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Energy & Fuels 2005, 19, 326-327
New Evaluation Method of Carbonaceous Structure on Coal Steam Gasification Yasushi Sekine,* Kiyohiro Ishikawa, Eiichi Kikuchi, and Masahiko Matsukata Department of Applied Chemistry, Waseda University, 55S-602, 3-4-1, Okubo, Shinjyuku, Tokyo 169-8555, Japan Received August 18, 2004. Revised Manuscript Received October 27, 2004 Introduction Compared to other fossil fuels, coal offers excellent economical efficiency and supply stability. In some countries, coal’s proportionate use as a primary energy source increases every year. Among the various coal-utilizing technologies, coal gasification has especially higher energy efficiency: it is known as a technology with a low environmental burden.1 Research and development of gasification plants is advancing briskly. For example, integrated coal gasification combined cycle (IGCC) and integrated coal gasification fuel cell combined cycle (IGFC) installations are planned in Japan.2-4 A salient feature of these plants is their operation under high temperatures and pressures. The coal gasification reaction can be classified into two steps: a pyrolysis step, which emits volatile matter content and generates char; and the gasification step, which reacts char with a gasifying agent. The latter step is the rate-determining one.5 The main components of coal char are fixed carbon and ash content. Generally, for combustion technology of pulverized coal, the existence of ash reduces the heating value and corrosion of combustion plants.6-8 However, a catalytic effect that is attributable to elements such as calcium is also reported in coal gasification processes.9-11 For full-scale utilization of coal gasification, many unknown facets exist regarding the gasification reaction and the fusion of the ash. Elucidation of these points is necessary for development of processes and troubleshooting during operation. Today, many studies of gasification reactivity specifically address the behavior of ash and the change of the surface area;12-17 however, the greater portion those * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) World Energy Outlooks2001 Insights Assessing Today’s Supplies to Fuel Tomorrow’s Growth; International Energy Agency (IEA): Paris, 2001; pp 243-306. (2) Shinada, O.; Yamada, A.; Koyama, Y. Energy Convers. Manage. 2002, 43, 1221-1233. (3) Huang, J.; Fang, Y.; Chen, H.; Wang, Y. Energy Fuels 2003, 17, 1474-1479. (4) Yoshiba, F.; Izaki, Y.; Watanabe, T. J. Power Sources 2004, 132, 52-58. (5) Jamil, K.; Hayashi, J.; Li, C. Fuel 2004, 83, 833-843. (6) Qui, J. R.; Zheng, Li, F.; Zheng, Y.; Zheng, C. G.; Zhou, H. C. Fuel 1999, 78, 963-969. (7) Reifenstein, A. P.; Kahraman, H.; Coin, C. D. A.; Calos, N. J.; Miller, G.; Uwins, P. Fuel 1999, 78, 1449-1461. (8) Wall, T. F.; Gupta, S. K.; Gupta, R. P.; Sanders, R. H.; Creelman, R. A.; Bryant, G. W. Fuel 1999, 78, 1057-1063. (9) Ohtsuka, Y.; Asami, K. Energy Fuels 1995, 9, 1038-1042. (10) Ohtsuka, Y.; Asami, K. Energy Fuels 1996, 10, 431-435. (11) Gopalakrishnan, R.; Bartholomew, C. H. Energy Fuels 1996, 10, 689-695. (12) Skodras, G.; Sakellaropoulos, G. P. Fuel Process. Technol. 2002, 77-78, 151-158.
Table 1. Proximate and Ultimate Analysis of South African Bituminous Coal (SS020) Proximate Analysis [a.d. %] water
ash
3.50
12.90
Ultimate Analysis [d.b. %]
volatile fixed matter carbon 30.90
52.70
C
H
N
S
O
71.10 4.23 1.76 0.65 8.89
Ash Composition [d.b. %] SiO2
Al2O2
Fe2O2
CaO
MgO
Na2O
K2O
SO3
TiO2
48.00
29.39
5.00
7.13
2.60
0.29
0.83
3.74
1.26
studies comprises investigations of the macroscopic perspective: studies of the entire coal char. Nevertheless, coal does not have a uniform composition; it is very difficult to grasp gasification behavior precisely from only an investigation of the macroscopic viewpoint. For that reason, a more intimate method of investigation is required. Consequently, we propose a new method of macroscopic viewpoint, which analyzes reactivity in the location of coal char. It is a method for overlaying both the distribution mapping of carbonaceous structure by laser Raman spectroscopy (LRS) and the distribution mapping of elements using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). Consequently, results clarified that ash partly affected the carbonaceous structural change. Experimental Section As for the coal samples, we used standard coals offered from CCUJ (Center for Coal Utilization, Japan). Table 1 shows the proximate and ultimate analysis of the coals. First, to analyze the carbonaceous structural change in different conversion rates, the gasification of South African semibituminous coal (SS020) was examined and an investigation of the macroscopic viewpoint was conducted. The experiment for coal char gasification was conducted with a flow-type apparatus. Coal char (20 mg) was set in a quartz tube with an outer diameter of 6 mm and an inner diameter of 4 mm. Argon gas was fed at a rate of 200 cm3/min; the reactor was heated with an infrared image gold furnace. After the temperature reached 573 K, steam was supplied, at a rate of 100 cm3/min, as a reaction gas. The heating rate was 500 K/min until the temperature reached a predetermined reaction level. After the reaction, at certain conversion rate, opening the furnace and cooling the coal quickly in the atmosphere stopped the reaction. Finally, the char (13) Lemaignen, L.; Zhuo, Y.; Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Fuel 2002, 81, 315-326. (14) Ye, D. P.; Agnew, J. B.; Zhang, D. K. Fuel 1998, 77, 1209-1219. (15) Ohme, H.; Suzuki, T. Energy Fuels 1996, 10, 980-987. (16) Alonso, M. J. G.; Borrego, A. G.; Alvarez, D.; Parra, J. B.; Menendez, R. J. Anal. Appl. Pyrolysis 2001, 58-59, 887-909. (17) Leboda, R.; Zieba, J. S.; Grzegorczyk, W. Carbon 1998, 36, 417425.
10.1021/ef049793r CCC: $30.25 © 2005 American Chemical Society Published on Web 11/10/2004
Communications
Energy & Fuels, Vol. 19, No. 1, 2005 327
Figure 1. Relationship between conversion and the ratio between the peak widths at half height for the graphite band and the disorder band (RG ) wG/wD) on the steam gasification of SS020 coal (1273 K, 0.1 MPa). after the reaction was weighed and the conversion rate was calculated. The conversion was defined by
x)
W0 - W W0
where x is the conversion, W0 the initial weight of char, and W the weight of char. Subsequently, the carbonaceous structure of each char at each conversion rate was evaluated using LRS (JASCO, Inc., model NRS-2100). The excitation wavelength was 532 nm; the laser spot was 2 µm. Three measurements were repeated for 10 s per point, and 200 arbitrary points were followed. These observations yielded a Raman spectrum with a graphite band (G-band) and disorder band (D-band),18,19 at 1590 and 1350 cm-1, respectively. The widths of each peak at halfheight were defined as wG and wD, respectively. The ratio of wD and wG (wD/wG) was defined as RG, which was the parameter for the graphitization of char.
Results and Discussion The average value, the maximum value, and the minimum value of RG were calculated for all measurement results of 200 arbitrary points. The relationship of RG with the conversion rate is shown in Figure 1. A monotonic increasing trend of the average value of RG was observed with increasing conversion rate. This phenomenon originated from the increase of graphitic carbon by gasification of highly reactive carbon. On the other hand, even though the conversion increased, the minimum value of RG was almost stable. This observation means that some sites reacted but others did not react at all. To discuss the carbonaceous structural change and its influence factor in great detail, we then conducted a microscopic investigation of raw SS020. To produce a correct map of carbonaceous structure, a field of the coal was flattened carefully using a grinder (model P320, Mecapol S.A., Poland) before pyrolysis. After evacuation drying for 1 day, the heating of coal under an argon atmosphere for devolatilization was continued at a rate of 1000 K/min, up to 1273 K; it was then pyrolyzed for 60 s. After pyrolysis, the carbonaceous structure was mapped using LRS. The distribution of carbonaceous structure was analyzed. Although the fundamental conditions were identical to those mentioned previously, the intervals of measurement were 2 µm. Measurements were performed at 50 points in the directions of the X-axis and Y-axis, (18) Nakamizo, M.; Kammereck, R.; Walker, P. L. Carbon 1974, 12, 259-267. (19) Nakamizo, M.; Honda, H. Carbon 1978, 16, 281-283.
Figure 2. Relationship between carbonaceous structure and silicon distribution: (a) mapping of the carbonaceous structure after pyrolysis, (b) mapping of the carbonaceous structure at a high conversion rate (X ) 0.7), and (c) mapping of the silicon distribution at a high conversion rate (X ) 0.7).
respectively. Thereby, Raman data for 2500 points were acquired. After RG ratios were calculated, data were classified using gradations for respective ratios. Next, steam gasification was performed for 180 s using the char after the first carbonaceous structure mapping, under the same conditions as the examination. After gasification, carbonaceous structural mapping was performed again in exactly the same portion that was mapped after pyrolysis. Finally, elemental distribution mapping was performed using SEM/EDX (model S-3000N, Hitachi High-Technologies Corp.) in precisely the same area as that of the carbonaceous mapping. The results from this microscopic viewpoint, using LRS and SEM/EDX, are shown in Figure 2. Panels a and b in Figure 2 show the distribution mapping of the carbonaceous structure after pyrolysis and that of at high conversion rate, respectively. These two figures show that the dark part of the image had a higher ratio of graphitic carbon. In the part of specification at high conversion rate, the ratio of graphitic carbon was increased; however, the difference for every part was hardly seen for char after the pyrolysis. On the other hand, Figure 2c shows a distribution mapping of silicon; the part with a white color shows that silicon existed. These diagrams demonstrate that the existence of silicon was related to the rate of nongraphitic carbon. When the distribution mapping of the carbonaceous structure and the distribution mapping of silicon at a high conversion rate were compared, it was considered that the existence of silicon was related to the rate of nongraphitic carbon. These results suggest that the existence of silicon suppressed the char conversion. One possibility should be considered to explain these phenomena: the surface of carbon had been covered with silicon, so the reaction gas could not come into contact with the carbon. Similar analysis for other elements revealed a similar tendency of aluminum; however, a conspicuous tendency was not shown by other elements. Conclusion This study clarified the influence of carbonaceous structural change and ash behavior affecting reactivity. An analysis method using microscopy was proposed. This method allowed detailed analysis to a greater extent than that allowed by conventional macroscopy. Consequently, the effect that ash had on the change of carbonaceous structuresnamely, the influence that ash had on the reactivity of charswas clarified. Acknowledgment. This study was financially supported in part by BRAIN-C (CCUJ/NEDO). EF049793R
