Coal gangue composite solid fuel and preparation method thereof

Abstract

The present application belongs to the technical field of solid waste utilization, in particular to a coal gangue composite solid fuel and a preparation method thereof, the solid fuel is composed of coal powder, coal gangue particles, composite binder, combustion improver, sulfur-fixing agent, tea polyphenol and water. The preparation method comprises the following steps: weighing each raw material according to the formula proportion and reserving; stirring uniformly after adding water to the coal powder, coal gangue particles, combustion improver and sulfur-fixing agent to obtain a mixture; mixing uniformly the tea polyphenol into the composite binder, and then adding into the mixture and punching into shape after stirring. The raw materials of the present application are common in the market, compared with raw coal, part of the cost can be reduced, compared with the extraction process of coal gangue, the production cost is greatly reduced under the approximate sulfur-fixing effect, the raw material conversion rate reaches 90%, and the production process is simple, so that the high-quality coal gangue briquette fuel can be efficiently prepared.

Description

Technical Field

[0001] This invention belongs to the field of solid waste utilization technology, specifically a coal gangue composite solid fuel and its preparation method. Background Technology

[0002] Coal gangue is an industrial waste generated during coal mining and washing. It is a dark gray rock with a low carbon content and harder than coal, which is associated with coal seams during the coal formation process. It includes tunneling gangue, roof and floor gangue, and washed gangue. Its chemical composition is complex, mainly composed of kaolin, quartz, montmorillonite, feldspar, illite, limestone, iron sulfide, aluminum sulfide, and dozens of other chemical elements. After years of mining, coal gangue has accumulated into mountains, occupying large amounts of land resources. Exposure to sun and rain causes weathering, spontaneous combustion, dust, exhaust gases, and rainwater leaching, polluting the land, atmosphere, and water environment.

[0003] Most existing methods for handling coal gangue come at the cost of atmospheric pollution. Some methods involve crushing coal gangue and mixing it with raw coal for sale as boiler fuel, resulting in excessive emissions and severe atmospheric pollution. Almost the same amount of solid waste is then discharged and stockpiled, merely relocating the coal gangue to a storage site. With increasing awareness of energy conservation and environmental protection, the large stockpiles of coal gangue in mining areas have attracted attention. my country currently has over 6 billion tons of coal gangue stockpiles, indicating a considerable potential for recycling and reuse. Furthermore, coal gangue contains sulfides that can leach or escape, polluting the atmosphere, farmland, and water sources. The carbon in coal gangue can also cause spontaneous combustion and fires. To rationally utilize resources, many countries have begun to focus on the treatment and utilization of coal gangue since the 1960s. Currently, coal gangue is mainly used as fuel in iron smelting, boiler operation, and lime production. In recent years, extensive research has been conducted in China on the utilization of coal gangue as fuel. For example, Chinese patent ZL201010136862.9, "Coal Gangue Solid Molded Fuel and its Preparation Method," discloses a coal gangue solid briquetted fuel. By weight percentage, this solid briquetted fuel consists of: coal gangue with a calorific value higher than 2000 kcal: 40-85%; agricultural organic waste: 10-50%; combustion improver: 1-5%; and binder: 1-5%. The preparation method involves crushing coal gangue with a calorific value higher than 2000 kcal, adding a certain proportion of agricultural organic waste, combustion improver, and binder, and then directly producing a solid briquetted fuel with a stable calorific value using molding machinery. This method rationally utilizes a large amount of agricultural organic waste, combining it with coal gangue in a certain proportion to produce a solid briquetted fuel with a stable calorific value. This biomass solid briquetted fuel can be used as fuel for boilers, stoves, and other combustion devices, offering significant social, environmental, and economic benefits. However, the selected combustion aids were not ideal for coal gangue, and the binders used were clay and polyvinyl alcohol, which increased production costs. In addition, the combustion produced smoke and contained toxic components, which caused some environmental pollution. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a coal gangue composite solid fuel and its preparation method.

[0005] The objective of this invention is achieved through the following technical solution: a coal gangue composite solid fuel, comprising the following raw materials in parts by weight:

[0006] Powdered coal: 90-120; Coal gangue particles: 70-100;

[0007] Composite binder: 15-32; Combustion accelerator: 7-18;

[0008] Sulfur-fixing agent: 15-27; Tea polyphenols: 1-3;

[0009] Water: 3-8;

[0010] The composite binder is a mixture of straw pulp and calcium chloride, with a solid-liquid ratio of 1-3 g:1 L.

[0011] As a preferred embodiment of the present invention, it comprises the following raw materials in parts by weight:

[0012] Powdered coal: 100; Coal gangue particles: 95;

[0013] Composite binder: 25; Combustion accelerator: 13;

[0014] Sulfur-fixing agent: 22; Tea polyphenols: 2;

[0015] Water: 5;

[0016] The composite binder is a mixture of straw pulp and calcium chloride, with a solid-liquid ratio of 1.5g:1L.

[0017] Furthermore, the combustion improver is composed of sodium peroxide, iron oxide, and lanthanum oxide, and the weight ratio of sodium peroxide, iron oxide, and lanthanum oxide is 4-8:3-5:1.

[0018] Furthermore, the sulfur-fixing agent is a mixture of calcium hydroxide and calcium oxide, and the weight ratio of calcium hydroxide to calcium oxide is 1:2 to 5.

[0019] Furthermore, the coal gangue particles have a particle size of 200-300 mesh and a calorific value of >8500 Kj / Kg.

[0020] The above-mentioned method for preparing solid briquettes from coal gangue includes the following steps:

[0021] S1. Preparation: Weigh each raw material according to the formula ratio and set aside;

[0022] S2. Mixing: Take coal powder, coal gangue particles, combustion aid and sulfur fixative, add water and stir evenly to obtain a mixture;

[0023] S3. Molding: Add tea polyphenols to the composite binder and mix evenly, then add to the mixture and stir before stamping and molding.

[0024] Furthermore, it also includes a step of pretreating the coal gangue particles, wherein the pretreating step specifically includes:

[0025] (1) Alkali washing: Take coal gangue particles, add calcium chloride to sodium hydroxide solution with a mass percentage of 8% at a ratio of 1L / 7g, stir at 40℃~50℃ for 25~30 min, and filter after alkali washing; sodium hydroxide solution can dissolve the grease impurities on the surface of coal gangue and remove some organic sulfur compounds, and can also remove some silica to improve the combustion performance of fuel and reduce the ignition point of fuel. Calcium chloride can change the small amount of sodium silicate colloidal structure remaining after filtration, making it change from a dense structure to a uniform loose network structure and have a binding effect, which can be used as a trace amount of binder. Secondly, the flame retardant effect brought about by reducing its dense structure can increase the contact area between the fuel and oxygen and improve the combustion performance.

[0026] (2) Pickling: Add the filtered coal gangue to the mixed acid solution at room temperature, stir and soak for 25-40 minutes. After soaking, rinse with clean water and dry. The mixed acid solution is a mixture of sulfuric acid, nitric acid and hydrofluoric acid.

[0027] Furthermore, the weight-to-volume ratio of the coal gangue particles to the sodium hydroxide solution is 1 g: 3 mL. The mixture of nitric acid and sulfuric acid can effectively react the sulfur and iron contained in the coal gangue to form acid-soluble substances, thus significantly reducing their content. The nitric acid solution can lower the activation energy required for combustion and oxidize large molecules into smaller molecules, thereby making thermal decomposition and combustion easier. The mixture of hydrofluoric acid and nitric acid can reduce the ash content of the fuel. Ash removal lowers the mass and heat transfer barriers, improves the reactivity, helps promote the pyrolysis reaction, and has a significant removal effect on Si, Al, and K elements.

[0028] Furthermore, the concentration of sulfuric acid in the mixed acid is 0.1 mol / L, the concentration of nitric acid is 1 mol / L, and the concentration of hydrofluoric acid is 1 mol / L.

[0029] Furthermore, the stamping process produces a cylindrical honeycomb-shaped fuel with a bottom diameter of 28–32 mm and a height of 25–30 mm.

[0030] In the formulation of this invention:

[0031] The combustion improver uses oxides of alkali metals, transition metals, and rare earth metals as a composite combustion improver. Since rare earth metal oxides are ineffective as combustion improvers on their own, requiring increased dosage and cost, they exhibit a significant synergistic effect when mixed with transition metal oxides. This significantly reduces the activation energy of the combustion reaction, improves combustion efficiency, and reduces pollutant emissions. La promotes the combustion of pyrolysis gases, and La₂O₃ accelerates the breaking of large molecular branches and aromatic ring branches in pulverized coal, increasing porosity and surface area in coal particles. Fe accelerates the combustion of semi-coke, and Na promotes coal pyrolysis. Furthermore, the synergistic effect of different metal oxides catalyzes and improves the combustion-enhancing process of single metal oxides. The combined use of sodium peroxide, iron oxide, and lanthanum oxide significantly enhances the overall effect of the combustion improver. Simultaneously, iron oxide, as a combustion improver, also acts as a sulfur-fixing agent, and its addition increases the activity of lanthanum oxide, resulting in a significant effect in reducing the activation energy of combustion. The reaction of Na2O2 with water vapor and CO2 at high temperature to produce sodium carbonate solves the problem of reduced combustion performance of coal gangue fuel due to the high melting point of SiO2 and its crystal structure. It lowers the ignition point of coal gangue, solves the problems of difficult combustion and incomplete combustion of fixed carbon inside coal gangue, and has less environmental pollution.

[0032] The desulfurizing agent is a mixture of calcium oxide and calcium hydroxide. Calcium oxide and calcium hydroxide can achieve the effect of desulfurization, and calcium hydroxide can also react with carbon dioxide to produce calcium carbonate, thereby reducing the lattice melting point of silicon dioxide in coal gangue and improving the overall combustion performance of the fuel.

[0033] Tea polyphenols are a general term for phenolic organic compounds from plants. As a high-carbon biomass, tea polyphenols begin to decompose at 200℃, producing volatile organic compounds such as CO, CH4, and other combustion-supporting substances that can compensate for the initial loss of combustion-supporting agents. During thermal decomposition, tea polyphenols generate expanded coke, which, due to its uniform dispersion in the fuel, increases the overall porosity of the fuel, allowing the fixed carbon inside to react fully with oxygen for combustion. Tea polyphenols also act as a catalyst for combustion-supporting agents, activating metal oxides.

[0034] The composite binder is made by adding calcium chloride to straw pulp. This composite binder fully utilizes the bonding advantages of modified biomass and inorganic curing agents, improving the compressive strength and drop strength of modified biomass fuel. An appropriate amount of calcium chloride forms a composite binder with modified biomass, enhancing the fuel's water resistance. The addition of calcium chloride as a modifier gives the fuel particles high mechanical strength and good water resistance. During the fuel drying and reheating process, the evaporation of surface moisture allows the unvolatile biomass and metal compounds to act as combustion improvers and sulfur fixatives, respectively.

[0035] The present invention has the following advantages:

[0036] (1) The composite binder in the formula of this invention serves as a fuel binder and combustion aid, possessing excellent adhesion and ignition performance. It does not alter the composition of the original fuel, nor does it increase ash content or raise the ash melting point, thus turning waste into treasure. After molding and drying, it leaves behind biomass components that can be used as combustion aids, and it can increase the porosity of the molded fuel. Experiments have shown that the porosity of the fuel under a stable molding structure is about 0.01-0.05 mm higher than that of the original coal fuel, which is beneficial to the full combustion reaction of the fuel with oxygen.

[0037] (2) The preparation method disclosed in this invention can significantly reduce the harmful sulfur dioxide gas produced by coal gangue fuel combustion and remove excessive organic oil and most of the silica from the minerals through hot alkaline washing and acid washing.

[0038] (3) The raw materials are all common in the market. Compared with raw coal, it can reduce some costs. Compared with the extraction process of coal gangue, it can significantly reduce production costs under similar sulfur fixation effects. The raw material conversion rate is 90%, and the production process is simple, which can efficiently produce high-quality coal gangue briquettes. Detailed Implementation

[0039] The present invention will be further described below with reference to embodiments, but the scope of protection of the present invention is not limited to the following description:

[0040] Example 1: A coal gangue composite solid fuel, comprising the following raw materials in parts by weight:

[0041] Powdered coal: 90; Coal gangue granules: 70;

[0042] Composite adhesive: 15; Combustion accelerator: 7;

[0043] Sulfur-fixing agent: 15; Tea polyphenols: 1;

[0044] Water: 3;

[0045] The composite binder is a mixture of straw pulp and calcium chloride, with a solid-liquid ratio of 1g:1L. The combustion aid consists of sodium peroxide, iron oxide, and lanthanum oxide, with a weight ratio of 4:3:1. The sulfur-fixing agent is a mixture of calcium hydroxide and calcium oxide, with a weight ratio of 1:2. The coal gangue particles have a particle size of 200-300 mesh, and the calorific value of the coal gangue is >8500 Kj / Kg.

[0046] Example 2: A coal gangue composite solid fuel, comprising the following raw materials in parts by weight:

[0047] Powdered coal: 120; Coal gangue granules: 100;

[0048] Composite binder: 32; Combustion accelerator: 18;

[0049] Sulfur-fixing agent: 27; Tea polyphenols: 3;

[0050] Water: 8;

[0051] The composite binder is a mixture of straw slurry and calcium chloride, with a solid-liquid ratio of 3g:1L. Alternatively, the composite binder can be a mixture of straw slurry and calcium chloride, with a solid-liquid ratio of 1g:1L. The combustion aid is composed of sodium peroxide, iron oxide, and lanthanum oxide, with a weight ratio of 8:5:1. The sulfur-fixing agent is a mixture of calcium hydroxide and calcium oxide, with a weight ratio of 1:5. The coal gangue particles have a particle size of 200-300 mesh, and the calorific value of the coal gangue is >8500 Kj / Kg.

[0052] Example 3: A coal gangue composite solid fuel, comprising the following raw materials in parts by weight:

[0053] Powdered coal: 100; Coal gangue particles: 95;

[0054] Composite binder: 25; Combustion accelerator: 13;

[0055] Sulfur-fixing agent: 22; Tea polyphenols: 2;

[0056] Water: 5;

[0057] The composite binder is a mixture of straw slurry and calcium chloride, with a solid-liquid ratio of 1.5 g:1 L. The combustion aid is composed of sodium peroxide, iron oxide, and lanthanum oxide, with a weight ratio of 5:4:1. The sulfur-fixing agent is a mixture of calcium hydroxide and calcium oxide, with a weight ratio of 1:3. The coal gangue particles have a particle size of 200-300 mesh, and the calorific value of the coal gangue is >8500 KJ / Kg.

[0058] Example 4: A method for preparing solid briquettes from coal gangue, comprising the following steps:

[0059] S1. Material Preparation: Weigh each raw material according to the formula ratio of Example 1 and set aside; pretreat the coal gangue particles, the pretreatment steps specifically being:

[0060] (1) Alkali washing: Take coal gangue particles, stir them with a sodium hydroxide solution with a mass percentage of 0.1% at 60℃ for 25 min, and filter them after alkali washing;

[0061] (2) Acid washing: Add the filtered coal gangue to the mixed acid solution at room temperature, stir and soak for 25 min, rinse with clean water and dry after soaking; wherein, the mixed acid solution is a mixture of sulfuric acid, nitric acid and hydrofluoric acid, the concentration of sulfuric acid in the mixed acid is 0.1 mol / L, the concentration of nitric acid is 1 mol / L and the concentration of hydrofluoric acid is 1 mol / L;

[0062] S2. Mixing: Take coal powder, coal gangue particles, combustion aid and sulfur fixative, add water and stir evenly to obtain a mixture;

[0063] S3. Molding: Take tea polyphenols and add them to the composite binder and mix them evenly. Then add them to the mixture and stir. After stirring, press and mold them. The pressing and molding process produces a cylindrical honeycomb briquette-shaped fuel with a bottom diameter of 28 mm and a height of 25 mm.

[0064] Example 5: A method for preparing solid briquettes from coal gangue, comprising the following steps:

[0065] S1. Material preparation: Weigh each raw material according to the formula ratio of Example 2 and set aside; pretreat the coal gangue particles, the pretreatment steps specifically being:

[0066] (1) Alkali washing: Take coal gangue particles, stir them in a sodium hydroxide solution with a mass percentage of 0.1% at 70℃ for 30 min, and filter them after alkali washing;

[0067] (2) Pickling: Add the filtered coal gangue to the mixed acid solution at room temperature, stir and soak for 40 min, rinse with clean water and dry after soaking; wherein, the mixed acid solution is a mixture of sulfuric acid, nitric acid and hydrofluoric acid, the concentration of sulfuric acid in the mixed acid is 0.1 mol / L, the concentration of nitric acid is 1 mol / L and the concentration of hydrofluoric acid is 1 mol / L.

[0068] S2. Mixing: Take coal powder, coal gangue particles, combustion aid and sulfur fixative, add water and stir evenly to obtain a mixture;

[0069] S3. Molding: Take tea polyphenols and add them to the composite binder and mix them evenly. Then add them to the mixture and stir. After stirring, press and mold them. The pressing and molding process produces a cylindrical honeycomb briquette-shaped fuel with a bottom diameter of 32 mm and a height of 30 mm.

[0070] Example 6: A method for preparing solid briquettes from coal gangue, comprising the following steps:

[0071] S1. Material preparation: Weigh each raw material according to the formula ratio of Example 3 and set aside; pretreat the coal gangue particles, the pretreatment steps specifically being:

[0072] (1) Alkali washing: Take coal gangue particles, stir with a sodium hydroxide solution with a mass percentage of 0.1% at 64℃ for 28 min, and filter after alkali washing;

[0073] (2) Pickling: Add the filtered coal gangue to the mixed acid solution at room temperature, stir and soak for 30 min, rinse with clean water and dry after soaking; wherein, the mixed acid solution is a mixture of sulfuric acid, nitric acid and hydrofluoric acid, the concentration of sulfuric acid in the mixed acid is 0.1 mol / L, the concentration of nitric acid is 1 mol / L and the concentration of hydrofluoric acid is 1 mol / L;

[0074] S2. Mixing: Take coal powder, coal gangue particles, combustion aid and sulfur fixative, add water and stir evenly to obtain a mixture;

[0075] S3. Molding: Take tea polyphenols and add them to the composite binder and mix them evenly. Then add them to the mixture and stir. After stirring, press and mold them. The pressing and molding process produces a columnar honeycomb briquette-shaped fuel with a bottom diameter of 30 mm and a height of 28 mm.

[0076] The following experiments illustrate the beneficial effects of this invention:

[0077] 1. Experiment on pretreatment of coal gangue particles

[0078] Preparation of the control group: The formula of Example 3 was used and prepared according to the method of Example 6, except that the coal gangue was not pretreated in the control group.

[0079] The fuels from Examples 6 and 3 were burned, and the experimental results are as follows:

[0080] Example 3 shows that in actual combustion tests, the combustion duration is 1.5-2 hours, with no odor. The combustion produces white smoke, is low in carbon, sulfur, and nitrogen, and is environmentally friendly. The combustion produces a calorific value of up to 4100 kcal, suitable for both industrial and residential use, including boilers and residential heating. The waste material after combustion can be used as raw material for non-fired bricks.

[0081] In actual combustion tests, Example 6 showed a sustained combustion time of 1.2-1.8 hours and a strong flame duration of 7-9 minutes. Compared to Example 3, the ignition time was reduced by one-third. The combustion speed was faster and there was no odor. The combustion produced white smoke. It was low in carbon, sulfur, and nitrogen, and environmentally friendly. The calorific value generated by combustion could reach 4350 kcal.

[0082] 2. Combustion oxidizer experiment:

[0083] Control group: The formula is the same as in Example 3, except that the combustion improver in control group 1 is sodium peroxide, the combustion improver in control group 2 is lanthanum oxide, and the combustion improver in control group 3 is lanthanum oxide and iron oxide, and the weight ratio of lanthanum oxide and iron oxide is 1:1. It is prepared according to the method in Example 6.

[0084] The fuels from Example 6, Control Group 1, Control Group 2, and Control Group 3 were burned, and the experimental results are as follows:

[0085] Compared with control group 1, Example 6, using a composite of sodium peroxide, iron oxide, and lanthanum oxide in a certain proportion as a combustion improver, showed that compared with using sodium peroxide alone, the ignition time was reduced by one-third, the flame was stronger, the ash content was lower, and the combustion duration was increased. Compared with control group 2, the ignition time was reduced by one-quarter, the flame was stronger, the ash content was lower, and the combustion duration was increased. Compared with control group 3, the ignition time was reduced by one-sixth, there was no significant difference in the size of the flame and the ash content, the duration was slightly increased, and the fuel ignition point was significantly lowered.

[0086] 3. Tea polyphenol experiment:

[0087] Control group: The formula is the same as in Example 3, except that the control group does not contain tea polyphenols and is prepared according to the method in Example 6.

[0088] The fuels from Example 6 and the control group were burned, and the experimental results are as follows:

[0089] Comparing Example 6 with the control experiment, it was found that the combustion of fuel in the early stage was the same. When the fire was strong, the fuel volume of Example 6 expanded slightly, the structural gaps increased, the fire was more intense and the duration was almost the same. The white smoke produced by combustion was not significantly different. The ash produced by the combustion of Example 6 was not significantly different from that of the control experiment.

[0090] 4. Adhesive Experiment

[0091] Control group: The formula is the same as in Example 3, except that the binder in control group 1 is straw pulp and the binder in control group 2 is sodium humate. They are prepared according to the method in Example 6.

[0092] The fuels from Example 6, Control Group 1, and Control Group 2 were subjected to compression tests, post-molding resilience observations, and combustion tests. The experimental results are as follows:

[0093] Compared with Control Experiment 1, Example 6 shows that calcium chloride, as a curing agent and activator, improves the fuel's molding ability. The compressive strength test shows that it has better mechanical properties and molding stability, and the degree of fuel combustion collapse is smaller in the combustion experiment.

[0094] Compared with control group 2, it can be found that Example 6, when using composite adhesive, can have the same bonding effect as the sodium humate adhesive in control group 2. After the fuel is formed, the bonding effect is the same, and it can still maintain a good shape without falling off or collapsing. However, the evaporation of water in the surface straw pulp in Example 6 creates a void of about 0.01-0.05 mm on the surface, thereby increasing the full reaction and combustion of the fuel with oxygen. In terms of combustion performance, since the carbon-containing substances remaining after the straw pulp volatilizes act as a combustion aid, the ignition time is reduced by one-sixth. Under the condition of maintaining stable combustion at high fire, the calorific value is higher, and the amount of white smoke produced is not significantly different. During combustion, the degree of fuel collapse is not significantly different in the early stage of combustion. At high fire, Example 6 collapses faster and the degree of collapse is slightly deeper.

Claims

1. A coal gangue composite solid fuel, characterized in that, The raw materials consist of the following parts by weight: Powdered coal: 90-120; Coal gangue particles: 70-100; Composite binder: 15-32; Combustion accelerator: 7-18; Sulfur-fixing agent: 15-27; Tea polyphenols: 1-3; Water: 3-8; The composite binder is a mixture of straw pulp and calcium chloride, with a solid-liquid ratio of 1-3 g:1 L; the combustion aid is composed of sodium peroxide, iron oxide, and lanthanum oxide, with a weight ratio of sodium peroxide, iron oxide, and lanthanum oxide of 4-8:3-5:

1. It also includes a step of pre-treating the coal gangue particles, the pre-treatment step specifically being: (1) Alkali washing: Take coal gangue particles, stir with a sodium hydroxide solution with a mass percentage of 0.1% at a temperature of 60-70℃ for 25-30 min, and filter after alkali washing; (2) Pickling: Add the filtered coal gangue to the mixed acid solution at room temperature, stir and soak for 25-40 minutes. After soaking, rinse with clean water and dry. The mixed acid solution is a mixture of sulfuric acid, nitric acid and hydrofluoric acid. The mixed acid contains sulfuric acid at a concentration of 0.1 mol / L, nitric acid at a concentration of 1 mol / L, and hydrofluoric acid at a concentration of 1 mol / L.

2. The coal gangue composite solid fuel as described in claim 1, characterized in that, The raw materials consist of the following parts by weight: Powdered coal: 100; Coal gangue particles: 95; Composite binder: 25; Combustion accelerator: 13; Sulfur-fixing agent: 22; Tea polyphenols: 2; Water: 5; The composite binder is a mixture of straw pulp and calcium chloride, with a solid-liquid ratio of 1.5g:1L.

3. A coal gangue composite solid fuel as described in claim 1 or 2, characterized in that, The sulfur-fixing agent is a mixture of calcium hydroxide and calcium oxide, and the weight ratio of calcium hydroxide to calcium oxide is 1:2 to 5.

4. A coal gangue composite solid fuel as described in claim 1 or 2, characterized in that, The coal gangue particles have a particle size of 200-300 mesh and a calorific value of >8500 Kj / Kg.

5. A coal gangue composite solid fuel as described in claim 1, characterized in that, The weight-to-volume ratio of the coal gangue particles to the sodium hydroxide solution is 1 g: 3 mL.

6. The method for preparing a coal gangue composite solid fuel as described in claim 1, characterized in that, It includes the following steps: S1. Preparation: Weigh each raw material according to the formula ratio and set aside; S2. Mixing: Take coal powder, coal gangue particles, combustion aid and sulfur fixative, add water and stir evenly to obtain a mixture; S3. Molding: Add tea polyphenols to the composite binder and mix evenly, then add to the mixture and stir before stamping and molding.

7. The method for preparing a coal gangue composite solid fuel according to claim 6, characterized in that, The stamping process produces cylindrical honeycomb-shaped fuel with a bottom diameter of 28–32 mm and a height of 25–30 mm.