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Ash Behavior in Fluidized-Bed Combustion and Gasification of Biomass and Waste Fuels

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Ash Behavior in Fluidized-Bed Combustion and Gasification of Biomass and Waste Fuels Thesis for the Degree of Doctor of Philosophy Ash Behavior in Fluidized-Bed Combustion and Gasification of Biomass and Waste Fuels Experimental and Modeling Approach Farzad Moradian Copyright © Farzad Moradian Swedish Centre for Resource Recovery University of Borås SE-501 90 Borås, Sweden Digital version: http://urn.kb.se/resolve?urn=urn:nbn:se:hb:diva-9563 ISBN 978-91-88269-14-0 (printed) ISBN 978-91-88269-15-7 (PDF) ISSN 0280-381X, Skrifter från Högskolan i Borås, nr. 78 Cover photo: fire flames Borås 2016 ii Abstract Over the past few decades, a growing interest in the thermal conversion of alternative fuels such as biomass and waste-derived fuels has been observed among the energy-producing companies. Apart from meeting the increasing demand for sustainable heat and power production, other advantages such as reducing global warming and ameliorating landfilling issues have been identified. Among the available thermal conversion technologies, combustion in grate-fired furnaces is by far the most common mode of fuel conversion. In recent years, Fluidized-Bed (FB) technologies have grown to become one of the most suitable technologies for combustion and gasification of biomass and waste-derived fuels. In spite of the benefits, however, some difficulties are attributed to the thermal conversion of the alternative fuels. Ash-related issues could be a potential problem, as low-grade fuels may include considerable concentrations of ash-forming elements such as K, Na, S, Ca, Mg, P, Si and Cl. These elements undergo many undesirable chemical and physical transformations during the thermal conversion, and often cause operational problems such as deposition- related issues, slag formation in furnaces, corrosion of the heat transfer surfaces, and bed agglomeration of the fluidized-beds. Ash-related problems in the utility boilers are a major concern that may result in decreased efficiency, unscheduled outages, equipment failures, increased cleaning and high maintenance costs. This thesis investigated the ash behavior and ash-related problems in two different FB conversion systems: a Bubbling Fluidized-Bed (BFB) boiler combusting solid waste, and a Dual Fluidized-Bed (DFB) gasifier using biomass as feedstock. Full-scale measurements, chemical analysis of fuel and ash, as well as thermodynamic equilibrium modeling have been carried out for the BFB boiler (Papers I-IV), to investigate the impact of reduced-bed temperature (RBT) and also co-combustion of animal waste (AW) on the ash transformation behavior and the extent of ash-related issues in the boiler. For the DFB gasifier (Paper V), a thermodynamic equilibrium model was developed to assess the risk of bed agglomeration when forest residues are used as feedstock. The experimental results showed that the RBT and AW co-combustion could decrease or even resolve the ash-related issues in the BFB boiler, resulting in a lower deposit-growth rate in the superheater region, eliminating agglomerates, and a less corrosive deposit (in RBT case). Thermodynamic equilibrium modeling of the BFB boiler gave a better understanding of the ash transformation behavior, and also proved to be a reliable tool for predicting the risk of bed agglomeration and fouling. The modeling of the DFB gasifier indicated a low risk of bed agglomeration using the forest residues as feedstock and olivine as bed material, which was in good agreement following the observations in a full-scale DFB gasifier. Keywords: Fluidized-bed, combustion, gasification, waste-derived fuels, biomass, ash-related problems, deposit, fouling, slagging, bed agglomeration, thermodynamic equilibrium modeling iii iv List of Publications This thesis is based on the results presented in the following articles: I. Pettersson A, Niklasson F, Moradian F. Reduced bed temperature in a commercial waste to energy boiler — Impact on ash and deposit formation. Fuel Processing Technology 2013;105(0):28-36. II. Moradian F, Pettersson A, Svärd S, Richards T. Co-Combustion of Animal Waste in a Commercial Waste-to-Energy BFB Boiler. Energies 2013;6(12):6170-6187. III. Moradian F, Pettersson A, Richards T. Bed Agglomeration Characteristics during Cocombustion of Animal Waste with Municipal Solid Waste in a Bubbling Fluidized-Bed Boiler—A Thermodynamic Modeling Approach. Energy & Fuels 2014;28(3):2236-2247. IV. Moradian F, Pettersson A, Richards T. Thermodynamic Equilibrium Model Applied To Predict the Fouling Tendency in a Commercial Fluidized-Bed Boiler, Combusting Solid Waste. Energy & Fuels 2015;29(5):3483-3494. V. Farzad Moradian, Placid A. Tchoffor, Kent O. Davidsson, Anita Pettersson, Rainer Backman. Thermodynamic Equilibrium Prediction of Bed Agglomeration Tendency in Dual Fluidized-Bed Gasification of Biomass. Manuscript. v Statement of Contributions Farzad Moradian’s contributions to the appended papers: Paper I: Co-author of the paper, contributed to the full-scale measurements, and SEM- EDX analysis of the ashes. XRD analysis of the ashes was performed by Anita Pettersson. Standard chemical analysis of the fuel and ashes were performed by an authorized laboratory. Paper II: Principal author of the paper, contributed to the full-scale measurements, and performed the chemical fractionation and SEM-EDX analyses of the ashes. XRD analysis of the ashes was performed by Anita Pettersson. Standard chemical analysis of the fuel and ashes were performed by an authorized laboratory. Lab-scale bed agglomeration tests were carried out by SP. Paper III: Principal author of the paper, contributed to the full-scale measurements, and performed the chemical fractionation of the ashes, SEM-EDX analyses of the ashes, as well as the thermodynamic equilibrium modeling of the BFB boiler. XRD analysis of the ashes was performed by Anita Pettersson. Standard chemical analysis of the fuel and ashes were performed by an authorized laboratory. Lab-scale bed agglomeration tests were carried out by SP. Paper IV: Principal author of the paper, contributed to the full-scale measurements, and performed the chemical fractionation of fuel, as well as the thermodynamic equilibrium modeling of the BFB boiler. XRD analysis of the ashes was performed by Anita Pettersson. Standard chemical analysis of the fuel and ashes were performed by an authorized laboratory. Paper V: Principal author of the paper, and performed the chemical fractionation of the fuels, as well as the thermodynamic equilibrium modeling of the DFB gasifier. Lab-scale steam gasification of the biomass was carried out by Placid A. Tchoffor. Standard chemical analyses of the fuels were performed by an authorized laboratory. vi Licentiate Thesis and Conference Contributions Licentiate thesis: Moradian F., 2013. Co-Combustion of Municipal Solid Waste and Animal Waste: Experiment and Simulation Studies. Thesis for the degree of Licentiate, School of Engineering, University of Borås, ISSN 0280-381X Conference contributions: Moradian F, Pettersson A, Richards T. Co-combustion of animal waste in a commercial waste-to-energy BFB boiler, 21st International Conference on Fluidized Bed Combustion in Naples (Italy) June 3-6, 2012, proceedings volume 1 (oral presentation) Moradian F, Pettersson A, Richards T. Bed agglomeration and fouling during waste combustion in a bubbling-fluidized bed boiler – A thermodynamic modeling approach, 22th International Conference on Fluidized Bed Conversion in Turku (Finland) June 14-17, 2015 (oral presentation) International meeting abstract: Moradian F, Pettersson A, Richards T. Full-Scale Research on Co-Combustion of Animal Waste as Additional Fuel in a Waste to Energy BFB Boilers Joint Meeting of the Scandinavian-Nordic Sections of the Combustion Institute, Trondheim (Norway), November 28-29, 2011 (oral presentation) vii viii Acknowledgments Living in Sweden since 2007 and carrying out my Masters and Ph.D. studies at the University of Borås have been wonderful opportunities and a challenging journey for me. As a student with an educational background in chemistry and a short but valuable practical experience in the energy industry, it was a great opportunity for me to start a Master’s program in Energy and Resource Recovery and then pursue my Ph.D. within the field of Energy Engineering. I would like to take this moment to acknowledge those who with their contribution, help, and guidance made this long journey possible. First and foremost, I would like to express my deepest gratitude to Associate Professor Anita Pettersson, under whose supervision I have completed my Masters and Ph.D. thesis. Thank you for providing me this opportunity to become a Ph.D. student and getting involved in the world of research where I have met so many leading experts. I have no words to express how much I appreciate your support, advices, and constant encouragement. I wish all the best for you and your lovely family. I would also like to acknowledge my supervisor Professor Tobias Richards for his contribution in the publications, and his concern about the status of my research and organization of my Ph.D. study. I extend my acknowledgment to my examiner Professor Bengt-Åke Andersson, an inspiring leader with extensive expertise in industry and academia and the founder of the combustion and thermal processes research group at the University of Borås. You have been a source of great support and inspiration to me. I owe my deepest gratitude to Professor Rainer Backman for his support and priceless aid, which kept me going further in the interesting field of thermodynamic modeling. Thank you for giving me your precious time. It has been a great privilege to have had the opportunity to work with and learn from you. I am extremely grateful to the Swedish Centre for Resource Recovery, University of Borås for accepting me as a Ph.D. student. I would like to express my profound gratitude to all the past and present colleagues, teachers, administrative staff, and Ph.D. students at the University of Borås for being helpful and creating a pleasant working environment. A special thanks is dedicated to Dr. Peter Therning, Dr. Tomas Wahnström, Kamran Rousta, Dr. Martin Bohlén, Dr. Karthik Rajendran, Susanne Borg, Sari Sarhamo, Louise Holmgren, Thomas Södergren, Solveig Klug, and Jennifer Tydén.
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