Using national inventories for estimating environmental impacts of products from industrial sectors: a case study of ethanol and gasoline

In order to understand the environmental outcomes associated with the life cycle of a product, to compare these outcomes across products, or to design more sustainable supply chains, it is often desirable to estimate results for a reference supply chain representative of the conditions for a sector in a specific region. This paper, by examining ethanol and gasoline production processes, explains how choices made in the calculation of sector-representative emission factors can have a significant effect on the emission estimates used in life cycle assessments.

This study estimates reference emission factors for United States dry-grind corn ethanol production and gasoline production processes suitable for use in baseline life cycle assessment unit processes. Based on facility-specific emissions and activity rates from the United States National Emissions Inventory, the Energy Information Administration, and an ethanol industry trade publication, the average emissions per unit energy content of fuel are computed using three different approaches. The Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI) characterization factors are used to estimate impact potentials for six environmental and three human health categories. Sector-specific direct emissions and impact potentials are compared across the three approaches and between the two sectors. The system boundary for this analysis is limited to the fuel production stage of these transportation fuel lifecycles.

Findings from this work suggest that average emission factors based on total emissions and total production may significantly under estimate actual process emissions due to reporting thresholds and otherwise unreported emissions.

Because of the potential for unreported emissions in regional inventories, it is more appropriate to estimate sector reference emission factors based on matched sets of facility or process level emissions and activity rates than to use aggregated totals. This study demonstrates a method which can be used for inventory development in cases where multiple facilities producing the same product are involved.