US3500934A - Fly ash injection method and apparatus - Google Patents

Description

fa if w" i March 17, 1970 M. o. MAGNUSON 3,500,934 I U 7. FLY ASH INJECTION METHOD AND APPARATUS 'l Filed Sept. 9, 1968 mg; 7 Z I I //v VENTOR MALCOLM 0. MAG/VUSQW BY 1? M 4 SLAM A TTORNEYS United States Patent FLY ASH INJECTION METHOD AND APPARATUS Malcolm O. Magnuson, Pittsburgh, Pa., assignor to the United States of America as represented by the Secretary of the Interior Filed Sept. 9, 1968, Ser. No. 758,440 Int. Cl. A62c 3/00 U.S. Cl. 169-2 Claims ABSTRACT OF THE DISCLOSURE This invention resulted from work done by the Bureau of Mines of the Department of the Interior, and domestic title to the invention is in the Government.

Coal remaining in inactive or abandoned coal mines will burn insidiously and the fire will spread over extensive areas if not controlled. Such fires constitute a health hazard, cause fire and subsidence damage to property above the coal seam and consume vast amounts of coal reserves.

If coal is heated to above about 200 F. and if oxygen is available, then the coal will undergo an accelerating self-oxidation until it reaches ignition temperatures of 800900 F. Thus, ignition sources of coal mine fires need not be direct flame but can be any sustained source of heat. Usually this heat is provided by trash or brush fires burning near a coal outcrop but may also be provided by the decay of organic material such as vegetation or garbage. Chemical action resulting in spontaneous ignition may start a fire in a low-rank coal but is not a likely source of ignition in bituminous or anthracite coal.

Fires in active coal mines can be relatively easily attacked. Usually the fire is accessible and is discovered at an early stage. In these cases, conventional methods of fire fighting, such as loading-out burning material and water extinguishing may be employed. Fires in inactive or abandoned workings present much more formidable problems. Often there is no safe access through the workings. Usually the fire is too extensive to be controlled by loading-out or other direct extinguishing techniques even if access were possible. In many cases, the only practical approach is to seal off from the fire all sources of oxygen and to maintain the fire area in a sealed condition for a time sufficient to allow the heat in the strata and coal to dissipate. If this heat is not dissipated and the coal cooled to below about 200 F., the fire will rekindle when oxygen is again admitted. Since sealing also prevents heat removal by convection, the first area may have to remain sealed for a number of years to permit the heat to dissipate by conduction.

The most practical approach to scaling passageways and voids in an inactive or abandoned coal mine so as to seal off a fire area is by drilling from the surface to intersect the workings and then injecting sealing materials through the borehole. When a water slurry of a finely divided particulate material is used, the technique is known as flushing. Detailed information on coal mine fire control in general and on the technique of flushing may be found in the Bureau of Mines Bulletin 590 (1960).

Flushing has been successful in the control of small fires in relatively flat beds. The technique is not particularly effective in pitching beds or in interconnected,

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multiple beds because drainage of the injected slurry cannot be adequately controlled. It does have the advantage though of providing substantial cooling of the fire area.

It has also been proposed to pneumatically inject particulate solid material, such as dry sand, through boreholes to fill cavities created by in situ gasification of coal. In situ coal gasification is eflectively a controlled coal mine fire. This technique is illustrated by US. Patent No. 2,710,232. Pneumatic injection of mineral wool and sand through a borehole into a mine passageway to effect a remote sealing has also been studied. This work has been published in the Bureau of Mines Report of Investigations 6453 (1964). These investigators were unable to form a full plug (complete sealing) using dry sand or slag. While full plugs could readily be formed using mineral wool, the fibrous nature of the material percluded air-tight sealing and water percolating from the roof caused matting and compression.

It has now been found that finely divided fly ash can be pneumatically injected into voids or caved mine areas through boreholes drilled from the surface. The finely divided state and the chemical properties of the fly ash allow it to readily penetrate crevices and rubble to form a stable, non-settling plug. Fly ash is particularly adapted to the sealing of large void spaces due to its extremely low angle of repose, excellent roofing characteristics, low density, non-settling properties and its tendency to swell and harden when exposed to water.

It is an object of this invention to provide a process and apparatus for the filling of voids and caved areas with fly ash.

A further object of this invention is to control and contain coal mine fires.

DETAILED DESCRIPTION OF THE INVENTION The invention will be more clearly understood from the following description of a preferred embodiment wherein reference is made to the accompanying drawlngs.

FIG. 1 is a partial sectional view of the system used for injecting fly ash into a mine void.

FIG. 2 is a representation of a casing and capping device used in the injection of fly ash through a borehole.

Referring now to FIG. 1, there is shown a void space 10 created by extraction of coal from the seam. Overlaying the coal seam with its contained voids is overburden sequence 11 which generally consists of sandstones and shales and which may vary in thickness from a little as 30 ft. to as much as several thousand ft.

Borehole 12 is drilled from the ground surface 13 through the overburden to intersect a mine void. The borehole may be of any convenient size, but a 6-in. diameter has been found to be about optimum. Casing and capping device 14, which is more fully illustrated in FIG. 2, is inserted into the top of the borehole. Casing device 14 is connected to a pneumatic source by means of adapter 15 and conduit 16.

The pneumatic source preferably comprises a bulk pneumatic tank truck 17 such as those conventionally used to transport finely divided particulate material such as cement and flour. It is preferred that the tank truck be equipped with an air compressor and aeration pads to provide motive power for the fly ash injection. Injection pressures depend upon the length of conduit connecting the pneumatic source with the borehole, the depth of the borehole and the openness of the void area penetrated by the borehole. Generally, pneumatic injection is accomplished at a few psi. and is continued until refusal occurs. At refusal, that point at which the borehole will not accept additional material, the pressure climbs rapidly to the maximum attainable by the pneumatic source, which is generally about 20 p.s.i.

Conduit 16 preferably comprises a flexible hose and may be connected to casing device 14 by means of any convenient coupling or adapter. -A 4-in. flexible hose connected to the casing device by means of a Kam-lock coupling attached to a street ell has been found especially convenient.

FIG. 2 illustrates a capping and easing device which has been found particularly useful in the injection of fly ash into open boreholes. The device consists of a length of casing 20 having a flange 21 attached thereto near the upper end. A threaded pipe cap 22 is used to protect the borehole prior to pneumatic injection. The casing device may be conveniently constructed of 4-in. pipe and have a total length of about ft. when used with a 6-in. borehole. Flange 21 is of suflicient size to rest on the borehole lip and to seal the casing-borehole annulus. Pipe cap 22 is removed and replaced with an adapter fitting connecting casing with a conduit leading to the pneumatic source during the injection operation. After injection is completed, the casing device may be removed and reused.

In those cases where a fire is burning in multiple, vertically interconnected beds of coal, then somewhat different techniques must be employed in order to selectively fill voids in each or all of the mine levels. A borehole is drilled from the surface to penetrate all mine levels having void spaces which require filling. The borehole is then cased to the bottom level as is conventional in water or oil wells. Injection of fly ash preferably proceeds from the bottom level to the uppermost level. After the lowermost level has been filled to refusal, then the casing may be raised to a point where the end is in communication with the next Succeeding level and the process continued until all levels have been filled. Alternatively, after the casing has been run, it may be perforated at the various void levels using shaped charges or other techniques common in oil field practice. Fly ash may be then injected into any selected void area through a tubing string. Isolation of a particular perforated section of casing may be accomplished by the use of removable packers carried on the tubing string as is conventional in oil well servicing and completions.

Characteristics of the fly ash are not critical. Fly ash normally recovered from the stack gas of power plants has been found satisfactory. Generally such ash has less than 10% combustible material and is in a very finely divided form. It is preferred that the fly ash used have a carbon content of less than about 8% and have a size distribution such that more than 90% will pass a SO-mesh screen and more than 75% will pass a 325-mesh screen. One typical fly ash used in the process contained from 2 to 4% combustible material. All of this ash passed through a -mesh screen and 86% passed through a 325-mesh screen.

The following examples will illustrate the flexibility and usefulness of the process.

EXAMPLE 1 An open-ended pipe was laced vertically in the center of a pile of slag crushed to a nominal 4-in. size which simulates the rubble filling of a caved mine passageway. The slag pile was about 6 ft. high and 10 ft. in diameter and the pipe terminated about 8 in. above the base of the pile.

Fly ash was pneumatically injected through the pipe until the outside of the pile was covered. The pile was then torn down and it was found that fly ash completely permeated the rubble and all interstices were completely filled. Unsupported sections of the pile stood almost vertically.

EXAMPLE 2 A rubble-fiilled chute pitching at 55 to the horizontal was constructed in order to simulate the steeply-pitching coal veins often found in anthracite mines. Fly ash was then pneumatically injected into the rubble. The fly ash permeated and filled the interstices in the rubble. An effective seal was formed.

EXAMPLE 3 Particulate dry sand and slag were pneumatically injected into a mine void from the surface. The suspension was conveyed down a pipe to a discharge nozzle within the void space. Both the sand and slag coned and injection refusal occurred before a full plug had been formed.

EXAMPLE 4 Experiments were conducted to determine the effect that a wet borehole would have on the pneumatic injection of dryfly ash. Metered amounts of water were introduced near the top of both cased and uncased boreholes during fly ash injection. It was found that water flow up to about 5 g.p.m. in an uncased borehole and up to about 10 g.p.m. in a cased borehole could be tolerated without disrupting the injection process.

EXAMPLE 5 A 6-in. borehole was drilled through 600 ft. of overburden to intersect a mine entry which was 18 ft. wide and 6 ft. high. The borehole actually terminated about 18 in. into the rib on one side of the entry but was opened by handwork. Dry fly ash was blown from a pneumatic tank truck through 300 ft. of 4-in. steel surface pipe and then down the cased borehole. Most of the fly ash was injected using air but a portion was injected using nitrogen. No differences in injection behavior were observed between the two gases.

An effective seal was obtained after injecting 320 tons of fly ash. An additional tons of fly ash was injected without refusal. Physical inspection of the plug formed showed that the fly ash had roofed for 40 ft. along the near rib and for 20 ft. along the far rib. Angle of repose of the fly ash was on the order of 10-12". Subsequent inspections failed to detect any signs of settling.

EXAMPLE 6 Dry fly ash was injected, using air as a carrier gas, into two cased boreholes which penetrated the abandoned mine workings in or near a fire area. One; of the holes showed elevated subsurface temperatures while the other showed normal ground temperatures. It was determined that the hot hole was located near the centerof the fire area while the cold hole was located in an area not yet affected by the fire.

The fly ash contained from about 2 to 4% combustible material and typically about 86% passed through a 325- mesh screen. Ten tons of fly ash was injected into the hot hole before refusal while the cold hole took 120 tons without refusal. Previous experience indicated that the hot hole would have taken about 1 ton of water-flushed crushed slag while the cold hold would have taken about 20 tons of the same material before refusal. Thus, the amount of dry fly ash which may be pneumatically injected into a borehole is 6 to 10 times greater than the amount of crushed slag which may be injected by water flushing. This ratio becomes even more significant when considered on a volume basis. Dry fly ash has a density of about 70 pounds per cubic feet while the density of crushed slag is about pounds per cubic feet. Volume of mine voids filled using dry fly ash is about 8 to 14 times that filled using water-flushed slag.

EXAMPLE 7 A fire had been burning in abandoned workings in the Pittsburgh seam for about 15 years. The active fire area was about 5 acres and temperatures within the mine ranged to 780 F. A major sinkhole had developed causing damage to 5 structures and threatening an additional 44 homes with subsidence and fume damage.

A total of 116 boreholes of 6-in. diameter were drilled in single and double rows on 25-ft. centers from the surface into the abandoned mine workings. Depth of the boreholes ranged from 30 to 62 ft. The borehole pattern extended for a distance of about 1200 ft. between outcrops of the coal seam and completely cut off the fire. A major seat of the fire appeared to be the area beneath the sinkhole which was of oval shape, approximately 75 ft. by 125 ft. Thirteen boreholes were drilled in the sinkhole.

In spite of the fact that no mine maps were available, the great majority of boreholes penetrated either mine voids or caved areas. Each of the boreholes was prepared for pneumatic injection by inserting a short length of surface casing such as is shown in FIG. 2. The casing consisted of a IO-ft. length of 4-in. pipe fitted with a 10--in. diameter circular steel flange to seal the borehole-casing annulus. Attached to the casing, which extended about 6-in. above ground level, was an adapter fitting for connection to the discharge line of a pneumatic tank truck. The casing could be sealed with a standard pipe cap for protection prior to installation of the adapter and injection of the fly ash.

A total of 6,237 tons of fly ash was injected into the boreholes. Injection was continued on each hole until refusal at a final pressure of about 20 p.s.i.g. Amount of fly ash injected per borehole varied from zero, in those holes terminating in unmined solid coal, to 660 tons in one borehole which apparently had penetrated a large void. Boreholes in caved areas accepted an average of about 18 tons of fly ash before refusal while boreholes terminating in voids accepted an average of about 135 tons. An overall average of 54 tons per hole was achieved.

The injected fly ash barrier appears to have completely and permanently sealed the fire area. No indications of continuing fire activity have so far appeared.

As is demonstrated in the examples, the advantages derived from the pneumatic injection of fly ash are manyfold as compared to the previous practices of water flushing or pneumatic injection of other particulate materials. Due to its chemical properties and its size and porosity, fly ash readily absorbs water. Upon absorbing water, the fly ash expands and hardens. Since water is usually present to some extent in most coal mines, this property contributes to a tighter and more permanent seal than can be obtained with other materials.

Due to its finely divided state, the fly ash penetrates crevices and rubble much more readily than does conventionally used flushing or injection materials. Far more fly ash may be injected per borehole thus requiring less drilling to inject a given volume of material. Since fly ash is generally available as a waste material in most coal mining areas, it often enjoys an economic advantage over conventional materials as well as providing superior sealing qualities.

In some circumstances, it may be advantageous to inject the fly ash in admixture with other materials. For example, cement premixed with the fly ash in amounts of l to 10% or more and thereafter injected into void spaces will form a low grade concrete upon contact with water absorbed by the mixture. Expansion of the fly ash upon contact with water may be enhanced by mixing with the fly ash small amounts of materials such as swelling bentonitic clays prior to injection.

What is claimed is:

1. A method of remotely filling and sealing voids and caved areas within a mine which comprises drilling holes from the surface to intersect said mine voids and caved areas and thereafter pneumatically injecting a gaseous suspension of particulate material comprising fly ash in admixture with a relatively minor amount of a material chosen from the group consisting of cement and swelling clays through said holes and into said voids and caved areas within the mine.

2. The method of claim 1 wherein said mine is a coal mine having a fire burning therein.

3. The method of claim 2 wherein said holes are drilled in a pattern intersecting all voids and caved areas communicating between said fire and external sources of oxidizing gas.

4. The method of claim 3 wherein said particulate material is pneumatically injected into all the holes making up said pattern so as to for-m a gas-impervious barrier between said fire and all external sources of oxidizing gas.

5. The method of claim 4 wherein particulate material injection into each of said holes is continued until refusal.

6. The method of claim 1 wherein said particulate material comprising fly ash contains less than about 8% combustible material and has a size distribution such that at least 90% will pass a SO-mesh screen and at least will pass a 325-mesh screen.

7. The method of claim 1 wherein said particulate material is injected in admixture with about 1 to about 10% of a material chosen from the group consisting of cement and swelling clays.

8. A mine having voids and caved areas at least partially filled with a particulate material comprising fly ash in admixture with a relatively minor amount of a material chosen from the group consisting of cement and swelling clays, said particulate material being emplaced within the mine by means of pneumatic injection through boreholes communicating between the surface and the voids and caved areas within the mine.

9. The product of claim 8 wherein the admixture of fly ash and a material chosen from the group of cement and swelling clays is reacted in situ with mine waters to form an expanded and hardened plug.

10. The method of claim 2 wherein the injection gas used to suspend and transport said particulate material does not support combustion.

, References Cited UNITED STATES PATENTS 1,391,678 9/1921 Francois 61-35 2,710,232 6/1955 Schmidt et a1 302-66 3,421,587 1/1969 Heavilon et al 1692 OTHER REFERENCES The Latrobe Bulletin, Aug. 1, 1967, Lloydsville Mine Fire Action Set.

EVERETT W. KIRBY, Primary Examiner U.S. Cl. X.R. 61-35; 299-12

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