METHOD FOR REMOVING HYDROGEN SULFIDE FROM SOUR GAS AND CONVERTING IT TO HYDROGEN AND SULFURIC ACID A DISSERTATION SUBMITTED TO THE DEPARTMENT OF AERONAUTICS AND ASTRONAUTICS AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Melahn Parker June 2010 © 2010 by Melahn Lyle Parker. All Rights Reserved. Re-distributed by Stanford University under license with the author. This dissertation is online at: http://purl.stanford.edu/ww988vc1913 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. Reginald Mitchell, 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. Ilan Kroo, Co-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. Aldo DaRosa 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. George Springer 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. Gar Hoflund 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 ABSTRACT A method for removing hydrogen sulfide from a sour-gas stream is investigated and found to be promising. The method oxidizes hydrogen sulfide to sulfuric acid by reducing aqueous bromine to hydrobromic acid in solution. The aqueous bromine solution does not react with hydrocarbon components common to natural gas including methane and ethane. This allows the process to both sweeten sour- gas and convert its hydrogen sulfide content to sulfuric acid in a single step. Specific reactor conditions were found to produce sulfur instead of sulfuric acid. In the process, sulfuric acid is concentrated to eliminate its bromine content prior to being removed from the system, while the remaining hydrobromic acid solution is electrolyzed to regenerate aqueous bromine and produce hydrogen. Hydrobromic acid electrolysis requires less than half the energy required by water electrolysis and is an inherently flexible load that can shed or absorb excess power to balance supply and demand. Thus the electrolysis of hydrobromic acid may provide a route to producing less expensive hydrogen and improving the electric grid. The key contributions of this thesis are confirmations that: • hydrogen sulfide reacts with aqueous bromine irreversibly and immediately to produce sulfuric and hydrobromic acids, • methane and ethane do not react with aqueous bromine, • hydrobromic acid can be electrolyzed in a multi-cell stack at less than 1 Volt The process removes hydrogen sulfide from increasingly important sour-gas streams such as biogas, natural gas, and refinery waste gases, and converts it into hydrogen. It can therefore improve the utilization of existing natural gas resources, and encourage the development of new hydrogen and biogas technologies. iv The unique thermo-electro-photochemical properties of bromine promise additional solutions to our energy and environmental problems by treating our legacy pollution sources, expanding our current clean energy resources, reducing our society’s wastes, producing renewable fuels, and allowing a solar solution to our electrical, cooling and heating needs. While a deeper dive into each of these technologies was beyond the scope of this work, it is my hope that this thesis inspires others to further investigate applying bromine’s unique properties to our energy and environmental needs. v ACKNOWLEDGEMENTS The work presented in this dissertation would not have been possible without the support, guidance and inspiration of many friends and colleagues. Thanks to these people, my time as a graduate student at Stanford University has not been long enough. I would like to thank my adviser, Professor Reginald Mitchell for his insightful and kind supervision through the endeavor. His broad interests, enthusiasm, accessibility and excellent advice made working with him a consistently rewarding experience. I very much hope to continue collaborating on fundamental solutions to our energy problems. Professor Aldo V. da Rosa is an excellent mentor and role model. His dedication and perseverance have pressed huge footsteps that I aspire to follow. He made this PhD possible by both believing in what I set out to do, and granting me the privilege of being the teaching assistant to his Fundamentals of Renewable Energy class. Professor Gar Hoflund made this thesis possible by sharing in its planning and overseeing the experimental decisions. He gave me the opportunity to experiment with my hands, and guided me through the journey from theoretical to experimental result. I could not have been more fortunate in terms of collaborators. Three to whom I owe enormous applaus are Michael Everett, for his tireless efforts running experiments and reporting results, Louis Oh, for teaching me everything I know about Gas Chromatography, and Jerry Kaczur, for showing me how “real” engineers put together and operate multi-kW electrolyzers and that the difference between men and boys is the price of their toys. I also extend sincere thanks to my reading committee members; Professors Ilan Kroo and George Springer, for providing me with a fresh perspective and valuable feedback that has improved the final product. The Department of Aeronautics and Astronautics is a great department for allowing me to apply the engineering foundation they helped lay to my interests in chemistry and energy vi in the Mechanical Engineering department. I am indebted to Jayanthi Subramanian and Lynn Kaiser for their efforts behind the scenes. The camaraderie of friends has been a source of strength, and I extend much appreciation to them. Particular thanks must go to Sohan Dharmaraja and Susannah Joy Oberdorf, both of whom kept me going in the final stretches of finishing this document. My family has been extremely important to me throughout this endeavor. I thank them for being fun, dependable, classy, inspirational, and encouraging. You know who you are! Last but certainly not least, I offer immeasurable thanks to my mother and father for being an unfaltering source of understanding, and for pushing me to do what is difficult. vii Table of Contents ABSTRACT ................................................................................................................... iv ACKNOWLEDGEMENTS ........................................................................................... vi List of Figures ............................................................................................................... xii List of Tables .............................................................................................................. xvii 1. INTRODUCTION .................................................................................................... 1 1.1. Description of Process .............................................................................. 1 1.2. Application of Technology ....................................................................... 3 1.3. Electrolyzer Types .................................................................................... 4 1.4. Previous Work .......................................................................................... 6 1.4.1. Reaction of Sulfur Dioxide with Bromine and Water ...................... 7 1.4.2. Reaction of Hydrogen Sulfide with Bromine and Water ................. 8 1.4.3. Electrolysis of Hydrogen Bromide and Hydrobromic Acid ............. 8 1.4.4. The Opportunity as Found in Nature.............................................. 12 1.5. Objectives of Work ................................................................................. 14 1.6. Consequences of Success ........................................................................ 15 2. COMMERCIAL RELEVANCE AND EXISTING SOLUTIONS ........................ 17 2.1. Hydrogen Sulfide Description ................................................................ 17 2.2. Sources of Hydrogen Sulfide .................................................................. 18 2.2.1. Refineries ....................................................................................... 18 2.2.2. Sour-Natural Gas Wells and Treatment Plants .............................. 19 2.2.3. Anaerobic Digesters, Landfills and Other Sources ........................ 20 2.2.4. Coal-Bed Methane .......................................................................... 20 2.2.5. Typical Gas Compositions ............................................................. 21 2.3. Present Solution to Hydrogen Sulfide..................................................... 23 2.3.1. Hydrogen Sulfide Capture .............................................................. 23 2.3.2. Hydrogen Sulfide Conversion ........................................................ 25 2.3.3. Flaring ...........................................................................................