With increasingly stringent environmental emission requirements, the efforts of enterprises to control pollution continue to increase, and the treatment of coke oven flue gas is receiving growing attention. The coking production process generates waste gas containing dust, SO2, NOx and other harmful substances that pollute the environment. To reduce the emission of harmful substances such as SO2 and NOx in coke oven flue gas and meet environmental requirements, while better improving ambient air quality, many advanced methods have been applied in actual projects. Studies have found that SCR denitrification technology has good sulfur resistance in low-temperature environments, with a flue gas denitrification rate above 85%. Jin Hui et al. [2] applied SCR technology to a project of Jiangsu Yizhou Coal Coke Chemical Co., Ltd., overcoming the difficulty of treating coke oven flue gas at low temperature. Wang Yan et al. [3] believe that coke oven flue gas treatment should effectively integrate source control, low-NOx combustion, and end-of-pipe purification, and attach importance to them.
After purification by the desulfurization, denitrification, and dust removal process, the coke oven flue gas emission concentration reaches SO2 ≤ 30 mg/m3, NOx ≤ 150 mg/m3, and dust concentration ≤ 15 mg/m3, meeting the special emission limit requirements in GB 16171-2012 'Emission Standard of Pollutants for Coking Chemical Industry', and achieving ultra-low emission standards.
1 Coal Oven Flue Gas Desulfurization and Denitrification Process
1.1 Process Flow
The coke oven flue gas is drawn from the underground machine-side and coke-side flues respectively, switched and merged through the bypass flue damper and the new inlet duct damper into the main flue. At the same time, the high-efficiency desulfurizer (particle size 20-25 μm) is injected into the main flue through the SDS dry deacidification injection and distribution device and activated by heating in the flue. Its specific surface area rapidly increases and, after full contact with the coke oven flue gas, physical and chemical reactions occur, and the acidic substances such as SO2 in the flue gas are absorbed and purified. The dust-laden flue gas after absorption and drying enters the baghouse dust collector for further desulfurization reaction and dust purification. The desulfurized and dust-removed flue gas is purified by denitrification in the SCR denitrification reactor. The NOx in the flue gas and the NH3 ejected from the ammonia injection grid are fully mixed in the static mixer, and react with NH3 under the action of the medium-low temperature catalyst in the SCR reactor to generate N2 and H2O, thereby removing NOx from the flue gas. The cleaned flue gas is drawn by the booster fan and discharged to the atmosphere through the outlet flue to the original coke oven chimney. The flue gas returned to the original coke oven chimney meets the coke oven hot-standby temperature requirement, ensuring that the thermal draft of the coke oven chimney remains normal in case of accidents.
1.2 Comprehensive Utilization of By-products
The desulfurizer for SDS dry desulfurization uses a high-efficiency composite desulfurizer. Because the SDS process has a very small over-spray amount, the amount of desulfurization by-product is small compared with other desulfurization methods. The by-product has a high proportion of Na2SO4, which is convenient for comprehensive utilization. The by-product is a dry powder material in which Na2SO4 accounts for about 80%-90% of the total mass and Na2CO3 accounts for about 10%-20%. In addition to being used as a raw material for mine tailings solidifying agents, the coke oven desulfurization by-product can also be applied in the following fields: mixed into cement to accelerate the formation of hydration product ettringite and speed up cement hydration and hardening; used in the glass industry to replace soda ash; used in the paper industry as a cooking agent for making sulfate pulp; used in the chemical industry as a raw material for manufacturing sodium sulfide, sodium silicate, and other chemical products; used in the textile industry to prepare the vinylon spinning coagulant; and also used in non-ferrous metallurgy, leather, etc. The spent catalyst from the updated denitrification system is recovered by the catalyst manufacturer.
2 Process Technology
Common coke oven flue gas desulfurization and denitrification methods mainly include SDS dry desulfurization plus medium-low temperature SCR denitrification, SDA(Na) semi-dry desulfurization plus medium-low temperature SCR denitrification, SDA(Ca) semi-dry desulfurization plus GGH-medium-low temperature SCR denitrification, and activated carbon dry desulfurization and denitrification process, etc.
2.1 SDS Dry Desulfurization Process
The high-efficiency desulfurizer (particle size 20-25 μm) is injected into the flue through the SDS dry deacidification injection and distribution device and activated by heating in the flue. Its specific surface area rapidly increases. [This technology was originally developed for the HCl removal dry system in the waste incineration industry, whose main by-product is NaCl, recoverable as a raw material for soda ash production. Subsequently, the SDS dry deacidification technology developed rapidly in Europe, and its supporting injection and grinding systems were successively developed. Currently in the European market this process is mainly used for flue gas deacidification of waste incineration furnaces, but it has also achieved good results in other industries including coking, glass manufacturing, coal-fired power plants, hazardous waste incineration, diesel power generation, biomass power generation, and cement.] After full contact with the flue gas, physical and chemical reactions occur, and the acidic substances such as SO2 in the flue gas are absorbed and purified.
The advantages of the SDS dry desulfurization plus medium-low temperature SCR denitrification process are high desulfurization and denitrification efficiency, no temperature drop, dry operation, low investment, small footprint, little by-product, low power consumption, no corrosion, simple equipment, easy operation and maintenance, and small desulfurization by-product generation with high sodium sulfate content. The disadvantage is that a small amount of desulfurization by-product is generated, which needs comprehensive utilization.
2.2 SDA Semi-Dry Desulfurization Process (including Na and Ca methods)
The rotary spray drying (SDA) desulfurization technology was developed by the Danish Niro Company in the early 1970s. The desulfurization process is to prepare CaO or Na2CO3 into a Ca(OH)2 slurry or Na2CO3 solution with a solid content of 20%-25%, which is atomized into 30-80 μm droplets by the high-speed rotating atomizer and sprayed into the absorption tower. The Ca(OH)2 slurry or Na2CO3 solution droplets (absorbent) in the tower rapidly absorb SO2 in the flue gas, achieving the removal of SO2 and other acidic media. At the same time, the coke oven flue gas heat instantly dries the droplets injected into the tower, turning them into dry powder solids collected by the baghouse filter.
The SDA desulfurization process is simple, with an empty-tower absorption tower structure. The advantages of SDA(Na) semi-dry desulfurization plus medium-low temperature SCR denitrification are high desulfurization efficiency, no wastewater generation, low water consumption, low power consumption, and no corrosion; the disadvantages are that the desulfurizer easily crystallizes, maintenance is difficult, and the by-product is hard to recover. The advantages of SDA(Ca) semi-dry desulfurization plus GGH-medium-low temperature SCR denitrification are medium desulfurization efficiency, no wastewater generation, low water consumption, low power consumption, and no corrosion; the disadvantages are large footprint, flue gas temperature first drops then rises, high energy consumption, and difficult by-product utilization.
2.3 Activated Carbon Dry Desulfurization and Denitrification Process
Based on the physical-chemical adsorption principle, activated carbon adsorbs SO2, H2O, and O2 in the flue gas and catalyzes a reaction to form sulfuric acid, which then migrates into the micropores for storage, while the NOx in the flue gas, under the catalysis of activated carbon, undergoes a reduction reaction with the ammonia water sprayed into the flue gas to form N2 and H2O. The activated carbon releases active adsorption sites through the regeneration system to continue adsorbing SO2, and the SO2-containing flue gas emitted by the regeneration system enters the by-product recovery system, where SO2 can be processed into various sulfur chemical products.
Activated carbon wears and is consumed chemically during regeneration, so new activated carbon needs to be supplemented regularly, and the worn activated carbon powder can be returned to the coal blending section for reuse.
The activated carbon dry desulfurization and denitrification process uses the adsorption of activated carbon to adsorb SO2, particulate matter, and NOx in the flue gas, thereby achieving simultaneous desulfurization, denitrification, and dust removal. The disadvantages are that the flue gas temperature must be reduced below 150 °C; the desulfurization by-product contains sulfuric acid and generates polluted wastewater; and the one-time investment is large with high operating costs.
In summary, whether from the advancement of the process technology (desulfurization and denitrification efficiency), or from a comprehensive analysis and comparison of the practicality of the process technology, footprint, investment cost, wastewater, and by-product utilization, the SDS dry desulfurization and medium-low temperature SCR denitrification process is the technology suitable for coke oven flue gas purification. Its configuration is reasonable, its control level reaches the international advanced level, and it can ensure the long-term, safe, stable, and continuous operation of the desulfurization and denitrification system.
3 Process Principle
3.1 SDS Process Principle
The SDS dry deacidification injection technology uniformly injects the high-efficiency desulfurizer (particle size 20-25 μm) into the pipe. The desulfurizer is activated by heating in the pipe, its specific surface area rapidly increases, and it fully contacts the acidic flue gas, undergoing physical and chemical reactions, and the acidic substances such as SO2 in the flue gas are absorbed and purified.
The main chemical reactions are:
2NaHCO3 +SO2+1/2O2 → Na2SO4 +2CO2+H2O
2NaHCO3 +SO3 → Na2SO4 +2CO2+H2O
The main reactions with other acidic substances (such as SO3, etc.) are:
NaHCO3 +HCl → NaCl +CO2+H2O
NaHCO3 +HF → NaF +CO2+H2O
4 Process Features
4.1 Technical Features of the SDS Desulfurization Process
The SDS desulfurization process has good regulation characteristics. The operation and shutdown of the desulfurization device do not affect the continuous operation of the coke oven. The load range of the desulfurization system coordinates with the load range of the coke oven, ensuring reliable and stable continuous operation of the desulfurization system. The technical features of this process are as follows:
(1) Simple system, convenient operation and maintenance;
(2) Low one-time investment, small footprint;
(3) Low operating cost;
(4) Fully dry system, no water required;
(5) High desulfurization efficiency;
(6) Reasonable desulfurizer distribution device;
(7) High flexibility, adjustable at any time according to emission target requirements;
(8) Good removal effect on acidic substances;
(9) Strong adaptability to coke oven conditions;
(10) Small amount of by-product, high sodium sulfate purity, easy to recover and utilize;
(11) The system is equipped with a rapid-switching accident bypass device, which does not affect the normal production of the coke oven in case of failure.
5 Composition of the Desulfurization and Denitrification Process System
The coke oven flue gas desulfurization and denitrification system consists of the following parts:
(1) SDS desulfurizer dosing and distribution device (standby considered for key equipment);
(2) Dust removal equipment and auxiliary equipment;
(3) Denitrification reactor system and auxiliary equipment;
(4) Desulfurization and denitrification system utilities, including nitrogen supply system, circulating water supply, etc.;
(5) Instruments, communication, power supply and distribution, online monitoring, fire protection, and control systems, etc.
With increasingly stringent environmental regulations, the dry deacidification process using sodium bicarbonate developed by Peman, incorporating advanced European and American technology, is finding increasing application. Using sodium bicarbonate as an adsorbent, the dry method removes various pollutants contained in flue gas. Its purification effect is comparable to other known methods, such as the spray adsorption method using lime milk. Dry flue gas purification can be used not only in coal-fired power plants, hazardous waste treatment, municipal waste or alternative fuel incineration plants, but also widely in industrial furnaces in the glass, cement, metallurgy and other industries. The dry flue gas purification method can economically remove gases containing acidic substances, such as SO2 and HCl, meeting national flue gas emission standards. In recent years, our company's sodium bicarbonate dry desulfurization and deacidification grinding mills have served many environmental protection companies across the country, successfully completing flue gas desulfurization retrofit projects for many steel group owners, thereby playing the role of desulfurization, deacidification, and flue gas purification. Today, our company, together with
an environmental protection technology Co., Ltd. in Hubei, has completed the grinding and pulverizing system for the hot-blast stove flue gas desulfurization retrofit project of a joint special steel Co., Ltd. in Tianjin. The equipment has been manufactured and will contribute to a more beautiful blue sky.
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