Coal-like Thermal Behavior of a Carbon-Based Environmentally

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Coal-like Thermal Behavior of a Carbon-Based Environmentally Benign New Material: Woodceramics Riko Ozao,*,† Wei-Ping Pan,‡ Nathan Whitely,‡ and Toshihiro Okabe§ North Shore College of SONY Institute, Atsugi, Kanagawa, 243-8501, Japan, Materials Characterization Center, Department of Chemistry, Western Kentucky University, Bowling Green, Kentucky 42101, and Environmental Technology Division, Aomori Industrial Research Center, Aomori, 030-0113, Japan Received September 4, 2003. Revised Manuscript Received January 15, 2004

Woodceramics prepared from apple pomace, which were obtained by impregnating it with phenolic resin and sintering the material at different temperatures of 1073 K (sample AWC800) and 1473 K (sample AWC1200), were investigated mainly by X-ray diffraction (XRD) and simultaneous differential scanning calorimetry-thermogravimetry (DSC-TG). The as-prepared samples were amorphous; using XRD, broad peaks with d-spacings of ca. 0.57-0.52 and ca. 0.360.38 nm were observed. This result suggested the presence of at least two different phases: the former, which is attributed to a structure derived from aliphatic chains, and the latter, which is attributed to a structure derived from aromatic rings (graphene-like layers). By heating to 1273 K, the aromatic-ring stacking or graphene-like layers developed in three dimensions, whereas the aliphatic chains, which are more prone to oxidization, disappeared when heated in air. Therefore, it was suggested that combustible fragments with aliphatic chains develop via pyrolysis and that these undergo combustion in an oxidizing atmosphere. Furthermore, sample AWC1200 contained additional phases with longer d-spacings, which disappear on heating to 1273 K. Thus, in case of sample AWC1200, the main combustion is preceded by pyrolysis and the combustion of additional matter having a long-range ordering. Woodceramics are structurally different from charcoal or coal, and an ordered structure with a characteristic graphene-like layer is developed through further heat treatment to 1273 K.

Introduction Woodceramics are a new type of environmentally benign material that is produced from plant-based carbon-containing wastes, and it is a carbon/carbon hybrid material in the sense that cellulose- or ligninbased carbon is reinforced by phenolic-resin based glassy carbon.1 Originally, woodceramics were prepared by impregnating wood boards such as medium-density fiberboards (MDFs) with a thermosetting resin such as phenolic resin under vacuum and then heating the resin-impregnated board in a furnace under an atmosphere that is substantially free of oxygen.2 Thus, it can be understood that woodceramics differ from simple charcoal in that they have higher mechanical strength3 and are resistant to higher temperatures.4 It is reported that the density increases as the sintering temperatures increase, up to 1073 K.3 Thus, it is now well-accepted * Author to whom correspondence should be addressed. E-mail address: [email protected]. † North Shore College of SONY Institute. ‡ Western Kentucky University. § Aomori Industrial Research Center. (1) Okabe, T.; Saito, K. Production of Wood Ceramics. Jpn. Patent Publication A, No. H04-164806, 1992. (2) Okabe, T.; Saito, K.; Fushitani, M.; Otsuka, M. Mechanical Properties of Porous Carbon Material: Woodceramics. J. Porous Mater. 1996, 2, 223-228. (3) Saitoh, K.; Hokkirigawa, K.; Ohtsuka, M.; Fushitani, M. Mechanical Properties of Woodceramics (in Jpn.). In Woodceramics (in Jpn.); Okabe, T, Ed.; Uchida Rokakuho Publishing: Tokyo, 1996; pp 130-157.

that sintering at 1073 K provides woodceramics with favorable mechanical strength. Furthermore, woodceramics have another advantage: they are ecological materials that can be prepared from lignocellulosic industrial wastes such as sawdusts, apple wastes, and olive wastes. For instance, the apple product industry in Aomori Prefecture, Japan, generates ∼4250 t/year of apple pomace, which corresponds to ∼20% of the raw material (average taken for the years 1997-2001). Attempts have been made to produce woodceramics from such waste materials.5 However, to date, no data concerning thermal properties and structure have been reported on such types of so-called “ecoceramics”. The authors have reported thermogravimetry-differential thermal analysis/mass spectrometry (TG-DTA/MS) results on woodceramics that were based on apple pomace (hereafter referenced simply as “apple woodceramics”) prepared by sintering at different temperatures of 1073 and 1473 K.6 This (4) Kano, M.; Momota, M.; Okabe, T.; Saito, K.; Yamamoto, R. New Porous Carbon Material, Woodceramics: Thermophysical Properties. In Proceedings of the 5th European Conference on Advanced Materials and Processes and Applications (EUROMAT ‘97); The Federation of European Materials Societies: Maastricht, The Netherlands, 1997; Vol. 2, pp 431-434. (5) Okabe, T. Report on Recyclable Nonused Resources in Aomori Prefecture (in Jpn.); Industrial Research Institute of Aomori Prefecture: Aomori, Japan, 2002. (6) Ozao, R.; Okabe, T.; Arii, T. Thermoanalytical Characterization of Apple-Based Woodceramics Using TG-DTA/MS. Trans. Mater. Res. Soc. Jpn. 2003, 28, 1075-1078.

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Coal-like Thermal Behavior of Woodceramics

material shows promising characteristics as a sorbent.7 When heated in a helium gas flow, the samples sintered at 1073 K exhibited two-step gas evolution; i.e., an evolution of gaseous CO2 occurred simultaneously with, or after that of, H2O at temperatures lower than ca. 500 K. For samples that were sintered at 1473 K, H2O evolved in one step, almost simultaneously with CO2. Furthermore, CO2 gas evolution was observed again at higher temperatures (∼973 K) for the products that were sintered at 1473 K, which was ∼100 K higher as compared to that observed for products sintered at 1073 K (873 K). The CO2 gas evolution was followed by CO gas evolution in both samples, which suggested progressive carbonization. These results suggested that different phases were formed by sintering at different temperatures. However, the details are yet to be clarified. In reference to other noncrystalline carbon materials, active carbons and activated carbons are produced from a variety of precursors. It is also well-known that active carbons are readily available from low-cost biomasses such as palm-shell olive stones and other naturally occurring lignocellulosic materials.8-11 The precursors, which are mostly charcoal, are activated either chemically or physically at high temperatures, to modify the pore structure. In regard to other heterogeneous carbon compounds, numerous reports have been made on the thermal change of coal. Coal is thought to be as a polymeric structure that forms a large macromolecular network.12 The macromolecular network consists of aromatic clusters that are linked and cross-linked to other aromatic structures by bridges. Such bridge structures have a large distribution of bond strengths.13 When heated under an oxygen-free atmosphere, coal first undergoes pyrolysis, i.e., devolatilization, before it is combusted. However, because coal has such a complex nature, devolatilization is thought to consist of various overlapping reactions such as breaking the bonds of the bridges and the side chains, and binding aromatic clusters together. Because woodceramics are believed to be porous carbon composites that consist of cellulose- and ligninoriginated carbon reinforced by the three-dimensional network of aromatic clusters generated by heating phenolic resin,3 similarities with coal may be found. For instance, Pan et al.14,15 studied different types of coals, using TG and DTA in air and in nitrogen, and found a (7) Ozao, R.; Arii, T.; Okabe, T. Thermal Characterization of AppleBased Woodceramics. In Proceedings of IUMRS-ICAM, 2003, Paper No. PPA-C-3-035. (8) Rodriguez-Reinoso, F.; Molina-Sabio, M.; Gonzalez, M. T. The Use of Steam and CO2 as Activating Agents in the Preparation of Activated Carbons. Carbon 1995, 33, 15-23. (9) Gonzalez, M. T.; Rodriguez-Reinoso, F.; Garcia, A. N.; Marcilla, A. CO2 Activation of Olive Stones Carbonized under Different Experimental Conditions. Carbon 1997, 35, 159-162. (10) Daud, W. M. A. W.; Ali, W. S. W.; Salaiman, M. Z. The Effect of Carbonization Temperature on Pore Development in Palm-ShellBased Activated Carbon. Carbon 2000, 38, 1925-1932. (11) Jia, G.; Lua, A. C. Preparation of Activated Carbons from OilPalm-Stone Chars by Microwave-Induced Carbon Dioxide Activation. Carbon 2000, 38, 1985-1992. (12) van Krevelen, D. W. Coal: Typology, Chemistry, Physics, and Constitution; Elsevier: New York, 1981. (13) Solomon, P. R.; Serio, M. A. Evaluation of Coal Pyrolysis Kinetics. In Fundamentals of the Physical Chemistry of Pulverized Coal Combustion; Lahaye, J., Prado, G., Eds.; Martinus Nijhoff: Dordrecht, The Netherlands, 1987; pp 126-151. (14) Chen, Y.; Mori, S.; Pan, W.-P. Estimating the Combustibility of Various Coals by TG-DTA. Energy Fuels 1995, 9, 71-74.

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high-reactivity combustibles region and a low-reactivity combustibles region in the coal char combustion process. When lignite coal that contained a large quantity of volatile matter was heated in air, an exothermic peak was observed on the DTA curve at ca. 573 K, bceause of the release of volatile matter and ignition. In the case of anthracite coal, which contains little volatile matter, no such exothermic peak appeared in this temperature region. However, for all types of coals, a combustion reaction that yielded a sharp exothermic peak with a rapid loss of mass was observed in the temperature range of ca. 673-773 K. This stage was followed by char combustion. Similarly, Liu et al.16 provided TG-DTG (derivative TG) curves in air and in nitrogen gas flow for a typical coal. Inherent moisture evolved at temperatures of 370 K and light gases such as CO, CO2, and light hydrocarbons are released at 470-770 K; however, at lower heating rates of 1 K/s (60 K/min), the light gases evolve at 670 K or higher. It is also known that tar is formed at ∼600-800 K in low heating rates; at a heating rate of 30 K/min, early crosslinking begins in the temperature range of 670-770 K, which is succeeded by later crosslinking. We report the structural change that occurs with heating of a newly developed environmentally benign carbon/carbon hybrid material, i.e., woodceramics produced from apple pomace. The effect of sintering temperature for the production of apple woodceramics is also shown. Experimental Section Samples. Samples of apple woodceramics were obtained by mixing commercially available apple fiber (Nichiro Corporation) with a phenolic resin (BELLPEARL S890, a product of Kanebo, Ltd.) at a ratio of 6:4 (by mass) and sintering at different temperatures of 1073 K (hereafter referenced as sample AWC800) and 1473 K (hereafter referenced as sample AWC1200), according to the method described previously.3 The granularity of the samples was 250 µm or less. Apparent Density Measurement. The apparent density of apple woodeceramics was obtained using an automatic gas (15) Chen, Y.; Mori, S.; Pan, W.-P. Studying the Mechanisms of Ignition of Coal Particles by TG-DTA. Thermochim. Acta 1996, 275, 149-58. (16) Liu, K.; Gao, Y.; Riley, J. T.; Pan, W.-P.; Mehta, A. K.; Ho, K. K.; Smith, S. R. An Investigastion of Mercury Emission from FBC Systems Fired with High-Chlorine Coals. Energy Fuels 2001, 15, 11731180. (17) Saxena, S. C. Devolatilization and Characteristics of Coal Particles. Prog. Energy Combust. Sci. 1990, 15, 55-94. (18) Hodek, W.; Kramer, M.; Juntgen, H. Reactions of Oxygen Containing Structures in Coal Pyrolysis. Fuel 1990, 70, 424-428. (19) Ibarra, J. V.; Molier, R.; Gavilan, M. P. Functional Groups Dependence of Cross-Linking Reactions During Pyrolysis of Coal. Fuel 1991, 70. (20) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. General Model of Coal Devolatilization. Energy Fuels 1988, 2, 405-422.

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Figure 1. Simultaneous DSC-TG results for apple woodceramics sintered at 1073 K (sample AWC800) obtained under a flow of N2 gas (100 mL/min) and a flow of air gas (100 mL/min) superimposed. Temperature ramp was 5 K/min. Derivative TG (DTG) plots are also shown. pycnometer (Quantachrome Instruments, model Ultrapycnometer 1000), using 10-cm3 volume cells under a helium gas flow. The samples were each scanned 10 times at 295 K. The last three data points were used for the calculation. The results are given in Table 1. Table 1. Apparent Density for Apple Woodceramics Sintered at 1073 K (Sample AWC800) and 1473 K (Sample AWC1200) Apparent Density (g/cm3) sample AWC800

sample AWC1200

first stage second stage third stage

2.39795 2.35148 2.41844

2.07169 2.08750 2.09031

average standard deviation

2.38929 0.028014

2.08317 0.008196

Simultaneous Differential Scanning CalorimetryThermogravimetry (SDT) Measurements. Differential scanning calorimetry (DSC) was performed simultaneously with thermogravimetric (TG) analysis, using a simultaneous DSC-TG analyzer (model SDT 2960, TA Instruments) and ∼10-15 mg of each sample over a temperature range from room temperature (RT) to 1273 K at a heating rate of 2, 5, 10, and 20 K/min. The measurements were conducted under an air flow of 100 mL/min (Airgas compressed air (breathing grade), Type I, Grade D, 21% O2 certified), and under a dry nitrogen gas flow (Airgas NI ED300 nitrogen, extra dry) at a rate of 100 mL/min. XRD Identification. X-ray diffraction (XRD) analysis on the as-received samples and heated samples (in air and in a nitrogen gas flow) were made using an X-ray diffractometer (Scintag model X’TRA AA85516, ThermoARL) that was equipped with a Peltier cooled silicon solid detector. Monochromatized Cu KR1 radiation (0.15054 nm) was used. Dif-

fraction patterns were collected at 45 kV, 40 mA, in steps of 0.01° step and a count time of 0.500 s over a range of 1.00° to 90.00° (2θ), at a step scan rate of 1.20°/min.

Results and Discussions Apparent Density of Samples Sintered at Different Temperatures. Table 1 shows the apparent density of apple woodceramics that have been sintered at 1073 K (sample AWC800) and 1473 K (sample AWC1200). The density of apple woodceramics sample AWC800 is higher than that sintered at 1473 K (sample AWC1200) by ∼15%. According to TG data, the mass losses of samples AWC800 and AWC1200 up to ca. 473 K are 7.08% and 3.07%, respectively. By taking the effect of physisorbed matter into account, the apparent densities for samples AWC800 and AWC1200 can be calculated as 2.22 and 2.02 g/cm3, respectively. The change in density of MDF-based woodceramics is reported up to a sintering temperature of 1373 K, and although not mentioned in the text, maximum density is attained at 1073 K.3 Kawamura et al.21 reported a volume expansion of glasslike carbon with increasing heat-treatment temperature, which suggests broadening of the mean interlayer spacing of the stacking layers. This results in a decrease in density with increasing heating temperature. Kercher and Nagle22 reported a (21) Kawamura, K.; Ozawa, S.; Endo, H. Volume Expansion of Glasslike Carbon upon High Temperature Heat Treatment. Carbon 2003, 41, 191-194. (22) Kercher, A. K.; Nagle, D. C. Microstructural Evolution During Charcoal Carbonization by X-ray Diffraction Analysis. Carbon 2003, 41, 15-27.

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Figure 2. Simultaneous DSC-TG results for apple woodceramics sintered at 1473 K (sample AWC1200) obtained under a flow of N2 gas (100 mL/min) and a flow of air gas (100 mL/min) superimposed. Temperature ramp was 5 K/min. Derivative TG (DTG) plots are also shown. Table 2. Average Mass Loss and Integrated Peak Area Obtained from Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) under Air Flow sample

integrated peak area (J/g)

mass loss (%)

AWC800 AWC1200

23397 21812

89.06 91.58

decrease in density for carbonized MDF with increasing carbonization temperature to >1273 K, although their helium density is not in agreement with their densities calculated from mass loss and volumetric expansion. Because woodceramics are composites of lignocellulosicoriginated carbon and glassy carbon that originated from phenolic resin, the aforementioned decrease in density with increasing sintering temperature from 1073 to 1473 K is in conformity with the reports. Simulataneous DSC-TG Results under Different Atmospheric Conditions. Figures 1 and 2 show DSC-TG results for samples AWC800 and AWC1200, respectively, obtained simultaneously at a temperature ramp of 5 K/min. Derivative TG (DTG) plots are also shown in the figures. The results obtained under a gaseous N2 flow of 100 mL/min and under an air flow of 100 mL/min are superimposed. The details of thermal change and heat resistance will be reported elsewhere,23 and a brief summary is given here. The mass loss up to ca. 473 K (first stage) is mainly due to the desorption of H2O and CO2. At temperatures of ca. 473-900 K (the second stage), pyrolysis occurs, which generates CO2 (23) Ozao, R.; Pan, W.-P.; Nishimoto, Y.; Toshihiro, O. Thermoanalytical Characterization of Carbon/Carbon Hybrid Material, Apple Woodceramics, submitted to Carbon.

Figure 3. Mass loss of sample AWC1200 for 773 K or higher and a total mass loss in N2 gas flow, plotted against heating rate. Additional data obtained for heating rate of 1 K/min are also plotted.

and light gases, as well as volatile matter. Under an oxidizing atmosphere, ignition and sequent combustion occur, as observed by the exothermic peak in DSC and a mass loss of ca. 90%-95%. In an oxidizing atmosphere, remaining char is combusted at temperatures higher than ca. 900 K (the third stage). At the beginning of the second stage, sample AWC1200 yields a small peak in DSC, which accompanies a loss in mass, as observed by TG and DTG. This is presumed to be due to the overlapping of pyrolysis and the combustion of volatile matter.6,23 The integrated peak area and mass loss obtained in an air gas flow for the second stage, which are averaged

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Figure 4. XRD patterns in the 2θ range of 1°-50° for as-received sample AWC800, sample AWC800 heated to 1273 K in an air flow of 100 mL/min, and sample AWC800 heated to 1273 K in a N2 gas flow of 100 mL/min.

Figure 5. XRD patterns in the 2θ range of 1°-50° for as-received sample AWC1200, sample AWC1200 heated to 1273 K in an air flow of 100 mL/min, and sample AWC1200 heated to 1273 K in a N2 gas flow of 100 mL/min.

for the four heating rates (2, 5, 10, and 20 K/min), are given in Table 2. As stated previously, in relation to density measurement, sample AWC800 contains higher amounts of physically adsorbed matter; this partially accounts for the lower mass loss. The integrated peak areas for both are higher than that of simple charcoal (the calculation was performed for the change in mass

and integrated). However, the value for sample AWC1200 is lower than that for sample AWC800. As shown in the DSC curve taken in N2 gas flow, pyrolysis is an endothermic reaction, but combustion is an exothermic reaction. Thus, by overlapping the pyrolysis and combustion reactions, the overall heat results are less than that of simple combustion. Accordingly, the results also

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Table 3. d-spacings for As-Received Samples and Samples Heated to 1273 K in Air and in Nitrogena sample

d002 (nm)

AWC800 AWC800 heated in air AWC800 heated in N2

0.3617 (br,w) 0.3523 (s) 0.3516 (s)

AWC1200 AWC1200 heated in air AWC1200 heated in N2

0.3824 (br,m) 0.3545 (s) 0.3531 (s)

d-spacing for phase with 2θ ≈ 16° (nm)

d-spacing for others (nm)

0.5712 (br,m) 0.5723 (br,m) ∼1.7-8.8 (br,s)

0.5205 (br,w) 0.5507 (br,m)

a

The abbreviations “br”, “s”, “m”, and w, given in parentheses denote the qualitative intensities (broad, strong, medium, and weak, respectively).

support the aforementioned presumption of overlapping pyrolysis and combustion that occurs at the beginning of the second stage for sample AWC1200. Figure 3 shows the mass loss of sample AWC1200 at temperatures of 773 K or higher and the total mass loss in N2 gas flow plotted against the heating rate. Additional data were obtained for a heating rate of 1 K/min. The plots can be expressed by two different linear regressions, depending on the heating rate. It seems that the mass loss increases drastically as the heating rate decreases to 5 K/min or less. We already proposed that this reaction could be expressed by a single reaction rate function.23 However, such a seemingly drastic change may account for the different devolatilization mechanisms. XRD Idenitification of As-Prepared and Heated Samples. Figures 4 and 5 show XRD patterns in the 2θ range of 1°-50° for as-received samples, as well as for samples heated to 1273 K in air and in a flow of N2 gas. As-received samples of AWC800 and AWC1200 show a somewhat amorphous pattern, with broad peaks at ca. 25° and 16° for sample AWC800 and ca. 23° and 17° for sample AWC1200. The broad peaks at ∼25° presumably represent the {002} planes of graphene-like layers,24 or the so-called “π-bands” in coal.25 The broad peaks at ∼16° are attributed to the so-called “γ-band”, which is believed to be derived from aliphatic chains25 and is typically observed in charcoals. Table 3 shows a summary of the d-spacings for the samples. The d002 spacing for sample AWC1200 is greater than that for sample AWC800, and this is similar to the case of glassy carbon that had been heated to various temperatures.21 Moreover, the XRD pattern for sample AWC1200 suggests that a long-range ordered phase with a d-spacing of 1.7-8.8 nm is present. However, this phase is no longer observed after heating in nitrogen or in air. This may account for the exothermic peak that accompanies mass loss, as observed in SDT curves at the beginning of the second stage of thermal change for sample AWC1200. That is, this phase is initially pyrolyzed as

volatile matter. Furthermore, when heated in nitrogen, both samples undergo pyrolysis, and the char remains without being combusted. The broad peak at ∼16° remains, with increased intensity, and is shifted slightly to the lower-angle side (i.e., to a larger d-spacing). In contrast, the d002-spacings are decreased while the peak intensity is increased. This observation indicates that the graphene-like layers develop three-dimensionally by heating, resulting in a charcoal-like pattern.

(24) Hirsh, P. B. X-ray Scattering from Coals. Proc. R. Soc. London, A 1954, A226, 143-169. (25) Watanabe, K.; Sakanishi, K.; Mochida, I. Change in Coal Aggregate Structure by Heat Treatment and Their Coal Rank Dependency. Energy Fuels 2002, 16, 18-22.

Acknowledgment. R.O. thanks North Shore College of SONY Institute for financial support.

Conclusions Woodceramics prepared from apple pomace, which were obtained by impregnating it with phenolic resin and sintering the material at different temperatures of 1073 K (sample AWC800) and 1473 K (sample AWC1200), were investigated mainly by XRD and simultaneous differential scanning calorimetry-thermogravimetry (DSC-TG). The as-prepared samples were determined to be amorphous; XRD analysis showed broad peaks with d-spacings of ca. 0.57-0.52 nm and ca. 0.36-0.38 nm. This observation suggested the presence of at least two different phases: the former was attributed to a structure that was derived from aliphatic chains, and the latter was attributed to a structure that was derived from aromatic rings (graphene-like layers). By heating to 1273 K, the aromatic-ring stacking or graphene-like layers develop in three dimensions, whereas the aliphatic chains, which are more prone to oxidization, disappear when heated in air. Therefore, it is suggested that combustible fragments with aliphatic chains develop by pyrolysis, and that these undergo combustion in an oxidizing atmosphere. Furthermore, sample AWC1200 contained additional phases with longer d-spacings, which disappear when heated to 1273 K. Thus, in the case of sample AWC1200, the main combustion is preceded by pyrolysis and the combustion of additional matter that has long-range ordering. Woodceramics are structurally different from charcoal or coal, and an ordered structure with characteristic graphene-like layer is developed through further heat treatment to 1273 K.

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