Molecular growth and oxygenated species formation in laminar ethylene flames

Abstract

Ethylene constitutes a key intermediate in the oxidation of practical hydrocarbon fuels. Past studies of ethylene oxidation in shock tubes, flow/stirred reactors, and flames have provided an understanding of several key aspects of the fuel oxidation chemistry. However, the current trend, toward leaner premixed combustion in practical devices results in significant changes in the combustion regime, and studies under fuel-lean flame conditions have become essential. The changes in the main reaction channels are significant, and the chemistry of oxygenated species comes to the fore. The present work compares the main reaction paths in fuel-rich and fuel-lean ethylene flames, with particular emphasis on the relative importance of addition, abstraction, and isomerization paths in the context of the formation of oxygenated and aromatic species. It is shown that in the fuel-lean flame, ethylene is mainly consumed by addition reactions, while in the rich flame environment, abstraction reactions leading to vinyl radical formation dominate. The balancing of the destruction chemistry of the latter and the importance of the rate and product distribution of the dolecular oxygen attack are discussed in detail. It is noted that the channel C2H4+O=CH2CHO +H is the dominant route to vinoxy under lean flame conditions, and the present work supports the acetyl channel (CH2CHO→CH3CO→CH3+CO) for vinoxy destruction at elevated temperatures. Potential benzene formation paths are assessed, and it is concluded that vinyl radical addition to vinylacetylene and propargyl radical recombination constitute the major benzene formation paths in rich ethylene flames. The present work thus further emphasises the need to consider multiple benzene formation channels.