Background
Climate change, attributed to increased CO2 concentration in the atmosphere, is driving worldwide Zero Carbon. The marine industry, to reduce its carbon footprint, is pushing for carbon-neutral fuels and 50-percent carbon emissions reduction by 2050 compared to 2008. The marine industry is considering multiple paths toward a zero-carbon future. These paths generally rely on renewable hydrogen (hydrogen generated by renewable energy such as solar or wind) which is then transformed into a higher energy density fuel such as synthetic methane, methanol, or ammonia. The higher energy density fuels are better suited to high fuel usage mobile applications, including ships.
A dual-fuel approach (diesel pilot ignition) is the most likely approach to ignite and combust ammonia for marine applications. While a dual-fuel approach would not be carbon free, it would retain diesel capability for emergency use and would provide a robust ignition source for the ammonia while still achieving a >90% reduction in greenhouse gas (GHG) emissions.
This project was undertaken to develop an understanding of ammonia-diesel, dual-fuel combustion. The dual-fuel combustion process is complex, relying on the injected diesel fuel to ignite and combust the ammonia and air mixture. The combustion is a mixture of diffusion burning, in the case of the diesel pilot, and of flame propagation, in the case of the ammonia not entrained in the diesel jet. The objective of this project was to develop an understanding ammonia-diesel, dual-fuel combustion including limitations on ammonia substitution and methods for reducing ammonia emissions using combustion computational fluid dynamics (CFD) simulation.
Approach
A Cummins ISX15 (15L) engine was selected for the study. The ISX15 model geometry and calibrated model were available. Appropriate boundary conditions were selected for the simulation. An ammonia combustion kinetic mechanism was identified in the literature and merged with the diesel kinetic mechanism to form a dual-fuel mechanism for the simulation. The model was refined to better match the experimental data for the laminar flame speed of ammonia versus equivalence ratio published in the literature.
Accomplishments
The simulations identified multiple mechanisms critical to diesel-ammonia dual-fuel combustion. The dominant mechanism is dependent on the substitution ratio (SR) and air-ammonia equivalence ratio. At low substitution ratios, the air-ammonia equivalence ratios are too lean to propagate a flame, so the ammonia conversion efficiency is largely dependent on the effectiveness of the air “utilization” of the diesel combustion, i.e., how much of the in-cylinder mixture contacts the diesel jets and flame (1st mechanism). Any ammonia contacting the diesel flame will combust, but the remainder will exit the cylinder as ammonia emissions. Lower diesel injection quantities (lighter loads) will have lower air “utilization” as the injection duration is shorter and air-diesel ratio is high leading to lower ammonia conversion.
At moderate substitution ratios, the air-ammonia equivalence ratio is still too lean to propagate a flame, so the ammonia conversion efficiency is again dependent on the air “utilization” of the diesel combustion. Reducing air flow through throttling can enrich the air-ammonia mixture towards the flammability region of ammonia, however, because there is a significant quantity of diesel (at moderate and high loads) the mixture can become overall rich resulting in poor diesel combustion.
At high substitution ratios, the air-ammonia mixture may or may not be within the flammable zone for ammonia. In the case that the air-ammonia ratio is lower than the lean flammability limit (LFL), the air flow can be reduced via throttling to enrich the air-ammonia mixture and create a mixture suitable for flame propagation (2nd mechanism). Enriching the mixture above the LFL can improve the ammonia conversion efficiency to 96-percent. Beyond this point, the conversion of ammonia is limited by the unburned ammonia in the crevice regions and due to wall quenching (3rd mechanism).