Life cycle inventory development for corn and stover production systems under different allocation methods
Introduction
Biofuels may offer an opportunity to reduce consumption of fossil fuels and greenhouse gas (GHG) emissions from the transportation sector, but they also have the potential for negative environmental and economic impacts. Production of biomass for biofuel feedstock represents a significant fraction of total environmental and energy impacts for biofuel production.
Agricultural residues may be a source of biomass for energy that avoids many of the unintended consequences of purpose-grown energy crops [1]. Corn stover (the above-ground remainder of the corn plant after grain harvest) appears to be a promising source of biomass for biofuel production, since it does not directly compete with food and is grown in large quantities during corn production. The U.S. produces well over 300 million tonnes of corn year−1 [2], and stover is produced at approximately a 1:1 ratio to corn on a dry-mass basis. Recent research has highlighted opportunities to use corn stover as a feedstock for chemical processes, including both biofuel and biopolymer production [3], [4].
In the current corn production system, stover is rarely harvested and utilized, but when harvested, it becomes a co-product of the corn production system. In life cycle assessments (LCAs) of cultivation systems that produce multiple products, or co-products, by rotating crops or processing multiple parts of a plant for different uses, the whole system's impacts are typically allocated among co-products [5]. This need for allocation is not limited to cropping systems or agricultural products; in LCAs of any system that generates multiple products or services, some method for attributing environmental impacts to each co-product is required.
In this study the term allocation is used to describe all processes and methods that might be used to divide or otherwise attribute environmental impacts to a single co-product. This usage is broader than typical because, strictly speaking, allocation refers to division of the total environmental impacts between products in proportion to some formula. This definition would not include other common methods for treating co-products in LCA, such as subdivision and system expansion, which are often considered approaches that avoid allocation. Nevertheless, allocation is used here to refer to all methods for treating co-products.
Not surprisingly, allocation methodology can significantly affect the life cycle environmental impacts attributed to stover-derived products [6]. This paper describes three different approaches to allocating impacts from the corn and stover production system to stover alone, and produces life cycle inventories (LCIs) for the production of corn stover. The results of applying the three approaches are compared to evaluate their effect on the performance of corn stover as a biomass resource. This comparison helps characterize the variability introduced by the selection of a particular method for co-product treatment in LCA. In addition, this study creates LCIs for corn grain and stover production under different allocation approaches, which is available in the online Supplementary material.
Section snippets
Co-product methods in LCA
Allocation of environmental impacts to valorized waste or residual streams has been recognized as a challenge by a wide range of researchers [7]. The International Standards Organization (ISO) promulgated the most widely acknowledged guidelines for LCA, the ISO 14040 and 14044 standards, which dictate a preference for avoiding allocation by subdivision or system expansion when assessing systems that produce co-products [8], [9]. If allocation is required, because subdivision and system
Scope and system definition
The system boundary for this study is limited to the agricultural system for producing corn and stover, in essence on-farm activity and the production of fuels and fertilizer. The functional unit of analysis is 1 ha of corn and stover production. This study assesses the production of stover from U.S. corn cultivation by modeling an average field based on national statistics. All of the activity described in this section is considered the baseline value for corn cultivation in the U.S. and is
Results and discussion
One hectare of corn and stover production requires approximately 23 GJ of fossil energy inputs and results in the emissions of approximately 2.5 tonnes of carbon dioxide equivalent (CO2e, 100-year IPCC basis) (Table 3). Fertilizer production effects generally dominated total GHG and fossil energy impacts. GHG emissions from fossil energy production and combustion represent about 19% of total emissions from stover, in the subdivision case and about 17% for grain alone.
For most indicators,
Conclusion
Co-product allocation is often required in biofuel or bio-based production systems. The corn and stover production system was modeled and three different methods were evaluated: economic allocation, energy-based allocation and subdivision, which only allocated impacts of changes in the system from baseline (corn-only) production to stover. Economic allocation and subdivision assigned similar burdens to stover, both around half that of energy-based allocation.
As applied here, using a
Acknowledgments
The authors would like to thank Matthew MacLachlan for his contribution to the discussion on long-term market behavior and Dr. Bryan Jenkins for his assistance in characterizing agricultural production systems and as a reviewer of a late draft of the paper. This project was supported by a grant from the U.S. Department of Energy.
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