Effect of air and oxyfuel staged combustion on oil shale fly ash formation with direct in-furnace limestone addition for sulphur retention

•Staged oil shale combustion was performed in air and OF27 environments.•The combustion environment had a slight effect on the collected ash composition.•Desulphurisation was proved from S and SO3 contents in ash, and SEM and XRD analyses.•Sulphur retention in the convective section increased by 13% compared to the radiative section.•The main crystalline phases were the same regardless the combustion mode and environment. El-Lajjun oil shale fly ash formation and S retention during in-furnace limestone injection were investigated in this work. Staged air-firing and 27% O2/CO2 combustion was performed at 1200 °C in a vertical once through reactor. Ash samples were collected at 2.5 m far from the burner and from the conductive section of the reactor. Limestone was added with oil shale before burning in both combustion modes. Ultimate, proximate, and XRF analyses were done for all collected samples, while XRD and SEM analyses were performed for some of them. Sulphur retention in the convective section increased by 13% compared to the radiative section of the reactor. Sulphur retention in the collected ashes ranged from 22 to 32% and 35 to 45% at 2.5 m from the burner and at the candle filter, respectively. The combustion mode had a slight effect on the collected ashes chemical composition. In both combustion modes, the ash samples were enriched with lime and depleted in sulphur, which proved sulphur capture. Fly ash particles were spherical in shape with some aggregates. XRD analysis showed that the main crystalline phases formed in the produced fly ashes were the same regardless the combustion mode and environment, some differences in the relative intensities of anhydrite and lime peaks were present.