Oil shale, a potential source of liquid fuel, is a fine-grained, sedimentary rock composed of both organic and inorganic solid compounds. The production of liquid fuel from oil shale requires the retorting of the shale, in which oil and gas are destructively distilled from the shale rock, leaving behind a highly carbonaceous char within the spent shale matrix. The amount of char left depends on the richness of the raw shale. Typically, 23% by weight of the organic material originally present in the shale is left as char. Combustion of this char could potentially provide all or part of the heat requirement for retorting of the shale. There have recently been several studies dealing with the combustion behavior of spent shale. Although char in shale is not pure carbon, most studies have treated it as carbon with reasonable success. In the combustion of spent oil shale in an environment of air with other inert gases, there are three active components to be considered, O/sub 2/, CO/sub 2/ and CO and four major chemical reactions, namely: Direct oxidation of the residual char, a process which is limited by the diffusion of oxygen and which creates an ''unburned core''.