METHOD AND SYSTEM FOR UNDERGROUND GASIFICATION OF COAL OR BROWNCOAL

Abstract: A method for underground gasification of coal or browncoal in an inclined coal seam (1), in which a substantially uniform gasification or combustion front is maintained by filling the cavity (5) generated by gasification of coal with a filler (6) so as to drive said front in an upward direction through the coal seam (1), the gases for maintaining the gasification being introduced through a first borehole (2) and the combustion gases being discharged through a second borehole (3), one of these boreholes being used for introducing the filler, one of said boreholes following the coal seam, preferably in a more or less horizontal direction and the other borehole being connected in the coal to the lower end (4) of the first borehole.

Description: METHOD AND SYSTEM FOR UNDERGROUND GASIFICATION OF COAL OR BRO NCOAL The invention provides a method and system for underground gasification of coal (UGC) in an inclined coal seam, with filling of the gasified chambers by sedimentat¬ ion of a filler in a carrier liquid. NL-C 181941, EP-B 0053418 and EP-B 0089085 des¬ cribe a method of underground gasification of coal in which two boreholes follow an inclined coal seam in a downward direction and gradually approach each other. At or near the deepest point a connection is made between the boreholes and a chamber is gasified between them by UGC. The system is then filled with a liquid, after which a suspension of a filling material in this liquid is led through the chamber. Where the suspension enters the chamber, its speed is reduced and the filler precipitates. Thus, the front of the filler propagates from the inject¬ ion towards the discharge borehole and the chamber com¬ pletely fills with the filler, with the exception of a liquid-filled channel that runs from the injection bore¬ hole along the high coal face to the discharge borehole. The liquid can be removed from this channel by leading through a gas, preferably the oxygen-containing gas that is used for gasifying the coal. The gasification process is then restarted and a second chamber is gasified be¬ tween the injection and discharge borehole, updip of and roughly parallel to the first chamber. By repeating this process of alternately gasifying and filling a number of times, a large triangular coal area is finally gasified between both boreholes. An increase of coal recovery is possible by dril- ling both boreholes parallel to each other and connecting their lower ends with a third deviated borehole. The invention provides an improvement of the method described above, whereby approximately the same volume of coal is gasified as in the latter method, but in which only one or two boreholes have to be drilled. One borehole is deviated from the ground surface into an inclined coal seam and follows this seam for a large distance, prefera¬ bly in a more or less horizontal direction. This borehole is preferably cased down to the point where it enters the seam. The path of the other borehole can be freely chosen, as long as it reaches a point in the coal seam that is close enough to the bottom of the first, deviated, bore¬ hole to allow a connection to be made between them. It is also possible not to use a borehole as the second injection or discharge conduit, but a tubing that is installed inside the first deviated borehole that fol¬ lows the coal seam, which tubing extends from the ground surface to preferably the end of this first borehole in the coal seam. The invention will be elucidated hereafter by reference to a drawing. In this drawing: Fig. 1 and 2 show schematic represen ' tations of the known methods described previously. Fig. 3 10 show schematic representations to explain some embodiments of the invention. A first embodiment will be described by reference to fig. 3. An inclined coal seam 1 is entered and followed more or less horizontally for some distance by a borehole 2. A second borehole 3 penetrates the coal seam 1 at a point 4 that is close enough to the first borehole 2 to enable a connection to be made between them. A chamber 5 is then gasified between the boreholes 2 and 3 by introdu¬ cing an oxygen-containing gas through the borehole 2 and producing the combustible gases through the borehole 3. This chamber 5 will ultimately occupy the whole length of the deviated borehole 2 in the coal seam 1. After finish¬ ing the gasification process, the gas pressure is bled off to atmospheric and the chamber 5 and both boreholes 2 and 3 are filled with liquid, after which a suspension of a filler 6 in this liquid is led into borehole 2, through the chamber 5 and back to the ground surface through the borehole 3. The filler 6 precipitates from the liquid and S gradually fills the chamber 5 from the injection borehole 2 to the discharge borehole 3, with the exception of a channel 7 that, by the nature of the sedimentation process automatically develops and runs from the injection bore- hole 2 updip to the high coal face 8, follows this coal face 8 and then turns downdip towards the discharge bore¬ hole 3. Fig. 3 shows the filling process nearing its com¬ pletion, the direction of flow of the carrier liquid being indicated with heavy arrows. The liquid is then removed from the channel 7 by leading a high-pressure gas, pre¬ ferably the oxygen-containing gas that is used for gasifi¬ cation, into the injection borehole 2, through the channel 7 and back to the ground surface through the discharge borehole 3. If desired, the liquid can also be removed from the filled chamber 5 simply by leading a gas into this chamber 5 through the injection borehole 2 at such a small injection rate that it collects updip against the high coal face 8 and establishes a more or ess horizontal gas/liquid interface that is gradually pushed down in the filled chamber 5 to the level where the boreholes 2 and 3 enter the coal seam 1, liquid being produced from the dis¬ charge borehole 3. Gasification is then restarted by in¬ jecting an oxygen-containing gas into one of the boreholes 2 or 3 and a new chamber is gasified between them in the coal, updip of the previous one. By alternately creating a chamber by gasification and filling it with a filling material, the gasification front is gradually driven updip. Fig. 4 shows a plan view of a dipping coal seam 1 in which five chambers 3, 9, 10, 11 and 12 have been gasi- fied consecutively between two boreholes 2 and 3, start¬ ing alternately from each borehole, which chambers have been filled by the method described, with the filling process in progress in the fifth chamber 12. Fig. 5 schematically shows a three-dimensional picture of a gasification/filling operation in progress, with gasification taking place in the sixth chamber. With this borehole configuration it can be advantageous to introduce a drainpipe into the coal seam, through the SHEET borehole that follows the seam, before starting the pro¬ cess for the first time. This drainpipe is provided with openings opposite the coal seam or part thereof and ex¬ tends to the ground surface. It remains in place during subsequent filling and gasification operations. By em¬ ploying a sufficiently high gas pressure, carrier liquid, or water that is entering from surrounding sediments, can be removed from the filling material simply by opening up the drainpipe at the ground surface. Should the gas pres- sure be insufficient to drive the liquid to the ground surface, the removal process can be assisted by installing a pump in the drainpipe. It may be advantageous, after the sedimentation process has been completed, to enlarge the updip channel through which the gasification process must be restarted, e.g. to reduce the injection pressure of the oxygen- containing gas that is used for gasification. This can be achieved by leading through the pure carrier liquid, after filling has been finished, at a higher rate than that used during the sedimentation process. For this purpose it is also possible to mix the carrier liquid with a gas. As mentioned earlier, the gasification and filling process can also be carried out with one deviated bore- hole, that follows the coal seam, in which a tubing has been installed extending from the ground surface to pre¬ ferably its bottom in the seam. This embodiment of the invention is shown in fig. 6 and 7. Fig. 6 shows the filling of the first chamber in progress. Filling and prior gasification of this chamber, in this example, are carried out by injecting through the inner tubing. It will be clear that the annulus between tubing and borehole casing can also be used for this purpose. In this embodiment a connection need not be made in the coal seam. Fig. 7 shows a plan view of gasification taking place in a third chamber, after two previous chambers have been filled with a filler. In this example also, gasification is carried out every time with injection through the inner tubing. To avoid collapses of the lower roof sediments through which a supply/discharge conduit is running, the coal underneath this part of the lower roof sediments can remain ungasified, as shown in fig. 8 in top view for a configuration with inner tubing. Gasification must then every time be commenced by injection through the inner tubing. The progress of the first gasification cycle can be followed with temperature measurements inside the inner tubing . In a number of cases it will not be possible to avoid collapses of the lower roof sediments above a devel¬ oping chamber. These collapses can be detrimental to the gasification process. Fig. 9 shows a vertical cross-sect¬ ion along the dip of a chamber with caved-in roof section, at the beginning of the filling phase. In such a situation the channel in the fill will ultimately run at the top of the caved-in roof section at 1 and not along the high coal face at 2. In such cases the gasification process cannot be restarted after having removed the carrier liquid. This problem can easily be solved by not, or only partly, bleeding off the gas pressure at the termination of a gasification phase, before filling the system with the carrier liquid. While filling with the carrier liquid, a high-pressure gas bubble then develops updip in the cham¬ ber, with a gas/liquid interface as e.g. indicated with the dotted line 3. The filling process will then take place in that part of the chamber that is located below the dotted line 3, while the gas-filled space above the dotted line 3 will remain unfilled. At the level of the dotted line 3 the channel will change into a meandering river. In that case the connection consists of the updip and downdip running branches of the channel plus the gas bubble. In unfavourable cases the volume of the gas bubble, that has been created updip in a chamber, will decrease during the filling phase, as a result of leakage of gas through fissures or faults in the overburden. To calculate the rate of leakage, the volume of the gas bubble must be calculated at various points in time. To that end, the filling process must temporarily be halted, the injection conduit cleared of filler and the system closed off at the surface. After measuring the closed-in pressure, a certain amount of carrier liquid is pumped into the closed-off system and the closed-in pressure is measured again. If: P, = the closed-in pressure before adding the extra amount of carrier liquid, corrected to the depth of the gas bubble P- = the closed-in pressure after adding the extra amount of carrier liquid, corrected to the depth of the gas bubble V, = the in situ volume of the gas bubble ^V = the added volume of carrier liquid the foll P χ x V χ x Λ By measuring the in situ volume of the gas bubble at two or more different points in time, the rate of gas leakage can be calculated. The volume of the gas bubble can then be maintained by adding sufficient amounts of gas to the car¬ rier liquid during the filling phase, so that the leakage losses are replenished. After completing the gasification of a portion of a coal seam, the boreholes can be plugged back and their upper portions can be used to exploit other parts of the same seam, or other seams below or above the first seam. The exploitation of three seams with one pair of boreholes is schematically shown three-dimensionally in fig. 10. The borehole configurations that are shown in the drawing are, as such, not new. They are in use for gasi¬ fying horizontal coal seams without filling. A suitable filling material is e.g. sand. Clean sand is, however, becoming scarce and expensive in many places. A substitute for clean sand is .polluted river-, harbour- or seasand, which at present is difficult to dis¬ pose of and which would be available at low or no cost. Other suitable, filling materials are waste matter from Coal-fired power station or surface coal gasification E SHEET units, such as ash, slag, gypsum and the like, or tailings and/or slag from mining or metallurgical operations, or part of other industrial or domestic waste. All these materials might be treated, e.g. sintered, crushed and/or sieved, to make them suitable as filling material. It may be advantageous to use as filler a material or mixture of materials that is sieved to certain speci¬ fications, heat-treated or otherwise prepared to reduce compaction of the fill in the chambers as much as possible. Two chemical reactions that take place in UGC, one after the other, are: C + 0 2 *- CO- and CO- + C >2C0. The first reaction releases more heat (406 KJ/mol) than the second one absorbs (160 KJ/mol), so that the combined result produces an increase of temperature. This results in warming up of the sediments around the developing chamber and in a high temperature of the combustible gases in the discharge borehole. By substituting part of the oxygen in the injection gas by carbon-dioxyde, the tempe¬ rature in and around the chamber will decrease. The result will be that part of the heat, that otherwise would stay underground, is used to produce carbon-monoxyde , while at the same time the lower temperature of the combustible gases will give fewer corrosion and cooling problems in the discharge borehole. SUBSTITUTE SHEET

Claims: C L A I M S 1. Method for the underground gasification of coal or browncoal in an inclined coal seam, whereby two con¬ duits are introduced from the ground surface into the coal seam, after which the ends of the conduits are connected and the coal is ignited and, by supplying an oxygen-con¬ taining gas through one of the conduits, a chamber is created between the conduits, which chamber, * after bleed¬ ing off the gas pressure is filled with a filler that is supplied through one of the conduits, said filler being suspended in a liquid, said suspension having such a con¬ centration and flow rate that the filler, because of the speed reduction when entering the chamber, will precipi¬ tate, leading through this suspension being continued until the chamber is completely filled with the filler with the exception o-f a channel, that connects both con¬ duits and runs along the high coal face, after which the carrier liquid is removed from the channel with a gas, the gasification process is restarted and a second chamber is gasified updip of the previous one, which is then also filled, this process of alternately gasifying and filling being repeated a number of times, to drive the gasifica¬ tion front updip in the coal seam, the conduits remaining in contact with each other by means of the channel and the gasification chamber, characterized in that one of the conduits is a borehole that has been deviated from the ground surface into the coal seam and follows this seam for some distance, preferably in a more or less horizon¬ tal direction, whilst the other conduit is also a borehole that has been drilled from the ground surface into the coal seam. 2. The method of claim 1, characterized in that, before starting the gasification process, a drainpipe is inserted from the ground surface into the coal seam through the deviated borehole that follows the seam, which drainpipe is provided with openings over at least part of its coal section, and is used to produce liquid EET from the fill by the pressure of a gas, if necessary assisted by artificial lift. 3. The method of claim 1, characterized in that the other conduit is an inner tubing in the deviated borehole that follows the coal seam, which inner tubing extends from the ground surface to preferably the end of the coal section that has been drilled by this deviated borehole . 4. The method of claims 1 - 3, characterized in that a formed channel is enlarged after filling by lead¬ ing the pure carrier liquid therethrough, together or not with a gas, the flow velocity being adapted to the desired channel cross-section. 5. The method of claims 1 - 4, characterized in that before restarting the gasification, part or all of the liquid is removed from a channel and fill by intro¬ ducing a gas, in a controlled manner, to push down the gas/liquid interface in the fill. 6. The method of claims 1 - 5, characterized in that collapses of the lower roof sediments around a supply and/or discharge conduit are avoided by leaving the coal underneath these roof sediments ungasified. 7. The method of claims 1 - 6, characterized in that the gas pressure in a chamber and both conduits is not, or only partly, bled off before filling these with the carrier liquid. 8. The method of claim 7, characterized in that the gas volume in a chamber is replenished while filling by adding gas to the injected carrier liquid. 9. The method of claim 8, characterized in that the required amount of gas to be injected is determined by measuring the gas volume in the chamber at different points in time. SUBSTITUTE SHEET 10. The method of claims 1 - 9, characterized in that the filling material consists at least partly of polluted soil, polluted sand or silt from rivers, har¬ bours or the sea, of ash, slag, gypsum or other waste material from coal-fired power stations or surface coal gasification units, of tailings or other waste material from mining or metallurgical operations, of other indus¬ trial waste or of domestic waste. 11. The method of claims 1 - 10, characterized in that the composition and/or pre-treatment of the filling material is such that, once in place, the compaction of the fill by the overburden pressure is minimized. 12. The method of claims 1 - 11, characterized in that , , after gasification of a portion of a coal seam has been completed, the boreholes are plugged back and the upper part of one or more is used to gasify another part of this seam or other coal seams above and/or below the first one. 13. The method of claims 1 - 12, characterized in that carbon-dioxyde gas is added to the feed gas of the underground gasification process. 14. System for underground gasification of coal or browncoal, with the purpose of carrying out the method according to one or more of the preceding conclusions, as pictured in the drawing and elucidated in the des¬ cription. SUBSTITUTE SHEET

Inventors: GRUPPING ARNOLD WILLEM JOSEPHU, GRUPPING, ARNOLD, WILLEM, JOSEPHUS
Applicants: GRUPPING ARNOLD W, GRUPPING, ARNOLD, WILLEM, JOSEPHUS
Application year: 1991
Main classification: E21B43
IPC codes: E21B43, E21F15