Nevertheless, where the NOX-SOX combination works, it often works well. Southern is in the process of adding two giant SOX scrubbers costing hundreds of millions of dollars to power plants that already have NOX controls. When he throws the switch on the new controls in 2007 and 2008, Monroe expects that together these systems will capture about 80 percent of the mercury, too.
The nation's coal-fired power plants currently emit about 48 tons of mercury a year. The toxin, along with mercury blown in from other countries, settles in rivers and lakes and enters the food chain, concentrating in fish. There are mercury fish advisories in 44 states, and the toxin has been found in women of child-bearing age.
The new mercury rule represents the first time the US has regulated mercury emissions from power plants.
"In so doing, we become the first nation in the world to address this remaining source of mercury pollution," Steve Johnson, acting administrator of the US Environmental Protection Agency, said in a statement announcing the new rule.
Under it, power plants are expected to cut mercury emissions from 48 tons currently to 38 tons by 2010 - and cut to 15 tons by 2018, a 70 percent reduction, using mercury-specific technologies.
Much of that first-phase reduction is expected to occur through "cobenefits" - cuts that occur by doing nothing more than meeting another new set of clean air requirements announced last week for reducing NOX and SOX. Those reductions are called the Clean Air Interstate Rule (CAIR).
Relatively few power plants today have both NOX and SOX removal gear like Mount Storm's. But under the new rule there should be many more Mount Storm-like conversions, experts say.
A key feature of both sets of new EPA rules is the use of market-based cap-and-trade systems. Under these, pollution allowances for mercury, SOX, and NOX may be purchased or traded by utilities. Such NOX and SOX removal systems may be efficient enough that they one day generate mercury credits sold on the open market - or compensate for other more polluting plants.