Characterization of Tar Derived from Principle Components of

Oct 26, 2015 - The composition for tar derived from PVC and their mixture showed a dramatic ... In this paper, the rheologic characteristics of tar sa...
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Characterization of Tar Derived from Principle Components of Municipal Solid Waste Qunxing Huang, Yijing Tang, Shengyong Lu,* Xianhao Wu, Yong Chi, and Jianhua Yan State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ABSTRACT: It is of great interest and importance to have a comprehensive understanding of the characteristics of tar derived from major components of municipal solid waste to guide gasification syngas cleaning and upgrading. In this paper, cellulose and polyvinyl chloride (PVC) were chosen as principle components, and the surface tension, kinematic viscosity, and contact angle of tar derived from cellulose, PVC, and their mixture at temperatures of 400, 500, and 600 °C were experimentally studied. Gas chromatography−mass spectrometry (GC−MS) and 13C nuclear magnetic resonance (NMR) were employed to identify major tar species and their corresponding molecular bonds. Results indicated that the surface tension increased from around 51.4, 41.4, and 58.6 mN/m at 400 °C to 54.8, 47.2, and 64.9 mN/m at 600 °C for cellulose, PVC, and their mixture, respectively. Correspondingly, the kinematic viscosity changed from around 5.64, 20.37, and 10.37 cSt to 10.35, 34.21, and 15.51 cSt. The surface tension was affected by the co-pyrolysis of cellulose and PVC, while no obvious interaction was observed for kinematic viscosity. GC−MS results showed that the major species of tar generated from cellulose were miscellaneous hydrocarbons with a proportion between 69.7 and 96.3%. The composition for tar derived from PVC and their mixture showed a dramatic difference compared to that from cellulose. NMR spectra indicated that the function group of tar derived from the PVC and cellulose mixture was dominated by PVC.

1. INTRODUCTION Municipal solid waste (MSW) management has been and will continue to be a major issue worldwide.1 The annual growth rate of global MSW is 3.2−4.5% in developed nations and 2− 3% in developing nations.2 As the most promising thermal treatment method, pyrolysis/gasification has gained attention as an advanced and environmental friendly solution for energy and resource recovery from MSW. However, because of the diversity of feedstock, gasification syngas usually contains unacceptable levels of various impurities, especially the liquid tar, which can cause serious operational problems in downstream facilities by blocking gas coolers, filter elements, or engine suction channels3,4 because it is easily to condense and adhere to the surface. In the past few years, many technologies have been developed for tar removal, such as thermal cracking, catalytic cracking, non-thermal plasmas, and mechanism methods. Unfortunately, complete tar removal is still a big challenge as a result of the lack of comprehensive characterization of tar with respect to MSW components. Previous research has found that, generally, the major combustible components of MSW are biomass and plastics. The principal compositions of biomass are cellulose, hemicelluloses, and lignin.5−7 Generally, cellulose accounts for 42− 49% of all different biomass contained in MSW.8 The plastics in MSW include polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl dichloride (PVdC). Among all plastics, PVC has been the center of attention because it is the most widely used halogen-containing polymer, typically accounting for less than half of the hydrogen chloride emitted from waste incineration.9 The annual production of PVC was estimated to be more than 35 million tonnes in 1997, increasing globally by 3.8% p.a. to 500 million tonnes in 2012.10,11 The abundance of PVC wastes makes sustainable disposal a real challenge. The disposal of end © 2015 American Chemical Society

of life PVC by thermal methods, such as energy reclamation (incineration) and thermal degradation, has been hampered by the release of hazardous chlorinated compounds and dioxins at elevated temperatures, which are known to lead to process complications, such as catalyst poisoning and increased capital costs (as a result of corrosion of plant equipment).12 Matsuzawa et al.13 have investigated the pyrolysis of mixtures of cellulose and various polymers by thermal analysis to simulate the pyrolysis of MSW, and they found that only the mixture of cellulose and chlorinated polymer gave different results from those for pure ones. Pyrolysis of pure cellulose or PVC has been extensively studied.14−17 However, researchers have mainly focused on the formation and composition of derived tar, while the basic apparent properties that affect tar removal efficiency have not been well studied. In this paper, the rheologic characteristics of tar samples derived from the pyrolysis of cellulose, PVC, and their mixture with a weight ratio of 1:1 at temperatures of 400, 500, and 600 °C were experimentally investigated. The surface tension, kinematic viscosity, contact angle, and ζ potential were quantitatively measured, and the main compositions and corresponding molecular bonds were identified by gas chromatography−mass spectrometry (GC−MS) and 13C nuclear magnetic resonance (NMR). The understanding of tar composition, microcosmics, and surface property will benefit the comprehensive characterization of tar and provide very usefully information for tar removal. Received: May 24, 2015 Revised: October 26, 2015 Published: October 26, 2015 7266

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Energy & Fuels Table 1. Ultimate and Proximate Analyses (wt %) of the Model Components ultimate analysis

proximate analysis

component

Cad

Had

Nad

St,ad

Oad

Mad

Aad

Vad

FCad

Qgr,ad (kJ/kg)

cellulose PVC

40.03 39.14

5.93 4.10

0.11 0.07

0.15 0.83

47.48 38.64

6.24 0.81

0.06 16.41

88.43 66.90

5.27 15.88

16284 19211

volumetric rate of fluid flow through the capillary. The kinematic viscosity is

2. EXPERIMENTAL SECTION 2.1. Materials and Pyrolysis Setups. Powder cellulose with a size below 125 μm was purchased from Henan province. Raw PVC materials obtained from Zhejiang province were sieved into particles of size below 300 μm. Before the test, the materials were oven-dried at 105 °C for 6 h to remove moisture contents. The ultimate and proximate analyses are listed in Table 1. The pyrolysis experiments were carried out in a horizontal quartz tube reactor of 700 mm in length and 36 mm in inner diameter. The reactor was fixed in an electrically heated furnace. Nitrogen flow at a rate of 200 mL/min was used to keep the inert atmosphere and to avoid the back flow of the gas. For each experimental run, the reactor tube was heated to the set temperatures of 400, 500, and 600 °C and maintained for 10 min. About 5 g of sample was used for each test. The outlet of the reactor was connected to a round-bottomed flask with a reflux condenser. The reaction products are separated into gases, liquid, and residual coke. Tar (with a molecular weight larger than benzene)3 was extracted from the liquid products collected in the flask. Each test was carried out at least 3 times to eliminate measurement uncertainties. 2.2. Tar Characterization. GC−MS analysis, NMR spectra analysis, and the measurement of surface tension, kinematic viscosity, contact angle, and ζ potential of pyrolysis tar were carried out to characterize tar comprehensively. Tar composition was measured using a GC−MS analyzer (Trace GC, ISQ MS, Thermo Scientific Co.) equipped with a TR-5MS capillary column (30 m length, 0.25 mm inner diameter, and 0.25 μm film thickness). The injector and transfer line were set at 270 and 250 °C, respectively. The oven temperature was held at 60 °C for 5 min, increased to 270 °C with a rate of 15 °C/min, and then held at 270 °C for 10 min. In the mass spectrometer, an electron ionization (EI) energy was used for ionization mode. The ion source temperature was maintained at 200 °C. The volume of each injection was 0.2 μL, and the split ratio was 200:1. As we know, tar is a complex compound with many heavy components. In addition to widely used GC−MS and Fourier transform infrared (FTIR) spectroscopy,18,19 NMR has been found useful for identifying and quantifying structural features in complex mixtures beyond the measurement range of traditional GC.20 It can identify molecular chemical structures and their connections with other molecules according to the chemical shift values.21 In this paper, the 1H NMR (500 MHz) and 13C NMR (500 MHz) spectra were obtained by a BRUKER AVANCE III 500 superconducting NMR equipped with a 5 mm PABBO BB-1H/D Z-GRD probe. To improve the solubility of heavy tar components, CD4O and CDCl3 were used as the solvent for tar derived from cellulose and for tar derived from PVC and the mixture, respectively. Tetramethylsilane (TMS) used as an internal reference was added in CDCl3. During analysis, the sample concentrations were set at 0.1 g/mL for 1H NMR and 0.8 g/mL for 13C NMR. The surface tension of tar samples was determined by the pendant drop method22,23 on a JC2000D drop shape analyzer (Zhongchen Digital Technical Apparatus Co., Ltd., Shanghai, China). All tests were carried out under ambient conditions. The viscosity η of the collected tar sample can be deduced from the capillary mathematical models with Poiseuille’s equation24−26 as

υ=

πR p 8QL

(2)

where t is the time cost for fluid to flow through, V is the volume of fluid that flows through the capillary, h is the height of the fluid, and g is the gravity. When g, h, V, L, R, and the temperature are kept constant, the kinematic viscosity is proportion to time t.24 In this paper, the capillary tubes with a length of 200 mm and an inner diameter of 1.2 mm were used to ensure that the fluid can flow out slow enough. Before the test, the inner wall of the capillary was wetted with the fluid first. The tar samples and reference sample (99.7% pure ethanol) whose viscosity can be found in the chemical database flew through the same section of capillary tubes freely, and the time cost was recorded. To reduce the test uncertainties, each test was repeated at least 3 times until the result variation was less than 0.5%. The contact angle is one of the most common methods to measure the wettability of a surface or material. To obtain the contact angle variation, the volume of the drop was kept at 3 μL.27 The mutative shapes of tar were recorded every 40 ms by a JC2000D drop shape analyzer. Then, the contact angles were obtained via five-point fitting of the Young−Laplace equation. The volume of the tar sample was kept at 3 μL in each run, and all tests were carried out under the same ambient conditions repeated at least 3 times. The variation tendency of the contact angle for tar derived from cellulose at 400 °C is showed in Figure 1. As we can notice, the contact angle was decreasing quickly after dripped on the substrate.

Figure 1. Variation tendency of the contact angle of tar derived from cellulose at 400 °C. Previous research has declared that more than 99% dust and 40− 70% tar removal can be obtained by the electrostatic precipitation (ESP) for the updraft gasifier at Harboore, the downdraft gasifier at Wiener Neustadt, and the circulating fluidized-bed gasifier at ECN.28 Their test indicated that the efficiency of tar removal was strongly associated with its charge property or ζ potential, which was a natural parameter to describe electrochemical properties, such as electrode

4

η=

πR4gh η = t ρ 8VL

(1)

where R is the radius of the capillary, L is the length of the capillary, p is the pressure difference at the ends of the capillary, and Q is the 7267

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Energy & Fuels capacitance29 and surface charge.30 In this paper, ζ potential of tar samples was measured on a JS94H microelectrophoresis apparatus (Zhongchen Digital Technical Apparatus Co., Ltd., Shanghai, China). The measurements for a certain tar solution were repeated 5 times to eliminate measurement uncertainties.

Table 3. Relative Content of Different Substance Groups in Pyrolysis Tar tar composition (%) feedstock

3. RESULTS AND DISCUSSION 3.1. Tar Yield under Different Temperatures. Figure 2 shows optical microscopic images of the original tar samples.

cellulose

PVC

mixture

phenols and derivatives

miscellaneous HCs

400 500 600 400 500 600 400 500 600

0.8 1.2 3.0 12.5 18.3 32.8 7.4 24.6 43.0

1.7 1.3 10.9 45.8 41.6 30.4 50.4 32.1 20.8

1.1 2.4 16.5 0 0.7 1.3 0 3.0 4.5

96.3 95.1 69.7 41.7 39.3 35.5 42.2 40.3 31.7

(3)

where Ptar is the weight of tar generated from 1.0 g of feedstock (g/g), Mtar is the weight of tar collected (g), and Md is the mass of feeding material on a dry basis (g). As shown in Figure 3, when the temperature rose from 400 to 600 °C, the tar yield of cellulose, PVC, and their mixture decreased from 0.48, 0.10, and 0.15 g/g to 0.38, 0.05, and 0.11 g/g with a reduction rate of 20, 54, and 28%, respectively. This phenomenon was also reported by Wanignon, who found that the tar yield decreased from 50.34 to 30.38% when the pyrolysis temperature increased from 450 to 750 °C.31 If the feeding material was a mixture of cellulose and PVC, the tar yield was half of that obtained by linear combination, indicating that the co-pyrolysis of cellulose and PVC has the potential to reduce the tar yield. The caloric value of tar was also measured by a C6000 calorimeter. As the pyrolysis temperature rises from 400 to 600 °C, the caloric value of tar derived from the mixture decreased first and then increased. One possible reason is that the decrease is caused by the oxidation of tar, while the increase resulted from carbonization as more heavy tar is formed with the increase of the pyrolysis temperature. Moreover, when cellulose was pyrolyzed with PVC, the heating value of the produced tar was only half or less than the linear calculated heating value of the tar derived from pure cellulose and PVC. Matsuzawa et al.13 have reported that the addition of PVC caused the degradation of cellulose at a lower temperature and increased char compared to pure PVC and cellulose. Thus, there is less organic carbon in tar derived from the mixture. This may account for the obvious decrease of the caloric value for tar derived from the mixture. 3.2. Composition of Tar. To better understand tar composition and the effects of the temperature on the composition, this paper categorized tar components into four

Table 2. Groups of Tar Composition

BTEX phenols and its derivatives miscellaneous HCs

BTEX

Ptar = M tar /Md

Figure 3. Tar yields for all three feeding materials at different reaction temperatures and the caloric values of tar.

PAHs

PAHs

Solid particles (carbon black) and colloid substance participating in the continuous oil phase (brown and yellow) can be observed. According to these images, it can be discovered that the tar derived from cellulose contains little carbon, followed by the tar derived from PVC, which contains more larger carbon particles. The tar derived from their mixture has a much larger density of carbon black as well as the particle size. Therefore, the co-pyrolysis of cellulose and PVC may enhance the formation of carbon black obviously. The pyrolysis of cellulose, PVC, and their mixture at temperatures of 400, 500, and 600 °C was carried out to study the effect of the temperature on the tar yields, which can be calculated according to eq 314

Figure 2. Microscopic images of the tar derived from cellulose (left), PVC (middle), and their mixture (right) at different pyrolysis temperatures.

hydrocarbon groups

temperature (°C)

representative compounds naphthalene, biphenyl, acenaphtylene, acenaphtene, fluorene, anthracene, and phenanthrene benzene, toluene, ethylbenzene, and xylene isomers phenol, methylphenol, and 2,6-dimethylphenol ethers, esters, and furans

7268

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Figure 4. 1H NMR spectra of tars generated from cellulose at temperatures of 400, 500, and 600 °C.

Figure 5. 1H NMR spectra of tars generated from PVC at temperatures of 400, 500, and 600 °C.

groups according to their molecular weight and chemical properties, as shown in Table 2.32,33 The content of the substance groups is shown in Table 3. Miscellaneous hydrocarbons (HCs) represent the largest component of pyrolysis tar apart from the tar generated from the mixture at 600 °C. The relative mass fractions of miscellaneous HCs for cellulose, PVC, and their mixture decreased from 96.3, 41.7, and 42.2% to 69.7, 35.5, and 31.7%, respectively. Although miscellaneous HCs represent a majority of the tar, other substance groups, such as benzene, toluene, ethyl benzene, and xylene (BTEX), phenols and their derivatives, and PAHs still account for considerable proportions of the tar. PAHs are another major composition of tar. BTEX accounted for approximately 1−10% for cellulose. For PVC and the mixture, the proportion of BTEX decreased as the reaction temperature rose, from approximately 45−50 to 20−30%.

The relative percentage of PAHs for cellulose, PVC, and their mixture increased from 0.8, 12.5, and 7.4% at 400 °C to 3.0, 32.8, and 43.0% at 600 °C because the deoxidization and aromatization reactions of primary and secondary tar, generated at a lower temperature, were strengthened with an increasing temperature. Phenols and their derivatives account for little proportion of the tar. The same phenomenon was also observed by Yu et al.14 They reported that, at higher temperatures, the tar composition shifted toward higher molecular weight substances, such as polycyclic aromatic hydrocarbons (PAHs). For cellulose, the proportion of phenols and their derivatives was approximately 1−16%. For PVC and the mixture, phenols and their derivatives accounted for 0−5% of the tar. A notable phenomenon can be observed by comparing the tar compositions. Miscellaneous HCs are major components for cellulose, whereas PAHs, BTEX, and miscellaneous HCs account for a similar proportion for PVC 7269

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Figure 6. 1H NMR spectra of tars generated from the mixture at temperatures of 400, 500, and 600 °C.

Figure 7. 13C NMR spectra of tars generated from cellulose at temperatures of 400, 500, and 600 °C.

pyrolysis temperature increases to 600 °C, peaks of 2−2.3 and 7.5−8 ppm rise, while the proportion of peaks during 0.8−1.5 ppm declines, which indicates the increasing proportion of Hall, Halky, HCO, HCN, Hben, Hami, and Har and the reduction of Halka. As shown in Figure 6, when the mixture was pyrolyzed at 400 °C, the types of H bonds show a variety of Halka, Hall, Halky, HCO, HCN, Hben, Hami, Hvin, Har, and Hald, among which HCO, HCN, Hben, and Hami occupy a huge proportion. When the pyrolysis temperature increases to 500 °C, the types of H bonds seem to decrease, mainly forming Halky and Har, while Halky occupies the maximum proportion. When the pyrolysis temperature increases to 600 °C, the types of H bonds mainly consist of Halka, Hall, Halky, HCO, HCN, Hben, Hami, Hvin, and Har. Also, it shows a notable increase of aromatic hydrogen. Figures 7−9 illustrate the C bonds for tar derived from cellulose, PVC, and their mixture at different pyrolysis temperatures. For cellulose, the peaks between 10 and 40, 60

and the mixture. This phenomenon is a result of the interaction during co-pyrolysis. The proportion and variation tendency of tar derived from the mixture show extreme similarity with that of PVC, indicating that, when cellulose and PVC are pyrolyzed together, PVC plays a crucial effect on the formation of tar. Figure 4 shows the 1H NMR spectra for tar derived from cellulose at different pyrolysis temperatures. The peaks during 0.8−1.5, 2−2.5, 3.4−4.7, and 7.5−8.3 ppm indicate the existence of Halka, Hall, Hcl, Hbr, Halco|Heth, Hvin, and Har. The proportion of aromatic hydrogens is quite small. Figure 5 shows the composition of H bonds of tar derived from PVC at different pyrolysis temperatures. It indicates that the composition of H bonds does not change obviously with the pyrolysis temperature when it increases from 400 to 500 °C. The peaks during 0.8−1.5, 1.8−2.5, and 7−8.5 ppm indicate the existence of Halka, Hall, Halky, HCO, HCN, Hben, Hami, and Har, among which Halka occupies the largest proportion. When the 7270

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Figure 8. 13C NMR spectra of tars generated from PVC at temperatures of 400, 500, and 600 °C.

Figure 9. 13C NMR spectra of tars generated from the mixture at temperatures of 400, 500, and 600 °C.

derived from the mixture show a stronger similarity with that from PVC than that from cellulose. Therefore, when tar forms during the pyrolysis of the mixture, PVC plays a crucial effect on the types of its bonds. 3.3. Surface Tension and Viscosity of Tar. Surface tension of tar is shown in Figure 10. As we can see in this picture, the surface tension of tar increases with the pyrolysis temperature. Considering the difference of feeding materials, tar derived from PVC has the lowest surface tension, followed by tar derived from cellulose. Therefore, we know that the copyrolysis of cellulose and PVC leads to a much higher result of the surface tension of tar. On the basis of Hagen−Poiseuille’s method, the kinematic viscosity of tar was obtained, as listed in Table 4. The kinematic viscosities of tar derived from PVC and cellulose are the highest and lowest, respectively. In addition, the kinematic viscosity increases with the reaction temperature monotonously. When

and 80, 90 and 100, 150 and 160, 175 and 180, and 205 and 215 ppm are associated with C1, C2, C3, C4, Calco|Ceth, Cy, Car| Ce, Cam, Cacid|Cest, and Calde|CCO. For PVC, when the pyrolysis temperature increased to 600 °C, peaks between 170 and 220 ppm rose, while the proportion of peaks between 120 and 140 ppm declined, which indicates the increase of Cam, Cacid|Cest, and Calde|CCO and the reduction of CN and Cacid|Cest. Figure 9 shows the composition of C bonds for tar derived from the mixture. When the pyrolysis temperature increases from 400 to 500 °C, the peaks in the spectra indicate the possible existence of C1, C2, C3, C4, Calco|Ceth, Cy, CN, Cam, Car|Ce, Cacid|Cest, and Calde|CCO. When the pyrolysis temperature increases to 600 °C, the proportion of peaks during 10−40 and 110−150 ppm rises, which indicates the generation of much more C1, C2, C3, C4, Car|Ce, and CN. From the discussion above, it can be concluded that the composition and proportion of H bonds and C bonds for tar 7271

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Figure 10. Surface tension of tar derived from different reaction temperatures.

Table 4. Kinematic Viscosity of Pyrolysis Tar feeding material kinematic viscosity (cSt)

400 °C 500 °C 600 °C

cellulose

PVC

mixture

5.64 ± 0.02 7.97 ± 0.02 10.35 ± 0.03

20.37 ± 0.09 23.46 ± 0.09 34.21 ± 0.12

10.37 ± 0.05 12.80 ± 0.05 15.51 ± 0.07

the reaction temperature rises from 400 to 500 °C, the growth slope for tar derived from PVC is similar to the others. However, as the reaction temperature rises from 500 to 600 °C, the growth slope for tar derived from PVC booms obviously, which indicates a considerable increase of kinematic viscosity. The reason for this increase may be that, as the pyrolysis temperature rises, more heavy tar components, such as aliphatic and aromatic HCs, are formed. In addition, more highly viscous asphaltenes are formed with the increase of the pyrolysis temperature, especially for tar derived from PVC when the reaction temperature rises from 500 to 600 °C, which causes the rapid increase of viscosity. In comparison of the kinematic viscosity of tar derived from the mixture obtained from the experiment and calculation, the experimental value is a little lower than the calculation. Therfore, when cellulose and PVC are pyrolyzed together, there is not obvious interaction on kinematic viscosity of tar. 3.4. Contact Angle of Tar. The contact angle is one of the common ways to measure the wettability of a surface or material. Wetting refers to the study of how a liquid deposited on a solid (or liquid) substrate spreads out or the ability of liquids to form boundary surfaces with solid states. The contact angle of tar was obtained using five-point fitting of the Young−Laplace equation. The results are shown in Figure 11. As the pyrolysis temperature rises from 400 to 600 °C, the contact angle of tar derived from cellulose with glass increases first and then decreases, while the contact angle with iron shows the opposite tendency. The contact angle with iron is much higher than that with glass, which shows greater wettability with glass than iron. The contact angles are all smaller than 90°, indicating that tar derived from cellulose is a wetting liquid with glass and iron. According to Figure 11b, as the pyrolysis temperature rises from 400 to 600 °C, the contact angle of tar derived from PVC with glass decreases first and then increases, just opposite that of cellulose. However, the contact angle with iron shows a continuous decrease, which is also different from cellulose. The wettability for tar derived

Figure 11. Contact angle of tar derived from (a) cellulose, (b) PVC, and (c) mixture.

from PVC with glass and iron is similar. The contact angle of tar derived from the mixture with glass decreases monotonously, as shown in Figure 11c. The difference between the contact angle with iron and glass is ignorable. In comparison of all of these figures, when both value and variation tendency are taken into account, the contact angles derived from cellulose and PVC differ strongly from each other. 7272

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Energy & Fuels Table 5. Comparison of the Contact Angle and ζ Potential cellulose glass 400 °C 500 °C 600 °C

contact angle (deg) ζ potential contact angle (deg) ζ potential contact angle (deg) ζ potential

PVC iron

glass

15.61 ± 0.06 55.24 ± 0.16 −(9.54 ± 0.05) 18.16 ± 0.07 39.96 ± 0.14 −(7.97 ± 0.04) 15.16 ± 0.08 48.83 ± 0.19 −(9.45 ± 0.05)

mixture iron

36.25 ± 0.07 32.98 ± 0.11 −(4.41 ± 0.02) 17.07 ± 0.06 31.17 ± 0.12 −(4.06 ± 0.02) 31.52 ± 0.13 25.68 ± 0.12 −(5.23 ± 0.02)

glass

iron

34.93 ± 0.12 32.12 ± 0.11 −(4.08 ± 0.02) 22.23 ± 0.08 30.58 ± 0.11 −(4.03 ± 0.02) 14.75 ± 0.06 19.03 ± 0.07 −(3.80 ± 0.02)

neous HCs. The NMR spectra indicate that bonds of tar derived from the mixture show a stronger similarity with that of PVC, for both H bonds and C bonds. Therefore, PVC is the dominant contributor with respect to tar formation.

The contact angle of tar derived from the mixture shows great similarity with tar derived from PVC, indicating that, during the co-pyrolysis of cellulose and PVC, PVC plays a dominant role on the wettability property of tar. This may be caused by the composition of tar. According to the previous discussion, tars derived from the mixture and PVC own a similar composition as well as H and C bonds, which results in the similarity of wettability. Considering different solid materials, the contact angle of tar derived from cellulose with glass is smaller than that of the others, while that with iron is larger than that with the other two, which indicates that tar derived from cellulose occupies higher wettability with glass and poor wettability with iron than the other two kinds of tar. Meanwhile, tar generated from cellulose easily adheres to glass and is cleared away from iron. However, tar generated from PVC and the mixture owns the opposite characteristic. These results can help to guide the selecting of material of ESP and downstream facilities. As Table 5 shows, ζ potential values of tar derived from cellulose, PVC, and their mixture are all negative and the contact angle of tar with iron is proportional with the ζ potential. When solid contacts with liquid, the electron cloud in the solid metal can stretch to the solvent molecular layer, which is close to the solid interface to a certain extent. Consequently, the dipole of solvent molecules or molecules without charge will face toward the surface of the metal. Positive ions that are easily hydrated tend to stay in the media, while negative ions that are not easily hydrated tend to be attracted to the surface.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-571-87953125. Fax: +86-571-87952438. Email: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Acknowledgment is gratefully extended to the National Basic Research Program of China 973 Program (Grant 2011CB201500), the National High Technology Research and Development Program (2012AA063505), the National Natural Science Foundation of China (51576172), the Zhejiang Provincial Natural Science Foundation of China (R14E060001), and the Program of Introducing Talents of Discipline to University (B08026) for their financial support.

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REFERENCES

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4. CONCLUSION The properties of tar derived from principle components of combustible MSW at 400, 500, and 600 °C were quantitatively studied in this paper. The tar yield, composition, microcosmics, surface tension, kinematic viscosity, contact angle, and ζ potential were discussed. The results show that, as the temperature increases, the tar yield decreases, while the surface tension and kinematic viscosity increase. Tar derived from the mixture possessed the highest surface tension of 58.6 ± 0.3, 61.4 ± 0.3, and 64.9 ± 0.3 mN/m at the pyrolysis temperatures of 400, 500, and 600 °C, respectively. The results of kinematic viscosity do not show an obvious interaction of feeding materials, while the contact angle of tar derived from the mixture shows great similarity with tar derived from PVC, indicating the occurrence of the interaction. In addition, it is noticed that the contact angle of tar with iron shows a proportional relationship with the ζ potential values, indicating a certain relationship between them. Optical microscopic images of the tar droplet have shown that co-pyrolysis enhanced the formation of carbon black and promoted the cross-linkage during the formation. The GC−MS results show that the major components of tar generated from cellulose are miscellaneous HCs, while for tar derived from PVC and their mixture, BTEX shows an almost equal majority as miscella7273

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Energy & Fuels

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DOI: 10.1021/acs.energyfuels.5b01152 Energy Fuels 2015, 29, 7266−7274