January 01, 2024

Researchers investigate lean combustion of
 ammonia-hydrogen mixtures in a pre-chamber engine
 

Researchers from Shanghai Jiao Tong University, with colleagues from Shaanxi Diesel Heavy Industry, have investigated the impact of the hydrogen energy fraction (H_frac), equivalence ratio, and ignition timing on the combustion and emission characteristics of ammonia-hydrogen mixtures at lean combustion conditions in a pre-chamber (PC) marine engine by CFD Simulation.

In a paper published in the journal Fuel, they report that in H₂-fuel active PC mode, increasing the hydrogen content or achieving stoichiometric combustion within the PC will shorten the cold jet phase and accelerate the formation of a more intense hot jet.

As the H_frac and the equivalence ratio in the main-chamber increase, the combustion pressure, temperature, and heat release rate (HRR) rise, while the combustion duration shortens due to enhanced reactivity, potentially leading to end gas auto-ignition and knock due to lower auto-ignition energy of the mixture.

NO primarily forms near the flame front, and both NO concentration and the generation area increase with an increase in hydrogen content and equivalence ratio. Fuel-NO is likely to be the dominant factor in NO generation with lean combustion conditions.

N₂O, as a product of NH₃ low-temperature combustion, is mainly concentrated near the flame front. An increase in unburned NH₃ emissions corresponds to an increase in N₂O emissions. Delaying the ignition timing to reduce NO_x emissions is primarily achieved by controlling NO emissions, even though there is a slight increase in N₂O.

The blending combustion of ammonia and hydrogen harnesses the mutually advantageous effects of their individual combustion characteristics, offering a feasible solution for ammonia-powered ICEs. Comprehensive experimental results have conclusively demonstrated that the incorporation of hydrogen brings about remarkable modifications to the combustion properties of ammonia. For instance, as the hydrogen volume fraction in the ammonia-hydrogen mixtures increases from 0% to 60%, the flame propagation velocity of the mixture at normal conditions escalates from 6.5 cm/s to 66.3 cm/s.

… Nevertheless, with the increased ammonia content in the mixtures, it becomes necessary to advance the ignition timing to maintain both combustion stability and power output in the engine. … To ensure stable engine power output, the minimum hydrogen volume fraction should be 7 % at full loads, and this proportion should increase to 11 % at partial loads.

… Exploring ammonia-hydrogen ultra-lean combustion has the potential to achieve high thermal efficiency and low NO_x emissions. However, an over-lean fuel–air mixture leads to combustion instability and even misfire, especially for ammonia fuel. As a result, the practice of lean combustion imposes high demands on the engine ignition system. The conventional spark ignition, with limited ignition energy and single-point flame propagation, is not suitable for lean combustion in large-bore marine engines. The pre-chamber (PC) turbulent jet ignition (TJI) utilizes the injection of high-temperature gases enriched with hydrogen radicals into the main-chamber (MC), which generates multiple ignition sources and induces strong turbulence. As a result, the low-reactivity mixture in the MC is ignited, achieving stable and dependable ignition performance.

Hence, the PC-TJI mode make it possible to achieve ultra-lean combustion of ammonia-hydrogen mixtures. Moreover, compared to the passive PC without fuel injection, the active PC mode could further extend the lean combustion limit by an auxiliary fuel supply system. Considering that the hydrogen is characterized by fast flame propagation speed and high adiabatic flame temperature, ultra-lean combustion of ammonia-hydrogen can be achieved with H₂-fueled active PC engine. In addition, there is potential to further reduce the energy substitution rate of hydrogen in H₂-fueled active PC mode.

However, there are still limited researches on the ignition of lean ammonia-hydrogen mixtures using PC-JTI. Additionally, the influence of hydrogen blending ratio and equivalence ratio on NO_x emissions from ammonia-hydrogen mixtures still require further exploration. Therefore, this study numerically investigates the combustion and emissions characteristics of ammonia-hydrogen in H₂-fueled active PC marine engines at fuel lean conditions.

—Huo et al.

 

Resources

• Jinlu Huo, Tongbin Zhao, He Lin, Jinhua Li, Wei Zhang, Zhen Huang, Dong Han (2024) “Study on lean combustion of ammonia-hydrogen mixtures in a pre-chamber engine,” Fuel Volume 361 doi: 10.1016/j.fuel.2023.130773.