A method for in-situ low-pressure high-temperature coal-to-hydrogen production

Abstract

The present invention discloses a method for in-situ low-pressure high-temperature coal-to-hydrogen production, belonging to the technical field of coal gasification mining; this method injects low-pressure superheated steam and oxygen into the coal seam, utilizes the heat released by the oxidation reaction of oxygen and coal to further heat the coal seam to 800-1000 °C, and reacts with steam to generate hydrogen and carbon dioxide; the present invention combines the synergistic injection of superheated steam and oxygen, adopts distributed temperature and pressure monitoring and well pattern technology to carry out in-situ efficient hydrogen production in coal; this method can not only break through the technical bottleneck of traditional coal mining, but also provide clean hydrogen energy while reducing carbon dioxide emissions, providing a new technical path for the efficient and clean utilization of coal resources.

Description

Technical Field

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

[0002] With the gradual depletion of easily mined shallow coal resources in China, the mining of shallow low-value and difficult-to-mine coal seams and deep coal resources has gradually become the main direction of coal mine mining. However, the mining of shallow low-value and difficult-to-mine coal seams and deep coal resources faces a series of technical problems, such as high mining costs for shallow low-value and difficult-to-mine coal seams, high stress in deep coal seams, and rising ground temperature, which have greatly increased the mining difficulty and correspondingly increased the safety risks. Therefore, it is particularly urgent to find a technical solution that can efficiently develop shallow low-value and difficult-to-mine coal seams and deep coal resources while achieving environmental friendliness and maximizing resource utilization.

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

[0004] Patent CN118757133B discloses a method for in-situ hydrogen production by synergistically injecting supercritical water and oxygen into coal. This method requires in-situ hydrogen production in a supercritical water environment, so the requirements for equipment are extremely high. An additional technical support and investment are needed for the supercritical water injection system, fracturing system, high-temperature and high-pressure injection equipment, etc. When using supercritical water and high-pressure oxygen injection, potential safety hazards may be caused. Especially during the in-situ reaction process in the coal seam, a violent reaction may occur between oxygen and hydrogen, increasing the safety risks. In addition, although this patent adjusts the coal seam temperature through temperature monitoring and reaction control, it fails to fully utilize the heat generated during the reaction process to preheat other coal seams or improve the energy efficiency of the system. This method also requires the coal seam to have a relatively high burial depth and pressure, so its application scope is limited to a certain extent, 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 are complex to operate and costly.

[0005] Patent CN117211741A discloses a method for in-situ hydrogen production and enhanced oil recovery in medium-deep and deep water-invaded gas reservoirs. This method involves multiple complex technical steps, such as well pattern densification, volume fracturing, and the use of water-gas shift catalysts. Specifically, the method requires injecting oxygen-rich gas into the infill wells and using an igniter or chemical ignition agent to ignite the gas reservoir. These operations require complex equipment configuration and precise regulation, increasing the implementation difficulty and equipment investment. At the same time, using oxygen-rich gas and an igniter to ignite the gas reservoir will form a high-temperature and high-pressure environment underground. The mixing of this high-temperature gas and oxygen poses a potential explosion risk, which may lead to a runaway reaction. Especially in in-situ reactions in coal seams, the violent reaction between oxygen and hydrogen may cause safety hazards. Although the patent mentions generating reaction heat by gas ignition, it does not detail how to use this heat to preheat other coal seams or improve energy efficiency.

[0006] Patent CN112878978A discloses a method for enhanced hydrogen production by supercritical water fracturing in underground coal gasification. This patent uses deep coal seams with a burial depth exceeding 1500 meters as the transformation object, which limits the applicability of this method. Especially for the development of shallow or low-rank coal seams, it is more difficult. In this patent, using oxygen-rich gas and ignition equipment to ignite the coal seam to initiate the gasification reaction results in a high-temperature and high-pressure environment, with a certain explosion risk. Especially in deep coal seams, the risk of a runaway reaction is greater.

[0007] Patent CN114876438B discloses a coal mining method for in-situ hydrogen production in backfilled coal seams. Although the patent does not explicitly state that the core idea of the process is to use supercritical water (critical temperature 374.3°C, critical pressure 22.05 MPa) for hydrogen production, it emphasizes in the claims that the threshold temperature for coal seam oxidation shall not be lower than 374°C, and the pressure of water and gas in the reaction zone shall not be lower than 22.1 Mpa. Therefore, this technology also belongs to the technical category of in-situ coal hydrogen production by supercritical water. In this technology, the wellbore layout of the mining unit adopts the method of three parallel U-shaped wells, without detailed design of coal seam preheating and goaf monitoring during the mining process. Although the reaction process is simple, there may be a lag in temperature control and unstable reactions, and the heat energy recovery of the high-temperature surrounding rock in the goaf is not achieved, resulting in low overall energy efficiency.

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

[0009] Patent CN114876437A discloses a method for in-situ hydrogen production from 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 adjusts the permeability by blocking the pores of the coal seam with salt rock powder, and finally produces hydrogen. The selection, injection of salt rock powder, and changes in the coal seam reaction zone (such as the formation of the reacted zone, the reaction progress zone, and the to-be-reacted zone) all require precise monitoring and adjustment, which makes the process relatively complex and may lead to unstable reactions, with greater control difficulty. This patent uses supercritical water, oxygen, and a high-temperature heater to carry out reactions in a closed homogeneous cavity, posing risks of high pressure, flammability, and explosion. Especially in a closed environment of high temperature and high pressure, the reaction between oxygen and hydrogen may trigger accidents. Therefore, these safety hazards require strict protective measures and real-time monitoring.

[0010] Patent CN117780326A discloses a device and method for in-situ coal pyrolysis to produce hydrogen underground. This method involves multiple devices and complex systems, including multiple core components such as vertical shafts, horizontal wells, methanation catalyst filling layers, supercritical fluid generators, and hydrogen separation devices. Especially in the use of supercritical fluid generators, high-pressure and high-temperature equipment is required, increasing the complexity and investment cost of the system. In addition, the interconnection and coordination between devices require precise control and technical support, which makes the cost of method implementation relatively high, and the installation and maintenance of equipment are more difficult.

[0011] Patents CN117211741A, CN112878978A, CN114876438B, CN117823112B, CN114876437A, and CN117780326A all emphasize in-situ coal hydrogen production under supercritical water environment, and their main chemical reaction characteristics are as follows:

[0012] 1. C + H2O = H2 + CO, (temperature: 500 - 600 °C, pressure: > 25 MPa);

[0013] 2. CO + H2O = CO2 + H2, (Temperature: >550 °C, Pressure: no requirement).

[0014] As can be seen from the above chemical reaction formula, the temperature in the in-situ hydrogen production process by supercritical water only needs to be higher than 550 °C, but the requirement for pressure is relatively high, needing to be greater than 25 MPa. This poses very high requirements for the tightness of the reservoir, the stability of the supercritical water generator, and the wellbore stability of the injection well and production well.

[0015] Most of the existing in-situ coal-to-hydrogen technologies (Patents CN117211741A, CN112878978A, CN114876438B, CN117823112B, CN114876437A, and CN117780326A) have adopted the route of supercritical water under high pressure and high temperature, which leads to the following obvious common problems in these patents:

[0016] 1. High-temperature and high-pressure conditions requirements: Most technologies rely on high-temperature and high-pressure environments to promote chemical reactions in coal seams, especially using supercritical water or oxygen-rich gases to react with coal seams (such as hydrogen production with enhanced efficiency by supercritical water fracturing, co-injection of supercritical water and oxygen, in-situ gasification of coal seams, etc.). These high-temperature and high-pressure conditions are the key to ensuring coal seam gasification and hydrogen production, but they also bring potential safety hazards and pose higher requirements for equipment.

[0017] 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 equipment not only pose relatively high requirements for engineering design and installation, but also require high-precision operation and real-time control systems. The diversity and complexity of the equipment lead to increased capital expenditure and operating expenditure, thus increasing the implementation difficulty and technical threshold.

[0018] 3. Safety hazards: The reaction process under high-pressure and high-temperature environments has potential safety risks. In particular, the mixed reaction of oxygen and hydrogen is prone to violent reactions and even explosions. In addition, the use of supercritical fluids and oxygen-rich gases increases the danger of operation, and a strict safety monitoring system is required to prevent accidents.

[0019] 4. Coal seam adaptability problems: Most patents require coal seams to have relatively high pressure and temperature conditions. Therefore, their applicability is usually limited to deep or higher-rank coal seams (such as with a burial depth exceeding 1500 meters). This limits the wide application of these technologies, especially in the development of shallow or low-rank coal seams, where difficulties may be faced. Many technologies also rely on specific physical properties of coal seams (such as porosity and permeability), and these properties may vary greatly in different coal seams, affecting the universality and flexibility of the technologies.

[0020] 5. Insufficient energy efficiency and heat energy utilization: Although high-temperature reactions and gasification technologies are used in these patents, most technologies do not fully consider how to effectively recover and utilize the heat energy generated during the reaction process. For example, the heat generated during supercritical water and gasification processes can often only be used to maintain the reaction temperature, and cannot effectively preheat the coal seam or improve the overall energy efficiency. This results in low energy utilization efficiency and increases the demand for external energy.

[0021] Generally speaking, although these patents have put forward innovative ideas in in-situ coal hydrogen production and accelerated chemical reactions through high-temperature and high-pressure conditions, they generally have problems such as high equipment complexity, low energy efficiency, large potential safety hazards, and poor technical applicability. Summary of the Invention

[0022] The present invention overcomes the deficiencies of the prior art and provides a method for in-situ low-pressure high-temperature coal hydrogen production. The present invention is realized through the following technical solutions:

[0023] A method for in-situ low-pressure high-temperature coal hydrogen production, comprising the following steps:

[0024] Step 1: Arrange a number of wellbores on the surface in sequence towards the target coal seam. After the vertical sections of all wellbores reach the target coal seam, adjacent wellbores are connected in the form of horizontal wells, and the target coal seam is fractured to form coal seam fracture cracks.

[0025] Step 2: Select two adjacent wellbores as the injection well and the production well. Inject superheated steam with a temperature > 650 °C through the injection well to preheat the target coal seam. After the temperature of the coal seam around the injection well > 450 °C, oxygen and superheated steam are injected into the target coal seam synchronously. After the oxygen enters the target coal seam preheated to above 450 °C, it reacts with the coal to release heat and further increase the coal seam temperature.

[0026] Step 3: A coal seam hydrogen production reaction zone and a preheating zone are sequentially formed in the coal seam from the injection well towards the production well; after the temperature of the coal seam hydrogen production reaction zone is above 850 °C, stop injecting oxygen and continuously inject superheated steam into the coal seam hydrogen production reaction zone for hydrogen production reaction. The generated H2 and CO2 flow from the coal seam hydrogen production reaction zone and the preheating zone between the production well and the injection well to the production well and the gas product is produced through the production well; at the same time, the gas product preheats the coal seam in the preheating zone.

[0027] Step 4: After the highest temperature in 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, and then continue to inject oxygen to synchronously inject O2 and superheated steam into the coal seam hydrogen production reaction zone. Repeat Step 2 and Step 3 to continuously carry out hydrogen production reaction on the coal seam between the injection well and the production well, and continuously produce the gas generated by the reaction.

[0028] Step 5: When the coal seam at the front end of the goaf is 20 - 40 m away from the production well, close the production well and open the shaft of the well behind the production well, which serves 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 through the preparation well, and this process realizes the preheating of the coal seam in the next mining unit, i.e., the coal seam in the preparation area.

[0029] Step 6: After the hydrogen production reaction area and the preheating area of the coal seam between the current production well and the injection well have completed sufficient hydrogen production, use the current injection well as a grouting well and inject slurry into the coal seam cavity that has completed in-situ hydrogen production through the grouting well for filling; at the same time, use the current production well as the next injection well and the current preparation well as the next production well, and perform Steps 2 to 5 between the next injection well and the next production well until all the coal resources in the target coal seam are converted into hydrogen-rich gas products and the filling of all goafs is completed.

[0030] Furthermore, when the target coal seam is an inclined coal seam, first complete the hydrogen production of all the coal in the deepest layer of the target coal seam and complete the filling of the formed goaf, and then continue to arrange mining units along the coal seam strike in the upward coal seam adjacent to the goaf until the hydrogen production of the inclined target coal seam is completed from deep to shallow.

[0031] Even further, the mining units composed of production wells, injection wells, grouting wells, and preparation wells are arranged parallel to the coal seam strike in the inclined coal seam, and the shaft depths of each mining unit are the same; hydrogen production starts from the coal in the deepest layer of the target coal seam, and the hydrogen production of all coal bodies is gradually completed. During this process, the filling of the lower goaf relies on the self-weight of the slurry to fill downward, realizing the natural zoning of filling and hydrogen production. At the same time, the water vapor evaporated by the slurry under the preheating effect of the surrounding rock provides steam for the hydrogen production reaction in the upper part, realizing the recovery and utilization of the waste heat of the surrounding rock; the liquid water in the upper coal seam flows downward to the lower goaf under the action of self-weight, preventing the heat consumption of the hydrogen production reaction in the upper layer due to the existence of liquid water.

[0032] Further, when the target coal seam is a thick coal seam with a thickness of ≥ 10 m, horizontal wells are arranged in layers after the vertical sections of all shafts reach the lower part of the target coal seam. The horizontal wells include upper coal layer horizontal wells and lower coal layer horizontal wells; adjacent shafts are connected through the upper coal layer horizontal wells and the lower coal layer horizontal wells, and finally a horizontal well network with shafts as nodes is formed in the coal seam. Volume fracturing is implemented in all lower coal layer horizontal wells to form coal seam fracturing fissures in the lower layer of the target coal seam; the high-temperature coal-to-hydrogen extraction is carried out on the coal seam corresponding to the lower coal layer horizontal wells according to steps 2 to 6. After the coal-to-hydrogen work and grouting filling work of this coal seam are completed, the screen pipes corresponding to the segmented sections of all shafts are blocked, and the screen pipes in the segmented sections where the upper coal layer is located in all shafts are opened. Repeat steps 2 to 6 to carry out high-temperature coal-to-hydrogen extraction on the coal seam corresponding to the upper coal layer horizontal wells until the in-situ hydrogen production of the upper coal layer of the target coal seam and the filling work of all goafs are completed.

[0033] Further, distributed temperature and pressure sensors are arranged in the horizontal wells to monitor the coal seam temperature.

[0034] Further, the spacing of the distributed temperature and pressure sensors arranged in the horizontal wells is ≤ 2 m.

[0035] Further, seismic wave and settlement monitoring holes and explosion source holes are evenly distributed in the strata above the target coal seam. High-precision positioning sensors and small high-precision seismic geophones are placed at the bottoms of the seismic wave and settlement monitoring holes, and an appropriate amount of explosives is placed at the bottom explosion source of the explosion source holes.

[0036] Further, in step 4, to determine the distribution range of the goaf, the explosives in the bottom explosion source of the explosion source hole are detonated, and the volume and distribution range of the goaf are determined by receiving seismic wave signals and the settlement of the coal seam roof through the high-precision positioning sensors and small high-precision seismic geophones at the bottoms of the seismic wave and settlement monitoring holes. After the monitoring of the distribution characteristics of the goaf at this location is completed, an appropriate amount of explosives is reinstalled in the explosion source hole for the next detection of the goaf distribution.

[0037] Further, the injection well is connected to an oxygen injection system through an oxygen injection pump and an oxygen flow and pressure regulating valve, and the injection well is connected to a superheated steam injection system through a superheated steam flow and pressure regulating valve and a steam injection regulating pump; during the injection process, the O2 injection flow is controlled through the oxygen injection pump and the oxygen flow and pressure regulating valve to prevent the oxidation reaction of O2 with the unexhausted H2 in the coal-to-hydrogen reaction zone and the preheating zone of the coal seam.

[0038] The beneficial effects of the present invention compared with the prior art are as follows:

[0039] 1. Different from the existing in-situ hydrogen production methods using supercritical water, the present invention realizes in-situ coal hydrogen production by synergistically injecting low-pressure superheated steam and oxygen, which has lower requirements for surface boiler technology, cementing technology, and the sealing property of coal seam surrounding rock. Although the reaction temperature for coal hydrogen production under low-pressure conditions (>800 °C) is higher than that required for coal hydrogen production in a supercritical water environment (>600 °C), by in-situ oxidizing the coal seam through oxygen injection, a high temperature above 1000 °C can be easily achieved. Therefore, the present invention can realize the clean and efficient utilization of various complex and difficult-to-mine or low-rank and low-value coal resources with burial depths ranging from dozens of meters to over 1000 meters, and has a wider technical applicability.

[0040] 2. In the present invention, the hydrogen production process and the oxygen injection heating process are carried out separately and alternately, effectively preventing the direct mixing and contact of O2 and H2 in the high-temperature and airtight environment of the coal seam, reducing the risk of violent reaction between O2 and H2 in the high-temperature and airtight environment of the coal seam, and at the same time avoiding the consumption of H2 products by O2 and reducing the production. It ensures that the high-temperature environment required for in-situ coal seam hydrogen production is only provided by coal seam oxidation without consuming H2 products.

[0041] 3. The present invention realizes the efficient utilization of thermal energy through two measures. One is to inject slurry into the high-temperature goaf, and the water vapor evaporated from the slurry 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 in the goaf and at the same time reducing the steam injection volume of the injection well. The other is to adjust and control the group of injection wells, production wells, and preparation wells, so that high-temperature gases such as H2 and CO2 generated in the hydrogen production reaction zone flow directionally and preheat the coal seams in the front-end preheating zone and the preparation zone, making full use of the thermal energy carried by high-temperature gases such as H2 and CO2, and at the same time reducing the operating temperature of the production well and improving the stability of the system.

[0042] 4. The present invention has natural advantages for in-situ hydrogen production in inclined coal seams. The mining unit composed of production wells, injection wells, grouting wells, and preparation wells is arranged parallel to the coal seam strike in the inclined coal seam, and the wellbore depths of each mining unit are the same, which is easy to construct. Hydrogen production starts from the deep part of the coal seam and advances step by step upward in the dip direction in a stepped strip, and the strip is parallel to the strike direction, gradually completing the hydrogen production work for all coal bodies. During this process, the filling work in the lower goaf can rely on the self-weight of the slurry to fill the lower goaf, realizing the natural zoning of filling and hydrogen production. At the same time, the water vapor evaporated from the slurry 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 the waste heat of the surrounding rock. In addition, the liquid water in the upper coal seam will also flow downward to the lower goaf under its own weight, preventing the heat consumption in the hydrogen production reaction zone due to the presence of liquid water. Description of the Drawings

[0043] Figure 1 Schematic diagram of the in-situ low-pressure and high-temperature coal hydrogen production mining unit;

[0044] Figure 2 It is a diagram of the in-situ low-pressure and high-temperature coal-to-hydrogen system for nearly horizontal coal seams in Example 1;

[0045] Figure 3 It is a schematic diagram of well layout for in-situ low-pressure hydrogen production and mining of inclined coal seams in Example 2;

[0046] Figure 4 It is a schematic diagram of layered layout for in-situ low-pressure hydrogen production and mining of thick coal seams in Example 3;

[0047] Figure 5 It is a schematic diagram of integrated hydrogen production and filling advancement for the lower coal seam in in-situ low-pressure layered hydrogen production and mining of thick coal seams in Example 3;

[0048] Figure 6 It is a schematic diagram of integrated hydrogen production and filling advancement for the upper coal seam in in-situ low-pressure layered hydrogen production and mining of thick coal seams in Example 3;

[0049] Figure 7 It is a schematic diagram of the slotted casing in the wellbore coal seam section in Example 3;

[0050] Figure 8 It is a schematic diagram of the layout of distributed temperature and pressure sensors in horizontal wells.

[0051] Reference numerals in the figure: 1 - target coal seam, 2 - goaf, 3 - grouting well, 4 - horizontal well, 401 - upper coal seam horizontal well, 402 - lower coal seam horizontal well, 5 - distributed temperature and pressure sensors, 6 - injection well, 7 - coal seam hydrogen production reaction zone, 8 - preheating zone, 9 - production well, 10 - preparation area, 11 - preparation well, 1301 - high-precision positioning sensor, 1302 - small high-precision seismic geophone, 14 - explosion seismic source, 15 - seismic wave and settlement monitoring hole, 16 - explosion seismic source hole, 18 - oxygen injection system, 19 - superheated steam injection system, 24 - oxygen injection pump, 25 - oxygen flow and pressure regulating valve, 26 - superheated steam flow and pressure regulating valve, 27 - steam injection regulating pump, 36 - coal seam fracturing crack. Detailed implementation manners

[0052] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer and more understandable, the present invention will be further described in detail in combination with the embodiments and the drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. The technical solutions of the present invention will be described in detail below in combination with the embodiments and the drawings, but the protection scope is not limited by this.

[0053] Example 1

[0054] This example proposes a method for in-situ low-pressure and high-temperature coal-to-hydrogen, which is mainly used for in-situ high-temperature hydrogen production of various complex and difficult-to-mine or low-rank and low-value coal resources with burial depths ranging from dozens of meters to more than 1000 meters.

[0055] As Figure 1 and Figure 2 shown, for in-situ low-pressure and high-temperature coal-to-hydrogen extraction of a nearly horizontal coal seam with a burial depth of 500 m and a thickness of 5 m, the specific implementation steps are as follows:

[0056] Step 1: Arrange a group of wells with a spacing of 100 m to 500 m 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 the wellbores reach the middle of the target coal seam 1, the adjacent wellbores are connected in the form of a horizontal well 4. Finally, a horizontal well network with the wellbores as nodes is formed in the coal seam. Volume fracturing is implemented in all the horizontal well sections to form complex coal seam fracturing fissures 36 in the target coal seam 1. Then, distributed temperature and pressure sensors 5 are arranged in all the horizontal well sections (see Figure 8 ), and the horizontal sections of all the wellbores are sealed by grouting. The spacing of the distributed temperature and pressure sensors 5 arranged in the horizontal well 4 is ≤ 2 m to ensure accurate monitoring of each partition section of the target coal seam 1.

[0057] Step 2: Arrange seismic wave and settlement monitoring holes 15 with a spacing of 50 m and explosion source holes 16 in the strata 30 m above the target coal seam 1. Place a high-precision positioning sensor 1301 and a small high-precision geophone 1302 at the bottom of the seismic wave and settlement monitoring holes 15, and place an appropriate amount of explosive at the bottom explosion source 14 of the explosion source holes 16. The high-precision positioning sensor 1301 mainly has two functions. One is to assist in generating high-precision three-dimensional characteristics of the goaf 2 from the seismic wave data obtained by the small high-precision geophone 1302. The other is to monitor the settlement of the rock strata above the gasification cavity and timely adjust the hydrogen production and goaf 2 filling schemes according to the three-dimensional shape of the goaf 2 and the deformation characteristics of the overlying rock strata to ensure minimizing the impact of the in-situ coal-to-hydrogen process on the strata.

[0058] Step 3: Select two adjacent wellbores as the injection well 6 and the production well 9. The injection well 6 is connected to an oxygen injection system 18 through an oxygen injection pump 24 and an oxygen flow and pressure regulating valve 25, and the injection well 6 is connected to a superheated steam injection system 19 through a superheated steam flow and pressure regulating valve 26 and a steam injection regulating pump 27;

[0059] Superheated steam with a temperature higher than 650°C is injected through the injection well 6 and the superheated steam injection system 19 to preheat the target coal seam 1. The distributed temperature and pressure sensors 5 embedded in the horizontal well 4 are used to monitor the temperature change of the coal seam. After the temperature of the coal seam around the injection well 6 is higher than 450°C, the oxygen injection system 18 is opened to inject oxygen into the target coal seam 1 synchronously with the superheated steam. After the oxygen enters the target coal seam 1 preheated to above 450°C, it reacts with the coal to release heat and further increase the coal seam temperature. At this time, the temperature of the hydrogen production reaction zone 7 in the coal seam further increases, and the heat required is provided by the self-generated heat of the coal seam oxidation reaction. The superheated steam injected from the ground only serves as a reactant for coal hydrogen production and no longer serves as a heat-carrying fluid. Therefore, the temperature of the superheated steam injected from the ground is then lowered to 300°C to ensure the long-term stable operation of the wellbore.

[0060] Step 4: The area where the injection well 6 extends 30 m towards the production well 9 is the hydrogen production reaction zone 7 in the coal seam, and the area between the hydrogen production reaction zone 7 in the coal seam and the production well 9 is the preheating zone 8; after the temperature of the hydrogen production reaction zone 7 in the coal seam is above 850°C, the oxygen injection system 18 is closed and superheated steam is continuously injected into the hydrogen production reaction zone 7 in the coal seam for hydrogen production reaction. The generated H2 and CO2 flow from the hydrogen production reaction zone 7 and the preheating zone 8 in the coal seam between the production well 9 and the injection well 6 to the production well 9 and the gas products are produced through the production well 9; meanwhile, the high-temperature gas products will preheat the coal seam at the preheating zone 8.

[0061] Step 5: After the highest temperature of the hydrogen production reaction zone 7 in the coal seam is lower than 750°C and the H2 concentration in the product is lower than 20%, the explosive in the explosive source 14 at the bottom of the explosion source hole 16 is detonated. The high-precision positioning sensor 1301 and the small high-precision seismic geophone 1302 at the bottom of the seismic wave and settlement monitoring hole 15 are used to receive the seismic wave signals and the coal seam roof settlement conditions to determine the volume and distribution range of the goaf 2. After the distribution characteristics of the goaf 2 at this location are monitored, an appropriate amount of explosive is reloaded into the explosion source hole 16 for the next detection of the distribution of the goaf 2.

[0062] It should be noted that when the coal seam roof settlement amount 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, and the cavity formed by hydrogen production is first filled, and after controlling the subsidence of the coal seam roof, hydrogen production can be carried out again.

[0063] Step 6: After determining the distribution range of the current goaf 2, turn on the oxygen injection system 18 to inject O2 and superheated steam into the coal seam hydrogen production reaction zone 7 synchronously. During the injection process, strictly control the O2 injection flow through the oxygen injection pump 24 and the oxygen flow and pressure regulating valve 25 to prevent violent oxidation reactions between O2 and combustible gases such as unexhausted H2 in the coal seam hydrogen production reaction zone 7 and the preheating zone 8. After the overall temperature of the coal seam within 30 m around the coal seam hydrogen production reaction zone 7 reaches above 850 °C, stop the O2 injection, and continue to inject high-temperature steam 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 are extracted through the production well 9, while continuing to preheat the coal seam in the preheating zone 8 to be reacted. During this process, the flow rate of the superheated steam injected by the injection well 6 can be adjusted downward to ensure that the flow rate of the superheated steam can meet the requirements of the hydrogen production reaction.

[0064] Step 7: Repeat Step 5 and Step 6. When the coal seam at the front end of the goaf 2 is 30 m away from the production well 9, close the production well 9 and open the preparation well 11 behind the production well 9; the coal seam area between the production well 9 and the preparation well 11 is the preparation area 10; high-temperature gases such as steam, H2, and CO2 gas are discharged from the goaf 2 through the preparation area 10 and then through the preparation well 11. This process can preheat the coal seam of the next mining unit, that is, the coal seam in the preparation area 10 in advance, while ensuring that the temperatures of the production well 9 and the preparation well 11 do not become too high.

[0065] Step 8: After the coal seam hydrogen production reaction zone 7 and the preheating zone 8 between the current production well 9 and the injection well 6 are fully hydrogenated, use the current injection well 6 as the current grouting well 3, and inject slurry into the coal seam cavity that has completed in-situ hydrogen production through the current grouting well 3; at the same time, use the current production well 9 as the next injection well 6, and use the current preparation well 11 as the next production well 9, and perform Steps 2 to 8 between the next injection well 6 and the next production well 9 until all the 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.

[0066] It should be further noted that since the target coal seam 1 is nearly horizontal and the coal seam thickness is moderate, the layout direction of the mining unit can be mainly determined by the surface topography and the layout plan of the ground industrial square. Moreover, the in-situ hydrogen production of each mining unit can be carried out synchronously on the premise of not affecting each other, and 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.

[0067] It should be further noted that the following are the chemical reaction characteristics of low-pressure and high-temperature coal-to-hydrogen in this embodiment: C + H2O = H2 + CO, (temperature: 800 - 1000 °C, pressure: atmospheric pressure ~ 22 MPa);

[0068] CO + H2O = CO2 + H2, (Temperature: >550 °C, Pressure: no requirement).

[0069] As can be seen from the above chemical reaction formula, if the temperature is raised to above 800 °C, there is no need to require the reaction pressure to reach above 22 MPa, which greatly reduces the requirements for the tightness of the reservoir and the high-pressure resistance characteristics of the hydrogen production system. At the same time, by injecting oxygen to oxidize and release heat in the coal seam, it is very easy to achieve a high-temperature environment above 800 °C under in-situ conditions.

[0070] Example 2

[0071] See Figure 1 and Figure 3 For the in-situ high-temperature coal-to-hydrogen mining of a steeply inclined coal seam with a burial depth of 500 meters and a thickness of 5 meters, the specific implementation steps 1-8 are the same as those of step 1-8 in Example 1; the difference is that the target coal seam 1 is in a steeply inclined state; and when all the coal in the deepest layer of the steeply inclined target coal seam 1 has been converted to hydrogen and the gob 2 has been filled after the hydrogen production is completed, continue to arrange mining units along the strike of the coal seam in the up-going coal seam adjacent to the gob 2, and repeat steps 1 to 8 until all the steeply inclined target coal seam 1 has been converted to hydrogen from deep to shallow.

[0072] It should be noted that the in-situ hydrogen production of the present invention for inclined coal seams has natural advantages. The mining unit composed of the production well 9, the injection well 6, the grouting well 3 and the preparation well 11 is arranged parallel to the strike of the coal seam in the inclined coal seam, and the wellbore depths of each mining unit are the same, which is easy to construct; starting from the bottom of the coal seam to produce hydrogen, step by step upward in the dip direction, and gradually complete the hydrogen production work of all coal bodies. In this process, the filling work of the lower gob 2 can rely on the self-weight of the slurry to fill downward, realizing the natural zoning of filling and hydrogen production. At the same time, the water vapor evaporated by the slurry under the preheating effect of the surrounding rock can provide steam for the upper hydrogen production reaction zone, realizing the recovery and utilization of the waste heat of the surrounding rock. In addition, the liquid water in the upper coal seam will also flow downward to the lower gob 2 under the action of self-weight, preventing the heat consumption of the hydrogen production reaction zone in the upper layer due to the existence of liquid water.

[0073] Example 3

[0074] As Figure 1 and Figures 4 - 6 shown, for the in-situ high-temperature coal-to-hydrogen mining of a nearly horizontal thick coal seam with a burial depth of 500 meters and a thickness of 10 meters, the specific implementation steps are as follows:

[0075] Step 1: Arrange a group of wells with a spacing of 100 m to 500 m 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 the wellbores reach the lower part of the target coal seam 1, horizontal wells are arranged in a layered manner to form an upper coal seam horizontal well 401 and a lower coal seam horizontal well 402;

[0076] Connect adjacent wellbores through the upper coal seam horizontal well 401 and the lower coal seam horizontal well 402, and finally form a horizontal well network with the wellbores as nodes in the coal seam. Implement volume fracturing in all the lower coal seam horizontal wells 402 to form complex coal seam fracturing fissures 36 in the lower layer of the target coal seam 1. Then arrange distributed temperature and pressure sensors 5 in all the horizontal well sections, and grout to seal the horizontal sections of all the wellbores.

[0077] Carry out high-temperature coal-to-hydrogen extraction on the coal seam corresponding to the lower coal seam horizontal well 402 according to Steps 2 to 8 of Embodiment 1. After completing the coal-to-hydrogen production work and grouting filling work of this coal seam, block the screen pipes corresponding to the segmented sections of all the wellbores (see Figure 7 ), open the screen pipes of the upper coal seam corresponding segmented sections of all the wellbores, repeat Steps 2 to 8, and carry out high-temperature coal-to-hydrogen extraction on the coal seam corresponding to the upper coal seam horizontal well 401 until the in-situ hydrogen production of the upper coal seam of the target coal seam 1 and the filling work of all the goafs 2 are completed.

[0078] 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 goaf 2 is formed during hydrogen production in the lower layer, fissures will be generated in the upper coal seam under the action of self-weight, which is convenient for the flow of gas during subsequent hydrogen production in the upper layer. During hydrogen production in the lower layer, the upper coal seam is also preheated. After grouting the lower goaf 2, the heat of the surrounding rock of the goaf can be used to evaporate the water in the slurry, providing a steam atmosphere for hydrogen production in the upper coal seam. At the same time, during the hydrogen production process of the upper coal seam, when there is a small amount of liquid water in the coal seam or roof, it will also flow downward into the lower goaf 2 under the action of gravity, preventing the influence of liquid water on the temperature rise of the coal seam.

[0079] It should be noted that the target coal seam 1 can be an ordinary easily mined coal seam, or it can be residual coal caused by a large dip angle, high gas content, high rock burst tendency, weak surrounding rock, poor coal quality, extremely uneven coal seam thickness, or due to the limitations of past technologies that prevent all-round and three-dimensional mining of the coal seam. However, there should be no water-rich layer directly connected to the coal seam in the roof and floor of the coal seam.

[0080] The above content is a further detailed description of the present invention in combination with specific preferred embodiments. It cannot be determined that the specific embodiments of the present invention are limited to this. For those of ordinary skill in the technical field to which the present invention belongs, without departing from the premise of the present invention, several simple deductions or substitutions can still be made, which should all be regarded as belonging to the patent protection scope determined by the claims submitted for the present invention.

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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