A mechanistic study of the effects of CO2 on pyrrhotite oxidation

The high levels of CO2 in oxy-fuel combustion are expected to have important effects on the transformation of pyrite, a major contributor to ash deposition. As a successive work to the previous study that explored the chemical role of CO2 in pyrite decomposition, this paper is purposely designed to investigate the effects of CO2 on the oxidation of pyrrhotite generated from pyrite decomposition. Pyrrhotite oxidation in pure CO2 was respectively investigated at 900, 950 and 1000 °C on a well-designed thermo-gravimetric reactor (TGR). The time-resolved data of sample weight loss and gas evolution were collected online by a data collection module and a Horiba PG-350 gas analyzer, respectively. The solid products were characterized by X-ray diffraction (XRD). The results demonstrate that the influence of CO2 on the oxidation of pyrrhotite is chemical in nature. The transformation of pyrrhotite in CO2 consists of three stages, i.e., the fast weight loss stage, the slow weight loss stage and the slow weight gain stage. The mechanisms involving CO2 at these three distinct stages are quite different. At the first stage, ferrous sulfide is formed by the decomposition reaction of pyrrhotite with CO2. At the second stage, the oxidation reactions of ferrous sulfide with CO2 are responsible for the formation of magnetite or hematite. Hematite may also be formed through further oxidation of magnetite by CO2. The third stage is dominated by the oxidation of magnetite to form hematite with CO as the only gas product.