Evolution of the Char Structure of Lignite under Heat Treatment and Its

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Energy Fuels 2010, 24, 152–159 Published on Web 09/10/2009

: DOI:10.1021/ef900531h

Evolution of the Char Structure of Lignite under Heat Treatment and Its Influences on Combustion Reactivity† Xiaoling Zhu and Changdong Sheng* School of Energy and Environment, Southeast University, Si Pai Lou Number 2, Nanjing 210096, People’s Republic of China Received May 25, 2009. Revised Manuscript Received July 30, 2009

A lignite and its demineralized sample were pyrolyzed in a tube furnace under various temperatures from 773 to 1673 K. The resulting chars were systematically characterized with Raman spectroscopy for carbon microstructure, Fourier transform infrared spectroscopy for functional groups, and N2 adsorption isotherm for pore structure. The reactivity of char combustion was measured with thermogravimetric analysis based on a non-isothermal approach to derive the reactivity index and kinetic data. Over the range of heat-treatment temperatures studied, char structure evolution had different behaviors before and after 1073 K. Increasing the treatment temperature from 773 to 1073 K led to the increases in the concentrations and the growth of aromatic ring sizes indicated by the evolutions of Raman parameters, the decreases in functional groups, and the increase of smaller pores. When the treatment temperature was increased from 1073 to 1673 K, the evolutions of Raman band area ratios and pore structure indicated that the main change was the microstructure ordering. Mineral matter was found to have little impact on the structure evolution of the lignite char. Combustion reactivity index and activation energy were observed to increase with the treatment temperature because of char structure evolution and ordering. Mineral matter was found to have a catalytic effect on the reactivity but no influence on the activation energy. Fairly good linear correlations were found between the reactivity index and band area ratios of ID1/IG and IG/IALL when considering the structure evolution behaviors at lower and higher temperature regions, respectively.

reactivity of residual carbon by modeling. Nevertheless, it is well-known that the char combustion process significantly depends upon the physicochemical structure and reactivity of the initial char, which are the results of the devolatilization and its conditions. During char formation, many changes occur, including loss of functional groups on the carbon surface, ordering of the carbon microstructure to become more graphitic, and a decrease of the inorganic matter catalytic role under heat treatment, which had been recognized to be the reasons responsible for the decrease of reactivities of coal char to reaction gases.11 Because of the importance to the reactivity, microstructural change of coal char under heat treatment has been extensively studied by employing various techniques, such as X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. Raman spectroscopy was widely used to characterize the structure features of carbonaceous materials and coal-derived products.12-20 Raman spectra of carbonaceous

Introduction Unburned carbon in ash contributes to the reduction of combustion efficiency and limits the use of fly ash as construction material. It is one of the most important concerns in design and operation of coal-fired combustors. Therefore, the underlying phenomena affecting it have attracted extensive research interests.1-9 The combustion process of coal particles mainly consists of two steps: pyrolysis or devolatilization to release volatiles and yield char and burnout of char particles. In comparison to the fast devolatilization, char burnout is very slow and, thus, the dominant process of coal conversion. After exposed to high temperature, char undergoes thermal deactivation or loss of reactivity. The unburned carbon is commonly related to thermal deactivation, which has been studied by many investigators.3,6-9 Hurt et al.5,10 described thermal deactivation as a function of temperature history and explained the low † Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. Telephone: þ86-2583790317. Fax: þ86-25-57714489. E-mail: [email protected]. (1) Hurt, R. H.; Gibbins, J. R. Fuel 1995, 74, 471–480. (2) Hurt, R. H.; Gibbins, J. R. Fuel 1995, 74, 1297–1306. (3) Davis, K. A.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 31–40. (4) Cai, H.-Y.; Guell, A. J.; Chatzakis, I. N.; Lim, J.-Y.; Dugwell, D. R. Fuel 1996, 75, 15–24. (5) Hurt, R. H.; Sun, J.-K.; Lunden, M. Combust. Flame 1998, 113, 181–197. (6) Russell, N. V.; Gibbins, J. R.; Williamson, J. Fuel 1999, 78, 803–807. (7) Feng, B.; Bhatia, S. K.; Barry, J. C. Carbon 2002, 40, 481–496. (8) Lu, L.; Kong, C.; Sahajwalla, V.; Harris, D. Fuel 2002, 81, 1215–1225. (9) Zhang, S.-Y.; Lu, J.-F.; Zhang, J.-S.; Yue, G.-X. Energy Fuels 2008, 22, 3213–3221. (10) Sun, J.-K.; Hurt, R. H. Proc. Combust. Inst. 2000, 28, 2205–2213.

r 2009 American Chemical Society

(11) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221–270. (12) Li, X.; Hayashi, J.; Li, C.-Z. Fuel 2006, 85, 1509–1517. (13) Li, X.; Hayashi, J.; Li, C.-Z. Fuel 2006, 85, 1700–1707. (14) Beyssac, O.; Goffe, B.; Petitet, J.-P.; Froigneux, E. L.; Moreau, M.; Rouzaud, J.-N. Spectrochim. Acta 2003, 59, 2267–2276. (15) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Poschl, U. Carbon 2005, 43, 1731–1742. (16) Sekine, Y.; Ishikawa, K.; Kikuchi, E.; Matsukata, M. Energy Fuels 2005, 19, 326–327. (17) Sekine, Y.; Ishikawa, K.; Kikuchi, E.; Matsukata, M.; Akimoto, A. Fuel 2006, 85, 122–126. (18) Zickler, G.; Smarsly, B.; Gierlinger, N.; Peterlik, H.; Paris, O. Carbon 2006, 44, 3239–3246. (19) Keown, D.; Li, X.; Hayashi, J.; Li, C.-Z. Energy Fuels 2007, 21, 1816–1821. (20) Tay, H.-L.; Li, C.-Z. International Symposium on Gasification and Application, 2008; CG-3-02.

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materials are usually divided into first- and second-order regions. A first-order Raman spectrum of crystalline graphite is a single frequency line at ∼1580 cm-1 (designated as the G band) because of the stretching vibration mode with E2g symmetry in the aromatic layers. For disordered carbonaceous materials, “defect bands” appear at ∼1350 cm-1 (D1 band) and ∼1620 cm-1 (D2 band). For poorly organized materials, defect bands are also observed at ∼1530 cm-1 (D3 band) and ∼1150 cm-1 (D4 band). The D1 and D2 bands are associated with graphene layer defects.15 The D3 and D4 bands are suggested to originate from a poorly organized structure, such as amorphous carbon.21,22 Raman parameters, especially band area ratios, are often used to monitor the degree of carbon microstructure ordering and to reflect the effect of heat treatment. They were also employed to correlate the microstructure with combustion23-25 and gasification reactivity,16,17 although further development of this application is still required.24 Besides the microstructure ordering, other physicochemical structure changes, including the loss of functional groups, evolution of the pore structure, and decrease of the catalytic effect of inorganic matter during char formation, are also important but complex factors influencing the reactivity of coal char. Understanding these changes under heat treatment is essential for describing the reactivity evolution during char combustion. The present work was addressed to the evolution of the physicochemical structure of coal char under heat treatment and its influences on combustion reactivity. A lignite and its demineralized sample were pyrolyzed under various temperatures. The resulting chars were systematically characterized with Raman spectroscopy for carbon crystalline structure, Fourier transform infrared spectroscopy (FTIR) for functional groups on the carbon surface, and N2 adsorption isotherm for pore structure. The reactivity of char combustion was measured with thermogravimetric analysis (TGA) based on a non-isothermal approach to obtain reactivity parameters. Attempt was made to correlate the first-order Raman parameters with the reactivity to investigate the influences of microstructure changes on the combustion reactivity. The impacts of mineral matter on the structure evolution and the reactivity were also studied on the basis of the comparison between the chars of the raw and demineralized lignite.

Table 1. Properties of the Lignite properties

value

Proximate Analysis, Air-Dried Basis (wt %) moisture volatile matter ash

14.61 37.53 8.55

Ultimate Analysis, Air-Dried Basis (wt %) carbon hydrogen nitrogen sulfur oxygen (by difference)

63.09 4.17 0.96 0.33 16.67

Ash Composition (wt %) SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3

49.00 19.90 7.57 12.30 2.65 0.68 0.55 6.23

furnace to generate char samples. The pyrolysis was carried out in N2 with a flow rate of 45 mL/min. About 2 g of sample was held in an alumina crucible, which was placed in the center of the furnace tube. It was then heated at a heating rate of 10 K/min to a preset temperature and held for 2 h and finally cooled to room temperature. Although devolatilization in practical combustion is at a high temperature and heating rate, the very low heating rate and the wide range of treatment temperatures were employed to obtain the char samples with a wide range of structure order and to focus on the influences of single variable, i.e., temperature, on the evolutions of char structure and reactivity. N2 Adsorption Isothermal. The char samples were measured with a N2 absorption apparatus (ASAP2000) at 77 K to analyze the pore structure. The data obtained from N2 adsorption isothermal was calculated by the Brunauer-Emmett-Teller (BET) method for specific surface area and by the BoppJancso-Heinzinger method for pore size. FTIR Analysis. Both non-demineralized and demineralized char samples were prepared to form standard KBr pellets for FTIR analysis. The spectra were recorded on a Bruker Vector 22 instrument from 400 to 4000 cm-1 with a resolution of 4 cm-1. A total of 16 scans were conducted for each sample. The recorded spectra were first processed with baseline correction and absorbance-transmittance conversion and then analyzed by curve fitting. Accordingly, band area ratios of O-containing functional groups and aromatic CH functional groups (ACOOH/ Aarom and AOH/Aarom) were calculated. Raman Spectroscopy. Laser Raman spectroscopic analysis on char samples was performed with a Jobin Yvon Labram HR800 spectrometer. During the analysis, the laser power at the surface of char particles was controlled at ∼1 mW. The laser spot diameter reaching the sample was ∼1 μm, much larger than the size of carbon microcrystallites in the chars. As a result, the Raman microprobe actually acquired the averaged information of a large number of randomly distributed microcrystallites. Nevertheless, considering the heterogeneous nature of char particles, eight particles were randomly chosen and analyzed for each char sample. The spectra were recorded in the wavenumber range of 800-2000 cm-1 covering the first-order bands. A typical spectrum is presented in Figure 1. The spectra were curve-fitted with an Origin7.5/Peak Fitting Module. Each spectrum was resolved into four Lorentzian bands and one Gaussian band (for the D3 band) following Sadezky et al.15 and Sheng,25 as shown in Figure 1. The spectrum parameters obtained were peak position, full width at half-maximum, intensity, and integrated area of each band. The band area ratios, including those of the defect bands to the

Experimental Section Char Preparation. A Chinese lignite was pulverized and sieved, and the size cut of 60-150 μm was used as raw material for char preparation. The properties of the raw material are presented in Table 1. To study the influences of mineral matter on the evolutions of the structure and combustion reactivity of the lignite char, the raw material was also demineralized with acid washing. A small amount sample was consecutively treated with 5 N HCl, 48% HF, and 12 N HCl at 60 °C for 1 h, respectively. Most of the mineral matter was removed by the demineralization process. The raw and demineralized materials were pyrolyzed under various temperatures from 773 to 1673 K in a horizontal tube (21) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martinez-Alonso, A.; Tascon, J. Carbon 1994, 32, 1523–1532. (22) Jawhari, T.; Roid, A.; Casado, J. Carbon 1995, 33, 1561–1565. (23) Bar-Ziv, E.; Zaida, A.; Salatino, P.; Senneca, O. Proc. Combust. Inst. 2000, 28, 2369–2374. (24) Zaida, A.; Bar-Ziv, E.; Radovic, L. R.; Lee, Y.-J. Proc. Combust. Inst. 2007, 31, 1881–1887. (25) Sheng, C. Fuel 2007, 86, 2316–2324.

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Figure 2. Char conversion is plotted as a function of the temperature. The dense and thin curves represent the non-demineralized and demineralized chars formed at 1173 K, respectively. Figure 1. Typical Raman spectrum of the char generated from demineralized lignite at 1073 K and its resolved bands.

G band (ID1/IG, ID2/IG, ID3/IG, and ID4/IG) and that of the G band to the integrated area (IG/IALL), were calculated. The former four ratios were used to present the evolution of char microstructure, especially the defects and imperfect structure. IG/IALL was used mainly to reflect the variation of the G band intensity and also used as the base to present the variations of other ratios with the sample. Combustion Reactivity Measurement. Combustion reactivity was measured by a thermogravimetric analyzer (Setaram TGA92), following the non-isothermal procedure developed by Shim and Hurt.26 The reaction was performed in the O2/N2 mixture with 21% O2. About 5 mg of char sample was placed in the TGA pan and heated from room temperature to 383 K, held for 30 min to dry the sample, and then heated at 7 K/min to 1273 K. The sample temperature and weight were continuously recorded. The conversion of the char at any time t was calculated by m0 -m R ¼  100% m0

Figure 3. Char yields of the raw and demineralized lignite vary with the heat treatment temperature.

1173 K chars burned off at 813 and 870 K, respectively. Actually, it was observed that all chars were completely burned before 1050 K. The burnout temperature of each char was lower than the corresponding treatment temperature, ensuring no further thermal annealing during the conversion. A 50% conversion was set as the upper limit to exclude the possible diffusion effect on the kinetic data at very high conversion.

ð1Þ

where m and m0 are the sample weight and its initial on a dry ash-free basis, respectively. As an example, Figure 2 shows the conversions as a function of the temperature for the non-demineralized and demineralized chars prepared at 1173 K. It can be seen that the char of non-demineralized lignite started burning and completed conversion at lower temperatures, indicating a higher reactivity than the demineralized one. For comparison purposes, the temperature at the conversion of R = 20% was defined as the index to represent the global reactivity of char combustion. Moreover, the approach developed by Russell et al.27 was employed to determine the kinetic parameters of char combustion based on non-isothermal TGA analysis. The combustion rate is expressed as -

1 dm ¼ k0 expð-E=RTÞ m dt

Results and Discussion Char Yield. Figure 3 presents the char yields on a dry ashfree basis, plotted against the heat treatment temperature. When the raw and demineralized lignite were compared, there was almost no impact of mineral matter on the char yields, except for those generated at the temperatures higher than 1273 K. The general trend shown in Figure 3 is that, with an increasing treatment temperature, the char yields of both raw and demineralized lignite decreased gradually from 773 to 1173 K because a higher temperature enhanced the decomposition and polymerization, resulting in more volatile release. After the temperature was higher than 1273 K, the char yield of the demineralized lignite only decreased slightly with the temperature increasing because of more severe carbonization and further light gas release at higher temperature. In contrast, the char yield of the raw lignite decreased considerably with an increasing pyrolysis temperature. This can be attributed to the vaporization of inorganic matter because a large part of basic elements in the lignite was believed to be active and easy to vaporize under higher temperatures.

ð2Þ

where k0 and E are the pre-exponential factor and activation energy, respectively, T is the temperature, and R is the universal gas constant. The activation energy E was derived by fitting the Arrhenius plot with the reaction rate in the conversion range of 5-50% for each sample. The conversion of 5% was chosen as the low limit to avoid the measurement uncertainty at very low conversion, particularly for the raw lignite chars. Figure 2 shows that the (26) Shim, H.-S.; Hurt, R. H. Energy Fuels 2000, 14, 340–248. (27) Russell, N. V.; Beeley, T. J.; Man, C.-K.; Gibbins, J. R.; Williamson, J. Fuel Process. Technol. 1998, 57, 113–130.

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Figure 4. Effect of the heat treatment temperature on (a) N2 BET surface area and (b) average pore size.

Char Structure Evolution. BET Surface Area and Average Pore Size. The impact of the treatment temperature on the char pore structure is illustrated in Figure 4. It can be seen in Figure 4a that the BET surface area of non-demineralized chars increased from 773 to 1073 K and then decreased from 1073 to 1673 K, consistent with the observations from lignite and high-volatile bituminous chars formed in the temperature range.28-30 The BET surface area of the demineralized chars had a similar trend as the non-demineralized ones, despite the data scattering. Figure 4b shows that the average pore sizes of both the raw and demineralized chars varying with the treatment temperature also had a similar trend. The average pore sizes gradually decreased to a minimum from 773 to 1073 K, slightly increased between 1073 and 1373 K, and then increase greatly after 1373 K. The decrease of the average pore size and the increase of the BET surface area with the temperature from 773 to 1073 K was mainly due to the enhanced decomposition of the lignite and release of volatile matter, which resulted in the development of smaller pores (micro- and mesopores) and, consequently, a higher pore surface area; the increase of the pore size at the higher temperatures from 1073 to 1673 K was attributed to the ordering of aromatic units and crystalline structure to form a more compact stacking arrangement as the heat treatment temperature increased, leading to the enlargement and mergence of the micropores to form mesoand macropores with a lower surface area. Functional Groups. Functional groups on the carbon surface are believed to be active sites,11 which have intimate relation with char combustion reactivity.30-36 In previous studies,32,37 FTIR spectra were resolved into several bands and the band area ratios were used to reflect the evolutions of certain functional groups. We followed this approach of using curve

Figure 5. Relative content of O-containing functional groups of demineralized lignite chars vary with the treatment temperatures.

fitting to quantify the processed FTIR spectra. Because of the differences in the intensities of the FTIR signals, the concentrations and path lengths were also different in various analyses. Therefore, only the ratios of the band areas with respect to a reference zone could be compared. The zones of 900-700, 1700-1500, and 3500-3300 cm-1 in the spectra of the demineralized chars were analyzed with curve fitting. The reason for choosing the demineralized chars was to avoid the influences of mineral peaks upon curve fitting. The zone of 900-700 cm-1 was selected as a reference zone, and ratios of band areas of O-containing functional groups relative to the reference were calculated. Therefore, band area ratios ACOOH/Aarom and AOH/ Aarom represent the relative content of O-containing functional groups. The evolutions of band area ratios ACOOH/Aarom and AOH/Aarom with the treatment temperature are shown in Figure 5. It can be seen in Figure 5 that both ACOOH/Aarom and AOH/ Aarom ratios decreased with an increasing treatment temperature. This is because the chemical bonds connecting COOH and OH groups to the carbon matrix broke down when the temperature increased to a certain value. The COOH groups decomposed into CO2 or condensed with other groups, while the OH groups dehydrated with H to generate H2O. The ratio for COOH groups was initially high (ca. 5.33) but decreased rapidly to ca. 0.72, when the temperature increased to 873 K, and then stabilized between 0.2 and 1 from 983 to 1473 K. This was consistent with the previous observations that COOH group decomposition was the main source of CO238 and CO2 mainly released before

(28) Sakintuna, B.; Yurum, Y.; C-etinkaya, S. Energy Fuels 2004, 18, 883–888. (29) Suuberg, E. M.; Teng, H.; Aarna, I. Prepr. Symp.-Am. Chem. Soc., Div. Fuel Chem. 1996, 41, 160–164. (30) Chan, M.-L.; Jones, J. M.; Pourkashanian, M.; Williams, A. Fuel 1999, 78, 1539–1552. (31) Ahmed, M. A.; Blesa, M. J.; Juan, R.; Vandenberghe, R. E. Fuel 2003, 82, 1825–1829. (32) Ibarra, J.; Moliner, R.; Bonet, A. J. Fuel 1994, 73, 918–924. (33) Ibarra, J. V.; Munoz, E.; Moliner, R. Org. Geochem. 1996, 24, 725–735. (34) Ibarra, J. V.; Miranda, J. L. Vib. Spectrosc. 1996, 10, 311–318. (35) Ohki, A.; Xie, X. F.; Nakajima, T.; Itahara, T.; Maeda, S. Coal Prep. 1999, 21, 23–34. (36) Xie, X. F.; Ohki, A.; Morimatsu, K.; Nakajima, T.; Oda, H.; Maeda, S. J. Jpn. Inst. Energy 1998, 77, 478–484. (37) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X.; Chan, W. G.; Hajaligol, M. R. Fuel 2004, 83, 1469–1482.

(38) Serio, M.; Hamblen, D.; Markham, J.; Solomon, P. Energy Fuels 1987, 1, 138–152.

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Figure 6. Band area ratios of both non-demineralized and demineralized chars vary with the treatment temperatures. Error bars denote the standard deviation among the particles of each char sample.

873 K.39 The ratio for OH groups decreased gradually and then decreased sharply after 973 K because the OH groups were relatively stable and not easy to remove until the temperature reached above 973 K. Raman Structural Parameters. The average band area ratios, including ID1/IG, ID2/IG, ID3/IG, and IG/IALL, were plotted against the treatment temperature in Figure 6. It can be seen in Figure 6 that, similar to the variation of the pore structure (see Figure 4), the evolutions of the band area ratios with the treatment temperature also had different behaviors at lower and higher temperature ranges for both the two types of chars. In the low-temperature region (from 773 to 1073 K), ID1/IG and ID2/IG increased, while IG/IALL decreased significantly with the treatment temperature; in the high-temperature region (from 1073 to 1673 K), the variation trends were reversed. For ID3/IG, there was a sharp increase from 773 to 873 K; after that, ID3/IG showed a decreasing trend with an increasing treatment temperature. Such trends of the ratios were also observed by Zickler et al.18 and Keown et al.19 for biomass chars, by Zaida et al.24 for cellulose char, and by Li et al.13 for Australian brown coal chars produced over a wide range of pyrolysis temperatures. Figure 6 also shows that, with the standard deviations considered, there are not differences between the Raman parameters of the chars generated from the raw and demineralized lignite, implying little impact of mineral matter on Raman parameters. This was consistent with our observations from other high-volatile coals.25 It is recognized that certain metals or minerals may have catalytic influence on

the degree of graphitization of coal chars7,40 and cokes.41 The influence depends upon raw material, temperature, type, and amount of the metals and catalyst size. In the lignite studied, the potential catalytic metals are mainly Fe and Ca. Ca is expected to have little influence because CaO can carbide and have a catalytic effect only at the temperature of above 1723 K,40 higher than the treatment temperatures involved in this work. Fe may catalyze the char ordering at much lower temperatures.7,40 However, the Fe content in the lignite (see Table 1) was not very high. Its reactions with other minerals, such as silicates, might also decrease its catalytic effect. On the other hand, lignite is a non-graphitizable material. Because of the low propensity of the lignite toward graphitization, the catalytic role of mineral matter in the structure ordering was also expected not to be evident. Therefore, the mineral matter was observed to have an insignificant influence on the structure ordering of the lignite chars studied. The evolutions of the band area ratios shown in Figure 6 reflect the transformation of char microstructure during pyrolysis and the contributions to Raman scattering signals. At the temperatures lower than 1073 K, the graphitization did not really start or just started to form small crystallites. For the char formed, the D1 and G band strengths were suggested to be proportional to the probability of finding a 6-fold aromatic ring in the carbon cluster.42 Li et al.13 also suggested the increase in the ID1/IG ratio, indicating the relative increases in the concentrations of aromatic rings having six or more fused benzene rings resulted from the

(39) Barton, S. S.; Gillespie, D.; Harrison, B. H. Carbon 1973, 11, 649– 654. (40) Wang, J.; Morishita, K.; Takarada, T. Energy Fuels 2001, 15, 1145–1152.

(41) Wang, W.; Thomas, K. M.; Poultney, R. M.; Willmers, B. R. Carbon 1995, 33, 1525–1535. (42) Ferrari, A. C.; Robertson, J. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 64, 1–13.

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Figure 7. Variations of (a) reactivity index and (b) combustion activation energy of the chars with the heat treatment temperature.

dehydrogenation of hydroaromatics and the growth of aromatic rings during the char formation. As speculated by Zaida et al.,24 the crystallites formed were too small to effectively couple with the incident laser and, thus, have a small contribution to the scattering signal. With an increasing treatment temperature in the lower temperature range, Figure 3 showed significant decreases of char yields and Figure 4 indicated the increases of smaller pores, while Figure 5 demonstrated the significant release of functional groups. All of these suggested that the main processes were the decomposition and release of the loose constituents of the organic matrix. The temperatures were not high enough for significant growth of the ordered structure. For the chars generated in high-temperature region, Figures 3 and 5 indicated that there was no considerable amount of loose constituents left, implying that the carbon structure ordering or the graphitization process happened. The decreases of ID1/IG and ID2/IG with an increasing treatment temperature imply that various forms of the structural defects and imperfections of the carbon crystallites were gradually eliminated when subjected to higher treatment temperatures. The decrease of ID3/IG also implies that the poorly organized structure in carbon materials, such as amorphous carbon, gradually turned ordered under severer heat treatment. As a result, the char structure became more ordered, indicated also by the increase of IG/IALL. The decrease in the BET surface area and the increase in the average pore size (Figure 4) also reflected the structure ordering with an increasing treatment temperature. Char Combustion Reactivity. The combustion reactivity indexes of the demineralized and non-demineralized chars are plotted as function of the treatment temperature in Figure 7a. The general trends can be found that the reactivity indexes of both types of chars increased with an increasing temperature, implying the decrease of the global combustion reactivity. This was consistent with previous studies.7,23-27 When the chars generated at the same temperature were compared, the reactivity index of the raw lignite char was lower than that of the demineralized one but the difference gradually narrowed and finally disappeared at the temperatures higher than 1473 K, consistent with our previous work on coal chars.25 The variation of char combustion activation energy with the treatment temperature is presented in Figure 7b. The activation energy reflects the temperature dependence of char combustion. The char with higher activation energy is generally less reactive at lower temperatures. Figure 7b shows that the activation energies of both types of chars

increased with an increasing treatment temperature, also indicating the decrease of the reactivity, which was in agreement with the reactivity indexes shown in Figure 7a. It can be found that the activation energies increased a bit faster with the treatment temperature before 1073 K than after 1073 K. However, no difference was observed between the two types of chars, implying little influence of mineral matter on the activation energy of char combustion. For the chars formed from the demineralized lignite, the decrease of the reactivity as well as the increase of activation energy with the treatment temperature was attributed to the structural changes under heat treatment. Char surface function groups are believed to be very active to oxidation. Smaller size aromatic rings and crystallites with more function groups are also more reactive43 and have lower activation energy. Therefore, the significant loss of function groups (see Figure 5) and the increases in the concentrations and the growth in sizes of aromatic rings with the temperatures increasing led to a great increase in reaction activation energy and decrease in the reactivity before 1073 K. For higher temperature chars, the crystalline structure ordering with an increasing treatment temperature resulted in a decrease in the reactivity and an increase in the activation energy. For the chars generated from the raw lignite, the decrease in the reactivity resulted from not only the evolution of char structure but also the change in the catalytic effect of mineral matter. The lower reactivity index or higher reactivity than the demineralized chars (see Figure 7a) confirmed the contribution of the catalytic role of inorganic matter in the oxidation, particularly for the chars generated at lower temperatures. The narrowing and eliminating of the difference between the two types of chars indicated the gradual decrease in the catalytic effect with an increasing treatment temperature because of the transformation of mineral matter, such as the evaporation of active elements under higher temperatures. Nevertheless, Figure 7b demonstrated that the mineral matter did not obviously affect the activation energy, implying that the inorganic matter increased the active sites rather than apparently affecting the activation energy. Correlations between Char Combustion Reactivity and Raman Parameters. Reactivity indexes are correlated with the average band area ratios in Figure 8. IG/IALL and ID1/IG were chosen to represent the char microstructure evolution. It was (43) Keown, D. M.; Li, X.; Hayashi, J.; Li, C.-Z. Fuel Process. Technol. 2008, 89, 1429–1435.

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Figure 8. Correlations between the reactivity index and the band area ratios of ID1/IG and IG/IALL for the chars generated in the lowtemperature region (a and b) and high-temperature region (c and d), respectively. The lines show the trends.

noted that the evolutions of the band area ratios changed the trends at the temperature of around 1073 K (Figure 6), while the reactivity indexes increased monotonically with the treatment temperature (Figure 7a). It is not possible to establish single correlations between the reactivity and Raman band area ratios. Therefore, to connect the first-order Raman parameters with the reactivity for the chars generated over a wide range of treatment temperatures, the reactivity indexes were correlated with ID1/IG and IG/IALL for the chars generated at lower and higher temperatures, respectively. It can be seen that, in panels a and b of Figure 8, in the lower temperature region (from 773 to 1073 K), the reactivity indexes of the two types of chars increase linearly with the decrease of IG/IALL and the increase of ID1/IG, indicated by the trend lines. For chars formed in the high-temperature region (from 1073 to 1673 K), the linear correlations were also found but the trends of the reactivity indexes varying with IG/IALL and ID1/IG were opposite to those for lowtemperature chars (see panels c and d of Figure 8). These behaviors are expected. As discussed above, the decrease of IG/IALL and increase of ID1/IG represent the increases in the concentrations and sizes of aromatic rings for low-temperature chars and the increase of the crystalline structure order for high-temperature chars. Both the changes led to the decrease in the reactivity. Figure 8 shows that better linear correlations could be found between the reactivity indexes and the band area ratios for the demineralized chars, while the data were more scattering for the raw lignite chars. This may be attributed to the complexity of the inorganic matter catalytic effect on char reactivity. When the band ratios shown in Figure 8 were compared, ID1/IG had a better performance. It should be noted that the correlations between the reactivity and the band area ratios of the high-temperature

chars were consistent with those obtained in a previous work on high-volatile coal chars,25 confirming the applicability of the first-order Raman band ratios in connecting the carbon microstructure with combustion reactivity for high-temperature chars. Although separated correlations had to be proposed for the low-temperature chars because of the transition behaviors of the band area ratios at the temperature around 1073 K, the fairly good correlations enabled the application of the first-order Raman band ratios to lowtemperature chars. Conclusions A lignite and its demineralized sample were pyrolyzed from 773 to 1673 K. Char structural evolutions with the treatment temperature were systematically characterized with multitechniques. The combustion reactivity of the chars was measured with a non-isothermal TGA method and was correlated with the first-order Raman parameters. The following conclusions were drawn from the present work: (1) Char structure evolution behaved differently before and after 1073 K. Increasing the treatment temperature from 773 to 1073 K resulted in a significant decrease in functional groups, decrease in char yield, and increase of smaller pores. The increases in the concentrations and the growth in sizes of aromatic rings were reflected by the Raman parameters. The carbon structure ordering mainly happened at the temperature above 1073 K. When the treatment temperature was increased from 1073 to 1673 K, Raman band area ratios indicated that the structural defects and imperfections of the carbon crystallites were gradually eliminated and poorly organized structure in carbon materials gradually turned ordered. Consequently, the char structure turned to more ordered, which was also indicated by the evolution of the pore 158

Energy Fuels 2010, 24, 152–159

: DOI:10.1021/ef900531h

Zhu and Sheng

structure. Raman parameters and pore structure data suggested that mineral matter had little impact on the structure evolution of the lignite char studied. (2) Reactivity index of the demineralized char increased with an increasing treatment temperature, which was the result from the loss of functional groups and the growth of aromatic rings under lower temperature treatment and from carbon microstructure ordering under higher temperature treatment. In comparison to the demineralized chars, lower reactivity indexes or higher reactivity of the raw lignite chars indicated the catalytic effect of mineral matters on the combustion reactivity, which was eliminated with an

increasing temperature. Increasing the treatment temperature led to the increase of the combustion activation energy, upon which no impact of mineral matter was found either. (3) Fairly good linear correlations were found between the band area ratios of ID1/IG and IG/IALL and the reactivity indexes when considering the structure evolution behaviors in lower and higher temperature regions, respectively. Acknowledgment. The authors thank the partial support of the present work from the National Key Basic Research and Development Program of China (2006CB200302).

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