Looked it up. Propane can burn two ways: w/ excess oxygen: C3H8 + 5 O2 -> 3 CO2 + 4 H2O + heat incomplete burning (insufficient oxygen): C3H8 + 3.5 O2 -> CO2 + CO + C + 4 H2O + heat
It is probably the C that is smelled. Not enough oxygen. Perhaps a regulator is out of adjustment, or crud is prevent sufficient oxygen from getting to the reaction.
Very efficiently. New ventless models are 99.9% efficient and can run unvented.
You'll need many more formulas. Hydrocarbons burning will produce a lot of various products, some toxic, others harmless. I have not noticed any smell from charcoal, graphite nor diamonds, don't think carbon has a detectable smell in any form. Carbon dioxide and carbon monoxide are in equilibrium with the reaction favoring CO2 (if you have one, the other will be there too).
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