Further Insight into the Formation and Oxidation of CaCr₂O₄ during Solid Fuel Combustion

Abstract

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).

Effect of applied pressure on microstructure and properties of hot‐pressed Ca‐doped LaCrO3 ceramics

“…According to previous research, there are two main ways to generate CaCr 2 O 4 . One is produced by the direct reaction of CaCO 3 and Cr 2 O 3 at a temperature of approximately 1100°C, 29 and the other is the reaction between CaCrO 4 and Cr 2 O 3 when the temperature is about 1022°C ± 2°C, 37 the overall reaction in the formation of perovskite phase is as given…”

“…28 Interestingly, when we prepared calcium-doped and chromium-deficient samples (La 0.8 Ca 0.2 Cr 0.98 O 3 ) by hot pressing (HP) to obtain high-density samples, we found that when the sintering pressure increased to a certain extent, the second phase CaCr 2 O 4 will be formed in the sintered samples. Previous studies have only mentioned that CaCr 2 O 4 may be formed after the operation of SOFC, 29 however, the formation of CaCr 2 O 4 during the sintering process and its effect on the properties of sintered bodies have not been reported.…”

Mechanism and Prediction of Cr and Zn Immobilization from Industrial Residues by Glass-Ceramics

“…The formation of Cr 6+ could be explained by the reaction of Cr 2 O 3 as an acid oxide and CaO as an alkaline oxide with the aid of O 2 via eq . It should be noted that the formation of Cr 6+ was undesirable due to the higher toxicity and mobility of Cr 6+ than Cr 3+ . , This reaction mainly occurred on the surface of glass-ceramics, while it was unfavorable in the bulk phase due to the deficiency of oxygen. The absence of CaCrO 4 peaks in XRD patterns also confirmed its relatively low concentration.…”

The migration and transformation of chromium during co-processing of cement raw meal mixed with chrome-polluted soil

“…Cr 2 O 3 and CaO were heated in a muffle furnace. The X-ray adsorption near-edge spectroscopy spectra indicated that the initial oxidation of Cr 2 O 3 occurred at approximately 700 °C, and the intensity of the Cr(VI) peak increased dramatically with increasing temperature (Chen et al 2013;Hu et al 2018). During the heating process, CaCr 2 O 4 is formed as an intermediate product, and the oxidation of CaCr 2 O 4 is favored with increasing free Ca (Hu et al 2018).…”

“…The X-ray adsorption near-edge spectroscopy spectra indicated that the initial oxidation of Cr 2 O 3 occurred at approximately 700 °C, and the intensity of the Cr(VI) peak increased dramatically with increasing temperature (Chen et al 2013;Hu et al 2018). During the heating process, CaCr 2 O 4 is formed as an intermediate product, and the oxidation of CaCr 2 O 4 is favored with increasing free Ca (Hu et al 2018). In addition, CaO can capture Cr steam to form calcium chromite, thereby inhibiting Cr volatilization and promoting Cr enrichment in solid products (Chen et al 2013).…”