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In this article environmental and economic impact of firing oil shale of higher quality is analyzed. Fuel consumption, emission indicators (CO 2 , SO 2 , CO, NO x , N 2 O, particulates) and ash mass flow of a circulating fluidized bed (CFB) boiler firing oil shale of lower heating value (LHV) of 8.2-11.5 MJ/kg are presented. The investigation is based on full-scale firing tests. Based on test results the impact of transportation and operational costs of oil shale and ash handling on electricity price is analyzed. The pollution charges and CO 2 emission allowances are considered when analyzing the environmental impact on costs. Firing upgraded oil shale (10.5 MJ/kg) leads to substantial reduction of environmental impact and enables to save costs of electricity production. Reduction of CO 2 emission by 7%, ash mass flow by 25% and fuel consumption by 22% when firing upgraded oil shale instead of conventional one (8.4 MJ/kg) enables to save up to 3 EUR/MWh e , achieving the major savings from environmental costs, especially from reduced need for purchasing CO 2 emission allowances.

The ash balance and the extent of carbonate decomposition (ECD) for circulating fluidised-bed (CFB) boilers depending on oil shale lower heating value (8.5 and 11.1 MJ/kg) were analysed. As for the ash balance, ash mass flow rates (kg/s) for each ash separation port were determined. In calculations the ECD methodology developed at the Department of Thermal Engineering (DTE) on the basis of ash composition was used. Changes in the boiler ash balance occurred when firing oil shale of different quality. Additionally, the reduction of the CO 2 emission by 7% and of the total ash mass flow by 23% was obtained when firing upgraded oil shale instead of low-grade oil shale.

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.

Estonia has two of the world's largest oil shale firing circulating fluidized bed (CFB) units with a designed electrical capacity of 215 MW each. The units are based on double boiler CFB technology provided by Foster Wheeler Energia OY. The units are located at Eesti and Balti power plants (EPP and BPP). The paper presents analyses of data obtained from tests of oil shale and biomass co-combustion in the full-scale CFB boiler located at BPP. The tests were conducted at nominal boiler load: 100% (314 t/h), with a biomass thermal input of 15%. During the experiments ash samples from the furnace chamber (bottom ash), INTREX, super-/reheater (SH, RH), economizer (ECO), and air preheater (APH), and from all four fields of the electrostatic precipitator (ESP) were taken. Samples of fly ash for determining the mass division (total suspended particulates PM10 and PM2.5) were taken after the ESP. The gas analysis was performed at the ESP inlet. Analysis of the chemical composition of ash was carried out. The specific consumption of oil shale per useful heat and gross electricity were found and other techno-economic characteristics determined. It was found that oil shale and biomass co-combustion reduced CO 2 emission by 14.6% and ash formation by 16% when compared with conventional oil shale CFB combustion. The SO 2 emissions remained in the limits of 20-30 mg/Nm 3. Total suspended particulates PM10 and PM2.5 did not change compared to conventional oil shale CFB firing. The CFB boiler efficiency even increased slightly, when it is known that in case of coal and biomass co-combustion it decreases. Therefore, oil shale and biomass cocombustion can be considered as a viable option and near-term solution for reducing the environmental impact of oil shale-based power production.