Energy and environmental assessment and reuse of fluidised bed recycled carbon fibres
Introduction
Growing demand for carbon fibre reinforced polymers (CFRP) for lightweighting in aerospace applications and, to a lesser extent, automotive applications contributes to fuel efficiency objectives in the transportation sector. 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]. The generation of CFRP-based wastes is correspondingly increasing, arising from manufacturing (up to 40% of the CFRP can be waste arising during manufacture [2], [3], [4]) and end of life products/components. CF recovery from wastes is a priority due to the energy intensity and high financial cost of virgin CF (vCF) production. Boeing aims to recycle at least 90% of retired airplane materials [5], which will increasingly require CF recovery in the future. Existing EU regulations aim to reduce the quantities of all wastes sent to landfill [6], while automotive sector-specific policy requires the recycling of at least 85% of end-of-life materials from 2015 [7]. In contrast to industry and policy goals, the vast majority of CFRP waste at present is not recovered: in the UK, for example, up to 98% of composite waste is disposed of in landfill or incinerated [8], [9]. Recovery of metals from end-of-life aircraft has proven to be beneficial in terms of cost and energy intensity relative to virgin material production [10], however, there is no detailed energy and cost information of the CFRP recycling processes. Thus, there is a need to identify an environmentally beneficial recycling technique to address these issues.
Recycling techniques take different approaches to recovering fibres from the cross-linked thermoset matrix material, including mechanical size reduction [11]and thermal processes to partially or fully decompose matrix [3]. Pyrolysis is a widely used thermal method, being established in commercial operations, e.g., ELG Carbon Fibre Ltd [12], [13]. A related thermal process is the fluidised bed process, being the subject of this paper, which has been developed for the recycling of glass fibre and CF at the University of Nottingham for over 15 years [3]. Although it has shown a strength reduction of up to 50% [3], [14], [15], this continuous process has been shown to be particularly robust in dealing with varied polymer types containing mixtures of different materials and other contaminants. Very low residual char remains on the fibre surface as organic material is oxidised and any metallic material, such as aluminium honeycomb, rivets etc. remains in the fluidised bed and can be removed by regrading the bed.
Prior studies have estimated energy requirements of various CFRP recycling technologies, finding substantially lower energy requirements compared to vCF manufacture. For instance, industry reports claim that recovered CF (rCF) achieves about 95% energy reduction to manufacture compared to vCF while the mechanical performance is comparable [5], [10]. To be specific, recycling energy consumption has been reported to be 0.17–1.93 MJ/kg for mechanical recycling of GFRP [16], 0.27–2.03 MJ/kg for mechanical recycling of CFRP [11], 3–30 MJ/kg for pyrolysis recycling of CFRP [16], 19.2 MJ/kg for solvolysis recycling of CFRP [17] and 60–90 MJ/kg for chemical recycling of CFRP in Japan [18] compared to 198–595 MJ/kg for vCF production. However, no recycling capacity or other processing details were specified in most literature, nor the modelling methodology for the energy intensity. Little work was focused on energy demand and environment burden particularly for fluidised bed recycling of CFRP, which needs to be addressed.
In addition, to comprehensively assess the environmental performance of CF recycling, however, evaluations should extend beyond the recycling process and account for the reutilisation of rCF in place of current materials. Life cycle assessment (LCA) is an internationally accepted environmental assessing method to quantify and evaluate the environmental impacts such as energy use and greenhouse gas emissions as described by [19]. A number LCA studies evaluating the use of CFRP in lightweighting applications have been conducted, generally finding that weight reductions owing to the use of CFRP to replace conventional materials such as steel and aluminium potentially leads to both energy and greenhouse gas (GHG) emission reduction in either aerospace or automotive industries [2], [20], [21], [22], [23], [24], [25]. However, these studies have not considered the end-of- life of CFRP components and therefore do not completely assess environmental impacts. Very few studies have estimated the environmental impacts of a CFRP recycling technology [21], [26], however these have relied on hypothetical data regarding the energy intensity of the recycling process. The lack of data regarding CFRP recycling process inputs and impacts is a barrier to developing informative LCA models. While potential environmental benefits are claimed in technical studies of CFRP recycling processes and fibre reuse opportunities, these benefits have yet to be demonstrated in a comprehensive life cycle study.
Environmentally-beneficial recycling strategies are essential to support the role of CF-based materials in lightweighting applications to reduce transportation energy consumption. In this study, comprehensive life cycle models are developed to consider the fluidised bed CFRP recycling process and subsequent reuse of rCF in composite materials. Process models of the recycling process and composite manufacture with rCF are developed and validated against pilot plant data. Inventory data (material and energy inputs; direct emissions) are derived from the process models and input to the LCA models. Key environmental impacts (primary energy consumption, global warming potential) are assessed. Environmental impacts of composite production from rCF are estimated and compared with vCF-based composite material.
Section snippets
Methods
This study evaluates the life cycle environmental impacts of CFRP recycling by the fluidised bed process and subsequent manufacturing of composite material from rCF. Results are compared with vCF materials to determine the environmental impacts of using rCF in place of vCF in composite materials and the following composites are considered:
- (1)
Recycled CFRP (rCFRP): rCF recycled from fluidised bed process is processed by a wet papermaking process co-mingled with polyamide (PA) fibre to make a
Fluidised bed recycling process
CF can be recovered from CFRP with energy expenditure as little as 6 MJ/kg CF for the fluidised bed operating parameters considered. Fig. 3 shows the energy balance of the recycling process, including energy inputs (natural gas, electricity), energy release from resin oxidation, and heat losses, for a fluidised bed plant with 100 t/yr of annual throughput of rCF. The energy requirements of the fluidised bed recycling process are primarily dependent on two factors: the feed rate of CFRP processed
Conclusion
A life cycle analysis of CF recycling process has been carried out in this study, based on a novel process model of the fluidised bed recycling process. Key recycling plant operating parameters, including plant capacity, feed rate, and air in-leakage are investigated. The feed rate per unit bed area is identified as the most important parameter for achieving energy-efficient CF recycling. The energy model shows that energy requirement of rCF production is very low relative to vCF and robust
Acknowledgment
The energy analysis of the fluidised bed recycling process is based on data from a pilot plant funded by the Boeing Company at the University of Nottingham. Their support is gratefully acknowledged.
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