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