A modified drop-tube reactor that allows particle distribution over the reactor cross-sectional area, and oxidation of chars produced in situ, was used to study the conversion efficiency of char nitrogen to nitric oxide (alpha(NO)). The results confirm previous findings by other investigators that alpha(NO) decreases as the weight of char burned increases. alpha(NO) for coal was the same as (at 4% O-2) or lower than (at 20% O-2) that for an equal mass of char during oxidation. Since coal will yield approximately half its mass as fixed carbon, these results suggest that the local stoichiometry surrounding the particle is responsible for the observed reduction in alpha(NO) as sample size increases. The analysis of the exhaust gases showed increases in HCN concentration and a decrease in CO2/CO ratio as sample size increased, suggesting that local stoichiometry influences alpha(NO). Additional experiments showed that alpha(NO) decreased as the background NO concentration was increased, at rates that diminished as the oxygen concentration increased, independent of particle size. The steep reduction in NO production as the background NO concentration increased was explained by the destruction of NO in the gas phase. (C) 2003 The Combustion Institute. Published by Elsevier Inc. All rights reserved.