"Two types of oil shale combustors, a cascading combustor and a fluidized-bed combustor, were previously modeled using the ASPEN process simulator (Poku, 1987). In those models, combustion was assumed to occur instantaneously. As an enhancement, the rate of char combustion was modified to account for resistances due to mass transfer, intraparticle oxygen diffusion, and kinetic rate of reaction. Whereas the previous combustor models were ""semipredictive,"" these enhancements were added to improve the models'predictive capabilities. In the cascading combustor, retorted shale and spent shale enter the top of the reactor and fall downward by gravity, reacting with air that is driven horizontally across the reactor. In the fluidized-bed combustor, the air enters the bottom of the reactor and flows upward, fluidizing a bed of solids while reacting with combustible residual kerogen and char in the shale. The cascading combustor model was validated using information from the Lawrence Livermore National Laboratory (LLNL) RETORT model (Braun, 1981). Next, a series of parametric studies was conducted to demonstrate the differences between the previous models (Model I) and the enhanced models (Model II). For example, the effect of stoichiometric air ratio on the extent of char combustion was more pronounced in the case of the cascading combustor than in the fluidized bed ."