Ash samples were collected at the end of the reactor (2.5 m) via an oil-cooled sampling probe. The ash was separated from the flue gas at the end of the oil-cooled probe by means of a filter and then immediately collected in glass container for lab analysis. The proximate analysis and S content of some collected ash samples are reported in Table 8. It was found that Z-Lys-OH S content increases in the ashes when limestone addition was performed. Generally, the S content in the collected ashes during combustion with limestone addition increases as the molar Ca/S ratio increases in both combustion modes proving the sulphation results from the flue gas concentration analysis. Higher S content in the ash is sinoatrial (SA) node found after oxyfuel combustion compared to air-firing. The sulphur retention during combustion of oil shale under air-firing and oxyfuel conditions is calculated and shown in Fig. 6. The sulphur retention is the fraction of oil shale-S retained in the ashes and is calculated as:equation(9)Sretention=(γS,ash·γash,oilshale)γs,oilshalewhere γS, ash is the mass fraction of sulphur in the ash, γash, oilshale is the mass fraction of ash related to the oil shale and γS, oilshale is the mass fraction of sulphur in the oil shale.