Strategies to improve ammonia combustion in a dual fuel marine engine by using CFD

Maritime transportation is currently responsible for delivering most of the world trade, and it still plays a major role in the global economy and pollutant emissions. Due to their heavy loads and long distances, full electrification is generally assumed as an unsuitable pathway, leaving combustion engines as the dominant technology in the marine sector. For a long time, these engines have benefited from the low costs of heavy oils produced from oil refining, but these fuels present a difficult combustion and are associated with significant pollutant emissions and greenhouse gases. Today, carbon-free fuels such as hydrogen and ammonia are gradually getting a foothold in the shipping market. In the present paper, an investigation on several strategies to improve the oxidation of ammonia is carried out for a dual fuel (DF) medium speed marine engine. A CFD analysis using ANSYS FORTE® code is performed on an improved and more realistic bowl geometry by varying the premixed fuel according to methods validated in previous studies and introducing adequate kinetic mechanisms from the literature. Results show that the sole use of ammonia to replace natural gas, as premixed fuel, leads to incomplete combustion; the increase of diesel fuel can only mitigate this condition, but it cannot be considered as viable solution. Therefore, several blends of ammonia/hydrogen are tested to examine combustion development and pollutant emissions. The addition of hydrogen above 10% to the premixed charge composition demonstrates to promote the oxidation of ammonia and to effectively reduce CO2 emissions of 84% without a dramatic increase in nitrogen oxides. On the contrary, hydrogen percentages lower than 10% feature a worsening of the combustion of the premixed charge. Finally, the use of a fuel containing nitrogen promote a slight formation of NOx emissions.