Kiln torque becoming more variable during AFR firing — even when the absolute level looks acceptable — is a pattern that usually points at coating disturbance. AFR ash can change the chemistry of the protective coating in the burning zone, and partial coating falls release stored heat in surges that the torque trace catches. The pattern matters more than any single excursion: rising variability over a campaign is the kiln telling you that AFR ash is now part of the coating equation, whether anyone planned for it or not.
Why this matters in the afr / alternative fuels
Coating that becomes unstable under AFR ash is a slow-moving cost. The kiln drive sees more cyclic loading than its design budget allowed, refractory wears more aggressively in the affected zone, and free lime starts to drift up in step with each coating disturbance. None of these effects are acute, but together they shorten the campaign in ways that show up only in the next outage.
The AFR side also pays: substitution rates have to be moderated to keep coating stable, and the economics of higher-AFR operation weaken. Plants that monitor torque variability — not just torque level — usually catch the trend early enough to shift AFR mix or schedule a coating intervention before the cost compounds.