Tertiary air duct

A tertiary air duct (TAD) carries hot combustion air from the clinker cooler to the calciner in a modern cement kiln system. It allows fuel to be burned in the calciner using heat recovered from clinker cooling rather than drawing all combustion air through the rotary kiln.

The TAD is a large refractory-lined duct carrying hot, dusty, oxygen-rich air. It is essential to calciner stability and overall kiln heat balance.

Operational issues

Common TAD problems include coating build-up, alkali-sulphate deposits, refractory wear, abrasion from entrained clinker dust, thermal expansion damage, damper sticking, false-air leakage and local hot spots. Build-up can reduce cross-section, shift air distribution and destabilise calciner combustion.

Because TAD gas is hot and oxygen-rich, access is difficult and cleaning usually requires careful isolation and cooling. A small restriction can have a large effect on pressure balance between cooler, calciner and kiln.

Design and maintenance implications

Important design details include duct slope, refractory selection, expansion joints, inspection doors, meal or dust ingress control, damper access and how deposits can be removed without damaging refractory. Operators monitor TAD pressure drop, calciner temperature, cooler pressure, oxygen, CO and kiln stability to infer duct condition.

Alternative fuels can worsen TAD build-up if chlorine, sulphur and alkali cycles intensify. Changes in thermal substitution rate should therefore be reviewed against duct fouling history.

Cleaning

Sonic horns may help in cooler, dustier sections where deposits are dry and weakly attached, but they are not a cure for hard kiln coating or refractory-bonded build-up. Air cannons, manual cleaning, access redesign and chemistry control are often more important. In Sylio-style cement work, TAD acoustic cleaning is considered only after deposit strength, temperature and access have been confirmed.

Process role and design issues

In a cement kiln system, the tertiary-air duct carries hot secondary air from the clinker cooler to the calciner or preheater system. That air supplies oxygen and heat for calciner fuel combustion, so duct flow affects calcination, kiln stability, NOx formation, fuel burnout and cooler performance. The duct is large, hot, abrasive and often lined with refractory, which makes access and cleaning difficult.

Design variables include air temperature, dust loading, velocity, refractory type, expansion allowance, damper arrangement, support loads, bypass take-offs and how the duct connects to the calciner. Poor flow distribution can create dead zones where dust settles, while high-velocity regions erode refractory and steelwork. Alkali, chloride and sulphur cycles can make deposits sticky in cooler pockets and around dampers.

Fouling and acoustic-cleaning relevance

Typical problems include build-up at bends, damper fouling, refractory loss, false-air leakage, hot spots, pressure drop increase and unstable calciner air split. Inspections look for lining damage, ring formation, ash shelves, cracked expansion joints and access doors that no longer seal. Because the duct carries hot air and dust, cleaning work requires controls for burns, confined space, falling material and refractory instability.

Sonic horns can help only where deposits are dry and still weakly bonded, such as dust build-up in a pocket or on a ledge. They are not a substitute for refractory repair, bypass chemistry control or mechanical removal of hard rings. If a horn is considered, designers check whether the refractory and duct shape will absorb or block the sound field and whether loosened dust can move safely into the process rather than accumulating elsewhere.

Related terms

• Calciner
• Rotary kiln
• Clinker
• Alkali cycle