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A sustainable biomass power plant forms the heart of district heating in a residential area. The plant operates at full capacity and is designed for high temperatures and varying conditions. Nevertheless, damage to the casing of an internally insulated flue gas duct was observed.
This flue gas duct is partly shielded against weather influences, partly not. Serious corrosion occurred on the unshielded side, resulting in local perforations. Inspection showed that the corrosion did not come from outside but from within. The question arose as to how this damage could have occurred, and whether this was an incidental or structural problem
The affected flue gas duct consists of a carbon steel outer casing, with the inside lined with a fireproof material, secured with stainless steel anchors. The corrosion was mainly concentrated around the weld seams of the outer casing and at the connections between the carbon steel and stainless steel anchors. These so-called dissimilar metal joints are particularly susceptible to galvanic corrosion, especially in the presence of moisture or acids.
During inspection, it was determined that the column was only shielded on one side. This resulted in thermal asymmetry: on one side, the wind strongly cooled the surface, which locally caused a significant temperature gradient. Condensation occurs when the internal temperature of the column drops below the dew point — a process that, when combined with combustion residues, greatly accelerates galvanic corrosion.
Based on the inspection, several improvement measures were formulated aimed at stabilising the thermal conditions and reducing corrosion risks in this construction. Recommendations included both repair techniques and addressing temperature gradients across the circumference of the flue gas duct.
Additional recommendations were made concerning shielding, insulation, and wall thickness monitoring. This combination of measures contributes to extending the installation’s service life and minimising unplanned downtime.
