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.

This is a study of the combustion of Baltic oil (bituminous) shales with different heats of combustion in a boiler with fluidized bed combustion (CFB). The minimum heat of combustion was varied over 8.5 -11.5 MJ/kg in the experiments. The behavior of the mineral part of the shales during combustion was taken into account in determining the boiler efficiency, CO 2 emissions, and amount of ash production.The heat released during combustion of 1 kg of oil (bituminous) shale depends strongly on the endothermic and exothermic processes taking place in the mineral part of the fuel [1]. These processes include the following: decomposition of calcite and dolomite CaCO 3 ® CaO + CaO 2 (-179 kJ/mole); CaMg(CO 3 ) 2 ® CaO + MgO + 2CO 2 (-315 kJ/mole);(1) oxidation of marcasite FeS 2 4FeS 2 + 11O 2 ® 2Fe 2 O 3 + 8SO 2 (+871 kJ/mole); (2) sulfating of CaOand, the formation of new minerals 2CaO + SiO 2 ® 2CaO · SiO 2 (+139 kJ/mole);(The thermal effects of the reactions are given in parentheses.)The extent to which these reactions proceed to completion and produce changes in the amount of heat released during combustion are mainly determined by the fuel combustion technology.In the case where oil shale dust is burnt, essentially complete decomposition of the carbonates is observed (degree of dissociation k CO 2 » 0.97 [1]), mainly because of the relatively high temperature in the furnace chamber (1400 -1500°C), a low partial pressure of CO 2 lying below the equilibrium line for reaction (1) at that temperature, and the small size of the fuel particles. On the other hand, for combustion in circulating fluid bed conditions, incomplete decomposition of the carbonates (k CO 2 » 0.60 -0.85) is observed because of the low temperature in the furnace and the large size of the fuel particles. Despite a high molar ratio of calcium and sulfur in the Baltic oil shales (Ca/S = 8 -10; at least a factor of 2 -3 higher than necessary for rapid mass exchange with complete bonding of SO 2 ), when the dust is burnt the bonding efficiency for sulfur is only 0.70 -0.85. This is because at relatively high temperatures 60 -70% of the CaO is involved in the formation of new minerals and combines with the sand-clay part of the fuel where the CaO is less active with respect to SO 2 than free CaO; at the same time, in fluidized bed combustion the formation of new minerals containing CaO proceeds more slowly owing to the low combustion temperature, so that a large part of the CaO formed as a result of the decomposition of carbonates remains free, which leads to essentially complete bonding of the SO 2 .Thus, depending on the shale combustion technology, the degree of completion of reactions (1) -(4) will be different, so that there will be a different amount of heat release per 1 kg of fuel. In addition, the extent of chemical transforma-

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