title: Use of synchrotron XANES and Cr-doped coal to further confirm the vaporization of organically bound Cr and the formation of chromium(VI) during coal oxy-fuel combustion.
abstract: Through the use of synchrotron XANES and Cr-doped brown coal, extensive efforts have been made to clarify the volatility of organically bound Cr during oxy-fuel combustion and the mode of occurrence and leachability of Cr in resulting fly ashes. As the continuation of our previous study using raw coal, the Cr-doped coal has been tested in this study to improve the signal-to-noise ratio for Cr K-edge XANES spectra, and hence the accuracy for Cr(VI) quantification. As has been confirmed, the abundant CO(2) as a balance gas for oxy-firing has the potential to inhibit the decomposition of organically bound Cr, thereby favoring its retention in solid ash. It also has the potential to promote the oxidation of Cr(III) to Cr(VI) to a minor extent. Increasing the oxygen partial pressure, particularly in the coexistence of HCl in flue gas, favored the oxidation of Cr(III) into gaseous Cr(VI)-bearing species such as CrO(2)Cl(2). Regarding the solid impurities including Na(2)SO(4) and CaO, Na(2)SO(4) has proven to preferentially capture the Cr(III)-bearing species at a low furnace temperature such as 600 °C. Its promoting effect on the oxidation of Cr(III) to Cr(VI), although thermodynamically available at the temperatures examined here, is negligible in a lab-scale drop tube furnace (DTF), where the particle residence time is extremely short. In contrast, CaO has proven facilitating the capture of Cr(VI)-bearing species particularly oxychloride vapors at 1000 °C, forming Ca chromate with the formulas of CaCrO(4) and Ca(3)(CrO(4))(2) via a direction stabilization of Cr(VI) oxychloride vapor by CaO particle or an indirect oxidation of Cr(III) via the initial formation of Ca chromite. The fly ash collected from the combustion of Cr-doped coal alone has a lower water solubility (i.e., 58.7%) for its Cr(VI) species, due to the formation of Ba/Pb chromate and/or the incorporation of Cr(VI) vapor into a slagging phase which is water-insoluble. Adding CaO to coal increased the water-solubility of both Cr(VI) and Cr(III) by forming Ca chromite and chromate, respectively.
authors.affiliation: State Key Laboratory of Coal Combustion, Huazhong University of Science & Technology, Wuhan 430074, People's Republic of China.
authors.forename: Juan
authors.initials: J
authors.lastname: Chen
authors.forename: Facun
authors.initials: F
authors.lastname: Jiao
authors.forename: Lian
authors.initials: L
authors.lastname: Zhang
authors.forename: Hong
authors.initials: H
authors.lastname: Yao
authors.forename: Yoshihiko
authors.initials: Y
authors.lastname: Ninomiya
mesh_terms: D017610:Calcium Compounds / Q000737:chemistry; D002857:Chromium / Q000737:chemistry*; D003031:Coal; D060729:Coal Ash / Q000032:analysis; D004785:Environmental Pollutants / Q000032:analysis; D006851:Hydrochloric Acid / Q000737:chemistry; D007220:Industrial Waste; D010087:Oxides / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry; D011210:Power Plants; D013431:Sulfates / Q000737:chemistry; D017356:Synchrotrons; D014835:Volatilization; D056928:X-Ray Absorption Spectroscopy
chemical_list: D017610:Calcium Compounds; D003031:Coal; D060729:Coal Ash; D004785:Environmental Pollutants; D007220:Industrial Waste; D010087:Oxides; D013431:Sulfates; D002857:Chromium; C012036:sodium sulfate; C074702:chromium hexavalent ion; C016538:lime; D006851:Hydrochloric Acid; D010100:Oxygen