The Yates gas-conversion outages will be staggered, Schwartz says. "The same equipment supply and engineering work will be done on both units," but while they "are identical, there are subtle details that [prevent] a one-size-fits-all approach," he notes.

Chris Curow, project manager for the coal-to-gas conversion at Alabama Power's Gaston units 1 through 4, in Shelby County, says the situation there is much the same. Gaston's are "traditional Babcock & Wilcox drum-type units producing approximately 270 MW each," he says. "Right now, they use pulverized coal as the primary fuel." To comply with MATS, they need to be either retrofitted with emissions controls or converted to natural gas, he says.

The Gaston project includes "modifications to the existing boilers to accommodate natural gas. They include all electrical upgrades, piping, valves, upgrade to the control system and burner modifications. In addition, a 32.5-mile, 24-inch-diameter natural-gas pipeline and related equipment valves will be installed" to deliver the units' new fuel, says Curow. "Each burner is being modified to accept a gas spud," he says. "Electronics will remain mostly unchanged, except for a new flame scanner … for the additional [ultraviolet] light emitted from burning natural gas. We are also adding natural-gas detectors along the front and rear walls."

The Alabama Power project manager says the utility is "realizing some economies of scale, with installation being undertaken on four sister units at the same plant site." Workers will benefit from a lot of "lessons learned" as the project progresses, says Curow.

"Pipefitters will be able to assemble the first unit's … pipe and then use the lessons learned to improve productivity on the next few units. They will have already installed one, so they know how it's done and may have a better idea for the next one. We feel electricians will be able to do the same during unit operation with cable tray, cable and conduit [installation]."

He adds, "Installing burner equipment and flame scanners should be similar across the four units. Therefore, we would expect productivity to increase somewhat after the first unit's equipment is installed." The Gaston and Yates coal-to-gas conversion projects are expected to be completed in 2015.

Several other U.S. utilities also are converting older coal units to natural gas, again as a cost-effective way to comply with EPA's new MATS rule. For example, Kentucky Power, a subsidiary of American Electric Power, plans to convert to natural gas the 278-MW unit at its 1,078-MW Big Sandy station. The larger, 800-MW unit there will be retired.

In Wisconsin, We Energies recently secured state regulatory approval to convert its 280-MW Valley coal unit to natural gas. The utility said retiring the unit would have required about $170 million in transmission system upgrades; the planned coal-to-gas conversion will cost only $70 million.

Independent power companies with coal-fired assets are taking the conversion route, as well. NRG Energy, the largest such company, is planning two coal-to-gas conversions: one at its 753-MW Avon Lake station in Ohio and the other at its 330-MW New Castle station in Pennsylvania.