EXPERIMENTAL DETERMINATION OF RATE CONSTANTS FOR REACTIONS OF THE HYDROXYL RADICAL WITH ALKANES AND ALCOHOLS A DISSERTATION SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Genny Anne Pang August 2012 © 2012 by Genny Anne Pang. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/rh393tq1232 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Ronald Hanson, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Craig Bowman I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. David Golden Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii iv Abstract Over one quarter of the energy usage in the United States currently occurs in the transportation sector. Improvements in energy conversion efficiency and sustainabil- ity in transportation applications, therefore, can substantially contribute to improved energy security in our future. The design of advanced high-efficiency energy conver- sion devices for transportation applications can be facilitated with complex computer models of combustion processes. The development of these models requires a large experimental database to ensure accuracy of the computational predictions. This thesis discusses how experimental studies are utilized to create a database of rate constants for elementary reactions; these rate constants are integral components of any computational model of combustion chemistry. During a combustion process, the reaction of the hydroxyl (OH) radical, a highly reactive chemical intermediate, with a combustible fuel molecule is a major fuel consumption pathway under many combustion conditions. Thus, the rate con- stants for these types of reactions must be accurately known to develop a com- putational model that correctly describes the combustion chemistry. This thesis presents an experimental method for measuring rate constants in the reaction family of OH + Fuel −! Products using a shock tube reactor, laser diagnostics, and tert- butylhydroperoxide (TBHP) as an OH radical precursor. Important rate constant parameters describing subsequent reactions of TBHP decomposition are also studied. Current transportation fuels of interest in the combustion community include molecules in the alkane and butanol classes. Alkane molecules are a major compo- nent of many petroleum-derived fuels such as gasoline and jet fuel. Isomers of the v butanol molecule are gaining popularity as a potential renewable alternative to gaso- line because of their high energy density and the many known methods of production from biomass and agricultural byproducts. The rate constant measurement method is applied to the reaction of OH with three alkane molecules (n-pentane, n-heptane, and n-nonane) and four isomers of butanol (n-butanol, iso-butanol, sec-butanol, tert- butanol), and the results are reported in this thesis. Comparison of the rate constant results to estimation methods in the literature are presented, and, for several of the isomers of butanol studied, the measured data are also used to validate and/or suggest refinements to existing detailed kinetic mechanisms. vi Acknowledgment The work presented in this thesis would not have been possible without the extensive support provided by my primary advisor Prof. Ron Hanson. I thank him for hav- ing confidence in me when I was initially exploring my graduate school options, for without his encouragement I may not have spent my graduate studies working in his world-class laboratory. And I am thankful that his support for me has never faded throughout my time at Stanford. His constant push to strive for the best in quality of work and presentation has no doubt led me to be a become a better researcher, scholar, teacher, and mentor than I would have been without his guidance. I would also like to thank Profs. Tom Bowman and Dave Golden for being my reading committee members and weekly consultants for my work. Prof. Bowman's meticulous attention to detail has taught me what high-quality research is, and also how to achieve it in my work. I thank Prof. Golden for teaching me to think like a chemist. The direction of this thesis work would not have been the same without his inspiration. I owe thanks to Profs. Jen Wilcox for chairing my oral exam, and also for teaching me about about topics that have increased my depth of understanding in this work. I am very thankful to Prof. Mark Cappelli for serving on my oral exam committee on short notice, and also being a supportive faculty member all throughout my graduate studies, starting from my very first class at Stanford. I also want to acknowledge support from Prof. Justin Du Bois. His patience and enthusiasm in teaching organic chemistry helped me develop a foundation to base many parts of this work, and I thank him for many interesting and insightful discussions in his office. I am immensely grateful for Dr. Dave Davidson's presence in the laboratory, and vii also in my life as a mentor, colleague, and friend. I am especially thankful for his willingness to always make time for students, whether by getting up from his desk to examine a laboratory problem with me, or just to provide wisdom, laughter, and chocolate during both good and difficult times. I also thank Dr. Jay Jeffries for his assistance in managing the laboratory and for helping me with occasional tasks. I feel fortunate to have worked with the extraordinary group of students in the Hanson Group. I particularly want to acknowledge the experimental assistance from Venky Vasudevan and Rob Cook. I thank them for their patience in teaching me to use the laser equipment and helping troubleshoot problems; the experimental work presented in this thesis would have been much more difficult without their help. I am grateful for the rest of my friends and colleagues from the Hanson Group, past and present, as my memories of group lunches, coffee breaks, ski trips, and other activities with the group will always be highlights of my time at Stanford. Perhaps most importantly, I am indebted to my friends, family, and loved ones outside of the laboratory who have made my experience in graduate school special. Their company and support has enriched my life in ways that I never would have imagined; and for that, I will be forever grateful. Financial support for the specific research presented in this thesis was pro- vided by the U.S. Department of Energy, Office of Basic Energy Sciences, with Dr. Wade Sisk as Program Manager. The National Defense Science and Engineering Graduate Fellowship, awarded by the Department of Defense, also provided tuition and stipend support for the early years of my graduate studies. Countless faculty, students, and affiliates of the Combustion Energy Frontier Research Center, funded by the U.S. Department of Energy, are also acknowledged for their support. viii Contents Abstract v Acknowledgment vii 1 Introduction 1 1.1 Motivation . .1 1.2 Background . .2 1.2.1 Development of Kinetic Mechanisms for Combustion . .2 1.2.2 Alkane Combustion Kinetics . .6 1.2.3 Butanol Combustion Kinetics . .8 1.3 Scope and Organization of Thesis . 10 2 Experimental Setup 13 2.1 Introduction . 13 2.2 Kinetics Shock Tube Facility . 13 2.2.1 Shock Tube Overview . 14 2.2.2 Gas-mixing Facility Overview . 17 2.3 Laser Diagnostics . 20 2.3.1 OH Mole-fraction Diagnostic . 20 2.3.2 Organic Fuel Mole Fraction Diagnostic . 24 2.4 Test Mixture Chemicals . 26 3 Decomposition of tert-Butylhydroperoxide 27 3.1 Introduction . 27 ix 3.1.1 Background . 27 3.1.2 Objectives of the Current Chapter . 29 3.2 TBHP Kinetic Mechanism . 30 3.2.1 Mechanism Generation . 30 3.2.2 OH Sensitivity Analysis . 31 3.3 Experimental . 32 3.4 OH Mole Fraction Measurements . 34 3.4.1 OH Time-history . 34 3.4.2 OH Yield . 35 3.5 Rate Constant Determinations . 38 3.5.1 Determination of k3:1 ....................... 38 3.5.2 Determination of k3:3 ....................... 41 3.6 Summary . 44 4 Reactions of OH with n-Alkanes 47 4.1 Introduction . 47 4.1.1 Background . 47 4.1.2 Objectives of the Current Chapter . 48 4.2 Experimental . 49 4.3 Data Analysis . 49 4.3.1 Pseudo-first-order . 50 4.3.2 Kinetic Modeling . 51 4.4 Results . 54 4.4.1 Rate Constant Measurements . 54 4.4.2 Uncertainty Analysis . 59 4.5 Comparisons with Literature . 60 4.5.1 Previous Experimental Works . 60 4.5.2 Validation of Estimation Methods . 62 4.6 Conclusions . 66 5 Reaction of OH with n-Butanol 69 5.1 Introduction . 69 x 5.1.1 Background and Motivation . 69 5.1.2 Objectives of the Current Chapter . 71 5.2 Analysis of n-Butanol Kinetic Mechanisms . 72 5.2.1 Influence of TBHP Kinetics . 72 5.2.2 Sensitivity of k5:1 Determination to Mechanism .