The S content in the ash and S retention holds a linear relationship for the three investigated cases, and the highest retention is obtained for the case OF27 combustion. If the amount of SO3, H2S and the SKLB610 of sulphur compounds in condensed water is negligible, the retention of sulphur in ash can be calculated by the conversion of oil shale-S to SO2: Retention = 100% – Conversion. By comparing the S retention in ashes and the values calculated from S conversion using Table 6 (without limestone addition), there is a difference of almost 20% for air-firing and 20–22% for oxyfuel cases; this suggests that there are significant portions of SO3, H2S and other sulphur compounds in the flue gases since higher SO2 concentrations support higher levels of other sulphur species. Kiga et al.  showed that O2/CO2 conditions with recycled flue gas enhanced sulphur deposition throughout the combustion process leading to an unaccounted 14–30% of the sulphur balance. Stanger and Wall  reported flora the oxyfuel combustion produced less sulphur in the flue gas (on a mass emitted basis) compared to the air-firing resulting in higher levels of sulphur in the ash and producing a higher amount of unaccounted sulphur (mass by difference), suggesting that a higher SO3 deposition rate occurs along the cooler transport lines. The conversion of SO2 to SO3 occurs in homogenous and heterogeneous phases. SO3 formation is affected by many factors including SO2 and O2 partial pressures, temperature, residence time, and the content of catalytically active compounds in the ash (e.g. iron and vanadium) ,  and . Further investigations are required to have a realistic information about SO3 concentration in flue gas during the combustion of such high S-oil shale.