With the more and more stringent environmental emission requirements, the efforts of enterprises to control pollution are also increasing, and the treatment of coke oven flue gas is also more and more attention. In the process of coke oven production, 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, so that it can meet the requirements of environmental protection, and better improve the quality of atmospheric environment, many advanced methods have been applied to practical projects. It is found that SCR denitrification technology has good sulfur resistance in low temperature environment, and the denitrification rate of flue gas reaches more than 85%. Jin Hui et al. [2] applied SCR technology to a project of Jiangsu Yizhou Coal Coking Co., LTD., and solved the problem that coke oven flue gas could not be treated at low temperature. Wang Yan et al. [3] believe that coke oven flue gas treatment should effectively integrate three aspects: source control, low nitrogen combustion and end purification, and pay more attention to them.

After purification by desulfurization, denitrification and dust removal process, the emission concentration of coke oven flue gas reaches SO2 ≤ 30 mg/m3, NOx ≤ 150 mg/m3, dust concentration ≤ 15 mg/m3, meeting the special emission limit requirements in GB 16171-2012 "Emission Standard for Coking Chemical Industry Pollutants". And can meet the ultra-low emission standards.

1. Desulfurization and denitrification process of coke oven flue gas

1.1 Process Flow

Coke oven flue gas is extracted from the underground engine side and the coke side flue gas, switched and merged into the flue gas main through the bypass flue gas pipe valve and the new inlet pipe valve. At the same time, the high efficiency desulfurizer (particle size of 20~25 μm) is sprayed into the total flue by SDS dry deacidification jet and distribution device and is heated and activated in the flue. Its specific surface area increases rapidly, physical and chemical reactions occur after full contact with coke oven flue gas, and acidic substances such as SO2 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 dust purification. The flue gas after desulfurization and dust removal is denitrification and purification in the SCR denitration reactor. NOx in the flue gas is fully mixed with NH3 emitted from the ammonia spray grid in the static mixer, and chemical reaction occurs with NH3 under the action of medium and low temperature catalyst in the SCR reactor to generate N2 and H2O, so as to achieve the purpose of removing NOx in the flue gas. The net flue gas is drawn by the booster fan and discharged into the atmosphere through the outlet flue to the chimney of the original coke oven. The flue gas temperature of the coke oven chimney can meet the requirements of the hot standby temperature of the coke oven, which can ensure that the hot drawing force of the coke oven chimney remains normal under the accident condition.

1.2 Comprehensive utilization of by-products

The desulfurizer of SDS dry desulphurization is high efficiency compound desulfurizer. Compared with other desulphurization methods, the desulphurization by-products of SDS process are less because of the small overspray amount. The proportion of Na2SO4 in the by-product is very high, which is convenient for comprehensive utilization. The by-product is dry powder material, of which Na2SO4 quality accounts for about 80%~90% of the total quality, Na2CO3 quality accounts for about 10%~20% of the total quality. The desulfurization by-product of coke oven can be used as raw material for the production of curing agent of mine tailings, and can also be used in the following fields: incorporation into cement, the hydration product calcium sulfoaluminate is formed faster, and the hydration hardening speed of cement is accelerated; Used in the glass industry as a substitute for soda ash; A cooking agent used in the manufacture of kraft pulp in the paper industry; Used in the chemical industry as raw materials for the manufacture of sodium sulfide, sodium silicate and other chemical products; It is used to prepare vinylon spinning coagulant in textile industry. It can also be used in nonferrous metallurgy, leather and other aspects. The waste catalyst after the renewal of the denitration system is recovered by the catalyst manufacturer.

2 Process Technology

The commonly used desulfurization and denitrification methods of coke oven flue gas mainly include SDS dry desulphurization + medium-low temperature SCR denitration, SDA(Na) semi-dry desulphurization + medium-low temperature SCR denitration, SDA(Ca) semi-dry desulphurization + GGH-medium-low temperature SCR denitration and activated carbon dry desulphurization and denitration process.

2.1 SDS dry desulfurization process

High efficiency desulfurizer (particle size of 20~25 μm) is sprayed into the flue through SDS dry deacidification jet and distribution device and is heated and activated in the flue. Its specific surface area is rapidly increased. The background is the HCl dry removal system developed by waste incineration industry. After SDS dry process deacidification technology has been rapidly developed in Europe, its supporting jet system and after full contact with the flue gas, physical and chemical reactions occur, 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 successively. At present, the process is mainly used in the European market for waste incinerator tail gas deacidification, but the technology has achieved good application results in other industries, including coking, glass manufacturing, coal-fired power plants, hazardous waste incinerators, diesel power generation, biomass power generation, cement and so on.

SDS dry desulphurization + middle and low temperature SCR denitration process has the advantages of high desulphurization and denitration efficiency, no temperature drop, no water operation, less investment, small footprint, less by-products, low power consumption, no corrosion, simple equipment, easy operation and maintenance, small desulfurization by-products, high sodium sulfate content; 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 in the early 1970s by the Danish company Niro. The desulfurization process is to prepare CaO or Na2CO3 with water to form Ca(OH)2 slurry or Na2CO3 solution with solid content of 20%~25%, and atomize the solution into 30-80μm droplets by high-speed rotation of the atomizer. 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 is instantly dried into the liquid droplets in the tower, making it a powdery dry solid, which is collected by the bag dust collector.

The desulfurization process is simple and the absorption tower is empty. The advantages of SDA(Na) semi-dry desulfurization and middle and low temperature SCR denitration are high desulfurization efficiency, no waste water generation, low water consumption, low power consumption and no corrosion. The disadvantages are that the desulfurizer is easy to crystallize, difficult to maintain, and difficult to recycle by-products. The advantages of SDA(Ca) semi-dry desulfurization + GGH-medium-low temperature SCR denitration process are moderate desulfurization efficiency, no waste water generation, low water consumption, low power consumption, and no corrosion. The disadvantage is that it occupies a large area, the flue gas temperature decreases first and then increases, the energy consumption is high, and the by-products are difficult to use.

2.3 Activated carbon dry desulfurization and denitrification process

Based on the principle of physical-chemical adsorption, activated carbon adsorbed SO2, H2O and O2 in the flue gas and catalyzed the reaction to produce sulfuric acid, and then migrated to the micropore storage, while NOx in the flue gas under the catalysis of activated carbon, and the ammonia water sprayed into the flue gas to produce N2 and H2O. Activated carbon continues to adsorb SO2 by releasing the active adsorption site through the regeneration system, and the flue gas containing SO2 discharged from the regeneration system enters the by-product recovery system, and SO2 can be processed into a variety of sulfur chemical products.

Activated carbon will produce wear and chemical consumption during the regeneration process, so it is necessary to regularly supplement new activated carbon, 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, so as to achieve the purpose of desulfurization, denitrification and dust removal at the same time. The disadvantage is that the flue gas temperature needs to be reduced to below 150 ℃; The by-product of desulfurization contains sulfuric acid and produces polluted wastewater, which has large one-time investment and high operating cost.

To sum up, whether from the advanced nature of process technology (desulfurization, denitrification efficiency), or from the practical aspects of process technology, land area, investment cost, waste water, by-product utilization and other aspects of comprehensive analysis and comparison, SDS dry desulfurization and low-temperature SCR denitrification process is suitable for coke oven flue gas purification process technology, its configuration is reasonable. The control level has reached 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 Principles of SDS process

SDS dry process deacidification injection technology is the high efficiency desulfurizer (particle size of 20~25 μm) uniformly sprayed in the pipeline, the desulfurizer is heated in the pipeline, the specific surface area is rapidly increased, and the acidic flue gas is fully in contact with physical and chemical reactions, and the 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 reactions with other acidic substances (such as SO3, etc.) are:

NaHCO3 +HCl → NaCl +CO2+H2O

NaHCO3 +HF → NaF +CO2+H2O

4 Process Characteristics

4.1 Technical characteristics of SDS desulfurization process

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

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

(2) Less one-time investment, small footprint;

(3) Low operating cost;

(4) Full dry system, no need for water;

(5) High desulfurization efficiency;

(6) reasonable desulfurizer distribution device;

(7) High flexibility, can be adjusted at any time according to the requirements of emission indicators;

(8) It has a 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 a quick switching device for the accident channel, which will not affect the normal production of the coke oven once the fault occurs.

5 Composition of desulfurization and denitrification process system

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

(1)SDS desulfurizer dosing and distribution device (key equipment should be used);

(2) Dust removal equipment and ancillary equipment;

(3) Denitration reactor system and auxiliary equipment;

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

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

With the increasingly strict requirements of environmental regulations, the process of baking soda dry deacidification principle developed by Perman in combination with advanced technology in Europe and the United States has been more and more applied. Various pollutants in the flue gas were removed by dry adsorption with sodium bicarbonate as adsorbent. The purification effect can be compared with other known methods, such as spray adsorption using lime milk as 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 also widely used in glass, cement, metallurgy and other industrial furnaces. The use of dry gas purification can economically remove the gas containing acidic substances, such as SO2, HCI, etc., to meet the national standard of flue gas emission standards. In recent years, the baking soda dry desulphurization and deacidification grinding machine produced and developed by our company serves a number of environmental protection companies in the country, and successfully completes the flue gas desulphurization transformation project for a number of iron and steel group owners in the country, thus playing the role of desulphurization and deacidification and flue gas purification

Hubei Environmental Protection Technology Co., Ltd. is the grinding and pulverizing system of Tianjin United Special Steel Co., Ltd. hot blast furnace flue gas desulfurization renovation project. The equipment has been completed, contributing to the realization of a more beautiful blue sky and white clouds.