During rotary kiln operation, if the firing condition becomes abnormal, the temperature profile and product sintering curve deviate significantly from the theoretical one, resulting in unstable kiln operation and poor product quality.
Common abnormal conditions include:
Underfiring throughout the kiln
Large vertical temperature differences across kiln sections
Dead (silent) bricks or clinkers
Overfiring and melted (scorched) bricks
Unstable firing temperature control
Rehydration of preheated materials
This article focuses on the main causes of underfiring in rotary kilns and provides practical solutions to improve the sintering condition
When the entire kiln is underfired, the material bed appears red or dark red, and all fired bricks are insufficiently sintered — resulting in “green” or underfired bricks with low mechanical strength and poor product quality.
If the kiln operator fails to accurately judge the fire intensity — especially due to the difference between daytime and nighttime lighting — a small flame may be misinterpreted as a large one.
This visual misjudgment leads to an incorrect temperature estimate, directly affecting the firing stability and product sintering quality.
In rotary kiln operation, fuel feeding frequency and method are critical.
If operators fail to follow the principle of “frequent, small, and even additions”, the kiln temperature will fluctuate sharply:
Temperature spikes when fuel is added in bulk
Rapid temperature drops when fuel is depleted
Such temperature instability reduces firing efficiency and causes inconsistent product quality.
Fuel quality is another key factor.
If the fuel contains excessive ash, has low calorific value, or overall poor combustion performance, yet is added in the same quantity as high-quality fuel, the total released heat will be insufficient to reach the required kiln temperature.
Inadequate temperature leads directly to underfiring and low-strength products.
If kiln dampers or air valves are opened too wide, excessive air flow causes too much heat to be carried away.
As a result, kiln temperature cannot rise to the desired level, leading to inefficient firing and reduced product performance.
Conversely, if the air valves are set too low, the combustion air becomes insufficient, resulting in incomplete fuel combustion.
Without adequate oxygen, the fuel’s heat release per unit time drops significantly, and the kiln temperature remains low.
This directly results in inadequate sintering and underfired products.
When underfiring occurs, the following steps can be taken to restore proper kiln firing conditions:
Accurate and timely temperature measurement is essential.
Operators should closely observe temperature trends — whether rising or falling — to decide when and how much fuel to add.
This data-driven control helps maintain stable kiln temperature and consistent product quality.
Always follow the principle of “feed fuel according to the flame condition” and add small amounts frequently.
Fuel quantity should match its actual burning rate in the kiln:
Avoid underfeeding, which prevents temperature rise
Avoid overfeeding, which causes incomplete combustion and smoke generation
Proper control ensures stable combustion and uniform heat distribution.
Operators must know the heat value of each fuel batch and adjust the feed rate accordingly.
If the new fuel has higher calorific value, reduce feed slightly
If the new fuel has lower calorific value, increase feed appropriately
This ensures the kiln always maintains the necessary thermal balance for stable firing.
If the kiln has excessive draft and loses heat rapidly:
Reduce the opening of all dampers to achieve a slight positive pressure in the burning zone
Maintain visible backfire at each burner row to confirm positive pressure
Limit air intake at the lower part of the material bed to reduce cold “backwind”
Use dry, low-moisture, high-calorific fuel to raise temperature quickly
Gradually reopen dampers once temperature stabilizes
These steps help recover the fire intensity and restore normal sintering.
If ventilation is too weak and combustion incomplete:
Increase damper openings to establish proper pressure balance (negative pressure in front, positive in rear)
Ensure visible flame return (backfire) in the burning zone
Increase air supply to the insulation and cooling zones, especially the air beneath the material bed, to improve combustion efficiency and heat exchange
This helps restore proper firing temperature, stable combustion, and good product quality.
Underfiring in rotary kilns is primarily caused by poor fire control, uneven fuel feeding, low fuel quality, or incorrect air settings. To restore proper firing conditions:
By following these operational principles, you can achieve stable sintering, improved thermal efficiency, and higher product quality.
For expert advice on refractory selection, kiln insulation, and energy-saving materials, contact us:
📧 sns@firebirdref.com
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