M.L.Mastellone Resources,Conservation Recycling:X 4 (2019)100017 the following calculations,a fraction of liquid product (syncrude)equal particularly for plastics (Mastellone and Arena,2008;Sjostrom et al., to 80%and NCG equal to 10%.The rest of the matter is converted into 1999). char and tar (Mastellone et al.,2002;Scheirs and Kaminsky,2006) The heat released by exothermal reactions increases the environ. The PtO plant (based on the Pruvia Fuels GmbH technology)is ment temperature until values depending by the equivalence ratio, preceded by a pre-treatment that removes moistures and halogens from proximate and ultimate analysis of waste and its net chemical energy. the plastic feedstock.This pre-treatment is carried out by using a The operating temperature of the waste gasifiers is generally in the combination between densifiers and degassing extruders(AMUT Group, range 850-1200'C depending by the above recalled parameters and by 2019).The densifiers allow to manage polyolefins having very light heat losses.The oxygen supplied to the gasifier is generally 25-40%of bulk density (LLDPE)and heterogeneous mechanical properties.The the stoichiometric demand.The reducing environment promotes the extrusion is a pre-treatment that allows to remove moisture with 99% partial oxidation of carbon and hydrogen element in the fuel by pro- efficacy if the initial content is less than 10%,to remove organic viding the heat necessary to the cracking of waste/fuel component.The chlorine by breaking the C-Cl bonds in the initial part of machine by fraction between the oxygen provided for gasification reactions and the allowing the degassing together with moisture,to bring the tempera- stoichiometric demand is known as equivalence ratio and its value ture up to 350C so favouring the input to the thermolysis reactor depends on ultimate analysis of the waste.The main product of gasi. (pyrolizer).Downstream the extruder,the following equipment is in- fication process is the synthesis gas also identified as "synthetic gas or stalled: syngas".This term is used when it is used as a feedstock for production of synthetic natural gas(SNG),Fischer-Tropsch liquids(FTL),hydrogen 1)Heated/insulated feeder to bring the plastic feedstock in the reactor or any other material or fuel;the term "producer gas"is used when the over the melting temperature gas is used to energy production.Anyway,whatever is the final utili- 2)Thermolysis reactor zation,the term syngas can be used to indicate the gaseous product of 3)Carbon/ash discharge system gasification.The syngas composition can vary depending on waste and 4)Cooling of pyrolysis products operating conditions,but the main constituents are carbon monoxide 5)Fractionation of pyrolysis products hydrogen,methane,C2-C6 hydrocarbons,carbon dioxide and water. 6)Light fuel gas energy recovery (heat,electricity or both) The general layout of a gasification plant includes: 7)Auxiliaries (scrubber,cooling water production,...) 1)Mechanical feeder at ambient temperature The reaction enthalpy necessary to break the chemical bonds of 2)Gasification reactor polymers can vary (Brems et al,2011);a mean value of 250 MJ/t has 3)Bottom ash discharge system been used by considering a prevailing amount of polyethylene.Starting 4)Cooling of syngas from ambient temperature,a sensible heat of 1200 MJ/t needs to be 5)Cleaning upgrading of syngas (thermal/catalytic cracking,tar added to reach the reaction temperature (total energy:1450 MJ/t).It is absorption,.…) noteworthy that the sensible heat acquired by the molten polymer in 6)Syngas energy recovery (heat,electricity or both) the extruder is in the range 60-70%of the total need.This means that 7)Auxiliaries (scrubber,cooling water production,...) the electricity used to operate the extruder is used to increase the en- thalpy of the plastic flow and only a limited amount is transferred to the The data used to model the performance of the gasifier section in the pyrolizer by using waste heat from engines (e.g.by using heat ex- following paragraphs come from an extensive series of experiments changers crossed by hot diathermic oil as heat vector). carried on the plastic waste by using a pilot-scale fluidized bed gasifier (Arena et al.,2010,2008;Mastellone and Zaccariello,2013).The main 2.3.Gasification process description performance parameters for the gasification of plastic waste mixture are reported in the following($4.3)and are related to a specific pilot plant Gasification is a thermochemical process that transforms a carbon operated with an oxidizing stream constituted by air having an based material into a gaseous mixture of low molecular weight species. equivalent ratio of about 0.3.The tar content in the syngas has been What's left is a clean "synthesis gas"that can be converted into valuable considered negligible thanks to use of catalytic reaction in the gasifier products and electricity.In particular,gasification of waste is an eco- (Arena et al.,2009).In the case of gasification,the absorbed process nomical and environmental viable solution to produce cleaner energy energy can be evaluated by considering that the exothermic reactions together with a remarkable waste weight reduction.The most simplistic uses a fraction of the feedstock energy of the input waste to guarantee a way to consider gasification is as an alternative to the combustion to given temperature and to promote the endothermic reactions;this obtain heat and power with a lesser environmental impact.In this case fraction of energy is the complement of the Cold Gas Efficiency para- the gasification process is used to transform a heterogeneous fuel (solid meter (CGE). waste,sludge,biomasses,low-rank coal,...into a homogeneous syn- thetic gaseous fuel to be utilized in an industrial burner,an engine or a 3.Materials gas turbine to produce electricity and heat.The loss of chemical energy necessary to promote the endothermic reactions of gasification is ba- The input material to which this paper refers is the plastic waste lanced by the higher performance of the homogeneous combustion.The residual from the centralized sorting at material recovery facility general concept and the technologies utilized to gasify a material is (MRF).This plastic waste has a low content of PET and a high content of similar to that reported for incineration facilities,but the operating polyolefins,other polymers and foreign matter.The sorting of this parameters of the plant are completely different. waste with the only aim to recover the PET and the HDPE,that are the First of all,the gasification process cannot be represented by a single only polymers suitable for the mechanical recycling,would be not main reaction,as for the combustion process,but by reactions involving economically convenient because of the largest part of the flow(about different reactants (oxygen,carbon dioxide,water)and characterized 90%)that should be addressed to disposal (landfilling or incineration). by different reaction's enthalpies (Mastellone,2015).Moreover,the By referring to the integrated system under study,where the sorting is interaction between intermediates and minerals must be considered, finalized to prepare the feedstocks for PtO and gasification processes,the following calculations, a fraction of liquid product (syncrude) equal to 80% and NCG equal to 10%. The rest of the matter is converted into char and tar (Mastellone et al., 2002; Scheirs and Kaminsky, 2006). The PtO plant (based on the Pruvia Fuels GmbH technology) is preceded by a pre-treatment that removes moistures and halogens from the plastic feedstock. This pre-treatment is carried out by using a combination between densifiers and degassing extruders (AMUT Group, 2019). The densifiers allow to manage polyolefins having very light bulk density (LLDPE) and heterogeneous mechanical properties. The extrusion is a pre-treatment that allows to remove moisture with 99% efficacy if the initial content is less than 10%, to remove organic chlorine by breaking the C-Cl bonds in the initial part of machine by allowing the degassing together with moisture, to bring the tempera￾ture up to 350 °C so favouring the input to the thermolysis reactor (pyrolizer). Downstream the extruder, the following equipment is in￾stalled: 1) Heated/insulated feeder to bring the plastic feedstock in the reactor over the melting temperature 2) Thermolysis reactor 3) Carbon/ash discharge system 4) Cooling of pyrolysis products 5) Fractionation of pyrolysis products 6) Light fuel gas energy recovery (heat, electricity or both) 7) Auxiliaries (scrubber, cooling water production, …) The reaction enthalpy necessary to break the chemical bonds of polymers can vary (Brems et al., 2011); a mean value of 250 MJ/t has been used by considering a prevailing amount of polyethylene. Starting from ambient temperature, a sensible heat of 1200 MJ/t needs to be added to reach the reaction temperature (total energy: 1450 MJ/t). It is noteworthy that the sensible heat acquired by the molten polymer in the extruder is in the range 60–70% of the total need. This means that the electricity used to operate the extruder is used to increase the en￾thalpy of the plastic flow and only a limited amount is transferred to the pyrolizer by using waste heat from engines (e.g. by using heat ex￾changers crossed by hot diathermic oil as heat vector). 2.3. Gasification process description Gasification is a thermochemical process that transforms a carbon￾based material into a gaseous mixture of low molecular weight species. What’s left is a clean “synthesis gas” that can be converted into valuable products and electricity. In particular, gasification of waste is an eco￾nomical and environmental viable solution to produce cleaner energy together with a remarkable waste weight reduction. The most simplistic way to consider gasification is as an alternative to the combustion to obtain heat and power with a lesser environmental impact. In this case the gasification process is used to transform a heterogeneous fuel (solid waste, sludge, biomasses, low-rank coal, …) into a homogeneous syn￾thetic gaseous fuel to be utilized in an industrial burner, an engine or a gas turbine to produce electricity and heat. The loss of chemical energy necessary to promote the endothermic reactions of gasification is ba￾lanced by the higher performance of the homogeneous combustion. The general concept and the technologies utilized to gasify a material is similar to that reported for incineration facilities, but the operating parameters of the plant are completely different. First of all, the gasification process cannot be represented by a single main reaction, as for the combustion process, but by reactions involving different reactants (oxygen, carbon dioxide, water) and characterized by different reaction’s enthalpies (Mastellone, 2015). Moreover, the interaction between intermediates and minerals must be considered, particularly for plastics (Mastellone and Arena, 2008; Sjöström et al., 1999). The heat released by exothermal reactions increases the environ￾ment temperature until values depending by the equivalence ratio, proximate and ultimate analysis of waste and its net chemical energy. The operating temperature of the waste gasifiers is generally in the range 850–1200 °C depending by the above recalled parameters and by heat losses. The oxygen supplied to the gasifier is generally 25–40% of the stoichiometric demand. The reducing environment promotes the partial oxidation of carbon and hydrogen element in the fuel by pro￾viding the heat necessary to the cracking of waste/fuel component. The fraction between the oxygen provided for gasification reactions and the stoichiometric demand is known as equivalence ratio and its value depends on ultimate analysis of the waste. The main product of gasi￾fication process is the synthesis gas also identified as “synthetic gas or syngas”. This term is used when it is used as a feedstock for production of synthetic natural gas (SNG), Fischer-Tropsch liquids (FTL), hydrogen or any other material or fuel; the term “producer gas” is used when the gas is used to energy production. Anyway, whatever is the final utili￾zation, the term syngas can be used to indicate the gaseous product of gasification. The syngas composition can vary depending on waste and operating conditions, but the main constituents are carbon monoxide, hydrogen, methane, C2-C6 hydrocarbons, carbon dioxide and water. The general layout of a gasification plant includes: 1) Mechanical feeder at ambient temperature 2) Gasification reactor 3) Bottom ash discharge system 4) Cooling of syngas 5) Cleaning / upgrading of syngas (thermal/catalytic cracking, tar absorption, …) 6) Syngas energy recovery (heat, electricity or both) 7) Auxiliaries (scrubber, cooling water production, …) The data used to model the performance of the gasifier section in the following paragraphs come from an extensive series of experiments carried on the plastic waste by using a pilot-scale fluidized bed gasifier (Arena et al., 2010, 2008; Mastellone and Zaccariello, 2013). The main performance parameters for the gasification of plastic waste mixture are reported in the following (§4.3) and are related to a specific pilot plant operated with an oxidizing stream constituted by air having an equivalent ratio of about 0.3. The tar content in the syngas has been considered negligible thanks to use of catalytic reaction in the gasifier (Arena et al., 2009). In the case of gasification, the absorbed process energy can be evaluated by considering that the exothermic reactions uses a fraction of the feedstock energy of the input waste to guarantee a given temperature and to promote the endothermic reactions; this fraction of energy is the complement of the Cold Gas Efficiency para￾meter (CGE). 3. Materials The input material to which this paper refers is the plastic waste residual from the centralized sorting at material recovery facility (MRF). This plastic waste has a low content of PET and a high content of polyolefins, other polymers and foreign matter. The sorting of this waste with the only aim to recover the PET and the HDPE, that are the only polymers suitable for the mechanical recycling, would be not economically convenient because of the largest part of the flow (about 90%) that should be addressed to disposal (landfilling or incineration). By referring to the integrated system under study, where the sorting is finalized to prepare the feedstocks for PtO and gasification processes, M.L. Mastellone Resources, Conservation & Recycling: X 4 (2019) 100017 5