Numerical Investigation of Premixed and Non-premixed Ammonia Main Charge Configurations Ignited by a Hydrogen-Fired Prechamber

Ammonia-fired reciprocating engines have emerged as a promising technology in the maritime and power generation sector at medium-to-large scale (1–80 MW). The use of “on-the-fly” partial ammonia decomposition to produce a relatively small amount of hydrogen that can be used as combustion promoter, replacing fossil fuels in this function, enables this technology to provide carbon-free propulsion and power generation. In this context, it is envisioned that a hydrogen-fired prechamber ignition strategy offers significant advantages by accelerating the ammonia ignition and complete combustion process, increasing its reliability and robustness while still aiming to achieve low NO _x , N₂O, and NH₃ emissions. This study exploits an OpenFOAM-based Large Eddy Simulation (LES) numerical modeling framework to investigate the ignition and combustion behavior of an ammonia main charge ignited by a hydrogen-fired prechamber. First, a conventional port-injection premixed configuration for the ammonia main charge is considered whereas the hydrogen-fired prechamber is found to provide a sufficiently strong ignition source for all ammonia–air mixtures investigated. The effect of the main charge equivalence ratio and the wall temperature on combustion efficiency and emissions formation is evaluated. Second, considering a non-premixed configuration for comparison, an identically configured hydrogen-fired prechamber is used to study the ignition and combustion process for ammonia main charges directly injected as liquid sprays and modeled as Lagrangian particle tracking (LPT) in conjunction with the LES model. The LES results suggest that the relative timing and angle of injection between the liquid sprays and the hydrogen jet flames emerging from the prechamber play a major role in controlling the ignition and combustion process. Finally, the non-premixed ammonia main charge configuration is found to significantly reduce the formation of pollutants and extend the operating range to leaner global equivalence ratios, compared to the premixed ammonia main charge configuration.