Energy & Fuels 2007, 21, 2669-2672
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Catalytic Effect of Metallic Oxides on Combustion Behavior of High Ash Coal X. G. Li,* B. G. Ma, L. Xu, Z. T. Luo, and K. Wang Key Laboratory for Silicate Materials Science and Engineering of Ministry of Education, Wuhan UniVersity of Technology, Wuhan 430070, Hubei, China ReceiVed January 29, 2007. ReVised Manuscript ReceiVed May 16, 2007
By means of thermogravimetric analysis, the catalytic effect of metallic oxides (CuO, Fe2O3, and ZnO) on the combustion behavior of high-ash coal was investigated under nonisothermal conditions. Experiments were conducted from ambient temperature to 1000 °C at a heating rate of 20 °C‚min-1. The ignition temperature, burnout performance, and exothermic behavior were used to evaluate the catalytic effect. Moreover, the kinetics parameters (activation energy and pre-exponential factor) were determined using the Coats-Redfern method. It is indicated that, compared with the combustion characteristics of high-ash coal, the ignition temperature of the samples with metallic oxides decreases by 8-50 °C. Metallic oxides can speed up the combustion rate and burnout of the fixed carbon. The exothermic values of samples incorporating metallic oxides increase by 1530%, which may be due to the catalytic effect of metallic oxides on fixed carbon combustion. The activation energies of the samples decrease, and there is a linear connection between the activation energies and preexponential factors (ln A ) 0.2683 × E-12.807).
1. Introduction In China, the production and consumption of coal is very great and most of the raw coals mined every year are directly used as fuels. It is very important and necessary to improve the burning of the raw coals so as to save energy and control environmental pollution. In addition, how to fully utilize lowrank coals such as high-ash coal (HAC), which occupy many lands and pollute the environment, is a problem that has to be solved. In rotary kilns, low-ash coal burns well. However, compared with soft coal, there are severe problems such as ignition, combustion rate, and burnout if high-ash coal is used to produce cement in a rotary kiln or in a precalciner. In recent years, many more investigations on the coal burning process and catalytic combustion were performed successively to promote combustion efficiency and meet pollution emission requirements. The ignition temperature and activation energy of fly ash coal due to the incorporation of alkali metal salt decrease.1-3 The catalytic effect of coal-burning additives can increase the combustion rate and decrease the activation energy of coal.4-5 The action of catalysts on fuel ignition can be explained by the enhancement of volatiles emissions. It can be assumed that alkali causes weakening of the intermolecular inter* Corresponding author. Tel./fax: +86-27-87160951; e-mail: lxggroup@ 163.com. (1) Altun, N. E.; Hicyilmaz, C.; Kok, M. V. Effect of different binders on the combustion properties of lignite, TG/DTG study, Part I. effect on thermal properties. J. Therm. Anal. Calorim. 2001, 65 (2), 787-795. (2) Yaman S.; Kucukbayrak, S. Effect of oxydesulphurization on the combustion characteristics of coal. Thermochim. Acta 1997, 293 (2), 109115. (3) Matsuzawa, Y.; Mae, K.; Hasegawa, I. Characterization of carbonized municipal waste as substitute for coal fuel. Fuel 2007, 86 (1-2), 264272. (4) Hsisheng, T.; Yung-Fu, H.; Ying-Tsung, T. Reduction of NO with NH3 over carbon catalysts-the influence of carbon surface structures and the global kinetics. Appl. Catal., B 1999, 20 (2), 145-154. (5) Tan, Z. C.; Wang, S. D.; Li, L.; Wu, D Y. Thermogravimetric study about the accelerating effect of coal burning additive on combustibility of coal and gangue. Chin. J. Catal. 1999, 20 (3), 263-266 (in Chinese).
action of the polymeric chains and, at the same time, catalyzes not only intralink dehydration but promotes the processes of retro-aldol cleavage and condensation of the products produced.6 The catalyst Cu/V/K is reported to produce a strong increase in the rate of carbon combustion and a marked decrease in the apparent activation energy.7 The activity of different metal oxides in the catalytic combustion of diesel soot having any amount of adsorbed hydrocarbons has been investigated.8 However, these findings were only used in applications of electricity generation; the type and quantity of the chemical additives were not completely suitable for cement production. Therefore, in order to widely use HAC in the precalcining kiln and to widen the selection scope of the coal used in the cement industry, fundamental research work on the catalytic and accelerating effect of metallic oxides on combustion characteristics should be carried out in order to give scientific guides to the application of HAC and catalysts in the cement and other industries. Thermogravimetric analysis (TGA) profiles contribute to enhancing the knowledge of this process and, therefore, to establishing the optimum operational conditions to develop it. Accordingly, several authors have studied the behavior of the pyrolysis, combustion, and kinetics of soft coal, anthracite, and other carbonaceous materials by TGA.9-14 In this paper, the catalytic and accelerating effects of metallic oxides such as CuO, Fe2O3, and ZnO on the ignition and burnout characteristics of HAC are investigated by TGA, and the calori(6) Rustamov, V. R.; Abdullayev, K. M.; Samedov, E. A. Biomass conversion to liquid fuel by two-stage thermochemical cycle. Energy ConVers. Manage. 1998, 39 (9), 869-875. (7) Ciambelli, P.; Coro, P.; Gambino, M. Catalytic combustion of carbon particulate. Catal. Today 1996, 27, 99-106. (8) Neri, G.; Bonaccorsi, L.; Donato, A. Catalytic combustion of diesel soot over metal oxide catalysts. Appl. Catal., B 1997, 11 (2), 217-231. (9) Hurt, R. H.; Lunden, M. M.; Brehob, E. G. Kinetic model of carbon burnout in pulverized coal combustion. Combust. Flame 1998, 113 (2), 181197. (10) Ko¨k, M. V. An investigation into the thermal behaviour of coals. Energy Sources 2002, 24 (10), 899-906.
10.1021/ef070054v CCC: $37.00 © 2007 American Chemical Society Published on Web 08/31/2007
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fic values of the coal samples are also determined by differential scanning calorimetry (DSC). The kinetic parameters of coal samples due to the catalytic effect are also determined according to the Coats-Redfern method. The results may be used to enhance the understanding of the characteristics of high-ash coal and also provide a useful basis for further applying high-ash coal in cement kilns or other industries with high efficiency.
In general, RT/E , 1, so eq 5 becomes
[
ln -
] [ ]
ln(1 - R) T2
) ln
AR E βE RT
(6)
The curves of TGA can be used to calculate ln[-ln(1 - R)/T2]. It was plotted against 1/T according to eq 6, and linear regression was used to determine E and A.
2. Experiments 2.1. Materials and Sample Preparation. For the investigation, a high-ash coal with a Mad (moisture content) of 1.4 wt %, Vd (volatile matter) of 8.50 wt %, Ad (ash) of 44.22 wt %, and FCd (fixed carbon) of 46.18 wt % was used, where Mad (moisture) was the weight loss percentage on a dry air basis at 75 °C, Ad (ash) was the residue percentage on a dry basis after complete combustion at 800 °C, Vd (volatile) was the weight loss percentage on a dry basis after devolatilization at 700 °C for 10 min under a scarce oxygen atmosphere, and FCd (fixed carbon) was calculated by difference. A coal with a particle size of Fe2O3. 3.2. Influence on Ignition Temperature. A number of parameters are known to influence the ignition temperature and ignition mechanism; these include the sample mass, particle size, volatile matter yield, and oxygen concentration. In addition to these variables, a number of system parameters related to the experimental conditions are important for the ignition behavior. Clearly, therefore, the ignition temperature is not a physical property of a fuel.15 Our aim was not to propose a method for ignition temperature evaluation. This study was based on a comparison attempt; that is, we kept the same burnoff procedure for virgin and modified samples to see the metallic oxides’ impact. In this investigation, the ignition temperature is determined according to the literature.16-17 It is shown from Table 1 that the ignition temperature (Ta) of HAC is 458.5 °C; the ignition temperatures of the coal samples with 6% CuO, Fe2O3, and ZnO decrease by 50, 8.4, and 19.2 °C, respectively. It is also indicated that the metallic oxides used in this study can catalyze and accelerate the ignition of HAC. The active sequence of catalysts relative to the ignition temperature can be described as follows: CuO > ZnO > Fe2O3. (14) Chen, Y.; Shigekatsu, M.; Wei-Ping, P. Studying the mechanisms of ignition of coal particles by TG-DTA. Thermochim. Acta 1996, 275 (1), 149-158. (15) Pranda, P.; Prandova, K.; Hlavacek, V. Combustion of fly-ash carbon. Part I: TG/DTA study of ignition temperature. Fuel Process. Technol. 1999, 61, 211-221. (16) Nie, Q. H.; Sun, S. Z.; LI, Z. Q. Thermogravimetric study on the combustion characteristics of brown coal blends. J. Combust. Sci. Technol. 2001, 7 (1), 72-76. (17) Li, X. G.; Ma, B. G.; Xu, L. Thermogravimetric analysis of the co-combustion of the blends with high ash coal and waste tyres. Thermochim. Acta 2006, 441 (1), 79-83.
Catalytic Effect of Metallic Oxides
Energy & Fuels, Vol. 21, No. 5, 2007 2671
Figure 2. DSC profiles of combustion of the coal samples. Table 2. The Exothermic Values and Kinetics Parameters of the Coal Samples Containing Metal Oxidea no.
peak temperature °C
exothermic value J/g
E KJ‚mol-1
ln A s-1
a b c d
517.9 519.7 514.6 547.3
6480 5375 5596 4932
58.79 60.79 62.14 66.53
2.97 3.50 3.87 5.04
a a: HAC + 6% CuO. b: HAC + 6% Fe O . c: HAC + 6% ZnO. d: 2 3 HAC.
Figure 1. TG-DTG profiles of combustion of the coal samples. Table 1. The Ignition and Burnout Behavior of the Coal Samples Containing Metal Oxidea no.
Ta °C
ta min
(dw/dt)max % min-1
Tp °C
∆t1/2 min
tp min
tf min
Df ×10-4
a b c d
408.5 450.1 439.3 458.5
18.68 20.86 20.13 20.99
-8.55 -7.07 -7.75 -4.77
451.3 515.9 496.7 587.9
5.46 6.86 5.76 9.63
20.70 24.10 23.04 27.83
30.46 30.25 33.10 34.20
24.84 14.14 17.64 5.20
a a: HAC + 6% CuO. b: HAC + 6% Fe O . c: HAC + 6% ZnO. d: 2 3 HAC. Tp is the corresponding temperature of (dw/dt)max; Ta and ta are the ignition temperature and the corresponding time; tf and Df are the burnout time and the burnout index.
The reduction of ignition temperature is due to the catalytic effect of metallic oxides on the enhancement of the emission of volatile matters from HAC. To a certain degree, the catalytic effect promoted the coal decomposition reaction, resulted in the oxidative and transformed reaction of the matter which is difficult to oxidize and transform, accelerated and eased the decomposition of the tar and the crude benzene, and increased the release of the coal volatile matter.18 (18) Shen, B. X.; Qin, L. Study on MSW catalytic combustion by TGA. J. Fuel Chem. Technol. 2005, 33 (2), 189-193. (19) Xie, J. L.; He, F. Catalytic combustion study of Anthracite in cement kiln. J. Chin. Ceram. Soc. 1998, 26 (6), 792-795.
3.3. Influence on Burnout Performance. When the weight loss in TG curves was close to zero, the corresponding temperature or time was considered as the burnout temperature or time of the samples. As shown in Table 1, the influence of coal on the burnout time (tf) is slight, so the burnout time cannot completely reflect burnout performance of the samples. In this study, the burnout index is used to evaluate the burnout performance, which can be described as follows:19
Df )
(dw/dt)max ∆t1/2tptf
where (dw/dt)max is the maximum combustion rate, ∆t1/2 is the time zone of (dw/dt)/(dw/dt)max ) 1/2, tp is the corresponding time of (dw/dt)max, and tf is the burnout time. It is shown that the burnout indexes (Df) of the pure HAC and the coal samples containing CuO, Fe2O3, and ZnO were 5.20 × 10-4, 24.84 × 10-4, 14.14 × 10-4, and 17.64 × 10-4, respectively (Table 1). Compared with the burnout characteristics of the samples, it has been shown that the burnout indexes of the samples with CuO, Fe2O3, and ZnO increase by 378%, 172%, and 239%, respectively. Moreover, when a catalyst has been added (excluding the mass of the catalyst), the final masses of the control and the samples with CuO, Fe2O3, and ZnO are 42.6%, 38.2%, 39.9%, and 38.9%, respectively. The active sequence of catalysts relative to the burnout performance can be described as follows: CuO > ZnO > Fe2O3. The catalytic effect on burnout performance may be a carrier of oxygen that promotes the oxygen transfer to the char of HAC. Metallic oxides are deoxidized to metal or low-valency metallic oxides (M2O) by carbon; then, metal or low-valency metallic oxides absorb oxygen and turn into metallic oxides; this process circulates between oxidation and deoxidization. Due to the
2672 Energy & Fuels, Vol. 21, No. 5, 2007
oxygen transfer from the metal to the carbon atom, the diffusion of oxygen is accelerated, which contributes to the burnout and combustion of the fixed carbon. 3.4. Influence on the Exothermic Value. According to the DSC profile, the relation between the exothermic value and peak area can be described as follows:
∆Q ) k
∫c∞ [∆H - (∆H)c] dτ ) kS
where ∆Q is the exothermic value of coal combustion, in joules per gram, k is a constant, ∆H is the enthalpy difference between the sample and the reference, in milliwatts per milligram, τ is the heating time, in minutes, and S is the peak area between the DSC profile and baseline. The exothermic behaviors of the coal samples with or without metallic oxides according to the DSC profiles are reported in Figure 2. The exothermic values and the peak temperature of the samples are shown in Table 2. It can be seen that, compared with that of the pure coal, the peak temperature of DSC can be decreased about 30 °C with the incorporation of the metallic oxides; moreover, the exothermic value of the samples can be increased by 15-30%, which may be due to the catalytic effect of metallic oxides on the fixed carbon combustion. 3.5. Influence on Kinetics Parameters. According to the TG-DTG profiles of the coal samples containing different metallic oxides, the activation energies (E) and pre-exponential factors (A) are determined by the Coats-Redfern method. The results are shown in Table 2. As can be seen from Table 2, compared with those of the HAC, metallic oxides can decrease the activation energies of HAC, and these activation energies were found as 66.53
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KJ‚mol-1 for the pure HAC, 58.79 KJ‚mol-1 for HAC with CuO, 60.79 KJ‚mol-1 for HAC with Fe2O3, and 62.14 KJ‚mol-1 for HAC with ZnO (Table 2), respectively. Moreover, there is a linear connection between the activation energies and preexponential factors; the connection can be described as the following equation: ln A ) 0.2683 × E-12.807. 4. Conclusions The ignition temperature of the coal samples with 6% CuO, Fe2O3 and ZnO decrease by 50 °C, 8.4 °C and 19.2 °C, respectively. The active sequence of catalysts relative to the ignition temperature can be described as follows: CuO > ZnO > Fe2O3. The burnout indexes of the samples with CuO, Fe2O3, and ZnO increase by 378%, 172%, and 239%, respectively. The active sequence of catalysts realtive to the burnout performance can be described as follows: CuO > ZnO > Fe2O3. The peak temperature of DSC can be decreased about 30 °C with the incorporation of the metallic oxides; moreover, the exothermic value can also be increased by 15-30%, which may be due to the catalytic effect of metallic oxides on the fixed carbon combustion. The activation energies of the samples containing CuO, Fe2O3, and ZnO decreased to some extent. Moreover, there is a linear connection between the activation energies and pre-exponential factors; the connection can be described as the following equation: ln A ) 0.2683 × E-12.807. Acknowledgment. This research was carried out at the Key Laboratory for Silicate Materials Science and Engineering of Ministry of Education, Wuhan University of Technology. The financial support from the Key Laboratory of Silicate Materials Science and Engineering (Wuhan University of Technology), the Ministry of Education of China (No. SYSJJ2007-01), and the hi-tech research and development program of China (No. 2002AA335050) are greatly acknowledged EF070054V
