TG-MS analysis and kinetic study of co-combustion of ca-rich oil shale with biomass in air and oxy-like conditions

In this study, the combustion behavior of ca-rich oil shale, spruce biomass, and their blends under air and oxyfuel environments of 21/79 % and 30/70 % O2/CO2 like conditions were investigated. Non-isothermal thermogravimetric (TG) experiments coupled with a quadrupole mass spectrometer (MS) were conducted to study individual fuels and their blends at 1:0, 4:1, 3:2, 2:3, 1:4, and 0:1 (0, 20, 40, 60, 80 and 100 wt.% Biomass) under three different heating rates 10, 30 and 50°C/min. Co-combustion synergistic along with the kinetic analysis by the isoconversional Friedman method has been carried out to evaluate the combustion process.The results show that the addition of biomass enhanced combustion performance and reduced burnout temperatures. Under oxy conditions, the ignition temperatures stabilized with a biomass ratio > 40 %. Ash content was reduced with biomass addition and when switching from air to oxyfuel combustion, the temperature of blends’ carbonate decomposition was stable at ∼ 720 °C. A positive synergistic effect in the devolatilization and the combustion of light hydrocarbons occurred at higher biomass ratios. Yet, the char oxidation peaks were below zero indicating a negative effect under 21 % of inlet O2. With increasing the heating rate, the negative synergistic effect was weakened for the three combustion atmospheres. SO2 emissions were reduced with increasing biomass ratio and increased under oxy mode along with H2O release. The activation energy (E) was lower in oxy conditions than in air mode for individual fuel and their blends, compared to the OS sample, the reduction was higher with increasing BM ratios and for the BM fuel. Overall, this study determines the appropriate conditions for the co-combustion of oil shale and spruce biomass under air and oxy modes for future carbon-negative capture applications in industrial oil shale combustion boilers

Comparison of the most likely low-emission electricity production systems in Estonia

To meet targets for reducing greenhouse gas emissions, many countries, including Estonia, must transition to low-emission electricity sources. Based on current circumstances, the most likely options in Estonia are renewables with energy storage, oil shale power plants with carbon capture and storage (CCS), or the combination of renewables and either oil shale or nuclear power plants. Here we compare these different scenarios to help determine which would be the most promising based on current information. For the comparison we performed simulations to assess how various systems meet the electricity demand in Estonia and at what cost. Based on our simulation results and literature data, combining wind turbines with thermal power plants would provide grid stability at a more affordable cost. Using nuclear power to compliment wind turbines would lead to an overall levelized cost of electricity (LCOE) in the range of 68 to 150 EUR/MWh (median of 103 EUR/MWh). Using oil shale power plants with CCS would give a cost between 91 and 163 EUR/MWh (median of 118 EUR/MWh). By comparison, using only renewables and energy storage would have an LCOE of 106 to 241 EUR/MWh (median of 153 EUR/MWh)

Ash and Flue Gas from Oil Shale Oxy-Fuel Circulating Fluidized Bed Combustion

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 kWth 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 NOx 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 NOx 130 mg/Nm3 @ 6% O2 and for OXY21 SO2 23 and NOx 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