The influence of flue gas temperature on lead chloride induced high temperature corrosion

Firing of waste-based fuels increases the risk for heavy metal-induced corrosion in the furnace walls and in other low-temperature heat transfer surfaces, such as primary superheaters. Lead-containing compounds, especially alkali lead chlorides, have been detected in the boiler water walls, causing severe corrosion. Corrosion rate of chlorine-induced corrosion is known to be dependent on the material temperature and the objective of this work was to study the influence of the flue gas temperature on lead chloride-induced corrosion. The experiments were carried out with full-scale corrosion probe and deposit probe measurements in a recycled wood firing CFB boiler. The material used in the corrosion probe measurements was low alloy steel EN10216-2 16Mo3 and the material temperature was adjusted to 360 degrees C. Two corrosion and deposit probes were used in different locations in order to expose the probes towards hot, 800 degrees C, and cooler, 490 degrees C, flue gas temperatures. Changes of the wall thicknesses were measured and the samples were analysed with SEM/EDS and XRD for more detailed deposit characterisation. Corrosion was detected in both the hot and the cooler flue gas samples. A low melting (T-o = 368 degrees C) alkali-lead-chloride mixture was identified. Findings from these measurements strongly indicate this mixture to be the corrosion-causing compound at both flue gas temperatures. However, the corrosion rate was higher in the hot flue gas sample compared to the cooler flue gas sample. A much steeper deposit temperature gradient was calculated for the hot flue gas sample, suggesting that the alkali-lead-chloride mixture is in the molten form. These findings, together with the higher proportion of the present alkali lead-chloride mixture, are the potential factors for the higher corrosion rate in the hot flue gas sample compared to the cooler flue gas sample. (C) 2017 Elsevier Ltd. All rights reserved.