Rolling Out the Anthropogenic Aluminum Cycle: With Foci on Temporal, Geographical, and Emission Perspectives
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Anthropogenic metal cycles today confront three interconnected large challenges: an increasing global demand due to rising population and escalating industrialization and urbanization, a profound change of the global supply chain induced by globalization process, and growing pressures on climate change mitigation in a carbon constraint world. For example, the global demand for aluminum, the currently second most used metal worldwide, is anticipated to triple by 2050, by which time global greenhouse gas (GHG) emissions are advised to be cut by 50%-85% to avoid catastrophic climate impacts. Process efficiency improvement and recycling can contribute to reducing emissions per material output; however, long-term material demand and scrap availability for recycling depend fundamentally on the dynamics of societies’ stocks of products in use. Meanwhile, on a country level, growing trade of aluminum products leads to cross-boundary leakages of both GHG emissions and secondary materials. This thesis aims to address these interconnected sustainability challenges using a systems approach that characterizes material flows both along their technological life cycle and across national boundaries, that enables an integrated analysis of the materialenergy- emissions nexus, and that considers both system feedbacks and time lags. We are essentially rolling out the anthropogenic aluminum cycles to understand their dynamics from a temporal perspective, to explore the trade links and interdependencies of individual country cycles from a geographical perspective, and to investigate the environmental consequences and mitigation strategies from a GHG emissions perspective. We endeavor to support industry and government in strategic decisionmaking related to aluminum production, trade, and climate change mitigation. Simulation results show that the contemporary global aluminum stock in use (0.6 Gt or 90 kg/capita) has reached about 10% of that in known bauxite reserves and represents an embodied energy amount that is equivalent to three quarters of the present global annual electricity consumption. The largest proportions of in-use stock are located in the U.S. (28%), China (15%), Japan (7%), and Germany (6%) and in sectors of building and construction (40%) and transportation (27%). Industrialized countries have shown similar patterns of aluminum in-use stock growth: once the per-capita stocks have grown to a threshold level of 50 kg, they kept a near linear annual growth of 5-10 kg/capita; no clear signs of saturation can be observed yet. The present aluminum stocks in use vary widely across countries: approximately 100-600 kg/capita in industrialized countries and below 100 kg/capita in developing countries. The growing global aluminum in-use stock has significant implications on future aluminum demand and provides important recycling opportunities that will be critical for GHG emissions mitigation in the aluminum industry in the coming decades. The trade-linked multilevel contemporary global aluminum cycle reveals several important features of the aluminum production, trade, and use patterns. (i) While the Southern hemisphere is the main primary resource supplier, aluminum production and consumption concentrate in the Northern hemisphere, where we thus also find the largest potential for recycling; (ii) The varying magnitudes of aluminum stocks and flows at the country level strongly reflect the availability of resources, the state of development, the structure of industry and economy, and the wealth and lifestyle of the country; (iii) The global aluminum cycle depends substantially on international trade of aluminum in all forms, but trade patterns vary significantly by commodity and country; (iv) The major industrialized countries tend to have a substantial and increasing presence throughout the stages from aluminum production on, while developing countries rarely create value added in later stages after bauxite refining; (v) A small group of countries play a key role in the global redistribution of aluminum and in the connectivity of the network. These results provide potential insights to inform government and industry policies in resource and supply chain security, value chain management, and cross-boundary environmental impacts mitigation. Strategies to decarbonize the anthropogenic aluminum cycle can be divided into process technology improvements (e.g., best available technology, new technologies such as inert anode and wetted cathode) and system-wide improvements (e.g., electricity decarbonization, improvement of post-consumer scrap recycling and processing and manufacturing yield). Taking the U.S. aluminum cycle for example, the reduction potentials that may be theoretically achieved for all process technology improvements add up to a 61% reduction relative to the baseline in 2006, the majority of which lies within the smelting process. 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 reduction potential (83%) of all system-wide strategies. If we can recover the aluminum lost in landfills, scrap export, and obsolete product stock accumulation and export, significant reductions (in territorial emissions) can be achieved (81%, 47%, and 15%, respectively). Maximizing yield of semi-manufacturing and manufacturing processes (i.e., yield loss completely eliminated) can each reduce emissions by about 10%. However, combination of these strategies does not lead to additive reductions due to system interactions; challenges are likely to arise while approaching these theoretical potentials in practice. The global model results show that future aluminum in-use stock patterns set essential boundary conditions for future emission pathways and mitigation priorities. Higher stock saturation levels result in higher aluminum demand and higher emissions, and earlier saturation times lead to earlier emission peaks and higher cumulative emissions until 2050. If developing countries follow industrialized countries in their aluminum stock patterns, a 50% emission reduction by 2050 below 2000 levels cannot be reached even under very optimistic recycling and technology assumptions. The target can only be reached if future global per-capita aluminum stocks saturate at a level much lower than that in current major industrialized countries (200 kg/capita). As long as global inuse stocks are growing rapidly, radical new technologies in primary production (e.g., inert anode and carbon capture and storage) have the greatest impact in emission reduction; however, their ‘window of opportunity’ is closing once the stocks begin to saturate and the largest reduction potential shifts to post-consumer scrap recycling.