Correlations between the sodium adsorption capacity and the thermal behavior of modified kaolinite during the combustion of Zhundong coal

Abstract

Kaolin is often utilized as an additive to relieve fouling and slagging problems in boilers. In a previous work of the authors it was successfully employed the modification of intercalation-exfoliation method to improve the adsorption capacity of kaolinite at 1000 °C, and this work aimed to investigate the best adsorption performance and corresponding application temperature of modified kaolinite at different temperatures. The adsorption experiment with Zhundong coal was conducted from 650 °C to 1200 °C. The thermal behavior of modified kaolinite was characterized over a wide temperature range to reveal the mechanism from the perspective of structural change. Results showed that modified kaolinite exhibited the best adsorption performance at 800 °C, with the adsorption capacity increasing by 39% after modification. The effect of modification on kaolin was also reflected in the thermal behavior. About 43% of the hydroxyl groups were removed via modification, the amount of mullite above 1100 °C approximately decreased by 38%, and the expulsion of crystalline SiO2 was totally inhibited by modification, suggesting that modification affected the reconstruction of Si/Al-related compounds at high temperature. In addition, the enhanced yield of unsaturated Al (V) at 800 °C after modification was totally consistent with the improved adsorption capacity of modified kaolinite at 800 °C. It is concluded that kaolinite can be used at higher temperature with improved adsorption capacity through a modification of intercalation-exfoliation method, which was presumably related to the greater content of 5-coordinated Al in modified kaolinite. Moreover, this discovery indicated that Al (V) content might be a useful index for the selection of various aluminasilicates as efficient adsorbents in furnace.