THE CARNOT CYCLE SECOND LAW EFFICIENCY MODEL

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BACKGROUND AND MOTIVATION

The focus of the world is the reduction of greenhouse gases such as carbon dioxide. South Africa, together with most countries, has undertaken to reduce carbon dioxide emissions into the atmosphere by introducing the carbon tax system as a way of incentivising industrial emitters to reduce emissions. South Africa is not one of the top ten emitters of carbon dioxide in the world in absolute terms, but the country is one of the top emitters when measured on a per capita basis. South Africa currently generates about 93% of its electricity from coal combustion; however, the country plans to reduce this percentage to 46% by the year 2030 even though the electric power generated from coal will stay almost the same. Unfortunately, the carbon dioxide emitted, when the share of electricity generation from coal is reduced, will be more than the current levels because poor quality coal with between 45% to 60% ash content and lower heating values has been mined in the country for at least the last decade.

JUSTIFICATION FOR THE STUDY

Not much work has been done to analyse the optimum operating conditions that would make the combustion of wood a viable alternative to coal combustion. This could be due to the cost of setting up experimental facilities to carry out studies to this effect. This study is confined to using numerical and analytical methodologies to arrive at a better understanding of phenomena involved in the circulating fluidised bed combustor, in particular the combustor component.

AIM OF THE STUDY

The aim of the study is to analyse the optimum conditions under which the combustor component of a circulating fluidised bed combustor is to be operated, and to maximise the availability of the generated energy from the running of the system.

SCOPE OF THE STUDY

The study focuses on the steady-state operation of a circulating fluidised bed combustor, under fast fluidisation conditions. The nitrogen element in the air is assumed to be inert, there is no NOx included in the combustion modelling.

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ABSTRACT
DEDICATION
ACKNOWLEDGEMENTS
Chapter 1. INTRODUCTION
1.1 BACKGROUND AND MOTIVATION
1.1. LITERATURE REVIEW
1.2. JUSTIFICATION FOR THE STUDY
1.3. AIM OF THE STUDY
1.4. OBJECTIVES OF THE STUDY
1.5. SCOPE OF THE STUDY
1.6. ORGANISATION OF THE THESIS
Chapter 2. MATHEMATICAL MODELLING
2.1. INTRODUCTION
2.2. NUMERICAL MODELLING
2.2.1. Combustion numerical model
2.2.2. Porous media laminar flow model
2.3. THERMODYNAMIC MODELLING
Chapter 3. ANALYSIS OF THEORETICAL MAXIMUM WORK THAT CAN BE DONE BY A WOOD-FIRED ADIABATIC COMBUSTOR
3.1. INTRODUCTION
3.2. ADIABATIC COMBUSTOR
3.2.1. Fuel properties
3.2.2. Combustion process CONTEN
3.3. CFD MODEL
3.4. THE CARNOT CYCLE SECOND LAW EFFICIENCY MODEL
3.5. RESULTS AND DISCUSSION
3.6. CONCLUSION
Chapter 4. THERMODYNAMIC OPTIMISATION AND COMPUTATIONAL ANALYSIS OF IRREVERSIBILITES IN A SMALL-SCALE WOOD-FIRED CIRCULATING FLUIDISED BED ADIABATIC COMBUSTOR
4.1. INTRODUCTION
4.2. MATHEMATICAL MODEL
4.2.1. Exergy analysis
4.2.2. Entropy generation number and relative entropy generation rate
4.3. CFD MODEL
4.4. RESULTS AND DISCUSSION
4.5. CONCLUSION
Chapter 5. THERMODYNAMIC OPTIMISATION OF A WOOD-FIRED COMBUSTOR: THE INFLUENCE OF WALL HEAT FLUX, WALL THICKNESS AND AIR INLET TEMPERATURE
5.1. INTRODUCTION
5.2. Thermodynamic model
5.2.1. Exergy analysis
5.2.2. Adiabatic combustor
5.2.3. Combustor with heat flux wall condition and an infinitely thin wall
5.2.4. Combustor with heat flux wall condition and varying wall thickness
5.3. Numerical model
5.3.1. Part 1: Adiabatic combustors
5.3.2. Part 2: Combustors with incoming air at 400K, one with adiabatic and the other with wall heat flux conditions
5.3.3. Part 3: Combustors with wall heat flux condition and wall thickness
5.3.4. Mesh independence
5.3.5. Validation of CFD simulations
5.4. Results and discussion
5.4.1. Part 1: Case 1 vs. Case 2
5.4.2. Part 2: Case 1 vs. Case 3
5.4.3. Part 3: Combustors with wall heat flux condition and wall thickness
5.5. CONCLUSION
Chapter 6. OPTIMUM GEOMETRY OF SOLID POROUS SPHERES WITH HEAT GENERATION
6.1. INTRODUCTION
6.2. ANALYTICAL MODEL
6.3. NUMERICAL ANALYSIS
6.4. RESULTS AND DISCUSSION
6.5. CONCLUSION
Chapter 7. CONCLUSIONS AND RECOMMENDATIONS
7.1. INTRODUCTION
7.2. CONCLUSIONS
7.3. RECOMMENDATIONS
REFERENCES