Jiao, Facun, Wijaya, Niken, Zhang, Lian, Ninomiya, Yoshihiko, and Hocking, Rosalie (2011) Synchrotron-based XANES speciation of chromium in the oxy-fuel fly ash collected from lab-scale drop-tube furnace. Environmental Science and Technology, 45 (15). pp. 6640-6646.

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Abstract

Speciation of chromium (Cr) in the fly ash collected from oxyfiring of Victorian brown coal has been reported for the first time to address the potential formation of toxic Cr(VI) and the variation of the quantities of Cr(III)-bearing species with flue gas composition. Synchrotron-based X-ray absorption near-edge structure (XANES) was employed for Cr speciation. Apart from a pure O₂/CO₂ mixture (27/73, v/v) versus air, the O₂/CO₂ mixtures doped with SO₂, HCl, and steam individually or together to simulate real flue gas have also been tested. Under all of the conditions tested here, the fractions of Cr(VI) in the fly ashes are insignificant, constituting no more than 5% of the total Cr. The test of Cr-doped brown coal in pyrolysis further confirmed that the Cr(VI) formation preferentially occurred through a local oxidation of Cr(III) at the oxygen-containing functions sites within coal matrix, rather than through an oxidation by external bulk O₂. This reaction is also highly temperature-dependent and slower than the interaction between Cr(III) and other metals such as iron oxide. Increasing temperature to 1000 degrees C inhibited the oxidation of Cr(IIII) to Cr(VI). Shifting the combustion gas from air to O₂/CO₂ exerted little effect on the Cr(VI) formation. Instead, the formation of iron chromite (FeCr₂O₄) was facilitated in O₂/CO₂ probably due to a strong reducing microenvironment formed by the CO₂ gasification reaction within the char matrix. The accumulation of HCl in flue gas favored the vaporization of chromium as gaseous chloride/oxychloride, as expected. The coexistence of SO₂ inhibited this phenomenon by promoting the formation of sulfate. The presence of steam was even beneficial for the inhibition of water-soluble Cr sulfate through stabilizing the majority of Cr into alumina-silicate which is in the slagging phase.

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