5090
Ind. Eng. Chem. Res. 1997, 36, 5090-5095
Characterization of Coke Formed in the Pyrolysis of Polyethylene Valerio Cozzani† Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Universita` degli Studi di Pisa, via Diotisalvi n.2, I-56126 Pisa, Italy
The characteristics and the reactivity of coke formed in the pyrolysis of polyethylene (PE) were investigated. A laboratory-scale fixed-bed reactor was used for PE pyrolysis at different temperatures. The morphology of the coke collected was analyzed by scanning electron microscopy. Reactivity of the coke samples was studied by thermogravimetric analysis in oxygen and carbon dioxide. Kinetic parameters and reaction order with respect to gas partial pressure were estimated and compared to literature data. Introduction The thermochemical conversion of wastes to energy dense fuels by gasification processes appears to be a promising route for the energy recovery and the reduction of the quantity of wastes to be landfilled (Buekens and Schoeters, 1986; Kaminsky, 1992; Bridgwater, 1995). In the gasification of wastes, a pyrolysis step is always present, yielding three different product fractions: a gaseous fraction, a condensable fraction usually named tar, and a solid fraction. The pyrolysis products are usually heated with a gas stream to undergo gasification reactions. The gasification stage is the ratecontrolling step, since the gasification reactions, involving the oxidation of the solid products fraction, are slower than the pyrolysis process (Bridgwater, 1995). As the yields of condensable and solid products should be minimized in gasification processes, information on the mechanisms of formation of the solid fraction and on its reactivity is important for the optimization of gasification reactors. Lignocellulosic wastes and plastic wastes are the two main components of municipal solid wastes and have quite different pyrolysis and gasification behaviors. Several studies are present in the literature concerning the mechanism of formation and the characteristics of char formed in the pyrolysis of lignocellulosic materials. These were recently reviewed by Antal and Varhegyi (1995) and by Milosavljevic et al. (1996). On the other hand, less information seems to be present on the characterization of product yields obtained from the pyrolysis of plastics. The studies present in the literature that deal with the pyrolysis of polyethylene (PE), the main component of plastic wastes, are focused mainly on the global weight loss behavior (Darivakis et al., 1990; Jellinek, 1949; Madorsky, 1952; Oakes and Richards, 1949) and on the characterization of primary volatiles (Hodgkin, 1982; Khalturinskii, 1987; Madorsky, 1965; Sugimura and Tsuge, 1979). The formation of the solid product fraction in the pyrolysis of PE in a fixed-bed reactor was found to be mainly the result of the secondary tar-cracking process (Cozzani et al., 1997). This is confirmed by the absence of a solid residue after complete conversion of PE generally observed when pyrolysis is performed by techniques that do not allow tar-cracking reactions to take place, as in vacuum or flash pyrolysis (Darivakis et al., 1990; Madorsky, 1965). The formation of a solid residue by a secondary tarcracking process is a significant difference with respect to the pyrolysis of lignocellulosic materials, where char †
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is formed mainly in a primary pyrolysis process. Hence, the actual mechanism of formation of the solid fraction in the pyrolysis of PE and the influence of reaction temperature and geometry are still open problems. Their comprehension is important in order to minimize solid formation in gasification reactors that may cause relevant operating problems to these processes (Bridgwater, 1995). This study deals with the characterization of the solid fraction obtained from the pyrolysis of polyethylene, the major component of plastic wastes. PE was pyrolyzed in a laboratory-scale fixed-bed reactor at temperatures ranging between 500 and 800 °C. The morphology of the solid product fraction was investigated by scanning electron microscopy. The influence of the different mechanisms involved in the formation of the solid fraction with respect to pyrolysis temperature was discussed. The reactivity of the solid fraction and the kinetics of gasification reactions with oxygen and carbon dioxide were studied by thermogravimetric analysis. Experimental Section Materials. The solid product fractions obtained from the pyrolysis of low-density polyethylene at temperatures ranging between 500 and 800 °C were collected. The solid products of a pyrolysis process are usually named char in the literature (see for example Antal and Varhegyi, 1995). However, in the pyrolysis of lignocellulosic materials the formation of a solid residue is mainly a result of the primary pyrolysis process. On the other hand, in the pyrolysis of PE the formation of a solid residue is due mainly to secondary reactions of the volatiles generated by the primary pyrolysis process. Thus the solid residue obtained in the pyrolysis of PE was referred to as coke in the following, since this term is commonly used in the literature to indicate the solids formed in the thermal cracking of hydrocarbons by secondary reactions. The results of the ultimate analysis of the coke samples collected and of the PE used for coke production are reported in Table 1. As expected coke was found to be mainly composed of carbon, and the hydrogen content was found to decrease with the pyrolysis temperature. Coke yields for the collected samples were respectively 2.8% by weight at 500 °C and 24.1% at 800 °C (Cozzani et al., 1997). Techniques. A laboratory-scale fixed-bed reactor (FBR) was used for coke production. The reactor consisted of an electrically heated tubular furnace. Reactor characteristics and mode of operation, as well as mass balances and influence of pyrolysis conditions on product © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5091 Table 1. Ultimate Analysis of PE and Coke Samples (daf) % (daf)
PE
500 °C coke
800 °C coke
C H N
84.6 15.4 traces
96.1 3.9 traces
99.77 0.23 traces
yields from PE pyrolysis, were extensively discussed in a previous paper (Cozzani et al., 1997). Coke structure was studied by scanning electron microscopy (SEM). A Jeol T 3000 Electronic Microscope was used. Thermogravimetric (TG) runs were performed using a Mettler TG-50 thermobalance. A constant heating rate of 20 °C/min (0.33 °C/s) and typical sample weights of 2-5 mg were used in experimental runs. The TG runs were performed using various gaseous mixtures in order to test a wide range of oxidizing conditions. Oxygen-nitrogen mixtures with 6.1%, 11.7%, and 21.0% of oxygen by volume and carbon dioxide-nitrogen mixtures with 19.6%, 46.3%, and 100.0% carbon dioxide by volume were used in order to study the oxidation reaction kinetics and the rate dependence on the partial pressure of the oxidizing species. Runs in pure nitrogen were also carried out in order to identify a possible effect of the loss of volatile matter, in particular for the lower temperature (500 °C) coke. Results and Discussion Morphology and Mechanisms of Coke Formation. PE coke formed a layer on the reactor walls and was collected at the end of pyrolysis runs brushing the walls of the FBR. It is worthwhile to point out that the coke was collected mainly on the reactor walls and not in the sample holder where the PE sample is positioned at the beginning of the pyrolysis run. In a previous work it was shown that the coke yields could be correlated to the extension of the tar-cracking process with an Arrhenius kinetic rate equation (Cozzani et al., 1997). This confirms that in the pyrolysis of PE the formation of a solid residue is mainly the result of secondary gas-phase reactions. Figure 1a shows the SEM microphotograph of the PE coke formed at 500 °C pyrolysis temperature in the FBR. Along with the filamentous structure shown in Figure 1a, the 800 °C coke also shows the spherical structure of Figure 1b. Both of the structures of PE coke are mainly formed of submicronic particles that during the pyrolysis process in the FBR conditions cluster on the reactor walls. Filamentous coke is believed to be the result of a metal-catalyzed coking process that takes place on the reactor walls (Albright and Marek, 1988a). On the other hand, the spherical coke shown in Figure 1b is believed to be formed mainly by a gas-phase mechanism. Condensation and dehydrogenation reactions occur to the volatiles generated in the primary pyrolysis process, resulting in the formation of tar droplets and soot particles. The low gas velocities in the FBR allow the deposition on the reactor walls of the high-viscosity tar particles. Further dehydrogenation and clustering on the reactor walls may take place, resulting in the final aspect of coke observed in Figure 1b. This second coke formation process was found to be prevailing at higher temperatures (Albright and Tsai, 1983). However, several other complicating factors may affect the coke formation mechanism, as gas velocity, reactor material (for the metal-catalyzed process), reactor geometry, etc. (Trimm, 1983). Unfortunately, the data obtained from
Figure 1. SEM microphotographs of the PE coke. (a) 500 °C coke. (b) 800 °C coke. The scale is indicated by the black bar (1 µm) in the lower right-hand corner of each micrograph.
the collected coke samples did not allow the unambiguous identification of the prevailing coke formation mechanism with respect to operating variables in the FBR. In spite of this, the presence of both filamentous and spherical coke structures points out that coke formation in FBR pyrolysis runs is caused not only by catalytic wall effects but also by the presence of a gasphase coke formation mechanism. The structure of the coke obtained from PE pyrolysis is quite different with respect to that of the char formed in the pyrolysis of lignocellulosic materials and of refusederived fuel (RDF). SEM microphotographs of char obtained at 900 °C in the FBR from the pyrolysis of a RDF in the same experimental conditions reported in a previous study (Cozzani et al., 1995) show that the fibrous structure of the lignocellulosic material is retained by the char formed in the pyrolysis process. Thus, the analysis of the morphology of the “secondary” coke obtained from PE pyrolysis confirms the relevant difference with respect to “primary” char obtained from the pyrolysis of lignocellulosic wastes. Since a gas-phase mechanism is involved in PE coke formation, not surprisingly the morphology of the coke obtained in PE pyrolysis recalls that of the solid residues formed in pyrolysis units for the production of ethylene starting from saturated C2-C3 hydrocarbons or light
5092 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
Figure 2. TG weight loss curves. (a) 500 °C coke. (b) 800 °C coke.
naphtha fractions. As a matter of fact, both the filamentous structure shown in Figure 1a and the spherical structure shown in Figure 1b were observed for the coke formed in ethylene crackers (Albright and Marek, 1988b). This may suggest the presence of common mechanisms for the formation and the deposition of coke on reactor walls. Previous works dealing with the formation of coke in industrial pyrolysis units identified acetylenic compounds, diolefins, olefins, and aromatics as the gas-phase precursors of coke (Albright and Tsai, 1983; Trimm, 1983). All these compounds are present in the gas-phase during PE pyrolysis (Cozzani et al., 1997). In particular olefins were shown to be formed in relevant quantities during the primary pyrolysis process (Sugimura and Tsuge, 1979). The presence of coke precursors in the gas formed during primary PE pyrolysis strongly suggests the presence of mechanisms involving free radical reactions also for the formation of coke during PE pyrolysis. Characterization of PE Coke Reactivity. The study of PE coke gasification reactivity is of interest since relevant coke formation can take place in plastic wastes pyrolysis and/or gasification reactors. In the present work, the reactivity of the coke in gasification reactions with oxygen and carbon dioxide was characterized by TG analysis. TG runs using oxygen-nitrogen and carbon dioxide-nitrogen mixtures were performed. The partial pressure of the gasifying agent was varied using mixtures at different gas concentrations. Figure 2 reports the TG curves obtained in 100% nitrogen, 21% oxygen, and 100% carbon dioxide for the 500 °C coke (a) and for the 800 °C coke (b). At the heating rates used for experimental runs, the carbonoxygen reaction and the carbon-carbon dioxide reaction have quite different temperature ranges (450-650 °C and 800-1000 °C, respectively). As expected, the 800 °C coke undergoes negligible weight loss during TG runs in pure nitrogen. On the other hand, the curve obtained for the 500 °C coke in pure nitrogen shows that a slight mass loss process takes places at temperatures higher than 500 °C. This is probably caused by the release of adsorbed hydrocarbons or dehydrogenation reactions of the coke, as the decrease of the coke H/C ratio with respect to coke formation temperature may suggest (see the Experimental Section). In Figure 2b, the TG curves obtained for the 500 °C coke in oxygen and carbon dioxide are reported as dash-dotted lines. The direct comparison with the results for the 800 °C coke shows
that the temperature ranges of the weight loss due to the gasification reactions with oxygen and carbon dioxide are very similar for both the 500 °C and the 800 °C cokes. This possibly indicates that the gasification processes and kinetics are similar for the cokes obtained from PE pyrolysis in the FBR, probably due to the limited differences that are present in the chemical nature of the two cokes. Both the cokes have presumably an aromatic structure (Trimm, 1983). The differences in the hydrogen to carbon ratio evidenced in Table 1 are probably caused by the presence of adsorbed volatile compounds or aliphatic side chains in the 500 °C coke. This hypothesis is confirmed by the higher weight loss experienced by the 500 °C coke during TG runs in pure nitrogen at temperatures between 400 and 700 °C, as shown in Figure 2. Since the influence of the temperature of coke formation on PE coke reactivity resulted in being negligible, if present, only the 800 °C PE coke kinetics was fully investigated. TG data were used in order to obtain information on global reaction kinetics with oxygen and carbon dioxide. Experimental data were correlated using an nth order Arrhenius rate equation:
( )(1 - ξ)
dξ ) Ae dt
Ea
-
RT
m
Pgn
(1)
where A is the frequency factor, Ea the activation energy of the reaction, R the gas constant, T the temperature, Pg the gas partial pressure, n the order of reaction with respect to gas concentration, m the reaction order with respect to coke conversion, and ξ the sample conversion, defined as
ξ)
M0 - M M0 - Mf
(2)
where M is the sample mass, M0 the initial mass, and Mf the final mass (the ash content of the sample). Kinetic parameters (frequency factor, activation energy, and reaction order with respect to sample conversion and to gas partial pressure) were numerically estimated from the TG weight loss data using a nonlinear least-squares method (Himmelblau, 1970). In order to estimate the order of reaction with respect to sample conversion, m, only the values 0, 0.5, 1, 1.5, and 2 were tested. At each of these values of m the other
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5093 Table 2. Best-Fit Kinetic Parameters Obtained for the 800 °C Coke Reactions in Oxygen and in Carbon Dioxide A (s-1 atm-n) Ea (kJ/mol) m n
oxygen
carbon dioxide
5.3 × 1011 234 1 0
7.0 × 106 217 1 0.55
kinetic parameters were evaluated by fitting eq 1 to the TG curves. The set of parameters corresponding to the minimum residual error was chosen. Best-fit parameters obtained by this procedure are reported in Table 2. Obviously, the kinetic parameters obtained should be considered only as indicative estimates, since the experimental approach used may lead to an oversimplified kinetic model. However, this kind of approach, based on the evaluation of kinetic parameters and reaction order from TG data, has been widely used in the literature with fruitful results. In Figure 4 the experimental TG data are compared to the kinetic model predictions. The kinetic model for gasification reactions is obviously not able to take into account the weight loss due to devolatilization. Thus, mass values obtained from experimental runs in pure nitrogen at 450 and 600 °C were used (for oxygen and carbon dioxide, respectively) as initial values (M0) for the kinetic model. The best-fit value estimated for the reaction order with respect to sample conversion resulted in m ) 1 for both the oxygen and carbon dioxide reactions. These results are not surprising, since the global kinetics of low-medium temperature gasification and oxidation heterogeneous reactions of carbonaceous materials is believed to be the result of three sequential reaction steps: adsorption of the gaseous oxidant on an “active site”, chemical reaction on the site, and desorption of reaction product (Essenhigh, 1981). Hence, the rate of reaction is proportional to the concentration of active sites. If the active sites concentration is roughly constant on the inner surface area, the reaction rate results proportional to sample mass. The value of the kinetic constant can be directly evaluated from the experimental TG weight loss data. If the order of reaction with respect to sample conversion is unity (m ) 1), the following expression is obtained from eq 1 for the kinetic constant as a function of temperature, sample weight, and rate of weight loss (quantities that are all recorded during TG runs):
(3)
Figure 3. Arrhenius plot for 800 °C coke. Dots: experimental TG data. Lines: kinetic model predictions (using parameters reported in Table 2) and literature data. (a) In oxygen. (b) In carbon dioxide.
The logarithm of the experimental kinetic constant (K) calculated from TG data using eq 3 is reported as a function of the reciprocal of temperature (Arrhenius plot) in Figure 3. As clearly emerges from the figure, the order of reaction with respect to oxygen partial pressure resulted in n ) 0. The order with respect to carbon dioxide partial pressure resulted in n ) 0.55. Orders of reactions comprised between 0 and 1 have been reported in the literature for many carbonaceous materials in the range of temperatures used in experimental runs (Smith, 1982). More recent results obtained for coal oxidation indicate reaction orders near to zero for oxygen partial pressure (Essenhigh and Mescher, 1997), well in accordance with the present findings. The comparison of the PE coke reactivity to that of other carbonaceous materials is problematic since a
great variability is present in the kinetic data reported in the literature. An overview to data reported in a comprehensive review by Smith (1982) shows that PE coke reactivity falls well in the range of data reported for coal chars. Kinetic data reported for a bituminous coal char (Illinois n.6), anthracite and a petroleum coke are plotted in Figure 3a for the sake of comparison. The reactivity of these three chars is slightly higher than that of PE coke, in particular at low temperatures. Figure 3a also reports the kinetic data obtained by Marcuccilli et al. (1994) for diesel soot. In spite of the quite different systems and operating conditions involved in the formation of the two materials, the reactivities of diesel soot and of PE coke are very similar. This may confirm that the chemical structure and thus the basic mechanisms involved in the formation of PE coke during pyrolysis are similar to those causing soot formation in combustion systems.
dM dt K(T) ) (M - Mf)Pgn
( )
5094 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
Acknowledgment The author gratefully acknowledges Prof. Leonardo Tognotti for useful discussions and Mr. Piero Narducci for technical assistance in the SEM analysis of samples. Nomenclature A ) frequency factor (s-1 atm-n) Ea ) activation energy (J/mol) K ) kinetic constant (s-1) m ) reaction order with respect to sample conversion M ) sample mass n ) reaction order with respect to gas partial pressure Pg ) gas partial pressure (atm) R ) universal gas constant (8.31 J/mol) t ) time (s) T ) temperature (K) ξ ) conversion Subscripts 0 ) initial value f ) final value
Literature Cited
Figure 4. Comparison of TG experimental data (lines) and kinetic model predictions (dots) for the 800 °C coke. (a) In oxygen. (b) In carbon dioxide.
Conclusions Tar-cracking reactions were found to play the main role in the formation of coke in the PE pyrolysis process. The reaction mechanisms leading to coke formation in the pyrolysis of PE closely recall those present in the thermal cracking of hydrocarbons, since common precursors are present in the gas phase. The analysis of PE coke morphology indicated the contemporary presence of a wall catalytic mechanism and of a gas-phase mechanism in the formation of coke during the pyrolysis process. The presence of a gas-phase mechanism leading to coke formation may cause relevant sooting problems in the operation of PE gasification processes. PE coke reactivity was characterized by TG analysis. The reactivity in oxygen and carbon dioxide was found to be very similar even for cokes formed at different temperatures and with different macrostructures. The obtained kinetic data allowed the evaluation of the characteristic times of two important reactions active in the thermal gasification processes. The results were compared to literature data reported for the reactivity of various materials. PE coke reactivity was found to be slightly lower than that of coal chars. On the other hand, PE coke reactivity resulted in the same order of that of diesel soot, confirming the similarities in the formation mechanisms and in the resulting structure of the two materials.
Albright, L. F.; Tsai, T. C. Importance of surface reactions in pyrolysis unit. In Pyrolysis: theory and industrial practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983; p 233. Albright, L. F.; Marek, J. C. Mechanistic model for formation of coke in pyrolysis units producing ethylene. Ind. Eng. Chem. Res. 1988a, 27, 755. Albright, L. F.; Marek, J. C. Coke formation during pyrolysis: roles of residence time, reactor geometry, and time of operation. Ind. Eng. Chem. Res. 1988b, 27, 743. Antal, M. J., Jr.; Varhegyi, G. Cellulose pyrolysis: the current state of knowledge Ind. Eng. Chem. Res. 1995, 34, 703. Bridgwater, A. V. The technical and economic feasibility of biomass gasification for power generation. Fuel 1995, 74, 631. Buekens, A. G.; Schoeters, J. G. European experience in the pyrolysis and gasification of solid wastes. Conserv. Recycl. 1986, 9, 253. Cozzani, V.; Nicolella, C.; Petarca, L.; Rovatti, M.; Tognotti, L. A fundamental study on conventional pyrolysis of a refuse-derived fuel. Ind. Eng. Chem. Res. 1995, 34, 2006. Cozzani, V.; Nicolella, C.; Rovatti, M.; Tognotti, L. The influence of gas-phase reactions on the product yields obtained in the pyrolysis of polyethylene. Ind. Eng. Chem. Res. 1997, 36, 324. Darivakis, G. S.; Howard, J. B.; Peters, W. A. Release rates of condensables and total volatiles from rapid devolatilization of polyethylene and polystyrene. Combust. Sci. Tech. 1990, 74, 267. Essenhigh, R. H. Fundamentals of coal combustion. In Chemistry of coal utilization; Elliot, M. A., Ed.; Wiley: New York, 1981; Chapter 19. Essenhigh, R. H.; Mescher, A. M. Influence of pressure on the combustion rate of carbon. Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1997; in press. Hodgkin, J. H.; Galbraith, M. N.; Chong, Y. K. Combustion products from burning polyethylene. J. Macromol. Sci., Chem. 1982, A17 (1), 35. Kaminsky, W. Possibilities and limits of pyrolysis. Makromol. Chem., Macromol. Symp. 1992, 57, 145. Khalturinskii, N. A. High temperature pyrolysis of polymers. J. Therm. Anal. 1987, 32, 1675. Jellinek, H. H. G. Thermal degradation of polystyrene and polyethylene. Part III. J. Polym. Sci. 1949, 4, 13. Madorsky, S. L. Rates of thermal degradation of polystyrene and polyethylene in a vacuum. J. Polym. Sci. 1952, 9, 133. Madorsky, S. L. Thermal degradation of organic polymers. Interscience Publishers: New York, 1965. Marcuccilli, F.; Gilot, P.; Stanmore, B.; Prado, G. Experimental and theoretical study of diesel soot reactivity. Twenty-Fifth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; p 619.
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5095 Milosavljevic, I.; Oja, V.; Suuberg, E. M. Thermal effects in cellulose pyrolysis: relationship to char formation processes. Ind. Eng. Chem. Res. 1996, 35, 653. Oakes, W. G.; Richards, R. B. The thermal degradation of ethylene polymers. J. Chem. Soc., London 1949, 619, 2929. Smith, I. W. The combustion of coal chars: a review. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; p 1045. Sugimura, Y.; Tsuge, S. Pyrolysis-hydrogenation glass capillary gas chromatographic characterization of polyethylenes and ethylene-olefin copolymers. Macromololecules 1979, 12, 512. Trimm, D. L. Fundamental aspects of the formation and gasification of coke. In Pyrolysis: theory and industrial practice;
Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983; p 233.
Received for review May 28, 1997 Revised manuscript received September 2, 1997 Accepted September 16, 1997X IE970381Y
X Abstract published in Advance ACS Abstracts, November 1, 1997.
