Method for removing sulfur oxides and nitrogen oxides from gas flow

Abstract

A method for removing sulfur oxides and nitrogen oxides from air flow belongs to the technical field of air pollution control, and is characterized in that treated air flow is introduced into a desulfurization reactor, a nitric acid solution absorbent is introduced into the reactor, sulfur dioxide in the air flow and nitric acid in the absorbent are subjected to an oxidation-reduction reaction, and the sulfur oxides and the nitrogen oxides are removed. Nitric oxide gas generated in the reaction process of the sulfur dioxide and nitric acid is guided into a denitration reactor along with the gas flow, meanwhile, ferric chloride absorbent slurry is guided into the denitration reactor, and the nitric oxide gas in the gas flow and ferric chloride are subjected to a chemical reaction to be absorbed; nitric oxide gas released by regeneration of the saturated absorbent after absorption is oxidized, nitric acid is recycled as a desulfurization absorbent for recycling, and ferric salt absorbent slurry is recycled after regeneration.

Description

Technical Field

[0001] This invention relates to a method for removing sulfur oxides and nitrogen oxides from airflow, belonging to the field of air pollution control and related environmental protection technology. Background Technology

[0002] Sulfur oxides in the atmosphere (including sulfur dioxide, carbon oxysulfide, carbon disulfide, and hydrogen sulfide) mainly originate from the combustion of fossil fuels, followed by processes in metallurgy, sulfuric acid production, oil refining, and the chemical industry. Generally, the concentration of sulfur dioxide in flue gas from fossil fuel combustion at thermal power plants is approximately several hundred to several thousand mg / m³. 3 The concentration of sulfur dioxide in the exhaust gas from the metallurgical industry is even higher.

[0003] The lime / limestone alkaline solution absorption method is currently the main method for removing sulfur dioxide from flue gas (referred to as wet desulfurization). Compared with other desulfurization processes, its advantages are high absorption efficiency and low operating costs. Its disadvantages are that the system process is long, it occupies a large area, consumes a large amount of lime or limestone, and the corresponding by-products (mainly calcium sulfate) and wastewater need further treatment.

[0004] The inventors of this invention have disclosed a method for removing nitric oxide from airflow using ferric chloride slurry as a denitrification absorbent (CN111167263). The principle is to utilize the chemical reaction between solid ferric chloride in the slurry and nitric oxide gas in the airflow under certain conditions to obtain the corresponding compound nitrosyl ferric chloride, thereby removing it from the airflow.

[0005] Based on the above invention, this invention proposes a new method for removing harmful gases such as sulfur oxides and nitrogen oxides from airflow. Summary of the Invention

[0006] The technical solution adopted in this invention is as follows: a method for removing sulfur oxides and nitrogen oxides from an airflow, characterized by introducing the airflow to be treated into a first reactor (desulfurization reactor), and simultaneously introducing a nitric acid solution absorbent (hereinafter referred to as absorbent) into the reactor. The sulfur oxides in the airflow (taking sulfur dioxide as an example) undergo a redox reaction with the nitric acid in the absorbent, causing the sulfur dioxide to be oxidized to sulfur trioxide, which is then absorbed by the absorbent aqueous solution. The nitric oxide gas generated during the reaction of sulfur dioxide and nitric acid is then introduced into a second reactor (denitrification reactor) with the airflow. Simultaneously, a ferric chloride absorbent slurry is introduced into the reactor. The nitric oxide gas in the airflow reacts chemically with the ferric chloride crystals in the ferric chloride slurry and is absorbed, yielding the corresponding nitrosyl complex, thereby achieving the purpose of gas purification. The main reactions are: 3SO2 + 2HNO3 + 2H2O → 3H2SO4 + 2NO↑ (1) FeCl3 + mNO + nH2O → Fe(H2O)n(NO)mCl3 (2) Where m + n ≤ 6 (generally ≤ 3, depending on the reaction conditions).

[0007] The concentration of the nitric acid solution used as the desulfurization absorbent in the method described in this invention is generally below 65%, preferably 1-15%. Higher concentrations may produce nitrogen dioxide gas. The theoretical molar ratio of nitric acid to sulfur dioxide is 1:1.5. The actual dosage of the absorbent mainly depends on factors such as the concentration of nitric acid in the absorbent, the reaction temperature, the actual removal rate requirement, and the regeneration cycle, and there are no special requirements.

[0008] The reaction temperature of the desulfurization reactor is generally at or above room temperature, preferably 50℃ to 90℃. Appropriately increasing the temperature can accelerate the reaction rate.

[0009] A certain amount of metal ions can also be added to the absorbent as a catalyst. These metal ions mainly include alkali metal and transition metal ions, such as alkali metal ions (Na+). ＋ K ＋ Li ＋ and transition metal ions Fe 3+ / Fe 2+ Mn 2+ Cu 2+ Zn 2+ Co 2+ Ni 2+ Al 3+ and Se 3+ One or more of these (in the form of nitrates, sulfates, or hydrochlorides) are added, generally at a rate of less than 1% (by mass) of the initial absorbent, but without limitation. These substances can promote the redox reaction between nitric acid and sulfur dioxide. A certain amount of oxidizing agent, such as H₂O₂, can also be added to the absorbent to promote the oxidation of sulfur dioxide.

[0010] The residence time of the gas in the desulfurization reactor is generally above 0.2 seconds, preferably 1 to 2 seconds, but there is no limit. When the mass concentration of sulfuric acid in the absorbent reaches 30 to 50%, it can be discharged from the reactor to the sulfuric acid storage tank.

[0011] Desulfurization reactors can be gas-liquid contact reactors such as rotary, spray, bubble, and moving bed reactors, which are commonly used in chemical unit operations. They can be used in various forms such as co-current, counter-current, and cross-current, with roughly the same effect. For specific design parameters, please refer to the relevant chemical equipment design manual.

[0012] The gas after sulfur dioxide removal is introduced into the denitrification reactor, where nitric oxide gas produced during the reaction of sulfur dioxide and nitric acid, as well as nitric oxide gas originally present in the gas stream (which has little effect on sulfur dioxide removal), are absorbed by the iron salt absorbent slurry. The saturated iron salt absorbent is regenerated and recycled, with the regeneration byproduct nitric acid being recycled as an oxidizing absorbent for sulfur dioxide. Since the temperature of the gas decreases and its moisture content increases after desulfurization, the gas stream can be heated or dehumidified before being introduced into the denitrification reactor to meet the requirements of the nitric oxide removal process. Heating can be achieved using a hot air furnace to introduce high-temperature gas and heat exchangers; dehumidification can be achieved using condensation dehumidification, or absorption dehumidification with absorbents such as calcium chloride and lithium chloride.

[0013] The denitrification reactor and the preparation and regeneration of the absorbent slurry described in this invention can be found in the aforementioned patents published by the inventors. The nitric oxide gas released from the regeneration of the iron salt absorbent slurry after absorption saturation can be oxidized and recovered as nitric acid for recycling as a desulfurization absorbent. The iron salt absorbent slurry is also recycled after regeneration.

[0014] The airflow described in this invention is mainly flue gas containing sulfur oxides and nitrogen oxides produced by burning fossil fuels in thermal power generation, smelting, etc., and may also be related airflows produced by other industrial processes.

[0015] The method described in this invention is also suitable for removing harmful gases from airflows containing carbon disulfide (CS2), carbon oxysulfide (COS), hydrogen sulfide (H2S), formaldehyde, methanethiol, and other gases that can undergo redox reactions with nitric acid, including both inorganic and organic compounds. Possible reactions include: 3CS2 + 4HNO3 → 2H2O + 3CO2 + 6S↓ + 4NO↑ (3) 3COS + 2HNO3 → H2O + 3CO2 + 3S↓ + 2NO↑ (4) 3H2S + 2HNO3 → 4H2O + 3S↓ + 2NO↑ (5) 3HCHO + 4HNO3→ 5H2O + 3CO2↑ + 4NO↑ (6) Compared with the prior art, the advantages of this invention are: it uses nitric acid solution to oxidize and absorb harmful gases such as sulfur oxides in the gas stream, oxidizing them into sulfur trioxide which is then absorbed by the absorbent solution, thus achieving gas purification while recovering by-products such as sulfuric acid. The nitric oxide gas generated during the reaction process is absorbed by the subsequent ferric chloride absorbent slurry. The ferric salt absorbent is recycled after regeneration, and the regenerated by-product nitric acid is recycled as a sulfur dioxide oxidizing absorbent. Attached Figure Description

[0016] Figure 1This is a schematic diagram of the process flow of the method of the present invention.

[0017] In the diagram: 101 First reactor; 102 Absorbent storage tank; 103 Sulfuric acid storage tank; 201 Second reactor; 202 Iron salt absorbent slurry storage tank; 203 Iron salt absorbent regeneration reactor.

[0018] The process flow involves introducing a sulfur dioxide-containing gas stream into the first reactor 101, while simultaneously introducing an absorbent solution from the absorbent storage tank 102 into reactor 101. Inside the reactor, the sulfur dioxide gas in the gas stream undergoes a redox reaction with nitric acid in the absorbent liquid, generating sulfur trioxide which is absorbed by the absorbent liquid. Once the sulfuric acid concentration in the reactor solution reaches a certain value, it is introduced into the sulfuric acid storage tank 103. The nitric oxide gas generated during the reaction, along with the original nitric oxide gas in the gas stream, enters the second reactor 201, where it reacts chemically with the ferric chloride absorbent in the absorbent slurry introduced from the iron salt absorbent slurry storage tank 202 and is absorbed, thus achieving gas purification. The saturated iron salt absorbent slurry is then introduced into the iron salt absorbent regeneration reactor 203 for regeneration. The nitric oxide gas released during regeneration is oxidized and absorbed to obtain nitric acid for recycling, and the regenerated iron salt absorbent slurry is also recycled. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0020] Example 1: The system process flow diagram is shown in Figure 1. The first reactor 101 adopts a chemically common spray-packed absorption tower, made of carbon steel lined with PTFE, with a tower diameter of Φ300mm, a total tower height of approximately 3500mm, and an effective height of the packing layer of approximately 2000mm. The second reactor 202 is the same as the first reactor, also a spray absorption tower (without packing, empty tower). The absorbent storage tank 102, sulfuric acid storage tank 103, iron salt absorbent slurry storage tank 202, and iron salt absorbent regenerator 203 are all chemically common glass-lined reactors, each with an effective volume of approximately 0.3 m³. 3 .

[0021] The process flow is as follows: First, the gas stream containing harmful gases such as sulfur dioxide (or a mixture of sulfur dioxide and nitric oxide) is introduced into the first reactor 101 (packed absorption tower). At the same time, an absorbent nitric acid solution prepared in the absorbent storage tank 102 is pumped to the first reactor, allowing the gas stream to fully contact the absorbent liquid in the reactor. The sulfur dioxide gas in the gas stream undergoes an oxidation-reduction reaction with the nitric acid in the absorbent liquid to generate sulfur trioxide, which is absorbed by the absorbent. The nitric oxide gas generated during the reaction process enters the second reactor 201 (empty tower) with the gas stream and reacts chemically with ferric chloride in the absorbent slurry introduced from the iron salt absorbent slurry storage tank 202, and is absorbed. The saturated iron salt absorbent slurry is then introduced into the iron salt absorbent regeneration reactor 203 for regeneration. The nitric oxide gas released during the regeneration process is oxidized to prepare nitric acid for recycling. The regenerated absorbent slurry is also recycled. Once the sulfuric acid concentration in the absorbent solution in the first reactor reaches a certain value, it is introduced into the sulfuric acid storage tank 103. The oxidation of nitric oxide to nitric acid is a known and mature process. The removal efficiency of nitric oxide and the regeneration process of iron salt absorbent can be found in the aforementioned relevant patent literature, and will not be described here again.

[0022] The gas flow rate is approximately 200 m³ / h. 3 The gas composition was approximately 2000 ppm sulfur dioxide, 8% oxygen (by volume), 10% moisture, 12% carbon dioxide, and the remainder nitrogen. The reaction temperature (temperature of the absorbent solution in the absorption tower, with the liquid and gas temperatures roughly the same) was set between 25 and 95°C, and the gas residence time in the reactor was approximately 2–3 seconds. Approximately 100 kg of absorbent solution was added, with an absorbent circulation rate of approximately 300 kg / h. Approximately 300 kg of iron salt absorbent slurry (with a moisture content of approximately 10%, the remainder mainly being ferric chloride) was added, with a slurry circulation rate of approximately 900 kg / h. The sulfur dioxide removal rate (the average removal rate over 2 hours after system stabilization) is shown in Table 1.

[0023] Table 1. Effects of different absorbent concentrations and temperatures on sulfur dioxide removal.

[0024] Furthermore, after adding nitric oxide gas (concentration of approximately 500 ppm) to the original gas stream, the above experiment was repeated, and the removal effect on sulfur dioxide was basically the same.

[0025] Example 2: The sulfur dioxide removal rates of different amounts of metal ions added to the sulfur dioxide absorbent solution with a nitric acid concentration of 10% and a reaction temperature of 50°C are shown in Table 2 (other conditions are the same as in Example 1).

[0026] Table 2. Effects of adding different catalysts on sulfur dioxide removal

[0027] Example 3: In Example 1, the sulfur dioxide component in the gas composition was changed to contain approximately 150 ppm each of COS, CS2, H2S, HCHO and CH3SH. A 10% dilute nitric acid solution was used, the reaction temperature was 50°C, and other parameters were the same as in Example 1. The removal rates of COS, CS2 and H2S were 77%, 85%, 98%, 91% and 86%, respectively.

[0028] Example 4: 5% H2O2 was added to the absorbent solution, and the rest was the same as in Example 3. The removal rates of COS, CS2 and H2S were increased by about 10%.

[0029] Example 5: Fe was added to the absorbent solution respectively 3+ and Mn 2+ With the two catalysts combined (0.1% each), everything else remained the same as in Example 3. The removal rates of COS, CS2, and H2S each increased by approximately 20%.

Claims

1. A method for removing sulfur oxides and nitrogen oxides from an airflow, characterized in that... The gas stream to be treated is introduced into the first reactor, and nitric acid solution absorbent is introduced into the reactor simultaneously. Sulfur dioxide in the gas stream undergoes a redox reaction with the nitric acid in the absorbent, causing the sulfur dioxide to be oxidized into sulfur trioxide, which is then absorbed by the absorbent solution. Nitric oxide gas generated during the reaction of sulfur dioxide and nitric acid is then introduced into the second reactor with the gas stream. At the same time, ferric chloride absorbent slurry is introduced into the second reactor. Nitric oxide gas in the gas stream reacts chemically with ferric chloride in the ferric chloride slurry and is absorbed. After the ferric chloride absorbent slurry is saturated, the nitric oxide gas released during regeneration is oxidized and the nitric acid is recovered and recycled as a desulfurization absorbent. The ferric salt absorbent slurry is also regenerated and recycled.

2. The method according to claim 1, characterized in that... The concentration of the nitric acid solution used as the desulfurization absorbent is below 65%.

3. The method according to claim 1, characterized in that... The concentration of the nitric acid solution used as the desulfurization absorbent is 1-15%.

4. The method according to claim 1, characterized in that... The reaction temperature of sulfur dioxide and nitric acid is above room temperature.

5. The method according to claim 1, characterized in that... The reaction temperature of sulfur dioxide with nitric acid is 30℃~95℃.

6. The method according to claim 1, characterized in that... The nitric acid solution absorbent contains less than 1% of a metal ion catalyst, wherein the metal ions include alkali metal ions Na. ＋ K ＋ Li ＋ and transition metal ions Fe 3+ / Fe 2+ Mn 2+ Cu 2+ Zn 2+ Co 2+ Ni 2+ Al 3+ and Se 3+ One or more of them.

7. The method according to claim 1, characterized in that... The sulfur dioxide in the airflow is replaced with carbon disulfide (CS2), carbon oxysulfide (COS), hydrogen sulfide (H2S), formaldehyde, and methanethiol.

8. The method according to claim 1, characterized in that... After desulfurization in the first reactor, the gas flow is heated or dehumidified before being introduced into the second reactor to remove nitrogen monoxide. Heating is achieved using a hot air furnace or heat exchanger, and dehumidification is achieved using condensation dehumidification or absorption dehumidification with calcium chloride or lithium chloride desiccant.

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