February 15, 2007

Focus on O&M (February 2007)

Capacity enhancement

The previous example describes only one type of benefit of reducing air heater leakage. The North American Electric Reliability Council (NERC) and others continue to project generating capacity shortfalls in some parts of North America. Each capacity shortfall increases the value of incremental generating capacity during peak periods, such as on hot summer days.

However, many fossil-fired power plants are capacity-constrained during these periods. The reason is that air expands and becomes less dense in warm weather. As a result, fans must move a greater volume of air in warm weather to provide the same amount of air mass as in cooler weather. Lack of fan capacity is the reason why many generating units are capacity-limited during warm weather. This problem becomes more evident when plants add or upgrade pollution control equipment such as a selective catalytic reduction system, electrostatic precipitator, and/or baghouse, each of which increases loads on the fans.

Because the price of power tracks demand, revenue forfeited from lost or reduced power sales due to fan capacity limitations can be significant. In some cases, when power producers sell power on the spot market, the power sold during peak periods over two months of the year can represent a large percentage of total annual revenue. If a power producer is committed to a fixed contract and is unable to generate promised power during unusually hot weather, the producer may need to purchase power at a high price (potentially as much as $1,000/MWh) to fulfill its contractual obligation.

In many cases, hot weather capacity losses can be eliminated by reducing air heater leakage. Assuming 250 hours per year of fan limitations, an average load reduction of 10 MW, and a sale price for power during the limitation of $250/MWh, the potential revenue loss is $625,000 per year.

Despite these adverse effects and lost opportunities, most power producers continue to use air heater seal technology that has not appreciably changed for decades. Traditional air heater seals are little more than strips of thin-gauge steel, which cannot seal tightly and are often blown open or damaged by high differential pressures. In contrast, today's high-performance air heater seals have proven capable of reducing air heater leakage by 50% or more, compared with standard seals.

Some newer seals use a bellows to produce a spring force that keeps the seal in contact with its mating surface (the sector plate) over a wide range of differential pressures and rotor turndowns. Others, called interlocking circumferential seals, are used on the perimeter of the air heater's rotor. They reduce bypass leakage, and their structure is designed to resist the type of damage suffered by standard seals during rotor turndown.

—Contributed by John Guffre, chief research scientist of Paragon Airheater Technologies. He can be reached at 888-488-3100 or jguffre@paragonairheater.com.