Study on PAH and soot formation in 2,5-dimethylfuran counterflow diffusion flames with ammonia addition

• The effect of ammonia addition on polycyclic aromatic hydrocarbon and soot formation in 2,5-dimethylfuran flames was first studied. • Ammonia addition inhibits the formation of polycyclic aromatic hydrocarbon and soot in 2,5-dimethylfuran flames. • Two important polycyclic aromatic hydrocarbon formation pathways in 2,5-dimethylfuran flames were found: de-cyclization and isomerization. • The addition of ammonia leads to a decrease in the soot inception rate and the soot surface growth rate, which further results in a decrease in the soot volume fraction. The influence of ammonia addition on the formation of polycyclic aromatic hydrocarbon (PAH) and soot in 2,5-dimethylfuran (DMF) counterflow diffusion flames was investigated by means of laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques. The results indicated that PAH emerged earlier and were more densely distributed in DMF flame than in n-heptane flame. Moreover, the addition of ammonia inhibited the formation of PAH (such as A2, A3, and A4) and soot in DMF flames. In DMF flames, there were two crucial pathways for PAH formation. Path 1 involved small molecules like C 2 H 2 , which were derived from the ring-opening reaction of DMF, as the key building blocks. Path 2 (via the reaction R2451: 2C 5 H 5 = A2 + 2H) utilizes C 5 H 5 , formed through the dehydrogenation and isomerization of DMF, as the key building block. Kinetic analysis revealed that the addition of ammonia initially heightened the competition for H and CH 3 radicals in the vicinity of the fuel nozzle. This, in turn, inhibited the dehydrogenation and isomerization of DMF in Path 2 and promoted the ring-opening reaction of DMF in Path 1 (initial benzene ring formation). Further downstream, the increased H radicals inhibited the formation of A2 by suppressing reaction R2451 above. Finally, after the addition of ammonia, both the soot inception rate and the soot surface growth rate were reduced, resulting in a decrease in the soot volume fraction SVF.