Energy, Environment and New Materials: Volume 1 Hydrogen Production and Energy Transition 9783110596250, 9783110596229

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Marcel Van de Voorde (Ed.) Hydrogen Production and Energy Transition

Also of Interest Ethics in Nanotechnology Emerging Technologies Aspects Marcel Van de Voorde, Gunjan Jeswani (Ed.),  ISBN ----, e-ISBN ----

Ethics in Nanotechnology Social Sciences and Philosophical Aspects Marcel Van de Voorde, Gunjan Jeswani (Eds.),  ISBN ----, e-ISBN ----

Handbook of Nanoethics Gunjan Jeswani, Marcel Van de Voorde (Eds.),  ISBN ----, e-ISBN ----

Nanoscience and Nanotechnology Advances and Developments in Nano-sized Materials Marcel Van de Voorde (Ed.),  ISBN ----, e-ISBN ----

Hydrogen Production and Energy Transition Volume I Edited by Marcel Van de Voorde

Volume Editor Professor Dr. Dr. h. c. mult. Marcel Van de Voorde University of technology DELFT (NL) Rue du Rhodania, 5, BRISTOL – A, App. 31 3963 CRANS – MONTANA Switzerland

ISBN 978-3-11-059622-9 e-ISBN (PDF) 978-3-11-059625-0 e-ISBN (EPUB) 978-3-11-059405-8 Library of Congress Control Number: 2021933268 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2021 Walter de Gruyter GmbH, Berlin/Boston Cover image: Petmal/iStock/Getty Images Plus Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Hydrogen will play a decisive role in future attempts to solve the challenges in connection with global warming. This requires enormous technological and political attempts. Professor Gerhard ERTL, Nobel Prize in Chemistry 2007 Hydrogen is often called the missing link of the energy transmission but equally important it is also a unique opportunity to create the necessary prosperity in our society Bart BIEBUYCK Executive Director EU Research Programme FCHJU (Fuel Cell and Hydrogen Joint Undertaking)

Series editor preface The decarbonization of the energy system is critical to reach the European climate objectives for both 2030 and 2050. In this respect, the “European Green Deal” put forward the need to ensure a smart integration of renewable energy sources, energy efficiency, and other sustainable solutions such as carbon capture. The “European Green Deal” also recognized the production of hydrogen, hydrogen storage, hydrogen networks, and the utilization of hydrogen as one of the important technology options that can ensure that the EU industry remains at the technology forefront and delivers breakthrough technologies in key industrial sectors. The book provides compiling reviews on these aspects from internationally recognized researchers, industrialists, and government agencies, and assembles topclass contributions. The topical scope of the book is broad, ranging from hydrogen production, storage of hydrogen, and the multiple applications but also the development of many new materials and new technologies and innovations will be necessarily developed in many fields of physics, chemistry, biology, and mechanical engineering. Common to all the books in the series is that they represent top-notch research and are highly application-oriented, innovative, and relevant for industry. The titles of the volumes in the series Hydrogen Technologies for Sustainable Economy are: – Hydrogen Production and Energy Transition – Hydrogen Storage for Sustainability – Utilization of Hydrogen for Sustainable Energy and Fuels They fall in the topics of energy, environment, and new materials but mobile applications such as automobiles, air, and space transport are becoming very popular. The book series appeals to a wide range of readers with backgrounds in physics, chemistry, biology, and engineering but also for those interested in renewables energy, environment, economy, and industrial safety. Students at universities to scientists at institutes and technologists in industrial companies may also have a great interest in this new hydrogen energy source. Ever since hydrogen was introduced many years ago it will greatly change our lives for the next generations. Developments are planned for many areas, which will result in a new “hydrogen economy”; in short, hydrogen technology is a hot topic! Marcel Van de Voorde, November 2020

https://doi.org/10.1515/9783110596250-202

Hydrogen Technology – Innovations and Applications Volume  Hydrogen Production and Energy Transition ISBN ---- e-ISBN ----

Volume  Hydrogen Storage for Sustainability ISBN ---- e-ISBN ----

Volume  Utilization of Hydrogen for Sustainable Energy and Fuels ISBN ---- e-ISBN ----

About the series editor Marcel Van de Voorde: Prof. Dr. Ing. ir. Dr. h. c., has many years’ experience in European Research Organizations, including CERN- Geneva and the European Commission research. He was involved in research, research strategies and management. He is emer. professor at the University of Technology, Delft (NL), holds multiple visiting professorships and is doctor honoris causa. He has been a member of numerous Research Councils and Governing Boards: e.a. CSIC (F), CNR (I), CSIC (E), NIMS (JP), of science and art academies, of the Science Council of the French Senate and the National Assembly, in Paris, and Fellow of multiple scientific societies. He has been honored by the Belgian King and received an award for European merits in Luxemburg by the former President of the European Commission. He is author of multiple scientific and technical publications and books.

Thanks to my wife for the patience with me spending many hours working on the book series through the nights and over weekend. The assistance of my son Marc-Philip related to the complex and large computer files with many sophisticated scientific figures is also greatly appreciated. Marcel Van de Voorde

https://doi.org/10.1515/9783110596250-203

Contents Volume I: Hydrogen Production and Energy Transition Series editor preface

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Volume editor: Marcel Van de Voorde List of contributors (for Volume I)

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Paolo Ciambelli, Marcel Van de Voorde Hydrogen: Present Accomplishments and Far-Reaching Promises

Forewords Louis Schlapbach Foreword 9 Alexander Wokaun Foreword 15

Extended Introductions Pierre Etienne Franc Hydrogen: why the times to scale have come Ad van Wijk Hydrogen key to a carbon-free energy system Paula Abreu Marques, Ruud Kempener The European hydrogen strategy 105 Andreas Züttel Introduction to the hydrogen books Václav Bartuška Geopolitics of hydrogen

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Contents

Volume I: Hydrogen production and energy transition Gaetano Iaquaniello, Emma Palo, Annarita Salladini 1 An overview of today’s industrial processes to make hydrogen and future developments’ trend 137 Paolo Ciambelli 2 Catalytic autothermal reforming for hydrogen production: from large-scale plant to distributed energy system 171 Oscar Daoura, Maya Boutros, Franck Launay 3 An overview of recent works on Ni silica-based catalysts for the dry reforming of methane 193 Maria Mikhail, Jacques Amouroux, Maria Elena Galvez, Stéphanie Ognier, Patrick Da Costa 4 CO2 hydrogenation by plasma-assisted catalysis for fuel production: power-to-gas application 213 Alberto Giaconia, Massimiliano Della Pietra, Giulia Monteleone, Giuseppe Nigliaccio 5 Development perspective for green hydrogen production 251 Long Han, Qinhui Wang 6 Hydrogen production from biomass pyrolysis Qinhui Wang, Long Han 7 Gasification of biomass and plastic waste

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Martin Paidar, Karel Bouzek 8 Water electrolysis as an environmentally friendly source of hydrogen Nicolas Grimaldos-Osorio, Kristina Beliaeva, Jesús González-Cobos, Angel Caravaca, Philippe Vernoux 9 Electrolysis for coupling the production of pure hydrogen and the valorization of organic wastes 359

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Stefano Campanari, Paolo Colbertaldo, Giulio Guandalini 10 Renewable power-to-hydrogen systems and sector coupling power-mobility 381 Paolo Ciambelli, Maria Sarno, Davide Scarpa 11 Photoelectrocatalytic H2 production: current and future challenges Dimitrios A. Pantazis 12 Biological water splitting

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Gunther Kolb 13 Fuel processing for fuel cells and energy-related applications

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Cheng Yi Heng, Susu Nousala 14 Emergent-based well-being design for a hydrogen-based community: social acceptance and societal evolution for novel hydrogen technology 493 Giuseppe Ricci, Maurizio Dessì, Marco Tripodi, Paolo Fiaschi, Roberto Palmieri, Luca Eugenio Basini, Thomas Pasini, Alessandra Guarinoni 15 Eni’s projects in Italy for hydrogen production 519 Marcel Van de Voorde, Paolo Ciambelli Conclusions and Recommendations: “The Future of Hydrogen” Index

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Volume II: Hydrogen Storage for Sustainability Paolo Ciambelli, Marcel Van de Voorde Hydrogen: Present Accomplishments and Far-Reaching Promises List of Contributors (for Volume II) Volume Editor: Marcel Van de Voorde

Forewords Louis Schlapbach Foreword Alexander Wokaun Foreword

Extended Introductions Pierre Etienne Franc Hydrogen: why the times to scale have come Ad van Wijk Hydrogen key to a carbon-free energy system Paula Abreu Marques, Ruud Kempener The European hydrogen strategy Andreas Züttel Introduction to the hydrogen books Václav Bartuška Geopolitics of hydrogen Volume II: Hydrogen Storage for Sustainability Romano Giglioli 1 Overview for hydrogen storage

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Volume II: Hydrogen Storage for Sustainability

Barbara Thijs, Maarten Houlleberghs, Lander Hollevoet, Gino Heremans, Jan Rongé, Johan A. Martens 2 Hydrogen fueling the future: introduction to hydrogen production and storage techniques Mieczysław Jurczyk, Marek Nowak 3 Materials overview for hydrogen storage Jean-Marc Bassat 4 Survey of SOFC cathode materials: an extended summary Ankur Jain, Shivani Agarwal, Takayuki Ichikawa 5 Ammonia: a promising candidate for hydrogen economy Tom Depover, Kim Verbeken 6 Hydrogen diffusion in metals: a topic requiring specific attention from the experimentalist Marek Nowak, Mieczysław Jurczyk 7 Nickel metal hydride batteries Zhao Zhang, Xianda Li and Omar Elkedim 8 Methods of preparing hydrogen storage materials Mieczysław Jurczyk, Marek Nowak 9 RE–Mg–Ni hydrogen storage alloys Dina Lanzi, Cosma Panzacchi, Christian Coti, Donatella Barbieri, Pierpaolo Ferraro, Francesco Maria Augusto Ghidoni, Matteo Scapolo, Sara Vassallo 10 Hydrogen storage Felipe Rosa, Alfredo Iranzo 11 An overview of technological research needs for a succcesfull hydrogen economy deployment Marcel van de Voorde, Paolo Ciambelli Conclusions and Recommendations: “The Future of Hydrogen”

Volume III: Utilization of hydrogen for sustainable energy and fuels Paolo Ciambelli, Marcel Van de Voorde Hydrogen: Present Accomplishments and Far-Reaching Promises List of Contributors (for Volume III) Volume Editor: Marcel Van de Voorde

Forewords Louis Schlapbach Foreword Alexander Wokaun Foreword

Extended Introductions Pierre Etienne Franc Hydrogen: why the times to scale have come Ad van Wijk Hydrogen key to a carbon-free energy system Paula Abreu Marques, Ruud Kempener The European hydrogen strategy Andreas Züttel Introduction to the hydrogen books Václav Bartuška Geopolitics of hydrogen

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Volume III: Utilization of hydrogen for sustainable energy and fuels

Volume III: Utilization of Hydrogen for Sustainability Energy and Fuels Gabriele Centi, Siglinda Perathoner 1 Applications of hydrogen technologies and their role for a sustainable future Tobias Christoph Brunner 2 Prospective of hydrogen in trucks Katsuhiko Hirose 3 Hydrogen for transport Laurent Allidières 4 Introduction to hydrogen energy: from applications to technical solutions Luigi Crema, Matteo Testi, Martina Trini 5 High-temperature electrolysis: efficient and versatile solution for multiple applications Luca Sementa, Fabio R. Negreiros, Alessandro Fortunelli 6 The use of hydrogen in ammonia synthesis, and in oxygen and carbon dioxide catalytic reduction – the reaction mechanisms Michel Noussan 7 The potential of hydrogen-fueled passenger cars in supporting the decarbonization of the transport sector Massimo Prastaro 8 The hydrogen as a fuel Urs Cabalzar, Christian Bach, Stefan Hiltbrand, Patrick Stadelmann 9 Hydrogen refueling of cars and light duty vehicles Thomas von Unwerth 10 Fuel cells for mobile applications Jens Mitzel, K. Andreas Friedrich 11 Hydrogen fuel cell applications

Volume III: Utilization of hydrogen for sustainable energy and fuels

Christophe Coutanceau, Marian Chatenet, Deborah Jones, Gael Maranzana 12 Materials for proton-exchange fuel cell for mobility and stationary applications Ciro Caliendo, Paola Russo, Paolo Ciambelli 13 Hydrogen safety: state of the art perspectives, risk assessment, and engineering solutions Giuseppe Ricci, Laura Prosperi, Maurizio Dessì, Marco Tripodi 14 Hydrogen application in Eni: from industrial application to power generation Marco Chiesa, Alessio Zolla 15 Hydrogen for mobility Paul E. Dodds, Daniel Scamman, Paul Ekins 16 Hydrogen distribution infrastructure Henning Zoz, Tejas Bopardikar 17 Power to gas to fuel® Paolo Ciambelli, Marcel Van de Voorde Conclusions and Recommendations: “The Future of Hydrogen”

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List of contributors Marcel Van de Voorde University of technology DELFT (NL) Rue du Rhodania, 5, BRISTOL – A, App. 31 3963 CRANS – MONTANA Switzerland [email protected] Paolo Ciambelli University of Salerno and Narrando srl Via Giovanni Paolo II, 132 84084 Fisciano Italy [email protected] Louis Schlapbach ETH & Empa, Zurich, Switzerland [email protected] Alexander Wokaun ETH Zurich Switzerland [email protected] Pierre Etienne Franc Hydrogen Energy World Business Line, Air Liquide Hydrogen Council Secretary Air Liquide, France, 75 quai d’Orsay Paris France [email protected] Ad van Wijk Department Process and Energy Faculty of Mechanical, Maritime and Materials Engineering. TU Delft Leeghwaterstraat 39 2628 CB Delft The Netherlands [email protected]

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Andreas Züttel Laboratory of Materials for Renewable Energy (LMER) Institute of Chemical Sciences and Engineering (ISIC) Basic Science Faculty (SB) École polytechnique fédérale de Lausanne (EPFL) Valais/Wallis Energypolis Rue de l’Industrie 17, CP 440 CH-1951 Sion Switzerland and Empa Materials Science and Technology, Dübendorf Switzerland [email protected] Paula Abreu Marques European Commission Directorate General for Energy DM24 4/138 B-1049 Brussels Belgium [email protected] Ruud Kempener Directorate General for Energy European Commission DM24 4/138 B-1049 Brussels Belgium Vaclav Bartuska Ambassador-at-Large for Energy Security Ministry of Foreign Affairs Loretanské nam. 5 118 00 Praha 1 Czech Republic [email protected]

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List of contributors

Gaetano Iaquaniello NextChem SpA Via di Vannina 88/94 00156 Rome Italy and KT – Kinetics Technology SpA Viale Castello della Magliana 27 00148 Rome Italy [email protected]

Campus Fanar, BP 90696 Jdeideh Lebanon

Emma Palo KT – Kinetics Technology SpA Viale Castello della Magliana 27 00148 Rome Italy [email protected]

Maria Mikhail Sorbonne Université Institut Jean le Rond d’Alembert CNRS UMR7190, Campus St Cyr 2 place de la gare de Ceinture 78210 Saint-Cyr l’Ecole France and Institut de Recherche de Chimie Paris UMR 8247 (CNRS – Chimie ParisTech) Equipe 2PM, 11 rue Pierre et Marie Curie 75005 Paris France

Annarita Salladini NextChem SpA Via di Vannina 88/94 00156 Rome Italy [email protected] Oscar Daoura Lebanese University Laboratoire de Chimie Physique des Matériaux (LCPM/PR2N) Faculté des Sciences II Campus Fanar, BP 90696 Jdeideh Lebanon and Sorbonne Université CNRS, Laboratoire de Réactivité de Surface, LRS Campus Pierre et Marie Curie 4, Place Jussieu 75005 Paris France [email protected] Maya Boutros Lebanese University Laboratoire de Chimie Physique des Matériaux (LCPM/PR2N) Faculté des Sciences II

Franck Launay Sorbonne Université CNRS, Laboratoire de Réactivité de Surface, LRS Campus Pierre et Marie Curie 4, Place Jussieu 75005 Paris France

Jacques Amouroux Institut de Recherche de Chimie Paris UMR 8247 (CNRS – Chimie ParisTech) Equipe 2PM, 11 rue Pierre et Marie Curie 75005 Paris, France Maria Elena Galvez Sorbonne Université Institut Jean le Rond d’Alembert CNRS UMR7190, Campus St Cyr 2 place de la gare de Ceinture 78210 Saint-Cyr l’Ecole France Stéphanie Ognier Institut de Recherche de Chimie Paris UMR 8247 (CNRS – Chimie ParisTech) Equipe 2PM, 11 rue Pierre et Marie Curie 75005 Paris, France

List of contributors

Patrick Da Costa Sorbonne Université Institut Jean le Rond d’Alembert CNRS UMR7190, Campus St Cyr 2 place de la gare de Ceinture 78210 Saint-Cyr l’Ecole France [email protected] Alberto Giaconia ENEA – Casaccia Research Center Via Anguillarese 301 00123 Rome Italy [email protected] Massimiliano Della Pietra ENEA – Casaccia Research Center Via Anguillarese 301 00123 Rome Italy massimiliano.della [email protected] Giulia Monteleone ENEA – Casaccia Research Center Via Anguillarese 301 00123 Rome Italy [email protected] Giuseppe Nigliaccio ENEA – Casaccia Research Center Via Anguillarese 301 00123 Rome Italy [email protected] Long Han Zhejiang University of Technology Hangzhou, Zhejiang China Qinhui Wang Zhejiang University Hangzhou, Zhejiang China [email protected]

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Martin Paidar Department of Inorganic Technology University of Chemistry and Technology Prague Czech Republic Karel Bouzek Department of Inorganic Technology University of Chemistry and Technology Prague Czech Republic [email protected] Nicolas Grimaldos-Osorio Université de Lyon Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS Université Claude Bernard Lyon 1 2 Avenue A. Einstein 69626 Villeurbanne France Kristina Beliaeva Université de Lyon Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS Université Claude Bernard Lyon 1 2 Avenue A. Einstein 69626 Villeurbanne France Jesús González-Cobos Université de Lyon Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS Université Claude Bernard Lyon 1 2 Avenue A. Einstein 69626 Villeurbanne France Angel Caravaca Université de Lyon Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS Université Claude Bernard Lyon 1 2 Avenue A. Einstein 69626 Villeurbanne France

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Philippe Vernoux Université de Lyon Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS Université Claude Bernard Lyon 1 2 avenue A. Einstein 69626 Villeurbanne France Stefano Campanari Department of Energy Politecnico di Milano Via Lambruschini, 4A 20156 Milan Italy [email protected] Paolo Colbertaldo Department of Energy Politecnico di Milano Via Lambruschini, 4A 20156 Milan Italy [email protected] Giulio Guandalini Department of Energy Politecnico di Milano Via Lambruschini, 4A 20156 Milan Italy [email protected] Maria Sarno Department of Physics “E.R. Caianiello” and Centre NANO_MATES University of Salerno Via Giovanni Paolo II, 132 84084 Fisciano Italy [email protected] Davide Scarpa Department of Industrial Engineering University of Salerno via Giovanni Paolo II, 132 84084 Fisciano Italy [email protected]

Dimitrios A. Pantazis Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1 45470 Mülheim an der Ruhr Germany [email protected] Gunther Kolb Fraunhofer Institute for Microengineering and Microsystems IMM Head of Business Division Energy Carl-Zeiss-Straße 18–20, 55129 Mainz, Germany Phone +49 6131 990-341 [email protected] www.imm.fraunhofer.de Yi Heng Cheng 0 Carb 2050 Platform College of Design and Innovation, Tongji University 281 Fu Xin Rd. 200092 Shanghai China [email protected] Giuseppe Ricci Chief Operating Officer Energy Evolution at Eni S.p.A. Rome, Italy [email protected] Maurizio Dessì Energy Evolution – Power Generation & Marketing at Eni S.p.A. [email protected] Marco Tripodi Energy Evolution – Power Generation & Marketing at Eni S.p.A Paolo Fiaschi Energy Evolution – Green&Traditional Refinery at Eni S.p.A. Roberto Palmieri Energy Evolution – Green&Traditional Refinery at Eni S.p.A. [email protected]

List of contributors

Luca Eugenio Basini Research & Technological Innovation at Eni S.p.A. Thomas Pasini Research & Technological Innovation at Eni S.p.A. [email protected] Alessandra Guarinoni Research & Technological Innovation at Eni S.p.A. [email protected]

Susu Nousala Creative Systemic Research Platform College of Design and Innovation, Tongji University 281 Fu Xin Rd. 200092 Shanghai China [email protected]

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Hydrogen: Presents Accomplishments and Far-Reaching Promises Despite the realistic analysis of the waving trend of the enthusiasm for hydrogen in the last 50 years, starting from the oil crisis in the 1970s to announce the future of a hydrogen economy, today the driving force of the global warming issue and a favorable convergence of interests by different stakeholders support the role of hydrogen as zero-carbon fuel much strongly than before. After the Paris Agreement on climate in 2015, many countries produced national road maps and collaboration projects involving hydrogen in order to reduce greenhouse gas emissions and achieve deep decarbonization. Pioneering countries in investing huge financial resources are Japan and Germany. To reach this goal, it is expected that hydrogen could be produced from renewable sources, consumed with no pollution, and universally used, for example, in transportation, energy storage, residential and industrial applications, and high-grade heat production. In the last 2 years, IEA [1] and WEC [2] reports confirmed the “unprecedented momentum for hydrogen,” the European Union (EU) has increased the financial support to research and innovation on hydrogen, and new investment plans are frequently announced, often at a gigawatt scale. At January 2021 Linde Company announced it will build, own and operate the world’s largest (24 MW) Proton Exchange Membrane electrolyzer plant in Germany The most concrete proof of this renewed interest is the publication by the EU Commission of two just published documents, inside the EU’s Green Deal and its Covid recovery plan, which assign to hydrogen a fundamental role to contribute to reach a climate-neutral and zero-pollution economy in 2050 [3, 4]. Even more surprising was the China’s President Xi Jinping announcement (September 2020) of pledge to achieve carbon neutrality before 2060, taking into account that China is the largest carbon emitter (28.6% share of global carbon emissions in 2018) and energy consumer. The target of European Green Deal (55% reduction in emissions by 2030 with reference to 1990 and carbon neutral by 2050) is mostly based at medium term on electrolyzers (40 GW at 2030). It is not an easy challenge, if one takes into account at least two main aspects: first, hydrogen today accounts for only a small fraction of the energy mix, while its contribution is expected to reach at least 15% in 30 years; second, more than 95% of 70 million tons of hydrogen yearly consumed for industrial purposes (mostly ammonia, methanol, and oil refining) is today still produced from fossil fuels (grey hydrogen from natural gas in Europe), giving a huge contribution to annual carbon dioxide release, close to 100 million tons. Therefore, the decarbonization goal requires a full change to cleaner hydrogen, especially green H2 (water, wind, and sun related). There is a need to decrease the cost of renewable hydrogen, although there is a continuous https://doi.org/10.1515/9783110596250-001

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improvement in this direction. Even more critical is the availability of all renewable energies necessary to produce all H2. Therefore, storage, especially on a medium-tolong term is a critical issue to be considered. The only solution is to produce H2 in remote areas where this can be cost-competitive, but here the problem is the transport from these regions. In terms of use, in the first phase, as indicated in the cited H2 strategy, this can be just the substitution of grey H2 with cleaner sources, but in the longer term (beyond about 2025) the issue is the development of new uses of H2, from mobility to industrial uses, in competition with the direct use of renewable electrical energy. There is thus the need of a full change in the energy system in relatively short term, requiring an impressive effort for this system transformation, which can be realized only in a concerted effort which catalyzes the transformation. With respect to the first aspect, the time to achieve the full change to green hydrogen, a realistic and objective analysis shows that in the short-to-medium term it will be necessary to resort to other forms of low-carbon hydrogen (blue hydrogen), to reduce emissions from actual hydrogen production, and to support the parallel and future growth of renewable hydrogen. Along this intermediate step, up to 90% of CO2 emissions can be captured during the production of grey hydrogen and stored in adequate places, for example, empty gas fields. However, the cost of going over 90% increases significantly, while it is possible that at 2030 the cost of hydrogen from electrolyzers could be competitive with that from methane. Therefore, the necessity of this step caused, as expected, criticism by influential international associations, such as WWF, European Environmental Bureau, Friends of the Earth Europe, and also from part of the scientific community, fearing that maintaining the blue option at least until 2030 could compromise the EU action on climate. There are alternatives, however, which can be considered, even maintaining the use of fossil fuels as a primary source. In fact, a better and more economical alternative to achieve the same goal of decreasing H2 production carbon footprint instead of producing CO2 and then sequestrating and storing it (carbon capture and storage (CCS)) is to intrinsically decrease its production. By using electrically heated reactors for steam reforming of methane, it is possible to achieve a reduction in CO2 emissions above 50–60%. Producing H2 from cracking of biomethane is another better possibility with respect to the use of CCS. Really, even if the priority of the EU is to develop green hydrogen, that is, hydrogen produced through the electrolysis of water powered by electricity stemming from renewable sources (wind and sun), it appears that it will be necessary to profit from intermediate opportunities for producing at least blue hydrogen, taking into account that they could be available in shorter time, contributing to progressively move toward the full change to the really green hydrogen. The European Clean Hydrogen Alliance, bringing together industry, national and local public authorities, civil society, and other stakeholders, aims at bringing together renewable and lowcarbon hydrogen production by 2030. It is obvious that to reach the final objective of decarbonization, a joint effort by the different stakeholders involved with different roles (industry, electricity, mobility,

Hydrogen: Presents Accomplishments and Far-Reaching Promises

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infrastructure building, government, research, and education) must be planned. This requires well-coordinated policies at national and European levels. The publication of the EU Commission indicates three time steps and relevant actions, starting from now, to achieve the goal through an investment agenda for the EU: – Boosting demand for and scaling up production – Designing an enabling and supportive framework: support schemes, market rules, and infrastructure – Promoting research and innovation in hydrogen technologies. With respect to the financial aspect, the reference to the next-generation EU plan will strongly support these actions. A regulatory framework for the hydrogen market will be assessed. Because of the complexity of the technology and the wish to achieve a breakthrough in the field, there is a great interest for a deep international collaboration between the United States, China, Canada, and EU countries, and some international organization between universities, research institutes, and industries. In addition, the EU, the International Energy Agency, and the countries themselves give hydrogen energy a top priority in their energy research and development plans. Therefore, with respect to the invitation of the Commission to all stakeholders from their specific point of view as to how to help support the fundamental role of hydrogen in a decarbonized economy, we offer this series of books as a small contribution. The series consists in three books treating, respectively, the three major areas of (i) processing technologies, (ii) storage technologies, and (iii) application technologies. The essential role of hydrogen for a sustainable development through decarbonization as fuel is analyzed and discussed in industry, in mobility (from cars to trains, to ships, to airplanes), for heating and cooling buildings, and in electricity demand and supply. Starting from the current, fossil-dependent, production processes, intermediate steps such as biomass and waste sources are presented, to arrive to updated progress in renewable hydrogen mostly based on water splitting and renewable energy. The second book deals with hydrogen storage. Together with an overview on technologies, most of the chapters are devoted to innovative materials for batteries and fuel cells, and others to stationary storage. The third book treats the whole range of applications in industry (power to gas to fuel, carbon dioxide conversion) and mobility (vehicles and refueling stations). One chapter is dedicated to a very critical issue: hydrogen safety and risk assessment, including the case of mobility. A review is available with respect to new materials, components, designs for the installations and facilities for the future, as well as for the production, storage, and multiple applications. Without materials research, design, and development it would be impossible to think on the new hydrogen technology applications. However, it is necessary to really integrate all R&D developments in a single vision to achieve fast the transition. This book series contributes to give this unitary vision.

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The pillars of the three books are “Role of Hydrogen Energy and Hydrogen Economy” reflected in the characteristics of the advanced technologies as well as their sustainability, that is, their implications on economy, society, including safety and public acceptability, and environment-friendly technologies. In addition, hydrogen technologies offer ways of handling some critical problems that current societies face, ways that could be preferable to other ways of coping with the problems. We invited several authors from academy, research centers, companies, and authorities to write a chapter keeping in mind mostly two keywords: research and technology. Therefore, you will find different contents, from basic research to innovation, to realizations, to scenarios, to safety and regulation aspects. All three books have an Introduction and are closed by Conclusions and recommendations. Their content typically includes chapters on the “State of Existing Technologies” and chapters on research and innovation in relevant fields from a theoretical, applied, innovative, and industrial point of view. Moreover, the current state of standardization and safety in the field of systems and devices for production, storage, transport, measurement, and use of hydrogen is analyzed in some chapters. The treatment of this aspect, a critical one for commercialization of hydrogen, is also required as an effect of cooperation of countries and continents on the roll out of hydrogen infrastructures, such as refueling stations or distribution networks.

Processing technologies In this book, most of the potential hydrogen processing technologies have been described in detail: the improvements of actually operating systems toward completely new and flexible techniques. Therefore, from various chapters the itinerary from grey to blue to green hydrogen is covered. Attention has been given to the theoretical aspects, thermodynamics, process calculations, and modeling approaches and reports of multiple successful new pilot systems.

Storage technologies This book focuses on new developments of hydrogen storage technologies with great attention to new advanced materials. Tools for the development of new materials focus on synthesis, kinetics, and thermodynamics and application of nanoscale hydrogen storage materials with status on existing technologies and perspectives. An impressive range of nanomaterials has been investigated: solid-state, metal, intermetallic, chemical, advanced carbon, and complex hydrides. Nanomaterials for hydrogen storage achieve an optimum compromise between having the hydrogen too weakly bonded to the storage material, resulting in a low storage capacity, and too strongly bonded to

Hydrogen: Presents Accomplishments and Far-Reaching Promises

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the storage material, requiring high temperatures to release hydrogen. For the storage of electric energy, nanostructured materials for positive and negative electrodes are used for advanced next-generation batteries.

Application technologies This book presents the current state of hydrogen economy with the focus on applications in the automotive technology and industry, aiming to reply to some key questions: Where are we in the implementation of hydrogen economy? What are the perspectives? Which are the directions to develop this field? This book provides compiling reviews on these aspects from internationally recognized researchers, industrialists, and government agencies. The mobile applications such as automobiles, air, and space transport are becoming very popular with a wide number of different types of fuel and electrolysis cells well described in the book. Large attention is being given to materials science and technologies with focus on the development of new materials for specific applications, as well as structured and functional materials, including research and innovations on electrolytes. Functional materials are key for advances in energy research in preparation of energy carriers from renewables, energy storage, and efficient energy conversion. Nanostructured materials offer high potentials, provided that they are tailored to exactly the right size of the nanometer scale. Catalysts for efficient energy conversion are a crucial enabling factor, but there is a need to avoid the use of critical raw materials such as noble metal catalysts in fuel cells. Hydrogen is a chemical widely used in various applications including ammonia and methanol production, oil refining, and energy. Hydrogen is widely regarded as an ideal energy storage medium, due to the ease with which electric power can convert water into its hydrogen and oxygen components through electrolysis and can be converted back to electrical power using a fuel cell. Power to gas is the conversion of electricity to a gaseous fuel such as hydrogen to be injected in methane distribution network. Via underground storage, hydrogen is stored in caverns, salt domes, and depleted oil and gas fields. Challenges and requirements for car industry are discussed presenting a status on existing technologies, particularly hybrid systems. Specific attention is devoted to mobility application discussing efficiency, technological development, and demonstration projects for fuel cell vehicles such as cars, heavy-duty vehicles, trains, and ships. Strictly related to mobility is the relation between vehicles and refuel stations, discussed in some chapters. One more critical aspect connected to automotive mobility is the safety analysis, risk assessment for both infrastructures and transport of persons or dangerous goods.

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The state of the art and development of portable applications is also presented. Some chapters gave an overview of the research and development of new components for hydrogen designs, and of newly developed monitoring and control equipment to assure reliable and efficient operation. Because of the great interest in “Hydrogen Technology” worldwide, the authors participating are university professors, research managers, industrialists, and government agencies from industrialized countries: China, Japan, the United States, Australia, Africa, South America, and many European countries. Focus is given to modernization of existing technologies to completely new developments. Developments are planned in many areas, which will result in a new “Hydrogen Economy” in the next decades given from the progresses in “Hydrogen Technology.” Breakthroughs are expected, and efforts are planned so that the new “Hydrogen Technology,” which has been impossible in the past, will finally become a reality.

References [1] [2]

[3] [4]

IEA, The Future of Hydrogen, IEA, 2019, Paris. https://www.iea.org/reports/the-futureofhydrogen Innovation Insights Brief – New Hydrogen Economy – Hype or Hope? Word Energy Council, 4 June, 2019. https://www.worldenergy.org/assets/downloads/WEInnovation-Insights-BriefNew-Hydrogen-Economy-Hype-or-Hope.pdf Powering a climate-neutral economy: An EU Strategy for Energy System Integration, COM/ 2020/299, Brussels, 8 July 2020. A hydrogen strategy for a climate-neutral Europe, COM/2020/301, Brussels, 8 July 2020.

Forewords

Louis Schlapbach

Foreword Overcoming the initiated climate crisis is the greatest challenge of humanity. Human activities like power generation and mobility, industrial processing, and building, and maintenance of infrastructure need to become carbon neutral, that is, to transform into a sustainable energy economy with a closed carbon cycle. Our attitude in handling of energy and materials must change from “just consume” to using renewable sources in a circular economy. Hydrogen is a key component in the sustainable handling of energy, materials and chemicals, and processing. Is the production of the three new volumes Hydrogen Production and Energy Transition: i) Synthesis and Processing ii) Storage and Transport iii) Hydrogen Applications timely? Yes, it is, as “Climate change is now detectable from any single day of weather at global scale” [1], and as the crucial function of hydrogen in all major low carbon energy technologies is well established, innovative solutions are needed leading us to a Hydrogen Based Low Carbon Society. The global acceptance that we are approaching directs us to serious challenges relative to our handling of energy and environment and thus of climate. All three subfields of the book series have gained a lot of international priority in science, technology, economics, and politics. Over the last decennies, it was shown that on a lab scale, hydrogen-based steps are able to make significant contributions to almost all non-nuclear energy technologies for main use in buildings, for mobility of persons and transport of goods including air and space, for industrial manufacturing and processing, as well as for communication and handling of big data. Hydrogen technology and economy covers production by traditional steam methane reforming and coal gasification and water splitting by electrolysis, transport and storage by high-pressure pipelines and tanks for gaseous hydrogen, by cryotechniques for liquid molecular hydrogen, and solidstate storage by reversible hydride formation, and finally the use and applications in thermal, mechanical, chemical, electrical, and electrochemical processes. Apart from space technology, wave-like ups and downs of enthusiasm and budgets for research, development, and innovation (R&D&I) characterized the chances for a major technoeconomic breakthrough. There are clear signs for a turning point: For those who want to set up climatefriendly energy technologies, closed hydrogen cycle and closed carbon cycle are central topics based on renewables. Concerning hydrogen, we approach asymptotically what Jules Verne was dreaming about in his “L’Ile mystérieuse” stating that “water will be the coal (fuel) of the future.” https://doi.org/10.1515/9783110596250-002

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After the Kyoto Protocol, 187 countries signed and ratified the Paris 2015 Climate Agreement COP21 for limiting greenhouse gas emissions. A Global Hydrogen Council was founded in 2017 in Davos by Toyota, Air Liquide, and Linde. IEAs 2019 statement by Director Fatih Birol, “Hydrogen is today enjoying unprecedented momentum; the world should not miss this unique chance to make hydrogen an important part of our clean and secure energy future,” supports the turning point. The European Academies Science Advisory Council EASAC published in September 2020 its strong Hydrogen and Synthetic Fuel support paper on the implementation of the EU Hydrogen Strategy on production and use of renewable hydrogen. Japan acts as a pioneering country in science, technology, and politics on hydrogen [Japanese Hydrogen Gamble, NATURE 591, 25 March 2021]: Tokyo Olympics 2020 (postponed to 2021) runs a hydrogen-based mobility system. JR East tests hydrogen FC trains. Fukushima Hydrogen Energy Research Field will soon operate a 10,000 kW hydrogen production facility. Many public R&D&I laboratories in PRC China focus their work on innovation in energy-efficient technologies for enhancing resource recycling and energy efficiency [2]. Reduced promotion of battery-electric cars in favor of methanol and hydrogenpowered cars is a crucial example. Hydrogen fuel cell pioneering car manufacturers in Japan and South Korea not only entered the car market (Toyota’s Mirai and Hyundai’s Nexo) but also have advanced plans for extensions to buses and trucks as hydrogen fuel cell electric vehicles including a 1,000 vehicle fleet in Switzerland. In Europe, Germany set a goal with the recent decision to push hydrogen-powered mobility. Fast recharging is a main advantage compared to battery-powered electric vehicles. Cost reductions by almost two orders of magnitude have been realized. Already in 2018, Germany started the commercial service of its Cordia iLint hydrogen fuel cell trains over 100 km rail. Hydrogen technology for marine applications based on H-fuel cells with metal hydrides or methanol reformers as providers of hydrogen are a hot navy topic [3]. The potential to transform nonmilitary transport of goods by open sea navigation – still exempted from Paris COP15 agreement – from today’s polluting heavy oil to sustainable hydrogen-based powering corresponds to around 4% of global CO2 emissions. We are used to transport goods over very large distances, for example, coal from Australia to Japan or crude oil from South America to everywhere. Why do we not (yet) transport solar power in the form of synthetic liquid hydrocarbons from Australia or other parts of the southern hemisphere to consumers further north, with the additional advantage of seasonal storage? Near-future energy scenarios will – in addition to the naturally used solar power – rely on decreasing amounts of fossil fuels (decarbonization) and classic fission-type nuclear energy and on rapid raising of different types of renewables (solar radiation, hydropower, wind power, biomass conversion, etc.) and some geothermal energy. The relative importance of the different contributing technologies is not yet fixed. However, it is clear that after the energy production and conversion, storage, especially seasonal storage to bridge the lower solar radiation of

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the winter period, will be the great challenge. In view of today’s low-cost solar and wind power, the old idea to use fission-based nuclear heat for the coproduction of electricity and hydrogen is reconsidered “to provide an outlet for the nuclear power that is now sold at a loss” [4]. Hydrogen (H) is a present of nature: with only one proton (zero neutrons) and one electron per hydrogen atom with strong (13.5 eV) binding energy, hydrogen has the highest energy per weight or mass of all chemical energy carriers. It is a carbonfree synthetic fuel; when burnt with oxygen, water vapor is produced and no CO2. Nature has quasi-nonlimited amounts of hydrogen chemically bound in water (H2O). Hydrogen atoms are not stable, so they combine to molecular H2. The challenge is that molecular hydrogen (H2) is a gas at room temperature. Cryogenic temperatures allow liquefaction, and additional very high pressure leads to solidification. For compact applications around room temperature densification of H2 gas, for example, compression by up to three orders of magnitude is needed. Important, still challenging options are the transformation of pure hydrogen into a hydrogen-rich synthetic fuel, a hydrocarbon or ammonia, preferentially in the liquid state. The extraordinary potential of hydrogen is first of all a strong driver for R&D&I for energy technologies but secondly for materials science and as a crucial chemical for semiconductor industry, metallurgy, and petroleum refinement. The following short list of selected recent achievements illustrates the progress and topicality of the themes. The publication “How the energy transition will reshape geopolitics; path to a low-carbon economy will create rivalries, winners and losers” by Goldthau et al. [5] elaborates societal impact by hydrogen energy and climate politics. Grolms [6] analyzed economic challenges of the realization of a sustainable hydrogen economy. The handling of safety and risks of new energy technologies requires reliable sensing. Based on nanostructured sensing devices, sensitive and fast response detectors were developed to handle hydrogen as a highly flammable, colorless, and odorless gas [7, 8]. Promising R&D&I work aims at larger scale synthesis of hydrogen by water splitting and at the production of synthetic liquid hydrocarbon fuels, both using solar, thermal, or photonic energy. Photo- or photoelectrochemical water splitting, solar thermal CO2 reduction, and CO2 hydrogenation reactions are steps in the direction of solar fuels. Hydrogen is always involved in an intermediate step. Laboratories like the Joint Center for Artificial Photosynthesis, an US-DOE Innovation Hub established in 2010 or GREEN at the National Institute for Materials Science in Tsukuba, Japan, started early as pioneers. Pham et al. [9] reviewed solar water splitting heterogeneous interfaces. Various hydrogenation reactions of CO2 based on intermediate H2 storage are well described by Bhanage and Arai [10]. The European H2020 project “From sun to liquid-fuels from concentrated sunlight” with a leading team at ETH Zurich is developing a solar thermochemical technology for synthetic hydrocarbon at large scale and competitive costs. Solar

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radiation is concentrated by an array of heliostats and efficiently absorbed in a solar reactor that thermochemically splits H2O and CO2 and converts via hydrogen to syngas, which is subsequently processed in a Fischer–Tropsch process into hydrocarbon fuels. Solar-to-syngas energy conversion efficiencies exceeding 30% can potentially be realized, thanks to favorable thermodynamics at high temperature and utilization of the full solar spectrum [11]. Such solar-based carbon-neutral fuels – produced in more than lab-scale quantities – will make aviation and maritime transport sustainable. Hydrogen-dominated materials remain a hot topic in materials science and technology. First of all, we refer to reversibly formed solid hydrides of metallic elements and compounds and their surfaces for hydrogen gas storage, as electrodes for rechargeable batteries, and as sources of atomic hydrogen, for example, in catalytic reactions [12]. An example is superconductivity of materials transporting electricity with zero resistivity and expulsing magnetic fields. At low temperature and very high pressure, pure molecular hydrogen gas transforms first into liquid (21 K) and then solid molecular hydrogen; under still higher pressure a transition to atomic metallic hydrogen, supposed to be superconducting, is expected (see, e.g., Dias et al. [13] and references therein). Rather than using pure hydrogen, it was demonstrated recently that the incorporation of hydrogen atoms into suitable solids (hydrogen compounds) induces superconductivity. Newest results report superconductivity in lanthanum hydride at 250 K under high pressure [14, 15]. Research will open more and attractive solutions, technology development will make it economically feasible, and the society will make its choice, hopefully respecting needs of mankind and nature. The well-advanced hydrogen technologies continue to offer great challenges for materials science and technology and are promising fields for young scientists and engineers with entrepreneurial spirit, for this and coming generations. And, on a medium term, it will be profitable for investors.

References [1] [2] [3] [4] [5]

[6] [7] [8]

Sippel S, et al. Nat Clim Chang 2020, 10, 35. “Clean energy in China”, Nature Spotlight and Advertisement. Nature 2020, 584(S1). Marine Forum 5, 2019, ISSN 0172–8547 “Could hydrogen bail out nuclear power?”, Kramer D, Physics Today, August 2020, p. 20. Goldthau A, Westphal K, Bazilian M, Bradshaw M. How the energy transition will reshape geopolitics; path to a low-carbon economy will create rivalries, winners and loosers. Nature 2019, 569, (29). Grolms M. Realizing a sustainable hydrogen economy. Adv Science News, Sept 2018. Gao M, et al. Small 2018, 14 (10), 1703691. Nugroho FA, et al. Metal-polymer hybrid nanomaterials for plasmonic ultrafast hydrogen detection. Nat Mater 2019, 18, 489.

Foreword

[9] [10] [11]

[12]

[13] [14] [15]

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Pham TA, et al. Review Modelling heterogeneous interfaces for solar water splitting. Nat Mater 2017, 16, 401. Bhanage BM, Arai M, eds. Transformation and utilization of carbon dioxide, Springer Series Green Chemistry and Sustainable Technology, 2014 (ISBN 978-3-642-44987-1) Marxer D, Steinfeld A, et al. Solar thermochemical splitting of CO2 into separate streams of CO and O2 with high selectivity, stability, conversion, and efficiency. Energy Environ Sci 2017, 10, 1142. Kim S, et al. A complex hydride lithium superionic conductor for high-energy-density allsolid-state lithium metal batteries. Nat Commun 2019, 10, 1081; Mohtadi, R., Orimo, S. “The renaissance of hydrides as energy materials” Nature Rev. Mater. 2, 16091 (2017); L. Schlapbach, A. Zuettel, “Hydrogen-storage materials for mobile applications”, Nature 414, 625 (2001); L. Schlapbach, ed., “Hydrogen in Intermetallics” I & II, Springer Series Applied Physics, 63 & 67 (1988, 1992). Dias RP, Silvestra IF. Science 2017, 355, 715, and references therein. Drozdov AP, et al. Nature 2019, 569, 528. Pickett W, Eremets M, Phys Today, May 2019, 52.

Alexander Wokaun

Foreword Why does hydrogen play such an eminent role as an energy carrier? From a chemist’s point of view, the answer is clear and unambiguous: it offers the best ratio of chemical binding energy to weight. Hydrogen consists of one electron, involved in chemical bonds, and one nucleon, the proton. By comparison, lithium features 1 binding electron per 7 nucleons,1 and sodium contains 1 binding electron per 23 nucleons. Consequently, dihydrogen offers the highest value of stored energy per weight: its heat of combustion (lower heating value) amounts to 120 MJ/kg, and rises to 142 MJ/kg with condensation of the produced water (higher heating value). This value is unsurpassed by any other chemicals. Besides, hydrogen offers many advantages: As we shall discuss, it may be produced by a variety of routes, is useful and required for a plethora of applications, and can be converted to other forms of energy by clean and efficient routes. Of course, there are also some disadvantages: For hydrogen as a light gas, the energy stored per volume is low (11 MJ/m3), as compared to methane (36 MJ/m3). For energy storage, hydrogen must therefore be compressed or liquefied, and pipeline transport needs to move larger volumes. As a reactive gas, hydrogen requires safe handling with adequate precautions. Fortunately, appropriate technical solutions are available to overcome these difficulties, such that society and industry can take advantage of hydrogen’s eminent properties. Henry Cavendish discovered the element of hydrogen in 1766. Soon thereafter in the eighteenth century, the light gas hydrogen was already used for lifting gas balloons, and its use for aerial transportation continued until the twentieth century. Unfortunately, this early career found an undeserved end with the accident of the “Hindenburg” zeppelin, although later it was established that the disastrous fire was due to burning of the aircraft’s skin, not of the hydrogen. The rising chemical industry continued to use hydrogen as a clean reducing agent, albeit in small quantities in the nineteenth century.

Hydrogen in the chemical industry The volume of hydrogen required and used increased drastically at the beginning of the twentieth century, mainly for two applications. The first is the chemical synthesis of nitrogen fertilizers for agriculture, when the need of the growing world population

1 Naturally occurring lithium is a mixture of two isotopes: 7Li with an abundance of 92.5% and 6Li with an abundance of 7.5%. https://doi.org/10.1515/9783110596250-003

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could no longer be covered from mineral or organic sources. Catalyzed ammonia synthesis (N2 + 3H2 → 2NH3) fixes nitrogen from air. This is accomplished by the famous Haber–Bosch process. The technical chemist Fritz Haber laid the foundations by establishing the reaction conditions of high pressure and moderate temperature, and chemical engineer Carl Bosch realized the reactors for large-scale production. The decisive role of the catalyst was elucidated by Nobel prize winner Gerhard Ertl, and search for advanced catalysts is still ongoing to afford ammonia synthesis at milder reaction conditions of temperature and pressure. Part of the ammonia is subsequently oxidized to yield nitric acid. Without the availability of ammonium nitrate, the so-called green agricultural revolution and the associated crop yield increases would definitely not have been possible. Today, the production of hydrogen for ammonia synthesis accounts for more than 2% of the global energy needs. The second large application of hydrogen is oil refining. Two of the associated processes rely on hydrogen, that is, the removal of sulfur from crude oil by hydrotreating, and the conversion of heavy (large) molecular weight fractions to lighter compounds by hydrocracking (breaking of carbon–carbon bonds with hydrogen). The amount of hydrogen used for these purposes matches the global volumes of transportation fuels (gasoline, diesel, and kerosene); hence, refineries handle very large volumes of hydrogen which is often transported in pipeline networks. In the early days of the Haber–Bosch ammonia synthesis, hydrogen was obtained as a side product of the chlorine–alkali electrolysis (2NaCl + 2H2O → Cl2 + H2 + 2NaOH), with chlorine as the main product. As the demand for hydrogen increased and natural gas became available, the steam reforming of the latter became the predominant source. In fact, the steam reforming of methane (overall reaction2 CH4 + 2H2O → CO2 + 4H2) is still the highest volume and least expensive production route of hydrogen, amounting for 48% of global use, followed by hydrogen from coal and oil. Together, these fossil sources still account for 96% of the hydrogen produced, which is termed brown hydrogen in the literature. Clearly, the production of hydrogen as an energy carrier in a future sustainable energy system cannot be based on fossil fuels, and hence we shall return to the production based on renewable energies, after briefly discussing another use of hydrogen.

Space applications Hydrogen played a pivotal role in NASA’s Apollo and Apollo-Soyuz space projects. The carrier rockets used in the launches between 1961 and 1975 were propelled by

2 The overall reaction consists of steam reforming (CH4 + H2O → CO + 3H2) and water gas shift (CO + H2O → CO2 + H2).

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hydrogen. For example, the fuel of Saturn II rocket consisted of an astounding quantity of 1,000,000 L of hydrogen, and 331,000 L of oxygen, both in liquefied form. Today, heavy-duty carriers like the Ariane rocket series use a combination of solid-state propellant boosters and hydrogen/oxygen engines, and carry more than 100 tons of fuel. Interestingly, the Apollo mission already featured another use of hydrogen: Electricity on board of the lunar command module was provided by fuel cells, and similarly the supply of the International Space Station is furnished by alkaline fuel cells (cf. section on transportation).

Hydrogen production by renewable energy Solar energy, wind, and hydropower are the renewable energies used for the production of so-called green hydrogen. There are three basic routes to achieve this aim. The first avenue harnesses renewable electricity and uses the latter in electrolysis to produce hydrogen and oxygen. The invention of electrolysis by Nicholson and Carlisle dates back to 1800. In the past three decades, tremendous advances have been achieved with respect to efficiency, power density, and cost reduction. There are three main variants, the choice among which depends on the application: – alkaline electrolysis is the industrial benchmark, usually the least expensive option for applications requiring semicontinuous hydrogen production; – high-temperature electrolysis is related to solid oxide fuel cells (see below3); it uses steam as the reactant, and the efficiency is increased by the possibility to use waste heat as secondary input; – polymer–electrolyte membrane electrolyzers excel by high current densities and fast switching times, and are therefore well matched to photovoltaic or wind energy sources with their inherently varying power; rapid start-up is possible as a lengthy warm-up phase of high-temperature electrolyzers is not required. At this point, it is appropriate to mention briefly another variant nicknamed blue hydrogen. The latter is again produced using (nearly) CO2-free electricity, but this time generated either by nuclear power or by coal/gas-fired power plants where CO2 is removed from the flue gases by carbon capture and sequestration. The second pathway uses concentrated solar power. The latter may again either be used to provide renewable electricity, or for the direct production of hydrogen using thermochemical cycles. A variety of cycles has been tested, and at present, the most promising candidates appear to be based on perovskite-type materials. In the high-temperature step, lattice oxygen is released from these materials, while in the subsequent low-temperature step the desired reduction of reactants takes place:

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The now oxygen-deficient perovskite is exposed to water vapor or water/CO2 mixtures, whereby either hydrogen or synthesis gas (H2/CO mixtures) are produced. The third route is termed “photocatalytic water splitting.” A suitable semiconducting material, or a sandwich of two judiciously chosen semiconductors, is doped with cocatalysts on either side, and exposed to sunlight. If the band gaps of materials and their surface chemical composition are adequately positioned in energy, hydrogen and oxygen gases are produced directly on opposite sides of the device, without the intermediacy of electricity generation. With respect to maturity, photocatalysis is still investigated at the laboratory scale, and hence considerably less advanced with respect to technological readiness, as compared to the abovementioned photovoltaics/electrolysis combination. Yet, a community of researchers considers photocatalysis as an option with highest long-term potential. In general, it mimics the photosynthesis process of nature in which water and CO2 are reduced; this is why one research team designated its realization as an “artificial leave.”

Energy storage using hydrogen Decarbonization of electricity generation is a high-level target in the European Union and worldwide. As the potential for an increase in hydroelectric power generation is limited on the continent, further substantial increases in photovoltaic generation and wind energy are elements of many national strategies. The intermittent generation due to these sources poses a well-recognized problem. While in summer, the peak of insolation may coincide with the maximum demand for air conditioning in warmer countries, photovoltaic electricity is not available in the later evening and night hours. The partially complementary profile of wind energy cannot yet be fully exploited due to a lack of high-voltage north–south transmission lines. Consequently, there is a need to store electricity on a daily, weekly, and seasonal timescale. Demand side measures (shifting consumption to match available power) and sectoral transfer to transportation are valid, but only partial responses that will likely not be sufficient. Batteries provide storage of up to megawatt-hours of electricity over timescales of hours or days. Chemical energy storage potentially offers large volume solutions of up to terawatt-hour quantities, operated during periods when an excess of renewable power in the grid cannot be absorbed otherwise, with the option of storing the produced chemicals over timescales of months and seasons. Conversion of the electrical energy to chemical energy stored in the form of hydrogen is the most efficient solution for long-term storage; this process is termed “power to gas” in the literature. Electrolyzers with efficiencies exceeding 70% are available for this purpose, and are being installed at increasing power ratings and decreasing cost.

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For longer term storage of the produced hydrogen, three strategies may be followed. Hydrogen could be injected directly into the natural gas grid with its inherently large storage capabilities. While the hydrogen content of the transported gas is presently limited to 2%, most pipelines could be safely operated with up to 10% hydrogen if regulations were adapted. Where direct injection is not feasible, conversion of hydrogen to methane is an option (cf. section on synthesis). Third, tests are ongoing to realize dedicated hydrogen storage caverns, for example, by repurposing salt caverns previously used as natural gas reservoirs.

Transport applications The decarbonization of the transportation sector is likely the most recalcitrant problem on the pathway to an energy system with “zero” CO2 emissions. The difficulty of replacing the standard fossil fuels by CO2-free fuels increases in the order from motorbikes, passenger cars, heavy-duty vehicles, to ships and airplanes. A wave of vibrant interest in hydrogen fuel cells for transportation started around 1990. In the fuel cell process, the reverse of electrolysis, hydrogen and oxygen (air) react electrochemically to produce electricity on board, which is then used to power an electric motor propelling the vehicle. As early as 1839 and 1845, respectively, Christian F. Schoenbein and William R. Grove had invented the principle of the fuel cell, but the technology was for long considered as too expensive for cars, and only used for space and military applications.3 Tremendous progress in increasing the fuel cell power per weight and volume, in decreasing the amount of required platinum, and in driving down the cost gave rise to the expectation that fuel cell passenger cars would be on the road by 2005. However, several challenges (mainly lifetime and cost) need to be addressed, and the development took until 2015 when the first fuel cell cars were sold to the general public. In the meantime, another form of electric propulsion had made headways, that is, the direct use of electricity in battery electric vehicles. This was made possible by advances in lithium ion battery technology – increasing the energy and power per weight, providing adequate safety, and decreasing the cost per stored kilowatt-hour to an unprecedented level. As of present, battery electric vehicles are on the market with ranges around 300 km, and premium models with ranges above 500 km. The consequence was a heated debate about the pros and cons of fuel cell and battery technologies, at times resembling a war of faith, with various countries and automotive companies taking different bets on the winning option. On the scientific

3 Polymer electrolyte fuel cells are predominantly used for transportation, as they are capable of rapid start-up and fast power-level changes. High-temperature solid oxide fuel cells, usually fueled with reformed natural gas, are suited for stationary combined heat and power applications.

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level, this was accompanied by careful life cycle analyses, emphasizing the importance of the source of electricity (for both), the materials used in the manufacturing of batteries or fuel cells, the weight increase of the car due to aggregates, the “refueling” time, the storage of the fuel on board, and the available utilities in summer and winter. Today, a consensus is slowly arriving that there is not an either/or decision between the two options but that the two technologies complement each other for defined purposes. For (lighter) vehicles mainly used in short-distance driving, battery electric vehicles offer unbeatable advantages due to the efficiency of charge/discharge cycles, albeit at the price of longer charging time. On the other hand, for heavy vehicles with longer driving distances per day, and the need for fast refueling, propulsion by fuel cells is the preferred option. Heavy-duty trucks and buses are prime candidates to be equipped with fuel cell aggregates. Delivery truck and bus fleets offer the advantage that they often depart from a central location, where the refueling infrastructure can be built up close by and thereby profit from predictable turnover. Several companies are investing in the development of fuel cell trucks. Particularly noteworthy is the concept of a Swiss consortium that focuses on the parallel buildup of fuel cell truck fleets and hydrogen fueling stations located close to their home base.

Synthesis of transportation fuels and base chemicals Two issues need to be addressed in the context of using hydrogen for transportation. First, the buildup of a hydrogen refueling infrastructure covering a country’s complete area is expensive, while a well-organized distribution network for liquid fuels already exists. Second, fueling by hydrogen for the most energy-demanding applications, such as large airplanes for long-distance flights, appears close to impossible, as here the high energy density of liquid fuels is irreplaceable. The synthesis of liquid fuels starting from hydrogen and CO2 offers a solution for aircraft and other heaviest duty applications. The simplest liquid fuel that can be produced by catalyzed synthesis is methanol. The advantages of methanol have been recognized early in the discussion around hydrogen, such that even the term “methanol economy” had been proposed. Methanol is a an important intermediate in chemical industry – today being produced from fossil sources in very large volumes, and used for a variety of applications, including the methanol-to-gasoline process. The latter might become an option for producing gasoline from renewables. A further option for production of a liquid fuel is Fischer–Tropsch synthesis. The basic technology was developed early in the twentieth century and is used to produce diesel-like hydrocarbon fuels from coal. Researchers improved the catalysts

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that promote the formation of carbon–carbon bonds and yield long-chain saturated hydrocarbons from synthesis gas (CO + 2 H2). With suitable modifications, the Fischer– Tropsch-type catalysts can be adapted to synthesize liquid hydrocarbons from hydrogen and CO2, a subject that received renewed and increasing interest from the late 1980s onward. For all of these options, the question about the source of CO2 remains to be answered. In the first option, CO2 is separated from the flue gases of fossil power plants, incinerators, cement plants, or steel production (carbon capture, storage, and use). The carbon moiety is thereby still of fossil origin but the emissions of the plant are “reused,” and the fossil transportation fuel is thereby replaced. The purist option is the extraction of CO2 from the ambient air (direct air capture), a cycle being closer to CO2 neutral, except for the energy expended for the capture. It offers the advantage that the synthesis plant can be erected wherever renewable hydrogen is available, not requiring the proximity of a flue gas separation facility. Another potential source of carbon is biomass. The combustion of biomass for power generation, combined with CO2 separation from the flue gases, has been proposed as a “negative emission” technology (biomass energy carbon capture and sequestration). If, instead of storing the separated CO2 underground, the latter is used for the synthesis of liquids, the overall cycle ending with combustion of the synfuel becomes CO2 neutral, as the biogenic carbon had before been extracted from the atmosphere by photosynthesis. At this point, we should mention another option, the synthesis of methane from H2 and CO2, which is also termed “power to gas” in a broader sense. It is legitimate to ask when this technology can make a meaningful contribution while, at the same time, hydrogen is produced in large volumes from fossil methane in today’s chemical industry. The answer requires careful argumentation: The target is the storage of “excess” renewable electricity. If the large volume of hydrogen, produced by electrolysis from this electricity, cannot be used in short time nor fed into the gas grid, then the production of CH4 offers a valuable solution. A prime application of the soproduced biogenic methane is the fueling of natural gas cars, thereby reducing their tailpipe emissions close to zero. Further, European directives target a content of 10% of renewable gas in the natural gas grid, and the methanation of CO2 will be needed to achieve this value. An attractive option for “power to methane” is a combination with the fermentation of biogas. The raw fermentation gas contains roughly equal amounts of CH4 and CO2, and conventionally the latter must be separated and released to the atmosphere before the biogas can be injected into the grid. If instead, the CO2 in the raw biogas stream is converted to methane using renewable hydrogen, the overall yield can be increased (technically by ~60%) while the separation step becomes superfluous. Widening the scope of syntheses from H2 and CO2, we focus on the feedstock for chemical industry. Presently, the base organic chemicals used in industry – olefins, aromatic compounds, and others –are almost exclusively derived from fossil fuels,

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mainly from natural gas and oil. The point has often been made that rather than burning oil for heating and transportation, one should save oil reserves as a resource for chemical industry as the chemical feedstock is difficult to replace. However, for a system targeting “zero” carbon emissions, other sources for feedstock are required. Biomass (“naturally regrowing raw materials”) can offer a partial replacement of the fossil base, provided that competition with food production can be avoided. For a larger volume, the synthesis of base chemicals and intermediates from H2 and CO2 provides a sustainable solution. The production of methanol, mentioned above, is a first option. Further, a plethora of small molecules exists that can be synthesized from the CO2 building block, and suitable routes have been developed and are available. In this way, renewable hydrogen opens a viable path to decarbonize the chemical industry.

Role of hydrogen in energy systems integration Sector coupling is key to achieving the goals of the energy strategies of many European countries, to reach their emission targets. The concept implies that the sectors of energy provision, industry, services, households, and transportation are no longer advanced separately but are considered as a connected entity and controlled together to reach a global optimum with minimum emissions. Examples of system integration have been amply discussed and have meanwhile become accepted concepts. For example, individual houses become prosumers that import and export electricity. The electricity grid and transportation may be coupled such that excess renewable electricity is used for charging electric vehicles, while the latter vehicles feed electricity stored in their batteries to the grid in periods of high demand (vehicle to grid). “Smart grids” implement demand side management by load shifting. The operational control of such a “smart” sector-coupling network has been termed “energy hub.” As one of key characteristics, these energy hubs, typically at the community or city quarter level, share a network of grids where not only electricity but also heat, cooling fluids, and gases circulate and can be exchanged. In such multienergy carrier grids, hydrogen will undoubtedly have to play an important role.

The future role of hydrogen Technology development often proceeds in “waves,” a concept proposed by Cesare Marchetti in other context. Reviewing the discussion of the preceding sections, one could discern the following phases that each implied a surge in the importance and use of hydrogen:

Foreword

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the first wave started with the massive need for hydrogen in the chemical industry, for fertilizer production, and refining; the second wave involved the highly visible use of hydrogen in space missions; the third wave, in the 1990s, was characterized by widespread enthusiasm for hydrogen in transportation and energy industries, such that hydrogen was considered the unique energy carrier of the future, and scientist formulated the vision of the hydrogen society.

Today, we are in the middle of a fourth, more realistic, yet more powerful phase that will engender a massively increased production and use of hydrogen. We no longer suppose a single solution and one unique predominant energy carrier, but realize that interconnected grids of electricity, heat, hydrogen, and other gases are required to achieve system integration and the needed coupling of sectors of our society and economy.

Conclusions and recommendations –

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Hydrogen is an indispensable energy carrier in the future energy system.As was demonstrated in the previous sections, hydrogen is urgently needed for energy storage, in transportation, as well as for the synthesis of renewable liquid fuels and feedstock for the chemical industry. European and global targets for energy efficiency, renewable energy shares, and decarbonization, and the associated climate protection goals, cannot be reached without hydrogen. For large-scale penetration, hydrogen technologies must become competitive in the energy markets. Therefore, research and development should focus on innovations that drive down costs. At the same time, governments need to agree on an adequate price for CO2 emissions. Several measures (CO2 tax and emission trading systems) have been discussed elsewhere. In order to reach competitiveness, initial support is required to mature the associated technologies. In particular, the entire chain from academic research via industrial development to pilot and demonstration should be financially encouraged by science policy. Hydrogen must become an integral part of teaching at institutions of higher education, in particular, at the universities of technologies. These institutions should also offer courses of continued education for practicing engineers and industrial leaders. Researchers are to be encouraged to reach out in communication to governments, to industry, to insurance companies, and to the public, to enter into a dialogue in the spirit of partnership, and to inform stakeholders about the advantages and

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unique properties of hydrogen, including the safety aspects. The financial support of such a dialogue process must be considered an integral part of public funding. Implementations at industrial scale are required to demonstrate to societal public the safe handling of hydrogen, according to the established standards. Capacity markets are an adequate measure to valuate hydrogen as a storage medium for intermittent renewable electricity (photovoltaics and wind energy). In transportation, strategies should aim for a concomitant build-up of hydrogen supply infrastructure and hydrogen demand, in order to avoid expensive upfront investments. The various variants of power-to-X technologies should be further advanced to increase the efficiencies of each step subsequent to hydrogen production. Innovative combined approaches such as coelectrolysis and coproduction of H2 and CO are to be pursued. Power to methane is a powerful option to store hydrogen by injection of CH4 into the national gas grids. This technology is required, in addition to biogas, in order to meet the European targets of renewable shares in the circulating and dispensed gas. In particular for air transportation, the synthesis of liquid fuels based on renewables should become a high-level target for research and development. Without the availability of these synthetic fuels, the decarbonization goals of transportation cannot be reached. The sectors of the economy must be coupled by implementing “smart” energy systems, including multi-energy carrier grids. Chemical industry should prepare for a change from fossil to renewable resource base. This constitutes a major challenge in view of the very large associated mass flows. Europe’s energy strategies and its decision to establish a CO2-neutral energy system by 2050 make it a prime mover to advance and demonstrate hydrogen technologies. The determinedness of Europe with respect to these goals provides a strategic advantage compared to the United States and China, countries that have partially other priorities. In view of Japan’s strategy of direct governmental financial support to industry, the European Union should act to secure its competitive advantage and thereby open up sustainable energy markets of the next decades.

In summary, hydrogen is thus truly indispensable to reach Europe’s high-level goals. It has a vital role to play on our ambitious climb toward a sustainable energy system. It offers the unique advantages of high energy per weight, long-term storability, pollution-free convertibility to other forms of energy, and flexibility in application. The quantity of hydrogen to be employed in society and the global energy system is only limited by the available renewable energy used for its generation, as the only other resource required for its production is water. Within a future-oriented, multipronged

Foreword

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energy system, our societies should be strongly encouraged to exploit this unique potential of hydrogen as one truly renewable energy carrier.

Further Reading For scholarly quotations on the individual aspects and technologies, the reader is kindly referred to the respective chapters where a large number of technical references are given. Here we restrict ourselves to a very few general references and compendia. [1] [2] [3]

Sperling D. Future Drive. Island Press, Washington D.C., 1995. Rivkin J. The Hydrogen Economy. Blackwell, Oxford, 2002. Olah GA, Goeppert A, Surya Prakash GK. Beyond Oil and Gas: The Methanol Economy. Wiley-VCH, Weinheim, 2006. [4] Ball M, et al. (eds.). The Hydrogen Economy: Opportunities and Challenges. Cambridge University Press, Cambridge, 2009. [5] Wokaun A, Wilhelm E. Transition to Hydrogen. Cambridge University Press, Cambridge, 2011. [6] Corbo P, Migliardini F, Veneri O. Hydrogen Fuel Cells for Road Vehicles. Springer, London 2011. [7] Kopernikus initiative, project. P2X (Power-to-X), https://www.kopernikus-projekte.de/ projekte/p2x. [8] Kopernikus initiative, project. SynErgie, https://www.kopernikus-projekte.de/synergie. [9] Stolten D, Scherer V (eds.). Transition to Renewable Energy Systems. Wiley-VCH, Weinheim, 2013. [10] Sherif SA, Goswami DY, Stefanakos EK, Steinfeld A (eds.). Handbook of Hydrogen Energy. CRC Press, Boca Raton, 2015. [11] Cox KE, Williamson KD (eds.). Hydrogen: Its Technology and Implication. CRC Press, Boca Raton, 2017. [12] Kober T, et al. Power-to-X: Perspektiven in der Schweiz. Swiss Competence Centers for Energy, 2019.

Extended Introductions

Pierre Etienne Franc

Hydrogen: why the times to scale have come Hydrogen technologies have been around for centuries. In the early decades of the eighteenth century, many engineers developed their first engines to use this magic molecule. However, the human history of energy utilization has – to now – chosen different routes to heat, move, and industrialize. A well-known graph shows, in a simplified way, that the history of our relationship with energy seems like the one of a quest to reduce its carbon content (even though it never was a conscious move). Each step of civilization seems to mean one level of carbon less, toward pure hydrogen-based solutions. Each step involves a further reach in technological knowledge to meet higher energy needs. We started our quest with wood, and then shifted to coal. The discovery of oil brought a fuel with far better energy density, boosting the transport revolution. Natural gas came next – getting us closer to hydrogen – but this shift to gas sources required us to master new storage, transportation, and distribution techniques. At the same time, each step has enabled a further sophistication of the materials and feedstocks used, shifting the use of fossil resources from a pure energy source to a far broader source of materials for building, equipment, and everyday life provisions for our needs.

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But the hydrogen economy, the hydrogen society, has not yet arrived. This long awaited next step into a hydrogen-driven energy model, where only electrons and hydrogen become the vectors of energy generation, storage, transport, and distribution, is not here yet. The key technologies needed to achieve such a hydrogen-based system are known, mastered. They still have their complexities and challenges, which are broadly discussed in this book and some are significant. But the issue of taking that next step is not technology related. It all depends on our goals.

Why can’t we say this time, therefore, that hydrogen is no longer the future, but the present? What has fundamentally changed to build conviction from the key hydrogen players and supporters? As noted, technologies have a major impact to progress in all key fields – power electronics, materials chemistry, thermodynamics, silicon technologies, miniaturization technologies, and digital – all enable performance improvements in the production, storage, distribution, and use in fuel cells of hydrogen-based solutions. Many solutions that exist today to store and transport high pressure or liquid hydrogen safely and competitively (heavy storage, supercritical transportation, leakage detection and leak preservation, safety measures, and control systems), to boost electrolysis performance (footprint, capex, usage of raw and precious materials in the catalyst and electrodes), and to capture carbon – all fields of global technological progress that enable the use of hydrogen in places, at densities and for public uses – could not have been considered when Cavendish, Rivaz, and other key scientists of the time first discovered hydrogen’s properties. In a way, hydrogen technologies take a full benefit of everything being developed around them, including the huge electrification wave currently under development in the automotive world. But more significantly, to reach the scale necessary to make a change, hydrogen technologies needed to serve a proper goal, one which outweighs all challenges and enables whole sectors to grasp it properly. This required three following steps: 1. Designing a shared vision against a major challenge. 2. Developing concrete and tangible proofs of the viability of hydrogen-based solutions. 3. Developing a concerted, coordinated multilateral ambition. This is what has been achieved over the past decade.

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The vision The vision, at present, results from the widely accepted challenge of fighting climate change. Whatever the economic options are considered, our world will not continue its current course of growth and energy use without significant life-threatening harm to our societies. Either through different growth models or a change in economic wealth patterns and goals, we need to shift to low-carbon energy sources, as renewable as possible, and this has to happen now. And it does. The share of renewable power in primary production continues to grow; new energy capacity additions are now higher from renewable sources than from fossil ones, and the levelized cost of electricity (LCOE) of those new sources of energy are in many places competitive with fossil base sources. Primary generation of renewable sources is on its way, but its transport, distribution, and use in downstream consuming sectors (transport, industry, district heating) are now the key challenges. Many analyses of future energy systems have now clearly demonstrated that hydrogen is an essential vector to enable a below 2 °C scenario. A key vector to store and transport vast amounts of renewable intermittent energy in places or times where they are in surplus to places and times where they are in need. A key vector to enable the continued use, for several decades, of fossil fuels that are as clean as possible, by using only the hydrogen part and sequestering the carbon through carbon capture and use or sequestration technologies. A key vector to decarbonize transport, alongside battery electric solutions, especially for large and intensive mobility needs. A key vector to decarbonize the use of feedstocks (by synthesis of gases) or fuels in high-energy-intensive industries. A key vector (in some geographies) to develop highly efficient low-carbon stationary heat and power generation solutions for district heating needs. These views have led several different organizations to converge on the view proposed by the Hydrogen Council, that hydrogen could represent up to 18% of final energy demand by 2050 and help achieve 20% of the CO2 reduction target of the planet. In other words, hydrogen is now increasingly seen as an unavoidable part of any future clean energy system. This vision, proposed by the coalition of CEOs on the Hydrogen Council, followed initial views pushed by leading countries on the matter – Germany, California, Japan. Recently, strong measures have also been adopted by South Korea, China, Australia, the European Union, and several Middle East countries. The International Energy Agency (IEA), the ETC, the International Renewable Energy Agency (IRENA) and Bloomberg, to name the most prominent consultants/commentators on the energy field, have all acknowledged the need to boost the deployment of hydrogen technologies if we are serious about energy transition.

1

Enable large-scale renewables integration and power generation

3

2

Act as a buffer to increase system resilience

Distribute energy across sectors and regions

Enable the renewable energy system

7

Serve as feedstock, using captured carbon

Help decarbonize building heating and power

Decarbonize industry energy use

5

6

Decarbonize transportation

4

Decarbonize end uses

The 7 roles of hydrogen in the energy transition

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This has also been possible because not only technologies but also implementations have progressed significantly over the decade. Back in 2010, a couple of hundred cars from several original equipment manufacturers (OEMs) were nicely demonstrating that it is possible to drive a hydrogen car. Stations were tested here and there, but nobody was seriously thinking yet that electrolysis could become a key node in the future energy system of the world. Ten large demonstration projects for buses were started in several cities. First forklift applications were considered in the United States. Now, technology deployment is catching up in all fields, and series have started to reach scale. The year 2020 saw more than 20,000 vehicles on the road, more than 500 stations up and running, several thousand trucks and buses, 30,000 forklifts in use every hour of the year, several GW of electrolysis capacity under construction and tens of GW of projects in development. The first large-scale trials for the use of hydrogen in power plants, in steel-making processes and of the first-liquid hydrogen carriers have been launched. The whole vision of a hydrogen-enabled energy system is being backed by a significant wave of deployments, experimentations, and tangible operations. Finally, this has also worked because not only have we seen a vision been built and the first tangible operational demonstrations being developed but also there is a clear alignment of views on the remaining key hurdles that the industry, jointly with policy makers, has to clear to make this real. First safety. Hydrogen, as any energy carrier, is dangerous. It needs to be handled with proper care and correct operating procedures. This includes stringent equipment, process, and behavioral rules that will have to pervade throughout the general public – so in a far broader and more public space than was the case when it was confined to industry-related topics. The more the technology spreads in its potential and use, the more players – large and small – will touch it, and the more chances we face for handling and safety issues. For this to be addressed properly, regulations have to be developed and harmonized between countries and a

2015

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Potential demand for hydrogen in a +2 °C Scenario (in EJ)

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2050

13%

12%

14%

21%

29%

12%

78

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Power generation, buffering

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New feedstock (CCU, DRI) Existing feedstock uses

Building heating and power

18% of final energy demand

Transportation

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Tomorrow’s use for the energy transition will unlock ten times bigger market

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Hydrogen: why the times to scale have come

Hydrogen mobility markets: Ready to scale TODAY

Ferries 1 T/day

Cruise ships 10 T/day

Material handling vehicles 100 kg/day per site

Trucks 100 kg/day per truck

Buses 20 kg/day per bus Trains 150 kg/day per train

Airplanes Applications

Individual cars 100-200 kg/day per station

Drones

Bicycles & scooters

very stringent program of training and education of all energy related players and OEMs will have to be developed. This is by far the most critical of the challenges ahead and it has to be tackled rapidly as many SMEs and new actors are entering the field. Education and normalization is to be developed to help them properly capture the challenges of this molecule in their activities and to benefit

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from competent resources to grasp those safely. But it is no different an issue than that which arose with the growth of oil-based activity, then gas, and now battery and electric uses. We are starting a new wave, developing uses of hydrogen technologies in much larger spaces for far more numerous applications, and we need to prepare the population for it. Second – low-carbon shift. Hydrogen technologies have to provide for a lowcarbon hydrogen source. If not, the hydrogen revolution will not happen, as it will not serve our common goal with respect to using clean energy vectors. Clean hydrogen technologies exist, but they are, by definition, more expensive today than fossil-based methods. In total, 95% or more of hydrogen produced today is based on natural gas, coal, or oil with an associated 10–20 kg CO2 produced/kg of hydrogen. The end game is to provide a kg of hydrogen at the point of use for less than 1 kg CO2. This is a huge technical and economic challenge. It is also the place where hydrogen reconciles all players, as it is the ultimate high-energy molecule if we are able to supply it without associated carbon. Developing the right methodology and goals for low-carbon hydrogen production, to be shared between geographies with agreed metrics and measurement methods, with a tracking system enabling hydrogen to be traded with its CO2 footprint and thus to contribute to decarbonizing energy in each geography is a key topic. Second, new disruptive technologies for low carbon or zero-carbon hydrogen production are still likely to emerge in coming decades and we shall see many future developments which could further accelerate the shift to hydrogen systems. In that context and in the light of the above, European 2021–2027 Research & Development program needs to continue to support Hydrogen technologies and be further strengthened, next to the need for broad deployment support schemes. Last, low-carbon hydrogen requires a low-carbon supply chain, and thus requires strong investment upstream with renewable sources, or with low-carbon energy sources to optimize the energy cycles and the capex cost of those solutions. Similarly downstream, as the book will show, because often only 10% of the hydrogen value chain relates to production, while up to 80% relates to the conditioning, transport, and distribution side where emissions and energy spending happen as well. Optimization of the supply chain solutions – on site, gas pipes re-purposing, liquid solutions, other hydrogen rich vectors as transportation means – will all have to be revisited over the coming decade to develop the best techno-economic compromise to enable safe, reliable, and competitive low-carbon pathways to hydrogen at the point of use. Third, cost. Hydrogen production through fossil means is relatively mature today, with a landed cost of ~ 1.5 €/kg (50 €/MWh). But the whole value chain goes from production, in a clean way, to the transition of all downstream processes to enable the use of hydrogen at the point of use. The gap to competitiveness with longstanding and mature incumbent technologies is significant. In each part of the value chain, efforts still have to be made to halve the cost of hydrogen-related technologies. In certain applications, this will still not be enough to make hydrogen competitive with incumbent fossil solutions, but in others, it will. The Hydrogen Council developed a strong fact-based

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analysis of 33 applications enabling hydrogen to cover 60% of world emitting sources. Of those, half can achieve competitiveness in the coming decade if the proper cost roadmaps are put in place. Those call for further progress in technologies, but primarily for scale deployments. This is especially the case for fuel cell technologies, electrolysis, and network development for hydrogen distribution. It is, however, widely acknowledged by all players that a pathway to cost competitiveness exists for most key transport applications, many heavy heat-intensive processes, and several district heating applications. Last but not least, this will not work without a paradigm change in the way industry and policy makers’ act and play. To date, the world has benefited from the dynamics of a very competitive environment, in which the economics of scale and the role of the free market has driven a fierce struggle to cost- and value-based solutions to the point that the satisfaction of individual needs comes with only a marginal cost to volume-based production. Digitalization, massive marketing efforts, and supply demand elasticity of production tools have enabled this. But, we are reaching the point of no return in terms of resource absorption, saturation of needs, deterioration of environment, livestock, and resources. The marginal cost of “things” brings with it the risk of the whole planet’s destruction! The next phase will require a paradigm shift to a more resilient and clean energy growth environment, for which hydrogen is one of the essential tools. It means more cooperation to reach scale, by sharing risks between industry players, strong alignment of development and deployment strategies between industry verticals, and a very stringent and sustainable regulatory and financial support scheme from the policy makers to ensure that efforts made will be valued over time. This also calls for different approaches to performance metrics from the financial markets and new accounting standards, to enable large industry players to commit significant amounts of investment to collaborative schemes, with adjusted returns at start. This would acknowledge the cost of developing competitive solutions using so many disruptive yet critical technologies as hydrogen to scale. What is a stake is a first, not only in the diversity of fields touched, but also in its criticality for the planet. In this context, it is of paramount importance that a comprehensive review of the current state of hydrogen technologies is put forward as proposed in this book, which would also help design properly the key expected pillars of the future European Research Program for the years to come. This will both explain to the public where we stand and what is ahead of us, and also enable opinion leaders, policy makers, investors, and industry leaders to see the amount of progress achieved already toward the successful realization of a clean – and hydrogen-based – energy system. For the European citizen, we have here a chance to prepare Europe early enough, not only to master and use the future key clean energy vector for its energy needs (it being coming from fossil or renewable sources) but also by making such moves early enough to also provide a new source of technologies, growth, and jobs

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for the continent. Energy policies for Europe need to match the famous tryptich of sovereignty, competitiveness, and sustainability. Hydrogen, by proposing a versatile source of energy as feed, will enable Europe further to build energy independence, as a way to use all possible sources of energy, including locally produced with its strong renewable resources. It creates a further element of network integration and interconnection between southern and northern parts of Europe, as possible feed for a clean hydrogen production and the central and continental Europe as a strong energy consumer. Last, starting early, the phases of deployments should help master the key technologies and reach competitiveness earlier than other geographies. As we said in a book done on the matter,1 hydrogen also provides Europe with another way to provide guidance to the world with a better policy for sustainability and climate change, and therefore provides a strong alignment with Europe’s political agenda in the global geopolitics.

1 Hydrogen, La transition énergétique en marche, PE Franc / P. Mateo, Preface from Pascal Lamy.

Ad van Wijk

Hydrogen key to a carbon-free energy system Abstract: Hydrogen has a key role to play as a carbon-free energy carrier alongside electricity. Hydrogen can be transported worldwide by ship and pipeline and can be stored underground in large volumes. This makes it possible to deliver cheap renewable energy, especially solar and wind, cost efficiently at the right time and place to the customers. Next to this systemic role, hydrogen is important to decarbonize energy use in hard to abate sectors in industry, mobility, electricity balancing, and heating. Future hydrogen systems will have similar characteristics as present day natural gas systems. Large-scale multi-GW renewable hydrogen production plants at good resources sites will produce a minimum of 1 million tonnes hydrogen. Hydrogen infrastructure can be realized by re-using the gas infrastructure, pipelines, and salt cavern storage, without major adaptations. As a transition, hydrogen produced from fossil fuels at the resource sites with Carbon Capture and Storage directly in the field below, can bring low-carbon hydrogen volume in the system. Such an approach can establish a fast, cheap, and reliable transition to a sustainable energy system, whereby hydrogen will fully replace natural gas, coal, and oil. The conversion technology used today is based on combusting technologies: boilers, furnaces, engines, and turbines. These combustion technologies can be easily and are fast adapted to combust hydrogen. In future, however, combustion technologies will be replaced by electrochemical conversion technologies including heat pump technologies. These technologies offer the promise to be cheaper, moreefficient with no harmful emissions to the air, land, or water. A smart symbiosis between electricity and hydrogen as zero-carbon energy carriers with electrochemical and heat pump technologies, will establish a clean, costeffective, reliable, fair, and circular energy system.

1 The role of hydrogen in a carbon-free energy system Hydrogen today (2020) is produced from natural gas and coal, emitting CO2 to the air. The use of hydrogen today is especially as a feedstock in the chemical and petrochemical industry. However, in a future zero greenhouse gas emissions, reliable, affordable, and fair energy system, hydrogen needs to play a more important role. Hydrogen is, like electricity, a carbon-free energy carrier, with a high mass energy density. The role of hydrogen in a future sustainable energy system is therefore to transport and store large volumes of energy and to decarbonize energy use in hard-to-abate energy sectors.

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1.1 Worldwide energy and hydrogen consumption 1.1.1 At present, hydrogen is mainly used as feedstock, representing 2% of primary energy consumption About 90% of the world’s primary energy consumption in 2016, amounting to 556 EJ or 155,000 TWh, is fossil energy: oil, gas, and coal. This fossil energy is transported around the world by ship or pipeline, and then converted into a useful energy carrier, electricity, gasoline/diesel, or a gas. The conversion to a useful energy carrier is often associated with energy losses. For example, the efficiency of a modern gasfired power plant is about 60%, which implies that 40% of the energy is lost as heat. These useful energy carriers are then distributed and used in houses, cars, factories, and so on. Energy is used in many parts of our modern life: For heating and cooling houses and buildings; for electricity production to power equipment, appliances, and lighting; for transport by vehicles, ships, or planes; and for industry where processes require high temperature of heat and steam. Moreover, fossil energy is also a feedstock, from which chemical products such as plastics or fertilizers can be made. The final energy consumption represents the energy use by companies, houses, cars and is the primary energy consumption minus conversion losses in power plants, refineries, and so on. A distribution of 2014 global final energy consumption [1] is given in the Fig. 1.

Fig. 1: Final energy consumption worldwide [1].

At present, hydrogen is mainly produced from natural gas and coal and primarily used as a feedstock to produce chemical products, ammonia (the main component of fertilizers), and methanol. And hydrogen is used in refineries to desulfurize oil and in the production of gasoline and diesel. The primary energy input by gas and coal is about 3.200 TWh, representing roughly 2% of worldwide primary energy

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consumption. Figure 2 presents the energy balance for worldwide hydrogen production and consumption [2]. At present almost all of the hydrogen is produced and used at or nearby chemical and petrochemical sites.

1.2 Solar and wind potential 1.2.1 In 1 h time the earth receives from the sun more energy than total worldwide energy demand The question is whether all the world’s fossil energy use can be replaced by renewable energy. From all renewable resources, solar and wind have by far the largest potential, therefore the question could be rephrased, “is it possible to generate worldwide energy use, by solar energy systems and wind turbines. The answer is simple: that is no problem. The potential of solar and wind energy is very large [1].” If worldwide energy demand, 155,000 TWh, had to be produced with solar PV systems only, it would require a surface area covering about 10% of Australia (see Fig. 3) or 8% of the Sahara Desert [1]. The Sahara Desert is about 9.2 million km2 in size, which is more than twice the area covered by the European Union. The global wind energy potential is also very high. In a scenario where the entire worldwide energy demand that would be produced with wind turbines would only require an area of 1.5% of the Pacific Ocean, see Fig. 3. It should be noted that surface, however, is used to a limited extent, with one large floating wind turbine at every kilometer. So there is more than enough space to produce all the necessary energy for the whole world with solar and wind. This is even the case with an increasing global population and rising prosperity level.

1.3 Development solar and wind levelized cost of electricity 1.3.1 At good resource locations, solar LCOE will be about 1 $ct/kWh and wind LCOE about 2 $ct/kWh before 2030 The development of electricity production cost by solar and wind energy, expressed in the LCOE, Levelized Cost of Electricity, has seen a steep decrease over the past 10 years. In the past couple of years, LCOE for onshore wind and solar PV at the best resource locations, are even below low cost electricity generation by fossil fuels, as showed in Fig. 4 [3]. Recent Tenders for 2 GW Solar PV in Abu Dhabi and 1 GW solar PV in Portugal has resulted in prices respectively of USD 1.35 cents/kWh [4] and Euro 1.12 cents/kWh [5]. Also wind onshore tenders have shown prices between USD 2 and 3 cents/kWh, for example in Mexico [6].

2 Mtoe

By-product hydrogen

48 Mt H2 of which Ni > Pt > Co was found. Trovarelli gave evidence for the promoting effect of ceria on Pt, Rh, and Pd toward CO and hydrocarbons conversion, due to its oxygen storage capacity [22]. This behavior was more marked for ceria–zirconia mixtures [23], attributed to higher stability at high

2 Catalytic autothermal reforming for hydrogen production

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temperature and larger surface area [24]. Improvements in stability and selectivity are achieved from bimetallic catalytic systems. In Ni–Pt catalysts, it was suggested that nickel catalyzes SR and platinum POX, and, when added to the same support, heat transfer between the two sites is enhanced [25]. In [26] the increased methane conversion catalyzed by Ni/γ-Al2O3 with the addition of small amounts of Pd, Pt, or Ir was attributed to the higher exposed Ni surface area favored by the noble metal under ATR conditions. Research on ATR catalysts has given also attention to perovskite systems of general formula ABO3, due to the possibility of substituting A and B to get a wide variety of mixed oxides, characterized by structural and electronic defects [27]. Interesting results of ATR of diesel fuels to hydrogen with perovskite-type oxides with B partially exchanged with ruthenium with respect to COx selectivity and sulfur tolerance during an aging test were reported in [28]. Cerium- and nickel-substituted LaFeO3 perovskites were investigated by Erri et al. [29] in the ATR of JP-8 fuel surrogate. Catalytic test carried out at high space velocity values showed improved hydrogen selectivity and beneficial effect on cerium as coking inhibitor. Great attention must be addressed to the catalyst structure (powder, pellets, honeycomb, foams, etc.). As an example, the requirement of high values of space velocity of compact ATRs suggests the choice of honeycomb monolith structured reactors, whose materials have good thermal conductivity properties. Moreover, previous studies have demonstrated that high thermal conductivity supports can play a relevant role in improving the performance of catalytic reactions, especially those strongly limited by reaction heat generation and transport, through the optimization of the temperature profile inside the reactor [30]. The open structure of these catalysts minimizes pressure drop along catalytic bed and the risk of hot-spot phenomena that can be responsible for coke formation and thus catalyst deactivation. We found that foam structured Ni catalysts are more active than honeycomb structured catalysts for CH4 ATR. Moreover, the temperature distribution along the foam catalytic bed showed a lower average temperature and a more uniform profile [31].

2.3 ATR-based process 2.3.1 Reaction system for reforming process We have highlighted that the common feature of hydrocarbon reforming and PO reactions is the generation of a syngas with different composition with respect to H2, CO, and CO2. Therefore, when the objective is hydrogen production at high yield and purity, the CO at the reactor outlet is converted to CO2 by WGS reaction (2.2), by which the maximum H2 yield is achieved from the co-reactant steam. However, because of the exothermicity of WGS, the high temperatures of SR and PO limit the WGS conversion, and, typically, equilibrium with the reverse WGS reaction is established,

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resulting in unconverted CO in the products. Moreover, low temperatures also favor other side reactions, such as methanation (the reverse of SMR) (2.8), fuel thermal decomposition (2.9), and the Boudouard reaction (2.3). The last two reactions both favor carbon deposition, resulting in catalyst coking and deactivation: Methanation ðMTHÞ: CO + 3 H2 ! CH4 + H2 O

(2:8)

Thermal decomposition: Cn Hm Ok ! ðn−kÞ C ðsÞ + 0.5m H2 + k CO

(2:9)

Boudouard reaction: 2 CO ! CO2 + C

(2:3)

To achieve both unwanted side reactions minimization and H2 yield maximization, SR and PO are performed in an ATR reformer at 800–1,200 °C, while WGS is carried out downstream at much lower temperature (5,000 t/day) based on ATR syngas technology [36]. Really, ATRs have still limited commercial experience. The most interesting applications are at SASOL in South Africa (a synthetic fuel plant collaborated with Qatar Petroleum, with a capacity of 34,000 barrels per day of GTL production, another at the same capacity with Chevron and Nigeria National Petroleum Corporation). ATRs have been installed in ammonia plants in Canada and China. It should be noted that ATR is much more energy efficient than SMR due to its chemistry. ATR requires much less steam than SMR and the WGS reaction is less prevalent. Its drawbacks, such as an oxygen plant to achieve higher H2 yield with respect to air and the management of high-temperature zone at reactor inlet, are evident.

2.4.2 ATR for distributed production of hydrogen In the previous paragraph we have mentioned examples of ATR application for large chemical industrial plants (ammonia, methanol, GTL processes). However, since hydrogen transport and storage suffer from critical issues such as cost and safety, the goal of developing a full hydrogen-based economy requires the development of distributed hydrogen generation through optimized processes, infrastructures, and operating conditions. A distributed energy system is seen to date as an efficient, reliable, and environmentally friendly alternative to the traditional energy system. In these systems small units for energy generation are located very close to energy consumers, like self-sufficient residential buildings in terms of heat, cooling, and electricity. A recent interesting paper reviewed recent advances in ATR production of hydrogen and distributed power generation through patents and papers, also suggesting novel research approach moving from hydrocarbons to alternative feedstock like oxygenated hydrocarbons, bio-oils, biodiesel, offer considerable room for further investigation [37]. The smaller size of ATR plants is a critical issue with respect to the growing interest in the last years to produce hydrogen for distributed energy production in either stationary or mobile application. In the area of mobility, ATR is considered as one of the most attractive options for on-board reforming of liquid fuels, such as kerosene and diesel for H2 feeding fuel cells [37]. Main superior characteristics are low energy duty and consumption, shorter contact time, flexibility of operating conditions, and CO2 recycling to achieve a given H2/CO ratio. Distributed PEM fuel cell power systems are focused on small scale and have capacities between 1 and 100 Nm3/h. A major issue of low-temperature PEM fuel cell application is the very high H2 purity requirement, in order to avoid the poisoning effect of CO, even at tens ppm impurity. Therefore, final purification step with pressure swing adsorption is necessary. Alternative solutions such as membranes or CO preferential oxidation are not yet ready for

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application. More resistant to the presence of CO and then more adequate to ATR option are the solid oxide fuel cells. More feasible is application of ATR in fueling station, simply considering that it could be profit of the present infrastructure network (methane, gasoline, diesel), progressively moving from a place where you fill the car tank with gasoline or diesel to a place where gasoline or diesel fuel are reformed to hydrogen for your car. In spite of the basic advantages of the ATR process, problems like heat management, reformer start-up, and catalyst improvements have to be addressed. More specifically, they ask for several requirements, reported in Tab. 2.1 together with instruments we have to answer. Tab. 2.1: Main features of ATR processor and instruments for optimization. Requirements

Available instruments

Fast start-up, shutdown, and energy request Wide feed versatility High system compactness Minimal external duties Minimal maintenance

Catalyst: chemical and physical formulation, hot-spot reduction, geometry effect Operating conditions: feed ratios, GHSV optimization Process optimization: thermal integration, process flow management

In the mentioned review [37] various patented solutions are reported to improve the performance of traditional ATR reformers with reference to heat management and efficiency of the reformer, catalyst formulation and catalyst structure, oxidant source, start-up method.

2.4.3 Chemistry, physics, and engineering contribution for small-scale ATR In general the whole ATR-based process for distributed energy systems, from feedstock to hydrogen as final product, is constituted by three main sections: a reforming stage to convert hydrocarbon to syngas, a WGS stage to convert CO to hydrogen, a purification stage to obtain the required hydrogen purity depending of the application (fuel cell, engine, distribution station, as examples). In this paragraph an example of approach and solutions to the issues of Tab. 2.1 are reported, based on personal research experience to the design of an ATR developed in collaboration with companies. We focused the attention to the optimization of chemical and physical formulation of the catalyst, thermal integration, catalytic reactor configuration, and start-up operation of a kW-scale ATR reactor designed and constructed in our labs. In particular, we patented a reactor configuration allowing a

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quiet flat temperature profile along the catalytic bed [38]. Then, the optimization was transferred to larger-scale processor design (up to 100 Nm3/h).

2.4.3.1 ATR catalyst Under 2.1 and from [37] it can be seen that noble metals (Pt, Rh, and Ru) are the most active ATR catalysts. Among non-noble metals, Ni, alone or with noble metals in bi-metallic formulation, is the major actor. However, given the major role of thermal management of the process both for thermodynamic and kinetic aspects, and for coke selectivity and aging, we addressed special attention to the catalyst support, including physical shape, texture, and structure. We compared two structured honeycomb shaped supports, loaded with Pt/Rh active species, one made of cordierite, the other one of metallic Fe–Cr Alloy [38]. Testing the performance of 5 Nm3/h methane ATR system in the range 45,000–150,000 h−1, clear differences were detected: in all GHSV range, the temperature profile of the catalytic bed was strongly flattened with the Fe–Cr alloy support and the temperature differences were about 50% lower than for cordierite. Moreover, methane conversion increased from 86% to 93% and hydrogen concentration in the reactor outlet gas from 38.9% to 42.5%. Finally, when loaded with catalyst on Fe–Cr Alloy the reactor responded very quickly and faster to variations of operating conditions with respect to cordierite supported catalyst. We concluded that the differences are related to the different thermal conductivity of the two supports. In particular from these results we reached the conclusion that the performance could have been furtherly improved by also optimizing the support macroporous structure and tortuosity. Finally, the influence of flow geometry in the ATR reactor (axial against radial flow) was also investigated. A clear positive effect of the radial flow on the temperature gradient was observed, but a limited effect on the ATR performance was obtained, likely due to the very high temperature generated at the initial part of the catalytic bed [38].

2.4.3.2 Multi-fuel solutions: diesel fuel ATR Since a liquid fuel has higher energy density than methane and other fuel gas, liquid hydrocarbon feed is expected to furtherly enhance the compactness of an ATR system. Then, one can think of a very flexible ATR, capable to be operated as a multi-fuel processor. We used dodecane as model fluid [39] and investigated possible solutions to achieve the goal. Since a major problem was to generate a homogeneous mixture of dodecane with air and steam in terms of temperature and composition as feed to the ATR reactor of, a mixing module was designed and constructed by optimizing the geometry of gas delivering, the spraying injector nozzle for liquid, and the stream flows,

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in order to get optimal mixing. Concerning the way of feeding liquid dodecane, we were inspired by the concept of common rail circuit of cars, by using a high pressure (1,000 atm) and alternate spray feeding system to generate dodecane microdroplets, whose vaporization is instantaneous. Pumps, electro-injector, valves, and pulsewidth modulator circuit complete the feeding device. Figure 2.2 shows the scheme of the device and the observed performance. Details of mixing module and dodecane delivery system are reported in [40]. Design Concept

Liquid fuel delivery system

The pulse generation tunes: Ø The opening width Ø The opening frequency

Injector Pulse Generator

Dodecane Vessel

100 V

Rail

Amplitude

Delivery Controller

High Pressure Pump

36 V

0V Width Frequency

Time

Linear correlation between dodecane rate and width

Cooling Fan Filter Filter Mass Flow Meter

Low Pressure Pump

Fuel Rate [g/min]

40

30

20

10

Pressure: 800 atm Frequency: 20 ms

0 400

500

600

700

800

900

1000

1100

Width [μs]

Fig. 2.2: Scheme of liquid feeding system to ATR reactor and measured performance (adapted from [40]).

2.4.3.3 Fast start-up The start-up time is a key issue of ATR reformers for propulsion and distributed power generation. Our solution was to start operating the reactor by feeding a combustion-rich methane/oxygen mixture in the first zone of the reactor, in order to increase the reactor temperature in a very short time, then changing the operating conditions to ATR modality by adding steam and methane to increase the methane/ oxygen ratio to values previously experimentally optimized. In the start-up phase, the methane–air mixture was ignited by a voltaic arc between two spark plugs, mounted on an SiC foam disk to optimize the radial distribution of the gas flow fed

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to the catalyst. Water was fed as liquid at room temperature from a 10 bar pressure tank. Table 2.2 shows a comparison between start-up times of fuel processors reported in the literature (more than 25 min) and our processor that allows reaching 80% of the steady state hydrogen production in less than 5 min. Tab. 2.2: Comparison of thermal efficiency of steam reforming and autothermal reforming-based fuel processors (from [41]). Processor

Literature

Start-up time (min)

Thermal efficiency (%)

SR based

Hajjaji et al.

.

SR based

Lee et al.

.

SR based

Seo et al.

.

.

SR based

Seo et al.

.

.

ATR based

Sheldon et al.

ATR based

Cipiti et al.

ATR based

Rabe et al.

ATR based

Di Bona et al.

ATR based

Our work

. .

. .

. .

.

2.4.4 Thermal integration Aiming at realizing a compact and self-sufficient ATR system without any external energy contribution to reactant preheating, the design and construction of a thermal integration module is a critical step. A very simple approach was to assure the preheating of air and water using the enthalpy of reactor outlet stream, while keeping the gas or liquid hydrocarbon feeding stream at room temperature. We found three as the minimum number of heat exchangers to be designed, two to vaporize water and overheated steam, one for air preheating. The three exchangers, made of stainless-steel tubes arranged as rectangular coils mounted in parallel on two manifolds to limit the pressure drop, were located inside a single module shell and tube heat exchanger, allowing uniform cross-flow section. The reactant stream flows in the tubes and the reactor outlet stream inside the shell. Details on the design of the thermal integration module are reported in [40, 42] Moreover, we also analyzed another available choice opportunity to optimize the efficiency of the heat integration module. In fact, since the sequence of the three heat exchangers was not univocally determined, we studied the effect of the relative position of the three exchangers along the chain of heat exchange. To compare the six possible alternatives, we

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calculated the heat exchange efficiency, defined as the ratio of the calculated and the highest available reactant temperature increase:   ε = ðTr,out − Tr,in Þ = Tp,in − Tr,in (2:10) where Tr,in is the inlet temperature of reactant (25 °C), Tr,out is the temperature of reactants at module outlet, and Tp,in is the exhaust gas temperature at the module inlet. We found that the best configuration is the sequence steam–air–water (ε = 71.4%), the worst air–water–steam (ε = 60.3%) for a given feed condition (O2/H2O/C = 0.6/1.2/1), but this conclusion seems to be independent of the feed condition [43]. The block scheme of the thermal module is shown in Fig. 2.3. The values of heat exchange surface area and of the thermal exchange coefficient are indicated. In Table 2.2, the thermal efficiency of our ATR processor is compared with results available in literature, relevant to SR and ATR processors showing the better performance achieved by us, despite the WGS stage of our process was not yet optimized with respect the conversion of CO.

Air

25 °C 100 °C Steam

h= 69 W/m2K A= 0,033 m2

h= 76 W/m2K A= 0,033 m2

h= 82 W/m2K A= 0, 065 m2

626 °C

487 °C

228 °C

Products 734 °C

384 °C

304 °C Catalytic Reactor

Methane 25 °C Water

Reactants Mixture

Fig. 2.3: Block scheme of integrated thermal module (adapted from [43]).

In conclusion, an ATR-based fuel processor thermally integrated was designed and built for 10 Nm3/h hydrogen production from methane or natural gas. First testing carried out with a commercial noble metal–based cylindrical honeycomb monolith catalyst (supplied by Johnson Matthey) gave very good performance in terms of hydrogen yield (94–95% fuel conversion, 35–36% hydrogen syngas composition before WGS) Fig.2.4. Moreover, the thermal efficiency is much higher than that reported in literature (Tabs. 2.2 and 2.3) [41].

185

2 Catalytic autothermal reforming for hydrogen production

WGS Catalyst

H2O Pressureized vessel

Air

ATR Catalyst

PLC

N2

ABB Analyzer

CH4

Ca→ldos H2 Magnos→ O2 Uras→CO, CO2, CH2

Natural Gas

Fig. 2.4: ATR-based thermally integrated pre-pilot plant (10 stp m3/h hydrogen production from methane or natural gas). Tab. 2.3: Hydrogen production and thermal efficiency of thermally integrated natural gas ATR processor (H2O/O2/NG = 0.80/0.60/1.0) (from [41]). GHSV (/h)



Produced hydrogen, (stp) m /h Thermal efficiency, %

,

,

,

,

.

.

.

.

.

.

.

.

2.5 From hydrocarbons to biomass: the case of ethanol In the last decades, research on fuel cell systems running on pure hydrogen such as PEM fuel cell has been addressed to hydrocarbons as reforming feedstock, mostly due to the high cost of hydrogen transport and storage [44]. In parallel, biomass conversion has attracted interest as a sustainable way for fuel production, especially through liquid biofuel such as ethanol, as fuel additive and for bio-fuel-powered fuel cell. Some years ago, there was a great excitation

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related to the development of second generation ethanol from cellulosic waste (first plant in the world opened in 2013 in Italy), but the process never took off, due to the decrease of oil/gas price. In 2015 the state of art of ethanol reforming was reviewed, with specific reference to perspectives of the autothermal solution [45], much less studied with respect to higher efficient SR alternative [46]. Inside the general approach to ethanol reforming is hydrogen production by lowtemperature SR: CH3 CH2 OH + 3 H2 O ! 6 H2 + 2 CO2

ΔHr = 173 kJ=mol

(2:10)

General attracting features are the availability of a hydrogen rich feedstock just containing all the water necessary as co-reactant (80–90 wt% in commercial ethanol) and then cheaper than high purity ethanol, which requires high distillation cost, easy storage, handling and transport, and no toxicity. The objective of reducing the ESR temperature is that of minimizing both the CO formation (favoring WGS conversion) and the thermal duty with respect to typical SR temperature. Moreover, the reaction network involved in ethanol SR is very complex, resulting in the formation of secondary products, mostly methane, CO, and coke, which reduce the overall hydrogen yield and cause catalyst deactivation, especially at low temperature ( Ni–KIT-6 > Ni–MCM-41 > conventional Ni/SBA-15. However, the GHSV tested in this work was relatively low (22.5 L/g h). For the interpretation, Zhang et al. have suggested that the solid-state grinding method improves the dispersion of the active component on carriers. However, the Ni dispersion was not measured [29].

3.2.2.3 Impact of the impregnation conditions Structured mesoporous materials offer high specific surface areas and pores volumes that are prone to favor high metal dispersions. However, the impregnation method used may be very determining as shown recently by various authors. Let us first mention, for example, the work of Kaydouh et al. (Tab. 3.1, entry 18) [47] dealing with the preparation of a series of Ni/SBA-15 catalysts with 2.5 < Ni wt%

205

3 An overview of recent works on Ni silica-based catalysts

60%. Thus, its cost depends on the electrical power source, that, for a cost of 30 €/MWh, it gives around 2,000 €/ton of hydrogen.

4.2 The Sabatier reaction 4.2.1 Description of the process According to Sabatier’s reaction [22], it is possible to perform hydrogenation of CO2 into methane (CO2 methanation), generating water as a by-product in the presence of nickel catalysts (M.B 2): CO2 + 4 H2 = CH4 + 2 H2 O

Mass balance

(4:2)

This is an exothermic reaction that takes place at high temperatures, optimally between 200 and 450 °C, and preferentially at atmospheric pressure or higher [24, 25]. In this interval of temperatures, the reaction enthalpy varies between −172.7 and −179.9 kJ/mol, respectively.

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Simultaneously with this reaction, secondary products may be formed. Table 4.2 shows a list of possible reactions than can occur in the presence of CO2 and H2. Tab. 4.2: List of reactions that can take place in the presence of carbon oxides and hydrogen [26]. Reaction

ΔH K (kJ/mol), at P =  atm −.

Reaction name

R

CO +  H ⇄ CH +  HO

R

CO + H ⇄ CO + HO

R

CO +  H ⇄ CH + HO

−.

CO methanation

R

 CO +  H ⇄ CH + CO

+.

Inversed methane CO reforming

R

 CO ⇄ C + CO

+.

Boudouard reaction

R

CH ⇄  H + C

+.

R

CO + H ⇄ C + HO

R

CO +  H ⇄ C +  HO

+.

CO methanation Reverse water–gas shift

Methane cracking

−.

CO reduction

−.

CO reduction

4.2.2 Thermodynamic and kinetic study After analyzing the free Gibbs energy of the exothermic hydrogenation of CO2, most of the related reactions are thermodynamically unfavorable. Indeed, since the values of ΔG° are more positive than the corresponding values of ΔH°, they are less favorable at 298 K and 1 atm. As a result, only a few reactions have negative ΔG° and ΔH° values. Most of them have ΔG < 0 values corresponding to the hydrogenation reaction. Thermodynamically, the transformation becomes more feasible when the process is done with high energy density reagents such as H2 as it supplies its chemical energy to transform CO2 by generating an overall Gibbs free energy value of ΔG° = −132.4 kJ/mol [27] by reacting one molecule of CO2 with four molecules of hydrogen. It is important to point out that the methanation of CO2 remains the most advantageous hydrogenation reaction thermodynamically compared to the production of other hydrocarbons or alcohols. The CO2 methanation reaction is exothermic but requires a large energy input to activate the highly stable CO2 molecule and displays serious kinetic limitations. The thermodynamic analysis gives important information such as type of thermodynamically stable products that may be expected, if the reaction is endo- or exothermal, or the effect parameters such as temperature, pressure, and inlet ratio of the reactants. This information is useful to understand the thermodynamic limits/allowances of each reaction and, hereby, provides guidance in the catalyst development or enhancement.

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Figure 4.6 reveals that the conversion of carbon dioxide is thermodynamically favored at low temperatures, due to its exothermicity, and at high pressures as the hydrogenation of CO2 is a volume-reducing reaction. (a)

(b)

Fig. 4.6: Equilibrium conversion of CO2 (a) and selectivity of CH4 (b) at different temperatures and pressures. Adapted from [28].

Due to the reversibility of the reaction, Chatelier’s principle [29] can be used to explain why high pressure and low temperatures favor the formation of products by displacing the reaction toward the formation of CH4 and H2O. Studies of equilibrium product fraction at atmospheric pressure have shown that CH4 selectivity is favored at temperatures below 350 °C, while other products such as CO and C at temperatures above 500 °C (R2, R5, and R6) [12]. However, although this latter reaction is thermodynamically favored, the reduction of fully oxidized carbon (CO2) to methane is an eight-electron process with significant kinetic limitations that requires a catalyst that can reach acceptable levels and selectivity for industrial use potential. A challenge in reactor design is the removal of the heat produced by the exothermic reaction and thus maintaining a relatively low process temperature (about 350 °C) without generating hot spots [30, 31]. Notice that hot spots are responsible of the thermal degradation of catalysts and sintering of the metallic sites. The heterogeneous catalysts increase the reaction rates and make it possible to obtain a selectivity close to 100% for the Sabatier reaction.

4.2.3 Catalysts used for such reaction Despite being thermodynamically favorable and having considerably high equilibrium conversions in this temperature range, this reaction requires a catalyst and

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some heating (≈200 °C) in order to take place. Due to the great stability of the CO2 molecule, high activation energy is necessary to start the reaction. Catalysts have been developed in order to overcome this barrier. Some of the most used catalysts in the formation of methane according with reaction R1 (methanation reaction) include nickel and ruthenium [32] supported on zirconia or alumina catalysts, since these metals have high selectivity toward methane. They activate H2 by its dissociative adsorption on the reduced metal particles surface. In general, metals of the group VIII, IX, X, and XI transition metals can be used in the hydrogenation of CO2. The incompletely filled d-orbitals of transition metals provide an easy electron exchange between the metal atoms and the molecules. It has been reported that the more active metals in COx methanation catalysis in decreasing activity order are Ru, Rh, Ir, Ni, and Co [33]. Nickel is the preferred metal due to the relatively low cost compared with other metals. Some examples of conversion and selectivity values obtained for the methanation of CO2 using different Ni catalysts are given in Tab. 4.3. Multi-metal catalysts that seek to use the functionality of different metals in a compatible manner are also of interest. The metal charge of a catalyst can be fixed to the dispersion, porous volume, pore size, and micropore blockage caused by agglomeration of species leading to a reduced surface area. The supported catalysts are composed of a support and an active metal center, as defined. The support is used to obtain the greatest dispersion of the catalytically active metal and to stabilize its center with respect to the sintering. The supports most often encountered in CO2 recovery processes are alumina, silica, glasses, clays, zirconium, and zeolites. In their excellent review, K. Hashimoto et al. have been showed that using Ni supported on tetragonal zirconia (a crystallographic phase of zirconia, which is more stable than monoclinic) gives an excellent performance for CO2 methanation, especially when the stabilization of tetragonal ZrO2 was carried out by inclusion of Sm3+ ions. This can be due to the fact that increasing Sm3+ ions in the ZrO2 lattice increase oxygen vacancies, thus the strong interaction between the oxygen vacancy and oxygen in CO2 seems to weaken the C–O bond strength and to enhance the hydrogenation of CO2 to form CH4 and H2O [34, 35]. Several families of catalysts have been tested including nickel-based (e.g., Ni/ γAl2O3 or Rh-CZ, or nickel-based supported on mixed cerium–zirconium oxides with various proportions of cerium and/or zirconium) [19]. The doping of the catalyst with cerium oxide allows the storage and the mobility of oxygen. The catalysts based on Ni supported on mixed oxides of cerium–zirconium (CexZr1xO2), on hydrotalcites, or various mesoporous oxides have the dual advantage of having a sufficient surface area and a high thermal stability in the temperature range studied [17].

4 CO2 hydrogenation by plasma-assisted catalysis for fuel production

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Tab. 4.3: Summary of CO2 conversion and CH4 selectivity for the Sabatier reaction using different active phases. Active phase catalyst

CO conversion (%)

CH selectivity (%)

Temperature (°C)

Reference

Pd/SiO

.

.



[]

Pd–Mg/SiO

.

.



[]

Pd–Fe/SiO

.

.



[]

Pd–Ni/SiO

.

.



[]

Pd–Li/SiO

.

.



[]

Ru/Ce

.

.



[]

Ru/CeAl

.

.



[]

Ru/CeAl

.

.



[]

Ru/Al

.

.



[]

%Ni/CeZrOx

.

.



[]

LaNiAl

.

.



[]

%Ni/CeZ

.

.



[]

%Ni/SiC

.

.



[]

%Ni/HT

.

.



[]

Ni/SiO

.

.



[]

Pd

Ru

Ni

4.2.4 The challenge in the Sabatier reaction An interesting challenge in the Sabatier process is the control of temperature, since selectivity for the desired products (H2O and CH4) is favored at low temperatures. However, it is difficult to initiate the reaction because the kinetic reactions are slow at low temperature. Therefore, there is still a need for a process for obtaining methane at low temperature and atmospheric pressure with a high conversion rate (at least >80%) and high selectivity (>95%) and without any secondary reaction. Several authors have focused their research on the thermodynamics of CO2 methanation [24, 28, 38–41]. Operating conditions must be maintained at temperatures below 350 °C in order to obtain a high conversion to CO2 while maximizing the selectivity of methane [42]. In this system, at less than 200 °C, the conversion becomes almost zero, and

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from 350 °C a secondary reaction is formed, which decreases the conversion efficiency: it is the RWGS in which the gas reacts with the water vapor, which causes a dominant appearance of carbon monoxide carbon. This water is also a Lewis base that will block the active sites of the catalyst [20]. Increasing the reaction temperature or that of the catalyst would allow desorbing water from the catalyst surface but leading to carbon deposition that leads to deactivation. We are still looking for a low energy consuming process operating at atmospheric pressure and at a low temperature. The hybrid plasma-catalysis process can then be the suitable solution [17, 19, 41, 43, 44].

4.3 Plasma processes Plasma processing offers another applicable clue to the climate change challenge. Plasma discharges can be chosen to produce concrete operational conditions of characteristic temperatures and charged species densities within the discharge that promote a given set of chemical reactions. It is necessary to first understand the essential properties of plasma in order to identify the proper plasma discharge for a given process from a wide selection of types of plasma sources.

4.3.1 Definition of plasma The term “plasma” was first introduced by Irving Langmuir (1928). Plasma is an ionized gas, which means that at least one electron is unbound, creating positively charged ions. In practice, the ionization degree in plasma can vary from fully ionized gases (100%) to partially ionized gases (e.g., 10−4–10−6). Besides the various types of ions (both positive and negative), plasma also consists of a large number of neutral species, for example, different types of atoms, molecules, radicals, and excited species. The latter can lead, among other things, to the emission of light. More importantly, all these species can interact with each other, making plasma a highly reactive and complex chemical cocktail, which is of interest to many potential applications. Like gas, plasma does not have a fixed shape or volume. It consists in an electrically neutral medium of unbound charged particles. Overall, the number of positive charged ions should be paired with the negative charged. As these particles move, they generate electric current with magnetic fields, thus affecting all the nearby particles, creating a very complex collective behavior. Unlike gas, in which there is only one independent acting kind of specie, in plasma there is more than one kind of particles interacting with each other. Based on the particle charge and its value different behaviors are obtained. Ionization of particles is necessary to create plasma. Practically, this can

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be done by heating a gas or applying a strong electromagnetic field to it. This will ionize the gas particles and break existing molecular bonds. Therefore, any gas can potentially become plasma when applied to it to create a significant density of electrons and ions [45]. The average charge by negatively charged species (electrons and ions negative) and positive ions gives plasmas the characteristic of near-neutrality [46]. Plasma is naturally available throughout the universe and contains ~99% of the visible cosmos, including the solar corona, solar wind, and nebula. Plasma is also present in the upper part of the Earth’s atmosphere (at levels >100 km) [46] where interactions with cosmic radiation cause the dissociation of atmospheric gas molecules.

4.3.2 Classification of plasma Plasma can be classified in different types depending on the way it is generated and on its properties. Some of the more known examples of industrial or commercial plasma are summarized in Tab. 4.4 along with their main characteristics and applications [47]. – Thermal plasmas, the electron and bulk gas temperatures as well as the energy content are very high and the temperatures of the electrons and particles (neutrons, protons, etc.) are the same. These plasmas are also characterized by a higher power and higher density as well as low selectivity of chemical processes and are partially ionized but sufficiently conductive. As examples of thermal plasma, we can give the sun, the plasma torch, and the arc discharge [48, 49]. – Nonthermal plasma (NTP) is any kind of plasma that is not in thermal equilibrium; they are also called cold-plasma. In NTPs, the electrons are indeed characterized by a much higher temperature than the heavy particles, leading to the non-LTE condition, as mentioned above. As a result of all the different species, the relationship between all their different temperatures can become quite complex, but conventionally the temperature of the electrons (Te) is the highest, followed by the vibrationally excited molecules (Tv), while the lowest temperature is shared by the neutral species (T0), or simply the gas temperature, Tg), the ions (Ti) and the rotational degrees of freedom of the molecules (Tr); hence, the temperature order is Te ≫Tv > Tr ≈ Ti ≈ T0. In most cases, the electron temperature is in the order of 1 eV, while the gas temperature remains close to room temperature. This high electron temperature is due to the small mass of the electrons, allowing them to be easily accelerated by the applied electromagnetic fields, whereas the heavy particles – even the ions – are not easily accelerated. Furthermore, due to the large mass difference, the electrons lose less energy during elastic collisions with heavy particles, so they can easily keep their high energy gained from the electric field. As mentioned above, the electrons can be considered as the initiators of the highly reactive chemical mixture. This is obviously one of the key advantages of NTP technology: it allows gases even as unreactive as CO2 to be activated at room temperature by the highly energetic electrons.

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Accordingly, there is no need to heat the entire reactor or the gas, because the discharge and the associated reactions are easily initiated by applying an electromagnetic field. This scalability and flexibility makes this technology highly recommended in different applications [41, 48, 50, 51]. As such, both production on demand and remote production become possible, which is a critical point for CCU. It has additionally been reported by E. Jwa et al. [52] that NTP-assisted catalytic hydrogenation of carbon oxides improves from COx conversion values lower than 15% at 180–260 °C to values near 90% in the same T range.

Tab. 4.4: Types of plasma and its applications.

Low pressure

Glow-discharge plasma

Nonthermal plasma (NTP) generated by the application of direct current or low-frequency electric field between two electrodes.

Fluorescent light tubes.

Capacity coupled plasma (CCP)

Similar to glow-discharge, but generated with high frequency RF electric fields (=. MHz).

Microfabrication and integrated circuit manufacturing industry; plasma-enhanced chemical vapor decomposition.

Cascade arc plasma

High-density NTP produced by a Wastewater treatment well-stabilized thermal electric arc discharge.

Inductively coupled plasma (ICP)

Similar to CCP with the electrode consisting in a coil wrapped around the chamber where the plasma is created.

Metal content analysis in alimentary industry.

Wave heated plasma

Microwave generated lowpressure discharge such as helicon discharge or electron cyclotron resonance (ECR).

Fusion research purposes.

Arc discharge

High power thermal discharge (=, K) that can be generated with different kinds of power supply.

Metallurgical processes.

Corona discharge

NTP discharge generated by applying high voltage to sharp electrode tips.

Ozone generators; Particle precipitators.

Atmospheric Dielectric barrier NTP discharge generated by pressure discharge (DBD) applying high voltages across small gaps coated with nonconducted material.

Web treatment of fabrics.

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Tab. 4.4 (continued)

Capacitive discharge

NTP discharge generated by application of RF power (=. MHz) to one powered electrode with a grounded electrode held at a distance of approximately  cm.

Piezoelectric direct discharge plasma

NTP discharge generated on the Ozone generators; medical high side of a piezoelectric technology. transformer.

Electronic technology industry.

The most used method for the formation of NTP is by the application of an external electric field between two electrodes surrounded by a volume of gas. The breakdown voltage (Vb) defines the minimum voltage required to breakdown a gas (or mixture of gases) to form a plasma discharge. Vb is dependent on the gas pressure (p) and the distance between the electrodes (d). This relationship is described by Law of Paschen, where a and b are constants that are dependent on the gas type and a represents the minimal ionization energy [46]: a = (E/p)min = (Vb/p × d)min E is the electric field (V/m) Vb =

a×p×d lnðp × dÞ + b

(4:3)

The most important parameters that characterize a plasma are the temperature of plasma species and the density of electrons [48]. Usually, energy is transferred to electrons during acceleration by an applied electric field, which in turn is transferred to heavy particles by collision processes. With an equilibrium temperature determined by the collision processes, the electron temperature may be higher than the heavy particle temperatures. If the collision frequency is too low, the electron and heavy particle temperatures will never equilibrate [48]. Therefore, to sum up, free charges make the plasma electrically conductive, active, and highly sensitive to electromagnetic fields [49]. Thus, plasma offers three major characteristics [45]: – The ability to reach very high temperatures and high densities compared to chemical processes. – Plasmas produce energetic and chemically active species radicals, ions, electrons, atoms photons, and vibrational species. – They can maintain the temperature of the low mass (ambient temperature) and simultaneously be out of equilibrium.

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4.3.3 Dielectric barrier discharge (DBD) Dielectric barrier discharges (DBD, silent discharges) (due to the absence of noisy sparks), are very colorful discharges, composed of multiple micro-discharges and used on a large industrial scale [49]. They combine the advantages of out-of-equilibrium plasma properties with ease of operation at atmospheric pressure. An important feature is the simple evolution from small laboratory reactors to large industrial plants with megawatt input powers [53]. The DBD discharge is driven by a sinusoidal alternating voltage in the frequency range of 0.05–500 kHz [54] and is capable of forming stable discharges in a range of different gases at relatively high discharge powers. DBD is one of the highly unbalanced plasmas that provide high-density active species (radicals, electrons, and energy ions) but always have a moderate gas temperature. Siemens has developed a process for generating ozone from oxygen or air in a DBD reactor and reported the first experimental research in 1857 [55]. A new feature of the Siemens discharge device was the fact that the electrodes were placed outside the chamber and were not in contact with the plasma. Industrial applications include ozone generation, pollution control, surface treatment, CO2 lasers, high power, ultra-mini-excimer lamps, excimer-free mercury-free fluorescent lamps, and flat plasma displays [56, 57]. The DBD reactor consists of two electrodes (with one or two dielectric barriers) mounted in the discharge space. Current dielectric materials with a high relative permittivity are quartz, glass, enamel, ceramic, silica glasses, and Teflon. Therefore, to operate a DBD reactor, AC voltages are required and the electric field in the discharge space must be high enough to cause failure. In Fig. 4.7, some possible DBD configurations are illustrated, including planar, cylindrical, and surface discharges. In general, one electrode is grounded while the other electrode is powered by an alternating high voltage current [53].

Fig. 4.7: Diagrams of planar, coaxial, and surface DBD configuration. Adapted from [56].

4.3.4 Plasma processes and molecules activation Plasma processes use a high-voltage electromagnetic field to modify the electrical conductivity of a neutral gas. In our study, we used a high voltage between 10 and

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20 kV, a frequency of 40–70 kHz, and specific electrode that is able to introduce the electrical field or the electromagnetic field within the gas phase. By that way, we produce a mixture of ions, electrons, excited species, and high-polarized molecules and radicals. Notice that all the electrons produced by the ionization mechanism are with very low rate less than 1%. To summarize, the excited states from “nonequilibrium plasma,” such as DBD in our study, open the way to a nonthermal process with very high reactivity of the gas mixture, and this is achieved by applying high voltage with pulse of nanoseconds. In this chapter, our aim is to valorize and recycle CO2 by combining it with H2, and the goal is to introduce the reactivity of the mixture at “room temperature,” or a translational temperature close to 298 K, in order to decrease the energy loss that takes place during the thermal conventional heating of the gas and the catalyst. It should be noticed that carbon dioxide is a very stable molecule with a linear configuration and without any polar properties. Thus in order to increase its adsorption on the catalytic sites, this needs to increase the O–C–O angle by vibration and excitation, while for hydrogen molecules, its adsorption and dissociation on nickel sites need an increase of its excitation state such as the vibrational one.

4.4 Plasma catalysis hybrid process Since the last two decades of the twentieth century, plasma-chemical and plasmacatalytic processes using low-temperature plasma have been of interest to the scientific community [41, 50, 58–60]. NTP offers a unique way of initiating and enhancing the performance of chemical reactions. However, due to the nonselective nature of plasma processes, the addition of a catalyst may be implemented in order to increase conversion of reactants, selectivity toward desired products, and, therefore, the process yield. The use of plasma in combination with heterogeneous catalysis may provide an interesting solution because it combines low temperature activity, fast response, and compactness of plasma reactors with high selectivity of catalytic reactions. It allows obtaining a better efficiency for many processes. The types of plasma, which are used in plasma-catalytic systems, are glow discharges, when the reactions are carried out under partial vacuum and DBDs for atmospheric or higher pressures. In plasma-catalytic processes, just like in the regular catalytic cases, the catalyst can be used in different forms such as pellets, foam, layers, or coating. Processes using nonthermal atmospheric plasmas (also called “cold plasmas” or thermodynamic off-equilibrium plasmas) are developing strongly, particularly in the areas related to the environment and energetics. These plasmas have a very high energy efficiency, especially because of the low heating of the gas. Crown discharges, DBDs, and glow discharges are the most commonly used in the fields of

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depollution, the development of new energy vectors (hydrogen), and the methanation process [55, 61–64].

4.4.1 Synergetic effect of this hybrid process The interaction of the DBD plasma discharge with the catalytic bed results in many phenomena. It is known that, due to the high electric field within the packed bed, microdischarges tend to happen between the voids of the catalyst particles. These microdischarges are favored by the existence of sharp-edged pellets and are more frequent in small particles [65]. It has been reported that plasma-treated catalysts may have their properties enhanced. Meng Guo et al. verified that plasma treatment of the catalyst results in the enhancement of the dispersion of active components [54, 65]. The activation of the catalyst obtained by influence of NTP is different from the one obtained by classical heating. The improvements are also verified in the catalytic stability and activity as plasma activity prevents the catalyst from poisoning due to the enhancement of specific species desorption [54, 66] and sintering by decreasing the reaction temperature when plasma is used. Besides, the specific surface area of the sample can be increased after prolonged plasma activity, as it causes changes in the structure of the catalyst [54, 67]. The discharge may affect the performance of the catalyst and vice versa [68] due to the direct interaction of the catalyst placed inside the discharge zone with the plasma. The plasma will provide a gas stream enriched in radicals and excited species, which can accelerate the thermal catalysis of the reagents and thus increase the efficiency of the process. The plasma-treated catalyst can also result in a reduction in the size of the metal particles and a greater dispersion of the metals, both leading to higher catalyst reactivity and greater durability. Due to the elastic and inelastic collisions between electrons and molecules, as well as vibrational-translational energy relaxation processes that occur during NTP activity the temperature of the gas increases, which results in a heating of the catalyst surface. This temperature increase, which was found in [69] to be dependent on the energy density of the medium, usually is not high enough to the thermal activation. However, the localized heating that may occur inside the catalytic packed bed due to electric microdischarges (hot spots) [50] is very likely to provide temperatures high enough to stimulate catalytic activity. This factor should not be neglected in plasmacatalytic studies. The energy density is given by the ratio between the input power and the flow rate. Plasma–catalyst contact, due to a combination of factors already referred to such as enhancement of the electric field of the material [69], temperature increase, structure modification of the catalyst, and dissociation and ionization of molecules, also influences the adsorption–desorption equilibrium of the existing species in the catalyst. Then, depending on the plasma–catalyst interaction outcome on the

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equilibrium, different results may be obtained. In their excellent review, Chen et al. [68] have distinguished effects in which the catalyst will influence the plasma discharge characteristics from those in which the plasma will affect the catalyst properties. In the former category, the introduction of a packing material (the catalyst) into the plasma region will change the electrical properties of the discharge (e.g., the electrical field strength), which will change the composition of the reactive species generated or the nature of the discharge from filamentary microdischarges to surface discharges. In the second category, the plasma may generate additional reactive species or change the nature of the catalyst. The result of these and other interactions may be to improve the efficiency, selectivity, and stability of the processing. A synergistic effect is reported sometimes where the outcome of employing plasma catalysis exceeds the combined effect of thermal catalysis and plasma operated separately, giving further enhancement in pollutant conversions. Amouroux et Cavadias [44] performed several tests for CO2 methanation: (i) in thermal process with presence of the catalyst where no significant production of methane has been obtained, (ii) in plasma alone (absence of catalyst) at low temperature, CO2 conversion was found to be about 5%, with a negligible yield of CH4, and (iii) combining the plasma with a catalyst it has given rise to a sharp increase in the conversion of CO2 to over 97%, with a methane selectivity of always greater than 90% at temperatures below 260 °C. Indeed, these results confirm the synergetic effect of a hybrid plasma catalysis system.

4.4.2 Plasma types existed for CO2 hydrogenation As mentioned above, in recent years, there has been an increasing interest in the use of plasma technology for CO2 conversion. Experiments have been carried out in several types of plasmas. The most common types reported in the literature are DBDs, microwave (MW) and gliding arc (GA) discharges, although other types have been used as well (e.g., radiofrequency, corona, glow, spark, and nanosecond pulse discharges). A DBD is a typical example of NTP, where the gas is more or less at room temperature, and the average energy of electrons is between 2 and 5 eV by the strong electric field in the plasma. The MW and GA discharges are other kind of electrons energy transfer to molecules depending for each case of the electron energy distribution. Each of these three major plasmas has different operating conditions and characteristic features mainly depending on the mixing way between gas and electrons in the electric field.

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Fig. 4.8: An overview of the possible effects of the catalyst on the plasma and vice versa possibly leading to synergism in plasma catalysis. Adapted from [76].

First, it is clear that, although DBDs are among the most extensively studied for CO2 conversion, and indeed are already successfully applied for commercial O3 production and VOC removal [56]; they appear to be unsuitable for the efficient conversion of CO2. Their energy efficiency remains a factor of four too low, even when combined with a packing, in order to justify them as industrially competitive. Second, the best results for GA plasmas are capable of reaching the set energy efficiency target, namely, 60% for CO2 splitting [48]. Moreover, almost all of the results obtained are far above the thermal equilibrium limit, which is especially interesting, keeping in mind that the GA plasmas operate at atmospheric pressure. This demonstrates the nonequilibrium character of this type of plasma, even at atmospheric pressure, and the benefits of being able to exploit this behavior through the energy-efficient dissociation of the CO2 vibrational levels. Moreover, modeling has revealed that this nonequilibrium character could (and should) be further exploited, to further enhance the energy efficiency. To date, the main challenge is the limited conversion, which remains below 20% because only a limited fraction of the gas passes through the active arc plasma. Finally, if, for now, we ignore the fact that most MW discharges used for CO2 conversion operate at reduced pressure, in contrast to the commercially more interesting atmospheric pressure of GA plasmas, it is clear that MW discharges offer a wide variety of possibilities. Even up to conversions of 40%, the energy efficiency target is easily crossed and they clearly operate in a nonequilibrium regime, thus favoring the stepwise vibrational-dissociation mechanism. Conversions in the range of 40–90% are also possible, albeit with maximum energy efficiencies of only up to

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40% [48]. Nevertheless, this shows the wide variety of both conversions and energy efficiencies achievable with MW discharges for the conversion of pure CO2.

4.4.3 CO2 hydrogenation by plasma-assisted catalysis The combination of NTP and catalysis, known as plasma catalysis, thus can be regarded as an attractive and promising solution to convert CO2 and renewable H2 into higher value chemicals at low temperatures and atmospheric pressure. Plasmacatalytic processes have great potential to generate a synergistic effect, which can reduce the activation energy of the reaction, enhance the conversion of reactants, and improve selectivity and yield toward the desired products. All of these contribute in different ways to increase the energy efficiency of the plasma process, as well as the activity and stability of the catalyst. A wide range of supported metal catalysts have been investigated for thermal catalytic CO2 hydrogenation at high temperatures (300–500 °C). As shown before, CO2 can be valorized and transformed into different energy vectors with heterogeneous catalysis such as methanol and methane.

4.4.3.1 CO2 hydrogenation to methanol CO2 hydrogenation to methanol is a promising process for CO2 conversion and utilization [23]. Despite a well-developed route for CO hydrogenation to methanol, the use of CO2 as a feedstock for methanol synthesis remains underexplored, and one of its major challenges is high reaction pressure (usually 30–300 atm). Wang et al. studied this reaction by assisting plasma with catalysis. In their work, the synthesis of methanol from CO2 and H2 has been successfully achieved using a DBD with and without a catalyst at atmospheric pressure and room temperature (∼30 °C). They observed that the combination of the plasma with Cu/γ-Al2O3 or Pt/γ-Al2O3 catalyst significantly enhanced the CO2 conversion and methanol yield compared to the plasma hydrogenation of CO2 without a catalyst. The maximum methanol yield of 11.3% and methanol selectivity of 53.7% were achieved over the Cu/γ-Al2O3 catalyst with a CO2 conversion of 21.2% in the plasma process, while no reaction occurred at ambient conditions without using plasma. These results have successfully demonstrated that this unique plasma process offers a promising solution for lowering the kinetic barrier of catalytic CO2 hydrogenation to methanol instead of using traditional approaches (e.g., high reaction temperature and high-pressure process), and has great potential to deliver a step change in future CO2 conversion and utilization [70].

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4.4.3.2 Water gas shift Xin Tu et al. have studied and developed a DBD reactor for plasma-catalytic CO2 hydrogenation at low temperatures and atmospheric pressure. They have found that reverse water–gas shift reaction and carbon dioxide methanation are dominant in the plasma CO2 hydrogenation process. Their results show that the H2/CO2 molar ratio and the different γ-Al2O3 supported metal catalysts (Cu/γ-Al2O3, Mn/γ-Al2O3, and Cu– Mn/γ-Al2O3) significantly affect the CO2 conversion and the yield of CO and CH4. The combination of plasma with these catalysts enhances the conversion of CO2 by 6.7– 36% and that Mn/γ-Al2O3 catalyst shows the best catalytic activity for CO production, followed by the Cu–Mn/γ-Al2O3 and Cu/γ-Al2O3 catalysts. The presence of the Mn/γAl2O3 catalyst in the plasma process significantly increases the yield of CO by 114%, compared with the plasma reaction in the absence of a catalyst, and that combining plasma with the Mn/γ-Al2O3 catalyst significantly enhances the energy efficiency of CO production by 116%, while others increase it slightly [47].

4.4.3.3 CO2 hydrogenation for the cogeneration of CO and CH4 Zeng et al. studied the plasma-catalytic CO2 hydrogenation over a Ni/Al2O3 catalyst for the cogeneration of CO and CH4 in a DBD reactor at 150 °C. The presence of the Ni catalyst in the DBD reactor has clearly demonstrated a plasma-catalytic synergistic effect at low temperatures, as the reaction performance of the plasma-catalytic CO2 hydrogenation is significantly higher than that of the sum of the individual processes (plasma process and thermal catalytic process) at the same temperature. They have found that the addition of argon (up to 60%) in the reaction enhances the conversion of CO2, the yield of CO and CH4, and the energy efficiency of the plasma process. This is due to formation of metastable argon (Ar*) in the plasma, the decrease of breakdown voltage of the feed gas, and the promotion of charge transfer through the reactor. In addition, they found that the selectivity of CO is almost independent of the Ar content in the feed gas, while increasing the Ar content from 0 to 60% enhances the CH4 selectivity by 85%. This phenomenon suggests that the presence of Ar* might promote the methanation of CO and CO2 with hydrogen at low temperatures [71].

4.4.3.4 CO2 to CH4 Amouroux et al. studied the process of reducing carbon dioxide to methane by DBD plasma activated by a catalyst at atmospheric pressure [41, 44]. They showed that under adiabatic conditions, DBD plasma could greatly improve the conversion of CO2 at low temperatures [39]. They have shown that catalysts combined with DBD plasma

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led to the high yields and conversion of CO2 (85%) under adiabatic conditions with the ideal gas ratio for CH4 generation [17, 19, 44]. Much research has focused on CO2 methanation, but significant efforts still need to be made in the coming years to understand fundamental reaction mechanisms to improve the activity and selectivity of catalysts for methane [17]. Benrabbah et al. were tested a hybrid plasma-catalytic process for low-temperature CO2 hydrogenation at atmospheric pressure. OES was used in their study to determine the excited species present in the plasma during plasma–catalyst interaction. The coupled plasma DBD-Ni/CeZrO2 system leads to high CO2 conversions into methane without any external heating source. This low-temperature activity can be explained by the formation of highly reactive species from CO2 and H2 in the presence of the plasma, further able to react on the surface of the Ni/CeZrO2 catalyst. The methanation reaction involving the consumption of the gaseous excited species is possible only in the presence of the Ni-containing catalyst. This finding points to the presence of Ni and to its physicochemical properties as the crucial issues in the design of active and selective catalysts for this coupled plasma-catalytic CO2 methanation process [43]. Mikhail et al. had focused in their study on the hybrid plasma catalytic process for CO2 methanation at atmospheric pressure. They found that this hybrid process, based on the combination of a DBD plasma and Ni/CeZrO2 catalyst, has several advantages over conventional catalysis: it operates at ambient conditions and requires no external heating. By optimization of the process considering the effect of the different operational parameters such as voltage, GHSV, catalyst mass, flow rate, and discharge length, they retrieved that at temperatures between 230 and 270 °C, CO2 conversion rate is about 75%, with a CH4 selectivity greater than 95%. These values are obtained with the following conditions (GHSV ≈ 43,000 h–1, total inlet flow = 200 mL/min and with a mass of catalyst = 300 mg) [72].

4.4.4 Mechanism of the reaction of CO2 methanation Although there are many hypotheses of mechanisms that have not yet been confirmed, it is well known that the ratio of CO2 and H2 used in the reaction plays an important role in the definition of the final product. The ideal gas ratio for CH4 generation has been found to be H2/CO2 = 4:1 [30]. The catalyst support also plays an important role in the performance of the catalyst through its interactions with the metal active phase and the specific surface it deploys, on which the active phase is dispersed. Depending on its properties, the support can also interact significantly with certain reagents and thus influence the mechanism of the reaction. The method of manufacturing the catalyst is also important because it also conditions the interactions between the support and the active

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phase. Therefore, these two factors influence the activity, the selectivity, and the stability of the catalyst [28, 73]. The catalytic mechanism of the reaction of CO2 methanation has only recently been studied extensively. Two types of mechanisms are now evoked: the first involves the passage by carbon monoxide as an intermediate, which would be then converted into methane via the mechanism of methanation of CO. The second involves the direct conversion of CO2 to methane via the formation of carbonates and formates on the surface of the catalyst support. These two mechanisms can further coexist on the catalyst. In all cases, the hydrogen is adsorbed and dissociated on the surface of the active phase. In the first mechanism, the CO2 is first adsorbed and then dissociated on the surface of the active phase of the catalyst to form an adsorbed CO (or monocarbonyl) intermediate. This step has been demonstrated several times by infrared spectroscopy and photoeletronic spectroscopy X [28, 40, 73]. According to the bibliography, the release of this catalytic methanation that involves the dissociation of CO2 into CO* and O* in which * is an active site is low at low temperatures but thermodynamically favorable (ΔG298 K = −130.8 kJ/mol) [70]. However, the reduction of the fully oxidized carbon to methane is an eight-electron process with significant kinetic limitations that requires a good catalyst that can achieve acceptable rates and selectivity [39]. The next steps in methanation are the reaction of this dissociated species with hydrogen to produce methane as the same mechanism as the hydrogenation of CO. It is well known that plasma can help the dissociation and the decomposition of molecules, which will lead to an improvement of the final conversion to CO2 and the CH4 yield. Thus, at low temperature and in the presence of the hybrid catalyst nickel metal supported on ceria–zirconia (NiCZ) and plasma system, the formation of these adsorbed active molecules is favored over the process using the catalyst alone in which this reaction does not take place at T < 250 °C. At high temperature, the catalysts are sufficiently active to activate the formation of CO* and O* species [73], and the plasma becomes ineffective and not even recommendable since the CO appears as a significant by-product. Though the details of this reaction mechanism are not yet entirely understood, it is known that once the Ni particles are dispersed in the support and reduced to an oxidation state of zero, they becomes highly reactive. This extremely active Ni0 can adsorb and eventually dissociate the H2 molecules that have been passed through the reactor. At the same time, the porous support (where the active metal is dispersed) will adsorb the CO2 molecules thanks to the previously mentioned oxygen vacancies of the support (cerium) and promote its dissociation into adsorbed O, CO, and eventually C. The combination of these now dissociated elements will allow the formation of CH4 and H2O, which will then desorb from the catalyst leaving open active sites for more reagents to transform into products.

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Thus, according to Choe et al. [74], the mechanism of the methanation of CO2 can be assumed to be done in five steps, and proceeds via a CO intermediate on the surface in step 1. Step 2 for the CO dissociation is irreversible owing to rapid removal of surface O by hydrogenation and lastly step 5 for desorption of methane is irreversible. Steps 3 and 4 are steps occurring after the rate-determining steps [74]. While a second family of mechanisms propose a CHxO-type reaction intermediate (x = 1.2) without passing through an adsorbed carbon. The exact nature of the reaction mechanism seems to differ depending on the active phase of the catalyst and the reaction conditions [73]. On catalysts with low acid (basic) support, the CO2 hydrogenation mechanism involves the formation of carbonates on the surface of the catalyst support. This mechanism is particularly preferred when the catalyst support has a less acidic acid character than alumina or silica supports. This is the case for ceria- and zirconia-based substrates, for example. In their study of a nickel-based catalyst supported on ceria–zirconia oxide, Aldana et al. proposed a mechanism involving the formation of a carbonate, hydrogenated to formate, and in turn hydrogenated to methane (see Fig. 4.9) [73]. In addition, according to Ocampo et al. [19], the mechanism of the methanation reaction in catalysis conditions can take place without CO as an intermediate (only CH4 is produced). The fact that the adsorption and the dissociation/hydrogenation of CO2 takes place on the surface of the support and not on the metallic phase, therefore left free for hydrogen, can explain the better activity of these catalysts in comparison with silica or alumina supports, where there is competition between the adsorption of CO2 and hydrogen on the active phase. Thus these catalysts using more basic supports appear very promising for the hydrogenation of CO2, whereas they seem less advantageous for the methanation of CO, the latter always adsorbing on the active phase [73]. Moreover, Amouroux et al. [44] showed in their study based on the process of reducing CO2 to CH4 by DBD plasma activated by a catalyst NiCeZr, the polarization of the support/catalyst by the electric field. The catalyst used has two specific sites, one for each species, the site CeO2/ZrO2 (N-type semiconductors) for the adsorption of CO2 and the site NiO (P-type semiconductor) for the adsorption of H2. The polarization resulting from their PN junction could explain the high CO2 conversion, at low temperature. The steps for the CO2 methanation, under plasma conditions inside nanopores of the catalyst particle are: First, the vibrational activation of CO2 (bending) and H2 in the gas phase into the nanopores by the electrons of the plasma, followed by the redox processes due to the polarization of the catalyst by the plasma, and finished by the desorption of the water produced by the methanation reaction. In the first step of CO2 methanation, during the diffusion of the reactants into the nanopore, the plasma activates reactants before they reach the catalyst. Thus, the “task” of the catalyst is to weaken the strength of the bond of the molecule.

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According to them, the presence of plasma enhanced: – the adsorption of excited CO2 molecules (activated in gas phase by the plasma) inside the micropores of the catalyst, – the control of the desorption of water molecule from the active sites of the pore, and – the rate of methane production at lower temperature and the electrochemical promotion of the catalyst by the transfer of the labile oxygen (O2), in the type N semiconductor CeO2 at room temperature. The support CeO2/ZrO2 is an N-type semiconductors with excess of e– (types of Lewis base), the reaction that can be formed is CO2 ðlinearÞ + e − ! CO2 ðv − shapedÞ

(4:4)

The NiO is a P-type semiconductor with default of e– (types of Lewis acid). The reaction that can be done from H+ coming from NiO is (eq. (4.6)), and then we produce CH4 by liberation of H2O molecules (eq. (4.7)): H2 ! 2 H + + 2 e − +

−

CO2 ðgÞ ðv − shapedÞ + 8 H ðgÞ + 8 e ! CH4 ðgÞ + 2 H2 OðlÞ

(4:5) (4:6)

Fig. 4.9: Simplified diagram of potential mechanism of the Sabatier reaction, without taking CO as an intermediate. Adapted from [73].

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4.4.5 Different configurations of plasma catalysis process As shown in Fig. 4.10 when the catalysts are combined with plasmas, they are generally incorporated into an NTP, in one or two-stage configurations. – In-plasma catalysis (IPC) (Fig. 4.10a): The plasma contacts with the catalyst, partially or totally, in one single step; plasma is generated across the catalytic bed, being in contact with reactants and catalyst as well as products of the reaction. This creates a great variety of active species that interact with each other and with the catalyst. The catalyst is in contact with the discharge and also in contact with the short-lived active species such as excited atoms and molecules, radicals, photons, and electrons; the electric current that is applied to create the plasma also interacts with the catalyst [39, 40, 66]. – Post-plasma catalysis (PPC) (Fig. 4.10b): The plasma and catalysis steps can happen under independent conditions and the plasma step is usually done upstream the catalytic reactor. The plasma can be used to provide energy for catalyst activation and can also provide the reactive gas species required for the reactions on the catalyst surface. Plasma can be created before the catalytic step (the outer electrode didn’t cover the catalytic bed, it is wound on the quartz tube before the catalytic bed, keeping its length always equal to the length of the catalytic bed). As the plasma is used for creating chemically active species that can be sent to the catalyst afterwards in order to improve the catalytic activity. With this configuration, the contact between the catalyst and short-living active species is avoided as well as the contact with plasma itself, which can affect the surface of the catalyst [50] and therefore change properties such as adsorption capacity [75], active sites, stability, or activity [54]. Another characteristic of this configuration is that plasma processes are by nature nonselective, therefore, by having a catalytic step downstream the plasma step can combine both reactant conversion provided by plasma and high selectivity inherent to catalyst utilization. Its purpose is solely to modify the composition of the inlet stream to the catalyst bed.

Fig. 4.10: Plasma-catalytic configurations. Adapted from [72].

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As commented before, in plasma-assisted catalysis, species are activated by the plasma due to excitation, ionization, or dissociation by electrons in the gas phase or on the catalyst surface [46, 76]. The major difference between the one-stage and two-stage configuration is the kind of species the catalyst can be exposed to. In the two-stage configuration, the end products and the long-life intermediates will interact with the catalyst, while in the one-stage configuration, the catalyst can also interact with all the short-life species, including excited species, radicals, photons and electrons. Mikhail et al. studied the influence of the type of configuration under the following conditions, that is, total flow of 200 mL/min (160 mL/min H2 ; 40 mL/min CO2) and 300 mg of catalyst (15NiCe58Zr42) corresponding to a length of the outer electrode of 6.5 mm. This experiment was done at frequency of 40 kHz and at atmospheric pressure [72]. The catalyst was reduced in situ (by a plasma discharge) during 20 min and under pure H2 (160 mL/min) at a voltage of 15 kV. Figure 4.11 depicts the conversion of CO2, selectivity of CH4 and the power of the discharge as a function of the voltage applied, for this in-plasma configuration with a range of temperature between 200 and 300 °C. The results obtained have been already discussed within this text. Briefly, conversion increases with the applied voltage, remaining then more or less stable, while power increases constantly with increasing voltage.

Fig. 4.11: Evolution of the conversion of CO2, selectivity of CH4 and the power of the discharge versus different voltages for the in-plasma configuration. Adapted from [72].

Figure 4.12 presents the results obtained when using the two-stage, off-plasma configuration. The resulting power communicated to the system notably differs from that measured for the in-plasma configuration. The discharge can more easily takes place in an empty reactor instead of in the bulk of a solid catalytic bed, and this will depend on the porosity and particle size of the catalyst [65]. In in-plasma system, the presence of catalyst seems to improve significantly the conversion and the

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selectivity toward methane. Inside the catalytic bed, the formation of hot spots inside the catalytic bed can contribute to the improvement of CO2 ionization. Using the offplasma configuration at 15 kV lower CO2 conversion (11%) and CH4 selectivity (about 50%) are obtained, in comparison to those obtained in the in-plasma system at the same voltage (conversion of 63% and a selectivity to CH4 of 100%). However, at higher voltages, that is, 16 kV, the results are approximately the same except for the power, which is higher for the off-plasma configuration. To conclude, both configurations give roughly the same performances in terms of CO2 conversion and CH4 selectivity. However, the in-plasma system is preferable because these performances are obtained with an energy consumption approximately twice lower than that of the post-plasma system. This could be explained by (i) the role of short-life reactive species and/or (ii) a higher temperature in the in-plasma system compared to the post-plasma system (at the same applied power). Notice that the electric field created in the mesoporous catalyst support is responsible for the vibrational and electronic states, for the electrons mobility, and for the adsorption and desorption of species, and this is due to the alternative DBD discharges that modify the polarization of the catalyst placed in situ.

Fig. 4.12: Evolution of the conversion of CO2, selectivity of CH4 and the power of the discharge versus different applying voltages. Adapted from [72].

4.4.6 A good choice of the plasma source and the catalyst: high conversion and yield of methane According to the bibliography, it appears that Ni is the best possible candidate among other catalysts because of its ability to adsorb dissociatively the hydrogen and its high selectivity for the production of methane. The role of the support dominates the design of the catalyst, mainly with regard to its ensuring a better metal

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dispersion. Since the methanation reaction is exothermic, the excessive heat of reaction induces metal sintering, which lowers the total metal surface and leads to the poor activity observed for conventional carriers. Therefore, it is essential to develop an efficient and low-temperature methanation catalyst with high thermal stability. It should have also the capacity to resist to the formation of coke especially when the feed is a mixture of CO and CO2. NTP has many advantages over conventional catalysis; in particular, it can be generated under ambient conditions and does not require any complex system. Some collision processes affect the energy exchange of charged and neutral species. The addition of catalytic material can help increase the conversion rate without changing the energy input and move the process temperature much lower, leading to an energy-efficient reaction more than plasma alone. During the hybrid process, there is no evolution of the catalytic phases, which leads us to optimism for the scaling up of this type of process [17]. It is well known that during the thermal methanation of CO2, the presence of water in the products leads to the sintering of the Ni particles, which results in the deactivation of the catalysts. In addition, side reactions promote the formation of CO and H2 at higher temperature (>300 °C). Thus, a hybrid combination of active and stable catalyst with the DBD plasma can enable us to overcome all the disadvantages mentioned above.

4.5 Conclusion Carbon dioxide valorization has been intensively investigated in recent years, since reducing greenhouse gas emission and finding a way of energy storage has become an international challenge. Thus, the air pollution needs to be solved by political decision. Moreover, the development of carbon dioxide capture technologies has to be connected with a new industrial business and the main one is, for the future, the energy storage for large megalopolis town. By that way the carbon dioxide could be recycled in the same site where we do energy storage and carbon capture; it gives us a great advantage for a flexible production of electricity from oxy-combustion gas power station during the spot time. Thus, carbon dioxide recovery as a raw material opens an industrial revolution; new processes are starting around the world, mainly in China, the United States, and Germany [77]. Nevertheless, CO2 conversion using a hydrogen source as a co-reactant should certainly be further pursued, since when successful in the long run it offers us the possibility of producing a wide variety of value-added chemicals and fuels (methane, methanol, etc.), starting from the same building block and allowing the flexibility to tune the output depending on the market’s needs.

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The conversion of CO2 into value-added chemicals and fuels is considered as one of the great challenges of the twenty-first century. Due to the limitations of the traditional thermal approaches, several novel technologies are being developed such as electrochemical, solar thermochemical, photochemical, biochemical, catalytic, and plasma conversions. One promising approach in this field, which has received little attention to date, is plasma technology. Its advantages include mild operating conditions, easy upscaling, and gas activation by energetic electrons instead of heat. This allows thermodynamically difficult reactions, such as CO2 conversion, splitting, and the dry reformation of methane, to occur with reasonable energy cost. Plasmas possess some important advantages over certain other novel technologies: (i) they can replace the thermal effect by electronic excited species and electron flow to start the excitation, dissociation, adsorption, and desorption of the species on the surface of the catalyst, at room temperature, in DBD discharge; (ii) they have a large flexibility in terms of the feeds that need to be processed; (iii) they provide an extremely flexible “turnkey” process, which allows for the efficient storage of energy, peak shaving and grid stabilization; (iv) the reactors have low investment and operating costs; (v) they have a simple scalability both in size and in applicability; and (vi) last but not least, the technology does not rely on rare earth materials – making it rather unique at this point. This unprecedented combination of features gives plasmachemical conversion a very high overall flexibility, making it an extremely useful and valuable technology for CCU techniques. The catalytic process of methanation of CO2 produces two molecules of water as a by-product. A current limitation in the CO2 methanation is the ageing of catalysts, mainly due to water adsorption during the process. To avoid this adsorption, the process is operated at high temperature (300–400 °C), leading to carbon deposition on the catalyst and its deactivation. To overcome this problem, a methanation plasma-catalytic process has been developed, which achieves high CO2 conversion rate (80%), and a selectivity close to 100%, working from room temperature to 200 °C, instead of 300– 400 °C for the thermal catalytic process. The main characteristics of this process are high-voltage pulses of few nanoseconds duration, activating the adsorption of CO2 in bent configuration and the polarization of the catalyst. The key step in this process is the desorption of water from the polarized catalyst. The high CO2 conversion at low temperature could be explained by the creation of a plasma inside the nanopores of the catalyst. Thus, the use of NTPs combined with catalysts opens new perspectives in the research for the valorization of CO2, by the study of new bi-functional catalysts of type p and n, low-cost and environmentally friendly, operating at low temperature and with electroconductive properties to take advantage of plasma creation by “field emission.” Moreover, the discharge power of the high voltage generator should be decreased in order to achieve low energy consumption. This could be achieved through the decrease of the discharge voltage (to about 5–10 kV) simultaneously using a pulsed plasma.

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All these results indicate a promising future for DBD plasma as an alternative energy source and reaction enhance for the catalytic methanation of CO2. The development of processes such as this one represents a positive transformation of industrial emissions and hence a step in the right direction in our fight against climate change.

Nomenclature SCH4 XCO2 AC NTP PPC IPC DBD

Methane selectivity (%) Conversion of carbon dioxide (%) Alternating current Nonthermal plasma Post-plasma catalysis In-plasma catalysis Dielectric barrier discharge

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Alberto Giaconia, Massimiliano Della Pietra, Giulia Monteleone, Giuseppe Nigliaccio

5 Development perspective for green hydrogen production Abstract: The growing interest in hydrogen requires evaluating all possible conversion processes for its production. Mature technologies today allow hydrogen production from renewable (carbon-free or carbon-neutral) sources using electrochemical, thermochemical or biochemical processes; however, there are opportunities for further improvement of green hydrogen production in terms of costs reduction and efficiency in the deployment of primary sources. This chapter presents four attractive routes for green hydrogen production, characterized by original features in terms of design, plant layout, system integration, operative parameters, and so on. Research and innovation challenges and perspectives are highlighted. First, advanced water electrolysis at higher pressures or temperatures is presented, using proton electrolyte membrane or molten carbonate electrolyzers, respectively. Then, solar thermochemical water-splitting cycles are introduced, being on an earlier development stage but showing wide techno-economic potentials to be competitive especially for massive hydrogen production in sites with high solar radiation. Finally, solar reforming of (bio)methane is introduced as an example hydrogen production from the combination of renewable sources such as biomass and solar energy.

5.1 Introduction It is well recognized that hydrogen can play different roles in the energy and industrial system and, as hydrogen appears to be a potential solution for a carbon-free society, its production represents a critical topic to fulfill the criteria of being environmentally benign and sustainable. In general, hydrogen can be produced from several H-containing feedstocks, such as water, hydrocarbons, alcohols, biomass-derived materials, hydrogen sulfide, and boron hydrides. Since hydrogen is not widely available in the nature in the form of H2 molecule, it needs to be separated from the aforementioned sources, for which energy is necessary to do this disassociation. The primary energy required for the conversion is generally electrical power and/or heat, which can be obtained from a primary energy source (fossil, nuclear, and renewable energy) or recovered from different production processes. Therefore, it is important to identify the energy sources to deploy and the technologies to exploit to satisfy the demand for environmentally benign hydrogen. Nowadays around 275 MTOE of energy is used for the production of hydrogen today (2% of global total primary energy demand). Natural gas is currently the primary https://doi.org/10.1515/9783110596250-013

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source for hydrogen production, and steam methane reforming (SMR) of natural gas is the technology mainly applied for the ammonia and methanol industries and in refineries: natural gas accounts for around three-quarters of the annual global dedicated hydrogen production of around 70 million tonnes of hydrogen, with around 205 billion cubic meters of natural gas consumption (6% of global natural gas use). The dependence on natural gas and coal leads to significant CO2 emissions associated to H2 production: 10 tonnes of carbon dioxide per tonne of hydrogen (tCO2/tH2) from natural gas, 12 tCO2/tH2 from oil products, and 19 tCO2/tH2 from coal. This results in total CO2 formation of about 830 MtCO2/year most of it emitted to the atmosphere. There are several alternative options for hydrogen production in terms of resources (energy and materials) and conversion processes. Hence, there are relevant potentials for hydrogen to become a key component in a sustainable economy aiming at 100% decarbonization and energy security. This transition implies the exploitation of reliable technical solutions being efficient, cost effective, and, at the same time, providing positive impacts in terms of economic growth, safety, protection of local environment, social acceptance, climate change mitigation, and so on. Research and innovation (R&I) will support this transition by tackling technology challenges along the pathway to the achievement of strategic goals. In the past, there have been spikes of interest in hydrogen, but not enough to boost an effective penetration of hydrogen technologies in the market. Today, renewable and low-carbon hydrogen is not yet cost-competitive compared to fossil-based hydrogen. Current mature technologies allow hydrogen production from renewable energy sources (RES) technologies (solar PV, wind, biomass, etc.) using electrochemical, thermochemical, or biochemical processes; however, advanced solutions have been proposed to improve the production processes in terms of costs (reduction of fixed and operative costs) and efficiency in the RES deployment. This chapter has the main goal to shade light on new perspective of green hydrogen production technologies, analyzing the state of the art and presenting four explanatory cases of innovative green hydrogen production processes developed by ENEA, shown in Fig. 5.1. These routes have original features in terms of design, plant layout, system integration, operative parameters, and so on. The four innovative approaches presented here show potential improvements to different hydrogen production routes and increase the sustainability of the whole system. Two concepts introduce innovations on water electrolysis: – Proton electrolyte membrane (PEM) electrolysis for high-pressure hydrogen production; – High-temperature molten carbonate electrolysis. Other two concepts are pure thermochemical processes using high-temperature solar heat: – New thermochemical water-splitting cycles (TWSC); – Solar reforming of biogas is presented as an example hydrogen production using combined renewable sources such as biomass-derived feedstock and solar energy.

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Fig. 5.1: Advanced technology pathways for green hydrogen production.

5.2 PEM Electrolysis for high-pressure hydrogen production 5.2.1 Basic principles and concepts In a world based on intermittent renewable energy resources, the storage of energy will be a critical component of a future energy economy. Hydrogen is recognized as a possible energy vector as it can be produced cleanly by the electrolysis of water and stored under pressure prior to transformation into electrical energy in a network of fuel cells. For water electrolysis to be significant part of such an economy will mean producing a meaningful percentage of the annual hydrogen production rate. PEM water electrolysis is a well-established and demonstrated technology that has excellent performance and stability and has established itself in the market place in certain niche applications [1]. In PEM electrolysis, the anode catalysts are typically IrO2 and Pt respectively. An acidic membrane is used as solid electrolyte (Nafion® – DuPont) instead of a liquid electrolyte. The membrane conducts H+ cations from the anode to the cathode and separates the hydrogen and oxygen produced in the reaction (Fig. 5.2).

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PEM water electrolysis

–

e

Power Supply

Cathode

e– Anode

H2

O2 H

+

H2O

PEM Anode:

2H2O ® 4H+ + O2 + 4e–

Cathode: 4H+ + 4e– ® 2H2 Fig. 5.2: Simplified diagram of a PEM polymer cell.

Among the advantages of this technology we can include a faster kinetics of the evolution reactions of hydrogen and oxygen than alkaline electrolysis, due to the low pH of the electrolyte and the metal surface of the electrodes (Pt and IrO2), and a safer process due to the absence of any caustic electrolyte. The use of a Nafion membrane in PEM electrolysis technology allows working at higher current densities, therefore to have more compact devices, and to obtain a hydrogen with a high degree of purity. Hydrogen use for transportation or injection in the gas network requires gas compression. For this reason, in recent years an important interest has shifted to the production of hydrogen by means of water electrolysis systems operating under pressure. PEM electrolysis can operate with high pressure on the cathode, while the anode can be operated at atmospheric pressure.

5.2.2 R&I challenges and perspectives PEM electrolyzers generally operate at a current density of 2 A/cm2 at 90 °C with approx. 2.1 V. This entails the use of suitable materials to resist to the highly corrosive effect of a low pH environment, due to the corrosive acidic nature of the membrane, and to sustain the high overvoltage applied to the anode. Corrosion resistance applies not only for the catalysts used, but also for current collectors and separator plates. Only a few materials can be selected that can perform

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in this harsh environment. This demands the use of scarce, expensive materials and components such as noble metal catalysts (e.g., Pt, Ir, and Ru), titanium-based current collectors, and separator plates [2]. Iridium is one of the rarest elements in the Earth’s crust, having an average mass fraction of 0.001 ppm in the earth’s crust. Conversely, gold and platinum are 40 times and 10 times more abundant, respectively. The major commercial sources of iridium are found in pyroxenite and the sulfide ore laurite in South Africa, as well as pentlandite from nickel mining regions in Russia and Canada [3]. Iridium demand has recently increased due to its use in crucibles employed to fabricate LEDs for smartphones, tablets, televisions, and automobiles. It is consequently expected that a high penetration of the PEM electrolysis technology in the market would considerably affect the demand for iridium and consequently the price. Advanced systems have recently been developed in order to produce hydrogen at high pressure with an efficient, clean, and safe production process. This device permits elevated hydrogen supply pressures without the need for mechanical compression. Hydrogen molecules are reduced electrochemically at the electrode surface and raise the pressure in the cathode chamber, provided that mechanical support is guaranteed from the membrane [4]. The pressure increases the passage of gas from one compartment to another (crossover), and the passage of water from the anodic compartment to the cathode compartment. Pressures above 100 bar require the use of thicker membranes (although more resistant), and internal gas recombiners to maintain critical concentrations (mostly H2 in O2) under safety thresholds (4 vol% H2 in O2). Lower gas permeability through the membrane can be obtained by incorporating miscellaneous fillers inside the membrane material, but this normally leads to less conducting materials. The increase in operating pressure also has consequences on the energy performance of the electrolytic cell; in particular it leads to a decrease in the potential, and this effect increases when operating at higher current densities (Fig. 5.3). A different approach provides for the two electrolytic compartments to operate under pressure and without pressure differential. In this case the system exerts an equal pressure on both side of the membrane, and it is therefore possible to use thinner membranes. According to a theoretical analysis, elevating the pressure decreases the volume of gas bubbles evolved during electrolysis. This in turn facilitates water transport, decreases ohmic losses in the catalytic layer, and improves electric contact between the layer and the current collector. Moreover, elevating the pressure makes it possible to run electrolysis at temperatures in excess of 100 °C, thus reducing the energy consumption owing to a decrease in the membrane resistance and the overvoltage [5].

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Fig. 5.3: Influence of pressure in systems with differential between anode and cathode.

The activities of the ENEA Laboratories concern both the development and optimization of the electrochemical aspects of polymeric devices, and the design and implementation of systems for the production of hydrogen under pressure without a differential between anode and cathode. The possibility of producing hydrogen under pressure allows reducing consumption for the mechanical compression of the gas by eliminating the first stage of gas compression, which is the most energy intensive. For what has been said the current commercial standard is 20–30 bar, even if there are manufacturers who have certified devices in their catalogue able to work even at higher pressures. If the operating pressure of the electrolyzer were to reach values close to 70 bar, mechanical compression could be completely eliminated for some applications, such as the introduction of gas to the network, with great plant and energy advantages. The possibility of developing a system for the production of hydrogen under pressure which also allows increasing the performance of the device would allow a reduction in the overall efficiency of the production and storage system. This represents an important target and the development of this technology would constitute a real technological breakthrough, guaranteeing for some applications the possibility of eliminating also the mechanical compression of the gas, with undoubted plant advantages.

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Polarization Curve at 60°C and different Pressure values (P = 1, 10, 30 bar) Voltage [V] 2,1 Experimental Model results point (medium values) 10 bar 10 bar 30 bar 30 bar

2,0

P atm 1,9

1,8

1,7

1,6

1,5

1,4

0

100

200

300

400 500 600 Current destiney [mA/cm2]

700

800

900

1000

Fig. 5.4: Polarization curve at 60 °C and different pressures (1, 10, 30 bar).

5.3 Molten carbonate electrolysis 5.3.1 Basic principles and concepts Due to their properties of low toxicity, electrical conductivity, catalytic activity and moderate corrosively, the use of molten alkali carbonate (MAC) salts as electrolyte has a long history of industrial and academic research interest for the development of high-temperature electrochemical processes in the field of energy generation/conversion and of energy-related sectors [6, 7]. Although MAC salts are in general considered much less aggressive and corrosive with respect to other molten salt classes (chlorides, fluorides, etc.), the corrosive nature of MAC salts may deeply change according to the conditions of use and to the gas atmospheres in equilibrium with the carbonate liquids. Without entering into details, suffice to say that acid–base properties (in terms of luxflood acidity definition) depend critically on the supernatant gaseous atmosphere. In general, molten carbonate electrochemical (but also chemical) processes are conducted under a CO2-containing gas atmosphere to keep the carbonate chemically stable. Thus, carbonate melts behave as neutral or mildly acidic liquids if CO2 in the gas atmosphere

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is kept at low levels, namely, under CO2/air mixture atmospheres. However, very acidic liquids (and therefore highly corrosive) may form under pure CO2 or CO2-prevalent gas atmospheres. For this reason, carbonate liquids are in general deemed as not very aggressive in molten carbonate fuel cell (MCFC) systems, since MCFCs operate under prevalent CO2/air gas atmospheres. On the other hand, melt corrosivity increases dramatically in molten carbonate electrolysis (MCE) environments, where poorly aerated and CO2-rich gas is in general the dominating atmosphere. Apart from fuel cells, MAC salts are recently attracting growing attention also as electrolyte for MCE processes in an attempt to develop a versatile MCE technology platform for the implementation of future sustainable industrial processes. Interest for MCE processes is developing following three main application directions: low-carbon manufacturing, energy storage, and CO2 capture and chemical conversion. The possibility of enabling low-carbon and sustainable manufacturing technologies for the production of high-value chemicals (i.e., carbon nanotubes) and key commodities (i.e., iron, cement) [8, 9] via MCE is at the basis of most of these novel studies. The possibility of easy integration of MCE technologies into concentrated solar power (CSP) systems to solar-powered MCE manufacturing processes is considered a potential key advantage, according to most studies. Chemical energy storage of intermittent RES via hydrogen or syngas production is another field with significant opportunities for MCE processes. Energy storage applications are the main subject of our interest and research studies on MCE processes. In particular, our interest is directed to the development of MCE processes for hydrogen production from steam in CO2/H2O gas mixtures (wet CO2) and at lowered temperatures (below 600 °C) to avoid loss of H2 by syngas formation [7]. Finally, the interest for using molten carbonates for CO2 capture, separation, and chemical conversion can be easily understood since the carbonate is anything else but a liquid concentrated CO2 system. Thus, several studies focus on using MCE process for capturing and electrochemically converting CO2 into high-value carbon products (e.g., carbon nanotubes, ultra-fine carbon powders) [7]. Figure 5.5 presents the primary components and reactions occurring inside a MCE cell. Water, carbon dioxide, heat, and electricity are required to perform the reduction reaction in the fuel electrode, producing H2 and carbonate ions (CO23 − Þ. The reduction reaction is expressed as H2 O + CO2 + 2e − ! H2 + CO23 −

(5:1)

The carbonate ions are conducted through the electrolyte to be oxidized at the oxygen electrode, producing carbon dioxide and oxygen. No flow is strictly required at the oxygen electrode inlet; nevertheless, some flow, air mixed with a little amount of carbon dioxide, is supplied to sweep out the formed gases, besides avoiding degradation of the electrode. The oxidation reaction is stated as

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. Q

C.V.

H2 + CO2 « CO + H2O

Reverse Water Gas Shift H2O CO2

H2O + CO2 + 2e− ®

Fuel electrode, Ni

CO32−

Matrix, Electrolyte Oxygen electrode, NiO O2, N2

H2 + CO32−

CO32− ® −12 O2 + CO2 + 2e−

CO2

H2 CO

ẇ Air CO2

Fig. 5.5: Primary components and reactions occurring inside a MCE cell.

CO23 − ! CO2 þ 1=2O2 þ 2e

(5:2)

The molar flow-rate of hydrogen produced by eq. (5.7) is determined by the reaction rate of the cell. It depends on the current demanded and the number of electrons involved in the reaction, ne, as eq. (5.3) shows n_ H2 =

I ne F

(5:3)

The electrical power in the system is a function of voltage and current. The voltage depends on the composition of the gases, the current density, and the operating temperature and pressure. Thus, operating the system under a certain electric current, the voltage would change depending on the electrical overpotentials present in the system that include ohmic, activation, and concentration losses. Figure 5.6 shows the voltage losses of a cell operating in reversible mode. Hence, the system voltage is predicted, adding (in electrolysis mode) or subtracting (in fuel cell mode) the electrical over-potentials from the ideal voltage.

5.3.2 R&I challenges and perspectives Owing to different application areas, MCE processes are also being investigated in different temperature ranges: intermediate temperatures (>600 °C) are prevalently studied for manufacturing and CO2 conversion applications. Temperatures lower than 600 °C may be in general of greater interest for H2 production by steam electrolysis. The research on MCE is still lying at fundamental and basic levels with almost no studies scaled up to significant maturity levels. Lack of corrosion resistance of commonly used metallic materials in the strong acidic carbonate melts is one of the

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Voltage Cell Voltage, Vcell Activation Overpotentials

Concentration Overpotentials

Open Circuit Voltage, VOCV Gas conversion Ohmic losses

Nernst Voltage, VN

Activation Overpotentials Nernst Voltage, VN Cell Voltage, Vcell

–j

Current density

Polarization losses Concentration Overpotentials +j

Fig. 5.6: Voltage losses of an MCE cell operating in reversible mode.

key issue impeding technology development of MCE processes in any of the abovementioned application areas. So far, in general all these basic studies have been conducted only for very short-term times (ten of hours), using alumina crucible as container for the carbonate liquids and electrodes of noble metals, Ni and (in some cases) of Fe. Although sufficient for the short-term purposes, the corrosion resistance of these non-noble electrode materials appears to be not adequate for longer exposure times. MCE has been investigated as a potential technology for hydrogen production only at lab scale in button cells (3 cm2) [10, 11]. Only recently a scale up has been done in the size of the tested prototypes from button cells to single cells (81 cm2) [12]. Particularly results obtained using button cells by Hu et al. showed how a molten carbonate fuel cell can be operated in a reversible mode switching from fuel cell mode to electrolysis mode preserving good performance in both modalities. Figure 5.7 shows electrochemical results obtained when an MCFC button cell was operated as power generator and hydrogen generator alternatively, highlighting that cell performances were not affected by the reversible operation. This statement is supported by both: the symmetrical behavior of polarization curves and the impedance spectra obtained in fuel cell mode and electrolysis mode (Fig. 5.7). Thanks to the preliminary and pioneeristic work carried out by Hu et al. [10, 11] a scale up from button cells (3 cm2 active area) to single cells (81 cm2 active area) was made by Perez et al. [12, 13] demonstrating the feasibility of the concept in a relevant environment. Particularly Perez performed a long-term test (1,000 h) in electrolysis mode feeding the electrolyzer with a realistic composition (CO2 and H2O with a small amount of hydrogen in the fuel electrode) and monitoring cell performance periodically during the experimental campaign. This long-term test (Fig. 5.8) represents the most relevant experiment carried out in the field of MCE so far, demonstrating the feasibility

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of the concept under realistic operating condition, showing how fluctuations in the voltage (see Fig. 5.8) of the cell are mainly related to the water management inside the MCE. 0.10 (a) Overpotential / V

0.05 Electrolysis cell mode 0.00 Fuel cell mode –0.05

–0.10 –0.5 –0.4 –0.3 –0.2 –0.1

0.0

0.1

0.2

0.3

0.4

0.5

Current density / A.cm–2 0.15 - Im (Z) / Ω∙cm2

(b)

OCV Fuel cell (OCV + 0.05V)

0.10

Electrolysis cell (OCV – 0.05V)

0.05 2 kHz 0.00

–0.05 0.00

2 Hz

200 Hz

0.05

0.10

0.2 Hz 20 Hz

0.15

0.20

0.25

0.30

0.35

0.40

Re (Z) / Ω∙cm–2

Fig. 5.7: Results obtained when an MCFC button cell was operated as power generator and hydrogen generator alternatively.

There are few research works related to MCE cells and a more detailed and structured research is needed to optimize the process and the overall systems. However, this type of technology is a good candidate to contribute to the energy transition being a reversible system able to store energy via high-temperature electrolysis and produce heat and power operating in fuel cell mode. Moreover, MCE can be used in the carbon capture, utilization, and storage (CCUS) field being able to convert CO2 and water into a hydrogen rich syngas that can be used as it is or as a building block to generate renewable fuels via methanation or Fischer–Tropsch reactions.

0.0

0.4

0.8

1.2

1.6

2.0

0

100

50 mA/cm2

100 mA/cm2

200

300

400 Time / h

500

600

shortage

700

150 mA/cm2

Fig. 5.8: Long-term test results obtained with a molten carbonate electrolysis single cell (81 cm2).

Voltage / V

800

900

electrolyte added

1000

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5.4 New thermochemical water-splitting cycles 5.4.1 Basic principles and concepts TWSC consist of a series of chemical reactions whose overall (net) effect is the splitting of the water molecule in hydrogen and oxygen. Therefore, intermediate reactants are recirculated in a closed chemical loop where one or more endothermic reactions require heat at temperatures usually in the range of 800–1,500 °C [14]. In general, a TWSC consists of two main chemical steps: 1) redox reaction between a reductant agent (M) and water/steam to generate hydrogen and the corresponding oxidized compound (H(2x–2)MOx); 2) high-temperature endothermic decomposition of the oxidizing agent (H(2x–2)MOx) to generate oxygen and the reductant agent (M) that will be recycled in the hydrogen generation reactor. Therefore, a general chemical cycle can be represented by the following reactions: xH2 O þ M ! Hð2x2Þ MOx þ H2

ðreaction 1; exothermic; Qout Þ

Hð2x2Þ MOx ! M þ ðx  1ÞH2 O þ 1=2O2 ðreaction 2; endothermic; 800  1500  C; Qin Þ H2 O ! H2 þ 1=2O2 If the high-temperature heat (Qin) to sustain reaction 2 is provided by concentrating solar (CS) technology, the overall water-splitting process will generate “green hydrogen” through a pure thermochemical pathway (i.e., without electrochemistry). Figure 5.9 shows a general solar-TWSC cycle concept scheme. It is noteworthy that hydrogen and oxygen production are carried out in separate units usually as highly pure gas streams. In principle, any conjugate redox pair Mred/Mox with suitable redox potentials could be adopted to develop a TWSC. Thus, hundreds TWSCs have been proposed and preliminarily evaluated so far, but only few cycles underwent further developments with flowsheet analysis, laboratory investigations, and prototyping [14]. Today, one broadly studied solar-TWSC is based on ceria oxides Ce2O3/CeO2 [15]: H2 O þ Ce2 O3 ! 2CeO2 þ H2

ðreaction 1; exothermic; < 1; 400  C; Qout Þ

2CeO2 ! Ce2 O3 þ 1=2O2

ðreaction 2; endothermic; 1; 400  1; 500  C; Qin Þ

H2 O ! H2 þ 1=2O2 The highest readiness level has been reached with the hydrosol process [16], based on the use of nickel-ferrites as reactive species operating at 1,400 °C for the endothermic oxygen generation step (2) and at 1,100 °C for the water-splitting step. In this

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Fig. 5.9: General scheme of a solar-driven thermochemical water-splitting cycle.

case, the TWSC has been proved in a 750 kW (thermal) solar receiver reactor built on a solar tower in Spain [16]. The above TWSCs are based on more or less complex metal oxides used in chemical loops, either as fluidized solid particles or as fixed beds integrated in the high-temperature solar receiver. A different kind of TWSCs are those belonging to the so-called sulfur family [14] where the intermediate agent “M” is not a metal element but sulfur and the conjugate redox pair is SO2/SO3: 2 H2 O + SO2ðaqÞ ! H2 SO4ðaqÞ + H2

ðreaction 1, exothermic, < 120  C, Qout Þ

H2 SO4ðaqÞ ! H2 O + SO2ðgasÞ + 1=2O2

ðreaction 2, endothermic, 800 − 900  C, Qin Þ

H2 O ! H2 + 1=2O2 In this case, the high-temperature step (2) is the decomposition of sulfuric acid: liquid H2SO4 from reaction (1) is first vaporized (>300 °C) and decomposed (400–520 °C) to SO3/H2O; then, SO3 is reduced to SO2 with oxygen production in a high-temperature catalytic reactor driven by solar energy [17, 18]. Among the main advantages of sulfur-based TWSCs are the lower temperatures of the endothermic step (≤900 °C) and the use of low-cost reactants without significant deactivation issues (differently from complex metal oxides whose activity or recyclability may decay after several cycles). The main issues with sulfur-based TWSCs usually deal with the chemical and physical operations carried out at lower temperatures, specifically with reaction 1 and downstream separation units. First, heat demanding operations are needed to separate liquid phases produced from reaction 1 (e.g., aqueous sulfuric acid distillation and evaporation). Additionally, reaction 1

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unaided is not thermodynamically effective but requires particular expedients to be completed [14]. One option is to drive it electrochemically, with relatively low voltage. However, a “pure thermochemical” pathway would be preferable as an alternative solution to water electrolysis (i.e., to avoid drawbacks of power-demanding and electrochemical routes). A proposed solution is to introduce a third reaction and perform reaction 1 in two steps, using iodine as intermediate: 2H2 O þ SO2ðaqÞ þ I2 ! H2 SO4ðaqÞ þ 2HIðaqÞ ðreaction 1a; exothermic; < ; 120  C; Qout Þ ðreaction 1b; slightly endothermic; < ; 500  CÞ

2HI ! I2 þ H2 2H2 O þ SO2ðaqÞ ! H2 SO4ðaqÞ þ H2

The above TWSC composed of three reactions (1a, 1b, 2) is known as the “iodine–sulfur” (IS) cycle, deeply studied in the past also to be combined with new generation nuclear reactors, but also considered for solar-hydrogen production [19–21].

5.4.2 R&I challenges and perspectives Thermochemical cycles may represent an appealing water-splitting alternative to electrolysis if substantial benefits will be demonstrated in terms of overall solar-to-H2 efficiency and costs (levelized cost of hydrogen, CAPEX and OPEX). Such a comparison is uneasy due to the current lower maturity level obtained by TWSCs compared to alkaline electrolysis. Solar-to-H2 efficiency (ηsolar − H2 ) in TWSC is the product between the thermochemical efficiency of the cycle (ηTWSC ) and the optical performance of the CS system (ηopt ) defined as the solar heat effectively absorbed by the receiver (Q_ abs, react , i.e., heat transferred to process fluids and reactants) divided by total solar irradiance on the solar field (Q_ sun ): ηsolar − H2 = ηopt × ηTWSC =

Q_ H2 Q_ abs, react × Q_ sun Q_ abs, react

(5:4)

where Q_ H2 is the heating value of the produced hydrogen (usually defined on HHV basis). The efficiency of a CS system (ηopt ) is in the range of 40–70%, depending on the technology (linear or point focusing), the optical performance and the temperature of the receiver; specifically, the receiver’s efficiency is highly sensible to temperature, higher the temperature of the receiver the lower is ηopt [22]. Point focusing system may have optical efficiency of 40–50% when the temperature of the receiver is >900 °C, while linear focusing technologies with 40% is claimed; therefore, R&I on the solar receivers would make the metal oxide TWSCs highly attractive in the long term. Differently, sulfur-based thermochemical cycles are expected to reach lower ηTWSC (75% of the heat can is requested at relatively mild temperatures 55% and, therefore, ηsolar − H2 > 15%. Several factors can improve the economic competitiveness of solar-TWSCs compared to solar-electrolysis for green hydrogen production. First, it is expected that the worldwide diffusion of the CSP technology will significantly reduce the cost of the CS system (collectors, receivers, etc.), being the major fixed cost item, with resulting decrease of the green hydrogen production costs. Additionally, while electrochemical routes that require special surface devices integrated in modular stacks (membranes, electrocatalysts, etc.), TWSCs are “bulky” processes for massive hydrogen production that could make use of typical process engineering methodologies; this feature will improve the economy of scale (CAPEX reduction) and also decrease also operation and maintenance costs (OPEX reduction) especially for large-scale green hydrogen production. Finally, the possibility to apply suitable thermal energy storage (TES) systems from the CS system will allow driving more or less complex units of the TWSC on steady state (24 h), especially those working at lower temperatures (120 °C), thus reducing material corrosion issues. Differently from other TWSCs, in the NIS cycle hydrogen is obtained in a simple lowtemperature step. Finally, there are several options to buffer intermittent solar-driven reactors (for endothermic reactions 3 and 5) by means of intermediate solid reactants that can be easily stored. Preliminary studies made by ENEA showed that the NiS cycle can reach ηTWSC = 20% without heat recoveries [23]; therefore, by optimized thermal integration the NIS process has the potential to compete with other watersplitting routes for its efficiency and techno-economic performances. In conclusion, the potentials of TWSCs have been demonstrated in laboratory or small prototypes. Optimized thermal integration, engineering, and demonstration of different TWSC options in fully integrated plants in representative environment are needed to prove the effective performance of the technology.

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Fig. 5.10: Simplified block diagram of the NIS water-splitting cycle, including low-temperature reactors (blue), high-temperature solar receiver/reactors (red), and solid storage systems (yellow). Downstream purification/dehydration units not shown.

5.5 Solar reforming of biogas 5.5.1 Basic principles and concepts The previous sections introduced different water-splitting processes using renewable heat and power. Alternative options for green hydrogen production consist in the conversion of carbonaceous feedstock derived from biomass, such as biogas, through thermochemical pathways driven by renewable heat. Figure 5.11 shows a general scheme of this thermochemical approach. In this case, widely proven hydrocarbon conversion pathways are modified to embed solar energy: carbon-based reactants (CnH2n+2) are converted to syngas or, in general, H2/COx mixtures. A key feature of these systems is that the chemical products of the reactions contain more energy than the feedstock; such net increase in energy content is the effectively embedded solar energy. Solar steam reforming of methane is the most widely studied solar thermochemical option as it leads to maximum conversion efficiency of the carbonaceous feedstock to hydrogen. A conventional steam reforming plant contains several process steps. The core of the process is represented by catalytic reactors where the SMR reaction (1), which is highly endothermic, and the water–gas shift (WGS) reaction (2), which is slightly exothermic, take place:

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CH4 + H2 O ! CO + 3 H2

 ΔH298 K = + 206 kJ=mol

ðSMR reaction, 750 − 950  C, 1Þ

CO + H2 O ! CO2 + H2

 ΔH298 K = − 41 kJ=mol

ðWGS reaction, 250 − 450  C, 2Þ

Fig. 5.11: General scheme of the solar aided thermochemical conversion of a carbonaceous feedstock CnH2n+2.

Typically, the SMR is carried out in a gas-fired furnace which provides the necessary high-temperature process heat. The purified methane feedstock is mixed with a controlled quantity of steam according to the selected value for the steam-to-carbon molar ratio (S/C) and preheated at 550 °C in the convection section of the reformer furnace. In general, S/C is 3 or higher, that is, steam is the excess reactant, in order to improve the hydrocarbons conversion and prevent any carbon deposition over the catalyst. The hot CH4/steam mixture is sent to the SMR tubular reactor usually filled in with Ni-based catalytic pellets. Usually, hydrogen is purified by pressure swing adsorption (PSA): the purge gas stream from the PSA unit is mixed to the primary gas fuel to provide the required heat duty to the SMR furnace. The flue gases (containing CO2) are discharged to the atmosphere. The heat-demanding process leads to significant fuel consumption with resulting high operating costs and massive greenhouse gas emissions. Since solar reforming does not involve any combustion in the process to supply the high-temperature process heat (the gas-fired furnace to drive SMR reaction is replaced by a solar reactor) the process will not involve flue gas emissions to the atmosphere as produced CO2 only derives from the reactions (1 and 2) conversion: specific CO2 emissions typically in the range of 8–11 kgCO2/kgH2 will be reduced to the 5.5 kgCO2/kgH2 as obtained from the stoichiometry of the two reactions (SMR and WGS). Typically, this displacement leads to a “combustion/flue gas free” solar SMR resulting in a reduction of CH4 deployment (or CO2 production) in the range of 30–50%. Additionally, produced CO2 can be easily captured and stored/reused and, hence, the whole process represents a solar-driven pre-combustion decarbonization of the primary fuel [27].

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Over the last 25 years, there has been an extensive research and development effort worldwide on the solar reforming of methane [28, 29]. Indeed, solar reforming technology is widely considered a low-cost and low-carbon option to boost the introduction of hydrogen and support the transition from a fossil-based to a renewable energy economy. If the carbonaceous feedstock is a biomass-derived fuel, the overall process will lead to green (100% renewable) hydrogen production also for a long-term carbonneutral economy. This is the case where a biogas or bio-methane is used to feed the solar SMR process: solar reforming of biogas can represent an alternative route for green hydrogen production in the long term, to increase energy security by the diversification of resources and technologies complementing the direct renewable (solar and wind) water-splitting routes. Additionally, the biogas reforming represents a waste-tohydrogen route to be applied in a circular economy perspective.

5.5.2 R&I challenges and perspectives The major challenge in the development of solar reformers is the integration between the steam reforming process and the available CS technology, which can be achieved through different methods. A first approach to solar reforming is based on “direct solar reforming” where solar receiver/reactors capture and convey the incoming highly concentrated solar radiation to the catalytic reactor operating at temperatures close to conventional reformers, that is, >750 °C [28]. In this case, the main challenge is the management of transients that affect the operation of the reactor and downstream units. First, it is necessary to quantify the impact of daily thermal cycling on the receiver reactor components, including both catalysts and the tubular receiver materials. This can be addressed with operational experience in a demonstration facility as a precursor to commercial-scale operations. Second, for the integration with downstream processes to form marketable products, all of the downstream processes rely on a specific syngas composition to form these usable products, so methods of buffering the daily solar and seasonable variations are required. More recently, ENEA has developed a different approach consisting on the development of a low-temperature steam reformer (

.

Methane reforming I

CH4 + H2 O ! 3 H2 + CO

.

.

Methane reforming II

CH4 + 2 H2 O ! 4 H2 + CO2



.

CO + H2 O ! H2 + CO2

−.

.

Char partial oxidation

C + 1=2 O2 ! CO

−

.

Water gas

C + H2 O ! H2 + CO

.

.

Boudouard reaction

C + CO2 ! 2 CO



.

Methanation

C + 2 H2 ! CH4

−.

.

Reaction

Equation

Biomass pyrolysis

WGS

a

a

WGS, water–gas shift.

7.1.3 Classifications of biomass gasification Gasification can be categorized into three types, which are air gasification, oxygen gasification, and steam gasification. Table 7.2 compares different gasification processes. Tab. 7.2: Comparison of different gasification processes. Oxygen gasification

Air gasification

Steam gasification

Product heating value, MJ/Nm

High –

Low –

High –

Products

H, CO, CO, HCa, HO, tar

H, CO, CO, HCa, HO, tar, N

H, CO, CO, light HCa, tar

Average product gas composition

H – %, CO – % H – %, CO – %, CO – %, H:CO: CH – %,CO – %, N – %, H:CO:.

H – %, CO – %, CH – %, CO – %, N – %, H:CO:.

Reactor temperature, °C

,–,

–,

–,

Cost

Costly

Cheap

Medium

a

HC, hydrocarbons.

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(1) Oxygen gasification. Biomass gasification is an endothermic process. Energy is required to meet the demand for biomass heating, gasification agent heating, and various endothermic reactions. The simplest way to provide this energy is partial combustion of biomass fuels. The ratio of amount of introduced oxygen to the value that can satisfy absolute combustion of biomass is defined as equivalent ratio (ER). A reasonable ER is generally not larger than 0.2. Otherwise, the heating value and quality of gasification syngas will be lowered because combustible components such as CO, H2, and CH4 decrease while exhaust components like CO2 and H2O increase in the final product gases. Typical heating value of oxygen gasification syngas is about 10–15 MJ/Nm3 and the H2 volume concentration can be up to 40%. (2) Air gasification. The largest disadvantage of oxygen gasification is that pure oxygen production cost is very high. Air gasification is an economical option because air is the cheapest gas and easiest to be utilized. However, syngas produced by air gasification is diluted with large amount of N2 in air. The N2 concentration in syngas is typically as high as 48 vol% for air gasification. As a result, the syngas heating value is only about 4–6 MJ/Nm3 while the H2 concentration is only 15 vol% for air gasification. (3) Steam gasification. Adding steam to the gasifier is favorable for many reactions such as tar and steam CH4 reforming, water gas, and WGS reactions. Steam gasification can promote the concentration and yield of H2 and realize maximum H2 production. The H2 concentration is higher than 40 vol% and the heating value is around 15–20 MJ/Nm3 for the syngas produced by steam gasification. It should be noted that air or O2 is also needed in steam gasification, which is used for partial combustion to provide energy for gasification reactions. It is of significance to accurately calculate and control the steam flow rate because steam generation consumes a large amount of energy. Excessive steam will condense to be water downstream gasifier and result in decreased gasification efficiency. Thus, the mass ratio of steam to biomass is a very important operational parameter in biomass steam gasification.

7.2 Factors influencing biomass gasification 7.2.1 Type of feedstock Chemical components of biomass have apparent effects on gasification. Biomass with higher cellulose and hemicellulose tends to yield more gaseous products from gasification. In contrast, higher lignin content is not favorable to produce more gases due to high yield of bio-char. Biomass proximate properties also influence gasification. Generally, moisture content of biomass for gasification should be in the range of 15–30%. Too high moisture content will increase biomass drying cost. If biomass moisture content is higher than 30%, gasification temperature will be reduced due to

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excessive steam in gasifier. High moisture content may also lead to the increase in tar yield and negatively affect gasification efficiency. Besides moisture, biomass ash also plays an important role for operation and maintenance of gasifier. Low-ash biomass is favorable for gasification. For biomass with high ash content, it is easier to have fouling and slagging problems when gasification temperature is beyond 600 °C because biomass ash consists of components with low melting temperature such as Ca, Mg, K, and Na silicates [2].

7.2.2 Temperature and pressure Reaction temperature is a decisive factor that influences the components of gasification syngas. Temperature is also a major parameter that affects char conversion and H2 concentration. Higher temperature results in promoted char gasification and production of H2. Increasing reaction temperature accelerates various gasification reactions and makes complete conversion of reactants, being beneficial to increase gases yields and syngas heating value. The yields of tar species and char are both reduced at high gasification temperatures. Reaction pressure has apparent influences on the performance of gasification. Increasing pressure promotes reaction rate of char gasification and elevated the capacity of gasifier. However, pressurized gasification leads to more complex system configurations and increases total capital cost. In addition, pressurized gasification favors methanation reaction (eq. (7.10)) which may result in the rise of CH4 concentration while also resulting in a drop of H2 concentration.

7.2.3 Gasification agent and ratios As an essential gasification agent, the flow rate of pure O2 (or O2 in air) should be maintained at a reasonable value, which ensures the released energy from partial combustion can meet the demand of gasification. Reasonable increase of O2 flow can elevate reaction temperature which is beneficial to enhance the yields of H2 and CO. Excessive O2 will reduce the contents of combustible gases and H2 yield. Using steam as gasification agent favors water gas reaction, WGS reaction, and reforming reactions of hydrocarbons and tar species. Suitable amount of steam benefits production of syngas and H2. But use of excessive steam should be avoided in order to maintain acceptable energy consumption and gasification efficiency.

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7.2.4 Reactor configuration (1) Fixed bed gasifier. Fixed bed reactor has a simpler design and operation, which has been verified as a mature technology. Figure 7.2 shows the diagram of two configurations of fixed bed gasifier. The left one is an updraft gasifier. It is suitable for biomass feedstock with low moisture and high ash content. Updraft gasifier has a small aptitude for slag formation, but the produced syngas contains considerable amounts of tar species which requires implementing expensive syngas cleanup and tar reduction unit downstream gasifier. Downdraft gasifier (the right one) has high thermal conversion efficiency and lower char production and tar concentration in syngas. It also works well for temperature control. However, the yields of H2 and CO are relatively low for downdraft gasifiers [3] (Fig. 7.2).

Fig. 7.2: A diagram of typical fixed bed gasifiers (left: updraft; right: downdraft).

(2) Fluidized bed gasifier. Fluidized bed gasifiers have advantages of high biomass conversion, less char residue, and low tar content in syngas. Figure 7.3 shows the diagram a circulating fluidized bed gasifier. Compared with a bubbling fluidized bed, circulating fluidized bed has higher gas velocity in the gasifier. Solid particles including heat carriers and unreacted char are carried out from the gasifier, which are then separated by cyclones and transferred back to the gasifier by circulation gas in a loop seal. Different from a fixed bed gasifier, fluidized bed reactors have improved heat and mass transfer in bed and require biomass particles smaller than 100 mm (Fig. 7.3).

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Fig. 7.3: A diagram of circulating fluidized bed gasifier.

(3) Entrained flow gasifier. This type of gasifier can adopt either dry fine particles or slurry feedstock. Figure 7.4 shows the diagram of an entrained flow gasifier with slurry fuel feeding [2]. Entrained flow gasifiers have very high reaction temperature and are often operated at elevated pressures. As a result, rapid biomass conversion and high throughput can be realized. It is also very easy for scaling up. However, entrained flow gasifiers require very fine biomass particles or using slurry fuels, which largely increases the processing cost of raw biomass. Besides, slurry feedstock contains a large amount of water, which reduces gasification efficiency (Fig. 7.4).

Fig. 7.4: The diagram of entrained flow gasifier.

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7.2.5 Biomass particle size Biomass with large particle size usually has increased resistance of heat and mass transfer. Large biomass particles are liable to have low conversion rate and produce more chars. In general, smaller particles provide increased surface area per unit mass, which benefits heat and mass transfer among biomass particles. Gasification reactions including water gas reactions, Boudouard reaction, and char secondary cracking reaction are promoted because of the improved heat and mass transfer for smaller particles. The yield of H2 and the energy conversion efficiency of biomass are also increased.

7.2.6 Catalyst Various groups of catalysts have been attempted to enhance biomass gasification. Typical catalysts include dolomite, nickel-based catalyst, alkali metal catalyst, noble metal catalyst, inorganic salts such as Na2CO3, K2CO3, and ZnCl2. Using catalysts can accelerate reaction rates of the Boudouard reaction, water gas reactions, WGS reaction, methane steam reforming reaction, secondary cracking of tar species, and so on. The adoption of gasification catalyst is beneficial to enhance syngas yield and H2 production.

7.3 Supercritical water gasification of biomass for H2 production A novel supercritical water gasification technology for H2 production from biomass has gained much attention in recent years [4]. This technology is safe, nontoxic, readily available, cost-competitive, and environmentally friendly due to the use of supercritical water. Other advantages of the technology over conventional gasification include: (1) very high reaction rate and being able to produce high concentration gaseous products; (2) perfect heat and mass transfer and complete conversion of feedstock; (3) minor char product and convenient separation of products. Figure 7.5 shows a flowchart of supercritical water gasification technology for H2 production from woody biomass (Fig. 7.5). Figure 7.5 presents a simplified flowchart of the mechanism for supercritical water gasification to produce H2. Lignocellulosic biomass firstly decomposes into sugars, guaiacols, syringols, phenolics, and others. C5 and C6 sugars are formed from the hydrolysis of cellulose and hemicellulose, while guaiacols, syringols, phenolics, and others are generated by lignin hydrolysis. In the supercritical water gasification regime, the products of the initial hydrolysis are further converted to compounds

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Fig. 7.5: A flowchart of supercritical water gasification technology for H2 production from woody biomass. Note: catalysts A (e.g., Ni, Ru, Rh, Pt, Pd, Ni/Al2O3, Ni/C, Ru/Al2O3, Ru/C, and Ru/TiO2); catalysts B (e.g., Ni, Ru, Pt, and activated carbon); catalysts C (e.g., Ni, Rh, Ru, Pt, and activated carbon), and catalysts D (e.g., Ni, Ru, NaOH, KOH, K2CO3, and Trona).

with much simpler molecule structure under the effects of suitable catalysts (catalysts A); these compounds include acids (carboxylic, succinic, acetic, etc.), alcohols (coumaryl, coniferyl, sinapyl, etc.), phenols, aromatics, and aldehydes. In the next step, these simple compounds are catalyzed to form H2 and CO (catalysts B). With the catalysis of different heterogeneous catalysts (catalysts C or D), H2 and CO can be transferred to either CH4/H2O or H2/CO2 through enhanced methanation or WGS reaction. Separation of CO2 from H2 and CO2 mixtures will produce a stream of pure H2. Currently there have been many investigations on supercritical water gasification of biomass. Biomass materials of single pure component and actual biomass are both adopted as the feedstock. Cellulose gasification in supercritical water was performed in a pilot plant by using a continuous tube reactor in University of Valladolid, Spain [5]. Supercritical water gasification of xylose, a principal sugar in hemicellulose, was conducted by Ege University, Turkey [6]. Gasification of organosolv-lignin in supercritical water was completed under the catalysis of charcoal supported noble metal salt in the National Institute of Advanced Industrial Science and Technology, Japan [7]. Given the importance of the difference in biomass compositions, supercritical water gasification of real biomass was also performed by many institutes in order to

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understand the interference of the various components in mixtures. Gasification of potato, corn starch, and potato wastes were conducted in University of Hawaii at Manoa, USA [8]. Supercritical water gasification of olive mill waste-water was performed in Yıldız Technical University, Turkey [9]. Assessment of sugarcane bagasse gasification in supercritical water for H2 production was performed in Xi’an Jiaotong University, China [10]. Comparison of Cassava root gasification with starch, cellulose, and glucose was completed in University of Leeds, UK [11].

7.3.1 Physicochemical properties of supercritical water The main difference between supercritical water gasification and other conventional gasification technologies is the gasification medium. Supercritical water plays dual roles as reactant and gasification medium in supercritical water gasification. When water temperature and pressure reach their critical point (374 °C, 22.1 MPa), a new state – super-critical state can be observed. Compared with normal liquid or gas phase of water, physicochemical characteristics of supercritical water such as ion concentration, density, dielectric constant, and viscosity are rather different [12]. Near its critical point, the density and ion concentration of water are much lower than normal state. The density of water is within the range of 100–600 kg/m3, which is much lower than normal water of 1,000 kg/m3. The decrease of water density causes the drop of ion product in water. When temperature is higher than that of critical point, a lot of free radicals are produced and the reaction mechanism transfers from ion mechanism to free radical mechanism. The dielectric constant of water also changes with state. At atmospheric temperature and pressure, water dielectric constant is relatively large (about 80) because there is a strong effect of hydrogen bond. However, dielectric constant of water decreases to only 5 at critical point. This makes supercritical water easy to dissolve biomass feedstock and organic gasification products such as various organic hydrocarbons because the dielectric constants of these organic compounds are quite close to that of supercritical water. In contrast, dissolving capacity of supercritical water for high polar inorganic compound, such as biomass ash or char, sharply drops. Supercritical state of water also results in the change of viscosity. Water viscosity is lowered to 2.98 × 10−5 Pa s which is close to that of vapor (2 × 10−5 Pa s). The decrease in viscosity creates perfect reaction conditions for higher reaction rate because lower viscosity favors high diffusion coefficient of water.

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7.3.2 Main factors influencing biomass supercritical water gasification (1) Biomass composition Biomass composition is a determining factor for supercritical water gasification. The contents of biomass components such as cellulose, hemicellulose, lignin, and inorganic ash all have influences on reaction rate and H2 yield. Research reveals that the supercritical water hydrolysis of cellulose is very rapid that can generate glucose and sugar oligomers. Identical gas yields can be obtained by supercritical water gasification of cellulose and glucose. However, it is still difficult to predict products yields of mixtures of different biomass components or real biomass, though single biomass component gasification characteristics and components portions can be well known. (2) Temperature Reaction temperature has significant effects on supercritical water gasification of biomass. At the high temperature of critical point, gasification is controlled by free radical mechanism instead of ionic mechanism. Higher temperature of supercritical water gasification results in higher concentration of free radicals, which are beneficial to promote gasification efficiency and H2 yield. Reaction temperatures of supercritical water gasification can be classified into two ranges, which are low temperatures (350–500 °C) and high temperatures (500–800 °C). At low temperatures, CH4 is the main gasification product because methanation reaction is profound. While at high temperatures, H2 production is largely promoted due to the enhancement of endothermic reforming reactions. However, in order to reduce operation and maintenance cost, lower reaction temperature is favorable. Therefore, it is essential to develop suitable catalysts to reduce reaction temperature while retaining high gasification efficiency. It is very interesting that glucose can be completely gasified within 3.6 sec at supercritical water of 25 MPa and 650 °C. At higher temperature of 767 °C, glucose can approach a H2 yield of 11.5 mol for each mole of feedstock within 60 seconds, indicating that it almost realizes the upper limit (12 mol H2 for each mole of glucose) for H2 production. These investigations demonstrate the great potential in maximum H2 production by biomass supercritical water gasification [4]. (3) Pressure The influences of pressure are relatively complicated. With rising pressure, dielectric constant and ionic product of supercritical water gasification also increase; thus, the reaction of water at elevated pressures tend to be controlled by ionic reaction mechanism other than free radical mechanism. Meanwhile, the density of supercritical water also increases with increasing pressure. Higher density has dual roles in the reactions. First, high density hinders the interactions between the

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solute molecules, resulting in slower coking and polymerization reactions of solute molecules. Second, high density is favorable for reactions involving both solute and solvent molecules such as reforming and WGS reactions. The dual roles of pressure on supercritical water gasification can be used to explain why there are adverse experimental results in literature. For example, increase of gasification efficiency and H2 yield was observed for the pressurized hydrolysis of lignin when reaction pressure increased from 15 to 27.5 MPa. However, H2 production of glucose gasification was found to be lowered with increasing pressure, within the temperature range of 400–600 °C [4]. In order to reduce facility capital and operation costs, the pressure of supercritical water gasification should be controlled at a reasonable level. (4) Feed concentration During supercritical water gasification, too high biomass concentration will increase the difficulty in gasification. Both gasification efficiency and gas yields decline with increasing biomass concentrations. This conclusion has been verified in the experiments for glucose, glycerol, lignin, and so on. For example, when the feeding concentration of glucose increased from 1.8% to 15%, H2 yield decreased from 11.2 to 5.7 mol/mol [4]. Moreover, higher feeding concentration of biomass also results in difficulties in pumping and leads to plugging problems. (5) Residence time Biomass conversion rate and gasification efficiency increase with increasing residence time, usually in the order of seconds. Moreover, the increment of residence time also enhances H2 yield. For example, during the supercritical water gasification of glycerol (10% of concentration) at 487 and 25 MPa, H2 yield increased from 0.3 mol/mol to 1.3 mol/mol with the increment of residence time from 5.2 to 9 s [4]. (6) Catalysts The addition of catalysts is helpful to reduce reaction temperature and pressure of supercritical water gasification, which can reduce operation and maintenance costs. Suitable catalysts are also beneficial to reduce the formation of char and tar species and simultaneously improve H2 production selectivity. The criteria of catalysts selection include the abilities to cleave C–C and C–O bonds, to enhance WGS reaction, and so on. Different catalysts such as alkali, transitional metals, and activated carbon have been used for biomass supercritical water gasification. (7) Reactor configurations Laboratory reactor systems such as batch, continuous flow, and quartz capillary reactors have been developed for biomass supercritical water gasification. Batch reactors are normally high-pressure autoclaves with volumes from several milliliters to 1 L. This type of reactor is widely adopted presently. However, it is difficult to control

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the reaction pressure for such reactors. Besides, gasification also proceeds during the preheating stage; the heating rate may affect product distributions. Continuous flow reactors can be operated at high heating rate, which can be realized by using preheater, multiple zone furnaces, a swirl generator at the reactor entrance, and so on. This type of reactor was mostly adopted for feeding soluble organic materials because pumping accurate amounts of feedstock into a high-pressure gasifier is rather difficult in practice. Capillary quartz tubes can realize safe and cheap operation at elevated pressure and heat biomass feedstocks rapidly. It is quite useful to provide additional information for visual observation and gases in catalyst-free tests. However, these capillary tubes may be unsuitable for gasification tests using catalyst powders because the distribution of catalyst along reactor is nonuniform due to the small inner diameter.

7.4 H2 production from CaO-based calcium looping gasification of biomass 7.4.1 H2 production with in situ CO2 capture Conventional steam gasification of biomass is beneficial to promote the concentration and yield of H2, but there is still large quantity of undesirable products in the syngas, such as tar species and CO2. CaO-based calcium looping gasification of biomass attains much attention in recent years. This technology has the potential to produce a stream of pure H2 sustainably and in situ capture CO2. Experimental results reveal that, even at atmospheric pressure and moderate temperatures (lower than 800 °C), a syngas having 71 vol% of H2 and almost without CO2 can be generated [13]. Figure 7.6 shows the mechanism of CaO-based calcium looping gasification of biomass (Fig. 7.6).

Fig. 7.6: A diagram of CaO-based calcium looping gasification of biomass.

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There are two reactors in CaO-based calcium looping gasification, i.e., the gasifier (or carbonation reactor) and the combustor (or calcination reactor). In the gasifier, biomass reacts with steam in the presence of CaO. Different from traditional steam gasification, the produced CO2 is in situ captured by CaO carbonation, which forms CaCO3 via eq. (7.11). The absorption of CO2 enhances WGS reaction (eq. (7.6)) and water gas, steam reforming reactions, which are all beneficial to promote H2 production. CaO carbonation also releases much reaction heat that is used to provide energy for endothermic gasification. In ideal conditions, the partial oxidation of biomass fuels becomes unnecessary. In addition, CaO also plays the role of tar reforming catalyst that also benefits tar reduction and H2 production. The product of CaO carbonation, that is, CaCO3, is transported to the combustor with unreacted bio-char. The apparent heat of CaO and bio-char also provides energy to the gasifier besides CaO carbonation. In the combustor, CaCO3 is calcined and decomposed to release CO2 gas (eq. (7.12)). It is the combustion of bio-char (eq. (7.13)) that provides the energy for calcination and retains the high temperature of 800–900 °C in combustor. In this way, CaO is regenerated and then recycled to the gasifier. When pure O2 is used as the oxidation medium, a stream of pure CO2 can be obtained from the combustor, which is very convenient for subsequent compression, transportation, and storage: CaO + CO2 ! CaCO3

ΔH0 = − 170.5 kJ=mol

(7:11)

CaCO3 ! CaO + CO2 ΔH0 = 170.5 kJ=mol

(7:12)

C + O2 ! CO2

(7:13)

ΔH0 = − 394.5 kJ=mol

Extensive research has been conducted from thermodynamic investigations to industrial-scale experiments for CaO-based calcium looping gasification of biomass. The implications of thermodynamic equilibrium on H2 production were analyzed in the University of Sydney, Australia [14]. The influences of various factors on H2 production at pressurized conditions were also thermodynamically investigated in Zhejiang University, China [15]. Laboratory-scale fixed bed and circulating fluidized bed gasification experiments were conducted in Dalhousie University, Canada [16, 17]. Exergy analysis of the circulating fluidized calcium looping gasification system was performed and the gasification efficiency approached 78.77% [17]. Both atmospheric and pressurized gasification in fluidized bed reactor have also been investigated in Zhejiang University, China [18, 19] and a maximum H2 fraction of 67.7% was realized at conditions of 680 °C and 4 bar. Furthermore, pilot scale of about 100 kWth and industrial scale of 8 MWth experimental tests have also been conducted in Vienna University of Technology, Austria [20, 21].

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7.4.2 Main factors influencing CaO-based Calcium looping gasification (1) Temperature Increasing reaction temperature favors endothermic reactions such as biomass pyrolysis (eq. (7.1)), steam reforming (eqs. (7.2)–(7.5)), and char gasification (eq. (7.8)), and thus benefits higher yields of gaseous products. However, too high temperature hinders exothermic CaO carbonation (eq. (7.11)) and WGS reaction (eq. (7.6)). In order to retain high CO2 capture efficiency, gasification temperature should not be higher than the equilibrium temperature relative to CO2 partial pressure in the gasifier, depending upon eq. (7.14). In this equation, PCO2, eq, and T indicate the CO2 equilibrium pressure and reaction temperature of CaO carbonation, respectively: log10 PCO2 , eq ½atm = 7.079 −

8;308 T

(7:14)

(2) Pressure The increase of reaction pressure can enhance CaO-based calcium loop gasification. Firstly, pressurized condition favors CaO carbonation for enhanced CO2 capture. Meanwhile, the increase of reaction pressure extends the upper limit of reaction temperature for carbonation or gasification, which is beneficial to promote endothermic reactions such as biomass pyrolysis, steam reforming, and gasification reactions. For biomass gasification, previous research indicates that char gasification plays a minor role because biomass has more volatiles and much less fixed carbon. As a result, the reaction pressure for biomass gasification should not be too high. Reasonable reaction pressure is also beneficial to avoid too complex system configuration and retain reasonable capital cost. (3) CaO absorbent to biomass ratio When adopting very high ratio of CaO absorbent to biomass, the CO2 in syngas can be absolutely absorbed and syngas with very high H2 concentration can be obtained. In a previous fixed bed gasification test, the H2 concentration in syngas approached 80 vol% and almost no CO2 could be detected when the mole ratio of CaO to carbon in biomass was 4 at operation conditions of 650 °C and 5.92 atm [13]. The value of CaO to biomass ratio should also consider the operation costs caused by CaO absorbents. Costs of CaO preparation, replacement, and disposal are important factors for industrial application.

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(4) Steam-to-biomass ratio Increasing steam-to-biomass ratio is beneficial for water gas, WGS, and steam reforming reactions and enhancing the production of H2. Increased ratio of steam to biomass also favors higher CO2 capture efficiency because the reactivation of CaO absorbent by steam can be promoted at higher steam partial pressures. Enhanced CaO carbonation will further improve water gas and WGS reactions. Similar to conventional steam gasification, excessive steam will cause the increase in energy consumption and cost, leading to decreased overall gasification efficiency.

7.5 Co-gasification of biomass and plastics The elemental hydrogen content in feedstock has a decisive influence on H2 concentration in final gasification syngas. Higher hydrogen content is favorable to produce more H2 through gasification. Biomass feedstock has quite low content of elemental hydrogen, which is only about 5–8 wt% of dried biomass. However, the elemental hydrogen content of organic plastics is quite high. Co-gasification of biomass with plastics is beneficial to promote concentration and production of H2 in syngas. There has been much research for co-gasification of biomass with plastics. Five to twenty percent of polypropylene (PP), high-density polyethylene (PE), polystyrene (PS), and a mixture of real plastics were mixed with saw dust and gasified in steam atmosphere in the University of the Basque Country, Spain [22]. The effects of Ni/Al2O3 catalysts were compared. Co-gasification of pine wood and plastics were performed in a fluidized bed in Portugal since 2002 [23]. A well-known 100 kWth dual fluidized bed pilot plant was used to gasify plastics with biomass in Vienna University of Technology, Austria [24]. Four types of plastic materials have been used and feedstock with 75% or even 100% of plastics has been successfully gasified into gases products. Using the same 100 kWth fluidized bed facility, co-gasification tests were also performed by researchers from the University of Ljubljana, Slovenia [25]. They found that co-gasification of biomass–plastic mixture is more successful than using only plastics. Steam gasification of various pellets made of wood, biomass/plastics, and olive husks were conducted in a fluidized bed in Istituto di Ricerche sulla Combustione, Italy [26]. It was concluded that adding plastics and catalysts are beneficial to elevate concentration and yield of H2 in syngas. This section introduces the significance of co-gasification of biomass and plastics. The synergic interactions between biomass and plastics as well as the main influencing factors are also summarized.

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7.5.1 Significance of plastics recycling and sustainable utilization Plastic materials such as PP, PE, and PS are widely available in municipal solid wastes (MSW). Landfill is a popular option for MSW and plastics disposal, but it results in severe pollutions. On the other hand, plastics contain very high energy and are rich in elemental hydrogen. The recovery and/or reuse of the energy in waste plastics are desirable instead of landfill disposal. Co-gasification of plastics with biomass has many advantages: (1) Reducing plastics pollutions caused by landfill and recovery plastics energy (2) Increasing energy density of gasification feedstock and reducing biomass cost (3) Promoting the elemental hydrogen content in biomass feedstock and favoring H2-rich syngas production (4) Ensuring stable supply of biomass and eliminating fluctuation with seasons (5) Avoiding operational problems such as feeding difficulty and large amounts of black fines caused by using only plastics

7.5.2 Synergetic interactions during co-gasification When co-gasification of biomass and plastics is conducted, synergetic interactions among intermediate species derived from plastics and biomass will play important roles in promoting the properties and quality of the final gaseous products. The adoption of mixed feedstock produces more types of free radicals during gasification; the interactions between these radicals and steam largely enhance reforming reactions. Compared with plastics, the lower stability of biomass affects the radical degradation and promotes polymers’ degradation. Bio-char also has positive effects on the decomposition of plastic polymers, steam reforming and tar reduction. The decomposition temperature of plastics is lowered under co-gasification condition. Besides, when fluidized bed is used as the gasifier, the enhanced contact between plastic volatiles and bed materials is also beneficial to catalytic reduction of gasification tars.

7.5.3 Main factors influencing co-gasification (1) Temperature Increasing reaction temperature is beneficial to enhance tar decomposition and reforming reactions, resulting in decreased concentrations of hydrocarbons and increased H2 concentration. When reaction temperature was elevated from 740 to 880 °C, the gas yields of co-gasification of pine and 10% of PE were doubled and energy conversion increased by 40%. When pine was mixed with 90% of PE, the conversion of mixtures to gaseous products was up to 90% [23].

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(2) Plastics’ share in mixtures The amount of plastics added into biomass apparently affects the syngas components. When only pine wood was gasified, the CO concentration was much higher than that of H2 (about 35%). Adding 10% of PE resulted in similar concentrations of CO and H2. If 20–40% of PE was mixed with biomass, H2 concentration (larger than 50%) would be higher than that of CO [23]. PE decomposition and steam reforming both promote the concentration and yield of H2. (3) Steam-to-feedstock ratio Steam-to-feedstock ratio has minor influences on gasification than reaction temperature. Increasing steam mass flow will enhance WGS reaction, resulting in decreasing CO concentration and increased concentrations of H2 and CO2. The increment of steam flow also benefits reforming reactions of CH4 and heavier hydrocarbons. Concentrations of CH4 and heavier hydrocarbons declined with reasonable increase of steam mass flow. However, excessive steam plays minor effects on reforming reactions and causes severe energy consumption. (4) Pelletization Pelletization is a useful strategy to improve the ease and reliability of fuel feeding and to avoid segregation of feedstock mixtures in hoppers and feeding devices. Pelletization also improves the energy density and fuel characteristics. Taking advantages of pelletization, co-gasification of biomass and plastics is able to run smoothly and avoid problems in fuel handling and metering devices. Moreover, fluctuation in feeding rate and agglomeration problem in hot parts of feeding system can be avoided in any operation conditions [23]. (5) Catalysts The addition of catalysts can enhance reforming and WGS reactions and reduce the formation of tar species during co-gasification. The yield of H2 and the H2-to-CO ratio in syngas are both increased in the presence of catalysts.

7.6 Typical industrial applications of biomass gasification 7.6.1 MILENA gasification technology in the Netherlands MILENA gasifier was mainly developed by the Energy research Centre of the Netherlands (ECN). The gasification system adopts two fluidized bed reactors [27]. Figure 7.7 shows the diagram of an industrial-scale plant with 800 kWth capacity,

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which is being operated in ECN. A concept of 10 MWth demonstration plant based on MILENA gasifier has also been proposed (Fig. 7.7).

Fig. 7.7: A diagram of MILENA gasification system in Netherlands.

Two fluidized beds are used as the reactors. One is the gasification reactor, which is operated as a fast fluidized bed. The other is the combustion reactor that runs as a bubbling fluidized bed. The gasification reactor runs as a riser where biomass particles are entrained together with the bed materials of sand and olivine. After the riser, bed particles are separated from gaseous products and fall into the bubbling fluidized bed through the downcomer section. This facility requires less steam and can reach higher cold gas efficiency of gasification. But the residence time of gaseous products and catalysis bed materials is relatively short because gasifier operates as a fast fluidized bed. Consequently, the contact time between gasification volatiles and catalysis bed materials is quite limited, which is not beneficial to reduce tar content in gaseous product. When Austrian olivine was used as bed materials, the typical syngas components of MILENA steam gasification are presented in Tab. 7.3. It is seen that the concentration of H2 in syngas is about 27–28% in volume (at 800 °C). Tab. 7.3: Typical product gas compositions for MILENA gasification. Components

Values

Units

H

–%

m/m

CO

–%

m/m

CO

–%

m/m

CH

–%

m/m

Tars

–

g/m

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A so-called OLGA system is used for syngas cleanup where tar species are removed through an oil scrubber. The obtained syngas can be combusted as fuel gas in a boiler. An additional tar adsorption and regeneration system can be employed for further removal of tar components, by which a stream of clean gas can be produced for engines or turbines. The sensible heat of high-temperature syngas and flue gas is recovered by circulating water to generate steam, which can be used as the gasification agent.

7.6.2 A 8 MWth dual fluidized bed gasification plant in Vienna University of Technology, Austria The concept of dual fluidized bed (DFB) gasification was first proposed by TU Wien, Austria [27]. The DFB gasification system was developed in the 1990s by Vienna University of Technology, Austria. A pilot plant of 100 kWth was initially constructed and tested within the framework of EU-Project [20]. Afterward, an industrial-scale gasification plant of 8 MWth was implemented in 2001 in Guessing, Austria [21]. The industrial plant is successfully used for decentralized energy supply and demonstrates the feasibility of DFB gasification system. This facility has been operated for more than 15 years and used for various types of gasification tests such as conventional biomass steam gasification, CaO-based biomass calcium looping gasification, and co-gasification of biomass and plastics, and it accumulates important scientific knowledge and industrial experience for future development of DFB technology. (1) Conventional biomass steam gasification Figure 7.8 shows a diagram of the DFB gasification system. A large amount of biomass steam gasification tests have been performed using this system (Fig. 7.8). This DFB system is different from the MILENA gasification technology in the Netherlands. The gasifier is operated as a bubbling fluidized bed with steam as gasification agent. The lower gas velocity in bubbling fluidized bed increases gas reactants’ residence time. The combustor is separated from the gasifier, which runs as a fast fluidized bed. The gasification char and bed materials are transported together to the combustor, where the bed materials are heated by the released heat from char combustion and then sent back to gasifier by a circulation unit. The fluidized loop seals ensure the separation of gasification syngas from combustion flue gas. The produced syngas is free of nitrogen and the heating value is higher than 12 MJ/m3. Olivine is adopted as bed materials for biomass steam gasification and its advantages include: (1) It is a suitable material as heat carriers; (2) Olivine can be used as catalysts to enhance steam reforming reactions of hydrocarbons and tar species as well as WGS reaction. Table 7.4 shows the typical syngas components of DFB gasification. It is seen that the ratio of H2 to CO is within the range between 1.5:1 and 2:1, which indicates that the generated syngas is quite suitable for methanol or FT synthesis.

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Fig. 7.8: A scheme of the dual fluidized bed gasification system in Guessing, Austria.

Tab. 7.4: Typical product gas compositions of dual fluidized bed for conventional biomass steam gasification. Components

Values

Units

H

–%

m/m

CO

–%

m/m

CO

–%

m/m

CH

About %

m/m

CH

–%

m/m

Tar

–

g/m

The gasification syngas is cooled to lower than 200 °C and then passes a filter where bio-char is separated from gaseous products. Afterward, syngas enters a scrubbing unit where water and tar species are cooled and separated. Rapeseed methyl ester is used to absorb tar species from the liquid mixtures. The obtained tar and biooil are not discharged as wastes, but are recycled back to the combustor where they are burnt to release heat for extra energy supply. The syngas after cleanup is introduced into gas engine to provide regional heat and power.

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The flue gas from the combustor passes through cyclones to separate ash. Then flue gas is cooled to recover sensible heat for generating steam, which is used for preheating air or regional heat supply. Flue gas filters are used to further separate fine ash. Design data of the DFB plant is presented in Tab. 7.5. Tab. 7.5: Design data of the dual fluidized bed gasification plant in Guessing, Austria. Thermal fuel power (basis LHV)

,

kW

Net producer gas power (basis LHV)

,

kW

Generator output

,

kW



kW

Net electric output

,

kW

Net heat production

,

kW

Electric consumption of the plant

(2) CaO-based biomass calcium looping gasification CaO-based biomass calcium looping gasification has also been successfully conducted on the 8 MWth plant without modifications to the devices. Pretreated limestone was used as bed material in place of olivine. Operation conditions were also different from conventional steam gasification and they were compared in Tab. 7.6. During CaObased biomass calcium looping gasification, a certain temperature difference between gasifier and combustor is necessary to afford the reactions of carbonation or calcination. In order to adapt this temperature difference, the circulation of the bed material between two reactors was kept at a low level and reduced by a factor of about 10. Tab. 7.6: Comparison in operation conditions of CaO-based chemical looping gasification with conventional gasification in 8 MWth dual fluidized bed gasification plant. Chemical looping

Standard gasification

Gasification

– °C

– °C

Combustion

~ °C

~ °C

Bed material

Limestone

Olivine

Tar concentration in the raw syngas prior to the scrubber was apparently reduced to only about 1 g/Nm3. Table 7.7 shows the product gas composition during steady operation at an average gasification temperature of 675 °C. The H2 concentration increased up to 50 vol% of dry syngas, which was attributed to enhanced WGS reaction caused by CO2 absorption through CaO carbonation. The concentrations of

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Tab. 7.7: Product gas compositionsa during steady state of CaO-based chemical looping gasification in 8 MWth dual fluidized bed gasification plant. Component vol % dry a

H

CO

CO

CH

CH

CH

CH

.

.

.

.

.

.

.

Low amount of N2 and O2 (

By-product**

* The price of hydrogen is highly dependent on raw material availability; significant differences exist among the countries. The carbon emission trading system (ETS) is not included in the prices presented. ** In the case of chlorine and caustic soda production, hydrogen represents a by-product. It is not, therefore, a technology designed primarily for hydrogen production and it is listed here only because of its significant production capacity share. The price of produced hydrogen strongly depends on the sales conditions of the primary products.

This table highlights the three predominant technologies today, namely, natural gas steam reforming, hydrocarbon partial oxidation, and coal gasification. These technologies are all based on fossil fuels. The economic profitability of the individual technologies is directly related to local accessibility of the necessary raw material and to national regulations. The evident economic dominance of fossil fuel-based technologies has had a detrimental effect on research and development in the field of water electrolysis. Thus, electrolysis cells currently used in industry were designed before 1970 [4]. At the time, and in agreement with the conditions of their foreseen application, the emphasis was mainly on the robustness of the cell during long-term operation under optimum load. Renewed interest in water electrolysis technology came later. Initially, of all the hydrogen technologies, fuel cells experienced the most intensive development. The motivation for fuel cell development, except highly specific applications like space

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programs, originated from the oil crisis in 1973. Alternative energy carriers were sought to reduce the dependence of industrial societies on oil imports from politically sensitive regions of the globe. Only later, with the rapid development of renewable energy sources, like wind and photovoltaics, did water electrolysis become the pivotal technology of the entire scheme of the “hydrogen economy.” The first idea consisted in converting the excess electrical energy produced by intermittent renewable sources into hydrogen energy. It can be used either as a green fuel for cars powered by fuel cells, or stored and converted back into electrical energy in periods of need. With the gradually increasing importance of climate change and the growing need to rapidly reduce CO2 emissions, the idea of the hydrogen economy, including advanced water electrolysis, has become more and more attractive. All the above-mentioned challenges clearly indicated the need for a new generation of water electrolysis satisfying different requirements than the traditional ones. One of the crucial issues is high flexibility, allowing a process to cater for the intermittent production of renewable energy sources. The second issue is to increase the efficiency of the electrolysis and energy density, saving both the capital expenditure (CapEx) and operating expenditure (OpEx) of the technology. Originally, the main hopes were pinned on a new type of water electrolysis process utilizing a proton-exchange polymeric membrane based on perfluorinated sulfonated acid (PFSA), referred to as PEM technology. The first electrolysis cells were developed in the 1980s directly following the development of the above-mentioned PFSA material. The first reported cells showed very promising properties for all the above-mentioned aspects [5], that is, efficiency, energy density, and flexibility, and rapidly became the hot topic of research and development. Later on, the third important water electrolysis technology came into play, namely solid oxide steam electrolysis (SOEC), based on solid oxide fuel cell (SOFC) technology. This hightemperature process was introduced in the 1970s. The high theoretical efficiency was attractive for companies, such as Westinghouse in the USA [6] and Dornier in Europe [7]. The solid electrolyte used in this type of cell is based on oxide ceramics conductive for oxygen anions. It utilizes a typical electrolyte based on ZrO2 stabilized by the addition of Y2O3. This material becomes ionically conductive at temperatures above 600 °C. The main accelerator of the development of this technology was originally its possible combination with high-temperature nuclear reactors in the framework of the HOT ELLY project [7]. A brief summary of all three above-mentioned water electrolysis technologies is provided in Tab. 8.2. As can be seen from this comparison, alkaline technology formally represents the best option in terms of a compromise between the CapEx and efficiency of the process. Besides the investment costs, another important consideration is process flexibility which is required in combination with renewable power sources. From this point of view, PEM technology is able to provide a significantly faster response to changes in

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Tab. 8.2: Overview of water electrolysis technologies currently considered as promising candidates for energy conversion processes [1]. Water electrolysis technology

Operating temperature (°C)

CapEx () (EUR/kWel)

Efficiency related to LHV (%)

Alkaline electrolysis

–

–,

–

PEM electrolysis

–

–,

–

–,

,–,

–

SOEC (solid oxide)

cell load and thus represents a more suitable option. Due to the high operational temperature, SOEC is characterized by the highest efficiency of the process. Additionally, it is distinguished by its ability to operate in a reversible mode, that is, both as a water electrolysis cell and as a fuel cell. On the other hand, the flexibility of this technology is extremely limited. It requires keeping the operating temperature constant; therefore, a complete shutdown represents a very limited option. For all the technologies discussed, it is crucial from the point of view of process economy to operate it under full load for as high a share of its lifetime as possible. It is considered that the cells need to be operated for at least a minimum of 2,500 h/year. In connection with the policy of decarbonizing energy, transportation, and industry; however, these aspects have to be considered from a different perspective. With the new innovation plan for the European economy within the framework of the European Green Deal [8] policy this becomes especially attractive. Water electrolysis technology offers not only the possibility of green energy conversion and storage, but also one of the few options to convert green, renewable energy directly into raw materials for subsequent industrial technologies. At the same time, it offers an opportunity to convert CO2, captured either from the atmosphere or from technological streams, into synthetic gas and, in the next step, via Fischer-Tropsch synthesis into useful chemicals. This represents a significant advantage. At the same time it is, of course, important to ensure that all processes remain competitive, at least within the regulatory framework. Further technological advances and optimization of water electrolysis technology and the economic parameters are, therefore, the main task for research and development in the coming years. The aim of this chapter is to summarize the main aspects of this endeavor.

8.2 Theoretical background Prior to a more detailed discussion of the individual water electrolysis technologies, it is important to summarize the fundamentals in order to better understand the approaches discussed and their limitations.

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8.2.1 Thermodynamics of the electrolysis cell Electrolytic water decomposition is described by the overall reaction given as follows: 1 H 2 O = H 2 + O2 2

(8:1)

Individual electrode reactions depend on the type of electrolysis and are not listed here. Reversible voltage of the cell, that is, the minimum voltage at, or above which, the water decomposition process is possible from a thermodynamic point of view, can be expressed as the difference between the reversible potentials of the cathode and the anode. This can be written in a general form as follows: ΔEd,rev = Erev.cathode − Erev,anode

(8:2)

Value of ΔEd,rev can be obtained using the change in Gibbs free energy connected with the water decomposition process as follows: Δr G = − nFΔEd,rev

(8:3)

where n stands for the number of electrons transferred in the reaction. Under standard conditions (293 K and 101.3 kPa), individual variables take the following values. The change in reaction Gibbs free energy Δr G0 is equal to 237.2 kJ/mol for the water splitting reaction. After substituting corresponding values for the equation, 0 = −1.229 V. Since the cell voltage of the electrolyzer (Ucell,rev) is the value of ΔEd,rev generally given as a positive value, it can be written as 0 0 = − ΔEd,rev = 1.229V Ucell,rev

(8:4)

As follows from the second law of thermodynamics, reversible energy is not the only type involved in the process, and the energy needed to achieve the reaction is, in general, different from ΔrG. The entire energy needed is called the reaction change of enthalpy (ΔrH) and it is defined as Δr H = Δr G + TΔr S

(8:5)

For the water electrolysis reaction under standard conditions ΔrH = 285.8 kJ/mol (at 293 K and 101.3 kPa). The difference to the ΔrG value provided above corresponds to the change in system entropy. This energy can also be supplied to the system in the form of heat. In reality, however, the electrolysis is typically slowed down by the kinetics of the electrode reactions unless very high temperatures are used. Therefore the cell voltage used is typically higher than Ucell,rev. Every form of electric energy delivered to the system or consumed by the system above the value corresponding to ΔrG or to Ucell,rev is converted into heat. 0

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The issue of the electrode reaction kinetics will be discussed in more detail in Section 2.2 of this chapter. Here, the impact of the operational temperature on the reversible, that is, on ΔrG, and total, that is, on ΔrH, value of energy needed to perform water electrolysis will be evaluated. Considering ΔrS = 163.16 J/mol K for the liquid water and 44.38 J/mol K for the steam and water evaporation heat ΔHev. of 40.6 kJ/mol (at 100 °C), the following dependence of the process thermodynamic quantities on the operational temperature is obtained.

Fig. 8.1: Thermodynamics of water electrolysis in relation to the system temperature (pressure 101.3 kPa, activity of all components equal to 1).

In the case where all the energy needed to decompose the water molecule is supplied in the form of electric energy, eq. (8.3) has to be modified. By substituting ΔrG for ΔrH, instead of reversible cell voltage ΔEd, rev the value of the electrode potential difference, corresponding to the thermoneutral operation of the cell ΔEd, tmn is obtained. This can now be converted into thermoneutral voltage UTMN using eq. (8.4) by replacing ΔEd, rev by ΔEd,tmn . The thermoneutral voltage corresponds to the cell voltage value at which, from a thermodynamics point of view, all the energy needed to decompose the water molecule is delivered in the form of electric energy and during the process the cell does not exchange any energy with the surroundings. If the voltage value is lower than this value, the cell has to absorb heat from the surroundings. Conversely, if the voltage is higher, the cell has to dissipate heat into the surroundings. Thermoneutral voltage significantly differs for liquid water or steam used as a starting medium. The resulting difference of approx. 200 mV then corresponds to the energy needed to change the water state from liquid to gaseous. It is at the same time identical to the difference between the low heating value (LHV) and the high heating value of the hydrogen energy content.

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8.2.2 Kinetics of the electrode reactions The previous section 2.1 of this chapter dealt with the equilibrium properties of the water electrolysis cell. Under equilibrium conditions; however, no macroscopic current flows through the cell and there is no production of hydrogen, or any other products, taking place. In order to trigger the desired reaction, a corresponding driving force has to be put in place. As follows from general chemical kinetics, the reaction is moved out of equilibrium by a change in activity of at least one of the reactants or products. In the case of an electrode reaction, electrode potential is directly related to the “activity” of the electrons at the electrode-electrolyte interface. The extent of the change in electrode potential is called overvoltage (η) and is defined by the following equation: η = Ej − Erev

(8:6)

Here Ej corresponds to the potential of the electrode under current load and Erev stands for the potential of the electrode in equilibrium, that is, under open-circuit conditions. Overvoltage represents an irreversible contribution to the energy costs of the cell operation which explains the copious efforts to reduce its value to a minimum. The main approaches to achieving this task are: (i) Selection of appropriate reaction system (ii) Utilization of an active electrocatalyst/electrode material (iii) Increase the operational temperature Considering the two desired electrode reactions of water electrolysis, that is, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), the latter represents the rate-determining step with sluggish kinetics. On the other hand, the HER is considered to be a relatively rapid reaction. Taking into account the finitely fast electrode reaction kinetics, the real voltage needed to operate the electrolysis cell under current load can be described as follows:   (8:7) Ucell,j = Ucell,rev + ηanode + ηcathode  + IRcell The last term of this equation is directly related to voltage losses on the ohmic resistance of the cell. Although it includes all cell components, including an external electric circuit, the dominant one is typically the ohmic loss on the electrolyte. The extent of the irreversible losses determines the voltage efficiency of the water electrolysis process: χU =

Ucell,j Ucell,rev

(8:8)

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When considering thermoneutral cell voltage UTMN, defined above as the reference one, eq. (8.8) can be reformulated: χU,THM =

Ucell,j UTMN

(8:9)

This now defines the thermoneutral electrolysis efficiency. It considers the entire ΔrH necessary to decompose the water molecule to be supplied in the form of electrical energy. As already mentioned, an important aspect regarding the electrode reaction kinetics is the operating temperature. Although ΔrH increases with operating temperature, see Fig. 8.1, a positive effect of rising temperature on the electrode reaction kinetics prevails. The overvoltage terms ηanode and ηcathode decrease rapidly with increasing temperature. In the case of high-temperature SOEC electrolysis, the temperature is sufficiently high to reduce the cell operating voltage to below UTMN. Hence, part of the ΔrH is delivered to the cell in the form of heat, resulting in a value of χU,THM exceeding 1. This is one motivation to further develop the high-temperature water electrolysis process. The second contribution determining the efficiency is Faradaic efficiencywhich corresponds to the volume of hydrogen produced in relation to the electric charge consumed: .

χI =

nH2 I 2F

nH = Rτ 2 0

I dτ

(8:10)

2F

Faradaic efficiency losses are usually considered negligible compared to voltage efficiency. The reason is that the first type of efficiency mentioned is related more or less exclusively to the penetration of the product, that is, hydrogen, into the counter-electrode reaction compartment. This is, therefore, connected not only with a loss of product, but more importantly, with the potential formation of an explosive mixture of oxygen and hydrogen, if the hydrogen content in the oxygen exceeds 4 vol%. All precautions have to be taken in the cell construction to avoid such an event and to keep the penetration of hydrogen into the oxygen stream to a minimum value. The second mechanism leading to Faradaic efficiency loss is the occurrence of parasitic current. This is directly related to the cell design. It is only relevant in the case of a bipolar electric connection to the cell. This undesired effect only arises if electrically conducting pathways exist which permit the occurrence of such a parasitic current. This is typically connected with the feeding and collecting channels of the circulating medium/electrolyte. Of the three types of water electrolysis process, therefore, it only concerns alkaline water electrolysis. The remaining two technologies (PEM and SOEC) do not use an electrically conducting medium in these parts of the cell. This undesired effect becomes particularly important at low current loads.

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8.2.3 Fundamentals of the electrolysis cell construction Although the individual water electrolysis technologies differ in numerous aspects, their construction is based on a plate-and-frame arrangement. This follows from the fact that the preferred electrical connection of the cell is the bipolar one. This requires the individual cells to be electrically connected in series. A set of individual cells connected in this way is called an “electrolysis cell stack.” The basic structure of such a stack is schematically shown in Fig. 8.2.

Fig. 8.2: Schematic sketch of the electrolysis cell stack. n – repeating unit in the stack.

Although the general scheme of the cell construction and principles of system design and control is valid for all three process types, the individual cell and system components may differ significantly according to the requirements imposed by each individual type of electrolysis. This will be discussed in greater detail in the next chapter focusing on the individual water electrolysis technologies.

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8.3 Types of water electrolysis processes The basic classification of water electrolysis technologies into three fundamental types is provided in Tab. 8.2. In this chapter, the aim is to provide a more detailed characterization of the individual types of water electrolysis processes.

8.3.1 Alkaline water electrolysis As already mentioned, the alkaline water electrolysis process utilizes a circulating liquid electrolyte, typically based on concentrated KOH or NaOH solution. This represents a traditional approach to water electrolysis technology. The first industrial cells were set up at the end of the nineteenth century. Due to material constraints, a porous diaphragm was used to separate the cathode and anode electrode compartments and thus to prevent a mixture of hydrogen and oxygen from forming. A concentrated caustic solution has to be used as an electrolyte to provide sufficient ionic contact between the electrodes. The electrolyte is streamed from the bottom to the top of the vertically oriented electrodes to carry away the gases produced and thus to reduce the partial pressure of the gases at the surface of the separator and, therefore, their penetration into opposite compartments. At the same time, it reduces the content of the gaseous phase in the electrolyte, decreasing its active cross-section. It thus reduces the electrolyte ohmic resistance caused by an accumulation of gaseous phase in the intra-electrode space. Such an arrangement predetermines some aspects of cell construction and operation, especially the following ones. First, the porous separator strictly demands symmetric pressure on both sides, that is, identical pressure is required in both electrode compartments. The appearance of any pressure difference leads to an excessive cross-flow of the electrolyte, including a dissolved and dispersed gas phase. This is an extremely undesirable effect both from a safety and from an economic point of view. Therefore, the supply of electrolyte to the cell is provided from a common reservoir in order to ensure the symmetric pressure requirement. In addition, absolute symmetric operating pressure is limited, since it is connected with an increase in the amount of gases dissolved in the electrolyte and thus in their penetration into the opposite electrode compartments. A further aspect is the distance between the electrode and porous separator of the electrode compartments, discussed in the previous paragraphs. This necessarily entails additional limitations and automatically leads to the need to utilize highly conductive liquid electrolyte in order to reduce ohmic cell voltage losses. Another aspect impacting the electrolyte conductivity is the operating temperature. Elevated operational temperature increases electrolyte ionic conductivity. At the same time, it reduces its viscosity and thus pumping costs. Last, but not least, an elevated

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341

temperature and low electrolyte solution viscosity allow fast separation of the gas phase from the liquid one. At the same time, it reduces the solubility of the produced gases in the electrolyte solutions and hence its losses. However, there is an important negative aspect connected with an operating temperature in the order of 80 °C. It is either limited flexibility of the cell needing substantial time to warm up its bulk before reaching optimal operating conditions, or excessive energy losses connected with maintaining an operating temperature during periods with low operational intensity. The approach chosen during the last decade to minimize the impact of the above aspects on cell flexibility consists in reducing the cell and stack dimensions, in contrast to traditional industrial technology. Smaller cell dimensions together with advanced porous separator materials enable not only the distance between the electrodes and separator to be reduced, but also the time needed to heat it up to optimum operating temperature. An overview of the main suppliers of alkaline electrolyzers with unit capacity exceeding 1 MW electricity is summarized in Tab. 8.3. Information on utilizing water electrolysis as a hydrogen source in traditional industrial technologies is not readily available. This is mainly because there is only little or no public funding. Nevertheless, the case of the following installations provides a good idea of the general situation. Industrias de Aceite Fino, part of the Romero Group in Peru, is an industrial manufacturer of edible oils and fats, margarine and soap. This company traditionally utilizes electrolytically produced hydrogen. In 2012 the company modernized its hydrogen-production technology. Five smaller electrolyzers originating from different manufacturers were replaced by an overhauled NEL A•300 (300 Nm3/h) plant from NEL Hydrogen. Interestingly, the new electrolysis technology was originally in operation at another Peruvian factory [20]. Another interesting example comes from the petrochemical industry. Reliance Industries Ltd (RIL) is the largest private sector company in India with activities in a vast number of sectors, such as oil and gas refining, petrochemicals, textiles, retail, and communication. Since 1996 RIL have purchased four electrolyzer units from NEL Hydrogen. The first three are installed in Hazira, the last unit in Dahej, both in Gujarat State. The electrolyzers installed at RIL’s plants serve as a dependable backup and alternative hydrogen source for PTA (purified terephthalic acid) production [20]. In recent years, projects relating to hydrogen production from renewable energy sources have been booming, and this new trend is reflected in increasingly frequent reports on the use of alkaline electrolysis as a hydrogen source for hydrogen refueling stations [21] or for energy storage [22].

* Flexibility is defined as an operation range in which an electrolyzer operates economically and safely.

McLyzer-

. atm



EV 

(DE)

HT-Hydrotechnik []

. 



L-size

(CH)

Ener-Blue []

. 



(FR)

McPhy []

. 

. 





NH-

(CA)

NextHydrogen []

. 



. atm

. 

,



. 

,

. 

 atm

,

.

.

.

.–.

.–.

.–.

.

.–.

.

.

.

 –

 n.a.

 n.a.

– –

– –

– n.a.

 n.a.

– –

 –

 –

 –

Capacity Power pmax (bar) Energy consumption LHV efficiency (%) Flexibility* (%) (Nm/h) (MW) (kWh/Nm)



A

(IN)

MVS Engineering Pvt Ltd. []

S-

(NO)

(CH)

IHT []

FDQ-

NEL []

(CN)

THE []

CDQ-

AHD 

(CN)

PERIC []

MW

Airox Nigen Equipments Pvt Ltd. [] (IN)

(DE)

Country Type code

ThyssenKrupp []

Manufacturer

Tab. 8.3: Overview of alkaline water electrolyzer stacks of minimum size 1 MW and their manufacturers.

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8.3.2 Proton-exchange membrane water electrolysis Proton-exchange membrane (PEM) water electrolysis represents the most innovative type of water electrolysis applied on an industrial scale. Most of the installations currently operated represent technology demonstrations. This is a vital step in identifying the advantages and drawbacks of existing technological solutions in order to make a selection both for further development and for the design of specific applications. The development of PEM water electrolysis is closely connected with the discovery of the PFSA ionomer and the related membrane. Due to its extraordinary properties, this material paved the way for a new approach to the construction of cells based on a zero-gap arrangement. With this set-up, electrodes are attached directly to the surface of the polymer electrolyte by which the solid electrolyte and the electrodes are united in one composite. This arrangement has several advantages, of which the main ones are: – Distance between the electrodes, and thus the ohmic resistance of the electrolyte, is minimized. – Presence of liquid electrolyte is theoretically not required, since all the charge transport by the movement of ions can be provided by the polymer electrolyte. – Accumulation of the gas phase, produced during electrolysis, in the space between the electrodes is excluded, which has a positive impact on ohmic loss reduction and on the homogeneity of the electrolysis cell operation. – Presence of a dense, non-porous separator enhances the safety of the process and allows differential pressure to be applied between the electrode compartments. – Phenomena on the liquid electrolyte-solid separator interface are eliminated, thus preventing potentially undesired effects, such as mass transfer limitation. As already mentioned, this approach is enabled by the specific properties of the PFSA material, including high ionic conductivity, high mechanical stability, and chemical resistance. Zero-gap cell design was first developed for application in fuel cell technology, where, especially due to the fact that gaseous reactants are used, it offers important advantages. The design of gas diffusion electrodes becomes significantly simpler. At the same time, the issue of circulating and treating liquid electrolyte is eliminated. At first sight these advantages are not as evident for the water electrolysis process. Nevertheless, it quickly becomes obvious that the benefits reaped by the application of PFSA are significant. Fundamental aspects of the PEM WE cell and stack are based on the knowledge acquired during the development of PEM fuel cells. A sandwich structure of the membrane-electrode assembly (MEA) is used which, in combination with bipolar plates, forms the repeating unit of a single cell. The main differences consist in the construction materials used and in the geometry of the flow fields. The selection of construction materials, especially on the anode side, is impacted by significantly

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higher anodic potentials used during operation, which is related to the instability of carbon-based materials. The geometry selection is influenced by the nature of the streaming medium. The fact that the streaming medium is water, instead of gases which serve as reactants in PEM fuel cells, changes the situation substantially. The first consideration is the question of the construction materials. To satisfy the stability condition on the anode side, gas diffusion or, in the case of PEM WE, the porous transport layer, is typically based on titanium in the form of felt, sintered particles or mesh [23]. Unlike the fuel cell, here the porous transport layer is in direct contact with the catalyst, the reason being that the microporous layer known from fuel cell technology is not available for PEM WE due to material restrictions. The second consideration concerns the catalyst and the composition and production of the catalyst layer. IrO2 represents the state-of-the-art anodic oxygen evolution catalyst because oxides covering Pt in the domain of the OER do not have suitable catalytic properties for the desired reaction. Once again, due to the absence of suitable supporting material, a catalytic layer characterized by a highly developed surface is not used. A very thin layer of IrO2 layer is deposited directly either on the membrane or on the porous transport layer surface, see Fig. 8.3.

Fig. 8.3: SEM picture of (a) blank Ti porous transport layer and (b) the same layer activated by IrO2.

The absence of a microporous, sufficiently thick and electrically conductive catalyst layer has a considerable impact on the anode properties. Electric contact between the porous transport layer and the catalyst and/or the membrane is affected only through a relatively small area represented by point contacts. As mentioned above, two strategies are possible: (i) a catalyst-coated membrane (CCM) and (ii) a catalyst-coated electrode (CCE) [24]. Since demineralized water is circulated as a reactant through the cell, the CCM option is clearly more efficient. IrO2 is a good electron conductor and, although it is present in a very thin layer on the top of the membrane, it provides a satisfactory distribution of electric charge from the contact points with the porous transport layer. At the same time, it is in direct contact with the membrane; therefore, ionic contact with the electrolyte is

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instantaneous. The main problem in this case is the dimensional changes of the membrane connected with its swelling that can result in disintegration of the catalyst layer and thus endanger the electric charge distribution. On the other hand, the CCE alternative ensures stable electric contact from the porous transport layer to the catalyst, independent of the electrode dimensions, degree of swelling, and so on. An important factor, however, is the ionic contact between the membrane and the catalyst. The contact points only represent a small portion of the membrane area. Since circulating demineralized water does not provide a conductive pathway, most of the catalyst is in contact with the membrane through the narrow film of PFSA binder fixing its particles on the surface of the titanium fibers or particles of the porous transport layer. PFSA is a far inferior electric charge conductor than IrO 2. Therefore, only a small part of the catalyst and membrane surface is active. The stability issue also concerns the bipolar plate on the anode side. Stainless steel covered by a protective coating to ensure its corrosion stability is one choice. Titanium offers greater stability, but at a higher cost. At the same time, it is important to bear in mind the fact that, as a valve metal, titanium is covered by a dense passive layer responsible for important contact resistance hindering electric charge transport. This concerns both the porous transport layer and the bipolar plate. Suitable surface modification is necessary to eliminate this undesired effect. Often Ti plating with a Pt group metal is used. This clearly has a negative consequence for the cell’s CapEx. Regarding the construction of the bipolar plate, the effect of circulating water as a reason for the change in the flow field geometry has already been mentioned. This is due to the fact that water viscosity is significantly higher than that of the gaseous reactants used in the PEM FC. At the same time, however, the high heat capacity of water circumvents the need for any kind of temperature control to be integrated into the stack. This can be accomplished via a corresponding heat exchanger forming part of the hydraulic circuit outside the cell. In summary, all these aspects have an important outcome. The stack as such is compact and lowweight when related to its electric power input/hydrogen production capacity. At the same time, the circulation rate of demineralized water as a reactant can be controlled, provided that the removal of gaseous products is sufficiently effective. This, together with the relatively low viscosity of demineralized water, results in high flexibility of the system. The stack load can be changed quite rapidly with no significant impact on electrolysis efficiency and stack life-time. This is what makes this technology very attractive regarding applications in connection with renewable energy sources which are characterized by intermittent operation. Intensive research activities have resulted in the emergence of a series of application-ready units. In this chapter, the focus is on electrolyzers exceeding a capacity of 1 MW. The rationale, similar to the section devoted to alkaline water electrolysis, is to filter out systems used predominantly as local hydrogen generators and to concentrate mainly on cells suitable for application on an industrial scale. Currently available cells, together with their manufacturers, are listed in Tab. 8.4.

(CA)

(GB)

(NO)

(US)

(DE)

Hydrogenics []

ITM Power []

NEL (Proton OnSite) []

Plug Power (Giner Inc.) []

Siemens AG []

SILYZER 

Allagash

MC

HGASXMW

HyLYZER ,

Type code

 

  .



,

,



Power (MW)

,

Capacity (Nm/h)











pmax (bar)

.

.

.

.

.–.

Energy consumption (kWh/Nm)

* Flexibility is defined as an operational range in which an electrolyzer operates economically and safely.

Country

Producer name

Tab. 8.4: Overview of PEM water electrolyzer stacks of a minimum size of 1 MW and their manufacturers.









–

LHV efficiency (%)

–

–

–

n.a.

–

Flexibility* (%)

346 Martin Paidar, Karel Bouzek

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In the case of PEM water electrolysis, commercial applications are not yet sufficiently developed and the majority of installations are based on projects co-financed by public funding. Nevertheless, these installations well document the fact that this technology has already reached a significant level of maturity. At the same time, they serve to collect the operational data necessary to develop a new cell generation and thus to increase the economic competitiveness of this technology. Of the demonstration projects, H2FUTURE, for example, is testing PEM electrolysis technology on an industrial scale (6 MW). The main focus is on simulating rapid load changes in electricity generated from intermittent renewable energy sources connected with electric arc steelmaking furnaces (grid balancing). At the same time, the hydrogen generated can be used as a reducing agent in the process [30]. Another example of a high capacity industrial PEM water electrolysis installation is the REFHYNE project [31]. Within the framework of this project SHELL is currently collaborating with ITM Power to build the world’s largest PEM water electrolysis plant with a peak capacity of 10 MW at Shell’s Rhineland refinery. The hydrogen produced is planned to be used for the processing and upgrading of products at the refinery’s Wesseling site as well as for testing the technology and exploring applications in other sectors. These are just a few representative examples of ongoing projects and of the capacity of PEM electrolyzers available for industrial applications.

8.3.3 Anion-exchange membrane water electrolysis The previous two chapters very briefly discussed two of the most mature water electrolysis technologies, together with their main characteristics. A comparison of these two technologies naturally leads to the idea of combining their individual advantages. The principle entails using an alkaline anion-selective polymer membrane and corresponding binder in the electrolysis cell instead of PFSA. An alkaline environment allows working with much more abundant materials, similar to alkaline electrolysis. On the other hand, membrane technology theoretically enables working with demineralized water and a zero-gap cell design and is thus more flexible and efficient. The main problems encountered are related to the alkaline polymer electrolyte. The first challenge is the absence of a stable alkaline polymer electrolyte satisfying requirements related to ionic conductivity and mechanical properties [32]. Currently, there are two main strategies to overcome this problem. The first is based on the development of anion-selective polymer electrolytes. This type of ion-selective polymer is typically based on a quaternized nitrogen functional group which is sensitive to nucleophilic attack by OH− ions. To avoid this attack, structural modifications of the polymer are made to sterically hinder such an attack. Additionally, a relatively low operating temperature (lower than 50 °C) and possibly a low concentration of OH− ions in the circulating medium serve to keep the polymer stable. A second strategy is based

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on utilizing solvating membranes which allow the use of a high concentration of alkaline electrolyte solution because of the absence of classical functional groups. Both strategies are suitable for zero-gap cell design. Both approaches, however, have drawbacks. As already mentioned, functionalized polymer electrolytes require a low concentration of liquid alkaline electrolyte in combination with a low temperature, typically below 50 °C, in order to prevent ionomer degradation. The first aspect is in line with the original plan, that is, to circulate only demineralized water through the electrolysis cell. The second aspect, that is, temperature limit, is, however, negative. Since an increase in temperature improves the conductivity of the polymer electrolyte and the electrode reaction kinetics, as high an operating temperature as possible is preferred. Ionic conductivity is also a limiting aspect with respect to the low concentration in the circulating medium. OH− represents the charge transferring ion in the polymer electrolyte. In general, its mobility in an aqueous environment has approximately half the value of that of H+. This has direct consequences on the conductivity value of a membrane with otherwise identical properties. Additionally, protons cause higher swelling of the membrane and, due to its unique internal structure, PFSA supports the high ionic conductivity of the membrane. It is, therefore, clear that anion-selective polymer electrolytes will be characterized by lower ionic conductivity. This does not represent an important issue solely with respect to ohmic loss on the membrane itself. This problem is more crucial in the case of catalyst layer production. With regard to CCE, a thin layer of binder does not provide sufficient ionic contact between the developed catalyst layer structure and the membrane itself, which results in a very low degree of electrode utilization and thus electrolysis efficiency. CCM seems more promising from this point of view. It is, however, important to note that it was only quite recently that the first successful trials to prepare CCM based on an alkaline polymer electrolyte were reported [33]. Moreover, non-platinum-group-metalbased catalysts are often characterized by relatively low electronic conductivity, which, once again, limits catalyst utilization and hence electrolysis efficiency. Therefore, currently the CCE approach utilizing diluted liquid electrolyte as the circulating medium represents the state of the art in this field. The change to deionized water and thus to a system with properties similar to PEM electrolysis is, at this stage, the midterm target of further research and development. Solvating membranes, on the other hand, require a relatively high concentration of alkaline electrolyte in the circulating medium. Problems concerning the degree of catalyst utilization have, to a considerable extent, been overcome. The main remaining challenge concerns the reduced flexibility of the system, since the conductivity and viscosity of concentrated caustic solution strongly depend on temperature. In addition, the separation of the gases produced from the circulating medium at a low temperature may pose a problem. Last, but not least,

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a concentrated caustic solution as the circulating medium imposes different requirements on the qualification of electrolysis cell operators and on safety measures. One last point should be mentioned at this juncture. When considering demineralized water (or even diluted caustic solutions) as the circulating medium in alkaline electrolysis, the stability of the materials used is often overlooked. Therefore, knowledge of non-platinum-group-metal materials utilizable under conditions of neutral pH in the alkaline water electrolysis cell is still extremely limited. This concerns not only the electrode reaction catalysts, but also the electrode substrates used. Despite its high application potential, this technology has not yet reached the stage of large demonstration installations, which is mainly because it is still a relatively new field of research. At the same time, it is also, to a certain extent, limited by materials research in the field of alkaline polymer electrolytes.

8.3.4 Solid oxide steam electrolysis Similar to PEM WE, solid oxide steam electrolysis or solid oxide electrolytic cell (SOEC) also builds on the knowledge generated historically during the development of SOFC technology. The motivation for the development of SOFC was a high operating temperature conducive to attaining high kinetics of the electrode reaction in the absence of catalysts based on precious metals and at the same time to burn low purity fuel, like natural gas reformate, or, under selected conditions, even hydrocarbons directly. Additionally, due to its high operating temperature, this type of fuel cell represents an ideal option for electrical energy and heat cogeneration, thus attaining much higher overall efficiency of the process. Once, the question of efficient water electrolysis became pressing, the use of SOEC for this purpose was obvious. The motivation was manifold. Once again, the first reason was based on the rapid electrode reaction kinetics and thus the possibility to avoid precious metals as electrocatalysts. The second aspect is closely related to the first one. Rapid electrode reaction kinetics allows cell operation in reversible mode, that is, as electrolysis as well as a fuel cell. This results in substantial CapEex savings in the case of energy storage and the production of regenerative systems. Rapid electrode reaction kinetics, high operating temperature, and transport of oxide ion across the electrolyte membrane allow direct syngas production by electrolyzing a mixture of steam and CO2. Again, this represents substantial savings compared to the low temperature approach leading to the need to split this process into two independent steps. The last of these main aspects is related to the thermodynamics of the water decomposition/formation system described by reaction (8.1) and discussed in section 2 of this chapter. This concerns a reduction of the reversible cell voltage with increasing temperature and thus with the possibility to replace part of the electrical energy needed to decompose water by heat.

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The SOEC process is typically operated at a temperature of around 800 °C. This is mainly on account of the materials used for the cell construction. Traditional yttria-stabilized zirconia (YSZ) is used as an electrolyte and Ni–YSZ cermet serves as a hydrogen electrode. The oxygen electrode is usually based on a perovskite-structured oxide ceramic material like La1−xSrxMnO3 (LSM). Several alternatives are possible to replace these materials by material more resistant to degradation processes or electrolytes with higher conductivity [34]. On the other hand, novel materials containing Sc and Gd show better performance, but are significantly more expensive. Proton-conductive ceramic materials manifesting certain advantages compared to oxide-conducting ceramics also exist. Nevertheless, their application in SOEC technology is still predominantly in the research stage. Two types of cells are distinguished depending on the construction of the MEA: (i) electrolyte-supported and (ii) electrode-supported cells. The main difference consists in the part of the MEA that provides it with mechanical stability. In the case of the electrolyte-supported cell, the electrolyte obviously plays the role of the mechanical support, thus it has to be at least 100 µm thick. A direct consequence is the relatively high ohmic resistance of such cells, resulting in the use of lower current loads. On the other hand, this type of MEA is mechanically more robust and exhibits a slower degradation rate. The second type of MEA, that is, electrode supported, typically utilizes a Ni–YSZ layer as the mechanical support of the MEA. It allows the YSZ electrolyte to be reduced to a thickness of 10 µm. The advantage of this approach is reduced ohmic losses and thus higher current loads. This advantage is compensated by lower mechanical stability (the supporting electrode phase is porous) and a higher degradation rate (the thin electrolyte results in a high percentage change of the properties, already caused by a relatively thin layer of degradation products). Furthermore, with respect to the SOEC stack construction, two fundamental options exist, namely (i) tubular and (ii) planar. The tubular form represents the traditional type that was originally developed by a Siemens-Westinghouse consortium [6] with the electrolyte taking the form of tubes. Electrodes covered the inner and outer surface of the tubes. This concept was used because it allows effective separation of the electrode compartments. It is, however, less attractive regarding the space it requires. For this reason, the second concept of stacked planar cells is nearly always selected for large SOEC stacks. High chromium steel plates serve as the interconnects (analogous to bipolar plates at alternative water electrolysis options). The high operating temperature, however, still poses a serious challenge to sealing the cell, as only a strictly limited number of materials satisfy the process requirements [35]. Despite the wide variety of SOFC stacks of various origins reported, the number of SOEC manufacturers is very limited. Currently available cells, together with their manufacturers, are listed in Tab. 8.5. In fact, only Sunfire GmbH offers a SOEC stack on a commercial base. Their stack is based on electrolyte-supported cells in a planar configuration.

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Tab. 8.5: Overview of SOEC stacks and their manufacturers. Manufacturer Country Type code

Sunfire []

(DE)

OxEon Energy, LLC. []

(US)

RSOC

LHV Flexibility* Capacity Power pmax Energy (Nm/h) (MW) (bar) consumption efficiency (%) (%) (kWh/Nm) 

.



.



−+

 .

n.a.

.



n.a.

* Flexibility is defined as an operational range in which the electrolyzer operates economically and safely.

In contrast to the low-temperature technologies, SOEC is still significantly less advanced in terms of large-scale industrial applications. That is why the GrInHy 2.0 project [38] is targeting scale-up of the cell to 720 kW. A pressurized SOEC stack enabling operation at a pressure of up to 30 bar has been developed under the Helmet project [39]. The main target of the project was to allow methanation of CO2 directly in the SOEC, thus rendering this power-to-X technology more cost-effective.

8.3.5 Recently developed alternative processes Alternative processes under development are currently based on the idea of utilizing a combination of the advantages of the above options, while avoiding their drawbacks. This entails utilizing temperatures high enough to abandon electrodes based on precious metals, while at the same time maintaining high electrode reaction kinetics and low construction material requirements. Operating temperatures in the order of 300–400 °C are under consideration. One typical obstacle for this technology is the availability of a corresponding electrolyte satisfying process requirements for this temperature range. The following two options reported in the literature are representative examples: (i) molten KOH fixed in a porous support [40] and (ii) molten dihydrogen phosphates of, for example, potassium [41]. To summarize, these alternative approaches are still in the basic research phase or in the early stages of transition to applied research. Therefore, they only represent potential options for a new generation of water electrolysis processes, since their way to application is still relatively long. They should, however, be considered for the mid- and long-term outlook.

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8.4 Perspectives for future development 8.4.1 Bottle necks of current technologies and their abatement The bottlenecks characteristic of the individual processes were discussed in the framework of the relevant chapters; therefore, only a general summary is provided here. The following main aspects are nowadays considered when optimizing the current state of the art: (i) economy, (ii) efficiency, and (iii) flexibility. CapEx and OpEx expenses can be consigned to the economy category [1]. CapEx is generally related to the costs of the construction materials. Although it seems to be obvious that technologies requiring the use of precious metals predominate, the situation is not so clear. Besides the unit costs of the materials used, the intensity of the process and thus the size of the unit required to achieve the desired production capacity need to be considered. With respect to CapEx, economies of scale are often mentioned as an important aspect leading to cost reductions. This argument does not contradict the CapEx part related to labor costs. When applied to large-scale series production, the product, that is, the electrolysis cell, becomes significantly less cost-intensive. On the other hand, in the case of critical raw materials, like precious metals, the effect may be the exact opposite on account of the limited resources. With increasing demand, the price can be expected to increase. Therefore, economies of scale are also focusing on a reduction of critical raw material demands. In the case of OpEx, the trend is to run the installation fully automatically using pure water as the reactant/circulating medium. Avoiding any electrolyte solution reduces the hazards connected with leakages, the demands on the construction materials and on the preparation of the process liquids, and the demands on the operator’s qualifications are also lower. The efficiency of the conversion of electrical energy into the chemical energy of hydrogen represents a key parameter of the water electrolysis process. In the case of the two low-temperature technologies (alkaline and PEM), anodic oxygen evolution is a critical aspect. It is a sluggish process determining the overall cell voltage and, therefore, the voltage efficiency of the process. Therefore, research is necessary on electrocatalysts permitting the kinetic losses of this reaction to be reduced; they should preferentially not be based on platinum or other critical raw materials. In the case of high-temperature processes, the main source of the efficiency losses is the electrolyte and its limited ionic conductivity. This can be improved by increasing the temperature. This, however, has a detrimental impact on the lifetime of the cell as well as on the construction materials used. One alternative is to reduce the thickness of the electrolyte layer. A second alternative targets developing new types of ceramic electrolytes, whereby proton-conducting ceramics are attracting a great deal of interest. The last of these issues is process flexibility. This directly concerns the intended use in connection with intermittent renewable electric energy sources. Flexibility is

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directly related to the ability of the electrolysis cell to adapt to the strongly fluctuating performance of these power sources without negatively impacting the efficiency of the energy conversion process and the cell lifetime. Such an adaptability is typically linked to the sensitivity of the cell performance to fluctuations in operating temperature and hence to the range of the optimum operating parameters, that is, cell voltage and/or current load, enabling efficient operation. The operating temperature impacts several aspects of the process, the main ones being the electrode reaction kinetics, electrolyte conductivity, and the separation of the gases produced. Sensitivity to temperature can be reduced by using highly active electrocatalysts for the electrode reactions, a highly conductive solid electrolyte and low viscosity circulating medium, that is, demineralized water. Utilizing a low-viscosity circulating medium allows a compact system of as low a heat capacity as possible to be designed. In such a case, it can rapidly react to a change of on/off state. The operational temperature of the high-temperature SOEC is a special case. The high temperature makes cooling down to ambient temperature difficult. The reason is that the related dimensional changes of the construction materials lead to delamination of the individual components and/or breakage of the thin layers. Nowadays the preferred option is to maintain operational temperature even during no-load periods. The lifetime of the cell can be impaired if the chemical state of a cell component, typically the electrode, is connected with its polarization. This means that, during low polarization, the content of a specific phase at the cell component may change, resulting in its potential degradation. In some cases this may be related to cell re-polarization during switching off. This may also lead to the destruction of the electrode (delamination of the surface layers due to the volume change of the original material, etc.).

8.4.2 Potential of individual process types to satisfy green deal demands It is clear that water electrolysis represents one of the key technologies in the scheme of the decarbonization of the European economy. Contrary to the usual statements, this does not only concern energy and transportation, but also industry. Whereas the question of both short-term and intersessional energy storage has been widely discussed for quite a long time, similar to fuel cell-powered electric vehicles, its broader application in different branches of industry has only recently been mooted. In the case of industry, the entire spectrum of options has still to be explored. As typical and traditional examples, the domains of industrial fertilizers and steel industry should be mentioned. These heavy-industry representatives consume large amounts of hydrogen produced from fossil fuels or utilize fossil fuels as a heat source and reducing agent, respectively. The implementation of hydrogen produced by means of renewable energy enables CO2 emissions to be substantially reduced.

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However, these are just illustrative examples and, as mentioned above, the spectrum of options is significantly broader. This is documented by the fact that recently a strategy was initiated by the EU, planning the installation of 2 × 40 GW capacity of water electrolyzers to promote further development of hydrogen technologies and their break-through into industrial practice [42]. One important new and rapidly developing topic is power-to-X technology. In contrast to previous technology based on a combination of hydrogen production via electrolysis with a subsequent separate catalytic process, the solid oxide electrolysis approach allows these two technologies to be combined in a single step. Besides the conversion of CO2 by co-electrolysis with water to form synthetic gas (a mixture of CO and H2), direct conversion to methane is being tested. However, up to now, there is a serious problem caused by the formation of carbon deposits blocking the cathode. In parallel to this, another trend is to explore direct cathodic reduction of CO2 to lower carbonaceous compounds at low temperature, that is, typically in a water solution environment. An alkaline environment is preferred due to the weak acid properties of CO2. However, other options are also being explored. Here, the reduction does not generally go so far and typical products reported are formic acid and also low molecular weight alcohols.

8.4.3 Plans for further development Future development will clearly follow the needs of industry and will be related to the foreseen applications. Currently the trend is to increase the dimension and production capacity of the cells in order to allow as simple installation as possible to satisfy the requirements of high-capacity production for large-scale industrial application. In a subsequent step, closer specialization depending on the particular application is to be expected. This, however, does not concern the fundamentals, but rather the size of the cells, the catalyst loading and/or the operating pressure.

8.5 Conclusion and recommendations As the above brief summary shows, water electrolysis is the pivot of the hydrogen economy scheme. This technology enables the conversion of renewable carbon-neutral electrical energy into chemically bound energy, thus opening up the way to its future broad use in diverse applications. Today the term water electrolysis stands for quite a broad range of technologies with different degrees of technological maturity and differing fields of application. It can also be stated that it represents a field with booming research and development activities and a significant variety of potential solutions for successful large-scale water electrolysis deployment. The priorities differ depending on

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the nature and operating conditions of each technology concerned. In this section a few basic suggestions are summarized. In the case of traditional alkaline electrolysis with circulating concentrated KOH solution, the focus should primarily address the improvement of the porous separator properties. The aim is to reduce ohmic energy losses and to reduce its permeability for gases, in the first instance for hydrogen. The design of an improved electrode structure and activation is also desirable in order to optimize process efficiency. At the same time, a new generation of the process based on an alkaline anion-selective polymer electrolyte has to be developed with the target of achieving a state as close as possible to the PEM process, that is, circulating demineralized water instead of KOH solution. At the same time, however, the main process advantage, that is, avoiding the use of platinum group metals, has to be maintained. For both alkaline processes, optimizing flexibility still poses a problem. In contrast to this, in the case of PEM electrolysis the main aim should be to focus on decreasing the demands of platinum group metals, while maintaining the efficiency and intensity of this process and not compromising durability. SOEC requires the most intensive research and innovation to reach a status comparable to that of its competitors. The chief concern remains the long-term stability of the cell components and their sensitivity to thermal cycling. This primarily entails intensive materials research and at the same time optimization of MEA preparation, stack assembly, and operation. There is no clear recommendation regarding the type of water electrolysis process to be developed. Depending on the current state of the art, each technology is clearly suitable for specific types of application which can be summarized as follows. The alkaline process is suitable for use in cases with less, or at least with less rapid, fluctuations in performance. By contrast, PEM water electrolysis is able to accommodate such rapid changes in production intensity much more effectively and with less impact on lifetime. Not forgetting SOEC, which is currently suitable for application in places where high potential waste heat is available and/or where a reversible system represents an important advantage. From a longer-term perspective, direct carbon dioxide conversion to syngas, or even to methane, is an important advantage of this particular process. For all technologies, further progress is, of course, crucial to increasing the safety of installations, together with the development of a variety of scales: from family houses (energy storage), to apartment blocks, industrial technologies, and grid balancing. In the case of installations in houses and apartment blocks, a significant degree of autonomy is required, since the operators are not qualified and only access the technology irregularly. With respect to alternative processes of hydrogen production, water electrolysis is the only type directly utilizing electrical energy that can be produced by means of green energy sources. Additionally, it produces high-purity hydrogen without the need for subsequent, complex purification technology. Only drying is needed for compression of the hydrogen produced. As already indicated, in the case of SOEC even direct conversion of carbon dioxide to syngas is possible by means of carbon

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dioxide–neutral electrical energy. This fact ensures water electrolysis a special place among alternative hydrogen production technologies. Now, water electrolysis technology is undergoing an important step toward broad implementation, as represented by ongoing demonstration projects. These projects assist in collecting necessary data on the operation of large-scale electrolysis cells under conditions of various industrial as well as domestic applications. This is an important move toward the design of a new generation of water electrolysis technology characterized by higher economic competitiveness.

References [1] [2] [3] [4] [5]

[6] [7] [8] [9]

[10] [11] [12] [13] [14] [15] [16] [17]

IEA. The Future of Hydrogen. IEA, Paris, 2019. Kreuter W, Hofmann H. Electrolysis: The important energy transformer in a world of sustainable energy. Int J Hydrogen Energy 1998, 23(8), 661–666. IRENA. Hydrogen from Renewable Power: Technology Outlook for the Energy Transition. International Renewable Energy Agency, Abu Dhabi, 2018. Santos DMF, Sequeira CAC, Figueiredo JL. Hydrogen production by alkaline water electrolysis. Quim Nova 2013, 36(8), 1176–1193. Hashimoto A, Hashizaki K, Shimizu K. Development of PEM water electrolysis type hydrogen production system for WE-NET. in Proceedings of the 14th World Hydrogen Energy Conference on CD. 2002. Isenberg AO. Energy conversion via solid oxide electrolyte electrochemical cells at high temperatures. Solid State Ionics 1981, 3–4(C), 431–437. Doenitz W, et al., Hydrogen production by high temperature electrolysis of water vapour. Int J Hydrogen Energy 1980, 5(1), 55–63. European Commission. The European Green Deal. European Commission, Brussels, 2019. Thyssenkrupp Uhde Chlorine Engineers. Hydrogen from large-scale electrolysis. 2020 [cited 10.09.2020]; Available from: https://www.thyssenkrupp-uhde-chlorine-engineers.com/en/ products/water-electrolysis-hydrogen-production/alkaline-water-electrolysis. PERIC. Alkaline type hydrogen generation system. 2020 [cited 10.09.2020]; Available from: http://www.peric718.com/Alkaline-Type-Hydrogen-G/r-85.html. TianJin Mainland Hydrogen Equipment Co. FDQ800. 2020 [cited 10.09.2020]; Available from: http://www.cnthe.com/en/product_detail-35-43-30.html. Industrie Haute Technology SA. Benefits of IHT’s electrolysers. 2020 [cited 10.09.2020]; Available from: http://www.iht.ch/technologie/electrolysis/industry/about.html. MVS Engineering. Hydrogen Generators – Bipolar Water electrolysis. 2020 [cited 10.09.2020]; Available from: https://www.mvsengg.com/products/hydrogen/water-electrolysis/. Airox Nigen Equipments Pvt. Ltd. Hydrogen gas generator. Bipolar pressurized water electrolysis. 2020 [cited 10.09.2020]; Available from: https://www.airoxnigen.com/Hydrogen-By-Water. Nel ASA. Atmospheric Alkaline Electrolyser. 2020 [cited 10.09.2020]; Available from: https:// nelhydrogen.com/product/atmospheric-alkaline-electrolyser-a-series/. Next Hydrogen. Next Hydrogen NH-450 Hydrogen Generator. 2020 [cited 10.09.2020]; Available from: https://www.nexthydrogen.ca/product. McPhy Energy S.A. McLyzer range: 100 to 800 Nm3/h at 30 bar. 2020 [cited0 10.09.2020]; Available from: https://mcphy.com/en/equipment-services/electrolyzers/.

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[18] Ener Blue SA. Indoor Industrial Hydrogen Generators. 2020 [cited 10.09.2020]; Available from: http://www.ener-blue.com/hydrogen-generators/indoor. [19] HT Hydrotechnik WasserElektrolyse GmbH. Elektrolyseure Typ EV 50. 2020 [cited 10.09.2020]; Available from: http://www.ht-hydrotechnik.de/fileadmin/Datenblaetter/Produktdatenblatt_ EV_05-13c.pdf. [20] Nel ASA. The World’s Most Efficient and Reliable Electrolysers. 2019 [cited 2020]; Available from: https://nelhydrogen.com/wp-content/uploads/2020/03/Electrolysers-Brochure-Rev-C. pdf. [21] WaterstofNet. WaterstofNet develops sustainable hydrogen projects. 2020 [cited 2020]; Available from: https://www.waterstofnet.eu/en. [22] STORE&GO. STORE&GO Power-to-Gas Roadmap. 2020 [cited 2020]; Available from: https:// www.storeandgo.info/. [23] Omrani R, Shabani B. Review of gas diffusion layer for proton exchange membrane-based technologies with a focus on unitised regenerative fuel cells. Int J Hydrogen Energy 2019, 44(7), 3834–3860. [24] Immerz C, et al., Effect of the MEA design on the performance of PEMWE single cells with different sizes. J Appl Electrochem 2018, 48(6), 701–711. [25] Verkoeyen G., Hydrogenics – review of world and European hydrogen initiatives in transport and energy sectors. HydrogenDays 2019, 27.-29.3. 2019, Prague, Czech Republic. [26] ITM Power. HGasXMW. 2020 [cited 2020]; Available from: https://www.itm-power.com/ images/Products/HGasXMW.pdf. [27] Nel ASA. Proton PEM Electrolyser M series. 2020 [cited 2020]; Available from: https:// nelhydrogen.com/product/m-series-3/. [28] Giner ELX. PEM Electrolyzer Systems. 2020 [cited 2020]; Available from: https://www. ginerelx.com/electrolyzer-systems. [29] Siemens Energy. SILYZER 300 The next paradigm of PEM electrolysis. 2020 [cited 2020]; Available from: https://www.siemens-energy.com/global/en/offerings/renewable-energy/ hydrogen-solutions.html. [30] H2FUTURE. Production of Green Hydrogen 2020 [cited 2020]; Available from: https://www. h2future-project.eu/technology. [31] REFHYNE. REFHYNE Clean Refinery Hydrogen for Europe. 2020 [cited 2020]; Available from: https://refhyne.eu/. [32] Miller HA, et al., Green hydrogen from anion exchange membrane water electrolysis: A review of recent developments in critical materials and operating conditions. Sustainable Energy and Fuels 2020, 4(5), 2114–2133. [33] Hnát J, et al., Development and testing of a novel catalyst-coated membrane with platinumfree catalysts for alkaline water electrolysis. Int J Hydrogen Energy 2019, 44(33), 17493–17504. [34] Mogensen MB. Materials for reversible solid oxide cells. Curr. Opin. Electrochem. 2020, 21, 265–273. [35] Fergus JW. Sealants for solid oxide fuel cells. J Power Sources 2005, 147(1–2), 46–57. [36] Sunfire. Sunfire-HyLink 2020 [cited 2020]; Available from: https://www.sunfire.de/en/ products-and-technology/sunfire-hylink. [37] O’Brien JE, et al., A 25 kW high temperature electrolysis facility for flexible hydrogen production and system integration studies. Int J Hydrogen Energy 2020, 45(32), 15796–15804. [38] GrInHy 2.0. Green Industrial Hydrogen 2020 [cited 2020]; Available from: https://www.greenindustrial-hydrogen.com/. [39] Brabandt J, et al., Pressurised high temperature co-electrolysis as effective source for Power-to-X applications. HydrogenDays 2016, 6.-8.4. 2016, Prague, Czech Republic.

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[40] Allebrod F, Chatzichristodoulou C, Mogensen MB. Alkaline electrolysis cell at high temperature and pressure of 250 °c and 42 bar. J Power Sources 2013, 229, 22–31. [41] Nikiforov AV, et al., Specific electrical conductivity in molten potassium dihydrogen phosphate KH2PO4 – An electrolyte for water electrolysis at ∼300 °C. Appl Energy 2016, 175, 545–550. [42] Wijk AV, Chatzimarkakis J. Green Hydrogen for a European Green Deal A 2x40 GW Initiative. 2020 11.9.2020; Available from: https://hydrogeneurope.eu/news/2x40gw-green-hydrogeninitiative-paper.

Nicolas Grimaldos-Osorio, Kristina Beliaeva, Jesús González-Cobos, Angel Caravaca, Philippe Vernoux

9 Electrolysis for coupling the production of pure hydrogen and the valorization of organic wastes 9.1 Introduction Green hydrogen will play a major role in the future decarbonized energy systems. Green hydrogen is an energy vector for grid balancing, power-to-liquid, and powerto-gas technologies [1]. Hydrogen is the most abundant element but does not exist in the nature in its molecular formula. It is usually associated with different molecules such as methane CH4 or water H2O. To this date, hydrogen is mainly produced by the steam reforming of natural gas, a fossil fuel, at high temperatures 500–800 °C on Ni-based catalysts [2]. However, further purification steps are necessary to obtain pure hydrogen with a concomitant emission of carbon dioxide. The most promising technology for the green hydrogen production is the water electrolysis that involves breaking water molecule into H2 and O2 using only renewable electricity [3] in an electrochemical cell without any carbon source. The intermittent primary renewable energy sources such as wind, solar, and tidal can be ideally combined with electrolyzers [4]. Green hydrogen can substitute fossil fuels for the industry, the transport or the electricity production without greenhouse gas emissions. Electrolyzers can operate below 100 °C [2–5] when water is liquid or at high temperatures (500–1,000 °C) [6] depending on the nature of the electrolyte. Alkaline water electrolyzers (AWE), based on liquid alkaline electrolytes such as potassium hydroxide, are a mature technology with many advantages such as a low capital cost, non-noble metal based electrodes and a long-term stability (30–40 years). A diaphragm permeable for OH− separates the two electrode compartments and the product gases. It is commonly made of either glass reinforced polyphenylene sulfide (Ryton) or polysulfone-bonded zirconia (Zirfon). Hydrogen is produced at the cathode (HER, hydrogen evolution reaction) while oxygen is released at the anode (OER, oxygen evolution reaction) (Fig. 9.1a). The mean AWE drawback deals with their low achieved current densities [7] (in the range 250–450 mA/cm2) due to the slow ionic conduction of the liquid electrolyte (high ohmic losses) and the formation of gas bubbles which can block the electrodes surface. Furthermore, AWEs exhibit a slow response upon application of transient voltages due to the liquid electrolyte high volume making this technology difficult to combine with the oscillating behaviors of renewable energy sources. In addition, AWEs cannot produce hydrogen at high pressure for safety reasons. Therefore, a compressor is required for hydrogen storage and transportation. https://doi.org/10.1515/9783110596250-017

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Fig. 9.1: Schematic drawing of different types of electrolyzers: (a) AWE, (b) PEM electrolyzer, (c) AEM electrolyzer, and (d) SOE.

The other types of electrolyzers are equipped with a solid electrolyte membrane to separate the anodic and the cathodic compartments. Proton-exchange membrane (PEM, Fig. 9.1b) and anion-exchange membrane (AEM, Fig. 9.1c) electrolyzers both operate at low temperature (typically 20–100 °C) with a polymeric electrolyte membrane conducting H+ cations and OH− anions, respectively. On the other hand, solid oxide electrolytic membranes are implemented in solid oxide electrolyzers (SOE, Fig. 9.1d) working at high temperatures (500–1,000 °C) with O2− anions.

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PEM electrolyzers based on protonic polymeric membranes capitalize the development of PEM fuel cells, including an optimized compact design (low thickness of the membrane of around 60–200 µm) and a very short start-up time. PEM electrolyzers can achieve high current densities (>1 A/cm2) and can operate at a higher pressure than AWE which is more suitable for further hydrogen compression and storage. Even if they are already commercialized, they undergo the high cost of the polymeric membranes. In addition, water has to be pure (deeply deionized) to avoid any poisoning of the proton-conductive polyfluorosulfonic acid membranes. Furthermore, the very acidic media limits the choice of electrodes to expensive noble metals (platinum at the cathode, IrO2 at the anode) since the OER and HER kinetics are lower by 2–3 orders of magnitude than in alkaline conditions. These harsh conditions also limit the lifetime between 5 and 20 years [5]. AEM electrolyzers could overcome these problems but it is a developing technology and actual AEMs show a poorer chemical and mechanical stability as well as a lower conductivity than PEMs. A major drawback of AEW and PEM electrolyzers, which run at low temperatures with the system H2–O2–H2O, is the high energy demand to split the water molecules under standard conditions considering liquid water (ΔG° = 237.1 kJ/mol). Consequently, the required cell voltage for water splitting is 1.23 V under standard conditions. Furthermore, electrode overpotentials, especially for the OER, are high on noble metals in acid media and on non-noble metals in alkaline media due to the low kinetics of the electrochemical reactions. This increases the required cell voltage up to around 1.6–2 V for high current densities (1 A/cm2), leading to low energy efficiencies of PEM and AEM electrolyzers resulting in a minimum electrical energy demand of around 45 kWh/kg H2. In addition, the anode product (O2) has a little economic value. All these issues lead to a high cost of green hydrogen. To overcome this problem, two possibilities are currently explored. The first is to develop high-temperature electrolyzers such as SOEs as the electrical energy demand (ΔG) and the electrode overpotentials significantly decrease with the temperature, leading to a theoretical SOE cell voltage of around 1.3 V. However, the net efficiency of SOEs, taking into account the heating energy demand, is in the range 40–60% compared to 59–70% and 65–82% for AWE and PEM electrolyzers, respectively [8]. The second solution is to replace the OER in low-temperature electrolyzers with a less-energy intensive reaction both from a thermodynamic and kinetic point of view. The electrochemical oxidation of much more oxidizable organic molecules such as biomass-derived alcohols [9–12] is reported in the literature. Considering the total oxidation of alcohols into CO2 (eq. (9.1)), the theoretical standard cell potential is 0.016 V for methanol, 0.084 V for ethanol and 0.0029 V for glycerol compared to 1.23 V needed for water electrolysis [13]. In practice, cell voltages to achieve significant hydrogen production rates are higher mainly due to the slow kinetic of the alcohol electro-oxidation at the anode but still significantly lower than 1.23 V. Methanol was the most studied alcohol in PEM electrolyzer but this compound is toxic and mainly produced from natural gas [13]. Except for methanol, the electro-oxidation is never complete, as the C–C bond breaking is quite difficult at low temperature, leading to

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the formation of by-products that limits the production of hydrogen. For this specific case of alcohols, the concomitant production of chemicals at the anode and hydrogen at the cathode is denoted as electrochemical reforming in the literature. The by-products hinder the production of hydrogen but valuable chemicals can be generated at the anode such as lactate (from ethanol), glycolate, and oxalate (from ethylene glycol), dihydroxyacetone, glycerate, and tartronate (from glycerol) [14, 15] Cn H2nþ1 OH + ð2n − 1Þ H2 O ! nCO2 + 3nH2

(9:1)

The electro-oxidation of bio-sourced ethanol and glycerol (a surplus by-product of biodiesel fabrication) at the anode of PEM electrolyzers could be promising but only low current densities (few hundred mA/cm2) are reported on expensive Pt-based anodes, making this approach economically unattractive. Recently, F.M. Sapountzi et al. [16] have suggested that higher performances could be achieved in alkaline media because anodic and ohmic overpotentials are lower. For instance, electrochemical reforming of various alcohols (ethanol, ethylene glycol, glycerol and 1,2-propanediol) was investigated in a AEM electrolyzer between 25 and 80 °C in NaOH [17]. The anode was based on Pd nanoparticles supported on a 3D titania nanotubes (1.7 mg Pd/cm2). Current densities larger than 1 A/cm2 were obtained at 80 °C, corresponding to an energy consumption in the range 18–20 kWh/kg H2 instead of 47 kWh/kg H2 if the electrolyzer is only alimented with water. The recent advances in electrochemical reforming of alcohols, in general, has been recently reviewed by Linares et al. [18] and Coutanceau and Baranton [13], while other reviews deal with the use of such technology for the specific case of glycerol electrolysis [15]. This idea to replace the OER with a less-energy-intensive oxidation reaction in order to depolarize electrolyzers can also be combined with the treatment and valorization of wastes without emissions of harmful products. Indeed, organic wastes containing H atoms can be fully or partially oxidized at the anode of electrolyzers to simultaneously produce pure hydrogen and valuable compounds. The aim of this chapter is to review recent developments in the utilization of organic wastes in electrolyzers. The first part will describe the valorization of biomass and plastic wastes in low-temperature electrolyzers, while the second part will report high-temperature oxidation processes in SOEs.

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9.2 Low-temperature electrolyzers for the valorization of organic wastes 9.2.1 Valorization of biomass wastes Among biomass wastes, lignin is a major component of woody wastes and is the second most abundant natural polymer. Therefore, lignin is a good potential anodic fuel. Lignin is a tridimensional biopolymer that is a waste of the Kraft paper-making industry with an annual production over 50 million tons. Lignin is usually incinerated to produce heat. The incineration is accompanied by the generation of toxic emissions [19–22]. Only a small part of lignin waste is valorized into valuable chemicals by pyrolysis, gasification, catalytic oxidation, and reduction [15–17]. Depending on the technique, temperature, and catalytic materials, lignin could be valorized into phenols, aldehydes, or syngas. Production of valuable chemicals such as vanillin and vanillic acid from lignin was also mentioned in literature [22–24]. In lignin electrolysis, still an early-state exploratory technology, two main families of studies can be identified in literature: some solely focused on its electrochemical depolymerization, leading to the production of high-value products, namely, vanillin and vanillic acid, without considering the simultaneous production of hydrogen. These studies have been recently summarized in some review articles by Zirbes and Waldvogel [25] and Du et al. [26]. Most of those works were performed in a high range of electrical potentials (sometimes as high as 12 V [27] and 15 V [28]) where the electro-oxidation of lignin (lignin + OH− → ligninox + e−) competes with the OER. The main reason to work in such drastic energy-demanding conditions is most likely attributed to the challenge to cleave the C–C bonds of organic molecules at low electrochemical potentials and temperatures below 90 °C, as it was observed in some studies dealing with the electro-oxidation of ethanol [29–31] and glycerol solutions [15, 32]. Nevertheless, several studies focused on H2 as the main product of interest [19, 33–40]. This process is potentially less energy-intensive, considering that the depolarizing effect of lignin allows producing H2 from electrical cell potentials as low as 0.2 V. However, the valorization of the lignin electro-oxidation products and the yield of H2 might be limiting, considering that the depolymerization (i.e., the cleavage of C–C bonds) of lignin under such conditions is unlikely. The recent research efforts for lignin electrolysis can be classified according to the temperature. Below 100 °C, experiments can operate in PEM electrolyzers using the actual mature technology (membrane, electrodes, design, etc.) or in AEM electrolyzers exploiting the recent developments on OH− conducting membranes. However, as previously mentioned, below 100 °C, the kinetics for the electrochemical oxidation reactions are slow and the depolymerization of lignin is tricky [41]. Another route is to perform electro-oxidation of lignin at temperatures higher than 100 °C, typically between 100 and 200 °C (intermediate temperature range) to boost

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the kinetics and the lignin depolymerization even if new alternative materials such as polymer membranes have to be develop.

9.2.1.1 Low-temperature (150 bar) or fiberglass reinforced aluminum cylinder (350/700 bar), or pipeline (>70 bar). Or common concrete building, which should have a standard strength of 20 MPa (×10 kg/cm2 or bar), that should be sufficient to sustain the hydrogen explosion. Should the storage or processing of hydrogen (such as compressors to raise the pressure) be indoors, the guidance of an explosion towards the sky would be preferable. For example, a light roof with much less strength of material, such as metal sheets, so as not to explode into the surroundings, where the residents or human activities maybe. We will see from the accidents happened (Section 14.3.3), though the explosion pressure is not high, however, it generates a shock wave, which is quite strong, even 5 km away, people could still feel it. The wave transmitted through the ground is also very strong, people experiencing the phenomena, compare the feeling similar to that of an earthquake.

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14.3 Non-financial Risks in Handling of Hydrogen Risk evaluation for handling hydrogen Due to different modes of transportation, storage, and operation, it’s necessary to apply extra-individual risk evaluation. Each evaluation will deepen understanding on different, individual consequences and results from direct and indirect contact, impacts in close and long distances, in small or large areas, and engagement with human (lives), properties (building, constructions, equipment and facilities), and environment (ecology). Using hydrogen as an energy source is new to the society; thus, risk evaluation discussed in this chapter is based on experiences gained from hydrogen production facilities, and accidents that occurred up to and including July 2020. Hydrogen can be transported in gas and liquid. Gas filled in high-pressure gas cylinder at room temperature and deep-cooled liquid containers needs to be kept well insulated. High-pressure gas can also be transported via pipeline. Each type of packaging and transportation mode has to be assessed separately. Due to the very nature of hydrogen, and any devices used to contain it, we can calculate and design safety assessments based on supply requirements. These requirements include: supply in the form of gas or liquid and subsequent supply devices, such as pipes, pumps, compressors, liquidization equipment, storage tanks, high-pressure bottles, liquid containers, and all related fittings, plugs, instruments and miniatures (hardware). From the user side or even community side (who bear the consequences), we can calculate the risk assessment on the emergency response system, HAZMAT team (a specialist team that has the knowledge and capability to deal with the incident), training of all personnel engaged in the fueling station operators, training of personnel in surrounding institutions and facilities, including schools, hospitals, offices, shopping malls, bus stations, housing estates, stadium, and car parks.

14.3.1 Overall risk assessment Risk assessment is based on two main parts: One is about the nature of the material dealt with, which is so-called hazard; The other part is the frequency of the hazardous operations. They are independent factors; thus, the risk assessment can be expressed in a multiplied product of both. As the only chemical we are dealing with is hydrogen, the hazards come in when hydrogen is in different transportation modes including packaging and different forms of mobility. The procedures of hazard operation can be summarized as follows: – vehicle loading, unloading, transportation; – high pressure gas cylinder filling, transporting, and plugin to gas supply network;

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– deep cooled liquid container filling, loading, unloading, plug-in to gasifier and supply to gas network; – plug-in to storage tank; – gas pipeline monitor and maintenance; – compressor maintenance and operation; – gas dispenser (nozzle and communication system) operation; – others. In this way, risk assessments need to be done in the community, for each individual and process for equipment or each operational procedure. This includes training required for all related operators and personnel. It is not necessary to reinvent the wheel, as this could all be developed from the standard safety reviews from the supplier side, i.e., hydrogen production site.

14.3.2 Job risk assessment When hydrogen is contained in any device, such as high-pressure pipeline, high-pressure bottles, and insulated cooled containers, it is in a safe status, risk can be described or set at the “level of safe”. When jobs or tasks are allocated or carried out with any of these devices, such as a routine maintenance, checkups, major changes on pipe racks, high-pressure bottles downloaded bundles from a truck trailers, or plugins onto designated platforms, or liquid hydrogen trailers for correct positioning for discharge operations, all these tasks are hazardous by nature thus, all need risk assessments (see Section 14.3.4.1). A job classified as “frequently repeatable” then after risk assessment, (such as discharge or unloading), handling can be regulated by a “standard operational procedure” (SOP). This approach is considered as linear risk assessment. Any task of a hazardous nature, such as the use of an open flame (welding etc) on the pipe rack, requires not only a fire work permit, (which is similar to the plant operation), but also the possible response to consequences, such as informing firefighting department, or even alerting emergency commanding center, or ask fire fighters and HAZMAT team to be present. This is considered as a dynamic risk assessment. There are emergent expected yet unknown, thus all relevant personnel are requested to be in the scene. The last job type has a link to the community and society in a broader sense. Further more, if the hydrogen leak did happen during a job, and the hydrogen concentration of 18% leading to explosion is expected, in order to reduce the shock impact to residents and surroundings, a dynamic risk assessment should be conducted beforehand, to work out emergency measures, such as closing the highway, evacuating the schools, or giving notice to hospital (see Sections 14.3.4.2 and 14.3.4.3).

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14.3.3 Risk minimization strategy A standard risk assessment inside the factory starts with the hazard identification and hazard operation procedure analysis (HAZOP). Both result in formulation of the risk minimization strategy. From the community’s point of view, we can consider the whole area of the community, where the fueling station is located, as a “factory.” Thus, the following risk minimization strategy used in a typical chemical plant can also be valid, with some adjustments wherever necessary. While a hydrogen fueling station deals with only one hazard, hydrogen, a combination of gas and oil filling station would increase the overall hazard and risk classification. In China, due to hazardous classification, gas filling stations including LNG, LPG, CNG (compressed natural gas), liquid fuel filling, gasoline, and diesel filling stations all have higher levels of rules and regulations established. This is inclusive of liquid fuel, LPG, and LNG tanks; all need to be buried underground [3]. Adding hydrogen to this situation, as either combined with gas, or liquid fuel, or both, as a standard approach, would increase the hazardous classification level to a higher one. This approach would need to be translated into the “permitted quantity” as the total volume of storage at any given station is classified into three categories: the minimum safe distance among gas, liquid fuel, and hydrogen storage tanks as well as its auxiliaries [4].

14.3.3.1 Avoiding exposure to potential hazards Same concept should be valid outside the factory. This means, next to the devices of hydrogen transportation (vehicles, bottles, and pipelines) and storage (tanks, bottles, and pipes), there should be no flammable materials (potential hazards). Every item used for transportation, storage, or operation needs to be well earthed (no electrostatic discharge), and should be free of any additional device above it, in order to avoid any possibility of hydrogen accumulation.

14.3.3.2 Minimizing the potential or frequency of hazardous events In the factory, this means keeping the devices away from human activities (such as next to plant tour walking path, or equipment or pipe rack maintenance access route, or driveways for non-EVs) and reducing the possibilities of contact or coming closer (such as manual checkout point with certain frequency of monitoring). Outside the factory, for stationary devices (such as pipe racks, compressor room, and fueling station), the safety requirements could still be the same. Movable or transportable devices such as hydrogen gas or liquid carriers or bundles of cylinders (not individual) are required

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to be kept in a designated “safe place”, an area where the requirements is equivalent to that of stationary devices. In a petrochemical complex, hydrogen production unit sitting next to the oil refining section or the hydrogenation unit is a common practice. The only difference is the operators or transportation vehicles in the factory will be under stringent surveillance. That also needs to be integrated in the operational procedures for the combined fueling station.

14.3.3.3 Controlling or mitigating the consequences of hazards Within the confines of the factory work space, controlling or mitigating refers to the action of restricting potential hazards in a predetermined area or routes. Outside the factory, this can still be valid. Moreover, it’s necessary to be able to determine the area and routes instantaneously. This means that all emergency response procedures and handling need to be well documented, matched with any necessary training. Any necessary training should be applied to any operators designated to stationary or movable devices. This training should also extend to the surrounding residents and workers in the institutions, or even the whole communities. In addition to the fire protection distance and volume stored, it is essential to conduct a thorough HAZOP study, including any additional hazardous operations that combine gas and liquid fuel filling stations. Future actions could be based on these extensive HAZOP studies, that would require any fueling station operating company to develop its own safety reviews and procedures for any type of combined stations.

14.3.3.4 Controlling impact to the environment By restricting to only hydrogen, the impact of environment becomes minimal. Water is the only substance produced by hydrogen, if no other materials, such as hydrocarbons, get involved, which are “accidentally” there. Even for a combined fueling station, it does not increase or add any additional harmful material to burden the bio-capacity of environment; we need to take a look on the bigger scope. The consideration should be on the production and application of hydrogen. In the production, there are three types of hydrogen, namely grey, blue, and green hydrogen. Grey hydrogen is based on fossil fuel such as gas, oil, and coal as raw material and energy source; they are deemed to have carbon emissions. Blue hydrogen with nonrenewable energy source such as nuclear, and green from renewable energy are those with low carbon emissions. For gray hydrogen, it should consider the carbon recovery and resourcing, for example, hydrogenation, or CCS and CCUS.

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On-station hydrogen generation could be on base of natural gas, methanol steam reforming (gray hydrogen), or ammonia splitting (gray hydrogen if use fossil fuel, green if use renewable energy) that will increase the hazard classification similar to the industrial evaluation with environmental impact [4]; or water electrolysis and photocatalytic water splitting, which will not have additional environmental impact (even green hydrogen if use renewable energy). Also for the compression and liquidization of hydrogen, its energy source needs to be taken care of. In the application, it needs to consider the recycle and reuse of packaging materials such as fiberglass-reinforced aluminum cylinder, or even the fuel cell itself.

14.3.3.5 Providing means to ensure safety of life involved in hazardous event In the factory, this means get the firefighting department and emergency medical assistance involved. In addition, close-by hospitals need to be informed of the nature of hazards, so that they can be ready for saving lives when emergency happened. Outside the factory, this becomes even more necessary; the emergency call center needs to be ensured that they understand the hazards. Also, a set of emergency medical response procedures need to be easily accessible at the stationary and moveable devices. Since most communities would not normally have an “on-site” emergency response team, they would typically rely on any available public emergency response hotline and any available regional HAZMAT team. These responses would usually link with firefighting departments supported by the chemical manufacturers association. Thus, from the outset, the safety precondition requires emergency call-centers, emergency response systems, HAZMAT teams, emergency medical centers to all be in place. In addition, the safety of life before, during and after any hazardous event is a mindset issue, rather than a technical issue. Thus, the strategy needs to adjust to include the technical, societal, and socio- technical issues. We will further discuss these in Sections 14.4.3 and 14.4.4.

14.3.4 Selected recent incidents Three cases are selected in this section: two are clearly classified as human error, and one is still under investigation. Nevertheless, we can learn from all three cases.

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14.3.4.1 Transportation: liquid hydrogen delivery to Veridam 14.3.4.1.1 Facts and description In August 2018 a truck carrying liquid hydrogen for a delivery to Veridam, El Cajon, CA caught fire on site due to improper docking. Total volume of the liquid hydrogen carried was 1,500–2,000 gal (around 8,000–9,000 L). HAZMAT team was there and let the vented hydrogen burnt, then shut off the valve. The venting system was functioning according to the safety design, and released gas gradually. Surrounding schools were evacuated. Emergency response system was well in place. The incident started at 8:10 AM, and ended at around 12 PM. No casualty or other damages were reported [5]. The incident happened due to miscommunication between the carrier driver and the plant site worker. Hence, it was a human error. 14.3.4.1.2 Lessons learned – Venting system of the truck loading and unloading area is critical. – Hydrogen burning from vented gas need to be under controlled. – National Emergency Response System is in place, nearby school, or shopping center needs to be evacuated through the emergency response commanding center. – Well-trained HAZMAT team is necessary.

14.3.4.2 Fueling station: Uno-X fueling station at Kjørbo 14.3.4.2.1 Facts and description On 10 June 2019, a fueling station owned by Uno-X in Norway had an explosion. The fueling station is owned by a JV of Nel ASA and Nippon Gas. The first investigation has clarified that the water alkaline electrolysis installed on site and fueling station had nothing to do with the incident. Thus the supply and business of electrolyzer and dispenser can proceed [6]. On 27 June 2019, Nel ASA [7] announced the final report from a third-party safety consulting company, Gexcon; from the root cause analysis, it was identified as an assembly error of a specific plug in the high-pressure hydrogen storage unit. This event caused a temporary shutdown of all Uno-X fueling station in Norway and four stations in Germany. Also, the HFC vehicle sales of Toyota and Hyundai in Norway stopped. Consequently, this event is classified as human error. Previous fueling stations supplied by Nel ASA to the United States and South Korea had a different design; thus, they were not affected by this event.

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The rundown of the whole event is as follows: 5:30 PM Confirmation of hydrogen leak at the fueling station 5:37 PM Arrival of emergency response team 5:40 PM Nel ASA got the event report (within 10 min) 5:41 PM Close down highway E16 and E18 5:47 PM About 500 m radius was classified as dangerous zone 7:28 PM Robot came in to cool down the production devices 8:14 PM Confirmation of event under control and reopen the highway

14.3.4.2.2 Lessons learned – Hydrogen leakages do not lead to explosions if hydrogen has not accumulated (to 18%) and burn upward at all times – To keep the other storage tank cooled down is necessary – Fueling station incident can cause business stoppage on the supply chain, such as alkaline electrolyzers, dispensers, and HFC car sales. The cease of operation and business activities can be longer than the third-party report, even if it is an human error; – Making a clear cut distinction between previous and current designs for the fuel station, in order to eliminate any doubts from customers; – There were two cars blown off their airbag due to the explosion; this gives the hint that car park should be no less than 5 km away

14.3.4.3 Production: OneH2 hydrogen fuel production hub 14.3.4.3.1 Facts and description On 7 April 2020, an explosion happened at OneH2 hydrogen fuel plant site located in Long View, NC, USA. The blast was felt several miles away (at least 5 km), had caused damages of 60 houses. The company made announcements after the accident, saying that the event had nothing to do with hydrogen. However, judging from the video filmed on the day, and the specific wording collected from the people on the ground, such as: got a shock, felt like an earthquake; broken window glasses shown via shock wave of the explosion; followed by no heat wave; it is more likely that the explosion was caused by hydrogen than any other flammable material including hydrocarbons. No injury was reported [8]. From all the available evidence, particularly that of the entrance door of the production hall being blown off, a high probability prevails, that hydrogen leaked and accumulated in the hall through the night, and was ignited only when the production activities resumed at 8:00 in the morning, when the explosion also happened. The cause of this accident could be either insufficient SOP after working hours, or human errors of mishandling. The root cause of this incident is still under investigation while this chapter is written.

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14.3.4.3.2 Lessons learned – Shock wave caused by hydrogen explosion shattered window glasses. Strength of glass need to be confirmed according to distance. – Shock wave generated by hydrogen explosion felt miles away, like earthquake. – Inflammable material to be kept away from hydrogen escape route and space.

14.3.5 Conclusion of incident-based risk evaluation From all listed incidents recently engaged with hydrogen listed under hydrogen safety [9], there were two in 2018, five in 2019, and one in 2020. Except the latest one, all were caused by human error. Leak of hydrogen can cause fire; only when a huge amount of hydrogen were accumulated (to around 18%), explosion could happen. Hydrogen explosion has a shock wave effect that can shatter window glasses nearby (up to 5 km); the shock wave transmitted through ground is a shake-like earthquake. Equipment failures were found in none of the cases; furthermore, new improvements are still ongoing. All cases showed an emergency response system in place is essential and HAZMAT team is absolutely necessary, especially when there is an increase in HFC cars and trucks on the road. The location of the incidents when on the outskirt of a town should prove to have less impact than those inside the town. This is especially important when there are schools, hospitals, and shopping malls within the 5 km safety radius zone. Within this zone, the necessary evacuation would be difficult, as the shockwaves could cause technical problems on precision of certain instruments. More importantly, the longer-term aftermath of trauma within the community and peoples’ minds could remain as a collective memory if there are such facilities and institutions within such radius. Conducting extra risk assessments specifically for those facilities or institutions within any 5 km radius is recommended. Since the human factor is the main cause of all incidents to date, it would also be advisable to conduct risk assessments and follow-up any risk evaluations in the workplace, where any handling of hazardous materials is required, including at the fueling station or dispenser or the production areas. To date there has been little mentioning about the resilience of the community, which will become more critical, when the deployment of fueling stations become more common place. The resilience in this case is not only about what has happened, but also about any present impacts , influencing what is currently occurring, and in turn, inform any future actions. The above conclusions look like “reverse risk assessment,” and all incidents are deemed to have a consequent value; once these value are known, we can use them

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to evaluate other risks. As already mentioned, the consequent value for resilience is not covered in any case. Please see Sections 14.4.3 and 14.4.4.

14.4 Adoption of new technology Despite the fact that hydrogen is the oldest molecule in the world or even the whole universe, to apply hydrogen comfortably at home as an energy resource, or even to welcome a new era of hydrogen economy, requires very new approaches before we can fully comprehend what are the necessary next steps. Traditionally, a risk management will follow certain established pattern, which may be true in finance, insurance, even safety and security; since all are interlinked, it also becomes a concept of company policy. Adoption of new technology begins with the willingness of people, household, or residential unit. To achieve harmonious community while adopting new technology, hydrogen FC has quite different aspects from HEV (hybrid), pure EV, and also, hydrogen-based power is more hazardous in comparison to solar and wind power. Hydrogen as pure gas deployed to each individual household would have an acceptance period but not for hydrogen as a blended component with town gas, or natural gas, that would not change the habits of the users at all. As pure gas, hydrogen may experience resistance from residential units or cluster as the sole means for central power and a heat generation resource. This communal resistance could include hydrogen fuel cell personal cars, parking in the public place of residential area, and parking house, underground garage, and so on. Any safety issues could create concern. Since hydrogen as a resource for the community is new to the community or society, we use a newly developed fact-based quantitative matching qualitative methodology to tackle the adoption. While there is no actual example, we will use a case in the next chapter, which we are planning to do for illustration. To introduce a new technology, we will normally look at its safety concerns, which involve only the technical aspects. Thus, focus is mostly on the technical standards, which is valid for all ICTs. For a community or even a person, it is more than that. The whole process of adoption of a new technology may roughly be divided into the following: acceptance, optimization, and evolution. Acceptance deals with the old pattern of thinking and lifestyle, while optimization depends on the degree of familiarization, and the evolution goes beyond the knowledge we have already gained. Resilience covers the whole process: before adoption; it is about the preparedness and prevention; during the optimization, resilience is playing the role of balancing; as for evolution, resilience can be remediation, recovery, and healing.

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14.4.1 Multi-signal alarm or tipping point of general opinion Industrial accidents are collected and analyzed. Accidents could happen when: the material or equipment (hazardous by nature), are used for jobs and tasks that are frequently carried out and in direct contact with people. Based on proactive incident prevention (behavior-based safety) summarized by one of the major international chemical manufacturing groups, the occurrence number of happenings has the ratio – Unsafe Acts: Near Misses: Minor Injuries: Lost Time Injuries – roughly 1,000:100:10:1. Thus, if the hazardous nature of hydrogen handling can be controlled, i.e. integrity of equipment are regularly checked and confirmed, operations are well distributed, and persons involved and engaged in sensitive areas are well informed and proactively activated, then potential accidents can be avoided by sufficient and thorough precautionary measures. Technical malfunction of equipment can be found out through regular check or maintenance, or even through online data analysis of total maintenance program. Frequency or overlapping of operations can be optimally allocated via programming. Human error is mostly linked with mindset, and the measures are more than training alone. Proactive becomes more essential, not only during operation but far before that. With regards to the question of human error, the case of COVID-19, poses some interesting questions, including the initial question, to decide whether COVID-19 was infectious from human to human. Regardless of this initial question, was there already sufficient uncertainties among the first group of doctors involved with initial contacts, who were assuming it could be a SARS-like epidemic? Further and subsequent questions of why were people not alerted from whistle blowing actions, moreover, how, why was these actions stopped? How can such signs or signals be collected to formulate an understanding of a general feeling? It would be possible to approach such a question through a dynamic multi-signal network alarm, via decentralized network, independent to the official, with long lasting impacts, finally avoiding multiple level human control, that is, essentially a linear reporting mechanism. This multi-signal alarm somehow reflects unlinked processes that are unpredictable with regards to their communication, for example, like the case of COVID-19, which started with ever increasing patients presenting with similar or unknown symptoms, or suspicions reports from non-virus medical specialists, such as dentist or eye doctors. In the case of the hydrogen based community, alerts could be summoned from non-linear, holistic sources, including natural signals, such as abnormal animal behaviors, or when traffic jams on highways present a concentration of HFC cars, being exposed to bad weather with accompanying thunder and lightening or any other potentially hazardeous, unknown reason. We can call this as an emergent-based multi-signal alarm. AI may to some degree, assist in this aspect.

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14.4.2 Well-being out of dynamic balance New technology often brings different ways of lifestyle. That also needs to be thought through. The purpose of adopting a new technology has to link to be beneficial to the human being, not only to the long-term future of mankind, such as climate change, but also to the short-term target, such as comfortableness of living. Hydrogen fueling stations cannot be round the corner of residential areas, also not at the shopping centers. They need to be some 5 km away from these places. The nature of hydrogen usage requires and has to take into consideration all special rules and regulations. So what is considered as convenient? What makes hydrogen different from fossil fuels, despite its cleanness? Considering the nature of hydrogen as a fuel, if a driver for example, considers that she is carrying a bomb at all times, and that one gram of hydrogen is equivalent to 24 g of TNT (only focusing on the possibility of leakage, not at the 1 full tank of 350 or 700 bar bottle), will the diver remain confident with a potential bomb, even with children on board? Whilst hydrogen is environmentally cleaner than gasoline or diesel, or even electricity from the grid (with mix pattern of energy sources, for example, typical rounded ratio for China 70% Fossil Fuel, 15% hydro, 5% nuclear, 10% solar and wind), technically, it seems safer than LNG, LPG, and CNG, the current concerns regarding hydrogen remain. How is it possible to competently address the feelings and concerns of any impacted communities, to ensure better standards of living than before? In a democratic world, it seems all regions are unique and may have their own value systems, whereas in a nationstate, it may have a very uniform one. Nevertheless, there are universal values that may be valid for everyone globally to achieve as the goal of well-being, these are [10]: – Human dignity – Common good that transcends individual interest and promote fairness – Ethics embedded in technologies and the applied institutions – Recognition and follow the age of surveillance capitalism – Growth within planetary boundaries

14.4.3 Acceptance and evolution In terms of technological adoption, we have grown accustomed to the novelties of newer versions of smart phones as they are continuously launched into the market place. Adoption of novelties is quite normal, if the applied technologies are well proven in other applications and fields, hence, smart phones are easily integrated. Yet there may be growing evidence (this is also a continuous debate) that shows that the electromagnetic wave is unsafe, yet it is still accepted and tolerated. When the concept and prototypes of the self-driving car was introduced, its disruptive nature

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caused a great deal of concern. Many questions around the unknown were voiced, regarding its practical applications including automatic reactions and response times, but also the impact on human behavior and other ethical considerations. The adoption of hydrogen as an energy source belongs within the same category of disruptive technologies. Thus, acceptance and tolerance of such potentially disruptive technology, relates directly to the well-being aspect of the community. This needs to be viewed from different levels of any community facing an unknown technological adoption, as there are multiple, simultaneous viewpoints, which are not static and can oscillate and shift according to the adoption of the community as any new technological mode is introduced. Any adoption has the potential to be successful if multiple, simultaneous viewpoints are considered. Then emergent changes from the communities or even regions themselves can play a big role toward positive outcomes. It takes a village to raise a child; it takes a community to successfully navigate its future technological adoptions. It needs and requires a more holistic prospective. The question of acceptance and tolerance is an intriguing and important one with regard to the resilience and long-term wellbeing of any community. When communities or regions have or are undergoing the many levels of processes involved with any change, it is important to view the phenomena as a spectrum of perspectives. These multiple, simultaneous viewpoints can oscillate and shift according to external constraints, and the emergent changes from within the communities and regions themselves. Armed with the understanding of the complexities and the dynamics that make up any community or region, it would be wise not to assume that they would not necessarily respond in the same way when adopting new technologies. A top-down approach either through authorities or investors, via a typical pattern summarized from other marketing practices, and repeatedly applied would most probably result in hostile rejections. While it is necessary to acknowledge regulatory constraints, it is also necessary to acknowledge bottom-up approaches are equally important, as these phenomena emerge from the community level, and represent long-term range slower rhythms of change. However, these slower rhythms of change have more lasting impacts for behavioral interaction between social, technological, and geological/environmental for communities and their regions. The lack of importance placed on both bottom-up and top-down approach within risk assessments for socio-technical community impacts have to date had unintended setbacks due to a lack of comprehension of differing properties hidden with layers of processes of dynamic socio-technical changes. These socio-technical approaches needs to cross over many boundaries (community, scientific, economic, industrial) of understanding to achieve a successful outcome (well-being). Between introduction and adoption for the community and its region, hydrogen-based energy can be explained in biological terms of evolving through emergent long-term action to produce more stable outcomes.

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The adoption of hydrogen-based energy source can be considered as the process of allopatric speciation, a biological and informatics commonly used phenomena. It needs both bottom-up and top-down socio-technical processes, to coevolve along “the branch” for the combination of mutual outcomes. The “art” of adopting such disruptive hydrogen-related technologies may evolve and branch off into different living examples. Adopting hydrogen-based energy as a new energy source may create unseen (tacit) and known (explicit) combinations of phenomenon that ultimately impact and influence communities. These impacts create specific sets of relational rules or protocols, which eventually help to move the thinking and action beyond the point of no return, a breakaway, to totally different states – allopatric speciation states (11). This is relevant since, as stated before, no community or region is exactly the same, so there must be the possibility for breakaway emergent adoptions and solutions for longer-term success.

14.4.4 Resilience Communities like organizations are not linear; their interactions of levels and layers are complex and dynamic [11, 13, 14]. In order to address and support communities of resilience, bottom-up approaches and observations of long-term equilibrium are necessary. Communities may act like organizations, constantly evolve and change via their individuals, who link or create interdependent relationships on multiple levels [15]. The ability for communities to sustain or improve living standards, requires compatible and renewable ways for individual comprehend their environments. The interconnections both human and non-human are constantly impacted unseen and known actions, resulting in emergent outcomes, of a non-linear nature. The result of community learning gained from process and practices, can present collective memory, that has a better chance of being retained [9, 13, 14]. A community is both the golden eggs and the goose that lays them [15]. Thus, regaining the community’s continuous growing and changing capacity via examining tacit knowledge transfers to achieve balanced environments or space for communities, requires the nurturing of individuals and collective intelligence, which are the energy, spirit and hope of the whole system [13–15].

14.5 Proposal for a hydrogen-based community To further illustrate how to achieve and maintain well-being during adoption of hydrogen-based community, we have used one case from Shanghai, China. The district is located close to the sea and has an international container port and a petrochemical complex nearby. A huge coal-fired power station (2 × 1,000 MW) is also within the

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limit of the district. Thus, hydrogen production based on fossil fuel is in place, so could be expanded too. With future investments of hydrogen in liquid, high-pressure gas cylinder form, the existing hydrogen gas pipeline option would also be possible. The whole area is around 600 km2, with a population of 800,000. The petrochemical complex has 8 existing supplies of hydrogen, and one at IGCC in planning, with an overall total capacity isof 650 kt/a. Natural gas used at the petrochemical complex is supplied by a West–East gas pipeline network. The container port has a turnover of 1.8 mio standard container units per year, with more than 1,000 trailers in and out of the district every day, another 300 plus vehicles for deliveries, including transportation and commuting purposes within the district. All these are planned to convert into hydrogen fuel cell driving. Facilities include shopping malls, residential areas, with schools and exhibition hall being planned as additions to existing areas, but focusing on hydrogen as the energy base. All are prepared to welcome the future development as the suburb of Shanghai. With all these plans for the district, beyond the technical feasibility, how do we prepare the community for the setting up of top-down decisions? What needs to be done for the preparedness and acceptance of the new technology? How to gain knowledge through the adoption of hydrogen based technologies and evolve further?

14.5.1 A top-down design of a district A typical design (see Fig. 14.1) would be based on advantages (including education), and future planning of the district [16]. As a pioneer of the hydrogen-based community, there would be three major principles for Jin Shan to focus on: – Geographical and logistical advantage: on top of the Yangtze Delta – Existing hydrogen availability: solid petrochemical industrial basis and supply of renewable energy – Demonstration for industrial, societal, and ecological integration [17] Jin Shan district would be designed according to the “One Belt One Road” concept: – One Road: one green corridor partly follows the hydrogen pipeline, partly follows the high-pressure gas cylinder and liquid container transportation route. This will be further connected to the Yangtze Delta Hydrogen Highway network (in planning) – One Industrial Development belt: with five centers (innovation center, energy control center, exhibition center, R&D center, and theme park) and six areas (incubation area, lab area, service area, leisure area, training area, residential area)

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Fig. 14.1: Jin Shan hydrogen-based community: 1. Location of Jin Shan district; 2. hydrogen-based community in planning; 3. hydrogen pipelines (existing), fueling stations (in planning), hydrogen production facilities (existing and planning); 4. one industrial development belt with five centers, in planning (see Section 14.4.2); 5. one green corridor connecting to Yangtze Delta in planning (see Section 14.4.2).

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Following the concept as previously outlined, the location of the hydrogen fueling stations would be selected according to the priority setting and weighing system.

14.5.2 The base of bottom-up survey: innovative conceptual framework and design We can assume that the prevailing system in a community such as Jin Shan is inefficient when adopting new technologies like hydrogen, which will be impacted by any ongoing self-sustaining systems that may emerge within such a complex system hierarchy [18]. This corresponds to situations where there is a gap between what people in a local area need and want in terms of affordances from their environment and what the social supersystem is providing. The complex and diverse variety of communication required to demonstrate an individual’s tacit concept or concepts is often shown or demonstrated by the original individual and observed by the others in the outside group or environment [14]. Thus, we suggest the following working groups to investigate the knowledge network impact: – Local community members of the region (such as residents committee members and housing owners committee) – Groups that operate with different life and regional experiences (such as teachers, workers, migrants) – Small and medium-sized enterprise (SME) companies and their various industry clusters (hydrogen fuel cell manufacturers, hydrogen distributors, etc.) – Engineering-based working groups both in educational and in industry settings (firefighters, emergency responders, engineers and chemists, HAZMAT team) – Creative practice-based working groups (such as new setups and ventures) – Groups that operate across different national and cultural communities (governmental agencies, religion and culture committee members, NGOs) Fundamentally the bottom-up approach should have at least two or more phases. Grouped within each phase is a series of practical case study stages. Phase I stage represents a rigorous audit [12] (concluded from previously conducted practical cases in South China close to Guangzhou), which would form a typical common data set for subsequent stages. The proposed phase II stage provides specific data set characteristics that form the basis of multilayered comparative analysis. This approach is specifically designed for investigating tacit knowledge threads within multiple scales through various embedded participants in their specific fields of endeavor. The bottom-up and top-down approach allows for multiple comparative analyses of key concepts to explore both horizontally and vertically across the community and their networks.

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14.5.3 Learning via emergent from adoption Hydrogen is a hazardous material, equipment and human beings dealing with it would be exposing themselves to certain risk. Despite its simplest structure and early discovery in periodic table, we are still not familiar with handling hydrogen in vast quantities outside a factory including the dangers that accompany applications. We need to learn quickly and thoroughly through emergent experiences, especially during the adoption phase. Globally, it comes as no surprise that current educational approaches of “learning” are more about instructing (dos and don’ts) and compliance (regulations and standards), supporting the institutional accounting matrix, enforced by economic, political and other societal external pressures. The difficulties with the limits of current educational abilities to support new technological approaches in terms of realworld applications need new types of viewpoints and mindsets to navigate these new conditions. Otherwise, we may unwittingly seed unwanted allopatric speciation for modern communities and their evolving environments due to impact of socio-technical miscommunication over longer periods of time. From both sides of the fence, community and technology, it can be understood that instructions while valuable on fundamental levels are simply not enough as our world’s boundaries blur into mixtures and various levels of ubiquitous classifications between physical and virtual data (big data and data based on both social and digital network) [16, 20].

Fig. 14.2: Showing the bottom-up and top-down key concepts exploring both horizontal and vertical actions within the community and their networks [19]. This figure is modified from [13].

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The various knowledge sets (as it does not seem that there were ever anything or initial transition points) and conditions that oscillated between and across the socio-technical space need to be expressed and expose clearly the different sets of protocols that operate as a part of these evolutionary relationships (See Figure 14.2). The question of protocols also lends itself to other questions about the various states of evolutionary developments for any community dealing with socio-technical change.

14.5.3.1 Top-down matching bottom-up Any kind of representational dynamic modeling [14, 16] can be based on the dynamic actions of the system and reflects of its many subsystems. The model example discussed here considers three basic forces: dynamic: (a) capacity, (b) robustness, and (c) adaptability, and their initial preconditions of the community in question, including any subsequent behaviors. These factors need to be considered in a simultaneous fashion, taking into account external forces from any constraints, as these may function as systemic catalysts of change. When considering representational models, they can inform from both qualitative and quantitative analysis, and offer understanding of resiliency behaviors within the physical or virtual community. Investigating communities as complex adaptive systems needs an approach that clearly defines the function and sub-component features of the system, including functional scope. For example, corresponding quantifiable systemic constraints may be basically identified as natural, sociocultural, and political-economic constraints. Analysis from a qualitative perspective offers understanding of resiliency from the socio-cultural that supports the socio-technical aspects. Analysis of this sort provides markers that identify aspects within any mapping process to provide a “story line” of the dynamic pathways within a complex adaptive system, in other words a way to show the interactions among component elements within the system. When considering any social systems, “mapping,” key elements can include emergence and/or dissipation of non-emotive adaptive processes. These maps or representations can provide a means for navigation of dynamic social pathways that also provide the foundations for understanding pathways of social cognitive change.

14.5.3.2 Technology matching community Matching new technology to community is a top-down process, while adjusting community to new technology is a bottom-up process. Since there are systems within different levels of a community system to consider, focusing on the relationships, dynamics, and adaptive nature of constraints and attributes is important for any integrated model

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[15, 20]. As such, the following points regarding demographics and population behaviors are of concern [20]: – The concept of adaptive demography, “All true societies are differentiated populations. When cooperative behaviors evolve it is put to service by one kind of individual on behalf of another, either unilaterally or mutually.” – The concept of distribution of demographic, age, and size being components of social behavior. – The concept of a non-adaptive (more linear) demographic, follows from literature regarding the behavior and lifecycles of individual, in contrast to adaptive demography requiring holistic analysis before the behavior and the meaning of the lifecycles can be comprehensively understood. – The concept of the adaptive setting, where moments of demographic frequency and distribution can take on new significance.

14.5.3.3 System of system: community–technology–sustainability When we put community–technology–sustainability together, it needs positioning and understanding of the needs for longitudinal sustainable development and management concerning systems-of-systems, in other words the inclusion of a synthesis from a sociobiology [22] perspective. The focus could be considered as the following: – Context – new technology is discussed and understood within the context of community and sustainability. – Relationship – new technology is the problematic of the landscape that provides the relationship with community and sustainability (fundamental to longitudinal approaches). – Perspectives – that new technology carries divergent perspectives within the context of community and sustainability. – Application/fieldwork – Jin Shan District and surroundings districts are included. This approach would provide a construct from which to build and develop a model to expand understanding of the dynamic viability of resilience and pressure points or measurable changes in the state and why they may be relevant or truly worth our attention. This could lay the basis for multi-signal alarm mentioned in Section 14.3.1.

14.5.4 Development for community well-being Jin Shan District was once an economically booming area, with a modern refinery and petrochemical complex was built in the 1980s–1990s, and had an international standard of safety and environmental management by Cao Jin Chemical Park – with many multinational production sites settled in beginning of Millennium. Jin Shan

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had experienced some protests against PX (Paraxylene) – Oxidation unit in the 2010s, due to suspicion of toxicity and explosion risk. Today, the local residents still have some reservations against chemical production. Despite the advantages, developing Jin Shan into a Hydrogen based community, there may have certain bad feelings and emotions against it. It is necessary to work with these former concerns, and achieve wellbeing for all towards a cleaner future. The fundamentals of well-being for communities lies in the sound, long-term development tacit knowledge networks of communities of practice, industry clusters working groups, cultural communities and conscious awareness of multicultural groups [10, 15, 16, 20, 25]. It is important that key individuals forming groups to construct, share, and apply knowledge relating to the problematic (hydrogen, hazardous chemicals, explosion) should coalesce around a “human attractor” [15] or a competence center (instead of community center) that attracts relevant individuals (not allocated individuals). The gathering of relevant and needed community members would include responsive and reactive communities, including grass roots lobbies, advocacy groups, attracting beyond the individual to share common goals and purpose. The attractor could offer a way, through emergence, a type of structure (blueprint or a DNA) to keep the organization self-replicating. People could recognize that they share interests with the attractor within the center and begin to exchange knowledge with her/him within Competence Center, and having been brought together by the attractor, they also begin to network amongst themselves through various knowledge transfer processes. This form of self-replication could serve as a basis of the Competence Centre, and could continue in the form of research and development, expanding the basis for a scientific, societal and socio-technical knowledge and emergent action for community well-being.

References [1] [2] [3] [4] [5] [6] [7]

, PDF file presented by BOC, now a member of LINDE. https://en.wikipedia.org/wiki/Adiabatic_flame_temperature Chinese national standard: GB 50156–2012, Code for design and construction of Gas and Liquid Fuel filling stations, 2014. Chinese national standard: GB 50516–2010 Technical Code for Hydrogen Gas Fuelling Station, 2010, currently under revision; https://www.10news.com/news/emergency-crews-respond-to-explosion-report-in-el-cajonneighborhood https://insideevs.com/news/354223/hydrogen-fueling-station-explodes/ https://news.cision.com/nel-asa/r/nel-asa–status-update–5-regarding-incident-at-kjorbo, c2852275

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Giuseppe Ricci, Maurizio Dessì, Marco Tripodi, Paolo Fiaschi, Roberto Palmieri, Luca Eugenio Basini, Thomas Pasini, Alessandra Guarinoni

15 Eni’s projects in Italy for hydrogen production 15.1 Foreword Eni is one of the global energy companies employing over 31,000 people in 66 countries in the world. Eni is working to build a future where everyone can access energy resources efficiently and sustainably. Eni’s work is based on passion and innovation, on unique strengths and skills, on the quality of the people, and in recognizing that diversity across all aspects of the operations and organization is something to be cherished. Eni believes in the value of long-term partnerships with the countries and communities where it operates. Eni’s commitment aims to respond with concrete, rapid, and economically sustainable solutions, to the challenge of improving access to reliable and clean energy, fighting climate change. Eni recognizes the Intergovernmental Panel on Climate Change’s scientific evidence on climate change and was among the signatories of the Paris Pledge for Action, supporting the objectives of the Paris Agreement to limit temperature increases to well below 2 °C. Eni’s strategy combines the objectives of continuous development in a rapidly evolving energy market with a significant reduction in the Group’s carbon footprint with ambitious net lifecycle emission (scope 1, 2, and 3) reduction targets: −30% by 2035 and −80% by 2050. Eni believes in the importance that hydrogen will have in the European economy and has defined a strategy that comprises all clean hydrogen production pathways, including natural gas (NG) reforming with carbon capture and sequestration (CCS), circular economy technologies for production of low-carbon hydrogen from plastic waste and from renewable electricity. The following sections describe a circular economy project on hydrogen production currently under investigation in Eni: hydrogen from plastic waste in Venice biorefinery. In the second part, the focus is on recent Eni’s project on short contact time (SCT) catalytic partial oxidation named kGas technology. In the final section, there are some insights on hydrogen production technologies with CCS.

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15.2 Hydrogen production from plastic waste: waste to H2 project in Venice Paolo Fiaschi, Roberto Palmieri

15.2.1 Introduction The purpose is to describe the basics of a hydrogen production plant based on hightemperature conversion technology of plastic and secondary recovery fuel (SRF) waste. Waste to hydrogen project, the realization of which is at the moment under evaluation, would have an estimated production equal to 25,000 Nm3/h of hydrogen, to feed Eni’s biorefinery of Porto Marghera (VE), in particular the ecofining process aimed at the production of HVO (Hydrotreated Vegetable Oil). The plant is capable of producing hydrogen from a plastic waste, identified by the technical term plasmix and SRF. The total amount of waste treated is between 170,000 and 200,000 ton/year, with small fluctuations depending on the amount of calorific power of incoming waste. The conversion of plastic waste into hydrogen is achieved through a series of conversion and purification phases, aimed at producing a hydrogen-rich syngas and removing residual pollutants such as metals, chlorine, and sulfur. The process can be divided into three macrounits as shown in the block diagram in Fig. 15.1: – Section of waste gasification, washing, and stabilization of syngas – Syngas compression and purification section – Section of conversion of syngas into hydrogen and its purification

15.2.2 The waste scenario in Italy and Veneto region In total, more than 30 million tons of urban solid waste are produced annually in Italy, including organic fraction, recyclable materials, and SRF. The percentage of separate collection is particularly variable among the Italian regions and reaches an average value of more than 58%. Only a part of the waste produced is now recycled, while the remaining quantities are destined for incinerators or landfills in Italy and abroad. In Italy, there is a strong need to limit landfill in order to reach the European target of 10% by 2030. In particular, among the wastes of interest for Eni, there is plasmix, consisting of nonrecyclable plastic waste. Plasmix is a mixture of mixed plastics obtained at the end of the selection of plastics from the differentiated collection, also called the end-of-tape waste. Every year in Italy about 500,000 tons of plasmix are produced and destined for incineration or landfill (source: Corepla).

Vitrified inerts

Wastewater

Wastewater treatment

Gasification, washing and storage

Syngas compression

Fig. 15.1: Waste to hydrogen process scheme.

PLASMIX SRF

Oxygen Fuel gas

Metals removal HCI removal

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SRF is the secondary solid fuel, which is the separate residual fraction downstream of the mechanical treatment of undifferentiated waste. Around 1.3 million tons of SRF are produced in Italy each year, and today they are mainly used in traditional incinerators. Veneto has collection levels in line with average collection percentages in Italy. In 2018, Veneto region saw a total production of urban-only waste amounting to about 2.4 million tons, of which about 1.8 million tons are differentiated, while the remaining part, about 0.6 million tons, constitutes the undifferentiated harvest. The vast majority of the waste collection confluences with material recovery (about 1.55 million tons), while the remaining part (0.25 million tons) is destined for landfill or incineration because it consists mostly of nonrecyclable materials. With a view to reducing landfill waste, it can be estimated that around 0.14 million tons from recycling and nonrecycled waste can be used in incineration systems. For the undifferentiated collection, however, the situation is much less virtuous. Although there is an excess of mechanical biological treatment plants in Veneto, to which the undifferentiated collection is usually sent to be selected and destined for correct treatments and disposals, there is a shortage of incineration plants. For this reason, except for a small portion of undifferentiated harvesting that manages to flow, after the necessary selection, to the recovery of matter (0.004 million tons) a large part of the undifferentiated collection ends up in landfill (about 0.2 million tons). Some of these are estimated to be directed, rather than to landfill, to incineration plants. On the other hand, some 0.25 million tons of undifferentiated harvesting are already earmarked for incineration. In total, the potential for undifferentiated collection incineration is estimated at around 0.45 million ton/year. Taking into account the flows from differentiated collection and undifferentiated collection, the incineration potential of Veneto is about 0.7 million ton/year. Considering Veneto’s incinerator capacity, between 0.25 and 0.3 million tons in 2018, it is clear how Veneto has to fill a plant gap even today (0.45 million ton/ year). The current gap will only be partially offset by a hypothetical growth in recycling and therefore a greater flow toward the recovery of matter. Landfilling will, of course, not be able to compensate for the plant gap present today, both for the delivery targets of not more than 10% by 2035 and for the saturation of the landfills available today.

15.2.3 Technological features and location of the plant To find an alternative and innovative solution to incineration or landfill, new technologies have been introduced over the years based on the concept of “indirect combustion” and in particular on the gasification process. The preliminary transition from solid to gas reduces formation of dangerous elements, improves combustion control, and allows to work with high calorific power materials such as nonrecyclable plastic residues.

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The technology first involves the conversion of waste into synthetic gas (syngas) through a process of thermal conversion and then the production of hydrogen from purified synthetic gases. The plant that is expected to be built will have a waste treatment potential of about 200,000 ton/year for a production of about 25,000 Nm3/h of hydrogen. The choice to locate the plant within the perimeter of the Venice refinery has the following advantages: – no subtraction of soil compared to the area occupation currently used for production/industrial use; – presence of railway line and docks near the refinery, which can be used to transport waste that constitutes the “charge” of the process of production of hydrogen; – the refinery is particularly suitable as it makes available many services such as available areas, roadways, and the presence of warehouses/buildings and workers trained for maintenance; and – the refinery allows plant synergies as the plants present are suitable to purify industrial waters and is also able to supply electricity to the hydrogen production plant and utilities needed for the process (steam, cooling water, etc.).

15.2.4 Process description 15.2.4.1 PLASMIX/SRF gasification section, syngas purification, and stabilization 15.2.4.1.1 PLASMIX/SRF gasification The first step in the process is the conversion of waste into a syngas stream, rich in H2 and CO and free of CH4 and upper organic compounds (tar). To meet the required hydrogen capacity during both normal operating conditions and maintenance phases, three gasification lines are required that run in parallel in the 3 × 50 solution. This results in an operating capacity per line of about 7.5 ton/h and a project capacity of 10 ton/h. Waste from the plant is fed to each of the gasification reactors through a system of cochlea and inert nitrogen channels. The latter allows the plant to balance the existing pressure inside the reactor, thus avoiding any loss of syngas during the waste loading phases. Due to the high temperatures inside the reactor, the walls of the charging duct flow with demi water to ensure temperatures of the order of 40 °C near the unloading screws. The gasifier uses pure oxygen as a gasification agent. Temperature control is carried out by modulating oxygen injections using injectors positioned at different heights along the reactor. Depending on the level of temperatures reached, two different zones can be distinguished within the reactor: a higher section, where temperatures of about 1,100 °C can be reached to ensure that complex molecules are completely broken, and a lower section affected by higher temperatures where the inert components are completely

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melted. The high temperatures also ensure the complete volatility of the organic fractions that flow to the upper section and at the same time avoid the formation of dioxins and furans as they are stable at lower temperatures. The inert fraction is collected at the bottom of the reactor and flows into water from which it is discharged in the form of granules. The melting phase does not produce ash and given the temperatures reached it makes the vitrified material completely inert. Fuel gas or other fuel is fed to ensure the temperature required in the melting zone along with oxygen. The main products of gasification are therefore a syngas (CH4 free and tar) and an inert product vitrified in the form of granules. The resulting vitrified product is sent at the battery limits to be disposed in landfill even if its carbon content is almost zero. The temperature level reached at the top, closely related to the quality of syngas, is monitored through a series of thermocouples. The high-temperature syngas is subsequently subjected to a strong cooling with direct injection of water. The sudden drop in temperatures to about 90 °C freezes the bonds that have formed at high temperatures, thus avoiding, during the cooling phase, the formation of unwanted compounds such as dioxins and furans. Leaving the quench stage, both the syngas and the condensed phase are sent to the sedimentation unit. The sedimentator operates at controlled pH through the dosage of chemicals (NaOH and/or HCl) in order to ensure the precipitation of heavy metals present in the syngas. The sedimented material is extracted from the bottom and sent to the treatment phase. The syngas coming out of the sedimentation tank after a further quench is powered by a cleaning train consisting of – acid washing, – cyclone, – alkaline washing, – dust removal with wet electrostatic filters, and – cutting drags with undercooled washing. Acid washing units and cyclones are present in each of the gasification lines, while basic washing, electrostatic filtration, and washing with cooled water are common to the three lines. In the acid drainage column, the gas is cooled down, causing part of the saturation steam to condense. The water recovered from the bottom of the acid wash column is partly recirculated in the sedimentation tank and partly cooled and sent back to the head of the washing column itself. The gas is then sent to the cyclone. The syngas coming out of the three lines is conveyed and sent to an alkaline wash column, where through the injection of NaOH the pH is brought back to alkaline, in order to minimize corrosion problems in downstream equipment. The water recovered from the bottom is partly recirculated at the head of the column after cooling in a plate exchanger and partly sent to the water treatment unit.

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In order to limit the volumes to be sent to disposal to battery limits, internal treatment is provided for the concentration of waste and the recovery of water to be recirculated within the process. The syngas leaving the column undergoes an additional cleaning stage consisting of two wet electrostatic filters placed in series for the reduction of dust and heavy metal residues. To ensure an intensive cleaning, at the exit of electrostatic filters, the gas is sent to an undercooled washing column, which cools down the gas, through the use of cold chilled water. As with the alkaline washing column, the water recovered from the bottom is partly recirculated at the head of the column and partly sent to the water treatment unit. The syngas from the washing section is heated through a thermal recovery and then sent to the stabilization section consisting of a water gasometer. 15.2.4.1.2 Syngas compression and purification section Syngas compression Since syngas is available at pressure little more than the atmospheric pressure, compression is required before starting it to the next sections. The syngas is then powered by a multistage compression unit with intermediate cooling. Condenses formed during cooling phases are sent to internal wastewater treatment as a precaution. The pressure of the syngas current is then increased up to 20 bargs through three compression stages. Pollutants removal The compressed syngas is initiated to a purification train aimed at eliminating residual pollutants, including particulate matter, HCl, and sulfurous compounds, for which a specific culling is necessary. Downstream of the compression section the syngas is sent to a particulate removal system followed by an HCl fueling system. Both systems have two guard beds working in lead lag mode to ensure continuity of the process. Polluting compounds such as H2S, COS, and HCN remain to be eliminated. To remove COS and HCN, a hydrolysis stage that turns them into H2S and NH3, respectively, according to the following reactions, is needed: COS + H2 O $ H2 S + CO2

(15:1)

HCN + H2 O $ NH3 + CO

(15:2)

The syngas coming out of the HCl removal system is preheated and then fed to the hydrolysis reactor, where organic-sulfurous compounds (mainly COS and CS2) are hydrolyzed in hydrogen sulfide and hydrogen cyanide in ammonia. Steam can eventually be used to promote and push hydrolysis reaction conversions.

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The outgoing gas from the hydrolysis reactor is sent to the H2S removal stage. The selected system, given the quantities of H2S present in the syngas, consists of a Lo-Cat able to break down the H2S content in syngas at the level of a few ppm and turn hydrogen sulfide into solid elementary sulfur. The gas must first be cooled to about 40 °C and sent to a separator to remove the formed condenses that are then sent to the stripping column. The sulfur produced in the Lo-Cat unit, depending on the intended use, may be available as a pure sulfur or with a certain degree of moisture. The residual H2S content present in the gas from the Lo-Cat system must be brought to a few hundred ppb in order to preserve the downstream catalysts. This additional stage of purification ensures a thrust removal of the residual H2S at the desired level through the following reaction: H2 S + ZnO ! ZnS + H2 O

(15:3)

15.2.4.2 Syngas conversion to hydrogen and purification 15.2.4.2.1 Syngas conversion to hydrogen The syngas current purified from sulfurous compounds at 200 °C is subjected to a high-temperature water gas shift treatment in order to increase the hydrogen content according to the following reaction: CO + H2 O $ CO2 + H2

(15:4)

The stream is then preheated through a thermal recovery with the effluent coming out of the shift reactor itself and added steam overheated to about 300 °C, in control of such scope as to ensure a steam/dry gas ratio of 1.5. The stream is then fed to the shift reactor. Given the exothermally of the reaction, the temperature coming out of the reactor reaches temperature above 400 °C. The catalyst used in the high-temperature shift reactor is sulfur-sensitive, so a thrust purification unit was required, as above. In order to maximize hydrogen production, an additional high-temperature shift reactor should be inserted, allowing an overall conversion of CO in the two shift stages higher than 90%. 15.2.4.2.2 Hydrogen purification and compression The hydrogen-rich current from the second water gas shift reactor is cooled with subsequent condensation separation. The syngas is then sent to a pressure-swing adsorption (PSA) unit to purify the hydrogen it contains. The gas purge available at the pressure of 0.3 barg is used as fuel in an auxiliary combustor/boiler along with a fraction of spiked syngas. The amount of syngas is such that the steam requirement of the whole process is closed. In the event that the

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calorific power of the charge does not meet the capacity of the plant to obtain a certain production of steam the boiler is also fed with fuel gas from the battery limits. Hydrogen is separated from the PSA unit with a purity higher than 99 mol% and low CO content.

15.2.4.3 Section of treatment of liquid effluents from the gasification section The main liquid effluents that require cleaning treatment are as follows: – sludge from the sedimentator and the acid wash column; – effluent of the alkaline washing column; – effluent of the subcooling column; and – condensate. The liquid current discharged from the sedimentator consists mainly of the water produced during the quench phase and those resulting from acid washing. The two operations, conducted at acid pH environment, result in the transition of almost all heavy metals and chlorine into solution. The wash also brings into resolution the carbon powder dragged by the syngas current. The effluent produced during the basic washing phase where pH is brought back above 7 to limit corrosion problems may also contain, to a much lesser extent, suspended metals and suspended solids. In the context of the Venice refinery, taking into account the parameters required for the delivery in the consortium system of waste treatment, especially with regard to the level of metals (unspecified) and ammonia nitrogen, it excludes the possibility of giving in it the washing water resulting from the gasification section. There is therefore an internal treatment aimed at recovering the carbon fraction and minimizing the volumes of waste to be disposed of externally. The treatment sequence consists of the following steps: – separation of the carbon material and its recycling to the gasifier; – pH adjustment to create a basic environment conducive to precipitation of heavy metals in the form of hydroxide; – reducing the residual water content of precipitated phases by using filter press; and – treatment of the remaining water phase through evaporation in order to maximize the fraction to be recycled within the process.

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15.2.4.4 Focus on feedstock 15.2.4.4.1 What is secondary recovery fuel (SRF) In Europe, and especially in Italy, there is a context with a high level of industrialization, but very sensitive toward public health and the environment. In particular, in terms of waste management, the principle behind modern philosophies, in variants that the various most developed countries have proposed and adopted, is that of the so-called 3R rule: reduce, reuse, and recycle. The aim is to maximize recycling and minimize the environmental impact of installations. With this in mind, therefore, extensive use of the differentiated collection of recyclable product fractions has been implemented: glass, plastics, metals, paper, and kitchen waste. Waste not intercepted by recycling systems is usually started at mechanical–biological treatment plants, where, through selection processes, a second phase of differentiation is carried out in which they are further separated into: – recyclable materials that escape recycling, – organic fraction, and – the residual fuel fraction. The first part follows the same path as the selected materials at the source from the separate collection. The organic fraction can be stabilized by aerobic or anaerobic treatment systems. The last fraction, which is not further recyclable or usable, usually has a good energy content and constitutes the so-called SRF. Recyclable materials, both collected in a differentiated manner and selected by the undifferentiated part, need processing and refinements before being reintroduced into their respective production cycles, which in turn generate significant volumes of waste. This applies to both urban solid waste and all those categories of waste produced by commercial and industrial activities. These fractions, which are not effectively recyclable and not dangerous, have usually a high energy content and, where they own chemical and physical characteristics set by the regulations, they are also classified as SRF. Currently, SRF in Europe is mainly used in traditional incinerators. 15.2.4.4.2 What is nonrecyclable plastic waste? Nonrecyclable plastic waste is also referred to as plasmix. Plasmix is a mixture of mixed plastics, obtained downstream of the selection tape, also known as ETW (end-of-tape waste). Depending on how the treatment of plastics is driven, a distinction can be made between – ETW or plasmix, which is the end-of-tape scrap; – ETW treated, which can be obtained through a pretreated implantation system; – SRA (secondary reducing agent), that is, the ETW obtainable, thanks to a series of physical and mechanical pretreated operations to transform it into SRA, generally provided as scales of aggregate material.

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According to a report by COREPLA (Consorzio Nazionale per la Raccolta, il Riciclo e il Recupero degli Imballaggi in Plastica), the national consortium for the collection, recycling of plastic waste for the appropriate enhancement of plastics, an appropriate preliminary selection, is necessary in order to distinguish the type and possible recycling, but above all an appropriate mechanical treatment to distinguish in a distinct way the share of plastic packaging that can no longer be valued with mechanical recycling from that which can be sent to a process of thermovalorization or other process. The aim is to maximize recycling and minimize the environmental impact of installations. According to the COREPLA consortium, the SRA is used in steel mills as a substitute for metallurgical coke in the dual role of fuel and reducing agent in oxidation reactions of ferrous materials. Currently, however, plastics in Europe are mainly disposed of in traditional incinerators. 15.2.4.4.3 Why SRF and plastic waste are currently being treated in incinerators Incineration developed in the post-war period due to the volumetric reduction of large quantities of urban wastes is precisely reduced to ashes before they are sent to landfill. This system has been modified and updated over time, both to overcome the environmental inconveniences involved, adding, for example, sections of purification of fumes, and to make it more rational and economical, adding, for example, the energy recovery section, but it always remains a system with inherent problems related to the applied process. In Japan, geographical and demographic conditions have long severely hampered the use of landfills and forced the installation of waste treatment plants in residential areas, demonstrating that environmental issues are very much felt. In this context, the development of waste treatment systems has been greatly encouraged, and the country’s wealth and its basic technological level have enabled extensive research and development. In order to overcome the drawbacks of traditional heat-tract systems, new technologies have been introduced based on the concept of “indirect combustion” and in particular on the gasification process. Since 2000, new orders for gasification plants in Japan have exceeded those of incinerators and there are more than 130 orders currently under construction. The concept arises from the observation that the combustion of a gas is simpler and more effective than that of a solid. Gasification is a process of high-temperature -induced molecular dissociation. In an environment of severe oxygen deficiency, the combustible fractions of the treated material are released into a gas, called “synthesis” or “syngas,” rich in hydrogen.

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Gas has now been used exclusively for the production of electricity. The preliminary switch from solid to gas dramatically reduces the formation of hazardous elements, improves combustion control, and works on high energy content materials such as nonrecyclable plastic residues. This is reflected in very low pollutant emissions, especially for dioxins and furans that are often under the measurability. Efficiency in energy production and inerts rendered in a potentially reusable form complement the strengths of this choice. But the system, even inferior to other solutions, still generates emissions. In addition, energy recovery, as with traditional incinerators, has a relatively low efficiency due to the small size of the plants, which means that the single kWh of electricity is produced with a high environmental “price” in terms of both pollutants and CO2. These assessments, together with new European waste management regulations, have led to a search for alternatives that favor the recovery of matter over energy. In this context, studies have been under way for some years on the possibility of using waste-derived synthetic gas not for energy production but for the synthesis of chemicals such as urea, methanol, hydrogen, methane, and ethanol.

15.2.4.5 Reasons for the new Eni’s waste to hydrogen project The waste to hydrogen project that Eni intends to carry out within the area of the Venice biorefinery is aimed at the production of hydrogen from plasmix and SRF from the treatment of urban solid waste. Waste and the material treated by the waste to hydrogen plant, in technical jargon called “charge,” are in the first phase plasmix and SRF. This project aims to recover a good part of the waste that Veneto region allocates to incineration or landfill: Eni proposes a project aimed at the recovery and enhancement of the material, which can be used for the production of mobility products. Unlike the incinerator, where fumes are treated and then released into the atmosphere, the waste to hydrogen project allows the production of synthetic gases with a controlled thermal reaction, in a closed environment and therefore without direct emissions into the chimney and takes place at temperature conditions that vitrify pollutants and make them inert and stable.

15.2.4.6 What is the potential of this technology compared to an incinerator? Compared to waste to energy, the waste to hydrogen project has the following advantages: – lower CO2 production, including on the basis of Eni’s internal studies of life cycle analysis (generically defined by the acronym LCA), with a high degree of purity to allow its commercialization and reuse in the industrial field;

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– high energy efficiency of the gasification process that takes place in the presence of pure oxygen with no harmful emissions (SOX and NOX); – a new product (hydrogen) is generated from the treated waste, which can be classified as recycled carbon fuel under the Renewable Energy Directive II. The environmental benefits in terms of greenhouse gas emissions were calculated through an LCA analysis, carried out by Eni’s research and development, to assess through a comparison the efficiencies and emissions of energy and fuel technologies, both upstream and downstream of the technologies and treatment processes considered. This analysis provides a complete picture of real emissions as it considers both the entire production chain and the phase of use of the final product. For this reason, the comparison of gasification technology is carried out with conventional waste treatment technology (heating) in combination with conventional technology of production of the chemicals in question (steam reforming (SR) by methane).

15.2.4.7 Employment impacts and investment Investment requirement estimation is currently ongoing and commits a refinery area of about 3.5 hectares. The activities planned to carry out the project include the execution of works aimed at making available the necessary spaces for the housing of the plants and the ancillary and connected structures and the subsequent commissioning. The main operational phases of the project can be divided into: – Site phase, which includes the adaptation of the existing areas and structures identified for the installation of the plants, as well as the mechanical and electroinstrumental activities for the subsequent production. – Operating phase, which includes both the operation of the hydrogen production plants and the inbound waste management and charge preparation phase. 15.2.4.7.1 Legislative framework Now that the rules on the termination of the waste qualification are once again in place, thanks to the early adoption of the European standard (Article no. 6 of the Waste Directive 2018/851/EU), the initiative could also contribute to achieve the target of renewable sources in transport by highlighting the contribution of the circular economy in this area. The Renewables Directive (EU 2018/2001 – RED II) allows Member States to consider recycled carbon fuels (products from waste or gas from waste treatment) to meet transport targets set at 14% by 2030 by the Directive and raised to 22% by the Italian PNIEC.

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15.3 H2 and its production technologies: the kGas solution Luca E. Basini, Alessandra Guarinoni, Thomas Pasini Hydrogen is mainly (approx. 94 wt%) produced from synthesis gas via SR and noncatalytic partial oxidation (POx), from hydrocarbons, such as NG, heavy refinery residues, naphtha (mainly in India), and coke (mainly in China) [1–5]. Other feedstock having a minor utilization includes those having a biomass origin such as biogas and biomethane. Noteworthy, the lowest CO2 emissions are achieved when hydrogen is obtained from NG, the hydrocarbon feedstock with the lowest carbon content. As already mentioned, SR is largely the preferred technology when NG is used as a feedstock and POx is almost exclusively utilized for processing high-carbon content feedstock. Hydrogen is also produced in minor amounts through water or steam electrolysis in small or medium capacity plants. In these cases, for improving the sustainability of the production, the use of electric energy surplus (e.g., from the grids and from nuclear energy plants) or from renewable sources is clearly advantageous.

15.3.1 Steam methane reforming SR technical elements include a furnace, reforming tubes filled with catalysts , a purge gas boiler for cooling the synthesis gas rising high-pressure steam and a convective heat recovery section. The furnace provides the heat for the occurrence of the endothermic reactions occurring inside the tubes at temperatures between 700 and 1,000 °C and pressures comprised between 5 and 30 atm. Although the technical elements of the technology (furnace, reforming tubes, and heat recovery sections) have been continuously improved, this technology concept has remained the same during the last 80 years. The synthesis gas at the exit of the reforming tubes includes the hydrogen, carbon monoxide, and a comparatively small amounts of carbon dioxide and unreacted methane. This synthesis gas is then “shifted” with a water–gas shift reactor converting carbon monoxide into additional hydrogen and carbon dioxide. A final physical process step “PSA” separates the shifted synthesis gas into a high-pressure pure hydrogen flow and a low-pressure flow mainly including carbon dioxide and unreacted methane: Steam−methane reforming reaction CH4 + H2 Oð+ heatÞ ! CO + 3H2 Water−gas shift reaction CO + H2 O ! CO2 + H2 ð+ small amount of heatÞ

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The production of syngas through reforming (SR) is based on four main technical elements: – Furnace – Reforming tubes filled with catalysts – Convective heat recovery section – Purge gas boiler As already mentioned, POx is instead utilized for producing hydrogen from heavy residues or coke, burning these feedstocks with pure oxygen in a substoichiometric flame for obtaining a synthesis gas that is often very rich in carbon monoxide and typically with a hydrogen versus carbon monoxide ratio lower than 1 on a volumetric basis. Subsequently, in a water–gas shift reaction, the carbon monoxide reacts with water to form carbon dioxide and more hydrogen. Again, at the end of this process, hydrogen is recovered with a PSA step. POx is an exothermic process – it gives off-heat which is recovered with a syngas cooler step by rising high-pressure steam. The flame-based process is typically much faster than SR process and requires a much smaller reactor vessel. As can be seen in chemical reactions of partial oxidation, this process initially produces less hydrogen per unit of the input fuel than what is obtained by SR of the same fuel: Partial oxidation of methane reaction Cx Hy + x=2O2 ! xCO + y=2H2 ð+ heatÞ Water−gas shift reaction CO + H2 O ! CO2 + H2 ð+ small amount of heatÞ Another synthesis gas production technology that has never been applied for producing hydrogen is the autothermal reforming that combines non-catalytic POx and catalytic steam methane reforming (SMR) reactions into a single reactor [6]. The process is instead mainly applied for producing syngas intermediate from NG for applications in methanol and ammonia synthesis and in the Fischer–Tropsch production of fuels: Initial auto thermal reforming reaction CH4 + 3=2 O2 ! CO + 2H2 O This reaction is exothermic and provides the heat required for the following intermediate reaction where the CO2 produced reacts with NG: Secondary auto thermal reforming reactions CH4 + CO2 ! 2CO + 2H2 CH4 + H2 O ! CO + 3H2

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15.3.2 Short contact time – catalytic partial oxidation (SCT-CPO): the kGas technology This is a novel entirely catalytic technology aiming at producing H2 and CO through the direct oxidation of hydrocarbons and its progress toward industrialization initiated in 1992, with initial fundamental observation by L.D. Schmidt [7–9]. Since then, several industrial environments (including Shell [10], Conoco [10], Praxair [11], Haldor Topsoe A/S [12], and Eni [13]) have deepened the possibilities of applying the fundamental discoveries for developing and industrial technology. Recently, after more than 20 years of R&D work, Eni has announced the construction of a demonstrative plant to be operated within the Taranto refinery. The development project will sustain the most efficient exploitation of NG, the reduction of greenhouse gas (GHG) emission in a number of syngas-related processes. This technology, named kGas, is not an improvement of the existing ones but a radically new way of making the synthesis with very small and simple reactors and syngas cooler system integrating into same elements more technical functions, as shown in Fig. 15.2. Noteworthy, in kGas the hydrocarbon oxidation occurs when gaseous premixed reactant flows collide for few milliseconds with extremely hot catalytic surfaces. This originates a fast and selective chemistry confined inside a thin (