Corrosion in Recycled Wood Combustion—Reasons, Consequences, and Solutions

Combustion of recycled wood leads to increased corrosion problems of furnace walls. One of the causes is known to be the elevated concentrations of heavy metals and chlorine and rather low levels of sulfur in the fuel. A detailed understanding of the boiler environment, deposit formation, and corrosion mechanisms are essential factors in finding the most cost-effective solutions for reducing corrosion problems for full-scale boilers. The presence and behavior of lead in bubbling fluidized bed boilers firing recycled wood were studied using a fine particle measurement technique and short-term deposit probe measurements. The main compounds present in the fine particles were determined with the use of ion chromatography and inductively coupled plasma mass spectrometry. Deposit samples were analyzed with scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and an X-ray diffractometer. The results were compared to the similar deposit samples taken from a circulating fluidized bed boiler. The analyses revealed that the major part of lead was combined with chlorine and, in most cases, also with potassium. Two compounds were identified (KPb2Cl5 and K2PbCl4) in the boiler environment as well as in laboratory-scale corrosion tests. Thermodynamic calculations also showed the formation of these compounds. The corrosivity and the corrosion mechanism of potassium lead chlorides were further studied with the laboratory scale measurements. The results showed that iron, potassium, lead, and chlorine will form sintered particles together. This supports the theory that corrosion observed below the first melting temperature of the deposit could be driven by the melt formation between the deposit and the corrosion product.