Fuels have varying carbon contents—coal has a higher carbon content than does oil; oil has a higher carbon content than natural gas. Since emissions of greenhouse gases are directly related to carbon content, it follows that combusting coal produces more emissions than does burning the equivalent amount of natural gas. Roughly speaking, combusting gas produces 60% less carbon than combusting coal of equivalent energy (Btu) content; combusting oil emits approximately 30% less than coal on an equivalent energy basis.

Therefore, electric utilities can reduce carbon emissions by substituting lower carbon fuels for higher carbon fuels throughout its operations. Switching from oil- and coal-based boilers to gas can be economically viable in many regions. Cleaning coal used at utility sites can also reduce carbon emissions by reducing the coal transportation needs (by reducing the quantities and volumes of impurities shipped with the coal) and by improved boiler performance. Likewise, using low- or zero-carbon fuels can further reduce utility carbon emissions.

These fuel options offer a wide range of opportunities to economically reduce carbon emissions while meeting site specific constraints. This section describes several of the principal fuel alternatives currently employed.