Geology Reference
In-Depth Information
Analysis of the data led to the conclusion that combustion was occurring in the roof coal and carbonaceous shales. It
was unlikely that the water reached these areas and no significant conversion to steam occurred. Temperature and gas
concentration changes did indicate that local cooling occurred. But not enough heat was removed to slow or stop the
combustion process. A better delivery system for placing the water in contact with the heated material was required.
Foam Injection
I
n the Renton fire, it was determined that unless the injected water is rapidly converted to steam, the movement of
the water through heated areas of the mine cannot be controlled. The use of foam as a water transport agent had two
advantages: it would keep the water in place long enough for it to be entrained in the air stream, and in voids the
foam could build upon itself to reach the mine roof where combustion may be seated. Foam is a dispersion of gas in
water; it is made of water, a suitable gas, either air or nitrogen, and a surfactant. Depending on its purpose, it may
also contain fire-inhibiting chemicals (Gross, 1991). Through its contract research program, the Bureau funded two
tests of foam injection to control subsurface fires.
A small-scale test of medium-expansion foam, which is stable and capable of filling voids, was conducted at the
Bureau
s Lake Lynn facility (Maustellar and Gross, 1996). The test was intended to determine the distribution of
injected foam in a 1.8m
3
(64 ft
3
) block of mine refuse. A 1-in pipe was used to inject the foam into the center of the
block. The foam followed the path of least resistance and flowed back through the interstitial space along the
injection tube. Dispersion through the refuse was limited, and it appeared that a better injection system would be
required before foam could be tested as a heat transfer medium at a waste bank. Subsequent laboratory tests in a
simulated waste bank indicated that the use of a polyurethane foam barrier to control foam dispersion and foam
injection beneath the fire zone could increase heat transfer from the coal waste to the heat-absorbing foam (Jones
et al., 1994).
'
In cooperation with the Colorado Division of Minerals and Geology (CDMG), the Bureau funded a demonstration
of foamed grout technology at the IHI Mine fire in Rifle, CO (Feiler and Colaizzi, 1996). The injected medium was
a cement and fly ash grout mixed with foam. The heat resistant material was injected through boreholes; it could
penetrate small cracks and fill large voids. The primary fire-containment mechanisms were oxygen exclusion and
heat removal.
During 42 calendar days, 1911 m
3
(2500 yd
3
) of foamed grout was injected into the fire zone. At the end of the
injection period, the fire was estimated to be 75% controlled. Injection in the area of the old mine portal, believed to
be the primary air source for the fire, did not completely terminate the influx of oxygen. At the end of the injection
project, the majority of borehole temperatures were above 93.3°C (200°F) and as high as 704.4°C (1300°F). The
project was initially considered to have controlled some portion of the fire, but, as of 2005, CDMG lists the fire as
active (Renner, 2005).
Cryogenic Slurry Injection
T
he use of cryogenic liquids as a heat-removal medium in coal fires has the potential advantages of uniform
distribution of the fluid and isotropic expansion of a cold gas. If water is injected into a waste bank, gravity causes
it to flow down dip, and erosion causes the size of the drainage channel to increase. The distribution of the water
affects a relatively small area, and cannot be controlled. If a cryogenic liquid is injected, moisture in the material
freezes, displacing the injected liquid to another area. This increases the size of the area affected by the injected
fluid. Also, as the temperature of the gas increases, the gas expands, creating a cold pressure front which should
move uniformly from the point of injection to the surface of the bank.
In small-scale tests of cryogenic injection, liquid CO
2
was used as the heat transfer liquid. The injected CO
2
formed
a solid, and in a relatively short period of time, blocked the flow of liquid. In medium-scale tests with liquid
nitrogen as the heat transfer medium, the nitrogen acted like a liquid and flowed to the bottom of the box.
In order to overcome the flow constraints, the Bureau developed and patented (Chaiken et al., 1994) an apparatus to
produce a pumpable slurry of liquid nitrogen and granular CO
2
at a temperature of ~
180°C. The change in state
from the solid or liquid to the gas phase produces a cold pressure wave that moves isotropically away from the
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