1442
Energy & Fuels 2007, 21, 1442-1447
Kinetic Study in Modeling Pyrolysis of Refuse Plastic Fuel Binlin Dou,† Sungjin Lim,† Pilsun Kang,† Jungho Hwang,*,† Soonho Song,† Tae-U Yu,‡ and Kyoon-Duk Yoon§ Department of Mechanical Engineering, Yonsei UniVersity, Seoul 120-749, Korea, Korea Institute of Industrial Technology, Chunan 330-825, Korea, and Korea Testing Laboratory, Seoul 152-718, Korea
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ReceiVed NoVember 23, 2006. ReVised Manuscript ReceiVed February 13, 2007
For the pyrolysis of refuse plastic fuel (RPF), the typical particle size is large and the time required for pyrolysis is long. Therefore, the rate-limiting mechanisms of gas diffusion and chemical reaction might be important. In this paper, the kinetics of RPF pyrolysis was investigated through a thermogravimetry analysis under isothermal conditions between 300 and 600 °C. A kinetic model was used to examine the effects of the surface chemical reaction and gas diffusion on the rate-limiting steps of RPF pyrolysis. The results show that the rate was controlled by a combination of the surface chemical reaction and gas diffusion through the solid product layer. The activation energies for the surface chemical reaction and gas diffusion were determined to be 70.2 and 65.9 kJ mol-1, respectively. The weight loss of RPF pyrolysis occurred mainly at temperatures higher than 400 °C and increased with temperatures. Concentrations of pyrolysis gases including H2, CO, and hydrocarbons were analyzed through a real-time gas analyzer. Gas yields from pyrolysis were sensitive to temperatures higher than 300 °C, while a very small amount of gas was released at 300 °C.
1. Introduction Studies of refuse derived fuel (RDF) that contains different types of solid material, such as paper, cardboard, plastic, wood, and fiber, began in the mid-1990s. These works have revealed the pyrolysis characteristics and emission mechanisms for flue gas.1-4 Refuse plastic fuel (RPF) is manufactured as a way of treating waste plastic and has been considered as a viable renewable energy source because of its higher heating value than RDF. The pyrolysis and/or gasification of RPF produce products that may serve as alternative sources of energy and chemical raw materials, such as syngas and char. Even though the pyrolysis of industrial waste plastics, including PE, PP, PS, and some PVC, has been extensively studied in various conditions by several researchers, there are little data available for RPF pyrolysis.5-11 Thus, the understanding of the pyrolysis process is necessary for both the design of pyrolysis reactors and the design of RPF gasifiers. * To whom correspondence should be addressed. Telephone: 82-2-21232821. E-mail:
[email protected]. † Yonsei University. ‡ Korea Institute of Industrial Technology. § Korea Testing Laboratory. (1) Kobyashi, N.; Itaya, Y.; Piao, G. L.; Mori, S.; Kondo, M.; Hamai, M.; Yamaguchi, M. Powder Technol. 2005, 151, 87. (2) Caputo, A. C.; Palumbo, M.; Scacchia, F. Appl. Therm. Eng. 2004, 24, 2171. (3) Liu, G. Q.; Itaya, Y.; Yamazaki, R.; Mori, S.; Yamaguchi, M.; Kondoh, M. Waste Manage. 2001, 21, 427. (4) Lin, K. S.; Wang, H. P.; Liu, S. H.; Chang, N. B.; Huang, Y. J.; Wang, H. C. Fuel Process. Technol. 1999, 60, 103. (5) Demirbas, A. J. Anal. Appl. Pyrolysis 2004, 72, 97. (6) Bockhorn, H.; Hornung, A.; Hornung, U. J. Anal. Appl. Pyrolysis 1998, 46, 1. (7) Knuemann, R.; Bockhorn, H. Combust. Sci. Technol. 1994, 101, 285. (8) McGhee, B.; Norton, F.; Snape, C. E.; Hall, P. J. Fuel 1995, 74, 28. (9) Matsuzawa, Y.; Ayabe, M.; Nishino, J.; Kubota, N.; Motegi, M. Fuel 2004, 83, 1675. (10) Mastral, F. J.; Esperanza, E.; Berrueco, C.; Juste, M.; Ceamanos, J. J. Anal. Appl. Pyrolysis 2003, 70, 1. (11) Montaudo, G.; Puglisi, C.; Scamporrino, E.; Vitalini, D. J. Polym. Sci. 1986, 24, 2.
Figure 1. Principle of pyrolysis for a single RPF particle: (I) dry zone, (II) pyrolysis zone, and (III) diffusion zone.
Useful information on kinetic parameters, such as the rate constant, activation energy, and reaction order, can be obtained by analyzing thermogravimetric data. These kinetic parameters are typically determined by adopting a kinetic model for the weight loss of a sample exposed to a certain heating rate and isothermal condition.12-17 The kinetic parameters, however, depend upon the experimental conditions, especially the heating rate in dynamic experiments and the final temperature in isothermal operations. A temperature gradient inside the sample of the dynamic experiments occurs because of nonstationary heat conduction. Then, the temperature profile is dependent upon the heat conductivity and the size of the sample as well as the heating rate. This nonhomogeneous temperature causes a faster reaction in the outer layer of the sample. Therefore, a thermo(12) Radmanesh, R.; Courbariaux, Y.; Chaouki, J.; Guy, C. Fuel 2006, 85, 1211. (13) Kenneth, M. B.; Mathew, J. H. Fuel 2003, 82, 1633. (14) Marcilla, A.; Beltrain, M. Polym. Degrad. Stab. 1995, 48, 219. (15) Reina, J.; Velo, E.; Puigjaner, L. Ind. Eng. Chem. Res. 1998, 37, 4290. (16) Holland, B. J.; Hay, J. N. Thermochim. Acta 2002, 388, 253. (17) Bockhorn, H.; Hornung, A.; Hornung, U. J. Anal. Appl. Pyrolysis 1999, 50, 77.
10.1021/ef060594c CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007
Kinetic Study in Modeling Pyrolysis of RPF
Energy & Fuels, Vol. 21, No. 3, 2007 1443
gravimetric signal always describes the integral weight loss of the sample that is dependent upon the prevailing temperature gradient over the sample. Even small errors in the temperature and heating rate can cause noticeable deviations in the kinetic parameters.16,17 Thus, it is necessary to improve the data evaluation of the dynamic method. The isothermal measurements at different temperatures require a large number of samples that should be instantaneously heated to the isothermal temperature. Isothermal measurements have numerous advantages, one of which is that changes in the mechanism are detectable because decomposition rates are obtained for a single temperature. Another advantage is the homogeneous sample temperature after attaining the isothermal reaction temperature.17 Few kinetic models include the impact of shrinkage on both pyrolysis and gas diffusion through the solid product layer,13,18-20 as well as the gasification effects of char in gases, such as H2O and CO2.21 The gas-solid reaction of pyrolysis may occur at an interface within each particle, which moves with time.22 This paper reports the kinetic results under isothermal conditions and considers the chemical reaction and gas diffusion through solid product layers. Also, the paper discusses the evolution of gases, including H2, CO, and hydrocarbons. 2. Kinetic Modeling At high temperatures, volatile matters, such as hydrocarbons and gaseous components, are released, leaving behind solid residue. At the same time, the cracking of hydrocarbons occurs. The reaction of char with H2O or CO2, which converts the char into gaseous species, may be important.21 The principle of pyrolysis is explained in Figure 1. When the devolatilization at the outer surface happens, a layer of char is being formed. H2O and CO2 are released by the devolatilization of the inner parts of the particle; further cracking of hydrocarbons occurs; and these gases pass through the surface layer of the solid product. In the pyrolysis of RPF, the rate-limiting mechanisms include several steps occurring in succession during the reactions: surface chemical reaction, diffusion of gaseous products through the ash back to the exterior surface of the solid, diffusion of gaseous products through the gas film back into the main body of the fluid, and so on. Gas film diffusion could be safely ignored because the resistances of gas-phase diffusion through the ash layer and surface chemical reactions are usually much greater than that of gas diffusion through the gas film surrounding the particle. Because the pyrolysis experiments were conducted with small-sized particles under isothermal conditions to avoid temperature profiles inside each particle,17,23 the ratelimiting mechanisms of gas diffusion through the solid product layer and surface chemical reaction were studied in this paper. In the thermogravitmetric analysis, the conversion of a RPF particle is defined as follows:
w0 - w x) w0 - wf
For the surface chemical reaction, the rate of the conversion of particle is defined as24,25
dx ) -bksc dt
(2)
where ks is the rate constant, b is the stoichiometric coefficient, and c is the percentage concentration of gas products. The rate of gas diffusion through the solid product layer is given by24,25
S0dc dx ) -De dt dr
(3)
where De is the effective diffusion coefficient in a porous structure, S0 is the surface area of the particle, and r is the radius of the particle. A model of the reaction core allows for a change in the radius of the particle to be given by
r3 ) 1 - Rx r30
(4)
where r0 is the initial particle radius and the shrinkage factor R is defined as
R ) ∆Vi/∆Vt)0 i
(5)
where i refers to a single volume element and ∆Vi and ∆Vt)0 i represent the volumetric shrinkage rate at time t and t ) 0, respectively. It is presumed that the macromolecular chains of the plastic compounds decompose into lighter products and volatile compounds and then the products react with char or each other, leading to various gaseous products. If the pyrolysis is controlled by the chemical rate of conversion at the particle surface, then the data can be modeled through the use of the shrinking core model expression. The time required for particle conversion is given by
( )
Fsr0 1 tg )
r r0
bksc0
(6)
where Fs represents the density of the particle and c0 is the initial percent concentration of gas. The characteristic time τg required for the complete conversion of the particle can be obtained using the following equation:
τg )
Fsr0 bksc0
(7)
Thus, eq 8 can be obtained
(1)
where w, w0, and wf are the actual, initial, and final masses of a sample, respectively. (18) Babu, B. V.; Chaurasia, A. S. Chem. Eng. Sci. 2004, 59, 1999. (19) Babu, B. V.; Chaurasia, A. S. Energy ConVers. Manage. 2004, 45, 297. (20) Galgano, A.; Blasi, C. D. Ind. Eng. Chem. Res. 2003, 42, 2101. (21) Jarvinen, M. P.; Zevenhoven, R. Combust. Flame 2002, 131, 357. (22) Hastaoglu, M. A.; Berruti, F. Fuel 1989, 68, 1408. (23) Ceamanos, J.; Mastral, J. F.; Millera, A.; Aldea, M. E. J. Anal. Appl. Pyrolysis 2002, 65, 93.
tg ) g(x) ) 1 - (1 - Rx)1/3 τg
(8)
We shall also take into account that each volume element is producing gases from the reactions occurring during the pyrolysis process. The RPF pyrolysis process is considered to be dependent upon hot gas access to the solid core surface and (24) Levenspiel, O. Chemical Reaction Engineering; New York, 1999; p 565. (25) Dou, B. L.; Gao, J. S.; Baek, S. W.; Sha, X. Z. Energy Fuels 2003, 17, 874.
1444 Energy & Fuels, Vol. 21, No. 3, 2007
Dou et al.
gas diffusion through the solid product layer. If the reaction is controlled by this step, it may follow the expression given by24
-Fs
∫r)rr
0
( )
∫0t dt
1 1 2 - r dr ) bDec0 r r0
p
(9)
or
tp )
[
]
Fsr20 r2 r3 1-3 2+2 3 6bDec0 r r 0
(10)
0
At last, we can obtain
tp ) p(x) ) 1 - 3(1 - Rx)2/3 + 2(1 - Rx) τp
(11) Figure 2. Weight change versus time.
where
τp )
Fsr20 6bDec0
Table 1. Characteristics of the RPF Sample
(12)
proximate analysis (wt %) moisture volatile matter fixed carbon ash ultimate analysis (wt %) C H O N S Cl calorific value (kcal kg-1) LHV
If the pyrolysis is assumed to be a combination of rate-limiting mechanisms, such as chemical reactions and gas diffusion, the time required for a certain conversion of a solid particle is the sum of two parcels and corresponds as follows:24-26
t ) tg + tp
(13)
t ) φ(x) ) g(x) + δ2p(x) τg
(14)
or
δ2 )
τp ksr0 ) τg 6De
(15)
The eqs 8, 11, and 14 are used to model the x-t data, and τg, τp, and δ2 can be estimated by minimizing the equation for the sum of the squares of the errors (SSE) N
[τgg(xi) + τpp(xi) - ti]2 ) Q(τg,τp) ∑ i)1
(16)
The value of δ2 represents the ratio of the diffusion resistance to the chemical reaction resistance. When δ2 , 1, the pyrolysis can be assumed to be controlled by the rate of the chemical reaction. When δ2 > 10, it is safely assumed that the pyrolysis is under the control of the product layer diffusion. An intermediate value of δ2 suggests that the pyrolysis is controlled by the chemical reaction and the product layer diffusion.25,26
0.04 73.34 7.32 19.30 63.10 8.59 11.45 0.19 0 0.98 6403.86
where
(
HHV ) 8100C + 34 000 H -
O + 2500S 8
)
(18)
The characteristics of RPF samples are summarized in Table 1. 3.2. Apparatus. RPF pyrolysis was studied by measuring the weight loss histories of a sample, which consists of many particles, subjected to a constant temperature in an inert atmosphere of pure nitrogen. Weight loss curves during pyrolysis were obtained with a thermogravimetric analysis (TGA) system. Each sample of 100 g was held by a metallic net and placed on the platinum pan of a thermobalance. The TG furnace was heated to a certain temperature, and then, the pan was inserted into the furnace and the sampleweight variations were recorded. The temperature of the sample was measured using a K-type thermocouple. Pure nitrogen at a constant flow rate [100 L min-1 at standard temperature and pressure (STP)] was used as the inert purge gas, both to prevent the presence of air in the pyrolysis zone and to remove gaseous products evolved during pyrolysis. The exiting gases including H2, CO, and gaseous hydrocarbons (GHC), which were the sum of CiHj (i < 6), such as CH4, C2H4, C3H6, etc., were detected by a realtime gas analyzer (GreenLine 9000).
3. Experimental Section 3.1. Materials. Samples of RPF were prepared to obtain fine particles and sieved to ∼1.0-2.0 mm. The proximate analysis was carried out using the method described by American Society for Testing and Materials (ASTM) D5142. The ultimate analysis was conducted using an element analyzer (EA1110, CE instrument). The calorific value of the sample (LHV) was calculated by the Dulongs equation LHV ) HHV - 600(9H + W)
(17)
(26) Dou, B. L.; Gao, J. S.; Sha, X. Z. Fuel Process. Technol. 2001, 72, 23.
4. Results and Discussion 4.1. Analysis of Thermogravimetry and Gas Production. First, the isothermal runs for the pyrolysis of RPF were carried out at four different temperatures of 300, 400, 500, and 600 °C. Figure 2 shows the degree of the weight change versus time at different temperatures, and Figure 3 shows the degree of the overall weight loss. It can be observed from Figure 2 that the weight decreased sharply at 500 and 600 °C and the weight change mainly occurred between 1 and 7 min. The final solid residues obtained at 500 and 600 °C were 28 and 25% of the initial weight, respectively. At 300 °C, the weight loss was quite
Kinetic Study in Modeling Pyrolysis of RPF
Figure 3. Overall weight loss at four temperatures.
Figure 4. Yield of GHC.
low and the residual weight fraction was 86%, even after the pyrolysis time of 30 min. Typically, at 300 °C, many parts of the plastic components are not decomposed and remain within the solid residue. This solid residue has several potential applications, such as active carbon for adsorption materials or char using combustion and gasification.27 The data shown in Figures 2 and 3 imply the importance of final temperatures in the isothermal operation. The weight loss at a higher temperature is very large compared with that at a lower temperature, and it is also attributed fundamentally to the cracking of residual organic components that are decomposed to produce gases and to the partial gasification of the char. The residual organic matters at high temperatures perpetuate the pyrolysis process and form more gases and less solid products.16,17,27,28 In this study, concentrations of different gaseous products, including H2, CO, and GHC, at different temperatures were measured using the gas analyzer. The flow rate of each gas component was calculated by a simple mass balance of the inert gas and then normalized on the basis of the initial weight of the RPF. This was done to eliminate the effect of changes in the initial mass in various experiments. CO2 was the most abundant gas in all of the experiments. Figures 4-6 show the concentrations of H2, CO, and GHC, respectively. (27) Berrueco, C.; Esperanza, E.; Mastral, F. J.; Ceamanos, J.; Garcı´aBacaicoa, P. J. Anal. Appl. Pyrolysis 2005, 74, 245. (28) Huang, H.; Tang, L.; Wu, C. Z. EnViron. Sci. Technol. 2003, 37, 4463.
Energy & Fuels, Vol. 21, No. 3, 2007 1445
Figure 5. Yield of H2.
Figure 6. Yield of CO.
Figure 7. Rate curves at different temperatures.
It was observed that the operating temperature had a great influence on gaseous products. While the maximum concentrations of H2, CO, and GHC were detected at 600 °C, very small amounts of gases of H2, CO, and GHC were released at 300 °C. It seems that the pyrolysis at 300 °C led only to depolymerization of plastic components with a low molecular weight. Some studies for the pyrolysis of biomass, waste tires, and some polyvinyl chloride (PVC) also indicate that the evolution of all gases begins at around 300 °C and gaseous products increase
1446 Energy & Fuels, Vol. 21, No. 3, 2007
Dou et al. Table 3. Activation Energies and Frequency Factors parameters ks De
Table 2. Kinetic Parameters for Pyrolysis of RPF, r ) 1 model
SSE
τg (min)
300
eq 8 eq 11 eq 14 eq 8 eq 11 eq 14 eq 8 eq 11 eq 14 eq 8 eq 11 eq 14
0.035 0.012 0.006 0.006 0.017 0.003 0.009 0.013 0.005 0.011 0.015 0.002
1335.8
400 500 600
830.5 74.4 50.3 23.0
A
70.2 65.9
1.6 × 105 s-1 7.5 × 104 m2 s-1
reaction control model (eq 8) and the diffusion control model (eq 11) at four temperatures. The results also show that the value of δ2 is ∼1.30-1.61. Therefore, fitting based on a model derived from the shrinking core models suggests that the pyrolysis of RPF is governed by a combination of the chemical reaction and product layer diffusion. The predicted rate curves are shown in Figure 7 and indicate that the reaction rates rapidly decrease during the initial period and reach nearly 0 after 20 min. We obtained further information about kinetic parameters. The activation energies for the chemical reaction and gas diffusion through the solid product layer were determined by eqs 7 and 12, respectively, using the following Arrhenius equations:
Figure 8. Arrnerius plots for De and ks.
temperature (°C)
E (kJ mol-1)
Er
τp (min)
δ2
ks ) Are-RT
6031.4 1335.0
1.61
130.7 65.9
De ) Aee-RT
1.30
27.9 18.5
1.34
18.7 12.4
1.37
13.8 16.9 9.06
as the temperature increases.5,12,15,27,28 For the pyrolysis of RPF, the following overall reactions are likely important:
RPF f char + HC (heavy and light hydrocarbons) + gas (H2, CO, H2O, CO2, CH4, C2H4, etc.) heavy hydrocarbons f light hydrocarbons + gas (H2, CO, H2O, CO2, CH4, C2H4, etc.) In addition, the further decomposition of light hydrocarbons is also possible
light hydrocarbons f gas (H2, CO, H2O, CO2, CH4, C2H4, etc.) The reaction of char with H2O and CO2 may convert char into some gaseous species at temperatures higher than 500 °C21,29
char + H2O f CO + H2 + solid residue char + CO2 f CO + solid residue
(19)
Ed
(20)
The results are summarized in Table 3. The linear relationships between ks, De, and 1/T are well-satisfied for all runs, as shown in Figure 8. The difference between the two activation energies is small, which implies that the pyrolysis reaction of RPF is governed by a combination of both the chemical reaction and gas diffusion through the product layer. 5. Conclusions The isothermal experiments of RPF pyrolysis using TGA, in which nitrogen was used as an inert gas, were performed to determine the kinetic parameters. Experimental results showed that the operational temperature had an important influence on the pyrolysis of RPF. As the temperature increased, the weight loss and gas production increased and the solid residue decreased. The kinetic results showed that the reaction rates rapidly decreased in the initial period of RPF pyrolysis and reached nearly 0 after 20 min. The rate-limiting mechanism was a combination of surface chemical reactions and gas diffusion through the solid product layer. The determined activation energy for the surface chemical reaction was 70.2 kJ mol-1, and the determined activation energy for gas diffusion in the solid phase was 65.9 kJ mol-1. Acknowledgment. This work was supported by the Seoul Development Institute project (Grant number 20060124-2-1-001), Korea.
Nomenclature 4.2. Kinetic Fitting. The rate expressions of g(x) and p(x) (refer to eqs 8 and 11, respectively) were applied to eq 14 by minimizing eq 16 first with respect to one of the τg and τp values by setting the other to 0 and then with respect to both τg and τp simultaneously. The computed parameters are shown in Table 2. Because the value of R was not sensitive to the model, R ) 1.0 has been used in the model to simplify the computations. The kinetic results show that the estimated SSE for the combined model (eq 14) was less than those for the chemical (29) Smagˇ, A.; Sinek, K.; Tekes¸ , A. T.; Misirhogˇlu, Z., Canel, M.; Wang, L. Chem. Eng. Process. 2003, 42, 1027.
Ae and Ar ) frequency factor (s-1) b ) stoichiometric coefficients c ) gas percentage concentration (%) De ) effective diffusion coefficient in a porous structure (m2 s-1) Er, Ed, and Ei ) activation energy (kJ mol-1) g(x) ) conversion function under chemical control ks ) intrinsic reaction rate constant (s-1) p(x) ) conversion function under diffusion control φ(x) ) conversion function under the combination of chemical control and product layer diffusion Q ) function of the sum of the squares of the errors r and r0 ) radius of the particle (mm)
Kinetic Study in Modeling Pyrolysis of RPF S0 ) surface area of the solid sample (m2/m3) SSE ) sum of the squares of the errors t, tg, and tp ) time (min) T ) temperature (K) ∆Vi and ∆Vit)0 ) volumetric shrinkage rate at t and t ) 0, respectively w, w0, and wf ) actual, initial, and final masses of the sample (kg) x ) conversion (%)
Energy & Fuels, Vol. 21, No. 3, 2007 1447 R ) shrinkage factor Fs ) solid particle density (kg m-3) δ2 ) ratio of the diffusion to the chemical reaction resistance τg ) characteristic time for chemical reaction control (min) τp ) characteristic time for diffusion control (min) EF060594C
