Ignition of in situ oil-shale retorts with hot inert gas

For controlled-combustion retorting of oil-shale rubble in situ, preheating with hot, inert gas appears to be a promising method to prepare the rubble for ignition. By thus setting up an initial separation between the combustion and retorting fronts, oil burning should be reduced and temperatures should remain below those at which spent shale fuses. Inert-gas preheating has been investigated by simulation, using the LLL computer model of in situ retorting. The modeling explored the effects of: temperature and flow rate of preheating gas, depth that ignition-range temperatures penetrated before changeover to retorting, temperature at the given depth of changeover, and staged changeover. Several sets of conditions gave retort temperatures below 1000/sup 0/C with low fuel usage. The choices of preheating factors strongly affect the maximum temperatures reached shortly after changeover. The cost of inert-gas startup appears to be small.