The paper examines the impact of reduction zone stoichiometric ratio, residence time, and coal characteristics on NOx emissions in the case of air-staged combustion. A comprehensive NOx reduction index Sz was proposed to correlate the maximum NOx reduction rate and coal characteristics. The study employs the Entrained Flow Reactor with Multiple Reaction Segment (EFRM) and focuses specially on low rank coal (four lignite and one sub-bituminous coal) and two high volatile bituminous coal in China. The results show that in air-staged combustion, the level of NOx emitted drops as the reduction zone stoichiometric ratio decreases, resulting in a high reduction rate. The longer the residence time, the higher the reduction rate will be. Coals with higher volatile content have a lower conversion rate of fuel nitrogen to NOR, benefiting the reduction of NOx emissions. The higher the fuel comprehensive NOx reduction index, the higher is the NOx reduction rate. In particular, satisfactory NOx reduction for the low rank coal is achieved at the residence time between 1 s and 1.5 s, depending on coal. For high volatile bituminous coal, the gain in performance by increase residence time after it is greater than 1 s is generally minimal. Moreover, air-staged combustion does not significantly reduce the burnout rate of the high volatile coal. (C) 2014 Elsevier B.V. All rights reserved.