Waste tires are known as &#;black pollution&#;, which is difficult to degrade. The safe handling and recycling of waste tires have always been the focus of and difficulty for the global rubber industry. Pyrolysis can not only solve the problem of environmental pollution but also completely treat the waste tires and recover valuable pyrolysis products. This paper summarizes research progress on the pyrolysis of waste tires, including the pyrolysis mechanism; the important factors affecting the pyrolysis of waste tires (pyrolysis temperature and catalysts); and the composition, properties, and applications of the three kinds of pyrolysis products. The composition and yield of pyrolysis products can be regulated by pyrolysis temperature and catalysts, and pyrolysis products can be well used in many industrial occasions after different forms of post-treatment.
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Keywords: waste tires, pyrolysis, temperature, catalysts, products
Tires are made of natural rubber (NR) and synthetic rubber (SR). NR is an important strategic resource and is composed of an elastic polymer from latex (cis-1,4-polyisoprene). NR is a variety of polymer compounds and is composed of different monomers under the action of the trigger agent, and there are different types of monomers, such as butadiene, styrene, propylene, isobutene, neoprene, and so on [1,2]. Tires are prepared through NR and SR being mixed and then vulcanized. After vulcanization, tires themselves have three-dimensional cross-linked chemical structures, which makes it difficult for the tires to biodegrade and photochemically decompose under natural conditions [3]. Tires have a high carbon content and are a type of high-calorific-value fuel (the calorific value is about 35 MJ/kg, which is equivalent to the calorific value of coal). Tires made of rubber and various rubber products have been widely used throughout the world, resulting in a large amount of hard-to-decompose waste rubber. Unreasonable handling of this material will cause economic and environmental problems.
The main methods of waste tire (WT) treatment are tire retreading, rubber powder production, heat energy utilization, pyrolysis, and so on, as shown in Figure 1. Rubber products can no longer be used to produce rubber products after being recycled two to three times, so they must be disposed of eventually. The production cost of rubber powder is high, and the demand is limited. The production process of regenerated rubber is complicated, and the waste gas produced will do great harm to the environment if not treated properly. The accumulation of WT not only occupies land resources but also easily breeds mosquitoes, which spread diseases. Although the heat energy utilization of WT can be utilized with its high calorific value properties, it will cause secondary pollution to the environment. Pyrolysis can decompose WT completely, and its products such as oil, gas, carbon black and steel wire can be used, which not only realizes the regeneration of resources but also solves the problem of pollution. Therefore, pyrolysis technology is an important method and direction of WT recycling at present.
Main methods of WT treatment.
Pyrolysis consists of heating the WTs to a certain temperature so that the pyrolysis reaction occurs; the large molecular chains are broken into small molecular chains, and finally, three types of pyrolysis products are obtained, namely pyrolysis oil, pyrolysis gas, and pyrolysis carbon black [4]. Pyrolysis can thoroughly dispose of a large amount of waste rubber and at the same time obtain high-value pyrolysis products, such as oil, chemicals, carbon black, and fuel gas with high calorific value, which is considered a more promising treatment method, not only protecting the environment but also increasing economic benefits [5].
Due to the obvious advantages of pyrolysis in WT treatment, there are many related studies, and some related reviews have been published. The published reviews on WT pyrolysis have focused on some aspects, such as pyrolysis reactors [6,7,8], mathematical models of pyrolysis process [9], modification of pyrolysis carbon black [10], catalysts applied during pyrolysis [7], and so on. The key to the industrialization of WT pyrolysis is to obtain high-quality pyrolysis products while minimizing energy consumption in the pyrolysis process. This review focuses on the important factors affecting the pyrolysis of WTs (pyrolysis temperature and catalysts) and the composition, properties, and applications of the three types of pyrolysis products. The composition and yield of pyrolysis products can be regulated by pyrolysis temperature and catalysts, and pyrolysis products can be well used in many industrial applications after different forms of post-treatment.
The polymer material used in making tires is mainly composed of NR and SR. Due to the special high elasticity, excellent wear resistance, shock absorption, insulation, and sealing performance, rubber products are widely used in industry and our lives, bringing great convenience to people&#;s lives. Today, the surge in the number of cars and the booming rubber industry has led to a growing global demand for rubber. Approximately 1.5 billion tires are sold worldwide each year, of which 50 percent are discarded without any treatment [11]. It is widely believed that every new tire sold on the market will have another tire scrapped. The annual production of WTs in some countries is depicted in Figure 2. It is estimated that by , million WTs will be produced each year [12]. Therefore, both now and in the future, the world is faced with the worldwide problem that it is difficult to recycle the large number of WTs produced each year.
The annual production of WTs in some countries [13] (adapted with permission from Elsevier).
Tires are mainly composed of rubber, carbon black, and a variety of organic and inorganic additives (including plasticizers, anti-aging agents, sulfur and zinc oxide, etc.) [14]. There is NR (20&#;25%), styrene butadiene rubber (SBR) (30&#;50%), butyl rubber (BR) (up to 30%), carbon black (30%), sulfur (1&#;2.5%), and a small amount of organic and inorganic additives in the general tread rubber on the market. The proportions of ingredients in the formula vary mainly depending on the purposes of use [9,15]. The SR used in tread compound formula is mainly SBR and BR. In the production process of tires, NR and SR were mixed in a certain proportion and cross-linked through sulfur reaction to form a very stable three-dimensional cross-linked chemical structure, which plays a bearing, damping, and anti-wear role.
At present, the main treatment methods of WTs are direct use, landfill, direct incineration, old tire refurbishment, reclaim, and pyrolysis [12,16,17]. Vulcanized rubber consists of long-chain polymers (isoprene, butadiene, and styrene), which are cross-linked with sulfur bonds and are further protected by antioxidants and anti-ozone agents [18]. Improper treatment can cause great pollution of the environment. Landfilling or directly discarding will contaminate land and water sources as the rubber can take centuries to degrade [19], and the accumulation of tires will provide mosquitoes with breeding grounds, spread disease, and become fire hazard sources, causing serious pollution to the environment and threatening human survival [20]. Direct incineration will release dioxins, polycyclic aromatic hydrocarbons, and volatile toxic pollutants [21,22]. Large amounts of harmful gases will pollute the atmosphere, and waste residues and some heavy metals will seriously pollute soil and water resources [23]. Direct use and tire refurbishment are our preferred resource-saving methods, and direct use can be used in pendants, playgrounds, shoes, etc., but the consumption is small and not enough to deal with the WTs produced in large quantities every year. Tire refurbishment requires the tire body to remain intact and is limited by the number of renovations. There are some problems in the preparation of the reclaimed rubber, such as low profit, high energy consumption, and serious pollution. At the same time, reclaimed rubber only realizes the reuse of resources; the product will eventually become waste rubber [24].
At present, pyrolysis is considered to be the most efficient and thorough method of treating WTs; it can not only recover high-value pyrolysis products and realize the regeneration of resources but also solve the problem of environmental pollution [25,26,27]. Therefore, the pyrolysis technology is an important method for handling waste rubber.
WT consists of 60% NR and SR, 30% carbon black, and 10% organic and inorganic fillers. After pyrolysis, waste tires are decomposed into 60% volatile fraction and 40% solid fraction, as shown in Figure 3. In other words, WT pyrolysis mainly includes the pyrolysis of two organic components, namely NR and SR. For NR, when the pyrolysis temperature reaches 326 °C, the incomplete pyrolysis reaction begins. At this time, the pyrolysis products are mainly dimer and trimer. As the temperature increases, a large number of isoprene monomers are produced, and cyclization pyrolysis products&#;such as xylene&#;are generated at the same time. As the temperature continues to rise, the secondary reaction of pyrolysis products will happen and the product composition will become more complex [28]. The whole pyrolysis process of NR is shown in Figure 4.
The scheme of WT pyrolysis [6] (Adapted with permission from Elsevier).
The pyrolysis process of NR.
SR in WTs is mainly composed of butadiene rubber (BR) and styrene&#;butadiene rubber (SBR). The pyrolysis mechanism of BR and SBR was proposed based on TG-FTIR/MS and Py-GC-TOF/MS [29]. For BR, there were four paths for compound transformation. Once heated, free radicals would appear. The first path was the formation of 1,3-butadienes via scission and dehydrogenation. The second method encompassed the rearrangement and cyclization of free radicals, ultimately triggering 4-vinyl-1-cyclohexenes. The other two methods occurred at almost the same time. One was the formation of 1,3-cyclopentadienes via dehydrogenation and cyclization. The other was the process of cyclization, forming 1,4-cycloheptadienes. The procedure for BR thermal cracking is shown in Figure 5a. For SBR, the process of pyrolysis covered a wide temperature range from 180 °C to 500 °C. Free radicals with C4 mainly went to 1,3-butadienes. Other free radicals of benzene derivatives mainly turned into styrene. Then, reaction between benzene derivatives occurred. The transformation path of SBR under hyperthermic and anoxic conditions is shown in Figure 5b.
The mechanism of SR pyrolysis; (a) BR, (b) SBR [29] (adapted with permission from Elsevier).
In the process of WT pyrolysis, there are two main reactions: main chain degradation and cross-bonding disconnection to form active chain segments. First of all, the broken chain occurs in C-C bonds, and the fracture is accompanied by hydrogen transfer, resulting in a decrease in the molecular weight of the chain segment [30]. Second, active bond fragments containing sulfur radicals may recombine to form a new network. It can be seen from mass spectrometry (MS) analysis of rubber pyrolysis, the pyrolysis products break from the main chain to form monomer or dimer of polymer, which indicates that the dissociation energy of C-C bonds is lower than that of C-H bonds. When heated, C-C bonds are prone to fracture, and free radicals are formed. Then, the free radicals react with each other to extract hydrogen or conduct disproportionation reactions to form a variety of products. The results of magnetic resonance imaging (MRI) showed that the fracture of single sulfur bonds mainly occurred at 300 °C, and with the extension of pyrolysis time, the isoprene unit in NR changed from cis to trans isomerization. At the range from 280 °C to 300 °C, the cross-linked bond fracture of NR occurred most frequently, followed by a rapid increase in the degradation of the main chain, and the higher the temperature, the higher the efficiency of this process. This indicated that the cross-linking fracture occurred earlier in the rubber pyrolysis, mainly because the dissociation energy of S-S bonds is lower than that of C-S bonds. However, with the extension of pyrolysis time, the main chain fracture became more obvious [31]. The pyrolysis of NR is dominated by the fracture of main chain and crosslinked bonds, with a low probability of recombination in the chain fracture [32].
In short, the pyrolysis reaction is a very complex process which cannot be completely described by one or several chemical reactions. However, the process is based on the mass conservation law, and the empirical formula with a guiding value can still be obtained. In the process of pyrolysis, WTs were decomposed into solid carbon black and volatile products. Martínez et al. [6] assumed that the solid conversion rate is 40% and that there was only interaction between organic components in the pyrolysis reaction. Based on this assumption, the pyrolysis process was expressed in the form of elemental analysis (as shown in Figure 6). On the basis of element analysis, the enthalpy of reaction of tire pyrolysis can be added to obtain the energy content of volatilization through heat balance.
Elemental analysis of tire pyrolysis process (daf means dry ash-free basis) [6] (adapted with permission from Elsevier).
In addition, the formation mechanism of the pyrolysis products was also studied. Xu et al. [33] conducted pyrolysis on waste bicycle tires and found that the pyrolysis process could be divided into two stages. In stage I, the primary pyrolysis of tires occurred at 285&#;531 °C, while in stage II, the secondary pyrolysis of pyrolysis products occurred mainly at 663&#;847 °C. According to thermogravimetric analyzer coupled with Fourier-transform infrared spectrometry (TG-FTIR), in stage I, small molecule alkenes and cycloalkenes were produced when rubber chains were broken at low temperatures. With the increase in pyrolysis temperature, the alkenes and cycloalkenes produced underwent Diels&#;Alder reaction, cyclization, aromatization and other reactions to form benzene and benzene derivatives. At the same time, more alkenes and cycloalkenes were ring-opened and recycled. In stage II, as the temperature rose again, benzene and its derivatives underwent further pyrolysis to obtain more aromatic compounds. Wei et al. [32] combined reactive molecular dynamics (RMD) simulation and TG-FTIR experiment to study the formation process of NR pyrolysis gas. The simulated results showed that CH3 was separated from the main chain and that H was extracted from other molecules to generate CH4, while C2H4 was generated mainly through C-C bond fracture from the long chain. Other small gas molecules were produced by breaking down large alkenes or low-activity free radicals. Seidelt et al. [34] also detected the thermal decomposition of SBR, NR, and BR by gas chromatographic/mass spectrometric (GC/MS). The results showed that the main pyrolysis products of NR were xylene and isoprene, while the main pyrolysis products of SBR were ethylbenzene, styrene, and cumene. Ding et al. [35] found that chain olefins were mainly derived from 2-pentene, 1-3-butadiene, and isoprene formed through the depolymerization of NR. Lopez et al. [36] found that cyclic olefin was mainly derived from the degradation of NR or the cyclization of chain olefin. The degradation process and secondary reactions mainly included terminal chain breaking, random chain breaking and cross-linking. The degradation process was studied via thermogravimetric analysis (TGA), and the results showed that with the increase in temperature and heating rate, the chain breaking rate of cross-linking decreased. Studies have shown that the activation energy of rubber polymer decreased with the increase in heating rate (171.06&#;136.51 kJ/mol) [37]. Under the condition of low pyrolysis temperature, chain uncoupling and chain breaking played a leading role. With the increase in pyrolysis temperature and activation of coupling reaction, the cross-linking mechanism played a more effective role in the pyrolysis process, and the products were transformed into cyclic compounds, which was consistent with other research results [38].
Pyrolysis is a complex decomposition process of organic matter, including both chemical and physical reactions [39]. In the process of pyrolysis, different organic compounds start to decompose at different temperatures, and the reaction and product composition are also different at different pyrolysis temperature. The pyrolysis of macromolecular organic matter includes macromolecular bond breaking, molecular isomerization and small molecular organic polymerization. The thermal stability of polymer mainly depends on the formation of bonds and the bond energy between atoms. The greater the bond energy, the harder it is to break and the higher its thermal stability. The smaller the bond energy, the easier it is to decompose and the lower its thermal stability. The composition and yield of the pyrolysis products depend on the raw material, pyrolysis temperature, pyrolysis rate, catalyst, and so on. The revelation of pyrolysis mechanism of NR and SR can not only help us understand the pyrolysis process but also effectively guide the regulation of pyrolysis products.
WT pyrolysis is the process in which the rubber polymers that make up WTs are decomposed into gas, oil and carbon black at the right temperature in the absence of oxygen or presence of inert gas. There are many factors that influence the pyrolysis process, such as reactor structure, raw material composition, pyrolysis temperature, pyrolysis pressure, pyrolysis residence time, heating rate, catalyst, waste rubber particle size, etc. Among them, pyrolysis temperature and catalyst are important factors influencing the pyrolysis reaction [40,41,42,43,44,45]. The influences of these two factors on the pyrolysis process and pyrolysis products of WTs are reviewed as below.
The long rubber polymer chains can only undergo condensation, polymerization, depolymerization, hydrogenation, and aromatization reactions at appropriate temperatures, and an important factor influencing this series of reactions is temperature [18]. In the following section, the pyrolysis process of rubber at different temperatures and the influence of temperature on the pyrolysis products are summarized.
At present, researchers mainly use thermogravimetric (TGA) and MS to analyze the pyrolysis characteristics of WTs. Tires are usually made of NR and SR. According to the results of TGA, the initial temperature of the pyrolysis reaction of NR was 326 °C, the pyrolysis speed reached its maximum at 375 °C, and the pyrolysis process was completely finished when the temperature reached 455 °C; when the pyrolysis temperature reached 286 °C, the pyrolysis reaction of SBR started. The pyrolysis speed reached its maximum at 452 °C, and the pyrolysis process was completely finished when the temperature reached 491 °C. The initial pyrolysis temperature of BR was 374 °C, the pyrolysis speed reached the maximum at 483 °C, and when the pyrolysis temperature reached 497 °C, the pyrolysis process was basically completed [46]. According to the above results, NR is decomposed first, followed by SBR and BR over the entire pyrolysis process.
Han et al. [47] divided the WT pyrolysis process into four stages according to MS and TG curves. In the first stage, the temperature was lower than 320 °C, the water in the tire was evaporated, and the plasticizer was decomposed; the second stage is at the temperatures between 320 °C and 400 °C, when the decomposition of NR occurred; the third stage occurred at 400&#;520 °C, and the decomposition of SBR and BR took place; the fourth stage occurred when the temperature was above 520 °C, with a small reduction in mass. Meanwhile, Kan et al. [48] found through experiments that there were three stages of WT pyrolysis: the first stage (about 200&#;350 °C) was the decomposition of volatile substances (such as oil, plasticizer, additive, etc.) in rubber, the second stage was the decomposition of NR component at about 300&#;450 °C, and the third stage was the degradation of SBR and BR components at about 400&#;500 °C. Islam et al. [49] believed that the decomposition temperatures of organic additives, such as oil and plasticizer, in tires were about 150&#;350 °C, 330&#;400 °C for NR, and 400&#;480 °C for SBR and BR. It can be seen from the above that the pyrolysis sequence of WTs can be divided into four main stages successively: at about 200 °C, due to the decomposition of additives at low boiling point, waste tires began to lose weight; NR began to decompose at about 300 °C; SR did not begin to decompose until 400 °C; pyrolysis was basically completed at about 500 °C.
Different types of tires, different experimental conditions, experimental equipment, experimental operations, and different catalysts may lead to different or even opposite experimental results. Even if the same type of waste rubber is pyrolyzed at the same temperature, different experimental results will be obtained. Most of the researchers found that with the increase in pyrolysis temperature, pyrolysis oil production of WTs decreased and gas production increased [36,50,51]. Other researchers found that pyrolysis oil production would reach the maximum or minimum at a certain temperature [52,53]. The former had a high degree of recognition, but there was also a group of the researchers who found that the production of pyrolysis oil increases with the increase in pyrolysis temperature [54]. In a word, temperature has a great influence on the oil production of waste tire pyrolysis. Pyrolysis oil production at different temperatures is shown in Table 1.
Oil production at different pyrolysis temperatures.
Ref. Temperature (°C) Oil (wt%) Pressure (Pa) [55] 450 58.1 101,325 525 56.9 600 53.1 [56] 450 49 101,325 500 45 600 40 [41] 400 36 101,325 500 44 600 45 [57] 350 30 101,325 450 33 550 38 [58] 450 43 &#; 550 44.6 650 42.9 [43] 500 55.4 600 52.2 700 36.6 Open in a new tabWhen the WTs are pyrolyzed at different temperatures, the main pyrolysis products also change. When the temperature is low, the secondary reaction of macromolecular organic compounds can be reduced and more molecular chains above C5 can be generated, so the pyrolysis oil content will be higher. When the pyrolysis temperature is higher, the large rubber molecular chain will have a large-scale stable bond breaking reaction without enough time to decompose at the weakest molecular chain node, and the small molecular chain generated by pyrolysis will have a secondary reaction. Therefore, low molecular organic compounds are mainly generated, resulting in an increase in olefin gases and a decrease in oil content. In general, increasing pyrolysis temperature will increase gas production and decrease oil production. The higher the pyrolysis temperature is, the faster the secondary pyrolysis reactions will be&#;such as cyclization, dehydrogenation, and aromatization&#;which will reduce the aliphatic components and increase the aromatic components in the pyrolysis oil [52]. Berrueco et al. [59] conducted pyrolysis of WTs at 400&#;700 °C. When the pyrolysis temperature varied from 400 °C to 500 °C, oil production increased, but when the pyrolysis temperature reached above 500 °C, the oil production did not increase any more. Gas production increased slightly at pyrolysis temperatures from 400°C to 700 °C. Williams et al. [55] found that when the pyrolysis temperature increased from 450 °C to 600 °C, the aromatic content increased and the aliphatic content decreased. At 475 °C, the maximum oil yield was 58.2 wt%, while at 600 °C, the oil yield decreased to 53.1 wt%.
Miranda et al. [60] studied the dynamic residues in the pyrolysis processes of NR, BR, and SBR components and found that the lower temperature (<390 °C) was conducive to the formation of olefins, while the higher temperature was conducive to the formation of aromatics. Rodriguez et al. [42] pyrolyzed automobile tires in autoclaves filled with nitrogen at 300&#;700 °C and found that when the temperature was above 500 °C, the quantity and quality of pyrolysis products did not change. When the temperature was lower than 500 °C, the main compounds in pyrolysis products were isoprene and limonene. Menares et al. [18] found that when the temperature was above 600 °C, it was conducive to the formation of single aromatics and gases. Laresgoiti et al. [61] conducted pyrolysis of the entire automobile tire in a high-pressure kettle filled with inert gas at 400&#;700 °C and found that carbon oxides ( COx) and lighter gases at high temperature were generated due to secondary pyrolysis of inorganic components and products. Yazdani et al. [62] pyrolyzed WTs in a rotary kiln filled with nitrogen (N2), and the pyrolysis temperature was 400&#; °C. The results showed that the highest yield of pyrolysis oil was 44 wt% at 550 °C.
Although the pyrolysis temperature of WTs is the main experimental control variable, the experimental results are also directly affected by other important factors, such as pressure, pyrolysis equipment, heating rate, catalyst, residence time, etc. Pyrolysis oil production may decrease or increase at certain temperatures and is also associated with the addition of specific catalysts, which may also contribute to increased gas production or oil production. Next, the effects of different types of catalysts on the pyrolysis process and the products are described in detail.
The pyrolysis reaction takes place at a certain temperature, which is called the critical temperature of pyrolysis. The critical temperature of the maximum molecular chain of rubber is about 380 °C, and when the pyrolysis temperature is higher than 420 °C, it is called high-temperature pyrolysis. Polycyclic aromatic hydrocarbons (PAH) (commonly known as dioxins), which are readily produced by high-temperature pyrolysis, are powerful carcinogens. Generally speaking, the higher the pyrolysis temperature, the more noncondensable combustible gas produced by pyrolysis, the lower the oil yield and&#;at the same time&#;the higher the proportion of aromatic hydrocarbons in oil products, resulting in the quality of oil products and carbon black decreased. The output and quality of pyrolysis oil and gas obtained from pyrolysis of WTs not only depend on the type of tires but also on the type and conditions of pyrolysis process [63]. Catalysts have great influence on tire pyrolysis. In general, compared with noncatalytic pyrolysis, catalytic pyrolysis is beneficial in increasing gas production and reducing pyrolysis oil, but the use of catalysts has little effect on coke yield.
Catalysts can accelerate the reaction rate and reduce the reaction activation energy and the wastage of the energy used in pyrolysis. Additionally, their own chemical properties do not change before and after reaction, and they themselves have no consumption, so they can be used repeatedly. At the same time, they can also reduce the requirements for pyrolysis equipment. In addition, for complex reaction, catalytic pyrolysis can choose to speed up the main reaction rate, restrain side effects, and improve the yield of the target products. In the catalytic pyrolysis of WTs, the material, aperture, structure, performance, and stability of the catalysts have great influence on the pyrolysis process. Catalysts with large pore size and low Si-Al ratio have better extraction effect on aromatics, but the use of such catalysts can reduce the output of pyrolysis oil [7].
The low-temperature catalytic pyrolysis of WT is a pyrolysis reaction at a temperature slightly higher than the critical pyrolysis temperature. The presence of a catalyst reduces the reaction temperature and time required for pyrolyzing. Most important is that the use of catalysts can improve the quality of pyrolysis oil and carbon black basically without PAHs in pyrolysis products and with no harm to the environment. Zhang et al. [64] reported that the use of alkaline additives and catalysts reduced the reaction temperature. Miranda [60] believed that the activation energy of rubber pyrolysis was 127.4&#;176.0 kJ/mol and that the use of different types of catalysts would reduce the activation energy of WT pyrolysis to different degrees. Another advantage of catalytic pyrolysis over thermal pyrolysis was that the catalytic pyrolysis of the polymerization chain resulted in a narrower variety of products [65]. The results of GC-MS showed that there were up to 93.3% mixed aromatic compounds and a small amount of aliphatic hydrocarbons in the thermal pyrolysis oil. The use of catalysts reduced the concentration of aromatic compounds in pyrolysis oil [66]. The use of fluid catalytic cracking (FCC) catalysts made it less likely to produce aromatic compounds [38]. Miandad et al. [66] found that in the absence of a catalyst, the yield of liquid oil could reach 40%; under the catalyzed conditions of activated alumina, activated calcium hydroxide, natural zeolite, and synthetic zeolite, the yields of pyrolysis oil were 32 wt.%, 26 wt.%, 22 wt.%, and 20 wt.%, respectively. Table 2 shows the content of pyrolysis products when different types of catalysts are used for waste tire pyrolysis.
Distribution of pyrolysis products under different catalysts.
Temperature (°C) Catalysts Yield (wt%) Ref. Oil Char Gas 450 - 50.47 36.47 13.06 [67] 500 - 51.98 36.09 11.92 550 - 52.61 35.69 11.70 600 - 54.10 36.30 9.61 500 Ca(OH)2 40 48 12 [68] 500 Na2CO3 47.8 37.6 14.6 [64] 500 ZSM-5 55.6 37.6 6.5 [69] 500 USY 53.5 36.5 10 450 HZSM-5 50.2 33.1 16.7 [70] 450 HY 54.9 33 12.1 450 Hβ 47.8 33.1 19.1 Open in a new tabAmong various catalysts used in WT pyrolysis, zeolites are the most common. Zeolite catalysts are popular because of their acidity and unique pore structure (there are six main types of zeolites). Zeolite can be used as a catalyst alone or can be mixed with precious metals to form a synthetic catalyst containing a variety of material components. Different catalysts have different functions, and some may even allow several chemical mechanisms to occur simultaneously. Zeolite catalysts can be used in the pyrolysis of WTs to prepare compounds with high aromatic content, which has great application value. Next, the influence and mechanism of some zeolite catalysts on the composition and content of pyrolysis products are summarized.
In general, the use of acid catalysts can reduce the yield of liquid oil and increase the yield of gas. Ultra-stable Y zeolite (USY) is often used as catalyst for upgrading chemical products due to its high activity and stability. Williams et al. [71,72] found that the gas yield obtained from USY and HZSM-5 catalyzed pyrolysis increased. Wang et al. [73] studied the effects of USY zeolite catalysts with different SiO2/Al2O3 molar ratios on the formation of aromatic hydrocarbons. The results showed that USY with SiO2/Al2O3 ratio of 5.3 was more conducive to the formation of aromatic hydrocarbons, and USY with a high SiO2/Al2O3 molar ratio (11.5) was more conducive to the formation of olefins. Vichaphund et al. [74] adopted the HZSM-5 zeolite processed through three different modes as catalysts and carried out the catalytic pyrolysis of waste rubber at the pyrolysis temperature of 500 °C. It was found that the increase in HZSM-5 catalyst content significantly promoted the formation of aromatic hydrocarbon compounds&#;especially an increase in the yield of benzene, toluene, and xylene&#;but the content of nonaromatic components in the pyrolysis products significantly decreased. Santos et al. [75] found that in the catalytic pyrolysis of waste rubber, USY zeolite had higher surface area and larger pore size than HZSM-5 zeolite, so the usage of USY zeolite was more conducive to the generation of aromatic hydrocarbon in catalytic pyrolysis. Manchantrarat et al. [76] used various zeolites for catalytic pyrolysis of waste rubber. They found that Y-type zeolites significantly increased the yields of saturated and monoaromatic hydrocarbons and reduced the yields of diaromatic, polyaromatic, and polar aromatic hydrocarbons. Boxiong et al. [67,77] found that through the usage of the molecular sieve USY catalyst and the HZSM-5 catalyst, high concentrations of monocycle aromatics&#;such as benzene, toluene, and xylene&#;could be prepared, and at the same time, the oil yield could be reduced and the gas phase yield improved. Higher concentrations of aromatic hydrocarbons could be obtained from the catalytic pyrolysis with the USY of waste tires, so USY was considered to be a more suitable catalyst for the preparation of raw chemical materials. Other researchers [70] found that the concentrations of benzene, toluene, and xylene in the pyrolysis products was high, while the concentration of limonene was decreased when USY zeolite catalyst was used. The reason was that the larger the USY pore size, the lower the acidity and catalytic activity and the higher the selectivity for limonene to decompose into aromatic hydrocarbons. Li et al. [69] found that the catalytic pyrolysis gas yield with different catalysts was also different, and the order of gas yield was roughly SAPO-11 > USY > Hβ > HZSM-5 > HZSM-22 > non-catalyst. The use of HZSM-5 catalyst resulted in the highest yield of pyrolysis oil (55.65%), while the use of SAPO-11 catalyst resulted in the highest gas production (10.45%) and the lowest carbonization rate (34.43%).
Metal or metal oxide doping can enhance the activity of a catalyst, so this method is increasingly used in the catalytic pyrolysis of WTs. Hijazi et al. [78] studied the catalytic pyrolysis of WTs catalyzed by Hβ and Pd/Hβ zeolite. The results showed that the gas yield of noncatalytic pyrolysis was 20% and that the gas yield increased to 28% and 37% by adding Hβ and Pd/Hβ, respectively. The stronger catalytic activity of Pd/Hβ was due to the dehydro-hydrogenation reaction caused by Pd metal sites. The results of GC-MS showed that under the action of Pd/Hβ catalyst, the composition of oil had significantly shifted towards a low carbon number (C9&#;C13), and the carbon number of hydrocarbons in pyrolysis oil decreased from diesel (C12&#;C20) to the narrow range of gasoline and naphtha (C9&#;C12). Additionally, Hijazi et al. [79] also prepared polyphase photocatalysts by TiO2 doped with metal Pd, Pt and metal oxide Bi2O3/SiO2, respectively. The results of WT pyrolysis with and without catalysts at 550&#;570 °C showed that the gas production rate of noncatalytic pyrolysis was 20%, and that of pyrolysis catalyzed by TiO2 was 27%. It was also found that doping TiO2 with precious metals and metal oxide would improve the catalytic capacity of catalysts. The gas production of pyrolysis catalyzed by Pd-Pt/TiO2 and Pd/TiO2 has maximum values of 40% and 41%, respectively. Metal doping changed the morphology of TiO2, resulting in the increase in nano grain size, pore volume and specific surface area. Pd can induce hydrogenation/dehydrogenation reaction. By converting alkanes to olefins, the olefins are isomerized and cracked at acidic sites near zeolite so as to help improve the catalytic activity. Basagiannis et al. [80] found that ruthenium (Ru) can also be doped with zeolite, which helped to increase catalytic activity, reduce pyrolysis temperature, and increase hydrogen production rate. Zeolite catalysts with noble metal carriers could catalyze the hydrogenation of raw materials, enhance the pyrolysis efficiency of WTs, and promote the removal of heteroatoms (sulfur and oxygen) [81]. Yu et al. [27] modified zeolite by doping copper and found that the use of strong acid sites could help reduce the sulfur content of pyrolysis products and improve oil quality. The Y-type zeolite catalyst has a high coking rate. Doping cerium into the zeolite structure through ion exchange technology to replace protons can change the activity and pore properties of the catalyst and finally reduce the formation of coke in pyrolysis reaction. The coking rate of large porous Y-type zeolite decreased from 8.1% to 5.7% after cerium ion exchange.
Mesoporous MCM-41 zeolite inhibited the formation of polycyclic aromatic and polar aromatic, and promoted the formation of monoaromatic and saturated hydrocarbons [82]. Khalil et al. [83] used a two-stage fixed bed for the catalytic pyrolysis of waste rubber and found that microporous zeolite catalysts increased the yield of aromatic compounds by 23.7% and that mesoporous McM-41 catalysts increased it by 18.7%. MCM-48 is a mesoporous material with a cubic crystal structure and has a better catalytic effect. The content of light olefin produced by Ru/McM-48 catalyst was twice that of noncatalytic cracking, while the proportion of light oil also increased, which improved the quality of oil products. In addition, the sulfur content of polyaromatic compounds and polar aromatic compounds in pyrolysis oil decreased [53]. Dũng et al. [82] studied the MCM-41 and Ru/MCM-41 in the catalytic activity of waste tire pyrolysis. The results showed that gas production increased while liquid production decreased, and lighter oil was produced after adding the two catalysts. The yield of light olefin catalyzed by Ru/MCM-41 was four times that of noncatalytic pyrolysis.
Apart from zeolites, some natural catalysts or some acid/alkali additives were also used in the catalytic pyrolysis of WTs. Miandad et al. [66] pyrolyzed WTs in a 20 L small semi-industrial scale reactor. The results of GC-MS confirmed that the aromatic compound content in the oil prepared from non-catalyst pyrolysis was as high as 93.3%. After the pyrolysis process was catalyzed by active calcium hydroxide, natural zeolite, and activated alumina, the concentrations of aromatic compounds in the obtained pyrolysis oil were reduced to 60.9%, 71.0%, and 84.6%, respectively. Ibrahim et al. [84] added 5% nickel to the calcined dolomite as a catalyst for the catalytic cracking of WTs; the gas yield increased from 30.3 to 49.1 wt.%, and the hydrogen yield doubled. At this time, the coke deposited on the surface of the catalyst reached a minimum of 0.9 wt.%. Itkarnka et al. [85] used HNO3-treated pyrolysis carbides of waste rubber as catalysts during waste rubber pyrolysis. Acid-treated carbides had higher acidity and larger surface area and pore size, and the use of this catalyst for catalytic pyrolysis of waste rubber increased gas production and promoted greater pyrolysis activity. In addition, Williams et al. [86] synthesized carbon nanotubes and hydrogen from the pyrolysis products using Ni/Al2O3 as a catalyst in a two-stage fixed bed reactor, and the results showed that at the tire:catalyst weight ratio of 1:1, the highest yield of filamentous carbons was produced at 253.7 mg/g tire.
Through WT pyrolysis, three kinds of valuable products can be obtained, and their applications are rather wide, as shown in Figure 7.
Applications of WT pyrolysis products.
Pyrolysis oil is the liquid product condensed from the volatile fraction of WT pyrolysis, which is a black opaque liquid with pungent smell. It is a very complex mixture. According to the experimental verification, there are more than 100 compounds in it, which can be converted into gasoline (C5&#;C10), diesel (C14&#;C18), and heavy oil (>C18) after purification. The pyrolysis oil contains aliphatic, aromatic, heteroatom, and polar components. Among them, aromatic hydrocarbons&#;including benzene, toluene, xylene, styrene, limonene, ninhydrin, and their alkylated homologues and 2&#;5 ring polycyclic aromatic hydrocarbons&#;account for a large proportion (about 62.4%). Aliphatic hydrocarbons account for a relatively small proportion(about 31.6%). The main aliphatic compounds are alkanes, the straight-chain hydrocarbons formed from C6&#;C37, and the olefin at a lower concentration [87]. The main components of aromatic hydrocarbons are monocyclic aromatic hydrocarbons, which account for about 43&#;58% of the total weight of aromatic hydrocarbons. The main components of aliphatic hydrocarbon include light aliphatic hydrocarbon and heavy aliphatic hydrocarbon. The composition of light aliphatic hydrocarbons is mainly olefins, while the composition of heavy aliphatic hydrocarbons is mainly n-alkanes. In the pyrolysis process, the chemical bonds of rubber are thermally decomposed under inert gases, and the pyrolysis products range from light alkane gases to heavy complex aromatic hydrocarbons. The composition of WT pyrolysis oil is not invariable, which is mainly affected by the raw material composition of WTs, pyrolysis temperature, pressure in the pyrolysis reactors, residence time, and so on [88,89,90,91].
Dai et al. [92] found that the pyrolysis oil obtained from WTs in a circulating fluidized bed contained alkane (26.77 wt%), aromatics (42.09 wt%), non-hydrocarbons (26.64 wt%), and asphalt (4.05 wt%). Nisar et al. [28] analyzed the pyrolysis products of WTs via GC-MS, and the results showed that the hydrocarbon carbon chain of noncondensable gas and liquid compounds was mainly distributed in C1&#;C5 and C16&#;C19, respectively. Laresgoiti et al. [93] found that pyrolysis oil contained a complex mixture of C6-C24 organic compounds, consisting mainly of aromatic compounds (53.4&#;74.8%), some nitrides (2.47&#;3.5%), and some oxygen compounds (2.29&#;4.85%), with a sulfur content of about 1.0&#;1.4%.
Pyrolysis oil is highly acidic, with high sulfur content and low thermal stability. In addition, other physical properties, such as viscosity and the flash point of pyrolysis oil, make it unable to burn directly or replace engine fuel. The sulfur content of pyrolysis oil is generally 1.0&#;2 wt%, while that of general commercial diesel oil is less than 0.05 wt%. The components of pyrolysis oil are complex, and the existence of low-boiling-point compounds leads to the flash point of pyrolysis oil generally being lower than 30 °C. The lower the flash point, the lower the safety. The pyrolysis oil must be further refined to improve its performance. As shown in Table 3, the physical properties of the pyrolysis oil produced under different conditions are also different.
Physical properties of pyrolysis oil under different conditions.
Pyrolysis Conditions 550 °C, NP 550 °C, NP 520 °C, VP 550 °C, VP 650 °C, NP S/(wt%) 0.6 0.58 0.8 1.26 1.35 H/C 1.6 1.60 1.5 1.36 1.42 Density/(kg.m&#;3) 900 900 950 987 943 Flash point/°C 20 20 28 30 <30 Heat value/(MJ/kg) 43.27 43.27 43.7 41.0 41.6 Ref. [94] [95] [20] [58] [96] Open in a new tabWT pyrolysis oil must be treated before it can be used as fuel oil. Currently, the commonly used treatment methods include hydrotreating, catalytic treatment, fractionation, copyrolysis, activated carbon adsorption, etc. [97,98]. Costa et al. [99] extracted light fuel fraction from tire pyrolysis oil via steam distillation. Light fuel fraction (LFF) is a light-yellow translucent liquid with a specific gravity of 0.76 g/cm3 and a dynamic viscosity of 0.4 MPa.s at 20 °C. The light component is mainly composed of benzene compounds (62.06%), ethyl benzene (14.84%), and methyl benzene derivatives (13.02%). It was found that the components of light fuel are very similar to those of gasoline extracted from petroleum, and it was feasible to replace traditional gasoline with light fuel. According to Miandad et al. [66], the physical properties of waste rubber pyrolysis oil, such as high heat value (HHV) (42&#;43.5 MJ/kg), kinematic viscosity (1.9 cSt), density (0.9 g/cm3), pour point (&#;2 °C), and flash point (27 °C), were close to the standard value of conventional diesel oil. Moreover, the liquid oil had higher calorific value, which is the same as conventional diesel oil (42.7 MJ/kg). WT pyrolysis oil contains large amounts of sulfide (1.15 wt%), so it is not suitable for internal combustion engines. Jantaraksa et al. [100] found that the WT pyrolysis oil could be improved by catalytic hydrodesulfurization of cobalt molybdenum supported by Molybdenum (Mo), nickel molybdenum (NiMo) or alumina oxide (AL2O3). The reaction took 30 min, and the maximum desulfurization rate is 87.8%. The calorific value of hydrolyzed pyrolysis oil (44 MJ/kg) is similar to that of commercial diesel fuel (45 MJ/kg) and gasoline fuel (47 MJ/kg). The results of fuel properties show that the properties of pyrolysis oil are basically the same as those of industrial diesel oil. However, to obtain high-quality fuels, further treatment such as hydrogenation, desulfurization or the use of catalysts are required. The treated pyrolysis oil can be mixed with diesel or other fuels in different proportions to meet the standard and serve as a fossil fuel substitute for motor vehicles. The treated pyrolysis oil can also provide high-calorific fuel for large fuel equipment, such as internal combustion engine boilers and heating furnaces. In summary, pyrolysis oil has a strong potential to replace traditional fuels.
After distillation, pyrolysis oil can be divided into three parts: naphtha fraction, medium fraction, and heavy fraction. Distillation can make pyrolysis oil more valuable for use. Li et al. [92] found that pyrolysis oil produced from waste tire pyrolysis in a rotary kiln contained 39.2&#;42.3 wt% of light naphtha (below 200 °C), a medium fractionation of 32.4&#;33.2 wt% (200&#;350 °C), and a heavy fractionation of 25.5&#;28.5 wt% (above 350 °C). Researchers generally found that higher temperatures promoted the formation of lighter pyrolysis oil, such as gasoline and kerosene. Table 4 reflects the fractions of pyrolysis oil at different pyrolysis temperatures.
Contents of pyrolysis oil fractions at different temperatures.
Reactors Temperature (°C) Fraction Content (vol%) Ref. LightNaphtha fractions obtained from the distillation of pyrolysis oil can be used to extract valuable chemical products, such as benzene, toluene, xylene, limonene, and phenolic compounds [103]. Among them, limonene has high economic value and high yield and can be used as solvent and aromatic agent. In the process of waste rubber pyrolysis, the C-C bonds in the double bonds of polyisoprene tend to break to form allyl radicals. limonene is generated after the cyclization of allyl radicals. Limonene is unstable and decomposes easily at high temperatures. Limonene production is increased at lower cracking pressures and shorter residence times. Limonene yield decreases with the increase in temperature, with the highest yield at 400&#;500 °C [104]. Other pyrolysis oil distillates are also important and widely used. Benzene is used in drugs, surfactants, and dyes. Toluene is used in the production of pesticides, dyes, surfactants, and solvents. O-xylene is used in the production of plasticizers, dyes, and pigments. M-xylene derivatives are used in the fiber industry, and p-xylene derivatives are used in the production of polyester fibers [93]. It can be seen from the above that the chemical products extracted from pyrolysis oil are of high economic value and are widely used in industry.
Pyrolysis gas is the gaseous product after rubber pyrolysis, and it is a noncondensable gas formed in the pyrolysis process. The gaseous products of WT pyrolysis are alkanes (C1&#;C4), olefins (C1&#;C4), hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and sulfur and nitrogen compounds in low concentrations [105,106,107,108,109,110]. The contents of H2 (30.4%) and methane (CH4) (23.3%) is highest in the pyrolysis gas, with a calorimetric value of about 38.5 MJ/Nm3 [111].
Based on the analysis and determination of GC, the main components of pyrolysis gas are CH4, ethane (C2H6), ethylene (C2H4), propylene (C3H6), propane (C3H8), acetylene (C2H2), butane (C4H10), butane (C4H8), 1,3-butadiene (C4H6), pentane (C5H12), benzene (C6H6), toluene (C7H8), xylene (C8H10), H2, nitrogen (N2), CO, CO2, and hydrogen sulfide (H2S) [64].
The composition of pyrolysis gas mainly depends on the composition of raw materials for tire preparation, such as SBR, NR, nitrile rubber, neoprene rubber, polybutadiene rubber, and so on. The gas composition is also related to pyrolysis temperature, pressure, etc. The long rubber polymer chain is broken at high temperatures to produce short-chain gas products. With the increase in temperature, the pyrolysis products will have secondary reactions, producing lighter gas and higher contents of hydrogen, methane and C1&#;C4 hydrocarbons. Wei et al. [32] studied the production process of pyrolysis gas by combining RMD and TG-FTIR. The results showed that CH3 separated from the main chain extracted H from other molecules to produce CH4, while C2H4 was mainly produced by the C=C bond fracture on the long chain. Other small gas molecules are produced by the decomposition of large alkenes or low-activity free radicals. Xu et al. [33] found that the generation of CH4 and C2H4 mainly occurred in the range of 350&#;600 °C. The formation of CH4 is generally attributed to the decomposition of -CH3 and -CH2 on aliphatic hydrocarbons. The formation of C2H4 is mainly attributed to the decomposition reaction of free radicals resulting from the bond-breaking of the olefin ring and aromatic ring on the straight chain and the side chain [112]. The total gas production generated by pyrolysis of tires will increase with the increase in pyrolysis temperature. The higher the temperature, the higher the gas production and the lower the oil production [52,113]. Kaminsky et al. [52] found that as pyrolysis temperature increased from 598 °C to 700 °C, pyrolysis yield increased from 20 wt% to 33 wt%, the hydrogen yield increased from 0.59 wt% to 1.1 wt%, the CH4 content increased from 2.9 wt% to 6.9 wt%, C2 hydrocarbons increased from 2.8 wt% to 5.8 wt%, and C3 hydrocarbons increased from 2.96 wt% to 5.03 wt%.
Pyrolysis gas contains valuable olefins, such as ethylene and propylene, which can be purified as an important chemical raw material. The content of pyrolysis gas and the valuable olefins can be increased through controlling factors, such as pyrolysis temperature [114] and catalysts [115], during the pyrolysis process. Li et al. [116] found that high H2 yield could be obtained by pyrolysis of WTs with Ni supported on activated carbon. Luo et al. [117] added blast furnace slag into the reactor during the process of WT pyrolysis and found that blast furnace slag could act as a dehydrogenation catalyst, significantly increasing the production of gas products and the contents of H2 and CO in the gas. Hydrogen can be purified through the hydrogen production process when hydrogen content is high [118,119,120]. Kuznetsov et al. [121] conducted plasma gasification of tires at a maximum temperature of K and used calcium oxide as catalyst to improve the production of hydrogen, and the hydrogen content increased from 58% to 99%. Portofino et al. [120] conducted catalytic pyrolysis of WTs in a rotary kiln and found that when the pyrolysis temperature was 823 K, the concentration of CH4 (42%) was the highest&#;followed by that of H2 (30%)&#;and when the pyrolysis temperature was K, the concentration of H2 (57%) was the highest&#;followed by that of CH4 (21%).
The pyrolysis gas has stable physical properties and low sulfur content, and the calorific value is equivalent to that of natural gas, which can reach &#;10,230 kJ/kg. Raman et al. [119] obtained a maximum pyrolysis gas yield of 0.76 Nm3/kg at K, and the gas calorific value was 39.6 MJ/Nm3. Galvagno et al. [51] obtained the pyrolysis gas using an FBR at 550 and 680 °C, and the calorific values were 22 and 29 MJ/kg, respectively. Laresgoiti et al. [61] found that the calorific value of pyrolysis gas prepared from the autoclave reactor under N2 atmosphere was higher. When the pyrolysis temperature was 400 °C, the gas calorific value was 81 MJ/m3, and when the pyrolysis temperature was 700 °C, the gas calorific value was 69.5 MJ/m3.
At present, pyrolysis gas is most commonly used in supplying the heat required by the pyrolysis process, and many self-sufficient pyrolysis devices have been developed successfully [20,51,61,122]. Furthermore, after simple treatment, pyrolysis gas can be used as industrial fuel for heating various large fuel equipment or factories, which has good market development prospects.
The solid products of WT pyrolysis are carbon black and ash composed of inorganic compounds, which account for about 35&#;40 wt% of the total weight of WTs. The carbon black accounts for 80&#;90 wt% of the solid products [6]. The main elements of pyrolysis carbon black are carbon (81.5&#;82.8 wt%), hydrogen (0.32&#;1.0 wt%), sulfur (1.7&#;3.3 wt%), and nitrogen (0.2&#;0.5 wt%) [110,123]. Meanwhile, there are still some other pollutants in the solid products, such as dust, heavy metals, volatile substances, and trace oil [124].
Carbon black is an important reinforcement filler of tires and plays the functions of coloring, reinforcing, conducting electricity, thermal conducting, anti-ultraviolet rays, and other functions in rubber refining and has an important influence on the physical and mechanical properties and processing technology of rubber compounds. Pyrolysis carbon black is a recyclable resource with high economic value, but it cannot achieve the reinforcing effect of industrial carbon black. The low-boiling-point material content and ash content of tire pyrolysis carbon black are higher than those of commercial carbon black (N550). High ash content and the presence of impurities have adverse effects on the curing characteristics, cross-linking density, and mechanical properties of carbon black [125]. There are functional groups on the carbon black&#;s surface, such as carboxyl, phenols, and ketones, which enhance the carbon black surface activity and improve the strength of the interaction of carbon black and rubber. However, the ash attached to the carbon black surface covers active sites of the carbon black surface [126], hindering the interaction between the carbon black and the polymer, affecting the reinforcement, which becomes the main obstacle to the recovery of pyrolysis black carbon. Tang [127] prepared carbon black by plasma pyrolysis of tire particles and compared it with commercial carbon black. It was found that the surface area of pyrolytic carbon black was 64.8 m2/g and that the ash content was 15.14 wt%, while the surface area of commercial carbon black (N330) was 80 m2/g, and the ash content was 0.4 wt%. Compared with other pyrolysis equipment, the surface area of pyrolysis carbon black produced by FBR was lower [96]. Meanwhile, pyrolysis pressure and temperature also have certain effects on the quality of pyrolysis carbon black. Sahouli et al. [128] compared the surface chemistry and morphology characteristics of carbon black recovered under different pyrolysis conditions and found that the surface characteristics of carbon black recovered via low-pressure pyrolysis were similar to those of the corresponding raw material carbon black. It was found that when the pyrolysis temperature was 425 °C, the specific surface area of pyrolysis carbon black was 46.5 m2/g, and when the pyrolysis temperature rose to 600 °C, the specific surface area of pyrolysis carbon black significantly increased to 116.30 m2/g. The reason was that with the increase in pyrolysis temperature, the hydrocarbons on the pyrolysis carbon surface were decomposed, reducing the coking on the carbon black surface [36]. In addition to the specific surface area, pyrolysis temperature also has a certain effect on some other important parameters of pyrolysis carbon black, as shown in Table 5.
Parameters of pyrolysis carbon black at different pyrolysis temperatures.
Temperature (°C) Carbon (wt%) Ash (wt%) S (wt%) Specific Surface Area (m2/g) Ref. 500 82.18 14.6 3.6 43.1 [129] 550 77.22 14.58 2.41 89.1 [58] 550 88.0 13.2 2.5 65.7 [130] 550 86.3 12.5 2.8 64 [94] 600 86.6 7.10 2.10 116.3 [36] 650 82.60 14.80 2.30 63.5 [96] 700 83.0 14.8 2.7 83 [131] Open in a new tabAccording to the data in Table 5, the unprocessed pyrolysis carbon black cannot be used in rubber processing. At present, the pyrolysis carbon black of WT pyrolysis is mainly applied in the following aspects: Firstly, when the particle size and surface active structures of pyrolysis carbon black meet the requirements after treatment, it can be used as a substitute for commercial carbon black and as a reinforcing agent and filler for rubber products. Secondly, it can be added to asphalt for asphalt modification. In the third aspect, activated carbon is prepared via physical and chemical methods. Pyrolysis carbon black is not used in large quantities in other applications, such as in coatings for the automotive industry&#;where it is protected from ultraviolet light&#;or in inks&#;where it is used to provide pigmentation.
The solid carbon produced by waste tire pyrolysis contains some high temperature pyrolysis products of additives. Elemental analysis shows that the solid residue contains 71 wt% C and some other elements&#;such as Fe, S, Zn, etc.&#;as well as a large amount of ash [132]. Therefore, solid carbon is usually treated with acid and alkali to remove inorganic elements and reduce ash and sulfur content. Jitkarnka et al. [85] found that acid treatment increased the surface area and pore size of carbon. The increase in total acidity of carbonates is due to the enhancement of the surface carboxyl (-COOH) groups. In addition, HNO3 treatment can make coke demineralize and significantly reduce the content of sulfur compounds in coke. Differently from other acids, HNO3 can maintain the pore structure of coke so that the coke has a great advantage in dust removal. HNO3 and NaOH were used to chemically leach the solid carbon black generated in the continuous pyrolysis process. The ratio of reagent/pyrolysis carbon was 10 mL/g, and 4.9 wt% of the ash content could be removed by soaking at 60 °C for 60 min [130]. Chaal et al. [129] acid pickled and alkaline washed cracked carbon black with H2SO4 and NaOH, respectively, under vacuum conditions. The results showed that: after acid&#;base treatment, the content of C element in pyrolysis carbon black increased significantly, and the ash content of pyrolysis carbon black decreased to 3.1 wt% (14.6 wt% before treatment). The pyrolysis carbon black was pickled and alkaline washed to remove ash on its surface and then reacted with stearic acid. The carboxyl group of stearic acid could esterify the hydroxyl group on the surface of carbon black, producing a long-chain alkyl group at one end of carbon black molecule, which enhanced its bonding with rubber. Tian et al. [133] prepared pyrolysis carbon black/rubber composites based on a novel approach called atomization dispersion and high temperature sputtering drying (ADSD method) (as shown in Figure 4). Before the ADSD mixing method, the pretreatment of pyrolysis carbon black had been performed, i.e., the pyrolysis carbon black was mechanically ground. Through the ADSD method, the uniform dispersion and instantaneous drying of the pyrolysis carbon black in the latex were realized. The performance of this carbon black/rubber composite was even better than that of composites prepared via traditional methods. In a word, after the improvement of surface structure of pyrolysis carbon black, it can be used in rubber products instead of industrial carbon black, which is of great significance for the recovery and utilization of waste tire pyrolysis solid products.
Studies have shown that the addition of commercial-grade carbon black to asphalt can effectively improve the photo-oxidation aging rate of asphalt [134,135]. The addition of carbon black obtained from waste rubber pyrolysis to asphalt also had a positive effect on the rheological properties of asphalt [136,137]. Feng et al. [138] added the treated waste tire pyrolysis carbon black to the asphalt for the asphalt modification treatment, and the results showed that if the pyrolysis carbon black content was not more than 10%, the pyrolysis carbon black can significantly improve its high temperature performance, the ability of permanent deformation at high temperature, can obviously improve the thermal aging and photo-oxidation aging. Modified asphalt can be used in road construction, which is of great significance to improve the pavement performance.
Pyrolysis carbon can also be used as an adsorbent, but due to its low surface area, which is approximately 30&#;90 m2/g, it is required to be activated to increase its porosity and surface area [139]. The porous activated carbon made from WT pyrolysis carbon black has a high adsorption capacity and can also remove various pollutants, such as heavy metals, dyes, pesticides, and other pollutants from water media [140]. Shah et al. [141] found that the adsorption capacity of acid treated pyrolysis carbon black was even higher than that of commercial activated carbon.
Traditional activated carbon preparation is divided into two methods: physical and chemical activation. During physical activation, the carbon material reacts with steam, carbon dioxide, or other oxidizing gases at a high temperature to make the disordered carbon in the carbon material partially oxidized and etched into pores, forming a developed microporous structure inside the material. It had been experimentally confirmed that the surface area of activated carbon produced via steam activation method is 20% higher than that produced by the carbon dioxide activation method [87]. Chemical activation is the preparation of activated carbon by mixing an appropriate proportion of activator (H2SO4, KOH, H3PO4) with raw materials and directly activating it [142]. Chemical activation has many advantages over physical activation, such as low activation temperature, high yield, high surface area, and large pore volume. Rambau [139] adopted a mechanochemical method to activate the pyrolysis carbon. Before activation, the pyrolysis carbon was compacted by mechanical methods with activator to increase its active site, and then the pyrolysis carbon was treated with water, HF, and HNO3. According to data analysis, the surface area of pyrolysis carbon was the largest after washing by HNO3, which was 955.20 m2/g. The purpose of acid treatment was to remove inorganic elements and generate more pores on the surface of carbon, thus increasing its surface area. Acosta et al. [143] activated pyrolysis carbon black with KOH in a temperature range of 600&#;800 °C to prepare activated carbon. The surface area of activated carbon prepared was up to 814 m2/g, and the surface area, micro porosity, and medium porosity were greatly improved. Gupta et al. [144] used microwave-assisted processing prepared high quality activated carbon from pyrolysis carbon black. Rahmani et al. [145] investigated that the activated carbon prepared from pyrolysis carbon black had a good adsorption effect on lead metal cations from lead-containing aqueous solutions. Trubetskaya et al. [146] used pyrolysis carbon black for the cleaning of wastewater and removed up to 95% of phenol and chloride. Therefore, after necessary means, pyrolysis carbon black is expected to replace commercial activated carbon [36,147,148].
The WT pyrolysis technology has been widely studied. At present, catalytic pyrolysis is the best treatment method for WTs. Through this method, a large number of WTs can be thoroughly treated to reduce the environmental pollution and obtain high-value pyrolysis products. WT pyrolysis includes physical and chemical reaction, that is, condensation, polymerization, depolymerization, and aromatization of rubber occur at an appropriate temperature. Usually, the progress of WT pyrolysis is divided into the following four steps: first, water vapor and additives decompose; NR begins to decompose at about 300 °C; around 400 °C, SR begins to decompose; at 500 °C, pyrolysis is basically completed. The yield and composition of pyrolysis products depend largely on temperature and catalysts. With the increase in pyrolysis temperature, the content of pyrolysis gas increases and the content of pyrolysis oil decreases. The higher the pyrolysis temperature, the faster the secondary reaction will occur, leading to an increase in the content of aromatic compounds. Zeolite catalysts are beneficial to the generation of aromatic hydrocarbons (especially monocyclic aromatic hydrocarbons, such as benzene, toluene, and xylene). At the same time, the use of zeolite catalysts makes the yield of oil reduce and the gas production increased. Pyrolysis oil and gas obtained from WT pyrolysis have a high calorific value (35&#;44 MJ/kg). After being treated, pyrolysis oil can be used as alternative fuel for engines or factory fuel. After distillation, the pyrolysis oil also can be purified as chemical raw materials, such as phenylxylene, toluene, and limonene. The pyrolysis carbon can be used as commercial carbon black or activated carbon or used in asphalt modification. Pyrolysis gas can be used directly for industrial fuel and hydrogen production. In a word, WTs can be converted into valuable products via pyrolysis. At present, the pyrolysis process and continuous equipment are not perfect, and the profitability of the pyrolysis of WTs depends on the main products obtained. Therefore, the pyrolysis products should be further refined and processed to produce products of higher value so as to make the pyrolysis process more economically feasible.
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The future research on WT pyrolysis should focus on the influence mechanism of reaction conditions on reaction path so that the pyrolysis products can achieve directional regulation. More novel methods of tests and experiments should be applied to the research of WT pyrolysis so that the pyrolysis mechanism can be explored in detail. In addition, researchers should also focus on the relationship between product selectivity and pollutant reduction. Only considering various factors, the environmental and economic benefits of WT pyrolysis can be further improved, and its degree of industrialization can be continuously expanded.
Conceptualization, W.H. and H.C.; investigation, W.H. and H.C.; writing&#;original draft preparation, W.H.; writing&#;review and editing, D.H.; supervision, D.H.; project administration, H.C.; funding acquisition, W.H. All authors have read and agreed to the published version of the manuscript.
Not applicable.
The data presented in this study are available on request from the corresponding author.
The authors declare no conflict of interest.
This research was funded by [National Natural Science Foundation of China] grant number []. And The APC was funded by this project.
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This section collects any data citations, data availability statements, or supplementary materials included in this article.
The data presented in this study are available on request from the corresponding author.
The purpose of this review is to understand the possibilities of using pyrolysis for waste treatment and to create a workflow for pyrolysis experimentation. It is therefore essential to know which feedstocks can be pyrolyzed and what results can be obtained from them. Moreover, it is essential to have a comprehensive understanding of the various types of pyrolytic reactors, as well as the operating conditions and the effects of catalysis.
Early work on biomass pyrolysis is known from the 18th century. During the s, scientists discovered that the yield of pyrolysis liquid could be increased by fast pyrolysis, where the feedstock is heated rapidly and the resulting vapors are also rapidly condensed [ 19 ]. Since then, and to date, the discoveries and advances in the use of pyrolysis have been significant. The commercial applications offered by pyrolysis promote the creation of systems to exploit this technology. One of the most recent patents, made in in Japan, proposes a process and a system for converting biomass into high-carbon bioreagents, suitable for various commercial applications [ 20 ].
Pyrolysis is a thermochemical cracking process that breaks down long-chain hydrocarbons into molecules with lower molecular weight [ 16 ]. The process is carried out under oxygen-free conditions and in temperature ranges between 300 and 700 °C. The final products depend on various factors, i.e., thermal decomposition rate, feedstock, particle size, and temperature [ 17 ]. The word &#;pyrolysis&#; is derived from the Greek words &#;pyro&#;, meaning fire, and &#;lysis&#;, meaning decomposition. It has been used for centuries for charcoal production and, in ancient Egypt, for making tar and embalming agents [ 18 ].
Biomass refers to any organic matter that is derived from living organisms, including plants and animals. Biomass can be used as a source of energy and is considered to be a renewable energy source, because it can be replenished relatively quickly. Examples of biomass include wood, crops, and agricultural waste, such as straw, as well as organic waste from the food and paper industries. Biomass can be converted into a variety of energy sources, e.g., heat, electricity, and biofuels. Biofuels mainly include ethanol, methanol, and biodiesel, which can be used as a replacement or supplement to traditional fossil fuels. Hydrocarbons are compounds that consist of hydrogen and carbon atoms. Some hydrocarbons, such as those found in fossil fuels like coal, oil, and natural gas, are non-renewable resources. However, biomass is mainly composed of carbohydrates; some examples include the following:
Municipal waste is only part of the problem in waste management. Discarded plastics have a major environmental and landfill impact. Accelerated consumption has led to an increase in single-use products, which, coupled with slow growth in recycling, results in a high rate of accumulation of plastic waste [ 9 10 ]. A recent study by Zhao et al. [ 11 ] analyzed plastic imports and exports from to . The plastic trade has been growing steadily since (84,500 t of imports), but it is between and that the highest growth was experienced. The highest point of consumption was achieved in , with 11,386,200 t of imported plastic waste. It is from onwards that trade has slowed down sharply. China has been the largest importer and exporter of plastic waste in the world for decades. However, in it announced a zero plastic waste import policy, significantly reducing the volume of the plastic trade, although the export volume has not changed [ 12 ]. The massive use of plastics has a negative impact on the environment. Despite its versatility and durability, non-biodegradable plastic takes centuries to decompose and ends up polluting the oceans. The full extent of plastics in the oceans and seas is not yet known, but the impact on marine wildlife is notorious, as they ingest the waste, which can then be consumed by humans [ 13 ].
Approximately 1.9 billion tonnes of municipal solid waste (MSW) is produced annually worldwide, and almost 30% is not collected by municipal waste management systems [ 5 ]. In addition, MSW generation in is projected to increase to 3.4 billion tonnes. In this context, the EU has included sustainable resource use as a priority area in the European Green Pact. Moreover, the EU has reiterated its commitment to implement the Agenda for Sustainable Development, with the aim of protecting the environment, reducing land degradation, and preventing biodiversity loss. This includes reducing the EU&#;s dependence on the use of natural resources from fossil origin [ 6 ]. Excessive and uncontrolled consumption leads to large generation of waste, and consumption extends to fuels. The transport sector is a major consumer of fossil fuels. The road transport sector accounts for 72% of total greenhouse gas emissions in transport [ 7 ]. Through the correct treatment of waste, it is possible to obtain eco-fuels, contributing to mitigating the problem. In the field of thermochemical processes, pyrolysis occupies a unique position for the conversion of coal and biomass into both energy and non-energy applications [ 8 ]. In this sense, pyrolysis plays an important role in potential resource and waste management pathways.
The Paris Agreement: Adopted in during the UN Conference of the Parties on Climate Change (COP21), it aims to limit global temperature rise to less than 2 °C above pre-industrial levels. Moreover, it works towards a more ambitious target of 1.5 °C [ 2 ].
Global energy demand is steadily increasing due to economic and population growth. At the same time, traditional fossil fuels, such as oil and coal, are being depleted and significantly contributing to environmental problems, such as climate change. It is therefore necessary to look for new renewable fuels. In this context, waste-to-energy technologies may help in meeting energy demand while reducing dependence on fossil fuels. Governments around the world are taking steps to encourage research and development of these technologies, i.e., establishing policies and investment programs. In addition, to reduce greenhouse gas emissions while achieving higher energy efficiency, long-term goals and targets are also being set. The United Nations Framework Convention on Climate Change (UNFCCC) is an international treaty, signed in , to address the problem of climate change. The treaty, which has been ratified by nearly all countries, establishes a framework for reducing greenhouse gas emissions and mitigating their effects. The goal of the UNFCCC is to stabilize greenhouse gas concentrations in the atmosphere at a level that will prevent dangerous human interference with the climate system. To oversee the implementation of the convention, it also established the Conference of the Parties (COP) as its supreme body [ 1 ]. In recent years, to address climate change while reducing greenhouse gas emissions, several international agreements have been reached. Some of the most prominent include the following:
Waste tires are an important source of raw materials for pyrolysis, as they contain rubber and steel, which can be recycled into fuels and other products. In the research of Berrueco et al. [ 51 ] on tire pyrolysis, it was pointed out that gas production is favored by long residence times at high temperatures. Tires are notable for their contents of hydrocarbons and gaseous fractions, i.e., H, CO, and CO. However, the calorific value of the gas obtained from the pyrolysis was lower than expected, although it was still valid for use in gas engines. According to Williams [ 52 ], the oil from tire pyrolysis is chemically complex and contains aliphatic, aromatic, heteroatomic, and polar compounds. This oil&#;s properties allow its use as a fuel, as its properties are similar to those of diesel fuel or light fuel oil.
Other industrial waste, such as paper and wood, can also be processed by pyrolysis to produce biofuels and other chemicals. Potential landfill waste can include newsprint and cardboard, which contain nitrogen, sulfur, and oxygen, as highlighted by Fekhar et al. [ 45 ], who also highlighted the difference in moisture content between plastics and paper waste. The latter contains considerably more moisture than plastics, which hardly include any moisture at all. It is important to take this characteristic into account before a pre-treatment process is selected. After pyrolysis, the authors noted that the liquid product from newsprint and cardboard resulted in water and various oxygenated compounds. Ahmed and Gupta [ 46 ] investigated the gasification and pyrolysis of paper, underlining that gasification offers better results in terms of higher material destruction, hydrogen production, and chemical energy. Yao et al. [ 47 ] focused on the treatment of paper sludge, emphasizing the problems that it poses in terms of industrial pollution in China. Moreover, sludges do not come with paper alone; they also contain heterocyclic compounds, polycyclic aromatic hydrocarbons, amino acids, and organic fluorinated compounds. The authors proposed pyrolysis treatment of this waste to reduce air pollution and carbon emissions, compared to direct burning of waste. Determining the pyrolysis temperature is important for optimal results. Kim et al. [ 48 ] sought the optimal temperature to achieve the maximum bio-oil yield from the pyrolysis of construction wood waste. Carlson et al. [ 49 ], in their study on the production of aromatics and olefins from wood in a fluidized bed reactor, showed that propylene is more reactive than ethylene and produces higher quantities of aromatics. They also noted that the lower the temperature, the lower the methane production. The study of experimental kinetics is interesting in pyrolysis studies. Slopiecka et al. [ 50 ] conducted a kinetics study of the devolatilization of aspen wood, finding that its thermal decomposition proceeds in three stages.
Plastic waste is an important source of feedstock for pyrolysis, as it contains carbons and can be converted into fuels, e.g., diesel fuel and natural gas [ 10 ]. In fact, liquids with a high calorific value can be obtained from the pyrolysis of plastics. In other words, they can be useful as fuels. Some plastics commonly used for pyrolysis include polyethylene (PE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), polystyrene (PS), and polypropylene (PP) [ 34 ]. Gaurch and Pramanik [ 35 ] studied the aromatization of PE plastic waste with fly ash (FA) as a catalyst. From this work, potential interest in and application of the catalytic pyrolysis process as an option to produce aromatics (benzene, toluene, ethylbenzene, and xylene (BTEX)) can be extracted. LDPE has been used as a raw material in numerous works, from which various conclusions can be drawn. Aguado et al. [ 36 ] highlighted the increase in the conversion rate by integrating catalysts in LDPE pyrolysis in a continuous screw reactor. Marcilla et al. [ 37 ] also analyzed the impact of catalysts. In their study, the authors investigated the polymer structure of products obtained from thermal and catalytic pyrolysis of LDPE and HDPE, in the presence of HZSM5 and HUSY zeolites. They focused on analyzing the composition of the gaseous and liquid fractions and found that the liquid fraction contained higher amounts of 1-olefins and n-paraffins. Investigations by Alonso-Morales et al. [ 38 ], on LDPE pyrolysis in batch feed reactors with slow and fast heating, did not provide optimal results in terms of solid char production, despite the use of activation additives and pyrolysis materials. However, the use of a semi-continuous feed reactor with fast heating achieved a high yield (15&#;52%,) of solid coals due to the longer residence times of the pyrolysis products. In addition, pyrolysis in a metal-free quartz reactor produced very high solid carbon yields (15&#;43%,). Fan et al. [ 39 ] focused their investigation on LLDPE conversion using both continuous-stirred microwave pyrolysis (CSMP) and batch microwave pyrolysis (BMP) systems. Reactions took place in the presence and absence of an ex situ catalytic bed with HZSM-5. The authors observed significant differences in product yields for the non-catalytic processes, where CSMP produced a higher condensate yield and a lower gas yield compared to BMP. Pyrolysis of HDPE was studied by Sogancioglu et al. [ 40 ], together with pyrolysis of LDPE, characterizing the resulting fractions. The char obtained was analyzed for its use as an additive in epoxy composites. Epoxy composites with HDPE carbon additives at 300 °C showed improved elongation at break and tensile strength performance. Kim et al. [ 41 ] performed kinetics tests on the pyrolysis of a mixture of waste automobile lubricating oil (WALO) and PS using thermogravimetric analysis (TGA). From this study, the analysis of the carbon number distribution of the oil produced at different heating rates is noteworthy. Decreasing the heating rates resulted in a slight shift in the carbon number of the produced oil towards light hydrocarbons. Park et al. [ 42 ] studied PP pyrolysis with a novel activator-assisted process. Increasing the activator temperature and the bubbling zone significantly increased the gas and oil yields, respectively. The use of nitrogen and a short residence time were found to increase the olefin yield. Degradation of activated PP molecules took place through different mechanisms. In this context, Kasar and Ahmaruzzaman [ 43 ] found that co-pyrolysis of crude oil with PP produced 80% pyrolytic oil. Furthermore, homogeneous catalysis has been proposed as an alternative for plastic waste treatment and high-value chemical production [ 44 ]. Aside from everything mentioned so far, plastics are ideal for pyrolysis because of their abundance, low density, and calorific value, among other properties. In summary, plastic waste, once considered to be an environmental problem, has become a valuable pyrolysis raw material.
Old and discarded furniture can also be processed by pyrolysis to produce biofuels and other chemicals. Uzun and Kanmaz [ 32 ] found that pine sawdust was a promising feedstock for bio-oil production, with maximum production rates of 42% (). The importance of particle size is highlighted in both the above study and the one carried out by Heo et al. [ 33 ]. In the latter study, with respect to furniture sawdust pyrolysis, it was found that a higher gas flow, together with a higher feed rate, was favorable for bio-oil production, as vapor residence times were reduced.
Agricultural and forestry industry residues: Pyrolysis can be used to convert them into biofuels [ 21 22 ]. These feedstocks include pine pruning waste, rice straw, corn stover, sunflower waste, and olive waste, among others. Chen et al. [ 23 ] carried out pyrolysis at different temperatures with pine needles. The pine needles were pre-treated by cleaning, air-drying, and baking for subsequent grinding. In this case, the authors sought to avoid burning and charring. Garcia-Perez et al. [ 24 ] used two types of pine chips, of different origins and sizes, in different pyrolysis reactors. This allowed for a comparative analysis, finding similar results, with slightly better liquid fraction yields in the auger-type reactor compared to the batch one. Another work, by Yildiz et al. [ 25 ], analyzed the product composition of catalytic and non-catalytic fast pyrolysis of pine wood, itself a low-ash feedstock. The ash that accumulates can affect the catalyst efficiency by influencing the composition of the resulting pyrolysis vapors. The authors concluded that ash accumulation has an impact comparable to that of other catalyst problems that can affect the pyrolysis process, e.g., catalyst deactivation. Nam et al. [ 17 ] experimented with the possibilities of rice residue pyrolysis with different reactors. The rice residue was subjected to pre-treatment by air drying, and then it was chopped into smaller particles. The authors emphasized the importance of the moisture content, and they observed better yields in slow processes (auger and batch) for biochar, in addition to higher bio-oil quantity in fluidized bed reactors. Huang et al. [ 26 ] discussed the recovery of rice straw into resources and energy, using microwave-induced pyrolysis. They sought constant moisture by tanning the rice residue for 10 days, before it was crushed and sieved. The efficiency of this process depended on the microwave power and the size of the rice straw particles. They concluded that, for a satisfactory result using very small particles, a lower microwave power would be necessary. Zabaniotou et al. [ 27 ] compared the results of pyrolysis of different agricultural-based materials, i.e., maize, sunflower, and olive residues. Using fixed-bed reactors with and without catalysis, cellulose- and hemicellulose-based wastes produced higher amounts of hydrogen-rich gas than those based on lignin. Ren et al. [ 28 ] investigated the integration of microwave torrefaction and pyrolysis of corn stover. Torrefaction oils are noted for their high-value-added chemicals (furans and phenols), making them potentially interesting as a fuel source. The authors highlighted the use of torrefaction as a pre-treatment, combined with pyrolysis, to improve bio-oil quality. Colantoni et al. [ 29 ] based their study on the pyrolysis of grape and sunflower residues in the search for sustainable alternative fuels. The results demonstrated that torrefaction and pyrolysis of pelletized agricultural residues was an effective method to produce high-calorific-value biochar. Lajili et al. [ 30 ] studied olive residue pyrolysis, implementing biomass gasification, although the results were inconclusive. Kabakci et al. [ 31 ], in addition to studying the characteristics of olive residue pyrolysis, also investigated pyrolysis kinematics. The results were compared with those from refuse-derived fuel (RDF) pyrolysis, finding that olive residue decomposition started at lower temperatures but showed a higher maximum temperature; the temperature range of olive residue devolatilization was larger than that for RDF.
The selection of the pre-treatment method and its feasibility are highly influenced by the type and humidity of the raw material. For this reason, it is crucial to classify the raw material for the desired production. Some of the most common feedstocks for pyrolysis include the following:
The horizontal tubular reactor is a pyrolyzer that consists of a horizontal tube where the feedstock to be pyrolyzed is introduced. The horizontal arrangement of the reactor allows for uniform heat distribution and higher efficiency in the decomposition of the materials [ 86 ]. The combination of different systems is being studied to obtain better results. Chen et al. [ 87 ] integrated coal pyrolysis and volatiles optimization in an integrated reactor with two sections to improve tar quality. Their article compares the results of three cases, the results of which are shown in Table 3 . In the first case, only the drop tube was used. In the second case, the pyrolysis time was prolonged, favoring the release of volatiles. They observed the decrease in the solid fraction and the increase in the liquid and gaseous fractions. Finally, the third case optimized the moving bed inside the reactor. The volatiles generated during pyrolysis in the drop tube passed through the carbon bed to be upgraded in the isothermal zone. A lower fraction of solids and a higher fraction of liquids were obtained.
The drop-tube reactor stands out for its versatility to work with all types of fuels, and in having good control over mass balances and residence times [ 84 ]. It is a reactor that operates at high pressures. As observed in [ 85 ], the char yield increased with pressure, while the tar yield decreased. At the same time, the liquid and gas yields remained relatively stable. The higher coal yield was due to the fact that the vapor pressure of tar and liquid is lower than the experimental pressure, causing them to remain solid.
Vacuum reactors can be used in batch or continuous conditions. This is a promising technology for the production of bio-oil [ 82 ]. Vacuum conditions are produced with a mechanical pump and by diffusion [ 83 ].
The wire mesh reactor (WMR) is characterized by the heating process using an electric current flowing through a mesh. It features minimized side reactions between primary volatile particles, along with relative control of the particles in terms of time and temperature [ 80 ]. The latter authors noted that the minimal side reaction results in a purer tar. The WMR also minimizes secondary reactions of the primary volatiles in the vapor phase [ 81 ].
In addition to the abovementioned reactors, there are variants that do not completely fit into the above classification. Some of these reactors have been studied in recent years, the results of which are given in Table 3
In summary, the ideal temperature is mainly sought to obtain bio-oil. To determine viability and yields, without forgetting to consider speed and the size of the reactor feed, reactors and waste are compared. For this, studies in pilot reactors are highlighted as an alternative prior to industrial use.
Some studies have been carried out in pilot plants, seeking the optimal design. Martinez et al. [ 74 ] carried out a study of the pyrolysis of tires in an auger-type pilot reactor. One of the most interesting conclusions drawn from this experimentation was that the gaseous fraction of the tire gas was the most efficient. They also highlighted the importance of COemissions produced during the process, which were lower than those generated in the direct combustion of used tires, which is a point in favor of the pyrolytic process. Fernandez-Akarregi et al. [ 69 ] used a conical bed pilot reactor, which is characterized by a non-porous draft tube that allows its operation at low gas flow rates and under stable conditions, ensuring high bio-oil yields. Similarly, the arrangement of the reactor&#;s hydrodynamic system enables uninterrupted char suppression and ensures a high heat transfer rate, preventing the accumulation of residues in the bed and ensuring temperature uniformity. Temperature uniformity is desirable for a homogeneous process. The system studied by Yang et al. [ 76 ] successfully generated intermediate pyrolysis oils, gases, and coals. They used wood pellets and barley straw as feedstocks in two types of reactors (twin co-axial and auger), with very similar results. The authors focused their study on determining the efficiency of the process once the energy yields of the pyrolysis products were known. Determining the efficiency of a process is an essential point in knowing its viability before it can be used on a commercial scale.
Nam et al. [ 17 ] carried out the experiment with three different reactors (batch, bed, and auger). The results of rice straw pyrolysis with the batch reactor are listed in Table 1 , while the rest are depicted in Table 2 . The authors found that the choice of reactor type, among other factors, may influence the final product. In this sense, the highest biochar yields were achieved with auger and batch reactors. Also, the highest amount of bio-oil was obtained in the fluidized bed reactor. The qualities of the products will also depend on the type of reactor. The bio-oil composition varied according to reactor type, while the biochar composition was similar in the three reactors. Based on the results, the authors suggested using the batch and auger reactors for biochar production. The interest in pyrolysis of waste tires has been mentioned above, but there are other automotive waste products that are also of interest. Haydary et al. [ 72 ] studied the suitability of ASR (automotive waste) for thermal processing to recover valuable materials and energy. It is possible to estimate the parameters required to design a thermal process from those associated with individual ASR components.
As can be seen in Table 1 and Table 2 , a slight improvement in the yields of pyrolysis liquid fractions can be observed in continuous reactors, in comparison to batch reactors. From a quick glance at Table 2 , it can be seen that the tendency is to pyrolyze small particles, in search of homogeneity, to improve the results. Temperatures range from 400 to 800 °C, with 500 and 550 °C being the ones that, in most cases, provide the best results in terms of the desired end products. Jung et al. [ 78 ] reaffirmed that the ideal pyrolysis temperature is around 400&#;550 °C, depending on the pyrolyzed material. This research also highlighted the influence of feed size and speed, with better bio-oil results for smaller feed size and higher speed. In the same way, Heo et al. [ 33 ] found that increasing the gas flow and feed rate resulted in more effective bio-oil production, due to decreased vapor residence time. In addition, the use of non-condensable gas as a fluidization medium showed significant potential to improve bio-oil production, achieving a maximum yield of 65% (). However, from Table 2 , it is not so evident that the yield of the liquid fraction is directly proportional to the temperature increase. Kim et al. [ 48 ], in addition to the pyrolysis of construction wood waste in a batch reactor, used a fluidized bed reactor. For both reactors, the maximum oil yield was reached at 500 °C. When the fluidized bed reactor was used, an increase in oil yield and a decrease in its temperature sensitivity were observed. Thus, it can be concluded that the fluidized bed reactor is an advantageous option for bio-oil production.
A circulating bed reactor (CFB) is a reactor characterized by operating at high gas velocities. It also allows for the recovery of light solids and recirculation of solids that have been dragged to the bottom of the bed. Its operation is pseudo-homogeneous, with high mass and heat transfer rates. Continuous recirculation of hot solids at high rates promotes temperature uniformity throughout the bed [ 71 ].
The bubbling fluidized bed reactor (BFB) is a common alternative to the fluidized bed reactor, due to its simple operation/design and high heat and mass transfer efficiency to biomass particles. However, it has drawbacks in heat transfer to the bed [ 69 ]. The bubbles play a key role in particle mixing and the thermal decomposition of the biomass [ 70 ].
In the case of fast pyrolysis, the most common alternative to the fluidized bed reactor for biomass and waste pyrolysis is the conical spouted bed reactor (CSBR). The particles&#; cyclic movement results in high rates of mass and heat transfer between phases. This characteristic makes the reactor able to handle fine materials, sticky solids, and particles with irregular texture. Energy consumption tends to be exponential with particle size; however, a spouted bed reactor can operate with larger particle diameters. The size of these reactors is smaller than that of a fluidized bed reactor, while keeping the same capacity, and without the need for a gas distribution plate [ 68 69 ].
The auger reactor is a variant for conveying feedstock to the reaction vessel and evacuating solid residues. To improve particle mixing and heat transfer between solid heat carriers and reactants, an appropriate design is essential. The material transport in the auger reactor allows for a good axial dispersion, which favors uniformity during the application of the thermal conditions [ 66 ]. A variant can be configured with two screws intercalated inside the reactor, known as a twin-auger or double-screw pyrolyzer. This configuration favors a constant undulation and agitation of the raw material, due to the interference of the propellers of one screw with those of the other [ 67 ].
The fluidized bed reactor is noted for its high efficiency due to its high particle heating rate. In fact, it maintains constant particle movement, which increases the heat transfer of the particles. The appropriate mass and heat transfer between individual particles and gas ensures the high uniformity of the product quality [ 65 ].
From the research collected so far, it is possible to observe the involvement of researchers in the search for alternatives for the management of these wastes, the possibilities of the compounds of the fractions produced by pyrolysis, and how it is important to take into account the parameters affecting pyrolysis, such as reactor type, raw material, particle size, and temperature.
Some of the studies listed in Table 1 performed pyrolysis on feedstocks such as plastics and synthetic waste. Several conclusions can be drawn from these studies. Berrueco et al. [ 51 ] pyrolyzed tire waste, highlighting the heating value of the obtained gas, between 5.5 and 9.0 MJ/m, valid for use in gas engines. Alonso-Morales et al. [ 38 ] studied LDPE pyrolysis in reactors with different fabrication materials: one with Hastelloy (cobalt&#;chromium&#;nickel&#;molybdenum alloy) and the other with quartz. The use of a semi-continuous loading reactor with a fast heating process can significantly increase the yield of carbonaceous solids, due to extended residence times. It has also been found that the material used in the construction of the reactor has an important influence on the yield and structure of the obtained solids. The use of a Hastelloy tube instead of quartz can increase the structural order of the resulting solids. Marcilla et al. [ 37 ] compared LDPE and HDPE pyrolysis. From this comparison, they highlighted that a higher proportion of gases was produced in the pyrolysis of HDPE than in that of LDPE. The researchers also concluded that the composition of the liquid fraction is influenced by the type of PE used, and that the observed disparities in results seemed to be more related to the polymer and zeolite properties than to the experimental conditions. The study by Kim et al. [ 59 ] examined the isothermal pyrolysis of PS at low temperatures using a batch-operated stirred tank. The main liquid products were single- and double-aromatic ring species, with styrene being the main product, with a yield of 70% (). Other studies, such as the one carried out by Lopez-Urionabarrenechea et al. [ 57 ], focused on the influence of temperature and time on the products obtained in the pyrolysis of plastic waste. This study established an optimal reaction time of 15&#;30 min; longer times had no effect on conversion or on the characteristics of the obtained products. Gaurh and Pramanik [ 64 ] investigated the thermal and catalytic pyrolysis of PE in a nitrogen medium, using three different types of catalysts. The possibilities and benefits exposed by this article on the implementation of catalysts will be discussed later, but the authors concluded that the same process can be purely thermal (without catalysis) and still yield valuable products, protecting the environment and saving energy.
In the study by Kim J. et al. [ 48 ], the maximum oil yield from construction waste wood pyrolysis was achieved at a temperature of 500 °C (54.2%,). They also found that, at higher temperatures, the decomposition reaction produced higher gas-phase yields, rather than liquid-phase yields. This deduction can be extrapolated to the pyrolysis of other feedstocks, and it can be stated that pyrolysis should ideally be carried out at the optimal temperature for each feedstock. Sometimes, not all of the liquid fraction from pyrolysis is useful. This mainly depends on the pyrolysis feedstock. In the study of Fekhar et al. [ 45 ], the liquid yield consisted of two parts: light oil, and water. In another study by Shadangi and Singh [ 62 ], pyrolysis of polanga seed oil was carried out. The results showed that this pyrolytic oil contained other bio-oils, such as oleic acid, hexadecanoic acid, and octadecanonic acid, in addition to presenting a characteristic functional group. It was found that this oil has a high calorific value and can be used as a fuel substitute, as well as in other applications, i.e., as an adsorbent and solid fuel. Garcia-Perez et al. [ 24 ] pyrolyzed pine chips and found it difficult to produce crude bio-oils with appropriate fuel properties. In any case, the combination with other fuels, i.e., biodiesel, could provide great economic and environmental improvements. Furthermore, it was stated that biodiesel&#;s properties, after mixing, do not seem to be highly altered by the soluble fractions of bio-oils, although neutralization with a weak base, e.g., NaHCO, is required to remove the soluble organic acids from biodiesel.
Batch-type stirred vessels are characterized by the agitation of the material during the heating process, at a stable speed and temperature [ 59 ]. In contrast, the static-bed batch reactor keeps the raw material immobile during the process [ 51 ]. Semi-continuous reactors consist of a tube reactor in a vertical position, with an automatic raw material loader. Quartz and Hastelloy are used for the tube [ 38 ].
The reactor can be modified to allow the addition of reagents to the process, making it a semi-batch reactor. Another variant is the batch microwave reactor, which is widely used but has some problems. It makes homogeneous reaction difficult, and due to its low thermal and mass efficiency the pyrolytic products are negatively affected. But it also shortens the reaction time by improving energy consumption and the quality of the resulting fuel [ 39 ].
Batch reactors are strictly discontinuous. A batch reactor is characterized as a simplified process with the capacity to process all types of polymeric waste. However, the batch reactor is irregular in the products of each batch, labor-intensive, and highly dependent on the heating and cooling times [ 45 ].
It is important to note that the exact composition of the fractions obtained during pyrolysis will depend on the process conditions and the organic material being used. As mentioned, pyrolysis reactors mainly differ in how the feedstock is fed and how the process conditions are controlled. The batch reactor loads a fixed amount of material, whereby the process is stopped at the end of the reaction. This leads to cleaning, emptying, and reloading for another batch. In contrast, in a continuous reactor, the material is continuously fed into the reactor and is constantly processed, without interruption. As for the process conditions, in a batch reactor, they must be controlled for the whole batch, while in a continuous reactor the conditions can be adjusted as the reactor is fed. From these differences, it follows that the continuous reactor allows for greater process control and optimization. In addition, the time required to load, process, and unload the material in a batch reactor restricts the production capacity and efficiency compared to a continuous reactor. The main advantage of the continuous reactor over the batch reactor is its ability to process homogeneous materials and small-scale processes. The different reactor types are further developed in the following sections.
Solid: The solid fraction of pyrolysis mostly consists of charcoal and other residues, e.g., ash. If the pyrolyzed residue is biomass, it is called vegetable charcoal. As with the other fractions, the pyrolysis temperature influences this final fraction. It has been found that the higher the temperature in the pyrolysis of plastics, the higher the amount of charcoal [ 38 ]. Secondary repolymerization reactions are responsible for carbon formation. Slight hydrogen production is also observed. In all cases, pyrolysis solids are carbon-rich materials with a high calorific value that can be used as a substitute for solid fossil fuels [ 57 ]. The pyrolysis of cellulose-containing biomass results in the formation of dehydrated saccharides, furans, furanones, benzenes, and cyclopentanones. The highest yield in cellulose pyrolysis is obtained from the saccharides [ 58 ].
Liquid: The liquid fraction resulting from pyrolysis depends on the type of pyrolyzed material and other parameters, such as temperature or the type of pyrolysis reactor. As highlighted in their research, Uzun and Kanmaz [ 32 ] found that the liquid product from biomass pyrolysis was a mixture of multiple organic compounds. It consisted of two phases: an aqueous phase, containing low-molecular-weight oxygenated organic compounds (acetic acid, methanol, and acetone), and a non-aqueous phase, containing aromatic hydrocarbons and organic compounds (aliphatic alcohols, carbonyls, acids, phenols, cresols, benzenediols, guaiacol, and its alkylated derivatives). Aromatic hydrocarbons include single-ring aromatic compounds (such as benzene, toluene, indene, and alkylated derivatives) and polycyclic aromatic hydrocarbons (PAH, such as naphthalene, furans, phenanthrene, and their alkylated derivatives). Water is also released from biomass pyrolysis [ 33 ]. On the other hand, the liquid fraction resulting from the pyrolysis of polymers results in aromatic hydrocarbons as the main compound [ 42 ]. In the liquid fraction resulting from the pyrolysis of polymers, iso-paraffins, aromatics, n-paraffins, and 1-olefins were also observed. The n-paraffins are more frequent at low temperatures, while 1-olefins are more frequent at high temperatures [ 37 ].
Gas: The gaseous fraction, like all fractions, depends on the composition of the pyrolyzed material and the type of reactor. Biomass pyrolysis results in gases such as CO, CO, or hydrocarbons, but gases like acetic acid, methanol, furfural, acetaldehyde, ethanol, propane, or hydroxymethylfural (HMF) can also be released. The increase in COcontent indicates further degradation of cellulosic and hemicellulosic components. Also, the presence of CHand CO suggests secondary cracking of the volatile compounds released during the process. Nam et al. [ 17 ] demonstrated that the composition of the resulting gases depends on the reactor. Hydrogen formation is characteristic of biomass containing paper and cardboard, whereas in the case of pyrolysis of plastics, mainly hydrocarbons can be identified [ 45 ]. In the study by Marcilla et al. [ 37 ], pyrolysis was carried out with LDPE and HDPE waste, resulting in the production of 1-olefins, n-paraffins, olefins, iso-paraffins, and aromatics.
Understanding the different pyrolysis techniques and available reactors is essential. The choice of the right reactor is linked to economic availability, reaction time, and the desired results. Pyrolytic reactors are divided into two main groups: batch and fluid reactors [ 48 ]. However, some changes in the cycle allow for differentiation between subtypes of pyrolytic processes. Some preliminary concepts that must be understood before addressing the different types of reactors are developed below. To create an oxygen-free environment, the reactor is purged with nitrogen. In some cases, the flow of N is maintained only at the beginning of the process [ 53 ], while in others, the flow is kept throughout the entire process [ 37 ]. The main parameters affecting the yield and final product properties are reaction time, pressure, temperature, particle size, heating rates, and feedstock initial moisture content [ 54 55 ]. During pyrolysis, the feedstock is subjected to high temperatures and fragmented into a series of simpler chemical compounds. These compounds can be divided into different fractions, each of which is made up of a specific set of chemical compounds. Some of the common fractions that can be obtained during pyrolysis include the following [ 56 ]:
Other authors have investigated the influence of roasting on hemicellulose, cellulose, and lignin pyrolysis [ 96 ]. The results indicated that, during torrefaction, O-acetyl and pentose units present in the hemicellulose were thermally degraded into acetic acid and furfural. As a result, acetic acid and furfural acid levels significantly decreased in the pyrolysis of torrefied hemicellulose. However, the impact of torrefaction on cellulose is small, due to the high thermal stability of its crystalline structure. Torrefaction at a temperature of 300 °C has a large impact on lignin pyrolysis. Torrefied lignin leads to an increase in the quantity of aromatic compounds during pyrolysis.
The effects of both roasting and pyrolysis of corn stover have also been studied [ 28 ]. Torrefied biomass had similar calorific values to coal and better than those of raw biomass. Moreover, torrefied oils contained valuable chemical compounds, such as furans and phenols. Although torrefaction reduced the pyrolytic oil yield, the total bio-oil yield obtained through torrefaction and pyrolysis was similar to that obtained through pyrolysis of unprocessed biomass.
As a result, three types of products are generated, namely, solid products, permanent gases (H, CO, CO, and CH), and a condensable mixture containing water, organic compounds, and lipids. The solid product is the main product, representing approximately 70% of the mass and 90% of the energy of the raw biomass. The results indicate that torrefaction with COand Oinstead of Nreduces the number of obtained solids. However, if HO is added, the negative effects of non-inert gases may be reduced [ 95 ].
Dry torrefaction, or soft pyrolysis, is a process in which biomass is heated to a temperature of 200&#;300 °C, in an inert atmosphere, for a period from 30 min to several hours [ 54 ]. In pyrolysis processes, torrefaction is useful to reduce the oxygen content and improve the bio-oil quality. It is carried out as a raw material pre-treatment [ 94 ].
The advantages of fast pyrolysis are the simplicity of the process, operation at atmospheric pressure, low production cost, high thermal efficiency, low fossil fuel inputs leading to COneutrality, and the production of a main liquid product that is easily stored and transported. At the same time, it is a process with very specific requirements, such as a suitable pyrolysis temperature, fast heating rate of biomass particles, fast condensation, and short residence time of volatiles in the reactor. The heating rate of the particles must be high enough for fast pyrolysis, thus increasing the gaseous products [ 19 91 ]. The residence time of volatiles must be limited to minimize their cracking to non-condensable gases or carbon resulting from secondary reactions [ 78 ]. In fast pyrolysis, at high temperatures, secondary cracking reactions of volatile compounds dominate, leading to a reduction in pyrolysis oil yield and, subsequently, increasing the gas yield [ 93 ]. In Table 2 , the results of fast pyrolysis of sawdust reflect an improvement in oil yield from 400 °C to 500 °C. However, from 550 °C onwards, the opposite effect is shown, due to secondary cracking of volatiles [ 68 ].
Slow or conventional pyrolysis is considered to be an efficient conversion technology for energy production, producing liquid and solid char, which facilitates its storage [ 89 ]. The biochar yield from slow pyrolysis is higher than that from fast pyrolysis; that is, slow pyrolysis has a lower degree of ablation of the feedstock [ 90 ]. Slow pyrolysis can work at lower temperatures, between 350 and 450 °C, as well as longer residence times for larger biomass particle sizes [ 89 ]. The low temperatures, together with the slow heating rate, mainly produce charcoal [ 91 ]. In terms of the economic cost of the system, slow pyrolysis reactors are less expensive [ 92 ].
There are two main types of pyrolysis, depending on the operating conditions, namely, fast and slow pyrolysis. Each has its advantages and disadvantages, and they are used for different purposes. In the following sections, the types of pyrolysis, along with their characteristics and applications, are discussed.
Gaurh and Pramanik [ 35 ] carried out an investigation on the pyrolysis and aromatization of PE plastic waste (present in municipal solid waste) using an FA catalyst, which was synthesized in two different arrangements. The results showed that the FA-800 catalyst was very efficient in the aromatization of the pyrolysis product in the reactor, obtaining the highest amount of aromatics/BTEX (22.10%,). Additionally, the study showed that this process can be scalable to treat large amounts of municipal PE waste and convert it into energy. In another investigation on the thermal and catalytic pyrolysis of PE in a nitrogen medium, three catalysts were used [ 64 ]. It was found that the ZSM-5 catalyst showed a high yield in the aromatization of the pyrolysis product in the reactor. In addition, pyrolysis with the multiphase catalyst (liquid and vapor phases) resulted in the highest number of aromatics (35%,). It was also observed that the pyrolysis oil had physicochemical properties that made it suitable as an alternative fuel and source of valuable chemicals, such as benzene, toluene, or xylene. The gas produced during pyrolysis may be used to supply energy in the process industry, while the surplus can be used to generate additional energy. Wang et al. [ 79 ], when applying a Ni-Fe catalyst in the pyrolysis of sawdust, observed that the gaseous fraction increased remarkably, while the liquid and solid fractions decreased significantly.
The HZSM-5 and ZSM-5 zeolite catalysts have been used in numerous studies, as shown in Table 4 . Marcilla et al. [ 37 ] compared them with a hierarchical Y-type zeolite catalyst (HUSY). They highlighted the increase in gas generation when the catalysts were used, especially with HZSM-5. In the case of HUSY, the increases in liquids and coke deposition were more remarkable. Fan et al. [ 39 ], in addition to comparing a continuous microwave pyrolysis reactor with a batch reactor, compared results with or without the presence of the HZSM-5 catalyst. Significant differences in product yields between catalytic and non-catalytic processes were found ( Table 1 Table 2 and Table 4 ). The continuous-stirred microwave pyrolysis (CSMP) process was found to be more effective in condensate generation and less effective in gas production, compared to batch microwave pyrolysis (BMP). Subsequent catalytic upgrading reduced the carbon number distribution and facilitated the formation of aromatics. Compared to BMP, CSMP in the downstream catalytic configuration demonstrated a narrower carbon number distribution, a higher selectivity of hydrocarbons in the gasoline range, and a larger higher heating value (HHV). Persson and Yang [ 61 ] carried out a catalytic batch pyrolysis of demineralized biomass. Using a bench scale at higher temperatures, higher yields of aromatic hydrocarbons were obtained. The findings indicated that secondary reactions of demineralized biomass pyrolysis produced a vapor composition conducive to aromatic hydrocarbon generation in the presence of HZSM-5. Table 4 shows how temperature affected the coke yield for these cases. The yield decreased as the temperature increased, but a much greater increase in coke yield was observed at higher temperatures. Kim et al. [ 48 ] reaffirmed that the use of the HZSM-5 catalyst increased the yield of aromatics. In addition, an increase in light phenols was also observed, improving the quality of the produced oil.
Compared to thermal cracking, catalytic pyrolysis has a higher yield of the gaseous fraction and a lower liquid fraction. This is due to the properties of the ZSM-5 catalyst (based on silica and alumina), which, due to its strong acidity and microporous crystalline structure, exhibits excellent performance in catalytic cracking efficiency, isomerization, and aromatization of larger hydrocarbon molecules [ 35 64 ]. Yildiz et al. [ 24 ] studied the effects of alkali and alkaline earth metals (AAEMs) as intrinsic catalysts for the thermal decomposition of biomass. AAEMs influenced cracking and repolymerization that occurred with biomass devolatilization. AAEMs with 0.5% () ash contents have been shown to be sufficient for the large alteration of pyrolysis products, affecting chemical speciation.
Catalytic pyrolysis of biomass has been studied in depth in recent years, focusing mainly on evaluating the influence of zeolitic catalysts, which have a strong presence in the petrochemical industry, as they promote cracking, deoxygenation, and aromatization reactions [ 61 ]. To remove oxygen from organic compounds and convert them to hydrocarbons, vapor-enhanced pyrolysis using zeolite catalysis is a potentially promising approach. ZSM-5 has been shown to be particularly effective in converting methanol to hydrocarbons in the gasoline range [ 97 ]. Fluid catalytic cracking (FCC) is the most used process in refineries to convert heavy crude oil into gasoline and other hydrocarbons [ 98 ].
The presence of catalysts in pyrolysis processes has been discussed by many authors. The aim of this section is to find out how catalysts may influence each fraction&#;s yield. In addition, their influence is compared and assessed in comparison to non-catalytic processes.
The higher heating value is one of the main quality guarantees of a pyrolytic fuel, as it determines its energy value [ 32 ]. However, the presence of products that interfere with combustion must be considered. Higher carbon and hydrogen contents translate into higher combustion energy and, as demonstrated in Table 7 , this combustion capacity is intrinsically related to the HHV. In cases where cellulose is present, the presence of hydrogen is common, as shown in the research of Fekhar et al. [ 45 ]. When the aim of pyrolysis is to obtain gaseous products to be used as fuels, the presence of hydrogen is beneficial, as it is a flammable gas with a high HHV. However, if the objective is to obtain liquid or solid products, the presence of hydrogen may decrease the quality and properties of these products. Results from Shadangi and Singh [ 62 ] indicate that pyrolytic polanga oil possesses suitable liquid fuel properties. The presence of a higher amount of oxygen in bio-oils produced by pyrolysis indicates the presence of oxygenated chemical compounds, as shown by Nam et al. [ 17 ]. A high oxygen content in the biofuel can have several drawbacks, such as low calorific value, or instability and immiscibility with other hydrocarbons.
The influence of temperature on the resulting compounds is also shown in Table 5 . Based on the investigations of Haydary et al. [ 73 ], the Hand CO contents in the gas increased with temperature, while the HCx content decreased. The use of catalysts has a significant impact on the liquid fraction (tar) content and the composition of the pyrolytic gases, especially for temperatures above 750 °C. The increase in phenols with temperature can be a sign of good-quality oil, and this is more common in continuous reactors. Kim et al. [ 48 ] found that the contents of oxygenates, acids, and N-compounds in bio-oil decreased as the temperature increased. In contrast, the phenol content increased, which was associated with a higher quality of the produced oil. In addition, the phenolic contents in the continuous reactor were found to be higher than those in the batch reactor, suggesting that the former is more effective in breaking down lignin. As indicated by Lopez-Urionabarrenechea et al. [ 57 ], gases produced during pyrolysis can be used to supply the energy demand of the process, and the surplus can be used to produce additional energy. In other words, pyrolysis, especially of plastic waste, is an energetically sustainable process. Jung et al. [ 78 ], in the pyrolysis of rice straw and bamboo, also assessed the possibility of using waste charcoal as an energy source, thus making the process more sustainable as a waste treatment method. In the same research, some bio-oil compounds were highlighted, namely, phenolics, ketones, and aldehydes. Phenolics and aldehydes can be used in the manufacture of resins, while ketones can be used as solvents. Considering bio-oil from the pyrolysis of furniture sawdust residues, Heo et al. [ 33 ] also identified the presence of phenols.
The composition of the individual fractions depends, among other things, on the type of pyrolysis and the pyrolyzed feedstock. The knowledge of the composition of the elements resulting from pyrolysis is crucial for finding possible applications. Nevertheless, it is difficult to obtain an overview of the individual compounds and sub-compounds, due to their wide variety. Considering the research mentioned above, Table 5 Table 6 and Table 7 reflect the chemical characteristics of the different pyrolyzed raw materials.
Once the feedstock has been analyzed, the sample must be processed. In other words, the material to be pyrolyzed must be crushed, cut, dried, or any other pre-treatment required to provide the best results. Next, the type of pyrolytic reactor must be selected beforehand. Then, there would be two possible routes, pyrolysis with or without a catalyst. Regardless of the process followed, the next step would be to analyze the obtained fractions. The analysis of the compounds derived from pyrolysis is mostly carried out by gas chromatography&#;mass spectrometry (GC-MS), as shown in Table 5 Table 6 and Table 7 . As an example of the use of GC-MS, Table 7 includes the study of polanga pyrolysis by Shadangi and Singh, which, in addition to making use of TGA, also performed an analysis of the chemical composition with GC-MS [ 62 ]. Another example of the use of GC-MS can be seen in the research of Jung et al. [ 78 ], where two different materials (rice straw and bamboo) were characterized, thus being very useful to differentiate the chemical properties resulting from pyrolysis under similar conditions from different materials.
This manuscript provides an overview of pyrolysis techniques and reactors, highlighting the importance of choosing the right reactor for achieving the desired results. Residues used in pyrolysis include agricultural and forestry residues, old furniture, plastic waste, industrial (paper, wood) residues, and waste tires. Pre-treatment effectiveness significantly impacts the pyrolysis efficiency and yield, highlighting the need for adapted strategies for each type of feedstock. Agricultural residues require specific pre-treatments, such as air-drying and grinding, to improve yields. Moreover, pre-treatment of paper and wood, which have high moisture contents, is critical. Plastic waste, due to its carbon content, is suitable for conversion to high-calorific-value fuels. Waste tires, rich in hydrocarbons, are preferred for gas and oil production.
The two main types of pyrolytic reactors are batch and continuous ones, with variations within each category. Parameters such as reaction time, pressure, temperature, particle size, heating rates, and the initial moisture content of the feedstock play a crucial role in determining the yield and properties of the final products. During the pyrolysis process, the feedstock is fragmented into a series of simpler chemical compounds that can be divided into different fractions, namely, gas, liquid, and solid. The composition of these fractions depends on the type of material being pyrolyzed, the process conditions, and the type of reactor. In the case of biomass pyrolysis, the resulting gaseous fraction typically includes CO2, CO, and hydrocarbons, but it may also include other compounds, e.g., acetic acid, methanol, and furfural. Hydrogen formation is characteristic of biomass containing paper and cardboard, while in the case of plastics, hydrocarbons are mainly identified. The liquid fraction resulting from biomass pyrolysis consists of multiple organic compounds, including aliphatic alcohols, carbonyls, acids, phenols, cresols, benzenediols, guaiacol and its alkylated derivatives, and aromatic hydrocarbons. In contrast, the liquid fraction resulting from polymer pyrolysis is dominated by aromatic hydrocarbons. The solid fraction mostly consists of charcoal and other residues, e.g., ash, with the composition depending on the process conditions and the organic material being used.
The type of pyrolysis significantly impacts the efficiency and generated products. Slow pyrolysis is effective for cost-efficient biochar production. Fast pyrolysis excels in converting organic materials into liquid fuels, offering simplicity, high thermal efficiency, and low production costs. However, minimizing secondary reactions that reduce oil yields is necessary. Additionally, torrefaction provides a beneficial pre-treatment, improving the bio-oil quality. It also generates high-energy solid products and valuable chemicals, despite potentially reducing the overall oil yield. The choice of pyrolysis type should align with the production goals and operational conditions, maximizing efficiency and product quality.
Previous studies reveal that increasing the reaction temperature improves the biomass conversion efficiency. The choice of reactor type, feed size, and speed are also crucial factors affecting the yield and quality of the final product. Studies also show that different waste materials require different reactors for optimal conversion to bio-oil or biochar. The fluidized bed reactor has been found to be an advantageous option for bio-oil production, while batch and auger reactors are suggested for biochar production. The pyrolysis of automotive waste and used tires is also a suitable process for the recovery of energy and valuable materials. The efficiency of the pyrolysis process is crucial in determining the process&#;s commercial viability.
The use of catalysts has been shown to significantly increase the formation of aromatics, which have a high combustion capacity and are, therefore, of interest as fuel-quality-enhancing additives. However, the toxicity of some of these compounds must be considered. The influence of temperature on the resulting compounds exhibits different behavior, either increasing or decreasing. Previous research highlights the importance of a fuel&#;s calorific value, which is determined by its carbon and hydrogen contents. The presence of hydrogen can be beneficial for gaseous products, but it may decrease the quality of liquid or solid products.
Catalysts present significant potential for improving waste pyrolysis, opening up future lines of research. The development of new, more efficient and specific catalysts may help in optimizing the conversion and selectivity of the desired products, e.g., high-quality bio-oil. In addition, exploring multifunctional, sustainable, and cost-effective catalysts, as well as studying their stability and durability, is crucial. The integration of catalytic processes may maximize energy efficiency, thus reducing operating costs. This research could lead to significant advances, making waste pyrolysis efficient and economically viable.
Also, it is worth noting that pyrolysis may be an energy-sustainable process when used to supply energy in the process industry. Overall, research demonstrates the potential of pyrolysis as a waste treatment method that can generate valuable products, including energy (bio-oil, biochar) and material recovery. Further research is needed to optimize the process parameters for different waste materials and reactor types. Further kinetics studies should also be implemented.
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