M.L.Mastellone Resources,Conservation Recycling:X 4(2019)100017 referring to the gate fee only (i.e.transportation not included),reached (Adrados et al.,2012;Haufe et al.,2004;Sogancioglu et al.,2017). very high values such as 145C/t(C.E.A.SpA,private communication, 2019). The huge increase of cost to dispose this kind of waste in Europe was 1.2.Feedstock recycling of polymers due to the so called "plastic ban"of China,following the so-called Green Fence,that introduced,for the first time after decades,quality Polymers are the main component of the "plastics family";they are specifications for secondary materials imported from Europe so dra- constituted by a repeating structure of monomers basically composed matically limiting the plastic waste amount export from European by carbon and hydrogen and,in some cases,by heteroatoms like Countries (Brooks et al.,2018).The restriction of use of landfilling for oxygen,nitrogen,chlorine,...they are generally classified,according plastic waste imposed by the European regulation was another key to their structure and properties,on the basis of thermal-mechanical point in the raising of disposal economic cost. behaviour and on the basis of their processing characteristics in ther Nowadays,it is urgent to strengthen the industrial network devoted moplastics,elastomers and thermosets.They can be also be classified to the plastics'recovery and recycling by including processes that asks according to their mechanism of polymerization as either addition or for a lower degree of purity.The feedstock and the chemical recycling condensation,where: processes,once integrated in the recycling system,allow to use the same "equivalent petroleum amount"several times:as material,feed- a polyaddition consists in the repeating of the same monomer along stock and fuel. the chain; The mechanical recycling of plastics should be preferred when a b condensation requires instead the bond between two different mo- mono-material collection of plastics must be treated,since the cost of lecules. the separation processes,as carried out in the material recovery facil- ities,is very high.Mechanical recycling consists of a series of physical Thermoplastic polymers such as PE,PP,PVC,PS are examples of operations where the recovered material is shredded,washed,melted polyaddition polymers;PET is an example of polycondensed polymer. and re-pelletized.In the case the mechanical recycling is not possible or The polyaddition polymers,with the exception of PVC that has a convenient it is possible to refer to the feedstock and chemical recycling peculiar behaviour(Sheirs and Kaminsky,2006)can undergo thermo- and,as last option,to the energy recovery processes.This last option is lysis in a controlled environment by producing a large spectrum of largely applied today for all the plastics that are not separately collected hydrocarbons having a number of carbon ranging from 1 (methane)to and for plastics that cannot be mechanically recycled.In fact,the het- around 20.The thermolysis of plastic waste is in fact oriented to recover erogeneous mixture of plastic contaminated with other components raw materials for petrochemical industries by means of processes such (such as paper,biowaste,textiles,etc.)is sent to combustion process as liquid and gas phase hydrogenation,steam-cracking,catalytic due to their large high-heating value(about 31.8 MJ/kg for a house. cracking,pyrolysis,coking and gasification. hold plastic mixture)(Themelis et al.,2011).Once energy recovery is A classification of thermolysis process into "feedstock recycling" applied,no other recovery is possible;in order to increase the number and "chemical recycling"(sub-category of feedstock recycling)also of life of fossil carbon,the preferred option is the material recovery that exists with reference to the different process outputs that are obtained. can be obtained by applying mechanical reprocessing and feedstock/ Chemical recycling consists in the depolymerization of certain con- chemical processes densation or addition polymers back to monomers.The chemical re- In all cases the mechanical recycling cannot be applied it is possible cycling allows the re-creating of the chemicals from which the polymers and convenient use the above cited alternative routes (Czajczynska were initially made.If the treatment breaks the polymers into an as- et al.,2017;Demirbas,2004;Panda et al.,2010;Perugini et al.,2005). sortment of chemical species,it can be decided whether to recover In particular,thermolysis processes of selected polymers and plastic specific chemicals for feedstock use or to use the assortment of chemical waste mixture can lead to very good performances in term of energy species for fuel or to use some combination of both end products;in this recovery with a limited environmental impact.Most important,the case the process lays in the more general definition of feedstock re- pyrolysis and gasification processes can be applied even at smaller scale cycling.A special class of feedstock recycling processes yields an im- by making possible the integration with other facilities;for instance, portant raw material called syngas (=synthesis gas,a mixture of hy- gasifiers and pyrolizers can be installed with thermal input capacities drogen and carbon monoxide):in this case the common name to from 250 kW to several megawatts,by requiring small footprints and by indicate the thermal conversion process is "gasification".This latter favouring the real circular economy at local and regional scale.Several process is a carried out in an oxidative environment where the oxygen studies have been published on these processes applied to plastic waste content is much less than the stoichiometric demand for complete and waste in general.Gasification processes differ for the applied combustion (Gartzen et al.,2018;Mastellone,2015). technology of main reactor (gasifier),the method to minimize the tar Hydrocarbons and syngas can be used as chemical feedstocks for formation,the cleaning/conditioning of syngas and its use.Gasification further upgrading to commercial products at oil refineries and chemical can be applied to heterogeneous plastic waste with good performances plants. in term of syngas yield and cold gas efficiency (Gershman and B.I. The plastic conversion into a sort of synthetis crude oil (syncrude) 2013;Lopez et al.,2018).Pyrolysis of plastics aims to obtain preferably can be obtained by using commercially available technologies that are materials instead of energy or fuels.In this case the feeding composition reported and compared in term of reactor technology,process type is limited by strict specifications.The most applied and studied process (thermal/catalytic),yields of products,capacities.A list of suppliers for material recovery from plastic waste is pyrolysis of polyolefins that developed the above cited catalytic and non-catalytic thermolysis is reported in the Table 1.The common point of the largest part of the listed technologies is the plastic feedstock specifications:all the poly- 1High-heating value (HHV)indicates the energy content of one unit mass of olefins can be accepted,polycondensed polymers must be avoided, matter that can be released during oxidation.HHV is an intrinsic property of cellulosic materials and moisture must be limited as much is possible. matter since it is correlated to chemical composition.The low-heating value (LHV)is obtained starting from the HHV value by taking into account that the The differences between the technologies are related to the reactor used hydrogen contained in the matter is transformed into water at standard con- for thermolysis,the presence or not of a catalyst and the maximum ditions(25'C)but,since the real temperature reached by oxidation is much capacity of a single reactor that normally does not exceed 25.000 ton/ larger than 100'C,the water is actually present under form of gas.The phase year. transition requires an amount of heat (2257J/g)that is subtracted to HHV by leading to the LHV.LHV is also an intrinsic property of molecules/compounds.referring to the gate fee only (i.e. transportation not included), reached very high values such as 145€/t (C.E.A. SpA, private communication, 2019). The huge increase of cost to dispose this kind of waste in Europe was due to the so called “plastic ban” of China, following the so-called Green Fence, that introduced, for the first time after decades, quality specifications for secondary materials imported from Europe so dra￾matically limiting the plastic waste amount export from European Countries (Brooks et al., 2018). The restriction of use of landfilling for plastic waste imposed by the European regulation was another key point in the raising of disposal economic cost. Nowadays, it is urgent to strengthen the industrial network devoted to the plastics’ recovery and recycling by including processes that asks for a lower degree of purity. The feedstock and the chemical recycling processes, once integrated in the recycling system, allow to use the same "equivalent petroleum amount" several times: as material, feed￾stock and fuel. The mechanical recycling of plastics should be preferred when a mono-material collection of plastics must be treated, since the cost of the separation processes, as carried out in the material recovery facil￾ities, is very high. Mechanical recycling consists of a series of physical operations where the recovered material is shredded, washed, melted and re-pelletized. In the case the mechanical recycling is not possible or convenient it is possible to refer to the feedstock and chemical recycling and, as last option, to the energy recovery processes. This last option is largely applied today for all the plastics that are not separately collected and for plastics that cannot be mechanically recycled. In fact, the het￾erogeneous mixture of plastic contaminated with other components (such as paper, biowaste, textiles, etc.) is sent to combustion process due to their large high-heating value1 (about 31.8 MJ/kg for a house￾hold plastic mixture) (Themelis et al., 2011). Once energy recovery is applied, no other recovery is possible; in order to increase the number of life of fossil carbon, the preferred option is the material recovery that can be obtained by applying mechanical reprocessing and feedstock/ chemical processes. In all cases the mechanical recycling cannot be applied it is possible and convenient use the above cited alternative routes (Czajczyńska et al., 2017; Demirbas, 2004; Panda et al., 2010; Perugini et al., 2005). In particular, thermolysis processes of selected polymers and plastic waste mixture can lead to very good performances in term of energy recovery with a limited environmental impact. Most important, the pyrolysis and gasification processes can be applied even at smaller scale by making possible the integration with other facilities; for instance, gasifiers and pyrolizers can be installed with thermal input capacities from 250 kW to several megawatts, by requiring small footprints and by favouring the real circular economy at local and regional scale. Several studies have been published on these processes applied to plastic waste and waste in general. Gasification processes differ for the applied technology of main reactor (gasifier), the method to minimize the tar formation, the cleaning/conditioning of syngas and its use. Gasification can be applied to heterogeneous plastic waste with good performances in term of syngas yield and cold gas efficiency (Gershman and B.I., 2013; Lopez et al., 2018). Pyrolysis of plastics aims to obtain preferably materials instead of energy or fuels. In this case the feeding composition is limited by strict specifications. The most applied and studied process for material recovery from plastic waste is pyrolysis of polyolefins (Adrados et al., 2012; Haufe et al., 2004; Sogancioglu et al., 2017). 1.2. Feedstock recycling of polymers Polymers are the main component of the “plastics family”; they are constituted by a repeating structure of monomers basically composed by carbon and hydrogen and, in some cases, by heteroatoms like oxygen, nitrogen, chlorine, …; they are generally classified, according to their structure and properties, on the basis of thermal-mechanical behaviour and on the basis of their processing characteristics in ther￾moplastics, elastomers and thermosets. They can be also be classified according to their mechanism of polymerization as either addition or condensation, where: a polyaddition consists in the repeating of the same monomer along the chain; b condensation requires instead the bond between two different mo￾lecules. Thermoplastic polymers such as PE, PP, PVC, PS are examples of polyaddition polymers; PET is an example of polycondensed polymer. The polyaddition polymers, with the exception of PVC that has a peculiar behaviour (Sheirs and Kaminsky, 2006) can undergo thermo￾lysis in a controlled environment by producing a large spectrum of hydrocarbons having a number of carbon ranging from 1 (methane) to around 20. The thermolysis of plastic waste is in fact oriented to recover raw materials for petrochemical industries by means of processes such as liquid and gas phase hydrogenation, steam-cracking, catalytic cracking, pyrolysis, coking and gasification. A classification of thermolysis process into “feedstock recycling” and “chemical recycling” (sub-category of feedstock recycling) also exists with reference to the different process outputs that are obtained. Chemical recycling consists in the depolymerization of certain con￾densation or addition polymers back to monomers. The chemical re￾cycling allows the re-creating of the chemicals from which the polymers were initially made. If the treatment breaks the polymers into an as￾sortment of chemical species, it can be decided whether to recover specific chemicals for feedstock use or to use the assortment of chemical species for fuel or to use some combination of both end products; in this case the process lays in the more general definition of feedstock re￾cycling. A special class of feedstock recycling processes yields an im￾portant raw material called syngas (=synthesis gas, a mixture of hy￾drogen and carbon monoxide): in this case the common name to indicate the thermal conversion process is “gasification”. This latter process is a carried out in an oxidative environment where the oxygen content is much less than the stoichiometric demand for complete combustion (Gartzen et al., 2018; Mastellone, 2015). Hydrocarbons and syngas can be used as chemical feedstocks for further upgrading to commercial products at oil refineries and chemical plants. The plastic conversion into a sort of synthetis crude oil (syncrude) can be obtained by using commercially available technologies that are reported and compared in term of reactor technology, process type (thermal/catalytic), yields of products, capacities. A list of suppliers that developed the above cited catalytic and non-catalytic thermolysis is reported in the Table 1. The common point of the largest part of the listed technologies is the plastic feedstock specifications: all the poly￾olefins can be accepted, polycondensed polymers must be avoided, cellulosic materials and moisture must be limited as much is possible. The differences between the technologies are related to the reactor used for thermolysis, the presence or not of a catalyst and the maximum capacity of a single reactor that normally does not exceed 25.000 ton/ year. 1 High-heating value (HHV) indicates the energy content of one unit mass of matter that can be released during oxidation. HHV is an intrinsic property of matter since it is correlated to chemical composition. The low-heating value (LHV) is obtained starting from the HHV value by taking into account that the hydrogen contained in the matter is transformed into water at standard con￾ditions (25°C) but, since the real temperature reached by oxidation is much larger than 100°C, the water is actually present under form of gas. The phase transition requires an amount of heat (2257J/g) that is subtracted to HHV by leading to the LHV. LHV is also an intrinsic property of molecules/compounds. M.L. Mastellone Resources, Conservation & Recycling: X 4 (2019) 100017 2