Waterwall corrosion in pulverized coal burning boilers: root causes and wastage predictions

Abstract

Waterwall corrosion has become a serious problem in the USA since the introduction of combustion systems, designed to lower NOx emissions. Previous papers have shown that the main cause of the increased corrosion is the deposition of corrodants, iron sulfides and alkali chlorides, which occurs under reducing conditions. In this paper, the contribution of various variables such as the amount of corrodant in the deposit, the flue gas composition and the metal temperature, is further quantified in laboratory tests, using a test furnace allowing thermal gradients across the deposit and the metal tube samples. Approximate deposit compositions were calculated from the coal composition, its associated ash constituents and corrosive impurities. A commercially available thermochemical equilibrium package was used, after modifications to reflect empirical alkali availability data. Predictions from these calculations agreed reasonably well with the alkali chloride and FeS content found in actual boiler deposits. Thus approximate corrosion rates can be predicted from the chemical composition of the coal using corrosion rates from laboratory tests, adjusted to account for the short duration (100 hours) of the laboratory tests. Reasonable agreement was again obtained between actual and predicted results.