Corn stover cannot simultaneously meet both the volume and GHG reduction requirements of the renewable fuel standard

Abstract

Corn stover is expected to supply much of the cellulosic biomass required to meet the 61 billion liters per year target under the US Energy Independence and Security Act. The Act also requires that cellulosic biofuels achieve a greenhouse gas (GHG) reduction of 60% compared to gasoline. If corn stover is harvested for biofuels, it can no longer help replenish soil organic matter, and net soil carbon emissions increase. So meeting the GHG reduction target is a concern. We studied the effect of stover removal on overall GHG emissions of corn stover ethanol systems in the 12-state Corn Belt region. Even at a stover removal rate of 66%, no more than 20 billion liters can be annually produced while simultaneously satisfying the 60% GHG reduction. Moreover, no GHG reduction relative to gasoline occurs in short time periods. The GHG benefits of corn stover ethanol only appear after longer time periods.

EISA (Energy Independence and Security Act) compliant ethanol fuel from corn stover in a depot‐based decentralized system

“…About 66% of corn stover in a corn‐soybean rotation under historical tillage management practices in the midwestern USA is assumed to be potentially available as cellulosic feedstock in the decentralized system . The county‐level corn stover availability is aggregated from the pixel‐level results at the 50‐year time horizon in the Kim et al .…”

“…The county‐level corn stover availability is aggregated from the pixel‐level results at the 50‐year time horizon in the Kim et al . study . In that study, SOC changes of the entire soil profile and soil nitrogen losses (or credits) are modeled at a 56 m spatial resolution via the EPIC model.…”

“…No replacement nitrogen nutrients are required in the soybean production years. The county‐level carbon emissions from SOC changes and soil N 2 O emissions are aggregated from the pixel‐level results in the 50‐year time horizon from the EPIC simulations . The process data for the depot and the decentralized biorefinery estimated from Kim and Dale's study are used to calculate GHG emissions associated with the depot and the decentralized biorefinery (Table S1 in the Supporting Information).…”

“…EISA also requires that cellulosic biofuels achieve a 60% reduction in lifecycle greenhouse gas (GHG) emissions compared with 2005 conventional gasoline. If corn stover ethanol is also required to meet a 60% GHG emission reduction requirement, then the potential corn stover ethanol volume is reduced by about 36% …”

“…Although carbon emissions from soil organic carbon (SOC) losses were not taken into account in their analysis, some decentralized biorefineries nonetheless did not meet the 60% GHG reduction requirement. Carbon dioxide released from SOC losses in corn stover removal systems is the primary GHG source in corn stover ethanol fuel, accounting for around 80% of GHGs of corn stover ethanol in the centralized system …”