Vertical roller mills are often operated at lower capacities to safeguard the process against variables in feed characteristics. Using intelligent automation tools for mill control, operators can consistently obtain a good quality product while running the mill at its optimum capacity.
The kiln is at the heart of the cement manufacturing process and justifiably attracts most attention from plant operators. Grinding mills, on the other hand, are considered secondary and are not as frequently monitored as the kilns. Most of the time, the grinder is operated at less than its optimum capacity. This is done to get some margin for errors that may arise due to variations in mill process variables, for e.g., variations in feed material characteristics. Continuous monitoring and timely adjustments of the grinder feed can significantly impact overall productivity. An advanced vertical roller mill (VRM) controller from Loesche GmbH, helps operators to do just that.
Loesche (www.loesche.com) is one of the leading suppliers of roller mills for more than a century and caters to both the hardware and software needs of cement manufacturers. The company has been providing automation tools such as mill simulators and SCADA solutions to cement manufacturers. Recently, they have launched an advanced mill controller called LMmaster for VRM control. It is the standalone real-time process optimisation solution for an optimal operation of the Loesche grinding systems. The focus of the LMmaster is on efficient performance, increase mill throughput, ensure product consistency and increase plant availability. There is no additional engineering work necessary after the implementation of the solution.
Predictive control
The LMmaster applies Model Predictive Control (MPC), a specific mathematical model for imaging of dynamic processes with multiple interacting variables. With the model-based control, all PID-controllers are contained in one model so the process can be considered as a whole with respective influences of each variable. The LMmaster calculates the future state of the system and uses these values to define the new operaing points of each controller. For the calculation of operating points to the current set points, the current actual values, the disturbances as well as historical data are taken into account. This model calculates the new set points for the PLC, which are taken from the necessary control variables and outputs. This happens continuously and in real time to avoid any delays.
The system displays monitored variables and the current mill configuration continuously to improve operational visibility. The tool automatically initiates accurate control actions several times per minute and substantially increases mill performance.
The technology has an inherent capability for disturbance rejection. This feature is highly desirable in the grinding circuit control as the material hardness and nature of feed streams can vary rapidly and disturb the control circuit. The mill automatically adjusts its settings as per the changes in feed hardness, to promote consistent grinding.
System safety
The software is installed on a PC and it seamlessly integrates with the existing PLC system. The flexibility of setting-up detailed and accurate mill control models ensures that the in-house proprietary knowledge and intellectual property remains protected. The system allows multiple authorised users to access the system and monitor performance through a browser-based interface. Grinding mills have several components that can be damaged due to out- of -control vibrations. The LMmaster plays a key role in minimising these vibrations by calculating fresh set points several times a minute to maintain mill performance at very high levels. By specifying the overall – and competing – requirements for production rate, product consistency and energy consumption, the optimal combination of set points can be calculated on a high-frequency cycle to drive the process to the economic optimum continuously. The system is capable of taking corrective actions in time and hence, highly effective in preventing mechanical damage to the mill. Operators rarely get the opportunity to understand such problems in advance and reconfigure the mill parameters.
The LMmaster is valuable in such situations as not only does the mill operate at lower vibration levels, it also effectively reduces wear on gearboxes and other major components. Continuous online monitoring of the system reduces the likely hood of mill trip. The software applies Loesche’s expertise in mill design to run the mill to its maximum capacity without putting the machinery to any unnecessary mechanical stress.
Holistic control
In conventional PID systems, a single variable set point is compared to the controlled variable at regular time intervals. Different variables are monitored and are a part of separate control loops. In conventional systems, these loops remain disconnected with each other, leading to sub-optimal control. Accurate control and complete optimisation can be obtained only if the responses from the loop are integrated to work as a single system. The dynamically interrelated loops have to be fine- tuned to configure the entire system and achieve higher process efficiency.
Under stable conditions, the controller helps to increase production, while under conditions of stress, such as high vibration, the controller simultaneously acts to reduce vibration, protect process stability and restore production without compromising on product quality.
Conclusion
Mill performance can be significantly improved if process variables are looked at holistically and system configurations are changed dynamically to meet set objectives of the throughput. The system takes complete charge, running the mill to peak performance without compromising on product quality, while simultaneously lowering specific power consumption and vibration levels. The mill is consequently protected from mechanical stress which can lead to wear or damage gearboxes.
Case Study VRM optimisation for Nuh Cimento Sanayi
Founded in 1966, Nuh Cimento Sanayi, a Turkish cement company produces in-house quarry cement for the local Turkish market and for export. The high export quota has forced manufacturers towards more efficient production methods.
The plant manufacture uses conventional control technology and an efficient mill at a high level of raw meal for the subsequent production of cement. Nuh Cimento Sanayi’s goal was to increase the throughput of the mill with reduced energy use and increased availability of the system. The company decided to increase the plant efficiency by investing in a Loesche LMmaster . The solution was applied first to the raw material mill RM3, a vertical roller mill LM 63.4 with a capacity of 505 t/h and a specific power consumption of the main drives (mill motor, mill fan and classifier) of 15,8 kWh/t.
The results
The application at Nuh Cimento Sanayi in Hereke decreased specific energy consumption significantly. The plant now runs in a more stable and smoother fashion, with higher availability of the grinding system and reduced variation in product quality. SinceNuh Cimento owns the power plant on site, the reduction of the specific energy consumption is combined with reduced CO2 emissions.
"The LMmaster is taking over the grinding process and this gives our operators more time to concentrate on other processes," says Ismail Dogan, Clinker Production Engineer at Nuh Cimento Sanayi.
Key benefits realised
- Rise in capacity of 5.8 % (29.4 t/h).
- Reduction in specific power consumption of 4.95 % (0.78 kWh/t).
- Improved consistency in operations.
- Mill vibration reduced by 17.25 % (1.04 mm/s).
- LMmaster controller utilisation greater than 90 %.
- Substantial relief of the operator.
Since The LMmaster is installed on a separate industrial PC with its own software platform, there is no security threat to the installed control system. Another safety aspect is the switch back option that allows the operator to switch smoothly to the original PLC/DCS mode. Tellingly, the company never did switch back to the original PLC/DCS mode.
Key benefits of automating mill control
- Improvement in production rates.
- Better drying capacity via mill exit temperature control.
- Low product size variability achieved by consistent grinding irrespective of variations in feed hardness.
- Lower specific power consumption.
- Improved mechanical stability and reduced vibrations.
- Corrective actions taken on the fly to prevent damage to mill components and prevent a mill trip.