e. The third step is the selection of carbon content factors for each fuel/product type and estimation of the total carbon content of fuels consumed. There is considerable variation in the energy and carbon content by weight and volume of fuels. However, expressing carbon content on a per unit of energy basis reduces this variation because of the close link between the carbon content and energy value of the fuel.

Carbon content factors may vary considerably both among and within primary fuel types:

• For natural gas , the carbon content factor depends on the composition of the gas, which in its delivered state is primarily methane, but can also include small quantities of ethane, propane, butane, and heavier hydrocarbons. Natural gas flared at the production site will usually be "wet" (i.e., contain far larger amounts of non- methane hydrocarbons), and its carbon content factor will be different. Methane is 75 percent carbon by weight. A typical natural gas sample on an NCV basis has a carbon content of 15 to 17 t C/TJ.
• Carbon content per unit of energy is usually less for light refined petroleum products such as gasoline than for heavier products such as residual fuel oil. Petroleum products vary between 5.6 degrees API gravity (dense products such as asphalt and road oil) and 247 degrees (ethane). This is a range in density of 60 to 150 kilograms per barrel, or +/-50 percent. The variation in carbon content, however, is much smaller (+/-5 to 7 percent): ethane is 80 percent carbon by weight (16.8 t C/TJ), while petroleum coke is 90 to 92 percent carbon (27.5 t C/TJ). The range of carbon contents can be explained by basic petroleum chemistry. A typical sample of crude oil is about 85 percent carbon by weight or 20 t C/TJ.
• For coal, its carbon content per tonne varies considerably depending upon the coal's rank and composition of hydrogen, sulfur, ash, oxygen, and nitrogen. The carbon content of coal can vary between 25 t C/TJ for some bituminous coals to 28 t C/TJ for some lignite coals.