IMPROVED UNDERGROUND COAL GASIFICATION METHOD AND APPARATUS

TECHNICAL FIELD

[0001] This invention relates to a method and apparatus for increasing the rate of coal combustion within a combustion zone of an underground coal gasifier. In particular, a method and apparatus for introducing a metal salt solution into a combustion zone of an underground coal gasifier are disclosed.

BACKGROUND ART

[0002] Underground coal gasification (UCG) 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 to allow for oxidant injection and product gas extraction. The wells are linked or extended to form an in-seam well channel (also referred to as a "linkage channel") to facilitate oxidant injection, cavity development and product gas flow. 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 can also utilise one or more vertical wells (service wells) located between the injection and production wells.

[0004] A coal seam having an injection well and a production well, with a well channel linking the two wells, is typically referred to as an underground 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 pyro lysis zone in which pyro lysis 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.

[0005] The product gas generated by UCG typically includes syngas as well as other components, and the constituency will depend on various factors, including the type of oxidant used for UCG (e.g., air or oxygen), water conditions (both ground water and exogenous water), coal quality, and UCG operating temperature and pressure.

[0006] Typically, UCG product gas will contain: (1) main syngas components (e.g., CO, H ₂ , C0 ₂ , N ₂ , and CH ); (2) solid particles/particulates (e.g., soot, ash, and coal particles); (3) water; (4) minor components such as C ₂ -C6 hydrocarbons, oxygen, argon, sulphur containing components (e.g., H ₂ S, COS, CS ₂ , mercaptans, and thiophenes), nitrogen based components (e.g., NH ₃ and HCN), hydrocarbon components (e.g., coal condensate, BTEX (benzene, toluene, ethylbenzene and xylenes), and PAHs (polycyclic aromatic hydrocarbons)); and (5) trace components such as heavy metals (arsenic and mercury) and chlorides.

[0007] Disadvantages of standard UCG methods utilising air as the oxidant include: due to a low rate of oxidation within the main combustion zone of an underground coal gasifier, additional oxidation reactions tend to take place downstream of the main combustion zone along the length of a well channel, thereby increasing the propagation rate of the combustion front. Consequently, there is less gasification zone development and gasifier cavity expansion in a lateral (i.e., cross sectional) direction relative to the length of the well channel and direction of gas flow. This leads to poor utilisation of coal seam reserves situated between well channels extending alongside one another (such as in a knife-edge gasifier

configuration), and to the necessity of having to decrease the distance between such well channels. That is, the low rate of oxidation reduces the effectiveness of coal resource recovery by standard UCG methods.

[0008] Additionally, when a well channel of an underground coal gasifier is short, an increase in the air injection rate does not increase the production of desirable product gas components (e.g., CO and H ₂ ), because the length of the gasification zone of the gasifier is reduced.

[0009] Finally, although using oxygen as an oxidant results in a greater rate of oxidation than using air as the oxidant, the cost of UCG product gas production increases multi-fold because oxygen production (or purchase) is costly.

[0010] Thus, there is a need for methods and systems of increasing the rate of coal combustion and appropriate gasification zone development in an underground coal gasifier to improve coal resource recovery. SUMMARY OF INVENTION

[0011] An object of the present invention is to provide a method and apparatus for UCG that minimises one or more of the disadvantages of the prior art.

[0012] 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.

[0013] The present inventors have now developed a method and apparatus for increasing coal combustion and appropriate gasification zone development within an underground coal gasifier, with concomitant improvement of coal resource recovery.

[0014] According to an aspect of the present invention, there is provided a method of increasing the rate of coal combustion within a combustion zone of an underground coal gasifier, the method including the step of introducing a metal salt solution into the combustion zone of the underground coal gasifier so as to increase the rate of combustion of the coal in the combustion zone over that achievable using an injected oxidant alone.

[0015] The metal salt solution can be introduced into the combustion zone of the underground coal gasifier in any suitable way and in any suitable form. For example, the metal salt solution can be injected into the combustion zone together with an oxidant such that the oxidant acts as a carrier for the metal salt solution. The injected oxidant can mix with the metal salt solution to form a dispersion (i.e., metal salt solution particles dispersed in gaseous oxidant) that promotes combustion of coal.

[001 ] The dispersion can include metal salt solution drops/particles of any suitable size. Preferably, the metal salt solution drops/particles have an average size of anywhere between about 10 and 40 microns, including about 15, 20, 25, 30, or 35 microns (i.e., a metal salt solution mist).

[0017] Preferably, the metal salt solution is dispersed within a pressurised oxidant and the pressurised metal salt solution dispersion is used for injection into the combustion zone of the underground coal gasifier.

[0018] Preferably, the oxidant is supplied to the combustion zone of the underground coal gasifier at a pressure of between about 10 and 30 atmospheres, although pressures of about 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, or 40 atmospheres can be used.

[0019] Any suitable type of metal salt solution can be used. The metal salt can be consumed in a chemical reaction occurring in the combustion zone of the gasifier, or can function as a catalyst (i.e., the metal salt is not necessarily consumed). Preferred types of metals include sodium and potassium, as well as other alkali metals of Group 1 of the periodic table (IUPAC numbering). Preferred types of metals also include barium and calcium, as well as other alkali earth metals of Group 2 of the periodic table. Yet other preferred types of metals include nickel and other transition metals of Group 10 of the periodic table (nickel family metals).

[0020] The metal salt can be a metal carbonate, sulphate or nitrate for example.

Preferably, the metal salt solution is an aqueous solution of potassium carbonate (K ₂ CO ₃ ), sodium carbonate (NaiCOs), barium carbonate (BaC0 ₃ ), or nickel carbonate (NiC0 ₃ ), as well as mixtures thereof. The preferred catalytic metal salt is sodium carbonate.

[0021] Although any suitable concentration of metal salt can be introduced into the combustion zone of the underground coal gasifier, preferably about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 ,13, 14, 15, 16, 17, 18, 19, 20, 21, 22 , 23, 24, 25, or even up to about 50%

weight/weight metal salt can be injected with oxidant; preferably as a pressurised metal salt solution dispersion.

[0022] The metal salt solution can be mixed with oxidant at any suitable ratio, including a ratio of approximately 95-80% volume/volume oxidant to 5-20% volume/volume metal salt solution.

[0023] The injected oxidant, which can act as a carrier stream for the metal salt solution, can be any suitable type of fluid or fluids. 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., C0 ₂ and/or nitrogen in any desired ratio) enriched with oxygen (greater than or equal to 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.

[0024] The metal salt solution can be introduced into the combustion zone of the underground coal gasifier by way of an injection well or a service well.

[0025] According to another aspect of the present invention, there is provided a pressurised metal salt solution dispersion supply system for increasing the rate of coal combustion within a combustion zone of an underground coal gasifier, the system including: a) a source of pressurised metal salt solution dispersion and b) a supply pipe having an inlet connected to the source of pressurised metal salt solution dispersion and an outlet connectable to a well head of a well of the underground coal gasifier for injecting the pressurised metal salt solution dispersion into the combustion zone of the underground coal gasifier.

[0026] According to a further aspect of the present invention, there is provided a pressurised metal salt solution dispersion supply system for increasing the rate of coal combustion within a combustion zone of an underground coal gasifier, the system including: a) a source of metal salt solution, b) a source of pressurised oxidant, and c) a supply pipe having (i) an oxidant inlet connected to the source of the pressurised oxidant, (ii) a metal salt solution inlet connected to the source of the metal salt solution and including a fogger for forming a metal salt solution mist that mixes with the pressurised oxidant within the supply pipe to form a pressurised metal salt solution dispersion, and (iii) an outlet connectable to a well head of a well of the underground coal gasifier for injecting the pressurised metal salt solution dispersion into the combustion zone of the underground coal gasifier.

[0027] The fogger (also known as a mister or sprinkler) can be of any suitable size, shape and construction, and can produce a metal salt solution mist (i.e., drops/particles) of any suitable size. Preferably, the fogger produces metal salt solution drops/particles having an average size of anywhere between about 10 and 40 microns, including about 15, 20, 25, 30, or 35 microns.

[0028] The fogger can spray a metal salt solution mist at a controlled rate into the supply pipe, such that a ratio of approximately 95-80% volume/volume oxidant to 5-20%

volume/volume metal salt solution mist is achieved.

[0029] The pressurised metal salt solution dispersion supply system can include at least one tank containing the metal salt solution and this tank can be of any suitable size, shape and construction. For example, the tank can have a capacity of about 150 to 200 litres. The inlet for the metal salt solution can extend to within the supply pipe from the bottom of the tank. A control valve of the system associated with the inlet for the metal salt solution can control the flow rate of the metal salt solution there through. The tank can have an oxidant inlet connected to the supply pipe for receiving the oxidant so as to place the source of the metal salt solution under pressure, as well as to ensure proper mixing of the tank's contents. A control valve associated with the oxidant inlet can regulate the flow of compressed oxidant into the tank for the metal salt solution.

[0030] The pressurised metal salt solution dispersion supply system can include two or more tanks containing the metal salt solution, such that one tank can be used whilst the other tank is being filled (including refilled) or serviced.

[0031] The supply pipe can be of any suitable size, shape and construction. The supply pipe can have an inner diameter of about 150 to 300 mm, including about 175, 200, 225, 250, or 275 mm. Preferably, the supply pipe has an inner diameter of about 160 mm. The supply pipe also includes an outlet that can connect to a well head of a well of an underground coal gasifier. This coupling can be achieved in any suitable way. The supply pipe preferably feeds the well of the underground coal gasifier a pressurised metal salt solution dispersion

(approximately 5-20% weight/weight metal salt solution concentration; approximately 95- 80% volume/volume oxidant to 5-20% volume/volume metal salt solution) at a rate of about 2 to 10 m ³ /minute (including, about 3, 4, 5, 6, 7, 8, or 9 mVminute) over the required time period (usually for as long as the gasifier remains in operation).

[0032] According to yet a further aspect of the present invention, there is provided a pressurised metal salt dispersion including a metal salt solution dispersed in a pressurised oxidant for increasing the rate of coal combustion within a combustion zone of an underground coal gasifier.

[0033] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures.

BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is a cross sectional view of a pressurised metal salt solution dispersion supply system, according to an embodiment of the present invention.

[0035] Figure 2 is a transverse cross-sectional view (through plane A- A) of that shown in Figure 1. [0036] Figure 3 is an enlarged cross sectional view of a fogger of the supply system shown in Figure 1.

[0037] Figure 4 depicts use of the supply system of Figure 1 for increasing the rate of coal combustion within a combustion zone of an underground coal gasifier, according to an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

[0038] In the figures, like reference numerals refer to like features.

[0039] The present inventors have now developed a method and apparatus for increasing the rate of coal combustion in an underground coal gasifier by introducing a metal salt solution (also referred to herein as a "metal catalyst") into a combustion zone of the gasifier in an oxidant carrier stream (e.g., an air stream). A result of this development is that the length of the combustion zone along a well channel of the gasifier is decreased and, as a

consequence, the lateral width of a gasification zone of the gasifier within the coal seam is increased. In addition, the amount of energy per unit of time is also increased, which ultimately provides more energy for reactions within the gasification zone, where the formation of combustible gases (particularly CO and H ₂ ) occurs byway of the following chemical reactions:

C0 ₂ + C = 2CO

H ₂ 0 + C = CO + H ₂

2H ₂ 0 + C = C0 ₂ + 2H ₂

[0040] The effect of various metal salt (e.g., carbonate) catalysts on the time taken to gasify coal samples (of uniform size) was investigated. Table 1 below shows the duration of complete gasification of coal samples as depending of the presence of metal carbonate catalysts.

TABLE 1

Metal catalyst(s) Amount of metal catalyst within Monochromatometre reading for time taken pressurised air (% mass) for complete gasification (seconds)

Catalyst- free - 0.6563

NiCOj 1.0 - Nico ₃ 2.5 0.5488

BaC0 ₃ 2.5 0.5053

CaC0 ₃ 5.0 0.4629

CaC0 ₃ + Na ₂ C0 ₃ 2.5+2.5 0.4349

Na ₂ C0 ₃ 5.0 0.4284

Na ₂ C0 ₃ + K ₂ C0 ₃ 2.5+2.5 0.3787

K ₂ C0 ₃ 5.0 0.3711

[0041] The results of Table 1 show that various metal salt catalysts shorten the time taken to gasify coal samples. The catalytic activity can be ranked as follows (from highest to lowest activity): K ₂ C0 ₃ > Na ₂ C0 ₃ > BaC0 ₃ > NiC0 ₃ .

[0042] A combination of metal carbonate catalysts was not found to provide an advantage over single metal carbonate catalysts.

[0043] The effect of various metal salt solution catalysts on combustion/gasification temperature was investigated. Table 2 below shows the temperature profile during the complete gasification of coal samples (of uniform size) as depending of the presence of metal salt catalysts.

TABLE 2

[0044] The results of Table 2 support that, in the presence of metal salt solution catalysts, the amount of energy per unit of time increases, thereby ultimately providing more energy for reactions within the gasification zone where the formation of combustible gases CO and H ₂ occurs.

[0045] Therefore, the present invention includes using metal salt catalysts (such as a ₂ C0 ₃ ) in an oxidant carrier stream (such as pressurised air) as a means of increasing the rate of coal combustion to: (i) increase the coal recovery rate, (ii) increase the CO and H ₂ content of product gas and (iii) reduce the volume of oily water by-product (coal/gas condensate) that is normally isolated from a raw product gas stream.

[0046] The inventors' pressurised metal salt solution dispersion supply system 1 for increasing the rate of coal combustion within a combustion zone of an underground coal gasifier (over that achievable using an injected oxidant alone) is shown in Figures 1-4. The system 1 includes a source of metal salt solution catalyst 4a, 4b, a source of pressurised air 5 (i.e., oxidant), a supply pipe 6 and a fogger 7

[0047] The source of metal salt solution 4a, 4b includes two cylindrical 170 litre tanks 8a, 8b, each capable of containing a metal salt solution (5-20% weight/weight sodium carbonate) and operable independently of one another. Each tank 8a, 8b has an inlet 50a, 50b for receiving the metal salt solution and an isolation valve 51a, 51b. Each tank 8a, 8b has an inlet 9a, 9b extending to the supply pipe 6 for receiving compressed air so as to place the metal salt solution under pressure and to ensure proper mixing of the tank contents. A control valve 10a, 10b of the system 1 associated with each inlet 9a, 9b regulates the flow of compressed air into each tank 8a, 8b. Each inlet 9a, 9b has a plurality of outlets 41a, 41b for air positioned along a lower region of the tank 8a, 8b.

[0048] The source of pressurised air 5 is an air compressor 13.

[0049] The supply pipe 6 has an inlet 11 connected to the air compressor 13, a branched inlet pipe 14a, 14b with a branch extending to each tank 8 a, 8b, and an outlet 15 connectable to a well head 17 of a well 30 of an underground coal gasifier for injecting the pressurised metal salt solution dispersion into a combustion zone of the gasifier of a coal seam 2 (as seen in Figure 4).

[0050] The supply pipe 6 has an inner diameter of about 160mm (but it could equally be about 180 to 200 mm). The supply pipe outlet 15 can couple with the well head 17 of a well 30 of an underground coal gasifier, as depicted in Figure 4.

[0051] The fogger 7 (also known as a sprinkler or mister) is connected to an end of the inlet 14a located within the supply pipe 6. Further details of the fogger 7 can be seen in figure 3 (which is a third party commercially available fogger). The fogger 7 has a tubular body 70, a channel 71 and a head 72. As the metal salt solution flows through the channel 71 of the body 70 and past the head 72, it is converted into a mist. The fogger 7 converts the pressurised metal salt solution into a mist of particles having an average size of anywhere between about 10 and 40 microns.

[0052] The metal salt solution mist mixes with the compressed air within the supply pipe 6 at a ratio of approximately 95-80% volume/volume pressurised air to 5-20%

volume/volume metal salt solution mist. However, this ratio may be varied according to the task at hand, as will be understood by one of ordinary skill in the art. Control valves 20a, 20b of the system 1 associated with the inlet 14a, 14b regulate the flow of the metal salt solution to the fogger 7 and can be adjusted to vary the ratio according to conditions within the underground gasifier and the composition of combustibles within the produced product gas.

[0053] The following explains how the delivery system 1 can be used to increase the rate of coal combustion within a combustion zone of an underground coal gasifier.

[0054] As seen in Figure 4, the delivery system 1 is coupled to the well head 17 of the well 30 of an underground coal gasifier. The well 30 includes a 128-300 mm metal pipe 22 that is encased in concrete. The coal seam 2 is, for example, located approximately 180-250 m below ground level.

[0055] One or more of tanks 8a, 8b are filled with a metal salt solution via inlet 50a, 50b. Tank 8a is placed under pressure by feeding in compressed air from compressor 13 via inlet 9a, with valve 20a closed. Compressed air is able to flow freely through supply pipe 6 to the well 30 via the well head 17. Control valve 20a is opened such that the metal salt solution can pass through inlet 14a and be converted to a mist via the fogger 7. The metal salt solution mist then mixes with compressed air within the supply pipe 6 at a ratio of approximately 95-80% volume/volume pressurised air to 5-20% volume/volume mist to form a pressurised metal salt solution dispersion. The supply pipe 6 injects into the coal seam 2 the pressurised metal salt solution dispersion at a rate of about 2 to 10 mVminute and at a pressure of between about 10 to 30 atmospheres. Having reached the combustion zone 53, the pressurised metal salt solution dispersion increases the rate of combustion of the coal and provides greater heat for gasification to take place downstream of the combustion zone within the gasification zone 54.

[0056] The invention as exemplified (or as generally described) has at least the following advantages: (i) the invention increases the efficiency of recovering coal via underground gasification by promoting gasification zone expansion laterally; (ii) the invention enables calorific value control of the product gas during gasification; (iii) the invention ensures that the quality of the product gas remains stable when aiming for a particular CO/H ₂ ratio; (iv) the invention is easy to implement; (v) the invention improves the efficiency of underground gasification of coal by increasing the content of useful product gas components; and (vi) the invention reduces the cost of energy production by improving the quality of UCG product gas and reducing the cost of drilling.

[0057] 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.

[0058] 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.