Key points:
- EPA data show power plant sulfur dioxide down 96 percent and nitrogen oxides down 90 percent since 1990.
- By 2016, 82 percent of coal capacity was scrubbed and 54 percent had catalytic NOx control.
- Independent tests at New Hampshire’s Merrimack Station found 96 percent to 98 percent sulfur dioxide removal and 97 percent to 98 percent mercury removal.
- Deaths linked to coal air pollution fell from about 55,000 in 1999 to 1,600 in 2020, with the drop tied to scrubbers and retirements.
- Real problems remain, including cost overruns, mercury hot spots and coal ash, but they are engineering and governance challenges, not reasons to ignore the progress.
- Coal’s on-site fuel stockpiles make it a reliability asset as data centers and industry push demand higher.
From soot to scrubbers
The story begins with soot. Before the Clean Air Act of 1970, American power plants vented sulfur dioxide and ash with little restraint, and the fallout, acid rain, scarred lakes and forests across the Northeast. Congress answered in 1990 with amendments that created the Acid Rain Program, a cap-and-trade system with a hard ceiling of 8.95 million tons of annual power plant sulfur dioxide. EPA reported in 2005 that compliance had run close to 100 percent. According to Stephen Heins, writing for MasterResource, wet sulfate deposition - the fingerprint of acid rain - has since fallen more than 70 percent.
The hardware did the heavy lifting. A wet scrubber sprays limestone slurry through the flue gas, where it binds with sulfur dioxide and leaves gypsum behind. Selective catalytic reduction passes ammonia or urea over a catalyst bed, splitting nitrogen oxides into harmless nitrogen and water. Baghouses and electrostatic precipitators strip out fine ash, and activated carbon injection traps mercury. The Energy Information Administration counted 91 gigawatts of coal capacity retrofitted with scrubbers between 2005 and 2011 alone.
Real plants tell the tale better than fleet averages. At Merrimack Station in Bow, New Hampshire, independent stack tests in 2012 showed the new scrubber removing 96 percent to 98 percent of sulfur dioxide and 97 percent to 98 percent of mercury. That was well beyond the 80 percent mercury target written into state law. Heins reports that Ameren’s Duck Creek, Coffeen and Sioux stations in Illinois posted sulfur dioxide removal of 99.83 percent, 99.95 percent and 99.65 percent in performance tests. Operators of Illinois’s Prairie State Generating Station say their equipment removes more than 98 percent of sulfur dioxide and nitrogen oxides and more than 95 percent of mercury. That claim comes from the plant’s own promotional material, so it deserves a grain of salt, but it fits the pattern seen elsewhere.
So why has the record vanished from public debate? “Modern coal plants are not the plants of the acid-rain era,” Heins wrote. He also wrote that “the air got cleaner because engineers installed and operated the hardware, not because coal was abolished.” The federal data back the first half of that history. EIA found that coal generation set a record in 2007, when sulfur dioxide reductions were already well underway. Natural gas and plant retirements have since taken a larger share of the credit, but the foundation was laid by steel, limestone and catalysts.
What the cleanup bought, and what it cost
The health payoff is measurable. A 2023 study in the journal Science linked each coal plant’s emissions to Medicare death records. It estimated that deaths attributable to coal air pollution fell from roughly 55,000 in 1999 to 1,600 in 2020. Deaths dropped when plants installed scrubbers or shut down. The study’s authors stress that the earlier toll was larger than scientists had realized, which is precisely the point. The controls are what bent the curve.
Mercury tells a similar tale. Researchers at Harvard report that power plant mercury is down about 90 percent since the 2011 Mercury and Air Toxics Standards took effect. “The MATS regulation is another wonderful success story linked to the Clean Air Act Amendments of 1990,” said Elsie Sunderland, a Harvard environmental chemistry professor.
Honest accounting requires some caveats. Merrimack’s scrubber ballooned from a 2006 estimate of $250 million to $422 million, and a prolonged fight over who would pay for it ended with a settlement spreading the cost across ratepayers. About 230 of the 507 coal plants operating in 2010 had fully retired by 2020, so shutdowns, not just scrubbers, explain part of the mercury drop. Texas and North Dakota, where plants burn local lignite under looser standards, remain mercury hot spots. Coal ash and scrubber wastewater also demand tighter handling, and federal coal ash rules from 2015 were barely enforced until 2022. These are management failures that engineers and regulators can fix, not indictments of the technology.
Another point: Coal plants store months of fuel on site, Heins notes, which has proved its worth when winter storms tighten natural gas deliverability. With data centers and industrial load rising and interconnection queues stretching for years, shutting down dispatchable plants that already carry their pollution controls is foolish. The cost lands on ratepayers, and in extreme cases on families sitting in the dark. That's why today's clean coal should be part of a diversified sustainable energy grid.
Sources include:
EPA.gov [PDF]
EPA.gov [PDF]
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