Elsevier

Waste Management

Volume 113, 15 July 2020, Pages 404-412
Waste Management

Waste tire valorization by intermediate pyrolysis using a continuous twin-auger reactor: Operational features

https://doi.org/10.1016/j.wasman.2020.06.019Get rights and content

Highlights

  • The twin-auger reactor is suitable for the intermediate pyrolysis of waste tires.

  • The pyrolysis process was assessed using the central composite design.

  • The process was assessed to maximize the yield of tire pyrolysis oil.

  • The process was also assessed to minimize the yield of recovery carbon black.

  • Reactor temperature and mass flow rate are the most influential parameters.

Abstract

Pyrolysis can be regarded as a roadmap towards a circular and sustainable economy for waste tires (WT). This work investigates the operational characteristics of a novel twin-auger reactor to transform WT by intermediate pyrolysis into tire pyrolysis oil (TPO), recovery carbon black (rCB), and tire pyrolysis gas (TPG). The influence of four operating parameters: reactor temperature (X1), WT mass flow rate (X2), solid residence time (X3) and N2 volumetric flow rate (X4), was assessed in order to maximize the TPO yield (Y1), while keeping the rCB one (Y2) as low as possible. The experimental campaign was conducted based on central composite design (CCD). The analysis of variance (ANOVA) showed that X1 and X2 exhibit the highest statistical influence. An optimization of both responses resulted in TPO, rCB, and TPG yields of 45, 40 and 15 wt%, respectively, when the pyrolyzer is operated at 475 °C, 1.16 kg/h, 3.5 min and 300 mL/min. At these conditions, the resulting TPO showed contents of C, H, S, N and O around 88.2, 9.7, 1.3, 0.7 and <0.1 wt%, respectively, along with a heating value of 42.02 MJ/kg. The rCB is comprised of moisture, volatile matter, fixed carbon, and ash around 2.5, 3.7, 75.5, and 18.3 wt%, respectively; while the TPG was mainly composed of H2 (23.7 vol%) and CH4 (28.2 vol%). Overall, these results suggest that twin-auger pyrolyzers are well suited for valorizing WT by intermediate pyrolysis.

Introduction

The proper management of all the streams of wastes generated daily by humankind could be regarded as one of the largest challenges in the world. The increasing generation of wastes, along with the lack of suitable management practices, are leading to severe problems associated with the environment and human health. As such, a reasonable solution for the handling of municipal solid wastes (MSW) is their proper valorization. From a chemical/energetic standpoint, there is an intrinsic value in such materials given the contents of both carbon and hydrogen. Therefore, instead of treating MSW as waste materials, they should be considered as beneficial resources. This shift in the categorization of MSW would contribute to the circular economy framework, whereby their value is retained either by reuse, recycling, repurposing and up-cycling with aspirations towards zero waste (Osman et al., 2019). MSW comprise a number of solid waste streams, a large proportion of which include waste tires (WT).

Approximately 17 M tons of WT were generated in 2012 worldwide (Forrest, 2014, Martínez et al., 2013a). Currently, strong economies such as the US, the EU (including Norway, Serbia, Switzerland and Turkey), and Japan are generating around 4.0, 3.9 and 1.0 M tons, respectively (WBCSD, 2018). These numbers are expected to increase, since the generation of MSW is directly proportional to population and gross domestic product growth (Rajaeifar et al., 2017). For instance, the tire production market alone is projected to reach 2500 M units by 2022 (Global Industry Analyst Inc, 2018). Although developed economies possess effective management systems to deal with the WT generated, including energy and material recovery, disposal into illegal stockpiles and landfills is still a common practice in developing economies. Furthermore, given the feedstocks used for their manufacture, WT are quite valuable to be merely used for heat production in combustion facilities.

In addition to occupying a large amount of space and exhibiting non-biodegradable qualities, WT are very difficult to transform since they comprise several complex materials, including non-melting thermoset polymers. As such, the same properties that render tires as a desirable product also encumber their final disposal and reprocessing. Hence, there is a growing desire for developing sustainable methods not only to properly dispose WT, but also to valorize them in the circular economy framework. Waste-to-energy (WtE) processes such as combustion, gasification, and pyrolysis, are beneficial ways to deal with the economic and environmental concerns related to MSW, while recovering energy, materials, and/or chemical products. Therefore, these processes can become a key instrument for circular economy implementation when wastes (e.g. WT) are transformed into useful products and services with minimal environmental effects. Stimulating the transition towards a circular economy in order to boost sustainable and resource-efficient policies for long-term socio-economic and environmental benefits is currently a global priority (Maina et al., 2017), in line with the sustainable development goals of the United Nations.

If done properly, the pyrolysis of WT can pave the way towards a sustainable economy for this type of waste. The problems associated with: i) waste generation, ii) rapid depletion of non-renewable resources, and iii) rising public concerns related to climate change have attracted the attention of pyrolysis as a green path for the conversion of WT into useful products. Pyrolysis is a thermochemical process performed under an oxygen-free atmosphere from which not only energy, but also materials can be recovered. When applied to WT, this process produces liquids and gases from both the natural and synthetic rubbers used in tire manufacture. In addition, a carbonaceous solid fraction is obtained, in which all carbon blacks (CB) and minerals incorporated into the tire are trapped. In short, the pyrolysis of WT produces: i) tire pyrolysis oil (TPO), ii) tire pyrolysis gas (TPG) and iii) recovery CB (rCB). Further, process conditions and degree of control over the process drastically influence the yields and characteristics of products derived from pyrolysis.

Several reactor designs have been reported to carry out the pyrolysis of WT (Lewandowski et al., 2019). Likewise, one of the most important specifications is the mode of operation, which can be either batch or continuous. The former commonly uses the fixed bed reactor, while the latter can be performed through fluidized beds, spouted beds, rotary kilns, or auger reactors, among others. The advantages of the continuous pyrolysis in steady state are multifold, including: the ease of operation, consistent product characteristics, low labor cost, compact design, short processing time, and flexibility in adjusting processing conditions (Qureshi et al., 2018). To date, the continuous pyrolysis of WT has been mainly performed through fluidized beds (Kaminsky and Mennerich, 2001, Raj et al., 2013), rotary kilns (Li et al., 2004, Yazdani et al., 2019), and spouted beds (Lopez et al., 2009, López et al., 2010). Even though some studies have considered the single-auger configuration for conducting the pyrolysis of WT (Aylón et al., 2010, Martínez et al., 2013b), the twin-auger configuration has yet to be explored.

Auger reactors are currently receiving special attention in the field of pyrolysis due to their potential to transform a wide range of feedstocks. In addition, they have been recognized as one of the technologies with better strengths not only for fast pyrolysis, but also for intermediate or slow pyrolysis (Campuzano et al., 2019). An essential advantage of auger pyrolyzers is the low specific size and ease of portability, thus enabling their use where the feedstock is generated/concentrated. Mobile pyrolysis reactors enable a decentralized transformation of feedstock; hence, this technology seems promising for reducing the costs associated with transportation, management, and logistics of the feedstock. In addition, the operating parameters in auger pyrolyzers are easy to control, and they require little-to-no carrier gas and minimal energy requirements (Campuzano et al., 2019, Lewandowski et al., 2019, Qureshi et al., 2018). These reactors also present multiple advantages for conducting co-pyrolysis or catalytic in-situ pyrolysis processes not only due to the mixing and heat transfer conditions, but also to the excellent control of catalyst-to-feedstock ratio.

The aim of the present investigation was to determine the operational characteristics of a novel twin-auger reactor to transform WT by intermediate pyrolysis. Accordingly, a set of experiments, established based on an experimental design, was performed in order to optimize the operating parameters for maximizing TPO yield while minimizing rCB yield. In addition, some basic properties of the derived TPO, rCB, and TPG at the optimized conditions were determined. According to the best authors’ knowledge, this is the first time that the operational features of this pyrolyzer are addressed when WT are used as feedstock.

Section snippets

Feedstock characterization

The feedstock used in the experimental campaign was a non-specific mixture of granulated WT supplied by a Colombian WT recycling company. The feedstock was mainly comprised of rubber without steel thread and textile netting and the particle size was between 2 and 4 mm. Elemental analysis was carried out in a Thermoscientific flash 2000 instrument using the standards ASTM-D5622-95 and UNE-EN-15407. Proximate analysis was conducted according to EPA-160.3 SM-2540-G, ASTM D1506, for determining

Feedstock characterization

As expected, the WT used in this work shows remarkable contents of carbon (82.4 wt%, dry basis) and hydrogen (8.2 wt%, dry basis), resulting in a HHV (38.69 MJ/kg), which is greater than those of middle and high-rank coals. For instance, anthracite coals exhibit a heating value around 35 MJ/kg (Mastalerz et al., 2011). It is worth to point out the low content of oxygen (0.8 wt%, dry basis), which strongly favors the WT conversion by pyrolysis, leading to excellent physical–chemical properties

Conclusions

This investigation sought to optimize the operational features of a twin-auger pyrolyzer used to valorize WT into TPO, TPG and rCB. As such, an experimental design was conducted in order to identify and quantify the effects of four key operational features (reactor temperature, WT mass flow rate, solid residence time, and N2 volumetric flow rate) on two key products: the yields of TPO and rCB. The process herein studied was addressed aimed at maximizing the TPO yield, while minimizing the rCB

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to express their sincere gratitude to COLCIENCIAS for the financial support given by the research project (1210-715-51742). F. Campuzano is in debt to COLCIENCIAS for the PhD scholarship (757-2016). D. Muñoz-Lopera expresses his gratitude to CIDI-UPB and its program “Formación Investigativa” for the MSc scholarship. The authors are also deeply grateful to GIMEL group from UdeA and Mr. Cristian Ortiz-Córdoba for their valuable support in the experimental campaign.

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