Fluidized bed combustion

FLUIDIZED BED COMBUSTION Written by: Simeon N. Oka Laboratory for Thermal Engineering and Energy VIN^A Institute of Nuclear Sciences Belgrade, Serbia and Montenegro Technical Editor: Dr. Edward J. Anthony CANMET Energy Technology Center (CETC) Natural Resources Canada Ottawa, Ontario, Canada Published by MARCEL DEKKER, Inc. New York, Basel, September 2003 Fluidized bed combustion is new, but industrially proven, clean energy technology enabling efficient and environmentally acceptable burning of low grade coals, biomass and industrial and municipal wastes. Book is devoted to the processes in bubbling fluidized bed combustion furnaces and boilers, suitable for small and medium size industrial units: • giving, from one side, deep fundamental explanation of the main physical and chemical processes in bubbling fluidized beds, book is useful for researchers and post diploma studies; • based on the fundamental knowledge of the processes main problems in the choice of the boiler concept and design, and main data for the calculation of regime parameters and boiler dimensions are explained giving engineering tools for design engineers; • in the part of the book discussing behavior of the furnaces and boilers in operation, advantages and deficiencies, book is useful for operational engineers explaining possible event in exploitation; and • finally, book is useful for all persons thinking or making decisions about the choice of the most suitable combustion technology, available on the world market, for energy production using wide range of solid fuels, low grade fuels, biomass, industrial and municipal wastes, in distributive energy production, and small and medium size utility units. Three chapters of the book are devoted to the recent results of the fundamental and applied research of the hydrodynamics of bubbling fluidized beds, heat and mass transfer and combustion of solid fuels. A special chapter is devoted to the concepts of the bubbling FBC furnaces and boilers, choice of concept, brief calculation of the main parameters and influence of fuel characteristics on boiler concepts and parameters. Separate chapter is devoted to the investigation of solid fuel behavior during fluidized bed combustion and determination of the engineering data for the choice of boiler concept and design. Last chapter defines emission characteristics of the bubbling FBC furnaces and boilers. In the first chapter, and throughout the whole book, place of the bubbling FBC technology among the conventional combustion technologies is discussed, pointing out the niche markets in which bubbling FBC evidently is technology of choice. The most important feature of this book is that in one book are gathered present knowledge important both for researchers and engineers, giving to the researchers insight in the problems of the boiler concept choice, problems in calculation of the main 105 parameters and dimensions, and operational problems, and to the engineers insight in fundamental processes determining boiler design and behavior in exploitation. Secondly, there is no other book in this field giving detailed discussion of the influence of fuel characteristics, both on the fundamental processes in the boiler furnace and boiler concept. Also, unique is the detailed description of the original methodology for investigation of the fuel suitability for combustion in fluidized bed, and determination of the main fuel characteristics and data necessary for the choice of boiler concept and boiler design. Emission characteristics and environmental impact are described and discussed in details giving possibility to the researchers to think about process parameters and engineers about boiler concept most suitable to the fuel characteristics and environmental standards. Book structure gives possibility to the readers with different interest and background knowledge to choose those parts of the book giving him necessary information, without necessity to reed other parts. Book gives from one side, deep fundamental explanation of the main physical and chemical processes in bubbling fluidized beds, and so can be useful for researchers and post diploma studies. Based on the fundamental knowledge of the processes main problems in the choice of the boiler concept and design, and main data for the calculation of regime parameters and boiler dimensions are explained giving engineering tools for design engineers. Part of the book discussing behavior of the furnaces and boilers in operation, advantages and deficiencies, some important practical FBC problems as erosion of immersed surfaces and bed agglomeration issues, is useful for operational engineers explaining possible event in, exploitation. Finally, book is useful for all persons thinking or making decisions about the choice of the most suitable combustion technology, available on the world market, for energy production using wide range of solid fuels, low grade fuels, biomass, industrial and municipal wastes, in distributive energy production, and small and medium size utility units. In the Part I of the book (Chapter 1) an overview is provided on the characteristics and reasons for the development of FBC technology, the current state of the art and prospects for the use of fluidized bed technology in a range of fields. Further, many of the ideas necessary to understand the issues for practical solid fuel combustion in fluidized beds in industrial FBC boiler are also provided here. This chapter gives possibility to think about place of the FBC technology among conventional combustion technologies, and to find fields of applications for this technology in the light of the modern trends to use unconventional fuels, low grade colas, biomass industrial and municipal wastes, in efficient and environmentally acceptable manner. The Part II of the book is devoted to those readers, which are engaged in the research on fluidized beds combustion in particular, but this material will also be useful to those dealing with the research in the field of fluidized beds in general, and also, to those interested in coal combustion itself. This part of the book also presents a critical review of the current knowledge and investigation techniques employed in the field of fluidized bed combustion, as well as the trends in investigation of such processes. The information provided about fluidized beds in this part of the book should also serve as a good basis for R&D of the bubbling fluidized bed technology and understanding of the advantages and disadvantages of the bubbling fluidized bed combustion, as well as to suggest methods for optimizing and controlling FBC boilers. Without such knowledge it is not possible to understand the details and characteristics of the boiler design, and behavior 106 in real full-scale FBC boilers. Engineers faced with problems in defining FBC boiler parameters in calculation and boiler design, will benefit, also, from reading Part II of the book, which presents the fundamentals of the processes in real full-scale FBC boilers. Chapter 2 is devoted to fluidized bed hydrodynamics. The main point presented here is that hydrodynamics lies at the basis of all other processes. Due to this, particular attention has been paid to bubble motion, and to particle and gas mixing processes. This chapter is not only basis for understanding heat and mass transfer and combustion processes in boilers, but also processes in chemical reactors. Chapter 3 is devoted to a consideration of the heat transfer processes in fluidized beds. Given the challenges of this field it is understandable that much effort has been devoted to understanding heat transfer to immersed surfaces. Many empirical and experimental correlations for calculation of heat transfer coefficients heat exchangers in fluidized bed boilers are presented here, and compared with available experimental results. However, in contrast to other books dealing with heat transfer in fluidized beds, this chapter pays special attention to heat transfer between large moving particle (fuel particle) and the fluidized bed media of course particles, which are key to the performance and problems associated with full-scale FBC boilers. Chapter 4 looks at the processes and changes experienced by a fuel particle, from its introduction in hot a fluidized bed until its complete burn-up. Special attention has been given to the effect of the complex and heterogeneous nature of coal on the boiler design and also on the performance of such fuels in a fluidized bed. As such this chapter will be of interest to anyone studying coal combustion processes. Chapter 5, is the first chapter of Part III of this book, and provides a description and historical review of the concepts and designs of FBC hot-gas generators and boilers for different applications. It also gives method for the choice of boiler concept and sizing, and describes auxiliary boiler systems and their characteristics. Particular attention is given to the issue of combustion efficiency and its dependence on fuel characteristics and boiler design. Methods for achievement a wide range of load following are presented, and some important practical FBC problems are also discussed such as erosion of immersed surfaces and bed agglomeration issues. Chapter 6 is devoted to the detailed analysis of the influence of coal type and coal characteristics on the choice of FBC boiler parameters and design. This analysis is mainly based on extensive investigations on a wide range of different Yugoslav coals for fluidized bed combustion, and gives unique discussion about suitability of different solid fuels for FBC combustion. Chapter 7, is the last chapter in the book, and looks at the critical issues around emission control for SO2, NOx, CO and particulates in fluidized bed combustion boilers. Fundamental understanding of those issues is based on the knowledge about processes described in the Part II of the book, and behavior of the fuel during FBC described in Chapters 5 and 6. At the end of each chapter devoted to the basic processes in fluidized beds (Chapter 2, Chapter 3 and Chapter 4), there are sections looking at mathematical modeling of specific phenomena. A high importance has been given to mathematical modeling because in the author’s opinion modern, differential mathematical models are now the inevitable engineering tool for calculation of and optimization of the many parameters relevant to both the operation and design of FBC boilers. Dr. Borislav Grubor 107