Interpretation of Char Reactivity Profiles Obtained Using a Thermogravimetric Analyzer Prabhat Naredi and Sarma V. Pisupati* Department of Energy and Mineral Engineering, Penn State UniVersity, 110 Hosler Building, UniVersity Park, PennsylVania 16802 ReceiVed July 17, 2007. ReVised Manuscript ReceiVed October 18, 2007 Accurate knowledge of char oxidation rate parameters is of paramount importance in modeling coal combustion behavior. Generally, an isothermal thermogravimetric analyzer (TGA) is used to determine the intrinsic rate parameters. However, despite operating in a kinetic controlled regime, i.e., at relatively lower reaction temperatures, the char reactivity profile (reaction rate versus burnoff) has often been observed to go through a maximum. The existence of a maximum in the reactivity profile poses a difficulty in extracting the intrinsic rate parameters. This present paper attempts to investigate the conditions that may lead to such a maximum and recommends a method to deduce the kinetic parameters from such a profile. To address this matter, a high volatile bituminous coal is pyrolzyed in a drop tube reactor (DTR) at different furnace temperatures and in a TGA. Physical and structural properties of the char samples generated in DTR were measured, and the reactivity of the resultant char samples was determined in a TGA. The results demonstrate that both the pyrolysis history and char oxidation temperature have a significant effect on the shape of the rate profile obtained in a TGA. It is proposed that such an occurrence of a maximum is due to a transition from a intraparticle diffusion controlled zone to a kinetic controlled zone. This was further confirmed by observation of similar estimated activation energy values at several conversion levels after the maximum. 1. Introduction Thermogravimetric analysis is one of the most convenient and widely used methods for analyzing the kinetics of gas–solid reactions. 1 This method relies on the measurement of temporal variation of the sample mass as the reaction occurs, using a thermogravimetric analyzer (TGA). The resulting curve of variation of the sample mass with time is then represented in terms of a rate-conversion curve (reactivity profile). Although extrapolation to other systems on a larger scale cannot be directly performed, the TGA is very useful from a fundamental viewpoint and for a comparison of reactivity between samples treated under different conditions. Isothermal reactivity at a lower temperature is widely used to characterize the char reactivity to predict the behavior at higher temperatures. Nonetheless, even while operating at lower temperatures and establishing a kinetic controlled zone in a TGA, in most of the reactivity plots, a maximum is observed. The rate is observed to increase with conversion up to a certain burnoff and then decrease with conversion. However, from a fundamental point of view, the rate should only decrease with conversion in a kinetic controlled zone. The initial part of the burnoff curve up to the maximum has been suggested to be due to the building up of partial pressure, 2 opening of the previously closed pores, 2–6 or a balance between the mass gain because of stable complex formation and the mass loss because of carbon gasification. 7,8 Because of the presence of a maximum in the rate profile, various reactivity parameters, such as average reactivity, 9 maximum reactivity, 2,10 and reactivity at a fixed conversion 11,12 have been used in the literature to obtain the rate parameters. This issue is raised by many researchers, 9,13 and the uncertainty of a reliable method may result in a significant scattering of * To whom correspondence should be addressed. Telephone: 1-814- 8650874. Fax: 1-814-8653248. 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