1. Introduction In the cement manufacturing industry, the high power consumption of certain high-power, continuously operating equipment is a significant factor contributing to the persistently high cost of cement production. Statistics show that electricity costs account for approximately 30% of the total cost of cement manufacturing. Therefore, applying frequency conversion technology to perform energy-saving process modifications on power-consuming equipment can reduce cement manufacturing costs and improve profit margins and market competitiveness. The engineering and technical personnel of Nanjing Wanhui Technology Co., Ltd. conducted on-site investigations and analyses of production equipment in several cement plants based on cement manufacturing processes. During production, most cement plant equipment, especially high-power equipment, does not operate at full load most of the time, resulting in a very low degree of automation. Examples include vertical shaft kiln air supply systems, pelletizing pre-processing systems, raw material homogenization feeding systems, cement classifier systems, vertical shaft kiln unloading systems, chain conveyors, dryers, drying fans, crushers, and slip differential speed control systems. These devices and systems exhibit considerable energy surplus and waste, creating opportunities for energy-saving modifications. 2. Cement Production Process Introduction The production of cement mainly involves several important processes: crushing, storage and pre-homogenization, grinding, raw meal homogenization and storage, calcination, and cement grinding. Grinding involves a mill with a large internal metal shield containing many spherical steel balls. As the mill rotates, these steel balls crush the raw material into small particles, called raw meal. Calcination occurs after homogenization. The raw meal is transported through a cyclone system called a preheater, undergoing gradual heat treatment to a temperature of 900℃ before entering a rotary kiln. The rotary kiln bakes the material. The kiln body is a long cylinder, 50–150m long and 3–50m in diameter, with refractory bricks lining the inner wall. The rotation of the kiln and its drive angle cause the material to move towards the outlet, raising the temperature to 1400℃. Because this process takes place inside the kiln, the raw meal is transformed into granular hard material called clinker. Clinker is the main component of cement, and it determines the quality of the final product. Cement, as a finished product, is a very fine powder. The powder must meet the requirements for producing clinker, gypsum, and some natural and artificial materials (e.g., volcanic ash). Cement mills are similar to raw meal mills, strictly controlling and continuously monitoring the mixing of materials. The level of cement type and most important characteristic, compressive strength, depends on the chemical composition of clinker, the duration of grinding, and the addition or non-addition of various admixtures. Its process flow diagram is shown in the attached figure. Attached figure : Cement production process flow diagram 3. Variable frequency transformation of vertical kiln air supply system (1) Fan operation status Generally, the adjustment method of low-pressure vertical kiln fans is damper adjustment, the air supply adjustment method of Roots fans is exhaust adjustment or return air adjustment; the air supply adjustment method of centrifugal fans is inlet valve adjustment. During normal operation, the valve opening degree is about 70% to 90%, that is, a part of the air volume is wasted. Saving the wasted airflow translates to energy saving. Blower systems are designed based on maximum air supply demand (maximum operating conditions plus backup), and the blower's operating conditions are also the same (i.e., based on the maximum air supply demand of a single unit). In actual use, the blower often needs adjustment based on actual operating conditions. The traditional method is to adjust by opening and closing dampers. This method increases the throttling losses of the air supply system, incurs a starting inrush current when starting the blower, and the system adjustment itself is only phased, slow, and has limited ability to reduce losses, causing the entire system to operate in a fluctuating state. However, by adding a variable frequency drive (VFD) to the blower, these problems can be solved once and for all, resulting in a smooth and stable system operation, and the investment can be recovered through energy savings from the VFD. (2) From general experience in the use of fans, it can be known that: under the power frequency operation state (AC380V/50Hz), the load of the fan that uses the damper (valve) to regulate the air volume fluctuates between 15% and 100% during use; the smaller the load, the greater the throttling loss of the damper (valve), and the lower the efficiency of the fan. However, by changing to the frequency conversion regulation method (i.e., the motor changes the power supply frequency), there is almost no throttling loss of the damper. At the same time, the frequency converter adopts the soft start method and there is no starting inrush current. For the power supply system of the factory with limited power distribution capacity, it can also improve its safety factor. (3) The modified frequency conversion air supply system is based on the original air supply system, with an additional frequency conversion circuit connected in parallel with the original circuit to form a dual-circuit convertible control system. The speed control device of the frequency converter is installed on the upper part of the kiln, and the air volume during sintering is adjusted by adjusting the speed of the motor (fan). Its characteristics: ● Good energy saving effect. Since the power consumed by the motor is proportional to the cube of the motor speed, after the modification, the motor can meet the air volume requirements by running at around 35-43 Hz most of the time, achieving an energy saving rate of more than 25%; ● It has a soft start function, reducing load intensity, extending equipment life, and has a small starting current, improving the electrical safety factor and reducing grid capacity; ● The air volume adjustment is accurate, timely, and convenient; ● Closing the return air vent or exhaust vent reduces cement dust pollution, etc. The test results of Jixi Hongzhan Cement Co., Ltd. after adopting the Fuleng frequency converter modification are shown in Table 1: Table 1 Energy Saving Effect Analysis of Vertical Kiln Blower Frequency Conversion Modification Table 4. Cement Classification System The working principle of the cement classification system is to adjust the speed of the classifier and the classifier fan according to the different grades of cement produced, thereby selecting cement products of different fineness. Traditional air classifiers require adjustments to the number and angle of the fan blades, and comparative tests are needed to achieve the required particle fineness. Newer air classifier systems consist of an air classifier and an air classifier fan. The air classifier is speed-regulated by a slip-ring (electromagnetic speed-regulating) motor, while the airflow of the air classifier fan is adjusted by changing the angle of the baffle plate. Both systems suffer from complex operation, poor adjustment precision, and energy waste. Slip-ring motors, in particular, are not only energy-intensive but also have a high failure rate due to the severe dust in the cement manufacturing environment, making maintenance frequent and difficult. After frequency conversion upgrades, Wan Hui Company has made technical modifications to both the old and new systems. This allows the motor to be adjusted to a specific speed to select particles of the required fineness, saving energy while achieving continuous and automated production. This improves labor efficiency, reduces labor intensity, and yields significant overall benefits. The analysis of the Xuzhou Longshan Cement Plant after the upgrade is shown in Table 2. Table 2. Energy-Saving Effect Analysis of Frequency Conversion Retrofit for Air Classifier Fans Table 5. Vertical Kiln Unloading System To achieve balance between feeding, air supply, and unloading during cement sintering, vertical kilns commonly use slip-ring motors (electromagnetic speed-regulating motors) as the power source for the disc-type unloading device. These motors not only fail to meet the protection requirements of cement production sites, but also consume approximately 20% more power than asynchronous motors at the same output speed. Furthermore, slip-ring motors are even less efficient at lower speeds, consuming more power than asynchronous motors. Therefore, replacing slip-ring speed regulation with a frequency conversion speed regulation system overcomes these shortcomings. The speed regulation performance is far superior to that of slip-ring speed-regulating motors, saving electricity while significantly reducing maintenance costs. This technology is now widely used in the industry. Table 3 shows the energy-saving effect of frequency conversion retrofit for the unloading process at companies such as Jixi Hongzhan Cement Co., Ltd. Table 3. Energy-Saving Effect Analysis of Frequency Conversion Retrofit for Unloading Table 6. Stone Chain Plates and Clinker Chain Plates Due to the slow speed of the chain plates, after the frequency converter retrofit, the operating frequency is generally around 15Hz, resulting in an energy saving rate of over 50%. 7. Pelletizing System The raw meal pelletizing process is one of the key processes affecting the sintering quality of cement clinker. The water-to-material ratio directly impacts pelleting quality. Using a frequency converter allows for stepless speed regulation of the pre-pumping water pump by tracking the raw meal supply, achieving fully automated closed-loop control. Stable material-water ratio results in good pelleting and significantly improves cement sintering quality. This system upgrade primarily focuses on improving automation and manufacturing processes; energy saving due to low power consumption is secondary. 8. Raw Meal Homogenization Feeding System After upgrading this system with a frequency converter, all feeding motors at the feeding ports are steplessly speed-regulated, ensuring proportional feeding and improving homogenization. This is also a manufacturing process consideration. 9. Other Equipment Dryers, drying fans, crushers, and other slip-ring speed-regulating motors, after analysis and comparison by Nanjing Wanhui Company's engineering technicians, all achieve energy savings of over 20% after frequency conversion upgrades. 10. Conclusion The application of frequency conversion technology in the cement industry not only saves energy, reduces maintenance and repair workload, and lowers cement production costs, but also stabilizes the process to improve cement quality. The application of frequency conversion technology in electromechanical equipment and production lines across various industries is an inevitable necessity for enterprise development. More and more enterprises recognize the competitive advantages that frequency conversion technology brings and are upgrading their traditional equipment to frequency conversion, thereby promoting enterprise development.