Elsevier

Energy Conversion and Management

Volume 142, 15 June 2017, Pages 523-532
Energy Conversion and Management

Waste truck-tyre processing by flash pyrolysis in a conical spouted bed reactor

https://doi.org/10.1016/j.enconman.2017.03.051Get rights and content

Highlights

  • Flash pyrolysis improves TPO and limonene yields vs. slow pyrolysis.

  • The most suitable temperature for TPO and limonene maximization was 475 °C.

  • Operating at 575 °C gas yields and char quality was enhanced.

  • Temperature showed a limited effect on sulphur distribution among products.

Abstract

The flash pyrolysis of waste truck-tyres was studied in a conical spouted bed reactor (CSBR) operating in continuous regime. The influence of temperature on product distribution was analysed in the 425–575 °C range. A detailed characterization of the pyrolysis products was carried out in order to assess their most feasible application. Moreover, special attention was paid to the sulphur distribution among the products. The analysis of gaseous products was carried out using a micro-GC and the tyre pyrolysis oil (TPO) by means of GC-FID using peak areas for quantification, with GC/MS for identification and elemental analysis. Finally, the char was subjected to elemental analysis and surface characterization. According to the results, 475 °C is an appropriate temperature for the pyrolysis of waste tyres, given that it ensures total devolatilisation of tyre rubber and a high TPO yield, 58.2 wt.%. Moreover, the quality of the oil is optimum at this temperature, especially in terms of high concentrations of valuable chemicals, such as limonene. An increase in temperature to 575 °C reduced the TPO yield to 53.9 wt.% and substantially changed its chemical composition by increasing the aromatic content. However, the quality of the recovered char was improved at high temperatures.

Introduction

Polymers have become a basic product that guarantee the current standard quality of living. Among them, and due to the growing transport necessities of modern societies, a continuous increase in the use of rubbers for tyre production has been reported [1]. According to recent estimations, the annual global tyre production is around 1.4 billion units [2], which means one car tyre is approximately discarded per inhabitant per year in developed countries. Moreover, around 4 billion waste tyres are currently in landfills worldwide [3].

Therefore, proper waste tyre management represents a major environmental challenge, since the inappropriate disposal of this non-biodegradable polymer could lead to significant issues. Accidental fires are especially risky due to the emissions of hazardous compounds and the great difficulty of their extinction [3]. Moreover, dumped waste tyres promote breeding of various pests and insects.

Due to the relatively high heating value of waste tyres, direct combustion may be a promising energy recovery route, but this alternative faces significant restrictions due to the emissions of polycyclic aromatic hydrocarbons (PAHs), dioxins and particulate matter [4]. Accordingly, valorisation of waste tyres by pyrolysis has gained increasing attention over the last years due to its environmental and economic advantages [5]. More recently, the co-pyrolysis of waste tyres with other residues has also been regarded as a promising valorisation route [6], [7].

The pyrolysis or thermal degradation under inert conditions of waste tyres gives way to three main products; gases, tyre pyrolysis oil (TPO) and residual pyrolysis char. The gases produced from waste tyres are mainly made up of hydrocarbons and some H2, CO2, CO and H2S. Consequently, their main interest is energy production given that their heating value is high, usually above 35 MJ m−3 [1]. Gas fraction yield is enhanced at high temperatures and long residence times, as these conditions maximize secondary cracking at the expense of TPO. The main product obtained in the pyrolysis of waste tyres is the TPO, being a complex mixture of aliphatic and aromatic hydrocarbons and heteroatomic compounds [8]. This oil contains interesting chemicals in high concentration, in particular dl-limonene, if produced under suitable pyrolysis conditions [9]. Limonene is a valuable chemical and its market price is estimated at US$ 2 kg−1 [9]. The TPO can be used directly as a fuel, be it with some minor limitations, mostly related to its sulphur and aromatics content. However, its upgrading in existing refinery units allows overcoming these limitations [5]. The char derived from waste tyres consists of the carbon black added to tyre formulation, with varying adulteration degrees, depending on the pyrolysis conditions. In spite of the fact that its direct re-utilization is an attractive option, its sulphur and ash contents, as well as its morphology, particle size distribution and porosity, are far from commercial carbon black specifications [10].

A wide range of reactor configurations have been applied to the pyrolysis of waste tyres. Fixed bed reactors operating in batch regime have been commonly used in the literature [11], [12], [13], [14]. Despite their simple design and operation, this technology involves serious drawbacks for its large-scale development. Fluidised bed reactors are a more appropriate alternative for waste tyre pyrolysis, as they ease continuous operation and process scale-up. Moreover, their operation under fast pyrolysis conditions improves TPO yield [1], [3]. Therefore, this technology has been widely used in the literature [15], [16], [17]. Another commonly proposed reactor design is rotary kiln since it allows good control of the process variables, especially waste tyre residence time in the reactor [18], [19], [20].

The conical spouted bed reactor (CSBR) has demonstrated adequate performance in the pyrolysis of different waste materials [21], [22], [23], [24], [25]. This reactor is characterized by its vigorous solid circulation, which allows operating under isothermal condition, with almost perfectly mixed regime for the solid and high heat and mass transfer rates. Accordingly, the CSBR allows for handling sticky and irregular materials without operational problems. This versatility for handling solids of different nature makes the CSBR suitable for the utilization of a catalyst in situ, as was demonstrated in previous studies dealing with catalytic pyrolysis of waste tyres [26], [27]. Moreover, its short residence time for volatiles minimises secondary reactions in the gas phase, which enhances oil production in pyrolysis processes [21], [25]. The main advantages and drawbacks of the CSBR in relation to other pyrolysis technologies have been discussed in detail elsewhere [28].

In the present study, continuous pyrolysis of waste truck-tyres was performed in a CSBR fitted with a non-porous draft tube. The use of the draft tube significantly improved spouting regime performance, given that bed stability was increased and fluidizing gas requirements reduced [29]. In fact, its use is critical for the full-scale application of the spouted bed technology. The main aim was to determine the influence of temperature on product yields and their composition, with the intention of determining their most suitable application. Thus, a sulphur mass balance has been carried out to determine its distribution among pyrolysis products and the limitations caused by its content on their possible uses. In addition, the results were compared with those in a previous study carried out in a fixed bed reactor operated at the same pyrolysis temperature [13]. Given that in both studies the same tyre material was used, the influence of pyrolysis conditions, specially heating rate and residence time, on process performance can be determined. Furthermore, the interest and originality of the study lies in the use of waste truck-tyre as they have a different composition in relation to the more studied light vehicle tyres.

Section snippets

Equipment

Continuous tyre pyrolysis runs were carried out in a bench scale plant, whose scheme is shown in Fig. 1. The main component of the plant is the CSBR, whose design is based on the prior application of this technology to the pyrolysis and gasification of different solid wastes, such as biomass [21], [22], [30], plastics [23], [31] and waste tyres [24], [25]. The spouted bed reactor was heated by a two-independent section radiant oven that provides the heat to operate up to 900 °C. The lower

Effect of temperature on product distribution in the CSBR

The influence of temperature on the product distribution obtained from the continuous pyrolysis of waste truck-tyres was studied in the 425–575 °C range. The products were grouped into three fractions; (i) the gaseous products, including C1 to C5 hydrocarbons, CO, CO2 and H2S, (ii) the liquid fraction or TPO, which is made up of hydrocarbons heavier than C6, and (iii) char or residual carbon black.

Fig. 4 shows the evolution of the yields of pyrolysis products in the 425–575 °C range. The main

Conclusions

The continuous pyrolysis of waste truck-tyres was studied in a bench scale plant provided with a conical spouted bed reactor in the 425–575 °C range. This reactor is an interesting alternative for waste tyre valorisation given that it allows attaining flash pyrolysis conditions (high heating rates and short residence times), and consequently obtain high TPO yields. This fact was pointed out when the results were compared with a previous study performed with the same tyre rubber but under slow

Acknowledgements

This work was carried out with financial support from the Ministry of Economy and Competitiveness of the Spanish Government (CTQ2016-75535-R and CTQ2014-59574-JIN), the European Regional Development Fund (ERDF), the Basque Government (IT748-13) and the University of the Basque Country (UFI 11/39). Jon Alvarez also thanks the University of the Basque Country UPV/EHU for his postgraduate Grant (ESPDOC 2015). This work was also supported by the Recycling and Economic Development Initiative of

References (73)

  • C. Diez et al.

    Pyrolysis of tyres. Influence of the final temperature of the process on emissions and the calorific value of the products recovered

    Waste Manage

    (2004)
  • A. Cunliffe et al.

    Composition of oils derived from the batch pyrolysis of tyres

    J Anal Appl Pyrolysis

    (1998)
  • N.M. Mkhize et al.

    Effect of temperature and heating rate on limonene production from waste tyre pyrolysis

    J Anal Appl Pyrolysis

    (2016)
  • T. Kan et al.

    Fuel production from pyrolysis of natural and synthetic rubbers

    Fuel

    (2017)
  • W. Kaminsky et al.

    Pyrolysis of synthetic tire rubber in a fluidised-bed reactor to yield 1,3-butadiene, styrene and carbon black

    J Anal Appl Pyrolysis

    (2001)
  • R. Edwin Raj et al.

    Optimization of process parameters in flash pyrolysis of waste tyres to liquid and gaseous fuel in a fluidized bed reactor

    Energy Convers Manage

    (2013)
  • S. Luo et al.

    The production of fuel oil and combustible gas by catalytic pyrolysis of waste tire using waste heat of blast-furnace slag

    Energy Convers Manage

    (2017)
  • J. Alvarez et al.

    Sewage sludge valorization by flash pyrolysis in a conical spouted bed reactor

    Chem Eng J

    (2015)
  • M. Amutio et al.

    Flash pyrolysis of forestry residues from the Portuguese Central Inland Region within the framework of the BioREFINA-Ter project

    Biores Technol

    (2013)
  • G. Lopez et al.

    Thermochemical routes for the valorization of waste polyolefinic plastics to produce fuels and chemicals. A review

    Renew Sustain Energy Rev

    (2017)
  • J. Makibar et al.

    Pilot scale conical spouted bed pyrolysis reactor: draft tube selection and hydrodynamic performance

    Powder Technol

    (2012)
  • A. Erkiaga et al.

    Steam gasification of biomass in a conical spouted bed reactor with olivine and g-alumina as primary catalysts

    Fuel Process Technol

    (2013)
  • G. Lopez et al.

    Effect of polyethylene co-feeding in the steam gasification of biomass in a conical spouted bed reactor

    Fuel

    (2015)
  • T. Ishikura et al.

    Hydrodynamics of a spouted bed with a porous draft tube containing a small amount of finer particles

    Powder Technol

    (2003)
  • B. Lah et al.

    Pyrolysis of natural, butadiene, styrene-butadiene rubber and tyre components: modelling kinetics and transport phenomena at different heating rates and formulations

    Chem Eng Sci

    (2013)
  • B. Danon et al.

    Determining rubber composition of waste tyres using devolatilisation kinetics

    Thermochim Acta

    (2015)
  • G. Lopez et al.

    Kinetics of scrap tyre pyrolysis under vacuum conditions

    Waste Manage

    (2009)
  • E. Aylon et al.

    Assessment of tire devolatilization kinetics

    J Anal Appl Pyrolysis

    (2005)
  • G. Choi et al.

    Non-catalytic pyrolysis of scrap tires using a newly developed two-stage pyrolyzer for the production of a pyrolysis oil with a low sulfur content

    Appl Energy

    (2016)
  • G. Choi et al.

    Total utilization of waste tire rubber through pyrolysis to obtain oils and CO2 activation of pyrolysis char

    Fuel Process Technol

    (2014)
  • G. Lopez et al.

    Continuous pyrolysis of waste tyres in a conical spouted bed reactor

    Fuel

    (2010)
  • S. Ucar et al.

    Evaluation of two different scrap tires as hydrocarbon source by pyrolysis

    Fuel

    (2005)
  • M. Banar et al.

    Characterization of pyrolytic oil obtained from pyrolysis of TDF (Tire Derived Fuel)

    Energy Convers Manage

    (2012)
  • N. Akkouche et al.

    Heating rate effects on pyrolytic vapors from scrap truck tires

    J Anal Appl Pyrolysis

    (2017)
  • J.D. Martínez et al.

    Demonstration of the waste tire pyrolysis process on pilot scale in a continuous auger reactor

    J Hazard Mater

    (2013)
  • Z. Song et al.

    Microwave pyrolysis of tire powders: evolution of yields and composition of products

    J Anal Appl Pyrolysis

    (2017)
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