The emissions of NOx, SO2, CO and decomposition of carbonates during oxyfuel combustion of low heating value semicoke in CFB pilot facility

•The Experiments of oil shale semi-coke oxy-fuel combustion at CFB pilot scale.•Low calorific preheated semi-coke oxy-fuel combustion.•NOx, SO2, CO emissions during combustion of semi-coke under O2/CO2 atmosphere.•The NOx emissions (per MJ) reduces during oil-shale semi-coke oxy-fuel combustion.•Carbonates decomposition during oxyfuel combustion of oil shale semicoke in CFB. Semicoke oxyfuel combustion is a technology that has the potential to play a role in transitioning to cleaner and more sustainable energy generation in shale oil industry. The current experimental study investigates semicoke combustion behavior in air and O2/CO2 environments. Semicoke burning experiments both in air and in oxygen-rich environments 21 % and 30 % of O2/CO2 were carried out in a 60 kWth circulating fluidized bed (CFB) test facility in the Department of Energy Technology of Tallinn University of Technology. The results show that the temperature distributions are similar when burning in air and O2/CO2 (21/79 %) environments. Under these conditions, the temperature distribution is characterized by a temperature increase in the upper part of the riser. When increasing the oxygen content in the supplied O2/CO2 mixture to 30 %, the heat release is redistributed and the temperature in the lower part of the riser increases. Under air combustion modes, at temperatures below 800 °C, the thermal decomposition of carbonates does not occur and the content of free lime (CaOfree) in the external heat exchanger (EHE) is 3 %. At higher temperatures > 800 °C, most of the carbonates decompose thermally and the content of CaOfree increased to 18 %. When switching from air to oxyfuel combustion, the temperature of the thermal decomposition of carbonates shifts to higher values lying in the range of 870 – 900 °C depending on the partial pressure of CO2. Oxyfuel combustion did not influence the release of SO2, the concentration remained within a few ppm since the high content of carbonates in the semi-coke still ensures almost complete binding of sulfur formed during combustion. NOx emissions are reduced by up to 40 % with the same oxygen excess ratio in dense bed under oxygen enriched environment compared to air combustion. The actual CO emissions (mg/MJ) are decreasing, and a greater reduction is observed at higher O2 content of the supplied O2/CO2 mixture. These results are compatible with other studies using oil shale fuel.