An investigation of the pyrolysis kinetics of American, Chinese and Estonian oil shales was conducted applying a non-isothermal thermo­gravi­metric analysis (TGA). TGA weight loss curves clearly indicate that the pyrolysis of all oil shales tested is independent of their geographic origin, and is mainly taking place in the temperature range of 300 to 500 ºC. As expected, at temperatures above 700 ºC mass loss due to the decomposition of oil shale carbonates was detected, except for the Kentucky and Chinese 2 oil shale samples. The kinetic decomposition rate parameters such as activa­tion energy and pre-exponential factor were calculated applying the Coats-Redfern integral and direct Arrhenius methods. Independent of the oil shale kerogen origin, pyrolysis occurs in two consecutive temperature zones with slightly dissimilar kinetic parameter values. The activation energy values obtained were in the range of 14 to 31 kJ/mol for the low and 70 to 149 kJ/mol for the high temperature zone.

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 CO₂ emission by 14.6% and ash formation by 16% when compared with con­ventional oil shale CFB combustion. The SO₂ emissions remained in the limits of 20–30 mg/Nm³. 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 co-combustion can be considered as a viable option and near-term solution for reducing the environmental impact of oil shale-based power production.

Carbon dioxide emissions are considered a major environmental threat. To enable power production from carbon-containing fuels, carbon capture is required. Oxy-fuel combustion technology facilitates carbon capture by increasing the carbon dioxide concentration in flue gas. This study reports the results of calcium rich oil shale combustion in a 60 kW_th circulating fluidized bed (CFB) combustor. The focus was on the composition of the formed flue gas and ash during air and oxy-fuel combustion. The fuel was typical Estonian oil shale characterized by high volatile and ash contents. No additional bed material was used in the CFB; the formed ash was enough for the purpose. Two modes of oxy-fuel combustion were investigated and compared with combustion in air. When N₂ in the oxidizer was replaced with CO₂, the CFB temperatures decreased by up to 100 °C. When oil shale was fired in the CFB with increased O₂ content in CO₂, the temperatures in the furnace were similar to combustion in air. In air mode, the emissions of SO₂ and NO_x were low (<14 and 141 mg/Nm³ @ 6% O₂, respectively). Pollutant concentrations in the flue gas during oxy-fuel operations remained low (for OXY30 SO₂ < 14 and NO_x 130 mg/Nm³ @ 6% O₂ and for OXY21 SO₂ 23 and NO_x 156 mg/Nm³ @ 6% O₂). Analyses of the collected ash samples showed a decreased extent of carbonate minerals decomposition during both oxy-fuel experiments. This results in decreased carbon dioxide emissions. The outcomes show that oxy-fuel CFB combustion of the oil shale ensures sulfur binding and decreases CO₂ production.