Further Insight into the Formation and Oxidation of CaCrO during Solid Fuel Combustion.

The control of toxic chromate (Cr) formation is still a significant challenge in solid fuel combustion. In particular, the mechanism of chromium transformation from Cr to chromate or other unoxidized forms remains unclear. The present study confirms the formation of a significant unoxidized Cr-containing compound CaCrO(Cr) during solid fuel combustion. Experiments were conducted, for the first time, to clarify the mechanism of CaCrO oxidation, which is quite different from CrO oxidation. The findings demonstrate that CaCrO was formed at temperatures above 1200 K, through rapid decomposition of CaCrO or slow and direct interaction between CaO and CrO. Compared to CrO, CaCrO could be oxidized at lower temperatures under the influence of free CaO. In the absence of free CaO, the oxidation of CaCrO was minimal; however, in the presence of CaSO, calcium in the form of CaCrO participated in the oxidation of CaCrO. Thus, chromium in the form of CaCrO was more likely to be oxidized when CaCrO-containing fly ash was reheated. Fortunately, CaCrO showed slight basicity on the surface, allowing it to react with acidic gases. Accordingly, measures were proposed to suppress the oxidation of CaCrO by stimulating the reactions between CaCrO and acidic substances, like SO and Si/Al-compounds. These compounds competed with chromium at high temperatures to react with calcium in the fly ash and in CaCrO. As a result, the unoxidized chromium was transformed into highly stable CrO or CaCr (SiO).