(584c) Investigation of Sulfur Adsorption on Fly Ash for Oxy-Combustion

Oxy-combustion proposes to incorporate carbon capture and storage technologies into existing coal-fired power plants to lower carbon emissions.  In an oxy-fired power plant, nitrogen is separated out prior to combustion, resulting in concentrated carbon dioxide as a final product that can be easily captured and stored.  Sulfur oxide (SO_X) emissions can be lowered due to high concentrations of sulfur trioxide in the oxy-combustion flue gas, which results 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 magnesium (Mg) and calcium (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 most prominent AAEM species, calcium and magnesium oxide.  Firstly, density functional theory (DFT) calculations were performed using Vienna ab initio Simulation Package (VASP) to investigate the binding mechanism for SO_X on CaO and MgO surface.  Calculations were also run with various other co-adsorbates to examine the effect their presence would have on any SO_X reactions on the surface.  Secondly, combustion experiments were employed to investigate the AAEM oxides after exposure to a SO₂/SO₃ mixture plus a variety of other flue gas components.  Samples were exposed to simulated flue gas of air and oxy-combustion at various temperatures with the resulting products being analyzed using x-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).  In addition to the investigation of individual oxides, samples of fly ash will also be characterized. Overall, the experimental results along with DFT calculations will be used to create a more complete picture of the SO_X binding mechanism on the fly ash metal oxides and compare air and oxy-combustion environments for sulfur capture.