Introduction:
The 6000 tpd cement production line of Hefei Southern Cement includes a 12 MW waste heat power generation system, which have been put into operation in 2022. The article provides a detailed introduction to the design characteristics of the production line in terms of roller press selection and configuration, exhaust gas channel design and configuration，and powder selection system configuration. More than half year of continuous production assessment shows that the raw material final grinding system of the system runs smoothly，with stable process indicators such as output and power consumption. The raw material self desulfurization rate can reach 50％-80％, and the synchronous operation rate of the SP boiler, kiln, and grinding system is over 90％. There has been no shut down due to severe fluctuations in production conditions. Eventhough vertical roller mill is also a good choice for clinker grinding but this article mainly focus on the other type which is roller press.
Roller press process selection and configuration and raw material final grinding system application analysis
Roller press process selection and configuration
The raw material final grinding system includes extrusion, powder selection, dust collection, iron removal, and comprehensive utilization of kiln tail exhaust gas SP boiler, SCR denitrification and other processes, with many design parameters, complex matching and connection, and distinct process characteristics. According to the maximum capacity of the burning system, the matching capacity of raw material finished products is about 534 t/h. At present, the maximum single output of the roller press is 520 t/h, and the matching capacity of a single machine is relatively small. Therefore, the design adopts the commonly used HFCG180-160 roller press and V-type classifier, high-efficiency powder selection machine, cyclone to form 2 independent grinding systems. The single set design capacity is 380 t/h, and the total matching capacity can meet the supply of the kiln system and is slightly surplus. In terms of process layout, the two roller press systems are adjacent and share a kiln tail bag dust collector, high-temperature fan, kiln tail exhaust fan and its supporting frame. Its design and selection are mainly based on the actual working conditions of the production line.
Why use high pressure grinding roller in this case?
- The main raw materials such as limestone, shale, and iron slag used for raw material batching have a maximum moisture content of 4.6%, a minimum of 0.15%, and a total moisture content of < 4%. The raw material has a low moisture content and no sticky materials are involved in the batching, which is particularly suitable for the roller press final grinding system.
- The measured raw material has good bond grindability, and the fluctuation value of each raw material ratio within the allowable range is only 12.5-13.2 kWh/t. The fluctuation value of grindability is not large, which can ensure the balance of kiln and mill capacity.
- In response to market factors and current capacity control, peak and valley electricity price and other policies, the two roller press systems are flexible and conducive to staggered production. For example: during valley electricity and sales peak season, two mills can be fully opened, and during peak electricity and sales off-season, only one mill can be opened or none of them can be opened; if it runs for 14 hours a day, a single mill can be opened, and if it runs for 10 hours, both mills can be fully opened, etc., the operation control is extremely convenient, and the system inspection and maintenance is also more favorable.
- According to the changes in performance indicators such as the moisture content and sulfur content of the raw fuel, the synchronous operation of the kiln and the mill is easy to control. For example, when the sulfur content is high, the raw material passes through the kiln tail exhaust gas SO and reacts with CaCO to form solidified CaSO. , thereby achieving white desulfurization in the grinding process and greatly reducing the impact on the burning system. The SP boiler is the main channel for the kiln tail exhaust gas of the two roller press systems. Its start and stop states are also easy to synchronize. The system has strong response capabilities, the kiln and the mill are closely connected, and the process is smooth.
- The two sets of grinding system equipment have the same specifications and models, and the spare parts can be used universally, with fewer procurement links and convenient replacement.
Roller Press Grinding System Close Circuit Equipment List:
| No. | Main Equipment Name | Model / Specification | Quantity |
| 1 | V-type Static Airflow Classifier | HFV7500-SL, classification air volume 480,000 m³/h | 2 |
| 2 | Roller Press | HFCG180-160 (column pin roller), throughput 1,200–1,350 t/h, motor power 2×1,600 kW | 2 |
| 3 | High-Efficiency Dynamic Separator | LAX8000, classification air volume 500,000 m³/h | 2 |
| 4 | Cyclone | 2 × Φ6,000 mm | 2 |
| 5 | Circulating Fan | Air volume 540,000 m³/h, inlet static pressure 7,200 Pa, motor power 1,400 kW | 2 |
| 6 | High-Temperature Fan | Air volume 950,000 m³/h, inlet static pressure 8,500 Pa, motor power 2,900 kW | 1 |
| 7 | Baghouse Dust Collector | Air volume 1,000,000 m³/h, dust emission concentration (standard) ≤ 5 mg/m³ | 1 |
| 8 | Tail Exhaust Fan | Air volume 1,020,000 m³/h, inlet static pressure 3,800 Pa, motor power 1,400 kW | 1 |
| 9 | Pipeline Electromagnetic Iron Remover | Specification: 900 × 1,100 mm | 2 |
| Self-Unloading Permanent Magnetic Iron Remover | Applicable belt width B 800 mm |
Configuration characteristics of powder selection system
The powder selection system equipment includes V-type powder selection machine, LAX8000 dynamic powder selection machine, qb6 m double series cyclone, bag dust collector and its supporting fan, drive motor, etc. In addition to meeting the process performance parameter requirements, its design selection also focuses on the structure of practical new, simple and reliable operation.
LAX8000 dynamic powder selector
LAX type separation high-efficiency dynamic powder selector is the result of innovative development and application in recent years. Under the action of airflow, the first rough separation can settle and discharge most of the coarse particles, the second dynamic separation can select most of the qualified particle size materials, and the third fine selection can accurately control the 0.2 mm particle size screen residue, which has a practical effect on improving the burnability of raw materials. Its functional structure is shown in Figure 2. The roller press, V-selection and air selection circulation elevator are all equipped with small low-pressure dust collectors separately, and the dust-containing gases in each place are treated by independent dust collectors without affecting each other. Compared with the conventional method of introducing dust collection air into the powder selector, it can effectively reduce the load and power consumption of the high-pressure circulation fan.
Cyclone Design
The material and gas separation efficiency of the cyclone for the flue gas at the outlet of the powder selector directly affects the system output and power consumption. The dust concentration of the flue gas at the outlet of the dynamic powder selector is as high as 600-800 g/m3. The more dust (as raw material finished product) collected by the cyclone, the higher the separation efficiency, which depends on the size of the vortex resistance. The traditional cyclone is still insufficient in this regard. This design uses a new type of high-efficiency and low-resistance cyclone for matching. The structures of the two cyclones are shown in Figure 3. The main features of the new high-efficiency and low-resistance cyclone are that the number of tubes is reduced to 2, the pipeline is smooth and the structure is compact. The spiral structure in the large volute can smoothly introduce the airflow into the tube, the vortex resistance in the inlet area is small, and the collision of the airflow and the backflow is avoided, so that the dust-containing gas can achieve rapid separation of gas and material under the action of inertia and centrifugal force, and the separation efficiency can reach more than 92%, and the resistance is as low as < 1 200 Pa.
Bag Dust Collector at Kiln Tail
The bag dust collector at the kiln tail is shared by two roller press systems. The filter bag is made of glass fiber coating, and the air volume can reach 1,000,000 m3/h, which can ensure the dust emission concentration of 45 mg/m3. When one of the roller presses stops running, the exhaust gas at the kiln tail entering the dust collector can also reach below the safe temperature. Even if the SP boiler is suspended for maintenance, opening the cold air valve on the exhaust gas air path can reduce the exhaust gas temperature to a safe range, so the dust collector will not have bag burning accidents and is always in a safe state. Two roller presses share one dust collector, the frame civil construction investment is small, the longitudinal distance and air duct length are small, and the engineering cost is also reduced accordingly. The dust concentration of the three air paths, such as the high-temperature fan, the circulating fan to the bag dust collector at the kiln tail, is high. These three air ducts are designed to be arranged longitudinally on the same plane, with fewer elbows and short lengths, which not only reduces system resistance, but also has a simple and beautiful appearance.
Iron removal device
The metallic iron in the raw material comes from foreign matter mixed in the raw material yard, crushing, homogenization and other links. The iron slag of the batching also contains a small amount of iron filings and their inclusions, which is very harmful to the roller surface of the roller press. Under normal conditions, most people use the method of installing a metal separator at the head of the conveyor belt to remove iron, but when the material is sticky or the conveying volume of the belt conveyor is large, the effect is not ideal, which is obviously not suitable for the conveying volume of 380 t/h.
The designed raw material iron removal process uses a four-stage self-unloading permanent magnetic iron remover and a two-stage pipeline electromagnetic iron remover to remove iron as much as possible and reduce equipment wear.
Self-unloading permanent magnetic iron remover
The self-unloading permanent magnetic iron remover has a bandwidth of 800 mm per stage and is set in the horizontal section after the trough belt feed. The material can be evenly stirred by turning the belt and reversing it, and the iron removal is relatively thorough.
Pipeline electromagnetic iron remover
The pipeline electromagnetic iron remover has a specification of 900mm×1100mm, and foreign matter is removed by magnetic adsorption. Due to the large flow of materials in the pipeline, a large amount of materials are often mixed with the primary iron removal and discharged at the same time, so a secondary iron removal is configured for separation, and the material loss is extremely small. Since the roller press has been running, there is no obvious impact on the roller surface, indicating that the iron removal effect is good.
Conclusion:
Through more than half a year of continuous production assessment, the raw material final grinding system operates smoothly, and the process indicators such as output and power consumption are stable. The raw material self-desulfurization rate can reach 50%~80%, and the synchronous operation rate of the SP boiler, kiln and grinding system is more than 90%. There is no shutdown due to severe fluctuations in production conditions, showing a good design effect.
Renovation case analysis
Case 1: After the renovation, under the same cement ratio and finished product fineness, the No. 1 mill increased production by 38%, and the system power consumption decreased by 3 kwh/t; the No. 2 mill increased production by 22%, and the system power consumption decreased by 2.6 kWh/t. The renovation effect is significant. According to the characteristics of the project, the grinding disc liner and the grinding roller sleeve are modified by online surfacing according to local conditions. The renovation cycle is short and the return on investment is high.
Case 2: A cement plant in Xinjiang has a cement vertical mill. Due to the poor strength of clinker, the specific surface area of the finished cement product is required to be greater than 400 m2/kg. The stability of the vertical mill has always been poor, and the capacity of the mill cannot be fully utilized. In 2022, the company transformed the system. In addition to replacing the original grinding components with a new grinding area structure, the wind ring and system circulation duct were also transformed to completely solve the problem of mill stability and optimize the problems of large vertical mill body resistance and unreasonable resistance distribution in the original system. After the transformation, the stability, grinding efficiency and system resistance of the vertical mill have been significantly improved. Under the same conditions before and after the transformation, the output increased by 20 t/h and the system power consumption decreased by 4 kWh/t. The transformation effect exceeded customer expectations, see Table 2.
Table2: Comparison table of utility consumption before and after modeification
| Item | Before Modification | After Modification |
| Feed Rate (t/h) | 155 | 175 |
| Specific Surface Area (m²/kg) | 400 | 400 |
| System Power Consumption (kWh/t) | 36.5 | 32.5 |
| System Power Consumption (converted to 350 m²/kg) (kWh/t) | 30.28 | 26.9 |
Analysis of waste gas treatment design
Basic parameters of preheater and waste gas in the firing system
The firing system adopts a 6-stage preheater, and its C1 outlet waste gas parameters are: standard flue gas volume 430,000 m³/h, maximum temperature 260℃, pressure - 5,200 Pa. The air flow temperature is not too high, so the kiln tail waste gas sent to the raw material mill system is changed from conventional pipeline humidification to SP boiler, SCR denitrification and then sent to the mill system through a high-temperature fan. Compared with the pipeline humidification system, this design has fewer pipeline elbows and lower operating resistance. The support platform where the SP boiler is located is raised to 45.600 m, shortening the length of the air duct between the C1 outlet and the SP boiler, reducing the heat dissipation loss of the waste gas, and is conducive to increasing the power generation of the SP boiler.
SP boiler design and operation safety guarantee
The SP boiler and the system are built and put into operation simultaneously, which can ensure that the exhaust gas temperature at the C1 outlet is lowered. During normal operation, the exhaust gas at the kiln tail of the boiler outlet is usually 170-260℃. Even if only one of the two roller press systems is working, the exhaust gas entering the bag dust collector at the kiln tail can be reduced to below the safe temperature. In extreme cases, even if the SP boiler is not working, opening the cold air valve on the exhaust gas air path can also reduce the temperature to a safe range, thereby ensuring the safety of the dust collector and the continuous and stable operation of the grinding system.
SCR denitrification process technical parameters
SCR denitrification adopts a medium-temperature medium-dust process, with a flue gas temperature of 180℃ and a dust concentration of 100 g/m³. It uses a TiO₂-V₂O₅ catalyst with a quantity of 205 m³, which can reduce the NOₓ concentration (under standard conditions) from 200 mg/m³ to 50 mg/m³, and control the ammonia escape at 3 mg/m³.
Design of countermeasures for SP boiler working conditions
The SP boiler is the main channel for the kiln tail exhaust gas to send raw material to the mill system. The design considers 17 working conditions and their start/stop countermeasures, see Table 2. Table 2 basically covers the working conditions of the grinding system running normally or abnormal fluctuations that may occur. The mill can respond at any time and make corresponding adjustments. For example: when the working conditions are normal or in order to generate more electricity, it can be fully opened during valley electricity and sales peak seasons, and can be opened or completely stopped during peak electricity and off-seasons; when the SP boiler abnormally requires short-term maintenance, resulting in excessive moisture content and flue gas temperature of the raw materials, the mill can still be fully opened by opening the cold air valve, and production is not affected; when the kiln output is less than 70%, the mill will be completely stopped, and the raw material supply can be met by using inventory, and the kiln and the mill are closely connected.
Maturity, reliability, high efficiency and low carbon are the company's unswerving pursuit. Although the company has ranked among the top in the industry in the field of low-carbon energy conservation, there is no end to the technological innovation of energy conservation and emission reduction, and the optimization and upgrading of vertical mill final grinding technology equipment is also a long way to go. Based on the power consumption composition of the cement vertical mill grinding system, in the future, we will continue to explore technical paths to reduce the power consumption of the vertical mill main machine and the fan system:
- Vertical mill main machine power consumption optimization: focus on improving grinding efficiency, such as developing a vertical mill structure that can operate stably under thin material layer conditions, subdividing and strengthening the functions of each section of the grinding area, exploring technical routes such as dual drive of grinding rollers and grinding discs, and breaking through energy consumption bottlenecks from the mechanical design level.
- Fan system power consumption control: Given that the fan power consumption accounts for about 25% of the vertical mill system power consumption, on the one hand, by optimizing the flow field distribution inside the vertical mill and the system pipeline, the economic wind speed at each point is determined; on the other hand, the fan air volume and total pressure are accurately selected according to the project characteristics to ensure that the fan always operates in the high-efficiency range.
In addition, the deep implantation of digital and intelligent technologies will become the core driving force for the industry's transformation to high efficiency, green and low carbon. The company's cement vertical mill system one-button start and stop, intelligent control, automatic driving and other technologies have been successfully implemented, achieving a comprehensive improvement in the utilization rate, stability and cement quality qualification rate of the full-condition system, while reducing power consumption by 1 kWh/t.
