WASHINGTONVILLE -- At 700 feet, the new smokestack at PPL's Montour Steam Electric Station is the tallest structure on the site, and with its completion, the 1,552-megawatt plant is one of the cleanest coal-fired electric generating plants in the country, utility officials said.

During a tour Thursday morning, plant manager Michael G. Munroe described in detail the three-year, $600 million construction project intended to remove sulfur dioxide and other by-products that result from burning bituminous coal from the flue gas before it reaches the atmosphere.

Sulfur dioxide is a major component of acid rain, haze and particulates in coal smoke. The flue gas emanating from the smokestack is little more than water vapor, Munroe said.

A separate system removes particulate matter and nitrous oxide from the flue gas.

"This was not a required project," he said. "We've done this ahead of the regulations."

The plant already meets federal environmental standards, Munroe said, and the scrubber system removes 97 percent of the sulfur dioxide.

A limestone-based scrubber system removes from the smoke sulfur dioxide, the result of burning 4 million tons of coal annually. It works by spraying a solution of powdered limestone and water on the flue gas. The calcium in the limestone reacts with the sulfur dioxide to form calcium sulfite, which is collected at the base of the smokestack. Air is pumped through the calcium sulfite, which changes it chemically into calcium sulfate, or synthetic gypsum, which is removed and dried.

The gypsum product is moved by a series of conveyor belts to a nearby storage site. In a few months, however, the material will be used by U.S. Gypsum, which will make wallboard across the road from the power station once its plant is built. When that plant comes on-line, the gypsum will go directly from the scrubber to the plant by conveyor.

Munroe estimated the scrubber will produce about 120,000 tons per year of synthetic gypsum.

Limestone from local quarries, supplemented by additional material brought in by rail, is ground into a "ball mill," a rotating cylinder with a quantity of baseball-sized steel balls inside that roll around and crush the limestone, after which water is added to produce a slurry the consistency of a milkshake.

That material is pumped to the scrubber and sprayed in.

The enormous smokestack and the adjacent buildings include 26,000 cubic yards of concrete, placed in three separate 24-hour continuous pours.

"We had a truck in here every six minutes around the clock for the pours," Munroe said.

The 25-foot tall scrubber includes about 1 million pounds of reinforcing bars, and the smokestack has another 1.2 million pounds of steel reinforcement. The fiberglass liners for the stack were made on site in sections, which were added to the bottom as the previous sections were jacked up.

The two original red-and-white striped smokestacks are no longer in use, but they will remain in place, Munroe said. It is cost-prohibitive to demolish them, and the stacks are also home to several peregrine falcons, which keep the pigeon population around the plant in check.