Reducing CO2 emissions from oil shale semicoke smoldering combustion by varying the carbonate and fixed carbon contents

One technique used to recover oil from ground oil shale, or to burn oil shale semicoke, consists of propagating a smoldering front through a packed bed. One drawback of this technique is that the mineral structure of the shale is decarbonated due to the high temperature of the front. This phenomenon causes 70% of the CO2 emissions released during such processes. The remaining 30% result from the fixed carbon oxidation. With the aim of decreasing the front temperature and thus avoiding decarbonation at the front passage, the impact of two parameters was experimentally tested in this work: first, increasing the amount of carbonates, as they may play the role of a heat sink, and second, decreasing the amount of fixed carbon in the medium. It is shown that increasing the amount of carbonates can only decrease the front temperature to 800 °C but not lower, which is still too high to avoid decarbonation. On the other hand, the front temperature can be decreased enough for decarbonation to be almost completely avoided by reducing the amount of fixed carbon. At the low temperatures reached, almost all the fixed carbon is oxidized, but not all the oxygen transported in the air is consumed by the chemical front. The velocity of the front is consequently decreased.