Analysis of Factors Affecting the Length of Rotary Kiln Flames:

1. Coal powder fineness: The finer the coal powder, the faster the combustion rate. As coal powder becomes finer, its surface area increases, leading to greater contact between the coal and oxygen in the air—and thus accelerating the combustion process, which in turn shortens the flame length.
2. Secondary air temperature: As the temperature rises, combustion speeds up, and the flame becomes shorter.
3. The shape of the coal nozzle affects the mixing of air and coal—the more uniform the air-coal mixture, the faster the combustion rate.
4. The effect of coal volatile matter on flame length: Bituminous coal with high volatile-matter content ignites quickly even at locations relatively close to the nozzle, resulting in a longer flame. Conversely, coal with low volatile-matter content behaves differently (since ignition temperature decreases as volatile matter increases, leading to varying distances between the ignition point and the nozzle—higher-volatile-coal types ignite earlier and sustain the combustion process over a longer distance, thus producing a longer flame). On the other hand, low-volatile-coal types release most of their thermal energy within a very short distance, causing the flame to remain concentrated and relatively short; in some cases, this can even lead to localized overheating.
5. The Influence of Gas Flow Velocity Inside the Kiln on Flame Length

The faster the gas flow rate, the longer the flame; yet the gas velocity is influenced by both the primary air speed and the exhaust airflow at the kiln's tail end. Increasing the primary air speed, on one hand, enhances the effective range of the pulverized coal, causing the flame to elongate. On the other hand, it ensures a more uniform mixing of air and fuel, leading to faster combustion and, consequently, a shorter flame—these are two opposing effects. Meanwhile, to prevent "backfire," the ejection velocity must exceed the flame propagation speed. Notably, the ejection velocity is closely tied to the kiln’s diameter: larger-diameter kilns require higher ejection speeds, while smaller kilns should operate at lower ejection velocities. This approach helps create optimal conditions for efficient heat exchange between the flame and the material inside the kiln. Additionally, increasing the exhaust airflow at the kiln’s tail end boosts the negative pressure there, introducing more secondary air and accelerating the outer gas flow around the flame, which in turn lengthens the flame. In normal production, operators typically adjust the kiln’s tail-end exhaust or reduce the tertiary air supply to lower the kiln’s tail-end negative pressure, thereby controlling the flame length as needed.

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