Analysis of the life cycle environmental impact reductions of remanufactured turbochargers

Recently, remanufacturing approach/technology, which includes a series of the processes of disassembling, cleaning, inspecting, repairing/reconditioning, reassembling components for resale, is becoming more popular as companies look for a way to combat the current climate crisis, and as it allows companies to reduce environmental impacts and to save energy and resources. This study analyzed the reduction effect of six environmental impacts and the savings effect of energy and resources by turbocharger remanufacturing compared to its newly manufacturing using a life cycle assessment (LCA) methodology. The results show that the most significant benefit of the turbocharger remanufacturing related to environmental impacts was the global warming potential (GWP), which could be reduced by 52.2%, followed by the abiotic depletion potential (ADP), terrestrial ecotoxicity potential (TETP), human toxicity potential (HTP), freshwater aquatic ecotoxicity potential (FAETP) and marine aquatic ecotoxicity potential (MAETP) which could be reduced by 51.9%, 44.8%, 44.2%, 42.6%, and 36.7%, respectively. Also, its resource saving could be obtained from 21.7 to 73.5% depending on the type of resources. Furthermore, turbocharger remanufacturing offered a significant energy saving of 83.9%. The results obtained from this study could be used for national policy-making to a net-zero carbon transition.


Introduction
Amid the growing global awareness of climate change issues, advanced countries are introducing greenhouse gases (GHGs) reduction policies along with a declaration of carbon neutrality. The circular economy, one of major tasks in the 2050 carbon neutral strategy, is a model that minimizes the use of natural resources and maximizes value-adding through reusing and recycling. It is closely related to the achievement of the GHGs emissions reduction target [1,14,34]. In particular, remanufacturing is one of the best effective ways to save energy and resources and to achieve equivalent performance as a new product through the processes of disassembling, cleaning, inspecting, repairing/reconditioning, reassembling, and final testing using used parts (cores) as its raw materials. In terms of resource efficiency and value-adding, it can also promote accomplishing sustainable production [5, 10, 12, 15-17, 19-21, 28].
The automotive parts remanufacturing industry is the most active sector which accounts for 70 to 90% of the entire remanufacturing industry of Korea [18,19]. Steinhilper reported that the remanufacturing of automotive parts required 80% less natural resources than the manufacturing of the new parts [32]. Recent related studies on the automatic transmission remanufacturing have shown similar results with 79.2% resource saving [22]. In addition, from the consumer's point of view, remanufactured products have a specific benefit : the resulting products are 20 to 80% cheaper than new products but with equivalent performance [21].
According to data from automobile remanufacturers, the turbocharger is the most popular traded component among automobile remanufacturing parts, and its sale is as much as 20% of the whole automobile remanufacturing parts sales.
Under these circumstances, the necessity of a study on the life cycle environmental impact reductions of remanufacturing turbochargers has been raised.
The frequently used methodology for conducting environmental assessment studies of remanufacturing is the life cycle assessment (LCA). LCA is a "cradle-to-grave" approach for assessing industrial products and systems, which enables the estimation of the cumulative environmental impacts resulting from all stages in a product life cycle [33]. In this study, the six environmental impacts categories including global warming potential (GWP), abiotic depletion potential (ADP), human toxicity potential (HTP), terrestrial ecotoxicity potential (TETP), freshwater aquatic ecotoxicity potential (FAETP), and marine aquatic ecotoxicity potential (MAETP) were selected. The reason why GWP and ADP were selected is that a turbocharger mainly uses steel resources and the steelmaking process emits lots of GHGs. Also, HTP, TETP, FAETP and MAETP were selected because carcinogenic toxic gases are emitted in the casting process of turbocharger components [3]. However, LCA studies associated with ADP and toxicity potentials in the life cycle of a turbocharger have never been conducted.
In 2017, Gao conducted an environmental impact assessment study of turbocharger remanufacturing [9]. Brand-new turbochargers were compared with remanufactured turbochargers from an environmental perspective using six environmental impacts categories: global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), ozone layer depletion potential (ODP), photochemical ozone creation potential (POCP), and primary energy demand (PED). The system boundary for new turbocharger manufacturing was divided into four stages: raw materials production, materials transport, manufacturing, and assembling. On the other hand, that for turbocharger remanufacturing was divided into five stages: recycling, disassembling, cleaning, remanufacturing, and assembling. As a result of the environmental impact assessment of turbochager remanufacturing, 73.3% of GWP, 11.9% of AP, 59.8% of EP, 90.8% of ODP, 73.2% of POCP, and 82.6% of PED were reduced compared to that of new turbocharger manufacturing [9]. However, Gao's research had the following limitations. First, the environmental impact reduction effect was thought to be over-calculated because it was assumed that all turbocharger components were remanufactured. Thus, it is necessary to re-analyze the environmental impacts by applying the remanufactured actual portion rather than fully remanufactured. Second, the resource-saving effect caused by turbocharger remanufacturing was not considered in Gao's research. So, ADP expressing resource depletion needs to be analyzed. Third, despite the significantly high recycling portion in the end-of-use/life stage, it was not considered. Thus, the material recycling effect should also be considered in the environmental impact assessment. Fourth, emissions of carcinogenic toxic gases such as phenol, benzene, and formaldehyde during the casting process when manufacturing new turbocharger components were not considered. Polycyclic aromatic hydrocarbons (PAHs) are generated from the incomplete combustion of fossil fuels while manufacturing a turbocharger [3]. So, the toxicity potentials (HTP, TETP, FAETP and MAETP) need to be analyzed.
Turbocharger is also related with the steel industry which is a highly carbon intensive industry [35]. The amount of GHGs emitted from the steel industry was 101.2 million tons CO 2 -eq. in 2018, comprising 38.8% of the total industrial sector GHGs emissions in Korea [30]. When remanufacturing a turbocharger, the reduction of the amount of steel used as its raw-material has an effect on reducing GHGs emission. Thus, GWP needs to be considered.
While the transition from internal combustion engine vehicles (ICEVs) to electric vehicles (EVs) continues, ICEVs still make up a significant proportion of the entire vehicle market. Casper and Sundin [4] predicts that EVs will account for 33% of total vehicle sales by 2040, meaning that ICEVs will continue to record high sales for the next 20 years. In this situation, remanufacturing ICEV parts is a quite suitable solution to promote carbon neutrality and support the circular economy industry.
Therefore, this study focuses on quantifying six environmental impacts, which are ADP, GWP, HTP, TETP, FAETP and MAETP, of remanufacturing turbochargers considering the replacement portion of new parts in the entire life cycle, i.e., raw material acquisition, production, and end-of-use/life stages compared to those of newly manufacturing turbocharger, and analyzing resources and energy savings effects of remanufacturing turbocharger.

Turbocharger remanufacturing process
The turbocharger is an internal combustion engine component classified as a suction device that consists of an intake housing, exhaust housing, bearing housing, turbine wheel, variable nozzle turbine (VNT), and operating device. Turbocharger primarily consists of steel due to requiring excellent heat resistance and durability against vacuum because they are exposed to a severe heat load and receive thermal expansion and contraction [27]. Figure 1 presents the operating procedure of a turbocharger. The turbine is rotated with the power of the exhaust gas, and the intake air is compressed with the rotational force and sent to the cylinder to increase the power. Figure 2 presents the assembly procedure of a turbocharger. The turbocharger mainly consists of six parts, i.e., intake housing, exhaust housing, bearing housing, turbine wheel, VNT, and actuator.
Used inner parts (cores) in a turbocharger, such as the bearing housing, turbine wheel, VNT and actuator, are usually replaced by new parts due to the damage of cores. Especially the turbine wheel, which is a part mounted inside the bearing housing, is thinner and worn faster than other parts. Owing to these characteristics of turbine wheels, most remanufacturing companies use new ones of them. Nevertheless, laser cladding technology has recently been applied to restore damaged blades of the turbine wheel. On the other hand, the other used inner parts are almost reused for remanufacturing the turbocharger.

Research methods
This study evaluated the environmental impact reduction effects of the remanufactured turbocharger compared to the new turbocharger using the life cycle assessment (LCA) methodology. LCA is an internationally standardized methodology specified in ISO 14,040 series [13]. LCA is a "cradle to grave" approach for assessing industrial products and systems, which enables the estimation of the cumulative environmental impacts resulting from all stages in a product life cycle, often including impacts not considered in more traditional analyses (e.g., raw material extraction, material transportation, ultimate product disposal, etc.) [33]. Figure 4 presents the procedure of this study. The target of LCA was selected. Second, the system boundary for LCA of the target was defined. Third, the data of each stage were collected and calculated, and their inventory data were analyzed. Fourth, environmental impacts were assessed based on the inventory data. Finally, the environmental impact reduction effect caused by remanufacturing was analyzed.

Data collection and calculation
The data collection went through the following procedures.
In order to check the specifications of the target turbocharger and investigate the remanufacturing process, a site survey was conducted at the turbocharger remanufacturing company C in Korea which has common remanufacturing process and equipment. The collected data includes the parts of the target turbocharger, the materials and new components replacement proportion of each part, the amount of the remanufactured turbocharger production and the utility usage in a remanufacturing process of the turbocharger. Also, the literature review has been conducted to collect the data on the new turbocharger manufacturing process which includes the consumed amount of the utilities such as electricity and industrial water.
Remanufacturing has a technical feature that utilizes used components(cores) as a raw material in the raw material acquisition stage.
Accordingly, three assumptions for a comparative analysis between remanufacturing process and new manufacturing process were made in this study: First, in the remanufacturing process, the weight of parts that can be remanufactured after the use of a new turbocharger is used in the remanufacturing stage. This is based on the avoided burden approach as the allocation method. This approach assumes that remanufacturing avoids the environmental burdens of raw material acquisition in the future. It is acknowledged that material not recycled needs to be replaced by primary material feedstock, consistent with ISO 14,044 [2,8,29]. Second, the remanufacturing is not applicable to the new manufacturing process. Third, a comparative analysis between remanufacturing process and new manufacturing process is conducted for only one cycle.

Raw material acquisition
The raw material acquisition stage refers to a stage in which a raw material is an input to produce a product. In this study, all components of new and remanufactured turbochargers were classified into different materials. Weights and materials of each component were collected based on 1 turbocharger. In the case of remanufacturing turbocharger, the used parts (cores) generated at the end-of-use/life stage were basically reused in the raw material acquisition stage. However, the proportion replaced by new parts due to their defects was excluded in the calculation of the reuse rate.

Production
The production stage refers to the stage, in which resources and energy are input in newly manufacturing and remanufacturing turbochargers. In this study, only electricity consumption for producing new and remanufactured turbochargers was considered. The electricity consumption for producing a new turbocharger used in this study was as suggested by Gao [9]. The electricity consumption in remanufacturing was calculated based on the annual electricity consumption and the data provided by the turbocharger remanufacturing company C.

End-of-use/life
Disposal after use is divided into end-of-use and end-of-life stages [31]. In this study, the end of use stage is defined as remanufacturing or scrapping, and the end-of-life stage is defined as material recycling, incineration, and landfill. the material recycling rate is based on the waste statistics for the environmental product declaration [24]. On the other hand, the remanufactured turbocharger uses cores as raw materials.

Selection of impact assessment methodology and impact category
An impact assessment was carried out at the characterization stage to evaluate the brand-new turbocharger and the remanufactured turbocharger objectively by applying the mid-point impact assessment methodology [23]. It is necessary to select the impact categories when analyzing the environmental impacts. Table 1 lists the selected impact categories. The reason for the selection of six impact categories is that ADP is directly related to the use of resources, and GWP is GWP (Global warming potential) kg CO 2 -eq. HTP (Human toxicity potential) kg 1,4-DCB-eq. TETP (Terrestrial ecotoxicity potential) kg 1,4-DCB-eq. FAETP (Freshwater aquatic ecotoxicity potential) kg 1,4-DCB-eq. MAETP (Marine aquatic ecotoxicity potential) kg 1,4-DCB-eq. used to derive the GHGs reduction effect through resource reduction. In addition, HTP, TETP, FAETP, and MAETP are closely linked to the environmental impacts caused by hazardous substances generated in the life cycle of the brand-new and remanufactured turbochargers.

Results and discussion
Setting target and scope Table 2 lists the target and scope of this study. The new turbocharger model evaluated in this study was "4A480" made by Company H which is the automobile parts manufacturer, and the remanufactured turbocharger model was given by company C. The functional unit (FU) in this study was defined as a 1 turbocharger weighing 6.5 kg.
Setting system boundary Figure 5 shows the system boundary of this study. This study included the raw materials acquisition, the production, and end-of-use/life stages. The transportation stage was excluded according to cut-off criteria. In general, all processes and flows that are attributable to the analyzed system are to be included in the system boundaries. However, not all these processes and elementary flows are quantitatively relevant: for the less relevant ones, low quality data can be used, limiting the effort for collecting or obtaining high quality data for those parts. Among these, the irrelevant ones can be entirely cut-off (under 5%) [7]. According to Gao's research, the transportation stage accounts for only 0.2% of environmental impact in the whole life cycle of turbocharger manufacturing process. This implies that the transportation stage is relatively neglectable [9]. The use stage is also excluded because the weight of input materials for maintenance was insignificant. Table 3 lists the composition of the turbocharger and the new parts replacement portion in the turbocharger remanufacturing process. This data was provided by the remanufacturing    Tables 4 and 5 show the material list required for the new and remanufactured turbochargers, which is referred to the constituent materials weight of each part in Table 3. The input material weight of each part of the brand-new turbocharger was calculated by applying the material weight proportion of each part, whereas that of the remanufactured one was calculated by multiplying the material weight proportion and the new part replacement proportion of each part.

Production
In the production stage, only the electricity consumption was considered for producing the brand-new and remanufactured turbochargers, as listed in Tables 6 and 7, respectively. The calculated electricity consumption in the production of the brandnew turbocharger was 4.2 kWh by converting the electricity consumption (28.5 kWh/44.5 kg) presented in Gao's study [9] into 6.5 kg, which is the weight of a turbocharger in this study. Its converting was applied by the allocation guidelines in the LCA methodology [13]. The calculated electricity consumption when remanufacturing the turbocharger was 0.7 kWh/6.5 kg according to the remanufacturing company C's internal data. It was calculated based on the annual electricity consumption (80 kWh/year) and the annual production of turbochargers (120 turbochargers/year).

End-of-use/life
In the case of the end-of-use/life stage, two assumptions were applied. First, the material recycling rate of turbocharger parts in the end-of-use/life stage is based on the industrial waste statistics given by KEITI [24], as shown in Table 8. Second, the remanufactured turbocharger is reused as raw materials for remanufacturing depending on its cores (used parts) usage ratio, and treatment ways of the remaining parts which are not used for remanufacturing at their end-of-use/life were as applied in the first assumption.  Table 9 lists the treated or recycled amount of the brand-new turbocharger in the endof-use/life stage. Assuming that a new turbocharger is not remanufactured. The amount of material recycling, landfill, and incineration for each part was calculated by applying the first assumption. Table 10 summarizes the profile of the remanufactured turbocharger in the end-ofuse/life stage. The cores of the remanufactured turbocharger were assumed to be fully or partly reused at their end-of-use/life. Intake and exhaust housings were assumed to be fully remanufactured. The same assumptions for estimating the recycling rate of the new turbocharger were applied to calculate the weight of CHRA as the mentioned above because used CHRAs were fully replaced by new parts when remanufacturing a turbochager. In the case of the VNT and actuator, 50% of the weight of used VNTs and 70% of the weight of the actuator (core) are reused as raw materials, and the remaining weight was treated by using the same assumptions as calculating CHRA above. Table 11 shows the list of the life cycle inventory databases ("LCI DBs") applied in the raw material acquisition, the production, and the end-of-use/life stages of the new and  [25,26] were applied to calculate the environmental impacts of the stainless steel, carbon steel, electricity production, and disposal methods. In addition, the ecoinvent DBs [6] were also applied to calculate the environmental impacts of the cast iron, silicone, and aluminum production and a few disposal ways that have not been established in Korea so far. Figures 6 and 7 illustrate the process charts of the entire life cycles of new and remanufactured turbochargers, respectively. Table 12 presents the results of an environmental impact assessment of new and remanufactured turbochargers at various stages, including raw material acquisition, production, and end-of-use/life.

Environmental impact assessment of new and remanufactured turbochargers
The results show that the largest environmental impact in all categories, including ADP, GWP, HTP, TETP, FAETP, and MAETP, is caused by the raw material acquisition stage, which accounts for between 91.1% and 105.8% of the total impact for both new and remanufactured turbochargers. It implies that the environmental impacts from the extraction and processing of metal ores are significantly larger than the environmental impacts caused by the manufacturing of products using those metals. However, remanufacturing turbochargers has much smaller environmental impact than manufacturing new parts, as shown in Table 12. The values exceeding 100% in Table 12 indicate the environmental impact that could be reduced by recycling materials at the end-of-use/life stage.
Electricity usage at the production stage contributes to the higher portion of GWP and ADP than any other environmental impact categories. When newly manufacturing a turbocharger, the production stage accounts for 10.4% and 10.3% of ADP and GWP, respectively. In contrast, when remanufacturing a turbocharger, the production stage only accounts for 3.5% of both ADP and GWP, which is a much smaller proportion than newly manufacturing turbocharger. Table 7 Electricity consumption of the remanufactured turbocharger in the production stage Annual production of turbocharger Electricity consumption Calculated electricity consumption per the functional unit of this study 120 turbochargers/yr 80 kWh/yr 0.7 kWh/FU The portion of toxicity potentials occurred in the production stage for four toxic impacts categories, i.e., HTP, TETP, FAETP, and MAETP, was less than 0.01%, suggesting little environmental impacts.
It is also seen that the recycling process at the end of life stage brought about the larger reduction of environmental impacts, especially ADP and TETP, ranging from − 2.0% to − 5.8%, whereas it created relatively smaller reduction of environmental impacts regarding other impact categories, GWP, HTP, FAETP, and MAETP, ranging from − 0.2% to − 1.4%, along with manufacturing and remanufacturing options. The fact that TETP was highly reduced by − 5.8% at the end-of-use/life stage when newly manufacturing the turbocharger was attributed to the higher amount of metallic material recycled in its end-of-use/life stage, compared to when remanufacturing the turbocharger. In other words, 64.1% of the total weight of the turbocharger is reused at the raw material acquisition stage when remanufacturing the turbocharger, and the amount(proportion) recycled in its end-of-use/life stage was thus relatively low compared to the amount recycled when newly manufacturing the turbocharger.

Analysis of the resource and energy saving effects by turbocharger remanufacturing
The resources and energy savings rates by turbocharger remanufacturing are shown in Table 13. It is seen that iron and silicone could be saved more than 70%; moreover, electricity could be saved more than 80% by its remanufacturing. However, the saving effect of aluminum alloy was shown to be relatively low (21.7%). This was because the aluminum alloy-contained parts were easily worn out, and thus replaced by new parts rather than reused.

Conclusion
This study analyzed the environmental impact of the new and remanufactured turbochargers using LCA. The results showed that turbocharger remanufacturing could reduce GWP and ADP by 52.2% and 51.9%, compared to its newly manufacturing, respectively. TETP, HTP, FAETP, and MAETP also could be reduced by 44.8%, 44.2%, 42.6%, and 36.7%, respectively. It was also found that resources and energy savings due to turbocharger remanufacturing could be obtained from 73.2% on iron, 73.5% on silicone, 21.7% on aluminum-magnesium alloy, and 83.9% on electricity. Data on electricity usage of newly manufacturing turbocharger for LCA is collected from the literature review. However, this study has an obvious distinction which considered the new parts replacement proportion at the material acquisition stage and the recycling proportion at the end-of-use/life stage. The distinctions are as follows.
(1) This study applied the actual replacement portion of new parts for turbocharger remanufacturing. But, Gao's study assumed all core parts can be remanufactured. By applying the replacement portion of new parts in the remanufacturing process, the environmental advantages of turbocharger remanufacturing compared to newly manufacturing were derived more practically and precisely. (2) The resource-saving effect of turbocharger remanufacturing was quantified. Remanufactured products are re-merchandized to their original performance levels or better levels through a series of processes such as non-destructive disassembling, cleaning, inspection, repair/reconditioning, reassembling, and final testing. It is well-known as an appropriate and efficient resource circulation method [18,19,21]. As the results of this study, the resource and energy saving effect through a turbocharger (6.5 kg) remanufacturing was 3,735 g on iron, 180 g on silicone, 250 g on aluminum-magnesium alloy, and 3.50 kWh on electricity. (3) The environmental impact was quantified by applying the recycling portions at the endof-use/life stage of newly manufacturing and remanufacturing turbocharger, respectively. In addition, the actual reduction effect of the environmental impacts by turbocharger remanufacturing was quantified more precisely by excluding the mass of cores (used parts) in their end-of-use/life stage moving back to the raw material acquisition stage. The results on quantified environmental impacts reveal that new parts replacement proportion has a great effect on the environmental impacts. This implies that the improvement of remanufacturing technology is directly linked with the lifting of the reuse rate of used parts(cores), and expected to reduce the overall environmental impacts. (4) The benefits of remanufacturing were quantified by considering the four toxic effects of HTP, TETP, FAETP, and MAETP, which were not addressed in previous studies.
In recent years, carbon neutrality has been issued in Korea and even the world widely. The circular economy agenda became a key tool to achieve carbon neutrality. Particularly, remanufacturing business is praised for being more ecologically friendly in Korea, coping with GHGs emissions and resources shortage/depletion.
Thus, new findings from this study, such as quantitatively analyzed results on environmental and economic effects caused by automotive parts remanufacturing, may be used in making circular economy or carbon neutrality policy from the perspective of environmental impact reduction and resources/energy savings.
Nevertheless, GHGs-related remanufacturing research results are scarce. So, economic and environmental benefits from remanufacturing should be stressed and appealed in order that the government may promote the remanufacturing industry and strengthen financial support in this sector.