"Five general classifications of oil-shale retorting processes can be made according to the method used to heat the shale. These classifications arbitrarily have been designated as follows: Class I. Heat transfer through a wall. Class II. Gas-to-solid heat transfer where hot gas is produced from combustion of shale within the retorting vessel. Class III. Fluid-to-solid heat transfer where the recycled fluid (gas or liquid) is externally heated. Class IV. Solid-to-solid heat transfer. Class V. Heating by electrical methods. A process engineering study has been made of retorting processes utilizing the Class II or internal-combustion principle of heating the shale. For Simplification, this study is divided into two parts - one in which air alone is used for retorting; the other utilizing a combination of air and recycle gases. This report is concerned with internal combustion retorting using air alone. Process engineering studies of the various basic retorting methods are being made, preliminary to engineering cost evaluation, to determine which retorting systems are impractical cal from process considerations alone ; and to allow selection of the "" most feasible processes for"" cost evaluation. It is believed that the results of these studies will be of interest to others concerned with oil-shale retorting process development."