Rotary regenerative air heaters capture and recycle about 60% of the heat energy exiting the boiler—energy that would otherwise go up the stack. For a 500-MW coal-fired plant, the recycled energy amounts to about 1.5 billion Btu per hour, and reusing it reduces fuel consumption by about 1,500 tons per day.
Although most performance engineers acknowledge that the air heater is among the most important contributors to a plant's thermal efficiency (perhaps second only to the condenser), many are unaware of the extent of air heater performance problems and their impact on plant operation and efficiency. That impact often is grossly underestimated by a too-narrow definition of air heater leakage in a procedure commonly used to measure it.
The "official" definition of air heater leakage is: "The weight of air passing from the air side to the gas side. It is assumed [italics mine] in calculations to leak directly from the air inlet to the gas outlet." The industry uses this narrow definition because direct leakage is the only kind that can be easily measured in practical terms. However, use of this inaccurate definition produces the erroneous conclusion that the only negative impact of air heater leakage on plant performance is the increase in fan power needed to pump the leaking air. Although a higher fan power requirement is significant (as much as 3 MW per unit), it's only part of the story.
Underestimating air heater leakage is a shame, because reducing it is relatively inexpensive and provides a big performance bang for the buck. Properly sealing an air heater can improve (reduce) a plant's heat rate by up to 75 Btu/kWh, increasing power production and revenues as a result.
Excessive air heater leakage can cause any or all of the following additional problems, to name just a few:
- Severe degradation of the performance of downstream air pollution control equipment (electrostatic precipitators, baghouses, and scrubbers).
- Lowered primary air temperatures and consequent reduction of coal mill capacity—particularly in the case of wet coals and Powder River Basin coals.
- Increased potential for mill fires or explosions.
- Load limitations and missed power sales opportunities due to inadequate fan capacity, especially during warm weather.
- Increased NOx production and loss-on-ignition.
- Poor flame stability at lower load and unattached flame ignition points, as well as flame pulsation.
- Excessive desuperheating due to increased convective heat transfer caused by increased mass gas flow.
- Increased heat rate and air heater plugging.
- Increased rate of cold end air heater basket corrosion.
With so much at stake, how can plant engineers be sure their air heater leakage measurements are correct? This article explains ways to be sure and provides steps that can be taken to reduce leakage to acceptable levels.
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