Ten fundamentals about bioenergy: Part 8
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Fundamental #8: Bioenergy feedstock crops have a wide range of energy efficiency ratings.
This is the eighth in a series of articles on bioenergy. The previous
installment focused on the need to have a broad range of bioenergy crops
available to best fit local growing conditions, markets, biorefinery
needs and environmental constraints. This article will focus on the
inherent variability in the energy efficiency of various bioenergy
feedstock crops. Biofuels have the potential to be cost competitive with
fossil fuels. One metric used to compare the efficiency of a particular
bioenergy feedstock crop is based on the amount of energy the crop
produces in its final fuel form compared to the cost of the feedstock.
This metric is expressed as dollars per giga-Joule of energy produced ($
per GJ). For reference, a GJ is equivalent to 0.948 million Btu.
Recently, scientists (Lynd et al., 2008) estimated the cost of liquid transportation fuels derived from cellulosic feedstocks to be at $3.00 per GJ compared to $8.70 for gasoline. Additionally, the authors project costs of $13.80 and $6.60 per GJ for soy biodiesel and corn grain ethanol respectively. Determining the best feedstock for biofuel production is confounded by several issues. A wide disparity between reported economic and energy input costs exists in the literature. My own experience in working with farmers the past 20 years has shown a tremendous diversity in management between farms and even between years for the same farm. Decisions on which tillage operations to perform, which and how much fertilizer, pesticide, and seed traits to use are often based on a myriad of temporal economic, climatic, and environmental factors and interactions. Biological systems, including farming, are naturally fraught with variability. Therefore, significant variability in the cost per GJ of bioenergy produced will continue to be observed. However, some generalizations can be made. When comparing enterprise cropping budgets for input costs and energy requirements, several items consistently rank near the top. These include nitrogen fertilizer, seed, and field machinery operations. Therefore, cropping systems that minimize these primary input cost items while maintaining yield will generally result in being more efficient on a cost per GJ of bioenergy produced basis. For example, perennial grass crops such as switchgrass have the potential for lower cost per GJ produced by virtue of their perennial life cycle (lower planting costs since a stand will last about 10 years) and lower nitrogen fertilizer costs (perennials will translocate some nutrients to root system in fall) compared to an annual grass crop such as corn. On soils responsive to reduced tillage, switching to no-till management can improve economic and energy returns with annual crops such as corn. Furthermore, the efficiency of annual crop systems can be improved by double-cropping a winter annual biomass crop such as winter cereal rye, with corn or soybean.
Lynd et al. 2008. How biotech can transform biofuels. Nature Biotechnology 26:169-172.
Figure 1 shows the estimated $ per GJ cost for several bioenergy crop feedstocks grown under Michigan conditions. To date, oilseed feedstocks for biodiesel such as soybean have tended to be less competitive than starch feedstock for ethanol when evaluated on an energy metric such as $ per GJ. This is primarily due to competition for oilseed feedstock from food markets which drives the price for the feedstock up. Additionally, the value of a co-product is not generally factored into an energy-based metric such as $ per GJ. Because soybean is considered more of a protein crop than a bioenergy crop, it tends to compare less favorably with other crops when measured on its energy value alone without regard to the value of the soymeal protein co-products.