THE INFLUENCE OF PARTLY COMBUSTED CHAR IN RECYCLED SOLIDS ON THE LIFT-PIPE COMBUSTION OF SPENT OIL-SHALE

A model is presented which allows for a distribution of flow rates and compositions over a range of particle sizes in the recycled solids to a lift-pipe combustor for spent oil shale. It takes into account the contribution of partly combusted char in solids recycled between the retort and the combustor. A modified version of Dung's model to include the decomposition of clay and carbonate minerals in the spent shale and the effect of temperature on the system component properties was used to describe the lift-pipe combustion. Simulations with conditions prevailing in a hypothetical plant producing 50,000 barrels of shale oil per day from Australian Stuart oil shale were undertaken. We found that: (1) the combustion of partly combusted char in the recycled solids contributed more than 50% of the combustor heat load, and the combustor sizes were therefore less than half those predicted when the recycled solids were assumed to be "inert"; (2) the total heat load contributed by the combustion of char became less than 1% only after the shale particles had been recycled more than nine times; (3) the air flow rate and recycle ratio had very small effects on the combustor performance; (4) a single-size particle approximation of the multi-size particle system using the weight mean particle diameter provided about a five-fold reduction in computer time while accuracy was maintained to within 7%. © 1990.