PROCESS INTEGRATION
A Life Cycle Assessment of Mechanical and Feedstock Recycling Options for Management of Plastic Packaging Wastes Floriana Perugini, Maria Laura Mastellone, and Umberto Arena Department of Environmental Sciences, University of Naples, II Via Vivaldi, 43– 81100 Caserta, Italy;
[email protected] (for correspondence) Published online 11 April 2005 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10078
Life cycle assessment (LCA) methodology is generally considered one of the best environmental management tools that can be used to compare alternative ecoperformances of recycling or disposal systems. It considers the environment as a whole, including indirect releases, energy and material consumption, emissions in the environment, and waste disposal and follows each activity from the extraction of raw materials to the return of wastes to the ground (cradle-to-grave approach). The study refers to the whole Italian system for recycling of household plastic packaging wastes. The aim was to quantify the overall environmental performances of mechanical recycling of plastic containers in Italy and to compare them with those of conventional options of landfilling or incineration and of a couple of innovative processes of feedstock recycling, low-temperature fluidized bed pyrolysis, and high-pressure hydrogenation. The results confirm that recycling scenarios are always preferable to those of nonrecycling. They also highlight the good environmental performance of new plastic waste management schemes that couple feedstock and mechanical recycling processes. © 2005 American Institute of Chemical Engineers Environ Prog, 24: 137–154, 2005
Keywords: life cycle assessment; plastic waste; plastic recycling; mechanical recycling; feedstock recycling
© 2005 American Institute of Chemical Engineers
Environmental Progress (Vol.24, No.2)
INTRODUCTION
Over the years, plastics have undergone a spectacular evolution in development and use. Plastics are now used in countless applications, from cars to packaging, from medical equipments to mobile phones and buildings. The packaging sector remained the largest consumer of plastics, accounting for 14,525 kton, or 38.1% of all plastics consumed in 2001–2002. Plastics continue to be the material of choice for packaging, increasingly substituting other materials because they are lightweight, flexible, and easy to process. Over 50% of all Europe’s goods are now packaged in plastics, even though these plastics account for only 17% by weight of all packaging [1]. One of the main negative consequences associated with this wide use—and reflecting changes in production and consumption—is the often emphasized question of plastic waste. Municipal solid waste is the main source of waste plastics, about two thirds of the total generated, whereas the second waste plastics stream comes from the distribution and industrial sectors [2]. The solution of this problem must necessarily include a large-scale use of the various recycling techniques for materials and/or energy recovery. The scheme in Figure 1 shows an industrial ecology for polymers, that is, how different forms of plastic waste treatment are related to the production cycle [3]. ● Reuse makes possible a new use of the original object with the same function for which it was originally produced; unfortunately reuse can be adopted in only a limited range of applications [4]. July 2005
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Figure 1. An industrial ecology for polymers (adapted from Clift [3]).
●
Mechanical recycling is the European plastics industry’s preferred recycling technique, being used for the 51.5% of the total plastics waste recovered in Western Europe in the year 2002 [1]. This technique directly recovers clean plastics for reuse in the manufacturing of new plastic products: the difficulties are mainly related to the degradation of recyclable material and heterogeneity of plastic wastes [4, 5]. ● Chemical recycling is the term used to identify some commercial processes able to recover synthesis monomers or feedstock chemicals by depolymerization. Several plastics, such as polymethylmethacrylate (PMMA) and polyethylene terephthalate (PET), certain polyamides, and polyurethanes, are advantageous for such treatment [5]. ● Feedstock recycling indicates a family of advanced recycling technologies that breaks down the solid polymeric materials into a spectrum of basic chemical components; these latter can be used as raw materials in the production of new petrochemicals and plastics, without any deterioration in their quality and without any restriction regarding their application. These processes involve the use of high temperatures to cleave the bonds in the backbone of the polymer; they can be carried out in the absence of air (pyrolysis), in the presence of a high partial pressure of hydrogen (hydrocracking), or of a controlled amount of oxygen (gasification). ● Plastics can also be conveniently used in energy recovery processes, particularly if they cannot be mechanically recycled because of excessive contamination, separation difficulties, or polymer property deterioration [4, 5]. A proper understanding and an objective quantification of all the environmental impacts related to different recycling and recovery processes are necessary to define an environmentally sustainable management system. Life cycle assessment (LCA) methodology is generally considered one of the best environmental management tools that can be used to compare alter138
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native eco-performances of recycling or disposal systems [6, 7]. It considers the environment as a whole, including indirect releases, consumption of raw materials, resources, and energy, and waste disposal. It offers a full picture of system interactions with the environment and avoids shifting of environmental impacts from one life cycle stage to another. The study refers to the whole Italian system for recycling of household plastic packaging wastes. The phases of the different management options were individually analyzed and quantified in terms of energy and material consumptions as well as of emissions in the environment. More specifically, the study is an extension of a previous research project [8]: it aims to compare the already quantified overall environmental performances of mechanical recycling of plastic containers in Italy with those of conventional options of landfilling or incineration and those of a couple of innovative processes of feedstock recycling: the lowtemperature pyrolysis carried out in fluidized bed reactors and high-pressure hydrogenation. LCA APPROACH TO THE ANALYZED PLASTIC WASTE MANAGEMENT
Life cycle assessment is an objective methodology, developed from chemical engineering principles and energy analysis [9, 10], that is able to account for upstream and downstream inputs and emissions related to the life cycle of a product or a service. The term life cycle indicates that every stage of the life cycle of the service, from resource extraction to ultimate end-of-life treatment, is taken into account. It is generally considered the best environmental management tool that can be used to move from a generic statement about the environmental benefit of a given recycling or disposal system to reach an objective quantification of its environmental sustainability. The type of study carried out here is a conventional or engineering LCA, characterized as “a process to evaluate the environmental burdens associated with a product, process, or activity by identifying and quantifying energy and materials used Environmental Progress (Vol.24, No.2)
Figure 2. System boundaries and principal environmental burdens for mechanical recycling scenario.
and wastes released to the environment; to assess the impact of those energy and material uses and releases to the environment; and to identify and evaluate opportunities to effect environmental improvements” [11]. Following the international standards ISO 14040-43 [12], the LCA’s structure consists of four distinct phases that contribute to an integrated approach. 1. Goal and scope definition, which serves to define the purpose and extent of the study and its boundaries, to indicate the intended audience, and to describe the system studied as well as the options that will be compared. In particular, during the definition of system boundaries it is useful to distinguish between the Foreground system and the Background system. The first is defined as the set of processes Environmental Progress (Vol.24, No.2)
whose selection or mode of operation is affected directly by decisions based on the study, in this case the plastic waste management activities; the Background is that instead defined as all other processes that interact with the Foreground, usually by supplying or receiving material or energy [6]. 2. Inventory analysis or LCI (life cycle inventory), which collects all the material and energy inputs and outputs (the so-called environmental burdens or interventions) that cross the boundary between the product or service system and the environment over its whole life cycle. The recommended way to report the LCI for a waste management scheme is [6, 13]: direct burdens, associated with the waste management operations themselves; plus indirect burdens, July 2005
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Figure 3. System boundaries and principal environmental burdens for mechanical recycling scenario and low-
temperature pyrolysis process.
associated with providing materials and energy to the waste management operations; minus avoided burdens, associated with economic activities that are displaced by materials and/or energy recovered from the waste. Direct burdens are usually definable at least on a regional or national level; the location of indirect and avoided burdens cannot normally be defined and their numerical estimates should be obtained from a reliable database [14]. 3. Impact assessment or LCIA (life cycle impact assessment), which aims at understanding and evaluating 140
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the magnitude and significance of potential environmental impacts of a system. It organizes the LCI inputs and outputs into specific, selected impact categories and models the inputs and outputs for each category into an aggregate indicator [15, 16]. 4. Interpretation, which identifies, qualifies, checks, and evaluates information from the results of the LCI analysis and/or LCIA, and present them to meet the requirements of the application as described in the goal and scope of the study. Interpretation involves a review of all of the phases in the LCA process and Environmental Progress (Vol.24, No.2)
Figure 4. System boundaries and principal environmental burdens for mechanical recycling scenario and
hydrogenation process.
a check that all the assumptions are consistent. In particular, a data quality check and a sensitivity analysis should be performed to establish the significance of data uncertainty on the results of the study. The purpose of applying LCA to waste management is principally to ensure that the balance between the local impacts of waste collection and treatment are offset clearly and systematically against any benefits of recovering materials and/or energy from the waste [13]. However, LCA is not the only tool for structuring and presenting information on environmental performance. Other approaches such as environmental impact assessment must be used, in conjunction with LCA, to Environmental Progress (Vol.24, No.2)
describe local impacts on public health and natural ecosystems, land use, occupational health, and safety, plus disamenity impacts such as odor, noise, and visual intrusion. For instance, the impact of transportation is analyzed from the perspective of emissions to the air and water, and from an energy-use perspective, without including the risk of accidents or infrastructural saturation (attributed to increased traffic congestion) or disamenity effects, such as odor, visual pollution, and noise (again, attributed to increased traffic). To cover these issues, methodological bases other than those used so far in LCA studies are required [17, 18]. July 2005
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Figure 5. System expansion method used in the study, with the indication of functional output of primary
interest and of avoided burdens coming from each system.
Goal and Scope Definition The primary audience for this effort is the Italian National Consortium for Packaging (Conai), which was interested in assessing energy and environmental profile of the current plastic waste management in comparison with some alternatives. However, the considerations and tools developed through the study are also of value to the several industries active in the field of plastic waste management, which are interested in defining economically convenient and environmental sustainable ways for a complete process integration for plastic waste recycling. The function of the system under study is to manage (and recycle) plastic containers for liquids, mainly made of PET or PE. Therefore, the functional unit—that is, the common basis on which modifications and alternative systems are to be compared— has been defined as the management of postconsumer PE and PET liquid containers (obtained by means of a monomaterial collection), which leads to the production of 1 kg of flakes of (recycled or virgin) PET. It is assumed that there is a market demand for all recycling products under investigation and that virgin and recycled material are equivalent for the market [8]. The proposed life cycle approach uses (1) for generic data (mainly indirect and avoided burdens) one of the most valued international databanks (Boustead 142
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Consulting Ltd., West Sussex, UK) and (2) for specific data (mainly direct burdens), only those derived from on-site investigations or deduced from official documents [8, 19]. Scenarios and System Boundaries The study set out herein was carried out to compare five different scenarios: 1. Landfilling: no recycling and landfill disposal of all the collected plastic wastes 2. Combustion with energy recovery: no recycling and all the collected plastic wastes sent to combustion with energy recovery 3. Mechanical recycling: mechanical recycling of all the collected plastic wastes and all the process wastes sent to combustion with energy recovery 4. Mechanical recycling ⫹ low-temperature pyrolysis process: mechanical recycling of the PET fraction and low-temperature pyrolysis of the polyolefins fraction 5. Mechanical recycling ⫹ hydrocracking: mechanical recycling of the PET fraction and hydrocracking of the polyolefins fraction The mechanical recycling scenario represents the base case assumed as the reference because it is very close to that applied in Italy. The nonrecycling scenarEnvironmental Progress (Vol.24, No.2)
Figure 6. Quantified mass flow of plastic packaging waste along the Italian chain for mechanical recycling (sort ⫽ efficiency of sorting stage; rep ⫽ efficiency of reprocessing stage).
ios were defined to quantify the performance of a hypothetical management scheme that uses only landfilling and of another scheme that refers only to energy recovery by combustion. The two scenarios of feedstock recycling were defined as possible alternatives to mechanical recycling. It is noteworthy that for these two management schemes the PET fraction was never considered as an input for thermolysis processes because its use in mechanical recycling appears absolutely convenient, from both economical and environmental perspectives [4, 5]. Figures 2– 4 show the principal operations that constitute the foreground system and the principal direct burdens and exchanges with the Background system for mechanical and feedstock recycling scenarios. It was assumed that the plastic container for liquids enters the system boundaries when it is delivered to a monomaterial collection site, whether it is a residential curbside, apartment collection site, or rural drop-off site. The foreground includes all the activities to manage the waste and the interunit transport phase [8]. For landfilling, combustion, and mechanical recycling scenarios all the data of interest were derived from on-site investigations and the data quality was furthermore increased by taking into account the composition of the specific waste generated at the various stages [8, 19]. Primary inventory data describing feedstock recycling processes were collected from official documents related to the BP Chemicals Polymer Cracking process and the Veba Combi-Cracking process [20, 21]. Environmental Progress (Vol.24, No.2)
The approach of system expansion, recommended in ISO 14041 to avoid the problem of allocation, that is, the attribution of environmental interventions and impacts between the outputs or services of multifunction systems, was applied. Figure 5 summarizes the expanded systems describing the analyzed scenarios, showing the principal avoided burdens in each case. As mentioned earlier, the Italian system for plastic packaging collection is strongly focused on plastic containers for liquids, mainly made of PET and PE: the system boundaries were accordingly drawn to include PET and PE reprocessing, and then the production of 0.39 kg of recycled PE flakes together with that of 1 kg of recycled PET flakes (Figure 6) [8]. ENVIRONMENTAL BURDENS OF PLASTIC WASTE MANAGEMENT OPTIONS
Some details about the processes and the environmental burdens of each of the five selected scenarios are reported in the following. Landfilling This scenario refers to the transport and disposal in landfill of 2.35 kg of plastic waste as well as the production of 1 kg of PET and 0.39 kg of PE from traditional activities to provide the same products obtained by a mechanical recycling scenario (Figure 5). In the specific case under study, an overwhelming majority of the wastes are made of polymeric scraps. As a consequence, landfill gas and leachate are negligible because only 1–3% [22] of the hydrocarbon content can be July 2005
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Table 1. Inventory of direct environmental burdens related to the mechanical recycling scenario.
Input
Output Collection
Plastic waste Road transport
1 kg 0.026 km
Products Plastic waste
1 kg
Products Plastic waste
1 kg
Compaction Plastic waste Auxiliary energy Electric energy
1 kg
Plastic waste Auxiliary energy Diesel Electric energy
1 kg
0.09 MJ Sorting Products PET PE Residues Scraps
0.084 MJ 0.122 MJ
0.56 kg 0.19 0.25
PET reprocessing Sorted PET Auxiliary materials Water Sodium hydroxide Auxiliary energy Methane Electric energy
1.32 kg
Products Recycled PET Residues Scraps
2.96 kg 3 ⫻ 10⫺3 kg
1 kg 0.32 kg
2.5 MJ 1.05 MJ PE reprocessing
Sorted PE Auxiliary materials Water Auxiliary energy Methane Electric energy
1.14 kg
Transported plastic Average length
1 kg
Products Recycled PE Residues Scraps
1.78 kg
1 kg 0.14 kg
0.6 MJ 2 MJ Transport 0.025 km
degraded during the considered time period of 100 years [8]. Combustion This scenario covers monocombustion of 2.35 kg of collected plastic waste with recovery of electricity dispatched by the Italian distribution grid; it also includes conventional production of 1 kg of PET and 0.39 kg of PE. A net calorific value of 27 MJ/kg has been evaluated on the basis of the composition of the collected plastic waste. The environmental burdens were estimated by assuming a process overall electric efficiency of 25% [8, 14]. Mechanical Recycling Figure 6 reports the layout of the different stages of the Italian recycling chain: a monomaterial collection, a sorting process, and a reprocessing phase producing recycled PET and PE flakes. The inventory of direct and avoided burdens has been obtained by averaging those measured during on-site investigations in some of the main companies currently active in Italy [8, 19]. The collection process is the first stage of the recycling system and its environmental interventions de144
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pend on the density of population and on the characteristics of the area where the collection is carried out. The sorting process has been carried out in dedicated plants where the collected material is separated into the different fractions (PE and colored, transparent, and blue PET) by means of a series of manual and automatic operations. The reprocessing plants are obviously more complex. The PET reprocessing process consists in a first section to remove impurities and in a second to recover PET and byproducts (HDPE and fines). The first section constitutes a prewashing to separate stickers and labels, a magnetic separation to separate ferrous material, and an X-ray separation to remove polyvinyl chloride (PVC). In the second one the selected material is sent to a washing and to a flotation stage to recover HDPE, after which it is dried, sent to a fine screening, and finally stored. The PE reprocessing is essentially similar to that of PET but it also includes the extrusion of fine and coarse PE to produce PE pellets. Details about both processes can be found elsewhere [8, 19]. Table 1 reports the inventory of direct environmental burdens related to the mechanical recycling scenario. It was assumed that all scraps of the recycling Environmental Progress (Vol.24, No.2)
Table 2. Inventory of direct environmental burdens related to the feedstock recycling scenarios.
Input
Output
Plastic waste Road transport
1 kg 0.026 km
Plastic waste Auxiliary energy Electric energy
1 kg
Plastic waste Auxiliary energy Diesel Electric energy
1 kg
Collection Products Plastic waste Compaction Products Plastic waste
1 kg 1 kg
0.09 MJ Sorting
Sorted PET Auxiliary materials Water Sodium hydroxide Auxiliary energy Methane Electric energy Polyolefines fraction Auxiliary materials Sand CaO Water Auxiliary energy Naphtha Electric energy
Polyolefines fraction Auxiliary materials Hydrogen CaO Auxiliary energy Natural gas Electric energy Steam
0.084 MJ 0.122 MJ 1.32 kg
Products PET Polyolefines fraction Residues Scraps PET reprocessing Products Recycled PET
0.56 kg 0.39 kg 0.05 kg 1 kg
2.96 kg 3 ⫻ 10⫺3 kg 2.5 MJ 1.05 MJ 1 kg 0.0085 kg 0.046 kg 0.002 m3 0.131 MJ 0.212 MJ
1 kg 0.011 kg 0.001 kg 4.62 MJ 0.96 MJ 0.112 MJ
BP process Products Gas fraction Heavy fraction (waxes) Light fraction (liquid) CaO/CaCl2 Sand Air Emissions CO2 NOx SO2 Residues Waxy filter to incineration Veba process Products Syncrude E-gas HCl CaCl2 Air Emissions NH3 Hydrocarbons Residues Solid waste Residue to incineration
chain are used to recover electric energy in dedicated combustion plants. Note also that diesel used for the sorting stage is essentially that for forklift vehicles and bulldozers, whereas the various apparatuses of plastic reprocessing plants use methane or electric energy. The burdens related to transport stage are expressed in term of average length; the specific energy consumptions have been evaluated as an average of those for each of the different transport vehicles. Environmental Progress (Vol.24, No.2)
0.147 0.448 0.265 0.057 0.076
kg kg kg kg kg
0.345 kg 0.0003 kg 0.002 kg 0.046 kg 0.822 kg 0.09 kg 0.005 kg 0.0041 kg 0.006 g 2.23 g 0.05 kg 0.066 kg
Feedstock Recycling Scenarios These two management scenarios include alternative options to the valorization of the polyolefins fraction after a limited preliminary treatment (necessary to provide sufficient handling properties to the material) [4]. As mentioned earlier, this study assumes that the PET fraction is separated from the remaining plastic waste and sent to the mechanical recycling process July 2005
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Figure 7. Simplified flow sheet for the BP polymer cracking process (from Arena and Mastellone [23]). Table 3. Products of the low-temperature pyrolysis process and the equivalent refinery products for which they can serve as substitutes.
Low-temperature pyrolysis process: products 0.448 kg heavy fraction (waxes) 0.265 kg light fraction (liquid) 0.147 kg gas fraction 1.48 MJ p-steam 0.076 kg sand and coke 0.057 kg CaO/CaCl2
(Figures 3 and 4). Table 2 reports the inventory of direct environmental burdens related to the considered feedstock recycling scenarios. Some details about the two processes considered are reported in the following. Low-Temperature Pyrolysis
The collected and sorted plastic mixture undergoes a treatment of size reduction and of separation of undesirable materials according to the process specification (max 4% contaminants, max 4.5% ash, max 2.5% chlorine, and max 1% moisture content [21]). The prepared mixed-waste plastics are then introduced into a bubbling fluidized bed reactor, where the low-temperature cracking reaction takes place (Figure 7). As the plastics enter the reactor, they quickly melt and coat the sand particles with a thin layer of polymer. This amalgam undergoes thermal cracking and produces lighter hydrocarbons that leave the bed with the fluidizing gas [4]. The gaseous products are purified first in a cyclone, which removes the bulk of the entrained fines, and then in a successive guard bed, which apprehends the chlorine content coming from PVC destruction by the reaction of CaO with HCl. The main result is a spent CaO/CaCl2 that has to be landfilled. The gas exiting the guard bed is purified by cyclones and collection hopper to remove the CaO/CaCl2 fines. An intermittent withdrawal of material from both fluidized bed reactor and guard bed is necessary to purge the bed from accumulated unwanted materials. The gas arising from the process is collected in a two-stage process. A ven146
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Equivalent products 0.448 kg atmospheric residue 0.265 kg naphtha 0.147 kg C3/C4 compounds 1.48 MJ heat from industrial heating station 0.076 kg sand 0.040 kg CaO ⫹ 0.017 kg CaCl2
turi loop system partially condenses the gas to give a waxy hydrocarbon product. This is condensed first and filtered to remove remaining fine solid particles: its high degree of purity allows mixing with naphtha at levels up to 20% so that the resulting mixture can be directly used in a steam cracker to give classic petrochemical products. The noncondensed gas stream is compressed, reheated, and returned to the cracking reactor as fluidizing gas: excess gas is continuously removed as a product to keep the process operating at a pressure of about 4 bar and partially used as fuel for indirect heating of the fluidized reactor [20, 23]. The polymer cracking process has an efficiency of nearly 80% conversion of plastic waste to petrochemical products, with an additional 10 –15% used as fuel gas in the process itself [4]. The obtained hydrocarbon feedstock is of high quality, whereas the waxy product can be used as feedstock material for steam crackers or can be fed into refinery units [such as the fluid catalytic cracking (FCC) unit]. Table 3 details the products obtained from 1 kg of plastic waste sent to the BP cracking process and indicates the refinery products that they can substitute. As shown in Figure 5 (Scenario 4), these latter have been considered in this study as avoided burdens. It should be noted that the recent process developments [24] seem to work in the direction of an improved energy efficiency that could change some of the data reported in the table and used for LCIA. These developments (and the related data of performance) are not available for reasons of industrial secrecy; as a Environmental Progress (Vol.24, No.2)
Figure 8. Simplified flow sheet for the Veba Combi-Cracking process (from Dijkema and Stougie [21]).
Table 4. Products of the hydrocracking process and
equivalent products for which they can serve as substitutes. Hydrocracking process: products 0.822 kg syncrude 0.09 kg gas fraction 0.032 kg HCl
Equivalent products 0.822 kg crude oil 0.09 kg natural gas 0.032 kg HCl
consequence, this study used only the officially published data. Hydrocracking
In hydrocracking, heat breaks molecules into highly reactive free radicals that are saturated with molecular hydrogen as they form. Partial pressure of hydrogen must be high enough (about 200 bar) to suppress undesirable coking or repolymerization. Figure 8 reports a flow diagram of the Veba process for converting plastic waste in fragments of hydrocarbons, in appearance and composition similar to crude oil. The diagram refers to an improved configuration of the plant installed in Bottrop, Germany, which has a depolymerization capacity of 200 kton/ year [21]. Mixed plastic waste (with an inorganic matter content limited to 4.5% and a PVC content ⬍ 10%) is fed to a depolymerization unit where it is thermally cracked to a light top product (71 wt %) and a heavy bottom product. The top product is composed of condensate (C5⫹, boiling range 400° C) and noncondensable (C3–C4) gases together with the 2 wt % of HCl. A water-wash column, where excess ammonia is added for neutralizing, removes the HCl content; the noncondensable gases are routed to the vortex-cyclone combustion (VCC) unit and the condensed gas is routed to a hydrotreater. Environmental Progress (Vol.24, No.2)
The hydrotreater operates with a partial pressure of 50 bar of hydrogen and 60 bar of total pressure and at a temperature ranging from 300 to 400° C. The main function of this unit is to convert in HCl the remaining organic chlorine content, which is removed by the last water wash. The syncrude produced is a naphtha/diesel mixture acceptable for steamcracker feed. The depolymerizate is mixed with vacuum residue prior hydrogenation and it is sent, by means of high-pressure pumps, to a VCC unit where high pressure of hydrogen (300 bar) is added. The resulting feed/hydrogen mixture is routed into the process furnace where it is heated to the reaction temperature (450 – 490° C). The hydrogenation reactions [5] take place in a liquid-phase reactor and the products are then separated in a hot separator. The top of the vacuum distillation tower is routed in a second reactor, a fixed-bed hydrotreater operating at high pressure but at lowered temperature, to recover a synthetic crude oil that can be processed in any refinery. Light cracking products end up as off-gas, which is sent to a treatment section for H2S removal. A bottom hydrogenation residue, composed of hydrocarbons contaminated with ashes, metals, and inert salt, is also produced by separation. The syncrude produced by the Veba process is of good quality, being composed almost entirely of liquefied petroleum gas (LPG), naphtha, kero, and gasoil fraction. Table 4 reports the products obtained from 1 kg of plastic waste sent to the Veba Combi-Cracking process and indicates the products for which they can serve as substitutes. As mentioned earlier and shown in Figure 5 (Scenario 5), they have been considered as avoided burdens. ENVIRONMENTAL COMPARISONS
The system expansion method of Figure 5, which defines the analytical comparison between the five selected scenarios, focuses on the functional output July 2005
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Figure 9. Net energy consumption related to each scenario with the indication of the contribution coming from
the different stages (A) and the same data reported in terms of primary sources (B).
of primary interest, that is, the management of sufficient postconsumer plastic waste, 2.35 kg of PE and PET liquid containers collected as a single material stream, to produce 1 kg of recycled or virgin PET flakes and 0.39 kg of recycled or virgin PE flakes. It also takes into account that avoided burdens (associated with economic activities displaced by material and/or energy recovered in each scenario) represent the “environmental credits” for recycling or energy 148
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from waste that have to be subtracted in the environmental burdens inventory [13]. This means, for instance, that electricity generated from combustion of plastic waste is taken to reduce output from other sources. Diagrams reported in the following describe the LCIA results with respect to some crucial impact categories. The negative values in the figures indicate the predominance of avoided environmental burdens. Environmental Progress (Vol.24, No.2)
Figure 10. Consumption of nonrenewable resources related to each scenario (A) and the crude oil consumption
with the indication of the contribution coming from different stages (B).
Energy and Resource Consumption Figure 9A shows the net energy consumption for all five scenarios and indicates the contributions from different stages. Note that the contributions related to collection and pretreatments are shown together to make the figure more readable. In any case, the collection contribution is the same (and almost negligible) for all the scenarios so that the differences can be ascribed to the sorting stage (for mechanical recycling) and to the specific process pretreatment (for feedstock recycling processes). The data show that the mechanical recycling sceEnvironmental Progress (Vol.24, No.2)
nario is always preferable to nonrecycling scenarios, landfilling, and combustion. This is explained by the fact that the life cycle for the production of 1 kg of PET recycled flakes produces an overall net energy saving of 5.2 MJ (taking into account the energy recovered from process wastes), whereas the production of the same quantity of virgin PET requires about 38.8 MJ. The energy (and then environment) saving is so remarkable even for PE, which is 0.5 MJ of the energy saved along the life cycle by producing 1 kg of recycled PE and about 31.4 MJ that required to produce the virgin polyolefin. The best July 2005
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Figure 11. Water consumption related to each scenario.
Figure 12. Generation of greenhouse gases (expressed as kilograms of CO2 equivalent) related to each scenario with the indication of contributions coming from the different stages.
energy performance is shown by Scenario 5 (mechanical recycling ⫹ hydrocracking), which allows a remarkable saving of more than 11 MJ, essentially 150
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attributed to the avoided burdens related to the products recovered from the hydrogenation of the polyolefins fraction. On the other hand, the benefits reEnvironmental Progress (Vol.24, No.2)
Figure 13. Air and water emissions related to each scenario.
lated to Scenario 4, including the low-temperature pyrolysis process, are lower and not sufficient to offset the energy consumption of the collection, sorting, and PET reprocessing. Figure 9B reports the same data of total net energy consumption or saving in terms of primary sources. In this form, the results are strongly affected by the Italian, carbon-intensive, energy mix [19, 25]. Figure 10A reports the fuel and feedstock consumptions for all the considered scenarios. The recycling scenarios are characterized by a remarkable saving of crude oil: ⫺75 g when the collected plastic Environmental Progress (Vol.24, No.2)
waste is mechanically recycled, ⫺143 g when the mechanical recycling of PET is associated with pyrolysis of polyolefins, and ⫺208 g when the hydrocracking process is used. Taking into account the carbon-intensive character of the Italian energy mix, the saving of crude oil is attributed (1) for only the mechanical recycling scenario, to the electric energy recovered from the combustion of process scraps, and (2) for the mechanical ⫹ feedstock recycling scenarios, to the combustion of PET scraps and to the production of different petrochemical products (Figures 5 and 10B). July 2005
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Figure 14. Solid waste generation related to each scenario with the indication of the origin of the different
streams. Table 5. Indicators of principal environmental impact categories, as evaluated for the five scenarios for plastic waste management.*
Impact category Energy consumption, MJ/kgrecycled or virgin PET Crude oil consumption, g/kgrecycled or virgin PET Water consumption, kg/ kgrecycled or virgin PET CO2-equivalent, kg/kgrecycled or virgin PET Air emissions of organic compounds, g/kgrecycled or virgin PET Waste production, kg/ kgrecycled or virgin PET
Mechanical recycling
MR ⴙ L-T pyrolysis
MR ⴙ hydrocracking
Landfill
Combustion
51.59
6.45
⫺5.41
12.14
ⴚ11.40
1462
995
⫺74.76
⫺145.86
ⴚ210.65
47.11
45.92
3.48
14.06
25.26
5.3
7.3
1.4
1.7
2.02
26.8
14.3
ⴚ0.05
1.42
4.78
2.49
0.19
0.09
0.20
0.26
*Values in boldface type indicate the best environmental performance, whereas values in italic type indicate the worst.
The three recycling scenarios appears to provide the best performance even with respect to water consumption (Figure 11). The base scenario, in particular, allows a saving of more than 92% of water necessary for landfilling and combustion options, these latter being charged by the environmental burdens related to the traditional petrochemical processes for the production of the equivalent amount of PET and PE flakes. 152
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Climate Change Figure 12 quantifies the generation of greenhouse gases (GHGs), expressed as kilograms of CO2-equivalent emissions for a time horizon of 100 years, for each plastic waste management scenario, with the indication of contributions related to the different stages of the life cycle. Also in this case, the (mechanical and feedstock) recycling scenarios show a Environmental Progress (Vol.24, No.2)
remarkable decrease in emissions of GHGs with respect to the nonrecycling scenarios (about 70 – 80%). Emissions in the Environment The LCA study also quantified emissions in air and water of some pollutants as well as the production of solid wastes (Figures 13 and 14). For all of these impact categories, the mechanical recycling scenario presents the best environmental performance, even though the other recycling options show rather good results. Overall Evaluation Table 5 summarizes the indicators of principal environmental impact categories, as evaluated for each of the selected scenarios. One immediately notes the good environmental performance of the plastic management scheme dominated by mechanical recycling. The two scenarios that include feedstock recycling processes for polyolefins fraction show a remarkable saving in terms of crude oil consumption and, for Scenario 5 including the hydrocracking process, also in terms of energy consumption. The nonrecycling scenarios show poorer performances with respect to all the evaluated impact categories. CONCLUSIONS
Five alternative plastic waste management options that are, or could be, used in Italy have been assessed by means of the standardized LCA procedure. In particular, a scenario, completely focused on mechanical recycling and close to the present management policy of plastic packaging wastes in Italy, has been compared with some alternatives. The results suggest the most convenient options from the energy and environmental perspective. In particular, they: ● confirm the poor performance of landfilling option ● indicate that the mechanical recycling option is always environmentally preferable, with the only exception of energy consumption ● indicate that feedstock recycling (particularly under conditions of the hydrocracking process) has a number of valuable environmental indices This result together with the potential further environmental improvements that can be achieved by this family of new recycling technologies indicate the environmental sustainability of plastic waste management schemes that couple mechanical recycling with feedstock recycling processes. These should be preferred for plastic waste fractions for which the 1:1 substitution ratio is not possible or convenient, that is, for polymers for which virgin and recycled material are not equivalent for the market. LITERATURE CITED
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