Natural Resources Research

, Volume 15, Issue 4, pp 255–270 | Cite as

A First-Law Thermodynamic Analysis of the Corn-Ethanol Cycle

Article

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.

KEY WORDS:

Net energy biorefinery efficiency coproduct emissions environment cost 

Notes

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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Copyright information

© Springer Science+Business Media, LLC 2007

Authors and Affiliations

  1. 1.Department of Civil and Environmental EngineeringUniversity of CaliforniaBerkeleyUSA

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