Page 198 - Synthetic Fuels Handbook

184 CHAPTER SIX

retorting temperature. Once the reaction is complete, recovering sensible heat from the hot
rock is very desirable for optimum process economics.
This leads to three areas where new technology could improve the economics of oil
recovery.
1. Recovering heat from the spent shale.
2. Disposal of spent shale, especially if the shale is discharged at temperatures where the
char can catch fire in the air.
3. Concurrent generation of large volumes of carbon dioxide when the minerals contain
limestone, as they do in Colorado and Utah.
Heat recovery from hot solids is generally not very efficient. The major exception to this
generalization is in the field of fluidized bed technologies, where many of the lessons of fluids
behavior can be applied. To apply fluidized bed technologies to oil shale would require grinding
the shale to sizes less than about 1 mm, an energy intensive task that would result in an expensive
disposal problem. However, such fine particles might be used in a lower temperature process for
sequestering CO , with the costs of grinding now spread over to the solution of this problem.
2
Disposal of spent shale is also a problem that must be solved in economic fashion for the
large-scale development of oil shale to proceed. Retorted shale contains carbon as a kind
of char, representing more than half of the original carbon values in the shale. The char
is potentially pyrophoric and can burn if dumped into the open air while hot. The heating
process results in a solid that occupies more volume than the fresh shale because of the
problems of packing random particles. A shale oil industry producing 100,000 bbl/day,
about the minimum for a world-scale operation, would process more than 100,000 tons of
3
shale (density about 3 g/cc) and result in more than 35 m of spent shale; this is equivalent
to a block more than 100 ft on a side (assuming some effort at packing to conserve volume).
Unocal’s 25,000 bbl/day project of the 1980s filled an entire canyon with spent shale over
several years of operation. Some fraction of the spent shale could be returned to the mined-
out areas for remediation, and some can potentially be used as feed for cement kilns.
Unocal’s process relied on direct contact between hot gases passing downward through
a rising bed of crushed shale. This required that the retorting shale be pumped upward
against gravity. Retorted shale reaching the top of the retort spilled over the sides and was
cooled as it left the vessel. Oil formed in the process trickled down through the bed of shale,
exchanged its heat with fresh shale rising in the roughly conical retort, and was drawn from
the bottom. Unocal produced 4.5 million barrels from 1980 until 1991 (AAPG, 2005) from
oil shale averaging 34 gal/ton. The major problem that had to be overcome was formation
of fine solids by decrepitation of the shale during retorting; the fines created problems in
controlling solids flow in the retort and cooling shafts.
The TOSCO process used a rotating kiln that was reminiscent of a cement kiln in which
heat was transferred to the shale by ceramic balls heated in an exterior burner. Retorted shale
was separated from the balls using a coarse screen and the balls were recovered for recycle.
Emerging vapors were cooled to condense product oil. The system was tested at the large pilot
scale, but construction of a commercial retort was halted in 1982. One problem with the sys-
tem was slow destruction of the ceramic balls by contact with the abrasive shale particles.

6.4.3 In Situ Technologies
In situ processes introduce heat to the kerogen while it is still embedded in its natural
geologic formation. There are two general in situ approaches; true in situ, in which there is
minimal or no disturbance of the ore bed, and modified in situ, in which the bed is given a
rubblelike texture, either through direct blasting with surface up-lift or after partial mining