Abstract: Recycle gas for the indirect retorting of oil shale, is heated after the oil has been recovered from the gas, utilizing the residual carbon in hot oil depleted shale. The hot oil depleted shale is charged into a vertical shaft furnace where it is combusted by the introduction of air into the upper portion of the furnace. The heat so generated is recovered from the lower part of the furnace for heating the recycle gas and cooling the spent shale. A portion of the coarse fraction of the cooled spent shale discharged from the bottom of the furnace is recycled to the top to regulate the temperature of combustion. The recycle gas may be heated directly by countercurrent contact with the hot, spent shale in the lower portion of the furnace or indirectly in a heat exchanger by an inert gas which is circulated through the lower portion of the furnace.