Study credits combined-cycle gas plants with cutting emissions

Thanks to the growing role of natural gas, emissions from fossil fuel power plants in the United States were 23% lower in 2012 than they would have been if coal power were still as dominant as in 1997.

That’s a key finding from a report drafted by officials at the National Oceanic and Atmospheric Administration (NOAA) in Colorado and made public Jan. 8. The American Geophysical Union (AGU) peer-reviewed journal, Earth’s Future, featured an analysis of the study.

Further reductions in these emissions can follow by converting a larger fraction of U.S. electric power production to natural gas, and by ensuring that all natural gas plants are equipped with the latest combined cycle technology.

The study used data from continuous emissions monitoring systems (CEMS) to support its findings. The study notes that combined-cycle gas plants emit on average 44% of the CO2 compared to coal plants.

In addition, combined-cycle gas plants emit less NOx and far less SO2 per unit energy produced than coal power plants. The increased use of natural gas has therefore led to emissions cuts of NOx (40%) and SO2 (44%), in addition to those obtained from the implementation of emissions control systems on coal power plants.

These benefits, however, should be weighed against the increase in emissions of methane, volatile organic compounds and other trace gases that are associated with the production, processing, storage and transport of natural gas.

But when natural gas is used for electric power generation it clearly results in less CO2 and pollutants than coal-fired power plants, according to the research.

“In general accord with gross load, the total CO2 emissions from fossil-fuel power plants in the U.S. increased until 2007, but then decreased more rapidly between 2008-2012. Since the CO2 emission intensity of coal is so much higher than that of natural gas, particularly with combined cycle technology, a significant fraction of this decrease in CO2 emissions can be attributed to the switch from coal towards natural gas,” according to the peer-reviewed article.

Several changes occurred over the 1997 – 2012 period. The number of coal power plants gradually decreased. The number of gas plants grew significantly over this same period.

As time went on, combined-cycle gas plants also yielded a reduced CO2 footprint than other types of natural gas power technology, according to the research. “From 1997-2012, the CO2 emission intensity of coal power plants decreased slightly. The CO2 emission intensity of natural gas power plants with combined cycle technology decreased by about one-third,” according to the journal article.

Due to the implementation of emissions controls, the NOx and SO2 emissions intensities for coal and natural gas power plants all decreased between 1997 and 2012. For coal, the average decreases in NOx (72%) and SO2 (71%). This is important because coal plants are the chief emitters of those two pollutants.

More electricity coming from cleaner gas plants

“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 lead author Joost de Gouw, an atmospheric scientist with the NOAA Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder.

To compare pollutant 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-based power plants emitted 915 grams (32 ounces) of carbon dioxide per kilowatt hour of energy produced;

Natural gas power plants emitted 549 grams (19 ounces) carbon dioxide per kilowatt hour; and

Combined cycle natural gas plants emitted 436 grams (15 ounces) carbon dioxide per kilowatt hour.


About Wayne Barber 4201 Articles
Wayne Barber, Chief Analyst for the GenerationHub, has been covering power generation, energy and natural resources issues at national publications for more than 20 years. Prior to joining PennWell he was editor of Generation Markets Week at SNL Financial for nine years. He has also worked as a business journalist at both McGraw-Hill and Financial Times Energy. Wayne also worked as a newspaper reporter for several years. During his career has visited nuclear reactors and coal mines as well as coal and natural gas power plants. Wayne can be reached at