Abstract
Effects of simultaneous, together with separate addition of hydrogen to the fuel and water vapour to the oxidizer on soot formation in laminar, counterflow ethylene diffusion flames at atmospheric pressure were studied numerically. A polycyclic aromatic hydrocarbon (PAH)-based sectional soot model coupled with detailed gas-phase chemistry, was used to investigate the underlying chemical pathways of soot formation. In agreement to available studies, the results showed that, in addition to the dilution effects, the fuel-side enrichment of hydrogen and/or oxidizer side dilution of water vapour suppress the soot formation through chemical effects. The reduction in soot formation through chemical effects of H2 and H2O addition is mainly attributed to the reduced rates of soot surface growth due to suppression of the H-abstraction reaction in the hydrogen-abstraction-C2H2-addition (HACA) sequence as a result of an increased molecular hydrogen concentration. The simulations indicate that the chemical effects tend to increase PAH concentration within the soot formation region, which results in increased soot nucleation rates and number density. However, the contribution of soot nucleation in the overall soot formation process becomes secondary, while the soot surface growth predominantly governs the chemically inhibiting effects of H2 and H2O addition. Compared to separate addition, simultaneous addition of hydrogen to the fuel and water vapour to the oxidizer proves to be more effective in soot suppression for the same dilution level, since there exist weak synergistic effects between H2 and H2O. The numerical analysis further demonstrates that the separate, as well as simultaneous addition of H2 and H2O, tend to decrease the number density of larger-sized particles, causing the reduction in average soot particle size.
Highlights
Considering the severe environmental and health impacts of soot, the legislative regulations on their emissions from combustion systems have become stringent
The important kinetic pathways associated with the chemical effects of adding either hydrogen to ethylene (SepH2 cases) or water vapour to oxidizer (SepH2O cases) are discussed first
It is evident that both H2 and i-H2 suppress the soot formation, with H2 being more effective in soot reduction compared to i-H2
Summary
Considering the severe environmental and health impacts of soot, the legislative regulations on their emissions from combustion systems have become stringent. Blending of reactants with various non-fuel diluents, such as N2, CO2, H2O, He, Ar and reactive additives, such as H2, CO, dimethyl ether (DME) remains one of the most effective approaches to suppress the soot formation in combustion devices. In principle, such diluents influence the soot formation via four important mechanisms [1]. (d) The radiative exchange, through which diluents modify flame temperatures, and affect the overall soot production These effects are strongly coupled, which makes it almost impossible to quantify and investigate the isolated effects through experiments. Given the intricacies of physico-chemical interactions associated with soot formation, understanding inherent mechanisms through which various additives influence the flame properties, PAH formation, soot inception, and surface growth stages, is very challenging, and continues to be an active area of research
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