Effect of the typical coal ash-analogous minerals on the migration of heavy metals during coal combustion

This study investigated the influence of various additives (CaO, Al 2 O 3 , SiO 2 , kaolin, and calcite) on the volatilization and immobilization of heavy metals during the coal combustion in a horizontal tube furnace (700–1100 ℃). Results showed that the Si/Al-based sorbents had less effect on the mineral changes of the combustion ash of coal than Ca-based sorbents which tend to react with the Si/Al components in coal ash to form Ca-containing aluminosilicates and affect the heavy metal retention. With the increase in combustion temperature, the volatilization of heavy metals gradually increased. At lower temperatures (700–900 ℃), Ca-based additives (CaO and calcite) enhanced Se and Pb retention due to the formation of Ca-Se/Pb compounds and competitive mineral incorporation. However, their efficacy diminished above 900 ℃ due to compound decomposition. The Si/Al-based additives (SiO 2 , Al 2 O 3 , and kaolin) exhibited superior immobilization of Zn, Cu, and Pb at elevated temperatures (1000–1100 ℃), attributed to the formation of aluminosilicates. In addition, when the combustion temperature increased, the toxicity of Se decreased more significantly than other elements as the proportion of acid extractable/exchangeable Se decreased. In contrast, the form distribution of Cr, Ni, and Mn are more influenced by Ca-based minerals. Specially, the proportion of Cr in acid extractable/exchangeable form almost doubled after the addition of CaO and calcite. This work was conducive to the subsequent control of heavy metal emissions in the coal burning process.