Thermal and kinetic behaviors of biomass and plastic wastes in co-pyrolysis

Energy Conversion and Management 75 (2013) 263–270

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Thermal and kinetic behaviors of biomass and plastic wastes in co-pyrolysis Özge Çepeliog˘ullar a,⇑, Aysße E. Pütün b a b

Department of Chemical Engineering, Faculty of Chemical and Metallurgical Engineering, Istanbul Technical University, 34469 Istanbul, Turkey Department of Chemical Engineering, Faculty of Engineering, Anadolu University, 26555 Eskisßehir, Turkey

a r t i c l e

i n f o

Article history: Received 2 April 2013 Accepted 22 June 2013

Keywords: Co-pyrolysis Biomass Plastic TGA Kinetics

a b s t r a c t In this study, co-pyrolysis characteristics and kinetics of biomass-plastic blends were investigated. Cotton stalk, hazelnut shell, sunflower residue, and arid land plant Euphorbia rigida, were blended in definite ratio (1:1, w/w) with polyvinyl chloride (PVC) and polyethylene terephthalate (PET). Experiments were conducted with a heating rate of 10 °C min1 from room temperature to 800 °C in the presence of N2 atmosphere with a flow rate of 100 cm3 min1. After thermal decomposition in TGA, a kinetic analysis was performed to fit thermogravimetric data and a detailed discussion of co-pyrolysis mechanism was achieved. Experimental results demonstrated that the structural differences between biomass and plastics directly affect their thermal decomposition behaviors. Biomass pyrolysis generally based on three main steps while plastic material’s pyrolysis mechanism resulted in two steps for PET and three steps for PVC. Also, the required activation energies needed to achieve the thermal degradation for plastic were found higher than the biomass materials. In addition, it can be concluded that the evaluation of plastic materials together with biomass created significant changes not only for the thermal behaviors but also for the kinetic behaviors. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Technological advances and financial growth have become possible with the unreasonable consumption of the natural resources. Consequently, these changes resulted in an ecological and economical imbalance besides an increase in consumption habits demanding on the enhancement in population. This depletion brings several problems such as treatment of wastes which needs to be solved urgently. Especially, the consumption of plastic materials due to their practical usage, low-cost and high-resistance has increased drastically. In our daily lives, plastics are used in a number of applications from greenhouses to coating, wiring, packaging, films, covers, bags and containers [1,2]. Among different types of wastes from different areas, especially package wastes made from plastic materials create a great controversy [3]. As a result of that plastics have become an important environmental issue even though they do not generally possess a direct hazard to the environment [4]. Because of their increasing cost and decreasing space of landfills, the traditional methods for the removal of plastic wastes such as landfilling and incineration which cause the irreversible environmental problems are not constituted a certain solution from an environmental standpoint [3]. Instead of using ⇑ Corresponding author. Tel.: +90 (0) 212 285 67 63; fax: +90 (0) 285 29 25. E-mail addresses: [email protected] (Ö. Çepeliog˘ullar), [email protected] (A.E. Pütün). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.06.036

traditional disposal methods, recycling these wastes by using alternative technologies in order to produce energy or chemicals is certainly a better way via today’s advanced technology [5–7]. When considering the most popular plastic materials both in daily and industrial usage, two plastics stand out among other polymers: polyethylene terephthalate (PET) and polyvinyl chloride (PVC). PET has a wide range of application areas such as prepaid cards, films, fibers and tapes besides being used as bottle material. That is the reason why PET has become the most encountered plastic material in daily life and recycling plastic material [8]. Also, polyvinyl chloride (PVC) is the second most used plastic behind polyethylene around the world. Due to its good chemical and physical resistance, it can be used in different applications like construction, packing, wires and cables, transport, furniture, etc. As a result, the PVC waste amount is increasing significantly, and the elimination of these waste materials has become a major environmental problem [9]. When it comes to creating clean energy, biomass still protects its place and importance in the past decade regarding the industrial development of thermochemical conversion plants [10]. In general, biomass is (i) clean, (ii) low-cost, (agricultural wastes, forest residues, food industry wastes, and daily garbage of houses), (iii) abundant, (iv) easy to grow and (v) renewable [11]. Being an agricultural country, Turkey has abundant sources of agricultural wastes and crop residues which can be evaluated as biomass resources instead of burning them in order to destroy after harvest.

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Nomenclature W0 Wt Wf A E T

initial sample mass (mg) sample mass at time t (min) or T (mg) final sample mass at the end of pyrolysis (mg) pre-exponential factor (min1) activation energy (kJ/mol) temperature (K)

In recent years, many studies have been carried out in order to evaluate biomass, such as forest residues, fruit shells, agricultural waste residues, paper and food industry residues in addition to tea waste and olive husk for energy production in Turkey. One of the promising ways to utilize the energetic and organic value of these wastes is co-processing technologies such as co-gasification, co-combustion, co-firing and co-pyrolysis. Especially, combustion and co-pyrolysis allow the researchers to observe and interpret if any resultant synergistic effects occurred during the process [12–14]. Thermogravimetric analysis (TGA) comes in the first rank for the pyrolysis of solid raw materials such as coal, plastics and biomass. After the thermogravimetric analysis experiments, thermal and kinetic behaviors of the pyrolysis and experimental parameters can be determined with the obtained TG data [15]. In this study, in order to compare the thermal and kinetic behaviors of individual raw materials with the mixtures, biomass-plastic materials were blended in definite ratio (1:1, w/w) and pyrolyzed with a heating rate of 10 °C min1 from room temperature to 800 °C in the presence of N2 atmosphere with a flow rate of 100 cm3 min1 in thermogravimetric analyzer. Cotton stalk, hazelnut shell and sunflower residue which are the most important agricultural wastes from different parts of Turkey and arid land plant Euphorbia rigida were chosen as biomass materials. Additionally, polyvinyl chloride (PVC) and polyethylene terephthalate (PET) as the most encountered plastic materials used in both daily life and industry were used as plastic waste materials. With the obtained TG data, kinetic equations among pyrolysis process were derived and decomposition temperatures and thermal behaviors of mixtures were determined.

2. Experimental 2.1. Raw materials In this study, three agricultural wastes, which constitute an economical importance for Turkey, cotton stalk (CS), hazelnut shell (HS), sunflower residue (SFR) were preferred besides, E. rigida (ER). The first biomass material CS is selected for the study because of its agricultural importance for Turkey, one of the eight countries producing 85% of the world’s cotton. CS used in the experiments was provided from Mediterranean Region of Anatolia. Turkey still holds the first place in the world for hazelnut production. Annual hazelnut production of Turkey is approximately 500,000 tons and half of this is shell as by-product of nut processing factories [16]. This amount of industrial by-product comprises an important biomass potential which needs to be evaluated for energy production. HS used in the thermal process was obtained from Black Sea Region. Another agricultural waste used in the study is SFR. Turkey stands at the top of the world sunflower production together with Russia, the USA, India, Argentina and China [17]. In this study, in order to evaluate the residues of this plant, SFR was selected and provided from Marmara Region of the country. Additionally, ER was used for the experiments. ER which grows in the arid lands of Anatolia and contains latex compound found in

R t x H R2

universal gas constant (J/mol K) time (min) weight loss fraction or pyrolysis conversion heating rate correlation factor

secretory cells referred to as laticifers including proteins, rubbers, sugars, alkaloids, terpenoids, tannis and lipids that may provide a renewable source of feedstocks and fuels [18]. These characteristics make ER a significant energy plant. Mediterranean and Central Anatolia Regions have large amounts of ER potential due to their hot climate. So, ER (the main body together with its stalks) used in the pyrolysis experiments was obtained from Mediterranean Region. As plastic materials, used water PET bottles were collected while PVC wastes were obtained from a local plant and used as the polymer source. All raw materials were dried at room temperature, grounded in a high-speed rotary cutting mill and then sieved. The particle size used in the experiments was <0.25 mm. Ultimate and component analysis results of biomass materials were given in Table 1 and proximate analyzes of all materials were given in Table 2. 2.2. Experimental procedure The pyrolysis experiments were carried out in a TGA (SETARAM-LABSYS evo) under an inert atmosphere to investigate thermal events of the raw materials and their mixtures during pyrolysis. About 5–15 mg of sample was pyrolyzed under 100 cm3 min1 N2 flow at a heating rate of 10 °C min1 from room temperature to 800 °C. This nitrogen flow ensured an inert atmosphere on the sample during the run, while the small amount of sample and the slow heating rate ensured that the heat transfer limitations can be ignored. 2.3. Kinetic study Kinetic parameters such as activation energy and pre-exponential factor of the pyrolysis mechanism were derived with the obtained TGA data. To begin with, some assumptions needed to be made. In the literature many researchers assumed that solid fuel pyrolysis is a first order reaction and explained the biomass-plastic pyrolysis reaction as the following formula:

dx ¼ A exp dt

   E ð1  xÞ RT

ð1Þ

Table 1 Proximate and component analysis of raw materials (wt.%, db).

a

Raw materials

CS

HS

SFR

ER

Proximate analysis Moisture Ash Volatiles Fixed Ca

7.46 5.52 64.92 22.10

10.94 0.711 68.98 19.36

6.05 9.34 65.26 19.35

3.02 6.72 75.05 15.21

Component analysis Holocellulose Oil Extractives Hemicellulose Lignin

72.75 6.80 5.63 21.68 22.16

72.61 5.01 5.36 25.48 23.46

65.45 4.25 14.04 35.18 20.94

48.67 5.15 12.55 29.50 37.92

By difference, db: dry basis.

Ö. Çepeliog˘ullar, A.E. Pütün / Energy Conversion and Management 75 (2013) 263–270 Table 2 Ultimate analysis of raw materials (wt.%, daf).

3. Results and discussion 3.1. Thermal decomposition of raw materials and their mixtures

Raw materials

C H N O H/C O/C Calorific valuea (MJ/kg)

CS

HS

SFR

ER

PET

PVC

47.95 5.50 3.24 43.31 1.38 0.68 16.3

56.37 5.62 5.96 32.05 1.19 0.43 21.4

47.91 5.27 8.65 38.17 1.32 0.59 16.9

54.17 5.70 1.30 38.3 1.25 0.52 19.8

75.21 3.90 4.89 16.01 0.62 0.16 28.2

55.98 6.14 0.54 37.34 1.32 0.50 21.1

daf: dry and free ash basis. a Calculated by using (338.2  C) + (1442.8  (H  (O/8))) + (94.2  S).

where A (min1) is pre-exponential factor, E (kJ/mol) is activation energy, T (K) is temperature, t (min) is time, R (J/mol K), universal gas constant, x is weight loss fraction or pyrolysis conversion which can be calculated by:

x¼

W0  Wt W0  Wf

ð2Þ

where W0 (mg) is the original mass of the test sample, Wt (mg) is the mass at time t (min) or T (K) and Wf (mg) is final mass at the end of pyrolysis. For a constant heating rate H during pyrolysis, H = dT/dt, rearranging Eq. (1) and integrating gives:

ln

   lnð1  xÞ T

2

¼ ln



  AR 2RT 1 HE E

ð3Þ

From these assays, the evolution with temperature of weight loss (TG) and the weight loss rate (DTG) were obtained for pyrolysis. The weight loss rate was calculated by the expression:

  dW 1 dWt ¼ dt W 0 dt

265

ð4Þ

where W0 (mg) is the initial sample mass, Wt (mg) is the amount of the sample at time t or T (K), t (min) is the time and T (K) is the temperature [19].

TGA results of raw materials are shown in Fig. 1. From the curves, it is obviously seen that the weight losses increased with the raising pyrolysis temperature for all raw materials and mixtures. From the comparison of TGA curves, it can be noted that biomass and plastic materials have different thermal behavior trends due to their structural differences. It is generally accepted that biomass pyrolysis includes three main stages; moisture drying, main degradation of the more unstable polymers and continuous slight devolatilisation [20,21]. d’Almeida et al. reported that lignocellulosic biomass is thought to be stable until 200 °C, with minor mass losses associated with the removal of moisture and the hydrolysis of some extractives [22]. According to the weight loss–temperature curves, small amount of weight loses occurred until 150 °C which were the indicative of inherent water being released within the biomass samples as reported in the literature [23]. It was observed that biomass samples continued to lose water, until all the moisture in their structure had disappeared. At the intermediate stage, cellulose which constitutes the main structure of biomass started to decompose within the temperature range of 250–450 °C. Second step of the biomass pyrolysis is known as the most complicated part; since at the high temperature regions chemical structure of the materials begins to decompose. All reactions, which will be yielded as the pyrolysis products, started and generated the pyrolysis mechanism. White et al. [21] separated cellulosic decomposition of biomass into two basic routes. The first route which comes into prominence at lower temperatures starts after the inherent water released and is completed at 280 °C. This part involves reactions that lower the degree of polymerization via bond scission, dehydration, free radical formation, and creation of oxygenated moieties (e.g., carbonyls, carboxyls, and peroxides), evolution of CO and CO2, and ultimately the production of carbonaceous residues. They pointed out that at higher temperatures (280–500 °C) cellulose degradation follows a different pathway. In this temperature region, depolymerization reactions are predominated with the integration of the bonds. This situation caused liquid product of pyrolysis containing wide range of organic compounds and chemicals [22]. Between this temperature range, CS, HS, SFR and ER also lost 44.32%, 46.75%, 50.1% and 48.48% of their initial weight, respectively due to cellulose breakdown and volatile removal.

Fig. 1. Weight loss–temperature curves of biomass and plastics.

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Fig. 2. Degradation process of PET.

At the terminal stage of the pyrolysis mechanism, lignin as the most ample and complex structural fraction of the biomass began to decompose. Thermal decomposition step of the lignin is of the vital importance for the pyrolysis mechanism because it leads in charcoal formation and has a wide decomposition temperature range between 200 °C and 800 °C as can be seen from Fig. 1. It was observed that after light volatiles has been removed, CS, HS, SFR and ER lost 17.22%, 17.02%, 23.77% and 18.05% of their initial weight due to in this temperature range on account of lignin degradation. It can be said that reactions were caused by the combustion of fixed carbon, decomposition of tars and other concerns between the temperature ranges of 500–800 °C. It is known that the remaining residue after thermal treatment has brought about the char and the ash [24]. After TGA experiments, remaining residues of CS, HS, SFR and ER were determined as 29.97%, 26.77%, 15.34%, and 26.33% respectively. Fig. 1 also shows the weight loss of plastics as a function of temperature. It is already known that the structure of plastic materials is not as complicated as biomass materials. Besides, the lack of inherent water in their nature, thermal decomposition of plastics occurred at higher temperatures compared with the biomass and completed in shorter-time period. It is obvious in Fig. 1 that the

structural breakdown of PVC starts nearly at 220 °C. In TGA curve of PVC, two main peaks which represent the weight loss due to increase in the temperature can be seen easily. Maximum decline in the initial weight loss (62.25%) was observed between 280 and 385 °C. The increase in the temperature led the increase in the weight loss and PVC also lost 21.74% of its initial weight in the temperature range of 285–520 °C. Ma et al. explained this change with the volatilization of adherent HCl in the main structure of PVC. With the effect of heat, at temperature above 340 °C, the polymer has already become the dechlorinated product and further pyrolyzed to low hydrocarbons of linear or cyclic structure (more than 170 C1–C7 products have been identified) [3]. Thermal degradation of PET started at higher temperatures (360 °C) when compared with PVC. The most notable peak was occurred at 427.7 °C and the material lost 79.78% of its initial weight due to decomposition of the structural backbone. In the literature, many researchers reported almost the same amount of weight loss occurred in this temperature range. For instance Girija et al. denoted that PET undergoes single stage degradation with a single peak and most of the total weight loss (approximately 60% of the total losses) detected was at the temperature of 440 °C [25]. As expected, it is harder to explain the thermal decomposition mechanism of PET at high temperatures due to its aromatically structure. Holland and Hay explained that thermal degradation of PET which leads to the production of volatile material has been rationalized into two degradation processes, intramolecular back biting, and b-C–H hydrogen transfer. Mechanism of the process is given in Fig. 2. Also, another important point which needs to be emphasized is as the reaction proceeds, PET loses more aliphatic parts than aromatic parts [26]. Fig. 3 shows the thermal degradation behaviors of biomass–PVC mixtures. It should be noted that thermal degradation of the blends comprises the behavior of individual component. At the first step, degree of weight loss started to increase slowly due to inherent water in the structure of biomass materials in the mixture. However, in this phase, weight loss was limited between 2.42% and 5.40% of the initial weight of the samples. This increase continued until 100–120 °C. When temperature reached nearly 200–250 °C, the cellulosic structure of the biomass materials started to decompose. For biomass–PVC mixtures, it was observed that the structural breakdown of PVC began to occur simultaneously. The significant peaks among biomass–plastic mixtures were determined at 284.1, 283.0, 274.4, 274.3 °C, for CS-PVC, HSPVC, SFR-PVC and ER-PVC, respectively. In addition, between the

Fig. 3. Weight loss–temperature of biomass–PVC mixtures.

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267

Fig. 4. Weight loss–temperature of biomass–PET mixtures.

Fig. 5. Plots of ln(ln(1  x)/T2) vs. 1/T of CS, PVC, CS-PVC.

temperature ranges of 200–350 °C, the mixtures in the given order lost 50.32%, 51.19%, 48.75%, and 54.13% of their initial weights. Fig. 4 shows the thermal degradation of biomass–PET mixtures as a function of weight loss–temperature. However, due to the above mentioned structural differences, higher temperatures were needed during experiments in order to decompose the biomass– PET mixtures. Maximum weight losses of the mixtures happened

at 427.4, 427.3, 425.2 and 420.4 °C, respectively. When temperature reached nearly 600 °C, only small changes in the weight loss observed. The remaining residues on account of biomass materials were achieved as between 16.25% and 26.71% of the mixtures. It can be said for biomass–PVC mixtures when compared with the thermal degradation of individual raw materials that the rate of decomposition was slow at the beginning, however with the increasing temperatures weight losses started to increase at the medium temperature regions. And, the thermal degradation was completed earlier, nearly at 500–600 °C. This means that biomass–PVC mixtures showed similarity with the thermal behaviors of PVC which decomposes mostly as gaseous products. On the other hand, for biomass–PET mixtures the opposite case occurred. Since, PET is a complex polymer which decomposes at two stages; the temperatures for the thermal decomposition of biomass–PET mixtures were higher than the individual materials. 3.2. Kinetic analysis In order to determine the parameters of the pyrolysis (preexponential factor and activation energy), Eq. (3) was used. Since it may be shown that for most values of E and for the temperature range of the pyrolysis, the expression ln[AR/HE(1  2RT/E)] in Eq. (3) was essentially constant, if the left side of Eq. (3) was plotted

Table 3 Kinetic parameters for the pyrolysis of individual materials. Sample name

Temperature (°C)

Conversion (%)

E (kJ/mol)

R2

A (min1) 9

CS

42–102 107–202 207–382

0.6–12 13–19 20–99

67.42 38.9 79.48

6.63  10 1.26  104 3.63  106

0.9742 0.9358 0.9636

HS

28–108 113–203 208–383

0–14 15–19 19–99

62.81 38.53 82.45

1.11  109 1.21  104 5.02  106

0.9667 0.9267 0.9714

SFR

37–77 82–192 197–362

1–9 10–25 27–99

72.67 30.64 74.2

2.25  1011 1.4  103 2.6  106

0.9732 0.9298 0.9637

ER

39–99 104–224 229–359

0–8 9–25 26–99

74.66 36.13 88.87

7.85  1010 2.49  103 3.82  107

0.9836 0.9416 0.9606

PET

373–443 448–503

0.4–81 88–99

347.4 172.6

6.4  1025 9.31  1011

0.9905 0.9703

PVC

222–292 297–387 392–522

0.8–48 53–75 75–99

246.78 108.12 191.32

1.8  1023 8.3  108 1.2  1013

0.9942 0.9683 0.9718

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Table 4 Kinetic parameters for the pyrolysis of biomass–PET mixtures. Mixture

Temperature (°C)

Conversion (%)

E (kJ/mol)

A (min1) 10

R2

CS-PET

34–89 94–144 149–214 219–369 374–504

0–4.7 5.1–6.9 7–7.9 8.1–39 40–99

68.59 72.95 76.57 89.02 171.48

1.78  10 3.86  109 1.14  108 2.82  107 1.52  1012

0.9935 0.9534 0.9635 0.9623 0.9179

HS-PET

38–88 93–133 138–208 213–348 353–503

0–4.9 5.2–6.1 6.2–6.8 7–35 37–99

73.94 85.5 61.79 93.36 139.04

1.46  1011 3.36  1011 3.23  106 7.35  107 6.36  109

0.9865 0.9752 0.9389 0.9721 0.9724

SFR-PET

43–98 103–188 193–303 308–378 383–443 448–548 553–648

0–4.5 4.6–6.7 7–29 30–40 41–83 85–92 93–99

66.63 50.24 91.2 123.55 315.82 104.64 261.32

7.8  109 3.01  105 2.45  109 9.65  109 3.73  1023 2.81  106 2.62  105

0.9642 0.9565 0.9716 0.9461 0.9863 0.9379 0.9470

ER-PET

42–97 102–177 182–292 297–377 382–447 452–557 562–637

0–3.9 4.2–6.5 6.7–24 26–44 45–88 89–95 96–99

63.36 52.19 86.43 115.28 277.45 112.8 316.34

2.14  109 1.11  106 9.79  107 2.16  109 4.96  1020 8.02  106 3.89  1018

0.9749 0.9550 0.9706 0.9393 0.981 0.9382 0.9510

Table 5 Kinetic parameters for the pyrolysis of biomass–PVC mixtures. Mixture

Temperature (°C)

Conversion (%)

E (kJ/mol)

A (min1)

R2

CS-PVC

67–137 142–187 192–322 327–387 392–502

0–3.1 3.2–3.6 3.7–65 68–79 80–99

51.08 108.82 139.11 157.78 190.13

6.13  107 4.01  1012 4.37  1012 5.72  1012 1.75  1013

0.9386 0.9674 0.9933 0.9464 0.9736

HS-PVC

38–138 143–188 193–333 338–408 413–503

0–6.4 6.5–6.9 7–72 74–82 83–99

45.06 96.31 135.99 127.17 23.29

1.53  106 1.55  1011 1.65  1012 8.33  107 3.46  1015

0.9417 0.9563 0.9853 0.9403 0.9523

SFR-PVC

33–113 118–203 208–293 298–393 398–508

0–6 6.1–9.6 10–66 68–83 84–99

45.88 66.24 146.98 95.05 197.72

3.6  106 1.6  107 7.47  1013 3.8  107 5.68  1013

0.9716 0.9884 0.9942 0.9343 0.9548

ER-PVC

33–113 118–193 198–328 333–408 413–503

0–6.4 6.6–9.4 9.8–74 75–83 84–99

42.1 65.41 113.41 135.46 228.6

8.4  105 8.27  107 2.31  1010 3.34  1010 9.56  1015

0.9748 0.9768 0.9782 0.948 0.9346

vs. 1/T, straight lines could be obtained. By using the slope of these straight lines (E/R) of the obtained straight lines, the activation energy (E) of the process was figured out. Also, by taking the temperature at which Wt = (W0 + Wf)/2 in the place of T in the intercept term of Eq. (3), the pre-exponential factor was calculated [22]. Fig. 5 shows the pyrolysis mechanisms of CS, PVC and CS-PVC and was plotted (ln(ln(1  x)/T2) vs. 1/T) to compare the pyrolysis mechanism paths of raw materials. It was observed that pure materials and co-components have different pyrolysis trends. It is clear from the figure that the curve of pyrolysis mechanism of CS-PVC mixture was placed between curves of the pure materials because it represents the characteristics of both individual raw materials. In order to determine the reactions occurring at high temperature region during pyrolysis, obtained curves were divided into straight lines which constitute the main steps of the pyrolysis. From the slope of each lines E and A values for all pyrolysis steps

were calculated and given in Tables 3–5 besides the conversion rates of the materials. When the calculated activation energies of each step for biomass and plastic materials were compared in Table 3, it is noteworthy that the energy needed to breakdown the structure of plastic materials is higher than the biomass. In general, it can be said that the start of the decomposition process of biomass materials was observed at lower temperatures. Their decomposition mechanisms divided into three main steps, and the activation energy need was found lower than the plastics. This difference can be explained with the structural differences between biomass and plastics as mentioned above. Thermal degradation of biomass starts at lower temperatures, takes longer, and it can be describe as gradual. As a result of that the conversion of individual biomass materials completed mostly at the final step of the pyrolysis which is obvious in Table 3 (nearly 20–99% completed at the 3rd step). Also, this thermal behavior of

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CS

269

energy need is higher for the decomposition of biomass–PET mixtures than the biomass–PVC mixtures. Since PET is a complex compound with an aromatic ring in the structure, energy barrier needed to be brought through requires more energy. CS, PVC and CS-PVC were taken as an example for an explanation and the pyrolysis mechanism of both were plotted in Fig. 6. Every step represents a different part of the pyrolysis process which was explained above. For all stages Eq. (2) was applied to each step separately and conversions were recalculated. Activation energies were determined between the range of 38–80 kJ/mol for CS, 108– 247 kJ/mol for PVC and 50–190 kJ/mol for CS-PVC mixture. Varying activation energies shows that biomass–plastic mixtures have different pyrolysis reactivities at different temperatures ranges. Also, the good correlation coefficient indicates that the corresponding independent first-order reaction model fits the experimental data very well.

PVC 4. Conclusion

CS-PVC

In this study, thermal behaviors of biomass–plastic mixtures at high temperature regions were investigated in thermogravimetric analyzer. Experimental results demonstrated some main differences between the thermal behaviors of biomass and plastics. Thermal degradation of biomass materials took longer than the plastic materials and higher temperatures were needed in order to provide structural breakdown. In addition, biomass decompositions were divided into three main stages. These are; moisture drying, main devolatilisation and continuous slight devolatilisation. On the other hand, plastic decomposition followed different paths. Plastics lost nearly 80–85% of their initial weight within the temperature range of 250–600 °C unlike biomass materials. In parallel with these experimental results, mixtures had the characteristics of both biomass and plastic materials during thermal decomposition. According to calculated activation energies, it was determined that the energy barrier of plastic materials in order to start the pyrolysis reactions were higher. The experimental results obtained from TG have an important role in the determination of the pyrolysis mechanism and process conditions while designing/implementing a thermochemical conversion method where biomass– plastic materials were preferred as raw materials.

References

Fig. 6. (a) CS, (b) PVC and (c) CS-PVC.

biomass caused the critical energy needed to start the pyrolysis reactions of biomass is less than the energy necessity of plastics. On the other hand, at the lower temperatures of the thermal process, nearly no weight loss occurred; this is the result of the bonds in the structure of the polymers needed higher temperatures and activation energy. From this point of view, it should be noted that the energy barrier of plastic material is higher, because sudden weight losses occurred in high temperature regions unlike biomass decomposition [27]. In parallel with, the conversion rates also verified these results. A major part of the polymer conversion completed at the second step due to exceeding of this energy barrier (see Table 3 for PET 81%, for PVC 75% of the conversion completed before 3rd step). Also, from Tables 4 and 5, it can be seen that the additive of polymers caused the most of the completion of the conversion at earlier stages. When it comes to comparing the calculated kinetic parameters of the mixtures, it can be clearly seen that the temperature and

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