Characteristics of high-pressure spray combustion of liquid ammonia blended with methanol/propane in a marine engine

• Effect of blending lower hydrocarbons with NH 3 on high-pressure dual-fuel combustion in a marine engine. • Experimental investigation of spray combustion of NH 3 /CH 3 OH and NH 3 /C 3 H 8 fuel blends using an RCEM. • CH 3 OH blending improved the flammability and combustion stability of high pressure NH 3 spray. • C 3 H 8 blending deteriorated NH 3 spray diffusion combustion due to increased air demand. • Sustained main fuel combustion achieved through hydrocarbon blending or extended pilot injection duration. Blending lower hydrocarbons with ammonia (NH 3 ) improves the ignition characteristics and burning velocity of NH 3 in premixed flames. However, the effect of blending lower hydrocarbons with NH 3 in the high-pressure dual-fuel (HPDF) mode of marine engines, involving intricate physical and chemical processes, has not yet been clarified. To investigate the effect of lower hydrocarbon blending with NH 3 on spray combustion in the HPDF mode of marine engines, this study conducted combustion tests on NH 3 /methanol (CH 3 OH) or NH 3 /propane (C 3 H 8 ) fuel blends in a rapid compression and expansion machine that can simulate a medium-speed marine engine. Fuel blends of NH 3 /CH 3 OH and NH 3 /C 3 H 8 were injected directly into the cylinder. The ignition was assisted by a pilot diesel injection and the diffusion combustion of the fuel spray of NH 3 /lower hydrocarbons was captured using a shadowgraph method. In addition to the heat-release analysis, exhaust emissions were analyzed using a Fourier-transform infrared (FTIR) exhaust gas analyzer. The use of NH 3 /CH 3 OH fuel blends improved the flammability of NH 3 by shortening the ignition delay and main combustion duration, improving the heat-conversion rate, and reducing the nitrogen oxide emissions. NH 3 /C 3 H 8 blends occasionally exhibited deteriorated combustibility, especially at C 3 H 8 blending ratios of 30 wt%, with prolonged combustion duration and reduced heat-conversion rates. This phenomenon can be attributed to an increase in the stoichiometric air requirement and shortening in flame liftoff length. The blending of CH 3 OH or C 3 H 8 with NH 3 , or an extended pilot injection, enable sustained combustion of the main fuel, thereby preventing a drop in the heat-release rate.