Transcript fossil fuels - physicspedagogics

Energy, Power, and Climate Change
8.3 Fossil Fuel Power Production
Plant material subjected to great pressure over a long
time becomes coal and oil - the fossil fuels.
COAL
There is evidence that man used coal as a fuel since
at least the bronze age, 4000 years ago.
However, for most of that 4000 years, wood has been
the prevalent fuel. Why?
With modern civilization's growing need for energy,
coal became the fuel of choice. Recall:
Fuel
Fuel Type
Energy Density (MJ/kg)
Coal
Fossil
32.5
Wood
Fossil
17.0
Cow Dung
Fossil
15.5
FYI: Note that coal has about twice the energy yield as wood per kilogram.
Energy,
Power,
and
Climate
Change
FYI: Since
you only need
about half
the weight
of coal, and
since it is
denser, it8.3
can more
easily beFuel
carried on
the locomotive
than wood.
Fossil
Power
Production
FYI: If you could somehow use a nuclear reactor to run this locomotive
you would
need 0.0009
kilograms
grams)
of uranium
since U-235 has
time
becomes
coal and
oil (0.9
- the
fossil
fuels.
an energy dnsity of 90,000,000 MJ / kg!
Plant material subjected to great pressure over a long
COAL
Suppose a steam locomotive is rated at 6000 horsepower.
How much wood must be burned in a 5 hour trip?
Recall that 1 h.p. = 750 watts, so that
P =
6000 h.p.
E = Pt
1 J/s
750 W
1 W
1 h.p.
3600 s
= 4.5 MJ 5 h
1 h
s
81000 MJ
1 kg Wood
17 MJ
= 4.5 MJ/s
= 81000 MJ
= 4765 kg Wood
How much coal must be burned for the same trip?
81000 MJ
1 kg Coal
32.5 MJ
= 2492 kg Coal
Waste Heat and Gaseous
and Particulate Pollution
Energy, Power, and Climate Change
8.3 Fossil Fuel Power Production
COAL-FIRED ELECTRICITY GENERATION
So how do we obtain useable
The steam generated in
energy from a coal-fired
plant?
The coal is burned in a boiler
to heat water into steam.
the boiler turns the
turbine.
The steam is turned back
into water in the
condenser.
The generator is turned
by the turbine, producing
electricity.
Turbine
Generator
Coal
Condenser
Boiler
Question: What is the approximate efficiency of this particular power
Energy,
Power,
Climate
AboutChange
25% to 33%.
plant, assuming
the Sankey
diagramand
is to scale?
8.3designs
Fossil
Fuel efficiencies.
Power Production
FYI: Different
have different
COAL-FIRED ELECTRICITY GENERATION
Chemical Energy
Motion
Hot Steam
Electricity
Friction
Waste Heat
Waste
Exhaust
Boiler
Turbine / Generator
Condenser
FYI: There is further energy loss due to transmission line resistance. The
farther the user is from the source, the greater this I2R loss.
Energy, Power, and Climate Change
8.3 Fossil Fuel Power Production
OIL-FIRED ELECTRICITY GENERATION
The only difference between coal- and oil-fired
plants is the energy density of the fuel:
Fuel
Energy Density (MJ/kg)
Crude Oil
41.9
Coal
32.5
Suppose a coal-fired plant supplies 100 MW of power?
How much coal is burned per day?
1 day is 24 h (3600 s / h) = 86400 s. Thus
E = Pt = (100 MW)(86400 s) = 8640000 MJ
8640000 MJ
1 kg Coal
= 266000 kg Coal
32.5 MJ
How much oil must be burned per day?
8640000 MJ
1 kg Oil
41.9 MJ
= 206000 kg Oil
Energy, Power, and Climate Change
8.3 Fossil Fuel Power Production
GAS-FIRED ELECTRICITY GENERATION
Gas-fired plants use a fuel that is already in
gaseous form, like natural gas.
This allows the fuel itself to gain kinetic energy
during combustion.
Then two turbines can be run simultaneously: The
steam turbine (just like the coal and oil power
plants) and a gas turbine, which runs on the hot gas.
Both turbines produce electricity, increasing the
efficiency of the gas-fired power station.
Energy, Power, and Climate Change
8.3 Fossil Fuel Power Production
GAS-FIRED ELECTRICITY GENERATION
Friction
Chemical Energy
Hot Steam
Motion
Electricity
Hot Gas
Motion
Electricity
Waste Exhaust
Boiler
Friction
Waste Heat
Waste Heat
Turbine / Generator
Condenser
Question: What is the approximate efficiency of this particular power plant,
About 50%.
assuming the Sankey diagram is to scale?
FYI: Different designs have different efficiencies.