A Model-based Phenomenological Investigation of Char Combustion Kinetics through Thermogravimetry

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【刊名】Chinese Chemical Letters


Thermogravimetry (TG) can be used in the study of the reaction kinetics of coal chars1.It works through measuring the continuous mass loss of the char sample held in thethermobalance in a temperature-program-controlled furnace. By fitting the data of themass and the temperature, the kinetic parameters of combustion, i.e. char reactivity, canbe obtained. In this paper, the combustion kinetics of five char samples were studied byusing thermogravimetry and the kinetic triplet was also given.ExperimentalThe samples were prepared from parent coals devolatilized in a drop tube furnace system(DTFS) in a stream of nitrogen at temperature of 1273 K. The proximate and ultimateanalytic data of these five coal samples were listed in Table 1. The average diameter ofthe particles was about 75 μm. The char samples were burnt in Dupont Thermal Analyst 2100 (TA Instruments).The ramping heat rate β remained at 30 K/min in each test. The test mass of eachsample was about 2.0~3.0 mg, dispersed flatly in the pan (sample carrier) of about 8 mmin diameter and 1 mm in depth. The samples were all burnt in atmospheric air, whoseflow rate was about 135 mL/min. All the char samples were burnt below 1000 K. Thecombustion reaction of the char samples was therefore all in kinetic-controlled zone2.Table 1 The proximate and ultimate analytic data of parent coal samples proximate analysis(air dry basis) ultimate analysis(dry basis) heating Samples inherent volatile fixed C H N O values ash S moisture matter carbon kJ/g % % % % % % % % % Adaro 24.87 14.2 1.0 43.8 41 72.3 5.2 0.8 20.4 0.1 Chaohua 30.89 1.72 14.3 13.0 71.0 77.1 3.9 1.2 3.2 0.4 Hebi 29.17 1.40 16.8 14.2 67.6 74.7 3.6 1.3 3.2 0.3 Luoyang 28.80 1.70 16.2 11.5 70.6 75.1 3.3 1.3 3.5 0.4 Shanxi 29.34 2.70 14.5 10.2 72.6 77.9 3.4 0.8 2.8 0.4 To obtain the information of pore structures, these char samples were also testedwith mercury intrusion with Atuoscan3 porosimetry (QUANTACHROME), whose workpressure was about 0~220 MPa. The pores above 3.5 nm in radius were measuredaccording to Young-Laplace equation3. The porosity and pore surface area of charsamples were obtained. The expression of char combustion rate was derived as follows: dα/dt = ATB exp(-E/RT)αm1 (1-α)m2 (1)According to the kinetic theory of gases, this equation can finally be expressed asfollows: da/dt = AS0C0T-1/2 exp(-E/RT)αm1 (1-α)m2 (2)where K=AS0C0, is referred to as pre-exponential factor. S0 is the initial specific surfacearea of the char sample, C0, the concentration of oxygen in the air under standardambient condition. E is the apparent activation energy, m1 and m2 are two exponents ofthe kinetic model function and A is a constant. α is the conversion of carbon in charsample. To explore the char reactivity of combustion, K, m1, m2 and activation energy Ein eqn (2) can be solved out with fitting curves to the TG data of each char sample.Results and DiscussionThe results of mercury intrusion experiments are listed in Table 2, which shows theporosity and specific surface area of each char samples. Table 3 shows the results ofthe kinetic parameters of the char samples from the curve fitting with the square-leastmethod. In the kinetic model, the pre-exponential factor K is derived from the kinetic theoryof gases and therefore is a known constant, while the pre-exponential factor andactivation energy have to be determined simultaneously in many other models. Themacropore surface area Sm (listed in Table 2) is deliberately taken as the initial specificsurface area S0 in eqn (2). The result of mercury intrusion indicates that the poresurface 5 and the pore radius r have the relationship dS/dr r1-D, where D is referred tothe fractal dimension. Different values of D are found between the micropore and themacropore from the experiment results. rtr in Table 2 is the transition radius betweenmicropore and macropore. It is known that the total surface of char particles consists ofmacropore surface and micropore surface. Although most surface area is contributedby micropore surface, it has been found that much micropore surface area is inaccessibleto oxygen during the char combustion, even when the reaction is controlled in kineticregime4. Besides, C0 in eqn (2) represents the oxygen concentration of both the bulk flowand gases in macropores. Therefore, the surface area affecting the char combustion isequivalent to the sum of macropore surface area. The products of A and Sm of the allsamples are listed in Table 4. Table 3 shows that, except for sample Adaro, the activation energies of other fourchar samples are almost in the same values, which fall within the range of that obtainedby Gopalakrishnan and Bartholemew5 (109~134 kJ/mol). These values are also close tothe values given by Suuberg6, Charpeney7 et al.. The Adaro char may still have somevolatile matter on account of the higher volatile matter content of its coal sample (seeTable 1). As a result, the activation energy value of Adaro char sample is a bit lowerthan others.

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