Thermodynamics of the Corn - Ethanol, Notas de estudo de Engenharia Química

Baixe Thermodynamics of the Corn - Ethanol e outras Notas de estudo em PDF para Engenharia Química, somente na Docsity! 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 Critical Reviews in Plant Sciences, 23(6):519-567 (2004) 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 July 22, 2006 CRPS, 23(6), 2004 T. W. PATZEK iii 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 77 E Cumulative Exergy Consumption in Steel Production 79 E.1 Steel component manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 VI Tables 87 List of Tables 1 Corn kernel composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 2 Application rates of field chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 3 Specific energies and application rates of nitrogen fertilizer . . . . . . . . . . . . . . . 87 4 Energy consumption in superphosphate production . . . . . . . . . . . . . . . . . . . 88 5 Specific energy and application rate of phosphorus fertilizers . . . . . . . . . . . . . . 88 6 Energy consumption in potassium fertilizer production . . . . . . . . . . . . . . . . . 88 7 Specific energy and application rates of potassium fertilizer . . . . . . . . . . . . . . 88 8 Specific energy and application rates of calcinated lime . . . . . . . . . . . . . . . . . 89 9 Specific energy and application rates of herbicides . . . . . . . . . . . . . . . . . . . 89 10 Specific energy and application rates of insecticides . . . . . . . . . . . . . . . . . . . 89 11 Average high and low heating values of fuels . . . . . . . . . . . . . . . . . . . . . . . 90 12 Calorific values and specific volumes of gasoline . . . . . . . . . . . . . . . . . . . . . 90 13 Calorific values and specific volumes of diesel fuel . . . . . . . . . . . . . . . . . . . . 91 14 Calorific values and specific volumes of LPG fuel . . . . . . . . . . . . . . . . . . . . 91 15 Calorific values and specific volumes of methane . . . . . . . . . . . . . . . . . . . . 91 16 Electricity use in corn farming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 17 Energy used in transportation related to corn farming . . . . . . . . . . . . . . . . . 93 18 Corn kernel composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 iv Thermodynamics of corn-ethanol biofuel. . . Web Version 19 Specific CO2 emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 20 Chemical exergies of compounds in the ideal corn-ethanol cycle . . . . . . . . . . . . 94 21 Product exergies after each step of the ideal corn-ethanol cycle . . . . . . . . . . . . 94 22 Product exergies after each step of the ideal corn-ethanol-hydrogen cycle . . . . . . . 94 23 CExC of major non-renewable resources used in the industrial corn-ethanol cycle . . 95 24 First Law summary of the U.S. corn-ethanol production in 2004 . . . . . . . . . . . . 95 25 Second Law summary of the U.S. corn-ethanol production in 2004 . . . . . . . . . . 96 26 Primary energy consumption in steel production from ore . . . . . . . . . . . . . . . 96 27 Primary energy consumption in steel production from scrap . . . . . . . . . . . . . . 96 28 Estimates of primary energy embedded in steel and its constituents . . . . . . . . . . 97 29 Estimates of CExC in steelmaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 List of Figures 1 Starch molecule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 Mean ethanol yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 History of energy efficiency of ammonia production . . . . . . . . . . . . . . . . . . . 8 4 Seven largest ammonia plants in the U.S. . . . . . . . . . . . . . . . . . . . . . . . . 9 5 Various estimates of the unit energy consumption to produce ammonium nitrate . . 10 6 The overall fertilizer application rates . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7 The overall application rates of herbicides and insecticides . . . . . . . . . . . . . . . 13 8 The overall fossil fuel volumes used in corn farming . . . . . . . . . . . . . . . . . . . 14 9 By-state and average use of methane in corn farming . . . . . . . . . . . . . . . . . . 14 10 By-state and average use of electricity in corn farming . . . . . . . . . . . . . . . . . 15 11 Energy use in labor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 12 Specific energy use in transport related to corn farming . . . . . . . . . . . . . . . . 17 13 Major fossil energy inputs into corn farming . . . . . . . . . . . . . . . . . . . . . . . 18 14 Solar energy dominates all other energy inputs to corn farming . . . . . . . . . . . . 20 15 soil nutrient losses with corn grain and stover removal . . . . . . . . . . . . . . . . . 21 16 Mass balance of dry corn and efficiency of ethanol production . . . . . . . . . . . . . 23 17 Average wet and dry corn yields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 18 Average fossil energy inputs to ethanol production in a wet milling plant . . . . . . 24 19 The overall energy balance of ethanol production . . . . . . . . . . . . . . . . . . . . 26 20 Fossil energy gain/loss in corn ethanol production . . . . . . . . . . . . . . . . . . . 27 21 Net energy yield in corn production . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 22 The boundary, process, and environment . . . . . . . . . . . . . . . . . . . . . . . . . 29 23 The Second Law efficiency of copper production . . . . . . . . . . . . . . . . . . . . . 32 24 The 2001 per capita energy consumption in the world . . . . . . . . . . . . . . . . . 33 25 The 1990 per capita water consumption in the world . . . . . . . . . . . . . . . . . . 33 26 The 1999 per capita carbon emissions in the world . . . . . . . . . . . . . . . . . . . 34 27 August 14, 2003, power blackout in New York . . . . . . . . . . . . . . . . . . . . . . 35 28 A linear process in industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 29 A linear process in industrial agriculture . . . . . . . . . . . . . . . . . . . . . . . . . 37 30 Energy and mass flow in an ecosystem . . . . . . . . . . . . . . . . . . . . . . . . . . 38 31 Thermodynamic cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 32 Exergy balance in an isothermal, ideal flow machine . . . . . . . . . . . . . . . . . . 42 33 Exergy balance in ideal and real nonisothermal industrial process . . . . . . . . . . . 43 CRPS, 23(6), 2004 T. W. PATZEK v 34 The ideal corn-ethanol cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 35 Average U.S. corn yield from 1866 to 1939 . . . . . . . . . . . . . . . . . . . . . . . . 45 36 The industrial corn-ethanol cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 37 The carbon cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 38 Specific CO2 emissions in the industrial corn-ethanol cycle . . . . . . . . . . . . . . . 48 39 Total CO2 emissions in the industrial corn-ethanol cycle . . . . . . . . . . . . . . . . 49 40 The water cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 41 Exergy flow the ideal CO2-Glucose-EtOH cycle . . . . . . . . . . . . . . . . . . . . . 52 42 Exergy flow the ideal CO2-Glucose-EtOH-H2 cycle . . . . . . . . . . . . . . . . . . . 54 43 Some of the useful work from the industrial corn-ethanol cycle is diverted to “undo” mining of the environment by this cycle . . . . . . . . . . . . . . . . . . . . . . . . . 55 44 CExC by the industrial corn-ethanol cycle. . . . . . . . . . . . . . . . . . . . . . . . 56 45 The minimum cumulative exergy consumption by the industrial corn-ethanol cycle and its maximum useful work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 46 Societal costs of ethanol production in the U.S. . . . . . . . . . . . . . . . . . . . . . 61 47 The cumulative one-hour exceedances of maximum legal ozone level in Southern California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 48 Electricity obtained from solar cells and ethanol powered fuel cells . . . . . . . . . . 76 2 Thermodynamics of corn-ethanol biofuel. . . Web Version word “sustainable” appears 130 times in this Primer, without ever being defined3. What will hap- pen 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 thermo- dynamics 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). 3The endlessly repeated harvest of whole plants that grow on the same soil would have to be sustainable. 4The energy of dry corn grain minus the fossil energy inputs per hectare and per crop. 5The reported net energy yields of perennial grasses, sugarcane, etc., seem somewhat high to me. 6“Ethanol production makes huge amounts of the nation’s corn disappear – some 1.4 billion bushels went into ethanol production in 2004 – and that affects overall corn supply and helps shore up corn prices nationwide.” National Corn Growers Association, http://www.ncga.com/ethanol/main/economics.htm, accessed July 2, 2005. 7Source: USDA NASS database: www.usda.gov/nass/ CRPS, 23(6), 2004 T. W. PATZEK 3 • 42% of world’s 708 million tonnes of moist corn grain8 in 2004 was produced in U.S. • All of the U.S. corn fields are fertilized. • Corn requires more fertilizer than any other major crop; 40% of all nitrogen fertilizer goes to corn (Frink et al., 1999). • Corn erodes soil much faster than it can rejuvenate by natural processes. • Corn needs ∼100 cm water, 15% of corn is irrigated. • Between 1995 and 2003, USDA distributed $37.4 billions, or ∼$2 – $7 billions per year, in corn crop subsidies. Recipients of payments made through most cooperatives, and the amounts, have not been made public9. • From 1995 to 2003, the top 10 percent of corn subsidy recipients were paid 68 percent of all corn subsidies. The mean payments were $465 172 each for the top first percent, and $176 415 each for the top tenth percent of recipients. The bottom 80% of farmers received mean payments of $4763 each. • Over 12 billion liters of corn ethanol was produced in the U.S. in 2004. • U.S. goal: Produce 20 billion liters of ethanol from corn annually. • Ethanol producers receive ∼$3 billion annually from the federal government and state gov- ernments, and extract ∼$2 billion from the environment. 1.2 Energy Inputs to Corn Production Fossil energy is essential to industrial agriculture. The following are the major energy inputs to industrial corn farming: • Nitrogen fertilizers (all fossil energy) • Phosphate, potash, and lime (mostly fossil energy) • Herbicides and insecticides (all fossil energy) • Fossil fuels: diesel, gasoline, liquified petroleum gas (LPG), and natural gas (NG) • Electricity (almost all fossil energy) • Transportation (all fossil energy) • Corn seeds and irrigation (mostly fossil energy) • Infrastructure (mostly fossil energy) • Labor (mostly fossil energy) Corn produced at a large expense of fossil energy is then transformed, with even more fossil energy, into pure ethanol. 8Source: 2004 world production of corn: www.cbot.com/cbot/pub/static/files/c − wprod.gif. 9Source: Farm Subsidy Database, http://www.ewg.org/farm, accessed July 2, 2005 4 Thermodynamics of corn-ethanol biofuel. . . Web Version 1.3 Layout This paper is divided into five parts, each of which can be read more-less independently. In Part I, I discuss the mass balance of corn processing, and the energy and mass balances of corn farming and ethanol production. Any First Law analysis of the corn-ethanol production process is fundamentally incomplete, and gives rise to confusion and arguments, which become moot once a more complete Second Law analysis is performed. Therefore, in Part II, I overview the fundamentals of thermodynamics, define the linear processes and cycles, irreversibility and sustainability, as well as exergy (the free energy available relative to the environmental conditions). In Part III, I apply the concepts developed in Part II to the industrial corn-ethanol cycle and answer the following questions: 1. Is ethanol production from corn a sustainable process? 2. If it is not sustainable, how unsustainable is it? 3. Can process changes result in making ethanol production from corn sustainable? In particular, in Part III, I discuss the Carbon Cycle, the Water Cycle, the Ideal and Indus- trial Corn-Ethanol Cycles, and calculate the minimum work required to restore the nonrenewable resources consumed to produce corn ethanol. In Part IV, I estimate the various subsidies lavished on the transnational agribusiness corporations by the U.S. federal and local governments, and the huge subsidy extracted by these corporations from the U.S. environment: the rural population, soil, groundwater, rivers, the Gulf of Mexico, air, plants, and wildlife. Part V lists all major conclusions from this work. Through my analysis, I hope to put to rest the sweeping statements made by some scientists, such as the following (Deluga et al., 2004): Fast and efficient fuel reforming is one of the critical steps in producing H2 for fuel cells and the “hydrogen economy,” and ethanol is now the most available and economically renewable fuel.. . . . . . Recent studies indicate that the energy in the fuel-ethanol is at least 1.34 times the energy used in its production. CRPS, 23(6), 2004 T. W. PATZEK 7 2000 2001 2002 2003 2004 2005 2.4 2.5 2.6 2.7 2.8 E tO H y ie ld , g al lo n/ w et b us he l Industry−reported yields Subtract Brazil imports Subtract 5% gasoline Mean = 2.48 gal/bu USDA estimate Figure 2: The mean of industry-reported ethanol yields. Source: (Patzek, 2006), Section 3.3. 3 Major Energy Inputs to Corn Production Most energy inputs will be expressed in MJ/kg of active ingredient in the input. For example, ammonia contains 82% nitrogen (active component); therefore, the specific energy input in MJ to obtain one kg of ammonia will be divided by 0.82. 3.1 Field Chemicals • Nitrogen is a component of many important structural, genetic and metabolic compounds in plant cells. It is a major component of chlorophyll, amino acids, cell energy carriers (ATP/ADP), and genetic material (DNA/RNA). • Phosphorus is one of the primary structural components of cell membranes. It is involved in the photosynthesis (ADP/ATP), synthesis of proteins and vitamins, and it occurs in im- portant enzymes. • Potassium activates enzymes that produce proteins and sugars. It maintains water content and, hence, the turgor (rigidity) of plant cells. • Calcinated lime is used to increase the pH of soil acidified by nitrogen fertilizer. The ideal pH for corn is 6.6. • Herbicides, such as Atrazine, Acetochlor, S-Metolachlor, Dicamba, Nicosulfuron, etc. are used to protect corn from weeds. • Insecticides, such as Chlorpyrifos, Terbufos, Carbofuran, Tefluthrin, etc. are used to protect corn from insects. The average application rates of major field chemicals used in corn farming are reported in Table 2. 8 Thermodynamics of corn-ethanol biofuel. . . Web Version 3.1.1 Specific Energy Requirements for Nitrogen Fertilizer 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 30 35 40 45 50 55 Year M J / k g N in N H 3 Figure 3: History of energy efficiency of ammonia production in MJ/kg N. Source: G. Kongshaug (1998). Nitrogen fertilizers are derived from ammonia, nitric acid, and carbon dioxide. Practically all ammonia is produced from natural gas and nitrogen in the Haber-Bosch process (Worrell et al., 1994; Kongshaug, 1998; Worrell et al., 2000). The energy efficiency of the Haber-Bosch process has been improved by 1/3 over the last 60 years, see Figure 3. Therefore, the age of the ammonia- producing plant does matter. Ernst Worrell et al. (2000) have compiled the ages and outputs of the 44 largest U.S. ammonia plants, see Figure 4. Most of these plants were built in the 1960’s, and some were later modernized and expanded. The fact is that the major U.S. plants were built 40 years ago, and some were revamped 20-30 years ago. Another example comes from Europe: In 1995, ammonia synthesis in modern European plants consumed approx. 36.93 MJ/kg N, while older plants needed about 43.08 MJ/kg N (Biermann et al., 1999). Remark 3 For nitrogen fertilizer production, I will use the average efficiency of 30-year old plants. I will also assume that all nitrogen fertilizer applied to the U.S. corn fields is represented by ammonium nitrate. 2 Kongshaug (1998) analyzed energy efficiency of ammonia production and divided ammonia plants into three classes: “Modern,” “Average European plants,” and ”30-years old plants.” Using his terminology, the major nitrogen fertilizers are produced with the following specific energy inputs per unit mass of nitrogen in them. Ammonia, NH3, has 82% of nitrogen by mass. Following Kongshaug (1998), I will assume the following net energy consumption to produce ammonia: 30 Years Old 47 MJ/kg N Average Europe 39 MJ/kg N Modern 34.5 MJ/kg N (2) CRPS, 23(6), 2004 T. W. PATZEK 9 0 0.5 1 1.5 2 1966?/1971 1965? 1966? 1975/1986 1966?/1977 1968/1977 1966 Mt of Ammonia/Year Figure 4: Together, these seven largest plants produce 40% of the U.S. ammonia. The first dates refer to plant opening. Some of the plants were later expanded and revamped, as indicated by the second dates. Source: Ernst Worrell et al., (2000). The November 1981 process description for Haldor Topsøe plants published in Hydrocarbon Pro- cessing (p. 129) gives 35.6 MJ/kg N as the total energy requirement for ammonia production. According to Prof. Vaclav Smil (1985), total energy expanded on ammonia production in the U.S. is 55 MJ/kg N for the largest new plants, and up to 65 MJ/kg N for small pre-1969 units, the weighted average for all plants being 58 MJ/kg N (see p. 163-165). The European and Asian plants are more energy-efficient. Urea, CO(NH2)2, has 45% of nitrogen by mass, and is obtained from ammonia and carbon dioxide: 2NH3+CO2 → CO(NH2)2+H2O. The net energy consumption (Kongshaug, 1998) is: 30 Years Old Ammonia + 10 = 57 MJ/kg N Average Europe Ammonia + 9 = 48 MJ/kg N Modern Ammonia + 7.2 = 42 MJ/kg N (3) Smil’s (1985) estimate (p. 164) of the U.S. urea production costs is 70 MJ/kg N. Ammonium Nitrate, NH4NO3, has 35% nitrogen by mass, and is produced from nitric acid and ammonia: HNO3+NH3 → NH4NO3. Nitric acid is obtained by burning ammonia over catalyst to produce NOx. One of the by-products of ammonium nitrate production is nitrous oxide N2O, a potent greenhouse gas. With 97% conversion of ammonia to AN, the energy consumption is 30 Years Old Ammonia + 4 = 51 MJ/kg N Average Europe Ammonia + 2 = 41 MJ/kg N Modern Ammonia + 0.43 = 35 MJ/kg N (4) Smil’s (1985) estimate (p. 164) of the U.S. ammonium nitrate production costs is 72-90 MJ/kg N. 12 Thermodynamics of corn-ethanol biofuel. . . Web Version Lime application rate is not commonly reported by the USDA. The suggested application rate is 1.8 times the application rate of nitrogen (Tisdale et al., 1985), but there are reports of several times higher application rates, e.g., (Pimentel, 2003). The total application rates of nitrogen, phosphorus and potash fertilizers, as well as that of calcinated lime are shown in Figure 6. 0 100 200 300 400 500 600 700 800 900 1000 Wang et al., 1997 Shapouri et al., 2002 Berthiaume et al., 2001 Patzek, 2004 Pimentel, 2003 N P K Ca kg/ha Figure 6: The total fertilizer application rates listed in Tables 3, 5, 7, and 8. 3.1.6 Specific Energy Requirements for Herbicides and Insecticides There are many active ingredients in commercial herbicides and insecticides, but all have very high specific energy requirements for their production. Here I have used the data collected in Table 5 in Shapouri et al.’s report (2002a). The specific energy inputs and application rates of herbicides and insecticides are listed in Tables 9 and 10. The overall application rates of herbicides and insecticides are also shown in Figure 7. 3.2 Specific Energy Requirements for Fossil Fuels A unit mass of a fossil fuel gives out a specific amount of heat (its calorific value) when burned. The Low Heating Value (LHV), or Net Calorific Value (NCV), of a fossil fuel assumes that combustion products contain the water of combustion as vapor. The heat contained in this water is not recovered. Outside of power stations and fuel cells, water remains as vapor after combustion. The High Heating Value (HHV), or Gross Calorific Value (GCV), assumes that combustion water is entirely condensed. The heat contained in this water is recovered. Pimentel (2003), Shapouri et al. (2002a) seem to use HHV for all fuels. Wang et al. (1997) give only the total amount of energy for each fossil fuel; therefore, their specific values are back-calculated for each fuel and agree with my estimates. CRPS, 23(6), 2004 T. W. PATZEK 13 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Berthiaume et al., 2001 Pimentel, 2003 Wang et al., 1997 Patzek, 2004 Shapouri et al., 2002 Herbicides Insecticides kg/ha Figure 7: The total herbicide and insecticide application rates listed in Tables 9 and 10. In prior work, I used the low heating values of fuels in combustion engines to perform the First Law energy balance calculations (Patzek, 2004; Patzek et al., 2005). More recently, however, I was swayed by Bossel (2003b) to use the high heating values. The reason is simple: Regardless of a machine (an internal combustion engine or a fuel cell) we use to burn a fuel, the fuel’s full energetic potential could be realized if we improved this machine. In other words, in determining sustainability, we check what possibly could be done, not what actually is done. Remark 4 (Use of High Heating Values) From now on, the high heating values of all fuels will be used to determine whether a technological process is sustainable according to the First and Second Law of thermodynamics. 2 After deciding on a high heating value for each fossil fuel, one needs to find the standard values for “average” gasoline, diesel fuel, LPG, and natural gas. Finding consensus was more difficult than I expected. The International Energy Agency’s (IEA) standards up to the year 2000 are published in (IEA, 2000). IEA changed its standards for gasoline equivalent and diesel fuel equivalent in 2003. Finally, I decided to use the high heating values measured and compiled by Professor Dietram Castorph et al. at the Technical University of Munich (Castorph et al., 1999), see Table 11. For reference, the standard densities of liquid fuels used in this work are: gasoline, 0.74 kg/L; diesel fuel, 0.84 kg/L; LPG, 0.58 kg/L; natural gas, 0.84 kg/sm3, and ethanol 0.79 kg/L. The calorific values and average volumes of fossil fuels used in corn farming are listed in Tables 12 – 15. The cumulative volumes of all fossil fuels are shown in Figure 8. Notice that not all sources account for all five fossil fuels, especially for LPG and natural gas. Both LPG and natural gas are used for corn drying and as fuel to power water pumps in irrigation. Their uses vary greatly from one state to another, and from one season to another, see Figure 9. 14 Thermodynamics of corn-ethanol biofuel. . . Web Version 0 50 100 150 200 250 300 350 Wang et al., 1997 Pimentel, 2003 Patzek, 2004 Shapouri et al., 2002 Berthiaume et al., 2001 Diesel Gasoline LPG Liters/ha Figure 8: The total fossil fuel volumes listed in Tables 12 - 15. 0 5 10 15 20 25 30 35 40 Pimentel, 2003 Berthiaume et al., 2001 Shapouri et al., 2002 Patzek, 2004 Wang et al., 1997 MN OH SD IA IL MI WI IN NE Standard m3/ha Figure 9: By-state and average use of methane in corn farming. The 1996 methane volume data from the largest corn-producing states are from Shapouri et al. (2002a). Note the large variability of methane use depending on wet/dry weather. 3.3 Use of Electricity The average electric energy spent on farming 1 hectare of corn is listed in Table 16. Shapouri et al. (2002a) have attempted to include the efficiency of fossil energy conversion into electricity in CRPS, 23(6), 2004 T. W. PATZEK 17 0 200 400 600 800 1000 1200 Berthiaume et al., 2001 Shapouri et al., 2002 Wang et al., 1997 Patzek, 2004 Pimentel, 2003 MJ/ha Figure 12: Specific energy use in transport related to corn farming. Note that Pimentel’s estimate may contain an additional single commute to and from the field. • Only 15% of crop is irrigated, USDA-NASS, 1997, (Pimentel, 2003). • On average 8.1 cm of water is used per acre, USDA-NASS, 1997, (Pimentel, 2003). • Water is pumped on average from the depth of 100 m. • Pump efficiency, etc., is 0.75. Then the specific energy requirement for irrigation is 1 m 100 cm × 10,000 m2 ha × 1000 kg m3 × 100 m × 9.81 m s2 /0.75 = 131 MJ/cm-ha (7) I have lowered Pimentel’s 2003 estimate of irrigation energy to my estimate. Both Shapouri et al., and Wang et al. have buried the irrigation energy in their use of electricity and methane, so they account for the irrigation indirectly, for the particular mixture of states and weather they considered. Since I use Shapouri et al.’s estimates for the fossil fuels, I have not included the irrigation energy in my calculations. 3.8 Energy in Transportation The specific energy use in transportation related to corn farming has been estimated by Wang et al. (Wang et al., 1997), see Table 17. The total energy use is about 720 kJ/kg of field chemicals (fertilizers, lime, fuel, etc.) transported into the field (∼400 MJ/ha-crop), plus personal commutes, see Figure 12. This estimate is sensitive to the number of commutes to and from the field by personnel using motor vehicles. 18 Thermodynamics of corn-ethanol biofuel. . . Web Version 3.8.1 Personal Commute At 6.2 hr/ha/crop of labor, 20 l/100km gasoline use, and a 30 km round trip, the energy cost of commuting is 6.2 9 hr/ha/crop hr work day × 60 km × 20 liter 100km × 1 100 × 0.74 kg liter gasoline × 47 MJ kg gasoline = 288 MJ/ha-crop (8) Therefore a single commute nearly doubles the overall transportation energy costs. This issue should be investigated further. 3.9 Machinery & Infrastructure Industrial agriculture requires heavy machinery (trucks, tractors, ploughs, cranes, railroad cars, airplanes, locomotives, barges, ships, etc.), which must be replaced periodically. Industrial agri- culture also requires an extensive infrastructure with a large environmental footprint (spare parts, machine shops, machine manufacturing factories, access roads, railroad tracks, ports, silos, pumps, driers, electricity generators, air-conditioners, etc.). Industrial corn is the single largest crop in the U.S., and its share of this infrastructure should be highest. The energy inputs as hardware have been estimated in Appendix E at 68-168 MJ/kg of operational machinery, close to the 110 MJ/kg estimated by Pimentel (2003). The mass of hardware assigned by Pimentel to corn farming is 55 kg/ha. Note that this estimate includes only a tiny part of the huge infrastructure listed above. Both Shapouri et al. (2002a) and Wang et al. (1997) omit this input altogether, but I have not amended their calculations. 0 5 10 15 20 25 30 35 Berthiaume et al., 2001 Wang et al., 1997 Shapouri et al., 2002 Patzek, 2004 Pimentel, 2003 GJ/ha Fossil Fuels Irrigation Nitrogen P−K−Ca Herbi−/Insecticides Electricity Transportation Repairs/Maintenance Seeds Machinery Figure 13: Major fossil energy inputs into corn farming. CRPS, 23(6), 2004 T. W. PATZEK 19 3.10 Fossil Energy Inputs into Corn Production The specific fossil energy requirements in industrial corn farming are shown in Figure 13. A few comments are in order. • The lowered Pimentel’s 2003 estimate of fossil fuel energy plus irrigation is identical with that of Shapouri et al. • The lowered Pimentel’s 2003 estimate of nitrogen fertilizer energy is higher than the uncor- rected one by Shapouri et al., which is too low. My estimate is in the middle. • Pimentel’s 2003 lime application rate is twice those of everyone else’s. It reflects the 1997 USDA average. • Pimentel’s 2003 transportation energy is higher than everyone else’s. It may reflect 1-2 more commutes/ha/crop. • Shapouri and Wang et al. have underestimated the fossil energy in seeds and left out the machinery and infrastructure. • I have added my seed energy estimate to their inputs, but left the machinery out. • Berthiaume et al. (2001) have not included several of the energy inputs in corn farming, so their estimate is presented here only for comparison. • The estimates of fossil energy inputs range from 19 GJ/ha (Wang) to 33 GJ/ha (Pimentel). My estimate is 28 GJ/ha. • Wang et al.’s estimates are consistently too low. Shapouri et al.’s and my estimates are almost identical. The only significant difference is the inclusion of machinery into my estimates. • The fossil energy use in corn farming is large and equivalent to 0.4 (Wang et al.) to 0.7 (Pimentel) metric tonne of gasoline per hectare and per crop. • The average energy use in corn farming does not tell the whole story because of the very large variability of energy use by state, depending on the local weather conditions. • All estimates, including mine, have had errors and/or omissions at one stage or another. I hope that by bringing the approaches of all authors into a common framework, most of the deficiencies of the prior analyses have been eliminated. 3.11 Solar Energy Input into Corn Production The amount of solar energy that irradiates 1 average hectare of corn field in the U.S. during the growth season is gigantic, and it dwarfs all fossil energy inputs and the calorific value of the 8600 kg of corn grain harvested from this hectare, see Figure 14. In fact, during the 120-day growth season, roughly only ∼0.7% of the solar energy is converted by corn plants into biomass (Biermann et al., 1999). In contrast, solar cells can collect sunlight all year long, see Appendix C. On an annual basis, the solar efficiency of corn plants drops by a factor of 3, i.e., only ∼0.2% of the solar energy is captured by an average corn crop15. In summary, the solar energy does not limit corn 15Two tenth of one percent is 20 parts in 10000 parts of mean insolation. Roughly half of these 20 parts becomes corn grain. 22 Thermodynamics of corn-ethanol biofuel. . . Web Version translocated from vegetative plant parts to the developing grain later in the season. According to Figure 15, a corn crop harvested with no recycling removes more than 1.5 times as much nitrogen, 1.6 as much phosphorus, 4 times as much potassium, 13 times as much calcium and 6 times as much magnesium as when this crop is harvested for grain. Other estimates are even more unfavorable (Wheaton et al., 1993). Whole plant harvest also removes most of the soil metals essential to the well-being of corn plants. The need to recycle plant parts and limit soil erosion largely negates the now fashionable attempts to produce ethanol from whole corn plants by harvesting everything from the corn field, see e.g., (NREL, 2002; Sheehan et al., 2004). Every ecosystem on the earth is highly optimized to recycle almost all mass it generates; otherwise life would not persist. 4 Major Energy Inputs to Ethanol Production Conversion of corn grain into 100% ethanol (EtOH) is a fossil energy-intensive process, which also generates significant gas emissions, as well as liquid and solid waste. Here I will consider only wet-milling of corn to convert it into glucose, which is subsequently fermented to industrial beer, and distilled to 96% ethanol. The final water removal is achieved in molecular sieves that exclude water, or by distillation with benzene, see Eq. (9). Fermentation is a slightly exothermic catalytic burning of aqueous glucose, in which 49% of its mass is converted to carbon dioxide gas. The main liquid reaction product, ethanol, retains most of the free energy of the glucose. Dry milling is energetically similar, and need not be considered. Corn Grain ︸ ︷︷ ︸ Steeping Grinding Germ Separation → Starch ︸ ︷︷ ︸ Gluten Liquefaction Saccharification → Glucose ︸ ︷︷ ︸ Fermentation CO2 → Ethanol ︸ ︷︷ ︸ Distillation Dehydration (9) 4.1 Corn Mass Balance Revisited In Section 2, I calculated the theoretical efficiency of corn conversion into ethanol, in which every step is 100% efficient. Here, in agreement with the USDA estimate (Shapouri et al., 2002b), I will assume that the conversion of corn grain into 100% ethanol incurs 8.5% losses18 by mass, see Figure 16. Two important conclusions can be drawn from Figure 16: 1. The average yield of anhydrous ethanol from corn is now 0.435 L EtOH/kg dry corn grain, or 2.914 gallons of EtOH per 56 lbs of dry corn grain (“dry bushel”), or 2.477 gallons of EtOH per nominal wet bushel with 15% moisture. 2. The reported field corn yields must be multiplied by 0.85 to convert the harvested corn to water-free or “dry” corn, see Figure 17. 3. In the literature, the USDA estimate of 2.682 gallons EtOH/bushel has been multiplied by the moist corn grain yields; this is incorrect and leads to an overestimation of the corn-ethanol yield by 8% (∼1/4 of the positive fossil energy balance claimed by USDA). 18This 8.5% overall loss lumps the losses in broken corn kernels and foreign matter (nominally 3% by mass for No. 2 yellow corn), in starch separation and hydrolysis, fermentation, distillation/rectification, and ethanol trans- portation and distribution. The fermentation process has several byproducts: n-propyl, isobutyl, amyl, iso-amyl, 1,2,3-propanetriol (glycerol) and higher alcohols; acetic aldehyde and acid; etc., see (White and Johnson, 2003), page 710; also see Appendix D for more details. The fermentation selectivity to ethanol can be less than 90%. CRPS, 23(6), 2004 T. W. PATZEK 23 0 1000 2000 3000 4000 5000 6000 7000 8000 Fermentation Corn Feed Starch Corn Oil Gluten meal Gluten feed Ethanol CO2 Losses kg/ha Figure 16: The result of practical corn conversion into ethanol with 8.5% losses is 0.435 L EtOH/kg dry corn grain = 2.48 gal EtOH/wet bushel with 15% moisture. Note that the dry starch is swollen by a factor of 180/162 caused by hydrolysis to glucose. 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Berthiaume et al., 2001 Shapouri et al., 2002 Wang et al., 1997 Pimentel, 2003 Patzek, 2004 Dry Grain Water kg/ha Figure 17: Average wet and dry corn yields. 24 Thermodynamics of corn-ethanol biofuel. . . Web Version 4.2 Transport in Ethanol Refineries Transport of materials and people in-and-out of an ethanol plant requires energy, and there is some disagreement between Pimentel and Patzek on one hand, and Shapouri et al. and Wang et al. on the other. Here it suffices to state that • Corn grain (8600 kg/ha) and fuel (e.g., ∼1200 kg/ha of coal) must be transported in. • Ethanol (∼2200 kg/ha), gluten meal and feed (∼2600 kg/ha) must be transported out • Workers must travel in-and-out. • Both Shapouri et al. and Wang et al. seem to underestimate these transport costs by a factor of 3-4. 0 2 4 6 8 10 12 14 16 18 Wang et al., 1997 Shapouri et al., 2002 Corn Handbook, 2003 Pimentel, 2003 Patzek, 2004 Berthiaume et al., 2001 MJ/Liter of Ethanol Processing Transport Credits HB Total Figure 18: The average fossil energy inputs to ethanol production in a wet milling plant. The length of each bar is the total energy outlay to produce 1 liter of EtOH, and the blue parts denote the size of energy credits assumed by the different authors. The modern dry mill plants use 11.36 MJ/L as steam and 3.12 MJ/L as electricity, 14.5 MJ/L total, not counting transportation costs. 4.3 Fossil Energy Inputs to Ethanol Because transportation is but a small fraction of the total energy outlay in ethanol production, there is little disagreement in the various estimates of the total energy used to produce ethanol from corn, which are all close to 15 MJ/L EtOH, see Table 18. This is easily seen when the total lengths of the bars in Figure 18 are compared19. 19For comparison, a recent feasibility study for a new ethanol plant (International, 2001) projects 13.08 MJ/L EtOH in methane, and 1.675 MJ/L EtOH in electricity, for the total of 14.8 MJ/L EtOH, excluding transport and CRPS, 23(6), 2004 T. W. PATZEK 27 −15 −10 −5 0 5 10 Pimentel, 2003 Patzek, 2004 Berthiaume et al., 2001 Shapouri et al., 2002 Wang et al., 1997 Fossil Energy Gain/Loss, GJ/ha Figure 20: Fossil energy gain/loss in corn ethanol production. Note that the dubious energy credits described in Section 4.4 do not eliminate the use of fossil fuels in the first place, but present alternative useful outcomes of this use. 0 20 40 60 80 100 120 140 Berthiaume et al., 2001 Shapouri et al., 2002 Pimentel, 2003 Patzek, 2004 Wang et al., 1997 Net Energy Yield, GJ/ha Figure 21: The net energy yield in industrial corn grain production is relatively small, 100 – 135 GJ/ha-crop. The HHV of dry corn grain is 18.8 MJ/kg, based on the mean of the values reported by Schneider & Spraque (1955), p. 496, 2033 kcal/lb; and Miller (1958), p. 639, 2059 kcal/lb. 1 thermochemical kcal = 4.184 kJ. 28 Thermodynamics of corn-ethanol biofuel. . . Web Version Part II Sustainability & Renewability 1 Introduction The following type of reasoning (Sheehan et al., 2004) is not uncommon in environmental literature: (Page 118: . . . Sustainability is fundamentally an ethical issue, the technological context. . . is not adequate to fully assess the sustainability of ethanol or any other fuel choices. . . . The stakeholders21 established a list of indicators that they felt should be used to measure the relative sustainability of switching from gasoline to [corn] stover- derived ethanol to fuel our cars22. More broadly, an informal check of Amazon.com, performed on August 16, 2003, revealed 4454 book titles containing the word “sustain⋆.” In particular, there were 573 book titles with “sustain⋆” and “⋆culture”23.” The phrase sustainable development is firmly rooted in our consciousness. Therefore, one must ask the following question: Is sustainable anything possible in nature? In the economy? Also, how sustained are the processes deemed by some as “sustainable”? Human nature, being what it is, destines us to choose a “truly great but brief, not a long and dull, career24” on the earth. After our eventual demise, the earth will be home to other less ambitious and impatient species. The name of the game, therefore, is to make the human presence on the earth as happy as possible, albeit not too short25. These two tasks require careful thought and delicate balance of human actions. No country has demonstrated an adequate implementation of either. In fact the opposite may be true. As the entropy on the earth increases, the actions of governments and societies resemble more and more episodic spasms, with ever less forethought and deliberation. The current hot button issues: the Hydrogen Economy, Ethanol from Corn, and the War on Terrorism are good examples. 2 Disclaimer The next eight sections of this paper are punctuated with verbatim quotations from, and my digestion of the most important book I have read in decades: The Entropy and the Economic Pro- cess by Nicholas Georgescu-Roegen (1906-1994), who was a great twentieth century thinker, economist, mathematician, historian, and philosopher. The impact of this book on my thinking has been profound. 21“A group of farmers, environmentalists, automakers, grain processors, and government researchers.” 22Therefore, any fuel or technology can be declared as “sustainable,” whenever there exists a group of people who feel good about it, and say that it is! 23My favorite: Permaculture: Principles and Pathways Beyond Sustainability by David Holmgren, published by Holmgren Design Services (July 2003). 24Georgescu-Roegen (1971), page 304. 25Preaching alone will not do. People will never choose less fulfilling life styles without coercion. This is why communism, or any other totalitarian “ism,” can never work; they strive to convert the thinking individuals into slaves or working animals. Says Percy Williams Bridgman, (1955), p.114, italics mine: “The individual is the unit in terms of which all our social concepts ultimately find their meanings.” CRPS, 23(6), 2004 T. W. PATZEK 29 Process Inputs Outputs Boundary Figure 22: The boundary separates the process from the environment at any time (it is the inter- face), and it defines the duration of the process. We may not describe a process by what happens inside or outside of it, but only by what crosses its boundary. 3 Preliminaries In science we divide actuality into two slices: one representing the partial process determined by our interest, and the second, its environment, see Figure 22. These two parts are separated by an analytic26 boundary. The boundary has two attributes. The first separates the process from the environment at any time (we can call this attribute the interface, or the frontier), and the second defines the duration of the process27. Often the terms process and environment are used interchangeably with system and surroundings. We may not describe a process by what happens inside or outside of it, but only by what crosses its boundary. Anything of interest crossing the boundary from the environment into the process is an input, and anything crossing the boundary in the opposite direction is an output. Solar energy is a typical example of only an input for any terrestrial process. The various materials abbreviated as “waste” are examples of only outputs. 4 Laws of Thermodynamics The three empirical laws of Classical Thermodynamics28 were originated by Joule, Clausius, Thomson, Planck and Nernst, and are often formulated as follows: First Law or the Energy Conservation Law (Joule, Clausius, Thomson) • Energy can neither be created or destroyed; • The energy of the universe remains constant; or • You can’t win. Second Law or the Entropy Law (Clausius) • Without the compensating changes elsewhere, heat can flow only from a hotter to a colder body; or • With passing chronological time, the entropy of the universe tends towards a maximum; or • You can’t break even. 26The word analytic means well-defined mathematically in space and/or time. 27The process is not defined outside its time interval. 28Started in 1824 with a memoir, Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance, on the efficiency of steam engine by a French engineer, Sadi Carnot (1943). 32 Thermodynamics of corn-ethanol biofuel. . . Web Version 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 Entropy efficiency = (Sin − Sout)/Sproduction Flash Smelter Converter Anode Furnace Electrolysis Figure 23: The Second Law efficiency of copper production is incredibly small. The steps in sorting the copper atoms are: Ore Concentration 2% → 30% Cu (not shown); Smelter 30% → 63% Cu; Converter 63% → 99.1% Cu; Anode furnace 99.1% → 99.55% Cu; and Electrolysis 99.55% → 99.99% Cu. Source: Stefan Gößling, Entropy Production as a Measure for Resource Use, University of Hamburg, 2001. Maxwell demon41, we have merely sorted the copper atoms from all others, but in order to achieve this end-result we have used up irrevocably a greater amount of low entropy than the difference between the entropy of the copper metal and that of the copper ore. In view of Figure 23, it would be a great mistake to compare just the latter two entropies and exclaim: “Lo! Man has created low entropy from high!” (Georgescu-Roegen, 1971). This claim, in effect, is made by all those who say that copper can be manufactured sustainably. The copper scrap recycling programs are successful, only because scrap copper (and aluminum) consumes less free energy than any other way of reconstituting metallic copper42. Nevertheless, insofar as fossil energy is used, by recycling we only postpone the inevitable exhaustion of low entropy in the environment. 6 Economic Activity Economists have a tendency to view the economic process as a closed system, while ignoring the continuous inflow of low entropy from the environment43. From a physical point of view, the economic process is entropic; it neither consumes nor creates mass or energy, but only transforms low entropy to high. To make things worse, the parallel entropy generation process in the environment is spontaneous, and goes on by itself without human intervention. 41J. Clerk Maxwell (2001) imagined a tiny demon posted near a microscopic swinging door separating two gases A, and B of equal temperature. The demon is instructed to open and close the door “so as to pass only the swifter molecules from A to B, and only the slower molecules form B to A.” Clearly this demon can make the gas in B hotter and in A cooler. Therefore, Maxwell’s demon creates low entropy – or does he? 42According to Stefan Gößling, entropy generation per ton of copper produced from ore is 52 MJ/K, and only 12 MJ/K for copper produced from scrap; for reference, see the caption of Figure 23. 43Georgescu-Rogen, (Georgescu-Roegen, 1971), Chapters IX and X. CRPS, 23(6), 2004 T. W. PATZEK 33 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Less DC Other DC U.S. Liters of crude oil equivalent per year per person Figure 24: The 2001 per capita energy consumption in the U.S., other Developed Countries (DC), and the less Developed Countries. Source: The U.S. DOE Energy Information Agency. 0 1000 2000 3000 4000 5000 6000 Africa S. America Asia Europe U.S. Liters of water per year per person Figure 25: The 1990 per capita total (personal + industrial) water consumption in the U.S., and elsewhere. Source: Water Quality Association, 151 Naperville Road Lisle, IL 60532-1088, USA. The material production process, in contrast, depends on the intervention of humans, who like the Maxwell demon, sort and direct environmental entropy according to the process rules44. This sorting activity is not a part of natural environmental processes and creates high entropy, i.e., waste, at a (much) faster rate than the biological life processes. From a purely material point of view, the economic process always transforms low entropy into waste. So what could be the justification for 44When watching an SUV commercial, I often see a monstrous truck carelessly damaging a low-entropy fragile ecosystem, a pristine meadow, river bed, or an alpine mountain slope. Thus the SUV commercials are a good metaphor for the interactions of the present-day economics with the environment. 34 Thermodynamics of corn-ethanol biofuel. . . Web Version 0 1000 2000 3000 4000 5000 6000 India China World Top 20 EU U.S. Kilograms of carbon per year per person Figure 26: The 1999 per capita carbon emission estimates in the U.S., and elsewhere. Source: World Resources Institute, U.S. EIA. economic activity? As described by Georgescu-Roegen, the true output of an economic process is not merely waste, but the enjoyment of life. It is not a coincidence that the very country, which on July 4, 1776 declared: “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness,” uses over twice as much free energy per capita than any other country on the earth, see Figures 24- 26. In general, abundant free energy equals enjoyment of life. The converse statement is as true in real life as it is in Logic, see Figure 27. 7 Agriculture The following statement is made surprisingly often: “Properly used, [the plants on the earth] can by their reproductive powers supply us indefinitely with the food, the wood, and the other natural products we require.45” Even though the intensity of sunlight that reaches the earth has not changed appreciably over the human scale of Time, the apparent dominance of solar energy in agricultural production should not obscure the importance of the entropic soil degradation by continuous cultivation. Soil degradation can be severe over a human life span. Even the earliest farmers knew that manuring a soil does not remove its degradation, and to farm always meant to mine, in part, the soil. Clean water is necessary for agriculture. Water is inevitably polluted by agricultural waste; therefore, water too is mined. It would be a mistake to believe that the practice of fertilizing soil can defeat the Entropy Law and transform food production into an everlasting process. Life feeds not only on sunlight but also on the low entropy of an ecosystem46. With time, draft animals, oxen, buffalo and horse, were replaced by machines. A tractor is made of iron, other metals, oil and coal, and it feeds on oil. The natural manure fertilizer from farm 45(Cépède et al., 1964), p. 309. 46A process that cycles living organisms. Only the solar energy and waste heat flow across the ecosystem boundary, everything else is recycled, cf. Section 10. CRPS, 23(6), 2004 T. W. PATZEK 37 everywhere the exact initial state when the irreversible process has once taken place (Planck, 1926). 2 Corollary 1 From the definition above, a linear process that converts the low entropy of fossil fuels into waste is irreversible and cannot be sustainable. 2 Stock of soil/water fossil fuels 200 years? Chemical waste Waste heat Figure 29: Current industrial agriculture is another example of a linear process, which by definition cannot be sustainable. In a linear process, see Figure 28 and 29, a finite stock of fossil fuels is rapidly depleted and burned to serve as a collective heat source for all heat engines employed by our civilization, see Figure 31a. In addition, the atmosphere, which acts as a heat sink, becomes polluted by chemical waste from combustion (chemical entropy), as well as by waste heat (thermal entropy). The earth can only export thermal entropy through its atmosphere, see Appendix A. In addition to the atmosphere, the earth, which is the system in Appendix B, also accumulates chemical entropy. As a result, the linear fossil fuel process accumulates chemical entropy in the earth and the atmosphere, and irreversibly degrades our planet on a time scale of our civilization, measured in hundreds of years. In contrast, a cyclic ecosystem can be sustainable, see Figure 30. A natural cycle uses the sun as its source of energy and low entropy, and it expels only waste heat into the atmosphere and, ultimately, into the universe, see Figure 31b. Most importantly, all materials involved in an ecosystem are recycled, and when the natural cycle is completed, only waste heat, or thermal entropy is generated. In order to discuss the existence and constraints on sustainability, I need first to define it. Definition 4 (Sustainability) A cyclic process is sustainable if and only if 1. It is capable of being sustained, i.e., maintained without interruption, weakening or loss of quality “forever,” and 2. The environment on which this process feeds and to which it expels its waste is also sustained “forever.” Corollary 2 A cyclic process, which is also “sustainable,” must not reject chemicals into the en- vironment, i.e., its net mass production must be “close” to zero “forever.” Corollary 3 A sustainable cyclic process must not reject heat into the environment at a rate that is too high for the earth to export this heat to the universe; otherwise, the environment properties will change. 38 Thermodynamics of corn-ethanol biofuel. . . Web Version Other life Death & Decay H2O, CO2 Nutrients Plant Matter Waste heatWaste heat Sun energy “Forever” Figure 30: An ecosystem transforms the sun energy (low thermal entropy) into waste heat (high thermal entropy). The waste heat is continuously exported to the universe. Everything else is completely reused, or recycled. Work Heat source Heat sink Life Sun Universe (a) (b) Figure 31: Thermodynamic cycles: (a) A heat engine, and (b) An ecosystem. 10.1 The Earth is an Open System to Heat Flow Attributes (1)-(2) of a sustainable cyclic process would be a thermodynamic contradiction if the earth were approximately a closed system with respect to the infrared radiation (heat). These two attributes would then make a sustainable process a perpetual machine of the second kind. Luckily for us, the earth can be treated as an open system with respect to visible and infrared light, and a CRPS, 23(6), 2004 T. W. PATZEK 39 sustainable cyclic process may generate thermal entropy at a rate per unit area of the earth surface (specific entropy rate or flux ) which is no more than the average flux of entropy export from the earth to the universe, jES , calculated from Eq. (34) in Appendix A, minus the specific rate of entropy generation in the atmosphere due to export of the solar energy, calculated from Eq. (39) in Appendix A. To quantify sustainability, I first assert that a cyclic process always converts all forms of entropy to thermal entropy. Thermal entropy is the ultimate waste from all “sustainable” cyclic processes on the earth, and it should be used for comparisons. Second, per unit area of the earth, we know that (1) the always positive specific rate of thermal entropy generation due to everything happening on the earth is σ > 0, (2) the rate of increase of the specific thermal entropy of the atmosphere due to all these happenings is σa > 0, and (3) the specific rate of thermal entropy generation due to the energy transport from the earth to the universe is σt > 0. Then, for cyclic processes, strong sustainability can be defined mathematically, see Appendix B, as σ + σa ≤ j E S − σt (12) at every point on the earth, and at all times. Over an arbitrary time interval [τ1, τ2], we can write the global condition of sustainability of all cyclic processes on the earth as (Eq. (55) in Appendix B) [Sa(τ2) − Sa(τ1)] ︸ ︷︷ ︸ Increase of atmospheric entropy + [S(τ2) − S(τ1)] ︸ ︷︷ ︸ Increase of earth entropy −SE(τ1, τ2) + St(τ1, τ2) ≤ 0 (13) where SE(τ1, τ2) is the total thermal entropy exported by the earth over the time interval [τ1, τ2], and St is the corresponding thermal entropy generation in the atmosphere due to the solar energy export. 10.2 Conclusions The immediate observations from the above discussion are: • To the extent that humans use 80-90% of fossil and nuclear energy to run the heat engines that power the global economy, our civilization is 80-90% unsustainable. • If the atmosphere dissipates more energy due to the increased greenhouse gas loading by human (and natural) activities (σt increases), there will be less room for all other human activities to remain sustainable. Stahl estimates, (1996), Table 1, the specific entropy gen- eration rate in the atmosphere to be σt = 0.2 W/K-m 2. So only 1 W/K-m2 of thermal entropy generation is left to all human and other natural activities, see Appendix A. • Only energy generation directly from the sun, sun-driven wind, and water can be sustainable. • Burning or extracting large quantities of wood or green matter requires chemical fertilization and cannot be sustainable to the extent that growing plants mines fossil fuels as well as low entropy from the soil, see Section 7. • Industrial agriculture can never be sustainable because it relies on the irreversible burning and chemical transformations of fossil fuels, see Section 7. 42 Thermodynamics of corn-ethanol biofuel. . . Web Version • Chemical exergy, Bch, is the work obtained by taking a substance at the pressure and temperature of the environment to the state of thermodynamic equilibrium with the datum levels of components of the environment. • Thermal exergy, Bth, is the sum of physical and chemical exergies: Bth = Bph + Bch (15) H1, S1 H2, S2 T0 Q0r Wmax = −∆Bth Figure 32: Exergy balance in an isothermal, ideal flow machine. The maximum possible shaft work from this machine is equal to the negative change of thermal exergy. 2.2 Change of Bth between Two States Consider an ideal (reversible) flow machine, see Figure 32. An exergy carrier with enthalpy H1, and entropy S1 enters the machine. After physical and/or chemical changes, the effluent has enthalpy H2, and entropy S2. Heat is transferred between the environment and the working fluid at the ambient temperature T0. The first and second law of thermodynamics are simply: Wmax = Bth1 − Bth2 = H1 − H2 + Q0r (I Law) S2 − S1 − Q0r T0 = 0 (II Law) Bth1 − Bth2 = −∆Bth = H1 − H2 − T0(S1 − S2) (I+II Law) (16) Physical exergy can be calculated immediately from Eq. (16) Bph = H − H0 − T0(S − S0) = Hph − T0Sph (17) 2.3 An Industrial Flow Process Consider now an industrial steady-state flow process, which can occur in a heat engine, corn field, or ethanol plant, see Figure 33. The input to this irreversible process is an exergy carrier with the enthalpy H1, and entropy S1. The process is also supplied with the quantity of heat Q1 from the source having temperature T1 > T0. The process effluent has enthalpy H2, and entropy S2. The rejected amount of heat Q0 is transferred to the environment. The useful outcome of the process CRPS, 23(6), 2004 T. W. PATZEK 43 H1, S1 H2, S2 Wu or Hu, Su T0 Q0 T1 Q1 Figure 33: Exergy balance in ideal (reversible) and real (irreversible) nonisothermal industrial process. can be mechanical work Wu or a chemical product having parameters Hu and Su. The effect of irreversibility is studied by comparing the industrial process with a reversible process with the same inflow and outflow parameters, and the same amount of driving heat. The only difference between these two processes is the amount of heat rejected to the environment. For the reversible process this heat is Q0r, and for the irreversible one, it is Q0. The first and second law balances for the two processes are: Hu = H1 − H2 + Q1 − Q0 Real process Hur = H1 − H2 + Q1 − Q0r Reversible process Hur − Hu = Q0 − Q0r (18) The increased useful effect of the reversible process causes the amount of rejected heat to be smaller than that in the industrial process, Q0r < Q0. The sum of all entropy increases in the industrial process is ∑ ∆S = − Q1 T1 + S2 − S1 + Q0 T0 + Su > 0 (19) while that in the reversible process is ∑ ∆Sr = − Q1 T1 + S2 − S1 + Q0r T0 + Sur ≡ 0 (20) From Eqs. (19) and (20) it follows that Q0 − Q0r = T0( ∑ ∆S + Sur − Su) (21) and we obtain (Hur − T0Sur) ︸ ︷︷ ︸ Bth,r − (Hu − T0Su) ︸ ︷︷ ︸ Bth ≡ δB = T0 ∑ ∆S (22) The left hand-side of Eq. (22) represents the difference of the useful thermal exergy in the reversible and industrial process, δB. It therefore represents the exergy loss due to the irreversibility of the industrial process under consideration. We have recovered, again, the famous Guoy-Stodola law, also derived in Appendix B. 44 Thermodynamics of corn-ethanol biofuel. . . Web Version 2.4 Cumulative Exergy Consumption (CExC) All steps of a production process leading from natural resources taken from the environment to the final product result in exergy losses or exergy consumption. Definition 6 (CExC) The cumulative exergy consumption (CExC) is the sum of the exergy of all natural resources in all the steps of a production process. 2 The problem of cumulative energy consumption (CEnC), discussed in Part I, is better known, but calculation of CExC is more informative because it accounts for the exergy of non-energetic raw materials (soil, water, air, minerals) extracted from the environment. 3 The Ideal and Real Corn-Ethanol Cycle Heat Ethanol Corn Solar Radiation Heat Work Heat Combustion Figure 34: The ideal corn-ethanol cycle. Adapted from Figure 2 in Berthiaume et al. (2001). Ideally, see Figure 34, the corn-ethanol system and cycle consist of three parts: (1) Sustainable corn farming, (2) Sustainable ethanol production, and (3) Ethanol combustion to produce useful work. The cycle is driven only by solar energy, and all its chemical by-products are fully recycled. Only the low quality heat is rejected by the ideal corn-ethanol cycle into the environment, and this heat is exported through the atmosphere into the universe. All carbon dioxide is recycled, and so is all water. This low-rate ideal cycle cannot deliver the massive quantities of ethanol fuel from (bi)-annual corn crops, see Figure 35. Remark 6 Between 1866 and 1939 (NASS, 2004b), the average yield of corn in the U.S. hovered around 26 ± 3 bushels per acre, or 1600 kg/ha, 1/5 of the average yield today. I will assume that 1600 kg/ha is the almost sustained corn yield using manuring, composting, crop rotation, and other not quite sustainable field practices. Note that between 1906 and 1937, the average corn yield declined, most likely due to the progressing soil deterioration50, cf. Section 7. 2 50Pimentel (2004c) observes: “Between 1900 and 1938, the early [U.S.] farmers were probably mining the soil of nutrients and soil erosion was quite severe. At that time, most farmers kept livestock and were applying manure to CRPS, 23(6), 2004 T. W. PATZEK 47 Pure EtOH CO2 Corn+stalk, leaves, roots Dried corn EtOH+H2O Combustion Distillation F er m en ta ti on Harvesting & Drying In d u strial farm in g Stalk, leaves, roots Gluten Waste H2O Soil humus Decomposition Figure 37: The internal carbon cycle in the industrial corn-ethanol cycle can be closed only by re- cycling most of the corn-plant and corn-grain components. Adapted from Figure 6 in Berthiaume et al. (2001). 5.1 Net CO2 Emissions As shown in Figure 36, our corn field-ethanol plant-combustion engine system uses fossil fuels as inputs, and outputs their combustion products into the environment. Therefore the industrial corn-ethanol cycle generates extra CO2 and other greenhouse gases, which will all be translated into equivalent CO2 for simplicity. The question now is as follows: Does the industrial corn-ethanol cycle generate more equivalent CO2 from its fossil fuel inputs than the gaseous emissions from replacing the cycle’s ethanol with gasoline, methane or diesel fuel? To make this comparison fair, I will account for the cumulative exergy consumption in production of the fossil fuels by adding another 15% to their calorific values, in agreement with Szargut et al. (1988) and Sheehan et al. (1998). By asking and answering this question, I seek to dispel common misconceptions about the industrially-manufactured biofuels, best summarized by the following quotation: About 70 million barrels52 of ethanol are included in annual U.S. gasoline consumption after 1992. Because ethanol is a biofuel, the carbon it contains should not be counted as an emission. Hence, carbon from ethanol is deducted from transportation gasoline consumption53. EIA is right, but then the CO2 emissions associated with the consumption of non-renewable resources in the industrial ethanol-corn cycle should be added to the transportation gasoline con- sumption. The question now is: What is the net balance? To answer this question, I will use the EIA and the European Fertilizer Manufactures’ Asso- ciation (EFMA) data on the specific carbon dioxide emissions from the fossil fuel inputs into the industrial corn-ethanol cycle, see Table 19. These specific emissions, in kg of CO2 per MJ in a fossil fuel, will be multiplied by the respective energy input fluxes in MJ ha−1 crop−1, established 52About 11 giga liters. 53The U.S. Energy Information Agency (EIA) (2002), Appendix A, page A3, my italics. 48 Thermodynamics of corn-ethanol biofuel. . . Web Version in Part I. Electricity is treated differently, and its specific CO2 emissions account for the average U.S. efficiency of conversion of thermal energy into electricity. 0 5 10 15 20 25 30 35 40 45 P2O5 KCl NG Herbicides Transportation Electricity Gasoline Custom work LPG Seeds Diesel CaO Wastewater BOD EtOH Plant Transport Machinery Nitrogen as Ammonia Humus Oxid. EtOH Plant Fuel Equivalent Carbon Dioxide Emissions, g/MJ in EtOH Figure 38: Equivalent CO2 emissions from each major non-renewable resource consumed by the industrial Corn-EtOH cycle. To convert the NOx emissions from the industrial corn-ethanol cycle to the equivalent CO2 emissions, I will follow the guidelines of EIA (2002), the Intergovernmental Panel on Climate Change (IPCC) (1997), and EFMA (Biermann et al., 1999): • 1.25% of applied N fertilizer escapes into the air as N2O. • 30% of applied N escapes from the field, and 2.5% of that quantity is converted to N2O in surface water. • 10% of applied N escapes as NH3 into the air, and 1% of that becomes N2O. • Nitrous oxide is 300 times more potent as a greenhouse gas (GHG) than CO2. • An average ammonia plant emits 0.03 kg N2O/kg N in nitric acid, which is used to make ammonium nitrate. The equivalent CO2 emissions from the fertilized corn fields are then ∼950 kg/ha. The equivalent CO2 emissions from the production of ammonium nitrate are ∼150×0.03×300×63/80 = ∼1000 kg/ha54. 54One may dispute this last number to the extent that ammonium nitrate is not used as fertilizer. CRPS, 23(6), 2004 T. W. PATZEK 49 The CO2 emissions resulting from electricity use in the removal of Biological Oxygen Demand (BOD) in wastewater from ethanol refineries, cf. Section 6, are also included. In addition I have made the following assumptions: • Emissions from humus oxidation in soil eroded by wind are included with the following as- sumptions: (a) topsoil contains 4% of humus (Stevenson, 1982), (b) humus contains 50% of C by weight (Stevenson, 1982), wind-eroded soil is enriched in humus by (1.3 + 5)/2 ≈ 3 on average (Allison, 1973), (c) wind erodes only 1 mm of topsoil per year, or 10 tonnes/ha-yr (Pimentel, 2006), and (d) the airborne humus is completely oxidized. • To make the comparisons meaningful, methane, gasoline, and diesel fuel are now charged with incremental 17% (2% more than before) emissions for their recovery, transportation, and processing. 0 20 40 60 80 100 120 140 NRR in EtOH−Corn Cycle Diesel Fuel Gasoline Fuel Methane Fuel Equivalent Carbon Dioxide Emissions, g/MJ in Fuel Fossil Fuels Corn Farming Ethanol Plant BOD Treatment Figure 39: The total equivalent CO2 emissions from the consumption of nonrenewable resources by the industrial corn-ethanol cycle. The CO2 emissions from the energy-equivalent amounts of methane, gasoline and diesel fuel were increased by 17% to account for their recovery, transport, and refinement. 5.2 Conclusions The results of my calculations, shown in Figures 38 and 39, lead to the following conclusions: 1. According to my estimates, 1 ha of industrial corn-for-ethanol generates 8955 kg of equivalent CO2 from the fossil fuel inputs and humus oxidation. 52 Thermodynamics of corn-ethanol biofuel. . . Web Version S o la r h ν , ∆ b0 c h = + 2 8 3 3 C6H12O6+6O2 6CO2 ∆b0 ch = +119 ∆b0 ch = −161 2CO2+2C2H5OH+6O2 6CO2 C 2 H 5 O H C o m b u st io n ∆ b0 c h = − 2 6 7 2 6H2O6H2O Figure 41: Exergy diagram of the ideal CO2-Glucose-EtOH cycle. Remark 9 Industrial agriculture uses a huge land area, and it mines and contaminates huge amounts of soil, water, and air. The environmental damage it causes is much more widespread and more difficult to reign in than that from the highly-concentrated industrial sources. In addition, industrial agriculture invades and destroys large ecosystems. In other words, twenty-first century industrial agriculture poses a more acute threat to life on the earth, than the nineteenth century smoke stacks ever did57. 2 7 Exergy Analysis of the Ideal Corn Ethanol Cycle 7.1 Chemistry of the CO2-Glucose-EtOH Cycle The ideal CO2-Glucose-EtOH cycle consists of three steps: Step 1 Photosynthesis of glucose from atmospheric CO2: 6CO2 + 6H2O + Solar radiation → C6H12O6 + 6O2 + Heat (24) Step 2 Production of ethanol from glucose: C6H12O6 + 6O2 → 2C2H5OH + 2CO2 + 6O2 + Heat (25) Step 3 Combustion of ethanol: 2C2H5OH + 2CO2 + 6O2 → 4CO2 + 2CO2 + 6H2O + Heat (26) The compounds that appear on both sides of the chemical reactions in Steps 2 and 3 do not participate in these reactions, but appear to close the cycle. The chemical exergies of all compounds are listed in Table 20. The chemical exergies of the products of each step of the cycle are listed in Table 21, and the exergy flow is depicted in Figure 41. 57For an in-depth analysis of the deadly industrial agriculture, see Kimbrell et al. (2002). CRPS, 23(6), 2004 T. W. PATZEK 53 7.1.1 The Maximum Cycle Output per Unit Mass of Corn To calculate the maximum possible energy output from the ideal CO2-Glucose-EtOH cycle, I made the following assumptions: • Dry corn is 66% glucose (100% hydrolyzed starch) by mass. • Starch is converted into ∼100% ethanol with 0% losses. • Corn delivered to an ethanol plant is 15% water. • The net chemical exergy of an ideal ethanol cycle per kg of moist corn grain is 15.74 MJ kg glucose × 0.66 × 180 162 kg glucose kg dry corn × 0.85 kg dry corn kg wet corn = 9.81 MJ kg wet corn (27) • At ∼8600 kg of moist corn per hectare58, the chemical exergy from an industrial ethanol cycle is 9.81 MJ kg wet corn × 8590 kg wet corn ha = 84.4 GJ/ha (28) • With 91.5% overall conversion efficiency of starch into 100% ethanol, the chemical exergy is 84.4 GJ/ha × 0.915 = 77.1 GJ/ha (29) Remark 10 The output of the industrial CO2-Glucose-EtOH cycle is the chemical exergy of ethanol equal to 77.1 GJ/ha-crop. This exergy can be transformed into useful work (e.g., shaft work or electricity) by different devices. 2 For example, the efficiency of an excellent internal combustion engine is 35% (usually it is 20%) Wu = 77.1 × 0.35 = 27.0 GJ/ha (30) For fuel cell/electric motor vehicles the efficiency of conversion of chemical exergy to shaft work is higher. Suppose that we could reform ethanol to hydrogen, and use a fuel cell with 60% efficiency59 (Deluga et al., 2004) Wu = 77.1 × 0.60 = 46.3 GJ/ha (31) to obtain electricity, and shaft work. 8 Exergy Analysis of the Modified Ideal Corn-Ethanol Cycle Now let us look at the useful exergy production in the modified ideal corn-ethanol-hydrogen cycle discussed by Deluga et al. (2004). This cycle is essentially the same as the cycle described in Eqs. (24 – 26). The only difference is in Step 3, which is moderately endothermic, and lowers the cycle efficiency by one percent, see Figure 42. The chemical exergies of the products of each step of the modified cycle are listed in Table 22. Step 3, reforming ethanol to hydrogen, is a catalytic variant of water-shift reaction: 58See Remark 6. Without synthetic fertilizers, I would have to use 1500 kg/ha-crop of corn as the average yield. 59As shown by (Bossel, 2003a) and in Appendix D, a 60%-efficient PEM fuel cell-powered car cannot exist! The real-life efficiency of such a car is about 38%. The reckless promoters of a hydrogen economy neglect to mention this important downward correction. 54 Thermodynamics of corn-ethanol biofuel. . . Web Version S o la r h ν , ∆ b0 c h = + 2 8 3 3 C6H12O6+6O2+6H2O 6CO2 ∆b0 ch = +119 ∆b0 ch = −161 2CO2+2C2H5OH+6H2O+6O2 ∆b0 ch = +184 4CO2+12H2+6O2+2CO2 6CO2 H 2 C o m b u st io n ∆ b0 c h = − 2 8 5 7 12H2O12H2O Figure 42: Exergy diagram of the ideal CO2-Glucose-EtOH-H2 cycle. Steps 3ab Ethanol oxidation to CO, and then CO2: 2C2H5OH + 2H2O → 4CO + 8H2 3a 4CO + 4H2O → 4CO2 + 4H2 3b (32) Note that Deluga et al. (2004) use a somewhat different stoichiometry with 10H2 and 10H2O, but this difference is insignificant. Remark 11 The ideal CO2-Glucose-Ethanol-H2 cycle discussed in Deluga et al. (2004) produces practically the same amount of useful chemical exergy as the ideal CO2-Glucose-Ethanol cycle. Therefore, all conclusions pertinent to the latter hold for the former. 2 9 Resource Consumption and Waste Generation in the Industrial Corn-Ethanol Cycle Now I will focus my attention on the industrial corn-ethanol cycle depicted in Figure 36. In contrast to the sun-driven ideal cycle, the industrial cycle relies heavily on fossil energy. Therefore, a part WR of the useful work Wu, must be diverted to restore the non-renewable resources depleted by the cycle, see Figure 43. As long as the useful work exceeds the restoration work, Wu > WR, the industrial corn-ethanol cycle is beneficial, otherwise it is indefensible. Remark 12 The depletion of concentrated natural resources is irrevocable, cf. Part II. Without causing changes in the environment, we cannot remanufacture the depleted amounts of oil, methane and coal in a reversible process, and put these fuels back into their deposits. Therefore, the reversible restoration work calculation provides the lowest estimate of the degree to which the irreversible industrial corn-ethanol cycle is also unsustainable. 2 From Definition 6, it follows directly that the minimum restoration work is equal to the sum of the cumulative exergy consumption (CExC) by all the processes that convert natural resources into CRPS, 23(6), 2004 T. W. PATZEK 57 0 20 40 60 80 100 120 Restoration Work Fuel Cell Efficient Car Engine Average Car Engine W u W R Corn Lowest W R EtOH BOD Treatment Increment to Patzek’s W R EtOH CExC, GJ/ha Figure 45: The minimum cumulative exergy consumption by the industrial corn-ethanol cycle and its maximum useful work, Wu. For comparison, the cycle’s ethanol is burned in an average car engine, an efficient car engine, and in an ideal fuel cell. This comparison demonstrates that the industrial corn-ethanol cycle is unsustainable by a factor of 2.3-7. No adjustment of process parameters I can think of will change this terrible situation. Note that the WR EtOH bar is the lowest fossil energy use (34 000 Btu/gal+0.75 kWh/gal advertised in 2004 by ICM, 310 North First Street, Colwich, Kansas 67030, www.icminc.com). The rightmost purple bar shows the difference between the ICM value and my estimate of fossil energy use to produce anhydrous ethanol. Better fertilization practices (Worrell et al., 1995), and artificial wetlands (Horne, 1991; Horne and Gregg, 1993; Horne et al., 1994; Horne, 1994) around the corn fields could significantly help in containing and removing the pervasive contamination these fields generate. 9.3 Restoration Work Now we are ready to estimate the restoration work of the non-renewable resources mined by the industrial corn-ethanol cycle. The results for each major input are shown in Figure 44. The three main sources of exergy consumption are ethanol plant fuel, nitrogen fertilizer, and the removal of biological oxygen demand in the ethanol plant wastewater. Note that I have not yet included the potentially huge restoration work of the High Plains aquifer (which underlies many of the Corn Belt states), other aquifers, the numerous streams and rivers which drain the field wastewater, the Mississippi River, and the Gulf of Mexico at the Mississippi River mouth. The bottom-line comparison is shown in Figure 45. Here I compare the as yet incomplete 58 Thermodynamics of corn-ethanol biofuel. . . Web Version cumulative exergy consumption by the industrial corn-ethanol cycle with the cycle’s maximum useful work performed by three different machines. This comparison reveals that the corn-ethanol cycle consumes 2.3-7 times more exergy than it replaces. The lowest deficiency of the cycle, by a factor of 2.3, is realized by employing an imaginary 60%-efficient Proton-Exchange Membrane (PEM) fuel cell to power a car. As shown in Appendix D and elsewhere, such a cell simply cannot exist (Bossel, 2003b). Real fuel cells are 2-3 orders more expensive than a car engine, 10 times less reliable, and may never be mass-produced (Keith and Farrell, 2003; Dresselhaus et al., 2003; Bossel et al., 2003; Davis et al., 2002). A 35%-efficient internal combustion engine produces 4 times less useful work than the restoration work, and today’s average car engine produces 7 times less work. Note that if one uses the lowest advertised (but not necessarily true) value of fossil fuel consumption to produce anhydrous ethanol, these factors are 2, 3, and 6 respectively. Remark 14 No matter how efficient the engine is that transforms the industrial corn-ethanol cycle’s output into shaft work, the cycle remains utterly unsustainable and unattractive as a source of fossil fuel. 2 10 Conclusions • Excluding the restoration work of decontaminating aquifers, rivers, and the Gulf of Mexico, the minimum cumulative exergy consumption in restoring the environment polluted and depleted by the industrial corn-ethanol cycle is over 7 times higher than the maximum shaft work of a car engine burning the cycle’s ethanol. • This unfavorable ratio decreases to ∼4, when an efficient internal combustion engine is used to burn the ethanol, and to 2.3 when an imaginary hydrogen fuel cell is used. • The industrial corn cycle is not renewable, and is unsustainable by a wide margin (at least 2.3 – 7 times). • No process changes can make this cycle more viable. • The annual corn-ethanol biofuel production is a human assault on geologic processes and the geologic time scale, and it can never work. • The limiting factors, nutrient-rich humus and water that carries the dissolved nutrients to plant roots are augmented by chemicals obtained in the linear, irreversible fossil fuel-based processes. • Over the last fifty years, corn yield has grown five-fold, mostly because of the steep increases in fertilization rate of corn fields. • Sunlight is not a limiting factor, and could be used to great benefit if we were in less of a hurry, cf. Appendix C. CRPS, 23(6), 2004 T. W. PATZEK 59 In vain, through every changeful year, Did Nature lead him as before; A primrose by a river’s brim A yellow primrose was to him, And it was nothing more. — WILLIAM WORDSWORTH, Peter Bell (1819) The outside world is something independent from man, something absolute, and the quest for the laws which apply to this absolute appeared to me as the most sublime scientific pursuit in life — MAX PLANCK Scientific Autobiography, and Other Papers (1949) 62 Thermodynamics of corn-ethanol biofuel. . . Web Version of corn-ethanol into useful work, the difference between the minimum restoration work and the maximum useful work by the cycle will vary. This difference in GJ/ha, can be translated roughly into $/ha, and into the cumulative environmental cost of the industrial corn-ethanol cycle. The hidden cost of mining the environment by the industrial corn-ethanol cycle is real, but rarely mentioned. According to the RFA President Bob Dinneen, ethanol displaces imported crude oil. Therefore, to arrive at an estimate of corn-ethanol’s environmental costs, I assume that the total exergy deficit will be “paid” with the imported crude oil, whose price in the first half of 2004 was close to $35/barrel. I also assume that 1 barrel of oil is 136 kg of 350 API oil, with the specific energy content of 45 MJ/kg. The results are listed in Table 25. In 2004, the environment has contributed an estimated 1.8 billion dollars per year by being continuously and irrevocably damaged and depleted. This huge gift to the corporate coffers from the U.S. rural population, soil, water, air, plants, and wildlife is as real as the federal tax subsidies. The 60%-efficient fuel cell car does not exist now, or in the future (Bossel, 2003b; Patzek and Pimentel, 2006). But even if in the next 20-years we were to replace all existing cars with efficient fuel cell cars, the environment’s contribution would still be $1.2 billion per year. If 10% of fuel consumption in the U.S. were supplied by corn-ethanol, the annual contribution from the environment would be $12 billion. Again, my current estimate should be viewed as the lowest bound on the environmental costs for two reasons: (1) The true restoration work is irreversible and significantly larger than the reversible restoration work, and (2) I have not yet calculated the minimum reversible work of restoring surface and ground water, and soil contaminated by the corn field runoff water. 31/01 28/02 31/03 30/04 31/05 30/06 30/07 31/08 30/09 31/10 30/11 31/12 0 5 10 15 20 25 30 35 40 45 50 2000 Max 0.18 2001 Max 0.19 2002 Max 0.17 2003 Max 0.23 E x ce ed an ce , % Figure 47: The cumulative one-hour exceedances of maximum legal ozone level in Southern Cali- fornia. Source: Cal Hodge, President of A 2nd Opinion, Inc. All the subsidies to corn growers, ethanol producers and distributors are compared in Figure 46. CRPS, 23(6), 2004 T. W. PATZEK 63 4 Public Health Problems The stated goal of adding ethanol from corn to gasoline was to help in cleaning the air we breathe and lessen the U.S. dependence on foreign oil. The opposite is achieved. Air becomes more polluted, and as much oil and more methane are burned as without the corn-ethanol. At the same time, additional health hazards are created by the agricultural chemicals, fertilizers, insecticides and herbicides, and by the waste water streams. For example, in 2002, twelve Minnesota ethanol refineries agreed to spend $2 million per plant, pay penalties of $29,000-$39,000, and limit the following air emissions64 • Volatile organic compounds by 2400 - 4000 tons per year, • Carbon monoxide emissions by 2000 tons per year, • Nitrogen oxides emissions by 180 tons per year, • Particulate matter by 450 tons per year, • Other hazardous air pollutants by 250 tons per year. Ethanol-in-gasoline seriously pollutes the air (Hodge, 2002). The reactivity of the combined exhaust and evaporative emissions using the ethanol-blended reformulated gasoline is estimated to be about 17% larger than those using the MTBE-blended reformulated gasoline (NRC, 1999). Ethanol does reduce the carbon monoxide emissions, but increases those of nitrogen oxides (NOx), acetaldehyde, and peroxy-acetyl-nitrate (PAN) (Rice et al., 1999). The negative effects of using gasoline-ethanol blends are clearly seen in Southern California, where ozone levels in the air ex- ceeded the one-hour legal limits more often, see Figure 47. By 2003, over 70% of gasoline produced in Southern California was blended with ethanol. In 2004, the California Air Quality Board completed a study (Hancock, 2005) in which the fuel systems of several vehicles were tested for diurnal evaporative permeation emissions with fuels containing MTBE and EtOH. The results were applied to the existing fleet in the South Coast Air Basin and Sacramento. Their analysis showed a 17-ton-per-day (ton/d) increase in the South Coast Air Basin on an ozone episodic day, an increase of 14% for evaporative emissions. For the Sacramento Metropolitan area, the increase in evaporative emissions due to ethanol was estimated to be 18% and 2.4 ton/d. In an earlier draft report, the California Air Quality Board concluded (ARB, 2005) that in the South Coast Air Basin alone, the removal of ethanol oxygenate from the reformulated gasoline CaRFG3 would decrease hydrocarbon emissions by 27.4 ton/d, NOx emissions by 6.7 ton/d, and increase CO emissions by 1.4 tons/d. So much for the cleaner air. . . Friends, Romans, countrymen, lend me your ears; I come to bury Cæsar, not to praise him. The evil that men do lives after them; The good is oft interred with their bones; — WILLIAM SHAKESPEARE Julius Cæsar Act 3, Scene 2, (1599) 64Cat Lazaroff - ENS, 3 Oct 2002. 64 Thermodynamics of corn-ethanol biofuel. . . Web Version Part V Summary & Conclusions The purpose of this paper was to prove beyond any reasonable doubt that the industrial corn-ethanol cycle accelerates the irrevocable depletion of natural resources: fossil fuels, minerals, top soil, surface and subsurface water, and air, while creating wide-spread environmental damage throughout the continental United States. My arguments relied entirely on the First and Second Law of thermodynamics, and on the Law of Mass Conservation. I have tried to avoid political questions, but at some point one should ask how it was possible for a poor agri-industrial technology to grow so explosively in the last four years? The only plausible answer lies in politics. The recent growth of ethanol production could occur only because of the massive transfer of money from the collective pocket of the U.S. taxpayers to the transnational agricultural cartel, represented most notably by Archer Daniel Midlands Co., Cargill Inc., Monsanto Co., and A. E. Stanley Manufacturing Co. This flow of billions of dollars from the pockets of the many to the pockets of the few was accomplished by federal subsidies of corn producers, and the federal and state tax subsidies of ethanol producers. It was spearheaded by many powerful, and I would like to think, thoroughly misinformed politicians. More ominously, as a country, we have diverted our collective attention from the most important issue of this century: energy conservation and increased reliance on the only renewable source of energy, the sun, and its weak derivative, the wind, see Appendix C. Instead, we have somewhat accelerated the rate of depletion of the precious natural gas and crude oil deposits, in exchange for the significantly more wide-spread pollution of water, soil and air over roughly 1/2 of the area of the United States, the incremental carbon dioxide emissions, the substandard ethanol fuel, and the continuous drain of taxpayers’ money. To make things worse, the scientific community in the U.S. seems to be preoccupied with promulgating empty illusions of a future global energy bliss brought about by the new and sexy, but inherently unsustainable technologies. The ethanol biofuel for hydrogen (Deluga et al., 2004), the fossil fuel-based “hydrogen economy” (Davis et al., 2002; Bossel et al., 2003; Dresselhaus et al., 2003; Keith and Farrell, 2003; Tromp et al., 2003; DOE, 2003), the subsurface carbon dioxide sequestration (Celia, 2002), etc., come to mind. I suggest that we – the scientists – should instead be advocating the simpler and less expensive, but painful, real solutions of the overwhelming energy problems facing the world. These solutions must involve far more energy conservation in every aspect of the U.S. economy, and the significantly increased reliance on the sun. The philosophical, ethical, and political arguments ought to be developed further, but I will leave this task to the others, see e.g., the transcript of an excellent speech by Nicholas E. Hollis, Ethics and Agribusiness – In Search of the New Food Security, given in Newcastle-on-Tyne, United Kingdom, March 15, 2004. Here I will only reiterate the following: 1. The industrial corn-ethanol cycle brings no energy savings and no lessening of the U.S. energy dependency on foreign crude oil, natural gas, and liquified petroleum gas. The opposite happens, (a) we import somewhat more methane, LPG, and crude oil; (b) we burn these fossil fuels to produce corn and ethanol; and (c) we burn the corn ethanol in car engines. All three steps of this cycle increase the extent of environmental damage beyond that caused by burning the same fossil fuels directly in the cars. 2. The industrial corn-ethanol cycle generates more carbon-dioxide equivalents than would be generated by the energy-equivalent quantity of gasoline or diesel fuel penalized by 15% to CRPS, 23(6), 2004 T. W. PATZEK 67 25. One hectare of solar cells placed anywhere can free 100 hectares of fertile agricultural land from industrial corn, and allow for the low-intensity, diversified, and almost sustainable agriculture. 17 Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them. 18 For they that are such serve not our Lord Jesus Christ, but their own belly; and by good words and fair speeches deceive the hearts of the simple. — THE EPISTLE OF PAUL THE APOSTLE TO THE ROMANS, (The New Testament) 68 Thermodynamics of corn-ethanol biofuel. . . Web Version Acknowledgements More than 60% of the work presented in this paper was performed at TU Delft, where I was a Visiting Professor at the Earth Sciences Department between February and June 2004. I would like to thank Prof. Jan-Dirk Jansen for hosting me and arranging financial support from TU Delft. I thank the bright and intellectually engaged Freshman Students at U.C. Berkeley, who attended my Freshman Seminars in Spring and Fall 2003, and greatly contributed to the ideas presented here. I thank Prof. David Pimentel of Cornell for patiently answering my endless questions, critique of my arguments, reviews of the evolving manuscript, and kind words of encouragement. I thank Prof. Clayton J. Radke of U.C. Berkeley for his critique of the early versions of the manuscript and suggestions of numerous improvements. Prof. Radke’s persistent skepticism has sharpened my arguments immeasurably. I also thank my son Lucas J. Patzek, a biochemistry senior at U.C. Santa Cruz, for meticulously correcting and improving the manuscript, as well as mitigating many of my statements. Subsequent to the CRPS article publication, Dr. Drew Ronneberg of Technology and Man- agement Services, Inc. has pointed out my error in calculating the theoretical efficiency of ethanol production from corn, kindly provided me with additional information about corn ethanol refineries, and highlighted the incompleteness of Jevons’ arguments about coal production. CRPS, 23(6), 2004 T. W. PATZEK 69 A Examples of Entropy Production and Disposal The Rate of Entropy Export by the Earth In the simplest model, the earth is in thermal equilibrium; continuously heated by the sun’s radia- tion, and cooled by the infrared radiation into the universe. The solar constant is the power collected at the top of the earth atmosphere by a unit area (1 m2) perpendicular to the light path. This power is remarkably constant, see e.g., (Hickey et al., 1980), and equal to q̇ = 1370 Watts65 per square meter (W/m2). The projection of the sun-lit earth hemisphere in the direction perpendicular to the sun light is πr2e , where re = 6371 km is the mean volumetric earth radius, or 1/2 of the hemisphere area, and 1/4 of the earth surface area (Ae ≈ 510 × 10 6 km2). The Planck temperature of the sun’s radiation is Ts ≈ 5700 K, and the Planck temperature at which the earth radiates its heat is Te ≈ 254 K 66. The earth reflects about 30% of the sun radiation, so its surface is reached by only 0.7 of the solar energy. Therefore, the time-averaged flux of entropy exported by the earth into the universe is jES = ∆Ṡ 4πr2e = 4 3 0.7q̇ 1 4 ( 1 Te − 1 Ts ) = 1.20 W/K-m2 (34) where the factor 4/3 comes from the Stefan-Boltzmann law67. This estimate agrees very well with the jES = 1.2 W/K-m 2 reported by Prof. Arne Stahl (1996). The Simplest Climate Model The earth is in thermal equilibrium: Rate of energy input from the sun = Rate of energy radiation by the earth Ėin = Ėout = P = const (35) Similarly to Frondel, Oertel and Rübbelke (2002), I assume that the earth’s atmosphere is a heat-transporting68 gas layer. The surface temperature of the earth is T0 and the Planck temperature of its radiation is Te. The stationary heat flow through the atmosphere occurs at a constant rate: P = kAe(T0 − Te) (36) where Ae is the surface area of the earth. In this simple model, the steady-state rate of energy export from the earth depends on the mean temperature difference between the earth surface and the uppermost atmosphere. The overall heat transfer coefficient, k, depends on how effectively the atmosphere transports heat. With the increasing concentration of heat absorbers (the greenhouse gases) this coefficient decreases, and the earth temperature must go up. At steady state, Second Law of thermodynamics requires the entropy flow rate to satisfy the following equation: P = T0Ṡ0 = TeṠe (37) 651 Watt = 1 Joule/second. 66The actual temperature of the earth surface is about 34 K higher due to the greenhouse effect. Therefore there is entropy generation in the atmosphere, see the section below. 67See Szargut’s monograph, (1988), page 72, Eq. (2.53). 68Heat transport through the atmosphere proceeds through turbulent convection and mixing, water evaporation and condensation, thermal conduction, and radiation. 72 Thermodynamics of corn-ethanol biofuel. . . Web Version B Availability and Irreversibility in Thermal Systems Because the earth can export entropy by infrared radiation from the outer layers of the atmosphere, in this appendix I define any thermodynamic system on the earth as interacting with the atmosphere only. The atmosphere will be treated as being in stable dynamic equilibrium, and characterized by the constant absolute temperature T0, volume Va, and the hydrostatic pressure, p0 = 1 atm, at sea level. By including within the system as much surface land area, surface water, groundwater, minerals, material, plants, machinery, etc.72, as affected by the process of interest, one is always able to construct the system that interacts with the atmosphere only. For simplicity, chemical entropy generated by the system is not considered here. Gibbs73 showed that for any process which can occur under these circumstances the quantity defined as Φ = E + p0V − T0S (43) decreases ∆Φ ≤ 0 (44) where E is the total energy of the system, V its volume, S its entropy, and the increment of Φ, ∆Φ, is taken in the direction of increasing chronological Time. The process of interest can only occur until the system pressure is uniformly hydrostatic and its uniform temperature is T0. Therefore, the state from which no spontaneous change can occur is the state in which the system has the hydrostatic pressure (p0 at sea level) and the atmospheric temperature T0, and for which Φ has the smallest possible value, Φmin. If only one state of the system results in this minimum value, the system is in stable equilibrium. Otherwise, if there are several states corresponding to the minimum value of Φmin, the system is in neutral equilibrium of maximum stability. Gibbs74 referred to the difference Φ − Φmin, (45) where Φ corresponds to the state in question, as the “. . . available energy of the body (our system) and the medium (our surroundings).” Joseph H. Keenan (1951) later showed that for the system undergoing change from an earlier state 1 to a later state 2, the amount of useful work Wu is Wu ≤ Φ1 − Φ2 ≤ Φ1 − Φmin (46) Therefore, for any state 1, the maximum possible useful work done by the system is Φ1 − Φmin. Keenan proposed to call this maximum value availability, Λ. It may be said75 that for any system in the stable atmosphere Λ ≥ 0 (47) and that for the most stable state of the system Λ = 0 (48) From Eq. (46) it also follows that ∆Λ = ∆Φ Wu ≤ Wu,max = Φ1 − Φmin = −∆Φ = −∆Λ (49) 72The entire globe, if necessary. 73(Gibbs, 1994), p. 40. 74(Gibbs, 1994), p. 53; my comments in italics. 75(Keenan, 1951), Eqs. (12) and (13). CRPS, 23(6), 2004 T. W. PATZEK 73 Keenan also quantified the irreversibility76, I, of a process executed by the system-atmosphere combination: I =Wu,max − Wu = − ∆Φ − Wu = − ∆Λ − Wu (50) and showed that I =T0∆S + ∆Ea + p0∆Va =T0∆(Sa + S) (51) Of course, Keenan’s irreversibility was discovered much earlier and independently by Gouy (1889) and Stodola (1898; 1927)77. Regardless, the irreversibility of a process is equal to the increase of entropy of everything involved in the process multiplied by the temperature of the atmosphere. One may use Eq. (51) in the differential form, and per unit area of the earth surface, by writing 1 Ae dI dt =T0 ( 1 Ae dSa dt + 1 Ae dS dt ) σI =T0(σa + σ) (52) where σI is the specific rate of irreversibility, σa is the specific rate of entropy increase in the atmosphere generated by the process, and σ is the specific rate of entropy increase of the system. Note that as the atmospheric temperature increases, so does the irreversibility of any process on the earth. From Appendix A it follows that we can treat the atmosphere as an open system that exports entropy to the universe with the flux jES calculated from Eq. (34). The energy transport through the atmosphere generates entropy at the specific rate of σt calculated from Eq. (39). Thus, we may rewrite Eq. (52) as σnetI /T0 = (σa + σ) ︸ ︷︷ ︸ Thermal entropy from Earth processes + (−jES + σt) ︸ ︷︷ ︸ Thermal entropy from Solar processes (53) Using equation (53), we can define sustainability as σnetI ≤ 0 σa + σ − j E S + σt ≤ 0 For all places on the earth, and at all times (54) For sustainability, equation (54) requires that the net rate of increase of entropy of everything at every place on the earth and for all times be less or equal to zero! Note that the process and energy transport increase the entropy of the earth and the atmosphere, and low-temperature heat radiation decreases it. Remark 15 As derived, Eq. (54) is quite deceiving. The anthropogenic part of the thermal entropy generation rates, σ + σa, can only be sustainable if this entropy is generated in cycles in which all process materials are completely recycled, and all chemical entropy is transformed into thermal entropy. If we rely on a finite stock of fossil energy, then even if the entropy generation rate in a process is much lower than the entropy export flux, the process is never sustainable. Therefore, sustainability can only be discussed in the context of cyclic processes. 2 76(Keenan, 1951), Eqs. (31) - (36). 77In particular, Volume II of Stodola’s monograph (1927), pp. 1271-1330, brings a thorough discussion of The Highest Possible Conversion into Work on the Basis of the Second Law of Thermodynamics. 74 Thermodynamics of corn-ethanol biofuel. . . Web Version Over an arbitrary time interval [τ1, τ2], we can write the global condition of sustainability of all cyclic processes on the earth as [Sa(τ2) − Sa(τ1)] ︸ ︷︷ ︸ Increase of atmospheric entropy from earth processes + [S(τ2) − S(τ1)] ︸ ︷︷ ︸ Increase of earth entropy −SE(τ1, τ2) + St(τ1, τ2) ≤ 0 For the entire earth, and arbitrary τ1, τ2 (55) where SE(τ1, τ2) = j E S Ae(τ2 − τ1) is the total thermal entropy exported by the earth over the time interval [τ1, τ2], and St(τ1, τ2) = σtAe (τ2 − τ1) is the corresponding thermal entropy generated in the atmosphere by the steady-state energy transport to the universe. CRPS, 23(6), 2004 T. W. PATZEK 77 D Efficiency of a Fuel Cell System In their Science paper, Deluga et al. (2004) claim the following: . . . Further, combustion used for transportation has ∼20% efficiency as compared with up to 60% efficiency for a fuel cell. . . The efficiency of these processes for a fuel cell suggests that it may be possible to capture >50% of the energy from photosynthesis as electricity in an economical chemical process that can be operated at large or small scales. (p. 996). Following Deluga et al.’s paper and common chemical engineering knowledge, I will assume the following: 1. The catalyst is made of a rare-earth metal, rhodium79, and a Lanthanoid, cerium80. 2. The catalytic reaction has 100% selectivity and >95% conversion efficiency. I will assume here the conversion efficiency η1 = 0.96. 3. The ethanol-water reactant mixture for the catalytic conversion to hydrogen is ultra pure (on the order of 99.9999% pure C2H5OH and H2O). Otherwise, any and all of the impurities listed in Footnotes 81 and 82, and carbon deposition, will destroy the catalyst and reactor. Therefore, one must separate all impurities from the 8 − 10% alcohol81-yeast-rest beer82 by multiple distillations, rectification, and molecular sieve exclusion (Maiorella, 1985). As described in Part II, all these processes consume irreversibly large amounts of free energy. 4. The reaction products are hydrogen, carbon dioxide, carbon monoxide, water vapor, plus whatever other impurities. The hydrogen fuel separation from all other components of reactor effluent is almost perfect. Compared with the reference hydrogen of 99.9999999% purity, even tiny amounts of impurities can cause a noticeable drop in performance of a fuel cell83. Some of these impurities are CO (at 5-10 ppm), SO2 (at 2 ppm), H2S (at 1-2 ppm), HCHO (at 10-20 ppm), and HCOOH (at 50-100 ppm). 5. In summary, by the time we are ready to use the ethanol-derived hydrogen in a fuel-cell/electric car, we have spent a lot of free energy on picking ethanol, water, and hydrogen molecules; much more than on distilling the relatively dirty ethanol to be mixed with gasoline. The latter free energy expenditure is ∼15 MJ/L EtOH, or >50% of the ethanol’s high heating value. 79Rhodium is a precious metal whose price is about US$30 000/kg, 3×more expensive than gold, http://- www.kitco.com/charts/rhodium.html. 80The nanoparticles of cerium dioxide are called ceria, and cost $250/kg, http://www.advancedmaterials.us/58N- 0801.htm 81Alcohol dissolves a large number of substances insoluble in water and acids, such as many inorganic salts, phosphorus, sulphur, iodine, resins, essential oils, fats, coloring matters, etc. (Wright, 1994). 82The beer obtained by mashing and fermenting consists of volatile components, such as water, alcohols (mostly ethyl, but also traces of methyl, propyl, butyl, amyl, and other alcohols – depending on the impurities in glucose), essential oils, and a little acetic acid; and of non-volatile substances, such as cellulose, dextrine (an intermediate product of starch hydrolysis), unaltered sugar and starch, mineral matter, lactic acid, glycerol, etc. 83Japan Automotive Research Institute (JARI), DOE Hydrogen Codes and Standards, Coordinating Commit- tee Fuel Purity Specifications Workshop, April 26, 2004, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/- fuel − purity − notes.pdf 78 Thermodynamics of corn-ethanol biofuel. . . Web Version After Bossel (2003a), I will summarize efficiency of a Proton Exchange Membrane (PEM) fuel cell as follows. In fuel cells, gaseous hydrogen is combined with oxygen to water. This process is the reversal of the electrolysis of liquid water and should provide an open circuit voltage of 1.23 V (Volts) per cell. Because of polarization losses at the electrode interfaces the maximum voltage observed for PEM fuel cells is between 0.95 and 1.0 V. Under operating conditions the voltage is further reduced by ohmic resistance within the cell. A common fuel cell design voltage is 0.7 V. The mean cell voltage of 0.75 V may be representative for standard driving cycles. Consequently, the average energy released by reaction of a single hydrogen molecule is equivalent to the product of the charge current of two electrons and the actual voltage of only 0.75 V instead of the 1.48 V corresponding to the hydrogen high heating value84. Therefore, in automotive applications, PEM fuel cells may reach mean voltage efficiencies of η2 = 0.75 V 1.48 V = 0.50 (56) However, there are more losses to be considered. The fuel cell systems consume part of the generated electricity. Typically, automotive PEM fuel cells consume 10% or more of the rated stack power output to provide power to pumps, blowers, heaters, controllers, etc. At low power demand the fuel cell efficiency is improved, while the relative parasitic losses increase. The small-load advantages are lost by increasing parasitic losses. Let us assume optimistically that for all driving conditions the net power output of an automotive PEM fuel cell system is about η3 = 0.9 of the power output of the fuel cell stack. Depending on the chosen drive train technology, the DC power is converted to frequency- modulated AC or to voltage-adjusted DC, before motors can provide motion for the wheels. Energy is always lost in the electric system between fuel cell and wheels. The overall electrical efficiency of the electric drive train can hardly be better than η4 = 0.9. By multiplying the efficiency estimates, one obtains for the maximum possible tank-to-wheel efficiency of a hydrogen fuel cell vehicle η = η1η2η3η4 = 0.96 × 0.50 × 0.90 × 0.90 = 0.38, (57) or 38%. This optimistic estimate agrees with another analysis (31-39%) (Fleischer and Ørtel, 2003), and is significantly less than the 60% used by the reckless promoters of a hydrogen economy and hydrogen fuel cell vehicles. 84According to Faraday’s Law, the standard enthalpy of combustion of hydrogen, ∆H0f = −285.9 kJ/mol, can also be expressed as an electrochemical potential (“standard potential”) U0 = −∆H0f/neF = 1.48 V with ne = 2 being the number of electrons participating in the conversion and F = 96485 Coulomb/mol the Faraday constant. CRPS, 23(6), 2004 T. W. PATZEK 79 E Cumulative Exergy Consumption in Steel Production The primary commercial iron ores in the world are hematite (Fe2O3) and magnetite (Fe3O4). Taconite, the principal iron ore mined in the United States, has a low (20 – 30%) iron content and is found in hard, fine-grained, banded iron formations. About 99% of iron ore is used in the iron and steel industry. Scrap can be considered a supplement to iron ore in the steelmaking process but is limited as a major feed material owing to inadequate supply of high-quality scrap. Alternatives, such as direct reduced iron (DRI), are also available, and their use continues to grow. Ore is put into a blast furnace and smelted to produce molten iron, which is then converted to steel by removing most of the remaining carbon in a basic oxygen furnace (BOF). Almost all molten iron goes directly to the BOF, eliminating the molds. The blast furnace product is usually referred to as pig iron. Iron ore consumption in the U.S. in 2003 was 61 million metric tonnes (Mt), a rise of slightly more than 1 Mt from that of 2002. There was an average of 30 blast furnaces active during 2003, up slightly from that of 2002 when the average number of blast furnaces operating was 29, the lowest since 1961. Accordingly, pig iron production at 40.6 Mt in 2003 was slightly above that of 2002, which had been the lowest since 1982. Crude steel production at 94 Mt increased by 2% compared with that of 2002. Steel demand remained constant at revised 2002 levels of 107 Mt. The large difference between ore production and steel demand is explained by examining the minimill sector and net imports of iron ore substitutes. In 2003, the minimill sector of the steel industry produced more than 50% of the crude steel in the United States. Minimills do not use iron ore as feedstock; instead they use iron and steel scrap, and some DRI in electric arc furnaces (EAF). For a more detailed summary, see Jorgenson & Kirk (2003). Several literature estimates of primary energy consumption in producing steel via integrated U.S. mills and BOF are listed in Table 26. The same-source estimates of primary energy consump- tion in producing steel from 100% scrap in EAF are listed in Table 27. The embedded energy and equivalent CO2 emissions in SAE 1045 and SAE 15V45 steel are listed in Table 28. E.1 Steel component manufacturing As an example, the track roller shaft in a Caterpillar tractor has been investigated in (Beloff et al., 2004). This track roller is conventionally produced with medium carbon steel, typically SAE 1045 or modified SAE 5038. Due to physical and cost benefits at least within the “shape and treat” manufacturing stage of the lifecycle, the part is now produced with microalloyed steel, namely SAE 15V45. In addition to the primary energy contained in the energy sources, exergy includes work that is available in other materials that contribute to the lifecycle. For simplicity, only iron and minerals that make up the compositions of the steels have been included in (Beloff et al., 2004) in addition to the primary energy, i.e., wastes and auxiliary materials have been ignored. With these assumptions, the primary energy embedded in the track roller is 30 and 27 MJ/kg-part, respectively, for SAE 1045 and SAE 15V45 steels. The standard chemical exergy embedded in these two steels is 6.7 MJ/kg. The incomplete cumulative exergy consumption (CExC) is 37 and 34 MJ/kg-part respectively. The GHG emissions are about 2.7 kg CO2 equiv./kg-part. Other, apparently more complete estimates of cumulative exergy consumption in steelmaking (Szargut et al., 1988) are listed in Table 29. They range from 46 to 84 MJ/kg of finished steel products. 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PATZEK 87 Part VI Tables Table 1: Average dry mass composition of corn grain (White and Johnson, 2003) Component % by mass Starch 66 Oil 3.9 Gluten feed (21% protein) 24 Gluten meal (60% protein) 5.7 Losses 0.4 Table 2: Average application rates of corn field chemicals in 2001 (NASS, 2002) Compound True Fraction ha Mean kg/ha w/ applied kg/ha N 148.8 0.96 142.8 P2O5 a 62.5 0.79 49.4 K2O a 93.5 0.65 60.8 Herbicides 2.54 0.98 2.49 Insecticides 1.08 0.29 0.31 aUSDA (NASS, 2002) reports “P” and “K” but, according to Ms. Barbara Tidwell of the NASS/MISO Customer Service, they mean P2O5 and K2O. Table 3: Specific energy consumption and application rates of nitrogen fertilizer Active Specific Application Source Ingredient Energy MJ/kg Rate kg/ha N 63.43 148.0 Pimentel, 2003 N 54.43 148.8 Patzek, 2004 N 43.00 140.0 Shapouri et al., 2002 N 49.06 153.0 Wang et al., 1997 N 54.43a 150.0 Berthiaume et al., 2001 a Berthiaume et al. (2001) do not give the specific N, P, K, Ca fertilizer energies, only the specific exergies. I have assumed that the specific energies of Berthiaume et al. are equal to my estimates, and used their reported application rates. 88 Thermodynamics of corn-ethanol biofuel. . . Web Version Table 4: Energy consumption in superphosphate production (Kongshaug, 1998) Process MJ/kg P2O5 Phosphate mining > 0.3 Apatite mining 2.9 Dihydrate process 2.5 Hemihydrate process 6.5 Table 5: Specific energy consumption and application rates of phosphorus fertilizers Active Specific Application Source Ingredient Energy MJ/kg Rate kg/ha P2O5 17.44 53.0 Pimentel, 2003 P2O5 6.80 62.5 Patzek, 2004 P2O5 4.76 54.0 Shapouri et al., 2002 P2O5 11.40 56.0 Wang et al., 1997 P2O5 6.80 55.0 Berthiaume et al., 2001 Table 6: Energy consumption in potassium fertilizer production (Kongshaug, 1998) Fertilizer K% MJ/kg K2O Chloride 52 6.8 (additive to phosphates) Sulphate 49 Not reported Nitrate 45 13.5 (KNO3 solution evaporation) 43 MJ/kg N Table 7: Specific energy consumption and application rates of potassium fertilizer Active Specific Application Source Ingredient Energy MJ/kg Rate kg/ha K2O 13.77 57.0 Pimentel, 2003 K2O 6.80 93.5 Patzek, 2004 K2O 8.71 85.0 Shapouri et al., 2002 K2O 5.30 66.0 Wang et al., 1997 K2O 6.80 85.0 Berthiaume et al., 2001 CRPS, 23(6), 2004 T. W. PATZEK 89 Table 8: Specific energy consumption and application rates of calcinated lime Active Specific Application Source Ingredient Energy MJ/kg Rate kg/ha CaO 1.33 699.0 Pimentel, 2003 CaO 1.75 333.0a Patzek, 2004 CaO 1.70 276.0 Shapouri et al., 2002 CaO 1.70 276.0b Wang et al., 1997 CaO 1.75 270.0 Berthiaume et al., 2001 aShapouri et al’s data (Table 2 in (Shapouri et al., 2002a)) with the two zero entries omitted. bWang et al. (1997) does not report lime use, I have corrected their estimates by adding the lime use reported by Shapouri et al. (2002a). Table 9: Specific energy consumption and application rates of herbicides Specific Application Source Energy MJ/kg Rate kg/ha 422.00 2.10 Pimentel, 2003 261.00 2.54 Patzek, 2004 261.00 4.73 Shapouri et al., 2002 237.30 3.07 Wang et al., 1997 Table 10: Specific energy consumption and application rates of insecticides Specific Application Source Energy MJ/kg Rate kg/ha 422.00 0.15 Pimentel, 2003 268.40 1.08 Patzek, 2004 268.40 0.22 Shapouri et al., 2002 243.00 0.22 Wang et al., 1997