The effects of chemical and physical properties of chars derived from inertinite-rich, high ash coals on gasification reaction kinetics Gregory Nworah Okolo B.Eng (Chem. Eng.) (ESUT, Enugu, Nigeria) Dissertation submitted in partial fulfilment of the requirements for the degree Master of Engineering in Chemical Engineering at the Potchefstroom Campus of North-West University, South Africa. Supervisor: Prof. R. C. Everson Co-supervisor: Prof. H. W. J. P Neomagus November 2010 Potchefstroom i Any fact facing us is not as important as our attitude towards it, for that determines our success or failure -Norman Vincent Peale- ii DEDICATION This dissertation is gratefully dedicated to the loving memory of my late sister, Miss Juliana Okolo, who passed on to glory on the 13th of April, 2008. May her gentle soul rest in perfect peace! Eternal rest grant unto her Oh Lord, And let perpetual light shine upon her, May she rest in peace. Amen iii DECLARATION I, Gregory Nworah Okolo, do hereby declare that the dissertation with the title: The effects of chemical and physical properties of chars derived from inertinite-rich, high ash coals on gasification reaction kinetics, submitted in partial fulfilment of the requirements for the degree of Master of Engineering (Chemical Engineering) is my work and has not been submitted at any other university either in part or as a whole. Signed at Potchefstroom on the................day of ....................................................2010. .............................. Gregory N. Okolo iv ACKNOWLEDGEMENT The author expresses his appreciation and gratefully acknowledges the following people for their help, contribution and assistance during the course of this research: The Almighty God and our Mother Mary, for the spiritual support, guidance, courage and wisdom to persevere to the end. Prof. Ray Everson and Prof. Hein Neomagus for their excellent foresight, guidance and assistance, invaluable suggestions, criticisms and magnanimous supervisorship without which this investigation would not have been successful. Prof. Harold Schobert of Penn State University for his advice, suggestions and discussions on coal characterisation and carbon crystallite analysis. Prof. Frans Waanders and Prof. John Bunt for their priceless, suggestions and discussions and for editing the original draft of the dissertation. Dr. Sabine Verryn (XRD Analytical and Consulting cc), for carbon crystallite analyses; and Mrs Vivien du Cann (Petrograhic SA), for petrographic analyses of the samples and interpretation of the results. My Mum and Dad, brother and sisters: Ejiyke, Juliet, Vero, Agatha and Paschal; for their prayers, love, morale support, perseverance, and patience. My former boss Engr. Charles Chidebelu and Engr. Patrick ThankGod for their motivation and support. Mr. Jan Kroeze and Mr. Adrian Brock for keeping the experimental apparatus in excellent and safe condition. The Coal Research Group for their co-operation and lively arguments during the weekly presentations. All the personnel of the School of Chemical and Minerals Engineering. My big friends in Joburg: Ezebuilo Hyginus A., Alum Gerald O., Nweke Humpery N., Eluwa Nnamdi S., Okereke Chidozie P., Duru Nnamdi K., Ogbonna Kelechi Micheal, Anyaoku Kingsley C., Ifeanyi Blessing Emmanuel.. Jide ka unu ji !!!!!!!!! v ABSTRACT With growing energy demand across the world, where the good quality coals are gradually going into extinction creating great opportunity for low grade coals, a better understanding of the important properties of these coals and the subsequent chars, that control their behaviour in various utilisation processes, becomes pertinent. An investigation was therefore undertaken, to study the effects of chemical and physical properties imparted on chars during pyrolysis on the subsequent gasification reaction kinetics of typical South African inertinite-rich, high ash Highveld coals. Attempt was made at following these changes in the transition from coals to chars by a detailed characterisation of both the parent coals and the respective chars. These changes were determined using various conventional and advanced techniques, which included among others, carbon crystallite analysis using XRD and char carbon forms analysis using petrography. Three of the four original coals were characterised as Bituminous Medium rank C (B, C and C2), while coal D2 was found to be slightly lower in rank (Bituminous Medium rank D). The coals were rich in inertinites (> 54 vol. %, mmb with coal C2 having as high as 79 vol. %, mmb) and high in ash contents (> 26.7 wt. %, db) and cabominerite and minerite contents (26 - 39 vol. %, mmb). Inertinite-vitrinite ratios of the coals were found to range from 1.93 to 26.3. Characterization results show that both volatile matter and inherent moisture contents decreased, while ash, fixed carbon and elemental carbon contents increased from coals to chars. A good indication that the pyrolysis process was efficient. Elemental hydrogen, oxygen and nitrogen contents decreased, whereas total sulphur content increased from coals to chars. This reveals that the total sulphur contained in the char samples were associated with the char carbon matrix and the minerals. Hydrogen-carbon and oxygen-carbon ratios decreased considerably from coals to chars showing that the chars are more aromatic and denser products than the original coals. Despite the fact that mineral matter increased from coals to chars, the relative abundance of the different mineral phases and ash components did not exhibit significant variation amongst the samples. However, the alkali index was found to vary considerably among the subsequent chars. Petrographic analysis on the coals and char carbon forms vi analysis on the chars reveal that total reactive components (TRC) decrease while the total inert components (TIC) increase from coals to chars. The 0% gain in TIC observed in char C2 was attributed to its relatively high content of partially reacted maceral char carbon form. Total maceral reflectance shifted to higher values in the chars (4.43 - 5.28 Rsc %) with respect to the coals (1.15 - 1.63 Rsc %); which suggests a higher structural ordering in the chars. Carbon crystallite analyses revealed that the chars were condensed (smaller in size) relative to the parent coals. Lattice parameters: inter-layer spacing, d002, increased, while the average crystallite height, Lc, crystallite diameter, La, and number of aromatic layers per crystallite, Nave, decreased from coals to chars. Carbon aromaticity generally increased whereas the fraction of amorphous carbon and the degree of disorder index decreased from parent coals to the respective chars. Both micropore surface area and microporosity were observed to increase while the average micropore diameter decreased from coals to chars. This shows that blind and closed micropores were opened up during the charring process. It was generally observed that, although the original coal samples did not show much variations in their properties (except for their maceral content), the subsequent chars exhibited substantial differences both amongst themselves and from the parent coals. The increasing orders of magnitude of micropore surface area, microporosity, fraction of amorphous carbon and structural disorderliness were found to change in the transition, a good indication that the chars properties varies from that of the respective precursor coals. Isothermal CO2 gasification experiments were conducted on the chars in a Thermax 500 thermogravimetric analyser in the temperature range of 900 - 950 C with varying concentrations of CO2 (25 - 100 mol. %) in the CO2-N2 reaction gas mixture at ambient pressure (0.875 bar in Potchefstroom). The effects of temperature and CO2 concentration were observed to be in conformity to established trends. The initial reactivity of the chars were found to increase in the order: chars C2 < C < B < D2, with char D2 reactivity greater than the reactivity of the other chars by a factor > 4. Gasification reactivity results were correlated with the parent coals and chars properties. Except for the rank parameter (the vitrinite reflectance), no significant trend was observed with any other coal petrographic property. Correlations with char vii properties gave more significant and systematic trends. Major factors affecting the gasification reactivity of the chars as it pertains to this investigation are: parent coal vitrinite reflectance, and: aromaticity, fraction of amorphous carbon, degree of disorder and alkali indices, micropore surface area, microporosity and average micropore diameter of the chars. The random pore model (chemical reaction controlling) was found to adequately describe the gasification reaction experimental data (both conversions and conversion rates). The determined activation energy ranged from 163.3 kJmol-1 for char D2 to 235.7 kJmol-1 for char B; while the order of reaction with respect to CO2 concentration was 0.52 to 0.67 for the four chars. The lower activation energy of char D2 was possibly due to its lower rank, lower coal vitrinite reflectance and higher alkali index. The estimated kinetic parameters of the chars in this study correspond very well with published results in open literature. It was possible to express the intrinsic reactivity, rs, of the chars (rate of carbon conversion per unit total surface area) using kinetic results, in empirical Arrhenius forms. Keywords: Inertinite-rich coal, Coal and char properties, Char carbon forms, Carbon crystallite analysis, Carbon dioxide gasification, kinetic modelling. viii OPSOMMING Die toeneemlike gebruik van lae graad steenkool word al hoe meer n realiteit, as gevolg van ho kwaliteit steenkole wat verkwis word om te voorsien in die behoeftes van die groeiende energiekwessie dwarsoor die wreld. Dit is dus van uiterste belang dat n goeie begrip rondom die eienskappe en gedrag van lae graad steenkole en hulle gevolglike sintels gevorm word om sodanig gebruik te word in verskeie kommersile prosesse. n Studie was dus onderneem om die effek van chemiese- en fisiese eienskappe van die gevormde sintels van verskeie lae graad steenkole op die gevolglike vergassings reaksie kinetika van tipiese Suid-Afrikaanse inertiniet ryk, ho as steenkole te ondersoek. n Poging was aangewend om die verandering in die karakteristieke eienskappe van steenkool tot die vorming van sintels te monitor, deur gebruik te maak van n gedetaileerde karakteriserings ondersoek op beide die rou steenkole en hulle gevormde sintels. Konvensionele- en gevorderde metodes soos koolstof kristallografie met behulp van XRD en sintels koolstof vorm analise met behulp van petrografie is ingespan. Drie van die vier steenkole wat gebruik is, is gekarakteriseer as Bitumineuse Gemiddelde Rang C (steenkole B, C en C2) steenkole, terwyl dit gevind is dat steenkool D2 egter n effense laer rang gehad het (Bitumineuse Gemiddelde Rang D). Al vier steenkole het ho inhoude van inertiniet (> 54 vol. %, mineraal basis met steenkool C2 wat die hoogste inhoud gehad het met 79 vol. % mineraal basis), as (> 26.7 wt. %, dro basis), karbomineriet en mineriet (26-39 vol. %, mineraal basis) bevat. Die inertiniet-vitriniet verhoudings van die vier steenkole het gewissel tussen ongeveer 1.93 en 26.3. Vanuit die karakteriserings resultate was dit duidelik dat beide die inherente waterinhoude en vlugtige stofinhoude afneem, terwyl die aswaardes, vaste koolstofinhoude en elementre koolstof inhoude dienooreenkomstig toeneem vanaf steenkool tot sintels. Hieruit kon dit afgelei word dat die pirolise proses, vir die generering van sintels, effektief was. Hiermee saam het die elementre waterstof-, suurstof- en stikstof inhoude ook afgeneem, terwyl die swawel inhoud toegeneem het. Die verhoogde swawel vlakke in die sintels toon aan dat dit hoogs waarskynlik meer geassosieer is met die koolstof/sintels koolstofmatriks as met die anorganiese ix minerale. Die waterstof-koolstof- en suurstof-koolstof verhoudings het ook n noemenswaardige afname getoon van steenkool tot sintel, wat aandui dat die gevormde sintels meer aromaties en digter is as die oorspronklike steenkole. Ten spyte van die feit dat minerale inhoud toeneem van steenkool na sintels kon daar geen noemenswaardige variasie tussen die relatiewe voorkoms van die verskeie mineraalfases en askomponente onderskei word nie. Dit is egter gevind dat die alkali indeks noemenswaardig varieer tussen die verskillende sintels. Petrografiese analise op die steenkole en sintels koolstof vormanalise op die sintels het getoon dat die totale reaktiewe komponente (TRC) afneem terwyl die totale inerte komponente (TIC) toeneem van steenkool na sintels. Die 0% toename in TIC vir sintels C2 kan toegeskryf word aan sy relatiewe ho inhoud van parsiel gereageerde maserale sintels koolstofvorm. Totale maserale reflektiwiteit was aansienlik hor vir die sintel (4.43 5.28 Rsc %) as vir die oorspronklike steenkole (1.15 1.63 Rsc %), wat op n hor strukturele geordenheid van die sintels dui. Vanuit koolstof kristallografie analise was dit duidelik dat die sintels meer gekondenseer (kleiner in grootte) is as die oorspronklike steenkole. Kristalstruktuur parameters soos die inter-laag spasiering (d002) het toegeneem, terwyl die gemiddelde kristalhoogte (Lc), kristaldiameter (La) en aantal aromatiese lae per kristal afgeneem het van steenkool na sintels. Koolstof aromatisiteit het gevolglik toegeneem, terwyl die fraksie van amorfe koolstof en graad van wanordelikheidsindeks afgeneem het van steenkool na sintels. Vanuit n fisiese perspektief het beide die mikroporieuse oppervlakarea en mikroporositeit toegeneem terwyl die gemiddelde mikroporieuse diameter afgeneem het vanaf steenkool tot sintels. Hieruit kan afgelei word dat blinde en geslote mikroporie geopen het gedurende die pirolise proses. Algeheel was daar geen duidelike verskil tussen karakteristieke eienskappe van die oorpsronklike steenkole nie, behalwe vir die verskil in maserale inhoud. In kontras het daar groot karakteristieke verskille bestaan tussen die verskillende sintels asook van hulle oorpronklike steenkole. Die toenemende orde van grootte van mikroporieuse oppervlakarea, fraksie van amorfe koolstof en strukturele ongeordenheid dui grotendeels daarop dat die sintels grootliks verskillend is van hulle oorspronklike steenkole. x Isotermiese CO2 vergassing is gedoen om die reaktiwiteit van die gevormde sintels te toets. Vir hierdie doeleinde is gebruik gemaak van n Thermax 500 termogravimetriese analiseerder. Temperature tussen 900 en 950 C en CO2 konsentrasies van 25 to 100 mol. % (CO2-N2 reaksie gasmengsel) by atmosferiese druk is gebruik (0.875 bar in Potchefstroom) om die sintelsreaktiwiteit te assesseer. Die effek van temperatuur en CO2 konsentrasie op die reaktiwiteit van die sintels het ooreengestem met wat gevind is in literatuur. Hiermee saam het die aanvanklike (inisile) reaktiwiteit van die sintels afgeneem in die volgende orde: C2 < C < B < D2 met sintel D2 wat se reaktiwiteit n faktor 4 groter was as die ander sintels. Vergassings reaktiwiteitsresultate is verder gekorreleer met die karakteristieke eienskappe van die oorspronklike steenkole en die sintels. Geen ander noemenswaardige korrelasie is verkry tussen die petrografiese eienskappe van die steenkole nie, behalwe vir die vitriniet reflektiwiteit. Korrelasies met die sintels-eienskappe het meer sistematiese verduidelikings gelewer. Vir die betrokke studie was die belangrikste faktore wat n rol gespeel het in vergassingsreaktiwiteit: oorspronklike steenkool vitriniet reflektiwiteit; aromatisiteit-, fraksie amorfe koolstof-, graad van wanordelikheidsindeks-, mikroporieuse oppervlakte-, mikroporositeit- en gemiddelde mikroporie diameter van die sintels. Die eksperimentele vergassingsresultate (beide omsetting en reaksietempo) kon akkuraat beskryf word deur die willekeurige poriemodel (chemiese reaksie beherend). Die bepaalde aktiveringsenergie het gewissel tussen 163.3 kJmol-1 vir sintel D2 tot 235.7 kJmol-1 vir sintels B, terwyl die reaksie-orde met betrekking tot CO2 konsentrasie gewissel het tussen 0.52 en 0.67 vir die vier sintels. Die lae aktiveringsenergie van sintels D2 kan heel waarskynlik toegeskryf word aan die steenkool se lae vitriniet reflektiwiteit en hor alkali indeks. Die beraamde kinetiese parameters van die sintels toon goeie ooreenstemming met wat bevind is in literatuur. Dit was verder ook moontlik om die intrinsieke reaktiwiteit, van die sintels (tempo van koolstofomsetting per eenheidsoppervlakarea) uit te druk deur gebruik te maak van kinetiese resultate in empiriese Arrhenius vorms. xi TABLE OF CONTENTS Dedication ..................................................................................................................... ii Declaration ...................................................................................................................iii Acknowledgement ....................................................................................................... iv Abstract ......................................................................................................................... v Opsomming ................................................................................................................viii Table of contents .......................................................................................................... xi List of figures ............................................................................................................. xvi List of tables ............................................................................................................... xix Nomenclature ............................................................................................................ xxi Greek symbols ......................................................................................................... xxiv Abbreviations ........................................................................................................... xxv Conference Presentation Resulting from this Investigation ..............................xxviii Chapter 1: GENERAL INTRODUCTION ......................................................... 1 1.1 Introduction ..................................................................................................... 1 1.2 Background Information and Motivation ........................................................ 2 1.2.1 Clean Coal Technologies .......................................................................... 4 1.3. Hypotheses of the Study .................................................................................. 6 1.4 The Objectives of the Study ............................................................................ 7 1.5 Scope of the Research Work ........................................................................... 9 Chapter 2: LITERATURE REVIEW ................................................................ 11 2.1 Introduction ................................................................................................... 11 2.2 Coal Gasification ........................................................................................... 12 2.2.1 The Industry Context of Coal Gasification ............................................. 13 2.2.2 History of Coal Gasification ................................................................... 14 2.2.3 Modern Coal Gasification ....................................................................... 15 2.3 The Coal Gasification Process ....................................................................... 17 2.4 Chemical and Physical Structure of Coal and Char ....................................... 18 2.4.1 Chemical and Physical Structure of Coal ............................................... 19 2.4.2 Chemical and Physical Structure of Coal Char ....................................... 22 2.4.3 The Crystallite Structure of the Carbon Basic Structural Unit (BSU) ..... 27 2.5 Coal, Char and Gasification Reactivity ......................................................... 30 2.6 Factors Influencing Gasification Reactivity .................................................. 31 2.6.1 Properties of the Parent Coal .................................................................. 32 xii 2.6.1.1 Volatile Matter Content .................................................................... 32 2.6.1.2 Fixed Carbon Content ...................................................................... 32 2.6.1.3 Petrographic Properties of Coal ...................................................... 33 2.6.2 Pyrolysis Conditions and Heat Treatment .............................................. 34 2.6.3 Chemical Structure and Composition of Coal and Char .......................... 38 2.6.4 Changes in Carbon Crystallite Properties ............................................... 39 2.6.5 Catalysis by Mineral Matter .................................................................... 42 2.6.6 Physical Structural Properties of Chars .................................................. 45 2.6.6.1 Total Surface Area ........................................................................... 45 2.6.6.2 Active Surface Area .......................................................................... 46 2.6.6.3 Surface Complex Concentration during Reaction ............................ 47 2.7 Methods of Measuring Gasification Reactivity ............................................. 48 2.7.1 Thermogravimetric Analysers ................................................................ 48 2.8 Char-CO2 Gasification Reactions .................................................................. 50 2.8.1 Char-CO2 Reaction Mechanism .............................................................. 50 2.9 Heterogeneous Char-Gas Kinetics ................................................................. 51 2.9.1 Reaction Rate Models ............................................................................. 51 2.9.2 Overall CO2 Gasification Kinetics .......................................................... 54 2.10 Homogeneous Gas-Phase Reactions .......................................................... 55 2.11 Structural Kinetic Models .......................................................................... 56 2.11.1 The Volume Reaction Model .............................................................. 57 2.11.2 Shrinking Core Model ......................................................................... 58 2.11.3 Random Pore Model ............................................................................ 59 Chapter 3: COAL AND CHAR CHARACTERISATION ............................... 60 3.1 Introduction ................................................................................................... 60 3.2 Origin of Coal Samples ................................................................................. 61 3.3 Sample Preparation ........................................................................................ 61 3.4 Char Preparation at 900 C ............................................................................ 62 3.4.1 Charring Apparatus and Procedure ......................................................... 63 3.5 Coal and Char Characterisation Analyses ..................................................... 65 3.6 Coal and Char Characterisation Equipment and Techniques ......................... 66 3.6.1 Chemical Analyses .................................................................................. 66 3.6.2 X-ray Diffraction (XRD) Mineral Analysis ............................................ 67 3.6.3 Ash Analysis (XRF) ................................................................................ 68 3.6.4 X-ray Diffraction (XRD) Carbon Crystallite Analysis ............................ 69 3.6.5 Petrographic Analysis ............................................................................. 73 3.6.6 Structural Analysis .................................................................................. 74 3.6.6.1 CO2 Adsorption Analysis .................................................................. 75 3.6.6.2 Helium Pycnometry .......................................................................... 76 3.7 Characterisation Results and Discussion ....................................................... 77 3.7.1 Chemical Analyses .................................................................................. 77 3.7.2 XRD Mineral Analyses ........................................................................... 80 xiii 3.7.3 Ash Analysis (XRF) ................................................................................ 83 3.7.4 X-ray Diffraction (XRD) Carbon Crystallite Analysis ............................ 85 3.7.4.1 Determination of Aromaticity of Coal and Char Samples ................ 91 3.7.4.2 Determination of Fraction of Amorphous Carbon of the Coal and Char Samples ....................................................................................... 92 3.7.5 Petrographic Analyses ............................................................................ 98 3.7.5.1 Reflectance Properties ..................................................................... 98 3.7.5.2 Maceral Analysis of Parent Coals ................................................. 101 3.7.5.3 Microlithotype Analysis of Parent Coals ........................................ 103 3.7.5.4 Carbominerite and Minerite Analysis of Parent Coals ................... 105 3.7.5.5 General Condition of Coal Samples .............................................. 106 3.7.6 Char Carbon Forms Analysis ................................................................ 107 3.7.7 Physical Structural Analysis: Coal and Char Samples .......................... 118 3.8 Summary ...................................................................................................... 124 Chapter 4: EXPERIMENTAL: CHAR GASIFICATION WITH CARBON DIOXIDE .................................................................................................................. 128 4.1 Introduction ................................................................................................. 128 4.2 Materials Used ............................................................................................. 129 4.2.1 Coals and Subsequent Chars ................................................................. 129 4.2.2 Reactant Gases ..................................................................................... 129 4.3 Reactivity Equipment: Thermogravimetry ................................................. 130 4.3.1 Thermax 500 Thermogravimetric Analyser (TGA) .............................. 130 4.3.2 Gas Supply ............................................................................................. 134 4.3.3 Data Acquisition Interface .................................................................... 134 4.4 Experimental Procedure ............................................................................. 135 4.5 TGA Experimental Programme ................................................................... 136 Chapter 5: GASIFICATION WITH CARBON DIOXIDE: RESULTS AND DISCUSSION ........................................................................................................... 138 5.1 Introduction ................................................................................................. 138 5.2 Normalisation of the Experimental Results ................................................. 139 5.3 Reproducibility of the Experimental Results ............................................... 141 5.4 Effect of Operating Conditions on Char-CO2 Gasification Reactivity ......... 141 5.4.1 Effect of Isothermal Temperature of Reaction ..................................... 141 5.4.2 Effect of CO2 Concentration in the Reaction Gas ................................ 143 5.5 Determination of the CO2 Reactivity of the Chars ....................................... 144 5.6 Effect of Coal and Char Properties on CO2 Reactivity of the Chars ............ 147 5.6.1 Effect of Parent Coals Petrographic Properties ..................................... 147 5.6.1.1 Effect of Maceral Index and Modified Reactive Maceral Index of the Parent Coals ...................................................................................... 149 5.6.1.2 Effect of Rank Parameter of the Parent Coals ................................ 152 xiv 5.6.2 Influence of Char Properties on Char-CO2 Reactivity ........................... 153 5.6.2.1 Influence of Char Petrography (Char- TRC and TIC) .................... 153 5.6.2.2 Influence of Char Carbon Crystallite (Chemical Structural) Properties ........................................................................................... 155 5.6.2.2.1 Influence of Char Aromaticity .................................................... 155 5.6.2.2.2 Influence of Fraction of Amorphous Carbon in Char ................ 156 5.6.2.2.3 Influence of Degree of Disorder Index of chars ......................... 157 5.6.2.3 Inherent Catalytic Effects of Ash Components of Chars ................. 158 5.6.2.4 Effect of Physical Structural Properties of Chars ......................... 159 5.6.2.4.1 Effect of Micropore Surface Area of Chars ................................ 159 5.6.2.4.2 Influence of Average Micropore Diameter of Chars .................. 160 5.6.2.4.3 Influence of Char Porosity .......................................................... 160 5.7 Comparison of the CO2 Reactivity of the four Chars ................................... 161 5.8 Summary ...................................................................................................... 164 Chapter 6: CHAR GASIFICATION WITH CARBON DIOXIDE: KINETIC MODELLING AND PARAMETERS EVALUATION ......................................... 166 6.1 Introduction ................................................................................................. 166 6.2 The Random Pore Model ............................................................................. 167 6.3 The Random Pore Model Equation ............................................................. 168 6.4 Validation Procedure ................................................................................... 171 6.5 Evaluation of Kinetic Parameters ............................................................... 173 6.5.1 Evaluation of the Structural Factor, ................................................ 174 6.5.2 Determination of the Time Factor, tf ..................................................... 176 6.5.3 Determination of Activation Energy, Ea ............................................... 178 6.5.4 Determination of Order of Reaction, m ................................................ 181 6.5.5 Determination of Lumped Pre-exponential Factor, 'sok ........................ 183 6.6 Validation of Kinetic Model and Associated Parameters ............................. 184 6.7 Summary ...................................................................................................... 189 Chapter 7: CONCLUSION AND RECOMMENDATIONS .......................... 191 7.1 Introduction ................................................................................................. 191 7.2 General Conclusions .................................................................................... 192 7.3 Contributions to Knowledge Base of Coal Science and Technology ....... 195 7.4 Recommendations for Future Studies ...................................................... 196 REFERENCES ......................................................................................................... 197 APPENDICES .......................................................................................................... 218 APPENDIX A ........................................................................................................... 219 Coal and Char Characterisation and Results .................................................... 219 xv A-1 Description of Standard Methods Used for Characterisation .................... 219 A-2 Vitrinite Reflectance Scan Histograms of Coal Samples .......................... 220 A-3 Total Maceral Reflectance Scan Histograms of Coal and Char samples .. 221 A-4 Outline of Classification System for Char Carbon Forms. ....................... 223 APPENDIX B ........................................................................................................... 224 Char-CO2 Gasification Reactivity Results .......................................................... 224 B-1 Reproducibility of Experimental Results and Reactivity of the Chars ..... 224 B-2 Determination of CO2 Reactivity of the Chars .......................................... 226 B-3 Effect of Isothermal Temperature of Reaction on the Char Reactivity ..... 228 B-4 Effect of CO2 Concentration in the Reaction Gas on Char Reactivity ...... 230 B-5 Comparison of CO2 Reactivity of the Four Chars ..................................... 232 APPENDIX C ........................................................................................................... 234 Evaluation of Kinetic Parameters and Gasification Modelling ........................ 234 C-1 Summary of Structural Parameter, Time Factor and Initial Reactivity of the Chars .............................................................................................................. 234 C-2 Dimensionless Plots for Chars C, C2 and D2. ............................................... 237 C-3 Comparison of Experimental and Model Gasification Results for Chars C, C2 and D2 ............................................................................................................ 238 APPENDIX D ........................................................................................................... 240 Model Validation: Random Pore Model (RPM) ................................................ 240 D-1 RPM Fitting to the Experimental Data of Chars B, C, C2 and D2. ............... 240 D-2 RPM Fitting of Char Conversion Rate to Experimental Results for the Chars ... ................................................................................................................... 242 xvi LIST OF FIGURES Figure 1.1: Total world energy supply and generation by fuel respectively in 2006 .... 2 Figure 1.2: Scope of the research work. ........................................................................ 9 Figure 2.1: Coal gasification products ......................................................................... 15 Figure 2.2: Molecular model for the inertinite-rich Highveld coal. ............................ 22 Figure 2.3: Molecular model for the vitrinite-rich Waterberg coal ............................. 22 Figure 2.4: A schematic representation of the structural changes that occurs upon heating of coal. ............................................................................................................. 24 Figure 2.5: Geometry optimised structural conformations of average coal and char molecules and intermediates, in the coal to char pyrolysis reaction. ........................... 26 Figure 2.6: Schematic representation of a crystallite of graphite ................................ 27 Figure 3.1: Experimental setup for char preparation ................................................... 63 Figure 3.2: Raw diffractograms of coal and char samples. .......................................... 87 Figure 3.3: Corrected and smoothened diffractograms of coal and char samples. ...... 88 Figure 3.4: Comparison of coal and char diffractograms for samples B and C. .......... 90 Figure 3.5: Determination of area under d002 and - band using HighScore Plus for coal B and char C2. ...................................................................................................... 92 Figure 3.6: Determination of amorphous fraction of carbon, XA, from (002) profile of coal C2 and char C. ...................................................................................................... 93 Figure 3.7: Relationship between aromaticity and fraction of amorphous carbon and the atomic ratios of hydrogen and oxygen to carbon in coal samples. ........................ 96 Figure 3.8: Relationship between various crystallite parameters of char samples. ..... 97 Figure 3.9: Photomicrographs of different categories of char carbon forms. ............ 111 Figure 3.10: Photomicrographs of different categories of char carbon forms. .......... 112 Figure 3.11: Photomicrographs of different categories of char carbon forms. .......... 113 Figure 3.12: Comparison of parent coals macerals and their specific char carbon forms in the chars. ................................................................................................................ 117 Figure 3.13: Comparison of the total inert and reactive components in the parent coals and the resultant chars. ............................................................................................... 118 Figure 3.14: Skeletal density of coal and char samples. ............................................ 118 Figure 3.15 Micropore surface area of coal and char samples. ................................. 119 Figure 3.16: CO2 adsorption isotherm plots for coal and char samples. .................... 122 Figure 3.17: Comparison of the CO2 adsorption isotherm plots for coals and chars: B, C, C2 and D2. ............................................................................................................. 123 Figure 4.1: Schematic representation of Thermax 500 TGA showing the essential parts and gas flow system. ......................................................................................... 131 Figure 4.2: Photograph of Thermax 500 TGA showing the essential parts. .............. 132 Figure 5.1: Typical mass loss curve for char C2 at 900 C, 100% CO2, 0.875 bar. .. 139 Figure 5.2: Conversion-time plot for char C2 at 900 C, 100% CO2, 0.875 bar. ...... 140 Figure 5.3: Effect of temperature on the CO2 reactivity of the chars at different constant CO2 concentrations, 0.875 bar. ................................................................... 142 xvii Figure 5.4: Effect of CO2 concentration on the char reactivity at various constant isothermal temperatures, 0.875 bar. .......................................................................... 143 Figure 5.5: Rate of reaction versus fractional conversion for the chars at 25% CO2 concentration, 0.875 bar. ........................................................................................... 145 Figure 5.6: Relationship between the initial reactivity of the chars and the petrographic properties of the parent coals at 100% and 75% CO2, 0.875 bar. ........ 148 Figure 5.7: Relationship between initial reactivity of the chars and the MI and RMI* of the parent coals at 100% and 75% CO2, 0.875 bar. .............................................. 152 Figure 5.8: Relationship between the initial reactivity of the chars and the vitrinite reflectance (Rr %) of the parent coals at 100% and 75% CO2, 0.875 bar. ................ 153 Figure 5.9: Relationship between the initial reactivity of the chars with the char TRC at 100% and 75% CO2, 0.875 bar. ............................................................................. 154 Figure 5.10: Relationship between the initial reactivity of the chars and the aromaticity of the char samples at 50% and 25% CO2, 0.875 bar. ............................ 155 Figure 5.11: Relationship between the initial reactivity of the chars and the fraction of amorphous carbon in chars at 50% and 25% CO2, 0.875 bar. ................................... 156 Figure 5.12: Relationship between the initial reactivity of chars and the degree of disorder index, DOI, at 50% and 25% CO2, 0.875 bar. ............................................. 157 Figure 5.13: Influence of the alkali index on the initial reactivity of the chars at 50% and 25% CO2, 0.875 bar. ........................................................................................... 158 Figure 5.14: Influence of the D-R micropore surface area of chars on their initial reactivity at 100% and 75% CO2, 0.875 bar. ............................................................. 159 Figure 5.15: Influence of the average micropore diameter of chars on the initial reactivity at 100% and 75% CO2, 0.875 bar. ............................................................. 160 Figure 5.16: Influence of porosity of char on the initial reactivity at 100% and 75% CO2, 0.875 bar. .......................................................................................................... 161 Figure 5.17: Comparison of CO2 reactivity of the chars at various temperatures and CO2 concentrations, 0.875 bar. .................................................................................. 162 Figure 6.1: Comparison of the gasification experimental and model results for char B, 0.875 bar. ................................................................................................................... 176 Figure 6.2: Arrhenius plots of the char-CO2 gasification reaction at 100, 75, 50 and 25% CO2 concentrations, 0.875 bar. ......................................................................... 180 Figure 6.3: Determination of char-CO2 gasification reaction order for chars B and C at various temperature and constant CO2 concentrations, 0.875 bar. ............................ 181 Figure 6.4: Determination of char-CO2 gasification reaction order for chars C2 and D2 at various temperature and constant CO2 concentrations, 0.875 bar. ........................ 182 Figure 6.5: Parity plot of predicted versus actual tf values for all four char samples.184 Figure 6.6: Comparison between experimental and model gasification results for ... 186 char B. ........................................................................................................................ 186 Figure 6.7: RPM fitting of char conversion at different CO2 concentrations. ........... 187 Figure 6.8: RPM fitting of char conversion rate for chars B, C, C2 and D2 at 25% CO2 concentration and different temperatures. .................................................................. 188 Figure A-1: Vitrinite reflectance scan histogram of the coal samples. ..................... 220 xviii Figure A-2: Total maceral reflectance scan histogram for coals and chars: B and C. .................................................................................................................................... 221 Figure A-3: Total maceral reflectance scan histogram for coals and chars: C2 and D2. .................................................................................................................................... 222 Figure B-1: Reproducibility results for chars B, C, C2 and D2 at different experimental conditions, 0.875 bar. .......................................................................... 224 Figure B-2: Rate of reaction versus fractional conversion for chars B and C at different experimental conditions, 0.875 bar. ............................................................ 226 Figure B-3: Rate of reaction versus fractional conversion for chars C2 and D2 at different experimental conditions, 0.875 bar. ............................................................ 227 Figure B-4: Effect of temperature on the CO2 reactivity of chars B and C, 0.875 bar. .................................................................................................................................... 228 Figure B-5: Effect of temperature on the CO2 reactivity of chars C2 and D2, 0.875 bar. ............................................................................................................................. 229 Figure B-6: Effect of CO2 concentration on the reactivity of chars B and C, 0.875 bar. .................................................................................................................................... 230 Figure B-7: Effect of CO2 concentration on the reactivity of chars C2 and D2, 0.875 bar. ............................................................................................................................. 231 Figure B-8: Comparison of the CO2 reactivity of chars at 100 and 75% CO2 concentration, 0.875 bar. ........................................................................................... 232 Figure B-9: Comparison of the CO2 reactivity of chars at 50 and 25% CO2 concentration, 0.875 bar. ........................................................................................... 233 Figure C-1: Dimensionless plots of conversion versus reduced time for chars C and C2. .............................................................................................................................. 237 Figure C-2: Dimensionless plot of conversion versus reduced time for char D2. ..... 238 Figure C-3: Comparison between the experimental and model gasification results of char C. ........................................................................................................................ 238 Figure C-4: Comparison between the experimental and the model gasification results of chars C2 and D2. ................................................................................................... 239 Figure D-1: RPM fitting of the experimental results of chars B and C at 0.875 bar. 240 Figure D-2: RPM fitting of the experimental results of chars C, C2 and D2 at 0.875 bar. ............................................................................................................................. 241 Figure D-3: RPM fitting of the experimental results of char D2 at 0.875 bar. ......... 242 Figure D-4: RPM fitting of the char conversion rates for char B at 25% and 50% CO2 concentration and different temperatures. .................................................................. 242 Figure D-5: RPM fitting of the char conversion rates for chars B, C, C2 at different experimental conditions, 0875 bar. ........................................................................... 243 Figure D-6: RPM fitting of the char conversion rates for chars C and D2 at different experimental conditions, 0.875 bar. .......................................................................... 244 xix LIST OF TABLES Table 2.1: Gasification based power generating plants. .............................................. 16 Table: 2.2: Some of the Plants using FBDB as Sasol. ............................................ 17 Table 2.3: Basic structures and functional groups in coal ........................................... 38 Table 3.1: Size requirements for coal and char characterisation analyses. .................. 62 Table 3.2: Char production conditions. ........................................................................ 63 Table 3.3: Char yield after production. ........................................................................ 64 Table 3.4: Characterisation analyses conducted on the coal and char samples. .......... 65 Table 3.5: Analytical methods used for chemical and mineralogical analysis. ........... 66 Table 3.6: Analysis parameters and settings on the XRD system for mineral analysis. ...................................................................................................................................... 67 Table 3.7: Analysis parameters and settings on the XRD system for carbon crystallite analysis. ........................................................................................................................ 70 Table 3.8: Result of proximate and chemical analyses of coal and char samples. ...... 79 Table 3.9: Pecentage of graphite and total crystalline mineral phases of the coal and char samples from XRD results. .................................................................................. 81 Table 3.10: Mineral abundance of coals and chars (graphite free bases, (wt. %, gfb)). ...................................................................................................................................... 82 Table 3.11: Char sample ash chemistry on LOI and sulphur free basis (wt. %, lfb and sfb ). .............................................................................................................................. 84 Table 3.12: Proximate analysis of raw and demineralised coal and char samples (wt. %, db). .......................................................................................................................... 86 Table 3.13: Comparison of aromaticity results from HighScore Plus and Origin 6.1. 91 Table 3.14: Determination of amorphous fraction of carbon for coal C2 and char C. 94 Table 3.15: Result on carbon crystallite analysis using XRD. .................................... 94 Table 3.16: Reflectance properties of coal and char samples. ................................... 100 Table 3.17: Maceral component summary of the coal samples (vol. %, mmb). ........ 101 Table 3.18: Maceral compositions of coal samples (vol. %, mmb). .......................... 102 Table 3.19: Microlithotype analysis of coal sample (vol. %,mmb). .......................... 104 Table 3.20: Carbominerite and minerite results as percentage of total carbominerite and minerite (vol. %, mmb). ....................................................................................... 106 Table 3.21: Char carbon forms analysis result (vol. %, mmb). .................................. 109 Table 3.22: Total reactive and inert components of coal and char samples (vol. %, mmb) .......................................................................................................................... 116 Table 3.23: Physical structural properties of coal and char samples. ........................ 120 Table 4.1 Specifications of gaseous reagents. ........................................................... 129 Table 4.2: Thermax 500 TGA specifications. ............................................................ 133 Table 4.3: Reaction conditions for char-CO2 gasification experiments. ................... 137 Table 5.1: Determined initial gasification reactivity, R of the char at various operating conditions, 0.875 bar. ................................................................................................ 146 xx Table 5.2: Results used to evaluate the maceral index (MI) and the modified reactive maceral index (RMI*) of the parent coals on mineral matter free basis (mmfb). ....... 151 Table 6.1: Dimensionless structural parameters for the char pores. .......................... 174 Table 6.2: Summary of the structural parameter, time factors and initial reactivity for char B, 0.875 bar. ...................................................................................................... 177 Table 6.3: Details of results of activation energy for the char samples at different CO2 concentrations in the reaction gas, 0.875 bar. ........................................................... 179 Table 6.4: Details of result for reaction order with respect to CO2 concentration at different temperatures and constant CO2 gas composition at 0.875 bar. ................... 182 Table 6.5: Determination of lumped pre-exponential factor for the four char samples, 0.875 bar. ................................................................................................................... 183 Table 6.6: Summary of structural and kinetic parameters for chars B, C, C2 and D2 .................................................................................................................................... 184 Table 6.7: Models used, structural parameter, and kinetic parameters obtained for char-CO2 gasification reaction by other investigators. .............................................. 185 Table B-1: Analysis of reproducibility and experimental error. ................................ 225 Table C-1: Summary of the structural parameter, time factor and initial reactivity for char C, 0.875 bar. ...................................................................................................... 234 Table C-2: Summary of the structural parameter, time factor and initial reactivity for char C2, 0.875 bar. .................................................................................................... 235 Table C-3: Summary of the structural parameter, time factor and initial reactivity for char D2, 0.875 bar. .................................................................................................... 236 xxi NOMENCLATURE Symbol Description Unit A Breadth of X-ray beam mm A() Absorption factor - A, kso Pre-exponential factor min-1 bar-m A002 Area under the (002) peak 2 Af Final ash content of coal or char after demineralisation wt. % Ai Original ash content of coal or char before demineralisation wt. % AI Alkali index - Ax Breadth of X-ray cm A Area under the gamma side band of (002) peak 2 Cg Concentration of gaseous reactant mole m-3 CV Calorific value MJ kg-1 d002 Inter-layer spacing for a group of Nave parallel layers dp Average diameter of coal or char particles m, mm Dp Pore diameter / average pore diameter E Activation Energy kJ mol-1 Ed Effectiveness of demineralisation % f(X) Structural factor m-1 fa Carbon aromaticity - GCV Gross calorific value MJ kg-1 Ha Hydrogen aromaticity - I X-ray reduced intensity / X-ray intensity Atomic units / counts I002 Reduced intensity due to (002) reflection atomic units Iam X-ray reduced intensity due to amorphous carbon atomic units Icr X-ray reduced intensity due to crystalline carbon atomic units Imax Maximum reduced intensity of (002) peak atomic units K Constant depending on X-ray refection plane - K Absolute temperature scale K k, k1, k2, k3 Reaction rate constant min-1 kso Lumped pre-exponential factor min-1 bar-m xxii Symbol Description Unit kso Pre-exponential factor m min-1 bar-m kv Intrinsic rate constant of the volume reaction model. min-1 K1 X-ray radiation from Cobalt due to 1 K counts K2 X-ray radiation from Cobalt due to 2 K counts La Crystallite diameter Lc Crystallite height Lo Total pore length per unit volume m m-3 M Molarity of acid M m Order of reaction with respect to CO2 concentration - mash mass of ash mg MI Maceral index - mo Initial mass of char mg mt Mass of char at time, t mg Nave Average number of aromatic layers per carbon crystallite - Pn Fraction of aromatic carbon contained within the d002 peak - R Ideal gas constant J K mol-1 R Initial reactivity of the chars min-1 r1, r2, r3 Reaction rates min-1 RMI Reactive maceral index - RMI* Modified reactive maceral index - Rr Mean random vitrinite reflectance % rs reaction rate m min-1 Rsc Mean random maceral reflectance % s 2sin/ -1 Smax Value of s (2sin/) at which Imax occurs -1 So Initial surface area m2 m-3 T Temperature C or K t Time min t0.5 Time for fractional carbon conversion of 50% min t0.9 Time for fractional carbon conversion of 90% min tf Time factor min-1 xxiii Symbol Description Unit V Volume per unit mass m3 g-1 X Fractional conversion of carbon - XA Fraction of amorphous carbon - COy Molar fraction / partial pressure of CO - / bar 2COy Molar fraction / partial pressure of CO2 - / bar xxiv GREEK SYMBOLS Symbol Description Unit Wavelength of incident X-ray Full width at half maximum of the corresponding peak or band degrees () o Initial porosity of char samples % Peak position / XRD angle of scan degrees () 002 Peak position of (002) peak degrees () 10 Peak position of (10) peak degrees () 11 Peak position of (11) Peak degrees () / Absorption coefficient for Cobalt-K radiation - Skeletal density / density of coal or char samples kg m-3 ', c Bulk density of coal or char sample kg m-3 Standard deviation various unit Dimensionless time - aEs Slope of ) ln(ft against T-1 at constant CO2 concentration K-1 ts Slope of the plot of real time t , against 1 ) 1 ln( 1 X , min 9 . 0 Dimensionless time at 90% conversion - Dimensionless structural parameter for char pores - xxv ABBREVIATIONS Acronym Meaning % Ave. Dev. Percent average deviation A.S.M.E. American Society of Mechanical Engineers ACT Advanced Coal Technology, Pretoria adb Air dry basis afb Ash free basis Afrox African Oxygen AI Alkali index ASA Active surface area ASAP Accelerated surface area and porosimetry ASTM American Society for Testing Materials Ave. Average value Ave. Dev. Average deviation BET Brunauer-Emmett-Teller Method BFBC Bubbling fluidised bed combustion Bit. Med. Bituminous Medium Rank BSU Basic structural unit Cat. Category CCT Clean Coal Technology CDM Clean Development Mechanism CFBC Circulating fluidised bed combustion daf Dry ash free basis db Dry basis Demin Demineralised coal or char sample DME Department of Minerals and Energy dmmb Dry mineral matter basis DOI Degree of disorder index D-R Dubinin-Radushkevich method DTF Drop tube furnace EFR Entrained flow reactor xxvi Acronym Meaning ESKOM South African Electricity Supply Commission ESS Error sum of squares FBC Fluidised bed combustion FBDB Fixed bed dry Bottom gasifier FBG Fluidised bed gasification FC Fixed carbon Fig. Figure FWHM Full width at half maximum gfb Graphite (carbon) free basis H/C Hydrogen-carbon atomic ratio HCL Hydrochloric acid HF Hydrofluoric acid H-K Horvath-Kawazoe method HP Helium pycnometry HPTGA High pressure thermogravimetric analyser HRTEM High resolution transmission electron microscopy HTR Horizontal tube reactor ID Identity IGCC Integrated Gasification Combined Cycle IR Infra-red ISO International Standard Organisation lfb LOI free basis LMO Local molecular orientation LOI Loss on ignition LTB Lithium tetraborate MIP Mercury intrusion porosimetry mmb Visible mineral matter basis mmfb Visible mineral matter free basis MOD Molecular orientation domain NMR Nuclear magnetic resonance NOX Oxides of nitrogen NWU North-West University xxvii Acronym Meaning O/C Oxygen-carbon atomic ratio PBBR Packed bed balance reactor PCC Pulverised coal combustion PCI Pulverised coal injection PDTF Pressurised drop tube furnace PF Pulverised fuel PFB Pressurised fluidised bed Pp Page number / pages PSD Position sensitive detectors rpm Revolution per minute RPM Random pore model SA South Africa SABS South African Bureau of Standards SCM Shrinking core model SOX Oxides of sulphur sfb Sulphur free basis TGA Thermogravimetric Analyser TIC Total inert components TPD Temperature programmed desorption TRC Total reactive components TSA Total surface area UCG Underground Coal Gasification UNFCCC The United Nations Framework Convention on Climate Change VM Volatile matter content vol. % Volume percent VRM Volumetric reaction model VTR Vertical tube reactor WCI World Coal Institute wt. % Weight percent XRD X-ray diffraction XRF X-ray fluorescence The nomenclatures for the petrographic analyses are provided in the relevant sections of Chapter 3 and Appendix A. xxviii Conference Presentation Resulting from this Investigation Okolo, G.N., Everson, R.C. and Neomagus, H.W.J.P. (2010). The effects of chemical and physical properties of chars derived from inertinite-rich, high ash coals on CO2 gasification reaction kinetics. Presented at the Fossil Fuel Foundation of Africa 15th Southern African Conference on Clean Coal Energy, 17-18th November, 2010, Johannesburg, South Africa. 1 CHAPTER CHAPTER CHAPTER CHAPTER 1 11 1 1.0 GENERAL INTRODUCTION 1.1 Introduction A brief introduction of the research project is given in this chapter. The basic management question of what, why, and how are concisely treated here. Sections 1.1 - 1.2.1 give the background information and motivation for engaging in this study. The working hypotheses and objectives of the research work are presented in Sections 1.3 - 1.4, while the scope of the research is laid down in Section 1.5. Chapter 1: 1.2 Background Information Coal is the most abundant fossil fuel and it will be available long after petroleum and natural gas wells are dry (Podolski fuel energy sources and resources. Currentlysource for power generation as well as for industrial processes and this will remain unchanged at least till 2030 (Cloke other fossil fuels. It is safe to transpachieving a diverse and balanced energy mix. It is also the major source of energy for the developed and the developing economies; it provides 26% of world primary energy needs and 41% of worlds electricity is generated from coal (WCI Coal Statistics, 2007). This is illustrated in Fig Figure 1.1: Total world e South Africa generated 95% of its electricity from coal (E2008) and is ranked 5th in of South Africas primary energy needs (DME: Digest of South African Energy Statistics, 2006). ESKOM fact Sheet General Introduction2 Background Information and Motivation ndant fossil fuel and it will be available long after petroleum and natural gas wells are dry (Podolski et al., 2008). It is a major contribution of the fossil fuel energy sources and resources. Currently, it is the most important primary energy power generation as well as for industrial processes and this will remain unchanged at least till 2030 (Cloke et al., 2003). Coal is comparatively cheaper than other fossil fuels. It is safe to transport and storage is easier. Thus, it remains vital in hieving a diverse and balanced energy mix. It is also the major source of energy for the developed and the developing economies; it provides 26% of world primary energy needs and 41% of worlds electricity is generated from coal (WCI Coal Statistics, in Figure 1.1. energy supply and generation by fuel respectively in 2006Coal Statistics, 2007) South Africa generated 95% of its electricity from coal (ESKOM in the world coal export market. Coal also accounted for 77% South Africas primary energy needs (DME: Digest of South African Energy Statistics, 2006). ESKOM fact Sheet, (2007) also noted that this trend is not likely to General Introduction ndant fossil fuel and it will be available long after petroleum and ., 2008). It is a major contribution of the fossil it is the most important primary energy power generation as well as for industrial processes and this will remain Coal is comparatively cheaper than easier. Thus, it remains vital in hieving a diverse and balanced energy mix. It is also the major source of energy for the developed and the developing economies; it provides 26% of world primary energy needs and 41% of worlds electricity is generated from coal (WCI Coal Statistics, uel respectively in 2006 (WCI Annual report, Coal also accounted for 77% South Africas primary energy needs (DME: Digest of South African Energy also noted that this trend is not likely to Chapter 1: General Introduction 3 change significantly in the next decade due to the relative lack of suitable alternatives to coal as an energy source. An average of 224 million tonnes of coal is produced annually in South Africa of which 25% is exported and the remainder used locally by various coal utilisation industries (DME Coal Statistics, 2006). Among these users are: Eskom, 53% for electricity generation; Sasol, 33% for transport fuel and petrochemical production; 12% was consumed by the metallurgical industries and the remaining 2% was used for domestic cooking and heating. These show the energy intensity level of South Africa and its heavy dependency on coal, more so as it produces 45% of total electricity in the African continent (ESKOM Fact Sheet, 2007). The Eskom Annual Report (2008) also estimated coal reserves in South Africa at 53 billion tonnes and with the present production rate there should be almost 200 years of coal supply left. South Africas extractable coals are located in widely separated coal provinces stretching interruptedly from the border with Botswana in the North-West, through the Limpopo and Mpumalanga provinces and into Kwazulu-Natal in the east (Keaton Energy, 2009). These coal provinces are themselves divided into distinct coalfields in which most of the commercially mineable resources are contained in the Permian-aged Vryheid formations of the Ecca Group (Highveld and Witbank coalfields) (Snyman, 1989; Falcon, 1989; Snyman and Botha, 1993; Keaton Energy, 2009). Other coalfields of emerging importance are: Waterberg, Soutpansberg and Ermelo coalfields (Keaton Energy, 2009). Most of the countrys coal (about 83%) is currently mined in the Highveld, Witbank and Ermelo coalfields located in the Mpumalanga province (Cairncross, 2001). Geology has determined that the Witbank and Highveld coalfields are by far the most important source of South Africas mined coal at present. However, the Waterberg deposits, which extend into Botswana, are widely expected to become the countrys principal future coal resource (Snyman and Botha, 1993, Cairncross, 2001); particularly as this is the region expected to be home to many of the new generation of thermal power stations (Eskom Annual Report, 2008; Keaton Energy, 2009). The Ultretch and Klip River coalfields in Kwazulu-Natal are comparatively small and production has been slowly declining overall. However, the area produces most of the Chapter 1: General Introduction 4 countrys anthracites as well as a fair part of its coking coals (Snyman and Botha, 1993; Cairncross, 2001). Falcon et al. (2010) reported that the concentrated coal mining operation in some of the coalfields (especially the Highveld coalfield), has had the effect that the current products from this area are of low quality, Grade D (Calorific value (CV) < 25.5 kJmol-1); with ash content up to or greater than 40%. The application of these low grade coals in conversion and utilisation processes (combustion and gasification), in conventional existing commercialized facilities is therefore, limited by its low efficiency, poor burnout characteristics, and increased equipment investment (greater than 30%) (Hu et al., 2004). Furthermore, an indisputable huge amount of emissions (SOX and NOX and particulate matter) that are environmentally unfriendly are produced which require expensive downstream processing (Marban, et al., 1995; Spalding-Fecher et al., 2000; Hu et al., 2004; Kaitano, 2007). Current stringent legislation has put fossil fuel utility plants to pollutant emission limits. This puts immense pressure on coal utility plants and coal resource users to evolve technologies and ways of reducing emissions drastically. One of these legislations is the United Nations Framework Convention on Climate Change (UNFCC) Clean Development Mechanism (CDM), which is an offshoot of the Kyoto protocol (UNFCC Website). The coal utilisation industries and the government are continuously investigating, assessing and encouraging various technologies for the utilisation of these low-grade coals. In this regard, the government and major industries dependent on coal offer their support to researches and studies on new technologies to efficiently use this vast reserve in the Highveld, Witbank, Waterberg, Soutpansberg, and Ermelo coalfields. 1.2.1 Clean Coal Technologies The growing worldwide awareness on pollutants emanating from coal usage has had the result that processes reliant on coal as feedstock had to evolve new technologies not as harmful to the environment as the older processes. Such processes are commonly referred to as Clean Coal Technologies (CCT) and defined as technologies Chapter 1: General Introduction 5 designed to enhance both the efficiency and the environmental acceptability of coal extraction, preparation and utilisation (WCI, Ecoal, 2003). The main motive for developing these new coal conversion technologies is the need to achieve significant improvements in the important areas of fuel effectiveness, technical performance and environmental impact protection (Osborne et al, 1996; Koornneef et al, 2007). Various Clean Coal Technologies had emerged of which fluidised bed combustion (FBC) and fluidised bed gasification (FBG) are more suited to the utilisation of these South African low grade coals. These technologies reduce emissions and waste and increase the amount of energy produced from each tonne of coal processed (Grainger and Gibson, 1981). FBC and FBG have gained rapid acceptance and commercialisation; can be operated at atmospheric and pressurised conditions and has been shown to be a viable alternative to Pulverised Coal Combustion (PCC) (Koornneef et al., 2007). One major advantage of the FBC is that most of the pollutants produced during the conversion processes as oxides of nitrogen- NOX and N2O can be removed by in-situ reduction and sorbents such as dolomite can be added to capture XSO in the process (Marban et al, 1995). The disposal of the associated ash from the high ash feedstocks can also be done more effectively due to the fluidised state of the ash particles. FBC has two main categories: the Bubbling Fluidised Bed Combustion (BFBC) and the Circulating fluidised Bed (CFBC) and both categories have been developed operating at pressures between atmospheric and 100 atmospheres and temperatures in the range of 750 C - 950 C. Particle sizes 2 in the transition which was more significant in chars D2 and C2. Quartz in Char D2 increased significantly from 6.19 wt. %, gfb in the parent coal to 33.2 wt. %, gfb. The same trend was observed for char C2 with a quartz content of 19.9 wt. %, gfb up from 6.38 wt. %, gfb value of the precursor coal. This may be attributed to the transformation of kaolinite and muscovite in the parent coal. Calcite was not observed in char B and only very little abundance (0.61 - 4.0 wt. %, gfb) was observed in the other chars. Calcite and quartz were the only mineral species that were present in both the parent coals and the chars (except char B). Clay minerals: illite and microline; and silicates: quartz and cristobalite were the most abundant minerals in the chars. Pyrite were reduced to alpha iron and some interactions with siderite in the parent coal yielded troilite in the chars. Other mineral species in the chars are oldhamite and traces of sodalite. 3.7.3 Ash Analysis (XRF) The normalised proportions (Loss on Ignition (LOI) and sulphur free basis) of major inorganic composition of the char ashes from the XRF spectroscopy analysis are presented in Table 3.11. Considering the LOI and the total ash content, it can be seen that the ash content from proximate analysis corresponds well with the total ash content from XRF result. This is due to the similarity of the ashing process of the two techniques at high temperature of 815 C. The XRF technique used is regarded as only semi-quantitative for sulphur Chapter 3: Coal and char Characterisation 84 (Matjie, 2008; Loubser and Verryn, 2008), hence the sulphur contents were not reported. From the result, SiO2 and Al2O3 are the most abundant chemical components in the ashes of the chars which compare well with the XRD mineral analysis results. These are derived from the clay minerals: quartz and kaolinite which form the bulk of the mineral in coal and subsequent chars (Spears, 2000). The presence of Fe2O3 in al the char samples can be attributed to the pyrite and siderite in the original coal as well as troilite and iron alpha detected in the chars. CaO and MgO were also detected in all of the four char samples which is a reflection of the dolomite and calcite phases in the parent coal samples and oldhamite in some appreciable quantise in the chars, with a larger contribution to CaO content of char B suspected to be due to the high content of gypsum in the parent coal B. The lowest value of CaO content was observed in char C2 at 3.02 wt. %, lfb. Table 3.11: Char sample ash chemistry on LOI and sulphur free basis (wt. %, lfb and sfb ). Sample ID Mineral Species (wt. %, lfb and sfb) Char B Char C Char C2 Char D2 SiO2 50.9 49.9 50.8 46.9 Al2O3 29.4 31.7 39.7 34.5 Fe2O3 5.03 3.59 2.50 4.75 CaO 8.78 8.16 3.02 8.18 MgO 1.90 2.16 0.68 2.30 MnO 0.06 0.05 0.03 0.08 TiO2 1.68 1.91 2.38 2.29 Na2O 0.23 0.23 0.26 0.14 K2O 0.74 0.98 0.37 0.48 P2O5 1.18 1.31 0.23 0.33 Cr2O3 0.09 0.04 0.05 0.04 Total (wt. %, lfb) 99.99 100.03 100.01 99.99 LOI (wt. %, adb) 71.3 67.5 68.9 67.3 Ash content of chars (wt. %, adb) 28.5 33.2 34.0 36.8 Alkali Index (-) 5.92 6.15 2.55 7.17 Chapter 3: Coal and char Characterisation 85 It is worthy to note that this analysis was confined to the chars only as the parent coals were not used for the reactivity experiments; while the motivation for the analysis is to study the inherent catalytic influence, if any, of the high ash components of the chars on their CO2 reactivity. The presence of K2O in all the chars with the highest value in char C is related to the muscovite in the parent coals and illite and microcline in the chars, while Na2O and TiO2 are derived from the sodalite in the chars and rutile in the parent coals respectively. A significant contribution of this analysis is the study of the catalytic effects due to the presence of these elemental components by determining the respective lumped parameter, alkali index AI, of the chars which are also presented in Table 3.11. It had been established by various investigators: Walker and Hippo (1975), Miura et al. (1989), Httinger and Natterman (1994), Tomita (2001), Zhang et al. (2006), and Lee (2007) reported that, elemental components such as: Fe2O3, CaO, MgO, Na2O, K2O impact some catalytic effect on the gasification reaction of coals and chars. The AI, refers to the ratio of the total weight fraction of the basic species in the ash (CaO, MgO, K2O, Na2O and Fe2O3) to the total weight fraction of the acidic compounds (SiO2 and Al2O3) in the ash, multiplied by the ash contents in weight percent of the chars. This formulation is shown as Equation 3.12; while the AI results of the chars are presented in Table 3.11. %3 2 23 2 2 2ashO Al SiOO Fe O Na O K MgO CaOAI |||

\|++ + + += (3.12) 3.7.4 X-ray Diffraction (XRD) Carbon Crystallite Analysis The second aspect of analysis involving the use of the X-ray diffraction technique was the study of the carbon crystallite properties of the coal and char samples. To reduce noise and the effects of mineral matter on the XRD diffractogram and for a simplified study of the carbon fraction, a three stage HCl-HF-HCl demineralisation was conducted on the samples. The outcome of this, presented as proximate analysis of the samples including the effectiveness of the process, is summarised in Table 3.12. Chapter 3: Coal and char Characterisation 86 Table 3.12: Proximate analysis of raw and demineralised coal and char samples (wt. %, db). Sample ID VM (wt. %, db) Ash (wt. %, db) Fixed Carbon (wt. %, db) Ed 1 (%) Coal B Raw 24.3 26.6 49.0 96.7 Demin2 27.0 0.88 72.2 Char B Raw 1.21 28.8 70.0 89.1 Demin 5.79 3.13 91.1 Coal C Raw 22.1 30.7 47.1 97.9 Demin 25.3 0.66 74.0 Char C Raw 0.81 33.6 65.6 93.9 Demin 5.59 2.06 92.4 Coal C2 Raw 18.9 29.7 51.3 98.3 Demin 20.3 0.5.0 79.2 Char C2 Raw 3.25 34.5 62.2 89.8 Demin 5.49 3.52 91.0 Coal D2 Raw 22.4 28.8 48.7 96.7 Demin 27.2 0.94 71.8 Char D2 Raw 2.71 36.8 60.5 90.5 Demin 5.55 3.48 91.0 1- Ed Effectiveness of demineralisation 2- Demineralised sample It is obvious from the result that, higher effectiveness of demineralisation was achieved on the parent coals (96.7- 98.3%) than on the chars (89.1- 93.9%) with the corresponding ash contents reduced to 0.5 - 0.94 wt. %, db and 2.06 - 3.52 wt. %, db respectively. This is to be expected since the pyrolysis reaction leading to the chars at 900 C, apart from driving off the volatiles, hardens the chars which may prevent the demineralisation agent from reaching the mineral inclusions easily. This will also culminate to an increase in both the skeletal and bulk density of the resulting chars. An increase in both the fixed carbon and volatile matter content was also observed for the demineralised coal and char samples. Similar results were reported by Lu et al. (2001), Maity and Mukherjee (2006), Kawakami et al. (2006), and Van Niekerk (2008). Chapter 3: Coal and char Characterisation 87 The raw diffractograms, corrected for polarisation and geometrical factors from the XRD analysis for the demineralised coal and char samples, are shown in Figure 3.2. The background due to the amorphous carbon fraction was further removed and the diffractograms smoothened with the HighScore Plus peak analysis tool and are presented in Figure 3.3. The diffractograms of all the demineralised coal and char samples investigated in this study possess the same graphitic features as those reported in literature (Franklin, 1950 and 1951; Hirsch, 1954; Alexander and Sommer, 1959; Shiraishi and Kobayashi, 1973; Kumar and Gupta, 1995; Lu et al., 2001, 2002a and 2002b; Aso et al., 2004; Kawakami et al., 2006; Maity and Mukherjee, 2006; Wu et al., 2008). Figure 3.2: Raw diffractograms of coal and char samples. 01000200030004000500060000 20 40 60 80 100 120Intensity, I(counts)2 () CoKCoal BCoal CCoal C2Coal D20100020003000400050000 20 40 60 80 100 120Intensity, I(counts)2 () CoKChar BChar CChar C2Char D2Chapter 3: Coal and char Characterisation 88 The graphitic features were established by the presence of the (002) band corresponding to the (00l) position of graphite. The (00l) position is related to the inter-layer spacing of graphite and its resemblances in the diffractograms ((002) band) occur at 29.56 2 30.05 for the coals; and 28.66 2 29.71 for the chars. The (10) and (11) bands were also observed, which correspond to the (hk0) lines of graphite, related to the hexagonal ring structure. The (10) bands occur at position 50.90 2 51.60 and 51.90 2 52.90, while the (11) band was observed at 98.21 2 98.59 and 96.70 2 97.45 for the coals and chars respectively. These peak positions are significant in the calculation of the crystallite lattice parameters. Figure 3.3: Corrected and smoothened diffractograms of coal and char samples. 050010001500200025000 0.2 0.4 0.6 0.8 1Intensity, I(counts)s=2Sin/ (-1)Coal BCoal CCoal C2Coal D203006009001200150018000 0.2 0.4 0.6 0.8 1Intensity, I(counts)s=2sin/ (-1)Char BChar CChar C2Char D2(002) (10) (11) (002) (10) (11) -band -band Chapter 3: Coal and char Characterisation 89 The assymetry exhibited by the (002) band had necessitated the dilineation of the peak into the d002 peak and the -side band which appears as a shoulder bands in the difractograms (Franklin, 1950 and 1951; Hirsch, 1954; Ergun and Tiensuu; 1959; Schoening, 1983; Lu et al., 2001; Lu et al., 2002a). The former is associated with the aromatic ring stacking while the later is due to the aliphatic side chains. The small spikes on the diffractogram are peaks of traces of minerals still remaining in the demineralised samples as indicated by the effectiveness of demineralisation, Ed, (Table 3.12). The inter-layer spacing was calculated from the (002) peak position using the Braggs equation; while the crystallite height, Lc, and diameter, La, were determined from the peak positions, and the full width at half maximum (FWHM) of the (002) and (10) peaks respectively. The (11) band was not used due to its diffuse and obscure nature which make quantitative analysis difficult. The characteristics of the diffractograms and the annealing effects of the transition from parent coal to char at 900 C (samples B and C) are expounded in Figure 3.4. The (002), (10) and (11) peaks of the chars are broader than that of the precursor coals, while the -shoulder-band is more prominent in the coals than in the chars. The broadening of the (002) peak in the diffractograms obtained for the chars is due to the closer packing and structural re-ordering, re-orientation and better alignment of the aromatic carbon layers, which results in an increase in the inter-layer spacing and a decrease in both the crystallite height and diameter of the chars, compared to that of the original coals. Broadening of peaks is evidence of a decrease in carbon crystallite size, while sharper peaks are signs of growth of the carbon crystallite (Kuroda and Akamatu, 1959; Short and Walker, 1963; Kumar and Gupta, 1995; Lu et al., 2002a and 2002b; Wu et al., 2008). Crystallite condensation, as observed in this study, was also observed by Takagi et al. (2004) for their chars produced at 760 C and 900 C. A careful analysis of the scatter of the results reported by Kawakami et al. (2006), also show an increase in inter-layer spacing between chars prepared at 900 C and 1200 C. It should be noted that significant crystallite growth usually starts from 1600 C (Kuroda and Akamatu, 1959; Kumar and Gupta, 1995; Lu et al. 2002a and 2002b; Wu et al. 2008), with the Chapter 3: Coal and char Characterisation 90 experiments conducted by these investigators, confined to temperatures above 1200 C. Takagi et al. (2004) however noted that broadening of peaks at lower temperatures of heat treatment (350 - 920 C) is more significant for lower rank coals, while for higher rank coals, sharper peaks may be observed. Figure 3.4: Comparison of coal and char diffractograms for samples B and C. The diffuse and in some cases almost absent -sideband in the diffractograms of the chars is due to the loss of the aliphatic side chain during the char production process. This culminates in a more ordered structure (Russell et al. 1999; Davis et al. 1995; Lu et al. 2001; Van Niekerk, 2008), which impacts on the chemical properties of the 03006009001200150018000 0.2 0.4 0.6 0.8 1Intensity, I(counts)s=2sin/ (-1)Coal BChar B03006009001200150018000 0.2 0.4 0.6 0.8 1Intensity, I(counts)s=2Sin/ (-1)Coal CChar C-band (10) (11) -band (10) (11) (002) (002) Chapter 3: Coal and char Characterisation 91 char, as aromaticity and the fraction of amorphous carbon and other crystallite parameters. Van Niekerk (2008), however reported a contrasting result for the investigated inertinite-rich Highveld coal sample, as a prominent -side band was observed, which was entirely missing in the diffractogram of the vitrinite-rich Waterberg coal. This means that the vitrinite-rich coal is structurally more ordered than the inertinite-rich sample. The result contradicts the high aliphatic content of vitrinite-rich coals (Choi et al., 1989; dela Rosa et al., 1992) and may have led to his inability to determine the aromaticity of the studied vitrinite-rich coal sample using XRD technique. 3.7.4.1 Determination of Aromaticity of Coal and Char Samples The aromaticity of both coal and char samples were determined from the areas under the (002) peak and the -side band (Lu et al., 2001, 2002a and 2002b; Maity and Mukherjee, 2006). This was done using a curve fitting analysis tool on the HighScore Plus application. A confirmation was conducted on the result by using the peak analysis, curve fitting and the data analysis tool of Origin 6.1 to determine the areas under the (002) peak and the -side band. The curve fitting and analysis of the two peaks to get the respective peak areas for coal B and Char C2, using HighScore Plus, is shown in Figure 3.5. A comparison of the results from the two methods described above is summarised in Table 3.13. It can be seen from the table that the results obtained from the two different data analysis applications are very similar to each other, thus validating the results. Table 3.13: Comparison of aromaticity results from HighScore Plus and Origin 6.1. Determination of Aromaticity (fractional values (-)) Coal B Char B Coal C Char C Coal C2 Char C2 Coal D2 Char D2 By HighScore Plus 0.7993 0.8822 0.8106 0.9091 0.8527 0.9513 0.7539 0.8489 By Origin 6.1 0.8011 0.8839 0.8113 0.9192 0.8517 0.9483 0.7513 0.8470 Chapter 3: Coal and char Characterisation 92 Figure 3.5: Determination of area under d002 and - band using HighScore Plus for coal B and char C2. 3.7.4.2 Determination of Fraction of Amorphous Carbon of the Coal and Char Samples The determination of the fraction of amorphous carbon contained in both the coal and char samples were carried out by normalising the diffractograms shown in Figure 3.4 to obtain the reduced intensity curve of the (002) peak (Figure 3.6), according to the method proposed by Franklin (1950) and variously used by Ergun and Tiensuu (1959); Short and Walker (1963); Lu et al. (2001, 2002a, and 2002b); Kawakami et al. (2006). Chapter 3: Coal and char Characterisation 93 The symmetrical section of the (002) band used for this analysis for coal C2 and char C is shown in Figure 3.6. In Table 3.14, the Imax, Smax, and other parameters for the calculation of the fraction of amorphous carbon in coal C2 and char C are presented. It shoul