Comprehensive analysis of measures to improve the recovery rate of coking tar in chemical production
Thread Content
Coke costs are primarily determined by the level of coal blending costs, but chemical products, acting as a deduction factor, also have a significant impact on them. The article discusses the factors affecting the tar recovery rate from the three systems of coal, coke, and chemical processing, and analyzes and explores measures to improve the tar recovery rate from these three aspects. 1 Factors affecting tar recovery rate 1.1 Influence of the coal system on tar recovery rate 1.1.1 Influence of blended coal on tar recovery rate Coking is the process in which coal is dried and distilled at high temperatures in the carbonization chamber of a coke oven, resulting in the production of coke and coke oven gas. The yield of tar depends on the volatiles content of the coal used for coking; the higher the volatiles content of the coal during high-temperature coking, the higher the tar yield. 1.1.2 Influence of coal moisture and fineness on tar recovery rate: Changes in the moisture content of the coal fed into the furnace affect the heat required for coking, which in turn impacts the stability of the overall furnace temperature. For every 1% change in the moisture content of the coal, the average furnace temperature changes by 7–8 degrees Celsius, affecting the uniformity of the furnace temperature. This can lead to localized formation of coke, thereby affecting the coking time and impacting the tar recovery rate. Excessively fine coal particles in the feed coal can increase the amount of coal dust in the gas. It also leads to a higher concentration of high-viscosity tar in the gas, thereby increasing the resistance of the primary cooler. This, in turn, affects the condensation of tar from the gas and consequently reduces the tar recovery rate. 1.2 Influence of the heating system on tar recovery rate 1.2.1 Influence of standard temperature Tar is a mixture composed of various organic compounds; its composition and properties vary depending on the temperature. The temperature at which coking takes place has a direct impact on the quality of the tar. An increase in the amount of toluene-insoluble substances in the tar can affect the tar recovery rate. 1.2.2 Effects of furnace wall temperature and roof space temperature: An increase in the temperature of the furnace wall in the carbonization chamber leads to a greater degree of secondary pyrolysis of the initial products. This results in a decrease in the yield of phenols and neutral oils in the tar, as well as an increase in the content of free carbon. Therefore, a high furnace wall temperature reduces the tar recovery rate. The temperature in the furnace roof area increases, which intensifies the secondary pyrolysis of the coking chemical products, resulting in a decrease in the tar yield. 1.2.3 Effect of pressure: A high pressure in the carbonization chamber increases the likelihood of gas leakage; if a negative pressure is created in the chamber, air will be drawn into it, and some of the gas will burn there, thereby affecting the yield of gas and all other chemical products. 1.3 Influence of the chemical system on tar recovery rate 1.3.1 Influence of the primary cooler resistance The primary cooler performs multiple functions, including cooling the gas, removing tar, and removing naphthalene; it is an important component in gas production. An increase in the resistance of the primary cooler significantly affects the condensation of tar in the gas, and the formation of tar residues and other scale-like substances during steam purging, thereby impacting the tar recovery rate. 1.3.2 Effect of gas temperature after primary cooling: If the temperature of the gas after the primary cooler is too high, the condensation rate of tar gases decreases. This not only affects the yield of tar but also leads to an increase in the tar content after electrostatic precipitation, thereby affecting the performance of subsequent washing processes. 1.3.3 Use of mechanical clarifiers and the impact of oil content The abnormal operation of mechanical clarifiers results in the circulating ammonia water not having sufficient time for natural sedimentation and separation within these devices; as a consequence, the residence time of the circulating ammonia water is short, which leads to poor separation of tar from the ammonia water. This results in an excessive oil content in the circulating ammonia water, affecting both the effectiveness of ammonia water spraying at the furnace top and the yield of tar.
2.2 Control of coking operations
2.2.1 Control of the temperature in the oven crown space
By increasing the excess air coefficient, which is originally set within the standard range of 1.15–1.25 for coke oven gas heating and 1.15–1.2 for blast furnace gas heating, the target is to maintain it at around 1.3, thereby reducing the temperature in the oven crown space. By increasing the mixing ratio and using blast furnace gas for heating, when the proportion of coke oven gas mixed in reaches 8%, the temperature in the roof space can be reduced by 15–20°C; when the coking time is 22–24 hours, this value can be kept at 6%–8%. When the coking time is 25–30 hours, it should be kept at 3%–5%. Optimize the heating regime by reducing the standard temperature while ensuring uniform maturation of the coke. A roof space temperature of 780°C is an ideal value that meets the requirements for coke maturation as well as environmental protection considerations, and it is conducive to improving the tar yield. 2.2.2 Coal loading control: Adjust the timing and sequence of coal feeding by the screw to reduce blockages. Optimize the coking coal handling process to steadily increase the amount of coal fed into each furnace per hole. Modify the flat coal rod reinforcement plates to increase the height of the coal line. Add support plates under the flat coal rod to eliminate the reduction in the coal level caused by the forward end being planted. 2.2.3 Graphite cleaning control: Graphite scrapers are installed at the two corners above the pusher rod head to remove the graphite from the top of the carbonization chamber during the pushing process. Used in the head air compressor purging system to remove graphite formed on the furnace top. To carry out graphite burning properly, close the water seal flap of the bridge tube 30 minutes in advance according to the coke pushing schedule, open the lid of the rising pipe, and open the lid of the coal loading port located away from the rising pipe; air is then drawn in to burn off the graphite at the top of the carbonization chamber, thereby ensuring clear space at the top of the furnace. 2.2.4 Gas collection tank pressure control: The pressure control of the gas collection tank is determined by maintaining the pressure at the bottom of the carbonization chamber, directly below the air intake pipe, at 5 Pa at the end stage of coking. Carry out efforts to control the pressure in the gas collection pipes, reduce pressure fluctuations in these pipes, and thereby ensure the appropriate pressure within the carbonization chamber. 2.2.5 Other coking operation controls: Ensure the furnace door is properly cleaned; repair or replace any damaged parts on the edge of the door to maintain a tight seal, thereby reducing gas losses. Keep the ammonia nozzle unobstructed to prevent clogging of the nozzle, which could lead to pressure fluctuations in the gas collection pipe as well as tar deposition at the bottom of the pipe. 2.3 Operation control for chemical product recovery 2.3.1 Control of the primary cooler resistance: Ensure the quality of the circulating water; discharge wastewater promptly if standards are exceeded to reduce scaling inside the primary cooler. Regularly purge the primary cooler to prevent the formation of deposits within it. Ensure that the spraying pipe remains unobstructed and that the condensate contains an appropriate level of oil, thereby effectively controlling the resistance in the primary cooler. 2.3.2 Control of gas temperature after primary cooling: For the renovation of the desulfurization cold water system, fresh water is used as a substitute for the cold water. Removal of old equipment is carried out followed by the installation of refrigerators, to ensure that the cold water capacity meets the requirements of the pre-cooler, while striving to keep the gas outlet temperature within acceptable limits. 2.3.3 Use of mechanical clarifiers and oil pressing: Establish maintenance standards for mechanical clarifiers to eliminate equipment failures and improve their utilization rate. In the oil pressing operation of the tar-ammonia water clarification tank, it is necessary to ensure the continuity of oil pressing, control the oil-water interface appropriately, and guarantee that the circulating ammonia water is free of oil. The appropriate oil-water interface is determined to be the 6th valve in the mechanical clarifier, and the position of the oil feed port is adjusted accordingly based on this height as well as the actual measured height of the ammonia solution level. Increase the temperature and pressure of the circulating ammonia water to achieve a better atomization effect of the ammonia water in the gas collection tank, improve the heat exchange between the gas and the ammonia water, and maximize the amount of tar that condenses in the gas.
2.2.1 Control of the temperature in the oven crown space
By increasing the excess air coefficient, which is originally set within the standard range of 1.15–1.25 for coke oven gas heating and 1.15–1.2 for blast furnace gas heating, the target is to maintain it at around 1.3, thereby reducing the temperature in the oven crown space. By increasing the mixing ratio and using blast furnace gas for heating, when the proportion of coke oven gas mixed in reaches 8%, the temperature in the roof space can be reduced by 15–20°C; when the coking time is 22–24 hours, this value can be kept at 6%–8%. When the coking time is 25–30 hours, it should be kept at 3%–5%. Optimize the heating regime by reducing the standard temperature while ensuring uniform maturation of the coke. A roof space temperature of 780°C is an ideal value that meets the requirements for coke maturation as well as environmental protection considerations, and it is conducive to improving the tar yield. 2.2.2 Coal loading control: Adjust the timing and sequence of coal feeding by the screw to reduce blockages. Optimize the coking coal handling process to steadily increase the amount of coal fed into each furnace per hole. Modify the flat coal rod reinforcement plates to increase the height of the coal line. Add support plates under the flat coal rod to eliminate the reduction in the coal level caused by the forward end being planted. 2.2.3 Graphite cleaning control: Graphite scrapers are installed at the two corners above the pusher rod head to remove the graphite from the top of the carbonization chamber during the pushing process. Used in the head air compressor purging system to remove graphite formed on the furnace top. To carry out graphite burning properly, close the water seal flap of the bridge tube 30 minutes in advance according to the coke pushing schedule, open the lid of the rising pipe, and open the lid of the coal loading port located away from the rising pipe; air is then drawn in to burn off the graphite at the top of the carbonization chamber, thereby ensuring clear space at the top of the furnace. 2.2.4 Gas collection tank pressure control: The pressure control of the gas collection tank is determined by maintaining the pressure at the bottom of the carbonization chamber, directly below the air intake pipe, at 5 Pa at the end stage of coking. Carry out efforts to control the pressure in the gas collection pipes, reduce pressure fluctuations in these pipes, and thereby ensure the appropriate pressure within the carbonization chamber. 2.2.5 Other coking operation controls: Ensure the furnace door is properly cleaned; repair or replace any damaged parts on the edge of the door to maintain a tight seal, thereby reducing gas losses. Keep the ammonia nozzle unobstructed to prevent clogging of the nozzle, which could lead to pressure fluctuations in the gas collection pipe as well as tar deposition at the bottom of the pipe. 2.3 Operation control for chemical product recovery 2.3.1 Control of the primary cooler resistance: Ensure the quality of the circulating water; discharge wastewater promptly if standards are exceeded to reduce scaling inside the primary cooler. Regularly purge the primary cooler to prevent the formation of deposits within it. Ensure that the spraying pipe remains unobstructed and that the condensate contains an appropriate level of oil, thereby effectively controlling the resistance in the primary cooler. 2.3.2 Control of gas temperature after primary cooling: For the renovation of the desulfurization cold water system, fresh water is used as a substitute for the cold water. Removal of old equipment is carried out followed by the installation of refrigerators, to ensure that the cold water capacity meets the requirements of the pre-cooler, while striving to keep the gas outlet temperature within acceptable limits. 2.3.3 Use of mechanical clarifiers and oil pressing: Establish maintenance standards for mechanical clarifiers to eliminate equipment failures and improve their utilization rate. In the oil pressing operation of the tar-ammonia water clarification tank, it is necessary to ensure the continuity of oil pressing, control the oil-water interface appropriately, and guarantee that the circulating ammonia water is free of oil. The appropriate oil-water interface is determined to be the 6th valve in the mechanical clarifier, and the position of the oil feed port is adjusted accordingly based on this height as well as the actual measured height of the ammonia solution level. Increase the temperature and pressure of the circulating ammonia water to achieve a better atomization effect of the ammonia water in the gas collection tank, improve the heat exchange between the gas and the ammonia water, and maximize the amount of tar that condenses in the gas.
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