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

Abstract

• Experimental analysis of soot in ammonia-diesel engine exhaust is presented. • Smaller primary particles indicate reduced growth of diesel soot by adding ammonia. • Ammonia addition yields more graphitic diesel soot that is less reactive to growth. • The nitrogen content of the soot surface increases linearly with ammonia addition. • Higher potential for forming nitrogenated polyaromatic hydrocarbons is found. The impact of ammonia (NH 3 ) 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 NH 3 is proposed as a near-term decarbonization solution. In this study, the effect of NH 3 port injection on the GHG emissions and the characteristics of the soot formed in a NH 3 -diesel dual fuel CI engine is investigated. Soot is sampled from the engine exhaust operating in dual-fuel mode at 20% and 40% NH 3 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 (CO 2 ) is reduced while N 2 O emissions increases with NH 3 addition. The engine soot emissions yield is significantly reduced with the average primary particles size and number concentration decreasing as the NH 3 energy fraction increases. The soot nanostructure is impacted by NH 3 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 NH 3 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 NH 3 addition. This indicates an increased potential of NH 3 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.