In-situ low-pressure high-temperature method for producing hydrogen from coal

Abstract

The invention discloses an in-situ low-pressure high-temperature coal hydrogen production method, and belongs to the technical field of coal gasification mining. According to the method, low-pressure superheated water vapor and oxygen are injected into a coal seam, heat is released through the oxidation reaction of the oxygen and coal, the coal seam is further heated to 800-1000 DEG C, and hydrogen and carbon dioxide are generated through the reaction of the oxygen and the water vapor; in-situ efficient hydrogen production is carried out in coal by combining cooperative injection of superheated water vapor and oxygen and adopting a distributed temperature and pressure monitoring and well pattern technology; according to the method, the technical bottleneck of traditional coal mining can be broken through, clean hydrogen energy can be provided while carbon dioxide emission is reduced, and a new technical path is provided for efficient and clean utilization of coal resources.

Description

Technical Field

[0001] The invention belongs to the technical field of coal gasification mining and relates to an in-situ low-pressure and high-temperature coal hydrogen production method. Background Art

[0002] With the gradual depletion of domestic easy-to-mine shallow coal resources, the mining of shallow low-value difficult-to-mine coal seams and deep coal resources has gradually become the main direction of coal mining. However, the mining of shallow low-value difficult-to-mine coal seams and deep coal resources faces a series of technical difficulties, such as high mining costs of shallow low-value difficult-to-mine coal seams, high stress in deep coal seams, and increased ground temperature, which greatly increase the difficulty of mining and increase safety risks accordingly. Therefore, it is particularly urgent to find a technical solution that can not only efficiently develop shallow low-value difficult-to-mine coal seams and deep coal resources, but also achieve environmental friendliness and maximize resource utilization.

[0003] Traditional coal mining technology mainly relies on mechanized means, and is often accompanied by high environmental pollution and energy waste. In recent years, in-situ mining technology has become an important technological innovation. It directly extracts minerals and transforms resources underground by changing the physical and chemical state of the coal seam. This method can not only improve resource utilization, but also reduce ground disturbance and environmental pollution.

[0004] Patent CN118757133B discloses a method for producing hydrogen by synergistic injection of supercritical water and oxygen in situ into coal. This method requires in-situ hydrogen production in a supercritical water environment, so the requirements for equipment are extremely high. The supercritical water injection system, fracturing system, and high-temperature and high-pressure injection equipment all require additional technical support and investment. When supercritical water and high-pressure oxygen are injected, safety hazards may occur, especially during the in-situ reaction of the coal seam, where a violent reaction may occur between oxygen and hydrogen, increasing the safety risk. In addition, although the patent adjusts the temperature of the coal seam through temperature monitoring and reaction control, it fails to make full use of the heat generated during the reaction to preheat other coal seams or improve the energy efficiency of the system. This method also requires the coal seam to have a high burial depth and pressure, so its scope of application is subject to certain restrictions, and the applicability of the technology is relatively single. Although the patent proposes an electromagnetic flow device and a filling system for monitoring and controlling the filling process of the goaf, this process relies on high-precision electromagnetic induction technology and high-pressure grouting technology, which is complicated to operate and costly.

[0005] Patent CN117211741A discloses a method for in-situ hydrogen production and improved recovery in medium-deep and deep water-invaded gas reservoirs. The method involves multiple complex technical steps, such as well network densification, volume fracturing, and the use of water-gas conversion catalysts. Specifically, the method requires the injection of oxygen-rich gas into the densified wells and the use of igniters or chemical igniters to ignite the reservoirs. These operations require complex equipment configuration and precise regulation, which increases the difficulty of implementation and equipment investment. At the same time, the use of oxygen-rich gas and igniters to ignite the reservoirs will form a high-temperature and high-pressure environment underground. This mixture of high-temperature gas and oxygen poses a potential risk of explosion, which may lead to a runaway reaction, especially in the in-situ reaction of coal seams. The violent reaction of oxygen and hydrogen may cause safety hazards. Although the patent mentions the generation of reaction heat by gas ignition, it does not describe in detail how to use this heat to preheat other coal seams or improve energy efficiency.

[0006] Patent CN112878978A discloses a supercritical water fracturing method for underground coal gasification to produce hydrogen. This patent uses deep coal seams with a burial depth of more than 1,500 meters as the transformation object, which limits the applicability of this method, especially for shallow or low-rank coal seams. In this patent, oxygen-rich gas and ignition equipment are used to ignite the coal seam to stimulate the gasification reaction, resulting in a high temperature and high pressure environment, which poses a certain risk of explosion, especially in deep coal seams, where the risk of reaction runaway is greater.

[0007] Patent CN114876438B discloses a coal mining method for in-situ hydrogen production from coal with filling. Although the patent does not clearly state that the core idea of ​​the process is to use supercritical water (critical temperature 374.3℃, critical pressure 22.05MPa) to produce hydrogen, the patent claims emphasize that the threshold temperature for coal seam oxidation shall not be lower than 374℃, and the pressure of water and gas in the reaction zone shall not be lower than 22.1Mpa. Therefore, this technology also belongs to the technical category of in-situ coal hydrogen production with supercritical water. In this technology, the shaft arrangement of the mining unit adopts the method of three parallel U-shaped wells, and the coal seam preheating and goaf monitoring in the mining process are not refined. Although the reaction process is simple, there is the possibility of temperature control lag and reaction instability, and the heat recovery of the high-temperature surrounding rock in the goaf is not achieved, and the overall energy efficiency is low.

[0008] Patent CN113982555A discloses a system and method for underground in-situ pyrolysis of coal. The patent uses microwaves to preheat coal seams. Although microwave heating can provide precise temperature control, its control of heating depth and area is relatively limited, especially in large-scale mining processes. The penetration and heating uniformity of microwaves are not as extensive and stable as the flow control of water vapor and oxygen. When the coal seam is preheated to above 700°C, heat is released through the gas combustion cavity, and the product is processed using a three-phase separation device. The gas product is mixed with saturated air and burned in the gasification cavity. In this process, a large number of gas products directly participate in the combustion, resulting in a waste of energy. In addition, the well layout of the patent is extremely complex, which is not conducive to large-scale industrial application. At the same time, the patent does not describe in detail the management of goafs and the monitoring and control of rock stability, which is particularly important for underground coal mining.

[0009] Patent CN114876437A discloses a method for in-situ hydrogen production in coal seams using supercritical water. The reaction process in this patent relies on the complex interaction of supercritical water, oxygen and salt rock powder, and the permeability is adjusted by plugging the pores of the coal seam with salt rock powder, and finally hydrogen is generated. The selection and injection of salt rock powder and the changes in the reaction zone of the coal seam (such as the formation of the reacted zone, the reaction zone and the zone to be reacted) all require precise monitoring and adjustment, which makes the process more complicated and may lead to unstable reactions and greater control difficulties. This patent uses supercritical water, oxygen and a high-temperature heater to react in a closed homogeneous chamber, which poses the risk of high pressure, flammability and explosion. Especially in a closed environment of high temperature and high pressure, the reaction of oxygen and hydrogen may cause accidents. Therefore, these safety hazards require strict protective measures and real-time monitoring.

[0010] Patent CN117780326A discloses an underground coal in-situ pyrolysis hydrogen production device and method, which involves multiple devices and complex systems, including vertical wells, horizontal wells, methanogenic catalyst filling layers, supercritical fluid generators, hydrogen separation devices and other core components. In particular, the use of supercritical fluid generators requires high-pressure and high-temperature equipment, which increases the complexity and investment cost of the system. In addition, the interconnection and coordination between equipment requires precise control and technical support, which makes the cost of implementing the method high and the installation and maintenance of the equipment difficult.

[0011] Patents CN117211741A, CN112878978A, CN114876438B, CN117823112B, CN114876437A and CN117780326A all emphasize in-situ coal-to-hydrogen production in a supercritical water environment, and their main chemical reaction characteristics are as follows: 1. C+H2O=H2+CO, (temperature: 500-600℃, pressure:>25MPa); 2. CO+H2O=CO2+H2, (temperature: >550℃, pressure: no requirement).

[0012] From the above chemical reaction formula, it can be seen that the temperature of the supercritical water in-situ hydrogen production process only needs to be higher than 550°C, but the pressure requirement is relatively high, which needs to be greater than 25MPa. This puts high demands on the airtightness of the reservoir, the stability of the supercritical water generator, and the wellbore stability of the injection well and production well.

[0013] Most of the existing in-situ coal-to-hydrogen technologies (patents CN117211741A, CN112878978A, CN114876438B, CN117823112B, CN114876437A and CN117780326A) use high-pressure and high-temperature supercritical water, which leads to the following common problems in these patents: 1. High temperature and high pressure conditions: Most technologies rely on high temperature and high pressure environments to promote chemical reactions in coal seams, especially the use of supercritical water or oxygen-rich gas to react with coal seams (such as supercritical water fracturing to enhance hydrogen production, supercritical water and oxygen synergistic injection, in-situ coal seam gasification, etc.). These high temperature and high pressure conditions are the key to ensuring coal seam gasification and hydrogen production, but they also bring safety hazards and put forward higher requirements for equipment.

[0014] 2. Equipment complexity and high cost: All of these patents require multiple complex equipment configurations, including supercritical fluid generators, high-pressure injection systems, catalyst filling layers, etc. These devices not only place high demands on engineering design and installation, but also require high-precision operation and real-time control systems. The diversity and complexity of the equipment leads to increased capital expenditures and operating expenses, thereby increasing the difficulty of implementation and the technical threshold.

[0015] 3. Safety hazards: The reaction process under high pressure and high temperature environment has potential safety risks, especially the mixed reaction of oxygen and hydrogen is prone to violent reaction and even explosion. In addition, the use of supercritical fluid and oxygen-rich gas increases the danger of operation, and a strict safety monitoring system is required to prevent accidents.

[0016] 4. Coal seam adaptability issues: Most patents require coal seams to have high pressure and temperature conditions, so their applicability is usually limited to deep or higher-rank coal seams (such as those buried more than 1,500 meters deep). This limits the widespread application of these technologies, especially in the development of shallow or low-rank coal seams, which may face difficulties. Many technologies also rely on specific physical properties of coal seams (such as porosity and permeability), which may vary greatly in different coal seams, affecting the universality and flexibility of the technology.

[0017] 5. Insufficient energy efficiency and thermal energy utilization: Although these patents use high-temperature reaction and gasification technologies, most technologies do not fully consider how to effectively recover and utilize the heat energy generated during the reaction. For example, the heat generated during supercritical water and gasification processes can often only be used to maintain the reaction temperature, but cannot effectively preheat the coal seam or improve overall energy efficiency. This leads to low energy utilization efficiency and increases the demand for external energy.

[0018] In general, although these patents have proposed innovative ideas for in-situ hydrogen production from coal and accelerated chemical reactions through high temperature and high pressure conditions, they generally have problems such as high equipment complexity, low energy efficiency, great safety hazards and poor technical applicability. Summary of the invention

[0019] The present invention overcomes the shortcomings of the prior art and proposes an in-situ low-pressure and high-temperature coal hydrogen production method. The present invention is achieved through the following technical solutions: A method for producing hydrogen from coal at low pressure and high temperature in situ, comprising the following steps: Step 1: Arrange several wellbores on the surface toward the target coal seam in sequence. After the vertical sections of all wellbores reach the target coal seam, connect the adjacent wellbores by means of horizontal wells to fractur e the target coal seam to form coal seam fracturing fissures; Step 2: Select two adjacent wellbores as the injection well and the production well, inject superheated water vapor with a temperature of more than 650°C through the injection well to preheat the target coal seam, and when the temperature of the coal seam around the injection well is more than 450°C, inject oxygen and superheated water vapor into the target coal seam simultaneously. After the oxygen enters the target coal seam preheated to more than 450°C, it undergoes an oxidation reaction with the coal to release heat and further increase the temperature of the coal seam; Step 3, forming a coal seam hydrogen production reaction zone and a preheating zone in sequence from the injection well to the production well; after the temperature of the coal seam hydrogen production reaction zone is above 850°C, stop injecting oxygen and continue to inject superheated water vapor into the coal seam hydrogen production reaction zone to carry out hydrogen production reaction, and the generated H2 and CO2 flow from the coal seam hydrogen production reaction zone and preheating zone between the production well and the injection well to the production well, and the gas product is extracted through the production well; at the same time, the gas product preheats the coal seam in the preheating zone; Step 4: After the maximum temperature of the coal seam hydrogen production reaction zone is lower than 750°C and the H2 concentration in the product is lower than 20%, determine the distribution range of the goaf area, and then continue to inject oxygen, so that O2 and superheated water vapor are synchronously injected into the coal seam hydrogen production reaction zone, repeat steps 2 and 3, continue to carry out hydrogen production reaction on the coal seam between the injection well and the production well, and continue to produce the gas generated by the reaction; Step 5: When there is 20 to 40 meters of coal seam left at the front end of the goaf from the production well, close the production well and open the shaft behind the production well as the preparation well; the coal seam area between the production well and the preparation well is the preparation area; water vapor, H2 and CO2 gases are discharged from the goaf through the preparation area and then discharged from the preparation well. This process realizes the early preheating of the next mining unit coal seam, that is, the coal seam in the preparation area; Step 6: After the coal seam hydrogen production reaction zone and preheating zone between the current production well and the injection well have fully produced hydrogen, the current injection well is used as a grouting well, and mud is injected into the coal seam cavity that has completed in-situ hydrogen production through the grouting well for filling; at the same time, the current production well is used as the next injection well, and the current preparation well is used as the next production well, and steps 2 to 5 are performed between the next injection well and the next production well until all coal resources in the target coal seam are converted into hydrogen-rich gas products and the filling of all goafs is completed.

[0020] Furthermore, when the target coal seam is an inclined coal seam, first complete the hydrogen production of the deepest coal in the target coal seam, and complete the filling of the formed goaf, and then continue to arrange mining units along the direction of the coal seam in the upper coal seam adjacent to the goaf until all the hydrogen production of the inclined target coal seam from deep to shallow is completed.

[0021] Furthermore, the mining units consisting of production wells, injection wells, grouting wells and preparation wells are arranged parallel to the direction of the coal seams in the inclined coal seams, and the wellbore depth of each mining unit is consistent; the deepest coal in the target coal seam begins to produce hydrogen, and the hydrogen production work of all coal bodies is completed step by step. In this process, the filling work of the lower goaf relies on the deadweight of the mud to fill downward, realizing the natural zoning of filling and hydrogen production. At the same time, the water vapor evaporated by the mud under the preheating effect of the surrounding rock provides steam for the upper hydrogen production reaction, realizing the recovery and utilization of waste heat from the surrounding rock; the liquid water in the upper coal seam flows to the lower goaf under the action of its own weight, preventing the presence of liquid water from consuming the heat of the hydrogen production reaction in the upper layer.

[0022] Furthermore, when the target coal seam is a thick coal seam with a thickness of ≥10m, after the vertical sections of all wellbores reach the lower part of the target coal seam, horizontal wells are arranged in layers, and the horizontal wells include upper coal horizontal wells and lower coal horizontal wells; the adjacent wellbores are connected by means of upper coal horizontal wells and lower coal horizontal wells, and finally a horizontal well network with wellbores as nodes is formed in the coal seam, and volume fracturing is carried out in all lower coal horizontal wells to form coal seam fracturing fissures in the lower layer of the target coal seam; according to steps 2 to 6, the coal seam corresponding to the lower coal horizontal well is subjected to high-temperature coal hydrogen production mining, and after the coal hydrogen production work and grouting filling work of the coal seam are completed, the flower pipes of the corresponding sections of all wellbores are blocked, and the flower pipes of all wellbores in the section where the upper coal is located are opened, and steps 2 to 6 are repeated to carry out high-temperature coal hydrogen production mining for the coal seam corresponding to the upper coal horizontal well until the in-situ hydrogen production of the upper coal of the target coal seam and the filling of all goafs are completed.

[0023] Furthermore, distributed temperature and pressure sensors are arranged in the horizontal well to monitor the temperature of the coal seam.

[0024] Furthermore, the spacing between the distributed temperature and pressure sensors arranged in the horizontal well is ≤2m.

[0025] Furthermore, seismic wave and settlement monitoring holes and explosion source holes are evenly distributed in the upper strata of the target coal seam, high-precision positioning sensors and small high-precision seismic detectors are placed at the bottom of the seismic wave and settlement monitoring holes, and a proper amount of explosives is placed at the explosion source at the bottom of the explosion source hole.

[0026] Furthermore, in step 4, the distribution range of the goaf is determined by detonating the explosives in the explosion source at the bottom of the explosion source hole, and the volume and distribution range of the goaf are determined by receiving the seismic wave signal and the settlement of the coal seam roof through the high-precision positioning sensor and the small high-precision seismic detector at the bottom of the seismic wave and settlement monitoring hole. After completing the monitoring of the distribution characteristics of the goaf at that location, an appropriate amount of explosives is reloaded into the explosion source hole for the next detection of the distribution of the goaf.

[0027] Furthermore, the injection well is connected to an oxygen injection system via an oxygen injection pump and an oxygen flow pressure regulating valve, and the injection well is connected to a superheated steam injection system via a superheated steam flow pressure regulating valve and a steam injection regulating pump; during the injection process, the O2 injection flow rate is controlled by the oxygen injection pump and the oxygen flow pressure regulating valve to prevent O2 from oxidizing with the unexhausted H2 in the coal seam hydrogen production reaction zone and the preheating zone.

[0028] The beneficial effects of the present invention compared with the prior art are as follows: 1. Different from the existing supercritical water in-situ hydrogen production method, the present invention adopts low-pressure superheated water steam and oxygen synergistic injection to realize coal in-situ hydrogen production, and has lower requirements on ground boiler technology, cementing technology and coal seam surrounding rock sealing. Although the reaction temperature of coal hydrogen production under low-pressure environment (>800°C) is higher than the temperature required for coal hydrogen production in supercritical water environment (>600°C), high temperature of more than 1000°C can be easily reached by injecting oxygen to oxidize the coal seam in situ. Therefore, the present invention can realize the clean and efficient utilization of various complex, difficult to mine or low-order and low-value coal resources with a burial depth ranging from tens of meters to more than 1000 meters, and the technical applicability is wider.

[0029] 2. In the present invention, the hydrogen production process and the oxygen injection and heating process are carried out separately and alternately, which effectively prevents O2 and H2 from directly mixing and contacting in the high-temperature and closed environment of the coal seam, reduces the risk of violent reaction between O2 and H2 in the high-temperature and closed environment of the coal seam, and avoids the H2 product being consumed by O2 to reduce the output, ensuring that the high-temperature environment required for in-situ hydrogen production in the coal seam is only provided by coal seam oxidation without consuming H2 products.

[0030] 3. The present invention adopts two measures to achieve efficient utilization of thermal energy. First, by injecting mud into the high-temperature goaf, the water vapor evaporated from the mud participates in the hydrogen production process in the front-end hydrogen production reaction zone, making full use of the waste heat of the surrounding rock of the goaf, while reducing the steam injection amount of the injection well; second, through the group well regulation of injection wells, production wells and preparation wells, the high-temperature gases such as H2 and CO2 generated in the hydrogen production reaction zone are directed to flow and preheat the coal seams in the front-end preheating zone and the preparation zone, making full use of the heat energy carried by high-temperature gases such as H2 and CO2, while reducing the operating temperature of the production well and improving the stability of the system.

[0031] 4. The in-situ hydrogen production of the present invention in inclined coal seams has natural advantages. The mining units consisting of production wells, injection wells, grouting wells and preparation wells are arranged in parallel to the direction of the coal seams in the inclined coal seams. The shaft depth of each mining unit is consistent, which is easy to construct. Hydrogen production starts from the deep part of the coal seam and advances upward in stepped strips in the dip direction, and the strips are parallel to the direction of the strike, gradually completing the hydrogen production of all coal bodies. In this process, the filling work of the lower goaf can rely on the deadweight of the mud to fill the lower goaf, realizing the natural zoning of filling and hydrogen production. At the same time, the water vapor evaporated by the mud under the preheating effect of the surrounding rock can provide steam for the hydrogen production reaction zone in the upper mining unit, realizing the recovery and utilization of waste heat from the surrounding rock. In addition, the liquid water in the upper coal seam will also flow to the lower goaf under the action of its own weight, preventing the presence of liquid water from consuming the heat of the hydrogen production reaction zone. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of an in-situ low-pressure and high-temperature coal-to-hydrogen production unit; Figure 2This is a diagram of the in-situ low-pressure and high-temperature coal-to-hydrogen production system for a near-horizontal coal seam in Example 1; Figure 3 This is a schematic diagram of the well layout for in-situ low-pressure hydrogenation mining of inclined coal seams in Example 2; Figure 4 This is a schematic diagram of the layered layout of in-situ low-pressure hydrogenation mining in thick coal seams in Example 3; Figure 5 This is a schematic diagram of the integrated advancement of hydrogen production and filling of lower coal layer by in-situ low-pressure layered hydrogen production in a medium-thick coal seam in Example 3; Figure 6 This is a schematic diagram of the integrated advancement of upper coal-filling hydrogen production and mining with low-pressure layered hydrogen production in situ in a medium-thick coal seam in Example 3; Figure 7 This is a schematic diagram of the flower pipe in the coal seam section of the shaft in Example 3; Figure 8 Schematic diagram of the layout of distributed temperature and pressure sensors in horizontal wells.

[0033] Numbers in the figure: 1-target coal seam, 2-goaf, 3-grouting well, 4-horizontal well, 401-upper coal horizontal well, 402-lower coal horizontal well, 5-distributed temperature and pressure sensor, 6-injection well, 7-coal seam hydrogen production reaction zone, 8-preheating zone, 9-production well, 10-preparation zone, 11-preparation well, 1301-high-precision positioning sensor, 1302-small high-precision seismic detector, 14-explosion source, 15-seismic wave and settlement monitoring hole, 16-explosion source hole, 18-oxygen injection system, 19-superheated water vapor injection system, 24-oxygen injection pump, 25-oxygen flow pressure regulating valve, 26-superheated water vapor flow pressure regulating valve, 27-steam injection regulating pump, 36-coal seam fracturing fissure. DETAILED DESCRIPTION

[0034] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer, the present invention is further described in detail in conjunction with the embodiments and the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. The technical solutions of the present invention are described in detail below in conjunction with the embodiments and the accompanying drawings, but the scope of protection is not limited thereto.

[0035] Example 1 This embodiment proposes an in-situ low-pressure and high-temperature coal hydrogen production method, which is mainly used for in-situ high-temperature hydrogen production of various complex, difficult-to-mine or low-grade and low-value coal resources with burial depths ranging from tens of meters to more than 1,000 meters.

[0036] like Figure 1 and Figure 2 As shown, for a nearly horizontal coal seam with a burial depth of 500 meters and a thickness of 5 meters, in-situ low-pressure and high-temperature coal-to-hydrogen production is carried out. The specific implementation steps are as follows: Step 1: On the surface of the target coal seam 1, a group of wells are arranged with a spacing of 100m to 500m, including a grouting well 3, an injection well 6, a production well 9 and a preparation well 11. After the vertical sections of all wellbores reach the middle of the target coal seam 1, the adjacent wellbores are connected by means of horizontal wells 4, and finally a horizontal well network with wellbores as nodes is formed in the coal seam. Volume fracturing is carried out in all horizontal well sections to form a complex coal seam fracturing fissure 36 in the target coal seam 1, and then distributed temperature and pressure sensors 5 are arranged in all horizontal well sections (see Figure 8 ), and grouting is performed to seal all horizontal sections of the wellbore. The spacing between the distributed temperature and pressure sensors 5 arranged in the horizontal well 4 is ≤ 2m to ensure accurate monitoring of each sub-section of the target coal seam 1.

[0037] Step 2: Seismic wave and settlement monitoring holes 15 and explosion source holes 16 are arranged at intervals of 50 m in the stratum 30 m above the target coal seam 1, high-precision positioning sensors 1301 and small high-precision seismic detectors 1302 are placed at the bottom of the seismic wave and settlement monitoring holes 15, and appropriate amounts of explosives are placed at the explosion source 14 at the bottom of the explosion source hole 16. The high-precision positioning sensor 1301 has two main functions: one is to assist the small high-precision seismic detector 1302 in generating high-precision three-dimensional features of the goaf 2 with seismic wave data; the other is to monitor the settlement of the rock formation above the gasification cavity, and timely adjust the hydrogen production and goaf 2 filling plan according to the three-dimensional morphology of the goaf 2 and the deformation characteristics of the overlying rock formation, to ensure that the impact of the coal in-situ hydrogen production process on the stratum is minimized.

[0038] Step 3: Select two adjacent wellbores as the injection well 6 and the production well 9, the injection well 6 is connected to the oxygen injection system 18 through the oxygen injection pump 24 and the oxygen flow pressure regulating valve 25, and the injection well 6 is connected to the superheated water vapor injection system 19 through the superheated water vapor flow pressure regulating valve 26 and the steam injection regulating pump 27; The target coal seam 1 is preheated by injecting superheated steam with a temperature higher than 650°C through the injection well 6 and the superheated steam injection system 19. The temperature change of the coal seam is monitored by the distributed temperature and pressure sensor 5 pre-buried in the horizontal well 4. After the temperature of the coal seam around the injection well 6 is higher than 450°C, the oxygen injection system 18 is turned on to inject oxygen and superheated steam into the target coal seam 1 synchronously. After the oxygen enters the target coal seam 1 preheated to above 450°C, it undergoes an oxidation reaction with the coal to release heat and further increase the temperature of the coal seam. At this time, the temperature of the coal seam hydrogen production reaction zone 7 is further increased, and the heat required is provided by the self-heating of the coal seam oxidation reaction. The superheated steam injected on the ground is only used as a reactant for coal hydrogen production, and no longer as a heat carrier fluid. Therefore, the temperature of the superheated steam injected on the ground is subsequently adjusted to 300°C to ensure the long-term stable operation of the wellbore.

[0039] Step 4: The area extending 30m from the injection well 6 to the production well 9 is the coal seam hydrogen production reaction zone 7, and the area between the coal seam hydrogen production reaction zone 7 and the production well 9 is the preheating zone 8; after the temperature of the coal seam hydrogen production reaction zone 7 is above 850°C, the oxygen injection system 18 is closed and superheated water vapor is continuously injected into the coal seam hydrogen production reaction zone 7 for hydrogen production reaction, and the generated H2 and CO2 flow from the coal seam hydrogen production reaction zone 7 and the preheating zone 8 between the production well 9 and the injection well 6 to the production well 9 and the gas product is extracted through the production well 9; at the same time, the high-temperature gas product will preheat the coal seam in the preheating zone 8.

[0040] Step 5: After the maximum temperature of the coal seam hydrogen production reaction zone 7 is lower than 750°C and the H2 concentration in the product is lower than 20%, detonate the explosives in the explosion source 14 at the bottom of the explosion source hole 16, and determine the volume and distribution range of the goaf 2 by receiving the seismic wave signal and the settlement of the coal seam roof through the high-precision positioning sensor 1301 and the small high-precision seismic detector 1302 at the bottom of the seismic wave and settlement monitoring hole 15. After completing the monitoring of the distribution characteristics of the goaf 2 at this location, reload an appropriate amount of explosives into the explosion source hole 16 to prepare for the next detection of the distribution of the goaf 2.

[0041] It should be noted that when the amount of coal seam roof settlement obtained by the high-precision positioning sensor 1301 at the bottom of the seismic wave and settlement monitoring hole 15 is large, the hydrogen production process needs to be interrupted, the cavity formed by hydrogen production needs to be filled first, and hydrogen production can be carried out after the coal seam roof settlement is controlled.

[0042] Step 6: After the current distribution range of the goaf 2 is determined, the oxygen injection system 18 is turned on to inject O2 and superheated water vapor into the coal seam hydrogen production reaction zone 7 synchronously. During the injection process, the oxygen injection pump 24 and the oxygen flow pressure regulating valve 25 are used to strictly control the O2 injection flow rate to prevent O2 from reacting violently with the unexhausted H2 and other combustible gases in the coal seam hydrogen production reaction zone 7 and the preheating zone 8. After the overall temperature of the coal seam 30m around the coal seam hydrogen production reaction zone 7 reaches above 850°C, the O2 injection is stopped, and high-temperature water vapor is continuously injected into the coal seam for hydrogen production reaction. The generated H2 and CO2 flow from the coal seam hydrogen production reaction zone 7 and the preheating zone 8 between the production well 9 and the injection well 6 to the production well 9 and the gas product is extracted through the production well 9, while the coal seam in the preheating zone 8 to be reacted continues to be preheated. In this process, the flow rate of the superheated water vapor injected into the injection well 6 can be reduced to ensure that the flow rate of the superheated water vapor can meet the needs of the hydrogen production reaction.

[0043] Step 7: Repeat steps 5 and 6. When there is only 30m of coal seam left at the front end of goaf 2 from production well 9, close production well 9 and open preparation well 11 behind production well 9; the coal seam area between production well 9 and preparation well 11 is preparation area 10; high-temperature gases such as water vapor, H2 and CO2 pass through preparation area 10 from goaf 2 and are discharged from preparation well 11. This process can realize the early preheating of the next mining unit coal seam, that is, the coal seam in preparation area 10, while ensuring that the temperature of production well 9 and preparation well 11 is not too high.

[0044] Step 8: After the coal seam hydrogen production reaction zone 7 and preheating zone 8 between the current production well 9 and the injection well 6 have fully produced hydrogen, the current injection well 6 is used as the current grouting well 3, and mud is injected into the coal seam cavity that has completed in-situ hydrogen production through the current grouting well 3 for filling; at the same time, the current production well 9 is used as the next injection well 6, and the current preparation well 11 is used as the next production well 9, and steps 2 to 8 are performed between the next injection well 6 and the next production well 9 until all coal resources in the target coal seam 1 are converted into high-value hydrogen-rich gas products and the filling of all goafs 2 is completed.

[0045] It should be further explained that, since the target coal seam 1 is nearly horizontal and has a moderate thickness, the layout direction of the mining units can be mainly determined by the surface topography and the layout plan of the ground industrial square, and each mining unit can simultaneously carry out in-situ hydrogen production without affecting each other. When conditions permit, the injection well 6 and the production well 9 can also be shared to achieve large-scale synchronous hydrogen production and improve the recovery rate.

[0046] It should be further explained that: the following are the chemical reaction characteristics of low-pressure and high-temperature coal-to-hydrogen production in this embodiment: C+H2O=H2+CO, (temperature: 800-1000°C, pressure: normal pressure~22MPa); CO+H2O=CO2+H2, (temperature: >550℃, pressure: no requirement).

[0047] From the above chemical reaction formula, it can be seen that if the temperature is raised to above 800℃, the reaction pressure does not need to be above 22MPa, which greatly reduces the requirements for the tightness of the reservoir and the high-pressure resistance of the hydrogen production system. At the same time, it is very easy to achieve a high temperature environment of above 800℃ under in-situ conditions by injecting oxygen to oxidize the coal seam and release heat.

[0048] Example 2 See also Figure 1 and Figure 3For a steeply inclined coal seam with a burial depth of 500 meters and a thickness of 5 meters, in-situ high-temperature coal-to-hydrogen production is carried out, and the specific implementation steps 1-8 are the same as steps 1-8 of Example 1; the difference is that the target coal seam 1 is in a steeply inclined state; and when the deepest coal in the steeply inclined target coal seam 1 is completely hydrogenated, the formed goaf 2 is filled, and then the mining units are continued to be arranged along the direction of the coal seam in the upper coal seam adjacent to the goaf 2, and steps 1 to 8 are repeated until the steeply inclined target coal seam 1 is completely hydrogenated from deep to shallow.

[0049] It should be noted that the present invention has natural advantages in in-situ hydrogen production in inclined coal seams. The mining unit composed of the production well 9, the injection well 6, the grouting well 3 and the preparation well 11 is arranged in parallel to the direction of the coal seam in the inclined coal seam. The shaft depth of each mining unit is consistent, which is easy to construct. Hydrogen production starts from the bottom of the coal seam, and the hydrogen production of all coal bodies is completed step by step in the inclined direction. In this process, the filling work of the lower goaf 2 can rely on the deadweight of the mud to fill the lower part, realizing the natural zoning of filling and hydrogen production. At the same time, the water vapor evaporated from the mud under the preheating action of the surrounding rock can provide steam for the upper hydrogen production reaction zone, realizing the recovery and utilization of waste heat from the surrounding rock. In addition, the liquid water in the upper coal seam will also flow to the lower goaf 2 under the action of its own weight, preventing the presence of liquid water from consuming the heat of the upper hydrogen production reaction zone.

[0050] Example 3 like Figure 1 and Figure 4-6 As shown, for a nearly horizontal thick coal seam with a burial depth of 500 meters and a thickness of 10 meters, in-situ high-temperature coal-to-hydrogen production is carried out. The specific implementation steps are as follows: Step 1: Arrange a group of wells with a spacing of 100m to 500m on the surface of the target coal seam 1, specifically including a grouting well 3, an injection well 6, a production well 9 and a preparation well 11 arranged in sequence. After the vertical sections of all wellbores reach the lower part of the target coal seam 1, a layered arrangement of horizontal wells is adopted to form an upper coal horizontal well 401 and a lower coal horizontal well 402; Adjacent wellbores are connected by means of upper coal horizontal wells 401 and lower coal horizontal wells 402, and finally a horizontal well network with wellbores as nodes is formed in the coal seam. Volume fracturing is carried out in all lower coal horizontal wells 402 to form complex coal seam fracturing fissures 36 in the lower layer of the target coal seam 1. Distributed temperature and pressure sensors 5 are then arranged in all horizontal well sections, and all horizontal sections of the wellbores are sealed by grouting.

[0051] According to step 2 to step 8 of embodiment 1, the coal seam corresponding to the lower coal horizontal well 402 is mined for high temperature coal hydrogen production. After the coal hydrogen production work and grouting filling work are completed, the flower pipes corresponding to the sections of all wellbores are blocked (see Figure 7), open the flower pipes of the upper coal sections of all shafts, repeat steps 2 to 8, and carry out high-temperature coal hydrogen production on the coal seam corresponding to the upper coal horizontal well 401 until the in-situ hydrogen production of the upper coal of the target coal seam 1 and the filling of all goafs 2 are completed.

[0052] It should be noted that for the in-situ hydrogen production of thick coal seams, a layered mining method is adopted. The advantage of such construction is that when the lower layer produces hydrogen to form a goaf 2, its own weight will cause cracks in the upper coal seam, which is convenient for the flow of gas during the subsequent upper hydrogen production. The upper coal seam is also preheated during the lower hydrogen production. After grouting of the lower goaf 2, the waste heat of the surrounding rock of the goaf can be used to evaporate the moisture in the mud, providing a water vapor atmosphere for hydrogen production in the upper coal. At the same time, during the hydrogen production process of the upper coal seam, if there is a small amount of liquid water in the coal seam or roof, it will also flow to the lower goaf 2 under the action of gravity, preventing the influence of liquid water on the temperature rise of the coal seam.

[0053] It should be noted that the target coal seam 1 can be an ordinary easy-to-mine coal seam, or it can be a relict coal caused by the inability to carry out all-round and three-dimensional mining of the coal seam due to a large inclination, high gas, high impact ground pressure, soft surrounding rock, poor coal quality, extremely uneven coal seam thickness, or due to the limitations of past technology. However, there must be no water-rich layers directly connected to the coal seam on the top and bottom plates of the coal seam.

[0054] The above content is a further detailed description of the present invention in combination with a specific preferred embodiment. It cannot be determined that the specific embodiments of the present invention are limited to this. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the present invention, which should be regarded as belonging to the present invention and the scope of patent protection determined by the submitted claims.

Claims

1. A method for producing hydrogen from coal at low pressure and high temperature in situ, characterized in that: The following steps are involved: Step 1: a plurality of wellbores are arranged on the surface toward the target coal seam (1) in sequence, and after the vertical sections of all wellbores reach the target coal seam (1), adjacent wellbores are connected by means of horizontal wells (4), and the target coal seam (1) is fractured to form coal seam fracture cracks (36); Step 2: Select two adjacent wellbores as an injection well (6) and a production well (9), inject superheated water vapor with a temperature of more than 650° C. through the injection well (6) to preheat the target coal seam (1), and when the temperature of the coal seam around the injection well (6) reaches more than 450° C., inject oxygen and the superheated water vapor into the target coal seam (1) simultaneously. After the oxygen enters the target coal seam (1) preheated to more than 450° C., it undergoes an oxidation reaction with the coal to release heat and further increase the temperature of the coal seam; Step 3: The coal seam in the direction from the injection well (6) to the production well (9) sequentially forms a coal seam hydrogen production reaction zone (7) and a preheating zone (8); after the temperature of the coal seam hydrogen production reaction zone (7) is above 850°C, the injection of oxygen is stopped and superheated water vapor is continuously injected into the coal seam hydrogen production reaction zone (7) to carry out a hydrogen production reaction, and the generated H2 and CO2 flow from the coal seam hydrogen production reaction zone (7) and the preheating zone (8) between the production well (9) and the injection well (6) to the production well (9) and the gas product is extracted through the production well (9); at the same time, the gas product preheats the coal seam in the preheating zone (8); Step 4: After the maximum temperature of the coal seam hydrogen production reaction zone (7) is lower than 750°C and the H2 concentration in the product is lower than 20%, determine the distribution range of the goaf (2), and then continue to inject oxygen, so that oxygen and superheated water vapor are simultaneously injected into the coal seam hydrogen production reaction zone (7), repeat steps 2 and 3, continue to carry out hydrogen production reaction on the coal seam between the injection well (6) and the production well (9), and continue to produce gas generated by the reaction; Step 5: When there is only 20 to 40 m of coal seam left at the front end of the goaf (2) from the production well (9), the production well (9) is closed, and the shaft behind the production well (9) is opened as a preparation well (11); the coal seam area between the production well (9) and the preparation well (11) is the preparation area (10); water vapor, H2 and CO2 gases are discharged from the goaf (2) through the preparation area (10) and then from the preparation well (11). This process realizes the early preheating of the coal seam of the next mining unit, that is, the coal seam of the preparation area (10); Step 6: After the coal seam hydrogen production reaction zone (7) and preheating zone (8) between the current production well (9) and the injection well (6) have fully produced hydrogen, the current injection well (6) is used as a grouting well (3), and mud is injected into the coal seam cavity that has completed in-situ hydrogen production through the grouting well (3); at the same time, the current production well (9) is used as the next injection well (6), and the current preparation well (11) is used as the next production well (9), and steps 2 to 5 are performed between the next injection well (6) and the next production well (9), until all coal resources in the target coal seam (1) are converted into hydrogen-rich gas products and the filling work of all goafs (2) is completed.

2. The method for producing hydrogen from coal at low pressure and high temperature in situ according to claim 1, characterized in that: When the target coal seam (1) is an inclined coal seam, hydrogen production is first completed in the deepest coal of the target coal seam (1), and after the filling work is completed in the formed goaf (2), mining units are continuously arranged along the direction of the coal seam in the upper coal seam adjacent to the goaf (2) until hydrogen production is completed in the inclined target coal seam (1) from deep to shallow.

3. The method for producing hydrogen from coal at low pressure and high temperature in situ according to claim 2, characterized in that: The mining unit consisting of a production well (9), an injection well (6), a grouting well (3) and a preparation well (11) is arranged in parallel with the direction of the coal seam in the inclined coal seam, and the shaft depth of each mining unit is consistent; hydrogen production begins from the deepest coal in the target coal seam (1), and hydrogen production work is completed step by step in all coal bodies. In this process, the filling work of the lower goaf (2) relies on the deadweight of the mud to fill the lower part, realizing the natural partition of filling and hydrogen production. At the same time, the water vapor evaporated by the mud under the preheating effect of the surrounding rock provides steam for the upper hydrogen production reaction, realizing the recovery and utilization of the waste heat of the surrounding rock; the liquid water in the upper coal seam flows to the lower goaf (2) under the action of its own weight, preventing the existence of liquid water from consuming the heat of the hydrogen production reaction in the upper layer.

4. The method for producing hydrogen from coal at low pressure and high temperature in situ according to claim 1, characterized in that: When the target coal seam (1) is a thick coal seam with a thickness of ≥10 m, after the vertical sections of all wellbores reach the lower part of the target coal seam (1), horizontal wells (4) are arranged in layers, and the horizontal wells (4) include upper coal horizontal wells (401) and lower coal horizontal wells (402); adjacent wellbores are connected by means of the upper coal horizontal wells (401) and the lower coal horizontal wells (402), and finally a horizontal well network with wellbores as nodes is formed in the coal seam, and volume fracturing is performed in all lower coal horizontal wells (402), and in the lower layer of the target coal seam (1), a horizontal well network is formed. A coal seam fracturing fissure (36) is formed in the coal seam; according to steps 2 to 6, the coal seam corresponding to the lower coal horizontal well (402) is subjected to high-temperature coal-to-hydrogen production mining. After the coal-to-hydrogen production work and grouting filling work of the coal seam are completed, the flower pipes of all wellbore sections corresponding to the sections are blocked, and the flower pipes of all wellbore sections where the upper coal is located are opened. Steps 2 to 6 are repeated to carry out high-temperature coal-to-hydrogen production mining on the coal seam corresponding to the upper coal horizontal well (401), until the in-situ hydrogen production of the upper coal of the target coal seam (1) and the filling of all goafs (2) are completed.

5. The method for producing hydrogen from coal at low pressure and high temperature in situ according to any one of claims 1 to 4, characterized in that: Distributed temperature and pressure sensors (5) are arranged in the horizontal well (4) to monitor the temperature of the coal seam.

6. The method for producing hydrogen from coal at low pressure and high temperature in situ according to claim 5, characterized in that: The spacing between the distributed temperature and pressure sensors (5) arranged in the horizontal well (4) is ≤2 m.

7. The method for producing hydrogen from coal at low pressure and high temperature in situ according to any one of claims 1 to 4, characterized in that: Seismic wave and settlement monitoring holes (15) and explosion source holes (16) are evenly distributed in the upper strata of the target coal seam (1), a high-precision positioning sensor (1301) and a small high-precision seismic detector (1302) are placed at the bottom of the seismic wave and settlement monitoring hole (15), and an appropriate amount of explosives is placed at the explosion source (14) at the bottom of the explosion source hole (16).

8. The method for producing hydrogen from coal at low pressure and high temperature in situ according to claim 7, characterized in that: In step 4, the distribution range of the goaf (2) is determined by detonating the explosives in the explosion source (14) at the bottom of the explosion source hole (16), and receiving the seismic wave signal and the settlement of the coal seam roof through the high-precision positioning sensor (1301) and the small high-precision seismic detector (1302) at the bottom of the seismic wave and settlement monitoring hole (15) to determine the volume and distribution range of the goaf (2). After completing the monitoring of the distribution characteristics of the goaf (2), a suitable amount of explosives is reloaded into the explosion source hole (16) for the next detection of the distribution of the goaf (2).

9. The method for producing hydrogen from coal at low pressure and high temperature in situ according to claim 1, characterized in that: The injection well (6) is connected to an oxygen injection system (18) via an oxygen injection pump (24) and an oxygen flow pressure regulating valve (25), and the injection well (6) is connected to a superheated steam injection system (19) via a superheated steam flow pressure regulating valve (26) and a steam injection regulating pump (27); during the injection process, the oxygen injection flow rate is controlled by the oxygen injection pump (24) and the oxygen flow pressure regulating valve (25) to prevent the oxygen from reacting with the unexhausted H2 in the coal seam hydrogen production reaction zone (7) and the preheating zone (8) to cause an oxidation reaction.

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