1. Why does a strong primary air flow result in a shorter flame? The main reasons are as follows:

In actual cement kiln operation, a large primary air flow produces a short flame, which is the opposite of the theoretical expectation that greater primary air should yield a longer flame. Why is this? Because when the primary air volume is high, the jet velocity is strong and the flame reach is long—this holds true under idealized conditions. However, the kiln flame is influenced by numerous factors, so its length also depends on other considerations. Moreover, the flame length in the kiln is not solely determined by the primary air rate; many additional factors come into play—for example, the combustion rate of the coal powder significantly affects flame development. When the primary air is abundant, both the flame reach and the combustion rate increase; conversely, with reduced primary air, the flame reach shortens and the combustion rate slows down. In short, flame reach and combustion rate are both directly proportional to the primary air volume. Yet one factor tends to elongate the flame while the other tends to shorten it. Acting simultaneously on the flame in opposing directions, these effects partially cancel each other out, leaving the net change in flame length to be governed by whichever factor predominates.

Typically, the rate of fuel combustion exerts a greater influence on flame characteristics than the flame’s reach. Consequently, in practical operation, when the primary air is excessive and the fuel receives sufficient oxygen for reaction, the flame is short; conversely, when the primary air is insufficient and the kiln‑head fuel lacks adequate oxygen, the flame tends to be long—this is entirely consistent with physical principles. However, this observation applies under normal conditions. In special cases—such as high air flow with low coal or low air flow with high coal—the situation differs, because the combustion rate depends not only on the excess‑air ratio but also on the temperature of the combustion air.

2. Why does the flame sometimes fail to enter the rotary kiln after it has been shut down? The main reasons are as follows:

After the rotary kiln is shut down and before resuming continuous operation, it sometimes happens that the flame fails to extend into the kiln and is instead forced back, resulting in flames erupting from the kiln head and the cooler. If this condition persists, calcination becomes impossible, leading to secondary overheating of the raw meal and another shutdown. The primary cause is the entry of incompletely decomposed material into the burning zone, where it absorbs substantial heat and undergoes vigorous decomposition, generating large amounts of carbon dioxide. This surge of CO₂ into the front of the kiln lowers the combustion temperature—essentially, the required heat exceeds the available heat, causing the flame to be pushed back. Additionally, during the shutdown, to maintain a controlled flame length and prevent excessive tail‑end temperatures, the draft is reduced too much, leading to incomplete coal‑powder combustion and inadequate, timely exhaust gas removal. When the kiln is restarted, the draft remains insufficient, further preventing the flame from penetrating and causing it to be compressed back, which in turn triggers flame ejection at the kiln head. Therefore, in… After shutdown and before resuming continuous operation, the exhaust ventilation should be appropriately increased. , or at least a certain excess air coefficient.

Rotary Kiln Burner–Related Technical Information

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