Energy and Exergy Analysis of Boiler Systems

ABSTRACT 

In this work, the results of the analysis of the NBC boiler plant using energetic and exergetic methods are presented. The main aim of this study is to investigate the effects of boiler rotary burner cup speed, oil nozzle size, excess air and fuel types on its performance and emissions with a view to identifying and quantifying components having greatest losses of energy and exergy efficiencies. Optimization of the boiler operating system is also carried out. The specific objectives of the work are to: (i) analyze the process plant’s (boiler) performance from energy and exergy perspectives and spot areas having highest energy loss and exergy destruction for different fuel types (ii) determine the influences of excess air level on the boiler components’ energetic and exergetic efficiencies and its cost implications, (iii) determine the influence of boiler burner rotary cup speed on emissions of CO and NOx and (iv) determine the influence of burner rotary cup fuel nozzle size on emission. The combustor yielded the highest energy efficiency of 99.8% while the heat exchanger gave the lowest energy efficiency of 94% when run on both fuel oil and natural gas. These values are also within the ranges of results published in the literature. It was also noted that at constant fuel flow rate, burner rotary cup speed and ambient temperature, the energy and exergy efficiencies of the combustor, heat exchanger and over all boiler energy and exergy efficiencies decreased with increase in the excess air level supplied to the burner. v A reduction of excess air level from 26.67% to 12.36% eliminates the presence of CO and NOx and improves the boiler overall energy efficiency by 11.67% and its overall exergy efficiency by 3.14% This improved efficiency translates to a saving of approximately 89 million naira annually for a boiler operating at 46% availability yearly. The results of the effects of burner rotary cup speed and fuel nozzle sizes on emissions show that for boiler having liquid fuel nozzle sizes between 4mm and 6mm and with constant fuel inlet temperature of 82o C, air and fuel flow rates of 4.01 kg/s and 0.234 kg/s respectively, fuel atomization increases with increase in the speed of the rotary cup burner. This was observed for the nozzle sizes within the range of 4mm to 6mm but produced a strange result for a 3mm nozzle size. With burner of nozzle size of 3mm, the fuel analysis showed a mixture of both CO and NOx emissions. However, for the burner of nozzle size of 3mm, regulating the air flow rate or the rotary cup speed below 3100 rev/min reduces (to a large extent) or completely eliminates the presence of NOx but not CO. On the other hand, it was found that a reduction in the fuel flow rate with same level of air flows greatly reduced the level of formation of CO. A small proportion of NOx (less than 23 ppm) was obtained. This suggests a great deal of influence of fuel jet velocity on the degree of atomization and hence complete combustion of the fuel as evidenced by the reduced CO formation.

In conclusion, it was observed that with nozzle size of 4mm diameter, the burner rotary cup can operate at speed of 2920 rev/min to 3200 rev/min generating minimum emission while burning fuel oil of 82o C temperature with excess air of 12.36%. Similarly, with nozzle size of 6mm, the rotary cup burner can operate between 3950rev/min to 4400 rev/min at 12.36 % excess air without CO and NOx emissions with fuel oil supplied at 82o C.

TABLE OF CONTENTS

Cover page i

Certification ii

Approval page iii

Abstract iv

Dedication vii

Acknowledgement viii

Table of contents ix

List of table captions xiv

List of figure captions xv

Nomenclature xviii

Subscripts xx

CHAPTER ONE: INTRODUCTION 1

1.1 Why this research? 2

1.2 Boilers 6

1.2.1 Fire-tube boiler 6

1.2.2 Water-tube boiler 10

1.3 Elements of a boiler 10

1.3.1 The fuel burner 10

1.3.2 Combustion chamber 12

1.3.3 Heat exchanger 14

1.3.4 Fuel supply system 15

1.3.5 Water/steam system 17

CHAPTER TWO: LITERATURE REVIEW 19

2.1 Forms of exergy 25

2.1.1 Physical Exergy 26

2.1.2 Chemical Exergy 26

2.1.3 Potential Exergy 29

2.1.4 Kinetic Exergy 29

2.2 Transfer of Exergy 30

2.2.2 Exergy Transfer by Work 29

2.2.3 Exergy Transfer by Mass 31

2.2.4 Efficiency Calculation 31

2.2.5 Mass Balance 33

2.2.6 Energy Balance 33

2.2.7 Exergy Balance 34

2.2.8 Conventional Exergetic Efficiency 35

2.2.9 Rational Exergetic Efficiency 36

CHAPTER THREE: METHODOLOGY 37

3.1 Modeling adiabatic temperature of combustion 37

3.2 Evaluation of other terms in the equation 41

3.3 Analysis of process plant component 43

3.3.1 Boiler 44

3.3.2 Combustor 44

3.3.2.1 Mass Balance 44

3.3.2.2 Energy Balance of Combustor 45

3.5.2.3 Exergy Balance of Combustor 45

3.5.3 Boiler Heat Exchanger 46

3.5.3.1 Mass Balance of heat exchanger 46

3.5.3.2 Energy balance of Heat Exchanger 47

3.5.3.3 The exergy balance of Heat Exchanger 47

3.5.4 Overall Boiler Efficiency 48

3.5.4.1 Boiler Energetic Efficiency 48

3.5.4.2 Boiler Exergetic Efficiency 49

3.5.5 Vaporizer 49

3.5.5.1 Mass Balance 50

3.5.5.2 Energy Balance of Vaporizer 50

3.5.5.3 Exergy Balance of Vaporizer 50

3.5.6 Pump 51

3.5.6.1 Energy Balance 52

3.5.6.2 Exergy Balance 52

3.5.7 Compressor 52

3.5.7.1 Mass Balance 53

3.5.7.2 Energy Balance of Compressor 53

3.5.7.3 Exergy Balance of Compressor 54

3.5.7.4 Exergy efficiency of the Compressor 54

3.5.8 Condenser 54

3.5.8.1 Mass balance 55

3.5.8.2 Energy Balance 56

3.5.8.3 Energy efficiency of Condenser 56

3.5.8.4 Exergy Balance 56

3.5.8.5 Exergy Efficiency of Condenser 56

3.6 Optimization 56

3.7 Experimental procedure 57

3.8. A -15/ 10 ton boiler 57

3.9 Flue gas analyzer 59

3.10. Field instruments 60

3.11. Effects of excess air on the boiler performance 62

3.12 Data analysis 64

CHAPTER FOUR: DATA ANALYSIS AND RESULTS

4.1 Influence of excess air on the boiler performance: natural gas as fuel 64

4.1.1 Experiment 1 calculation 64

4.2 Heavy fuel oil 81

4.2.1 Influence of excess air on the boiler performance: heavy fuel oil 81

4.2.2 Experiment 1 82

4.3 Variation of excess air with adiabatic temperature 89

4.4 Influence of burner cup speed on NOx and CO emissions 90

4.5 Plants normal operating condition 99

4.6 Grassman diagram for the initial operating conditions 100

4.7 Grassman diagram for the boiler at optimal operating conditions 100

4.8 Grassman diagram for the boiler at the optimal operating conditions 101

4.9 Annual fuel and cost savings 102

CHAPTER FIVE: CONCLUSION AND RECOMMENDATION 104

REFERENCES 106

APPENDIX 114

Data Analysis programme (SCILAB software) 114