Underground reservoir method for sealing carbon dioxide and fluoride through cooperation of high-alkalinity mine water and fly ash

Abstract

The invention discloses an underground reservoir method for synergistically storing carbon dioxide and fluoride through high-alkalinity mine water and fly ash, and relates to the cross technical field of environmental engineering, carbon capture, utilization and storage and mining area ecological restoration. The method comprises the following steps: selecting a waste coal mine goaf which is stable in structure, provided with a waterproof top plate and a waterproof bottom plate and buried in the depth of 200-800m as an underground reservoir, and carrying out anti-seepage curtain reinforcement treatment on the underground reservoir by adopting a cement paste grouting process to form a closed isolation space. According to the underground reservoir method for synergistically sealing and storing the carbon dioxide and the fluoride through the high-alkalinity mine water and the fly ash, collaborative coupling of five resources including the CO, the fly ash, the coal gangue, the high-fluorine and high-alkalinity mine water and the abandoned goaf is achieved for the first time, and the four problems of carbon emission, solid waste stockpiling, water pollution and geological disasters are synchronously solved; and a closed-loop system of waste recycling, pollutant sealing and storage and geological environment restoration is formed.

Description

Technical Field

[0001] This invention relates to the interdisciplinary fields of environmental engineering, carbon capture, utilization and storage, and ecological restoration of mining areas, specifically a method for the co-storage of carbon dioxide and fluorides in underground reservoirs using high-alkalinity mine water and fly ash. Background Technology

[0002] In my country's major coal-producing areas in Northwest China, the "coal-power integration" development model is widely adopted, with coal mines and coal-fired power plants built and operated adjacent to each other, forming a closed-loop energy production system. However, in the long-term development process, these industrial parks have accumulated multiple interconnected environmental governance problems, which have become a key bottleneck restricting the region's green development. Firstly, coal mining has resulted in a large number of abandoned mines and goaf areas. In the Northwest region alone, the cumulative volume of goaf areas has exceeded 10 billion m³. These goaf areas not only pose risks of geological disasters such as ground subsidence and ground fissures, but also continuously emit large amounts of mine water. This type of mine water is generally characterized by "high fluoride, high alkalinity, and high hardness," with fluoride ion concentrations typically ranging from 5 to 50 mg / L, far exceeding the 1.0 mg / L limit stipulated in the "Surface Water Environmental Quality Standard" (GB3838-2002). Traditional treatment processes, such as chemical precipitation, require the addition of large amounts of reagents, resulting in large amounts of sludge and treatment costs as high as 15 to 30 yuan / m³. Adsorption methods suffer from limited adsorption capacity and difficulties in regeneration. Membrane separation methods face the dilemma of severe membrane fouling and high operating energy consumption. All of these methods struggle to achieve stable compliance with standards and low-cost treatment, posing a serious threat to the fragile aquatic ecosystems in arid regions.

[0003] Secondly, coal-fired power plant industrial parks emit massive amounts of pollutants annually. Coal-fired power plants alone emit over 5 million tons of carbon dioxide per plant annually, and the national annual production of fly ash has exceeded 600 million tons, with accumulated coal gangue stockpiles exceeding 5 billion tons. Fly ash and coal gangue are rich in active components such as SiO2, Al2O3, CaO, and MgO, theoretically possessing CO2 mineralization potential and fluorine adsorption capacity. However, their current comprehensive utilization rate is less than 40%, with most disposal occurring through open-air stockpiling or landfill. This not only occupies valuable land resources but also easily generates secondary environmental problems such as dust dispersion and leaching water pollution, exacerbating regional ecological pressure.

[0004] Third, CO2 geological sequestration is considered a key carbon reduction pathway, but existing technologies mostly rely on deep saline aquifers or depleted oil and gas reservoirs, which presents challenges such as limited site selection, long CO2 transportation distances (usually exceeding 50 km), high injection costs (approximately 200–300 RMB / ton CO2), and complex long-term monitoring. Although some studies have explored using mineralization reactions to convert CO2 into stable carbonate minerals, or using adsorption / precipitation methods alone to treat high-fluoride water, there is still no technology that can systematically integrate and synergistically utilize the four core elements within a coal-fired power plant industrial park: power plant CO2, power plant fly ash, high-fluoride and high-alkali mine water, and abandoned goaf space resources. Summary of the Invention

[0005] The purpose of this invention is to provide a method for co-sealing carbon dioxide and fluorides in underground reservoirs using high-alkalinity mine water and fly ash, in order to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash, comprising the following steps: (a) Within the coal-power integration park, select an abandoned coal mine goaf with a stable structure, a waterproof roof and floor, and a burial depth of 200-800m as an underground reservoir. Use cement grouting technology to reinforce the underground reservoir with a seepage-proof curtain to form a closed and isolated space. (b) The fly ash produced by the coal-fired power plant and the coal gangue produced by the coal mine are crushed, screened and pre-treated by water washing to remove soluble salts and loose impurities, and then mixed to prepare composite functional filler. The composite functional filler layer is then arranged in layers inside the underground reservoir. (c) Select high-alkalinity mine water with a total alkalinity ≥300mg / L and a fluoride ion concentration ≥20mg / L after two-stage reverse osmosis concentration, and mix it with carbon dioxide gas from the flue gas carbon capture unit of the coal-fired power plant in a mixing tank to form a carbonated mine water solution. No additional lime, sodium hydroxide or other strong alkalis are added to adjust the pH during the mixing process. (d) The carbonated mine water solution is injected into the underground water reservoir at a pressure of 0.5–3.0 MPa through the injection well, so that carbon dioxide reacts with calcium and magnesium ions in the mine water and filler in a high alkaline environment to form carbonate minerals. At the same time, fluoride ions are stably fixed through a dual mechanism of co-precipitation with calcium ions to form calcium fluoride and adsorption on the surface of aluminosilicate. (e) Install a multi-parameter sensing system inside and around the underground reservoir to monitor pH value, fluoride ion concentration, carbon dioxide partial pressure, rock mass strain and temperature parameters in real time, and evaluate the safety and efficiency of the sealing based on the monitoring data.

[0007] Furthermore, the mine water has a pH value of 7.5–9.0, a calcium ion concentration of ≥50 mg / L, a total hardness of ≥200 mg / L, and a magnesium ion concentration of ≥20 mg / L.

[0008] Furthermore, the mass ratio of fly ash to coal gangue is (5–7):(3–5), the particle size range of coal gangue is 3–15 mm, the particle size range of fly ash is 50–200 μm, and the thickness of the composite functional filler layer is 1.0–2.5 m.

[0009] Furthermore, the composite functional filler layer is arranged in a gradient within the underground reservoir: the upper layer is a fluorine adsorption layer, made of coal gangue and modified fly ash mixed at a mass ratio of 1:1, with a thickness of 0.3–1.0 m; the lower layer is a carbon dioxide mineralization reaction layer, made of high-calcium fly ash alone or mixed with steel slag at a mass ratio of (8-9):(1-2), with a thickness of 0.7–1.5 m, wherein the modified fly ash is fly ash activated by a 3%–5% hydrochloric acid solution.

[0010] Furthermore, the carbon dioxide is compressed to 8–15 MPa and then transported to a mixing tank in a supercritical or liquid state to participate in mixing. The gas-liquid contact time in the mixing tank is 20–60 min, and the gas-liquid volume ratio is (0.8–1.2):1.

[0011] Furthermore, the injection process adopts a pulsed, intermittent, or cyclic reinjection mode, with a single injection duration of 2–24 hours, an interval of 12–72 hours, and an injection flow rate controlled at 50–200 m³ / h.

[0012] Furthermore, the pH value of the carbonated mine water solution is controlled between 7.5 and 9.5. The pH dynamic balance is achieved by adjusting the carbon dioxide injection rate or the mine water replenishment in real time. When the pH is below 7.5, the carbon dioxide injection rate is reduced, and when the pH is above 9.5, the carbon dioxide injection rate is increased.

[0013] Furthermore, after the underground reservoir is sealed and put into operation, 30%–60% of the pores in the internal rock mass are filled with calcium carbonate, magnesium carbonate and calcium fluoride precipitates, the uniaxial compressive strength of the rock mass is increased by more than 10%, and the surface subsidence rate is reduced by more than 30%.

[0014] Furthermore, this includes setting up a grouting barrier curtain 30–50m outside the underground reservoir and deploying 2–4 emergency pumping wells around the underground reservoir; when the monitoring system detects that the fluoride ion concentration rises to above 1.5mg / L, the abnormal carbon dioxide migration rate exceeds 0.1m / d, or the rock mass deformation exceeds 5mm, the pumping and secondary treatment procedures will be automatically initiated.

[0015] Furthermore, in step (a), the permeability coefficient of the impermeable curtain is ≤1×10⁻ 7 The flow rate is cm / s, the curtain thickness is 2–5m, and the double-row hole grouting method is used for construction. The hole spacing is 1.5–2.5m. In step (b), the solid-liquid ratio of the water washing pretreatment is 1:5–1:10, the water washing time is 30–60min, and the moisture content of the filler after water washing is controlled at 15%–25%.

[0016] This invention provides a method for co-sealing carbon dioxide and fluoride in underground reservoirs using high-alkalinity mine water and fly ash, which has the following beneficial effects: This invention is the first to achieve the synergistic coupling of five major resources: CO2, fly ash, coal gangue, high-fluoride and high-alkali mine water, and abandoned goaf areas. It simultaneously solves four major problems: carbon emissions, solid waste storage, water pollution, and geological disasters, forming a closed-loop system of "waste recycling, pollutant sealing, and geological environment restoration." All treated materials come from within the coal-power integrated industrial park, eliminating the need for long-distance transportation and reducing the overall system cost compared to traditional methods. The natural high alkalinity of the mine water drives CO2 mineralization, eliminating the need for alkaline agents such as lime, further reducing operating costs. CO2 is converted into thermodynamically stable carbonate minerals through mineralization, eliminating the risk of leakage. Fluoride ions are deeply removed through a dual mechanism of precipitation and adsorption. Fly ash and coal gangue, after simple pretreatment, are used as functional fillers, eliminating the need for complex processes such as high-temperature activation and avoiding secondary pollution caused by open-air stockpiling. The mineralized products fill the pores of the goaf areas, increasing the uniaxial compressive strength of the rock mass, reducing the surface subsidence rate, and effectively preventing geological disasters. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the process for a method of co-sealing carbon dioxide and fluorides in an underground reservoir using high-alkalinity mine water and fly ash according to the present invention. Detailed Implementation

[0018] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0019] Example 1: Demonstration Project of Coal-Power Integration Industrial Park The technical solution of this invention was implemented in a coal-power integrated industrial park in Inner Mongolia. The park includes a modern coal mine with an annual output of 8 million tons of raw coal and a 2×660MW supercritical coal-fired power plant. The power plant emits approximately 5 million tons of CO2 and produces 600,000 tons of fly ash annually. The normal water inflow of the mine is 3000 m³ / d. The mine water quality indicators are: fluoride ion concentration 25 mg / L, total alkalinity 610 mg / L (calculated as CaCO3), calcium ion concentration 140 mg / L, magnesium ion concentration 35 mg / L, pH 8.5, and total hardness 420 mg / L (calculated as CaCO3).

[0020] Specific implementation steps: (a) Select the closed eastern wing mining area within the park as an underground reservoir. This mining area has a depth of 300m and a volume of 4.2×10 4 The structure has a volume of m³, with a complete sandstone waterproof top and bottom slab. A cement grouting double-row hole injection process was used to construct the seepage barrier curtain, with a hole spacing of 2.0m and a curtain thickness of 3m. The permeability coefficient test result is 5×10⁻⁻⁻⁴. 8cm / s. (b) Select fly ash (CaO content 17%) collected by the power plant dust collector and coal gangue from the coal washing plant. Crush and screen the coal gangue to ≤10mm and screen the fly ash to 50–150μm. Perform water washing pretreatment at a solid-liquid ratio of 1:8 for 40min to remove soluble salts and loose impurities. Then mix them at a mass ratio of 7:3 to prepare composite functional filler. Lay a 1.8m thick composite functional filler layer in the underground reservoir, of which the upper 0.6m is a fluorine adsorption layer (coal gangue and 3% hydrochloric acid activated fly ash mixed at a 1:1 ratio) and the lower 1.2m is a CO2 mineralization reaction layer (high calcium fly ash prepared separately). (c) 95% pure CO2 is obtained from the power plant flue gas via amine capture, compressed to 10 MPa and stored in liquid form. Mine water is introduced into a surface mixing tank, and liquid CO2 is introduced at a gas-liquid volume ratio of 1:1. After stirring and contacting for 30 minutes, a carbonated mine water solution with a pH of 8.2 is formed. (d) The carbonated mine water solution is injected into an injection well through a reverse modification of the existing drainage pipeline at a pressure of 1.8 MPa and a flow rate of 120 m³ / h, using an intermittent injection mode, with each injection lasting 20 hours and an interval of 48 hours. (e) Five monitoring points are set up in the underground reservoir. Each monitoring point integrates a fiber optic strain sensor, a fluoride ion selective electrode, a CO2 partial pressure probe, a pH / conductivity composite electrode, and a microseismic detector. Data is collected every 2 hours and transmitted wirelessly to the surface central control unit and connected to the cloud platform. At the same time, a grouting barrier curtain is set up 30m outside the underground reservoir, and two emergency pumping wells are deployed.

[0021] Monitoring results after 90 days of operation: the effluent fluoride concentration stabilized at 0.5–0.9 mg / L, with a removal rate of 96.4%–98.0%; approximately 5200 tons of CO2 were cumulatively sealed, with a CO2 mineralization efficiency of 91.5%; XRD analysis of the packing material showed the formation of large amounts of calcite (CaCO3), dolomite (CaMg(CO3)2), and calcium fluoride (CaF2), and amorphous aluminum-silicon phase was detected; the uniaxial compressive strength of the goaf rock mass increased from 32 MPa to 38 MPa, an increase of 18.75%; the surface subsidence rate above the goaf decreased from 8 mm / month to 4.8 mm / month, a decrease of 40%; the overall system treatment cost was 8.6 yuan / m³ of mine water, a reduction of 35% compared to traditional processes.

[0022] Example 2: Gradient packing and cyclic injection optimization An optimization plan was implemented in a coal-power integrated industrial park in Ningxia. The fluoride concentration in the mine water in the park was 32 mg / L, the total alkalinity was 720 mg / L (calculated as CaCO3), and the pH was 8.8. The fly ash of the power plant had a CaO content of 21% and an annual CO2 emission of approximately 4.5 million tons.

[0023] The composite functional packing layer adopts a three-layer gradient layout: the upper 0.4m fluorine adsorption layer (coal gangue, modified fly ash, and bentonite mixed in a mass ratio of 5:4:1), the middle 0.8m transition layer (fly ash and coal gangue mixed in a mass ratio of 6:4), and the lower 1.0m mineralization reaction layer (high-calcium fly ash and steel slag mixed in a mass ratio of 8:2); the CO2 compression pressure is 12MPa, the gas-liquid contact time in the mixing tank is 45min, and the gas-liquid volume ratio is 1.2:1; The system adopts a cyclic reinjection mode of "24 hours of injection, 48 hours of stagnation, and 2 hours of pumping back for testing". The pumping back water volume is 10% of the injected water volume, and the system is re-injected after passing the test.

[0024] Results after 120 days of operation: Fluoride ion removal rate reached 98.2%, effluent fluoride ion concentration ≤0.58mg / L; CO2 sequestration efficiency reached 93.0%, and the sequestration capacity per unit volume of packing increased by 15%; system operating energy consumption decreased by 20% compared to Example 1; surface settlement rate decreased by 45%, and there were no signs of any pollutant leakage.

[0025] Example 3: Verification of Intelligent Monitoring and Emergency Response An emergency response verification test was conducted in a coal-fired power plant in Shaanxi Province. Using the same basic parameters as in Example 1, on the 60th day of operation, an abnormal rise in fluoride ion concentration was artificially simulated (a high-fluoride solution was injected near a monitoring point, causing the fluoride ion concentration at that point to suddenly increase to 1.8 mg / L).

[0026] Emergency response results: The cloud platform AI analysis system identified the anomaly within 15 minutes and automatically triggered an early warning; the emergency pumping system was activated within 30 minutes to pump back the water in the abnormal area; after the pumped water was treated by activated carbon adsorption on the ground, the fluoride ion concentration dropped to 0.8 mg / L and was reinjected into the underground reservoir; the entire emergency response process did not cause any spillage of pollutants, and the system returned to normal operation within 48 hours.

[0027] The embodiments of the present invention are given for illustrative and descriptive purposes only, and are not intended to be exhaustive or to limit the invention to the forms disclosed. Many modifications and variations will be apparent to those skilled in the art. The embodiments were chosen and described in order to better illustrate the principles and practical application of the invention, and to enable those skilled in the art to understand the invention and to design various embodiments with various modifications suitable for a particular purpose.

Claims

1. A method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash, characterized in that, Includes the following steps: (a) Within the coal-power integration park, select an abandoned coal mine goaf with a stable structure, a waterproof roof and floor, and a burial depth of 200-800m as an underground reservoir. Use cement grouting technology to reinforce the underground reservoir with a seepage-proof curtain to form a closed and isolated space. (b) The fly ash produced by the coal-fired power plant and the coal gangue produced by the coal mine are crushed, screened and pre-treated by water washing to remove soluble salts and loose impurities, and then mixed to prepare composite functional filler. The composite functional filler layer is then arranged in layers inside the underground reservoir. (c) Select high-alkalinity mine water with a total alkalinity ≥300mg / L and a fluoride ion concentration ≥20mg / L after two-stage reverse osmosis concentration, and mix it with carbon dioxide gas from the flue gas carbon capture unit of the coal-fired power plant in a mixing tank to form a carbonated mine water solution. No additional lime, sodium hydroxide or other strong alkalis are added to adjust the pH during the mixing process. (d) The carbonated mine water solution is injected into the underground water reservoir at a pressure of 0.5–3.0 MPa through the injection well, so that carbon dioxide reacts with calcium and magnesium ions in the mine water and filler in a high alkaline environment to form carbonate minerals. At the same time, fluoride ions are stably fixed through a dual mechanism of co-precipitation with calcium ions to form calcium fluoride and adsorption on the surface of aluminosilicate. (e) Install a multi-parameter sensing system inside and around the underground reservoir to monitor pH value, fluoride ion concentration, carbon dioxide partial pressure, rock mass strain and temperature parameters in real time, and evaluate the safety and efficiency of the sealing based on the monitoring data.

2. The method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash according to claim 1, characterized in that, The mine water has a pH of 7.5–9.0, a calcium ion concentration of ≥50 mg / L, a total hardness of ≥200 mg / L, and a magnesium ion concentration of ≥20 mg / L.

3. The method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash according to claim 2, characterized in that, The mass ratio of fly ash to coal gangue is (5–7):(3–5), the particle size range of coal gangue is 3–15 mm, the particle size range of fly ash is 50–200 μm, and the thickness of the composite functional filler layer is 1.0–2.5 m.

4. The method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash according to claim 3, characterized in that, The composite functional filler layer is arranged in a gradient within the underground reservoir: the upper layer is a fluorine adsorption layer, made of coal gangue and modified fly ash mixed at a mass ratio of 1:1, with a thickness of 0.3–1.0 m; the lower layer is a carbon dioxide mineralization reaction layer, made of high-calcium fly ash alone or mixed with steel slag at a mass ratio of (8-9):(1-2), with a thickness of 0.7–1.5 m. The modified fly ash is fly ash activated by a 3%–5% hydrochloric acid solution.

5. The method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash according to claim 4, characterized in that, The carbon dioxide is compressed to 8–15 MPa and then transported to the mixing tank in a supercritical or liquid state to participate in mixing. The gas-liquid contact time in the mixing tank is 20–60 min, and the gas-liquid volume ratio is (0.8–1.2):

1.

6. The method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash according to claim 5, characterized in that, The injection process adopts a pulsed, intermittent, or cyclic reinjection mode, with a single injection duration of 2–24 hours, an interval of 12–72 hours, and an injection flow rate of 50–200 m³ / h.

7. A method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash according to claim 6, characterized in that, The pH value of the carbonated mine water solution is controlled between 7.5 and 9.

5. The pH dynamic balance is achieved by adjusting the carbon dioxide injection rate or the mine water replenishment in real time. When the pH is below 7.5, the carbon dioxide injection rate is reduced, and when the pH is above 9.5, the carbon dioxide injection rate is increased.

8. A method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash according to claim 7, characterized in that, After the underground reservoir is sealed and put into operation, the pores of the internal rock mass are filled with calcium carbonate, magnesium carbonate and calcium fluoride precipitates by 30%-60%, the uniaxial compressive strength of the rock mass is increased by more than 10%, and the surface subsidence rate is reduced by more than 30%.

9. A method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash according to claim 8, characterized in that, It also includes setting up a grouting barrier curtain 30–50m outside the underground reservoir and setting up 2–4 emergency pumping wells around the underground reservoir; when the monitoring system detects that the fluoride ion concentration rises to above 1.5mg / L, the abnormal carbon dioxide migration rate exceeds 0.1m / d, or the rock mass deformation exceeds 5mm, the pumping and secondary treatment program will be automatically started.

10. A method for co-sealing carbon dioxide and fluoride in an underground reservoir using high-alkalinity mine water and fly ash according to claim 9, characterized in that, In step (a), the permeability coefficient of the impermeable curtain is ≤1×10⁻ 7 The flow rate is cm / s, the curtain thickness is 2–5m, and the double-row hole grouting method is used for construction. The hole spacing is 1.5–2.5m. In step (b), the solid-liquid ratio of the water washing pretreatment is 1:5–1:10, the water washing time is 30–60min, and the moisture content of the filler after water washing is controlled at 15%–25%.

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