On the Failure Analysis of an Air Preheater in a Steam Power Plant

Abstract

The air heater is a rotary regenerative heat exchanger which recovers heat from the outgoing hot gases of the boiler and transforms it to the incoming air needed for combustion process. This system is composed of large numbers of heat transfer elements made from corrosion-resistant material (Corten) or enamel coated in order to resist acid dew-point corrosion. In this study, a root cause failure analysis of heating elements was carried out. Visual inspection, chemical analysis, and microscopic examinations of the heating elements, enamel coating, and depositions were conducted. Sulfuric acid dew-point temperature (ADPT) was determined to be 138–142 °C based on the sulfur dioxide of the flue gases. Moreover, temperature distribution across the air heater matrix was simulated using Fluent software. The failures of the heating elements were mainly due to sulfuric acid dew-point corrosion and under-deposit pitting corrosion. The corrosion products were mainly iron sulfate, iron oxide, and iron sulfide. In the case of enamel-coated elements, the failure was attributed to the existence of a large number of cracks and big bubbles in the coating, allowing the penetration of corrosive elements into and beneath the surface of coating and thus the detachment of coating.