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From what I've read, the production of soot in a flame is still an open research topic. But some hydrocarbon fuels produce much more soot than others. Are there (simple) rules that tell which fuels produce more or less soot? Especially for aliphatics?

Some things that influence soot production that I am aware of:

  • The amount of oxygen available and if this is pre-mixed with the fuel. I want to limit this question to diffusion flames (non-pre-mixed) with plenty of air available.
  • According to Wikipedia, naphthalenes produce more soot than benzenes which produce more soot than aliphatics.
  • Longer carbon chains generally produce more soot than shorter carbon chains.

But now I read that ethanol and diethylene glycol produce less soot than methane, even though both contains two-carbon chains and methane only a single carbon atom. Does it apply in general that hydrocarbons with oxygen atoms produce less soot, or are these an exception? And are there more general rules to predict which fuels produce more or less soot, short of setting up a full chemical simulation?

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  • $\begingroup$ Generalities, though not rules: more hydrogen, less soot. E.G. ethane has 3 times as much H as sooty acetylene. Because of the H "end-cap", the proportion of H :C decreases for longer alkanes. More oxygen, less soot, also. $\endgroup$ – DrMoishe Pippik Nov 2 '17 at 23:11
  • $\begingroup$ Thank you for asking this question. Can you provide some references? I can't track this down: "According to Wikipedia, naphthalenes produce more soot than benzenes which produce more soot than aliphatics." Also "ethanol and diethylene glycol produce less soot than methane", "the production of soot in a flame is still an open research topic". I am interested in this subject, and even though I can't answer the question, I think it would help me if you added more references. $\endgroup$ – Metamorphic Sep 24 '18 at 21:39
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    $\begingroup$ @metamorphic I added a link to the wikipedia article, which itself has some references to chemistry publications. The methane/ethanol statement is based on rules for bioethanol and natural gas fireplace burners. Gas burners require a chimney (unless they pre-mix the gas and air but then you only get blue flames which doesn't look as nice), bioethanol does not because it burns much cleaner. I'll try to find my source on diethylene glycol, can't find it now. $\endgroup$ – JanKanis Sep 24 '18 at 22:08
  • $\begingroup$ Possible duplicate of chemistry.stackexchange.com/questions/78623/… $\endgroup$ – matt_black Sep 25 '18 at 12:21

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