.HYDROGEN FUEL Production, Transport, and Storage

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The two most important environmental hazards faced by humankind today are air pollution and global warming. Both have a direct link with our current overdependence on fossil fuels. Pollutants produced from combustion of hydrocarbons now cause even more health problems due to the urbanization of world population

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HYDROGEN FUEL Production, Transport, and Storage
HYDROGEN FUEL Production, Transport, and Storage Edited by Ram B. Gupta Boca Raton London New York CRC Press is an imprint of the Taylor & Francis Group, an informa business
CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-4575-8 (Hardcover) This book contains information obtained from authentic and highly regarded sources Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the valid- ity of all materials or the consequences of their use. The Authors and Publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or uti- lized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopy- ing, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http:// www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For orga- nizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Hydrogen fuel : production, transport, and storage / Ram B. Gupta, editor. p. cm. Includes bibliographical references and index. ISBN 978-1-4200-4575-8 (hardcover : acid-free paper) 1. Hydrogen as fuel. 2. Fuel cells. I. Gupta, Ram B. II. Title. TP359.H8H89 2008 665.8’1--dc22 2008000265 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com
Contents Preface .............................................................................................................................. vii Editor ................................................................................................................................. ix Contributors ...................................................................................................................... xi Section I: Production and Use of Hydrogen 1 1 Fundamentals and Use of Hydrogen as a Fuel .....................................................3 K. K. Pant and Ram B. Gupta 2 Production of Hydrogen from Hydrocarbons ..................................................... 33 Nazim Z. Muradov 3 Hydrogen Production from Coal ........................................................................ 103 Shi-Ying Lin 4 Hydrogen Production from Nuclear Energy ..................................................... 127 Ryutaro Hino and Xing L. Yan 5 Hydrogen Production from Wind Energy ......................................................... 161 Dimitrios A. Bechrakis and Elli Varkaraki 6 Sustainable Hydrogen Production by Thermochemical Biomass Processing .............................................................................................. 185 Wiebren de Jong 7 Use of Solar Energy to Produce Hydrogen ........................................................ 227 Neelkanth G. Dhere and Rajani S. Bennur 8 Hydrogen Separation and Purification ............................................................. 283 Ashok Damle Section II: Transportation and Storage of Hydrogen 325 9 Targets for Onboard Hydrogen Storage Systems: An Aid for the Development of Viable Onboard Hydrogen Storage Technologies............... 327 Sunita Satyapal and George J. Thomas 10 Hydrogen Transmission in Pipelines and Storage in Pressurized and Cryogenic Tanks ................................................................................................... 341 Ming Gao and Ravi Krishnamurthy v
vi Contents 11 Hydrogen Storage in Metal Hydrides ................................................................ 381 K. K. Pant and Ram B. Gupta 12 Hydrogen Storage in Carbon Materials............................................................. 409 K. K. Pant and Ram B. Gupta 13 Hydrogen Storage in Organic Chemical Hydrides on the Basis of Superheated Liquid-Film Concept ..................................................................... 437 Shinya Hodoshima and Yasukazu Saito Section III: Safety and Environmental Aspects of Hydrogen 475 14 Hydrogen Codes and Standards ......................................................................... 477 James M. Ohi 15 Hydrogen Sensing and Detection ...................................................................... 495 Prabhu Soundarrajan and Frank Schweighardt 16 Hydrogen Safety ................................................................................................... 535 Fotis Rigas and Spyros Sklavounos 17 Carbon Sequestration .......................................................................................... 569 Ah-Hyung Alissa Park, Klaus S. Lackner, and Liang-Shih Fan Index ................................................................................................................................ 603
Preface The two most important environmental hazards faced by humankind today are air pol- lution and global warming. Both have a direct link with our current overdependence on fossil fuels. Pollutants produced from combustion of hydrocarbons now cause even more health problems due to the urbanization of world population. The net increase in envi- ronmental carbon dioxide from combustion is a suspect cause for global warming, which is endangering the Earth—the only known place to support human life. In addition, the import of expensive hydrocarbon fuel has become a heavy burden on many countries, causing political and economic unrest. If we look at the past 2000 years’ history of fuels, usage has consistently moved in the direction of a cleaner fuel: wood → coal → petroleum → propane → methane as shown on the next page. With time, the fuel molecule has become smaller, leaner in carbon, and richer in hydro- gen. The last major move was to methane, which is a much cleaner burn than gasoline. Our future move is expected to be to hydrogen, which has the potential to solve both the environmental hazards faced by humankind. Through its reaction with oxygen, hydrogen intensely releases energy in combustion engines or quietly releases it in fuel cells to pro- duce water as its only by-product. There is no emission of smoke, CO, CO2, NOx, SOx, or O3. In fact, the health costs for urban populations can be reduced by switching to hydrogen automobiles. Hydrogen can be produced from water using a variety of energy sources including solar, wind, nuclear, biomass, petroleum, natural gas, and coal. Since renewable energy sources (solar, wind, and/or biomass) are available in all parts of the world, all countries will have access to hydrogen fuel. Hence, a greater democratization of energy resources will occur. Also the use of solar, wind, or biomass in producing hydrogen does not add to environmental CO2. Before widescale use of hydrogen fuel can be accomplished, key technological challenges need to be resolved, including cost-effective production and storage of hydrogen. During the early adoption of hydrogen fuel, government incentives will be needed, which may be recovered from savings in the health care expenditures and carbon credits. This book is organized into three sections: Chapters 1 through 8 deal with production and use aspects; Chapters 9 through 13 cover transportation and storage aspects, and Chapters 14 through 17 discuss safety and environmental aspects of hydrogen fuel. The hydrogen molecule is the smallest and lightest of all the fuel molecules, with unique properties and uses (Chapter 1). Hydrogen can be produced from a variety of primary ener- gies including hydrocarbons (Chapter 2), coal (Chapter 3), nuclear (Chapter 4), wind (Chap- ter 5), biomass (Chapter 6), and solar (Chapter 7). Wind, solar, and nuclear electrolyses can produce pure hydrogen ready for use in fuel cells or in internal combustion engines. However, hydrogen derived from the other energy sources will require separation and purification (Chapter 8). A major technical challenge with hydrogen fuel is its transportation and storage. The U.S. Department of Energy has specified technical targets for storage (Chapter 9). Hydrogen can be transported using pipelines and tankers (Chapter 10) and stored using compressed tanks (Chapter 10), as metal hydrides (Chapter 11), adsorbed on carbons (Chapter 12), and as chemical hydrides (Chapter 13). Proper codes and standards need to be adopted for effective utilization of hydrogen fuel (Chapter 14). Fuel and safety properties of hydrogen are different from conventional vii
viii Preface Wood ↓ Coal O ↓ H3C CH3 CH3 Petroleum C C ↓ H3C C CH3 H2 H Propane CH3 ↓ CH3 CH2 Methane CH4 ↓ Hydrogen H2 (future fuel) fuels. Hence, proper monitoring (Chapter 15) and safety designs need to be incorporated (Chapter 16). Finally, if hydrogen is produced from fossil fuels, the by-product CO2 needs to be sequestered (Chapter 17). Preparation of this book would not have been possible without the valuable contribu- tions from various experts in the field. The timely contributions and support from the Alabama Center for Paper and Bioresource Engineering, Auburn University and the Consortium for Fossil Fuel Science are deeply appreciated.
Editor Ram B. Gupta is an alumni (chair) professor of chemical engineering at Auburn University. He has published numer- ous research papers and holds several patents on hydrogen fuel and supercritical fluid technology, and is the recipient of the Distinguished Graduate Faculty Lectureship Award (2007) from Auburn University, the Science and Engineer- ing Award (2002–2004) from DuPont, the Junior and Senior Research Awards (1998, 2002) from the Auburn Alumni Engi- neering Council, the James A. Shannon Director’s Award (1998) from the National Institutes of Health, and the Young Faculty Career Enhancement Award (1997) from Alabama NSF-EPSCoR. Dr. Gupta is a consultant to several energy companies. He received his BE (1987) from the Indian Institute of Technology, Roorkee; an MS (1989) from the University of Calgary, Canada; and his PhD (1993) from the University of Texas at Austin, in chemical engineer- ing. He joined Auburn University in 1995, after two-year postdoctoral work at the University of California, Berkeley. His recent books are Nanoparticle Technology for Drug Delivery (2006, Taylor & Francis), Solubility in Supercritical Carbon Dioxide (2007, CRC Press), and Hydrogen Fuel: Production, Transport, and Storage (2008, CRC Press). ix
Contributors Dimitrios A. Bechrakis Shinya Hodoshima Hellenic Transmission System Department of Industrial Chemistry Athens, Greece Tokyo University of Science Tokyo, Japan Rajani S. Bennur Department of Biochemistry Ravi Krishnamurthy Karnataka University Blade Energy Partners Dharwad, India Houston, Texas Klaus S. Lackner Ashok Damle Department of Earth and Environmental Techverse, Inc. Engineering Cary, North Carolina Columbia University New York, New York Wiebren de Jong Department of Process and Energy Shi-Ying Lin Delft University of Technology Japan Coal Energy Center Delft, the Netherlands Tokyo, Japan Nazim Z. Muradov Neelkanth G. Dhere Florida Solar Energy Center Florida Solar Energy Center University of Central Florida University of Central Florida Cocoa Beach, Florida Cocoa Beach, Florida James M. Ohi Liang-Shih Fan Hydrogen Technologies and Systems Department of Chemical and National Renewable Energy Laboratory Biomolecular Engineering Golden, Colorado The Ohio State University Columbus, Ohio K. K. Pant Department of Chemical Engineering Ming Gao Indian Institute of Technology Blade Energy Partners Delhi, India Houston, Texas Ah-Hyung Alissa Park Department of Earth and Environmental Ram B. Gupta Engineering Department of Chemical Engineering Columbia University Auburn University New York, New York Auburn, Alabama Fotis Rigas Ryutaro Hino School of Chemical Engineering Japan Atomic Energy Agency National Technical University of Athens Ibaraki-Ken, Japan Athens, Greece xi
xii Contributors Yasukazu Saito Prabhu Soundarrajan Department of Industrial Chemistry H2scan Corporation Tokyo University of Science Valencia, California Tokyo, Japan Sunita Satyapal George J. Thomas Office of Hydrogen, Fuel Cells, and Office of Hydrogen, Fuel Cells, and Infrastructure Technologies Infrastructure Technologies U.S. Department of Energy U.S. Department of Energy Washington, DC Washington, DC Frank Schweighardt Elli Varkaraki Process Analytical Technology Centre for Renewable Energy Consultant Sources Allentown, Pennsylvania Attiki, Greece Spyros Sklavounos School of Chemical Engineering Xing L. Yan National Technical University of Athens Japan Atomic Energy Agency Athens, Greece Ibaraki-Ken, Japan
Section I Production and Use of Hydrogen
1 Fundamentals and Use of Hydrogen as a Fuel K. K. Pant and Ram B. Gupta CONTENTS 1.1 Introduction.............................................................................................................................4 1.2 Physical Properties .................................................................................................................5 1.3 Chemical Properties ...............................................................................................................7 1.4 Fuel Properties ........................................................................................................................8 1.4.1 Energy Content ............................................................................................................9 1.4.2 Combustibility Properties ......................................................................................... 9 1.4.2.1 Wide Range of Flammability .................................................................... 10 1.4.2.2 Low Ignition Energy .................................................................................. 11 1.4.2.3 Small Quenching Distance ........................................................................ 11 1.4.2.4 Autoignition Temperature ......................................................................... 11 1.4.2.5 High Flame Speed ...................................................................................... 11 1.4.2.6 Hydrogen Embrittlement .......................................................................... 12 1.4.2.7 Hydrogen Leakage ..................................................................................... 12 1.4.2.8 Air/Fuel Ratio ............................................................................................. 12 1.5 Hydrogen Internal Combustion Engine ........................................................................... 12 1.5.1 Premature Ignition and Knock ............................................................................... 13 1.5.2 Fuel Delivery Systems .............................................................................................. 14 1.5.2.1 Central Injection ......................................................................................... 14 1.5.2.2 Port Injection ............................................................................................... 14 1.5.2.3 Direct Injection............................................................................................ 15 1.5.3 Ignition Systems ........................................................................................................ 15 1.5.4 Crankcase Ventilation .............................................................................................. 15 1.5.5 Power Output ............................................................................................................ 15 1.5.6 Hydrogen Gas Mixtures .......................................................................................... 16 1.5.7 Current Status ............................................................................................................ 16 1.6 Hydrogen Fuel Cells ............................................................................................................ 17 1.6.1 Types of Fuel Cells .................................................................................................... 17 1.6.2 Major Challenges ...................................................................................................... 20 1.7 Supply of Hydrogen ............................................................................................................. 21 1.7.1 Cost of Hydrogen Production ................................................................................. 21 1.7.2 Environmental Aspects ............................................................................................ 24 1.7.3 Hydrogen Storage ..................................................................................................... 25 1.7.3.1 Compressed Hydrogen .............................................................................. 25 1.7.3.2 Liquid Hydrogen ........................................................................................ 26 1.7.3.3 Metal Hydrides ........................................................................................... 26 3
4 Hydrogen Fuel: Production, Transport, and Storage 1.7.3.4 Organic Chemical Hydrides ..................................................................... 26 1.7.3.5 Carbon Materials ........................................................................................ 27 1.7.3.6 Silica Microspheres..................................................................................... 27 1.8 Current Challenges .............................................................................................................. 27 1.9 Future Outlook ..................................................................................................................... 28 1.10 Conclusions ........................................................................................................................... 29 References ...................................................................................................................................... 29 1.1 Introduction Owing to an increasing world population and demands for higher standards of living and better air quality, the future energy demand is expected to increase significantly. To meet this demand poses great challenges. Currently, most of the world energy requirement for transportation and heating (which is two-third of the primary energy demand) is derived from petroleum or natural gas. These two fuels are generally favored due to the ease of transport of liquid or gaseous forms. Unfortunately, the combustion of hydrocarbon fuels for transportation and heating contributes over half of all greenhouse gas emissions and a large fraction of air pollutant emissions. Hence, today’s world is facing an urgency in developing alternative fuels. Among various alternatives, hydrogen fuel offers the highest potential benefits in terms of diversified supply and reduced emissions of pollutants and greenhouse gases. For the past 40 years, environmentalists and several industrial organi- zations have promoted hydrogen fuel as the solution to the problems of air pollution and global warming. The key criteria for an ideal fuel are inexhaustibility, cleanliness, conve- nience, and independence from foreign control. Hydrogen possesses all these properties, and is being evaluated and promoted worldwide as an environmentally benign replace- ment for gasoline, heating oil, natural gas, and other fuels in both transportation and nontransportation applications. A number of reports are now available on several aspects of hydrogen [1–25]. Similar to electricity, hydrogen is a high-quality energy carrier, which can be used with a high efficiency and zero or near-zero emissions at the point of use. It has been technically demonstrated that hydrogen can be used for transportation, heating, and power generation, and could replace current fuels in all their present uses [2–6]. Hydrogen can be produced using a variety of starting materials, derived from both renewable and nonrenewable sources, through many different process routes. At present, two basic process technologies— (1) reformation of natural gas and (2) electrolysis of water—are widely used. In the advent of hydrogen economy, the principal focus of hydrogen technology has shifted to the safe and affordable utilization of hydrogen as an alternative fuel based on seamless integration of generation, distribution, and storage technologies. Inaccuracies, inconsistencies, and contradictions abound in the seemingly persuasive arguments tar- geting the general public and politicians regarding the merits of the hydrogen case. These inaccuracies tend to create the global perception that hydrogen will become an active source for our energy needs, replacing today’s relatively less-efficient machines with clean fuel cells, which will efficiently power cars, trucks, homes, and businesses, ending global warming and air pollution. The key assertions of the initiative for hydrogen production and utilization are based on the premise that the fuel cell is a proven technology and hydrogen is in abundant supply on Earth [10–12], but unfortunately, most of the hydrogen
Fundamentals and Use of Hydrogen as a Fuel 5 TABLE 1.1 United States and World Hydrogen Consumptions by End-Use Category United States World Total U.S. Share of 3 Captive Users Billion m Share (%) Billion m3 Share (%) World Total (%) Ammonia producers 33.7 38 273.7 61 12 Oil refiners 32.9 37 105.4 23 31 Methanol producers 8.5 10 40.5 9 21 Other 3.4 4 13.6 3 25 Merchant users 10.8 12 16.1 4 67 Total 89.3 100 449.3 100 20 Source: Adapted from SRI Consulting Inc., Chemical Economics Handbook 2001, Menlo Park, CA, July 2001; Wee, J.H., Renewable Sustainable Energy Rev., 11, 1720–1738, 2007. on Earth is in the fully oxidized form as H2O, which has no fuel value, and there are no natural sources of desirable molecular hydrogen (H2). At present, hydrogen production is a large and growing industry. Globally, some 50 million t of hydrogen, equivalent to about 170 million t of petroleum, were produced in 2004. And the production is increasing by about 10% every year. As of 2005, the economic value of all hydrogen produced worldwide was about $135 billion per year [3]. The cur- rent global hydrogen production is 48% from natural gas, 30% from petroleum, 18% from coal, and 4% from electrolysis [4]. Major end users of the hydrogen are listed in Table 1.1. Hydrogen is primarily consumed in two nonfuel uses: (1) about 60% to produce NH3 by the Haber process for subsequent use in fertilizer manufacturing [14] and (2) about 40% in refinery, chemicals, and petrochemical sectors. If nonconvenentional resources, such as wind, solar, or nuclear power for hydrogen production were available, the use of hydrogen for hydrocarbon synfuel production could expand by 5- to 10-fold [4]. It is estimated that 37.7 million t per year of hydrogen would be sufficient to convert enough domestic coal to liquid fuels to end U.S. dependence on foreign oil imports, and less than half this figure to end dependence on Middle East oil. Figure 1.1 shows various application areas of hydrogen energy, out of which the use of hydrogen energy for vehicular application is of current focus [26]. 1.2 Physical Properties Hydrogen atom is the lightest element, with its most common isotope consisting of only one proton and one electron. Hydrogen atoms readily form H2 molecules, which are smaller in size when compared to most other molecules. The molecular form, simply referred to as hydrogen is colorless, odorless, and tasteless and is about 14 times lighter than air, and diffuses faster than any other gas. On cooling, hydrogen condenses to liquid at −253°C and to solid at −259°C. The physical properties of hydrogen are summarized in Table 1.2. Ordinary hydrogen has a density of 0.09 kg/m3. Hence, it is the lightest substance known with a buoyancy in air of 1.2 kg/m3. Solid metallic hydrogen has a greater electrical con- ductivity than any other solid elements. Also, the gaseous hydrogen has one of the highest heat capacity (14.4 kJ/kg K).
6 Hydrogen Fuel: Production, Transport, and Storage Fuel cells Hydrogen energy Internal combustion engines Vehicle Combustion Fuel cells applications Efficiency Gas turbines Applications for improvement Hydrogen plants power generation Defense industry Transport Heating Power generation Cooking Domestic Ship engines Air conditioning applications Navigation Defense Pumping applications Communication Transportation Tourism Pollution control Ammonia synthesis Energy storage Fertilizer production Industrial Petroleum refineries applications Metallurgical applications Gas turbines Energy storage Jet engines Flammable mixtures Space Defense industry Electronic industry applications Rockets Glass and fiber Antimissile production Space industry Nuclear reactors Energy storage Power generation systems FIGURE 1.1 Application areas for hydrogen energy. (Reproduced with permission from Elsevier; Midilli, A., Dincer, I., and Rosen, M.A., Renewable Sustainable Energy Rev., 9(3), 255–271, 2005.) TABLE 1.2 Properties of Hydrogen Property Value Molecular weight 2.01594 Density of gas at 0°C and 1 atm. 0.08987 kg/m3 Density of solid at −259°C 858 kg/m3 Density of liquid at −253°C 708 kg/m3 Melting temperature −259°C Boiling temperature at 1 atm. −253°C Critical temperature −240°C Critical pressure 12.8 atm. Critical density 31.2 kg/m3 Heat of fusion at −259°C 58 kJ/kg Heat of vaporization at −253°C 447 kJ/kg Thermal conductivity at 25°C 0.019 kJ/(ms°C ) Viscosity at 25°C 0.00892 centipoise Heat capacity (Cp) of gas at 25°C 14.3 kJ/(kg°C) Heat capacity (Cp) of liquid at −256°C 8.1 kJ/(kg°C) Heat capacity (Cp) of solid at −259.8°C 2.63 kJ/(kg°C) Source: Adapted from Kirk-Othmer Encyclopedia of Chemical Technology. Fundamentals and Use of Hydrogen as a Fuel. 3rd ed., Vol. 4, Wiley, New York, 1992, 631p.
Fundamentals and Use of Hydrogen as a Fuel 7 The hydrogen atom (H) consists of a nucleus of unit positive charge and a single electron. It has an atomic number of 1 and an atomic weight of 1.00797. This element is a major constituent of water and all organic matters, and is widely distributed not only on the earth but also throughout the Universe. There are three isotopes of hydrogen: (1) protium—mass 1, makes up 99.98% of the natural element; (2) deuterium—mass 2, makes up about 0.02%; and (3) tritium—mass 3, occurs in extremely small amounts in nature, but may be produced artificially by various nuclear reactions. The ionization potential of hydrogen atom is 13.54 V [7]. Hydrogen is a mixture of ortho- and para-hydrogen in equilibrium, distinguished by the relative rotation of the nuclear spin of the individual atoms in the molecule. Mole- cules with spins in the same direction (parallel) are termed ortho-hydrogen and those in the opposite direction as para-hydrogen. These two molecular forms have slightly different physical properties but have equivalent chemical properties. At an ambient temperature, the normal hydrogen contains 75% ortho-hydrogen and 25% para-hydrogen. The ortho-to-para conversion is associated with the release of heat. For example, at 20 K, a heat of 703 kJ/kg is released for ortho-to-para conversion. The conversion is slow but occurs at a finite rate (taking several days to complete) and continues even in the solid state. Catalysts can be used to accelerate the conversion for the production of liquid hydrogen, which is more than 95% para-hydrogen. The vapor pressure of liquid normal hydrogen is given by P (Pa) = 10 [ ] 44.9569 −_______+6.79177+0.0205377 (K) T (K) Hydrogen has a low solubility in solvents; for example, at ambient conditions, only 0.018 and 0.078 mL of gaseous H2 dissolves into each milliliter of water and ethanol, respectively. However, the solubility is much more pronounced in metals. Palladium is particularly notable in this respect, which dissolves about 1000 times its volume of the gas. The adsorp- tion of hydrogen in steel may cause “hydrogen embrittlement,” which sometimes leads to the failure of chemical processing equipment [4]. 1.3 Chemical Properties At ordinary temperatures, hydrogen is comparatively nonreactive unless it has been activated in some manner. On the contrary, hydrogen atom is chemically very reactive, and that is why it is not found chemically free in nature. In fact, very high temperatures are needed to dissociate molecular hydrogen into atomic hydrogen. For example, even at 5000 K, about 5% of the hydrogen remains undissociated. In nature, mostly the hydrogen is bound to either oxygen or carbon atoms. Hence, to obtain hydrogen from natural compounds, energy expenditure is needed. Therefore, hydrogen must be considered as an energy carrier—a means to store and transmit energy derived from a primary energy source. Atomic hydrogen is a powerful reducing agent, even at room temperature. For example, it reacts with the oxides and chlorides of many metals, including silver, copper, lead, bismuth, and mercury, to produce the free metals. It reduces some salts, such as nitrates, nitrites, and cyanides of sodium and potassium, to the metallic state. It reacts with a number of elements, both metals and nonmetals, to yield hydrides such as NH3, NaH, KH, and PH3. Sulfur forms a number of hydrides; the simplest is H2S. Combining with oxygen, atomic