A coal underground gasification filling method

Abstract

The present application belongs to the technical field of coal mining, and discloses a coal underground gasification filling method. After gasification starts, the established signal of filling is judged according to the total volume content of effective components of the gas from the gas outlet, the output per unit time or the expected value of the cavity volume of the combustion empty area; the filling slurry is injected into the combustion empty area through the gasification agent injection hole, the gasification mode is switched to the filling mode in real time or step by step, and the gas outlet of the gas outlet hole is kept; whether the expected filling target is reached is judged by real-time monitoring of the temperature, effective component content or gas production rate of the gas outlet hole, or real-time monitoring of whether the maximum filling amount of single filling is reached; after the expected filling target is reached, the filling mode is switched to the gasification mode in real time or step by step; the above steps are repeated until the gasification coal seam is mined out. The present application combines gasification and filling, improves the coal recovery rate, and fills the combustion empty area in time to eliminate the possible existing or future geological environment damage in the larger combustion empty area.

Description

Technical Field

[0001] This invention relates to the field of coal mining technology, and more specifically to a method for underground coal gasification and backfilling. Background Technology

[0002] Underground coal gasification is a chemical mining method that directly converts underground coal seams into coal gas. This method involves igniting the coal seam through boreholes or tunnels, injecting gasifying agents such as air or oxygen-enriched air, and then generating combustible gases such as hydrogen, carbon monoxide, and methane through a thermochemical reaction, which are then extracted to the surface for use. After gasification, the underground coal seam forms a goaf zone. The existence of this goaf zone causes stress redistribution in the rock mass, leading to deformation, damage, and movement of the roof, surrounding rock, and reserved isolation coal seam within the goaf zone. This inevitably disturbs and damages the surface environment, posing significant safety hazards and requiring timely remediation of the goaf zone.

[0003] Underground coal gasification with shafts (where people can go down into the mine to approach the coal seam to be gasified) is usually used for isolation coal zones or residual coal at the edges and corners reserved by the physical mining method in mining. The treatment method for the goaf formed by this underground gasification method is the same as the treatment method for the goaf formed by physical mining, that is: to transport solid waste to the goaf through the mine to fill the goaf.

[0004] Unlike well-type underground coal gasification, wellless underground coal gasification connects the underground coal seam with surface equipment through boreholes. Humans and large equipment cannot enter the underground space. Consequently, the combustion voids formed by wellless underground coal gasification have many unknown factors, such as their three-dimensional shape, expansion direction, and volume, making remediation more difficult. Currently, the remediation of combustion voids formed by wellless underground coal gasification mainly involves backfilling after gasification. For example, patents such as (application number CN201710462088.2) on determining and backfilling the volume of underground combustion voids and (application number CN202111297472.4) on a method for strip-backfilling underground coal gasification mining both disclose remediation methods that involve obtaining the volume of the combustion voids after gasification and then performing grouting backfilling. There are some problems with gasification followed by backfilling: In order to reduce various costs, an underground gasifier will gasify as much coal as possible and produce more gas, resulting in a large combustion zone. Within the large combustion zone, there may have been roof collapse, surrounding rock collapse, local blockage, etc. It is difficult to eliminate the existing geological damage after gasification followed by backfilling.

[0005] Therefore, developing a method for simultaneous underground coal gasification and backfilling is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the present invention proposes a wellless underground coal gasification method, which improves the coal recovery rate by combining gasification with backfilling, and timely backfilling of the combustion goaf to eliminate the geological environmental damage that may already exist or will occur in the large combustion goaf. At the same time, the backfill material disclosed in the present invention can encapsulate the pollutants remaining in the combustion goaf, delay the migration of pollutants, and reduce groundwater pollution.

[0007] To achieve the above objectives, the present invention provides a method for underground coal gasification backfilling, comprising the following steps:

[0008] (1) After gasification begins, the predetermined signal for filling is determined based on the total effective component volume content of the gas from the gas outlet, the output per unit time, or the expected value of the cavity volume of the combustion zone.

[0009] (2) Inject filling slurry into the combustion zone through the gasifying agent injection hole, and switch the gasification mode to the filling mode immediately or gradually to maintain gas production at the outlet hole;

[0010] (3) By real-time monitoring of the temperature, effective component content or gas production rate of the gas produced from the gas outlet, or by real-time monitoring of whether the maximum filling amount of a single filling has been reached, it can be determined whether the expected filling target has been achieved.

[0011] (4) After the expected filling target is achieved, the filling mode shall be switched to the gasification mode immediately or gradually.

[0012] (5) Repeat steps (1)-(4) until the gasified coal seam is mined out.

[0013] Furthermore, in step (1), when the total volume content of the effective components in the gas decreases by 5% to 15% compared to the stable period, preferably 10%, filling begins; or, when the gas yield per unit time decreases by 20% to 40% compared to the stable period and shows a continuous downward trend, filling begins; or, based on the coal seam parameters, geological conditions, and the length and width of the combustion zone of the gasified coal seam, an empirical estimate is first made, and then a certain multiple of the empirical estimate is used as the expected volume value of the combustion zone. When the cavity volume of the combustion zone reaches the expected value, filling begins.

[0014] Preferably, the multiplier is 0.6 to 0.8 times, and more preferably 0.7 times.

[0015] Furthermore, in step (2), the filling grout is a single-component cement grout, a clay cement grout, or a composite grout.

[0016] Preferably, the dry material in the composite slurry is one or more of fly ash, gravel, sand, and waste slag added to cement and clay; the composite slurry also includes a certain amount of water, which is mixed with the dry material to form a slurry.

[0017] Furthermore, in step (3), filling is stopped when the temperature of the gas is below 80°C; or, filling is stopped when the total volume content of the effective components CH4, H2 and CO of the gas is below 20% and the O2 content is greater than 5%; or, filling is stopped when the pressure difference between the gasifying agent injection hole and the gas outlet hole exceeds the expected value.

[0018] Furthermore, in step (3), the specific operation for estimating the maximum filling amount of a single filling is as follows: first determine the stacking shape of the filling material and calculate its volume, then the maximum filling amount is ≤ the volume of the stacking shape of the filling material minus the volume of the coal ash produced by gasification; or, estimate the maximum filling amount of a single filling to be 60%~100% of the cavity volume of the combustion zone when filling is carried out.

[0019] More preferably, in step (3), the filling adopts a three-stage filling method, which corresponds to the front filling grout, the middle filling grout and the final filling grout respectively; wherein, the proportion of the front filling grout and the final filling grout is the same, and the mass fraction of cement is higher than that of the middle filling grout.

[0020] Furthermore, in step (4), the operation of switching to gasification mode is to swap the gasification agent injection hole and the gas outlet hole, that is, to inject the gasification agent from the original gas outlet hole and to produce gas from the original gasification agent injection hole.

[0021] Further preferably, the switching to gasification mode includes: maintaining the pressure of the underground gasifier unchanged, gradually reducing the injection rate of the filling slurry and the discharge volume of the raw gas outlet; when the gas production rate is less than one-third of the normal gasification production rate, injecting carbon dioxide, water vapor and / or inert gas into the raw gas outlet, and discharging gas from the original gasifying agent injection hole; when the gas injection volume is greater than or equal to half of the underground space volume (including the remaining cavity of the combustion zone and the inlet / outlet holes), gradually increasing the oxygen concentration in the injected gas to the level of the original gasifying agent; and when the exhaust from the original gasifying agent injection hole approaches the composition of the gas during the stable gasification period, the interchange between the original gasifying agent injection hole and the gas outlet is successfully achieved.

[0022] The technical concept of this invention is as follows: a method for underground coal gasification without a shaft, which involves constructing an underground coal gasifier without a shaft and implementing underground gasification; in conjunction with the gasification process, when a predetermined signal indicating the need for filling occurs, the gasifying agent in the gasifying agent injection hole is switched or gradually replaced with filling slurry, that is, the gasification mode is switched to the filling mode, and filling slurry is injected into the combustion air zone through the gasifying agent injection hole, while maintaining the gasification furnace to produce coal gas. Water in the filling slurry is separated in the underground combustion air zone. After the expected filling target is achieved, the filling mode is switched or gradually replaced with the gasification mode: either the slurry injection hole can be switched to a gasifying agent injection hole to continue forward gasification, or it can be switched to a coal gas production hole to start reverse gasification.

[0023] As can be seen from the above technical solution, compared with the prior art, the technical effects achieved by the present invention are as follows:

[0024] This invention improves coal recovery rate by combining gasification with backfilling, and timely backfilling of combustion goaf areas eliminates potential or future geological environmental damage within large combustion goaf areas. At the same time, the backfill material disclosed in this invention can encapsulate residual pollutants in combustion goaf areas, delay the migration of pollutants, and reduce groundwater pollution. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0026] Figure 1 This is a process flow diagram of the present invention.

[0027] Figure 2 This is a schematic diagram of an inclined coal seam.

[0028] Figure 3 This is a schematic diagram of the spatial form after gasification.

[0029] Figure 4 It is a single-filling and stacking pattern.

[0030] Figure 5 This refers to the ideal packing morphology of the combustion zone filler formed by multiple filling processes.

[0031] Figure 6 This is a schematic plan view of a single gasification unit in Example 1.

[0032] Figure 7 This is a graph showing the relationship between the total volumetric content of effective components in coal gas and time in Example 1.

[0033] Figure 8 This is a schematic diagram of the flow angle of the filling material in Example 1. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] like Figure 1As shown, a method for underground coal gasification without a shaft is described. This involves constructing an underground coal gasifier without a shaft and implementing underground gasification. During gasification, when a predetermined signal indicating the need for filling occurs, the gasifying agent in the injection port is switched or gradually replaced with filling slurry. The filling slurry is injected into the combustion chamber through the injection port while maintaining coal gas production from the gasifier. Water in the filling slurry is separated in the underground combustion chamber. After achieving the desired filling target, the filling mode is switched or gradually replaced with the gasification mode.

[0036] The established signal for the need for filling can be a significant decrease in gas quality (total volume content of effective components, output per unit time), such as a decrease of 5% to 15% (preferably 10%) in the total volume content of effective components, or a decrease in gas output per unit time to 60% to 80% of the stable period, with a continuous downward trend. In underground coal seam gasification, if the operating conditions or parameters remain unchanged, the total effective component content (or calorific value) or gas output per unit time of the gas produced by stable gasification will fluctuate slightly within a certain range. As gasification progresses, the combustion goaf gradually increases, and the effective contact surface area between the gasifying agent and the coal seam or coal on the gasification working face decreases, affecting the combustion-gasification process and causing a decrease in gas quality. When the gas quality (total volume content of effective components, output per unit time) shows a significant decrease compared to the stable period, such as a decrease of 5% to 15% (preferably 10%) in the total volume content of effective components, or a decrease in gas output per unit time of 20% to 40%, with a continuous downward trend, it can be determined that the requirement for filling the combustion goaf has been met.

[0037] The predetermined signal for the need for filling can also be based on estimating the cavity volume of the corresponding goaf zone based on the gas production at that stage, indicating that it has reached the expected value. The cavity volume of the goaf zone refers to the volume of coal burned or gasified minus the volume of coal ash produced and the volume of filling slurry injected. The amount of coal burned can be calculated using the carbon conservation law based on the produced gas volume, and the volume of the goaf zone can be calculated using the density of the local coal. These calculations are relatively easy. However, determining the optimal cavity volume at which roof collapse or surrounding rock collapse might occur requires empirical estimation based on the coal seam parameters, geological conditions (especially the bearing capacity of the coal seam roof), and the length and width of the goaf zone. A multiple of the estimated result is used as the expected value, for example, 0.6 to 0.8 times, preferably 0.7 times. When the cavity volume of the goaf zone reaches this expected value, the predetermined signal for filling can be considered to have occurred.

[0038] The filling grout can be single-component cement grout, clay-cement grout, or composite grout. Single-component cement grout is a grout of various concentrations made by mixing cement and water. It has advantages such as high strength of the solidified body, good impermeability, simple process equipment, and convenient operation. The disadvantages are long setting time, which cannot be controlled according to needs, low early strength, and high cost. Clay-cement grout is made by adding an appropriate amount of clay to cement grout. After being injected into the combustion zone, under certain pressure and time, it undergoes a process of diffusion-dehydration-gelling-consolidation. It not only strengthens the roof and seals water-conducting fissures, but also has the advantages of low cost, low water separation rate, and good diffusion-flow effect compared with single-component cement grout. Composite grout is a grout formed by adding new aggregates (such as fly ash, gravel, sand, and / or waste slag) to a grout formed by cement and clay. Clay itself is a highly injectable water-blocking material with fine particles that can enter even the smallest cracks. Clay grout can also act as a carrier suspension, carrying other materials without easily settling. It is low in cost and widely available. Cement can improve the strength of the aggregate and ensure the filling effect, but it is more expensive. Fly ash is a waste material from power plants, with abundant raw materials, low cost, and uniform particles. Using clay grout as a suspension makes it less prone to settling, has a large diffusion radius, and good injectability, which is beneficial for filling larger cracks.

[0039] The preferred filling grout is a composite grout, in which the following components are present in the dry material: cement accounts for 20% to 40% of the total mass, clay accounts for 5% to 20%, fly ash, gravel, sand and / or waste slag accounts for 50% or more, and lime, with or without, accounts for 5% to 20%. Water of not less than 20% of the total mass is added to the dry material and mixed to form a grout. Additives (including forming agents and quick-setting agents, which are used to accelerate the gelation and solidification of the grout) of 1% to 3% of the total grout volume can be added to continue mixing to form the filling grout.

[0040] When injecting filling slurry into the combustion air zone through the gasifying agent injection port, a small amount of gasifying agent can be injected simultaneously or the injection of gasifying agent can be stopped while maintaining the gasification furnace's gas production. On the one hand, the need to stop filling can be determined by monitoring the parameters of the produced gas (temperature, composition, gas production rate, etc.). Preferably, the total volume content of effective components (CH4, H2, CO) in the produced gas is less than 20%, and the O2 content is greater than 5%, or the temperature of the produced gas is below 80℃, or the pressure difference between the inlet and outlet exceeds the expected value, such as 0.5 MPa, indicating that filling needs to be stopped. On the other hand, it is essential to ensure the continuity between the injection port (i.e., the slurry injection port) and the outlet port to avoid affecting subsequent gasification operations.

[0041] The decision to stop filling can also be made by monitoring whether the filling volume of a single filling has reached the maximum filling volume. The main influencing factors to consider when estimating the maximum filling volume include coal seam thickness, coal seam dip angle, coal quality, inlet / outlet gas hole spacing, single gasification coal volume, and filling material accumulation morphology.

[0042] The gas inlet / outlet ports are switched, alternating between gasification and filling. When it is determined that filling can be stopped, the original gasifying agent injection port and gas production port of the underground gasifier are swapped (i.e., gasifying agent is injected from the original gas production port, and gas is produced from the original gasifying agent injection port), implementing reverse gasification. Specifically, the injection volume of filling slurry and / or gasifying agent is gradually reduced, and the discharge volume of gas (strictly speaking, it is already substandard gas in terms of composition) is gradually reduced, maintaining the pressure of the underground gasifier constant. When the gas production rate is less than one-third of the normal gasification production rate, carbon dioxide, water vapor, and / or nitrogen and other "inert gases" can be injected from the original gas outlet into the coal seam to drive the gas towards the gasification working face and injection port. Gasifying agent cannot be injected directly, otherwise the gasifying agent will burn or even explode upon contact with the gas. At the same time, gas can be discharged from the original gasifying agent injection port. When the injection volume of carbon dioxide, water vapor and / or nitrogen is greater than or equal to half of the underground space volume (including the remaining cavity of the combustion zone and the inlet / outlet vents), it is equivalent to having an "inert gas" barrier between the gas and the gasifying agent. The oxygen concentration in the injected gas is gradually increased, reaching the original gasifying agent level within 1 hour, thus completing the switching between the gasifying agent injection vent and the gas production vent.

[0043] When estimating the maximum filling volume for each filling operation, first determine the packing shape of the initial filling material and calculate its volume. The maximum filling volume (volume) is less than or equal to the volume of the initial filling material's packing shape minus the volume of coal ash produced by gasification. This depends on the coal seam, especially those with a certain dip angle, such as... Figure 2 As shown, the expansion pattern of the gasification combustion zone morphology is as follows: Figure 3 The shape of the filling material accumulation is determined based on the combustion zone. Preferably, from a perpendicular angle to the coal seam, after each filling, considering factors such as the flow angle of the filling material and its concentration and particle size, the filling material accumulates into an inverted isosceles triangle shape, or a large half of a long rhombus, with its cross-section parallel to the coal seam as shown in the figure. Figure 4 As shown, its volume can be approximated as follows:

[0044] ;

[0045] Wherein, l—distance between inlet / outlet vents, in meters; a—horizontal distance of filling material flow, in meters; f—maximum width of filling area or distance of gas injection point retreat, in meters; b—half of the base of the unfilled (hypothetical) small triangle, in meters; n—reverse expansion width of gasification combustion, in meters; h′—average equivalent height of the combustion air zone in the coal seam thickness direction, in meters.

[0046] Through alternating vaporization and filling via inlet / outlet vents, a relatively ideal combustion zone filling material accumulation morphology is formed, such as... Figure 5 As shown, the stacking order of the filling material is: right 1 → left 1 → right 2 → left 2 → right 3... stacking upwards in sequence.

[0047] In the initial and final stages of a single backfilling process, the mass ratio of cement in the backfill grout should be appropriately increased to encapsulate pollutants in the combustion zone within a denser, more stable solid, thus delaying pollutant migration. For example, the cement mass ratio can be increased by 5% to 30%. This is primarily to improve the density and stability of the solid formed in the initial and final stages of backfilling, ensuring that the pollutants in the combustion zone are fixed within the solid and do not easily migrate out of the combustion zone under long-term immersion and scouring by groundwater, thereby isolating the pollutants in the combustion zone.

[0048] A water collection area is set up in the gasified coal seam or combustion zone to collect and drain groundwater during the filling or gasification process. Especially during filling, as the filling slurry accumulates, water contained in the slurry will precipitate out. This water needs to be immediately drained from the gasification face; otherwise, it will affect the temperature of the gasifier, and in severe cases, may extinguish the flame at the gasification face. This water flows through the fissures and cracks in the filling slurry and coal seam to the water collection area for temporary storage. After each filling, the temporarily stored water is pumped to a surface water storage tank for reuse in subsequent fillings.

[0049] Example 1

[0050] Taking an underground coal seam gasification backfilling project at a certain pilot industrial-scale coal gasification base without a shaft as an example:

[0051] Basic information about the target coal seam for gasification: burial depth 309.16m, thickness 12m, dip angle 30°~40°, average 35°, the coal seam contains several thin layers of gangue. The distribution of the basic gasification unit facilities is as follows: Figure 6 As shown. The inner diameter of the injection hole (i.e., the right airflow channel) is DN219, and the inner diameter of the outlet hole (i.e., the left airflow channel) is DN177. The distance between the two holes is 50m.

[0052] Gasification operation conditions: Air gasification, gas injection rate: 1200-3000 m³ 3 / h, injection pressure: 0.65-2.45MPa, exhaust volume: 1620-4500 m³ / h 3 / h; the surface temperature of the vent is 150-250℃.

[0053] Step 1: Determining the timing of gasification and filling

[0054] After the gasifier has been running for a period of time, under unchanged operating conditions, the total volumetric content of the effective components of the gas begins to decrease from approximately 30% during the stable period, and shows a continuous downward trend (as follows). Figure 7As shown in the diagram, the effective contact surface area between gas and solid decreases, affecting the combustion-gasification process between the gasifying agent and coal, thus causing a decline in coal gas quality. The direct cause is the increased volume of the combustion gob. Based on this, it is determined that the combustion gob needs to be filled. However, the coal gas production rate is still maintained at over 85% of the stable period. Therefore, the criterion for whether to start filling is to determine whether the coal gas quality decline reaches the expected level.

[0055] Step 2: Estimation of maximum filling volume

[0056] ① Shape of the primary filling material:

[0057] Viewed from a perspective perpendicular to the coal seam, after each filling, the filling material accumulates into the shape of an inverted isosceles triangle, or a large half of a long rhombus.

[0058] like Figure 4 As shown, the shape of the pile after one filling (viewed perpendicular to the coal seam) is basically a narrow isosceles triangle (with an empty triangle inside).

[0059] Let the height of the small triangle be a and the width be 2b, and let a = 15m. [(20 + 2b) / 2] / (50 - 15) = b / a = b / 15, so b = 150 / 30 = 5m. The gasifying agent injection point is moved back 20m each time, and the width of the base of the triangle is 20 + 2b = 30m.

[0060] With a hole spacing of 50m, the flow of the filling material to one side is 50-15=35m;

[0061] The length of the oblique line AC = (35) 2 +15 2 ) 0.5 =38.08m.

[0062] tgβ = 15 / 35 = 0.45857;

[0063] Therefore, the pseudo-flow angle β = 23°.

[0064] ② Flow angle of the filling material during filling:

[0065] The coal seam dips at an angle of 30° to 40°, with an average of 35°.

[0066] Considering one point, its flow trajectory is as follows Figure 8 In AC, sin35° = AE / DE = AE / 15;

[0067] AE=sin35°×15=0.57358×15=8.61m;

[0068] sinα=AE / AC=8.61 / 38.08=0.22604;

[0069] The flow angle (angle of accumulation) α ≈ 13°;

[0070] In other words, the flow angle of the filler is 13°, meaning that 22% of the drop flows downwards.

[0071] That is, under the impact force of 3~4MPa, the filling material can flow 38m along AC.

[0072] In addition, the flow of the filling material is related to various factors such as its concentration and particle size, all of which must be considered in conjunction with the actual situation.

[0073] ③ Maximum filling volume:

[0074] Each filling operation aims to completely fill a triangular space without overfilling. Therefore, there exists a maximum filling amount, which is calculated as follows:

[0075] 15×35×6-5×15×6+2×20(approx.)×6 =3150-450+240=2940m 3 ;

[0076] The coal seam has an ash content of approximately 14.25%. After subtracting the space occupied by the ash, the maximum filling capacity is:

[0077] 2940-2940×14.25%=2940-418.95=2526.05m 3 ;

[0078] Because a portion of the immediate roof will collapse, the existing roof may subside slightly, and there are other factors such as the angle of the coal seam, the actual maximum filling volume will be less than the calculated value, estimated at around 2000 m³. 3 about.

[0079] Step 3: Pulping - Gasification Filling, Pumping

[0080] Fly ash from a nearby thermal power plant, with particles of 0.1-0.01 mm in diameter screened out, along with quicklime (mainly composed of calcium oxide and magnesium oxide) and / or ordinary 425# silicate cement as dry materials, plus 25% of the total dry material mass of non-acidic wastewater (discharged from the underground gasification furnace), are sent to a pulping machine in proportion. The pulping machine is immediately stirred and mixed into a uniform slurry, which is then output by a grouting pump.

[0081] Preparation of three-stage filling grout:

[0082] The initial filling grout used in a single filling operation is estimated to be 20% of the maximum filling volume (approximately 400m³). 3 The mixture consists of 50% fly ash, 30% 425# ordinary Portland cement, 5% clay, and 15% quicklime. After thorough mixing, add approximately 80-100 ml of the mixture.3 Wastewater discharged from the gasifier is proportionally transported to the pulping machine, and then approximately 20m³ of total wastewater is added proportionally. 3 The molding agent and quick-setting agent are quickly stirred and mixed to make a filling slurry.

[0083] Intermediate-stage filling grout for a single filling: Estimated usage is 60% of the maximum filling volume (approximately 1200m³). 3 The mixture consists of 70% fly ash, 20% 425# ordinary Portland cement, and 10% clay. After thorough mixing, approximately 240-400 ml of the mixture is added. 3 Wastewater discharged from the gasifier is proportionally transported to the pulping machine, where it is quickly stirred and mixed to produce filling slurry.

[0084] The final stage of a single filling process uses the same mix ratio as the previous stage filling grout.

[0085] When it is determined that filling needs to be started, connect the grouting pump to the air inlet of the underground gasifier through a pipeline, gradually reduce the injection rate of the gasifying agent, and reduce it to below 20% after 30 minutes. Then, start the grouting pump and gradually increase the injection rate of the grout into the combustion zone through the air inlet, increasing it to the maximum injection rate per unit time (15m³) after 30 minutes. 3 ( / min), while maintaining ventilation through the vent to ensure connection between the vent and the inlet. Inject the front-stage filling grout, the middle-stage filling grout, and the final-stage filling grout sequentially.

[0086] During grouting, closely monitor the changes in the parameters of the gas discharged from the vent holes and the total amount of grout injected to determine whether grouting needs to be stopped.

[0087] Maintain the underground gasifier's drainage (storage) water system in normal operation and appropriately drain the accumulated water from the underground gasifier.

[0088] Step 4: Switch to reverse gasification

[0089] During the filling process of the combustion zone, the temperature of the exhaust gas dropped rapidly from 125°C at the beginning of filling to 98°C and maintained a fluctuation range of 5°C. When the last one-third of the filling grout was injected, the total volume percentage of hydrogen, carbon monoxide and methane in the exhaust gas had dropped to below 20%, and the oxygen content reached 5.0%, which served as a signal to stop filling.

[0090] Close the vent to release air, reduce the injection rate of the filling slurry, and maintain a constant pressure inside the gasifier. If the pressure rises, continue to reduce the injection rate of the filling slurry. After closing the vent for 30 minutes, stop grouting and inject nitrogen into the vent. Begin slowly venting gas from the original injection hole, injecting approximately 500 ml of nitrogen over 30 minutes. 3By replacing nitrogen with air while maintaining a constant injection rate per unit time, the content of hydrogen, carbon monoxide, and methane in the gas discharged from the original injection hole gradually increased, while the oxygen content gradually decreased. After 24 hours, the content of each component in the produced gas was close to that of the coal gas during the stable gasification period. This indicates that the switch from filling grouting to air gasification has been successfully completed, and the original injection hole and the original gas outlet hole have been successfully interchanged.

[0091] Step 5: Repeat steps 1-4 in Example 1 above.

[0092] Using air as the gasifying agent, underground coal gasification was carried out, repeating steps 1-4 of Example 1 above. The filling effect of the combustion zone in the underground coal gasification was as follows: Figure 5 As shown.

[0093] In the estimation of the maximum filling volume in step 2 of Example 1, an alternative estimation scheme is as follows:

[0094] Based on empirical estimates of the local coalfield geological conditions, coal seam / coal quality parameters of the gasified coal seam, and the length and width of the combustion chamber formed by gasification, the maximum volume of the combustion chamber cavity in the underground gasifier reaches 3600 m³. 3 At this time, there is a high possibility of roof collapse and surrounding rock collapse in the combustion zone cavity. To avoid collapse, the cavity volume should be 0.7 times its maximum value (i.e., the cavity volume is approximately 2500 m³). 3 (When) filling is started.

[0095] The real-time calculation of the cavity volume is as follows:

[0096] V = (1000(Q / S) / d)(1-k) – V 已填 ;

[0097] In the formula, V is the cavity volume of the combustion air zone, in meters. 3 Q represents the total gas production up to the time of calculation, in Nm³. 3 S represents gas production per ton of coal, in Nm³. 3 / t; d is the specific gravity of coal, in kg / m³ 3 ; k is the influence coefficient of gangue-ash in the gasifier, generally taken as 0.10-0.25; V 已填 This represents the total amount of filling grout injected up to the time of calculation.

[0098] Based on the coal quality of this underground coal gasification experimental base without a shaft, the specific gravity of the coal is 1430 kg / m³. 3 The ash content (k) is 12%, and the gas production per ton of coal during air gasification is approximately 3300 Nm³. 3 ,but:

[0099] V=1.82×10 -4 Q – V 已填 .

[0100] Based on empirical estimates, when the total gas production reaches 13,000,000 Nm³... 3 At this time, the first filling can be initiated (V) 已填 =0), the estimated maximum filling volume for this filling is the cavity volume (approximately 2500 m³). 3 80% of ), that is, 2000m 3 .

[0101] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for underground coal gasification backfilling, characterized in that, Includes the following steps: (1) After gasification begins, the predetermined signal for filling is determined based on the total effective component volume content of the gas from the gas outlet, the output per unit time, or the expected value of the cavity volume of the combustion zone. (2) Inject filling slurry into the combustion zone through the gasifying agent injection hole, and switch the gasification mode to the filling mode immediately or gradually to maintain gas production at the outlet hole; (3) By real-time monitoring of the temperature, effective component content or gas production rate of the gas produced from the gas outlet, or by real-time monitoring of whether the maximum filling amount of a single filling has been reached, it can be determined whether the expected filling target has been achieved. (4) After the expected filling target is achieved, the filling mode shall be switched to the gasification mode immediately or gradually. (5) Repeat steps (1)-(4) until the gasified coal seam is completely mined; In step (2), the operation of switching to the filling mode is as follows: when a predetermined signal that filling is required appears, the gasifier in the gasifier injection hole is switched or gradually replaced with filling slurry, and filling slurry is injected into the combustion zone through the gasifier injection hole, while maintaining the gasification furnace to produce coal gas. In step (4), the operation of switching to gasification mode is as follows: maintain the pressure of the underground gasifier unchanged, gradually reduce the injection rate of the filling slurry and the discharge volume of the original gas outlet. When the gas production rate is less than one-third of the normal gasification production rate, inject carbon dioxide, water vapor and / or inert gas into the original gas outlet and discharge gas from the original gasifying agent injection hole. When the gas injection volume is greater than or equal to half of the underground space volume, gradually increase the oxygen concentration in the injected gas to the level of the original gasifying agent. When the gas composition discharged from the original gasifying agent injection hole is close to the composition of the gas during the stable gasification period, the exchange between the original gasifying agent injection hole and the gas outlet hole is successfully realized.

2. The method for underground coal gasification backfilling according to claim 1, characterized in that, In step (1), filling begins when the total volume content of the effective components in the gas decreases by 5% to 15% compared to the stable period; or, When the output of the gas per unit time decreases by 20% to 40% compared to the stable period, and there is a continuous downward trend, filling shall begin. or, First, an empirical estimate is made based on the coal seam parameters, geological conditions, and length and width of the combustion zone of the gasified coal seam. Then, a certain multiple of the empirical estimate is used as the expected volume value of the combustion zone. When the cavity volume of the combustion zone reaches the expected value, filling begins.

3. The method for underground coal gasification backfilling according to claim 2, characterized in that, The multiplier is 0.6 to 0.8 times.

4. The method for underground coal gasification backfilling according to claim 1, characterized in that, In step (2), the filling grout is a single-component cement grout, a clay cement grout, or a composite grout.

5. A method for underground coal gasification backfilling according to claim 4, characterized in that, The dry material in the composite slurry is one or more of the following: fly ash, gravel, sand, and waste slag added to cement and clay. The composite slurry also includes a certain amount of water, which is mixed with the dry material to form a slurry.

6. The method for underground coal gasification backfilling according to claim 1, characterized in that, In step (3), when the temperature of the gas is below 80°C, filling is stopped; or, When the total volumetric content of the effective components CH4, H2, and CO in the gas is less than 20%, and the O2 content is greater than 5%, filling shall be stopped; or, When the pressure difference between the gasifying agent injection port and the gas outlet port exceeds the expected value, the filling process should be stopped.

7. The method for underground coal gasification backfilling according to claim 1, characterized in that, In step (3), the specific operation for estimating the maximum filling volume of a single filling is as follows: first, determine the stacking shape of the filling material and calculate its volume; then, the maximum filling volume is ≤ the volume of the stacking shape of the filling material minus the volume of coal ash produced by gasification; or, The maximum filling volume for a single filling is estimated to be 60% to 100% of the volume of the combustion zone cavity during filling.

8. The method for underground coal gasification backfilling according to claim 1, characterized in that, In step (3), the filling adopts a three-stage filling method, corresponding to the front filling grout, the middle filling grout, and the final filling grout, respectively; wherein, The proportions of the front-stage filling grout and the final-stage filling grout are the same, and the mass fraction of cement in the latter is higher than that in the middle-stage filling grout.

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