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Cally Carswell | Jun 13, 2012 06:00 AM

I've done a few stories on air pollution in the last year, and many a source has told me this: When it comes to pollution, all fossil fuel power plants are not created equal. It's a principle enshrined in the Clean Air Act. Power plants that began generating electricity before 1978 are grandfathered into the act's pollution control regulations -- unless they undergo substantial upgrades, they don't have to outfit themselves with the emission-cutting equipment that the Environmental Protection Agency requires new facilities to have. It doesn't take much of a stretch of the imagination to predict the result: Older power plants pollute more than newer ones. 

In a new report, the Government Accountability Office quantified the disparity, and the specifics are pretty interesting. Among fossil fuel power plants -- those burning coal, natural gas or oil -- older units generate 45 percent of the electricity. Yet those same plants spew out 75 percent of sulfur dioxide emissions, 64 percent of emissions of nitrogen oxides, and 54 percent of carbon dioxide emissions.

A couple of charts tell the story a different way. 

GAO emissions by decade

 

GAO SO NOX by decade

 

Pretty striking, eh? There are a few explanations for why newer power plants -- and especially the newest -- emit so much less sulfur dioxide and nitrogen oxides. One is that in the 2000s, many more new natural gas plants came online than coal plants. They don't emit sulfur dioxide -- which can aggravate asthma and other respiratory ailments; emit far less nitrogen oxides, a contributor to ozone; and release about half as much carbon dioxide. Before the 1990s, almost all new power plants to come online were coal-fired, and in the 1990s the split between gas and coal was about even. 

Another reason for the difference is the Clean Air Act. New coal fired power plants are required to install what's called "best available control technology" to curb emissions of pollutants like sulfur dioxide. What's considered the best available technology changes and improves over time. For instance, these days new coal plants are outfitted with scrubbers that can eliminate more than 90 percent of sulfur dioxide emissions. Sixty-three percent of the coal-fired power plants built after 1978 use this technology, while only 26 percent of those built before then do. 

As for carbon dioxide, aside from being more efficient at converting coal into electricity, new coal plants aren't much better than old because, as we reported on this blog recently, technology to capture carbon emissions is still in its infancy. 

So, the good news: We've gotten a lot better at generating cleaner power, even from fossil fuels. The bad news: Those old, dirty power plants are still an important part of our energy mix -- and, for the most part, they aren't getting any cleaner. 

Cally Carswell is HCN's assistant editor. 

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