Review Article | Open Access
Julius I. Osayi, Sunny Iyuke, Samuel E. Ogbeide, "Biocrude Production through Pyrolysis of Used Tyres", Journal of Catalysts, vol. 2014, Article ID 386371, 9 pages, 2014. https://doi.org/10.1155/2014/386371
Biocrude Production through Pyrolysis of Used Tyres
A review of the pyrolysis process of used tyre as a method of producing an alternative energy source is presented in this paper. The study reports the characteristics of used tyre materials and methods of recycling, types and principles of pyrolysis, the pyrolysis products and their composition, effects of process parameters, and kinetic models applied to pyrolysis. From publications, the proximate analysis of tyre rubber shows that it is composed of about 28.6 wt.% fixed carbon, 62 wt.% volatile material, 8.5 wt.% ash, and 0.9 wt.% moisture. Elemental analysis reveals that tyre rubber has an estimated value of 82 wt.% of C, 8 wt.% of H, 0.4 wt.% of N, 1.3 wt.% of S, 2.4 wt.% of O, and 5.9 wt.% of ash. Thermogravimetry analysis confirms that the pyrolysis of used tyre at atmospheric pressure commences at 250°C and completes at 550°C. The three primary products obtained from used tyre pyrolysis are solid residue (around 36 wt.%), liquid fraction or biocrude (around 55 wt.%), and gas fraction (around 9 wt.%). Although there is variation in the value of kinetic parameters obtained by different authors from the kinetic modeling of used tyre, the process is generally accepted as a first order reaction based on Arrhenius theory.
One of the main challenges of modern society is the rising rate of solid waste generated by man’s activities which has poised a major environmental concern [1–3]. The disposal of used tyres and other polyisoprene based products is a large fraction of such problems as 1.3 billion tyres are estimated to reach their end of life cycle annually worldwide [4–9]. This is because of their excellent properties which have made them useful in all areas of human life . However, these excellent properties also place them at a disadvantage [3, 11, 12] as they are not biologically degradable leading to problems with their disposal [2, 7, 11, 13–16]. Over the years landfill and open dumping (stock piling) were the common ways in handling the problem of used tyres. However, landfills take up valuable land space due to the bulky nature of tyres which cannot be compacted neither does it degrade easily [7, 11, 17–19]. Dumped used tyres in massive stockpiles do not only occupy a large land space but also serve as a potential health and environmental hazard due to the possibility of a fire outbreak with high emissions of toxic gases and as a breeding ground for disease carrying vectors [7, 15, 16, 20, 21]. Despite other tyre recycling options such as reclaiming, grinding, incineration, retreading, and so forth being used, these processes have their setback and do not fully utilize used tyres as an excellent material for energy recovery [8, 22]. For this reason, pyrolysis has received renewed attention as the process conditions can be optimized to produce high valued products [4–6, 11, 23–27]. Pyrolysis mainly involve the decomposition of polyisoprene materials to low molecular weight component at high temperature (250°C–900°C) in an inert atmosphere [3, 7, 22, 28]. The process being an excellent energy recovery route can be performed under atmospheric or reduced pressure. It is an environmentally friendly technique to thermally decompose a wide variety of wastes, including used tyres [2, 4, 6, 11, 15, 16, 20, 27, 29–44]. The three basic products of pyrolysis are solid residue (char), liquid (biocrude), and gases . Solid residue may be used in the manufacture of activated carbon, reinforcement in rubber industry, or as solid fuel. The pyrolytic liquid can serve as replacements for conventional liquid fuel due to its high gross calorific value of about 41–44 MJ/kg. It can also be used as petroleum refinery feedstock or a source of chemicals for wide industrial applications. Gaseous fraction can be used as fuel or as a source of energy for the pyrolysis process [22, 28, 29, 45–49].
This paper presents a literature review of the pyrolysis process with a focus on the characteristics of used tyres materials and methods of recycling, types and principles of pyrolysis, properties of the pyrolysis products, effects of process parameters, and kinetic models applied to pyrolysis.
2. Used Tyres
Used tyres also referred to as waste tyres can be defined as tyres that have expired as a result of exceeding their production life span or are no longer safe for usage due to defects, such as degradation of its physical composition/structure from use and cannot be retreaded. It is one of the most challenging hazardous solid wastes facing modern society, particularly in developing countries [3, 10, 11]. It is estimated that the EU, Japan, and USA generate 6 × 106 tons of waste tyres annually [6, 22, 23, 46, 64]. According to a report, 160,000 tons of waste tyres are generated in South Africa annually and up to 28 million used tyres are dumped unlawfully or burnt . This figure is projected to increase by 9.3 million yearly . The nonbiodegradable nature of this polymer material makes its disposal difficult. Disposal by massive stockpiling and landfilling is one of the common ways of handling waste tyre, but this requires a large space as the volume of tyres cannot be compacted. These also poise the danger of the possibility of fire outbreak with the emission of harmful gases [1, 7, 19, 21, 22, 64, 71–75].
2.1. Methods of Recycling of Used Tyres
The various methods in which used tyres are recycled are discussed below.
2.1.1. Retreading Method
Retread also referred to as “remold” involves buffing away of the remaining tread in a spent tyre and replacing it with a new tread rubber strip by vulcanization. It is carried out only on used tyres casing that have been inspected and repaired. Although it is economically advantageous, product quality confidence is a major challenge of the process.
2.1.2. Mechanical or Cryogenic Method
Both mechanical and cryogenic (grinding of waste tyres at temperatures of −80°C to –100°C using liquid nitrogen) recycling of used tyres involve the milling of tyres to produce ground rubber of different particle sizes. The fine grounded rubber can be used in varieties of application such as an additive in road asphalting, sports and children playground surfacing, carpets, and other rubber products. However, the limited market for the product and a high cost of running the process are a disadvantage.
2.1.3. Reclaiming Rubber Raw Materials Methods
Different methods and processes for reclaiming rubber have been developed. The most relevant of them are thermomechanical reclaiming , microwave reclaiming , mechanical shearing process , reclaiming by biotechnological process, ultrasonic reclaiming , reclaiming by renewable resource materials, reclaiming by use of different chemical agents , and pyrolysis of waste tyre . These methods actually help in transforming used tyres from a three dimensionally interlinked thermoset polymer state to a two-dimensional polymer exhibiting the properties of a virgin rubber. The high cost of the process, quality of products, and lack of acceptance of reclaim rubber by industries as a raw material are a setback on the process.
2.2. Characteristics and Composition of Used Tyres
Tyres are made up of different types of rubber elastomers (natural or synthetic rubber) with varying composition, carbon black, stabilizers, antioxidants, sulphur, hydrocarbon oils, zinc oxides, textile or steel cords [4, 6, 11, 69], and so forth. Proximate and ultimate analyses of used tyres reported by various authors are shown in Tables 1 and 2. The difference in values can be attributed to the manufacturers’ formulation which is closely guarded and determines to a large extent the weight percent of the various components of tyres.
2.3. Study of Thermal Degradation of Used Tyre
Thermogravimetry analysis is used to study the general behavior of thermal degradation of used tyres . Both the thermogravimetry (TG) and differential thermogravimetry (typical curve is shown in Figures 1 and 2) are standard techniques used in the study of mass change of tyre rubber sample as a function of temperature and time [73, 74, 78]. With the results data (shown in Table 3) obtained in this study and those of other authors in the literature, it can be understood that there is more than one degradation temperature zone during tyre rubber pyrolysis. This also gives an idea of the commencement and end point temperature for the different rubber components in the waste tyre [3, 10]. For example, NR (natural rubber), SBR (styrenebutadiene rubber), and BR (butadiene rubber) decompose at a temperature range of 375°C to 500°C.
3. Pyrolysis, Process Conditions, and Reactors
Pyrolysis mainly involves the thermal degradation of tyre rubber at high temperatures (250–900°C) in an oxygen absent environment. It can be performed under vacuum or atmospheric pressure [23, 29]. Amongst other methods such as combustion and gasification used to extract energy from biomass, pyrolysis has received more attention in the area of research because the process conditions can be optimized to produce high energy density liquids, char, and gas. Also, the condensable fraction (biocrude) can be stored and easily transported to where it can be most proficiently utilized. Tyre pyrolysis can be said to be made up of three stages, that is, the release of volatile and moisture at lower temperature succeeded by the thermal decomposition of natural rubber (NR) and the decomposition of polybutadiene (BR) and polybutadiene-styrene rubber at higher temperature, respectively [79, 80].
The process conditions that can influence the percentage weight of each fraction of pyrolysis include pressure, heating rate, temperature, feed particle sizes, catalysis, flow rate of inert carrier gas, configuration of reactor used , and so forth.
3.1. Pyrolysis Reactors
The reactor is a very important part of the pyrolysis process. Over the years, the innovation and technological advancement of pyrolysis has advanced considerably. Researchers have developed and studied different reactors and processes to the point where pyrolysis is now an acceptable technique to the deriving of gaseous, liquid, solid fuels, and chemicals. Table 4 presents the chronology of pyrolysis process from the 1970s to date. Reactors can greatly influence the pyrolysis performance, product quality, and yield [2, 46]. However, each reactors type has its benefits and limitations. Table 5 shows different types of reactors and their heating methods.
|n.r.: not reported.|
3.2. Types of Pyrolysis
Pyrolysis process can be performed under different operating conditions which can be used in classifying it. They are differentiated by residence time of the pyrolysed material in the reactor, process temperature, feed particle size, heating rate [81–83], and so forth. These include the following.
3.2.1. Slow Pyrolysis
The solid residence time(s) in the reactor is 450–550, heating rate (°C) is 0.1–1, and feed particle size (mm) is 5–50 with temperature (°C) of 550–950. This process enhances char production and is unlikely to be unsuitable for high quality bio-oil production. Also, due to high residence time secondary reaction is favorable as cracking of primary product occurs which could adversely affect bio-oil yield and quality  (Table 6).
3.2.2. Fast Pyrolysis
Fast pyrolysis involves the rapid heating of the feed material to a high temperature in the absence of oxygen with a short residence time of the condensable vapor in the reactor. Its operating parameters are solid residence time between 0.5 and 10s, heating rate of 10–200°C, feed particle size less than 5 mm, and reaction temperature of 550–1200°C. The technology has received much popularity in producing liquid fuels and a range of specialty and commodity chemicals. Typically on weight basis, fast pyrolysis yields 60%–75% pyro oil with 15%–25% compared to other processes; it has reasonably low investment costs and high energy efficiencies particularly on a small scale .
3.2.3. Flash Pyrolysis
This process is characterized by residence time of less than 0.5 s, high heating rate of more than 200°C, particle size of less than 0.2 mm, and high reaction temperature of more than 1000°C. However, the major technological challenge of the process is poor thermal stability, solids in the oil, and production of pyrolytic water [86, 87].
3.2.4. Catalytic Pyrolysis
Catalytic pyrolysis is a pyrolysis process that includes the use of a catalyst. The catalyst helps enhance the pyrolysis reaction kinetics by cracking down higher molecular weight hydrocarbon compounds to lighter hydrocarbon products. It has been reported that the use of catalyst in tyre pyrolysis systems can greatly influence the composition, quality, and yield of products [6, 88]. Examples of catalysts used in tyre pyrolysis include Na2CO3, NaOH, MgO, CaCO3, aluminium-based catalyst, perlite, CaC2, Cu(NO3)2, zeolite-based catalyst, and so forth [6, 7, 89]. Operating conditions can determine different product distribution for different catalysts. Pyrolysis catalyst can be grouped based on their method of application. The first group is when the catalyst is added to the feedstock before being fed into the reactor. The second group is when catalyst is added after the feed is already heated up in the reactor allowing it to have immediate contact with vapors, solid, and char. The third group is when the catalyst is placed in another reactor located downstream from the pyrolysis reactor [46, 90].
3.3. Chemical Reaction of the Catalytic Pyrolysis
The chemical reaction of catalytic pyrolysis is an endothermic process. Three stages involved include dehydration, fragmentation, and product formation which occur at different temperatures during the process. It is carried out in an inert environment in order to avoid combustion. The mechanism of the reaction is shown below: where = heat.
3.4. Pyrolysis Products and Their Composition
The three primary products obtainable from tyre rubber pyrolysis are listed and discussed below.
Solid residue or char (34–38 wt.% of feedstock) is made up of carbon black and other nonvolatile materials such as tyre rubber additives like zinc, sulphur, silica, clays, and so forth, initially present in the tyre. This char can be used as solid fuel, carbon black, or as precursor for activated carbon manufacturing . From the literature, elemental analysis shows that the solid residue contains 71 wt.% of C, 13.3 wt.% of O, 5.4 wt.% of Fe, 2.8 wt.% of S, 2.3 wt.% of Zn, 1.3 wt.% of Ca, and 0.3 wt.% of Al.
Liquid product (48–58 wt.% of feedstock) also known as pyrolysed tyre oil or biocrude is the most significant product of the process. It is gotten from the condensation of vapor of a pyrolysis reaction. Several publications on the study of the properties of the pyrolytic oil abound [18, 45]. It is reported that the calorific value of the oil is 42 MJ/kg, percentage content of sulphur ranges between 1.2 and 1.36, and the oil is a mixture of C6–C24 organic compounds (53.4–74.8%) specified to be paraffins, olefins, and aromatic compounds with some nitrogenated (2.47–3.75%) and some oxygenated compounds (2.29–4.85%) [17, 18, 28]. Such pyrolytic oil can be used directly as low-sulphur-emission fuels or blend with petroleum products such as gasoline, feedstock for petroleum refinery, and as an important source for chemicals in chemical industries due to high concentration of benzene, toluene, xylene, and limonene . Elemental analysis of the tyre pyrolysis oil shows that it contains 86.871 wt.% of C, 10.07 wt.% of H, 1.169 wt.% of N, 0.906 wt.% of S, and 1.169 wt.% of O.
Gas fraction (9–14 wt.% of feedstock) from the pyrolysis process is mainly composed of H2, CO, CH4, CO2, C2H4, H2S, and other light hydrocarbons with SO2, NOx, CO, and PAHs reported as the main pollutant gases. It has a high heating value of about 37 MJ/m3, an amount of energy enough for the pyrolysis process.
4. Pyrolysis Kinetics
Kinetic models for used tyre pyrolysis have been presented by various authors [91–97]. Generally, the degradation of the tyre rubber is split into two or more steps, while thermogravimetry data and Arrhenius equation form the basis of the models. In the kinetic analysis of the data for the tyre decomposition, the Arrhenius theory based on first order reaction is mostly assumed by researchers: where is the rate constant, is the preexponential factor (min−1), is the activation energy (J/mol), and and are the universal gas constant (8.314 Jmol−1 k−1) and temperature of reaction (K), respectively. Equation (3) can be used to represent the rate of tyre decomposition: where is the fraction of reactant decomposed at time and is the order of reaction. Application of the above given equations for tyre pyrolysis under different conditions along with experimental conditions and obtained kinetic parameters published in the literature by some authors is given below.
Leung and Wang . Consider the following.
Model: where (min−1) is normalized mass loss rate, is preexponential factor, is activation energy, is ideal gas constant, is absolute temperature, is time, and index is reaction.
Experimental Conditions. Temperature: 20–600°C, particle size: 0.355–0.425 mm, heating rate: 10–60°C/min, and used mobile gas: N2. The kinetic parameters obtained by Leung and Wang are given in Table 7.
Williams and Besler . Consider the following.
Model: where is the weight of sample at time , is the weight of residue at the end of the reaction, and represents the rate constant defined by Arrhenius equation.
Experimental Conditions. Temperature: 720°C, sample sizes: <1 mm, heating rate: 5°C/min, and used mobile gas: N2.
Table 8 shows the kinetic parameters obtained.
Biocrude production from used tyres and natural rubber is a viable means of an alternative renewable energy source that can help caution the fast depletion of crude oil reserves, its fast rising cost due to high demand, along with its adverse negative environmental impact. This review reports the present stage of research in used tyre pyrolysis. From the literature, the proximate analyses of tyre rubber show that it is composed of about 28.6 wt.% fixed carbon, 62 wt.% volatile material, 8.5 wt.% ash, and 0.9 wt.% moisture. Elemental analyses reveal that tyre rubber has an estimated value of 82 wt.% of C, 8 wt.% of H, 0.4 wt.% of N, 1.3 wt.% of S, 2.4 wt.% of O, and 5.9 wt.% of ash. Both the proximate and ultimate analysis value of the used tyres is dependent on the tyre formulation by the tyre manufacturers. Thermogravimetry analysis confirms that the pyrolysis of used tyre at atmospheric pressure commences at about 250°C and completes at around a temperature of 550°C. Usually, during tyre pyrolysis, there exist different degradation temperatures. This was also confirmed by the thermogravimetric analysis performed in our laboratory. It was also observed that pyrolysis products yields and their characteristics are greatly affected by the composition of the feed, operating conditions, and the specific properties of the system used.
Thus, results vary from different researchers making it difficult to compare results. Nevertheless, the three primary products obtained from used tyres pyrolysis are solid residue or char (around 36 wt.%), liquid fraction or biocrude (around 55 wt.%), and gas fraction (around 9 wt.%) with average higher heating values (HHV) of 28 MJ kg−1, 42 MJ kg−1, and 36 MJ N−1 m−3, respectively.
Studies have also shown that the main components of the liquid fractions are aliphatic and aromatic hydrocarbons and hydroxyl compounds while the gases that make up the gas fractions are H2, CO, CH4, CO2, C2H4, H2S, and other light hydrocarbons with SO2, NOx, CO, and polycyclic aromatic hydrocarbons (PAHs) reported as the main pollutant gases. The char contains high fixed-carbon content and inorganic matter.
Although there is a wide difference in results of kinetic modeling of used tyres by different authors in the value of kinetic parameters obtained, generally it is accepted that the process is a first order reaction based on Arrhenius theory.
Therefore, there is the need to investigate the effect of copyrolysis of natural rubber (biomass obtained from rubber tree) and used tyre on the pyrolysis product yields in relation to the following.(i)Composition and quantity of gas yield.(ii)Enhancing the possibility of more valuable chemical products from the liquid fraction such as limonene which is closely related to polyisoprene (NR) present in used tyre.(iii)Analyzing the amount of sulphur content in the biocrude (liquid fraction).(iv)The quality of char produced in comparison with commercial carbon black will be the area of our research.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Financial support from the National Research Foundation (NRF) under the South Africa NRF Focus Area and student bursaries made available by the University of the Witwatersrand are deeply appreciated.
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