Technology for controlling NOx emissions from coke oven flue gas by combining upstream prevention and control with downstream treatment

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Technology for controlling NOx emissions from coke oven flue gas by combining primary prevention and secondary treatment Tang Liang1,2 (1. German Weikon Consulting and Engineering Company； 2. Changsha Weikang Engineering Technology Co., Ltd.) 1 Introduction: Coking ovens generate many pollutants that are harmful to the environment during the production of coke, and nitrogen oxides (NOx) are one such pollutant. With the implementation of GB16171-2012 \"Emission Standards for Pollutants in the Coking Chemical Industry\", as of January 1, 2015, existing enterprises as well as newly established enterprises are required to keep the NOx concentration in the flue gases emitted from coke ovens below 500 mg/m3; in areas with stricter emission limits, this concentration must be below 150 mg/m3. The implementation of this standard has attracted great attention from professionals in China’s coking industry. Reducing NOx emissions from coke ovens is an urgent task for coke manufacturing enterprises. It is understood that industry insiders have conducted a preliminary assessment: under normal production conditions, coking enterprises must ensure that the NOx concentration in the exhaust gases emitted from their coke oven stacks remains below 500 mg/m³. This poses tremendous technical challenges; as a result, only very few coking enterprises are able to meet this standard. In areas with strict emission limits, it is even more difficult to keep the NOx emission concentration below 150 mg/m3. To ensure that the nitrogen oxide emissions from coke oven flue gas meet regulatory standards, relevant treatment technologies must be employed. However, at present, managing these technologies is difficult, the overall investment costs are high, and the maintenance costs in the long term are significant. To this end, this paper explains the control of nitrogen oxide emissions from coke oven flue gas by combining the company’s technologies for preventive measures at the source and treatment at the end point, for reference by peers in the industry. 2 Sources of nitrogen oxides in coke oven flue gas: NOx is generated during the combustion of the fuel used to heat the coke ovens in their vertical flues. There are three main sources of such NOx: the first is temperature-induced thermal NOx ； Second, the nitrogen compounds contained in coke oven gas release NOx upon combustion ； Third is the rapid NOx from hydrocarbon fuels. Among the NOx generated during combustion, NO accounts for 90%–95%, while NO2 makes up about 5%. Therefore, by combining mature front-end control technologies to regulate the NOx concentration at the source, overall investment costs can be significantly reduced. 3 NOx pre-treatment technologies and their applications. The pre-treatment technology for nitrogen oxides in coke oven flue gas developed by the German company Wecon Consulting & Engineering consists of two main components. The first is the coke oven temperature measurement system technology, and the second is the coke oven thermal system control technology. 3.1 Technology of Coke Oven Temperature Measurement System 3.1.1 Introduction to the Coke Oven Temperature Measurement System As shown in Figure 1, the coke oven temperature measurement system consists of a measuring rod, a gas pipeline system, a temperature control box, an electrical control system, and an analysis system. Among these, the hardware components are installed on the coke pusher. During the coke-pushing process, as the pusher moves in and out rapidly, multiple probes (indicated by red dots in Figure 1a) measure the temperature inside each carbonization chamber. The data is then transmitted back via the temperature control system and the electrical control system for analysis. Using a computer-based analytical model, it is possible to obtain a comprehensive understanding of the temperature distribution within each carbonization chamber of the coke oven, thereby visualizing the overall temperature distribution across the entire wall structure ； Secondly, inspect the furnace top graphite ； Third, it can detect damage to the heating flue and the walls of the carbonization chamber. Through detection and analysis, a comprehensive understanding of the coke oven’s condition can be obtained, providing the necessary basis for adjusting its thermal system. As shown in Figure 2, these are the operating conditions obtained by the temperature measurement system through computer-based analysis models. Among them, in Figure 2a, green indicates a normal temperature, red indicates an excessively high temperature, and blue indicates a low temperature. In Figure 2b, the temperature distribution throughout the entire coke oven is shown. file:///C:/Users/TL/AppData/Local/Temp/msohtmlclip1/01/clip_image002.jpg file:///C:/Users/TL/AppData/Local/Temp/msohtmlclip1/01/clip_image004.jpg a Schematic diagram of the temperature measurement system b Diagram of the industrial installation 1 Temperature measurement system for coke ovens file:///C:/Users/TL/AppData/Local/Temp/msohtmlclip1/01/clip_image005.png file:///C:/Users/TL/AppData/Local/Temp/msohtmlclip1/01/clip_image006.png a Temperature distribution in the carbonization chamber b Graph showing actual temperature readings in the coke oven Figure 2 Analysis of the temperature measurement system 3.1.2 Technical advantages of the coke oven temperature measurement system (1) Provides a necessary basis for regulating the thermal system of coke ovens, enabling control and reduction of the overall energy consumption of these ovens ； (2) Provide a necessary basis for the regulation techniques of the coke oven thermal system, in order to control and reduce concentrations of pollutants such as NOx in flue gases ； (3) By analyzing the temperature of the coke oven, the carbonization process is optimized to achieve higher-quality coke ； (4) Detecting damage to the furnace walls facilitates early prevention and repair, thereby reducing gas leaks in the coke oven ； (5) Extend the service life of the coke oven. 3.1.3 Industrial application: The coke oven temperature measurement system can be directly modified and installed on the pusher rod, as shown in Figure 1b. It has been successfully applied in over 70 coking plants in countries such as Germany, Austria, and India. 3.2 Regulation technology for the coke oven thermal system: Based on the data provided by the coke oven temperature measurement system, this regulation technology is used to directly control the concentration of NOx in the exhaust gases from the coke oven. (1) Reduce the temperature of the vertical flame channel. The high-temperature area in the vertical flue of the combustion chamber is the direct cause of NOx generation; based on the data provided by the temperature measurement system, the standard temperature of the vertical flue can be reduced while ensuring the quality of the coke ； (2) Reduce the air excess factor. Adjust the ratio of combustion-supporting air to heating gas to reduce the amount of air required for the combustion of the heating gas, thereby allowing the gas to burn at a lower excess air coefficient ； (3) Adjustment of the thermal operating regime of coke ovens. Adjust the temperature uniformity in the straight and cross rows of the coke oven to avoid localized high or low temperatures, so as to achieve a more balanced temperature distribution in the vertical flues ； (4) Adjust the air flow rate and pressure values of the actuator ； (5) Adjust the amount of exhaust gas circulating in the combustion chamber ； (6) Adjust the pressure difference of flue gas waste gas. 3.3 Industrialization of front-end prevention and control technologies and their application effects: Through these front-end prevention and control technologies, the NOx concentration in coke oven flue gas can be kept below 500 mg/m3. Without the need for any additional equipment, it is possible to meet the requirement specified in GB16171-2012, the Emission Standards for Pollutants from the Coking Chemical Industry, which mandates that the NOx concentration in the flue gases emitted by existing as well as newly built enterprises must be less than 500 mg/m3. At the same time, it can also reduce the overall energy consumption of the coke oven. 4 End-of-pipe treatment technologies for NOx: According to standard GB16171-2012, in areas with stringent emission limits, the NOx concentration in coke oven exhaust is required to be less than 150 mg/m3. Front-end prevention and control technologies alone are no longer sufficient to meet the standards; therefore, new technologies and devices are needed to effectively treat NOx in the flue gas from coke ovens. 4.1 Process flow The schematic diagram of the process flow for the integrated desulfurization, dust removal, and denitrification technology for coke oven flue gas developed by our company is shown in Figure 3. file:///C:/Users/TL/AppData/Local/Temp/msohtmlclip1/01/clip_image008.jpg Figure 3: Schematic diagram of the flue gas desulfurization and denitration process. The flue gas from the coke oven is first subjected to desulfurization to remove SO2; the temperature of the gas after desulfurization drops by approximately 10°C. Subsequently, it enters the dust removal unit, where desulfurization and denitration take place in one integrated process. The flue gas after denitration experiences a certain degree of temperature rise; depending on the actual requirements of each coking plant, it is possible to add a heat recovery unit. The coke oven flue gas, after desulfurization, dust and nitrogen oxide removal as well as heat recovery, is directly discharged into the atmosphere. 4.2 Principle of the SCR denitration process: In the presence of a catalyst, NOx in coke oven flue gas reacts with NH3 through a reduction reaction to produce N2 and H2O, thereby removing NOx. The denitration reaction equations are: 4NO + 4NH3 + O2 → 4N2 + 6H2O (1), 2NO2 + 4NH3 + O2 → 3N2 + 6H2O (2), NO + NO2 + 2NH3 → 2N2 + 3H2O (3). Equations (1) and (3) are the main reactions; over 90% of the NOx in coke oven flue gas exists in the form of NO. 4.3 Process Characteristics (1) The desulfurization step is carried out before denitration, reducing the SO2 content in the flue gas to below 30 mg/m3 to ensure efficient denitration thereafter ； (2) Integrated flue gas dust removal and denitration – no need to construct an additional SCR denitration reactor ； (3) It enables low-temperature denitrification of coke oven flue gas, meeting the operating conditions of a flue gas temperature of 200–240°C, without the need for an additional preheater ； (4) Reduces overall investment, has low operating costs, and requires less space. 5 Conclusions (1) Front-end control technologies for NOx can meet the standard of keeping NOx concentrations below 500 mg/m3. Front-end control methods have low costs and mature technology, but they cannot meet the emission requirement of keeping NOx concentrations below 150 mg/m3 in restricted areas; in such cases, end-stage treatment technologies must also be employed. (2) By using front-end prevention and control technologies to reduce the initial NOx concentration in the flue gas from coke ovens to below 500 mg/m3, and then applying integrated desulfurization, dust removal, and denitration technologies, the overall investment costs as well as subsequent operation and maintenance costs can be significantly reduced. Author profile: Tang Liang, Master’s degree; main research areas: coke oven machinery, environmental protection technologies. Contact number: 13755064691 Address: Economic and Technological Development Zone, Changsha City, Hunan Province