The concept of reactive surface area applied to uncatalyzed and catalyzed carbon (char) gasification in carbon dioxide and oxygen.

摘要

The main objective of this investigation was to explain and/or predict the reactivity variations with conversion (or reactivity profiles) of carbons (chars) gasified in carbon dioxide and oxygen. The virtues of, and/or problems with, utilizing the concepts of total and active surface area to explain the reactivity profiles were evaluated and discussed. An alternative approach, involving the concept of reactive surface area (RSA), was introduced and results based on the direct measurement of RSA were presented. Here, reactive surface area is defined as the concentration of carbon atoms on which the carbon-oxygen C(O) surface intermediate forms and subsequently decomposes to give gaseous products. The transient kinetics (TK) approach gave a direct measurement of RSA for chars gasified in CO{dollar}sb2{dollar} and O{dollar}sb2{dollar}, i.e., gasification rates normalized with respect to RSA were essentially constant over the entire conversion range. A temperature-programmed desorption technique was also used to determine the amount of reactive surface intermediate formed on these chars during gasification in CO{dollar}sb2{dollar} and O{dollar}sb2{dollar}; the results were in good agreement with those obtained by TK. The application of these two independent, but complementary, techniques provided the heretofore elusive quantitative understanding of char reactivity variations with conversion in CO{dollar}sb2{dollar} and O{dollar}sb2{dollar}. A comparison of turnover frequencies for different chars gasified in 1 atm CO{dollar}sb2{dollar} suggested that char gasification may be a structure sensitive reaction.; The concept of RSA was also used to achieve a better quantitative understanding of catalyzed char reactivity variations with conversion in CO{dollar}sb2{dollar}. For a calcium-exchanged lignite char gasified in 1 atm CO{dollar}sb2{dollar}, a poor correlation was found between RSA and reactivity, suggesting that in addition to the direct decomposition of the reactive C(O) intermediate, other processes, e.g., oxygen spillover, contributed to the transient evolution of CO. An extensive study of Saran char loaded with calcium, potassium or nickel by impregnation to incipient wetness (IW) or ion exchange (IE) was undertaken. An excellent correlation was found between reactivity and RSA variations with conversion for both IW and IE K-catalyzed chars, suggesting that TK indeed titrates the reactive K-O-C complexes formed during gasification in CO{dollar}sb2{dollar}.