Assessing the chromium mobility in ashes through SiO2–Al2O3–Fe2O3–CaO system: The role of composition

Abstract

Given the millions of coal-fired power stations worldwide and the generalization of co-firing technologies in the near future, the aqueous extraction experiments were taken to study the effects of oxides on Cr mobility in ashes through SiO2–Al2O3–Fe2O3–CaO system. The results identify that both the component and the species of Cr in samples are vital factors to govern the fate of Cr during combustion. Although Cr-oxide hardly reacts with Al2O3, SiO2, and mixtures at 900 °C, the immobilization of Cr in Si–Al glass is ascribable to the heat-driven phase transformation of Cr-rich clay. The strong capture of Cr-oxide by CaO leads to the primary extraction of active Cr with the high toxicity and mobility; however, the opposite effect is found by Fe2O3. But the interactions of Al-/Si- oxides with others can trigger some changes in Cr mobility, wherein there is the lowest mobility of Cr in the case of Cr entering into the structures of Fe-rich SiO2/Si–Al glass during combustion. Thus, without regard to the sample source, it is effective to reduce the environmental risk of Cr in ashes through raising SiO2 and reducing MCaO/MFe2O3 < 5/4 prior to combustion.