Occurrence and release of cadmium, chromium, and lead from stone coal combustion

Stone coal, which has high ash yield, low carbon content, and low calorific value, releases toxic elements during the combustion process. In this study, samples were collected from an open-pit stone coal mine in the southern Shaanxi province, China. A simulated combustion experiment and sequential chemical extraction were applied to investigate the enrichment, occurrence, and release of Cr, Cd, and Pb in raw stone coal and ash. The contents of Cr, Cd, and Pb in raw stone coal were larger than those reported for other coals in China and worldwide. The enrichment factors of Cr, Cd, and Pb in ash at different temperatures exceeded 0.7, with the exception of Cd at 1000 degrees C. This indicates that Cr, Cd, and Pb were first enriched at low temperature (below 600 degrees C) and then volatilized at high temperature due to the decomposition of clay minerals. The dominant forms of toxic elements in raw stone coal were Fe-Mn-bound Cr and Cd and carbonates of Pb. As the temperature increased, the organic forms decreased obviously, and the residual forms increased; however, the Fe-Mn-bound forms remained the dominant forms of Cr, Cd, and Pb.