Evaluation of the Accuracy of Selected Syngas Chemical Mechanisms

Abstract

The implementation of reduced syngas combustion mechanisms in numerical combustion studies has become inevitable in order to reduce the computational cost without compromising the predictions' accuracy. In this regard, the present study evaluates the predictive capabilities of selected detailed, reduced, and global syngas chemical mechanisms by comparing the numerical results with experimental laminar flame speed (LFS) values of lean premixed (LPM) syngas flames. The comparisons are carried out at varying equivalence ratios, syngas compositions, operating pressures, and preheat temperatures to represent a range of operating conditions of modern fuel flexible combustion systems. NOx emissions predicted by the detailed mechanism, GRI-Mech. 3.0, are also used to study the accuracy of the selected mechanisms under these operating conditions. Moreover, the selected mechanisms' accuracy in predicting the laminar flame thickness (LFT), species concentrations of the reactants, and OH profiles at different equivalence ratios and syngas compositions are investigated as well. The LFS is generally observed to increase with increasing equivalence ratio, hydrogen content in the syngas, and preheat temperature, while it is decreased with increasing operating pressure. This trend is followed by all mechanisms understudy. The global mechanisms of Watanabe–Otaka and Jones–Lindstedt for syngas are consistently observed to over-predict and under-predict the LFS up to an average of 60% and 80%, respectively. The reduced mechanism of Slavinskaya has an average error of less than 20%, which is comparable to the average error of the GRI-Mech. 3.0. It however over-predicts the flame thickness by up to 30% when compared to GRI-Mech. 3.0. The NO prediction by Li mechanism and the reduced mechanisms are observed to be within 10% prediction range of the GRI-Mech. 3.0 at intermediate equivalence ratio (φ=0.74) up to stoichiometry. Moving toward more lean conditions, there is significant difference between the GRI-Mech. 3.0 NO prediction and those of the reduced mechanisms due to relative importance of the prompt NOx at lower temperature compared to thermal NOx that is only accounted for by the GRI-Mech. 3.0.