AU2013101616A4 - Oxidant injection method - Google Patents

Abstract

The invention provides a method for carrying out hydrogen peroxide fed underground coal gasification.

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant: Linc Energy Ltd Invention Title: OXIDANT INJECTION METHOD The invention is described in the following statement: 1 OXIDANT INJECTION METHOD TECHNICAL FIELD [0001] This invention relates to a method of carrying out underground coal gasification (UCG). In particular, a method of hydrogen peroxide (H 2 0 2 ) fed UCG is disclosed. BACKGROUND ART [0002] Underground coal gasification is a process by which product gas is produced from a coal seam by combusting and gasifying the coal in situ in the presence of an oxidant. The product gas is typically referred to as synthesis gas or syngas and can be used as a feedstock for various applications, including clean fuels production, chemical production, and electricity generation. [0003] Wells are drilled into the coal seam to allow for oxidant injection and product gas extraction. The wells are linked or extended to form an in-seam well channel to facilitate oxidant injection, cavity development and product gas flow. [0004] The well allowing the injection of oxidant is called an injection well. The well from which product gas emerges is called a production well. Both horizontal and vertical well regions can be used for injection and production. Underground coal gasification may also utilise one or more vertical wells (service wells) located between the injection and production wells. [0005] A coal seam having an in-seam well channel is typically referred to as a coal gasifier. The gasifier will have a combustion zone within which coal is combusted in the presence of an oxidant, a gasification zone located downstream of the combustion zone in which coal is gasified and partially oxidized to produce product gas, and a downstream pyrolysis zone in which pyrolysis of coal occurs. Hot product gas flows downstream from the gasification zone and exits the ground from a well head of the production well. As coal is consumed or gasified, a gasifier (gasification) cavity within the coal seam develops and grows in size.

2 [0006] The product gas (raw syngas) generated by UCG typically comprises syngas as well as other components, and the constituency will depend on various factors including the type of oxidant used for UCG (air or other oxidant, such as oxygen or oxygen-enriched air), water presence (both ground water and exogenous water), coal quality, and UCG operating temperature and pressure. [0007] Challenges of UCG include controlled combustion and gasification of coal seams, particularly in areas where there is limited available water (geological and/or above ground). SUMMARY OF INVENTION [0008] An object of the present invention is to provide a method for UCG that minimises one or more of the problems of the prior art. [0009] In one aspect, the invention provides a method of UCG in a coal seam provided with an injection well, a production well and an in-seam channel linking the injection well and the production well, including the steps of a) inserting a perforated liner into the linkage channel, b) igniting the coal seam, c) injecting hydrogen peroxide into the linkage channel through the injection well to maintain combustion/gasification of the coal seam, and d) withdrawing product gas from the production well. [0010] In one embodiment, the coal seam is further provided with one or more service wells. The service wells are typically vertical wells located between the injection and production wells, and can be used as ignition wells and/or auxiliary injection/production wells. [0011] In another embodiment, the liner further includes a combustible sheath. Suitably, the sheath covers at least some of the perforations in the liner. [0012] In a further embodiment, one or more thermocouples are attached to the liner for collection of temperature data from the liner during underground coal gasification.

3 [0013] In another embodiment, igniting the coal seam includes positioning an ignition tool in the linkage channel at one or more of the perforations in the liner, providing an ignition fuel to the ignition tool, and igniting the coal seam using the ignition tool, wherein the ignition tool includes ignition means. [0014] In yet another embodiment, the method further includes the step of injecting an oxidant into the linkage channel. [0015] In another embodiment, the method further includes the step of injecting carbon dioxide into the linkage channel. BRIEF DESCRIPTION OF DRAWING [0016] Figure 1 is a side section view of a portion of an underground coal gasifier illustrating certain aspects of the present invention. DESCRIPTION OF EMBODIMENTS [0017] The present invention relates to a method of hydrogen peroxide fed UCG. [0018] Throughout this specification, unless the context requires otherwise, the words "comprise", "comprises" and "comprising" will be understood to mean the inclusion of a stated integer, group of integers, step, or steps, but not the exclusion of any other integer, group of integers, step, or steps. [0019] In one aspect, the invention provides a method of UCG in a coal seam provided with an injection well, a production well and an in-seam channel linking the injection well and the production well, including the steps of a) inserting a perforated liner into the linkage channel (also referred to as a "well channel"), b) igniting the coal seam, c) injecting hydrogen peroxide into the linkage channel through the injection well to maintain combustion/gasification of the coal seam, and d) withdrawing product gas from the production well.

4 [0020] The liner (also referred to as a "well liner") inserted into the linkage channel can be of any suitable size, shape and construction, and can be made of any suitable material or materials. [0021] Preferably, the well liner extends from adjacent a heel of the injection well to adjacent a heel of the production well. [0022] The size, shape and construction of the well liner is selected to ensure that it can be installed into a well channel of an underground coal gasifier and remains intact during service (i.e., it keeps the in-seam channel linking the injection well and the production well open). Preferably, the well liner is strong enough to be inserted into the linkage channel using traditional drilling service equipment, as will be known to one of ordinary skill in the art. [0023] The well liner can be of unitary construction or can include a plurality of connectable units (i.e., segments). The well liner or segments can be of any suitable length, including, metres, tens of metres, hundreds of metres, and kilometres. Accordingly, well liner segments can be connected together to form a well liner being tens of metres long, hundreds of metres long, or even several kilometres in length, depending on the length of the linkage channel. Each well liner segment can be, for example, about 1 to 10 metres in length, including about 3, 5, 6, 7, or 9 metres in length. [0024] The well liner segments can be connected together in any suitable way. For example, the ends of each segment can be threaded, and the well liner can include one or more threaded collars for connecting the ends of adjacent well liner segments together. [0025] The well liner will have an outside diameter (or width) appropriate for the linkage channel into which it is being inserted. Typically, the well liner will have an outside diameter of anywhere between about 5 to 10 inches, more preferably about 5 to 8 inches, and even more preferably about 5.5 to 7 inches.

5 [0026] The well liner is preferably resistant to chemical attack from the products of coal gasification and pyrolysis (e.g., sulfur), as well as attack from the syngas itself (including, CO, H 2 , C02, and H 2 0) which may be corrosive. [0027] The well liner can be made of any suitable material, including, for example, metal (including steel, such as carbon steel, and aluminium), fibreglass, carbon fibre, plastic, and combinations thereof. [0028] The well liner (including segments thereof) can be manufactured in shapes and sizes to suit the specific application. Preferably, the well liner has a round cross section to provide an annular passage, although other cross-section shapes are possible, as will be understood by one of ordinary skill in the art. [0029] The well liner perforations can be of any suitable size, shape and arrangement as required to mitigate a number of technical and operational issues associated with a fully closed-in well liner design. For example, the perforations allow the ignition of a coal seam from within a linkage channel using an ignition tool located within the well liner to ignite the surrounding coal seam. Additionally, perforations in the well liner allow oxidant and water/steam to pass through the liner and contact the coal while preventing coal and ash from blocking the path of production gases. [0030] The perforations can be in periodic symmetry in both circumferential and axial directions. The perforations can be in the form of circular or other shaped holes (e.g., hexagonal or octagonal), or slots. The perforations can be, for example, circular having a diameter of about 10 mm to about 25 mm. The perforations can be in a rectangular or diamond-shaped grid pattern, or both, for example. Preferably, the perforations are in a staggered arrangement (diamond spacing) as this provides the well liner with the greatest structural integrity. [0031] The well liner can have anywhere between about 10% to about 60% of its surface area in an open configuration (i.e., perforated), provided that the structural integrity of the well liner meets operational, in-seam requirements. Although the perforations can comprise about 10%, about 15%, about 20%, about 25%, about 30%, about 35%, about 40%, about 45%, about 50%, about 55%, or about 60% of 6 the well liner's surface area, about 10% to about 50% appears to be optimal, as adequate structural integrity is retained by the well liner. [0032] Preferably, the perforations in the well liner are grouped together in one or more regions along the length of the liner, alternating with non-perforated sections of the liner. [0033] The well liner can further include a sheath configured to cover some or all of the perforations in the liner. Similar to the well liner, the sheath can be of unitary construction or can include a plurality of units (i.e., segments). For example, sheath segments can be utilised to cover the perforations that are grouped together in one or more regions along the length of the well liner, leaving the intervening non perforated sections of the liner uncovered. The sheath or segments can be of any suitable length as required by the well liner, including, metres, tens of metres, hundreds of metres, and kilometres. Each sheath segment can be, for example, about 1 to 10 metres in length, including about 3, 5, 6, 7, or 9 metres in length. [0034] The sheath (including segments thereof) can be of any suitable size, shape, and construction, and can be made of any suitable material or materials, including, for example, metal (including steel, such as carbon steel, and aluminium), fibreglass, carbon fibre, plastic, and combinations thereof. The sheath can be about 1 mm to about 20 mm thick, including about 2 mm, about 5 mm, about 10 mm, or about 15 mm thick. [0035] The sheath can be, for example, a membrane, sheet, or film that wraps around the outside or the inside of the well liner at least once and covers some or all of the perforations in the liner. [0036] In one embodiment, the well liner is made of metal and further includes a combustible sheath configured to cover some or all of the perforations in the liner. Such a sheath can be made of any suitable combustible material, including plastic or fibreglass, and acts as an accelerant during ignition of the coal seam.

7 [0037] Preferably, the combustible sheath is made of high density polyethylene (HDPE). The HDPE sheath can have a thickness of about 10 mm. [0038] In another embodiment, the sheath is made of a material with a low melting temperature, so that it degrades quickly upon commencement of the ignition process in the coal seam. For example, the sheath can be made of aluminium. [0039] The sheath can be attached to or associated with the well liner in any suitable way. For example, fasteners, such as screws, ring clamps, or straps (including stainless steel straps), can be used to bind the sheath to the well liner. Additionally, the sheath can be welded to the liner or integrated into it (e.g., sandwiched into a double-layer liner). [0040] Alternatively, where the sheath is made of plastic, it can be hot-rolled in place around the outside or the inside of the well liner, particularly in association with some or all of the perforations in the liner (i.e., the sheath covers the perforations). [0041] The well liner can include associated instrumentation such as one or more sensors for sensing and reporting conditions in the well liner, the linkage channel, and/or the surrounding coal seam. Any suitable type of sensor can be used. For example, the sensor can be a thermocouple for sensing the temperature, a gas sensor for sensing the nature of the product gas, a pressure sensor for sensing pressure, an optical sensor for viewing the well liner and/or the linkage channel, or a position sensor for reporting the location of the well liner within the linkage channel or the location of one or more tools in the lined well. [0042] The associated instrumentation can be connected to the well liner in any suitable way. For example, the perforations in the well liner can be used in the connection of the associated instrumentation. [0043] As will be understood by one of ordinary skill in the art, the step of igniting a coal seam preferably includes using an ignition tool, whereby an ignition tool that includes ignition means is inserted into the coal seam via the injection well, a service 8 well, and/or the production well. The ignition tool can optionally inject hydrogen peroxide into the coal seam. [0044] Once introduced into the coal seam, the ignition tool is used to ignite the coal seam and establish a combustion zone. The ignition tool can be positionable and retractable, so that ignition (including re-ignition) of the coal seam and the formation of combustion zones can be established in sequence. A preferred method is utilising the controlled retracting injection point (CRIP) concept. [0045] Positioning of the ignition tool can be achieved utilising coiled tubing connected to the ignition tool and extendible within the lined linkage channel to position the ignition tool at a desired location within the linkage channel. [0046] The ignition tool can be connected to the coiled tubing in any suitable way. The ignition tool can be connected to the coiled tubing in a fluid-tight manner. The ignition tool can be releasably connected or permanently connected to the coiled tubing. Preferably, the ignition tool is connected to an end of the coiled tubing by way of a quick connector (such as grapple, torque thru, dimple, etc.), screw thread, or weld. [0047] The coiled tubing can be of any suitable size, shape and construction and can be made of any suitable material or materials. More particularly, the coiled tubing can be of any suitable length and diameter. Preferably, the coiled tubing is made of metal, such as stainless steel, carbon steel, or copper. The coiled tubing can be of unitary construction or can include two or more connectable tube segments/pieces. A preferred outside diameter for the coiled tubing is 2.0 inches. [0048] The coiled tubing can include a single tube (line) connected to the ignition tool. The coiled tubing can alternatively include at least one inner tube (inner line) extending within an outer tube (outer line), wherein one or both of the inner and outer tubes are connected to the ignition tool. That is, the coiled tubing can include at least one inner tube and an outer tube that extend concentrically relative to one another. More than one inner tube may extend within the same outer tube. A preferred 9 diameter for the outer tube is 2.0 inches, whereas a preferred diameter for the inner tube is 0.75 inches. [0049] The coiled tubing can be maintained on a spool from which the coiled tubing is unspooled. Preferably, the spool can unspool coiled tubing having a length of about 300 metres, about 400 metres, about 500 metres, about 600 metres, about 700 metres, about 800 metres, about 900 metres, about 1,000 metres, about 1,200 metres, or about 1,500 metres. [0050] The ignition tool can ignite the coal seam in any suitable way. For example, the ignition tool can directly ignite the coal seam at any one of the perforations in the well liner, or ignite a combustible fuel (i.e., an ignition fuel) resident in the linkage channel (either inside or outside of the well liner) or supplied to the well channel (e.g., supplied as a gas, liquid, or solid). Suitable ignition fuels include, but are not limited to, hydrocarbon gases, for example, methane, propane, butane, and mixtures thereof. Furthermore, high purity hydrogen peroxide of the appropriate concentration can be used in the ignition process to enhance the likelihood of success, owing to its reactive nature. [0051] Alternatively, the ignition tool can indirectly ignite the coal seam via primary ignition of a combustible sheath configured to cover some or all of the perforations in the well liner. [0052] In one embodiment, the ignition means includes an electrical spark generator (e.g., a spark plug) and a power supply for generating the spark. The power supply can be located above ground or the spark generator can be powered by an in-seam turbine and transformer electrically connected to the spark generator. [0053] In another embodiment, the ignition means includes an electrical heat resistor (e.g., a glow plug) and a power supply for electrifying the resistor. The heat resistor can, for example, generate about 180 kW of heat. The power supply can be located above ground or the electrical heat resistor can be powered by an in-seam turbine and transformer electrically connected to the heat resistor.

10 [0054] In a further embodiment, the ignition means includes at least one type of ignition chemical. The ignition chemical can be a pyrophoric substance (e.g., a liquid, such as triethylboron (TEB), a gas, such as silane, a solid, such as phosphorus or an alkali metal), a pyrophoric substance and a hydrocarbon mixture, such as TEB vaporised in methane, or a pyrophoric substance and an inert gas, such as TEB and nitrogen. The hydrocarbon or inert gas flow can help transport/vaporise the pyrophoric substance to the ignition tool. [0055] Once the coal seam has been ignited (or re-ignited), the ignition tool is retracted to a safe distance within the linkage channel and hydrogen peroxide is injected into the linkage channel through an injection well (or a service well) to fuel/maintain combustion/gasification of the coal seam. Alternatively, the ignition tool can be withdrawn from the linkage channel following successful ignition (or re ignition) of the coal seam. [0056] According to an important aspect of the present invention, hydrogen peroxide injection into an in-seam channel in an underground coal seam as part of a method of underground coal gasification serves to provide both an oxidant (i.e., 02) and water for the UCG process. This is a particular advantage in areas where there is limited water (e.g., geological and/or above ground) available for the UCG process, as hydrogen peroxide decomposes exothermically into water and oxygen spontaneously (2 H 2 0 2 -- 2 H 2 0 + 02). Various concentrations of hydrogen peroxide can be injected downhole, depending on stoichiometric requirements of the reaction and how much oxygen/water is required for the UCG process. That is, the concentration of hydrogen peroxide can be adjusted to match the required rates of oxidant/water injection necessary to achieve appropriate downhole reaction conditions. [0057] As will be understood by one of ordinary skill in the art, the rate of hydrogen peroxide decomposition is temperature, concentration, and pH dependent The presence of impurities and stabilizers can also affect the rate of decomposition. [0058] Commercial grades of high concentration/purity hydrogen peroxide are readily available, including 30 wt% and higher concentrations (e.g., 68 to 98 wt%).

11 Additionally, hydrogen peroxide can be made by the electrolysis of sulphuric acid, which can optionally be produced as part of a downstream facility associated with an underground coal gasifier (i.e., on-site). Sulphur is extracted in such a downstream facility, oxidised to S03, and added to water to produce sulphuric acid. [0059] Hydrogen peroxide can also be produced by bubbling compressed air through a solution of derivatised anthracene, whereby the oxygen present in the air reacts with the labile hydrogen atoms (of the hydroxy group), giving hydrogen peroxide and regenerating anthraquinone. Hydrogen peroxide is then extracted and the anthraquinone derivative is reduced back to the dihydroxy (i.e., anthracene) compound using hydrogen gas (which is a component of UCG product gas) in the presence of a metal catalyst. [0060] A hydrogen peroxide source can include a tank/cylinder of liquid H 2 0 2 , as will be understood by one of ordinary skill in the art. The step of injecting hydrogen peroxide into a linkage channel in an underground coal seam can be achieved in any suitable manner (including, for example, via an ignition tool, as discussed herein). [0061] In one embodiment, injecting hydrogen peroxide into a linkage channel in an underground coal seam includes positioning a hydrogen peroxide injection device in the linkage channel via an injection well (or a service well) and providing H 2 0 2 to the injection device. The hydrogen peroxide injection device can be positionable and retractable. [0062] As with the ignition tool, positioning of the hydrogen peroxide injection device can be achieved utilising coiled tubing connected to the hydrogen peroxide injection device and extendible within the lined linkage channel to position the hydrogen peroxide injection device at a desired location within the linkage channel. The source of the hydrogen peroxide can be connected directly or indirectly to the coiled tubing associated with the hydrogen peroxide injection device in a fluid-tight manner, for introduction of the hydrogen peroxide into the linkage channel via the coiled tubing.

12 [0063] Alternatively, a source of hydrogen peroxide can be connected directly or indirectly to a well head of an injection well, such that the H 2 0 2 is injected/introduced into the linkage channel via an injection well. [0064] In one embodiment, the method of UCG disclosed herein further includes the step of injecting an oxidant into the linkage channel. [0065] The oxidant is preferably a gas such as air (approximately 20% oxygen), oxygen-enriched air (greater than 20% oxygen), or a gas/gas mixture (e.g., carbon dioxide and/or nitrogen in any desired ratio) enriched with oxygen (greater than 20% oxygen), or substantially pure oxygen. The oxidant source can include an air compressor, a tank/cylinder of compressed air or oxygen, an air separation unit, or a tank/cylinder of liquid oxygen, for example. [0066] The source of the oxidant can be connected directly or indirectly to the coiled tubing associated with the ignition tool and/or hydrogen peroxide injection device in a fluid-tight manner, for introduction of the oxidant into the linkage channel via the coiled tubing. As discussed herein, the coiled tubing can include two or more tubes (lines), in which case the tubes can extend concentrically (one within the other). An inner tube can feed an ignition fuel, an ignition chemical, and/or hydrogen peroxide to the ignition tool and/or hydrogen peroxide injection device, and an outer tube can feed an oxidant to the ignition tool and/or hydrogen peroxide injection device or vice versa, depending on the percent concentration of oxygen. [0067] Alternatively, the source of the oxidant can be connected directly or indirectly to a well head of an injection well such that the oxidant is injected/introduced into the linkage channel via an injection well rather than the coiled tubing. Accordingly, for an ignition tool and/or hydrogen peroxide injection device having a single tube (i.e., single coil tubing), oxidant injected into the linkage channel via an injection well can flow externally of the single coil tubing. [0068] In addition to injecting hydrogen peroxide and, optionally, oxidant into a linkage channel through an injection well (or a service well), the disclosed UCG method optionally further includes the step of injecting carbon dioxide into the linkage 13 channel and/or the production well. The carbon dioxide can be injected into the linkage channel via the injection well and/or a service well. The carbon dioxide can be recycled from a downstream processing facility that separates carbon dioxide from product gas. [0069] In one embodiment, the injected carbon dioxide acts as a quenching fluid to reduce the temperature of the product gas to less than about 200-600 0C, including less than about 550 C, less than about 500 C, less than about 450 C, less than about 400 C, less than about 350 C, less than about 300 C, and less than about 250 C. [0070] The quenching fluid (i.e., carbon dioxide) can be injected into the linkage channel and/or the production well in any suitable way. For example, the quenching fluid can be delivered using a quenching fluid delivery system to deliver the quenching fluid and this can be of any suitable size, shape and construction. [0071] In one embodiment, the quenching fluid delivery system can include an injection well, a service well, and or a production well for conveying the quenching fluid to the product gas stream. The delivery system can further include a circulation pump and fluid reservoir connected to a well head of the well for pumping the quenching fluid into the well. Alternatively, the delivery system can further include a gas compressor connected to the well head for injecting quenching gas into the well. [0072] In another embodiment, the quenching fluid delivery system can include a tube for conveying the quenching fluid and optionally a nozzle or pig tail fitted to a lower end of the tube for spraying the quenching fluid into the product gas stream. The tube can be flexible such that it can be unwound from a spool. The delivery system can further include a circulation pump and fluid reservoir connected to an upper end of the tube. Alternatively, the delivery system can further include a gas compressor connected to an upper end of the tube. [0073] As will be understood by one of ordinary skill in the art, reducing the temperature of the product gas via a quenching fluid may require that the temperature of the product gas in-seam and/or within a well (e.g., a production well) 14 be monitored, and the injection rate and quantity of quenching fluid be regulated according to the temperature reading. To that end, the quenching fluid delivery system can include at least one thermocouple (located in-seam or within a well) electrically connected to a computer-operable valve for regulating flow of the quenching fluid. [0074] In one embodiment, the injection of carbon dioxide and/or hydrogen peroxide can be used to adjust the composition of the product gas. For example, adjusting the ratio of injected carbon dioxide to oxidant (including H 2 0 2 ) can be used to maintain hydrogen to carbon monoxide ratio in the product gas of between about 1.5 and 4.0, including between about 1.6 and 3.0 and between about 1.8 and 2.5. [0075] As will be understood by one of ordinary skill in the art, adjusting the composition of the product gas via injection of carbon dioxide and/or hydrogen peroxide may require that the composition of the product gas be monitored, and the injection of one or more of carbon dioxide and/or hydrogen peroxide be regulated according to the product gas composition. To that end, a gas analyser coupled with a microprocessor can be employed to monitor the composition of the product gas and/or control the injection of one or more of carbon dioxide and/or hydrogen peroxide. [0076] In certain embodiments, the disclosed method optionally further includes the step of repositioning the ignition tool to a new ignition site in the linkage channel and/or the step of repositioning the hydrogen peroxide injection device to a new location in the linkage channel. That is, in order to continue the UCG process through the coal seam, it may be necessary to reposition the ignition tool to a new ignition site and/or reposition the hydrogen peroxide injection device to a new location in the linkage channel. The coal in the vicinity of the new ignition site can be ignited (or re ignited) and a new combustion zone created to progress the combustion along the coal seam to produce optimum consumption of the coal seam. [0077] Referring to Figure 1, there is generally depicted an underground coal gasifier 10 illustrating certain aspects of the invention. A coal seam 11 is located underground and surrounded by overburden 12 and underburden 13, and includes 15 an 8.5" inside diameter in-seam well channel 16 linking an injection well 15 and a production well (which is not shown) further downstream. The coal seam 11 is ignited using an ignition tool (which is not shown). An oxidant flow 14 is optionally injected into the well channel 16 via the injection well 15 to maintain combustion/gasification of the coal seam 11. The injection well 15 includes a 9.625" outside diameter steel casing 17, cement 18, and a 5.5" outside diameter steel casing 19. [0078] The well channel 16 is lined at the injection end with a 5.5" outside diameter steel well liner 20, having perforations that are grouped together in one or more regions along the length of the liner, alternating with non-perforated sections of the liner (which is not shown). An optional 5.5" outside diameter firebreak casing section 21 is inserted between the 5.5" outside diameter steel casing 19 and the 5.5" outside diameter steel well liner 20. At the production end, the well channel 16 is lined with a 7.0" outside diameter steel perforated well liner 22. [0079] A hydrogen peroxide 23 injection device 24 is positioned in the well channel 16 and provides H 2 0 2 delivery 25 to the coal seam 11 and/or a gasifier cavity (which is not shown). Positioning of the hydrogen peroxide injection device 24 is accomplished utilising a 2.0" outside diameter coiled stainless steel tubing 26 connected to the hydrogen peroxide injection device 24 and extendible/retractable within the well channel 16 to a desired location within the well channel 16. The coiled tubing 26 is maintained on a spool 27 from which the coiled tubing 26 is unspooled/respooled. [0080] The coal seam 11 is further provided with one or more service wells 28. The service wells 28 are vertical wells located between the injection well 15 and the production well (which is not shown), and can be used as ignition wells and/or auxiliary injection/production wells. The service wells 28 include an 8.5" inside diameter bore hole 29 into the coal seam 11, 5.5" outside diameter steel casing 19, and cement 18. [0081] In use, any combination of oxidant 14 and hydrogen peroxide 23 can be used for combustion/gasification of the coal seam 11. That is, combinations of oxidant 14 and hydrogen peroxide 23 can be adjusted to achieve desired downhole 16 reaction conditions for efficient resource recovery. As described herein, oxidant 14 and hydrogen peroxide 23 can be delivered to the well channel 16 via the ignition tool (which is not shown) and/or the hydrogen peroxide injection device 24. [0082] The underground coal gasifier 10 produces syngas, which moves through the well channel 16 to a production well (which is not shown) further downstream. [0083] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics can be combined in any suitable manner in one or more combinations. [0084] Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. It will therefore be appreciated by those of skill in the art that, in light of the instant disclosure, various modifications and changes can be made in the particular embodiments exemplified without departing from the scope of the present invention.

Claims (22)

1. A method of underground coal gasification in a coal seam provided with an injection well, a production well and an in-seam channel linking the injection well and the production well, including the steps of: a. inserting a perforated liner into the linkage channel; b. igniting the coal seam; c. injecting hydrogen peroxide into the linkage channel through the injection well to maintain combustion/gasification of the coal seam; and d. withdrawing product gas from the production well.

2. The method of claim 1, wherein the coal seam is further provided with one or more service wells.

3. The method of claim 1 or claim 2, wherein the liner is made of metal, fibreglass, carbon fibre, plastic, or combinations thereof.

4. The method of any one of claims 1 to 3, wherein the perforations in the well liner are grouped together in one or more regions along the length of the liner, alternating with non-perforated sections of the liner.

5. The method of any one of claims 1 to 4, wherein the liner is made of metal and further comprises a combustible sheath.

6. The method of claim 5, wherein the combustible sheath is made of high density polyethylene.

7. The method of claim 5 or claim 6, wherein the combustible sheath covers at least some of the perforations in the liner.

8. The method of any one of claims 5 to 7, wherein the combustible sheath is fastened into place around the outside of the liner with straps. 18

9. The method of any one of claims 5 to 7, wherein the combustible sheath is hot-rolled in place inside or outside the liner.

10. The method of any one of claims 1 to 9, wherein one or more thermocouples are attached to the liner for collection of temperature data from the liner during underground coal gasification.

11. The method of any one of claims 1 to 10, wherein igniting the coal seam comprises positioning an ignition tool in the linkage channel at one or more of the perforations in the liner, providing an ignition fuel to the ignition tool, and igniting the coal seam using the ignition tool, wherein the ignition tool comprises ignition means.

12. The method of claim 11, wherein the ignition fuel comprises a hydrocarbon gas.

13. The method of claim 12, wherein the hydrocarbon gas is methane.

14. The method of any one of claims 11 to 13, wherein the ignition means comprises an electrical spark generator and a power supply for generating the spark.

15. The method of any one of claims 11 to 13, wherein the ignition means comprises an electrical heat resistor and a power supply for electrifying the resistor.

16. The method of any one of claims 11 to 13, wherein the ignition means comprises an ignition chemical.

17. The method of claim 16, wherein the ignition chemical is a pyrophoric substance.

18. The method of any one of claims 1 to 17, further including the step of injecting an oxidant into the linkage channel.

19. The method of claim 18, wherein the oxidant is air. 19

20. The method of claim 18 or claim 19, wherein the oxidant is injected into the linkage channel via the injection well and/or a service well.

21. The method of any one of claims 1 to 20, further including the step of injecting carbon dioxide into the linkage channel.

22. The method of claim 21, wherein the carbon dioxide is injected into the linkage channel via the injection well and/or a service well.