A method for simultaneous removal of nitrogen monoxide and sulfur dioxide

Abstract

A method for simultaneously removing nitric oxide and sulfur dioxide, belonging to the field of air pollution control technology, is characterized by introducing the gas stream to be treated into a reactor, simultaneously introducing an absorbent into the reactor. Sulfur dioxide in the gas stream undergoes a redox reaction with ferric chloride in the absorbent, causing the sulfur dioxide to be oxidized to sulfate ions, which are then absorbed by the absorbent. Nitric oxide gas in the gas stream reacts chemically with solid ferric chloride crystals in the ferric chloride slurry of the absorbent and is absorbed. The absorbent, after saturation, is regenerated and recycled. This method can also remove harmful gases such as carbon disulfide, carbon oxysulfide, and hydrogen sulfide from the gas stream separately.

Description

Technical Field

[0001] This invention relates to a method for simultaneously removing harmful gases such as nitrogen monoxide and sulfur dioxide, 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 exhaust gases from industries such as metallurgy 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 low cost, while its disadvantages include a long process flow, large land area required, consumption of large amounts of lime or limestone, and the need for further treatment of corresponding byproducts (mainly calcium sulfate). Furthermore, harmful gases such as carbon oxides and carbon disulfide generated in industrial processes are difficult to treat due to their relatively stable chemical structure and poor water solubility.

[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, thereby removing it from the airflow.

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

[0006] The technical solution adopted in this invention is as follows: a method for simultaneously removing nitric oxide and sulfur dioxide from an airflow, characterized in that the airflow to be treated is introduced into a reactor, and an absorbent ferric chloride slurry (hereinafter referred to as absorbent) is introduced into the reactor. Nitric oxide gas in the airflow reacts chemically with ferric chloride crystals in the absorbent ferric chloride slurry and is absorbed. At the same time, sulfur dioxide reacts with ferric chloride in the absorbent in a redox reaction, causing sulfur dioxide to be oxidized into sulfate ions and absorbed. The absorbent after absorption saturation is regenerated and recycled.

[0007] The water content of the ferric chloride slurry absorbent of the present invention is 0.5% to 60% by mass, and it contains any one or more compounds of ferric chloride crystals, hydrated ferric chloride crystals, or complexes of ferric chloride and chloride ions.

[0008] The reaction between nitric oxide and ferric chloride during the absorption process is as follows: FeCl3 + mNO + nH2O → Fe(H2O)n(NO)mCl3 (1) Where m + n ≤ 6 (generally ≤ 3).

[0009] The main reaction between sulfur dioxide and ferric chloride is as follows: 2FeCl3+ SO2+ 2H2O → 2FeCl2+ H2SO4+ 2HCl (2) The ferrous ions produced by the reaction of sulfur dioxide and ferric chloride can be re-oxidized to ferric ions by an oxidizing agent (such as oxygen in an airflow), as shown in the following reaction equation: 4FeCl2 + O2 + 4HCl → 4FeCl3 + 2H2O (3) The oxidation of ferrous ions consumes hydrogen chloride, which should be replenished in a timely manner to prevent hydrolysis of ferric chloride due to a rise in the pH of the absorbent slurry. Generally, the pH of the absorbent slurry should be less than 6, preferably 1-3. Hydrogen chloride can be replenished by directly adding hydrogen chloride gas into the gas stream, chlorinating the slurry by introducing hydrogen chloride gas or chlorine gas, or by adding hydrochloric acid solution.

[0010] Generally, the reaction temperature of the reactor is 20℃ to 110℃, with the effective temperature range for removing sulfur dioxide being approximately 20℃ to 100℃ and the effective temperature range for removing nitric oxide being approximately 30℃ to 110℃, preferably 50℃ to 90℃. The lower the moisture content and the higher the ferric chloride content in the absorbent, the higher the operating temperature of the absorbent slurry. When the moisture content in the absorbent is less than 0.5%, the absorbent slurry cannot be obtained within the temperature range described in this invention. When the moisture content is greater than 60%, and the slurry does not contain any anhydrous ferric chloride or other hydrated ferric chloride crystals, or any compound of ferric chloride and hydrochloric acid, it has no absorption effect on nitric oxide, but is still effective for absorbing sulfur dioxide. The preferred moisture content is 4% to 35%.

[0011] The preparation of the ferric chloride slurry absorbent described in this invention can be found in the inventor's published patents. The reactor can be a gas-liquid (solid) contact reactor commonly used in chemical unit operations, such as rotary, spray, bubbling, and moving bed reactors. Various flow configurations, including co-current, counter-current, and cross-current, can be employed with generally comparable results. Specific design parameters can be found in relevant chemical equipment design manuals. The gas residence time in the reactor is generally above 0.1 s, preferably 1–2 s, but there are no limitations.

[0012] A rotary reactor mainly includes a horizontally placed reactor cylinder, with a gas inlet at one end and a gas outlet at the other end. The upper part of the reactor cylinder is provided with a material inlet, and the lower part is provided with a material outlet. The reactor cylinder is driven to rotate by a transmission system.

[0013] The processing flow involves adding the absorbent into the reactor cylinder through the material inlet, introducing the gas stream to be treated into the reactor through the gas inlet, and simultaneously rotating the reactor cylinder via a transmission system. This causes the absorbent slurry within the reactor cylinder to flow along the inner wall and fully contact the gas. Nitrogen monoxide and sulfur dioxide in the gas stream are absorbed by the absorbent slurry, and the purified gas stream is discharged from the gas outlet at the other end of the reactor. The absorbent slurry, after absorption saturation, can be discharged from the material outlet.

[0014] A countercurrent spray absorption tower mainly includes an absorption tower body, with a gas inlet at the lower end and a gas outlet at the upper end. The upper part of the absorption tower is provided with an absorbent slurry material inlet and a slurry sprayer, and the lower part is provided with a material circulation tank. The material circulation tank is connected to the material inlet of the absorption tower through an absorbent slurry circulation pump and a connecting pipeline. The slurry after absorption saturation can be sent to a regeneration treatment unit for treatment and recycling through a bypass pipeline.

[0015] Because the content of ferric chloride crystals that can chemically react with nitric oxide in the absorbent is less than the total ferric chloride content of the absorbent (generally below 10%), the absorbent usually reaches nitric oxide absorption saturation first during operation. When the content of sulfate ions in the solution generated after the absorbent absorbs sulfur dioxide is low (generally below 10%), it has little impact on the absorption of nitric oxide. After the absorbent is saturated with nitric oxide, it is regenerated to release nitric oxide gas. The absorbent is then regenerated and recycled. The regeneration process can be found in the inventor's published patent.

[0016] When the sulfate content in the absorbent reaches or exceeds 10%, further sulfate ion removal can be performed. The process involves first removing the nitric oxide gas absorbed by the absorbent through heating or dilution with water, ensuring the absorbent's water content reaches at least 30%, preferably 40-60%. Then, reactants that can chemically react with sulfate ions to form a solid are added to the absorbent solution, primarily calcium chloride and barium chloride. The reaction is as follows: 2Fe 3+ + 3SO4 2- + 3Ca 2+ + 6Cl - → 3CaSO4↓ + 2FeCl3 (4) 2Fe 3+ + 3SO4 2- + 3Ba2+ + 6Cl - → 3BaSO4↓ + 2FeCl3 (5) Then, after solid-liquid separation to remove the solid precipitate calcium sulfate or barium sulfate, the absorbent solution is regenerated by dehydration and chlorination (by passing hydrogen chloride gas or hydrochloric acid, etc.). The regeneration process is the same as the preparation process of the absorbent that absorbs nitric oxide alone.

[0017] To improve the absorption efficiency of sulfur dioxide, nitric acid or nitrates can be added to the absorbent slurry. The nitrates are generally alkali metal or transition metal nitrates, including alkali metals such as sodium, potassium, and lithium, and transition metal ions such as manganese and copper. These metal ions are added along with the nitrates and also play a catalytic role in promoting the oxidation of harmful gases. The nitric acid or nitrate ions can undergo a redox reaction with sulfur dioxide, causing the sulfate ions of sulfur dioxide oxides to be absorbed. 3SO2 + 2HNO3 + 2H2O → 3H2SO4 + 2NO↑ (1) During the reaction, nitric acid or nitrate ions are reduced to nitric oxide and then absorbed by ferric chloride crystals in the absorbent. The mass content of the nitric acid or nitrate ions is generally below 10%, preferably 1-5%, but there is no limit. If the concentration of added nitric acid or nitrate ions is too high, nitrogen dioxide gas may be generated during the reaction. In the presence of nitric acid or nitrate ions, the redox reaction with sulfur dioxide proceeds preferentially.

[0018] The regeneration process after the absorbent containing nitric acid or nitrates is saturated can be carried out by heating in a closed container. First, the absorbed nitric oxide gas is released, and then an oxidant such as oxygen is added to oxidize the released nitric oxide gas into nitrogen dioxide, which is then absorbed and dissolved in the absorbent, thus achieving the regeneration effect.

[0019] The absorbent described in this invention can simultaneously or individually absorb and remove harmful gases in the airflow, such as carbon disulfide (CS2), carbon oxysulfide (COS), and hydrogen sulfide (H2S), which can undergo redox reactions with ferric chloride, nitric acid, or nitrate ions. 3CS2 + 4HNO3 → 2H2O + 3CO2 + 6S↓ + 4NO↑ (6) 3COS + 2HNO3 → H2O + 3CO2 + 3S↓ + 2NO↑ (7) 3H2S + 2HNO3 → 4H2O + 3S↓ + 2NO↑ (8) The elemental sulfur obtained in the above reaction can be recovered after solid-liquid separation.

[0020] Compared with the prior art, the advantages of this invention are: while removing nitric oxide, it also removes sulfur dioxide, hydrogen sulfide and other harmful gases from the gas stream. The addition of nitric acid or nitrate to the absorbent enhances the removal effect of harmful gases, and the generated nitric oxide is absorbed by the absorbent. The absorbent can be recycled after regeneration, overcoming the disadvantages of limestone / lime desulfurization systems such as complexity, large footprint and difficulty in treating other harmful gases. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a rotary reactor.

[0022] Figure 2 This is a schematic diagram of the structure of a spray reactor.

[0023] In the diagram: 1. Gas inlet; 2. Rotary reactor; 3. Absorbent inlet; 4. Gas outlet; 5. Absorbent outlet; 6. Slurry pump; 7. Connecting pipe; 8. Absorbent circulation tank; 9. Bypass pipe; 10. Absorbent delivery pipe; 11. Absorbent tower slurry inlet; 12. Sprayer; 13. Absorbent tower body. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0025] Example 1: Using Figure 1 The reactor shown is mainly composed of a horizontally placed reactor cylinder 2. One end of the reactor cylinder 2 is a gas inlet 1, and the other end is a gas outlet 4. The upper part of the reactor cylinder is provided with a material inlet 3, and the lower part is provided with a material outlet 5. The reactor cylinder 2 is driven to rotate by a transmission system.

[0026] The reactor cylinder has a diameter of Φ300mm and a length of approximately 1500mm. Both ends are conical, and the inlet and outlet diameters are 150mm. The material is Hastelloy. The flow rate of the gas being processed is approximately 120m³. 3 The gas flow rate is approximately 25–110°C per hour, the gas residence time within the reactor is approximately 3 seconds, and the cylinder rotation speed is approximately 45–60 rpm. The inlet gas flow contains approximately 500 ppm NO, 800 ppm SO2, approximately 8% oxygen (by volume), 10% moisture, 12% carbon dioxide, and the remainder is nitrogen. Approximately 30 kg of absorbent is added.

[0027] The process involves adding the absorbent slurry into the reactor through the material inlet 3, and introducing the gas stream to be treated into the reactor through the gas inlet 1. Simultaneously, a transmission system rotates the reactor cylinder 2, causing the absorbent material to flow along the inner wall of the reactor cylinder, ensuring full contact with the gas. Nitric oxide and sulfur dioxide in the gas stream react chemically with the absorbent slurry and are absorbed. The purified gas stream exits from the gas outlet 4 at the other end of the reactor. The saturated product is periodically discharged from the material outlet 5 to the regeneration system. The average removal rates of nitric oxide and sulfur dioxide after one hour are shown in Table 2 below.

[0028] Table 2. Effects of different absorbent ratios and reaction temperatures on the removal of sulfur dioxide and nitric oxide. Absorbent serial number Initial absorbent composition (mass %) Airflow temperature (°C) <![CDATA[Average SO2 removal rate (%)]]> Average NO removal rate (%) 1 <![CDATA[95%FeCl3, 4%H2O, 1%HCl]]> 100~110 10 65 2 <![CDATA[90%FeCl3, 8%H2O, 2%HCl]]> 85~95 45 80 3 <![CDATA[85%FeCl3, 10%H2O, 5%HCl]]> 70~80 60 90 4 <![CDATA[80%FeCl3, 15%H2O, 5%HCl]]> 55~65 80 85 5 <![CDATA[80%FeCl3, 15%H2O, 5%HNO3]]> 55~65 95 55 6 <![CDATA[65%FeCl3, 35%H2O]]> 40~50 85 15 7 <![CDATA[60%FeCl3, 40%H2O]]> 30~35 80 5 8 <![CDATA[35%FeCl3, 60%H2O, 5%HNO3]]> 25~30 85 / Example 2: Using Figure 2 The absorption reactor shown is a slurry reactor. The reactor mainly consists of an absorption tower body 13, with a gas outlet 4, an absorption tower slurry inlet 11, and a slurry sprayer 12 at the top. A gas inlet 1 is located at the lower side. The lower part of the tower body is a slurry circulation tank 8, with a material outlet 5 (also serving as a chlorine gas inlet) at the bottom. An absorbent slurry material inlet 3 is located at the top. The lower part is connected to an absorbent slurry pump 6 via a connecting pipe 7. The slurry pump is connected to the absorption tower slurry inlet 11 via a slurry delivery pipe 10. A bypass pipe 9 is also installed on the slurry delivery pipe 10 for the recovery and regeneration of the absorbent slurry. The absorption tower is an empty tower made of Hastelloy alloy, with a tower diameter of Φ300mm and a total tower height of approximately 3500mm. The effective spray height is approximately 2000mm, and the entire piping system is insulated.

[0029] The treatment process involves pumping the absorbent slurry to the top of the absorption tower, where it is sprayed from top to bottom by a slurry sprayer. The gas stream containing sulfur dioxide is then introduced into the absorption tower through the gas inlet at the bottom. Inside the tower, the gas stream comes into full contact with the slurry droplets, and the sulfur dioxide and nitrogen monoxide in the gas stream react chemically with the ferric chloride in the absorbent slurry and are absorbed. The purified gas stream is discharged from the top of the absorption tower, and the absorbent slurry is recycled. Saturated slurry can be sent for regeneration through a bypass port, and chlorinated gas from the absorbent slurry can be introduced through the material outlet 5 at the bottom of the circulation tank to replenish the chloride ions consumed during the slurry reaction.

[0030] The flow rate of the gas being processed is approximately 150 m³ / h. 3The gas composition was the same as in Example 2. The gas temperature inside the absorption tower was approximately 65-75°C, and the slurry temperature was roughly the same as the gas temperature. The gas residence time in the reactor was approximately 2 seconds. Using absorbent number 3 from Example 1 (85% FeCl3, 10% H2O, 5% HCl), approximately 50 kg of absorbent was added, and the slurry pump circulation rate was approximately 350 kg / h. After one hour, the average removal rates of sulfur dioxide and nitrogen monoxide in the gas stream were approximately 85% and 90%, respectively.

[0031] Example 3: In Example 2, when the sulfate content in the absorbent reaches 10%, absorbent regeneration and desulfurization are performed. First, the absorbent saturated with nitric oxide is introduced into the absorbent regeneration reactor, then water is added to approximately 45% to release nitric oxide (which is then discharged and recovered). Next, calcium chloride of the same molar amount as sulfate ions is added, causing it to react with sulfate ions to form calcium sulfate precipitate. After solid-liquid separation, the absorbent solution is then dehydrated and chlorinated according to the absorbent preparation method. When the water content in the liquid phase is approximately 10% and the hydrogen chloride content is 5%, the regeneration of the absorbent is complete.

[0032] Example 4: Experiment on the removal of harmful gases alone: ​​The reactor of Example 2 was used, but the gas composition was not nitric oxide. The sulfur dioxide composition was changed to contain approximately 150 ppm each of COS, CS2, and H2S. The absorbent number 5 in Example 1 (80% FeCl3, 15% H2O, 5% HNO3) was used. The reaction temperature was approximately 50-60℃. Other conditions were the same as in Example 2. The removal rates of COS, CS2, and H2S were 80%, 85%, and 99%, respectively.

[0033] Example 5: Absorbent Regeneration Experiment: In Example 4, after the nitric oxide gas generated during the reaction process is saturated by the absorbent, the absorbent needs to be regenerated. First, the saturated absorbent (after filtering to remove solids) is introduced into the absorbent regeneration reactor (sealed). Then, after evacuation, it is heated to approximately 110-130°C to release the absorbed nitric oxide gas. Simultaneously, an appropriate amount of oxygen is introduced to oxidize the released nitric oxide gas into nitrogen dioxide. The mixture is then stirred to allow absorption by the absorbent, and the temperature is lowered to 50-60°C. Once the nitrogen oxides in the gas phase are completely absorbed, the regeneration of the absorbent is complete.

[0034] The above embodiments are only used to illustrate the technical solutions of the present invention. Any modifications to the technical solutions described in the embodiments, or any equivalent substitutions, modifications, changes, and improvements to some of the technical features within the spirit and principles of the present invention, should be included within the protection scope of the present invention.

Claims

1. A method for simultaneously removing nitric oxide and sulfur dioxide from an airflow, characterized in that... The gas stream to be treated is introduced into the reactor, and at the same time, ferric chloride slurry is introduced into the reactor. Sulfur dioxide in the gas stream undergoes a redox reaction with ferric chloride in the absorbent, causing sulfur dioxide to be oxidized into sulfate ions, which are then absorbed by the absorbent. Nitric oxide gas in the gas stream undergoes a chemical reaction with solid ferric chloride crystals in the ferric chloride slurry and is absorbed. The absorbent is regenerated and recycled after absorption saturation.

2. The method according to claim 1, characterized in that... The absorbent contains 0.5% to 60% water by mass.

3. The method according to claim 1, characterized in that... The absorbent contains any one or more of the following compounds: anhydrous ferric chloride crystals, hydrated ferric chloride crystals, or complexes of ferric chloride and chloride ions.

4. The method according to claim 1, characterized in that... The reaction temperature is 20~110℃.

5. The method according to claim 1, characterized in that... The absorbent contains nitric acid or nitrate ions.

6. The method according to claim 5, characterized in that... The mass content of the nitric acid or nitrate ions is less than 10%.

7. The method according to claim 1, characterized in that... The absorbent contains alkali metal or transition metal ions such as sodium, potassium, lithium, manganese, and copper.

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

9. The method according to claim 1, characterized in that... After the absorbent is saturated, it is regenerated by heating or by diluting with water to release nitric oxide gas. Then, calcium chloride and barium chloride, which can react with sulfate ions, are added to obtain insoluble sulfate. After solid-liquid separation, it is dehydrated and regenerated.

10. The method according to claim 1, characterized in that... The regeneration of the absorbent after absorption saturation is achieved by heating in a sealed container to release the absorbed nitric oxide gas. At the same time, oxygen is added to oxidize the released nitric oxide gas into nitrogen dioxide, which is then absorbed and dissolved in the absorbent, thereby achieving regeneration.

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