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Topic: Wondering about autoignite temps for various fuels  (Read 2480 times)

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Offline TheSpark

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Wondering about autoignite temps for various fuels
« on: February 26, 2013, 06:01:55 PM »
hi! i'm a noob here, but forums are great mines of information, and i am in need of facts!  :P

i'm a fire performer organizing the safety for a large festival event this summer. we spin things on fire in the middle of the woods, so safety is vital! in order to provide a better answer as to why some fuels are not allowed, and to provide the best safety possible, i am researching the chemistry behind the various flammables and combustibles. do any of you know where i can go to find the flash points of about 10 different chemicals? the wiki will only go so far, and after a while i feel like i'm going in circles..

thank you!!!!

Offline Enthalpy

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Re: Wondering about autoignite temps for various fuels
« Reply #1 on: March 01, 2013, 12:22:21 PM »
These are two different notions, which are only loosely related, including the corresponding temperatures.

For the autoignition, you heat the air+fuel mix slowly and check when it catches fire. For instance Diesel oil has a low autoignition temperature, while high octane shall resist autoignition (and detonation as well).

Above its flash point, the fuel makes vapour concentrated enough that this vapour can sustain a flame in air, whatever the ignition method is - lighter, spark... This is more linked with fuels being volatile, like ether for instance.

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To estimate the risk of ignition, the flash point is the primary information. You may check the subtle distinction between "open cup" and "closed cup" flash point, but essentially, if the fuel is:

- Under its flash point, it's hard to light: cold Diesel oil, heating oil, cold cooking oil...
- Above the flash point, easy: gasoline, butane (gas, hence already fully vapour state), hot cooking oil...

Fuels can be ignited under the flash point:
- If they form a mist. Diesel oil or kerosene in the engine, but also hydraulic fluids in a high-pressure leak. Fire eaters use that.
- If a wick is available, voluntarily or not. Wax, vegetable oil, kerosene lamp... Dead leafs can be a wick!
- If they spat, for instance water boiling in cooking oil
- Probably more possibilities!

Solid fuels may ignite before they melt, for instance because they give enough vapour or decomposition gases directly from the solid (Hexamine) or because they're so loose that they get heated immediately (cotton wool) or burn as a solid (metal powders)...

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The flash point of individual substances - not only pure chemical substances - is often given in their Material Safety Data Sheet or MSDS. Sigma-Adldrich give theirs online.

One list for pure chemical substances:
http://www.nist.gov/data/PDFfiles/jpcrd669.pdf

Very generally, be careful about chemical data found on the Web. Most is not measured, but estimated by a software instead, and can be extremely false. If you see tenth of a degree precision in a flash point it's likely from a software.

And of course, experiment by yourself to check - but you don't need an exhortation for that, do you?

Offline Stepan

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Re: Wondering about autoignite temps for various fuels
« Reply #2 on: March 23, 2013, 10:16:06 AM »
i'm a fire performer organizing the safety for a large festival event this summer. we spin things on fire in the middle of the woods, so safety is vital! in order to provide a better answer as to why some fuels are not allowed, and to provide the best safety possible, i am researching the chemistry behind the various flammables and combustibles. do any of you know where i can go to find the flash points of about 10 different chemicals? the wiki will only go so far, and after a while i feel like i'm going in circles..

Flash points heavily depend on chemical composition. Let say diesel fuel may have flash point of 96C. If it was accidentally contaminated with only 2% of gasoline, the temperature drops to 50C. The only way to know for sure is to run the test. Send them to my lab, and I will test them for you for flash point temperature. :-) We are specialized in those kind of tests. http://www.lcsairtest.com/msds-tests.html

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