Flame Structure and Soot-Precursor Formation of Coflow n-Heptane Diffusion Flame Burning in O2/N2 and O2/CO2 Atmosphere

Abstract

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 CO2 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% O2 (molar basis) with N2 or CO2 as diluent. The chemical effect of CO2 diluent was isolated numerically from its total thermal effect using an inert counterpart fictitious CO2. Results demonstrated that increasing O2 concentration significantly increased the flame temperature and soot precursor species, whereas a notable reduction of flame height occurred with increasing O2. Soot precursors were suppressed by replacing N2 with CO2 in the coflow oxidizer through not only thermal effects but also through chemical effects. The primary pathway for the chemical effect of CO2 is the reaction CO2+H=CO+OH, which decreases H radicals. The results also showed that CO2 the effect of CO2 was more profound at lower oxygen conditions, indicating that the inhibition effect of CO2 could be restricted in higher oxygen conditions.