Page 200 - Synthetic Fuels Handbook

186 CHAPTER SIX

The process involves use of ground-freezing technology to establish an underground bar-
rier called “freeze wall” around the perimeter of the extraction zone. The freeze wall is created
by pumping refrigerated fluid through a series of wells drilled around the extraction zone. The
freeze wall prevents groundwater from entering the extraction zone, and keeps hydrocarbons
and other products generated by the in situ retorting from leaving the project perimeter.
High yields of liquid products, with minimal secondary reactions, are anticipated (Mut,
2005; Karanikas et al., 2005).
In situ processes avoid the spent shale disposal problems because the spent shale remains
where it is created but, on the other hand, the spent shale will contain uncollected liquids
that can leach into ground water, and vapors produced during retorting can potentially
escape to the aquifer (Karanikas et al., 2005).
Modified in situ processes attempt to improve performance by exposing more of the tar-
get deposit to the heat source and by improving the flow of gases and liquid fluids through
the rock formation, and increasing the volumes and quality of the oil produced. Modified
in situ involves mining beneath the target oil shale deposit prior to heating. It also requires
drilling and fracturing the target deposit above the mined area to create void space of 20
to 25 percent. This void space is needed to allow heated air, produced gases, and pyrolized
shale oil to flow toward production wells. The shale is heated by igniting the top of the
target deposit. Condensed shale oil that is pyrolized ahead of the flame is recovered from
beneath the heated zone and pumped to the surface.
The Occidental vertical modified in situ process was developed specifically for the
deep, thick shale beds of the Green River Formation. About 20 percent of the shale in
the retort area is mined; the balance is then carefully blasted using the mined out vol-
ume to permit expansion and uniform distribution of void space throughout the retort
(Petzrick, 1995).
In this process, some of the shale was removed from the ground and explosively shattered
the remainder to form a packed bed reactor within the mountain. Drifts (horizontal tunnels
into the mountain) provided access to the top and bottom of the retort. The top of the bed was
heated with burners to initiate combustion and a slight vacuum pulled on from the bottom
of the bed to draw air into the burning zone and withdraw gaseous products. Heat from the
combustion retorted the shale below, and the fire spread to the char left behind. Key to success
was formation of shattered shale of relatively uniform particle size in the retort, at reasonable
cost for explosives.
If the oils shale contains a high proportion of dolomite (a mixture of calcium carbonate
and magnesium carbonate; for example, Colorado oil shale) the limestone decomposes
at the customary retorting temperatures to release 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.

6.5 REFINING SHALE OIL

Shale-retorting processes produce oil with almost no heavy residual fraction. With upgrad-
ing, shale oil is a light boiling premium product more valuable than most crude oils.
However, the properties of shale oil vary as a function of the production (retorting) pro-
cess. Fine mineral matter carried over from the retorting process and the high viscosity and
instability of shale oil produced by present retorting processes have necessitated upgrading
of the shale oil before transport to a refinery.
After fines removal the shale oil is hydrotreated to reduce nitrogen, sulfur, and arsenic con-
tent and improve stability; the cetane index of the diesel and heater oil portion is also improved.
The hydrotreating step is generally accomplished in fixed catalyst bed processes under high