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Emergy assessment of the benefits of closed-loop recycling accounting for material losses
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Emergy assessment of the benefits of closed-loop recycling accounting for material losses Bruno Lacarrière, Kévin Ruben Deutz, Nadia Jamali-Zghal, Olivier Le Corre ∗ Ecole des Mines de Nantes, Dept. Energy Systems and Environment 4 rue Alfred Kastler, 44307 Nantes Cedex 3, France
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Emergy Closed loop recycling Aluminum
a b s t r a c t Emergy analysis is applied to closed-loop recycling processes. The emergy balance is written under the form of a discrete-time equation following a comparable approach to the Lagrangian particle tracking in fluid mechanics. Two material losses were taken into consideration, the first one at the level of treatment (collection, dismantling, recycling etc.) and the second one at the transformation level. As expected, the emergy of a product increases with the number of cycles (after the transformation processes). To assess the environmental stress at each recycling stage, relative emergy of the product Ep (n) was used after discussing the limitation of others solutions like the environmental loading ratio (ELR) or the recycle benefit ratio (RBR) which are not suitable for use in the cases of multiple recycling analysis. The values obtained showed that the Ep (n) has an increasing trend for successive recycles. Additionally, this approach allowed the study of different cases of theoretical closed-loop recycling and an application on aluminum. The results indicated that the several cycles in the first stage of recycling have a significant impact, and an asymptotic behavior of Ep (n) can be noted. In any cases recycling remains a better option compared to raw material use to compensate the material losses. Furthermore, the presented equations illustrate the impact of the material recycle rate and the percentage of losses during the both recycling and transformation processes. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recycling is generally identified as a good practice to limit the use of raw materials and energy, and to limit the environmental impacts of an industrial activity. The assessment of recycling has been widely studied in the literature with several methods and appropriate tools: life cycle analysis (Shen et al., 2010; Heijungs and Guinée, 2007), ecological footprint (Herva and Roca, 2013; Rees and Wackernagel, 1994), exergy analysis (Szargut et al., 1988; Castro et al., 2007), emergy analysis (Odum, 1996) etc. The latter is an accounting method that enables analysts to quantify the flows of resources (used in processes, systems, products or goods) by using the common reference of solar energy (Brown and Herendeen, 1996). Numerous studies were carried out to develop the theory of emergy analysis and various case studies have been performed over the last few decades. These different assessment approaches have been compared one by one by Kharrazi et al. (2014) or in combinations (Duan et al. (2011)). These approaches are not
∗ Corresponding author. Tel.: +33 2 51 85 82 57; fax: +33 2 51 85 82 99. E-mail addresses: [email protected], [email protected] (O. Le Corre).
mutually exclusive, and on the contrary, they should complement each other. Ulgiati et al. (2004) suggested that life cycle analysis is more user-side oriented and presented emergy accounting as a donor-side oriented approach. This aspect of comparison and/or integration of emergy and LCA was also studied by Raugei et al. (2014). For these authors the emergy approach has the potential to be an addition to LCA. As it is donor-side orientated emergy provides an indication of the work of the environment that would be needed to replace what was consumed. Emergy accounting for the sustainability assessment of recycling has been applied to various sectors. Brown and Buranakarn (2003) studied the recycling potential for building materials and comparison of three recycle trajectories (standard recycle where materials are used again as the same material, by-product-use and adaptative re-use). Marchettini et al. (2007) used the emergy indices to compare the different solutions for municipal solid waste (MSW) treatment (the solutions investigated were incineration, landfilling and composting). Bakshi (2002) introduced an emergy analysis method applied to the industrial waste treatment, considering both ecological and economical input to analyze the industrial processes. Recycling was taken into account by considering waste as a source of material in the Energy Systems Language diagram. A similar accounting for recycling has been done by Yang et al. (2003) who included the classical emergy
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Please cite this article in press as: Lacarrière, B., et al., Emergy assessment of the benefits of closed-loop recycling accounting for material losses. Ecol. Model. (2015), http://dx.doi.org/10.1016/j.ecolmodel.2015.01.015
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Fig. 1. Multiple recycling accounting (adapted from Amponsah et al., 2011).
indices (Ulgiati et al., 2004, 1995; Brown and Ulgiati, 1997) to assess the impact of industrial waste on the environment. In the recycling, two kinds of loops are distinguished: closed-loop recycling and open-loop recycling. According to the U.S. Environmental Protection Agency (Environmental Protection Agency of the USA), the former is a recycling system in which the particular mass of material is remanufactured into the same product (e.g. works of Meng et al., 2006). Open-loop recycling is a recycling system in which a product made from one type of material is recycled into a different type of product (e.g. works of Shen et al., 2010). Williams et al. (2010) followed the International Standard on Life Cycle Assessment (BS EN ISO 14044, 2006) and proposed a more precise definition to distinguish the recycling loops. The authors suggested that on one hand, closed loop recycling corresponds to a process where a product is recycled into the same product or into another product with no change in the inherent material properties (i.e. loss of quality). On the other hand, open-loop recycling was defined as the use of recycled materials with a quality loss (a change in inherent properties). Most of the studies that deal with recycling using emergy analysis do not consider the number of reuses for the material. Amponsah et al. (2011) and Amponsah et al. (2012) analyzed the effect of multiple cycles of material reuse on the product’s emergy. Recycling during the lifetime of a product can be represented with a schematic Energy Systems model (Fig. 1). In this figure Ei is the emergy of the raw material that enters the process, Ep is the emergy of the product, Et is the emergy required for transformation and Ec is the necessary additional emergy for recycling. As represented in Fig. 1, Ei , Ec and Et are the sum of three emergies corresponding to the classical distinction between non-renewable, renewable and purchased materials (subscripts N, R and F, respectively). After its use, part of the material of the product is landfilled, whereas a fraction q of it is reintroduced into the production process through recycling. For the authors (Amponsah et al., 2011), the successive use of a material corresponds to an iterative reuse of a fraction q of the output (at end of each cycle). As a result, a fraction of the input to the production process consists of new raw material, and the complement is derived from material recycled from the previous cycle. The then, the accounting (and the associated equations) of
the recursive reuse of the output ‘composite’ material have been described. In their work, the equations were developed with a discrete approach of time (contrary to Tilley, 2011a,b; Tilley and Brown, 2006 for instance) along with a sampling time which corresponds to the product lifespan. Their equations describe the case of closed-loop recycling where Ei , Ec and q are assumed constant. These authors then showed monitored the material through its different life stages, in a comparable approach as the Lagrangian tracking of particles that is used in fluid mechanics. This means that at each new material transformation (e.g. each recycle) the additional emergy required for recycling should be accounted for, as it will be discussed later in the text. This accounting thus completes the existing results by integrating the additional emergy due to the processes used for recycling without double counting the initial stages of the material (contribution of the geobiosphere, extraction, refining, and delivery). As a case study, Amponsah et al. (2011) used two materials, one metallic and one non-metallic commonly used in industry. A specific complementary work applied this approach to building materials (Amponsah et al., 2012). This recycling accounting is based on the assumption that the recycling pathway is ideal in a sense that no material losses take place along the different steps of the material’s reuse (in the chain of the processes shown in Fig. 1). In real recycling processes, considering that the lack of pure material has to be compensated with additional raw material input (Deutz et al., 2014), losses must be taken into account. Following the initial works of Amponsah et al. (2011, 2012), the aim of this study is to broaden multi-recycle emergy accounting. In particular, the new developments proposed in this paper take into account the material losses at each cycle in the case of closedloop recycling, following the International Standard on Life Cycle Assessment (BS EN ISO 14044, 2006) definition. Additionally, an application to recycled aluminum is investigated. 2. Methodology Mass losses take place during the different steps of a process each time when the cycle of the material is replayed. In the proposed methodology the equations are developed assuming that the same recycling technology is used within each cycle.
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B. Lacarrière et al. / Ecological Modelling xxx (2015) xxx–xxx Table 1 Combination of mass losses. Case 1: Mass losses at the recycling step
εc > 0, εt = 0
Case 2: Mass losses at the transformation step
εc = 0, εt > 0
Case 3: Mass losses at the recycling and the transformation steps
εc > 0, εt > 0
q: inlet mass rate at the recycling step q(1 − εc ): outlet mass rate at the recycling step 1 − q(1 − εc ): inlet mass rate at the transformation step coming from the raw material q: inlet mass rate at the recycling step q: outlet mass rate at the recycling step 1 − q + εt ): inlet mass rate at the transformation step coming from the raw material q: inlet mass rate at the recycling step q(1 − εc ): outlet mass rate at the recycling step 1 + εt − q(1 − εc ): inlet mass rate at the transformation step coming from the raw material
3
mass losses at the recycling step only, (2) mass losses at the transformation step only and (3) mass losses at both steps. Furthermore, in the description proposed in this work, the emergy of the transformation step (Et ) is separated from the emergy of the material coming from the mine (Ei ), which was not initially considered by Amponsah et al. (2011). The mass balance of each case is described in the third column of Table 1. Accordingly, the emergy of the product Ep can be written as a function of εc and εt by writing the mass balance at each stage for each recycling cycle of the fraction q. For the general case (case 3), the emergy of the product can be written with the following time discrete equations:
Ep (n) = (1 − q + εt + qεc ) Ei + q Ep (n − 1) + Ec + (1 + εt ) Et
(1)
Ep (0) = (1 + εt )Ei + (1 + εt )Et
(2)
where Ep (0) corresponds to the first time that the transformation is considered (n = 0), and thus without any additional emergy due to the recycling. After the first cycle of recycling, Ep becomes: Ep (1) = (1 + εt + q (εc + εt )) Ei + (1 + εt ) (1 + q) Et + qEc After the second cycle of recycling:
(3)
Ep (2) = 1 + εt 1 + q + q2 + qεc (1 + q) Ei
+ (1 + εt ) 1 + q + q2 Et + q (1 + q) Ec
(4)
2.1. Closed-loop recycling (Single recycled material origin) After the nth cycle: The system described in Fig. 2 corresponds to a closed-loop recycling as the recycled material coming from the product is used to produce the same type of product, even if it is a long time after the end of the product use. To use similar example as Williams et al. (2010), this application corresponds to a plastic yoghurt pot recycled into an identical plastic yoghurt pot. Added to the fraction of material recycled q, introduced by Amponsah et al. (2012), the ratios εt and εc proposed in Fig. 2 correspond to the fraction of material losses occurring at the transformation step and the recycling step, respectively. They are defined as the percentage of mass losses for their respective stage during the process. Thus the mass balance can be written for each of the three possible cases (Table 1): (1)
Ep (n) =
1 + εt + q (εc + εt )
+ (1 + εt )
qn − 1
qn+1 − 1 q−1
q−1
Et + q
Ei
qn − 1 q−1
Ec
(5)
Table 2 summarizes the equations giving the emergy of the product for the three possible cases defined in Table 1. It can be noted that Eq. (5), which is the most general, includes the specific case (εt = εc = 0 and Et = 0) studied by Amponsah et al. (2011).
Fig. 2. Closed-loop recycling (single recycled material origin) with mass losses.
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Table 2 Emergy of the product for the different combination of mass loses (closed-loop recycling). Case 1:
Case 2:
qn −1 E + qn+1 −1 qnq−1 i −1 E + q E c q−1 q−1 t n −1 Ep (n) = 1 + εt + qεt qq−1 Ei + (1 + qn+1 −1 qn −1 Ep (n) = 1 + qεc
εt ) Case 3:
- Step 3: Closed-loop recycling with different mixes of recycled material coming from the different origins (composite reservoirs). This case corresponds to the most general one.
q−1
Et + q
q−1
Ec
Ep (n) = n −1 1 + εt + q(εc + εt ) qq−1 E + (1 + qn+1 −1 qn −1 i
εt )
q−1
Et + q
q−1
Ec
2.2. Closed-loop recycling-multi recycled material origins In this section closed-loop recycling with multiple origins of recycled material is considered. The reservoir is made-up of mixed materials that were recycled several times but the representative mass percentage in all reservoirs is known (e.g. 10% of the material of the reservoir was recycled 15 times, 20% was recycled 10 times, and 50% only once). The addition of raw material is possible when necessary. For easier understanding, emergy analysis calculations were conducted in three steps: - Step 1: Closed-loop recycling with mixed recycled material flows coming from the same origin (e.g. N mass flows coming from a similar-process as described in Section 1-with each recycled different number of times). - Step 2: Closed-loop recycling with a reservoir discharge without addition of raw material. This case is purely theoretical and must be considered as a necessary step to develop the next one.
Step 1. Closed-loop recycling with mixed recycled material coming from the same origin. In Fig. 3, the source of recycled material is a composite one (made up of material with different levels of recycling). xj is the mass fraction of recycled material (recycled for j times) and used for the transformation. This fraction after the nth cycle can be written as xj,n with j ∈ {0, . . ., n − 1}. Then, for the general case of material losses (case 3, defined in Table 1), the emergy of the product can be written with the time discrete equations:
E¯p (n) = (1 + εt − q) Ei + q E¯p (n − 1) + Ec + (1 + εt ) Et
(6)
E¯p (0) = (1 + εt ) Ei + (1 + εt ) Et
(7)
where E¯p is defined as the average emergy from the material coming from the recycled reservoir: E¯p (n − 1) =
n−1
xj,n Ep (j)
(8)
j=0
After the nth cycle, the average emergy can be written under its developed form (similar to the equations developed by Amponsah et al. (2011):
⎛
E¯p (n) = E¯p (0) + ⎝
n−1 n−1
⎞
xj,n qk+1 ⎠ ((εc + εt ) Ei + (1 + εt ) Et + Ec )
k=0 j=k
(9)
Fig. 3. Closed-loop recycling with mixed recycled material coming from the same origin.
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B. Lacarrière et al. / Ecological Modelling xxx (2015) xxx–xxx Table 3 Emergy of the product for the different combination of mass loses (Open-loop recycling—reservoir made of mixed recycled material). Case 1:
E¯p (n) =
Case 2:
E¯p (0) + k=0 E¯p (n) = E¯p (0) +
n−1 n−1
n−1 n−1 k=0
Case 3:
j=k
j=k k+1
xj,n q
E¯p (n) = E¯p (0) +
xj,n qk+1 (εc Ei + Et + Ec )
(ε E + (1 + εt )Et + Ec )
i n−1 c n−1 k=0
εt )Ei + (1 + εt )Et + Ec )
j=k
k+1
xj,n q
((εc +
In these equations, past processes (i.e. where the recycled material comes from) are supposed to be the same as the one considered in the present accounting method (e.g.; recycling several times through a Closed-loop with a Single Source). Otherwise Et and Ec should be the result of successive accountings associated with all the processes that have used the material. Similar to Tables 2 and 3 summarizes the three possible cases of mass losses and the corresponding products’ emergy. Step 2. Closed-loop recycling with discharge from a reservoir without addition of raw material. In this case, the raw material enters the process for the first time on its transformation; then, only the recycled material is used and it originates from a reservoir for which the material that has been previously recycled is homogeneous (see Fig. 4). The material is supposed to come from the same process. This case is an intermediary one, and aims to explain the most general case of the next section. The simplifications chosen help to focus the equations governing the discharge of the reservoir: raw materials entering the process only once (flip-flop symbol (Odum and Peterson, 1996) on Fig. 4) and a single homogeneous reservoir of recycled material. The mass balance gives the mass of material (as defined in Eq. (10)), the emergy of the product is given by Eq. (11) (both are time
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discrete equations), whereas the specific emergy (relative to the mass defined in Eq. (10)) is defined by Eq. (12). m (n) =
1 − εc qm (n − 1) 1 + εt
(10)
Ep (n) = qm (n − 1) ep (n − 1) + ec + (1 − εc ) et ep (n) =
Ep (n) m (n)
(11) (12)
In these equations, the variables ep , ec and et correspond to the specific emergy of the product, the specific emergy for recycling and the specific emergy for transformation, respectively. Eqs (13)–(15) correspond to the first time that the raw material enters the process: m (0) = 1
(13)
Ep (0) = (1 + εt ) (Ei + Et )
(14)
ep (0) = (1 + εt ) (ei + et )
(15)
In Eq. (15), ei corresponds to the specific emergy for the raw material. By developing these equations for each recycling stage, it can be found that: m (n) =
1 − ε n c
1 + εt
q
(16)
n
Ep (n) = q
(1 + εt ) ei + (1 + εt ) et
+ ((1 − εc ) et + ec )
(1 − εc ) / (1 + εt )
n −1
(1 − εc ) / (1 + εt ) − 1
(17)
Fig. 4. Closed-loop recycling with a discharge from a reservoir without addition of raw material. The flip-flop symbol (Odum and Peterson, 1996) is used to illustrate that the raw material enters the system only once (the first time).
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Fig. 5. Closed -loop recycling with different mixed recycled material coming from different origins (composite reservoirs).
ep (n) =
1 + ε n
t
1 − εc
+ ((1 − εc ) et + ec )
The detailed form of S¯p can be used to rewrite Eq. (19) as follows:
⎛
(1 + εt ) ei + (1 + εt ) et (1 − εc ) / (1 + εt )
n −1
(1 − εc ) / (1 + εt ) − 1
e¯p (n) = (1 + εt − q + qεc ) ei + q ⎝
n−1
⎞
xj,n sp (j)⎠ + ec + (1 + εt ) et
j=0
(18)
(21) Then, a development for each stage of recycling (as done previously) gives the specific emergy of the product:
e¯p (n) = (1 + εt − q + qεc )ei + qec + (1 + εt )et + q
⎧ ⎨
n−1
(1 + εt )(ei + et )
⎩
j=0
xj,n
1 + ε j t
1 − εc
+
(22)
1 − εcet + ec1 + 1 − εc1 + εtj = 0n − 1xj, n1 + εt1 − εcj
Step 3. Closed-loop recycling with a different mix of the recycled material coming from different origins (composite reservoirs). This case of closed-loop recycling generalizes the previous ones with multiple reservoirs made-up of different levels of recycled material and with the addition of raw materials when it is necessary (see Fig. 5). The material is supposed to originate from the same type of process. Thus, et , ec and q, can be assumed to be the same throughout the present process. ¯ has been introduced as the mean specific emergy of the (sp )(n) storage. This emergy is given by the discharge description of a reservoir as was done in the previous step. Its mathematical expression is obtained by solving Eqs. (16)–(18). Thus, the mean specific emergy of the product for closed-loop recycling (in general case) can be written (for the case εc > 0, εt > 0) with the time discrete equation:
e¯p (n) = (1 + εt − q + qεc ) ei + q s¯p (n) + ec + (1 + εt ) et
(19)
e¯p (0) = ei + et
(20)
The last equation is written under the assumption of a constant recycling rate q. In case of the variable recycling rate Eq. (19) must be used to determine the specific emergy of the product at each cycle. 2.3. Recycling follow-up The best method of accounting for recycled materials is not obvious and it is still under discussion in the literature (Ulgiati et al., 2004; Amponsah et al., 2011; Agostinho et al., 2013). Constructing emergy indices is one of the possible ways to improve this methodology, as suggested by Brown and Ulgiati (1997). Depending on the case studied, emergy ratios for recycling can be adapted as in the study of Agostinho et al. (2013), who defined a Modified Recycle Yield Ratio (‘modified’ to account the emergy of the refined material). Brown and Buranakarn (2003) proposed a set of indices and ratios to assess different options for material cycles and recycling. Among them, the environmental loading ratio (ELR) is defined as the ratio of the purchased and non-renewable inputs to the renewable emergy. A small value of ELR indicates a small environmental stress whereas this ratio increases for larger stresses (Brown and Ulgiati, 1997). Mu et al. (2011) proposed a modified definition of ELR in order to take into account the additional
Please cite this article in press as: Lacarrière, B., et al., Emergy assessment of the benefits of closed-loop recycling accounting for material losses. Ecol. Model. (2015), http://dx.doi.org/10.1016/j.ecolmodel.2015.01.015
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Fig. 6. Influence of the recycling mass rate on Ep (n) (Closed-loop recycling with mixed recycled material coming from the same origin, Case 3, εc = εt = 10%, ei , et , ec are constant as defined in Table 5).
emergy purchased for waste treatment, the additional investment for environmental protection and the environmental service to dilute waste. Considering that the waste treatment can recycle some valuable resources, Song et al. (2012, 2013) proposed another improvement to this ratio by considering separately the emergy for recycling. For these authors, ELR is thus defined as the ratio of purchased emergy, non-renewable emergy and emergy of the service (to dispose the waste) to the emergy of renewable resources and recycled resource emergy. Furthermore the authors applied their accounting approach to a case study that considered the e-waste treatment. However, this definition is not in accordance with the analysis done by Lu et al. (2014) who consider the ELR as a “characterization of the effect of an input in increasing the load on the environment or in enhancing the capacity of the environment to process that load”. Consequently, the recycled resource emergy should not be at the denominator of ELR. The recycling benefit ratio (RBR) is also a possible solution to account for the benefit of recycling. In its original definition, RBR is the ratio of emergy used in providing a material from raw resources to the emergy used in recycling (Brown and Ulgiati, 1997). However, this definition is relevant only for recycling accounting without variation of the recycled rate of the material at each cycle. To avoid any confusion or new adaptation of existing ratios, the benefit of using (multi) recycled material is expressed following the comparison of the emergy of the product (including its history of recycling) with a reference chosen as the emergy of the same product produced with raw material only. Thus the indicator used for the follow-up is:
Ep (n) = Ep (n) − Ep (0)
(23)
3. Case study In this section, the proposed equations are applied to the Closedloop recycling with mixed recycled material coming from the same origin, corresponding to Fig. 2. Then, the Closed -loop recycling with different mixed recycled material coming from different origins (composite reservoirs) is applied to the aluminum recycling (corresponding to Fig. 5). Considering both resource optimization and the economical point of view, aluminum recycling is considered to have great potential. The efficiency of metal recycling depends on various parameters: scrap purchasing cost, environmental regulations, metal recovery, metal yield and metal quality (Xiao and Reuter, 2002). The different situations presented above were considered. Recycling rates for the aluminum industry used for the calculations over the past years were deduced from Bertram et al. 2009. The specific emergy and the input resources required for conventional aluminum production were obtained from Amponsah et al. (2011). 4. Results and discussion Closed-loop recycling with mixed recycled material coming from the same origin To illustrate the usefulness of the equations proposed in this work (Eqs. (5)–(7)) Ep (n) was used to identify the influence of the recycle rate q (see Fig. 6). In this figure, each curve corresponds to a fixed value of q which is used at each cycle n. The mass losses rates (εc and εt ) are kept constant (equal to 0.1 each) for all the cases in this figure. The values chosen for q are theoretical and cover a large range of variation. The general trend of Ep (n) increases similarly altogether with n (whatever the value for q) before reaching a horizontal asymptote. The greater the recycling rate (q) the later the asymptote is reached. This constant value corresponds to a limit in
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the increasing benefit of recycling. This figure is in accordance with and broadens the results found by Amponsah et al. (2011) to the case of multiple recycling. The figure represents the emergy of the product taking into account recycling along the successive cycles which characterizes the memory of the material from the past cycles. The use of raw material only implies to account for emergy of stages (extraction, refining, etc.) which is no longer necessary with the use of recycled material. However, this benefit of recycling has to reflect the necessary additional emergy for the recycling process. Even if recycling remains a solution to lighten the loads on the environment, it is of importance to include the recycling memory of a product for a more accurate emergy accounting of its production. The accounting of additional contribution of environment for recycling could be tackled using the transformity rather than the emergy as the recycling does not deliver the additional emergy to the final product. However, the equations developed in the present work are written for a unit of material and consider that there is not quality loss (loss of exergy) in the different processes (recycling and transformation). Thus, the emergy analysis and the results obtained (e.g. Fig. 6) then amount to broadening what occurs for a system operating under evolutionary transformation which is summarized by Odum (1996) by “The more energy transformations there are contributing to a product, the higher is the transformity”. In Fig. 7, Ep (n) is used to compare the three cases of mass losses described in Table 1. In this case, εc and εt were set to 10% when they are not equal to zero, and q was set to 40%. It can be verified that a double source of losses (during the recycling process and the transformation one) has a stronger impact on the emergy of the product compared to only one origin of losses. There is no significant difference between the individual effects of c and t . Whereas Fig. 6 shows the influence of q on the asymptotic behavior, Fig. 7 tends to show that εc and εt have little influence on it as the constant is reached after five cycles. This trend is also visible in Fig. 8 that shows the influence of the mass losses on Ep (n). In this
Fig. 7. Influence of the different cases of mass losses on Ep (n) (Closed-loop recycling with mixed recycled material coming from the same origin, q = 40%, ei , et , ec are constant as defined in Table 5).
figure, for the three cases defined in Table 1, four different values were set to the mass loss rates εc and εt (when they are not equal to zero), and the recycling rate q was fixed at 40%. It can be verified that for a selected case (n fixed), the more losses occur in the processes (recycling and/or transformation), the greater global value for E(n). - Closed-loop recycling with different mixed recycled material coming from different origins (composite reservoirs)
Fig. 8. Influence of the mass losses on Ep (n) (Closed-loop recycling with mixed recycled material coming from the same origin, q = 40%, ei , et , ec are constant as defined in Table 5).
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Table 4 Aluminum recycling rates (Bertram et al., 2009). Year
1950 (%)
1960 (%)
1970 (%)
1980 (%)
1990 (%)
2000 (%)
2010
Recycling rate* xj,n
18 1
20 2
22 7
25 15
28 25
35 50
q = 31% Present time
*
Bertram et al. 2009.
Table 5 Emergy evaluation of conventional aluminum production and reuse of aluminum, source: Amponsah (2011). Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Conventional aluminium sheet production Primary aluminium (ingot) Electricity Labour Annual yield Recycling process Used aluminium can Primary aluminium (ingot) Aluminium scrap Used Al. can collection Used Al. Can separation Electricity Transport (Truck) Labour Annual yield Specific emergy for recyclinga Specific emergy for transformationb Specific emergy for raw materialc
Unit/year
Input resource
Specific emergy (sej/unit)
Emergy (sej/year)
g J $ g
4.17E+11 1.08E+15 2.09E+07 4.00E+11
1.17E+10 1.74E+05 1.15E+12 1.27E+10
4.88E+21 1.88E+20 2.40E+19 5.08E+21
g g g g g J ton-mile $ g Sej/g Sej/g Sej/g
2.29E+11 1.25E+11 6.25E+10 2.29E+11 2.29E+11 1.08E+15 2.82E+07 2.90E+07 4.00E+11 – – –
1.17E+10 1.17E+10 1.17E+10 2.51E+08 8.24E+06 1.74E+05 9.65E+11 1.15E+12 1.29E+10 7.69E+08 5.30E+08 1.17E+10
2.68E+21 1.46E+21 7.31E+20 5.75E+19 1.89E+18 1.88E+20 2.72E+19 3.34E+19 5.18E+21 – – –
a
ec = Em(8+9+11)/input(13). et = Em(2+3* )/input(4). c ei = Em(1)/input(4). Counted at the plant level. b
*
In the equation used in this section (Eq. (22)) the variables xj,n represent the mass fractions of the recycled material that have been recycled j times and used in cycle n. The equation was implemented with a theoretical variation of the recycling rate (35% for j = 1, 30% for j = 2, 20% for j = 3, 10% for j = 4, 5% for j = 5) and the accounting was
done at the fifth cycle (n = 5). Thirty five percent must be understood as the material in the reservoir that has been already recycled once, 30% is the material in the reservoir that has been already recycled twice, and 20% is rate of the material in the reservoir that has been already recycled three times etc.
Fig. 9. Emergy accounting with follow-up of recycling history (Closed-loop recycling with different mixed recycled material coming from different origins-composite reservoirs); εc = εt = 10%.
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By applying the results on the theoretical values of the recycled mass fraction recycled, it was possible to test the proposed work. Results provided by the implementation of the equations proposed ((19)–(22)) are in accordance with the physical analysis. Thus, the same equations are now applied to the case of the aluminum recycling rates over the past decades, as it is represented in Table 4 (Bertram et al., 2009). In this table, the year 2010 is considered as the initial set for the reservoir of material. Consequently the fraction of recycled material used in the equation is q = 31%. Values of the xj,n are chosen according to the data in Table 4. The input resources for conventional aluminum production were obtained from the Amponsah’s work (Amponsah et al., 2011) (our Table 5). Using the values given in this table, the specific emergy for recycling ec was calculated as the sum of the items 8, 9 and 11 divided by the item 13; the specific emergy for transformation et is calculated as the sum of the items 2 and 3 divided by item 4; the specific emergy of raw material ei was calculated as the ratio of the item 1 to the item 4. In Fig. 9, it can be seen how the equations proposed could help to follow up the materials taking into account the history of its recycling. It can be seen that 17% of the products emergy corresponds to the emergy of the material that has been recycled once, 10% to the emergy is accounted for by a material that has been recycled twice, etc. The results are given for εc = εt = 10% and the 6th cycle of recycling (n = 6). The proposed method requires known values for xj,n (fixed a priori in this work). Thus, a continuous follow-up of the reservoir composition is necessary. If the 17% of the recycle emergy of the product are associated to15.5% (q × x1,6 = 0.31 × 0.5, see Table 4) of the recycled material used, then the 10% correspond to 7.75% (q × x2.6 = 0.31 × 0.25, see Table 4) of the recycled material used, etc. 5. Conclusion Recycling is commonly considered as a suitable process in terms of sustainability. Numerous methods for its assessment already exist in the literature, such as the life cycle analysis (a multicriteria method, user-side oriented), or the emergy assessment (a mono-criteria method, donor-side oriented) and they are considered more as complementary rather than competitive among themselves. Recycling can be done under the two configurations: open-loop or closed-loop. Applying emergy assessment, discrete time emergy equations were set according to the recycling configuration for different cases of closed-loop recycling. The relative emergy of the product Ep (n), including the different previous cycles of the recycled material, is used to assess the impact of recycling. It has been shown that the Ep (n) is an increasing function of the number of cycles illustrating the accumulation emergy for previous cycles of recycling. The equations proposed help to integrate this history of recycling of the material and to follow-up this history. This approach to the emergy accounting for recycle also helps to show the impact of compensating for material losses with recycled material (whatever the type of loss considered and the rate of recycled material use) rather than using raw material. This potential usefulness of the equations developed was illustrated through the example of the aluminum in the most general case of a reservoir made-up of mixed recycled material forms over time. It was demonstrated how the proposed equations could follow-up the history of recycling even in this general case. Acknowledgement The authors thank the reviewers and the chief editor for their valuable comments during the review process.
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Contents lists available at ScienceDirect
Ecological Modelling journal homepage: www.elsevier.com/locate/ecolmodel
Emergy assessment of the benefits of closed-loop recycling accounting for material losses Bruno Lacarrière, Kévin Ruben Deutz, Nadia Jamali-Zghal, Olivier Le Corre ∗ Ecole des Mines de Nantes, Dept. Energy Systems and Environment 4 rue Alfred Kastler, 44307 Nantes Cedex 3, France
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Emergy Closed loop recycling Aluminum
a b s t r a c t Emergy analysis is applied to closed-loop recycling processes. The emergy balance is written under the form of a discrete-time equation following a comparable approach to the Lagrangian particle tracking in fluid mechanics. Two material losses were taken into consideration, the first one at the level of treatment (collection, dismantling, recycling etc.) and the second one at the transformation level. As expected, the emergy of a product increases with the number of cycles (after the transformation processes). To assess the environmental stress at each recycling stage, relative emergy of the product Ep (n) was used after discussing the limitation of others solutions like the environmental loading ratio (ELR) or the recycle benefit ratio (RBR) which are not suitable for use in the cases of multiple recycling analysis. The values obtained showed that the Ep (n) has an increasing trend for successive recycles. Additionally, this approach allowed the study of different cases of theoretical closed-loop recycling and an application on aluminum. The results indicated that the several cycles in the first stage of recycling have a significant impact, and an asymptotic behavior of Ep (n) can be noted. In any cases recycling remains a better option compared to raw material use to compensate the material losses. Furthermore, the presented equations illustrate the impact of the material recycle rate and the percentage of losses during the both recycling and transformation processes. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recycling is generally identified as a good practice to limit the use of raw materials and energy, and to limit the environmental impacts of an industrial activity. The assessment of recycling has been widely studied in the literature with several methods and appropriate tools: life cycle analysis (Shen et al., 2010; Heijungs and Guinée, 2007), ecological footprint (Herva and Roca, 2013; Rees and Wackernagel, 1994), exergy analysis (Szargut et al., 1988; Castro et al., 2007), emergy analysis (Odum, 1996) etc. The latter is an accounting method that enables analysts to quantify the flows of resources (used in processes, systems, products or goods) by using the common reference of solar energy (Brown and Herendeen, 1996). Numerous studies were carried out to develop the theory of emergy analysis and various case studies have been performed over the last few decades. These different assessment approaches have been compared one by one by Kharrazi et al. (2014) or in combinations (Duan et al. (2011)). These approaches are not
∗ Corresponding author. Tel.: +33 2 51 85 82 57; fax: +33 2 51 85 82 99. E-mail addresses: [email protected], [email protected] (O. Le Corre).
mutually exclusive, and on the contrary, they should complement each other. Ulgiati et al. (2004) suggested that life cycle analysis is more user-side oriented and presented emergy accounting as a donor-side oriented approach. This aspect of comparison and/or integration of emergy and LCA was also studied by Raugei et al. (2014). For these authors the emergy approach has the potential to be an addition to LCA. As it is donor-side orientated emergy provides an indication of the work of the environment that would be needed to replace what was consumed. Emergy accounting for the sustainability assessment of recycling has been applied to various sectors. Brown and Buranakarn (2003) studied the recycling potential for building materials and comparison of three recycle trajectories (standard recycle where materials are used again as the same material, by-product-use and adaptative re-use). Marchettini et al. (2007) used the emergy indices to compare the different solutions for municipal solid waste (MSW) treatment (the solutions investigated were incineration, landfilling and composting). Bakshi (2002) introduced an emergy analysis method applied to the industrial waste treatment, considering both ecological and economical input to analyze the industrial processes. Recycling was taken into account by considering waste as a source of material in the Energy Systems Language diagram. A similar accounting for recycling has been done by Yang et al. (2003) who included the classical emergy
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Fig. 1. Multiple recycling accounting (adapted from Amponsah et al., 2011).
indices (Ulgiati et al., 2004, 1995; Brown and Ulgiati, 1997) to assess the impact of industrial waste on the environment. In the recycling, two kinds of loops are distinguished: closed-loop recycling and open-loop recycling. According to the U.S. Environmental Protection Agency (Environmental Protection Agency of the USA), the former is a recycling system in which the particular mass of material is remanufactured into the same product (e.g. works of Meng et al., 2006). Open-loop recycling is a recycling system in which a product made from one type of material is recycled into a different type of product (e.g. works of Shen et al., 2010). Williams et al. (2010) followed the International Standard on Life Cycle Assessment (BS EN ISO 14044, 2006) and proposed a more precise definition to distinguish the recycling loops. The authors suggested that on one hand, closed loop recycling corresponds to a process where a product is recycled into the same product or into another product with no change in the inherent material properties (i.e. loss of quality). On the other hand, open-loop recycling was defined as the use of recycled materials with a quality loss (a change in inherent properties). Most of the studies that deal with recycling using emergy analysis do not consider the number of reuses for the material. Amponsah et al. (2011) and Amponsah et al. (2012) analyzed the effect of multiple cycles of material reuse on the product’s emergy. Recycling during the lifetime of a product can be represented with a schematic Energy Systems model (Fig. 1). In this figure Ei is the emergy of the raw material that enters the process, Ep is the emergy of the product, Et is the emergy required for transformation and Ec is the necessary additional emergy for recycling. As represented in Fig. 1, Ei , Ec and Et are the sum of three emergies corresponding to the classical distinction between non-renewable, renewable and purchased materials (subscripts N, R and F, respectively). After its use, part of the material of the product is landfilled, whereas a fraction q of it is reintroduced into the production process through recycling. For the authors (Amponsah et al., 2011), the successive use of a material corresponds to an iterative reuse of a fraction q of the output (at end of each cycle). As a result, a fraction of the input to the production process consists of new raw material, and the complement is derived from material recycled from the previous cycle. The then, the accounting (and the associated equations) of
the recursive reuse of the output ‘composite’ material have been described. In their work, the equations were developed with a discrete approach of time (contrary to Tilley, 2011a,b; Tilley and Brown, 2006 for instance) along with a sampling time which corresponds to the product lifespan. Their equations describe the case of closed-loop recycling where Ei , Ec and q are assumed constant. These authors then showed monitored the material through its different life stages, in a comparable approach as the Lagrangian tracking of particles that is used in fluid mechanics. This means that at each new material transformation (e.g. each recycle) the additional emergy required for recycling should be accounted for, as it will be discussed later in the text. This accounting thus completes the existing results by integrating the additional emergy due to the processes used for recycling without double counting the initial stages of the material (contribution of the geobiosphere, extraction, refining, and delivery). As a case study, Amponsah et al. (2011) used two materials, one metallic and one non-metallic commonly used in industry. A specific complementary work applied this approach to building materials (Amponsah et al., 2012). This recycling accounting is based on the assumption that the recycling pathway is ideal in a sense that no material losses take place along the different steps of the material’s reuse (in the chain of the processes shown in Fig. 1). In real recycling processes, considering that the lack of pure material has to be compensated with additional raw material input (Deutz et al., 2014), losses must be taken into account. Following the initial works of Amponsah et al. (2011, 2012), the aim of this study is to broaden multi-recycle emergy accounting. In particular, the new developments proposed in this paper take into account the material losses at each cycle in the case of closedloop recycling, following the International Standard on Life Cycle Assessment (BS EN ISO 14044, 2006) definition. Additionally, an application to recycled aluminum is investigated. 2. Methodology Mass losses take place during the different steps of a process each time when the cycle of the material is replayed. In the proposed methodology the equations are developed assuming that the same recycling technology is used within each cycle.
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B. Lacarrière et al. / Ecological Modelling xxx (2015) xxx–xxx Table 1 Combination of mass losses. Case 1: Mass losses at the recycling step
εc > 0, εt = 0
Case 2: Mass losses at the transformation step
εc = 0, εt > 0
Case 3: Mass losses at the recycling and the transformation steps
εc > 0, εt > 0
q: inlet mass rate at the recycling step q(1 − εc ): outlet mass rate at the recycling step 1 − q(1 − εc ): inlet mass rate at the transformation step coming from the raw material q: inlet mass rate at the recycling step q: outlet mass rate at the recycling step 1 − q + εt ): inlet mass rate at the transformation step coming from the raw material q: inlet mass rate at the recycling step q(1 − εc ): outlet mass rate at the recycling step 1 + εt − q(1 − εc ): inlet mass rate at the transformation step coming from the raw material
3
mass losses at the recycling step only, (2) mass losses at the transformation step only and (3) mass losses at both steps. Furthermore, in the description proposed in this work, the emergy of the transformation step (Et ) is separated from the emergy of the material coming from the mine (Ei ), which was not initially considered by Amponsah et al. (2011). The mass balance of each case is described in the third column of Table 1. Accordingly, the emergy of the product Ep can be written as a function of εc and εt by writing the mass balance at each stage for each recycling cycle of the fraction q. For the general case (case 3), the emergy of the product can be written with the following time discrete equations:
Ep (n) = (1 − q + εt + qεc ) Ei + q Ep (n − 1) + Ec + (1 + εt ) Et
(1)
Ep (0) = (1 + εt )Ei + (1 + εt )Et
(2)
where Ep (0) corresponds to the first time that the transformation is considered (n = 0), and thus without any additional emergy due to the recycling. After the first cycle of recycling, Ep becomes: Ep (1) = (1 + εt + q (εc + εt )) Ei + (1 + εt ) (1 + q) Et + qEc After the second cycle of recycling:
(3)
Ep (2) = 1 + εt 1 + q + q2 + qεc (1 + q) Ei
+ (1 + εt ) 1 + q + q2 Et + q (1 + q) Ec
(4)
2.1. Closed-loop recycling (Single recycled material origin) After the nth cycle: The system described in Fig. 2 corresponds to a closed-loop recycling as the recycled material coming from the product is used to produce the same type of product, even if it is a long time after the end of the product use. To use similar example as Williams et al. (2010), this application corresponds to a plastic yoghurt pot recycled into an identical plastic yoghurt pot. Added to the fraction of material recycled q, introduced by Amponsah et al. (2012), the ratios εt and εc proposed in Fig. 2 correspond to the fraction of material losses occurring at the transformation step and the recycling step, respectively. They are defined as the percentage of mass losses for their respective stage during the process. Thus the mass balance can be written for each of the three possible cases (Table 1): (1)
Ep (n) =
1 + εt + q (εc + εt )
+ (1 + εt )
qn − 1
qn+1 − 1 q−1
q−1
Et + q
Ei
qn − 1 q−1
Ec
(5)
Table 2 summarizes the equations giving the emergy of the product for the three possible cases defined in Table 1. It can be noted that Eq. (5), which is the most general, includes the specific case (εt = εc = 0 and Et = 0) studied by Amponsah et al. (2011).
Fig. 2. Closed-loop recycling (single recycled material origin) with mass losses.
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Table 2 Emergy of the product for the different combination of mass loses (closed-loop recycling). Case 1:
Case 2:
qn −1 E + qn+1 −1 qnq−1 i −1 E + q E c q−1 q−1 t n −1 Ep (n) = 1 + εt + qεt qq−1 Ei + (1 + qn+1 −1 qn −1 Ep (n) = 1 + qεc
εt ) Case 3:
- Step 3: Closed-loop recycling with different mixes of recycled material coming from the different origins (composite reservoirs). This case corresponds to the most general one.
q−1
Et + q
q−1
Ec
Ep (n) = n −1 1 + εt + q(εc + εt ) qq−1 E + (1 + qn+1 −1 qn −1 i
εt )
q−1
Et + q
q−1
Ec
2.2. Closed-loop recycling-multi recycled material origins In this section closed-loop recycling with multiple origins of recycled material is considered. The reservoir is made-up of mixed materials that were recycled several times but the representative mass percentage in all reservoirs is known (e.g. 10% of the material of the reservoir was recycled 15 times, 20% was recycled 10 times, and 50% only once). The addition of raw material is possible when necessary. For easier understanding, emergy analysis calculations were conducted in three steps: - Step 1: Closed-loop recycling with mixed recycled material flows coming from the same origin (e.g. N mass flows coming from a similar-process as described in Section 1-with each recycled different number of times). - Step 2: Closed-loop recycling with a reservoir discharge without addition of raw material. This case is purely theoretical and must be considered as a necessary step to develop the next one.
Step 1. Closed-loop recycling with mixed recycled material coming from the same origin. In Fig. 3, the source of recycled material is a composite one (made up of material with different levels of recycling). xj is the mass fraction of recycled material (recycled for j times) and used for the transformation. This fraction after the nth cycle can be written as xj,n with j ∈ {0, . . ., n − 1}. Then, for the general case of material losses (case 3, defined in Table 1), the emergy of the product can be written with the time discrete equations:
E¯p (n) = (1 + εt − q) Ei + q E¯p (n − 1) + Ec + (1 + εt ) Et
(6)
E¯p (0) = (1 + εt ) Ei + (1 + εt ) Et
(7)
where E¯p is defined as the average emergy from the material coming from the recycled reservoir: E¯p (n − 1) =
n−1
xj,n Ep (j)
(8)
j=0
After the nth cycle, the average emergy can be written under its developed form (similar to the equations developed by Amponsah et al. (2011):
⎛
E¯p (n) = E¯p (0) + ⎝
n−1 n−1
⎞
xj,n qk+1 ⎠ ((εc + εt ) Ei + (1 + εt ) Et + Ec )
k=0 j=k
(9)
Fig. 3. Closed-loop recycling with mixed recycled material coming from the same origin.
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B. Lacarrière et al. / Ecological Modelling xxx (2015) xxx–xxx Table 3 Emergy of the product for the different combination of mass loses (Open-loop recycling—reservoir made of mixed recycled material). Case 1:
E¯p (n) =
Case 2:
E¯p (0) + k=0 E¯p (n) = E¯p (0) +
n−1 n−1
n−1 n−1 k=0
Case 3:
j=k
j=k k+1
xj,n q
E¯p (n) = E¯p (0) +
xj,n qk+1 (εc Ei + Et + Ec )
(ε E + (1 + εt )Et + Ec )
i n−1 c n−1 k=0
εt )Ei + (1 + εt )Et + Ec )
j=k
k+1
xj,n q
((εc +
In these equations, past processes (i.e. where the recycled material comes from) are supposed to be the same as the one considered in the present accounting method (e.g.; recycling several times through a Closed-loop with a Single Source). Otherwise Et and Ec should be the result of successive accountings associated with all the processes that have used the material. Similar to Tables 2 and 3 summarizes the three possible cases of mass losses and the corresponding products’ emergy. Step 2. Closed-loop recycling with discharge from a reservoir without addition of raw material. In this case, the raw material enters the process for the first time on its transformation; then, only the recycled material is used and it originates from a reservoir for which the material that has been previously recycled is homogeneous (see Fig. 4). The material is supposed to come from the same process. This case is an intermediary one, and aims to explain the most general case of the next section. The simplifications chosen help to focus the equations governing the discharge of the reservoir: raw materials entering the process only once (flip-flop symbol (Odum and Peterson, 1996) on Fig. 4) and a single homogeneous reservoir of recycled material. The mass balance gives the mass of material (as defined in Eq. (10)), the emergy of the product is given by Eq. (11) (both are time
5
discrete equations), whereas the specific emergy (relative to the mass defined in Eq. (10)) is defined by Eq. (12). m (n) =
1 − εc qm (n − 1) 1 + εt
(10)
Ep (n) = qm (n − 1) ep (n − 1) + ec + (1 − εc ) et ep (n) =
Ep (n) m (n)
(11) (12)
In these equations, the variables ep , ec and et correspond to the specific emergy of the product, the specific emergy for recycling and the specific emergy for transformation, respectively. Eqs (13)–(15) correspond to the first time that the raw material enters the process: m (0) = 1
(13)
Ep (0) = (1 + εt ) (Ei + Et )
(14)
ep (0) = (1 + εt ) (ei + et )
(15)
In Eq. (15), ei corresponds to the specific emergy for the raw material. By developing these equations for each recycling stage, it can be found that: m (n) =
1 − ε n c
1 + εt
q
(16)
n
Ep (n) = q
(1 + εt ) ei + (1 + εt ) et
+ ((1 − εc ) et + ec )
(1 − εc ) / (1 + εt )
n −1
(1 − εc ) / (1 + εt ) − 1
(17)
Fig. 4. Closed-loop recycling with a discharge from a reservoir without addition of raw material. The flip-flop symbol (Odum and Peterson, 1996) is used to illustrate that the raw material enters the system only once (the first time).
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Fig. 5. Closed -loop recycling with different mixed recycled material coming from different origins (composite reservoirs).
ep (n) =
1 + ε n
t
1 − εc
+ ((1 − εc ) et + ec )
The detailed form of S¯p can be used to rewrite Eq. (19) as follows:
⎛
(1 + εt ) ei + (1 + εt ) et (1 − εc ) / (1 + εt )
n −1
(1 − εc ) / (1 + εt ) − 1
e¯p (n) = (1 + εt − q + qεc ) ei + q ⎝
n−1
⎞
xj,n sp (j)⎠ + ec + (1 + εt ) et
j=0
(18)
(21) Then, a development for each stage of recycling (as done previously) gives the specific emergy of the product:
e¯p (n) = (1 + εt − q + qεc )ei + qec + (1 + εt )et + q
⎧ ⎨
n−1
(1 + εt )(ei + et )
⎩
j=0
xj,n
1 + ε j t
1 − εc
+
(22)
1 − εcet + ec1 + 1 − εc1 + εtj = 0n − 1xj, n1 + εt1 − εcj
Step 3. Closed-loop recycling with a different mix of the recycled material coming from different origins (composite reservoirs). This case of closed-loop recycling generalizes the previous ones with multiple reservoirs made-up of different levels of recycled material and with the addition of raw materials when it is necessary (see Fig. 5). The material is supposed to originate from the same type of process. Thus, et , ec and q, can be assumed to be the same throughout the present process. ¯ has been introduced as the mean specific emergy of the (sp )(n) storage. This emergy is given by the discharge description of a reservoir as was done in the previous step. Its mathematical expression is obtained by solving Eqs. (16)–(18). Thus, the mean specific emergy of the product for closed-loop recycling (in general case) can be written (for the case εc > 0, εt > 0) with the time discrete equation:
e¯p (n) = (1 + εt − q + qεc ) ei + q s¯p (n) + ec + (1 + εt ) et
(19)
e¯p (0) = ei + et
(20)
The last equation is written under the assumption of a constant recycling rate q. In case of the variable recycling rate Eq. (19) must be used to determine the specific emergy of the product at each cycle. 2.3. Recycling follow-up The best method of accounting for recycled materials is not obvious and it is still under discussion in the literature (Ulgiati et al., 2004; Amponsah et al., 2011; Agostinho et al., 2013). Constructing emergy indices is one of the possible ways to improve this methodology, as suggested by Brown and Ulgiati (1997). Depending on the case studied, emergy ratios for recycling can be adapted as in the study of Agostinho et al. (2013), who defined a Modified Recycle Yield Ratio (‘modified’ to account the emergy of the refined material). Brown and Buranakarn (2003) proposed a set of indices and ratios to assess different options for material cycles and recycling. Among them, the environmental loading ratio (ELR) is defined as the ratio of the purchased and non-renewable inputs to the renewable emergy. A small value of ELR indicates a small environmental stress whereas this ratio increases for larger stresses (Brown and Ulgiati, 1997). Mu et al. (2011) proposed a modified definition of ELR in order to take into account the additional
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Fig. 6. Influence of the recycling mass rate on Ep (n) (Closed-loop recycling with mixed recycled material coming from the same origin, Case 3, εc = εt = 10%, ei , et , ec are constant as defined in Table 5).
emergy purchased for waste treatment, the additional investment for environmental protection and the environmental service to dilute waste. Considering that the waste treatment can recycle some valuable resources, Song et al. (2012, 2013) proposed another improvement to this ratio by considering separately the emergy for recycling. For these authors, ELR is thus defined as the ratio of purchased emergy, non-renewable emergy and emergy of the service (to dispose the waste) to the emergy of renewable resources and recycled resource emergy. Furthermore the authors applied their accounting approach to a case study that considered the e-waste treatment. However, this definition is not in accordance with the analysis done by Lu et al. (2014) who consider the ELR as a “characterization of the effect of an input in increasing the load on the environment or in enhancing the capacity of the environment to process that load”. Consequently, the recycled resource emergy should not be at the denominator of ELR. The recycling benefit ratio (RBR) is also a possible solution to account for the benefit of recycling. In its original definition, RBR is the ratio of emergy used in providing a material from raw resources to the emergy used in recycling (Brown and Ulgiati, 1997). However, this definition is relevant only for recycling accounting without variation of the recycled rate of the material at each cycle. To avoid any confusion or new adaptation of existing ratios, the benefit of using (multi) recycled material is expressed following the comparison of the emergy of the product (including its history of recycling) with a reference chosen as the emergy of the same product produced with raw material only. Thus the indicator used for the follow-up is:
Ep (n) = Ep (n) − Ep (0)
(23)
3. Case study In this section, the proposed equations are applied to the Closedloop recycling with mixed recycled material coming from the same origin, corresponding to Fig. 2. Then, the Closed -loop recycling with different mixed recycled material coming from different origins (composite reservoirs) is applied to the aluminum recycling (corresponding to Fig. 5). Considering both resource optimization and the economical point of view, aluminum recycling is considered to have great potential. The efficiency of metal recycling depends on various parameters: scrap purchasing cost, environmental regulations, metal recovery, metal yield and metal quality (Xiao and Reuter, 2002). The different situations presented above were considered. Recycling rates for the aluminum industry used for the calculations over the past years were deduced from Bertram et al. 2009. The specific emergy and the input resources required for conventional aluminum production were obtained from Amponsah et al. (2011). 4. Results and discussion Closed-loop recycling with mixed recycled material coming from the same origin To illustrate the usefulness of the equations proposed in this work (Eqs. (5)–(7)) Ep (n) was used to identify the influence of the recycle rate q (see Fig. 6). In this figure, each curve corresponds to a fixed value of q which is used at each cycle n. The mass losses rates (εc and εt ) are kept constant (equal to 0.1 each) for all the cases in this figure. The values chosen for q are theoretical and cover a large range of variation. The general trend of Ep (n) increases similarly altogether with n (whatever the value for q) before reaching a horizontal asymptote. The greater the recycling rate (q) the later the asymptote is reached. This constant value corresponds to a limit in
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the increasing benefit of recycling. This figure is in accordance with and broadens the results found by Amponsah et al. (2011) to the case of multiple recycling. The figure represents the emergy of the product taking into account recycling along the successive cycles which characterizes the memory of the material from the past cycles. The use of raw material only implies to account for emergy of stages (extraction, refining, etc.) which is no longer necessary with the use of recycled material. However, this benefit of recycling has to reflect the necessary additional emergy for the recycling process. Even if recycling remains a solution to lighten the loads on the environment, it is of importance to include the recycling memory of a product for a more accurate emergy accounting of its production. The accounting of additional contribution of environment for recycling could be tackled using the transformity rather than the emergy as the recycling does not deliver the additional emergy to the final product. However, the equations developed in the present work are written for a unit of material and consider that there is not quality loss (loss of exergy) in the different processes (recycling and transformation). Thus, the emergy analysis and the results obtained (e.g. Fig. 6) then amount to broadening what occurs for a system operating under evolutionary transformation which is summarized by Odum (1996) by “The more energy transformations there are contributing to a product, the higher is the transformity”. In Fig. 7, Ep (n) is used to compare the three cases of mass losses described in Table 1. In this case, εc and εt were set to 10% when they are not equal to zero, and q was set to 40%. It can be verified that a double source of losses (during the recycling process and the transformation one) has a stronger impact on the emergy of the product compared to only one origin of losses. There is no significant difference between the individual effects of c and t . Whereas Fig. 6 shows the influence of q on the asymptotic behavior, Fig. 7 tends to show that εc and εt have little influence on it as the constant is reached after five cycles. This trend is also visible in Fig. 8 that shows the influence of the mass losses on Ep (n). In this
Fig. 7. Influence of the different cases of mass losses on Ep (n) (Closed-loop recycling with mixed recycled material coming from the same origin, q = 40%, ei , et , ec are constant as defined in Table 5).
figure, for the three cases defined in Table 1, four different values were set to the mass loss rates εc and εt (when they are not equal to zero), and the recycling rate q was fixed at 40%. It can be verified that for a selected case (n fixed), the more losses occur in the processes (recycling and/or transformation), the greater global value for E(n). - Closed-loop recycling with different mixed recycled material coming from different origins (composite reservoirs)
Fig. 8. Influence of the mass losses on Ep (n) (Closed-loop recycling with mixed recycled material coming from the same origin, q = 40%, ei , et , ec are constant as defined in Table 5).
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Table 4 Aluminum recycling rates (Bertram et al., 2009). Year
1950 (%)
1960 (%)
1970 (%)
1980 (%)
1990 (%)
2000 (%)
2010
Recycling rate* xj,n
18 1
20 2
22 7
25 15
28 25
35 50
q = 31% Present time
*
Bertram et al. 2009.
Table 5 Emergy evaluation of conventional aluminum production and reuse of aluminum, source: Amponsah (2011). Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Conventional aluminium sheet production Primary aluminium (ingot) Electricity Labour Annual yield Recycling process Used aluminium can Primary aluminium (ingot) Aluminium scrap Used Al. can collection Used Al. Can separation Electricity Transport (Truck) Labour Annual yield Specific emergy for recyclinga Specific emergy for transformationb Specific emergy for raw materialc
Unit/year
Input resource
Specific emergy (sej/unit)
Emergy (sej/year)
g J $ g
4.17E+11 1.08E+15 2.09E+07 4.00E+11
1.17E+10 1.74E+05 1.15E+12 1.27E+10
4.88E+21 1.88E+20 2.40E+19 5.08E+21
g g g g g J ton-mile $ g Sej/g Sej/g Sej/g
2.29E+11 1.25E+11 6.25E+10 2.29E+11 2.29E+11 1.08E+15 2.82E+07 2.90E+07 4.00E+11 – – –
1.17E+10 1.17E+10 1.17E+10 2.51E+08 8.24E+06 1.74E+05 9.65E+11 1.15E+12 1.29E+10 7.69E+08 5.30E+08 1.17E+10
2.68E+21 1.46E+21 7.31E+20 5.75E+19 1.89E+18 1.88E+20 2.72E+19 3.34E+19 5.18E+21 – – –
a
ec = Em(8+9+11)/input(13). et = Em(2+3* )/input(4). c ei = Em(1)/input(4). Counted at the plant level. b
*
In the equation used in this section (Eq. (22)) the variables xj,n represent the mass fractions of the recycled material that have been recycled j times and used in cycle n. The equation was implemented with a theoretical variation of the recycling rate (35% for j = 1, 30% for j = 2, 20% for j = 3, 10% for j = 4, 5% for j = 5) and the accounting was
done at the fifth cycle (n = 5). Thirty five percent must be understood as the material in the reservoir that has been already recycled once, 30% is the material in the reservoir that has been already recycled twice, and 20% is rate of the material in the reservoir that has been already recycled three times etc.
Fig. 9. Emergy accounting with follow-up of recycling history (Closed-loop recycling with different mixed recycled material coming from different origins-composite reservoirs); εc = εt = 10%.
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By applying the results on the theoretical values of the recycled mass fraction recycled, it was possible to test the proposed work. Results provided by the implementation of the equations proposed ((19)–(22)) are in accordance with the physical analysis. Thus, the same equations are now applied to the case of the aluminum recycling rates over the past decades, as it is represented in Table 4 (Bertram et al., 2009). In this table, the year 2010 is considered as the initial set for the reservoir of material. Consequently the fraction of recycled material used in the equation is q = 31%. Values of the xj,n are chosen according to the data in Table 4. The input resources for conventional aluminum production were obtained from the Amponsah’s work (Amponsah et al., 2011) (our Table 5). Using the values given in this table, the specific emergy for recycling ec was calculated as the sum of the items 8, 9 and 11 divided by the item 13; the specific emergy for transformation et is calculated as the sum of the items 2 and 3 divided by item 4; the specific emergy of raw material ei was calculated as the ratio of the item 1 to the item 4. In Fig. 9, it can be seen how the equations proposed could help to follow up the materials taking into account the history of its recycling. It can be seen that 17% of the products emergy corresponds to the emergy of the material that has been recycled once, 10% to the emergy is accounted for by a material that has been recycled twice, etc. The results are given for εc = εt = 10% and the 6th cycle of recycling (n = 6). The proposed method requires known values for xj,n (fixed a priori in this work). Thus, a continuous follow-up of the reservoir composition is necessary. If the 17% of the recycle emergy of the product are associated to15.5% (q × x1,6 = 0.31 × 0.5, see Table 4) of the recycled material used, then the 10% correspond to 7.75% (q × x2.6 = 0.31 × 0.25, see Table 4) of the recycled material used, etc. 5. Conclusion Recycling is commonly considered as a suitable process in terms of sustainability. Numerous methods for its assessment already exist in the literature, such as the life cycle analysis (a multicriteria method, user-side oriented), or the emergy assessment (a mono-criteria method, donor-side oriented) and they are considered more as complementary rather than competitive among themselves. Recycling can be done under the two configurations: open-loop or closed-loop. Applying emergy assessment, discrete time emergy equations were set according to the recycling configuration for different cases of closed-loop recycling. The relative emergy of the product Ep (n), including the different previous cycles of the recycled material, is used to assess the impact of recycling. It has been shown that the Ep (n) is an increasing function of the number of cycles illustrating the accumulation emergy for previous cycles of recycling. The equations proposed help to integrate this history of recycling of the material and to follow-up this history. This approach to the emergy accounting for recycle also helps to show the impact of compensating for material losses with recycled material (whatever the type of loss considered and the rate of recycled material use) rather than using raw material. This potential usefulness of the equations developed was illustrated through the example of the aluminum in the most general case of a reservoir made-up of mixed recycled material forms over time. It was demonstrated how the proposed equations could follow-up the history of recycling even in this general case. Acknowledgement The authors thank the reviewers and the chief editor for their valuable comments during the review process.
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