Ammonia, as a carbon-free fuel, is a promising alternative fuel source for decarbonization of the shipping industry. Nevertheless, the poor flammability and low flame speed restrict its potential application as marine engine fuel. In order to explore the ammonia application feasibility and methods in marine engines, the effects of two combustion promoters, including n-heptane and hydrogen, on improving the ammonia combustion characteristics were compared and discussed theoretically in this study, in terms of flammable intake boundary conditions and laminar flame speed under engine operating conditions. A new detailed reaction mechanism of ammonia/n-heptane dual fuel was developed and validated to characterize the combustion of ammonia and diesel. The results demonstrate that the flammability of ammonia is more sensitive to intake temperature as compared to equivalence ratio and intake pressure. The introduction of n-heptane or hydrogen has been observed to have a noteworthy impact on the combustion characteristics of ammonia, resulting in a decrease in the necessary intake temperature. The promotion effect is more pronounced with an increase in the volume fraction of the combustion promoter. However, n-heptane exhibits a superior ability to support combustion in comparison to hydrogen, which allows significantly lower intake temperature requirement for ammonia combustion, even reaching room temperature levels. The reaction pathway of the ammonia/n-heptane mixture indicated that the OH radicals produced by the low-temperature oxidation of n-heptane play a crucial role in facilitating ammonia combustion. Furthermore, the addition of n-heptane has the potential to significantly enhance the laminar flame speed of ammonia up to 34 cm/s, when a 10% volume fraction of n-heptane is employed. Ultimately, the map of NOx and unburned hydrocarbon emissions was illustrated, followed by the proposal of a stratified charge combustion approach aimed at the simultaneous reduction of unburned hydrocarbon and NOx emissions.