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LCA study finds carbon intensity of corn ethanol decreasing, gasoline rising; ethanol estimated 43-60% lower than oil by 2022

Top: Weighted CI (g CO2 e/MJ) of petroleum fuels and corn ethanol consumed in the US over time. Bottom: Weighted CI of petroleum fuels consumed in the US and California over time. Click to enlarge.

The carbon intensity (CI) of corn ethanol—i.e., the greenhouse gas emissions produced via the production of a volume of the fuel—is declining, while the average CI of gasoline produced from petroleum sources is gradually increasing, according to a recent report prepared by Life Cycle Associates, LLC for the Renewable Fuels Association (RFA). Life Cycle Associates has completed numerous life cycle analysis studies, including those to establish fuel pathway carbon intensities (CI) for the California Low Carbon Fuel Standard (LCFS).

According to the study, the average corn ethanol reduced GHG emissions by 32% compared to average petroleum gasoline in 2012—including prospective emissions from indirect land use change (ILUC) for corn ethanol. When compared to fuel produced from unconventional petroleum sources such tight oil from fracking and oil sands, average corn ethanol reduces GHG emissions by 37% compared to the former and 40% to the latter.

Further, the study expects that average corn ethanol will reduce GHG emissions by 43% compared to average petroleum gasoline in 2022. If some amount of corn stover fed to livestock is appropriately treated as a co-product, the weighted average GHG emissions reduction for corn ethanol rises to more than 60%.

“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.”
Carbon Intensity of Marginal Petroleum and Corn Ethanol Fuels

A number of factors factors contribute to the reduction in emissions from corn ethanol, the study found; notably, estimates of land use conversion (LUC) have declined with recently published studies from Purdue University and Argonne National Laboratory.

Other factors include energy efficiency and fuel switching as well as an expansion of co-productsOther feed options such as corn oil also displace products with high LUC emissions. The production of cellulosic ethanol from stover will further reduce the average CI of ethanol from the corn crop.

Carbon Intensity (g CO2 e/MJ) of petroleum fuels versus consumption for the US in 2000, 2012 and 2022.   Carbon intensity (g CO2 e/MJ) of petroleum fuels consumed in California in 2000, 2012 and 2022.

Source: “Carbon Intensity of Marginal Petroleum and Corn Ethanol Fuels”. Click to enlarge.

The US Environmental Protection Agency (EPA) published an updated analysis of GHG emissions for the Renewable Fuel Standard (RFS2) in 2010. 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. LCFS used a 2006 petroleum baseline gasoline blending component. 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.

However, the study notes, 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.

The question arises as to whether policy and industry efforts to reduce the CI of transportation fuels by substitution and blending has had any impact at all? When alternative fuels are viewed as an incremental resource, several marginal petroleum options represent the effect of these new energy resources.

… Extrapolating from current policy and production scenarios to determine drivers for future growth, generates two potentially significant scenarios. These are the approval of the Keystone XL pipeline and the continuance of the U.S. shale boom. Both of these scenarios would increase the shares of unconventional oil in the domestic slates and shift the weighted GHG emissions accordingly. Under these drivers, significant quantities of marginal oil would be fed into U.S. refineries, generating corresponding emissions penalties, that would be further aggravated in the absence of renewable fuel alternatives. Projecting towards 2022, the effect of stripping renewable fuels from the slates immediately earns emissions penalties and leads to an increase in the overall weighted GHG emissions from petroleum fuels.

—“Carbon Intensity of Marginal Petroleum and Corn Ethanol Fuels”

The LCA authors of the study made a number of recommendations based on the findings:

  • Refine the GHG emissions from petroleum pathways. This will include continuing to monitor crude oil production by resource type; comparing local emission inventory reports to LCA model inputs; refining emission estimates from fracking; improving the integration of oil refining with crude oil type; including crude oil upstream results for diesel in crude oil LCA; and examining methane emissions from crude oil production.

  • Consider avoidance of marginal petroleum GHG emissions as an indirect effect of biofuels substitution. Incorporating co-product effects of CRF and soy oil into RFS2 and LCFS ratings for corn ethanol will improve the analysis. Further, there should be ongoing monitoring of corn ethanol production by production technology.

  • Consider corn from starch and cellulose as a single feedstock/fuel pathway when assessing the national impact of renewable fuels.




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