Catalysts’ Influence on Thermochemical Decomposition of Waste Tires Sana Kordoghli,a,b Maria Paraschiv,c,d Radu Kuncser,e Mohand Tazerout,a and Fethi Zagroubaf a Ecole des Mines de Nantes, GEPEA UMR 6144, rue Alfred Kastler, Nantes 44307, France b Ecole Nationale d’Ingenieurs de Monastir, Avenue Ibn EL Jazzar, Monastir 5019, Tunisia c National Institute of R&D for Biological Sciences, 296 Spl. Independentei, Bucharest 060031, Romania;
[email protected] (for correspondence) d Products and Processes (CAMPUS), Research Center for Advanced Materials, 313 Spl. Independentei, Bucharest 060042, Romania e National Research and Development Institute for Gas Turbines COMOTI, 220D Iuliu Maniu Blvd, Bucharest 061126, Romania f Institut Superieur des Sciences et Technologies de l’Environnement de Borj Cedria, Hammam Lif, Tunisia Published online 30 April 2017 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12605 In this article, correlation between the influence of catalysts on waste tires pyrolysis at small- and laboratory-scale installation are highlighted. Kinetic and thermodynamic parameters of tires rubber thermochemical transformations were evaluated using thermogravimetric analyses (TGA). For that purpose, Zeolite (ZSM-5), alumina (Al2O3), calcium carbonate (CaCO3), and magnesium oxide (MgO) were used as catalyst. It was found that all catalysts induce a delayed onset of pyrolytic process and MgO and CaCO3 significantly reduced the activation energy (Ea) from 246.89 kJmole21 (thermal pyrolysis) to 121.82 and 128.34 kJ mole21, respectively. At laboratory scale, a fixed-bed reactor was used to distinguish how the contact manner between tires and catalysts influences the yield of pyrolysis products as well as the gas quality. It was proved that CaCO3 and Al2O3 are the most suitable catalysts for increasing the gas fraction, while the MgO promotes the C 2017 American Institute of Chemiformation of liquid fraction. V cal Engineers Environ Prog, 36: 1560–1567, 2017
Keywords: kinetics, catalytic pyrolysis, TGA/DTG, waste tire INTRODUCTION
The amount of used tires have increased remarkably in recent times, which need to be recycled and valorized to reduce their deposit in landfills, and thus meet the latest regulations on waste disposal. There are different pathways that can be employed to appropriately approach this topic, which may allow these materials to become valuable resources for energy or new materials. The evaluation of waste tires rubber thermochemical and chemical decompositions has been approached by several authors to improve as much as possible the recycling process. From economic and environmental points of view, thermochemical processes such as pyrolysis and gasification are promising transformation routes as all resulted fractions can be used as alternative fuels. Moreover, the obtained oil has a high added value as chemical feedstock due to its rich content of benzene, toluene, and xylenes [1–6]. Co-pyrolysis of C 2017 American Institute of Chemical Engineers V
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different polymer-based wastes with biomass is also a sustainable route for optimal use of their energetic potential [7–9]. To promote the formation of low-molecular-weight aromatic compounds in high yields, catalytic systems are employed [10–13]. Under catalytic reactions, the oil yield decreases with a corresponding increase of gas yield. Unfortunately, the process is accompanied by the formation of carbonaceous coke which covers the catalyst surface. Among most used catalytic systems, Ni-based compounds have been reported as promising catalysts for oil secondary cracking with hydrogen production in steam gasification processes due to their good catalytic effect and attractive cost. Williams et al. (2010) [14] investigated the production of hydrogen and other gases from the bench-scale pyrolysisgasification of waste tires. The experiment was carried out using a two-stage system consisting of pyrolysis of the waste tires at 5008C followed by catalytic steam gasification in a second reactor at 8008C using Ni–Mg–Al (molar ratio 1:1:1) as a catalyst. Evidently, the presence of H2 in large amount has a major interest, and many research works have been concentrated on identifying the role played by different catalysts in pyrolysis-gasification processes for maximizing hydrogen production. Thus, Ibrahim et al. (2010) [15] investigated a waste tire and three of its elastomer constituents, natural rubber, styrene–butadiene rubber, and butadiene rubber, for hydrogen production through a pyrolysis-catalytic gasification process by using a two-stage fixed bed reaction system with a laboratory prepared Ni-based catalysts. Another catalyst class largely used for increasing the gas fraction during thermochemical process is the zeolites. Extensive studies using zeolites were carried out by Shen et al. [16–18] and William and Brindle [12]. For catalytic conversion of pyrolytic oil into aromatic hydrocarbons, they used zeolite-based catalysts such as Y-zeolite (CBV-400) and zeolite ZSM-5. The results showed that the catalyst reduces the yield of oil with a consequent increasing of gas amount accompanied by coke deposition on the catalyst surface. Arabiourrutia et al. [19] tested HZSM-5 and HY zeolites for tires pyrolysis in a conical spouted bed reactor at 425 and 5008C.
Environmental Progress & Sustainable Energy (Vol.36, No.5) DOI 10.1002/ep
Figure 1. Schematic representation of the bench-scale installation. [Color figure can be viewed at wileyonlinelibrary.com]
They proved that HZSM-5 zeolite catalyst produces an increase of gases yield, more specifically, a larger amount of propene and butadiene. Shah et al. [20] studied the influence of an acidic catalyst (SiO2), a basic catalyst (Al2O3), and their mixture (Al2O3/ SiO2) on the pyrolysis of used tire rubber. They found that oil derived from catalytic pyrolysis with Al2O3 contains higher concentration of polar compounds (40%) and lower concentration of aliphatic hydrocarbons while the aromatic hydrocarbons amounts appear not to be influenced by the catalyst type. Also, catalytic pyrolysis in presence of Al2O3 gave the highest amount of liquid fraction. Shah et al. [21] studied the waste tires degradation through catalytic pyrolysis using basic catalysts (MgO and CaCO3) in a batch reactor under atmospheric pressure. Tires conversion to gas, liquid, and char fractions at 3508C and for 2 h of catalytic pyrolysis was 24.4/39.8/35.8 (%wt) for MgO, and 32.5/32.2/35.2 wt % for CaCO3. They found that both catalysts produced about 25%wt of aliphatic hydrocarbons in liquid fraction, while the use of MgO generates an increasing aromatic hydrocarbons amount (55%wt). Co-pyrolysis of waste tires is another pathway that many researchers have investigated. Onenc et al. (2012) [9] carried out experiments on scrap tire catalytic co-pyrolysis with oily wastes. They used commercial catalysts such as FFC (used in refinery for catalytic cracking) and Red Mud with mass ratio catalyst:feedstock of 1:5. The result showed that these catalysts did not have significant effects on the yields of pyrolysis products. As presented above, published papers looked into the use of catalysts for energy utilization of waste tires paying attention mostly on how they act to produce more gas or liquid products. The purpose of this work was to identify the influence factors that intervene during the thermochemical degradation of French waste tires. In the first part of the study [22], the influence of textile fibers and metal wires used as reinforced materials on thermochemical behaviors of rubber was identified and discussed. In this part, the influence of catalysts is approached at TGA and laboratory bench scales. The influence of contact between pyrolysis vapors and catalyst surface on pyrolysis products yield is also studied. Aiming to establish the most effective catalyst and catalytic system to increase the gas fraction, at laboratory bench scale, two contact ways were investigated and discussed. In TGA test, only direct contact between rubber in decomposition stage and catalysts particles was possible. EXPERIMENTAL WORK
Materials The materials tested in this study were supplied by Aliapur, a French company dealing with waste tires, and they
are a mixture of light duty vehicles used tires. Before pyrolysis, the samples were washed and dried at room temperature in a well-ventilated area. Four catalysts were used in this work: zeolite (ZSM-5), alumina (Al2O3), calcium carbonate (CaCO3), and magnesium oxide (MgO). All catalysts were Sigma Aldrich analytically pure compounds, in powder form. Thermogravimetric Analysis Tire rubber samples used in this study were cut in small pieces (2 3 3 mm) and weighing between 20 and 25 mg. Setaram SETSYS Evolution TGA-DTG/DSC analyzer was used. Parameters used for thermogravimetric (TG) tests were as follows: heating rate: 108C/min; nitrogen flow: 20 mL/min; temperature range: 25–6008C. The nitrogen gas was used as a carrier gas. The tests were performed to extract specific information on the evolution of mass loss intensity and on changes of specific thermal ranges. Bench-Scale Installation The experiments were conducted at atmospheric pressure in a batch reactor (Figure 1) operated within the temperature range of 25–5508C. The average heating rate was kept between 8 and 118C/min, and the operation time varied between 55 and 65 min. The reactor (1) was made of stainless steel, with a total volume of 0.3 L and heated by an electrical heating system (2). The temperature inside reactor was measured and regulated by using three K-type thermocouples (T1, T2, and T3) placed in the center of the reactor at the bottom (T1, measuring the temperature of tire in different decomposition stages), at catalyst level (T2, measuring the temperature of catalytic reaction), and exhaust pipe level (T3, measuring the temperature of vapors leaving the reactor). The acquisition device (3) registered the values of temperatures evolution inside the reactor (T1, T2, and T3) and the reference temperature (TR), and allowed the control of thermal profile along the process. Two reaction systems were used: Case I and Case II. In Case I, the catalyst was placed over the tire samples in the same manner as TGA curves were recorded. The system allowed direct contact of catalysts with carbonaceous surface but also with primarily pyrolysis vapors resulted from rubber degradation. Using this system, the cracking reactions occurred in both homogenous (solid/solid) and heterogeneous (vapor/solid) conditions. In Case II, a fixed catalytic bed (4) was placed at the mid height of the reactor. This catalytic system allowed direct contact only between catalyst and pyrolysis vapors resulted from rubber degradation. Using this system, the cracking reactions are expected to occur only through heterogeneous reactions.
Environmental Progress & Sustainable Energy (Vol.36, No.5) DOI 10.1002/ep
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Table 1. Information on thermal range of rubber degradation during pyrolysis tests.
Sample Tire Tire Tire Tire Tire
rubber (no catalyst) rubber 1 CaCO3 rubber 1 Al2O3 rubber 1 ZSM-5 rubber 1 MgO
Ti (8C)
T* (8C)
T** (8C)
Tf (8C)
Dt (min)
Total conversion (%wt)
329 371 340 402 371
388 – 387 – –
439 460 455 452 460
494 505 501 496 504
16.40 13.33 17.38 9.36 13.21
54.59 63.92 63.32 53.85 62.68
by difference. The experimental tests have been repeated several times to ensure a good repeatability of the values presented further in this article and also used to design the next level of pyrolysis reactor. The experimental standard deviation is lower than 5% for results obtained in bench installation and lower than 2% in TGA. 3000 Micro-GC/TCD (Agilent Technologies), a 2-channel gas chromatograph with thermal conductivity detection (8) was used for the analysis of pyrolysis gases. The standard deviation of results obtained by GC/TCD is
