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

Abstract

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°C. Two corrosion and deposit probes were used in different locations in order to expose the probes towards hot, 800°C, and cooler, 490°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 (T0=368°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.