By Glenn McGrath
This article was published by the US Energy Information Administration on June 9, 2021.
Over the past 15 years, the US 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 US electric power sector produced 1,724 million metric tons (MMmt) of CO2, 32 per cent 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 per cent of US electricity generation; that share declined to 23 per cent in 2019. Conversely, natural gas increased from 19 per cent of total generation in 2005 to 38 per cent 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, nine per cent of the electricity generated in the United States came from renewable sources. The renewable share of generation rose to 18 per cent in 2019, largely driven by growth in wind and solar generation. Nuclear generation, a zero-emission energy source, made up about 20 per cent of US 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 per cent) of that decline is attributable to the increase in renewable generation. In comparison, almost 532 million metric tons (65 per cent) 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.