How much air is required to burn 1 KG of Coal?

Thank you for the invitation

 it appears that the above said coal is high volatile Bituminous coal, Typically for this of coal the quantity of air required will be approximatley 8-11 kg air/kg coal (This value can be theorotically calculated using dulongs formula).

There appears to be anomaly, Why there is no sufficient secondary air in kiln? (Which should have been sufficiently designed, considering said coal). More than quantity of air, explore the possibilitis of "very high leakages" or "insufficinet draught" in PH fan..