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Combustion of Solid Fuel Ina Fluidized Bed Combustor

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Combustion of Solid Fuel Ina Fluidized Bed Combustor COMBUSTION OF SOLID FUEL INA FLUIDIZED BED COMBUSTOR A Thesis Presented to The Faculty ofthe Fritz J. and Dolores H. Russ College ofEngineering and Technology Ohio University In Partial Fulfillment Ofthe Requirement for the Degree Master ofScience --,-,*, \ ., by ,m Abu Noman Hossain June, 1998 i ACKNOWLEDGEMENTS I would like to express and extend my heartfelt appreciation and deep gratitude to the many people who have, in one way or other, helped and supported me in my work. To Dr. David J Bayless, my honorable advisor, whom I respect and admire for the excellent advice and guidance he has given me throughout the entire course ofmy study. I will always remember him for his advising, academic, and financial support at Ohio University. To Dr. H. Pasic, who generously served on my thesis committee. His suggestions and hearty attitude made my work easier and enjoyable. To Dr. M. Prudich for his keenness and initiative to serve on my thesis committee. To Len Huffman, for his excellent help and cooperation in the laboratory and mechanical workshop. To Matt Eckels, Ben Reineck and Kyle Wilson for their collaborative help and work in the design and drafting process. To Muhammad Turjo and Shatkat Shah for their endless support and continuous cooperation. To my beloved parents and my family, for their love, support, and encouragement. To Almighty Allah who has blessed me with the opportunity to come to Ohio University and has given this chance to work with these people. ii ABSTRACT The emissions ofpollutants from power plants have become the subject of increasing public concern. Legislation limiting the amount ofemitted pollutants has made control ofpollutants such SOx, NOx, CO, and particulates a major concern in designing and operating coal fired power plants. Another important environmental issue "addressed here is the recent surge in landfill closure. Environmental concern, cost, and availability ofnew land are major causes for the recent escalation oflandfill costs. An alternative to the waste disposal in landfill is the solid waste incineration. Disposal ofsolid waste through incineration, while offering numerous advantages, produces numerous pollutants. Operating and initial costs associated with the control ofthese emissions are high. As a result ofboth engineering and regulatory concerns, researchers are looking for combustion processes with higher efficiency and improved pollutant control. Fluidized Bed Combustion (FBC), which utilizes the phenomenon offluidization for the purpose of more efficient combustion, has shown the potential to meet these demands at lower costs. FBC is studied in this thesis for the possible reduction ofair emissions from coal and solid waste combustion. The evaluation ofdesign, development ofa fluidized bed combustor, the analysis ofthe fluidizing phenomena, and the combustion process for an unspecified fuel are discussed in this work. In general, the fuel must be fed into a heated bed for combustion. The entrained particles need to be separated from the exhaust gas and unburned particles have to be returned to the bed for complete combustion. The whole system must be iii integrated to perform the desired operations. However, the prediction ofthe particle's behavior, the rates ofmass and heat transfer, combustion reactions, and distribution of fuel and sorbent within the bed is difficult to predict accurately. Therefore, the design process requires the modeling ofthe bed in finding the key design parameters. Analytical models are needed to simulate the behavior ofthe bed in the desired operating conditions and be solved by computer codes to find the design parameters. An acrylic prototype model is also needed to verify analytical data. Agreement between experimental data and analytical predictions would indicate that the corresponding data would besuitable for design. Actual construction and equipment specifications necessary to meet constraints, both engineering and economic, are also needed to complete the design description iv TABLE OF CONTENTS ACKNOWLEDGEMENTS i ABSTRACT. ·.. ·.. ·· ii TABLE OF CONTENTS iv LIST OF FIGURES vii LIST OF TABLES ix NOMENCLATURE x CHAPTER 1. Introduction 1 1.1 Background 1 1.2 Significance 4 1.3 Combustion Process 6 1.4 Outline ofthe Work 8 2. Fluidization and Components ofthe Fluidized Bed Combustor 10 2.1 Fludization Phenomenon 10 2.2 Circulating Fludized Bed Combustor 12 2.3 Components ofFluidized Bed Combustor 13 2.3.1 Combustor 14 2.3.2 Particulate Collection Device: Cyclones 15 2.3.2.1 Cyclone In.let 17 v 2.3.2.2 Cyclone Body and Cone 17 2.3.2.3 Dust Discharge System (Hopper) 18 2.3.2.4 Cyclone Gas Outlet 19 2.3.3 Particle Fluidizing Device 19 2.3.4 Fuel Feeding Device 20 3. Particle Dynamics 21 3.1 Free Gas Stream Particle Velocity 21 3.2 Minimum Fluidization Velocity 24 3.3 Terminal Settling Velocity 25 4. Numerical Modeling ofthe Particle Behavior in the Bed 27 4.1 Effect ofTerminal Settling Velocity and Minimum FluidizationVelocity 28 4.2 Effect ofGas Velocity 29 4.3 Effect ofGas Density 28 4.4 Effect ofGas Viscosity 29 5. Heat Transfer Modeling 35 5.1 Heat Transfer Process in Fluidized Bed Combuster 35 5.1.1 Convection 36 5.1.2 Conduction 38 5.1.3 Radiation 39 5.2 Thermal Model ofthe Heat Transfer Process 39 5.3 Solution Techn.ique 43 vi 5.4 Result and Discussions 45 6. Experimental Analysis 49 6.1 Validation ofNumerical Model 49 6.2 Experimental Results 50 6.3 Discussion 51 7. Component Design 53 7.1 Combustor 54 7.2 Designing the Cyclone 56 7.3 Fluidizing Device: Blower 59 7.3.1 Bed Pressure Drop 60 7.3.2 Gas Flow Rate 62 7.4 Fuel Feeding Device 62 7.5 Ash Removal System 68 8. Conclusion 69 8.1 Summary- 69 8.2 Conclusions 70 8.3 Recommendations 71 VII LIST OF FIGURES Figure 2.1 Schematic Diagram ofBasic Fluidized Bed Combustor System 12 Figure 2.2 Schematic Diagram ofthe Burner Enclosure 15 Figure 2.3 Schematic Diagram ofCyclone 16 Figure 2.4 Schematic Diagram ofthe Air Introduction in the Bed 19 Figure 3.1 Direction ofForces acting on a Particle in the Bed 22 Figure 3.2 Direction ofForces acting on a Free Falling Particle 25 Figure 4.1 Terminal Settling Velocity Behavior for Different Particle 31 Figure 4.2 Particle Velocity Behavior for Different Gas Velocity 32 Figure 4.3 Particle Velocity Behavior for Different Gas Density 33 Figure 4.4 Particle Velocity Behavior for Different Gas Viscosity 34 Figure 5.1 Heat Transfer ofa Single Fuel Particle in the Bed 41 Figure 5.2 Effect ofConduction Surface Area 47 Figure. 5.3 Comparison ofDifferent Modes ofHeat Transfer in the Bed 48 Figure 6.1 Drawing ofthe Acrylic Prototype 50 Figure 7.1 Fluidized Bed Combustor Assembly 53 Figure 7.2 Combustor Detail Drawing 56 Figure 7.3 Standard Cyclone Dimensions 58 Figure 7.4 Detail Drawing ofthe Cyclone 59 Figure 7.5 Bed Pressure Drop along the Flow Path 61 viii Figure 7.6 Geometrical Dimension ofThread 63 Figure 7.7 Detail Drawing ofScrew Thread 64 Figure 7.8 Detail Drawing ofthe Fuel Hopper 65 Figure 7.9 Detail Drawing ofFuel Feeder Assembly 66 Figure7.10 Detail Drawing ofthe Screw Feeder Assembly 67 Figure7.11 Detail Drawing ofFuel Return Line 68 IX TABLES Table 6.1 Comparison ofAnalytical and Experimental Data 51 Table 7.1 Total Pressure Drop in the Fluid Flow Path 60 x NOMENCLATURE Ab Cross sectional area ofthe bed Ap Projected area ofthe particle Aev Convective area ofthe particle Aad Conductive surface area ofparticle Arid Radiative surface area ofparticle At Cross sectional area ofthe thread a Length ofthe thread cross sectional area Bi Biot number b Width ofthe thread cross sectional area Cd Drag coefficient Cp Specific heat ofthe sphere CVe Calorific value ofcarbon Dc Cyclone body diameter Dd Diameter ofdust outlet De Diameter ofthe gas exit dm Major diameter ofthe thread dp Diameter ofthe particle Fb Buoyancy force Fd Drag force Fg Gravitational force H Height ofthe cyclone inlet HCVr Higher calorific value ofthe fuel h Convective heat transfer coefficient hr Radiative heat transfer coefficient K Geometric configuration parameter kr Thermal conductivity ofthe gas kp Thermal conductivity ofthe particle I Distance traveled by thread in one revolution mfr Mass feed rate offuel Lb Length ofthe body Le Length ofcyclone cone me Mass ofthe carbon unburned mr Mass ofthe fuel mp Mass ofthe particle N Rpm ofthe thread Nu Nusselt number p Pitch ofthe thread L1Pb Bed pressure drop Pr Prandtl number xi Q Fluid flow rate qConv Convective heat transfer rate ofthe bed qCond Conductive heat transfer rate ofthe bed qr Radiative heat transfer rate ofthe bed rp Radius ofthe particle S Length ofthe vortex frnder T Instantaneous particle temperature Tb Bed temperature Ti Initial temperature ofsphere To Ambient temperature Twall Wall temperature t Time Umf Minimum fluidizing velocity V Thread inside volume V p Volume ofthe particle V Volumetric flow rate offuel vg Velocity ofthe gas vp Velocity ofthe particle W Width ofthe cyclone inlet ~U Particle/Fluid relative velocity pg Density ofgas Pp Density ofthe particle Jl Gas dynamic viscosity c Stefan-Boltzman constant Em Apparent value ofemissivity ofsurface 11 co Combustion efficiency CO Angular velocity ofthe screw llc Cyclone collection efficiency 1 CHAPTER 1 Introduction 1.1 Background An important aspect ofcombustion engineering is the efficient use offuel in the production ofelectrical power. The importance ofefficient fuel use is evident when costs, environmental concerns, and future fuel availability are considered. Fossil fuels account for almost 90% ofthe U.S. energy production, with coal accounting for over 55% ofthe fuel used to produce electrical power [1]. The supply ofcoal in the US is estimated to be 474 billions tons, which is approximately 94% ofthe proven U.S. fossil energy reserves [1].
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