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  • UNIVERSITY OF CINCINNATI

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    I,______________________________________________,hereby submit this as part of the requirements for thedegree of:

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    in:________________________________________________

    It is entitled:________________________________________________

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    Approved by:________________________

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  • INVESTIGATION OF THE CHEMICAL PATHWAY OF GASEOUS NITROGEN

    DIOXIDE FORMATION DURING FLUE GAS DESULFURIZATION WITH DRY

    SODIUM BICARBONATE INJECTION

    A dissertation submitted to the

    Division of Research and Advanced Studies Of the University of Cincinnati

    In partial fulfillment of the

    Requirements for the degree of

    DOCTOR OF PHILOSOPHY (PH.D)

    In the Department of Civil and Environmental Engineering Of the College of Engineering

    2001

    by

    Antoinette Weil Stein

    B.S., University of Wisconsin, Madison, 1983 M.S.E, Milwaukee School of Engineering, MSOE, 1988

    Committee Chair: Dr. Timothy C. Keener

  • ABSTRACT

    The chemical reaction pathway for the viable flue gas desulfurization

    process, dry sodium bicarbonate injection, was investigated to mitigate

    undesirable plume discoloration. Based on a foundation of past findings, a

    simplified three-step reaction pathway was hypothesized for the formation of the

    plume-discoloring constituent, NO2. As the first step, it was hypothesized that

    sodium sulfite formed by sodium bicarbonate reaction with flue gas SO2. As the

    second step, it was hypothesized that sodium nitrate formed by sodium sulfite

    reaction with flue gas NO. And as the third step, it was hypothesized that NO2

    and sodium sulfate formed by sodium nitrate reaction with SO2.

    The second and third hypothesized steps were experimentally investigated

    using an isothermal fixed bed reactor. As reported in the past, technical grade

    sodium sulfite was found to be un-reactive with NO and O2. Freshly prepared

    sodium sulfite, maintained unexposed to moist air, was shown to react with NO

    and O2 resulting in a mixture of sodium nitrite and sodium nitrate together with a

    significant temperature rise. This reaction was found to proceed only when

    oxygen was present in the flue gas.

    As reported in the past, technical grade sodium nitrate was shown to be

    un-reactive with SO 2. But freshly formed sodium nitrate kept unexposed to

    humidity was found to be reactive with SO2 and O2 resulting in the formation of

    NO2 and sodium sulfate polymorphic Form I. The NO 2 formation by this

    reaction was shown to be temperature dependant with maximum formation at

    175C.

  • Plume mitigation methods were studied based on the validated three-step reaction

    pathway. Mitigation of NO2 was exhibited by limiting oxygen concentration in the flue

    gas to a level below 5%. It was also shown that significant NO2 mitigation was achieved

    by operating below 110C or above 250C. An innovative NO2 mitigation method was

    patented as a result of the findings of this study. The patented process incorporated a

    process step of sodium sulfite injection to remove flue gas NO prior to sodium

    bicarbonate injection.

  • Copyright 2001

    Antoinette Weil Stein

  • ACKNOWLEDGEMENTS

    I wish to thank my advisor Dr. Keener for his support to accomplish this research.

    It was a special honor and opportunity for me to have worked under Dr. Keener on this

    project particularly since Dr. Keener was an early developer of sodium bicarbonate

    sorbent systems. I would also like to thank Dr. Keener for obtaining sufficient research

    funding from the Electric Power Research Institute and from the Department of

    Education (DOE) Fellowship that he encouraged.

    I wish to thank my thesis committee members, Professor Soon-Jai Khang,

    Professor Paul Bishop, Professor Riley Kinman and Professor Boerio for their

    participation and support. I wish to thank Barbara Toole ONeil from the Electric Power

    Research Institute for her project leadership in funding this project. I also would like to

    thank her for her mentorship and technical writing guidance. The reference books that

    she suggested were very helpful.

    I would like to thank a number of key people who technically contributed to this

    work. From the University of Cincinnati, I would like to thank a number of others in our

    interdepartmental energy and pollution control research group including Guang Li for his

    patience in teaching me the use of many laboratory pieces of equipment including the

    BET and the total sulfur analyzer and his willingness to explain and share ideas on

    achieving product conversion from elemental analysis data. Also I thank him for

    maintaining a professional and positive attitude and being very supportive on a day to day

    basis in the laboratory. Additionally I would like to thank Kasemsan Manomaiphiboon

    for his TGA and wet chemistry analysis of the sodium pyrosulfite decomposition

    samples. I would like to thank Dr. Wayne Bresser from the Electrical and Computer

  • Engineering and Computer Science Department for the extensive X-Ray diffraction

    analysis that his lab provided. I would like to thank Elvin McCorvey for his electrical,

    mechanical and computer expertise in setting up and maintaining suitable operation of the

    extensive network of equipment used for laboratory experimentation. I would like to

    thank Professor Boerio from the Material Science and Engineering Laboratory and his

    laboratory students for their XPS, and IR material analysis used in reaction product

    identification. I would like to thank Ernie Clark from the SEM laboratory in the Material

    Science and Engineering Department for his expert skill in providing SEM analysis and

    photography included in this dissertation. I wish to thank Jim Layden, the librarian in the

    geology and physics library for his help in obtaining excellent literature on the structural

    transformations of sodium sulfate with sodium carbonate. I wish to thank the librarian in

    the Chemistry Department library, and in the Geophysics library for their technical

    expertise in running successful data-base literature searches that found key background

    information used in this study. Additionally I wish to thank a number of people from

    outside the University of Cincinnati for their technical contributions. I would like to

    thank Malcolm W. Chase from the National Institutes of Science and Technology, editor

    of the Third edition of the JANF Thermochemical Tables, for his assistance in obtaining

    hard to find thermo-data. I would like to thank Pat McKeown and John Moulder of

    Physical Electronics Corporation for their generous donated hours of technical services in

    running SIMS and ESCA material analysis for product identification that was key

    evidence in the findings for this research. I wish to thank the technical support from the

    Porter instrument team, in particular, John Nealy, Art Schell, and Mike Marcio, for their

    generous donated time, concern, and quick turn around time in resolving maintenance

  • issues that arose with their manufactured mass flow controllers operated in the extremely

    corrosive conditions. I wish to thank Ted Jennings from Witco Surfactants group for

    their input on sodium nitrate additives used in manufacturing. And I wish to thank M.W.

    Hott Company, including Duane Lorenz for the help constructing the test reactor with

    their specialized components.

    Finally I would like to thank my family for their support. In particular I would

    like to thank my sister and husband for their encouragement in undertaking these studies;

    my husband for his patience and interest in this subject to partake in engaging discussions

    related to my research findings. It was through these discussions that many of my

    research findings were realized. Also, I would like to thank Morris Brown, Mary Kenney

    and Joe Pasqua and my husband for the technical support in the converting this document

    from a Word file to a PDF format. A special thank you to my son Adam and my

    daughter Mia, for their daily inspiration and energy.

    It was the contributions of all of the above persons that made it possible for me to

    persevere and complete these studies.

  • 1

    TABLE OF CONTENTS

    CHAPTER 1...................................................................................................................... 22

    INTRODUCTION............................................................................................................ 22

    1.1 Background ........................................................................................................... 22

    1.2 The Problem .......................................................................................................... 23

    1.3 Prior study of this problem.................................................................................... 25

    1.4 Goals and objectives of this study......................................................................... 26

    1.5 Approach and scope .............................................................................................. 26

    CHAPTER 2...................................................................................................................... 31

    LITERATURE REVIEW................................................................................................. 31

    2.1 Literature review of plume discoloration .............................................................. 31

    2.1.1 History of plume discoloration...................................................................... 31

    2.1.2 Reported observations of plume discoloration during sodium sorbent

    flue gas desulfurization trials. ....................................................................... 35

    2.1.3 The discovery that NO2 was the plume discoloring agent for sodium

    bicarbonate bag-house injection.................................................................... 37

    2.2 Literature review of sodium bicarbonate reaction chemistry................................ 38

    2.2.1 Literature review of sodium bicarbonate decomposition. ............................. 39

    2.2.2 Literature review of sodium bicarbonate reaction with SO2 flue gas

    without NOx................................................................................................... 41

    2.2.2.1 Review of Erdos investigations of sodium bicarbonate reaction with

    SO2 flue gas without NOx.............................................................................. 41

  • 2

    2.2.2.2 Review of Howatson et al. (1980)[8] investigations of sodium

    bicarbonate reaction with SO2 flue gas without NOx. ................................... 43

    2.2.2.3 Review of Keener (1982)[9] investigations of sodium bicarbonate

    reaction with flue gas with SO2 but without NOx. ........................................ 46

    2.2.3 Literature review of sodium bicarbonate and sodium carbonate

    reaction with NOx without SO2. .................................................................... 47

    2.2.4 Literature review of sodium bicarbonate reaction with flue gas with

    SO2 and NOx.................................................................................................. 49

    2.2.4.1 Review of Blands (1990) [16] investigations of the reaction of sodium

    bicarbonate with SO2 flue gas with NO......................................................... 50

    2.2.4.2 Review of Verlaeten et al.s (1991)[20] investigations of the reaction

    of sodium bicarbonate with flue gas with SO2 and NO. ............................... 54

    2.2.4.3 Review of Bortzs (1994)[45] investigations of the reaction of sodium

    bicarbonate with flue gas with SO2 and NO. ................................................ 55

    2.2.5 Past Hypotheses for NO2 formation and NOx removal reactions.................. 57

    2.2.5.1 Review of Millers (1986)[19] hypothesis for NO2 formation........................ 57

    2.2.5.2 Review of Blands (1990)[16] hypothesis for NO2 formation. ....................... 60

    2.2.5.3 Review of Verlaeten et al.s (1991)[20] hypothesis for NOx removal. ........... 65

    2.2.5.4 Review of Bortzs (1994)[45] hypothesis for NO2 formation......................... 69

    2.2.6 Review of experimental validation findings for the hypothesized

    reactions. ....................................................................................................... 72

    2.2.6.1 Review of sodium sulfite reaction findings................................................... 72

    2.2.6.1.1 Millers (1986)[19] sodium sulfite reaction findings. .....................................72

  • 3

    2.2.6.1.2 Blands (1990)[16] sodium sulfite reaction findings.......................................74

    2.2.6.1.3 Verlaeten et al.s (1991)[20] sodium sulfite reaction findings. .......................74

    2.2.6.1.4 Lai and Yesavage (1994)[21] sodium sulfite reaction findings. .....................75

    2.2.6.2 Review of sodium pyrosulfite experimental findings. .................................. 76

    2.2.6.2.1 Blands (1990)[16] sodium bisulfite and sodium pyrosulfite reaction

    findings..........................................................................................................76

    2.2.6.2.2 Verlaeten et al.s (1991)[20] sodium pyrosulfite reaction findings. ...............77

    2.2.6.2.3 Lai and Yesavage (1994)[21] sodium pyrosulfite reaction findings. ..............77

    2.2.6.3 Review of sodium sulfate reaction findings. ................................................. 78

    2.2.6.4 Review of sodium nitrite and sodium nitrate reaction findings. ................... 79

    2.2.6.5 Review of gas phase interactions of SO2 and NOx........................................ 80

    2.3 Conclusions ........................................................................................................... 82

    CHAPTER 3...................................................................................................................... 85

    THEORETICAL DEVELOPMENT................................................................................ 85

    3.1 Introduction to theoretical development ............................................................... 85

    3.2 Summary of key undisputed reactions and end products...................................... 86

    3.3 The criteria for assessment.................................................................................... 91

    3.3.1 Criteria for SO2 removal ............................................................................... 91

    3.3.2 Criteria for NOx removal. ............................................................................. 92

    3.3.3 Criteria for NO2 formation. ........................................................................... 97

    3.4 Conformity of each of the past hypothesized pathways with the criteria. ............ 99

    3.4.1 Conformity of Millers hypothesized reaction pathway to the criteria

    for SO2 removal, NO2 formation, and NOx removal................................... 100

  • 4

    3.4.1.1 Conformity of Millers pathway to the SO2 removal criteria...................... 100

    3.4.1.2 Conformity of Millers (1986)[19] pathway to the NOx removal

    criteria.......................................................................................................... 101

    3.4.1.3 Conformity of Millers (1986)[19] pathway to the NO2 formation

    criteria.......................................................................................................... 102

    3.4.1.4 Validity of Millers hypothesized pathway. ................................................ 103

    3.4.2 Conformity of Blands (1990) [16] original hypothesized reaction

    pathway to the criteria for SO2 removal, NOx removal and NO2

    formation. .................................................................................................... 104

    3.4.2.1 Conformity of Blands (1990) [16] original pathway to the SO2

    removal criteria. .......................................................................................... 105

    3.4.2.2 Conformity of Blands (1990) [16] original pathway to the NOx

    removal criteria. .......................................................................................... 106

    3.4.2.3 Conformity of Blands (1990) [16] original pathway to the NO2

    formation criteria. ........................................................................................ 108

    3.4.2.4 Validity and value of Blands (1990) [16] original hypothesized

    pathway. ...................................................................................................... 110

    3.4.3 Conformity of Blands (1990) [16] final hypothesized reaction pathway

    to the criteria for SO2 removal, NOx removal and NO2 formation. ............ 110

    3.4.3.1 Conformity of Blands (1990) [16] final pathway to the SO2 removal

    criteria.......................................................................................................... 111

    3.4.3.2 Conformity of Blands (1990) [16] final pathway to the NOx removal

    criteria.......................................................................................................... 112

  • 5

    3.4.3.3 Conformity of Blands (1990) [16] final pathway to the NO2 formation

    criteria.......................................................................................................... 115

    3.4.3.4 Validity and value of Blands (1990) [16] final hypothesized pathway. ....... 116

    3.4.4 Conformity of Verlaeten et al.s (1991)[20] hypothesized reaction

    pathway to the criteria for SO2 removal, NO2 formation, and NOx

    removal........................................................................................................ 118

    3.4.4.1 Conformity of Verlaeten et al.s (1991)[20] pathway to the SO2

    removal criteria. .......................................................................................... 118

    3.4.4.2 Conformity of Verlaeten et al.s (1991)[20] pathway to the NOx

    removal criteria. .......................................................................................... 120

    3.4.4.3 Conformity of Verlaeten et al.s (1991)[20] pathway to the NO2

    formation criteria. ........................................................................................ 122

    3.4.4.4 Validity and value of Verlaeten et al.s hypothesized pathway. ................. 122

    3.4.5 Conformity of Bortzs hypothesized reaction pathway to the criteria

    for SO2 removal, NO2 formation, and NOx removal................................... 126

    3.4.5.1 Conformity of Bortzs pathway to the SO2 removal criteria....................... 126

    3.4.5.2 Conformity of Bortzs pathway to the NOx removal criteria. ..................... 127

    3.4.5.3 Conformity of Bortzs pathway to the NO2 formation criteria. .................. 129

    3.4.5.4 Validity and value of Bortzs hypothesized pathway.................................. 131

    3.4.6 Assessment of Lai and Yesavages experimental findings. ........................ 131

    3.5 Summary of conformity of all of the hypothesized reaction pathways to

    the criteria for SO2 removal, NOx removal, and NO2 formation................. 133

    3.6 Summary of key controversial reactions and end products................................. 134

  • 6

    3.7 Development of a hypothesized reaction pathway.............................................. 137

    3.7.1 Conformity of the foundation of the hypothesized reaction pathway. ........ 137

    3.7.1.1 Conformity of the foundation of the hypothesized reaction pathway to

    the SO2 removal criteria. ............................................................................. 138

    3.7.1.2 Conformity of the foundation of the hypothesized reaction pathway to

    the NOx removal criteria. ............................................................................ 140

    3.7.1.3 Conformity of the foundation of the hypothesized reaction pathway to

    the NO2 formation criteria........................................................................... 145

    3.7.2 Assembling the overall hypothesized reaction pathway ............................. 146

    3.7.2.1 Subsets of the overall hypothesized reaction pathway with respect to

    temperature.................................................................................................. 147

    3.7.2.1.1 Overall hypothesized reaction pathway Subset for T< 50C. .....................148

    3.7.2.1.2 Overall hypothesized reaction pathway, Subset for 50C

  • 7

    4.2.2 The simplified three-step reaction pathway for NO2 formation.................. 170

    4.2.3 Objective and scope of study ...................................................................... 170

    4.3 Apparatus ............................................................................................................ 173

    4.3.1 Thermal gravimetric analyzer. .................................................................... 173

    4.3.2 Isothermal fixed bed reactor........................................................................ 174

    4.3.2.1 The flue gas mixing station ......................................................................... 176

    4.3.2.2 The gas pre-heater. ...................................................................................... 181

    4.3.2.3 Powder injection module............................................................................. 182

    4.3.2.4 The main isothermal reaction zone. ............................................................ 183

    4.3.2.5 The quick-quench transfer zone for sample removal. ................................. 186

    4.3.2.6 The gas analyzer station for SO2 and NOx monitoring. .............................. 188

    4.3.2.7 The data acquisition computer system. ....................................................... 189

    4.3.3 Precision and accuracy ................................................................................ 190

    4.4 Analytical methods.............................................................................................. 190

    4.4.1 Elemental analysis....................................................................................... 190

    4.4.2 X-ray diffraction analysis............................................................................ 193

    4.4.3 BET surface area measurements ................................................................. 194

    4.4.4 Scanning electron microscope microstructure analysis .............................. 195

    4.4.5 Time-of-flight static secondary ion mass spectrometry .............................. 195

    4.4.6 X-ray photoelectron spectroscopy /electron spectroscopy for

    chemical analysis......................................................................................... 197

    4.5 Test Procedures ................................................................................................... 198

    4.5.1 Procedures for preliminary tests.................................................................. 198

  • 8

    4.5.1.1 Test procedure to investigate the effect of temperature on NO2

    formation from the reaction of sodium bicarbonate and SO2 flue gas

    with NO. ...................................................................................................... 198

    4.5.1.2 Test procedures for material characterization studies. ................................ 199

    4.5.1.2.1 Conversion tracking test procedures for reactions related to sodium

    bicarbonate decomposition inhibition. ........................................................199

    4.5.1.2.2 Conversion tracking test procedures for the study of the finite time

    period of sodium nitrate accumulation........................................................201

    4.5.1.2.3 Procedure to study surface area changes for the reaction of sodium

    bicarbonate with SO2 flue gas with NO at 150 and 250C. ........................205

    4.5.2 Procedures for reaction validation tests. ..................................................... 206

    4.5.2.1 Test procedure to validate the reaction of sodium sulfite with NO and

    O2................................................................................................................. 206

    4.5.2.1.1 Special preparations of sodium sulfite for reactivity tests. .........................208

    4.5.2.1.2 Method of producing reaction product for material analysis. .....................209

    4.5.2.2 Test procedure to validate the reaction of sodium nitrate with SO2 and

    O2................................................................................................................. 210

    4.5.3 Procedures for mitigation study tests. ......................................................... 211

    4.5.3.1 Test procedure to investigate the effect of oxygen percent on NO2

    formation from the reaction of sodium bicarbonate and SO2 flue gas

    with NO. ...................................................................................................... 211

    CHAPTER 5.................................................................................................................... 213

    PRELIMINARY TESTS................................................................................................ 213

  • 9

    5.1 Introduction ......................................................................................................... 213

    5.2 Test for proper operation of apparatus.- Measurements of NO2 versus

    temperature for the reaction of sodium bicarbonate with SO2 flue gas

    with NO. ...................................................................................................... 214

    5.3 Material characterization tests............................................................................. 217

    5.3.1 Sodium bicarbonate decomposition inhibition in SO2 flue gas with

    NO. .............................................................................................................. 217

    5.3.2 Sodium nitrate accumulation by sodium bicarbonate reaction with

    SO2 flue gas with NO. ................................................................................. 236

    5.3.2.1 The effect of flue gas water vapor on sodium nitrate accumulation for

    sodium bicarbonate reaction with SO2 flue gas with NO............................ 262

    5.3.3 Sodium bicarbonate surface area and microstructure changes that

    occur during reaction with SO2 flue gas with NO....................................... 271

    5.3.4 Sulfation product analysis for sodium bicarbonate reaction in SO2

    flue gas without NO. ................................................................................... 276

    5.3.5 Sodium bisulfite and sodium pyrosulfite decomposition products. ............ 282

    5.4 Conclusions ......................................................................................................... 284

    5.5 Follow on tests .................................................................................................... 288

    CHAPTER 6.................................................................................................................... 290

    VALIDATION OF THE REACTION OF SODIUM SULFITE WITH NO AND

    O2................................................................................................................. 290

    6.1 Introduction ......................................................................................................... 290

  • 10

    6.1.1 Examination of past studies of the reaction of sodium sulfite with NO

    and O2. ......................................................................................................... 291

    6.2 Experimental approach........................................................................................ 295

    6.3 Experimental results of the reaction tests............................................................ 296

    6.3.1 Experimental result of blank test................................................................. 297

    6.3.2 Experimental results of technical grade sodium sulfite reaction with

    NO and O2. .................................................................................................. 298

    6.3.3 Experimental results of freshly made sodium sulfite reaction with NO

    and O2. ......................................................................................................... 298

    6.3.3.1 Analysis of reaction product formed by specially prepared freshly

    made sodium sulfite with NO and O2.......................................................... 300

    6.3.3.1.1 Elemental Analysis......................................................................................302

    6.3.3.1.2 Time of flight static secondary ion mass spectrometry analysis. ................303

    6.3.3.1.3 Electron Spectroscopy for chemical analysis..............................................309

    6.4 Conclusions ......................................................................................................... 311

    6.5 Areas for further studies...................................................................................... 314

    CHAPTER 7.................................................................................................................... 316

    VALIDATION OF THE REACTION OF SODIUM NITRITE WITH SO2................. 316

    7.1 Introduction ......................................................................................................... 316

    7.1.1 Examination of past studies of the reaction of sodium nitrate with

    SO2. ............................................................................................................. 332

    7.2 Experimental approach........................................................................................ 333

    7.3 Experimental results of reaction tests.................................................................. 335

  • 11

    7.3.1 Experimental results of blank test. .............................................................. 336

    7.3.2 Experimental results of SO2 and O2 reaction with technical grade

    sodium nitrate. ............................................................................................. 336

    7.3.3 Experimental results of SO2 and O2 reaction with specially prepared

    sodium nitrate prepared fresh by reaction of sodium carbonate with

    NO2.............................................................................................................. 336

    7.3.3.1 The effect of temperature on NO2 formation from the reaction of the

    specially prepared sodium nitrate and SO2 and O2. .................................... 337

    7.3.4 Experimental results of SO2 and O2 reaction with specially prepared

    sodium nitrate prepared fresh by reaction of sodium sulfite with NO

    and O2. ......................................................................................................... 339

    7.3.4.1 The effect of temperature on NO2 formation by the SO2 reaction with

    specially prepared sodium nitrate prepared fresh by reaction of

    sodium sulfite with NO and O2. .................................................................. 341

    7.3.5 Experimental results of SO2 and O2 reaction with specially prepared

    sodium nitrate prepared fresh by the reaction of the three-step product

    with NO and O2. .......................................................................................... 343

    7.4 X-ray diffraction analysis of sulfate reaction products from specially

    prepared sodium nitrate reaction with SO2 and O2. .................................... 343

    7.5 Conclusions ......................................................................................................... 352

    7.6 Areas for further studies...................................................................................... 355

    CHAPTER 8.................................................................................................................... 356

  • 12

    MITIGATION OF PLUME DISCOLORATION FOR SODIUM

    BICARBONATE INJECTION IN SO2 FLUE GAS WITH NO ................ 356

    8.1 Introduction ......................................................................................................... 356

    8.2 Plume discoloration mitigation options............................................................... 358

    8.2.1 Plume discoloration mitigation by limiting oxygen concentration in

    the flue gas. ................................................................................................. 358

    8.2.2 Plume discoloration mitigation by adjusting the injection flue gas

    temperature.................................................................................................. 360

    8.2.3 Plume discoloration mitigation by injecting flue gas additives. ................. 362

    8.2.4 Plume discoloration mitigation by removing NO from the flue gas

    prior to sodium bicarbonate injection.......................................................... 363

    8.2.5 Plume discoloration mitigation by injecting sodium carbonate after

    the sodium bicarbonate flue gas desulfurization injection step................... 364

    8.2.6 Conclusions ................................................................................................. 365

    CHAPTER 9.................................................................................................................... 367

    CONCLUSIONS............................................................................................................ 367

    9.1 Chapter Summaries ............................................................................................. 367

    9.1.1 Summary of Chapter 1 ................................................................................ 367

    9.1.2 Summary of Chapter 2 ................................................................................ 367

    9.1.3 Summary of Chapter 3 ................................................................................ 371

    9.1.4 Summary of Chapter 4 ................................................................................ 372

    9.1.5 Summary of Chapter 5 ................................................................................ 375

    9.1.6 Summary of Chapter 6 ................................................................................ 379

  • 13

    9.1.7 Summary of Chapter 7 ................................................................................ 382

    9.1.8 Summary of Chapter 8 ................................................................................ 385

    9.2 Overall Conclusions ............................................................................................ 387

    BIBLIOGRAPHY .......................................................................................................... 388

    APPENDIX A ................................................................................................................394

    A.1 Introduction .........................................................................................................394

    A.2 Gibbs free energy equation .................................................................................394

    A.3 Reactions thermodynamically evaluated.............................................................396

    A.4 Equilibrium calculations from 27 to 627C for each of the reactions.................396

    A.5 Summary of all equilibrium calculations from 27 to 627 C................................406

    APPENDIX B CALCULATIONS OF CONVERSION FOR SODIUM

    BICARBONATE AND SODIUM CARBONATE TRACKING

    REACTION EXPERIMENTS ....................................................................410

    B.1 Equations for molar percentage of each compound in the product.....................410

    B.2 Equations for elemental weight percent of hydrogen, nitrogen and sulfur. ........412

    B.3 Determining the weight fractions of the respective compounds in the

    product.........................................................................................................414

    B.4 Calculating the moles of each compound in the product. ...................................417

    APPENDIX C UNITED STATES PATENT OF NITROGEN DIOXIDE

    MITIGATION PROCESS...........................................................................418

  • 14

    LIST OF FIGURES

    Figure 1-1 Dry sorbent baghouse injection method for coal burning utility

    applications. ................................................................................................... 22

    Figure 1-2 Development history of the dry sodium bicarbonate bag-house

    injection method............................................................................................. 24

    Figure 2-1 Spectra of NO2 absorbency, base e as a function of wavelength. .................. 32

    Figure 2-2 Schematic of the sodium bicarbonate decomposition reaction. ..................... 40

    Figure 2-3 Chemical structure of sodium carbonate. ....................................................... 41

    Figure 2-4 Schematic of the reaction pathway for of sodium carbonate with SO2.......... 42

    Figure 2-5 Schematic of Erdos postulated reaction pathway of sodium

    bicarbonate with flue gas with SO2................................................................ 43

    Figure 2-6 Sulfation products formed for the reaction of sodium bicarbonate with

    SO2 without NOx............................................................................................ 44

    Figure 2-7 Total weight percent of sodium sulfite versus temperature............................ 45

    Figure 2-8 Schematic of Keener and Khang.s (1982,1993)[9,52]reaction pathway

    for SO2 removal by sodium bicarbonate. ....................................................... 47

    Figure 2-9 Schematic of Knights (1977)[18] observed reaction of sodium

    carbonate with NO2........................................................................................ 48

    Figure 2-10 Weight percent sodium sulfite, Na2SO3, and weight percent sodium

    sulfate, Na2SO4, in product as a function of temperature. ............................. 51

    Figure 2-11 NO2 formation as a function of reactor temperature for sodium

    bicarbonate. .................................................................................................... 52

  • 15

    Figure 2-12 Schematic of Millers (1986)[19] Hypothesized Reaction Pathway (a)

    for sodium carbonate with flue gas with SO2 and NO, and (b) for

    sodium hydroxide with flue gas with SO2 and NO........................................ 59

    Figure 2-13 Schematic of Blands(1990)[16] first hypothesized reaction pathway

    for NO2 formation and NOx removal. ............................................................ 62

    Figure 2-14 Schematic of Blands (1990)[16] final theory on the reaction

    pathway for SO2 and NO removal and NO2 formation.................................. 65

    Figure 2-15 Schematic of Verlaeten et al.s (1991)[20] hypothesis on SO2 and NOx

    removal........................................................................................................... 68

    Figure 2-16 Schematic of schematic of Bortzs (1994)[45] hypothesis of SO2 and

    NOx removal and NO2 formation................................................................... 71

    Figure 2-17 Schematic of mixed SOx/NOx compounds formed in the chamber

    process for the manufacture of sulfuric acid. ................................................. 82

    Figure 3-1 Compilation of essential reaction pathway segments from past

    researchers.................................................................................................... 137

    Figure 3-2 The reaction pathway of sodium carbonate with SO2 flue gas with NO

    reduced from the foundation of the hypothesized reaction pathway. .......... 142

    Figure 3-3 The overall hypothesized reaction path........................................................ 147

    Figure 3-4 Schematic of overall hypothesized reaction pathway subset for T<

    50C ............................................................................................................. 149

    Figure 3-5 Schematic of overall hypothesized reaction pathway, Subset for

    50C

  • 16

    Figure 3-6 Schematic of overall hypothesized reaction pathway, for 120C 250C............ 159

    Figure 3-9 Compilation of essential reaction pathway segments from past

    researchers.................................................................................................... 160

    Figure 4-1 Apparatus...................................................................................................... 175

    Figure 4-2 Nitrogen rotometer calibration curve ........................................................... 177

    Figure 4-3 Carbon dioxide rotometer calibration curve................................................. 177

    Figure 4-4 Oxygen rotometer calibration curve. ............................................................ 178

    Figure 4-5 SO2 Mass flow controller calibration curve ................................................. 179

    Figure 4-6 NO mass flow controller calibration curve .................................................. 179

    Figure 4-7 Calibration curve of heater controller 1........................................................ 185

    Figure 4-8 Calibration curve of heater controller 2........................................................ 186

    Figure 4-9 Calibration curves for elemental analysis. ................................................... 192

    Figure 5-1 Validation test results of peak NO2 formation versus temperature. ............. 215

    Figure 5-2 Snapshot of ash layers from reaction sequence............................................ 216

    Figure 5-3 Conversion of sodium bicarbonate in inert nitrogen gas at 111C............... 220

    Figure 5-4 Conversion of sodium bicarbonate in SO2 flue gas without NO at

    111C. .......................................................................................................... 222

    Figure 5-5 Conversion of sodium bicarbonate in SO2 flue gas with NO at 111C........ 224

  • 17

    Figure 5-6 Conversion of sodium bicarbonate in nitrogen gas with NO, O2 at

    111C. .......................................................................................................... 227

    Figure 5-7 Conversion of sodium bicarbonate in nitrogen gas with NO2 at 111C....... 229

    Figure 5-8 Conversion of sodium bicarbonate in SO2 flue gas with NO2 at 111C. ..... 231

    Figure 5-9 Conversion of sodium carbonate in nitrogen gas with NO2 at 111C.......... 233

    Figure 5-10 Conversion of sodium carbonate in nitrogen gas with NO and O2 at

    111C. .......................................................................................................... 235

    Figure 5-11 Conversion of sodium bicarbonate in inert nitrogen at 150C................... 239

    Figure 5-12 Conversion of sodium bicarbonate in SO2 flue gas with NO at

    150C. .......................................................................................................... 241

    Figure 5-13 Conversion of sodium bicarbonate in nitrogen with NO and O2 at

    150C. .......................................................................................................... 247

    Figure 5-14 Conversion of sodium bicarbonate in nitrogen with NO2 at 150C for

    120 minutes. ................................................................................................. 250

    Figure 5-15 Conversion of sodium bicarbonate in SO2 flue gas with NO2 at

    150C. .......................................................................................................... 253

    Figure 5-16 Conversion of sodium carbonate with NO2 at 150C................................ 257

    Figure 5-17 Conversion of sodium carbonate in SO2 flue gas with NO at 150C. ........ 259

    Figure 5-18 Conversion of sodium carbonate in SO2 flue gas with NO2 at 150C........ 260

    Figure 5-19 Conversion of sodium bicarbonate in humidified inert nitrogen at

    150C. .......................................................................................................... 263

    Figure 5-20 Conversion of sodium bicarbonate in humidified inert nitrogen with

    5% H2O at 250C. ........................................................................................ 265

  • 18

    Figure 5-21 Conversion of sodium bicarbonate in humidified SO2 flue gas with

    NO at 150C................................................................................................. 267

    Figure 5-22 Conversion of sodium bicarbonate in SO2 flue gas with NO at

    250C. .......................................................................................................... 270

    Figure 5-23 BET surface area measurements versus time for the reaction products

    of sodium bicarbonate in inert nitrogen. ...................................................... 272

    Figure 5-24 Photo Micrographs of product particle surface from the reaction for

    sodium bicarbonate with SO2 flue gas with NO at 150C (a) after 10

    minutes, (b) after 300 minutes. .................................................................... 273

    Figure 5-25 BET surface area measurements versus time for the reaction products

    of sodium bicarbonate in inert nitrogen and in SO2 flue gas with NO

    at 250C. ...................................................................................................... 274

    Figure 5-26 Photo Micrographs of product particle surface from the reaction for

    sodium bicarbonate with SO2 flue gas with NO at 250C (a) after 10

    minutes, (b) after 60 minutes. ...................................................................... 275

    Figure 5-27 Phase diagram[80] of sodium nitrate and sodium sulfate............................. 276

    Figure 5-28 X-ray diffraction analysis of product from reaction of sodium

    bicarbonate with SO2 flue gas without NO at 150C. The scan in (a) is

    matched with the standard files for sodium sulfite in (b), and sodium

    bicarbonate in (c). Identical product was found for the reaction in

    SO2 flue gas without NO at 111C. ............................................................. 279

    Figure 5-29 X-ray diffraction analysis of product from reaction of sodium

    bicarbonate with SO2 flue gas without NO at 250C................................... 281

  • 19

    Figure 5-30 Thermal gravimetric analysis of sodium pyrosulfite.................................. 283

    Figure 6-1 NOx analysis for blank run........................................................................ 297

    Figure 6-2 NOx versus time for the reaction of freshly made sodium sulfite with

    NO and O2 at 175C..................................................................................... 299

    Figure 6-3 Diagram of reacted particle from experiment .............................................. 301

    Figure 6-4 SIMS Positive ion scan of high intensity peak............................................. 305

    Figure 6-5 SIMS positive ion scan of high intensity peak ............................................. 306

    Figure 6-6 SIMS negative ion scans of high intensity peaks ......................................... 307

    Figure 6-7 SIMS negative ion scans of high intensity peaks ......................................... 308

    Figure 6-8 ESCA scan of product from the reaction of sodium sulfite with NO

    and O2........................................................................................................... 310

    Figure 7-1 NO2 formation versus temperature for the reaction of NaNO3 with

    SO2 and O2. .................................................................................................. 338

    Figure 7-2 NO formation versus temperature for the three-step reaction sequence. ..... 340

    Figure 7-3 NO2 formation from the three step reaction sequence versus

    temperature................................................................................................... 342

    Figure 7-4 X-ray diffraction analysis of product from reaction of sodium

    bicarbonate with SO2 flue gas with NO at 111C........................................ 349

    Figure 8-1 NO2 Formation versus O2 percent in flue gas. ............................................. 360

    Figure APPENDIX A-1 Summary of Gibbs free energy versus temperature for all

    of the reaction equations. ............................................................................. 408

    Figure APPENDIX B-1 Details of numerical computations for weight fractions.......... 416

  • 20

    LIST OF SYMBOLS

    f NaHCO3 weight fraction of the product that is sodium bicarbonate

    f Na CO2 3 weight fraction of the product that is sodium carbonate

    32SONaf weight fraction of the product that is sodium sulfite

    42SONaf weight fraction of the product that is sodium sulfate

    f NaNO3 weight fraction of the product that is sodium nitrate

    Gr Gibbs free energy of the reaction

    Kr equilibrium constant

    Kr equilibrium constant

    Kf( i ) equilibrium constant for each reactant and product substances

    mNaHCO3 moles of sodium bicarbonate per 100 grams of product (mol/100g)

    mNa CO2 3 moles of sodium carbonate per 100 grams of product (mol/100g)

    mNa SO2 3 4/ moles of sodium sulfite or sulfate per 100 grams of product

    (mol/100g),

    mNaNO3 moles of sodium nitrate per 100 grams of product (mol/100g)

    MWH molecular weight of hydrogen (grams/mole)

    MWN molecular weight of nitrogen (grams/mole)

    MWS molecular weight of sulfur (grams/mole)

    MWNaHCO3 molecular weight of sodium bicarbonate (grams/mole)

    MWNa CO2 3 molecular weight of sodium carbonate (grams/mole)

  • 21

    32SONaMW molecular weight of sodium sulfite (grams/mole)

    42SONaMW molecular weight of sodium sulfate (grams/mole)

    MWNaNO3 molecular weight of sodium nitrate (grams/mole)

    R universal gas constant

    T temperature

    i is the stoichiometric coefficient of substance i

    wt%(H) weight percent hydrogen

    wt%(N) weight percent nitrogen

    wt%(S) weight percent sulfur

    X NaHCO3 molar percent of Na in the form of sodium bicarbonate in the product

    X Na CO2 3 molar percent of Na in the form of sodium carbonate in the product

    X Na SO2 3 molar percent of Na in the form of sodium sulfite or sulfate in the product

    X NaNO3 molar percent of Na in the form of sodium nitrate in the product

  • 22

    CHAPTER 1

    INTRODUCTION

    1.1 Background

    The Environmental Protection Agency (EPA) through the Acid Deposition Programs of

    Title IV of the Clean Air Act has promulgated stringent regulations limiting sulfur

    dioxide, SO2, and nitrogen oxide, NOx, emissions from US electric utilities.[1] In the year

    2000, more than 1000 utilities will be required to comply with Phase II requirements of

    the Acid Rain Rule. There are several options available to meet these regulations

    including an installation of end-of-pipe SO2 and NOx removal device.[2]

    Dry sorbent baghouse injection is an attractive end-of-pipe SO2 removal option.

    This method is economically desirable since it involves low capital investment, is very

    simple to operate, and can be easily retrofitted into existing hardware configurations.[2]

    Figure 1.1 shows a representative diagram of the scrubbing method.

    Flue Gas: N2, CO2,O2, H2O, SO2,NO

    Spent Sorbent andFly Ash for recycle

    CombustionZone

    Stack

    Dry SorbentInjection

    AirPreheater

    Coal andPreheated

    Air

    CleanedFlue Gas

    AshFly Ash

    BaghouseParticulateCollector

    Figure 1-1 Dry sorbent baghouse injection method for coal burning utility applications.

  • 23

    In this method, dry pulverized sorbent is injected into the coal combustion flue gas duct

    downstream of the air preheater. The sorbent is then collected in a baghouse collector.

    The injected sorbent reacts with gaseous SO2 in the flue gas during injection flight and

    within the baghouse. The solid reaction product that is captured on the sorbent is either

    regenerated for reuse or disposed of.

    Lime based dry alkali sorbents are commonly used for scrubbing in the eastern

    regions of the US since they are readily available in these regions. Sodium based dry

    alkali sorbents extracted from the Four Corners region of the western US, Wyoming,

    Idaho, Utah, and Colorado, may potentially be used for scrubbing in the western regions

    of the US.[3]

    1.2 The Problem

    Dry sodium bicarbonate sorbent injection has been under development since the

    early 1960s in the US and Europe.[4] It has not yet been fully implemented as a viable

    technology option for use by electric utilities because of issues surrounding plume

    discoloration that were discovered during full-scale demonstration tests.[5] The plume

    discoloration has not yet been resolved for this scrubbing method.

    Figure 1-2 traces the development history of the dry sodium bicarbonate baghouse

    injection method.

  • Figure 1-2 Development history of the dry sodium bicarbonate bag-house injection method. The dashed line represents the steps that are made within this study. The circle with the back slash indicates the route that was not taken; instead the detour drawn was taken.

    Note that the ultimate goal of the development of dry sodium bicarbonate baghouse

    injection is full-scale implementation of the technology into the marketplace. Figure 1.2

    indicates that initial laboratory research and development occurred between 1960 and the

    mid-1980s.[4,6,7,8.9.10,11] Primarily, these laboratory investigations established the

    fundamentals of the reaction of sodium bicarbonate with SO2. Pilot and full-scale coal

    combustion demonstration tests were conducted between 1970 to 1986.[12]

    From 1984 to 1986 the Electric Power Research Institute (EPRI) successfully

    demonstrated in fu removal ra 90% for compliance with New

    Source Performanc s.[13,14 ] It w ese 1986 full-scale demonstration

    tests that plume discoloration was noticed desirable reddish-brown

    discoloration of the exit stack gas was observed when sodium njected

    into t ring dem lthough the disco ot violate

    stack

    Initial

    Laboratory

    Full Scale UtilityDemonstration Tests 1970-1986

    Plume Discoloration

    Observed 1986

    Determine Reaction

    Pathway of NO2 2001

    Devise Method to Abate NO2

    2001

    Enable Full Scale Implementation to Electric Utilities he duct duopacity regulationonstration tests. All scale SO2

    e Standards and was fully cotes of 70 to

    as during th

    .[14] The un24

    mpliant with all SO bicarbonate was i

    lored exit gas did n2 and NOx emission

  • 25

    regulations, it poses a serious public relations problem since discoloration of any kind is

    typically aesthetically unacceptable and a nuisance to local communities. As a result to

    date the technology has not been fully implemented into the marketplace.

    Instead research and development of the process has continued in the laboratory

    to determine the cause and the exact reaction pathway responsible for the plume

    discoloration. The reaction pathway in an effort to devise an abatement method[15] to

    ultimately allow the technology to reach full-scale implementation for use by utilities.

    Very shortly after the discoloration was first noticed investigators determined that

    the discoloration was caused by the formation of a small concentration of NO2.[16] It was

    common knowledge that coal combustion flue gas contained colorless NO[17]. But it was

    unclear how NO2 was formed from the flue gas NO during sodium bicarbonate injection.

    The cause of the NO2 formation has not to date been determined. Prior to learning of

    plume discoloration by NO2 formation, NO in flue gas was assumed to be non-

    participatory in desulfurization reactions based on studies by Knight and others that

    found NO to be unreactive with sodium bicarbonate alone.[18]

    1.3 Prior study of this problem.

    As a result prior to 1986, the reaction of sodium bicarbonate with SO2 flue gas

    was experimentally studied in the absence of NO, under the assumption that NO had no

    role in desulfurization. After the discoloration findings in 1986, NO was included in the

    SO2 flue gas mixture for these sodium bicarbonate reaction studies. The role of NO in

    the reactions began to be closely examined in an attempt to reveal that there was in fact a

    complex interaction of NO in the reaction pathway. NO2 formation was found to occur

    by the reaction but little more of this complex interaction was known.

  • 26

    One theory hypothesized that NO2 was formed by the reaction of NO with a

    sulfation reaction product or sulfation product intermediate compound.[12] Between 1986

    and 1994 numerous researchers investigated possible reactions of NO with various

    desulfurization products and intermediate product compounds.[19,20,21,22] But none of

    these studies successfully revealed any sulfation compound to be responsible for the NO2

    formation.

    1.4 Goals and objectives of this study

    The goal of this study was to enable full-scale implementation of the dry sodium

    bicarbonate bag-house injection method for SO2 removal allowing this technology to

    enter the marketplace for utilities to use for Phase II Acid Rain regulations. The specific

    objectives were to:

    1) Determine the reaction pathway for NO2 formation during dry sodium bicarbonate

    injection.

    2) Devise a method of abatement for plume discoloration during dry sodium bicarbonate

    injection based on knowledge of the NO2 formation reaction pathway.

    1.5 Approach and scope

    Chapter 2 presents relevant literature on plume discoloration and reaction

    chemistry of sodium bicarbonate flue gas desulfurization. The plume discoloration

    section includes a general historical understanding of plume discoloration and plume

    discoloration reports from sodium bicarbonate and sodium sorbent flue gas

    desulfurization field trials. Also Chapter 2 provides a description of how NO2 was found

    to be the discoloring agent. The reaction chemistry section of Chapter 2 includes a review

  • 27

    of reaction studies of sodium bicarbonate decomposition, sodium bicarbonate reaction

    with flue gas containing SO2 without NO, sodium bicarbonate and sodium carbonate

    reaction with NO and NO2 but without SO2, and sodium bicarbonate reaction with flue

    gas containing SO2 and NO. Additionally past investigators hypotheses on NO2

    formation and NOx removal are reviewed. Illustrative schematic diagrams of the past

    hypothesized reaction paths are provided wherever possible to assist in pathway

    visualization. Experimental validation findings for these hypotheses are also reviewed

    including findings on reactions of sodium sulfite, sodium pyrosulfite, and sodium sulfate

    with NO and O2, reactions of sodium nitrite and sodium nitrate with SO2 and O2, and gas

    phase interactions of SO2 and NOx.

    Chapter 3 provides the theoretical development of the overall reaction pathway

    for NO2 formation and NOx removal. Criteria for SO2 removal, NOx removal, and NO2

    formation are established based on past undisputed experimental findings reported in

    Chapter 2. The past proposed hypothesized reaction pathways from past researchers

    described in Chapter 2 are assessed in Chapter 3 for conformity with the established

    reaction criteria. The conforming portions of the past researchers hypothesized reaction

    pathways then serve as a foundation for a new hypothesized reaction pathway constructed

    for investigation in this study. This new hypothesized reaction pathway is checked for

    thermodynamic feasibility at the end of Chapter 3 with the supporting calculations

    described in Appendix A.

    Chapter 4 includes a description of the experimental methodology used to verify

    and validate the new hypothesized reaction pathway proposed for study. The

    experimental approach and objectives of the study are included. The laboratory

  • 28

    apparatus is also described in Chapter 4. In addition the special methods used in the

    laboratory for reaction reagent preparation and handling are described. The particular

    experimental test procedures are explained. The specific analytical test methods used to

    characterize and determine the reaction products are given. And the quality assurance

    steps employed are also included in Chapter 4.

    Chapter 5 presents the results of the preliminary tests. In particular the reaction of

    sodium bicarbonate with flue gas with SO2 and NO is investigated with respect to

    temperature in the reactor setup to show that the apparatus produces NO2 formation

    comparable to NO2 formation previously documented by past investigators. Chapter 5

    also includes several preliminary material characterization studies. Sodium bicarbonate

    decomposition inhibition in SO2 flue gas with NO is studied by conversion tracking

    reactions. Sodium nitrate accumulation during sodium bicarbonate reaction in SO2 flue

    gas with NO is studied including the effect of added humidity by conversion tracking

    reactions. A description of the conversion tracking calculations is given in Appendix B.

    Sodium bicarbonate surface area and microstructure changes during reaction with SO2

    flue gas with NO are also studied. Sulfation product formation by the reaction of sodium

    bicarbonate and SO2 flue gas without NO is also studied. And sodium bisulfite and

    sodium pyrosulfite decomposition is studied including the decomposition product

    identification.

    In Chapter 6 the key un confirmed reaction of sodium sulfite with NO and O2that

    is in the hypothesized reaction pathway is experimentally validated. This reaction, which

    is one of the three key reactions in the hypothesized reaction for NO2 formation, is

    validated. Past studies of this reaction are closely examined in Chapter 6 to determine the

  • 29

    reason why different researchers obtained different experimental results in the past. The

    effect of sodium sulfite freshness on the reaction is experimentally studied. In particular,

    various different sodium sulfite preparations are experimentally tested to demonstrate the

    experimental importance of sodium sulfite freshness on the reaction. Analytical results of

    product formation from the reaction of freshly made sodium sulfite with NO and O2 are

    presented including elemental analysis, X-ray diffraction (XRD), infrared (IR), X-ray

    photoelectron spectroscopy, Time-of-Flight Secondary Ion Mass Spectrometry (TOF-

    SIMS), and Electron Spectroscopy for Chemical Analysis (ESCA).

    Additionally in Chapter 7 the reaction of sodium nitrate with SO2 and O2, another

    key reaction in the hypothesized reaction pathway for NO2 formation is validated.

    Various different sodium nitrate preparations are studied to demonstrate the importance

    of sodium nitrate freshness in the reaction for NO2 formation. Chapter 7 provides X-ray

    diffraction (XRD) analysis of the reaction product formed by the reaction of freshly made

    sodium nitrate with SO2 and O2.

    Chapter 8 investigates optional mitigation methods for plume discoloration during

    sodium bicarbonate injection into SO2 flue gas with NO. Changes in flue gas oxygen

    concentration are experimentally investigated for its effect on NO2 formation. Adjusting

    the duct temperature is contemplated, as well as injecting flue gas additives to avoid NO2

    formation. And finally, removing NO from the flue gas prior to sodium bicarbonate

    injection is considered as a mitigation option. A novel process using sodium bicarbonate

    reaction chemistry was devised to remove NO from the flue gas prior to the

    desulfurization step of sodium bicarbonate injection and is described in Appendix C.

  • 30

    And finally Chapter 9 is a summary of the main findings from the previous

    chapters.

  • 31

    CHAPTER 2

    LITERATURE REVIEW

    This chapter reviews relevant literature on plume discoloration and sodium bicarbonate

    reaction chemistry.

    2.1 Literature review of plume discoloration

    The following areas of plume discoloration are reviewed:

    1) General historical understanding of plume discoloration.

    2) Plume discoloration reports from sodium bicarbonate and sodium sorbent flue gas

    desulfurization trials.

    3) A description of how NO2 was found to be the discoloring agent during sodium

    bicarbonate injection demonstration tests.

    2.1.1 History of plume discoloration

    Around the early part of the 20th century plume coloration was studied by the

    original air pollution scientists and engineers in the smoke and soot era of the industrial

    revolution in Europe and in the US. In the early 1900s ordinances prohibiting dense

    black smoke prompted study of smoke and methods of prevention. Soon smoke

    abatement legislation emerged with adopted definition of smoke color intensity.

    According to Stern (1977)[23] smoke darker then a #3 on a Ringelmann scale, equivalent

    to 60% opacity, was prohibited in most communities. As smoke prevention measures

    were put into place by the 1950s, such as improved fuel utilization methods, plumes of

    black smoke largely disappeared and plumes took on a new appearance.

  • 32

    The U.S. Edison Electric Institute and the Public Health Services and the Central

    Electricity Generating Board in Great Britain conducted fundamental opacity studies of

    non-black emission plumes in the 1960s.[24] By the 1980s the contributions of NO2

    absorption and particulate matter (aerosol sulfates or fly ash) scattering on visibility were

    well established by Charlson and Ahlquist (1969)[25], Horvath (1971)[26], Husar and

    White (1976)[27], and White and Patterson (1981)[24]. Fundamentally, light attenuation by

    NO2 within the visible range was reported as early as the 1800s by Brewster (1834)[28].

    The NO2 absorbency spectra in the visible light range occurs from wavelength 250 nano-

    meters (nm) to 650 nm with its peak near 400 nm according to Davidson et al.(1988)[29]

    and others[30,31,32]. Figure 2.1 shows this light absorbency spectrum.

    250 300 350 400 450 500 550 600 650Wavelength (nm)

    0.0

    0.4

    0.8

    1.2

    1.6

    2.0

    2.4

    Abs

    orba

    nce

    Figure 2-1 Spectra of NO2 absorbency, base e as a function of wavelength. Measurement from Davidson et al.(1988)[29] at 9.7C (50F), [NO2] = 7.20 x1014 molecules/cm3, path length equal to 48.6 meters.

  • 33

    Under fixed meteorological and stack conditions Lindau (1991) showed that NO2

    followed the Beer-Lambert rule in which the light absorption was proportional to the NO2

    concentration.[33]

    White and Patterson (1981)[24] report that Middleton in 1952 found a visual threshold,

    above which plume contrast was perceptible to the eye, between 2 to 5 %. Likewise,

    Hardison (1970)[34] found a threshold of an NO2 laden plume from a nitric acid

    processing plant to be approximately 2%. Hardison (1970)[34] formulated a correlation

    relation equation for threshold with respect to NO2 concentration of plume visibility and

    stack diameter:

    60 9.D

    T= (2.1)

    where

    D is stack diameter in meters

    T is the threshold NO2 concentration for plume visibility in ppm

    According to this equation a plume is visible to the eye from a 2 meter stack when NO2

    concentrations exceeded 50 ppm. White and Patterson (1981)[24] additionally showed that

    plume visibility depended on other factors besides NO2 concentration. For example,

    plume visibility was shown to depend also on background sky contrast to the stack.

    Melo and Stevens (1981)[35] explained that NOx in flue gas from fossil-fired power plant

    plumes is produced as an unwanted side effect of fuel combustion where the nitrogen

    from the air and the fuel is oxidized in the flame by excess oxygen and temperature.

    They showed that NOx formed in the combustion flame was in the form of colorless NO.

    Kircher and Hougen (1957)[36] showed that in some cases the colorless NO species

  • 34

    converted to the reddish brown colored NO2 species by the following equilibrium tri-

    molecular oxidation reaction with oxygen in the atmosphere,

    2NO (g) + O2(g) 2NO2(g) (2.2)

    According to Duecker and West (1959)[37] the tri-molecular gas phase conversion of NO

    to NO2 was non-catalytic and possessed a negative temperature coefficient.

    At typical stack emitted temperature the NO to NO2 conversion is very slow and does not

    occur instantaneously but requires a relatively long time period. Duecker and West

    (1959)[37] provided estimates of the steady state conversion time for NO to convert to

    NO2 based on the findings of Stevenius-Nielsen. They report that over 1 hour was

    required to convert 50% of the NO to NO2 at an exit temperature of 141C (286F) for a

    gas mixture of 500 ppm NO and 6.5% O2 at atmospheric pressure. England and

    Corcoran (1975) also reported very slow reaction rates for the conversion of NO to NO2.

    Exit flue gas from coal combustion sources such as electric utilities generally contains

    small quantities of NO due to combustion. Because the reaction rate of steady state

    conversion of NO to NO2 is slow at stack gas temperatures, electric utility plumes at or

    near the stack are typically colorless. However with sufficient time the emitted NO is

    converted to NO2 according to the steady state equilibrium reaction (equation 2.2). As a

    result, colorless exit flue gas under stable meteorological conditions discolors at a

    distance away from the stack. These unattached visible plumes may be seen when

    plumes drift distances without significant lateral mixing (i.e. within an inversion). Under

    these conditions the NO in the maintained plume converts to NO2 a distance away from

    the stack. Melo and Stevens (1981)[35] reported the occurrence of unattached reddish

    brown NO2 plumes at distances of 0.5 to 5 km from respective power plant stacks. These

  • 35

    unattached plumes stretched in length from 10 to 100 km depending on the amount of

    mixing from meteorological conditions.

    2.1.2 Reported observations of plume discoloration during sodium sorbent flue

    gas desulfurization trials.

    To reiterate, coal combustion flue gas is generally colorless as it exits the stack because

    the NOx in the exit flue gas is in the form of colorless NO. Since SO2 scrubbing of coal

    combustion flue gas by dry sodium bicarbonate injection was not initially known or

    anticipated to have any effect on the colorless NO in the flue gas, plume discoloration

    and NO interactions were not a focus during early development of the injection process.

    Although some reports of plume discoloration during dry sodium bicarbonate injection

    exist as far back as the 1960s through the mid-1980, the discoloration was never

    attributed to the sodium bicarbonate injection process or any interaction of NO with

    desulfurization by sodium bicarbonate. Reexamining the record however, reveals that

    there were some early indications that plume discoloration was directly associated with

    dry sodium bicarbonate injection. For example, yellow discoloration of scrubbed exit gas

    was observed during dry sodium bicarbonate bag-house injection trials at the Air

    Preheater Company for the National Air Pollution Control Administration as early as

    1969 by Liu and Chaffee (1969)[38]. Reports of plume coloration varied from trial to

    trial. In some cases no color was observed and in other cases faint yellow color was

    observed and in other cases a deep reddish brown color was observed. But because the

    discoloration occurrences were sporadic and inconsistent they were initially overlooked

    as a concern. For example Muzio et al. (1985)[13] reported no plume discoloration for the

    pilot scale 22MW demonstration tests at Cameo Station. Confounding the situation

  • 36

    further was the fact that discoloration was reported to change its color intensity during

    demonstration testing even though combustion conditions and sodium bicarbonate

    injection conditions were held constant.[14,15,16] Because of these inconsistencies sodium

    bicarbonate injection was not immediately charged as cause of the discoloration. The

    cause of plume discoloration was instead initially attributed to other factors besides

    sodium bicarbonate injection such as atmospheric humidity or temperature.

    It is pertinent to note that plume discoloration was reported in the literature to occur for

    other sodium related flue gas desulfurization processes besides dry sodium bicarbonate

    injection. Felsvang et al. (1983)[39], for example, reported plume discoloration at

    Riverside Station during lime spray dryer, Niro SOx/NOx ,trials. Importantly, these trials

    employed sodium-based additives. Markussen et al. (1986)[40] at Pittsburgh Energy

    Technology Center reported plume discoloration during lime spray dryer laboratory trials

    when caustic, NaOH, additive was used. Similarly NO2 formation was found by Argonne

    National Laboratory when caustic additions were used in a lime spray dryer trials.[41]

    Also, in the Oxley (1984)[42] interviews, observations of plume discoloration were said to

    have occurred at Coyote Station during wet sodium carbonate spray dryer trials. The

    Coyote Station trials involved burning of high sodium lignite coal which may have also

    contributed to the plume discoloration. Plume discoloration was noted at Antelope

    Valley Station which burned high sodium lignite coal but did not use any sodium based

    additives in its lime spray dryer flue gas desulfurization system.[42]

  • 37

    2.1.3 The discovery that NO2 was the plume discoloring agent for sodium

    bicarbonate bag-house injection.

    The connection between plume discoloration and dry sodium bicarbonate bag-

    house injection was definitively made during Phase II of the full scale demonstration tests

    at R.D. Nixon Station in 1986 when it was observed that plume discoloration terminated

    when sodium bicarbonate injection was terminated.[15] Although it was not initially

    known what specific compound caused the plume discoloration, Fuchs et al.(1990)[14] and

    Bland (1990)[15] hypothesized that NO2 formation was the agent responsible for the

    plume coloration based on a number of factors:

    1) The reddish brown coloration matched the typical discoloration from NO2. A

    clue that NO2 was the responsible agent for plume discoloration was the

    distinguishing reddish brown plume color of the plume. This reddish brown

    color was the same color observed in detached downwind NO2 plumes

    discussed in the previous section (Section 2.1.2). This reddish brown color

    corresponded to the familiar color of smog where a build up of NO2 forms.

    The reddish brown color also corresponded to the color of the NO2 fumes

    evolved from laboratory beakers of fuming nitric acid.

    2) Other possible candidates such as sulfates and fly ash were not present as a

    result of the desulfurization and particulate collection process in place. NO2

    was hypothesized as the responsible agent for the discoloration, partially by

    process of elimination, since it was known that the sulfates were not forming

    since the sodium bicarbonate flue gas desulfurization process removed 70 to

    90% of the flue gas SO2. Additionally, fly ash was eliminated as a possible

  • 38

    coloration candidate since the bag-house filter assured that fly ash was

    removed.

    The hypothesis that NO2 was the plume-coloring agent was substantiated by

    quantitative NOx measurements within multiple investigations [14,15,43,44] including those

    at the Arapaho Test Facility. NO2 concentration prior to dry sodium bicarbonate injection

    was compared with the NO2 concentration after injection and collection and found to

    form by the sodium bicarbonate injection. However the exact chemistry and reaction path

    responsible for the NO2 formation was not defined.

    A complex interaction of the flue gas NO with the sodium bicarbonate injection

    process was thought to be responsible for the NO2 formation. NO2 formation was

    thought to be due to an associated reaction. It seemed plausible that sodium bicarbonate

    or a related by-product from the sodium bicarbonate desulfurization process was related

    to the NO oxidation to NO2.

    2.2 Literature review of sodium bicarbonate reaction chemistry.

    As a foundation for understanding the reaction chemistry of the reaction of sodium

    bicarbonate and flue gas with SO2 and NO, the following is a review of the literature on:

    1) Sodium bicarbonate decomposition.

    2) Sodium bicarbonate reaction with flue gas with SO2 but without NOx

    including the studies by Mocek and Wald (1988)[4], Howatson et al. (1980)[8],

    and Keener (1982)[9].

    3) Sodium bicarbonate reaction with NOx without SO2 including the study by

    Knight (1980)[18].

  • 39

    4) Sodium bicarbonate reaction with flue gas with SO2 and NOx including the

    studies of Bland (1990)[16], Verlaeten et al. (1991)[20] and Bortz (1994)[45].

    5) Hypothesis for NO2 formation and NOx removal reactions suggested by

    different investigators including Miller (1986)[19], Bland (1990) [16], Verlaeten

    et al. (1991)[20] and Bortz (1994)[45]. Illustrative schematic diagrams of the

    hypothesized reaction chemistry are provided wherever possible to assist in

    pathway visualization.

    6) Experimental validation findings of hypothesized reactions of:

    a) Sodium sulfite as conducted by Miller (1986)[19], Bland (1990) [16],

    Verlaeten et al. (1991)[20], and Lai and Yesavage (1994)[21].

    b) Sodium pyrosulfite as conducted by Bland (1990) [16], Verlaeten et al.

    (1991)[20], and Lai and Yesavage (1994)[21].

    c) Sodium sulfate as conducted by Bland (1990) [16].

    d) Sodium nitrite and sodium nitrate conducted by Kozlowski et al (1970)[22]

    and Bland (1990) [16].

    e) Gas phase interactions of SO2 and NOx.

    2.2.1 Literature review of sodium bicarbonate decomposition.

    According to Barrall and Rogers (1966)[46], the decomposition reaction of sodium

    bicarbonate was first discovered by Lescoeur in the late 1800s. The following reaction

    equation was established,

    2NaHCO3 Na2CO3 + H2O + CO2 (2.3)

    Because water vapor is a by-product in the reaction, the reaction is often referred to as a

    dehydration reaction rather than a decomposition reaction. For the remainder of

  • 40

    discussion in this work, equation 2.3 will be referred to as a decomposition reaction.

    Besides Lescoeur, many other researchers including Barrall (1966)[46], Subramanian et al.

    (1972)[47], Templeton (1978)[48], Keener et al. (1985)[49] and Hu et al. (1986)[50] studied

    this decomposition reaction has been studied by The schematic diagram of the

    decomposition reaction, equation 2.3, is shown in Figure 2.2 as a visual aid for reaction

    pathway development.

    Figure 2-2 Schematic of the sodium bicarbonate decomposition reaction.

    The past studies of this reaction revealed that the onset of fast sodium bicarbonate

    decomposition ranged from 67C to 125C depending upon the partial pressure of CO2

    and H2O. These studies also revealed that decomposition dramatically changed the

    material microstructure. Theoretically, a mole of sodium bicarbonate with molar volume

    of 38.9 cm3, is decomposed to a half a mole of sodium carbonate with molar volume of

    20.7 cm3. A porous popcorn-like microstructure with high surface area forms from the

    sodium bicarbonate decomposition because of the molar volume decrease. Figure 2.3

    shows the chemical structure of sodium carbonate.

    NaHCO3 Na2CO3

    H2O

    T>67C

    H2O

  • 41

    Figure 2-3 Chemical structure of sodium carbonate.

    Surface area measurements have shown an increase in surface area from less than 1 m2/g

    before decomposition to 7-10 m2/gram after decomposition.[51]

    2.2.2 Literature review of sodium bicarbonate reaction with SO2 flue gas

    without NOx.

    This section reviews the early sodium bicarbonate reaction studies with SO2 flue

    gas without NOx. These studies were primarily conducted to establish the reactivity of

    sodium bicarbonate with SO2 for flue gas desulfurization processes. At the time NO was

    not included in the flue gas for these trials because at that time NO was not known to

    have any role in the reaction chemistry of SO2 removal. These studies were conducted

    prior to the discovery of plume discoloration and the realization that NO had a role in the

    process. The findings of Mocek and Wald (1988)[4], Howatson et al.(1980)[8], and

    Keener (1982)[9] are reviewed.

    2.2.2.1 Review of Erdos investigations of sodium bicarbonate reaction with SO2

    flue gas without NOx.

    According to Mocek and Wald (1988)[26], as early as 1964 Erdos at the

    Czechoslovakia Academy of Sciences was the first to establish that sodium carbonate

    was a reactive alkali with acidic gas concentrations of SO2:

    O C

    O

    O

    Na

    Na

  • 42

    Na2CO3 + SO2 + O2 Na2SO4 + CO2 (2.4)

    Figure 2.4 shows a schematic diagram of the reaction pathway.

    Na2CO3 Na2SO4

    CO2

    SO2,O2

    Figure 2-4 Schematic of the reaction pathway for of sodium carbonate with SO2.

    Erdos discovered that SO2 was more reactive with sodium bicarbonate compared to

    sodium carbonate. Erdos attributed the improved reactivity to activation from the

    decomposition of sodium bicarbonate.

    Erdos conceptualized that the reaction between sodium bicarbonate and SO2

    occurred in two steps:

    1) Erdos postulated that decomposition first activated the freshly formed sodium

    carbonate (activation is indicated by the ! symbol),

    2NaHCO3 Na2CO3 + H2O + CO2 (2.5)

    2) Erdos postulated that SO2 then reacted with activated sodium carbonate to form

    sodium sulfate,

    Na2CO3 + SO2 + O2 Na2SO4 + CO2 (2.6)

    Note that the sum of these two steps, equations 2.5 and 2.6, makes up Erdos overall

    reaction equation,

    2NaHCO3 + SO2 + O2 Na2SO4 + H2O + 2CO2 (2.7)

  • 43

    As a visual aid, Figure 2.5 shows a schematic diagram of Erdos two-step reaction

    pathway.

    NaHCO3 Na2CO3!!!!

    H2O

    Na2SO4

    CO2

    T>67C

    SO2,O2

    CO2

    Figure 2-5 Schematic of Erdos postulated reaction pathway of sodium bicarbonate with flue gas with SO2.

    Other researchers including, Knight (1977)[18], Howatson et al. (1977)[6], Stern (1978)[31],

    Carson (1980)[7] accepted and used Erdos conceptual model for theoretical development

    of flue gas desulfurization reactions.

    2.2.2.2 Review of Howatson et al. (1980)[8] investigations of sodium bicarbonate

    reaction with SO2 flue gas without NOx.

    Howatson et al. (1980)[8] conducted experiments measuring the composition of

    reaction products from the sodium bicarbonate reaction with SO2 flue gas without NO

    (the mixture contained 14%CO2, 5%O2, 4% H2O, 1000ppm SO2) from 90C to 300C.

    In particular Howatson et al.(1980)[8] found that the reaction product formed depended on

    the reaction temperature:

    1) Below 150C, sodium sulfite was found to be the main product formed with

    very little sodium sulfate product formation.

    2) Above 250C, the reverse was found; sodium sulfate was found to be the

    primary product formed with very little sodium sulfite product.

  • 44

    3) Between 150 and 250C, the conversion product was found to depend on

    temperature, where sodium sulfite was formed at the lower end of the

    temperature scale and sodium sulfate at the higher end of the temperature

    scale.

    Howatson et al.(1980)[8] did not postulate the chemical reactions involved in the reaction

    pathway. The product formations were grap