Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, TN 406, 1992 Sep; :1-2
Objectives: Develop a safer, more effective, and more efficient method to control fires in abandoned coal mines and waste banks. Maintain the costs of this new method at less than or comparable to conventional AML fire control methods. How It Works: Cryogenic extinguishment of subsurface fires is a method of heat transfer. Bureau-designed injection equipment produces a pumpable slurry of liquid nitrogen and solid particles of carbon dioxide. Liquid or gaseous carbon dioxide from a pressurized tank flows to a bell-shaped nozzle where the change in pressure causes the carbon dioxide to form a granular snow. The solid carbon dioxide is mixed with liquid nitrogen. A jet pump is used to move the slurry through the delivery lines and into a simple injection probe. At injection, the nitrogen vaporizes and the carbon dioxide sublimes. This phase change produces a cold gas (-180" C) that, as it expands and moves in all directions, removes heat and also displaces oxygen. This combined process inhibits the combustion process and lowers the temperature below the reignition point. When the liquid nitrogen and solid carbon dioxide are converted to gases, the expanding gas acts like a piston, forcing warm air out of the porous bed. Although smoke and hot gases are forced from the burning coal, generally there is no significant increase in the odor associated with burning coal. Application: Injection tests were conducted in the Bureau's surface trench facility at Lake Lynn, FA. This facility consists of a 540-ft3 bed containing 40,000 lb of coal. Sixty-three thermocouples were placed in the bed to monitor temperature changes. In a waste bank application, the probe, which is a 1-in stainless steel pipe with a closed and pointed end and a row of 3/16-in holes located approximately 1 in behind the end, is driven into the coal waste. The maximum temperature change at each probe is shown in figure 1. In the area surrounding the injection point, temperatures dropped to -180 degrees C in less than 5 min. After injection was completed, temperatures in the bed remained below ambient for up to 4 months. To illustrate a potential field application, preliminary cost estimates have been completed for cryogenic slurry injection at a waste bank fire. At this site, the waste bank is approximately 225 ft in length and has an oval cross section 100 ft wide and 40 ft high. On this basis, it contains approximately 25,000 tons of coal refuse. The heat capacity of this coal is approximately 0.4 (cal/g)/degrees-C or 0.72 (Btu/lb)degrees-C. Generally, in a waste bank fire, less than 5% of the material may be actively burning at any given time. Some of the material will be heated by the fire and some of the material will be at or near normal temperatures. To extinguish the fire, it is necessary to stop combustion and to remove stored heat. For purposes of estimating the heat content, the average temperature of material in the bank is assumed to be 20 degrees C above normal. On this basis, the bank has a heat content of 7.2 x 10(8) Btu. The cryogenic slurry has an estimated heat removal capacity of 250 Btu/lb, and 3 million lb of slurry would be needed to extinguish a fire of this magnitude. Benefits: The benefits of cryogenic slurry are: 1. It is effective in removing heat from the fire area; 2. It acts as an encapsulating material around the fire area; 3. It eliminates the availability of oxygen to the fire area; 4. The injection of a slurry of liquid nitrogen and solid carbon dioxide overcomes the distribution problems that are associated with the injection of a simple liquid extinguishing agent; 5. It is as cost effective as conventional fire control and extinguishment methods.
Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, TN 406