The characteristics and mechanism of NO formation during pyridine oxidation in O2/N2 and O2/CO2 atmospheres

Highlights: • The experiments and simulation of the oxidation of pyridine in O₂/CO₂ atmospheres. • CO₂ has limited effect on the conversion of pyridine to HCN. • Under fuel-rich conditions, the higher availability of HNO promotes NO formation. • Under stoichiometric conditions, the inhibition of NH + ONO + H inhibits NO formation. • Under fuel-lean conditions, reduction by NCO radicals suppresses NO formation. -- Abstract: The characteristics and mechanism of NO formation during pyridine oxidation in O₂/CO₂ atmospheres are investigated both experimentally and numerically. Comparison experiments in O₂/N₂ and O₂/CO₂ atmospheres are performed in a flow reactor at atmospheric pressure covering fuel-rich to fuel-lean equivalence ratios with temperature ranging from 773 K to 1573 K. Experimental results indicated that HCN is completely consumed in CO₂ atmospheres, whereas significant amounts remain in N₂ atmospheres under fuel-rich conditions. Compared with O₂/N₂ atmospheres, the formation of NO in O₂/CO₂ atmospheres is reduced by 8.85% and 5.8% under stoichiometric and fuel-lean conditions respectively, whereas it is 5.15% greater under fuel-rich conditions. A newly developed chemical kinetic mechanism based on our previous studies satisfactorily reproduced the main features of CO, HCN, and NO formation. The conversion differences of pyridine to NO between O₂/CO₂ and O₂/N₂ atmospheres are mainly due to the differences of conversion of HCN to NO. The conversion ratio discrepancies of pyridine to HCN are all less than 2%. The conversion ratios of HCN to NO in O₂/N₂ and O₂/CO₂ atmospheres are 7.2% and 15.6% under fuel-rich conditions, 65.3% and 57.4% under stoichiometric conditions, and 83.5% and 76.3% under fuel-lean conditions, respectively.