title: Elucidating the mechanism of Cr(VI) formation upon the interaction with metal oxides during coal oxy-fuel combustion.
abstract: The thermodynamics underpinning the interaction of Cr-bearing species with basic metal oxides, i.e. K2O, Fe2O3, MgO and CaO, during the air and oxy-fuel combustion of coal have been examined. The synchrotron-based X-ray adsorption near-edge spectroscopy (XANES) was used for Cr speciation. For the oxides tested, Cr(VI) formation is dominated by the reduction potential of the metals. The oxides of Ca(2+) with high reduction potential favored the oxidation of Cr(III), same for K(+). The other two basic metals, Fe2O3 and MgO with lower reduction potentials reacted with Cr(III) to form the corresponding chromites at the temperatures above 600°C. Coal combustion experiments in drop-tube furnace have confirmed the rapid capture of Cr vapors, either trivalent or hexavalent, by CaO into solid ash. The existence of HCl in flue gas favored the vaporization of Cr as CrO2Cl2, which was in turn captured by CaO into chromate. Both Fe2O3 and MgO exhibited less capability on scavenging the Cr(VI) vapor. Particularly, MgO alone exhibited a low capability for capturing the vaporized Cr(III) vapors. However, its co-existence with CaO in the furnace inhibited the Cr(VI) formation. This is beneficial for minimizing the toxicity of Cr in the coal combustion-derived fly ash.
authors: Department of Chemical Engineering, Monash University, Clayton, GPO Box 36, Victoria 3800, Australia; State Key Laboratory of Coal Combustion, Huazhong University of Science & Technology, Wuhan 430074, China.
authors: Juan
authors: J
authors: Chen
authors: Facun
authors: F
authors: Jiao
authors: Lian
authors: L
authors: Zhang
authors: Hong
authors: H
authors: Yao
authors: Yoshihiko
authors: Y
authors: Ninomiya
mesh_terms: D002857:Chromium / Q000737:chemistry*; D003031:Coal; D060729:Coal Ash / Q000737:chemistry*; D004785:Environmental Pollutants / Q000737:chemistry*; D010087:Oxides / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D003031:Coal; D060729:Coal Ash; D004785:Environmental Pollutants; D010087:Oxides; D002857:Chromium; C074702:chromium hexavalent ion
keywords: Cr capture; Cr(VI); Oxy-fuel combustion; XANES