We have investigated an index for modeling a NOx reaction mechanism of pulverized coal combustion. The reaction mechanism of coal nitrogen was examined by drop-tube furnace experiments under various burning conditions. We proposed the gas phase stoichiometric ratio (SRgas) as a key index to evaluate NOx concentration in fuel-rich flames. The SRgas was defined as: SRgas equivalent to amount of fuel required for stoichiometry combustion/amount of gasified fuel where, the amount of gasified fuel was defined as the amount of fuel which had been released to the gas phase by pyrolysis, oxidation and gasification reactions. When SRgas < 1.0, NOx concentration was strongly influenced by the value of SRgas. In this condition, the NOx concentration was hardly influenced by coal type, particle diameter, or reaction time. We developed a model to analyze NOx and XN(HCN, NH3) concentrations for pulverized coal/air combustion and coal/CO2/O-2 combustion, based on the index. NOx and XN concentrations did not reproduce the experimental results without considering reactions between hydrocarbons and NOx. The hydrocarbon reaction was important for both NOx and XN, especially for air combustion. In the present model, an empirical formula was used to estimate the total concentration of hydrocarbons in coal flame. The reaction of heavy hydrocarbons which had plural aromatic rings was very important to analyze the reaction mechanism of hydrocarbons for coal combustion in detail. When burning temperature and SRgas were the same, total hydrocarbon concentration in a coal flame was larger than that of a light gaseous hydrocarbon flame. Total hydrocarbon concentration in oxy-fuel combustion was lower than that in air combustion. We verified the proposed model by experimental results obtained for a drop-tube furnace and a laboratory-scale furnace that had an installed low-NOx burner. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.