Low metamorphic high-chlorine coal multi-stage countercurrent washing and dechlorination method

Abstract

The application discloses a low metamorphic high-chlorine coal multi-stage countercurrent washing and dechlorination method, and belongs to the technical field of clean coal utilization. The method comprises the following steps: S1, crushing and screening low metamorphic high-chlorine coal with a chlorine content of no less than 0.30wt% into coal materials with a particle size of no more than 13mm; S2, feeding the coal materials into N-stage (3-6 stages) washing units in series for countercurrent washing, the water flow direction is opposite to the coal flow, the i-stage washing liquid is used as the washing water for the i-1-stage, fresh water is introduced into the N-stage, the chlorine-containing concentrated washing liquid is discharged from the 1-stage, the washing temperature is increased along the coal flow and is 15-90 DEG C, the temperature of the last stage is 40-90 DEG C, the liquid-solid ratio is 2:1-8:1, and the single-stage time is 5-60 min; S3, dehydrating to obtain dechlorination coal products; and S4, desalting the chlorine-containing concentrated washing liquid to recover chloride salt, and recycling water. The method does not need chemical agents, promotes the dissolution of chlorine through the concentration gradient and warm water strengthening in countercurrent, the dechlorination rate is no less than 70%, water consumption is reduced by more than 70%, and zero discharge of waste water and recycling of chlorine resources are realized.

Description

Technical Field

[0001] This invention relates to the field of clean coal utilization and coal chemical technology, and in particular to a multi-stage countercurrent washing and dechlorination method for low-rank, high-chlorine coal. The method of this invention is applicable to the pre-combustion dechlorination treatment of low-rank coal (including lignite, long-flame coal, and other low-rank coals) with a chlorine content of not less than 0.30 wt%. It can significantly reduce the chlorine content in coal, alleviate chlorine corrosion and flue gas chlorine pollution problems during coal combustion, and realize the recycling of washing water and the resource recovery of chloride salts. Background Technology

[0002] Chlorine in coal is one of the most harmful trace elements. Large quantities of low-rank coal (mainly lignite and long-flame coal) are found in regions such as Zhundong in Xinjiang, Ningxia, and Inner Mongolia in my country. In some mining areas, the chlorine content of the coal is as high as 0.3wt%~1.5wt% (dry basis), classifying it as typical high-chlorine coal. During combustion or pyrolysis, the chlorine in high-chlorine coal is released into the flue gas primarily in the form of HCl, Cl2, and small amounts of organic chlorine. This not only causes severe acid corrosion and fouling of boiler heating surfaces, tail flue ducts, and flue gas purification equipment, but also increases the risk of generating persistent organic pollutants such as dioxins, seriously hindering the safe and clean utilization of low-rank high-chlorine coal. Therefore, efficiently removing chlorine from coal before it enters the furnace is of great significance for expanding the utilization pathways of low-rank high-chlorine coal, ensuring the safe operation of coal-fired equipment, and protecting the environment.

[0003] The occurrence form of chlorine in coal is a key factor determining the choice of dechlorination method. Existing studies have shown that chlorine in coal mainly exists in two forms: inorganic chlorine (water-soluble salts such as sodium chloride, potassium chloride, and calcium chloride) and organic chlorine. Inorganic chlorine is mainly distributed in the pore water, fissures, and surface adsorption layer of coal particles, while organic chlorine is combined with the macromolecular structure of coal (see the article "Changes of Chlorine in Coal under Subcritical Water Conditions" published in the Journal of Fuel Chemistry in 2007 and the literature on the influence of chlorine form analysis and leaching conditions on chlorine removal published in Fuel Journal in 2020). International research on dechlorination of high-chlorine coal by washing has shown that washing temperature, coal particle size, and leaching time have a significant impact on chlorine removal efficiency. For example, a high-temperature leaching study of high-chlorine coal in Illinois, USA (see the US Department of Energy technical report "Removal of chlorine from Illinois coal by high-temperature leaching", 1987) confirmed that washing at higher temperatures can significantly improve the chlorine removal rate from coal. However, this study used a single-stage high-temperature leaching method, which consumes a lot of water and energy and did not involve the recycling of washing water. For highly coalified bituminous coal and anthracite, which have low porosity and strong hydrophobicity, chlorine mainly exists in the form of bound minerals. In contrast, for low-rank coals such as lignite and long-flame coal, which have well-developed pores, abundant oxygen-containing functional groups, and strong hydrophilicity, chlorine mainly exists in the form of water-soluble inorganic chlorine in pores and fissures, and the proportion of organic chlorine is relatively low. This inherent characteristic makes water washing a natural fit for low-rank, high-chlorine coal: water can penetrate the well-developed pore network of low-rank coal, dissolving and carrying away water-soluble chlorine. However, the efficiency of water washing dechlorination is controlled by the mass transfer process, with the diffusion rate of chlorine from the pores of coal particles to the aqueous phase being the decisive factor. This diffusion rate, in turn, depends on the chlorine concentration difference at the coal-water interface, the diffusion coefficient of chlorine in water, and the washing time. Existing single-stage water washing processes and ambient temperature washing processes have failed to systematically optimize for this mass transfer characteristic of low-rank coal, resulting in low dechlorination rates or excessive water and reagent consumption. This is precisely the root cause of the technical problem that this invention aims to solve.

[0004] Currently, various technical approaches for chlorine removal from coal have been reported and documented in patent literature both domestically and internationally. Based on the different dechlorination stages, these can be broadly categorized into post-combustion flue gas dechlorination and pre-combustion coal dechlorination. Post-combustion flue gas dechlorination requires the addition of a dechlorination system in the power plant's tail flue. For example, Chinese patent CN111408255A discloses a coal-fired flue gas dechlorination system and process, which achieves dechlorination by injecting an alkaline dechlorinating agent into the flue gas. However, this method cannot fundamentally solve the corrosion problem of HCl on high-temperature heating surfaces and increases the investment and operating costs of the flue gas purification system. Pre-combustion coal dechlorination mainly includes thermal dechlorination, chemical leaching dechlorination, and physical washing dechlorination.

[0005] Regarding thermal dechlorination, Chinese patent application CN117448021A discloses a low-temperature pyrolysis upgrading method and system for high-chloride coal in Xinjiang. This method involves crushing the high-chloride coal, mixing it with alkali metal oxide additives, and heating it to 400-600℃ in a rotary pyrolysis furnace for low-temperature pyrolysis. This converts the chlorine in the coal into water-soluble chlorides, which are then removed by water washing, achieving a dechlorination rate of over 98%. However, this method suffers from high heating temperatures and energy consumption. The pyrolysis process also results in the loss of some volatile matter and calorific value from the coal, and the use of additives increases operating costs. Furthermore, the pyrolysis system requires significant investment and is complex, making it unsuitable for large-scale, low-cost fuel pretreatment in coal-fired power plants. Chinese patent application CN118909657A discloses a device and method for dechlorinating and upgrading medium-to-high-chloride coal pyrolysis oil and gas, which also suffers from high energy consumption and system complexity due to high-temperature pyrolysis. Chinese patent application CN120865978A discloses an efficient dechlorination method for high-chlorine coal in Xinjiang, which uses microwave radiation pretreatment followed by water washing for dechlorination. Although microwaves can promote the precipitation of chlorine in coal, the industrial scale-up of microwave equipment is difficult, the processing capacity is limited, and the control precision of pretreatment time and temperature is required.

[0006] In the field of chemical leaching dechlorination, Chinese patent CN112625771A discloses a water washing dechlorination method for chlorinated coal. A weak acid (such as carbonic acid or acetic acid) and an eluent (such as ethanol) are dissolved in water to prepare an eluent. The eluent is mixed with coal powder at a mass ratio of 10-100:1 and stirred at 20-40°C for 3-12 hours. The process utilizes the H₂ provided by the weak acid... +Lowering the pH of the eluent promotes the removal of chlorine-containing compounds, while simultaneously enhancing the affinity between coal and the eluent with organic solvents. Although this method achieves good dechlorination results, it has significant drawbacks: First, it requires the use of chemical agents such as weak acids and organic solvents, resulting in high agent costs. Furthermore, the presence of acids and organic solvents in the eluent makes wastewater treatment difficult and puts significant environmental pressure on the environment. Second, the mass ratio of eluent to coal powder is as high as 10-100:1, leading to extremely high water consumption and a massive volume of washing wastewater. Third, the single-stage static stirring and washing time is as long as 3-12 hours, resulting in low treatment efficiency and making it difficult to meet the needs of continuous industrial production. Fourth, this method does not consider the differences in coal types; for low-rank, high-chlorine coals with well-developed pore structures and strong hydrophilicity, the introduction of organic solvents may actually exacerbate the coal sliming problem.

[0007] In the field of physical washing and dechlorination, Chinese patent application CN118454887A discloses a deep deashing and dechlorination process and system for high-chlorine coal. This involves pre-screening and crushing the high-chlorine coal, then using a five-stage jig, a two-stage jig, a flotation machine, and a liquid-solid fluidized bed to separate and dechlorinate the coal of different particle sizes. Chinese patent application CN119186794A discloses a dechlorination jig equipment and process for coal preparation, achieving dechlorination through jigging washing. The aforementioned gravity separation and flotation methods mainly utilize the density difference and surface property differences between coal and gangue for separation. For low-rank high-chlorine coal, where water-soluble inorganic chlorine is mainly present on the particle surface and in fissures, although the washing process can remove some chlorine-containing coal slurry, it is difficult to deeply remove the chlorine present inside the coal pores. The dechlorination rate is generally low, and the jigging and flotation systems have long process flows and large equipment footprints. In addition, Chinese patent CN104762120A discloses a method and equipment for desodiuming high-sodium coal. The method uses a three-stage countercurrent washing process to desodium Xinjiang high-sodium coal. The raw coal is crushed to 1-4mm and then fed into a three-stage coal washer. The coal is washed countercurrently at room temperature and pressure with an aqueous solution from the next stage coal washer, and the liquid-solid ratio is controlled at 1.25:1. Although this patent discloses a general approach to multi-stage countercurrent washing, it targets the removal of soluble sodium salts from high-sodium coal. The washing is carried out at room temperature. It does not disclose the warm water-enhanced washing and temperature gradient control methods necessary for the chlorine content characteristics of low-rank high-chlorine coal (chlorine exists not only in the form of water-soluble chlorides in the pores, but also in some organically bound forms, and low-rank coal has well-developed pores and strong water retention). At the same time, its liquid-to-solid ratio (1.25:1) is low and the washing time varies greatly at each stage. If this method is directly applied to the dechlorination of low-rank high-chlorine coal, on the one hand, room temperature washing is difficult to effectively promote the dissolution and diffusion of chlorine in the pores, resulting in a low dechlorination rate. On the other hand, low-rank coal is prone to mudding and swelling when exposed to water. Under low liquid-to-solid ratio conditions, the coal and washing water are not mixed evenly, the mass transfer effect is poor, and solid-liquid separation is difficult after coal mudding, making it impossible to achieve deep dechlorination.

[0008] In addition, some technical solutions for enhancing dechlorination by utilizing external energy fields or combined processes have emerged in recent years. For example, Chinese patent application CN120574617A discloses a method and system for efficient dechlorination and upgrading of high-chlorine coal using multi-field coupling. This method involves ultrasonically heating a crushed high-chlorine coal sample, followed by reaction with a complexing agent to generate a metal chloride complex to remove chlorine. While this method has high dechlorination efficiency, it requires the use of chemical agents such as complexing agents, and the ultrasonic heating and complexation reaction processes are costly and complex. Furthermore, the waste liquid contains complexing agents, making environmental treatment difficult. Chinese patent application CN121450371A discloses a dechlorination method for chlorine-containing coal. This method involves mixing chlorine-containing coal powder with water and drying it to obtain chlorine-containing coal powder A. Then, chlorine-containing coal powder A is mixed with a washing liquid, subjected to solid-liquid separation, and dried to obtain dechlorinated coal. This method involves multiple mixing and drying steps, making the process lengthy. The selection and preparation of the washing liquid further complicates the process, and it does not involve the concept of multi-stage countercurrent washing and temperature gradient-enhanced dechlorination. Chinese patent application CN120944603A discloses a system and method for the comprehensive utilization of high-chlorine coal through dechlorination, pyrolysis and cascade resource utilization. It couples the dechlorination and pyrolysis processes of high-chlorine coal, resulting in a high degree of system integration, but also a large overall investment and complex operation.

[0009] In summary, existing coal dechlorination methods generally suffer from drawbacks such as high energy consumption, high cost, high water consumption, the need for chemical additives, low dechlorination rates, or poor adaptability to specific coal types. In particular, there is a lack of a multi-stage countercurrent washing dechlorination method for low-rank, high-chlorine coals (lignite, long-flame coal) that requires no chemical additives, consumes little water, achieves high dechlorination rates, and can be operated continuously in a chemical industry. Therefore, developing a new dechlorination method for low-rank, high-chlorine coal that balances dechlorination efficiency, economy, and environmental friendliness has significant practical implications and broad application prospects. Summary of the Invention

[0010] To address the shortcomings of existing technologies, the present invention aims to provide a multi-stage countercurrent washing and dechlorination method for low-rank, high-chlorine coal. The technical problem this invention addresses is the aforementioned deficiencies in existing dechlorination methods for low-rank, high-chlorine coal. This method provides a multi-stage countercurrent washing and dechlorination method that eliminates the need for any chemical reagents. By combining multi-stage countercurrent washing with enhanced temperature gradient in warm water, the chlorine concentration gradient in the washing liquid formed by countercurrent washing drives the mass transfer and diffusion of chlorine from the coal pores into the washing water. Simultaneously, warm water increases the solubility and diffusion rate of chlorine in water, achieving highly efficient dechlorination of low-rank, high-chlorine coal, significantly reducing washing water consumption, and recovering chloride salt byproducts.

[0011] The inventive concept of this invention lies in the fact that low-rank coal (lignite, long-flame coal) has a low degree of coalification, well-developed pore structure, large specific surface area, and strong hydrophilicity. Its chlorine element mainly exists in the form of water-soluble inorganic chlorine (sodium chloride, potassium chloride, calcium chloride, etc.) in the pore water, fissures, and surface adsorption layer of coal particles, with some chlorine weakly bound to the organic matter in the coal. For this type of coal, water washing dechlorination is essentially a mass transfer process of chlorine from the coal phase to the aqueous phase. The mass transfer rate depends on the concentration difference of chlorine in the two phases and the diffusion coefficient of chlorine in water.

[0012] This invention enhances the mass transfer process through two synergistic methods: First, it employs multi-stage countercurrent washing, causing the washing water and coal to move in opposite directions. The chlorine concentration of each stage of the washing solution forms a gradient from high to low along the water flow direction, while the coal contacts the washing solution with decreasing chlorine concentration at each stage in its forward direction. This maintains a positive concentration difference driving force at the coal-water interface throughout the entire washing path, avoiding mass transfer stagnation caused by the concentration difference approaching zero in the later stages of single-stage washing. Second, it uses warm water to enhance washing, making the washing temperature increase gradually along the coal flow direction. The washing temperature of the final stage (Nth stage) washing unit is controlled at 40~90℃. The increased temperature increases the solubility of chloride in water and significantly improves the diffusion coefficient of chloride ions in water (the diffusion coefficient increases by about 10%~20% for every 10℃ increase in temperature). At the same time, the low viscosity of warm water helps it penetrate into the micropores of coal particles, promoting the dissolution of chlorine within the pores. Furthermore, the gradient setting with increasing temperature along the coal flow direction allows the coal to come into contact with the washing water with the highest temperature and lowest chlorine concentration in the later stage of dechlorination (final washing unit). This enables the residual chlorine to be removed deeply with the maximum mass transfer driving force, achieving precise control of the chlorine content of the product coal. This is the essential difference between this invention and existing ambient temperature countercurrent washing (such as CN104762120A) and single-stage chemical water washing (such as CN112625771A).

[0013] The above-mentioned objective of this invention is achieved through the following technical solutions: A multi-stage countercurrent washing and dechlorination method for low-rank, high-chlorine coal includes the following steps: Step S1, Crushing and Screening: The low-rank high-chlorine coal with a chlorine content of not less than 0.30 wt% is crushed and screened into coal material with a particle size of not more than 13 mm; the low-rank high-chlorine coal is a low-rank coal with a coalification degree of lignite or long-flame coal, and its maximum vitrinite reflectance Rmax is not more than 0.60%; Step S2, multi-stage countercurrent washing: the coal material is sequentially fed into N stages of washing units arranged in series for countercurrent washing, where N is an integer of 3 to 6. The coal flow direction is the 1st-stage washing unit → the 2nd-stage washing unit → ... → the Nth-stage washing unit, and the flow direction of washing water is opposite to the coal flow direction. Wherein, the washing liquid discharged from the i-th stage washing unit is conveyed to the (i-1)-th stage washing unit to be used as washing water, i=2, 3, ..., N; fresh washing water is introduced into the Nth-stage washing unit, and the chlorine-containing concentrated washing liquid is discharged from the 1st-stage washing unit; the washing temperature of each stage of washing unit is 15 to 90°C, and along the coal flow direction, the washing temperature of the 1st-stage to the Nth-stage washing units presents an increasing gradient, wherein the washing temperature of the Nth-stage washing unit is 40 to 90°C; the liquid-solid ratio of each stage of washing unit is 2:1 to 8:1, and the single-stage washing time is 5 to 60 min; Step S3, solid-liquid separation: dewatering the dechlorinated coal material discharged from the Nth-stage washing unit to obtain a dechlorinated coal product; Step S4, washing liquid desalination and reuse: the chlorine-containing concentrated washing liquid discharged from the 1st-stage washing unit is subjected to desalination treatment to recover chloride salts, and the water after desalination treatment is reused as washing water in the Nth-stage washing unit.

[0014] Further, in step S1, the particle size of the coal material is 1 to 6 mm; in step S2, the washing temperature of each stage of washing unit is 50 to 80°C, and the washing temperature difference between two adjacent stages of washing units is 5 to 15°C.

[0015] Further, in step S2, N is 4 or 5; along the coal flow direction, the 1st-stage washing unit and the 2nd-stage washing unit constitute a normal-temperature pre-washing section with a washing temperature of 15 to 35°C, and the 3rd-stage to the Nth-stage washing units constitute a heating washing section with a washing temperature of 60 to 90°C; the sum of the single-stage washing time of all stages of washing units is 30 to 240 min.

[0016] Further, in step S2, the liquid-solid ratio of each stage of washing unit is 3:1 to 5:1, and along the coal flow direction, the liquid-solid ratio of each stage of washing unit decreases step by step; the single-stage washing time is 10 to 40 min.

[0017] Further, in step S1, the crushed coal material is sieved and classified into a coarse-grained coal material and a fine-grained coal material, the particle size of the coarse-grained coal material is 3 to 13 mm, and the particle size of the fine-grained coal material is not more than 3 mm; the coarse-grained coal material and the fine-grained coal material are respectively fed into two independent countercurrent washing systems for washing, wherein the coarse-grained coal material is subjected to N-stage countercurrent washing, the fine-grained coal material is subjected to M-stage countercurrent washing, M is an integer of 2 to 4 and M < N; the temperature of each stage of washing unit during washing the fine-grained coal material is 5 to 15°C higher than the temperature of the corresponding washing unit for the coarse-grained coal material.

[0018] Furthermore, in step S2, each washing unit is equipped with a stirring device and an ultrasonic generator. During the washing process, the mixture of coal and washing water is stirred and ultrasonically treated. The stirring speed is 50~200 r / min, the ultrasonic frequency is 20~80 kHz, and the ultrasonic power density is 0.1~1.0 W / mL.

[0019] Further, in step S4, the desalination treatment is an evaporation crystallization treatment, which adopts multi-effect evaporation or mechanical steam recompression evaporation, with an evaporation temperature of 70~100℃ and an evaporation pressure of 20~80kPa absolute pressure. The chloride salt recovered by evaporation crystallization includes at least one of sodium chloride, potassium chloride and calcium chloride, and the evaporation condensate is reused as washing water in the Nth stage washing unit.

[0020] Furthermore, in step S2, the fresh washing water is the power plant condensate, recycled water after softening of circulating wastewater, or reverse osmosis concentrate; in step S3, the chlorine content of the dechlorinated coal product is not greater than 0.15 wt%, and the dechlorination rate is not less than 70%.

[0021] Furthermore, in step S3, the dewatering is a two-stage dewatering process. First, the dechlorinated coal discharged from the Nth stage washing unit is dewatered through a vibrating dewatering screen, and then dewatered through a centrifugal dewatering machine. The dewatered water produced by the two stages of dewatering is collected and reused in the Nth stage washing unit.

[0022] Further, in step S2, the coal material is conveyed between each washing unit using a belt conveyor or scraper conveyor, and the washing liquid is conveyed between each washing unit using a pump. The washing liquid discharged from each washing unit is removed by a hydrocyclone separator to remove fine coal slime before being conveyed to the next washing unit. In step S4, the desalination treatment is a combination of nanofiltration membrane separation and evaporation crystallization. The chlorine-containing concentrated washing liquid is first separated and concentrated by nanofiltration membrane to obtain chloride concentrate and permeate. The chloride concentrate is evaporated and crystallized to recover chloride salts. The permeate and evaporation condensate are combined and reused in the Nth washing unit.

[0023] In summary, compared with the prior art, the present invention has at least one of the following beneficial technical effects: High dechlorination rate: This invention addresses the characteristics of low-rank, high-chlorine coal, where chlorine is mainly water-soluble inorganic chlorine and exists in the pores and fissures of coal particles. It employs warm water enhanced washing (final stage washing temperature 40~90℃) to significantly improve the solubility and diffusion rate of chlorine in water, promoting the mass transfer and dissolution of chlorine from the pores into the washing water. Simultaneously, multi-stage countercurrent washing creates a gradient distribution of chlorine concentration in each stage of the washing solution along the water flow direction. That is, the coal material comes into contact with washing solutions with increasingly lower chlorine concentrations in its forward direction. In the Nth stage washing unit, the coal material comes into contact with the fresh washing water with the lowest chlorine concentration. Thus, throughout the entire washing process, a positive concentration difference driving force is maintained, where the chlorine concentration on the coal surface is higher than that in the washing water. This avoids backmixing and mass transfer stagnation caused by the increase in chlorine concentration in the washing solution in single-stage washing, achieving a dechlorination rate of not less than 70%, and under preferred conditions, it can reach 75%~85%.

[0024] Extremely low water consumption and zero wastewater discharge: This invention adopts a counter-current washing method, in which the washing water is reused in stages along the opposite direction of the coal flow. The fresh washing water entering at the Nth stage is recycled in stages after counter-current, and the chlorine it carries is concentrated in the first stage washing unit to form a high-concentration chlorine-containing washing liquid. The water volume is only 1 / 3 to 1 / 5 of that of a single-stage washing. After desalination treatment (evaporation crystallization or membrane separation + evaporation crystallization), the chlorine-containing washing liquid recovers chloride salts. All the desalinated water is reused in the washing system, realizing closed-loop circulation of washing water and zero wastewater discharge. It is particularly suitable for water-scarce regions such as Xinjiang and Inner Mongolia.

[0025] No chemical reagents required, environmentally friendly: This invention uses only water as the washing medium throughout the entire process, without adding any chemical reagents such as acids, alkalis, organic solvents or complexing agents. There is no secondary pollution during the washing process, and the waste liquid is clean after desalination treatment and can be safely reused. The recovered chloride salts (sodium chloride, potassium chloride, calcium chloride, etc.) can be sold as industrial by-products, realizing the resource utilization of chlorine, which meets the requirements of circular economy and green development.

[0026] The process is simple, energy-efficient, and easy to industrialize: The process flow of this invention is short, mainly consisting of four stages: crushing and screening, multi-stage countercurrent washing, dewatering, and washing liquid desalination and reuse. Each washing unit can use conventional stirred tanks or mechanically stirred tower equipment, resulting in low equipment investment. The washing temperature is relatively low (final stage washing temperature 40~90℃, the preceding stage can be at room temperature), which is far lower than the energy consumption level of thermal dechlorination (400~600℃), and does not lose volatile matter and calorific value in coal, resulting in high-quality coal products. The method can be operated continuously, with short single-stage washing time (5~60min), and the total washing time is significantly reduced compared to single-stage static water washing (3~12h), resulting in high processing efficiency and easy industrial application in coal-fired power plants and coal chemical enterprises.

[0027] Highly targeted: This invention is designed to address the special properties of low-rank, high-chlorine coal (lignite, long-flame coal, with a maximum vitrinite reflectance Rmax ≤ 0.60%) (well-developed pores, strong hydrophilicity, and chlorine mainly in water-soluble form). Through the coordinated control of temperature gradient, liquid-solid ratio, number of stages, and time, it ensures efficient chlorine dissolution while avoiding the problems of easy mud formation and difficult dewatering of low-rank coal when exposed to water through reasonable stage and dewatering process configuration.

[0028] Furthermore, there is a synergistic relationship among the various process parameters of this invention, and the parameter ranges mentioned above are not arbitrarily selected: When the particle size is too large (greater than 13 mm), the diffusion path of chlorine from the interior of the particle to the surface is long, making it difficult for chlorine in the pores to dissolve in a short time, resulting in a significant decrease in dechlorination rate; when the particle size is too small (less than 1 mm), the specific surface area of ​​the coal increases sharply, the tendency to become muddy intensifies, solid-liquid separation becomes difficult, and the coal mud carries away a large amount of washing water, increasing water consumption. When the washing temperature is too low (the final washing temperature is below 40°C), the dissolution and diffusion rate of chlorine is low, making it difficult to achieve deep dechlorination; when the final washing temperature is above 90°C, steam energy consumption increases significantly and approaches the atmospheric boiling point, resulting in poor operational safety and reduced economic efficiency. When the liquid-to-solid ratio is below 2:1, the coal cannot be fully wetted and dispersed, resulting in a small effective mass transfer area; when it is above 8:1, washing water consumption and treatment load increase significantly, weakening the advantage of countercurrent water saving. When the single-stage washing time is less than 5 minutes, chlorine dissolution is insufficient; when it is above 60 minutes, treatment efficiency decreases and equipment investment increases. When the number of stages N is less than 3, the mass transfer advantage of the countercurrent concentration gradient is not significant; when there are more than 6 stages, the increase in equipment investment and process complexity no longer leads to a significant improvement in dechlorination rate. Therefore, this invention controls parameters such as particle size, temperature gradient, liquid-solid ratio, single-stage washing time, and number of stages within the above ranges. This is an optimized selection based on a comprehensive trade-off between the chlorine occurrence characteristics of low-rank high-chlorine coal, mass transfer kinetics, and industrial economics. The various parameters work together to ensure the high efficiency and economy of this invention. Attached Figure Description

[0029] Figure 1 This is a process flow diagram of the multi-stage countercurrent washing and dechlorination method for low-rank, high-chlorine coal according to the present invention.

[0030] Figure 2 This is a schematic diagram of the multi-stage countercurrent washing system of the present invention.

[0031] Figure 3 This is a schematic diagram of the structure of the washing unit of the present invention.

[0032] Figure 4 This is a schematic diagram of the principle of the washing liquid desalination and reuse system of the present invention. Detailed Implementation

[0033] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0034] The method of this invention can be implemented in the following general manner: First, the low-rank, high-chlorine raw coal is crushed and screened to control the coal particle size to be no greater than 13 mm. If necessary, it is screened and graded into coarse-grained coal (3~13 mm) and fine-grained coal (no greater than 3 mm). The coal is fed into a multi-stage countercurrent washing system consisting of N washing units (N is an integer from 3 to 6) connected in series. The coal is conveyed from the first-stage washing unit to the Nth-stage washing unit by a belt conveyor or scraper conveyor. The washing liquid discharged from each washing unit is pumped to the next washing unit. Fresh washing water is introduced into the Nth-stage washing unit, and the first-stage washing unit discharges concentrated chlorine washing liquid. During the washing process, the washing temperature is controlled by the heating jackets of each washing unit. The washing temperature is increased from 15 to 90°C along the coal flow direction, with the final (Nth) washing unit's temperature controlled at 40 to 90°C. The liquid-to-solid ratio is controlled by the feed rate to be 2:1 to 8:1, and the single-stage washing time is controlled by the residence time of the coal in each washing unit to be 5 to 60 minutes. If necessary, a stirring device and an ultrasonic generator are activated to enhance mass transfer. After washing, the dechlorinated coal discharged from the Nth washing unit is dehydrated and dried by a vibrating dewatering screen and a centrifugal dewatering machine to obtain the dechlorinated coal product. The concentrated chlorine-containing washing liquid discharged from the first washing unit is treated by evaporation crystallization or a combination of nanofiltration membrane separation and evaporation crystallization to recover chloride salts. The desalinated water is reused in the Nth washing unit, forming a closed-loop cycle. The above general implementation method is similar to... Figure 1 Corresponding to the process flow shown, those skilled in the art can select specific operating parameters within the above parameter range based on the characteristics of the coal and the requirements for dechlorination indicators.

[0035] It should be noted that in the multi-stage countercurrent washing and dechlorination method of the present invention, each washing unit can be a stirred tank type washer or a mechanically stirred tower type washer, the structure of which is as follows: Figure 3 As shown; the series arrangement of multi-stage washing units is as follows Figure 2 As shown; the overall process flow is as follows Figure 1 As shown; the washing solution desalination and reuse system is as follows Figure 4 As shown. Those skilled in the art will understand that equivalent substitutions or modifications can be made to the above-described device structure without departing from the concept of the present invention.

[0036] Example 1 This embodiment provides a multi-stage countercurrent washing and dechlorination method for low-rank, high-chlorine coal. The coal sample used is long-flame coal produced in the Zhundong mining area of ​​Xinjiang, with an air-dried basis chlorine content of 0.62 wt%, a vitrinite maximum reflectance (Rmax) of 0.52%, a total moisture content of 21.5%, an ash content of 8.6%, and a volatile matter content of 31.2%. After being crushed by a crusher and screened by a vibrating screen, coal with a particle size of no more than 6 mm is obtained.

[0037] like Figure 1 , Figure 2 and Figure 3 As shown, the coal is continuously fed into a multi-stage counter-current washing system consisting of five washing units (from the first to the fifth stage) arranged in series. The coal enters from the first washing unit via a belt conveyor, passes through the second, third, and fourth washing units in sequence, and is finally discharged from the fifth washing unit. Each washing unit is a vertical stirred washing tank with a heating jacket and a stirring device. The tank is equipped with a heating jacket on the outside, which is heated by steam. The tank is equipped with stirring blades driven by a stirring motor, and the stirring speed is 120 r / min.

[0038] Washing water flows in the opposite direction to the coal flow: Fresh washing water (power plant condensate) at a temperature of 80℃ is introduced into the 5th stage washing unit. The washing liquid discharged from the 5th stage washing unit is pumped to the 4th stage washing unit after fine coal slime is removed by a hydrocyclone separator. The washing liquid discharged from the 4th stage washing unit is pumped to the 3rd stage washing unit. The washing liquid discharged from the 3rd stage washing unit is pumped to the 2nd stage washing unit. The washing liquid discharged from the 2nd stage washing unit is pumped to the 1st stage washing unit. The chlorine-containing concentrated washing liquid discharged from the 1st stage washing unit enters the washing liquid desalination and reuse section.

[0039] The operating parameters for each washing unit are as follows: The washing temperature for the first washing unit is 40℃, the liquid-to-solid ratio is 5:1, and the washing time for a single stage is 20 min; the washing temperature for the second washing unit is 50℃, the liquid-to-solid ratio is 4.5:1, and the washing time for a single stage is 20 min; the washing temperature for the third washing unit is 60℃, the liquid-to-solid ratio is 4:1, and the washing time for a single stage is 20 min; the washing temperature for the fourth washing unit is 70℃, the liquid-to-solid ratio is 3.5:1, and the washing time for a single stage is 20 min; the washing temperature for the fifth washing unit is 80℃, the liquid-to-solid ratio is 3:1, and the washing time for a single stage is 20 min. The washing temperature increases gradually along the coal flow direction, with a temperature difference of 10℃ between adjacent stages; the liquid-to-solid ratio decreases progressively along the coal flow direction.

[0040] After washing, the dechlorinated coal discharged from the fifth-stage washing unit is first dewatered by a vibrating dewatering screen, and then dewatered by a centrifugal dewatering machine. After dewatering, the coal is dried to obtain dechlorinated coal products. The dewatering water generated from the two stages of dewatering is collected and reused in the fifth-stage washing unit.

[0041] The concentrated chlorine-containing washing liquid discharged from the first-stage washing unit is pumped to the concentrated washing liquid storage tank, and then sent to the mechanical vapor recompression evaporation (MVR) crystallization device for evaporation and crystallization treatment. The evaporation temperature is 85℃ and the evaporation pressure is 50kPa absolute pressure. The evaporation crystallization yields chloride salt (a mixed salt with sodium chloride as the main component). The evaporation condensate is reused in the fifth-stage washing unit as washing water.

[0042] The chlorine content of the dechlorinated coal product was determined using high-temperature hydrolysis-ion chromatography, referring to the national standard GB / T3558-2014 "Determination of Chlorine in Coal", and the dechlorination rate was calculated. The results showed that the chlorine content of the dechlorinated coal product was 0.11 wt%, and the dechlorination rate reached 82.3%. The total washing time for the coal through five stages of countercurrent washing was 100 min, and the fresh water consumption was 0.9 t / t (tons of fresh washing water consumed per ton of dry coal), which is more than 70% lower than that of conventional single-stage washing (3-5 t / t of fresh water). No chemical agents were added during the washing process, and approximately 11.8 kg / t (tons of dry coal) of chloride salt was recovered through evaporation and crystallization, achieving resource recovery of chlorine.

[0043] Example 2 This embodiment provides another method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal. The coal sample used is lignite produced in a mining area in Inner Mongolia, with an air-dried basis chlorine content of 0.85 wt%, a vitrinite maximum reflectance (Rmax) of 0.38%, a total moisture content of 28.4%, and an ash content of 10.2%. After crushing and screening, the coal sample yields coal with a particle size not exceeding 13 mm.

[0044] In this embodiment, the multi-stage countercurrent washing system consists of four washing units connected in series (N=4). For example... Figure 1 and Figure 2 As shown, the coal enters from the first-stage washing unit and is discharged after passing through the second, third, and fourth-stage washing units in sequence; the washing water flows in the opposite direction, with fresh washing water introduced into the fourth-stage washing unit, and the washing liquid discharged from the i-th-stage washing unit being transported to the (i-1)-th-stage washing unit (i=2,3,4), while the first-stage washing unit discharges concentrated chlorine-containing washing liquid.

[0045] The operating parameters for each washing unit are as follows: The first and second washing units constitute a pre-wash section at room temperature, with a washing temperature of 30℃ (within the range of 15~35℃), liquid-to-solid ratios of 5:1 and 4.5:1 respectively, and a single-stage washing time of 30 min for each. The third and fourth washing units constitute a heated washing section, with washing temperatures of 70℃ and 85℃ respectively, liquid-to-solid ratios of 4:1 and 3.5:1 respectively, and a single-stage washing time of 20 min for each. The stirring speed within each washing unit is 100 r / min. The sum of the single-stage washing times for each washing unit is 100 min, falling within the range of 30~240 min.

[0046] The washed coal material is subjected to two-stage dewatering via a vibrating dewatering screen and a centrifugal dewatering machine, followed by drying to obtain a dechlorinated coal product; the chlorine-containing concentrated washing liquid discharged from the first-stage washing unit is desalted through a multi-effect evaporation crystallization device (triple-effect evaporation) at an evaporation temperature of 95°C and an absolute evaporation pressure of 40kPa, to recover mixed chloride salts mainly composed of calcium chloride and sodium chloride, and the evaporated condensate water is reused in the fourth-stage washing unit.

[0047] The test results show that the chlorine content of the dechlorinated coal product obtained in this example is 0.18wt%, and the dechlorination rate reaches 78.6%. Although the original chlorine content of this coal sample is relatively high (0.85wt%), the dechlorination rate still reaches over 78%, and the chlorine content of the product coal drops below 0.2wt%, which meets the general control requirement for the chlorine content of coal fed into furnaces in coal-fired power plants (≤0.2wt%); the consumption of fresh washing water is 1.1t / t, which is significantly lower than that of single-stage water washing; the normal-temperature pre-washing section can remove the soluble chlorine on the surface of coal particles and the chlorine in coal slime in advance, and the heated washing section further removes the chlorine inside the pores. The two-stage temperature configuration balances the dechlorination effect and steam energy consumption.

[0048] Example 3 This example provides a dechlorination method that performs multi-stage countercurrent washing respectively after classifying coal materials by particle size, and the coal sample used is the same as that in Example 1 (Xinjiang Zhundong long-flame coal, with a chlorine content of 0.62wt%).

[0049] In this example, the crushed coal material is sieved and classified into coarse particle size coal material (particle size 3~13mm) and fine particle size coal material (particle size no more than 3mm), and the two particle sizes of coal material are respectively sent to two independent countercurrent washing systems for washing. Among them, the coarse particle size coal material is sent to a countercurrent washing system (N=5) with 5 stages of washing units connected in series, the temperature of each stage of washing unit is 45°C, 55°C, 65°C, 75°C, 85°C in sequence, and the liquid-solid ratio is 5:1, 4.5:1, 4:1, 3.5:1, 3:1 in sequence, with a single-stage washing time of 25min; the fine particle size coal material is sent to a countercurrent washing system (M=3, M<N) with 3 stages of washing units connected in series, the temperature of each stage of washing unit is 60°C, 70°C, 90°C in sequence (5~15°C higher than the temperature of the corresponding washing unit for the coarse particle size coal material), the liquid-solid ratio is 6:1, 5:1, 4:1 in sequence, with a single-stage washing time of 15min. Due to the large specific surface area and fully exposed pores of the fine particle size coal material, deep dechlorination can be achieved under the conditions of higher temperature, shorter time and higher liquid-solid ratio.

[0050] The dechlorinated coal materials discharged from the two washing systems are respectively subjected to two-stage dewatering and drying, and then combined into a dechlorinated coal product; the chlorine-containing concentrated washing liquids discharged from the first-stage washing units of the two systems are combined and then sent to the washing liquid desalination and reuse section for treatment.

[0051] The test results showed that after five stages of countercurrent washing, the chlorine content of coarse-grained coal decreased from 0.61 wt% to 0.11 wt%, with a dechlorination rate of 81.5%; after three stages of countercurrent washing, the chlorine content of fine-grained coal decreased from 0.63 wt% to 0.084 wt%, with a dechlorination rate of 86.7%; the combined dechlorinated coal product had a chlorine content of 0.10 wt%, with an average dechlorination rate of approximately 83.5%. Compared with the ungraded Example 1 (dechlorination rate of 82.3%), the dechlorination rate was further improved by using matched washing parameters for coarse and fine-grained coal after particle size classification. Furthermore, the fine-grained coal only required three stages of washing to achieve a high dechlorination rate, saving on equipment investment and steam energy consumption.

[0052] Example 4 This embodiment adds ultrasonic-assisted enhancement treatment to the existing embodiment 1 to examine the effect of ultrasound on improving dechlorination efficiency. The coal sample used is the same as in embodiment 1, and the parameters such as the number of stages, temperature gradient, liquid-to-solid ratio, and single-stage washing time of the multi-stage countercurrent washing system are also the same as in embodiment 1.

[0053] In this embodiment, an ultrasonic generator (such as an ultrasonic generator) is installed at the bottom of the tank of each washing unit. Figure 3 As shown), during the washing process, the mixture of coal and washing water is stirred (stirring speed 120 r / min) while ultrasonic treatment is applied. The ultrasonic frequency is 40 kHz and the ultrasonic power density is 0.5 W / mL. The single-stage washing time is shortened to 15 min, and the sum of the single-stage washing times of each washing unit is 75 min.

[0054] The test results show that the chlorine content of the dechlorinated coal product obtained in this example is 0.092 wt%, and the dechlorination rate reaches 85.1%, which is about 2.8 percentage points higher than that in Example 1 (without ultrasound, the dechlorination rate is 82.3%). Simultaneously, due to the cavitation effect of ultrasound, the desorption and mass transfer of chlorine within the micropores of coal particles can be enhanced. Under the premise of achieving a similar dechlorination rate, the total washing time is shortened from 100 min to 75 min, and the processing efficiency is increased by 25%. It is evident that ultrasonic-assisted treatment and multi-stage countercurrent warm water washing have a synergistic effect.

[0055] Example 5 This embodiment focuses on examining the operational performance of the washing liquid desalination and reuse section. The coal sample used is the same as in Example 1, and the multi-stage countercurrent washing parameters are the same as in Example 1. The difference lies in the fact that the chlorine-containing concentrated washing liquid discharged from the first-stage washing unit is desalinated and reused using a combined treatment method of "nanofiltration membrane separation + evaporation crystallization" (e.g., Figure 4 (As shown).

[0056] Specifically, after being pumped to the concentrated washing liquid storage tank, the chlorine-containing concentrated washing liquid is first sent to a nanofiltration membrane separation device for concentration and separation: under the conditions of operating pressure of 2.0 MPa and temperature of 35°C, the nanofiltration membrane has a chloride ion rejection rate of about 85%. After nanofiltration concentration, the chlorine-containing concentrated washing liquid is divided into chloride concentrate (concentration factor of about 3 times) and permeate. The chloride concentrate enters the MVR evaporation and crystallization device, where it is evaporated and crystallized under the conditions of evaporation temperature of 85°C and evaporation pressure of 50 kPa absolute pressure to recover chloride salts, mainly sodium chloride. The permeate and evaporation condensate are combined and then enter the recycled water tank, and are pumped back to the 5th stage washing unit as washing water.

[0057] Operational results show that when using the combined treatment method of "nanofiltration membrane separation + evaporation crystallization", nanofiltration pre-concentration can significantly reduce the amount of washing liquid entering the evaporation crystallization device, and the steam consumption of the evaporation crystallization device is reduced by about 45% compared with direct evaporation (Example 1). The entire desalination and reuse section achieves a water recovery rate of over 98%. During continuous operation of the washing system, there is no need to add a large amount of fresh water, only to replenish the water lost through evaporation and carried away by the product coal. The purity of the recovered chloride salt is 85%~92%, which can be used as a raw material for industrial salt. This embodiment shows that the washing liquid desalination and reuse section of the present invention can achieve efficient closed-loop circulation of washing water and resource recovery of chlorine, with good economic and environmental benefits.

[0058] Example 6 This embodiment investigates the effect of the number of countercurrent washing stages N on the dechlorination effect. The coal sample used is the same as in Example 1 (Xinjiang Zhundong long-flame coal, chlorine content 0.62wt%, particle size no greater than 6mm). Three, four, five, and six-stage countercurrent washing systems were used respectively. The washing temperature for each stage was uniformly distributed along the coal flow direction in an increasing gradient within the range of 40~80℃ (for example, for stage 5, the temperatures were 40℃, 50℃, 60℃, 70℃, and 80℃ respectively; for stage 4, they were 40℃, 53℃, 67℃, and 80℃ respectively; for stage 3, they were 40℃, 60℃, and 80℃ respectively; and for stage 6, they were 40℃, 48℃, 56℃, 64℃, 72℃, and 80℃ respectively). The liquid-to-solid ratio for each stage was 4:1, and the single-stage washing time for each stage was 20 min. All other operating conditions were the same as in Example 1.

[0059] The test results show that: with 3-stage countercurrent washing, the dechlorination rate is 70.2%, and the product coal chlorine content is 0.18 wt%; with 4-stage countercurrent washing, the dechlorination rate is 76.8%, and the product coal chlorine content is 0.14 wt%; with 5-stage countercurrent washing, the dechlorination rate is 82.3%, and the product coal chlorine content is 0.11 wt%; with 6-stage countercurrent washing, the dechlorination rate is 83.1%, and the product coal chlorine content is 0.105 wt%. It is evident that as the number of stages N increases from 3 to 5, the dechlorination rate significantly improves (from 70.2% to 82.3%), but when further increasing to 6 stages, the improvement in dechlorination rate narrows significantly (only increasing by 0.8 percentage points), while equipment investment and process complexity increase significantly. Therefore, this invention preferably uses 4-5 stages N to achieve the best balance between dechlorination effect and equipment investment. This selection is based on the diminishing marginal returns of dechlorination rate with increasing stage number, and is a result of comprehensively considering technical effectiveness and economic efficiency.

[0060] Example 7 This embodiment investigates the effect of the washing temperature gradient setting on the dechlorination effect. The coal sample used is the same as in Example 1 (chlorine content 0.62wt%, particle size not greater than 6mm), and both are subjected to 5-stage countercurrent washing with a liquid-to-solid ratio of 4:1 and a single-stage washing time of 20min. The difference lies in the temperature configuration of each washing unit: (a) constant temperature washing: the temperature of all 5 washing units is 60℃; (b) decreasing gradient washing: the temperature of each stage is 80℃, 70℃, 60℃, 50℃, and 40℃ respectively (decreasing along the coal flow direction); (c) increasing gradient washing (the method of this invention): the temperature of each stage is 40℃, 50℃, 60℃, 70℃, and 80℃ respectively (increasing along the coal flow direction).

[0061] The test results show that the dechlorination rate is 73.5% under constant temperature washing, with a product coal chlorine content of 0.16 wt%; the dechlorination rate is 68.9% under decreasing gradient washing, with a product coal chlorine content of 0.19 wt%; and the dechlorination rate is 82.3% under increasing gradient washing (the method of this invention), with a product coal chlorine content of 0.11 wt%. The reason for this is that the increasing gradient washing method ensures that the coal comes into contact with the washing water with the highest temperature (80℃) and lowest chlorine concentration at the end of the dechlorination stage (stage 5). At this point, the small amount of chlorine remaining in the pores of the coal particles can be dissolved to the maximum extent under high temperature and high concentration difference conditions, thus obtaining the lowest product chlorine content. In contrast, under decreasing gradient washing, the coal comes into contact with the washing water with the lowest temperature at the end of the dechlorination stage, resulting in insufficient dechlorination power in the final stage and a higher product chlorine content. While constant temperature washing has no temperature gradient, it fails to form the strongest dechlorination power in the final stage. Therefore, the increasing temperature gradient setting along the coal flow direction is one of the key features of this invention, and its synergistic effect with the countercurrent concentration gradient produces unexpected technical results.

[0062] Comparative Example 1 This comparative example uses a single-stage, room-temperature static water washing method for dechlorination. The coal sample used is the same as in Example 1 (Xinjiang Zhundong long-flame coal, chlorine content 0.62wt%). The coal (particle size no larger than 6mm) is mixed with room-temperature clean water (25℃) at a liquid-to-solid ratio of 5:1 in a stirring tank. The stirring speed is 120 r / min, and after stirring and washing for 120 min, it is filtered, dehydrated, and dried. This comparative example simulates the existing single-stage water washing dechlorination process (without adding chemical agents).

[0063] The test results showed that the chlorine content of the coal sample obtained in Comparative Example 1 was 0.40 wt%, and the dechlorination rate was only 35.2%, far lower than the 82.3% of Example 1. The reason for this is that during single-stage washing, the chlorine concentration in the washing water continuously increases as washing progresses, and the chlorine concentration difference between the coal particle surface and the washing water gradually decreases until it approaches equilibrium. This significantly weakens the mass transfer driving force, preventing a large amount of chlorine stored inside the pores of the coal particles from dissolving. Furthermore, the low dissolution and diffusion rate of chlorine at room temperature further limits the dechlorination effect. Therefore, the synergistic effect of multi-stage countercurrent washing (maintaining the chlorine concentration gradient) and warm water enhancement (increasing the diffusion rate) used in this invention significantly improves the dechlorination rate.

[0064] Comparative Example 2 This comparative example simulates the three-stage countercurrent washing conditions for high-sodium coal disclosed in Chinese Patent CN104762120A and applies them to the dechlorination of low-rank, high-chlorine coal to examine the effectiveness of directly applying existing countercurrent washing technology to the dechlorination of high-chlorine coal. The coal sample used was the same as in Example 1, crushed to 1-4 mm, and fed into a three-stage coal washer (3-stage countercurrent washing). All washing was carried out at room temperature (25℃) and normal pressure, with a liquid-to-solid ratio controlled at 1.25:1. The first-stage washing time was 30 min, and the second and third-stage washing times were both 5 min. The washing water flowed counter-currently; clean water was introduced in the third stage, and the washing liquid was discharged from the first stage. After washing, the coal was dehydrated, dried, and the chlorine content was measured.

[0065] The test results showed that the chlorine content of the coal sample obtained in Comparative Example 2 was 0.35 wt%, and the dechlorination rate was only 42.8%, lower than the 82.3% of Example 1. The reasons for this are as follows: First, the dissolution and diffusion rate of chlorine in water is low during room temperature washing, making it difficult for chlorine to fully dissolve in the pores of low-rank coal; second, the liquid-to-solid ratio was only 1.25:1, resulting in insufficient water to fully wet and penetrate the pores of the coal particles when the coal and washing water were mixed, and the surface of low-rank coal became muddy after contact with water, leading to uneven mixing and poor mass transfer under low liquid-to-solid ratio conditions; third, the total washing time was short (only 40 min), leaving insufficient time for chlorine to dissolve in the pores. This comparative example demonstrates that the existing room temperature countercurrent washing method for high-sodium coal cannot be directly applied to the deep dechlorination of low-rank, high-chlorine coal. This invention overcomes the above technical obstacles and achieves unexpected dechlorination results through a temperature gradient in the water, a higher liquid-to-solid ratio, and a reasonable stage-time configuration.

[0066] Comparative Example 3 This comparative example simulates the jigging washing and dechlorination process route disclosed in Chinese patent application CN119186794A, using the same coal sample as in Example 1. The coal (particle size 0-13mm) is fed into a jig for jigging washing. The jigging cycle frequency is 1.2Hz, the jigging amplitude is 30mm, and the throughput is 8t / (m³). 2 After washing and selection, the clean coal is dehydrated, dried, and its chlorine content is tested.

[0067] The test results showed that the chlorine content of the clean coal obtained in Comparative Example 3 was 0.38 wt%, and the dechlorination rate was only 38.5%, lower than the 82.3% of Example 1. The reason for this is that jigging mainly relies on the density difference between coal and minerals such as gangue for separation, and its ability to remove water-soluble chlorine stored inside the pores of coal particles is limited. During jigging washing, only some chlorine on the particle surface and in the cracks is removed with the coal slurry water; chlorine inside the pores is difficult to dissolve into the wash water in a short time. This comparative example shows that physical washing dechlorination is ineffective in removing porous chlorine from low-rank, high-chlorine coal, and the method of this invention has significant technical advantages.

[0068] Comparative Example 4 This comparative example simulates the single-stage water washing dechlorination process route of weak acid + organic solvent disclosed in Chinese Patent CN112625771A. The coal sample used is the same as in Example 1 (chlorine content 0.62wt%). According to the preferred conditions of the patent: acetic acid is used as the weak acid and ethanol is used as the eluent. An eluent with an acetic acid molar concentration of 0.005mol / L and an ethanol volume fraction of 10% is prepared. Coal powder (particle size not greater than 6mm) is mixed with the eluent at a mass ratio of 1:50. After stirring at 25°C for 6 hours, the mixture is filtered, dehydrated, dried, and the chlorine content is measured.

[0069] The test results show that the chlorine content of the coal sample obtained in Comparative Example 4 was 0.14 wt%, and the dechlorination rate was 77.4%. The dechlorination effect was similar to or even slightly lower than that of Example 1 of this invention (dechlorination rate 82.3%). However, the cost was as follows: chemical agents such as acetic acid and ethanol were used, resulting in high agent costs; the mass ratio of the eluent to coal powder was as high as 50:1, and the fresh water consumption was about 6 t / t, which is about 6.7 times that of Example 1 of this invention (0.9 t / t); the single-stage stirring and washing time was as long as 6 hours, which is about 3.6 times that of this invention (100 min); and the eluent contained acid and organic solvents, requiring specialized wastewater treatment facilities. The above comparison fully demonstrates that this invention achieves a dechlorination effect superior to or equivalent to chemical washing without using any chemical agents, and with significantly reduced water consumption and time, demonstrating significant economic benefits and environmental advantages.

[0070] The dechlorination effects of the examples and comparative examples are summarized in Table 1.

[0071] Table 1 Comparison of dechlorination effects between each embodiment and the comparative example ; As shown in Table 1, the dechlorination rates of all embodiments of the present invention reached over 78%, significantly higher than those of the comparative examples (35%~43%). Specifically, Example 1 (5-stage countercurrent warm water washing) achieved a dechlorination rate 47.1 percentage points higher than Comparative Example 1 (single-stage ambient temperature static water washing), 39.5 percentage points higher than Comparative Example 2 (ambient temperature three-stage countercurrent washing), and 43.8 percentage points higher than Comparative Example 3 (jigging washing). Simultaneously, the fresh water consumption was reduced from 5.0 t / t in single-stage washing to 0.9 t / t, saving over 80% of water. These comparative results demonstrate that the technical solution of the present invention, characterized by "multi-stage countercurrent + warm water temperature gradient + higher liquid-to-solid ratio + synergistic configuration of stages and time," achieved a significant and unexpected dechlorination effect on the chlorine occurrence characteristics of low-rank, high-chlorine coal, while substantially reducing water consumption.

[0072] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal, characterized in that, comprising the following steps: Step S1, crushing and screening: crushing low-metamorphism high-chlorine coal with a chlorine content of not less than 0.30 wt%, and screening it into coal material with a particle size of not more than 13 mm; the low-metamorphism high-chlorine coal is a low-rank coal whose coalification degree belongs to lignite or long-flame coal, and its maximum vitrinite reflectance Rmax is not more than 0.60%; Step S2, multi-stage countercurrent washing: feeding the coal material sequentially into N stages of washing units arranged in series for countercurrent washing, where N is an integer of 3 to 6, the coal flow direction is the 1st-stage washing unit → the 2nd-stage washing unit → ... → the Nth-stage washing unit, the flow direction of washing water is opposite to the coal flow direction, wherein the washing liquid discharged from the i-th-stage washing unit is transported to the (i-1)-th-stage washing unit to be used as washing water, i=2, 3, ..., N; fresh washing water is introduced into the Nth-stage washing unit, and the 1st-stage washing unit discharges chlorine-containing concentrated washing liquid; the washing temperature of each stage of washing unit is 15 to 90°C, and along the coal flow direction, the washing temperature of the 1st-stage to Nth-stage washing units presents an increasing gradient, wherein the washing temperature of the Nth-stage washing unit is 40 to 90°C; the liquid-solid ratio of each stage of washing unit is 2:1 to 8:1, and the single-stage washing time is 5 to 60 min; Step S3, solid-liquid separation: dehydrating the dechlorinated coal material discharged from the Nth-stage washing unit to obtain a dechlorinated coal product; Step S4, washing liquid desalination and reuse: subjecting the chlorine-containing concentrated washing liquid discharged from the 1st-stage washing unit to desalination treatment to recover chloride salts, and the water after desalination treatment is reused as washing water in the Nth-stage washing unit.

2. The method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal according to claim 1, characterized in that, In step S1, the particle size of the coal material is 1 to 6 mm; in step S2, the washing temperature of each stage of washing unit is 50 to 80°C, and the difference in washing temperature between two adjacent stages of washing units is 5 to 15°C.

3. The method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal according to claim 1, characterized in that, In step S2, N is 4 or 5; along the coal flow direction, the 1st-stage washing unit and the 2nd-stage washing unit constitute a normal-temperature pre-washing section with a washing temperature of 15 to 35°C, and the 3rd-stage to Nth-stage washing units constitute a heated washing section with a washing temperature of 60 to 90°C; the sum of the single-stage washing time of all stages of washing units is 30 to 240 min.

4. The method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal according to claim 1, characterized in that, In step S2, the liquid-solid ratio of each stage of washing unit is 3:1 to 5:1, and along the coal flow direction, the liquid-solid ratio of each stage of washing unit decreases stage by stage; the single-stage washing time is 10 to 40 min.

5. The method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal according to claim 1, characterized in that, In step S1, the crushed coal material is screened and classified into coarse-grained coal material and fine-grained coal material, the particle size of the coarse-grained coal material is 3 to 13 mm, and the particle size of the fine-grained coal material is not more than 3 mm; the coarse-grained coal material and the fine-grained coal material are respectively fed into two independent countercurrent washing systems for washing, wherein the coarse-grained coal material adopts N-stage countercurrent washing, the fine-grained coal material adopts M-stage countercurrent washing, M is an integer of 2 to 4 and M<N; the temperature of each stage of washing unit during the washing of the fine-grained coal material is 5 to 15°C higher than the temperature of the corresponding washing unit for the coarse-grained coal material.

6. The method for multi-stage countercurrent washing and dechlorination of low-rank high-chlorine coal according to claim 1, characterized in that, In step S2, each washing unit is equipped with a stirring device and an ultrasonic generator. During the washing process, the mixture of coal and washing water is stirred and ultrasonically treated. The stirring speed is 50~200 r / min, the ultrasonic frequency is 20~80 kHz, and the ultrasonic power density is 0.1~1.0 W / mL.

7. The method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal according to claim 1, characterized in that, In step S4, the desalination process is an evaporation crystallization process. The evaporation crystallization process adopts multi-effect evaporation or mechanical steam recompression evaporation. The evaporation temperature is 70~100℃ and the evaporation pressure is 20~80kPa absolute pressure. The chloride salt recovered by evaporation crystallization includes at least one of sodium chloride, potassium chloride and calcium chloride. The evaporation condensate is reused as washing water in the Nth stage washing unit.

8. The method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal according to claim 1, characterized in that, In step S2, the fresh washing water is the power plant condensate, recycled water after softening of circulating wastewater, or reverse osmosis concentrate; in step S3, the chlorine content of the dechlorinated coal product is not greater than 0.15 wt%, and the dechlorination rate is not less than 70%.

9. The method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal according to claim 1, characterized in that, In step S3, the dewatering is a two-stage dewatering process. First, the dechlorinated coal discharged from the Nth stage washing unit is dewatered through a vibrating dewatering screen, and then dewatered through a centrifugal dewatering machine. The dewatered water produced by the two stages of dewatering is collected and reused in the Nth stage washing unit.

10. The method for multi-stage countercurrent washing and dechlorination of low-rank, high-chlorine coal according to claim 1, characterized in that, In step S2, the coal material is conveyed between each washing unit using a belt conveyor or scraper conveyor, and the washing liquid is conveyed between each washing unit using a pump. The washing liquid discharged from each washing unit is removed by a hydrocyclone separator to remove fine coal slime before being conveyed to the next washing unit. In step S4, the desalination treatment is a combination of nanofiltration membrane separation and evaporation crystallization. The chlorine-containing concentrated washing liquid is first separated and concentrated by nanofiltration membrane to obtain chloride concentrate and permeate. The chloride concentrate is evaporated and crystallized to recover chloride salts. The permeate and evaporation condensate are combined and reused in the Nth washing unit.

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