High-temperature transformation of pyrite in CO2: Effects of residence time and the presence of O2

Abstract

Listen

Translate article

To understand the behavior of pyrite (a key slag-inducing coal mineral) in oxyfuel combustion (O2/CO2 combustion), our previous work firstly investigated the effects of pure CO2 on fixed bed reactors and at low temperatures (<1273 K) and long residence time (order of minutes). As a consecutive contribution, this work explores pyrite transformation in CO2 with the presence of O2 on a drop tube furnace (DTF) and at a high temperature of 1573 K. These conditions are more relevant to practical oxyfuel combustion. Pulverized pyrite samples of sizes between 63 and 75 µm were tested in CO2 with varying O2 from 0 to 3%, mimicking the conditions near the boiler burners. The changes of pyrite transformation with residence time were investigated by examining the intermediate products, collected by a high-temperature sampling probe. For a clear clarification of the effects of CO2, tests were also carried out in O2/N2 for comparison. The solid products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). The gas products were analyzed online by a gas analyzer. Results showed that, in the absence of O2, CO2 chemically accelerated pyrite decomposition and oxidation when compared with N2. This finding was consistent with that in previous work performed on fixed-bed reactors. In the presence of the same O2 concentration, the positive roles of CO2 persisted, enhancing the formation of magnetite (Fe3O4) and SO2. In either O2/CO2 or O2/N2, increasing O2 concentration also favored the formation of magnetite and SO2. Significant changes of SO2 and S in pyrrhotite (FeSx) were observed between 0.5–0.7 s, and longer residence time did not seem to have significant effects. However, the amount of magnetite formed increased with increasing residence time. The obtained knowledge is new and very useful to further understanding of pyrite behavior in real oxyfuel combustion systems.