Unearthing Potentials for Decarbonizing the US ... - ACS Publications

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Unearthing Potentials for Decarbonizing the U.S. Aluminum Cycle Gang Liu, Colton E. Bangs, and Daniel B. M€uller* Industrial Ecology Programme and Department of Hydraulic and Environmental Engineering, Norwegian University of Science and Technology, S.P. Andersens vei 5, 7491 Trondheim, Norway

bS Supporting Information ABSTRACT: Global aluminum demand is anticipated to triple by 2050, by which time global greenhouse gas (GHG) emissions are advised to be cut 50 85% to avoid catastrophic climate impacts. To explore mitigation strategies systematically, a dynamic material flow model was developed to simulate the stocks and flows of the U.S. aluminum cycle and analyze the corresponding GHG emissions. Theoretical and realistic reduction potentials were identified and quantified. The total GHG emissions for the U.S. aluminum cycle in 2006 amount to 38 Mt CO2-equivalence. However, the U.S. has increasingly relied on imports of aluminum embodied in various products. The in-use stock is still growing fast in most product categories, which limits current scrap availability for recycling and emissions saving. Nevertheless, there is still large emission mitigation potential through recycling. The potentials from “100% old scrap collection” and “low emission energy” were each calculated to be higher than all process technology potential. Total emissions will decrease dramatically and mitigation priorities will change significantly under a stock saturation situation as much more old scrap becomes available for recycling. The nature of in-use stock development over the coming decades will be decisive for the aluminum industry to reach deeper emission cuts.

1. INTRODUCTION Aluminum is the second most used metal worldwide and global aluminum demand is anticipated to triple at least by 2050.1 Primary production of aluminum from bauxite is very energy and greenhouse gas (GHG) emissions intensive. While the IPCC advises cutting global GHG emissions by 50 85% by 2050 to avoid catastrophic climate impacts that would accompany a 2 °C global average temperature increase,2 aiming for such a reduction in the aluminum industry would be extremely challenging only through technology improvements.3 For example, the perfluorocarbon (PFC) emissions intensity has limited scope for further improvement after an 86% reduction over the past twenty years,4 so the growing demand for primary aluminum would result in a dramatic increase of total emissions. Recycling of aluminum scrap requires up to 95% less energy than the production of primary aluminum; however, currently scrap availability is limited due to high accumulation of aluminum in products in use. Understanding the dynamics of the entire aluminum cycle would facilitate the exploration of mitigation strategies from a systems approach that goes far beyond the potential of process technology improvements. Several studies on the emissions of the aluminum industry applied a life cycle assessment (LCA) approach5 and concentrated mainly on primary aluminum production6,7 and lightweighting use of aluminum in vehicles.8,9 These studies neglect aggregate effects, interactions within the entire cycle, and the time dimension, and thus cannot provide a sectoral or regional context for discussions on absolute emissions reduction. These shortcomings can be avoided by employing a material/substance r 2011 American Chemical Society

flow analysis (MFA/SFA) approach. The aluminum cycles of Denmark,10 Italy,11 the U.S.,12 and China 13 for a single year or selected years have been characterized. Dynamic models for calculating aluminum scrap generation using historical consumption data and product lifetimes were introduced in the 1970s,14 and recently further refined for Germany,15 the UK,16 and the U.S.17,18 These models primarily focus on mass stocks and flows, and only a few have been integrated with an environmental dimension to further enhance the policy relevance for long-term issues of environmental impact mitigation.19 21 Schwarz et al.22 and Allwood et al.3 built material flow models on the global scale to discuss future aluminum mass flows and emissions. However, their models do not include in-use stocks explicitly, and therefore have limited potential for explaining changes in scrap availability, which is critical for insights into emissions reduction potential through recycling. In this paper, we develop a model which simulates the dynamic anthropogenic aluminum cycle and allows an integrated analysis of material flows and corresponding energy use and GHG emissions. The U.S. was selected as a case country due to its large market and long history for aluminum use. We first analyze and quantify all relevant stocks and flows of the U.S. aluminum cycle for the period of 1900 2008, then present a detailed analysis of the emissions for the cycle in 2006. Subsequently, Received: June 28, 2011 Accepted: October 4, 2011 Revised: September 5, 2011 Published: October 04, 2011 9515

dx.doi.org/10.1021/es202211w | Environ. Sci. Technol. 2011, 45, 9515–9522

Environmental Science & Technology

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Figure 1. System definition of the U.S. anthropogenic aluminum cycle and its consequent energy use and GHG emissions. Four types of semis α = a d (rolled, extruded, cast, and other mill products); twelve types of final and obsolete products β = a l (BC, TAU, TAE, TOT, PCA, POT, ME, ECA, EOT, CD, OTN, and OTD); two types of scrap γ = a b (can scrap and other general scrap); seven types of energy carriers δ = a d (U.S. smelting contract mix, U.S. grid mix, natural gas, heavy oil, hard coal, propane, and diesel and light fuel oil). P12 and P13 are placed outside the system boundary to portray that only emissions allocated to the U.S. aluminum cycle is considered for these two processes.

Table 1. Product Categories, Codes, and Examples Used in This Study code

product category

product examples

BC TAU

building and construction transportation: automobiles and light trucks

roofing, cladding, window and door frames engine blocks, suspension components, automobile frames and body panels, wheel rims

TAE

transportation: aerospace

aircraft frames and decking

TOT

transportation: others

railway cars, marine vessels, motorcycles and bicycles

PCA

packaging: cans

beverage cans, aerosol cans

POT

packaging: others

foil for flexible packaging, semirigid food containers

ME

machinery and equipment

irrigation pipe, ladders, office and hospital equipment

ECA

electrical: cables

wire, cables

EOT CD

electrical: others consumer durables

transformers and capacitors, electric lamps air conditioners, refrigerators, dishwashers, cookware

OTN

other uses: non-destructive use

other uses except destructive use

OTD

destructive uses

metallurgical products for steelmaking

theoretical emission reduction potentials are quantified and several realistic options are discussed.

2. METHODS The historic U.S. aluminum cycle was calculated using a system definition described in Figure 1. Primary aluminum is produced by electrolytic reduction of alumina (Hall Heroult smelting). Molten aluminum from smelters is alloyed, cleaned, and cast into different kinds of ingots. These ingots are further transformed into different semiproducts mainly by rolling, extrusion, and casting, and eventually manufactured into final products. New scrap from all stages of production and manufacturing and old scrap from products leaving use are recycled in refiners and remelters. The processes Manufacturing and Use are divided into 12 subprocesses reflecting different product categories (Table 1). All flows and stocks were calculated as aluminum metallic equivalent using a dynamic MFA model as detailed in the

Supporting Information. Three approaches were used to calculate the flows: use of industry and government statistical data, calculation by transfer coefficients estimated from literature and expertise, and derivation by the mass balance principle. Aluminum in products leaving use and accumulated in in-use stock was calculated based on historic consumption and lifetime for each product category. To get more accurate consumption data, the statistical shipment data of different product categories to manufacturing were adjusted by assumed yield ratios, and historic trade statistics of around 110 products were used to account for the import and export flows of aluminum contained in parts and final products (“indirect trade”). Recent years after 2006 were excluded because of incomplete trade data. Due to a lack of historical data regarding the age of products upon disposal, a parameter variation was performed for the lifetimes (normal distribution) of all product categories. Energy use and GHG emissions corresponding with the U.S. aluminum cycle were calculated using coefficients based on the 9516

dx.doi.org/10.1021/es202211w |Environ. Sci. Technol. 2011, 45, 9515–9522


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Figure 2. Historic aluminum flows in the U.S. assembled by markets, 1900 2008. Mt/a = million metric tons per annum.

output of each process. All processes were considered except Manufacturing and Use, because emissions from these two processes are difficult to allocate to a single material and instead reported by other sectors. Energy use was differentiated between that occurring on-site at each process (primary), and that occurring off-site from “well-to-gate” in the energy infrastructure (secondary). Besides the consequent primary and secondary energy related emissions, a third type of GHG emissions is called process emissions, which mainly includes CO2 emissions from anode production and consumption and PFC emissions from smelters (CF4 and C2F6). Emissions from transportation of raw materials and products, which were estimated to have a 5% share of total emissions for the global aluminum cycle,23 were excluded in the model, except for those related to energy carriers. Emission mitigation options were analyzed based on the result for 2006. Theoretical reduction potentials for those identified process technology and system-wide improvements were each calculated as the difference between current total emissions and emissions with the entire mitigation potential achieved.

The entire mitigation potentials for the major processes were calculated as the according emissions of theoretical minimum energy requirements reported by the U.S. Department of Energy.24 Several realistic options were identified and quantified using a sensitivity analysis approach. A hypothetical saturated in-use stock situation is compared to the current situation to illustrate the importance of stock dynamics on total emissions and mitigation options.

3. RESULTS AND DISCUSSION 3.1. Historic Flows and Stocks. The historic U.S. aluminum flows are assembled by markets (bauxite, alumina, aluminum, semiproducts, final products, and scrap) in Figure 2. All markets show a slow initial development and a rapid penetration after World War II. Thereafter the production curves from upstream to downstream show a progressive decoupling, which demonstrates the substitution of domestic production by increasing net import of aluminum-containing goods (aluminum, semis, and 9517



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Figure 3. Historic U.S. aluminum in-use stock: absolute (left) and per capita (right), 1900 2006. The thick lines indicate the medium lifetime scenario, and the bottom and top dot lines are for the long and short lifetime scenarios, respectively. See lifetime scenarios in detail in Tables S3 and S4 in the Supporting Information.

final products) and the recovery of scrap. While total domestic production growth of all markets slowed down since the 1970s, the total net import has doubled to satisfy the growing demand for final products. In general, the U.S. is a net importer of aluminum in all preconsumer markets and a net exporter of scrap, a phenomenon similar to the U.S. iron cycle.25 The import dependency has gradually shifted along the production chain, i.e., from bauxite before 1970s, to alumina around 1980s, and aluminum and final products after 1990s, which increases not only foreign dependence but also the potential leakage of embodied GHG emissions of the U.S. aluminum industry to other countries. Since 2001, secondary recovery exceeds primary production, and castings production, which predominantly uses recycled aluminum, approaches the level of extrusion. However, a large share of secondary production still comes from new scrap (around 60% in recent years), which reflects inefficiency of the whole aluminum processing and production chain and increases GHG emissions as a result of additional energy use for new scrap remelting. The absolute and per-capita U.S. aluminum in-use stocks in 2006 were estimated to be 146 Mt and 490 kg, respectively, for the medium lifetime scenario (Figure 3). The categories BC, TAU, and TOT constitute the largest components of the in-use stock, representing about two-thirds of the total in 2006. The packaging categories (PCA and POT), though having a high share in annual consumption, form a negligible in-use stock due to short product lifetimes. Our absolute in-use stock result of 2006 generally agrees with the crude estimation of the United States Geological Survey (USGS) for 2002, 142 Mt,26 but is higher than the estimation of Hatayama et al. with 120 Mt for 2003 17 and McMillan et al. with 91.1 97.6 Mt for 2007.18 The differences most probably come from our explicit consideration of manufacturing scrap and full inclusion of indirect trade. The total net import relative to the total apparent consumption of aluminum final products varies from 0 to 15% since 1962 and remains around 20% in recent years in our model (Figure S3 in the Supporting Information). This falls within a similar range as U.S. indirect trade of other metals,27 and is higher than the aforementioned two studies.17,18 Simulation results also show a growing trend of U.S. aluminum in-use stocks in most product categories, with a relatively small impact from lifetime assumptions. Results of the short and long lifetime scenarios differ from that of the medium by only about 18% 25%, indicating the fact that aluminum in-use stocks are still growing rapidly in the U.S. This differs from a

previous observation for the U.S. iron in-use stock, which saturated and has remained stable on a per-capita level since the 1980s.28 The stock increase was initially dominated by the penetration of aluminum applications in the BC category before the late 1980s, and has since gradually shifted to transportation, especially in the TAU category. This reflects the increasing use of aluminum in automobiles for weight reduction and consequent energy savings. Although aluminum has historically been employed in automobiles primarily in the form of castings, a wider variety of products including extrusions, stamped sheet parts, and forgings have started to penetrate the markets in recent years, and are expected to be increasingly used in car body and other structural applications.9 Nevertheless, the per-capita in-use stocks in some categories such as ECA, EOT, and PCA witnessed saturation after the 1980s, which may reflect the mature state of aluminum use in the electrical engineering and packaging sectors in the U.S. The average plateau levels for ECA and EOT are approximately 40 and 12 kg/cap, respectively. Used beverage cans (UBCs, obsolete PCA) form the largest source of obsolete scrap in the U.S., followed by end-of-life vehicles (obsolete TAU), and retired consumer durables (obsolete CD) (Figure 4). Aluminum in UBCs was estimated to be 1.4 Mt in 2006, making up around 26% of the total in all obsolete products. This can be explained by their high apparent consumption and short lifetimes. The obsolete BC flow, on the contrary, only comes in fifth due to its long lifetime, despite having the largest in-use stock. However, the continuously declining collection ratio of UBCs and the fast growing net export of old scrap in recent years resulted in a substantial decrease of domestic recovery. Additionally, since the obsolete product markets are poorly understood and thus difficult to be quantified, the simulated aluminum flow leaving use is generally higher than the aluminum flow entering collection (the sum of landfilled29 and collected30). The difference indicates an important gap of the generation and use of obsolete products, e.g., 0.4 Mt in 2006, which could be further explored from two aspects: (i) the stock growth of obsolete products, e.g., uncollected used products in the back yard, which is not yet understood sufficiently; and (ii) the net export of obsolete products, especially second-hand vehicles, which is not tracked by trade statistics. Hence, the fraction of the two unknown flows could not be determined directly, but it was estimated that the U.S. exported 1.21 million old passenger cars in 2005,31 which is approximately equal to 0.15 Mt aluminum, or 35% of the gap. 9518



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Figure 4. Simulated U.S. historic aluminum flows in obsolete products and comparison with USGS reported recovery:30 different categories (left) and the total (right), 1900 2006.

Figure 5. The U.S. anthropogenic aluminum cycle in 2006. Detail may not add to totals due to rounding.

3.2. Energy Use and GHG Emissions of the Contemporary U.S. Aluminum Cycle. The stocks and flows presented above

were assembled to generate the historic U.S. aluminum cycle at a detailed level. A snapshot for 2006 is visualized in Figure 5 and its consequent energy use and GHG emissions were calculated. The results indicate that 464 PJ or 129 TWh of energy was expended in total in 2006, which is 31% lower than the value calculated by the U.S. Department of Energy for 2003.24 This is reasonable considering primary production fell by 16% in the U.S. between 2003 and 2006, and most of the process energy data used in that study were from 1995 while data in our model are mostly from 2005 or more recent. Thus the difference may reflect process improvements in that ten-year span. The total GHG emissions amount to about 38 Mt CO2-equivalence, which is equal to 0.53% of total U.S. GHG emissions in 2006.32 This proportion is lower than the world average, as global aluminum production was estimated to cause 1% of global GHG emissions.1 However, it should be noted that emissions embodied in trade are not considered in our territorial (or production-based) model. Because the U.S. aluminum cycle is highly dependent on imports, the results of a consumption-based approach are expected to be higher. This assertion is supported by the study of primary aluminum ingot in North America.6 When broken down by process and source (Figure 6), the dominating smelting process (71%) and refining and anode

Figure 6. GHG emissions of the U.S. aluminum cycle in 2006, by process and emission source.

production together comprise 83% of total emissions, which is much higher than the semimanufacturing processes (11%) and scrap remelting and refining (5%). Secondary emissions are the most significant source at 67%, followed by natural gas and process emissions, indicating that a large share of emissions is induced by electricity use. With the other fossil fuels contributing 9519



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Figure 7. Theoretical reduction potentials and realistic measures on global warming potential (GWP-100) of the U.S. aluminum cycle under the current situation and a hypothetical in-use stock saturation situation.

relatively much less, it appears that fuel switching has limited potential left as a mitigation measure. 3.3. Options for GHG Emissions Mitigation. The emission reduction potentials of several identified mitigation options are presented in Figure 7. They are listed with theoretical potentials serving as umbrella categories for more realistic short-term and longterm measures. Emission reductions are displayed across four aggregated process categories (mining and refining, smelting, semimanufacturing, and waste management and recycling) of the aluminum life cycle to visualize where emission savings are occurring. The theoretical reduction potential for all process technology improvements adds up to a 61% reduction relative to the baseline situation in 2006. The majority of reduction potential lies within smelting due to its high electricity intensity and 58% coal power share in the contract mix. While upgrading U.S. refiners to the world best available technology (BAT) is not expected to make

a significant difference, BAT in smelting has the largest potential of all the strategies deemed available in the short-term (13% reductions), though this may be difficult and costly considering the long lifetime and capital investment of smelting potlines. The inert anode cell paired with the wetted cathode is expected to bring emission reductions reaching to 23% of the total, though technical and commercialization feasibility has not yet been proven.1 Continuous strip casting allows molten metal to be directly cast into slab or strip and has demonstrated 25% energy savings for sheet and foil production,24 though when applied to the system the emission savings amount to only a 1.5% reduction. Finally, oxy-fuel combustion, in which natural gas is burned with pure oxygen to increase energy efficiency, has been demonstrated in an aluminum remelting furnace with overall energy savings of 50 60%.2 When this technology is applied to all scrap remelting and refining in the system, 2% emission reductions are achieved. 9520


Environmental Science & Technology Whereas process technology improvements deal typically with improving process energy efficiency and end-of-pipe treatment, system-wide improvements capture synergies between processes and sectors that facilitate more radical material efficiencies, smarter energy sourcing, and better integration of technological, social, and policy factors. Low emission energy, which implies that all electricity and fuels for the aluminum cycle can be produced with near-zero emissions, shows the highest theoretical potential (83% reduction) of those analyzed. Specific options include fuel switching to natural gas in the short term, and decarbonizing the electricity supply and applying carbon capture and storage (CCS) to coal power plants in the long term. The next three potentials address the most significant sources of unrecovered metal in the system: landfilling, scrap export, and obsolete product stock accumulation and export. Each has a large theoretical reduction potential (81%, 47%, and 15%, respectively), since all additionally recovered old scrap was assumed to replace metal that would have come from primary production. Cans and durable goods are the major sources of aluminum entering landfills, with the 2006 can collection rate at a staggeringly low 43%29 compared to the 70% world average33 and the durable goods collection at a negligible level.29 Improving can collection to 90% and durable goods collection to 50% are two significant measures that can reduce system-wide emissions by 19% each. Keeping scrap and obsolete products within the U.S. formal recycling system has moderate reduction potentials, while it is less practical and makes no difference from a global perspective. Next, improving semimanufacturing and manufacturing yield reduces scrap generation at these processes which reduces both energy required for scrap melting and recasting and metal loss to unrecovered dross. Each can achieve around 10% theoretical reductions with yield maximization. Avoiding scrap remelting in a “nondestructive recycling” (e.g., extrusion by solid bonding of new scrap) system34 is a potential practical measure here, though this strategy does not address the actual generation of scrap. If the stock approaches saturation, total emissions will decrease dramatically and mitigation priorities will change significantly as much more old scrap becomes available for recycling. In the case of full saturation (no net addition to stock) on the 2006 level, secondary production has the potential to replace domestic primary production, decrease the current reliance on imported ingot, and reduce total emissions by 81%. This would help eliminate domestic emissions from primary production and shift the focus to semimanufacturing and recycling processes. Therefore the nature of in-use stock development over the coming decades will be critical for industry and government to prioritize relevant innovation investments and policy instruments for deeper emission reductions. Ironically, increasing domestic recycling will actually increase territorial based emissions under this situation (in the range of 1 6%), because the consequent recovery increase further exceeds demand, which will therefore either be exported or replace imports. This demonstrates that a territorial based emissions accounting provides little incentives for global emissions reduction. It is worth mentioning that in many cases emission reduction potentials between different measures cannot be summed due to complex system interactions. The short-term and long-term scenarios illustrate how recent and future mitigation potential can be determined by combining measures. It is evident that short-term measures alone (23% reduction) are not significant enough to meet the IPCC target with the current system, but

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with long-term potentials (87% reduction) the cycle is able to meet the target. Under the stock saturation situation, the longterm scenario achieves reductions of 58%. This is due largely to the application of CCS on coal power for semimanufacturing electricity consumption, a measure that brings relatively modest reductions in the current situation. Challenges are likely to arise, however, to achieve those realistic potentials and approach those theoretical potentials in practice. Besides the massive investment requirement and uncertain feasibility of new process technologies, the lock-in effect would further hamper technology penetration and the consequent emissions reduction. Practical accessibility of old scrap is highly dependent on the socio-economic context, consumer behaviors, and recycling infrastructure. And more importantly, the thermodynamic barriers and accumulation of tramp elements over time through repeated recycling will introduce huge challenges to materials engineering and product design.35 The remanufacturing of products36 and intelligent use of aluminum such as lightweighting for automobiles are acknowledged to reduce GHG emissions as well. However, the sectoral approach applied here is unable to include the energy use and emissions in Manufacturing and Use due to complexities with allocation arising from the intricate web of materials, processes, and sectors these phases contain. Nevertheless, the model we developed is an example of how the intimate connection between material metabolisms and associated energy and emission flows can be explored using a dynamic MFA framework. The analyzed patterns of U.S. aluminum cycle and in-use stock growth can shed light on other countries and materials. The immense importance of in-use stock dynamics for recycling and emission reduction illustrated here provides a new perspective to inform long-term industry and government policy.

’ ASSOCIATED CONTENT

bS

Supporting Information. Model details, parameter estimations and data sources, and details of different mitigation strategies. This information is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; tel: + 47 73594754; fax: +47 73591298.

’ ACKNOWLEDGMENT We thank Georg Rombach from Norsk Hydro for fruitful discussions. Special thanks also go to Henry F. Sattlethight and Nicholas A. Adams from the Aluminum Association and E. Lee Bray from United States Geological Survey for providing part of the data and helpful clarification. ’ REFERENCES (1) IEA. Energy Technology Transitions for Industry: Strategies for the Next Industrial Revolution; The International Energy Agency (IEA): Paris, France, 2009. (2) IPCC. Climate Change 2007: The Fourth Assessment Report of the Intergovernmental Panel on Climate Change; Cambridge University Press: Cambridge, U.K. and New York, 2007. 9521


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