June 9, 2021

Electric power sector CO₂ emissions drop as generation mix shifts from coal to natural gas

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Over the past 15 years, the U.S. electricity generation mix has shifted away from coal and toward natural gas and renewables, resulting in lower CO₂ emissions from electricity generation. In 2019, the U.S. electric power sector produced 1,724 million metric tons (MMmt) of CO₂, 32% less than the 2,544 MMmt produced in 2005.

Lower CO₂ emissions have largely been a result of a shift from coal to natural gas in the electricity generation mix. In 2005, coal made up 50% of U.S. electricity generation; that share declined to 23% in 2019. Conversely, natural gas increased from 19% of total generation in 2005 to 38% in 2019.

For the next few years, this trend may be changing. In the recent releases of our Short-Term Energy Outlook, we forecast that higher natural gas prices will lead to less natural gas-fired generation and more coal-fired generation in 2021. However, in 2022, we expect both coal and natural gas to lose a portion of their shares to renewables.

When generating electricity, coal emits significantly more CO₂ than natural gas. In 2019, coal-fired generation produced 2,257 pounds of CO₂ per megawatthour (MWh) of electricity. Natural gas-fired generation produced less than half that amount at 976 pounds of CO₂/MWh.

CO₂ emissions associated with generating electricity from coal and natural gas differ because of differences in the fuels themselves—coal has more carbon content per unit of energy. In addition, coal-fired plants and natural gas-fired plants differ in how efficiently they convert their respective fuels to electricity. The amount of CO₂ produced when a fuel is burned depends on a fuel’s carbon content. Coal produces more CO₂ per unit of energy than natural gas does when burned. Coal consumption for electricity generation produces 209 pounds of CO₂ per million British thermal units (MMBtu), compared with 117 pounds of CO₂/MMBtu for natural gas.

Natural gas-fired generators, especially those that operate in a combined-cycle configuration, are also more efficient than coal-fired generators. On average, natural gas-fired generators produce electricity with significantly less energy input than coal, also helping to lower CO₂ emissions. A lower heat rate indicates a more efficient plant. In 2019, the conversion efficiency for natural gas-fired generation was 7,731 British thermal units per kilowatthour (Btu/kWh) and 10,551 Btu/kWh for coal-fired generation.

The increased use of renewables has also reduced emissions from generating electricity in the United States. In 2005, 9% of the electricity generated in the United States came from renewable sources. The renewable share of generation rose to 18% in 2019, largely driven by growth in wind and solar generation. Nuclear generation, a zero-emission energy source, made up about 20% of U.S. generation in both 2005 and 2019.

Although both the increased use of renewables and the shift from coal-fired to natural gas-fired generation contributed to reductions in electric power sector CO₂ emissions, the shift from coal to natural gas had a larger effect. Of the 819 million metric ton decline in CO₂ emissions from 2005 to 2019, approximately 248 million metric tons (30%) of that decline is attributable to the increase in renewable generation. In comparison, almost 532 million metric tons (65%) of the decline in CO₂ emissions is attributable to the shift from coal-fired to natural gas-fired electricity generation. Decreased petroleum-fired generation largely influenced the remaining decrease in CO₂ emissions.

As the rate of coal-to-gas switching reverses in the short-term, the trend of declining power sector CO₂ emissions may change. Annual changes in power sector CO₂ emissions from natural gas and coal depend first on changes in the shares of those fuels in electricity generation and second on improvements in natural gas-fired generation efficiency.

Principal contributor: Glenn McGrath