Nuclear Engineer

If the Earth had no atmosphere, its average surface temperature would be about −18◦C. The Earth is kept at its relatively warm temperature by molecules in the atmosphere, including water molecules and carbon dioxide molecules, that absorb some of the infrared radiation emitted by the Earth and prevent its escape from the Earth’s environment. This is the natural “greenhouse effect.” Since the beginning of the industrial era, additional gases have been emitted into the atmosphere—particularly carbon dioxide (CO2)—which add to this absorption and are believed to further increase the Earth’s temperature.

This increment is referred to as the anthropogenic greenhouse effect. Warnings about the effects of CO2 emissions date to the 19th century, but they have become a matter of widespread concern only since the 1970s. The anticipated consequences are described as “global warming” or, more broadly, as “global climate change.” The production of CO2 is the inevitable accompaniment of any combustion of fossil fuels. The amount released per unit energy output varies for the different fuels, due largely to differences in their hydrogen content. Natural gas is primarily methane (CH4) and a considerable fraction of its combustion energy comes from the chemical combination of hydrogen and oxygen. Its ratio of carbon dioxide production to energy production is the lowest among the fossil fuels.

The releases are usually specified in terms of the mass of carbon (C), not CO2. Even for a given fuel type, the amount produced per unit energy is not a constant because the chemical composition of the fuels is nonuniform. However, for many purposes, approximate average values are adequate. Approximate coefficients, in megatonnes (Mt) of carbon per exajoule (EJ) of energy, are 24.6 Mt/EJ for coal, 18.5 Mt/EJ for petroleum, and 13.7 Mt/EJ for natural gas.11 These numbers illustrate the benefit of switching from coal to natural gas, when possible.