CHM 101 GENERAL CHEMISTRY
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FALL
QUARTER 2008 |
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Section
2 |
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Lecture
Notes |
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(last revised: |
6.5
Present Sources of Energy (We covered the following parts of this section in
the previous lecture.)
- But First, a Little History:
o
Solar Power The Ultimate Source of All Energy:
o
The Pre-Industrial Age:
o
The Industrial Revolution:
o
Petroleum:
o
Sources of Energy in the
- Petroleum and Natural Gas:
o
Natural Gas:
o
Petroleum:
(This is the point where we
left off last time.)
- Coal: Coal has formed over geological time from buried biomass.
Depending on the age of the biomass deposit and the pressures and
temperatures it has encountered over that time, it has by now been
converted to (in order of the extent of transformation) lignite, subbituminous coal, bituminous coal, or anthracite.
This is also their order of carbon content, as the transformation
processes tend to eliminate carbon, hydrogen, and oxygen. See Table 6.5,
where I have included the mass elemental composition of wood (assumed to
be the same as cellulose) for comparison purposes.
- Uses of Fossil Fuels: Lets return to Figure 6.11 and talk about
sources and uses of energy in
the
In 1850, the
By 1900, the
In more modern times,
since around 1950, we see that petroleum (together with natural gas) has become
our dominant source of energy. It now powers nearly all of our transportation,
cars, trucks, airplanes, ships, and even our railroads. (When
the railroads converted from steam to diesel in the 1940s and 1950s, their energy
efficiency increased by a factor of 4.) The only significant market
remaining for coal is electric power generation; coal accounts for about half
of this market.
- Problems with Fossil Fuels Supply: If we refer back to Figure 6.11 (repeated
here), we see that we derive most of our current energy consumption from
petroleum (including natural gas). Indeed, it is said that the
- Problems with Fossil Fuels Pollution: In an ideal world, coal would be 100% carbon
and would burn completely to produce energy, carbon dioxide, and nothing
else:
C (s) + O2
(g) > CO2 (g) ΔH° = 394 kJ
Of course, the world is
not ideal, and coal combustion produces several unwanted effects:
o
Incomplete Combustion: Incomplete combustion produces toxic soot and
carbon monoxide.
o
By-Products: The high temperatures reached during combustion promote the reaction
of nitrogen and oxygen in the combustion air to form various oxides of
nitrogen. (Recall our discussion about acid rain in Section 5.10.)
o
Impurities: Any sulfur present in the coal will be oxidized to sulfur dioxide.
Other impurities present in the coal (even uranium!) can be vaporized or they
can be converted into fine particulates.
These various unwanted
products of coal combustion tend to be swept into the stream of exhaust gases,
and unless they are trapped in some way, they will pass into the environment,
where they can cause tremendous pollution problems. (Think
Pittsburgh in the early 1900s or Eastern Europe during the Communist Era.)
These kinds of problems
are not unique to coal:
o
Gasoline:
Photochemical smog from the unburnt hydrocarbons and
nitrogen oxides in car exhaust.
o
Diesel Oil: Soot and sulfur dioxide from diesel engines.
o
Wood:
Wood smoke. Need I say more?
The above discussion is
limited to problems that occur at the point of use. Pollution also can occur at
the point of extraction, during transport, and during processing. Some
examples:
o
Oil Spills at Sea
o
Toxic Releases from Oil Refineries
o
Open Pit Coal Mining
- The Greenhouse Effect: The sun radiates copious amounts of energy
that impinges on the earth, mostly in the form of visible light. Lets
examine what happens to this energy:
o
Reflection by the Atmosphere: About 30% of solar energy gets reflected back to
space by the atmosphere (mostly by cloud tops).
o
Photosynthesis: Plants absorb light and use its energy to convert CO2 and
H2O to sugars and other biomass.
o
Hydrological Cycle: Solar energy powers the hydrological cycle by
evaporating water (mostly from the sea).
o
Heating:
Most of the solar energy reaching the surface becomes absorbed by soil, rocks,
and water and increases the temperature at the surface.
Reradiation from the Surface: What happens to solar energy absorbed at the
earths surface?
o
Atmospheric Heating: Some of the energy absorbed at the surface goes
into heating near-surface air and causing it to rise. Thus this energy sets the
atmosphere into motion and drives our weather.
o
Infrared Radiation: Most of the energy that goes into heating the
earths surface is reradiated back toward space in the form of infrared
radiation. Not all of it gets there. Several atmospheric gases such as carbon
dioxide, water vapor, and methane are opaque to infrared and absorb it, only to
reradiate it. The process is illustrated in Figure 6.12:
The result is that the
earths surface and the lower atmosphere are significantly warmer than they
would be, were there no infrared absorption by atmospheric gases. The role of
these gases is analogous to the role of the glass in a greenhouse, thus the
phenomenon is called the greenhouse
effect, and the infrared-absorbing gases are called greenhouse gases.
- Carbon Dioxide and Global Warming: Water vapor and carbon dioxide are the
principal contributors to the greenhouse effect. Since infrared absorption
is proportional to concentration, fluxuations
and long-term changes in the atmospheric contents of these gases should be
of concern.
o
Water Vapor: The concentration of water vapor in the atmosphere has a marked
effect on heat retention. Indeed, under conditions of high humidity (high
atmospheric moisture content), there is little cooling at night, and we
experience warm, muggy weather conditions. In contrast, cooling is extensive on
low-humidity nights when the air is clear. Fortunately, the amount of water
vapor in the atmosphere is controlled by the hydrological cycle, and although
it fluxuates over short periods of time, it does not
change in the long run.
o
Carbon Dioxide: The carbon dioxide content of the atmosphere shows only small,
short-term fluxuations. But what about the long run?
Figure 6.13 shows a significant long-term increase in the CO2
content of the atmosphere since 1750. This increase is thought to have come
about from humankinds use of fossil fuels starting at the dawn of the
Industrial Revolution around 1750.
o
Global Temperature Trends: The Wikipedia article on global warming shows a graph of
global temperatures for the period 1850-2008. The trend is unmistakably toward
rising temperatures; increases of 0.8 C° since 1850 and 0.4 C° versus the
global mean for 1961-1990.
o
Cause and Effect: Although global warming has been a controversial
subject, and although increases in atmospheric carbon dioxide are not its only
cause, nearly all scientists now agree that carbon dioxide emissions from use
of fossil fuels are a significant cause of global warming, and that humankind
needs to curtail these emissions in order to forestall catastrophic further
increases in global temperatures.
6.6
New Energy Sources: Our present energy economy, based primarily on
petroleum, is unsustainable in the long run. World oil shortages approach, and
if we are to continue to supply enough energy to meet future demand, we need to
look elsewhere than petroleum. But even if petroleum were in unlimited supply,
the threat of global warming would still force us to curtail our exploitation
of fossil fuel resources unless and until we can develop technology for
sequestering CO2 before it can enter the atmosphere.
- Practicalities
o
Pollution
o
Technical Feasibility
o
Energy Efficiency
o
Ease of Distribution
- Coal Conversion: Coal creates environmental problems at every
stage of its production, refinement, transport, and use cycle, at least
while it remains a solid. Is it possible to control some of these problems
by using it a feedstock to produce a liquid or a gas fuel?
o
Gasification: Coal gasification is hardly a new process. The earliest gas utility
companies in the latter half of the 19th century produced their gas
from coal, but later switched to natural gas, as long-distance pipelines
enabled it to be distributed. The most important chemical reactions in the
process are:
|
C (s) +
½ O2 (g) |
> |
CO
(g) |
ΔH°= |
111 kJ |
(1) |
|
C (s) +
O2 (g) |
> |
CO2
(g) |
ΔH°= |
394 kJ |
(2) |
|
C (s) +
H2O (g) |
> |
H2
(g) + CO (g) |
ΔH°= |
131 kJ |
(3) |
|
C (s) +
2H2 (g) |
> |
CH4
(g) |
ΔH°= |
75 kJ |
(4) |
The feedstocks
are coal, air, and water. Some of the coal is burned in reactions (1) and (2) to
supply energy to make steam and drive the endothermic reaction (3) which produces
a desirable product stream, a mixture of hydrogen and carbon monoxide (and air)
called synthetic gas, syngas for short,
or originally, water gas. Often, some
of the hydrogen from the syngas stream is used to
treat additional carbon to produce methane (4).
Under carefully
controlled conditions, the process runs without requiring additional inputs of
energy. The process, however, has some disadvantages. Carbon dioxide is an
unavoidable by-product of generating the energy needed to drive the process. See
reaction (2). And direct use as fuel of the gases produced by reaction (3) is
complicated by the presence of air in the mixture. This product stream is
sometimes called low BTU (for British Thermal Unit) gas because it
has only about 30% the energy content of natural gas. Moreover, the toxicity of
CO is also of concern.
o
Liquefaction: Syngas can be directly converted to the
useful fuel, methanol:
CO (g) +
2H2 (g) > CH3OH
(l) ΔH°= 128 kJ
The process is already
commercial; about half the motor vehicle fuel used in
The reaction, however,
does not consume all of the CO in the feed stream.
Would it be feasible to use it to generate additional methanol? On paper, one
might consider reacting it with steam. We can look up
the heats of formation of the components in Appendix A4 to compute the enthalpy
of this reaction:
|
|
CO (g) |
+ H2O (l) |
> |
H2 (g) |
+ CO2 (g) |
|
ΔHf°: |
111 kJ |
286 kJ |
|
0 kJ |
394 kJ |
|
x |
1 |
1 |
|
1 |
1 |
|
ΔH°= |
111 kJ |
286 kJ |
|
0 kJ |
394 kJ |
|
ΔH°= |
3 kJ |
||||
I have no information
concerning whether it gets put to use, but it does look like an energy-neutral
way to convert a potentially unwanted by-product into something useful.
- Hydrogen as a Fuel: Many people advocate using hydrogen as a
clean, CO2-free fuel. Its combustion
reaction looks very attractive. It produces lots of energy and absolutely
no carbon dioxide:
H2 (g)
+ ½ O2 (g) > H2O
(l) ΔH°= 286 kJ
However, there are some
little (I mean big.) problems.
o
Supply and Production: There is very little naturally-occurring hydrogen
gas. The standard manufacturing process is by reacting methane with steam:
|
|
CH4 (g) |
+ H2O (g) |
> |
3H2 (g) |
+ CO (g) |
|
ΔHf°: |
75 kJ |
242 kJ |
|
0 kJ |
111 kJ |
|
x |
1 |
1 |
|
3 |
1 |
|
ΔH°= |
75 kJ |
242 kJ |
|
0 kJ |
111 kJ |
|
ΔH°= |
206 kJ |
||||
This reaction is highly
endothermic. Using the methane directly as fuel would be much more energy
efficient:
|
|
CH4 (g) |
+ 2O2 (g) |
> |
CO2 (g) |
+ 2H2O (g) |
|
ΔHf°: |
75 kJ |
0 kJ |
|
394 kJ |
242 kJ |
|
x |
1 |
2 |
|
1 |
2 |
|
ΔH°= |
75 kJ |
0 kJ |
|
394 kJ |
484 kJ |
|
ΔH°= |
803 kJ |
||||
Now one might argue that
there is all the hydrogen we need. All we need to do is extract it from water, using
the reaction:
H2O (l)
> H2 (g) + ½ O2
(g) ΔH°= 286 kJ
The fallacy in this is
that we must as much energy into this reaction as we can get out of the
hydrogen it produces. Even though water is an abundant source of hydrogen,
there is no energy to be gained by turning it into hydrogen.
However, this reaction is
potentially useful as a means of storing
energy. It is quite possible to convert solar energy into electricity by
capturing it in photovoltaic panels. We could use this electricity as it is
produced, or we could use it to charge a storage battery, or we could use it to
electrolyze water to make hydrogen gas and oxygen gas. Now the hydrogen is
available for us to use as a fuel.
Indeed,
o
Energy Density: The energy content of hydrogen gas appears to be high. One mole has a
mass of just over 2 grams and a combustion energy of 286
kJ/mol.
Comparison with Gasoline: This works out to 143 kJ/g
compared to 48 kJ/g for gasoline (computed as octane). But gasoline is a
liquid at room temperature, wheaeas one needs to cool
hydrogen to 253 °C in order to liquefy it. Even then its density is
less than 10% that of gasoline (0.07 g/cm3 versus 0.8 g/cm3).
So a given volume of gasoline contains about 3 times the energy of an equal
volume of liquid hydrogen.
Comparison with Methane: We just computed the molar heat of combustion of
methane to be 803 kJ/mole. Now one mole of methane
occupies the same volume as one mole of hydrogen under equal conditions of
temperature and pressure, but the molar heat of combustion of hydrogen is only 286 kJ/mol. Thus methane contains nearly 3 times the
energy as an equal volume of hydrogen.
o
Reactivity with Metals: Hydrogen gas will dissolve in certain metals and
react to form metal hydrides. Since metal hydrides tend to be much more brittle
than the original metals, if one makes a storage tank out of such a metal and
tries to use it for hydrogen, the tank will become brittle and may rupture.
(Maybe I should have said, GO BOOM!)
- Wind Power
- Solar Power
- Biomass