The Kinetics Of The Steam Activation Of Coal Char

Author : E. M. Suuberg
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (316 download)

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Download or read book STUDY OF ACTIVATION OF COAL CHAR. written by E. M. Suuberg and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: This is the final report on a project whose aim is to explore in a fundamental manner the factors that influence the development of porosity in coal chars during the process of activation. It is known that choices of starting coal, activating agent and conditions can strongly influence the nature of an activated carbon produced from a coal. This project has been concerned mainly with the process of physical activation, which in fact involves the gasification of a char produced from a coal by oxidizing gases. This is of interest for two reasons. One is that there is commercial interest in production of activated carbons from coal, and therefore, in the conditions that can best be used in producing these materials. Much is already known about this, but there is a great deal that is in the realm of ''trade secret'' or just ''industry lore''. The second reason for interest in these processes is that they shed light on how porosity develops during any gasification process involving oxidizing gases. This has implications for understanding the kinetics and the role that ''surface area'' may play in determining kinetics. In earlier reports from this project, several conclusions had been reached upon which the present results rest. There is an often-cited difference in use of nitrogen and carbon dioxide as molecular probes of carbon porosity when using vapor adsorption techniques. Carbon dioxide is often ''preferred'' since it is argued that it offers greater access to fine microporosity, due to the higher temperature of carbon dioxide as opposed to nitrogen measurements. The early phases of this work revealed that the extreme differences are observed only in chars which are not much activated, and that by a few weight percent burnoff, the difference was negligible, provided a consistent theoretical equation was used in calculating uptake or ''surface area''. In another phase of this study, it was noted in a preliminary way how the use of different oxidizing environments would lead to very different porosity development in the same char. There did not seem to be a link to the overall inherent reactivity of the gas-char combination to the pattern of porosity development. In another portion of this study, it was observed that the expected pattern of porosity development could be seen, as a function of whether the process was carried out in a pure chemical kinetic control regime (Zone I) or in a partially mass transfer control regime (Zone II). This portion of the study was useful in suggesting that the unburned carbon from many practical pulverized coal combustion processes had actually emerged from a Zone II environment. This confirms other published hypotheses, and strongly suggests that the material does not survive the boiler environment because it was produced in a purely oxygen mass transfer limited zone (so-called Zone III) or because it was simply so unreactive that it could not burn up in the allotted time (a pure Zone I argument). Moreover, it is believed that the very rapid initial opening of porosity that is revealed by the rapid disappearance of nitrogen and carbon dioxide accessible porosity may be associated with a very thin surface layer of pyrolytically-formed carbon that effectively blocks the bulk char structure from nitrogen. Once removed by low extent of burn-off this phenomenon disappears. Finally, the project turned to comparing the relative influences of the starting coal and the oxidizing environment on the nature of porosity that was developed. Once again, the Argonne Premium coal suite served as a source of chars that would be representative of the broad range of coals found an utilized in the US. The conclusion is that the starting coal has a profound influence upon the ability of an oxidizing agent to develop porosity in the char. This is the single most important factor. Beyond this, however, there was a surprise to the extent that the ordering of porosity development did not follow a simply predictable pattern related to the reactivity of the activating agents. Oxygen is a very effective activating agent, if operation can be kept under control under so-called Zone I conditions. Its effectiveness is comparable to that of the more widely-employed steam.