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This study explores the challenges and possibilities of using ammonia (NH3) as a carbon-free fuel for marine propulsion in a Wärtsilä ammonia/diesel dual-fuel engine using a reactivity-controlled compression ignition (RCCI) concept. The main issues in ammonia RCCI engines are high ammonia slip and high emissions of nitrogen oxides and nitrous oxide. A joint experimental and computational investigation was conducted to understand the combustion process, pollutant and greenhouse gas (GHG) formation mechanisms, and the effects of injector configuration and injection timing on engine performance. A comparative assessment between the ammonia/diesel RCCI engine and a baseline natural gas/diesel RCCI engine showed that, upon replacing premixed natural gas with ammonia and maintaining the same energy share of diesel (8.5% of the total energy from diesel), the engine yielded considerably poor combustion efficiency. Increasing diesel usage to 24% share of the total energy allowed a successful engine operation, cutting GHG by 70%. However, higher diesel usage increased CO and CO2 emissions. N2O emission was attributed to the slow premixed ammonia/air flame propagation and near-wall flame quenching. The primary sources of NO emissions in ammonia/diesel RCCI engines were identified as fuel-NOx from premixed ammonia oxidation and thermal NOx in the diesel flame region. A recommendation was put forth for enhancing the operating conditions and injection strategies of ammonia RCCI engines. The proposed operation and injection strategy results in a 45% reduction in CO emission, a 60% reduction in total GHG emissions, and, notably, an 89% decrease in CO2 emissions compared to the LNG/diesel engine.
Xu et al. (Thu,) studied this question.