THE BEHAVIOR OF SWIRLING FLAMES UNDER VARIABLE FUEL COMPOSITION A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN MECHANICAL ENGINEERING AT THE SCHOOL OF ENGINEERING CARDIFF UNIVERSITY JONATHAN LEWIS 2014 DECLARATION AND STATEMENTS Declaration This work has not been submitted in substance for any other degree or award at this or any other university or place of learning, nor is being submitted concurrently in candidature for any degree other award. Signed: ................................................. (Candidate) Date: ................................................. Statement 1 This thesis is being submitted in partial fulfilment of the requirements for the degree of PhD in Mechanical Engineering. Signed: ................................................. (Candidate) Date: ................................................. Statement 2 This thesis is the result of my own independent work/investigation, except where otherwise stated. Other sources are acknowledged by explicit references. The views expressed are my own. Signed: ................................................. (Candidate) Date: ................................................. Statement 3 I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter-library loan, and for the title and summary to be made available to outside organisations. Signed: ................................................. (Candidate) Date: ................................................. PAGE |II SUMMARY This thesis is concerned with the swirl stabilised combustion of gases with variable composition, primarily those derived from the gasification of carbonaceous material, and secondarily those that occur naturally, such as shale gas. During the course of this research the temporal composition of producer gas, derived from the gasification of biomass, was studied in order to ascertain the effect its variable fuel composition had on its combustion properties. Its variation was highly dependent on gasifier operation, and despite the stoichiometric air-to-fuel ratio and Wobbe Index of the fuel being consistent, high throat temperatures resulted in high hydrogen content and laminar flame speeds. Alterations in flame speed are linked to thermo-acoustic instabilities, flame extinction and damaging flame propagation. Acoustic response under combustion conditions was investigated, to determine how it altered over a flames stability range. Indicators of impending flame flashback and blowoff were found, which could be utilised to prevent such events from occurring in an appropriate control system, without the need for real time gas analysis. Flames with high hydrogen content display a propensity for flashback, especially in high turbulence burners, such as those found in gas turbines, where thermo-acoustics are also a significant problem. Variation in fuel composition, particularly in the proportion on hydrogen, exacerbates these problems. The diffusive injection effects of three gases on reacting flow structures were investigated as a method of improving the stability of pre-mixed flames. Carbon dioxide was found to improve flame stability, whilst reducing emissions during the combustion of syngas mixtures in a development gas turbine combustor. Monitoring acoustic response and diffusive injection are thus suggested as additional stabilisation methods for the combustion of gases with variable composition. PAGE |III ACKNOWLEDGEMENT Firstly I would like to thank my supervisors; particularly Dr. Richard Marsh, who over the past three and a half years has provided me with support, encouragement and pragmatic advice whenever it was required, and Professor Anthony Griffiths, whose input has significantly improved the quality of this thesis. My thanks also go to Refgas UK, who part funded my research, in chiefly George and Paul Willacy. I would also express my gratitude to Steve Morris, who has challenged my perceptions of both life, and swirl combustion. Yura Sevcenco, who, with little reward, put up with me and my taste in music on our many exciting, combustion related, escapades. Agustin Valera-Medina, who provided insight and impetus to my research, and to all the other staff at the GTRC and Cardiff University who have been influential, for varying reasons over the past three years, including; Professor Philip Bowen, Malcolm Seaborne, Terry Treherne, James Lyons, Sally Hewlett, Jack Thomas, Gina Goddard- Bell, Professor Nick Syred, Dr. Tony Giles and Dr. Andrew Crayford. My final thanks go to my family and friends, particularly my mother and father, who have never wavered in their love and support. PAGE |IV CONTENTS DECLARATION AND STATEMENTS ................................................................................................... II SUMMARY ................................................................................................................................. III INDEX OF FIGURES ...................................................................................................................... IX INDEX OF TABLES ....................................................................................................................... XV NOMENCLATURE ...................................................................................................................... XVI ABBREVIATIONS ....................................................................................................................... XIX CHAPTER 1: INTRODUCTION ....................................................................................... 1 1.1 BACKGROUND .................................................................................................................. 2 1.2 CENTRALISED AND DISTRIBUTED POWER GENERATION ........................................................... 10 1.3 TYPES OF GASIFIERS ......................................................................................................... 11 1.4 COMBINED HEAT AND POWER (CHP) SYSTEMS .................................................................... 12 1.4.1 INTERNAL COMBUSTION ENGINES .................................................................................... 13 1.5 GASIFICATION OF BIOMASS AND REFUSE DERIVED FUEL ......................................................... 15 1.6 INTEGRATED GASIFICATION COMBINED CYCLE POWER PLANTS ................................................ 18 1.6.1 OXYGEN PRODUCTION .................................................................................................... 18 1.6.2 GAS CLEANING AND PROCESSING ..................................................................................... 19 1.6.3 GAS TURBINES .............................................................................................................. 20 1.7 SUMMARY AND THESIS AIMS............................................................................................. 22 1.8 THESIS STRUCTURE .......................................................................................................... 23 CHAPTER 2: SWIRLING FLOWS, THERMO-ACOUSTICS AND FUEL COMPOSITION .... 24 2.1 INTRODUCTION ............................................................................................................... 25 2.2 SWIRL FLOW CHARACTERISTICS .......................................................................................... 27 2.2.1 SWIRL NUMBER............................................................................................................. 27 2.2.2 TYPES OF SWIRL BURNERS ............................................................................................... 29 2.2.3 VORTEX BREAKDOWN .................................................................................................... 31 2.2.4 FLOW STRUCTURES ........................................................................................................ 32 2.2.5 COMBUSTION INDUCED VORTEX BREAKDOWN ................................................................... 40 2.3 THERMO-ACOUSTIC INSTABILITIES ...................................................................................... 41 2.3.1 MECHANISMS AND SATURATION ...................................................................................... 43 PAGE |V 2.3.2 CONTROL ..................................................................................................................... 45 2.3.3 GAS TURBINE THERMO-ACOUSTICS................................................................................... 48 2.4 FUEL COMPOSITIONS ....................................................................................................... 52 2.4.1 NATURAL GAS ............................................................................................................... 52 2.4.2 BIOMASS ...................................................................................................................... 55 2.4.3 COAL AND COAL DERIVED SYNGAS ................................................................................... 56 2.5 CHAPTER SUMMARY ........................................................................................................ 59 CHAPTER 3: EXPERIMENTAL RIGS AND METHODOLOGY .......................................... 61 3.1 INTRODUCTION ............................................................................................................... 62 3.2 DIAGNOSTIC TECHNIQUES ................................................................................................. 62 3.2.1