Chemical Kinetics of Ethanol Oxidation

Experimental profiles of stable species mole fractions are reported for ethanol oxidation in a Variable Pressure Flow Reactor (VPFR) at initial temperatures of 800-950K and constant pressures of 3 to 12atm. A detailed mechanism for ethanol combustion is developed in a hierarchical manner, and at each level, the newly added portions of the mechanism are tested by thorough comparison of model predictions and experimental results found in laminar premixed flames, shock tubes, and flow reactors. The present ethanol mechanism predicts reasonably well the major species profiles of the VPFR oxidation experiments, and significantly improves the predictions of the experimental targets originally investigated by a most recent ethanol mechanism [1]. * Corresponding author: [email protected] Associated Web site: http://www.princeton.edu/~combust Proceedings of the European Combustion Meeting 2005 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 oxygen-additive 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 [2] 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 carbon monoxide. 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 chemistry of gas-phase oxidation and pyrolysis of ethanol have been the subjects of numerous studies over the last five decades. Data have been reported from shock tubes, static reactors, flow reactors, diffusion flames, and laminar premixed flame experiments. Norton and Dryer [3] conducted a series of ethanol oxidation experiments in an Atmospheric Pressure Flow Reactor (APFR). In the modeling efforts of Norton and Dryer, as well as those of several other investigators [4,5] emphasized the importance of including all three isomeric forms of C2H5O produced by H-atom abstraction from ethanol. More recently, Marinov [1] carried out an extensive detailed kinetic modeling study of ethanol combustion at intermediate and high temperatures. His computational results indicated that ethanol oxidation exhibits strong sensitivity to branching ratio assignments of the H-atom abstraction reactions of ethanol as well as to the kinetics of its unimolecular decomposition. The mechanism performed reasonably well under the experimental conditions found in laminar premixed flames, shock tubes, and the APFR. However, few experiments to validate the pyrolysis and highpressure oxidation characteristics of ethanol at flow reactor conditions were available for comparison. In recently published works [6,7], our laboratory has produced new pyrolysis data, and new rate determinations for ethanol uni-molecular decomposition reactions. The present paper reports new flow reactor data of ethanol oxidation at high-pressure conditions (3-12atm with initial temperatures of 800-950K). Since the Marinov mechanism does not predict well either the earlier pyrolysis or the new oxidation data, a revised, new detailed oxidation mechanism was developed. The mechanism is described and compared here with a wide range of experimental data, including the present highpressure flow reactor results, shock tube ignition delay data, and laminar flame speed measurements. Experimental Methods Experiments were conducted in the Princeton Variable Pressure Flow Reactor (VPFR). A schematic of the flow reactor is shown in Figure 1. Detailed information about the VPFR instrumentation and experimental methodology can be found in other publications [8,9], and only a brief description of these issues is given here. Carrier gas (N2 in this study) is heated by a pair of electrical resistance heaters and directed into a reactor duct. Oxygen is also introduced at the duct entrance. The carrier gas/oxygen mixture flows around a baffle plate into a gap serving as the entrance to a diffuser. The vaporized fuel (ethanol) flows into the center tube of a fuel injector, and injected radially outward into the gap where it rapidly mixes with the carrier gas and oxygen. The reacting mixture exits the diffuser into a constant area test section. Near the exit of the test section, a sampling probe is positioned on the reactor centerline to continuously extract and convectively quench a small portion of the flow. At the same axial location, the local gas temperature is measured with a type R thermocouple.