A method for pyrolysis and gasification of coal seams by using microwave energy-assisted heating

Abstract

The present application discloses a method for pyrolysis and gasification of coal seams assisted by microwave energy heating, which comprises the following steps: drilling an injection well, a horizontal well, and a production well in a selected coal seam, with the horizontal well close to the bottom of the coal seam, and the injection well and the production well being connected through the horizontal well; introducing microwave into the coal seam, heating the coal seam by the microwave, thereby causing medium-temperature pyrolysis of the coal, then introducing oxygen and igniting to cause the coal to burn, and when the heat generated in the gasification cavity reaches a set value, introducing water, and the volatile matter and coke generated by medium-temperature pyrolysis react with water and oxygen under an oxygen-deficient state to undergo coal gasification reaction, and the coal gasification reaction generates raw gas; the raw gas is transported to a ground treatment system through the production well. The present application uses microwave energy to heat the coal seam, replacing partial oxygen combustion for heating the coal seam, reducing the oxygen consumption, and solving the problem of excessive carbon emissions caused by a large oxygen demand in the existing underground coal gasification technology.

Description

Technical Field

[0001] This application belongs to the technical field of underground coal gasification, and particularly relates to a method for pyrolysis and gasification by using microwave energy to assist in heating coal seams. Background Art

[0002] Underground coal gasification technology (UCG) is a new type of clean chemical coal mining energy technology that creates appropriate process reaction conditions underground for coal without mining, enables controlled combustion of coal, and generates combustible gases such as hydrogen, carbon monoxide, and methane through the pyrolysis of coal and a series of chemical reactions between coal and oxygen and steam, realizing clean coal mining. This technology has advantages such as ensuring safety, reducing manpower, reducing pollution, and increasing coal mining rate, and has become a hot topic in coal energy research at home and abroad. As a new type of green, safe, and economical coal mining method, underground coal gasification technology is expected to become an effective way for China to solve energy security and environmental pollution. This revolutionary technology of underground coal gasification can achieve "artificial gas production" and conforms to the direction of clean coal utilization. Before new energy reaches large-scale supply, it can open up a new strategic way for the effective supply of methane and hydrogen with characteristics.

[0003] For well-type medium-deep underground direct coal gasification, the existing technology is to add oxygen and coal to undergo an oxidation reaction to provide energy for the gasification reaction of coal and water, which requires the preparation of the gasification agent oxygen. The conventional nitrogen and oxygen production process has a low nitrogen-oxygen regulation ratio, poor reliability of high-pressure pure oxygen equipment, and a large amount of carbon dioxide is generated during the reaction. There are many influencing factors for repeated ignition of the gasification process, and it is difficult for the coal seam to ignite. Problems such as corrosion, carbon dioxide separation, emission, and gasification ignition need to be solved. Summary of the Invention

[0004] By providing a method for pyrolysis and gasification by using microwave energy to assist in heating coal seams, the embodiments of this application solve the problem of excessive carbon emissions caused by a large demand for oxygen in the existing underground coal gasification technology.

[0005] To achieve the above object, the embodiments of the present invention provide a method for pyrolysis and gasification by using microwave energy to assist in heating coal seams, including the following steps:

[0006] Set the drilling position and the initial gasification area in the selected coal seam, drill an injection well, a horizontal well, and a production well at the drilling position, the horizontal well is close to the bottom of the coal seam, and the injection well and the production well are connected through the horizontal well;

[0007] Microwaves are introduced into the gasification position of the coal seam, and the microwaves heat the coal seam, thereby causing medium-temperature pyrolysis of the coal. Then, oxygen is introduced and ignition occurs to make the coal burn. When the generated heat reaches a set value, water is introduced. The volatile matter and coke generated by the medium-temperature pyrolysis undergo a coal gasification reaction with water and oxygen in an oxygen-deficient state, gradually forming a gasification cavity, and the coal gasification reaction generates raw gas.

[0008] The raw gas generated by the coal gasification reaction is transported to the ground treatment system through the production well.

[0009] In a possible implementation manner, when water, oxygen, and microwaves are introduced into the gasification position through the injection well, nitrogen is injected simultaneously, and the nitrogen isolates the gasification cavity from the external air.

[0010] In a possible implementation manner, the injection well is arranged with a double-layer coiled tubing, a coal seam casing, and an injection technology casing from the inside out in sequence. The double-layer coiled tubing includes an inner tube and an outer tube sleeved with each other, and a microwave transmission cavity is formed between the inner tube and the outer tube.

[0011] The microwaves are introduced into the gasification position of the coal seam through the microwave transmission cavity, oxygen is injected through the inner tube, water is injected through the annulus between the outer tube and the coal seam casing, and nitrogen is injected through the annulus between the coal seam casing and the injection technology casing.

[0012] In a possible implementation manner, the copper plating layers on the outer wall of the inner tube and the inner wall of the outer tube form the microwave transmission cavity.

[0013] In a possible implementation manner, the raw gas generated by the coal gasification reaction is transported to the ground treatment system through the production tubing arranged in the production well. During transportation, the raw gas in the production tubing is cooled by spraying through a spraying mechanism in the gap between the production tubing and the production technology casing, and the water absorbs heat and vaporizes to cool the raw gas in the production tubing.

[0014] One or more technical solutions provided in the embodiments of the present invention have at least the following technical effects or advantages:

[0015] An embodiment of the present invention provides a method for pyrolysis and gasification of coal seams by using microwave energy-assisted heating. In this method, microwave, water, and oxygen are simultaneously introduced into the gasification chamber. Water and oxygen serve as gasifying agent and oxidizing agent respectively. Under the action of microwave and the combined action of the gasifying agent oxygen and coal combustion, the temperature of the coal seam is increased to obtain the energy required for coal pyrolysis, and the coal seam is heated for pyrolysis. In the gasification chamber, the coal reacts with water to generate raw gas. The raw gas is transported to the ground treatment system through the production well. The present invention uses microwave to provide energy for the gasification reaction of coal and water, reduces the demand for oxygen, thereby saving energy and improving the safety of the process. At the same time, the oxidation reaction of microwave, oxygen, and carbon provides energy for the gasification reaction together, enabling the optimal allocation of resources. The injection of less oxygen greatly reduces the generation amount of CO2 and significantly increases the generation amounts of H2 and CH4, thus solving the problems of corrosion, carbon dioxide separation, and emission, and improving the quality of syngas. The central temperature of the object heated by microwave is higher than the surface temperature, and its heat transfer and mass transfer directions are the same, both from the inside to the outside. The volatile matter passes through the medium-temperature zone, which can minimize the secondary reaction of the volatile matter. It avoids the problems in the prior art that ignition liquid and ignition fuel are introduced into the gasification chamber, not only with a high demand for oxygen but also requiring ignition operation, so there are cumbersome steps. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0017] Figure 1 It is a schematic diagram of the implementation state of the method for pyrolysis and gasification of coal seams by using microwave energy-assisted heating provided by the embodiment of the present invention.

[0018] Figure 2 It is a schematic diagram of the structure of the injection well pipeline system provided by the embodiment of the present invention.

[0019] Figure 3 It is a flow chart of the coal reaction provided by the embodiment of the present invention.

[0020] Figure 4 It is a schematic diagram of the microwave release structure formed by slitting the end of the outer pipe provided by the embodiment of the present invention.

[0021] Reference numerals: 1 - coal seam; 2 - injection well; 3 - production well; 4 - gasification cavity; 5 - coiled tubing tool; 6 - double coiled tubing; 61 - inner tube; 62 - outer tube; 621 - microwave release structure; 7 - coal seam casing; 8 - microwave transmission cavity; 9 - injection technology casing; 10 - production tubing; 11 - surface treatment system; 12 - spraying mechanism; 13 - microwave generator; 14 - air separation unit; 15 - temperature and pressure monitoring sensor; 16 - production technology casing. Detailed implementation manners

[0022] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the embodiments of the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation of the present invention. The terms "first", "second", "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance. In addition, the terms "mounted", "connected", "connected" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected, or indirectly connected through an intermediate medium, and it may be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present invention can be understood according to specific circumstances.

[0024] As Figures 1 to 3 shown, the method for pyrolysis and gasification of a coal seam assisted by microwave energy provided by the embodiments of the present invention includes the following steps:

[0025] Set the drilling position and the initial gasification area in the selected coal seam 1, drill an injection well 2, a horizontal well, and a production well 3 at the drilling position. The horizontal well is close to the bottom of the coal seam 1, and the injection well 2 and the production well 3 are connected through the horizontal well.

[0026] Microwaves are introduced into the gasification position of the coal seam 1 through the microwave transmission cavity 8 formed between the double-layer coiled tubing 6. The microwaves heat the coal seam 1, thereby causing medium-temperature pyrolysis of the coal. Then, oxygen is introduced and ignited to make the coal burn. When the generated heat reaches the set value, water is introduced. The volatiles and coke generated by medium-temperature pyrolysis undergo a coal gasification reaction with water and oxygen in an oxygen-deficient state, gradually forming the gasification cavity 4, and the coal gasification reaction generates raw gas.

[0027] The raw gas generated by the coal gasification reaction is transported to the surface treatment system 11 through the production well 3.

[0028] The reaction process of the coal is monitored by the temperature and pressure monitoring equipment at the end of the coiled tubing. After the gasification reaction is completed, the coiled tubing is controlled to retreat, so that a new gasification cavity 4 is formed in the coal seam 1 between the end of the coiled tubing 7 and the production well 3. The new gasification cavity 4 is gasified according to the same steps.

[0029] The remaining coal seam 1 to be gasified is gasified according to the same steps.

[0030] It should be noted that the microwave energy is generated by the microwave generator 13. The microwave generator 13 is installed on the ground, and the microwaves generated by the microwave generator 13 in this embodiment are introduced into the underground target coal seam 1 through the microwave transmission cavity 8.

[0031] The microwave heating of the coal seam 1 has the following characteristics: First, rapid heating: The traditional heating method mainly heats the material through heat transfer methods such as heat conduction, convection, and radiation. Microwaves directly penetrate into the interior of the material and are absorbed and converted into heat energy, without the need for a heat transfer process. Second, overall heating: Microwave heating uses microwave energy as heat energy, heating evenly inside and outside, with the temperature rising overall, and the temperature distribution being high inside and low outside. Third, selective heating: The ability of the material to absorb microwave energy is directly related to its dielectric properties. Substances with strong dielectric properties in the material absorb microwave energy strongly, while substances with poor dielectric properties absorb microwave energy weakly. Fourth, high-efficiency heating: The microwave heating equipment is a microwave source, such as a magnetron that generates microwaves, and then the microwaves are fed into a metal cavity to heat the material. The equipment and the metal cavity hardly absorb microwaves, with extremely little energy loss.

[0032] In this embodiment, the oxygen can be produced by the air separation device 14, and the air separation device 14 separates oxygen-rich or pure oxygen from the air and introduces it into the gasification cavity 4. This embodiment can adopt a wind-solar power generation system and the surplus electricity of the power grid, and through an intelligent power distribution system, the electric energy is supplied to the microwave generator 13 and the air separation device 14 respectively. Adopting a wind-solar power generation system can solve the problems of green and low-carbon and energy storage.

[0033] In the present invention, microwaves, water, and oxygen are simultaneously introduced into the gasification cavity 4, and the coal in the coal seam 1 in the gasification cavity 4 undergoes a gasification reaction. The main process of the coal gasification reaction is as Figure 3 shown.

[0034] Under the action of microwave energy, the temperature of the coal begins to rise. When the temperature of the coal rises to 350 - 450 °C, the pyrolysis of the coal starts, and volatile substances such as tar and gas are released. The pyrolysis products are as follows:

[0035]

[0036] The gasification reaction of coal refers to the complex reaction of the volatile matter generated by pyrolysis and the residual coke particles with the gasifying agent. Since this reaction is carried out under an oxygen-deficient state, the main products of the coal gasification reaction include combustible CO, H2, and CH4. Only a small part of the carbon is oxidized to CO2, and there may also be a small amount of H2O. The main chemical reactions in this process are:

[0037]

[0038] It can be seen from the reaction process that due to the use of microwaves, the amount of oxygen required for coal gasification is greatly reduced. The raw gas generated after gasification is transported to the surface treatment system through the production well system, and the surface treatment system conducts fine processing to obtain the required products.

[0039] In the present invention, microwaves, water, and oxygen are simultaneously introduced into the gasification chamber 4. Water and oxygen serve as the gasifying agent and oxidizing agent respectively. Under the action of microwaves and the combined action of the gasifying agent oxygen and coal combustion, the coal seam 1 is heated to obtain the energy required for coal pyrolysis, and the coal seam 1 is heated for pyrolysis. In the gasification chamber 4, the coal undergoes a gasification reaction with water to generate raw gas. The raw gas is transported to the surface treatment system 11 through the production well 3.

[0040] The present invention uses microwaves to provide energy for the gasification reaction of coal and water, reducing the demand for oxygen, thereby saving energy and improving the safety of the process. At the same time, the oxidation reaction of microwaves, oxygen, and carbon provides energy for the gasification reaction together, enabling the optimal allocation of resources. The injection of less oxygen significantly reduces the generation amount of CO2 and greatly increases the generation amounts of H2 and CH4, thereby solving the problems of corrosion, carbon dioxide separation, and emission, and improving the quality of the syngas. The central temperature of the object heated by microwaves is higher than the surface temperature, and its heat transfer and mass transfer directions are the same, both transferring from the inside to the outside. The volatile matter passes through the medium-temperature zone, which can minimize the secondary reaction of the volatile matter. It avoids the prior art of injecting the ignition liquid and ignition fuel into the gasification chamber 4, which not only has a high demand for oxygen, poor reliability of the oxygen production equipment, but also requires ignition operation, so there are problems of cumbersome steps.

[0041] In this embodiment, when water, oxygen, and microwaves are introduced into the gasification position through the injection well 2, nitrogen is simultaneously injected, and the nitrogen isolates the gasification chamber 4 from the external air.

[0042] It should be noted that the air separation device 14 separates nitrogen in the air and injects it into the gasification chamber 4 through the injection well 2. Nitrogen can prevent external air from entering the gasification chamber 4, thus ensuring the smooth progress of the reaction in the gasification chamber 4.

[0043] In this embodiment, the injection well 2 is arranged with a double-layer coiled tubing 6, a coal seam casing 7, and an injection technology casing 9 from the inside to the outside in sequence. The double-layer coiled tubing 6 includes an inner pipe 61 and an outer pipe 62 sleeved together, and a microwave transmission cavity 8 is formed between the inner pipe 61 and the outer pipe 62.

[0044] Microwaves are introduced into the gasification position of the coal seam 1 through the microwave transmission cavity 8. Oxygen is injected through the inner pipe 61, water is injected through the annulus between the outer pipe 62 and the coal seam casing 7, and nitrogen is injected through the annulus between the coal seam casing 7 and the injection technology casing 9.

[0045] It should be noted that the injection well 2 includes a vertical section and a horizontal section that are connected. Among them, the horizontal section covers all the coal seam 1 areas to be operated, thereby maximizing the exploitation of coal resources. When the reaction in the first operation area is completed, the pipeline system of the injection well 2 is retracted, so that a second operation area is formed between the end of the pipeline system of the injection well 2 and the production well 3, and the medium-temperature pyrolysis and coal gasification reactions in the second operation area are carried out according to the same steps. Until the gasification reactions in all the coal seam 1 areas to be operated are completed.

[0046] Water can be in the form of liquid water or water vapor. Water and oxygen are injected through the inner pipe 61 and the annulus between the outer pipe 62 and the coal seam casing 7. In actual application, oxygen or a mixture of water vapor and oxygen can be input into the inner pipe 61, and liquid water can be injected into the annulus between the outer pipe 62 and the coal seam casing 7. Or a mixture of water vapor and oxygen is injected into both the inner pipe 61 and the annulus between the outer pipe 62 and the coal seam casing 7. Or a mixture of liquid water or water vapor and oxygen is input into the inner pipe 61, and oxygen is injected into the annulus between the outer pipe 62 and the coal seam casing 7. The specific conveying method and conveying amount are formulated according to the actual reaction requirements.

[0047] Setting the microwave transmission cavity 8 between the inner pipe 61 and the outer pipe 62 of the double-layer coiled tubing 6 can enable the double-layer coiled tubing 6 to absorb the heat dissipated by the microwave transmission cavity 8 when conveying water or oxygen. On the one hand, it heats the water or oxygen to achieve preheating during the reaction. On the other hand, it can cool the microwave transmission cavity 8 and play a cooling role. Water, oxygen, nitrogen, and microwaves are sent underground through the pipeline system of the injection well 2, and this method is highly efficient and easy to operate.

[0048] In the first embodiment, the copper plating layers on the outer wall of the inner pipe 61 and the inner wall of the outer pipe 62 form the microwave transmission cavity 8.

[0049] It should be noted that the copper plating layer forms the microwave transmission cavity 8. The copper plating layer reduces the energy loss during microwave transmission, thereby improving the utilization efficiency of microwave energy.

[0050] An inclined slit is circumferentially arranged on the front side wall of the outer tube 62 to form a microwave release structure. The width of the slit is less than or equal to 0.1 times the wavelength, the length is less than or equal to 0.5 times the wavelength, and the slit spacing is equal to the wavelength. The slit forms the microwave release structure 621, which is conducive to the escape of microwaves and reduces the time required for microwave pyrolysis of coal. Figure 4 medium λ is the microwave wavelength.

[0051] In this embodiment, the raw coal gas generated by the coal gasification reaction is transported to the ground treatment system 11 through the production tubing 10 disposed in the production well 3. During transportation, the raw coal gas in the production tubing 10 is cooled by spraying through the spraying mechanism 12 in the gap between the production tubing 10 and the production technical casing 16. During the process of water absorbing heat and gasifying, the raw coal gas in the production tubing 10 is cooled.

[0052] It should be noted that the raw coal gas is transported to the ground through the production tubing 10. Water is introduced from the ground into the production technical casing 16, and the raw coal gas is cooled by the spraying mechanism 12. The cooled raw coal gas is transported to the ground treatment system 11 for fine processing to obtain products. In this embodiment, a heat exchanger can also be used to cool the raw coal gas, and the heat energy collected by the heat exchanger can be recycled.

[0053] In this embodiment, for those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention.

Claims

1. A method for pyrolysis and gasification of coal seams by using microwave energy-assisted heating, characterized in that, It includes the following steps: Set the drilling position and the initial gasification area in the selected coal seam (1). Drill an injection well (2), a horizontal well, and a production well (3) at the drilling position. The horizontal well is close to the bottom of the coal seam (1), and the injection well (2) and the production well (3) are connected through the horizontal well; Introduce microwaves into the gasification position of the coal seam (1). The microwaves heat the coal seam (1), causing the coal to undergo medium-temperature pyrolysis. Then, introduce oxygen and ignite it to make the coal burn. When the generated heat reaches the set value, introduce water. The volatile matter and coke generated by the medium-temperature pyrolysis undergo a coal gasification reaction with water and oxygen in an oxygen-deficient state, gradually forming a gasification cavity (4), and the coal gasification reaction generates raw gas; The raw gas generated by the coal gasification reaction is transported to the ground treatment system (11) through the production well (3); When introducing water, oxygen, and microwaves into the gasification position through the injection well (2), inject nitrogen at the same time. The nitrogen isolates the gasification cavity (4) from the external air; The injection well (2) is arranged with a double-layer coiled tubing (6), a coal seam casing (7), and an injection technology casing (9) from the inside out. The double-layer coiled tubing (6) includes an inner pipe (61) and an outer pipe (62) sleeved together. A microwave transmission cavity (8) is formed between the inner pipe (61) and the outer pipe (62); Introduce microwaves into the gasification position of the coal seam (1) through the microwave transmission cavity (8), inject oxygen through the inner pipe (61), inject water through the annulus between the outer pipe (62) and the coal seam casing (7), and inject nitrogen through the annulus between the coal seam casing (7) and the injection technology casing (9); The copper plating layers on the outer wall of the inner pipe (61) and the inner wall of the outer pipe (62) form the microwave transmission cavity (8).

2. The method for pyrolysis and gasification of coal seams by using microwave energy-assisted heating according to claim 1, wherein: Transport the raw gas generated by the coal gasification reaction to the ground treatment system (11) through the production tubing (10) arranged in the production well (3); during transportation, cool the raw gas by spraying through a spraying mechanism (12) in the gap between the production tubing (10) and the production technology casing (16). The water absorbs heat and vaporizes, cooling the raw gas in the production tubing (10).

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