Propanol isomers: Investigation of ignition and pyrolysis time scales

This study presents an investigation of the ignition and pyrolysis kinetics of the two propanol isomers. Experiments are carried out behind reflected shock waves at average pressures of 3.5, 5.0, and 11 atm over a temperature range of 1150-1550 K. Oxidation experiments are carried out using mixtures with equivalence ratios of 0.5, 1.0, and 2.0. Laser absorption measurement of CO is carried out near 4.6 µm on the basis of which a characteristic pyrolysis time scale is defined. By means of this pyrolysis time scale, the effects of reactor temperature, pressure, and fuel concentration on pyrolysis are established. Contrast is further made between the global kinetics of pyrolysis and purely oxidative kinetics during ignition. The measured ignition and pyrolysis time scales are also compared with predictions of three chemical kinetic models from the literature. The expected lower reactivity of iso-propanol relative to n-propanol during ignition and pyrolysis is observed. The study also reveals a distinctly higher global temperature sensitivity of the pyrolysis process (global activation energy > 60 kcal/mol) compared to the typical global activation energy of ignition (global activation energy in the range of 20-40 kcal/mol). This difference leads to a cross-over effect at a characteristic reactor temperature between ignition that is controlled by pyrolysis and one that is controlled by predominantly oxidative kinetics. While this is observed for the chosen molecular systems, the cross-over effect is a general feature of high-temperature fuel reaction kinetics. At lower temperatures the chemical time scales of predominantly pyrolytic reactions are much longer than the time scales of oxidative radical processes which dominate the ignition process whereas at higher temperatures the time scales of pyrolytic processes become shorter and dominate the global ignition kinetics. Selected CO concentration profiles obtained during pyrolysis are also compared with predictions of current chemical kinetic models, revealing varying degrees of agreement and discrepancies among the models. The study advances characterization of propanol combustion and demonstrates the use of global chemical time scales to characterize fuel pyrolysis kinetics.