Equilibrium analysis of the affect of temperature, moisture and sodium content on heavy metal emissions from municipal solid waste incinerators

An equilibrium analysis was performed to determine the impact of the variations in the moisture and sodium contents in the solid waste feed on heavy metal speciation in municipal solid waste (MSW) incinerators. This analysis was based on the speciation of heavy metals among chlorides, oxides and other species under various incinerator conditions. The Gibbs free energy of the reaction mixture was minimized by the method of chemical potentials combined with atom population constraints using a thermodynamic equilibrium computer code. Fifty eight species were considered, including compounds of six trace metals, aluminum, cadmium, chromium, mercury, sodium, and lead. Changes in the moisture and sodium contents in the feed waste were found to significantly impact the speciation of heavy metals among chloride and oxide species. Estimates have been carried out on the impact of these speciation changes on the bottom ash, fly ash and flue gas emissions of heavy metals. The results indicate that, at the typical municipal incineration temperature of 950 degrees C, a decrease in the waste moisture content from 37% to 5% results in a modest increase in the percentage of the lead in the fly ash from about 54% to 68% of the total. Similarly, a decrease in the waste sodium content from 6560 ppm (upper limit for MSW content) to 4500 ppm (MSW average) results in a more significant increase in the percentage of lead in the fly ash from 36% to 60% at the average waste moisture content. Several other heavy metals have also been studied, but the impacts are much smaller. A qualitative discussion and resulting implications of environmental impact due to the shifts in lead speciation are discussed. (C) 1997 Elsevier Science B.V.