SO2 Emissions from Oil Shale Oxyfuel Combustion in a 60 kWth Circulating Fluidized Bed

Carbon capture, utilization, and storage (CCUS) have emerged as pivotal technologies for curtailing emissions while maintaining fossil fuel. Estonia faces a challenge due to its dependence on carbon-intensive oil shale, but the need for energy security, highlighted by the war in Ukraine, makes reducing CO2 emissions a priority while maintaining energy independence. In this context, the presented study determines the environmental impact of combustion of the Estonian oil shale from the release of SO2 emission and compares sulfur retention in the ash between different oxyfuel combustion campaigns in a 60 kWth CFB test facility. The pilot was operated under air, O2/CO2, and with recycled flue gas (RFG), and we tested the application of extremely high inlet O2 up to 87%vol. The key objective of this study is to examine how different combustion atmospheres, operating temperatures, and excess oxygen ratios influence SO2 formation. Additionally, the research focuses on analyzing anhydrite (CaSO4), calcite (CaCO3), and lime (CaO) in ash samples collected from the dense bed region (bottom ash) and the external heat exchanger (circulating ash). The results indicate that increased inlet O2% does not significantly affect SO2 emissions. Compared to air-firing, SO2 emissions were higher than 40 mg/MJ under a 21/79%vol O2/CO2 environment but were significantly reduced, approaching zero, as the inlet O2% increased to 50%. Under O2/RFG conditions, higher SO2 concentrations led to increased sulfur retention in both the bottom and circulating ash. The optimal temperature for sulfur retention in air and oxyfuel combustions is below 850 °C. This study for the first time provides a technical model and discusses the effects of operating parameters on sulfur emissions of the Estonian oil shale CFB oxyfuel combustion.