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Signature Redacted Signature of Author: Department of Chemical Engineering January 1976 Signature Redacted Certified By: Adel F THE BEHAVIOR OF ASH IN PULVERIZED COAL UNDER SIMULATED COMBUSTION CONDITIONS by ASHOK S. PADIA B.E. (Hons.), Birla Institute of Technology and Science Pilani, India (1970) S. M., Massachusetts Institute of Technology (1971) Ch.E., Massachusetts Institute of Technology (1973) Submitted in partial fulfillment of the requirements for the degree Doctor of Science at the Massachusetts Institute of Technology January 1976 Signature Redacted Signature of Author: Department of Chemical Engineering January 1976 Signature Redacted Certified by: Adel F. Sarofim, Thesis Supervisor- Signature Redacted J~ck B. Howard, Thesis Supervisor Approved by: Signature Redacted Archives Glenn C. Williams, Thesis Supervisor Chairman, Departmental Committee on Graduate Students MAY 28 1976 2. THE BEHAVIOR OF ASH IN PULVERIZED COAL UNDER SIMULATED COMBUSTION CONDITIONS by ASHOK S. PADIA Submitted to the Department of Chemical Engineering on January 14, 1976 in partial fulfillment of the requirements for the degree of Doctor of Science. ABSTRACT A systematic study of the physical and chemical behavior of mineral matter during rapid combustion of pulverized coal particles was performed by first completely characterizing the distribution of mineral matter in the coal with respect to its composition and particle size distribution, and then oxidizing the coal particles in a preheated drop-tube furnace. Two types of coals, a Montana lignite and a Pittsburgh seam bituminous, pulverized and size graded into two fractions 38-45pm and 75-90pm, were used in the study. In order to obtain the distribution of mineral matter in the original coal, mineral matter was separated from coal samples in its essentially unaltered state by low temperature ashing. Major minerals present in the coals were identified by x-ray diffraction patterns. Examination of the mineral matter under a scanning electron microscope with simultaneous energy dispersive x-ray analysis to obtain elemental composi- tion made it possible to characterize the composition and particle size distribution of the mineral matter in coal. The major minerals in coal consisted of kaolinite [Al2 Si 0 (OH)A] ranging from 1 to 5pm platelets (2.5ym mass median Mie), carbonates and sulfates mainly of calcium of about 2pm mass median size and pyrites [FeS 2 ] about lym mass median size. The overall mass median size of low temperature ash was determined to be about 2pm. The high temperature behavior of mineral matter from both coals was studied in a drop tube furnace operated at tempera- tures up to 1830*K with an oxidizing atmosphere. The particle residence time was about 1 second and the heating rates were of the order of 10 5 *C/second. The weight loss of mineral mat- ter and the particle size distribution of ash were measured as a function of temperature, coal type and coal size. 3. Loss of about 30 percent by weight of the mineral matter at temperatures up to the ASTM ashing temperature (7500 C) can be attributed to the thermal decomposition of kaolinite and carbonates and the oxidation of pyrites. Additional loss of about 25 percent by weight of ASTM ash for lignite and about 6 percent for bituminous was observed when the ash was heated to 1830*K. This additional loss was contributed mainly by CaSO 4 decomposition and some (about 3 percent) silica vapori- zation. These results of thermal decomposition of kaolinite, carbonates and sulfates and oxidation of pyrites are supported by successive diffraction patterns obtained for each sample. Significant effects of combustion conditions (oxidizing or inert) are observed on the weight loss of ash. During heating the individual mineral particles fuse at about 1000-1200 0 K and agglomerate as the carbon burnout pro- gresses. Surface tension forces are high enough to retain the fused ash particle on the burning char surface. If all the mineral matter particles in one coal particle agglomerate without separation from char surface during combustion, this would result in the formation of 1 ash/coal particle. The experimental observationssuggest that about 3 ash particles/ coal particle for lignite and 5 ash particles per coal parti- cle for bituminous coal are produced. Combustion temperature from 1250 to 1830 0 K did not have any noticeable effect on the resulting particle size distribution. The mean particle size of ash produced was proportional to the mean particle size of parent coal particles. Physical observations of burning be- havior of lignite and bituminous coals provide evidence of fragmentation of char particles and formation of hollow spheres (cenospheres) of char in the case of bituminous coals. The formation of more than 1 ash particle per coal particle is consistent with the observed physical behavior of the char during oxidation. Ash also formed cenospheres due to evolution of gas by thermal decomposition. The formation of cenospheres resulted in a distribution of specific gravities of ash ranging from less than 1 to 3.5. The formation of cenospheres is governed by the viscous relaxation of the fused ash and the kinetics of gas evolution. The temperature range in which cenospheres were formed could be explained by use of a simple theoretical model. Some experiments performed with silica and carbon in inert atmosphere up to 2250*K temperature showed that SiC formed at high temperatures. Silica also seemed to volatilize in the form of more volatile SiO which disproportionates into SiO 2 and Si. Condensation products about 0.5pm in size, probably due to disproportionation of SiO, were observed on the cooler portions of the furnace. 4. The implications of the above findings to ash vaporization and heat transfer pertinent to MHD combustors are also outlined. Recommendations have been made for further work. Thesis Supervisors and Titles: Professor Adel F. Sarofim, Professor of Chemical Engineering Professor Jack B. Howard, Professor of Chemical Engineering Professor Glenn C. Williams, Professor of Chemical Engineering 5. Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts 02139 January 1976 Professor Irving Kaplan Secretary of the Faculty Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Dear Professor Kaplan: In accordance with the regulations of the faculty, I herewith submit a thesis entitled "The Behavior of Ash in Pulverized Coal under Simulated Combustion Conditions", in partial fulfillment of the requirements for the degree of Doctor of Science in Chemical Engineering at the Massachusetts Institute of Technology. Respectfully submitted, Signature Redacted Ashok S. Padia 6. ACKNOWLEDGEMENTS I wish to express my deepest gratitude to my thesis supervisors, Professor A.F.Sarofim, Professor J.B.Howard and Professor G.C.Williams. Their invaluable contribu- tions and continued encouragement are most appreciated. Thanks are also due to Professor J.F.Elliott and Profes- sor H.C.Hottel for their valuable comments and suggestions during committee meetings. The Office of Coal Research made funds available for this research. Their support is gratefully acknowledged. Professor J. P. Longwell first introduced me to low temperature ashing technique, for which I am extremely grateful. Thanks are due to Professor C. Hamrin of the University of Kentucky for providing me with low tempera- ture ash samples of our coals and due to LFE Corporation, Waltham, Massachusetts, for letting me use their low temperature asher. I am grateful also to Professor R.E. Ogilvie, Dr. Kaplesh Kumar and Joe Adario of the Depart- ment of Metallurgy for their assistance on x-ray diffrac- tion studies. Leonard Suddenfield and Dr. Allen Parks are acknowledged for their assistance in scanning electron microscopy and electron microprobing, respectively. Thermosystem Incorporation demonstrated their laser doppler anemometer which made it possible to measure particle velocities. Their courtesy is sincerely appreciated. 7. I am extremely grateful to my colleague Hisashi Kobayashi for his assistance and superb cooperation during the whole thesis. Group efforts with Hisashi Kobayashi, John H.Pohl and Jerry Mandel during experimental investigations were en- joyable and fruitful. Robert Sherry provided assistance in atomic absorption spectroscopy, his help is kindly noted. Discussion and associations with Dr. Benno L. Wersborg, Dr. Sanjay Amin, Dr. Parvez Wadia, Dr. J. Shukla, Franklin Wong, Dr. Reggie Mitchell et al were most helpful and rewarding. Stanley R. Mitchell is to be thanked for his many technical suggestions and his efforts in the preparation of this manuscript. Sincere thanks are extended to Mrs. Elenore D. Kehoe for the excellent typing of my thesis. I am most indebted to my parents and parents-in-law for their continuous confidence and support throughout my studies. Finally, my deepest praise is for my wife, Usha, whose patience, understanding and encouragement were an immeasurable support. Not only did she spend considerable time performing particle size analyses, but provided me the love, affection and encouragement during difficult times. 8. Dedicated to my family 9. TABLE OF CONTENTS Page CHAPTER 1 SUMMARY 23 1.1 Introduction 23 1.1.1 Background and Motivation 23 1.1.2 Previous Studies 24 1.1.3 Method of Approach 25 1.2 Apparatus and Procedure 26 1.2.1 Mineral Matter Characterization 26 1.2.2 Drop-tube Furnace 26 1.2.3 Crucible Experiments 28 1.2.4 Analysis 30 1.2.5 Coals Studied 30 1.3 Results 31 1.3.1 Characterization of Mineral Matter in Pulverized Coal 31 1.3.2 Chemical Transformations 41 1.3.3 Physical Transformations
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