Impact of Geographic Scope and Allocation Methods on Primary Aluminum Production Carbon Footprints in the United States
Quantifying greenhouse gas (GHG) emissions from the electricity sector is highly dependant upon the fuel mix of the electricity generating assets. That fuel mix can also vary geospatially and temporally, complicating the use of uniform emissions factors. We explored the effects of different geospatial and distributional electricity allocation protocols by modeling primary aluminum production GHG emissions in the U.S. Primary aluminum production has large energy requirements, so emissions accounting for aluminum-intensive products are heavily influenced by electricity allocation. We calculated cradle-togate GHG emissions factors for the nine U.S. primary aluminum smelters operating in 2010 using smelter-specific and industry-average data. This modeling effort utilized location-specific electricity consumption emissions factors at multiple levels of geographic and distributional aggregation to capture variations in emissions associated with different electricity allocation protocols. The electricity consumed during electrolysis represents the largest GHG burden in the aluminum production process; accounting for ~75%. The results were dramatically impacted by localized electricity emission factors and displayed a range of 4.28 to 29.99 kg-CO2-eq-per-kg-aluminum-ingot. This seven-fold difference between smelters occurred when electricity consumption was allocated to the most localized generating resources. When electricity allocation protocols associated electricity consumption with a larger number of geographically proximate generating resources, emissions factors normalized between smelters. The production-weighted average of all GHGs emitted during primary aluminum production in the U.S. in 2010 ranged from 15.67 to 19.80 kg-CO2-eq-per-kg-aluminum-ingot, depending on the electricity allocation protocol. Previous studies characterized the emissions as ranging from 10.05 to 11.07 kg-CO2-eq-per-kg-aluminum-ingot. Our results depart significantly from previous studies and suggest both a larger absolute value for GHG emissions, as well as an increased range of variation within the U.S. These results could also change the outcome of existing life cycle assessments as emissions associated with aluminum material production increase to match those indicated by our work.