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Energy & Fuels 2002, 16, 752-755
Pyrolysis Behavior of Nickel-Loaded Loy Yang Brown Coals: Influence of Calcium Additive Kenji Murakami,† Masahiko Arai,‡ and Masayuki Shirai*,§ Department of Materials-Process Engineering & Applied Chemistry for Environments, Faculty of Engineering and Resource Science, Akita University, Akita, 010-8502, Japan, Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan, and Institute for Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan Received September 24, 2001. Revised Manuscript Received January 14, 2002
The pyrolysis behavior of nickel-calcium-loaded Loy Yang brown coals was studied. Two peaks of CO evolution appeared from nickel-calcium-loaded Loy Yang brown coal (Ni loadings: 4-7 wt %), one at 695 K and the other at 920 K, compared with those at 670-690 K and at 920 K from nickel-loaded coals. The amounts of CO evolved at 695 K were increased by adding calcium species to nickel-loaded Loy Yang coals. The calcium loadings did not affect the temperature and amount of CO evolution. EXAFS analysis showed that nickel metal particles were formed at 695 K in the nickel-calcium-loaded Loy Yang brown coals, while they were formed at 670-690 K in the nickel-loaded coals. The catalysis behavior of nickel species in the nickel-calcium-loaded Loy Yang coals is explained by the aggregation behavior of nickel species.
Introduction It is well accepted that metal catalysts loaded by ionexchange methods have high activity for coal conversion processes,1-8 because the catalysts are highly dispersed and have large surface areas. To enhance the activities of the catalysts loaded on coals, the addition of secondary species as promoters have been tried.9-11 One probable effect of promoters is to suppress the agglomeration of main catalytically active species during coal conversion processes. Haga et al. reported that the yield of methane evolution from nickel-impregnated coals during hydrogasification was enhanced by addition of calcium species.12,13 They also reported that smaller * Corresponding author. Tel. and Fax: +81-22-217-5631. E-mail:
[email protected]. † Akita University. ‡ Hokkaido University. § Tohoku University. (1) Hippo, E. J.; Jenkins, R. G.; Walker, P. L., Jr. Fuel 1979, 58, 338-344. (2) Hengel, T. D.; Walker, P. L., Jr. Fuel 1984, 63, 1214-1220. (3) Agnew, J. B.; Jackson, W. R.; Larkins, F. P.; Rash, D.; Rogers, D. E.; Thewlis, P.; White, R. Fuel 1984, 63, 147-152. (4) Takarada, T.; Nabatame, T.; Ohtsuka, Y.; Tomita, A. Energy Fuels 1987, 1, 308-309. (5) Salinas-Martinez de Lecea, C.; Almela-Alarcon, M.; LinaresSolano, A. Fuel 1990, 69, 21-27. (6) Joseph, J. T.; Forrai, T. R. Fuel 1992, 71, 75-80. (7) Taghiei, M. M.; Huggins, F. E.; Mahajan, V.; Huffman, G. P. Energy Fuels 1994, 8, 31-37. (8) Murakami, K.; Shirato, H.; Nishiyama, Y. Fuel 1997, 76, 655661. (9) Lawson, J. D.; Rase, H. F. Ind. Eng. Chem. Prod. Res. Dev. 1970, 9, 317-324. (10) Inui, T.; Funabiki, M.; Takegami, Y. J. Chem. Soc., Faraday Trans. 1 1980, 76, 2237-2250. (11) Murakami, K.; Shirato, H.; Hanada, N.; Nishiyama, Y. Energy Fuels 1998, 12, 843-848. (12) Haga, T.; Nishiyama, Y. J. Catal. 1983, 81, 239-246. (13) Haga, T.; Nishiyama, Y. Ind. Eng. Chem. Res. 1987, 26, 12021206.
nickel particles were obtained after hydrogasification in nickel-calcium-loaded coal samples than those in nickel-loaded coal samples by scanning electron microscope (SEM) and X-ray diffraction (XRD) techniques, indicating that calcium species suppress the agglomeration of nickel metal particles. They measured the state of coal samples after the reaction; however, they did not precisely study the effect of calcium (the amount of loadings and the agglomeration behavior of nickel species). Although XRD technique is universally applicable, it is not useful for determining the structure of smaller particles than a few nanometers. X-ray absorption fine structure (XAFS) is a powerful technique to determine the local structure of a specific element in complex systems under in-situ conditions. Using an EXAFS technique we have shown that well-dispersed nickel particles were obtained on Loy Yang brown coal by an ion-exchange method and these small nickel metal particles enhanced the evolution of carbon monoxide and methane during pyrolysis and hydrogasification.14-17 In this paper, we report the additive effect of calcium for nickel-exchanged Loy Yang brown coals on their pyrolysis behavior. Experimental Section Sample Preparation. Loy Yang brown coal from Victoria, Australia, was ground below 250 µm particle size, washed with deionized water, dried at 323 K under vacuum, and stored (14) Shirai, M.; Murakami, K.; Nishiyama, Y. Energy Fuels 1997, 11, 1012-1018. (15) Shirai, M.; Arai, M.; Murakami, K. Energy Fuels 1999, 13, 465470. (16) Shirai, M.; Arai, M.; Murakami, K. Energy Fuels 2000, 14, 1038-1042. (17) Murakami, K.; Arai, M.; Shirai, M. Energy Fuels 2000, 14, 1240-1244.
10.1021/ef010235b CCC: $22.00 © 2002 American Chemical Society Published on Web 03/26/2002
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Energy & Fuels, Vol. 16, No. 3, 2002 753
Table 1. Preparation of Nickel-Calcium-Loaded Coals amount of cation species sample number sample code 1 2 3 4 5 6
4.0Ni-0.41Ca 4.3Ni-0.18Ca 6.5Ni-0.54Ca 6.7Ni-0.14Ca 3.7Ni-1.1Ca 5.9Ni-0.5Ca
Ni/ wt %
Ca/ wt %
preparation method
4.0 4.3 6.5 6.7 3.7 5.9
0.41 0.18 0.54 0.14 1.1 0.5
co-exchange co-exchange co-exchange co-exchange successive exchange mixture of 6.4 wt % nickel-loaded coal (0.93 g) and 6.8 wt % calcium-loaded coal (0.07 g)
in a desiccator. The analyses for this coal are as follows: C 67.6% (dry ash free), H 5.2% (dry ash free), N 0.8% (dry ash free), O 26.4% (difference), and ash 0.2% (dry). Acid-washed coal was prepared by stirring the raw coal in 0.5 M hydrochloric acid for 24 h. The concentration of carboxyl groups was 3.7 mmol/g.18 Nickel- or calcium-loaded Loy Yang brown coals were obtained with an ion-exchanging method reported previously.19-21 Nickel-calcium-loaded coals were prepared with two methods. One is a co-exchanging method: a coal sample was immersed in 500 mL of aqueous solution containing various concentrations of nickel and calcium chlorides. The pH of the solution was adjusted by adding ammonia or hydrochloric acid. After the exchange reaction, the exchanged coal sample was filtered, washed with deionized water, dried at 323 K under vacuum, and stored in a desiccator. The other is a successive exchanging method. Calcium ions were loaded on nickel-exchanged coals by an ion-exchanging method. Nickel-exchanged coals were immersed in an aqueous solution containing calcium chloride at adjusted pH. After the calciumexchange reaction, the resulting coals were washed, dried, and stored. For reference, samples of the physical mixture of nickelexchanged and calcium-exchanged coals were also used. The samples loaded with both nickel and calcium are summarized in Table 1. C1 Gas Analysis. The pyrolysis experiment was carried out in a fixed bed type pyrolyzer under helium flow at a heating rate of 5 K/min from 373 to 1173 K. The C1 gases (CO, CO2, and CH4) evolved were analyzed at every 4 min by a gas chromatograph attached to the reactor. EXAFS. Details of EXAFS analysis were described previously.14 After a sample was pyrolyzed in a glass tube under vacuum at a heating rate of 5 K/min to the desired temperatures, it was rapidly cooled to room temperature and transferred into a XAFS cell without exposure to air. We measured EXAFS spectra at room temperature to reduce the effect of Debye-Waller factor (thermal disorder) for the exact estimation of coordination numbers of the X-ray absorbing atoms. Data were collected at beam lines at the Photon Factory of High Energy Accelerator Research Organization, Tsukuba. The storage ring was operated with an electron energy of 2.5 GeV. Data were recorded in the transmission mode in the region of the Ni K edge (8331.7 eV) using ion chambers filled with nitrogen gas. EXAFS oscillation was extracted from the EXAFS raw data by using a cubic spline method and normalized with the edge height.22 The k3-weighted EXAFS spectra were Fourier transformed to R space. The inversely Fourier filtering data were analyzed by a curve-fitting technique on the basis of the single-scattering plane-wave theory.23 Experi(18) Murakami, K.; Ozaki, J.; Nishiyama, Y. Fuel Process. Technol. 1995, 43, 95-110. (19) Murakami, K.; Shirato, H.; Ozaki, J.; Nishiyama, Y. Fuel Process. Technol. 1996, 46, 183-194. (20) Murakami, K.; Yamada, T.; Fuda, K.; Matsunaga, T.; Nishiyama, Y. Fuel 1997, 76, 1085-1090. (21) Murakami, K.; Yamada, T.; Fuda, K.; Matsunaga, T. Fuel 2000, 80, 599-605. (22) Iwasawa, Y. X-ray Absorption Fine Structure for Catalysts and Surfaces; World Scientific: Singapore, 1995.
Figure 1. C1 gas evolution profile (CO: solid line; CO2: dotted line; CH4: broken line) for various coal samples; (a) acid-washed, (b) 4.2 wt % nickel-loaded, (c) 6.4 wt % nickelloaded, (d) 4.3Ni-0.18Ca, (e) 6.7Ni-0.14Ca, (f) 3.7Ni-1.1Ca, and (g) 5.9Ni-0.5Ca.
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Figure 2. Relation between the peak top temperature of R-CO evolution and the nickel loading. Nickel-loaded samples (O), nickel-calcium-loaded samples with co-exchange (4) and successive exchange (3), and physical mixture of nickel-loaded and calcium-loaded coals (2). The numerals indicate the sample numbers shown in Table 1. mentally determined phase shifts and backscattering amplitudes for Ni-Ni and Ni-O were obtained from EXAFS data for Ni foil (Ni-Ni; coordination number (N) ) 12, distance (R) ) 2.488 Å), and NiO (Ni-O; N ) 6, R ) 2.098 Å), respectively.
Results and Discussion Influence of the Coexisting Calcium on the C1 Gas Evolution during Pyrolysis of Nickel-Exchanged Coal. Figure 1 shows the C1 gas evolution profiles of the acid-washed coal and several nickelexchanged coals. The C1 gas evolution profiles of the nickel-loaded coals were affected by the amount of nickel species. Two peaks of CO evolution appeared for nickelloaded coals (Figure 1b,c), compared with a broader peak at 880 K for the acid-washed coal (Figure 1a). We have reported that two CO evolution peaks appeared and they can be deconvoluted by two Gaussian functions, one at 650-770 K, depending on the amount of exchanged nickel species (hereafter referred to as R-CO) and the other at ca. 880 K (hereafter referred to as β-CO), independent of the nickel loading.16 The influence of the presence of nickel ions on the CH4 evolution was little. A small CO2 peak appeared at 850 K for the samples exchanged with the nickel ions. The C1 evolution profile of the 0.5 wt % calcium-loaded coal was similar to that of the acid-washed coal. The pyrolysis results of the samples containing both nickel and calcium ions are also shown in Figure 1. In the pyrolysis of 6.7Ni-0.14Ca (Figure 1e), two peaks appeared at 700 K (R-CO) and 920 K (β-CO). The peak top temperature of R-CO was higher than that for the 6.4 wt % nickel-loaded sample (670 K). The CO evolution profiles of the 3.7Ni-1.1Ca (Figure 1f) and 5.9Ni-0.5Ca (Figure 1g) samples were similar to that of the 6.7Ni0.14Ca sample, indicating that calcium ions affect the evolution of R-CO; however, the effect of calcium ions is independent of the amounts of calcium and nickel species and of the sample preparation methods. Figure 2 shows the relation between the peak top temperatures of R-CO evolution and the nickel loadings. These peak top temperatures were determined by the
Figure 3. (a) Total CO yields, (b) R-CO yields, and (c) β-CO yields against the amount of nickel loaded. Nickel-loaded samples (O), nickel-calcium-loaded samples with co-exchange (4) and successive exchange (3), and physical mixture of nickel-loaded and calcium-loaded coals (2).
Figure 4. Fourier transforms for EXAFS oscillations of nickelloaded and nickel-calcium-loaded brown coals pyrolyzed at various temperatures; (a) 6.4 wt % nickel-loaded coal, (b) 6.5Ni-0.54Ca, and (c) 6.7Ni-0.14Ca.
deconvolution of the CO evolution profile into two Gaussian peaks. The peak top temperatures of R-CO for nickel-calcium-loaded Loy Yang coals with 4-6 wt % nickel were about 695 K, independent of the amount and loading method of calcium species. Figure 3 shows the yields of total, R-, and β-CO evolved during the pyrolysis of nickel-loaded and nickelcalcium-loaded samples. The total-CO yields from the nickel-calcium-loaded samples were higher than those from the nickel-loaded samples. As shown in Figure 3b,c, these increases in total-CO yields are mainly due to R-CO. The amount of CO evolved did not depend on the calcium loadings. The preparation method (coexchange, successive exchange, and physical mixing) did not affect the amount of CO evolved.
Pyrolysis of Ni-Loaded Loy Yang Brown Coals
Energy & Fuels, Vol. 16, No. 3, 2002 755
Table 2. Curve-Fitting Results for EXAFS Data for Nickel-Loaded and Nickel-Calcium-Loaded Loy Yang Brown Coal for N ( 20 % and R ( 0.02 Å parameter range
Ni-O shell
Ni-Ni shell
sample name
FT range of k/Å-1
back FT range of R/Å
curve-fitting range of k/Å-1
N
R/Å
E0/eV
∆σ2/Å2
N
R/Å
E0/eV
∆σ2/Å2
ref
6.4Ni-623K 6.4Ni-663K 6.4Ni-703K 6.4Ni-708K 6.4Ni-713K 6.4Ni-723K 4.3Ni-0.18Ca-703K 4.3Ni-0.18Ca-723K 4.3Ni-0.18Ca-743K 6.5Ni-0.54Ca-643K 6.5Ni-0.54Ca-713K 6.5Ni-0.54Ca-723K 6.5Ni-0.54Ca-743K 6.7Ni-0.14Ca-713K 6.7Ni-0.14Ca-748K 6.7Ni-0.14Ca-753K
2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45 2.0-12.45
1.0-2.0 1.0-2.7 1.0-2.7 1.4-2.7 1.4-2.7 1.4-2.7 1.0-2.7 1.0-2.7 1.4-2.7 1.0-2.0 1.0-2.7 1.0-2.7 1.0-2.7 1.0-2.7 1.4-2.7 1.4-2.7
3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0 3.0-12.0
5.3 5.2 3.6 nd nd nd 5.7 4.6 nd 5.2 4.0 3.3 3.0 4.2 nd nd
2.01 2.02 2.00 nd nd nd 1.99 2.01 nd 2.02 1.99 1.98 1.99 1.99 nd nd
-2.20 -0.15 -1.00 nd nd nd 1.21 3.46 nd -1.70 1.20 -0.63 -0.63 1.80 nd nd
0.0021 0.0042 0.0073 nd nd nd 0.0060 0.0102 nd 0.0027 0.0070 0.0050 0.0126 0.0063 nd nd
nd 0.7 3.4 3.9 4.1 5.0 0.8 3.0 5.1 nd 2.3 2.8 5.1 2.1 5.2 5.4
nd 2.51 2.48 2.46 2.46 2.47 2.50 2.48 2.47 nd 2.48 2.48 2.47 2.48 2.47 2.47
nd -5.08 -12.44 -16.91 -16.72 -14.60 -6.41 -11.34 -14.55 nd -12.47 -13.06 -13.03 -11.20 -13.70 -13.66
nd 0.0001 0.0003 0.0008 0.0009 0.0004 -0.0007 0.0004 0.0007 nd 0.0002 0.0001 0.0004 0.0000 0.0007 0.0005
14 15 15 15 15 14 this work this work this work this work this work this work this work this work this work this work
Figure 5. Coordination number of Ni-Ni bonds as a function of pyrolysis temperature; 6.4 wt % nickel-loaded coal, O; 4.3Ni-0.18Ca, 4; 6.5Ni-0.54Ca, 3; 6.7Ni-0.14Ca, 2.
Structure Changes of the Nickel Species during Pyrolysis. Figure 4 shows several results of EXAFS Fourier transforms of the 6.4 wt % nickel-loaded coal, 6.5Ni-0.54Ca, and 6.7Ni-0.14Ca samples pyrolyzed at different temperatures. All data are presented without correction for phase shift. The Fourier transform for the EXAFS spectra of the 6.4 wt % nickel-loaded sample pyrolyzed up to 623 K exhibits only one peak between 1 and 2 Å, which is ascribed to the Ni-O bond. No peak was found for the Ni-O-Ni bond. After pyrolysis up to 663 K, peaks assigned to Ni-Ni metal bonds were observed in addition to the appearance of the Ni-O bond peaks. With increasing temperature of pyrolysis, the peak intensity assigned to Ni-Ni metal bonds increased, indicating that the nickel species agglomerated. However, the temperature at which Ni-Ni metal bonds appeared in the nickel-calcium-loaded samples were higher than that in the nickel-loaded sample. Table 2 summarizes the EXAFS curve-fitting analysis results. Figure 5 shows the dependence of Ni-Ni coordination number on the pyrolysis temperature. Nickel species in the 6.4 wt % nickel-loaded sample began to aggregate to metal particles at 660 K. The experimental errors of agglomeration temperatures are within (10 K. On the other hand, nickel species aggregated at a higher temperature of 695 K in the (23) Yokoyama, T.; Hamamatsu, H.; Ohta, T. Program EXAFSH ver. 2.1; The University of Tokyo: Tokyo, 1994.
nickel-calcium-loaded coals, suggesting that the presence of calcium ions delayed the agglomeration of nickel species. The amount of calcium and preparation method little change the starting temperature of the agglomeration of nickel metal particles in nickel-calcium-loaded samples. Relation between the Agglomeration of Ni Species and the C1 Gas Evolution. We have reported that the peak top temperatures of R-CO evolved from the nickel-exchanged coals corresponded to the agglomeration temperatures of nickel species and that small nickel metal particles promoted the R-CO evolution during pyrolysis.15,16 The peak top temperature (695 K) of R-CO evolution and the temperature (695 K) of nickel agglomeration in the nickel-calcium-loaded samples were higher than those of the nickel-loaded samples. Also, the amounts of CO evolved from the nickel-calcium-loaded samples were larger than those from nickel-loaded samples. The amount of calcium loading and preparation method did not affect the peak top temperature of R-CO evolution and the agglomeration temperature of nickel metal particles. These results indicate that calcium cations prevent the aggregation of nickel particles in the nickel-calcium-loaded coals up to 695 K and small nickel metal particles formed at 695 K affect the R-CO evolution. Conclusions The amounts of R-CO evolved from nickel-loaded Loy Yang coals during pyrolysis were increased by adding calcium species. The peak top temperature of R-CO evolution and aggregation temperature of nickel metal particles in the calcium-nickel-loaded samples (Ni loading: 4-7 wt %) shifted up to 695 K from those of nickel-loaded samples (670-690 K). These effects of the calcium additives were independent of the amount of calcium loaded. Calcium species suppress the agglomeration of nickel metal particles. The small nickel metal particles enhance the R-CO evolution during pyrolysis. Acknowledgment. This work was carried out under the approval of the PF advisory committee (No. 99G252). EF010235B
