To solve the problem of the very high SO2 emissions during oil shale combustion, direct sorbent injection was investigated in this work, limestone was used as sorbent. In this section, the efficiency of desulphurisation during unstaged El-Lajjun oil shale air-firing and oxyfuel (OF27 and OF35) combustion is studied. The desulphurisation efficiency of O2/CO2 combustion and that of conventional combustion is compared based on the same Ca/S ratio. Three molar ratios of Ca/S were used, namely 1, 2 and 3. The SO2 emissions over the three ratios used for the three investigated conditions (air-firing, OF27 and OF35) are shown in Fig. 3. It arises from the presented results that as the Ca/S molar ratio increases the SO2 emission decreases significantly (Ca/S = 0 means no limestone addition with oil shale). During OF27 combustion, SO2 emission reduced from 5136 mg/MJ to 2645 mg/MJ when limestone is added with molar ratio of 1. As the Ca/S molar ratio is increased to 3, SO2 emission is reduced to 16 mg/MJ. Similarly for air-firing, when the Ca/S molar ratio is increased to 3, the SO2 emission is reduced to 297 mg/MJ. For OF35 combustion, at Ca/S molar ratio of 3, the SO2 emission reduced to 419 mg/MJ. The significant CX-5461 in SO2 emissions by limestone addition in both combustion modes (air-firing and OF combustion) can be explained by the high SO2 concentration for this high-S oil shale. CaSO4 decomposition is inhibited owing to the high SO2 concentration inside the furnace . The particle size of the limestone has an important role as well for the significant sulphation found, since smaller particle size means higher surface area, better mixing with the fuel and then with the produced SO2 gas. Under conditions of fine sorbent particles and short residence times, such as in furnace-sorbent injection, the pore size distribution model can explain most of the factors affecting the reaction. The model indicates the dominant factors to be pore size distribution, particle size and the partial pressure of SO2. Therefore, efficient in-furnace desulphurisation for oil shale air-firing and under oxyfuel conditions is obtained. Chen et al.  reported that direct sulphation of limestone was only pronounced when an extra SO2 was introduced into flue gas and excess Ca was present in flue gas; the doping of extra SO2 through the recirculation of flue gas mainly increased the sulphation of limestone and concurrently reduced the unreacted limestone fraction through direct sulphation.