Model-Free Adaptive Control
of Rotary Kilns
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| • Anti-Delay
MFA handles large time delays in
kiln temperature loop. |
• Temperature control is improved by at
least a 50% reduction in temperature variability. |
| • Prevents product overheating. |
• Fuel or coal consumption is sharply reduced. |
| • Feedforward and robust MFA controllers
deal with large load changes. |
• Temperature and other key process variables
remain under automatic control all the time. |
| • Reduces variation in temperature and product
quality variable. |
• Product quality, product grade, and production
efficiency are improved and scrap is reduced. |
| • Improves efficiency & productivity. |
• Full investment is returned in months
if not sooner. |
An Anti-delay
MFA controller with feedforward
and robust functions manipulates coal flow
intelligentlyto achieve tight kiln temp (blue) control.
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Case History:
MFA Control at Zhong-Zhou Alumina Plant
A sintering process uses low-grade ore
to manufacture alumina (aluminum oxide). A rotary kiln
treats slurry mixed with lowgrade ore, limestone and
caustic soda at a high temperature. The kiln is coal-fired
to a temperature of 1200-1300 °C. The slowly rotating
kiln is tilted to allow material to exit. The material
goes through a digestion/clarification process to extract
aluminum ore from bauxite and refine it to produce alumina.
Large time delays and other disturbances make control
a challenge. Kiln temperatures must be kept in spec,
although temperature fluctuations as high as +/-100
to 200 °C are common. This is critical: too high
a temperature causes excessive reactions and damages
the refractory lining, yet too low a temperature causes
insufficient reactions and poor yield. A long time delay
exists between the coal feeder speed change to the time
it takes for burning coal to affect kiln temperature.
The delay also varies when the material load and rotation
speed change.
The process has a large Tau/T ratio of 5 (Tau=delay
time, T=time constant). Since PID controller can only
deal with a Tau/T ratio of 1 without de-tuning, kiln
temperature loops are typically detuned or in manual
control. The process also has disturbances from variations
in ore inflow, suction airflow, and coal BTU value.
The rotary kiln is now controlled by a PLC monitored
by HMI software running on PCs. MFA
control software provides advanced regulatory
control for the kiln, networked via the HMI software.
An Anti-Delay
MFA controller maintains kiln temperature
by manipulating screw speed of the coal-feeding machine.
It compensates for disturbances and uncertainties and
implements robust bounds to prevent the furnace from
running too hot or too cold.
A Feedforward
MFA controller adjusts for slurry feed
variations, and another MFA controller manipulates airflow
based on the cross-limited control scheme to adjust
the fuel-air ratio.
With MFA control, the kiln can be automatically controlled
under all conditions. Temperature is controlled within
the +/-30 °C specification. Major energy savings
are achieved due to less temperature variations and
better combustion.
Manual control often produces low quality product or
scrap. MFA control results in higher efficiency, quality
and yield. The plant verifies that MFA control produces
high-grade product in each batch.
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