Combustion behaviors and pollutant emission characteristics of low calorific oil shale and its semi-coke in a lab-scale fluidized bed combustor

•Co-combustion performances of oil shale and its semi-coke were studied in a lab-scale bubbling fluidized bed combustor.•Synergic reactions between oil shale and its semi-coke were identified in terms of emissions.•It was recommended to increase the blending ratio of oil shale up to 50%.•The ultra-low emission of SO2 and NO was able to be achieved.