Comprehensive Comparison of Chemical Kinetics Mechanisms for Syngas/Biogas Mixtures

The performance of the most commonly used chemical kinetics mechanisms is compared against a large set of experimental data, accumulated for syngas mixtures diluted with other gases such as CO₂, H₂O, N₂, and CH₄. These dilutes are pertinent to syngas/biogas mixtures and could have significant effects on the combustion characteristics. The laminar flame speed and ignition delay are used as quantitative metrics to compare the simulation predictions with the experimental data. The experimental data include ignition measurements obtained from shock tubes and rapid compressions, and flame speed measurements obtained from spherical bombs as well as counterflow configurations covering a wide range of temperature, pressure, and equivalence ratios. The mixture compositions are systematically categorized in this study. After exhaustive comparisons, it is found that the NUIG2013 mechanism is in closest agreement with the measured ignition delay and laminar flame speed for the investigated mixtures. However, the predictions deviated significantly from the measured ignition delays for the H₂/CO/CO₂, H₂/CO/CH₄, and H₂/CO/CH₄/CO₂/H₂O mixtures. We performed a sensitivity analysis on the ignition delay time of the H₂/CO/CO₂, H₂/CO/CH₄, and H₂/CO/CH₄/CO₂/H₂O mixtures, using the NUIG2013 mechanism to identify the reactions that are responsible for the deviations. We found that by adopting a much lower rate constant expression [<i>k</i>₀(CO₂) = 8.82 × 10¹⁹ × T­[K]^−1.40 (cm⁶/mol²/s)] for H + O₂ (+CO₂) = HO₂ (+CO₂) (R2), the NUIG2013 mechanism could accurately predict the ignition delay time of the H₂/CO/CO₂ mixture at low pressures (<i>P</i> = 1.24–2.36 atm) and low temperatures (<1025 K) range. A new rate constant expression [<i>k</i>₃ = 2.3 × 10⁸ × <i>T</i>[<i>K</i>]^1.40 exp­(−2850/T) (cm³/mol/s)] for the CH₄ + OH = CH₃ + H₂O R3 reaction in the NUIG2013 mechanism is required to reconcile the discrepancy observed between the measured and simulated ignition delay time of the H₂/CO/CH₄ and H₂/CO/CH₄/CO₂/H₂O mixtures at <i>P</i> = 32 atm. In addition, the newly fitted rate constant expression for R3 improved the performance of the NUIG2013 mechanism in predicting the ignition delay of H₂/CH₄ at <i>P</i> = 5, 10, and 20 atm. More importantly, the modifications made to the reactions (R2) and R3 did not substantially affect the NUIG2013 mechanism’s excellent predictions of the flame speed. Hence, it is recommended that the rate constant expressions for (R2) and R3 be incorporated into the NUIG2013 mechanism for any study of the combustion characteristics of biogas/syngas mixtures