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The Renewable Fuel Standard May Limit Overall Greenhouse Gas Savings by Corn Stover-Based Cellulosic Biofuels in the U.S. Midwest: Effects of the Regulatory Approach on Projected Emissions

Kim, Seungdo, Dale, Bruce E., Zhang, Xuesong, Jones, Curtis Dinneen, Reddy, Ashwan Daram, Izaurralde, Roberto Cesar
Environmental science & technology 2019 v.53 no.5 pp. 2288-2294
biofuels, biorefining, carbon, corn, corn stover, emissions, energy, ethanol, greenhouse gases, market prices, supply chain, California, Midwestern United States
The Renewable Fuel Standard (RFS) program specifies a greenhouse gas (GHG) reduction threshold for cellulosic biofuels, while the Low Carbon Fuel Standard (LCFS) program in California does not. Here, we investigate the effects of the GHG threshold under the RFS on projected GHG savings from two corn stover-based biofuel supply chain systems in the United States Midwest. The analysis is based on a techno-economic framework that minimizes ethanol selling price. The GHG threshold lowers the lifecycle GHG of ethanol: 34.39 ± 4.92 gCO₂ MJ–¹ in the RFS-compliant system and 46.30 ± 10.05 gCO₂ MJ–¹ in the non RFS-compliant system. However, hypothetical biorefinery systems complying with the RFS will not process the more GHG-intensive corn stover, and thus much less biofuel will be produced compared to the non RFS-compliant system. Thus, taken as a whole, the non RFS-compliant system would achieve more GHG savings than an RFS-compliant system: 10.7 TgCO₂ year–¹ in the non RFS-compliant system compared with 4.4 TgCO₂ year–¹ in the RFS-compliant system. These results suggest that the current RFS GHG reduction threshold may not be the most efficient way to carry out the purposes of the Energy Security and Independence Act in the corn stover-based biofuel system: relaxing the threshold could actually increase the overall GHG savings from corn stover-based biofuels. Therefore, the LCFS-type regulatory approach is recommended for the corn stover-based cellulosic biofuel system under the RFS program. In addition, our calculation of the GHG balance for stover-based biofuel accounts for SOC losses, while the current RFS estimates do not include effects on SOC.