AN ABSTRACT OF THE THESIS OF Viboon Sricharoenchaikul for the degree of Master of Science in Chemical Engineering presented on February 24, 1995. Title: Sulfur Species Transformations and Sulfate Reduction During Pyrolysis of Kraft Black Liquor. Redacted for Privacy Abstract approved: William J. Frederick Sulfur species in the char/ash residue and the product gases were measured during rapid pyrolysis of black liquor solids. The experiments were carried out in a laminar entrained-flow reactor (LEFR) in which particle heating rates in excess of 10,000 °C/s were obtained. The experimental conditions were 700 - 1100 °C and 0.3 - 1.7 s residence time, in a nitrogen atmosphere. The solid residue produced was collected in a nominal 3 pm cutoff cyclone. The water-soluble species were analyzed for hydrosulfide, sulfate, sulfite, and thiosulfate ions with a CES. The product gases were analyzed for COS, CS2, SO2, and organosulfur compounds with a FT-IR. H2S was analyzed by adsorption in a cadmium acetate solution and titration. Thiosulfate decomposed rapidly and completely at a reactor temperature of 900 °C and above within the shortest residence time (0.3 s) that can be achieved with this LEFR setting. Sulfite was present at short residence times and then decomposed after that. Sulfate was not reduced at 700 °C and an induction period was observed for the reduction at 900 and 1000 °C. No sulfide was detected at 700 °C while it formed rapidly after an induction period at 900 and 1000 °C. Sulfur is apparently converted from gaseous species to sulfide. In gaseous sulfur products, large amounts of mercaptans and H2S were formed rapidly. Much smaller amounts of CS2, COS, and SO2 were also detected. All of those species were formed at short residence times and then disappeared afterward. It was proposed that the mechanism of sulfur species transformations in black liquor involves rapid decomposition of thiosulfate to mostly elemental sulfur. The elemental sulfur produced then reacts with hydrocarbons to produce organic sulfur compounds, mostly mercaptans. A number of subsequent reactions produce H2, H2S, (CH3)SH, (CH3)2S, CS2, and other R-CH2SxH. For the sulfate reduction mechanism, it is likely that sulfate exchanges oxygen atoms with fully reduced catalytic sites and forms sulfite as an intermediate. The sulfite produced is further reduced to sulfide. The rate of sulfate reduction obtained was similar to the rate calculated using a sulfate reduction model developed by Cameron and Grace. Sulfur Species Transformations and Sulfate Reduction During Pyrolysis of Kraft Black Liquor by Viboon Sricharoenchaikul A THESIS submitted to Oregon State University in partial fulfillment of the requirement for the degree of Master of Science Completed February 24, 1995 Commencement June 1995 Master of Science thesis of Viboon Sricharoenchaikul presented on February 24, 1995 APPROVED: Redacted for Privacy Major Pro sor, representing Chemical Engineering Redacted for Privacy hair of ent of Chemical Engineering Redacted for Privacy Dean of Graduat6 chool I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Redacted for Privacy Viboon Sricharoenchaikul, Author ACKNOWLEDGMENT I would like to take this opportunity to express my appreciation to the following people and organization, this work would never be accomplished without their support. Dr. Frederick, for his valuable time and superb guidance throughout the project. Narongsak Jivakanun, who encouraged me to start on this work and helped me on FT-IR measurement. Scott Sinquefield and Kaj Wag, for helping me with the experiments. My parents, brothers, sister and Natty for always being there for me. Finally, this work is funded by US Department of Energy. TABLE OF CONTENTS Page 1. INTRODUCTION 1 2. THESIS OBJECTIVES 6 3. LITERATURE REVIEW 7 3.1 CHARACTERISTIC AND COMPOSITION OF BLACK LIQUOR SOLIDS 7 3.2 BLACK LIQUOR DROPLET COMBUSTION 8 3.3 THE CHEMISTRY OF SULFUR DURING PYROLYSIS OF BLACK LIQUOR 9 3.3.1 Sulfur Volatilization During Pyrolysis of Black Liquor 10 3.3.2 Reactions of Sodium Sulfide 13 3.3.3 Reactions of Sodium Thiosulfate 14 3.3.4 Reactions of Sodium Sulfate 16 3.3.5 Reactions of Sodium Sulfite 17 3.3.6 Reaction of Organic Sulfur 18 3.4 REDUCTION OF SULFATE 19 4. ANALYTICAL METHODS 22 4.1 CHAR COLLECTION AND SULFUR EXTRACTION 22 4.2 CAPILLARY ELECTROPHORESIS ANALYSIS OF SULFUR SPECIES 23 4.3 TESTING FOR LINEARITY OF THE CALIBRATION CURVE OF SULFUR ION SPECIES 27 4.4 COMPARING BETWEEN ANTIOXIDANT AND DEOXYGENATED WATER 31 TABLE OF CONTENTS (Continued) Page 4.5 FOURIER TRANSFORM INFRARED SPECTROMETER FOR ANALYSIS OF SULFUR GASEOUS SPECIES 33 5. EXPERIMENTAL PROCEDURE FOR PYROLYSIS EXPERIMENTS 41 5.1 EXPERIMENTAL SET-UP 41 5.2 EXPERIMENTAL CONDITIONS 44 5.3 EXPERIMENTAL PROCEDURE 45 5.3.1 Pre-run Preparation 46 5.3.2 Pyrolysis Runs 47 5.3.3 Collecting Data, Char, and Gas Sample 48 6. RESULTS AND DISCUSSION 50 6.1 CHAR SAMPLE ANALYSIS 50 6.2 GAS SAMPLE ANALYSIS 51 6.3 SULFUR MASS BALANCE CLOSURE ANALYSIS 57 6.4 DETAILED DISCUSSION ON CHAR AND GASEOUS SULFUR SPECIES 59 6.4.1 Thiosulfate 59 6.4.2 Sulfate 61 6.4.3 Sulfite 62 6.4.4 Sulfide 63 6.4.5 Hydrogen sulfide (H2S) 65 TABLE OF CONTENTS (Continued) Page 6.4.6 Mercaptans and their derivatives 67 6.4.7 Carbon disulfide (CS2) 69 6.4.8 Carbonyl sulfide (COS) 71 6.4.9 Sulfur dioxide (SO2) 72 6.5 SULFATE REDUCTION 72 6.6 PROPOSED SULFUR SPECIES TRANSFORMATION MECHANISM (PATHWAY) 75 7. SUMMARY AND CONCLUSIONS 78 8. RECOMMENDATIONS AND FUTURE WORK 81 BIBLIOGRAPHY 82 APPENDICES 86 LIST OF FIGURES Figure Page 1.1 Fuel production world-wide per year 2 1.2 Simplified schematic diagram of pulping and kraft recovery process 2 1.3 Recovery boiler based on conventional Tomlinson design 3 1.4 Major components of black liquor solids 4 3.1 Stages of black liquor droplet combustion 12 3.2 The sulfate-sulfide cycle 20 4.1 Basic schematic diagram of capillary electrophoresis 23 4.2 Test for sulfate peak with different voltage between poles 24 4.3 Comparison between using 50 vs 60 cm capillary length 26 4.4 Comparison among 190, 210, and 250 nm wavelengths 28 4.5 Calibration curve for thiosulfate ion 29 4.6 Calibration curve for sulfate ion 29 4.7 Calibration curve for sulfite ion 30 4.8 Calibration curve for hydrosulfide ion 30 4.9 Schematic diagram of FT-IR spectrometer 35 4.10 Typical spectrogram acquired from scanning pyrolysis gas product 36 LIST OF FIGURES (Continued) Figure Page 4.11 C2H4 spectrum overlaps with CH3OH spectrum 38 4.12 C2H4 standard spectrum 38 4.13 Spectrum after subtraction of C2H4 spectrum 38 4.14 Spectrogram prior to subtraction of the water vapor spectrum 39 4.15 Water vapor standard spectrum 39 4.16 Final spectrogram, revealing others hidden peaks 39 4.17 Methyl mercaptan standard spectrum 40 4.18 Dimethyl sulfide standard spectrum 40 5.1 A simplified schematic diagram of the experimental set-up 42 5.2 Laminar entrained flow reactor 43 5.3 Black liquors solids temperature history 45 6.1 Sulfur ion species transformation at 700 °C 53 6.2 Sulfur ion species transformation at 900 °C 53 6.3 Sulfur ion species transformation at 1000 °C 54 6.4 Sulfur ion species transformation at 1100 °C 54 6.5 Sulfur gas species formed at 700 °C 55 LIST OF FIGURES (Continued) Figure Page 6.6 Sulfur gas species formed at 900 °C 55 6.7 Sulfur gas species formed at 1000 °C 56 6.8 Sulfur gas species formed at 1100 °C 56 6.9 Sulfur mass balance closure 57 6.10 Thiosulfate vs. residence time 59 6.11 Sulfate vs. residence time 61 6.12 Sulfite vs. residence time 63 6.13 Sulfide vs. residence time 64 6.14 H2S formed vs. residence time 66 6.15 Mercaptans formed vs. residence time 67 6.16 CS2 formed vs. residence time 69 6.17 COS formed vs. residence time 70 6.18 SO2 formed vs. residence time 71 6.19 Comparison between Cameron and Grace's model and experimental results 73 6.20 Reaction diagram for sulfate reduction involving carbonate decomposition 74 6.21 The proposed mechanism of sulfur species transformation 76 LIST OF TABLES Table Page 3.1 Elemental analysis of a typical southern pine black liquor 7 3.2 Analysis of sulfur species from southern pine black liquor 8 3.3 Rate constants and empirical values for equation 3.33 21 4.1 Analysis condition for CES operation 27 4.2 Typical sulfur ion species used in the experiments 29 4.3 Comparing between antioxidant and deoxygenated water 32 4.4 Analysis results from replicate runs for sulfur species in char for runs at 900 °C and 0.7 s particle residence time 33 4.5 Operating specification for FT-IR 34 4.6 Gas products detected from pyrolysis of black liquor solids 36 5.1 Experimental conditions 44 LIST OF APPENDICES Page APPENDICES 86 APPENDIX A MEASUREMENT OF H2S IN THE PRODUCT GAS BY ABSORPTION IN AQUEOUS CADMIUM ACETATE 87 APPENDIX B PREPARING CALIBRATION CURVE FOR FTIR ANALYSIS OF SULFUR GASEOUS SPECIES 90 APPENDIX C FTIR CALIBRATIONS 92 FTIR COS Calibration (carbonyl sulfide) 92 FTIR (CH3)2S Calibration (dimethyl sulfide) 94 FTIR CH3SH Calibration (methyl mercaptan) 96 APPENDIX D GAS ANALYSIS DATA 98 APPENDIX E CHAR ANALYSIS DATA 118 1. INTRODUCTION The kraft pulping recovery process is the most important chemical recovery process world-wide.