A Three-step Global Kinetic Mechanism for Predicting Extinction Strain Rate of Syngas-air Nonpremixed Flames

Abstract

ABSTRACT A systematic survey of chemical kinetic mechanisms (both simplified and global) shows an absence of a robust kinetic mechanism that can accurately predict characteristics of syngas-air nonpremixed flames. In this work, a three-step global kinetics mechanism is proposed for predicting the extinction strain rate (ag ) of syngas-air nonpremixed flames. The available experimental ag data-set from the Tsuji type burner was used to evaluate the performance of six syngas kinetics models (four simplified and two global). For this, 2D-planar computations were performed to obtain ag values using four CO/H2 ratios (1, 3, 5, and 29). The results showed that all the simplified kinetics mechanisms overpredicted the ag values in the range of 16–224% for the four CO/H2 ratios. In the case of the global kinetics models namely, CFFR and WO, except for CO/H2 ratio of 29, CFFR underpredicted ag values by 22–55%. WO underpredicted ag by 10–44% for CO/H2 = 1 and overpredicted ag by 19–58% for CO/H2 = 3 and 5. For CO/H2 = 29, WO overpredicted ag values by 151–205%. For CO/H2 = 1, WO underpredicted ag due to two reasons, (1) double accounting of CO hydroxyl oxidation path and, (2) suppression of H2 oxidation pathways in four-step global JL mechanism, a source for the construction of WO mechanism. From the overall perceptive, WO has performed better compared to other mechanisms. Hence, WO was chosen for further optimization using experimental ag data as a target. Sensitivity analysis in terms of ag and maximum flame temperature (TF ) was performed for WO using two CO/H2 ratios (1 and 5). It was found that for CO/H2 = 1 and 5, ag is most sensitive to Rxn1 (H2 + 0.5O2↔ H2O) and Rxn2 (CO + 0.5O2 → CO2). Results from sensitivity analysis, 0D (PSR) computations, and 2D computations have shown that the improper quantification of CO and H2 oxidation pathways leads to inaccurate ag prediction by the WO model. Based on these new insights, a modified three-step WO syngas kinetics model (WO-Opt) is proposed for predicting the extinction strain rate of syngas-air nonpremixed flames. The WO-Opt model was validated against ag experimental data. Compared to the WO model, ag predictions obtained from the modified WO-Opt model were in good agreement with the syngas-air nonpremixed flames experiments.