Reduced chemical kinetic mechanisms for NOx emission prediction in biomass combustion

Because of the complex composition of biomass, the chemical mechanism contains many different species and therefore a large number of reactions. Although biomass gas‐phase combustion is fairly well researched and understood, the proposed mechanisms are still complex and need very long computational time and powerful hardware resources.

A reduction of the mechanism for biomass volatile oxidation has therefore been performed to avoid these difficulties. The selected detailed mechanism in this study contains 81 species and 703 elementary reactions. Necessity analysis is used to determine which species and reactions are of less importance for the predictability of the final result and, hence, can be discarded.

For validation, numerical results using the derived reduced mechanism are compared with the results obtained with the original detailed mechanism. The reduced mechanism contains much fewer reactions and chemical species, that is, 35 species and 198 reactions, corresponding to 72% reduction in the number of reactions and, therefore, improving the computational time considerably.

Yet, the model based on the reduced mechanism predicts correctly concentrations of NOx and CO that are essentially identical to those of the complete mechanism in the range of reaction conditions of interest, especially for the medium‐temperature range. The reduced mechanism failed to predict the concentrations in the high‐ and low‐temperature range.

Therefore, two more reduced mechanisms are also proposed for the high‐ and low‐temperature range with 26 and 52 species, respectively. The modeling conditions are selected in a way to mimic values in the range of temperature 700–1400°C, excess air ratio 0.8–3.3, and four different residence times: 1, 0.1, 0.01, and 0.001 s, since these variables are the main affecting parameters on NOx emission. © 2012 Wiley Periodicals, Inc.

Int J Chem Kinet 44: 219–231, 2012