A computational analysis of soot precursor generation: Propargyl chemistry and permanently blue flames at high Z_st

A detailed computational study was performed on the effects of the flame structure on the formation and destruction of soot precursors during ethylene combustion using the USC Mech Version II mechanism. The contribution of different pathways to the formation of benzene and phenyl was determined in a wide domain of Z_st values via an original reverse pathway analysis. It was shown that for conventional ethylene-air flames two reversible reactions play a primary role in the propargyl (C₃H₃) chemistry, namely C₂H₂ + CH₃ = p C₃H₄ + H (1) p C₃H₄ = C₃H₃ + H (2) with the corresponding overall endothermic reaction of propargyl formation C₂H₂ + CH₃ = C₃H₃ + 2H. (3) At low Z_st these reactions contribute significantly to propyne (pC₃H₄) and propargyl formation on the fuel side of the radical pool at temperatures greater than approx. 1600 K. At higher local temperatures near the radical pool where the concentration of H is significant, the reverse reactions begin to dominate resulting in net propyne and propargyl destruction. As Z_st is increased the region of net propyne and propargyl formation associated with these reactions diminishes and eventually becomes a region of net destruction. In the low Z_st flame propargyl self-combination leading to benzene and phenyl accounts for nearly 60% of the total propargyl consumed; however, at high Z_st the reverse of Reaction 2 becomes the dominant propargyl destruction reaction. Consequently, at sufficiently high Z_st the pathway to benzene and phenyl through propargyl is essentially cut-off. A physico-chemical explanation of this phenomenon is presented. At high Z_st the flame structure is altered resulting in lower temperatures and more H in the region associated with propargyl production. The calculated dependence of the reaction quotient, Q_r, of Reaction 3 vs. temperature at different Z_st suggests the existence of a "permanently blue flame boundary", i.e. a Z_st value above which soot will not be observed. For this system such a boundary was observed near Z_st = 0.3, which compares well with the experimental results for permanently blue counterflow flames in the literature.