Transcript Title

CHAPTER 6
ENERGY RELATIONSHIPS
From Green Chemistry and the Ten Commandments of
Sustainability, Stanley E. Manahan, ChemChar Research,
Inc., 2006
[email protected]
Energy
Energy is the capacity to do work or to transfer heat.
Heat is the form of energy that flows from a warmer to a colder
object and is due to the motion of atoms and molecules.
The internal combustion piston engine used in automobiles (below)
converts chemical energy (in fuel) to heat energy, then to mechanical
energy in rotation of a crankshaft. Fuel is taken in with air or
injected near the top of the compression stroke.
Energy
Conversion of chemical energy to heat energy
C16H34 + 33O2  16CO2 + 17H2O + heat energy
(6.1.1)
In a piston engine, the hot gases in the engine’s cylinders push the
pistons down, some of this heat energy is converted to mechanical
energy.
The standard unit of energy is the joule, abbreviated J.
• 4.184 J of heat energy will raise the temperature of 1 g of liquid water
by 1˚ C.
• This amount of heat is equal to 1 calorie of energy (1 cal = 4.184 J)
Thermodynamics
Thermodynamics is the science that deals with energy in its various
forms and with work.
The first law of thermodynamics (law of conservation of energy)
states that energy is neither created nor destroyed.
• Concentrated, useful energy in the form of hydrocarbon fuel
becomes dissipated as a slight warming of the surroundings—no
use.
The first law of thermodynamics must always be kept in mind in the
practice of green chemistry.
• Green chemistry requires the most efficient use of energy as energy
goes through a system
• If enough energy is available, almost anything can be
accomplished.
6.2. RADIANT ENERGY FROM THE SUN
Solar flux, 1,340 watts per square meter
Solar energy from thermonuclear fusion of hydrogen in the sun:
1
1
4 H
4
0
2He + 2 +1e + ener gy
Solar Energy
Most of the solar energy that actually reaches Earth’s surface does so
as visible light and infrared radiation.
• Forms of electromagnetic radiation, which includes ultraviolet
radiation, visible light, infrared radiation, microwaves, and radio
waves.
Amplitude
Wa veleng th
Sho rter wa veleng th,
hig her frequency,
g reater energ y
Wavelength (, Greek lambda), amplitude, and frequency (, “nu”)
 = c
(6.2.2)
•  in meters (m)
•  is in cycles per second (s-1), hertz, Hz
The wavelength is the distance required for one complete cycle and
 is the number of cycles per unit time.
Electromagnetic Radiation
Energy, E, is associated with electromagnetic radiation.
• Packets of energy called quanta
• According to the quantum theory, electromagnetic radiation, can
be absorbed or emitted only in discrete quanta, also called
photons.
• Photons have energy, E
• E = h where h is Planck’s constant, 6.63 x 10-34 J-s (joule x
second).
Outbound energy from Earth is in the infrared region above 700
nanometers (nm 1 x 10-9 meters).
• Reabsorbed by greenhouse gases, such as carbon dioxide and
methane, delaying its eventual exit from Earth.
• Beneficial greenhouse effect
• Detrimental in excess
Electromagnetic Radiation (Cont.)
Ultraviolet radiation below 400 nm cannot be seen.
• Ultraviolet photons are sufficiently energetic to “excite” the valence
electrons of molecules to higher levels.
• Excited molecules can split apart to form very reactive species
resulting in photochemical reactions.
Photochemical Energy
Direct photochemical energy
• Warmth from sunlight • Photovoltaic cells generate electricity
Indirect photochemical energy
• Chemical energy from photosynthesis using solar energy, h:
6CO2 + 6H2O  C6H12O6 (biomass) + 6O2
• Biomass energy utilized by organisms in aerobic respiration
• C6H12O6 + 6O2  6CO2 + 6H2O + energy
• Biomass energy in fossil fuels
• Indirect solar energy in movement of wind and water
• Wind electrical generators
• Hydroelectric power
6.3. STORAGE AND RELEASE OF ENERGY BY
CHEMICALS
Chemical potential energy to heat energy
CH4 + 2O2  CO2 + 2H2O + energy (6.3.1)
Energy is released because of bond energy in chemical bonds.
Calculate the energy released in kilojoules (kJ) when 1 mol of CH4
reacts with 2 mols of O2 to produce1 mol of CO2 and 2 mols of H2O
(next slide):
Chemical Energy Release Calculation
H C-H 411 kJ/mol O=O 494 kJ/mol
H C H + O O O O
H
O H O H
O C O
O-H 459 kJ/mol
C=O 799 kJ/mol
H
H
In the products: 1 mol CO  2 mol C=O  799 kJ = 1598 kJ
2
mol CO 2 mol C=O
2 mol H 2O  2 mol O-H  459 kJ = 1836 kJ
mol H 2O
mol O-H
Total bond energy in products = 1598 kJ + 1836 kJ = 3434 kJ
In the reactants:
Total bond energy in reactants = 1644 kJ + 988 kJ = 2632 kJ
The difference in bond energies between products and reactants is
3434 kJ - 2632 kJ = 902 kJ
Bond Energy Calculation (Cont.)
From the preceding slide, based upon considerations of bond energy,
alone, the energy released when 1 mole of CH4 reacts with 2 moles
of O2 to produce 1 mole of CO2 and 2 moles of H2O, is 902 kJ.
The reaction is an exothermic reaction in which heat energy is
released, so it is denoted as negative, -902 kJ.
6.4. ENERGY SOURCES
Until about 1800, virtually all the energy used in the world was from
biomass sources.
• Energy for heating from burning wood
• People and goods moved on land, as well as cultivation of soil,
mostly by means of humans, horses, and oxen walking
• The energy above from biomass
A significant amount of energy for transportation was provided by
wind, which drove sailing boats and ships.
All the above energy was from renewable sources.
The Development of Fossil Fuel Energy Sources
The use of coal for energy grew spectacularly during the 1800s and
by the end of that century coal had become the predominant source
of energy in the United States, England, Europe, and other countries
that had readily accessible coal resources.
Major shift from renewable biomass energy sources to coal, a
depletable resource that had to be dug from the ground.
By 1950 petroleum had surpassed coal as a source of energy in the
U.S.
By 1950 natural gas had become a significant source of energy
lagging behind petroleum in its rate of development.
Other Contributors to Energy
Hydroelectric power had become significant by 1900, and retained a
significant share of energy production through the 1900s.
By around 1975, nuclear energy had become a significant source of
electricity and now is several percent of world energy.
Miscellaneous sources including geothermal and, more recently,
solar and wind energy now make contributions to total energy
supply.
Biomass still contributes a little to the total of the sources of energy
used.
Current Sources of Energy
Figure 6.5. U.S. (left) and world (right) sources of energy.
Fossil fuel sources account for the vast majority of energy used.
• Coal • Petroleum • Natural Gas
Two problems with fossil fuels
• Running out (petroleum peaked around 2000)
• Greenhouse gas source from CO2
Carbon Dioxide Emissions from Fossil Fuels
Variable amounts of CO2 added by burning various fossil fuels
(greater the H2O/CO2 ratio, less CO2 per unit energy produced)
• Natural gas: CH4 + 2O2  CO2 + 2H2O + energy
1 CO2 for each 2 2H2Os
• Petroleum: CH2 + 3/2O2  CO2 + H2O + energy
1 CO2 for each H2O
• Coal: CH0.8 + 1.2O2  CO2 + 0.4H2O + energy
1 CO2 for each 0.4 H2O
6.5. CONVERSIONS BETWEEN FORMS OF ENERGY
The most abundant sources of energy are usually not directly useful
and must be converted to other forms.
Much of what is done with energy involves converting it from one
form to another.
To get energy from fission of uranium
1. Enrich in the isotope whose nucleus can undergo fission
2. Place this isotope in a nuclear reactor where fission occurs,
converting the nuclear energy to heat
3. Use this heat to produce steam
4. Run the steam through a turbine to produce mechanical energy
5. Couple the turbine to a generator to convert its mechanical energy
to electrical energy.
Energy Conversion Efficiencies
Energy Conversion Efficiencies
The preceding slide illustrates major forms of energy and
conversions between them.
• Vast differences in conversion efficiencies
• Photosynthesis is less than about 0.5% efficient in converting light
energy to chemical energy, leaving room for substantial
improvement, such as by genetically engineered plants.
• Fluorescent bulbs are 5-6 times more efficient than incandescent
bulbs in converting electrical energy to light.
The Carnot equation describes efficiency of converting heat energy
Superheated
to mechanical energy:
steam in, T
T1 - T 2
Percent efficiency =
100
T1
Higher peak temperature
means higher efficiency
Reason for high efficiency of
diesel engines
1
Rotating shaft connected
to electrical generator
Cooler steam
out, T2
Turbine vanes
6.6. GREEN ENGINEERING AND ENERGY CONVERSION
EFFICIENCY
Early fossil-fueled electrical power generating plants from around
1900 were only about 4% efficient in converting chemical energy to
electrical energy; modern ones exceed 40%.
Change from steam locomotives to efficient diesel locomotives
during the 1940s and 1950s resulted in an approximately four-fold
increase in the energy-use efficiency of rail transport.
Improvements such as these due to
• Improved materials that can tolerate higher peak temperatures
• Advances in engineering
• Now, computerized control
Combined Power Cycles for Overall Energy Efficiency
District heating, is commonly practiced in Europe and can save
large amounts of fuel otherwise required for heating.
6.7. CONVERSION OF CHEMICAL ENERGY
Conversion of chemical energy from one form to more useful form
Generation of hydrogen gas from fossil fuels is an important
chemical energy conversion process that may become much more
widely practiced as fuel cells, which use elemental hydrogen as a
fuel, come into more common use
Coal gasification burns part of the carbon in the coal,
C(coal) + O2  CO2 + heat
(6.7.1)
leaving a solid residue of very hot carbon from the unburned coal.
This material reacts with water in steam,
C(hot) + H2O  H2 + CO
(6.7.2)
to generate elemental H2 and CO in a reaction that absorbs heat. The
CO can be reacted with more steam over an appropriate catalyst,
CO + H2O  H2 + CO2
(6.7.3)
Used for well more than a century in the coal gasification industry.
Coal Gasification
Before natural gas came into common use, steam blown over heated
carbon was used to generate a synthesis gas mixture of H2 and CO
that was piped into homes and burned for lighting and cooking.
• In addition to forming treacherous explosive mixtures with air, it
was lethal to inhale because of the toxic carbon monoxide.
Coal gasification may have a future for the generation of elemental
hydrogen for use in fuel cells.
• By using pure oxygen as an oxidant, it produces greenhouse gas
carbon dioxide in a concentrated form that can be pumped
underground or otherwise prevented from getting into the
atmosphere, a process called carbon sequestration.
The synthesis gas mixture of H2 and CO2 is a good raw material for
making other chemicals, including methanol or hydrocarbons that
can be used as gasoline or diesel fuel.
Methanol: CO + H2  H3COH + CO2
(6.7.4)
6.8. RENEWABLE ENERGY SOURCES
Renewable energy resources do not pollute and never run out.
Solar Energy is the best—when the sun shines
Sunshine offers widespread availability, an unlimited supply, and
zero cost up to the point of collection.
• Does not cause air, heat, or water pollution.
• Intense and widely available in many parts of the world.
• At a collection efficiency of 10%, approximately one-tenth of the
area of Arizona would suffice to meet U.S. energy needs.
Solar Energy
Utilization of solar energy
• Solar heating, solar-heated houses, and solar water heaters
• Solar boilers that generate steam from sunlight reflected from
mirrors
• Direct conversion to electricity in photovoltaic cells (next slide),
around 12–15% efficient generating electricity at a cost 4–5 times
that of electricity generated in fossil fuel power plants and could be
used for up to 15% of electricity on existing power grids.
• Means of storing solar generated electricity include batteries (small
scale), extremely high-temperature/high-pressure supercritical
water stored deep underground, or mechanical energy stored in the
extremely rapid rotation of flywheels.
• Generation of hydrogen gas usable in fuel cells by electrolysis of
water
2H2O + electrical energy  2H2(g) + O2(g)
(6.8.1)
Solar Voltaic Cells
Wind Energy
The Growing Success of Wind Power
World wind electricity generating capacity exceeded 60,000
megawatts in 2006 up from less than 5,000 mw in 1995 (1,000 mw is
the size of a large conventional coal-fired or nuclear power plant)
Biomass Energy
Production of biomass from photosynthesis could in principle
provide all needed carbonaceous fibers and materials and a large
percentage of liquid fuels
Photosynthesis is only about 0.4% efficient in converting solar
energy to chemical energy
Utilization of biomass energy does not add any net carbon dioxide to
the atmosphere
Wood is the most commonly used cooking fuel in many societies
Finland gets about 15% of its energy from wood
Oil seeds can provide direct sources of hydrocarbons, such as those
that can be used in diesel fuel
Ethanol and Biodiesel Fuels from Biomass Sources
Ethanol, C2H6O, is produced by fermentation of sugars by yeasts
• Used to supplement and substitute for gasoline
In the U.S., most of the raw material for ethanol production is sugar
derived from corn grain
Little net energy is obtained from ethanol made from grain
Sugarcane returns much more energy than grain by ethanol
fermentation and has enabled Brazil to become largely independent
of imported petroleum for fuel
Efforts are underway to obtain sugars to make ethanol from
cellulose sources, such as wheat straw
Biodiesel fuel is made by converting long-chain organic acids from
oil-bearing grains to liquid esters that can fuel diesel engines
Oils for biodiesel fuel production include rapeseed, sunflower,
soybean, palm, coconut, jatropha
• Rapeseed predominant in Europe • Soybean predominant in U.S.
Coconut and jatropha are attractive because they grow in the tropics
and come from perennial plants
Potential of Lignocellulose Fuels
Lignocellulose in stalks, straw, leaves, and corncobs provides much
more potential fuel than grain-derived fuels such as ethanol and
biodiesel fuel
Crop byproduct biomass is a large potential source of lignocellulose
fuel
A much larger potential source is from dedicated crops
• Hybrid poplars that can re-grow from stumps after harvesting
• Switchgrass
• Sawgrass
Lignocellulose can be burned directly for energy
Lignocellulose can be partially burned to generate heat, then reacted
with steam to produce CO and H2
Mixtures of CO and H2 can be reacted together to produce methane
(CH4) as well as hydrocarbons including gasoline, diesel fuel, and jet
fuel
• Byproduct CO2 from these processes can be sequestered
Biogas
Biodegradable biomass, represented {CH2O}, can be fermented in
the absence of oxygen to produce biogas methane, the gas present
in natural gas
2{CH2O}  CH4 + CO2
• Generates gas from sewage sludge
• Biogas from livestock wastes
• Potential from biomass produced for fermentation
Geothermal Energy
Geothermal energy from hot steam underground is used to generate
electricity in Iceland, Japan, Russia, New Zealand, the Phillipines, at
Larderello, Italy, and at the Geysers in northern California.
Dry steam is best, but the steam is often mixed with superheated
water.
Toxic hydrogen sulfide from underground sources can cause
pollution problems.
Injecting water into hot, dry underground water formations to
produce steam has the potential to increase resources of geothermal
energy ten-fold.
18.12. Nuclear Energy
Nuclear energy is generated by the splitting of nuclei of atoms of
fissionable uranium-235 (only 7.1% of natural uranium) or plutonium
from unfissionable uranium-238 when it absorbs neutrons
Generation of Electricity from Controlled Nuclear Fission
Advantages and Disadvantages of Nuclear Energy
Advantages:
• No greenhouse gas emissions
• Reliable, steady source of power (with modern design)
• Relative abundance of fuel (uranium)
• Passive stability and simplicity of latest designs
Disadvantages:
• Generation of radioactive byproducts
• Need to decommission radioactive old reactors
• Possible catastrophic failure (Chernobyl)
Nuclear Fusion Energy
Nuclear fusion energy is that released when two nuclei fuse together,
such as one of deuterium and one of tritium (both forms of
hydrogen):
Despite its promise of a low-pollution process with a virtually
inexhaustible source of fuel, a practical nuclear fusion reactor has
not been developed, and probably will not be