Combustible casing for underground coal gasification channel and preparation method thereof

Abstract

The present invention discloses a combustible casing for an underground coal gasification channel and a preparation method thereof, belonging to the technical field of petroleum industry pipe manufacturing. The preparation process of the combustible casing is simple. The combustible casing is composed of 0.2-1.0Zr, 1.0-5.0Zn, 1.0-5.0Al, 7.0-13.0Gd, 3.0-6.0Y, 3.0-6.0Nd, and the remainder being Mg and impurities in mass percentage. After being cast and turned into an ingot, the ingot is solution treated and extruded, and then subjected to aging treatment. Finally, the combustible casing is prepared, which has sufficient space for normal operation and has the characteristic of being burned and consumed above a specific temperature. The casing has a rationally designed alloy element composition, low density, high specific strength, excellent room temperature and high temperature strength, and the characteristics of being burned and consumed above the specific temperature.

Description

Technical Field

[0001] The invention belongs to the technical field of petroleum industry pipe manufacturing and relates to a combustible casing for an underground coal gasification channel and a preparation method thereof. Background Art

[0002] Underground coal gasification (UCG) technology is a chemical coal mining method that creates appropriate underground process conditions for controlled combustion of coal. Through pyrolysis and a series of chemical reactions between coal, oxygen, and water vapor, combustible gases such as hydrogen, carbon monoxide, and methane are produced, achieving clean coal mining. UCG is a new approach to natural gas development that integrates coal mining and surface coal-to-gas production. It offers a shortened process, high safety, and environmentally friendly natural gas extraction method. UCG enables sealed underground coal production, producing a crude gas rich in methane, carbon monoxide, and hydrogen, without solid waste emissions. It integrates well construction, coal mining, and gasification processes, transforming solid coal into a clean energy source, improving the energy structure. UCG not only effectively gasifies residual coal from mining, but also allows for the recovery of unminable medium- and deep-seated coal, as well as unminable high-sulfur, high-ash, and high-gas coal, significantly increasing recoverable coal resources.

[0003] During the underground coal gasification process, the temperature in the combustion zone of the gasification chamber can reach as high as 1000°C, the wellbore temperature is 300-800°C, and multi-component gases such as hydrogen, carbon dioxide, and water vapor exist in the wellbore. The underground working conditions are harsh, and there are also "bottleneck" problems such as the lack of specialized tools and equipment for the gasification channel, the lack of related control technologies and tools. At present, the tools used in the underground coal gasification process, on the one hand, do not have sufficient high-temperature strength to withstand the load of the coal seam, resulting in insufficient space for the tools and equipment inside the wellbore to operate normally. On the other hand, they do not have the characteristics of reaching combustion consumption above a specific temperature, and the front end of the gasification channel does not automatically retreat, which cannot ensure that the ignition tool fully contacts the coal seam. Summary of the Invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a combustible casing for an underground coal gasification channel and a preparation method thereof, so as to solve the problem that the casing in the combustion area of the gasification channel in the prior art does not have sufficient high-temperature strength and does not have the characteristics of reaching combustion consumption above a specific temperature.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] The present invention discloses a method for preparing a combustible casing for an underground coal gasification channel, comprising the following steps:

[0007] S1: melting charge Gd-Y rare earth ferrosilicon, neodymium iron, ferrozirconium, pure zinc and pure aluminum to form a liquid alloy, the element composition of which is 0.2-1.0% by weight of Zr, 1.0-5.0% by weight of Zn, 1.0-5.0% by weight of Al, 7.0-13.0% by weight of Gd, 3.0-6.0% by weight of Y, 3.0-6.0% by weight of Nd, and the remainder being Mg and impurities;

[0008] S2: Liquid metal is cast to form alloy rod-shaped ingots;

[0009] S3: Turning finishing to process the outer surface of the alloy rod ingot;

[0010] S4: The alloy rod-shaped ingot after turning finishing is kept at 500-550°C for 5 hours for solution treatment;

[0011] S5: The alloy rod-shaped ingot after solution treatment is extruded to form a tube;

[0012] S6: Perform aging treatment on the pipe at an aging temperature of 200°C to eventually form a combustible casing for underground coal gasification channels.

[0013] Preferably, S1 adopts a 10kg vacuum induction melting furnace for smelting, and starts heating after Gd-Y rare earth ferrosilicon, neodymium iron, and ferrozirconium are filled in the furnace. When the temperature reaches 800°C, it starts to be kept warm for 3 minutes, and then pure zinc and pure aluminum are added and kept warm for 3 minutes.

[0014] Further preferably, in S2, the casting is performed in a preheated metal mold.

[0015] More preferably, in S4, the solution treatment is performed at 500° C. and kept for 5 hours.

[0016] Further preferably, in S5, the extrusion pressure is 700t, the extrusion temperature is 490°C, the extrusion ratio is 11, and the extrusion speed is 4mm / s; and in S6, the holding time is 50h.

[0017] Preferably, a 100 kg vacuum induction melting furnace is used for melting in S1. After Gd-Y rare earth ferrosilicon, neodymium iron, ferrozirconium, pure zinc and pure aluminum are loaded into the furnace, heating is started. When the temperature reaches 800° C., insulation is started for 3 minutes.

[0018] Further preferably, in S2, the casting is carried out in a preheated dry sand mold.

[0019] More preferably, in S4, the solution treatment is performed at 530° C. and kept for 5 hours.

[0020] Further preferably, in S5 , the extrusion is performed at 1200t, the extrusion temperature is 580° C., the extrusion ratio is 8, and the extrusion speed is 1 mm / s; and in S6 , the holding time is 60 h.

[0021] The present invention also discloses a combustible casing for an underground coal gasification channel prepared by the above method, which is composed of 0.2-1.0% Zr, 1.0-5.0% Zn, 1.0-5.0% Al, 7.0-13.0% Gd, 3.0-6.0% Y, 3.0-6.0% Nd and Mg in mass percentage.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention discloses a method for preparing a combustible casing for an underground coal gasification channel. The method comprises the following steps: heating and melting furnace charge to form a liquid alloy having a mass percentage of 0.2-1.0 Zr, 1.0-5.0 Zn, 1.0-5.0 Al, 7.0-13.0 Gd, 3.0-6.0 Y, 3.0-6.0 Nd, and the remainder being Mg and impurities; the alloy is then cast; and after casting to form an alloy rod-shaped ingot, the outer side of the ingot is finely turned to ensure the outer shape of the combustible casing. The method adopts a solution treatment at 500-550° C. and holding for 5 hours. At this temperature and holding time, the metallographic structure of the tube body can fully complete the phase transformation strengthening process, while avoiding the effect of coarse grains caused by excessively high temperature and excessively long holding time on subsequent processing performance. The mechanical properties of the alloy are initially improved by solution treatment, and then the ingot after solution treatment is extruded and then aged to improve its mechanical properties for a second time. The method disclosed in the present invention reasonably distributes various processing operations, taking into account the machinability of the combustible casing material before forming and the comprehensive performance after forming. During underground coal gasification, the casing in the coal seam combustion area needs to be consumed synchronously with the coal seam so that the internal equipment and preparations of the casing can fully contact the coal seam. The chemical properties of magnesium alloys provide feasibility for gasification conditions. Therefore, magnesium is used as the main component of the combustible casing. Adding rare earth elements to magnesium alloys can significantly improve the mechanical properties of the alloy, especially the high-temperature mechanical properties. Gd has a high solubility in Mg. When added to magnesium alloys, it can form a second phase with excellent high-temperature strength within the alloy structure. Y, in addition to improving the high-temperature strength of magnesium alloys while maintaining room-temperature strength, also has high solubility in Mg and significantly increases the alloy's age-hardening strength. Nd has a far greater effect on improving the alloy's high-temperature strength than rare earth elements such as La and Pr, and this improvement does not conflict with that of Gd and Y. The present invention utilizes a Gd-Y-Nd rare earth element combination to ensure the high-temperature strength of the combustible casing, maximizing the high-temperature strength-enhancing effect of the rare earth elements. Zr preferentially forms compound particles with Mg during alloy solidification, serving as a heterogeneous nucleation core for alloy solidification, refining the grain size and improving mechanical properties. Al in magnesium alloys forms a preferential solid solution with Mg, exhibiting significant solid-solution strengthening effects, ensuring the alloy's high strength. Zn, with a slightly lower solid-solubility than Al in magnesium alloys, also exerts a solid-solution strengthening effect when dissolved in the alloy, and its effect can be additive with that of Al. Therefore, the present invention utilizes both Zn and Al as solid-solution strengthening elements.

[0024] Furthermore, a 10kg vacuum induction melting furnace was used to melt the master alloy, which helps prevent oxidation and burning of the charge during the melting process due to prolonged heating time and the presence of atmospheric air. The furnace was first loaded with Gd-Y rare earth ferrosilicon, neodymium iron, and ferrozirconium and heated to 800°C. Pure zinc and aluminum were then added and held at this temperature for 3 minutes. Pure zinc and aluminum are volatile in a vacuum environment, so they should be added after the melt has heated up to shorten the holding time.

[0025] Furthermore, the use of a metal mold to cast the master alloy is conducive to rapid cooling and molding of the melt.

[0026] Furthermore, a 100kg vacuum induction melting furnace is used to melt all the charges. Melting each charge at the same time is beneficial to shorten the melting time and prevent the charge from volatilization loss.

[0027] Furthermore, compared to the master alloy, the final ingot has a larger cross-sectional area, and the solidification cooling rate cannot be too fast. Therefore, dry sand mold casting is used to control the melt cooling rate within a reasonable range, preventing too slow a rate from causing coarse grains and too high a rate from causing cracking in the casting. After solidification, the outer surface of the ingot is first processed by turning to remove surface impurities and low-performance areas. Then, solution treatment is performed at 500-550°C for 5 hours to improve the ingot's processing performance.

[0028] Furthermore, the extrusion temperature is 700-1200t, the extrusion temperature is 490-580℃, the extrusion ratio is 8-11, and the extrusion speed is 1-5mm / s. This parameter range is helpful to prevent the tube body from being loose, the tube body size deviation or the waste caused by excessive extrusion ratio, too low extrusion temperature, too small extrusion force and too high extrusion speed.

[0029] The present invention discloses a combustible casing for underground coal gasification channels, characterized by low density, high room-temperature and high-temperature strength, and ignition and combustion performance above a specific temperature. It also features high specific strength, good workability, and a high yield rate. Mg, the primary material of the combustible casing, ensures extremely low density and combustion above a certain temperature. Appropriate amounts of Zr, Zn, and Al ensure sufficient room-temperature strength and corrosion resistance. High levels of Gd, Y, and Nd ensure sufficient high-temperature strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a physical diagram of Example 1 of the present invention;

[0031] Figure 2 This is a physical diagram of Example 2 of the present invention;

[0032] Figure 3 This is the mechanical property test result curve of the present invention. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0034] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0035] The present invention is described in further detail below with reference to the accompanying drawings:

[0036] The specific chemical composition of the present invention is as follows (wt%): 7.0-13.0Gd, 3.0-6.0Y, 3.0-6.0Nd, 0.2-1.0Zr, 1.0-5.0Zn, 1.0-5.0Al, and the rest is Mg and impurities.

[0037] The performance of combustible casing for underground coal gasification channels is mainly determined by its composition and heat treatment process. The design reasons of the present invention are as follows:

[0038] Magnesium is the primary element in combustible casing specifically designed for underground coal gasification (UCG) tunnels. The greatest advantages of magnesium alloys are their low density, high specific strength, and high specific stiffness. The equivalent bending strength and stiffness of magnesium alloys are significantly higher than those of high-strength steel and aluminum alloys, and are nearly comparable to carbon fiber-reinforced polymers. Magnesium alloys offer excellent casting, machinability, vibration and noise reduction, and damping properties. Their damping performance is 300 times that of titanium alloys and over 100 times that of aluminum alloys. Under the same load conditions, they can dissipate greater deformation work and withstand significant shock and vibration loads. Furthermore, magnesium alloys exhibit strong reducing properties and are capable of combustion in a variety of atmospheres, including air, nitrogen, and even carbon dioxide. During the UCG process, the casing in the combustion zone of the coal seam must be consumed simultaneously with the coal seam to ensure adequate contact between the equipment and agents within the casing. The chemical properties of magnesium alloys make them suitable for gasification conditions. Therefore, magnesium is used as the primary element in UCG casing.

[0039] Gadolinium: The addition of rare earth elements to magnesium alloys can significantly improve the alloy's mechanical properties, especially its high-temperature mechanical properties. Although rare earth elements are a series of elements with similar chemical properties, their effects on the properties of magnesium alloys vary. For example, while Ce can improve the high-temperature strength of magnesium alloys, the room-temperature strength is reduced due to the addition of Ce. Gd has a high solubility in Mg and, when added to magnesium alloys, can form a second phase in the alloy structure with excellent high-temperature strength. Therefore, the present invention selects Gd as one of the rare earth element additives, with the content controlled between 7.0-13.0wt%.

[0040] Yttrium: Y not only improves the high-temperature strength of magnesium alloys while maintaining room-temperature strength, but also has high solubility in magnesium and significantly enhances the alloy's age-hardening strength. This invention selects Y as one of the rare earth elements in the additive combination, with a suitable content of 3.0-6.0 wt%.

[0041] Neodymium: As the third element in the rare earth element combination added in this invention, Nd significantly enhances the alloy's high-temperature strength, surpassing other rare earth elements like La and Pr. This enhancement doesn't conflict with the effects of Gd and Y. This invention utilizes a Gd-Y-Nd rare earth element combination to ensure the high-temperature strength of the combustible casing, maximizing the high-temperature strength-enhancing effects of the rare earth elements. The optimal Nd content is 3.0-6.0 wt%.

[0042] Zirconium: One of the main reasons for the reduction in room-temperature strength of magnesium alloys after the addition of rare earth elements is that rare earth elements cause grain coarsening, which deteriorates the alloy's mechanical properties. Zr preferentially forms compound particles with Mg during alloy solidification, acting as a heterogeneous nucleation core for the alloy's solidification, refining the grains and improving mechanical properties. The present invention uses Zr to refine the alloy's grains, with a suitable content of 0.2-1.0 wt%.

[0043] Aluminum: Al is one of the most commonly used alloying elements in magnesium alloys. It forms a preferential solid solution with Mg in magnesium alloys, exhibiting significant solid solution strengthening, ensuring the alloy's high strength. Therefore, the optimal Al content in the present invention is 1.5-5.0 wt%.

[0044] Zinc: Zn has a slightly lower solubility in magnesium alloys than Al. When dissolved in the alloy, it also provides solid solution strengthening, and its effect can be additive with that of Al. Therefore, the present invention uses both Zn and Al as solid solution strengthening elements, with the Zn content controlled between 1.5-5.0 wt%.

[0045] Example 1:

[0046] Melting is performed in a 10kg vacuum induction melting furnace. The charge consists of Gd-Y rare earth ferrosilicon, neodymium iron, ferrozirconium, pure zinc, and pure aluminum. All charges are crushed into approximately 15mm blocks. After the furnace is loaded with Gd-Y rare earth ferrosilicon, neodymium iron, and ferrozirconium, heating begins. When the temperature reaches 800°C, a holding period begins. After the holding period lasts for three minutes, pure zinc and pure aluminum are added. Three minutes later, the preheated mold is poured.

[0047] After obtaining the alloy rod-shaped ingot, the outer surface of the ingot is turned and finished. The treated ingot undergoes a solution treatment at 500°C for 5 hours to initially improve the alloy's mechanical properties. The solution-treated ingot is then extruded using an extrusion force of 700 tons, an extrusion temperature of 490°C, an extrusion ratio of 11, and an extrusion speed of 4 mm / s.

[0048] The extruded pipes are subjected to aging treatment to improve their mechanical properties for the second time. The aging temperature is 200℃ and the holding time is 50h. Figure 1 Measured dimensions: Φ30mm, wall thickness 2mm. Mechanical properties tests and chemical composition analysis were conducted on this batch of combustible casings. The results are shown in Tables 1 and 2.

[0049] Example 2:

[0050] A 100kg induction melting furnace was used. The charge consisted of Gd-Y rare earth ferrosilicon, neodymium iron, ferrozirconium, pure zinc, and pure aluminum. All charges were crushed into approximately 15mm blocks. After loading, heating began, and when the temperature reached 800°C, the heat was maintained. After 3 minutes, the preheated dry sand mold was poured.

[0051] After obtaining the alloy rod-shaped ingot, the outer surface of the ingot is turned and finished. The treated ingot undergoes a solution treatment at 530°C for 5 hours to initially improve the alloy's mechanical properties. The solution-treated ingot is then extruded at a pressure of 1200 tons, an extrusion temperature of 580°C, an extrusion ratio of 8, and an extrusion speed of 1 mm / s.

[0052] The extruded pipes are subjected to aging treatment to improve their mechanical properties for the second time. The aging temperature is 200℃ and the holding time is 60h. Figure 2 Measured dimensions: Φ114.3mm, wall thickness 9.65mm. Mechanical properties tests and chemical composition analysis were conducted on this batch of combustible casings. The results are shown in Tables 1 and 2.

[0053] Table 1 Mechanical properties of combustible casing

[0054] Performance indicators Yield strength / MPa Tensile strength / MPa Elongation / % Example 1 279 302 5 Example 2 380 415 5

[0055] Table 2 Chemical composition of combustible casing (wt.%)

[0056] Element type Gd Y Nd Zr Al Zn Mg Example 1 7.0 3.7 5.0 0.4 4.3 3.8 margin Example 2 9.0 4.0 3.0 0.2 4.5 1.4 margin

[0057] See also Figure 3 , which is a comparison chart of the mechanical properties of the same batch of pipes produced in two embodiments of the present invention. It can be seen that the basic mechanical properties of the pipes produced in the two embodiments are at a relatively high level. Combined with the chemical composition comparison results given in Table 2, the pipe produced in Example 2 has a higher content of rare earth elements and solid solution strengthening elements. Therefore, the mechanical properties of the pipe produced in Example 2 are higher than those of the pipe produced in Example 1.

[0058] In summary, the present invention, based on magnesium alloys, more rationally designs the types and amounts of alloying elements added, as well as the preparation process. Through this novel combination of alloying elements, the material achieves excellent room-temperature and high-temperature mechanical properties. Furthermore, the material boasts low production costs and a simple production process, making it highly valuable for widespread application.

[0059] The above content is only for explaining the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a combustible casing for an underground coal gasification channel, characterized in that: The following steps are involved: S1: Melting the charge Gd-Y rare earth ferrosilicon, Nd-Fe, ferrozirconium, pure zinc and pure aluminum to form a liquid alloy, the elemental composition of which is 0.2-1.0 Zr, 1.0-5.0 Zn, 1.0-5.0 Al, 7.0-13.0 Gd, 3.0-6.0 Y, 3.0-6.0 Nd, and the remainder is Mg and impurities; Melting is carried out in a 10kg vacuum induction melting furnace, and the furnace is charged with Gd-Y rare earth ferrosilicon, Nd-Fe and ferrozirconium and then heated. When the temperature reaches 800°C, the temperature is maintained for 3 minutes, and then pure zinc and pure aluminum are added and maintained for 3 minutes; S2: Liquid metal is cast to form alloy rod-shaped ingots; S3: Turning finishing to process the outer surface of the alloy rod ingot; S4: The alloy rod ingot after turning finishing is kept at 500~550℃ for 5h for solution treatment; S5: The alloy rod-shaped ingot after solid solution treatment is extruded to form a tube; S6: performing aging treatment on the pipe at an aging temperature of 200° C., and finally forming a combustible casing for an underground coal gasification channel; the combustible casing is synchronously burned and consumed along with the coal seam during the gasification process.

2. The method for preparing a combustible casing for an underground coal gasification channel according to claim 1, characterized in that: In S2, the casting is performed in a preheated metal mold.

3. The method for preparing a combustible casing for an underground coal gasification channel according to claim 1, characterized in that: In S4, the solution treatment is carried out at 500°C for 5 hours.

4. The method for preparing a combustible casing for an underground coal gasification channel according to claim 1, characterized in that: In S5, the extrusion pressure is 700t, the extrusion temperature is 490℃, the extrusion ratio is 11, and the extrusion speed is 4mm / s; in S6, the holding time is 50h.

5. The method for preparing a combustible casing for an underground coal gasification channel according to claim 1, characterized in that: S1 uses a 100kg vacuum induction melting furnace for smelting. After Gd-Y rare earth ferrosilicon, neodymium iron, ferrozirconium, pure zinc and pure aluminum are loaded into the furnace, heating begins. When the temperature reaches 800°C, insulation is started and the insulation is maintained for 3 minutes.

6. The method for preparing a combustible casing for an underground coal gasification channel according to claim 5, characterized in that: In S2, casting is performed in a preheated dry sand mold.

7. The method for preparing a combustible casing for an underground coal gasification channel according to claim 5, characterized in that: In S4, solution treatment was performed at 530°C for 5 hours.

8. The method for preparing a combustible casing for an underground coal gasification channel according to claim 5, characterized in that: In S5, the extrusion was performed with 1200t, extrusion temperature 580°C, extrusion ratio 8, and extrusion speed 1mm / s; and in S6, the holding time was 60h.

9. A combustible casing for an underground coal gasification channel prepared according to any one of claims 1 to 8, characterized in that: The combustible sleeve is composed of 0.2-1.0% Zr, 1.0-5.0% Zn, 1.0-5.0% Al, 7.0-13.0% Gd, 3.0-6.0% Y, 3.0-6.0% Nd and Mg in percentage by mass.

Images