If you interfere with the flame, by taking away heat or limiting airflow so it can't get enough oxygen, you will get partial combustion, where you have incomple

If you interfere with the flame, by taking away heat or limiting airflow so it can't get enough oxygen, you will get partial combustion, where you have incomplete combustion byproducts and vaporized fuel in the mix as well. Same reason why a can of gasoline on fire will produce huge billows of black smoke (insufficient oxygen) while when burned under precise conditions in an engine it's much cleaner, even without exhaust filtering.