5000t/d production line kiln tail high temperature fan frequency conversion retrofit

1 Introduction

Under the policy environment of the State Council's call to accelerate the construction of a resource-conserving society, improving the level of resource-conserving manufacturing and application in the cement industry and establishing a resource-conserving cement industrial system is of great significance. Given the current prominent contradiction between domestic and international energy supply and demand, cement production enterprises must reduce energy consumption through various means to achieve optimal economic benefits and maximum labor productivity. Cement production enterprises are major energy consumers in national economic production, and the cement industry has been listed as one of the key areas for national resource conservation.

In cement production, the power consumption of electric motors accounts for nearly 30% of the cost, and the high-voltage motors driving the fans constitute a large proportion of the total motor consumption. In a cement production line, 25% to 30% of the electrical energy is used to drive various types of fans. Therefore, reducing energy consumption and increasing efficiency for fan motors is extremely important. Utilizing variable frequency speed control technology to change the operating speed of the equipment and adjust the airflow can meet production requirements while saving energy. It also reduces economic losses caused by wear and tear on baffles and pipes due to baffle adjustments and frequent downtime for maintenance.

With the development of power electronics and electronic technology, frequency conversion technology has become increasingly mature. Internationally, the method of adjusting the opening of matching dampers for regulating the air volume and pressure of fans has been largely abandoned, replaced by variable-speed electrical drive regulation. Variable frequency speed control has become the best and preferred electrical drive solution for energy saving and consumption reduction in fans and pumps. High-voltage frequency converters have become equipment required by national cement energy-saving design standards. Article 4.3.5 of the mandatory standard GB50443-2007 Cement Plant Energy-Saving Design Code, promulgated on October 25, 2007 and implemented on May 1, 2008, stipulates that "high-temperature fans at the kiln tail should adopt variable frequency speed control devices."

2. On-site process conditions and speed regulation method selection

After ten years of continuous and healthy development, Zibo Luzhong Cement Co., Ltd. has initially formed a circular economic entity integrating cement manufacturing, thermal power generation, and fluoride salt production. It has been awarded the title of "Top 100 Industrial Enterprises in Zibo City" for three consecutive years and has entered the top 50 cement companies in China. Figure 1 shows the factory area.

Figure 1 Factory area scene

Zibo Luzhong Cement Co., Ltd. has recently installed a new 5000t/d dry-process cement production line. The waste heat generated during combustion in the rotary kiln is preheated and homogenized by a preheater using a high-temperature blower at the kiln tail. The cooled waste heat is then extracted by the high-temperature blower for waste gas treatment (dust removal and discharge). After homogenization, the preheated raw material is calcined into clinker in the rotary kiln. The rotary kiln requires suitable air pressure and temperature to allow sufficient suspension time for complete combustion of the pulverized coal, ensuring optimal heat treatment of the raw material. Due to significant changes in material accumulation within the kiln, the instantaneous air pressure fluctuates frequently. The high-temperature blower at the kiln tail is used to regulate the kiln pressure. Additionally, the high-temperature clinker produced after calcination in the rotary kiln generates waste gas containing ash. This waste gas is drawn out by the high-temperature blower at the kiln tail, where an electrostatic precipitator removes the ash before the waste gas is discharged. Due to the process requirements for controlling the pressure of the kiln tail preheater, the air volume of the high-temperature fan is adjusted through DCS control. Previously, the motors of the high-temperature fans were generally speed-regulated using methods such as hydraulic couplings or wound-rotor motors with water resistance in series. After careful consideration, the cement plant leadership compared hydraulic coupling speed regulation with high-voltage variable frequency speed regulation, and concluded that:

(1) The speed regulation range of a high-voltage frequency converter is 0-100%, while the speed regulation range of a hydraulic coupling is generally 40-95%. That is, the speed loss caused by the high speed section is 5%, and the hydraulic coupling can generally only reach 40% of the rated speed.

(2) High-voltage frequency converters have high efficiency (greater than 96%) throughout the entire speed range, while hydraulic couplers have lower efficiency at lower speeds, resulting in significant losses and greatly reducing the energy-saving effect of speed regulation.

(3) High-voltage frequency converters enable true soft starting of motors and load machinery. If the process requires it, the motor can be started repeatedly in a short period of time. Hydraulic couplings cannot solve the motor starting problem. The motor is still directly started, requiring a starting device. The starting impact is also large, and it cannot be started frequently.

(4) High-voltage frequency converters are high-tech products with high reliability and are basically maintenance-free. In the event of a frequency converter failure, the motor can still be directly connected to the power grid and run at the industrial frequency without causing production losses. In contrast, hydraulic couplings contain multiple systems such as oil and water circuits, and have a high failure rate. Once a hydraulic coupling fails, the load machinery will be unable to operate and will have to be shut down for repairs. This results in a large workload for maintenance and a reduction in effective working time.

(5) High-voltage frequency converters do not increase costs due to the speed of the motor, while the cost of using hydraulic couplings to regulate the speed of low-speed, high-capacity motors will increase significantly, and in some cases, it may not even be possible.

(6) Replacing the hydraulic coupling with a variable frequency speed control requires a larger initial investment but has a very short payback period. From an economic perspective, it is worthwhile, and from the perspective of energy conservation and emission reduction, it has even greater social benefits.

Due to its superior performance compared to other speed control technologies and its flexibility in installation site conditions, the price of high-voltage frequency converters has gradually decreased, making their cost-effectiveness increasingly prominent. Furthermore, frequency conversion retrofitting is now very common in the cement industry. Through a bidding process, the JD-BP38-2500F (2500KW/10KV) high-voltage frequency converter manufactured by Shandong Xinfengguang Electronic Technology Development Co., Ltd. was selected for frequency conversion drive control of the kiln tail high-temperature fan. The retrofit was successful. This article briefly introduces the application of the Fengguang brand JD-BP38-2500F high-voltage frequency converter in a 5000t/d kiln tail high-temperature fan.

3. Parameters of the high-temperature blower and motor at the kiln tail

3.1 Motor Parameters

3.2 Parameters of high-temperature blower at kiln tail

4 Key Features of Shandong Xinfengguang Electronics JD-BP38 Series

In September 2007, the Fengguang JD-BP38 series high-voltage variable frequency speed control system was awarded the title of "China Famous Brand". Shandong Xinfengguang is a national high-tech enterprise. Its Fengguang JD-BP38 series high-voltage variable frequency speed control system uses a high-speed DSP as the control core, employing speed vector control technology and power unit series multi-level technology. It is a high-high voltage source type frequency converter, with harmonic performance far below the IEEE 519-1992 national harmonic standard. It has a high input power factor and good output waveform quality, eliminating the need for input harmonic filters, power factor compensation devices, and output filters. It also avoids problems caused by harmonics such as additional motor heating, torque pulsation, noise, output dv/dt, and common-mode voltage, allowing the use of ordinary asynchronous motors. The product was listed as a national key new product in 2003, and the high-voltage elevator variable frequency speed control system was listed as a national torch program project in 2005, receiving funding from the Ministry of Science and Technology's SME Technology Innovation Fund. In 2007, the high-voltage variable frequency speed control system won the Shandong Provincial Major Energy-Saving Achievement Award.

(1) Using a high-speed DSP as the central processing unit, the computing speed is faster and the control is more precise.

(2) Fly-start function: It can identify the speed of the motor and start the car directly without stopping the motor.

(3) Complete automatic switching technology between power frequency and variable frequency: Current high-voltage variable frequency speed control systems generally have a power frequency bypass switching cabinet. When the frequency converter fails, it can switch the high-voltage motor to power frequency operation. There are two types of bypass switching: manual bypass and automatic bypass switching. Manual bypass requires manual operation and takes a long time, suitable for operating conditions without backup devices or for less important conditions. Automatic bypass can directly and automatically switch to power frequency operation after the frequency converter fails. The automatic bypass switching cabinet provided by our company can not only automatically switch from variable frequency to power frequency operation in the event of a frequency converter failure, but also instantly switch from power frequency to variable frequency after the frequency converter maintenance is completed. The entire switching process will not affect the operation of the user's equipment.

(4) Restart function during rotation: If the high voltage is momentarily lost during operation and then restored within 30 seconds, the high voltage inverter will not stop. After the high voltage is restored, the inverter will automatically run to the frequency before the power outage.

(5) Automatic line voltage balancing technology (star point drift technology): When a unit in a phase of the inverter has a fault, in order to balance the line voltage, the traditional method is to reduce the voltage of the other two phases to the same voltage as the faulty phase. However, the automatic line voltage balancing technology adjusts the angle between the phases to ensure the maximum line voltage balance output under the premise that the phase voltage output is maximum and not equal.

(6) Equipped with sudden phase-to-phase short-circuit protection function. If an output short circuit occurs due to equipment failure or other reasons, and the frequency converter does not have phase-to-phase short-circuit protection function, it will lead to a major accident. Our frequency converter can immediately block the frequency converter output when such a problem occurs, protecting the equipment from damage and avoiding the occurrence of an accident.

5. Major problems and solutions in the application of high-voltage frequency converters in the cement industry

5.1 Problem of pipe "collapse" in high-temperature fans leading to motor overcurrent or even shutdown

The high-temperature blower at the kiln tail is a crucial load for maintaining negative pressure within the cement production line kiln. Even in the past, when high-temperature blowers operated at industrial frequency and used hydraulic couplings for speed regulation, it was common for the blower motor to trip due to overload caused by pipe "material collapse," leading to production interruptions. Furthermore, due to the limitations of the power electronic components in the frequency converter, such overload shutdowns caused by material collapse are unavoidable without specialized technical expertise, causing further damage to the cement production line.

Overload of high-temperature fans due to "material collapse" occurs because dust adhering to the preheater tube walls during the rotary kiln cement production process collapses and falls off when it reaches a certain thickness. This increases dust concentration within the pipes, leading to increased resistance and negative pressure, thus increasing the load on the exhaust fan. Similarly, if there is material collapse or crusting in the vertical flue or preheater, it can also cause airflow fluctuations, disrupting the airflow within the exhaust pipe and causing the high-temperature fan to overload and shut down. Frequent occurrences of this phenomenon damage the high-temperature fan motor. In actual use, the "material collapse" phenomenon can intermittently cause the motor operating current to exceed the normal current by several times for very short periods. If using general-purpose high-voltage frequency converters from domestic and international manufacturers, this may lead to frequent inverter trips during operation, directly affecting the normal operation of the high-temperature fan and the cement production line.

Xinfengguang Company's cement industry-specific frequency converters employ speed-free vector control technology. Addressing the motor overload protection issue caused by material collapse in cement plant high-temperature fans, technicians have utilized vector control principles. When collapse occurs, the frequency converter, based on measured voltage and current signal changes and pre-measured motor parameters, calculates the motor's magnetic flux position, flux amplitude, output torque, and motor speed using a built-in motor model. Then, based on the deviation between this speed and the given speed, the output torque is controlled, ensuring that the frequency converter continues to operate without overload even during collapse. This effectively avoids the repeated tripping caused by material collapse in general-purpose high-voltage frequency converters from other manufacturers, guaranteeing continuous production.

5.2 Power supply fluctuations and operational reliability issues of high-temperature fan frequency converters

The continuous production nature of the cement industry dictates that high-voltage frequency converters used in cement plants need to have high reliability to ensure safe production. Many domestic enterprises are often equipped with multiple production lines, each consisting of main and auxiliary equipment. Whenever large-capacity auxiliary equipment starts up, the power bus voltage experiences a significant drop (exceeding -15%), affecting the normal operation of other equipment on the same bus. To address this issue, Fengguang high-voltage frequency converters have undergone special software design to ensure that the frequency converter continues to operate without shutdown when the grid voltage fluctuates between -35% and 15%.

5.3 Environmental and Heat Dissipation Issues in Inverter Applications

Due to the high dust levels and relatively harsh environment at cement plants, electronic and power electronic devices, which generate approximately 3-4% heat loss and require heat dissipation, typically employ air cooling. Xinfengguang Company's cement industry-specific variable frequency speed control system features a large margin in component selection and design, a unique unit layout, and optimized radiator and air duct design to improve unit heat dissipation, ensuring low system temperature rise and strong environmental adaptability. Practical experience has proven its ability to withstand the harsh environment of cement plants.

6 Project Implementation and Renovation Plan

Based on the actual conditions at the user's site, the bypass cabinet adopts a one-to-one automatic frequency converter switching scheme, as shown in Figure 2. The bypass cabinet contains two high-voltage disconnect switches and two vacuum contactors. To ensure no reverse power is fed to the frequency converter output, KM3 and KM4 are electrically interlocked. When K1, K3, KM1, and KM3 are closed and KM4 is open, the motor operates at the variable frequency; when KM1 and KM3 are open and KM4 is closed, the motor operates at the mains frequency. In this case, the frequency converter is isolated from the high voltage, facilitating inspection, maintenance, and commissioning.

The bypass cabinet must be interlocked with the upstream high-voltage circuit breaker DL. When DL is closed, it is absolutely forbidden to operate the frequency converter output isolating switch to prevent arcing and ensure the safety of operators and equipment.

Figure 2 Automatic bypass cabinet

If the user selects an automatic bypass cabinet and chooses "manual" mode, the specific conversion process is as follows:

Step 1: Perform the shutdown operation of the variable frequency speed control system.

Step 2: Disconnect the AC incoming circuit breaker.

Step 3: Disconnect the input-side disconnector K1 of the variable frequency speed control system (see Figure 2).

Step 4: Disconnect the output side switch K3 of the variable frequency speed control system.

Step 5: Connect the bypass switch KM4 of the variable frequency speed control system.

Step 6: Close all doors (except the control cabinet).

Step 7: Close the AC incoming line circuit breaker.

When the user selects "Automatic" mode, the variable frequency speed control system will automatically switch to mains frequency bypass operation based on the fault condition. The specific switching process is as follows:

When a fault occurs in the JD-BP38 series frequency converter during frequency conversion operation, the controller sends a node signal to control the incoming vacuum contactor KM1 and the frequency converter output vacuum contactor KM3 to disconnect. Then, the controller sends a frequency conversion to power frequency signal to the PLC. After a certain delay, the bypass switch KM4 closes.

Protection: Maintain the original protection for the motor and its setting values ​​unchanged.

The commissioning of the wind-powered high-voltage frequency converter is very convenient. After confirming that the on-site wiring is correct, the basic parameters can be set using the converter's human-machine interface, and the setup is complete. The high-voltage frequency converter was successfully installed and commissioned on the first attempt and put into normal use in September 2009. After nearly half a year of operation and observation, the modification has achieved the expected results.

7. Running effect

The main advantages of using a high-voltage frequency converter to control the high-temperature fan at the kiln tail are as follows:

(1) Stable operation, safe and reliable.

Since the inverter was successfully put into operation, regular inspections have been conducted on various parameters, including the temperature rise of the input transformer, the temperature rise of the power unit, the output voltage, and the output current, and all parameters have been found to be completely normal. The inverter has been operating very stably, providing a strong guarantee for the safe and reliable production of the cement plant. For the user, only periodic dust removal of the inverter is required, without shutdown, ensuring the continuity of cement production.

(2) The motor and unit operate smoothly and all indicators meet the process requirements.

Throughout the entire operating range, the motor ran smoothly with normal temperature rise. The fan started with very little noise and starting current, and there was no abnormal vibration or noise.

(3) The three-phase output waveform of the frequency converter is perfect and very close to the standard sine wave.

After on-site testing, the three-phase output voltage and current waveforms of the frequency converter were found to be very standard, indicating that the frequency converter can fully control the operation of ordinary motors without any special requirements for the motors.

(4) Improved operating conditions reduce the labor intensity of workers.

With the increasing demand for high-temperature blowers in cement plants, adjusting the blower's speed and airflow is crucial to meet production requirements, significantly reducing workload. The unique fly-start function automatically identifies the motor's direction of rotation and tracks its speed to start the blower directly, eliminating the need to stop and restart the motor – a highly convenient feature of variable frequency drive (VFD) starting.

(5) The human-machine interface adopts a full Chinese interface, which displays voltage, current, speed, frequency, various operating status indicators, and fault alarm indicators. The operation is simple and intuitive, which is convenient for Chinese users.

(6) Significant energy-saving effect.

To evaluate the effect of the frequency conversion retrofit of the kiln tail high-temperature blower system, the actual use of the equipment and energy saving were measured and analyzed after the system was put into normal operation. The operating conditions of the power frequency and frequency conversion were basically the same. On a randomly selected normal working day, the system was switched to power frequency operation, and the air volume was adjusted using dampers. The operating data was recorded on the grid side. Then, the system was switched to frequency conversion operation, and the operating data was recorded. The specific data is shown in Table 3 below:

Power consumed when using the mains frequency:

P_{power frequency} = 1.732 * 10.1 * 159 * 0.90 = 2503.3 KW

Power consumed when using frequency conversion speed control:

P_inverter = 1.732 * 9.8 * 96.8 * 0.99 = 1627.1 KW

Energy saving rate: η = (P_power frequency - P_converter frequency) / P_power frequency = (2503.3 - 1627.1) / 2503.3 = 35%.

Assuming 300 days of operation per year, 20 hours of operation per day, and a price of 0.3 yuan per kilowatt-hour, the annual electricity savings for users can be calculated as follows: (2503.3-1627.1)*20*300*0.3 yuan = 1,577,160 yuan.

8. Conclusion

Zibo Luzhong Cement Co., Ltd. has improved its production process and achieved significant economic benefits by retrofitting the high-temperature kiln tail blower with frequency converters. The application of high-voltage frequency converters for energy conservation and consumption reduction in the cement industry is a technological approach strongly promoted by the state. As high-voltage frequency converter technology matures further, more and more cement plants will join the retrofitting effort.

References

[1] Shandong Xinfengguang Electronics User Manual [Z]. Shandong Xinfengguang Electronics Technology Development Co., Ltd.

About the Author

Wang Lei, male, is a technical support engineer employed by Shandong Xinfengguang Electronic Technology Development Co., Ltd.