Effects of H2 addition on soot formation in counterflow diffusion flames of propane: A comparative analysis with He and N2 addition

Abstract

Co-firing hydrogen (H2) with hydrocarbon fuels is widely seen as an effective pathway to reduce soot emissions. Nevertheless, the soot-reducing potential of H2 can be strongly dependent on the type of the co-fired hydrocarbon and the combustion conditions. In extreme cases H2 enrichment could even promote soot production in hydrocarbon flames, thus highlighting the necessity of an in-depth understanding on the roles of H2 addition. The present study aims to clarify the effects of H2 addition on soot formation in non-premixed flames of propane. The effects of He and N2 addition are included for a cross-comparison to identify the different effects of H2 (i.e., direct chemical roles, dilution or transport effects associated with fuel stream's physical properties). The technique of laser-induced incandescence (LII) was used for soot volume fraction measurement, and accompanying soot modeling analysis was performed for additional kinetic insights into the experimental results. The measurements and modeling results consistently show that H2 is a more effective soot inhibitor than the chemical inert additive of He. Considering the comparative transport properties between H2 and He, we concluded that H2 has additional chemically-inhibiting effects on soot formation; kinetic analysis indicates that the chemical roles of H2 mainly occur in the stages of soot precursor formation and soot nucleation process. Another interesting finding is that while both He and N2 are chemically inert, He has notably stronger soot-reducing ability than N2, suggesting the favorable physical properties of He (i.e., the high diffusivity) in reducing soot formation. The major contributions of this work include: 1) experimentally identify the different effects of H2 (chemical, dilution and physical) on soot formation in counterflow flames of propane, 2) unravel the chemical roles of H2 in the different sooting stages including soot precursors formation, soot inception and surface growth process, 3) provide new datasets for mechanism validation and refinement related to sooting characteristics of H2/hydrocarbon mixture fuels.