Volatility of cadmium during pyrolysis of typical low rank coals

Abstract

The distribution of cadmium (Cd) occurrence modes in a Naomaohu coal (NMH) and an Inner Mongolia high sulfur coal (GL) was investigated using the sequential chemical extraction method. The influence of the occurrence modes, final pyrolysis temperature, heating rate and residence time on the release behavior of Cd during the pyrolysis of two raw coals at 400–800°C in a horizontal/vertical tubular furnace was examined, respectively. The simulation of the migration and transformation of Cd under ideal conditions by FactSage software was carried out. The results show that Cd in NMH and GL is present in an organic matter state by 46% and 37%, a carbonate state by 32% and 24%, a disulfide state by 12% and 1%, and an aluminosilicate state by 10% and 38%, respectively. The distribution of occurrence modes seriously affects the volatilization behavior of Cd, in which the Cd in the organic matter state is easy to volatilize at low temperature, while those in the carbonate, aluminosilicate and sulfide state can volatilize at medium and high temperature; and reducing pyrolysis rate and extending residence time are conducive to the release of Cd. The FactSage simulation shows that Cd, CdO, Cd(OH)x and CdS are the major gaseous products of Cd during the pyrolysis of NMH and GL coal, and the difference of volatilization behavior of Cd in two kinds of coal is mainly determined by pyrolysis temperature, distribution of occurrence modes and coal rank, which are in good agreement with the experimental results.