Numerical study of Ammonia combustion in dual-fuel engine

The drive to reduce the carbon intensity of the energy system has generated much interest in applying carbon-free fuels such as ammonia (NH3) in combustion systems. The high hydrogen density and well-established production processes make NH3 a valuable chemical energy carrier to address and sustain the energy shift toward renewable energy source integration. However, some difficulties can be highlighted in the NH3 practical application. The combustion of NH3 is prone to producing harmful nitric oxides. In addition, NH3 has lower reactivity than most hydrocarbon fuels, which makes ignition challenging. Also, admixing NH3 with highly reactive fuels such as DME will facilitate ignition. The partnerships of this proposal are very interested in applying renewable NH3 as fuel in combined heat and power engines, and this research proposal suggests simulating a dual-fuel engine with NH3 as its primary fuel. The results of this research will help determine the optimum operating conditions for performing an experimental study.

This study investigates the impact of ammonia on dual-fuel engines, focusing on achieving stable combustion and minimizing emissions. Ammonia, while carbon-free, has poor ignition quality due to its low flame speed and high activation energy, resulting in long ignition delay times (IDTs) that can destabilize combustion. However, blending ammonia with pilot fuels like hydrogen or dimethyl ether (DME) significantly improves its ignition performance.

Hydrogen enhances ignition by reducing IDTs, mainly through accelerated radical formation and chain-branching reactions. However, this can lead to higher peak combustion temperatures, increasing NOₓ emissions, particularly under lean conditions. DME, with its high cetane number, improves ignition while limiting peak combustion temperatures, reducing thermal NOₓ formation, and aiding in ammonia oxidation, minimizing unburned ammonia in exhaust gases.

Simulations show that adding more than 20-30% pilot fuel offers fewer benefits, indicating an optimal mix for fuel efficiency without using too much pilot fuel. The model developed in this study was validated against experimental data, showing strong agreement, especially at temperatures above 1000 K and under stoichiometric conditions.

The findings highlight ignition delay time (IDT) as a critical factor for both engine stability and emissions. Ammonia can achieve stable ignition in dual-fuel systems with as little as 5-10% hydrogen or DME, reducing IDTs by up to 60-80% compared to pure ammonia.

In conclusion, ammonia can be a viable fuel in dual-fuel engines if blended with appropriate pilot fuels. DME stands out as the more practical option for reliable ignition and lower emissions, making ammonia a promising, carbon-free energy source for future internal combustion engine applications.