A multi-alkali method for grading treatment of tail gas from airflow drying of artificial board

Abstract

The application discloses a kind of artificial board airflow drying tail gas multi-base method grading processing method, it is related to artificial board industry waste gas processing technical field, aiming at the condition that artificial board airflow drying tail gas temperature is higher, and contains wood fiber dust and adhesive mist drop, and the problems that drying tail gas contains formaldehyde, phenolic, methane carbon hydrogen compound, carbon monoxide and other multiple VOCs components, with dust collector, cooling tower is preprocessed to tail gas, remove particulate matter in tail gas and cooling, then adopt circulating lye countercurrent spray multi-stage spray to remove VOCs and particulate matter in tail gas, different spray area is sprayed with multiple different formula alkali agent and VOCs and occurs multiple chemical reactions to produce different precipitates, improve the removal efficiency of each component in VOCs, reduce VOCs export concentration to meet emission requirements;The product after processing is used as fuel to participate in boiler combustion, releases heat for production and utilization, compared with RTO catalytic combustion system of burning natural gas, reduce operating processing cost.

Description

Technical Field

[0001] This invention relates to the field of waste gas treatment technology in the wood-based panel industry, specifically to a multi-alkali graded treatment method for exhaust gas from airflow drying of wood-based panels. Background Technology

[0002] With increasingly stringent national environmental protection requirements, the emission standards for pollutants from the wood-based panel industry are constantly being raised. According to the "Emission Standard for Pollutants from Wood-based Panel Industry" (draft for comments), the concentration of particulate matter in the exhaust gas from wood-based panel drying needs to be reduced from 120 mg / m³ to 10 mg / m³, formaldehyde from 25 mg / m³ to 5 mg / m³, phenols from 100 mg / m³ to 10 mg / m³, and non-methane total hydrocarbons from 120 mg / m³ to 60 mg / m³.

[0003] In the production of engineered wood products, the airflow drying process is one of the main sources of VOCs emissions. The VOCs emissions from the drying stages of fiberboard and particleboard account for approximately 75% of the total emissions from these enterprises. Due to the complex composition of the drying exhaust gas, which contains various VOCs such as formaldehyde, phenols, methane hydrocarbons, and carbon monoxide, and the significant fluctuations in VOC concentrations (ranging from 2000-8000 mg / m³) caused by production process variations, coupled with the relatively high temperature (80-120℃) and the presence of wood fiber dust and adhesive droplets, the removal efficiency of the complex VOCs components in the engineered wood products industry is low, making it difficult to meet emission standards for VOCs outlet concentrations. Summary of the Invention

[0004] The main objective of this invention is to provide a multi-alkali graded treatment method for exhaust gas from airflow drying of engineered wood products, which addresses the problems mentioned in the background section.

[0005] To achieve the above objectives, the present invention provides a multi-alkali graded treatment method for exhaust gas from airflow drying of engineered wood products, comprising the following steps: S1: Dust removal and cooling. The exhaust gas from the airflow drying of the artificial board is introduced into the dust collector to remove particulate matter, and then cooled by the cooling tower to further remove fine particulate matter. S2: Spray demisting. The exhaust gas after dust removal and cooling enters from the bottom of the spray tower and passes through the first-stage spray and the second-stage spray in sequence. The first-stage spray uses circulating alkaline solution in countercurrent spray. The second-stage spray uses fresh alkaline solution in countercurrent spray. The exhaust gas after spray absorption is demisted by the demister and discharged from the top of the tower. S3: Multi-alkali reaction: Collect the spray absorption liquids from the first-stage spray and the second-stage spray respectively, and pump them into the multi-alkali mixing reaction tank; add various alkali agents according to the VOCs concentration in the absorption liquid; S4: Solid-liquid-gas separation. After the reaction is completed, the reaction liquid is pumped into the sedimentation tank for settling. The precipitate is dehydrated by the solid-liquid separator and sent to the storage silo. The supernatant is returned to the first-stage spray area for recycling. After recycling, it is treated as waste liquid or added to the reaction tank. The gaseous VOCs released during the reaction are collected by the gas collection hood and transported to the boiler furnace through pipeline. S5: Fuel utilization, solid sediment in the storage silo is continuously or intermittently fed into the boiler via a screw conveyor; inside the boiler, the solid sediment is burned together with the released gaseous VOCs, and the heat released by the combustion is used for boiler steam production, which is used in the artificial board production process; the ash residue after combustion is used as building material raw material or landfill disposal. In step S1, the exhaust gas from the artificial board airflow drying process is cooled to 50-70°C via a cooling tower. In step S2, the first-stage spray zone uses circulating alkaline solution with a pH of 8-9, and the second-stage spray zone uses fresh alkaline solution with a pH of 10-12. In step S3, the concentration of VOCs in the absorbent is determined by COD or TOC; various alkaline agents include calcium hydroxide, magnesium oxide, sodium carbonate, and sodium hydroxide; the reaction temperature is 40℃-60℃, and the stirring time is 20-40 minutes; In step S4, the reaction liquid is pumped into a sedimentation tank and allowed to stand for 1-2 hours for gravity sedimentation. The water content of the precipitate after dehydration by the solid-liquid separator is <30%. The pH value of the supernatant to be reused is 8-9. The supernatant is recycled 5-10 times. In step S5, the combustion temperature is controlled at 850-1000℃.

[0006] As a further improvement of the present invention, a cyclone dust collector is used in step S1, with a processing air volume of 55,000 m³ / h, a dust removal efficiency of 85%, and an outlet particulate matter concentration of 120 mg / m³; the cooling tower has a cooling water volume of 100 m³ / h and an outlet exhaust gas temperature of 60°C.

[0007] As a further improvement of the present invention, a multi-stage spray tower is used in step S2, wherein a first-stage spray zone and a second-stage spray zone are arranged sequentially from bottom to top in the spray tower.

[0008] As a further improvement of the present invention, the circulating alkaline solution formula used in the first-stage spray zone in step S2 is calcium hydroxide solution (concentration 5-10 g / L) + magnesium oxide (concentration 2-5 g / L) + surfactant (concentration 0.1-0.5 g / L).

[0009] As a further improvement of the present invention, the fresh alkaline solution used in the second-stage spray zone in step S2 is sodium hydroxide solution (concentration 10-20 g / L) + sodium carbonate (concentration 5-10 g / L).

[0010] As a further improvement of the present invention, in step S3, the multi-alkali mixing reaction tank is equipped with a first alkali inlet, a second alkali inlet, multiple sets of alkali agent addition ports, a stirring device, and a reaction temperature control device; the first alkali inlet is connected to the absorbent outlet of the first-stage spray zone; the second alkali inlet is connected to the absorbent outlet of the second-stage spray zone; the multiple sets of alkali agent addition ports are respectively used to add calcium hydroxide, magnesium oxide, sodium carbonate, and sodium hydroxide.

[0011] As a further improvement of the present invention, the ratio of alkali in the multi-alkali mixing reaction vessel in step S3 is: calcium hydroxide: magnesium oxide: sodium carbonate: sodium hydroxide = 10:3:2:1 (mass ratio).

[0012] As a further improvement of the present invention, the liquid-to-gas ratio in the spray tower in step S2 is 1:600-800 (volume ratio), and the gas residence time in the spray tower is 3-5 seconds.

[0013] As a further improvement of the present invention, the reaction time in the multi-alkali mixing reaction vessel in step S3 is 20-40 minutes; the stirring speed of the stirring device is 60-100 rpm.

[0014] As a further improvement of the present invention, it also includes an online monitoring unit and an automatic control unit; the online monitoring unit is used to monitor in real time the concentration and composition of VOCs in the inlet air, the concentration of VOCs in the outlet air, the pH value and VOCs concentration of the spray liquid, and the temperature and pH value of the reaction tank; the automatic control unit automatically adjusts the alkaline spray volume and pH value, the amount of alkaline agent added, and the reaction temperature according to the online monitoring data.

[0015] The beneficial effects of this invention are reflected in: To address the issue of high-temperature exhaust gas from the airflow drying of engineered wood products, which contains wood fiber dust and adhesive droplets, a dust collector and cooling tower are used for pretreatment to remove particulate matter and lower the temperature. A first-stage spraying process using a circulating alkaline solution in counter-current flow removes acidic and water-soluble VOCs, while a second-stage spraying process using fresh alkaline solution in counter-current flow absorbs residual VOCs and particulate matter. Different alkaline formulations are added to the absorbent solution in different spraying zones to induce multiple chemical reactions with VOCs, producing various precipitates. This addresses the problem of removing multiple VOCs, including formaldehyde, phenols, methane hydrocarbons, and carbon monoxide, from the dried exhaust gas, improving the removal efficiency of each component and reducing the VOC outlet concentration to meet emission requirements. The entire system is simple and reliable, requiring no additional energy consumption. Furthermore, the treated products can be used as fuel in boiler combustion, releasing heat for production use. Compared to RTO catalytic combustion systems that burn natural gas, this reduces operating costs. Attached Figure Description

[0016] Figure 1 This is an overall schematic diagram of a multi-alkali graded treatment method for exhaust gas from airflow drying of engineered wood products according to the present invention. Explanation of reference numerals in the attached figures: 1. Cyclone dust collector; 2. Cooling tower; 3. Spray tower; 4. First-stage spray zone; 5. Second-stage spray zone; 6. Demister; 7. Multi-alkali mixing reaction tank; 8. First alkali inlet; 9. Second alkali inlet; 10. Multiple alkali additive inlet; 11. Stirring device; 12. Reaction temperature control device; 13. Sedimentation tank; 14. Solid-liquid separator; 15. Supernatant recovery pipeline; 16. Gas collection hood; 17. Conveying pipeline; 18. Boiler; 19. Screw conveyor; 20. Storage silo. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the described embodiments are merely some, not all, of the embodiments of this invention. Unless otherwise specified, the embodiments and features described in this application can be combined with each other. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0018] In one embodiment, see Figure 1 The present invention provides a multi-alkali graded reaction treatment system for exhaust gas from airflow drying of engineered wood products, comprising the following components connected in sequence: (1) Pretreatment unit: including cyclone dust collector and cooling tower, used to remove large particulate matter in dry exhaust gas and reduce the exhaust gas temperature to 50-70℃; (2) Spray absorption unit: includes multi-stage spray towers, which are arranged sequentially from bottom to top inside the towers: First-stage spray zone: Circulating alkaline solution (pH 8-9) is used for spraying, mainly to remove acidic VOCs and water-soluble VOCs from the exhaust gas; Second-stage spray zone: Fresh alkaline solution (pH 10-12) is used for spraying to further absorb residual VOCs and particulate matter; Demisting area: Equipped with a demister to remove liquid droplets from the exhaust gas; (3) Multi-base reaction unit: including a multi-base mixing reaction vessel, the vessel is equipped with: First alkaline solution inlet: connected to the absorbent outlet of the first-stage spray zone; Second alkaline solution inlet: connected to the absorbent outlet of the second-stage spray zone; Multiple alkali addition ports: used for adding alkalis such as calcium hydroxide, magnesium oxide, sodium carbonate, and sodium hydroxide; Stirring device: to ensure that the alkaline solution reacts fully with the absorbed VOCs; Reaction temperature control device: maintains the reaction temperature at 40-60℃; (4) Precipitation separation unit: including a sedimentation tank and a solid-liquid separator, used to separate reaction products: Sedimentation tank: Uses gravity settling to separate solid precipitates generated during the reaction; Solid-liquid separator: Uses pressure filtration or centrifugation to dehydrate the precipitate to a moisture content of <30%; Supernatant recovery pipeline: Returns the supernatant to the spray absorption unit for recycling; (5) Gas collection unit: including gas collection hood and conveying pipeline, used to collect gaseous VOCs released during the reaction process and transport them to the boiler for combustion; (6) Fuel conveying unit: including screw conveyor and storage bin, used to convey dehydrated solid precipitate to boiler as auxiliary fuel.

[0019] The working principle of the multi-base reaction unit is as follows: The VOCs (mainly formaldehyde, phenols, alcohols, aldehydes, etc.) in the absorbent solution react with various alkaline agents as follows: Formaldehyde reacts with calcium hydroxide to form calcium formate precipitate: HCHO + Ca(OH)2 → HCOOCa↓ + H2O Phenolic compounds react with sodium hydroxide to form sodium phenoxide: C6H5OH + NaOH → C6H5ONa + H2O Organic acids react with sodium carbonate to produce sodium salts of organic acids and CO2: RCOOH + Na2CO3 → RCOONa + H2O + CO2↑ Some sparingly soluble VOCs are oxidized and decomposed under strongly alkaline conditions. The precipitate produced by the reaction mainly consists of calcium formate, organic acid salts, and calcified waste residue, with a calorific value of approximately 3000-5000 kcal / kg, which can be used as boiler fuel.

[0020] Preferably, the circulating alkaline solution formula used in the first-stage spray zone is: calcium hydroxide solution (concentration 5-10 g / L) + magnesium oxide (concentration 2-5 g / L) + surfactant (concentration 0.1-0.5 g / L). Preferably, the fresh alkaline solution used in the second-stage spray zone is formulated as follows: sodium hydroxide solution (concentration 10-20 g / L) + sodium carbonate (concentration 5-10 g / L). Preferably, the alkali ratio in the multi-alkali mixing reaction vessel is: calcium hydroxide: magnesium oxide: sodium carbonate: sodium hydroxide = 10:3:2:1 (mass ratio). Preferably, the liquid-to-gas ratio of the spray absorption unit is 1:600-800 (volume ratio), and the gas residence time in the spray tower is 3-5 seconds; Preferably, the reaction time of the multi-base reaction unit is 20-40 minutes, and the stirring speed is 60-100 rpm; Preferably, the system further includes an online monitoring unit for real-time monitoring: Intake VOCs concentration and composition; VOCs concentration in exhaust gas; pH value and VOCs concentration of the spray solution; Reactor temperature and pH; Preferably, the system further includes an automatic control unit that automatically adjusts based on online monitoring data: Alkali spray volume and pH value; Alkali addition amount; Reaction temperature; The method for treating the exhaust gas from the airflow drying process of the above-mentioned system includes the following steps: Step 1: Preprocessing The exhaust gas from the airflow drying process of the engineered wood products is introduced into a cyclone dust collector to remove 80-90% of the particulate matter (particle size > 10μm). The temperature is reduced to 50-70℃ in the cooling tower, while further removing fine particulate matter; Step 2: Spray absorption The pretreated exhaust gas enters from the bottom of the spray tower and passes through the first-stage spray zone and the second-stage spray zone in sequence; The first-stage spray zone uses a counter-current spray of circulating alkaline solution (pH 8-9) to absorb 60-70% of VOCs. The second-stage spray zone uses fresh alkaline solution (pH 10-12) for countercurrent spraying to absorb 20-25% of VOCs. After being demisted by the demister, the purified exhaust gas is discharged from the top of the tower, with a total VOCs removal rate of 85-90%. Step 3: Multi-base reaction The first and second stage spray absorption liquids were collected separately and pumped into the multi-alkali mixed reaction tank. Based on the VOCs concentration in the absorbent (determined by COD or TOC), add various alkaline agents according to the formula; Control the reaction temperature at 40-60℃ and stir the reaction for 20-40 minutes; During the reaction, VOCs in the absorbent undergo multiple chemical reactions with the alkaline agent: a) Formaldehyde reacts with calcium hydroxide to form calcium formate precipitate; b) Phenolics react with sodium hydroxide to form sodium phenolate, which then reacts further with calcium salts to form calcium salt precipitate; c) Alcohols, aldehydes, etc., are oxidized and decomposed under strongly alkaline conditions; d) Some gaseous VOCs are released from the liquid phase. Step 4: Solid-liquid-gas separation After the reaction is complete, the reaction solution is pumped into a sedimentation tank and allowed to stand for 1-2 hours for gravity settling. The precipitate is dehydrated to a moisture content of <30% by a solid-liquid separator and then sent to a storage silo. The pH value of the supernatant should be tested (it should be maintained at 8-9), and then returned to the first-stage spray area for recycling. After 5-10 cycles (judged according to the cumulative concentration of VOCs), it should be treated as waste liquid or added to the reaction tank. The gaseous VOCs (mainly methane, CO2, etc.) released during the reaction are collected by a gas collection hood and transported to the boiler furnace through pipelines; Step 5: Fuel Utilization Solid sediments (mainly calcium formate, organic acid salts, and calcified waste residue) in the storage silo are continuously or intermittently fed into the boiler via a screw conveyor. Inside the boiler, solid precipitates and precipitated gaseous VOCs combust together, undergoing the following reactions: a) Combustion of organic matter: C n H m O x a) Combustion of calcium formate: (HCOO)₂Ca → CaO + CO₂ + CO + H₂O + heat c) Combustion of carbides: C + O₂ → CO₂ + heat The combustion temperature is controlled at 850-1000℃ to completely decompose and oxidize VOCs, generating CO2 and water vapor; The heat released by combustion (approximately 3000-5000 kcal / kg of precipitate) is used to produce boiler steam for use in the production process of engineered wood products. The ash residue after combustion (mainly inorganic oxides such as CaO and MgO) is used as building material raw material or disposed of in landfills.

[0021] Preferably, in step 3, the total amount of alkali added to the multi-alkali mixing reaction vessel is calculated according to the following formula: Mbase = CVOCs × Vliquid × Kreaction × αexcess in: Malkali: Total mass of alkali (kg) C_VOCs: Concentration of VOCs (mg / L) in the absorption solution, determined by COD or TOC. V_liquid: Volume of absorbent liquid (L) K-reaction: The reaction equivalence coefficient, determined based on VOCs composition, with a typical value of 1.2-1.5. α excess: Excess coefficient, typically 1.1-1.3. Preferably, in step 4, the conditions for recycling the supernatant are as follows: Maintain the pH value between 8 and 9; COD < 1000 mg / L; Suspended solids <50 mg / L; when the above index is exceeded, fresh alkali solution must be added or the solution must be treated as waste liquid.

[0022] Preferably, in step 5, the feeding rate of the solid precipitate is controlled in the following manner: Calculate the feeding rate based on the boiler heat load and the calorific value of the precipitate; Use continuous or intermittent feeding methods to avoid boiler temperature fluctuations; Feeding rate: F = Q_boiler × (1 - η_coal) / (Q_sediment × η_combustion) Where: F: Sediment feed rate (kg / h) Q: Boiler: Rated heat load of the boiler (kcal / h) η_coal: thermal efficiency of coal combustion Q: Precipitate: Calorific value of the precipitate (kcal / kg) η combustion: Precipitate combustion efficiency (typical value 0.85-0.90).

[0023] Example 1: Treatment of drying exhaust gas from a medium-density fiberboard production line A medium-density fiberboard (MDF) manufacturer has a drying exhaust gas volume of 50,000 m³ / h, a temperature of 100℃, a VOCs concentration of 3,500 mg / m³ (of which formaldehyde accounts for 40%, phenols for 25%, methane hydrocarbons for 30%, and others for 5%), and a particulate matter concentration of 800 mg / m³.

[0024] The system of this invention is used for processing, and the specific parameters are as follows: (1) Preprocessing unit Cyclone dust collector: handles an air volume of 55,000 m³ / h, has a dust removal efficiency of 85%, and an outlet particulate matter concentration of 120 mg / m³; Cooling tower: cooling water flow rate 100 m³ / h, outlet exhaust gas temperature 60℃.

[0025] (2) Spray absorption unit Spray tower: 3.5m in diameter, 12m in height, and 8m in packing layer height (using polypropylene saddle ring packing). First-stage spray zone: circulating alkaline solution flow rate 85 m³ / h, pH 8.5, number of nozzles 24 (spiral nozzles). Circulating alkali solution formula: Ca(OH)₂ 8 g / L + MgO 3 g / L + surfactant 0.3 g / L; Second-stage spray zone: Fresh alkaline solution flow rate 25 m³ / h, pH 11, number of nozzles 12; Fresh alkali solution formula: NaOH 15 g / L + Na2CO3 8 g / L; Demister: Plate type demister, demisting efficiency 98%; Liquid-to-gas ratio: (85+25) / 50 = 2.2 m³ liquid / 1000 m³ gas; Dwell time: 4.2 seconds; VOCs removal rates: Level 1: 65%, Level 2: 23%, Total removal rate: 88%.

[0026] (3) Multi-base reaction unit Multi-base mixing reaction vessel: volume 15 m³ (effective volume 12 m³), ​​stirring speed 80 rpm; First-stage absorbent: VOCs concentration 2270 mg / L (measured by TOC), flow rate 2.5 m³ / h; Second-stage absorbent: VOCs concentration 420 mg / L, flow rate 0.8 m³ / h; Alkali dosage (calculated using the formula): Malkali = (2270 × 2500 + 420 × 800) × 1.35 × 1.2 / 1000 = 10.8 kg / h Alkali ratio: Ca(OH)2:MgO:Na2CO3:NaOH = 10:3:2:1 (mass ratio) Actual dosage: Ca(OH)₂ 6.75 kg / h, MgO 2.0 kg / h, Na₂CO₃ 1.35 kg / h, NaOH 0.7 kg / h Reaction temperature: 50℃ (controlled by jacket cooling water); Reaction time: 30 minutes; The COD of the liquid after the reaction decreased from the initial 5800 mg / L to 850 mg / L, with a removal rate of 85%.

[0027] (4) Solid-liquid-gas separation unit Sedimentation tank: volume 20 m³, retention time 1.5 hours; Precipitate yield: 8.5 kg / h (dry basis); Solid-liquid separator: Plate and frame filter press, moisture content after dewatering is 28%, wet basis output is 11.8 kg / h; Supernatant: pH 8.6, COD 850 mg / L, returned to the first-stage spray area for recycling (replaced after 7 cycles); Evolved gas: flow rate 80 m³ / h, main components CH4 45%, CO2 40%, others 15%.

[0028] (5) Fuel Utilization Unit Solid precipitate: calorific value 4200 kcal / kg (dry basis), feed rate 8.5 kg / h (dry basis); Gas released: calorific value 3500 kcal / m³, feed flow rate 80 m³ / h; Total heat recovery: 4200×8.5 + 3500×80 / 1000 = 35700+280000 = 315700 kcal / h = 1.32×10 6 kJ / h; Boiler rated heat load: 10 t / h steam boiler, heat load 6×10 6 kcal / h; The system recovers heat at a rate of 315700 / 6000000 = 5.3% of the boiler's heat load, saving approximately 120 kg / h of standard coal.

[0029] Processing effect: VOCs concentration at the outlet: 15 mg / m³ (concentration after spraying: 420 mg / m³, removal rate after combustion: 96.4%). Outlet particulate matter concentration: 8 mg / m³; Export formaldehyde concentration: 1.5 mg / m³; Total VOCs removal rate: (3500-15) / 3500 = 99.6%.

[0030] Economic benefits: Initial investment: 850,000 yuan for equipment, 150,000 yuan for installation, totaling 1 million yuan; Annual operating costs: Electricity consumption: 120 kW × 7200 h × 0.6 yuan / kWh = 518,000 yuan Alkali agent: 10.8 kg / h × 7200 h × 2 yuan / kg = 156,000 yuan Water consumption: 5 m³ / h × 7200 h × 3 yuan / m³ = 108,000 yuan Maintenance: 50,000 yuan Total: 832,000 yuan Annual energy savings: Savings in standard coal: 120 kg / h × 7200 h = 864 tons / year Value: 864 × 800 yuan / ton = 691,000 yuan Annual net operating cost: 83.2 - 69.1 = 14.1 million yuan Compared to the RTO system (annual operating cost of approximately 1.4 million yuan), it saves 1.259 million yuan annually. Investment payback period: 100 / (140-14.1) = 0.8 years.

[0031] This invention is particularly suitable for the following scenarios: (1) Wood-based panel industry: exhaust gas treatment for airflow drying of medium density fiberboard, particleboard, plywood and other production lines; (2) Timber processing industry: VOCs treatment in timber drying and hot pressing sections; (3) Furniture manufacturing industry: VOCs treatment in the painting and drying process (pretreatment is required); (4) Paper industry: Pulp drying exhaust gas treatment; (5) Other industrial exhaust gases containing water-soluble VOCs.

[0032] This invention has the following advantages: 1) Significantly reduce investment costs It eliminates the high investment in RTO equipment (heat storage medium, high temperature fan, switching valve, control system, etc.), and the equipment cost is only 1 / 5 to 1 / 10 of that of RTO. The system has a simple structure, and the main equipment consists of conventional chemical equipment (spray towers, reaction tanks, sedimentation tanks, etc.), requiring no high-temperature corrosion-resistant materials. It occupies a small area and has low construction costs.

[0033] 2) Extremely low operating costs, achieving energy saving and efficiency improvement. Zero fuel consumption: No auxiliary fuels such as natural gas and diesel are required, saving 1-1.5 million yuan in fuel costs per year compared to RTO; Low power consumption: Only equipment such as spray pumps and mixers consume power, and the power consumption is only 1 / 10 of that of RTO; Energy recovery: The treated precipitate and released gas are used as boiler fuel with a calorific value of 3000-5000 kcal / kg. The recoverable energy accounts for 60-80% of the total energy of VOCs in the dried tail gas. The alkali solution is recycled, the alkali consumption is low, and the annual operating cost is only 1 / 10 to 1 / 20 of that of RTO.

[0034] 3) High treatment efficiency and emissions meet standards. The total VOCs removal rate reaches 85-90% (spray absorption) + 95-98% (combustion) = 99.3-99.8%; The VOCs concentration at the outlet is <20 mg / m³, far below the national standard requirement (60 mg / m³); the particulate matter removal rate is >95%, and the outlet concentration is <10 mg / m³; the formaldehyde removal rate is >98%, and the outlet concentration is <2 mg / m³.

[0035] 4) Specifically designed for the engineered wood products industry, highly adaptable Specifically designed for the characteristics of exhaust gas from the drying process of engineered wood products (including particulate matter, sticky substances, concentration fluctuations, etc.); It has low requirements for intake conditions and can handle exhaust gas containing particulate matter and temperature fluctuations. It can handle a wide range of air volumes (10,000-100,000 m³ / h) and can adapt to production lines of different sizes. The system is easy to operate, requires no professional technicians, and is easy to maintain.

[0036] 5) High security It operates at normal temperature and pressure, with no risk of high-temperature combustion or explosion. It uses water-based alkaline solution for absorption, eliminating the risk of fire. It contains no toxic catalysts or adsorbents, and there is no problem of poisoning or inactivation.

[0037] 6) Environmentally friendly, with no secondary pollution It has a high combustion temperature (850-1000℃), and VOCs are completely decomposed and oxidized into CO2 and H2O; No NOx is generated (because high-temperature air is not used for combustion); No risk of dioxin formation (no high-temperature synthesis conditions for chlorine-based VOCs); Ash slag is an inorganic oxide (CaO, MgO, etc.) and can be used as a building material raw material.

[0038] 7) Achieve a circular economy VOCs can be transformed from pollutants into energy, thus turning waste into treasure; The alkali solution is recycled, reducing wastewater discharge; Solid precipitates can be used as fuel, reducing coal consumption; Ash slag is used as a building material raw material, achieving zero emissions.

[0039] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method, characterized in that, Includes the following steps: S1: Dust removal and cooling. The exhaust gas from the artificial board airflow drying is introduced into the dust collector to remove particulate matter. Then it passes through the cooling tower (2) to cool down and further remove tiny particulate matter. S2: Spraying and demisting, the exhaust gas after dust removal and cooling enters from the bottom of the spray tower (3) and passes through the first stage spray and the second stage spray in sequence; the first stage spray uses circulating alkaline solution countercurrent spray; the second stage spray uses fresh alkaline solution countercurrent spray; the exhaust gas after spraying and absorption is demisted by the demister (6) and discharged from the top of the tower. S3: Multi-alkali reaction: Collect the spray absorption liquids from the first-stage spray and the second-stage spray respectively, and pump them into the multi-alkali mixing reaction tank (7); add various alkali agents according to the VOCs concentration in the absorption liquid; S4: Solid-liquid-gas separation. After the reaction is completed, the reaction liquid is pumped into the sedimentation tank (13) for static settling. The precipitate is dehydrated by the solid-liquid separator (14) and sent to the storage silo (20). The supernatant is returned to the first-stage spray zone (4) for recycling. After recycling, it is treated as waste liquid or added to the reaction tank. The gaseous VOCs released during the reaction are collected by the gas collection hood (16) and transported to the boiler (18) furnace through the pipeline. S5: Fuel utilization, solid sediment in storage bin (20) is continuously or intermittently fed into boiler (18) via screw conveyor (19); in boiler (18), solid sediment is burned together with precipitated gaseous VOCs, and the heat released by combustion is used for boiler (18) steam production for use in artificial board production process; the ash residue after combustion is used as building material raw material or landfill disposal. In step S1, the exhaust gas from the artificial board airflow drying process is cooled to 50-70°C by a cooling tower (2); In step S2, the first-stage spray zone (4) uses a circulating alkaline solution with a pH of 8-9, and the second-stage spray zone (5) uses a fresh alkaline solution with a pH of 10-12. In step S3, the concentration of VOCs in the absorbent is determined by COD or TOC; various alkaline agents include calcium hydroxide, magnesium oxide, sodium carbonate, and sodium hydroxide; the reaction temperature is 40℃-60℃, and the stirring time is 20-40 minutes; In step S4, the reaction liquid is pumped into the sedimentation tank (13) and left to stand for 1-2 hours for gravity sedimentation. The water content of the precipitate after dehydration by the solid-liquid separator (14) is <30%. The pH value of the supernatant to be reused is 8-9. The supernatant is recycled 5-10 times. In step S5, the combustion temperature is controlled at 850-1000℃.

2. The method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method according to claim 1, characterized in that: In step S1, a cyclone dust collector (1) is used, with a processing air volume of 55,000 m³ / h, a dust removal efficiency of 85%, and an outlet particulate matter concentration of 120 mg / m³; the cooling tower (2) has a cooling water volume of 100 m³ / h and an outlet exhaust gas temperature of 60℃.

3. The method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method according to claim 2, characterized in that: In step S2, a multi-stage spray tower (3) is used, and the first-stage spray zone (4) and the second-stage spray zone (5) are set up from bottom to top in the spray tower (3).

4. The method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method according to claim 3, characterized in that: In step S2, the circulating alkaline solution used in the first-stage spray zone (4) is calcium hydroxide solution (concentration 5-10 g / L) + magnesium oxide (concentration 2-5 g / L) + surfactant (concentration 0.1-0.5 g / L).

5. The method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method according to claim 4, characterized in that: In step S2, the fresh alkaline solution used in the second-stage spray zone (5) is sodium hydroxide solution (concentration 10-20 g / L) + sodium carbonate (concentration 5-10 g / L).

6. The method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method according to claim 5, characterized in that: In step S3, the multi-alkali mixing reaction tank (7) is equipped with a first alkali inlet (8), a second alkali inlet (9), multiple alkali agent addition ports, a stirring device (11), and a reaction temperature control device (12); the first alkali inlet (8) is connected to the absorbent outlet of the first-stage spray zone (4); the second alkali inlet (9) is connected to the absorbent outlet of the second-stage spray zone (5); the multiple alkali agent addition ports are used to add calcium hydroxide, magnesium oxide, sodium carbonate, and sodium hydroxide, respectively.

7. The method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method according to claim 6, characterized in that: In step S3, the ratio of alkali in the multi-alkali mixing reaction vessel (7) is: calcium hydroxide: magnesium oxide: sodium carbonate: sodium hydroxide = 10:3:2:1 (mass ratio).

8. The method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method according to claim 7, characterized in that: In step S2, the liquid-to-gas ratio in the spray tower (3) is 1:600-800 (volume ratio), and the gas residence time in the spray tower (3) is 3-5 seconds.

9. The method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method according to claim 8, characterized in that: The reaction time in the multi-alkali mixing reaction vessel (7) in step S3 is 20-40 minutes; the stirring speed of the stirring device (11) is 60-100 rpm.

10. The method for graded treatment of exhaust gas from airflow drying of engineered wood products using a multi-alkali method according to claim 9, characterized in that: It also includes an online monitoring unit and an automatic control unit; the online monitoring unit is used to monitor the concentration and composition of VOCs in the inlet air, the concentration of VOCs in the outlet air, the pH value and VOCs concentration of the spray liquid, and the temperature and pH value of the reaction tank in real time; the automatic control unit automatically adjusts the alkaline spray volume and pH value, the amount of alkaline agent added, and the reaction temperature based on the online monitoring data.

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