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Energy & Fuels 1995,9, 71-74
Estimating the Combustibility of Various Coals by TG-DTA Yong Chen* and Shigeikatsu Mori Department of Chemical Engineering, Nagoya University, Furo-Cho Chikusa-Ku, Nagoya 464, Japan
Wei-Ping Pan Center for Coal Science and Department of Chemistry, Western Kentucky University] Bowling Green, Kentucky 42101 Received July 18, 1994@
To estimate the combustibilityof 12 different coals involving anthracite, bituminous, and lignite, experiments of the coal combustion were carried out by using thermogravimetry (TG) and differential thermal analysis (DTA). High-reactivity combustibles region and low-reactivity combustibles region were observed during coal char combustion process. Also, two reaction rate constants (kl and kz) for these two combustibles were determined. The value of the reaction rate constant of the low-reactivity combustibles (kz) is much smaller than that of the highreactivity combustibles (kl) in each coal and also i t 2 shows lower temperature dependence. These are reasons why 5 1 0 % unburned carbon remains in the ash discharged from most of the coalfired power plants. The ignition temperature increases with decreasing volatile matter content of coal. Three ranks of coals as anthracite, bituminous, and lignite show the different ignition ways.
Introduction Thermal analysis methods, such as thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC), have been employed extensively in investigations relating to coal utilization. Frazier et al. studied the effectiveness of SO2 sorbents by thermogravimetry-combustion with coal.' In another research, TGDTA was used to study the characteristics of high-carbon fluidized-bed cyclone ashes.2 Tognotti et al. measured the ignition temperature of coal by using TG e q ~ i p m e n t .Rusianova ~ et al. have investigated the petrographic characteristics, caking properties, and structural parameters of Russian coals by TMA and DTGa4 In the study presented by Qzyuguran et al., the techniques of the thermogravimetry and derivative thermogravimetry were applied to 25 Turkish lignite.5 Shao and Pan studied the behavior of chlorine and sulfur during coal combustion using both simultaneous TG/FTIR and TG/IC techniques.6 In authors' previous works, the thermal analysis methods were employed to investigate the effect of the fuel ratio on pyrolysis characteristics of coals,7 behaviors of ni-
trogen release during coal pyr0lysis,8?~ and the properties of carbon containing in fly ash discharged from pulverized coal combustion boiler.1° In this study, thermal analysis methods (TGDTA) are used to investigate the combustibility of 12 coals involving anthracite, bituminous, and lignite. Based on the observed mechanisms of coal combustion, two reaction rate constants (kl and kz) are estimated. Furthermore, the effects of the volatile content and coal rank on the coal ignition are discussed.
Reaction Model Determination of Reaction Rate Constants. In this study, the global reaction model was adopted for describing the rate expression of coal combustion as follows: h l d t = d.ddT(dT/dt) = k( 1 - x)"
(1)
k = A exp(-E/RT)
(2)
Abstract published in Advance ACS Abstracts, November 15,1994. (1) Frazier, G. C.; Mason, C.; Badin, E. J. Fuel 1982, 61, 1226. (2) Frazier, G. C.; Mason, C.; Badin, E. J. Fuel 1984,63,499. ( 3 )Tognotti, L.; Malotti, A.; Petarca, L.; Zanelli, S. Combwt. Sci. Technol. 1985,44, 15. (4) Rusianova, N. D.; Maksimova, E. E.; Butakova, V. I. Fuel 1990,
where x is the coal conversion, it is the reaction rate constant, n is the reaction order, E is the apparent activation energy, and A is the preexponential factor involving effects of both the specific surface area of coal and the partial pressure of ambient oxygen in the system . In the determination of the kinetic parameters by use of the above equations, various analysis methods have been suggested, such as Borchardt-Daniels method,ll
(5) Qzyuguran, A.; Karatepe, N.; Kucukbayrak, S. Proc. 7th Znt. Conf. Coal Sci. 1993,2, 121. (6) Shao, D.; Pan, W. P. Proc. 7th Int. Conf. Coal Sci. 1993,1,656. (7) Chen, Y.; Matsuda, H.; Hasatani, M. J. Jpn. Znst. Energy 1992, 71 (2), 85.
(8) Chen, Y.; Matsuda, H.; Hasatani, M. J . Jpn. Znst. Energy 1992, 71 (8), 766. (9) Chen, Y.; Matsuda, H.; Hasatani, M. Proc. 7th Znt. Conf. Coal Sci. 1993,1, 55. (10) Chen, Y.; Mori,S. J . Jpn. Znst. Energy 1994, 73 (8), 748.
* Author to whom correspondence should be addressed.
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0 1995 American Chemical Society
Chen et al.
72 Energy &Fuels, Vol. 9,No. 1, 1995
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IiT Il/K] Figure 1. Estimating the reaction order.
Freeman-Carroll method,12 Coats-Redfern method,13 Ozawa method,14 and the nonisothermal analysis method.15 In the present study, the nonisothermal analysis method proposed by Ito et al.15 was used. Introducing logarithm into the above equations, the following equation is obtained. ln((1 - x)-"(&/dt)) = In A - E/RT
(3)
The values of bcldt and x can be directly obtained from the TG curves. To determine the apparent kinetic parameters in eq 3, the reaction order n will be assumed, at first. By using this n value, the calculated results of the left-hand term in eq 3 are plotted against the reciprocal of absolute temperature. The n value is determined as the value which shows a straight line in ln((1 - x)-"(dx/dt)) vs 1/T (see Figure 11, and then the reaction rate constant k is obtained from eq 1.
Experimental Section Samples. The 12 different coals studied here were Kaixi (KX), Sangei (SG), Kaipin (KF'), Jinjin (JJ), Datong (DT) (China), Loy Yang (LY), Newlands (NL), Blair Athol (BA) (Australia), Kangra (KG)(South Africa), Hongay (HG) (Vietnam), IBC-106 (106), IBC-109 (109) (American). Hongay coal is anthracite, Loy Yang coal is lignite, and other coals are the bituminous. Proximate and ultimate analysis of these coals were measured by electric furnace (1200PKP, Motoyama) and CHN-corder (MT2, Yanaco), respectively, and they are shown in Table 1. The samples were air dried at 383K for 24 h before the combustion test. Equipment. Figure 2 shows a schematic diagram of TGDTA system (TAS-100, RIGAKU). The coal combustion experiment was carried out in a n atmosphere of air flow. In order to measure the ignition temperature of coal, the TG curves of coal pyrolysis were obtained under an atmosphere of nitrogen. Experimental Conditions. For using the global reaction model, it is important to determine the experimental condi-
gas in
9 1uance
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1 Figure 2. Schematic diagram of experimental apparatus employed. tions where both heat- and mass-transfer resistances in coal particles can be neglected. The experimental conditions were determined by the preliminary combustion experiment of coals by changing the heating rate, the particle size, the air flow rate, and the sample size until a TG curve could be obtained irrespective of the experimental condition. The typical examples of these examinations are shown in Figure 3, a and b. From Figure 3a, it can be seen that the reaction rate constant show same value at heating rate under 5 Wmin. From Figure 3b, it can be seen that TG curves obtained converge to a constant TG curve under several experimental conditions, such as particle size
~
~~
10 20 30 40 50 60 Volatile Matter Content [%] Figure 7. Relation between the volatile matter content and the ignition temperature.
0
volatile matter, and the volatile matter almost burned out when the char ignited. For the bituminous coals, the char ignite after a part of the volatile matter is released from the coal. For the anthacite, the particles ignite heterogeneously by direct oxygen attack on the whole particle, because the volatile matter almost does not release from the coal before the coal ignites.
Conclusions Combustion experiments of 12 coals were carried out using thermal analysis methods, such as TG and DTA, and the following results were obtained. 1. The kinetic constant of low-reactivitycombustibles 122 is much smaller than that of the high-reactivity combustibles k1 in the coal. Also, kz shows lower temperature dependence. 2. The ignition temperature increases with decreasing the volatile matter content of coal. Three ranks of coals, anthracite, bituminous, and lignite, show different ways of ignition. Acknowledgment. This paper is a part of work carried out by the International Research Team "Interfluid" supported by the New Energy Development Organization (NEDO), Japan, a t the universities of Nagoya, Chubu, and Hamburg-Harburg. EF9401435
