Characterization of the Liquid Obtained in Tyre

I NTERNATIONAL J OURNAL OF C HEMICAL R EACTOR E NGINEERING Volume 5

2007

Article A96

Characterization of the Liquid Obtained in Tyre Pyrolysis in a Conical Spouted Bed Reactor

∗

Miriam Arabiourrutia∗

Gartzen Lopez†

Gorka Elordi‡

Martin Olazar∗∗

Roberto Aguado††

Javier Bilbao‡‡

University of the Basque Country, [email protected] University of the Basque Country, [email protected] ‡ University of the Basque Country, [email protected] ∗∗ University of the Basque Country, [email protected] †† University of the Basque Country, [email protected] ‡‡ University of the Basque Country, [email protected] ISSN 1542-6580 c Copyright 2007 The Berkeley Electronic Press. All rights reserved. †

Characterization of the Liquid Obtained in Tyre Pyrolysis in a Conical Spouted Bed Reactor∗ Miriam Arabiourrutia, Gartzen Lopez, Gorka Elordi, Martin Olazar, Roberto Aguado, and Javier Bilbao

Abstract Used tyres pose a serious environmental problem and pyrolysis is considered one of the more feasible solutions that may be economically profitable on a large scale. In this study the pyrolysis of tyres has been carried out in a conical spouted bed reactor at 500 ◦ C and the liquid product has been characterized taking into account composition, heat value and simulated distillation. The tyre mass feed in each run was 2 g and the bed was made up of 15 g of sand. Pyrolysis of scrap tyre at the 500 ◦ C gives way to a yield of 3.1% of gases, 37.6% of liquid fraction (C5-C10 range hydrocarbons), 25.6% of tar (C11+) and 33.7% of char. The liquid fraction is of suitable quality for its use as fuel but the char requires activation for its upgrading. KEYWORDS: tyre pyrolyis, spouted beds, liquid characterization

∗

This work was carried out with the financial support of the University of the Basque Country (Project GIU06/21), the Ministry of Science and Education of the Spanish Government (Project CTQ2004-01562/PPQ), the Department of Industry of the Basque Government (Project IE05149), and the Ministry of Environment of the Spanish Government (Project 5.3-085/2005/3-B).

Arabiourrutia et al.: Characterization of the Liquid Obtained in Tyre Pyrolysis

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1. INTRODUCTION At present, 2.5 million tonnes per year of waste tyres are generated in Europe, a similar amount in USA and approximately 1 million tonnes in Japan (Shulman, 2000). Consequently, urgent solutions are required for their valorisation. Landfilling of tyres is declining as a disposal option, since tyres do not degrade easily in landfills. Open dumping may result in accidental fires with high pollution emissions and tyres can be a breeding ground for insects and a home for vermin. Alternative waste management options to landfilling and open dumping have included, tyre retreading, crumbing to produce rubber for applications such as carpets, sports surfaces and childrens’ playgrounds. Incineration of tyres with energy recovery is also a growing option since it utilises the high calorific value of tyres (Sharma et al., 2000). A different alternative is the recovery of the tyre components by hydrogenation (Sugano et al., 2005), liquefaction (Money and Harrison, 1999), or pyrolysis. Pyrolysis is considered to be one of the more feasible solutions at large scale. The interest is centred on the fact that the products obtained by this process may be easily handel, stored and transported and, consequently, they may be transformed in other units that are not near the recycling plant (Lee et al., 1995). Pyrolysis liquid products may be used directly as fuel or they may be added to a refinery feed. In addition to their use as fuels, the oils have been shown to be a potential source of light aromatics such as benzene, toluene and xylene, which command a higher market value than the raw oil (Cunliffe and Williams, 1998; Williams and Brindle, 2003). Similarly, the oils have been shown to contain limonene, a high value product used in industrial applications including formulation of industrial solvents, resins and adhesives, as a dispersing agent for pigments, as a fragrance in cleaning products and as an environmentally acceptable solvent (Stanciulescu and Ikura, 2006; Benallal et al., 1995). The gases obtained may be used as fuel for making the process autothermal. The carbonaceous material may be used as a quality fuel, carbon black or active carbon (Murillo et al., 2005; Gonzalez et al., 2006).

2. EXPERIMENTAL Based on hydrodynamic studies in a cold unit, on the experience acquired in prior pyrolysis of other type of agroforest residues and plastics (Aguado et al., 2000; Olazar et al., 2000; Aguado et al., 2002) and on the information collected in the literature, a pilot plant for pyrolysis and gasification of scrap tyre has been designed, built and fine tuned. Figure 1 shows a diagram of the unit, which consists of the following components: 1) solid feeder; 2) a system for feeding the inert gas; 3) the pyrolysis reactor; 4) a system for fine separation from the gaseous stream; 5) condensers and filters for liquid product collection; 5) a system for the analysis by gas chromatography of the gases produced; 6) a control unit (Labtech under Windows). The char (carbon black) outlet pipe has been designed for continuous operation and has been closed for carrying out the discontinuous operation required for the kinetic study. The char is collected from the fountain of the bed and leaves the reactor through a 6 mm pipe to a vessel located below the reactor. The reactor is the main component of the unit and is of conical geometry provided with an upper cylindrical section. The total height of the reactor, HT, is 34 cm, the height of the conical section, Hc, 20.5 cm, and the angle of the conical section, γ, 28º. The diameter of the cylindrical section, Dc, is 12.3 cm, the diameter of the base, Di, 2 cm and the gas inlet diameter, Do, 1 cm. These dimensions guarantee bed stability in a wide range of process conditions, particularly regarding gas velocity, and they have been established in previous hydrodynamic studies. The hydrodynamic regime may be changed from that corresponding to the minimum spouted bed to a vigorous jet spouted bed (or dilute spouted bed), with a wide transition regime between them both This implies great versatility in the gas-solid contact conditions, especially in the gas residence time. Consequently, starting with a given stagnant bed height, Ho, and increasing the gas flowrate, the residence time may be decreased from a few seconds to values near 20 ms. This fact, together with an excellent heat transfer capacity, makes this reactor especially suitable for flash pyrolysis processes.

Published by The Berkeley Electronic Press, 2007

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International Journal of Chemical Reactor Engineering

Vol. 5 [2007], Article A96

Figure 1. Diagrammatic representation of the pyrolysis unit. Below the reactor, there is a cartridge containing a ceramic resistance, which is inside a metallic casing and thermally insulated. This resistance heats the nitrogen stream up to the reaction temperature, which is measured with a thermocouple placed at the upper end of the resistance (below the reactor inlet). In the conical section of the reactor, there is another resistance surrounding the wall of this section, which is controlled by measuring the temperature at a point near the wall by means of another fixed thermocouple. Furthermore, the reactor is insulated in order to minimise energy loss. The main components of the tyre material studied are (in %wt): natural rubber (SMR 5CV), 29.59; styrenebutadiene rubber (SBR 1507), 29.59; carbon black (ISAF N220), 29.59. Other components are (in %wt): stearic acid, 0.59; IPPD (n-isopropyl-n´-phenyl-p-phenylendiamine), 0.89; zinc oxide, 2.96; phenolic resin, 2.37; sulpher, 0.89; CBS (n-cyclohexyl-2-benzothiazol-sulfenamide), 0.89; H-7 (hexamethylentetramine), 0.18; PVI (ncyclohexylthio)-phthalimide), 0.12; aromatic oil, 2.37. The density is 1140 kg m-3 and the high calorific value (determined in a Parr 1356 isoperibolic bomb calorimeter) 38847 kJ kg-1. This material has been ground to a particle size lower than 1 mm in a Retsch ZM 100 mill. The industrial implementation of the process for waste tyre valorisation requires removing the steel cord prior to grinding or it may also be collected from the reactor as nonpyrolysable material together with the char. The bed was initially made up of 15 g of 1 mm sand and 2 g of tyre material. The nitrogen flowrate entering the reactor was 1.2 times that corresponding to the minimum spouting velocity, which was 6.5 L min-1 (Olazar et al., 1993, 2006). The tyre amount used allows for obtaining a homogeneous mixture in the bed and for minimizing the time required to stabilise temperature. The tyre material was fed as a pulse once the temperature of the sand bed was uniform. The volatile products leave the reactor together with the inert gas and the finest carbon black particles. These particles are retained in a high efficiency cyclone followed by a 25 µm sintered steel filter, both placed at the reactor outlet. The gases leaving this filter circulate through a volatile condensation system consisting of two condensers and a coalescence filter. The first condenser is a double shell tube cooled by tap water. The second is a gas trap and is filled with iced water. The gaseous stream is then passed through a coalescence filter,

http://www.bepress.com/ijcre/vol5/A96


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where the small drops that are not condensed are retained. There is a bubble flow-meter at the filter outlet, whose purpose is to record the flowrate of non-condensable gases. Product identification has been carried out by GC/MS (Shimadzu UP-2010S) using NIS library. The chromatograph is provided with a TRB-1 (20 m x 0.10 mm) column. Product quantification has been carried out by gas chromatography with one piece of equipment for analysing noncondensable gases and another for condensable gases. The former is an Agilent microGC 3000 with three channels provided with thermal conductivity detectors (TCD): a channel with a Plot U (3 m x 0.32 mm) pre-column and a Plot molecular sieve 5A (10 m x 0.32 mm) capillary column for permanent gases, and two channels with Pora Plot Q (10 m x 0.32 mm) capillary column for oxygenate compounds and light hydrocarbons (up to C4). The condensed liquid product has been analysed by GC/MS in the same equipment used for product identification.

3. EXPERIMENTAL RESULTS Thermal pyrolysis of tyre particles has been studied at 500 ºC. This temperature is required to guarantee full conversion and corresponds to the maximum yield of the liquid product (de Marco et al., 2001; Aguado et al., 2005; Murillo et al., 2006). Furthermore, a higher temperature favours carbon black degradation by deposition of carbonaceous material. The components have been grouped into five lumps: Char, gas (C1-C4 hydrocarbons), nonaromatic liquid fraction (non-aromatic C5-C10 hydrocarbons), aromatic liquid fraction (single ring C10- aromatic hydrocarbons) and tar (which includes C11+, independently of their aromatic or non-aromatic nature). Table 1 shows the yields obtained for the five lumps. These values correspond to the arithmetic average of the results of three runs, with a standard error lower than 2 wt% in the yields of the five lumps and lower than 3 wt% in the yields of the individual components. Mass balance closure is at 98.0 wt. Table 1. Yields obtained for the five lumps Gas 3.1

Non-aromatic liquid 29.8

Aromatic liquid 7.8

Tar 25.6

Char 33.7

The content of C and H in the material to be pyrolysed is 82.50 and 6.40 (elemental analysis), whereas that corresponding to the products is: gases, C, 85.71 wt% and H, 14.29 wt%; non-aromatic C5-C10, C, 88.66 wt% and H, 10.36 wt%; aromatic C10-, C, 89.73 wt% and H, 8.91 wt%; carbon black (elemental analysis), C; 80.20 wt% and H, 1.40 wt%. The gases are made up of mainly methane and C2-C4 olefins and, given that their yield is very low, their best use is to burn in order to produce energy for the pyrolysis process. Operation under the conditions described gives way to a high yield of liquid fraction (37.6 wt% of C5-C10 range hydrocarbons). This fraction may be processed in a refinery or directly used as fuel, but in this case the content of aromatics is rather high, which may limit its application as automobile fuel. The yield of aromatics is favoured by high temperatures (due to secondary reactions) and by the presence of aromatics in the original formulation of the tyre, as is our case (styrene-butadiene rubber). Figure 2 shows the product distribution in the C5-C10 fraction expressed as a function of the number of carbon atoms and divided in aromatics and non aromatics. The high values obtained for the more abundant fractions are due to two individual components. Thus, the high value obtained for C10 fraction is due to the presence of limonene and in the case of C5 fraction to the monomer isoprene. The liquid fraction contains interesting hydrocarbons in a high proportion, such as isoprene, limonene, styrene and BTX. Tables 2 and 3 show the yields of identified non aromatic and aromatic compounds respectively in the C5-C10 fraction in the thermal pyrolysis of tyres at 500 ºC. Separation of these components may also be economically justified in certain cases, as that of limonene, whose yield reaches 15 wt%.



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40 35

Aromatics Non-aromatics

Yields (%)

30 25 20 15 10 5 0

C5

C6

C7

C8

C9

C10

Figure 2. Product distribution according to the number of carbon atoms Table 2. Yields of identified non aromatic compounds in the C5-C10 fraction in the pyrolysis of tyres at 500 ºC. Compound C5 isoprene 3-methyl-1-butene 2-methylbutane 1-pentene 2-methyl-1-butene pentane 2-pentene 2-methyl-2-butene cyclopentene C6 2-methylpentane hexane 1,1,2-trimethylcyclopropane 3-methyl-2-pentene methylcyclopentane 2-methyl-1,3-pentadiene 3-hexen-1-yno 5-methyl-1,3-cyclopentadiene 1,3-hexadiene 4-methylcyclopentene 1,3-cyclohexadiene cyclohexene unidentified

.


wt% 6.82 5.21 0.07 0.02 0.03 0.05 0.22 0.85 0.24 0.12 1.23 0.05 0.06 0.02 0.11 0.04 0.03 0.03 0.25 0.08 0.16 0.18 0.11 0.09


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Table 2. Continued. Compound C7 isopropylcyclobutane 3-methylcyclohexene 1,3,4-trimethylcyclobutane 1-ethylcyclopentene 1-methyl-1,4-cyclohexadiene 2-methyl-1,3,5-hexatriene 2,3-dimethyl-1,3-pentadiene 3-methyl-1,3,5-hexatriene 5,5-dimethyl-1,3-cyclopentadiene unidentified C8 2,5-dimethyl-1,4-hexadiene 3-methyl-1,5-hexadiene 2,4,6-octatriene 3-methyl-1,5-heptadiene 3-methyl-1,5-heptadiene 5,5-dimethyl-1,3-hexadiene 3,5-dimethylcyclohexene 3-ethyliden-1-methylcyclopentene 1,3-dimethylcyclohexene 4-ethenylcyclohexene ethenylcyclohexane 2-methylbicyclo[2.2.1]heptane unidentified C9 1,6-dimethylhepta-1,3,5-triene 5-(1,1-dimethylethyl)-1,3-cyclopentadiene 1-methylethylidencyclohexane 2,6-dimethyl-1,3,6-heptatriene 1,4-dimethyl-2-methylencyclohexane 1,1-dimethyl-4-methylencyclohexane 7,7-dimethyl-1,3,5-cycloheptatriene 2-methylbicyclo[3.2.1]octane unidentified C10 2,5,6-trimethyl-1,3,6-heptatriene 5,5-dimethyl-1-propyl-1,3-cyclopentadiene 2,7-dimethyl-1,3,7-octatriene l-limonene 1-methyl-3-(1-methylethenyl)cyclohexane 1,3,5,5-tetramethyl-1,3-cyclohexadiene 2,6-dimethyl-1,6-octadiene 3,3,5-trimethyl-1,5-heptadiene 2,7,7-trimethylbicyclo[2.2.1]hept-2-ene 2,4,6-trimethyl-1,3,6-heptatriene 1,2,4,5-pentamethyl-1,3-cyclopentadiene 1-methyl-4-(1-methylethyl)cyclohexene dl-limonene 1-methylen-4-(1-methylethenyl)cyclohexane 1,3-butadienylidencyclohexane 1-methyl-4-(1-methylethyliden)cyclohexene 3-ethenyl-1,2-dimethyl-1,4-cyclohexadiene 5-methyl-3-(1-methylethyliden)-1,4-hexadiene 2,6-dimethyl-2,4,6-octatriene 5-ethenyl-1,2-dimethyl-1,4-cyclohexadiene unidentified total C5-C10 non aromatics

wt% 1.03 0.04 0.06 0.03 0.01 0.08 0.14 0.23 0.23 0.10 0.12 2.19 0.06 0.07 0.02 0.03 0.03 0.07 0.05 0.03 0.13 1.21 0.10 0.22 0.21 0.76 0.04 0.01 0.21 0.19 0.03 0.04 0.09 0.05 0.11 34.94 0.08 0.03 0.28 1.27 0.51 0.01 0.96 0.34 0.20 0.82 0.41 0.91 25.53 0.40 1.02 0.70 0.16 0.34 0.47 0.13 0.37 46.97


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Table 3. Yields of the aromatic compounds in the C5-C10 fraction obtained in the pyrolysis of tyres at 500 ºC. Compound benzene toluene ethylbenzene xylenes styrene propylbenzene 1-ethyl-2-methylbenzene 1-methylethenylbenzene 1,2,3-trimethylbenzene 1-methyl-4-(1-methylethyl)benzene 1H-indene 1-ethyl-2,3-dimethylbenzene 1-methyl-4-(1-methylethenyl)benzene napthalene total C5-C10 aromatics

wt% 0.21 0.78 0.53 1.23 2.17 0.10 0.05 1.56 0.40 0.69 3.08 0.14 1.13 0.24 6.62

The heavy fraction or tar, made up of C11+ components, accounts for 25.6 wt% of the original tyre. The characterization of the heavier fraction of the liquid is much more complex than the light fraction. In the C5-C10 fraction the unidentified compounds represent less than 1 wt% but in the C11+ fraction are about 7 wt%. Tables 4 and 5 show the yields of identified non aromatic and aromatic compounds respectively in the C11+ fraction in the thermal pyrolysis of tyres at 500 ºC. Figure 3 shows the yield obtained in the tar fraction according to six groups of components: non-aromatics, aromatics, sulphur compounds, nitrogen compounds, oxygenates and unidentified components To complete the characterization of the pyrolysis liquid Table 6 shows the hetereoatomic compounds obtained in tyre pyrolysis of tyres at 500 ºC, and also the simulated distillation and the gross calorific value have been studied. The simulated distillation has been carried out in a Perkin-Elmer 8500 gas chromatograph provided with a HT Simdist simulated distillation column. Figure 4 shows the results obtained. Thus, the light fraction (boiling point under 200 ºC) is the most important in the pyrolysis liquid and accounts for 60 wt% of the whole liquid fraction. The medium fraction, 200 < BP < 350 ºC, accounts for 30 wt% and the heavier fraction with a boiling point above 350 ºC only accounts for 5 wt%. These results are similar to those obtained by other authors (Cunliffe and Williams, 1998; Laresgoiti et al., 2002). -1

The gross calorific value obtained for the tyre pyrolysis liquid is 43 MJ kg , which is a result of the same order as those reported in the literature (Laresgoiti et al., 2002; Roy et al., 1990). The amount of char obtained is slightly higher than the sum of the original carbon black plus the inorganic components of the tyre. This implies that there is a certain degree of coking or degraded tyre deposition on the carbon black. The char obtained is of low quality (surface area of 80 m2g-1) for its use as carbon black or active carbon. For this purpose, it requires activation with steam of carbon dioxide (Murillo et al., 2005; Gonzalez et al., 2006; Pantea et al., 2003). The aforementioned results for yields and composition of gaseous, liquid and solid products lie between those of the literature corresponding to the fluidized bed under atmospheric pressure (Conesa et al., 1996; Kaminsky and Mennerich, 2001) and those of vacuum pyrolysis (Roy et al., 1999). This is explained by a more efficient gassolid contact than in a fluidized bed, with higher values for heat and mass transfer coefficients. In addition to achieving high yields at temperatures slightly lower than those of a fluidized bed reactor, the reduced residence time of the gas contributes to minimizing the secondary reactions, as pursued in vacuum pyrolysis.



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Table 4. Non aromatic compounds in the C11+ fraction. Compound 6,6-dimethyl-2-vinylidenbicyclo[3.1.1]heptane 3-[1,3-buthadienyl]-4-vinylcyclopentene 1,5-diethenil-3-methyl-2-methylencyclohexane 5,6-diethenylcyclooctene 1,3-cyclododecadiene dodecane 7-isopropenylbiciylo[4.2.1]nona-2,4,7-triene 1,2-di-buth-2-enylcyclohexane 2,3,5,8-tetramethyl-1,5,9-decatriene 2,6,10-trimethyl-1,5,9-undecatriene 1-ethenyl-1-methyl-2,4-1-methylethenylcyclohexane 1,5-diethenyl-2,3-dimethylcyclohexane tridecane 10-methyl-endotricyclo[5.2.1.0(2,6)]decane bi-1-cycloocten-1-yl tetradecane pentadecane 4-methylen-2,8,8-trimethyl-2-vinylbicyclo[5.2.0]nonane hexadecane 1-ethenyl-1-methyl-2-(1-methylethenyl)-4-(1-methylethyliden)cyclohexane octadecane nonadecane eicosane unidentified total C11+ non-aromatics

wt% 0.35 0.32 1.60 0.03 0.90 0.04 0.05 0.76 0.74 0.43 0.59 0.21 0.57 0.25 0.20 0.65 0.65 0.97 1.20 1.05 0.26 0.37 0.28 10.47 22.92

Table 5. Aromatic compounds in the C11+ fraction. Compound 4-pentenylbenzene 1-ethenyl-4-ethylbenzene 3-pentenylbenzene pentylbenzene 1,2-dimethyl-1-propenylbenzene 1-methyl-3-(1-methyl-2-propenyl)benzene 2-methyl-1-butenylbenzene cyclopentylbenzene 4-hexenylbenzene 3-methylcyclopentylbenzene 4,7-dimethyl-1H-indene hexylbenzene 3-methyl-4-pentenylbenzene 4-pentynylbenzene 1-cyclopentynylbenzene 1-methylnaphthalene 1-(1,1-dimethylethyl)-4-ethenylbenzene 3-cyclohexenylbenzene biphenyle 1-metylpenta-1,3-dienylbenzene 1-phenylbicyclo[2.1.1]hexane 2-cyclohexen-1-ylbenzene 2,3,6-trimethylnaphthalene octahydro-3,5,5-trimethyl-9-methylen-1H-benzocycloheptene 1-phenyl-3,4-divinylcyclohexane 1,1'-(1,3-propanodiyl)benzene 1-methyl-7-(1-methylethyl)phenanthrene total C11+ aromatics

wt% 0.52 0.87 0.69 0.05 0.44 0.86 0.17 0.86 0.05 0.53 0.22 0.45 0.21 0.85 0.28 0.08 0.13 0.20 0.03 0.01 0.04 0.03 0.14 0.18 0.43 0.61 0.06 9.01



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Table 6. Heteroatomic compounds obtained in tyre pyrolysis at 500 ºC. Compound Sulphur Compounds 2-methylthiophene benzothiazol Nitrogen compounds 1,2-dihydro-2,2,4-trimethylquinoline 4,8,12-trimethyl-3,7,11-tridecatrienonitrile N-phenyl-2-naphthalenamine N-(1,3-dimethylbuthyl)-N'-phenyl-1,4-benzenediamine Oxygen compounds Hexadecanoic acid Octadecanoic acid Octahydro-1,4a-dimethyl-7-(1-methylethyl)-1-phenanthrenocarboxilic acid 3,7,11-trimethyl-1,6,10-dodecatrien-3-ol 3,7,11-trimethyl-2,6,10-dodecatrien-1-ol ethenylester hexadecanoic acid total heteroatomic compounds

wt% 1.39 0.18 1.21 1.33 0.33 0.03 0.67 0.30 6.08 1.91 0.47 0.31 1.72 1.31 0.36 8.80

14 12

Yields (%)

10 8 6 4 2

A

ro

m

at ic U s ni d Su en tif lfu ie r N d co itr m og pu en nd co s m po un ds O xy ge na te s

N on

-a ro

m

at ic s

0

Figure 3. Product distribution in the tar fraction for different groups of components .



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70 Bp < 200ºC

60

Yields

50 40

200ºC < Bp < 350ºC

30 20 10

Bp > 350ºC

0 Figure 4. Simulated distillation of the tyre pyrolysis liquid obtained at 500 ºC.

4. CONCLUSIONS The conical spouted bed reactor (CSBR) is an interesting technology for the pyrolysis of scrap tyres, due to the excellent hydrodynamic qualities and to the efficient heat transfer between phases in the spouted bed, apart from other characteristics of the conical geometry of the reactor, such as its versatility in gas and solid flowrates under stable and isothermal conditions in the bed. Moreover, pyrolysis in a CSBR does not have problems related to particle fusion or agglomerate formation and it only requires a small amount of sand to help solid flow. Batch operation at 500 ºC gives way to a yield of 3.1 wt% of gases, 30 wt% of non aromatic C5-C10 range hydrocarbons, 16.1 wt% of C5-C10 aromatics, 8.8 wt% of tar and 36 wt% of char. Although the liquid fraction has a rather high amount of aromatic components, its gross calorific value allows for using as a fuel of suitable quality by blending with standard commercial fuels.

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