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Effect of steel wires on the microwave pyrolysis of tire powders Zhanlong Song, Yecheng Yan, Xu Lv, Yaqing Yang, Li Liu, and Xiqiang Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03337 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018
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Effect of steel wires on the microwave pyrolysis of tire powders Zhanlong Songa,b, Yecheng Yana,b, Xu Lva,b, Yaqing Yanga,b, Li Liuc, Xiqiang Zhaoa,b,∗ a
Shandong Provincial Key Lab of Energy Carbon Reduction and Resource Utilization, Shandong University, 17923 Jingshi Road, Jinan 250061, China b National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University, 17923 Jingshi Road, Jinan 250061, China c School of Information Science and Engineering, Shandong Normal University, 88 East Wenhua Road, Jinan 250014, China
Abstract The discharge caused by metals triggered by microwave radiation is a key issue, whereas little attention was paid to the pyrolysis of waste tires by using the interaction between microwave and metals. In this work, the effect of such factors, i.e., the microwave power, the steel wire (SW) mass fraction, and its spatial position in the tire powders (TPs), as well as the flow rate of the carrier gas, on the pyrolysis of the samples were experimentally examined in a laboratory-scale microwave oven. It was observed that the addition of SWs significantly accelerated the pyrolytic reaction of the TPs with regard to the conversion rate (product yields), even at rather low power, which may lead to a more complete degradation and significant reduction in the economic cost of the process. In addition, the presence of SWs has significant effects on the composition of products with different phases, especially, gas yield (up to 35.3%) and also the H2 concentration in tires (up to 16.11%) were extremely high. The research on the discharge triggered by metals subjected to microwave irradiation will be instructive for promoting the industrial applications of discharges due to microwave radiation. Keywords: Steel wires; Metal discharge; Microwave pyrolysis; Tire powder. INTRODUCTION The production of waste tires is keeping increasing with the rapid development of the auto industry, and the waste tires have become a hazardous “black pollution” because their improper disposal may generate dangerous pollutants. Therefore, the treatment of waste tires has become an urgent environmental and social issue. Nowadays there are several methods to deal with these problematic waste tires, such as tire retread, reclaimed rubber production, rubber powder production, and thermal treatment for energy and resources recovery.
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Considering the tires in
fact contain valuable resources, i.e., rubber, steel wire (SW), carbon and other additives in their production, if the scrap tires are pyrolyzed in an oxygen free atmosphere to generate potential *
Corresponding author. Tel.: +86 531 88399372; fax: +86 531 88395877. E-mail address:
[email protected].
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useful products, i.e., a gas, a liquid and a char, then these products after proper treatment can be used as high value-added chemicals and resources for other fields.1 Considerable research on tire pyrolysis has been carried out based on the laboratory-scale or pilot-scale experiments as well as the kinetic models.1-13 Product yields and compositions depend on the operational conditions applied and the reactor used. A range of different reactors, such as fixed-bed (batch), screw kiln, spouted and fluidized-bed have been used.14 Cracking experiments are done in batch reactors in 95% of cases; Only a few researchers have investigated the cracking of waste polymers under continuous or semi-continuous conditions.15 It is noted that even if the reactor is the same, different conditions and feedstock types will lead to a marked difference in the yields and compositions according to the specific requirement. For instance, for the fixed bed reactor (working temperature 400–700°C), the maximum yield of oil was in the range of ~20–60%, and char and gas yields were in the range of 30–40% and 5–20%, respectively.5, 16–22 Likewise, these data are ~35–55%, 30–40%, and 10–20%, respectively, for the screw kiln reactor (working temperature 500–700°C).23,24 While for the fluidized bed and spouted bed reactors (working temperature 360–810°C), the corresponding results are approximately ~30–60%, 30–40%, and 3–40%, respectively.5,26 It is noted that all these reactors have advantages and disadvantages in terms of technical, economical and ecological parameters and are used for different energy applications. In order to get a small particle size, the crushing and grinding of tires, which will spend much cost, are often needed in conventional pyrolysis technologies and the entire tires are not used directly except for some unique fields such as cement production. Additionally, the nichetargeting consideration is not made with regard to the different effect of SWs as well as carbon black on the pyrolysis of tires. In a word, it is appealing to explore a new method that makes the various compositions of the tires utilize more efficiently during the pyrolysis process. As an alternative, microwave heating is characterized by selectivity, penetrability, high efficiency, and instantaneity; and it is widely used in many fields. As we all know, the composition of tires is relatively heterogeneous. Besides the main component of rubber, rubbers have low dielectric constants and loss factors and hence are not suitable for direct microwave heating. However, the presence of carbon black within the tire formulation makes this waste a perfect candidate for microwave pyrolysis. The carbon absorbs the microwaves, heating up and transferring the heat to the rubber, producing more carbon, which in turn absorbs microwaves and heats up.15 Moreover, inner temperatures of the samples are usually higher than the surface temperatures under microwave radiation, thus microwave heating is therefore suitable for materials with either low heat conductivities or low heat transfer coefficients, like tire rubber. At
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the same time, the further degradation of pyrolytic products may be less likely to happen compared with conventional pyrolysis because of the homodromous transfer of heat and mass, thus inhibiting the unexpected secondary reaction. Therefore, more high value-added pyrolytic products may be generated than those obtained at the conventional pyrolysis. These unique features make the microwave heating a promising method to realize the efficient pyrolysis. In contrast with conventional pyrolysis, research on waste tire pyrolysis under microwave is relatively scarce, and there was few literature available regarding the effects of different operating variables on the distribution of products of waste tires. Anyway, Undri et al. performed a pioneering work. They reported that: 1) microwave power (P) and sample mass (M) have significant effects on the pyrolysis of waste tires and a new parameter of P/M2 was proposed as a descriptive parameter;
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2) Pyrolysis solids contained large amounts of amorphous carbon and
inorganic compounds added during tire manufacturing process, as well as some ZnS formed during pyrolysis;28 3) The oil product was rich in aromatics and olefins, and different tires affected the microwave pyrolysis results.29 Besides, Pringle provided an envisage about destructive distillation of a whole waste tire using microwave and recycled products, but lacking follow-up progress.30 Yatsun et al. reported that pyrolysis gases derived from waste tire pyrolysis under microwave mainly consisted of H2, C1–C5 hydrocarbon, CO, CO2, and traces of H2S gases.31 As is known to all, high-frequency induction current and discharge phenomena will probably generate when the metal material is put in the microwave field. As a result, the high temperature and hence the highly reactive plasma caused by microwave-metal discharge may generate, which provides a very favorable atmosphere for the pyrolysis.32–34 Consequently, the disadvantage of unexpected metal discharge may be converted to the advantage of the inner heat source to accelerate the pyrolysis of organic matter. Based on these amazing findings, considering the fact that the SWs is also a commonly used additive of tires; if the tires including the SWs are put in the microwave field then, the metal discharge possibly promotes the tires pyrolysis efficiently. As a consequence, microwave pyrolysis possibly requires simpler tire pretreatment by virtue of this promotion of metal discharge, even the whole tire without crushing and grinding can be used directly, which is of great significance in energy conservation potentially involved in the crushing and grinding operation. Furthermore, with the aid of the sparks, more materials, covering the microwave transparent media and microwave reflecting media, can be used under microwave heating, thus the discharges can be widely used to intensify some chemical reactions. Obviously, discharges stimulated by microwaves on the metal surface can significantly extend the scope of applications of microwave heating, such as microwave pyrolysis of waste plastics, polymers, automotive engine oils, etc.35–37
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Even for metal-free substances, e.g., organic matter, by adding some specific metals into these kinds of materials, a similar discharge effect is likely to result. As a promising issue, the efficient utilization of microwave discharge will be well worth expecting in the future. In this work, the effect of SWs on the product yields of microwave pyrolysis of tire powders (MPTP) was investigated in a modified household microwave oven. The factors affecting the product yield such as SW ratio and spatial position were experimentally examined. Tests on the influence of microwave power and the flow rate of carrier gas were also carried out in the presence of SW. Moreover, the effects of the SWs on the composition of the products were investigated experimentally. Considering that the microwave parameter was only applicable to the facility employed in the specific research work, we instead used the power per one gram of sample as a new criterion, i.e., the specific microwave power (SMP).38The investigation was expected to clarify the effect of SWs on the microwave degradation of tire rubber and unveil the interaction between microwaves and metal, with the aim of providing more information for the practical application of microwave heating technology. In addition, this study provides an appealing approach to economically realizing the efficient utilization of waste tires, electronic waste, and other materials that exhibit weak absorption of microwaves, such as tar, volatile organic compounds, and so on.
EXPERIMENTAL SECTION Materials and characterization The sample used in this experiment was acquired from a company at Zouping County, China. The tire powder (580°C). At high temperatures, a secondary reaction of oil may be increased and more gas are likely to be formed, which was consistent with the previous statement,1,24 where pyrolysis reaction can be classified as a) primary pyrolysis reaction (250–520°C) and b) secondary pyrolysis of volatiles compounds (600–800°C). As the power was further increased to 24 W/g, the char content further decreased by 3 pp. This is because, on the one hand, the discharge of metal and the absorption of the generated energy are strengthened under high power, resulting in more thorough cracking of the residual rubber powder; on the other hand, the high temperature caused by the discharge of SW under high power may lead to the decomposition of inorganic salts such as CaCO3 added in the tire production process,42 and thus a small amount of CO2 is produced, also contributing to the decrease of the solid product mass to some extent. At this point, the yield of char (38.7%) was close to the theoretical value (the sum of ash and fixed carbon in the raw rubber powder was 37.6%), which indicates that the TP pyrolysis was nearly complete at this power level. At the same time, the amount of oil continued to decrease at 24 W/g and the gas content (35.3%) increased significantly than before. Undri et al. also carried out the relative studies at the similar power (~24 W/g) without carrier gas and the product yields for the gas, liquid and char were 14.8–26, 27.8–42.8, and 42.4–46.2%, respectively.29 After comparison of both studies, it was inferred that the more complete degradation of tires was realized in the present work. Besides, it is noted that the gas yield (35.3%) was extremely higher than that in the above literature.27,29 As compared to the data obtained in cases of absence of SWs under 24 W/g (the gas yield = 18%), which shows that the addition of SWs is beneficial for the formation of gas; therefore, it can be concluded that the discharge produced by adding SWs is an efficient method for generating combustible gas by pyrolysis of waste tires. To be pointed out, it is recognized that the tire type has a significant effect on the pyrolysis of waste tires and truck tire has different composition from car tire. Usually car tire were used in literature and truck tire were employed in the present work. As a result, the different results as
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mentioned above from other literature could also be attributed to the material employed.
Effect of fraction of SWs in the TPs The mass fraction of SWs in the TPs was found to be another factor affecting the pyrolytic reaction. The test conditions were as follows: the specific microwave power was 9 W/g, the carrier gas flow rate was 0.4 L/min, the pyrolysis time was 30 min, and the SWs were placed under VER mode. The effects of SWs mass contents (16.7, 9.1, and 4.8%) on the yields of the products are depicted in Fig.5.
It is revealed from Fig.5 that when the mass ratio of the wire was increased from 4.8% to 9.1%, the promoting effect on pyrolysis was remarkable. The char content was reduced from 45% to 42.7%, whereas the yield of gas and oil increased. This is because when the appropriate amount of SW is added, the discharge effect of the SW in the microwave field is obvious and the pyrolysis rate of the rubber powder is enhanced. Further, when the fraction of SWs was increased to 16.7%, the char content also decreased slightly (char content=41.6%) and only 1 percent point from the data in the condition of 9.1% of the wire ratio. Upon increasing the number of SWs, the mutual effect was strengthened and it should achieve more powerful dissociation of tires. However, the degree of tire cracking in terms of the char content exhibited a slight decline. The possible reason may be that the wires form a circle in the tire powders and hence a circle of wires may cause a “shielding effect”, obstructing the penetration of microwaves into the circled samples, which could partially weaken microwave penetration and discharge.43 With two parts working together, the char content in the high wire ratio (16.7%) still showed positive effect on the dissociation of tires. Considering the trend of slow growth in the pyrolysis effect, the proper content of SWs is thought to be approximately 16.7%, and there is no need to increase the steel content excessively. Effect of SWs on the pyrolysis products The addition of SWs has important effects not only on the yield of the product but also on the composition of the product, and the components of the pyrolysis products have significant effects on the comprehensive utilization of the tires. Therefore, it is necessary to analyze the changes in the composition of the pyrolysis products before and after the addition of the SWs. Gas products As mentioned earlier, pyrolysis of TP can result in the release of appreciable amounts of gas; especially when adding SWs, the maximum gas yield can reach 35.3% at 24 W/g (Fig.4), which is evidently higher than the value in the literature,14 among which the gas yields in a wide range of temperature via different reactors are
