A set of conditions was simulated with ASPEN PLUS software, including air and oxyfuel (OF) combustion of carbonate-rich unconventional fuels: Estonian oil shale (EOS) and Utah White River oil shale (UOS). For OF combustion cases, two different flue gas recycle (FGR) strategies were applied by controlling the O-2 percentage in the flue gas on the basis of maintaining similar temperatures at the outlet of combustion reactor with those of air combustion. Differences between the fuels and simulated cases have been identified, focusing on the flue gas volumetric flow rates, heat capacities, flue gas compositions, and boiler efficiencies. The results show that it is possible to reach similar temperature levels in OF cases, as it was in air combustion by increasing the O-2 concentration in the recycled flue gas (RFG) mixture for both fuels. When the O-2 concentration is increased to almost 26% for wet recycle and 28% for dry recycle in the case of EOS combustion, the calculated temperatures are similar to the temperature in air combustion. The heat capacity of flue gases and heat duties in combustion reactors are higher for OF combustion cases, compared to the air combustion case, showing that the amount of heat output and boiler efficiency can be increased if conventional combustion of EOS is changed to an OF combustion process.