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In this numerical study we investigate the combustion characteristics of ammonia-air mixtures at elevated pressure and lean conditions which are encountered in gas turbine combustors. The Konnov mechanism is implemented to predict the laminar burning velocity, autoignition, species concentrations and the sensitivity analysis of ammonia decomposition and laminar flame speed. A laminar premixed freely propagating flame model is implemented to calculate burning velocity, mole fractions of species, and contribution of reactions in molar conversion of specific species. Also a homogenous reactor model is used to analyze the temporal mole fraction of radicals and ignition delay time. Effects of adding hydrogen to the fuel mixture is investigated and compared to the pure ammonia case. The improvement in combustion characteristics such as laminar burning velocity and ignition delay time is noticeable. Based on the sensitivity analysis, it is found that under high pressure and lean conditions, the OH radicals have leading role in fuel mole conversion and laminar flame speed. The sensitivity analysis of laminar flame speed shows key influence of some specific reaction pathways on the flame propagation speed. Also, NOx conversion sensitivity is performed and the major reactions contributing in NOx formation under these conditions are determined. As the final part, homogenous rector model is used to study ignition delay time and temporal concentrations of important species. It is verified that under high pressure lean conditions autoignition is dominated by a sharp increase in some radicals such as OH, NH2, O and H.