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Energy & Fuels 2007, 21, 1816-1821
Characterization of the Structural Features of Char from the Pyrolysis of Cane Trash Using Fourier Transform-Raman Spectroscopy Daniel M. Keown,† Xiaojiang Li,†,‡ Jun-ichiro Hayashi,§ and Chun-Zhu Li*,† Department of Chemical Engineering, PO Box 36, Monash UniVersity, Victoria 3800, Australia, and Centre for AdVanced Research of Energy Technology, Hokkaido UniVersity, N13-W8, Kita-ku, Sapporo 060 8628, Japan ReceiVed January 28, 2007. ReVised Manuscript ReceiVed March 11, 2007
Structural features of chars from the pyrolysis of cane trash in a fluidized-bed/fixed-bed reactor under both slow and fast heating rate conditions were investigated using Raman spectroscopy. Chars from the pyrolysis of coals were also investigated for comparison. Spectra were curve-fitted using 10 Gaussian bands representing different structural features of the chars. Differences in the total Raman intensity between cane trash chars and the chars of 3 coals of varying rank were great at low pyrolysis temperature (600 °C) but decreased as the pyrolysis temperature was increased to 900 °C. Both low (10 K min-1) and high (>103-104 K s-1) particle heating rates resulted in increased aromatization and relative aromatic ring size with increasing pyrolysis temperature from 600 to 900 °C. However, pyrolysis at slow and fast heating rates gave chars of very different structural features as revealed by Raman spectroscopy.
1. Introduction Reforming/gasification of biomass will play an important role for the efficient utilization of biomass as a renewable source of energy and chemical feedstock. The ultimate efficiency of a reformer/gasifier would depend on the conversion level of char that can be achieved through gasification. For a gasifier of a given volume, the reactivity of char is an important factor determining the conversion level of char. The gasification reactivity of char is influenced by the structure of the char as well as the concentration of catalysts such as alkali and alkaline earth metallic (AAEM) species.1-4 The structure of the char also helps determine the physicochemical form in which the catalysts exist in the char.5 Clearly, the understanding of the structural features of char is important to the development of efficient biomass utilization technologies. Several techniques have been used to investigate the carbon skeleton structural features of biomass chars. NMR spectroscopy has identified different biomass char structures such as aromatics, aliphatic carbons, and O-containing structures at low temperatures, but it lacks the sensitivity to identify these structures at temperatures greater than 400 °C,6,7 even though FTIR studies show their existence.8,9 While FTIR spectroscopy
is useful in examining various bonds in O-containing functional groups,7-10 it is of limited use in exploring the less-polar aromatic structures as well as sp3-sp3 or sp3-sp2 cross-linkages between structures. Raman spectroscopy has been used to study the structural features of chars from the pyrolysis and gasification of biomasses.8,11-13. In these studies, the Raman spectral parameters derived from highly ordered carbon materials, mainly the widths, position, and intensities of the G (graphite) and D (defect) bands,14 were used to investigate the biomass/char structure and its correlation to other characteristics, for example, graphene crystallite size, pore size, and surface area. However, unlike the spectra of highly ordered carbon materials such as graphite that show two clearly distinct and well-resolved peaks of D and G,14,15 the spectra of biomass chars show much broader bands near G and D than the highly ordered carbon materials8,11-13 The overlaps between the D and G bands as well as the shoulders at the two sides of the G and D bands for the Raman spectra of biomass chars contain rich information about the structural features of biomass chars. In other words, the Raman spectra of highly disordered carbonaceous materials such as amorphous carbon or chars differ considerably from that of
* Corresponding author. Phone: +61 3 9905 9623. Fax: +61 3 9905 5686. E-mail:
[email protected]. † Monash University. ‡ Present address: GE (China) Research and Development Center Co., Ltd, 1800 Cailun Road, Zhangjiang High-Tech Park, Pudong New District, Shanghai, 201203, P. R. China. § Hokkaido University. (1) Takarada, T.; Tamai, Y.; Tomita, A. Fuel 1985, 64, 1438-1442. (2) Miura, K.; Hashimoto, K.; Silveston, P. L. Fuel 1989, 68, 14611475. (3) Wu, H.; Hayashi, J.-i.; Chiba, T.; Takarada, T.; Li, C.-Z. Fuel 2004, 83, 23-30. (4) Quyn, D. M.; Wu, H.; Hayashi, J.-i.; Li, C.-Z. Fuel 2003, 82, 587593. (5) Wu, H.; Li, X.; Hayashi, J.-i.; Chiba, T.; Li, C.-Z. Fuel 2005, 84, 1221-1228.
(6) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Hajaligol, M. R. Fuel 2001, 80, 1825-1836. (7) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X.; Chan, W. G.; Hajaligol, M. R. Fuel 2004, 83, 1469-1482. (8) Theodoropoulou, S.; Papadimitriou, D.; Zoumpoulakis, L.; Simitzis, J. Anal. Bioanal. Chem. 2004, 379, 788-791. (9) El-Hendawy, A.-N. A. J. Anal. Appl. Pyrolysis 2006, 75, 159-166. (10) Li, X.; Hayashi, J.-i.; Li, C.-Z. Fuel 2006, 85, 1509-1517. (11) Theodoropoulou, S.; Papadimitriou, D.; Zoumpoulakis, L.; Simitzis, J. Diamond Relat. Mater. 2004, 13, 371-375. (12) Paris, O.; Zollfrank, C.; Zickler, G. A. Carbon 2005, 43, 53-66. (13) Darmstadt, H.; Pantea, D.; Suemmchen, L.; Roland, U.; Kaliaguine, S.; Roy, C. J. Anal. Appl. Pyrolysis 2000, 53, 1-17. (14) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126-1130. (15) van Doorn, J.; Vuurman, M. A.; Tromp, P. J. J.; Stufkens, D. J.; Moulijn, J. A. Fuel Process. Technol. 1990, 24, 407-413.
10.1021/ef070049r CCC: $37.00 © 2007 American Chemical Society Published on Web 04/10/2007
Char from the Pyrolysis of Cane Trash
“ideal” polycrystalline graphite:16,17 the presence of a wide variety of aliphatic structures and O-containing structures in biomass char means that the biomass char will be distinctly different from the well-structured carbon materials. Thus, the methodology for the study of highly ordered/structured carbon materials such as graphite is flawed when applied directly to the investigation of structural features of such highly disordered carbon materials such as biomass chars. Recent Raman studies of chars from Victorian brown coal10,18,19 were able to reveal the evolution of char structure under pyrolysis and gasification conditions by deconvoluting the entire Raman spectra into 10 bands representing chemical structural features of the chars, and in this way, the information contained in the “overlap” and shoulders of the G and D bands was revealed. The purpose of this study was to investigate the structural features of cane trash char produced with different heating rates and temperatures using Fourier transform-Raman (FT-Raman) spectroscopy. The Raman spectra were deconvoluted into 10 bands representing the typical structures to be found in the chars from biomass and other low rank fuels. The results showed differences between the structures of chars produced at different heating rates as well as the effect of the contents of AAEM species on the structural features of biomass char. 2. Experimental 2.1. Biomass Samples and Pyrolysis. A cane trash sample (mainly cane residues left in the field), with a particle size range of 125-210 µm, from Queensland (Australia) was used in this study. Analysis of the sample gave an ash yield of 7.6% (db) with the following elemental analysis: C, 49.5; H, 6.1; N, 0.31; S, 0.08; and O, 44.0 wt % (daf). Pyrolysis of the cane trash was carried out in a one-stage quartz fluidized-bed/fixed-bed reactor. The details of this reactor have been given elsewhere.20,21 Different from a normal fluidized-bed reactor, a quartz frit was installed in the freeboard to separate char from evolved volatiles at the reaction temperature. The reactor could be operated in either fast or slow heating rate modes. In the fast heating rate mode, about 2 g of biomass particles were entrained at approximately 130 mg min-1 (giving a total feeding time of about 15 min) in argon through a water-cooled feeding arm into the hot sand bed where the biomass particles were heated up at a rate in excess of 103-104 K s-1.22 As soon as the feeding was completed, the reactor was raised out of the furnace and allowed to cool naturally. While the volatiles passed through the frit in the freeboard and exited the reactor, the char particles were retained by the frit to form a thin char bed within the reactor. The reactor thus has combined features of a fluidized-bed reactor and a fixed-bed reactor. In the slow heating rate mode, the biomass particles (∼2 g and accurately weighed) were charged into the reactor (with sand) at room temperature and the reactor was heated slowly (nominally 10 K min-1) to the desired temperature. The sand and biomass particles were separated during the early stages of heating (before major volatile release occurred) with the biomass particles forming a fixed-bed underneath the top frit and the sand acting only as a heating medium for the argon gas. The reactor was held the desired peak temperature for 15 min before being raised out of the furnace and allowed to cool naturally with argon gas flowing continuously. (16) Johnson, C. A.; Patrick, J. W.; Mark Thomas, K. Fuel 1986, 65, 1284-1290. (17) Schwan, J.; Ulrich, S.; Batori, V.; Ehrhardt, H.; Silva, S. R. P. J. Appl. Phys. 1996, 80, 440-447. (18) Li, X.; Hayashi, J.-i.; Li, C.-Z. Fuel 2006, 85, 1700-1707. (19) Li, X.; Li, C.-Z. Fuel 2006, 85, 1518-1525. (20) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 143-149. (21) Keown, D. M.; Favas, G.; Hayashi, J.-i.; Li, C.-Z. Bioresour. Technol. 2005, 96, 1570-1577. (22) Tyler, R. J. Fuel 1979, 58, 680-686.
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Figure 1. Effects of char concentration in the char-KBr mixture on the total observed Raman intensity for the char prepared from the pyrolysis of cane trash (fast heating rate) at a peak temperature of 900 °C.
2.2. FT-Raman Spectroscopy. The FT-Raman spectra of chars were recorded with a Perkin-Elmer Spectrum GX FTIR/ Raman spectrometer following the procedures outlined previously.18 An InGaAs detector operated at room temperature was used to collect Raman scattering using a back-scattering configuration. The excitation Nd:YAG laser wavelength was 1064 nm. A laser power of 100 mW was used. Each spectrum represents the average of 1000 scans. The spectral resolution was 4 cm-1. A curved baseline was considered for each spectrum, and the baseline correction was carried out with the software provided by Perkin-Elmer with the spectrometer. Char particles are near black bodies that can be heated up easily in the laser beam, resulting in the emission of Planck radiation23 and char structural damage. To reduce the heat up of the char sample, char was mixed with spectroscopic grade KBr and ground manually. KBr prevents sample degradation by allowing the dissipation of heat to prevent the char from heating up during Raman spectrum acquisition. Figure 1 shows the effects of the char concentration in a char/KBr mixture on the observed Raman intensity for a char sample prepared at high temperature: the total peak area between 800 and 1800 cm-1 after baseline correction was used a measure of the Raman intensity. As was the case of previous studies on Victorian brown coal char, the observed Raman intensity increased with the increasing char concentration and then approached plateau values18 within a concentration of 0.05 wt %. All spectra to be reported here were then recorded with a char concentration of 0.05 wt % in the char-KBr mixture. This concentration was chosen to avoid the complications associated with the heating of char samples by the excitation laser while allowing sufficiently strong signals to be recorded. As is shown in Figure 2, the shapes of the Raman spectra of char, measured as the ratios of peak areas of various Raman bands, remained relatively unchanged with an increasing char concentration. This was expected because the Raman spectral shape reflects the structural feature of char, which in turn is independent of the concentration of char used to record the spectrum. The Raman spectra in the range between 800 and 1800 cm-1 were curve-fitted using the GRAMS/32 AI software (version 6.00) with 10 Gaussian bands (Table 1) representing the typical structures to be found in chars from low-rank fuels such as brown coal and biomass. A detailed discussion on the band assignment has been given previously;18 however, assignment of the main bands (G, Gr, Vl, Vr, D, and S) will be discussed briefly. The G band at 1590 cm-1 mainly represents aromatic ring quadrant breathing and the graphite E22g vibration. As the graphite structure gives relatively low intensity,18 the observed G band is mainly due to the aromatic
1818 Energy & Fuels, Vol. 21, No. 3, 2007
Keown et al.
Figure 2. Raman intensity ratios versus char concentration in the charKBr mixture for the char prepared from the pyrolysis of cane trash (fast heating rate) at a peak temperature of 900 °C.
Figure 3. Typical example of Raman spectral deconvolution using the 10 bands listed in Table 2. The char was prepared from the pyrolysis of cane trash at 800 °C at a fast heating rate.
ring systems. The D (1300 cm-1) band represents defect structures in the highly ordered carbonaceous materials and, more importantly, aromatics with not less than six rings. The overlap between the D and G bands has been deconvoluted into three bands: Gr (1540 cm-1), Vl (1465 cm-1), and Vr (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 S (1185 cm-1) band mainly represents Caromatic-Calkyl, aromatic (aliphatic) ethers, C-C on hydroaromatic rings, hexagonal diamond carbon sp3, and C-H on aromatic rings. Figure 3 shows a typical example of the spectral deconvolution using the 10 bands; all other Raman spectra in this study showed similar success of spectral deconvolution.
3. Results and Discussion 3.1. Total Raman Integrated Intensity of Chars. Figure 4 illustrates the effects of pyrolysis temperature on the total Raman intensity for cane trash chars prepared with a heating rate of 10 K min-1. The total Raman intensity is taken as the total peak area in the region 800-1800 cm-1. Also shown in Figure 4 are the total Raman intensities of chars from the pyrolysis of three coals (prepared in a thermogravimetric analyzer at 20 K min-1) from a previous study.24 The carbon contents of the coals (in wt %, daf) were: Pocahontas, 91.0 (low-volatile bituminous coal); Drayton 82.1 (bituminous coal); and Loy Yang, 68.5 (brown coal). It can be seen clearly that the differences in total Raman intensities between all samples reduced significantly with increasing pyrolysis temperature, regardless of rank or substrate
Figure 4. Total Raman peak areas (800-1800 cm-1) with increasing temperature for the chars from the slow heating rate pyrolysis of cane trash in a fluidized-bed/fixed-bed reactor (10 K min-1) and various coals in a thermogravimetric analyzer (TGA) (20 K min-1).
type. This data suggests that the carbon skeletal structure of chars from biomass and coal of varying rank may have similar features under severe conditions.24
Table 1. Summary of Raman Peak/Band Assignment18 band name
band position
(cm-1)
Gl G Gr Vl
1700 1590 1540 1465
Vr
1380
D
1300
Sl S
1230 1185
Sr R
1060 960∼800
description
bond type
Carbonyl group CdO Graphite E22g; aromatic ring quadrant breathing; alkene CdC Aromatics with 3∼5 rings; amorphous carbon structures Methylene or methyl; semicircle breathing of aromatic rings; amorphous carbon structures Methyl group; semicircle breathing of aromatic rings; amorphous carbon structures D band on highly ordered carbonaceous materials; C-C between aromatic rings and aromatics with not less than 6 rings Aryl-alkyl ether; para-aromatics Caromatic-Calkyl; aromatic (aliphatic) ethers; C-C on hydroaromatic rings; hexagonal diamond carbon sp3; C-H on aromatic rings C-H on aromatic rings; benzene (ortho-di-substituted) ring C-C on alkanes and cyclic alkanes; C-H on aromatic rings
sp2 sp2 sp2 sp2, sp3 sp2, sp3 sp2 sp2, sp3 sp2, sp3 sp2 sp2, sp3
Char from the Pyrolysis of Cane Trash
Total Raman intensity is affected by the Raman scattering ability of the char and the light absorptivity of the char.18,25 As the pyrolysis temperature increases, the chars produced would become increasingly aromatic. As the sp3 carbons condense into the sp2 carbons present in the aromatic structures, their Raman scattering ability would increase, increasing the Raman intensity. However, as the overall aromaticity of the char increased, the light absorptivity of the char would increase, decreasing the Raman intensity.26 The trends seen in Figure 4 suggest that increasing light absorptivity is much more important than increasing the effective light scattering ability in terms the effect on observed Raman intensity with increasing temperature. Electron-rich structures such as those containing O tend to have high Raman scattering ability and thus have an increasing effect on total Raman intensity.25 The cane trash used in this study contains 44.0 wt % O, far more than the sample with the next highest content, Victorian brown coal with 25.7 wt % O. As the rank of the coal increases, the abundance of O (and hence O-containing structures) decreases, partially explaining the differences seen at 600 °C. Also, as the pyrolysis temperature increases, the loss of these O-containing structures would contribute to the decrease (and convergence) of total Raman intensity observed in Figure 4. The O-containing functional groups could also influence the Raman intensity indirectly by exerting a resonance effect:27 while the O-containing groups are not greatly Raman-sensitive, they could increase the Raman intensity of (hetero)aromatic ring systems to which they are attached, e.g., through conjugation. The presence of O in the char structure is also likely to hinder the growth of the lamellar aromatic structures in char, reducing the char’s absorptivity in the near infrared (NIR) region,26 in which both excitation laser and Raman scattering lie, and enhancing the observed Raman intensity. The differences between the total Raman intensities of all samples decreased with increasing temperature. Pyrolysis reactions such as the loss of substitutional groups and condensation of ring systems only become intensified at temperatures greater than 600 °C.28 The fact that the Raman intensities converge at higher temperatures suggests that the chars from coals of varying ranks and of the cane trash sample have more similar carbon skeletal features at increased pyrolysis temperatures.24 The cane trash sample used in this study had a lower carbon content and higher oxygen content [44 wt % (daf) and 49 wt % (daf), respectively] than the brown coal. Since the lightabsorbing capacity of the pyrolysis char increases with increasing rank,26 and since O-containing structures contribute to increased Raman intensity, cane trash chars should have exhibited higher total Raman intensity than the coal chars across the temperature range studied. Figure 4 shows that, in fact, the total Raman intensity for cane trash chars were not greater; they were similar to those of Loy Yang coal char from 700 to 900 °C and lower than Loy Yang coal char at 600 °C. A possible reason for this difference is the high levels of AAEM species in the cane trash [Na, 0.04; K, 0.53; Mg, 0.19; Ca, 0.37 (all wt %, daf)]. It has been shown that the presence of Na and Ca can have a decreasing effect on total Raman intensity.18 The AAEM (23) Schrader, B. Tools for infrared and Raman spectroscopy; VCH: Weinheim, 1995; pp 63-188. (24) Li, X.; Li, C.-Z. J. Fuel Chem. Technol. 2005, 33, 385-390. (25) McCreery, R. L. Raman spectroscopy for chemical analysis; WileyInterscience: New York, 2000. (26) Ito, O. Energy Fuels 1992, 6, 662-665. (27) Leites, L. A.; Bukalov, S. S. J. Raman Spectrosc. 2001, 32, 413424. (28) Li, C.-Z.; Nelson, P. F. Energy Fuels 1996, 10, 1083-1090.
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Figure 5. Total Raman peak areas (800-1800 cm-1) with increasing temperature for the chars from the pyrolysis of cane trash.
species, particularly if present in the forms of carboxylates in the biomass substrate, would be involved in the pyrolysis reactions involving radicals.29 They can enhance the breakage of weak bonds to result in the formation of stronger bonds. In other words, the concentrations of radicals during pyrolysis would be increased due to the presence of these AAEM species. The repeated bond-breaking and bond-formation reactions involving AAEM species would lead to the enhanced release of O-containing functional groups. These reactions could also result in the activation of ring systems due to the presence of radicals at increased concentration and thus in the formation of more condensed aromatic ring systems. Clearly, the loss of O-containing groups and the formation of more condensed aromatic ring systems would increase the light absorptivity and decrease the observed Raman intensity. 3.2. Effect of Heating Rate on Total Intensity. Figure 5 shows the total Raman intensity for cane trash chars prepared under slow heating rate and fast heating rate pyrolysis conditions. While both sets of chars exhibited the same decreasing trend for total Raman intensity with increasing pyrolysis temperature, the chars formed at a fast heating rate (103-104 K s-1) showed lower Raman intensities at all temperatures. There are likely to be two competing effects during the char formation at a fast heating rate affecting the Raman intensity of the chars. First, the heating rate has a direct effect on the volatiles interacting with the char. In a previous paper it was shown that the char yield from the fast pyrolysis of cane trash decreased as the temperature increased from 600 to 900 °C, while that from the slow pyrolysis of cane trash was maintained.21 Selfgasification of the chars by reactive components in the volatiles was thought to be a possible cause of this decrease in char,21 and this21 will likely have an increasing effect on the observed Raman intensity.10 At reaction temperatures greater than 500 °C, more than 80 wt % (db) of the cane trash mass is released as volatiles formed during the primary pyrolysis of the cane trash.21 With a heating rate of 10 K min-1, these volatiles would have been swept out of the reactor at or near this temperature. However, in experiments where char was formed at a fast heating rate, the release of volatiles would occur at the reaction temperature, allowing inherent moisture in the raw sample as (29) Li, C.-Z.; Sathe, C.; Kershaw, J. R.; Pang Y. Fuel 2000, 79, 427438.
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well as H2O and CO2 from the primary pyrolysis of the cane trash to partially gasify the char. At temperatures greater than 700 °C, thermal cracking of light hydrocarbons and tar in the volatiles would also produce H2O and CO2 that would also gasify the char. This gasification would result in a relative increase in O-containing functional groups on the surface of the remaining char,10 which, as discussed earlier, would have an increasing effect on the observed Raman intensity. However, it has also been shown that the presence of Ca and Na alters the char/gasifying-agent reaction pathways.10 It was believed that the O-containing functional groups resulting from the partial gasification of chars from Na- or Ca-rich substrates were not closely associated with Raman-active carbon structures, thereby failing to affect the observed Raman intensity.10 Second, as discussed earlier, the presence of Na and Ca has been shown to have a negative effect on the observed total Raman intensity in brown coal chars by enhancing radical-based pyrolysis reactions.18 During slow heating rate pyrolysis, >80 wt % of the sample mass is volatilized below 500 °C,21 a temperature too low for volatile reforming reactions to produce significant quantities of radicals. Thus, the effect of AAEM species on char formation reactions would be magnified under fast heating rate conditions because the radical concentration on the char surface would be much higher. This higher radical concentration would result in enhanced ring condensation reactions, making the char structure more condensed with higher light absorptivity and thus lowering the total observed Raman intensity. As discussed earlier, the AAEM species contents in the cane trash sample were significant (K is likely to have a similar effect as Na, as is Mg-Ca). The data in Figure 5 suggests that the removal of Ramanactive structures via radical-based pyrolysis reactions had more of an effect on total Raman intensity than did the formation of O-containing structures on the char surface through selfgasification. 3.3. Peak Area Ratios of Some of the Major Bands. The deconvolution of the biomass char spectra revealed that the main Raman bands were G, Gr, Vl, Vr, D, and S bands (also see Figure 3). The ratios among these bands provide detailed information about the changes in the structure of biomass char during pyrolysis. 3.3.1. ID/IG with Increasing Extent of Graphitization. Figure 6 shows the intensity (band area) ratios between the D and G bands for the chars from the pyrolysis of biomass. In previous studies, the G band and D band have been assigned to graphitelike and defects in the graphite carbon structure, respectively; accordingly, a decrease in the ID/IG ratio is normally expected with increasing extent of condensation/ graphitisation, e.g., due to increases in pyrolysis temperature.14 However, ID/IG showed an increase from 600 to 700 °C before showing a decrease above 800 °C. Experiments using graphite under similar conditions gave very low Raman signals, lower than those of the chars prepared at 900 °C shown in Figure 6.24 In fact, X-ray diffraction (XRD) studies showed that true graphite structures did not exist in chars from the pyrolysis of Victorian brown coal.18 From this, it is unlikely that the observed Raman signals for the biomass chars at the positions of the D and G bands were due to graphite structures; rather, it is more likely that they were due to other structural features of the biomass chars. The low total intensity observed with graphite24 compared with that of all biomass and coal char samples means that for biomass and coal chars the major component of the G band is likely to be aromatic ring breathing rather than E22g vibrations
Keown et al.
Figure 6. Intensity ratios between bands D and G with increasing temperature for the chars from the pyrolysis of cane trash.
of crystalline graphite. Accordingly, the D band is likely to represent aromatic ring systems with at least six rings rather than defects in the graphite structure. The increase in ID/IG from 600 to 700 °C seen in Figure 6 indicates that a relative increase in the concentration of larger aromatic rings (with a minimum of six rings) has occurred. This increase transpires through ring condensation reactions during pyrolysis that allow the ring systems to grow. 3.3.2. OVerlap between G and D Bands. The overlap between the G and D bands is represented by the Gr, Vl, and Vr bands (Table 1) due to the typical structures found in amorphous carbon. They represent particularly the smaller aromatic ring systems although the large aromatic ring systems (g6 benzene rings) also make small contributions to the signals in this area.18 Therefore, the ratio I(Gr+Vl+Vr)/ID can be considered as a rough relative measure of the abundance ratio between the smaller (equivalently
