Abstract: Lime, as an important clarification raw material, directly affects the quality and utilization rate of sugar produced from sugar beets. To improve lime yield and quality, a sugar beet factory in Xinjiang used a Siemens S7-300 PLC as the controller and built a distributed control system using the Profibus bus. This system controlled the weighing, feeding, and unloading processes of the lime kiln, and a supervisory control interface was designed using WinCC. Operational results show that the control system is stable and reliable, while reducing labor costs and fuel consumption, and increasing lime yield. It is worthy of promotion in related fields.
Keywords : sugar lime kiln; Profibus; S7-300 PLC; WinCC
0 Introduction
my country is the earliest country to produce and utilize lime. Lime is widely used in steelmaking and construction, and plays an important role in beet sugar production. In the process of producing white sugar, a very important step is the purification of sugar juice. At present, beet sugar factories at home and abroad generally use lime and CO2 as clarifying agents to produce high-quality white sugar. The amount used is generally 2-3% of the beet processing volume. It is mainly used to adsorb and remove harmful elements such as N, P, and organic matter from sugar raw materials (beets) after producing light calcium carbonate in sugar production. The light calcium carbonate produced by high-quality, fast-reacting, and thorough lime and CO2 can improve the purification speed, shorten the purification time, reduce the consumption of limestone per ton of sugar, and reduce the residue of pigments and pseudo-melanin, thereby effectively improving the quality of white sugar products [1]. Therefore, the demand for lime in beet sugar factories is relatively large. In order to reduce production costs and increase product price competitiveness, beet sugar factories generally build lime kilns to produce lime and CO2 gas needed for sugar production.
A sugar beet processing enterprise in Xinjiang has a daily processing capacity of 3,100 tons of sugar beets and an annual processing capacity of 400,000 tons of sugar beets. The lime required for its sugar production is mainly produced by a 150m³ mechanical vertical kiln, and the main fuel is coal gas [2]. Since the lime kiln is mainly operated manually, it leads to adverse results such as high lime production costs and unstable product quality. In order to solve these problems, it was decided to adopt an advanced control method to centrally control the entire production process of the lime kiln, improve production quality and yield, and reduce production costs.
1. Production process of lime kilns
The production of lime kilns has a strict process, and it can vary depending on the geographical environment and weather conditions. Based on the experience of technicians over many years, the main operating steps in lime kiln production are as follows [3]:
(1) Kiln Drying: First, clean the inside of the kiln thoroughly. Then, install the kiln drying grate frame, place about one cubic meter of wood on the grate, and ignite it with an open flame. Continue burning the flame, intermittently adding more wood to gradually raise the temperature inside the kiln, drying out the moisture and preventing cracks in the brick walls. The kiln drying process takes 5 to 7 days. The temperature is controlled at around 50 degrees Celsius on the first day, around 100 degrees Celsius on the second day, around 200 degrees Celsius on the third day, and around 500 to 600 degrees Celsius on the fourth day, until the kiln is completely dried. Afterward, extinguish the fire, allow the temperature to drop, and remove the kiln drying grate. The entire kiln drying process is now complete.
(2) Kiln loading: First, clean the inside of the kiln and the surface of the ash removal pan. Then, pour lime blocks into the ash removal pan and fill it to about 80cm below the first layer of fire holes. Then, place the wood on top of the lime blocks in the kiln. First, fill one layer (about 20 to 30cm). Then, leave a 40 to 50cm wide passage between the two ends of the first layer of fire holes. Continue to neatly place the wood on both sides of the passage upwards until it is above the first layer of fire holes, 20 to 30cm above the fire holes. Then, completely seal the top with wood and continue to place wood until it is 1 to 2 meters above the second layer of manholes. After the kiln is lit, completely seal the manholes.
(3) Ignition: First, soak the wood shavings in diesel fuel (approximately 50 kg of shavings and 15 kg of diesel fuel) to facilitate ignition. Insert the soaked shavings into the pre-reserved wood channels through the first-layer burner. Before ignition, spray a mixture of flammable diesel and gasoline (5 kg of diesel fuel and 5 kg of gasoline) into the first-layer burner. Prepare two wooden handles (approximately 1.5 meters long) and two large pieces of cloth. Wrap one end of the wooden handle with the cloth and soak it in diesel fuel. After the gas pump is turned on and the gas extraction rate in the kiln is normal, ignite the oil-soaked wooden handles and insert them into the wood channels through both ends of the first-layer burner to begin burning. The wood will continue to burn, and the temperature inside the kiln will gradually rise. The stone material inside the kiln will gradually decrease. Depending on the situation inside the kiln, gradually add material to maintain the material level within the normal range (approximately 2 meters below the material distributor) until the wood is completely burned. Then, gas will begin to enter, and the fire level will gradually rise. Once the fire level is maintained at layer 5, unloading can begin, with the unloading temperature controlled at approximately 100 degrees Celsius.
The next step after kiln ignition is to increase and maintain the calcination temperature. In lime kilns, the feeding and ash removal processes are mostly controlled by direct digital logic. Replacing manual control with PLC control is expected to optimize efficiency, so this will not be elaborated upon here. Controlling the calcination temperature within the kiln is the focus of this study. The temperature control requirements and steps for the lime kiln are as follows: The first layer temperature is controlled at approximately 200 degrees Celsius; the second layer at approximately 500 degrees Celsius; the third layer at approximately 900 degrees Celsius; the fourth layer at approximately 1000 degrees Celsius; and the fifth layer at approximately 900 degrees Celsius. The kiln gas outlet temperature is controlled at approximately 100 degrees Celsius. After normal combustion, the first layer temperature can be maintained at approximately 100 degrees Celsius. The calcination layers are controlled within the range of 3 to 5, with the calcination temperature controlled within the range of 1000 ± 12 degrees Celsius. The basic structure of the pump station is shown in Figure 1. To ensure the system's efficiency and stability, two parallel water supply lines are used for selective operation. Meanwhile, before the water from the source enters the pumping system, it passes through a filtration tank to remove various impurities present in the water, ensuring the safety of the system during operation.
2. Composition of the lime kiln control system
According to the different roles of each part in the system, the entire control system is divided into three layers: management layer, control layer, and equipment layer. The main function of the host computer IPC is to manage the production equipment and its operating status at the bottom layer through online monitoring of the configuration software; each module of the S7-300 PLC acts as the core controller, completes the collection of equipment status information, and realizes automated production through logical judgment and data analysis; the main functions of the bottom equipment are signal conversion of field status information, process execution, etc. The communication method between the host computer and the S7-300 CPU module and between the CPU and the remote interface ET200M station is Profibus bus. The three-layer structure of the system is shown in Figure 1 [4][5].
Figure 1. Structure of the lime kiln control system
The on/off logic control of the lime kiln temperature control system includes feeding and ash unloading. It also uses actuator error alarms, material mass upper limit alarms, and air pressure signal alarms as interruption conditions, and implements interlocking control with operating motors and other equipment. For example, during the lime kiln feeding process, the weighing hopper gate opens; when the material mass exceeds the upper limit, the material conveying hopper automatically closes. These logic controls are programmed in STEP7, and a partial variable table is shown in Figure 2.
Figure 2 Variable table in STEP7
The feeding program is written according to production requirements. This is the foundation of temperature control because accurate feeding quantity makes the calcination process more stable and reduces fluctuations, thereby reducing under-calcination and over-calcination and improving the utilization rate of limestone. As shown in Figure 3, the program on the right is the stone weighing program for the large kiln. The PIW336 is the stone weight sensor that transmits the stone weight signal to the PLC, which is then converted and placed in DB3.DBD0. The program on the left is the automatic interruption program for the small kiln, which cuts off the hoist and other equipment when the material exceeds the upper limit.
Figure 3 shows the material feeding process.
3. Monitoring screen design
After the control system is completed, all control and monitoring can be operated on the monitor of the host computer. WinCC, which is fully compatible with Siemens PLC, is used. WinCC is a software tool for monitoring and SCADA. It can provide functional modules such as display of industrial dynamic flowcharts, alarm processing, display of real-time and historical curves, production reports and storage. With high-performance process coupling, fast screen updates and reliable data, WinCC has high visualization and practicality[6].
WinCC integrates technologies such as graphics, human-machine interface, database, and communication, enabling developers to complete the design of monitoring software through a dialog-based configuration method. Users can easily communicate by combining PLC with WinCC, and it is easy to shorten the debugging time in the project by combining STEP7 with WinCC [7]. In addition, WinCC software is an open software suitable for monitoring applications, and each software provider can develop its own application software to connect with WinCC's open interface.
The basic main flow chart of the system process is shown in Figure 4, which dynamically displays the on-site process of the lime kiln, including the feeding process and the status of operating equipment. This allows for timely and accurate monitoring of the limestone calcination and decomposition, and enables real-time fault diagnosis.
Figure 4 shows the material feeding process.
The actual field instrument monitoring system implements both automatic and manual modes. Manual operation is used for step-by-step verification during the installation and commissioning of all system hardware and software. Once the system is operational and running normally, automatic operation is employed, utilizing the designed ladder diagram control program to achieve precise control, particularly for analog data such as weighing and flow rate.
During the control process, important analog control parameters such as wind pressure, material quality, and fuel flow rate are monitored and alarmed in real time on the main screen. This includes audible and visual alarms for parameters exceeding limits, prompting the cause of the fault via a dialog box, and automatically adjusting back to normal monitoring status. The entire process not only predicts changes but also saves historical curve data for each time period. After analyzing and comparing the curves, corresponding predictions are made regarding the trend of temperature variables, as shown in Figure 5.
Figure 5. Calcination temperature curve of lime kiln
4. Conclusion
The lime kiln control system adopts a typical industrial control scheme. The hardware and software are respectively S7-300PLC and WinCC. The control system is built using Profibus. It completes a series of steps in lime kiln production, such as feeding, weighing, and unloading, reducing labor costs, improving the working efficiency of sugar lime kilns, and realizing real-time status monitoring and data management of the entire lime kiln production. It provides a reference for lime kiln production in related sugar enterprises.
References:
[1] Ma Ji. Control and Regulation of Vertical Lime Kiln in Sugar Factory [J]. Guangxi Light Industry, 2010, (1): 13-14
[2] Shi Hao. Optimization Design and Application of Technical Upgrading Project of Guitang 270m3 Lime Kiln [D]. Guangxi University, 2009
[3] Dalian Light Industry College, Qiqihar Light Industry College. Beet Sugar Making Technology [M]. Beijing: Light Industry Press, 1982, 55-240.
[4] Wang Huaqiang. Lime Kiln Production Process Control System [J]. Journal of Hefei University of Technology (Natural Science Edition), 2006, (2): 31-33
[5] Zhang Yungang. From Beginner to Expert: Siemens S7-300/400 PLC Technology and Application [M]. Posts & Telecom Press, 2009, 100-320.
[6] Su Kunzhe. Siemens WinCC V6 Explained in Simple Terms [M]. Beijing University of Aeronautics and Astronautics Press, 2005, 1-19
[7] Liu Huabo, Wang Xue, He Wenxue. WinCC Configuration Software and Its Applications [M]. Machinery Industry Press, 2009, 32-120.
Class 085, Electrical Engineering Department, South Campus, Xinjiang University, Tianshan District, Urumqi, Xinjiang (830047)
13579414730
