Merrimack Station, a power plant in Bow, N.H., owned and operated by Public Service of New Hampshire (PSNH), burns 4,000 tons of coal a day, as it generates enough power to satisfy the energy needs of one-third of New Hampshire's population. The fly ash produced during the combustion of coal contains a trace amount of mercury that is emitted into the atmosphere. The plant's operators sought to reduce these emissions to improve the environment and public health, and to meet strict air-quality legislation recently passed in New Hampshire.
PSNH executives chose to install a wet, limestone-based flue-gas-desulphurization (FGD) system to limit mercury and sulfur dioxide emissions.
Chicago Bridge & Iron Company (CBI) built the primary processing system for the FGD system. It included six carbon-steel tanks located side-by-side. The tanks range from 24 ft to 56 ft in diameter.
Once constructed, the interior of the tanks required a special coating to ensure protection against constant exposure to abrasive and corrosive conditions.
CBI hired G.C. Zarnas, a national industrial and commercial painting contractor based in Bethlehem, Pa., to blast all six tanks and apply a Ceilcoate fiber-glass inlay coating to the metal. Bob Mudri, northeast superintendent with G.C. Zarnas, oversaw the project.
"The major threat to the integrity of the coating was the possibility of formation of rust bloom—corrosion that occurs immediately on steel surfaces between the blasting phase and application of the paint during moist conditions," Mudri said.
For the formation of rust to be prevented, and the coating to be applied and cured properly, the temperature of the metal had to be maintained at a minimum of 50°F, and humidity levels inside of the tank had to be less than 50 percent.
Adding to the challenge was that the project was scheduled to begin during winter in northern New Hampshire, where temperatures can reach as low as -15°F.
"I needed a system that allowed us to blast the entire tank without having to apply the coating the same day," Mudri said. "The process of blasting and coating in sections is costly because of the labor and time involved, and leaves overlaps in the coating that can become failure points over time."
Mudri contacted Jeff Cox, lead industrial technician for the northern United States region for Polygon, to design a system to control moisture and temperature during the blasting and coating process.
Cox analyzed the project's parameters and recommended a combination of dehumidification and heat to maintain ideal blasting and coating conditions.
Polygon technicians delivered a 4500-cfm desiccant dehumidifier and two inline electric heaters from the company's Plaistow, N.H., office and configured the equipment near the tanks. Polygon personnel used ductwork to connect the climate-control equipment to the tanks via manways at the base of each structure.
Once operational, the equipment provided a dew-point differential of 17°F and steel-surface temperatures of 50°F. This allowed G.C. Zarnas to blast an entire tank before coating it.
"The larger tanks took up to a week to sandblast," Cox said. "After sandblasting, the tank was cleaned and then the coating was applied."
"Without the proper climate, we would have had to put a coat of paint or primer on each day after finishing to prevent rust," Mudri said. "It would have taken much longer— possibly twice as long—because we would have to sandblast, clean, and then paint."
Mudri said work continued even with temperatures dropping significantly during the harsh New England winter.
"It was a brutal winter with temperatures below zero at times," Mudri said. "Even so, conditions inside the tank were always at the necessary standard and the project was completed as planned."