Detailed Chemical Kinetic Study of Acetaldehyde Oxidation and Its Interaction with NOx

This work entails a detailed modeling and experimental study for the oxidation kinetics of acetaldehyde (CH₃CHO) and its interaction with NO_x. The ignition behavior of CH₃CHO/O₂/Ar has been investigated in a shock tube over the temperature range of 1149 to 1542 K, with equivalence ratios of 0.5 and 1.0 and pressures near 1.2 bar. Absorbance–time profiles of acetaldehyde were recorded using a mid-IR laser during the autoignition measurements. A comprehensive kinetic model has been developed to quantitatively predict the oxidation of acetaldehyde and its interaction with NO_x. The kinetic model has been validated using experimental data of this work and available literature data from shock tube, plug flow, and jet-stirred reactors, freely propagating, and burner-stabilized premixed flames. For better accuracy of the kinetic model, the thermochemistry of 14 important species in the acetaldehyde submechanism was calculated using ab initio methods. The heat of formation of these species was computed using atomization and isodesmic reaction schemes. For the first time, this modeling study examines the effect of NO on acetaldehyde oxidation behavior over a wide range of experimental conditions. In most cases, the proposed kinetic model captures the experimental trends remarkably well. Interestingly, the doping of NO in CH₃CHO did not perturb the NTC behavior of CH₃CHO in contrast to other fuels, such as n-heptane and dimethyl ether. However, for flow reactor conditions at 1 atm, doping with 504 ppm of NO was found to promote the reactivity of acetaldehyde by lowering the onset temperature for CH₃CHO oxidation by ∼140 K. The hydroxyl radical is the main cause of this shift, which originates from the NO + HO₂ = OH + NO₂ reaction. Further evolution of hydroxyl radicals occurs via the “NO–NO₂” looping mechanism and expedites the reactivity of the system. This experimental and modeling work sheds new light on acetaldehyde oxidation behavior and its interaction with NO_x under combustion-relevant conditions.