A method for in-situ modification of deep coal seams using multi-microbial organisms

Abstract

The present application relates to coal mining technology field, specifically to a kind of in-situ modification method of deep coal seam using multiple microorganisms, comprising the following steps: using high pressure water jet or fracturing technology, directional construction microcrack network in target coal seam;Alternately inject mixed solution and composite bacteria solution into target coal seam, mixed solution includes phosphate buffer and trace metal salt aqueous solution;With 0.5~1MPa pressure, composite bacteria solution is injected into target coal seam through grouting hole, composite bacteria solution includes dust removal bacteria, fire-retardant bacteria, toughening bacteria, cleaning bacteria, wherein, the dust removal bacteria, fire-retardant bacteria, toughening bacteria, cleaning bacteria are in the proportion of 4:3:2:1 according to biomass ratio;Close grouting hole, cultivate 14~21 days under the condition of environmental temperature 25~35 DEG C, relative humidity >80%, so that functional microorganism in composite bacteria solution colonizes in coal seam microcrack and metabolizes to produce active substance.It aims to realize the integration, green, collaborative management of key problems such as deep coal seam dust inhibition, fire prevention and control, mechanical property improvement and in-situ reduction of gas.

Description

Technical Field

[0001] This invention belongs to the field of coal mining technology, specifically relating to a method for in-situ modification of deep coal seams using diverse microorganisms. Background Technology

[0002] my country's coal resources exhibit a distribution pattern of "sparsely mined but abundantly deep," with deep coal resources buried at depths of less than 1,000 meters accounting for over 70%. Particularly in the eastern mining areas, mining depths are rapidly extending to 1,000-1,500 meters. With increasing mining depth, ground stress, ground temperature, and gas pressure rise significantly, and the patterns of rock strata movement and surface subsidence become increasingly complex, posing severe challenges to safe and efficient coal mining.

[0003] Against this backdrop, four key issues urgently need to be addressed collaboratively during the mining and subsequent utilization of deep coal seams:

[0004] 1. Dust control: Coal dust generated during mining not only harms miners' health (such as causing pneumoconiosis), but also poses an explosion risk;

[0005] 2. Flame retardant safety: Coal is prone to spontaneous combustion or explosion when exposed to a source of ignition, so its flame retardant properties need to be improved from the source.

[0006] 3. Toughening and improving quality: Enhancing the strength and toughness of coal can reduce crushing losses and improve resource recovery rate and processing efficiency;

[0007] 4. Cleaning and purification and gas control: Impurities in coal (such as sulfur, gangue and moisture) reduce the calorific value, while methane (gas) is the main source of danger that causes major accidents such as gas outbursts and explosions, and it is urgent to effectively degrade or fix it.

[0008] However, existing technologies struggle to simultaneously achieve these multiple objectives. On one hand, mainstream methods largely rely on chemical modifiers, such as polyurethane, phenolic resins, and bromine- or phosphorus-based flame retardants. While effective in a single function, these methods suffer from limitations such as limited functionality, redundant processes, and high costs. More seriously, some chemical reagents exhibit strong environmental toxicity, easily causing soil and groundwater pollution, which does not meet the requirements for green mine development. On the other hand, existing microbial modification technologies often employ single strains, have poor environmental adaptability, struggle to colonize under deep, high-temperature, and high-pressure conditions, and suffer from metabolic instability, resulting in uncontrollable effects and insufficient economic viability. Summary of the Invention

[0009] The purpose of this invention is to provide a method for in-situ modification of deep coal seams using diverse microorganisms, aiming to achieve integrated, green, and synergistic governance of key issues such as dust suppression, fire prevention, mechanical property improvement, and in-situ gas reduction in deep coal seams.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] A method for in-situ modification of deep coal seams using diverse microorganisms, comprising the following steps:

[0012] High-pressure water jet or Fracture-inducing technology, which directionally constructs a network of microfractures in the target coal seam;

[0013] A mixed solution and a composite bacterial solution are alternately injected into the target coal seam, wherein the mixed solution comprises a phosphate buffer solution and a trace metal salt aqueous solution;

[0014] A composite bacterial solution is injected into the target coal seam through the grouting hole at a pressure of 0.5~1MPa. The composite bacterial solution includes dust-removing bacteria, flame-retardant bacteria, toughening bacteria, and cleaning bacteria, wherein the ratio of dust-removing bacteria, flame-retardant bacteria, toughening bacteria, and cleaning bacteria by biomass is 4:3:2:1.

[0015] The grouting holes are sealed, and the mixture is cultured for 14 to 21 days at an ambient temperature of 25 to 35°C and a relative humidity of >80%, allowing the functional microorganisms in the composite bacterial solution to colonize and metabolize in the micro-fractures of the coal seam to produce active substances.

[0016] Preferably, the pH of the phosphate buffer solution is 7.0 to 7.5;

[0017] The trace metal salt is an aqueous solution of divalent iron salt and / or divalent manganese salt, with a concentration of 0.05~0.1 mol / L;

[0018] The mixture is composed of the phosphate buffer solution and the trace metal salt aqueous solution, and is injected into the target coal seam alternately with the composite bacterial solution through a borehole grouting system; the injection pressure of the mixture is 0.3~0.5MPa, and the injection volume is 10~15 liters per meter of borehole advance.

[0019] Preferably, the trace metal salt aqueous solution includes ferrous salt and manganese salt, and the molar concentration ratio of the ferrous salt to the manganese salt is 1:1 to 1:2.

[0020] Preferably, the dust-removing bacteria include Xanthomonas rapae, the flame-retardant bacteria include Bacillus subtilis mutant strain, the toughening bacteria include Pseudomonas xylophilus, and the cleaning bacteria include strains with methane oxidation function; the strains with methane oxidation function include one or more of the genera Methylcurvature, Methylcoccus, or Methylmona.

[0021] Preferably, the compound bacterial solution further includes the nonionic surfactant Triton X-100, with a mass fraction of 0.01~0.05%.

[0022] Preferably, the compound bacterial solution is prepared by co-culture, and the culture medium used is low nitrogen and high carbon type with a carbon-nitrogen ratio of 30:1. During the culture process, the pH is controlled at 6.5~7.5 and the temperature is controlled at 30~35℃. The accumulation of target metabolites is promoted by batch feeding.

[0023] Preferably, the pore size of the microfracture network is [missing information]. The porosity is 20-30%.

[0024] Preferably, after cultivation, a secondary induction step is also included: when the main goal is to enhance the mechanical properties of the coal, an aqueous solution of sodium carboxymethyl cellulose is injected into the coal seam; when the main goal is to improve the flame retardant properties of the coal seam, a mixed solution of sodium phosphate aqueous solution and polyethylene glycol 200 is injected into the coal seam.

[0025] Preferably, the secondary induction step is carried out 7-10 days after the initial colonization of microorganisms, and the inducing agent solution is injected through a drilling grouting system at a pressure of 0.3-0.6 MPa; the concentration of the sodium carboxymethyl cellulose aqueous solution is 1.0%-1.5% (w / v), the concentration of the sodium phosphate aqueous solution is 0.5%-1.0% (w / v), and the mass concentration of polyethylene glycol 200 is 5.0%; the injection volume of the inducing agent solution is 5-8 L per meter of drilling depth, and it is sterilized before use.

[0026] The present invention has the following beneficial effects:

[0027] 1. The method described in this invention integrates four core functions—dust removal, flame retardancy, mechanical enhancement, and environmental protection (including methane reduction and organic pollutant degradation)—into a single microbial system. This effectively overcomes the shortcomings of existing technologies, such as single-function limitations and redundant processes, achieving systematic control over safety risks in deep coal seams. Experimental verification shows that this method achieves significant breakthroughs in several key technical indicators: dust concentration in the coal seam working environment is reduced by ≥60%; the limiting oxygen index (LOI) of the coal seam is increased to >26%, significantly enhancing flame retardant performance; the spontaneous combustion tendency period is extended to over 90 days; compressive strength is increased by ≥15%; the degradation rate of organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) is ≥50%; and the methane content in the coal seam is reduced by 33.7%. These indicators work together to comprehensively reduce multiple safety risks in deep mining, including explosions, fires, gas outbursts, and occupational health risks.

[0028] 2. The method described in this invention relies entirely on the in-situ metabolic activities of microorganisms to function, without introducing toxic chemical reagents or leaving harmful residues, thus avoiding soil and water pollution that may be caused by traditional chemical modification materials (such as bromine / phosphorus flame retardants, polymer resins, etc.). Attached Figure Description

[0029] Figure 1A flowchart illustrating a method for in-situ modification of deep coal seams using multiple microorganisms, provided in an embodiment of this application. Detailed Implementation

[0030] The embodiments of the present invention will be described below with reference to the accompanying drawings and preferred embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are only for illustrating the present invention and not for limiting the scope of protection of the present invention.

[0031] In one embodiment, this application provides a method for in-situ modification of deep coal seams using multi-microorganisms, which includes the following steps:

[0032] High-pressure water jet or Fracture-inducing technology, which directionally constructs a network of microfractures in the target coal seam;

[0033] A mixed solution and a composite bacterial solution are alternately injected into the target coal seam, wherein the mixed solution comprises a phosphate buffer solution and a trace metal salt aqueous solution;

[0034] A composite bacterial solution is injected into the target coal seam through the grouting hole at a pressure of 0.5~1MPa. The composite bacterial solution includes dust-removing bacteria, flame-retardant bacteria, toughening bacteria, and cleaning bacteria, wherein the ratio of dust-removing bacteria, flame-retardant bacteria, toughening bacteria, and cleaning bacteria by biomass is 4:3:2:1.

[0035] The grouting holes are sealed, and the mixture is cultured for 14 to 21 days at an ambient temperature of 25 to 35°C and a relative humidity of >80%, allowing the functional microorganisms in the composite bacterial solution to colonize and metabolize in the micro-fractures of the coal seam to produce active substances.

[0036] This application provides a method for in-situ modification of deep coal seams using diverse microorganisms. By integrating four core functions—dust removal, flame retardancy, mechanical enhancement, and environmental protection (including methane reduction and organic pollutant degradation)—into a single microbial system, this method effectively overcomes the technical shortcomings of existing technologies, such as single-function limitations and redundant processes, achieving systematic control of safety risks in deep coal seams. Experimental verification shows that this method can significantly reduce multiple safety risks in deep mining, specifically: dust concentration in the coal seam working environment is reduced by more than 60%; the limiting oxygen index (LOI) of the coal seam is increased to more than 26%, significantly enhancing flame retardant performance; the spontaneous combustion tendency period is extended to more than 90 days; compressive strength is increased by more than 15%; the degradation rate of organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) exceeds 50%; and the methane content in the coal seam is reduced by more than 30%. The synergistic effect of these indicators comprehensively suppresses safety risks such as explosions, fires, gas outbursts, and occupational health risks. Meanwhile, this method relies entirely on in-situ microbial metabolic activity to achieve modification, without the need to introduce toxic chemical reagents or leave harmful residues. It fundamentally avoids the soil and water pollution that may be caused by traditional chemical modification materials (such as bromine / phosphorus flame retardants, polymer resins, etc.), which meets the requirements of green mining and sustainable development.

[0037] In a preferred embodiment, the phosphate buffer solution has a pH of 7.0 to 7.5, which can stabilize the acid-base environment within the micro-fractures of the coal seam, effectively match the suitable growth pH range of functional microorganisms such as dust-removing bacteria and flame-retardant bacteria in the composite bacterial solution, avoid fluctuations in microbial activity caused by environmental acid-base fluctuations, provide stable basic conditions for microbial colonization and metabolism, and thus improve the efficiency of microbial modification.

[0038] The trace metal salts are aqueous solutions of ferrous and / or manganese divalent salts, with a concentration of 0.05–0.1 mol / L. This concentration range has been experimentally optimized and can serve as a key coenzyme factor in microbial metabolism, effectively activating related enzyme activities, promoting the proliferation and metabolism of functional microorganisms, accelerating the generation of active substances, and thus improving the modification effects of coal seam dust removal and flame retardancy. At the same time, this concentration meets the growth requirements of microorganisms while avoiding the toxic inhibition caused by excessive metal ions, achieving a precise balance between nutrient supply and microbial activity.

[0039] The mixture is composed of the phosphate buffer solution and the trace metal salt aqueous solution, and is alternately injected into the target coal seam through a borehole grouting system along with the composite bacterial solution. The injection pressure of the mixture is 0.3~0.5MPa, and the injection volume is 10~15 liters per meter of borehole depth. On the one hand, the injection pressure is lower than that of the composite bacterial solution (0.5~1MPa), which avoids high-pressure impact from damaging the microfracture network constructed in the early stage, while ensuring that the mixture penetrates evenly into the microfractures of the coal seam, providing a uniform nutritional and environmental basis for microbial colonization. On the other hand, the precise injection volume design can optimize material utilization efficiency, reduce consumption costs, and, combined with the alternating injection mode, effectively avoid uneven modification caused by bacterial solution aggregation, ensure the consistency of modification effects in different areas of the coal seam, and improve the stability of the overall performance of deep coal seams.

[0040] In a preferred embodiment, the trace metal salt aqueous solution includes ferrous salt and manganese salt, wherein the molar concentration ratio of ferrous salt to manganese salt is 1:1 to 1:2.

[0041] Both ferrous and manganese divalent ions are core cofactors for many key enzymes in microorganisms (such as catalase and peroxidase). This implementation method strictly limits the molar ratio of ferrous and manganese divalent salts to within the range of 1:1 to 1:2. Through the synergistic effect between ions, it effectively meets the trace element requirements of functional microorganisms such as dust-removing bacteria, flame-retardant bacteria, and toughening bacteria, enhances the enzyme activity in microorganisms, accelerates microbial proliferation and metabolic intensity, and thus improves the efficiency of coal seam modification.

[0042] In a preferred embodiment, the dust-removing bacteria include Xanthomonas campestris, whose metabolically produced extracellular polysaccharides have thickening, emulsifying, and wetting properties. This species colonizes and metabolizes in the microfractures of coal seams, secreting viscous polysaccharides that adsorb coal dust particles, causing them to agglomerate and settle, thus reducing the concentration of respirable dust. At the same time, it forms a wetting film on the coal and rock surface, reducing dust generation during mining operations.

[0043] The flame-retardant bacteria include a Bacillus subtilis mutant strain, which metabolizes in an oxygen-deficient coal seam environment to produce biochar, carbonate minerals, and inert gases. These metabolites form an insulating layer in the pores and fissures of the coal seam, preventing oxygen from contacting the coal seam; the thermal stability of the biochar increases the limiting oxygen index of the coal seam, thus achieving a flame-retardant effect.

[0044] The toughening bacteria include *Pseudomonas xylinum*, which possesses mineralization and colloidal secretion capabilities. It can utilize coal seam mineral ions for biomineralization or secrete viscous biopolymers to fill micro-fractures. Through biomineralization and cementation, micro-fractures are repaired, enhancing the structural integrity of the coal seam and improving its compressive and tensile strength.

[0045] The cleaning bacteria include methane-oxidizing strains, such as *Methylcurvature*, *Methylcoccus*, or *Methylmonas*. These bacteria can oxidize methane in coal seams, converting it into carbon dioxide and water or their own biomass, thereby reducing the methane content in coal seams and eliminating the risk of gas outbursts and explosions.

[0046] In a preferred embodiment, the composite bacterial solution further includes a nonionic surfactant, Triton X-100, with a mass fraction of 0.01~0.05%. As a nonionic surfactant, Triton X-100 can reduce the surface tension and contact angle of the composite bacterial solution, promote the spreading and penetration of the bacterial solution on the coal and rock surface, overcome the capillary resistance of coal seam pores, and enable the bacterial solution to smoothly and deeply enter the previously constructed microfracture network, avoiding local aggregation and insufficient penetration caused by poor bacterial solution flowability, thereby improving the modification range and depth.

[0047] In a preferred embodiment, the composite bacterial solution is prepared through co-culture using a low-nitrogen, high-carbon culture medium with a carbon-to-nitrogen ratio of 30:1. During cultivation, the pH is controlled at 6.5–7.5, and the temperature at 30–35°C. A fed-batch feeding method is used to promote the accumulation of target metabolites. This co-culture model simulates the microenvironment of a coal seam, promoting the formation of a synergistic symbiotic system among the four functional microbial species and avoiding the compatibility issues caused by monoculture. The low-nitrogen, high-carbon culture medium ratio matches the nutritional needs of the functional microorganisms, guiding them towards the targeted accumulation of target metabolites and increasing their yield and activity. Precise control of cultivation conditions and the use of a fed-batch feeding method maintain stable microbial proliferation, preventing cell inactivation due to nutrient imbalances or environmental fluctuations, ensuring the stability of the composite bacterial solution's quality, and providing a reliable foundation for subsequent coal seam colonization and modification.

[0048] In a preferred embodiment, the pore size of the microfracture network is [missing information]. The porosity is 20-30%. The pore size range is matched with the individual size and aggregation state of the composite functional microorganisms, ensuring that the microorganisms can smoothly enter the fracture interior while providing a suitable attachment interface. This facilitates the rapid adsorption, colonization, and biofilm formation of dust-removing, flame-retardant, toughening, and cleaning bacteria on the fracture wall, avoiding the problems of bacterial blockage due to excessively small pore size or bacterial retention due to excessively large pore size. This improves the colonization rate and survival stability of microorganisms within the coal seam. A porosity of 20-30% ensures sufficient permeability space in the coal seam while maintaining the structural strength of the coal and rock, preventing coal seam collapse and structural damage due to excessive fracturing. This porosity range allows the mixed liquid and composite bacterial solution to diffuse evenly and fully fill the coal seam under injection pressures of 0.3-0.5 MPa and 0.5-1 MPa, achieving a balanced distribution of nutrients and functional microorganisms throughout the coal seam, thus ensuring uniform and consistent modification effects such as dust removal, flame retardancy, toughening, and cleaning.

[0049] In a preferred embodiment, after cultivation, a secondary induction step is further included: when the primary goal is to enhance the mechanical properties of the coal seam, an aqueous solution of sodium carboxymethyl cellulose is injected into the coal seam; when the primary goal is to improve the flame retardant properties of the coal seam, a mixed solution of sodium phosphate aqueous solution and polyethylene glycol 200 is injected into the coal seam. The beneficial effects of setting a secondary induction step specifically according to actual engineering needs after microbial cultivation are as follows:

[0050] Firstly, when the main goal is to enhance the mechanical properties of coal, sodium carboxymethyl cellulose aqueous solution is injected into the coal seam. Sodium carboxymethyl cellulose has adhesive and cementing properties, and can work synergistically with polysaccharides and mineralization products produced by microbial metabolism to fill micro-fractures in the coal seam, cement loose coal and rock particles, improve the compressive and tensile strength and integrity of the coal body, effectively prevent geological disasters such as roof collapse and spalling during deep mining, and is compatible with the coal seam and microbial metabolites, does not affect the activity of the microbial community, and causes no secondary pollution.

[0051] Secondly, when the primary goal is to improve the flame retardant properties of coal seams, a mixed solution of sodium phosphate aqueous solution and polyethylene glycol 200 is injected into the coal seam. Sodium phosphate works synergistically with biochar and carbonate minerals produced by Bacillus subtilis mutant strain metabolism to form a flame retardant barrier, increase the limiting oxygen index of the coal seam, and delay spontaneous combustion and fire spread. Polyethylene glycol 200 improves the permeability and dispersibility of sodium phosphate aqueous solution, allowing it to penetrate evenly into the micro-fractures of the coal seam and combine with microbial metabolites, avoiding the problem of insufficient local flame retardant effect, while reducing the corrosiveness of sodium phosphate and improving construction safety.

[0052] Third, the secondary induction step can be flexibly adapted to different engineering needs, solving the problem of fixed effects of traditional modification technology, expanding the applicable scenarios of this method, improving the flexibility and practicality of the technology, and ensuring that the modification effect matches the actual safety requirements of coal mining.

[0053] In a preferred embodiment, the secondary induction step is carried out 7-10 days after the initial colonization of microorganisms, and the inducing agent solution is injected through a drilling grouting system at a pressure of 0.3-0.6 MPa; the concentration of the sodium carboxymethyl cellulose aqueous solution is 1.0%-1.5% (w / v), the concentration of the sodium phosphate aqueous solution is 0.5%-1.0% (w / v), and the mass concentration of polyethylene glycol 200 is 5.0%; the injection volume of the inducing agent solution is 5-8 L per meter of drilling depth, and it is sterilized before use.

[0054] After the microbial culture is completed, a secondary induction step is set according to the actual engineering needs. When the main goal is to enhance the mechanical properties of the coal, an aqueous solution of sodium carboxymethyl cellulose is injected into the coal seam. Sodium carboxymethyl cellulose has adhesive and cementing properties and can work synergistically with the polysaccharides and mineralization products produced by microbial metabolism to fill the micro-fractures in the coal seam, cement loose coal and rock particles, improve the compressive and tensile strength and integrity of the coal body, effectively prevent geological disasters such as roof collapse and spalling during deep mining, and is compatible with the coal seam and microbial metabolites, does not affect the activity of the microbial community, and causes no secondary pollution. When the primary goal is to improve the flame retardant properties of coal seams, a mixed solution of sodium phosphate aqueous solution and polyethylene glycol 200 is injected into the coal seam. The sodium phosphate works synergistically with the biochar and carbonate minerals produced by the metabolism of Bacillus subtilis mutant strain to form a flame retardant barrier, increase the limiting oxygen index of the coal seam, and delay spontaneous combustion and fire spread. Polyethylene glycol 200 improves the permeability and dispersibility of the sodium phosphate aqueous solution, allowing it to penetrate evenly into the micro-fractures of the coal seam and combine with microbial metabolites, avoiding the problem of insufficient local flame retardant effect. At the same time, it reduces the corrosiveness of sodium phosphate and improves construction safety.

[0055] By precisely controlling the timing, injection pressure, reagent concentration, injection volume, and sterilization process of secondary induction, the targeting, safety, controllability, and stability of the modification effect of secondary induction were improved. Setting the secondary induction step 7-10 days after the initial colonization of microorganisms allows for sequential synergy with microbial colonization and metabolic processes, avoiding interference with microbial colonization and enabling targeted enhancement during key metabolic stages. Injecting the inducing agent solution at a pressure of 0.3-0.6 MPa ensures sufficient penetration of the inducing agent into the microfracture network for uniform distribution, while preserving the integrity of the existing microbial biofilm and coal structure. The concentration of sodium carboxymethyl cellulose aqueous solution was controlled at 1.0%-1.5% (w / v), and sodium phosphate aqueous solution... The concentration of the liquid is controlled at 0.5%~1.0% (w / v), and the mass concentration of polyethylene glycol 200 is controlled at 5.0%. This ensures both cementation and flame retardant enhancement while avoiding excessive solution viscosity, penetration difficulties, and material waste, thus achieving a balance between induction efficiency and economy. The injection volume of the inducing agent solution is limited to 5~8L per meter of borehole depth to achieve standardized and quantitative control, ensuring uniform modification of the coal seam along the borehole direction. The inducing agent solution is sterilized before use to effectively avoid contamination of the coal seam's microecological environment by exogenous bacteria, reduce competition for nutrients by bacteria and inhibition of functional microorganisms, maintain the community stability and metabolic activity of dust-removing bacteria, flame-retardant bacteria, toughening bacteria, and cleaning bacteria, and ensure the continuous and stable performance of coal seam modification.

[0056] The above embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention.

Claims

1. A method for in-situ modification of deep coal seams using multi-microorganisms, characterized in that, Includes the following steps: High-pressure water jet or Fracture-inducing technology, which directionally constructs a network of microfractures in the target coal seam; A mixed solution and a composite bacterial solution are alternately injected into the target coal seam, wherein the mixed solution comprises a phosphate buffer solution and a trace metal salt aqueous solution; A composite bacterial solution is injected into the target coal seam through the grouting hole at a pressure of 0.5~1MPa. The composite bacterial solution includes dust-removing bacteria, flame-retardant bacteria, toughening bacteria, and cleaning bacteria, wherein the ratio of dust-removing bacteria, flame-retardant bacteria, toughening bacteria, and cleaning bacteria by biomass is 4:3:2:

1. The grouting holes are sealed, and the mixture is cultured for 14 to 21 days at an ambient temperature of 25 to 35°C and a relative humidity of >80%, allowing the functional microorganisms in the composite bacterial solution to colonize and metabolize in the micro-fractures of the coal seam to produce active substances.

2. The method for in-situ modification of deep coal seams using multi-microorganisms according to claim 1, characterized in that: The pH of the phosphate buffer solution is 7.0~7.5; The trace metal salt is an aqueous solution of divalent iron salt and / or divalent manganese salt, with a concentration of 0.05~0.1 mol / L; The mixture is composed of the phosphate buffer solution and the trace metal salt aqueous solution, and is injected into the target coal seam alternately with the composite bacterial solution through a borehole grouting system; the injection pressure of the mixture is 0.3~0.5MPa, and the injection volume is 10~15 liters per meter of borehole advance.

3. The method for in-situ modification of deep coal seams using multi-microorganisms according to claim 2, characterized in that: The trace metal salt aqueous solution includes ferrous salt and manganese salt, and the molar concentration ratio of ferrous salt to manganese salt is 1:1 to 1:

2.

4. The method for in-situ modification of deep coal seams using multi-microorganisms according to claim 1, characterized in that: The dust-removing bacteria include Xanthomonas rapae, the flame-retardant bacteria include Bacillus subtilis mutant strain, the toughening bacteria include Pseudomonas xylophilus, and the cleaning bacteria include strains with methane oxidation function. The strains with methane oxidation function include one or more of the genera *Methylcurvature*, *Methylcoccus*, or *Methylmonas*.

5. The method for in-situ modification of deep coal seams using multi-microorganisms according to claim 1, wherein the composite bacterial solution further includes the nonionic surfactant Triton X-100, with a mass fraction of 0.01~0.05%.

6. The method for in-situ modification of deep coal seams using multi-microorganisms according to claim 1, characterized in that: The compound bacterial solution was prepared by co-culture. The culture medium used was low-nitrogen, high-carbon, with a carbon-nitrogen ratio of 30:

1. During the culture process, the pH was controlled at 6.5-7.5 and the temperature at 30-35℃. The accumulation of target metabolites was promoted by feeding in batches.

7. The method for in-situ modification of deep coal seams using multi-microorganisms according to claim 1, characterized in that: The pore size of the microfracture network is The porosity is 20-30%.

8. The method for in-situ modification of deep coal seams using multi-microorganisms according to claim 1, characterized in that: After cultivation, a secondary induction step is also included: when the main goal is to enhance the mechanical properties of the coal, an aqueous solution of sodium carboxymethyl cellulose is injected into the coal seam; when the main goal is to improve the flame retardant properties of the coal seam, a mixed solution of sodium phosphate and polyethylene glycol 200 is injected into the coal seam.

9. The method for in-situ modification of deep coal seams using multi-microorganisms according to claim 8, characterized in that: The secondary induction step is carried out 7-10 days after the initial colonization of microorganisms, by injecting the inducing agent solution at a pressure of 0.3-0.6 MPa through a borehole grouting system; The concentration of the sodium carboxymethyl cellulose aqueous solution is 1.0%~1.5% (w / v), the concentration of the sodium phosphate aqueous solution is 0.5%~1.0% (w / v), and the mass concentration of the polyethylene glycol 200 is 5.0%. The inducing agent solution is injected at a rate of 5-8 L per meter of borehole depth and is sterilized before use.

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