Carbon Intensity of Marginal Petroleum and Corn Ethanol Fuels

Greenhouse gas (GHG) emissions from petroleum and alternative fuels have been the subject of research for several decades. Petroleum gasoline and corn ethanol are the most widely used automotive fuels and arguably the most closely examined. From production and refining to end use in vehicles, each stage of the fuel supply chain contributes to the fuel’s carbon footprint. These well to wheel GHG emissions are expressed in grams of carbon dioxide emitted per megajoule of fuel (g CO2 e/MJ). Substitution and blending of renewable fuels into petroleum gasoline is one key strategy to reduce the carbon footprint of transportation fuels. However, both the methods for examining GHG emissions as well as the technologies for resource and fuel production have evolved over the decades. In order to better understand the evolving trends in GHG emissions from petroleum gasoline and corn ethanol, this study examines the trends in crude oil based gasoline and corn ethanol in the U.S. and California.

In 2010, the U.S. EPA published the updated analysis of GHG emissions for the Renewable Fuel Standard (RFS2). Under the regulation, U.S. transportation fuel suppliers are required to include specified volumes of renewable fuels in transportation fuels through 2022. The RFS2 established mandatory emission reduction thresholds for renewable fuel categories based on reductions from a 2005 baseline. California and other states introduced Low Carbon Fuel Standards (LCFS), which require a declining carbon intensity (CI) of the average on-road transportation fuel. The California Air Resources Board established a 2006 petroleum baseline gasoline blending component in 2009. Since that time, the emissions from crude oil production have been examined further by the Air Resources Board to reflect the changing mix of crude oil resources utilized in the state.

As unconventional sources of crude oil have grown in recent years, the CI of petroleum fuels has increased above the baseline levels initially identified in the above fuel polices. This study examines the resource mix of petroleum options over time encompassing conventional and unconventional crude oil sources. Resource types are ranked by cost and CI in order to show the effect of marginal crude oil resources. As the average CI of petroleum is gradually increasing, the CI of corn ethanol is declining. Corn ethanol producers are motivated by economics to reduce the energy inputs and improve product yields. Incentives for lower CI also motivate the industry to adopt new technologies, including feedstock and technological innovations as they roll out.

Figure S.1 shows the volume weighted carbon intensities (g CO2 e/MJ) of U.S. petroleum gasoline and corn ethanol over time based on the historical crude oil and ethanol plant resource mixes and future projections. The mix of crude oil resources is based on Energy Information Administration estimates combined with crude oil type by country. The mix of corn technologies includes stover based ethanol contributing to RFS2 targets. Additionally, projections of stover used as corn replacement feed (CRF) and corn oil used as feed are included as a co-product credit. The baseline values used in the RFS2 and LCFS are presented alongside for comparison.