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Effect of Microstructural Changes on Gasification Reactivity of Coal Chars during Low Temperature Gasification Atul Sharma, Hayato Kadooka, Takashi Kyotani, and Akira Tomita* Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Received July 5, 2001. Revised Manuscript Received September 27, 2001
An attempt has been made to quantitatively investigate the effect of microstructural changes on gasification reactivity of coal chars. Pocahontas No. 3, Illinois No. 6, and Beulah-Zap coal char samples were gasified in 1% O2 at 500 °C or 600 °C up to 90% (daf) conversion, and their structure were observed under a high-resolution transmission electron microscope (HRTEM). A quantitative structural analysis was done based on HRTEM images to obtain structural parameters such as number of stacks, layer size diameter, and their distributions using an image analysis algorithm. Effect of mineral matter on structural changes was also studied by carrying out gasification experiments with demineralized chars. The gasification reactivity of these chars was correlated with the structural parameters.
Introduction There has been considerable interest in the study of structural change of coal char during combustion or gasification. Over the years, the presence of residual or unburned carbon in fly ash during pulverized coal combustion and high-temperature coal gasification has attracted much attention, since it is directly related to the efficiency of combustion as well as the quality of ash in its post-utilization in the cement industry.1-8 The char reactivity generally becomes low as the conversion increases, and this has been attributed to the change in carbon structure and/or to the loss of catalytic activity of mineral matter. The relative importance of these factors depends on the nature of the coal sample as well as on the reaction conditions. The structural change of coal chars due to gasification has been studied by using various techniques. Among all, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy, and optical microscopy are the most powerful techniques. The lattice fringe image detected by HRTEM gives direct information on the structure of char at an atomic level, although we need to keep in mind that “things are seldom what they seem (Oberlin9)”. Also we should
try to avoid dangers implicit in the use of microscopy; they are eclecticism and tendentiousness.10 So far, several qualitative to quantitative analyses through TEM observation have been proposed. Davis et al.6 developed a quantitative analysis method, which consists of the filtration of raw image using Fourier transform (FT) to eliminate nonperiodical structure and the analysis of the filtered image. From this analysis they obtained average fringe lengths and relative amounts of crystalline structure. Furuta et al.11 reported the structural change of resin char during CO2 gasification at 900 °C. Rouzaud et al.12 evaluated the evolution of microtexture in cokes during gasification and correlated it to their reactivity behavior using 002-dark field TEM technique. Wornat et al.13 used HRTEM to study the structural transformation of biomass chars during combustion. Palota´s et al.14 used HRTEM and an image analysis system to quantitatively study the structural changes of soot and carbon black particles during combustion. They also used the method of FT of TEM image for extraction of structural data, followed by reverse transform. Their results concluded that the changing structure of the carbon black during oxidation impacts its rate of oxidation. Shim and Hurt15 further developed their algorithm to determine mean crystallite
* Corresponding author. Fax: +81-22-217-5626. E-mail: tomita@ tagen.tohoku.ac.jp. (1) Shibaoka, M. Fuel 1985, 64, 263-269. (2) Vleeskens, J. M.; Nandi, B. N. Fuel 1986, 65, 797-802. (3) Sha, X.-Z.; Kyotani, T.; Tomita, A. Fuel 1990, 69, 1564-1567. (4) Lin, S. Y.; Hirato, M.; Horio, M. Energy Fuels 1994, 8, 598606. (5) Beeley, T. J.; Gibbins, J. R.; Hurt, R.; Man, C. K.; Pendlebury, K. J.; Williamson, J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39 (2), 564-568. (6) Davis, K. A.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. H. Combust. Flame 1995, 100, 31-40. (7) Hurt, R. H.; Gibbins, J. R. Fuel 1995, 74, 471-480. (8) Hurt, R. H.; Davis, K. A.; Yang, N. Y. C.; Headley, T. J.; Mitchell, G. D. Fuel 1995, 74, 1297-1306.
(9) Oberlin, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 22, pp 1-143. (10) Thomas, J. M. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1965: Vol. 1, pp 122-198. (11) Furuta, T.; Yamashita, Y.; Shiraishi, M. Tanso 1989, 140, 241247. (12) Rouzaud, J. N.; Duval, B.; Leroy, J. In Fundamental issues in control of carbon gasification reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publisher: The Netherlands, 1991; pp 257-267. (13) Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 131-143. (14) Palota´s, A Ä . B.; Rainey, L. C.; Sarofim, A. F.; Sande, J. B. V.; Ciambelli, P. Energy Fuels 1996, 10, 254-259. (15) Shim, H.-S.; Hurt, R. Proceedings of the 23rd Carbon Conference; Penn State University, 1997; pp 438-439.
10.1021/ef0101513 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/28/2001
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diameter, nongraphitic amorphous index, orientational order parameter, and tortuosity. In these studies, they correlated the crystalline transformation of coal chars with the decrease of global oxidation reactivity. Zielin˜ska-Blajet et al.16 characterized the structure of activated carbons using XRD and TEM techniques, and observed a qualitative agreement between the results from the two techniques. In another study, Endo et al.17,18 used HRTEM combined with image processor for observation and analysis of pores in the activated carbon fibers. Very recently, Yoshizawa et al.19 used a similar FT technique to analyze an HRTEM pattern and evaluate the layer length and stacking number distribution of KOH-activated carbons prepared at various activation temperatures. We20,21 developed a new filtration technique and statistical image analysis algorithm to obtain quantitative information such as graphene layer size, the interlayer spacing, the number of layers per stack (or the stacking number), and their distributions from HRTEM images. We applied this technique to investigate the transformations in the carbon layer ordering of Pocahontas No. 3 coal char (POC) during the hightemperature gasification,20 structural parameters of raw coals,22,23 and the structural transformations in four different coals during heat treatment.24 In this study we have attempted to correlate, for the first time, the low-temperature gasification behavior of three coal chars with the change in the carbon structure based on quantitative HRTEM image analysis technique. Also, the effect of mineral matter on structure of chars and gasification rate has been investigated by comparing the results for raw chars and demineralized chars. Experimental Section Samples. Argonne Premium Coal Samples: Pocahontas No. 3 (POC), Illinois No. 6 (IL), and Beulah-Zap (BZ), were selected for investigation. The elemental analysis and ash analysis of these coals are shown in Table 1. The coal char samples were prepared by devolatilizing the samples at 800 °C for 5 min in a fluidized bed reactor with He gas as a fluidizing agent. The char samples thus obtained were then gasified in a thermogravimetric analyzer with 1% O2-He mixture at 500 °C or 600 °C up to 60% and 90% (daf) conversion and the resultant chars were termed as POC-R60, POC-R90, IL-R60, IL-R90, BZ-R60, and BZ-R90, respectively. Reference chars (ungasified) were prepared by heat-treating the chars at 500 °C or 600 °C in pure He for 5 min, and they were termed as POC-R0, IL-R0, and BZ-R0. To understand the effect of mineral matter on carbon structure and gasification reactivity, all the three coals were demineralized. For demineralization, 1 g of coal samples (16) Zielin˜ska-Blajet, M.; Yoshizawa, N.; Furuta, T.; Maruyama, K.; Yamada, Y. Proc. Int. Conf. Coal Sci., Essen, 1997, 793-796. (17) Endo, M.; Oshida, K.; Kogiso, K.; Matsubayashi, K.; Takeuchi, K.; Dresselhaus, M. S. Proc. Mater. Res. Soc. Symp. 1995, 371, 511515. (18) Endo, M.; Furuta, T.; Minoura, F.; Kim, C.; Oshida, K.; Dresselhaus, G.; Dresselhaus, M. S. Supramol. Sci. 1998, 66 (5), 261266. (19) Yoshizawa, N.; Yamada, Y.; Shiraishi, M. J. Mater. Sci. 1998, 33, 199-206. (20) Sharma, A.; Kyotani, T.; Tomita, A. Fuel 1999, 78, 1203-1212. (21) Sharma, A.; Kyotani, T.; Tomita A. Carbon 2000, 38, 19771984. (22) Sharma, A.; Kyotani, T.; Tomita, A. Energy Fuels 2000, 14 (2), 515-516. (23) Sharma, A.; Kyotani, T.; Tomita A. Energy Fuels 2000, 14 (6), 1219-1225. (24) Sharma, A.; Kyotani, T.; Tomita A. Fuel 2001, 80, 1467-1473.
Energy & Fuels, Vol. 16, No. 1, 2002 55 Table 1. Elemental and Ash Analysis of Coals Investigated POC
IL
BZ
C (%, daf) H (%, daf) N (%, daf) O (%, daf) ash (%, dry)
91.1 4.4 1.3 2.5 4.8
77.7 5.0 1.4 13.5 15.5
72.9 4.8 1.2 20.3 9.7
SiO2 Al2O3 CaO Fe2O3 K2O MgO Na2O SrO TiO2 BaO MnO P2O5 SO3 others
32.0 20.1 12.8 15.8 0.6 2.0 2.0
43.7 18.3 7.9 18.0 2.9 1.2 0.0
1.9
1.0
0.4 12.4
0.2 6.8
18.4 10.2 24.7 8.0 0.9 0.1 7.8 1.1 0.5 0.8 0.1 0.5 17.6 1.9
were mixed with 46% concentrated hydrofluoric (HF) acid and kept for 24 h at room temperature. The HF-treated coal samples were then treated with 6 N hydrochloric (HCl) acid and kept for 12 h at 60 °C. Finally the coal samples were washed with excess demineralized water. The ash content in the demineralized coals was found to be 0.5%, 2.3%, and 0.5% (dry basis) for POC, IL, and BZ coals, respectively. The char samples were then prepared from demineralized coals at 800 °C with a soaking time of 5 min in pure He flow. The demineralized chars were gasified under conditions similar to those for raw chars. These chars were differentiated from raw chars by putting a symbol D between the coal name and char conversion in %. The samples were observed under a 200 kV TEM (JEOL, JEM-2010), and several pictures were taken from different spots to get a general view. A quantitative structural analysis is done to obtain different structural parameters such as number of stacks, layer size diameter, and their distributions. HRTEM Observation. HRTEM fringe imaging requires thin samples that partly transmit the electron beam. For poorly ordered structures, the thickness of sample is the most dangerous cause of errors as it is very difficult to eliminate the superimposition of lattice fringes.9 In the present study, the samples were hand-ground to fine powder in ethanol and sprayed over a copper microgrid for TEM observation. TEM observation was done on a 200 kV transmission electron microscope (JEOL, JEM-2010), and several pictures were taken for each sample from different spots to get a general view. The transmission electron microscope was equipped with a computerized imaging system and EDS for elemental analysis. For disordered structures or small crystallite sizes, spherical aberration influences on TEM lattice images must be considered. The phase transfer function was calculated for λ ) 0.00251 nm (electron beam wavelength) and CS ) 0.5 mm (spherical aberration coefficient) which are the conditions for the present observations. The transfer function is used to obtain the defocus position at which smoothness in contrast was guaranteed for 0.3 nm and higher basal spacing. In all cases, submicron size particles were first examined at moderate magnification to find the wedge-shaped particles that are optically thin at the edge and its diffraction pattern was taken and elemental analysis was carried out. A number of such regions were then imaged at high magnification (×500000). Structural Parameters. The TEM images were then subjected to image analysis for semiquantitative information such as interlayer spacing, d, stacking distribution, and layer size distribution. The methodology has already been described elsewhere.20,21 However, we will describe it here briefly. The digitized TEM images were converted to skeletonized images
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that showed a network of layers connected to each other by Y- or T-shaped links. The Y- and T-shaped links were mathematically removed to separate the layers. The resultant image is subjected to a layer identification program to obtain the characteristics of layers which are the inputs to image analysis algorithm. As no commercial software is available that can evaluate the desired structural parameters from HRTEM images, we developed an algorithm in FORTRAN 77 for this purpose by ourselves. Briefly, the algorithm makes use of four parameters: aspect ratio, parallelism, overlap view parameter, and the interlayer spacing to evaluate the structural parameters. The validity of algorithm was established by using a rather simple image; a good agreement was obtained between computed parameters and those counted manually.
Results The gasification reactions for raw chars and demineralized chars were examined with a thermogravimetric analyzer. To verify the reaction regime for each gasification reaction, the relation between conversion (X) and dimensionless t/τ (time/total time) was examined.25
(
X)1- 1-
t3 τ
)
(1)
The results at 600 °C showed that gasification reactions for POC-R, IL-R, POC-D and IL-D were in the chemical reaction-controlled regime, while those for BZ-R and BZ-D were controlled by gas diffusion to some extent. When the temperature for these chars was decreased to 500 °C, the reactions became chemically controlled ones. Figure 1 shows the weight loss vs time profiles for all the chars. It should be noted that the reactivity of BZ-R is especially high even at 500 °C. Figure 2 shows a typical TEM and the corresponding skeletonized image for POC-R0 char. The skeletonized image very closely represents the HRTEM image. Therefore, for other chars only skeletonized images are shown here for simplicity. Figures 3-5 show skeletonized images of POC, IL, and BZ raw and demineralized chars at different conversions, respectively. All the skeletonized images were then subjected to image analysis algorithm to obtain different structural parameters such as number of stacks, layer size diameter, and their distributions.20,21 Figure 6 shows stacking number distribution for POC, IL, and BZ raw and demineralized chars from 11 to 12 different observed spots for each sample. Similarly, the layer size distribution is shown in Figure 7. It can be seen that only POC raw char shows a wide range of distribution in stacking number and layer size. To establish a relationship among reactivity, conversion, and structural transformation, it is important to discuss the gasification rate together with the structure (average stacking number) and microtexture (local orientated domains) of chars. The results given in Table 2 include both structure and microtextural information. The average stacking number gives the structural information, while the standard deviation gives a sort of microtextural information. The average number of layers per stack and average layer diameter obtained from the above results are shown in Tables 2 and 3, respectively. The BZ and IL (25) Levenspiel, O. In Chemical Reaction Engineering; John Wiley & Sons: New York, 1962; pp 357-408.
Figure 1. O2-gasification profiles of the raw and demineralized POC, IL, and BZ chars. Gasification temperature: 600 °C for POC and IL chars; 500 °C for BZ char. Table 2. Average Stacking Number of Raw and Demineralized Coal Chars at Different Conversions X (%) POC-R
average stacking number (-) POC-D IL-R IL-D BZ-R
BZ-D
0 3.8 ( 1.3 2.8 ( 0.3 3.0 ( 0.4 3.0 ( 0.4 2.7 ( 0.3 2.5 ( 0.2 60 3.5 ( 1.0 3.0 ( 0.4 2.7 ( 0.3 2.7 ( 0.2 2.7 ( 0.5 2.4 ( 0.2 90 4.6 ( 0.9 3.3 ( 0.4 3.3 ( 0.3 2.6 ( 0.2 3.1 ( 0.7 2.5 ( 0.3 Table 3. Average Layer Size of Raw and Demineralized Coal Chars at Different Conversions X (%) POC-R
POC-D
average layer size (nm) IL-R IL-D BZ-R
BZ-D
0 1.0 ( 0.1 1.0 ( 0.1 1.0 ( 0.1 1.0 ( 0.0 1.0 ( 0.1 1.0 ( 0.0 60 1.1 ( 0.2 1.0 ( 0.1 1.0 ( 0.1 1.0 ( 0.0 1.0 ( 0.3 0.9 ( 0.0 90 1.2 ( 0.1 1.0 ( 0.1 1.0 ( 0.1 1.0 ( 0.0 1.0 ( 0.1 0.9 ( 0.0
coal chars show a small increase in average stacking number and no change in layer size with gasification. The POC shows a significant increase in stack and layer size, suggesting a significant increase in ordering.
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Energy & Fuels, Vol. 16, No. 1, 2002 57
Figure 2. A typical HRTEM and skeletonized image of POC-R0 char.
Figure 3. Typical skeletonized images of (a) POC-R0, (b) POC-R60, (c) POC-R90, (d) POC-D0, (e) POC-D60, and (f) POCD90 chars.
Figure 4. Typical skeletonized images of (a) IL-R0, (b) ILR60, (c) IL-R90, (d) IL-D0, (e) IL-D60, and (f) IL-D90 chars.
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Figure 7. Average layer size distribution in POC, IL, and BZ raw and demineralized chars at 0, 60, and 90% conversions.
shown in Figure 8b. The most prominent species in this region was Ca. Similar results were obtained for other encircled areas. The d spacing value was evaluated to be 0.25 nm by assuming the d-spacing of partially graphitized char to be 0.34 nm. It is very close to the value for CaO (0.241 nm). This result is in contrast with the results of Yamashita et al.26 and Ohtsuka et al.27 They reported that the main Ca species under similar gasification conditions was CaCO3 (d-spacing: 0.304 nm). It should be noted that we only examined, in the present study, a couple of Ca particles among million of particles in the sample. In this sense, TEM technique is not an adequate tool to study catalysis by mineral matter. Discussion
Figure 5. Typical skeletonized images of (a) BZ-R0, (b) BZR60, (c) BZ-R90, (d) BZ-D0, (e) BZ-D60, and (f) BZ-D90 chars.
Figure 6. Average stacking number distribution in POC, IL, and BZ raw and demineralized chars at 0, 60, and 90% conversions.
In Figures 6 and 7, BZ-R60 char exhibits one exceptional point with large stacking number and layer size. The photograph shown in Figure 8a corresponds to these points. Highly ordered bundle-type structure was observed, usually in close vicinity of mineral matter. The EDX analysis of the spot indicated by a white circle is
Several studies have been carried out to elucidate the role of carbon structure and catalytic activity of mineral matter on the gasification reactivity of the coal chars.1-8 In general, the most important structural parameters that can statistically account for the structural transformations are average stacking number and layer size. Most of the earlier studies that correlated the structural changes with gasification reactivity have obtained these parameters from XRD technique. Few have attempted to draw quantitative information on char structure during gasification or combustion with the aid of HRTEM observation.8,11,14,19 The present study for the first time evaluated the stacking number and layer size as well as their distribution during char transformation. Moreover, the approach we have introduced here is very new, and one can get both average stacking number and size of microtextural domains from HRTEM images. Generally, microtextural information is obtained by using 002-dark field images,12 which we have not used here, because we think that microtexture information can also be obtained by HRTEM analysis. Carbon Structure of Pyrolyzed Char. First, we will discuss the structural parameters of chars determined by HRTEM analysis. In the present study, chars have been prepared at 800 °C prior to further heattreatment at 500 °C or 600 °C. As the char preparation temperature is higher, the structure of POC-R0, IL-R0, and BZ-R0 chars might be more or less similar to the (26) Yamashita, H.; Nomura, M.; Tomita A. Energy Fuels 1992, 6, 656-661. (27) Ohtsuka, Y.; Tomita, A. Fuel 1986, 65, 1653-1657.
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Energy & Fuels, Vol. 16, No. 1, 2002 59
Figure 8. (a) HRTEM image showing highly graphitized bundle-like structures in BZ-R60 char; (b) EDX analysis results for the enclosed white circle in the above image.
chars prepared at 800 °C. In fact, the structural parameters for these chars are very close to those obtained in our previous study24 for the chars heattreated at 800 °C. Among the six chars, only POC-R0 char shows a large standard deviation for stacking number, suggesting a big heterogeneity in carbon structure. The deviation is much less for POC-D0. This suggests that the development of graphitic ordering is significantly affected by the presence of mineral matter. It is known that only some species of mineral matter are catalytically active for graphitization. The ordering of the carbon layers in the close vicinity of such species would be greatly developed in comparison to the carbon that does not have such species in close vicinity. Therefore the layer ordering of such char will vary greatly. POC-R0 is likely to contain such catalytic mineral matter. Although the char preparation temperature (800 °C) is not high enough for catalytic graphitization, such function cannot be neglected. Mineral matter is known to affect the char structure during the char preparation step and the char gasification step.3,15,20,28 Species such as Ca and Fe have an ability to increase the ordering of carbon layers.3 POC contains a significant fraction of Ca and Fe as shown in Table 1. However, the species that is responsible for the crystal growth of POC-R0 char is not clear only from the present data. In the case of IL-R0 and BZ-R0 chars, the average stacking number shows a little difference from IL-D0 and BZ-D0 chars, respectively, suggesting that mineral matter does not affect the carbon structure during the char preparation step. This may be in contradiction with the observation in Figure 8, where some evidence of catalytic effect is clearly seen. This apparent contradiction is due to the fact that the data in Table 2 are based on the average value of many images. Transformation of Carbon Structure during Gasification. It is known that the coal char structure undergoes transformation during gasification and it becomes more ordered as gasification proceeds.3,6-8,14,15,20 (28) Senneca, O.; Salatino, P.; Masi, S. Fuel 1998, 77, 1483-1493.
There have been two frequently suggested reasons for increase in ordering during the later stage of gasification. One possibility is that highly ordered portions were developed during the gasification due to thermal annealing effect and/or catalytic effect of mineral matter. Another explanation is that both ill-ordered and wellordered portions are present in the original char, and that the former would be preferentially lost in the earlier stage while the latter remains until the later stage.1-3,6-8 First, we would like to discuss the result of demineralized char, since for these cases it is unnecessary to pay attention to the effect of mineral matter. The average layer size was always around 1.0 nm and there is no significant change during gasification. The change of average stacking number with conversion is not straightforward. The stacking number showed some increase for POC-D char while it decreases for IL-D char. There was little change in stacking number for BZ-D chars. Therefore, the above statement, that the coal char becomes more ordered as gasification proceeds, does not always hold true for the demineralized chars under investigation. Since the temperature for gasification in the present study is not high enough to cause thermal annealing, the structural change is likely to be governed by the balance between the following two opposing factors. As reaction proceeds, layers are lost due to consumption of carbon, and this would results in the decrease of the number of stacking. On the contrary, if layers from less ordered portions are preferentially lost over more ordered portions, this would result in the increase of the extent of stacking. For POCD, where the average stacking number increased with conversion, the reason would be the preferential loss of less ordered portions during gasification. On the other hand, for IL-D, where the opposite trend was observed, the loss of carbon layers due to gasification would be more important than the above effect. Unlike POC-D, in case of IL-D there may not be any preferential loss of carbon layers from any specific domains that will result in an overall decrease in stacking number. These
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two opposite effects might cancel each other in the case of BZ-D char. The effect of mineral matter can be elucidated by comparing the results for raw chars with those for demineralized chars. The change of average layer size with conversion is insignificant for raw chars, except a slight increase in the case of POC-R (Table 3). The discussion will be thus focus on the stacking number. Figure 6 and Table 2 show that the stacking numbers for all three chars POC-R, IL-R, and BZ-R increased to some extent. The increase in POC-R char is higher than the other two chars. As discussed in the previous section, POC-R is likely to have some mineral matter with catalytic activity for graphitization. The gasification temperature is as low as 600 °C, and therefore the structural development of carbon would be far from socalled graphitization, but some rearrangement of carbon layers would take place under the influence of catalyst. A more plausible explanation for the increase in ordering during gasification is the preferential loss of less ordered portions over more ordered portions. POC-R0 already has some well-ordered structure, and therefore the reactivity of such portion would be smaller than the other portion. For example, POC-R0 has 3.8 average stacking and 1.3 as standard deviation. This means that we have a distribution of locally orientated domains and the size of these domains is from 2.5 to as high as 5.1. In POC-D0 we find that 2.8 is average and 0.3 standard deviation which means we have a much narrower distribution of microtexture in POC-D0 than in POCR0. This clearly suggests that the gasification rate of POC-D will be larger than that for POC-R, a fact we observed experimentally. In the case of BZ-R char, the stacking number increased with conversion, but to a lesser extent than that for POC-R. However, we observed a special case like Figure 8. It is very clear that BZ-R char contains Ca catalyst that is active for graphitization. Such graphitization seems to take place in the char preparation stage at 800 °C but not during the gasification stage at 500 °C, even though we could not find any graphitized portion in BZ-R0 char. The failure to find such area before gasification may be due to the drawback of our study, where only 10-12 areas were examined for each char sample. Figure 8 strongly suggests that Ca is responsible for graphitization, but we could not find catalytic graphitization by Ca contained in POC and IL coal samples. The reason is again that the number of observation is not enough. Relationship between Carbon Structure and Gasification Rate. It is well-known that the gasification rate of coal chars depends on several parameters, among which is the carbon structure of chars. However, as discussed in the previous section, in some cases carbon structure is significantly affected by catalytic mineral matter. Mineral matter can affect the gasification rate by two ways: (1) catalyzing the reaction thus increasing the rate, and (2) changing the carbon structure, i.e., increasing the ordering of carbon layers. It is therefore not reasonable to discuss the relationship between carbon structure and gasification rate without considering the mineral matter effect. First, we will discuss the overall gasification rate of coal chars with
Sharma et al.
Figure 9. Relationship between specific rate and conversion for POC, IL, and BZ raw and demineralized chars.
respect to carbon structure and mineral matter effect by comparing the data for raw and demineralized chars. For POC char, the reactivity has increased on demineralization (Figure 1). If mineral matter catalyzes the reaction, one would expect a decrease in rate on demineralization. However, the observed increase in rate on demineralization suggests that some other factor is controlling the rate. One possible explanation is the difference in crystallite ordering between POC-R and POC-D. As discussed before, the average stacking number of carbon structure of POC char has drastically reduced on demineralization. It can be said that POC-D char is less ordered than POC-R char is. Less ordered carbons are in general more reactive than well-ordered carbons. Another explanation has been proposed by Jenkins et al.29 They observed a similar rate increase upon demineralization of high rank coal, and they attributed this phenomenon as the result of increasing porosity. The average stacking number of IL char did not change much on demineralization. In accordance with (29) Jenkins, R. G.; Nandi, S. P.; Walker, P. L., Jr. Fuel 1973, 52, 288-293.
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this, the rate of IL-D char was a little higher than ILR. Unlike POC and IL chars, BZ coal shows a great decrease in rate on demineralization. This is not surprising because it is well-known that ion-exchanged metals on low rank coals act as excellent gasification catalysts. Usually such species are also active as a graphitization catalyst. The average stacking number for BZ-R is larger than BZ-D to some extent. Especially the area corresponding to Figure 8a gave an exceptionally high value for both stacking number and layer size as is seen in Figures 6 and 7. The above results discuss only the overall gasification rate, but the gasification rate changes throughout the course of the gasification reaction. To correlate the gasification reactivity with the structural changes, the gasification rate per unit weight of remaining char was plotted against char conversion, and the results are shown in Figure 9. As the conversion increases, all the chars, raw and demineralized, show an increase in reactivity relative to initial reactivity. The determination of rate at a later stage, such as at a conversion of >90%, is rather difficult, but it can be safely said that the reactivity at a conversion of 90% is, at least, not smaller than that at the initial stage. This tendency is not uncommon, as found recently by Yamada,30 who determined the reactivities of 40 coal chars in % CO2 reaction environment at 900 °C and found that the reactivities of 17 coals at the final stage are larger than those at the initial stage. Table 2 shows that the average stacking number increases with conversion for all chars except IL-D char. If the carbon microstructure only controls the reactivity, the gasification reactivity would decrease with conversion. It is interesting to note that BZ-R char shows a decrease in reactivity after 80%. This decrease in reactivity only with BZ-R is likely to be attributed to the presence of a highly ordered bundlelike structure (Figure 8a). It is possible that at about 80% conversion, these highly graphitized bundle-like structures will become predominant and thus decreasing the reactivity even though the reactivity during this (30) Yamada, T. Private communication.
Energy & Fuels, Vol. 16, No. 1, 2002 61
stage is still higher than the initial reactivity. Microstructural change is not the predominant parameter controlling the reactivity. The reactivity of char depends on multiple factors among which carbon structure is one of them.3,14,26 Conclusions The development of quantitative HRTEM image analysis technique provides an important added dimension in our understanding of variation of reactivity with conversion. The technique provides statistically significant results that are based on verifiable structural parameters. This can characterize the feature of coal char gasification. The effect of mineral matter on carbon structure was significant upon pyrolysis of POC, while marginal for IL and BZ coals as a whole. A wide distribution of stacking number observed in POC char was thought to be brought by the catalytic graphitization effect of mineral matter. The Ca species is likely to be responsible for development of such highly ordered carbon structures. There are many patterns on the variation of carbon structure during gasification. POC-R contains both ordered and disordered parts in the initial stage, and the ordered part dominates after gasification. BZ-R0 mainly consists of less ordered carbon, and the crystallite ordering increases upon gasification. On the other hand, IL-R and all the demineralized chars do not show big change before and after gasification, most of carbon is less ordered and rather homogeneous throughout the reaction. The relationship between variation in reactivity with gasification and crystallite ordering of char is not straightforward. The reactivity is governed by multiple factors. Increase in ordering does not necessarily mean a decrease in reactivity. Acknowledgment. The authors gratefully acknowledge the financial support provided by the Japan Society for the Promotion of Science under the “Research for the Future” project for this study. EF0101513
