In this work comparisons and validations of several simplified ammonia oxidation mechanisms at conditions relevant for combustion of biomass have been performed. The aim of the investigation waste clarify the weakness and the strength of the available simplified mechanisms for computational fluid dynamics (CFD) based prediction of NO, emissions. The mechanisms have been tested at typical conditions for biomass combustion, considering temperature and gas composition. All the simplified mechanisms have been compared against a validated comprehensive elementary reaction mechanism containing 340 reversible reactions between 56 chemical species. In comparing the simplified mechanisms, the mixing pattern between the species was described by a continuous stirred tank reactor, with typical residence times in eddy dissipation concept fine structure. The results show that, for substoichiometric conditions at low temperatures (<1000 degrees C), all tested simplified mechanisms have poor agreement with the comprehensive mechanism. This is especially evident at short residence times. Typically, the simplified mechanisms fail to predict any nitric oxide formed at those conditions, whereas the comprehensive mechanism indicates up to 30% oxidation of ammonia to nitric oxide. The study shows that more accurate simplified models are necessary for a better description of ammonia oxidation when modeling furnaces in which these conditions are present. Black liquor recovery boilers and other combustors with staged combustion belong to that group of furnaces. At higher temperatures (> 1000 degrees C) and at oxidizing conditions the mechanisms worked better. The deviation from the results obtained with the comprehensive mechanism are in the range of 20% in nitric oxide levels and ammonia consumption at longer residence times, whereas the deviations increases up to 90% at shorter residence times.