When free lime rises but burning zone temperature, kiln speed, and feed rate all look normal, AFR ash chemistry has usually moved underneath the operating loop. Sulfates from tyres, phosphates from sewage sludge, heavy metals from various AFR streams — each can interfere with the formation of C₃S in subtle, cumulative ways. The kiln looks stable on the panel because the controls were never tuned for this chemistry. Free lime is the symptom that arrives in the lab before the panel ever sees the cause.
Why this matters in the afr / alternative fuels
Free lime that rises without a kiln-parameter signal is one of the harder symptoms to diagnose, precisely because every panel-level indicator says the kiln is fine. The cause sits in the raw mix-and-AFR-ash chemistry, and the lab — not the control room — usually catches it first. Operators who do not have a feedback path from quality to AFR acceptance criteria can lose entire campaigns to a creeping problem they could not see in real time.
For downstream cement performance the cost is direct: soundness margin shrinks, strength performance softens, and customer complaints tend to follow. Quality teams treating AFR-period free-lime drift as an alert to the AFR programme — not just a kiln issue — usually catch it earliest.