Experimental analysis of the combustion characteristics of Estonian oil shale in air and oxy-fuel atmospheres

A non-isothermal experimental study using thermogravimetric analysis and differential scanning calorimetry coupled with a quadrupole mass spectrometer was conducted to investigate Estonian oil shale combustion characteristics. The analyses were performed in air (N2/O2) and oxy-fuel (CO2/O2) atmospheres with various oxygen ratios (10, 20 and 30 vol.%). Our experimental results in TGA show that combustion in the CO2/O2 atmosphere is delayed compared to that in the N2/O2 atmosphere. Carbonate minerals in oil shale decompose in air in one step and in the oxy-fuel atmosphere in two separate steps: the decomposition of dolomite (CaMg(CO3)2) and the subsequent decomposition of calcite (CaCO3). An increased oxygen ratio in combustion in the oxy-fuel atmosphere increases the overall combustion rate, whereas the CO2 emission volumes decrease because of the lower decomposition extent of carbonates. The quadrupole mass spectrometer measurements indicate several combustion products. A higher CO reading is registered in the CO2/O2 atmosphere, but there is no other significant difference. Based on the measurement results, a combustion model for Estonian oil shale is proposed. Combustion in the oxy-fuel atmosphere is similar to combustion in air, which eases the design of oxy-fuel combustors.