Effect of CO₂/H₂O addition on laminar diffusion flame structure and soot formation of oxygen-enriched ethylene

Oxygen-enriched combustion technology has an important application prospect, in which Oxy-steam combustion is a new generation of potential carbon capture technology. In this paper, the effects of O₂/CO₂ and O₂/H₂O atmosphere on soot formation in laminar diffusion flame were studied. The flame image was taken by a color (CCD) camera in the visible band, the directional emission power in R and G bands was calibrated, and the two-dimensional distribution of temperature and soot volume fraction in the flame was reconstructed. The experimental results are compared with the numerical results, and the numerical results well predict the temperature of oxygen-enriched flame in O₂/N₂/CO₂ atmosphere, but overestimate the soot volume fraction. The laminar diffusion flame of ethylene in two kinds of oxygen-rich atmosphere was numerically studied by using a step-by-step decoupling method to separate the density, transport, thermal and chemical effects of the diluent by introducing four virtual substances. The calculation results show that combustion in O₂/H₂O and O₂/CO₂ atmosphere reduces the flame peak temperature, but the effect on the flame centerline temperature distribution is different. The addition of H₂O enhanced the concentration of OH through the reverse reaction rate of OH + H₂=H + H₂O, while the addition of CO₂ decreased the concentration of OH.O₂/CO₂ combustion leads to the increase of flame height and radius, while in O₂/H₂O atmosphere, the flame height is shortened. Both CO₂ and H₂O have a good inhibitory effect on soot formation. CO₂ in the oxidant affects soot nucleation and surface growth rate mainly by reducing flame temperature and H mole fraction, but not by promoting soot oxidation process. Water vapor also inhibits soot formation by increasing OH concentration through chemical effect.