An energy-saving and environment-friendly technology for debromination of plastic waste: Novel models of heat transfer and movement behavior of bromine

Highlights

• Infrared vacuum heating was applied for debromination of brominated resin.

• The critical temperature of brominated resin was determined.

• Heat transfer models were constructed to study the heat distribution of pyrolysis.

• The movement behavior of bromine after pyrolysis was studied.

• The energy consumption of infrared heating for debromination was estimated.

Abstract

The recovery and reuse of waste brominated resin, which is a typical plastic waste, is troublesome because it contains toxic brominated flame retardants. Conventional pyrolysis of brominated resin was suggested to be an effective approach for debromination. However, conventional pyrolysis caused high energy consumption and high yield of toxic volatiles. An energy-saving and environment-friendly technology called infrared heating was reported in this study. According to computation of the developed heat transfer models, the critical debromination temperature was 260 °C in infrared heating, which was 271 °C lower than conventional pyrolysis. Meanwhile, no volatile product appeared in the reported technology. In the pyrolysis residue after infrared heating, bromine concentrated orientationally in the fixed and limited area on the resin particles. Free radicals, such as •CH₃, H•, and Br•, were combined with Br• generated in infrared heating to form the concentrated bromine. Compared to the chaotic distribution of bromine in conventional pyrolysis, the orientational concentration of bromine was a progress for removing and collecting bromine in infrared heating. Moreover, compared to conventional pyrolysis, infrared heating could decrease 76.2% energy consumption. This work contributed to provide the novel technology for recovery of plastic wastes

Introduction

The production of global plastics was estimated to be 359 million tonnes in 2018 (Tournier et al., 2020). With low degradability and recyclability, about 150–200 million tonnes of plastic waste accumulated both on land and in the oceans (Geyer et al., 2017, Jambeck et al., 2015). On one hand, waste plastic debris directly killed hundreds of thousands of marine animals by ingestion or entanglement (Rochman et al., 2013). On the other hand, toxic additives in plastic waste, such as flame retardants and heavy metals, entered into environmental medium, even in human bodies (Tang et al., 2014, Imhof et al., 2016). Therefore, recovery of plastic waste became a significant research direction. Recent research pointed several ways towards recovery of plastic waste, including granulating, reusing as fillers, biodegradation, chemical decomposition and pyrolysis (Al-Salem et al., 2009). Only thermoplastics can be recovered by granulation, while thermosetting plastics cannot be granulated (Garcia and Robertson, 2017). Waste brominated resin from crushed waste printed circuit boards was a typical thermosetting plastic waste, containing resins, glass fibers, flame retardants and curing agents (Guo et al., 2008, Guo et al., 2010). About 200 thousand tons of brominated resin from crushed waste printed circuit boards are generated annually worldwide, of which China accounts for a fifth (Ghosh et al., 2015). Open incineration, landfill, and piling of brominated resin lead to migration of pollutants to soil, water, and air (Li et al., 2018, Liu et al., 2020). The environmental risks are mainly induced by brominated flame retardants which are bioaccumulative and could have negative effects on the secretory and nervous systems of human bodies (Li et al., 2019). Therefore, debromination was paid much attention to in the recovery and reuse field of brominated resin of plastic waste.

Waste brominated resin was reused as fillers by mechanical approaches or as superabsorbent resin by chemical synthesis, but the leaching of brominated flame retardants made fillers risky to be applied (Zheng et al., 2009, Guo et al., 2009, Guo et al., 2012, Zhang and Zhang, 2018). Chemical recovery could remove bromine by depolymerization of brominated resin (Onwudili and Williams, 2009, Xing and Zhang, 2013, Zhang and Zhang, 2012). However, the reaction condition needs high temperature and the waste chemical liquid is difficult to treat (Chien et al., 2000). The parameters of mechanochemical treatment of brominated resin are hard to control, showing an unstable debromination rate (Cagnetta et al., 2016; Chen et al., 2020b). Pyrolysis was considered as a promising method to recover brominated resin due to high efficiency and low pollution (Chen et al., 2018a; Ma et al., 2016). The components of pyrolysis product mainly depend on pyrolysis temperature and holding time (Chen et al., 2021). Pyrolysis at high temperature of 500–800 °C with holding time of more than 30 min was more likely to make bromine move to gas and oil (Yang et al., 2013). In order to improve the efficiency of fixing bromine, additives, such as metal oxides, calcium carbonate, and biomass, were added for co-pyrolysis (Chen et al., 2018b; Chen et al., 2020a; Gao and Xu, 2019; Wu and Qiu, 2015). On one hand, high temperature increases the yield of volatiles, of which components were complex and actually difficult to separate and reuse. On the other hand, high temperature and long holding time result in high energy consumption. Low pyrolysis temperature would decrease the yield of volatiles and energy consumption at the same time. Thus, it is necessary to find critical temperature between debromination and low yield of volatiles. Moreover, an energy-saving and environment-friendly technology have to be developed for debromination of brominated resin.

Compared with conventional pyrolysis, infrared heating has characteristics of high efficiency, low thermal inertia, and accurate temperature (Ozen and Singh, 2020, Huang et al., 2020). The high heating rate of infrared vacuum heating shortened the time of temperature rising so that key lime juice could maintain color and taste with a short holding time of 2.5 min (Aghajanzadeh et al., 2016). In recent years, researchers began to study the infrared heating process of biomass. When the holding time was 15 min, it was possible to complete the pyrolysis process of cedar and improve the proportion of glucose derivatives in pyrolytic products (Zhu et al., 2021). However, little research has studied heat transfer of infrared heating and no study has reported on applying infrared heating for recovery of brominated resin.

In this study, we developed a green and low-cost infrared heating technology for debromination of brominated resin. The critical temperature between debromination and low yield of volatiles was determined by analyzing thermogravimetric characteristics of brominated flame retardants and brominated resin. The heat transfer models were constructed for studying heat distributions in pyrolysis processes. The movement behavior of bromine after pyrolysis was studied. Finally, we compared the energy consumption of infrared vacuum heating with conventional vacuum pyrolysis. The aim of this study was to provide an energy-saving and environment-friendly technology for debromination of brominated resin of plastic waste.