A First-Law Thermodynamic Analysis of the Corn-Ethanol Cycle
Tad W. Patzek1,2
Received 14 June 2006; accepted 10 August 2006 Published online: 22 February 2007
This paper analyzes energy efﬁciency of the industrial corn-ethanol cycle. In particular, it critically evaluates earlierpublications 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 carbonfuels 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 ofethanol 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.
INTRODUCTION This paper analyzesenergy efﬁciency of the industrial corn-ethanol cycle. In particular, it critically reviews the report by Farrell and others (2006a). This report is based on an Excel spreadsheet with cells containing numbers from three peer-reviewed papers (Patzek, 2004; Pimentel and Patzek, 2005; de Oliveira, Vaughan, and Rykiel, 2005) and four3 gray-literature reports (Wang, 2001; Graboski, 2002;
1 Department ofCivil and Environmental Engineering, University
of California, 425 Davis Hall, MC 1716, Berkeley, CA 94720. whom correspondence should be addressed; e-mail: patzek@ patzek.berkeley.edu. 3 The terse, 5-page report (Shapouri and McAloon, 2004) is incomplete and, e.g., corn ethanol yield must be inferred from (Shapouri, Dufﬁeld, and Wang, 2002).
Shapouri, Dufﬁeld, and Wang, 2002; Shapouriand McAloon, 2004). The authors cite the only relevant peer-reviewed paper by Shapouri, Dufﬁeld, and Wang (2003) merely to rationalize their Footnote (6) that disposes of the caloriﬁc value of corn grain. The report’s supporting online material (Farrell and others, 2006c) is a Users’ Manual that explains the spreadsheet assumptions and contents. The authors then perform certain arithmeticoperations on the numbers they have stored in the spreadsheet, from which they draw their conclusions. In order to arrive at their conclusions about corn ethanol, the authors selected the following path: C1. Corn Grain Has No Energy. Caloriﬁc value of corn grain is omitted and not subtracted as a raw energy input to ethanol reﬁneries. 255
2007 International Association forMathematical Geology
Figure 1. Schematic of dry grind corn ethanol plant. Note that after starch hydrolysis (liquefaction) to glucose, solid part of corn mash and oil could be separated physically from glucose liquor, and go straight to centrifuge and drying.
C2. Ethanol Yield Is High. Average yield of corn ethanol is increased by more than possible. C3. Co-Products...