Experimental and Numerical Studies of Ethanol Decomposition Reactions

Ethanol pyrolysis experiments at 1.7 – 3.0 atm and 1045 – 1080 K were performed in the presence of radical trappers using a variable pressure flow reactor. Stable species time histories were determined using continuous sampling, on-line Fourier Transform Infrared Spectrometry, and off-line Gas Chromatography. The rate constant k1 of the molecular decomposition reaction, C2H5OH → C2H4 + H2O (R1), was determined experimentally. The obtained result agrees very well with extrapolation of the recent shock tube data of Herzler et al. The multi-channel unimolecular decomposition of ethanol was also investigated theoretically based upon RRKM/master equation calculations. The effects of the hindered rotations in C2H5OH and quantum tunneling on the molecular decomposition reaction were taken into account. The reaction (R1) was found to be strongly dependent on temperature and the dominant channel over the range of temperatures from 300 to 2500 K at 1 atm. The calculated k1 is in excellent agreement with the recent theoretical work of Tsang as well as with the experimental measurements of Herzler et al. and the present data. The influence of tunneling on the shape of the falloff is discussed. In addition, the RRKM/master 2 equation results were fit to modified Arrhenius expressions to facilitate chemical kinetic modeling applications of the results. INTRODUCTION. Ethanol (C2H5OH) is a very important energy carrier that can be produced from renewable energy resources. It can be used as a fuel extender, octane enhancer, and oxygenadditive in, or as an alternative, neat fuel to replace reformulated gasoline. Ethanol also has potential as a hydrogen carrier for fuel cell applications. The 1990 Clean Air Act Amendments presently require the addition of oxygenates to reformulated gasoline, with seasonal adjustments, on the premise that oxygen content decreases automotive emissions, particularly smog generation participants and CO. Ethanol is favored to replace methyl tertiary butyl ether (MTBE), another widely used oxygenate additive that has become unpopular based upon ground water contamination and human health effects. While most ethanol is currently generated by fermentation (grain alcohol), recent developments suggest that ethanol fuel can be derived more efficiently from other biomass, thus offering potential to reduce dependence on fossil fuel energy resources. The chemical kinetics of ethanol related to combustion has been extensively studied in many previous works, with the most recent detailed modeling studies being those of Marinov. His work emphasized the high sensitivity of experimentally measured ignition delay during shock-induced decomposition of rich ethanol mixtures to the rate constants of ethanol decomposition reactions. Moreover, his analyses showed that high temperature ethanol oxidation is strongly sensitive to the falloff kinetics of the ethanol decomposition process, and to the branching ratio assignments among the ethanol abstraction reactions. Unfortunately, there were few ethanol pyrolysis data available for comparison at the time of this modeling work. While our recent ethanol pyrolysis experiments using the same variable pressure flow reactor employed here showed that H2O and C2H4 are the major products of ethanol thermal decomposition, we found that Marinov's model underestimated their production rate as well as the overall ethanol consumption rate. We also confirmed that ethanol pyrolysis is very sensitive to the decomposition reactions: C2H5OH → C2H4 + H2O, (R1), 3 C2H5OH → CH3 + CH2OH, (R2), as well as to hydrogen abstraction reactions with CH3 radicals, which primarily come from reaction (R2). Since the decomposition and abstraction pathways are coupled during both pyrolysis and oxidation, accurate description of the unimolecular decomposition process over a wide range of conditions is needed to further understand the contributions of the abstraction reactions in comprehensive kinetic models for describing ethanol combustion. Recently, Tsang conducted a theoretical study of the ethanol decomposition reactions. His predictions for the rate coefficient of reaction (R1) agree within 30% of those of Marinov at our experimental conditions. However, Tsang pointed out that Marinov's analyses of the other dissociation reactions were flawed since each was treated individually in his calculations. As a consequence, the competition among the different channels was not considered properly and the derived rate coefficients for the dissociation reactions including that of reaction (R2) were grossly overestimated. Additionally, Park et al. have conducted low temperature static reactor pyrolysis experiments and high temperature shock tube experiments to determine the rate constants of ethanol decomposition reactions. Park et al also performed a theoretical study of ethanol decomposition reactions. Their prediction for the rate coefficient of reaction (R1) is about one-third of Tsang's at 1100 K and 1 atm, i.e. at conditions attainable using flow reactors. In the present study, we conducted ethanol pyrolysis experiments in a variable pressure flow reactor in the presence of either toluene or 1,3,5-trimethylbenzene as a radical trapper to determine the rate constant of reaction (R1). The multi-channel thermal decomposition of ethanol was also studied theoretically by using the RRKM/master equation approach. Both the experimental data and the theoretical results are compared with the prior literature. EXPERIMENTS WITH RADICAL TRAPPERS. The ethanol decomposition reactions via C-C or C-O bond dissociation generate radicals, including CH3. These radicals can further react with the fuel by abstracting H-atoms, producing intermediates leading to C2H4, CH4, etc. Therefore, both the