The white plume rising up from Constellation Energy's Brandon Shores plant can be seen from miles away.

Billowing out of a wide exhaust stack, it looks like it could be a major source of air pollution.

But that water vapor actually is a sign that the Pasadena plant is one of the cleanest coal-fired plants in the country, company officials say.

It's part of the company's long-awaited flue gas desulfurization - or wet scrubbers - that cut sulfur dioxide emissions by an estimated 95 percent and mercury emissions by 90 percent.

"It's a step forward not only for us, but for the state of Maryland and...