The effect of benzene on the structure of low-pressure premixed H2/CH4/CO-air flames and related NO formation at different equivalence ratios

Coke oven gas (COG) is a valuable by-product of coke-making process, due to its high H2 content and calorific value. The potential exploitation of COG as an energy carrier requires a proper characterization of its kinetic behavior. Moreover, the residual presence of aromatic compounds in COG can potentially affect both its combustion efficiency and pollutant emissions. In this work, the chemical effect of benzene addition (up to 1.12% by vol.) to H2/CH4/CO mixtures, representative of COG, is assessed. Flame structure and speciation are extensively investigated through a combined experimental and modeling study, carried out in low-pressure (7.5 kPa) premixed laminar flames. Different equivalence ratios (0.8, 1.0, 1.2) are considered, with air as oxidant and a constant cold-gas velocity. It is found that NO formation progressively increases with the amount of benzene at all the investigated equivalence ratios, and such increase mostly occurs at the flame front region. Through a detailed kinetic analysis, it is found that benzene addition increases the NO formation in the H2/CH4/CO/C6H6 flames. This is mainly due to the enhancement of the prompt and thermal NO routes. The kinetic study allowed to identify the sources of the interactions between benzene oxidation and prompt NO chemistry, ultimately involving the ketenyl radical (HCCO) and acetylene (C2H2). This results in an increase of singlet CH2 (CH2(S)) and CH2 with consequent modification of CH profile, via HCCO→CH2(S)→CH2→CH and C2H2→CH2→CH routes. As a result, the higher available CH radical enhances NO formation through the established prompt mechanism, coherently with the experimental results.