Dissertation/Thesis Abstract

A flow-reactor study of vitiated ethane oxidation at intermediate temperatures
by Walters, Kevin M., Ph.D., Stanford University, 2008, 285; 3313825
Abstract (Summary)

Vitiated combustion processes such as Flameless oxidation (FLOX), Moderate Intensive Low-Oxygen Dilution (MILD) combustion, High-Temperature Air Combustion (HiTAC) and Homogeneous Charge Compression Ignition (HCCI) offer the potential to improve the thermodynamic efficiency in hydrocarbon-fueled combustion systems, providing a subsequent decrease in energy-specific CO2 emissions. As an added benefit, these systems typically have low emission levels of nitrogen oxides (NOx) and particulate matter. However, the need remains for the fundamental study of oxidation kinetics of vitiated mixtures, especially at elevated pressure and intermediate temperatures where a significant amount of chain-branching can be attributed to peroxy species. This dissertation comprises a combined experimental and modeling study of vitiated oxidation in a high-pressure flow reactor (HPFR). Ethane (C2H 6), which is the second-most abundant constituent in natural gas, is used as the fuel surrogate in this study.

A systematic parameter study investigating the effect of pressure (1–6 bar) and O2 mole fraction (3.5–7.0%) on lean C2H 6 oxidation in vitiated air is performed at temperatures of 1075–1100K. The vitiated gas mixtures contain 1518 mole % H2O. Concentrations of stable gas-phase species are measured by extractive sampling and on-line analysis. Time-history measurements of species (C2H6, C2H4, CO, CO2 H2, CH2O and O2) are used to characterize the overall rate of reaction and track the fuel carbon through intermediate and product species.

A consistent framework is presented to mode the chemically reacting flow in the HPFR. A 1-dimensional mixing-reacting model that accounts for partial oxidation during reactant mixing is used to implement a large detailed kinetic mechanism. This model is then implemented as part of a Solution Mapping optimization of an existing reaction mechanism using experimental targets from this work.

Experimental results show the evolution of the parent fuel (C2H 6) to products (CO and CO2) over residence times ranging from 35–100 ms. The stable-species measurements represent more than 95% of the injected fuel carbon and are consistent with the measured temperature rise. The optimized mechanism results in a modest improvement in the agreement with the experimental data. The largest remaining discrepancy is for CH 2O, which may be attributed to polymerization in the sample transferring system. The simulation is used to identify the elementary reaction pathways for the oxidation of ethane. Changes in these competing pathways due to variation sin pressure and O2 mole fraction five rise to the complex pressure dependence seen in the experiments.

A validation study is performed for the optimized mechanism including a variety of C2H6 oxidation experiments reported in the literature. The performance of the mechanism for a selection of flow reactor, shock tube, stirred reactor and laminar flame speed measurements is presented. Simulation results using the optimized mechanism are generally in good agreement with available experimental data.

Indexing (document details)
Advisor: Bowman, Craig T.
School: Stanford University
School Location: United States -- California
Source: DAI-B 69/05, Dissertation Abstracts International
Subjects: Chemical engineering, Mechanical engineering
Keywords: Ethane oxidation, Flow reactor, Kinetics, Mechanism optimization, Vitiated combustion
Publication Number: 3313825
ISBN: 978-0-549-62998-6
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