Page 24 - Synthetic Fuels Handbook

12 CHAPTER ONE

or thermal decomposition. The pyrolysis reaction is endothermic in nature, requiring heat,
and produces lighter molecules thereby increasing the pressure.
In addition to the kerogen pyrolysis reaction, carbonate decomposition reactions are
also included here as principal chemical reactions, due to their abundant existence and also
to their reaction temperature ranges that overlap the kerogen pyrolysis temperature range.
Other mineral matters of oil shale that are worthy of note are alumina, nahcolite, and daw-
sonite. Some of the processes are designed to recover these mineral matters for economic
benefit to the overall process. The pyrolysis reaction is quite active at a temperature above
400°C, where most of the commercial retorting processes are operated. Most of the ex situ
processes utilize the spent (processed) shale as a char source to supply the process heat, thus
accomplishing higher energy efficiency for the process. The typical temperature required
to carry out such pyrolysis reaction is in the range of 450 to 520°C. In order to make the
efficiency of oil extraction higher, oil shale rocks need to be ground to finer particle sizes,
thus alleviating mass transfer resistance and at the same time facilitating smoother flow for
cracked hydrocarbons to escape out of the rock matrix. Due to the poor porosity or perme-
ability of oil shale rock, the rock matrix often goes through stress fracture during pyrolysis
operation, typically noticed as crackling.
Major drawbacks of this type of process involve (a) “mining first” operation, which
is costly, (b) transportation or conveying the mined shale to retorting facilities, (c) size
reduction such as rubblizing, grinding, or milling, and (d) returning the spent shale back
to the environment. In the current energy market of the globe, any major transportation of
unprocessed (raw) shale would be economically unfavorable, unless the raw shale contain
very high levels of oil contents or liquid fuel price from conventional petroleum source
is substantially higher. In ex situ operations, heavy reliance on earth-moving equipment
(EME) and rock-handling equipment, such as rock pump and heavy-duty hammer mill, is
noticeable. Furthermore, it is likely to be constrained by the underlying market principle
that the recoverable value including any tax credits or incentives from oil shale has to be
favorable after considering all the cost factors including mining, transportation, and pro-
cessing. Often, the mass percentage of oil content of oil shale or the volume of recoverable
oil from unit mass of oil shale is used as a measuring parameter. The latter is called Fischer
assay, which is based on the ASTM standard under a prescribed condition of retorting.
However, this value should not be considered as the maximum recoverable oil content for
the shale or the oil content itself in the shale.
Several of the ex situ retorting processes have been commercially tested on large
scales and also proven effective for designed objectives. Some of the successfully demon-
strated processes include: (a) Gas Combustion retort process, (b) TOSCO (The Oil Shale
Corporation) process, (c) Union Oil retorting process, (d) Lurgi-Ruhrgas (LR) process,
(e) Superior Oil’s multi-mineral process, and (f) Petróleo Brasileiro (Petrobrás) process.
Partial combustion of residual char provides the thermal energy for heating the shale via
direct contact, thus achieving energy efficiency. TOSCO process uses heated ceramic balls
to provide the thermal energy for heating the shale by direct contact, and also successfully
implements multi-levels of heat recovery and energy integration strategy. The Union Oil
retorting process is unique and innovative with utilizing well-designed rock pumps and
adopting a number of designs for heating shale in the retort. The LR process produces
hydrocarbons from oil shale by bringing raw shale in contact with hot fine-grained solid
heat carrier, which can be just spent shale. The Petrobrás process was operated for about
10 years in southern Brazil, treating over 3,500,000 tons of Irati (Permian age) oil shale to
produce more than 1,500,000 bbl of shale oil and 20,000 tons of sulfur.
In situ retorting of oil shale does not involve any mining operation, except starter holes
and implementation digging. Therefore, in situ retorting does not require any transportation
of shale out of the oil shale field. In situ retorting is often called subsurface retorting. The
advantages of in situ retorting processes include: (a) no need for mining, (b) no need for