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Utah environmental regulators on Wednesday unveiled a revised plan for clearing the skies over the state's national parks, but to environmentalists' dismay it would do nothing to further cut emissions from aging coal-fired power plants.

Environmental groups and national park advocates had hoped the plan would require Rocky Mountain Power to retrofit two of its biggest Utah plants with the best technology available today for capturing nitrogen oxide emissions.

Instead, Division of Air Quality scientists sided with the utility and stuck with a dated pollution control plan, which they say will clean up the air enough. Anything more, they insist, is just cost-prohibitive.

"These required investments have been a significant contributing factor in recent price increases for our customers," RMP spokesman David Eskelsen said. "These controls significantly reduced emissions of nitrogen oxides, sulfur dioxide and particulates. The substantial additional expense of selective catalytic reduction on these units would likely not result in noticeable improvement to visibility in areas targeted by the Regional Haze program."

Utah has until January to submit a plan for cutting regional haze to the U.S. Environmental Protection Agency for approval.

Under that court-ordered deadline, the Air Quality Board on Wednesday endorsed a revised plan for Rocky Mountain Power's 1970s-vintage Hunter and Huntington stations in Emery County and opened up a 30-day public comment period.

Developed as a component of the Clean Air Act, the Regional Haze Rule is meant to protect 156 national parks and wilderness areas — including Utah's "Mighty Five" national parks — from visibility-impairing haze.

The federal law calls for the states to require energy companies to install "best available retrofit technology" on old coal-fired power plants because their sulfur and nitrogen oxides are leading contributors to the haze that obscures vistas from the Grand Canyon to the Great Smoky Mountains.

Utah is one of the last states in the country to fully comply with this section of the Clean Air Act. The state sued two years ago after the U.S. Environmental Protection Agency rejected parts of its first attempt at a regional haze implementation plan. The 10th U.S. Circuit Court of Appeals in Denver ultimately rejected Utah's suit and set a deadline of January 2015 for a new plan.

The state's original plan, implemented in 2008, required Rocky Mountain Power to spend $588 million installing low-NOx burners and other measures at the Emery plants' four units.

The upgrades have already achieved significant emissions reductions, DAQ scientist Colleen Delaney said Wednesday, and yet there has been little improvement in visibility over Utah's parks.

Accordingly, she said, the revised state plan does not require the power company to install selective catalytic reduction equipment (SCR), considered industry's gold standard for cutting NOx from plants' exhaust. But it also doesn't require any other upgrades.

"This is a widely available technology and we think the Clean Air Act requires it," said Christopher Thomas of HEAL Utah, one of four environmental groups denouncing the plan. "We think most Utahns would support more investments to cut air pollution."

Donna Spangler, a spokeswoman for the Department of Environmental Quality, stressed that when the state first ordered emission cuts, it required technology that was the most reasonable at the time.

"The problem HEAL is finding is that today, newer technology is available that could result in deeper cuts," she wrote in an email. "However, while those cuts may be reasonable going from where Utah's plants were in the 70s to today, they are very unreasonable going from where those plants are today to where they could be with the latest technology."

Environmentalists countered that Utah seems to be content with falling behind neighboring states, where old coal-fired plants are being required to use SCR. Installing such equipment at Hunter and Huntington would take $673 million and would cut NOx emissions by 80 percent, from 17,000 to 3,300 tons per year. Framed another way, RMP would be spending between $8,800 and $11,600 per ton to cut these emissions.

State regulators say that's too expensive.

At the same time, past reductions have yet to result in much clearing over the parks, despite modeling that predicted improvement.

Not requiring up-to-date technology is "bad news" for Utah's parks, according to Cory MacNulty of the National Parks Conservation Association. An estimated 9 million visitors travel to Utah's national parks each year. Another $12 billion is spent in Utah on outdoor recreation annually.

"Travel and tourism is Utah's second largest economic engine and a great deal of what makes that engine run are its national parks, monuments, recreation areas and the gateway communities that support and benefit from them," MacNulty said.

"The state can ill afford to play Russian roulette when it comes to air quality."

Meanwhile, RMP plans to shut down a relic, the Carbon Power Plant in Helper, by next April, a move the company and the state say could exceed any new reductions forced on the nearby Emery County plants.