Causes of low-temperature corrosion in CFB combustion of bituminous coal

Maximizing energy recovery by keeping flue gas temperatures as low aspossible is of great interest in the power business, as higher recovery leadsto higher plant efficiency. However, minimum allowable temperature is often limitedby the presence of sulfuric acid. Condensation of sulfuric acid on steelsurfaces could lead to severe corrosion. Hygroscopic deposits may also causesevere corrosion by taking up water and thus leading to wet corrosion. In thisstudy, the risk of sulfuric acid corrosion and corrosion caused by hygroscopicdeposits was studied in a full-scale circulating fluidized bed (CFB) boiler.The boiler was run both with and without limestone addition to reveal theeffect of limestone on sulfuric acid corrosion and the effect of limestone onwet deposit corrosion. Furthermore, the effect of load on corrosion wasstudied. The measurements were performed before and after the electrostaticprecipitator (ESP). A KCl salt method, which has been developed to detect traceconcentrations of gaseous sulfuric acid, was used to study the risk of sulfuricacid corrosion. Furthermore, an acid dew point meter was used to determine thedew point temperature in the flue gases. Additionally, an air-cooled short-termcorrosion probe was used to measure the corrosion rate of carbon steel atvarious material temperatures. Windward, side, and leeward side deposits werecollected from the probe and analyzed by means of scanning electron microscopywith energy dispersive X-ray spectroscopy (SEM-EDX). Fuel and fly ash samplesfor the different test cases were collected and analyzed. The work revealed newinteresting findings on the causes of low-temperature corrosion in combustionof bituminous coal. The different operational parameters of the boiler had asignificant impact on deposit composition and on expected corrosion risk. In thispaper, we discuss the risks of sulfuric acid corrosion as well as corrosioncaused by hygroscopic deposits. Furthermore, the influence of boiler parameterson deposit composition and chemistry will be discussed.