A first evaluation of butanoic and pentanoic acid oxidation kinetics

Despite the recent interest in large carboxylic acid oxidation due to their presence in pyrolysis bio-oils, their kinetics of pyrolysis and oxidation has not been experimentally addressed. For the first time, this paper reports a new set of experimental data for the oxidation in a jet-stirred reactor of two high molecular weight carboxylic acids: butanoic (butyric) and pentanoic (valeric) acids. This work was performed at 106.7 kPa (800 Torr) over a range of temperatures from 800 to 1100 K. The experiments were carried out under highly diluted conditions (inlet fuel mole fraction of 0.005) for three equivalence ratios: 0.5, 1 and 2. During this study a wide range of products has been identified and quantified from CO and CO2 to C-5 species: 36 for pentanoic acid and 18 for butanoic acid. An interpretative kinetic model has been developed based on a recent theoretical study on the pyrolysis and oxidation of acetic acid (Cavalotti et al. PROCI, 37 (2019) 539-546) and on alkane rate rules (Ranzi et al. Combust. Flame, 162 (2015) 1679-1691). This new kinetic subset has been implemented in the CRECK kinetic framework covering the pyrolysis and oxidation of molecules from syngas up to heavy fuels, including PAHs formation. The mole fractions of fuel and product species were compared with results from model simulations over the experimental temperature range, providing reasonable agreement. A flow rate analysis allowed a better understanding of the most important degradation pathways of these acids, including a small contribution of low-temperature oxidation channels.