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Oil shale (OS) is an unconventional low-calorific-value fossil fuel, the usage of which is increasing due to increasing energy demand. Today, Estonia's OS usage is the largest in the world. Approximately 90% of the electricity consumed is produced from Ca-rich OS. Most of the OS, approximately 12 million tons yearly, is used for power generation utilizing pulverized combustion (PC) and circulating fluidized bed combustion (CFBC) technologies that produce nearly 6 million tons of ash. As a result, Estonia has one of the world's highest CO 2 emission rates per capita. The current study is focused on determination of the amount of CO 2 bound by CFBC and PC boiler OS ashes in an ash field. The binding ability toward CO 2 of the ash stored in an ash field is observed. Based on the in situ experimental data, when looking the binding abilities of ashes from both the technologies separately, it is found that approximately 9.7% of the CO 2 emitted is bound by the CFBC ash and about 3.4% by the PC boiler ash in an ash field. On the basis of experimental data it is found that approximately 5-6% of the CO 2 emitted is bound back by oil shale power plant ash fields and sediment ponds.

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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 , CO 2 and H 2 O [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 1 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 CO 2 over the equilibrium state line of calcite decomposition reaction at the bed temperature. The decrease of ECD was observed up to CO2-min 0.28. k = The decrease of CO2 k leads to an additional increase in the amount of heat released during oil shale combustion per 1 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 CO 2 per input fuel energy for oil shale is expected to be even smaller compared with those of the considered coals.