Experimental and kinetic modeling of nitrogen oxide formation in ammonia/N-dodecane dual-fuel combustion

The use of diesel to ignite ammonia combustion is a key approach for utilizing ammonia as an alternative fuel in large compression ignition engines. This study develops a simplified reaction mechanism for ammonia/diesel mixtures using a decoupling modular approach, incorporating the chemical kinetics of n-dodecane and ammonia, as well as key steps in the formation of polycyclic aromatic hydrocarbons (PAH) and nitrogen oxides (NOx). Laminar flame speeds of ammonia-n-dodecane mixtures under various conditions are experimentally measured to validate the mechanism. The results show that ammonia reduces thermal NOx formation by lowering combustion temperatures in laminar flames. Fuel NOx mainly arises from ammonia decomposition products (NH2, H radicals). As flame temperature increases, free radical generation promotes fuel NOx consumption and thermal NOx formation. In-cylinder, increasing ammonia energy ratio (AER) from 0 % to 45 % increases fuel NOx production, with 45 % AER yielding three times higher fuel NOx than 25 %. The peak thermal NOx formation is delayed, and high-temperature zone distribution is reduced, indicating ammonia's role in inhibiting thermal NOx formation.