ABSTRACT

 

With the ever-growing energy demand over the globe, fluidised bed combustion (FBC) technology is continuously gaining importance due to its ability to burn different low-grade coals and biomass as source of fuel. This study presents a waste-to-energy process by incineration of corncob as an agricultural waste in a fluidised bed boiler for thermal energy production. The X-ray Diffractometry profile and Thermo-gravimetry profile of the corncob calcined at 800oC proves its viable options for use of this fuel in fluidised boiler. Hence, the performance evaluation of the developed miniature fluidised bed boiler at bed height of 77mm, 47mm and 27mm using 250μm granual material recorded stability in saturation temperature of steam at 121oC from bed height of 77mm at 50 minutes, 144oC from bed height of 47mm at 45 minutes and 153oC from bed height of 27mm at 30 minutes. In addition, the superheated temperature of 141oC at 55 minutes, 147oC at 45 minutes and 163oC at 30 minutes was obtained for bed height of 77mm, 47mm and 27mm respectively. Furthermore, the maximum superheated pressures obtained were 2.1 bar at 55 minutes for bed height of 77mm, 2.6 bar from bed height of 47mm at 45 minutes and 4.0 bar to 4.2 bar from bed height of 27mm from 45 to 55 minutes. The maximum capacity of steam generated throughout the experimental methods was 6.6kg/h that is capable to run a small steam turbine to meet the rural electrification of a small community like shika located in zaria, Kaduna state. Lastly, biomass as a promising energy source due to its abundant, carbon-fixing, and carbon-neutral properties has proven to be efficient in a fluidised bed boiler as the emission analysis of the flue gas has shown to be low in various percentages of 0.0003% of NOx, 0.001% HC, 0.02% of CO and 0.93% of Nitrogen respectively.

 

TABLE OF CONTENTS

Contents Pages
Title page i
Declaration ii
Certification iii
Dedication iv
Acknowledgment v
Abstract vi
Table of Contents vii
List of Figures xi
List of Tables xii
List of Plates xiii
List of Appendices xiv
Nomenclatures xv
CHAPTER ONE 1
1.0 INTRODUCTION 1
1.1 Background of the Study 1
1.2 Statement of Research Problem 3
1.3 Present Research 4
1.4 Aim and Objectives 4
1.5 Justification of the Study 5
1.6 Research Scope 5
CHAPTER TWO 6
2.0 LITERATURE REVIEW 6
2.1 Introduction 6
2.2 Boiler Terms 7
2.3 Types of Boilers 8
viii
Contents Pages
2.3.1 Fire tube boilers 9
2.3.2 Water tube boiler 10
2.3.2.1 “A” type boilers 11
2.3.2.2 “D” type boilers 12
2.3.2.3 “O” types boilers 12
2.4 Fluidised Bed Boiler 12
2.4.1 Bubbling fluidised bed (BFB) boiler 13
2.4.2 Circulating fluidised bed (CFB) boiler 14
2.5 Energy from Biomass 14
2.6 Regimes of gas-solid Fluidization 16
2.7 Water Circulation 18
2.7.1 Principle of natural circulation 18
2.7.2 Principle of forced circulation 19
2.8 Review of Related Past works 20
2.9 Conclusion from the Review and Justification for the Present Work 24
CHAPTER THREE 25
3.0 MATERIALS AND METHODS 25
3.1 Materials 25
3.1.1 Combustion chamber 25
3.1.2 Steam drum 26
3.1.3 Steam tubes 26
3.1.4 Insulation 26
3.1.5 Instruments and equipment 27
3.2 Methodology 28
3.2.1 Design considerations 28
3.2.2 Design analysis 28
3.2.2.1 Operating temperature and pressure 28
3.2.2.2 Internal design pressure of a boiler 28
3.2.2.3 Stresses in tubes and drums 29
3.2.2.4 Design of the steam drum 30
ix
Contents Pages
3.2.2.5 Design of the steam tube 30
3.2.2.6 Design height of the combustion chamber 30
3.2.2.7 Minimum wall thickness of tubes and drum 31
3.2.2.8 Change in boiler dimension due to internal design pressure 31
3.2.2.9 Velocity of fluid inside tubes, pipes and drum 32
3.2.2.10 Quantity flow rate of fluid inside tubes 33
3.2.2.11 Bed material and particle size of a fluidised bed boiler 33
3.2.2.12 Bed voidage 33
3.2.2.13 Superficial velocity 34
3.2.2.14 Minimum fluidization velocity 34
3.2.2.15 Terminal velocity 35
3.2.2.16 Combustion of fuel 36
3.2.2.17 Calorific value of fuel 37
3.2.2.18 Thermal load of the combustion chamber 37
3.2.2.19 Rate of heat transfer (conduction and convection) 38
3.2.2.20 Boiler efficiency 38
3.2.3 Design Calculation of the Boiler 39
3.2.3.1 Internal design pressure of the boiler 39
3.2.3.2 Stresses in the tubes and drum 39
3.2.3.3 Design of steam drum 40
3.2.3.4 Design of steam tubes 40
3.2.3.5 Design of combustion chamber 40
3.2.3.6 Design of minimum wall thickness 41
3.2.3.7 Velocity of fluids in tubes, pipes and drum 41
3.2.3.8 Quantity flow rate of fluid inside tubes 42
3.2.3.9 Superficial velocity 42
3.2.3.10 Terminal velocity 42
3.2.3.11 Design of frame support 43
3.2.4 Construction process 43
3.2.4.1 Fabrication of parts 43
x
Contents Pages
3.2.5 Experimental procedure 50
3.2.5.1 Water quality/quantity 50
3.2.5.2 Bed height and fluidization 50
3.2.5.3 Feeding of fuel 50
3.2.5.4 Pressure measurement 50
3.2.5.5 Temperature measurement 50
3.3 Bill of Engineering Measurement and Evaluations 52
3.4 Combustion of Fuel 54
3.5 Calorific Value of Fuel 54
CHAPTER FOUR 55
4.0 RESULTS AND DISCUSSION 55
4.1 Introduction 55
4.2 TG/DSC Profile 55
4.3 XRD of Corncob 56
4.4 Saturation Temperature of Steam at Steam Drum 57
4.5 Superheated Temperature of Steam at Super heater Tube 58
4.6 Saturation Pressure of Steam at Steam Drum 58
4.7 Superheated Pressure of Steam at Super heater Tube 59
4.8 Bed Temperature at Combustion Chamber 60
4.9 Flue Gas Temperature at Exhaust Pipe 61
4.10 Amount of Steam Generated 62
4.11 Analysis of the Flue Gas Emission 63
4.12 Rate of Steam Generation 64
4.13 Rate of Heat Transfer (Conduction and Convection) 66
4.14 Boiler Efficiency 66
CHAPTER FIVE 68
5.0 CONCLUSION AND RECOMMENDATIONS 68
5.1 Conclusion 68
5.2 Recommendations 69
References 70
Appendices 75
xi

 

 

CHAPTER ONE

NTRODUCTION
1.1 Background of the Study
Nowadays, technology offers several solutions and incentives to use different kinds of biomass fuels to produce energy. Technological developments over time have increased the ability to burn different kind of fuels by means of specific boiler concepts and the flexibility to burn different fuels in a boiler. A waste-to-energy plant takes profit from useless waste by converting it to energy, usually by incineration in a boiler in order to convert water to steam. There are different kinds of waste being incinerated in boilers such as household garbage from a society or by-products from the process industry and organic matters from agricultural waste.
Steam is the technical term for the gaseous phase of water, which is formed when water boils.
Steam is a critical resource in today’s industrial world; it is used in the production of goods and foods, the heating and cooling of large buildings, the running of equipment, and the production of electricity (Ohijeagbon et al., 2013).
Heating water at any given pressure will eventually cause it to boil and steam will be released. When water is boiling, both water and steam have the same temperature and for each boiling pressure there is only one saturation temperature (Folayan, 2014) as long as water and steam are in contact, temperature will remain at saturation point for that pressure.
Temp. Steam
Boiling water
Water
Enthalpy
Figure 1.1: Temperature – Enthalpy diagram showing the state of water and steam
2
A system in which steam is generated is called a boiler or steam generator. Boilers are pressure vessels designed to heat water or produce steam, by combustion of fuel which can then be used to provide space heating and/or service water heating to a building (Odigure et al.,2004). Steam is preferred over hot water in some applications, including absorption cooling, kitchens, laundries, sterilizers, and steam driven equipment. Steam is therefore important in engineering and energy studies. According to American Society of Mechanical Engineers (ASME), a steam generating unit is defined as a combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the heat so made available to the fluid being heated and vaporized (Rajput, 2010).
Boilers are classified into different types based on their working pressure and temperature, fuel type, draft method, size and capacity, and whether they condense the water vapour in the combustion gases. Boilers are also sometimes described by their key components, such as heat exchanger materials or tube design. Two primary classifications of boilers are Fire tube and Water tube boilers. In a Fire tube boiler, hot gases of combustion flow through a series of tubes surrounded by water. Alternatively, in a water tube boiler, water flows in the inside of the tubes and the hot gases from combustion flow around the outside of the tubes.
Fluidised bed combustion (FBC) is a combustion technology used to burn solid fuels. In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone, etc.) through which jets of air are blown to provide the oxygen required for combustion. The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed. FBC plants are capable of burning a variety of low-grade solid fuels, including most types of coal and woody biomass, at high efficiency and without the necessity for expensive fuel preparation (e.g., pulverising).
Deterioration of coal quality and pollutant gases (NOx) arising out of burning coal in conventional utility boilers lead to the development of fluidised bed combustion boilers. The
3
main advantages of the fluidised bed combustion boilers are: reduced NOx, SOx due to relatively low combustion temperature, better efficiency and reduction in boiler size and design. It also has the ability to burn low grade coal and it is less corrosive as the combustion temperature is less when compared to that of an utility boiler (Thenmozhi & Sivakumar, 2013).
Fluidised bed combustion (FBC) reduces the amount of sulfur emitted in the form of SOx emissions. Limestone or sand is used to precipitate out sulfate during combustion, which also allows more efficient heat transfer from the boiler to the apparatus used to capture the heat energy usually water tubes. The heated precipitate coming in direct contact with the tubes (heating by conduction) increases the efficiency. Since this allows coal plants to burn at cooler temperatures, less NOx is also emitted. However, burning at low temperatures also causes increased polycyclic aromatic hydrocarbon emissions. FBC boilers can burn fuels other than coal, and the lower temperatures of combustion (800 °C / 1500 °F) have other added benefits as well. In addition to all of these, the startup and shut down operation of FBC boilers are much easier.
1.2 Statement of Research Problem
In science and engineering laboratories, there is sometimes the need to utilize steam or hot water to generate power, to carry out tests or for other heating applications. This steam or hot water can be obtained using boilers (Ohijeagbon et al., 2013). Moreover, in Nigeria a significant volume of agricultural waste is generated within the rural areas, which are potential sources of fuel for power and heat generation. Thus, the energy that could be generated is wasted by dumping or burning in an open air, it therefore become necessary to generate useful energy using suitable waste-to-energy technologies. Furthermore, efforts have been made in the past by different researchers to utilize locally sourced materials in the design and developments of equipment. Hence, this research wishes to achieve a locally
4
made FBC steam generator using agricultural waste remains (corncob) for sizing of a steam turbine.
1.3 Present Research
Today’s process and heating applications will continue to be powered by steam and hot water. The mainstay technology for generating heating or process energy is the boilers. This present research study focused on the utilization of local materials for development of a miniature fluidised bed water tube boiler for steam generation that operated between 0.5 and 4.2 bar of steam pressure and a maximum superheated temperature of 168oC was obtained. Conversely, the capacity of the steam generated is adequate for medical sterilization, soil steaming and stands the purpose of practical demonstrations and teaching aid. Stresses, stoichiometric air-fuel ratio and efficiency of the boiler were calculated using the required relationships and expressions.
1.4 Aim and Objectives
The goal of this research is to design, construct and test a locally made fluidised bed boiler using agricultural waste as source of fuel for steam generation to be used for the purpose of energy production.
Therefore, the specific objectives of this research are:
i. To design a miniature fluidised bed boiler.
ii. To construct the miniature fluidised bed boiler.
iii. To carry out the feasibility study of using agricultural waste (corncob) for energy production.
iv. To test the developed fluidised bed boiler to determine its efficiency with respect to steam turbine selection.
5
1.5 Justification of the Study
The fluidsied bed combustion has the advantage of fuel flexibility and capacity to burn broad spectrum of fuels at high combustion efficiency with minimum emissions of greenhouse gases. Essentially, the biomass energy resource base of Nigeria is expected to be 144 million tonnes per year. In Kaduna state alone, production estimates for rice and maize were 364,170MT and 1,027,790 MT respectively (NAERLS and NFRA, 2009). This shows enough potential of biomass waste to generate minimum of 90MW of electricity. In recent study, Shika community generates about 3400 tonnes of agricultural waste annually, which is capable of producing a minimum of 1.9MJ/s, sufficient to generate 200kW of electricity from a power plant.
Consequently, fluidised bed boiler was no doubt justifying the incineration of corncob into the system to generate steam at any desire capacity and pressure, and have higher efficiency than conventional boiler. In addition, it has proven to be highly efficient and cost effective measure in generating energy for process and heating applications since the waste were appropriately utilized which serve as an awareness to the need to develop an environmentally waste-to-energy processes.
1.6 Research Scope
The scope of this work covers the utilization of locally sourced materials to design, develop and test the fluidised bed boiler for steam generation by carrying out technical feasibility of using corncob as fuel and varying the bed height while keeping the superficial velocity of fluidising gas constant. More so, the characteristic of the steam generated was used in the selection of an applicable steam turbine.

 

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