Evaluating the Environmental Impact of Corn Stover Collection for Biofuels Production Thesis Presented in Partial Fulfillment O

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Evaluating the Environmental Impact of Corn Stover Collection for Biofuels Production

Thesis

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science

in the Graduate School of The Ohio State University.

Asmita Khanal, B.E.

Environmental Science Graduate Program

The Ohio State University

2018

Master’s Examination Committee:

Dr. Ajay Shah, Advisor

Dr. Harold Keener

Dr. Frederick Michel

Dr. Shauna Brummet

Asmita Khanal

2018

Abstract

Corn stover is the most readily available lignocellulosic feedstock for cellulosic biofuel production in the Midwestern U.S. Biofuels produced from corn stover could offset the environmental impact of fossil fuel combustion. Corn stover harvest for biofuels production removes potentially recyclable nutrients and carbon, increases the vulnerability of the soil to soil erosion, and produces emissions during harvest and collection due to operation and manufacture of the machinery, thus challenging the sustainability of the process. The main objective of this research was to evaluate the environmental impact of corn stover removal for biofuel production by first determining physical and compositional attributes of the corn stover in terms of dry matter, structural carbohydrates, lignin and nutrients, and then quantifying the environmental impact of corn stover collection for biofuel production. In 2016 and 2017, stover fractions below and above ear-level, and cobs contributed 42-56%, 31-38% and 13-18% to the total non-grain aboveground dry matter, respectively. Based on this dry matter contribution of the different fractions, three different corn stover removal scenarios were established: 1) removal of cobs, 2) removal of stover above ear level excluding cobs, and 3) removal of stover above half way between ground and ear level excluding cobs, to analyze the environmental impact in terms of greenhouse gases emissions in comparison to the base case scenario, i.e., no stover removal. The system boundary of the research included the harvest, collection and stacking of corn stover bales at the field edge, and the emissions produced by corn stover decomposition and fertilizer volatilization in the field. The nitrogen and phosphorus content in stover above ear, stover below ear and cobs were uniform across all fractions and were in the range 0.44-2.03% and 0.02-

0.15%, respectively. Potassium concentration was significantly higher (1.49-2.41%) in the stover ii

fraction below ear compared to the other two fractions (0.15-1.15%). Using the nutrient concentrations thus obtained, fertilizer requirements for the different corn stover removal scenarios were determined. Experimental data as well as secondary data from the literature were used for the estimation of greenhouse gases for the different corn stover removal scenarios. Net greenhouse gas emissions for Scenarios 1, 2 and 3 increased by 76-258, 218-546 and 277-675 kg-CO2e/ha, respectively, compared to the base case scenario (492-2,355 kg-CO2e/ha). The outcome of this study shows that harvesting corn stover above ear, including cobs, will allow biomass removal with higher sugar concentrations while retaining ~50% of the dry matter and nutrients in the field for maintaining soil organic matter and soil erosion prevention. In addition, the greenhouse gas emissions for the different corn stover removal scenarios are similar to each other, which does not provide clear indication on the sustainable amount of corn stover to be removed from the field based on their greenhouse gas emissions footprint.

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Acknowledgements

I am extremely grateful to Dr. Ajay Shah, my advisor, for providing me with this opportunity to pursue my Master’s degree and providing me with the necessary support and guidance throughout my program. Dr. Shah not only advised me throughout my program but has also motivated me to be involved in different academic activities as a result of which I am not only acquainted with a Master’s degree but a diverse graduate school experience. Without his encouragement to try different types of research, my experience would not have been as exciting as it has been. I would also like to thank my committee members, Dr. Harold Keener, Dr. Shauna

Brummet and Dr. Frederick Michel for their guidance and feedback.

I would also like to express my sincere thank you to my fellow graduate students and staff members in Dr. Shah’s research group for their continued support throughout my Master’s program in academic as well as personal matters. As I transition into my PhD program in the same group, I could not be more excited for more interesting research projects in such a supportive and fun loving research group. I would also like to thank other staffs, friends and colleagues in the department for such a wonderful experience and look forward to my PhD.

I am extremely grateful to my family members for their continued love and support, and motivation to pursue my goals. With the accomplishment of this degree, I hope to have made them proud.

Vita

2016……………………………………B.E. Mechanical Engineering, Tribhuvan University,

Institute of Engineering

2016 to present ……………………...... Environmental Science Graduate Program,

Home Department: Department of Food, Agricultural

and Biological Engineering,

The Ohio State University

Publications

Khanal, A., Manandhar, A., & Shah, A. Evaluating distributions of dry matter, structural carbohydrates, lignin and nutrients in corn residue for cellulosic biofuel production- In preparation.

Khanal, A., Mousavi-Avval, S.H., Khanal, S., & Shah, A. Life Cycle greenhouse gas emissions from corn residue collection for cellulosic biofuel production- In preparation.

Field of Study

Major Field: Environmental Science Graduate Program

Table of Contents Abstract ...... ii

Acknowledgements ...... iv

Vita ...... v

List of Tables ...... viii

List of Figures ...... ix

Chapter 1: Introduction ...... 1

1.1 Background ...... 1 1.2 Objectives...... 5 1.3 Thesis Organization ...... 6 Chapter 2: Evaluating Distributions of Dry Matter, Structural Carbohydrates, Lignin and

Nutrients in Corn Residue for Cellulosic Biofuel Production ...... 7

2.1 Abstract ...... 7 2.2 Introduction ...... 8 2.3 Methods ...... 11 2.3.1 Corn plants collection and sampling ...... 11 2.3.2 Experimental design ...... 12 2.3.3 Measurements ...... 13 2.4 Results and discussion ...... 15 2.4.1 Physical parameters ...... 15 2.4.2 Structural carbohydrates and lignin...... 19 2.4.3 Nutrients ...... 24 2.4.4 Carbon ...... 28 2.5 Conclusions ...... 28

Chapter 3: Life Cycle Greenhouse Gas Emissions from Corn Residue Removal for

Cellulosic Biofuel Production ...... 29

3.1 Abstract ...... 29 3.2 Introduction ...... 30 3.3 Methodology ...... 33 3.3.1 Goal and scope ...... 33 3.3.2 Modeling overview ...... 35 3.3.3 Inventory analysis ...... 38 3.3.4 Impact assessment ...... 42 3.3.5 Uncertainty analysis ...... 43 3.4 Results and discussion ...... 43 3.4.1 Fertilizer, fuel and machinery requirements ...... 43 3.4.2 Greenhouse gas emissions for different stover removal scenarios ...... 45 3.4.3 Sensitivity analysis ...... 49 3.4.4 Comparison of emissions in different stover removal scenarios to literature ..... 52 3.5 Conclusions ...... 52 Chapter 4: Conclusions and Recommendations for Future Work ...... 54

References ...... 57

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List of Tables

Table 2.1 Lengths (in m) of the below and above ear stover fractions of the corn plant ...... 16

Table 2.2 Harvest index for 2016 and 2017 corn production ...... 18

Table 3.1 Input data for resource requirements and the greenhouse gas estimations and their distributions...... 36

Table 3.2 Specifications of the farm machineries used for corn stover collection upto stacking at the field edge ...... 39

Table 3.3 Resource requirements for different corn stover collection scenarios ...... 45

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List of Figures

Figure 2.1 Dry matter distributions of the non-grain fractions of the corn plant...... 17

Figure 2.2 Distribution of (a) glucose, (b) other sugars, and (c) lignin across different corn stover fractions and cobs ...... 23

Figure 2.3 Distribution of (a) nitrogen, (b) phosphorus, and (c) potassium across corn stover fractions and cobs ...... 27

Figure 3.1 System boundary for the study indicating the different scenarios ...... 34

Figure 3.2 Emissions for the base case scenario of no stover removal from the field ...... 46

Figure 3.3 Emissions for different amounts of corn stover removal in addition to the base case 48

Figure 3.4 Sensitivity analysis of emissions at different corn stover removal rates ...... 51

Chapter 1: Introduction

1.1 Background

With the significant increase in energy demand in the U.S. since the mid-1900s, the consumption of petroleum fuels has increased drastically (U.S. EIA, 2018). At the current rate of consumption, oil reserves are forecasted to exhaust in 50 years from 2016 (BP, 2017).

Renewable sources of energy is therefore a pressing need of the present context since the share of renewables in the total energy supply in the U.S. is minimal (~10%) (Institute for Energy

Research, 2016). The energy supply from renewable sources in 2015 consisted of hydropower

(25%), wood (21%), biomass waste, i.e., municipal solid waste, sludge waste, landfill gas, agricultural byproducts (5%), biofuels (22%), wind (19%), geothermal (2%) and solar (5%).

Approximately half of the total renewable energy supply is from biomass sources, which alludes to the fact that biomass is the most abundant source of renewable energy in the U.S. in the present context. In addition, the transportation sector consumes about 1/3rd of the total energy produced in the U.S. with 95% of it being petroleum, this provides a strong motivation for substituting some share of petroleum fuels used in transportation with alternative fuels so that

U.S. dependence on petroleum based fuel can be reduced (U.S. EIA, 2018).

In order to reduce the U.S. dependence on petroleum based fuel, the Energy Independence and

Security Act (EISA) of 2007 mandated production of 36 billion gallons of renewable fuels by

2022 through the Renewable Fuel Standard (RFS) (U.S. EPA, 2017a). The volume of the biofuels target set by RFS consists of 21 billion gallons of advanced biofuels consisting of 16 billion gallons of cellulosic biofuels, 1 billion gallons of biodiesel, 4 billion gallons of other 1

advanced biofuels, including renewable jet fuel, and the remaining 15 billion gallons of conventional biofuel that is mainly ethanol, produced from corn grain. The target for conventional biofuel production has already reached the mandated target of 15 billion gallons in

2016 (U.S. EIA, 2017). However, the production of advanced biofuels is minimal. According to a report produced by NREL (National Renewable Energy Laboratory), 177.5 million gallons of cellulosic ethanol and 812.5 million gallons of hydrocarbon fuel are anticipated to be produced as advanced biofuels by 2020 (Schwab et al., 2016). This volume sums to only about 5% of the target set by the EISA. Therefore, cellulosic feedstock and their prospect in advanced biofuel production is being vigorously researched. One of the main challenges with cellulosic biofuel lies in the recalcitrant structure of the lignocellulosic biomass caused by the presence of lignin that creates a physical barrier to accessing the sugars that are fermented into ethanol (Hood,

2016). Also, the utilization of mainly the cellulosic fraction of the feedstock, which contributes to only 1/3rd of the biomass composition, limits the production of biofuels from lignocellulosic feedstock (Balan, 2014).

Lignocellulosic biomass that can be used for energy generation can be classified as agricultural residues, energy crops, woody biomass, forest residues, and blends and mixture of these biomass sources (U.S. Department of Energy, 2016). Among different biomass sources, agricultural residues are desirable feedstock because they are the by-products of other higher value crops, such as corn grain in the case of corn stover, which means that the inputs used for production are mostly allocated to the higher value product. Therefore, corn residue is of high importance in the

U.S. because corn is the major agricultural product and the corn plants produce approximately

the same amount of residue as the grain. In addition, corn grain yield has increased significantly in recent decades (Nielson, 2017), and the amount of corn residue produced has also increased

(Lorenz et al., 2010). With this realization, the U.S. Department of Energy assessed the economic and environmental feasibility of different biomass sources for biofuels production and concluded that corn stover is the most dominant lignocellulosic feedstock that will be available in the near term (U.S. Department of Energy, 2016). Corn stover is an ideal feedstock for biofuel production in the present context.

There are environmental concerns associated with the removal of corn stover from the field.

Nutrient removal, increased vulnerability of the soil to erosion, and removal of organic carbon are the main issues associated with complete corn stover removal (Cruse and Herndl, 2009;

Johnson et al., 2010; Karlen et al., 2015; Wilhelm et al., 2007). Given these potential issues, not all the stover that is produced needs to be left in the field, as it can interfere with planting equipment (Jeschke and Heggenstaller, 2012) and can increase the time required for seed germination in the spring as the soil takes longer to warm up (Sindelar et al., 2013). It has been shown that corn emergence rate is higher when partial stover is removed from the field compared to no stover removal from the field (Sindelar et al., 2013). As an organic matter with a high carbon nitrogen ratio (~40:1), corn stover decomposition in the field can immobilize nitrogen in the field and provide food for pathogens (Shah et al., 2016), which can have a negative impact on corn yield. Decomposition of corn stover can also produce significant amounts of greenhouse gases (Jin et al., 2014). These examples illustrate that there are environmental pros and cons associated with the utilization of corn stover for biofuel production.

Several studies have estimated the agronomic impact of corn stover removal on the field conditions (Adler et al., 2015; Blanco-Canqui and Lal, 2007; Edgerton et al., 2010; Johnson et al., 2014; Karlen et al., 2011; Kenney et al., 2015; Lal, 2009; Lindstrom, 1986; Wilhelm, 2010;

Wilhelm et al., 2007; Wu et al., 2015). These studies have identified the sustainable amount of stover that can be removed from the field based on different sustainability criteria, primarily soil organic carbon maintenance and soil erosion risks, which also depend on other factors such as the topography of the field and management practices such as tillage, cover crop and crop rotation (Karlen et al., 2011; Wilhelm, 2010). Based on these studies, the sustainable amount of corn stover removal ranges from no removal to 50% removal of the total corn stover available in the field. These studies suggest that higher corn stover retention in the field is required for maintaining soil organic carbon than for protection from soil erosion. Some of these studies

(Adler et al., 2015; Blanco-Canqui and Lal, 2007; Karlen et al., 2011; Kenney et al., 2015;

Nafziger, 2011; Wilhelm et al., 2007) have also quantified the effect of corn stover removal on corn yield. While many studies did not find significant differences in corn grain yield because of stover removal (Adler et al., 2015; Blanco-Canqui and Lal, 2007; Karlen et al., 2011; Kenney et al., 2015), some studies (Pantoja et al., 2015; Sindelar et al., 2013) found increase in grain yields with partial corn stover removal for the same nitrogen fertilization rate and tillage practice. The evidence for the sustainable rate of corn stover removal in the literature is inconsistent indicating the need to quantify the impact that corn stover removal has on the environment.

In addition to the agronomic impact of corn stover removal, several researchers have estimated the environmental impact of producing cellulosic ethanol from corn stover (Canter et al., 2016;

Jenkins and Alles, 2011; Kim et al., 2009; Kim and Dale, 2005; Liu et al., 2018; Murphy and

Kendall, 2015; Whitman et al., 2011). This helps estimate the net environmental impact of biofuel production from renewable sources, such as corn stover, in comparison to petroleum fuels. These studies have estimated the complete life cycle environmental impact of cellulosic ethanol production from the production of corn stover to the distribution of ethanol (Jenkins and

Alles, 2011; Kim et al., 2009; Liu et al., 2018; Murphy and Kendall, 2015; Wang et al., 2012;

Whitman et al., 2011). Only one of these studies provides estimates for the environmental impact of cellulosic ethanol production for varying corn stover removal rates (Whitman et al., 2011).

Determining the suitable corn stover removal rate that provides agronomic benefits, such as organic carbon maintenance and protection from soil erosion, and produces minimal environmental impact during the different processes undertaken for conversion of corn stover to ethanol and its distribution is an important task.

1.2 Objectives

The overall goal of this research is to evaluate the environmental impact of corn stover removal from the field for cellulosic biofuel production. To achieve the overall goal of this research, there are two specific objectives:

1) Evaluate the distributions of dry matter, structural carbohydrates, lignin and nutrients in

corn residue for cellulosic biofuel production.

2) Estimate the life cycle greenhouse gas emissions of corn residue removal from the field for

cellulosic biofuel production.

1.3 Thesis Organization

This thesis is organized in four chapters. The first chapter consists of the overall background on the importance of corn stover in cellulosic biofuel production and the challenges associated with the process. It also presents the objectives and the organization of the thesis. The second chapter consists of the results and analysis from the experimental work done for the thesis in 2016 and

2017 in Ohio Agricultural Research and Development Center (OARDC) at The Ohio State

University Wooster Campus. This chapter discusses the methods used for the determination of physical (length, dry matter) and compositional (sugars, lignin, nutrients) characteristics of the corn stover fractions, and presents the statistical analysis of the same. This chapter also provides recommendations on the quantity and fractions of corn stover removal for cellulosic biofuel production. The third chapter consists of estimates for greenhouse gas emissions for different corn stover removal scenarios. Using some of the information obtained from Chapter 2 and from literature, this chapter presents the methods used for model development and discusses the results for different corn stover removal scenarios. This chapter also provides recommendation for suitable amount of corn stover removal based on the environmental impact. The fourth chapter summarizes the main findings of the research and makes suggestions for future research that could be done in this area.

Chapter 2: Evaluating Distributions of Dry Matter, Structural Carbohydrates, Lignin and Nutrients in Corn Residue for Cellulosic Biofuel Production

2.1 Abstract

Corn stover removal for biofuels production removes potentially recyclable nutrients and carbon challenging the sustainability of the process. This study focused on quantifying the distributions of dry matter, nutrients, sugars and lignin in corn stover and cobs. Corn plants were collected in

2016 and 2017, and the non-grain fraction was separated into stover fractions below and above ear-level, and cob, which contributed to 42-56%, 31-38% and 13-18% of the non-grain dry matter, respectively. Glucose, lignin, nitrogen and phosphorus concentrations across the fractions were 32-40%, 8-15%, 0.44-2.03% and 0.02-0.15%, respectively. Other sugars (xylose, mannose, arabinose, galactose) were significantly higher (~36%) in cobs compared to other fractions (21-

30%). Stover below ear contained the highest potassium concentration (1.49-2.41%) compared to other fractions (0.15-1.15%). Harvesting stover above ear, including cobs, will allow sufficient biomass removal with higher sugar concentrations while retaining ~50% of the dry matter and nutrients in the field.

Keywords: Cellulosic biofuels; corn residues; nutrients; sugars; lignin

2.2 Introduction

Corn stover is the primary feedstock source for cellulosic biofuel production as identified in the

Billion Ton Report (U.S. Department of Energy, 2016). Several studies (Adler et al., 2015;

Hoskinson et al., 2007; Lal, 2009; Shah et al., 2016; Wilhelm, 2004) have identified the benefits and challenges associated with corn stover removal for biofuels production. There are agronomic benefits to removal of corn stover, such as decreased feed for pathogens and reduced nitrogen immobilization from reduced stover decomposition in the field (Jeschke and Heggenstaller,

2012; Shah et al., 2016). Also, removal of stover leads to reduced in-field greenhouse gas (GHG) emissions, such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), due to stover decomposition (Jin et al., 2014). On the other hand, removal of corn stover removes the nutrients

(i.e., nitrogen (N), phosphorus (P), and potassium (K)) and carbon (C) that are present in various concentrations in corn stover fractions and would otherwise be available for the next cropping cycle (Johnson et al., 2010; Karlen et al., 2015; Mourtzinis et al., 2016). Increased vulnerability of the soil to erosion because of the lack of soil coverage caused by corn stover removal is also concerning (Cruse and Herndl, 2009).

The amount of corn stover that can be sustainably removed from the field while meeting the biorefinery feedstock demand depends upon different field and environmental conditions.

Different sustainability criteria constrain the amount of stover removal from the field. For instance, Graham et al. (2007) recommended collection of 30% corn stover for soil erosion control, soil moisture maintenance and nutrient replacement, which represents corn stover removal of 2 metric tons per hectare. Another study (Nafziger, 2011) suggested that 50%-60%

(~6 metric tons per hectare) corn stover removal at the grain yield of 12-13 metric tons per hectare is not detrimental to soil health as higher removal of stover in this study decreased the nitrogen fertilization requirement in a continuous corn cropping system without tillage. Another study (Blanco-Canqui and Lal, 2007) recommended removing less than 25% stover (1.25 metric tons per hectare) from sloping and erosion prone soils, which sets another criteria for stover removal based on the topography and soil type. The same study further showed that the removal of up to 50% corn stover (2.5 metric tons per hectare) did not reduce the soil organic carbon content significantly for soil with 2% slope. Therefore, even at the similar stover yield, there is no consensus on the sustainable amount of corn stover that can be removed from the field, which is primarily governed by the field conditions. It is important to find the appropriate region- specific balance between the removal of corn stover for biofuels production, and its impact on the field conditions as well as on the environmental sustainability.

Several studies (Blanco-Canqui and Lal, 2009, 2007; Graham et al., 2007; Jin et al., 2014;

Johnson et al., 2014; Lal, 2009; Wilhelm, 2004) have been carried out to determine recommendations for the amount of corn stover that needs to be retained in the field for maintaining soil health. Retaining corn stover in the field helps maintain soil quality by returning valuable nutrients, carbon, as well as dry matter for soil cover to prevent soil erosion from the field. There are very few studies (Johnson et al., 2010; Karlen et al., 2015) that have evaluated the distributions of nutrients, carbon and dry matter in different fractions along the height of the corn plant. It will be helpful to distinguish different fractions of corn stover based on their composition, so that suitable fractions can be removed from the field for further processing while

retaining the rest in the field for maintaining soil health.

Compositions of the corn stover and cob fractions have been determined by many studies

(Aboagye et al., 2017; Barten, 2013; Duguid et al., 2009; Gao et al., 2009; Garlock et al., 2009;

Hoskinson et al., 2007; Mourtzinis et al., 2016, 2014; Templeton et al., 2009; Weiss et al., 2010;

Ye et al., 2006), as it is one of the major feedstock sources for biofuel production and its composition directly impacts the productivity of biofuel. The studies report the composition in terms of the monomeric sugars, i.e., glucose, xylose, mannose, arabinose and galactose, or their structural carbohydrates, cellulose and hemicellulose. Glucose is a six carbon sugar monomer, which forms the cellulose component of the plant and can be fermented to ethanol. Other five and six carbon sugars, including glucose, xylose, mannose, galactose and arabinose, bond together to form the hemicellulose structure of the plant. These sugars, except glucose, are not currently utilized for bioethanol production but are being researched for production of higher value chemicals and products (He et al., 2016).

These studies also report the lignin concentrations since it is important during conversion.

Lignin is a highly complex polymer structure formed from several compounds, and is the resistant structure that needs to be broken down to access the sugars that are used for biofuel production (Chen, 2014). Lower lignin concentration is therefore desirable in feedstock used for biofuel production. Few of these studies (Barten, 2013; Mourtzinis et al., 2016, 2014) have reported the concentrations of sugars and lignin along the height of the corn plant, which is useful information to evaluate the stover cutting height during harvest, especially for a single-

pass system. Single pass harvesting includes harvesting of grain and corn stover at the same time, so that multiple passes of heavy harvesting equipment over the soil surface can be avoided and biomass with low soil contact can be collected.

The objective of this study was to investigate the distribution of nutrients, carbon, structural carbohydrates, lignin and dry matter across the different non-grain fractions of the corn plant

(stover fractions above and below ear, and cobs) in Ohio. Different varieties of corn plants were used for 2016 and 2017, and were sampled in October and November each year. The information gathered in this study can be used to identify the quantity and the parts of the corn plant that are suitable to be removed from the field for biofuels production in terms of dry matter, sugars and nutrient concentrations.

2.3 Methods

2.3.1 Corn plants collection and sampling

Corn plants used in this study were collected in the fall of 2016 and 2017 from a research plot

(silt loam soil, 2-6% slope, moderate drainage and 2-3% organic matter) at Ohio Agricultural

Research and Development Centre (OARDC), Ohio State University. The corn variety used in

2016 was a commonly used 105 days maturity hybrid, and the variety used in 2017 was a commonly used genetically modified hybrid for ethanol production with 105 days maturity. In

2016 and 2017, 50 corn plants were collected randomly from different locations in the field in mid-October and early-November to obtain the plant samples representing two harvest dates. 40 plants at each harvest date were used for dry matter determination and among the remaining 10

plants some were used for estimating the sugars and lignin concentrations and others were used for estimating nutrient concentrations. The grain moisture at the first and the second harvest dates in 2016 were in the range of 15.6-25.0% and 13.5-22.3%, respectively, and those in 2017 were in the range 18.8-29.8% and 20.0-24.7%, respectively.

The corn plants were first cut at the ground level, and further cut just below the ear to divide the plants into two fractions, i.e., below ear and above ear. In 2016, the stover fraction below ear was further divided into two equal halves (stover-below ear (bottom) and stover-below ear (top)).

Grains, cob, husk and silk were separated from the ear, and the husk and silk were included in the stover fraction above ear. Stover-below ear was not split in two halves in 2017 because the characterization results for the two fractions below ear in 2016 were not statistically significantly different (see results section for details).The corn plants in 2017 had only 3 fractions, i.e., stover- above ear, stover-below ear and cobs. The fractions were separately analyzed for dry matter, sugars, lignin, carbon and nutrients.

2.3.2 Experimental design

A full factorial experimental design was used for this study. The statistical analysis was done separately for 2016 and 2017 samples, and was not compared between the two years because the objective of this study was not to compare the varieties, but to observe the distributions of their different fractions at two different harvest dates. Each of the different fractions (four in 2016, and three in 2017) at both harvest dates were analyzed separately for dry matter, sugars, lignin,

nutrients (N, P, K), and carbon content. 40 plants at each harvest date from both years were used for determining the below and above ear height of the corn plant and the same plants were then used for dry matter determination. In addition, 3 plants at each harvest date in 2016, and 5 plants at each harvest date in 2017 were used for quantifying the other parameters, including sugars, lignin, carbon and nutrients (N, P, K), for each fraction. The statistical analysis was performed using SAS® software (SAS Institute Inc., 2011) at the significance level of 0.05, and Tukey-

Kramer test was used for means comparison.

2.3.3 Measurements

2.3.3.1 Physical parameters

The lengths of the below and above ear fractions of all the corn plants were measured to estimate the height of the corn ear from the ground. This is useful information for determining the combine setting for corn header height for grain harvest. Different fractions of corn plants were then dried separately in a convection oven (Model 1690/Controller JUMO dTRON 308, VWR,

PA, USA) at 105°C until their weights were stable. The dry weight of the non-grain fractions of the corn plant included the weights of the different stover fractions and the cob. Harvest index was then estimated as the ratio of the dry weight of the grains to the total dry matter of the above ground biomass, including the grains (Pennington, 2013).

2.3.3.2 Structural carbohydrates and lignin

Different fractions were ground separately using a knife mill. Each fraction was analyzed for glucose, other sugars (xylose, mannose, arabinose and galactose) and lignin using the NREL

(National Renewable Energy Laboratory) laboratory procedure for the determination of structural carbohydrates (cellulose and hemicellulose) and lignin in biomass (Sluiter et al., 2012).

The biomass was first extracted with water and ethanol using Accelerated Solvent Extraction

(ASE 200 Accelerated Solvent Extractor, Dionex, CA, USA). Cellulose and hemicellulose present in the biomass were broken down into their monomeric sugars using sulfuric acid digestion, and then quantified using HPLC (High Performance Liquid Chromatography, 1200 series, Agilent, CA, USA). Even though lignin is a complex structure and cannot be fractionated into individual compounds using this method, some of the lignin dissolves in the acid. Acid soluble lignin was quantified using a spectrophotometer (BioMate™ 3S Spectrophotometer, MA,

USA). The acid insoluble lignin was determined by calculating the difference between the mass of the solids remaining after acid digestion and after ashing.

2.3.3.3 Nutrients (NPK)

Different plant fractions were ground and used for the determination of nutrients, specifically N,

P and K, as these are the major macro-nutrients that are applied for the growth of the corn plants.

Since removal of corn stover removes these essential nutrients from the field, these were quantified in the different fractions of the plants. Nitrogen was quantified using the Carbon

Nitrogen Sulfur (CNS) analyzer (Vario Max CNS Macro Elemenal Analyzer, Elementar, Hanau,

Germany), which uses the Dumas combustion method for combustion of biomass at a high

temperature of ~1000°C and measures the concentration of nitrogen in gaseous form (Gonick et al., 1945). P and K for the different plant fractions were determined according to the EPA

Method 3051 (US EPA, 2007) using an ICP (Inductively Coupled Plasma) analyzer after nitric acid digestion of the material at the Services Testing and Research Laboratory (STAR) lab at

The Ohio State University.

2.3.3.4 Carbon (C)

The carbon content of different fractions was also determined along with nitrogen using the CNS analyzer (Vario Max CNS Macro Elemenal Analyzer, Elementar, Hanau, Germany), as stated above. Knowing the carbon content is essential because the quantity of stover that can be sustainably removed from the field is also constrained by the soil organic carbon. Carbon present in corn stover can add to the soil organic carbon in the field. Organic carbon benefits the plants by improving soil structure, enhancing water exchange and aeration and sustaining microbial biomass in the soil (Blanco-Canqui and Lal, 2009; Mann et al., 2002; Wilhelm et al., 2007).

2.4 Results and discussion

2.4.1 Physical parameters

The lengths of the stover fractions below and above ear for 2016 at the first harvest date were not significantly different from each other (Table 2.1). At the second harvest date, the length of the stover fractions below ear was significantly different from the stover fraction above ear (Table

2.1). In 2017, the length of the below-ear fraction was significantly different from the length of

the fraction above ear at both harvest dates. The average total plant height for corn variety used in 2016 was 2.55 m (standard deviation: 0.18; range 2.16-2.97) and for that used in 2017 was

2.80 m (standard deviation: 0.22; range 2.29-3.73). Even though the corn plants in 2017 were taller, the average length of their below ear fraction (0.69-1.88 m) was similar to that in 2016

(0.76-1.55 m). Since the main purpose of measuring the length of the fractions was to determine the harvest height for the combine, it provides suitable estimation of harvest height, and can be used in conjunction with the dry matter results to set the harvest height to remove the known quantity of dry matter from the field.

Table 2.1 Lengths (in m) of the below and above ear stover fractions of the corn plant1,2,3 Harvest First harvest date Second harvest date Year Below ear Above ear Below ear Above ear 2016 1.26±0.14b 1.32±0.13ab 1.11±0.15c 1.39±0.18a 2017 1.13±0.12b1 1.71±0.18a1 1.07±0.19b1 1.69±0.16a1 1 40 plants were collected at each harvest date for each year. 2 The values in the table represent the ‘average’ ± ‘standard deviation’. 3 Dissimilar letters indicate the statistically significant difference (at the significance level of 0.05) among different fractions of the corn plants sampled each year. The means comparison was done separately for each year. Letters without number ‘1’ indicate differences for 2016, and those with ‘1’ for 2017.

The dry matter of the two below ear fractions in 2016 was lower at the second harvest date compared to the first (Figure 2.1). Dry matter of the same fractions in 2017 were not significantly different between the two harvest dates. In 2016 and 2017, the dry matter contributions of the fractions below ear were 40-64% (average: 52%; standard deviation: 6%), and 34-59% (average: 45%; standard deviation: 5%) of the total non-grain dry matter, respectively, which means that stover harvest just below the ear level would leave approximately half of the total available biomass in the field. The reason for the suggestion to leave the below ear fraction in the field will become clear with the discussion of other parameters, sugars and nutrients. Biomass removal of 50% is towards the higher end of the recommended proportion of dry matter to be left in the field for sustainability, as suggested by different studies (Johnson et al., 2014; Wilhelm et al., 2007; Wu et al., 2015).

Figure 2.1 Dry matter distributions of the non-grain fractions of the corn plant 17

(Note: Vertical bars represent mean and error bars represent standard deviation. Dissimilar letters indicate the statistically significant difference at the significance level of 0.05. The means comparison was done separately for each year. Letters without subscript indicate the differences for 2016, and those with subscript ‘1’ for 2017.)

The average harvest index in 2016 was higher at the second harvest date compared to the first

(Table 2.2). The harvest index for 2017 remained the same between the two harvest dates. The harvest index for 2016 increased from the first harvest date to the second because the dry matter of the stover fractions below ear significantly decreased at the second harvest date (Figure 2.1).

The harvest index was not significantly different in 2017 because the dry matter of the different fractions stayed the same between the two harvest dates (Figure 2.1). The decrease in dry matter observed in the stover fractions below ear in 2016 may have been because of physical loss of biomass as they became more dry and brittle after grain maturity and vulnerable to environmental factors such as wind and rain (Huang et al., 2012). The information on dry matter can be used to determine the amount of corn stover that can be removed from the field depending on sustainability criteria set by different studies.

Table 2.2 Harvest index for 2016 and 2017 corn production1,2 Harvest Year First harvest date Second harvest date 2016 0.53±0.03b 0.56±0.04a 2017 0.58±0.03a1 0.58±0.06a1

1 1 The values in the table represent the ‘average’ ± ‘standard deviation’. 2 Dissimilar letters indicate the statistically significant difference in harvest index at the significance level of 0.05. The means comparison was done separately for each year. Letters without number ‘1’ indicate differences for 2016 and those with‘1’ for 2017.

2.4.2 Structural carbohydrates and lignin

The amounts of glucose, other sugars (i.e., xylose, mannose, galactose and arabinose) and lignin

in the different plant fractions are summarized in Figure 2.2. In 2016, glucose concentrations

across the fractions ranged from 32-40% and 33-40% at the first and second harvest dates,

respectively (Figure 2.2a). The glucose concentrations for the same fractions did not change with

the change in harvest dates. The glucose concentrations in different fractions for the same

harvest date were significantly different in some cases. At the first harvest date, stover-above ear

had significantly lower glucose concentration compared to stover-below ear (bottom). Glucose

concentrations were not significantly different among other fractions. At the second harvest date,

stover-above ear and cobs had significantly low glucose concentrations compared to stover-

below ear (bottom). In 2017, the glucose concentrations ranged from 35-37% and 34-38% across

the stover fractions and cobs at the first and second harvest dates, respectively, and neither

harvest dates nor the fractions had an effect on the glucose concentration.

Other studies (Aboagye et al., 2017; Garlock et al., 2009) show different concentrations of

glucose for different fractions when the fractionation is based on the plant parts, i.e., stalk, leaf,

husk and cobs. Since, the fractionation in this study was done based on the height of the plants, 19

each fraction consisted of a combination of leaves and stalk, and stover-above ear also included husk, which led to similar concentrations of glucose in the different fractions. The values of these results are consistent with similar studies done on the characterization of corn stover

(Barten, 2013; Mourtzinis et al., 2016, 2014). Glucose is the monomer of cellulose, which is one of the structural carbohydrates that forms the cell wall structure along with hemicellulose and lignin. Since the structure of stover fractions is the same along the height of the plant, the uniform distributions of the glucose throughout the stover fractions is reasonable.

Other sugars, which includes xylose, mannose, galactose and arabinose, were significantly higher in cobs compared to all the stover fractions (Figure 2.2b). In 2016, the concentration of other sugars did not change between the two harvest dates for any of the fractions. Differences among the fractions at the same harvest date were significant. Stover-below ear (bottom) had significantly lower concentration of other sugars compared to stover-above ear and cobs at both harvest dates. In 2017, the concentration of other sugars followed a similar trend to 2016. On an average, cobs had the highest concentration of other sugars, 35.11% and 36.12%, respectively at the first and second harvest dates. The stover fractions below and above ear at each harvest date had similar concentrations of the other sugars, and were not significantly different from each other at both harvest dates.

This finding is consistent with the other studies (Aboagye et al., 2017; Duguid et al., 2009;

Garlock et al., 2009; Mourtzinis et al., 2014), which also found the concentration of the other

sugars to be higher in cobs compared to the other stover fractions. This might be because cobs are not the support structure of the plants, and perform different functions in the plant compared to the stover fractions. The higher concentrations of the other sugars in cobs could be beneficial to the production of chemicals, other than ethanol, such as lactic acid, as suggested by He et al.

(2016). Current technologies used for obtaining glucose degrade these other sugars present in the hemicellulose fraction. For this reason several studies (Gao et al., 2009; He et al., 2016; Lu and

Mosier, 2007) focused on the fractionation of cellulose, hemicellulose and lignin, and separately upgrading them to various higher-value products.

Lignin content in the stover fractions and cobs had different trends in 2016 and 2017 (Figure

2.2c). In 2016, the lignin content of the fractions was in the range 10.8-15.5% and 8.7-15.1% at the first and second harvest dates, respectively. Lignin concentration in the stover fractions and cobs at the first harvest date was not significantly different from each other, while lignin concentration in cobs at the second harvest date was significantly lower than in the stover-below ear fraction. The results showed that harvest dates did not affect the concentration of lignin in the fractions. In 2017, lignin concentration for the fractions was in the range 14.3-16.2% at the first harvest date and 14.9-15.6% at the second, and was not significantly different among the fractions at the same harvest date. Neither harvest date nor fractions had an effect on the concentration of lignin in 2017.

Uniform concentration of lignin in all stover fractions is justifiable because lignin is a supporting structure of the plant, and, since all the stover fractions in this study consisted mainly of the stalk that supports the plant, no significant difference was observed in the lignin concentrations among the stover fractions. Cobs do not form the supporting structure of the plant, and the concentration of lignin in the cobs was significantly different from the stover-below ear (bottom) fraction at the second harvest date in 2016 but this difference was not observed at the first harvest date in the same year or at either harvest dates in 2017. The difference in results could be due to the different corn varieties used or the environmental conditions under which the corn grew in the two years (Templeton et al., 2009).

Figure 2.2 Distribution of (a) glucose, (b) other sugars, and (c) lignin across different corn stover fractions and cobs (Note: Vertical bars represent mean and error bars represent standard deviation. Dissimilar letters indicate the statistically significant difference at the significance level of 0.05. The means comparison was done separately for each year. Letters without subscript indicate the differences for 2016 and those with subscript ‘1’ for 2017.) 23

2.4.3 Nutrients

In 2016, stover-below ear (bottom) at the second harvest date had lower average concentration of nitrogen (0.44%) compared to the first harvest date (1.10%) (Figure 2.3a). This difference was not the main effect of harvest date or fraction since the difference is not consistent in other fractions at different harvest dates. Nitrogen concentration in the other stover fractions, including cobs, was uniform at the first and second harvest dates and was in the range 0.56-0.91% and

0.65-0.9%, respectively. In 2017, nitrogen concentration ranged from 1.07-1.73% and 1.42-

2.03% at the first and second harvest dates, respectively, and there was no effect of harvest date or fraction on the nitrogen concentration. There was no trend in the distribution of nitrogen among the fractions or between harvest dates. Since the translocation of nitrogen between the fractions occurs before physiological maturity (Whitehead, Viets, and Moxon 1948), the distribution of nitrogen across the fractions at the two harvest dates did not show a particular trend since the corn plants for this study were collected after maturity.

Phosphorus was distributed uniformly throughout the fractions in both years of the study. In

2016, the concentration of phosphorus in all fractions at the first and second harvest dates was in the range 0.07-0.15% and 0.08-0.13%, respectively (Figure 2.3b). For 2017, phosphorus concentration in the fractions at the first and second harvest dates was in the range 0.02-0.07% and 0.03-0.09%, respectively. The concentration of phosphorus is found to be high in surface organs, such as leaves, because phosphorus is a mineral carried by water and water transpiration occurs through the leaves and causes deposition of phosphorus there (Li et al., 2014). Since all

the stover fractions consisted of leaves and stalks, no differences were seen in the phosphorus concentrations.

Potassium content on the other hand, was the highest in the stover fractions below ear for both years 2016 and 2017. In 2016, stover-below ear (bottom) had the highest average potassium concentration of 2.07% and 1.49% at the first and second harvest dates, respectively (Figure

2.3c). Stover-below ear (top) had the second highest concentration of potassium (1.40%) at the first harvest and was significantly higher than the potassium concentration for stover-above ear

(0.74%) and cobs (0.76%). Potassium concentrations between stover-below ear (top), stover- above ear and cobs at the second harvest date was not significantly different from each other

(0.57%-0.97%). In 2017, stover-below ear had the highest average concentrations of potassium

(2.41% at the first harvest date and 1.68% at the second). Potassium concentrations at the first and second harvest dates in stover-above ear was 1.15% and 0.92%, and in cobs was 0.55% and

0.51%. Statistical analysis showed that potassium concentration in the stover fraction below ear was significantly higher than for stover-above ear and cobs at both harvest dates.

Potassium concentration was higher in stover fractions below ear compared to fractions above ear because potassium is taken up by the corn plants and stored in the older tissues that are towards the bottom of the plants and supplied to the newer tissues only when required (Smith,

2010). Other studies (Johnson et al., 2010; Karlen et al., 2015) have also quantified potassium concentration in stover fractions below and above ear, and have reported similar values. The

results from this study indicate that stover fraction below ear is more important for retention in the field for nutrient recycling.

Figure 2.3 Distribution of (a) nitrogen, (b) phosphorus, and (c) potassium across corn stover fractions and cobs (Note: Vertical bars represent mean and error bars represent standard deviation. Dissimilar letters indicate the statistically significant difference at the significance level of 0.05. The means

comparison was done separately for each year. Letters without subscript indicate the differences for 2016 and those with subscript ‘1’ for 2017.)

2.4.4 Carbon

Carbon concentrations in the fractions ranged from 38.92% to 41.67% in 2016 and from 44.23% to 46.46% in 2017, and were not significantly different from one fraction to another within the same year at the same harvest or within the same fraction at the two harvest dates. The carbon content of all fractions was found to be uniform which is reasonable because the tissues are composed primarily of cellulose, hemicellulose and lignin in similar concentrations, and carbon is the primary element that forms cellulose, hemicellulose and lignin (Chen, 2014).

2.5 Conclusions

This study provides knowledge on suitable cellulosic fractions to harvest for bioproducts and biofuels production while retaining biomass for nutrients and carbon in the field, providing valuable information for evaluating the sustainability of the harvesting process. Collecting corn stover above ear with cobs will retain approximately half of the overall aboveground biomass, containing higher potassium concentration in the field. This harvesting strategy will remove stover fractions and cobs with a similar cellulose and lignin content compared to stover fraction below ear. Cobs alone, with their high cellulose and hemicellulose content, can become desirable feedstock for value added products.

Chapter 3: Life Cycle Greenhouse Gas Emissions from Corn Residue Removal for

Cellulosic Biofuel Production

3.1 Abstract

Cellulosic biofuel produced from readily available agricultural residue, such as corn stover, in the Midwestern U.S. could offset the environmental impact of fossil fuel combustion. However, quantification of this offset is mitigated by the greenhouse gas emissions caused by corn stover harvest and collection, and direct in-field emissions from corn stover and fertilizer in the field.

The main objective of this study was to quantify the environmental impact of corn stover collection for biofuel production by modelling three different corn stover removal scenarios: 1) removal of cobs, 2) removal of stover above ear excluding cobs, and 3) removal of stover above half way between ground and ear level excluding cobs and comparing them to the base case scenario of no stover removal. The system boundary for the analysis consists of the field operations for corn stover harvest up to stacking of corn stover bales at the field edge, and quantification of the in-field emissions from corn stover decomposition and volatilization from fertilizers. Experimental data as well as secondary data from literature were used for the analysis.

The net increase in greenhouse gas emissions for scenarios 1, 2 and 3 (90% central range) were

76-258, 218-546 and 277-675 kg-CO2e/ha, respectively compared to the base case scenario (492-

2355 kg-CO2e/ha). The outcome of this study showed that the three corn stover removal scenarios considered did not give a clear indication of the sustainable amount of corn stover that can be removed from the field based on their greenhouse gas emissions footprint.

Keywords: Cellulosic biofuel, corn stover removal, greenhouse gas emissions, global warming

3.2 Introduction

Corn stover is one of the major agricultural residues in the U.S. because of the large-scale corn production in the country. The maximum possible utilization of corn stover for biofuel production is ideal because it is a by-product from corn grain production that has similar yield as the corn grain, based on the harvest index. Due to the established infrastructure for corn grain production in the Midwestern U.S., a majority of starch based ethanol facilities are located in this region (Nebraska Government, 2018). All three commercial scale cellulosic biorefineries that use corn stover as their feedstock are also located in the Midwest (Jayasinghe, 2018). Because of its abundance, corn stover is an important feedstock for cellulosic biofuels. The Renewable Fuel

Standard (RFS) Act mandates the production of at least 16 billion gallons of cellulosic biofuel by

2022 (U.S. EPA, 2017a). As of May 2018, the total production capacity of cellulosic ethanol in the U.S. is ~90 million gallons per year (Nebraska Government, 2018), which is <1% of the mandated biofuels volume for 2022. A significant effort will be required towards the utilization of corn stover in order to materialize the targets set by the RFS Act. One of the strong motivations for cellulosic biofuel production is the significant reduction of greenhouse gas emissions target set by RFS, i.e., 60% below that of the 2005 petroleum baseline (U.S. EPA,

2017a).

U.S. Agriculture, while producing feedstock for energy generation, also uses a significant amount of energy and contributes to greenhouse gas emissions. Agricultural activities alone consumed 2% of the total energy used in the US in 2014 (USDA, 2017). About 60% of this energy was directly consumed in the form of gasoline, diesel, electricity and natural gas, resulting in direct emissions from the consumption of fossil fuel during farm operations, such as 30

tillage, planting, fertilizer and pesticide application, and harvesting. In this study, farm operations include residue chopping, applying fertilizers, windrowing, baling and stacking of the corn stover bales at the field edge. About 40% of the energy consumed is indirect, from the energy consumed during the production of inputs such as seeds, fertilizer, pesticides and machinery. In this study, fertilizers are the major material inputs because they are the inputs that are most affected by varying corn stover removal rate. Another category of emissions, which are significant in agricultural systems, are those from the field itself because of the different biogeochemical processes occurring in the field at all times, and are affected by environmental factors. Some of these emissions are produced as a result of interaction of the chemical fertilizers with the environment and conversion into gases such as nitrous oxide (N2O), ammonia

(NH3), mitrogen oxides (NOx) and carbon dioxide (CO2) (Klein et al., 2006). In addition, emissions are produced from the decomposition of crop residues, such as corn stover, which are retained in the field for maintaining soil health (Jin et al., 2014). Similarly, when corn stover is removed from the field, nutrients are removed and must be replenished by mineral fertilizers or animal manures (Johnson et al., 2010; Karlen et al., 2015). The direct and indirect emissions from the synthetic fertilizers account for 29% of the total emissions from agriculture (USDA,

2017). Removal of corn stover from the field will contribute to the emissions in various ways.

In order to estimate the environmental impact of biofuel production from corn stover, life cycle analysis of the corn stover production system has been done by several researchers (Canter et al.,

2016; Jenkins and Alles, 2011; Kim et al., 2009; Kim and Dale, 2005; Liu et al., 2018; Murphy and Kendall, 2015; Whitman et al., 2011). These studies take into account the different inputs

that are required for the production of corn grain and stover, and the emissions that occur in the field, in order to quantify the environmental impact of the production system (Jenkins and Alles,

2011; Kim et al., 2009; Liu et al., 2018; Murphy and Kendall, 2015; Wang et al., 2012; Whitman et al., 2011), including the environmental impact of transportation and conversion of corn stover to biofuel. Some of these studies (Kim et al., 2009; Liu et al., 2018; Wang et al., 2012; Whitman et al., 2011) have identified that direct in-field emissions contribute significantly to the total emissions during corn stover production and harvest. However, only one of these studies compares the environmental impact of different corn stover removal scenarios on cellulosic ethanol production (Whitman et al., 2011).

The environmental benefits that corn stover provides as a feedstock for biofuel production and the environmental impact due to its removal from the field should be quantified in order to make informed decisions about stover harvest for biofuel production. The main objective of this study is to quantify the greenhouse gas emissions for different corn stover removal scenarios, including removal of cobs, stover above ear and stover above midway between ground and ear level. Since corn stover left in the field produces significant amount of greenhouse gases, and the focus of this study is to quantify the greenhouse gas emissions for corn stover removal from the field, this study considers the in-field emissions (N2O, NH3, NOx) along with the emissions from machinery operations for stover harvest and collection (CO2, CH4, N2O), and direct and indirect emissions from supplemental fertilizer (CO2, N2O, NH3, NOx).

3.3 Methodology

3.3.1 Goal and scope

The goal of this study is to compare the greenhouse gas emissions for three different corn stover removal scenarios from the field: 1) just cobs, 2) stover above the ear level excluding cobs

(~35% of dry matter in cobs, stalks, and leaves), and 3) stover above half way between ear and ground level excluding cobs (~60% of dry matter) (Figure 3.1). The base case scenario is the conventional system, where no stover is removed from the field. The system boundary for the study includes the operations required from stover harvest to stacking of corn stover bales at the field edge, application of supplemental fertilizers and the in-field emissions caused by the corn stover retained in the field. This system boundary is chosen to estimate the greenhouse gas emissions for different corn stover removal scenarios for biofuel production. The main differences among the scenarios exist within the field, and are the same in the downstream processes where biomass is converted to biofuel. The downstream processing for conversion of corn stover to ethanol is the same once it leaves the field. The geographic scope of the study is the Midwestern U.S. because it is the major corn producing region and most of the ethanol plants are concentrated in this region (U.S. DOE, 2018). The functional unit considered for this study is one hectare of corn field.

Figure 3.1 System boundary for the study indicating the different scenarios

3.3.2 Modeling overview

A model was developed to estimate the greenhouse gas emissions for the base case scenario and the three corn stover removal scenarios. The general governing equation for the greenhouse gas emissions is presented in expression (1).

퐺푟푒푒푛ℎ표푢푠푒 𝑔푎푠 푒푚𝑖푠푠𝑖표푛푠 = (퐷1 + 퐷2 + 퐷3) + (퐼1 + 퐼2 + 퐼3) − (퐶1 + 퐶2) ------(1)

Where,

D1 = Emissions from fuel combustion during field operations

D2 = Emissions from corn stover decomposition

D3 = Emissions from fertilizer volatiles released in the field

I1 = Emissions from fuel manufacture

I2 = Emissions from machinery manufacture

I3 = Emissions due to fertilizer manufacture

C1 = Emissions avoided due to corn stover removal

C2 = Emissions avoided due to reduced residue management need

The general equation (1) can be applicable for estimating the greenhouse gas emissions associated with the base case and the three scenarios, as follows:

Base Case Scenario:

D3, I3, C1 and C2 are not applicable to this scenario.

Scenario 1:

D2 and C2 are not applicable to this scenario.

Scenario 2 and 3:

D2 is not applicable to both of these scenarios.

The main inputs to the model (Table 3.1) are obtained from Chapter 2 and literature review. The modeling approach used for this study is based on building sub-models for different processes, which include different operations required from stover harvest to stacking of bales at the field edge, the in-field emissions from corn stover left in the field, and the direct and indirect emissions from fertilizer supplemented due to corn stover removal. The emissions associated with the conventional practice (i.e., no stover removal) are used as a baseline, and the emissions estimated for the other scenarios are changes from the baseline estimates for the conventional practice. The emissions avoided due to corn stover removal in Scenarios 1, 2 and 3 and, by avoiding shredding operation in Scenarios 2 and 3, are classified as emissions credits and subtracted from the total footprint for the respective scenarios to which they are applicable.

Table 3.1 Input data for resource requirements and the greenhouse gas estimations and their distributions Parameters Units Average Std. Range References dev

Harvest index 0.56 0.05 Chapter 2 Grain yield t/ha 9.98 (USDA NASS, 2017)

Dry matter g/plant Chapter 2 Cobs 19.61 4.82 Stover above ear 41.28 8.01 Stover below ear 52.06 14.92 Nutrient content in stover % Chapter 2 fractions Nitrogen Cobs 1.05 0.51 Stover above ear 1.30 0.43 Stover below ear 1.88 0.37 Phosphorus 36

Parameters Units Average Std. Range References dev Cobs 0.06 0.04

Stover above ear 0.11 0.06 Stover below ear 0.06 0.03 Potassium

Cobs 0.68 0.20 Stover above ear 0.85 0.24 Stover below ear 2.05 0.55 Machinery (Yao, Z., LE Manguer, Shredder field efficiency % 80 75-85 1996) Shredder speed kmph 8 5-10 Fertilizer spreader field % 70 60-80 Fertilizerefficiency spreader speed kmph 11 8-16 Cob cart field efficiency % 70 65-80 Cob cart speed kmph 5 3-6.5 Windrower field efficiency % 80 70-85 Windrower speed kmph 8 5-13 Baler field efficiency % 75 60-85 Baler speed kmph 6.5 4-10 Stacker field efficiency % 95 (Shah and Darr, 2017) Stacker speed kmph 5 4-6 (Highline Manufacturing Limited, 2018) Emission factors kg/kg (Klein et al., 2006) N to N2O-N 0.01 0.003-0.03 N to NH3-N (stover) 0.2 0.05-0.5 N to NH3-N (fertilizer) 0.1 0.03-0.3 NH3-N to N2O-N 0.01 0.002-0.05 N to NO3-N 0.3 0.1-0.8 NO3-N to N2O-N 0.0075 0.0005-0.025 Urea to CO2-C 0.2 N2O-N to N2O 44/28 CO2-C to CO2 44/12

3.3.3 Inventory analysis

3.3.3.1 Corn grain and stover yield

The corn grain yield is fixed at 9.9 t/ha for each scenario as it was the average grain yield in

2016 for Ohio (USDA NASS, 2017). Stover yield is calculated based on the grain yield (dry basis) using an average harvest index of 0.56 (standard deviation: 0.05), which was obtained from the experimental data presented in Chapter 2. Nutrient and dry matter distributions of the stover fractions are also from Chapter 2. The dry matter distributions of the fractions were used to estimate the amount of corn stover removed and retained in the field for the three scenarios.

The dry matter and nutrient contents of the fractions are used to estimate the in-field emissions from corn stover for the three scenarios.

3.3.3.2 Machinery requirements

Previous studies (Pantoja et al., 2015; Sindelar et al., 2013) recommend that with partial stover removal reduced or no tillage should be implemented because with stover removal more of the soil is exposed to the wind and water, and additional tillage might make the soil more prone to erosion. No-till was the soil management practice used in this study. The common management practice for no-till corn production of chopping stover residue was used in this study (Butzen et al., 2018). Since Scenario 1 involves removal of cobs from the field, a cob cart was driven next to the combine for collecting cobs (Erickson et al., 2011; Maung and Gustafson, 2011). A cob cart is an addition to Scenario 1 compared to the base case scenario. Another operation required for Scenario 1 is fertilizer application. Due to the removal of cobs, mineral fertilizer needs to be

applied to replenish the nutrients removed with cobs. A tractor pulled fertilizer spreader was considered for this operation.

The processes required for Scenarios 2 and 3 include fertilizer application as in Scenario 1, windrowing of corn stover, and baling and stacking of corn stover bales at the field edge. The machineries considered for the operations include tractor pulled baler and stacker, and a self- propelled windrower. Since the windrower considered in this study also chops the residue, no chopper was considered for Scenarios 2 and 3, which involve stover removal. A tractor pulled baler and stacker are considered for baling of stover, and collection and stacking of stover bales at the field edge. The specifications of different machineries used for the different operations are summarized in Table 3.2.

Table 3.2 Specifications of the farm machineries used for corn stover collection upto stacking at the field edge Machinery Make Model Capacity Useful life Weight References (hours) (kg) Tractor John Deere 6155R 90 kW 16,000 7723 (TractorData, 2016) Shredder John Deere 115 4.6 m, 90 kW 1,200 1497 (John Deere, 2012) Fertilizer Eurospand 733R 5 m 1,200 (Eurospand, spreader 2018) Cob Cart Brent V700 112.5 kW 16,000 5152 (Brent, 2018)

Machinery Make Model Capacity Useful life Weight References (hours) (kg) Windrower John Deere 500R 5 m, 193 kW 4,000 7348 (John Deere, 2018a) Baler John Deere 328 108 kW 2,000 9210 (John Deere, 2018b) Stacker ProAG 12SR 93 kW 2,000 11241 (ProAG, 2018)

For the machineries used to perform the operations, greenhouse gas emissions associated with their manufacture, repair and maintenance, and operation were estimated. The emissions associated with the manufacture, assembly and maintenance of the machineries were estimated based on the weight of the equipment. The equipment is considered to be manufactured completely of steel as suggested by other studies (Hill et al., 2006; Shah and Darr, 2017). The weight of the machinery (kg) associated with each operation is calculated by multiplying the total weight of the machinery with the ratio of service life of the machinery (h) to field capacity

(ha/h), because it is assumed that machinery will be uniformly depreciated over its life span. The energy required for the production of 1 kg of steel is considered to be 25 MJ (Hill et al., 2006;

Shah and Darr, 2017). The amount of CO2 equivalent emissions associated with one MJ of energy used in the manufacture of farm machinery is considered to be 75.3 g-CO2e/MJ (Shah and Darr (2017)). The greenhouse gas emissions associated with the assembly of the equipment is considered to be 50% of that required for manufacture (Shah and Darr, 2017). Emissions associated with the repair and maintenance of the machinery is assumed to be 55% of that for manufacture and assembly (Shah and Darr, 2017).

The emissions due to the operations of the different machineries were estimated based on their fuel consumption. Different machinery parameters used for this estimation are field capacities and the power required for their operation. The power required for operation was calculated using the methodology provided in the ASAE (American Society of Agricultural Engineers) standard (Yao, Z., LE Manguer, 1996). Based on the total PTO power provided in the machinery brochure and the calculated power requirement, the fuel consumption was calculated using the procedure in the ASAE standard (Yao, Z., LE Manguer, 1996). The machinery parameters used in the calculations are listed in Table 3.1.

3.3.3.3 Fertilizer requirements

Because some of the recyclable nutrients were removed from the field with the removal of corn stover and cobs, the fertilizer requirement considered in this study was only the supplemental fertilizer used to replenish the nutrients removed with the corn stover and cobs from the field

(Canter et al., 2016; Wang et al., 2012; Whitman et al., 2011). The amount of supplemental fertilizer increases with the increase in corn stover removal rates. The amount of supplemental fertilizer to be applied was estimated based on the nutrient concentrations of the corn stover fractions, which was obtained from Chapter 2. The fertilizer emissions associated with the manufacture and distribution of nitrogen, phosphorus and potassium fertilizers to the field were taken from Kim and Dale (2003), and were 3,270 g-CO2/kg of nitrogen fertilizer consisting of

61% ammonia and 39% urea, 34 g-CO2/kg of phosphorus fertilizer, and 642 g-CO2/kg of potassium fertilizer.

3.3.3.4 In-field emissions

The in-field emissions from the supplemental fertilizer and corn stover left in the field were estimated as a sum of the direct and indirect emissions from the corn stover and nitrogen fertilizer as suggested in the IPCC (Intergovernmental Panel on Climate Change) guidelines for calculating greenhouse gas emissions from agricultural land (Klein et al., 2006). The direct emissions refer to the direct conversion of nitrogen, present in nitrogen fertilizer and corn stover, to N2O-N, and were calculated using the procedure in IPCC and the emission factors in Table

3.1. The indirect emissions refer to the production of N2O from indirect sources, such as NH3,

- NOx and NO3- as nitrogen first converts to NH3, NOx, and NO3 then to N2O during nitrification and denitrification. The indirect emissions were also estimated using the IPCC guideline and the emission factors given in Table 3.1. The amount of CO2 emissions from urea, which is present in the nitrogen fertilizer was calculated using the IPCC guideline and the emission factor of 0.2

(Klein et al., 2006). The in-field emissions savings estimated in Scenarios 1, 2 and 3 include the emissions that could have occurred from the respective amount of corn stover removed from the field in each of the scenarios.

3.3.4 Impact assessment

Greenhouse gas emissions (CO2, N2O and Methane (CH4)) were estimated for the three scenarios. CH4 and N2O have 25 and 298 times the 100 year global warming potential of CO2

(U.S. EPA, 2017b).These factors were used to convert CH4 and N2O into CO2 equivalent emissions.

3.3.5 Uncertainty analysis

Emissions from corn stover production has a wide range of uncertainties. The input parameters and their distributions are given in Table 3.1. The distribution of nutrients and dry matter were obtained from Chapter 2 and a normal distribution has been assumed. Field efficiency and speed for the different farm equipment used were obtained from machinery brochures and a triangular distribution was used to describe those parameters. Similarly, emissions factors were obtained from the IPCC standard (Klein et al., 2006) and were assumed to have a triangular distribution.

Thus, to analyze the impacts of distributions on different parameters, uncertainty analyses were conducted through Monte Carlo simulations (10,000 trials), and sensitivity analysis (at 10th and

90th percentiles for different inputs).

3.4 Results and discussion

3.4.1 Fertilizer, fuel and machinery requirements

The estimated resources required for different processes include fertilizer (kg/ha), fuel (L/ha) and the depreciated weights of the machineries (kg/ha), and are summarized in Table 3.3. The different types of fertilizers considered in this study are nitrogen, phosphorus and potassium fertilizers. The amount of all three types of fertilizers required for Scenarios 1, 2 and 3 increased with the increase in corn stover removal rate because the amount of nutrients removed was directly proportional to the amount of corn stover removed from the field. The supplemental amounts of nitrogen and potassium fertilizer was comparatively significantly higher than the phosphorus fertilizer because of the higher nitrogen and potassium concentration in corn stover compared to phosphorus.

Stacking and baling are the most fuel intensive operations within the system boundary of the study. The fuel requirement for shredding is the same for the base case scenario and Scenario 1 because the difference in the two scenarios is the removal of cobs, which contribute to only a small (~15%) fraction of the dry matter. Even with the removal of cobs in Scenario 1, a significant amount of stover is left in the field for shredding, which resulted in the same fuel requirement for this operation. No additional shredding was required for Scenarios 2 and 3 because shredding was done during the windrowing operation. The fuel required for windrowing in Scenarios 2 and 3 was the same because all the corn stover present in the field was considered to be shredded and windrowed. Since the corn stover yield was considered to be the same for all the scenarios, the fuel requirement for shredding was also the same. The average fuel requirement for baling and stacking increased with the increase in corn stover removal rates. The fuel requirement for fertilizer application was also the same in all scenarios because the difference in the amount of fertilizer in the three scenarios was minimal to affect the productivity of the fertilizer spreader. Fuel requirement for collection of cobs in Scenario 1was the lowest out of all the operations and did not apply to Scenarios 2 and 3.

The depreciated weights for the equipment used for different operations were calculated to be the same in the different scenarios because it was assumed that the field capacity (ha/h) for the same equipment did not change across different corn stover removal scenarios.

Table 3.3 Resource requirements for different corn stover collection scenarios

90% Central Range Category Units Base Case Scenario 1 Scenario 2 Scenario 3 Fertilizer kg/ha Nitrogen 2.0-31.2 12.7-71.7 24.7-88.4 Phosphorus 0-1.9 0.4-7.0 1.6-10.2 Potassium 4.1-21.3 11.2-21.3 30.9-95.7 Fuel L/ha Fertilizer spreader 3.2-4.2 3.2-4.2 3.2-4.2 Cob cart 2.2-4.5 Shredder 5.0-6.6 5.0-6.6 Windrower 3.8-6.1 3.8-6.0 Baler 8.0-12.4 8.6-13.4 Stacker 9.3-11.6 10.1-12.8 Depreciated weight kg/ha Fertilizer spreader 1.2-2.1 1.2-2.0 1.2-2.0 Cob cart 0.1-0.2 Shredder 0.2-0.4 0.2-0.4 Windrower 0.3-0.8 0.3-0.8 Baler 0.9-1.8 0.9-1.8 Stacker 2.1-2.7

3.4.2 Greenhouse gas emissions for different stover removal scenarios

The emissions associated with the base case scenario were estimated by modelling only those processes that would produce different amount of emissions when no corn stover removal takes place. The processes consist of residue management, i.e., residue shredding and in-field emissions from corn stover decomposition. With removal of corn stover from the field, the amount of residue to be shredded would decrease and the amount of corn residue left in the field would also decrease. The emissions associated with the different removal rates would be different. For the base case scenario (i.e., when no stover is removed from the field), the 90%

central range of emissions was 483-2356 kg-CO2e/ha (Figure 3.2). In-field emissions contributed to about 95% of the total emissions. This trend was consistent with other modeling studies (Kim et al., 2009; Whitman et al., 2011) that quantified emissions from corn production systems.

These estimates were lower than field measured data (Guzman et al., 2015; Jin et al., 2014), which were in the range 10,000-20,000 kg-CO2e/ha. These studies considered total emissions in the field throughout the year, consisting of emissions from fertilizers added for corn grain and emissions due to tillage, which were not applicable to this study. Thus, the emissions in these experimental studies were higher than the emissions in this study because this study only quantified emissions due to corn stover removal.

Figure 3.2 Emissions for the base case scenario of no stover removal from the field

Compared to the base case scenario, in Scenario 1, the total emissions increased by 94-554 kg-

CO2e/ha (90% central range) (Figure 3.3). The emissions embodied with fertilizer was 46

considerably high (26%) than the emissions due to field operations, and was consistent with other studies (Kim et al., 2009; Luo et al., 2009; Shah and Darr, 2017). In-field emissions from fertilizer were other major contributors (52%) to this increase in the total emissions. Emissions credits from the in-field emissions avoided because of removal of cobs was significant (16-311 kg-CO2e/ha) in this scenario.

Emissions in scenarios 2 and 3 increased by 362-1211 kg-CO2e/ha and 518-1480 kg-CO2e/ha

(90% central range), respectively, compared to the base case scenario (483-2356 kg-CO2/ha)

(Figure 3.3). Emissions embodied with the fertilizers (30%) and in-field emissions from fertilizer

(40% and 50% for Scenarios 2 and 3, respectively) were the significant contributors to the increase in emissions in Scenarios 2 and 3. All corn stover were considered to be shredded in the base case scenario and windrowed in Scenarios 2 and 3. Since only a fraction of the corn stover needs to be windrowed for Scenarios 2 and 3, the emissions produced for windrowing corn stover left in the field were considered emissions credits. Although not significant this results in

12-25 kg-CO2e/ha and 3-11 kg-CO2e/ha (90% central range) of emissions credits in Scenarios 2 and 3, respectively. Emissions credits are mostly from emissions avoided due to corn stover removal and were 109-734 kg-CO2e/ha and 171-913 kg-CO2e/ha (90% central range) for

Scenarios 2 and 3, respectively.

Figure 3.3 Emissions for different amounts of corn stover removal in addition to the base case

3.4.3 Sensitivity analysis

In the base case scenario, the net change in emissions was most sensitive to the rate of conversion of nitrogen present in the stover to N2O. The other sensitive parameters include harvest index, nitrogen content and dry weight of the stover fraction below ear, and the rate of conversion of NH3 to N2O (Figure 3.4a). This is reasonable because in-field emissions were the highest contributors to greenhouse gas emissions in the base case scenario (Figure 3.2), and the in-field emissions were highly dependent on the amount of nitrogen present in the stover left in the field and the conversion rate of this nitrogen to N2O.

In Scenario 1, the net change in emissions was most sensitive to the nitrogen content in cobs

(Figure 4b). The nitrogen content in cobs determined the amount of supplemental fertilizer required. Since the fertilizer-related emissions contribute to ~80% of the total emissions in this scenario, there is a reasonable directly proportional relationship between nitrogen content in cobs and net change in emissions. Harvest index was the second most sensitive parameter because the corn stover yield was based on the harvest index and the in-field emissions depend on the amount of corn stover left in the field. Since in-field emissions from corn stover were the major contributors in this scenario, the sensitivity of total emissions to harvest index is reasonable.

Similarly, dry weight of corn stover below and above ear, and the conversion rate of NH3

(volatilized from nitrogen present in the stover) to N2O are the other sensitive parameters in this scenario. The five most sensitive parameters directly affect the in-field emissions, strongly indicating that in-field emissions are the main source of emissions for cobs collection.

For Scenarios 2 and 3, the net changes in emissions were sensitive to harvest index, nitrogen content in the corn stover fraction above ear level, conversion rate of nitrogen to NO3, NO3 to

N2O, and NH3 to N2O (Figures 4c, 4d). Because the stover fraction above ear is removed in both these scenarios, and the nitrogen content in the stover fraction is directly proportional to net emissions, it indicates that emissions due to supplemental fertilizer application outweigh the emissions credits due to removal of this fraction. The sensitivity analysis for Scenario 3 also shows that the net changes in emissions were directly proportional to the conversion of NH3-N to

N2O-N, which suggests that fertilizer related emissions are more impactful in this scenario.

Conversion of N to NO3-N and NO3-N to N2O-N is inversely proportional to total emissions, suggesting that emissions credits due to corn stover removal are higher than the emissions associated with the supplemental fertilizer required for replenishing the removed nutrients. All of these factors contribute to either in-field emissions or emissions credits due to avoiding in-field emissions via stover removal. The sensitivity analysis of these scenarios also shows that the parameters that affect the in-field emissions and fertilizer requirements are the major contributors of emissions in the life cycle of biofuel production from corn stover.

Figure 3.4 Sensitivity analysis of emissions at different corn stover removal rates 51

3.4.4 Comparison of emissions in different stover removal scenarios to literature

The total greenhouse gas emissions per metric ton of corn stover for Scenarios 1, 2 and 3 are 83-

295, 108-324 and 120-335 kg-CO2e (90% central range), respectively. These values are close to the emissions reported in the literature for the system boundary, scenarios and the quantities of corn stover removal that closely match the ones in this study. Whitman et.al. (2011) estimated the emissions for a multiple pass corn stover harvesting system for the dry grain yield of 6.1 t/ha for removal of 15% (0.9 t/ha), 45% (2.7 t/ha) and 75% (4.5 t /ha) corn stover collection scenarios to be 101, 163 and 227 kg-CO2e/t, respectively. Another study (Murphy and Kendall, 2013) evaluated greenhouse gas emissions for removal of 38% stover (4 dry t/ha) with system boundary including embodied and in-field emissions from fertilizers, and stover harvest and collection operations. Their estimated greenhouse gas emissions were 88 kg-CO2e/t. There exists large uncertainties in the estimation of emissions because of differing allocation methods adopted in different studies, and in system boundary and uncertainties with in-field emissions that contribute significantly to the overall emissions.

3.5 Conclusions

Greenhouse gas emissions for removal of cobs, stover above ear level and stover above mid-way between ear and ground level were estimated using the in-field emissions and emissions due to different operations for harvest, collection and stacking of corn stover bales at the field edge and compared to the scenario with no stover removal. The different operations consisted of shredding of corn stover for the base case scenario and Scenario 1, fertilizer application, cobs collection for

Scenario 1 and, fertilizer application, windrowing, baling and stacking for Scenarios 2 and 3.

Average estimated emissions for the base case scenario were 1253 kg-CO2e/ha mainly from in- field emissions from corn stover. The average net increase in emissions for Scenario 1,2 and 3 were 150, 353 and 448 kg-CO2e/ha in addition to the emissions in the base case scenario. In-field emissions from fertilizer contribute significantly to the total emissions in Scenario 1 while emissions credits from stover removal are considerably high in Scenarios 2 and 3. This study suggests that net emissions for different corn stover removal rates are similar to each other and does not provide a clear indication on the amount of corn stover that should be removed from the field based on the environmental impact.

Chapter 4: Conclusions and Recommendations for Future Work

Through this research, important physical and compositional attributes of different corn stover fractions were determined. It was found that the height of the ear level from the ground was similar among corn plants from two different varieties in two different years, 2016 and 2017.

The dry matter contribution of the different stover fractions were determined, which was important when different corn stover removal scenarios were created for determining the environmental impact. It was found that cobs contribute the least to overall non-grain dry matter among the corn stover fractions analyzed in this study. Stover below ear and stover above ear including cobs each contribute to approximately 50% of the total available non-grain dry matter.

Some compositional attributes such as the glucose, lignin, nitrogen and phosphorus were similar between the different stover fractions. The different stover fractions did differ in the concentrations of other sugars and potassium. Cobs had the highest concentration of other sugars and the stover fractions below ear had the highest concentration of potassium compared to the other fractions analyzed in this research.

The dry matter contribution of the different stover fractions suggests that retention of the corn stover fractions below ear is a suitable strategy. This means that approximately half of the total available aboveground biomass in the field will be retained in the field, which is the recommended amount of corn stover for maintaining organic carbon for soil health and avoiding soil erosion, according to the literature. The higher concentration of potassium in the stover fraction below ear also suggests that retaining stover fraction below ear is a suitable harvesting 54

strategy, since this can help retain nutrient rich fractions in the field. Similarly, higher concentrations of other sugars in cobs make them a desirable feedstock for higher value bioproducts other than ethanol and thus provides another valid reason for removing the stover fraction above ear from the field. There were no statistically significant differences in the compositional attributes of the different fractions at the two different harvest dates suggesting that harvesting can be done at any time after the physiological maturity of the corn plant and will not affect the composition of corn stover for bio-refining or for sustainability purposes in the field.

The second half of the research consisted of the estimation of life cycle greenhouse gas emissions for different corn stover removal scenarios in order to determine the suitable amount of corn stover to remove from the field. The system boundary for the research consisted of corn stover harvest and collection up to stacking of corn stover bales at the field edge, and in-field emissions from corn stover and fertilizer in the field. Three corn stover removal scenarios were modelled to estimate the increase in greenhouse gas emissions compared to the base case scenario, i.e., no stover removal. The three scenarios modelled were 1) removal of cobs, 2) removal of corn stover above ear level excluding cobs and 3) removal of stover above half way between ear and ground excluding cobs. The functional unit for the analysis was 1 hectare of corn field. In all three scenarios, there was an increase in emissions due to corn stover removal compared to the base case scenario, although the net emissions between the different scenarios were similar to each other.

It can be concluded that as long as enough stover is retained in the field for the agronomic benefits that corn stover provides such as soil organic carbon maintenance and soil erosion prevention, the removal of corn stover should not limited by the environmental impact due to the field operations for corn stover harvest. It will be appropriate to remove the maximum amount of corn stover from the field that allows for soil organic carbon maintenance and soil erosion prevention.

The in-field emissions in this study were estimated using emission factors given in the IPCC standards. Estimation of in-field emissions based on a crop model, which would allow environmental factors such as temperature and precipitation to be included, would be an interesting extension of this study. Comparison of the results obtained from these two approaches would allow for evaluation of differences between the two methods and identify the most accurate method for estimation. The remaining processes in the cellulosic biofuel production pathway such as the storage, transport, conversion and distribution can be added to the model for estimating the overall environmental impact of cellulosic ethanol production from corn stover.

Since there are not many studies that quantify the overall environmental impact of cellulosic biofuel production at different corn stover removal rates, it would be an important addition to the existing literature.

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