Fate of sulfur in coal-direct chemical looping systems.

The fate of sulfur in the coal-direct chemical looping system was investigated in the sub-pilot reactor system. The sulfur balance was successfully closed during the injection of high sulfur coal. More than 69% of the total amount of atomic sulfur in coal was released as SO 2 and H 2 S from the reducer flue gas stream while less than 5% was emitted as SO 2 from the combustor spent air. The remaining atomic sulfur was retained in coal ash as inorganic sulfur compounds. The finding suggests an acid gas removal system targeting both H 2 S and SO 2 is required to meet the recommended quality of CO 2 stream for sequestration and transportation. Using the determined ratio of SO 2 and H 2 S, a properly designed Claus plant can enable the recovery of elemental sulfur as a value-added byproduct. 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 slope 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 affirms the effectiveness of the counter-current moving bed design for 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%, respectively. 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. Compared with chemical looping systems using circulating fluidized bed configuration, the use of a moving bed reducer has the additional benefit of minimizing slippage of char into the combustor due to the use of large oxygen carrier; resulting in lower sulfur emission in the combustor spent air. The findings demonstrate the robustness of the coal-direct chemical looping system to handle high sulfur coal without a complicated acid gas cleaning scheme or severe performance penalties. [ABSTRACT FROM AUTHOR]