Study: Combined cycle natural gas power plants produce less emissions

Power plants that use natural gas and new technology to squeeze more energy from fuel emit less carbon dioxide than coal-fired plants, according to new analysis. So-called “combined cycle” natural gas power plants also emit less nitrogen oxides and sulfur dioxide.

“Since more and more of our electricity is coming from these cleaner power plants, emissions from the power sector are lower by 20, 30 even 40 percent for some gases since 1997,” said Joost de Gouw, lead author of the analysis.

De Gouw works as an  atmospheric scientist with National Oceanic and Atmospheric Administration Cooperative Institute for Research in Environmental Sciences at the University of Colorado in Boulder.

De Gouw and his colleagues analyzed data from systems that continuously monitor emissions at power plant stacks around the country. Previous aircraft-based studies have shown these stack measurements are accurate for carbon dioxide, nitrogen oxides and sulfur dioxide.

To compare emissions from different types of power plants, the scientists calculated emissions per unit of energy produced for all data available between 1997 and 2012. During that period of time, on average:

  • Coal-fired power plants emitted 32 ounces of carbon dioxide per kilowatt hour of energy produced.
  • Natural gas power plants emitted 19 ounces of carbon dioxide per kilowatt hour.
  • Combined cycle natural gas plants emitted 15 ounces of carbon dioxide per kilowatt hour.

In combined cycle natural gas power plants, operators use two heat engines in tandem to convert a higher fraction of heat into electrical energy.

For context, U.S. households consumed 11,280 kilowatt hours of energy, on average, in 2011, according to the U.S. Energy Information Agency. This amounts to 11.4 metric tons per year of carbon dioxide per household if all of that electricity was generated by a coal-fired power plant, or 5.4 metric tons if it all came from a combined cycle natural gas plant.

The researchers reported that between 1997 and 2012, the share of electricity generated in the United States from coal decreased from 83 percent to 59 percent, while the proportion of energy from combined cycle natural gas plants rose from none to 34 percent.

That shift in the energy industry meant that power plants, overall, emitted 23 percent less carbon dioxide into the atmosphere last year than they would have had coal been providing about the same proportion of electric power as in 1997, de Gouw said.

The switch led to even greater reductions in emissions of nitrogen oxides and sulfur dioxide, which dropped by 40 percent and 44 percent, respectively.

The new findings are consistent with recent reports from the Energy Information Agency that substituting natural gas for coal in power generation lowered power-related carbon dioxide emissions in 2012.