Energy & Fuels 2001, 15, 841-847
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Pyrolysis Products of Tetra Pack in Different Oxygen Concentrations Chao-Hsiung Wu*,† and Yu-Fen Liu‡ Department of Environmental Engineering, Da-Yeh University, Changhwa 515, Taiwan, and Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan Received October 18, 2000. Revised Manuscript Received March 2, 2001
The tetra pack is one of the principal materials in municipal solid wastes (MSW). The amount of waste tetra pack is over 6000 tons per year in Taiwan. The components of tetra pack are kraft paper (about 70 wt %), low-density polyethylene (LDPE, about 25 wt %), and aluminum foil (about wt 5%), generally. Accordingly, its conversion to useful fuels as well as the recycling aluminum foil has become a worthy goal from not an economic, but also from an environmental standpoint. In this study, the experiments for the partial oxidation of tetra pack were carried out with a thermogravimetric analysis (TGA) reaction system. The pyrolysis reactions of tetra pack were performed in four different oxygen concentrations (CO2) of 5.4, 10.4, 14.8, and 21.0% for analyzing the pyrolysis products. The gaseous products were collected at room temperature (298 K) and analyzed by gas chromatography (GC). The residues were collected at the pyrolysis temperatures 575, 656, 700, 730, and 1073 K and analyzed by elemental analyzer, and X-ray powdered diffraction (XRPD). The cumulated masses and the instantaneous concentrations of gaseous products corresponding to the different oxygen concentrations were obtained under the experimental conditions. The recovery of total gaseous products based on the mass of tetra pack sample was over 88% for four different oxygen concentrations. The major gaseous products of tetra pack pyrolysis reaction were CO2, CO, H2O, and hydrocarbons (HCs). The hydrocarbons mainly consist of low molecular paraffins and olefins (C1-5). The percentages of C1-5 relative to the total hydrocarbons gases were about 69.9, 71.1, 74.3, and 68.8 wt % for tetra pack pyrolysis in CO2 ) 5.4, 10.4, 14.8, and 21.0%, respectively. In the XRPD analysis, the results indicated that a high-purity aluminum foil could be obtained from the final residues.
Introduction The major components of tetra pack are kraft paper (about 70 wt %), low-density polyethylene (LDPE, about 25 wt %), and aluminum foil (about 5 wt %), generally. Cellulose is a main component of kraft paper. Several studies investigated the pyrolysis mechanisms and the effects of reaction conditions on the pyrolysis products of cellulose.1-8 For the pyrolysis of paper, Agrawal and * Author to whom correspondence should be addressed. † Da-Yeh University. ‡ National Taiwan University. (1) Fairbridge, C.; Ross, R. A.; Sood, S. P. A Kinetic and Surface Study of the Thermal Decomposition of Cellulose Powder in Inert and Oxidizing Atmospheres. J. Appl. Polym. Sci. 1978, 22, 497-510. (2) Shafizadeh, F.; Bradbury, A. G. W. Thermal Degradation of Cellulose in Air and Nitrogen at Low Temperatures. J. Appl. Polym. Sci. 1979, 23, 1431-1442. (3) Antal, M. J.; Friedman, H. L., Jr.; Rogers, F. E. Kinetics of Cellulose Pyrolysis in Nitrogen and Steam. Combust. Sci. Technol. 1980, 21, 141-152. (4) Simkovic, I.; Durindova, M.; Mihalov, V.; Konigstein, J. Thermal Degradation of Cellulose Model Compounds in Inert Atmosphere. J. Appl. Polym. Sci. 1986, 31, 2433-2441. (5) Bilbao, R.; Arauzo, J.; Millera, A. Kinetics of Thermal Decomposition of Cellulose. Part I. Influence of Experimental Conditions. Thermochim. Acta 1987, 120, 121-131. (6) Gullett, B. K.; Smith, P. Thermogravimetric Study of the Decomposition of Pelletized Cellulose at 315 °C - 800 °C. Combust. Flame 1987, 67, 143-151. (7) Agrawal, R. K. Kinetics of Reactions Involved in Pyrolysis of Cellulose. II. The Modified Kilzer-Broido Model. Can. J. Chem. Eng. 1988, 66, 413-418.
McCluskey explored the thermal degradation of acid washing newsprint at low pressure ( 730 K, the C/H weight ratios decreased rapidly from 22.4 (at 730 K) to 1.2 (at 1073K). This may be ascribed to the gasification of C to CO and CO2. Gaseous Products. The gaseous products were analyzed by GC/TCD (for O2, N2, CO, and CO2), psychrometer (for H2O), and GC/FID (for HCs). The retention times of non-hydrocarbons (O2, N2, CO, and CO2) and hydrocarbons are listed in Tables 4, 5. The components of HCs were classified into five groups (C1-5 alkanes and alkenes, hexene and hexane, benzene and heptane, toluene, and octane) according to the retention times as shown in Table 5. The GC/FID spectrogram of gaseous products for tetra pack pyrolysis at HR ) 5.25 K/min and CO2 ) 5.4%, for example, is shown in Figure 2. It was noted from the GC/FID spectrogram that the major gas hydrocarbon components for the pyrolysis of tetra pack were the low molecular compounds (C1-5). Quantitative analysis of gaseous products was based on the calculation using the linear calibration response equations of standards. The equation was generated for each of gas and liquid standards using a minimum of five different concentrations with three replicates at each concentration. All correlation coefficients (r2) of linear calibration response curves were great than 0.997. The instantaneous concentrations of gaseous products (discrete samples) at various reaction temperatures are shown in Figures3 and 4. The results indicated that the amount of gaseous products was negligible for temperatures lower than 450 K. The gaseous products released mainly at the temperature ranges of 498-656 K and 656-775 K. There were two peak concentrations for CO, CO2, and HCs in the pyrolysis reaction temperature range (498-1073 K), while there was only one for H2O.
d
ND:
Table 5. Retention Times of Major Components of HCs Measured by GC/FID species group 1 methane ethene ethane propene propane butene butane pentene pentane average group 2 hexene hexane average group 3 benzene heptane average group 4 toluene average group 5 octane average
empirical formula
molecular weight
retention time (min)
CH4 C2H4 C2H6 C3H6 C3H8 C4H8 C4H10 C5H10 C5H12 C1-5
16 28 30 42 44 56 58 70 72
2.42 2.41 2.42 2.45 2.46 2.55 2.58 2.82 2.89 0-3.22
5.47 × 10-4 1.96 × 10-4 5.47 × 10-4 2.02 × 10-4 2.98 × 10-4 2.32 × 10-4 5.69 × 10-4 2.43 × 10-4 1.06 × 10-3 4.18 × 10-4
C6H12 C6H14 C6
84 86
3.56 3.71 3.22-4.12
3.01 × 10-4 2.09 × 10-3 1.19 × 10-3
C6H6 C7H16 C6-7
78 100
4.51 5.86 4.12-6.99
2.33 × 10-4 2.92 × 10-4 2.63 × 10-4
C7H8 C7
92
8.12 6.99-9.82
2.79 × 10-4 2.79 × 10-4
C8H18 C8
114
11.53 9.82-12
2.05 × 10-4 2.05 × 10-4
µg/L per area
Figure 2. GC/FID spectrogram of gaseous products for partial oxidation of tetra pack at HR ) 5.25 K/min and CO2 ) 5.4%. t: retention time.
The maximum instantaneous concentrations occurred at about 598 and 748 K for CO and CO2, 623 and 723 K for HCs, and 748 K for H2O, respectively. For CO and CO2, the concentrations at 748 K (about 26 000 and 93 000 µg/L) were higher than that at 598 K (about 22 000 and 51 000 µg/L). The secondary reaction of volatiles and/or tar to CO and CO2 was an important reaction in the temperature range of 656-775 K. This was similar to the results obtained by Cozzani et al.17 Major gaseous products were CO2, CO, H2O, and HCs for the pyrolysis of tetra pack at CO2 ) 21.0%. Group 1
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Figure 3. Instantaneous concentrations of HCs (C) for tetra pack pyrolysis at HR ) 5.25 K/min and CO2 ) 21.0%.
Figure 4. Instantaneous concentrations of major gaseous products (C) for tetra pack pyrolysis at HR ) 5.25 K/min and CO2 ) 21.0%.
Figure 5. Total yields of gaseous products of tetra pack pyrolysis at HR ) 5.25 K/min.
(C1-5 alkanes and alkenes) was the major product of HCs. On examining the TGA curves, there were two principal reactions as distinguished by the two significant and distinct mass changes over the temperature range of 498-1073 K. The devolatilization of tetra pack to low molecular weight compounds (CO2, CO, H2O, and HCs, etc.) was significant from 498 to 656 K. The results indicated that the first stage was contributed by the primary reaction of kraft paper. The HCs were the main products for the thermal degradation of LDPE. The pyrolysis reactions of intermediates/LDPE and the secondary reactions of volatiles were significant from 656 to 775 K. The total gaseous products of the tetra pack pyrolysis at different oxygen concentrations are shown in Figures 5 and 6. From Figure 5, the recovery of total gaseous products were 88.5, 98.2, 107.1, and 108.5 wt % for CO2 ) 5.4, 10.4, 14.8, and 21.0%, respectively. The recovery was based on the mass of tetra pack sample (not included O and ash). For CO2 ) 14.8 and 21.0%, the recovery of total gaseous products more than 100 wt % may be ascribed to the summation of experimental errors of various analysis methods, i.e., O2, N2, CO, and CO2 measured by GC/TCD, HCs by GC/FID, and H2O
Wu and Liu
Figure 6. Percentages of groups 1-5 relative to total HCs for tetra pack pyrolysis at HR ) 5.25 K/min.
by psychrometer. The recovery of major gaseous products at CO2 ) 5.4% were 76.6, 7.7, 2.9, and 1.3 wt % for CO2, CO, HCs, and H2O, respectively. The recovery of major gaseous products at CO2 ) 10.4% were 85.3, 8.9, 2.8, and 1.1 wt % for CO2, CO, HCs, and H2O, respectively. The recovery of major gaseous products at CO2 ) 14.8% were 93.3, 9.5, 2.7, and 1.5 wt % for CO2, CO, HCs, and H2O, respectively. The recovery of major gaseous products at CO2 ) 21.0% were 96.8, 7.8, 2.4, and 1.6 wt % for CO2, CO, HCs, and H2O, respectively. CO2 was the main component of gaseous products in the partial oxidation reaction. The incineration efficiency (weight ratio of CO2/(CO2 + CO)) was about 0.91 for the tetra pack oxidation in the four oxygen concentrations (CO2 ) 5.4, 10.4, 14.8 and 21.0%). The recovery of HCs decreased from 2.9 wt % (at CO2 ) 5.4%) to 2.4 wt % (at CO2 ) 21.0%). It was noted that O2 enhanced the oxidation reaction and resulted with high incineration efficiency. The recovery of H2O was in the range of 1.11.6 wt %. From Figure 6, one noted that Group 1 (C1-5 alkanes and alkenes) was the major product of HCs. The percentages of group 1 relative to the total HCs gases were about 69.9, 71.1, 74.3, and 68.8 wt % for tetra pack pyrolysis in CO2 ) 5.4, 10.4, 14.8, and 21.0%, respectively. The yields of non-hydrocarbon gases (CO2, CO, and H2O) and of hydrocarbon gases (HCs) relative to the initial sample were 85.6 and 2.9 wt % for CO2 ) 5.4%, 95.4 and 2.8 wt % for CO2 ) 10.4%, 104.4 and 2.7 wt % for CO2 ) 14.8%, 106.1 and 2.4 wt % for CO2 ) 21.0%, respectively. The results indicated that the non-hydrocarbons were the major gaseous products. The amount of liquid products may be negligible in the partial oxidation reaction. The pyrolysis process is an endothermic reaction. The energy needed in the pyrolysis reaction may be supplied by the partial oxidation of tetra pack. However, the differential scanning calorimetry (DSC) experiment was not done in this study. It would be helpful to investigate the heat released in various oxygen conditions and/or at different reaction temperatures. The formation of polycyclic aromatic hydrocarbons (PAHs) is a serious issue in the thermal treatment processes. Some possible PAH compounds, such as naphthalene, acenaphthylene, acenathene, fluorene, phenanthrene, anthracene, fluouanthene, pyrene, benzo[a]-anthracene, and chrysene, were not detected under the experimental conditions. This suggests that PAHs may be not be released in the oxidation reaction for tetra pack pyrolysis.
Pyrolysis Products of Tetra Pack
Conclusion Pyrolysis experiments of the tetra packs were carried out in a TGA reaction system at four different oxygen concentrations. The gaseous products were collected at room temperature (298 K) and analyzed by gas chromatography. The residues were collected at some significant pyrolysis reaction temperatures and analyzed by elemental analyzer, and X-ray powdered diffraction (XRPD). The cumulated masses and the instantaneous concentrations of gaseous products were obtained under the experimental conditions. The major gaseous products included non-hydrocarbons (CO2, CO, and H2O) and hydrocarbons (C1-5, alkanes and alkenes). In the XRPD analysis, the results indicated that a high-purity aluminum foil could be obtained from the final residues
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even in oxygen environment. Since the synthetic gases (CO, CO2, H2O, HCs) contained a high calorific value, their use as marketable fuels greatly supported the importance for resource recycling of the waste tetra packs. The gaseous products distributions as well as the results in the elemental analyses of residues provided useful data for determining the pyrolysis mechanisms. Acknowledgment. We express our sincere thanks to the National Science Council of Taiwan for the financial support under projects NSC 88-2211-E-212002. We also thank the Tetra Pak corporations for providing the samples. EF000231R
