Innovations in Air Heater Design Produce Performance and Reliability Improvements
By Stephen K. Storm; John Guffre, PE; and Andrea Zucchelli, PhD
(Courtsy of Coal Power Mag)
Advancements in air quality control system design and sootblower technology have allowed the power industry to become more fuel flexible—but, in some cases, at the expense of plant reliability. Fuel blends with challenging mineral ash constituents can accelerate deactivation of SCR catalysts, NH3 slip, and/or sorbent injections for SO3 control that are the cause of boiler tube fouling, plugging, and/or corrosion of the air heater.
Air heater problems often lead to unit derates caused by lack of fan capacity, improper combustion temperatures, and/or unacceptable gas temperatures entering the stack. Higher-than-desirable air heater gas outlet temperatures can reduce the collection efficiency of electrostatic precipitators and have negative effects on the reliability of baghouses.
However, recent technical advancements in air heater design have been shown to successfully improve air heater and overall unit efficiency while reducing forced outages related to the air heater. In this article, we’ll address these technical advancements in terms of:
- Equipment operation and maintenance
- Mechanical component design
- Materials and coatings
Operation and Maintenance
Both air heater leakage and upstream air in-leakage from a boiler’s penthouse can significantly affect the thermal performance of a regenerative air heater, given its relatively large surface area (Figure 1). Most regenerative air heaters were originally designed for 6% to 10% air heater leakage and account for more than 10% of a unit’s overall thermal efficiency.
1. Heating surface comparison for a typical large utility boiler. Source: Stephen Storm Inc. and Paragon Airheater Technologies
Leakage occurs on the hot and cold ends of a regenerative air heater (Figure 2). However, most of the leakage often occurs on the cold end, where corrosion is more likely to occur due to the low air and gas temperatures, which produce larger clearances between the sealing surfaces. However, it’s also important to understand that air in-leakage upstream of the air heater can temper the incoming flue gas temperature while also increasing the volume of gas going through the air heater on a balanced draft unit.
2. Regenerative air heater basic flow diagram. Source: Stephen Storm Inc. and Paragon Airheater Technologies
An air heater in poor condition will typically exhibit air in-leakage in excess of 15% to 20%. Considering this, correction of the gas outlet temperatures is required to determine the “no-leakage” gas outlet temperature. On a typical unit, each 30F to 35F is worth ~1% in unit efficiency.
Regenerative air heater design features, air-to-gas ratios, and air-to-gas pressure variations have a major impact on regenerative air heater performance. Note that the mechanical condition of the air heater and minimizing air-to-gas leakage is vital for the health of the air heater and overall plant performance. Any flue gas diluted with air leakage on the cold end of the air heater will lower the overall gas outlet temperature as well as change the gas temperature gradients downstream of the air heater outlet. Furthermore, it must be understood that high air in-leakage results in lower gas outlet temperatures and, when combined with S03, accelerated corrosion is likely to occur.
Numerous boiler operational parameters can influence SO3 formation. These variables are fuel sulfur content, ash content and composition, convective pass surface area, gas and tube surface temperature distributions, excess air level, firing mechanism, and coal fineness.
Leakage control in conjunction with the control of sulfuric acid (H2SO4) production is especially important when managing flue gas that is a byproduct of high-sulfur fuels. SO3 combines with flue gas moisture to form vapor-phase sulfuric acid at temperatures below about 600F. Therefore, any sulfuric acid in the flue gas can lead to significant problems, such as boiler air heater plugging and fouling, and corrosion in the air heater and downstream ductwork/equipment.
To prevent condensation of the SO3 (and thus limit formation of sulfuric acid) the exit gas temperature coming from the air heaters must be kept above the dew point of the sulfuric acid. The higher required exit gas temperatures translate directly into a loss of system efficiency, which imposes a significant heat rate penalty. However, the more SO3 is formed, the higher the dew point. The sulfuric acid dew point temperature depends on the SO3 and water vapor concentrations in the flue gas: Higher concentration of either species raises the acid dew point temperature.
Although a significant portion of the SO3 will condense on ash particles and be collected along with the fly ash, the noncondensed SO3 can have significant side effects. Excess SO3 leaving the stack can result in a noticeable “blue plume,” which consists primarily of sulfuric acid that has condensed into tiny droplets. Those same droplets may also condense on the cold end of the air heater or in the downstream ductwork, causing corrosion and plugging.
In addition, excess SO3 can combine with ammonia slip from a selective catalytic reduction (SCR) system to form ammonium bisulfate (ABS), which has a notorious reputation for plugging air heater heat transfer elements. This ABS plugging also adversely affects the distribution of the air and flue gas by elevating air temperature differentials and gas leakage, as a result of the elevated pressure drop across the air heater. Essentially, the excess ammonia combines with excess SO3 and water vapor, which starts to condense on the air heater element surfaces at temperatures below about 450F (230C).
In some plants, to remove excess SO3, dry powder or water slurry mixes of alkaline sorbents (hydrated lime, limestone, magnesium oxide, sodium bisulfate, and trona) are sometimes injected upstream or downstream of regenerative air heaters. Though these chemicals are quite effective in adsorbing excess SO3 and reducing blue plume and corrosion, the effect of these sorbents on the air heater and its operation are still being evaluated.
Whether you are referencing burner performance, boiler performance, air heater performance and/or the air pollution control equipment, all of these components are influenced by their inputs with regard to their operational efficiency (see table). Variables such as the air heater gas outlet temperatures and velocity gradients, leakage/mass flow, coal fineness, sorbent particle sizing and distribution, excess air setting, calcium-to-SO3 molar ratio, ash content, and ash resistivity can have a significant effect on the air pollution control equipment…….
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