Fossil fuel consumption for electric power generation fell to 22.5 quadrillion British thermal units (quads) in 2017. The Energy Department's Energy Information (EIA) says that was the lowest level since 1994.

The trend has been driven by a drop in the use of coal and petroleum. That drop was offset somewhat by an increase in the use of natural gas. Changes in the fuel mix and improvements in electricity generating technology have also led the power sector to use fewer fossil fuels, EIA says.

Declining trend in fossil fuel use for power generation. Source: EIADeclining trend in fossil fuel use for power generation. Source: EIAIndeed, a December 2017 paper from the St. Louis Federal Reserve Bank said that cheaper alternative energies are the main factors driving the decline of U.S. coal consumption and production.

“Some have attributed coal’s misfortune to increased regulation on carbon emissions,” wrote the authors. “Although regulation increases costs for coal producers, coal’s main problem is technology that has reduced the cost of competing sources of energy.”

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Depressed global demand for coal has driven down prices and hurt producer profits. This, in turn, has hit U.S. coal mining employment, which is down 42 percent from its high in 2011.

Lower heat rates equate to higher efficiencies at power plants. Source: EIALower heat rates equate to higher efficiencies at power plants. Source: EIAEIA says that in 2017, coal consumption by the electric power sector reached its lowest level since 1982. Petroleum consumption in the sector was the lowest since record-keeping began in 1949. Recent natural gas consumption in the power sector has been rising, but EIA says that 2017 consumption was slightly lower than the record-high 2016 level.

In energy-equivalent terms, more coal was consumed in the power sector than natural gas in 2017, at 12.7 quads and 9.5 quads, respectively. However, in terms of electricity generation, natural gas-fired power plants produced more electricity than coal-fired plants, at 31 percent and 30 percent of the U.S. total, respectively, in 2017. Natural gas-fired units tend to be more energy efficient, requiring less energy content to produce a unit of electricity.

As recently as 2000, natural gas-fired power plants were on average about as efficient as coal-fired plants. Since then, EIA says that new natural gas-fired power plants have tended to use combined-cycle generators. These are more efficient because the waste heat from the gas turbine is routed to a steam turbine that generates additional power.

Emissions have dropped as the fuel mix has changed. Source: EIAEmissions have dropped as the fuel mix has changed. Source: EIACombined-cycle units now make up most of the natural gas-fired electricity generation capacity. EIA says that by the end of 2018, natural gas combined-cycle units may surpass conventional coal-fired power plants to become the most prevalent technology for generating electricity in the United States.

As natural gas-fired technology has expanded and become more efficient, the generation-weighted average efficiency among fossil fuel-fired electricity generation has improved. In 1994, fossil fuel power plants required 10,400 British thermal units (Btu) of primary energy to produce each kilowatthour; by 2017 that rate had fallen to 9,400 Btu/kWh, indicating a more efficient use of each unit of energy.

EIA says that these changes in energy consumption and efficiency have also affected carbon dioxide emissions from the electric power sector, which in 2017 were the lowest since 1987. Because coal combustion is much more carbon intensive than natural gas combustion, CO2 emissions from coal were more than double those from natural gas in 2017, even though natural gas provided more electricity generation.