Thermodynamic equilibrium calculations were performed in order to determine the removal behaviour of the heavy metals Cd, Cr, Cu, Ni, Pb and Zn from sewage sludge ash by adding Cl and performing a thermal treatment process. FactSage 5.3.1 and ChemSheet 1.5 are thermodynamic equilibrium calculation programs which were used to design a multicomponent thermodynamic system on the basis of the mass flows (sewage sludge ash-additive mixture and gases) obtained from experiments with a methane-fired laboratory-scale rotary kiln. In a first step the thermodynamic equilibrium calculations performed were validated by the experimental results and in a second step the calculations should serve for the understanding of the heavy metal removal processes in order to realise a technical application. The thermodynamic equilibrium calculations were performed in the temperature range from 500 to 1000,C using three variations of Cl addition (0, 5 and 10 wt%) in the sewage sludge ash. With regard to their calculated removal results, the heavy metals investigated can be classified into three fractions, namely "easily volatile" (Cd and Pb). "semi-volatile" (Cu and Zn) and "low-volatile" heavy metals (Cr and Ni). The calculations indicate good compliance with experimental results for "easily volatile" and "low-volatile" heavy metals (Cr and Ni). For the "semi-volatile" fraction (Cu and Zn) the removal calculations showed the largest deviations from the measurement results due to kinetic effects. Nevertheless, calculations give clear removal trends for all heavy metals regarded and allow recommendations for a technical application of industrial-scale. According to calculations and the finding that Cu. Pb and Zn indicate higher removal Fates at higher temperatures, "easily" and "semi-volatile heavy metals" should be removed at a treatment temperature of 1000 degrees C in directly heated systems. In the case of "easily volatile heavy metals", chlorination is not implicitly necessary. They are volatilised in form of hydroxides and/or oxides. In contrast to this fraction,"semi-volatile heavy metals" require high amounts of Cl addition (at least 10 wt% in the ash-additive mixture) for their volatilisation as chlorides. Furthermore, a flue gas poor in H2O and sewage sludge ash rich in Si may positively influence the removal. Even a Cl addition of 10 wt% in combination with a treatment temperature of 1000 degrees C in directly heated systems is not sufficient for the removal of "low-volatile heavy metals". Both Cr and Ni require a flue gas without H2O and a specific chemical composition (high contents of Si, low contents of Fe) of the sewage sludge ash used for high-removal rates. Following, indirectly heated systems (where the reactor atmosphere can be adjusted more easily) and a treatment temperature of 1000 degrees C are recommended for the removal of Cr and Ni. (C) 2008 Elsevier B.V. All rights reserved.