Oxy-fuel combustion is considered as one of the promising carbon capture and storage (CCS) technologies for Coal-fired boilers. In oxy-fuel combustion, the combustion gases are oxygen and the recirculating flue gas, and the main components of the combustion gas are O-2, CO2 and H2O [1]. The paper presents the results of the calculation of the flue gas amount during combustion of oil shale using oxy-fuel technology in a circulated fluidized bed (CFB) mode. The calculations were performed for different oil shale heating values and different recycled flue gas (RFG) ratios. Oxy-fuel combustion with flue gas recycling was found to enable the decrease of the extent of carbonate minerals decomposition (ECD), thereby increasing the amount of heat released per I kg of fuel. To minimize ECD, the recycled flue gas ratio should be maintained at a level higher than 0.7. This condition allows an increase of the partial pressure of CO2 over the equilibrium state line of calcite decomposition reaction at the bed temperature. The decrease of ECD was observed up to k(CO2-min) = 0.28. The decrease of k(CO2) leads to an additional increase in the amount of heat released during oil shale combustion per I kg and, depending on the mean lower heating value (LHV), the heat can be increased up to 0.34 MJ/kg. A comparison with the bituminous and anthracite coals revealed that the specific emission of CO2 per input fuel energy for oil shale is expected to be even smaller compared with those of the considered coals.