Oxyanions forming metals in waste incineration

摘要

The sources of the oxyanion forming elements arsenic, antimony, chromium and molybdenum and their behaviour during the waste combustion process have been investigated. The evaluation of equilibria of probable chemical reactions inside the combustion chamber on the basis of the Gibbs free formation enthalpy revealed potential species for volatilisation out of the fuel bed. Thermogravimetric and calorimetric measurements as well as X-ray diffractometric analyses of mixtures of metal salts and calcium oxide provided evidence that the elements form oxyanionic salts during the combustion process which cannot be volatilised. Tests in a pilot plant for waste incineration documented a transfer of approx. 50 % of the arsenic and antimony inventory into the raw gas. Chromium showed a very low and molybdenum a moderate transfer. A strong influence of the primary air supply oh the volatilisation of antimony was observed. This effect was less pronounced for arsenic and did not show for molybdenum and chromium. For arsenic, antimony, and molybdenum a real transfer through evaporation could be verified whereas for chromium only mechanical carryover is responsible for the transfer into the raw gas.