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Every once in a while I hear in the news that a grain elevator exploded and burned down because of a static shock that ignited the dust in the air. I also hear about the danger of the same thing happening in people's woodworking shops.

What are the minimum necessary conditions in order for a static shock to ignite wood or grain dust in the air? For example, does the static discharge need to have a certain voltage or amperage, or does it need to arc across a certain minimum distance?

I imagine humidity and/or the concentration of combustible grain or wood dust are important factors, but for the purposes of this question you can make whatever assumptions are within reason (but please include them in your answer). In that case, what are the minimum required characteristics for a static electric discharge to ignite the dust in the air?

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  • $\begingroup$ Conditions for combustion might be more of a chemistry question, but I think it'd be reasonable to leave this here for a few days to see if you get any interesting responses. Welcome to Physics.SE, @rob! I hope you hang around and post many interesting questions and answers so that people become unable to tell us apart. $\endgroup$ – rob May 18 '15 at 21:04
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    $\begingroup$ This is a great question. If you want to see what it looks like, almost any fine-particulate matter will combust. Coffee creamer for example, and sawdust. $\endgroup$ – tpg2114 May 18 '15 at 21:05
  • $\begingroup$ @rob D'oh! You're probably right. I've been lurking, dropping in every once in a while to read some interesting questions. I'm not sure I'll have much to contribute in terms of answers, but that could be entertaining if I post questions and you answer them. $\endgroup$ – rob May 18 '15 at 21:08
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    $\begingroup$ FWIW, I think this is perfectly on topic here. I think the chemistry.SE would be more interested in the chemical kinetics involved than in the physical processes that lead to something ordinarily safe exploding. $\endgroup$ – tpg2114 May 18 '15 at 21:08
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    $\begingroup$ The concept of "minimum ignition energy" is well established and tabulated for many gas mixtures in technical publications. I am sure there is some theory for it (using the chemical activation energy, density and specific heat of substances), but I haven't seen any of it. $\endgroup$ – CuriousOne May 18 '15 at 21:09
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Explosion is essentially a fast cascading combustion reaction. Combustion requires three things: Fuel, oxygen, and heat. In an explosion or any continuous combustion (candle, campfire), the heat requirement is satisfied by the reaction itself and only an initial ignition source is required. The difference between a continuous combustion and an explosion is basically the rate of reaction. Continuous reactions have their reaction rates limited in some way. These are the factors I can think of affecting a potential explosion:

  • Oxygen-fuel mixture: In a campfire, combustion is limited by the supply of oxygen, which cannot penetrate into the fuel source (wood). In a dust cloud, oxygen will be mixed with the fuel. There is some optimum ratio of fuel to oil outside of which your explosion will happen less completely or not at all.
  • Homogeneity: If the fuel-oxygen mixture is "lumpy", then any explosion could stop at localized areas of high or low fuel-oxygen ratio.
  • Fuel quality: Things like dampness or contamination by incombustible material will raise the activation energy of ignition, making an explosion weaker and less likely to occur.
  • Temperature: Likewise, fuel/air temperature will also affect activation energy of ignition. If something is already near combustion temperature, it's easier to ignite. Hot fuels also tend to be dryer.
  • Fuel energy: How much energy is released when your fuel burns? If it's a lot, it's more likely to explode.

You asked specifically about the ignition requirement. If everything else is optimized (for volatility, or explosiveness), an extremely tiny spark would be able to ignite the whole mess. The spark in your car's engine is only a couple millimeters long. But as with most things in life, bigger is better. Having a larger, hotter ignition source increases the chance of ignition.

That's all I can think of. With that many factors, the exact requirements for explosiveness are way too complex to go into here but I hope this gives you a better idea of how such a system works.

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