A relatively low electrical efficiency of 20-25% is obtained in typical west European waste boilers. Ash species released from the grate combustion zone form boiler deposits with high concentrations of Cl, Na, K, Zn, Pb, and S that cause corrosion of superheater tubes at high temperature. The superheater steam temperature has to be limited to around 425°C, and thereby, the electrical efficiency remains low compared to wood or coal-fired boilers. If a separate part of the flue gas from the grate has a low content of corrosive species, it may be used to superheat steam to a higher temperature, and thereby, the electrical efficiency of the plant can be increased. In this study, the local temperature, the gas concentrations of CO, CO 2 , and O 2 , and the release of the volatile elements Cl, S, Na, K, Pb, Zn, Cu, and Sn were measured above the grate in a waste boiler to investigate if a selected fraction of the flue gas could be applied for increased steam superheating. On a 26 ton/h grate-fired waste boiler, Vestforbraending unit 5 in Denmark, local probe measurements were performed in five ports along the grate and in the top of the boiler chamber. New extraction probe equipment were designed and used to extract a flue gas with high contents of tar. Gas concentration measurements of O 2 , CO, and CO 2 showed that the waste experienced pyrolysis and combustion on grate sections 1 and 2, some char combustion takes place on section 3, and the slag was cooled on sections 4 and 5. The measurements showed that the waste grate combustion process can provide a flue gas with a high energy content and a relatively low concentration of corrosive species. This opens up for the possibility of using an additional superheater section to increase the steam temperature and, thereby, increase electrical efficiency.
