A municipal solid waste incineration full-resource system and method coupling fly ash water washing dechlorination and molten salt pyrolysis

Abstract

The present application relates to solid waste treatment and resource utilization technical field, disclose a kind of coupling fly ash water washing dechlorination and molten salt pyrolysis of domestic waste incineration full resource system and method, method includes: fly ash is carried out multistage countercurrent water washing, and separation is obtained water washing fly ash and chlorine-containing water washing liquid;Chlorine-containing water washing liquid is recycled industrial grade sodium chloride and potassium chloride by evaporation crystallization, and produce evaporation condensate water;Water washing fly ash is dried and mixed with molten salt medium, and it is carried out segmented temperature control molten salt pyrolysis processing under anaerobic condition, and water vapor and carbon dioxide gas are passed in stages during pyrolysis process;Pyrolysis gas is recycled heat energy by high-temperature combustion;Pyrolysis residue and molten salt mixture are separated and recovered molten salt medium after cooling and solidification, and harmless residue is obtained;Evaporation condensate water is recycled for water washing unit;System includes water washing unit, evaporation crystallization unit, drying unit, pyrolysis unit, combustion unit, cooling separation unit and backwater pipeline.The present application realizes full resource utilization of fly ash.

Description

[0001] This invention relates to the field of solid waste treatment and resource utilization technology, specifically to a system and method for the full resource utilization of municipal solid waste incineration coupled with fly ash water washing and dechlorination and molten salt pyrolysis. Background Technology

[0002] With the rapid advancement of urbanization and the improvement of residents' living standards in my country, the amount of municipal solid waste generated continues to grow. Due to its core advantages such as significant volume and weight reduction, thorough harmlessness, and recyclable energy, municipal solid waste incineration has become the mainstream method for treating municipal solid waste. While the incineration industry is expanding rapidly, the disposal of fly ash, a byproduct of incineration, is becoming increasingly prominent, becoming a core bottleneck restricting the sustainable development of the industry.

[0003] Incineration fly ash is a collector and flue gas settling material in waste incineration flue gas purification systems. Fly ash is highly toxic, enriched with leached heavy metals such as lead, cadmium, and zinc, and contains dioxins, a potent carcinogen and persistent organic pollutant. Furthermore, the proportion of soluble sodium chloride and potassium chloride in fly ash is as high as 17.9%-22.1%, and the high chloride ion content easily leads to equipment corrosion. During its utilization in building materials, it reduces product strength and poses a risk of heavy metal migration. Traditional disposal methods mainly employ a combination of stabilization and solidification with secure landfilling. However, this model has the following problems: landfill capacity is increasingly strained; soluble chlorides in fly ash will leach out due to long-term contact with moisture in the landfill, leading to a risk of heavy metal leaching; dioxins are difficult to degrade naturally in the landfill environment, posing a long-term environmental hazard; and valuable components such as sodium chloride and potassium chloride, as well as residual heat energy, are directly discarded, resulting in resource waste. Summary of the Invention

[0004] The purpose of this invention is to provide a system and method for the complete resource recovery of municipal solid waste incineration by coupling fly ash water washing and dechlorination with molten salt pyrolysis, so as to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for the complete resource recovery of municipal solid waste from incineration by coupling fly ash water washing and dechlorination with molten salt pyrolysis, comprising the following steps: S1. The fly ash from municipal solid waste incineration is sent to the water washing unit, water is added for multi-stage countercurrent water washing, and the water-washed fly ash and chlorine-containing water washing liquid are separated. S2. The chlorine-containing washing solution obtained in S1 is sent to the evaporation and crystallization unit. Through evaporation and crystallization, industrial-grade sodium chloride and industrial-grade potassium chloride are recovered, and evaporation condensate is generated. S3. The water-washed fly ash obtained in S1 is sent to the drying unit and dried by hot air to obtain dry fly ash; S4. The dried fly ash obtained in S3 is mixed with molten salt medium and then sent to the pyrolysis unit for molten salt pyrolysis under anaerobic conditions to cause the dioxin-like organic pollutants in the fly ash to be cracked and mineralized, resulting in pyrolysis gas, pyrolysis residue and molten salt mixture. S5. The pyrolysis gas obtained in S4 is sent to the combustion unit for high-temperature combustion treatment. The high-temperature flue gas generated by combustion is used to recover heat energy or is fed back to the drying unit in S3. S6. The pyrolysis residue and molten salt mixture obtained in S4 are sent to the cooling and separation unit. After cooling and solidification, the molten salt medium is separated and recovered to obtain a harmless residue. S7. The evaporation condensate generated in S2 is returned to the water washing unit in S1 as a supplement to the water used for washing.

[0006] As a preferred embodiment of the present invention, the multi-stage countercurrent water washing in S1 specifically includes: First-stage water washing: Fly ash and recycled water are mixed at a liquid-solid ratio of 2:1 to 4:1 and stirred for 10-20 minutes. After solid-liquid separation, first-stage water-washed fly ash and first-stage water washing liquid are obtained. Second-stage water washing: Mix the first-stage water washing fly ash with fresh water at a liquid-solid ratio of 2:1 to 3:1, stir and react for 10-20 minutes, and then separate the solid and liquid to obtain the second-stage water washing fly ash and the second-stage water washing liquid. The second-stage washing solution is returned to the first-stage washing step as recycled water. Repeat the above operation up to the Nth stage of water washing, where N is 3, 4 or 5. The last stage of water washing uses pure water or evaporated condensate, and the last stage water washing solution is returned to the previous stage of water washing.

[0007] As a preferred embodiment of the present invention, the molten salt pyrolysis treatment in step S4 further includes segmented control of the pyrolysis temperature: First temperature stage: The mixture is heated from room temperature to 250-350℃ and held for 15-30 minutes to remove adsorbed water and crystal water from the fly ash, while the molten salt medium begins to melt. Second temperature range: continue heating to 400-550℃, hold for 20-40 minutes, causing the C-Cl bond in dioxin molecules to break, and chlorine to be captured by the alkaline components in the molten salt medium to form chlorides; The third temperature stage involves heating to 600-700℃ and holding for 10-20 minutes to completely decompose the remaining organic carbon skeleton, allowing heavy metals to react with the molten salt medium to form stable metal salts or metal oxides.

[0008] As a preferred embodiment of the present invention, the molten salt medium in S4 is a composite molten salt system, which consists of a main molten salt and an auxiliary molten salt. The main molten salt is sodium hydroxide or potassium hydroxide, accounting for 60%-80% by mass; The auxiliary molten salt is at least one of sodium carbonate, potassium carbonate, and sodium sulfate, accounting for 20%-40% by mass; The compound molten salt system forms a eutectic liquid phase in the range of 300-450℃. The liquid phase covers the surface of the water-washed fly ash particles, promoting the decomposition of organic matter and the fixation of chlorine.

[0009] As a preferred embodiment of the present invention, in step S4, different atmospheric media are introduced into the pyrolysis unit in stages during the molten salt pyrolysis process, specifically including: First stage atmosphere injection: When the temperature of the pyrolysis unit reaches 250-350℃, water vapor is introduced into the pyrolysis unit at a rate of 0.2-0.4 kg per kg of dry fly ash. The water vapor reacts with the molten salt medium to generate hydroxyl ions. The hydroxyl ions launch a nucleophilic attack on the chlorine atom in the dioxin molecule that is attached to the benzene ring, generating hydrogen chloride and hydroxyl-substituted dioxin intermediates. The hydrogen chloride is neutralized in situ by the alkaline components in the molten salt medium to generate sodium chloride or potassium chloride. Second stage atmosphere injection: When the temperature of the pyrolysis unit reaches 450-550℃, stop the introduction of water vapor and introduce carbon dioxide gas into the pyrolysis unit. The amount of carbon dioxide introduced is 0.1-0.3 kg per kg of dry fly ash. The carbon dioxide reacts with the molten salt medium to generate carbonate ions. The carbonate ions undergo transesterification with the hydroxyl-substituted intermediate generated in the first stage, causing the aromatic ring structure of dioxin to open and break, generating small molecule carboxylate and carbon monoxide. The third stage of atmosphere injection: When the temperature of the pyrolysis unit reaches 600-700℃, water vapor and carbon dioxide gas are introduced simultaneously. The volume ratio of the two gases is 1:1 to 2:1, and the total amount introduced is 0.2-0.5 kg per kg of dry fly ash. The water vapor and carbon dioxide work together to completely vaporize the residual organic carbon skeleton into hydrogen and carbon monoxide, while promoting the reaction of heavy metals with carbonate and hydroxyl groups to generate stable basic carbonates or hydroxyl oxides. Atmosphere injection sequence control: Water vapor, carbon dioxide, and a mixture of water vapor and carbon dioxide enter the pyrolysis unit through independent injection pipelines. Each injection pipeline is equipped with a shut-off valve and a flow regulating valve. The valves of the corresponding pipelines are opened in sequence according to the arrival order of the temperature range. A transition period of 1-3 minutes is set between two adjacent atmosphere injection stages. During the transition period, the injection of the two gases is closed at the same time, and the intermediate products are fully converted by utilizing the residual atmosphere inside the pyrolysis unit.

[0010] As a preferred embodiment of the present invention, the cooling separation unit in S6 specifically includes the following processing of the pyrolysis residue and molten salt mixture: Step 1: Cooling: Cool the mixture to 300-400℃ at a cooling rate of 10-30℃ / min, so that the molten salt medium changes from a liquid state to a semi-solid state; The second step is cooling: continue to cool down to 100-200℃ to completely solidify the molten salt medium. The pyrolysis residue and the solidified molten salt medium form two phases with different physical properties. Mechanical separation: The solidified molten salt medium is separated from the pyrolysis residue by vibrating screen or air classification. The separated molten salt medium is crushed and returned to the pyrolysis unit of S4 for reuse. Water washing and separation: The pyrolysis residue of the molten salt medium that remains after mechanical separation is sent to the water washing tank, pure water is added and stirred to wash and dissolve the remaining molten salt medium. After solid-liquid separation, the final harmless residue is obtained, and the washing liquid is returned to the water washing unit of S1.

[0011] A comprehensive resource recovery system for municipal solid waste incineration, coupled with fly ash washing and dechlorination and molten salt pyrolysis, includes: The water washing unit is used to receive fly ash from municipal solid waste incineration and add water for multi-stage countercurrent water washing, separating and outputting water-washed fly ash and chlorine-containing washing liquid; The evaporation and crystallization unit, connected to the water washing unit, is used to receive chlorine-containing washing liquid and perform evaporation and crystallization treatment, outputting industrial-grade sodium chloride, industrial-grade potassium chloride, and evaporation condensate. The drying unit, connected to the washing unit, is used to receive the washed fly ash and perform hot air drying treatment, and output the dried fly ash. The pyrolysis unit, connected to the drying unit, is used to receive a mixture of dried fly ash and molten salt medium, and to carry out molten salt pyrolysis treatment under anaerobic conditions, outputting pyrolysis gas, pyrolysis residue and molten salt mixture; The combustion unit, connected to the pyrolysis unit, is used to receive pyrolysis gas and perform high-temperature combustion treatment to output high-temperature flue gas; The cooling separation unit, connected to the pyrolysis unit, is used to receive the pyrolysis residue and molten salt mixture, and after cooling and solidification, separate and recover the molten salt medium, and output harmless residue. The return water pipeline connects the evaporation and crystallization unit and the water washing unit, and is used to return the evaporated condensate to the water washing unit.

[0012] As a preferred embodiment of the present invention, the pyrolysis unit is divided into a first heating zone, a second heating zone, and a third heating zone sequentially along the material flow direction: The heating power of the first heating zone is set to bring the material temperature to 250-350℃; The heating power of the second heating zone is set to bring the material temperature to 400-550℃; The heating power of the third heating zone is set to bring the material temperature to 600-700℃. The three heating zones are independently temperature-controlled, and heat insulation baffles are installed between each zone.

[0013] As a preferred embodiment of the present invention, the pyrolysis unit is provided with three independent gas injection ports, which are respectively connected to a steam generator, a carbon dioxide cylinder, and a steam and carbon dioxide mixer. Each gas injection port is connected to the interior of the pyrolysis unit by an independent injection pipeline. Each injection pipeline is provided with a shut-off valve and a flow regulating valve in sequence. The outlets of the three injection pipelines extend into the interior of the pyrolysis unit and face the surface of the material layer. The three gas injection ports are opened in sequence according to the temperature range within the pyrolysis unit.

[0014] As a preferred embodiment of the present invention, the cooling separation unit comprises the following components connected in sequence: The primary cooler, employing a jacketed water-cooled or air-cooled structure, is used to cool the pyrolysis residue and molten salt mixture to 300-400℃ at a cooling rate of 10-30℃ / min. The secondary cooler, which uses natural cooling or forced air cooling, is used to further cool the material to 100-200℃; Mechanical separators, such as vibrating screens or air classifiers, are used to separate solidified molten salt media from pyrolysis residues. Mechanical separators are equipped with molten salt media outlets and coarse residue outlets. The water washing separation tank is connected to the coarse residue outlet of the mechanical separator. The water washing separation tank is equipped with an agitator for washing the coarse residue to dissolve the residual molten salt medium. The water washing separation tank has a final residue outlet and a washing liquid outlet, and the washing liquid outlet is connected to the water washing unit.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention achieves full resource utilization of municipal solid waste incineration fly ash by coupling multi-stage countercurrent water washing dechlorination with molten salt pyrolysis for deep degradation. The water washing unit effectively removes most of the soluble chloride salts from the fly ash, avoiding corrosion of subsequent pyrolysis equipment by chlorine elements. At the same time, industrial-grade sodium chloride and potassium chloride products are recovered, realizing the resource utilization of chloride salts.

[0016] 2. In the molten salt pyrolysis treatment, the present invention adopts a compound molten salt system and performs segmented temperature-controlled pyrolysis under anaerobic conditions. The compound molten salt system forms a eutectic liquid phase in the low temperature range. The liquid phase covers the surface of fly ash particles, promoting the decomposition of organic matter and the chemical fixation of chlorine. The segmented control of the pyrolysis temperature allows the adsorbed water and crystal water in the fly ash to be removed in the low temperature range, the carbon-chlorine bonds in dioxin molecules to be broken in the medium temperature range, and the organic carbon skeleton to be completely decomposed in the high temperature range. At the same time, heavy metals react with the molten salt medium to generate stable metal salts or metal oxides, realizing the efficient degradation of dioxins and the simultaneous stable solidification of heavy metals.

[0017] 3. In the molten salt pyrolysis process, the present invention introduces water vapor, carbon dioxide, and a mixture of water vapor and carbon dioxide in stages. In the low-temperature section, water vapor reacts with the molten salt medium to generate hydroxyl ions. The hydroxyl ions launch a nucleophilic attack on dioxin molecules to remove and neutralize chlorine atoms. In the medium-temperature section, carbon dioxide reacts with the molten salt medium to generate carbonate ions. The carbonate ions undergo an ester exchange reaction with the hydroxyl-substituted intermediate, causing the aromatic ring to open and break. In the high-temperature section, water vapor and carbon dioxide synergistically vaporize the residual organic carbon skeleton. The three atmospheres are injected in stages and a transition period is set to fully convert the intermediate products, which significantly improves the degradation efficiency of dioxins.

[0018] 4. This invention combines two-step cooling and mechanical separation with water washing separation to process the pyrolysis residue and molten salt mixture through a cooling separation unit. The first step of cooling changes the molten salt medium from a liquid state to a semi-solid state. The second step of cooling completely solidifies the molten salt medium and forms two phases with different physical properties with the pyrolysis residue, which facilitates subsequent mechanical separation. The separated molten salt medium is crushed and returned to the pyrolysis unit for reuse, realizing the recycling of the molten salt medium. Water washing separation further recovers the residual molten salt medium, and the washing liquid is returned to the water washing unit, effectively reducing the amount of fresh water to be added.

[0019] 5. This invention sends pyrolysis gas into the combustion unit for high-temperature combustion treatment. The high-temperature flue gas generated by combustion is used to recover heat energy or is fed back to the drying unit, realizing the recovery and utilization of residual heat energy in fly ash. The evaporation condensate generated by evaporation and crystallization is returned to the water washing unit through the return water pipeline as a supplement to the water washing water, realizing the recycling of water resources throughout the entire process. Compared with the traditional stabilization and solidification landfill mode, this invention significantly reduces the cost of fly ash disposal, avoids the environmental risks of heavy metal leaching and dioxin diffusion, and realizes the full-chain recycling of chloride salts, molten salt media, heat energy and water resources in fly ash. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall process of a method for the complete resource recovery of municipal solid waste from incineration by coupling fly ash water washing and dechlorination with molten salt pyrolysis according to the present invention. Figure 2 This is a structural framework diagram of a municipal solid waste incineration full-scale resource recovery system that couples fly ash water washing and dechlorination with molten salt pyrolysis according to the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Example 1 Taking a municipal solid waste incineration power plant in East China as an example, the plant processes 1200 tons of municipal solid waste per day, generating approximately 48 tons of fly ash daily. The chloride ion mass fraction in the fly ash is 20.3%, and the dioxin toxicity equivalent concentration is 3.2 ng TEQ / g. Figure 1 As shown, the method for full resource recovery of municipal solid waste through incineration, which combines fly ash water washing and dechlorination with molten salt pyrolysis provided by this invention, is used for treatment.

[0023] Step S1: 48 tons of municipal solid waste incineration fly ash are sent to the washing unit, employing a four-stage countercurrent washing process. The first stage of washing involves mixing fly ash with recycled water at a liquid-to-solid ratio of 3:1 (liquid refers to the volume of recycled water, and solid refers to the mass of fly ash). The mixture is stirred and reacted for 15 minutes. After solid-liquid separation using a plate and frame filter press, first-stage washed fly ash and first-stage washing liquid are obtained. The second stage of washing involves mixing first-stage washed fly ash with fresh water at a liquid-to-solid ratio of 2.5:1. The mixture is stirred and reacted for 15 minutes. After solid-liquid separation, second-stage washed fly ash and second-stage washing liquid are obtained. The second-stage washing liquid is returned to the first-stage washing unit as recycled water. The washing process includes three stages: Third-stage water washing: Second-stage water washing fly ash is mixed with evaporative condensate at a liquid-to-solid ratio of 2:1 and stirred for 12 minutes. After solid-liquid separation, third-stage water washing fly ash and third-stage water washing liquid are obtained. The third-stage water washing liquid is returned to the second-stage water washing step. Fourth-stage water washing: Third-stage water washing fly ash is mixed with pure water at a liquid-to-solid ratio of 2:1 and stirred for 10 minutes. After solid-liquid separation, water washing fly ash and fourth-stage water washing liquid are obtained. The fourth-stage water washing liquid is returned to the third-stage water washing step. After four stages of countercurrent water washing, the chloride ion mass fraction in the water-washed fly ash is reduced to 0.8%, and the dechlorination efficiency reaches 96.1%.

[0024] Step S2: The chlorine-containing washing solution obtained in step S1 is sent to the evaporation crystallization unit. The chlorine-containing washing solution has a sodium chloride mass concentration of 85 g / L and a potassium chloride mass concentration of 42 g / L. Evaporation crystallization is carried out using a triple-effect evaporation crystallizer. The first effect evaporation temperature is 105℃, the second effect evaporation temperature is 90℃, and the third effect evaporation temperature is 75℃. After evaporation crystallization, the solution is centrifuged to recover industrial-grade sodium chloride and industrial-grade potassium chloride, respectively. The purity of sodium chloride is 98.5%, and the purity of potassium chloride is 97.8%. The evaporation condensate generated during the evaporation crystallization process is collected in a condensate storage tank. The conductivity of the evaporation condensate is less than 50 μS / cm.

[0025] Step S3: The washed fly ash obtained in step S1 is sent to the drying unit. The water content of the washed fly ash is 32%. It is dried by hot air using a rotary drum dryer. The hot air temperature is 180℃, the hot air velocity is 2.5m / s, and the drying time is 25 minutes to obtain dried fly ash. The moisture content of the dried fly ash is reduced to below 1.5%.

[0026] Step S4: The dried fly ash obtained in step S3 is mixed with molten salt medium at a mass ratio of 1:2 and then fed into the pyrolysis unit. The molten salt medium is a compound molten salt system, consisting of potassium hydroxide as the main molten salt and potassium carbonate as the auxiliary molten salt. The mass percentage of potassium hydroxide is 70%, and the mass percentage of potassium carbonate is 30%. The pyrolysis unit is an externally heated rotary kiln. Nitrogen gas is maintained in the pyrolysis unit to ensure an oxygen-free environment. The pyrolysis temperature is controlled in stages: In the first temperature stage, the mixture is heated from room temperature to 300°C at a heating rate of 15°C / min, with a residence time of 20 minutes. The adsorbed water and solidified water in the fly ash are then concentrated in the kiln. After the crystal water is removed, the molten salt medium begins to melt and form a liquid phase. In the second temperature stage, the temperature continues to rise to 500℃ at a rate of 10℃ / min and a residence time of 30 minutes. The C-Cl bonds in the dioxin molecules break, and chlorine is captured by the alkaline components in the molten salt medium to form potassium chloride. In the third temperature stage, the temperature rises to 650℃ at a rate of 8℃ / min and a residence time of 15 minutes. The remaining organic carbon skeleton is completely decomposed, and heavy metals react with the molten salt medium to form stable potassium metal acids or metal oxides. The pyrolysis unit outputs a mixture of pyrolysis gas, pyrolysis residue, and molten salt.

[0027] During the molten salt pyrolysis process, different atmospheres are introduced into the pyrolysis unit in stages. The first stage of atmosphere injection involves introducing steam when the pyrolysis unit temperature reaches 300℃. The steam injection rate is 0.3 kg per kg of dried fly ash. The steam is generated by an electrically heated steam generator at a pressure of 0.2 MPa. The steam reacts with the molten salt medium to generate hydroxyl ions. These hydroxyl ions launch a nucleophilic attack on the chlorine atom attached to the benzene ring in the dioxin molecule, generating hydrogen chloride and hydroxyl-substituted dioxins. In the first stage, hydrogen chloride is neutralized in situ by potassium hydroxide in the molten salt medium to form potassium chloride. The second stage involves atmospheric injection: when the pyrolysis unit temperature reaches 500℃, steam is stopped, and carbon dioxide gas is introduced instead. The carbon dioxide gas is supplied from industrial-grade carbon dioxide cylinders with a purity of 99.5%, and the injection rate is 0.2 kg per kilogram of dried fly ash. The carbon dioxide reacts with the molten salt medium to generate carbonate ions. These carbonate ions then undergo transesterification with the hydroxyl-substituted intermediate generated in the first stage. The aromatic ring structure of dioxins undergoes ring-opening and breakage, generating small-molecule potassium carboxylate and carbon monoxide. The third stage involves atmosphere injection: when the pyrolysis unit temperature reaches 650℃, water vapor and carbon dioxide are simultaneously introduced at a volume ratio of 1.5:1, with a total injection rate of 0.35 kg per kg of dry fly ash. The water vapor and carbon dioxide work synergistically on the residual organic carbon skeleton, completely vaporizing it into hydrogen and carbon monoxide. Simultaneously, they promote the reaction of heavy metals with carbonates and hydroxyl groups to generate stable basic carbonates or hydroxyl oxides. Atmosphere injection sequence control: water vapor, carbon dioxide, and mixtures of water vapor and carbon dioxide enter the pyrolysis unit through independent injection pipelines. Each injection pipeline is equipped with a shut-off valve and a flow regulating valve. The valves of the corresponding pipelines are opened sequentially according to the arrival order of the temperature ranges. A 2-minute transition period is set between the first and second temperature ranges, and between the second and third temperature ranges. During these transition periods, the injection of both gases is simultaneously shut off, utilizing the residual atmosphere inside the pyrolysis unit to complete the full conversion of the intermediate products.

[0028] Tests showed that the dioxin toxicity equivalent concentration in the pyrolysis residue and molten salt mixture after pyrolysis treatment was 0.05 ng TEQ / g, which is lower than the national standard limit of 0.1 ng TEQ / g, and the dioxin removal rate reached 98.4%.

[0029] Step S5: The pyrolysis gas obtained in step S4 is fed into the combustion unit. The main components of the pyrolysis gas are hydrogen, carbon monoxide, methane and a small amount of small molecule hydrocarbons. The combustion unit adopts a high-temperature combustion chamber with a combustion temperature of 950°C and a flue gas residence time of 2.5 seconds. The pyrolysis gas is completely burned in the combustion chamber, and the generated high-temperature flue gas enters the heat exchanger. The heat exchanger recovers the heat of the high-temperature flue gas to generate steam. The steam is incorporated into the plant's steam pipeline network. After heat exchange, part of the high-temperature flue gas is cooled to 200°C and used as hot air to be supplied back to the drying unit in step S3.

[0030] Step S6: The pyrolysis residue and molten salt mixture obtained in step S4 are fed into a cooling and separation unit. First cooling step: A jacketed water-cooled screw conveyor is used to cool the mixture to 350°C at a cooling rate of 20°C / min, causing the molten salt medium to change from a liquid to a semi-solid state. Second cooling step: The semi-solid mixture is fed onto a natural cooling conveyor belt for further cooling to 150°C, completely solidifying the molten salt medium. The pyrolysis residue and the solidified molten salt medium form two phases with different physical properties. Mechanical separation: A vibrating screen is used to separate the solidified molten salt medium from the pyrolysis residue. The screen mesh size is 2mm, and the molten salt medium particles... Particles larger than 2mm are retained, while pyrolysis residues smaller than 2mm pass through a sieve. The separated molten salt medium is crushed to a particle size of less than 5mm by a hammer crusher and returned to the pyrolysis unit in step S4 for reuse. Water washing separation: The pyrolysis residues of molten salt medium remaining after mechanical separation are sent to a water washing tank, pure water is added and stirred for washing. The liquid-to-solid ratio is 3:1, and the washing time is 20 minutes to dissolve the remaining molten salt medium. The residues are then separated by a plate and frame filter press to obtain the final harmless residue. The washing liquid is returned to the water washing unit in step S1 as supplementary water. The final harmless residues are tested and can be used as roadbed materials or landfill cover soil.

[0031] In step S7, the evaporation condensate generated in step S2 is returned to the water washing unit in step S1 through the return water pipeline as a supplement to the water washing water. According to calculations, the evaporation condensate reuse rate is 92%, and the fresh water replenishment amount for the entire process is only 8%, thus realizing the recycling of water resources.

[0032] In this embodiment, the above technical solution achieves full resource utilization of fly ash from municipal solid waste incineration: sodium chloride and potassium chloride are recycled as industrial-grade products, molten salt medium is recycled, pyrolysis gas is burned to recover heat energy, and the final harmless residue reduction rate reaches 85%. Compared with the traditional stabilization and solidification landfill mode, the fly ash disposal cost is reduced by about 40%.

[0033] Example 2 Taking a large-scale municipal solid waste incineration power plant in South China as an example, the plant processes 2,000 tons of municipal solid waste per day, generating approximately 80 tons of fly ash daily. The chloride ion mass fraction in the fly ash is 21.6%, and the dioxin toxicity equivalent concentration is 4.1 ng TEQ / g. Figure 2As shown, the municipal solid waste incineration full-scale resource recovery system provided by this invention, which combines fly ash water washing dechlorination and molten salt pyrolysis, is used for processing.

[0034] The system includes a water washing unit, an evaporation and crystallization unit, a drying unit, a pyrolysis unit, a combustion unit, a cooling and separation unit, and a return water pipeline. The water washing unit consists of four counter-current water washing reactors connected in series. Each reactor is a stirred tank with a volume of 10 m³. 3 The tank has a filtrate outlet at the bottom and a slurry outlet on the side. The evaporation and crystallization unit is a triple-effect evaporator and crystallizer, with steam pipelines connecting the first and second effects, and between the second and third effects. The drying unit is a rotary drum dryer with a length of 12m and a diameter of 2m, and lifting plates inside the drum. The pyrolysis unit is an externally heated rotary kiln with a length of 8m and a diameter of 1.5m, and three independent electric heating zones on the outer wall of the kiln. The combustion unit includes a high-temperature combustion chamber and a shell-and-tube heat exchanger. The outlet of the high-temperature combustion chamber is connected to the hot-side inlet of the shell-and-tube heat exchanger, and the cold-side inlet of the shell-and-tube heat exchanger is connected to a blower. The cooling separation unit includes a primary cooler, a secondary cooler, a mechanical separator, and a water washing separation tank. The primary cooler is a jacketed water-cooled screw conveyor, the secondary cooler is a natural cooling conveyor belt, the mechanical separator is a vibrating screen, and the water washing separation tank is a horizontal reaction tank with an agitator. The return water pipeline is a DN80 stainless steel pipe that connects the condensate outlet of the evaporation crystallization unit to the inlet of the last stage water washing reactor of the water washing unit.

[0035] Eighty tons of municipal solid waste incineration fly ash were fed into a washing unit employing a four-stage countercurrent washing process. In the first-stage countercurrent washing reactor, recycled water was added at a liquid-to-solid ratio of 3.5:1, with a stirring speed of 60 rpm and a reaction time of 18 minutes. After the reaction, the bottom valve was opened to discharge the slurry to a plate and frame filter press for solid-liquid separation, yielding first-stage washed fly ash and first-stage washing liquid. In the second-stage countercurrent washing reactor, fresh water was added at a liquid-to-solid ratio of 2.8:1, with a stirring speed of 60 rpm and a reaction time of 15 minutes. After solid-liquid separation, second-stage washed fly ash and second-stage washing liquid were obtained. The secondary washing liquid is returned to the first-stage countercurrent washing reactor through the filtrate return port. Evaporated condensate is added to the third-stage countercurrent washing reactor at a liquid-to-solid ratio of 2.2:1, with a stirring speed of 50 rpm and a reaction time of 12 minutes. After solid-liquid separation, the third-stage washing fly ash and the third-stage washing liquid are obtained. The third-stage washing liquid is returned to the second-stage countercurrent washing reactor. Pure water is added to the fourth-stage countercurrent washing reactor at a liquid-to-solid ratio of 2:1, with a stirring speed of 50 rpm and a reaction time of 10 minutes. After solid-liquid separation, the washing fly ash and the fourth-stage washing liquid are obtained. The fourth-stage washing liquid is returned to the third-stage countercurrent washing reactor.

[0036] The chlorine-containing washing solution enters the evaporation and crystallization unit. The operating parameters of the triple-effect evaporator and crystallizer are as follows: first effect evaporation temperature 108℃, pressure -0.05MPa; second effect evaporation temperature 92℃, pressure -0.07MPa; third effect evaporation temperature 78℃, pressure -0.09MPa. After evaporation and crystallization, industrial-grade sodium chloride and industrial-grade potassium chloride are obtained by centrifugation. The purity of sodium chloride is 98.2%, and the purity of potassium chloride is 97.5%. The evaporation condensate is collected in a condensate storage tank. The conductivity of the evaporation condensate is 45μS / cm.

[0037] The washed fly ash enters the drying unit. The rotary drum dryer rotates at 5 rpm, the hot air inlet temperature is 190℃, and the hot air flow rate is 5000 m³ / h. 3 The drying process was carried out at a rate of 1 / h for 22 minutes to obtain dried fly ash with a moisture content of 1.2%.

[0038] Dry fly ash and molten salt medium are mixed at a mass ratio of 1:2.5 and then fed into the pyrolysis unit. The molten salt medium is a compound molten salt system, consisting of sodium hydroxide as the main molten salt and sodium carbonate as the auxiliary molten salt. The mass percentage of sodium hydroxide is 65%, and the mass percentage of sodium carbonate is 35%. The pyrolysis unit is an externally heated rotary kiln. The interior of the kiln is divided into a first heating zone, a second heating zone, and a third heating zone along the material flow direction. The heating power of the first heating zone is set to reach a material temperature of 320℃, the heating power of the second heating zone is set to reach a material temperature of 520℃, and the heating power of the third heating zone is set to reach a material temperature of 680℃. The three heating zones are independently temperature controlled. A ceramic fiber heat insulation baffle with a thickness of 50mm is installed between each zone. The pyrolysis unit is kept under negative pressure with a negative pressure value of -50Pa, and an oxygen-free condition is maintained by an induced draft fan.

[0039] The pyrolysis unit has three independent gas injection ports, which are respectively connected to a steam generator, a carbon dioxide cylinder, and a steam-carbon dioxide mixer. The steam generator is an electrically heated steam boiler with a rated evaporation capacity of 50 kg / h and a steam pressure of 0.25 MPa. The carbon dioxide cylinder is a 40L standard steel cylinder filled with industrial-grade carbon dioxide with a purity of 99.5%. The mixer is a static mixer, with flow regulating valves installed at both the steam and carbon dioxide inlets. The mixer outlet connects to the third gas injection port. Each gas injection port is connected to an independent DN25 stainless steel injection pipeline between itself and the inside of the pyrolysis unit. Each injection pipeline is equipped with a shut-off valve and a flow regulating valve in sequence. The outlets of all three injection pipelines extend into the pyrolysis unit and face the surface of the material layer, with the outlets 50 mm away from the material layer surface. The three gas injection ports are arranged in order of the temperature range within the pyrolysis unit. Initial startup: When the material temperature in the first heating zone reaches 320℃, open the shut-off valve and flow regulating valve of the steam injection pipeline, with a steam injection rate of 0.35 kg per kg of dry fly ash; when the material temperature in the second heating zone reaches 520℃, close the valve of the steam injection pipeline and open the valve of the carbon dioxide injection pipeline, with a carbon dioxide injection rate of 0.25 kg per kg of dry fly ash; when the material temperature in the third heating zone reaches 680℃, close the valve of the carbon dioxide injection pipeline and open the valve of the mixed gas injection pipeline, with a steam to carbon dioxide volume ratio of 1.8:1 and a total injection rate of 0.4 kg per kg of dry fly ash. A 2.5-minute transition period is set between adjacent heating zones. During the transition period, the material moves between the heat insulation baffles, and all gas injection pipeline valves are closed simultaneously. The residual atmosphere inside the pyrolysis unit is used to complete the full conversion of intermediate products.

[0040] The pyrolysis unit outputs a mixture of pyrolysis gas, pyrolysis residue, and molten salt. The pyrolysis gas enters the high-temperature combustion chamber of the combustion unit, where the combustion temperature is 1000℃ and the flue gas residence time is 3 seconds. The high-temperature flue gas generated after complete combustion of the pyrolysis gas enters a shell-and-tube heat exchanger with a heat exchange area of ​​50 m². 2 High-temperature flue gas flows in the tube side, while cold air flows in the shell side. The cold air is heated to 250°C and then sent as hot air to the hot air inlet of the drying unit. The flue gas after heat exchange is discharged in compliance with emission standards through the exhaust gas treatment system.

[0041] The pyrolysis residue and molten salt mixture enter the cooling and separation unit. The primary cooler uses a jacketed water-cooled screw conveyor with an inlet water temperature of 25℃ and an outlet water temperature of 45℃. The screw conveyor rotates at 10 rpm, cooling the mixture to 380℃ at a cooling rate of 25℃ / min. The secondary cooler uses a natural cooling conveyor belt with a length of 8m and a conveying speed of 0.2m / s. The material is naturally cooled to 180℃ on the conveyor belt. The mechanical separator is a vibrating screen with a screen aperture of 1.5mm and a vibration frequency of 1400 times / min. The solidified molten salt medium is trapped above the screen and discharged through the oversize outlet. The pyrolysis residue passes through the screen and is discharged through the undersize outlet. The separated molten salt medium is crushed by a hammer crusher and returned to the pyrolysis unit. The water washing separation tank is a horizontal reaction tank with an agitator and a tank volume of 5m³. 3 The stirring paddle rotates at 40 rpm, and the pyrolysis residue after mechanical separation is sent to the water washing separation tank. Pure water is added and stirred for washing. The liquid-to-solid ratio is 4:1 and the washing time is 25 minutes. After washing, the bottom valve is opened to discharge the slurry to the plate and frame filter press for solid-liquid separation to obtain the final harmless residue. The washing liquid is connected to the fourth-stage countercurrent water washing reactor of the water washing unit through the washing liquid outlet and pipeline as a supplement to the water washing water.

[0042] Testing revealed that the final harmless residue achieved a volume reduction rate of 92%, with heavy metal leaching concentrations of 0.15 mg / L for lead, 0.02 mg / L for cadmium, and 0.35 mg / L for zinc. The dioxin toxicity equivalent concentration was 0.03 ng TEQ / g, lower than the national standard limit of 0.1 ng TEQ / g. The entire process recovered approximately 4.2 tons of industrial-grade sodium chloride and 2.1 tons of industrial-grade potassium chloride. The molten salt medium recycling rate reached 95%. Compared with the traditional fly ash stabilization, solidification, and landfill methods, the fly ash disposal cost in this embodiment was reduced by approximately 45%, while simultaneously achieving resource recovery of chloride salts from fly ash, complete degradation of dioxins, stabilization and solidification of heavy metals, and heat energy recovery and utilization.

[0043] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention.

Claims

1. A method for the complete resource recovery of municipal solid waste from incineration by coupling fly ash water washing and dechlorination with molten salt pyrolysis, characterized in that, Includes the following steps: S1. The fly ash from municipal solid waste incineration is sent to the water washing unit, water is added for multi-stage countercurrent water washing, and the water-washed fly ash and chlorine-containing water washing liquid are separated. S2. The chlorine-containing washing solution obtained in S1 is sent to the evaporation and crystallization unit. Through evaporation and crystallization, industrial-grade sodium chloride and industrial-grade potassium chloride are recovered, and evaporation condensate is generated. S3. The water-washed fly ash obtained in S1 is sent to the drying unit and dried by hot air to obtain dry fly ash; S4. The dried fly ash obtained in S3 is mixed with molten salt medium and then sent to the pyrolysis unit for molten salt pyrolysis under anaerobic conditions to cause the dioxin-like organic pollutants in the fly ash to be cracked and mineralized, resulting in pyrolysis gas, pyrolysis residue and molten salt mixture. S5. The pyrolysis gas obtained in S4 is sent to the combustion unit for high-temperature combustion treatment. The high-temperature flue gas generated by combustion is used to recover heat energy or is fed back to the drying unit in S3. S6. The pyrolysis residue and molten salt mixture obtained in S4 are sent to the cooling and separation unit. After cooling and solidification, the molten salt medium is separated and recovered to obtain a harmless residue. S7. The evaporation condensate generated in S2 is returned to the water washing unit in S1 as a supplement to the water used for washing.

2. The method according to claim 1, characterized in that, The multi-stage countercurrent water washing in S1 specifically includes: First-stage water washing: Fly ash and recycled water are mixed at a liquid-solid ratio of 2:1 to 4:1 and stirred for 10-20 minutes. After solid-liquid separation, first-stage water-washed fly ash and first-stage water washing liquid are obtained. Second-stage water washing: Mix the first-stage water washing fly ash with fresh water at a liquid-solid ratio of 2:1 to 3:1, stir and react for 10-20 minutes, and then separate the solid and liquid to obtain the second-stage water washing fly ash and the second-stage water washing liquid. The second-stage washing solution is returned to the first-stage washing step as recycled water. Repeat the above operation up to the Nth stage of water washing, where N is 3, 4 or 5. The last stage of water washing uses pure water or evaporated condensate, and the last stage water washing solution is returned to the previous stage of water washing.

3. The method according to claim 1, characterized in that, The molten salt pyrolysis process in S4 further includes segmented control of the pyrolysis temperature: First temperature stage: The mixture is heated from room temperature to 250-350℃ and held for 15-30 minutes to remove adsorbed water and crystal water from the fly ash, while the molten salt medium begins to melt. Second temperature range: continue heating to 400-550℃, hold for 20-40 minutes, causing the C-Cl bond in dioxin molecules to break, and chlorine to be captured by the alkaline components in the molten salt medium to form chlorides; The third temperature stage involves heating to 600-700℃ and holding for 10-20 minutes to completely decompose the remaining organic carbon skeleton, allowing heavy metals to react with the molten salt medium to form stable metal salts or metal oxides.

4. The method according to claim 1, characterized in that, The molten salt medium in S4 is a composite molten salt system, which consists of a main molten salt and an auxiliary molten salt. The main molten salt is sodium hydroxide or potassium hydroxide, accounting for 60%-80% by mass; The auxiliary molten salt is at least one of sodium carbonate, potassium carbonate, and sodium sulfate, accounting for 20%-40% by mass; The compound molten salt system forms a eutectic liquid phase in the range of 300-450℃. The liquid phase covers the surface of the water-washed fly ash particles, promoting the decomposition of organic matter and the fixation of chlorine.

5. The method according to claim 1, characterized in that, In the S4 molten salt pyrolysis process, different atmospheric media are introduced into the pyrolysis unit in stages, specifically including: First stage atmosphere injection: When the temperature of the pyrolysis unit reaches 250-350℃, water vapor is introduced into the pyrolysis unit at a rate of 0.2-0.4 kg per kg of dry fly ash. The water vapor reacts with the molten salt medium to generate hydroxyl ions. The hydroxyl ions launch a nucleophilic attack on the chlorine atom in the dioxin molecule that is attached to the benzene ring, generating hydrogen chloride and hydroxyl-substituted dioxin intermediates. The hydrogen chloride is neutralized in situ by the alkaline components in the molten salt medium to generate sodium chloride or potassium chloride. Second stage atmosphere injection: When the temperature of the pyrolysis unit reaches 450-550℃, stop the introduction of water vapor and introduce carbon dioxide gas into the pyrolysis unit. The amount of carbon dioxide introduced is 0.1-0.3 kg per kg of dry fly ash. The carbon dioxide reacts with the molten salt medium to generate carbonate ions. The carbonate ions undergo transesterification with the hydroxyl-substituted intermediate generated in the first stage, causing the aromatic ring structure of dioxin to open and break, generating small molecule carboxylate and carbon monoxide. The third stage of atmosphere injection: When the temperature of the pyrolysis unit reaches 600-700℃, water vapor and carbon dioxide gas are introduced simultaneously. The volume ratio of the two gases is 1:1 to 2:1, and the total amount introduced is 0.2-0.5 kg per kg of dry fly ash. The water vapor and carbon dioxide work together to completely vaporize the residual organic carbon skeleton into hydrogen and carbon monoxide, while promoting the reaction of heavy metals with carbonate and hydroxyl groups to generate stable basic carbonates or hydroxyl oxides. Atmosphere injection sequence control: Water vapor, carbon dioxide, and a mixture of water vapor and carbon dioxide enter the pyrolysis unit through independent injection pipelines. Each injection pipeline is equipped with a shut-off valve and a flow regulating valve. The valves of the corresponding pipelines are opened in sequence according to the arrival order of the temperature range. A transition period of 1-3 minutes is set between two adjacent atmosphere injection stages. During the transition period, the injection of the two gases is closed at the same time, and the intermediate products are fully converted by utilizing the residual atmosphere inside the pyrolysis unit.

6. The method according to claim 1, characterized in that, The specific treatment of the pyrolysis residue and molten salt mixture by the cooling separation unit in S6 includes: Step 1: Cooling: Cool the mixture to 300-400℃ at a cooling rate of 10-30℃ / min, so that the molten salt medium changes from a liquid state to a semi-solid state; The second step is cooling: continue to cool down to 100-200℃ to completely solidify the molten salt medium. The pyrolysis residue and the solidified molten salt medium form two phases with different physical properties. Mechanical separation: The solidified molten salt medium is separated from the pyrolysis residue by vibrating screen or air classification. The separated molten salt medium is crushed and returned to the pyrolysis unit of S4 for reuse. Water washing and separation: The pyrolysis residue of the molten salt medium that remains after mechanical separation is sent to the water washing tank, pure water is added and stirred to wash and dissolve the remaining molten salt medium. After solid-liquid separation, the final harmless residue is obtained, and the washing liquid is returned to the water washing unit of S1.

7. A comprehensive resource recovery system for municipal solid waste incineration that couples fly ash water washing and dechlorination with molten salt pyrolysis, characterized in that, include: The water washing unit is used to receive fly ash from municipal solid waste incineration and add water for multi-stage countercurrent water washing, separating and outputting water-washed fly ash and chlorine-containing washing liquid; The evaporation and crystallization unit, connected to the water washing unit, is used to receive chlorine-containing washing liquid and perform evaporation and crystallization treatment, outputting industrial-grade sodium chloride, industrial-grade potassium chloride, and evaporation condensate. The drying unit, connected to the washing unit, is used to receive the washed fly ash and perform hot air drying treatment, and output the dried fly ash. The pyrolysis unit, connected to the drying unit, is used to receive a mixture of dried fly ash and molten salt medium, and to carry out molten salt pyrolysis treatment under anaerobic conditions, outputting pyrolysis gas, pyrolysis residue and molten salt mixture; The combustion unit, connected to the pyrolysis unit, is used to receive pyrolysis gas and perform high-temperature combustion treatment to output high-temperature flue gas; The cooling separation unit, connected to the pyrolysis unit, is used to receive the pyrolysis residue and molten salt mixture, and after cooling and solidification, separate and recover the molten salt medium, and output harmless residue. The return water pipeline connects the evaporation and crystallization unit and the water washing unit, and is used to return the evaporated condensate to the water washing unit.

8. The system according to claim 7, characterized in that, The pyrolysis unit is divided into a first heating zone, a second heating zone, and a third heating zone along the material flow direction: The heating power of the first heating zone is set to bring the material temperature to 250-350℃; The heating power of the second heating zone is set to bring the material temperature to 400-550℃; The heating power of the third heating zone is set to bring the material temperature to 600-700℃. The three heating zones are independently temperature-controlled, and heat insulation baffles are installed between each zone.

9. The system according to claim 7, characterized in that, The pyrolysis unit is equipped with three independent gas injection ports, which are respectively connected to a steam generator, a carbon dioxide cylinder, and a steam and carbon dioxide mixer. Each gas injection port is connected to the interior of the pyrolysis unit by an independent injection pipeline. Each injection pipeline is equipped with a shut-off valve and a flow regulating valve in sequence. The outlets of the three injection pipelines extend into the interior of the pyrolysis unit and face the surface of the material layer. The three gas injection ports are opened sequentially according to the temperature range within the pyrolysis unit.

10. The system according to claim 7, characterized in that, The cooling separation unit comprises, in sequence: The primary cooler, employing a jacketed water-cooled or air-cooled structure, is used to cool the pyrolysis residue and molten salt mixture to 300-400℃ at a cooling rate of 10-30℃ / min. The secondary cooler, which uses natural cooling or forced air cooling, is used to further cool the material to 100-200℃; Mechanical separators, such as vibrating screens or air classifiers, are used to separate solidified molten salt media from pyrolysis residues. Mechanical separators are equipped with molten salt media outlets and coarse residue outlets. The water washing separation tank is connected to the coarse residue outlet of the mechanical separator. The water washing separation tank is equipped with an agitator for washing the coarse residue to dissolve the residual molten salt medium. The water washing separation tank has a final residue outlet and a washing liquid outlet, and the washing liquid outlet is connected to the water washing unit.

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