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Energy & Fuels 2008, 22, 4034–4038
Changes in Char Structure during the Gasification of a Victorian Brown Coal in Steam and Oxygen at 800 °C Xin Guo,†,‡ Hui Ling Tay,† Shu Zhang,† and Chun-Zhu Li*,† Department of Chemical Engineering, PO Box 36, Monash UniVersity, VIC 3800, Australia and State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology, Wuhan, Hubei, 430074, P. R. China ReceiVed July 2, 2008. ReVised Manuscript ReceiVed August 25, 2008
Char structure is an important factor influencing its reactivity during gasification. This study aims to investigate the changes in char structure during the gasification of brown coal. A Victorian brown coal was gasified in a fluidized-bed/fixed-bed reactor at 800 °C in atmospheres containing 15% H2O, 2000 ppm O2, or 15% H2O and 2000 ppm O2, respectively. Although the char gasification in 2000 ppm O2 was mainly rate-limited by the external diffusion of O2, the char-H2O reaction was mainly rate-limited by the chemical reactions. The structural features of char at different levels of char gasification conversion were examined with FT-Raman spectroscopy. Our results show that the chars from the gasification in the mixture of 2000 ppm O2 and 15% H2O had almost the same features as the chars from the gasification in 15% H2O alone when the same levels of char conversion were achieved. Both the thermal decomposition of char and the char gasification reactions could result in changes in char structure during gasification.
Introduction Gasification is a major route to clean energy for both fossil fuels (e.g., coal) and biomass.1 Low-rank fuels such as brown coal and biomass are particularly suitable for low-temperature gasification to achieve very high efficiencies.1-3 However, the full potentials of low temperature gasification can only be realized if a high level of char conversion can be achieved through gasification. As the gasification of char is normally the slowest step for the gasification of coal and biomass, maintaining high char reactivity is of paramount importance, especially for low-temperature gasification processes in which the char gasification reactions may be slow due to low temperature. The reactivity of a char from a low-rank fuel (brown coal and biomass) is mainly determined3-5 by the concentration and dispersion of (inherent) catalysts in char, the presence of inhibiting species, and the structure of char. The structure of char itself also affects the dispersion of catalysts in the char as well as the catalyst-char interaction; both are vital in determining the reactivity of char during gasification. Therefore, understanding the changes in char structure during gasification is indispensable for a better understanding of the char gasification mechanism. The quantification of the structural features of chars from brown coal has been a major challenge.3,18 Many commonly used analytical techniques can only provide very limited information about char structure. For example, Fourier transform * To whom correspondence should be addressed. Fax: +61 3 9905 5686; e-mail:
[email protected]. † Monash University. ‡ Huazhong University of Science and Technology. (1) Li, C.-Z. Process Saf. EnViron. Prot. 2006, 84, 407–408. (2) Hayashi, J.-i; Hosokai, S.; Sonoyama, N. Process Saf. EnViron. Prot. 2006, 84, 409–419. (3) Li, C.-Z. Fuel 2007, 86, 1664–1683. (4) Takarada, T.; Tamai, Y.; Tomita, A. Fuel 1985, 64, 1438–1442. (5) Miura, K.; Hashimoto, K.; Silveston, P. L. Fuel 1989, 68, 1461– 1475.
infrared (FT-IR) spectroscopy, although powerful for examining the functional groups in char, gives little information about the main carbon skeleton structure of the char. The lack of welldefined crystal structure in char, especially if produced at low temperatures (e.g., 103 K s-1) to release volatiles and to form char. However, the reactor differed from a normal fluidized-bed reactor by having another sintered quartz frit installed in its free board. Although the volatiles were able to pass through this frit in the freeboard and exit the reactor, the majority of char particles was elutriated out of the sand bed and was then retained underneath the frit to form a thin fixed bed. As coal particles were continuously fed into the reactor, the volatiles generated from the coal particles at a later time would have to pass through and interact with the char bed (underneath the frit) formed from the coal particles fed into the reactor at an earlier time. The volatile-char interactions have been shown3,20,21,26-29 to influence almost every aspect of brown coal gasification. At the conclusion of coal feeding, the char was then gasified in situ. The gasification of char could be terminated at any preset holding time (thus, preset char gasification conversion level) by lifting the reactor out of the furnace and turning off gasifying agents immediately to allow the char residue to cool down naturally. The char yield was determined by weighing the reactor and char before and after the experiment. The char yields at different holding times for each of the gasification atmospheres were determined in separate experiments. The char was then collected for further analysis. Three different gasification atmospheres were used: 2000 ppm O2, 15% H2O, and 2000 ppm O2 and 15% H2O. The balance gas in all experiments was argon (>99.999%). The steam required for gasification was generated inside the reactor by continuously feeding water into the reactor with an HPLC pump. Unless specified otherwise (e.g., in Table 1), the total gas flow rate was 1.3 L min-1 (measured under ambient conditions). Char Characterization. The FT-Raman spectra of chars were recorded with a PerkinsElmer Spectrum GX FT-IR/Raman spectrometer following the procedure outlined previously.18-21 To record a Raman spectrum, the char sample was ground and diluted to 0.25 wt % with spectroscopic grade KBr. An InGaAs detector operated at room temperature was used to collect Raman scattering with a back-scattering configuration. The excitation laser wavelength was 1064 nm. The spectral resolution was 4 cm-1, and the nominal laser power was 100 mW. The Raman spectra in the range between 800 and 1800 cm-1 were curve-fitted with 10 Gaussian bands using the GRAMS/32 AI software (v 6.00). A detailed discussion of Raman band (27) Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S.; Shimada, T.; Hayashi, J.-I.; Li, C.-Z.; Chiba, T. Fuel 2006, 85, 340–349. (28) Wu, H.; Quyn, D. M.; Li, C.-Z. Fuel 2002, 81, 1033–1039. (29) Wu, H.; Li, X.; Hayashi, J.-I.; Chiba, T.; Li, C.-Z. Fuel 2005, 84, 1221–1228.
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Figure 1. A typical example of Raman spectrum fitted with 10 bands. The char was prepared from the gasification of Loy Yang brown coal in 2000 ppm O2 and 15% H2O at 800 °C for 15 min.
assignments may be found elsewhere.18-21 A typical example of such curve-fitting is shown in Figure 1. Of the 10 bands, only 6 bands, that is, G, GR, VL, VR, D, and S bands, were significant for these chars, although the inclusion of the other 4 bands allowed a better curve fit and, more importantly, allowed reasonable band widths to be achieved. The G band at 1590 cm-1 mainly represents aromatic ring quadrant breathing, and the contribution of graphite E22g vibration is minimal because of the lack of true graphite structures in this type of chars.18,20,21 The D band (1300 cm-1) represents aromatic structures with not less than 6 fused benzene rings (or equivalent) and the contribution due to “defect structures” in the traditional sense is minimal: the absence of true graphite structure makes the concept of “defect structures” meaningless for these chars. The “overlap” between the D and G bands (Figure 1) has been deconvoluted into three bands: GR (“right of G band”) centered at 1540 cm-1, VL (“valley left band”) centered at 1465 cm-1 and VR (“valley right band”) centered at 1380 cm-1. These bands represent typical structures in amorphous carbon (especially smaller aromatic ring systems) as well as the semicircle breathing of aromatic rings. The shoulder peak, S band (1185 cm-1), mainly represents Caromatic-Calkyl, aromatic (aliphatic) ethers, C-C on hydroaromatic rings, hexagonal diamond carbon sp3, and C-H on aromatic rings.
Results and Discussion Char Yield as a Function of Holding Time. Char yields at different holding times for each of the gasification atmospheres (2000 ppm O2, 15% H2O, and 2000 ppm and 15% H2O) are presented in Figure 2. The char yield at time 0 represents the amount of char remaining in the reactor immediately after the completion of coal feeding. The char yields at zero holding time in all three atmospheres were almost the same. This observation was not unexpected. First, the strong interactions between volatiles and char during the time when coal particles were continuously fed into the reactor would have greatly inhibited the gasification of char.27 In particular, O2 would have been preferentially consumed by the volatiles. In any case, as the data in Figure 2 indicate, 2 min (i.e., the feeding time) seems to be too short to result in very significant gasification of char under the current experimental conditions. As expected, the char gasification rate in the mixture of 2000 ppm O2 and 15% H2O was quicker than that in 2000 ppm O2
Figure 2. Char yield as a function of holding time for the gasification of Loy Yang brown coal in 2000 ppm O2, 15% H2O, and 2000 ppm O2 and 15% H2O at 800 °C.
or in 15% H2O alone. However, a detailed examination of the data in Figure 2 indicates that there appears to be a relatively rapid weight loss (gasification) of char in 2000 ppm O2 at the initial stage, which is followed by further slower gasification of char by 2000 ppm O2. This rapid initial weight loss of char would also exist in atmospheres containing H2O (15% H2O or 15% H2O and 2000 ppm O2), although not as distinct as in the case of 2000 ppm O2. As will be discussed below, this initial rapid weight loss of char is at least partly due to the thermal decomposition (annealing) of char. Evolution of Char Structure during Gasification. Figure 3 shows the changes in the total Raman peak area between 800 and 1800 cm-1 during the course of char gasification in 2000 ppm O2, 15% H2O, and 2000 ppm O2 and 15% H2O. In all cases, there were significant initial decreases in the total Raman peak area, and the decrease was particularly profound for the char gasification in 2000 ppm O2. As was explained in detail
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Figure 3. Changes in Raman total peak area (800-1800 cm-1) during char gasification in 2000 ppm O2, 15% H2O, and 2000 ppm O2 and 15% H2O at 800 °C.
elsewhere,18,20,21 the total Raman peak area for this type of chars is determined by three major factors: the light absorption ability, the Raman scattering ability, and the content of oxygen that can act as a “sensitizer” (i.e., resonance effects) to enhance Raman intensity. In the cases of the chars shown in Figure 3, as soon as the feeding of coal was stopped, the gas atmosphere surrounding the char particles would change drastically: a mixture of volatiles, partially degraded volatiles, and gasifying agent(s) would be replaced by the gasifying agent(s) alone. Some structures that were relatively stable in the presence of volatiles might decompose as a result of this change in gas atmosphere surrounding the char particles. The initial decreases in the total Raman peak area in Figure 3 are believed to be partly due to the loss of oxygen from the char, for example, due to the thermal decomposition (annealing) reactions. The loss of hydrogen during char decomposition, which would also enhance ring condensation, may also contribute to the decreases in the total Raman peak area. Another possible factor contributing to the observed decreases in the Raman intensity may be the preferential consumption of smaller rings due to their gasification and/or conversion to larger ones. This argument is supported by the Raman band ratios I(GR+VL+VR)/ID and IGR/ID shown in Figure 4. The ratios reflect qualitatively the relative proportion of larger (>6 rings) aromatic ring systems (D band) and smaller ones (GR + VL + VR bands, especially the GR band18,20,21). Corresponding to the initial decreases in the Raman peak area in Figure 3 are the initial decreases in the Raman band ratios I(GR+VL+VR)/ID and IGR/ ID in Figure 4. It should be noted that although the GR band is more sensitive than the sum of GR + VL + VR bands to the smaller aromatic ring systems in char, relatively big uncertainties are expected for the peak area of GR than for the sum of GR + VL + VR bands due to the close proximity of these three bands (see Figure 1). For this reason, Figure 4b (GR band) shows somewhat bigger scatters than Figure 4a (the sum of GR + VL + VR bands). The preferential consumption of smaller ring systems (or their conversion to bigger ones) was also observed inourpreviousstudiesonthethermaldecomposition(pyrolysis)18,19,22 and gasification20,21 of brown coal/biomass char. Following the initial decreases, the total peak areas for the char gasification in 15% H2O and 2000 ppm O2 and 15% H2O increased again, whereas that in 2000 ppm O2 remained
Figure 4. Changes in ratios (a) I(GR+VL+VR)/ID and (b) IGR/ID during char gasification in 2000 ppm O2, 15% H2O, and 2000 ppm O2 and 15% H2O at 800 °C.
unchanged with further gasification (i.e., decreases in char yield in Figure 3). We will first discuss the case of char gasification in 2000 ppm O2. The data in Figures 3 and 4 indicate that there were little changes both in the total Raman peak area and in the Raman band ratios I(GR+VL+VR)/ID and IGR/ID when the char yield was further decreased from about 25 to 5 wt % through gasification in 2000 ppm O2. Little changes were also observed for other spectral properties (e.g., the relative intensity of the S band shown in Figure 5) within this char yield range. Different from our previous observations20 on the char-O2 reaction at 400 °C, these data in Figures 3-5 indicate that the char-O2 at 800 °C did not show profound preferential consumption of certain structures in char. This is related to the nature of char-O2 reaction at high temperatures (e.g., 800 °C in this study), where diffusion may be a rate-limiting step. Further experiments indeed prove that the char conversion in 1000-4000 ppm O2 was greatly affected by the overall gas flow rate (Table 1), indicating that the external diffusion was an important rate-limiting step for the char-O2 reaction at 800 °C under the present experimental conditions. This means that O2 was mainly consumed in its reaction with char as soon as it reached the char surface due to the very high rate of the char-O2 chemical reaction at
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Figure 5. Changes in the relative intensity of S band expressed as a percentage of the total Raman peak area during the char gasification in 2000 ppm O2, 15% H2O, and 2000 ppm O2 and 15% H2O at 800 °C.
800 °C. There was little chance for O2 to reach the internal surface of char structure. In other words, the char-O2 reaction at 800 °C would exert little influence on the char structure beyond the char surface. When the residual char was examined with Raman spectroscopy, little change due to the char-O2 reaction would have been observed. Any observed changes in char structure must have been due to the thermal decomposition of char itself that took place throughout the char matrix. This provides a plausible explanation for the data in Figures 3-5. The rapid initial decreases in the Raman total peak area (Figure 3) and the rapid initial decreases in the ratios of I(GR+VL+VR)/ ID and IGR/ID (Figure 4) for the case of char-O2 reaction were all mainly due to the thermal decomposition (annealing) of char. The thermal decomposition of char slowed down with increasing time and thus little further decreases in the Raman total area or the I(GR+VL+VR)/ID and IGR/ID ratios were observed at the later stages of char gasification with 2000 ppm O2. We now consider the gasification of char in 15% steam or in 15% H2O and 2000 ppm O2. The char-H2O chemical reaction rate would be a lot slower than the char-O2 chemical reaction rate. Therefore, the char-H2O reaction system was mainly ratelimited by the chemical reaction itself. Therefore, the char-H2O reaction must have taken place throughout the whole char particles. In agreement with our previous study21 on the char-H2O reaction, the data in Figure 4 confirm that the smaller aromatic ring systems were preferentially gasified. Also in agreement with the previous study,21 the relative S band intensity (mainly the stable cross-links21) remained almost unchanged (Figure 5) because these structures were not preferentially gasified by H2O. Despite decreases in the relative proportions of smaller ring systems (Figure 4), the total Raman intensity increased (Figure 3) after the initial decrease. It is suspected that the O-containing or H-containing structures in the initial/ nascent char were not the same as those preferred for the char-H2O reaction. After the initial destruction of these
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structures due to thermal decomposition (annealing) of char, new O-containing (or H-containing) structures might have introduced into the char to result in the increases in the total Raman peak area (Figure 3). All data in Figures 3-5 showed great similarities in char structural features between the chars from the char gasification in 2000 ppm O2 and 15% H2O and those from the gasification in 15% H2O when the same char yields were achieved. On the basis of the above discussion, it is clear that the char-O2 reaction took place mainly at the “external” char surface on which the char gasification rate due to the char-H2O reaction was negligible. Therefore, in terms of the observed structural features of char residual during the gasification in 2000 ppm O2 and 15% H2O, the char-O2 reaction would behave like the ablative removal of char surface nonpreferentially and would exert negligible effects on the structure of the residual char. On the other hand, the char gasification due to the char-H2O reaction mainly took place on the internal pore surface in the char and was dominant in determining the structure of the char residual. Therefore, the gasification in 2000 ppm O2 and 15% H2O gave the char with the same structural feature as that in 15% H2O alone. Conclusions A Victorian brown coal sample was gasified in a novel fluidized-bed/fixed-bed reactor at 800 °C in 2000 ppm O2, 15% H2O, and 2000 ppm O2 and 15% H2O. The char gasification rate was higher in the mixture of 2000 ppm O2 and 15% H2O than in 2000 ppm O2 or 15% H2O alone. The characterization of the char samples collected at different char conversion levels with FT-Raman spectroscopy has provided important information about the changes in char structure during gasification. The char-O2 reaction system was rate-limited by the mass transfer of O2 onto the char surface. This char-O2 reaction had little influence on the structure of the unreacted char. The char-H2O reaction tended to consume the smaller aromatic ring systems preferentially. During the gasification of char in 2000 ppm O2 and 15% H2O, the char-O2 reaction only consumed the char from its external surface, whereas the char-H2O reaction consumed the char mainly from its internal surface. As a result, the char from the gasification in 2000 ppm O2 and 15% H2O had almost the same features as the char from the gasification in 15% H2O alone when the same level of char conversion was achieved. Acknowledgment. The authors gratefully acknowledge the support of this study by the Victorian State Government under its Energy Technology Innovation Strategy program and the Latrobe Valley Generators (International Power Hazelwood, International Power Loy Yang B, Loy Yang Power, and TRUenergy). X. G. also acknowledges the financial support by the National Key Basic Research and Development Program, the Ministry of Science and Technology, China (2006CB705806), and the National Natural Science Foundation of China (NSFC) (90410017). EF800528C
