Decreased furnace wall corrosion in fluidised bed boilers - The influence of fuel lead content and sewage sludge additive

Abstract: Usage of used-wood fuel in the power industry has resulted in increased corrosion problems for metallic materials in the boilers, most probably due to relatively high amounts of chlorine, alkali metals and heavy metals present in the fuel.In this investigation, laboratory exposures were performed in order to study the influence of lead content in a wood-based fuel on the corrosion of selected materials. Field exposures were also performed to investigate whether co-firing sewage sludge with used-wood fuel could decrease the corrosion of materials in a boiler. Temperatures studied were 350 °C and 400 °C and the exposures were performed at furnace wall position for 8-14 h.The findings showed increased corrosion of the low-alloyed steel 16Mo3 when increasing the lead content in the fuel. At 350 °C, the corrosion was minor but at 400 °C it was considered severe. The explanation for the corrosion at 400 °C was proposed to be a reaction between lead oxide and/or metallic lead, present in the deposit, with iron chloride, formed in the initial corrosion process, resulting in the formation of iron oxide and lead chloride. The presence of lead chloride in the deposit together with, in this case, potassium chloride, results in a very corrosive deposit due to formation of a low melting temperature salt mixture, traces of which were found in the oxide of the corroded samples. For the other tested materials negligible corrosion was observed.  It was found that co-firing sewage sludge with a used-wood fuel decreased the corrosion for all tested materials.