Comprehensive evolution mechanism of SOx formation during pyrite oxidation

Pyrite (FeS2) oxidation during coal combustion is one of the main sources of harmful SO2 emission from coal-fired power plants. Density functional theory (DFT) study was performed to uncover the evolution mechanism of SOx formation during pyrite oxidation. The results show that chemisorption mechanism is responsible for O-2, SO2 and SO3 adsorption on FeS2 surface. The presence of formed oxidation layer (Fe2O3) weakens the interaction between O-2 molecule and FeS2 surface. The adsorbed O-2 molecule easily dissociates into active surface O atom for SOx formation. The dissociation reaction of O-2 is activated by 77.38 kJ/mol, and exothermic by 138.46 kJ/mol. Compared to the further oxidation of SO2 into SO3, SO2 prefers to desorb from FeS2 surface. The dominant reaction pathway of SO2 formation from the oxidation of the outermost FeS2 surface layer is a three-step process: surface lattice S oxidation, SO2 desorption and replenishment of S vacancy by activated surface O atom. The elementary reaction of surface lattice S oxidation has an activation energy barrier of 197.96 kJ/mol, and is identified as the rate-limiting step. SO2 formation from the further oxidation of bulk FeS2 layer is controlled by a four-step process: bulk lattice S migration, lattice S oxidation, SO2 desorption and surface O atom deposition. Migration of lattice S from bulk position to the outermost surface shows a high activation energy barrier of 175.83 kJ/mol. The deposition process of surface O atom is a relatively easy step, and is activated by 21.05 kJ/mol. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.