Page 207 - Synthetic Fuels Handbook

FUELS FROM OIL SHALE 193

lower temperature process for sequestering carbon dioxide. 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 char, representing more than half of the original car-
bon 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
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more than 100,000 t of 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. Part 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.
In situ processes such as Shell’s ICP avoid the spent shale disposal problems because
the spent shale remains where it is created (Fletcher, 2005). In addition, ICP avoids carbon
dioxide decomposition by operating at temperatures below about 350°C (662°F). On the
other hand, the spent shale will contain uncollected liquids that can leach into groundwater,
and vapors produced during retorting can potentially escape to the aquifer. Shell has gone
to great efforts to design barrier methods for isolating its retorts to avoid these problems
(Mut, 2005). Control of in situ operation is a challenge that Shell claims to have solved in
its work (Mut, 2005; Karanikas et al., 2005).
Shale (such as the Colorado shale) that contains a high proportion of dolomitic lime-
stone (a mixture of calcium and magnesium carbonates) thermally deposes under the condi-
tions of retorting and releases large volumes of carbon dioxide. This consumes energy and
leads to the additional problem of sequestering the carbon dioxide to meet global climate
change concerns.
In addition, there are also issues with the produced shale oil that also need resolution.
Shale oil is different to conventional crude oils, and several technologies have been
developed to deal with this. The primary problems identified were arsenic, nitrogen, and
the waxy nature of the crude. Nitrogen and wax problems were solved by Unocal and
other companies using hydroprocessing approaches, essentially classical hydrocracking.
Since that time, Chevron and ExxonMobil have developed technologies aimed at making
high-quality lube stocks, which require that waxy materials be removed or isomerized.
These technologies are well adapted for shale oils. However, the arsenic problem remains
(DOE, 2004b).
Unocal found that its shale oils contain several parts per million of arsenic. It developed
a specialty hydrotreating catalyst and process, called for Shale Oil Arsenic Removal (SOAR).
This process was demonstrated successfully in the 1980s and is now owned by UOP as part
of the hydroprocessing package purchased from Unocal in the early 1990s. Unocal also
patented other arsenic removal.
Arsenic removed from the oil by hydrotreating remains on the catalyst, generating a
material that is a carcinogen, an acute poison, and a chronic poison. The catalyst must be
removed and replaced when its capacity to hold arsenic is reached. Unocal found that its
disposal options were limited. Today, regulations require precautions to be taken when a
reactor is opened to remove a catalyst.
Thus several issues need to be resolved before an oil shale industry can be a viable
option. These issues are not insurmountable but require the search for viable alternatives.
For example, an alternative not much explored involves chemical treatment of shale to
avoid the high-temperature process. The analogy with coal liquefaction here is striking:
liquids can be generated from coal in two distinct ways: (a) by pyrolysis, creating a char
coproduct, or (b) by dissolving the coal in a solvent in the presence of hydrogen.