Simultaneous impacts of H2 and CO2 transport on NO formations of premixed and diffusion opposed flames fueled by biogas-H2 blends

Abstract

With the numerical simulation, the transport impacts of H2 and CO2 on the NO formations of premixed and diffusion opposed biogas-H2 flames were isolated quantitatively by using the fictious species method. The combined and individual impacts of H2 and CO2 transport on the NO concentrations, radical pools and critical reactions were compared and investigated comprehensively. The results show that the NO concentration of diffusion flame can be increased steadily with either H2 or CO2 transport, while the NO concentration of the premixed flame can be increased and decreased at fuel-lean and fuel-rich conditions respectively owing to the dominant role of H2 transport in the coupling transport influence. The CO2 transport is confirmed to have the effective influence on the NO formations in the fast reaction zones of the premixed flames based on the convection-diffusion-reaction (CDR) budget analysis, but its effects on the NO concentration are weakened effectively owing to that the NO production in the post-flame zone plays a more dominant role in the NO concentration of the premixed flame. Additionally, the H2 and CO2 transport have the opposite impacts on the major NO production/destruction reactions in the premixed flame but they exert negative or positive effects simultaneously in the diffusion flame. In either premixed or diffusion flame, the H2 and CO2 transport can influence the NO production primarily by affecting the NO/NO2 conversion rate, the thermal route (N + OHNO + H) and the NNH intermediate route (HNO + HH2+NO). Particularly, unlike the CO2 transport effects on the thermochemistry identified in our previous studies, the CO2 transport is found to be relatively insignificant to the NO generation of the premixed biogas-H2 flame. Meanwhile, for the NO production in the diffusion biogas-H2 flame, the CO2 transport effects could account for around 25% of total transport effects of H2 and CO2, suggesting that the CO2 transport should be not neglected when investigating the NO production in the diffusion biogas-H2 flame.