A wet desulfurization slurry purification and desulfurization efficiency improvement integrated system

Abstract

The present invention discloses an integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement, which relates to the technical field of flue gas desulfurization. The system includes a nanofiltration separation subsystem, a desulfurization efficiency improvement subsystem, and an integrated control subsystem. The nanofiltration separation subsystem includes a first nanofiltration unit and a second nanofiltration unit. The desulfurization efficiency improvement subsystem includes an upper limestone slurry injection pipeline, a lower limestone slurry injection pipeline, an upper aeration branch pipe, and a lower aeration branch pipe arranged along the height direction of the absorption tower. Through parameter linkage by the integrated control subsystem, when the chloride ion concentration increases, the nanofiltration separation subsystem starts dechlorination and retains calcium ions. At the same time, the desulfurization efficiency improvement subsystem adaptively adjusts the injection amount of limestone slurry in the upper pH regulation area and the supply amount of oxidation air in the lower oxidation air supply area, realizing the linkage adjustment of the two subsystems and solving the technical problem of the decline in desulfurization efficiency under the condition of high-chlorine coal.

Description

Technical Field

[0001] This invention belongs to the field of flue gas desulfurization technology, specifically an integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement. Background Technology

[0002] Limestone-gypsum wet desulfurization technology is the mainstream process for sulfur dioxide removal from flue gas in coal-fired power plants. This process uses limestone slurry as the absorbent, which comes into countercurrent contact with sulfur dioxide-containing flue gas in the absorption tower. Calcium sulfite is generated, and after oxidation, crystallization, and dehydration, gypsum is obtained as a byproduct. During system operation, the desulfurization circulating slurry continuously accumulates chloride ions, mainly from: chloride ions released as hydrogen chloride during combustion of coal and dissolved in the slurry; chloride ions carried by process water; and chlorides entrained in the limestone absorbent.

[0003] For units burning high-chlorine coal (chlorine content greater than or equal to 0.3% by mass), the chloride ion concentration in the desulfurization circulating slurry can reach over 20,000 mg / L within several weeks of operation, far exceeding the 8,000 to 12,000 mg / L under conventional coal operating conditions. High chloride ion concentrations have the following adverse effects on the desulfurization system: First, chloride ions compete with sulfite ions for alkaline absorption components such as hydroxide and carbonate ions in the liquid phase, reducing the driving force for sulfur dioxide absorption and mass transfer; second, chloride ions compete for catalytically active sites in the oxidation reaction, inhibiting the oxidative conversion of calcium sulfite to calcium sulfate, leading to an increase in sulfite concentration in the slurry, causing slurry foaming and difficulty in gypsum dewatering; third, high chloride ion concentrations increase slurry viscosity and density, reducing the dissolution rate of limestone particles and decreasing the supply of effective alkaline components.

[0004] The shortcomings of the existing technology are: the slurry purification subsystem and the desulfurization efficiency improvement subsystem operate independently, with no information exchange or parameter coupling between them; the loss of calcium ions during the slurry purification process is not compensated, resulting in insufficient supply of alkaline components for the desulfurization reaction; and the desulfurization efficiency improvement control parameters are not correlated with the chloride ion concentration in the slurry, making it impossible to adaptively adjust according to changes in chloride ion concentration. Summary of the Invention

[0005] In summary, a system is needed that integrates slurry purification and desulfurization efficiency improvement, using chloride ion concentration as a coupling variable to achieve coordinated regulation of the two subsystems and solve the technical problem of reduced desulfurization efficiency under high-chlorine coal conditions.

[0006] To solve the above problems, the present invention adopts the following technical solution:

[0007] Specifically, an integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement is proposed, including:

[0008] A nanofiltration separation subsystem includes a first nanofiltration unit and a second nanofiltration unit. The inlet of the first nanofiltration unit is connected to the suction branch of the desulfurization circulating slurry pipeline via a variable frequency drive pump. The permeate outlet of the first nanofiltration unit is connected to the purified liquid reinjection pipeline. The concentrate outlet of the first nanofiltration unit is connected to the inlet of the second nanofiltration unit. The concentrate outlet of the second nanofiltration unit is connected to the concentrate treatment pipeline. The concentrate treatment pipeline is equipped with a three-way diverter valve. The first outlet of the three-way diverter valve is connected to the desulfurization wastewater treatment unit, and the second outlet of the three-way diverter valve is connected to the desulfurization circulating slurry reinjection port.

[0009] A desulfurization efficiency improvement subsystem is installed inside the absorption tower. The subsystem includes an upper limestone slurry injection pipeline and a lower limestone slurry injection pipeline arranged along the height of the absorption tower. The upper limestone slurry injection pipeline is equipped with a first variable frequency metering pump, and the lower limestone slurry injection pipeline is equipped with a second variable frequency metering pump. The subsystem also includes an upper aeration branch pipe and a lower aeration branch pipe arranged along the height of the absorption tower. The upper aeration branch pipe is equipped with a first flow regulating valve, and the lower aeration branch pipe is equipped with a second flow regulating valve.

[0010] An integrated control subsystem includes a chloride ion concentration detection probe, a density detection probe, an oxidation rate detection probe, and a control module. The chloride ion concentration detection probe is installed on the desulfurization circulating slurry pipeline in the section before the inlet of the variable frequency drive pump. The density detection probe and the oxidation rate detection probe are installed in the slurry pool of the absorption tower. The control module is equipped with a first threshold memory and a second threshold memory.

[0011] The signal input terminals of the control module are communicatively connected to the chloride ion concentration detection probe, the density detection probe, and the oxidation rate detection probe, respectively. The first signal output terminal of the control module is communicatively connected to the variable frequency drive pump. The second signal output terminal of the control module is communicatively connected to the first variable frequency metering pump. The third signal output terminal of the control module is communicatively connected to the second variable frequency metering pump. The fourth signal output terminal of the control module is communicatively connected to the first flow regulating valve. The fifth signal output terminal of the control module is communicatively connected to the second flow regulating valve. The sixth signal output terminal of the control module is communicatively connected to the three-way diverter valve.

[0012] Furthermore, the operating pressure of the first nanofiltration unit is 0.8 MPa to 1.2 MPa, and the operating pressure of the second nanofiltration unit is 1.5 MPa to 2.0 MPa;

[0013] Both the first nanofiltration unit and the second nanofiltration unit are made of polyamide composite nanofiltration membranes with a molecular weight cutoff of 200 Da to 300 Da.

[0014] The first nanofiltration unit has a chloride ion rejection rate higher than 0.90 and a calcium ion rejection rate lower than 0.30.

[0015] The second nanofiltration unit has a chloride ion rejection rate of over 0.95.

[0016] Furthermore, the injection area of ​​the upper limestone slurry injection pipeline is the upper pH control zone, and the target pH value of the upper pH control zone is 5.2 to 5.8;

[0017] The injection area of ​​the lower limestone slurry injection pipeline is the lower pH control zone, and the target pH value of the lower pH control zone is 4.6 to 5.0;

[0018] The upper pH control zone and the lower pH control zone are each equipped with an online pH detection probe, and the online pH detection probe is communicatively connected to the control module.

[0019] Furthermore, the aeration area of ​​the upper aeration branch pipe is the upper oxidation air supply area, and the aeration area of ​​the lower aeration branch pipe is the lower oxidation air supply area.

[0020] The amount of oxidation air supplied per unit volume in the upper oxidation air supply zone is 1.3 to 1.8 times that in the lower oxidation air supply zone.

[0021] Furthermore, the control module includes a comparator unit, a signal processing unit, and six signal output terminals. The first input terminal of the comparator unit is communicatively connected to the output terminal of the chloride ion concentration detection probe, the second input terminal of the comparator unit is communicatively connected to the output terminal of the first threshold memory, the third input terminal of the comparator unit is communicatively connected to the output terminal of the second threshold memory, the output terminal of the comparator unit is communicatively connected to the input terminal of the signal processing unit, and the output terminal of the signal processing unit is communicatively connected to the six signal output terminals respectively.

[0022] The comparison result signal output by the comparator unit includes a first comparison result signal and a second comparison result signal. The first comparison result signal is the difference signal between the detection value of the chloride ion concentration detection probe and the value stored in the first threshold memory. The second comparison result signal is the difference signal between the detection value of the chloride ion concentration detection probe and the value stored in the second threshold memory.

[0023] Furthermore, the threshold stored in the first threshold memory is determined experimentally by measuring the effect curve of chloride ion concentration on sulfur dioxide removal efficiency. The experiment involves preparing a simulated desulfurization slurry with a chloride ion concentration gradient ranging from 5000 mg / L to 30000 mg / L in a constant-temperature water bath reactor, while controlling the flue gas sulfur dioxide concentration at 2000 mg / m³. 3 The liquid-to-gas ratio is 15 L / m³. 3 The reaction temperature was 50 degrees Celsius and the pH value of the slurry was 5.5. The concentration of sulfur dioxide at the outlet was measured and the removal efficiency was calculated. The chloride ion concentration value corresponding to the removal efficiency decreasing by 0.05 compared with the baseline efficiency of 5000 mg / L was taken.

[0024] The threshold stored in the second threshold memory is determined by experimental measurement of the effect curve of chloride ion concentration on sulfite oxidation rate. The experimental measurement is carried out by adding calcium sulfite suspension to simulated slurry under the same conditions, passing in oxidizing air and measuring the conversion rate of calcium sulfite to calcium sulfate per unit time. The chloride ion concentration value corresponding to the oxidation rate decreasing by 0.10 compared with the baseline oxidation rate when the chloride ion concentration is 5000 mg / L is taken.

[0025] The threshold stored in the first threshold memory is lower than the threshold stored in the second threshold memory.

[0026] Furthermore, the control module is also equipped with a feedforward compensation unit, which has a boiler load signal receiving port and an inlet flue gas sulfur dioxide concentration signal receiving port, and the feedforward compensation unit is equipped with a prediction model memory.

[0027] The output of the feedforward compensation unit is communicatively connected to the signal processing unit of the control module.

[0028] Furthermore, the control module is equipped with a current split ratio calculation unit. The input terminal of the current split ratio calculation unit is communicatively connected to the density detection probe and the oxidation rate detection probe, respectively. The output terminal of the current split ratio calculation unit is communicatively connected to the sixth signal output terminal of the control module.

[0029] The diversion ratio calculation unit stores the diversion ratio calculation formula: x=x0+K1×(ρ-ρ0) / ρ0+K2×(η0-η ox ), where x0 is the baseline split ratio (0.30), ρ0 is the target density value (1100 kg / m³), η0 is the target oxidation rate value (0.96), K1 is the density adjustment coefficient (0.50), K2 is the oxidation rate adjustment coefficient (0.40), ρ is the real-time density detection value of the density detection probe, and η ox This refers to the real-time oxidation rate detection value of the oxidation rate detection probe.

[0030] Furthermore, the comparator unit is provided with a trend judgment module, which is provided with a sample-and-hold circuit and a subtractor. The input terminal of the sample-and-hold circuit is communicatively connected to the output terminal of the chloride ion concentration detection probe. The first input terminal of the subtractor is communicatively connected to the output terminal of the chloride ion concentration detection probe, and the second input terminal of the subtractor is communicatively connected to the output terminal of the sample-and-hold circuit.

[0031] The output of the subtractor is communicatively connected to the signal processing unit of the control module.

[0032] Furthermore, the nanofiltration separation subsystem and the desulfurization efficiency improvement subsystem share a common circulating cooling medium circuit, which includes a cooling water tank, a cooling water pump, a first heat exchanger, and a second heat exchanger.

[0033] The first heat exchanger is installed in the motor cooling jacket of the variable frequency drive pump, and the second heat exchanger is installed in the compressed air cooling jacket of the oxidation blower. The outlet of the oxidation blower is connected to the upper aeration branch pipe and the lower aeration branch pipe.

[0034] The technical solutions provided by the embodiments of this disclosure have at least the following beneficial effects:

[0035] By employing nanofiltration membrane separation technology through the nanofiltration separation subsystem, selective removal of chloride ions from the desulfurization circulating slurry is achieved. The chloride ion rejection rate is higher than 0.90, while the calcium ion rejection rate is lower than 0.30. This reduces the chloride ion concentration while retaining most of the calcium ions, avoiding the problem of insufficient alkaline components in desulfurization caused by the simultaneous loss of calcium ions during conventional reverse osmosis dechlorination.

[0036] Using chloride ion concentration as a coupled control variable, the nanofiltration separation subsystem and the desulfurization efficiency improvement subsystem are linked by an integrated control subsystem. When the chloride ion concentration increases, the nanofiltration separation subsystem starts dechlorination, while the desulfurization efficiency improvement subsystem lowers the pH target value in the upper pH control zone to compensate for the competitive inhibition of chloride ions on sulfur dioxide absorption, and increases the oxidation air supply in the lower oxidation air supply zone to compensate for the competitive inhibition of chloride ions on sulfite oxidation. The synchronous adjustment of the two subsystems enables the desulfurization system to maintain stable desulfurization efficiency under conditions of chloride ion concentration fluctuations.

[0037] The dual-threshold control strategy stores thresholds in the first and second threshold memories that correspond to a decrease in sulfur dioxide removal efficiency of 0.05 and a decrease in sulfite oxidation rate of 0.10, respectively. A buffer zone is formed between the two thresholds, enabling the nanofiltration separation subsystem to operate continuously, avoiding the frequent start-stop problems under single-threshold control, and extending the service life of the nanofiltration membrane.

[0038] A feedforward compensation unit is set up to form a feedforward control channel. The feedforward compensation unit receives the boiler load signal and the inlet flue gas sulfur dioxide concentration signal. When the boiler load or the inlet sulfur dioxide concentration fluctuates, the system completes the adjustment of operating parameters before the actual change in chloride ion concentration, so as to avoid large overshoot of chloride ion concentration and improve the stability of desulfurization efficiency of the system under the condition of boiler load change.

[0039] The nanofiltration concentrate is distributed and regulated using a three-way diversion valve and a diversion ratio calculation unit. The diversion ratio is determined in real time based on the density and oxidation rate of the desulfurization circulating slurry. When the slurry density is too high and the oxidation rate is too low, the reinjection ratio of the concentrate into the desulfurization circulating slurry is increased to replenish the solid phase components of the slurry using calcium ions and sulfite ions in the concentrate. When the slurry density is too low or the oxidation rate is too high, the discharge ratio of the concentrate into the desulfurization wastewater treatment unit is increased. This distribution method maintains the density of the desulfurization circulating slurry within a reasonable range, improving both the limestone dissolution rate and the gypsum crystallization quality. Attached Figure Description

[0040] Figure 1 This is a flowchart of the present invention. Detailed Implementation

[0041] 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.

[0042] like Figure 1 This invention illustrates an integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement, comprising:

[0043] A nanofiltration separation subsystem includes a first nanofiltration unit and a second nanofiltration unit. The inlet of the first nanofiltration unit is connected to the suction branch of the desulfurization circulating slurry pipeline via a variable frequency drive pump. The permeate outlet of the first nanofiltration unit is connected to the purified liquid reinjection pipeline. The concentrate outlet of the first nanofiltration unit is connected to the inlet of the second nanofiltration unit. The concentrate outlet of the second nanofiltration unit is connected to the concentrate treatment pipeline. The concentrate treatment pipeline is equipped with a three-way diverter valve. The first outlet of the three-way diverter valve is connected to the desulfurization wastewater treatment unit, and the second outlet of the three-way diverter valve is connected to the desulfurization circulating slurry reinjection port.

[0044] A desulfurization efficiency improvement subsystem is installed inside the absorption tower. The subsystem includes an upper limestone slurry injection pipeline and a lower limestone slurry injection pipeline arranged along the height of the absorption tower. The upper limestone slurry injection pipeline is equipped with a first variable frequency metering pump, and the lower limestone slurry injection pipeline is equipped with a second variable frequency metering pump. The subsystem also includes an upper aeration branch pipe and a lower aeration branch pipe arranged along the height of the absorption tower. The upper aeration branch pipe is equipped with a first flow regulating valve, and the lower aeration branch pipe is equipped with a second flow regulating valve.

[0045] An integrated control subsystem includes a chloride ion concentration detection probe, a density detection probe, an oxidation rate detection probe, and a control module. The chloride ion concentration detection probe is installed on the desulfurization circulating slurry pipeline in the section before the inlet of the variable frequency drive pump. The density detection probe and the oxidation rate detection probe are installed in the slurry pool of the absorption tower. The control module is equipped with a first threshold memory and a second threshold memory.

[0046] The signal input terminals of the control module are communicatively connected to the chloride ion concentration detection probe, the density detection probe, and the oxidation rate detection probe, respectively. The first signal output terminal of the control module is communicatively connected to the variable frequency drive pump. The second signal output terminal of the control module is communicatively connected to the first variable frequency metering pump. The third signal output terminal of the control module is communicatively connected to the second variable frequency metering pump. The fourth signal output terminal of the control module is communicatively connected to the first flow regulating valve. The fifth signal output terminal of the control module is communicatively connected to the second flow regulating valve. The sixth signal output terminal of the control module is communicatively connected to the three-way diverter valve.

[0047] It should be noted that:

[0048] The design of this system is based on the fact that the flue gas produced by the combustion of high-chlorine coal has a higher chlorine content than that of conventional coal. Chlorine is released as hydrogen chloride during combustion and accumulates as chloride ions after dissolving in the desulfurization circulating slurry. Increased chloride ion concentration has the following effects on the desulfurization process: chloride ions compete with sulfite ions for alkaline absorption components such as hydroxide and carbonate ions in the liquid phase, reducing the liquid-phase mass transfer coefficient of sulfur dioxide; chloride ions compete for active sites of dissolved oxygen in the oxidation reaction, inhibiting the oxidative conversion of calcium sulfite to calcium sulfate; high chloride ion concentration increases the ionic strength and viscosity of the slurry, reducing the surface dissolution rate of limestone particles. These effects deteriorate the sulfur dioxide removal efficiency and gypsum dewatering performance of the desulfurization system. This system uses chloride ion concentration as a coupling variable between the nanofiltration separation subsystem and the desulfurization efficiency improvement subsystem. The nanofiltration separation subsystem removes chloride ions and retains calcium ions through the selective separation of the nanofiltration membrane; the desulfurization efficiency improvement subsystem achieves zoned control of pH value and oxidation air supply in different height areas of the absorption tower through independent upper and lower limestone slurry injection pipelines and independent upper and lower aeration branch pipes; the integrated control subsystem acquires real-time concentration signals through a chloride ion concentration detection probe, compares them with preset thresholds in the first and second threshold memories, and outputs control signals to the variable frequency drive pump, the first variable frequency metering pump, the second variable frequency metering pump, the first flow regulating valve, the second flow regulating valve, and the three-way diverter valve through six independent signal output terminals, enabling the two subsystems to adjust their operating parameters in a correlated manner based on the chloride ion concentration detection value.

[0049] A variable frequency drive pump is installed in the pipe section before the inlet of the first nanofiltration unit to provide driving head for the desulfurization circulating slurry to enter the nanofiltration unit and to regulate the feed flow rate. The first signal output terminal of the control module outputs a speed regulation signal to the variable frequency drive pump, adjusting the nanofiltration throughput by changing the pump speed; the second and third signal output terminals output speed regulation signals to the first and second variable frequency metering pumps, respectively, adjusting the limestone slurry injection volume in the corresponding area by changing the speed of each pump, and controlling the pH value of each area; the fourth and fifth signal output terminals output opening signals to the first and second flow regulating valves, respectively, adjusting the oxidation air supply in the corresponding area by changing the valve opening; the sixth signal output terminal outputs an opening signal to the three-way diverter valve to adjust the diversion ratio of the concentrate.

[0050] As an optional embodiment, the operating pressure of the first nanofiltration unit is 0.8 MPa to 1.2 MPa, and the operating pressure of the second nanofiltration unit is 1.5 MPa to 2.0 MPa;

[0051] Both the first nanofiltration unit and the second nanofiltration unit are made of polyamide composite nanofiltration membranes with a molecular weight cutoff of 200 Da to 300 Da.

[0052] The first nanofiltration unit has a chloride ion rejection rate higher than 0.90 and a calcium ion rejection rate lower than 0.30.

[0053] The second nanofiltration unit has a chloride ion rejection rate of over 0.95.

[0054] It should be noted that:

[0055] The stepped operating pressure design of two-stage nanofiltration is based on the following principle: the separation effect of nanofiltration membranes on ions is determined by both the Donnan effect and the steric hindrance effect. The hydrated ionic radius of chloride ions is 0.332 nm, and that of calcium ions is 0.412 nm. When the molecular weight cutoff of the nanofiltration membrane is between 150 Da and 400 Da, the effective pore size of the membrane lies between the radii of these two hydrated ions. The membrane surface is typically negatively charged, and its electrostatic repulsion of divalent cations (calcium ions) is weaker than that of monovalent anions (chloride ions), resulting in a selective separation characteristic of high chloride ion cutoff and low calcium ion cutoff.

[0056] The operating pressure of the first nanofiltration unit is set to 0.8 MPa to 1.2 MPa. Within this pressure range, the permeation flux of the nanofiltration membrane to the desulfurization slurry is not less than 20 L / (m³). 2 (·h), with a calcium ion rejection rate of less than 0.30, which meets the water replenishment requirements of the desulfurization system and retains the effective calcium concentration in the circulating slurry.

[0057] The operating pressure of the second nanofiltration unit is set to 1.5 MPa to 2.0 MPa to further process the concentrate from the first nanofiltration unit. Since the chloride ion concentration in the concentrate has increased, deeper concentration of chloride ions can be achieved under higher pressure.

[0058] The pressure difference between the two nanofiltration units generates a sufficient driving head, allowing the concentrate from the first nanofiltration unit to flow naturally into the second nanofiltration unit without the need for an external feed pump, simplifying system configuration. The retention rate R is calculated using the formula: R = 1 - Cp / Cf, where Cp is the ion concentration in the permeate and Cf is the ion concentration in the feed solution. A specific calculation example is used: a polyamide composite nanofiltration membrane with a molecular weight cutoff of 200 Da is selected, the membrane surface zeta potential is -30 mV, and the operating pressure is 1.0 MPa.

[0059] The chloride ion concentration in the feed solution is 20000 mg / L, and the chloride ion concentration in the permeate is measured to be 1500 mg / L. Therefore, the chloride ion rejection rate R(Cl-) = 1 - 1500 / 20000 = 0.925. The calcium ion concentration in the feed solution is 800 mg / L, and the calcium ion concentration in the permeate is measured to be 600 mg / L. Therefore, the calcium ion rejection rate R(Ca2+) = 1 - 600 / 800 = 0.25. The first nanofiltration unit has a rejection rate higher than 0.90 and a calcium ion rejection rate lower than 0.30. The second nanofiltration unit has a chloride ion rejection rate higher than 0.95.

[0060] As an optional embodiment, the injection area of ​​the upper limestone slurry injection pipeline is an upper pH control zone, and the target pH value of the upper pH control zone is 5.2 to 5.8;

[0061] The injection area of ​​the lower limestone slurry injection pipeline is the lower pH control zone, and the target pH value of the lower pH control zone is 4.6 to 5.0;

[0062] The upper pH control zone and the lower pH control zone are each equipped with an online pH detection probe, and the online pH detection probe is communicatively connected to the control module.

[0063] It should be noted that:

[0064] The concentration of gaseous sulfur dioxide and the desulfurization reaction load exhibit a gradient distribution at different heights within the absorption tower. The zoning setting of the pH target value is based on the following analysis and calculations. The lower region, which is in contact with the inlet flue gas, has the highest concentration of gaseous sulfur dioxide. The desulfurization reaction consumes a large amount of alkaline components, and this region is also the main reaction zone for limestone dissolution.

[0065] The dissolution rate of limestone is related to pH as follows: r = k·(Cs - Ceq), where r is the dissolution rate, k is the mass transfer coefficient, Cs is the saturated concentration of limestone particles on the surface, and Ceq is the liquid-phase equilibrium concentration. The relationship between Ceq and pH is: Ceq = Ksp / [CO3^2-]. Since [CO3^2-] decreases as pH decreases, Ceq decreases and the dissolution rate increases at low pH conditions. Specifically, at pH 5.5, the liquid-phase carbonate ion concentration is 1.2 × 10^-5 mol / L, and Ceq = 3.8 × 10^-5 mol / L; at pH 4.8, the liquid-phase carbonate ion concentration is 3.0 × 10^-6 mol / L, and Ceq = 1.5 × 10^-4 mol / L. Under the same mass transfer coefficient k and surface concentration Cs, the dissolution rate at pH 4.8 is 1.5 to 2.0 times that at pH 5.5.

[0066] Therefore, the target pH value of the lower pH control zone is set to 4.6 to 5.0, which is beneficial for the rapid dissolution of limestone to replenish alkaline components. The sulfur dioxide concentration in the flue gas in the upper region has been significantly reduced, requiring a higher liquid phase alkalinity to maintain the absorption mass transfer driving force. Under pH conditions of 5.2 to 5.8, the total concentration of carbonate and bicarbonate ions in the liquid phase is higher than the corresponding value under pH condition 5.0, which is beneficial for improving the removal efficiency of residual sulfur dioxide. Zoned pH control is achieved through two independent limestone slurry injection pipelines. Each pipeline is equipped with a variable frequency metering pump and an online pH detection probe. The control module adjusts the speed of the variable frequency metering pump based on the deviation between the pH detection value and the target value, thereby controlling the limestone slurry injection volume in each zone.

[0067] As an optional embodiment, the aeration area of ​​the upper aeration branch pipe is the upper oxidation air supply area, and the aeration area of ​​the lower aeration branch pipe is the lower oxidation air supply area.

[0068] The amount of oxidation air supplied per unit volume in the upper oxidation air supply zone is 1.3 to 1.8 times that in the lower oxidation air supply zone.

[0069] It should be noted that:

[0070] The basis for the differentiated setting of the oxidation air supply zone is the relationship between the sulfite oxidation reaction rate and pH value and chloride ion concentration. The sulfite oxidation reaction is a liquid-phase catalytic reaction, and the relationship between the reaction rate constant k and pH value is: k = k0 × 10^(0.5 × pH - 4.0), where k0 is the reference rate constant, taken as 0.001 s^-1. Specific calculation steps: When pH = 5.5 in the upper pH control zone, k1 = 0.001 × 10^(0.5 × 5.5 - 4.0) = 0.001 × 10^(-1.25) = 5.62 × 10^-5 s^-1;

[0071] When pH=4.8 in the lower pH control zone, k2=0.001×10^(0.5×4.8-4.0)=0.001×10^(-1.60)=2.51×10^-5s^-1. The oxidation reaction rate r=k·[SO3^2-]·[O2]. Under the same sulfite concentration, the ratio of oxidation rates is equal to the product of the ratio of rate constants and the ratio of dissolved oxygen concentrations. Considering the inhibition coefficient α of chloride ion concentration on the liquid phase mass transfer coefficient of oxygen, α=1 / (1+0.01×[Cl-] / 1000). When [Cl-]=20000mg / L, α=1 / 1.2=0.83;

[0072] When [Cl-] = 30000 mg / L, α = 1 / 1.3 = 0.77. The chloride ion concentration in the upper region of the slurry is higher due to cumulative circulation, so α1 = 0.80 is taken; the chloride ion concentration in the lower region is relatively lower, so α2 = 0.90 is taken. The comprehensive calculation of the supply ratio is: Q1 / Q2 = (k1 / k2) × (α2 / α1) = (5.62 / 2.51) × (0.90 / 0.80) = 2.24 × 1.125 = 1.34; when pH = 5.8 and α1 = 0.75, Q1 / Q2 = (7.08 / 2.51) × (0.90 / 0.75) = 2.82 × 1.2 = 3.38, taking the upper limit of 1.8 times. Therefore, the supply ratio range is 1.3 to 1.8 times.

[0073] As an optional embodiment, the control module includes a comparator unit, a signal processing unit, and six signal output terminals. The first input terminal of the comparator unit is communicatively connected to the output terminal of the chloride ion concentration detection probe, the second input terminal of the comparator unit is communicatively connected to the output terminal of the first threshold memory, the third input terminal of the comparator unit is communicatively connected to the output terminal of the second threshold memory, the output terminal of the comparator unit is communicatively connected to the input terminal of the signal processing unit, and the output terminal of the signal processing unit is communicatively connected to the six signal output terminals respectively.

[0074] The comparison result signal output by the comparator unit includes a first comparison result signal and a second comparison result signal. The first comparison result signal is the difference signal between the detection value of the chloride ion concentration detection probe and the value stored in the first threshold memory. The second comparison result signal is the difference signal between the detection value of the chloride ion concentration detection probe and the value stored in the second threshold memory.

[0075] It should be noted that:

[0076] The comparator unit, signal processing unit, first threshold memory, and second threshold memory constitute the hardware foundation of the dual-threshold control strategy. The first input of the comparator unit receives the real-time detection value from the chloride ion concentration detection probe, the second input receives the first threshold stored in the first threshold memory, and the third input receives the second threshold stored in the second threshold memory. The comparator unit compares the real-time detection value with the first and second thresholds respectively, and outputs two difference signals to the signal processing unit.

[0077] The signal processing unit amplifies and shapes the difference signal, then outputs it to six signal output terminals to drive the corresponding actuators. The threshold stored in the first threshold memory is the warning activation threshold, and the threshold stored in the second threshold memory is the enhanced control threshold. When the real-time detected value is greater than or equal to the first threshold, the first comparison result signal output by the comparator unit is positive. This signal is processed by the signal processing unit and output to the frequency converter drive pump from the first signal output terminal to start the nanofiltration separation subsystem.

[0078] When the real-time detected value further increases to be greater than or equal to the second threshold, the second comparison result signal output by the comparator unit becomes positive. This signal, after processing by the signal processing unit, is output from the second signal output terminal to the first variable frequency metering pump and from the fifth signal output terminal to the second flow regulating valve, increasing the control amplitude. The coordination of the two thresholds ensures the nanofiltration separation subsystem has continuity after startup. Calculation of the buffer zone time width: Assume the total volume of the desulfurization circulating slurry is V = 500 m³. 3 Chloride ion input rate F = 120 kg / h, nanofiltration rated capacity Q = 25 m³ / h 3 / h, nanofiltration dechlorination efficiency Rc=0.80.

[0079] The rate at which the chloride ion concentration rises from the first threshold C1 to the second threshold C2 is dC / dt = (FQ × Rc × 40) / 500, where 40 is the ratio of the chloride ion concentration of the concentrate to that of the feed liquid. Considering fluctuations in the chloride release rate, the actual concentration rise rate is 800 to 1200 mg / (L·h). Taking C1 = 18000 mg / L and C2 = 30000 mg / L, then Δt = (30000 - 18000) / 1000 = 12h, which is much greater than the design constraint of 2h, thus meeting the requirements. Each of the six signal output terminals outputs a different control signal: the first signal output terminal outputs a variable frequency speed control signal to the variable frequency drive pump;

[0080] The second signal output terminal outputs a variable frequency speed control signal to the first variable frequency metering pump; the third signal output terminal outputs a variable frequency speed control signal to the second variable frequency metering pump; the fourth signal output terminal outputs an opening adjustment signal to the first flow regulating valve; the fifth signal output terminal outputs an opening adjustment signal to the second flow regulating valve; and the sixth signal output terminal outputs an opening adjustment signal to the three-way diverter valve.

[0081] As an optional embodiment, the threshold stored in the first threshold memory is determined experimentally by measuring the effect curve of chloride ion concentration on sulfur dioxide removal efficiency. The experiment involves preparing a simulated desulfurization slurry with a chloride ion concentration gradient from 5000 mg / L to 30000 mg / L in a constant-temperature water bath reactor, and controlling the sulfur dioxide concentration in the flue gas to be 2000 mg / m³. 3 The liquid-to-gas ratio is 15 L / m³. 3The reaction temperature was 50 degrees Celsius and the pH value of the slurry was 5.5. The concentration of sulfur dioxide at the outlet was measured and the removal efficiency was calculated. The chloride ion concentration value corresponding to the removal efficiency decreasing by 0.05 compared with the baseline efficiency of 5000 mg / L was taken.

[0082] The threshold stored in the second threshold memory is determined by experimental measurement of the effect curve of chloride ion concentration on sulfite oxidation rate. The experimental measurement is carried out by adding calcium sulfite suspension to simulated slurry under the same conditions, passing in oxidizing air and measuring the conversion rate of calcium sulfite to calcium sulfate per unit time. The chloride ion concentration value corresponding to the oxidation rate decreasing by 0.10 compared with the baseline oxidation rate when the chloride ion concentration is 5000 mg / L is taken.

[0083] The threshold stored in the first threshold memory is lower than the threshold stored in the second threshold memory.

[0084] It should be noted that:

[0085] The methods for determining the two thresholds and the complete calculation steps are as follows. First threshold determination: Six sets of simulated desulfurization slurries with chloride ion concentrations of 5000 mg / L, 10000 mg / L, 15000 mg / L, 20000 mg / L, 25000 mg / L, and 30000 mg / L were prepared in a laboratory constant-temperature water bath reactor. The sulfur dioxide concentration in the flue gas, Cin, was controlled at 2000 mg / m³. 3 The liquid-to-gas ratio is 15 L / m³. 3 The reaction temperature is 50 degrees Celsius and the pH value of the slurry is 5.5.

[0086] Simulated flue gas containing sulfur dioxide was introduced into each group of slurries, and the sulfur dioxide concentration Cout in the outlet flue gas was measured. The sulfur dioxide removal efficiency η of each group was calculated using the formula η=(Cin-Cout) / Cin. An influence curve was plotted with chloride ion concentration Cf as the x-axis and removal efficiency η as the y-axis. The baseline efficiency was taken as the efficiency value when Cf=5000mg / L. The Cf value corresponding to η=baseline efficiency-0.05 was found along the curve, and this Cf value was written into the first threshold memory.

[0087] The fitting formula for the measured data is:

[0088] η = 0.95 - 5 × 10^-6 × (Cf - 5000), let η = 0.90, and we get Cf = 0.05 / 5 × 10^-6 + 5000 = 15000 mg / L.

[0089] Second threshold determination: Simulated desulfurization slurries with the above six chloride ion concentration gradients were prepared in the same constant-temperature water bath reactor. Calcium sulfite suspension was added to each group of slurries to make the initial calcium sulfite concentration 0.05 mol / L. Oxidizing air was introduced into each group of slurries, and the decrease in calcium sulfite concentration ΔC within 30 minutes was measured. The sulfite oxidation rate r for each group was calculated using the formula r = ΔC / 1800, where 1800 is the number of seconds corresponding to 30 minutes. An influence curve was plotted with chloride ion concentration Cf as the abscissa and oxidation rate r as the ordinate. The baseline oxidation rate was taken as the rate value when Cf = 5000 mg / L. The Cf value corresponding to r = baseline oxidation rate × (1 - 0.10) was found along the curve, and this Cf value was written into the second threshold memory. The fitting formula for the measured data is: r / r0 = 1 - 4 × 10^-6 × (Cf - 5000). Let r / r0 = 0.90, then Cf = 0.10 / 4 × 10^-6 + 5000 = 30000 mg / L. The reduction ranges of 0.05 and 0.10 are determined based on the following: when the removal efficiency decreases by 0.05, the outlet sulfur dioxide concentration is close to the upper limit margin of the emission standard; when the oxidation rate decreases by 0.10, the sulfite concentration in the slurry accumulates to the foaming critical value. According to the measured data of high-chlorine coal, the first threshold usually falls in the range of 15000 mg / L to 20000 mg / L, and the second threshold usually falls in the range of 25000 mg / L to 35000 mg / L.

[0090] As an optional embodiment, the control module is further provided with a feedforward compensation unit, which is provided with a boiler load signal receiving port and an inlet flue gas sulfur dioxide concentration signal receiving port, and the feedforward compensation unit is provided with a prediction model memory.

[0091] The output of the feedforward compensation unit is communicatively connected to the signal processing unit of the control module.

[0092] It should be noted that:

[0093] The feedforward compensation unit constitutes the feedforward control channel, forming a composite control structure with the feedback control channel formed by the chloride ion concentration detection probe. The boiler load signal and the inlet flue gas sulfur dioxide concentration signal are leading indicators of chloride ion concentration changes. When the boiler burns high-chlorine coal, the boiler load is directly proportional to the amount of flue gas entering the desulfurization system per unit time and the total amount of chloride ions. When the boiler load increases, the flue gas volume and the total amount of chloride ions increase synchronously, but the increase in chloride ion concentration in the slurry has a lag time. The lag time τ consists of two parts: the residence time of the flue gas in the desulfurization system τ1 and the slurry circulation cycle τ2.

[0094] τ1=Vflue / Qflue, where Vflue is the free volume inside the desulfurization tower and Qflue is the flue gas volume flow rate.

[0095] τ2 = Vslurry / Qpump, where Vslurry is the slurry tank volume and Qpump is the circulation pump flow rate. Typical parameters: Vflue = 200 m³ / s. 3 Qflue=500000m 3 / h, then τ1=200 / 500000×3600=1.44s; Vslurry=500m 3 Qpump=3000m 3 / h, then τ2=500 / 3000×60=10min. The total lag time τ≈10min to 30min, considering the mixing inhomogeneity, τ=0.5h to 2h. The chloride ion concentration prediction formula stored in the prediction model memory is: d[Cl-] / dt=Kcl×S×(dL / dt) / Vslurry, where Kcl is the chloride content of coal (mass fraction), S is the coal consumption (t / h), dL / dt is the boiler load change rate (1 / h), and Vslurry is the slurry tank volume (m³). 3 ), d[Cl-] / dt is the predicted rate of change of chloride ion concentration (mg / (L·h)).

[0096] The feedforward compensation unit calculates the predicted value of d[Cl-] / dt based on the dL / dt value received from the boiler load signal receiving port and the formula stored in the prediction model memory. Typical parameters are: Kcl=0.003, S=100t / h, Vslurry=500m 3 When dL / dt = 0.10 / h, d[Cl-] / dt = 0.003 × 100 × 10^6 × 0.10 / 500 = 60 mg / (L·h). The output of the feedforward compensation unit is connected to the signal processing unit. The feedforward compensation unit converts the predicted value of d[Cl-] / dt into a pre-adjustment signal for the variable frequency drive pump. The switching conditions between feedforward control and feedback control are as follows: when the boiler load change rate is less than 5% / h and the inlet sulfur dioxide concentration fluctuation value is less than 0.20 of the nominal value, the system mainly uses chloride ion concentration feedback control; when the boiler load change rate is greater than or equal to 5% / h or the inlet sulfur dioxide concentration fluctuation value is greater than or equal to 0.20 of the nominal value, the pre-adjustment signal output by the feedforward compensation unit takes precedence over the feedback control signal.

[0097] As an optional embodiment, the control module is provided with a shunting ratio calculation unit. The input terminal of the shunting ratio calculation unit is communicatively connected to the density detection probe and the oxidation rate detection probe, respectively, and the output terminal of the shunting ratio calculation unit is communicatively connected to the sixth signal output terminal of the control module.

[0098] The diversion ratio calculation unit stores the diversion ratio calculation formula: x=x0+K1×(ρ-ρ0) / ρ0+K2×(η0-ηox (where x0 is the baseline diversion ratio, taken as 0.30, and ρ0 is the target density value, taken as 1100 kg / m³). 3 η0 is the target oxidation rate value, set to 0.96; K1 is the density adjustment coefficient, set to 0.50; K2 is the oxidation rate adjustment coefficient, set to 0.40; ρ is the real-time density detection value of the density detection probe; η ox This refers to the real-time oxidation rate detection value of the oxidation rate detection probe.

[0099] It should be noted that:

[0100] The diversion ratio calculation unit calculates the diversion ratio of the three-way diversion valve based on two real-time parameters: density and oxidation rate of the desulfurization circulating slurry, using the formula described above to regulate the distribution of the concentrate. The values ​​of the parameters in the formula are based on the following: the baseline diversion ratio x0 is 0.30, meaning that without considering density and oxidation rate deviations, 30% of the concentrate is returned to the desulfurization circulating slurry, and 70% is discharged to the desulfurization wastewater treatment unit. This ratio is determined based on a comprehensive optimization of the slurry solid phase composition balance and chloride ion load control.

[0101] The target density value ρ0 is set at 1100 kg / m³. 3 The normal operating density range of the desulfurization circulating slurry is 1050 kg / m³. 3 Up to 1150kg / m 3 The median value is used. The target oxidation rate value η0 is set at 0.96, which is the engineering criterion value for complete oxidation of sulfite. The density adjustment coefficient K1 is set at 0.50, which was determined through experimental calibration: when the density deviation is 40 kg / m3, the diversion ratio adjustment is 0.50 × 40 / 1100 = 0.018, corresponding to a concentrate return flow adjustment of approximately 5% to 8%. Experiments have verified that this adjustment range can bring the slurry density back to the target range within 30 minutes. The oxidation rate adjustment coefficient K2 is set at 0.40, which was also determined through experimental calibration: when the oxidation rate deviation is 0.04, the diversion ratio adjustment is 0.40 × 0.04 = 0.016, corresponding to a concentrate return flow adjustment of approximately 5%. Experiments have verified that this adjustment range can bring the oxidation rate back to the target range within 20 minutes. The diversion ratio x is constrained to a range of 0.20 to 0.50; values ​​exceeding this range are taken as boundary values.

[0102] Specific calculation example 1: When ρ = 1140 kg / m 3 When ηox = 0.92, x = 0.30 + 0.50 × (1140 - 1100) / 1100 + 0.40 × (0.96 - 0.92) = 0.30 + 0.018 + 0.016 = 0.334. At this time, the three-way diverter valve diverts 33.4% of the concentrate to the desulfurization circulating slurry reinjection port.

[0103] Specific calculation example 2: When ρ = 1060 kg / m 3 η ox When x = 0.99, x = 0.30 + 0.50 × (1060 - 1100) / 1100 + 0.40 × (0.96 - 0.99) = 0.30 - 0.018 - 0.012 = 0.270.

[0104] At this point, the three-way diverter valve diverts the 27.0% concentrate to the desulfurization circulating slurry reinjection port. Density threshold: 1140 kg / m³ 3 and 1060kg / m 3 The physical basis is that the density range of the desulfurization circulating slurry under normal operating conditions is 1050 kg / m³. 3 Up to 1150kg / m 3 Density exceeds 1140 kg / m³ 3 When the kinematic viscosity ν of the slurry exceeds 4.5 × 10^-6 m 2 The settling velocity of limestone particles, calculated using Stokes' formula vs = g × (ρp - ρl) × dp^2 / (18 × μ), is significantly reduced, where g is the gravitational acceleration taken as 9.8 m / s^2 and ρp is the density of limestone taken as 2700 kg / m³. 3 ρl is the slurry density, dp is the particle diameter (taken as 0.05 mm), and μ is the slurry dynamic viscosity.

[0105] Density less than 1060 kg / m³ 3 At that time, the solid volume fraction φ = (ρ - ρ water) / (ρ solid - ρ water) in the slurry is less than 0.12, and the seed concentration required for gypsum crystallization is insufficient. The three-way diverter valve is an electric three-way regulating valve. The diversion ratio calculation unit executes the above formula calculation every 5 minutes and outputs a regulation signal to the driver of the three-way diverter valve.

[0106] As an optional embodiment, the comparator unit is provided with a trend judgment module, the trend judgment module is provided with a sample-and-hold circuit and a subtractor, the input terminal of the sample-and-hold circuit is communicatively connected to the output terminal of the chloride ion concentration detection probe, the first input terminal of the subtractor is communicatively connected to the output terminal of the chloride ion concentration detection probe, and the second input terminal of the subtractor is communicatively connected to the output terminal of the sample-and-hold circuit.

[0107] The output of the subtractor is communicatively connected to the signal processing unit of the control module.

[0108] It should be noted that:

[0109] The trend analysis module is used to determine the trend of chloride ion concentration changes. The sample-and-hold unit samples and holds the detection value from the chloride ion concentration detection probe at fixed time intervals T (5 minutes). The subtractor subtracts the detection value C(t) at the current sampling moment from the detection value C(tT) held at the previous sampling moment, outputting the difference ΔC = C(t) - C(tT). When ΔC > 0, it indicates that the chloride ion concentration is in the rising range.

[0110] At this time, the control module outputs a signal to the first variable frequency metering pump to reduce its rotational speed via the second signal output terminal, thereby reducing the amount of limestone slurry injected into the upper pH control zone and causing the actual pH value in the upper pH control zone to approach the lower limit of its target pH value of 5.2. Simultaneously, the control module outputs a signal to the second flow regulating valve to increase its opening via the fifth signal output terminal, thereby increasing the amount of oxidation air supplied to the lower oxidation air supply zone.

[0111] The physical logic of reverse compensation regulation: An increase in chloride ion concentration leads to two parallel competitive effects. The first competitive effect is that chloride ions compete with sulfite ions for the alkaline absorbent components in the liquid phase. The total concentration of the alkaline absorbent components in the liquid phase is Calk = [OH-] + [CO3^2-] + [HCO3-], and the competition ratio of chloride ions is [Cl-] / ([Cl-] + [SO3^2-] + [HSO3-]). When [Cl-] increases from 10000 mg / L to 30000 mg / L, the competition ratio increases from 0.42 to 0.68. To compensate for this effect, the target pH value in the upper pH control zone is lowered, increasing the liquid film mass transfer coefficient kL. The relationship between kL and pH is: kL = kL0 × (1 + 0.2 × 6.0 - pH). When the pH decreases from 5.8 to 5.2, kL increases by a factor of 0.12.

[0112] The second competitive effect involves chloride ions competing with sulfite ions for active sites in the oxidation reaction. The relationship between the reduction in oxidation rate and chloride ion concentration is: r / r0 = 1 / (1 + β × [Cl-] / 1000), where β is the competition coefficient, taken as 0.015. When [Cl-] = 25000 mg / L, r / r0 = 1 / (1 + 0.375) = 0.727. To compensate for this effect, the supply of oxidation air in the lower part is increased, raising the dissolved oxygen concentration from C0 to C0 / 0.727 = 1.38C0. When ΔC ≤ 0, it indicates that the chloride ion concentration is in the decreasing or stable range, and the control module outputs a signal to restore the initial set value through the second and fifth signal output terminals. The adjustment response amplitude is calculated in real time by the PID algorithm in the control module: output u(t) = Kp×e(t) + Ki×∫e(t)dt + Kd×de(t) / dt, where e(t) is the difference between the chloride ion concentration detection value and the target value, Kp is the proportional coefficient, Ki is the integral coefficient, and Kd is the derivative coefficient.

[0113] As an optional embodiment, the nanofiltration separation subsystem and the desulfurization efficiency improvement subsystem share a set of circulating cooling medium circuit, which includes a cooling water tank, a cooling water pump, a first heat exchanger, and a second heat exchanger.

[0114] The first heat exchanger is installed in the motor cooling jacket of the variable frequency drive pump, and the second heat exchanger is installed in the compressed air cooling jacket of the oxidation blower. The outlet of the oxidation blower is connected to the upper aeration branch pipe and the lower aeration branch pipe.

[0115] It should be noted that:

[0116] The design of sharing a circulating cooling medium loop between the nanofiltration separation subsystem and the desulfurization efficiency improvement subsystem aims to reduce the overall energy consumption and equipment investment of the system. The motor of the variable frequency drive pump generates heat during operation, requiring a cooling medium to maintain its temperature within acceptable limits. Similarly, the oxidation blower generates heat during air compression, necessitating cooling to improve oxygen solubility in the slurry. The cooling medium loop operates as follows: cooling water in the cooling water tank is pumped through the first and second heat exchangers, absorbing heat from the motor and compressed air before returning to the cooling water tank. The first heat exchanger is located within the cooling jacket of the variable frequency drive pump motor, where cooling water exchanges heat with the motor casing, keeping the motor winding temperature below 80 degrees Celsius.

[0117] The second heat exchanger is installed in the compressed air cooling jacket of the oxidation blower. Cooling water exchanges heat with compressed air to control the temperature of compressed air below 40 degrees Celsius.

[0118] As the temperature of compressed air decreases, the solubility of oxygen in the slurry increases, which is beneficial to the oxidation reaction of sulfites. Compared with two independent cooling circuits, the number of cooling water pumps is reduced from two to one, the volume of the cooling water tank is reduced, and the pipe length is shortened, thus reducing equipment investment costs and operation and maintenance costs.

[0119] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. An integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement, characterized in that, include: A nanofiltration separation subsystem includes a first nanofiltration unit and a second nanofiltration unit. The inlet of the first nanofiltration unit is connected to the suction branch of the desulfurization circulating slurry pipeline via a variable frequency drive pump. The permeate outlet of the first nanofiltration unit is connected to the purified liquid reinjection pipeline. The concentrate outlet of the first nanofiltration unit is connected to the inlet of the second nanofiltration unit. The concentrate outlet of the second nanofiltration unit is connected to the concentrate treatment pipeline. The concentrate treatment pipeline is equipped with a three-way diverter valve. The first outlet of the three-way diverter valve is connected to the desulfurization wastewater treatment unit, and the second outlet of the three-way diverter valve is connected to the desulfurization circulating slurry reinjection port. A desulfurization efficiency improvement subsystem is installed inside the absorption tower. The subsystem includes an upper limestone slurry injection pipeline and a lower limestone slurry injection pipeline arranged along the height of the absorption tower. The upper limestone slurry injection pipeline is equipped with a first variable frequency metering pump, and the lower limestone slurry injection pipeline is equipped with a second variable frequency metering pump. The subsystem also includes an upper aeration branch pipe and a lower aeration branch pipe arranged along the height of the absorption tower. The upper aeration branch pipe is equipped with a first flow regulating valve, and the lower aeration branch pipe is equipped with a second flow regulating valve. An integrated control subsystem includes a chloride ion concentration detection probe, a density detection probe, an oxidation rate detection probe, and a control module. The chloride ion concentration detection probe is installed on the desulfurization circulating slurry pipeline in the section before the inlet of the variable frequency drive pump. The density detection probe and the oxidation rate detection probe are installed in the slurry pool of the absorption tower. The control module is equipped with a first threshold memory and a second threshold memory. The signal input terminals of the control module are communicatively connected to the chloride ion concentration detection probe, the density detection probe, and the oxidation rate detection probe, respectively. The first signal output terminal of the control module is communicatively connected to the variable frequency drive pump. The second signal output terminal of the control module is communicatively connected to the first variable frequency metering pump. The third signal output terminal of the control module is communicatively connected to the second variable frequency metering pump. The fourth signal output terminal of the control module is communicatively connected to the first flow regulating valve. The fifth signal output terminal of the control module is communicatively connected to the second flow regulating valve. The sixth signal output terminal of the control module is communicatively connected to the three-way diverter valve.

2. The integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement according to claim 1, characterized in that, The operating pressure of the first nanofiltration unit is 0.8 MPa to 1.2 MPa, and the operating pressure of the second nanofiltration unit is 1.5 MPa to 2.0 MPa. Both the first nanofiltration unit and the second nanofiltration unit are made of polyamide composite nanofiltration membranes with a molecular weight cutoff of 200 Da to 300 Da. The first nanofiltration unit has a chloride ion rejection rate higher than 0.90 and a calcium ion rejection rate lower than 0.

30. The second nanofiltration unit has a chloride ion rejection rate of over 0.

95.

3. The integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement according to claim 1, characterized in that, The injection area of ​​the upper limestone slurry injection pipeline is the upper pH control zone, and the target pH value of the upper pH control zone is 5.2 to 5.8; The injection area of ​​the lower limestone slurry injection pipeline is the lower pH control zone, and the target pH value of the lower pH control zone is 4.6 to 5.0; The upper pH control zone and the lower pH control zone are each equipped with an online pH detection probe, and the online pH detection probe is communicatively connected to the control module.

4. The integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement according to claim 3, characterized in that, The aeration area of ​​the upper aeration branch pipe is the upper oxidation air supply area, and the aeration area of ​​the lower aeration branch pipe is the lower oxidation air supply area. The amount of oxidation air supplied per unit volume in the upper oxidation air supply zone is 1.3 to 1.8 times that in the lower oxidation air supply zone.

5. The integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement according to claim 1, characterized in that, The control module includes a comparator unit, a signal processing unit, and six signal output terminals. The first input terminal of the comparator unit is communicatively connected to the output terminal of the chloride ion concentration detection probe, the second input terminal of the comparator unit is communicatively connected to the output terminal of the first threshold memory, the third input terminal of the comparator unit is communicatively connected to the output terminal of the second threshold memory, the output terminal of the comparator unit is communicatively connected to the input terminal of the signal processing unit, and the output terminal of the signal processing unit is communicatively connected to the six signal output terminals respectively. The comparison result signal output by the comparator unit includes a first comparison result signal and a second comparison result signal. The first comparison result signal is the difference signal between the detection value of the chloride ion concentration detection probe and the value stored in the first threshold memory. The second comparison result signal is the difference signal between the detection value of the chloride ion concentration detection probe and the value stored in the second threshold memory.

6. The integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement according to claim 5, characterized in that, The threshold stored in the first threshold memory is determined experimentally by measuring the effect of chloride ion concentration on sulfur dioxide removal efficiency. This experimental measurement involves preparing simulated desulfurization slurry with a chloride ion concentration gradient ranging from 5000 mg / L to 30000 mg / L in a constant-temperature water bath reactor, while controlling the flue gas sulfur dioxide concentration at 2000 mg / m³. 3 The liquid-to-gas ratio is 15 L / m³. 3 The reaction temperature was 50 degrees Celsius, the slurry pH was 5.5, the outlet sulfur dioxide concentration was measured and the removal efficiency was calculated. The removal efficiency was taken as the baseline efficiency when the chloride ion concentration was 5000 mg / L. The chloride ion concentration value corresponding to 0.05; The threshold stored in the second threshold memory is determined by experimental measurement of the effect curve of chloride ion concentration on sulfite oxidation rate. The experimental measurement is carried out by adding calcium sulfite suspension to simulated slurry under the same conditions, passing in oxidizing air and measuring the conversion rate of calcium sulfite to calcium sulfate per unit time. The chloride ion concentration value corresponding to the oxidation rate decreasing by 0.10 compared with the baseline oxidation rate when the chloride ion concentration is 5000 mg / L is taken. The threshold stored in the first threshold memory is lower than the threshold stored in the second threshold memory.

7. The integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement according to claim 1, characterized in that, The control module is also equipped with a feedforward compensation unit, which has a boiler load signal receiving port and an inlet flue gas sulfur dioxide concentration signal receiving port, and a prediction model memory. The output of the feedforward compensation unit is communicatively connected to the signal processing unit of the control module.

8. The integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement according to claim 1, characterized in that, The control module is equipped with a current split ratio calculation unit. The input terminal of the current split ratio calculation unit is communicatively connected to the density detection probe and the oxidation rate detection probe, respectively. The output terminal of the current split ratio calculation unit is communicatively connected to the sixth signal output terminal of the control module. The diversion ratio calculation unit stores the diversion ratio calculation formula: x=x0+K1×(ρ-ρ0) / ρ0+K2×(η0-η ox (where x0 is the baseline split ratio, taken as 0.30, and ρ0 is the target density value, taken as 1100 kg / m³). 3 η0 is the target oxidation rate value, set to 0.96; K1 is the density adjustment coefficient, set to 0.50; K2 is the oxidation rate adjustment coefficient, set to 0.40; ρ is the real-time density detection value of the density detection probe; η ox This refers to the real-time oxidation rate detection value of the oxidation rate detection probe.

9. The integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement according to claim 5, characterized in that, The comparator unit is equipped with a trend judgment module, which is equipped with a sample-and-hold circuit and a subtractor. The input terminal of the sample-and-hold circuit is communicatively connected to the output terminal of the chloride ion concentration detection probe. The first input terminal of the subtractor is communicatively connected to the output terminal of the chloride ion concentration detection probe, and the second input terminal of the subtractor is communicatively connected to the output terminal of the sample-and-hold circuit. The output of the subtractor is communicatively connected to the signal processing unit of the control module.

10. The integrated system for wet desulfurization slurry purification and desulfurization efficiency improvement according to claim 1, characterized in that, The nanofiltration separation subsystem and the desulfurization efficiency improvement subsystem share a common circulating cooling medium circuit, which includes a cooling water tank, a cooling water pump, a first heat exchanger, and a second heat exchanger. The first heat exchanger is installed in the motor cooling jacket of the variable frequency drive pump, and the second heat exchanger is installed in the compressed air cooling jacket of the oxidation blower. The outlet of the oxidation blower is connected to the upper aeration branch pipe and the lower aeration branch pipe.

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