POWER July 2011 - 37

CLEAN COAL TECHNOLOGY
methods employed on modern UCG projects
around the world.
However, Courtney pointed out that
" on balance, it's fair to say that the LVW
method is currently used more. " It is used
in projects in Uzbekistan, South Africa,
China, and New Zealand. On the other
hand, the company Carbon Energy uses
the parallel CRIP method in Australia;
Swan Hills in Canada is using the linear
CRIP method; and Linc Energy has
experimented with most of the available
techniques.
" As long as the techniques are deployed
in properly selected sites (site selection is
fundamentally important for UCG) and operated
correctly, there are no major technical
challenges to these techniques used for
UCG, " he said. " Much of the work nowadays
is in refining the techniques to increase
efficiency and reduce costs, rather
than in overcoming major technical challenges. "
UCG
plants can produce syngas by exploiting
coal resources located both onshore
and offshore (Figures 3 and 4).
The Linked Vertical Well Method
The LVW method uses reverse combustion
(RC): The coal is ignited from one of the
vertical wells and air/oxygen is introduced
into the coal from the other well. The combustion
front then moves toward the air/
oxygen, forming linkages between the wells
by progressively consuming small amounts
of coal and forming tube-like channels as it
goes. Once the linkage is established, forward
combustion (where the combustion
front moves in the same direction as the
injected air/oxygen toward the production
well) is used to gasify the coal.
" An advantage of the RC-LVW method
is that it is relatively inexpensive, as no expensive
directional drilling is required to
link the wells, " Courtney said.
It is also possible with RC-LVW to use
greater-diameter injection and production
wells than with the CRIP methods because
no deviated in-seam drilling (where large
borehole diameters are a disadvantage) is
required. This means that greater syngas
flow rates could be achieved using RCLVW
than with CRIP at shallow depths
(<300 meters or 984 feet).
A disadvantage of the RC-LVW method
is that, because it relies on the natural
permeability of coals, it is not particularly
well suited to low-permeability coal,
or deeper coal seams (>300 meters [m]),
which tend to be under great pressure and
consequently have reduced permeability,
according to Courtney. The reliance on
natural permeability may also force the
July 2011 | POWER
Overburden
Coal
Active
Underburden
CRIP cavity
Depleted
CRIP cavities
linkage to take unpredictable paths, as the
linkage will likely follow preexisting fractures
or paths of low permeability. Furthermore,
it is more difficult to ensure that
the linkage in the coal seam is maintained
as close as possible to the base of the seam
using this method.
" This is important because early UCG
trials using the RC-LVW method showed
strong evidence that maintaining the injection
point at a low position in the coal
seam is essential for obtaining good syngas
quality and high mining/gasification
efficiency, " he said.
3. Hitting pay dirt. This diagram illustrates the controlled retractable injection point (CRIP)
method being used to access a coal seam onshore. The CRIP method relies on a combination
of conventional drilling and directional drilling to access the coal seam and physically form the
link between the injection and production wells. Source: UCG Association
Low air emissions
Air separation
plant
Electricity
generation
plant
Gas
cleaning
plant
Injection well
O2, air, &
water
Production well
CO2, H2, CH4
& other minor
constituents
4. Going to the ocean depths. UCG plants can also be constructed with injection and
production wells that can access coal seams located offshore. Source: UCG Association
UCG surface
plant
Injection
well
Syngas end use
facility
Production
well
Offshore region
www.powermag.com
37
Direction of gasification
In-seam length
Coal seam
Underburden

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POWER July 2011

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