Underground gasification re-mining process and system for strip mine end slope residual coal

Abstract

The invention belongs to the technical field of strip mine residual coal resource recovery, and particularly relates to a strip mine end slope residual coal underground gasification re-mining process and system.The strip mine end slope residual coal underground gasification re-mining process comprises the following construction steps that S1, a directional drilling system is constructed at a strip mine end slope coal seam outcrop and comprises a gas injection drilling hole, an exhaust drilling hole and a drainage drilling hole, forming a gasifier type I suitable for a thick coal seam or a gasifier type II suitable for a thin-medium thick coal seam; s2, injecting a gasification agent into the gasification furnace to perform coal bed gasification to generate coal gas and capture CO2 in gasified tail gas; s3, pressurizing and liquefying the captured CO2, injecting the liquefied CO2 into a combustion space area, and performing mineralization and storage in combination with alkaline ash; wherein the gas injection drill hole and the exhaust drill hole are arranged at the bottom or the top of a coal seam according to the thickness difference of the coal seam, and part of accumulated water is reserved in the whole drainage drill hole to prevent gas leakage. Therefore, efficient recovery of residual coal and full-process low-carbon emission are realized through differential gasification furnace structural design, dynamic sealing of the drainage drill hole and collaborative filling technology of the combustion space area.

Description

Technical Field

[0001] The present invention belongs to the technical field of residual coal resource recovery in open-pit mines, and specifically relates to an underground gasification and re-mining process and system for residual coal at the end wall of open-pit mines. Through differentiated gasification furnace structure design, dynamic sealing of drainage boreholes, and coordinated filling technology of combustion and void areas, efficient recovery of residual coal and low-carbon emissions throughout the entire process are achieved. Background Art

[0002] It is urgent to explore a clean, low-carbon, safe and efficient mining model based on the deep coal resource conditions of these open-pit mines.

[0003] In this context, a process and system for underground gasification and re-mining of residual coal at the end of open-pit mines has been developed. It does not require manual tunneling and has strong adaptability to the occurrence conditions of different coal seams. It can achieve clean, low-carbon, safe and efficient recovery of residual coal resources at the end of open-pit mines, which is of great significance to improving the utilization rate of coal resources and promoting the low-carbon utilization of high-carbon energy. Summary of the Invention

[0004] In order to realize the mining and recovery of deep coal resources in open-pit coal mines safely, economically, low-carbon and efficiently, the present invention provides an underground gasification and recovery process for residual coal at the end wall of open-pit mines, comprising the following steps:

[0005] S1. Gasifier Construction Phase: A directional drilling system is constructed at the coal seam outcrop on the open-pit mine's sidewall, including gas injection holes, exhaust holes, and drainage holes, to form a Type I gasifier suitable for thick coal seams or a Type II gasifier suitable for thin to medium-thick coal seams.

[0006] S2. Underground coal gasification stage: Injecting gasifying agent into the gasifier to gasify the coal seam, generate coal gas and capture CO2 in the gasification exhaust gas;

[0007] S3. Filling the combustion void: The captured CO2 is pressurized and liquefied and then injected into the combustion void, where it is mineralized and sealed with alkaline ash.

[0008] The gas injection boreholes and the gas exhaust boreholes are arranged at the bottom or top of the coal seam according to the thickness difference of the coal seam, and the drainage boreholes retain some accumulated water throughout to prevent gas leakage.

[0009] In one embodiment, in the gasifier type I, the gas injection boreholes are arranged at the bottom of the coal seam, and the exhaust boreholes are arranged at the top of the coal seam;

[0010] In gasifier type II, both the gas injection boreholes and the exhaust boreholes are arranged at the bottom of the coal seam.

[0011] In one embodiment, in S2, the gasifying agent consists of air, oxygen, steam and captured CO2, wherein the volume proportion of CO2 is 10%-25%, the gasification reaction temperature is 800-1500°C, and the gasification pressure is 10-30kPa.

[0012] In one embodiment, the lowest point of the drainage borehole always retains a water layer with a height of ≥0.5m, which is used to seal the gasifier and prevent gas leakage.

[0013] In one embodiment, in S3, the combustion zone filling stage includes the following steps:

[0014] S31. The alkaline filling slurry is injected into the combustion zone through the slurry delivery system, with a slurry solid-liquid ratio of 1:3-1:5;

[0015] S32. Simultaneously inject pressurized liquefied CO2, with a mineralization reaction cycle of 15-30 days.

[0016] In one embodiment, in S1, the gasifier construction stage further includes injecting a flame retardant slurry at the coal seam outcrop, wherein the flame retardant slurry is a mixture of Portland cement, fly ash and water in a mass ratio of 1:2:4;

[0017] Among them, the length of the flame retardant grouting area is at least 10m, the grouting width is at least 10m on both sides of the combustion width of the gasification working face, and the grouting height is the thickness of the coal seam.

[0018] In one embodiment, in S1, the diameter of the directional drilling hole is 250-400 mm, the drilling trajectory extends along the direction of the coal seam, and the final hole position error is ≤1.2 m.

[0019] In one embodiment, the CO2 capture uses a two-stage absorption tower, the first stage uses NaOH solution for desulfurization, and the second stage uses a composite absorbent;

[0020] The composite absorbent consists of 20% methyldiethanolamine, 30% n-propanol and 50% water, with an absorption temperature of 30-40°C and a regeneration temperature of 90-110°C.

[0021] In another aspect, the present invention provides an underground gasification and mining system for residual coal at the end of an open-pit mine. The system is used in the above-mentioned underground gasification and mining process for residual coal at the end of an open-pit mine, comprising:

[0022] Gasifier construction system: including directional drilling rig, flame retardant slurry injection device and drainage control equipment;

[0023] Gasifying agent preparation system: including blower, steam boiler and gasifying agent mixing ratio controller;

[0024] Combustion space filling system: includes slurry preparation device, CO2 pressurized liquefaction pump and distributed optical fiber monitoring device.

[0025] In one embodiment, the distributed optical fiber monitoring device monitors the temperature, pressure and filling density of the combustion zone in real time, and the data is fed back to the control system to dynamically adjust the injection ratio of slurry and CO2.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. In view of the characteristics of exposed coal seams at the end of open-pit mines and poor stability of overburden, the present invention designs two types of gasification furnace structures, Type I and Type II, which are respectively suitable for thick coal seams with a thickness of more than 5m and thin to medium-thick coal seams with a thickness of 1-5m. Through the differentiated arrangement of gas injection drill holes and exhaust drill holes (Type I is "bottom injection and top discharge", Type II is "double bottom parallel"), the gas flow path in the gasification working face is optimized, reducing heat loss and reaction dead corners. The recovery rate of residual coal at the end of the wall is increased from 35%-40% in the traditional process to 75%, the resource utilization rate is more than doubled, and resource waste is significantly reduced. In addition, the entire mining process does not require secondary stripping, avoiding damage to the open-pit mine slopes and the ecological environment, and meeting the requirements of green mine construction.

[0028] 2. The present invention adopts CO2 recycling technology to reuse the captured CO2 in the gasification tail gas as a gasifying agent, and finally pressurizes and liquefies it and injects it into the combustion zone for mineralization and storage. In addition, the "fly ash + cement" alkaline slurry introduced in the combustion zone filling stage can synergize with CO2 to generate stable carbonates (such as CaCO3) through mineralization reaction. This achieves net negative carbon emissions throughout the life cycle, helps achieve the "dual carbon" goals, and the synergistic injection of slurry filling and CO2 further improves the CO2 mineralization rate in the combustion zone, and strengthens the long-term storage stability.

[0029] 3. This invention retains a 0.5-1.0m thick layer of water at the end of the drainage borehole. Integrating an ultrasonic water level sensor with an automatic pump control system, the water level is adjusted in real time to prevent gas leaks. Simultaneously, a Portland cement-based flame-retardant slurry is injected into the gasifier outcrop, forming a leak-proof layer ≥10m thick with a flame retardant efficiency ≥90%. This completely eliminates the explosion risk associated with gas leaks in traditional gasification processes, resulting in a zero gas accident rate and a monitored displacement of the combustion zone roof ≤3mm / year, well below the safety threshold, effectively ensuring the safety of surface structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for describing the embodiments or the prior art.

[0031] Figure 1This is the open pit end wall residual coal underground gasification furnace type I of the present invention (suitable for thick coal seams);

[0032] Figure 2 This is the open pit end wall residual coal underground gasification furnace type II of the present invention (applicable to thin to medium thick coal seams);

[0033] Figure 3 This is a schematic diagram of the underground gasification temperature propagation range simulated and calculated within one year by the present invention. DETAILED DESCRIPTION

[0034] The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0035] Take a certain open-pit coal mine re-mining project as an example.

[0036] 1. Detailed description of the specific process flow

[0037] 1. Gasifier construction stage

[0038] Drilling arrangement and construction:

[0039] Equipment selection: ZDY-4000 fully hydraulic directional drilling rig equipped with a φ300mm diamond composite drill bit. The drilling trajectory extends along the direction of the coal seam. Real-time correction is made through the while-drilling measurement system to ensure that the final hole position error is ≤1.0m.

[0040] Gasifier Type I (thick coal seams, such as Figure 1 shown):

[0041] Gas injection drilling: Drill along the coal seam floor from the outcrop step at an angle of 15° to the ignition point at the bottom of the gasification working face (80m horizontal distance from the outcrop);

[0042] Exhaust drilling: Constructed parallel to the gas injection drilling hole at 30m on the side of the opening, the bottom of the drilling hole extends to the top of the coal seam and penetrates the bottom of the gas injection drilling hole.

[0043] Gasifier Type II (thin-medium thick coal seams, such as Figure 2 shown):

[0044] The gas injection and exhaust boreholes are arranged along the coal seam floor with a horizontal spacing of 20m and extend vertically parallel to the same gasification working face.

[0045] Drainage drilling setting: construct an inclined drilling hole on the outside of the gasifier, with the end 0.8m below the ignition point of the gasification working face, and initially inject water to form a 0.5m water accumulation layer.

[0046] The area in the coal seam enclosed by gas injection boreholes and exhaust boreholes is the gasification working face.

[0047] Flame retardant slurry injection:

[0048] Slurry ratio: Portland cement (P.O42.5), fly ash (particle size ≤ 0.075mm) and water are mixed in a mass ratio of 1:2:4, with a stirring time of ≥ 10min. The slump test is used to control the slurry fluidity to 180-200mm;

[0049] Injection process: Use a high-pressure grouting pump to inject slurry at the outcrop of the gasifier at a pressure of 2MPa to form a flame retardant and leakproof layer with a thickness of 10m. The solidification time is ≤24h. Figure 3 The simulation results for the propagation range of gasification temperature in the coal seam show that the flame-retardant grouting zone length (distance from the stop-mining line to the outcrop) is set at 10m. The grouting width is at least the combustion width of the gasification working face plus 10m on both sides, which is the minimum requirement. The height is the coal seam thickness. The grouting dimensions shown above are only minimum requirements. Engineers can increase the 10m above the above 10m to adapt to different mining conditions based on the actual situation of each project.

[0050] The purpose of injecting flame-retardant slurry is to prevent the gasification working face from burning through the coal seam and causing gas leakage, while also preventing the temperature from spreading to the outside.

[0051] 2. Underground coal gasification stage

[0052] Gasifying agent configuration and injection:

[0053] The gasifying agent consists of: air (provided by a blower), oxygen (prepared by a VPSA oxygen generator, with an oxygen purity ≥90%), steam (provided by a steam boiler, with a steam pressure of 15kPa and a temperature of 250°C) and captured CO2 (20% by volume); the specific initial ratio can be 20% CO2, 45% oxygen and 35% steam by volume.

[0054] Specific injection parameters: total flow rate 2000Nm 3 / h, with an initial injection pressure of 15kPa. The gasification temperature is monitored in real time by a thermocouple and maintained between 800-1500°C by adjusting the oxygen content. Specifically, when the thermocouple detects that the gasification temperature is below 800°C, the volume fraction of oxygen in the gasifying agent can be increased, for example, the gasifying agent ratio can be 10% CO2, 50% oxygen, and 40% steam, and the injection pressure can be adjusted to 30kPa. When the thermocouple detects that the gasification temperature is above 1500°C, the volume fraction of oxygen in the gasifying agent can be reduced, for example, the gasifying agent ratio can be 25% CO2, 40% oxygen, and 35% steam, and the injection pressure can be adjusted to 10kPa, until the gasification temperature returns to the range of 800-1500°C.

[0055] Drainage and sealing control:

[0056] Water level monitoring: Install ultrasonic water level sensors (range 0-2m, accuracy ±1cm) in drainage boreholes to transmit data to the central control system in real time;

[0057] Automatic adjustment: When the water level is less than 0.5m, the water injection pump is started to replenish water;

[0058] When the water level is greater than 1.0m, start the drainage pump to lower the water level and always maintain the water layer height at 0.5-1.0m.

[0059] Gas purification and utilization:

[0060] Purification process: High-temperature raw coal gas at around 400°C is passed into a water-sealed scrubber, where it is sprayed with 50°C water to cool the temperature down to below 80°C. The gas then enters a three-stage desulfurization tower. The three-stage desulfurization tower employs a first-stage activated iron oxide desulfurization, a second-stage NaOH solution absorption, and a third-stage ceramic filter element filtration arrangement, ensuring a desulfurization efficiency of ≥95%.

[0061] Power generation and utilization: The purified coal gas is transported to a gas-fired internal combustion generator set, and can be used to capture CO2 in the power generation tail gas through a two-stage absorption tower. This purifies the tail gas, reduces greenhouse gas emissions, and realizes the recycling of CO2. The two-stage absorption tower can specifically adopt an absorption layout in which the first stage uses NaOH solution for desulfurization, and the second stage uses a composite absorbent to capture CO2. The composite absorbent can be composed of 20% methyldiethanolamine, 30% n-propanol, and 50% water, with an absorption temperature of 30-40°C. After absorption, the composite absorbent is then introduced into a regeneration tower and heated to 100°C to release the absorbed CO2, thereby completing the CO2 capture process. The composite absorbent can be reused after release.

[0062] 3. Filling stage of combustion void area

[0063] Preparation and injection of alkaline filling slurry:

[0064] Alkaline filling slurry ratio: fly ash (60%), cement (20%), water (20%), solid-liquid ratio 1:3, fluidity ≥180mm after mixing;

[0065] Injection process: Use a double-shaft mixer to prepare the slurry, and use a screw pump to pump the slurry at a rate of 10m 3 / h rate is injected into the fuel-air zone, and the filling rate is ≥90%.

[0066] CO2 storage and mineralization:

[0067] CO2 treatment: The captured CO2 is pressurized to 10MPa and liquefied (temperature -20℃) by a three-stage reciprocating compressor, and then injected into the combustion zone at a rate of 30t / h through the original gas injection borehole;

[0068] Mineralization reaction: Liquid CO2 reacts with CaO in the slurry to generate CaCO3, the reaction temperature is 80-100℃, the mineralization cycle is 25 days, and the mineralization rate is ≥85%;

[0069] Sealing treatment: After filling is completed, nano-modified cement slurry (compressive strength ≥35MPa, initial setting time 1.5h) is used to seal the end of the borehole, and the sealing length is ≥20m.

[0070] 2. Configuration of core equipment and specific parameter settings of the multiple mining system

[0071]

[0072] 3. Technical Effect Verification and Comparative Analysis

[0073]

[0074] The present invention optimizes the structure of the gasifier to increase the end-wall residual coal recovery rate by about 40% compared with the traditional process, thereby reducing resource waste. At the same time, during the mining process, the present invention stores 1.8 tons of CO2 per ton of coal. Combined with coalbed methane power generation instead of coal burning, the net emission reaches -0.2 tons, effectively reducing carbon emissions. In addition, the drilled holes are reused in the three stages of underground coal gasification and CO2 storage in the combustion zone, eliminating the secondary stripping process, reducing the cost per hole by 50%, and significantly shortening the project investment payback period.

[0075] In summary, the process and system of this invention, through core innovations such as differentiated gasifier design, dynamic drainage sealing, and CO2 co-sequestration, address the pain points of low safety, resource waste, and high carbon emissions in the recovery of residual coal from open-pit mines. This achieves a technological breakthrough in the four key areas of safety, efficiency, low carbon, and economy. Compared to existing technologies, this invention offers significant advantages in resource utilization, carbon emission reduction, cost control, and adaptability, providing an innovative solution for the green transformation of open-pit mines.

[0076] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made thereto. Therefore, such modifications and improvements, without departing from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. Underground gasification and mining process for residual coal at the end of open-pit mine, characterized by: The following steps are involved: S1. Gasifier Construction Phase: A directional drilling system is constructed at the coal seam outcrop on the open-pit mine's sidewall, including gas injection holes, exhaust holes, and drainage holes, to form a Type I gasifier suitable for thick coal seams or a Type II gasifier suitable for thin to medium-thick coal seams. S2. Underground coal gasification stage: Injecting gasifying agent into the gasifier to gasify the coal seam, generate coal gas and capture CO2 in the gasification exhaust gas; S3. Filling the combustion void: The captured CO2 is pressurized and liquefied and then injected into the combustion void, where it is mineralized and sealed with alkaline ash. The gas injection boreholes and the gas exhaust boreholes are arranged at the bottom or top of the coal seam according to the thickness difference of the coal seam, and the drainage boreholes retain some accumulated water throughout to prevent gas leakage.

2. The underground gasification recovery process for residual coal at the end of an open-pit mine according to claim 1, characterized in that: In the gasifier type I, the gas injection holes are arranged at the bottom of the coal seam, and the exhaust holes are arranged at the top of the coal seam; In gasifier type II, both the gas injection boreholes and the exhaust boreholes are arranged at the bottom of the coal seam.

3. The underground gasification and re-mining process for residual coal at the end of an open-pit mine according to claim 1, characterized in that: In the S2, the gasifying agent is composed of air, oxygen, steam and captured CO2, wherein the volume proportion of CO2 is 10%-25%, the gasification reaction temperature is 800-1500°C, and the gasification pressure is 10-30kPa.

4. The underground gasification recovery process for residual coal at the end of an open-pit mine according to claim 1, characterized in that: The lowest point of the drainage borehole always retains a water layer with a height of ≥0.5m, which is used to seal the gasifier and prevent gas leakage.

5. The underground gasification recovery process for residual coal at the end of an open-pit mine according to claim 1, characterized in that: In S3, the combustion zone filling stage includes the following steps: S31. The alkaline filling slurry is injected into the combustion zone through the slurry delivery system, with a slurry solid-liquid ratio of 1:3; S32. Simultaneously inject pressurized liquefied CO2, with a mineralization reaction cycle of 15-30 days.

6. The underground gasification recovery process for residual coal at the end of an open-pit mine according to claim 1, characterized in that: In said S1, the gasifier construction stage further comprises injecting a flame retardant slurry at the coal seam outcrop, wherein the flame retardant slurry is a mixture of Portland cement, fly ash and water in a mass ratio of 1:2:4; Among them, the length of the flame retardant grouting area is at least 10m, the grouting width is at least 10m on both sides of the combustion width of the gasification working face, and the grouting height is the thickness of the coal seam.

7. The underground gasification recovery process for residual coal at the end of an open-pit mine according to claim 1, characterized in that: In S1, the diameter of the directional drilling hole is 250-400 mm, the drilling trajectory extends along the direction of the coal seam, and the error of the final hole position is ≤1.2 m.

8. The underground gasification recovery process for residual coal at the end of an open-pit mine according to claim 1, characterized in that: The CO2 capture adopts a two-stage absorption tower, the first stage uses NaOH solution for desulfurization, and the second stage uses a composite absorbent to capture CO2; The composite absorbent consists of 20% methyldiethanolamine, 30% n-propanol and 50% water, with an absorption temperature of 30-40°C and a regeneration temperature of 90-110°C.

9. A system for underground gasification and re-mining of residual coal at the end of an open-pit mine according to any one of claims 1 to 8, characterized in that: include: Gasifier construction system: including directional drilling rig, flame retardant slurry injection device and drainage control equipment; Gasifying agent preparation system: including blower, steam boiler and gasifying agent mixing ratio controller; Combustion space filling system: includes slurry preparation device, CO2 pressurized liquefaction pump and distributed optical fiber monitoring device.

10. The system for underground gasification and re-mining of residual coal at the end of an open-pit mine according to claim 9, characterized in that: The distributed optical fiber monitoring device monitors the temperature, pressure and filling density of the combustion and air space in real time, and the data is fed back to the control system to dynamically adjust the injection ratio of slurry and CO2.

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