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Carbon Content and H/C Ratio
As fossil fuels are derived from organic matter, they naturally contain high percentages of carbon. Volatile fossil fuels, such as natural gas, have the lowest carbon content, around 14.4 tons of carbon per terajoule produced, followed by oil with 19.9 t/TJ, and coal with 25.4 t/TJ. What is desirable in a fossil fuel is a high hydrogen to carbon ratio, as this results in more energy released upon combustion. Therefore, the higher the saturated hydrocarbons content in a fuel, the more energy it produces. Coal, with its high carbon content, has the lowest H/C ratio.
Combustibility and Heating Value
Combustion refers to the burning of fossil fuels in the presence of air, producing energy as well as water, carbon dioxide and other compounds. A fuel's heating value, also known as the heat of combustion, is the total amount of energy produced by its complete combustion. Natural gas has the highest heating value, producing 54 kilojoules per gram. Gasoline and diesel, crude oil derivatives, also have relatively high heating values, producing 47 and 45 kJ/g respectively. Coal's low H/C ratio gives it a low heating value, around 15-27 kJ/g.
Flammability
Flammability is a property often confused with combustibility. It differs, however, in that flammability, an indication of how easily a substance ignites, is rather a measure of how safe a fuel is to handle. Coal, for instance, is combustible, but not flammable. Gasoline is one of the most highly flammable fossil fuels. It has a low ignition temperature and low concentration for flammability. Natural gas, on the other hand, has a narrow flammability range in addition to a high ignition temperature, making it one of the safest fossil fuels.
Emissions
The combustion of fossil fuels results in the production of carbon dioxide, water vapor and air pollutants, such as sulfur dioxide, nitrogen oxides and other volatile organic compounds, as well as heavy metals. The growing concern about greenhouse gas emissions is one of the main arguments against fossil fuels. Among the fossil fuels, coal produces the most carbon dioxide emissions, along with sulfur dioxide and nitrogen oxides, gases responsible for producing acid rain. Natural gas is the cleanest fossil fuel, as it produces less carbon dioxide per joule delivered -- 30 percent less than petroleum and 45 percent less than coal.