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SAMPE Europe Conference 2018 Southampton
AN ENVIRONMENTAL COMPARISON OF CARBON FIBRE
COMPOSITE WASTE END-OF-LIFE OPTIONS
Fanran meng1*, Stephen J. Pickering1, Jon McKechnie1
1 Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, United Kingdom
* Email: Fanran.Meng@nottingham.ac.uk
ABSTRACT
By 2020, annual global production of the widely used high performance carbon fibre
reinforced polymers (CFRP) is expected to be over 140,000 tonnes. However, the resulting
increased quantity of CFRP waste has highlighted the need for sustainable treatment options
as carbon fibre manufacture has high-energy intensity. The objective of this study is to assess
the environmental viability of several waste management options for CFRP waste. Life cycle
assessment (LCA) models are performed to quantify the environmental impacts of waste
recycling and disposal pathways, comparing the current recycling techniques including
mechanical, pyrolysis, fluidised bed and chemical recycling processes relative to
conventional landfill and incineration, based on best available literature data, process models
and experimental work. Overall, LCA results indicate that recycling scenarios are generally
the most environmentally preferable options for primary energy demand (PED) and global
warming potential (GWP) minimisation in dealing with CFRP wastes. Recycling processes
can achieve a representative reduction of GWP of 19 to 27 kg CO2 eq. and reduction of PED
of 395 to 520 MJ per kg CFRP. This indicates the potential benefits of CFRP recycling
compared to other disposal routes.
1. INTRODUCTION
A steady increase in the use of carbon fibre reinforced polymers (CFRP) across a wide range
of aerospace (e.g., Boeing 787 airplane wing structures), automotive (e.g., BMW i3 body
panels), energy (e.g., wind turbine blades), and sporting applications (e.g., fishing rods,
bicycles) has been seen as CFRP contributes to significant weight reduction of the product
while providing excellent performance. In the past 10 years, the annual global demand for
carbon fibre (CF) has increased from approximately 16,000 to 72,000 tonnes and is forecast
to rise to 140,000 tonnes by 2020 [1]. However, the resulting CFRP wastes from
manufacturing (up to 40% of the CFRP can be waste arising during manufacture) [2] and
end-of-life components are generated at an increasing speed in the next decades. Quantities of
CF in production and end-of-life waste are estimated to be 62,000 tonnes by 2020 [3].
Existing automotive sector-specific EU regulations requires the recycling of at least 85% of
end-of-life materials [4]. Moreover, the high cost and energy intensity of virgin CF (vCF)
manufacture (198–595 MJ/kg) also provide an opportunity to recover substantial value from
CFRP wastes. Therefore, there is a clear and significant requirement to develop a CFRP
waste management system dealing with CF recovery to comply with this legislation.
Recycling has been recognised as a desirable end-of-life option to deal with CFRP wastes as
recycling has the potential to recover value from the waste materials rather than being
disposed in landfill or incineration, fulfilling legislative and sustainability targets. Current
recycling methods vary from conventional mechanical recycling to thermal recycling (e.g.,
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pyrolysis and fluidised bed process) and chemical recycling [5, 6]. For instance, the fluidised
bed process has been developed for the recycling of glass and carbon fibres at the University
of Nottingham for over 15 years [5]. Recovered fibre shows almost no reduction in modulus
and 18-50% reduction in tensile strength of recovered CF (rCF) relative to vCF [2, 5].
Recycling processes are now transitioning from lab scale to commercial facilities (e.g., ELG
Carbon Fibre Ltd. in UK using a pyrolysis recycling process with an annual capacity of 2,000
t/yr). Recovered CF could reduce environmental and financial impacts relative to vCF
production, while the potentially lower cost of rCF can create new markets for lightweight
materials [7, 8]. However, to date there is no systematic study comparing the environmental
performance of different recycling technologies.
Life Cycle Assessment (LCA) has been widely recognized as a valuable tool to aid decision
making for waste management systems, or strategic decisions concerning resource use
priority. Prior studies [9-13] have estimated energy requirements of various CFRP recycling
technologies and found substantially lower energy requirements compared to vCF
manufacture. However, very few studies have been undertaken to quantify environmental
impacts of CFRP recycling processes. Meng et al. [7, 8] evaluated the energy and
environmental impacts of CF recycling by a fluidised bed process and reuse of rCF to
manufacture a CFRP material for the first time. While potential overall environmental
benefits are claimed in technical studies of CFRP recycling processes and fibre reuse
opportunities to replace vCF, these benefits have yet to be demonstrated in a comparative life
cycle study of different recycling techniques.
Overall, prior analyses indicate reduced energy consumption for rCF compared to vCF. Due
to the lack of inventory data on recycling processes in any LCA databases and the markets for
recycled materials, comparative LCA studies on CFRP recycling are not well established. In
this study, a comparative LCA analysis of waste treatment options for CFRP materials has
been developed in order to quantify the environmental impacts (primary energy consumption,
global warming potential in 100 years) of selected waste disposal alternatives in a life cycle
perspective. Comparisons of conventional landfill, incineration and systems for
material/energy recovery (mechanical recycling; fluidised bed recycling; pyrolysis recycling;
chemical recycling) are undertaken to identify relative environmental performance of
alternative waste treatment routes on the UK basis.
2. METHODOLOGY
The life cycle model begins where the waste CFRP has been collected. System expansion has
been applied in the model that includes credits from substituting production of energy and
virgin materials as potential outputs of waste treatments. “Gate-to-grave” models are
developed for CFRP waste treatment by landfilling and incineration including waste
processing (disassembly, shredding), transport (100 km) (between recycling facilities only),
and waste treatment (landfill, incineration) (see Fig. 1). For recycling, a “gate-to-gate”
approach is taken to include the production of composite materials from rCF. The production
and use of CFRP prior to end–of-life is identical regardless of the waste treatment route
selected, thus is not included in the model; i.e., the boundary of our LCA assumes availability
of CFRP waste as a feedstock without any prior associated/allocated environmental burdens.
Maintenance and facility construction are also excluded as the allocated emissions and energy
use per functional unit from these long-lived assets are likely to be small. The functional unit
is one tonne of CFRP waste treated, which is assumed to consist of 55wt% fibre content and
45wt% matrix (epoxy resin) content, to compare waste management options (i.e., landfill,
incineration, mechanical recycling, pyrolysis recycling, fluidised bed recycling, and chemical
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recycling) for CFRP waste. In order to complete the full life cycle model, data collected from
process model based on the pilot plant, experimental data, best available literature data and
LCA databases (e.g., Eco-invent [14]) are the major sources to provide process-specific data.
Primary energy consumption (PED) and global warming potential (GWP) are the two main
metrics examined from the life cycle models. Fig. 1 shows a schematic representation for
waste treatment routes considered in this study and final products of each process.
Figure 1 CFRP Waste treatment routes.
2.1 Waste treatment routes on CFRP
This study considers six potential waste treatment routes for CFRP: landfill, incineration and
mechanical, pyrolysis, fluidised bed, and chemical recycling processes. Each waste flow for
recycling processes only can be seen in the following sections, and shown in Fig. 1. For all
recycling processes, collected CFRP waste materials are transported to materials disposal
disposition facilities by 32 tonne standard dump truck for shredding and separation before
waste treatment. Transport distances of 100 km to landfill and 200 km to incineration or
recycling facilities are assumed. Waste residues arising from the recycling process, which is
either landfilled or incinerated, is assumed to be transported an additional 100 km distance
from the recycling site to the landfill site or an additional 200 km to incineration site.
Combustion ash generated from incineration processes is assumed to be transported 100 km
to landfill. Recycling credits either from electricity production or CF replacement are
allocated to the waste treatment of CFRP waste as the methods specified in Section 2.3.
The inventory data for landfill and incineration is obtained from LCA databases – Gabi and
EcoInvent database. The inventory data for rCF is based on data collection of all the
activities including recycling processes and transport followed by documentation of the data
obtained covering inputs and outputs. Outputs from mechanical recycling consists of fine
fibre (24 wt%), fine powder (19 wt%) and coarse recyclate fractions (57 wt%) [15]. Energy
use for mechanical recycling can be obtained from literature data [2, 11]. The mass data from
pyrolysis recycling of CFRP is primarily based on laboratory-scale experiment. Energy
related inventory data is obtained from ELG’s dataset [16]. Inventory data related to fluidised
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bed is obtained from process models developed previously [8]. For chemical recycling
process, data is obtained from a laboratory-scale process as in [17].
2.2 Carbon fibre replacement
vCF production energy requirement and sources vary significantly (198 to 595 MJ/kg from a
mix of electricity, natural gas, and steam) and GHG emissions associated with vCF
production have been estimated at 30-80 kg CO2 eq. [8]. In this study, the manufacture of
vCF is modelled based on existing data from literature and life cycle databases, with
parameters (e.g., mass yield in acrylonitrile, polyacrylonitrile and CF conversion steps)
selected based on literature consensus, expert opinion, and results from a confidential
industrial dataset. All of these data have been recalculated relative to 1 kg of CF, giving an
energy consumption of 149.4 MJ electricity, 177.8 MJ natural gas, and 31.4 kg steam [8].
Utilising CFRP recyclate to displace vCF in composite production is desirable for
maximising revenue from waste streams and minimising energy use and greenhouse gas
emissions.
In order to determine the quantity of rCF that replaces vCF, it is required to have an
equivalent material function. The replacement method is based on variable material thickness
as described previously [7].
3. RESULTS
Section 3.1 describes the environmental impacts of the various waste treatment options
covering gate-to-gate impacts of recycling processes while Section 3.2 shows the total life
cycle environmental impacts including the use of waste treatment products (recovered
energy, rCF, chemicals) to displace conventional products/production.
3.1 Environmental impacts of waste treatment options
The PED and GWP results, excluding avoided virgin material production from replacement,
are compared among selected CFRP waste treatment methods as shown in Fig. 2. Landfill
emits only minor GHG emissions of 0.14 kg CO2 eq./kg CFRP waste including shredding
process of the waste materials due to no further GHG emissions after landfilling of CFRP
while incineration produces larger GHG emissions of 3.12 kg CO2 eq./kg CFRP waste. The
large amount of GHG emissions are mainly from the combustion process as the carbon
content of CFRP is released to the environment as CO2.
Excluding the benefits of energy recovery or the use of rCF materials, recycling processes
require energy inputs to treat the CFRP wastes. Mechanical recycling with landfilling of the
remaining coarse fraction gives GHG emissions of 0.11 kg CO2 eq./kg CFRP waste and the
mechanical recycling process accounts for 0.035 kg CO2 eq. In comparison, mechanical
recycling with incineration produces larger GHG emissions of 1.80 kg CO2 eq./kg CFRP
waste due to combustion of coarse fraction but this option will have GHG deduction (0.57 kg
CO2 eq.) from energy outputs to displace grid electricity and heat.
Thermal recycling process - pyrolysis and chemical recycling - require greater energy inputs
and are associated with higher PED and GWP than the fluidised bed, primarily due to the
recovery of energy from oxidised matrix material in the fluidised bed process. Pyrolysis is
associated with 2.9 kg CO2 eq./kg CFRP waste [16], primarily from electricity and natural
gas consumption for recycling process. Transport accounts for only 1% of the total energy
consumption. Fluidised bed and chemical recycling processes have similar amount of GHG
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emissions arising from recycling process, producing 1.56 kg CO2 eq. and 1.53 kg CO2 eq. per
kg CFRP waste.
Figure 2 Primary Energy Demand and Global Warming Potential comparison of the carbon
fibre recovery and conventional landfill and incineration without credits from the use of
products of recycling processes.
3.2 Life cycle impact assessment
Including the recycling credits from CF replacement or electricity production, the PED and
GWP of different CFRP waste treatment methods are shown in Fig.3. Overall the LCA
results indicate that recycling scenarios are generally the environmentally preferable options
for PED and GWP considered in this analysis, though the exact net environmental impacts
vary depending the CF replacement method selected.
Conventional waste treatment processes (landfill, incineration) perform worst in terms of life
cycle PED and GWP. Even including energy credits, GHG emissions from incineration
exceeds the energy saving from electricity and heat power generation giving a net GHG
emission of 2.14 kg CO2 eq. while landfilling consumes more than 1.11 MJ energy due to no
environmental credit from displacement.
Mechanical recycling with landfilling of the coarse recyclate fraction exhibits a modest
global warming potential reduction relative to mechanical recycling with incineration. The
displacement of glass fibre production results in a GHG emissions credit of 0.48 kg CO2
eq./kg CFRP, giving a net global warming potential reduction of 0.37 kg CO2 eq./kg CFRP.
However, if the coarse recyclate fraction is incinerated, mechanical recycling produces a net
GHG emission increase of 0.76 kg CO2 eq./kg CFRP. Compared to the advanced thermal
recycling process, mechanical recycling was not favourable primarily due to the low
mechanical performance obtained in the recovery [18].
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Figure 3 Primary Energy Demand and Global Warming Potential comparison of the carbon
fibre recovery and conventional landfill and incineration including credits from the use of
products of recycling processes.
4. CONCLUSIONS
This study provides a complete life cycle assessment of different waste management options
of CFRP waste, presenting for the first time comparative analyses between advanced CFRP
recycling technologies based on literature currently available, process models or data
measured experimentally. CF recycling by pyrolysis, fluidised bed and chemical processes is
found to provide substantial advantages relative to conventional recycling methods (landfill,
incineration, mechanical recycling) in terms of life cycle energy use and greenhouse gas
emissions.
While CFRP recycling has a range of potential advantages, challenges and opportunities co-
exit. Future work should evaluate in more details for application fields of rCFRP materials,
such as aerospace, automotive, and renewable energy industries. Full consideration of their
environmental and financial impacts would enable inclusion of trade-off between CFRP
waste management strategies and help to ensure opportunities are pursued that provide the
greatest overall net benefit.
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