Why does methane burn clean?
Why does compressed natural gas, or methane, burn cleaner than regular fuel? It's still a hydrocarbon. Why shouldn't it combust into the same pollutants as everything else?
There are two sides to this and that's the amount of carbon monoxide, which is what we call CO, and also the amount of soot which is much closer to pure carbon. The amount that's formed depends on the carbon to oxygen ratio in the mixture. Coal, for example, and many long-chain hydrocarbons have a much higher carbon content and it's necessary to have a much higher oxygen content in the mixture in order to avoid the formation of what we call lower oxides or soot. So if there's lots of carbon there it's much easier to get something wrong and not have enough oxygen to take it all the way to CO2.