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Determination of the Effects Caused by Different Polymers on Coal Fluidity during Carbonization Using High-Temperature 1H NMR and Rheometry Miguel Castro Díaz, Lucky Edecki, Karen M. Steel, John W. Patrick, and Colin E. Snape* Nottingham Fuel and Energy Centre, School of Chemical, EnVironmental and Mining Engineering, Nottingham UniVersity, Nottingham NG7 2RD, United Kingdom ReceiVed August 2, 2007. ReVised Manuscript ReceiVed October 5, 2007
The effects of blending polyethylene (PE), polystyrene (PS), poly(ethyleneterephthalate) (PET), a flexible polyurethane (FPU), and a car shredded fluff waste (CSF) on fluidity development of a bituminous coal during carbonization have been studied by means of high-torque, small-amplitude controlled-strain rheometry and in situ high-temperature 1H NMR spectroscopy. The most detrimental effects were caused by PET and PS, which completely destroyed the fluidity of the coal. The CSF had a deleterious effect on coal fluidity similar to that of PET, although the deleterious effect on the viscoelastic properties of the coal were less pronounced than those of PET and PS. On the contrary, the addition of 10 wt % PE caused a slight reduction in the concentration of fluid hydrogen and an increase in the minimum complex viscosity, and the addition of 10 wt % FPU reduced the concentration of fluid hydrogen without changing the viscoelastic properties of the coal. Although these results suggest that these two plastics could potentially be used as additives in coking blends without compromising coke porosity, it was found that the semicoke strengths were reduced by adding 2 wt % FPU and 5 wt % PE. Therefore, it is unlikely that more than 2 wt % of a plastic waste could be added to a coal blend without deterioration in coke quality.
Introduction Since the consumption of plastics is on the increase, the effective recycling processes for plastic waste represent a major challenge in the protection of the environment and natural resources. According to Warmington,1 plastic wastes constituted 18 million tons of the 2.8 billion tons of total wastes in Western Europe in 1996. Disposing of wastes in a landfill is not a solution, essentially because, apart from it being increasingly difficult to find suitable places for building technically adequate landfills, it is still not well established what the long-term effects of their degradation could be.2 Incineration of plastic waste to produce heat may be a possibility, but its organic content would be totally destroyed. However, the increasing incineration capacity for polymer waste streams will raise environmental concerns, particularly the possibility of forming dioxins from chlorinated polymers and the fate of heavy metals.3 Also, while the legally binding Kyoto protocol requires the U.K. to reduce its greenhouse gas emissions by 12.5% by 2008–2010, the U.K. domestic goal of reducing carbon dioxide emissions by 20% by 20104 is also an enduring target. An EU directive has set * Corresponding author. Address: Nottingham Fuel and Energy Centre, SChEME, Nottingham University, University Park, Nottingham NG7 2RD, U.K. Phone: +44-115-9514166. Fax: +44-115-9514115. E-mail:
[email protected]. (1) Warmington, A. DSD Releases Plastics RecoVery Figures for Western Europe; Harriman Chemsult Ltd: 1996, 1997; Vol. 42, pp 12– 16. (2) Pinto, F.; Casta, P.; Gulyurtlu, I.; Cabrita, I. J. Anal. Appl. Pyrolysis 1999, 51, 39–55. (3) Uzumkesici, E. S.; Casal-Banciella, M. D.; McRae, C.; Snape, C. E.; Taylor, D. Fuel 1999, 78, 1697–1702. (4) DEFRA, Department for Environmental, Food and Rural Affairs Waste Strategy 2000 for England and Wales, Part 11, Chapter 5, www. defra.gov.uk/environment/waste/strategy/cm4693/index.htm.
the minimum recovery and recycling targets of polymer wastes as 50 and 25 wt %, respectively, which is to be met within the next few years.5 These stringent targets create the need for a number of effective alternative strategies. Mechanical recycling is clearly simple and has a low energy cost, but it is only applicable to thermoplastics (e.g., polythene in bottles and garbage bags). The thermal cracking in the absence of oxygen (or pyrolysis) of polymer waste back to monomers and other chemical feedstocks is currently perceived as being an environmentally desirable option for recycling. However, the scale of pyrolysis units will be limited by the high transport costs for low-density polymer wastes and, even after size reduction and pelletization, such plants will require significant subsidies to be economically viable.3 This scenario presents a major opportunity for the coke production process. The use of additives either from coal or petroleum precursors to partially substitute coal in coke making and to improve the properties of the coal blends has been widely investigated by various authors.6–10 This has given rise to the extensive literature available concerning the influence of potential additives (pitches, charcoal, delayed and calcined petroleum coke, coal-tar, breeze coke, anthracite, oil, etc.) on the thermoplastic properties of coal (5) Plast. Rubber Wkly. Issues for Jan 7 and 14, 1994. (6) Mochida, I.; Matsuoka, H.; Korai, Y.; Fujitsu, H.; Takeshita, K. Fuel 1982, 61, 587–594. (7) Swietlik, U.; Gryglewicz, G.; Machnikowska, H.; Mechnikouski, J.; Barriocanal, C.; Alvarez, R.; Díez, M. A. J. Anal. Appl. Pyrolysis 1999, 52, 15–31. (8) Menendez, J. A.; Pis, J. J.; Álvarez, R.; Barriocanal, C.; Canga, C. S.; Díez, M. A. Energy Fuels 1997, 11, 379–384. (9) Barriocanal, C.; Álvarez, R.; Canga, C. S.; Díez, M. A. Energy Fuels 1998, 12, 981–989. (10) Díez, M. A.; Dominguez, A.; Barriocanal, C.; Álvarez, R.; Blanco, C. G.; Casal, M. D.; Canga, C. S. J. Chromatogr., A 1998, 823, 527–536.
10.1021/ef7004628 CCC: $40.75 2008 American Chemical Society Published on Web 11/27/2007
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and the structure and physical properties of the resultant coke.11 However, interest in using plastic wastes in coking blends and studies of the effect of adding plastics to coking blends on the properties of the product coke are relatively recent.12–16 Since the development of coal fluidity in high-temperature carbonization is a prerequisite for the production of graphitizable carbon material, various analytical techniques such as Gieseler plastometry, dilatometry, optical microscopy, proton magnetic resonance thermal analysis (PRMTA), and thermogravimetric analysis have been used to elucidate fluidity development in coals and blends during carbonization. Nomura et al.14 investigated the effects of five different plastics on the coking properties of four coals. Polyethylene, polypropylene, and polyvinylchloride had little effect on fluidity or coke strength. However, polystyrene and polyethyleneterepthalate inhibited fluidity and decreased coke strength. Vivero et al.11 using Gieseler plastometry have also confirmed the detrimental effects of plastics on coal fluidity development. Due to the fact that Gieseler plastometry or dilatometry requires complex and assumption-rich modeling to predict the behavior of blends even in the absence of interaction,17,18 other more reliable and efficient methods have been used. In situ broadline 1H NMR has proved to be a highly successful technique for investigating the molecular motion in coals and pitches during carbonization.19–26 After the technique was first used by Sanada et al. in the late 1970s to identify the fluid materials from coal and pitch at the early stages of carbonization,19 Sakurovs and co-workers20–23 quantified both the rigid and fluid phases using a benchtop spectrometer from the early 1980s onward and have referred to their approach as proton magnetic resonance thermal analysis (PMRTA). They have used mainly the empirical parameter M2T16, corresponding to the second moment integration limited at a width of 16 kHz, for gauging the reductions in fluidity as a function of oxidation and solvent extraction and the increase due to pitch/model compound addition. Other authors have deconvoluted the 1H NMR spectra into Lorentzian and Gaussian distribution functions that represent (11) Vivero, L.; Barriocanal, C.; Álvarez, R.; Díez, M. A. J. Anal. Appl. Pyrolysis 2005, 74, 327–336. (12) Sakurovs, R. Fuel 2003, 82, 1911–1916. (13) Díez, M. A.; Álvarez, R.; Gayo, F.; Barriocanal, C.; Moinelo, S. R. J. Chromatogr., A 2002, 945, 161–172. (14) Nomura, S.; Kato, K.; Nakagawa, T.; Komaki, I. Fuel 2003, 82, 1775–1782. (15) Dominguez, A.; Blanco, C. G.; Barriocanal, C.; Álvarez, R.; Díez, M. A. J. Chromatogr., A 2001, 918, 135–144. (16) Díez, M. A.; Barriocanal, C.; Álvarez, R. Energy Fuels 2005, 19, 2304–2316. (17) Hashimoto, K.; Honma, M.; Hanaoka, K.; Igawa, K.; Sorimachi, K. Proceedings of the Third International Cokemaking Congress, Gent, Belgium, 1996. (18) Sakurovs, R.; Lynch, L. J.; Maher, T. P. Fuel Process. Technol. 1994, 37, 255–269. (19) Miyazawa, K.; Yokono, T.; Sanada, Y. Carbon 1979, 17, 223– 225. (20) Lynch, L. J.; Webster, D. S.; Sakurovs, R.; Barton, W. A.; Maher, T. P. Fuel 1988, 67, 579–583. (21) Clemens, A. H.; Matheson, T. W.; Lynch, L. J.; Sakurovs, R. Fuel 1989, 68, 1162–1167. (22) Sakurovs, R.; Lynch, L. J. Fuel 1993, 72, 743–749. (23) Lynch, L. J.; Sakurovs, R.; Webster, D. S.; Redlich, P. J. Fuel 1988, 67, 1036–1041. (24) Maroto-Valer, M. M.; Andresen, J. M.; Snape, C. E. Energy Fuels 1997, 11, 236–244. (25) Maroto-Valer, M. M.; Andresen, J. M.; Snape, C. E. Fuel 1997, 76, 1301–1308. (26) Maroto-Valer, M. M.; Andresen, J. M.; Snape, C. E. Fuel 1997, 77, 921–926.
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the fluid and rigid components in the sample, respectively.24–28 The fraction of fluid phase can be calculated from the areas of the spectrum peaks. The peak width of the Lorentzian distribution function is inversely proportional to the spin–spin relaxation time (T2L), which is a measure of the changes in mobility of the fluid phase. This behavior has been monitored as a function of temperature.29 Under most conditions, softening coals show characteristics of both elastic and viscous materials and can be treated as a viscoelastic material. To characterize such materials accurately, both elastic and viscous responses must be measured.30 In this manner, dynamic mechanical analysis is a unique powerful method capable of measuring both properties simultaneously. A high-torque rheometer has been used to measure the linear viscoelastic properties of coal samples. The measurement involves placing a sample between two parallel plates. One plate is connected to a motor which moves sinusoidally. The variable parameters in the measurement are the frequency (ω, number of oscillations per second) and the strain (γ, angular displacement from the zero position). The sample stress response to this oscillatory deformation (τ) is measured by a transducer connected to the other plate. The elastic or storage modulus (G′) is proportional to the elastic energy that is stored and recovered, and the viscous or loss modulus (G″) is proportional to the energy dissipated on flow. G′ and G″ are given by eqs 1 and 2, respectively.31 G′ )
τ0 γ0 cos δ
(1)
G′′ )
τ0 γ0 sin δ
(2)
τ0 is the maximum stress response measured in the sinusoidal motion, and γ0 is the maximum applied strain in the sinusoidal motion. The phase shift or phase angle (δ) provides information about the viscous and elastic character of the sample. If the sample is an ideal elastic material, the strain and stress curves over time are in phase, and if the sample is an ideal viscous material, the strain and stress curves over time are out of phase by 90°. Other important parameters are the complex viscosity (η*) and tan δ. The complex viscosity can be calculated using eq 3, and tan δ is given by eq 4. |η*| )
√(G′)2 + (G′′)2
(3) ω G′′ (4) tan δ ) G′ Where tan δ > 1, viscous properties dominate, and where tan δ < 1, elastic properties dominate. The objective of this study is to elucidate and compare the influence of different plastics in coking blends on fluidity development using in situ broadline high-temperature 1H NMR and rheometry. Experimental Section The bituminous coal (K6) used throughout this work and the light fraction of a plastic waste in the form of shredder fluff that is (27) Steel, K. M.; Díaz, M. C.; Patrick, J. W.; Snape, C. E. Energy Fuels 2004, 18, 1250–1256. (28) Díaz, M. C.; Steel, K. M.; Drage, T. C.; Patrick, J. W.; Snape, C. E. Energy Fuels 2005, 19, 2423–2431. (29) Snape, C. E.; Martin, S. C. Prepr.sAm. Chem. Soc., DiV. Fuel Chem. 2000, 45, 205–210. (30) Norinaga, K.; Iino, M. Energy Fuels 2000, 14, 929–935. (31) Steffe, J. F. Rheological Methods in Food Processing; Freeman Press: East Lansing, MI, 1996.
Coal Fluidity during Carbonization
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Table 1. Proximate and Ultimate Composition and Gieseler Data of Bituminous Coal K6 ultimate analysisa coal
C
H
N
proximate analysisb Oc
K6 82.8 4.4 1.5 11.3
Gieseler
volatile Tsoft max fluidity Tresol moisture ash matter (°C) (ddpm) (°C) 0.5
a
Expressed as wt % dmmf. difference.
7.7 b
20.5
435
199
Expressed as wt %.
c
505
Calculated by
Table 2. Ultimate Composition of Car Shredded Fluffs (CSFs) ultimate analysisa CSF
C
H
N
Ob
Cl
other elementsc
52.2
6.8
2.3
33.3
2.9
2.5
a
Expressed as wt % db. b Calculated by difference. c Each element accounts for less than 0.5 wt % in a dry basis. These elements comprise F, S, Pb, Cr, K, Cu, Mb, Na, Ni, Hg, and Zn.
used in car interiors were supplied by Voestalpine Stahl GmbH. The ultimate and proximate compositions of the coal are shown in Table 1. The car shredder fluff is basically a shredded polyurethane waste, and its ultimate composition is shown in Table 2. Polyethylene (medium density PE), polystyrene (PS), and poly(ethyleneterephthalate) (PET) were purchased from Sigma-Aldrich U.K. The flexible polyurethane (FPU) was supplied by Huntsman Polyurethanes. The particle sizes used for the coal and plastics were 2.8 mm percent index.
Results Thermogravimetric Analysis. Figures 1 and 2 show, respectively, the changes in weight and rate of weight loss with temperature for the individual coal and plastics. Figure 1 shows that the temperature at which the polymers start to be degraded increases in the order FPU < CSF < PS < PET < PE, and Figure 2 shows that the maximum value in the rate of weight loss for the plastics increases in the order CSF < FPU < PET < PE < PS. The residues remaining at 650 °C for CSF, PET, and FPU are, respectively, 34.4, 15.4, and 7.6 wt % within
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Figure 3. Weight loss with temperature for coal K6, polyethylene, polystyrene, poly(ethyleneterephthalate), the flexible polyurethane, and the car shredded fluffs and the calculated experimental trends for the 10 wt % mixtures.
experimental error. On the other hand, the residues at the same temperature for PS (0.4 wt %) and PE (0.3 wt %) can be considered negligible. These results indicate that the formation of polymer-derived chars, and therefore its contribution to the semicoke products from their cocarbonization with coal, varies significantly with different polymers. Further, the degradation or decomposition temperature of all the plastics occurs at a lower temperature than the coal softening temperature, i.e., below the temperature range where coal shows thermoplasticity. Such different thermal behavior between the coal and the polymers as well as the different chemical composition of the volatiles released during pyrolysis of the polymers are expected to influence fluidity development of the coal to different extents. Figure 3 shows the weight loss with temperature of mixtures of coal K6 with 10 wt % PE, PS, PET, FPU, and CSF. Moreover, the trends for the coal and the polymer alone together with the calculated weight loss value for the coal/plastic mixtures based on the formula given by Nomura et al.14 are also plotted for comparison purposes. The trends of the measured and calculated weights of the mixtures with PE and PET are very similar in the whole temperature range studied. However, the measured weights for the K6/PS and K6/FPU mixtures are greater than the calculated ones in the temperature ranges 360–450 and 250–500 °C, respectively. Also, the measured
weight for the K6/CSF mixture is lower than the calculated one in the temperature range 450–650 °C. This suggests that there is a strong chemical interaction between the coal constituents and the decomposition products evolving from PS and FPU during carbonization, which might influence the physical and structural properties of the resultant semicoke produced from the coprocessing of these polymers. In addition, the results obtained with the mixtures containing PE and PS are in good agreement with those of Nomura et al.14 Rheometry. Figure 4 shows the viscoelastic properties of coal K6 as a function of temperature at a heating rate of 3 °C min-1. At temperatures less than 425 °C, the storage modulus (G′) and loss modulus (G′′) remain constant, indicating that there is relatively little change taking place in the sample. As G′ > G′′ or tan δ < 1, elastic properties dominate. Then, G′ shows a maximum near the softening temperature (ca. 450 °C). As the temperature exceeds 450 °C, G′ and G′′ decrease rapidly while tan δ increases, indicating that the sample is softening and becoming less elastic. At approximately 465 °C, tan δ exceeds 1, indicating that viscous forces dominate over elastic forces and the material behaves more like a fluid. The maximum fluidity or minimum complex viscosity (η*) occurs at approximately 475 °C and reaches a value of 1.2 × 105 Pa · s. As the temperature increases above 475 °C, elasticity increases and
Coal Fluidity during Carbonization
Figure 4. Complex viscosity, loss and storage moduli, and tan δ for coal K6 as a function of temperature.
the sample begins to resolidify. Tan δ drops below 1 at 490 °C, and resolidification is almost complete at 525 °C. Figure 5 shows the complex viscosity of the coal and the mixtures with PE, PS, PET, FPU, and CSF with different blending ratios (2, 5, and 10 wt %) as a function of temperature. By adding 2 wt % PE, there is no evidence that the coal and PE interact in any way that affects the viscoelastic behavior of the coal. However, further addition of PE leads to a slight increase in complex viscosity (from 1.2 × 105 to ca. 1.8 × 105 Pa · s). On the contrary, the addition of just 2 wt % PS not only leads to an increase in complex viscosity from approximately 1.2 × 105 to 2.3 × 105 Pa · s but also leads to a shift in the temperature of minimum complex viscosity to a higher temperature (i.e., 475 °C cf. 483 °C). Further addition of PS leads to the complete destruction of coal fluidity, which is consistent with the findings previously reported by other authors.14 A similar effect to that caused by PS is found with PET, i.e., increase in complex viscosity and increase in the temperature of maximum fluidity. However, the increase in complex viscosity or loss in fluidity is much more pronounced with the addition of 2 wt % PET than with 2 wt % PS (from approximately 1.2 × 105 to 3.3 × 105 Pa · s). The addition of flexible polyurethane has little or no effect on the viscoelastic properties of the coal, even at 10 wt % addition to the blend. There is an increase in complex viscosity from 1.2 × 105 to 1.6 × 105 Pa · s when adding 2 wt % CSF, and a further increase to 5 wt % CSF causes an increase in the minimum complex viscosity (i.e., 3 × 105 Pa · s) without altering the temperature of maximum fluidity. Increasing the amount of the CSF from 5 to 10 wt % does not affect the minimum value in complex viscosity but lowers the temperature of maximum fluidity by approximately 10–15 °C and expands the thermoplastic temperature range by reducing the softening temperature from 450 to 425 °C. A comparison of all of these figures indicates that the addition of 10 wt % plastics causes a reduction in fluidity in the order FPU < PE < CSF < PS < PET. Figure 6 shows the elastic and viscous moduli as a function of temperature for the blends of coal K6 with 2, 5, and 10 wt % plastics. All of the blends with PE and FPU and the blend with 2 wt % CSF show G′/G′′ crossover in the thermoplastic region. An increase in the concentration of CSF causes a reduction in fluidity and the mixture behaves as a viscoelastic solid (i.e., tan δ < 1), as for all of the blends with PS and PET. The G′/G′′ crossover temperature during resolidification does not change with plastic and plastic concentration (ca. 485 °C). The plots show that G′′ has two maxima at approximately 460 and 490 °C. As the fluidity is reduced, the peak at 460 °C is reduced to a greater extent than the peak at 490 °C (e.g., for K6 + FPU). If the fluidity is further reduced, the two peaks
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merge into one peak at 485 °C (e.g., K6 + CSF), and the trend for G′′ becomes linear with temperature when the fluidity is completely destroyed (e.g., K6 + PET). On the other hand, G′ has a minimum at the temperature of maximum fluidity of the coal (i.e., 475 °C), which increases as fluidity is reduced and disappears when the fluidity is completely destroyed. These results show that there is a sequential effect on G′ and G′′ with fluidity reduction, and could be represented by the plots of the blends with PE followed by those of the blends with PET. Figure 7 shows tan δ of coal K6 and the mixtures with 10 wt % PE, PS, PET, FPU, and CSF as a function of temperature. Between 465 and 490 °C, tan δ values for coal K6 and for the mixtures with PE and FPU are greater than 1 while tan δ values for PS, PET, and CSF are less than 1 for the whole temperature range. Hence, it can be inferred that viscous properties dominate at one stage of the carbonization with mixtures containing up to 10 wt % PE and FPU. The reverse situation happens with 10 wt % PS, PET, and CSF, where elastic properties dominate throughout the carbonization process. 1H NMR Spectroscopy. Figure 8 shows the percentage of fluid phase as a function of carbonization temperature for the coal and the mixtures with 10 wt % PE, PS, PET, FPU, and CSF. At 350 °C, the percentage of fluid phase increases in the order of coal K6 < FPU (10 wt %) < CSF (10 wt %) ∼ PE (10 wt %) ∼ PET (10 wt %) < PS (10 wt %). While the percentage of fluid hydrogen increases steadily with temperature for coal K6 and the blends with PE and FPU, the trend for the blend with PS decreases steeply until it reaches 400 °C and then increases until it reaches a maximum fluidity at 440 °C. The higher fluid phase concentration present in the blend with PS in comparison to the other mixtures at low temperatures (
