The in-line burner process is a modification of the Gas Combustion retort in which process heat is supplied b hot flue gases generated by burning retort gas in a furnace external to the retort vessel. The process is illustrated in figure 1. In this sketch, which illustrates the Rifle pilot-plant equipment, cold recycle gas is used as burner fuel. In a commercial installation, to preheat the gas before it is burned, fuel gas for the burner would probably be withdrawn from the retort above the gas-preheating-retorted-shale-cooling zone (bottom zone of the retort).