Use of pyrolytic gas from waste tire as a fuel: A review

Dina Czajczyńska1,2, Renata Krzyżyńska1*, Hussam Jouhara2, Nik Spencer3

1. Faculty of Environmental Engineering, Wroclaw University of Science and Technology, Wyb. Wyspiańskiego 27, 50-370 Wro-claw, Poland

2. Institute of Energy Futures, College of Engineering, Design and Physical Sciences, Brunel University London, Uxbridge, Middlesex UB8 3PH, London, United Kingdom

3. Manik Ventures Ltd & Mission Resources Limited, Offenham Road, Worcestershire Evesham WR11 8DX, UK

ABSTRACT

Scrap tires are a burdensome and common kind of waste. After use they become a non-functional waste product, disposal of which is also difficult. Almost 1.5 billion tires are produced each year and each tire produced will eventually join the waste stream. According to European Union regulations, the disposal of waste tires is prohibited; as an alternative they should be recovered and recycled. Pyrolysis allows the dissolution of the waste and it also produces useful by-products. In this process gas, liquid and solid phases are formed. Pyrolytic gases have a high heating value, about 30 - 40 MJ/Nm3. The energy obtained from combustion of the pyrolytic gas is enough not only to perform the pyrolysis process but it can also be utilized for other applications. However, there is a big challenge: the concentration of SO2 in the flue gases is greater than regulatory limits. Similar situations could also arise with HCl, NO X and heavy metals. In order to meet regulatory requirements and maintain optimum pyrolysis, gas cleaning methods will be needed in order to remove those substances from the exhaust gases formed during waste tire pyrolysis. The main aim of this article is to review the properties of pyrolysis gas for energy recovery because it is a good gaseous fuel. In addition, possible implications will be identified.

Keywords: pyrolytic gas, emissions, gas cleaning, tire, waste tire pyrolysis, energy recovery

1. IntroductionNowadays energy supply and environmental pollution are important international issues. The vast quantity of used and

waste tires represent a particular problem. Each year about 1.5 billion tires are produced around the world, which corres -pond to an estimated 17 million tonnes of used tires. In 2013, the used tires in European Union countries were estimated at 3.6 million tonnes [1]. In the U.S. about 4 million tonnes was generated in 2015 [2]. China, the European Union countries, the USA, Japan and India produce the largest amounts of waste tires – together almost 88 % of the total [3]. Most of them are recycled or recovered [1]. Other methods of waste tire treatments, like gasification or pyrolysis, are still underutilised but looking at stricter EU environmental regulations, the energy crisis, fuel depletion and an increasing number of invest -ments in pyrolysis plants, they seem to be the future for waste tire treatment. Unfortunately, in some countries, waste tires are still being dumped. Disposal of waste tires is very problematic and dangerous for the environment and human health.

Tires properties, like resistance to mechanical damage, long life and safety, regardless of weather conditions, make disposal very difficult. The rubber is abrasion and water resistant. It is also resistant to heat, electricity, many chemicals and bacteria. Microorganisms need more than 100 years to destroy tires [4]. Moreover, spent tires are bulky waste. Waste tires dumps are also a high threat to the environment and human health because of the risk of fire and because they are habitats for mosquitos and rodents, which are strongly associated with many diseases.

Uncontrolled burning of tires generates smoke, oil and other toxic substances that pollute the atmosphere, soil, surface water and groundwater [5–7]. Apart from the problems with waste tires mentioned above, the tires also provide large oppor-tunities for resource conservation, because they are a source of great potential for valuable materials and fuels [8].

Increases in the quality of life and the general development in countries are leading to a growth in the number of car users. According to the statistics, the number of cars is increasing continuously. In Europe it could reach 347 million by 2025 (compared with 322 million in 2014). In China and India, the numbers are respectively 332 and 69 million in 2025 (compared with 107 and 28 million in 2014). In the latter cases, this represents more than a two-fold increase [9]. Addition-ally, the number of trucks also is growing quickly, because this kind of vehicle delivers materials and products, which people around the world – especially in developing countries - need more and more every year. It should be noticed, that truck tires have quite different qualities, because they must be more durable than the tires of cars and so their disposal could even be more difficult.

1 Corresponding author. Tel.: +48 71 320 43 86E-mail address: renata.krzyzynska@pwr.edu.pl (R. Krzyżyńska), Postal address: Wybrzeze Wyspianskiego 27, C6, r. 316, 50-370 Wroclaw, Poland

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The management of waste tires should follow the generally accepted hierarchy: prevention, minimization of waste, reuse, recycling, energy recovery and - eventually - landfilling. Applying the above-mentioned options could help to reduce the negative impact of waste on the environment. The legal prohibition of tire stockpiling in landfills in European Union counties was introduced by a Waste Landfill Directive in 1999[10]. Effects are impressive. In 1996, about 50% of waste tires were stored at landfill sites, and in 2010 only 4% [11]. Disposal of tires in landfill sites is also prohibited in 11 states in the U.S. Some states have emphasized the use of waste tires as a fuel supplement, while others, such as Arizona, place a very heavy emphasis on recycling tires through use as rubberized asphalt [12]. In 1990, about a billion scrap tires were in stockpiles in the U.S. By 2015, over 93% of those tires have been cleaned up [2]. Sienkiewicz et al. reviewed the methods of waste tires utilization in the European Union. They focused on legislation and short described and compared available methods such as retreading, energy recovery, pyrolysis, and product and material recycling [7]. Pyrolysis is considered one of the most promising methods for waste tire recycling and/or energy recovery and was considered several times. Martinez et al. widely described the waste tires pyrolysis parameters and their influence on product yields and composition. They also studied in detail the pyrolysis oils composition and properties. However, less attention was paid to the char and pyrolytic gas [13]. On the other hand, Williams more accurately described the types of reactors used in pyrolysis of waste tires. He also did not delve into the topic of pyrogas [14]. It is worth noticing, that this process, carried out in appropriate conditions, gets rid of problematic waste and obtains: 1) valuable liquid chemicals, 2) good quality char (raw material for activated carbon production), 3) gaseous fuels, which provide enough energy to run the process and additionally produces electricity. However, there are some important challenges faced by scientists and engineers: 1) the high sulphur content in pyrolysis products, 2) economic viability and 3) emissions standards, which must be fulfilled. The most important aim of this article is to accurately describe the properties of pyrolytic gas with a particular consideration for its use in energy recovery, since it is a gaseous fuel with good properties. However, possible problems related to this will also be addressed - especially in the context of air emissions.

2. Characteristics of tires

2.1. Composition A typical tire consists principally of three kinds of materials: rubber mixtures, metal and textiles. Each material has

specific properties, which used in the right combination provide the tire with the required strength and flexibility. Figure 1. compares the contents of typical materials used in car tires and truck tires manufactured in Europe.

natural rubber

22%

synthetic rubber 23%

fabric, fillers, accelators, antio-

zonants, etc.14%

steel13%

carbon black28%

Car tirenatural rubber30%

synthetic rubber 15%fabric, fillers, accela-

tors, antiozonants etc. 10%

steel25%

carbon black 20%

Truck tire

FIGURE 1. Composition of tires [7]

The rubber mixtures represent the main component of vehicle tires. They consist of natural and/or synthetic rubber, carbon black, amorphous silica, vulcanizing agents and a lot of additives. It can be said that more than one hundred com -pounds can be added to the tire depending on the specific use to be given to the tire [13]. Natural rubber is obtained from the

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sap of the Hevea brasiliensis tree and despite the invention of synthetic rubber, natural rubber is characterized by unique properties and it still is the most important element in the production of tires. Synthetic rubber is derived from petroleum-based products. The most widely used kinds of synthetic rubber are butyl rubber and styrene-butadiene rubber. Besides the rubber, important ingredients of the rubber mixtures are carbon black and amorphous silica, that make the tire durable and resistant to wear and tear [15]. Vulcanisation is a chemical process for converting natural rubber or related polymers into more durable materials via the addition of sulphur (including compounds). These additives modify the polymer by forming cross-links (bridges) between individual polymer chains. Vulcanised materials are less sticky and have better mechanical properties. Therefore, another two important substances appearing in the composition of rubber mixtures are sulphur (also its compounds) and zinc oxide, which are commonly used as vulcanising activators.

The second type of material used for the production of tires is metal. Usually it is high quality steel wire. The coating materials and activators includes brass, tin and zinc. The purpose of the metal is to provide rigidity and strength in tires [15].

The last type of material used in the pneumatic tire is textiles. Reinforcing fabrics are used to lend structural strength to the carcasses of car tires [15]. They could be synthetic or natural. The most widely used are polyester, rayon or nylon.

2.2. Structure Each tire is a product with a complex structure too. The most common types of tire structure are diagonal (cross-ply),

bias-belted and radial, but almost 80 % of all tires sold are radial [14]. A typical tire consists of elements shown in Figure2.They are briefly described below. Tread means the portion of a pneumatic tire designed to come into contact with the ground and hence it must be abrasion and traction resistant [16]. The space between the adjacent ribs or blocks in the tread pattern is called tread groove. Sidewall means the part of a pneumatic tire between the tread and the area designed to be covered by the rim flange. It is made of a mixture of natural and synthetic rubber with small amounts of carbon black and additives. Ply is a layer of rubber-coated parallel cords and its purpose is to stabilize the tire and cord means the strands forming the fabric of the plies in a pneumatic tire. The structural part of a pneumatic tire, which when inflated supports the load, is named carcass. Section width means the linear distance between the outside of the sidewalls of an inflated pneu-matic tire. Belt refers to a layer or layers of materials underneath the tread, laid substantially in the direction of the centre line of the tread to restrict the carcass in a circumferential direction. Bead means the part of a pneumatic tire that is shaped and structured so as to fit the rim and hold the tire onto it and chafer protects the carcass against chafing or abrasion by the wheel rim [15].

FIGURE 2. Components of a tire: 1. tread, 2. tread groove, 3. sidewall, 4.&5. ply, 6. cord, 7. carcass, 8. section width, 9. belt, 10. bead, 11. chafer [15]

2.3. Energy potential of tires

According to EU definition waste means any substance (object) which the holder discards or intends or is required to discard [17]. Therefore the decision about whether something is or is not a waste is made by the holder. Many product with high potential to material or energy recovery must be considered as a waste, because they are no longer useful for theirs

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holder. Municipal solid waste (MSW) is heterogeneous mixture with variable composition and the easiest way to utilize them is landfilling. Tires must not be treated that way. It is worth to consider the elemental and proximate analysis, and heating value of them in comparison with other solid fuels. This analysis is showed in Table 1. It can be observed, that tyres has high carbon content – usually more than 80 wt. %. The volatile matters, the fixed carbon and the ash content is about 65 wt. %, 30 wt. % and 5 wt. %. Additionally, the heating value of tires is about 30-40 MJ/kg and it is higher than other solid fuels. Moreover, waste tires has a very low moisture content comparing with alternative energy sources such as bio -mass or MSW. Tires composition of the tires is also stable and does not show significant differences. Taking into account those facts, used tires should be considered as a high valuable energy source rather than waste.

TABLE 1. Tires and other solid fuels propertiesMaterial Elemental analysis on dry basis, wt. % Proximate analysis on

as received basis, wt. %Heating value, MJ/kg

Ref.

C H N S O A A VM FC MTires 81.79 7.99 0.48 1.81 3.04 4.90 4.88 65.74 28.98 0.40 38.3 [18]Tires 86.70 8.10 0.40 1.40 1.30 2.10 8.00 61.90 29.50 0.70 36.2 [19]Tires 82.80 7.60 0.50 1.30 4.50 3.30 3.30 68.70 27.20 0.80 36.5 [20]Tires (car) 83.92 6.83 0.78 0.92 3.39 4.16 4.16 64.97 30.08 0.75 38.6 [21]Tires (truck) 83.20 7.70 1.50 1.44 6.16 5.00 66.10 27.50 1.40 33.4 [22]Tires (motorcycle) 75.50 6.75 0.81 1.44 15.50 20.10 57.50 20.85 1.53 29.18 [23]

MSW 15-30 2-5 0.2-1.0 0.02-0.1 12-24 10-30 30-60 3-15 10-40 8.9-13.4[24]Biomass

(wood) 49.5 6.0 0.2 0.1 42.7 1.5 0.6 68 11.3 20 16.7-19.0

Bituminous coal 73.1 5.5 1.4 1.7 8.7 9 9 35 45 11 34.0 [25]Lignite 56.4 4.2 1.6 18.4 5 6 29 31 34 26.8

A – ash, VM – volatile matter, FC – fixed carbon, M – moisture

Fuel consisting of shredded waste tires is known as tire derived fuel (TDF). In the European Union the cement industry is one of the greatest consumers of waste tires, which uses them as an alternative fuel combusted together with coal. Cement plants are able to use as a fuel even whole tires. This is possible because the temperature in cement kilns is above 1200 °C, which ensure the complete combustion of all components of the tires. The ash and steel cord are permanently bound to the clinker, but this does not seriously impair its properties. Furthermore, the combustion of tires in cement kilns is environ -mentally safe because of the lower emissions, compared to coal combustion. Apart from the cement industry, used tires also can be used as a fuel for the production of electrical energy, paper, lime and steel. This is because the co-combustion of coal with rubber wastes improves the thermal efficiency of boilers and furnaces, and the amounts of exhaust gases [7]. However, combustion of tires under uncontrolled or non-optimal conditions is a source of many pollutants that pose a serious threat to the environment and humans. Pyrolysis of waste tires may be consider as an option better than combustion because it gives a possibility to convert them into solid, liquid and gaseous products. They can be subsequently combusted with lower emis-sions comparing with direct tires combustion, or can be used as a source of chemicals.

3. Pyrolysis of waste tires The pyrolysis of waste tires has been widely considered [13,14,16] but it is necessarily to make a short explanation of

the process before moving to a discussion about pyrolysis products. Pyrolysis (thermal distillation, thermolysis) of waste tires is a thermo-chemical decomposition of the organic com-

pounds present in them by breaking apart chemical bonds. This process takes place in the absence of oxygen (inert atmo-sphere or vacuum) in temperatures from 400 °C to about 800 °C [13,26]. In practice, the processes of thermal treatment of waste often operate with a small amount of air present. Eventually this leads to a partial gasification. It can be assumed that pyrolysis occurs in the inner zone of the bed. These processes are often described as "quasi-pyrolysis" [27]. During the heating period many reactions take place including dehydration, cracking, isomerization, dehydrogenation, aromatization and condensation [28]. They result in the production of gas, vapour that can be collected as a liquid and solid char [29].

3.1. Types of pyrolysis It is possible to successfully operate the pyrolysis process under different conditions and so there any many classifica -

tions of pyrolysis. There are several types of pyrolysis according to the operating parameters: atmospheric, vacuum, cata -

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lytic, fast, ultra-fast (flash) and slow [30]. Atmospheric pyrolysis is carried out under atmospheric pressure and vacuum pyrolysis is conducted under very low pressure, e.g. approximately 4,0 kPa [21]. The use of a catalyst generally promotes the pyrolysis of waste tires and improves the yields or upgrades the properties of the products to obtain valuable chemicals for many applications. It also increases the reaction rate and shortens the reaction time. Accordingly, catalytic pyrolysis is recently being increasingly researched [21,31,32].

The simplest classification divides the pyrolysis processes into fast and slow. Table 2. simply compares these two kinds of the process.

TABLE 2. Comparison of slow and fast pyrolysisFeature Slow pyrolysis Fast pyrolysis

Decomposi-tion

rateslow rapid

Heating rate low highResidence

time long – minutes to hours

short –up to few seconds

Temperature lower higherParticles size relatively big small

Main product solid-phase liquid-phase

Typical reactors fixed bed rotary kiln,

fluidized

3.2. Pyrolysis products When whole used tires are processed, four output streams are produced: gas, liquid (oil), solid (char) and steel. On the

other hand, if rubber shreds are the raw material, steel does not appear in the output stream, because it should have been mechanically separated previously.

Lopez et al. proposed another classification of pyrolysis products. He grouped them into five fractions: gas (C1-C4 hy-drocarbons), non-aromatic liquid fraction (non-aromatic C5-C10 hydrocarbons), aromatic liquid fraction (single-ring C–

10 aro-matic hydrocarbons), tar (C+

11 hydrocarbons) and char (solid residue) [33]. The composition of each fraction depends on the pyrolysis conditions used and on the tire composition. González et al.

examined the effects of temperature and heating rate on the composition of the pyrolysis products. Increasing the temperat -ure decreases the yield of char and increases the yield of gas. The oil fraction yield passed through a maximum at 550–575 °C. Similar trends were obtained by increasing the heating rate [19]. Regarding the impact of pressure on the products of waste tire pyrolysis, reduced pressure or vacuum can result in a lower energy consumption during the process, a higher liquid yield, and better quality of char [8]. The majority of the research undertaken on waste tires has used tires from repro-cessing plants where the tires would be a mixture of different brands and types [14]. Kyari et al. checked the influence of the type of tires on the composition of the products obtained in the process of pyrolysis. The results of the research suggest that the yields of oil, gas and char are not significantly influenced by the type and origin of tires, but there were noticeable dif -ferences in the composition of the gases and oils collected [34].

Generally, the pyrolysis of waste tires aims to maximize the yield of the liquid-phase product, because of the valuable chemicals obtained from it. Another way of improving the economics of the process is the acquisition of activated carbon from char. It is worth considering that the high calorific value of the pyrolysis gas meets the energy requirements of the process and allows the production of electricity. However, there is an enormous challenge: the high sulphur content in pyro-lysis products because sulphur is the main ingredient in the process of rubber vulcanization. The liquid and solid products of the pyrolysis of waste tires are characterized below. The gas phase will be discussed in more detail in a separate section.

3.2.1. Solid-phase The pyrolysis solid residue is char, which is also called pyrolytic carbon black. It is mesoporous material with an aver -

age heating value of 30 MJ/kg, composed of reinforcing carbon black used in tire production and other inorganic com-pounds formed during the pyrolytic process [21]. The composition and properties of the char depend on the pyrolysis condi-tions and the waste tire composition. Figure 3.shows the characteristics of char obtained in four different types of pyrolysis reactors [31,33,35,36].

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The chars have a high carbon content (up to 90 wt.%) and sulphur (about 2 wt.%). In the pyrolysis a solid residue also appears, zinc (about 4 wt.%) and other metals (e.g. Ca, Fe, Al) [35]. What is important, char contains a lot of ash compared with an original carbon black (more than 5 wt. %, up to almost 20 wt.%). This amount of ash is created from metal addit -ives, added during the tire production and from dirt deposited on waste tires. In the rotary kiln, fixed bed reactor and conical spouted bed reactor the content of ash was 10,35 wt.%, 11,6 wt.% and 7,1 wt.%, respectively. In the fluidized bed some fine bed material is mixed into the carbon black, so that the authors obtained a high percentage of ash (18,95 % at 710°C) [36].

C H N S0

102030405060708090

100

Rotary kiln [31]

Conical spouted bed [33]

Element

wt.

%

Rea

ctor

type

[Ref

.]

400 500 600 7100

20

40

60

80

100

120

Rotary kiln [31]

Conical spouted bed [33]

Temperature, °C

BET

surfa

ce a

rea,

m2/

g

Rea

ctor

type

[Ref

.]

FIGURE 3. Characteristics of char.

The char yield varies between about 35 and 55 wt.%. However, Shah et al. obtained approximately 98.5 wt.% of solid-phase from catalytic (Al2O3+SiO2) pyrolysis in a batch reactor under atmospheric pressure at 300 °C [37].

Properties of the solid-phase greatly influence the economic viability of the whole pyrolysis process. The most wide -spread use of the char is the production of activated carbons. The potential of this product as a possible adsorbent for vari -ous pollutants has been assessed and its use was found to be very successful. Active carbons from waste tires have been used to absorb phenols, basic dyes and metals, butane, and natural gas [38]. Tire rubber start to decompose at 400 °C and this phase is practically complete at 500 – 600 °C. As a result, a char with limited porosity is obtained – usually below 100 m2/g. In order to form a good quality product, physical and chemical activation processes were widely considered. Physical activation using carbon dioxide or steam as an oxidizing agent in temperatures of 800-1000 °C allows activated carbons to be obtained with excellent BET surface area. Gonzalez et al prepared activated carbons with surface area values of 1317 and

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496 m2 /g by activation of a pyrolytic tire char at 900 and 850 °C in a steam/N2 mixture and CO2, respectively [39]. Chem-ical activation allows both pyrolysis and activation to be integrated into a single, lower temperature process. Acosta et al. used char from the pyrolysis of tires as activated carbon precursors by KOH activation at temperatures ranging from 600 to 800 °C. He obtained activated carbons with surface areas as high as 700 m 2/g and high mesopore volumes. Those carbons were successfully tested for tetracycline abatement, moreover tetracycline adsorption capacities were better than commer-cial activated carbons [40].

3.2.2. Liquid-phase

The liquid-phase of pyrolysis products is usually named pyrolysis oil. It is a dark, cloudy, dense liquid with a rather strong odour. Post-pyrolytic oil is a very complex mixture of hydrocarbons consisting of saturated and unsaturated linear or cyclic hydrocarbons with 7-20 carbon atoms per particle, resinous substances and the entrainment of black particles [41]. Oil yields vary between 38 and 56 wt.% and the heating value is about 40-43 MJ/kg [16].

The chemical composition of the pyrolysis oil could be complex depending on the type of tire rubber and processing additives. Choi et al. reported that aromatics were the main constituents, amounting to 65-79 wt.% of the pyrolysis oil they obtained. The main aromatic compounds were xylenes, trimethylbenzenes, dimethylstyrenes and dimethylindenes. Li-monenes were one of the main compounds and some heteroatom-containing compounds, such as benzothiazole and 2,4-di-methylquinoline, were also found in the pyrolysis oils [42]. Dębek & Walendziewski wrote, that in terms of the hydrocar-bon composition, their pyrolytic oil contained 47 wt.% of aromatic compounds, 13 wt.% of alkanes, 7 wt.% of dienes, 6 wt.% of cycloaliphatic and aromatic compounds, 5 wt.% of cycloalkenes, 2 wt.% of polymeric substances and other unidenti-fied compounds and 1 wt.% of compounds containing oxygen and nitrites [41]. Pyrolysis oil contains a relatively high amount of sulphur – usually more than 1 wt.%. which prevents it from being used directly as a fuel in many cases. These sulphur compounds have their origin in the thermal degradation of the vulcanization agents and accelerators added during the tire fabrication [43].

It is possible to use two parameters for a simple description of the quality of tire pyrolytic oil for use as a fuel: sulphur content (%) and energy yield. The energy yield (heating value (MJ/kg) × oil yield (%)) represents the amount of energy that can be obtained from each unit mass of tire from a particular pyrolysis process [44]. Figure 4. shows the quality of pyrolysis oil obtained by three researching groups [45–47]. The lower the sulphur content and higher energy yield, the better the qual-ity of pyrolysis oil. As the Figure 4. shows, the sulphur content was relatively high, which would create difficulties in using the oils directly as a fuel. Diesel oil usually has a similar heating value to that of pyrolysis oil, but the sulphur content is definitely lower, about 0,1 wt.%.

10 12 14 16 18 20 22 24 26 28 300

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Quality of pyrolysis oils

[45][46][47]

Energy yield, MJ%/kg

S co

nten

t, w

t. %

FIGURE 4. The quality of tire pyrolysis oil for use as a fuel: sulphur content and energy yield .

The major problem in using pyrolysis oil as a fuel is therefore the high sulphur content. There are many methods of desulphurization referred to in the literature, e.g. adding alkaline additives such as CaO, Ca(OH)2 or NaOH [48], distillation

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of oil [49], oxidation of sulphur compounds with hydrogen peroxide in the presence of an acidic catalyst [50] or hydrorefin-ing [41]. However, Choi et al. obtained a pyrolysis oil with the sulphur content 0.55 wt.% using a two-stage pyrolyzer, but without using any catalyst. It still appears to be high for its direct use as a fuel. However, the low sulphur content compared with those of oils obtained from the conventional non-catalytic tire pyrolysis would make it easier to further reduce the sulphur content by other chemical or physical methods [51]. Hita et al. reported that high sulphur content, concentration of aromatics and heavy molecules within the gasoil boiling point range (BP > 350 °C) make the waste tire pyrolysis oil diffi-cult to use directly as a fuel. They recommend hydrodesulfurization, hydrocracking and hydrodearomatization as methods to solve those problems [52].

Benzene, toluene and xylene are very important chemicals. They are used to produce plastics, resins, fibres, surfactants, dyestuffs and pharmaceuticals. Dipentene comprises the racemic mixture of the two enantiomers D- and L-limonene. It is used in the formulation of industrial solvents, resins, and adhesives and as a dispersing agent for pigments and replacements for chlorofluorocarbon solvents to clean electronic circuit boards or as an active ingredient in different pesticide products [34,53]. The price of dipentene currently varies between 1500 and 2400 US$ per tonne. In 2016 the price of benzene, tolu-ene and xylene was about 700-1000 US$ per tonne [54]. A noticeable yield of these compounds in the chemical composi-tion of the oil makes their recovery profitable and can have a significant impact on the economic viability of pyrolysis of waste tires.

4. Gas-phase products at waste tires pyrolysis

Gas obtained from the pyrolysis of waste tires is named pyrolytic gas, pyrogas or syngas. It can range from a few per cent to more than ten percent of the products depending on the technology used and the process conditions. It has a high heating value, up to about 84 MJ/Nm3 or 42 MJ/kg [55]. It can be said that gas-phase products from waste tire pyrolysis generally are a mixture of paraffins, olefins (other hydrocarbons also appear), carbon oxides, hydrogen and small amounts of sulphur and nitrogen compounds.

4.1. YieldsAs mentioned above, the yields of pyrolytic gas strongly depend on the parameters of the pyrolysis process. Overall gas

yield is known to increase with an increasing process temperature. At higher temperatures the thermal cracking of the pyro -lysis oil vapour occurs, thus more gases are produced. Also differences in heating rates and in pyrolysis gas residence time can have a significant impact on the relative yields of gas, where higher temperatures of pyrolysis and long gas residence times in the hot zone of the reactor can crack the oil to gas [14].

The fixed bed reactor is used mostly, because it is easy to construct and operate. Figure 5. shows the gas-phase yields obtained by different authors according to the pyrolysis temperature [42,45,48,56–58]. Choi et al. obtained the largest yields of gas, 30 wt.% at 800 °C, although the aim of the experiment was to obtain oils and activated pyrolysis char. Waste tires were pyrolyzed in a fixed bed reactor consisting of a stainless steel tube with a height of 410 mm and an inner diameter of 83 mm. The reactor was indirectly heated by an electric furnace at a heating rate of 10 °C/min. The N 2 flow rate was 2 NL/min. When the pyrolysis temperature reached the desired reaction temperature (500 – 800 °C), the reactor was main -tained at the temperature for 2 h [42].

Results obtained by different researchers are compared in Figure 5. Looking at the graph it may be seen that although the process temperature actually has a large effect on the amount of gas that is generated, it is definitely not the only key factor. If this were the case, then the amount of gas produced by the pyrolysis process in a fixed bed by individual research -ers should be comparable, but meanwhile the differences are significant. According to the Figure 5. and Table 3. it can be said, that heating rate, reaction time, particle size, the particular design of the device used and other conditions must influ-ence strongly. Table 3. presents the conditions selected for the pyrolysis process in fixed bed reactors.

8

200 300 400 500 600 700 800 900 1000 11000

5

10

15

20

25

30

35

Fixed bed reactors

[42][45][48][56][57][58]

Temperature, °C

Gas

yie

lds,

wt.

%

FIGURE 5. Gas yields according to the pyrolysis temperature

TABLE 3. Gas yields and pyrolysis conditions in the fixed bed reactors.

No.Temperature,

°CGas yields, wt.%

Heating rate, °C/

min

Reaction time

Atmosphere Particle size References

1.

500 22.59±1.02

10 2 h N2 1-2 mm [42]600 28.74±1.08700 29.47±1.21800 30.08±2.69

2.

300 7.6±3.9

15 0.5 h N22-3 cm

(~ 175 g)[45]

400 19.3±2.2500 17.2±1.8600 17.6±0.8700 17.8±1.2

3.

400 7.42

12 4 h N2 small slices [48]

450 10.39500 11.86550 13.7600 15.9650 17.85700 18.68

4.

400 2.4

15 2 h N2 20 mg [56]500 3.6550 3.6700 4.4

5.350 3,1

13 4 h N2 30-50 mesh [57]400 11.2450 17.3500 5 up to 1200 54 s N2 powder [58]

9

6.

600 8 ~ 45 s700 18 ~ 39 s800 20.5 ~ 20 s900 23 ~ 16 s

1000 23 8 s

The desire for achieving continuous processes resulted in the development of rotary kilns and fluidized beds. A rotary kiln pyrolyzer offers many advantages. For instance, the slow rotation of the inclined kiln enables a good mixing of wastes and thus uniform pyrolytic products are obtained. Additionally, the residence time can be easily regulated to provide the optimum conditions [46]. In turn, fluidized bed reactors are more difficult to operate, but they have also the following ad-vantages: long residence times which contribute to secondary reactions, lower temperatures and heating rates, which reduce oil yields [59][60].

Li et al., Antoniou & Zabaniotou and Galvagno et al. investigated pyrolysis of waste tires in rotary kiln reactors. At a temperature of 600 °C they obtained 18 wt.%, 10.80 wt.% and 8.16 wt.% of gas yields, respectively [46,59,61].

Raj et. al optimized the process parameters in flash pyrolysis to liquid and gaseous fuel in a fluidized bed reactor. They checked the influence of temperature, particle size and feed rate on the gas yield. The temperature changes had a key influ -ence on the gas yields. The percentage composition of methane and carbon monoxide increased up to 475 °C and hydrogen yield increased from 350 °C to 600 °C. The effect of particle size and heat rate was trivial [62].

4.2. Composition The tire pyrolysis gas mainly consists of methane and other hydrocarbons having from 2 to 6 carbon atoms per mo-

lecule, plus carbon oxides, hydrogen and some small amounts of sulphur and nitrogen compounds. The gases with four or fewer carbon atoms per molecule are CH4 (methane), C2 (ethene, ethane), C3 (propane, propene),

C4 (butane, butenes, etc.) and these are the predominant products. These gases come from the depolymerisation of rubber, for example, styrene-butadiene and from secondary cracking reactions (at higher temperatures) [18]. Generally, it can be said that the pyrolytic gas contains about 20 vol. % of methane (up to 40 %) and the concentrations of other hydrocarbons vary widely.

Also hydrogen is one of the most abundant pyrolytic gas components and it can range from few to dozen/ more than ten vol. %. Berruecco et al. obtained H2 contents of between 2.6 and 17.8 vol. % (temperatures between 400 °C and 700 °C) and Lopez et al. achieved 22.27 vol. % at 550 °C [18,56]. Leung et al. were able to get a pyrolytic gas containing about 25 % of hydrogen, but they needed a temperature of 1000 °C [58]. However, it should be remembered that increasing the temperat-ure makes the process more expensive. Some researchers have obtained pyrolytic gas containing even more than 30 vol. % of hydrogen (see Table 4.).

Other gaseous products derived from pyrolysis include COX (CO, CO2) and H2S. Other sulphur compounds may also occur, for example, COS and CS2 [21]. COX components are derived from the oxygenated organic and inorganic compounds contained in tires. COX concentration is usually a few percent. H2S comes from the decomposition of the sulphur links of the vulcanized rubber structure [63]. The concentration of H2S varies between a fraction of a percent and more than 4 % [13]. It can be lowered by adding a wide range of catalysers during the pyrolysis process. Nitrogen compounds like NH3 and NO2

also were reported in the pyrolytic gas [3,64]. The composition of pyrolytic gas obtained by different researchers is shown in Table 4. Additionally, the concentration of H2, CH4 and CO2 are shown in Figure 6.

TABLE 4. Composition of the pyrolytic gas obtained in fixed bed reactorsPyrolysis temperature, ° C

Gas concentration (vol. %) References

H2 CH4 C2H6 C2H4 C3 C4 C5 C6 CO CO2

350 24.00 20.00 29.00a 12.00a 5.70 3.00 1.00a 1.00 2.30 [65]400 2.608b 1.024b

0.432b 0.250b 0.51b 3.390 0.027 0.298 1.080 1.414 [56]

450 30.00 24.00 26.00 9.00 4.30 2.00 0.20 1.10 1.90 [65]500 21.50 17.30 8.20 8.70 7.30 5.70 n. r. n. r. 5.10a 26.20a [34]550 40.00a 26.00 20.00 6.00 2.80 1.10 0.10b 1.60 1.40 [65]550 17.86 5.64 1.75 0.833 1.49 1.95 0.021b 0.30 0.26b 0.78 b [56]550 7.90 9.90 5.90 5.30 38.10a n. r. n. r. 9.00 [66]550 22.27 21.32 4.19 2.32 5.43 35.39 <2 1.98 2.54 [18]600 30.40 23.27 6.20 4.45 7.65 10.74 n. r. n. r. 2.38 2.90 [67]

10

650 26.06 23.92 9.00 4.37 11.80 7.64 10.74a 0.19 1.15 2.41 [22]700 10.097 5.427 1.557 1.152 1.532 2.477 0.035 0.47

80.382 1.177 [56]

800 20.70 44.50a 4.40 17.30 3.90 1.30b n. r. n. r. 2.60 1.80 [58]900 8.40 24.1 10.3 6.2 24.8 n. r n. r. 13.00 [66]

n. r.: not reported; a: the highest concentration of particular component; b: the lowest concentration of particular component

350 [65] 400 [56] 450 [65] 500 [34] 550 [65] 550 [56] 550 [18] 600 [67] 650 [22] 700 [56] 800 [58]05

101520253035404550

Concentration of methane, hydrogen and carbon dioxide in pyrolytic gas

methane hydrogen carbon dioxide

Temperature, °C[Ref.]

Con

cent

ratio

n, v

ol. %

FIGURE 6. Concentration of methane, hydrogen and carbon dioxide in pyrolytic gas obtained in different conditions.

The concentration of the individual components is strongly dependent on the process conditions. Gonzalez et al. checked the influence of operating parameters on the composition of gaseous products from the pyrolysis of small tire pieces. Increasing the temperature (from 350 to 700 °C) or heating rate (from 5 to 20 °C/min) led to increases in practically all gases, especially CH4, C2H4 and C2H6 [19]. Leung et al. also checked the influence of operating parameters on the com-position of gaseous products from the pyrolysis of tire powder. It can be observed that the percentage of methane increases, but COX decreases with increasing temperature from 500 to 900 °C. Above 900 °C, CH4 begins to decrease and COX in-creases with increasing temperature up to 1000 °C. The percentage of H2 generally increases with increasing temperature. The C2 hydrocarbons increase with increasing temperature from 500 to 800 °C and then decrease, but C3 and C4 components increase with increasing temperature from 500 to 700 °C. Above 700 °C, the C3 and C4 components begin to decrease [58].

4.3. Properties The gas-phase product obtained during the pyrolysis of waste tires is a good fuel. Its most important properties are

heating value and the concentration of sulphur compounds, especially H2S. Hydrogen sulphide can pollute the environment and erode devices. It is also highly toxic at high concentrations. Moreover, during combustion it can be easily oxidized to SO2, which is also harmful for humans and the environment.

The most popular gaseous fuel is natural gas so other similar fuels should be compared with it. Table 5. shows the dif-ference in composition between wet natural gas and pyrolytic gas obtained in a fixed bed reactor at 650 °C from car tires [22]. It can be seen, that pyrolytic gas contains less methane than natural gas, but more hydrocarbons C2-C6. It is important to note that the gas-phase from pyrolysis contains much hydrogen but neither nitrogen nor argon.

TABLE 5. Comparison of natural gas and pyrolytic gas compositionSubstance Concentration, vol. %

11

Natural gas (wet) Pyrolytic gas

Hyd

roca

rbon

s

Methane 84.6 23.92Ethane 6.4 9.00Ethene - 4.37Propane 5.3 11.80Propene -Isobutane 1.2 7.64n-Butane 1.4Isopentane 0.4 10.74n-Pentane 0.2Hexanes 0.4 0.19Heptanes 0.1 -

Carbon dioxide <5 2.41Carbon monoxide - 1.15Hydrogen - 26.06Helium 0.5 -Hydrogen sulphide 5 4.18Nitrogen 10 -Argon 0.05 -

References [68] [22]

The heating value of waste tires has been checked many times and it is around 30 - 40 MJ/kg for car tires and it is quite different for other tires (truck, motorcycle etc.). Syngas obtained from them has a significant heating value and has been reported to be between 20 MJ/m3 and more than 65 MJ/m3 depending on the composition [14]. Compared with natural gas (heating value: 35-40 MJ/m3 depending on the origin) pyrolytic gas seems to be a promising fuel. Table 6.. shows the heat-ing value – from lowest to highest - of pyrolytic gas obtained by different researchers.

TABLE 6. Heating values of pyrolytic gas obtained in different conditions

Temperat-ure, °C Details

Heating Value,MJ/m3

References

400 fixed bed 11.97 [19]600 vacuum pyrolysis 17.30 [21]450 vacuum pyrolysis 19.80 [21]550 vacuum pyrolysis 20.50 [21]500 vacuum pyrolysis 20.70 [21]550 pilot plant 22.04 [61]600 pilot plant 23.98 [61]500 fixed bed 28.37 [19]680 pilot plant 29.03 [61]900 tire powder 34.90 [58]800 tire powder 38.10 [58]600 fixed bed 38.59 [19]500 test bench 40.50 [69]700 fixed bed 42.87 [19]700 tire powder 43.20 [58]900 fixed bed 57.50 [66]550 fixed bed 65.60 [66]550 pilot plant 68.70 [18]700 fixed bed 69.50 [63]600 fixed bed 73.80 [63]500 fixed bed 76.70 [63]400 fixed bed 81.60 [63]

12

Due to environmental concerns, the emission of sulphur compounds should be taken into account in the evaluation of waste tires pyrolysis. Ucar et al. obtained pyrolytic gas with 4.18 and 0.94 vol. % of H 2S for car and truck tires respectively (process temperature – 650 °C) [22]. Zhang et al. checked the concentration of H2S depending on the vacuum pyrolysis temperature (without any additives). They noticed that the maximum value was 33.48 mg/m3 at 480 °C [21], but the use of dolomite and limestone reduced the concentration of this compound. Also Kandasamy & Gökalp reported that the single emission peak of H2S occurred at around 480 °C for both car tires and truck tires [3]. A temperature about 500 °C is con-sidered to be optimal for the pyrolysis of tires and thus the H2S concentration is a serious problem.

4.4. Application The most common use for the pyrolytic gas is in combustion in order to provide the energy required by the pyrolysis

process. The enthalpy of pyrolysis (fixed bed reactor – the most widely used) was found to be circa 270 J/g. and the gross heating value of the pyrolytic gas is 2900 J/g. expressed as energy per rubber tire unit mass. There is therefore enough en-ergy to satisfy the reaction requirements and to compensate for heat losses or for use for other purposes [70].

However, there is an important issue for commercial tire pyrolysis plants located in the European Union. DIRECTIVE 2000/76/EC of The European Parliament and of the Council of 4 December 2000 on the incineration of waste accurately determines the emissions of selected compounds for waste incineration plants. Incineration plant is defined as any station-ary or mobile technical unit and equipment dedicated to the thermal treatment of wastes with or without recovery of the combustion heat generated. This includes the incineration by oxidation of waste as well as other thermal treatment pro-cesses such as pyrolysis, gasification or plasma processes in so far as the substances resulting from the treatment are sub -sequently incinerated [71]. That means the waste tire pyrolysis plant must abide by emission limits specified in Annex V. Those limits are showed in Table 7. Experimental plants used for research or development, which treat less than 50 tonnes of waste per year, are excluded from the scope of the Directive [71].

TABLE 7. Air emission limit values for incineration plants [71]

No. Substance

Air emission limit values, mg/m3

(concentration of oxygen: 11 %)

Daily average Half-hourly average

1 Total dust 10 30

2 Gaseous and vaporous organic substances. expressed as total organic car-bon (TOC) 10 20

3 Hydrogen chloride (HCl) 10 604 Hydrogen fluoride (HF) 1 45 Sulphur dioxide (SO2) 50 2006 Carbon monoxide (CO) 50 100

7

Nitrogen monoxide (NO) and nitrogen dioxide (NO2) expressed as nitro-gen dioxide for existing incineration plants with a nominal capacity ex-ceeding 6 tonnes per hour or new incineration plants

200 400

Nitrogen monoxide (NO) and nitrogen dioxide (NO2) expressed as nitro-gen dioxide for existing incineration plants with a nominal capacity of 6 tonnes per hour or less

400 -

8

Heavy metals and its compounds, expressed as metal

All average values over the sample period of a minimum of 30 minutes and a maximum of 8 hours.

Cadmium (Cd) + Thallium (Tl) 0.05Mercury (Hg) 0.05Antimony (Sb) + Arsenic (As) + Lead (Pb) + Chromium (Cr) + Cobalt (Co) + Copper (Cu) + Manganese (Mn) + Nickel (Ni) + Vanadium (V) 0.5

9Dioxins and furans (The emission limit value refers to the total concentration of dioxins and furans calculated using the concept of toxic equivalence in accordance

Average values over the sample period of a minimum of 6 hours and a maximum of 8 hours. 0.1 ng/m3

13

with Annex I.)

There is very limited information on pollutants generated from the combustion of pyrolytic gas in the literature. How-ever, this is an important topic for anyone who plans to invest funds in tire pyrolysis plants, because the cost of flue gas cleaning may be one of the highest in the different kinds of waste incineration plant. Aylón et al. checked the emissions from the combustion of gas-phase products from tire pyrolysis. They reported that total dust, CO, HF and NOX were under the limits, but TOC, HCl and especially SO2 (4780 mg/Nm3) were over the maximum allowed limits. Pyrolytic gas has a relatively high concentration of H2S, which during combustion is easily oxidized to SO2.The metal concentration was under the detection limits of Jobin Yvon 200 Ultrace (analytical technique: ICP-OES) and the concentration of dioxins and furans was 0.0063 ng/Nm3. Aylón et al. concluded that it would be necessary to incorporate an acid gas cleaning system[70]. Moreover, a flue gas cleaning unit should be considered by any potential owner of a tire pyrolysis plant already in the phase of investment planning to ensure that all health and safety requirements are met. There are a lot of different commercially available methods of flue gas purification. There are also some promising lab-scale purification methods, which may be applied in a pyrolysis plant. The type of method depends on the kind of pollution that needs to be removed, regulation limits and costs. However, it must be emphasized, that combustion of low-molecular-weight gases from pyrolysis is much cleaner than the combustion of raw feedstocks (in other words, a pyrolysis process is more similar to combustion of natural gas as opposed to combustion of coal). Pyrolysis processes use very little or no air or oxygen too. This makes the control of air emission less complicated and less costly than in incineration units [12].

5. Conclusions

Waste tires should not be treated as typical waste, because they are a rich source of energy (heating value up to 40 MJ/kg) and chemicals, which both can be recovered. Waste tires used in pyrolysis processes can deliver pyrolysis oil, char, calorific gases, process heat and electricity. Pyrolytic gas can be consider as a valuable energy source. It is a gaseous fuel composed mainly of paraffins, olefins, carbon oxides, hydrogen and small amounts of sulphur and nitrogen compounds; characterized by high heating value. Combustion of pyrolytic gas allows to meet the energy demand for pyrolysis process. Thus oil and char can be used in different ways (chemicals sources or activated carbon precursors) or also for energy recov-ery. Pyrolytic gas combustion is very promising but there are still some important challenges faced by scientists and engin-eers. Among them are the quality of products, the high sulphur content in pyrolysis products, economic viability and the emissions standards which must be met. Although there are some data suggesting that advanced tire pyrolysis processes are able to meet environmental permit limits for industrial processes, it is strongly suggested that an emphasis should be fo -cused on the collection of data on emissions and other process data as pyrolysis plants begin operation. Of course, this should not only include the main pollutants for which there are criteria (such as particulates and compounds of sulphur and nitrogen), but also trace materials such as heavy metals, dioxins, and furans.

AcknowledgementsThe paper was financial supported by Department of Air Conditioning, Heating, Gas Engineering and Air Protection; Wro-claw University of Science and Technology (No. 04010066/16).

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