Effects of sulfation on the ash fusibility and minerals evolution of corn straw during oxy-fuel combustion

Abstract

Oxy-biomass combustion is an important technology for achieving negative CO2 emission. However, during oxy-biomass combustion, a series of ash-related problems are induced because of the accumulation of recycled SO2. In this study, the effects of SO2 on the ash fusibility were systematically investigated through ash fusion temperatures (AFTs) tests, scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectrometer (ICP-OES), X-ray diffraction (XRD), and chemical thermodynamics calculation (FactSage). Besides, the relevance among minerals evolution, melting performance, and ash sulfation was established. It was found that the presence of SO2 suppressed the release of K and Ca species, thereby increasing the AFTs of corn straw, while this depended heavily on the SO2 concentration, the duration of ash sulfation, and the initial concentration of O2. Furthermore, two reacting mechanisms, including moderate and excessive sulfation, were proposed in this work. When the initial SO2 concentration was less than 1000 ppm, AFTs increased due to the transformation of alkaline chlorides to their sulfates and aluminosilicates with the presence of SiO2 and Al2O3 in ash samples, which was named as moderate sulfation. When the concentration of SO2 was higher than 1000 ppm, large amounts of KAlSi3O8, CaAl2Si2O8, Ca2Al2SiO7 and CaSO4 were generated, causing the simultaneous eutectic and CaO-assisted melting, thereby decreasing the AFTs of corn straw, which was named as excessive sulfation.