Modeling Temporal Aluminum Material Flows and Greenhouse Gas Emissions to Evaluate Metals Recycling Allocation in Life Cycle Assessment
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Although dynamic, time-dependent aspects mark its life cycle, aluminum has largely been treated as a static system in industrial ecology. Life cycle assessment (LCA) and material flows analysis (MFA) continue to expand beyond their initial purpose of providing single point-in-time results, but remain limited in their ability to capture the temporal nature of aluminum. As a result, this dissertation has developed more comprehensive and robust approaches for evaluating greenhouse gas (GHG) emissions and material flows of aluminum production, consumption, and recycling over time.
Dynamic MFA and LCA approaches are developed to analyze the effects of economic and technological trends on U.S. aluminum in-use stocks and global absolute and relative GHG emissions from primary aluminum production. A dynamic MFA model is developed to estimate in-use stocks and recovery from 1900 to 2007. Results show that 34% of apparent consumption since 1900 remains as in-use stocks in 2007. Time series analysis is used to quantify the relationship between gross domestic product and net additions to in-use stocks. A dynamic LCA is developed to quantify the spatial and temporal variation in the life cycle GHG emissions of global primary production, consumption, and trade from 1990 to 2005. Seven world regions are shown to have distinct GHG intensities; the largest difference in 2005 is between Asia (21.9 kg CO2-eq/kg) and Latin America (7.07 kg CO2-eq/kg).
The analysis of economic and technological trends is also used to provide a critical evaluation and counterargument for the metal industry’s position that metals are: widely recycled, recycled many times over, and constrained in secondary production by scrap availability. The position that primary metal production is displaced by secondary production is put into question by analyzing the U.S. aluminum market.
Lastly, four LCA recycling allocation approaches are evaluated for their capacity to accurately reflect the temporal nature of aluminum. The recycled content approach is recommended based on its ability to accurately account for the timing of material flows and GHG emissions, and to be used in a consequential LCA framework. Where appropriate, this approach should be extended with systems expansion methods that are based on sound economic theory.