Characterization of soot emissions formed in a compression ignition engine cofired by ammonia and diesel

Abstract

The impact of ammonia (NH3) co-firing with diesel on the greenhouse gases (GHG) and soot emissions of compression ignition (CI) engines is a current concern in large-scale agriculture, marine transportation and shipping applications for which NH3 is proposed as a near-term decarbonization solution. In this study, the effect of NH3 port injection on the GHG emissions and the characteristics of the soot formed in a NH3-diesel dual fuel CI engine is investigated. Soot is sampled from the engine exhaust operating in dual-fuel mode at 20% and 40% NH3 energy fraction and in diesel-only mode at a fixed engine speed, load, and diesel injection strategy. Detailed analysis of soot is done to investigate: the exhaust soot yield using gravimetric analysis, soot growth and inception through the average primary particles size and number concentration using analysis of Transmission Electron Microscopy (TEM) imaging, the soot nanostructure using Raman spectroscopy, and the chemical composition using X-ray Photoelectron Spectroscopy (XPS). The engine GHG emissions measurements shows that carbon dioxide (CO2) is reduced while N2O emissions increases with NH3 addition. The engine soot emissions yield is significantly reduced with the average primary particles size and number concentration decreasing as the NH3 energy fraction increases. The soot nanostructure is impacted by NH3 addition as it becomes more graphitic with high ratio of sp2 to sp3 carbon bonding. The XPS chemical composition analysis also shows an increase in the nitrogen bonding with carbon in the aromatic rings on the particles surface. The results suggest that the chemical interaction of NH3 with diesel results in soot with more graphitic nanostructure as nitrogen reaction with carbon at active defect sites leads to an increase in the nitrogen content on the soot surface with NH3 addition. This indicates an increased potential of NH3 co-firing with diesel to form nitrogenated PAHs (N-PAH) on the soot surface which would require further studies to identify the risks posed on the environment and human health.