The influence of nitrogen and hydrogen addition/dilution on soot formation in coflow ethylene/air diffusion flames

Abstract

Harmful emissions from combustion, such as soot, have severe adverse effects on the environment and human health. One way to mitigate these emissions and their dangerous influence is through fuel dilution. In the present study, the effects of adding different percentages of hydrogen and nitrogen to the fuel in a set of laminar coflow ethylene/air diffusion flames are investigated numerically by employing an in-house algorithm, the CoFlame code. The various influences of hydrogen, including dilution, density, transport, thermal, and chemical effects on soot formation and flame characteristics, are evaluated by introducing different fictitious species into the chemical mechanism. The numerical calculations of temperature, soot volume fraction, and average primary particle diameter of soot are in agreement with experimental measurements from the literature. Results show that blending hydrogen and nitrogen into the fuel decreases soot volume fraction and radiation. The hydrogen abstraction C2H2 addition (HACA) surface growth and polycyclic aromatic hydrocarbon (PAH) condensation rate reductions are responsible for the decrease in soot volume fraction. It is found that the soot formation/oxidation reaction rates are decreased under the dilution effect while increased under the transport and chemical effects. Among the various effects of hydrogen addition, the dilution effect has the most significant influence on soot formation/oxidation reaction rates. It is seen that the density and thermal effects have a negligible effect on soot volume fraction.