Formation and corrosivity of CaCl2 in the cold end of biomass- and waste-fired boilers

• CaCl 2 forms at low temperatures (100 °C) when Ca(OH) 2 or CaCO 3 are exposed to HCl in synthetic flue gas. • CaCl 2 is highly corrosive to carbon steel, especially below its deliquescence temperature. • The intermediate compound Ca(OH)Cl forms first and subsequently converts to CaCl 2 in the presence of excess HCl. • Limiting HCl and moisture and avoiding calcium-rich deposits is suggested to mitigate corrosion risks. The presence of hygroscopic chlorides in cold end deposits in boilers can lead to severe corrosion. One highly corrosive and hygroscopic salt is calcium chloride (CaCl 2 ). Measurements in full-scale BFB boilers burning biomass and waste streams have indicated the presence of CaCl 2 , however, its formation in flue gas conditions is not fully understood. This work studies the formation of CaCl 2 from Ca(OH) 2 and CaCO 3 in the presence of HCl(g) in flue gases. Ca(OH) 2 and CaCO 3 salts were exposed to 600 ppm v HCl, 5 vol% O 2 , 12 vol% CO 2 , and 15–25 vol% H 2 O at 80 or 100 °C. The compounds formed were analyzed using XRD, and detailed SEM-EDX analyses of their surface and cross-sections were conducted. Furthermore, carbon steel samples were exposed to different calcium salts (Ca(OH) 2 , CaCO 3 ) using a similar gas composition. The study revealed that Ca(OH) 2 first formed Ca(OH)Cl, while CaCl 2 formed on the outer surface once all Ca(OH) 2 had converted into Ca(OH)Cl. Surprisingly, CaCO 3 also converted to CaCl 2 , and its deliquescence occurred under the studied conditions. Corrosion of carbon steel was observed beneath the CaCl 2 deposits. Interestingly, corrosion also occurred around the deposit, indicating a direct reaction between HCl and carbon steel.