Results of mathematical modeling of an oil shale retort having a fluidized-bed pyrolyzer and a lift-pipe combustor

A commercial-size, hot-solids recycle retorting process, consisting of a staged fluidized-bed pyrolyzer and a lift-pipe combustor, was simulated using a previously developed mathematical model. The focus of this work at Lawrence Livermore National Laboratory (LLNL) is to show the effects of varying key process parameters. A design study shows the effects of certain design variables (such as pyrolyzer temperature, combustor pressure, etc.) on items that would affect the capital or operating costs of the plant (such as reactor sizes, utility requirements, yield, etc.). An operation study, in which the reactor dimensions were fixed, shows how the plant would respond to changes imposed on it. We evaluated the plant's sensitivity to some unavoidable changes and to the effectiveness of several strategies for controlling process conditions. Tabulated and graphical results from 22 individual parameter studies are presented. Conclusions drawn from these results include the following. Costs can be shifted from the combustor side of the process to the pyrolyzer side by decreasing pyrolysis temperature and/or increasing recycle ratio. Higher combustor pressure or oxygen/air enrichment would reduce the number and height of lift-pipes at the expense of large blower capacity or oxygen costs. Very large changes in shale grade or rate would not significantly upset plant operation. Plant operation would be extremely sensitive to the fraction of fines in the raw shale unless the fines are pyrolyzed completely before being fed to the combustor. Ranked from best to worst, the control options we considered are: manipulation of the solids recycle ratio, oxygen/air fraction, lift-pipe pressure, raw-shale preheat temperature, and extent of fines pyrolysis.