(499c) Binding of SO3 to Fly Ash Components: CaO, MgO, Na2O and K2O

Oxy-combustion proposes to incorporate carbon capture and storage technologies into existing coal-fired power plants to lower carbon emissions.  It does so by separating out the N₂ prior to combustion, resulting in a concentrated CO₂ as a final product that can be easily captured and stored.  Sulfur oxide (SO_X) emissions can be lowered due to high concentrations of SO₃in the oxy-combustion flue gas resulting in sulfur retention on fly ash and ash deposits in the furnace. It has been shown that the extent of sulfur retention is heavily dependent on the alkali and alkaline earth metal (AAEM) species in coal such as Na, K, Mg and Ca and the sulfur retention increases as the AAEM:S molar ratio increases. While higher sulfur retention on the ash particles reduces the emission rates, it creates problems utilizing the fly ash for cement and concrete production.

 Although there have been previous experimental studies, the mechanism of sulfur retention on fly ash is not well understood. In this study, a variety of computational and experimental methods were employed to examine the binding of SO_X species to the AAEM species.  Firstly, density functional theory (DFT) calculations were performed using Vienna ab initio Simulation Package (VASP) to investigate the binding mechanism for SO₃ on several metal oxides that compose fly ash, namely CaO, MgO, Na₂O, and K₂O.  Calculations were also run where SO₃ was co-adsorbed with H₂O to examine if its presence would facilitate further reaction of the SO₃ on the oxide surface.  Secondly, FTIR spectroscopy will be employed to investigate the AAEM oxides after exposure to a SO₂/SO₃ mixture plus a variety of other flue gas components. FTIR results along with DFT calculations will be used to create a more complete picture of the SO_X binding mechanism on the metal oxides.