What are the different types of fossil fuels?

Fossil fuels are hydrocarbons, which are complex compounds comprised of hydrogen and carbon atoms, combined with a variety of other elements, that have formed by the decomposition of dead animals and plants. When a hydrocarbon is burned, the hydrogen and carbon react with oxygen in the air to produce carbon dioxide (CO2), water (H2O), and heat (i.e., energy). The other elements result in a variety of emissions to the atmosphere, including nitrous oxides, sulfur dioxide, mercury and other heavy metals, and particulate matter (soot). The amount of CO2 created is a direct function of the fuel's carbon content. Natural gas, which is predominantly methane, has the highest ratio of hydrogen to carbon, and results in the least amount of CO2 emissions per Btu (British thermal unit) of energy when burned (see Figure 1). Coal is the most complex hydrocarbon and results in the most CO2 emitted per Btu, as well as the most other air pollutants as well.

The main categories of fossil fuels are:

• Natural gas is a mixture of hydrocarbon molecules with 1 to 4 carbon atoms per molecule. It is composed mainly of methane (CH4), generally between 70-90%, with the remainder being smaller quantities of ethane (C2H6), propane (C3H8), and butane (C4H10). Natural gas is the most commonly used fuel for conversion into energy for residential use. It is burned in furnaces to heat the home, water tanks to heat water, clothes dryers, and stoves. Propane is most commonly used in small tanks for mobile applications such as bbqs or campers. Liquefied Petroleum Gas (LPG) contains propane and butane and is used in larger tanks for residential use where there is no connection to utility gas. It has also become an option for a transportation fuel. Butane is used is cigarette lighters.
  
  
• Petroleum or crude oil is a broad range of hydrocarbons with components that can range from 5 to over 20 carbon atoms per molecule (see Figure 2). It can be further subdivided, from lightest to heaviest, as follows (note - categories overlap, and naturally occurring hydrocarbon accumulations always contain a combination of several fuels):
  
  • Napthas are a general name given to mixtures with 5 to 7 carbon atoms per molecule, including pentane (C5H12), liquid hexane (C6H14), and heptane (C7H16). These are used in a variety of solvents for paint, paint thinners, dry cleaning, and lighter fluids. They are easily vaporized and highly combustible.
  • Gasoline is a mixture of hydrocarbon molecules with 7 to 12 carbon atoms per molecule, including octane (C8H18), nonane (C9H20), and decane (C10H22). Gasoline's primary use is for transportation fuel.
  • Kerosene is mix of hydrocarbons with 12 to 16 carbon atoms per molecule. Kerosene is used as fuel for jet engines and tractors.
  • Diesel fuel is a mixture of hydrocarbons containing 15 to 18 carbon atoms per molecule and is normally produced by blending two or more refinery streams such as light gas oil, heavy gas oil and kerosene.
  • Fuel oil has a range of hydrocarbons with 20 to 40 carbon atoms. It is primarily used for home heating, shipping, and power plants.
  • Lubricating oil has a range of hydrocarbons with 16 to 20 carbon atoms. This oil does not vaporize at normal temperatures, which makes it ideal for lubricating engines.
  • Tar and asphalt bitumen are solids at room temperature and contain hydrocarbons with more than 20 carbon atoms. They can be used directly, e.g., asphalt road surfaces, or processed into a heavy crude oil.
  
  
• Coal is essentially a rock composed of hydrocarbons, but the geological process that produce coal tend to remove most of the hydrogen and oxygen atoms, leaving mostly carbon. For more information on coal, see OWOE: What is coal?.

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