environmental chemistry chapter19

Chapter 19SUSTAINABLE ENERGY: THE KEY TO EVERYTHING

Environmental Chemistry, 9th EditionStanley E. Manahan

Taylor and Francis/CRC Press2010

For questions, contact:Stanley E. Manahan

manahans@missouri.edu

19.1. THE ENERGY PROBLEMGiven sufficient energy

• Transportation: Electrified railways and vehicles can be used for most land transportation

• Fuels: Biomass sources of fixed carbon can be converted to hydrocarbon fuels to be used in applications for which there are no viable alternatives (such as aircraft) without emitting net amounts of carbon dioxide

• Water: Wastewater and saline water can be converted to water pure enough to drink

• Food: Marginal land can be reclaimed, water can be pumped long distances, greenhouses for growing vegetables can be heated

• Wastes: All hazardous wastes converted to benign forms

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19.2. NATURE OF ENERGYEnergy is the capacity to do work (move matter)Heat is energy in the movement of atoms and moleculesKinetic energy is contained in moving objects• Flywheels used for energy storagePotential energy is stored energy• Water in a reservoir that can be used in turbines• Chemical energy released in chemical reactions• Electrical energy in charged capacitorsMechanical energy in a spinning turbine and generator armatureElectrical energy converted from mechanical energy in a generator

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Units of Energy and PowerThe joule (J) is the standard unit of energy• 4.184 J raises temperature of 1g of liquid water 1˚C4.184 J = 1 calorie (cal)Energy often expressed in kilojoule• 1 kJ = 1000 JFood energy commonly stated in units of 1000 cal, commonly called “calories”• 1 food calorie = 1000 cal

PowerPower is energy per unit time

Watt = 1 joule per second (J•S-1)

• A typical compact fluorescent light bulb is 21 watts• A large electrical power plant is typically 1000 megawatts (mw)• Very large amounts of power in gigawatts (1 gw = 109 w)

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ThermodynamicsThermodynamics deals with energy in its various forms and with workFirst law of thermodynamics states that energy is neither created nor destroyed• Law of conservation of energyThe first law of thermodynamics is important in green science and technology the best practice of which requires the most efficient use of energy• According to thermodynamic laws only a fraction of chemical energy

can be converted to heat energy and then mechanical energy (as occurs in an internal combustion engine)

• Typically, the “unused” energy is dissipated as heat• Green technology utilizes this energy for heating

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19.3. SOURCES OF ENERGY USED IN THE ANTHROSPHEREBefore 1800 most energy from renewable sourcesBiomass sources• Wood for heating• Food for animals and humansWind-driven sailing ships and windmills• Wind from solar heating of air massesWater-driven waterwheels• Water from solar driven hydrologic cycleShift to coal energy in 1800s• Steam engine from around 1800Development of petroleum energy during 1900sNatural gas a major energy source since around 1950Hydroelectric a significant source by around 1950Nuclear energy a significant source by 1975Some geothermal energySmall but growing solar and wind energy

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Figure 19.1. Sources of Energy

U.S. World

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Figure 19.2. Original Amounts of World Fossil Fuels8

Relative Amounts of Water Produced Per Atom of Fossil Fuel C burnedLarger amounts of water produced per carbon atom burned reflect less greenhouse gas carbon dioxide released per unit energy produced

• Natural gas, CH4, produces the most H2O per C atom burned

CH4 + 2O2 CO2 + 2H2O + energy

• Petroleum, approximate simple formula CH2, produces half as much H2O per C atom burned as does natural gas

CH2 + 3/2O2 CO2 + H2O + energy

• Coal, approximate simple formula CH0.8, produces the least H2O per C atom burned of all fossil fuels CH0.8 + 1.2O2 CO2 + 0.4H2O + energy

• Biomass, approximate simple formula {CH2O}, produces the same amount of H2O per C atom burned as does petroleum, but the C was recently removed from the atmosphere by photosynthesis, so there is no net addition of CO2 to the atmosphere

{CH2O} + O2 CO2 + H2O + energy

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Figure 19.3. Examples of Energy Conversion Devices10

Figure 19.4. Energy Conversion Efficiencies

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Figure 19.5. Steam Turbine, a Heat Engine for Conversion of Heat to Mechanical Energy

The Carnot equation expresses the efficiency by which heat energy is converted to mechanical energy

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Figure 19.6. An Internal Combustion Piston Engine

Higher compression, as in a diesel engine, yields higher peak temperatures and greater efficiency

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Figure 19.7. Fuel Cells Produce Electricity Directly by the Reaction 2H2 + O2 H2O + electrical energy

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19.5. GREEN TECHNOLOGY AND ENERGY CONVERSION EFFICIENCYIncreased efficiency of chemical energy heat energy mechanical energyMuch of the gain due to increased peak temperatures in heat engines• Carnot equation• Improved materials and lubricants to withstand higher temperaturesComputerized control has greatly improved internal combustion engine efficiencies• Ignition timing • Valve timing • Fuel injection

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19.6. ENERGY CONSERVATION AND RENEWABLE ENERGY SOURCESFigure 19.8. Oil required per $1000 gross domestic product showing increased efficiencies of energy utilization

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Figure 19.9. Illustration of earlier gains in fuel economy in the U.S. followed by a lack of progress as fuel economy standards were not tightened• Fuel economy standards tightened in 2007

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Figure 19.10. The hybrid vehicle is a major advance in energy conservation

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19.7. PETROLEUM AND NATURAL GASLiquid petroleum occurs in porous rock formations• Primary petroleum recovery is only about 30%• Much greater recovery with secondary and tertiary techniquesShale oil cooked from organic-bearing oil shale is a potential petroleum substitute• Severe environmental effectsNatural gas (CH4) is a fossil fuel source that produces minimal greenhouse gas CO2

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19.8. COALCoal is a carbon-rich fuel, approximate formula CH0.8

Coal is an abundant fossil energy resource, but has a number of adverse environmental effectsCoal conversion can be used to convert coal to more environmentally friendly liquid and gaseous fuels and raw materials• Sequestration of byproduct CO2 in coal conversion

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Figure 19.11. Routes to Coal Conversion21

19.9. CARBON SEQUESTRATION FOR FOSSIL FUEL UTILIZATIONTrapping CO2 before it can enter the atmosphere enables utilization of fossil fuels without adding greenhouse gas to the atmosphere Potential sinks for carbon dioxide

• Oceans, potential problems from lowering pH• Deep saline water aquifers• Porous sedimentary formations

Sequestration easiest from concentrated sources• Fermentation to make ethanol• Coal conversion

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Figure 19.12. Coal Conversion with Carbon Sequestration23

19.10 INDUSTRIAL ECOLOGY FOR ENERGY AND CHEMICALS

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Fig 19.13 The Great Plains Coal Gasification Plant in North Dakota is an example of industrial ecology applied to energy conversion

19.11. NUCLEAR ENERGYFrom splitting of uranium-235 or plutonium nuclei• Uranium-235 is only 0.7% of naturally occurring uraniumFigure 19.14. Fission of a uranium-235 nucleus with production of energy and neutrons that sustain fission

Despite problems, nuclear energy may be the best source of sustainable electrical energy

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Figure 19.15. A Typical Nuclear Fission Plant26

Nuclear Fusion Energy from Fusion of Lighter Nuclei

Despite its high potential, practical production of energy from nuclear fusion has not been accomplished

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26.12. GEOTHERMAL ENERGYFrom underground steam, hot water, hot rockFirst utilized for energy at Larderello, Italy, in 1904Now being developed in many areas including• Iceland • Japan • Russia • New Zealand • Phillipines • Geysers in northern California

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19.13. THE SUN: AN IDEAL, RENEWABLE ENERGY SOURCESunlight is an ideal source of energy• Unlimited supply • Widely available • Non-polluting• Does not add to Earth’s total heat burdenRelatively small fraction of land area could provide for all of Earth’s energy needs• 1/30 - 1/10 Arizona land area for U.S. needs

Sunlight utilized by• Photovoltaic cells• Parabolic mirrors that focus sunlight onto heat collectors• Solar heated Stirling enginesIntermittent nature of solar energy requires energy storage• Example: Superheated supercritical fluids stored underground

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Figure 19.16. A Photovoltaic Cell30

Figure 19.17. High-Efficiency Thin-Film Solar Photovoltaic Cell Using Amorphous Silicon

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19.14. ENERGY FROM MOVING AIR AND MOVING WATERThe Surprising Success of Wind PowerLong history for grain drying and water pumpingWind energy is• Completely renewable• Non-polluting’• Indirect way of utilizing solar energyWind energy can be used to generate elemental H2 and O2

• Used to make synthetic fuels from biomass and NH3 fertilizer

Many areas of he world are suitable for generating wind energy

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Figure 19.18. Representation of a Wind Farm33

Energy from Moving WaterWaterwheel has been used since ancient timesFirst hydroelectric powerplant in the U.S. in 1882By 1980 hydroelectric was 25% of world electricity and 5% of total energyChina’s enormous Three Gorges powerplant will generate 22.4 gwHydroelectric is renewable and “free” but has some environmental problems• Impoundment of streams for hydroelectric power has caused problems

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19.15. BIOMASS ENERGYBiomass from photosynthesis could provide a large fraction of fuels and feedstocks now made from petroleumBiomass energy offers some definite advantages• Renewable• Locally available, such as in agricultural areas of the U.S.• Some plants produce hydrocarbons directlyBiomass energy has some problems• Photosynthesis is inefficient energy production• Land used to produce fuels is not used to produce food

Annual world production of biomass is around 146 billion metric tons2 metric tons per acre per year for terrestrial plantsLarger production of biomass from algaeBiomass has heating value of 5000-8000 Btu/Lb, about half of coal

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Ethanol FuelProduced by fermentation of sugar from cornstarch, but with little overall gain in energyFermentation of sugar from cane sugar in tropical regions, such as Brazil, yields much more energy than that used in productionEfforts to utilize fermentable sugars from crop byproduct biomass have not proven practicalDiversion of corn to ethanol production has greatly distorted agricultural markets in the U.S.

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Biodiesel FuelOriginal diesel engines ran on vegetable oils• Problem with oil solidification at colder temperaturesPetroleum-based diesel fuels became standardNow, biodiesel fuels are being synthesized by esterification of vegetable oils

Large areas of rain forest are now being used for growing palm oil and other biodiesel feedstocks causing food shortages and environmental problems

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The Unrealized Potential of Lignocellulose FuelsCrop byproducts, such as wheat straw, and other plant biomass are composed of complex lignocellulose materialPerennial plants, such as switchgrass, can be used for lignocellulose fuelThe U.S. and many other countries could probably produce all their fuel and organic raw materials now provided by petroleum from lignocellulose sources

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Production of Fuels and Feedstocks from Lignocellulose• Direct combustion to produce heat and to raise steam to generate

electricity• Fermentation to produce ethanol (from sugar) or methane• Pyrolysis to yield gaseous fuels (particularly methane), liquids

(including hydrocarbons and oxygenated species), and solid carbon• Thermochemical gasification to produce CO, H2, CH4, byproduct

liquids, and solid carbon• Fischer-Tropsch synthesis of hydrocarbons from CO and H2 derived

from biomass• Hydrogenation of oxygenated liquids from biomass to produce

hydrocarbons• Methyl esterification of oils to produce methyl ester biodiesel fluids

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BiogasAnoxic fermentation of organic matter, {CH2O}

2{CH2O} CH4 + CO2

In rural China backyard digesters generate methane from a variety of materials• Animal wastes • Human wastes • Vegetable and crop wastesBuried in ground with surface solar heated to accelerate fermentationEspecially useful for cooking fuel

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19.16. HYDROGEN AS A MEANS TO STORE AND UTILIZE ENERGYH2 gas is an ideal fuel that produces only H2O as a combustion product Most of the 6 million tons of H2 produced in the U.S. each year is made by steam reforming methane gas followed by reaction of CO with steam• CH4 + H2O 3H2 + CO

• CO + H2O CO2 + 2H2

The CO2 can be sequestered

A more sustainable source of H2 is the electrolysis of water made conducting by dissolved salts• 2H2O + electrical energy 2H2 + O2

• H2 can be used as a fuel in piston engines or gas turbines or combined with oxygen in a fuel cell to generate electricity directly

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19.17. COMBINED POWER CYCLESFigure 19.19. Illustration of a Combined Power Cycle in Which Fuel is Used with Maximum Efficiency

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19.18 A System of Industrial Ecology for Methane ProductionReactions Involved in Gasification of Biomass with Carbon Sequestration (assuming sequestration of all CO2)

Heat production: {CH2O} + O2 CO2 + H2O + heat

Partial oxidation: {CH2O} + 1/2O2 CO + H2O + heat

Pyrolysis: {CH2O} + heat C + H2O and {CH2O} + heat CO + H2

• CO/H2 mixture is synthesis gas

Hot carbon + steam synthesis gas: C + H2O + heat CO + H2

Water-gas shift to increase H2/CO: CO + H2O + O2 CO2 + H2

Methanation: CO + 3H2 CH4 + H2O

Fischer-Tropsch hydrocarbon synthesis: 8CO + 17H2 C8H18 + 8H2O

(Similar reactions to make methanol and ethanol)

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Fig 19.20 An Industrial Ecosystem Based upon Biomass 44