Pyrolysis, Combustion, and Steam Gasification of Various Types of

Dec 5, 2014 - combustion, and gasification characteristics of scrap truck and car ... samples were conducted in oxygen and steam ambiences, respective...
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Pyrolysis, Combustion, and Steam Gasification of Various Types of Scrap Tires for Energy Recovery Jayaraman Kandasamy*,† and Iskender Gökalp‡ ICARE-CNRS, 45071 Orleans Cedex 2, France ABSTRACT: The energy recovery from carbonaceous materials is considered a reliable energy source. In this context, pyrolysis, combustion, and gasification characteristics of scrap truck and car tire samples were investigated using a thermogravimetric analyzer coupled with mass spectrometer system. Pyrolysis behavior of the sample was estimated in argon atmosphere with the heating rate of 40 K/min over the temperature range of 100−1100 °C. The combustion and gasification studies of the tire samples were conducted in oxygen and steam ambiences, respectively. In addition, the size effect of the truck tire has been examined. The ignition temperature, temperature of maximum mass loss rate, and burn-out temperatures of both truck and car tires were measured. The pyrolysis gas mainly consisted of hydrogen, hydrocarbons, and carbon oxides, whereas in the condensable gases (pyrolytic liquids) mainly aromatic hydrocarbons and alcohols were detected in the temperature range of 300−500 °C. In the combustion processes, mostly CO and CO2 gases were released; besides some hydrocarbons were also evolved. Moreover, the release of sulfur compounds and nitrogen oxides were detected in pyrolysis and combustion stages, but the nitrogen oxide was predominant in the later stage. The complete gasification conversion of scrap tire is taking place at 950 °C during the isothermal condition in steam ambience. The main products detected during gasification were CO2, CO, and H2, indicating that oxidation, water gas, and water gas shift reactions were predominant. The emission of sulfur compounds during combustion and gasification reactions implied the presence of sulfur in pyrolyzed char. The results indicated that the scrap tires can be used as a potential energy resource. The activation energy (Ea) was calculated from the thermogravimetric test results by using an Arrhenius-type kinetic model.

1. INTRODUCTION Energy crisis and environmental degradation are the main problems that mankind is facing nowadays. This is due to the growing population, rapid industrialization, and disposal of diverse solid waste, which are generated on a regular basis. Considerable research has been done to recover energy from waste materials, including the materials that are not biodegradable. The continuous accumulation of used tires is one of the worst solid waste problems faced by several countries. According to reports from the largest associations of tire and rubber manufacturers, the annual global production of tires is quoted as 1.4 billion units, which corresponds to an estimated 17 million tonnes of used tires each year.1−6 China, the countries of the European Union (EU), the USA, Japan, and India produce the largest amounts of tire wastesalmost 88% of the total number of withdrawn tires around the world.7 Several policies have been introduced to minimize the environmental impact of waste tires that focus on lowering the number of tires requiring for disposal. Particularly, improvements in materials and manufacturing have made it possible to reduce the weight of tires and to extend their life cycle. Several methods were proposed to recover the energy from waste tire disposal along with being compatible with the ecosystem.8 Thermal valorization is emerging as a possible solution for reprocessing the huge amounts of these materials. The three main technologies for thermal valorization are pyrolysis, gasification, and combustion. Pyrolysis is a thermochemical process performed under inert atmosphere, while gasification and combustion are performed under mild and severe oxidative conditions, respectively. © 2014 American Chemical Society

Pyrolysis is considered to be an environmentally friendly process transforming waste tires into useful products. Pyrolysis involves the decomposition of the tire rubber at the temperature range of 200−900 °C in an inert atmosphere. The waste tire is decomposed into pyrolysis products in the form of solid char, liquid oil, and gases during the process. Temperature, heat, and mass transfer conditions, as well as the particle size, are the main factors affecting the behavior of thermal decomposition and the amount and composition of the pyrolysis products. The solid residue is a good candidate for solid fuel or for low grade carbon black. Several researchers have reported on the pyrolysis characterization of scrap tires and other carbonaceous materials using different methods.9−17 Rodriguez et al.9 examined the pyrolysis of scrap tires under nitrogen atmosphere at 300, 400, 500, 600, and 700 °C. They have found that there is no significant influence of temperature on the amount and characteristics of pyrolysis products over 500 °C. Some researchers9,18 have reported the composition of evolved pyrolysis gas fractions and mentioned the significant concentrations of methane, ethane, butadiene, and other hydrocarbon gases with a gross calorific value (GCV) of almost 37 MJ/m3. The yield and chemical composition of the pyrolysis products vary with the tire compositions that depend on tire type, age, and manufacturer.19,20 Different types of scrap tires are available in the society including the following: bicycle tires, motorcycle tires, car tires, tractor tires, bus tires, and truck tires. Received: October 10, 2014 Revised: December 5, 2014 Published: December 5, 2014 346

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high H2 concentrations, and the heating values reached were 8−10 MJ/m3 which were approximately 2.5 times higher by weight and 1.6 times by volume as compared to an air-blown gasification system. Galvagno et al.33 have reported the comparative analysis of steam gasification of refuse-derived fuel (RDF), poplar wood, and scrap tires in a laboratory-scale rotary kiln. The tiresderived syngas contains higher concentrations of hydrogen (almost 45% (v/v)), methane, ethylene, and ethane, but minor contribution from oxygenated products. Donatelli et al.34 reported on the production of high energy syngas using waste tires in steam gasification in rotary kiln pilot plant. They have reported that the gas energy content can be increased with the optimum steam/tire ratio of 0.33. Nicolas and Aldo35 have kinetically examined the synthesis gas production by steam gasification of carbonaceous waste materials with high volatile contents (e.g., sewage and industrial sludge, fluff, and scrap tire powder). Portofino and co-workers36,37 have performed the steam gasification of waste tire to assess the product yield and composition in the temperature range of 850−1000 °C. They have quoted that the production of the syngas is increased at elevated temperatures, typically with a greater amount of hydrogen. López et al.38 reported pressure effects on the gasification of the char derived from distillation of granulated scrap tires. They have reported the yield of H2 and CO contents and higher heating value (HHV) of the syngas produced from char gasification increases with pressure. Karatas et al.39 performed the gasification studies of waste tire using air and CO2, air and steam, and steam in a bubbling fluidized bed gasifier. They have reported that the average lower heating value (LHV) of the product gas was 15.21 ± 0.98 MJ/(N m3). Most of the works examined the pyrolysis and gasification characteristics of scrap tires independently, but there is limited research on pyrolysis, combustion, and gasification processes in an organized manner, whereas this work brings together all three processes. In this context, the tire samples were selected according to their potential to be used in thermochemical conversion processes. As reported in our earlier research,40,41 other research42 also recently stated that thermogravimetric analysis coupled with mass spectrometry (TGA-MS) technique could be used to obtain information in real time of mass loss and evolved gas analysis for pyrolysis, combustion, and gasification processes. This work presents the experimental investigation of the pyrolysis, combustion, and gasification of car and truck tires with different particle sizes using TGA studies to understand the basic phenomena associated with these processes. Particularly, volatile matter emission and burnout and char combustion details have been estimated. The appropriate temperature range for the gasification process was evaluated. Furthermore, this work pretends to establish and gain further understanding of the possible effects of rubber constituents such as natural and synthetic ones over various processes. In addition, evolved gases from the thermochemical conversion of scrap tires were also evaluated using the MS technique. The use of non-isothermal process associated with evolved gas analysis provides a qualitative explanation of the pyrolysis and gasification characteristics of the scrap tires. In addition, a preliminary kinetic analysis of the thermochemical conversion process was performed in order to obtain the apparent reaction rates by using basic Arrhenius kinetic method.

These tires comprise vulcanized rubber as well as rubberized fabric with reinforcing textile cords, fabric belts or steel, and steel wire reinforcing beads.21 Many different tire manufacturers and countless different tire formulations are available based on the applications; the composition of the tire varies depending on the tire grade and manufacturer. Accordingly, the tire pyrolysis products are varied in terms of yield and chemical composition influenced by the source and grade of the tires. Ucar et al.22 have explained the pyrolysis behavior of two different scrap tires using thermogravimetry and gas chromatography−mass spectrometric (GC-MS) study. They also found significant variations in the proximate and ultimate analyses and variations in the pyrolytic product yields, and liquids and gaseous product compositions, of two different tire wastes. Kyari et al.23 have investigated the pyrolysis characteristics of seven different brands of scrap car tires from several countries throughout the world and analyzed the product liquids from individual tires and a mixture of various categories of tire waste. Leung et al.24 have analyzed the gaseous composition of tire pyrolysis and reported that the heating value of the gaseous product was in the range of 20−37 MJ/ ́ and his co-workers have reported that Nm3. Recently, Martinez the co-pyrolysis of waste tire with biomass improved the stability of bio-oils with upgraded properties by radical interactions25 and also reported its application in diesel engines.26 The gases released from the combustion of waste tires are characterized by process conditions such as temperature, pressure, oxygen concentration, particle sizes, and reactor type, etc. Using thermogravimetric analysis, Atal and Levendis27 have noticed that tire particles has undergone an intense primary volatiles evolution and subsequent combustion phase, followed by a phase of simultaneous secondary volatile combustion of less intensity and the remaining char combustion. They have also reported that char burnout times were considerably shorter for tire particles than for coal. Mastral et al.28 have used the fluidized bed combustion system and reported that both gas superficial velocity and partial pressure of oxygen exert influence upon the overall fixed carbon combustion efficiency. Courtemanche and Levendis29 have reported the burned coal and waste tire crumb features using an electrically heated drop-tube furnace at high particle heating rates (104−105 K s−1) and elevated gas temperatures (1300− 1600 K). Some researchers30,31 have stated that combustion of coal generated four times more NOx than combustion of tire crumb, in proportion to their nitrogen content, whereas the emissions of SO2 are comparable. Gasification is a partial oxidation process in which oxidant streams reacts with the solid char in an endothermic reaction, producing gaseous carbon monoxide and hydrogen, along with carbon dioxide and light hydrocarbons. The gas composition mainly depends on the operating conditions such as temperature and gasifying agent concentrations (air, oxygen, and/or steam), etc. With the resulting gas mixture commonly specified as syngas, which can be transported and used as fuel in gas turbines and fuel cells following the cleaning stage, the difficulty and thoroughness depend on the composition of the raw fuel and its final application. Gasification studies of waste tires have been performed at both laboratory and pilot scales. The characteristics of syngas evolution during pyrolysis and steam gasification of waste tires have been investigated by several researchers.32−40 Lee et al.32 have shown that the steam gasification of carbonaceous materials produces syngas with 347

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ratios qualitatively. A semiquantitative analysis of the gases produced from the tire samples was performed by comparing the intensity peak areas obtained for each compound. Screening analyses were carried out in the selected-ion monitoring (SIM) mode. The ions characteristic of several molecules were monitored based on their m/z (molecular weight/charge) ratio. The experimental error of these measurements was calculated, the weight loss measurement is within ±0.5% and temperature measurement is within ±2 °C. Also, it is important to notice that the QMS spectrum of some mass values represents more than one molecule over the tested samples, which is explained in detail in Results and Discussion. 2.2. Kinetic Analysis. Arrhenius Method. In the Arrhenius method,43 the mass loss measured rate from the TG curves accounts for gross changes in the total mass, and the reaction model assumes that the reaction rate of mass loss of the total sample is dependent on the rate constant, the remaining mass of sample (W), and the temperature.

2. EXPERIMENTAL DETAILS 2.1. Materials, Thermogravimetric Analysis and Mass Spectrometric Analysis of the Gaseous Products. The demetallized and dereticulated car and truck tire samples were acquired from Phenix Machinery, Sancheville, France. The size of the car tire sample was around 10 mm, whereas the truck tire samples were of two sizes: finer particles with the size range from 500 to 800 μm; larger particles in the size range from 1 to 4 mm. The proximate analyses of the scrap tire samples were determined at ASTM standards which were used to estimate the percentages of moisture, volatile matter, fixed carbon (by difference), and ash. Ultimate analysis was used to obtain the mass fraction of carbon, hydrogen, nitrogen, oxygen, and sulfur. The calorific value was determined experimentally using a bomb calorimeter. The ultimate and proximate analyses of the samples are given in Table 1. The thermogravimetric tests were carried out using a

Table 1. Proximate Analysis, Ultimate Analysis, and Heating Values of Tire Samples unit moisture ash VM fixed carbon C H N S others (O + ash) calorific value

truck tire (dry basis)

Proximate Analysis 0.82 4.17 62.7 32.31 Ultimate Analysis % 80.30 % 7.18 % 0.5 % 1.19 % 10.83 Heating Value MJ/kg 30 % % % %

dW /dt = kW n The temperature dependence of the rate constant, k, is expressed by the following Arrhenius equation:

car tire (dry basis)

k = A r e−Ea / RT

0.8 8.6 61.8 28.8

With the assumption of first-order kinetics, dW /dt = A r e−Ea / RT W

[(dW /dt )/W ] = A r e−Ea / RT

79.1 6.7 0.6 2.2 11.4

The logarithm is taken on both sides,

log[(dW /dt )/W ] = log A r − Ea /(2.303RT ) where, dW/dt refers to the rate of mass change, Ea and T represent the activation energy and the temperature, respectively, Ar refers to Arrhenius constant, and n indicates the order of reaction. When log[(dW/dt)/W] is plotted vs 1/T, a straight line depicts the slope which is equal to Ea/(2.303R) and from the intercept, the Arrhenius constant is estimated. Knowledge of kinetic parameters, such as the activation energy, is one of the key parameters to determine the reaction mechanisms in the solid phase.

28.9

NETZSCH STA 429 thermal analyzer coupled with a quadrupole QMG 511 mass spectrometer in various ambience conditions. The experimental setup used for the gasification experiments was presented in detail elsewhere.40,41 A separate steam (water vapor, WV) generator is connected with the STA, in which the steam generator and transfer line were maintained at the temperatures of 180 and 150 °C, respectively. During the experiment, about 20 mg of sample was placed in a ceramic crucible and heated to 1150 °C with a heating rate of 40 K/min and isothermal sections were retained at 950 and 1000 °C in some cases. Argon was used as a protective gas with the flow rate of 20 mL/min and also used as the carrier gas for the steam; steam with the flow rate of 6 g/h is utilized, air with the flow rate of 2 mL/min was used, and oxygen was used with the flow rate of 2 mL/min in the steam gasification process and 50 mL/min in the combustion process. These flow rates are programmed and automatically controlled by the ATG system. The output of the TGA system is coupled to the mass spectrometer through a heated line with quartz capillary tube. Mass spectrometric analyzer is used to monitor flue gas composition during the thermal analysis. The TGA-MS runs were performed in a dynamic gas atmosphere. The sample temperature is measured with a type S (Pt−Rh10/Pt) thermocouple directly which is placed under the sample holder. The smaller volumes in the thermobalance microfurnace, transfer line, and gas measurement cell admit low carrier gas flow rates in the system which leads to good detection of the released gases from the pyrolysis, combustion, and gasification process. Online gas analyses are performed for the detection of released gases which are fed to the mass spectrometer, and experimental data are stored as a function of time. The balance adapter, transfer line, and MS gas cell are maintained at 250 °C, hence avoiding the condensation of the less volatile compounds. The excitation energy in the mass spectrometer is 1100 eV. The MS system is operated under a vacuum and detects the characteristic fragment ion intensity of the volatiles according to their respective mass to charge ratios (m/z). The mass spectrum intensity shows the relative gas release components based on mass to charge

3. RESULTS AND DISCUSSION 3.1. Pyrolysis Characterization of Scrap Tires. In this study, the samples were subjected to a dynamic heating rate of 40 °C/min until it reaches the temperature of 1100 °C in the presence of argon atmosphere. TG and DTG analyses on samples were conducted by measuring the decrease in mass of the tire with the increase in time and temperature, respectively, as shown in Figure 1. The car tire with the size of 20 mm and two sizes of truck tire samples, namely, 1−4 mm and 500−800 μm size ranges, were shown. Even though both tires displayed a single pyrolysis stage and virtually the same total carbon content (Table 1), the decomposition temperature differed between the two types of tire samples. The compositions can be assessed with use of the fact that the rubbers typically used in the manufacturing of tires are natural rubber (NR, polyisoprene), styrene−butadiene rubber (SBR), and polybutadiene rubber (BR). As reported elsewhere, the thermogravimetric analysis is to give information about the thermal decomposition process of the tire material,21 as well as its rubber component types.18 Besides, TG and DTG methods are commonly used in the investigation of thermal degradation behavior of the main components of the tire materials.44−46 The initial decomposition temperature (Tdo) refers to the temperature at which the decomposition of the sample starts and it is usually described to weight loss rates above 0.1%/min. The final pyrolysis temperature refers to the temperature where 348

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The maximum degradation rates (Tp) are noticed in the temperature range of 375 °C for NR, 445 °C for SBR, and 465 °C for BR. It can be seen that the DTG curve of the car tire exhibits the dominant peak at around 450 °C, such that this peak is an indication of the presence of a larger amount of SBR in car tire waste and agrees well with the results reported by other authors.46,48,49 In the truck tire, the DTG curve shows that the major peak is around 410 °C (Tp), which is the indication of natural rubber presence. The pyrolysis tests reflect that the major composition of truck tires consists of natural rubber when compared to the lower level in car tires. Marginally higher temperatures were required to pyrolyse the car tire compared to the truck tire. Similar trends were also observed in the reported literature by Berrueco et al.18 and Murillo et al.50 The ultimate char yield from the tire pyrolysis process is derived as 35% for car tires and 34% for truck tires, which is closer to the reported values of 33−38% by several researchers.49,50,52 Finally, a mass degradation of 3% is observed at 1050 °C in truck tire samples; this occurred due to the devolatilization of char and the decomposition of rubber constituents. Releases of gaseous species and products as a result of thermal decomposition of the tires were simultaneously monitored by mass spectrometry during the TGA tests. The mass spectra of the gases (major ions) evolved during pyrolysis are illustrated in Figure 3 and Figure 4 for the car and truck

Figure 1. TG curves of scrap tires in argon and oxygen ambience.

the pyrolysis rate vanishes (Tpf), and it is identified when the weight loss rate is below 0.1%/min. According to the decomposition profile (Figures 1 and 2), the thermal

Figure 2. TG (solid line) and DTG (dotted line) curves of scrap tires in argon and oxygen ambience.

decomposition of the car tire starts at about 280 °C (Tdo) and is practically complete at 530 °C (Tpf). Unlike the results of car tires, the thermal decomposition of smaller sized truck tires starts at 250 °C (Tdo) and finishes at around 500 °C (Tpf). In this context, two factors could be clarified: the first one is the size effect, and another is the rubber ingredients and additives. The size effect factor could be attributed to smaller particles containing larger surface area leading to an increased diffusion phenomenon for the pyrolysis reaction. On the other hand, the total amount of char produced (34 wt %) from this process remained constant even with different particle sizes as illustrated in Figure 1. The peak temperature (Tp) corresponds to the temperature when the maximum weight loss rate (dW/ dt)max is attained. The presence of oxygen slightly shifted the maximum pyrolysis phase of both truck and car tires into lower temperature ranges when compared to argon ambience. This may be caused due to surrounding gases diffusional effects over the evolved gases from pyrolysis. According to the reported results by Sułkowski et al.,47 the thermal decomposition progressively occurs through different peaks which account for the instantaneous degradation of the main components of the tire, principally NR, SBR, and BR.

Figure 3. TG and mass spectra of the evolved gases of truck tire in argon ambience.

Figure 4. TG and mass spectra of the evolved gases of car tire in argon ambience. 349

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tires, respectively. As reported elsewhere,9,14 the composition of the pyrolysis products is directly related to the temperature of the thermal treatment. It can be attributed to gaseous products mainly composed of light volatiles such as H2 and H2O (m/z = 2 and 18); hydrocarbons such as CH4 and C2H4+ (m/z = 16 and 28); carbon oxides (C−O) such as CO and CO2 (m/z = 28 and 44); alcohols (AL) such as CH2OH and C2H5OH (m/z = 31 and 46); isoprene (C5H8+; m/z = 68); nitrogen compounds such as NO2 ((m/z = 46); aromatic compounds such as C5H4+, C6H6, and C7H8 (m/z = 64, 78, and 92); and sulfur compounds (S) such as H2S, COS, and SO2 (m/z = 34, 60, and 66). The thermal decomposition of tires took place via depolymerization and decarboxylation of the hydrocarbon chains of the tire polymers and carbohydrates. The breakdown of this polymer molecule turned into the production of a great variety of low molecular weight products between 300 and 550 °C such as H2, CO, CH4, C2H4, C2H3+, CO2, H2S, and SO2 gases. Both tire samples showed production of a substantial amount of gases such as CH4, H2, CO, CO2, and C2H4, which agrees well with the reported literature.18,24,44,45 The release of H2O was observed at around 200−400 °C which is associated with moisture and combustion of volatile compounds and char. On the other hand, the H2O production given by the MS analysis can be slightly overestimated, as contribution of H2 and O2 fragmentation may exist. Williams and Taylor53 have shown that hydrogen, methane and ethane resulted from secondary aromatization reactions which produce aromatic hydrocarbons. As previously mentioned, the maximum production rate of hydrocarbons (HCs) (CH4, C2H3+, and C2H4) and COx took place at temperatures around 480 °C, when volatiles are released evidencing carbon removal chemistries such as decarboxylation and decarboxylation reactions. The gaseous product results were in good agreement with those reported in previous works focused on the study of the HC and COx gases from scrap tire pyrolysis.9,54,55 The COx gases are derived from oxygenated compounds such as stearic acid, extender oils which are used in tire manufacturing. Cypres et al.56 have reported that hydrogen and methane were released due to the secondary aromatization reactions. Two emission peaks were observed for CO2 and SO2: first was in the main devolatilization stage at 480 °C, whereas the second was at 780 °C for truck tires. The second stage emission of CO2, obtained in the temperature range of 650− 750 °C, was attributed to the thermal cracking of the pyrolysis oil vapors in the gas, as previously reported by Kyari et al.23 Moreover, the shape of MS curves for SO2 and COS gases were very similar pointing out that the evolution of sulfur compounds occurred at the same temperatures, despite the fact that the emission level of COS is relatively less. The second emission of sulfur compounds can be characterized by part of the sulfur remaining in the char which might be later gasified or oxidized turning it into the production of gases. However, the car tire showed the single and extended peak of CO2 and SO2 emission over the temperature between 300 and 500 °C. Similar trends were observed for CO emission also. However, the CO production given by the MS technique might be slightly overestimated. All signals for m/z = 28 were considered as the contribution of CO production, while contribution of C2H4 and CO2 fragmentation also existed.24,45 In addition, a single emission peak of H2S is presented at around 480 °C for both the tires. As it can be seen that H2S curve is overlapped on a NO2 curve; hence, both gases are released at the same level for both tires. The main peak obtained for NO2 could be

associated with the combustion of nitrogen with oxygen which has emerged as tire ingredients (Table 1). The total contents of hydrocarbon gaseous products were similar in both car and truck tires derived gaseous products. However, car pyrolysis gases contained a lower amount of CO2 but comparatively higher amount of sulfur compounds than that of truck tire pyrolysis product gases which is well correlated to the low carbon and high sulfur content calculated in the ultimate analysis (Table 1). Similar gaseous product releases have been reported for car and truck tires by Ucar et al.22 The smaller level of mass loss of the sample at 1050 °C shows the release of CO, CO2, and H2 gases. The formation of these products resembled well the degradation stages observed in the mass loss curves. Hence, it can be concluded that the tire pyrolysis mechanism and gaseous products were mainly based on its ingredients. Gaseous products generated during the pyrolysis of tires were composed by light-weight and condensable gases such as primary alcohols of CH2OH and C2H5OH and aromatic compounds of benzene (C6H6), toluene (C7H8), and isoprene (C5H8+) as stated by other researchers.18,24,44,45 The maximum evolution of these condensable gases were taken at temperatures from 420 to 480 °C for truck tires, whereas , for the car tires, the temperature range is widened from 400 to 500 °C. The condensable gases were detected near nonequilibrium conditions by MS sensor. As it can be seen from the ion intensity level of the MS curves of the condensable gases, the contribution of aromatics compounds were high and then followed by oxygenated compounds such as alcohols. This trend of product emission with the pyrolysis temperature range was in good agreement with those previously reported.16,34 Hence, it is confirmed that the representation of pyrolytic liquid production from the tire pyrolysis processes, such as the release of alcohols and aromatic compounds, took place during the pyrolysis stage. These results are similar to those reported by other authors.15,24,33,44,45 However, other pyrolytic liquids might also be formed, as stated by the researchers; here only the major contributions were reported. 3.2. Combustion of Scrap Tires and Evolved Gas Analysis. During combustion experiments 80% of oxygen by volume is supplied into the furnace. The thermogram (TGADTG profiles) of the car tire shows that the initial pyrolysis stage (280−530 °C) was not influenced by the oxygen concentration since the curves almost overlap, as shown in Figures 1 and 2. This phenomenon reveals that the thermal decomposition of the sample occurred in the kinetic control zone and was mainly affected by the temperature, and the effect of oxygen concentration is almost insignificant. However, there is a slight shift of TG-DTG curves to lower temperature for truck tire; the possible causes are explained in the pyrolysis section. The second stage was due to the oxidation of the remaining char after the volatiles were removed from the samples and the gradual diffusion of oxygen to the surface of the samples and the subsequent combustion which is clearly depicted in Figures 1 and 2; Ti and Tb are the ignition and burn-out temperatures of the samples. The burn-out temperature of combustion is also determined as the final pyrolysis temperature. The thermograms in Figures 1 and 2 show that the combustion of the car tire starts (Ti) at 500 °C, whereas that for the truck tire starts at 490 °C. In this process, fixed carbon, nonpyrolyzed sulfur, and nitrogen compounds present in the char, about 32% total mass fractions for truck tire and 29% total mass fractions for car tire, along with nonpyrolyzed 350

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H2S, estimated in the MS curve during the combustion stage. All signals for m/z = 46 were considered as the contribution of C2H5OH production while contribution of NO2 also existed in the gases. Therefore, it could be assumed that the MS curve (m/z = 46) in the combustion stage is due to a significant amount of NO2 as well as H2S. The release of CO is substantial in both tire combustion stages, as shown in Figures 5 and 6. Hence, it can be concluded that the partial oxidation of char (2C + O2 ↔ 2CO) and secondary reactions such as methanation (C + H2 ↔ CH4) have also occurred in the tested temperature range when compared to the CO and CH4 intensity levels. Similarly, other HCs such as C2H3+ and C2H4 are also formed and released marginally in this stage. Furthermore, the SO2 release took place continuously at the combustion process, which could be correlated to the decomposition of annealing components existing in the tire samples (Table 1). Atal and Levendis27 have also reported the various gaseous pollutants such as CO, CO2, SO2, and NOx oxides at the tire combustion under fuel-lean to fuel-rich conditions. Since this experiment is conducted at nitrogen-free gas, the NO2 emissions are detected; since it is overlapped with H2S, hence was not substantial as reported.27 The combustion of tires produced contaminant emission such as CO2 and SO2, which was being controlled and was provided with the necessary elimination system. Previous works have demonstrated that the direct combustion of the scrap tire is not a feasible solution due to the high amount of emissions generated; for example when coal is burnt at the same conditions, emissions are 2 orders of magnitude lower.45 Cofiring coal with tire appears to be a promising technique to control the overall emissions from the tire combustion process. Also, it was observed that the percentage of SO2 is around 700 ppm, and taking into account the increasing emission restrictions, schemes for gas cleaning should be implemented. 3.3. Gasification Characteristics of Scrap Tires. In the gasification process, the tire samples were subjected to dynamic heating rate until 950 °C and maintained at isothermal conditions for 25 min. The total sample conversion is defined as a function of temperature and run time. The steam is supplied inside the furnace for the gasification process, whereas argon was used as carrier gas. The char gasification process is more complex than the pyrolysis, as the former is a heterogeneous process where the chemical reactions occurred over the surface of the material. Also, it can be admitted that the heterogeneous rates of char conversion occurred by the fundamental components, represented by surface area, surface accessibility, carbon active sites, added inorganic matter, and the gasification agent composition. Figure 7 shows the conversion of the tire in the pyrolysis and gasification process versus time on stream for car and truck tires, for two truck tire particle sizes, an argon flow rate of 60 mL/min, a steam concentration of 70% by volume in argon, and a heating rate of 40 °C/min. Scrap tire powder revealed a clear separation of pyrolysis from gasification reactions, with more than 60% mass loss occurring below 550 °C in pyrolysis process. The mass loss and gas evolution can be identified as the result of pyrolysis, which usually occurs at the temperature range of 250−550 °C. After pyrolysis, no significant mass loss or gas evolution was observed at the temperatures between 600 and 750 °C. Important gasification reactions begin at around 750 °C in the steam ambience which was very close to the reported temperature by Wilson57 using coal char. Once the temperature is reached to 950 °C, constant temperature is maintained for

compounds (around 3% of the total mass fraction for both the tires) are combusted. The effect of the tire type is highly remarkable between 530 and 820 °C, where the oxidation of the char is taking place. Due to the difference in the natural rubber and synthetic rubber compositions of car and truck tire samples as reported earlier, Tb of the car and truck tires are occurred at 820 and 700 °C, respectively. It can be observed from the DTG curves that the combustion rate of the truck tire is relatively high. The truck tire turned out to be the most reactive material, followed by the car tire. Notably higher temperatures are required to combust the car tire compared to the truck tire. The size effect of the truck tire during combustion is not significant, as shown in Figure 1. The gases released during the tire combustion are shown in Figures 5 and 6. Significant amounts of CO, CO2, and SO2

Figure 5. TG and mass spectra of the evolved gases of truck tire in oxygen ambience.

Figure 6. TG and mass spectra of the evolved gases of car tire in oxygen ambience.

gases are released at both tire combustion processes. Marginally, H2 and H2O were released in the truck tire combustion. As was mentioned earlier the CH4 and oxygen fragment curves coincided with one another, but the intensity level of CH4 is relatively more in oxygen ambience when compared to argon ambience. A small amount of CH4 is released in truck tire combustion. However, the car tire combustion significantly releases CH4. It can be observed that the majority of condensable gases such as aromatic compounds and alcohols were released during the pyrolysis stage. However, the oxygenated compound such as C2H5OH was overlapped with 351

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Figure 7. TG and temperature curves of scrap tires gasification in steam ambience.

Figure 9. TG and mass spectra of the evolved gases of car tire in steam ambience.

the period of 25 min. In fact, a total mass conversion of 5% was obtained in the dynamic temperature from 750 to 950 °C for both tire types. Finally, the last stage (at 950 °C) corresponded to the gasification of the pyrolized char. At the end of this stage, it was observed that between 30 and 32 wt % of the char was gasified for the car and truck tires, respectively. After a further rise in temperature to 1000 °C, it did not support the char gasification reactions, as presented in Figure 7. The rate of gasification is almost similar for both the car and truck tires. The particle size effect on the gasification of the truck tire char was not as significant as pyrolysis. It can be observed that the char conversion efficiency of 99% is achieved in the present process with the residence time of 20 min, as shown in Figure 7. These results substantiated that the scrap tire char gasification can be carried out at ∼950 °C using the steam and also blended mixture of steam, oxygen, and air with efficient carbon conversion. The MS analysis shows the presence of the gas components; mainly CO, CO2, H2, and SO2 were demonstrated in Figures 8

the CH4 content is less significant at atmospheric pressures36 and at higher temperatures.38 Gasification appears to run within the isothermal regime at 950 °C, as indicated by the sustained peak of H2, CO, SO2, and CO2 gases. Scrap tire powders also achieved the complete reaction/carbon conversion within the isothermal regime at 950 °C; mass loss curves and mass spectrometric analysis of the revealed gases confirmed the perception. Several researchers have debated the scrap tire gasification temperatures; Piatkowski and Steinfeld35 mentioned the ranges from 875 to 1100 °C, López et al.38 indicated the constant temperature of 1000 °C, Portofino et al.36 reported from 850 to 1100 °C temperature ranges, and Karatas et al.39 reported from 790 to 820 °C. The conversion efficiency of 95% is achieved in the present process with the residence time of 20 min, as shown in Figure 7. These results show that the scrap tire gasification can be carried out at ∼950 °C using the steam and also steam-blended air with efficient carbon conversion. 3.4. Kinetics Analysis. The use of thermo gravimetric analysis to determine kinetic parameters for the pyrolysis, combustion, and gasification of tires is complex, in that tires contain a multi component mixture of polymers, carbon black, mineral fillers, curatives, plasticizers, and other components. In addition, the decomposition of tires comprises a large number of reactions in parallel and in series, whereas thermogravimetric analysis measures the overall weight loss due to these reactions. This provides general information on the overall kinetics rather than individual reactions. However, it is useful in providing comparative kinetic data under different decomposition and reaction conditions, as in this work. Kinetic analysis is performed to provide the theoretical basis for the behavior of low temperature pyrolysis, combustion, and gasification of the samples. The dependence of the logarithm of the reaction rate ((dW/dt)/W) on 1/T is linear, with a slope of m = Ea/R and with an intercept value of A, as shown in Figure 10. It shows the values of the activation energy (Ea) as functions of temperature. Ea of the samples is determined for lower temperature thermal decomposition, moderate and higher temperature combustion, and gasification processes respectively using the Arrhenius method (Figure 10) , and the results are given in Table 2. Activation energy calculated during the decomposition of car and truck tires is 77.5 and 135.9 kJ/mol, respectively. The value differences from the pyrolysis of truck tire and car tire is due to the difference in the type of additives used in tire manufacturing, as mentioned in the pyrolysis section. These values show better agreement with the observed values of

Figure 8. TG and mass spectra of the evolved gases of truck tire in steam ambience.

and 9. Hence, the significant amount of H2 and CO release pointed out that char gasification reactions (C + H2O ↔ CO + H2; C + 2H2O ↔ CO + 2H2) were substantial. Besides, the SO2 release took place at this process, which could be related to the oxidation of sulfur existing in the pyrolyzed char, as released in the combustion process. Some amounts of CH4 are also detected in this process, but it is not shown in the figures due to the unit level. This is accomplished by other researchers also as 352

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takes place at 950 °C during the isothermal condition in steam ambience. The on-line mass spectrometric analyses confirmed the syngas production in steam gasification. The apparent activation energy of tire combustion is smaller when compare to the pyrolysis process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. † Postdoctoral researcher. ‡ Director, ICARE-CNRS.

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ACKNOWLEDGMENTS Financial support for this work is provided by the European Commission: Project OPTIMASH FP7- ENERGY, 2011-1 Project No 283050.

Figure 10. Arrhenius parameters estimation.

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Table 2. Kinetic Parameters of Car and Truck Tire Samples at Different Atmospheres type of samples and ambient conditions

activation energy, kJ/mol

A, min−1

car tire, oxygen truck tire, oxygen car tire, argon truck tire, argon

64.1 73.3 135.9 77.5

544.5 3641 179871 1480

REFERENCES

(1) RMARubber manufacturers association (USA). Scrap tire markets in the united states 9th biennial report, 2009; http://www.rma. org/scrap-tires/ (May 2011). (2) ETRMAEuropean tyre & rubber manufacturers’ association (Belgium). A valuable resource with growing potential, 2010 ed., 2010; http://www.etrma.org/default.asp (May 2011). (3) ETRMAEuropean tyre & rubber manufacturers’ association (Belgium). Used tyres recovery 2010 (table) − ut/part worn tyres/elt’s europe, volumes situation 2010, 2010, http://www.etrma.org/default. asp (May 2011). (4) ETRMAEuropean tyre & rubber manufacturers’ association (Belgium). The Annual Report 2010/2011, 2011, http://www.etrma. org/default.asp (May 2011). (5) WBCSDWorld business council for sustainable development (Switzerland). The report managing end-of-life tyres, 2008, http://www. wbcsd.org/templates/TemplateWBCSD5/layout.asp?type= p&MenuId=MTYwNg&doOpen=1&ClickMenu=LeftMenu (May 2011). (6) WBCSDWorld business council for sustainable development (Switzerland). End-of-life tyres: A framework for effective management systems. 2010, http://www.wbcsd.org/templates/template_WBCSD5/ layout.asp?MenuID=1 (May 2011). (7) JATMAThe Japan automobile tyre manufacturers association nc (Japan). Tyre industry of Japan 2010, 2010, http://www.jatma.or.jp/ english/media/ (May 2011). (8) Sharma, V. K.; Fortuna, F.; Mincarini, M.; Berillo, M.; Cornacchia, G. Appl. Energy 2000, 65, 381−394. (9) Rodriguez, I. M.; Laresgoiti, M. F.; Cabrero, M. A.; Torres, A.; Chomán, M. J.; Caballero, B. Fuel Process. Technol. 2001, 72, 9−22. (10) Mastral, A. M.; Murillo, R.; Callen, M. S.; Garcia, T.; Snape, C. E. Energy Fuels 2000, 14, 739−744. (11) Nielsen, A. R.; Larsen, M. B.; Glarborg, P.; Dam-Johansen, K. Energy Fuels 2012, 26, 854−868. (12) Ahoor, A. H.; Zandi-Atashbar, N. Energy Convers. Manage. 2014, 87, 653−669. (13) Lanteigne, R.; Laviolette, J.-P.; Tremblay, G.; Chaouki, J. C. Energy Fuels 2013, 27, 1040−1049. (14) Choi, G.-G.; Jung, S.-H.; Oh, S.-J.; Kim, J.-S. Fuel Process. Technol. 2014, 123, 57−64. (15) Conesa, J. A.; Font, R.; Marcilla, A. Energy Fuels 1996, 10, 134− 140. (16) Quek, A.; Balasubramanian, R. J. Anal. Appl. Pyrolysis 2013, 101, 1−16. (17) Mui, E. L. K.; Lee, V. K. C.; Cheung, W. H.; McKay, G. Energy Fuels 2008, 22, 1650−1657. (18) Berrueco, C.; Esperanza, E.; Mastral, F. J.; Ceamanos, J.; GarciaBacaicoa, P. J. Anal. Appl. Pyrolysis 2005, 74, 245−253.

previous researchers17,51,58 with similar experimental conditions. The activation of the tire particles during the combustion process in the range of 64−73 kJ/mol is consistent with the conclusion of a previous author.36 But, these values are less when compared with other researchers’ observed values of 145 kJ/mol.58 This indicates that attention must be paid to scrap tire gasification under various reactive gases.

4. CONCLUSION The TGA, DTG, and MS from this study provide valuable information on the pyrolysis process, combustion mechanisms, and gasification reaction for the three different tire waste samples. In the pyrolysis stage, each of the DTG curves exhibits two different weight loss regions which are due to the various constituents of the tire materials. The results indicate that the ignition temperature, temperature of maximum mass loss rate, and burn-out temperatures of truck tires occurred at a lower temperature range when compared with those for car tires. The pyrolysis of tire rubber and the combustion char occurs at temperature ranges of about 250−500 and 490−700 °C, respectively, for truck tire, whereas for car tires it is in the temperature ranges of 280−530 and 500−820 °C, respectively. The pyrolysis of tire particles and the combustion of its char occur at different temperature ranges which overlap slightly; these two processes have taken place continuously. Among the tested samples, the percentage of total weight loss is higher for truck tire waste and is lower for car tire. The pyrolysis gases from the tire samples consisted mainly of hydrogen, hydrocarbons and carbon oxides, whereas in the condensable gases (pyrolytic liquids) mainly aromatic hydrocarbons and alcohols were present in the temperature range of 300−500 °C. During combustion, CO, CO2, H2, and NO2 gases are mainly released; also a small amount of CH4 is detected. The tire char gasification process started at around 800 °C in steam ambience. The complete gasification conversion of scrap tires 353

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(19) Benallal, B.; Roy, C.; Pakdel, H.; Chabot, S.; Poirier, M. A. Fuel 1995, 74, 1589−1594. (20) Williams, P. T.; Besler, S.; Taylor, D. I. Fuel 1990, 69, 1474− 1482. (21) Williams, P. T. Waste Treatment and Disposal; 2nd ed.; John Wiley & Sons; London, 2005. (22) Ucar, S.; Karagoz, S.; Ozkan, A. R.; Yanik, J. Fuel 2005, 84, 1884−1892. (23) Kyari, M.; Cunliffe, A.; Williams, P. T. Energy Fuels 2005, 19, 1165−1173. (24) Leung, D. Y. C.; Yin, X. L.; Zhao, Z. L.; Xu, B. Y.; Chen, Y. Fuel Process. Technol. 2002, 79, 141−155. (25) Martínez, J. D.; Veses, A.; Mastral, A. M.; Murillo, R.; Navarro, M. V.; Puy, N.; Artigues, A.; Bartrolí, J.; García, T. Fuel Process. Technol. 2014, 119, 263−271. (26) Martinez, J. D.; Ramos, A.; Armas, O.; Murillo, R.; García, T. Appl. Energy 2014, 130, 437−446. (27) Atal, A.; Levendis, Y. A. Fuel 1995, 74, 1570−1581. (28) Mastral, A. M.; Callen, M. S.; Garcia, T. Energy Fuels 2000, 14, 164−168. (29) Courtemanche, B.; Levendis, Y. A. Fuel. 1998, 77, 183−196. (30) Levendis, Y. A.; Atal, A.; Carlson, J.; Dunayevskiy, Y.; Vouros, P. Environ. Sci. Technol. 1996, 30, 2742−2754. (31) Singh, S.; Nimmo, W.; Javed, M. T.; Williams, P. T. Energy Fuels 2011, 25, 108−118. (32) Lee, U.; Chung, J. N.; Ingley, H. A. Energy Fuels 2014, 28, 4573−4587. (33) Galvagno, S.; Casciaro, G.; Casu, S.; Martino, M.; Mingazzini, C.; Russo, A. Waste Manage. 2009, 29, 678−689. (34) Donatelli, A.; Iovane, P.; Molino, A. Fuel 2010, 89, 2721−2728. (35) Piatkowski, N.; Steinfeld, A. Fuel 2010, 89, 1133−1140. (36) Portofino, S.; Donatelli, A.; Iovane, P.; Innella, C.; Civita, R.; Martino, M.; Matera, D. A.; Russo, A.; Cornacchia, G.; Galvagno, S. Waste Manage. 2013, 33, 672−678. (37) Portofino, S.; Casu, S.; Lovane, P.; Russo, A.; Martino, M.; Donatelli, A.; Galvagno, S. Energy Fuels 2011, 25, 2232−2241. (38) López, F. A.; Centeno, T. A.; Alguacil, F. J.; Lobato, B.; LópezDelgado, A.; Fermoso, J. Waste Manage. 2012, 32, 743−752. (39) Karatas, H.; Olgun, H.; Akgun, F. Fuel Process. Technol. 2012, 102, 166−174. (40) Jayaraman, K.; Gokalp, I. Int. J. Adv. Eng. Sci. Appl. Math. 2014, 6, 31−40. (41) Jayaraman, K.; Gokalp, I. Energy Convers. Manage. 2014, 89, 83− 91. (42) Zhang, H.; Zheng, J.; Xiao, R.; Jia, Y.; Shen, D.; Jin, B.; Xiao, G. Energy Fuels 2014, 28, 4294−4299. (43) Kök, M. V. J. Therm. Anal. Calorim. 2005, 79, 175−180. (44) Islam, M. R.; Tushar, M. S. H. K.; Haniu, H. J. Anal. Appl. Pyrolysis 2008, 82, 96−109. (45) Juma, M.; Korenova, Z.; Markos, J.; Annus, J.; Jelemensky, L. Pet. Coal 2006, 48, 15−26. (46) Seidelt, S.; Muller-Hagedorn, M.; Bockhorn, H. J. Anal. Appl. Pyrolysis 2006, 75, 11−18. (47) Sulkowski, W. W.; Danch, A.; Moczyński, M.; Radoń, A.; Sułkowska, A.; Borek, J. J. Therm. Anal. Calorim. 2004, 78, 905−921. (48) Islam, M. R.; Haniu, H.; Beg, M. R. A. Fuel 2008, 87, 3112− 3122. (49) Yang, J.; Kaliaguine, S.; Roy, C. Rubber Chem. Technol. 1993, 66, 213−229. (50) Murillo, R.; Aylón, E.; Navarro, M. V.; Callén, M. S.; Aranda, A.; Mastral, A. M. Fuel Process. Technol. 2006, 87, 143−147. (51) Senneca, O.; Salatino, P.; Chirone, R. Fuel 1999, 78, 1575− 1581. (52) Islam, M. R.; Haniu, H.; Fardoushi, J. Waste Manage. 2009, 29, 668−677. (53) Williams, P. T.; Taylor, D. T. Fuel 1993, 72, 1469−1474. (54) Laresgoiti, M. F.; Rodriguez, I. M.; Torres, A.; Caballero, B.; Cabrero, M. A.; Chomon, M. J. J. Anal. Appl. Pyrolysis 2000, 55, 43− 54.

(55) González, J. F.; Encinar, J. M.; Canito, J. L.; Rodriguez, J. J. J. Anal. Appl. Pyrolysis 2001, 58 − 59, 667−683. (56) Cypres, R.; Bettens, B. In Pyrolysis and gasification; Ferrero, G. L., Mariatis, K., Buckens, A., Bridgewater, A. V., Eds.; Elsevier Applied Science: Oxfordshire, U.K., 1989. (57) Wilson, M. W. Method for increasing steam decomposition in a coal gasification process. U.S. Patent No. 4786291, 1998. (58) Leung, D. Y. C.; Wang, C. L. J. Anal. Appl. Pyrolysis 1998, 45, 153−169.

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