Waste incineration on a grate is a well-established thermal treatment technology in Denmark and several other countries. The method is flexible with respect to operation, it allows for recovery of energy, it reduces the Volume of solid waste significantly (by a factor of 8-10). and advanced flue gas cleaning technologies ensures very low emissions from modern incineration plants. In 2005, 24% of the total reported Danish waste production was incinerated. However, the presence of inorganic constituents such as alkali metals, Cl. S, and heavy metals in the waste constitute all essential challenge in waste incineration, both with respect to operational issues and due to environmental concerns. Formation of acidic pollutants, high mass loading of aerosols, and deposition of potentially corrosive components on heat transfer surfaces are among the encountered problems caused by volatile alkali compounds and heavy metals during combustion, and the potential leaching of heavy metals from the solid residues upon disposal is another major en v iron mental concern. In this work the partitioning of trace elements in a waste-to-energy (WtE) boiler wits studied experimentally, based on results from a full-scale measuring campaign at a Danish WtE plant while applying changes in waste composition and grate operation, respectively. The changes in waste composition were applied by adding (one-by-one): dedicated waste fractions, comprising road salt (NaCl), household batteries, automotive shredder waste, CCA (copper-chromate-arsenate)-impregnated wood, PVC, and, shoes. to a base-load waste. Special focus in the present work was on the partitioning of the elements Pb, Zn, Cl, S, Na, and K, which are all considered problematic elements with respect to deposition and corrosion problems. The partitioning of the elements wits found to be influenced by various effects, and there was not necessarily a correlation between the input concentration of an element in the feedstock and the amounts recovered in the fly ash and flue gas tractions. The study indicated that Cl and S played an important role for the partitioning of Pb, and Maybe also Zn. When firing Cl-rich waste fractions (PVC, salt, shoes), the partitioning of Pb seemed to shift toward increased vaporization (and subsequent recovery in fly ash and aerosol fractions). The full-scale Study also implied that organically bound Cl (as in PVC, shoes) was preferably released (as HCl(g)) to the gas phase, whereas the inorganically hound G (salt) was recovered in the bottom ash and fly ash fractions (indicating alkali-chloride bonding). Overall, Cl was concluded to be a critical element with respect to deposition and corrosion problems as it (I) enhanced the vaporization of heavy metals, (2) caused increased mass-load of aerosols if present as alkali-chloride in the waste, and (3) caused increased deposition fluxes and increased concentrations of Cl in the deposits.