Prediction of the distribution of alkali and trace elements between the condensed and gaseous phases generated during clinical waste incineration

The mobilisation of alkali and trace elements present in clinical waste can lead to accelerated deterioration of the plant and to environmental damage. The damage can be caused by transfer of low levels of trace elements, which are difficult to monitor, and a model of the underlying processes which predicts the degree of mobilisation of each element from waste of specified characteristics is thus desirable. The Equilibrium module of the FACT suite of computer programs has been used to make predictions for alkali and trace element mobilisation from a typical waste composition with variation in the S/Cl ratio which influences the volatilisation/condensation processes. Although thermodynamic data for some of the potential melts are incomplete, predictions made using the various oxide melt models, matte, salt and solid solution models available in FACT are combined to allow meaningful comment on Pb, Cu, Zn, Ni and Cr distributions. Separate consideration is given to mobilisation in primary (pyrolysis) and secondary combustion (oxidation) chambers. Comparisons are made with published data from municipal waste incineration plants. An interesting feature of the predictions for condensation during cooling of the waste gases is that if solution of Pb, Zn and Cu chlorides is permitted in alkali chloride or Pb sulphate into sulphate melts then Pb and Cu are predicted to be largely removed from the gas stream into these melts. (C) 2001 Elsevier Science B.V. All rights reserved.