In the last years chlorination has become an important industrial technique for the removal of heavy metal impurities (especially cadmium, copper, lead and zinc) in raw materials. In order to estimate the chlorination effect of different chlorine donors on heavy metals in sewage sludge ashes and the influence of gas humidity, the thermal decomposition behaviour of the chlorine donors was investigated. Therefore, thermal analysis of calcium chloride between 20 degrees C and 1400 degrees C in synthetic dry and humid flue gas atmosphere simulating firing methane with air was conducted in this work. In the first case the gas composition contained 12 v% oxygen, 5 v% carbon dioxide and 83 v% nitrogen whereas in the second case water vapour was added to the gas mixture in an amount to equal 10 v% of the total gas. In both cases (dry and humid flue gas), the first mass losses of calcium chloride up to 260 degrees C are based on the decomposition of its hydrates. Chlorine release occurs above the melting temperature of calcium chloride (782 C) only: in the first case chlorine gas (Cl-2) by reaction of gaseous calcium chloride with oxygen and in the second case hydrogen chloride (HCl) by reaction with water vapour are formed. These different reaction products may influence the chlorination effect on heavy metals and further their removal in industrial processes. The solid reaction product is in both cases calcium oxide. (c) 2008 Elsevier B.V. All rights reserved.