Nova Scotia Power's Point Aconi plant overcomes CFB design problems to become rock of reliability

Dr. Robert Peltier, PE

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Nova Scotia, on Canada's southeast coast, is a thin peninsula. No matter where you go, you're never more than 34 miles from the sea. Residents of the province get 97% of their electricity from Nova Scotia Power Inc. (NSPI), a vertically integrated, regulated subsidiary of Halifax-based Emera Inc. NSPI and Bangor Hydro-Electric, a transmission and distribution company serving Maine, generate 90% of Emera's revenues.

In addition to operating 3,100 miles of transmission lines, 15,000 miles of distribution lines, and 200 substations, NSPI has an installed generating capacity of 2,293 MW. The generation resources (Figure 1) comprise four coal/pet coke-fired plants, one plant fueled by oil or natural gas, 33 small hydro plants on six rivers with a total capacity of 360 MW, seven simple-cycle combustion turbines, two LM 6000 gas turbines, a few wind turbines, and the well-known, 22-year-old, 20-MW Annapolis Tidal Power Plant, which is still the only facility of its kind in North America.

1. Fuel diversity, Canadian style. The Point Aconi plant, which burns a blend of coal and pet coke, is located on the eastern tip of the Nova Scotia peninsula. Source: Nova Scotia Power

Of NSPI's four coal plants, Point Aconi Thermal Generating Station (Figure 2), on Cape Breton Island at the east end of the peninsula, is most notable because it is powered by a circulating fluidized-bed (CFB) boiler. With original nameplate ratings of 183 MW (gross) and 165 MW (net), Point Aconi was the world's largest CFB plant—and the first to enter commercial service in North America—when it came on-line in 1993 (see box). Its boiler was supplied by Finland's Ahlstrom Pyropower, which was purchased by Foster Wheeler Corp. in 1995.

2. Early record holder. Rated at 183 MW (gross), Point Aconi had the largest circulating fluidized-bed boiler in North America when it went commercial in 1993. Courtesy: Nova Scotia Power

Inside Point Aconi Thermal Generating Station

Point Aconi was originally rated to produce 183 MW (gross) by burning 65 tons/hr of local coal and 15.5 tons/hr of limestone. The Ahlstrom Pyropower circulating fluidized-bed (CFB) combustion system (Figure 3) was designed to achieve a 90% reduction in SO₂ emissions and a 65% to 75% reduction in NO_x emissions—very respectable numbers in 1993, when the plant began operation. Tokyo-based Mitsui & Co., Ltd. was the turnkey contractor for Point Aconi; Sargent & Lundy was the project's architect/engineer.

3. Original equipment. The original configuration of Point Aconi's circulating fluidized-bed boiler and associated systems. Courtesy: Nova Scotia Power

The drum-type reheat CFB boiler was designed to produce 1,207,150 lb/hr of steam (maximum continuous rating) at 1,800 psi and 1,000F. Toshiba Corp. supplied the multicylinder, tandem-compound steam turbine-generator. The 3,600-rpm generator is totally enclosed and has a cylindrical rotor whose winding is directly cooled by hydrogen at 30 psi. The generator is rated at 217 MVA and has a power factor of 0.85.

Point Aconi's coal-handling system comprises an active 27,500-ton storage pile; an inactive storage pile of identical size; receiving hoppers, conveyors, crushing equipment; and four in-plant storage bunkers. Fuel is delivered to the station by truck. The conveying system can handle 440 tons of coal per hour or about 550,000 tons per year. The limestone-handling system consists of receiving hoppers and conveyors common to the coal system, crushers, dryers, storage silos, and a pneumatic conveyor leading to two in-plant storage bunkers. Limestone also arrives by truck, at the rate of about 165,000 tons per year.

The in-plant ash-handling system conveys flyash from hoppers under the baghouse, economizer, and air heater, as well as bottom ash from the combustor, to either of two silos located outside the powerhouse. Once in the silos, the ash is conditioned with water, loaded into trucks, and transported to a disposal site located next to the plant.

Point Aconi uses a once-through circulating seawater system. Seawater is drawn from 3,300 feet offshore at a depth of 36 feet through a velocity cap intake structure. A tunnel 130 feet below the seabed connects the offshore velocity cap to the onshore pump house. The two-pass, surface-type deaerating condenser operates at 1 inch HgA.

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