The Renewable Fuel Standard May Limit Overall Greenhouse Gas

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Policy Analysis

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 Seungdo Kim, Bruce E. Dale, Xuesong Zhang, Curtis Dinneen Jones, Ashwan Daram Reddy, and Roberto Cesar Izaurralde Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02808 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Environmental Science & Technology

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The Renewable Fuel Standard May Limit Overall Greenhouse Gas Savings by Corn

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Stover-based Cellulosic Biofuels in the U.S. Midwest: Effects of the Regulatory Approach

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on Projected Emissions

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Seungdo Kim1,2*, Bruce E. Dale1,2, Xuesong Zhang3, Curtis Dinneen Jones4, Ashwan Daram

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Reddy4, Roberto Cesar Izaurralde4,5

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1

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Toxicology Building, East Lansing, MI 48824

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2 Chemical

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Boulevard, Lansing, MI 48910

Great Lakes Bioenergy Research Center, Michigan State University, 164 Food Safety and

Engineering and Materials Science, Michigan State University, 3815 Technology

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3

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University Research Court, Suite 3500, College Park, MD 20740

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4

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7251 Preinkert Drive, College Park, MD 20742

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*Correspondence to: [email protected]

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Keyword: Biofuel, Corn stover, Ethanol, GHG reduction threshold, Lifecycle greenhouse gas,

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Renewable Fuel Standard program, Supply chain, U.S. Energy Security and Independence Act

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(EISA) of 2007

Joint Global Change Research Institute, Pacific Northwest National Laboratory, 5825

Department of Geographical Sciences, University of Maryland, 2181 Samuel J. LeFrak Hall,

Texas AgriLife Research and Extension, Texas A&M University, Temple, TX 76502.

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Abstract:

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The Renewable Fuel Standard (RFS) program specifies a greenhouse gas (GHG) reduction

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threshold for cellulosic biofuels, while the Low Carbon Fuel Standard (LCFS) program in

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California does not. Here, we investigate the effects of the GHG threshold under the RFS on

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projected GHG savings from two corn stover-based biofuel supply chain systems in the United

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States Midwest. The analysis is based on a techno-economic framework that minimizes ethanol

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selling price. The GHG threshold lowers the lifecycle GHG of ethanol: 34.39±4.92 gCO2 MJ-1 in

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the RFS-compliant system and 46.30±10.05 gCO2 MJ-1 in the non RFS-compliant system.

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However, hypothetical biorefinery systems complying with the RFS will not process the more

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GHG-intensive corn stover, and thus much less biofuel will be produced compared to the non

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RFS-compliant system. Thus, taken as a whole, the non RFS-compliant system would achieve

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more GHG savings than an RFS-compliant system: 10.7 TgCO2 year-1 in the non RFS-compliant

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system compared with 4.4 TgCO2 year-1 in the RFS-compliant system. These results suggest that

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the current RFS GHG reduction threshold may not be the most efficient way to carry out the

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purposes of the Energy Security and Independence Act in the corn stover-based biofuel system:

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relaxing the threshold could actually increase the overall GHG savings from corn stover-based

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biofuels. Therefore, the LCFS-type regulatory approach is recommended for the corn stover-

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based cellulosic biofuel system under the RFS program. In addition, our calculation of the GHG

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balance for stover-based biofuel accounts for SOC losses, while the current RFS estimates do not

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include effects on SOC.

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Table of Contents (TOC)/Abstract Art

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1. Introduction

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The Energy Security and Independence Act (EISA) of 2007 is over ten years old. Among other

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things, the purpose of EISA is “To move the United States toward greater energy independence

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and security, to increase the production of clean renewable fuels…”1. These are critically

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important goals. To achieve its objectives, the Renewable Fuel Standard (RFS) program2

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stipulates that specific volumes of several different types of renewable fuels are to be produced

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by the year 2022 and also specifies the greenhouse gas (GHG) reduction thresholds that are to be

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achieved by each fuel type. In particular, the RFS program requires that cellulosic biofuels

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reduce GHG emissions by 60% per unit of energy produced compared with the baseline gasoline

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and that 61billion liters (16 billion gallons) of cellulosic biofuel be produced by 2022. A similar

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program was enacted in the California-Low Carbon Fuel Standard (LCFS) program3. In the

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LCFS program, alternative fuels with carbon intensity (lifecycle GHG emissions) below the

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carbon intensity standards for gasoline or diesel are eligible for the LCFS program. Unlike the

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RFS program, the LCFS program does not specify GHG reduction thresholds for each alternative

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fuel type.

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The RFS and LCFS thus take different approaches to mitigate climate change. The RFS GHG

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reduction threshold lowers lifecycle GHG emissions on a per unit energy basis. However, this

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approach may also limit the volume of RFS-compliant biofuels. Since the overall GHG savings

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are the product of GHG emissions per unit volume of biofuel and the total volume of biofuels

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produced, the RFS GHG reduction threshold may actually not increase overall GHG savings

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from RFS-compliant biofuels. Here, we investigate the effects of the GHG reduction threshold

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on the overall GHG savings from corn stover-based cellulosic biofuel system in the U.S.

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Midwest. Our hypothesis is that relaxing the RFS threshold of 60% GHG reduction compared to 4 ACS Paragon Plus Environment

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gasoline would increase the overall GHG savings from corn stover-based biofuels because

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greater volumes of biofuels could become eligible for cellulosic biofuels under the RFS.

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We examine two corn stover-based cellulosic biofuel systems in the U.S. Midwest to understand

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how the GHG reduction threshold could influence cellulosic biofuel production volume as well

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as the overall GHG savings. The first system does not require compliance with the RFS standard,

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and is referred to as the “non RFS-compliant biofuel system”. In this scenario, each liter of the

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biofuel produced at a biorefinery is not required to meet the 60% GHG reduction standard. This

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approach is similar to the LCFS program. The second scenario, referred to as the “RFS-

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compliant biofuel system”, mandates that each liter of fuel meets the 60% reduction threshold.

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Economic factors, in particular the farm-gate price of cellulosic feedstock and its logistics costs,

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prevent some of the corn stover from being converted to ethanol fuel.4 That is, biorefineries

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avoid processing high-priced or remote corn stover in order to minimize their ethanol selling

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prices. The ethanol fuel volumes actually produced in each system are estimated based on corn

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stover supply chains which are established by minimizing ethanol selling price.

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2. Methods

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Corn stover collection. About 66% of the total available corn stover (a practical maximum) in

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corn-soybean rotations under no-tillage management practices in the Midwest is presumed to be

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collected as feedstock for ethanol fuel production in the two cellulosic biofuel systems. Farmers

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practicing no-till cultivation practices could likely collect corn stover without changing tillage

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practices. The corn-soybean rotation practice is the major cropping system in the Midwest. The

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fractions of cornfields converted from soybean fields (grown the previous year) in the Midwest

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are 62% in 2015, 59% in 2016 and 66% in 2017 (https://nassgeodata.gmu.edu/CropScape/).



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Recent studies5-7 show that GHG emissions associated with soil organic carbon (SOC) changes

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by corn stover removal could be a major GHG source of the lifecycle GHG emissions of ethanol

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fuel derived from corn stover under current agricultural practices. EISA1 states

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“The term ‘lifecycle greenhouse gas emissions’ means the aggregate quantity of

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greenhouse gas emissions (including direct emissions and significant indirect emissions

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such as significant emissions from land use changes)…”

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Therefore, GHG emissions associated with SOC changes should be included in the lifecycle

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calculations under the RFS program.2, 8 The current RFS life cycle analysis (LCA) assumes that

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certain levels of corn stover removal minimize impacts on SOC in order to estimate the

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availability of corn stover for biofuel production under the RFS. GHG emissions associated with

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SOC changes by corn stover removal at such removal rates are therefore assumed to be zero.2 In

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contrast, we believe changes in SOC due to stover removal should be calculated, and not simply

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assumed to be zero.

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Here, we use the Environmental Policy Integrated Climate (EPIC) model5, 6, 9, which has been

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extensively tested across 10 sites in the U.S. Midwest6, in order to estimate biomass production,

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SOC changes and soil nitrogen losses at 56-m spatial resolution over a 30-year time horizon.

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Since there is no pixel-level information on tillage management, the county-level fractions of no-

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tillage management10 are used to estimate the total hectares of no-till croplands as the product of

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the county level hectares of croplands for corn-soybean rotation and the fraction grown under no-

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till in each county (Figure S1).

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The EPIC simulation results combined with the no-till fraction show that about 40.9 Tg (1012 g)

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of dry corn stover per year grown under no-tillage management practices from 993 counties in

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the U.S Midwest is potentially available to produce cellulosic ethanol. Conversion of all this corn 6 ACS Paragon Plus Environment

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stover to ethanol would yield 12 billion liters of ethanol, equal to 20% of the total cellulosic

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biofuel volume mandated by the RFS2. Corn stover collection per hectare ranges from 1.5 to 6.9

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dry Mg ha-1 (Figure S2). Iowa produces the most corn stover, followed by Nebraska (Table S1).

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Illinois and South Dakota are the other major corn stover-producing states, and some counties

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can provide corn stover sufficient to operate a minimum commercial scale biorefinery11, 700 dry

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Mg day-1 (Figure S3).

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The farm-gate price of corn stover includes costs for fuel, labor and plant nutrients to replace

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those removed with the stover, and also the required profit for the farmer. The costs for fuel and

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labor are estimated based on a two-pass harvesting system12. The nutrient values of corn stover

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and the fertilizer price are obtained from literature13, 14. It is assumed that about 25% of the total

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corn stover collection costs are added to the farm-gate price as profit to the farmer. The farm-

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gate price of corn stover ranges from US$ 45 to US$ 68 dry Mg-1 (Figure S4), and the quantity-

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weighted average farm-gate price is US$ 47.58 ± 1.95 dry Mg-1. These values align well with the

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forecasts given by the 2016 Billion-Ton Report15 of between 27 Tg at US$ 44 dry Mg-1 and 82

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Tg at US$ 55 dry Mg-1.

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GHG emissions of corn stover production consist of GHG emissions associated with diesel fuel

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consumption in collecting and baling corn stover, GHG emissions due to the replacement

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nutrients required (phosphorus and potassium), carbon emissions from SOC changes, and soil

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N2O emissions. No replacement nitrogen nutrients are included because of corn-soybean rotation

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practices.5 Energy consumption in collecting and baling corn stover is estimated from the

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literature12. GHG emissions associated with SOC changes and soil N2O emissions are estimated

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by the marginal analysis approach5. In this approach, the differences in GHG emissions between

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corn-soybean rotations with corn stover removal and corn-soybean rotations with no corn stover 7 ACS Paragon Plus Environment


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removal are assigned to the corn stover. Since this approach is not currently practiced in the RFS

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program, sensitivity analyses are done to determine the differences between our method that

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accounts for GHG emissions from SOC changes and the EPA’s current practice that does not

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explicitly consider SOC losses.

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GHG emissions of corn stover range from 44 to 705 kg CO2 dry Mg-1 (Figure S5). Carbon

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releases from SOC changes are the major GHG source in the corn stover collection system,

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accounting for over 70% of the total GHG emissions from corn stover removal (Figure S7).

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Illinois, Indiana and Iowa can produce more low cost corn stover than other states. This stover is

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also less GHG-intensive compared to other states (Table S1).

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Biorefinery. The mass and energy balances within the biorefinery are estimated based on an

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Aspen Plus model developed by the National Renewable Energy Laboratory16 (NREL), in which

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dilute acid technology is used as the pretreatment process, and enzyme is produced on-site.

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However, neutralization of ammonia in the wastewater treatment facility is assumed to be done

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via nitrification and denitrification processes, instead of via a reaction with sodium hydroxide.

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Ethanol yield is 327 liter dry Mg-1, and surplus electricity is about 0.48 kWh liter-1 (Table S2).

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The minimum ethanol selling price (MESP) is calculated based on the NREL Aspen Plus

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model16, which estimates the MESP value by the discounted cash flow rate of return approach

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(Figure S8), with a cost year of 2013. The ethanol selling price includes feedstock cost

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(including corn stover and logistics costs), capital investment, labor and operating costs,

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chemical costs, and return on investment (ROI).

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Lifecycle GHG emissions. The lifecycle GHG emissions of corn stover ethanol fuel are

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normalized to one MJ of ethanol fuel. The system boundaries in the lifecycle GHG calculations

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include corn stover collection, transportation of baled corn stover, storage, the biorefinery, 8 ACS Paragon Plus Environment

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avoided grid electricity by surplus electricity generated at the biorefinery, transportation and

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distribution of ethanol fuel, combustion of ethanol fuel, and upstream processes (fossil fuels

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consumed, chemicals, etc.).

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It is assumed that the surplus electricity exported from the biorefinery will displace the grid

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electricity in the state in which the biorefinery is located. The state-level fuel mixes are based on

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data from the year 2012.17 The GHG emissions associated with the upstream processes (e.g.,

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diesel, electricity and materials, etc.) are obtained from the U.S. life cycle inventory database

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(http://www.nrel.gov/lci/) after updating the electricity fuel mixes and the GREET model13. Due

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to a lack of specific information, GHG emissions of transportation and distribution of ethanol are

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obtained from the GREET model13. GHG emissions of combusting ethanol fuel as given by the

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RFS2 are used in the analysis. Biogenic carbon dioxide emissions released from combusting

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ethanol fuel are not included as GHG emissions.

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Supply chain system. Baled corn stover in a given county is transported by truck from farms to

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a county-level storage facility. The collection radius assumed for the county-level storage facility

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is equal to the county radius. The county-level storage facility is assumed to be located at the

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centroid of the county and supplies baled corn stover to only one biorefinery. Trucks also haul

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baled corn stover from the county-level storage facilities to a biorefinery. Due to a lack of

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information about possible biorefinery locations, biorefineries are assumed to be located at the

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centroid of one county, and corn stover in counties where biorefineries are located is stored at the

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storage facility of the biorefinery. Hence corn stover is directly transported from farms to the

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biorefinery in such counties. Dry mass losses of baled corn stover are assumed to be 2% during

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transportation and 8.4% during storage.13



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Similar supply chain modeling approaches based on our previous studies4, 5 are used to analyze

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the corn stover biofuel system. Corn stover supply chains are modeled so as to minimize ethanol

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selling price subject to three separate constraints: biorefinery size, collection radius and lifecycle

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GHG emissions. The biorefinery size must be between 700 and 2000 dry Mg day-1. The

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collection radius is assumed to be less than the maximum distance for a daily trip by truck. In the

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RFS-compliant biofuel system, the lifecycle GHG emissions of the ethanol fuel must meet the

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GHG reduction threshold, while the lifecycle GHG emissions in the non-compliant biofuel

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system must be less than those of gasoline. The corn stover supply chain systems in the RFS-

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compliant biofuel system are estimated from the following optimization routine:

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Objective function: Constraints :

Minimize MESPk GHGk ≤ GHGgasoline ∙ 0.4 COLk,j ∙ TR ≤ Rmax for every j Smin ≤ Sizek ≤ Smax

(1)

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where MESPk is the minimum selling price for ethanol from the kth biorefinery (US$ lge-1; lge:

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liter of gasoline equivalent). GHGk is the lifecycle GHG emissions of ethanol fuel in the kth

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biorefinery (g CO2 MJ-1), and GHGgasoline is the lifecycle GHG emissions of the baseline

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gasoline, 93.08 g CO2 MJ-1 2. The first constraint is the GHG reduction threshold imposed by the

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RFS. COLk,j is the straight line distance between the kth biorefinery and the jth county-level

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storage facility. TR is the tortuosity factor for road travel, 1.354. Rmax is the upper limit of the

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collection radius, which is equal to the maximum length of a one-day trip by truck (~354 km).

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Sizek is the kth biorefinery size (dry Mg day-1). Smin is the minimum commercial scale11, 700 dry

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Mg day-1, and Smax is the upper limit of biorefinery size (~2000 dry Mg day-1) due to baled

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feedstock handling constraints in the biorefinery.


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In contrast, the corn stover supply chain systems in the non RFS-compliant biofuel system are

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not required to comply with the RFS GHG reduction threshold. The corn stover supply chain

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systems in the non RFS-compliant biofuel system are estimated from the following optimization:

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Objective function: Constraints :

Minimize MESPk GHGk ≤ GHGgasoline COLk,j ∙ TR ≤ Rmax for every j Smin ≤ Sizek ≤ Smax

(2)

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Detailed descriptions for size, MESP and lifecycle GHG emissions are given in the Supporting

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Information. In the corn stover supply chain system, the first biorefinery location is assumed to

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be in the county with the most available corn stover if the ethanol fuel produced in this

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biorefinery meets all the required constraints. The second biorefinery is located in the county

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with the most available corn stover among counties not participating in the corn stover supply

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chain for the first biorefinery. The remaining biorefinery locations are selected following the

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same procedure. A mixed integer nonlinear programming algorithm is used to solve the

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minimization problems.

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Sensitivity analysis. The following parameters are investigated in the sensitivity analysis: upper

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limit of biorefinery size, corn stover removal rate, upper limit of collection radius, and the effect

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of the time horizon considered. Effects of SOC changes as simulated by a different

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agroecosystem model (CENTURY) are also investigated. In the sensitivity analysis of the

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agroecosystem model, carbon emissions from the SOC changes in corn soybean rotations under

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no-tillage management practices are obtained from Argonne National Laboratory’s Carbon

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Calculator for Land Use Change from Biofuels Production model13. In this model, the county-

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level SOC changes due to collecting 60% of the available corn stover from corn-soybean

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rotations under no-tillage management practices are estimated using the CENTURY model.



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The major differences between the approaches used here and the current practices in the RFS

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lifecycle GHG calculations lie in GHG emissions associated with SOC changes, grid electricity

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displaced by surplus electricity from the biorefinery, and replacement nutrient required due to

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corn stover removal. Under the RFS practices, corn stover removal levels are assumed to

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maintain soil carbon, and U.S. grid electricity is displaced by the surplus electricity. Nitrogen-

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containing nutrients are included as replacement nutrients due to corn stover removal regardless

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of the crop rotation practices. In the sensitivity analysis, current RFS practices are applied in the

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lifecycle GHG calculations to determine the effects of the GHG reduction threshold on the

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ethanol volume produced and overall GHG savings. To check the assumption that corn stover

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removal does not affect SOC, as assumed by the RFS, a corn stover removal rate of 33% (below

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the removal rate assumed in the RFS program) is used to estimate the amount of corn stover

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available as feedstock for ethanol fuel in this sensitivity analysis.

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3. Results and Discussion

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RFS-compliant biofuel system. The RFS-compliant biofuel system produces about 3.6 billion

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liters per year of corn stover ethanol fuel from 17 biorefineries located in Indiana, Iowa,

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Michigan, Minnesota, Nebraska and Ohio (Figure 1A, Table S4). Annual ethanol production in

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these individual biorefineries ranges from 0.09 to 0.23 billion liters with an average annual

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ethanol volume of 0.21 ± 0.04 billion liters. A volume-weighted average collection radius in the

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RFS-compliant biofuel system is 82 ± 46 km. Approximately 187 counties among the 993 total

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counties in 12 Midwestern states participate in supplying corn stover to these 17 biorefineries.

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These biorefineries process only 31% of the total corn stover potentially collectible in these

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states. The lifecycle GHG emissions of corn stover ethanol in the RFS-compliant biofuel system

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range from 21.76 to 37.23 g CO2 eq. MJ-1 (Table S4). Iowa produces more RFS-compliant 12 ACS Paragon Plus Environment

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ethanol than any other state at 2.3 billion liters per year and likewise provides about 66% of the

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corn stover processed in the RFS-compliant system. Approximately 97% of the corn stover

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collected in Iowa can produce RFS-compliant ethanol fuel.

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Figure 1.

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The remaining 69% of corn stover that is potentially collectible in the U.S. Midwest does not

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participate in the RFS-compliant biofuel system due to high GHG emissions of the stover, low

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stover availability or high farm-gate price. The average farm-gate price and GHG emissions of

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corn stover are higher for areas that do not participate than those in the participating counties.

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Non-participating areas include Kansas, North Dakota, western Nebraska and South Dakota,

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northern Minnesota and Wisconsin, and southern Missouri (Table S1, Figure 1A, Figure S4,

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Figure S5). The logistics costs of corn stover in the low stover availability areas are much higher

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than the cost reductions achieved by economies of scale for the biorefinery. Hence stover

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produced in the low stover availability areas such as western North Dakota (Figure S3) does not

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participate in the supply chains for any RFS-compliant biorefineries. The low per hectare rate of

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corn stover collection leads to higher farm-gate price and reduced corn stover availability (Figure

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S2, Figure S4). The GHG emissions of corn stover in areas of low corn stover availability are

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also higher than those in high availability areas, and the farm-gate price in the low corn stover

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availability areas is also high.

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Non RFS-compliant biofuel system. If the GHG reduction threshold specified by the RFS is not

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a constraint in the corn stover supply chain system, 53 biorefineries can produce 11.3 billion

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liters of ethanol fuel annually from corn stover grown in 869 counties (Figure 1B, Table S5).

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Over 98% of the total corn stover collected in the 12 U.S. Midwestern states could potentially be

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processed in the non RFS-compliant biofuel system. Under these conditions, Iowa and Nebraska 13 ACS Paragon Plus Environment


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are the major corn stover ethanol producing states in the non RFS-compliant biofuel system, with

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each state producing over 2.0 billion liters per year of ethanol. In contrast to the results under the

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RFS-compliant biofuel system, Illinois and South Dakota are each able to support biorefineries

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and can produce over 1.0 billion liters per year of corn stover-derived ethanol fuel. In the RFS-

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compliant biofuel system, Illinois and South Dakota cannot support any biorefineries due to low

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GHG credits of the surplus electricity produced in the biorefineries. This is because nuclear

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power plants generate 49% of the total electricity in Illinois, and hydropower plants generate half

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of the total electricity in South Dakota17 (Table S3).

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The lifecycle GHG emissions of corn stover ethanol in the non-RFS compliant biofuel system

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range from 21.58 to 72.33 g CO2 eq. MJ-1 (Table S5). The ethanol volume-weighted average

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lifecycle GHG emissions in the non-RFS compliant biofuel system (46.30 ± 10.05 g CO2 eq. MJ-

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1)

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GHG reduction threshold imposed by the RFS greatly influences both the volume of ethanol

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produced and also the overall lifecycle GHG emissions of corn stover-derived ethanol fuel. Only

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22% of corn stover ethanol fuel from eleven biorefineries in the non RFS-compliant biofuel

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system meets the GHG reduction threshold.

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The major difference between the RFS-compliant and the non-compliant biofuel systems is the

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GHG emissions assigned to corn stover (Figure 2). The quantity-weighted average GHG

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emissions associated with corn stover are 197 ± 58 kg CO2 eq. dry Mg-1 in the RFS-compliant

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biofuel system and 261 ± 62 kg CO2 eq. dry Mg-1 in the non-compliant biofuel system. The RFS-

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compliant biofuel system does not process GHG-intensive corn stover in the low stover

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availability areas regardless of farm-gate price.

are higher than those in the RFS-compliant biofuel system (34.39 ± 4.92 g CO2 eq. MJ-1). The


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The GHG credits of the surplus electricity in the non-compliant biofuel system are slightly lower

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than those in the compliant biofuel system due to the biorefineries located in Illinois and South

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Dakota, states with relatively low GHG emissions from their electrical grids17. The differences

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between the two systems in GHG emissions due to transportation of baled corn stover are

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insignificant (Tables S6-S7).

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Figure 2.

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Overall GHG savings. Remarkably, despite higher lifecycle GHG emissions per MJ of ethanol

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fuel, the non RFS-compliant biofuel system achieves almost two and a half times more total

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GHG savings compared to gasoline than does the RFS-compliant biofuel system. The overall

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GHG savings are 10.7 Tg CO2 eq. year-1 from 11.3 billion liters of corn stover ethanol fuel in the

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non RFS-compliant system and 4.4 Tg CO2 eq. year-1 from 3.6 billion liters of corn stover

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ethanol fuel in the RFS-compliant system.

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As long as lifecycle GHG emissions of ethanol produced in each biorefinery are less than those

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of gasoline, the GHG emissions reductions grow as the volume of fuel produced grows. Multiple

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sensitivity analyses lead to the same conclusion: the non RFS-compliant biofuel system offers

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much greater overall GHG savings than the RFS-compliant system (Figure 3, Table S10). The

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RFS GHG emissions reduction threshold excludes large quantities of corn stover-derived biofuel

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with lifecycle GHG emissions higher than the threshold requirement. These large quantities of

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biofuel that do not meet the threshold would nonetheless help reduce total GHG emissions.

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Figure 3.

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When current RFS practices are applied to estimate the lifecycle GHG emissions, most corn

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stover-based ethanol fuels comply with the RFS GHG reduction threshold. Thus the overall



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GHG savings compared to gasoline is not reduced by the threshold. However, the non RFS-

315

compliant biofuel system achieves more GHG savings compared to gasoline than does the RFS-

316

compliant biofuel system when corn stover-based ethanol fuels are produced in high GHG-

317

intensive biorefinery processes and less electricity from coal is displaced by the excess electricity

318

from the biorefinery (Table S11).

319

Wilhelm et al.18 estimated the amount of corn stover that must be left on the land in order to

320

maintain soil organic carbon (5.25‒12.5 Mg ha-1). Based on Wilhelm’s estimate and data from

321

the National Agricultural Statistics Service (https://www.nass.usda.gov/), corn stover removal

322

rates required to sustain SOC in corn-soybean rotations in the Midwest under no-tillage

323

management practices were calculated. These stover removal rates range from 10 to 40% below

324

the proposed removal rates for no-tillage management practices (50%) in the RFS program2.

325

When spatial variations in SOC due to corn stover removal are fully accounted for in the RFS

326

lifecycle GHG calculations, the GHG reduction threshold currently imposed by the RFS will

327

likely: a) reduce the total RFS-compliant corn stover-based cellulosic biofuel volume produced,

328

b) limit the economic benefits associated with the RFS-compliant corn stover-based cellulosic

329

biofuel system, including new jobs generated, and c) decrease the overall GHG emission savings

330

achieved by the RFS-compliant corn stover-based cellulosic biofuel system. To avoid these

331

undesired and unfavorable outcomes in the future, the RFS program should consider a LCFS-

332

type approach for certain biofuels. For example, corn stover-based cellulosic biofuels that do not

333

meet the GHG reduction threshold, but are less than the lifecycle GHG emissions of gasoline,

334

should probably be allowed.

335

Using cover crop practices and manure application to compensate for corn stover removal can

336

also reduce GHG emissions associated with SOC losses compared to the current removal 16 ACS Paragon Plus Environment

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337

system.19, 20 Such mitigation practices would also enable the system to produce more RFS-

338

compliant corn stover ethanol fuel. Despite the SOC benefits, unless the cover crop is harvested

339

and used productively, employing cover crops would increase the farm-gate price of corn stover,

340

leading to higher ethanol selling prices. The current excess nutrient levels generated by manure

341

application in the Midwest21 can also prevent some croplands from applying manures. Therefore,

342

more detailed studies on mitigation practices are required to illuminate the best approaches for

343

low carbon biofuel policies.

344

Acknowledgments

345

This material is based upon work supported by the U.S. Department of Energy, Office of

346

Science, Office of Biological and Environmental Research under Award Number DE-

347

SC0018409, and work funded by the DOE Great Lakes Bioenergy Research Center (DOE BER

348

Office of Science DE-FC02-07ER64494) and the DOE BETO Office of Energy Efficiency and

349

Renewable Energy (DE-AC05-76RL01830). Professor Dale gratefully acknowledges support

350

from Michigan State University AgBioResearch and also from the USDA National Institute of

351

Food and Agriculture. Professor Izaurralde was partially supported by Texas AgriLife Research

352

and Extension, Texas A&M University, Temple, TX 76502. Dr. Xuesong Zhang acknowledges

353

support from NASA (NNX17AE66G and NNH13ZDA001N) and NSF (INFEWS 1639327)

354

Supporting information

355

Additional tables and figures; Detailed descriptions for size, minimum ethanol selling price and

356

lifecycle GHG emissions.

357

References



358

1. Energy Independence and Security Act of 2007. Public Law 110-140, Vol. 121, 2007;

359

https://www.gpo.gov/fdsys/pkg/PLAW-110publ140/pdf/PLAW-110publ140.pdf.

360

2. Regulation of Fuels and Fuel Additives: Changes to Renewable Fuel Standard Program;

361

Final Rule. Federal Register 2010 Part II. U.S. Environmental Protection Agency. 40 CFR

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Part 80, 2010; https://www.epa.gov/renewable-fuel-standard-program/renewable-fuel-

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standard-rfs2-final-rule-additional-resources.

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3. California Air Resources Board. Staff Report: Proposed Regulation to Implement the Low

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Carbon Fuel Standard - Initial Statement of Reasons Volume 1: Staff Report; California Air

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Resources Board, 2009; http://www.arb.ca.gov/fuels/lcfs/030409lcfs_isor_vol1.pdf.

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4. Kim, S.; Dale, B. E. All biomass is local: The cost, volume produced, and global warming

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impact of cellulosic biofuels depend strongly on logistics and local conditions. Biofuel

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Bioprod. Bioref. 2015, 9 (4), 422–434.

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5. Kim, S.; Zhang, X.; Dale, B. E.; Reddy, A. D.; Jones, C. D.; Cronin, K.; Izaurralde, R. C.;

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Runge, T.; Sharara, M. Corn stover cannot simultaneously meet both the volume and GHG

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reduction requirements of the Renewable Fuel Standard. Biofuel Bioprod. Bioref. 2018, 12

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(2), 203–212.

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6. Jones, C. D.; Zhang, X.; Reddy, A. D.; Robertson G. P.; Izaurralde, R. C. The greenhouse

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gas intensity and potential biofuel production capacity of maize stover harvest in the US

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Midwest. Glob. Change Biol. Bioenergy 2017, 9 (10), 1543–1554.

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7. Liska, A. J.; Yang, H.; Milner, M.; Goddard, S.; Blanco-Canqui, H.; Pelton, M. P.; Fang, X.

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X.; Zhu, H.; Suyker, A. E. Biofuels from crop residue can reduce soil carbon and increase

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CO2 emissions. Nat. Clim. Change 2014, 4, 398–401.


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8. Yacobucci, B. D.; Bracmort, K. Calculation of lifecycle greenhouse gas emissions for the

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renewable fuel standard (RFS); R40460; Congressional Research Service: Washington DC,

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2010; http://nationalaglawcenter.org/wp-content/uploads/assets/crs/R40460.pdf.

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9. Zhang, X.; Izaurralde, R. C.; Manowitz, D. H.; Sahajpal, R.; West, T. O.; Thomson, A. M.;

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Xu, M.; Zhao, K.; LeDuc, S. D.; Williams, J. R. Regional scale cropland carbon budgets:

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evaluating a geospatial agricultural modeling system using inventory data. Environ. Model

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Softw. 2015, 63, 199–216.

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10. West, T. O.; Brandt, C. C.; Baskaran, L. M.; Hellwinckel, C. M.; Mueller, R.; Bernacchi, C.

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J.; Bandaru ,V.; Yang, B.; Wilson, B. S.; Marland, G.; Nelson, R. G.; De la Torre Ugarte, D.

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G.; Post, W. M. Cropland carbon fluxes in the United States: increasing geospatial resolution

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of inventory–based carbon accounting. Ecol. Appl. 2010, 20 (4), 1074–1086.

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11. Integrated Biorefineries: Biofuels, Biopower, and Bioproducts; DOE/EE-0912; Office of Energy Efficiency & Renewable Energy: Washington, DC, 2013. 12. Economics of Harvesting and Transporting Corn Stover; PM 3053B, Iowa State University

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Extension and Outreach: Ames, IA, 2014;

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https://store.extension.iastate.edu/product/Economics-of-Harvesting-and-Transporting-Corn-

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Stover.

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13. The greenhouse gases, regulated emissions, and energy use in transportation (GREET)

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computer model version 2015; Argonne National Laboratory: Argonne, IL, 2015;

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https://greet.es.anl.gov/.

400

14. Fertilizer Use and Price; Economic Research Service: Washington, DC;

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https://www.ers.usda.gov/data-products/fertilizer-use-and-price.aspx.



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15. 2016 Billion-Ton Report: Advancing Domestic Resources for a Thriving Bioeconomy,

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Volume 1: Economic Availability of Feedstocks; ORNL/TM-2016/160; Oak Ridge National

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Laboratory: Oak Ridge, TN, 2016.

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16. Humbird, D.; Davis, R.; Tao, L.; Kinchin, C.; Hsu, D.; Aden, A.; Schoen, P.; Lukas, J.;

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Olthof, B.; Worley, M.; Sexton, D.; Dudgeon, D. Process Design and Economics for

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Biochemical Conversion of Lignocellulosic Biomass to Ethanol: Dilute–Acid Pretreatment

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and Enzymatic Hydrolysis of Corn Stover; NREL/TP–5100–47764; National Renewable

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Energy Laboratory: Golden, CO, 2011.

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17. Emissions & Generation Resource Integrated Database (eGRID); U.S. Environmental Protection Agency: Washington, DC, 2012; https://www.epa.gov/energy/egrid. 18. Wilhelm, W. W.; Johnson, J. M. F.; Karlen, D. L.; Lightle, D. T. Corn Stover to Sustain Soil

413

Organic Carbon Further Constrains Biomass Supply. Agron. J. 2007, 99, 1665–1667.

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19. Jones, C. D.; Oates, L. G.; Robertson, G. P.; Izaurralde, R. C. Perennialization and Cover

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Cropping Mitigate Soil Carbon Loss from Residue Harvesting. J. Environ. Qual. 2018, 47,

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710–717.

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20. Qin, Z.; Canter, C. E.; Dunn, J. B.; Mueller, S.; Kwon, H.; Han, J.; Wander, M. M.; Wang,

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M. Land management change greatly impacts biofuels’ greenhouse gas emissions. Glob.

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Change Biol. Bioenergy 2018, 10, 370–381.

420

21. Estimated Animal Agriculture Nitrogen and Phosphorus from Manure; U.S. Environmental

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Protection Agency: Washington, DC; https://www.epa.gov/nutrient-policy-data/estimated-

422

animal-agriculture-nitrogen-and-phosphorus-manure.

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Figure legends

424 425

Figure 1. Corn stover supply chains in the U.S. Midwest. A: RFS-compliant biofuel system and

426

B: Non RFS-compliant biofuel system. A factory symbol represents a biorefinery location, and a

427

particular color represents the specific corn stover supply chain feeding each biorefinery.

428

Counties with the same color supply corn stover to a biorefinery in that colored area. A black

429

bullet is a local stover storage facility located at the centroid of a given county. Farmers in

430

counties without local storage facilities do not practice no-till farming and are excluded from

431

supplying stover to either of the two systems considered here. Created with the Maptitude

432

mapping software, www.caliper.com.

433

Figure 2. Selected GHG sources in the corn stover ethanol fuel. The bottom and top of the box

434

are the first and third quartiles, and the horizontal line inside the box is the median value. The

435

ends of the whiskers represent 10th (lower) and 90th (upper) percentile.

436

Figure 3. Overall GHG savings from corn stover ethanol fuel compared to gasoline.

437 438



Figure 1. Corn stover supply chains in the U.S. Midwest. A: RFS-compliant biofuel system and B: Non RFScompliant biofuel system. A factory symbol represents a biorefinery location, and a particular color represents the specific corn stover supply chain feeding each biorefinery. Counties with the same color supply corn stover to a biorefinery in that colored area. A black bullet is a local stover storage facility located at the centroid of a given county. Farmers in counties without local storage facilities do not practice no-till farming and are excluded from supplying stover to either of the two systems considered here. Created with the Maptitude mapping software, www.caliper.com 159x57mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. Selected GHG sources in the corn stover ethanol fuel. The bottom and top of the box are the first and third quartiles, and the horizontal line inside the box is the median value. The ends of the whiskers represent 10th (lower) and 90th (upper) percentile. 154x109mm (300 x 300 DPI)



Figure 3. Overall GHG savings from corn stover ethanol fuel compared to gasoline. 191x138mm (300 x 300 DPI)


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The Renewable Fuel Standard May Limit Overall Greenhouse Gas

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