Thermal and chemical stabilities of arsenic in three Chinese coals

The forms and pyrolysis behavior of arsenic in three Chinese coals was studied. The arsenic in the coals and the coal-derived pyrolysis chars were characterized into five forms using a sequential extraction method where a series of solutions with increasing acidity and oxidizability were used. The five arsenic forms are named as ion exchangeable, bound to carbonates, bound to Fe–Mn oxides, bound to organic matter and remained in residue. XRD, SEM-EDX and TOF-SIMS techniques were also used to characterize the coals and the chars. Results show that bleeding ratio of arsenic in the three coals increases with increasing pyrolysis temperature. The type of coal and the arsenic distribution in the coal have significant effect on the pyrolysis behavior of arsenic. The different volatility of arsenic was attributed to its chemical stability and thermal stability. Ion exchangeable arsenic and the arsenic bound to carbonates are not present in the three coals. Other arsenic forms undergo transformation during the pyrolysis, from the solid phase to the vapor phase and from one chemical form to other chemical forms. The main arsenic remained in the residue may include resistant sulfides.