Combustion Chemistry Studies of Alternative Fuels with Applications in Marine Engines

Modern society faces climate challenges that will have a considerable impact on our environment if not addressed. The increasing use of fossil fuels exacerbates the greenhouse effect, which leads to extreme local weather conditions due to the increase in global temperatures. Considering the global CO₂ emissions, the transportation sector is known to have a major impact with the shipping industry solely accounting for 2.9% of total global CO₂ emissions. The ongoing electrification of the transport sector on the roads is not possible for long voyages overseas, which calls for innovative solutions.

The combustion community are facing these problems together with the industry and intensive research on alternative fuels is underway. At DTU, we are taking part in this development by conducting experimental and model investigations. We are collaborating with MAN Energy Solutions, who provide the large two-stroke engines used in the shipping industry. MAN Energy Solutions are interested in the model investigations we perform at DTU as they need accurate models in their development of two-stroke engines that can run on alternative fuels.

This PhD thesis investigates the oxidation of n-heptane, which is a surrogate fuel for the heavy fuel oil currently used in the industry, and dual fuel mixtures of methane/n-heptane and ammonia/n-heptane. These dual fuel mixtures are
relevant as methane represents the natural gas and ammonia in the future can be produced by CO2 neutral power-to-X sources. The experiments are conducted at pressures of up to 100 bar and intermediate temperatures (450-900 K), and they reveal useful information for kinetic model validation. The thesis identifies the best model for n-heptane oxidation predictions and develops a new improved detailed model for ammonia/n-heptane oxidation. Furthermore, the thesis presents a new approach to create a semi-global dual fuel model for methane/n-heptane mixtures.

In the aftermath of the PhD project, there remain research questions that need to be addressed. The detailed model of n-heptane overall is in good agreement with the experimental data across different conditions, but in the species profiles
of the oxidation intermediates, the model still shows rather large discrepancies. Furthermore, the oxidation of methane/n-heptane mixtures exhibits unexpectedly low temperature n-heptane conversion at 21 bar, which is completely neglected by the model. The new ammonia/n-heptane model needs further validation and possible further updates to the individual ammonia subset or the ammonia/nheptane oxidation interactions. The same is the case for the semi-global model approach, which is hopefully sufficient for kinetic use in CFD simulations and will be applicable for dual fuels other than methane/n-heptane.