Kinetics And Modeling Of The Oxidative Pretreatment Of Coal

Author : Guang Shi
Publisher :
ISBN 13 :
Total Pages : 364 pages
Book Rating : 4.:/5 (748 download)

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Download or read book Characterization of Early Stage Coal Oxidation and Effects of Drying and Supercritical CO2 Pretreatment on Chemical Structure of Coals written by Guang Shi and published by . This book was released on 2010 with total page 364 pages. Available in PDF, EPUB and Kindle. Book excerpt: To obtain representative temperature-programmed desorption (TPD) profiles of young oxidized chars up to 1650°C with minimal reactor wall interferences, the chemistry and physics of four ceramic materials has been critically reviewed. A two-staged experimental apparatus is then uniquely designed to produce chars in an Al 2 O 3 flow reactor with 1 to 21% O 2 followed by in-situ TPD with a SiC tube. Comparison of TPD profiles of oxidized chars with those from pyrolyzed chars and ashes suggests early stage char oxidation is profoundly influenced by oxygen from three sources: organics oxygen, mineral matters and gas phase O 2 . Young chars oxidized at 1000°C with less than 0.3 s residence time shows CO desorption peaks during TPD at three distinct temperatures: 730, 1280, and 1560°C. The peaks at 730°C are mainly caused by incomplete devolatilization. The peaks at 1280°C represent mainly desorption of stable surface oxides and incomplete devolatilization. Increasing the gas phase oxidants notably increases the amount of stable surface oxides. The broad peaks between 1400 and 1650°C are attributed to the reactions of oxidants decomposed from minerals and carbon in the char or SiC tube. Gas-phase oxygen shifts these reactions to lower temperatures. Detailed oxygen balance based on the CO and CO 2 yields and elemental compositions of both pyrolysis and oxidized chars reveals that oxygen uptakes are very high, +0.056 mg O per mg of carbon, in chars derived from bituminous coal, while lignite chars show negative oxygen uptake, -0.020 mg O per mg of carbon, in char. Indeed, lignite char seems to possess little amount of stable surface oxides other than those contributed by the minerals. The extensive emissions of CO from lignite chars during TPD seem to suggest that either O 2 or minerals promotes the oxygen transfer on char surface and subsequent carbon oxidation. The studies on the ignition, devolatilization and combustion kinetics of chars in oxy-coal combustion reflect the fundamental importance of oxidant-activated mechanisms in the early stage of char oxidation. Independently, temperature-programmed desorption (TPD) has revealed sensitive characteristics of chars oxidized by 1-21% O 2 . It demonstrated the existence of stable surface oxides that desorb between 1100-1650°C. Interactions between CO 2 and coal have been gaining interests due to its application in geologic sequestration of CO 2 and methane recovery in coalmines. Recently, we reported the changes in combustion behaviors, including NO emissions and carbon burnout, and physical structures of coals after treatment. The objective of the third part of my work is to investigate the changes in chemical structure of coals after supercritical CO 2 treatment. We found that drying and pretreatment can alter the CO and CO 2 emissions during coal pyrolysis. FTIR analysis shows that oxygen functional groups were found to be significantly changed after drying and/or pretreatment by supercritical CO 2 for both bituminous coal and lignite. These phenomena may significantly affect the conversion of coal gasification and liquefaction.