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Net energy of cellulosic ethanol from switchgrass
- *U.S. Department of Agriculture–Agricultural Research Service, University of Nebraska, 314 Biochemistry Hall, P.O. Box 830737, Lincoln, NE 68583-0737; and
- ‡Agricultural Economics Department, University of Nebraska, 314A Filley Hall, Lincoln, NE 68583-0922
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Edited by Pamela A. Matson, Stanford University, Stanford, CA, and approved November 21, 2007 (received for review May 21, 2007)
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Fig. 1.Switchgrass field locations managed for bioenergy (filled circle) and human-made prairie plots (+) with average annual precipitation zones for 2000–2005 (13).
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Fig. 2.Switchgrass agricultural inputs (GJ·ha−1) from the establishment year (Estab.) and postplanting harvest years (Post.) in a multilocation farm trial using known farm inputs. Agricultural inputs used were the embodied energy of switchgrass seed, fertilizer, herbicide, diesel, and other energy (farm machinery, farm labor, product transportation, electricity, and product packaging). Results are compared with agricultural input data from switchgrass energy balance studies (8, 10, 14) based on small plot data and input estimates.
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Fig. 3.Energy estimates for 10 switchgrass fields managed for bioenergy for the establishment year (filled circle) and second (open circle), third (yellow square), fourth (open square), and fifth years (red triangle), using input and biomass production data from 10 farms in the EBAMM model (9). (a) Comparison of net energy values (MJ·liter−1) from the fields based on known agricultural inputs with estimates from two simulated switchgrass studies (8, 10). NEV are not shown for one study (14), because they were negative for switchgrass at all ethanol yields due to the misassumption that nonrenewable energy will be used for all biorefinery energy needs. (b) PER, which is the biofuel output (MJ) divided by the petroleum (MJ) requirements for the agricultural, biorefinery, and distribution phases, for the 10 fields compared with three simulated studies (8, 10, 14). Blue line, Wang (10); green line, Farrell et al. (8); and red line, Pimental and Patzek (14).
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Fig. 4.Comparison of estimated ethanol yield and NEY from switchgrass fields managed as a bioenergy crop; low-input, high-diversity, human-made prairies (LIHD) on small plots (19); low-input switchgrass (LI-SW) small plots (19); and corn grain yields (ref. 20; 2000–2005) from Nebraska and South and North Dakota). (a) Mean ethanol yield (liter·ha−1) was greater for the three farms with low mean ethanol yields, mean ethanol yields of all farms, and three farms with high mean ethanol yields (≥2 yr after seeding) or established switchgrass plots (≥9 yr after seeding) grown in a higher precipitation zone and was comparable to corn grain ethanol yields for the three states. Conversion of corn grain and cellulosic biomass to ethanol was estimated at 0.4 liter·kg−1 and 0.38 liter·kg−1, respectively (9). (b) NEY from established switchgrass fields for all farms was consistently higher than human-made prairies or low-input switchgrass (19) grown in a higher precipitation zone.
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Fig. 5.Estimated displacement (%) of GHG emissions by replacing conventional gasoline (baseline) with cellulosic ethanol derived from switchgrass. Minimum (grey), mean (blue), and maximum (green) percent GHG displacement for each switchgrass harvest year is based on actual production data from 10 switchgrass fields. Estimated GHG values include the amount of CO2 sequestered in the soil (100 yr) by switchgrass, which was estimated to be 138.1 kg of CO2 Mg−1 of aboveground biomass yr−1 (28).
Footnotes
- †To whom correspondence should be addressed. E-mail: ken.vogel{at}ars.usda.gov
- © 2008 by The National Academy of Sciences of the USA
