This paper analyzes energy efficiency of the industrial corn-ethanol cycle. In particular, it critically evaluates earlier publications by DOE, USDA, and UC Berkeley Energy Resources Group. It is demonstrated that most of the current First Law net-energy models of the industrial corn-ethanol cycle are based on nonphysical assumptions and should be viewed with caution. In particular, these models do not (i) define the system boundaries, (ii) conserve mass, and (iii) conserve energy. The energy cost of producing and refining carbon fuels in real time, for example, corn and ethanol, is high relative to that of fossil fuels deposited and concentrated over geological time. Proper mass and energy balances of corn fields and ethanol refineries that account for the photosynthetic energy, part of the environment restoration work, and the coproduct energy have been formulated. These balances show that energetically production of ethanol from corn is 2–4 times less favorable than production of gasoline from petroleum. From thermodynamics it also follows that ecological damage wrought by industrial biofuel production must be severe. With the DDGS coproduct energy credit, 3.9 gallons of ethanol displace on average the energy in 1 gallon of gasoline. Without the DDGS energy credit, this average number is 6.2 gallons of ethanol. Equivalent CO2 emissions from corn ethanol are some 50% higher than those from gasoline, and become 100% higher if methane emissions from cows fed with DDGS are accounted for. From the mass balance of soil it follows that ethanol coproducts should be returned to the fields.
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Notes
Ethanol distillation is the single biggest expenditure of fossil energy in the corn-ethanol cycle, see Section Energy Balance of Ethanol Refinery.
With an unspecified moisture content.
The actual energy bills paid by ethanol refineries in South Dakota point to about 40,000 Btu and 1.95 kWh per gallon of ethanol. Source: The South Dakota Public Utilities Commission, private comm., 7 April 2006.
Note that the imprecise, ill-defined units used by the U.S. ethanol industry offer some leeway. For example, in a report on the Gopher dry mill plant in St. Paul, MN, (Anonymous, 2003), the mean ethanol yield is 2.5 gal/bu, but 1 bushel weighs 68.5 lbm, not 56 lbm, as it is supposed to (see page 4).
Consistently with my analysis, I will use Sheehan and others (1998) estimate of primary energy spent on crude oil production (domestic and foreign), transport, refining, and finished diesel fuel transport. This estimate is listed in their table 3, page 11. The Fossil Energy Ratio = 1 MJ Fuel Energy/1.1995 MJ of Fossil (Primary) Energy Input = 0.8337. The Fossil Energy ratio of crude oil refining is 0.065. figure 2, on page 13, rescales the components of table 1 to MJ/MJ of Fuel. In the rescaled units, the energy spent on refining is 0.12 of the energy in diesel fuel.
Estimating Ethanol Logistics Cost and Energy Use. David Hirshfeld, pers. comm. MathPro, Washington, D.C., 27 Jan. 2006.
Additional field area for hybrid corn breeding also should be included.
For definitions and details, see my Fall 2005 CE24 Lecture, http://petroleum.berkeley.edu/patzek/ce24/-Fall2005/Materials/PlantEfficiency.pdf
Farrell and others (2006a) write this about (Patzek, 2004; Pimentel and Patzek, 2005): “[T]wo studies also stand apart from the others by incorrectly assuming that ethanol coproducts (materials inevitably generated when ethanol is made, such as dried distiller grains with solubles, corn gluten feed, and corn oil) should not be credited with any of the input (sic!) energy and by including some input data that are old and unrepresentative of current processes, or so poorly documented that their quality cannot be evaluated (tables S2 and S3).”
It is difficult to grasp the scale of the ecosystem restoration problem. For example, in 1997, 20,500 gigatonnes/yr of N were injected into the U.S. environment from distributed human sources; one third was exported (Howarth and others 2002). More than 60 percent of our coastal rivers and bays in every coastal state of the continental United States are moderately to severely degraded by nutrient pollution. This degradation is particularly severe in the mid Atlantic states, in the southeast, and in the Gulf of Mexico (Howarth, 2000) Therefore, ground and surface water restoration will add to the Restoration Work Inputs another term that may be significantly larger than 1.
This energy cost was 2 times higher (Patzek, 2004).
The same argument applies with force to all large-scale schemes to cellulosic ethanol production (Patzek and Pimentel, 2006).
Emissions of other gases, mostly nitrous oxide N2O, ammonia NH3, and methane CH4, are converted to equivalent CO2 emissions using their relative potencies in creating the greenhouse effect (Anonymous, 2002).
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ACKNOWLEDGMENTS
My work on biofuels receives zero funding. I have received, however, heart-felt support from hundreds of people of different walks of life: students at Berkeley, Iowa State, and many other campuses, farmers, engineers, community activists, environmentalists, lawyers, scientists, academic teachers, business people, consultants, writers, journalists, and so on. I cannot overstate how much their encouragement has meant to me and I thank them all. In particular, I would like to thank my wife, Joanna, for putting up with my being absent almost every evening and weekend for five weeks. I want to thank Katie Schwartz, a graduate student in my group, for programming in pstricks the beautiful schematics of ethanol plant and pyruvate reaction pathways. Many people have provided important information; their personal communications are acknowledged in the footnotes, and I thank them here again. In particular, I would like to thank Dmitriy Silin and another person, who wishes to remain anonymous, for their meticulous reviews of the paper manuscript, and significant improvements to its content and style. Clayton Radke made me explain the crucial mass and energy balances so much better. David Andress has provided an insightful written critique of the manuscript, most of which has been incorporated. In addition, Kamyar Enshayan, David Hirshfeld, Richard Muller, David Pimentel, and Marvin Scott, and Nathan Hagens, Harry Blazer, and Jeff Webster reviewed the manuscript and provided valuable feedback. Finally, I am grateful to my cellular biologist son Lucas, currently with the Harvard Medical School, and my freshly minted biochemist daughter Sophie, for carefully editing the manuscript and providing the final touches.
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Patzek, T.W. A First-Law Thermodynamic Analysis of the Corn-Ethanol Cycle. Nat Resour Res 15, 255–270 (2006). https://doi.org/10.1007/s11053-007-9026-9
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DOI: https://doi.org/10.1007/s11053-007-9026-9