The formation of ammonium bisulfate (ABS) in the air preheater is a consequence of the operation of an ammonia based NOx reduction system in a flue gas environment containing sulfur tri-oxide (SO3) and water vapor (H2O). ABS forms in accordance with the following chemical reaction:
> NH3+ SO3 + H2O -» NH4HSO4
The reaction takes place in a temperature range generally between 400 and 550ºF and is dependent upon a number of parameters, including the concentrations of the constituents and temperature. The ABS will collect or condense on flyash particles, and surfaces it comes in contact with, including the heat transfer surfaces within the air preheater. Operating experience on over 200 high dust units indicates that the deposit collects within the air preheater on metal surfaces with temperatures generally between 300 and 375ºF. The deposit is in the form of a molten salt. In combination with flyash, the deposit is usually very sticky and difficult to remove.
The consequences of ABS deposits on air preheater operation include the increase of pressure differential across the air preheater, an increase in air to gas leakage within the air preheater, a reduction in the thermal performance of the air preheater and an increase in the corrosion rates of heating elements, cold end structures and down stream equipment.
The Ljungström® air preheater design can be reconfigured with an ABS Tolerant™ upgrade to incorporate several unique design features which increases the ABS tolerance of the unit. These features include:
- A deep high efficiency cold end layer of heating element to capture the ABS deposits within a single layer of heating element, move the deposits closer to the cold end cleaning device and improve the effectiveness of the cleaning device in removing the deposits. Experience has shown that if the ABS collects in a region which bridges across two layers of heating element, the rate of accumulation and subsequent consequences are accelerated. Our DNF® element profile was specially developed to lessen the impact of ABS deposits on air preheater operations.
- Porcelain enamel coating on the cold end heating element to minimize the quantity of ABS which collects, reduce the adhesion of the deposit and inhibit corrosion which may result because of the deposit. Experience has shown that the surface finish of the heating element has a significant impact on deposit buildup. The smooth surface finish associated with the porcelain enamel coating is much less susceptible to the collection of ABS than that of mild steel or low alloy steel, is more effectively cleaned and is more resistant to corrosion.
- Hot and cold end Multi-Media™ cleaning devices. ABS deposits are usually sticky and difficult to remove with normal sootblowing. The deposits are normally very water soluble. The Multi-Media™ cleaning device incorporates two sets of nozzles, a low pressure set and a high pressure set. The low pressure set can be connected to a normal sootblowing medium such as air or steam. The nozzle pattern concentrates and reinforces the sootblowing energy for improved ABS removal. The low pressure nozzles can also be connected to a medium pressure source of water, for hot end washing. The high pressure nozzle set is designed for high pressure water delivery, in the event that flyash accumulations occur on top of the ABS deposit.