Energy & Fuels 2006, 20, 1609-1613
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Effect of Experimental Conditions on the Yields during the Copyrolysis of Mustafa Kemal Pas¸ a (MKP) Lignite (Turkey) with Low-Density Polyethylene Ali Sınagˇ,* Melike Sungur, and Muammer Canel Department of Chemistry, Science Faculty, Ankara UniVersity, 06100 Bes¸ eVler-Ankara, Turkey ReceiVed March 17, 2006. ReVised Manuscript ReceiVed May 1, 2006
Copyrolysis of a turkish lignite with low-density polyethylene (LDPE) is conducted in a tubular reactor. The effect of experimental conditions (temperature of 400-700 °C, catalyst, LDPE contents of the mixture are 33, 50, and 67 wt %) on the formation of tar, gas, and char and their effects on the formation of phenol are investigated. The catalysts used are red mud (which is a waste product of an aluminum factory in Turkey), zeolite (Linde type A (LTA)), and K2CO3. Tar evolution is determined to be increased significantly by increasing the LDPE content of the coal-LDPE mixture during the pyrolysis. The effect of adding LDPE to the coal on the gas generation is not remarkable. An increase in temperature leads to increased gas yields. Phenol and phenol derivatives are the obstacles for the complete conversion of lignite to tar and gas. To investigate this negative effect of phenols on the yields, the phenols found in tar from coal pyrolysis are detected by gas chromatography-mass spectroscopy (GC-MS), and it is observed that phenolic structures detected in the tar obtained by individual pyrolysis of coal are dramatically decreased by adding polymer to the coal. The use of catalysts during the copyrolysis procedure leads to improved gas generation. The possible reasons of these variations are discussed. A remarkable synergetic effect between lignite and LDPE on the tar yields is also observed.
Introduction An increase in the usage and consumption of plastics such as polyethylene, polystyrene, polypropylene, and poly(ethylene terephthalate) is considered to be inevitable, because of their versatility, their myriad of uses, and the relatively small amount of energy required for their production (compared to that of other materials). At the same time, plastic wastes are becoming a serious environmental problem worldwide, because of their quantity, complexity (in terms of having multiple polymers and mineral additives), and inherent stability. Landfilling has long been the most widely used waste disposal method. Inert plastics will not degrade over thousands of years in a landfill; however, the additives and plasticizers, which contain lead, chlorine, and other toxic compounds, have aroused the growing concern of the public. Recycling is one of the more acceptable methods of the disposal of plastic waste. However, conventional plastic recycling is facing many problems, such as the existence of 100 classes of plastic polymer with 1000 specifications. Therefore, an advanced technology is required for sorting and separating post-consumer waste plastic for high-quality recycling.1,2 Another option for plastic waste disposal is incineration. Although incineration facilities are equipped with air pollution control devices, such as electrostatic precipitators, fabric filters for particulate control, and dry or wet scrubbers for acid gas
removal, there is still substantial public concern over environmental issues regarding incineration. Such concerns not only add to the high cost of pollution control but also result in significant uncertainty with regard to the construction and future operation of these facilities. Recently, there has been an increased interest in the coprocessing of waste plastics with coal. Coal is relatively plentiful throughout the world, including Turkey; this abundance provides a readily available hydrocarbon resource for conversion to liquid fuels. There is evidence that higher conversion efficiencies can be obtained by coprocessing the coal with waste plastics.2,3 Generation of hydrogen radicals or hydrocarbon radicals from polyolefins requires no such high pressure, as does molecular hydrogen in hydropyrolysis during copyrolysis of coal with polymers. However, the yields of the products are not great, because of the low hydrogen-to-carbon ratio content of coals. To increase the liquid yields obtained from coal pyrolysis, the stabilization of the radicals produced during the breaking reactions should be enhanced, and simultaneously, the crosslinking reactions increasing char formation should be decreased.4,5 For that reason, it is necessary to supply the radicalic hyrdogen to coal from other sources such as polymeric wastes, hydrocarbon materials, etc. The previous studies show that copyrolysis of lignite and polymer mixtures generates a synergism between the coal and the polymer6-9 and the quality of the products is improved, as explained previously. However,
* Author to whom correspondence should be addressed. Tel: +90 312 212 67 20/1022. Fax: +90 312 223 23 95. E-mail: sinag@ science.ankara.edu.tr. (1) Joo, H. K.; Curtis, C. W. Catalytic coprocessing of LDPE with coal and petroleum resid using different catalysts. Fuel Process. Technol. 1998, 53, 197-214. (2) Taghiei, M. M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Coliquefaction of Waste Plastics with Coal. Energy Fuels 1994, 8, 1228-1232.
(3) Palmer, S. R.; Hippo, E. L.; Tandan, D. Co-conversion of coal/waste plastic mixtures under various pyrolysis and liquefaction conditions. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 1995, 40 (1), 29-33. (4) Hayashi, H.; Mizuta, K.; Kusakabe, S.; Morooka. Flash copyrolysis of coal and polyolefin. Energy Fuels 1994, 8, 1353. (5) Asante, K. O.; Stock, L. M.; Zabransky, R. F. Pathways for the decomposition of linear paraffinic materials during coal pyrolysis. Fuel 1988, 68, 567.
10.1021/ef060108l CCC: $33.50 © 2006 American Chemical Society Published on Web 05/25/2006
1610 Energy & Fuels, Vol. 20, No. 4, 2006 Table 1. Ultimate Analysis of the MKP Lignite
Sınagˇ et al. Table 3. Composition of Red Mud
element
content (wt %, daf)
component
content (wt %)
C H N O S
69.62 4.87 1.83 19.30 4.38
Na2O MgO Al2O3 SiO2 P2O6 K2O CaO TiO2 Cr2O3 MnO Fe2O3
7.13 0.40 17.27 17.10 1.15 0.29 4.54 4.81 2.17 7.42 37.72
Table 2. Physical and Chemical Properties of Zeolite property
remark/value
tradename chemical formula for the zeolite content zeolite structure, according to the IUPAC pH value (20 °C) density (bulk) surface area
Ko¨strolith molecular sieves of type 5A (CaNaA, CaA) in the form of beads xCaO‚(1 - x)Na2O‚Al2O3‚2SiO2‚nH2O (for x g 6) Linde Type A (LTA) 9-11 (5 g of product in 100 g of water) 680-780 g/L 300 m2/g
there is limited information about the underlying reaction pathways and mechanisms that control coal liquefaction in the presence of polymeric materials. In the present study, lignite, low-density polyethylene (LDPE), and mixtures of lignite and LDPE are subjected to the pyrolysis process. The effect of experimental conditions such as temperature, the amount of LDPE in the coal-LDPE mixture, and the catalyst (red mud, zeolite, K2CO3) on the tar, gas, and residual coke yields is investigated. The contents of phenol and phenol derivatives of tar obtained via the pyrolysis of lignite and lignite-LDPE mixtures are determined to investigate whether the phenolic structures have an effect on the generation of tar. Experimental Section MKP lignite is obtained from the Mustafa Kemal Pas¸ a region of Bursa, Turkey. Ultimate analysis of the lignite is performed on a LECO model 932 CHNS elemental analyzer, using coal sample sizes of ∼1.5 mg, and the results are given in Table 1. The LDPE is supplied as a powder from the O ¨ zugˇur-Akc¸ im Plastic Company (Izmir, Turkey). The catalysts used are red mud, zeolite, and K2CO3. Eightmembered-ring zeolites, in particular, have been actively investigated for hydrocarbon conversions, because their window sizes are comparable to the molecular dimensions and they can provide high adsorption capacities. A typical example is the Linde type A (LTA) zeolite, which is characterized by a set of three-dimensional interconnected channels that have eight-membered-ring window apertures. The effective size of the windows can be controlled by appropriately selecting the type of charge-balancing cations. Zeolite was purchased from CWK Chemiewerk Bad Ko¨stritz GmbH. The zeolite used as a catalyst in the experiments is an LTA zeolite, the properties of which are given in Table 2. Zeolite is a solid-type catalyst, and it has alkali or alkaline-earth metal on its surface. Preheating the catalyst at a moderately high temperature can recover the active sites and thermal pretreatment of the zeolytic catalyst can efficiently improve the reaction conversion. Therefore, zeolite (6) Suelves, I.; Moliner, R.; La´zaro, M. J. Synergetic effects in the copyrolysis of coal and petroleum residues: influences of coal mineral matter and petroleum residue mass ratio. J. Anal. Appl. Pyrol. 2000, 55, 29-41. (7) Suelves, I.; La´zaro, M. J.; Moliner, R. Synergetic effects in the copyrolysis of samca coal and a model aliphatic compound studied by analytical pyrolysis. J. Anal. Appl. Pyrol. 2002, 65, 197-206. (8) Meesri, C.; Moghtaderi, B. Lack of synergetic effects in the pyrolytic characteristics of woody biomass/coal blends under low and high heating rate regimes. Biomass Bioenergy 2002, 23, 55-66. (9) Moliner, R.; Suelves, I.; La´zaro, M. J.; Synergetic effects in the copyrolysis of coal/petroleum residue mixtures by Pyrolysis/Gas Chromotoraphy: Influence of temperature, pressure, and coal nature. Energy Fuels 1998, 12, 963-968.
that is used as a catalyst in this work has been pretreated by heating to 200 °C for 2 h and then at 500 °C for an additional 2 h before use. Red mud, which is a waste product from an aluminum factory in Turkey, was supplied by Seydisehir Aluminum Company in Turkey. It was filtered and then dried at 110 °C. The chemical composition of red mud10 is shown in Table 3. Lignite and LDPE are mixed well in amounts of 33 wt % (6.66 g of lignite and 3.33 g of LDPE), 50 wt % (5 g of lignite and 5 g of LDPE), and 67 wt % (3.33 g of lignite and 6.66 g of LDPE) of LDPE. These weight percentages of LDPE in the mixtures are equal to lignite/LDPE ratios of 2:1, 1:1, and 1:2, respectively. Apart from coal-LDPE mixtures, coal (LDPE of 0%) and LDPE (100%) are also subjected to pyrolysis at temperatures of 400-700 °C in the system given in Figure 1. In each catalytic run, 50 wt % (4.9 g of lignite and 4.9 g of LDPE) of LDPE and coal are mixed with 0.2 g of catalyst and then charged into the reactor. At the beginning of this study, the effect of the different amounts of catalyst at the different temperatures on the yields is investigated. Maximum gas yields are obtained when 0.2 g of catalyst is used. This is the reason 0.2 g of catalyst is used during the experiments. After the reactor is inserted into the electrically heated furnace, the air of the reactor is purged with a nitrogen flow of 30 mL/min.The reactor is heated from room temperature to a determined temperature, depending on reaction temperature, at a heating rate of 10 °C/min, and held until no further liquid is produced. The outlet of the reactor is connected to a round-bottomed flask with a reflux condenser where condensation of the pyrolysate occurred. The reaction mixture is cooled to ∼0 °C with an ice-salt bath. The reaction products are classified into three groups: gases, liquid hydrocarbons (tar), and residual coke. The yield of tar is defined as the amount of liquid collected in the round-bottomed flask, and the yield of residual coke as the char remaining inside the reactor after the experiment. The degree of conversion is calculated by subtracting the residual coke yield (in terms of weight percent) from 100. The amount of gas was measured using a gas meter that was connected to the end of the reflux condenser. The water content of the tar was determined by distillation, according to the methods described in ASTM D95-05e1.11 Phenols presented in the tar were determined via gas chromatography-mass spectroscopy (GC-MS), using an Agilent model 6890 GC System 5973 MSD instrument. GC-MS conditions were as follows: column, HP1 (50 m × 0.32 mm × 0.52 µm); carrier gas, helium; flow rate of helium, 0.7 mL/min; and temperature program of oven, initial hold at 50 °C for 15 min, then ramp to 300 °C by a heating rate of 5 °C/min, and then hold for 17 min. Phenols and phenol derivatives are identified via comparison of their spectrum with that in the NIST library of the GC-MS system, in terms of their peak area (expressed as a percentage) in the total chromatogram. (10) Yanik, J.; Uddin, A.; Ikeuchi, K.; Sakata, Y. The catalytic effect of Red Mud on the degradation of poly (vinyl chloride) containing polymer mixture into fuel oil. Polym. Degrad. Stab. 2001, 73, 335-346. (11) Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation, ASTM D95-05e1, 1995 Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA.
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Energy & Fuels, Vol. 20, No. 4, 2006 1611
Figure 1. Experimental set up for pyrolysis.
Figure 2. Effect of the weight percentage of low-density polyethylene (LDPE) in the blends on the yields at a temperature of 400 °C.
Figure 4. Effect of the weight percentage of LDPE of the blends on the yields at a temperature of 600 °C.
Figure 3. Effect of the weight percentage of LDPE in the blends on the yields at a temperature of 500 °C.
Figure 5. Effect of the weight percentage of LDPE in the blends on the yields at a temperature of 700 °C.
Results and Discussion The effect of the pyrolysis temperatures (400, 500, 600, and 700 °C) on the tar, gas, residual coke, and water yields is presented in Figures 2, 3, 4, and 5, respectively. An increase in the LDPE content of the mixture leads to an increase in tar evolution at all temperatures studied. A slight decrease is observed in the tar yields at 400 °C between 67 wt % and 100 wt % of LDPE content. The residual coke amount is decreased by increasing the LDPE content of the mixture, which increases as the tar amount increases, as can be seen from Figures 2-5. The maximum tar yield is obtained by adding
LDPE to the lignite at 50 wt % LDPE at 600 °C. To increase tar yield, radical stabilization should be promoted and crosslinking should be suppressed. However, the hydrogen content of coal is not sufficient to stabilize fragment radicals by themselves. Thus, radical stabilizers such as hydrogen or methyl radicals should be supplied to coal at a comparable rate to that of the generation of fragment radicals. LDPE contains a rather high amount of hydrogen and provides hydrogen for coal.12-14 (12) Sharypov, V. I.; Kuznetsov, B. N.; Golovin, A. V.; Sidelnikov, V. N.; Doroginskaya, A. N.; Baryshnikov, S. V.; Beregovtsova, N. G. Liquid hydrocarbons obtained by thermal conversion of biomass/plastic mixtures. Chem. Sustain. DeV. 1997, 5, 201-207.
1612 Energy & Fuels, Vol. 20, No. 4, 2006
Figure 6. Effect of the weight percentage of LDPE in the blends on the degree of conversion.
Thus, coal conversion and tar yields during the pyrolysis of the coal-LDPE mixture are enhanced, compared to the yields obtained when coal alone is reacted. This situation is more remarkable at higher pyrolysis temperatures. There is not much variation in the amount of gas generated by increasing the LDPE content. The effect of temperatures and LDPE content of the blends on conversion degrees of coal, coal-polymer blends, and LDPE to products (tar, gas, and water) are shown in Figure 6. An increase in the LDPE content of the blends leads to an increase in the degree of conversion. The degree of conversion also increases as the experimental temperature increases. This shows that cracking and radikalic reactions are enhanced as a result of increased temperature. Radicals formed at higher temperatures are stabilized by hydrogen provided by LDPE, as explained below. Thus, the degree of conversion is increased. One of the effects that are being studied in this work is the effect of synergism. That is, when two or more agents acting together having a larger effect than each one of them acting separately. The effect may be reinforced positively or negatively. The effect of LDPE on copyrolysis process can be clearly observed by comparing the coprocessed tar yields to the hypothetical means that are calculated from the arithmetic means of the tar yields calculated after the pyrolysis of lignite and LDPE alone (the synergetic effect).15,16 When the difference between the experimental coprocessed value and the hypothetical mean is positive, then it can be concluded that adding LDPE to the lignite enhanced tar generation. When the difference is negative, then tar yields are higher in the individual pyrolysis of lignite and LDPE. Figure 7 shows the synergetic effects obtained for the tar yields during the copyrolysis of LDPE and lignite (the LDPE contents of the mixture are 33, 50, and 67 wt %). According to this figure, an increase in the LDPE content of the mixture leads (13) Ballice, L.; Yu¨ksel, M.; Sagˇlam, M.; Schulz, H.; Reimert, R. Classification of volatile products evolved during temperature-programmed co-pyrolysis of Turkish oil shales with low-density polyethylene. Fuel 1998, 77, 1431. (14) Ochoa, R.; Woert, H. V.; Lee, W. H.; Subramanian, R.; Kugler, E.; Eklund, P. C. Catalytic degradation of medium-density polyethylene over silica-alumina supports. Fuel Process. Technol. 1996, 49, 119. (15) Luo, M.; Curtis, C. W. Effect of reaction parameters and catalyst type on waste plastics liquefaction and coprocessing with coal. Fuel Process. Technol. 1996, 49, 177-196. (16) Ades, H. F.; Subbaswamy, K. R. Effect of reaction parameters and catalyst type on waste plastics liquefaction and coprocessing with coal. Fuel Process. Technol. 1996, 49, 177-196.
Sınagˇ et al.
Figure 7. Synergetic effects obtained for the tar after the co-pyrolysis of lignite-LDPE mixtures.
to the increased synergetic effect in the production of tar. Tar generation attains its maximum value at 400 °C, when 67 wt % of LDPE is added to the lignite. The effect of LDPE on the tar evolution is remarkable at all temperatures except 600 °C. This synergic effect can be explained by the atmosphere of hydrogen radicals released from the polymer at temperatures of >400 °C. These radicals may terminate the decomposition of polymers, and the volatile liquid products are leaving the reactor prior to their further cracking to gases. Coal, especially low-rank coal such as lignite, has numerous phenolic structures.17 These structures are also found in tar during the pyrolysis experiments. Phenol and phenol derivatives enhance the formation of polymerization products. This situation leads to a decrease in gas and tar yields, which is undesirable in the pyrolysis process. In the case of individual lignite pyrolysis [0 wt % LDPE], tar and gas yields are less than that obtained via the copyrolysis of lignite-LDPE mixtures at all the pyrolysis temperatures, as observed in Figures 2-5. Therefore, one aim of this study is to determine whether LDPE has an influence on the removal of phenolic structures during copyrolysis. Phenol and phenol derivatives detected in the tar via GCMS analysis from the pyrolysis of lignite alone and the mixture of lignite: LDPE of 50 wt % are presented in Table 4. Generally, phenol is very stable and presents no serious degradation. However, the amount of phenol and phenol derivatives is decreased dramatically by adding LDPE to the lignite, which is desirable, as explained previously, according to the GCMS results. The possible reason for this decrease is the hydrogen content of the polyethylene. This means that polyethylene acts as a hydrogenation medium for the coal product under the experimental conditions studied, yielding products such as cyclohexanol and cyclohexanone, as a result of the ring hydrogenation of phenol, as can be seen in Table 4. Figure 8 shows that all of the catalysts used in the experiments lead to improved gas generation. The effect of zeolite used in this study can be explained by its great microporous surface area. Zeolite (LTA) contains pores of relatively small size.18 (17) Dorrestijn, E.; Laarhoven, L. J. J.; Arends, I. W. C. E.; Mulder, P. The occurrence and reactivity of phenoxyl linkages in lignin and low rank coal. J. Anal. Appl. Pyrol. 2000, 54, 153-192. (18) Woo, H. C.; Lee, K. H.; Lee, J. S. Catalytic skeletal isomerization of n-butenes to isobutene over natural clinoptilolite zeolite. Appl. Catal., A 1996, 134, 147-158.
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Energy & Fuels, Vol. 20, No. 4, 2006 1613
Figure 8. Effect of catalyst on the yields at a temperature of 600 °C and 50 wt % LDPE. Table 4. Distribution of Phenol and Phenol Derivatives and Cyclohexanol and Cyclohexanone (Possible Hydrogenation Products of Phenols) in the Tars Collected at 600 °C and a Material Ratio of 1:1 Percentage of Peak Area phenol and phenol derivatives
lignite
lignite/LDPE
phenol 2-methyl phenol methyl phenol 2,6-dimethyl phenol 2-ethyl phenol 3,5-dimethyl phenol 2,4-dimethyl phenol 4-ethyl phenol 2,3-dimethyl phenol 2,4,5-trimethyl phenol 4-vinylphenol 2,3,6-trimethyl phenol 3-ethyl-5-methyl-phenol 4-(1-methylpropyl) phenol 4-(3-hydroxy-1-propenyl) phenol cyclohexanol cyclohexanone
5.8 2.1 0.65 0.2 0.4 1.15 0.52 1.23 1.55 0.23 0.22 0.42 0.3 0.21 0.16 0.3 0.5
0.43 0.15 0.48 0 0 0.06 0 0.25 0 0 0 0 0 0 0 3.7 4.8
This causes greater catalyst activity for zeolite. The primary cracking reactions of the polymer chain may proceed on the macroporous surface of the catalyst, while the smaller fragments are cracked on the microporous surface of the catalyst. Another approach to the effect of the catalyst used is also related to the micropore structure of zeolite, as proposed by Grieken et al.19 They suggested that the micropore nature of this catalyst, which prevents the primary cracking reactions from occurring in the internal surface of the material leads to a large external surface area. According to these researchers, the high acid strength of the zeolite active sites favors the coal-LDPE degradation, (19) Grieken, R.; Serrano, D. P.; Aguado, J.; Garcı´a, R.; Rojo, C. Thermal and catalytic cracking of polyethylene under mild conditions. J. Anal. Appl. Pyrol. 2001, 58-59, 127-142.
according to an end-chain cracking mechanism, which mainly leads to the evolution of gaseous hydrocarbons. Red mud is a mixture of various metal compounds, as can be seen in Table 3. Some of the metals in red mud have a catalytic effect on gas generation, as reported by Yanik et al.10 It is not easy to propose an exact adsorption mechanism for red mud, because it is not a synthetic catalyst. The role of alkali metal carbonates as catalysts during the pyrolysis of biomass has been widely known.20 Potassium, sodium, and lithium carbonates promote the gasification rate and also increase the volume of gas produced. It has been reported that the catalytic effect of alkali-metal carbonates involves an oxidation-reduction cycle with the intermediate formation of free alkali metal. Mims and Pabst have also shown that K2CO3 reacts with coal or char at ∼750 K, with the liberation of CO from the carbonate and the formation of surface salt complexes.21 They attributed the enhanced gas evolution rate to the formation of potassium complexes, for instance, phenoxide groups C-O-K, which may be considered as a type of highly dispersed potassium species. Conclusions The following conclusions can be made from this study: (1) A copyrolysis process conducted at various temperatures leads to the higher tar yields comparing individual pyrolysis results of coal and low-density polyethylene (LDPE), which indicates that LDPE has the role of a hydrogen donor solvent for the lignite under the experimental conditions. (2) The Linde type A (LTA) zeolite catalyst, red mud, and K2CO3 have a positive influence on gas generation during copyrolysis at 600 °C at a material ratio of 1:1 (50 wt % LDPE). The great microporous surface area of zeolite and the formation of potassium complexes in the case of K2CO3 are the main reasons for the increased gas generation. (3) The copyrolysis of lignite and LDPE blends seems to be an attractive recycling alternative for waste plastics and lowrank coals, giving very high tar yields. Adding more LDPE to the lignite results in increased tar yields, which indicates synergism between the lignite and the LDPE. (4) Phenols and phenol derivatives detected in the tar are significantly decreased in the presence of polyethylene, as a result of the ring hydrogenation of phenol and phenol derivatives (cyclohexanol and cyclohexanone are detected as hydrogenation products of phenol), because the polymer acts as a hydrogenation medium for the coal product. Acknowledgment. The authors would like to acknowledge the Ankara University BAP, Department of Scientific Research Projects, for supporting this research (under BAP Contract No. 2005 07 05 095). EF060108L (20) Maschio, G.; Lucchesi, A.; Stoppato, G. Production of syngas from biomass. Bioresource Technol. 1994, 48, 119-126. (21) Mims, C.; Pabst, J. K. Role of surface salt complexes in alkalicatalysed carbon gasification. Fuel 1983, 62 (2), 176-179.
