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Fluidized Bed Combustion – Modeling and Mixing THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Fluidized bed combustion – modeling and mixing David Pallarès Department of Energy and Environment Division of Energy Technology CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg (Sweden), 2008 Fluidized bed combustion – modeling and mixing David Pallarès ISBN 978-91-7385-142-8 © David Pallarès, 2008 Doktorsavhandlingar vid Chalmers Tekniska Högskola Ny serie nr. 2823 ISSN 0346-718X Department of Energy and Environment Division of Energy Technology Chalmers University of Technology SE-41296 Göteborg (Sweden) +46 31 7721000 Chalmers Reproservice Göteborg (Sweden), 2008 Fluidized bed combustion – modeling and mixing David Pallarès Department of Energy and Environment Division of Energy Technology Chalmers University of Technology Abstract First demonstrated on industrial scale in 1976, fluidized bed combustion (FBC) rapidly became an established technology for solid fuel combustion with a current installed capacity of about 30 GWe and power plants ranging from 10 to 460 MWe. The widespread use of the FBC technology both under so-called circulating and bubbling conditions (i.e. with and without external recirculation of solids) is generally considered to be due to two main advantages with respect to other competing combustion technologies: the high fuel flexibility and possibilities of cost efficient reduction of SOx and NOx emissions. Yet, the relative lack of knowledge within some of the phenomena governing the process represents a major obstacle for the further development of the FBC technology. Efficient scale-up and development of the FBC technology depends on the possibility to increase the knowledge on the main processes involved in FBC boilers. A combination of experimental work and formulation of mathematical models is important to increase the knowledge of the process. In this work, macroscopic submodels for key processes in FBC (fluid dynamics, combustion and heat transfer) are formulated based on experimental data. These submodels are then linked to form a comprehensive model for large-scale fluidized bed combustion which is compared to experimental data not used in the validation of the submodels. The submodels include treatment of important phenomena such as fluctuations in the gas phase, separate convective and radiative heat transfer accounting for absorption in the particle suspension, influence of the pressure drop across the gas distributor and corner effects in the solid phase. These phenomena were not included in previous comprehensive FBC models and are the focus of this thesis. The comparison between the model and experimental data from large-scale fluidized bed boilers have been carried out for a wide range of data categories covering the solid phases (inert bed material and fuel), the gas phase and the temperature field. The modeled data generally shows good agreement with experimental data. Although the model developed is primarily aimed at circulating fluidized bed boilers, its principles and formulation can also be applied to bubbling beds and gasifiers as well as new FBC technologies such as oxy-fuel or chemical looping combustion. Keywords: Fluidized bed combustion; Modeling; Mixing List of publications This thesis is based on the work contained in the following papers, referred to by Roman numerals in the text. I Pallarès, D., Johnsson, F. 2006. ”Macroscopic modelling of fluid dynamics in large-scale circulating fluidized beds”. Progress in Energy and Combustion Science, 32 (5-6), pp. 539-569. II Pallarès, D., Johnsson, F. 2008. ”Modeling of fuel mixing in fluidized bed combustors”. Accepted por publication in Chemical Engineering Science. III Pallarès, D., Johnsson, F. 2008. ”Macroscopic modeling of the gas phase in fluidized bed combustion”. Submitted por publication. IV Pallarès, D., Johnsson, F. 2006. ”A novel technique for particle tracking in cold 2-dimensional fluidized beds - simulating fuel dispersion”. Chemical Engineering Science, 61 (8), pp. 2710-2720. V Pallarès, D., Díez, P., Johnsson, F. 2007. ”Experimental analysis of fuel mixing patterns in a fluidized bed”. Proc. of the 12th Int. Conf. on Fluidization (Harrison, Canada). Appendixes A Macroscopic modeling of heat transfer in circulating fluidized bed units. David Pallarès is the principal author and investigator in all papers. Filip Johnsson has supervised the work in all papers and contributed to their writing. Pedro Díez carried out the experiments in Paper V. The following papers have also been published during the course of the work but are not included in this thesis since their content either is outside the scope of the present work or partly overlaps with the papers included in it. • Pallarès, D., Johnsson, F. 2008. ”A comprehensive model of CFB combustion”. Proc. of the 9th Int. Conf. on Circulating Fluidized Beds (Hamburg, Germany). • Pallarès, D., Johnsson, F. 2002. ”Fluid dynamic modeling of CFB units”. Proc. of the 7th Int. Conf. on Circulating Fluidized Beds (Niagara Falls, Canada), pp. 387-394. • Pallarès, D., Johansson, A., Johnsson, F. 2006. ”Interpretation of dynamics of pressure measurements”. Proc. of the 19th Int. Conf. on Fluidized Bed Combustion (Vienna, Austria). • Pallarès, D., Johnsson, F. 2007. ”Modeling fuel mixing in a fluidized bed combustor”. Proc. of the 12th Int. Conf. on Fluidization (Harrison, Canada). • Niklasson, F., Pallarès, D., Johnsson, F. 2006. ”Biomas co-firing in a CFB boiler - the influence of fuel and bed properties on in-furnace gas- concentration profiles”. Proc. of the 19th Int. Conf. on Fluidized Bed Combustion (Vienna, Austria). • Almendros-Ibáñez, J.-A., Pallarès, D., Johnsson, F., Santana, D. 2008. ”A novel approach to characterize fluidized bed dynamics combining PIV and FEM”. Submitted for publication in AIChE Journal. • Almendros-Ibáñez, J.-A., Pallarès, D., Johnsson, F., Santana, D. 2008. ”Porosity distribution around bubbles in a fluidized bed”. Submitted for publication in Powder Technology. Acknowledgements Foremost, I wish to express my most sincere gratitude to my supervisor Professor Filip Johnsson for his skillful guidance throughout this and other works. His deep knowledge of the field of FBC combustion, together with his confidence on this work, provides a privileged ground which I deeply appreciate. My gratitude must be extended to Associate Professor Henrik Thunman for the time and effort he has spent on valuable discussions which saved me by far more time and effort than otherwise would have been the case. Associate Professor Lars-Erik Åmand is acknowledged for his passion for the experimental field, which burns like no one else’s and grants him the ability to provide almost any experimental data one can think of (and explanations for it). Besides for wise advice, I am grateful to Professor Emeritus Bo Leckner specially for giving me the opportunity to regularly attend the highly rewarding workshops arranged by the Fluidized Bed Conversion group of the International Energy Agency, where this work has been presented and given further qualified feedback. Very special thanks must go to Adjunct Professor Bengt-Åke Andersson for his strong trust in the embryo of what you hold in your hands and for giving it the chance to develop within a successful framework of collaboration with the manufacturing industry. It has been and will continue to be a pleasure to work with METSO Power Oy; their flexibility and understanding for the nature of academic research is highly appreciated and Mr. Marko Palonen is specially acknowledged in this sense. Lennart Darell is thanked for always having a solution and a smile at hand for any difficulty encountered in the experimental field. Coworkers at the Thermal and Fluid Engineering Department of University Carlos III (Madrid, Spain) and José Antonio Almendros-Ibáñez in special as well as Pedro- Antonio Díez are thanked for fruitful cooperation. Financial support from the European Commission (under contract RFC-CR-03001), the Swedish Energy Agency and METSO Power Oy is acknowledged. Highly-skilled surrounding, long field experience and funding have been a necessary but not sufficient condition for the implementation of this work. Thus, last but not least come the following acknowledgements: The friendly and inspiring atmosphere at the division of Energy Technology has played an important role, and not only in pure academic terms: chats about weekend activities, movies, soccer, politics or life itself are a must to preserve some reasonable level of mental health during a work like this. A time ago Göteborg became my new home, and some people managed to make it feel home too. These friends know who they are and how much I enjoy them. My (increasing!) family at home in Barcelona has given me unconditional understanding and support for my move to Sweden, which I deeply value: moltes gràcies de debò. Friends I left there are also missed, even more now when distance confirmed them to be real ones (both day- and night-time). It is nothing but a blessing to share life with my love and best friend Emma and her everlasting warmth and loyalty. “All models are wrong. Some are useful”. - George Box, chemist and statistician Table of contents Chapter 1 - Introduction ………………………………………..1 1.1 Scope: CFB boilers 1 1.2 Aim: Modeling and experimental work 3 1.3 Background: Macroscopic models 4 Chapter 2 - Submodels …………………………………………6 2.1 Governing phenomena in CFB combustion 6 2.2 Theory 9 Chapter 3 - Comprehensive Model ………………………….24 3.1 Coupling of submodels 24 3.2 Computational flow scheme 30 3.3 Furnace mesh 32
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