Progress in Clean Energy: A Review of Ammonia-Hydrogen Blended Fuels for Internal Combustion Engine Applications

Because of the global increasing demand for energy, combustible fuels are widely used by both home and industry. Today hydrocarbon dependency has led to huge environmental and health risks from the emissions that they cause. As a result, hydrogen (H2) and ammonia (NH3) have been proposed as promising renewable and carbon-neutral energy carriers to mitigate CO2 emissions in transport. For long-range applications like marine diesel engines, typical alternatives, such as battery-electric, hybrid or fuel cell solutions, are simply too expensive and provide insufficient torque output. The low auto-ignition propensity of ammonia necessitates the utilization of highly reactive fuels to facilitate its combustion process, such as hydrogen. Ammonia as a marine fuel offers several benefits, including the elimination of emissions of sulfur oxides (SOx) and particulate matter (PM). However, newly published research suggests that significant combustion difficulties are faced while burning hydrogen-ammonia blends, primarily due to the various ignition delays of hydrogen and ammonia, which results in incomplete combustion and increased generation of nitrogen oxides (NOx) products like NO, NO2, and N2O. This review thoroughly examines the properties of ammonia and hydrogen as potential fuels for internal combustion engines (ICEs), discussing their physicochemical properties, blending methods, and fuel injection techniques, while also identifying critical knowledge gaps that need to be overcome to stimulate the evolution of H2-NH3 fuels.