Handling of co-products in life cycle analysis in an evolving co-product market: A case study with corn stover removal

Corn stover (cobs and residue) is an important part of the life cycle of corn, either as fuel or as animal feed, but most life cycle analysis (LCA) models treat them separately from starch ethanol. This paper compares four stover and corn grain based ethanol pathways to show how the greenhouse gas (GHG) impacts can differ depending on system boundaries. Corn stover can be used as a cellulosic feedstock for ethanol. It can also be used as a replacement for corn and hay or corn silage in animal feed. Life cycle GHG emissions for corn-based biofuels pathways were estimated in order to explore the life cycle boundary impact of corn replacement feed (CRF) on the carbon intensity of the studied pathways. CRF is a rapidly emerging agricultural practice where corn stover is removed from the fields in addition to the grain and substituted for corn grain and hay in cattle feed. Removing stover for CRF produces additional emissions from collection, transport, nutrient replacement, and digestibility pre-treatment but its use at feedlots results in GHG credits from the avoided corn and hay farming including reduced indirect land use change (LUC) emissions. Depending on the life cycle modeling boundaries, the treatment of these emissions credits and debits result in significantly different GHG estimates. The four pathways were modeled using data from two life cycle modeling frameworks: the GREET_1 2013/CCLUB and CA_GREET/GTAP BIO. GREET_1 is updated by Argonne National Laboratory and CA_GREET is used for the California Low Carbon Fuel Standard (LCFS) pathways. Co-examining stover as feed (30% stover removal rate) with starch ethanol predicted avoided LUC and feed displacement was comparable to the LUC calculated for corn grain ethanol. This case study based on corn stover removal demonstrated the differences of having a single pathway relative to a more holistic integrated approach resulting in a more accurate prediction of environmental performance. As cellulosic ethanol from stover and CRF from stover become more common, the current LCA procedures for corn ethanol will have to be modified, and we illustrate how this can be done.