Catalytic Pyrolysis-Gasification of Waste Tire and Tire Elastomers for

Jun 10, 2010 - Yeshui Zhang , Chunfei Wu , Mohamad A. Nahil , and Paul Williams ... Sana Kordoghli , Maria Paraschiv , Radu Kuncser , Mohand Tazerout ...
0 downloads 0 Views 2MB Size
Download PDF
Energy Fuels 2010, 24, 3928–3935 Published on Web 06/10/2010

: DOI:10.1021/ef100317b

Catalytic Pyrolysis-Gasification of Waste Tire and Tire Elastomers for Hydrogen Production Ibrahim F. Elbaba, Chunfei Wu, and Paul T. Williams* Energy & Resources Research Institute The University of Leeds, Leeds LS2 9JT, U.K. Received March 17, 2010. Revised Manuscript Received May 20, 2010

Hydrogen production from waste tires was investigated using a two-stage pyrolysis-gasification reactor and Ni-Mg-Al (1:1:1) as a catalyst. In addition, the elastomer constituents most commonly used in tires, natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR), were also investigated. Experiments were conducted at a pyrolysis temperature of 500 °C and gasification temperature of 800 °C. The results showed that the gas and hydrogen yield were increased for the tire and elastomer constituents during pyrolysis-gasification. However, there was a dramatic increase in H2 and CO concentrations as well as a consequent decrease in CH4 and C2-C4 concentrations when the Ni-Mg-Al catalyst was applied to the pyrolysis-gasification process. For example, hydrogen production increased from 0.68 to 5.43 wt % for the catalytic steam pyrolysis-gasification of waste tire in the presence of Ni-Mg-Al catalyst. The highest hydrogen production (15.26 wt %) was obtained for the BR feedstock. Reacted catalysts were characterized using a variety of methods, including temperature-programmed oxidation (TPO) and scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDXS). The SEM results showed that large amounts of filamentous carbons were observed for the reacted Ni-Mg-Al catalysts derived from all the feedstocks. The total amount of coke deposition on the reacted catalyst, calculated from TPO experiments, was lowest for the BR feedstock (7.0 wt %) and was 31, 21.8, and 18.4 wt % for the waste tire, NR, and SBR samples, respectively. belts, and steel-wire reinforcing beads.7 Natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR) are the most common rubbers used for tires.8,9 There are some other ingredients used to improve the performance and durability of tires, carbon black is used as a reinforcing agent, extender oil (a mixture of aromatic hydrocarbons) is used to soften and enhance the workability of rubber, and sulfur is used to vulcanize the rubber.10 Other chemicals (i.e.organosulfur compound, zinc oxide, and stearic acid) are used as accelerating and protective agents (i.e., antioxidizing, antiozonzant, stabilizer). There is a wide variation among regional and national governments in the methods used for dealing with waste tires. In Europe, the main methods for waste tire management are materials recovery (38.7%), energy recovery (32.3%), and retreading (11.3%).11 In the U.S. however, the main methods for waste tire management are tire-derived fuel (52.8%), ground rubber (16.8%), and civil engineering applications (11.9%).4 In the past, landfill was considered the most acceptable option for dealing with waste tires, but now most developed countries have either banned or are legislating against resorting to landfill. This has been motivated by reasons relating to their composition and energy value. First, because tires are difficult to be degraded due to their chemical and biological

1. Introduction Hydrogen is considered as a clean energy fuel with the potential to reduce the world’s dependence on fossil fuels. About 5  1011 N m3 of hydrogen is produced each year in the world, and about 96% is produced from fossil fuels.1 The principal production methods include methane steam reforming (48%), oil reforming (30%), coal gasification (18%), and electrolysis (3.9%).2 There is much interest in the use of alternative feedstocks for the production of hydrogen. The use of waste materials as a source of hydrogen is particularly of interest in that it would also solve a waste treatment problem. The use of waste tires offers a potential source of hydrogen. The generation rate of waste tires is increasing, especially with the continued increase in production of cars and trucks. For instance, nearly 3.4 million tonnes of used tires were generated within Europe in 2007,3 about 4.6 million tonnes within the U.S. in 2007,4 greater than 1 million tonnes in Japan,5 and about 1 million tonnes in China.6 Tires are formed from vulcanized rubber in addition to the rubberized fabric with reinforcing textile cords, steel or fabric *To whom correspondence should be addressed. Telephone: 44 1133432504. E-mail: [email protected]. (1) Ewan, B. C. R.; Allen, R. W. K. Int. J. Hydrogen Energy 2005, 30 (8), 809–819. (2) Tolga Balta, M.; Dincer, I.; Hepbasli, A. Int. J. Hydrogen Energy 2009, 34 (7), 2925–2939. (3) European Tire & Rubber Manufacturers’ Association. Annual Activity Report 2007-2008, 2008. (4) U.S. Rubber Manufacturers Association. Scrap Tire Markets in the United States: 2007, May, 2009. (5) The Japan Automobile Tire Manufacturers Association, Inc. Tire Industry of Japan, 2007. (6) Li, S. Q.; Yao, Q.; Chi, Y.; Yan, J. H.; Cen, K. F. J. Ind. Eng. Chem. Res. 2004, 43 (17), 5133–5145. r 2010 American Chemical Society

(7) Williams, P. T. Waste Treatment and Disposal; John Wiley & Sons: Chichester, U.K., 2005. (8) Williams, P. T.; Besler, S. Fuel 1995, 74 (9), 1277–1283. (9) Seidelt, S.; M€ uller-Hagedorn, M.; Bockhorn, H. J. Anal. Appl. Pyrolysis 2006, 75 (1), 11–18. (10) Sharma, V. K.; Fortuna, F.; Mincarini, M.; Berillo, M.; Cornacchia, G. Appl. Energy 2000, 65 (1-4), 381–394. (11) European Tire & Rubber Manufacturers’ Association (ETRMA). ELTs Treatment Data in 2007, October 2008.

3928

pubs.acs.org/EF

Energy Fuels 2010, 24, 3928–3935

: DOI:10.1021/ef100317b

Elbaba et al.

resistance, they would last in landfill for a long time. Moreover, the large size of many tires takes up a space within the landfill and can also act as nests for insects, rodents, and birds. Also, because of the high heating value of waste tire (about 32 MJ kg-1), it is considered a valuable energy source which is wasted in the landfill. Where natural rubber is found in tires, it represents a renewable source of hydrogen, since the rubber is harvested from the rubber tree without detriment to the life of the tree. In addition, the synthetic rubbers represent recovery of hydrogen from a fossil fuel based source. In both such cases an attractive environmental option for the production of hydrogen. Therefore, the European Union has legislated a series of regulations toward waste tire management including the European Commission’s Landfill Directive (1999),12 which has banned the disposal of waste tires to landfill from 2006 and the European End of Life Vehicle Directive 2000/53,13 which requires that 80% in weight of an end of life vehicle is reused or recycled. The pyrolysis and gasification of waste tires for the production of liquid fuels, chemical feedstocks, activated carbons, and gases has been extensively researched.14-20 Recently, it has been suggested that the thermal decomposition of waste tire at high temperature might be an alternative for production of hydrogen in future energy systems.17 Catalysts play an important role in pyrolysis-gasification processes for maximizing hydrogen production.21-23 Nickelbased catalysts have been reported as promising catalysts for tar removal and hydrogen production in steam gasification processes24-26 due to their good catalytic effect and comparatively low cost. In this paper, a waste tire and three of its elastomer constituents, natural rubber, styrene-butadiene rubber, and butadiene rubber, were investigated for hydrogen production through a pyrolysis-catalytic gasification process by using a two-stage fixed bed reaction system with a laboratoryprepared Ni-Mg-Al (1:1:1) used as a catalyst.

Figure 1. Schematic diagram of the two-stage pyrolysis-gasification experimental system.

catalyst (molar ratio 1:1:1) was prepared using the rising pH technique. The materials were obtained from Sigma-Aldrich, U.K. The precipitant, 1 M NH4OH, was added to 200 mL of an aqueous solution containing Ni(NO3)2 3 6H2O, Al(NO3)3 3 9H2O, and Mg(NO3)2 3 6H2O. The precipitation was carried out at 40 °C with moderate stirring until a pH of 8.3 was obtained. The precipitate was filtered with water at 40 °C and dried at 105 °C overnight followed by calcination at 750 °C for 3 h under an air atmosphere. The catalysts were sieved to granules with the size of less than 0.212 mm. 2.2. Characterization of Materials. The thermal degradation of waste tire and its elastomer constituents was carried out using a Stanton-Redcroft thermogravimetric analyzer (TGA). The sample was heated at 40 °C min-1 under an inert (nitrogen) atmosphere to a final temperature of 700 °C with a dwell time of 10 min. The carbon deposited on the used catalysts was examined by temperature-programmed oxidation (TPO) carried out using a Shimadzu Stanton-Redcroft thermogravimetric analyzer (TGA). The sample was heated at 15 °C min-1 in air, and the weight changes were recorded up to a sample temperature of 800 °C with a final hold time of 10 min. Scanning electron microscopy (SEM) (Phillips XL30 Environmental) coupled to an energy dispersive X-ray spectroscopy (EDXS) system was used to investigate the surface morphology of the catalysts and the carbon deposited on the used catalysts. 2.3. Experimental System. The pyrolysis-gasification was carried out in a two-stage fixed bed reactor. A schematic diagram of the experimental system is shown in Figure 1. The tire rubber sample was pyrolyzed in the first reactor, and the pyrolysis products were passed directly to the second reactor where steam catalytic gasification of the pyrolysis gases was carried out. Both of the reactors were constructed of stainless steel and were 2.2 cm internal diameter 16 cm length, and each was separately heated externally via an electrical heater with

2. Materials and Methods 2.1. Materials. The rubber tread of a waste passenger car tire and three different pure elastomer compounds including natural rubber (NR), styrene-butadiene rubber (SBR), and polybutadiene rubber (BR) were employed for this study. The Ni-Mg-Al (12) Council Directive 1999/31/EC, Landfill of Waste; Official Journal of the European Communities, Brussels, Belgium, 1999; p L182. (13) Council Directive 2000/53/EC, End-of-Life Vehicles; Official Journal of European Communities, Brussels, Belgium, 2000; p L269/34. (14) Raman, K. P.; Walawender, W. P.; Fan, L. T. Conserv. Recy. 1981, 4 (2), 79–88. (15) Leung, D. Y. C.; Wang, C. L. Fuel Process. Technol. 2003, 84 (1-3), 175–196. (16) Xiao, G.; Ni, M.-J.; Chi, Y.; Cen, K.-F. Energy Convers. Manage. 2008, 49 (8), 2078–2082. (17) Galvagno, S.; Casciaro, G.; Casu, S.; Martino, M.; Mingazzini, C.; Russo, A.; Portofino, S. Waste Manage. 2009, 29 (2), 678–689. (18) Williams, P. T.; Besler, S.; Taylor, D. T.; Bottrill, R. P. J. Inst. Energy 1995, 68, 11–21. (19) Cunliffe, A. M.; Williams, P. T. Environ. Technol. 1998, 19, 1177– 1190. (20) Williams, P. T.; Brindle, A. J. J. Anal. Appl. Pyrol. 2003, 67, 143– 164. (21) Luo, S.; Xiao, B.; Hu, Z.; Liu, S.; Guo, X.; He, M. Int. J. Hydrogen Energy 2009, 34 (5), 2191–2194. (22) He, M.; Hu, Z.; Xiao, B.; Li, J.; Guo, X.; Luo, S.; Yang, F.; Feng, Y.; Yang, G.; Liu, S. Int. J. Hydrogen Energy 2009, 34 (1), 195–203. (23) Yoon, S. J.; Choi, Y.-C.; Lee, J.-G. Energy Convers. Manage. 2010, 51 (1), 42–47. (24) Tomishige, K.; Kimura, T.; Nishikawa, J.; Miyazawa, T.; Kunimori, K. Catal. Commun. 2007, 8 (7), 1074–1079.

(25) Kimura, T.; Miyazawa, T.; Nishikawa, J.; Kado, S.; Okumura, K.; Miyao, T.; Naito, S.; Kunimori, K.; Tomishige, K. Appl. Catal., B: Environ. 2006, 68 (3-4), 160–170. (26) Wu, C.; Williams, P. T. Appl. Catal., B: Environ. 2009, 87 (3-4), 152–161.

3929

Energy Fuels 2010, 24, 3928–3935

: DOI:10.1021/ef100317b

Elbaba et al.

temperature control. Approximately 1.0 g of waste tire was held in the first pyrolysis reactor, and 0.5 g of catalyst, supported on quartz wool, was placed in the second gasification reactor. Water was introduced by a syringe pump with a flow rate of 4.74 mL h-1 between the pyrolysis stage and the catalyst bed to generate steam for the gasification process in the second stage reactor. During each experiment, the N2 was first introduced to purge the air out of the reaction system and was used as a carrier gas for all experiments. The gasification reactor was heated to 800 °C and stabilized, and then the first stage pyrolysis reactor was started to be heated at a heating rate of 40 °C min-1 to the final pyrolysis temperature of 500 °C. The pyrolysis gases derived from the pyrolysis reactor were passed directly to the second gasification reactor where the catalytic steam reforming took place. For the experiments where no catalyst was used, the catalyst was replaced with sand of similar size to the catalyst. According to the examination of the thermal degradation profile of waste tire and its elastomer constituents, the major weight loss of all samples began at about 300 °C and was largely completed at around 500 °C. Therefore, water was introduced when the temperature of the first reactor reached 300 °C with a reaction time of 20 min. The amount of reacted water in each experiment was calculated by the oxygen content obtained from the concentration of CO and CO2 gases; it was assumed that all the oxygen in CO and CO2 gases were derived from reacted water. Experiments were repeated to ensure the accuracy of experimental results. The gaseous products derived from the reaction system passed through an air-cooled condenser and two dry ice cooled condensers where liquid products were collected. The noncondensed gases were collected with a 25 L Tedlar gas bag and were analyzed offline by packed column gas chromatography. In order to ensure all of the gases were collected, the gases were collected for more than 20 min after each experiment. The gaseous product, collected in the gas bag, was analyzed using two gas chromatographs. Hydrocarbons from C1 to C4 were detected using a Varian 3380 gas chromatograph with a flame ionization detector (GC/FID) on a stainless steel 2 m length by 2 mm diameter column packed with 80/100 mesh Heysep packing. Nitrogen was used as the carrier gas. The temperature was programmed to start at 60 °C for 3 min, then was set at 100 °C at the heating rate of 5 °C min-1, and held for 3 min, finally ramped to 120 at 20 °C min-1 and held for 17 min. Permanent gases were detected using two packed columns with a second Varian 3380 gas chromatograph with two thermal conductivity detectors (GC/TCD). Nitrogen, hydrogen, oxygen, and carbon monoxide were detected with a stainless steel 2 m  2 mm column, packed with 60/80 mesh Heysep packing. Argon was used as the carrier gas. Carbon dioxide was detected on a stainless steel 2 m length  2 mm diameter column, packed with 80/100 mesh packing. The gas chromatograph oven was held isothermally at 40 °C for the analysis. The column oven temperature is operated at 120 °C with a filament temperature of 160 °C.

Figure 2. TG analysis of waste tire, natural rubber (NR), styrenebutadiene rubber (SBR), and polybutadiene rubber (BR).

Figure 3. DTG analysis of waste tire, natural rubber (NR), styrenebutadiene rubber (SBR), and polybutadiene rubber (BR).

The results show that one major decomposition peak (396 °C) was observed for NR; however, two decomposition peaks have been found for SBR (390 and 474 °C) and BR (378 and 482 °C). The DTG analysis for the waste tire shows that the first decomposition peak occurred at around 400 °C and the second decomposition peak was observed around 467 °C (Figure 3). Williams and Besler8 suggested that for the thermal decomposition of waste tires, the first decomposition peak of DTG corresponded to the thermal decomposition of natural rubber (NR) and the second peak to the thermal decomposition of styrene-butadiene rubber (SBR) and butadiene rubber (BR), these being the main rubber components of tires. However, three peaks of weight loss have been reported by Seidelt et al.9 for tire decomposition; they argued that the first peak was assigned to the volatilization of the added processing oil, the second peak was the thermal decomposition of NR, and the third peak might be the thermal decomposition of SBR. 3.2. Steam Catalytic Pyrolysis-Gasification of Waste Tires and Its Elastomer Constituents. 3.2.1. Product Yield. Table 1 shows the product yield from the pyrolysis-gasification of waste tire and its elastomer constituents in the absence of steam and catalyst. The solid residue and oil yield were measured by weight difference, and the gas yield was calculated from the appropriate gas concentration and molecular mass, rather than “gas by difference”. The results show that the major product from the pyrolysis-gasification experiments with pure rubbers was the oil and then the gas; the

3. Results and Discussion 3.1. Themogravimetry Analysis of Waste Tire and Its Elastomer Constituents. The thermogravimetric analysis (TGA) of waste tire rubber and three pure elastomers is presented in Figure 2. The TGA analysis shows that the major weight losses of the waste tire rubber and the three pure elastomers began at about 300 °C and was completed at about 500 °C. It seems that the natural rubber had a lower decomposition temperature than the waste tire and other pure rubbers. Williams and Besler8 have also reported the lower decomposition temperature for NR compared to SBR and BR. The derivative thermogravimetric analysis (DTG) of waste tire and the three main tire rubbers is shown in Figure 3. 3930

Energy Fuels 2010, 24, 3928–3935

: DOI:10.1021/ef100317b

Elbaba et al.

Table 1. Results of Pyrolysis-Gasification of Waste Tire, Natural Rubber (NR), Styrene-Butadiene Rubber (SBR), and Polybutadiene Rubber (BR) without Steam and Catalyst

Table 3. Results of Pyrolysis-Gasification of Waste Tire, Natural Rubber (NR), Styrene-Butadiene Rubber (SBR), and Polybutadiene Rubber (BR) with Steam and Catalyst steam with catalyst

gas/sample solid/sample oil/sample mass balance

tire

NR

22.2 36.7 33.0 91.9

Yield (wt %) 25.0 2.1 66.0 93.1

SBR

21.8 15.7 57.0 94.5

24.4 10.2 58.0 92.6

Table 2. Results of Pyrolysis-Gasification of Waste Tire, Natural Rubber (NR), Styrene-Butadiene Rubber (SBR), and Polybutadiene Rubber (BR) with Steam steam

tire

NR

SBR

Mass Balance in Relation to Sample þ Water gas/(sample þ water) (wt %) 25.9 26.6 23.3 solid/(sample þ water) (wt %) 32.0 9.4 14.2 oil/(sample þ water) (wt %) 41.5 55.8 54.7 mass balance 99.4 91.8 92.1

reacted water (g)

0.01

0.02

SBR

BR

Mass Balance in Relation to Sample þ Water gas/(sample þ water) (wt %) 36.2 55.1 64.7 solid/(sample þ water) (wt %) 41.6 14.8 17.4 oil/(sample þ water) (wt %) 17.1 21.2 9.5 mass balance 94.9 91.1 91.5

82.5 14.1 1.1 97.7

Mass Balance in Relation to Sample Only gas/sample (wt %) 43.1 78.3 97.7 solid/sample (wt %) 49.5 21.0 26.3 oil/sample (wt %) 20.3 30.2 14.3 mass balance 112.9 129.4 138.2

153.4 26.3 2.0 181.7

reacted water (g)

0.86

0.19

0.42

0.51

Table 4. Gas Composition and Hydrogen Production in the Produced Gases from Pyrolysis-Gasification of Waste Tire, Natural Rubber (NR), Styrene-Butadiene Rubber (SBR), and Polybutadiene Rubber (BR) without Steam and Catalyst

32.5 6.3 53.5 92.3 32.9 6.4 54.0 93.3

0.02

NR

BR

Mass Balance in Relation to Sample Only gas/sample (wt %) 26.5 27.1 23.6 solid/sample (wt %) 32.8 9.6 14.4 oil/sample (wt %) 42.6 56.8 55.6 mass balance 102.0 93.4 93.6 0.03

tire

BR

H2 (vol %) CO (vol %) CO2 (vol %) CH4 (vol %) C2-C4 (vol %) hydrogen production (wt %)

tire

NR

SBR

BR

21.9 0 0 44.3 33.8 0.52

24.6 0 0 44.8 30.6 0.70

26.7 0 0 37.6 35.7 0.66

25.5 0 0 36.1 38.4 0.67

The effect of the Ni-Mg-Al catalyst on the gasification of the waste tire and its elastomer constituents was investigated, and the results are presented in Table 3. The results show that the gas yield was greatly increased for all samples used in the presence of the catalyst. For example, without steam and catalyst (sand), only 24.4 wt % gas yield in relation to the mass of BR was obtained; however, gas yield in relation to the mass of BR was increased to 32.9 wt % with the introduction of water at the gasification temperature of 800 °C without catalyst. With the presence of the Ni-MgAl catalyst and steam, the gas yield in relation to the mass of BR increased to 153.4 wt %. It should be noted that yields of more than 100 wt % of gas yield reported in our work refer to presentation of the results in terms of the mass of gas production divided by the mass of only the feedstock, that is, without the amount of reacted water. 3.2.2. Gas Composition and Hydrogen Production. Table 4 shows the composition of the noncondensed gases obtained from the pyrolysis-gasification experiments of waste tire and its elastomer constituents in the presence of sand (no catalyst) and no steam. It should be noted that gas components such as C5 gases as well as sulfurous gases were not measured in our work and therefore would not contribute to our reported mass of measured gas during the steam pyrolysisgasification of waste tire and its elastomer constituents. The results show that the composition of the collected gases had a high methane and C2-C4 content which were between 36.1 and 44.8 vol %, 30.6 vol %, and 38.4 vol %, respectively, for the waste tire rubber and the pure rubbers used. The hydrogen content increased in the sequence tires < NR < BR < SBR, increasing from 21.9 to 26.7 vol %. CO and CO2 were not detected for the gases in the absence of catalyst and steam. Others have reported low CO and CO2 levels from pyrolysois of waste tires. Berrueco et al.29 reported that the

other product was solid residue. For example, for the pyrolysis-gasification of natural rubber in the absence of steam and catalyst, the oil yield was 66.0 wt %, the gas yield was 25.0 wt %, and the solid yield was 2.1 wt %. For waste tire however, the product yield of the solid residue was the highest at 36.7 wt % with an oil product yield of 33.0 wt % and gas product yield of 22.2 wt %. The tires contain a significant amount of carbon black and other additives that will contribute to the higher solid residue compared to the pure elastomers, for example, Williams7 reports a typical carbon black content of the rubber component of tires at 26 wt % and zinc oxide additive to the rubber at 2 wt %. Cunliffe and Williams27 conducted experiments in a staticbed batch reactor at a temperature range of 450-600 °C in a nitrogen atmosphere and found that 58.1-53.1 wt % of waste tire was converted into oil, 4.5-8.9 wt % into gas and 37.4-38.0 wt % into solid. The higher gas yields and lower oil used reported in our work are due to the thermal cracking of the pyrolysis oil vapors in the second gasification reactor at 800 °C. Others have shown high temperatures result in lower oil and higher gas yields, for example, Leung et al.28 pyrolyzed powdered tires that provided heating rates up to 1200 °C min-1 at a temperature range of 500-1000 °C in a nitrogen atmosphere and 44.0 wt % of oil, 21 wt % gas, and 35.0 wt % char yield were obtained at the reaction temperature of 800 °C. With the introduction of steam to the second reactor, a slight increase in gas yield from waste tire and its elastomer constituents was observed in the absence of any catalyst (Table 2). It is also indicated from Table 2 that less than 0.03 g of water was reacted during the pyrolysis-gasification of waste tire and its elastomer constituents in the absence of any catalyst. (27) Cunliffe, A. M.; Williams, P. T. J. Anal. Appl. Pyrolysis 1998, 44 (2), 131–152. (28) Leung, D. Y. C.; Yin, X. L.; Zhao, Z. L.; Xu, B. Y.; Chen, Y. Fuel Process. Technol. 2002, 79 (2), 141–155.

(29) Berrueco, C.; Esperanza, E.; Mastral, F. J.; Ceamanos, J.; Garcı´ a-Bacaicoa, P. J. Anal. Appl. Pyrolysis 2005, 74 (1-2), 245–253.

3931

Energy Fuels 2010, 24, 3928–3935

: DOI:10.1021/ef100317b

Elbaba et al.

was shown in Table 4 and 5, the hydrogen production was slightly increased for each of the samples, when the steam was introduced without the presence of the catalyst. As shown in Tables 5 and 6, hydrogen production was largely increased with the introduction of the Ni-Mg-Al catalyst. For example, hydrogen production increased from 0.68 to 5.43 wt % for the catalytic steam pyrolysis-gasification of waste tire in the presence of the Ni-Mg-Al catalyst. The highest hydrogen production (15.26 wt %) was obtained for the BR feedstock (Table 6). 3.3. Investigation of Reacted Ni-Mg-Al Catalyst. The reacted Ni-Mg-Al catalysts from different feedstocks were analyzed by SEM-EDXS to investigate their surface characteristics after the pyrolysis-gasification experiments. It is known that Ni-Mg-Al catalysts have been reported to be effective in the gasification process of different polymers in order to increase hydrogen production.26,30 The SEM and EDXS results are shown in Figures 4 and 5, respectively. It is demonstrated that filamentous carbons were observed on the surface of the reacted Ni-Mg-Al catalyst for all the feedstocks (Figure 4). Filamentous carbons have been extensively reported and observed using SEM31,32 For example, Wu and Williams33 investigated a Ni-Mg-Al catalyst for hydrogen production from the pyrolysis-gasification of polypropylene. They revealed that the filamentous carbon formed on the reacted Ni-Mg-Al catalyst was observed from SEM analysis. The reacted Ni-Mg-Al catalyst was further analyzed by TPO experiments. The results of TPO-TGA and TPODTG for the reacted Ni-Mg-Al catalyst from catalytic steam pyrolysis-gasification of waste tire and its elastomer constituents are shown in Figures 6 and 7, respectively. Figure 6 shows that the mass of reacted Ni-Mg-Al catalyst reduced first, then increased, and finally decreased to a stable content. The first reduction of the mass in the TGA-TPO result is suggested to be caused by water vaporization, the phenomena of mass increasing of the reacted Ni-Mg-Al catalyst in the TGA-TPO result might be due to the oxidation of metallic Ni and the final decrease could be due to the combustion of deposited coke on the catalyst.33 The amount of coke deposited on the catalysts could be calculated from the TGA-TPO experiments (Figure 6). The amount of coke equals the weight loss of sample divided by the initial sample weight in the TGA experiment. The calculated amount of coke deposited on the catalyst was 31, 21.8, 18.4, and 7.0 wt % in relation to waste tire, NR, SBR, and BR as the feed stock, respectively. In addition, the sequence of gas production corresponding to the mass of feedstock was tire < NR < SBR < BR (Table 3). It is suggested that the highest gas production for the steam catalytic pyrolysis-gasification of BR might be due to the highest carbon gasification rate among the different types of feedstocks; thus, the lowest carbon deposition was obtained for the pyrolysis-gasification of BR. The lowest catalytic activity of Ni-Mg-Al catalyst using the tire is suggested to be related to the sulfur content in the tire; the sulfur has been

Table 5. Gas Composition and Hydrogen Production in the Produced Gases from Pyrolysis-Gasification of Waste Tire, Natural Rubber (NR), Styrene-Butadiene Rubber (SBR), and Polybutadiene Rubber (BR) with Steam steam

tire

NR

SBR

BR

H2 (vol %) CO (vol %) CO2 (vol %) CH4 (vol %) C2-C4 (vol %) hydrogen production (wt %)

24.65 3.24 3.76 37.26 31.09 0.68

25.80 2.76 1.91 40.82 28.72 0.78

28.76 2.81 1.96 33.02 33.45 0.75

26.67 1.11 1.01 34.69 36.52 0.94

Table 6. Gas Composition and Hydrogen Production in the Produced Gases from Pyrolysis-Gasification of Waste Tire, Natural Rubber (NR), Styrene-Butadiene Rubber (SBR), and Polybutadiene Rubber (BR) with Steam and Catalyst steam with catalyst

tire

NR

SBR

BR

H2 (vol %) CO (vol %) CO2 (vol %) CH4 (vol %) C2-C4 (vol %) hydrogen production (wt %)

66.69 16.01 5.29 8.73 3.28 5.43

65.29 21.97 6.19 5.25 1.31 8.98

66.05 27.43 2.09 3.27 1.17 12.00

60.98 32.05 3.22 2.93 0.81 15.26

main gases of pyrolysis of shredded waste tires at temperatures between 400 and 700 °C were H2, CO, CO2, and hydrocarbons such as CH4, C2H4, C2H6, C3H6, C4H8, and C4H6. Leung et al.28 also found that the concentrations of the H2, CO, CO2, CH4, and C2-C4 were 20.7, 2.6, 1.8, 44.5, and 26.9 vol %, respectively, when powdered tire was pyrolyzed at 800 °C. After introduction of water into the second stage of the reactor in the absence of catalyst (sand), there was a small difference in the concentration of gases compared with those without steam (Table 5). A small amount of CO and CO2 was produced from all samples used. Galvagno et al.17 investigated the steam gasification of tire waste at 850 °C using a rotary kiln reactor. The result of their investigation showed that the hydrogen content was 51.5 vol %, the oxygenated products contents (CO, CO2) were 6.3 and 4.7 vol %, respectively, and the methane content was 27.6% and the C2-C4 content was 9.9 vol %. The Ni-Mg-Al catalyst was investigated with the aim of improving the catalytic steam reforming of gaseous products derived from the pyrolysis of the tire. The gas concentrations from the catalytic steam pyrolysis-gasification of the tire and rubber samples are presented in Table 6. The results show that a dramatic increase in H2 and CO concentrations as well as a marked decrease in CH4 and C2-C4 concentrations were achieved when the Ni-Mg-Al catalyst was applied to the pyrolysis-gasification process. For example, the H2 concentration increased from 26.67 to 60.98 vol %, CO increased from 1.11 to 32.05 vol %, CH4 decreased from 34.69 to 2.93 vol %, and C2-C4 concentrations decreased from 36.52 to 0.81 vol %, respectively, when the BR sample was processed for hydrogen production with the catalytic steam pyrolysis-gasification system. In this paper, the influence of water injection and catalyst introduction on the hydrogen production was also investigated for the different tire and tire elastomer feedstocks. The hydrogen production, presented in this work, was calculated by the mass of produced hydrogen divided by the mass of sample used in the experiment. The hydrogen production was less than 1.0 wt % during the pyrolysis-gasification experiment of waste tire and its elastomer constituents, when the Ni-Mg-Al catalyst was not used (Tables 4 and 5). As

(30) Garcia, L.; Benedicto, A.; Romeo, E.; Salvador, M. L.; Arauzo, J.; Bilbao, R. Energy Fuels 2002, 16 (5), 1222–1230. (31) Echegoyen, Y.; Suelves, I.; Lazaro, M. J.; Sanjuan, M. L.; Moliner, R. Appl. Catal., A: Gen. 2007, 333 (2), 229–237. (32) Alberton, A. L.; Souza, M. M. V. M.; Schmal, M. Catal. Today 2007, 123 (1-4), 257–264. (33) Wu, C.; Williams, P. T. Appl. Catal., B: Environ. 2009, 90 (1-2), 147–156.

3932

Energy Fuels 2010, 24, 3928–3935

: DOI:10.1021/ef100317b

Elbaba et al.

Figure 4. SEM results of the reacted Ni-Mg-Al catalyst used with pyrolysis-gasification of waste tire, natural rubber (NR), styrene-butadiene rubber (SBR), and polybutadiene rubber (BR).

Figure 5. EDXS results of the reacted Ni-Mg-Al catalyst used with pyrolysis-gasification of waste tire, natural rubber (NR), styrenebutadiene rubber (SBR), and polybutadiene rubber (BR).

known to cause serious deactivation of the catalyst.34,35 Moreover, as was shown in Figure 5, sulfur was detected for the used catalyst by the EDXS analysis, when the catalyst

was used for catalytic steam pyrolysis-gasification of the waste tire. The results of the DTG-TPO analysis for the reacted catalysts from the waste tire are compared with the different elastomer feedstocks are shown in Figure 7. It is shown that one carbon oxidation peak was observed for all the

(34) Bartholomew, C. H. Appl. Catal., A: Gen. 2001, 212 (1-2), 17–60. (35) Sehested, J. Catal. Today 2006, 111 (1-2), 103–110.

3933

Energy Fuels 2010, 24, 3928–3935

: DOI:10.1021/ef100317b

Elbaba et al.

Figure 8. Gas compositions from the deactivation test of the NiMg-Al catalyst for the pyrolysis-gasification of tire at 800 °C. Figure 6. TGA-TPO results of the reacted Ni-Mg-Al catalyst used with pyrolysis-gasification of waste tire, natural rubber (NR), styrene-butadiene rubber (SBR), and polybutadiene rubber (BR).

3.4. Deactivation of the Ni-Mg-Al Catalyst. In order to obtain information on the deactivation of the Ni-Mg-Al catalyst during the steam catalytic pyrolysis-gasification of waste tire, five experiments were carried out with the same Ni-Mg-Al catalyst while the raw feedstock was reloaded after each experiment. During the experiment, 1.0 g of waste tire and 0.5 g of the Ni-Mg-Al catalyst were used and the gasification temperature was 800 °C. The gas compositions for the catalyst deactivation test are shown in Figure 8. From Figure 8, it is seen that the H2 concentration decreased from about 67.35 to 47.03 vol % and the CO concentration decreased from about 16.54 to 9.66 vol %. It seems that the Ni-Mg-Al catalyst was deactivated after the steam catalytic pyrolysis-gasification of waste tire. It is suggested that the Ni-Mg-Al catalyst might be deactivated by the coke deposition and/or the poisoning from sulfur derived from the waste tire. However, we have investigated the Ni-Mg-Al catalyst for pyrolysisgasification of polypropylene with a continuous reaction system; the result turned out to be that the catalyst was not deactivated after about 8 h testing. Therefore, the deactivation of the Ni-Mg-Al catalyst in this work might not be mainly due to the deposition of coke on the catalyst but due to the sulfur poisoning.

Figure 7. DTG-TPO results of the reacted Ni-Mg-Al catalyst used with pyrolysis-gasification of waste tire, natural rubber (NR), styrene-butadiene rubber (SBR), and polybutadiene rubber (BR).

investigated feedstocks. Because of the large amounts of filamentous carbons deposited on the used catalysts (Figure 4), the weight loss peak in Figure 7 is suggested to be mainly oxidation of the filamentous type carbons. From Figure 7, the temperature of the oxidation peak was moved to lower temperature when the feedstocks investigated were changed from tire, NR, and SBR to BR. The sequence of oxidation peak temperatures for the different feedstocks were consistent in relation to the amount of coke deposition on each catalyst. The highest catalytic activity of the Ni-Mg-Al catalyst was obtained for BR among different investigated feedstocks. From Figure 2, the DTG result of temperature programmed decomposition indicated that both tire and NR have a decomposition peak at around 390 °C. Another small decomposition peak for tire observed in Figure 2 was at about 460 °C, where in addition, SBR showed a decomposition peak. Also, from the results of the TPO experiments of the reacted catalysts (Figures 6 and 7), the oxidation peak of the used catalyst from the tire experiments seemed to be close to that obtained for NR and SBR experiments. Therefore, the main rubbers in the tire sample investigated in this paper might be NR with SBR also suggested to be present as a major elastomer in tire.

4. Conclusions In this paper, a waste tire and three of its elastomer constituents (natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR)) were investigated for the production of hydrogen through a pyrolysis-catalytic gasification process in a two-stage reactor system by using a laboratory-prepared Ni-Mg-Al (1:1:1) catalyst. The following conclusions were observed: (1) The gas yield was greatly increased for all samples with the introduction of steam and/ or catalyst. For example, without steam and catalyst (sand), only 24.4 wt % gas yield in relation to the mass of BR was obtained. The gas yield in relation to the mass of BR was increased to 32.9 wt % with the introduction of water at the gasification temperature of 800 °C without the catalyst. The gas yield in relation to the mass of BR was further increased to 153.4 wt % in the presence of the Ni-Mg-Al catalyst and steam. In addition, the hydrogen yield was also increased with the introduction of catalyst in the process for all the researched tire and elastomer feedstocks. (2) A dramatic increase in H2 and CO concentrations as well as a marked decrease in 3934

Energy Fuels 2010, 24, 3928–3935

: DOI:10.1021/ef100317b

Elbaba et al.

CH4 and C2-C4 concentrations were achieved, when the Ni-Mg-Al catalyst was applied to the pyrolysis-gasification process. (3) Reacted catalysts were characterized using a variety of methods, including, thermogravimetric analysis and scanning electron microscopy with energy dispersive X-ray spectrometry. The results showed that the reacted Ni-Mg-Al was deposited with large amounts of filamentous carbons and a total coke deposition (calculated from the TPO experiment) of 31, 21.8, 18.4, and 7 wt % for the waste tire, NR, SBR, and BR, respectively. The results from the DTGTPO experiments showed that the reacted catalyst derived from the BR elastomer had the lowest oxidation peak tem-

perature compared with the other feedstocks. It is suggested that the highest catalytic activity of the Ni-Mg-Al catalyst was obtained for BR feedstock; in addition, the highest gas and hydrogen yields were also obtained for BR during the catalytic steam pyrolysis-gasification process. Acknowledgment. The authors would like to thank the Libyan Ministry of Higher Education and Omar Al-Mukhtar University, Al Beida, Libya, for support for Ibrahim Farag ElBaba. The authors would also like to thank Mr. Ed Woodhouse for his technical support and the analytical support from Dr Jude Onwudili. Support from EPSRC Grant EP/D053110/01 is also gratefully acknowledged.

3935