(29f) A Theoretical Investigation of Sulfur Adsorption on Calcium Oxide

Sulfur capture on coal fly ash plays a critical role in limiting SO_x emissions in power plants. The retention on the fly ash is heavily dependent on the alkali and alkaline earth metal (AAEM) species in coal such as Ca. In oxy-combustion, this mechanism becomes particularly important as the sulfur concentration (SO₂ + SO₃) in the flue gas can be several times greater than traditional air-combustion due to the unique staging required of oxy-fuel systems. While higher rates of sulfur capture on fly ash have been observed for oxy-combustion compared to air-combustion, this increased retention can create concerns in utilization of the fly ash for cement and concrete production.

Although there have been previous experimental studies, the surface mechanism of sulfur retention on Ca-based materials is not well understood. In this study, computational modeling was employed to examine the binding of SO_x species to the most prominent AAEM species, calcium oxide. Density functional theory (DFT) calculations were performed using Vienna ab initio Simulation Package (VASP) to investigate the binding mechanism for SO_x on a CaO(100) surface. Calculations were also run with various other co-adsorbates, including CO₂, H₂O and SO_x, that are pre-adsorbed to the surface to examine the effect of their presence on any subsequent SO_x reactions on the surface. The energetic, geometric and electronic properties of the simulated surfaces were analyzed to discern any possible trends in the SO_x binding mechanism with respect to the specific pre-adsorbed molecule.  The results were then compared to experimental data where CaO was exposed to a variety of flue gas compositions. In this way, a more complete picture of the SO_x binding mechanism on calcium oxide was determined allowing for a better understanding of the initial stages of sulfur capture in combustion systems.