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Technical and Economic Analysis of the Operation of Laigang’s No. 1 Dry Quenching Unit for Coke Yu Shiyu, Li Mingfu, Xu Hong (Coking Plant, Laiwu Iron and Steel Co., Ltd., Laiwu 271104, Shandong) Abstract: This paper presents an overview of the dry quenching process for coke at Laigang’s No. 1 unit, its technical characteristics, operational status, as well as the technical and economic benefits associated with it. After the 1# dry quenching system was put into operation, the dry quenching rate of coke exceeded 85%, the steam generation per ton of coke was over 0.54 tons, and 40 kg of standard coal could be saved per ton of dry quenched coke ； The quality of coke improved significantly: M40 increased by 3.1%, M10 decreased by 0.6%, CRI decreased by 3.1%, and CSR increased by 3.8% ； The cost of blending coal decreased by 13.1 yuan per ton, and the environmental conditions improved significantly. Keywords: dry quenching of coke ； Run ； Techno-economic indicators ； Coke Quality Chinese Library Classification Number: TQ522.16 Document Code: A Article Number: 1004-4620(2007)06-0010-02 Technical and Economic Analysis of 1# CDQ Installation Operation in Laiwu Steel YU Shi-you, LI Ming-fu, XU Hong (Coking Plant of Laiwu Iron and Steel Co., Ltd., Laiwu 271104, China) Abstract: This paper introduces the general process situation, technical features, operational status, as well as the technical and economic effects of the 1# CDQ system at Laiwu Steel. Since its commissioning, the dry quenching efficiency has remained above 85%, with steam production exceeding 0.54 tons per ton of coke produced; 40 kg of standard coal can be saved per ton of coke processed. The quality of the coke has improved significantly: the M40 value increased by 3.1%, the M10 value decreased by 0.6%, the CRI value decreased by 3.1%, while the CSR value increased by 3.8%. The cost of blended coal was reduced by 13.1 yuan per ton, and the environmental conditions were also markedly improved. Keywords: CDQ; operation; technical and economic indicators; coke quality 1 Introduction Energy conservation and emission reduction are current top priorities in the metallurgical industry. As a major advanced technology for energy conservation and emission reduction recognized both domestically and internationally in the coking sector, dry quenching technology offers advantages over traditional wet quenching methods in terms of improving the recovery rate of secondary energy, enhancing coke quality, and protecting the environment. Laiyuan Iron and Steel Company’s No. 1 dry quenching unit was put into operation on December 28, 2005. It has been operating stably over the past two years, playing an important role in helping the company reduce energy consumption and emissions as well as improve the quality of coke. 2 Technical Features of Laigang’s 1# Dry Quenching Process The 1# dry quenching unit at Laigang is designed to work in conjunction with the large 6 m coke ovens No. 5 and No. 6, which have an annual production capacity of 1.1 million tons of coke; its designed capacity for processing coke is 140 tons per hour. The basic process involves a locomotive pulling a rotating coke drum containing red coke to the bottom of the lifting jib, then lifting and moving it to the top of the dry quenching furnace. A loading device is used to insert the red coke into the furnace, where the hot red coke exchanges heat directly with an inert gas (primarily nitrogen). The coke is cooled to below 180 °C and is then removed via a coke discharge device for further processing. The inert gas, which has absorbed the sensible heat of red coke, enters the waste heat boiler where it undergoes indirect heat exchange with soft water to produce high-quality steam. The cooled inert gas is sent back to the dry quenching furnace for recycling via a circulation fan. 　　The No. 1 dry quenching unit at Laigang employs many advanced technologies to improve the cooling efficiency of hot red coke, achieve centralized control over the production process, and enhance operational safety and stability. 　　1) The dry quenching furnace features a short and robust structure, with a height-to-diameter ratio (H/D) of 0.9. By reducing H/D, the pressure loss of the cooling gas in the dry quenching furnace can be decreased, the head pressure of the circulation fan can be lowered, and investment can be saved. 2) The loading of red coke into the system is carried out using a rotating coke hopper, a loading bell, and a central air cap for regulating gas distribution. This ensures uniform particle size of the coke and even gas distribution within the dry quenching furnace, thereby improving its cooling efficiency. The gas-to-coke ratio reaches 1,250 m3/t, which is 15% lower compared to similar installations. 3) A continuous coking discharge device equipped with a gas-tight rotary seal valve can **reduce the leakage of circulating gas. Depending on the level of material in the pre-storage section of the dry quenching furnace, the amount of coke discharged can be controlled with great precision using a vibrating feeder. 4) The circulating fan is of a double-support, double-suction centrifugal fan type, with variable frequency speed control, which facilitates more precise regulation of the circulating gas flow, enhances operational flexibility, and helps to stabilize the boiler inlet temperature as well as the overall operation of the boiler. 5) An adjustable air intake device is specifically installed at the circular air duct; by adjusting the opening degree of the air intake valve according to the boiler inlet temperature and the levels of CO and H2 in the gas composition, the control over air intake is enhanced, which facilitates safe production management. 6) The secondary dust collector uses multi-tube cyclone dust removal, which offers good dust removal efficiency and requires less floor space. 7) The feedwater preheater further reduces the temperature of the circulating gas entering the dry quenching furnace, improving the cooling effect on coke and reducing the gas-to-material ratio in the dry quenching furnace; it also helps to decrease the amount of steam required by the deaerator. 8) The dry quenching boiler utilizes a combination of forced circulation and natural circulation: natural circulation operates between the steam drum, the membrane water wall, and the economizer, while forced circulation is used between the steam drum and the evaporator, achieving a thermal efficiency of over 85%. 9) The red coke conveying systems are all equipped with an automatic alignment APS device; the adjustment range of this device is ±100 mm, and its accuracy of adjustment is ±10 mm, ensuring high alignment precision ； The pre-storage chamber of the dry quenching furnace employs dual-interlock level control to prevent operational accidents. 10) The electrical and instrumentation systems share a single EI control system, enabling centralized control of the production process and ensuring the safe and efficient operation of the entire system. 3. Analysis of technical and economic indicators during operation: Laiyuan Iron and Steel Company’s No. 1 dry quenching unit was put into operation on December 28, 2005. After nearly 1.5 years of operation, its first annual maintenance was carried out on June 10, 2007. Production resumed on July 9, 2007, after completion of the repairs, and its overall operation has been stable. 3.1 Quenching Rate The No. 1 coke quenching unit at Laigang has been operating stably since its commissioning, and the coke quenching rate has increased steadily, reaching over 90% in normal months (see Table 1). Due to various internal and external factors, the dry quenching rate was low at certain times; for example, after the end of the heating season in April 2006, it was only 65.1% due to issues related to steam balance ； Affected by the annual maintenance of the dry quenching system, the dry quenching rate was low in June and July 2007. Statistics show that as of October 2007, the dry quenching rate was 82.7%; excluding the impact of those 3 months, the average dry quenching rate exceeded 86%, reaching a good level. According to statistics, since its commissioning until October 2007, Unit 1 for dry quenching of coke had processed a total of 1.623 million tons of coke through dry quenching. Table 1 Statistics on the dry quenching rate, steam production, and coke quality since the commissioning of Unit 1 for dry quenching of coke Time Dry Quenching Rate/% Steam Output/(t·h-1) Coke Quality/% M40 M10 CRI CSR Moisture 2006-01 82.7 53.1 86.9 6.1 28.4 64.1 2.6 2006-02 87.2 56.3 86.8 6.2 28.0 64.9 2.6 2006-03 86.2 62.2 86.7 6.0 29.2 64.5 2.4 2006-04 65.0 46.1 86.4 5.9 27.9 64.7 3.5 2006-05 89.2 66.8 87.8 5.8 26.5 65.9 2.4 2006-06 84.6 67.9 88.0 5.7 28.2 64.8 2.6 2006-07 87.3 67.1 87.8 5.6 25.7 67.4 2.6 2006-08 87.3 67.7 87.7 5.6 26.0 66.1 2.5 2006-09 87.9 68.6 88.0 5.6 27.3 65.1 2.9 2006-10 87.1 66.2 87.8 6.4 27.9 64.4 3.7 2006-11 89.7 72.0 87.9 6.3 27.9 63.8 2.7 2006-12 88.6 69.3 88.0 6.3 24.7 66.7 2.3 2007-01 88.6 67.0 88.0 6.1 27.2 63.9 3.2 2007-02 90.4 71.6 88.0 6.2 23.7 66.6 3.6 2007-03 89.9 73.7 87.8 6.3 29.6 62.2 3.8 2007-04 90.0 71.3 87.9 6.1 29.5 63.3 2.8 2007-05 90.5 72.3 87.9 6.1 28.9 63.4 2.0 2007-06 30.4 32.3 86.7 6.3 28.8 63.1 3.9 2007-07 48.8 32.8 87.2 6.3 28.7 63.9 2.9 2007-08 83.9 59.2 88.2 6.2 27.5 64.1 2.0 2007-09 90.5 64.4 88.1 6.1 25.8 66.6 1.9 2007-10 90.0 63.2 87.8 6.1 27.1 66.0 2.3 Average 82.7 61.9 87.6 6.0 27.5 64.8 2.8 3.2 Steam Production Statistics show that as of October 2007, Unit 1 for dry quenching of coke at Laiyuan Iron and Steel Group produced 875,590 tons of steam, with an average output of 61.9 tons per hour; the amount of steam per ton of coke produced was 539 kg. The steam production is greatly influenced by the steam balance at Laizhou Iron and Steel Plant; without taking into account steam losses, the average amount of steam generated per ton of coke in the dry quenching process exceeds 540 kg. 3.3 Coke Quality The quality of coke has improved significantly. According to the statistics in Table 1, the cold strength of the coke produced by Laiyuan Iron and Steel’s No. 5 and No. 6 coke ovens after dry quenching is 87.6% for M40 grade and 6.0% for M10 grade ； The reactive CRI at high temperature is 27.5%, the strength after reaction is CSR 64.8%, and the moisture content of the coke is 2.8% (due to the limited temperature that the belts in the Laiwu steel plant can withstand, the coke must be cooled further after dry quenching; hence, its moisture content is relatively high). Table 2 shows that when the coal ratio after dry quenching was **worse than before dry quenching, the M40 and M10 grades of coke obtained through dry quenching improved by 3.1 and 0.6 percentage points respectively, while CRI and CSR improved by 3.1 and 3.8 percentage points respectively; the moisture content decreased by 2.3%. Table 2 Comparison of coke quality before and after the commissioning of 1# dry quenching plant, in % Time Mechanical strength Thermal properties M40 M10 CRI CSR 2005: 84.5 6.6 30.6 61.0 October 2006–2007: 87.6 6.0 27.5 64.8 Differences: +3.1 -0.6 -3.1 +3.8 The microstructure of the coke improved significantly. Using an image-analysis optical microscope, the pore structure parameters of dry quenching and wet quenching for coke ovens No. 5 and No. 6 were analyzed respectively; the results are shown in Table 3. As can be seen from the data in Table 3, after dry quenching of coke, the porosity decreases, the average pore diameter shrinks, the average pore wall thickness increases, the average pore roundness improves, and the density rises. Table 3 Changes in pore structure parameters of dry quenching and wet quenching coke. Quenching method, Pore volume percentage/%, Average pore diameter/mm, Average pore wall thickness/mm, Average pore roundness, Density. Dry quenching: 0.57, 100.2, 86.1, 7, 088.5, 0.68; Wet quenching: 0.59, 128.3, 73.7, 6, 932.5, 0.67. 3.4 Energy consumption during the process: The energy consumption in the coke production process decreased significantly. After the commissioning of Laiyuan Iron and Steel’s No. 1 dry quenching unit, the energy consumption in the coking process dropped from 125.22 kg/t in 2005 to 109.01 kg/t from January to October 2007, a reduction of 16.21 kg/t. 3.5 Environmental Protection 　　Significant effects are achieved in environmental protection. Data show that in traditional wet coking processes, on an scale of 1 million tons of coke produced per year, the emissions of toxic gases such as phenol, cyanides, hydrogen sulfide, and ammonia (33, 4.2, 7.0, 14.0 kg/h respectively) will exceed 600 tons, causing severe pollution to the atmosphere and the surrounding environment. Dry quenching of coke does not pollute the environment to any significant extent, as it uses inert gases to cool the red coke in a sealed dry quenching tank, and is equipped with effective dust removal systems. On the other hand, the steam generated by dry quenching of coke prevents air pollution caused by boiler flue gas produced in the same quantity, thereby reducing emissions of SO2 and CO2. 　　Since its commissioning, Laiwu Iron and Steel Company’s No. 1 dry quenching unit has quenched 1.623 million tons of coke. Based on these figures, it is possible to reduce the emission of the aforementioned toxic gases by 969.6 tons. If the emission pollution of SO2 and CO2 into the atmosphere caused by burning thermal coal in steam boilers to produce the same amount of steam is taken into account, the indirect environmental protection benefit is even greater. 3.6 Cost of blending coal The dry quenching process can significantly improve the quality of coke; by maintaining the same level of coke quality, it is possible to significantly reduce the amount of highly caking coal used. After the commissioning of Laigang’s No. 1 dry quenching unit, the coal blending ratios for No. 5 and No. 6 coke ovens were appropriately adjusted (see Table 4). Among them, the price of low-cost bituminous coal increased by 13%, while that of lean coal rose by 2%; there was a 5% increase in the use of gas-enriched coal. Meanwhile, the use of high-priced and scarce coking coal and rich coal each decreased by 10%, resulting in a significant reduction in the cost of blended coal. Based on Laiwu Iron and Steel’s current planned prices, the cost of blending coal has decreased by an average of 13.1 yuan per ton; since the commissioning of Unit 1 for dry quenching of coke, the total reduction in the cost of blending coal amounts to 30.83 million yuan. 　　Table 4: Coal blending ratios for coke ovens No. 5 and No. 6 before and after the commissioning of Unit 1 for dry quenching of coke, in %; Energy savings value. Since the commissioning of Laiyuan Iron and Steel Company’s Unit 1 for dry quenching of coke, **875,590 t of steam has been generated, at an average rate of 61.9 t/h. For every 1 t of coke dried in this way, 0.54 t of medium-pressure steam is produced, which is equivalent to approximately 60 kg of standard coal per t of coke. The electricity, steam, water, nitrogen, etc., consumed in the dry quenching of coke amount to approximately 20 kg of standard coal per ton. Therefore, for every 1 ton of coke processed by dry quenching, approximately 40 kg of standard coal equivalent in terms of energy savings is achieved; since the system began operation, a total of 64,920 tons of standard coal has been saved. 4 Recommendations for the Operation of Laiwu Iron and Steel’s Dry Quenching Unit The No. 1 dry quenching unit at Laiwu Iron and Steel has been in operation for nearly 2 years, achieving good economic and social benefits. Regarding the operation of the dry quenching device, continuous research, improvement, and enhancement are still needed in areas such as process technology, equipment, and control. 　　1) The dry quenching rate is a comprehensive technical and economic indicator that reflects the operational performance of dry quenching coke units. The quenching rate of Laiyuan Iron and Steel’s No. 1 dry quenching unit still has considerable potential to be improved, influenced by various factors. Take measures to improve the quenching rate and further enhance the overall efficiency of the coking quenching unit’s operation. 　　2) Strengthen research on the equipment and automatic control systems for dry quenching of coke to reduce operational failure rates ； Understand the operating patterns of the equipment, standardize the maintenance of dry quenching coke plants, further reduce the time required for maintenance and annual overhauls, and extend the intervals between scheduled repairs. 　　3) The construction of a dry quenching unit takes a long time and requires significant investment; the core component of its process is the dry quenching furnace. Therefore, the lifespan of dry quenching furnaces will inevitably receive increasing attention. Strengthen research and management on the material distribution within the dry quenching furnace, the structure and composition of the refractory lining, the behavior of gas and materials inside the furnace, as well as the operation of various process technical parameters, in order to advance the development of dry quenching furnace technology and increase its service life. This post was last edited by gxh60 on 2009-2-25 09:38.]