Retorting and combustion processes in surface oil-shale retorts

Lawrence Livermore National Laboratory's mathematical model for a batch oil-shale retorting process has proved useful in understanding in situ retorting in a bed of oil-shale rubble. This model is based on an improved understanding of the physical and chemical processes involved in retorting. It has since been modified to calculate the retorting of shale moving through a retort, as required for a continuous process in surface equipment. Calculations were made in which the retorting process that releases oil from raw shale is physically separated from the combustion process that recovers energy from spent shale by burning the residual carbon. Calculations were also made with the retorting and combustion processes combined in one unit. The results of the calculations illustrate the advantages and disadvantages of inert-gas and combustion retorts. Temperature control and the efficient use of energy from the nonliquid products of pyrolysis are key problems in surface retorting processes. The amount of gas produced by the combustion of char with air is insufficient to retort shale without steep temperature gradients which, in turn, may make the temperature difficult to control and result in the loss of oil yield. We believe that application of this mathematical model can contribute to improved surface processing.