Modeling of NOx Formation in Diffusion Flame Combustors

Abstract

The model presented in this paper predicts the NO x formation under simultaneous droplet burning a nd partially premixed reaction processes. The model simulates various combustion zones with an arrangement of reactors and is coupled with a detailed chemical reaction scheme. In the model, the primary zone of the combustor comprises a reactor representing contribution from droplet burning under stoichiometric condition and a mixing reactor that provides additional air or fuel to the primary zone. The additional flow allows the formation of a fuel vapor/air mixture distribution that reflects the unmixedness nature of the fuel injection process. Expressions to estimate the spread in fuel/air ratios in the primary zone of the combustor around the mean value, and the duration of droplet burning under stoichiometric condition were derived. The derivation of the expressions utilized a database obtained for an experimental gas turbine combustor where primary zone equivalence ratio, combustor pressure drop, and pilot/main fuel split varied over a wide range. The calculated residence time for droplet burning increase d with a decrease in engine power. The larger contribution from droplet burning under low power modes is attributed to the lower evaporation rates and the leaner primary zone stoichiometry under these conditions. Model validation involved the application o f the calculation method to a production gas turbine combustor. Good agreement was achieved between the measured and calculated NO x emissions for the production combustor. This indicates that the simulation of the flame by a combined droplet burning and fu el vapor/air mixture distribution offers a promising approach for estimating NO x emissions in combustors, in particular for those with significant deviation from traditional stoichiometry in the primary combustion zone. Nomenclature