Waste gas purification method

Abstract

The invention discloses a waste gas purification method, and belongs to the technical field of environmental protection. The method comprises the following steps: converting nitrogen-containing substances into NOO and adding the NOO into a high-temperature non-combustion area at the tail end of a hearth or the front end of a flue in combustion equipment, wherein fuel is burnt out; nO is cracked in the region to generate active free radicals; the free radicals oxidize NO in the flue gas to generate NO, and oxidize SOO, CO and VOCs to generate oxidation products; continuously cracking the generated NOs to form chain circulation; and the oxidation product is removed in a subsequent purification system. The NOs comprise pure NOs and dimers NO (nitrogen tetroxide) thereof, and NO is decomposed into NOs to participate in the reaction at high temperature. According to the invention, by optimizing the NO adding position, collaborative purification of multiple pollutants such as NOx, SOs, CO, VOCs and the like is realized, a reducing agent does not need to be added, ammonia escape is avoided, the investment and operation cost is low, and the method is particularly suitable for treatment of high-concentration CO and VOCs flue gas in industries such as coking and the like.

Description

Technical Field

[0001] This invention belongs to the field of environmental protection technology, specifically relating to a method for synergistic purification of multiple pollutants in combustion flue gas. Background Technology

[0002] Flue gas emitted from combustion equipment such as boilers, kilns, and coke ovens contains pollutants such as nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), volatile organic compounds (VOCs), and particulate matter, causing serious environmental impacts. Existing denitrification technologies mainly rely on selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR), but these suffer from problems such as ammonia slip, catalyst poisoning, and high investment and operating costs. While ozone oxidation can oxidize NOx, it requires additional equipment and has high requirements for subsequent absorption.

[0003] For the coking industry, the concentration of CO and VOCs (especially benzene compounds and polycyclic aromatic hydrocarbons) in coke oven flue gas is relatively high. Existing technologies usually use catalytic combustion or activated carbon adsorption to treat these gases. However, these methods have problems such as catalyst deactivation, frequent replacement due to adsorption saturation, and high operating costs. Furthermore, separate treatment systems are required, which increases equipment investment and operating burden.

[0004] On August 30, 2024, the applicant submitted two patent applications: "A Method for Denitrification, Oxidation, Desulfurization, and Synergistic Purification of Combustion Flue Gas Based on Photoquantum Physical Pyrolysis and Free Radical Chemical Oxidation" (Application No. 202411204054.X) and "A Method for Ammonia Gas External Bypass Combustion and Co-firing Oxidation" (Application No. 202411204043.1). Based on the above research, the inventors further observed that in the early morning, as sunlight intensifies, the concentration of nitrogen dioxide (NO2) in ambient air decreases, while the concentration of ozone (O3) increases, and the changes in these two concentrations are correlated. This phenomenon suggests that NO2 may undergo transformation under photothermal conditions, generating reactive substances that can then affect other pollutants. However, when applying this phenomenon to flue gas purification solutions, the selection of the injection location has a decisive impact on the purification effect, and existing technologies have not yet provided clear guidance.

[0005] Furthermore, industrially produced nitrogen dioxide is typically stored and transported as dinitrogen tetroxide (N₂O₄). At room temperature, NO₂ and N₂O₄ exist in a dynamic equilibrium: 2NO₂ ⇌ N₂O₄. At low temperatures, the equilibrium shifts towards N₂O₄, while at high temperatures, N₂O₄ decomposes into NO₂. Commercially available "nitrogen dioxide" gas is actually a mixture of NO₂ and N₂O₄, and its conversion characteristics must be considered when using it. Summary of the Invention

[0006] The purpose of this invention is to provide a method for purifying waste gas, which optimizes the NO2 addition location and utilizes the conversion characteristics of NO2 to achieve synergistic purification of multiple pollutants without secondary pollution.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for purifying waste gas includes the following steps:

[0009] S1: Convert nitrogen-containing substances into NO2, and add NO2 to the high-temperature non-combustion zone at the end of the furnace or the front end of the flue where the fuel has been burned out in the combustion equipment;

[0010] S2: In the high-temperature non-combustion zone at the end of the furnace or the front end of the flue where the fuel has been burned out, NO2 undergoes a cracking reaction to generate active free radicals;

[0011] S3: The active free radicals oxidize NO in the flue gas to generate NO2, and at the same time oxidize SO2, CO and VOCs to generate oxidation products; the generated NO2 continues to decompose, forming a chain cycle;

[0012] S4: Remove the oxidation products generated in S3 in the subsequent purification system.

[0013] The NO2 mentioned in this invention includes pure NO2, as well as substances that can be converted into NO2 under certain conditions, such as dinitrogen tetroxide (N2O4) or a mixture thereof with NO2. In the high-temperature non-combustion zone (850-1100℃), N2O4 completely decomposes into NO2, which then participates in the cracking reaction to generate active free radicals.

[0014] The nitrogen-containing substances include one or more of ammonia, ammonia water, ammonium salts, nitrates, nitrites, urea, nitro waste residue, and nitrogen-containing waste liquid. The NO2 dosage can be dynamically adjusted according to the pollutant concentration in the flue gas: when the flue gas online monitoring system detects an increase in the outlet NOx concentration, the NO2 dosage is increased; when an increase in SO2 or CO concentration is detected, the NO2 dosage is increased accordingly to ensure a sufficient supply of free radicals.

[0015] This invention adds NO2 to the high-temperature non-combustion zone at the end of the furnace or the front of the flue where the fuel has been burned out. In this zone, there is no reducing atmosphere to interfere with the process, and NO2 can be fully decomposed to generate active free radicals, which in turn oxidize a variety of pollutants, achieving synergistic purification without the need for external reducing agents and without the risk of ammonia escape.

[0016] Beneficial effects

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. By optimizing the NO2 addition location (the end of the furnace where the fuel has been completely burned or the high-temperature non-combustion zone at the front of the flue), the consumption of NO2 by the reducing atmosphere is avoided, ensuring that the cracking reaction proceeds fully;

[0019] 2. By utilizing the active free radicals generated from NO2 cracking to oxidize NO, SO2, CO and VOCs in flue gas, the synergistic purification of multiple pollutants can be achieved without the need to set up multiple independent treatment systems;

[0020] 3. After NO in the flue gas is oxidized to NO2, it continues to participate in the cracking reaction, forming a chain cycle, reducing the need for external NO2 addition;

[0021] 4. No external reducing agent is required, eliminating the risk of ammonia escape and avoiding secondary pollution;

[0022] 5. Particularly suitable for the synergistic removal of CO and VOCs from flue gas in industries such as coking, and can replace or reduce the need for specialized equipment such as catalytic combustion and activated carbon adsorption;

[0023] 6. The raw material addition method is flexible. It can use pure NO2, or N2O4 (nitrogen tetroxide) or its mixture commonly used in industry, which is convenient for industrial application and storage and transportation.

[0024] Comparative Example

[0025] In a spray drying tower at a ceramics factory, an experiment was conducted to add NO2 at the air inlet. The NOx concentration in the outlet flue gas was measured, but no significant decrease was observed. Analysis suggests that unburned fuel or a reducing atmosphere existed in the air inlet area, causing the NO2 to be reduced and consumed, failing to undergo effective cracking. This comparison indicates that NO2 should be added in a non-combustion zone where the fuel has been completely burned and there is no reducing atmosphere.

[0026] Example 1 (Application in Coal-fired Power Plants)

[0027] In a coal-fired power plant, a nitrogen gasification station produces NO2 through ammonia catalytic oxidation. The NO2 is then compressed and liquefied, stored as N2O4, and gasified before being fed into the furnace. NO2 is added through a dedicated nozzle in the non-combustion zone at the end of the boiler furnace (flue gas temperature approximately 850-1100℃, no reducing atmosphere) where fuel has been completely burned. N2O4 rapidly decomposes into NO2 in the high-temperature zone. NO2 then decomposes in the heat and light environment of the flue gas, producing reactive substances such as oxygen free radicals. These free radicals oxidize NO in the flue gas to NO2, which continues to decompose, forming a chain reaction. Simultaneously, the free radicals oxidize SO2 to SO3, CO to CO2, and VOCs to CO2 and H2O. The oxidation product SO3 is adsorbed and removed by flue dust or injected alkaline substances in the subsequent dust collector. The NO2 dosage is dynamically adjusted based on CEMS monitoring data: the dosage is increased when the outlet NOx concentration increases, and correspondingly increased when the SO2 or CO concentration increases, thus simultaneously reducing the outlet NOx, SO2, and CO concentrations.

[0028] Example 2 (Co-removal of CO and VOCs from coking plant flue gas)

[0029] The flue gas from a coke oven in a coking plant has a flow rate of 200,000 Nm³ / h and a temperature of approximately 280°C. The initial concentrations of the main pollutants are: NOx approximately 400 mg / m³, SO₂ approximately 150 mg / m³, CO approximately 1,500-2,500 mg / m³, and VOCs (calculated as benzene) approximately 60-100 mg / m³. Since the flue gas temperature is lower than the temperature required for NO₂ cracking, this embodiment utilizes the high-temperature zone of the coke oven heating system as the NO₂ injection point.

[0030] NO2 injection nozzles are installed at the end of the coke oven combustion chamber (the fuel-burned zone, flue gas temperature approximately 900-1000℃, non-reducing atmosphere). The NO2 source is ammonia water from the plant, prepared in a small ammonia catalytic oxidation furnace and stored as N2O4. Upon injection, it is vaporized and transported to the injection point via pipeline, where it rapidly decomposes into NO2 in the high-temperature zone. A CEMS online monitoring system is used to monitor the concentrations of CO, VOCs, NOx, and SO2 in the flue gas in real time, dynamically adjusting the NO2 injection rate: when CO or VOCs concentrations increase, the NO2 injection rate is increased to ensure sufficient free radical supply; when NOx concentrations increase, the injection rate is also increased accordingly.

[0031] NO2 decomposes in the high-temperature non-combustion zone to produce oxygen free radicals (O·). These free radicals then undergo oxidation reactions with CO, VOCs, NO, and SO2 in the flue gas.

[0032] • CO is oxidized to CO2;

[0033] • VOCs are mineralized into CO2 and H2O;

[0034] • NO is oxidized to NO2, and the generated NO2 continues to decompose, forming a chain cycle;

[0035] • SO2 is oxidized to SO3.

[0036] After the reaction, the flue gas is cooled to approximately 150°C by a waste heat boiler before entering the existing baghouse dust collector. During the cooling process, SO3 forms an aerosol, which is neutralized and adsorbed by alkaline substances (CaO, MgO, etc.) in the coke powder and is then captured by the dust collector along with the dust.

[0037] Operational results show that, with an NO2 dosage approximately 1.2-1.5 times the NOx concentration in the flue gas (in molar terms), the CO concentration in the outlet flue gas drops below 200 mg / m³, the VOCs removal rate reaches over 85%, the NOx concentration is below 100 mg / m³, and the SO2 concentration is below 30 mg / m³, achieving synergistic purification of multiple pollutants. Compared with traditional methods, there is no need to add catalytic combustion or activated carbon adsorption devices, no ammonia escape, and significantly reduced operating costs.

[0038] Industrial applicability

[0039] The method of this invention is applicable to the purification of exhaust gas from combustion equipment such as coal-fired power plants, gas turbines, steel sintering plants, cement kilns, brick and tile kilns, ceramic and glass kilns, coking furnaces, and industrial boilers. It is particularly suitable for industrial kilns with high concentrations of CO and VOCs in flue gas, and is also applicable to the treatment of VOCs exhaust gas.

[0040] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. 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 method for purifying waste gas, characterized in that, Includes the following steps: S1: Convert nitrogen-containing substances into NO2, and add NO2 to the high-temperature non-combustion zone at the end of the furnace or the front end of the flue where the fuel has been burned out in the combustion equipment; S2: In the high-temperature non-combustion zone at the end of the furnace or the front end of the flue where the fuel has been burned out, NO2 undergoes a cracking reaction to generate active free radicals; S3: The active free radicals oxidize NO in the flue gas to generate NO2, and at the same time oxidize SO2, CO and VOCs to generate oxidation products; the generated NO2 continues to decompose, forming a chain cycle; S4: Remove the oxidation products generated in S3 in the subsequent purification system.

2. The method according to claim 1, characterized in that, The nitrogen-containing substances include one or more of the following: ammonia, ammonia water, ammonium salts, nitrates, nitrites, urea, nitro waste residue, and nitrogen-containing waste liquid.

3. The method according to claim 1, characterized in that, The amount of NO2 added is dynamically adjusted based on the NOx, CO, and SO2 concentration data from the flue gas online monitoring system. When the NOx concentration at the outlet is detected to increase, the NO2 addition is increased; when the SO2 or CO concentration is detected to increase, the NO2 addition is increased accordingly.

4. The method according to claim 1, characterized in that, The NO2 is produced by an ammonia catalytic oxidation furnace, an ammonia torch, a nitric acid reduction decomposition unit, or a nitrite-hydrochloric acid reaction device.

5. The method according to claim 1, characterized in that, The method is applicable to the purification of exhaust gases from coal-fired power plants, gas turbines, steel sintering plants, cement kilns, brick and tile kilns, ceramic and glass kilns, coking furnaces, and industrial boilers.

6. The method according to claim 1, characterized in that, The method is also applicable to the treatment of VOCs waste gas.

7. The method according to claim 1, characterized in that, The method requires no external reducing agent and has no ammonia escape.

8. The method according to claim 1, characterized in that, The oxidation product SO3 is adsorbed, neutralized, and removed by filtration in the subsequent dust removal system.

9. The method according to claim 1, characterized in that, The NO2 is added in the form of dinitrogen tetroxide (N2O4) or a mixture of it and NO2. After N2O4 decomposes into NO2 in the high-temperature non-combustion zone, it participates in the cracking reaction.