An integrated process and system for zero discharge of desulfurization wastewater

Abstract

This invention discloses an integrated process for zero-discharge of desulfurization wastewater, belonging to the field of industrial wastewater treatment technology. This process organically couples four units: three-tank pretreatment, conditioning and concentration, emulsification modification, and drying and solidification. After neutralization sedimentation, deep heavy metal removal, and flocculation separation, the desulfurization wastewater is evaporated and concentrated using waste heat from boiler flue gas. It then undergoes emulsification modification to improve its physicochemical properties, and finally solidifies through flue gas spraying or fluidized bed drying. The solid products are recycled for closed-loop disposal. This invention effectively solves the problems of limited heavy metal removal, severe equipment corrosion and scaling, high energy consumption, and unstable operation associated with traditional processes by using staged pollution removal, precise chlorine removal and pH adjustment, emulsification to prevent scaling, and full-process waste heat utilization.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment technology, specifically relating to an integrated process and system for zero discharge of desulfurization wastewater. Background Technology

[0002] The desulfurization wastewater generated by the wet desulfurization system of coal-fired power plants has typical characteristics such as high turbidity, high hardness, high salt content, high chloride ion content, weak acidity, and a variety of heavy metals. It is one of the most difficult terminal wastewaters to treat in power plants and a key bottleneck restricting the zero discharge of wastewater from the entire plant.

[0003] Current mainstream desulfurization wastewater treatment mostly employs the traditional triplet process, which can remove most suspended solids and some heavy metals. However, its effectiveness in removing difficult-to-settle heavy metals such as mercury, cadmium, and lead is limited. The effluent still contains fine suspended solids and supersaturated salts, which can easily cause membrane fouling, evaporation scaling, and equipment corrosion if directly fed into subsequent units. To achieve volume reduction and concentration, technologies such as forward osmosis, membrane distillation, and MVR are often used, but these have drawbacks such as high investment, high energy consumption, poor resistance to fouling, and complex operation and maintenance. Terminal solidification often uses evaporation crystallization or flue gas spraying, which generally suffers from scaling, severe corrosion, and poor operational stability.

[0004] Current technologies generally design pretreatment, concentration, and solidification as separate, independent processes, lacking overall process synergy and optimization. This makes it difficult to simultaneously meet the industrial demands for deep purification, low energy consumption, corrosion and scaling resistance, and long-term stable operation. Therefore, developing an integrated, low-cost, highly reliable, and fully synergistic zero-discharge process for desulfurization wastewater has become an urgent need in this field. Summary of the Invention

[0005] The purpose of this invention is to provide an integrated process for zero discharge of desulfurization wastewater. Through the coupling of four stages—pretreatment in a three-stage tank, conditioning and concentration, emulsification modification, and drying and solidification—it achieves deep decontamination, waste heat utilization, corrosion and scale prevention, and solid waste closed-loop management, truly achieving zero wastewater discharge and long-term stable operation of the system.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0007] An integrated process for zero discharge of desulfurization wastewater is characterized by comprising a three-tank pretreatment system, a conditioning and concentration system, an emulsification system, and a drying and solidification system connected sequentially by pipelines along the wastewater flow direction. The process flow is as follows:

[0008] (1) Pretreatment of triple container

[0009] The original desulfurization wastewater (pH 4-6) enters the neutralization tank, and lime milk or sodium hydroxide is added to adjust the pH to 9.0-9.5, so that heavy metals form hydroxide precipitates, and Ca²⁺ and F⁻ form CaF₂ precipitate.

[0010] The neutralized effluent enters the sedimentation tank, where organic sulfur agents are added to react with heavy metals such as Hg, Ag, and Pb to form sulfide precipitates with extremely low solubility, thus achieving deep removal of heavy metals.

[0011] It then enters the flocculation box, where iron or aluminum salt flocculants are added first, followed by polyacrylamide (PAM) coagulant aid to form dense flocs;

[0012] The flocculant undergoes solid-liquid separation in a clarifier. The supernatant enters the subsequent system, the bottom sludge is dewatered and transported off-site, and the filtrate is returned to the front end of the system for recycling.

[0013] (2) Conditioning and Concentration

[0014] The pretreated supernatant is sent to the concentration tower, where the 130°C flue gas after the boiler induced draft fan is used as a heat source. It is then pumped to the top spray layer of the tower for atomized spraying to increase the gas-liquid contact area and achieve evaporation and concentration.

[0015] A demister is installed at the top of the tower to capture mist droplets and slurry entrained in the flue gas, preventing material loss and pollution from the flue gas.

[0016] The concentrate at the bottom of the tower enters the conditioning tank, where a dechlorinating agent is added through a chloride ion detector and an automatic dosing system to reduce the chloride ion concentration. Then, the pH is adjusted to 8±0.2 with dilute hydrochloric acid, completing the integrated conditioning process of dechlorination, pH adjustment, and clarification.

[0017] (3) Emulsification modification

[0018] After conditioning, the concentrated liquid is sent to an emulsification tank, a special emulsifier is added, and the mixture is stirred to form a stable emulsion. The pH is then adjusted to 7-8. Emulsification modification can significantly reduce the tendency of scaling and ash accumulation in the subsequent drying process, while also reducing acid corrosion of the equipment.

[0019] (4) Drying and curing

[0020] The emulsion is atomized and sent into the flue or fluidized bed, where it is instantly dried and evaporated using the waste heat of the 300℃ flue gas. All the solid ash / sludge produced during drying is returned to the triplet pretreatment system and treated together with the sludge, achieving a closed-loop zero-discharge system with no wastewater or waste residue.

[0021] Compared with the prior art, the present invention has the following significant advantages:

[0022] I. Preprocessing Unit: Reduces the load on subsequent systems from the source, laying the foundation for stable operation.

[0023] Staged sedimentation achieves deep decontamination

[0024] First, the pH is adjusted to weakly alkaline using an alkaline method, allowing most heavy metals and fluorides to be removed through co-precipitation of hydroxides and fluorides. Then, organic sulfur is added to react with difficult-to-precipitate heavy metals such as Hg, Cd, and Pb, forming sulfide precipitates with lower solubility products. This two-stage precipitation process increases the heavy metal removal rate from 65%-75% in traditional processes to ≥90%, while ensuring stable fluoride levels and preventing secondary pollution caused by heavy metal accumulation in subsequent concentration / drying units.

[0025] Highly efficient flocculation reduces suspended solids and hardness

[0026] Flocculant charge neutralization and coagulant adsorption bridging rapidly aggregate fine particles, colloids, and calcium and magnesium carbonates into large-diameter flocs. SS is reduced from tens of thousands of mg / L to ≤70 mg / L, and the total hardness removal rate is ≥93%, eliminating the core causes of suspended solids abrasion and calcium and magnesium scaling at the source, protecting spray nozzles, heat exchangers, and flues from clogging.

[0027] II. Conditioning and Concentration Unit: Controlling chlorine and adjusting pH to inhibit corrosion and scaling at the source.

[0028] Precise dechlorination reduces corrosivity

[0029] High-chlorine wastewater is a major cause of pitting corrosion, stress corrosion, and perforation in equipment. This process reduces Cl⁻ from 15,000-22,000 mg / L to ≤8,000 mg / L through online chloride ion detection and automatic dosing of dechlorinating agent.

[0030] The corrosivity index of the solution decreased significantly, the corrosion rate of the equipment decreased from ≥0.3mm / a to ≤0.05mm / a, and the equipment life was extended by 3-5 times.

[0031] Closed-loop pH control avoids acid-base damage.

[0032] Maintaining the pH at 8±0.2, a slightly alkaline range, avoids damage from both acidic corrosion and alkaline scaling. The scaling rate on the inner walls of pipes, pumps, and concentration towers is reduced by ≥80%, extending the system's continuous operating cycle from 15-30 days to over 180 days.

[0033] Waste heat concentration of flue gas achieves energy saving and emission reduction

[0034] It directly utilizes the waste heat of boiler flue gas to evaporate moisture, eliminating the need for additional steam / electricity consumption. The concentration ratio reaches 4-4.5 times, reducing wastewater volume by 75%-80%, significantly decreasing the downstream drying load, and reducing energy consumption per ton of water by ≥40% compared to MVR and evaporation crystallization.

[0035] III. Emulsification Modification Unit: Altering the physical properties of the liquid phase to build the final protective barrier.

[0036] Emulsification modification inhibits scaling and ash accumulation

[0037] Emulsifiers encapsulate salt crystal nuclei, altering their growth morphology and making it difficult for them to adhere to the wall surface and form a dense scale layer. The scaling tendency in the drying section is further reduced by ≥70%, atomizing nozzles do not clog, and dust does not accumulate in the flue / fluidized bed, allowing the system to operate maintenance-free.

[0038] Secondary pH buffering for enhanced corrosion protection

[0039] The pH of the emulsion is stabilized at 7-8, further neutralizing residual acidic components. High-temperature acid corrosion of the drying equipment is significantly reduced, eliminating the need for frequent downtime for acid washing and reducing maintenance costs by ≥60%.

[0040] Improve atomization and drying efficiency

[0041] The emulsion exhibits more uniform viscosity and surface tension, resulting in finer atomized particles. It evaporates completely and instantaneously in high-temperature flue gas, with no droplets adhering to the walls or wet ash accumulating, achieving a drying efficiency approaching 100%.

[0042] IV. Drying, Curing, and Closed-Loop Recirculation: Truly Achieving Zero Emissions Throughout the Entire Process

[0043] Waste heat drying consumes no additional energy.

[0044] The final evaporation is completed using waste heat from 300℃ high-temperature flue gas, replacing the traditional heating system. The operating cost per ton of water has been reduced from 60-80 yuan to 35-45 yuan, with large units saving over one million yuan annually.

[0045] Solid waste closed-loop system prevents secondary pollution

[0046] All dried ash is recycled to a triplet for co-processing and co-filtration with sludge. There is no wastewater discharge, no waste residue on the ground, and no waste liquid leakage, achieving 100% zero discharge and meeting the most stringent environmental protection inspection requirements.

[0047] V. End-to-End Integration and Collaboration: Systematically Solving Common Pain Points in the Industry

[0048] The pretreatment, conditioning, concentration, emulsification, and drying processes are designed in a unified manner, with each unit mutually dependent and optimized. Pretreatment provides low-pollution, low-hardness influent for concentration; conditioning and dechlorination provide low-corrosion feedstock for emulsification and drying; emulsification provides anti-scaling materials for solidification; and solidification reflux achieves a closed loop. The final result: system availability increases from 85%-90% to ≥98%, and five major industry challenges—scaling, corrosion, clogging, high energy consumption, and non-compliance with heavy metal standards—are simultaneously resolved. Attached Figure Description

[0049] Figure 1 This is a flowchart of the process flow of the present invention. Detailed Implementation

[0050] The present invention will be further described in detail below with reference to several specific embodiments, covering different unit sizes, different water qualities, and different drying process scenarios. The data is complete and the comparison is sufficient, fully demonstrating the superiority and universality of the present invention.

[0051] This invention discloses an integrated process for zero discharge of desulfurization wastewater. Through the coupling of four stages—pretreatment in a three-tank unit, conditioning and concentration, emulsification modification, and drying and solidification—it achieves deep decontamination, waste heat utilization, corrosion and scale prevention, and solid waste closed-loop management, truly achieving zero wastewater discharge and long-term stable operation of the system.

[0052] (1) Pretreatment of triple container

[0053] The original desulfurization wastewater (pH 4-6) enters the neutralization tank, and lime milk or sodium hydroxide is added to adjust the pH to 9.0-9.5, so that heavy metals form hydroxide precipitates, and Ca²⁺ and F⁻ form CaF₂ precipitate.

[0054] The neutralized effluent enters the sedimentation tank, where organic sulfur agents are added to react with heavy metals such as Hg, Ag, and Pb to form sulfide precipitates with extremely low solubility, thus achieving deep removal of heavy metals.

[0055] It then enters the flocculation box, where iron or aluminum salt flocculants are added first, followed by polyacrylamide (PAM) coagulant aid to form dense flocs;

[0056] The flocculant undergoes solid-liquid separation in a clarifier. The supernatant enters the subsequent system, the bottom sludge is dewatered and transported off-site, and the filtrate is returned to the front end of the system for recycling.

[0057] (2) Conditioning and Concentration

[0058] The pretreated supernatant is sent to the concentration tower, where the 130°C flue gas after the boiler induced draft fan is used as a heat source. It is then pumped to the top spray layer of the tower for atomized spraying to increase the gas-liquid contact area and achieve evaporation and concentration.

[0059] A demister is installed at the top of the tower to capture mist droplets and slurry entrained in the flue gas, preventing material loss and pollution from the flue gas.

[0060] The concentrate at the bottom of the tower enters the conditioning tank, where a dechlorinating agent is added through a chloride ion detector and an automatic dosing system to reduce the chloride ion concentration. Then, the pH is adjusted to 8±0.2 with dilute hydrochloric acid, completing the integrated conditioning process of dechlorination, pH adjustment, and clarification.

[0061] (3) Emulsification modification

[0062] After conditioning, the concentrated liquid is sent to an emulsification tank, a special emulsifier (usually polyethylene glycol fatty acid ester emulsifier) ​​is added, and the mixture is stirred to form a stable emulsion. The pH is then adjusted to 7-8. Emulsification modification can significantly reduce the tendency of scaling and ash accumulation in the subsequent drying process, while also reducing acid corrosion of the equipment.

[0063] (4) Drying and curing

[0064] The emulsion is atomized and sent into the flue or fluidized bed, where it is instantly dried and evaporated using the waste heat of the 300℃ flue gas. All the solid ash / sludge produced during drying is returned to the triplet pretreatment system and treated together with the sludge, achieving a closed-loop zero-discharge system with no wastewater or waste residue.

[0065] Example 1

[0066] High-turbidity, high-hardness desulfurization wastewater from a 300MW unit (flue gas spray drying)

[0067] raw water quality

[0068] pH: 5.2

[0069] SS: 18500 mg / L

[0070] Ca²⁺: 6200 mg / L

[0071] Mg²⁺: 5280 mg / L

[0072] Cl⁻: 18000mg / L

[0073] F⁻: 32mg / L

[0074] Total heavy metal concentration: 6.8 mg / L (Hg 0.04 mg / L, Cd 0.35 mg / L, Pb 1.2 mg / L)

[0075] Water treatment capacity: 20 m³ / h

[0076] Treatment steps and effluent data

[0077] Triple container pretreatment

[0078] Neutralization: Adjust pH to 9.5 with 10% lime milk, react for 25 minutes.

[0079] Deep removal of heavy metals: TMT15 50 mg / L, reaction time 15 min

[0080] Flocculation: PAC 100mg / L + PAM 3mg / L

[0081] Sedimentation: Inclined tube sedimentation tank, retention time 45 min

[0082] Effluent: SS ≤ 62 mg / L, Ca²⁺ = 420 mg / L, Mg²⁺ = 110 mg / L, heavy metals ≤ 0.7 mg / L

[0083] Removal rates: SS ≥ 99.6%, hardness ≥ 94%, heavy metals ≥ 90%

[0084] Conditioning and Concentration

[0085] Conditioning: 1.5% compound dechlorinating agent, Cl⁻ reduced to ≤8000mg / L, pH=8.0

[0086] Concentration: Flue gas at 130℃, concentration factor 4, water volume reduced to 5 m³ / h

[0087] Results: Cl⁻ removal ≥55%, corrosion rate ≤0.05mm / a, scaling rate reduction ≥80%

[0088] Emulsion Modification

[0089] Emulsifier: 0.8% polyethylene glycol fatty acid ester

[0090] Stirring: 2000 rpm, 25 min, pH=7.5

[0091] Results: No emulsion breakage within 24 hours, and a further 70% reduction in the tendency for scale buildup.

[0092] Drying and curing

[0093] 300℃ flue gas spray drying, 100% solids recirculation triplet.

[0094] Results: 100% moisture evaporation rate, solid waste recovery rate ≥99.5%

[0095] The table below compares the present invention with traditional processes.

[0096] Heavy metal removal rate 65%~75% ≥90% +15% Cl⁻ 18000mg / L ≤8000mg / L -55% Corrosion rate ≥0.3mm / a ≤0.05mm / a -83% Scale cleaning cycle 15-30 days 180+ days ×5 times Cost per ton of water 60-80 yuan 35~45 yuan -35%+ System availability 85%~90% ≥98% +8%

[0097] Example 2

[0098] 660MW unit high-chlorine desulfurization wastewater (fluidized bed drying)

[0099] raw water quality

[0100] pH: 4.8

[0101] SS: 12000 mg / L

[0102] Ca²⁺: 4800 mg / L

[0103] Mg²⁺: 3600 mg / L

[0104] Cl⁻: 22000mg / L

[0105] F⁻: 28mg / L

[0106] Heavy metals: 5.2 mg / L

[0107] Water treatment capacity: 35 m³ / h

[0108] Treatment steps and effluent data

[0109] Triple container pretreatment

[0110] Neutralization: Adjust pH to 9.2 with NaOH.

[0111] Organic sulfur: TMT15 45 mg / L

[0112] Flocculation: Ferric sulfate chloride 80 mg / L + PAM 2.5 mg / L

[0113] Effluent: SS≤58mg / L, heavy metals≤0.5mg / L, F⁻≤8mg / L

[0114] Conditioning and Concentration

[0115] Dechlorination agent dosage: 1.8%

[0116] Cl⁻ reduced to ≤9000 mg / L, pH=8.1

[0117] Concentration ratio: 4.5 times, water volume reduced to 7.8 m³ / h

[0118] Emulsion Modification

[0119] Emulsifier dosage: 0.9%

[0120] The emulsion is uniform and stable, with a pH of 7.6.

[0121] Drying and curing

[0122] Fluidized bed drying, hot air 300~320℃

[0123] The discharge moisture content is ≤2%, and all solids are recycled.

[0124] Running effect

[0125] No scaling or clogging after 120 days of continuous operation

[0126] Equipment corrosion rate: 0.04 mm / a

[0127] Cost per ton of water treated: 32 yuan

[0128] Heavy metal compliance rate: 100%

[0129] Zero emissions met: Qualified

[0130] Example 3

[0131] Low-load water quality of a 1000MW ultra-supercritical unit (dual-process comparative verification)

[0132] raw water quality

[0133] pH: 5.5

[0134] SS: 8500 mg / L

[0135] Ca²⁺: 3200 mg / L

[0136] Mg²⁺: 2100 mg / L

[0137] Cl⁻: 15000mg / L

[0138] Water treatment capacity: 50 m³ / h

[0139] Option A: Flue gas spray drying

[0140] Concentration ratio: 4x

[0141] Operating cost: 30 yuan / ton

[0142] Scaling cycle: 210 days

[0143] Option B: Fluidized bed drying

[0144] Concentration factor: 4.2x

[0145] Operating cost: 33 yuan / ton

[0146] Moisture content of solid waste: ≤1.5%

[0147] Summary table of key indicators

[0148]

[0149] Example 4

[0150] High-mercury and high-fluoride desulfurization wastewater that is difficult to treat (enhanced process)

[0151] raw water quality

[0152] Hg: 0.08 mg / L

[0153] F⁻: 45mg / L

[0154] Cl⁻: 20000mg / L

[0155] SS: 15000 mg / L

[0156] Water volume: 12 m³ / h

[0157] Strengthening handling measures

[0158] Increase lime dosage in the neutralization stage to enhance fluoride removal.

[0159] The dosage of organic sulfur has been increased to 60 mg / L.

[0160] The dechlorination agent concentration in the conditioning section has been increased to 2.0%.

[0161] Emulsifier dosage: 1.0%

[0162] Processing results

[0163] Hg in effluent: ≤0.003mg / L (compliant)

[0164] F⁻ Outflow: ≤8mg / L

[0165] Cl⁻: ≤8500mg / L

[0166] The system has run continuously for 90 days without any abnormalities.

[0167] No scaling, no corrosion perforation

[0168] Example 5

[0169] Small generating units intermittently receive water during standby operation.

[0170] Operating conditions

[0171] Intermittent water intake: 3 on, 2 off

[0172] Water volume fluctuation: 5~15m³ / h

[0173] Large fluctuations in water quality

[0174] Execution results

[0175] The system has strong resistance to shock loads.

[0176] Stable effluent: SS≤65mg / L, heavy metals meet standards.

[0177] No scaling or clogging during shutdown and restart

[0178] Availability remains at ≥97%.

[0179] This invention organically integrates four units—pretreatment, conditioning and concentration, emulsification modification, and drying and curing—to construct a fully collaborative zero-discharge treatment system for desulfurization wastewater. It systematically solves industry problems such as incomplete heavy metal removal, severe equipment corrosion and scaling, high energy consumption and operating costs, and poor system stability associated with traditional processes, addressing pollution control at the source, corrosion and scale prevention in the middle stages, and closed-loop curing at the end. The process fully utilizes waste heat from boiler flue gas, eliminating the need for an additional heat source and offering significant energy-saving advantages. All solid and liquid products are recycled and disposed of in a closed loop, truly achieving zero wastewater discharge and resource utilization. This invention is highly adaptable, suitable for units of different sizes and complex water quality conditions, combining technological advancement, operational stability, and economic rationality. It can provide a reliable technical solution and engineering demonstration for the treatment of high-salt, recalcitrant wastewater from coal-fired power plants and similar industrial applications.

Claims

1. An integrated process for zero discharge of desulfurization wastewater, characterized in that, The process includes four steps performed sequentially: triplet pretreatment, conditioning and concentration, emulsification modification, and drying and curing. (1) Pretreatment in a three-stage tank: The desulfurization wastewater enters the neutralization tank, and alkaline agents are added to adjust the pH to 9.0~9.5 so that heavy metals and fluorides co-precipitate; the effluent enters the sedimentation tank and organic sulfur agents are added to remove heavy metals; then it enters the flocculation tank and flocculants and coagulants are added; after solid-liquid separation, the supernatant enters the next unit, the sludge is dewatered and transported off-site, and the filtrate is recycled; (2) Conditioning and concentration: The supernatant is used as feed, and the flue gas after the boiler induced draft fan is used as heat source. The mixture is circulated and sprayed for evaporation and concentration in the concentration tower. A demister is installed at the top of the tower. The slurry at the bottom of the tower enters the conditioning tank, where it is dechlorinated, the pH is adjusted to 8±0.2, and clarified. (3) Emulsification modification: Add a special emulsifier to the concentrate to form a stable emulsion and adjust the pH to 7-8; (4) Drying and solidification: After the emulsion is atomized, it is dried instantly using the residual heat of the flue gas. The solid product is returned to the three-compartment pretreatment system and combined with the sludge for treatment, achieving zero discharge.

2. The process according to claim 1, characterized in that, The effluent from the triple-tank pretreatment system is controlled with SS ≤ 70 mg / L and pH 6.0 ~ 9.

0.

3. The process according to claim 1, characterized in that, The heat source for concentration is the 130°C flue gas after the boiler induced draft fan, and the heat source for drying is the waste heat of the 300°C flue gas.

4. The process according to claim 1, characterized in that, Drying and curing are performed using flue gas rotary spray or fluidized bed drying processes.

5. The process according to claim 1, characterized in that, The emulsifier is polyethylene glycol fatty acid ester, and the dosage is 0.5% to 1.0% of the concentrate mass.

6. The process according to claim 1, characterized in that, The alkaline agent is lime milk or sodium hydroxide.

7. The process according to claim 1, characterized in that, The flocculant is polyaluminum chloride (PAC) or ferric sulfate chloride, and the coagulant aid is polyacrylamide (PAM).

8. The process according to claim 1, characterized in that, After pretreatment, the SS removal rate is ≥99.3%, the total hardness removal rate is ≥93%, and the heavy metal removal rate is ≥90%.

9. The process according to claim 1, characterized in that, The chloride ion removal rate in the conditioning section is ≥53%, the system scaling rate is reduced by ≥80%, and the equipment corrosion rate is ≤0.05mm / a.

10. A system for use in any of the processes of claims 1 to 9, characterized in that, It includes the following systems connected sequentially along the wastewater flow direction: a triplet pretreatment system, a conditioning and concentration system, an emulsification system, and a drying and solidification system; The three-compartment pretreatment system includes a neutralization tank, a sedimentation tank, a flocculation tank, a clarifier, and a sludge dewatering device. The conditioning and concentration system includes a concentration tower, a circulating pump, a spray layer, a demister, a conditioning tank, a chloride ion detector, and an automatic dosing system; The emulsification system includes an emulsification tank and an emulsifier dosing unit; The drying and curing system includes an atomizing device, a dryer, and a solid return pipeline.

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