The Institute of Paper Science and Technology Atlanta, Georgia Doctor's Dissertation The Depletion of Nitric Oxide by Reaction with Molten Sodium Carbonate and Sodium Carbonate/Sodium Sulfide Mixtures Laura M. Thompson January 1995 THE DEPLETION OF NITRIC OXIDE BY REACTION WITH MOLTEN SODIUM CARBONATE AND SODIUM CARBONATE/SODIUM SULFIDE MIXTURES A Thesis Submitted by Laura M. Thompson B.S. 1988, University of New Hampshire M.S. 1990, Institute of Paper Science and Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy from the Institute of Paper Science and Technology Atlanta, Georgia Publication Rights Reserved by the Institute of Paper Science and Technology January 1995 TABLE OF CONTENTS Page ABSTRACT 1 INTRODUCTION 3 OBJECTIVES 5 ORGANIZATION 6 BACKGROUND 8 THE KRAFT PULPING PROCESS 8 BLACK LIQUOR COMBUSTION 10 RECOVERY BOILER AIR EMISSIONS 12 FUME FORMATION 13 DECOMPOSITION OF SODIUM NITRATE 17 A REVIEW OF NOx FORMATION MECHANISMS IN RECOVERY FURNACES 19 SUMMARY 19 ARTICLE 20 DISCUSSION 26 The Form of Nitrogen in Black Liquor 28 Fuel NOx vs. Thermal NOX 28 Nitrogen Release During Black Liquor Pyrolysis and Char Burning 29 Depletion Mechanisms 30 A PROPOSED MECHANISM FOR THE DEPLETION OF NOx IN A KRAFT RECOVERY FURNACE 31 SUMMARY 31 ARTICLE 32 DISCUSSION 37 Homogeneous Depletion Reactions 37 Heterogeneous Depletion Reactions 39 KINETICS OF NO DEPLETION BY REACTION WITH MOLTEN SODIUM CARBONATE 41 SUMMARY 41 EXPERIMENTAL APPARATUS 41 ARTICLE 43 DISCUSSION 49 Two-film Theory 49 Heterogeneous Rate Expressions 50 Infinitely Slow Reaction 51 Instantaneous Chemical Reaction 51 Slow Reaction 52 Clues for Identifying Kinetic Regime 52 Derivation of the Pseudo First-order Rate Expression 54 Results 58 Sensitivity Analysis 58 THE RATE OF DEPLETION OF NO BY REACTION WITH MOLTEN SODIUM SALTS 61 SUMMARY 61 ARTICLE 62 DISCUSSION 75 Comparison of Reaction Rate Data 76 Error Analysis 78 THE FATE OF NITROGEN IN A KRAFT RECOVERY FURNACE 79 SUMMARY 79 ARTICLE 80 DISCUSSION 85 CONCLUSIONS 86 RECOMMENDATIONS FOR FUTURE WORK 89 ACKNOWLEDGMENTS 90 LITERATURE CITED 92 APPENDIX 1: NOMENCLATURE 98 APPENDIX 2: MECHANISM FOR SELECTIVE NONCATALYTIC REDUCTION (SNCR) OF NO BY REACTION WITH AMMONIA 101 APPENDIX 3: EXPERIMENTAL APPARATUS AND CALIBRATION METHODS 102 APPENDIX 4: DETERMINATION OF THE ACTIVATION ENERGY FOR THE DEPLETION OF NO BY REACTION WITH Na2CO 3 AND Na2CO3/Na 2S MIXTURES BASED ON AN INFINITELY SLOW REACTION MODEL 106 APPENDIX 5: A COMPARISON OF EXPERIMENTALLY DETERMINED AND CALCULATED VALUES FOR THE GAS FILM MASS TRANSFER COEFFICIENT 108 APPENDIX 6: SAMPLE CALCULATION OF THE PSEUDO FIRST-ORDER RATE CONSTANT FOR THE DEPLETION OF NO BY REACTION WITH MOLTEN SODIUM CARBONATE 112 APPENDIX 7: DETERMINATION OF THE PSEUDO FIRST ORDER-RATE CONSTANT FOR THE DEPLETION OF NO BY REACTION WITH MOLTEN SODIUM CARBONATE 116 APPENDIX 8: DEPLETION OF NO AT VARIOUS INLET CONCENTRATIONS 125 APPENDIX 9: DEPLETION OF NO BY REACTION WITH Na 2CO3 AND Na2CO 3/Na 2S MIXTURES; RATE DATA AND GAS ANALYSIS BY GC 131 APPENDIX 10: GC CALIBRATION DATA 154 APPENDIX 11: A COMPARISON OF PREDICTED AND MEASURED VALUES FOR THE EXIT CONCENTRATION OF NO 163 APPENDIX 12: AG,. FOR THE OVERALL REACTIONS FOR DEPLETION OF NO 168 1 ABSTRACT This study was an investigation of the depletion of NO by reaction with sodium species that are present in a kraft recovery furnace. Thermodynamic calculations identified many reactions that could occur to form sodium nitrate (NaNO3) as a product. Because NaNO3 begins to thermally decompose at temperatures greater than 450°C, it is believed that NaNO3 may not be a final product, but could play a role as an intermediate species in the depletion of NO. The rate of depletion of NO by reaction with molten sodium carbonate and mixtures of sodium carbonate and sodium sulfide has been determined over the temperature range, T = 860 to 973°C. The heterogeneous reaction system has been shown to fit a mixed control reaction model where both mass transfer and chemical reaction kinetics play an important role. The rate of depletion of NO has been shown to follow a pseudo first-order rate expression. Values for the rate constant have been determined experimentally for the depletion of NO by reaction with Na 2CO3 and mixtures of Na2CO 3/Na2S. Analysis of gaseous products indicated that all of the NO that is depleted by reaction with Na 2CO 3 is converted to oxygen (02) and nitrogen (N2). For Na 2CO 3/Na2S mixtures, no oxygen could be detected in the gas stream. It was shown that the oxygen formed by depletion of NO was consumed by the sodium sulfide to form sodium sulfate (Na 2SO 4). Analysis of the nitrogen content of samples taken from industrial furnaces showed that approximately half of the nitrogen entering the furnace in black liquor could not be accounted for. It was therefore assumed that nitrogen leaves the furnace as a gas phase species, e.g., N 2 or 2 NH 3. As noted previously, reactions of NO with Na2CO 3 or Na2S have been shown to reduce NO to N 2. Thus, nitrogen in black liquor could be reduced to N2 by the following pathway: + FuelN -- NO - '/2 N 2 /2 02. Oxidation Reduction by Na Species 3 INTRODUCTION Nitric oxide (NO) and nitrogen dioxide (NO2) can be formed during the combustion of fuels in the presence of nitrogen. These oxides of nitrogen (collectively referred to as NOx) have been shown to be key constituents in reactions leading to photochemical smog and acid rain.'12 For these reasons, the emission of NO, from combustion sources is regulated by both state and federal government agencies. In 1991, results of an analysis of the best available control technology (BACT) options for kraft recovery furnace NOx emissions were published.3 The authors evaluated several control methods including selective catalytic reduction, selective noncatalytic reduction, flue gas recirculation, and proper combustion control. The authors determined combustion control to be the BACT for NOx emissions from a recovery furnace. Therefore, there is a great need to understand recovery furnace operating conditions as they relate to NOx emissions. It has been an ongoing project at the Institute of Paper Science and Technology to develop a three-dimensional, fundamental mathematical model of a kraft recovery furnace. It is intended that the model will predict gas flow patterns, temperature profiles, and species concentration profiles, and ultimately allow the user to predict the effect of changing operational parameters on such things as combustion efficiency, reduction efficiency, corrosion potential, air emissions, etc. To include predictions for NOx emissions in the model, it is necessary to understand the rates of NOx formation and depletion reactions in a recovery furnace environment. 4 NOx emission models have been developed for many types of furnaces and fuels. However, due to the nature of the operation of recovery furnaces, other models would not be applicable for predicting NO, emissions from a recovery furnace. This is primarily due to some of the unique features of black liquor: low fuel nitrogen content (- 0.1%), high moisture content (15 to 35%), and high inorganic composition (- 33% of black liquor solids). The impact of sodium species on NOx emissions has essentially been ignored in the literature. However, black liquor solids are typically on the order of 15 to 20% sodium by weight. It is understood that sodium species react with SO, in a recovery furnace; it was therefore hypothesized that analogous reactions could occur with NO, species. This thesis is primarily a kinetic investigation of the reaction of nitric oxide with molten sodium species. 5 OBJECTIVES The overall objective of this thesis was to provide a better understanding of the depletion of nitric oxide by reaction with sodium species that are present in a kraft recovery furnace. Specific objectives included: * Determine the thermodynamic feasibility of reactions between NOx and different sodium species. * Determine the rate of depletion of NO by reaction with molten Na2CO 3 and mixtures of Na2CO 3/Na2 S over the temperature range, T = 860 to 973°C. * Identify a global mechanism for the depletion of NO by reaction with molten Na2CO3 and mixtures of Na2CO3 /Na2S. 6 ORGANIZATION This dissertation has been written as a series of five articles that have been published and/or presented in a variety of sources. An introduction to the kraft pulping process and overviews of black liquor combustion, recovery boiler emissions, and fume formation chemistry are provided as background material. Each article is preceded by a summary and followed by a discussion of the results presented in the article. The first article presents an overview of NO, formation mechanisms as they relate to black liquor combustion. The article also identifies several research needs that were considered important to provide a better understanding of NOx emissions from recovery furnaces. The concept of depletion of NOx by reaction with sodium species was first introduced in this article. It was suggested that nitrogen in black liquor could be oxidized to form nitric oxide, a fraction of which undergoes further reactions (possibly with sodium species) which reduces NO to N 2. The second article presents results of thermodynamic calculations which were made to illustrate the feasibility of reactions of NOx with different sodium species. While thermodynamics predicted that reactions with sodium species could provide a potential mechanism for the depletion of NO, it was not clear if these reactions occur fast enough to have any effect on NOx emissions from recovery furnaces. Thus, experiments were designed to investigate the rate of depletion of NO. The third and fourth articles present the results of experimental work conducted to determine the rate of depletion of NO by reaction with molten Na 2CO 3 and Na 2CO 3/Na2S 7 mixtures, respectively.