Flame Structure and Soot-Precursor Formation of Coflow $n$-Heptane Diffusion Flame Burning in ${\mathrm{O}}_2/{\mathrm{N}}_2$ and ${\mathrm{O}}_2/{\mathrm{CO}}_2$ Atmosphere

Oxycombustion allied with flue gas recirculation is a promising combustion technology owing to its potential to enhance combustion while reducing pollutant emissions. This numerical work investigated the effects of ${\mathrm{CO}}_2$ dilution under oxygen enrichment conditions on the flame structure and soot precursor formation in an $n$-heptane coflow diffusion flame. The numerical approach considered kinetic chemistry, thermal and transport properties, and a radiative heat transfer model. The oxygen content in the oxidizer stream was varied from 30% to 60% ${\mathrm{O}}_2$ (molar basis) with ${\mathrm{N}}_2$ or ${\mathrm{CO}}_2$ as diluent. The chemical effect of ${\mathrm{CO}}_2$ diluent was isolated numerically from its total thermal effect using an inert counterpart fictitious ${\mathrm{CO}}_2$. Results demonstrated that increasing ${\mathrm{O}}_2$ concentration significantly increased the flame temperature and soot precursor species, whereas a notable reduction of flame height occurred with increasing ${\mathrm{O}}_2$. Soot precursors were suppressed by replacing ${\mathrm{N}}_2$ with ${\mathrm{CO}}_2$ in the coflow oxidizer through not only thermal effects but also through chemical effects. The primary pathway for the chemical effect of ${\mathrm{CO}}_2$ is the reaction ${\mathrm{CO}}_2+\mathrm{H}=\mathrm{CO}+\mathrm{OH}$, which decreases H radicals. The results also showed that ${\mathrm{CO}}_2$ the effect of ${\mathrm{CO}}_2$ was more profound at lower oxygen conditions, indicating that the inhibition effect of ${\mathrm{CO}}_2$ could be restricted in higher oxygen conditions.