Causes of low-temperature corrosion in combustion of bituminous coal

Maximizing energy recovery by keeping flue gas temperatures as low as possible is of great interest in the power business, as higher recovery leads to higher plant efficiency. However, minimum allowable temperature is often limited by the presence of sulfuric acid. Condensation of sulfuric acid on steel surfaces could lead to severe corrosion. Hygroscopic deposits may also cause severe corrosion by taking up water and thus leading to wet corrosion. In this study, the risk of sulfuric acid corrosion and corrosion caused by hygroscopic deposits was studied in a full-scale circulating fluidized bed (CFB) boiler. The boiler was run both with and without limestone addition to reveal the effect of limestone on sulfuric acid corrosion and the effect of limestone on wet deposit corrosion. Furthermore, the effect of load on corrosion was studied. The measurements were performed before and after the electrostatic precipitator (ESP). A KCl salt method, which has been developed to detect trace concentrations of gaseous sulfuric acid, was used to study the risk of sulfuric acid corrosion. Furthermore, an acid dew point meter was used to determine the dew point temperature in the flue gases. Additionally, an air-cooled short-term corrosion probe was used to measure the corrosion rate of carbon steel at various material temperatures. Windward, side, and leeward side deposits were collected from the probe and analyzed by means of scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). Fuel and fly ash samples for the different test cases were collected and analyzed. The work revealed new interesting findings on the causes of low-temperature corrosion in combustion of bituminous coal. The different operational parameters of the boiler had a significant impact on deposit composition and on expected corrosion risk. In this paper, we discuss the risks of sulfuric acid corrosion as well as corrosion caused by hygroscopic deposits. Furthermore, the influence of boiler parameters on deposit composition and chemistry will be discussed.