University of Plymouth PEARL https://pearl.plymouth.ac.uk 04 University of Plymouth Research Theses 01 Research Theses Main Collection 1978 REACTIVITY OF LIME AND RELATED MATERIALS WITH SULPHUR DIOXIDE O'NEILL, PETER http://hdl.handle.net/10026.1/2065 University of Plymouth All content in PEARL is protected by copyright law. Author manuscripts are made available in accordance with publisher policies. Please cite only the published version using the details provided on the item record or document. In the absence of an open licence (e.g. Creative Commons), permissions for further reuse of content should be sought from the publisher or author. REACTIVITY OF LIME AND RELATED MATERIALS WITH SULPHUR DIOXIDE Peter O'NEILL A thesis presented for the Research Degree of DOCTOR OF PHILOSOPHY of the COUNCIL FOR NATIONAL ACADEMIC AWARDS LONDON John Graymore Chemistry Laboratories School of Environmental Sciences Plymouth Polytechnic Plymouth PL4 8AA Devon September 1978 Accfi. 5 5002165 CLCSS _ tin. tT 5"if^. 3? otsjs" ABSTRACT A study has been made of reactions between some alkaline earth metal compounds and sulphur dioxide. The compounds studied included naturally-occurring forms, limestone and dolomite, and industrial products, Limbux and'quick-lime. Particular attention was paid to reactions which could have proved useful in the desulphurisation of industrial flue gases. All the substances used reacted to some extent with sulphur dioxide, but often slowly. The greatest reactivity was shown by calcium-based compounds as the carbonate or hydroxide decomposed to form fresh, highly active calcium oxide. Reaction rates, below 400°C, were only appreciable for calcium oxide. The reaction products, depending upon conditions, included sulphites, sulphates, sulphides and oxides. At lower temperatures, sulphite was formed and the thermal stabilities of the alkaline earth metal sulphites were studied in greater detail. X-ray diffraction examination indicated that the structures of anhydrous calcium, strontium and barium sulphites were similar and closely related to the structure of calcium sulphite hemihydrate. Differences in orthorhombic unit cell dimensions were in agreement with changes in the cationic radii. The thermal stabilities of the alkaline earth metal sulphites varied with changes in the gaseous atmosphere surrounding the samples. In non-oxidising atmospheres there was competition between (a) reactions leading to oxide formation and (b) disproportionation reactions, leading to the formation of a mixture of sulphate and sulphide. The relative importance of the disproportionation reaction increased as the atomic number of the cation increased. A proposed mechanism suggested how the various phases found in different reactions could be formed. In oxidising atmospheres there was extensive formation of sulphate from both sulphite and sulphide. The use of dry absorbents, based on alkaline earth metal compounds, for the desulphurisation of flue gases appeared to be an expensive process. Acknowledgements The author wishes to express his very sincere thanks to Dr. D.R. Glasson for his helpful advice and guidance during the course of this work. He is grateful to Dr. G.H.W. Milburn for his introduction to single-crystal X-ray studies, to Mr. B. Lakey for discussions concerning the electron microscope, and to Mr. L.C. Anderson for his many useful conzDents. He would like to thank the Polytechnic for providing the research facilities and Imperial Chemical Industries Ltd. (Mond Division) for providing some of the materials used in these studies. He would also like to thank Mrs. G. Lane for her special care in typing the manuscript. CONTENTS P^e CHAPTER ONE INTRODUCTION 1-19 1.1 Historical Review 2 1.1.1 Dry injection processes 4 1.1.2 Comparison of natural limestone and magnesite absorbents with their calcined and hydrated products 5 1.1.3 Thermogravimetric studies 8 1.1.4 Efficiency of sulphur dioxide removal 8 1.1.5 Comparison of "dry" and *'wet" sulphur dioxide removal processes .. .. 15 1.2 The Sulphur Dioxide Problem 18 CHAPTER TWO EXPERIMENTAL TECHNIQUES 20-54 2.1 Introduction .. 20 2.2 Thermal Analysis 20 2.2.1 Thermogravimetry (TG) 21 2.2.2 Differential thermal analysis (DTA) 25 2.2.3 Simultaneous thermogravimetry and differential thermal analysis 28 2.2.4 The Massflow thermobalance 28 2.3 Surface Area, Porosity and Particle Size 32 2.3.1 Determination of surface area 32 2.3- 2 The sorption balance 35 2.3.3 Determination of porosity 36 2.3.4 Estimation of particle size. 38 2.4 Electron Microscopy 40 2.4- 1 The electron microscope 40 2.4.2 Hot-stage electron microscopy 42 2.4.3 Preparation of specimens 43 2.5 Optical Microscopy 44 2.6 X-ray Diffraction 44 2.6.1 General theory , 44 2.6.2 Procedure for X-ray powder diffraction .47 2.6.3 Procedure for single crystal X-ray diffraction 48 2.7 Fluidised Bed Studies 50 2.8 Chemical Analysis .. 54 CHAPTER THREE THE PREPARATION AND DETERMINATION OF SOME CRYSTALLOGRAPHIC PROPERTIES OF ALKALINE EARTH . METAL SULPHITES 55-86 3.1 Magnesium Sulphite 56 3.1.1 Preparation of magnesium sulphite hexahydrate .. 56 3.1.2 Structure of magnesium sulphite hexahydrate .. 57 3.2 Calcium Sulphite 57 3.2.1 Preparation of calcium sulphite hemihydrate .. 57 3.2.2 Calcium sulphite hemihydrate crystal structure determination 59 3.2.3 Powder diffraction patterns of hydrated and anhydrous calcium sulphite 63 3-3 Strontium Sulphite 69 3.3.1 Preparation of strontium sulphite 69 3.3.2 X-ray studies 70 3.4 Barium Sulphite • 77 3.4.1 Preparation of barium sulphite 77 3.4.2 X-ray studies 77 3.5 Comparison of Anhydrous Calcium, Strontium and Barium Sulphites .. 78 CHAPTER FOUR THERMAL STABILITY OF ALKALINE EARTH METAL SULPHITES 87-191 4.1 Magnesium Sulphite 104 4.1.1 Results 104 4.1.2 Discussion 117 4.2 Calcium Sulphite 136 4.2.1 Results 136 4.2.2 Discussion 148 4.3 Strontium Sulphite' 160 4.3.1 Results 160 4.3.2 Discussion 165 4.4 Barium Sulphite 175 4.4.1 Results 175 4.4.2 Discussion 179 4.5 General Discussion .. .. 181 CHAPTER FIVE REACTIVITY OF SOME ALKALINE EARTH METAL COMPOUNDS WITH SULPHUR DIOXIDE 192-235 5.1 Dynamic Thermogravimetric Studies 192 5.1.1 Efficiency of solid absorbents 210 5.1.2 Industrial applicability 217 5.2 Porosity Changes in Calcined Limbux 219 5.3 Fluidised Bed Studies 232 5.3.1 Introduction 232 5.3.2 Results and discussion 233 CHAPTER SIX THE USE OF ALKALINE EARTH METAL COMPOUNDS FOR INDUSTRIAL FLUE GAS DESULPHURISATION PROCESSES 236-246 6.1 Recent Trends in Flue Gas Desulphurisation 236 6.2 "Dry" Flue Gas Desulphurisation Systems 237 6.3 "Wet" Flue Gas Desulphurisation Systems 239 6.4 Economics of Flue Gas Desulphurisation Systems .. .. 243 CHAPTER SEVEN CONCLUSIONS 247-250 REFERENCES 251-258 APPENDICES Appendix 1 Computer program for Phillips spot shape correction Appendix 2 List of F> , , and F, . v values for calcium sulphite (obs) (calc) * hemihydrate Appendix 3 Powder diffraction data for strontium hydroxide Appendix 4 Publications based on work in this Thesis (a) Thermogravimetric studies of alkaline earth metal sulphites. First European Symp. Thermal Anal. 1976, Salford. (b) Hot-stage microscopy of alkaline earth metal sulphites, 2nd Anglo-French Symp. Thermal Anal. 1977, Plymouth. (c) Porosity of absorbents for desulphurisation of industrial flue gases, Symp. on the Characterisation of Porous Solids 1978, Neuchatel. Appendix 5 Private communication, 1978- Anderson L.C., Glasson D.R., Porosity of quicklimes and hydrated limes. CHAPTER ONE INTRODUCTION The work described in this thesis commenced in 1970 when a number of countries, particularly the U.S.A. and Japan, were introducing much stricter controls over the emission of sulphur dioxide from stationary sources, i.e. electricity generating stations and other industrial plant. At this time few desulphurisation methods were in operation for plants emitting gases containing less than IZ sulphur dioxide, and relatively little data had been published on the chemical reactions which seemed most likely to be readily applicable to the problems of flue gas desulphurisation. Of the many methods of achieving a reduction in sulphur dioxide emissions, it was decided to study some of the reactions and reactants associated with, the use of alkaline earth metal compounds. This was a natural continuation of the extensive work already carried out in these laboratories by Glasson^'^ and the more limited studies carried 2 3 out by the present author ' , Dry processes of flue gas desulphurisation have the advantage that it should be possible to treat the flue gas without causing a large reduction in temperature, as occurs with wet scrubbing methods; the problems associated with lack of plume buoyancy, v^ich occur when the gas temperature becomes too low, would be avoided. Calcium carbonate, oxide and hydroxide and the similar derivatives of dolomite are relatively abundant and low-cost materials; thus, their usage was likely to be favoured. The investigation was to examine: (a) The thermal stabilities of the likely products of the desulphurisation process in order to assess the possibilities of re-cycling the alkaline earth metal compounds and using the captured sulphur as a source of sulphur compounds. Particular attention was to be directed to the sulphites which were likely to be major products 4 at flue gas temperatures . (b) The applicability of utilizing a fluidised bed system. Published work^ had indicated that the reaction of sulphur dioxide with particles of-calcium carbonate, etc. occurred on the surface of the particles and that a surface layer of product was formed which prevented further reaction between the unreacted core of the particles and more sulphur dioxide. Fluidised beds, as well as increasing the particle/gas contact time^, often lead to abrasion^ between the fluidised particles and it was thought that this might help to remove the reacted surface layer from the particles.