The damage to air preheater (APH) tubes in the zones where air and gas temperatures are low discovered in CFB boilers at Estonian oil shale power plants was indicative of low-temperature corrosion (LTC). To establish damage causes, samples of ash deposits from boiler heating surfaces and APH corroded tubes were collected and chemically analysed, visual observations were conducted, dew point of flue gas was experimentally determined and deposit samples were collected also in the gas flue of a working boiler.

    The measured dew-point temperature of sulphuric acid in a pulverized firing (PF) boiler is 75–80 °C. The maximum chlorine content of the deposits collected from a probe tip inserted in a PF boiler’s gas flue is ~6%. The measured dew-point temperature in a CFB boiler’s gas flue is 55–60 °C. If the probe tip was contaminated, dew-point temperature rose to 75–80 °C. Chlorine content of the deposit samples collected from a CFB boiler was ~3%. The estimated dew-point temperature of water steam with no sulphur in flue gas was 52.0–53.4 °C in the following conditions: Q^r_i = 8.30–8.50 MJ/kg, W^r_i = 11–14% and a = 1.20 (O₂ = 3.5%).

    Possible LTC causes were analysed and recommendations were offered concerning implementation of measures to reduce LTC. It became evident that the temperature of APH tube walls fell below the dew-point temperature of oil shale flue gas, causing condensation of the water steam present in flue gas on the tubes. If there is water, well-soluble chlorides form an aqueous solution or become liquefied (CaCl₂ crystal hydrates), and that process causes intensive electrolytic LTC.