Pyrolysis of Plastic Waste for Feedstock Recovery

The different recovery options available for plastics waste might be summarized as follows  

The products are recovered from the MS W stream via curbside collection, separated/ sorted at MRFs, and then cleaned and ground into chips at plastic reclaiming facilities to be remelted into recycled resin pellets. The recycled resin is used, mixed with virgin plastics, in the fabrication of plastic products. Mechanical recycling works best when applied to source-separated streams of waste plastics products. 

Waste plastic is changed by heat or chemical agents into chemicals that might be used in the production of new polymers or as general chemical feedstocks (Al-Salem et al., 2010). Monomer recovery works well with source-separated plastics whereas general feedstock recovery is better suited for mixed plastic waste. 

Energy recovery can be either via incineration or via biological treatment  

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Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

Feedstock recycling is examined as a method of plastics recovery. The range of techno logics currently employed are described, and include pyrolysis, hydrogenation, gasification, and chemolysis. Methods for the recycling of mixed plastics wastes are discussed, which include work by BP Chemicals, VEBA Oil, Shell Chemicals and Leunawerke. 

Waste materials such as municipal solid waste, scrap tires, and waste plastics have traditionally been placed in sanitary landfills. However, with landfill space rapidly decreasing in the United States and worldwide, an alternative disposal method for these waste materials becomes imperative. The recycling of solid wastes is a challenging problem, with both economic and environmental constraints. Recently, two broad approaches have been attempted to reclaim solid wastes. The first approach relies on thermal or catalytic conversion of waste materials into fuel and valuable chemical feedstocks. Examples of this approach include gasification, pyrolysis, depolymerization, and liquefaction. The second approach relies on the physical recovery of valuable ingredients in the waste materials. 

Gasification and pyrolysis are the thermal conversion processes available for the thermal treatment of solid wastes. As shown in Figure 8.3, different by-products are produced from the application of these processes and different energy and matter recovery systems can be used to treat these products [16, 27]. Both pyrolysis and gasification produce three different phases a solid phase (char, 5-25 wt%), liquid phase (tars, 10-45 wt%) and gas phase [14, 22, 28]. The main disadvantages of plastic pyrolysis and gasification are the necessity to control the chloride content in the feedstock and the risk of bad fluidisation because of particle agglomeration [29]. 

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