New insights into HCl-induced low-temperature corrosion in biomass- and waste-fired boilers

This study investigates the risk of low-temperature corrosion on carbon steel tube surfaces located at the clean side of the flue gas channel downstream of a selective catalytic reduction (SCR) unit in a full-scale bubbling fluidized bed (BFB) boiler, where the HCl(g) concentration is typically very low (<5 ppm v ). Short- and long-term corrosion probe measurements, along with online deposit monitoring, were carried out in the flue gas channel to determine the causes of corrosion and establish safe material temperatures. To further investigate the results, laboratory tests were conducted in a quartz-tube furnace under simulated flue gas conditions. The results demonstrated that corrosion rates increased once the material surface temperature fell below 90 °C, with particularly severe attack evident at 80 and 70 °C. Notably, a considerable amount of chlorine was present in the corrosion products, indicating a high risk of chlorine-induced corrosion at cold-end surfaces, even at very low HCl(g) concentrations. Two potential corrosion mechanisms were identified, namely the absorption of HCl(g) into surface-adsorbed water monolayers above the water dew point, and the presence of corrosive NH 4 Cl(s,aq) on tube surfaces. HCl-induced corrosion was found to be the most likely mechanism. According to this mechanism, corrosion can occur even without bulk water condensation, and it depends on the local relative humidity (RH) at the steel surface. The findings collectively suggest that carbon steel surfaces on the clean side of the flue gas path should be maintained above approximately 90 °C (with an appropriate safety margin, depending on flue gas H 2 O(g) concentration) to mitigate the observed HCl-induced corrosion. A similar corrosion mechanism may also affect the dirty side of the flue gas path at similarly low temperatures. • In BFB boilers, cold-end surfaces are at risk of Cl-induced corrosion below ≈90 °C. • Corrosion is mainly due to HCl(g) adsorption into surface-adsorbed water monolayers. • Even <5 ppm v HCl(g) can cause corrosion on carbon steel under flue gas conditions.