ARTICLE pubs.acs.org/EF
Effect of Furnace Purging on Kinetic Rate Parameter Determination Using Isothermal Thermogravimetric Analysis Prabhat Naredi, Yaw D. Yeboah, and Sarma V. Pisupati* John and Willie Leone Family Department of Energy and Mineral Engineering and the EMS Energy Institute, The Pennsylvania State University, 110 Hosler Building, University Park, Pennsylvania ABSTRACT: In determining char reactivity using thermogravimetric analysis (TGA), typically the sample is heated in an inert atmosphere until the reaction temperature is achieved and the gas is switched to the reaction gas. The reaction gas initially purges the inert gas from the furnace. The main objective of this work is to determine the role of “furnace purging” in determining the rate parameters of porous solidgas reactions. Various tests were conducted in this study using char samples with different timetemperatureconversion histories and gas environments in TGA. Increasing the air flow rate to purge the inert gas resulted in achieving the maximum rate much earlier. Results showed that the observed maximum during the rate measurement is the result of a furnace purging effect, and a mathematical expression was developed to correct for the time it takes to purge the inert gas to build the partial pressure depending on the volume of the furnace. This correction, when applied to the reactivity profile, resulted in an increase in the activation energy of about 325 kJ/mol, depending on the extent of conversion. This result suggests that estimating rate parameters or deducing conclusions regarding char reactivity from the initial part of a rate profile is misleading if the purging effect is ignored.
’ INTRODUCTION For char burnout predictions in combustion conditions using an intrinsic approach, accurate knowledge of intrinsic activation energy is of utmost importance because this indicates the reaction rate’s sensitivity to temperature. Typically, a TGA furnace is brought to the reaction temperature in an inert gas environment and then switched to reactive gas for rate measurement. “Furnace purging” is referred to in this study as the time taken to build up the reactive gas partial pressure. Sometimes, an arbitrary temperature, for example 823 K, is used to compare the relative ranking of coals.1,2 However, ranking of coal samples’ reactivity will differ at other reference temperatures for the same samples if their activation energy values are different. A few studies have determined intrinsic rate parameters using combustion tests conducted in the high temperature conditions of an entrained flow reactor (EFR).35 However, apparently small differences in experimental conditions and modeling assumptions could lead to different parameters,6 and therefore, parameters deduced using this method may not be accurate. On the other hand, thermogravimetric analysis (TGA) potentially offers an attractive method to determine rate parameters by using controlled temperature and gas environments.7 This analysis can be carried out in standard, commercially available equipment and is rapid, inexpensive, easy to use, and gives repeatable results. For char-oxidation rate measurement, the TGA instrument may be operated in either an isothermal or nonisothermal manner. Typically, the nonisothermal method is used to compare the relative rank,8 whereas the isothermal method is used to derive the intrinsic rate parameters.9 In an isothermal method for kinetic parameter determination, experiments are performed at sufficiently low temperatures and the resulting curve of variation in the sample mass time is then represented as a rate-conversion curve (reactivity profile). However, even while operating at r 2011 American Chemical Society
reasonably lower temperatures in a TGA, several investigators have reported a maximum in the reactivity plot.10 The rate (normalized to initial sample mass) in these studies increased with conversion up to a certain conversion level, reached a maximum, and subsequently decreased monotonically thereafter. Because of the presence of a maximum in the rate profile, various methods, such as average reactivity,1113 maximum reactivity,14 and reactivity at a fixed conversion level,15,16 have been used in the literature to obtain the rate parameters. Sometimes, rate parameters were calculated at various conversion levels in the entire reactivity plots.1720 The initial part, up to the maximum in the reactivity plot, has generally been suggested to be due to the presence of intraparticle diffusion limitation or a balance between the mass gain (because of stable complex formation) and the mass loss (because of carbon reaction).18 Table 1 summarizes the activation energy values published in the last 25 years for char oxidation reaction. From Table 1, it is apparent that a wide range of activation energy values have been reported for oxidation of char and porous carbon samples. In fact, for the same rank coal, different studies reported a wide range of activation energy values. For example, activation energy values from 117 kJ/mol2 to 180 kJ/mol21 and from 110 kJ/mol13 to 140 kJ/mol22 are reported for an Illinois No. 6 bituminous coal and a North Dakota lignite coal, respectively. This point is further highlighted by the inconsistencies in reported activation energy values in the literature as described below: • In the same study, different activation energies are reported for similar ASTM rank coals.2 Received: June 29, 2011 Revised: October 6, 2011 Published: October 17, 2011 4937
dx.doi.org/10.1021/ef200949n | Energy Fuels 2011, 25, 4937–4943
Energy & Fuels
ARTICLE
Table 1. Summary of Operating Conditions, Methods Used, and Reported Activation Energies in the Literature for CharOxidation Reaction rate param. est. method
pyrolysis condit.
coal seam
ASTM rank
char treatment
oxidation
activation
temp. (K)
energy
ref
Isothermal Technique random pore model
na x = 0.5
TGA: 900 K, 15 min
Cerejjen
hvB
Dietz
SB
in situ
723923
blue
SB
136
Hiawatha
hvC
148
Illinois 6 Pittsburgh 8
hvC hvA
117 112
Ulan
hvB
116
Blair Athol
hvB
140
lower Kitt
lvb
TGA: 1273 K, 120 min TGA: 1273 K, 60 min
North Dakota Illinois 6
Lignite hvC
TGA: 925 K, 60 min
sucrose char
132
