Formation and Destruction of Hexavalent Chromium in a Laboratory Swirl Flame Incinerator

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Abstract
The partitioning of chromium (Cr) in combustion systems was investigated theoretically and experimentally. Theoretical predictions were based on chemical equilibrium, and suggested that hexavalent chromium [Cr(VI)] was favored by the presence of chlorine (Cl), and diminished by the presence of sulfur (S). Experimental studies employed a 59 kW laboratory-scale combustor with a swirling natural gas diffusion flame through which aqueous Cr solutions were sprayed. Three types of experimental data were obtained. First, and most important, the overall Cr(VI) fraction of the total Cr in the exhaust was measured as a function of initial Cr valence [trivalent (III) or hexavalent], and CI and S concentrations. Second, the size segregated distribution of Cr(VI) in the exhaust was explored for the Cr(III) waste feed with and without CI and S. Third, the influence of waste feed Cr valence on the exhaust aerosol particle size distribution was determined. Analytical determinations of Cr(VI) and total Cr are described. Results show that, for the high temperature, highly turbulent, gas-phase incinerator conditions examined, the relative ratio of Cr(VI)/total Cr is unaffected by the initial Cr valence of the waste, and ranged from near zero to approximately 8%, depending on the presence of S or Cl. CI addition increased the fraction of Cr(VI) found in small particles (smaller than approximately l.1µm) from approximately 30 to 70%. In contrast to the chemical analysis, the particle size distribution (PSO) measurements indicate that the initial form of the Cr waste does influence the resultant PSD. This mechanism is not well understood, but a possible explanation is that at combustion temperatures the Cr speciation chemistry is equilibrium controlled, while the PSD is determined by the aerosol dynamics which are dependent on physical transformations and specific chemical pathways.