, and the fire burns until all the fuel is consumed or all the oxygen is consumed. You can put out many fires by throwing water on them -- this takes away the heat needed to ignite unburnt fuel. Wet fuel (wood, for example) needs to dry out first before it can burn, and evaporating water takes 540 calories per gram.

Once the fuel and the oxygen are there, and it's not too wet, then just about any little addition of energy can get a fire going. Examples include:

1) a match -- this supplies heat energy to ignite fuel. Variations on this theme are cigarette lighters and pilot lights in older gas stoves. A forgotten cigarette can cause a house to burn down.

2) an electrical spark -- newer gas stoves ignite their fuel with electrical sparks. Spark plugs in cars get the gasoline burning in the engine's cylinders. Lightning strikes can cause forest fires and prairie fires.

3) Heat from focused sunlight. I'll confess I used to burn bugs with a magnifying glass when I was small. You can ignite paper or wood with this too.

4) Flint and steel. It's not the flint, but the steel, which makes hot sparks when little bits of steel are forcibly pulled off of a bigger chunk of steel. You also see this when grinding steel with a grinding wheel. Or in movies, metal parts of cars scraping on the ground during high-speed car chases make nice sparks. These little glowing-hot bits of steel are enough to ignite flammable substances.

5) Bacterial metabolism heat. Grain that's stored in a big silo or grain elevator has been known to get very hot because bacteria feed on the dust and produce heat as a byproduct. The big pile of grain makes an excellent insulator for heat, and the temperature can build up high enough to cause the grain dust to burn and even explode. Grain-storage silos often have cooling fans to help counteract this.

This effect probably was the cause of the disastrous explosion of a truck carrying flour through the Mont Blanc Tunnel between France and Italy in the late 1990's. The truck's cargo spontaneously exploded in the tunnel, creating a larger fire as the gasoline and diesel fuel of other cars which collided in the pile burned.

6) Meteors, when they strike the ground, can start fires too.

Can you think of other ways fires can be started? Or more importantly, stopped?

Tom

(published on 10/22/2007)

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