Coking Process Training Manual

Thread Content

University training handout: The process of heating coal in isolation from air to produce solid, liquid, and gaseous products is known as the carbonization of coal. Based on the final temperature to which the coal is heated, it is divided into low-temperature carbonization (500–550°C), medium-temperature carbonization (600–800°C), and high-temperature carbonization (900–1050°C), which is the coking process. In the early days of iron smelting, charcoal was used as both fuel and reducing agent. Starting in 1709, coke began to be used in place of charcoal for iron smelting, which in turn promoted the development of coke production and related technologies. 1. Development stages of coking technology: The four development stages are pile coking and kiln-type coking, counterflame ovens, waste heat coke ovens, and modern regenerative chamber coke ovens. The current stages of further development in coking technology are as follows: 1) enlargement of the volume of coke ovens; 2) coal preprocessing techniques: coal blending technologies, ramming processes, coal preheating processes, etc.; 3) environmental protection; 4) automation technologies in coking. 2. Functions and properties of coke: A blast furnace is a vertical furnace, which consists of five parts from top to bottom: the furnace throat, furnace top, furnace body, furnace waist, furnace belly, and furnace hearth. The raw materials include iron ore (or sinter), coke, and limestone, which are alternately fed into the furnace from the top through feeding devices. Carbon monoxide, produced by the incomplete combustion of coke and oxygen, is the main reducing agent in the blast furnace. The heat released from the combustion reaction of coke and oxygen is the main source of heat in the blast furnace smelting process. The purpose of adding limestone is to react it with the high-melting-point acidic oxides present in the ore and coke, thereby forming slag with a lower melting point and lower specific gravity, which separates from the iron and is discharged from the furnace bed. Since coke serves as a structural framework within the blast furnace to support the charge column and maintain a uniform distribution of the material as well as good air permeability, it is required to have high resistance to crushing and wear. It also needs to have a certain size; the more uniform this size is, the better. As blast furnaces become larger and coal injection technology is employed, higher requirements are placed on the strength and size of coke. The chemical composition of coke includes moisture, ash, volatile matter, sulfur, phosphorus, etc. The moisture content of coke is unrelated to the moisture content of the coking coal feed, nor does it depend on the coking process conditions; it is primarily influenced by the method used for quenching the coke. Additionally, the moisture content of coke should be kept as stable as possible, which is beneficial for stabilizing the blast furnace charge. The main components of the ash in coke are SiO2 and Al2O3. An increase in coke ash not only reduces the strength of coke, but also requires more limestone to be used in blast furnace production, resulting in a decrease in iron output. The volatiles content of coke is an indicator of its degree of maturity. Excessively high volatile matter in coke indicates that the coke has not fully matured, resulting in \"green coke\". When the volatile matter content of coke is too low, it indicates that the coke has been over-treated; as a result, there are more cracks in the coke, making it more fragile. The sulfur content in coke is influenced by the coking coal used; it is a major harmful impurity in pig iron. When the sulfur content in coke is high, more limestone must be added during blast furnace processing to remove sulfur, which results in a reduction in iron production. The phosphorus content in coke is very low. The level of phosphorus in coke depends on the coal used for coking; almost all of the phosphorus present in the coal ends up in the coke. Generally, the phosphorus content in coke is around 0.02%. During the iron smelting process, phosphorus enters the pig iron, causing it to become cold-sensitive. Industrial analysis and mechanical strength of coke: The industrial analysis of coke includes parameters such as moisture, ash content, sulfur content, and volatile matter content. Mechanical strength includes the impact resistance index M40 and the wear resistance index M10. Our country uses the Mikum drum test method to determine the mechanical strength of coke. The screening composition of coke refers to the calculation of the percentage content of various particle sizes of coke, such as those larger than 80 mm, 80–60 mm, 60–40 mm, and 40–25 mm. The uniformity coefficient k of coke lump size can be calculated using the screening composition of the coke. k can be calculated using the following formula: K = M40, M10 drum indices of coke strength are both cold-state properties of coke, whereas coke is used in blast furnaces under hot conditions at temperatures exceeding 1000°C. The main reason for the weakening of coke strength in the lower part of the blast furnace is the erosion of coke by CO2 at high temperatures. The carbon in the coke is consumed in the process of direct reduction, resulting in a loss of its strength at high temperatures and subsequent pulverization. This loss of the coke’s ability to support gas flow prevents the blast furnace from operating properly. Therefore, the high-quality coke required for modern large blast furnaces is one that is not easily eroded by CO2 at high temperatures. The reactivity of coke (CRI) and its strength after reaction (CSR) can be used as indicators to evaluate the high-temperature strength of coke. The measurement method adopted in our country is the same as that used by Nippon Steel in Japan, namely reacting pure CO2 with a coke sphere of 20 mm in diameter at a constant temperature of 1100°C for 120 minutes; the sample weight is 200 g, and the percentage weight loss after the reaction is used as the reactivity index (CRI). The reacted coke was placed in a Type I drum with a diameter of 130 mm and a length of 700 mm, rotated at 20 revolutions per minute for 600 revolutions, and then screened; the percentage of the material passing through a 1 mm sieve relative to the total weight of the sample fed into the drum was used as the strength after reaction (CSR). 3. The coking process in the carbonization chamber. The coking process of coal materials in the carbonization chamber has two main characteristics: one is unidirectional heating and layered coking ； Second, the heat transfer characteristics during coking change depending on the state and temperature of the charge. Before coal is loaded, the wall temperature in the carbonization chamber is generally around 1100°C. Once the coal at room temperature is loaded, it absorbs a large amount of heat from the walls, causing the temperature on the surface of the furnace walls to drop sharply. Meanwhile, the coal located right next to the furnace walls heats up rapidly, while the coal in the center of the carbonization chamber remains at room temperature. As the coking time progresses, the coal near the furnace wall has already formed coke 6 to 7 hours after coal charging. The temperature of the coal material on the central surface of the carbonization chamber is only 100–200°C; in other words, the material located between the walls of the carbonization chamber and its central surface is in various stages of the coking process. Right next to the furnace walls lies a layer of coke, followed successively by layers of semi-coke, plastic coal, dry coal, and wet coal. In a carbonization chamber, the coking process begins at the walls on both sides of the chamber and gradually moves toward the center in layers, a phenomenon known as \"layered coking\". At the late stage of coking, the coke layer gradually moves toward the central plane, and the entire carbonization chamber becomes coke. Therefore, the temperature at the central plane of the carbonization chamber at this late stage (the temperature at the center of the coke cake) can serve as an indicator of the degree of maturity of the coke cake, and thus constitutes the final temperature for coking. Near the walls of the carbonization chamber, the temperature difference between the two ends of the coke pieces is large; as a result, there are many deep cracks in these pieces. The particle size of the coke in this area is small. In the center of the coke cake, however, the temperature difference between the two ends of the coke pieces is small, so there are fewer and shallower cracks, and the coke pieces are larger in size. Furthermore, when the plastic zones on both sides at the center of the coke cake meet, the thickness of the plastic zone is at its maximum; the resistance to the emission of pyrolytic gases from this plastic zone is also at its highest, and the lateral pressure exerted by the plastic mass on the furnace walls on either side is greatest as well. In other words, the expansion pressure is at its peak at this time. 4. Refractory materials for coke ovens: Silica bricks belong to acidic refractory materials, with a SiO2 content of over 93%; they possess excellent resistance to acidic erosion. They also have good thermal conductivity and a high temperature at which they begin to soften, usually above 1620°C. The thermal conductivity of silica bricks increases as the operating temperature rises; there is no residual shrinkage, and during the heating process, the volume of silica bricks increases as the temperature goes up. Clay bricks refer to clay-based products that are aluminum silicate materials with an Al2O3 content of 30%~40%. The mineral composition is mainly kaolinite (Al2O3·2SiO2·2H2O) with 6%–7% impurities (oxides of potassium, sodium, calcium, titanium, and iron). The firing process of clay bricks is primarily a process in which kaolinite loses water continuously and decomposes to form mullite (3Al2O3·2SiO2) crystals. Clay bricks are weakly acidic refractory products that can resist the erosion of acidic slag and acidic gases, but they have slightly weaker resistance to alkaline substances. Clay bricks have good thermal properties and are resistant to sudden changes in temperature. The refractoriness of clay bricks is on par with that of silica bricks, reaching 1690–1730°C, but their load softening temperature is more than 200°C lower than that of silica bricks. This is because, in addition to mullite crystals with high refractoriness, clay bricks also contain nearly half a low-melting-point amorphous glass phase. In the temperature range of 0–1000°C, the volume of clay bricks expands uniformly as the temperature rises; the linear expansion curve is approximately linear, with a linear expansion rate of 0.6%–0.7%, which is only about half that of silica bricks. When the temperature reaches 1200°C and continues to rise, its volume will begin to shrink from the maximum expansion point. The residual shrinkage of clay bricks leads to the cracking of the mortar joints in masonry, which is a major drawback of clay bricks. Clay bricks can only be used in secondary parts of coke ovens, such as the lining bricks for regenerator walls, the lining bricks for small flues, the grid bricks in the regenerator, the lining bricks for furnace doors, the furnace roof, and the lining bricks for rising pipes. High-alumina bricks are refractory products made of alumina silicate or alumina with an Al2O3 content of over 48%. They feature high density, low porosity, high mechanical strength, and wear resistance. It is effective to use high-alumina bricks for constructing the burner parts of the coke oven combustion chamber and the bottom bricks in the carbonization chamber. 5. Structure of the coke oven body: At the very top of a modern coke oven is the oven roof; below this roof are combustion chambers and carbonization chambers arranged alternately. Below that are regenerative chambers, as well as inclined channels that connect the regenerative chambers to the combustion chambers. The small flues located beneath each regenerative chamber are connected to the main flue through exchange valves. The flue is located within or on either side of the coke oven foundation, with its end leading to the chimney. A coke oven consists of multiple combustion chambers and multiple carbonization chambers, arranged alternately. The carbonization chamber is where coal is loaded and coking takes place, while the combustion chamber is where gas burns, supplying heat to the carbonization chamber through the partition walls on both sides. The coal loaded into the furnace is converted into coke through high-temperature carbonization in the carbonization chamber. The wall temperature in the combustion chamber reaches 1300–1400°C, while that in the carbonization chamber is around 1000–1150°C. The wall temperature in the carbonization chamber changes significantly during coal loading and coke extraction, and the salts present in the coal are corrosive to the furnace walls. Modern coke ovens use silica bricks to construct the walls of the carbonization chamber. The combustion chamber is divided into many vertical flues, and the configuration of these vertical flues varies depending on the type of coke oven. The distance from the lower surface of the top brick of the firebox to the lower surface of the top brick of the carbonization chamber is referred to as the heating level intensity; it is an important dimension in the structure of the furnace. If this dimension is too small, the temperature in the furnace roof area will become too high, resulting in excessive deposition of carbon on the furnace roof ； Conversely, if the temperature in the furnace top space is too low, the upper part of the coke cake will not receive sufficient heat, which affects the quality of the coke. Furthermore, either too high or too low a temperature in the furnace roof space can have an adverse effect on the quality of coking chemical products. To recover the heat from the flue gases generated by the coke oven and use it to preheat the lean gas and air, regenerative chambers are installed at the lower part of the coke oven. Modern coke ovens all use horizontal regenerative chambers (whose center line is parallel to the center line of the combustion chamber) to facilitate individual adjustment. The walls of the regenerator are generally built with silica bricks, and grid bricks are placed inside the regenerator to fully recover the heat from the exhaust gases. Grid bricks have to withstand repeated temperature changes from sudden cooling and heating, so they are made from clay-based or semi-siliceous materials. There is a small flue at the bottom of the regenerator, whose function is to alternately introduce cold gas and air into the regenerator, or to discharge exhaust gases. Due to the large temperature differences, in order to withstand sudden changes in temperature and prevent the gas from corroding the small flue, it is necessary to line the small flue with clay bricks. The chute area bears the huge weight of the upper part of the coke oven, and at the same time is in a high-temperature range of 1100–1300°C; therefore, it is also built with silica bricks. The roof area is located at the very top of the coke oven structure. It is equipped with fire observation holes, coal loading holes, and rising pipe holes for guiding raw gas from the carbonization chamber, among others. The lowest layer at the top of the furnace is the top layer of the carbonization chamber cover, which is usually constructed from silica bricks to ensure uniform expansion of the entire carbonization chamber. To reduce heat loss from the furnace top, clay bricks, red bricks, and insulating bricks are used for construction above the carbonization chamber roof layer. The furnace roof surface is generally covered with cylinder bricks to increase its wear resistance. 6. Coke oven machinery and equipment 1) Furnace protection components: Furnace protection equipment includes: furnace columns, small furnace columns, protective plates, longitudinal and transverse struts, springs, etc. The function of furnace protection equipment is to apply a protective pressure to the masonry, thereby maintaining its integrity during furnace heating and the production process, and preventing damage caused by temperature changes and mechanical stress. The function of the longitudinal tie rod is to apply a certain amount of pressure, through a set of springs, to the concrete retaining walls at the longitudinal ends of the coke oven, thereby preventing cracks or deformation in the masonry due to free expansion or contraction. The cross braces, springs, as well as the furnace columns and smaller columns serve to apply a protective pressure laterally to the coke oven. 2) Gas heating equipment: The gas heating pipeline is used to supply gas to the coke oven. The blast furnace gas pipeline distributes the gas from the main pipe to the main pipes on the machine and coke sides, and then the gas enters the coke oven through branch pipes. The coke oven gas pipeline introduces the gas from the main pipe into the main duct in the basement, and then through branch pipes, horizontal pipes, and downward nozzles, it enters the vertical brick gas ducts. Gas shut-off valves are installed on the main pipe and branch pipes to regulate and cut off the gas supply to the entire furnace. The control unit is equipped with a automatic gas pressure regulating flap, which automatically maintains the gas pressure at the specified level according to production requirements. To prevent substances such as naphthalene and tar from condensing out of the gas and blocking pipes and fittings, and to stabilize the temperature of the gas and ensure consistent heating conditions, a gas preheater must be installed on the main pipeline when using coke oven gas for heating, in order to preheat the coke oven gas. On the branch pipe, there are adjustment cocks, switching cocks, and orifice plate boxes installed. The control valve is used to turn off or on the gas supply. The exchange cock is used to periodically exchange gas and introduce air for decarburization, thereby bringing the heating of the coke oven to a heating state at different positions. Orifice plates are used to control the amount of gas entering each combustion chamber. A condensate water seal tank is also installed on the coke oven gas pipeline. The water and tar that condense from the gas in the pipeline flow naturally into this water seal tank. 3) Exhaust gas equipment: Exhaust gas equipment includes exchange valves and flue dampers for main and branch ducts. The exchange switch is a device that controls the heating gas and air entering the regenerator, as well as the exhaust of waste gases generated by combustion. The main flue flap and the branch flue flap are devices used to stabilize the suction in the flue. 4) Switching equipment: Switching equipment is the power device and transmission mechanism used to change the direction of gas flow in the coke oven heating system, including switches and switching transmission devices. The switch is the power mechanism that drives the various transmission pull rods to switch. The steps for operating the switch are all three: turn off the gas → exchange exhaust gas with air → turn on the gas. The purpose of this procedure is to, after shutting off the gas supply, gradually reverse the flow direction of air and exhaust gases in the absence of gas flow. This prevents unburned gas from entering the flue and forming explosive mixtures. Once normal airflow of air and exhaust gases is established, gas can be supplied back to the combustion system, thus avoiding disruptions in airflow or the formation of explosive gas mixtures. 5) Off-gas extraction equipment: The equipment in the off-gas extraction system includes rising pipes, bridge pipes, water seal valves, gas collection pipes, suction pipes, ammonia spray systems, etc. The riser is directly connected to the carbonization chamber or connected to it through an iron riser base. The riser is welded from steel plates, with clay lining bricks installed inside. Its upper part is connected to the bridge tube. The bridge tube is a cast-iron component lined with clay bricks. The bridge tube is equipped with cleaning holes and high- and low-pressure ammonia nozzles. Low-pressure ammonia spraying uses hot ammonia at around 75°C to cool the raw gas discharged from the carbonization chamber to 80–100°C, thereby cooling down most of the tar present in it. Using hot ammonia water for spraying helps to utilize the heat absorption during ammonia vaporization in order to lower the temperature of the gas. Additionally, hot ammonia water is used to cool the tar to maintain its good fluidity, preventing the tar from solidifying and blocking the pipelines. High-pressure ammonia spray is applied only during coal loading; the thrust of the high-pressure ammonia spray creates a suction effect, generating negative pressure beneath the coal loading hole to prevent the escape of smoke and gas. The gas collection pipe is a circular tube or trough-shaped structure made by welding steel plates; it is placed along the length of the coke oven on the oven pillar supports, serving to collect the raw gas from each carbonization chamber. Each gas collection pipe is also equipped with two vent pipes, which are used to release pressure when the fans are stopped or ammonia solution supply is halted, as well as during startup. A double water seal valve is installed at the bottom of each exhaust pipe. To reduce air pollution caused by the release of coke oven gas, an automatic ignition device should be installed at the top of the release pipe to ignite the gas being released. The suction elbow is designed specifically for the discharge of raw gas, and it is equipped with manual and automatic control flaps to regulate the pressure in the gas collection pipe. 6) Coke oven machinery: Special machinery used in coke production; for top-loading coke ovens, these include coal charging cars, coke stoppers, coke pushers, and coke quenching cars ； In rammed coke ovens, a coal loading pusher (or a separate pusher and a ramming coal loader) is used in place of the coal loader and pusher, with rammers and smoke suppression vehicles added to ram the coal cakes and eliminate the dust generated during coal loading. 7) Coke oven production process: Definition and formulation principles of the coke pushing sequence. During coke pushing, the coke cake exerts a certain pressure on the walls of the carbonization chambers on both sides; if the stress on these walls is uneven, they will deform. Expansion pressure is generated during the carbonization of coal, and this pressure reaches its maximum at the middle stage of coke formation. In order for the pressures on both sides of the carbonization chamber wall to cancel each other out, it is necessary for the adjacent carbonization chambers on the two sides of the coke pushing carbonization chamber to be in the middle stage of coking.