Energy Fuels 2009, 23, 4605–4611 Published on Web 07/21/2009
: DOI:10.1021/ef900268y
Pyrolysis of Maize Stalk on the Characterization of Chars Formed under Different Devolatilization Conditions Peng Fu,† Song Hu,*,† Jun Xiang,† Lushi Sun,† Peisheng Li,‡ Junying Zhang,† and Chuguang Zheng† †
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China, and ‡ School of Power and Mechanical Engineering, Wuhan University, Wuhan 430074, China Received March 27, 2009. Revised Manuscript Received July 8, 2009
In this context, the work dealt with the characterization of chars obtained by the pyrolysis of maize stalk and the effects of pyrolysis temperature and heating rate on the char properties. The pyrolysis was performed at atmospheric pressure and temperatures ranging from 600 to 1000 °C under low heating rate (LHR) and high heating rate (HHR) conditions. The chars were characterized by ultimate analysis, X-ray diffraction (XRD), helium density measurement, N2 isothermal adsorption/desorption method, and Fourier transform infrared spectroscopy (FTIR). The results indicated that the char characteristics markedly depended upon pyrolysis conditions, particularly temperature. The char yield decreased from 22 to 16.3% with increasing temperature from 600 to 900 °C. The decrease in H/C was more than twice that in O/C. As the temperature increased, maize stalk chars had a slightly less amorphous structure and less aliphatic side chains and became more aromatic and ordered. At a HHR, progressive increases in porosity development with increasing pyrolysis temperature occurred, whereas a maximum development of micro- and mesopores appeared at 900 °C. The surface area of char reached a maximum of 81.6 m2/g at 900 °C and decreased slightly at higher temperatures. Over 900 °C, structural ordering, pore widening, and/or the coalescence of neighboring pores led to the decrease in the surface area values, resulting in thermal deactivation of the chars. The influence of the heating rate on the surface area was not significant. FTIR analysis showed that the hydroxyl, aliphatic C-H, and carbonyl and olefinic CdC groups were lost at high temperatures. The loss of ether groups led to a more ordered carbon structure.
constitutes an attractive option. The gasification process can be used to produce syngas, which has a direct application in all hydro-heating operations, ammonia production, and the synthesis of superclean liquid fuels. Biomass gasification is generally a complex thermochemical process, in which biomass fuels are converted into a gas mixture, leaving behind a residue known as ash. The gasification process primarily consists of two overlapping stages: (1) pyrolysis or release of volatiles and (2) char conversion. The char conversion is generally a much slower step and, therefore, the rate-determining step.4 Consequently, a clear understanding of the characteristics of chars formed during pyrolysis has interest in practical applications to improve the efficiency of the gasification system and reduce pollutant emissions. It is well-established that heterogeneous reactions between the char and gasifying agent (e.g., air, CO2, and H2O) are greatly affected by the char properties, because the amount and type of pores determines the gas accessibility to the active surface sites.5,6 Previous studies have shown that the properties of char are decisively affected, not only by properties of parent material but also by operating conditions used, mainly the heating rate, the maximum temperature experienced, and the residence time at this temperature.6-8 These variables
1. Introduction There is increasing interest in biomass as a source of renewable energy to reduce the potential environmental impact of fossil fuels. Biomass belongs to natural high-molecular organic substances of lignocellulosic structure.1,2 Being available in abundance, biomass including agricultural residues is recognized as one of the main renewable energy source used for electrical energy, thermal power, and syngas. Moreover, biomass fuels can be considered CO2-neutral fuels and give lower emissions of SO2, NOx, and heavy metals with respect to coals. Maize stalk is one of the major agriculture biomasses produced in large quantities in developing countries. In China, nearly 1.22-1.27 million tons of maize stalk as residue are produced every year.3 Part of these is consumed in a traditional way, such as fodder for livestock and household fuel for stoves. Until recently, the majority of maize stalks have not been used and disposed off by burning in open fields, causing environmental and public health problems. At present, China energy policies are strongly encouraging the use of maize stalk for energy purposes, mainly owing to three aspects: economic and social development, elimination of wastes, and reduction of CO2 emissions. In this situation, gasification of biomass, as a promising technology,
(4) Cetin, E.; Gupta, R.; Moghtaderi, B. Fuel 2005, 84, 1328–1334. (5) Liu, G.; Benyon, P.; Benfell, K. E.; Bryant, G. W.; Tate, A. G.; Boyd, R. K.; Harris, D. J.; Wall, T. F. Fuel 2000, 79, 617–626. (6) Guerrero, M.; Ruiz, M. P.; Alzueta, M. U.; Bilbao, R.; Millera, A. J. Anal. Appl. Pyrolysis 2005, 74, 307–314. (7) Guerrero, M.; Ruiz, M. P.; Millera, A.; Alzueta, M. U.; Bilbao, R. Energy Fuels 2008, 22, 1275–1284. (8) Onay, O. Fuel Process. Technol. 2007, 88, 523–531.
*To whom correspondence should be addressed. Telephone: 86-2787542417-8209. Fax: 86-27-87545526. E-mail:
[email protected]. (1) Luik, H.; Johannes, I.; Palu, V.; Luik, L.; Kruusement, K. J. Anal. Appl. Pyrolysis 2007, 79, 121–127. (2) McKendry, P. Bioresour. Technol. 2002, 83, 37–46. (3) Yang, S.; Ding, W.; Chen, H. Biomass Bioenergy 2009, 33, 332– 336. r 2009 American Chemical Society
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Energy Fuels 2009, 23, 4605–4611
: DOI:10.1021/ef900268y
Fu et al. Table 1. Analysis of Maize Stalk
proximate analysis (wt %, as received basis) moisture volatile matter fixed carbon ash BET surface area (m2/g) total pore volume (cm3/g) helium density (g/cm3)
ultimate analysis (wt %, dry and ash-free basis)
5.70 76.15 12.45 5.70 1.72 0.0084 1.40
carbon hydrogen nitrogen oxygen (by difference) micropore volume (cm3/g) mesopore volume (cm3/g)
49.27 6.55 1.56 42.62 7.75 10-4 0.0023
elemental compostion of ash (wt %) Na2O K2O MgO CaO Al2O3
0.54 8.34 4.03 10.34 6.42
Fe2O3 SiO2 TiO2 P2O5 other
1.37 57.65 0.23 7.21 3.87
Figure 1. Schematic diagram of the experimental appratus.
influence the amount and nature of volatiles produced during pyrolysis, as well as their rate of release, affecting char properties. These factors also determine both the macroscopic morphology and the microscopic porosity of the char.6,7 In the last 2 decades, the impacts of pyrolysis conditions on the physicochemical structure properties and reactivities of coal chars have been studied extensively. However, few studies can be found in the literature concerning the characterization of chars from biomass fuels and the effects of pyrolysis conditions on char properties, particularly for chars produced at high heating rates (HHRs).7 Among them, Pindoria et al.9 compared the pyrolysis and gasification of eucalyptus under different conditions and highlighted the importance of the structural modifications, which occurred after biomass devolatilization. Biagini et al.10,11 studied biomass char morphology under various devolatilization conditions and reported the occurrence of particle melting as a result of plastic deformation at HHRs. Cetin et al.4 reported that pyrolysis conditions had a notable impact on biomass char morphology. Therefore, it is still an area of interest to be explored.
In this study, maize stalk is used in this study as the representatives of agricultural residues, and the aim of the present work is to characterize the physical and chemical structure of maize stalk chars and study the influence of pyrolysis temperature and heating rate on the properties of chars. To accomplish this objective, ultimate analysis, X-ray diffraction (XRD), helium density measurement, N2 isothermal adsorption/desorption method, and Fourier transform infrared spectroscopy (FTIR) were applied. 2. Experimental Section 2.1. Material. Maize stalk was used in this study as the representatives of agricultural residues. The sample was first crushed and sieved. Fractions in the size range of 800 °C). This type of hysteresis can be explained by the presence of inkbottle pores. In pores of this shape, emptying of the wide portion will be delayed during desorption until the narrow neck can evaporate. With increasing pyrolysis temperature, the type E shape of the hysteresis loop becomes more obvious. Meanwhile, Tanev et al.22 proposed that the shape of the hysteresis curve related to the degree of pore blocking. Therefore, it can be deduced that pore characteristics in char particles change gradually during the reaction. The specific surface area SBET of the samples is evaluated using the multilayer adsorption model23 developed by Brunauer, Emmett, and Teller (BET). The BET model regards the surface of the solid as an array of adsorption sites having the same energy. Each of them can only accommodate one adsorbate molecule. An adsorption-desorption equilibrium between molecules reaching and leaving the solid surface is assumed. At the saturation pressure, the adsorbate condensates in an infinite number of layers. The most convenient form of the BET equation is given by P=P0 1 ðC -1Þ ¼ þ ðP=P0 Þ ð1Þ Vm C Vad ð1 -P=P0 Þ Vm C
Figure 6. N2 (-196 °C) adsorption isotherms of the maize stalk char obtained at 900 °C and different heating rates: LHR and HHR.
passages, occurs. Therefore, the structural shrinkage can affect pore narrowing. Pastor-Villegas et al.18 found a moderate structural shrinkage occurred when heating eucalyptus charcoal above 500 °C. The structural shrinkage in the chars may result from the loss of thermally unstable oxygen functional groups (specifically, ether-type structures). 3.2.3. Pore Structure Characteristics. Generally, the pore structure can be divided into three classes: micropores with a pore size smaller than 2 nm, mesopores with a pore diameter between 2 and 50 nm, and macropores wider than 50 nm.19 One of the methods for estimating the type of pores in a material is by analyzing the isotherm curve. The adsorption isotherms of maize stalk and char samples are illustrated in Figures 5 and 6. They provide information about macropores, mesopores, and the large micropores. The shape of the isotherms for the raw material and chars at 600 and 700 °C is similar to and resembles features between type I and II isotherms according to the Brunauer-Deming-DemingTeller (BDDT) classification.20 This fact indicates that only slight changes in the range of porosity evaluated by this technique occur at relatively low temperatures up to 700 °C. In contrast, pronounced changes in the shape of the isotherms are observed in the chars generated at higher temperatures (>700 °C). These changes may be attributed to
The micropore volumes VDR of the samples are obtained by applying the Dubinin-Radushkevich (DR) equation24
(18) Pastor-Villegas, J.; Rodrı´ guez, J. M. M.; Pastor-Valle, J. F.; Garcı´ a, M. G. J. Anal. Appl. Pyrolysis 2007, 80, 507–514. (19) Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. H.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Pure Appl. Chem. 1994, 66, 1739–1758. (20) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723–1732.
(21) de Boer, J. H. In The Structure and Properties of Porous Materials; Evereerr, D. H., Stone, F. S, Eds.; Butterworths: London, U.K., 1958. (22) Tanev, P. T.; Vlaev, L. T. J. Colloid Interface Sci. 1993, 160, 110– 116. (23) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982. (24) Dubinin, M. M. Carbon 1989, 27, 457–467.
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: DOI:10.1021/ef900268y
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Table 3. Characteristics of Porosity in Maize Stalk Chars Obtained under Different Pyrolysis Temperatures final temperature (°C)
BET surface area SBET (m2/g)
micropore volume Vmicro (cm3/g)
mesopore volume Vmeso (cm3/g)
total pore volume Vtot (cm3/g)
average pore diameter Dave (nm)
0.0029 0.0018 0.0088 0.0293 0.0275
0.0202 0.0179 0.0462 0.0827 0.0830
46.12 46.01 14.37 7.07 8.93
0.0241
0.0760
6.61
HHR Chara 600 700 800 900 1000
12.99 9.32 48.63 81.63 78.79
0.0062 0.0049 0.0228 0.0378 0.0367 LHR Charb
900 a
90.27
0.0420
Char obtained at a high heating rate. b Char obtained at a low heating rate (10 °C/min).
(eq 2) to P/P0 < 0.20,25 from which the micropore surface area is then determined.26 This equation based on the Polanyi thermodynamical theory of adsorption considers that adsorption in very fine pores involves a volume-filling process rather than layer-by-layer adsorption on the pore walls. It is frequently used to estimate the micropore volume of the samples from the low-pressure part of the isotherm.27 lgV ¼ lgVDR -Dlg2 ðP0 =PÞ
ð2Þ
where V is the amount adsorbed expressed as a liquid volume, VDR is the micropore volume of the sample, and D is the Dubinin coefficient. The experimental data plotted according to eq 2 are shown in Figure 7. They are linear over the relative pressure range for the maize stalk chars, indicating that the DR equation fits present data satisfactorily. For each sample, the micropore volume is obtained from the ordinate at the origin of the DR plots (Figure 7). The mesopore volume, Vme (cm3/g), is obtained by subtraction of the volume of nitrogen adsorbed at P/P0 = 0.10 from the volume of nitrogen adsorbed at P/P0 =0.95.28 The total volume is estimated by converting the amount of N2 gas adsorbed at a relative pressure of 0.99 to liquid volume of the adsorbate (N2).29 The density value taken for conversion of gas volumes into liquid volumes is 0.808 g/cm3 for nitrogen.29 Table 3 gives pore structure characteristic values obtained by the nitrogen adsorption/desorption method. As can be seen, the HHR chars formed below 700 °C have a small microand mesopore volume. Afterward, the micro- and mesopore volume increases dramatically to 0.0378 and 0.0293 cm3/g for the HHR char prepared at 900 °C and slightly decreases above 900 °C. The opposite trend applies to the average pore diameter. The above results indicate that the higher temperature char has a more developed pore structure. According to the pyrolysis results of maize stalk, the hemicellulose, cellulose, and lignin in the original sample would undergo dehydration, linkage breaking off reactions, the structural ordering process of the residual carbon, and finally, polymerization reaction during the carbonization process. With increasing pyrolysis temperature, the polymerization reaction would be deepened, the pore diameter would be lowered gradually, and the micro- and mesopore would be developed, giving rise to increases in the total volume and amount of micro- and mesoporosity of chars.
Figure 8. FTIR spectra of maize stalk and the HHR chars obtained at 600, 700, and 800 °C under HHR conditions.
The surface area of char may strongly influence the reactivity of the char. The devolatilization of biomass materials develops porosity in the chars, resulting in particles with an essentially micro-macropore structure. At higher temperature, the devolatilization is more intensive, making the char more porous. As can be seen from Table 3, at HHR, the BET-specific surface area follows a similar trend to that observed for the micro- and pore volume and reaches a maximum value of 81.6 m2/g at 900 °C. This observation is similar to that reported in the literature for some carbon materials, which also showed a maximum in the surface area with an increasing temperature.30,31 Between 600 and 900 °C, the increase in SBET may be attributed to the release of volatiles, which favors the development of some new porosities that become accessible to N2. Over 900 °C, structural ordering, pore widening, and/or the coalescence of neighboring pores seem to predominate, leading to the decrease in the surface area values and thermal deactivation of the chars. Thermal deactivation at high temperatures has also been suggested by other researchers.11,14 Besides, as a result of the softening, melting, fusing, and carbonization, pores in the chars might be partially blocked. This would prevent the access of the adsorption gas to the pores and lead to a lower surface area. High temperatures are detrimental to the development of the pore structure in the char. On the other hand, the heating rate influences the surface area of chars. At 900 °C, the HHR char has a lower surface area compared to the LHR char (81.63 and 90.27 m2/g, respectively).
(25) Lua, A. C.; Guo, J. Carbon 1998, 36, 1663–1670. (26) Dubinin, M. M. In Progress in Surface and Membrane; Cadenhead, D. A., Ed.; Academic Press: New York, 1975. (27) Rocca, P. A. D.; Cerrella, E. G.; Bonelli, P. R.; Cukierman, A. L. Biomass Bioenergy 1999, 16, 79–88. (28) Pastor-Villegas, J.; Duran-Valle, C. J. Carbon 2002, 40, 397–402. (29) Li, W.; Yang, K.; Peng, J.; Zhang, L.; Guo, S.; Xia, H. Ind. Crops Prod. 2008, 28, 190–198.
(30) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Hajaligol, M. R. Fuel 2001, 80, 1825–1836. (31) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X.; Chan, W. G.; Hajaligol, M. R. Fuel 2004, 83, 1469–1482.
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: DOI:10.1021/ef900268y
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Table 4. Main Atomic Groups and Structures of Maize Stalk -1
wavenumber (cm )
infrared absorption
atomic groups and structures
3200-3700 2800-3000 1650-1770 1610-1680 1450-1600 1420-1480 1360-1430 1200-1300 1000-1200 1000-1160 1050-1160 1070-1120 1000-1060 625-1000
O-H stretching C-H stretching CdO stretching CdC stretching CdC stretching C-H bending O-H and C-H bending C-O stretching C-H out-of-plane bending C-O stretching C-O stretching C-O stretching C-O stretching C-H out-of-plane bending
hydroxyl aliphatic structures carbonyl olefinic structures aromatic structures aliphatic structures hydroxyl, acid, phenol, olefins, and methyl unsaturated ethers aromatic structures saturated ethers tertiary hydroxyl secondary hydroxyl primary hydroxyl olefinic and aromatic structures
analyzed. The main conclusions obtained are summarized as follows: (1) The char yield decreased from 22 to 16.3% with an increasing temperature from 600 to 900 °C under HHR conditions. The char yield increased with a decreasing heating rate. The decrease in H/C was more than twice that in O/C, suggesting that some hydrogen was lost not only by dehydration but also by direct dehydrogenation or demethanation. (2) At HHR, maize stalk chars had a slightly less amorphous structure and less aliphatic side chains and became more aromatic and ordered with increasing temperatures. Higher temperature chars had a higher crystallite diameter in these solids. (3) Under HHR conditions, progressive increases in porosity development with increasing pyrolysis temperature occurred, whereas a maximum development of micro- and mesopores appeared at 900 °C. The surface area of char reached a maximum of 81.6 m2/g at 900 °C and decreased slightly at higher temperatures. Over 900 °C, structural ordering, pore widening, and/or the coalescence of neighboring pores led to the decrease in the surface area values, resulting in thermal deactivation of the chars. The influence of the heating rate on the surface area was not significant. (4) FTIR analysis showed that the hydroxyl, aliphatic C-H, carbonyl, and olefinic CdC groups were lost at high temperatures. The loss of ether groups led to a more ordered carbon structure. The spectrum of the HHR char obtained at 800 °C was nearly flat, suggesting that the raw sample could graphitize under the action of temperatures above 800 °C.
This observation is different from the results found in the literature.6 Guerrero et al.6 found that the eucalyptus char prepared at HHR had a higher surface area. 3.2.4. FTIR Analysis. FTIR analysis is a useful method for comparing qualitatively either vibrating absorption spectra of chars or relative intensities of the respective bands. A typical IR spectrum of maize stalk is shown in Figure 8. Band assignments, which are summarized in Table 4, indicate that maize stalk contains a number of atomic groupings and structures. Band intensities reveal that the most abundant chemical bonds are O-H, C-H, olefinic CdC, and C-O. CdO and aromatic CdC bonds are also detected in maize stalk. The spectra of the selected char samples are also presented in Figure 8. With the development of pyrolysis, there is an increasing drift in the baseline at high wavenumbers. In the opinion of Sharma et al., this may be an indication of the increase in the carbonaceous component content of chars.32 In the spectrum of the HHR char obtained at 600 °C, the bands attributed to aromatic vCdC and vC-O vibrations are clearly observed, suggesting that the char contains aromatic CdC bonds and ether structure as oxygen functional groups and becomes increasing polyaromatic. As the temperature increases, the ether groups decrease, which denotes that they are thermally unstable above 600 °C. The loss of ether groups leads to a more ordered carbon structure.33 When the temperature rises up to 800 °C, the spectrum of the HHR char obtained at 800 °C is nearly flat, which indicates that the raw sample can graphitize under the action of temperatures above 800 °C. Graphite spectra do not display infrared absorption bands because there is no change in the dipole moment when the atoms vibrate in this symmetric crystal build up of equal atoms.34
Acknowledgment. This work was supported by the National Natural Science Foundation of China (NSFC) (50776037 and 50721005), Program for New Century Excellent Talents in University (NCEF-07-0335), and the Major State Basic Research Development Program of China (2004CB217704). These supports are gratefully acknowledged. The authors also acknowledge the extended help from the Analytical and Testing Center of Huazhong University of Science and Technology.
4. Conclusions The influence of the pyrolysis temperature and heating rate on the properties of chars formed from maize stalk has been (32) Sharma, R. K.; Hajaligol, M. R.; Smith, P. A. M.; Wooten, J. B.; Baliga, V. Energy Fuels 2000, 14, 1083–1093. (33) Fu, P.; Hu, S.; Sun, L.; Xiang, J.; Yang, T.; Zhang, A.; Zhang, J. Bioresour. Technol. 2009, 100, 4877–4883. (34) Gomez-Serrano, V.; Pastor-Villegas, J.; Perez-Florindo, A.; Duran-Valle, C.; Valenzuela-Calahorro, C. J. Anal. Appl. Pyrolysis 1996, 36, 71–80.
Note Added after ASAP Publication. The footnotes of Tables 2 and 3 appeared incorrectly in the version of this paper published ASAP July 21, 2009; the corrected version published ASAP July 23, 2009.
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