The proposed project is aimed at fundamental understanding of behaviour of trace toxic elements during the fluidized bed combustion. In the experimental part of the project we are going to study the distribution of toxic trace elements (Cd, Hg, Pb, As, Be, Cr, Mn, Ni, Sb and Se) between solid residues (bottom ash, coarse and submicron fly ash) and flue gas when various additives such as alumina, silica, titania, limestone, hydrated lime, kaolinite, bentonite and diatomaceous earth are used as sorbents. In this part of the project the leaching of toxic elements from ashes will be also investigated. In the theoretical part the experimentally determined distributions of trace elements is compared with the theoretical predictions based on thermodynamic calculations. A general method based on minimisation of the total Gibbs energy of the system on a set of points satisfying the material balance conditions will be used.