In a modern dry-process cement precalciner kiln system, the primary air, secondary air, and tertiary air are all essential for fuel combustion, yet their sources, destinations, functions, and control priorities differ significantly.

Remember this in one sentence: primary air delivers the coal, secondary air fires the kiln, and tertiary air fires the furnace.

The essential differences among the three winds

Primary air: delivers coal and shapes the flame; the less, the better.

Primary air is supplied by the primary air fan. At the kiln head, primary air is delivered into the kiln via a multi‑stage burner, which typically comprises coal air, inner air, outer air, and central air. At the kiln tail, primary air enters the decomposition furnace through the kiln‑tail burner.

It serves three purposes:

1. Coal-powder conveying: delivers coal powder from the kiln head and kiln tail into the kiln and the combustion zone of the decomposition furnace.

2. Provide initial oxygen: helps the volatile matter of pulverized coal ignite.

3. Flame shape control: the inner air flow influences swirl, the outer air flow affects flame length, and the central air flow protects the burner nozzle.

The primary air is cold; once it enters the kiln, it carries away heat. Therefore, the guiding principle is: while ensuring adequate pulverized‑coal delivery, preventing backfire, and maintaining a stable flame, the primary air flow should be kept as low as possible—typically accounting for a fraction of the total combustion air. 8%-12% 。

If the primary air flow is too high, the flame elongates, the temperature drops, and heat consumption increases; if the primary air flow is too low, pulverized coal cannot be conveyed effectively, backfiring is likely, ignition becomes difficult, and it is hard to establish a stable circulating flame.

Secondary air: the main driver of kiln firing; higher temperatures lead to greater fuel savings.

The secondary air originates from the high-temperature zone of the grate cooler and is hot air that, after being heated during clinker cooling, is recirculated back into the kiln.

Its main function is:

1. It supplies the primary oxygen to the kiln-head pulverized coal and serves as the main combustion zone within the kiln.

2. It introduces a substantial amount of high-temperature heat, significantly reducing the system’s thermal consumption.

3. It participates in gas–solid heat exchange within the kiln, thereby influencing the temperature of the calcination zone and clinker sintering.

4. Reflects the cooling performance of the grate cooler: High secondary air temperature indicates effective clinker cooling and efficient heat recovery.

The secondary air temperature is generally desired to be maintained at 1000 Above a certain temperature, higher temperatures are more conducive to reducing coal consumption. If the secondary air temperature drops, common causes include an excessively thin material bed in the grate cooler, insufficient cooling, system air leakage, or abnormal kiln air usage; conversely, low secondary air temperature further lowers the firing temperature.

Third air: the main driving force of the kiln, determining the capacity of the decomposition furnace.

The tertiary air also originates from the grate cooler, but is delivered to the decomposition furnace via the tertiary air duct to supply oxygen for coal‑powder combustion within the furnace.

Its main function is:

1. Supports decomposition furnace combustion: The decomposition furnace typically undertakes 55%-65% The fuel combustion mission.

2. Kiln inlet decomposition rate: The normal kiln inlet decomposition rate should generally reach 92%-95% 。

3. Recovering waste heat from the grate cooler: to take 800-1100 ℃ Hot air is directly fed into the decomposition furnace, reducing the system’s thermal consumption.

4. Balancing kiln air: The ratio of secondary air in the kiln to tertiary air from the calciner is adjusted via the tertiary air damper.

The tertiary air damper is a critical component of the kiln system, determining how much hot air is directed to the decomposition furnace and how much is fed into the kiln.

The most critical logic: the secondary and tertiary air flows are inversely related.

Both the secondary and tertiary air streams originate from the high-temperature zone of the grate cooler. Under relatively stable total draft conditions, increasing the tertiary air flow reduces the secondary air flow within the kiln, while decreasing the tertiary air flow increases the secondary air flow.

Therefore, one should not consider the “degree of opening of the tertiary air damper” in isolation; instead, it is essential to assess whether the kiln’s oxygen content is properly matched.

• Decomposition furnace outlet O ₂: Generally kept at 2.0%-3.0% 。

• Kiln tail flue chamber O ₂: Generally kept at 1.5%-2.5% 。

If the kiln tail CO An increase in temperature and the presence of a reducing atmosphere indicate insufficient secondary air in the kiln; if the decomposition furnace is burning poorly, C5 An elevated outlet temperature and a reduced decomposition rate indicate insufficient tertiary air.

Key Points for On-Site Assessment

• Observe the flame through the primary air: Ensure the flame is smooth, vigorous, and of appropriate length, with no backdraft or excessive elongation.

• Secondary air temperature: High secondary air temperature, a stable burning zone, and an appropriate kiln outlet temperature.

• Three‑air flow decomposition rate: The decomposition furnace temperature is stable, the decomposition rate at the kiln inlet meets specifications, and the thermal load within the kiln is not excessive.

• Secondary and tertiary air: check oxygen balance at the decomposition furnace outlet. O ₂ and the kiln tail flue chamber O ₂ Only when both are properly balanced can the kiln’s air supply be considered well-matched.

The lower-level sequence of decisions is as follows: kiln inlet decomposition rate → distribution of thermal load between primary and secondary coal feeds → allocation of secondary and tertiary air. Air is not adjusted independently; it must follow the coal, which in turn follows the thermal load, ultimately determined by the kiln inlet decomposition rate.

Cement kiln burner, oxygen-enriched burner, low-NOx burner

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