| 摘要 |
A substantially flat combustion zone is established in a fragmented mass of particles containing oil shale in an in situ oil shale retort. By igniting a portion of the mass of particles, a heated zone including a combustion zone is established in the retort. For a first period of time, an oxidizing gas is introduced into the retort and heated zone at a rate sufficient to advance the heated zone through the fragmented mass. The locus of the combustion zone is monitored to determine if the combustion zone is substantially flat. If the combustion zone is not substantially flat, introduction of oxidizing gas into the retort is reduced temporarily for a second period of time to a rate such that the flow of heated gas through the retort for retorting oil shale in a retorting zone on the advancing side of the combustion zone is substantially reduced for a sufficient time to appreciably flatten the heated zone. Thereafter, introduction of gas comprising an oxidizing gas to the retort is resumed at a sufficient rate to advance the heated zone through the fragmented mass. Off gas withdrawn from the retort during the second period of time can be enriched having a heating value of at least about 75 BTU/SCF, and often in excess of about 150 BTU/SCF. To produce such enriched off gas, introduction of gas into the retort can be temporarily reduced even when it is not necessary to establish a substantially flat combustion zone in the retort. This enriched off gas can be withdrawn from the top of the retort and can be used for igniting another retort or for sustaining a secondary combustion zone in another retort.
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