Availability of corn stover as a sustainable feedstock for bioethanol production

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Abstract

The amount of corn stover that can be sustainably collected is estimated to be 80–100 million dry tonnes/yr (t/yr), a majority of which would be available to ethanol plants in the near term as only a small portion is currently used for other applications. Potential long-term demand for corn stover by non-fermentative applications in the United States is estimated to be about 20 million dry t/yr, assuming that corn stover-based products replace 50% of both hardwood pulp and wood-based particleboard, and that 50% of all furfural production is from corncobs. Hence, 60–80 million dry t/yr of corn stover should be available to fermentative routes. To achieve an ethanol production potential of 11 billion L (3 billion gal) per year (a target level for a non-niche feedstock), about 40% of the harvestable corn stover is needed. This amount should be available as long as the diversion of corn stover to non-ethanol fermentative products remains limited.

Introduction

A variety of biomass feedstocks can be used for near-term bioethanol production, e.g., mill wastes, urban wastes, agricultural residues, and forest residues. Among these resources, agricultural residues dominate in terms of tonnage and can serve as ethanol feedstocks. Particularly, this study discusses the suitability of corn stover as a non-niche feedstock and the attendant logistical issues.

Glassner and Hettenhaus (1997) considered various potential biomass feedstocks for bioethanol production. They assert that the most probable feedstocks for initial plants would be low-cost residues, by-products, and wastes from other processes such as

  • fine feed/gluten feed,

  • corn stover,

  • bagasse/sugar beet pulp,

  • wheat straw and rice straw,

  • pulp and paper mill residues,

  • sawdust,

  • forest thinnings,

  • municipal solid waste,

  • recycled paper that has been previously recycled.

Although specific available quantities were not given for these particular streams, Table 1 lists the availability data provided by Glassner et al. (1998) for selected feedstocks in million dry tonnes/yr (t/yr). Agricultural residues clearly dominate available biomass resources for near-term ethanol production as shown in Table 1; these are discussed below.

According to the USDA (2002), average total areas planted for major agriculture crops in the United States for 2002 are as follows in millions of ha (millions of acres): corn 31.9 (78.9), soybeans 29.6 (73.0), hay 26.2 (64.7), wheat 24.3 (60.1), cotton 5.8 (14.4), grain sorghum 3.8 (9.3), oats 2.1 (5.1), barley 2.0 (5.1), rice 1.3 (3.2), and rye 0.6 (1.4). As these data show, corn is the most widely planted crop, and as established later, corn stover is the most abundant agricultural residue.

While soybean is a major crop in terms of total acreage in production, residue generation is relatively modest. Moreover, soybean residue in the field can rapidly degrade, limiting its utility as a feedstock (Walsh et al., 2000). Wheat straw is also a potential ethanol feedstock based on planted wheat acreage. The estimated availability of wheat straw and its ethanol potential in different regions of the US are estimated in Table 2. The estimates in Table 2 were developed using data on average wheat production for 1997–2001 (USDA, 2002) and typical straw yields (Fiber Futures, 2000). The available straw was calculated assuming that 40% of the total can be sustainably harvested. About 33 million dry t/yr of wheat straw are available, which is equivalent to 9.6 billion L (2.5 billion gal) of ethanol per year assuming an ethanol yield of 292 L/t (70 gal/dry ton).

Based on sustainable harvest estimates of 0.8 t straw/t grain (University of Maryland, 1999), 44.7 million dry t/yr of wheat straw can be produced. This amounts to about 50% of the total in Table 2. Another rule of thumb for sustainable harvest is 2.25 t straw/ha (1 ton straw/acre), which is based on 4.5 t straw/ha (4000 lb/acre) of total straw and 50% ground cover to protect soil from erosion (Smith, 1996; Agri-Food Canada, 1999). For the year 2001, wheat was harvested on 19.7 million ha (48.7 million acres) of farmland (USDA, 2002). Using the above rule of thumb, 44 million dry t/yr of wheat straw, amounting to 54% of the Table 2 total, would be available. Although these higher harvest estimates increase the total domestic ethanol potential of wheat straw above 11 billion L/yr (3 billion gal/yr), a target level defining a non-niche feedstock, the combined ethanol potential for Midwest and North/Central West regions, in which wheat straw is concentrated, is still well below this target.

The theoretical ethanol yield is about 480 L/dry t (115 gal/dry ton) using an average corn stover composition (unpublished NREL data) and assuming that both hexose and pentose sugars are fermented. For 80 million dry t/yr of available corn stover (a conservative estimate, about half of the corn stover indicated in Table 1), this is equivalent to about 19 billion L (4.8 billion gal) of ethanol per year (240 L/dry t or about 50% of theoretical ethanol yield). Hence, although other residues such as wheat straw and soybean stubble are potential candidates, corn stover is arguably the feedstock of choice for large volume ethanol production as it represents 80% of the total agricultural residue and is concentrated in the Midwestern region of the United States.

Section snippets

Estimating corn stover availability

The various estimates for corn stover availability are given in Table 3. Estimates vary widely depending on assumptions about what fraction of agricultural residues can be sustainably collected. Some residue must be left in the field to protect the soil against water and wind erosion. The amount of stover left on the ground is dependent on many factors including: tillage practice, topography (especially the slope and extent of sloped land), soil type, and crop rotation.

Glassner et al. (1999)

Potential corn stover uses

Although about 80–100 million dry t/yr of corn stover are available based on the above recent estimates, the competing uses for corn stover would lower the amount accessible for ethanol production. In this context, the potential diversion of corn stover to competing applications is discussed next. The possible corn stover uses are listed below.

Corn stover usage for competing applications

Currently, <5% of the stover is physically collected for off-field use; this is similar to rice straw off-field usage in California (Kadam et al., 2000). Although no quantitative data are available for all the possible corn stover applications listed above, pulp and paper, fiberboard, and chemicals are the only applications for which corn stover may be needed in any substantial quantity. The estimates for their demand for corn stover need to be developed since when collected in an organized and

Corn stover availability for ethanol production

Making some assumptions about market penetration for the various products, the corn stover demand by non-fermentative routes can be estimated. The corn stover demand for strawboard production is calculated as 4.5 million dry t/yr for 50% particleboard production from stover. Satisfying half of the US furfural demand is equivalent to about 1 million t/yr of corncobs. A potential demand for corn stover is estimated at 17.5 million dry t/yr, if it were to totally replace 50% of hardwood pulp.

Limitations of the study

This study focuses on the current situation and uses a generalized model for stover production and removal. The amount of stover that would be available is dependent on market dynamics; as stover becomes a valuable coproduct, a track record of yearly sales would stimulate stover production. For example, improved crop economics could change the acreage planted and may justify higher plant populations. Also, narrow rows may become more popular and hybrids or cultivars would be selected based on

Conclusion

About 60–80 million dry t/yr of stover can be potentially available for ethanol production. However, with the development of inexpensive cellulase enzymes, the corn stover-derived sugars, at least partially, could be more economically utilized by other fermentative products. Although the impact of this diversion cannot be predicted at the present, at an ethanol yield of 330 L/dry t (80 gal/dry ton), only 33 million dry t/yr of corn stover are needed to achieve a capacity of 11 billion L (3

Acknowledgements

This work was funded by the Office of Biomass Programs of the US Department of Energy.

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