Impacts of CO2 on the pyrite–kaolinite interaction and the product sintering strength

Abstract

Interactions between pyrite and silicates are very critical to ash slagging in coal-fired boilers. However, no work has been reported regarding the impacts on such interactions of CO2, the dominant component in the oxy–fuel combustion gas. This was investigated in the present work by using mixtures of pyrite and kaolinite, a prevailing silicate mineral in coal. Furthermore, the sintering strength of the generated products was also evaluated. The pyrite–kaolinite mixtures were treated on a fixed bed reactor in both N2 and CO2 for comparison. The treating temperature was 1050, 1150 and 1250 °C while the reaction time was 3, 6 and 12 min, respectively. The solid products were characterized by techniques such as X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). Sintering tests of these products were carried out as well. It was found that some interactions between pyrite and kaolinite took place in the N2 atmosphere. This was evidenced by the formation of iron aluminosilicate and attributed to the effects of water vapor released from kaolinite dehydroxylation. Nevertheless, pyrite–kaolinite interactions in N2 were limited and had insignificant effects on product sintering strength development. In CO2, pyrite–kaolinite interactions were significantly enhanced, compared with those in N2. Although the kaolinite-derived water vapor had some effects, CO2 was found to play a dominant role. Enhanced pyrite–kaolinite interactions resulted in an increase of eutectic phases. Consequently, the product sintering strength development was greatly elevated. It was further found that, under the conditions investigated, the interactions between pyrite and kaolinite were actually through reactions between FeO, rather than other intermediates, and aluminosilicate. This new finding enabled us to develop the mechanisms for pyrite–kaolinite interactions in N2 and CO2.