Studies of the combustion characteristics of alcohols, aldehydes, and ketones

2011-07-06The combustion characteristics of oxygenated C₁-C₄ hydrocarbons were investigated both experimentally and numerically in laminar premixed and non-premixed flames. These characteristics included laminar flame speeds and extinction limits. Experimentally, flames were established in the counterflow configuration and flow velocity measurements were made using the digital particle image velocimetry technique. All experiments were conducted at an elevated unburned reactant temperature and at atmospheric pressure. A wide range of fuels were studied including the C₁-C₄ alcohols, C₃-C₄ aldehydes, and C₃-C₄ ketones. Numerically, laminar flame speeds and extinction limits were simulated using quasi-one-dimensional codes which integrated the conservation equations with detailed descriptions of molecular transport and chemical kinetics. ❧ Premixed flames of methanol, ethanol, and n-butanol were initially studied. Experimental results revealed that the laminar flame speeds of methanol/air flames are considerably higher than both ethanol/air and n-butanol/air flames under fuel-rich conditions. Additional measurements were conducted to determine the laminar flame speeds of methane, ethane, and n-butane flames in order to compare the effect of alkane and alcohol molecular structures on high-temperature flame kinetics. It was shown that laminar flame speeds of ethanol/air and n-butanol/air flames are similar to those of their n-alkane counterparts, and that methane/air flames have consistently lower laminar flame speeds than methanol/air flames. Two recently developed detailed chemical kinetic reaction models for n-butanol oxidation were used to simulate n-butanol/air laminar flame speeds and extinction limits. Numerous kinetic differences were revealed between these models regarding the consumption pathways of n-butanol and its intermediates. ❧ The combustion characteristics of premixed flames of the remaining three butanol isomers were then studied. Experimental results revealed that n-butanol/air flames propagate somewhat faster than both sec-butanol/air and iso-butanol/air flames, and that tert-butanol/air flames propagate notably slower compared to the other three isomers. Experiments were simulated using a recently developed chemical kinetic reaction model for the oxidation of the four isomers of butanol. Reaction path analysis of numerical simulations of tert-butanol/air flames revealed iso-butene to be a major intermediate, which subsequently reacts to form the resonantly stable iso-butenyl radical retarding thus the overall reactivity of these flames relatively to the other three isomers. ❧ A study similar to the first two was then conducted in which the combustion characteristics of the two propanol isomers and propane were investigated. Experimental results revealed, as expected, that the laminar flame speeds and extinction limits of n-propanol/air and propane/air flames are close to each other whereas those of iso-propanol/air flames are consistently lower. The chemical kinetic reaction model used in this study was found to overpredict the experimental results for fuel-rich n-propanol/air, iso-propanol, and propane/air flames. Analysis revealed that those discrepancies are most likely caused by deficiencies in the C₃ alkane kinetics. ❧ The final study focused on some of the key oxygenated intermediate species formed during the oxidation of the aforementioned alcohols, namely the C₃-C₄ aldehydes and ketones. Acetone/air flames were determined to propagate notably slower than butanone/air flames. For the aldehydes, between fuel-lean and stoichiometric conditions, propanal and n-butanal/air have very similar laminar flames speeds. For fuel-rich conditions propanal/air flames propagate faster than n-butanal/air flames. For all equivalence ratios considered iso-butanal/air flames propagate the slowest of flames of the aldehydes. It was also observed that flames of the aldehydes propagate significantly faster than their corresponding ketones, although this effect diminishes with increasing carbon chain length