Kinetic Modeling of Ammonia Reforming–Diesel Dual-Fuel Systems: Combustion Enhancement Pathways and NO Emission Analysis

Amid the global transition toward low-carbon energy, ammonia (NH3) is regarded as a promising clean fuel, though its use is limited by high autoignition temperature and low flame propagation speed. This study proposes a dual-fuel system integrating catalytically reformed ammonia (producing H2/N2-enriched gas) with n-heptane (n-C7H16) to improve combustion performance. An improved combustion reaction mechanism was developed. Numerical simulations conducted via Chemkin software, combined with sensitivity analysis optimization, achieved high-accuracy predictions of laminar burning velocity (LBV), ignition delay time (IDT), and NO emissions The improved mechanism demonstrates superior performance over the original mechanism in predicting mixed fuel behavior under the Premixed Laminar Flame-Speed Calculation module. The results demonstrate that ammonia reforming significantly enhances the LBV by promoting chain-branching reactions through hydrogen-enriched gas (H2/N2). As the reforming ratio increases from 0% to 90%, the peak flame speed of fuel containing 10% n-heptane rises from 18 to 60 cm/s. NO emissions exhibit strong correlations with equivalence ratio and reforming ratio: under lean conditions (Φ=0.8), the peak NO concentration exceeds that under rich conditions (Φ=1.2) by 300%. In lean combustion, the contribution of the NH3 oxidation pathway (NH3→NH2→HNO→NO) increases from 19.4% to 27.5% as the reforming ratio grows from 10% to 50%, resulting in over 50% elevation in NO concentration. Under high-temperature and high-pressure engine conditions, for a fuel containing 5% n-heptane, enhanced combustion should adopt Φ=1.1–1.3 combined with a reforming ratio of 50–70% to maintain the flame velocity greater than 30 cm/s. For optimized NO emissions, it is necessary to prioritize increasing the equivalent ratio on the premise of ensuring the combustion intensity, and second, to control the ammonia reforming ratio. The study demonstrates the application potential of ammonia reforming integrated with dual-fuel strategies in enhancing thermal efficiency and reducing carbon emissions.