Ash and Flue Gas from Oil Shale Oxy-Fuel Circulating Fluidized Bed Combustion

Carbon dioxide emissions are considered a major environmental threat. To enable power production from carbon-containing fuels, carbon capture is required. Oxy-fuel combustion technology facilitates carbon capture by increasing the carbon dioxide concentration in flue gas. This study reports the results of calcium rich oil shale combustion in a 60 kWth circulating fluidized bed (CFB) combustor. The focus was on the composition of the formed flue gas and ash during air and oxy-fuel combustion. The fuel was typical Estonian oil shale characterized by high volatile and ash contents. No additional bed material was used in the CFB; the formed ash was enough for the purpose. Two modes of oxy-fuel combustion were investigated and compared with combustion in air. When N-2 in the oxidizer was replaced with CO2, the CFB temperatures decreased by up to 100 degrees C. When oil shale was fired in the CFB with increased O-2 content in CO2, the temperatures in the furnace were similar to combustion in air. In air mode, the emissions of SO2 and NOx were low (<14 and 141 mg/Nm(3) @ 6% O-2, respectively). Pollutant concentrations in the flue gas during oxy-fuel operations remained low (for OXY30 SO2 <14 and NOx 130 mg/Nm(3) @ 6% O-2 and for OXY21 SO2 23 and NOx 156 mg/Nm(3) @ 6% O-2). Analyses of the collected ash samples showed a decreased extent of carbonate minerals decomposition during both oxy-fuel experiments. This results in decreased carbon dioxide emissions. The outcomes show that oxy-fuel CFB combustion of the oil shale ensures sulfur binding and decreases CO2 production.