Factors Affecting Flame Length of Rotary Kiln

The length of the rotary kiln flame directly influences the position of the burning zone, kiln coating and refractory lining service life. An excessively long flame results in an elongated burning zone, elevated kiln‑inlet temperature and a high tendency for rear ring formation. An excessively short flame causes concentrated high‑temperature zones, erosion of kiln coating and refractory bricks, and risk of over‑burned clinker.

1. Fuel‑related Factors

1. Pulverized coal fineness

Finer pulverized coal burns faster, producing a shorter flame. Coarse coal particles require longer combustion time, leading to a longer flame. Over‑coarse coal may cause post‑combustion and result in ring formation at the kiln inlet.

2. Volatile matter of pulverized coal

High volatile matter enables rapid ignition and a short flame. Low volatile matter delays ignition and lengthens the flame.

3. Moisture content of pulverized coal

High moisture consumes heat during coal pre‑heating and decomposition, delaying ignition and extending the flame.

4. Fuel consumption

Under constant air volume, increased coal feed lengthens the overall flame; reduced coal feed shortens the flame.

2. Primary Air (Burner Air)

1. Primary air volume

Large primary air volume ensures sufficient oxygen‑coal mixing and fast combustion, creating a shorter flame.

Insufficient primary air leads to inadequate oxygen supply and delayed combustion, lengthening the flame.

Note: Excessive primary air is not recommended, as it reduces kiln temperature and increases heat consumption.

2. Primary air velocity

High velocity produces strong jet penetration and elongates the flame.

Low velocity yields weak jet flow, so pulverized coal burns near the kiln mouth, forming a short and thick flame.

For multi‑channel burners, adjusting the ratio of inner and outer air is the key measure for flame control:

- Increase outer air (axial air): higher velocity, longer flame

- Increase inner air (swirl air): stronger swirl, entrainment of high‑temperature flue gas, earlier ignition, shorter flame

3. Secondary Air and Tertiary Air

1. Secondary air temperature

High secondary‑air temperature accelerates coal ignition and shortens the flame. Low secondary‑air temperature delays ignition and lengthens the flame.

2. Secondary air volume

Adequate secondary air provides sufficient oxygen for accelerated combustion and a shorter flame. Insufficient secondary air causes severe post‑combustion and a longer flame.

3. Tertiary air

High tertiary‑air flow lowers secondary‑air temperature and reduces oxygen at the kiln head, lengthening the flame. Reduced tertiary‑air flow raises secondary‑air temperature and shortens the flame.

4. Kiln Operating Conditions and System Ventilation

1. Kiln‑inlet negative pressure and total system air flow

High kiln‑inlet negative pressure means large system exhaust and high gas velocity inside the kiln, which drags the flame backward and lengthens it. Low negative pressure and weak ventilation result in a shorter flame.

2. Material filling ratio and kiln speed

Higher kiln speed moves materials faster, shifting heat demand rearward and relatively lengthening the flame. Lower kiln speed moves the burning zone forward, creating a shorter flame. Excessive material bed thickness obstructs gas flow and distorts flame shape.

3. Ring formation and coating build‑up

Rear ring formation reduces the kiln cross‑section for ventilation, increases flow resistance and gas velocity, and elongates the flame. Thick coating at the kiln mouth compresses the flame and shortens it.

5. Burner Position

- Extending the burner further into the kiln: longer flame

- Retracting the burner outward (closer to kiln mouth): shorter flame

Simple Adjustment Guidelines

To shorten the flame: increase swirl air, raise secondary‑air temperature, grind coal finer, moderately reduce kiln‑inlet negative pressure, retract burner outward.

To lengthen the flame: increase axial air, moderately coarsen pulverized coal, raise kiln‑inlet negative pressure, extend burner deeper into kiln.

Reminder: Flame length shall not be adjusted in isolation. Consider kiln coating status, clinker quality, NOₓ emissions and phosphate brick performance. The flame must not impinge directly on refractory bricks.