Effect of coal properties on scaffold formation in rotary kilns

The effect of fuel properties on ash deposition behaviour in a coal fired Rotary kiln, used for preparing iron rich pellets from virgin iron ore, was examined using simulated combustion conditions combined with a range of analytical techniques. A new index based on ash melting during thermo-mechanical analysis of coal ash is developed to predict ash deposition potential in a Rotary kiln, and related to ash deposition trend near the kiln entrance. Iron dust was found to consist of mainly hematite. Iron dust presence in coal increases ash agglomeration tendency such that agglomeration intensity increases with increasing concentration of iron dust. The study also established that intensity of iron dust impact is influenced by parent coal chemistry such that iron and calcium rich coals displayed greater intensity of iron dust impact. It seems external iron presence has a stronger effect on ash agglomeration compared to inherent coal iron minerals as confirmed by TMA index and ash agglomerate plot. The interfacial examination of iron dust and ash particles did not show any apparent evidence of chemical interactions which implies that iron dust mainly participated physically for the evolution of agglomerate at least under tested conditions. This suggests that in Rotary kiln, inertial impaction of iron dust may be determining intensity of deposit formation. Increasing kaolinite concentration in coal did not show any apparent change on the agglomeration tendency for low to medium slagging coals. It is possible that due to smaller particle size and lower density, their contribution to possible inertial impaction may be not significant. The study further highlighted that increase in mullite presence in high slagging coals mainly seems to be attributed to addition of external kaolinite in specimens. Like iron dust, kaolinite particles did not appear to show any chemical interaction with coal ash particles, even though occurs as mullite in the agglomerate. The interfacial studies supported the ash deposition prediction based on TMA index such that coals with high TMA index displayed high agglomeration index with exception of kaolinite rich coals.The study suggests that physical properties of coal minerals particularly iron bearing phases in coals may have strong effect on the growth of agglomeration. Therefore, future studies may focus on establishing the effect of physical properties of additive as well as variation of coal mineral chemistry with coal grain size