Oxyfuel combustion is a promising technology in terms of CO2 and NOX emissions control. Furthermore, CO2-rich atmospheric condition in a furnace leads to in-furnace desulphurisation. The high sulphur content in Jordanian oil shale is considered one of the biggest challenges for its utilisation. Direct sorbent injection studies for desulphurisation in O-2/CO2 pulverised coal combustion were very limited and there is none for oil shale combustion. In this study direct limestone injection has been investigated during Jordanian oil shale combustion under unstaged oxyfuel conditions as well as air-firing in a 20 kW vertical reactor. Different molar ratios of Ca/S were investigated in both firing modes. The oil shale-S conversion rates to SO2 are lower during unstaged oxyfuel combustion compared to air-firing; they were 61%, 49%, and 58% for air-firing, OF27, and OF35 combustion, respectively. Sulphur self-retention is more pronounced during oxyfuel combustion compared to air-firing due to the higher concentrations of SO2 and CO2. Significant reduction in SO2 emissions is obtained by limestone addition in both combustion modes. The desulphurisation efficiency increases with Ca/S molar ratio for both air-firing and oxyfuel combustion. At Ca/S molar ratio of 3, the desulphurisation efficiencies were 95%, 100% and 93% for air-firing, OF27, and OF35 combustion, respectively; there is a clear potential for zero SO2 emissions at high Ca/S molar ratios during unstaged combustion. (C) 2015 Elsevier Ltd. All rights reserved.