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Thermodynamics of the Corn-Ethanol Biofuel Cycle

Tad W. Patzek

Department of Civil and Environmental Engineering 425 Davis Hall

University of California, Berkeley, CA 94720 Email: patzek@patzek.berkeley.edu

This Web Version is being periodically updated

New: Appendix D on fuel cells, consistent use of fuel HHVs, corrected theoretical yield of ethanol from starch, equivalent CO2 emissions and CExC adjusted to Patzek’s, not Shapouri et al.’s inputs, added eroded soil humus oxidation

Increased ethanol yield to ∼2.5 gal/wet bushel, 91.5% of theoretical yield Appendix E on free energy consumed to produce machinery


1.1 Corn Highlights2
1.2 Energy Inputs to Corn Production3
1.3 Layout4

1 Introduction 1

I Mass & Energy Balance 5 1 Introduction 5 2 Mass Balance of Corn 5

3.1 Field Chemicals7
3.1.1 Specific Energy Requirements for Nitrogen Fertilizer8
3.1.2 Other Energy Inputs to Fertilizer Production10
3.1.3 Specific Energy Requirements for Phosphorus Fertilizers10
3.1.4 Specific Energy Requirements for Potassium Fertilizers1
3.1.5 Specific Energy Requirements for Calcinated Lime1
3.1.6 Specific Energy Requirements for Herbicides and Insecticides12
3.2 Specific Energy Requirements for Fossil Fuels12
3.3 Use of Electricity14
3.4 Averages Can Be Misleading15
3.5 Energy in Human Labor15
3.6 Energy in Corn Seeds16
3.7 Energy in Irrigation16
3.8 Energy in Transportation17
3.8.1 Personal Commute18
3.9 Machinery & Infrastructure18
3.10 Fossil Energy Inputs into Corn Production19
3.1 Solar Energy Input into Corn Production19
3.12 Soil Humus and Micro-Element Depletion by Corn Production20

3 Major Energy Inputs to Corn Production 7

4.1 Corn Mass Balance Revisited2
4.2 Transport in Ethanol Refineries24
4.3 Fossil Energy Inputs to Ethanol24
4.4 Energy Credits25
4.5 Overall Energy Balance of the Corn-Ethanol Process25

4 Major Energy Inputs to Ethanol Production 2

I Sustainability & Renewability 28 1 Introduction 28 2 Disclaimer 28 i Thermodynamics of corn-ethanol biofuel... Web Version

3 Preliminaries 29 4 Laws of Thermodynamics 29 5 Thermodynamics and Economics 31 6 Economic Activity 32 7 Agriculture 34 8 Industrial Production 35 9 Waste 36

10.1 The Earth is an Open System to Heat Flow38
10.2 Conclusions39

10 Sustainability 36

I Sustainability of Corn-Ethanol Cycle 41 1 Introduction 41

2.1 Introduction to Exergy41
2.2 Change of Bth between Two States42
2.3 An Industrial Flow Process42
2.4 Cumulative Exergy Consumption (CExC)4

2 Available Free Energy 41 3 The Ideal and Real Corn-Ethanol Cycle 4 4 System Boundary 46

5.1 Net CO2 Emissions47
5.2 Conclusions49

5 The Carbon Cycle 46 6 Water Cycle 50

7.1 Chemistry of the CO2-Glucose-EtOH Cycle52
7.1.1 The Maximum Cycle Output per Unit Mass of Corn53

7 Exergy Analysis of the Ideal Corn Ethanol Cycle 52 8 Exergy Analysis of the Modified Ideal Corn-Ethanol Cycle 53

9 Resource Consumption and Waste Generation in the Industrial Corn-Ethanol Cycle 54

9.1 Cleanup of BOD in Ethanol Plant Wastewater5
9.2 Cleanup of Contaminated Field Runoff Water56

10 Conclusions 58

IV Other Problems with Corn-Ethanol 60 1 Introduction 60 2 First-Law View of Corn-Ethanol Production in 2004 60 3 Second-Law View of Corn-Ethanol Production in 2004 61 4 Public Health Problems 63

V Summary & Conclusions 64 A Examples of Entropy Production and Disposal 69 B Availability and Irreversibility in Thermal Systems 72 C Is Economic Sustainability Possible? 75 D Efficiency of a Fuel Cell System 7

E.1 Steel component manufacturing79

E Cumulative Exergy Consumption in Steel Production 79

VI Tables 87

1 Corn kernel composition87
2 Application rates of field chemicals87
3 Specific energies and application rates of nitrogen fertilizer87
4 Energy consumption in superphosphate production8
5 Specific energy and application rate of phosphorus fertilizers8
6 Energy consumption in potassium fertilizer production8
7 Specific energy and application rates of potassium fertilizer8
8 Specific energy and application rates of calcinated lime89
9 Specific energy and application rates of herbicides89
10 Specific energy and application rates of insecticides89
1 Average high and low heating values of fuels90
12 Calorific values and specific volumes of gasoline90
13 Calorific values and specific volumes of diesel fuel91
14 Calorific values and specific volumes of LPG fuel91
15 Calorific values and specific volumes of methane91
16 Electricity use in corn farming92
17 Energy used in transportation related to corn farming93
19 Specific CO2 emissions93
20 Chemical exergies of compounds in the ideal corn-ethanol cycle94
21 Product exergies after each step of the ideal corn-ethanol cycle94
2 Product exergies after each step of the ideal corn-ethanol-hydrogen cycle94
23 CExC of major non-renewable resources used in the industrial corn-ethanol cycle95
24 First Law summary of the U.S. corn-ethanol production in 200495
25 Second Law summary of the U.S. corn-ethanol production in 200496
26 Primary energy consumption in steel production from ore96
27 Primary energy consumption in steel production from scrap96
28 Estimates of primary energy embedded in steel and its constituents97
29 Estimates of CExC in steelmaking97

iv Thermodynamics of corn-ethanol biofuel... Web Version

1 Starch molecule6
2 Mean ethanol yield7
3 History of energy efficiency of ammonia production8
4 Seven largest ammonia plants in the U.S9
5 Various estimates of the unit energy consumption to produce ammonium nitrate10
6 The overall fertilizer application rates12
7 The overall application rates of herbicides and insecticides13
8 The overall fossil fuel volumes used in corn farming14
9 By-state and average use of methane in corn farming14
10 By-state and average use of electricity in corn farming15
1 Energy use in labor16
12 Specific energy use in transport related to corn farming17
13 Major fossil energy inputs into corn farming18
14 Solar energy dominates all other energy inputs to corn farming20
15 soil nutrient losses with corn grain and stover removal21
16 Mass balance of dry corn and efficiency of ethanol production23
17 Average wet and dry corn yields23
18 Average fossil energy inputs to ethanol production in a wet milling plant24
19 The overall energy balance of ethanol production26
20 Fossil energy gain/loss in corn ethanol production27
21 Net energy yield in corn production27
2 The boundary, process, and environment29
23 The Second Law efficiency of copper production32
24 The 2001 per capita energy consumption in the world3
25 The 1990 per capita water consumption in the world3
26 The 1999 per capita carbon emissions in the world34
27 August 14, 2003, power blackout in New York35
28 A linear process in industry36
29 A linear process in industrial agriculture37
30 Energy and mass flow in an ecosystem38
31 Thermodynamic cycles38
32 Exergy balance in an isothermal, ideal flow machine42

List of Figures 3 Exergy balance in ideal and real nonisothermal industrial process . . . . . . . . . . . 43

34 The ideal corn-ethanol cycle4
35 Average U.S. corn yield from 1866 to 193945
36 The industrial corn-ethanol cycle46
37 The carbon cycle47
38 Specific CO2 emissions in the industrial corn-ethanol cycle48
39 Total CO2 emissions in the industrial corn-ethanol cycle49
40 The water cycle50
41 Exergy flow the ideal CO2-Glucose-EtOH cycle52
42 Exergy flow the ideal CO2-Glucose-EtOH-H2 cycle54

CRPS, 23(6), 2004 T. W. PATZEK v 43 Some of the useful work from the industrial corn-ethanol cycle is diverted to “undo”

mining of the environment by this cycle5
4 CExC by the industrial corn-ethanol cycle56
and its maximum useful work57
46 Societal costs of ethanol production in the U.S61

45 The minimum cumulative exergy consumption by the industrial corn-ethanol cycle 47 The cumulative one-hour exceedances of maximum legal ozone level in Southern 48 Electricity obtained from solar cells and ethanol powered fuel cells . . . . . . . . . . 76


In this paper I define sustainability, sustainable cyclic processes, and quantify the degree of non-renewability of a major biofuel: ethanol produced from industrially-grown corn.

First, I demonstrate that more fossil energy is used to produce ethanol from corn than the ethanol’s calorific value. Analysis of the carbon cycle shows that all leftovers from ethanol production must be returned back to the fields to limit the irreversible mining of soil humus. Thus, production of ethanol from whole plants is unsustainable. In 2004, ethanol production from corn will generate 1 million tonnes of incremental CO2, over and above the amount of

CO2 generated by burning gasoline with 115% of the calorific value of this ethanol. Second, I calculate the cumulative exergy (available free energy) consumed in corn farming and ethanol production, and estimate the minimum amount of work necessary to restore the key non-renewable resources consumed by the industrial corn-ethanol cycle. This amount of work is compared with the maximum useful work obtained from the industrial corn-ethanol cycle. It appears that if the corn ethanol exergy is used to power a car engine, the minimum restoration work is about 7 times the maximum useful work from the cycle. This ratio drops down to 2.4, if an ideal (but nonexistent) fuel cell is used to process the ethanol.

Third, I estimate the U.S. taxpayer subsidies of the industrial corn-ethanol cycle at $3.3 billion in 2004. The parallel subsidies by the environment are estimated at $1.9 billion in 2004. The latter estimate will increase manifold when the restoration costs of aquifers, streams and rivers, and the Gulf of Mexico are also included.

Finally, I estimate that (per year and unit area) the inefficient solar cells produce ∼100 times more electricity than corn ethanol. We need to rely more on sunlight, the only source of renewable energy on the earth.

KEY WORDS: biofuel, ethanol, fossil fuels, corn, sustainability, thermodynamics, energy, entropy, exergy, solar

Nullis in verba (Take nobody’s word) The motto of the Royal Society of London, 1662

1 Introduction

In the Preface to What is Life? – one of the great science classics of all times – Erwin Schrodinger (1967) observed: “A scientist is supposed to have a complete and thorough knowledge, at first hand, of some subjects and, therefore, is usually expected not to write on any topic of which he is not a master. This is regarded as a matter of noblesse oblige.”

triesThe IPCC’s2 biomass intensive future energy supply scenario includes 385 million hectares

The principle of non-interference with the far-away fields of science often precludes the scientists from seeking to explain the universal aspects of science, which are of paramount importance to the society at large. For example, the sophisticated technological models of biofuel production, e.g., Hemelinck (2004), cannot be formulated alone, without welding them first to a detailed analysis of the possibilities of depleting the environment in the long-term and destroying the valuable ecosystems1. This example is not merely of academic interest. A United Nations Bioenergy Primer (Kartha and Larson, 2000) states: “In the most biomass-intensive scenario, [modernized] biomass energy contributes...by 2050...about one half of total energy demand in developing counof biomass energy plantations globally in 2050 (equivalent to about one quarter of current planted agricultural area), with three quarters of this area established in developing countries.” The magic

1For a definition, see Footnote 3. 2Intergovernmental Panel on Climate Change.

2 Thermodynamics of corn-ethanol biofuel... Web Version word “sustainable” appears 130 times in this Primer, without ever being defined3. What will happen if the developing countries entrust their fragile ecosystems and societies to a fundamentally flawed, unsustainable energy supply scheme? What if the distributed generation of solar power is a significantly better alternative to biofuels?

So here I renounce the noblesse and embark on a synthesis of facts and theories related to the production of a common biofuel, ethanol from corn, albeit with second-hand knowledge of some of these facts – and at a risk of making a fool of myself. I hope that some or most of this paper will be read by the concerned farmers, engineers, environmentalists, and policymakers. In particular I wish to reach the fellow scientists, who – for most part – remain blissfully unaware of the astronomical real problems with supplying energy to over 6 billion people, but who often vigorously analyze the peripheral issues (which in addition are tackled in isolation and out of context).

Most traditional biofuels, such as ethanol from corn, wheat, or sugar beets, and biodiesel from oil seeds, are produced from classic agricultural food crops that require high-quality agricultural land for growth. A significant portion of the sunlight these crops capture is diverted to produce seeds and store sugar, and their growing seasons are short. The net energy yield of corn4, ∼100-130 GJ/ha-crop (Part I of this paper), is significantly lower that those5 of perennial crops and grasses (200-300 GJ/ha-crop), and sugarcane (∼400 GJ/ha-crop) (Rogner, 2000). Also, the environmental costs of annual crops are very high: they cause more soil erosion (up to 100-fold), require 7-10 times more pesticides, and more fertilizers than perennial grasses or wood (Berndes et al., 2003). Finally, industrial manufacturing of hybrid seeds is very energy-intensive.

In this paper, I will describe in some detail the unfavorable thermodynamics of the industrial production of ethanol from one particular food crop, corn. I will use the Second Law of thermodynamics to track what is happening to us (or, is it U.S.?) as mere years pass, and the precious resources the sun and the earth have been making and storing for millions of years are being squandered in front of our eyes.

1.1 Corn Highlights

The U.S. is the single largest corn producer in the world. Large overproduction of subsidized cheap corn forces corn producers and processors to invent new ingenious uses for their product6. In terms of their large negative impact on the society and the environment, two corn products – ethanol and high-fructose syrup – stand out (Pollan, 2002; Elliott et al., 2002). About 13% of the U.S. corn production is now diverted to produce ethanol. Hence, in this paper I will de facto argue that the U.S. corn production should be reduced by at least 13% with significant benefits to the taxpayers and the planet. A telegraphic description of the U.S. corn farming and processing is as follows:

• Corn is the single largest U.S. crop (a record 300 million tonnes of moist corn grain in 2004).

• Corn is harvested from ∼30 million hectares, roughly the area of Poland or Arizona, and a bit less than 1/4 of all harvested cropland in the U.S.

• The recent average yield7 of moist corn grain has been ∼8600 kg/ha (and a record 10100 kg/ha in 2004).

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