Low-temperature corrosion in a BFB boiler firing biomass and waste streams – Online measurement technique for monitoring deposit corrosivity

The cause of low-temperature corrosion was studied in a full-scale bubbling fluidized bed boiler burning waste streams and biomass. A measurement campaign was conducted at the boiler air preheaters. Samples of corroded air preheater tubes were collected and analyzed. Deposits were sampled with an air-cooled probe with material temperatures of 70–130 °C and analyzed with SEM-EDX and XRD. The initial corrosion rates were determined for carbon steel. Additionally, a novel probe that monitors deposit properties online was developed and tested. Analyses of corroded air preheaters revealed that the corrosion was chlorine-induced. The corrosion layer on the air preheater tube was rich in Fe, Ca, and Cl and contained iron chloride as a corrosion product. The corrosion probe measurements at the air preheaters showed that some corrosion occurred with a material temperature of 120 °C and increased significantly with the lowering of the material temperature. The deposits were rich in Ca and Cl, and the share of Cl increased with a decrease in material temperature, indicating the formation of highly hygroscopic calcium chloride. The online deposit monitoring probe showed the deposit corrosivity and deposit build-up tendency in situ. At the lowest material temperature of 80 °C, the current reading was the highest, indicating a wet and corrosive deposit. As the temperature was gradually increased, the current reading decreased, and the deposit fully dried when the temperature was increased above 120 °C. The online deposit monitoring probe can be used to find optimal material temperatures in the cold-end and determine how changes in the combustion conditions, fuel composition, or additives affect the deposit corrosivity and stickiness in situ.