A numerical study of NOX and soot emissions in ethylene-ammonia diffusion flames with oxygen enrichment

Abstract

Blending ammonia with hydrocarbon fuels is gaining interest due to its potential to significantly reduce carbon emissions from the combustion of these blends. Additionally, studies have provided strong evidence of oxygen enhanced combustion to reduce soot emissions and improve flame stability. The present work attempts to leverage the benefits of both strategies, ammonia doping and oxygen enhanced combustion, by considering ethylene diffusion flames. Simulations are performed using the CHEMKIN-Pro software. A reaction mechanism with 372 species and 13,847 reactions is developed by combining gas phase chemistries for ethylene, NH3 and NOX, along with a detailed soot mode. The mechanism is extensively validated for a range of fuel blends in terms of flame structure, intermediate hydrocarbons, soot precursors and soot. Oxygen enrichment is achieved by removing certain amount of nitrogen from oxidizer stream and adding it to fuel stream, and thereby maintaining a nearly constant flame temperature. Extensive analysis is carried out to characterize the chemical and dilution effects of ammonia doping on flame structure, soot precursors, soot, and NOX in ethylene flames with varying levels of oxygenation. Further, dominant reaction pathways for several soot precursors and NOx are identified through detailed rate of production analyses. Results indicate that ammonia doping in ethylene flames leads to substantial increase in NOX emissions, while oxygenation has negligible influence on NOX emissions. In contrast, both NH3 doping and oxygenation provide significant reduction in the formation of soot precursors and soot. Oxygenation achieves this reduction through both hydrodynamic and flame structure effects, as the flame shifts from oxidizer side to fuel side. On the other hand, NH3 doping reduces the rates of important reactions for the formation of soot precursors, as the concentrations of relevant species (C2H2, C3H3, C3H3-P, C3H3-A etc.) decrease due to their consumption through reactions with species like NCO and CN formed from NH3. Moreover, the effectiveness of NH3 doping is further enhanced when combined with oxygenation. Hence, NH3 doping and oxygenation are found to be effective and synergistic strategies in reducing soot emissions, and their proper combination could provide near zero soot emissions.