As environmental protection emission requirements become more and more stringent, enterprises are increasing their efforts to control pollution, and the control of coke oven flue gas is receiving more and more attention. During the coke oven production process, waste gas containing dust, SO2, NOx and other harmful substances will be produced, causing pollution to the environment. In order to reduce the emissions of harmful substances such as SO2 and NOx in coke oven flue gas to meet environmental protection requirements and better improve the quality of the atmospheric environment, many advanced methods have been applied to actual projects. Research has found that SCR denitrification technology has good sulfur resistance in low-temperature environments, and the flue gas denitrification rate reaches more than 85%. Jin Hui et al. [2] actually applied SCR technology to a project of Jiangsu Yizhou Coal Coking Co., Ltd., overcoming the problem that coke oven flue gas cannot be processed at low temperatures. Wang Yan et al. [3] believe that coke oven flue gas treatment should effectively integrate three aspects: source control, low-nitrogen combustion, and terminal purification, and pay attention to it.

After purification through 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 GB 16171-2012 "Coking Chemical Industry Pollutant Emission Standards" "Special emission limit requirements in ", and can meet the requirements of ultra-low emission standards.

1 Coke oven flue gas desulfurization and denitrification process

1.1 Process flow

The coke oven flue gas is led from the underground machine side and coke side flues respectively, and enters the flue gas main pipe after being switched and merged by the bypass flue gas pipe valve and the new inlet pipe valve. At the same time, the highly efficient desulfurizer (particle size is 20~25 μm) is sprayed into the main flue through the SDS dry deacidification injection and uniform distribution device and is heated and activated in the flue. Its specific surface area increases rapidly and is incompatible with the coke oven smoke. Physical and chemical reactions occur after full contact with the flue gas. SO2 and other acidic substances in the flue gas are absorbed and purified. The absorbed and dried powder-containing flue gas enters the bag dust collector for further desulfurization reaction and flue dust purification. The flue gas after desulfurization and dust removal is denitrated and purified in the SCR denitrification reactor. The NOx in the flue gas and the NH3 sprayed from the ammonia injection grille are fully mixed in the static mixer, and are mixed under the action of the medium and low temperature catalyst in the SCR reactor. It reacts chemically with NH3 to generate N2 and H2O, thereby achieving the purpose of removing NOx in the flue gas. The clean flue gas is drawn by the booster fan and discharged into the atmosphere through the outlet flue to the original coke oven chimney. The flue gas temperature of the original coke oven chimney meets the coke oven hot standby temperature requirements, which can ensure that the hot pulling force of the coke oven chimney remains normal in an accident.

1.2 Comprehensive utilization of by-products

The desulfurizer for SDS dry desulfurization uses high-efficiency composite desulfurizer. Since the overspray volume of the SDS process is very small, compared with other desulfurization methods, this scheme produces very few desulfurization by-products. The proportion of Na2SO4 in the by-product is very high, which is convenient for comprehensive utilization. The by-product is dry powdery material, of which the mass of Na2SO4 accounts for about 80% to 90% of the total mass, and the mass of Na2CO3 accounts for about 10% to 20% of the total mass. In addition to being used as raw materials for the production of mine tailings curing agents, coke oven desulfurization by-products can also be used in the following fields: blended into cement to generate the hydration product calcium sulfoaluminate faster and speed up the hydration and hardening of cement; In the glass industry, it is used to replace soda ash; in the paper industry, it is used as a cooking agent in the manufacture of sulfate pulp; in the chemical industry, it is used as a raw material for the manufacture of sodium sulfide, sodium silicate and other chemical products; in the textile industry, it is used for blending Vinylon spinning coagulant; can also be used in non-ferrous metallurgy, leather, etc. The spent catalyst after the denitrification system has been updated will be recycled by the catalyst manufacturer.

2 Process technology

Commonly used coke oven flue gas desulfurization and denitrification methods mainly include SDS dry desulfurization + medium and low temperature SCR denitrification, SDA (Na) semi-dry desulfurization + medium and low temperature SCR denitrification, SDA (Ca) semi-dry desulfurization + GGH- medium and low temperature SCR denitrification. As well as activated carbon dry desulfurization and denitrification processes.

2.1 SDS dry desulfurization process

The high-efficiency desulfurizer (particle size is 20~25 μm) is sprayed into the flue through the SDS dry deacidification spray and uniform distribution device and is heated and activated in the flue. Its specific surface area increases rapidly. The background is developed by the waste incineration industry. HCl removal dry system, the main component of the by-product is NaCl, which can be recycled as raw material and reused to produce soda ash. After that, SDS dry deacidification technology developed rapidly in Europe. Its supporting injection system fully contacts the flue gas and undergoes physical and chemical reactions. SO2 and other acidic substances in the flue gas are absorbed and purified. The opening system and grinding system of this technology have been developed one after another. Currently in the European market, this process is mainly used for deacidification of waste incinerator tail gas, but this technology has achieved great success in other industries including coking, glass manufacturing, coal-fired power plants, hazardous waste incinerators, diesel power generation, biomass power generation, cement, etc. Good application effect.

The advantages of SDS dry desulfurization + medium and low temperature SCR denitrification process are high desulfurization and denitrification efficiency, no temperature drop, waterless operation, low investment, small footprint, few by-products, low power consumption, no corrosion, simple equipment, easy Operation and maintenance, the production of desulfurization by-products is small, and the sodium sulfate content is high; the disadvantage is that a small amount of desulfurization by-products will be produced, which needs to be comprehensively utilized.

2.2 SDA semi-dry desulfurization process (including Na method and Ca method)

Rotary spray drying (SDA) desulfurization technology was developed by the Danish Niro Company in the early 1970s. The desulfurization process is to add water to CaO or Na2CO3 to form a Ca(OH)2 slurry or Na2CO3 solution with a solid content of 20%~25%. The solution is atomized into 30~80μm droplets through high-speed rotation of the atomizer and sprayed into the absorption tower. Inside, the Ca(OH)2 slurry or Na2CO3 solution droplets (absorbent) in the tower quickly absorb SO2 in the flue gas to achieve the purpose of removing SO2 and other acidic media. At the same time, the heat of the coke oven flue gas instantly dries the liquid droplets sprayed into the tower, turning them into powdery dry solids, which are captured by the bag dust collector.

 The desulfurization process is simple and the absorption tower has an empty tower structure. The advantages of SDA (Na) semi-dry desulfurization + medium and low temperature SCR denitrification are high desulfurization efficiency, no waste water production, low water consumption, low power consumption, and no corrosion; the disadvantage is that the desulfurization agent is easy to crystallize, difficult to maintain, and the by-products are difficult to recycle . SDA(Ca) semi-dry desulfurization + GGH- medium and low temperature SCR denitrification process has the advantages of medium desulfurization efficiency, no waste water production, low water consumption, low power consumption, and no corrosion; the disadvantage is that it occupies a large area and the flue gas temperature is lowered first Then it rises, the energy consumption is high, and the by-products are difficult to utilize.

2.3 Activated carbon dry desulfurization and denitrification process

Based on the principle of physical-chemical adsorption, activated carbon adsorbs SO2, H2O and O2 in the flue gas and then catalytically reacts to generate sulfuric acid, which is then migrated to the micropores for storage. The NOx in the flue gas is catalyzed by the activated carbon and sprayed. The ammonia water entering the flue gas undergoes a reduction reaction to generate N2 and H2O. Activated carbon releases active adsorption sites through the regeneration system to continue adsorbing SO2. The SO2-containing flue gas emitted by the regeneration system enters the by-product recovery system. SO2 can be processed into a variety of sulfur chemical products.

Activated carbon will cause wear and chemical consumption during the regeneration process, so new activated carbon needs to be replenished regularly. 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 effect of activated carbon to absorb SO2, particulate matter and NOx in the flue gas, thereby achieving the purpose of simultaneous desulfurization, denitrification and dust removal. The disadvantage is that the flue gas temperature needs to be reduced to below 150°C; the desulfurization by-products contain sulfuric acid and polluted wastewater is produced, resulting in large one-time investment and high operating costs.

In summary, no matter from the perspective of the advancement of process technology (desulfurization, denitrification efficiency), or from the practicality of process technology, floor space, investment cost, wastewater, by-product utilization, etc., SDS dry desulfurization can be comprehensively analyzed and compared. The medium and low temperature SCR denitrification process is a process technology suitable for coke oven flue gas purification. Its configuration is reasonable and the control level reaches the international advanced level, which can ensure the long-term, safe, stable and continuous operation of the desulfurization and denitrification system.

3 Process principle

3.1 SDS process principle

SDS dry deacidification injection technology sprays high-efficiency desulfurizer (particle size 20~25 μm) evenly into the pipeline. The desulfurizer is heated and activated in the pipeline, and the specific surface area increases rapidly, fully contacting the acidic flue gas, and occurs Through physical and chemical reactions, acidic substances such as SO2 in the flue gas are absorbed and purified.

Its main chemical reactions are:

2NaHCO3 +SO2+1/2O2 → Na2SO4 +2CO2+H2O

2NaHCO3 +SO3 → Na2SO4 +2CO2+H2O

Its main reaction with other acidic substances (such as SO3, etc.) is:

NaHCO3 +HCl → NaCl +CO2+H2O

NaHCO3 +HF → NaF +CO2+H2O

4 Process characteristics

4.1 Technical characteristics of SDS desulfurization process

The SDS desulfurization process has good adjustment characteristics. The operation and shutdown of the desulfurization device will not affect the continuous operation of the coke oven. The load range of the desulfurization system is coordinated with the load range of the coke oven to ensure reliable and stable continuous operation of the desulfurization system. The technical characteristics of this process are as follows:

(1) The system is simple and easy to operate and maintain; 

(2) Small one-time investment and small floor space; 

(3) Low operating costs; 

(4) Fully dry system, no water required; 

(5) High desulfurization efficiency; 

(6) Reasonable desulfurizer uniform distribution device; 

(7) It is highly flexible and can be adjusted at any time according to emission indicator requirements; 

(8) It has good removal effect on acidic substances; 

(9) Strong adaptability to coke oven working conditions; 

(10) The amount of by-products is small, the purity of sodium sulfate is high, and it is easy to recycle; 

(11) The system is equipped with an accident channel quick switching device, so that once a failure occurs, it will not affect the normal production of the coke oven.

5 Desulfurization and denitrification process system composition

The coke oven flue gas desulfurization and denitrification system consists of the following parts: 

(1) SDS desulfurization agent dosing and uniform distribution device (key equipment should be considered for backup); 

(2) Dust removal equipment and ancillary equipment; 

(3) Denitrification reactor system and ancillary equipment; 

(4) Public auxiliary equipment for the desulfurization and denitrification system, including nitrogen supply system, circulating water supply, etc.; 

(5) Instrumentation, communications, power supply and distribution, online monitoring, fire protection and control systems, etc.

With the increasingly stringent requirements of environmental protection regulations, the process based on the baking soda dry deacidification principle developed by Perlman in conjunction with advanced European and American processes has been increasingly used. Various pollutants contained in flue gas are removed by dry adsorption using sodium bicarbonate as adsorbent. Its purification effect is comparable to other known methods, such as the spray adsorption method using lime milk as the adsorbent. Dry flue gas purification can not only be used in coal power plants, hazardous waste treatment, municipal waste or alternative fuel incineration plants, but can also be widely used in industrial furnaces in glass, cement, metallurgy and other industries. Dry flue gas purification can economically remove gases containing acidic substances, such as SO2, HCI, etc., and meet the national flue gas emission standards. In recent years, the baking soda dry desulfurization and deacidification grinder produced and developed by our company has served many environmental protection companies across the country, and has successfully completed flue gas desulfurization transformation projects for many steel group owners across the country, thereby achieving desulfurization, deacidification, and flue gas purification. The role of our company today is to unite A Hubei Environmental Protection Technology Co., Ltd. has completed the production of the grinding and pulverizing system for the hot blast furnace flue gas desulfurization renovation project of a Tianjin United Special Steel Co., Ltd., contributing to the realization of more beautiful blue skies and white clouds.