Vertical heating of 4.3-meter coke ovens
Thread Content
The designed coking time is 18 hours, but the current coking time has reached 45 hours; how to control the vertical heating of the coke cake?
1’ Exhaust gas recirculation, prolongs the flame. 2’ Increase gas pressure and flue suction. 3’ uses high and low light fixtures.
Improvements in vertical heating of coke ovens 1 Introduction The surface area of the walls in the carbonization chamber is relatively large; in order to obtain coke that is uniformly matured and has a consistent level of volatiles, it is necessary for the heat supply to be adequate both vertically and along the length of the chamber, meaning that the temperature should be uniform throughout. If the coke cake is not heated evenly along the height of the coke oven, it will lead to reduced uniformity in coke particle size and decreased wear resistance, increased energy consumption, difficulties in operating the coke oven, as well as increased pushing resistance due to poor contraction in the areas of the coke cake that are not heated properly. In the coke ovens designed and put into operation today, the carbonization chambers are getting taller and taller. In our country, a number of coke ovens with carbonization chambers 6 meters high have been put into operation, while in the world, there are already coke ovens with carbonization chambers over 8 meters high in use. Therefore, studying the various factors that affect vertical heating in coke ovens, and determining how to select appropriate parameters for vertical heating based on different heights of the carbonization chambers, is one of the important tasks for those working in coke production. 2 Factors affecting upward heating The factors that influence upward heating in coke ovens include the speed of gas combustion, the velocity of air flow, heat transfer through the furnace walls, and the density of the coal charge as it is stacked vertically. Among them, the burning speed of gas plays a major role. 2.1 The speed of gas combustion: The length of the flame is related to the speed at which the gas burns. The slower the gas burns, the longer the flame; conversely, the faster it burns, the shorter the flame. Typically, gas and air are preheated before entering the highly heated combustion chamber, so that the combustible mixture is heated to its ignition temperature and burns, with the chemical reaction taking place in an instant. The combustion rate of gas in the vertical flue depends on the physical process of contact between combustible molecule and air molecules, namely the diffusion process. During diffusive combustion, the lack of oxygen in certain areas leads to the pyrolysis of hydrocarbons, resulting in the formation of free carbon. The presence of solid particles in the combustion zone generates intense light and heat radiation, giving rise to a bright flame. To lengthen the flame and improve the uniformity of vertical heating, gas and air should be allowed to mix slowly within the coke oven flue. The time required for the heated gas and air to burn in the vertical flame channel is the total time needed for all the components involved in the combustion to reach their ignition points, for the combustible components to combine with oxygen, and for the combustion reaction to take place. Under conditions of very high temperature in the vertical flame channel, gas and air are heated to their ignition point in the regenerative chamber before entering the vertical flame channel. The total time taken from the ignition point to the completion of the combustion process is actually the time required for the gas and air to mix. 2.2 Gas flow velocity The behavior of gas flow within the vertical flame channel and the combustion conditions are theoretically complex to analyze, but practical experience in production shows that the flame on the coke side is usually longer than that on the machine side; when operating with a shorter coking cycle, the flame becomes longer; exhaust gas recirculation can also prolong the flame, etc. This is because: (1) the cross-sectional area of the vertical fire channel on the coke side is smaller than that on the machine side, yet the gas flow rate is higher; as a result, the gas velocity inside the vertical fire channel on the coke side is greater, and the flame is therefore longer. (2) When the turnaround time is reduced, the amount of gas supplied per unit time increases, which raises the gas velocity and thus lengthens the flame, altering the heating conditions at the upper and lower parts of the coke cake. (3) Introducing an inert gas into the upward flame channel not only reduces the concentrations of flammable and oxidizing gases, thereby slowing down their combustion rate, but also causes the flame to elongate due to factors such as an increased gas flow velocity. 2.3 Heat transfer through the furnace wall: The heat generated by the combustion of the heating gas in the vertical flame channel is transferred to the coal material through the walls of the carbonization chamber. Therefore, the heat transfer in the furnace wall is closely related to the upward heating of the coke cake. The thickness of the furnace wall bricks, the shape of the vertical flue partitions, the level of heating, as well as segmented heating, all affect the uniformity of heating in the vertical direction of the coke cake. 2.4 Arrangement of gas, air, and exhaust outlets at the bottom of the vertical flue: In the vertical flue of a coke oven, the location and angle of the gas inlets and air inlets directly affect the mixing of gas and air as well as the combustion rate; the arrangement of these inlets is determined by the design. At present, for the dual-chamber waste gas recirculation bottom-firing coke ovens produced and designed in China, regardless of the height of their carbonization chambers, due to the structural requirements of the inclined channel area and the regenerator, the relative arrangement of the recirculation hole inclined channels and the nozzles at the bottom of the vertical flue is shown in Figure 1. Circulation hole, chute opening, lamp head. Figure 1: Layout of chute opening, lamp head, and circulation hole. With the chute opening, lamp head, and circulation hole arranged relative to each other as shown in Figure 1, when the height of the lamp head is less than half the height of the circulation hole, the effect of the exhaust gas circulation on the combustion of the flame in the burner depends on the type of gas being heated. When heated with coke oven gas, its high combustible content results in a fast burning rate. However, the exhaust gas that circulates through the circulation holes blows the coke oven gas emitted from the lamp head (the air emitted from the chute openings) toward the air (or gas) side, which helps to accelerate the mixing of air and gas, thereby shortening the combustion flame of the gas. When heated with blast furnace gas, the circulating exhaust gas is blown toward the middle of the tuyere opening (above the nose brick), separating the gas from the air, which helps to lengthen the flame currently burning in the vertical flue. Blast furnace gas has a low flammability content, a slow gas diffusion rate, and a long combustion flame. Due to the effect of the circulating exhaust gas, the combustion flame is stretched longer. As a result, when coke ovens are heated with different types of gas, there are significant differences in the heating pattern of the coke cake from top to bottom; moreover, as the height of the carbonization chamber increases and the amount of waste gas circulation rises, these differences become even greater. Due to the layout of the coke oven chutes and nozzles, when blast furnace gas with a higher content is used for heating in coke ovens, better uniformity in the vertical direction is achieved. 2.5 Effects of coal charging and leveling operations in coke ovens: If the openings for coal charging are blocked or not leveled properly, or if the leveling rods used in large coke ovens leave residue that carries coal to the side of the oven, this will result in insufficient maturation of the upper part of the coke cake. 3 Improvements in top heating during the heating of coke oven gas 3.1 Dilution of coke oven gas Dilution of coke oven gas refers to the addition of some blast furnace gas to coke oven gas in order to reduce the concentration of its combustible components. We know that coke oven gas has the combustion characteristics of a fast burning rate and a short flame, which are determined by its composition. Coke oven gas contains about 60% hydrogen alone, and this is the factor that determines its fast burning rate. Under conditions of diffusive combustion, the length of the flame is essentially determined by the burning rate of the gas. The combustion rate depends on the composition of the combustible gas and the diffusion rate of oxygen; the diffusion rate is inversely proportional to the square root of the molecular weight of the combustible gas. (According to the kinetic theory of gases, the diffusion coefficient of gaseous fuels is proportional to the root mean square speed of the molecules, namely D∝(8RT/πM)0.5, where M is the average molecular weight of the fuel and T is the absolute temperature of the gas.) Hydrogen has a low molecular weight, so its diffusion rate is high, which results in a fast combustion rate and a short flame. The combustible component in blast furnace gas is mainly CO, which has a slow combustion rate and produces a long flame. Guided by this combustion theory, we propose mixing a portion of blast furnace gas into coke oven gas, thereby reducing the concentration gradient and diffusion rate of the combustible gases, slowing down the combustion rate, and thus lengthening the flame. At the same time, due to the mixing in blast furnace gas, the pressure in the main pipes in the basement increases; this leads to an increased exit velocity of the gas at the nozzles, resulting in a stronger jet force. This can increase the amount of exhaust gas that is circulated, and it also helps to lengthen the flame. Tests have shown that heating with mixed gas can significantly improve the uniformity of vertical heating in the coke cake; meanwhile, as the mixing ratio increases, the temperature difference between the upper and lower parts of the coke cake decreases. But depletion will bring some side effects. For example: an increase in the main pressure in the basement, along with an increase in the pressure in the horizontal pipes, leads to an increased risk of gas leakage and a rise in CO levels in the basement. Furthermore, in the main pipes, hard lumps form due to the combination of blast furnace gas dust and tar, which accumulate in the main pipes and brick gas channels, affecting the temperature of the cross walls. The above problems should be addressed by taking the following measures: (1) Determine an appropriate mixing ratio ； (2) Ensure that the temperature of the coke oven gas after preheating is within 45±1°C ； (3) Regularly maintain the gas pipelines in the basement, especially at all connection points. At the same time, the CO level in the basement should be checked once a week ； (4) Regularly clean the main coke oven gas pipeline. Pangang conducted experiments on the dilution of coke oven gas, and the results are shown in Table 1. Table 1: Changes in the temperature at the center of the coke cake when using mixed gas
Heating conditions, coking time (hours), temperature at the center of the coke cake (°C), temperature difference between the upper and lower parts of the coke cake, temperature difference between the middle and lower parts of the coke cake – Machine side, Coke side, Upper, Middle, Lower, Upper, Middle, Lower. Machine side, Coke side, Machine side, Coke side. Pure coke oven gas: Heating time = 20 hours; temperatures: 960, 1044, 1121 for upper, middle, and lower parts respectively; temperature differences: 997, 1081, 1129. Differences: 161, 132, 77, 48. When 22% blast furnace gas is used in combination: Heating time = 20 hours; temperatures: 995, 1053, 1100 for upper, middle, and lower parts respectively; temperature differences: 1018, 1084, 1118. Differences: 105, 100, 47, 34. The differences are as follows: 35, 9, -21, 21, 3, -11, -56, -32, -30, -14. As can be seen from Table 1, the use of mixed gas leads to a significant improvement in the uniformity of heating across the upper and lower parts of the coke cake; the temperature difference between these two areas decreases by 44°C, while the difference between the middle and lower parts decreases by 22°C. The use of mixed gas also results in an increase in the amount of gas used for heating, which helps to reduce the temperature in the brick gas channels. This, in turn, reduces or even eliminates the problem of blockages in these channels caused by gas pyrolysis. 3.2 Reducing the diameter of the gas nozzle: The diameter of the gas nozzle in the horizontal brick gas channel should be selected to ensure uniform upward heating and proper distribution of the gas flow across all burners in the combustion chamber. The average diameter of the nozzle to the diameter of the brick gas channel is generally in the range of 0.55–0.65. As the average diameter of the nozzle decreases, the exit velocity of the gas flow increases, the thrust of the gas jet increases, and the amount of exhaust gas circulation increases, thereby improving the uniformity of heating across the top and bottom of the coke cake. Similar experiments have been conducted both domestically and internationally, with fairly noticeable results. However, the diameter of the nozzle should not be too small, otherwise the high resistance at the gas outlet will cause excessive pressure in the brick gas duct, leading to gas leakage and affecting normal heating. At the same time, the small pore size makes it more sensitive to the effect of deposited graphite, affecting the stability of the furnace temperature. 3.3 Raising the gas outlet position: To study the effect of the height of the lamp head on vertical heating in coke ovens, a lamp head 260 mm high was added to the original lamp head of the Ben-54 type coke oven (i.e., above the exhaust gas circulation holes), and comparative tests were conducted with the lamp head raised either near or far from the circulation holes; the test results are shown in Table 2. Table 2 Effect of raising the light heads on the upward heating in Ben-54 type coke ovens. Group, Light head status, Temperature difference above and below the vertical flame channel: 0.6–3.2 m; Temperature difference above and below the coke cake: 0.6–3.2 m; Air excess coefficient. I: Before testing, 110°C, 80°C, 1.08; After raising those near the circulation holes, 94°C, 18°C, 1.26; After raising all of them, 72°C, -8°C, 1.25. II: Before raising those far from the circulation holes, 87°C, 62°C, 1.18; After raising those far from the circulation holes, 75°C, 22°C, 1.15. It can be seen from Table 2 that: (1) After raising the light heads, there are significant changes in both the temperature of the flame channel and the temperature difference at the center of the coke cake, which is beneficial for upward heating. There are the following reasons: A. By raising the lamp head, the burning point moves upward, which prevents the circulating exhaust gases from affecting the gas and air, thereby reducing the temperature in the upper area. B. Before raising the lamp head, the circulating exhaust gas blows the gas toward the air side, or blows the air toward the gas side, accelerating the mixing of air and gas and thus shortening the flame. By raising the lamp head above the circulation hole, the aforementioned phenomenon is avoided; this reduces the oxygen concentration around the gas or blows air in a direction away from the gas. (2) Raising the lamp holder near the circulation hole yields a better effect than raising it away from the circulation hole. The former changes the temperature difference between the top and bottom of the coke cake by 62°C, while the latter changes it by 40°C℃ ； This is mainly due to the fact that when the lamp holder near the circulation hole is raised, the exhaust gas flowing back blows the air in a direction away from the gas; whereas farther from the circulation hole, the air is blown in the direction of the gas. 4 Methods to Improve Upward Heating When Heating Blast Furnace Gas 4.1 Enriching Blast Furnace Gas Enrichment refers to the addition of some coke oven gas to blast furnace gas in order to increase the concentration of its combustible components. The incorporation of a portion of coke oven gas into blast furnace gas to heat coke ovens is widely adopted in designs, and many plants have been operating using this method for a long time. The inclusion of blast furnace gas in coke oven gas increases the concentration of its combustible components, raises the diffusion coefficient, and reduces the gas flow velocity; as a result, the mixing of gas and air is accelerated and the combustion flame is shortened. At the same time, it can reduce the pressure difference between the rising gas stream and the top of the regenerator chamber, as well as decrease the air leakage rate from the single and main walls of the regenerator chamber. When the lower calorific value of the mixed gas ranges from 4000 to 4300 kJ/m3, the rising gas stream and the air regenerator can operate at constant pressure; as a result, the volume of the rising gas and the volume of air are equal, and the exhaust gas temperatures in the small ducts for the gas and air are also similar, which helps to improve the thermal efficiency of the coke oven. Enriching blast furnace gas is mainly done to reduce the resistance in the heating system and increase the draft in the chimney, as well as to stabilize the calorific value of the heating gas. 4.2 Changing the distance between the gas and air outlets and their outlet angle: The distance between the gas and air outlets is the width of the nose brick. The nosepiece is wide, making it more difficult for molecules of combustible gases and oxidizing gases to collide; as a result, the combustion proceeds more slowly and the flame becomes longer ； Conversely, it is short. Such experiments were conducted in China in the mid-1960s on a single-chamber test furnace with a carbonization chamber height of 6 meters. The width of the nose brick is 240 mm; it is heated with blast furnace gas, and the temperature at the center of the coke cake is about 500°C higher in the upper part than in the lower part ； Change the width of the nose bridge brick to 40mm, with all other conditions remaining basically unchanged. The temperature at the center of the coke cake is about 300°C higher in the upper part than in the lower part. This clearly shows that changes in the width of the nose brick have a significant impact on the uniformity of the temperature at the center of the focal cake. The angle between the gas and air outlets is related to the width of the bridge tile, its shape, and the placement of the adjustable tiles at the chute openings. The larger the angle, the lower the point of convergence between the molecules of the combustible gas and the oxidizing gas, resulting in faster combustion and a shorter flame ； When the angle between them is greater than 90 degrees, that is, when the gas and air outlets move in opposite directions, it is equivalent to increasing the distance between the outlets; this makes combustion more difficult and results in a longer flame. For Ansteel’s No. 15 and No. 16 coke ovens without waste gas recycling, when heated with blast furnace gas, experiments were conducted using adjustment bricks to increase the uniformity of heating in the upper part of the coke oven by enlarging the distance between the gas and air outlets and reducing the angle between them. The first approach involved placing the adjustment bricks directly against the nose brick ； The second option is to rotate the adjusting brick by 180 degrees and then rest it on the bridge brick. The test results are listed in Table 3. Table 3 Effect of Changing the Position of the Regulating Brick on Vertical Heating
Test Scheme, Measurement Time, Angle between Air and Gas Outlets, Distance between Air and Gas Outlets (mm), Standard Flue Temperature (°C), Temperature at the Center of the Coke Cake (°C), Height from the Carbonization Chamber (m), Temperature Difference Between Upper and Lower Parts, Machine Side, Coke Side: 0.6 (lower), 3.2 (upper).
I. Initial Conditions: 420, 40, 1288, 1326, 1138, 876, 262; After Testing: 00, 180, 1274, 1308, 1048, 962, 86.
II. Initial Conditions: 420, 40, 1294, 1336, 1099, 754, 345; After Testing: -290, 180, 1266, 1301, 1019, 1020, -1.
As can be seen from Table 3, whether it is the first or second scheme, changes in the distance and angle between the gas outlets result in a slower mixing process of gas and air, thereby lengthening the flame. The temperature difference between the top and bottom of the coke cake in the first solution was reduced by 176°C. In the second scheme, due to a greater change in the angle of the outlet airflow, the temperature difference between the upper and lower parts of the coke cake changed by 346°C. After implementing this solution across the entire furnace, the standard flue gas temperature was reduced by 45°C–50°C. It not only improves the quality of coke but also saves a large amount of energy. It should be noted that Ansteel’s coke ovens No. 15 and No. 16 are coke ovens without exhaust gas recycling; their combustion points are relatively concentrated, and they are sensitive to changes in the airflow exit conditions. On coke ovens with dual-chamber waste gas recirculation, changing the airflow outlet condition has a relatively smaller impact on the temperature difference between the upper and lower parts of the coke cake; based on the experience gained from 6m single-chamber test furnaces, a change of 1 mm in the width of the nose brick results in a temperature difference of around 1°C between the upper and lower parts of the coke cake. 4.3 Changing the height of the nose brick: By altering the height of the nose brick, it is possible to change the angle of the air flow outlet; simultaneously, this also allows for changing the location where ignition occurs. Increasing the height of the nose bridge brick can reduce the vertical temperature difference. To address the issue of low temperatures in the upper part of the coke cakes, Ansteel’s coke ovens No. 15 and No. 16 underwent experiments involving the raising of the nose brick height. It involves placing the nose brick provided in the vertical flue on top of the original nose brick, thereby increasing the height of the nose brick by 110 mm; as a result, the temperature difference between the upper and lower parts of the coke cake decreased from 240°C to 140°C. Opposite experiments were also conducted on the down-fired Type 58 coke oven with exhaust gas recirculation. This is due to overheating of the upper part of the coke cake at the burner area. The upper part is 58℃ higher than the lower part. After removing the nose brick of channel 27, the temperature at the bottom of the coke cake was made 3°C higher than that at the top, resulting in a temperature difference of 61°C between the upper and lower sections. 5 General methods: Whether using coke oven gas or blast furnace gas for heating, the general methods for improving upward heating are as follows: 5.1 Reduce the air excess coefficient. Combustion in which the combustible material reacts fully with oxygen, resulting in no combustible components in the combustion products, is known as complete combustion. The main reasons for incomplete combustion include insufficient air supply, poor mixing of fuel and air, or the pyrolysis of H2O and CO2 in the combustion products at high temperatures to produce CO and H2. The mixing of air and gas is ensured by the structure of the combustion chamber, and the excess oxygen in the combustion products can suppress the pyrolysis of H2O and CO2. To ensure complete combustion of the fuel, the amount of air supplied must be greater than the theoretical amount of air; the ratio between the two is called the air excess factor, denoted by a. a = actual air volume (L_actual) / theoretical air volume (L_theoretical). The selection of a is very important for the heating of coke ovens; its value reflects the level of oxygen concentration in the rising flue. The higher the oxygen concentration, the faster the combustion and the shorter the flame ； Conversely, the flame is long. The larger A is, the more exhaust gas there is, and thus the more heat is carried away by the exhaust gas ； When a is too low, incomplete combustion occurs, and the combustible components are discharged along with the exhaust gases; therefore, both insufficient a and excessive a increase gas consumption. When heated with coke oven gas, depending on the structure of the coke oven, a=1.20—1.25 ； When heated with blast furnace gas, due to the high content of inert components, a can be lower, at a value of 1.15–1.20. In production, a fluctuates with changes in gas temperature, calorific value, atmospheric temperature, etc., and requires regular monitoring and timely adjustment. The value of a can be calculated through exhaust gas analysis using the following formula: a=1+K(O2-0.5CO)/(CO2+CO), where K = VCO2/O2. In this formula, O2, CO, and CO2 represent the concentrations of oxygen, carbon monoxide, and carbon dioxide in the exhaust gas, measured as volume percentages ； V CO2 is the volume of CO2 produced theoretically when 1 m3 of gas is completely burned, in m3; O2 is the amount of oxygen theoretically required to burn 1 m3 of gas, in m3. The K value varies depending on the composition of the gas; generally, for coke oven gas, K = 0.43, and for blast furnace gas, K = 2.5. Regardless of the type of coke oven, reducing the air excess coefficient can lower the temperature difference in the upper part. This is because the amount of air decreases, slowing down the combustion rate of the gas in the vertical flue. However, this method has certain limitations. In coke ovens without exhaust gas recirculation, changes in the air excess factor have a more significant effect on vertical heating. Pangang conducted comparative tests by taking samples through the vertical flue during the heating with coke oven gas, and the results are shown in Table 4. Table 4 Effect of varying the air excess coefficient on upward heating. Setting time, air excess coefficient a, temperature at the center of the coke cake (°C), temperature difference between the machine side and the coke side, upper, middle, lower temperatures – machine side, coke side: 18 hours: 1.30–1.45, 960, 1040, 1060, 960, 1010, 1040, 100, 80; 18 hours: 1.20–1.30, 920, 960, 1010, 950, 990, 1020, 90, 70; 18 hours: 1.10–1.20, 940, 970, 990, 960, 990, 1000, 50, 40. As can be seen from Table 4, as a decreases, the temperature difference at the center of the coke cake gradually reduces, and the uniformity of cooking throughout the coke cake improves. Furthermore, it is ideal to maintain an air excess coefficient of around 1.25 for the vertical flue when using coke oven gas for heating. Therefore, reducing and maintaining an appropriate air excess factor is a simple and effective way to reduce the high temperature gradient in the coke cake. 5.2 Exhaust gas recirculation: Gas and air burn in the rising flue, generating exhaust gases which flow into the descending flue through the crossover holes. At this point, some of these exhaust gases are drawn back into the rising flue via the recirculation holes at the bottom of the dual flue system. This combustion method is known as exhaust gas recirculation. The momentum principle states that: \"In a steady flow, the sum of the external forces acting on a certain region of the fluid in a particular coordinate direction is equal to the change in momentum of the fluid passing through the ends of that region per unit time in that direction.\" ”Based on this principle and the equations for ascending and descending air currents, the basic equation for exhaust gas circulation in a dual-flue system can be derived: (V20coal*ρ0coal*Tcoal) / (273*Ffire+Fcoal) + (V20air*ρ0air*Tair) / (273*Ffire+Fair) – V20waste*(1+x)^2 *ρ0waste*Twaste/F2fire*273 + H*g*(ρWaste_lower–ρWaste_upper) = (PH–PB) + ∑(1–H)ΔP. Here, V0coal, V0air, and V0waste represent the flow rates of coal gas, air, and waste gas, respectively, in m3/s ； ρ0 is the gas density ； F-fire, F-coal slope (burner), and F-air slope represent the cross-sectional areas of the flame channel, blast furnace gas slope (burner), and air slope, respectively, in m2 ； Tcoal, Tair, and Tupwaste represent the absolute temperatures of the gas, air, and waste gas from the upward airflow flue at the exit of the inclined duct, in K ； H is the height of the flue ； ρ_lower and ρ_upper represent the exhaust gas densities in the descending and ascending airflow flues, respectively, in kg/m3 ； x = V_ring/V_waste is the percentage of the exhaust gas circulation volume to the total amount of exhaust gas generated during combustion, expressed in %. The terms 1–4 on the left-hand side of Equation (1) represent the gas injection force (△h_gas), air injection force (△h_air), residual injection force of exhaust gases in the flue (△h_exhaust), and the difference in buoyancy between the rising and descending flues (△h_buoyancy), respectively. On the right-hand side, (PH−PB) denotes the resistance at the circulation holes, while ∑1−H△P represents the resistance across the holes and flues; these are combined to give the total resistance ∑total△P. Thus, Equation (1) can be rewritten as: △h_gas + △h_air − △h_exhaust + △h_buoyancy = ∑total△P (2). In deriving the above equation, the resistance associated with the merging of recirculated exhaust gases and those in the flues was not taken into account, nor was the utilization rate of the injection forces considered. Consequently, the calculated volume of recirculated exhaust gases is greater than the actual value. Tests have shown that when the jet force utilization coefficient K is 0.75, the results obtained are consistent with reality; thus, equation (2) can be modified to: 0.75(△hcoal + △hair – △hwaste) + △hfloat = ∑total△P. The principle of exhaust gas recirculation can be briefly explained by the following three points: (1) Air and coal gas are ejected from the inclined ports and nozzles, and their velocity head generates a jet force that creates a suction effect at the bottom of the upward-flowing flue, thereby drawing in the downward-flowing exhaust gas. Since the cross-sectional area of the nozzle remains constant, the greater the gas flow rate and the higher the gas preheating temperature, the greater the jet force. (2) The temperature of the upward-moving air is higher than that of the downward-moving air; this creates a difference in buoyancy, which causes the upward-moving air to draw in the downward-moving air. The greater the temperature difference between the two burners in a dual-unit setup, the greater the buoyancy difference, and thus the suction force increases. (3) The difference in buoyancy and the jet force are the driving forces that generate exhaust gas recirculation. Due to this driving force, a portion of the exhaust gas in the downward flow is drawn into the upward-flowing flue, thereby increasing the resistance of the gas as it passes through the vertical flue, the crossover holes, and the circulation holes, thus achieving a balance between the driving force and the resistance. That is: K* (gas injection force + air injection force + buoyancy difference) = frictional resistance in the vertical flue + resistance at the crossing holes + resistance at the circulation holes. Under the current design dimensions of these holes, the resistance at the crossing holes is the dominant factor, accounting for 70%–80% of the total resistance, while the resistance at the circulation holes accounts for only about 10%. The so-called \"gas circulation rate\" that we commonly refer to means the ratio of the amount of gas circulated to the rising flue to the amount of exhaust gas discharged through the inclined duct in the descending flue. Strictly speaking, it should be called the exhaust gas recirculation ratio. The size of the exhaust gas recirculation ratio is related to factors such as the type of heating gas, the height of the carbonization chamber, the coking time, and the height of the burner head. Conducting measurements on production coke ovens is a tedious and difficult task. Chinese coking industry researchers conducted measurements on single-chamber test furnaces with a height of 6 m and 8 m. Some of the data are listed in Table 5. Table 5 Relationship between the exhaust gas recycling ratio and coal gas type, burner height, coking time, and carbonization chamber height. 6 meters, 8 meters; heating with coke oven gas, heating with coke oven gas, heating with blast furnace gas; burner height of 190 mm, burner height of 740 mm, burner height of 120 mm/120 mm, burner height of 120 mm/1200 mm. 20 hours: 112, 80.9, 123.5, 91.6, 89.6; 16.5 hours: 105.8. As can be seen from the data in Table 5, the recycling ratio is higher when using coke oven gas for heating compared to when using blast furnace gas ； The higher the lamp holder, the smaller the circulation ratio ； The longer the coking time, the greater the circulation ratio ； The higher the carbonization chamber, the greater the circulation ratio. This fully demonstrates that exhaust gas recirculation has a very good automatic regulating effect on the upward heating of coke ovens. When heated with blast furnace gas, the increase in the flow rates of the gas and exhaust gases leads to an increase in injection force and resistance, while the effect of the buoyancy difference decreases relatively, resulting in a reduced amount of exhaust gas circulation. In this way, when the furnace control device remains unchanged and heating is carried out using coke oven gas, the amount of exhaust gas circulation is high, which helps to improve the uniformity of heating in the upper regions ； When heated with blast furnace gas, the amount of exhaust gas circulation automatically decreases to accommodate the longer flame length of blast furnace gas. Furthermore, when the flow rate remains constant and the uniformity of upward heating deteriorates, the temperatures in the rising and descending flues increase, resulting in a greater buoyancy difference that automatically increases the amount of exhaust gas circulation, thereby improving the uniformity of upward heating. The amount of exhaust gas recycled is generally stable. However, when certain unstable factors arise, it may cause the gas and air in the upward flue gas flow to be drawn directly into the downward gas flow chute through the circulation holes for combustion, rather than burning in the vertical flue – a phenomenon known as short-circuiting (that is, the flame entering the downward flue directly through the circulation holes). This will damage the furnace body, and it should be prevented. The main reason for the short circuit is that the sum of the jet force and the buoyancy force is less than the resistance of the circulation hole. Situations that may cause a short circuit during production are as follows: (1) A short circuit is likely to occur when negative pressure appears at the viewing holes on the top of the furnace. This is because when the fire viewing cover is opened, external air is drawn into the vertical flue, increasing the resistance in the vertical flue (including the crossing opening) and the downward chute, which in turn reduces the exit jet force and increases the likelihood of short-circuiting. Furthermore, due to the high negative pressure in the combustion chamber, when the integrity of the furnace is poor, a large amount of external gas or raw gas leaking into the vertical flue also increases the likelihood of short circuits. (2) Wild gas leaks from the carbonization chamber. (3) During heavy rain, some water vapor penetrates into the vertical flue. (4) Immediately after the exchange, the temperature of the downflow flue is higher than that of the upflow flue, and the buoyancy difference between the upflow and downflow gases is negative. When the commutation time interval is long, the gas flow rate is low, and the temperature difference between the rising and descending flues is large, the negative value of the buoyancy difference at the beginning of commutation increases, making short circuits more likely; however, this effect disappears automatically after a certain period of time following commutation. (5) Due to heat dissipation from the furnace body, the temperature in the burner channel remains lower than that of the adjacent channels during upward airflow; as a result, the buoyancy difference is negative. Additionally, the larger cross-sectional area at the outlet of the burner channel reduces the exit velocity of the airflow, thereby decreasing the injection force. Moreover, cracks in the burner channel can lead to leakage of raw gas, which lowers the temperature and increases resistance, making short circuits more likely to occur. To prevent this phenomenon, waste gas circulation holes are no longer provided between the burners at the front end of JN-type coke ovens. (6) Debris in the flame channel increases the system resistance; when this reaches a certain level, a short circuit may occur. (7) Short circuits are likely to occur when the coke oven turnaround time is prolonged or during the heat retention period. This is because the amount of heated gas decreases, resulting in a reduced jet force; moreover, as the temperatures in the rising and descending flues tend to become equal, the difference in buoyancy also decreases significantly, thus making short circuits more likely to occur. (8) At the beginning of coal loading, if a large amount of raw gas leaks into the flue through cracks in the furnace wall or areas where the drying holes are not properly sealed, it increases the resistance in the flue. In such cases, the observation holes show positive pressure, and there is a possibility of short-circuiting in the flue. To eliminate this short circuit, the observation ports in the short-circuited flues on both sides of the coal charging chamber can be opened to allow some gas to escape, thereby reducing resistance, increasing buoyancy, and eliminating the short circuit. Methods to avoid short circuits: (1) Increase the air supply to boost the exit jet force ； (2) Reduce commutation time ； (3) Increasing the pressure at the viewing hole reduces the negative pressure in the combustion system, thereby decreasing the amount of gas that leaks into the vertical flue. 5.3 Length of commutation time: The reason why the length of the exchange interval affects the uniformity of heating in the cake lies in the fact that at the moment of commutation, the regenerator that was previously under downward airflow becomes one under upward airflow; at this point its temperature is the highest. The temperature of the preheated gas or air is also at its highest, so the combustion temperature is the greatest as well ； The vertical flue, on the other hand, has just shifted from a downward to an upward flow; at this point its temperature is the lowest. In other words, the temperature difference between the burning flame and gases and the lower part of the vertical flue is greatest, resulting in a large amount of heat being transferred to the lower section. As time passes after the exchange, the temperature of the flame combustion gradually decreases ； At the lower part of the three-flue structure, as the heating temperature gradually increases, the temperature difference between the two areas decreases, resulting in less heat being transferred to the lower part, while more heat is transferred to the upper part. Therefore, an increased exchange interval is beneficial for heating the upper part of the cake. Although the temperatures of the gas and air during preheating gradually decrease over the commutation cycle, the actual gas flow temperature in the vertical flue still rises gradually. The actual temperature rises because the temperature of the rear furnace wall increases gradually, resulting in a gradual decrease in the heat transferred to the furnace wall by radiation from the gases. Therefore, with a slight increase in the overall maturity of the coke cake, the standard temperature can be appropriately reduced. However, after the commutation period is extended, the temperature in the furnace top space during the later stage of commutation rises, the temperature of the raw gas increases, the efficiency of the regenerator decreases, and heat consumption goes up; therefore, all these factors need to be taken into consideration. 5.4 Appropriate heating level height – Choosing an appropriate heating level height can also improve the conditions of vertical heating, as the height of the heating level has a significant impact on the degree of maturity in the upper part of the cake. At a heating level of 600 mm, the maturation of the upper part of the coke cake is good, but the temperature in the space above the furnace is high. At a heating level of 900 mm, heating of the upper part of the coke cake is somewhat insufficient, but the temperature in the space above the furnace decreases. Therefore, current coke ovens all use a heating level height of 700–800 mm. 5.5 Reducing cycle time: The length of the cycle time has an impact on the vertical heating in coke ovens. Reducing the turnaround time increases the amount of gas supplied per unit time, which raises the gas flow velocity and thus lengthens the flame, improving the uniformity of the coke cake from top to bottom. 5.6 Strengthen the operational management of coal loading and leveling. Improper methods of coal loading and leveling can also affect vertical heating. Overfilling with coal causes the coal leveling rod to compress the coal layer above excessively, resulting in a high bulk density in that upper part of the coal pile. This also reduces the space at the top of the furnace, affecting the flow rate of the gas stream. As a result, the gas pressure inside the carbonization chamber increases, and insufficient heating of the coke at the top leads to the formation of unripe coke. If the coal loading is insufficient, the space above the furnace increases and the temperature in that space rises. This not only reduces the production capacity of the coke oven and the quality of the chemical products, but it also leads to an increase in graphite within the furnace; in severe cases, it can cause difficulties in pushing out the coke. 6 Conclusions At present, the main methods for achieving uniform heating in coke ovens in the vertical direction are four: different furnace wall thicknesses, segmented or even multi-segmented heating, high and low burner heads, and exhaust gas recycling. Different thicknesses of the furnace wall are employed to improve the heating at the upper part of the coke cake, by increasing the thickness of the furnace wall at the lower part of the carbonization chamber or by gradually varying and altering the shape of the partition bricks in the vertical flue ； Segmented heating involves introducing blast furnace gas and air into the vertical flue through holes in the partition walls, at different heights, thereby enabling segmented combustion. This approach allows the flame to be made very long, and the vertical heating can be adjusted by controlling the cross-sectional area at the exits of these holes. Multi-stage heating involves supplying air and blast furnace gas in several stages along the vertical direction within the vertical flue, thereby enabling multi-stage combustion to lengthen the flame. These methods are effective in improving the uniformity of vertical heating in coke cakes, but they increase the variety of brick types used in constructing coke ovens and complicate their structure, which is why they have not been widely adopted. The upper and lower burners use gas burners at different heights within adjacent flues, in order to change the height of the combustion point within the flue and thus ensure uniform heating from above. However, this method is only applicable to heating with coke oven gas; moreover, since the gas is released from the upper burner at a certain distance above the bottom of the flue, the air coming from the inclined channels can easily burn away the carbon deposits in the brick gaps beneath the upper burner, leading to leaks. Waste gas recirculation is currently a simple and effective method for achieving uniform high-temperature heating in the combustion chamber. It uses some of the exhaust gas from the descending flue to enter the ascending flue through the circulation holes, diluting the concentrations of air and gas and thereby reducing their combustion rate. At the same time, as the speed of the mixed airflow increases, the flame becomes further elongated. It helps to achieve uniform heating of the coke cake from top to bottom, improves the quality of coke, reduces the coking time, increases production, and lowers the heat consumption in coke production. It is also possible to increase the height and volume of the carbonization chamber, improve the labor productivity of coke ovens, and reduce the capital investment per unit of product; therefore, it is widely used in modern coke ovens. Since each of the above methods has its own advantages and disadvantages, modern large-capacity coke ovens often employ several methods simultaneously to achieve high uniformity in heating.
Heating conditions, coking time (hours), temperature at the center of the coke cake (°C), temperature difference between the upper and lower parts of the coke cake, temperature difference between the middle and lower parts of the coke cake – Machine side, Coke side, Upper, Middle, Lower, Upper, Middle, Lower. Machine side, Coke side, Machine side, Coke side. Pure coke oven gas: Heating time = 20 hours; temperatures: 960, 1044, 1121 for upper, middle, and lower parts respectively; temperature differences: 997, 1081, 1129. Differences: 161, 132, 77, 48. When 22% blast furnace gas is used in combination: Heating time = 20 hours; temperatures: 995, 1053, 1100 for upper, middle, and lower parts respectively; temperature differences: 1018, 1084, 1118. Differences: 105, 100, 47, 34. The differences are as follows: 35, 9, -21, 21, 3, -11, -56, -32, -30, -14. As can be seen from Table 1, the use of mixed gas leads to a significant improvement in the uniformity of heating across the upper and lower parts of the coke cake; the temperature difference between these two areas decreases by 44°C, while the difference between the middle and lower parts decreases by 22°C. The use of mixed gas also results in an increase in the amount of gas used for heating, which helps to reduce the temperature in the brick gas channels. This, in turn, reduces or even eliminates the problem of blockages in these channels caused by gas pyrolysis. 3.2 Reducing the diameter of the gas nozzle: The diameter of the gas nozzle in the horizontal brick gas channel should be selected to ensure uniform upward heating and proper distribution of the gas flow across all burners in the combustion chamber. The average diameter of the nozzle to the diameter of the brick gas channel is generally in the range of 0.55–0.65. As the average diameter of the nozzle decreases, the exit velocity of the gas flow increases, the thrust of the gas jet increases, and the amount of exhaust gas circulation increases, thereby improving the uniformity of heating across the top and bottom of the coke cake. Similar experiments have been conducted both domestically and internationally, with fairly noticeable results. However, the diameter of the nozzle should not be too small, otherwise the high resistance at the gas outlet will cause excessive pressure in the brick gas duct, leading to gas leakage and affecting normal heating. At the same time, the small pore size makes it more sensitive to the effect of deposited graphite, affecting the stability of the furnace temperature. 3.3 Raising the gas outlet position: To study the effect of the height of the lamp head on vertical heating in coke ovens, a lamp head 260 mm high was added to the original lamp head of the Ben-54 type coke oven (i.e., above the exhaust gas circulation holes), and comparative tests were conducted with the lamp head raised either near or far from the circulation holes; the test results are shown in Table 2. Table 2 Effect of raising the light heads on the upward heating in Ben-54 type coke ovens. Group, Light head status, Temperature difference above and below the vertical flame channel: 0.6–3.2 m; Temperature difference above and below the coke cake: 0.6–3.2 m; Air excess coefficient. I: Before testing, 110°C, 80°C, 1.08; After raising those near the circulation holes, 94°C, 18°C, 1.26; After raising all of them, 72°C, -8°C, 1.25. II: Before raising those far from the circulation holes, 87°C, 62°C, 1.18; After raising those far from the circulation holes, 75°C, 22°C, 1.15. It can be seen from Table 2 that: (1) After raising the light heads, there are significant changes in both the temperature of the flame channel and the temperature difference at the center of the coke cake, which is beneficial for upward heating. There are the following reasons: A. By raising the lamp head, the burning point moves upward, which prevents the circulating exhaust gases from affecting the gas and air, thereby reducing the temperature in the upper area. B. Before raising the lamp head, the circulating exhaust gas blows the gas toward the air side, or blows the air toward the gas side, accelerating the mixing of air and gas and thus shortening the flame. By raising the lamp head above the circulation hole, the aforementioned phenomenon is avoided; this reduces the oxygen concentration around the gas or blows air in a direction away from the gas. (2) Raising the lamp holder near the circulation hole yields a better effect than raising it away from the circulation hole. The former changes the temperature difference between the top and bottom of the coke cake by 62°C, while the latter changes it by 40°C℃ ； This is mainly due to the fact that when the lamp holder near the circulation hole is raised, the exhaust gas flowing back blows the air in a direction away from the gas; whereas farther from the circulation hole, the air is blown in the direction of the gas. 4 Methods to Improve Upward Heating When Heating Blast Furnace Gas 4.1 Enriching Blast Furnace Gas Enrichment refers to the addition of some coke oven gas to blast furnace gas in order to increase the concentration of its combustible components. The incorporation of a portion of coke oven gas into blast furnace gas to heat coke ovens is widely adopted in designs, and many plants have been operating using this method for a long time. The inclusion of blast furnace gas in coke oven gas increases the concentration of its combustible components, raises the diffusion coefficient, and reduces the gas flow velocity; as a result, the mixing of gas and air is accelerated and the combustion flame is shortened. At the same time, it can reduce the pressure difference between the rising gas stream and the top of the regenerator chamber, as well as decrease the air leakage rate from the single and main walls of the regenerator chamber. When the lower calorific value of the mixed gas ranges from 4000 to 4300 kJ/m3, the rising gas stream and the air regenerator can operate at constant pressure; as a result, the volume of the rising gas and the volume of air are equal, and the exhaust gas temperatures in the small ducts for the gas and air are also similar, which helps to improve the thermal efficiency of the coke oven. Enriching blast furnace gas is mainly done to reduce the resistance in the heating system and increase the draft in the chimney, as well as to stabilize the calorific value of the heating gas. 4.2 Changing the distance between the gas and air outlets and their outlet angle: The distance between the gas and air outlets is the width of the nose brick. The nosepiece is wide, making it more difficult for molecules of combustible gases and oxidizing gases to collide; as a result, the combustion proceeds more slowly and the flame becomes longer ； Conversely, it is short. Such experiments were conducted in China in the mid-1960s on a single-chamber test furnace with a carbonization chamber height of 6 meters. The width of the nose brick is 240 mm; it is heated with blast furnace gas, and the temperature at the center of the coke cake is about 500°C higher in the upper part than in the lower part ； Change the width of the nose bridge brick to 40mm, with all other conditions remaining basically unchanged. The temperature at the center of the coke cake is about 300°C higher in the upper part than in the lower part. This clearly shows that changes in the width of the nose brick have a significant impact on the uniformity of the temperature at the center of the focal cake. The angle between the gas and air outlets is related to the width of the bridge tile, its shape, and the placement of the adjustable tiles at the chute openings. The larger the angle, the lower the point of convergence between the molecules of the combustible gas and the oxidizing gas, resulting in faster combustion and a shorter flame ； When the angle between them is greater than 90 degrees, that is, when the gas and air outlets move in opposite directions, it is equivalent to increasing the distance between the outlets; this makes combustion more difficult and results in a longer flame. For Ansteel’s No. 15 and No. 16 coke ovens without waste gas recycling, when heated with blast furnace gas, experiments were conducted using adjustment bricks to increase the uniformity of heating in the upper part of the coke oven by enlarging the distance between the gas and air outlets and reducing the angle between them. The first approach involved placing the adjustment bricks directly against the nose brick ； The second option is to rotate the adjusting brick by 180 degrees and then rest it on the bridge brick. The test results are listed in Table 3. Table 3 Effect of Changing the Position of the Regulating Brick on Vertical Heating
Test Scheme, Measurement Time, Angle between Air and Gas Outlets, Distance between Air and Gas Outlets (mm), Standard Flue Temperature (°C), Temperature at the Center of the Coke Cake (°C), Height from the Carbonization Chamber (m), Temperature Difference Between Upper and Lower Parts, Machine Side, Coke Side: 0.6 (lower), 3.2 (upper).
I. Initial Conditions: 420, 40, 1288, 1326, 1138, 876, 262; After Testing: 00, 180, 1274, 1308, 1048, 962, 86.
II. Initial Conditions: 420, 40, 1294, 1336, 1099, 754, 345; After Testing: -290, 180, 1266, 1301, 1019, 1020, -1.
As can be seen from Table 3, whether it is the first or second scheme, changes in the distance and angle between the gas outlets result in a slower mixing process of gas and air, thereby lengthening the flame. The temperature difference between the top and bottom of the coke cake in the first solution was reduced by 176°C. In the second scheme, due to a greater change in the angle of the outlet airflow, the temperature difference between the upper and lower parts of the coke cake changed by 346°C. After implementing this solution across the entire furnace, the standard flue gas temperature was reduced by 45°C–50°C. It not only improves the quality of coke but also saves a large amount of energy. It should be noted that Ansteel’s coke ovens No. 15 and No. 16 are coke ovens without exhaust gas recycling; their combustion points are relatively concentrated, and they are sensitive to changes in the airflow exit conditions. On coke ovens with dual-chamber waste gas recirculation, changing the airflow outlet condition has a relatively smaller impact on the temperature difference between the upper and lower parts of the coke cake; based on the experience gained from 6m single-chamber test furnaces, a change of 1 mm in the width of the nose brick results in a temperature difference of around 1°C between the upper and lower parts of the coke cake. 4.3 Changing the height of the nose brick: By altering the height of the nose brick, it is possible to change the angle of the air flow outlet; simultaneously, this also allows for changing the location where ignition occurs. Increasing the height of the nose bridge brick can reduce the vertical temperature difference. To address the issue of low temperatures in the upper part of the coke cakes, Ansteel’s coke ovens No. 15 and No. 16 underwent experiments involving the raising of the nose brick height. It involves placing the nose brick provided in the vertical flue on top of the original nose brick, thereby increasing the height of the nose brick by 110 mm; as a result, the temperature difference between the upper and lower parts of the coke cake decreased from 240°C to 140°C. Opposite experiments were also conducted on the down-fired Type 58 coke oven with exhaust gas recirculation. This is due to overheating of the upper part of the coke cake at the burner area. The upper part is 58℃ higher than the lower part. After removing the nose brick of channel 27, the temperature at the bottom of the coke cake was made 3°C higher than that at the top, resulting in a temperature difference of 61°C between the upper and lower sections. 5 General methods: Whether using coke oven gas or blast furnace gas for heating, the general methods for improving upward heating are as follows: 5.1 Reduce the air excess coefficient. Combustion in which the combustible material reacts fully with oxygen, resulting in no combustible components in the combustion products, is known as complete combustion. The main reasons for incomplete combustion include insufficient air supply, poor mixing of fuel and air, or the pyrolysis of H2O and CO2 in the combustion products at high temperatures to produce CO and H2. The mixing of air and gas is ensured by the structure of the combustion chamber, and the excess oxygen in the combustion products can suppress the pyrolysis of H2O and CO2. To ensure complete combustion of the fuel, the amount of air supplied must be greater than the theoretical amount of air; the ratio between the two is called the air excess factor, denoted by a. a = actual air volume (L_actual) / theoretical air volume (L_theoretical). The selection of a is very important for the heating of coke ovens; its value reflects the level of oxygen concentration in the rising flue. The higher the oxygen concentration, the faster the combustion and the shorter the flame ； Conversely, the flame is long. The larger A is, the more exhaust gas there is, and thus the more heat is carried away by the exhaust gas ； When a is too low, incomplete combustion occurs, and the combustible components are discharged along with the exhaust gases; therefore, both insufficient a and excessive a increase gas consumption. When heated with coke oven gas, depending on the structure of the coke oven, a=1.20—1.25 ； When heated with blast furnace gas, due to the high content of inert components, a can be lower, at a value of 1.15–1.20. In production, a fluctuates with changes in gas temperature, calorific value, atmospheric temperature, etc., and requires regular monitoring and timely adjustment. The value of a can be calculated through exhaust gas analysis using the following formula: a=1+K(O2-0.5CO)/(CO2+CO), where K = VCO2/O2. In this formula, O2, CO, and CO2 represent the concentrations of oxygen, carbon monoxide, and carbon dioxide in the exhaust gas, measured as volume percentages ； V CO2 is the volume of CO2 produced theoretically when 1 m3 of gas is completely burned, in m3; O2 is the amount of oxygen theoretically required to burn 1 m3 of gas, in m3. The K value varies depending on the composition of the gas; generally, for coke oven gas, K = 0.43, and for blast furnace gas, K = 2.5. Regardless of the type of coke oven, reducing the air excess coefficient can lower the temperature difference in the upper part. This is because the amount of air decreases, slowing down the combustion rate of the gas in the vertical flue. However, this method has certain limitations. In coke ovens without exhaust gas recirculation, changes in the air excess factor have a more significant effect on vertical heating. Pangang conducted comparative tests by taking samples through the vertical flue during the heating with coke oven gas, and the results are shown in Table 4. Table 4 Effect of varying the air excess coefficient on upward heating. Setting time, air excess coefficient a, temperature at the center of the coke cake (°C), temperature difference between the machine side and the coke side, upper, middle, lower temperatures – machine side, coke side: 18 hours: 1.30–1.45, 960, 1040, 1060, 960, 1010, 1040, 100, 80; 18 hours: 1.20–1.30, 920, 960, 1010, 950, 990, 1020, 90, 70; 18 hours: 1.10–1.20, 940, 970, 990, 960, 990, 1000, 50, 40. As can be seen from Table 4, as a decreases, the temperature difference at the center of the coke cake gradually reduces, and the uniformity of cooking throughout the coke cake improves. Furthermore, it is ideal to maintain an air excess coefficient of around 1.25 for the vertical flue when using coke oven gas for heating. Therefore, reducing and maintaining an appropriate air excess factor is a simple and effective way to reduce the high temperature gradient in the coke cake. 5.2 Exhaust gas recirculation: Gas and air burn in the rising flue, generating exhaust gases which flow into the descending flue through the crossover holes. At this point, some of these exhaust gases are drawn back into the rising flue via the recirculation holes at the bottom of the dual flue system. This combustion method is known as exhaust gas recirculation. The momentum principle states that: \"In a steady flow, the sum of the external forces acting on a certain region of the fluid in a particular coordinate direction is equal to the change in momentum of the fluid passing through the ends of that region per unit time in that direction.\" ”Based on this principle and the equations for ascending and descending air currents, the basic equation for exhaust gas circulation in a dual-flue system can be derived: (V20coal*ρ0coal*Tcoal) / (273*Ffire+Fcoal) + (V20air*ρ0air*Tair) / (273*Ffire+Fair) – V20waste*(1+x)^2 *ρ0waste*Twaste/F2fire*273 + H*g*(ρWaste_lower–ρWaste_upper) = (PH–PB) + ∑(1–H)ΔP. Here, V0coal, V0air, and V0waste represent the flow rates of coal gas, air, and waste gas, respectively, in m3/s ； ρ0 is the gas density ； F-fire, F-coal slope (burner), and F-air slope represent the cross-sectional areas of the flame channel, blast furnace gas slope (burner), and air slope, respectively, in m2 ； Tcoal, Tair, and Tupwaste represent the absolute temperatures of the gas, air, and waste gas from the upward airflow flue at the exit of the inclined duct, in K ； H is the height of the flue ； ρ_lower and ρ_upper represent the exhaust gas densities in the descending and ascending airflow flues, respectively, in kg/m3 ； x = V_ring/V_waste is the percentage of the exhaust gas circulation volume to the total amount of exhaust gas generated during combustion, expressed in %. The terms 1–4 on the left-hand side of Equation (1) represent the gas injection force (△h_gas), air injection force (△h_air), residual injection force of exhaust gases in the flue (△h_exhaust), and the difference in buoyancy between the rising and descending flues (△h_buoyancy), respectively. On the right-hand side, (PH−PB) denotes the resistance at the circulation holes, while ∑1−H△P represents the resistance across the holes and flues; these are combined to give the total resistance ∑total△P. Thus, Equation (1) can be rewritten as: △h_gas + △h_air − △h_exhaust + △h_buoyancy = ∑total△P (2). In deriving the above equation, the resistance associated with the merging of recirculated exhaust gases and those in the flues was not taken into account, nor was the utilization rate of the injection forces considered. Consequently, the calculated volume of recirculated exhaust gases is greater than the actual value. Tests have shown that when the jet force utilization coefficient K is 0.75, the results obtained are consistent with reality; thus, equation (2) can be modified to: 0.75(△hcoal + △hair – △hwaste) + △hfloat = ∑total△P. The principle of exhaust gas recirculation can be briefly explained by the following three points: (1) Air and coal gas are ejected from the inclined ports and nozzles, and their velocity head generates a jet force that creates a suction effect at the bottom of the upward-flowing flue, thereby drawing in the downward-flowing exhaust gas. Since the cross-sectional area of the nozzle remains constant, the greater the gas flow rate and the higher the gas preheating temperature, the greater the jet force. (2) The temperature of the upward-moving air is higher than that of the downward-moving air; this creates a difference in buoyancy, which causes the upward-moving air to draw in the downward-moving air. The greater the temperature difference between the two burners in a dual-unit setup, the greater the buoyancy difference, and thus the suction force increases. (3) The difference in buoyancy and the jet force are the driving forces that generate exhaust gas recirculation. Due to this driving force, a portion of the exhaust gas in the downward flow is drawn into the upward-flowing flue, thereby increasing the resistance of the gas as it passes through the vertical flue, the crossover holes, and the circulation holes, thus achieving a balance between the driving force and the resistance. That is: K* (gas injection force + air injection force + buoyancy difference) = frictional resistance in the vertical flue + resistance at the crossing holes + resistance at the circulation holes. Under the current design dimensions of these holes, the resistance at the crossing holes is the dominant factor, accounting for 70%–80% of the total resistance, while the resistance at the circulation holes accounts for only about 10%. The so-called \"gas circulation rate\" that we commonly refer to means the ratio of the amount of gas circulated to the rising flue to the amount of exhaust gas discharged through the inclined duct in the descending flue. Strictly speaking, it should be called the exhaust gas recirculation ratio. The size of the exhaust gas recirculation ratio is related to factors such as the type of heating gas, the height of the carbonization chamber, the coking time, and the height of the burner head. Conducting measurements on production coke ovens is a tedious and difficult task. Chinese coking industry researchers conducted measurements on single-chamber test furnaces with a height of 6 m and 8 m. Some of the data are listed in Table 5. Table 5 Relationship between the exhaust gas recycling ratio and coal gas type, burner height, coking time, and carbonization chamber height. 6 meters, 8 meters; heating with coke oven gas, heating with coke oven gas, heating with blast furnace gas; burner height of 190 mm, burner height of 740 mm, burner height of 120 mm/120 mm, burner height of 120 mm/1200 mm. 20 hours: 112, 80.9, 123.5, 91.6, 89.6; 16.5 hours: 105.8. As can be seen from the data in Table 5, the recycling ratio is higher when using coke oven gas for heating compared to when using blast furnace gas ； The higher the lamp holder, the smaller the circulation ratio ； The longer the coking time, the greater the circulation ratio ； The higher the carbonization chamber, the greater the circulation ratio. This fully demonstrates that exhaust gas recirculation has a very good automatic regulating effect on the upward heating of coke ovens. When heated with blast furnace gas, the increase in the flow rates of the gas and exhaust gases leads to an increase in injection force and resistance, while the effect of the buoyancy difference decreases relatively, resulting in a reduced amount of exhaust gas circulation. In this way, when the furnace control device remains unchanged and heating is carried out using coke oven gas, the amount of exhaust gas circulation is high, which helps to improve the uniformity of heating in the upper regions ； When heated with blast furnace gas, the amount of exhaust gas circulation automatically decreases to accommodate the longer flame length of blast furnace gas. Furthermore, when the flow rate remains constant and the uniformity of upward heating deteriorates, the temperatures in the rising and descending flues increase, resulting in a greater buoyancy difference that automatically increases the amount of exhaust gas circulation, thereby improving the uniformity of upward heating. The amount of exhaust gas recycled is generally stable. However, when certain unstable factors arise, it may cause the gas and air in the upward flue gas flow to be drawn directly into the downward gas flow chute through the circulation holes for combustion, rather than burning in the vertical flue – a phenomenon known as short-circuiting (that is, the flame entering the downward flue directly through the circulation holes). This will damage the furnace body, and it should be prevented. The main reason for the short circuit is that the sum of the jet force and the buoyancy force is less than the resistance of the circulation hole. Situations that may cause a short circuit during production are as follows: (1) A short circuit is likely to occur when negative pressure appears at the viewing holes on the top of the furnace. This is because when the fire viewing cover is opened, external air is drawn into the vertical flue, increasing the resistance in the vertical flue (including the crossing opening) and the downward chute, which in turn reduces the exit jet force and increases the likelihood of short-circuiting. Furthermore, due to the high negative pressure in the combustion chamber, when the integrity of the furnace is poor, a large amount of external gas or raw gas leaking into the vertical flue also increases the likelihood of short circuits. (2) Wild gas leaks from the carbonization chamber. (3) During heavy rain, some water vapor penetrates into the vertical flue. (4) Immediately after the exchange, the temperature of the downflow flue is higher than that of the upflow flue, and the buoyancy difference between the upflow and downflow gases is negative. When the commutation time interval is long, the gas flow rate is low, and the temperature difference between the rising and descending flues is large, the negative value of the buoyancy difference at the beginning of commutation increases, making short circuits more likely; however, this effect disappears automatically after a certain period of time following commutation. (5) Due to heat dissipation from the furnace body, the temperature in the burner channel remains lower than that of the adjacent channels during upward airflow; as a result, the buoyancy difference is negative. Additionally, the larger cross-sectional area at the outlet of the burner channel reduces the exit velocity of the airflow, thereby decreasing the injection force. Moreover, cracks in the burner channel can lead to leakage of raw gas, which lowers the temperature and increases resistance, making short circuits more likely to occur. To prevent this phenomenon, waste gas circulation holes are no longer provided between the burners at the front end of JN-type coke ovens. (6) Debris in the flame channel increases the system resistance; when this reaches a certain level, a short circuit may occur. (7) Short circuits are likely to occur when the coke oven turnaround time is prolonged or during the heat retention period. This is because the amount of heated gas decreases, resulting in a reduced jet force; moreover, as the temperatures in the rising and descending flues tend to become equal, the difference in buoyancy also decreases significantly, thus making short circuits more likely to occur. (8) At the beginning of coal loading, if a large amount of raw gas leaks into the flue through cracks in the furnace wall or areas where the drying holes are not properly sealed, it increases the resistance in the flue. In such cases, the observation holes show positive pressure, and there is a possibility of short-circuiting in the flue. To eliminate this short circuit, the observation ports in the short-circuited flues on both sides of the coal charging chamber can be opened to allow some gas to escape, thereby reducing resistance, increasing buoyancy, and eliminating the short circuit. Methods to avoid short circuits: (1) Increase the air supply to boost the exit jet force ； (2) Reduce commutation time ； (3) Increasing the pressure at the viewing hole reduces the negative pressure in the combustion system, thereby decreasing the amount of gas that leaks into the vertical flue. 5.3 Length of commutation time: The reason why the length of the exchange interval affects the uniformity of heating in the cake lies in the fact that at the moment of commutation, the regenerator that was previously under downward airflow becomes one under upward airflow; at this point its temperature is the highest. The temperature of the preheated gas or air is also at its highest, so the combustion temperature is the greatest as well ； The vertical flue, on the other hand, has just shifted from a downward to an upward flow; at this point its temperature is the lowest. In other words, the temperature difference between the burning flame and gases and the lower part of the vertical flue is greatest, resulting in a large amount of heat being transferred to the lower section. As time passes after the exchange, the temperature of the flame combustion gradually decreases ； At the lower part of the three-flue structure, as the heating temperature gradually increases, the temperature difference between the two areas decreases, resulting in less heat being transferred to the lower part, while more heat is transferred to the upper part. Therefore, an increased exchange interval is beneficial for heating the upper part of the cake. Although the temperatures of the gas and air during preheating gradually decrease over the commutation cycle, the actual gas flow temperature in the vertical flue still rises gradually. The actual temperature rises because the temperature of the rear furnace wall increases gradually, resulting in a gradual decrease in the heat transferred to the furnace wall by radiation from the gases. Therefore, with a slight increase in the overall maturity of the coke cake, the standard temperature can be appropriately reduced. However, after the commutation period is extended, the temperature in the furnace top space during the later stage of commutation rises, the temperature of the raw gas increases, the efficiency of the regenerator decreases, and heat consumption goes up; therefore, all these factors need to be taken into consideration. 5.4 Appropriate heating level height – Choosing an appropriate heating level height can also improve the conditions of vertical heating, as the height of the heating level has a significant impact on the degree of maturity in the upper part of the cake. At a heating level of 600 mm, the maturation of the upper part of the coke cake is good, but the temperature in the space above the furnace is high. At a heating level of 900 mm, heating of the upper part of the coke cake is somewhat insufficient, but the temperature in the space above the furnace decreases. Therefore, current coke ovens all use a heating level height of 700–800 mm. 5.5 Reducing cycle time: The length of the cycle time has an impact on the vertical heating in coke ovens. Reducing the turnaround time increases the amount of gas supplied per unit time, which raises the gas flow velocity and thus lengthens the flame, improving the uniformity of the coke cake from top to bottom. 5.6 Strengthen the operational management of coal loading and leveling. Improper methods of coal loading and leveling can also affect vertical heating. Overfilling with coal causes the coal leveling rod to compress the coal layer above excessively, resulting in a high bulk density in that upper part of the coal pile. This also reduces the space at the top of the furnace, affecting the flow rate of the gas stream. As a result, the gas pressure inside the carbonization chamber increases, and insufficient heating of the coke at the top leads to the formation of unripe coke. If the coal loading is insufficient, the space above the furnace increases and the temperature in that space rises. This not only reduces the production capacity of the coke oven and the quality of the chemical products, but it also leads to an increase in graphite within the furnace; in severe cases, it can cause difficulties in pushing out the coke. 6 Conclusions At present, the main methods for achieving uniform heating in coke ovens in the vertical direction are four: different furnace wall thicknesses, segmented or even multi-segmented heating, high and low burner heads, and exhaust gas recycling. Different thicknesses of the furnace wall are employed to improve the heating at the upper part of the coke cake, by increasing the thickness of the furnace wall at the lower part of the carbonization chamber or by gradually varying and altering the shape of the partition bricks in the vertical flue ； Segmented heating involves introducing blast furnace gas and air into the vertical flue through holes in the partition walls, at different heights, thereby enabling segmented combustion. This approach allows the flame to be made very long, and the vertical heating can be adjusted by controlling the cross-sectional area at the exits of these holes. Multi-stage heating involves supplying air and blast furnace gas in several stages along the vertical direction within the vertical flue, thereby enabling multi-stage combustion to lengthen the flame. These methods are effective in improving the uniformity of vertical heating in coke cakes, but they increase the variety of brick types used in constructing coke ovens and complicate their structure, which is why they have not been widely adopted. The upper and lower burners use gas burners at different heights within adjacent flues, in order to change the height of the combustion point within the flue and thus ensure uniform heating from above. However, this method is only applicable to heating with coke oven gas; moreover, since the gas is released from the upper burner at a certain distance above the bottom of the flue, the air coming from the inclined channels can easily burn away the carbon deposits in the brick gaps beneath the upper burner, leading to leaks. Waste gas recirculation is currently a simple and effective method for achieving uniform high-temperature heating in the combustion chamber. It uses some of the exhaust gas from the descending flue to enter the ascending flue through the circulation holes, diluting the concentrations of air and gas and thereby reducing their combustion rate. At the same time, as the speed of the mixed airflow increases, the flame becomes further elongated. It helps to achieve uniform heating of the coke cake from top to bottom, improves the quality of coke, reduces the coking time, increases production, and lowers the heat consumption in coke production. It is also possible to increase the height and volume of the carbonization chamber, improve the labor productivity of coke ovens, and reduce the capital investment per unit of product; therefore, it is widely used in modern coke ovens. Since each of the above methods has its own advantages and disadvantages, modern large-capacity coke ovens often employ several methods simultaneously to achieve high uniformity in heating.
The speed of gas combustion, airflow velocity; reducing the nozzle diameter, increasing suction force
Adjust the opening of the air intake valve to increase suction
There are two methods: changing the opening degree of the air inlet – this is the most straightforward approach, but it does not cause a drop in furnace temperature; in fact, the furnace temperature might even rise. The other method is to change the suction force. It may cause the furnace temperature to drop. Generally speaking, two methods need to be used in combination.
After being cooked for such a long time, if charring occurs over time, it’s necessary to maintain the temperature of the burner; adding wire in the middle will solve the problem:lol
Increase the amount of air supplied to regulate the flue suction
I would like to ask the people upstairs why increasing the suction in the smoke ducts makes the flame longer