A circulating fluidized bed semi-dry desulfurization process

Abstract

The present application relates to a kind of circulating fluidized bed semi-dry desulfurization process, belong to flue gas desulfurization technical field, including S1, the flue gas from boiler air preheater enters bag filter and is pre-dusted, then sent into reaction tower, and real-time monitoring flue gas parameter is carried out to the flue gas entering reaction tower, S2, based on flue gas parameter automatic diagnosis flue gas state is standard working condition, extreme working condition, S3, according to the difference of flue gas state, flue gas is adaptively handled, S4, after reaction, flue gas is gas-solid separation, gas is discharged, and solid is recycled catalytic-absorption composite agent by magnetic separation;The present application has the advantages of efficient desulfurization, prevent scale formation, reduce operating cost and realize by-product resource.

Description

Technical Field

[0001] This invention belongs to the field of flue gas desulfurization technology, specifically relating to a circulating fluidized bed semi-dry desulfurization process. Background Technology

[0002] Semi-dry flue gas desulfurization processes (such as spray drying) are desulfurization technologies that fall between wet and dry methods. The principle involves spraying lime (CaO) and water into the absorption tower. When the flue gas comes into contact with the lime and water, it reacts to produce byproducts such as CaSO3. Simultaneously, the moisture in the slurry evaporates, forming dry desulfurization ash.

[0003] Although semi-dry desulfurization processes do not require wastewater treatment, their desulfurization efficiency is insufficient to meet increasingly stringent ultra-low emission requirements (e.g., SO2 emission concentration ≤35mg / m³). Meanwhile, scaling within the tower and nozzle wear increase maintenance costs, and the large amount of desulfurization ash is primarily disposed of through landfill, which is both wasteful of resources and poses environmental risks. Furthermore, traditional single-stage spray methods lack flexibility in responding to changes in flue gas load, easily leading to waste of desulfurizing agents or excessive emissions. Therefore, there is an urgent need for a semi-dry desulfurization process that can efficiently desulfurize, prevent scaling, reduce operating costs, and achieve byproduct resource utilization. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems existing in the prior art and to provide a circulating fluidized bed semi-dry desulfurization process.

[0005] The technical solution of the present invention is as follows:

[0006] A circulating fluidized bed semi-dry desulfurization process includes the following steps:

[0007] S1. The flue gas from the boiler air preheater enters the bag filter for pre-dust removal and is then sent into the reaction tower. The flue gas parameters are monitored in real time.

[0008] S2. Automatic diagnosis of flue gas status based on flue gas parameters, including standard and extreme operating conditions, where SO2 concentration < 600 mg / Nm³. 3 At that time, the flue gas condition is under standard operating conditions, and the SO2 concentration is ≥600mg / Nm³. 3 At that time, the flue gas condition was an extreme operating condition;

[0009] S3. Adaptive processing of flue gas is performed according to different flue gas states, wherein the adaptive processing includes:

[0010] When the flue gas is under standard operating conditions: quicklime and water mist are injected into the reaction tower to carry out the desulfurization reaction, and when the flue gas flow rate is less than 65% of the rated load, the clean flue gas recirculation is started;

[0011] When the flue gas is under extreme operating conditions: inject the catalytic-absorption composite agent and water mist into the reaction tower to carry out the desulfurization reaction, and start the clean flue gas recirculation when the flue gas flow rate is less than 65% of the rated load;

[0012] S4. Perform gas-solid separation on the flue gas after the reaction. The gas is discharged, and the solid is recovered as the catalytic-absorption composite agent through magnetic separation.

[0013] Furthermore, the gas discharged in step S4 is monitored for SO2 concentration. When the SO2 concentration remains above 35 mg / Nm³, further monitoring is conducted. 3 If the flue gas fails to meet the standards, 5-15% of it will be sent back to the inlet of the reaction tower for secondary treatment.

[0014] Furthermore, the baseline value for the standard operating condition described in step S2 is an SO2 concentration of 380-420 mg / Nm³. 3 .

[0015] Furthermore, the initial mass fraction ratio of the catalyst-absorption composite agent is as follows: 80%±3% highly active calcium-based absorbent, 10%±2% sodium-based promoter, 3%±1% nanocomposite catalyst, and 7%±1% carrier / flow aid.

[0016] Furthermore, the catalytic-absorption composite agent is effective when the SO2 concentration is higher than 600 mg / Nm³. 3 When the total feed amount of the catalyst-absorption composite agent is increased by 20-50%, the proportion of sodium-based promoter is increased to the upper limit of 15%; when the SO2 concentration is higher than 1000 mg / Nm³, 3 At that time, the proportion of nanocomposite catalysts in the catalytic-absorption composite increases by 0.5%-1%.

[0017] Furthermore, the SO2 concentration of the gas discharged in step S4 is detected. When the outlet SO2 concentration is higher than 35 mg / Nm³, the concentration is determined to be higher. 3 At that time, the dosage was 10 mg / Nm³ per deviation. 3 Increase the proportion of sodium-based accelerator by 1% to 2% and increase the total feed by 5% to 10%; when the outlet SO2 concentration is lower than the target value, reduce the addition ratio accordingly; based on magnetic separation and recovery data and circulating ash flowability monitoring, when the active components of the nanocomposite catalyst are lost, increase the replenishment ratio of the nanocomposite catalyst by 0.5% and activate the return combustion optimization.

[0018] Furthermore, the nanocomposite catalyst has a core-shell structure with Fe3O4 as the magnetic core and Mn-doped CeO2 as the shell, wherein the saturation magnetization of the magnetic core is not less than 50 emu / g, the shell thickness is 20±5 nm, and the SO2 oxidation rate of the nanocomposite catalyst is not less than 95% under the conditions of 150℃ and SO2 concentration of 1000ppm.

[0019] Furthermore, the recirculation flow rate of the clean flue gas recirculation is 20-30% of the total flue gas volume.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] The desulfurization process of this invention employs a catalytic-absorption composite agent under extreme operating conditions with SO2 concentration ≥600mg / Nm³. Its core nano-catalyst can significantly enhance the oxidation and absorption of SO2, ensuring that the desulfurization efficiency remains stable at over 95%, effectively addressing the challenges of high-sulfur fuels or load fluctuations. Furthermore, the sodium-based promoter in the catalytic-absorption composite agent can improve the reaction environment and reduce the formation of viscous intermediate products (such as calcium sulfite), while the nano-catalyst promotes the conversion of SO2 into easily treatable sulfates, reducing the risk of scaling in the tower and pipelines from the reaction mechanism perspective. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0023] A circulating fluidized bed semi-dry desulfurization process includes the following:

[0024] S1. The flue gas from the boiler air preheater enters the bag filter for pre-dust removal and is then sent into the reaction tower. The flue gas parameters are monitored in real time.

[0025] S2. Automatic diagnosis of flue gas status based on flue gas parameters, including standard and extreme operating conditions, where SO2 concentration < 600 mg / Nm³. 3 At that time, the flue gas condition is under standard operating conditions, and the SO2 concentration is ≥600mg / Nm³. 3 At that time, the flue gas condition was an extreme operating condition;

[0026] S3. Adaptive processing of flue gas is performed according to different flue gas states, wherein the adaptive processing includes:

[0027] When the flue gas is under standard operating conditions: quicklime and water mist are injected into the reaction tower to carry out the desulfurization reaction, and when the flue gas flow rate is less than 65% of the rated load, the clean flue gas recirculation is started;

[0028] When the flue gas is under extreme operating conditions: inject the catalytic-absorption composite agent and water mist into the reaction tower to carry out the desulfurization reaction, and start the clean flue gas recirculation when the flue gas flow rate is less than 65% of the rated load;

[0029] The recirculation flow rate of the clean flue gas recirculation is 20-30% of the total flue gas volume;

[0030] S4. Perform gas-solid separation on the flue gas after the reaction. The gas is discharged, and the solid is recovered by magnetic separation of the catalytic-absorption composite agent. Monitor the SO2 concentration of the discharged gas. If the SO2 concentration remains above 35 mg / Nm³, the gas is discharged. 3 If the flue gas fails to meet the standards, 5-15% of it will be sent back to the inlet of the reaction tower for secondary treatment.

[0031] Preferably, the baseline value for the standard operating condition is an SO2 concentration of approximately 380-420 mg / Nm³. 3 ;

[0032] In this embodiment, the initial mass fraction ratio of the catalyst-absorption composite agent is: 80%±3% highly active calcium-based absorbent, 10%±2% sodium-based promoter, 3%±1% nanocomposite catalyst, and 7%±1% carrier / flow aid;

[0033] Catalyst-absorption composite agent at SO2 concentrations above 600 mg / Nm 3 When the total feed amount of the catalyst-absorption composite agent is increased by 20-50%, the proportion of sodium-based promoter is increased to the upper limit of 15%; when the SO2 concentration is higher than 1000 mg / Nm³, 3 At that time, the proportion of nanocomposite catalysts in the catalysis-absorption composite agent increased by 0.5%-1%;

[0034] The nanocomposite catalyst has a core-shell structure with Fe3O4 as the magnetic core and Mn-doped CeO2 as the shell, wherein the saturation magnetization of the magnetic core is not less than 50 emu / g, the shell thickness is 20±5 nm, and the nanocomposite catalyst has an SO2 oxidation rate of not less than 95% under the conditions of 150℃ and SO2 concentration of 1000ppm.

[0035] During use, the SO2 concentration of the gas discharged in step S4 is detected. When the outlet SO2 concentration is higher than 35 mg / Nm³, the concentration is considered zero. 3 At that time, the dosage was 10 mg / Nm³ per deviation. 3 Increase the proportion of sodium-based accelerator by 1% to 2% and increase the total feed by 5% to 10%; when the outlet SO2 concentration is lower than the target value, reduce the addition ratio accordingly; based on magnetic separation and recovery data and circulating ash flowability monitoring, when the active components of the nanocomposite catalyst are lost, increase the replenishment ratio of the nanocomposite catalyst by 0.5% and activate the return combustion optimization.

[0036] In this embodiment, the preparation method of the nanocomposite catalyst includes the following steps:

[0037] Step (1) Synthesis of Fe3O4 magnetic cores: In a microchannel reactor, industrial grade FeCl3·6H2O and FeSO4·7H2O with a molar ratio of 2:1 were used as iron sources. The reaction was carried out continuously with 2M NaOH solution at 80±1℃ and pH=11.0±0.1 for 5 minutes. The reaction was carried out under an inert atmosphere. The reaction product was separated by an online electromagnetic separator and then dried by spray drying technology at an inlet temperature of 200℃ and an outlet temperature of 80℃ to obtain Fe3O4 nanoparticles with a particle size of 50±10nm.

[0038] Step (2) Coating and activation of Mn-doped CeO2 shell: Fe3O4 nanoparticles obtained in step (1) were placed in a fluidized bed reactor. Under the conditions of 60±2℃ and fluidization rate of 0.5m / s, a mixed precursor solution of Ce(NO3)3·6H2O and Mn(NO3)2·4H2O with a Ce to Mn molar ratio of 9:1 was atomized and sprayed. The precursor solution concentration was 0.5mol / L and the solvent was a water-ethanol mixture containing 0.5% polyethylene glycol-4000. The spraying rate was controlled at 2mL / min for 30 minutes. After spraying, the product was aged and hydrolyzed for 10 minutes under microwave power of 300W. Then, the product was heated to 350℃ in a continuous microwave calcination furnace under an air atmosphere and kept at that temperature for 5 minutes to obtain Mn-doped CeO2@Fe3O4 nanocomposite with a shell thickness of 20±5nm.

[0039] Step (3) Post-processing: Soak the nanocomposite obtained in step (2) in 3% dilute acetic acid for 10 minutes, wash with water until neutral, and dry at 110°C for 5 minutes to obtain the final product;

[0040] The effectiveness of the circulating fluidized bed semi-dry desulfurization process provided by this invention will be verified below;

[0041] I. Basic Experimental Conditions

[0042] Boiler model: 1025t / h subcritical circulating fluidized bed boiler;

[0043] Desulfurization reaction tower: Turbo circulating fluidized bed reaction tower (seven-hole Venturi hollow tower structure);

[0044] Dust collector: Special bag dust collector (filter material treated with PPS+PTFE);

[0045] Flue gas volume treated: approximately 946,800 m³ 3 / h (rated load);

[0046] To verify the performance of the desulfurization process under different operating conditions, the SO2 concentration in the flue gas was divided into categories: 380-420 mg / Nm³. 3650-850mg / Nm 3 And SO2 concentration 400 mg / Nm 3 Three stages, among which:

[0047] SO2 concentration should be controlled at 380-420 mg / Nm³. 3 Corresponding standard operating condition test;

[0048] SO2 concentration should be controlled at 650-850 mg / Nm³. 3 Corresponding to extreme working condition tests;

[0049] The flue gas flow rate was reduced to 60% of the rated load (approximately 568,080 m³ / h). 3 / h), SO2 concentration 400 mg / Nm 3 Corresponding standard operating condition test;

[0050] Flue gas temperature: 100-130℃ (remains constant);

[0051] Other equipment and systems: Mn-doped CeO2@Fe3O4 catalyst storage and feeding system, high gradient magnetic separator (magnetic field strength 1.2T), DCS intelligent control system, clean flue gas recirculation pipeline (maximum 30% recirculation rate).

[0052] II. Test Operation Process and Key Phenomena

[0053] The trial period lasted 45 days and was divided into three phases.

[0054] Phase 1:

[0055] Operating conditions: SO2 inlet concentration maintained at 400±20 mg / Nm³ 3 The boiler is operating at full load.

[0056] Process operation:

[0057] The system automatically diagnosed it as "standard operating condition".

[0058] The DCS intelligent control system controls the addition of quicklime according to the standard ratio and sprays water mist.

[0059] The temperature of the desulfurization reaction tower is controlled at 120±5°C, and the pressure drop of the fluidized bed is maintained at 900±100Pa.

[0060] During operation, the SO2 concentration in the outlet flue gas decreased from an initial level of approximately 80 mg / Nm³ within 4 hours. 3 Rapidly decreased to 22-28 mg / Nm 3 The interval;

[0061] Phase Two:

[0062] Operating conditions: By blending high-sulfur coal, the SO2 inlet concentration is increased in stages: Days 16-20: 650-700 mg / Nm³ 3 Days 21-25: 720-780 mg / Nm 3 Days 26-30: 800-850 mg / Nm 3 .

[0063] Process operation:

[0064] When SO2 concentration exceeds 600 mg / Nm 3 When the condition occurs, the DCS intelligent control system immediately alarms and automatically switches to the "extreme working condition" mode, and uses a high-gradient magnetic separator to magnetically recover the catalyst-absorption composite agent.

[0065] Intelligent dynamic response:

[0066] SO2 concentration is 650-700 mg / Nm³ 3 At that time, the total feed rate was increased by 25%, and the proportion of sodium-based accelerator was increased to 12%.

[0067] SO2 concentration rises to 720-780 mg / Nm³ 3 At the same time, the total feed rate will be increased by 35%, and the proportion of sodium-based accelerator will be increased to 14%.

[0068] SO2 concentration reached 800-850 mg / Nm³ 3 At its peak, the total feed rate is increased by 45%, the proportion of sodium-based accelerator is increased to the upper limit of 15%, and the proportion of nanocomposite catalyst is automatically increased to 3.8%.

[0069] Key phenomena:

[0070] During the second-stage, stepwise increase of SO2 inlet concentration, the DCS intelligent control system automatically executed a preset "extreme operating condition" mode based on concentration changes. By dynamically increasing the dosage of the catalytic-absorption composite agent and adjusting the component ratio, the average outlet SO2 concentration under the three high-sulfur concentration steps was successfully stabilized at 26.8, 27.5, and 28.9 mg / Nm³, respectively. 3 .

[0071] Meanwhile, the catalyst recovery and circulation system operated smoothly, and the high-gradient magnetic separator had a stable operating current, indicating that its separation load was uniform and there was no blockage. Material balance confirmed that the system's magnetic recovery rate reached 96.5%. The recovered catalyst was sampled and tested, and its catalytic activity (based on CaSO3 oxidation rate) was 94.2%, and the magnetic strength decreased by less than 5% compared with the initial value, proving that it has excellent chemical and physical stability.

[0072] Based on continuous measurement of the amount of fresh catalyst replenished and recovered, it was calculated that the net catalyst consumption under this extreme condition is only 10.5 kg / h.

[0073] Observation of the circulating ash samples showed that the proportion of CaSO4 in the circulating ash was 90.5%, and the texture was dry and loose with no sticking.

[0074] Phase Three

[0075] Operating conditions: High-sulfur coal is no longer blended, resulting in an SO2 concentration of 400 mg / Nm³. 3 And reduce the flue gas flow rate to 60% of the rated load.

[0076] Process operation:

[0077] The flue gas condition is confirmed to be standard operating condition based on SO2 concentration. Based on the detected flue gas flow rate, the system automatically starts clean flue gas recirculation, and the recirculation flow rate is controlled at 25% of the total flue gas volume. The DCS control adds quicklime and sprays water mist according to the benchmark ratio.

[0078] Key phenomena:

[0079] The recirculation system effectively maintained the fluidization state and temperature stability within the reaction tower.

[0080] During 15 days of continuous operation, despite reduced load, the outlet SO2 concentration remained stable at 20-30 mg / Nm³. 3 Within the specified range, and the average SO2 concentration at the outlet is 24.5 mg / Nm³. 3 No significant fluctuations were observed.

[0081] III. Test Data Results and Compliance Analysis

[0082] The key performance data collected after a complete 45-day test cycle are as follows:

[0083] Table 1 (Emission Performance Data)

[0084] Operating condition phase <![CDATA[Average inlet SO2 concentration (mg / Nm 3 )]]> <![CDATA[Average concentration of outlet SO2 (mg / Nm 3 )]]> Desulfurization efficiency (%) <![CDATA[Reaching assurance rate (proportion of time when concentration < 35mg / Nm 3 )]]> Standard operating conditions 400 25.2 93.7% 100% Extreme operating conditions (650-700) 675 26.8 96.0% 100% Extreme operating conditions (720-780) 750 27.5 96.3% 100% Extreme operating conditions (800-850) 825 28.9 96.5% 100% Standard operating conditions (flow rate is 60% of rated load) 400 24.5 93.9% 100%

[0085] Performance index test results, target values, and conclusions;

[0086] Magnetic recovery rate ≥95% (96.5%)

[0087] Catalyst activity (CaSO3 oxidation rate) 94.2%;

[0088] The catalyst consumption (net consumption) is 10.5 kg / h, and the recovery system is highly effective.

[0089] The recycled ash contains 90.5% CaSO4, and the by-products are of high quality, which is conducive to resource utilization.

[0090] IV. Verification Conclusion

[0091] The test data fully demonstrates that the desulfurization process of this application can ensure that the outlet SO2 concentration is consistently below 35 mg / Nm³ under all test conditions. 3 Ultra-low emission standards.

[0092] Under standard operating conditions (400 mg / Nm³) 3 Under these conditions, the outlet concentration remained stable at 25.2 mg / Nm³. 3 .

[0093] Under extreme operating conditions (up to 850 mg / Nm³) 3 Under these conditions, the highest outlet concentration was only 33 mg / Nm³. 3 Furthermore, through intelligent regulation, it can be rapidly stabilized at 30 mg / Nm³. 3 the following.

[0094] Under standard operating conditions (flow rate at 60% of rated load), the system ensured reaction conditions through recirculation, with an average emission concentration of 24.5 mg / Nm³. 3 It is even better than the full-load condition.

[0095] Throughout the entire testing period, the compliance rate was 100%, and no events exceeding the standards occurred, demonstrating the system's extremely high reliability.

[0096] Highly efficient oxidation by nanocomposite catalysts: The efficiency of converting reaction intermediate CaSO3 into CaSO4 exceeds 90%, fundamentally improving the completeness of the desulfurization reaction.

[0097] Multi-level protection design: Even under the most extreme conditions, the system has sufficient redundancy (such as double loop) to ensure reliability and stability.

[0098] In conclusion, the circulating fluidized bed semi-dry desulfurization process is fully feasible under the experimental conditions described above, and detailed experimental data proves that it can stably meet and exceed ultra-low emission standards. It is a mature, reliable, and efficient innovative technology.

[0099] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A semi-dry desulfurization method using a circulating fluidized bed, characterized in that: Includes the following steps: S1. The flue gas from the boiler air preheater enters the bag filter for pre-dust removal and is then sent into the reaction tower. The flue gas parameters are monitored in real time. S2. Automatic diagnosis of flue gas status based on flue gas parameters, including standard and extreme operating conditions, where SO2 concentration < 600 mg / Nm³. 3 At that time, the flue gas condition is under standard operating conditions, and the SO2 concentration is ≥600mg / Nm³. 3 At that time, the flue gas condition was an extreme operating condition; S3. Adaptive processing of flue gas is performed according to different flue gas states, wherein the adaptive processing includes: When the flue gas is under standard operating conditions: quicklime and water mist are injected into the reaction tower to carry out the desulfurization reaction, and when the flue gas flow rate is less than 65% of the rated load, the clean flue gas recirculation is started; When the flue gas is under extreme operating conditions: inject the catalytic-absorption composite agent and water mist into the reaction tower to carry out the desulfurization reaction, and start the clean flue gas recirculation when the flue gas flow rate is less than 65% of the rated load; The components of the catalytic-absorption composite agent include a highly active calcium-based absorbent, a sodium-based promoter, a nanocomposite catalyst, and a support / flow aid. The nanocomposite catalyst has a core-shell structure with Fe3O4 as the magnetic core and Mn-doped CeO2 as the shell. S4. Perform gas-solid separation on the flue gas after the reaction. The gas is discharged, and the solid is recovered as the catalytic-absorption composite agent through magnetic separation.

2. The circulating fluidized bed semi-dry desulfurization method according to claim 1, characterized in that: The gas discharged in step S4 is monitored for SO2. When the SO2 concentration remains above 35 mg / Nm³, further monitoring is conducted. 3 If the flue gas fails to meet the standards, 5-15% of it will be sent back to the inlet of the reaction tower for secondary treatment.

3. The circulating fluidized bed semi-dry desulfurization method according to claim 1, characterized in that: The baseline value for the standard operating conditions mentioned in step S2 is an SO2 concentration of 380-420 mg / Nm³. 3。 4. The circulating fluidized bed semi-dry desulfurization method according to claim 1, characterized in that: The initial mass fraction ratio of the catalyst-absorption composite agent is: 80%±3% highly active calcium-based absorbent, 10%±2% sodium-based promoter, 3%±1% nanocomposite catalyst, and 7%±1% carrier / flow aid.

5. The circulating fluidized bed semi-dry desulfurization method according to claim 4, characterized in that: SO2 concentration is 650-700 mg / Nm³ 3 At the same time, the total feed rate will be increased by 25%, and the proportion of sodium-based accelerator will be increased to 12%; SO2 concentration rises to 720-780 mg / Nm³ 3 At the same time, the total feed rate will be increased by 35%, and the proportion of sodium-based accelerator will be increased to 14%; SO2 concentration reached 800-850 mg / Nm³ 3 At peak levels, the total feed rate will be increased by 45%, and the proportion of sodium-based accelerators will be raised to the upper limit of 15%. When SO2 concentration is higher than 1000 mg / Nm 3 At that time, the proportion of nanocomposite catalysts in the catalytic-absorption composite increases by 0.5%-1%.

6. The circulating fluidized bed semi-dry desulfurization method according to claim 4, characterized in that: The SO2 concentration of the gas discharged in step S4 is detected. When the outlet SO2 concentration is higher than 35 mg / Nm³, the concentration is determined to be negative. 3 At that time, the dosage was 10 mg / Nm³ per deviation. 3 Increase the proportion of sodium-based accelerator by 1% to 2% and increase the total feed by 5% to 10%; when the outlet SO2 concentration is lower than the target value, reduce the addition ratio accordingly; based on magnetic separation and recovery data and circulating ash flowability monitoring, when the active components of the nanocomposite catalyst are lost, increase the replenishment ratio of the nanocomposite catalyst by 0.5% and activate the return combustion optimization.

7. The circulating fluidized bed semi-dry desulfurization method according to claim 4, characterized in that: The nanocomposite catalyst has a core-shell structure with Fe3O4 as the magnetic core and Mn-doped CeO2 as the shell, wherein the saturation magnetization of the magnetic core is not less than 50 emu / g, the shell thickness is 20±5 nm, and the SO2 oxidation rate of the nanocomposite catalyst is not less than 95% under the conditions of 150℃ and SO2 concentration of 1000ppm.

8. The circulating fluidized bed semi-dry desulfurization method according to claim 1, characterized in that: The recirculation flow rate of the clean flue gas recirculation is 20-30% of the total flue gas volume.