title: Chromium speciation in coal and biomass co-combustion products.
abstract: Chromium speciation is vital for the toxicity of products resulting from co-combustion of coal and biomass. Therefore, understanding of formation processes has been studied using a combination of X-ray absorption fine structure (XAFS) spectroscopy and thermodynamic equilibrium calculations. The influence of cofiring on Cr speciation is very dependent on the type of fuel. Cr(VI) contents in the investigated fly ash samples from coal and cofiring average around 7% of the total chromium. An exception is cofiring 7-28% wood for which ashes exhibited Cr(VI) concentrations of 12-16% of the total chromium. Measurements are in line with thermodynamic predictions: RE factors of Cr around 1 are in line with volatile Cr only above 1400 °C; lower Cr(VI) concentrations with lower oxygen content and Cr(III) dissolved in aluminosilicate glass. Stability of Cr(VI) below 700 °C does not correlate with Cr(VI) concentrations found in the combustion products. It is indicated that Cr(VI) formation is a high-temperature process dependent on Cr evaporation (mode of occurrence in fuel, promoted by organic association), oxidation (local oxygen content), and formation of solid chromates (promoted by presence of free lime (CaO) in the ash). CaCrO(4)(s) is a probable chemical form but, given different leachable fractions (varying from 25 to 100%), different forms of Cr(VI) must be present. Clay-bound Cr is likely to dissolve in the aluminosilicate glass phase during melting of the clay.
authors.affiliation: KEMA, PO Box 9035, 6800 ET Arnhem, The Netherlands. arthur.stam@kema.com
authors.forename: Arthur F
authors.initials: AF
authors.lastname: Stam
authors.forename: Ruud
authors.initials: R
authors.lastname: Meij
authors.forename: Henk
authors.initials: H
authors.lastname: Te Winkel
authors.forename: Ronald J van
authors.initials: RJ
authors.lastname: Eijk
authors.forename: Frank E
authors.initials: FE
authors.lastname: Huggins
authors.forename: Gerrit
authors.initials: G
authors.lastname: Brem
mesh_terms: D018533:Biomass; D002244:Carbon / Q000032:analysis* / Q000737:chemistry; D002857:Chromium / Q000032:analysis* / Q000737:chemistry; D003031:Coal / Q000032:analysis*; D060729:Coal Ash; D015386:Hazardous Substances / Q000032:analysis*; D052638:Particulate Matter / Q000032:analysis* / Q000737:chemistry; D011210:Power Plants; D014131:Trace Elements / Q000032:analysis* / Q000737:chemistry; D056928:X-Ray Absorption Spectroscopy
chemical_list: D003031:Coal; D060729:Coal Ash; D015386:Hazardous Substances; D052638:Particulate Matter; D014131:Trace Elements; D002857:Chromium; D002244:Carbon