February 15, 2007

Focus on O&M (February 2007)

AIR HEATER MAINTENANCE

Bypass losses squander big bucks

Many fossil plant engineers are unaware of the extent of their air heater performance problems or the revenues lost by failing to eliminate excessive gas bypass. Air heater leakage of 20% is not uncommon, and in some extreme cases a level of 40% has been measured. The impact on plant operations of air heater leakage is often grossly underestimated. It adversely affects the performance of the boiler and pollution control equipment, increases plant heat rate, and can constrain plant output during peak loads. These effects increase maintenance and fuel costs, put the plant owner at risk of paying noncompliance fees, and reduce revenue from power sales.

The biggest adverse impact of air heater leakage is on boiler fans. Fan motors are one of the largest electricity users in the plant, and air heater seal leakage can account for as much as 25% of total fan horsepower. Any plant engineer would jump at the chance to reduce fan power consumption by 25%. Surprisingly, that can be accomplished simply and inexpensively, by installing new air heater seals.

Seal the deal

Regenerative air heaters capture the heat in boiler exhaust gases by passing those gases through heat-absorbing metallic elements. The elements slowly rotate and alternately contact the hot gases and the cool inlet air from boiler fans. The captured waste heat is released into the inlet air, preheating it just before it enters the boiler (Figure 3).

3. Around the edges. Typical temperatures in a regenerative air heater. Source: Paragon Airheater Technologies

Sealing these types of heaters is extremely difficult due to their large diameter (up to 60 feet) and the large temperature difference between the hot and cold sides (about 400F), which produces dynamic thermal distortion of the rotor. The outer edge of a large hot air heater typically distorts by 2 inches or more (compared with the cold condition), providing a large gap through which plenty of air can escape.

Figure 4 shows common leakage paths for typical air heaters. Paths A and B illustrate radial leakage between the sector plates and the basketed element on the cold and hot sides, respectively. Radial leakage raises required boiler fan horsepower because it does not contribute to combustion, yet it still must be moved. Paths C and D show circumferential leakage, in which air leaks past the outside of the rotor and thus either fails to be preheated (path C) or fails to transfer its heat to the air heater (path D). Circumferential leakage does not increase fan horsepower because it still makes its way to the boiler. However, circumferential leakage does adversely impact heat transfer and boiler heat rate.

4. Down the drain. The sum of radial and circumferential leakage in air heaters typically exceeds 25%. Radial leakage can reach up to 350 feet/second. Source: Paragon Airheater Technologies

Quantify the losses

The amount by which radial leakage (paths A and B) can be reduced is directly proportional to avoided fan horsepower and, hence, reduced electricity consumption. Because fans primarily move a volume of air, the horsepower/volume relationship for the boiler fan is relevant. Although the slope of this fan curve is typically about 1.0 at the fan's design operating point, the slope can easily increase to 2.5 or more as the fan's operating point approaches maximum volume. This means that for every 1% reduction in radial air leakage, 1% less air must be moved by the boiler's forced-draft or induced-draft fans. This, in turn, translates into a 1% to 2.5% reduction in required fan horsepower (Figure 5).

5. Power hog. Power consumption of different classes of power plant fans as a percentage of air flow. Source: Paragon Airheater Technologies

A real-world example illustrates the magnitude of electricity (and cost) savings possible. It assumes the following:

• Initial radial leakage rate: 16% of airflow.
• Radial leakage rate after installation of more-effective air heater seals: 9%.
• Net reduction of radial seal leakage: 7%.
• Plant output: 500 MW.
• Plant heat rate: 10,500 Btu/kWh (net).
• Total installed fan hp: 16,000 hp.
• Fuel cost (coal): $2/mmBtu.

Assuming a fan curve slope of 1.5, a 7% reduction in air volume translates into a fan horsepower reduction of 1,680 hp, or 1,253 kW. At the assumed heat rate of 10,500 Btu/kWh, the reduction in air leakage would produce a fuel savings of 13.2 mmBtu/hr. At the assumed fuel cost of $2/mmBtu, the fuel cost savings would be $26.40/hr. If its capacity factor were 75%, a plant would reduce its annual fuel bill by $173,500. In other words, $173,500 less fuel would be needed to power the fans simply by reducing radial leakage in the air heater.