The large-scale coke oven is the inevitable trend of the development of coking technology in China. Over the past 10 years, China has achieved remarkable results in the operation of large-scale coke ovens through system optimization and equipment improvement, promoting the progress of coking technology.

1. Development history of coke oven large-scale in China

The definition of large coke oven changes with the continuous increase of coke oven chamber volume and the progress of coking technology. In the middle of 1980s, China's coke ovens were dominated by top loaded coke ovens with a carbonization chamber height of 4.3m. The top loaded coke ovens with a carbonization chamber height of 5.5m built by Panzhihua Iron and Steel Co., Ltd. and the top loaded coke ovens with a carbonization chamber height of 6m introduced by Baosteel in 1985 promoted the development of large-scale coke ovens. With the production of JN60 top loaded coke oven with 6m high carbonization chamber designed and built by our country in Beijing Coking Plant, 6m top loaded coke oven has gradually become the main type of coke oven in the 1990s.

In June 2006, Shandong Yankuang International Coking Company introduced 7.63m top loaded coke oven from Germany, which opened the prelude to the large-scale development of coke ovens in China. Subsequently, MCC Coke Resistance Engineering Co., Ltd. developed a 7m top mounted coke oven and a 6.25m stamp charged coke oven in Tangshan Jiahua. On this occasion, a 6m or more top mounted coke oven and a 5.5m or more stamp charged coke oven became the mainstream furnace in China. At present, the super large coke oven with a carbonization chamber height of 8m has been developed, which can realize uniform heating at low temperature along the height direction of the combustion chamber and reduce NOx generation, marking the maturity of China's large-scale coke oven coking technology.

According to the newly revised Access Conditions for the Coking Industry in 2014, the minimum threshold for the access of top mounted coke ovens is 6m high and 5.5m high for the coking chamber of stamp charged coke ovens. Therefore, it is appropriate to define top mounted coke ovens with a coking chamber height of more than 6m and stamp charged coke ovens with a coking chamber height of more than 5.5m as large coke ovens.

By the end of 2016, there were 66 7m top loaded coke ovens, 17 7.63m coke ovens and 22 6.0m and above stamp charged coke ovens in operation in China. The total coke production capacity accounted for 15.8% of the total national coke production in 2016.

2. Particularity of Large Coke Oven Technical Management

2.1 Advantage analysis of large coke oven

1) High labor productivity

The four cars of large coke oven adopt automatic positioning interlocking system, PLC adopts UPS power supply for alignment, and the operation mode of 7.63m coke oven four cars can realize full automatic, semi-automatic, interlocking and manual modes. The pressure of a single carbonization chamber has the function of self adjustment, and the labor productivity is high.

Due to the increase of the height and width of the coking chamber, the coke output of large coke ovens also increases. For the top mounted 4.3m, 6m, 7m and 7.63m coke ovens, the coke output of one hole coking chamber is different. The coke output per hole of 7m top mounted coke oven is 2.05-2.37 times that of 4.3m top mounted coke oven and 1.28-1.49 times that of 6m coke oven; The amount of coke pushed out from each hole of 7.63m coke oven is 2.04 times that of 6m top mounted coke oven and 1.38-1.59 times that of 7m coke oven. The coke output per hole of 6.25m stamp charged coke oven is 1.20 times that of 5.5m stamp charged coke oven.

2) High environmental protection level

Under the same production capacity, large coke ovens have fewer times of discharging, and the paroxysmal pollution of coal loading and coke pushing is relatively light. Take the coking plant with an annual output of 2 million tons of coke as an example: 7.63m, 7m, 6m and 4.3m top loaded coke ovens have 113, 172, 255 and 357 times of coke pushing per day, respectively. The less the times of discharging, the less the paroxysmal pollutant emissions of coal loading and coke pushing.

The environmental protection measures for leakage points such as furnace door, riser and coal charging hole are improved, with fewer leakage points and less non paroxysmal pollution. Taking the coke oven with an annual output of 1.1 million tons as an example, the number of coke oven leakage ports with a carbonization chamber height of 7 m is 23.6% less than that of 6 m coke oven, the length of sealing surface is 8.5% less, and the number of times each leakage port is opened every day is 31% less, thus greatly reducing harmful gas emissions and improving the coking production and operation environment.

JNX3-70 coke oven adopts the measures of increasing waste gas circulation, setting high and low lamp holders of coke oven gas and sectionalized air supply to reduce the generation of NOX, and the concentration of NOX in flue gas is less than 500mg/m3.

2.2 Structural difference of large coke oven body

China has become a world power in coking technology, and now has almost all types of top loaded coke ovens in the world.

For top mounted coke oven, the average width of the carbonization chamber of 4.3m coke oven is 0.407m, and the average width of the carbonization chamber of 6m coke oven is 0.45m, with an increase of 10.57%; The average width of 7m coke oven carbonization chamber is 0.50m, and the width is increased by 22.85%; Then to 7.63m coke oven, the average width of the carbonization chamber is 0.60m, and the width is increased by 47.42%. That is to say, compared with 4.3m coke oven, the width of 7.63m coke oven carbonization chamber has increased by nearly 1/2, the height of carbonization chamber has increased by 39.76%, and the effective volume is 3.53 times of 4.3m.

The modern large-scale coke oven is a thermal furnace with complex structure. The 7.63m coke oven and the 6m coke oven have a great difference in furnace body structure, which is mainly reflected in that the 7.63m coke oven is: lean gas and air are both side fed, and the regenerative chamber is divided into grids and single-sided flue; When mixed gas with low calorific value is used for heating, both gas and air are regulated by metal jet plate at the top of small flue; Single side small flue; Three sections supply air for staged combustion, and the coke pushing operation of coke oven adopts 2-1 series sequence.

The increase of coal loading of large coke oven is not a simple way to improve the furnace temperature. Because the coke shrinkage is a function of coking time and coking rate, the coking rate and the shrinkage performance of coke cake also change accordingly, so more and more scarce main coking coal needs to be added. Anshan Thermal Energy Institute of Sinosteel and Wuhan Iron and Steel Co., Ltd. have studied the influence of the width of the carbonization chamber on the charging coal, coke and coke pushing. The previous coal blending theory is no longer applicable. The width of the carbonization chamber is different, and the coal blending structure must be adjusted accordingly.

2.3 Complexity of large coke oven equipment

After the large-scale coke oven, the stress condition of the coke oven masonry becomes more complicated. During coke pushing, higher requirements are put forward for the mechanical vibration force of the furnace wall and the thermal stress of the furnace body structure, the materials of the three iron parts for furnace protection, the thermal resistance and the rationality of the provided protection force and its distribution.

Coke oven door is the most important sealing equipment of coke oven carbonization chamber, and its sealing effect directly affects the continuous smoke and dust emission of coke oven. With the enlargement of the coke oven, the deformation of the door itself will increase significantly, and the pressure of the gas in the furnace will be greater, which greatly increases the difficulty of sealing the door. The material and structure of the coke oven door must be optimized, especially for 7.63m and 7m top mounted coke ovens and 6.25m stamp charged coke ovens. The strength, stiffness and deformation of the key components such as the brick trough, web and knife edge of the door must have the best structural type under the high temperature environment.

The production process of large coke oven is very compact and the operation process is accurate and reliable, which requires perfect automation procedures and high-quality hardware equipment.

3.Difficulties in production technology management of large coke oven

The bulk specific gravity (dry coal) of charging coal for 7.63m and 6m top charging coke ovens is 0.778t/m3 and 0.740t/m3 respectively, which is 1.05 times of the latter. As a result, the coke oven heating system and adjustment means are quite different. If the traditional conventional adjustment method is used, it cannot meet the needs of coke oven production.

3.1 Some process coefficients are low

The coke pushing operation of large coke oven adopts a 2-1 series sequence. The uniformity of the fire flue horizontal temperature of the 7m and 7.63m coke ovens depends on the metal jet plate at the top of the small flue to adjust, which makes the straight temperature uniformity coefficient Kb of the 7.63m coke oven, the fire flue uniformity coefficient K of the combustion chamber horizontal wall and the total production operation coefficient K3 far lower than the indicators of small and medium-sized coke ovens.

2-1 The coke pushing sequence makes the distance between the coke pushing and carbonization chamber and the coal loading chamber smaller. For a 7.63m coke oven with 70 holes in Block A, more than 40 coke ovens need to be pushed out within 8 hours of a working shift. The coal loading chamber is concentrated in one coke oven, and the combustion chamber needs a lot of heat supply. The vertical flue temperature is relatively low; In addition, the coke cake of Block B coke oven is in semi coke or has matured, and the vertical flue temperature is relatively higher, which leads to uneven straight temperature Kb.

Large coke oven has high level of mechanization and automation, complex procedures, difficult maintenance, and frequent faults of coke oven locomotive. A little careless operation will cause K3 disorder.

There is no pressure measuring hole on the top of the regenerative chamber of large coke oven, and the temperature and suction cannot be measured. The amount of gas and air entering each vertical flue is mainly controlled by the coarse adjustment of gas orifice plate, nozzle plate and mixing ratio; Moreover, the blast furnace gas enters from the small flue at the coke side and gradually reaches the turbine side depending on the pressure of the gas branch pipe. When it reaches the 1 # side flue, the gas pressure head will be greatly reduced, which will directly affect the gas distribution of the side flue at the turbine side. The temperature curve of the transverse wall will produce "sawtooth type" and "multimodal type", and the K transverse wall coefficient is less than 0.85.

WISCO and TISCO have changed the size of nozzle plate, and TISCO has also changed the deformed checkered bricks of regenerator, which has slightly improved the Kb and K transverse wall coefficients. At the initial stage of operation of 7.63m coke oven in Shagang, when using blast furnace gas for heating, the temperature difference at the coke side is generally large, at 80-100 ℃. After adjusting the size of the second to seventh nozzle plates, the temperature difference gradually stabilized at 30-50 ℃.

3.2 Some thermal indexes cannot be measured and adjusted

7.63m and 7m coke ovens do not have temperature measuring holes at the top of the regenerator. The top suction, resistance, five point pressure and temperature of the regenerator in the six pressure systems and eight temperature systems required for coke oven regulation cannot be measured. Moreover, when the coke oven is heated with lean gas, the pressure difference between rising and falling the top of the regenerator is an important fine regulation means. Three of the six important pressure indicators of coke oven temperature regulation cannot be measured, which makes coke oven thermal management fall into the original stage of rough regulation.

3.3 The yield of products conditioned by furnace top space temperature

The research of Shagang shows that the temperature of furnace top space will increase by 10 ℃ for every 1% increase of coke cake shrinkage. This is because with the widening of the carbonization chamber, the coke shrinkage increases, so that the carbonization chamber has larger free space. This situation leads to the rise of the temperature in the free space of the carbonization chamber and the deposition of graphite in the furnace top area, thus affecting the tar quality, crude benzene output and quality, gas indicators, etc.

The heating level of 7.63m coke oven combustion chamber in Wuhan Iron and Steel Co., Ltd. and Masteel Co., Ltd. is 1210mm, while that in Taiyuan Iron and Steel Co., Ltd. is 1110mm. The temperature of furnace top space is generally as high as 900 ℃. The heating level of Shagang's 7.63m coke oven is 1500mm. At the initial stage of production, the temperature of the furnace top space is on the high side. After formulating a reasonable heating system, controlling the coal loading, improving the coal density and other methods, the temperature of the furnace top space is basically controlled at 830-850 ℃, and the phenomenon of carbon deposition on the furnace top is slowing down.

The operation data of most 7.63m coke ovens show that the tar yield is lower than 3.3%, the crude benzene yield is lower than 0.8%, and the tar quality is poor, and the content of dust and toluene insoluble matters in the tar is significantly increased.

3.4 Disordered production order of large tamping coke oven

In 1956, the first 3.2m stamp charged coke oven appeared in China, and in 1970, the 3.8m stamp charged coke oven appeared, which made the stamp charged coking technology develop rapidly in China. However, like foreign stamp charged coke ovens, there are many production problems. When the moisture content of coal is more than 13%, the collapse rate of coal cakes increases, and the corners of coal cakes fall and collapse; When loading soot and opening the furnace door at the machine side to push the coal cake, the machine side will smoke seriously; The expansion pressure of tamping coal charging is large, smoke and dust leak out and affect the life of furnace body.

4. Improvement practice of coking technology of large coke oven

4.1 Furnace operation must be "stable"

The furnace condition of modern large coke oven is closely related to the change of mixed coal index, the fluctuation of thermal system, the change of coking time and the impact of atmospheric factors. The stability of these external conditions must be ensured.

The practice of stabilizing the coking furnace at Shagang is to strictly follow the coking curve, not shorten or extend the coking time without reason, reasonably arrange the furnace discharge plan, reduce the abnormal maintenance time, and standardize the thermal management. In view of the high moisture content of coking coal, Shagang invested heavily in building 64 coal storage silos, improved the automatic coal blending process of electronic scales, significantly improved the accuracy of coal blending, and improved the coke quality. In recent three years, the crushing strength of coke has increased from 88.0% to 89.2%.

Maanshan Iron and Steel Coking Co., Ltd. applied statistical technology to control the process of 7.63m coke oven. All stages of the process were evaluated and monitored. The main operating parameters of the coke oven were checked and scored, the operating conditions of the coke oven were accurately judged, and the working conditions of the coke oven were effectively monitored at multiple levels and in all aspects. At the same time, standardized operations were strengthened, and the coal fineness was controlled at 73.5% ± 0.5%. The average value of the coke oven K3 coefficient in 2016 reached 0.87, a significant improvement over previous years. To ensure smooth and stable production of coke oven and stable coke quality.

Wuhan Iron and Steel Co., Ltd. and Maanshan Iron and Steel Co., Ltd. also measure the temperature and suction at the exhaust disc of the small flue of the coke oven to replace the measurement and adjustment of the temperature and suction at the top of the original regenerative chamber, so as to meet the uniformity requirements of the straight temperature of the coke oven. The temperature difference between the straight temperature machine and the coke side is 60 ℃

TISCO and Maanshan Iron and Steel Co., Ltd. reduced the temperature of furnace top space by lowering the standard temperature. Shagang and Maanshan Iron and Steel Co., Ltd. raised the coal line to less than 480mm. At the same time, Maanshan Iron and Steel Co., Ltd. also paid attention to controlling the growth of furnace top graphite by increasing the coal loading.

Shougang Jingtang Xishan Coking Co., Ltd. carried out a single factor coking time variance analysis to improve the cold and hot performance of coke with the best coking time. For three years, the crushing strength of coke has been more than 90%, the abrasion resistance strength is less than 6.0%, and the thermal strength of coke is in a good state. Referring to the production practice of Phase I coke oven, the heating level of Phase II 7.65m coke oven was raised to 1550mm to control the top space temperature.

Based on the operation practice of 6.25m stamp charging coke oven, Shoushan Coking of Pingdingshan Shenma Group has broken through the two bottlenecks of coal cake density and coal loading soot control, developed efficient elastic wheels and friction plates, adopted fixed small spacing tamping hammers and low noise and wear resistant guidance technology to increase the coal cake density to 1.1t/m3, and the height width ratio of coal cakes to 13.5; A reliable sealing frame is set on the charging car, and the flue gas is introduced into the adjacent carbonization chamber by virtue of the negative pressure generated by the riser Prove system of the adjacent carbonization chamber. The 7.63m top charging coke oven was successfully transformed into a tamping coke oven, which was put into operation in October 2013. It is the first super large tamping coke oven in the world, realizing the diversification of coal sources and reducing the coking cost.

4.2 Thermal indicators emphasize "fine tuning"

Thermal management of coke oven is a science. The pressure system and temperature system of coke oven are complementary. Each parameter index of pressure and temperature must be measured and adjusted frequently, and analyzed and adjusted in real time to ensure the uniformity of temperature system with the stability of pressure system.

Taking 7.63m coke oven as an example, due to low heating level, three-stage air heating and single carbonization chamber pressure regulating system, the temperature of furnace top space is higher than 850 ℃, coke is overheated, vertical brick gas channel leaks, and chemical product yield is low.

In the thermal regulation of WISCO, part of the second and third section air ports are closed, so that the air can enter completely from the bottom, reduce the flame burning height, stabilize the moisture and volatile of the coal, and appropriately expand the size of the jet plate; More than 20 leaking brick gas passages were grouted and sealed, and the semi dry gunning technology was used for gunning treatment. These technical measures made the straight temperature uniformity coefficient Kb reach 0.88.

In terms of horizontal heating temperature regulation of Jingtang Xishan Coke Oven of Shougang, supplemented by the intelligent expert control system of "fire falling", the coke oven fire falling temperature is automatically determined to avoid large fluctuations in production, which improves the daily production organization and management efficiency of coke oven and reduces the energy consumption of coking by 2.91%.

4.3 Guarantee "Perfection" of Environmental Protection Facilities

The key to the normal operation of the PROven system with monotonous pressure in the carbonization chamber of large coke oven is to maintain the system equipment, keep the circulating ammonia clean, and strengthen the cleaning of the bridge pipe. In addition, TISCO has added a filter for the circulating ammonia to ensure that the bottom of the carbonization chamber is