[0001 ] This invention relates to methods of treating synthesis gas in situ within an underground coal gasifier (UCG). One aspect concerns a method of promoting in situ methanation of synthesis gas (syngas) so as to yield product gas enriched in methane.

Another aspect concerns an in situ method of reducing the sulphur content of synthesis gas.

BACKGROUND ART

[0002] Methods of methanating synthesis gas and for removing sulphur from synthesis gas are known and typically require an above-ground gas processing facility. Methods of methanating synthesis gas above-ground usually include removing sulphur from the synthesis gas, adjusting the hydrogen to carbon monoxide ratio of the gas, heating the gas to reactor temperature, and methanating the gas using a catalyst (e.g., nickel).

SUMMARY OF INVENTION

[0003] The present inventors have now developed a method of promoting in situ methanation of synthesis gas within an underground coal gasifier as well as a method of reducing the sulphur content of synthesis gas in situ within an underground coal gasifier.

[0004] According to an aspect of the present invention, there is provided a method of promoting in situ methanation of synthesis gas within an underground coal gasifier, the method including the step of introducing a methanation catalyst into the gasifier so as to promote methanation of synthesis gas within the gasifier.

[0005] According to another aspect of the present invention, there is provided a method of promoting in situ methanation of synthesis gas within an underground coal gasifier, the method including the step of pneumatically conveying a dust or particulate methanation catalyst into a methanation cavity located downstream of a gasification cavity of the underground coal gasifier via a service well, wherein the methanation catalyst is iron ore/iron oxide (Fe ₂ 0 ₃ or Fe ₃ 0 ₄ ). [0006] According to yet another aspect of the present invention, there is provided a method of promoting in situ methanation of synthesis gas within an underground coal gasifier, the method including the step of introducing a methanation catalyst into a service well, a horizontal well passage or a production well of the underground coal gasifier, wherein the methanation catalyst is iron ore/iron oxide (Fe^CH or Fe30 ).

[0007] According to a further aspect of the present invention, there is provided an underground coal gasifier comprising a methanation catalyst for promoting in situ

methanation of synthesis gas within the gasifier.

[0008] According to yet a further aspect of the present invention, there is provided methane enriched product gas when produced by the method according to the aspects described herein or the gasifier according to the aspect described herein.

[0009] Any suitable type and form of methanation catalyst can be used, and the catalyst can be introduced into the gasifier in any suitable way. In one embodiment of the invention, the methanation catalyst can be introduced into the gasifier pneumatically as a dust or particulate. The methanation catalyst can be introduced into one or more cavities of the gasifier. In another embodiment of the invention, the methanation catalyst can be in the form of a fixed bed catalytic reactor or a well liner (including a part portion of a well liner), or a catalyst associated with a well liner (including a part or portion thereof), introduced into an ignition, production and/or service well, and/or cavity or horizontal well passage of the gasifier.

[0010] The catalyst can be, for example, of the type that is not readily poisoned by sulphur. The catalyst can be, for example, a metal. Such metals may include, but are not limited to, Group rV(B), V(B), VI(B), or VIII metals, such as, for example, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Reduced iron, for example, is a metal which catalyses methanation in the presence of sulphur and also catalyses water gas shift reactions.

[0011 ] The catalyst can be a mixture or combination of different metal types and/or metal compounds (e.g., different types of metal oxides). The catalyst can be, for example, a (reduced) metal which promotes methanation such that at least about 20% volume/volume of the product gas exiting the production well includes methane. Preferably, the catalyst is iron ore/iron oxide (Fe ₂ 0 ₃ or which is reduced in situ by hydrogen and carbon monoxide in the presence of steam resident within the gasifier.

[0012] The catalyst can be introduced into an ignition, production or service well, or cavity or horizontal well passage of the gasifier in any suitable way. If introduced into a cavity of the gasifier, it can be introduced by way of at least one service well, or the production well or injection well.

[0013] In another embodiment, the method includes the step of introducing the catalyst either directly or indirectly into a cavity of the gasifier in a fluid form. Preferably, the catalyst is pneumatically conveyed as a particulate (dust) within a fluid stream, allowing the catalyst to remain airborne/suspended within the cavity, mixing with synthesis gas resident within the cavity, and providing a large surface area for catalysing methanation of the synthesis gas. The catalyst is preferably pneumatically conveyed at a rate so as to continuously promote methanation of the synthesis gas.

[0014] Preferably, catalyst that is introduced pneumatically has an average particle size of about 150 microns, although the average particle size can be up to about ten-fold smaller (i.e., aboutl5 microns) or greater (i.e., about 1,500 microns) than this, or any range in between (e.g., about 25, 50, 75, 100, 125, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, 1,000, or 1,250 microns). The catalyst can be introduced into the cavity continuously (in the event that it has low residence time within the cavity or is poisoned or otherwise rendered inactive) at a rate of about 10 to 100 kg per hour, for example, including 20, 30, 40, 50, 60, 70, 80, or 90 kg/hour.

[0015] Methanation occurs optimally when the carbon dioxide to hydrogen ratio is 1 :4 or the carbon monoxide to hydrogen ratio is 1 :3. Therefore, the fluid stream that pneumatically conveys the catalyst can include any suitable gas or gas mixture optimised to produce these reagent ratios in the synthesis gas. That is, injected gas type(s) and ratios can be adjusted to achieve a target carbon dioxide to hydrogen ratio of 1 :4 and/or a target carbon monoxide to hydrogen ratio of 1 :3 in the synthesis gas. One of ordinary skill in the art will be able to optimise a suitable injection gas or gas mixture to produce these target reagent ratios. In practice, only minor adjustments may need to be made to the synthesis gas generated within a gasifier cavity.

[0016] The method can also include the step of injecting an oxidant mixture optimised to produce these reagent ratios. The oxidant mixture can include any suitable gas or gas mixture. The oxidant mixture can include, for example, 20%, 30%, 40%, or 50% volume volume oxygen and 80%, 70%, 60%, or 50% volume/volume carbon dioxide, respectively.

Alternatively, air or an inert gas such as nitrogen can be used, but the inert gas may need to be removed from the product gas above ground in a gas clean-up step.

[0017] In a further embodiment, the method includes the step of introducing the catalyst into an ignition, production or service well, or cavity or horizontal well passage of the gasifier in the form of a fixed bed catalytic reactor or well liner (including a part/portion of a well liner or associated with a well liner).

[0018] The well liner can be of any suitable size, shape and construction, and can be made of metal, or other appropriate material, as will be known to one of ordinary skill in the art. In one embodiment, the well liner can include a metal pipe containing, coated with and/or infused with catalyst for promoting methanation. The entire length of the pipe can be coated and/or infused with such catalytic material, or only with regard to a specific segment, such as on a vertical section of the production well. In another embodiment, the well liner can include a jacketed metal pipe having an inner pipe and an outer pipe with catalytic material contained between the inner and outer pipes.

[0019] Methanation within the gasifier/cavity can be performed at any suitable temperature. A suitable temperature range is about 200-600 °C (e.g., about 250, 300, 350, 400, 450, 500, or 550 °C), more preferably about 300-550 °C and even more preferably about 500 °C.

[0020] Methanation within the gasifier/cavity can be carried out at any suitable pressure. Generally speaking, the higher the pressure, the greater the degree of methanation. The pressure range can be anywhere from atmospheric pressure to any upper pressure level, which in practice is usually up to about 150 bar. A more preferred pressure range is about 4 bar to 150 bar (e.g., about 10, 15, 20, 25, 30, 35, 40, 45, 50, 75, 100, or 125 bar). [0021 ] Synthesis gas can have any suitable residence time within the gasifier/cavity to undergo methanation. For example, the residence time can be at least about 30 minutes. As will be understood by one of ordinary skill in the art, the residence time will depend on the gasifier/cavity size and the synthesis gas flow rate through the gasifier/cavity.

[0022] Typically, the cavity in which methanation occurs ("methanation cavity") will be located downstream of a gasification cavity/zone of the gasifier. Even more typically, the methanation cavity will be located downstream of a cavity in which synthesis gas cools as it moves from the gasification cavity/zone to the methanation cavity.

[0023] In another embodiment, the method can include the step of introducing a

methanation catalyst capable of promoting methanation into a second methanation cavity so as to further catalyse methanation of synthesis gas resident within that cavity. It is to be appreciated that the gasifier could have three, four or even more methanation cavities in total. The methanation catalyst can be as described herein.

[0024] If the gasifier has a second methanation cavity, that cavity can be located

downstream of the first methanation cavity and adjacent an upstream cooling cavity. This is because the methanation reaction is exothermic and produces heat within the methanation cavity and the heat of methanated synthesis gas exiting the first methanation cavity needs to be lowered before entering the second methanation cavity for further methanation.

[0025] Optimally, the sequence of cavities of an underground coal gasifier from upstream to downstream is as follows: (1) cooling cavity/zone downstream of the gasification cavity/zone; (2) methanation cavity; (3) additional cooling cavity; (4) second methanation cavity;

optionally, one or more repeats of (1) to (3); (5) production well. Such a sequence of cavities can be produced using a retractable injection point system (such as CRIPS) with an ignition burner that is moved from the production well towards the injection well (i.e., downstream to upstream direction). The catalyst can be introduced into each methanation cavity or cooling cavity via a service well or service wells.

[0026] In a further embodiment, the method can include the step of treating the methane enriched product gas which exits the production well, and this can be done in any suitable way. Typical gas processing steps include (1) removing or lowering the particulate dust content (although the majority is likely to be trapped in the underneath ash rubble), (2) removing or lowering the carbon dioxide content, (3) removing or lowering the trace carbon monoxide content, and/or (4) separating hydrogen from the product gas, so as to produce a product stream further enriched in methane.

[0027] Particulates/catalyst-containing ash can be removed from the product gas using a cyclone separator of water wash/scrubber, for example.

[0028] Carbon dioxide and hydrogen sulphide can be removed from the product gas using cryogenic technology, for example.

[0029] The treated product gas will primarily constitute methane and hydrogen, and can be used as a natural gas substitute or can be subjected to further separation, processing or treatment steps. For example, the hydrogen can be separated from the methane using pressure swing adsorption (PSA).

[0030] In one embodiment, the method preferably includes pneumatically conveying metal catalyst particulates/dust (e.g., iron ore/iron oxide (Fe ₂ C>3 or FejO-j)) into a cooling or methanation cavity via one or more service wells such that synthesis gas entering the methanation cavity mixes with suspended metal catalytic particles, undergoes methanation, and then moves to the production well as a product gas having at least about 20%

volume/volume methane. Typically the methanation cavity temperature will be about 200- 600 °C, the methanation cavity pressure will be up to about 150 bar and synthesis gas residence time will be at least 30 minutes (either in the one methanation cavity or within all methanation cavities of the gasifier combined).

[0031 ] According to a further aspect of the present invention, there is provided a method of reducing the sulphur content of synthesis gas in situ within an underground coal gasifier, the method including the step of introducing a sulphur reducer into the gasifier so as to reduce the sulphur content of synthesis gas within the gasifier. For example, sulphur content of synthesis gas can be reduced by 50-95%, including 60%, 70%, 75%, 80%, 85%, or 90%. [0032] According to another aspect of the present invention, there is provided an underground coal gasifler comprising a sulphur reducer for reducing the sulphur content of synthesis gas within the gasifier.

[0033] According to yet another aspect of the present invention, there is provided sulphur- reduced product gas when produced by the method according to the aspects described herein or the gasifier according to the aspect described herein.

[0034] Any suitable type and form of sulphur reducer can be used, and the sulphur reducer can be introduced into the gasifier in any suitable way. In one embodiment of the invention, the sulphur reducer can be a reagent or a catalyst, introduced into the gasifier pneumatically as a dust or particulate. The sulphur reducer can be introduced into one or more cavities of the gasifier. In another embodiment of the invention, the sulphur reducer can be in the form of a fixed bed catalytic reactor or a well liner (including a part portion of a well liner), or a catalyst associated with a well liner (including a part or portion thereof), introduced into an ignition, production and/or service well, and/or cavity or horizontal well passage of the gasifier.

[0035] In one embodiment, the sulphur reducer is a metal or a mixture or combination of different metal types and/or metal compounds (e.g., different types of metal oxides). .

Preferably, the metal is iron ore iron oxide (Έ^ _Ϊ or Fe30 ₄ ).

[0036] The sulphur reducer can be introduced into an ignition, production or service well, or cavity or horizontal well passage of the gasifier in any suitable way. If introduced into a cavity of the gasifier, it can be introduced by way of at least one service well or the production well or injection well.

[0037] In another embodiment, the method includes the step of introducing the sulphur reducer either directly or indirectly into a cavity of the gasifier in a fluid form. Preferably, the sulphur reducer is pneumatically conveyed as a particulate (dust) within a fluid stream, allowing the sulphur reducer to remain airborne/suspended within the cavity, mixing with synthesis gas resident within the cavity, and providing a large surface area for interacting with sulphur species within the synthesis gas. The catalyst is preferably pneumatically conveyed at a rate so as to continuously promote reduction of the sulphur content of the synthesis gas. [0038] Preferably, the sulphur reducer that is introduced pneumatically has an average particle size of about 150 microns, although the average particle size can be up to about tenfold smaller (i.e., about 15 microns) or greater (i.e., about 1,500 microns) than this, or any range in between (e.g., about 25, 50, 75, 100, 125, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, 1,000, or 1,250 microns). The sulphur reducer can be introduced into the cavity continuously, at a rate of about 10 to 100 kg per hour, for example, including 20, 30, 40, 50, 60, 70, 80, or 90 kg/hour.

[0039] In a further embodiment, the method includes the step of introducing the sulphur reducer into an ignition, production or service well, or cavity or horizontal well passage of the gasifier in the form of a fixed bed reactor or well liner (including a part portion of a well liner or associated with a well liner).

[0040] The well liner can be of any suitable size, shape and construction, and can be made of metal, or other appropriate material, as will be known to one of ordinary skill in the art. In one embodiment, the well liner can include a metal pipe containing, coated with and/or infused with a sulphur reducer. The entire length of the pipe can be coated and/or infused with such material, or only with regard to a specific segment such as on a vertical section of the production well. In another embodiment, the well liner can include a jacketed metal pipe having an inner pipe and an outer pipe with sulphur reducing material contained between the inner and outer pipes.

[0041] Sulphur reduction within the gasifier/cavity can be performed at any suitable temperature. A suitable temperature range is about 200-600 °C (e.g., about 250, 300, 350, 400, 450, 500, or 550 °C), more preferably about 300-550 °C and even more preferably about 500 °C.

[0042] Sulphur reduction within the gasifier/cavity can be carried out at any suitable pressure. The pressure range can be anywhere from atmospheric pressure to any upper pressure level, which in practice is usually up to about 150 bar. A more preferred pressure range is about 4 bar to 150 bar (e.g., about 10, 15, 20, 25, 30, 35, 40, 45, 50, 75, 100, or 125 bar). [0043] Synthesis gas can have any suitable residence time within the gasifier/cavity to undergo sulphur reduction. For example, the residence time can be at least about 30 minutes. As will be understood by one of ordinary skill in the art, the residence time will depend on the gasifier/cavity size and the synthesis gas flow rate through the gasifier/cavity.

[0044] Typically, the cavity in which sulphur reduction occurs ("sulphur reduction cavity") will be located downstream of a gasification cavity/zone of the gasifier. Even more typically, the sulphur reduction cavity will be located downstream of a cavity in which synthesis gas cools as it moves from the gasification cavity/zone to the sulphur reduction cavity.

[0045] In another embodiment, the method can include the step of introducing a sulphur reducer capable of reducing sulphur content of the synthesis gas into a second cavity so as to further promote sulphur reduction of synthesis gas resident within that cavity. It is to be appreciated that the gasifier could have three, four or even more sulphur reduction cavities in total. The sulphur reducer for these further cavities can be as described herein.

[0046] In a further embodiment, the method can include the step of treating the sulphur reduced product gas which exits the production well and this can be done in any suitable way. Typical gas processing steps include (1) removing or lowering the particulate/dust content (although the majority is likely to be trapped in the underneath ash rubble), (2) removing or lowering the carbon dioxide content, (3) removing or lowering the trace carbon monoxide content, and/or (4) separating hydrogen from the product gas, so as to produce a product stream further enriched in methane; as described herein.

[0047] In one embodiment, the method preferably includes pneumatically conveying metal particulates/dust (e.g., iron ore/iron oxide (Fe ₂ C>3 or Fe3C>4)) into a cooling or sulphur reduction cavity via one or more service wells such that synthesis gas entering the sulphur reduction cavity mixes with suspended metal particles, undergoes sulphur reduction, and then moves to the production well as a product gas having a greatly reduced sulphur content.

[0048] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 is a schematic of an underground coal gasifier having a methanation catalyst in the form of a suspended catalyst, according to an embodiment of the present invention.

[0050] Figure 2 is a block flow diagram for downhole in situ synthesis gas methanation and above-ground product gas treatment, according to an embodiment of the present invention.

[0051 ] Figure 3 is a schematic of an underground coal gasifier having a methanation catalyst in the form of a catalytic bed, according to an embodiment of the present invention.

[0052] Figure 4 is a view of a well liner with associated methanation catalyst for application in an underground coal gasifier, according to an embodiment of the present invention.

[0053] Figure 5 is a view of another type of well liner with associated methanation catalyst for application in an underground coal gasifier, according to an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

[0054] The inventors have determined that, by introducing a methanation catalyst into an underground coal gasifier, synthesis gas can be methanated downhole in situ and a product gas enriched in methane can be produced. After suitable minimal treatment above-ground, the methane enriched product gas can be piped into natural gas pipelines.

[0055] The inventors have further determined that a metal catalyst, such as iron ore/iron oxide (Fe ₂ 0 ₃ or Fe ₃ 0_j), can be used as a downhole methanation promoter (and/or sulphur reducer), either in the form of catalyst dust conveyed pneumatically into one or more gasifier cavities, or in the form of a fixed bed catalytic reactor or well liner (including a part/portion of a well liner or associated with a well liner), situated within an ignition well, a production well, a service well, a horizontal passage, and/or a cavity of the gasifier.

[0056] In either case, sulphur-tolerant iron ore/iron oxide (Fe ₂ 03 or Έ^Ο^) is used to convert carbon monoxide and hydrogen into methane inside the gasifier in a temperature range of between about 200 and 600 °C. Iron oxide has been found to both catalyse the methanation and water gas shift reactions.

[0057] The downhole methanation reaction is:

CO + 3H ₂ →CH4 + H ₂ 0 dH = -206.2 kJ/mol

CO2 + 4H2 -+ CH + 2 H2O dH = -165.0 kJ/mol

[0058] Equilibrium constants are listed in Tables 1 and 2 below: TABLE 1

Methanation reaction equilibrium constants

TABLE 2

Shift reaction equilibrium constants

Temperature (°C) K, Shift

227 137.2

327 28.4

427 9.38

527 4.2

627 2.29

727 1.43 [0059] The inventors have determined that the methanation reaction can be performed downhole in the temperature range of 200 to 600 °C, and more preferably 300 to 500 °C. From thermodynamics, the equilibrium reaction to produce methane downhole is favoured at high pressures and at low temperatures.

[0060] The inventors have determined that typically along with methanation, water gas shift is also catalysed by iron. This can be performed downhole in the temperature range of 200 to 400 °C. From thermodynamics, the equilibrium reaction to produce H ₂ downhole is favoured at low temperature. The equilibrium is not affected by pressure downhole.

[0061 ] As mentioned, the inventors have determined that iron-based catalysis of the methanation reaction is much less prone to sulphur poisoning than nickel-based catalysis and so can be used downhole in a sulphur-prone synthesis gas. This is primarily because part of the iron oxide reacts with hydrogen sulphide (¾S) and the remainder is available for methanation catalysis.

[0062] Iron oxide (Fe ₂ 0 ₃ ) removes the sulphur in synthesis gas via the reaction: 3Fe ₂ 0 ₃ + ¾→ 2Fe ₃ 0 ₄ + H ₂ 0 Fe ₃ 0 ₄ + H ₂ + 3H ₂ S→ 3FeS + 4 H ₂ 0

[0063] Thus, the amount of iron ore iron oxide (Fe ₂ 0 ₃ or Fe ₃ C>4) injected into the gasifier must exceed the amount that would react with hydrogen sulphide.

[0064] So that the invention may be readily understood and put into practical effect, the following non-limiting Examples are provided.

EXAMPLES

Example 1 : Downhole Suspended Catalyst

[0065] This example describes the use of iron ore/iron oxide (Fe ₂ 0 ₃ or Fe ₃ 04) as a downhole methanation catalyst in the form of dust conveyed pneumatically into gasifier cavities. [0066] A typical gasifier 1 is depicted in Figure 1. The gasifier 1 as depicted has (from upstream to downstream in coal seam 2) an injection well 3, an active gasification cavity 4, a first cooling cavity 5, a first service well 6, a first methanation cavity 7, a second cooling cavity 8, a second service well 9, a second methanation cavity 10, and a production well 11.

[0067] The cavities 4, 5, 7, 8, and 10 are produced by retracting the injection point from the production well 11 toward the injection well 3. As a forming cavity hits the overburden 12, a new cavity is ignited upstream. Thus the active cavity 4 is always the first cavity of the gasifier 1. Cavities 5, 7, 8, and 10 downstream of the active cavity 4 cool down due to water ingress, and likewise cool down any synthesis gas that passes there through.

[0068] Iron oxide was milled down to an average particle size of about 150 microns and pneumatically conveyed as a dust 13 in C(½ down service wells 6 and 9 to methanation cavities 7 and 10. Iron oxide was continuously conveyed to the cavities 7 and 10 at a rate of about 50 kg hour (for a 3000 Nm ³ /h synthesis gas gasifier which was air blown) so as to maintain good suspended catalytic activity. The iron oxide 13 was reduced by hydrogen gas and carbon monoxide in the presence of stream that was resident in the gasifier 1.

[0069] The gasifier 1 was at a pressure of about 60 bar. Coal was gasified in the active gasification cavity 4 at a temperature of about 800-1000 °C to produce synthesis gas. The injected oxidant included about 40% oxygen and about 60% carbon dioxide. Synthesis gas entering the first cooling cavity 5 cooled to about 400 °C due to the natural ingress of water into the cavity 5. Synthesis gas entering the first methanation cavity 7 mixed with suspended iron oxide dust 13, underwent methanation and raised the cavity 7 temperature by about 190 °C. The residence time of the synthesis gas within the cavity 7 was about 30 minutes.

[0070] Synthesis gas then entered the adjacent second cooling cavity 8 and cooled again to about 400 °C due to the natural ingress of water into the cavity 8. Synthesis gas entering the second methanation cavity 10 mixed with suspended iron oxide dust 13, underwent further methanation and raised the cavity 10 temperature by about 90 °C. The residence time of the synthesis gas within the cavity 10 was about 30 minutes before moving to the production well 11 as a final product gas. The product gas included about 20% volume/volume methane, along with carbon dioxide, hydrogen and long hydrocarbon chains (in minority) such as paraffins and olefins. (Product gas mole fractions: hydrogen 0.070; carbon monoxide 0.011; methane 0.229; and carbon dioxide 0.686.)

[0071] Assuming injected oxidant including about 40% oxygen (fed at about 80.4 kmol/hour) and about 60% carbon dioxide (fed at about 120.6 kmol/hour), product gas including hydrogen 7.4 mol%, 26.84kmol/hour; methane 22 mol%, 79.81 kmol/hour; and ^\ carbon dioxide 69.4 mol%, 251.74 kmol hour can be produced. The product gas, after separation, can yield 25% hydrogen and 75% methane, having a heating value of about 30MJ/ Nm ³ . This can be further separated into hydrogen and methane using PSA, as required.

[0072] Iron oxide acted as a suspended catalyst for the methanation and sulphur removal reactions. The iron oxide dust 13 remained airborne/suspended within the cavities 7 and 10, mixed well with synthesis gas resident within the cavities 7 and 10, and provided a large surface area for catalysing methanation of the synthesis gas and sulphur removal.

[0073] The bulk of the iron oxide was trapped underground in coal ash rubble while the remainder can be separated from the product gas as ash above ground with water wash column or cyclone treatment steps, prior to the product gas being piped into natural gas pipelines.

[0074] The inventors have determined that high gasifier pressure is desirable for methanation and water influx is not detrimental to the chemical process, although it will increase oxygen requirements.

[0075] The inventors have determined that it is advantageous to use empty cavities of a multicavity gasifier to maximise methane production, because the empty cavities downstream of active cavities have low temperatures (e.g., about 400-500 °C), which are favourable to these reactions, and maximum carbon monoxide concentration. Heat of reaction can be absorbed in old gasifier cavities without need for temperature regulation.

[0076] Figure 2 is a block flow diagram for downhole in situ synthesis gas methanation and above-ground product gas treatment, according to an embodiment of the present invention. [0077] The figure shows a remote underground coal gasification site 20 having an underground gasifier 23 having a minimal surface facility. An oxidant feed pipeline 21 extends from an oxidant compressor 22 to an injection well of the gasifier 23. An iron oxide feed pipeline 24 extends from an iron oxide injection facility 25 to a service well of the gasifier 23. A (methane rich) product gas pipeline 26 conveys product gas from a production well of the gasifier 23 to a central processing facility 27 over the life of the gasifier 23 whereby the product gas is treated.

[0078] At the central processing facility 27, the trace carbon monoxide component of the product gas can be converted to methane, residual sulphur and other contaminants can be removed as in any synthesis gas processing facility. The carbon dioxide component of the product gas can be separated and, together with newly added oxygen, can be piped to the oxidant compressor via an oxidant pipeline 28. The methane component of the product gas can be separated and conveyed to a natural gas pipeline 29 as finished product. The hydrogen can be separated using PSA and piped into a hydrogen pipeline. Any excess carbon dioxide 30 can be used for any other purpose.

Example 2: Downhole Catalytic Bed

[0079] This example describes the use of iron ore/iron oxide (Ρ¾θ3 or Fe3C>4) as a downhole methanation catalyst in the form of one or more beds of catalyst placed downhole in the synthesis gas flow path, either in a production well or service well, or a horizontal channel.

[0080] Figure 3 is a schematic of an underground coal gasifier 1 having a methanation catalyst in the form of a catalytic bed 15. An iron oxide catalyst bed 15 is delivered to the relevant part of the gasifier (production well 11 or service well 6 or 9, or horizontal channel

Ι ⁴ )·

[0081 ] The conditions and parameters can be substantially the same as described for Example 1.

Example 3 : Downhole Catalytic Well Liner [0082] This example describes the use of iron ore/iron oxide (Fe ₂ 0 j or e^Oi) as a downhole methanation catalyst in the form of a catalytic well liner, placed downhole in the production well in the synthesis gas flow path.

[0083] As shown in Figure 4, the well liner 40 is a metal pipe containing, coated with and/or infused with iron oxide 41. The entire length of the well liner 40 can be coated and/or infused with such catalytic material 41, or only with regard to a specific segment, such as on a vertical section of the production well.

[0084] Alternatively, as shown in Figure 5, the well liner 50 can include a dual concentric pipe arrangement having an inner pipe 51 and an outer pipe 52, with iron oxide 53 contained between the inner pipe 51 and the outer pipe 52.

[0085] The conditions and parameters can be substantially me same as described for Example 1.

[0086] 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.

[0087] 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.