Abstract: As hot oil depleted shale from an indirect retorting process passes downward through a vertical shaft furnace, the residual carbon in the shale is burned out in the upper portion by air fed into the top of the furnace. The temperature of combustion is moderated by an inert gas also fed into the top of the furnace. Additional inert gas fed into the bottom of the furnace flows upward in countercurrent contact with the descending hot shale to cool the shale and heat the inert gas. The downwardly flowing gases from the upper, combustion portion of the furnace and the upwardly flowing gas from the lower, cooling portion are withdrawn from the furnace intermediate the upper and lower portions. The inert gas can be recycle gas which is heated by direct contact with the hot shale, a separate inert gas which is circulated through the heat exchanger to heat the recycle gas or a combination of the two.

Abstract: Hot oil depleted shale from an indirect oil shale retorting process is charged into the top of a vertical shaft furnace together with sufficient oxygen to combust the residual carbon in the shale. Recycle process gas is fed into the bottom of the furnace for direct heating by the spent shale. The heated recycle process gas and combustion gases are withdrawn from the furnace separately to minimize dilution of the high BTU process gas. A selected quantity of coarse, cooled spent shale discharged from the bottom of the furnace is recycled to the top to moderate the temperature of combustion.