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A circular economy approach to green energy:
Wind turbine, waste, and material recovery
Siqi HAO1
University of Nottingham Ningbo China
Siqi.HAO@nottingham.edu.cn
Adrian T. H. KUAH, PhD 2 *
James Cook University Australia, Singapore Campus
adrian.kuah@jcu.edu.au
Christopher D. RUDD, PhD 2
James Cook University Australia, Singapore Campus
chris.rudd@jcu.edu.au
Kok Hoong WONG, PhD1
University of Nottingham Ningbo China
kok-hoong.wong@nottingham.edu.cn
Nai Yeen Gavin LAI, PhD1
University of Nottingham Ningbo China
gavin.lai@nottingham.edu.cn
Jianan MAO1
University of Nottingham Ningbo China
zy21955@nottingham.edu.cn
Xiaoling LIU, PhD1
University of Nottingham Ningbo China
xiaoling.liu@nottingham.edu.cn
Declarations of interest: none
1 University of Nottingham Ningbo China (UNNC), Ningbo, 315100, China
2 James Cook University (JCU), 149 Sims Drive, Singapore 387380, Singapore
*Corresponding author: Associate Professor Adrian Kuah (Adrian.Kuah@jcu.edu.au)
College of Business Law and Governance
James Cook University Australia, Singapore Campus
149 Sims Drive
Singapore 387380
2
Abstract
Wind energy has been considered as
one of the greenest renewable energy
sources over the last two decades.
However, attention is turning to
reducing the possible environmental
impacts from this sector. We argue
that wind energy would not be
effectively “green” if anthropogenic
materials are not given attention in a
responsible manner. Using the
concept of the circular economy, this
paper considers how anthropogenic
materials in the form of carbon fibers can reenter the circular economy system at the highest
possible quality. This paper first investigates the viability of a carbon-fiber-reinforced polymer
extraction process using thermal pyrolysis to recalibrate the maximum carbon fiber value by
examining the effect of (a) heating rate, (b) temperature, and (c) inert gas flow rate on char
yield. With cleaner and higher quality recovered carbon fibers, this paper discusses the
economic preconditions for the takeoff and growth of the industry and recommends the reuse
of extracted carbon fibers to close the circular economy loop.
Highlights
• Four economic preconditions ensure re-entry of recovered fiber into circular system
• Rapid heating rate and high temperature reduce char formation on carbon fiber
• Effect of inert gas flow on reduction of char residues is only obvious at 550°C
• Improved pyrolysis conditions increase char intrinsic reactivity and oxidation rate
Keywords:
circular economy, wind turbine, carbon fiber, pyrolysis, recovery, recycling
CItation:
Hao S, Kuah A.T.H, Rudd C.D., Wong K.H., Lai N.Y.G., Mao J and Liu X (2019) A circular
economy approach to green energy: wind turbine, waste, and material recovery. Science of the
Total Environment (In Press)
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1. Introduction
Wind energy has been considered as one of the greenest renewable energy sources over the
last two decades (Liu and Barlow 2017; Liu et al., 2019). As a result, national and regional
energy policies have encouraged the development of onshore and offshore wind farms, where
installed capacity has grown rapidly from 7,600 MW in 1998 to 591,000 MW in 2018 (Global
Wind Energy Council [GWEC], 2015; 2019). Amidst this growth, attention turns naturally to
the environmental impact of end-of-life turbine blades, especially when the end-of-life blades
and associated structures end up in landfills and negate the “green” credentials of the industry.
This is a pertinent challenge because the annualized growth rate in wind power over the
first decade of the 21st century exceeded 12% (GWEC, 2014) and based on projection, 14.9–
18% of global electricity demands will be supplied by wind energy between 2020 and 2050
(European Wind Energy Association [EWEA], 2014; International Energy Association [IEA],
2011). A steady growth scenario of new installation wind farms around the world has been
reported by Liu and Barlow (2017) in China, the United States, Europe, and the rest of the
world, as shown in Fig. 1. New global installation capacity grew to 51.7 GW in 2014, then 63.8
GW in 2015 but stayed fairly consistent for the next 3 years (54.9 GW in 2016, 53.5 GW in
2017, and 51.3 GW in 2018) in a report by the GWEC (2019).
Figure 1: Annual new wind turbine installations by region
Source: Adapted from Liu and Barlow (2017)
With the rapid growth in wind energy capacities, and considering the typical turbine design
lifespan of 20 years, Liu and Barlow (2017) have projected that the end-of-life waste from
turbines becomes a critical global problem by 2028. Albers (2009) predicted around 50,000
tons of blade waste in 2020, with the amount exceeding 200,000 tons by 2034. Similarly,
Andersen et al. (2014) predicted 400,000 tons of blade waste being generated between 2029
and 2033. Liu and Barlow (2017) estimate the blade material usage in China reaching
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1,500,000 tons by 2050. Therefore, there is a pressing need to consider this very significant
waste stream.
A typical horizontal-axis unit consists of four main components: a foundation, a tower, a
nacelle, and three blades. The nacelle is fabricated from steel and copper. The tower is
fabricated from concrete or steel and the foundation is made solely from concrete, with the
rotating blades made from composite materials to minimize inertial and windage losses.
Considering the anthropogenic materials used, composite is one of the most problematic
materials because there are currently no mature recycling channels (Job, 2013; Pimenta and
Pinho, 2011).
The composite found in the blades is of fiber-reinforced polymer composite for ease of
manufacture into aerodynamic shape and high mechanical performance. However, because of
the cross-linked polymer chains in the thermoset matrix, recycling remains a significant
challenge, particularly reusing the ingredients in other high-grade applications. Most of the
older blades are made of glass-fiber-reinforced polymer (GFRP) composites because of their
relatively low manufacturing and material costs. However, this imposes a constraint for
recycling options because cost must be tightly controlled to make the recycling process
economically viable. To date, the only recycling route that is commercially active is where
GFRP waste is shredded and consumed in cement kilns. The value of the waste stream is
reduced to that of calcium carbonate, making this approach only viable where landfill is
prohibited, as in the case of Germany (Job, 2013).
Because the wind power industry is working toward larger turbines capable of producing
10 MW or greater, weight saving is a primary concern because blade mass increases in
proportion to the cube of the rotor radius (Igwemezie et al., 2019). This makes carbon fiber an
ideal material because of its high specific stiffness and reduced fatigue sensitivity (Veers et al.,
2003). However, the main disadvantage is its high initial cost (Liu and Barlow, 2017). For this
reason, carbon-fiber-reinforced polymer (CFRP) has only displaced GFRP in manufacturing
structural elements, such as the spar, for blades longer than 45 m. For the next generation of
10 MW units with blades of length 100 m, Wood (2010) notes that the total mass can be reduced
by 30% if carbon fiber is used to make blade skins. This mass reduction can potentially mitigate
the high cost impact of the material (Veers et al., 2003). Thus, it is recognized that the
proportion of carbon fiber composite usage will increase and a trend toward fully carbon
composite blades is expected (McKenna et al., 2016). Because carbon fibers are energy
intensive to produce and have high intrinsic value, there are both environmental and economic
motivations for recovering carbon fibers from CFRP (Shuaib et al., 2015).
In this study, the concept of Circular Economy (CE) is used to consider how the valuable
carbon fiber can be recovered from the end-of-life blades and what economic preconditions are
required to allow the fiber to reenter the cycle at the highest possible quality. The CE is defined
as “an industrial system that is restorative or regenerative by intention and design. It replaces
the end-of-life concept with restoration, shifts towards renewable energy, elimination of toxic
chemicals which impair reuse and return to the biosphere, and aims for the elimination of waste
through the superior design of materials, products, systems, and business models” (Ellen
MacArthur Foundation, 2013). The CE creates a closed-loop system in which resources can be
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kept in a continuous cycle of production and utility, thereby allowing precious and finite
resources to generate more value for an extended period of time (United Nations Environment
Program [UNEP], 2006). Hence, moving toward CE necessitates changes in the way we design,
produce, consume, use (and reuse), and manage waste.
Some common CE approaches include: (1) recycling and recovery, where used materials
are processed or treated so that they can be reused (Hamzaoui-Essoussi and Linton, 2010); (2)
remanufacturing, in which worn-out, damaged, or end-of-life products are restored (Wang and
Kuah, 2018); (3) sharing or collaborative consumption for optimization of utility (Belk, 2014);
and (4) product life extension, in which products are ultimately designed to have a longer
lifetime (Tse et al., 2015). These practices require technological improvements and changes to
processes, hence most innovation is driven by industry.
This paper does not consider lifetime extension or collaborative consumption possibilities
covered by the CE concept, but rather how carbon fiber could reenter the circular economy
system at the highest possible quality—either in the forms of a product (reuse/repurpose or
resize/reshape) or as recycled “raw” or intermediate material (recycle, recovery, and
conversion). Sending end-of-life wind turbine blades to landfill is not a long-term viable
solution, where many European Union countries legislate against composite waste being sent
to landfills (Pickering, 2006). In response, Asmatulu et al. (2013) explored the reuse of these
materials as structural components in bridges, buildings, or artificial reefs. Other ways to
repurpose blades may involve bridges or urban furniture, but the key challenge remaining in
the reuse of composites in public amenity infrastructure is to ensure structural integrity. In
terms of composite blade recycling, the valuable output streams are fiber, filler, resin, and
energy recovered (Liu et al., 2019). Blades’ recycling typically involves jaw cutters for
sectioning before crushing or shredding. Shredding reduces fiber length and strength while
hammer milling reduces the composite to smaller fragments, generating noise and dust. The
recyclates still contain polymer residue, quality is variable, and applications therefore limited
to low-grade structures.
From the CE’s perspective, material loop needs to be closed and this very much depends
on the quality of the recovered carbon fiber and the technicalities involved (Hahladakis and
Iacovidou, 2018; Kasprzyk and Gajewska, 2019). Clean carbon fibers can be recovered
through three known thermal decomposition processes. First, a pyrolysis process, which
extracts fibers, energy, and pyrolysate at high temperature in an inert environment. The trade-
off of this process is the use of the lowest possible temperature to devolatilize the polymer to
avoid fiber degradation (Fraisse et al., 2016). Second is the “fluidized bed” process (Pickering
et al., 2015) to decompose the polymer composite thermally. The feedstock is heated to 450–
550 °C on a layer of silica sand, fluidized by a flow of hot air, thereby oxidizing and
decomposing the polymer matrix. Solvolysis is an alternative process performed using sub- or
supercritical fluids. This strips the polymer matrix via a chemical reaction in an aggressive
solvent attack. Pure carbon fibers, an inorganic residue, and low molecular weight
hydrocarbons are the typical output streams (Sokoli et al., 2018).
Among the three recovery options, pyrolysis has been the process of choice in recent
decades. Unlike the fluidized bed process technology, which burns off the organic matrix for
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energy recovery, the pyrolysis process recovers both fiber and a hydrocarbon stream for
potential reuse. Although low-cost solvents are used in solvolysis, a high energy intensity of
up to 101 MJ/kg (La Rosa et al., 2016) is required to achieve the high pressure and temperature
conditions, thus limiting their progress to the laboratory scale. In contrast, the energy
requirement for a typical pyrolysis process is much lower at around 30 MJ/kg (Witik et al.,
2013) and compares favorably to virgin carbon fiber production, which consumes 704 MJ/kg
(Das, 2011). Microwave-assisted heating may also yield energy saving compared with
conventional convective furnaces (Jiang et al., 2015). Fiber recovered from the pyrolysis
process is relatively clean, with low levels of char residue, and around 90% property retention
(McConnell, 2010). These fibers also bond well to epoxy resin (Jiang and Pickering, 2016),
making them reusable in new composites.
Despite its growing popularity, few reports address the effect of pyrolysis conditions on the
quality of the recovered fiber. Meyer et al. (2009) focused on the effects of pyrolysis
temperature, dwell time, and oven atmosphere on the performance of recovered carbon fibers,
while Lyon (1998) studied char residuals and their dependence on resin chemistry. A
conventional pyrolysis process will result in char formation, which requires a second oxidative
treatment (Meyer et al., 2007) because it inhibits free fiber handling and dispersion quality in
intermediate products such as nonwoven mat (Wong et al., 2012) and compromises adhesion
strength. Lower oxidation temperature and short oxidation times seem to assist char
minimization and fiber strength retention (Yang et al., 2015). Clearly, minimizing char residues
is critical, along with minimizing oxidative damage to the fiber.
It is evident that carbon fiber recovery from end-of-life blades is a critical issue for greener
and more sustainable wind energy production, where successful carbon fiber recovery through
pyrolysis is very promising to create and close the circular economy loop. The presence of char
residues affects the quality of recovered carbon fibers and posttreatment processes are needed,
which add additional cost and complexity to the recovery of fibers. To achieve this, we
determine the pyrolysis conditions that lead to ideal recovered fibers, which could reenter the
circular economy system at the highest possible quality and without any secondary cleaning.
Hence, this paper investigates the potential of CFRP recovery and the quality of the
recovered materials. To close the CE loop, the technicalities and economics of extraction are
considered, alongside the potential applications of recovered fibers. The paper is organized as
follows: Section 2 presents the method to determine the pyrolysis conditions that lead to ideal
recovered fibers, which could reenter the circular economy system at the highest possible
quality. Section 3 presents the results; while Section 4 discusses the recovery costs, economic
preconditions, and considerations for fiber applications. Section 5 concludes.
2. Methodology
The effects of pyrolysis temperature, heating rate, and nitrogen flow rate on char volume
were investigated using a thermogravimetric analyzer (TGA). Then, the oxidation rate of the
produced char, as expressed in terms of intrinsic reactivity, was measured via a nonisothermal
approach. The morphology of the char was then studied via a scanning electron microscopy
(SEM) analysis.
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2.1 Materials
Unidirectional prepreg Toray® T700s carbon fibers and 37 wt% epoxy resin were supplied
by Aojing Composite Company, Shanghai, China. The prepreg was cut to 200 mm by 200 mm
and cured at 140 °C in air for 2 h. The release film and backing paper were removed before the
prepreg was pyrolyzed using a TGA. To study the char oxidation rate, epoxy was squeezed out
from the as-received prepreg between hot platens at 5 MPa and 80 °C. The resin was cured at
the previous schedule and subjected to thermal analysis, as described below.
2.2 Thermal analysis
Thermal and degradation properties of the cured prepreg were investigated using an SDT
Q600 TGA from TA Instruments, Delaware, US, on approximately 20 mg samples in a nitrogen
environment according to the heating profiles summarized in Fig. 2. Samples were heated from
ambient to either 550 °C or 650 °C, after which the samples were held isothermally for 30 min
before cooling to room temperature. A range of heating rates were used in this study. The
slowest heating rate was decided according to the common practice in lab-scale pyrolysis
studies on composite waste, which is between 10 and 30 °C/min (Onwudili et al., 2016; Song
et al., 2017), thus it was set to 20 °C/min. The highest heat rate was determined by the capability
of the TGA unit, i.e., 200 °C/min. Other selected heating rates were 80 and 100 °C/min, which
are common in fast pyrolysis studies on biomass (Wang et al., 2019) and coal (Jiang et al.,
2019). The weight loss profile of the degrading sample under these four different heating rates
was recorded. These tests were undertaken at a constant nitrogen gas flow rate of 50 ml/min.
However, in the later stage of the study, the nitrogen gas flow rate was increased to 100, 200,
and 400 ml/min with other process variables unchanged.
Char oxidation kinetics was studied by subjecting the neat epoxy to the same thermal cycles
as shown in Fig. 3 to create different grades of char. The gas flow was maintained at 50 ml/min.
The pyrolytic chars were later dried and subjected to an intrinsic reactivity test and SEM
analysis, as detailed below.
Figure 2. Thermal treatment profile used in TGA test
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Figure 3. Effect of heating rates and pyrolysis temperatures on CFRP’s TG curves
2.3 Char analysis
The combustion characteristics of carbonaceous residue were determined using an intrinsic
reactivity analysis with nonisothermal heating in air (Unsworth et al., 1991). Pyrolytic char
samples were heated from ambient temperature to 105 °C inside an air-filled chamber at a
heating rate of 20 °C/min. The temperature was maintained for 30 min for moisture removal
and then ramped to 900 °C at the same heating rate to complete the intrinsic reactivity study.
Mass loss profile (TG) and the first derivative of the mass loss profile (DTG) were analyzed to
identify peak temperature (PT) and burnout temperature (BT) of the pyrolytic char sample. The
peak of the DTG curve was used to determine the PT value because it is defined as the
temperature at which the highest combustion rate occurs. The BT is defined at 1 wt%/minute
of combustion rate. A Zeiss@ Sigma VP scanning electron microscope was used to study the
morphology of pyrolytic char with a 10 kV accelerating voltage.
3. Results
3.1 Effects of heating rates and pyrolysis temperatures
TGA mass loss profiles are shown in Fig. 4, which clearly demonstrate that after a marginal
drop during early-stage heating, each of the profiles undergoes a sharp drop in mass before
finally reaching a plateau region. The initial drop was mainly due to moisture loss, while
conversion of the epoxy matrix into volatiles produced the sharp loss in mass and the rate of
conversion, which is accelerated at higher pyrolysis temperatures. A portion of the epoxy
matrix was converted into pyrolytic char and remained on the surface of the carbon fiber.
Together, they contributed to the final masses at the plateau region as shown in Fig. 3, which
vary with heating rates and pyrolysis temperatures. Because the carbon fiber reinforcement was
relatively unaffected by the pyrolysis process, the variations in final mass loss corresponded to
the extent of char retention.
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Figure 4: Effect of heating rates and pyrolysis temperatures on CFRP’s mass loss
The variations in mass loss are further illustrated in Fig. 5, recalling the initial epoxy
loading of 36.4 wt%. A mass loss exceeding 36.4 wt% suggests degradation of the carbon fiber,
but a lower value indicates the presence of pyrolytic char. At 550 °C, a mass loss of 28.0 wt%
was recorded at 20 °C/min and a further 5 wt% reduction was achieved by ramping the heating
rate to 200 °C/min, which suggested the fiber residue entrained 3.5 wt% of char. Lower char
contents were obtained at the higher pyrolysis temperature of 650 °C and again, higher heating
rates resulted in greater mass loss, but the rate effects were lower than that at 550 °C.
Bridgwater and Peacocke (2000) have reported the significance of these two factors on the
mass distribution of char and volatiles from biomass, typically a higher heating rate and
pyrolysis temperature favored the production of gaseous products and the reverse conditions
favored char formation due to secondary coking and repolymerization reactions. These agree
with the findings reported in Fig. 4, and because the aim of the project is to reduce char
formation, this can be achieved with higher heating rate and/or increasing the pyrolysis
temperature from 550 °C to 650 °C. The former factor is preferable because higher
temperatures are likely to degrade the carbon fiber performance.
3.2 Effect of nitrogen gas flow rate
Fig. 5 shows the mass loss curve for prepreg at different nitrogen gas flow rates for two
different pyrolysis temperatures. It can be seen in Fig. 5(a) that at a pyrolysis temperature of
550 °C, more volatiles were released, or fewer char residues were left on the fiber when the
gas flow rate was increased from 50 ml/min to 200 ml/min for both 20 °C/min and 80 °C/min
heating rates. Higher flow rate suggests shorter residence time within the heating chamber for
volatiles and this reduces secondary reactions that promote char formation (El-Harfi et al.,
1999; Pütün et al., 2006; Uzun et al., 2007) or cracking of the primary volatiles and
repolymerization in hot char particles (Lanzetta et al., 1997). However, no further significant
mass loss was observed with gas flow rate higher than 200 ml/min. In contrast, the dependency
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of mass loss on gas flow rate became less extensive at a heating rate above 100 °C/min.
Previous studies again support this finding, e.g., a shortened volatiles’ residence time was
observed by Montoya et al. (2015) in depolymerization reactions of cellulose and
hemicellulose. In another case, which focused on pyrolytic behavior of rapeseed, Haykiri-
Acma et al. (2006) found that a higher heating rate reduced volatiles’ residence time, which
could further reduce secondary reactions such as cracking, repolymerization, and
recondensation. Our results are consistent with these studies because greater mass loss
accompanied higher heating rates and the volatiles’ residence time was expected to be greatly
reduced and become independent of nitrogen flow rate for a heating rate above 100 °C/min.
Similar tests on the effect of gas flow rate were repeated at a higher pyrolysis temperature
of 650 °C. However, as plotted in Fig. 5(b), a rather complex relationship is observed. At 20
and 80 °C/min heating rates, char mass loss increased gradually to 32.4% and 33.6%,
respectively, with increasing gas flow rate to 200 ml/min, but the mass loss started to decline
with further increase in flow rate to 400 ml/min. A general trend toward higher mass loss,
despite not being as evident as the results at 550 °C, can be identified for 100 and 200 °C/min
heating rates. Overall, the impact of the gas flow rate was less apparent at 650 °C.
Figure 5. Effect of nitrogen gas flowrate s on CFRP’s mass loss with (a) 550°C and (b) 650°C
pyrolysis temperature
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3.3 Intrinsic reactivity analysis of char oxidation rate
Fig. 6 shows the effects of pyrolysis temperature and heating rate on chars’ intrinsic
properties; at either 550 °C or 650 °C, both PT and BT reduce with higher heating rates, which
indicates char resulting from higher heating rate was more reactive and could be oxidized at a
lower temperature. In addition, higher pyrolysis temperatures increased the BT value provided
the heating rate was less than 100 °C/min. Consistent with this, Chitsora et al. (1987) reported
such an effect in relation to German bituminous coal char produced in a fluidized bed, similarly
on lignite char by Ashu et al. (1978).
Figure 6. Effect of heating rates on intrinsic reactivity of char generated at 550 and 650 ºC.
3.4 Scanning electron microscopy
The effects of heating rates and pyrolysis temperatures on the morphologies of pyrolytic
char are depicted in SEM images shown in Fig. 7. It is evident that the combination of low
heating rate and low pyrolysis temperature, as shown in Fig. 7(a), created char with a rough
but continuous appearance. However, at 650 °C, as shown in Fig. 7(b), porosity became
apparent, increasing with higher temperature and heating rate. Fushimi et al. (2003) suggested
that a high heating rate caused a rapid evolution of volatiles, which in turn increased the porous
structure. Fast volatile release rate produced considerable overpressure, which encouraged void
coalescence and greater porosity levels (Guerrero et al., 2005). Septien et al. (2018) reported
that a high porosity level would enhance gas species diffusion within the char open structure,
which facilitated penetration of oxygen and better evacuation of reaction products from the
porous structure. Char created from the epoxy matrix in this study reinforced their findings
because a high intrinsic reaction was found from char with a high porosity.
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Figure 7. SEM images of pyrolytic chars produced at different temperatures and heating rates
(a) 550°C, 20°C/min (b) 650°C, 20°C/min (c) 550°C, 200°C/min and (d) 650°C, 200°C/min
4. Discussion
Section 3 reported that a rapid heating rate caused a substantial reduction of pyrolytic char
volume, particularly for pyrolysis at 550 °C. The inert-gas flow rate was another contributing
factor to char yield and the level at which it affected the char content depended on the pyrolysis
temperature and heating rate. Higher gas flow rate promoted devolatilization and reduced the
char content provided the heating rate was less than 100 °C/min and the pyrolytic temperature
was 550 °C. However, the positive effect became insignificant at higher temperature and
heating rate. The intrinsic reactivity of the char was significantly influenced by the pyrolytic
reaction conditions. Char with higher intrinsic reactivity was associated with high heating rates
and temperature and this implied the char had a faster oxidation rate. These findings are of
commercial relevance to the carbon fiber recycling industry with high priority in cost control
because with a lower char volume and faster oxidation kinetics, the energy-intensive oxidation
process can be shortened and the carbon fiber can potentially be recovered with a higher
mechanical performance due to the compressed thermal cycle. The importance of recovery cost
and the reuse options available for the recovered carbon fiber will be discussed in subsequent
sections.
4.1 Recovery costs
Presently, there is no industrial-scale recycling of end-of-life turbine blades; therefore, the
costs and actual commercialization procedure have not been well-defined (Larsen, 2009).
Research on recycling and remanufacturing of these items is still ongoing. Pyrolysis is a mature
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fiber recovery approach and has been considered suitable for mass-scale commercial efforts
use (Rybicka et al., 2016). Existing pyrolysis practices require size reduction and progressively
shorter fibers (and lower value) as the number of cycles increases. Thus, the hierarchy of
applications ranges from initial, continuous fiber composites, ultimately to milled fiber fillers
for lower grade structures. This potential circular economy flow is illustrated in Fig. 8.
Figure 8. Circular economy for end-of-life wind turbine blades
The materials CE loop sets some economic preconditions for the retrieval of carbon fibers.
At the initial stages, without a demand-side pull, legislative drivers, or standards in the reuse
of materials, private sector investments are unlikely. Furthermore, implementing recycling and
recovery comes at a price, including the collection costs, pretreatment and sorting costs, and
the costs of final recovery.
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However, the market value of the recovered carbon fiber and concomitant by-products
could offset many of these costs. This is because the production process of virgin carbon fibers
is energy intensive, and incurs high manufacturing cost, especially in the case of high-grade
carbon fiber (used for structural applications such as blades). Therefore, there is a greater
economic incentive to recover these carbon fibers. Moreover, the costs of commercially
available fibers reclaimed through pyrolysis have been reported by industry sources to be about
10 Euros per kg while the market value of virgin product is 18–50 Euros per kg (ELG Carbon
Fiber, 2016).
Industry perspectives also agree that the cost to recover carbon fiber will be a fraction of
that for producing virgin carbon fiber (Carberry, 2008). The energy requirement to recover
carbon fibers (Cherrington et al., 2012; Vo Dong et al., 2018) is typically <10% that of virgin
fiber production1. Previous studies also highlighted the importance of throughput in reducing
the unit cost through a recycling plant (Meng et al., 2018). Clearly, the energy requirements,
efficiency, and cost associated with recycling the carbon fibers from blades would improve
beyond the current reported (laboratory) figures in mature, mass production settings.
4.2 Economic preconditions needed
As the technology for recovery and up-scaling of recycling continues to be developed,
there are four considerations that need to be addressed to enable the takeoff and growth in the
recovery of carbon fibers from end-of-life blades.
First, there must be a network to ensure a consistent supply of feedstock for fiber recovery
that would deliver economies of scale. The current lack of infrastructure for collecting end-of-
life blades is a key challenge. Ideally, recycling facilities should be located close to wind farms;
alternatively, mobile recycling units have been trialed in some regions. Sorting and
classification will also improve value streams. Nonstandard construction (Brøndsted et al.,
2005) means that traceability would help to identify ideal processing parameters, likely yield,
etc.
Second, a marketplace must be created for secondary or recovered materials (Stahel, 2013)
to centralize demand for recyclates or fibers produced. The market demand for the materials
will help to offset the cost of decommissioning and collecting end-of-life blades. The concept
of CE necessitates that there is a ready market to receive and reintroduce the recovered
materials into the economic cycle (Wang and Kuah, 2018). There could be an issue if the cost
of virgin materials is already low as in the case of glass fibers. The recovered materials must
have a value higher than the cost needed to retrieve them, i.e., using recovered fibers must be
cheaper than the cost of using virgin materials directly. This is the most likely case for carbon
fibers.
The recent agreement in providing composite waste from Boeing’s aircraft manufacturing
facilities to ELG Carbon Fiber signaled both the value of composite waste supply and the
availability of a marketplace for the recovered fiber (Zazulia, 2018). Fiber recovered from the
manufacturing wastes and growing end-of-life parts can potentially help in mitigating the
1 286 MJ/kg (Suzuki and Takahashi, 2005) and 704 MJ/kg (Das, 2011)
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shortage in virgin fiber supply, particularly in the demand for discontinuous fibers. Recyclers
have been developing scalable conversion technologies to enlarge the supply-side capacity for
recovered fiber. For example, a new hybrid nonwoven mat containing recovered carbon fiber
and polyamide 6 resin was developed for making seatbacks for the high-volume automotive
applications (Milberg, 2017). Driven by the affordability (Nicolais and Pisanova, 2012) and
more environmentally friendly recovery process, more reuse applications in the near future are
anticipated.
Third, quality standards for the fibers or recyclates must be established to build confidence
(Carberry, 2008; Finnveden et al., 2013; Job, 2013; Pickering, 2006; Wood, 2010). The design
of a product, the material retrieval system, efficiency of sorting, and the recovery technology
are fundamental in increasing the quantity, quality, and usability of recovered materials
(Gregson et al., 2015).
Fourth, key legislation and government policy intervention need to mandate both operators
and end-users into the reuse of recovered carbon fiber with accompanying fiscal penalties and
benefits. Cherrington et al. (2012) outlined some of the key examples of legislation and
directives relevant to end-of-life blades. Landfill and incineration disposal are increasingly
penalized (Cherrington et al., 2012) whilst R&D incentives for sustainable product design and
technologies that enhance the recycling process (Söderholm and Tilton, 2012) are increasingly
important. Extended Producer Responsibility (EPR) is another important initiative to
encourage further recycling, where producers play a more proactive role in supporting recovery
and reuse. EPR has been successfully utilized for end-of-life vehicles and waste electrical and
electronic equipment (Cherrington et al., 2012).
4.3 Recovered carbon fibers’ applications and considerations
Low-grade application:
Granulation of CFRP scrap requires the lowest energy of all recovery methods (Wong et
al., 2017). These recyclates can be sorted into resin-rich and fibrous-rich groups, but both have
low commercial value because the recyclates still contain a high level of resin residues, limiting
their usage to low-grade applications, such as being used as a filler for polymer resin or
construction materials (Thomas et al., 2014) and concrete (Mastali and Dalvand, 2016).
Medium-grade application:
To maximize the value of recovered carbon fibers, they should be separated from the
polymer matrix and the fiber should retain enough mechanical performance for the next
application. To date, this can be achieved via the common pyrolysis process and with the use
of adequate pyrolytic conditions; as discussed in this paper, cleaner and stronger carbon fibers
can potentially be recovered. However, blades are bulky, and to reduce logistical cost,
decommissioned blades are sectioned in situ to a manageable size for transportation to
recycling facilities, at which, further size reduction has taken place prior to feeding to the
pyrolysis process.
As a result, the recovered fibers are generally short and fluffy and cannot be processed in
the same way as the virgin fibers. To allow the fibers to reenter the circular economy system
16
at the highest possible quality, they should be converted into intermediate forms suitable for
industrial molding processes.
Nonwoven mat is a common intermediate form widely offered by the recycling industries,
which can be made by carding and spinning or papermaking. Both are cost-effective processes,
suited to mass volume production and with versatile combinations of thermoplastic filaments
or powders suitable for thermoforming (Wolling et al., 2017). Because of the random
orientation of fibers, the fiber packing density is limited to around 30% (Wong et al., 2017).
Nonwovens are typically used in nonstructural applications, such as tooling for aerospace parts
(Gardiner, 2014), heating elements (Pang et al., 2012), and electromagnetic interference
shielding (Wong et al., 2010).
Higher-grade application:
Fiber alignment is a necessary intermediate step for higher-value applications because the
presence of a close-packed structure greatly increases the reinforcing potential of the fibers.
Hydrodynamic alignment was originally developed in the 1970s (Bagg et al., 1977) but more
recent innovations (e.g., van de Werken et al., 2019; Wong et al., 2009) optimize streamline
velocities to deposit an aligned fiber slurry onto a moving mesh. Clean, free-flowing filaments
are essential here, underlining the need for a char-free feedstock, because char carryover
inhibits uniform dispersion, hence the need for upstream control of pyrolytic conditions, as
reported here.
Other alignment technologies include electrostatics (Ravindran et al., 2018), air streaming
(Ericson and Berglund, 1993), and the dry carding process (Miyake and Imaeda, 2016). Fiber
alignment plays an important role in upgrading the value of the recovered carbon fiber but,
clearly, production economics remains to be established for any of these secondary operations.
5. Conclusions and Recommendations
This paper is the first to consider urban mining of carbon fiber from end-of-life wind turbine
blades to close the CE loop. Using the concept of CE in reusing, repurposing, recycling, and
recovering, this paper investigates CFRP recovery and the quality of the recovered materials.
Our investigation revealed that pyrolytic reaction conditions were important in controlling
char formation volume and its oxidation rate. A rapid heating rate caused a substantial
reduction of pyrolytic char volume, particularly for the pyrolysis process undertaken at 550 °C.
Nitrogen gas flow rate also affected the char content at a specific combination of pyrolysis
temperature and heating rate. At 550 °C and less than 100 °C/min heating rate, a higher gas
flow rate favored the devolatilization process and reduced the char content. High heating rate
and pyrolysis temperature produced char with higher intrinsic reactivity, suggesting a faster
oxidation rate. This is beneficial to shorten the post-processing step, thereby leading to lower
energy costs. These findings are of commercial significance to the carbon fiber recycling
industry with high priority in cost control as with lower char volume and faster oxidation
kinetics, and hence the carbon fiber can potentially be recovered with a higher mechanical
performance due to the shortened thermal cycle.
17
Creating a market and closing the CE loop requires several issues to be overcome so that
recovered carbon fibers can be accepted as an environmentally friendly, reliable, and cost-
effective material. The industry would require establishment of standards for the recycled
carbon fiber products and to regulate pyrolysis operations. In addition, a labeling scheme such
as those used in recycled plastics would yield greater user acceptance and support. This
addresses the demand-side conditions. Further fiscal incentives and penalties by governments
would also push the supply side so that companies might engage more responsibly in closing
the circular loop.
Our investigation also identifies scope for future studies. This investigation looked into
maximum carbon recovery for first-time recycled carbon fiber. Carbon fiber physical and
mechanical properties will degrade over time after multiple thermal treatments, hence affecting
their reuse value. Therefore, a more detailed study is recommended to encompass this complex
scenario of having different stages of recovered fiber content to ensure long-term sustainability.
Second, the thermal pyrolysis of carbon fiber produces two other by-products—oil and gas—
that have good calorific and some economic value. Clearly, further study of these by-products
would assist a full loop recycling solution for the composite wastes.
Acknowledgement
The experimental work was done in the “ACC TECH-UNNC joint laboratory in Sustainable
Composite Materials”. The authors would like to acknowledge the financial supported by
Ningbo S&T Bureau Industry collaboration Project (project code 2017D10030), and Ningbo
3315 Innovation team Scheme “Composites Development and Manufacturing for Sustainable
Environment”.
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