Coal-to-natural gas switch leads to 28% drop in emissions

April 27, 2021
TMI Staff & Contributors

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CO2 emissions in the U.S. electric power sector have dropped 28% over the last decade primarily due to coal-to-natural gas switching.

Despite year-to-year changes in economic activity and weather, total electric power generation from utility-scale electric power plants has remained nearly constant at just over 4,000 gigawatthours (GWh) annually since 2010. However, changes in the electric power generation mix have resulted in a significant decrease in CO2 emissions associated with electric power generation. In 2019, U.S. electric power generation produced 1,724 million metric tons (MMmt) of CO2, down from 2,389 MMmt in 2010.

CO2 emissions from electric power generation decreased by 664 MMmt from 2010 to 2019 mostly because of the shift from coal to natural gas in the electric power generation mix. The increased use of renewable resources also contributed to reduced emissions. Wind and solar energy represent the majority of current renewable resources, and renewable resources have continued to displace fossil fuels in the electric power generation mix. In 2019, renewable generation was 18% of total generation, an increase from 10% of total generation in 2010. In 2019, fossil fuel generation accounted for 62% of total generation, a decrease from 71% of total generation in 2010. Nuclear generation (another source of electric power generation that does not emit CO2) was approximately 20% of total generation in both 2010 and 2019 and has stayed relatively constant throughout the period. All other energy sources that are not renewables, fossil fuels, or nuclear have had little effect on the nation’s electric power generation mix.

Natural gas and coal emit different amounts of CO2 when generating electric power. In 2019, coal-fired generation produced 2,257 pounds (lb.) of CO2 per megawatthour (MWh), and natural gas-fired generation produced 976 lb. of CO2/MWh, about 43% of the CO2 emission rate of coal. Differences in chemistry and conversion efficiency help explain this divergence. In terms of chemistry, coal has a higher carbon content per weight than natural gas, and on average, coal consumption for electric power generation produces 209 lb. of CO2 per million British thermal units (MMBtu) each year. In contrast, natural gas consumption for electric power generation produces 117 lb. of CO2/MMBtu.

In terms of conversion efficiency, natural gas-fired generation technologies on average generate electric power with less energy input than coal, which helps lower CO2 emissions. In 2019, the conversion efficiency of natural gas-fired generation was 7,731 British thermal units per kilowatthour (Btu/kWh) and conversion efficiency has increased gradually since 2010 as more efficient generators have come online. In contrast, the conversion efficiency of coal-fired generation was 10,551 Btu/kWh in 2019 and has stayed relatively constant mostly because coal-fired generation technology has not changed much recently.

Because the share of natural gas has grown in the electric power generation mix, CO2 emissions for fossil fuel generation as a whole have declined. In 2019, the average blended CO2 emission rate for all fossil fuels was 1,479 lb./MWh, which is significantly lower than 1,833 lb./MWh in 2010.

Even though both the increased use of renewables and the shift from coal-fired to natural gas-fired generation have reduced CO2 emissions in the electric power sector, the coal-to-natural gas shift has had a larger effect on reduced emissions. Of the 664 million metric ton decline in CO2 emissions from electric power generation, about 200 million metric tons is attributable to the nearly 300,000 GWh increase in renewable power generation. In contrast, almost 411 million metric ton of CO2 emissions were saved by the 353 lb./MWh decline in the CO2 emission rate among fossil fuels.