Geology Reference
In-Depth Information
Wastebank
Heat
and fumes
Cold gas
Injection
probe
Slurry
Figure 16.5.5. Sketch of cryogenic slurry injection into a waste bank. Change in state from liquid/solid to gas
forces heat adsorbing cold front through the bank to the surface. From Kim 1994, Figure 1.
injection point, adsorbing heat and producing an inert atmosphere. It forces heat, smoke, and fumes from the
combustion zone to the surface (Figure 16.5.5.). The movement of the inert gas is controlled by pressure and
buoyancy.
To produce a slurry of CO
2
and N
2
, liquid or gaseous CO
2
pumped through a bell-shaped nozzle forms finely
powdered particles due to the rapid expansion of the gas. If at the same time, liquid N
2
is injected into the nozzle, it
cools the solid particles of CO
2
, and the 1iquid nitrogen and solid CO
2
form a slurry. The slurry was entrained
through a jet pump into another stream of liquid N
2
at a delivery pressure of 80 psi. A stainless-steel injection line
carried the slurry to the injection point. When the slurry is pumped into a bed of coal or coal waste, the temperature
near the probe drops to -180°C, and the expanding cold gas lowers the temperature in the surrounding material.
A test of the cryogenic method to extinguish subsurface coal fires was conducted at a coal waste bank in Ohio
(Kim, 2004). The bank was adjacent to a high wall, and extended east to west along the eroded southern slope to a
concrete foundation, believed to be the remains of a tipple. The flat top of the waste bank was ~29.7 m above the
base (see Geophysical Methods in Section 16.4 for a more complete description of the waste bank).
Temperature surveys indicated that there were two areas of combustion. A small fire zone in the western portion of
the site was apparently cooling. The more extensive combustion zone was located in the eastern portion of the site.
For logistical and economic reasons, the cryogenic injections were concentrated in the eastern zone. During the first
injection, the jet pump designed to inject the slurry failed. Only liquid nitrogen (about 12 600 kg) was injected.
Post-injection temperature surveys indicated that the average temperature in the bank had decreased by 6°C, and
there was a decrease of 296°C in the maximum temperature. During the second injection, a slurry of 4600 kg of N
2
and 1140 kg of CO
2
was injected at relatively low injection pressures into the eastern portion of the site (Figure
16.5.6.). For the duration of the trial, subsurface temperatures ranged from 19 to 450°C.
Temperatures near the injection point, showed a decrease followed by a return to the pre-injection temperature over
a 4-month period. Varying degrees of initial cooling followed by a long-term decline in temperature was indicated
in several areas, while there was initial cooling followed by a temperature increase in other areas. The variable
nature of the results indicates that combustion was reduced at hot spots near the injection point, but displacement of
hot gases could cause an increase in temperature in adjacent areas.
Temperatures measured more than 1 year after the injection of the slurry indicated lower temperatures throughout
the site, but a continuing low level of combustion in the interior of the bank. In a detailed analysis of the
temperature data, Chaiken et al. (1996) concluded that the second cryogenic injection apparently controlled or
eliminated hot spots in the East fire zone reducing heat generation within the waste pile. This allowed environ-
mental cooling rates to exceed heat generation rates. This implied that cryogenic slurry injections quenching
localized hot spots might be sufficient to extinguish a waste bank fire.
