Volatilization of Arsenic During Coal Combustion Based on Isothermal Thermogravimetric Analysis at 600–1500 °C

The volatilization characteristics of arsenic in different coals at 600–1500 °C were studied on an isothermal reaction system. Through thermogravimetric analysis similar to that used for coal analysis, the mass loss and mass loss rate of arsenic for coal samples were determined. Sequential chemical extraction method was used to measure the mode of occurrence of arsenic in the coals. TG-MS techniques were also carried out to study the relationship between sulfur and arsenic. Results show that the volatilization proportion of arsenic increases with temperature and 53– 99% of the total arsenic in coal is vaporized in the combustion zone at PC conditions (1500 °C). Coals with higher arsenic concentrations (As > 4 μg/g) tend to have larger arsenic volatility proportions than coals with lower arsenic concentrations (As < 4 μg/g) at a given temperature. In addition, three volatility zones with two mass loss rate peaks of arsenic are observed in all coals during coal combustion. Before 600 °C, the evaporation of organic-bound arsenic dominates; then the first arsenic mass loss peak at 800–900 °C is mainly from the decomposition or oxidation of arsenic in sulfide forms. The second peak after 1000 °C is probably generated through the decomposition of arsenates. For As > 4 μg/g coals, the first peak is higher than the second peak due to the larger proportion of sulfide-bound arsenic, while the second mass loss peak of arsenic is higher in As < 4 μg/g coals due to the larger proportion of arsenates. Furthermore, thermodynamic analysis by HSC chemistry6.0 software were carried out to prove that arsenates, like Ca 3 (AsO 4 ) 2, FeAsO 4, and Mg 3 (AsO 4 ) 2, are thermally stable and could only decompose at relatively high temperatures.