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

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 M_CaO/M_Fe2O3 < 5/4 prior to combustion.