JP7227605B2 - Coal underground gasification method - Google Patents

Description

The present invention relates to a method for underground gasification of coal.

Underground Coal Gasification (UCG) is a technique for gasifying coal while it is in an underground coal bed without mining and taking it out to the surface (see Patent Document 1).

As a general method of UCG, for example, first, two boreholes (blowing hole and discharge hole) are drilled from the ground to reach the coal bed, and the two are connected within the coal bed. In this way, the gas is made to flow from the blow hole to the discharge hole via the coal bed.
When the coal in the coal seam is ignited while air is in short supply through the blast holes, the coal generates heat and produces carbon dioxide. The carbon dioxide generated further reacts with the heated coal to form carbon monoxide. If oxygen is used instead of air, a higher calorie gas is obtained.

JP-A-57-212295

As UCG progresses, cracks may appear around cavities in coal beds (coal beds, bedrock, etc.). For example, when UCG is applied to a shallow coal seam with a depth of about 0 to 400 m, the cracks may cause gas to leak to the ground, pollute groundwater, or subside.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an underground coal gasification method capable of repairing cracks appearing in coal beds and bedrock as the underground gasification of coal progresses.

The present inventors have made intensive studies to achieve the above object. As a result, it was found that silica (SiO ₂ ), which occupies about 60% of the surface layer of the earth, prevents the development of cracks (that is, repairs the cracks) that appear with the progress of coal gasification. completed.

That is, the present invention provides the following [1] to [4].
[1] A gasifying agent is fed into the coal in the coal bed through air holes reaching the coal bed from the ground to gasify the coal, and the gas generated by the gasification of the coal is discharged from the coal bed to the ground. A method of underground gasification of coal with discharge through holes, characterized in that a silica-containing composition is fed into said coal bed through said air blow holes.
[2] The underground coal gasification method according to [1] above, wherein the silica-containing composition contains silica and an organic substance.
[3] The method for underground gasification of coal according to [1] or [2] above, wherein the silica-containing composition is at least one selected from the group consisting of moss plants, fern plants and gramineous plants.
[4] The underground coal gasification method according to any one of [1] to [3] above, wherein the silica-containing composition is rice husk.

According to the present invention, it is possible to repair cracks appearing in coal beds and bedrock as the underground gasification of coal progresses.

1 is a schematic diagram showing an example of UCG; FIG. FIG. 4 is a schematic diagram showing a variation of UCG; FIG. 11 is a schematic diagram showing another variation of UCG;

An embodiment of the present invention will be described below.
First, underground coal gasification (UCG) will be described based on FIGS. 1 to 3. FIG.

FIG. 1 is a schematic diagram showing an example of UCG.
In FIG. 1, a soil layer 2, a bedrock 3 which is a rock layer, and a coal bed 4 which is a coal layer are arranged in order of proximity to the ground 1 in the underground.
In the UCG, first, two boring holes are drilled from the ground 1 to the coal bed 4, and are used as the air blow hole 5 and the discharge hole 6. Blow hole 5 and discharge hole 6 are constituted by tube 7 and tube 8, respectively.
The blow hole 5 and the discharge hole 6 are connected inside the coal bed 4 . In FIG. 1 , the blow hole 5 and the discharge hole 6 are connected by a cavity 9 formed inside the coal bed 4 .
In this way, as indicated by white arrows in FIG. Gas can be expelled.

FIG. 2 is a schematic diagram showing a variation of UCG.
In FIG. 2, the same parts (portions having the same functions) as those in FIG.
In FIG. 2, the tubes 7 and 8 are so-called double tubes. That is, the pipe 7 forming the blow hole 5 is an inner pipe, and the pipe 8 forming the discharge hole 6 is an outer pipe. In this case, it is economical because only one boring hole needs to be drilled.
In FIG. 2 as well, as indicated by white arrows in FIG. The gas can be discharged to the ground 1.

FIG. 3 is a schematic diagram showing another variation of the UCG.
In FIG. 3, the same parts (portions having the same functions) as those in FIGS. 1 and 2 are denoted by the same reference numerals, and description thereof is omitted.
In FIG. 3 , a double pipe composed of pipes 7 and 8 is bent inside the coal bed 4 and extends along the coal bed 4 . In FIG. 3 as well, as indicated by white arrows in FIG. The gas can be discharged to the ground 1.

In such a configuration (FIGS. 1-3), the coal in the coal bed 4 is gasified.
First, a gasifying agent containing water vapor, oxygen, air, etc. is fed into the coal in the coal bed 4 (more specifically, the coal around the cavity 9) through the blowing holes 5 reaching the coal bed 4 from the ground 1. while heating (igniting) the coal. This gasifies the coal to produce a product gas containing carbon monoxide, hydrogen, and the like.
Then, the produced gas is discharged from the coal bed 4 through the discharge hole 6 reaching the ground 1 . The generated gas discharged from the discharge hole 6 to the ground 1 is appropriately recovered and used for various purposes.

An example of a basic reaction formula for coal gasification is shown below.
(1) Coal → H ₂ +C _{m Hn} +C
(2) C+ _O2 → _CO2
(3) C+1/ _2O2 →CO
(4) C+ _CO2 →2CO
(5) C+ _H2O →CO+ _H2
(6) C + _2H2O → _CO2 + _2H2
(7) CO+ _H2O → _CO2 + _H2
(8) C+ _2H2 → _CH4

The above formula (1) is the thermal decomposition reaction of coal.
Equations (2) and (3) above represent reactions with oxygen, combustion and partial combustion, providing the heat of reaction required for gasification.
Equations (4) and (5) above are the predominant gasification and endothermic reactions.
Along with these reactions, the reactions shown in the above formulas (6) to (8) also occur.

Gasification of coal starts at 100 and several tens of degrees Celsius, and if it is performed in a furnace lined with a heat-insulating refractory material, it is considered to reach about 1500 degrees Celsius.

The above is the outline of UCG.
Since the idea of UCG was announced in 1888 by Mendeleev of Russia, who is famous for the periodic table, research and development of UCG has progressed in many countries, and in 1956 it was put to practical use in the Soviet Union. Development is also underway in Australia and the United States.
Advantages of UCG include, for example, a significant reduction in hazardous underground operations. Even if the coal seam 4 is extremely thin or the grade is low, it is not economical to use a general coal mining method.

By the way, as UCG progresses, as shown in FIGS. 1 to 3, cracks 10 may appear in the coal bed 4 and the bedrock 3 around the cavity 9 where coal gasification is progressing.
For example, when the thickness of the coal bed 4 is approximately 1 to 3 m, the total length of cracks 10 appearing in the coal bed 4 is approximately 3 m at maximum.
Usually, hundreds of thousands of cracks 10 with a small surface area (small scale) having a total length of several centimeters appear, and other cracks 10 having a total length of several meters also appear.

Due to the appearance of such cracks 10 , gas in the cavity 9 may leak to the ground 1 through the cracks 10 . In addition, there is a risk of contamination of groundwater. Furthermore, the cracks 10 may cause the soil layer 2, the bedrock 3, the coal bed 4, and the like to collapse, resulting in ground subsidence.
These problems are particularly likely to occur when UCG is applied to shallow coal beds 4 .

Therefore, in the present embodiment, a silica-containing composition containing at least silica (SiO ₂ ) is put into the blowing hole 5 together with, for example, a gasifying agent.
As a result, the silica-containing composition is sent through the air blowing holes 5 to a portion of the coal bed 4 where coal gasification is progressing (the coal around the cavity 9 in FIGS. 1 to 3).

The delivered silica-containing composition penetrates into cracks 10 .
Due to the gasification reaction described above, the internal pressure of the cavity 9 is relatively high, so the silica-containing composition easily enters the cracks 10 .
The silica-containing composition is particularly likely to enter cracks 10 when gas is leaking through cracks 10 (when gas flow is present). If necessary, the pressure in cavity 9 may be varied, such as by adjusting the flow rate of the gasifying agent, to guide the gas flow to crack 10 .
The silica-containing composition that has entered the crack 10 adheres to the inner wall of the crack 10 and the like.

Here, as described above, since the gasification of coal in the coal bed 4 is performed at a high temperature, the inside of the crack 10 is also in a high temperature state.
Due to this high temperature state, the silica (amorphous silica) contained in the silica-containing composition is polymerized in the cracks 10 to close the cracks 10 . The crack 10 is thus repaired. That is, the development of cracks 10 can be prevented. Silica is one of the substances that form the earth's crust and accounts for about 60% of the surface layer of the earth, so the cracks 10 can be repaired.

Monitoring (sensing) the state of occurrence of the cracks 10 is more effective because the silica-containing composition can be introduced into the air blowing holes 5 at an appropriate timing.
Methods for sensing cracks 10 include, for example, methods for measuring destructive sounds (AE: Acoustic Emission) or microearthquakes. By this measurement, the number of cracks 10 generated, the size, the location of generation, the mode (tensile or shear), and the like are detected.

In addition, the silica-containing composition charged from the blow hole 5 may be discharged from the discharge hole 6 together with the produced gas without being used in the coal bed 4 . In this case, for example, a collecting part such as a cyclone is provided on the exit side of the discharge hole 6 to collect the silica-containing composition discharged from the discharge hole 6, and the collected silica-containing composition is returned to the blow hole. You can put it in 5.

The silica-containing composition used in the present embodiment preferably contains an organic substance in addition to silica. Examples of organic matter include carbohydrates such as cellulose, hemicellulose, and lignin. By introducing the organic matter into the blowing holes 5 together with the gasifying agent, the organic matter is combusted and gasification is facilitated, increasing the amount of generated gas.

Such a silica-containing composition is not particularly limited as long as it contains silica and an organic substance.
However, since drilling a borehole is very costly, it is preferable to consider the economic aspect of the silica-containing composition to be used.
Therefore, from the viewpoint of economy, the silica-containing composition is preferably a plant containing silica, and specific examples thereof include at least one selected from the group consisting of moss plants, fern plants and gramineous plants. be done.

The moss plants include, for example, Polytrichum, Sphagnum moss, Black moss, Luminous moss, Sturgeon's moss, Polytrichum, Marchantia moss, Jasmos, Urticococcus moss, Venus moss, Lycopodium moss, Corticaria, and Hornwort.

Pteridophytes include, for example, black bracken, bracken, horsetail, fern, fern, mizunira, lycopod, horsetail, and the like.

Examples of gramineous plants include wheat, durum wheat, rye, triticale, barley, oats, adlay, corn, rice, barnyard millet, millet, and millet.
As a gramineous plant, a gramineous plant seed may be used.
The gramineous plant seed includes, for example, the gramineous plant seed itself; the gramineous plant seed cut, crushed or powdered; the gramineous plant seed dried; the gramineous plant seed dried and then pulverized or powdered and the like.
As the Poaceae plant seed itself, for example, a seed coat can be mentioned, and a specific example thereof is preferably rice husk.

The husk is the outermost skin part of the paddy (unhulled rice) and is obtained by threshing.
Rice husks are generated all over the world, and the annual total amount generated is about 80 million tons, and the annual amount generated in Japan is 2 million tons.
Since rice husk is one of agricultural wastes and can be obtained at a relatively low cost, the use of rice husk as a silica-containing composition is very economical.

The composition of rice husk is about 70% by mass of carbohydrates such as cellulose, hemicellulose, lignin, etc., about 15-20% by mass of silica, and most of the remainder is water, and contains a small amount of alkali metal as an impurity.
At high temperatures (for example, about 800° C. or higher), carbohydrates such as cellulose in rice husks are gasified and disappear, and silica (amorphous silica) is polymerized.
Another advantage is that rice husks are easily combustible, and once they are burned, they burn rapidly at a high combustion rate and gasify well (rice husks have a calorific value of about 3,000 kcal/kg).

In the above description, the silica-containing composition closes the cracks 10 around the cavity 9, but the silica-containing composition may close the cavity 9 itself.
Cavities 9 of coal bed 4 where UCG is finished are usually left unblocked and may be used, for example, to store carbon dioxide.
By the way, even if the crack 10 does not appear when the UCG is finished, the coal bed 4 and the bedrock 3 may collapse over time, causing ground subsidence.
Therefore, a silica-containing composition is fed into the cavities 9 of the coal bed 4 where UCG has been completed. In this case, since the volume of the cavity 9 is also larger than the volume of the crack 10, a larger amount of silica-containing composition is delivered in total than when the crack 10 is repaired. Further, since the UCG has been completed, the cavity 9 is separately brought to a high temperature state in order to polymerize the silica. Thus, the silica is polymerized in the cavity 9 to block the cavity 9 .

1: Ground 2: Soil layer 3: Bedrock 4: Coal bed 5: Ventilation hole 6: Drain hole 7: Pipe 8: Pipe 9: Cavity 10: Crack

Claims (4)

A gasification agent is fed into the coal in the coal bed through air blow holes that reach the coal bed from the ground, the coal is gasified, and the gas generated by the gasification of the coal is discharged from the coal bed through the discharge holes that reach the ground. A method for underground gasification of coal, wherein
A method for underground gasification of coal, characterized in that a silica-containing composition is sent into the coal bed through the blow hole. 2. The underground coal gasification method according to claim 1, wherein the silica-containing composition contains silica and an organic matter. 3. The method for underground gasification of coal according to claim 1 or 2, wherein the silica-containing composition is at least one selected from the group consisting of moss plants, fern plants and gramineous plants. The underground coal gasification method according to any one of claims 1 to 3, wherein the silica-containing composition is rice husk.

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