POWER July 2011 - 36

CLEAN COAL TECHNOLOGY
Underground Coal Gasification:
Another Clean Coal Option
Underground coal gasification (UCG) is the gasification of coal in-situ, which
involves drilling boreholes into the coal and injecting water/air or water/
oxygen mixtures. It combines an extraction process and a conversion
process into one step, producing a high-quality, affordable synthetic gas,
which can be used for power generation. Still in the early stage of commercialization,
UCG is poised to become a future major contributor to the
energy mix in countries around the world.
By Angela Neville, JD
C
oal gasification is a well-known
chemical process that converts solid
carbonaceous material into synthetic
gas (syngas), which consists predominantly
of methane (CH4
ide (CO), carbon dioxide (CO2
(H2), and water (H2
), carbon monox),
hydrogen
O) steam. Gasification
differs from combustion (or burning) because
burning coal takes place in excess O2
and produces only CO2
and water steam.
In the underground coal gasification
(UCG) application, air and/or oxygen is introduced
to coal while it is still in the ground
by pumping it down boreholes (called injection
wells), which are drilled into the coal
seam from the ground surface (Figure 1).
Rohan Courtney, chairman of the trustees
of the UCG Association (UCGA, www
.ucgp.com) and also chairman of Clean
Coal Ltd., told POWER in May about the
UCG process. He explained that once syngas
is formed in the coal seam, the syngas
then flows back to the surface under pressure
via a second borehole (the production
well), which is linked through the coal
seam to the injection well (Figure 2). A
linked injection well and production well
is called a UCG " module, " which is the
cornerstone of UCG.
" In many ways, it is the relatively recent
perfection of drilling and methods for
linking the injection and production wells
that has led to the huge resurgence of interest
in UCG that we are now witnessing
across the globe, " he said. (See sidebar,
" Coal Gasification Pilot Projects. " )
Two UCG Methods
The two main methods used to carry out
UCG are often referred to generically as
the linked vertical well (LVW) method
and the controlled retractable injection
point (CRIP) method. Both of these methods
rely on a module of at least two linked
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boreholes to inject the oxidant and remove
the syngas.
The LVW method uses vertically drilled
wells to access the coal seam and different
techniques to link the boreholes.
In contrast, 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.
Evidence from previous and current trials
suggests that the two basic methods are
generally suited to exploit different coal
resources. LVW methods are more suited
to shallow coals seams, and CRIP methods
are more suitable for deeper coal seams.
There is a fairly equal distribution of
1. Making coal cleaner. The underground
gasification of steeply dipping coal
seams was demonstrated in a pilot project
near Rawlins, Wyo. In the underground coal
gasification process, air and/or oxygen is
introduced to the coal while it is still in the
ground by pumping it down boreholes (called
injection wells), which are drilled into the coal
seam from the ground surface. Courtesy: Paul
Ahner
2. Surfacing. After syngas is formed in the coal seam, it then flows back to the surface under
pressure via a second borehole (the production well), which is linked through the coal seam
to the injection well. Courtesy: Marc Mostarde

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POWER July 2011

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