Evaluation of chemical kinetic models for NH3/H2 fuel mixtures through laminar flame speed analysis

Abstract

The urgent shift toward sustainable energy has positioned ammonia (NH₃) as a carbon-neutral fuel, yet its low laminar flame speed (LFS) restricts practical combustion applications. To address this limitation, this study enhances NH₃ combustion by blending it with hydrogen (H₂) and evaluates five chemical kinetic mechanisms—Otomo, Li, Zhang, Zhou, and Zhu—through a detailed LFS analysis. These mechanisms are rigorously tested against experimental data spanning equivalence ratios ($\varphi$: 0.6–1.8), hydrogen mole fractions ($x_{H_2}$: 0.0–1.0), unburned temperatures ($T_u$: 298–473 K), and initial pressures ($p_u$: 0.5–2.0 atm). To evaluate their performance, a comprehensive approach was employed, using statistical metrics—root mean square error (RMSE), mean absolute error (MAE), mean absolute percentage error (MAPE), coefficient of determination ($R^2$), and Kling-Gupta efficiency (KGE)—to compute weighted normalized scores for assessing predictive accuracy. The Zhu mechanism consistently demonstrated the highest accuracy in capturing LFS trends across all conditions, closely followed by Zhou and Zhang, which showed reliable performance under most scenarios, while the Otomo mechanism exhibited significant discrepancies, particularly at elevated pressures and varying equivalence ratios. Sensitivity analysis further revealed that the chain-branching reaction

consistently governs LFS across all five kinetic mechanisms and under various initial conditions. In contrast, the modeling of nitrogen-containing intermediates (NH₂, NNH, etc.) exhibited significant inconsistencies, thereby underscoring existing gaps in nitrogen reaction sub-mechanisms. These findings position the Zhu model as the most reliable for NH₃/H₂ combustion systems, offering a robust guide for mechanism selection in practical applications like gas turbines and engines. However, challenges in nitrogen kinetics under high-pressure conditions necessitate further refinement. Future research should prioritize improving nitrogen reaction pathways and validating models with new experimental data to enhance combustion efficiency and minimize NO${}_x$ emissions, advancing

as a sustainable fuel.