Another way of looking at fuels is to consider the carbon content. All fuels contain carbon bound to hydrogen atoms. Oxidizing those compounds releases the carbon which is commonly referred to as CO2. More properly they are referred to as carbon equivalents to cover the broad spectrum of greenhouse gases that are created when fuel is burned. It's important to know that fuels for a given Btu content have different stack characteristics. Processed natural gas is a particularly clean burning fuel. It has very little sulfur, no ash and not much comes out of the stack but carbon dioxide, carbon monoxide and water vapor. Natural gas actually has no scent. The "rotten egg" smell that we all associate with natural gas is the addition of T-butyl Mercaptan which became a standard practice after the 1937 disaster at the New London School in Texas.
One look (and sniff) of a jar full of our alternate fuel (#6) will demonstrate it is a much different fuel. #6 oil is considered to be one step above the lowest grade of fuel called bunker oil commonly used in the shipping industry to fuel the massive engines found in ocean faring ships. Although the #6 fuel oil we use has been processed to contain less than 0.5% sulfur there are many other compounds and particulates that will not be found in lighter oils or natural gas. It has the consistency of molasses at room temperature and in fact has to be heated and injected at high pressure through a nozzle to burn properly. Even with a clean burn there is considerable fly ash and other particulates which precipitate out of the exhaust stream. Additionally, fuel oil when burned will have fairly high levels of nitrogen oxides and sulfur oxides both of which contribute to ozone formation.