KUNGL TEKNISKA ISRN KTH/MSE--02/12--SE+ENERGY/AVH ISBN 91-7283-301-7 HÖGSKOLAN Black Liquor Combustion in Kraft Recovery Boilers-Numerical Modelling Doctoral thesis by Reza Fakhrai STOCKHOLM DEPARTMENT OF MATERIAL SCIENCE AND ENGINEERING May 2002 DIVISION OF ENERGY AND FURNACE TECHNOLOGY ROYAL INSTITUTE OF TECHNOLOGY SE - 100 44 STOCKHOLM Black Liquor Combustion in Kraft Recovery Boilers-Numerical Modelling Doctoral thesis by Reza Fakhrai Department of Material Science and Engineering Division of Energy and Furnace Technology Royal Institute of Technology SE-100 44 Stockholm Sweden Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm, framlägges för offentlig granskning för avläggande av teknisk doktorsexamen, onsdagen den 29 maj 2002 kl. 10.00 i E1, Elektroteknik, Kungliga Tekniska Högskolan, Stockholm ISRN KTH/MSE--02/12--SE+ENERGY/AVH ISBN 91-7283-301-7 i Black Liquor Combustion in Kraft Recovery Boilers-Numerical Modelling Reza Fakhrai Dissertation for the degree of doctor of Philosophy in Energy and Furnace Technology (TeknD) 2000 Royal Institute of Technology Department of Material Science and Engineering Division of Energy and Furnace Technology S-100 44 Stockholm, Sweden Abstract Black liquor is a by-product, which results from digestion of wood chip in alkaline pulping processes. After the evaporation process the solid in black liquor increases up to 80% and it is combustible. Black liquor is conventionally burned in a large unit called Kraft recovery boiler, for the dual purposes of energy production and recovery of the pulping chemicals. Kraft recovery boiler model in the present context refers to the numerical simulation for solving partial differential equations governing the characteristics phenomenon in a Kraft recovery furnace. The model provides an analytical tool and it is best appreciated when the numerical simulations and the measurement techniques are linked to the real industrial problem and the industry that used it. The purpose of this study was to enhance the understanding of the processes involved in a Kraft recovery furnace through mathematical modelling and to keep the code in state-of-art. It is essential to consider the path of every drop in the cavity of a Kraft recovery furnace. In this process the liquor may accumulate on the walls and depending on the gravity or the flow pattern at the wall it periodically sloughs off and falls to the bed. As new components in the general framework of the Kraft recovery boiler model, the interaction of burning drops and walls in a recovery boiler considering the above mentioned was modelled. The importance of a bed model and its effect on the predicted temperature in the furnace cavity was examined. The heterogeneous conditions in a Kraft recovery furnace with significant local variation of concentration of constituent gas components and temperature level/gradient could affect NO production rate. The NOx model developed in this work considers the NO formation from fuel NO and prompt NO. It is assumed that the fuel nitrogen in black liquor is released either via devolatilization or char combustion. Further work focussed on estimation of the temperature level near/on the bed based on the mass distribution on the char bed. The model was also used to examine the effects of changes in black liquor properties (used in Kraft recovery furnace model) namely effect of the swelling and solid content versus the Kraft recovery furnace performance. The results illuminate also the potential of numerical modelling method as a promising tool to deal with the complicated combustion processes even for practical application in the industry. ISRN KTH/MSE--02/12--SE+ENERGY/AVH ISBN 91-7283-301-7 ii Supplements The work presented here this thesis is mainly based on the following publications, referred by Roman numerical I. Theoretical Analysis of Interaction between Fuel Drop and Walls during Black Liquor Combustion in A Kraft Recovery Furnace. Energy conversion & management- an International Journal, RAN2001 Special Issues, Nagoya, Japan Dec. 14-17, 2001 II. Combustion Performance of the Kraft Recovery Boiler Versus Black Liquor Properties – Numerical Study, Submitted to Energy Conversion & Management III. Use of a Computer Model for Evaluation of Combustion and NOx Control Alternatives in a Kraft Recovery Boiler, International Chemical Recovery Conference, Tappi, Tampa Florida June 1-4 1998 iii Acknowledgment The work in this thesis has been performed within the Energy and Furnace technology, Material Science department, Royal institute of technology (KTH) Stockholm Sweden, during the years 1997-2002. Energi Myndigheten (STEM), Ångpannföreningen forskningstiftelse (ÅF) and Värmeforsk financed the project. I wish to convey my sincerest thanks to the leader of the Energy and Furnace division, Associate Professor Wlodek Blasiak, for giving me the opportunity to work in the group and to write my thesis under his direction, and for constantly providing me encouragement, guidance and moral support. I appreciate his commitment and utmost professionalism in all regards. I also thank the other members of the Material Science department Prof. Seshadri Seetharaman, Prof. Pär Jönsson who made me feel like a colleague more than a student during the years I have worked at the department. I would also like to take the opportunity to thank my co-workers Jan Bong, Simon Lille who were my friends when I needed one. I would like to thank my family, particularly my father, Abas who passed away 1999 and my mother Golestan, for their support and encouragement throughout my long academic career. Finally, my heartfelt thanks to my dear wife “Neda” for her patience, concern and understanding, my son “Sam” and my daughter “Ella Mina” who inspired me to aim high and motivated me to achieve it. iv Table of Contents Abstract I Supplements II Acknowledgements III Table of Contents 1V Nomenclature VII Thesis Summary 1. Introduction 1 1.1 Black Liquor 2 1.2 Recovery Furnace 2 1.3 Objectives 4 2. Liteature Review 5 2.1 Flow Field 5 2.2 The In-flight Combustion of Balck Liquor Droplets 6 2.3 The Char bed 8 2.4 Pollutant Formation 9 2.5 Gird Generation and Geometry 11 2.6 Model Validation 12 2.7 Concluding Remarks 13 3. Development of Model of Recovery Furnace Used in This Work 14 3.1 Geometry of the Furnace Used in This work 14 3.1.1 Conventional Firing Strategy (CF) 15 3.1.2 Rotational Firing Strategy (RF) 17 3.2 Black Liquor Combustion 20 3.2.1 Spray 20 3.2.2 In-flight Drop Combustion 23 3.2.3 Char Bed Processes 24 3.2.4 The Interaction Between Drops and the Walls in the Recovery Furnace 24 3.3 NO Modeling 28 3.3.1 Thermal NO 28 3.3.2 Turbulence Chemistry Interaction 29 3.3.3 Prompt NO 30 3.3.4 Fuel NO 31 4. Result and Disussion 33 4.1 Estimation of Surface Temperature and Mass Distribution on a Char Bed 33 v 4.2 Effect of the Physical Properties of Liquor on The Furnace Performance 36 4.2.1 Effect of the Swelling on the Path of a Droplet 36 4.3 Effect of the Wall-burning Model on the Overall Recovery Furnace Model performance 40 4.4 Prediction of the NO Level in Recovery Furnace 42 5. Conclusions 46 References 48 Appendix vi Nomenclature 2 Aint Internal surface area of carbon in the bed (m ). The value of the internal surface area of Kraft chars was taken from the experimental data obtained with two experimental chars obtained 2 Asp Internal specific surface area of char, taken as 11000, m /kg 3 CC, bed Molar concentration of carbon in the bed, mol/m CSO4 Sulphate concentration, mol SO4/mol Na2 CC Carbon concentration, mol C/mole Na2 3 CNa2, bed Sodium concentration, moles Na2/bed volume, mol/m Md Mass of the drop/parcel, kg M char, Na Total amount of sodium in the drop parcels, kg M char, S Total amount of sulphur in the drop parcels, kg. Md0 Initial mass of the drop/parcel, kg Mi, Mj Molecular weight of corresponding gas species Pg Local pressure of gas mixture at the cell adjacent to the char bed, bar RO2, overall Overall oxidation rate, RCO2, overall Overall CO2 gasification rate RH2O, overall Overall H2O gasification rate 3 2 V bed Char bed volume per unit surface area (m /m ), XH20 Mass fraction of water XVM Mass fraction of volatiles XC Mass fraction of char XSmelt Mass fraction of smelt XNa, XS Initial mass fraction of sodium and sulphur in the drop parcel. YCO2, YH2O Molar fractions of carbon dioxide, water vapour, the cell adjacent to the char bed. YCO, YH2 Molar fractions of carbon monoxide, and hydrogen at the cell adjacent to the char bed. YO2 Local oxygen mole fraction at the cell adjacent to the char bed. β Multiplier for Cameron-Grace reduction rate η Sumnicht factor, for carbon surface area available for direct oxidation. and has a value of 0.6, which corresponds to 50% completion of sulphur reduction58 from the Institute of Paper Chemistry’s drop tube furnace, which had a value of about 11 m2/g. vii 1. Introduction In the 1970’s the paper industry in Sweden started to make large scales structural changes to the production of paper. These modifications continued during the 1980’s and 1990’s. The number of the paper making units was 47 in 1998, a reduction by 8 units since 1988. Nevertheless, the total energy used by the industry increased because of environmental regulation [1]. The industry is almost independent on oil, but it requires heat for evaporation plant. Higher electricity costs as a result of nuclear phase-out seems to be the real threat to the industry [2].