(212g) Fate of Sulfur in Coal-Direct Chemical Looping Systems

The fate of sulfur in the coal-direct chemical looping system (CDCL) was investigated in the sub-pilot reactor with a heat-traced gas analysis system. The CDCL process achieves 96.5% carbon capture efficiency with a 26.8% increase in cost of electricity compared to a conventional pulverized coal power plant without CO₂ capture. The sulfur balance was successfully closed during the injection of high sulfur coal. More than 69% of sulfur from coal exit the reactor as SO₂ and H₂S in the reducer flue gas while less than 5% exit as SO₂ in the combustor spent air. The remaining sulfur was retained in coal ash as inorganics. The finding suggests an acid gas removal system targeting both H₂S and SO₂ is required to meet the recommended quality of CO₂ stream for sequestration and transportation. Using the determined ratio of SO₂ and H₂S, a properly designed Claus plant can potentially enable the recovery of elemental sulfur. On the other hand, the combustor spent air was found to comply with the US EPA sulfur emission regulation and can be released to the atmosphere without a costly acid removal system. The relationship between the sulfur and carbon capture efficiencies was established experimentally and was found to be proportional to each other throughout the experiment at a ratio of 0.8 below 93% of carbon capture efficiency and near 1 above 93%. This was attributed to the delayed release of organic sulfur during incomplete char gasification in the reducer. The finding reaffirms the effectiveness of the counter-current moving bed design in minimizing the amount of carbon and sulfur emission in the combustor spent air with an average carbon and sulfur capture efficiency of 96.5 and 95%. Sulfur deposition on the iron based oxygen carriers did not affect the system performance and complete removal of deposited sulfur was observed during oxidation in a thermogravimetric analyzer. The findings demonstrate the robustness of the coal-direct chemical looping system to handle high sulfur coal without complicated acid gas cleaning scheme or severe performance penalties. A 250 kW_th pilot scale coal-direct chemical looping reactor has been constructed by Babcock and Wilcox Power Generation Group in Barberton, OH that is undergoing unit testing with both low and high sulfur coal in 2017.