LOI in clinker rising above the design band during AFR firing is the lab telling you that combustion finished somewhere it should not have — usually in the calciner, when AFR particles are not given the residence time and temperature they need to burn out. Unburnt carbon shows up directly in LOI, and once it does, the rest of the clinker chemistry is also at risk. The cause sits in fuel sizing, calciner residence time, or temperature, not at the kiln main burner.
Why this matters in the afr / alternative fuels
Clinker LOI is a quality indicator with downstream consequences. Cement made from high-LOI clinker can show inconsistent setting and strength behaviour, and quality teams may have to adjust grinding or gypsum dosing to compensate. None of these recoveries are free.
The more important signal, though, is what high LOI says about combustion. Unburnt AFR in clinker means the calciner is not finishing its work, and the same conditions are likely to be feeding CO, NOx, or volatile-cycle problems elsewhere. AFR programmes treating clinker LOI as a combustion KPI — not just a quality variable — typically find the AFR sizing or calciner-temperature fix before the other symptoms compound.