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61.66% and 71.29%. Moving-point gasification improved the changes in the cavity in
the coal seams or the effect of roof in-break (i.e., hole in roof for solar energy) on gas
quality. For steep seams, during oxygen–steam mixture gasification, the composition
of CO + H 2 remained within 58% and 72%. The average oxidation zone temperature
reached 1200°C, and it was higher for forward gasification than for backward gasification. In general, for both types of seams, hydrogen concentration increased and
carbon monoxide concentration decreased with an increase in steam-to-oxygen ratio.
The hydrogen concentration reached about 60% at the steam/oxygen ratio of about 3.
4.7.5.1 Underground Gasification reactors
A typical underground gasification reactor is illustrated in Figure 4.5 [1,3]. In this
type of reactor, the combustion process can be handled in either forward or reverse
mode. The forward combustion involves the movement of the combustion front and
injected air in the same direction. In the reverse combustion, the combustion front
moves in the opposite direction to the injected air. The process involves drilling
and subsequent linking of the two boreholes to enable gas flow between the two.
Combustion is initiated at the bottom of one borehole (called injection well) and
is maintained by the continuous injection of air and steam. A typical underground
reaction system involves linking of a series of such a unit reactor system.
There are two principal methods for underground steam gasification which have
been tried successfully: shaft methods and shaftless methods (and a combination of
two). Selection of a specific method depends on the parameters such as natural permeability of coal seam; the geochemistry of coal deposit; the seam thickness, depth,
width, and inclination; closeness to the metropolitan areas; and the amount of mining
desired. Shaft methods involve driving of shafts and drilling of other large diameter
openings which require the underground labor and shaftless methods use boreholes
for gaining access to the coal seam which do not require any underground labor.
The shaft method can be further divided into three subdivisions: (1) chamber or
warehouse method in which underground galleries are prepared and the coal panels
are isolated with brick wall, (2) borehole producer method in which parallel underground galleries are created about 500 ft apart within the coal bed, and (3) stream
method in which inclined galleries following the dip of the coal seam of steeply
pitched coal beds are constructed parallel to each other.
The shaftless method carries out gasification through a series of boreholes drilled
from the surface to the coal seam. The coal beds are made more permeable between
the inlet and outlet boreholes by a chosen linking method, ignite the coal seam, and
gasify it by passing air and steam from the inlet to the outlet borehole. In percolation or filtration method, multiple boreholes, at a distance that depends on the seam
permeability, are used to gasify the underground coal.
The potential problems in all of these methods include (1) high and constant quality of product gas; (2) high-percentage recovery of coal energy; (3) control of groundwater contamination; (4) combustion control; (5) roof structure control; (6) product
gas leakage control; (7) proper control of permeability, linking, and fracturing; and
(8) proper monitoring of underground processes. An ideal underground steam gasification system must be the following: (1) it is operable on large scale; (2) no large
deposit of coal remains ungassified; (3) the process is controllable and the quantity