Zhuojin Pipe Fitting Co., LtdQuantitative Coal Feeding Device and Microcomputer Control System for Three-Channel Coal Powder Burner
Overview and Technical Context
This 1989 paper by Li Rongfan from Liaoning Xiaotun Cement Plant describes a quantitative coal feeding system for a three-channel coal powder burner, incorporating a microcomputer-based control system. The system uses a weight signal from a metering hopper supported on load cells to automatically calibrate and correct the measurement error of an impulse-type flowmeter installed at the hopper inlet. This approach was presented as a method to improve the accuracy of the impulse flowmeter, which is inherently prone to drift and calibration errors.
System Architecture and Control Logic
The control system architecture can be described as follows:
| Component | Function | Signal Type |
|---|---|---|
| Impulse flowmeter | Measures volumetric flow of coal powder at hopper inlet | Pulse frequency |
| Metering hopper | Temporarily stores coal powder for weight-based verification | Analog weight signal |
| Load cells | Measure hopper weight to determine actual coal mass | Analog electrical signal |
| Microcomputer | Compares flowmeter output with weight-based measurement, calculates correction factor | Digital control signal |
| Feed actuator | Adjusts coal feed rate based on microcomputer command | Analog or digital |
The fundamental principle is a closed-loop feedback system where the weight-based measurement serves as a reference standard against which the flowmeter output is continuously validated. When a discrepancy is detected, the microcomputer adjusts the flowmeter calibration factor or directly corrects the feed rate signal to maintain the desired coal feed rate.
Engineering Significance and Analysis
The innovation in this system lies in its self-calibrating capability. Traditional coal feeding systems rely on periodic manual calibration of flowmeters, which is labor-intensive and cannot account for real-time changes in coal properties (moisture content, particle size distribution, bulk density). The microcomputer-based approach provides continuous, automatic correction, which is particularly important for rotary kiln operations where coal feed rate directly affects:
- Kiln temperature profile: Too much coal causes overheating, too little causes underfiring.
- Clinker quality: Incorrect coal feed leads to poor clinker mineralogy and poor setting properties.
- Fuel efficiency: Over-firing wastes fuel; under-firing reduces throughput.
- Burner and refractory life: Excessive coal feed can cause flame impingement and refractory degradation.
Integration with Process Control
From a process control perspective, this system represents an early example of instrument redundancy and cross-validation. The use of two independent measurement methods (volumetric flowmeter and gravimetric weighing) to cross-check each other is a principle that remains relevant in modern process control. In contemporary practice, this approach would be implemented using a DCS (Distributed Control System) with advanced process control (APC) algorithms, but the fundamental concept of cross-validation remains the same.
The paper also touches on the broader challenge of coal quality variability. Coal used in cement kilns can vary significantly in moisture content, volatile matter, ash content, and particle size distribution from batch to batch. These variations affect the bulk density of the coal powder, which in turn affects the volumetric-to-mass conversion factor used by the flowmeter. The automatic calibration system addresses this variability by continuously updating the conversion factor based on actual weight measurements.
Practical Considerations and Limitations
Several practical considerations arise from this system design:
- Load cell accuracy: The system's overall accuracy is limited by the accuracy and stability of the load cells. Drift in load cell output due to temperature changes, mechanical creep, or electromagnetic interference can introduce errors.
- Hopper dynamics: The metering hopper must be designed to minimize material segregation, bridging, and rat-holing, which can cause fluctuations in the weight signal that are not representative of actual feed rate changes.
- Microcomputer reliability: In 1989, microcomputers were less reliable and less robust than modern industrial controllers. The system would have required careful design for electromagnetic compatibility, environmental protection, and fail-safe operation.
- Response time: The weight-based measurement has an inherent lag compared to the instantaneous flowmeter reading. The control algorithm must account for this time delay to avoid oscillations or instability.
Study Insights
This paper, while focused on cement industry applications, demonstrates a universal engineering principle: redundancy and cross-validation in measurement systems improve reliability and accuracy. The concept of using a secondary measurement method to validate a primary one is applicable across all industries, from oil and gas pipeline flow measurement to pharmaceutical dosing systems. The use of a microcomputer for automatic calibration was innovative for its time and foreshadowed the widespread adoption of intelligent instrumentation in process industries.
Conclusion
The quantitative coal feeding system described in this paper represents a practical and effective approach to improving feed control accuracy in cement kiln burners. By combining impulse flowmeter measurement with gravimetric verification and microcomputer-based automatic correction, the system addresses the inherent limitations of single-instrument measurement in the face of variable coal properties. The principles of cross-validation, closed-loop feedback, and automatic calibration remain highly relevant in modern process control, and this early work serves as a valuable historical reference for understanding the evolution of industrial automation in cement manufacturing.