[Krzychu]

Hello, I have a question why the soot in this film does not catch fire and in general why is the soot very resistant to burning?

https://www.youtube.com/watch?v=_WtTag3Nm9Q&t=15s

[jeffmoonchop]

isnt soot the remains of something that has already burned?

[Borek]

isnt soot the remains of something that has already burned?

No, it is mostly carbon.

It does burn, but because it doesn't contain volatile fractions it is quite difficult to ignite. A bit like coke or char - once it is really hot and in mass it burns quite nicely and is capable of sustaining the required temperature, but getting there (especially with low temperature flame) is not easy.

[Krzychu]

You don't want to burn at high temperature and oxygen availability.

[Enthalpy]

I wish I knew that!

Soot is close to graphite, and the closer a material gets to graphite, the more difficult it is to ignite. Graphite blocks serve as a liner in high temperature ovens because they resist in air at very high temperature. But they must be graphitized for long at high temperature to remove any hydrocarbon.

You can try with lubricant graphite. It's a powder down to 30nm grain size. Such a fine powder of iron, aluminium... would catch fire explosively by mere contact with air at room temperature. But Graphite powder can fall through a candle flame and it lands unburned. I'm sure the tiny grains get really hot in the flame.

Once soot is produced somewhere in a flame, it's very difficult to complete the combustion elsewhere, despite soot is initially very fine and hot, and it receives new oxygen. That's a hard limit to super fuels with highly strained cage molecules that contain too little hydrogen.

Vague hypothesis: the vapour pressure of graphite is far too small to feed a flame even when hot, and the solid does not burn because no reaction path can rip carbon atoms from graphite.