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Recycling of Rubber Tires in Electric Arc Furnace Steelmaking: Simultaneous Combustion of Metallurgical Coke and Rubber Tyres Blends Magdalena Zaharia,*,† Veena Sahajwalla,† Byong-Chul Kim,† Rita Khanna,† N. Saha-Chaudhury,† Paul O’Kane,‡ Jonathan Dicker,‡ Catherine Skidmore,‡ and David Knights‡ School of Materials Science and Engineering, The UniVersity of New South Wales, Sydney, NSW 2052 Australia, and Onesteel, Rooty Hill, Sydney, Australia ReceiVed December 10, 2008. ReVised Manuscript ReceiVed February 15, 2009
The present study investigates the effect of addition of waste rubber tires on the combustion behavior of its blends with coke for carbon injection in electric arc furnace steelmaking. Waste rubber tires were mixed in different proportions with metallurgical coke (MC) (10:90, 20:80, 30:70) for combustion and pyrolysis at 1473 K in a drop tube furnace (DTF) and thermogravimetric analyzer (TGA), respectively. Under experimental conditions most of the rubber blends indicated higher combustion efficiencies compared to those of the constituent coke. In the early stage of combustion the weight loss rate of the blends is much faster compared to that of the raw coke due to the higher volatile yield of rubber. The presence of rubber in the blends may have had an impact upon the structure during the release and combustion of their high volatile matter (VM) and hence increased char burnout. Measurements of micropore surface area and bulk density of the chars collected after combustion support the higher combustion efficiency of the blends in comparison to coke alone. The surface morphology of the 30% rubber blend revealed pores in the residual char that might be attributed to volatile evolution during high temperature reaction in oxygen atmosphere. Physical properties and VM appear to have a major effect upon the measured combustion efficiency of rubber blends. The study demonstrates that waste rubber tires can be successfully co-injected with metallurgical coke in electric arc furnace steelmaking process to provide additional energy from combustion.
Introduction The steel industry is the largest energy-consuming industry in the world, responsible for approximately 5% of worldwide energy consumption.1 Economic and population growth have caused the demand for energy to increase dramatically leading to pressure on the steel industry to develop innovative technologies to reduce consumption of fossil fuels. In the electric arc furnace (EAF), predominantly scrap material is being charged saving the consumption of virgin raw materials and energy. On the basis of the current industrial practice, the energy input consists of approximately 70% electrical energy, while the remaining 30% is provided by the chemical energy, derived from oxy-fuel combustion and oxidation of carbon and other chemical reactions.2 Metallurgical coke is one of the sources of carbon in EAF steelmaking. The amount of MC injected in the electric arc furnace varies with the melt shop practice and equipment. However, coke consumption needs to be reduced due to problems associated with green house gas (GHG) emissions, and this can be achieved by replacing it with * Corresponding author. Phone: 61 2 9385 6597. Fax: 61 2 9385 5956. E-mail:
[email protected]. † The University of New South Wales. ‡ Onesteel. (1) Babich, A. I.; Senk, D.; Gudenau, H. W. Proceedings of the 3rd International Conference on Science and Technology of Ironmaking; Du¨sseldorf: Germany, 2003, S. 89-94. (2) Stubbles, J. R. In Energy Use in US Steel Industry: Historical PerspectiVe and Future Opportunities; U.S. Department of Energy: Washington, DC, 2000.
other sources of carbon. The rubber in a tire accounts for approximately 30% natural rubber, which is assumed to have no net greenhouse impact due to sequestering of carbon dioxide by rubber trees.3 Rubber tires are embodied with a high amount of carbon and energy and are available at lower costs.4 Additionally, the ever increasing use of rubber tires poses serious problems in disposal due to their poor biodegradable nature. Landfills and incineration are the general practices of disposal; however, they are becoming unattractive for legislative reasons.5 The management of waste rubber tires disposal is another challenging task, especially in industrialized and developed countries. The world’s rubber consumption in 2004 was about 20 million tons, with the annual figure of 1.2 billion tires consumed in 2004, expected to further increase. There are 290 million rubber tires discarded in the U.S. every year, roughly one tire per person per year, while in Australia 20.8 million EPU (equivalent passenger unit) or 197 000 ton of tires entered the waste stream only in 2004.5 Adding to this problem, disposal releases toxic compounds in the environment, such as PAHs (polycyclic aromatic hydrocarbons), benzene, and phenol, suspected to have carcinogenic properties. Landfills are not leak(3) Atech Group. A National Approach to Waste Tyres; Commonwealth Department of Environment: Australia, 2001. (4) Ghebremeskel, G. N.; Sekinger, J. K.; Hoffpauir, J. L.; Hendrix, C. Rubber Chem. Technol. 1996, 69, 874–884. (5) Houghton, N.; Tsolakis, D. ; Preski, K.; Rockliffe, N. Economics of Tyre Recycling; Department of Environment and Heritage-RC 3765: Australia, 2004.
10.1021/ef8010788 CCC: $40.75 2009 American Chemical Society Published on Web 04/03/2009
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proof, allowing the hazardous substances released during the decomposition of the material to filter down through the site into the surrounding area, polluting water courses and affecting the living organisms.5 Rubber tires can be regarded as a potential supplementary fuel in EAF steelmaking as they possess high calorific value and low moisture content. Other beneficial properties of these polymeric materials include their high volatile matter and carbon content.6,7 The phenomena involved in the introduction of polymeric materials as a source of carbon and energy in the steelmaking industry have been extensively studied by our research group.8,9 Electric arc furnaces in the U.S. began to use scrap tires as a source of carbon and steel by introducing the whole tire into the charge bucket,10 while in Europe, tires were charged in the EAF both for energy and material recovery.11 Rubber tires are used, at a high scale, in Japan in the scrap melting converter of Nippon Steel Corporation in its Hirohata mill12 as a substitute of coal and iron scraps since 1999. Their procedure involves cutting the waste tires to smaller dimensions and charging them into the scrap melting bath along with the steel cords contained in the tire. The recovery of these waste materials in EAF steelmaking is being extensively investigated in our studies.13 High temperature tests involving blending waste rubber tires with coke in EAF steelmaking has been done with the following aims: (1) as a supplementary energy source, as the heat content of rubber ranges between 26 and 36 MJ/Kg14 which is equal to or higher than most coals, cokes, and other solid residues like wood and municipal solid wastes, (2) as a carbon source:, because the generated carbonaceous residue can replace the role of coke in promoting and controlling a stable foaming slag which is highly desirable in EAF steelmaking.9 The current paper focuses on the combustion efficiency of the automobile tire wastes in their role as partial replacement of MC in EAF steelmaking. These studies are conducted in specially designed facilities simulating conditions of combustion reactions between the injected carbon material and the provided oxidizing atmosphere in an EAF. At high temperatures, when pore diffusion and reaction control occur, char combustion can finish in an extremely short time. Neither TGA nor fixed bed rectors have the capacity to withdraw the partly reacted sample in such a time frame; therefore, the flow reactor system in which a stream of carbonaceous particle is continuously entrained and combusted into a gas atmosphere is appropriate for this study. Bench scale reactors such as DTF (drop tube furnace) and TGA (thermogravimetric analyzer) have been used to compare the combustion behavior of the injected fuels. Blends of varying compositions of MC with worn out rubber tires were tested in (6) Alvarez, R.; Clemente, C.; Gomez-Limon, D.; Diaz-Bautista, M. A.; Mastral, A. M.; Callen, M. S.; Lopez, J. M. Proceedings of the 8th International Conference on EnVironmental Science and Technologies; Lemnos Island, Greece, 2003; p CD-1. (7) Zhou, L.; Wang, Y.; Huang, Q.; Cai, J. Fuel Process. Technol. 2006, 87, 963–969. (8) Sahajwalla, V.; Rahman, Khanna, R.; Knights, D.; O’Kane, P. Proceedings of the AISTech Conference; Cleveland, OH, 2006; p CD-2. (9) Sahajwalla, V.; Hong, L. Proceedings of the AISTech Conference; Charlotte, NC, 2005; p CD-2. (10) Rubber Manufacturers Association, Scrap Tire Markets US, 2004 Edition. (11) Gorez, J. P.; Gros, B.; Birat, J. P.; Huber, J. C.; Coq, X. L. ReV. Metall./Cah. Inf. Tech. 2003, 100, 17–24. (12) Nakao, Y; Yamamoto, K. Nippon Steel Technical Report; Japan, 2002; pp 21-24. (13) Sahajwalla, V.; Zaharia, M.; Rahman, M.; Knights, D.; O’Kane, P. Proceedings of the AISTech Conference; Pittsburgh, PA, 2008; p CD-2. (14) Larsen, M. B.; Schultz, L.; Glarborg, P.; Dam-Johansen, K.; Frandsen, F.; Henriksen, U. Fuel 2006, 85, 1335–1345.
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Figure 1. Illustration of the structure of styrene-butadiene rubber (SBR) molecule.
a DTF at a temperature of 1473 K under O2 and N2 (20:80) gas mixture for studying the effect of rubber concentration in the blend. The combustion efficiency is expressed in terms of blends burnout on the basis of ash conversion. The residence time of carbonaceous particles in a DTF is often on the order of seconds.15 The combustion of most solid fuels involves two steps: thermal decomposition (pyrolysis) accompanied by physical and chemical changes and the subsequent combustion of the solid residue. A TGA was employed to study the effect of blending on the pyrolysis and combustion behavior of the carbon based materials. The analyzer is used to generate sample weight loss versus time, reflecting the thermal behavior of the initial sample and the final residue. Since it was established that pore diffusion plays an important role in the kinetic regime of combustion reactions at the studied temperature,16 it is very important to provide data on opening of pores and their development. Micropore surface area measurements were carried out for the mixtures before and after combustion to study the influence of rubber, at various levels on the combustion efficiency. XRD studies and SEM analysis were also carried out to understand the transformations due to release of volatiles, structural ordering, and physical changes taking place during the combustion of MC-rubber blends. Effect of Rubber Chemical Structures. Styrene-butadiene is a copolymer of 1,3-butadiene and styrene mixed in a 3:1 ratio. The styrene and butadiene repeating units are arranged in a random manner along the polymer chain as is shown in Figure 1. With the combustion process involving an initial devolatilization reaction, the polymeric products are expected to decompose primarily into their monomers, dimers, and trimers. When polymers are subject to heating or combustion conditions, complicated reactions such as random-chain scission, end-chain scission, chain stripping, cross-linking, and char formation would take place.17 The behavior of the thermal decomposition of rubber is generally studied by thermogravimetric analysis. Previous works on combustion of tire derived fuels have focused on different aspects with the majority of the attempts carried out to design the thermal breakdown of tire wastes.7,18 Li19 found that rubber combustion takes place in 5 temperature stages: 523-623, 633-693, and 693-753 K attributed to the combustion of volatile matter which contributed to a total weight loss of 42%, followed by the combustion of the fixed carbon at 793-873 and 923-1073 K. Conesa et.al20 have proposed a kinetic model for the thermal decomposition of tire at various heating rates of 5, 10, and 20 °C/minute for temperatures up to 973 K with mass spectroscopy validation. Under oxygen (15) Liming, L.; Sahajwalla, V.; Harris, D. Energy Fuels 2000, 14, 869– 876. (16) Smith, I. W. 19th Symposium (International) on Combustion 1982, 19 (1), 1045–1065. (17) Beyler, C. L.; Hirschler, M. M. SFPE Handbook of Fire Protection Engineering 2002, 1–110. (18) Wang, C. L.; Leung, D. Y. C. J. Anal. Appl. Pyrolysis 1998, 45, 153–169. (19) Li, X. Thermochim. Acta 2006, 441, 79–83. (20) Conesa, J. A.; Font, R.; Fullana, A.; Caballero, J. A. Fuel Process. Technol. 1998, 77 (13), 1469–1475.
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Table 1. Proximate and Ultimate Analysis of MC, Rubber, and the Corresponding Rubber Blendsa
moisture ash volatile matter fixed carbon total carbon sulfur hydrogen a
metallurgical coke
rubber tire
10% rubber
20% rubber
30% rubber
1.30 18.3 3.00 73.6 77.7 0.28 1.11
0.9 5.7 63.2 30.2 83.8 2.00 7.6
2.00 17.6 10.30 71.1 79.21 0.46 1.26
1.00 15.7 14.8 68.5 79.72 0.55 1.86
0.9 13.1 20.9 65.10 80.23 0.97 2.68
Proximate (air-dry base) (%) and ultimate analysis (dry ash free)
(%).
atmosphere, rubbers undergo devolatilization and lose weight up to 89%. They concluded that the weight loss takes place in four steps, such that in the initial stage the oil fraction decomposes, then the natural rubber is subjected to degradation, SBR decomposes, and in the final stage the remaining carbonaceous fraction is prone to combustion. Levendis and Atal21 pointed out that tire particles underwent combustion in two phases, primary volatile phase combustion which occurred with flame temperature of 2100 K and secondary volatile/char phase combustion due to the evolution of heavier polystyrene pyrolysates with simultaneous char combustion. The combustion of SB rubber studies carried out earlier22 revealed that the decomposition reaction starts with an initial bond breakage which results in the formation of butadiene and styrene followed by hydrogen liberation. The backbone continues to be hydrogenated to various extents forming low molecular weight compounds such as 4-phenyl cyclohexane, 4-vinyl cyclohexane, styrene, ethyl benzene, methyl benzene, and methyl styrene.23 Since in the early thermal degradation studies the maximum temperature used was 973 K, which is sufficient for rubbers to become lower hydrocarbons, it can be inferred that the temperature used in our study leads to further decomposition into simpler carbon sources. Experimental Section Sample Selection and Preparation. MC samples, typically used as injecting materials in the EAF, are supplied by OneSteel Sydney Mill, Australia. Table 1 illustrates the chemical properties of the samples used in this study. The proximate (air-dry base, %) and ultimate (dry ash free, %) analyses of the samples were carried out at Amdel Laboratories and Technical Services, NSW, based on Australian standards. MC samples were ground and sieved to a particle size in the range 0.45-0.47 mm to minimize the effect of particle size on experimental results. Rubber tire wastes were cut into pieces with a mean diameter of 4 mm and further cryogenically ground by immersing in liquid nitrogen to a particle size less than 1.5 mm. Finally, they were crushed to smaller sizes by using a cutting mill “Pulverisette 15” and then sieved to a particle size similar to that of MC. The samples for the experiments were prepared by mixing MC and waste tire particles in various proportions: 10% rubber and 90% MC, 20% rubber and 80% MC, and 30% rubber and 70% MC. Table 1 shows that rubber tire contains a lower amount of ash, low moisture, and a high volatile content, as compared to the coke used in this study. The hydrogen content in the rubber is obviously higher than the hydrogen present in coke, while a significant amount, equal to 2.14% sulfur, is seen in the ultimate analysis of the rubber. Sulfur is used to cross-link the polymer chains in the process of rubber manufacturing (vulcanization). With increasing (21) Levendins, Y. A.; Atal, A. Fuel 1995, 74 (11), 1570–1581. (22) Castaldi, M. J.; Kwon, E.; Weiss, B. EnViron. Eng. Sci. 2007, 24, 1160–1178. (23) Michal, J.; Mitera, J. Fire Mater. 1985, 9 (33), 111–116.
rubber content in the blend an increase in sulfur is obvious. Environmental concerns must be addressed that the combustion flue gas may contain acidic compounds such as SO2.24 A laboratory study on the combustion emissions of waste tires in an electrically heated drop tube furnace at elevated gas temperatures revealed relatively low SO2 emissions. This was explained on the basis of the higher mass ratio of volatile to fixed carbon for tires. The higher mass of volatiles released from tires readily creates reducing conditions that do not favor the formation of SO2,25 and sulfur is retained in the carbonaceous matrix. More hydrogen, on the other hand, accounts for more active sites in the rubber structure, promoting oxygen reactivity of nearby carbon atoms.26-28 The majority of the hydrogen atoms in the carbon matrix are thought to reside at the edged sites by bonding to the edged carbons. These hydrogen bond sites are more susceptible to oxygen attack when compared to typical aromatic sites in the same carbon layer due to weaker C-H bonds.26,27,29 A high amount of volatiles are present in the composition of the rubber material that leads to the formation of pores in the residue, due to the thermal process the samples are being subjected to. A strong evolution of volatile species has an endothermic effect and might deplete the dense phase of oxygen and cause hot spots.30 However, when the rubber is blended with coke, the volatile content in the blend does not reach a detrimental limit as Table 1 indicates. A linear increase is expected in the burnout (%) values with increasing rubber content in the blends. These speculations are supported by the blends of volatiles which are expected to be released when subjected to high temperature conditions and fixed oxygen content, and this could improve the burnout of the blends. Thermogravimetric Analysis (TGA). In order to monitor the weight loss of MC and its blends during pyrolysis and combustion under inert (100% N2) and oxidizing (20% O2) atmospheres, respectively, a custom-made TGA furnace was used.31 The weight loss was continuously recorded every 5 s under isothermal conditions by a computer connected to a balance. The material was placed in a high temperature resistant glass holder and sealed in the cold zone for 30 min while being purged with nitrogen gas. The sample assembly was pushed into the hot zone of the furnace where the temperature was 1473 K. N2 (100%), and a mixture of 20% O2 and 80% N2, was used to pyrolyze and combust the samples, respectively. After completion of pyrolysis and combustion tests, the sample assembly was moved back to the cold zone and later removed from the furnace. A schematic diagram of a TGA furnace employed in this study is presented in a previous paper.31 Drop Tube Furnace (DTF) Test. The combustion tests were carried out using a DTF32 which consists of a feeding system, a sampling probe, an electrically heated furnace, and a gas distribution system. The DTF is a vertical furnace provided with two type-B thermocouples as can be observed in Figure 2; the external one is used for furnace ramping while the internal one is used for an improved response during the measurements. Two gas inlets are also provided; a primary one is for N2, which helps to carry the solid particles into the reaction zone, and a secondary one for O2, which is the gaseous reactant. Three mass flow controllers are used to adjust the flow rate and composition of the gases. The optimized experimental conditions for the DTF test are given in Table 2. (24) Spliethoff, H.; Heinn, K. R. Fuel Process. Technol. 1998, 54, 189– 205. (25) Courtemanche, B; Levendis, Y. A. Fuel 1998, 77, 183–196. (26) Khan, M. R. Fuel 1987, 66, 1626–1634. (27) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221–270. (28) Essenhigh, R. H. Fundamentals of Coal Combustion. In Chemistry of Coal Utilization; Wiley: New York, 1981; 2nd Supplementary Volume, pp 1153-1312. (29) Abd El, S.; Hampartsoumian, E. Fuel 1990, 69, 1029–1036. (30) Ogada, T.; Wether, J. Fuel 1996, 75, 617–626. (31) Kim, B.-C.; Sahajwalla, V.; Gupta, S.; Kim, S.-M. Energy Fuels 2008, 22, 514–522. (32) Jacob, W.; Gupta, S.; Sahajwallla, V. Energy Fuels 2006, 20, 2557– 2563.
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Figure 3. XRD patterns of 100% rubber, 100% MC, and its mixtures with rubber in proportions of 10%, 20%, and 30%.
measure the surface area for the raw mixtures as well as for the residue after combustion. The samples were degassed at 343 K for 24 h before CO2 gas adsorption was carried out. The DubininRadushkevich (DR) micropore surface area was calculated using NovaWin 1.12 software. Bulk density measurements performed in Particle and Surface Sciences laboratories, Gosford, are also provided in the present study for a better understanding of the physical changes following the combustion reaction. At the end of the combustion experiments, the samples were mounted on a double side carbon tape and gold coated to investigate their morphology using a Hitachi S3400X scanning electron microscope operating at a voltage of 20 kV. XRD patterns of MC and its mixtures with rubber tires were obtained using a Siemens D5000 X-ray diffractometer. The XRD spectra was aquired at a step size (0.02°) over the angular range of 10-50° (2θ) and a rate of 0.5°/min at an accelerating voltage and current of 30 kV and 30 mA, respectively. Figure 2. Schematic representation of DTF used for combustion experiments. Table 2. Operating Conditions of DTF operating parameters
values/conditions
temperature particle size material injection rate combustion air composition gas flow rate residence time
1473 K 0.45-0.47 mm 0.05 g/s 20% O2; 80% N2 1.00 L/min approximately 1-2 s
Char particles collected at the bottom of DTF were subsequently used to measure the residual carbon and ash content using a LECO analyzer and a muffle furnace, respectively. The combustion efficiency was calculated using eq 1 that is at the base of the ash tracer method29 often used for this purpose33-36
n ) (1 - (A0Ci)/AiC0) × 100%
(1)
where A0 and C0 are the ash and carbon content of the feed sample (%) and Ai and Ci represent the ash and carbon content of the combusted char samples. In this study, the percentage of carbon burnout of MC after the DTF test was used to compare the effect of blending ratio of rubber with MC on their combustion efficiency under the tested conditions. Since the temperature inside the DTF was 1473 K, breakdown of polymeric chains can be ensured, and an enhancement in the combustion performance of the coke/rubber mixture is expected. Original and Char Samples Characterization. A Quantachrome high speed gas adsorption analyzer, at UNSW, was used to (33) Bailey, J. G.; Tate, A.; Diessel, C. F. K.; Wall, T. F. Fuel 1990, 69, 225–239. (34) Haas, J.; Tamura, M.; Weber, R. Fuel 2001, 80, 1317–1323. (35) Suda, T.; Takafuji, M.; Hirata, T.; Yoshino, M.; Sato, J. Proc. Combust. Inst. 2002, 29, 503–509. (36) Moghtaderi, B.; Meesri, C.; Wall, T. F. Fuel 2004, 83, 745–750.
Results and Discussion Effect of Chemical Properties. XRD patterns were taken to show the effect of rubber content on its blends with MC. The position of the peak in the angular region (2θ ) 10-35°) corresponds to the (002) graphite peak which is considered to be the average stack height. The apparent asymmetry of this peak can be very clearly noticed in Figure 3, and this was previously attributed by Ergun37 to the existence of a γ band situated just to the left side of the (002) reflection. According to Ergun,37 this γ band usually occurs in the angular range (2θ ) 16-23°), and it is associated with packing of aliphatic side chains or condensed saturated rings. The aliphatic side chains are the volatiles contained in the carbonaceous material matrix which will be transported out of the carbon particle during the combustion process. From a qualitative point of view, 100% MC presents a well-defined γ band with a significant amount of highly disordered material. This disordered material is referred to as amorphous carbon by which is included all the nonaromatic carbon38 species and is well-known to influence combustion performance.39 According to visual observations, a much broader diffused spectrum is observed in Figure 3 demonstrating the existence of the above-mentioned γ band with a more amorphous structure developed by the 100% rubber sample, whereas the prepared blends showed peaks at the same angular values. However, due to the limited scope of this study, carbon structure parameters are not included in this paper. During combustion, all carbon atoms are exposed to oxidizing gas and the amorphous carbon which is more reactive is liberated as volatiles. (37) Ergun, S.; Tiensuu, V. H. Fuel 1959, 38, 64–78. (38) Robertson, J.; O’Reilly, E. P. Phys. ReV. B 1987, 35, 2946–2957. (39) Liming, L.; Sahajwalla, V.; Harris, D. Met. Mater. Trans. B 2001, 32, 811–820.
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Figure 4. Comparison of residual mass in TGA with time for 100% MC and its mixtures with rubber content at 1473 K combustion temperature.
Figure 5. (a) Weight loss curves of MC and its mixtures with rubber and (b) comparison of total weight loss of the mixtures in the final stage of pyrolysis.
Figure 6. Effect of blending MC with varying rubber contents on the combustion of the blends at 1473 K in the presence of 20% O2.
Effect of Blending on Pyrolysis and Combustion. Combustion of most carbon materials involves an initial thermal decomposition, where volatiles are released in presence of nitrogen gas accompanied by physical and chemical changes, followed by the subsequent combustion of the solid residue. The pyrolysis and combustion behavior of the sample was studied using a TGA. Figure 4 illustrates the effect of time on the weight loss of 100% raw MC particles and its blends up to 30% rubber. MC and the rubber tire mixtures were consumed by thermal decomposition (devolatilization) as well as by char oxidation; larger fraction of rubber is expected to be released as volatiles during the combustion process. This high amount of volatile yield occurs over a relatively short time and is
believed to influence the time required for complete combustion when compared to raw MC.40 It can be noticed in Figure 4 that the total weight loss occurred differently for all the samples; at an early stage of combustion the weight loss rate of the 30% rubber mixtures was much faster then the rate of 100% MC, following the decomposition rate of 20% rubber and 10% rubber blends. This is similar to the values expected on the basis of the high volatile yield of the blended materials, which was previously reported by different authors.31,41 Increasing attention on the initial stage of reaction (Figure 4b), a relatively short time, on the order of seconds, can be observed, in which volatiles were released followed by the sample combustion spreading over 45 min. At 1473 K, the temperature of the tests, the degradation time of MC decreased with increasing rubber content in the blend possibly due to the massive decomposition of the organic material leaving behind a residual mass which is believed to be ash. The total weight loss in the final stage of pyrolysis (see Figure 5) was found to be 7% for MC alone, reaching 10% for an initial addition of 10% rubber and increasing to 23% total weight loss when mixed with 30% rubber. This is in good agreement with the volatile matter content in the samples employed in this study. Under nitrogen atmosphere, the weight loss is mainly due to the volatile release, whereas under combustion conditions, the oxygen leads to oxidation of volatile matter and residual carbon as shown in Figure 4. The combustion efficiencies of coke and its mixtures with two different rubber tires at various ratios were calculated from (40) Rudiger, H.; Greul, U.; Splithoff, H.; Hein, K. R. G. In Final Report, APAS Clean Coal Technology Programme CT92-0001; 1995. (41) Kim, S. D.; Park, J. K.; Chun, H. D. EnViron. Eng. 1995, 121, 507–514.
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Figure 7. Effect of volatile matter on combustion performance of rubber/MC blends.
Figure 9. Effect of blending rubber with coke on the particle bulk density of the combusted blend. Figure 8. DR, CO2, micropore surface area for MC and its blends with different proportions of rubber.
eq 1. As rubber tires, light truck and passenger truck tire have been used in this study. Figure 6 compares the combustion efficiency of MC and its blends with waste light truck and passenger car tire in different proportions (10%, 20%, and 30%). As can be seen from Figure 6, there is no significant difference in the burnout values between the two types of waste tires used, which is expected considering the almost equal rubber content in these tires.10 The burnout of coke/rubber blends appears to develop a greater value then the burnout of MC alone, increasing with each stepwise addition of rubber in the mixture, rendering a two times increase when 30% rubber was blended with 70% MC. Extensive research has been done on the combustion performance of coal blends with polymeric materials such as plastics, likewise known for their high volatile content. On the basis of obtained XRD patterns, the burnout of coal/plastic blends increased compared to the constituent coal which was attributed to structural modifications in the mixture.19 Increasing the blending ratio of rubber in coke above 30% led to practical difficulties, causing blockage of the inlet/outlet of the DTF. Therefore, all experiments were limited to mixtures up to 30% rubber. Variations in combustion efficiency values can be attributed to a certain extent to differences in chemical properties for coke and its blends, such as volatile matter and carbon content. Proximate analysis of the samples suggests an increase in VM with increasing rubber content in the blend with a 4-fold increase when 30% of the total mass of coke was replaced by rubber (Figure 7a). As expected, the gross trend in combustion performance with respect to the volatile matter of rubber mixtures appears to record an almost linear increase with
increasing waste percentages in the blend, gaining an almost 2 times increase for 30% rubber mix (Figure 7b). The presence of rubber in the blends may have had an impact upon the structure during the release and combustion of their high VM and hence increased char burnout. Effect of Physical Properties. To gain a better insight into the overall mechanism of combustion, micropore surface area was measured for mixtures of MC with different proportions of rubber. The tests were performed on the raw samples, as well as on the residual chars collected after combustion in DTF at 1473 K under 20% O2 and 80% N2 gas mixture. It can be observed from Figure 8 that the surface area of the raw samples was found to increase with increasing rubber content in the blend and, as combustion proceeded, the micropore surface area developed higher values for all the samples. An increase in micropore surface area of the residual chars is attributed to pores opening up during the combustion process. Generally speaking, the surface area normally increases with char formation.42,43 It was suggested42 that the increase in surface area could be attributed to the enlargement of pores as volatile matter is being removed, leading to the creation of new pores and the opening of bottleneck pores. Summarizing, the surface area of rubber blends with MC increased after being exposed to combustion conditions. Carbon is removed during combustion; the oxygen molecules attack and eliminate carbon atoms, leaving behind numerous meso- and micropores especially for mixtures containing more than 10% rubber. On the basis of high temperature conditions available in the DTF and kinetic diffusion control, we may consider the surface area as a very important factor which contributes to a (42) Gale, T. K.; Fletcher, T. H.; Bartholomew, C. H. Energy Fuels 1995, 9, 513–524. (43) Kulaots, I.; Aarna, I.; Callejo, M.; Hurt, R. H.; Suuberg, E. M. Proc. Combust. Inst. 2002, 29, 495–501.
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Figure 10. SEM images of raw (a) MC and (b) rubber at 100× magnification and (c) raw MC and (d) 30% rubber at 1000× magnification.
Figure 11. SEM images of (a) 100% MC and (b) 30% rubber after combustion in DTF at 100× magnification and (c) 100% MC and (d) 30% rubber at 1000× magnification.
certain extent to the combustion performances. It is understood that the deeper the oxygen molecules penetrate the pores before they are completely depleted, the higher the char surface area. After reviewing the literature44 another measurement of the physical structure, which could affect combustion behavior, besides micropore surface area, is particle bulk density. When oxygen penetrates within the porous structure of a particle, combustion occurs with decreasing density. The penetration depth of oxygen is dependent on the particle radius and hence on its porous nature. Consequently, these particles burn with reduction in density. In our study, the smallest measured bulk density is recorded by 30% rubber blend with a value of 0.77 g/mL, followed by 20% rubber leading to a less dense structure when the amount of rubber in the blend is higher (see Figure 9). The least dense particles developed in the char structure might have contributed to an increase in combustion efficiency of the rubber mixtures when compared to raw coke. Char chemical structure plays an important role when its combustion is chemically controlled. Considering the elevated temperature and the short residence time of the DTF which continuously entrain the carbonaceous particles in a gas stream, we may assume that the reaction is likely limited by pore diffusion and possible by chemical reaction on the pore surface. In such cases, the char physical structure becomes increasingly significant and further investigations are required. SEM analyses (44) Smith, K. L. In The Structure and Reaction Processes of Coal; New York Division of Publishing Corporation 233: New York, 1994.
were carried out to compare the morphology of the samples before and after passing through the DTF in 20% O2 atmosphere at 1473 K. Figure 10 presents typical morphological features of 100% MC and 30% rubber at different magnifications. Images initially taken at 100× magnification show MC particles (Figure 10a) with a more irregular shape, sharp edged and very inhomogeneous. Several textural layers characterize particular coke samples (Figure 10b). Upon increased magnification (Figure 10d), a rough surface can be distinguished, with irregular shapes and more active sites. These sites could represent crystallite edges in contact with catalytically organic impurities.15 Morphologies of char samples, collected after reaction with oxygen in the DTF at 1473 K, are illustrated in Figure 11, where the images, initially taken at 100× magnification, present a very heterogeneous structure, consisting of particles with recognized carbon morphologies and many distinct inorganic rich particles that are either irregularly shaped agglomerates or fused spheres. In the surface morphology, after combustion MC particles had rather poorly developed structures with some pores present. Figure 11a shows the 100% MC which acquired a more round shape, and residual ash appears as bright white spots. MC has high ash content in its composition, which leads us to believe in a possible accumulation of the available ash at the receding surfaces of the burning char. Consequently, oxygen diffusion is impeded, and a lower combustion performance of this material is expected.
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The scanning electron micrographs of the developed structures in the 30% rubber mixture presents a char (Figure 11b) which appears to have acquired a very porous nature after reaction in the DTF. The char particles resulting from the rubber blends seem to contain a relatively greater number of large pores when compared to those in coke-char particles, which may be attributed to the evolution of volatile species from the interior of the particle during the combustion process. The morphology of the samples is consistent with the measured micropore surface area, bulk density, and calculated combustion efficiencies. The combined modification of pore and carbon structure of coal-char could be responsible for steady burnout values of the blends at high rubber levels, i.e., as the rubber tire proportion increased from 10% to 30%. Further investigations are in progress to shed light on the combustion behavior of the rubber blends. This study suggests that higher combustion efficiencies were aquired when coke was partially replaced by waste rubber, while at an industrial level a simultaneous injection of coke with rubber, in EAF steelmaking under oxygen and nitrogen atmosphere, is expected to increased furnace efficiency to a certain extent. Conclusions The combustion behavior of MC and its blends with rubber in different ratios was studied in a TGA and DTF in the presence of 20% O2 and 80% N2. The influence of blending waste rubber tires with coke on the combustion efficiency was the main target in this study. The X-ray diffraction patterns collected on the raw materials before combustion qualitatively show that rubber blends contain more aliphatic side chains bonded in the carbon structure as reflected by the γ band and the volatile content.
Zaharia et al.
The 002 peak diffusivity increases with increasing rubber content in the blend, and this might be further related to their combustion performance. The calculated burn-out was found to be higher for the rubber blends than for MC alone, increasing almost linearly with the rubber amount in the blend and recording the highest value for the 30% rubber mixture. The percent of rubber addition on the combustion efficiency of the mixtures could be influenced by the amount of volatiles present in the rubber which are being released over a relatively short time. The gross trend in combustion performance with respect to the volatile matter of rubber mixtures appears to record an almost linear increase with increasing waste percentages in the blend gaining an almost 2 times increase for 30% rubber mix. Micropore surface area was measured by using CO2 gas and DR method. The samples collected after reaction in the DTF showed an increase in micropore surface area with increasing rubber percent in the blend which influenced to a certain extent the higher combustion performance of the blended materials. The penetration depth of oxygen is dependent on the porous nature of the particles; consequently, these particles burn with reduction in density. In the present study, the smallest measured bulk density is developed by a 30% rubber blend, the density increasing with decreasing rubber content in the blend. The surface morphology of the 30% blend reveals particles with a rough surface, irregular shapes, and more active sites. Following the combustion reaction, the residue developed pores attributed to the evolution of volatile species from the interior of the particle during the combustion process. The combined modification of pore and carbon structure of coke-rubber char could be responsible for steady burnout values of the blends at high rubber levels. EF8010788
