AFS Technology

Suspension Burning for Tire Fuel Optimization in Precalciner Cement Kilns

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Courtesy of AFS Technology

Synopsis: A presentation on optimizing substitution rates of Tire Derived Fuel (TDF) in precalciner kilns using Suspension Burner technology.  Discussions will be made on the challenges faced by both preheater and precalciner kilns using TDF on the feed shelf, technical measures aimed at solving these limitations, achieving high TDF substitution rates while maintaining operational stability and observing environmental requirements.


Tire Derived Fuel (TDF) has conventionally been a solution aimed at lowering operational costs, ensuring environmental compliance, and maintaining product quality.  Facilities using mid-kiln firing for long kilns and feed shelf firing for preheater kilns have achieved TDF substitution rates of up to 30%.  However, with the introduction of precalciner kilns the maximum fuel substitution with whole or shredded tires on the feed shelf is typically limited to less than 5%.  Both preheater kilns and precalciner kilns have experienced sulfur buildup problems with tires on the feed shelf.  The development of Suspension Burner technology enables precalciner kilns to achieve substitution rates of up to 30%, while minimizing sulfur buildup in the tower.

Feed Shelf Firing vs. Suspension Burning

For Precalciner Kilns

Double Gate Arilock Valve Suspension Burner
Traditional Feed Shelf method using Double Gate Airlock Valve to feed whole tires Suspension Burner method suspends whole tires in calciner or riser duct

Figure 1


Precalciner kilns have observed limited results with the use of feed shelf firing primarily due to excessive sulfur buildup in the riser.  This can be attributed towards two main changes in the sulfur cycle:

  • Oxygen level
  • Localized reduction


Sulfur Cycle

Sulfur introduced by the fuel combusts to form fuel-sulfur (SO2 and SO3).  In the calcination zone, fuel sulfur begins to react with free lime at a temperature above 900°C to produce calcium sulfate (CaSO4).  The reaction reaches a maximum rate at 1050°C.  This raw material sulfur (CaSO4) travels downhill in the kiln feed to the burning zone.  At the burning zone 61%-64% of the sulfur volatilizes while the rest of it will leave the kiln with the clinker.  The concentration of sulfur can be 4 to 10 times higher in the space between the calcining zone and burning zone than in the rest of the kiln. This volatilized sulfur oxides gets trapped again in the calcining zone and the cycle repeats itself.

S + O2 → SO2

SO2 + O2 → SO3

SO2 + CaO → 3CaSO4 + CaS (at temperature below 1050°C)

3CaSO4 + CaS → SO2 + CaO (at temperature above 1050°C)

(Where=S: Sulfur, O: Oxygen, Ca: Calcium)

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