ABSTRACT

This study adopts the theory of fluidisation to design a combustor suitable for use in the rural communities of Nigeria. The Combustor, which will use sand particles as bed material, will burn maize cobs supplied at 5kg/h to generate heat energy for thermal applications including steam generation. Like most African countries, lack of adequate electricity supply in Nigeria has hampered economic activities, with less than 40% of the rural community connected to the national grid due to high cost of rural electrification exercise. Furthermore, the rural population continuously rely on direct burning of solid biomass (like fuel wood) as means of obtaining much needed heat energy for basic applications like cooking and heating, constituting an environmental nuisance. The study contends that using a bubbling fluidised bed combustor, it is possible to reduce energy poverty in the rural areas of Nigeria, while complying with the sustainable development goals. Chapter one discusses the background of the study, identifying the objectives, research problems, justification and scope of the work. Chapter two reviews the previous documentation on fluidised bed combustion including its history, principle of fluidization and advantage of fluidized beds over conventional methods of burning biomass. This chapter explains the theoretical background of the combustor and also introduces the Ergun 6.2 modeling software used in this work. The third chapter identifies the materials selected and the equations governing the design of the fluidised bed system. The simulation of the fluidised bed, construction procedure and equipment used are also defined in this chapter. Chapter four presents the result of the flue gas analysis, fluid bed and flue gas temperature measurements, with subsequent discussions. The fifth chapter summarizes and concludes the entire study.
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The temperatures of the fluid bed and flue gas during combustion were measured at 837 ºC and 220 ºC respectively using the MASTECH multipurpose clamp meter. The flue gas constituents were also analysed using the gas analyser which gave the concentrations of CO, NOx and SOx at 2ppm, 5ppm and 1ppm respectively.

 

 

TABLE OF CONTENTS

Title Page ……………………………………………………………………………………………………………………. i
Declaration…………………………………………………………………………………………………………………. ii
Certification ………………………………………………………………………………………………………………. iii
Dedication …………………………………………………………………………………………………………………. iv
Acknowledgements …………………………………………………………………………………………………….. v
Abstract …………………………………………………………………………………………………………………….. vi
Table of Contents …………………………………………………………………………………………………….. viii
List of Tables ……………………………………………………………………………………………………………. xii
List of Figures ………………………………………………………………………………………………………….. xiii
List of Plates ……………………………………………………………………………………………………………. xiv
List of Appendices …………………………………………………………………………………………………….. xv
Nomenclature…………………………………………………………………………………………………………… xvi
1.0 INTRODUCTION………………………………………………………………………………………………… 1
1.1Background ………………………………………………………………………………………………………….. 1
1.2Statement of the Problem ………………………………………………………………………………………. 2
1.3Justification of the Research ………………………………………………………………………………….. 3
1.4Aim and Objectives of the Study ……………………………………………………………………………. 4
1.5 Scope……………………………………………………………………………………………………………………. 5
2.0 LITERATURE REVIEW …………………………………………………………………………………….. 6
2.1Background History………………………………………………………………………………………………. 6
2.2Renewable Solid Fuels …………………………………………………………………………………………… 6
2.3Characteristics of Fluidised Bed Combustion …………………………………………………………. 7
2.4Types of Fluidised Bed Combustors ……………………………………………………………………….. 8
2.4.1Atmospheric fluidised bed combustor …………………………………………………………………….. 8
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2.4.2 Pressurized fluidised bed combustor ……………………………………………………………………… 8
2.4.3 Bubbling fluidised bed …………………………………………………………………………………………. 9
2.4.4 Circulating fluidised bed ………………………………………………………………………………………. 9
2.5 Advantages of Fluidised Bed Combustion ……………………………………………………………. 10
2.6 Applications of Fluidised Bed Combustion ………………………………………………………….. 10
2.7Principle of Fluidisation ………………………………………………………………………………………. 11
2.8Fluidisation Regimes ……………………………………………………………………………………………. 11
2.9Review of Related Works …………………………………………………………………………………….. 12
2.10 Theoretical Background ……………………………………………………………………………………. 14
2.10.1 Thermal stress analysis …………………………………………………………………………………….. 14
2.10.2 Geldart’s Classification …………………………………………………………………………………….. 15
2.11Energy Potential in Maize Cobs …………………………………………………………………………. 17
2.12Brief Description of ERGUN 6.2 Software ………………………………………………………….. 18
3.0 MATERIALS AND METHOD …………………………………………………………………………… 19
3.1 Description of Bubbling Fluidised Bed Combustor ………………………………………………. 19
3.2Materials …………………………………………………………………………………………………………….. 20
3.2.1 Maize cobs ……………………………………………………………………………………………………….. 20
3.3 Material Selection ………………………………………………………………………………………………. 20
3.3.1 Fluidised bed cylinder ………………………………………………………………………………………… 21
3.3.2 Distributor ………………………………………………………………………………………………………… 21
3.3.3 Cyclone ……………………………………………………………………………………………………………. 21
3.3.4 Feed hopper ………………………………………………………………………………………………………. 21
3.3.5Insulator ……………………………………………………………………………………………………………. 22
3.4Design Analysis……………………………………………………………………………………………………. 22
3.4.1 Bed temperature ………………………………………………………………………………………………… 22
3.4.2 Bed depth …………………………………………………………………………………………………………. 23
3.4.3 Bed material and particle size ……………………………………………………………………………… 23
3.4.4 Minimum fluidisation velocity ……………………………………………………………………………. 24
3.4.5 Terminal velocity ………………………………………………………………………………………………. 24
3.4.6 Superficial velocity ……………………………………………………………………………………………. 25
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3.4.7 Gas viscosity …………………………………………………………………………………………………….. 26
3.4.8 Gas density ……………………………………………………………………………………………………….. 26
3.4.9 Bed voidage ……………………………………………………………………………………………………… 26
3.4.10 Design of gas distributor …………………………………………………………………………………… 27
3.4.11 Distributor grids for bubbling fluidised beds ……………………………………………………….. 27
3.4.11.1 Pressure drop ………………………………………………………………………………………………… 28
3.4.11.2 Orifice velocity …………………………………………………………………………………………….. 28
3.4.11.3 Orifice number ……………………………………………………………………………………………… 29
3.4.12 Distributor thickness ………………………………………………………………………………………… 29
3.4.13 Plenum chamber………………………………………………………………………………………………. 29
3.4.14 Bed expansion design ………………………………………………………………………………………. 30
3.4.15 Bubble velocity ……………………………………………………………………………………………….. 31
3.4.16 Bubble diameter ………………………………………………………………………………………………. 31
3.4.17 Volume fraction of bubbles in the bed ………………………………………………………………… 32
3.4.18 Transport disengagement height (TDH) ……………………………………………………………… 33
3.4.19 Cylinder thickness ……………………………………………………………………………………………. 34
3.4.20 Insulation thickness ………………………………………………………………………………………….. 34
3.4.21 Entrainment …………………………………………………………………………………………………….. 35
3.4.21.1 Design of cyclone …………………………………………………………………………………………. 35
3.4.21.2 Cyclone diameter ………………………………………………………………………………………….. 35
3.5Design Consideration …………………………………………………………………………………………… 37
3.5.1 Calorific value …………………………………………………………………………………………………… 37
3.5.2 Combustion temperature …………………………………………………………………………………….. 37
3.5.3 Minimization of combustible losses …………………………………………………………………….. 37
3.5.4 Gravity chute feed hopper …………………………………………………………………………………… 38
3.5.5 Bed material and bed height ……………………………………………………………………………….. 38
3.5.6 Power Requirement ……………………………………………………………………………………………. 38
3.6Design Calculations ……………………………………………………………………………………………… 39
3.7 Equipment …………………………………………………………………………………………………………. 39
3.8Construction ……………………………………………………………………………………………………….. 40
3.8.1 Cylinder……………………………………………………………………………………………………………. 40
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3.8.2 Distributor grid………………………………………………………………………………………………….. 41
3.8.3 Cyclone ……………………………………………………………………………………………………………. 41
3.8.4 Feed hopper ………………………………………………………………………………………………………. 42
3.8.5 Blower ……………………………………………………………………………………………………………… 42
3.8.6 Insulation………………………………………………………………………………………………………….. 42
3.9Simulation of Fluidised Bed …………………………………………………………………………………. 42
3.9.1 Introduction ………………………………………………………………………………………………………. 42
3.9.2 Parameters for fluidized bed simulation ……………………………………………………………….. 44
3.9.3 Data requirement ……………………………………………………………………………………………….. 46
4.1Ignition/Testing Procedure ………………………………………………………………………………….. 49
4.2Bed Temperature ………………………………………………………………………………………………… 49
4.3 Flue Gas Temperature ………………………………………………………………………………………… 51
4.4Volume Flow Rate ……………………………………………………………………………………………….. 53
4.5 Results Of Simulation …………………………………………………………………………………………. 54
4.5.1 Particle …………………………………………………………………………………………………………….. 54
4.5.2 Grid …………………………………………………………………………………………………………………. 55
4.5.3 Bubbling …………………………………………………………………………………………………………… 56
4.5.4 Reh diagram ……………………………………………………………………………………………………… 57
4.5.5 Entrainment ………………………………………………………………………………………………………. 58
4.5.6 Cyclone ……………………………………………………………………………………………………………. 59
4.5.7 Fluidised bed modelling and expert analysis …………………………………………………………. 60
4.5.8 Comparing calculated and simulated values ………………………………………………………….. 61
4.5.9 Estimated Cost of a Fluidised Bed System ……………………………………………………………. 61
5.0 SUMMARY, CONCLUSION AND RECOMMENDATION ………………………………… 63
5.1Summary …………………………………………………………………………………………………………….. 63
5.2Conclusion ………………………………………………………………………………………………………….. 63
5.3 Recommendation………………………………………………………………………………………………… 64
REFERENCES ………………………………………………………………………………………………………… 65
APPENDICES …………………………………………………………………………………………………………. 72
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CHAPTER ONE

INTRODUCTION
1.1 Background
Global demand for energy has led to rapid depletion of non-renewable energy resources (fossil fuels). This demand, along with high crude oil prices are key factors driving renewable energy development and utilisation (Omer, 2013).
Renewable energy is the energy which comes from natural sources such as the sun, wind, water and plant or animal organic matter. It is replenished by natural processes at a rate equal to or faster than the rate at which it is consumed. Biomass, a renewable energy source, refers to biological material from living or recently living plants and animals. It can either be used directly, or converted into other energy products such as biofuel . While the discovery of fossil fuels has led to a decline in the use of biomass, recent data has indicated renewed efforts towards conversion of biomass to bioenergy either by combustion, gasification, conversion of biomass to biofuels or biomass briquettes. Biomass resources include dedicated energy crops, forest residues, municipal, animal and agricultural waste (DOE, 2007). Combustion of solid biomass fuels accounts for over 90% of the energy generated from biomass worldwide. This is mostly common in developing countries, where biomass combustion provides basic energy for cooking and heating in rural households (ECN, 2006). These traditional applications are relatively inefficient and promote environmental degradation.
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Standalone biofuel combustion, which is state-of-the-art, can provide an effective means of improving Biofuel efficiency. Such standalone systems include:
a. Diesel generators where biodiesel produced from energy plants like jatropha, replace diesel fuel to produce electricity for off-grid applications.
b. In biomass-based power plants, heat produced by direct biomass combustion can be used to generate electricity via a steam turbine. The electrical efficiency of this system is currently low, but provides the cheapest and most reliable means of producing power from biomass for standalone applications (IEA, 2009).
c. Co-generation is the process of producing two useful forms of energy, normally electricity and heat, from the same fuel source. Co-generation significantly increases the overall efficiency of a power plant and hence its competitiveness (IEA, 2009). The application of heat from biomass combustion for industrial processes like drying is well established in the pulp and paper industries, sugar mills and palm oil mills.
d. Waste-to-energy technology has been unexploited despite its potential in most countries. Waste-to-energy plants based on municipal and agricultural wastes require robust technologies and strict air emission controls, increasing the costs of waste-to-energy facilities (IEA, 2009).
The above waste-to-energy standalone systems can be realised through Fluidised Bed Combustion (FBC) technology, in which, a fluidised bed of sand like particles is initially heated and then the fuel (which can be agricultural waste) is thrown on the bed for combustion.
1.2 Statement of the Problem
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Every year, a significant volume of agricultural waste is generated within the rural areas in Nigeria. The waste is a potential source of fuel for heat and power generation. Consequently, the energy that could be generated is wasted by indiscriminate dumping and burning. It therefore becomes necessary to generate useful energy using suitable waste-to-energy technologies. While the use of fossil fuels has become synonymous with environmental concerns like global warming and acid deposition, emission of gases like carbon dioxide, sulphur and nitrogen oxides is also a major problem of the 21st century. Furthermore, poverty, unemployment and lack of electricity are key factors encouraging rural-urban migration. Public infrastructure in the urban centres becomes strained while shortage of labour in the agricultural sector develops within the rural areas (Ladan, 2012).
1.3 Justification of the Research
A significant part of global population relies on biomass fuel particularly for heating and cooking. Unfortunately, this comes at a high economic cost. Although biomass fuel is relatively cheap, many illnesses prevalent in villages especially those related to respiratory and heart diseases are attributed to indoor emissions from biomass-fired cooking. Furthermore, connection to the national electrical grid is a rare occurrence in rural areas of the developing world. In majority of the world’s poorer countries, it is estimated that significantly less than 5% of the rural population are connected to the national grid. In such communities, localized grid can be established using a local power source such as agricultural waste. Individual households can also be connected to standalone systems which can be powered by any of a wide variety of energy sources.
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There are several benefits of introducing electricity to rural communities. While obvious reasons include social gains like lightening, cooking and water pumping, electricity will help to stem the flow of rural-urban migration which is a common problem in many developing countries like Nigeria. Introduction of electricity also helps to provide productive employment in rural areas thereby creating a positive impact on economic as well as social growth. Fluidised bed combustor when combined with a boiler can provide efficient and affordable source of energy thereby boosting rural education and development, since it uses agricultural waste as a fuel source. Flue gases generated can either be used for electricity production in a gas turbine, generate steam in a boiler or simply burned indoors. Fluidised bed technology has high greenhouse gas mitigation potential since it eliminates sulphur and nitrogen oxide emissions, thereby curbing global warming and acid deposition. The biomass energy resource base of Nigeria is expected to be 144million tonnes per year. In Kaduna state alone, production estimate for maize was 1,027,790 MT (NAERLS and NFRA, 2009). This shows enough potential of biomass waste to generate electricity. Supplying electricity to rural areas will not only reduce poverty and unemployment, but also decentralize national grid connection where it exists.
1.4 Aim and Objectives of the Study
The aim of this project is to design, construct and simulate an agricultural waste bubbling fluidised-bed combustor. The specific objectives of the work are:
a. To design the various parts of the combustor
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b. To select appropriate materials for the fluidised bed combustor
c. To simulate the performance and validate the results by test on the fluidised bed combustor constructed
Estimate the cost of a prototype of the fluidised bed combustor.
1.5 Scope
The current work focuses on the design, construction and simulation of a bubbling fluidised-bed combustor, which will use agricultural waste as fuel to produce heat energy. The heat produced can then be used to generate steam for power and thermal applications. Thermal energy from the fluidised bed combustor can be used to generate steam in a boiler for power generation. Bubbling fluidised bed (BFB) technology is well suited for biomass fuels and will be utilised in this work. The agricultural wastes include maize cobs, groundnut shells, rice and millet husks, millet stalks, sorghum stalks, maize stalks and cotton stalks among others. However, this research will focus on maize cobs because of its availability and energy content.
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