ARTICLE pubs.acs.org/est
Environmental Impact of Pyrolysis of Mixed WEEE Plastics Part 2: Life Cycle Assessment Sue M. Alston* and J. Cris Arnold Materials Research Centre, Swansea University, Singleton Park, Swansea SA2 8PP, U.K.
bS Supporting Information ABSTRACT: Waste electrical and electronic equipment (WEEE) contains up to 25% plastics. Extraction of higher quality fractions for recycling leaves a mix of plastic types contaminated with other materials, requiring the least environmentally harmful disposal route. Data from trials of pyrolysis, described in part 1 of this paper set, were used in a life cycle assessment of the treatment of WEEE plastics. Various levels of recycling of the sorted fraction were considered, and pyrolysis was compared with incineration (with energy recovery) and landfill for disposal of the remainder. Increased recycling gave reduced environmental impact in almost all categories considered, although inefficient recycling decreased that benefit. Significant differences between pyrolysis, incineration and landfill were seen in climate change impacts, carbon sent to landfill, resources saved, and radiation. There was no overall “best” option. Landfill had the least short-term impact on climate change so could be a temporary means of sequestering carbon. Incineration left almost no carbon to landfill, but produced the most greenhouse gases. Pyrolysis or incineration saved most resources, with the balance depending on the source of electricity replaced by incineration. Pyrolysis emerged as a strong compromise candidate since the gases and oils produced could be used as fuels and so provided significant resource saving without high impact on climate change or landfill space.
’ INTRODUCTION The increasing growth in waste electrical and electronic equipment (WEEE),1 together with rising recycling and recovery targets, means that recovery of the metal fraction of WEEE is no longer sufficient and there is a need to also make use of the plastic fraction. Some plastic parts can easily be extracted from WEEE during dismantling, and can then be mechanically recycled. The remainder of the equipment is shredded and sorted. A proportion of the sorted plastic is of sufficiently high quality for mechanical recycling but the rest must be disposed of in some other way. Currently the most likely disposal options are landfill or incineration, with incineration plants incorporating energy recovery through generation of electricity and possibly heat. A possible alternative is pyrolysis, where the waste is heated without oxygen so that it breaks down into oils and gases, leaving a residue containing carbon and inorganic content from the original waste. Previous work has shown that more than 70% of the feedstock can be converted to oils and gases suitable for use as fuels,2 and the volume of remaining residue is considerably less than that of the original waste. Decisions on waste treatment tend to be based on legal requirements and economic factors; however the policy driving these should be guided by knowledge about the environmental impact of the various options. Although an old technology in principle, the commercial use of pyrolysis for waste treatment is still at the developmental stage. The economics of the process are r 2011 American Chemical Society
affected by external factors such as changing prices for alternative waste treatment methods and energy sources, but can be improved by optimization of the technology. Consequently each proposed process is different and produces different outputs. Knowledge of the environmental impact compared to other waste treatments could also affect the drive to improve performance. With these aims in mind, the outputs measured during work on a pilot plant developed and patented by ITEM Technology Solutions Ltd.,3 described in part 1 of this paper set,2 are assessed here via life cycle assessment (LCA) studies. LCA studies of pyrolysis of plastics, and in particular WEEE plastics, are limited and have studied rather different pyrolysis processes. A study by the UK Waste and Resources Action Programme 4 on treatment of mixed waste plastics included two types of pyrolysis, one for cracking polyolefins and the other producing diesel. In comparison to recycling and incineration alternatives, these scored highly for primary energy consumption, human toxicity, and ozone depletion impacts but poorly for global warming potential, solid waste, and abiotic depletion. Huisman et al.5 found that, for plastic dominated WEEE and using an aggregate score, it was worthwhile to recycle the plastic housing rather than incinerate it. However another study6 found Received: May 16, 2011 Accepted: September 22, 2011 Revised: September 21, 2011 Published: September 22, 2011 9386
dx.doi.org/10.1021/es2016654 | Environ. Sci. Technol. 2011, 45, 9386–9392
Environmental Science & Technology
Figure 1. Process flow for WEEE treatment.
that mechanical recycling could be less beneficial than other methods if the resulting recycled material was of low quality. Current WEEE plastics can contain brominated flame retardants (BFRs) which limits their direct recycling since many of these are now banned in the EU. Freegard et al.7 identified that the “Creasolv” solvent extraction method of recovering BFR-free plastics was potentially commercially viable in the UK at a throughput of 10 000 tonne/year. This combined with mechanical recycling of the resulting BFR-free plastic was found to have a significant net environmental benefit compared to landfill and incineration with energy recovery. A study of plastic packaging waste 8 suggested that for this particular mix, feedstock recycling through pyrolysis could give a comparable greenhouse effect benefit to a high level of mechanical recycling. Two studies9,10 on municipal solid waste also showed potential advantages to pyrolysis. In one the replacement of 55% recycling, 45% landfill with 17% recycling, 83% micropyrolysis gave significant extra benefit in respiratory inorganics and fossil fuels and small benefits in other categories. Azapagic10 compared large scale incineration with small scale pyrolysis and gasification, both with energy recovery. With a functional unit of amount of waste treated, incineration was slightly more beneficial in all categories except global warming where it was significantly worse. However if the functional unit was the amount of electricity produced, pyrolysis was more beneficial in all categories. The specific example of a small-scale pyrolysis process used to treat WEEE plastics is compared here with the alternatives of more or less recycling, incineration with energy recovery, and landfill.
’ MATERIALS AND METHODS Goal and Functional Unit. The goal was defined as “To compare the environmental impact of pyrolysis as a means of
ARTICLE
disposal of WEEE plastics with the alternatives of mechanical recycling, incineration and landfill.” The WEEE entering the system consisted of a full range of material including metals, plastics, glass, wood, batteries, capacitors, etc. However the aim of this study was to look specifically at treatment of plastics. The functional unit in this case was therefore taken to be the weight of WEEE containing 1 kg of plastic. Plastics were divided into three categories: a light fraction (less dense than water, typically polypropylene, suitable for mechanical recycling), a medium fraction (mainly styrenic polymers suitable for mechanical recycling) and a remaining heavy fraction containing plastics with high levels of additives, dense polymers found in small quantities, and contaminants. System Boundaries. The start point of the study was taken to be a designated WEEE collection facility (typically a civic amenity site). The impact of all waste treatment and disposal activities was included, through to final disposal of fractions that could not be recovered. The effect of recovery of energy and plastics was taken account of by including the impacts avoided as a result of not having to produce these from other sources. The infrastructure required for the various activities was included to provide consistency with the background data used. Process Flow. The process flow for the WEEE treatment route is shown in Figure 1. A certain level of “de-pollution” is required by the EU WEEE regulations, for example, removal of batteries, LCD backlights etc. This leads to a minimum level of dismantling which must always be carried out, and will lead to a certain amount of recovered metals and plastics. The depolluted WEEE can then be shredded and metals removed. The residues, mostly plastics with some contaminants, can either be sent to a plastics recycling facility or for pyrolysis, incineration or landfill. Rather than simply depolluting the WEEE, a greater level of dismantling can be carried out with more segregated parts removed and sent directly for recycling. The remainder can then be shredded, after which it is assumed it will go to a recycling facility. After removal of metals, the remainder is sorted, recyclable fractions are washed, dried, and extruded to pellets, and the remainder goes for pyrolysis, incineration or landfill. Analysis was carried out using Simapro (version 7) software from Pre Consultants. Allocation. The separation and recycling of metals was not included within the system boundary since the environmental benefits of these processes are very large and would have swamped the impacts of the treatment of plastics which was the area of interest. For example recycling of 1 kg of aluminum typically saves 5 10 kg CO2-eq of greenhouse gases compared to 1.5 2 kg for 1 kg of plastics.11 This meant that the environmental burdens associated with separating metal for recycling, that is, dismantling, shredding, and metal separation, needed to be excluded. Allocation of burdens was made using material mass. Inventory Data. The “ecoinvent” database from the Swiss Centre for Life Cycle Inventories was used for background data, including transport, energy sources, and production of virgin materials. Data applicable to the UK were used where available, and where this was not the case either European or Swiss data were used. Ecoinvent data for incineration and landfill were also used, with long-term being taken as infinite time. Typical WEEE composition was taken to be 45% metal, 23% plastic, 20% CRT components, 3.9% glass, 3.9% wood/fiber, 2% circuit boards and cables, 1.6% copper fines and 0.6% batteries.12 The CRT components, circuit boards and cables were assumed 9387
dx.doi.org/10.1021/es2016654 |Environ. Sci. Technol. 2011, 45, 9386–9392
Environmental Science & Technology
ARTICLE
Table 1. Environmental Impact Categories Used category
characterization unit
from method
normalization quantity
weighting factor
Human Health climate change
DALY
EI 99
2.382 10
3
0.4
ozone layer depletion
DALY
EI 99
2.193 10
4
0.1049
radiation
DALY
EI 99
2.99 10
respiratory organics
DALY
EI 99
6.978 10
respiratory inorganics
DALY
EI 99
0.01067
carcinogens
DALY
EI 99
1.994 10
human toxicity
kg 1,4-DB eq
CML
19936
0.0048
freshwater aquatic ecotoxicity
kg 1,4-DB eq
CML
1329
0.0048
marine aquatic ecotoxicity
kg 1,4-DB eq
CML
298686
0.0048
terrestrial ecotoxicity
kg 1,4-DB eq
CML
124.1
0.0048
acidification
kg SO2 eq
CML
71.89
0.0727
eutrophication
kg PO4 eq
CML
32.81
0.0557
land use
PDF m2yr
EI 99
3944
0.0182
carbon deposit
kg C
ES 06
116
0.0364
kg Sb eq
CML
39.05
0.0819
5
0.0727 5
0.0606
3
0.0048
0.0727
Ecosystem Quality
Resources abiotic depletion
to be removed during the depollution stage. The proportion of each material which could be reused or removed in whole parts for recycling was based on the transfer coefficients for manual treatment in the ecoinvent processes. The plastic was estimated to be made up of a 10% light fraction, a 60% medium fraction and a remaining 30% heavy fraction, based on figures from Freegard et al.7 The elemental composition of the heavy fraction was obtained from measurements carried out on samples obtained from Axion Polymers.13 These were combined with ecoinvent data to estimate the composition of the light and medium fractions. Most data for the operation of mechanical recycling processes were obtained from a range of manufacturers and an average taken. These included power, weight, size, air, water, salts (for density sorting), chemicals and emissions. Ecoinvent modules were used to estimate emissions from shredding and metal separation processes, and as a basis for extrusion of recycled plastic, assuming that the process would suffer 2.5% losses and that, in the case of ABS and HIPS, 0.5% of emissions would escape to the atmosphere. Plant infrastructure was taken as a proportion of the ecoinvent infrastructure module for a mechanical treatment plant based on equipment weight. The proportion of a particular fraction (light or medium) recovered by a sorting process was set to 77%. Data from the most effective process for removal of BFRs, “Creasolv”, was not publicly available, so data for steam and toluene used in the “Centrevap” process were used as representative.7 Most of the data for material inputs to and outputs from the pyrolysis process were obtained from experimental work on a pilot rig.2,13 Of the gases produced, 0.1% was assumed to escape and be emitted to air; the remainder substituted for fuels. Operational data for feeding and pyrolyzing the feedstock were obtained from design calculations for a production facility, including heat input of 6.8 MJ per kg feedstock.
Transport distances between a WEEE treatment center, a plastic recycling plant, and pyrolysis, incineration and landfill facilities were obtained from published data4 or estimated. It was estimated that 10% of material was transported by train, with wagons fully loaded and no return cost. The remaining transport was by lorry, fully loaded but with an empty return journey. Avoided Impacts. Recycling of plastics was considered to avoid the use of virgin polymer with a “substitution factor” λ, that is, One kg of recycled plastic replaced λ (
