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Impacto Ambiental causado pela geração de enegia usando combustível fóssil

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27/09/2013
1
Environmental impact of Electricity 
Generation using Fossil Fuels
Kevin Davis
KDavis CIT 2013 1
References
1. Renewable Energy in Power Systems by Leon Freris & 
David Infield (Wiley)
2. Electrical Machines, Drives and Power Systems by 
Theodore Wildi, 6th ed.(Pearson\Prentice Hall)
3. Energy System and Sustainability by Boyle, Everett, 
Ramage Pub: Oxford
KDavis CIT 2013 2
The Carbon Cycle - storing the energy
Approximately 1% of the Sun’s energy incident on the 
Earth’s surface is fixed by organic matter and 
becomes stored bioenergy (Biomass)
As biomass fossilises, the carbon content increases due 
to losses of volatile compounds mainly CO2 and CH4
leaving pure carbon compounds such as Coal and pure 
hydrocarbons such as mineral oil
Glucose
(sugar)
KDavis CIT 2013 3
The Carbon Cycle - releasing the energy again
During combustion the stored energy in hydrocarbons 
is released which releases CO2 as well
Hydrocarbon
KDavis CIT 2013 4
Level of 
Oxygen 
required for 
combustion
Carbon 
dioxide 
emissions
Water 
released
****
***
Electrical Power Generation is the largest source of atmospheric 
pollution
Hydrocarbons
Hydrocarbon
KDavis CIT 2013 5
Fuel
Molecular 
Formula AKA
Methane CH
4
Natural Gas
Propane C
3
H
8
Camping Gas
Butane C
4
H
10
LPG
Octane C
8
H
18
Gasoline
Dodecane C
12
H
26
Kerosane \Diesel
Triacontane C
30
H
62
Tar
Cellulose C
6
H
10
O
5
Wood (Biomass)
*
The Energy Balance in the Earths atmosphere
“Earth’s Annual Global Mean Energy Budget” Kiehl and Trenberth, 1997: Bull. Am. Met. Soc. 78, 197-208
KDavis CIT 2013 6
342 Wm-2 = 107 Wm-2 + 235 Wm-2 in balance power flows
**
**
**
27/09/2013
2
KDavis CIT 2013 7
Impact of Carbon Dioxide in the atmosphere
• CO2 along with water in the atmosphere absorb 
radiation and release heat.
• The earth’s average temperature is around 15oC. 
Removing all the CO2 and H2O would reduce this to 
minus18oC (-18oC)
• The radiative properties of water account for about 
18oC and CO2 about 15oC.
• Water is about 1% of the atmosphere, but CO2 is 
only 0.038% so changes in CO2 will have a greater 
relative effect.
KDavis CIT 2013 8
KDavis CIT 2013 9
Stoichiometric 
Reactions.
Number on Periodic table of element = Atomic number = # of 
protons
Atomic Mass = # of protons + # of neutrons in atom (the 
numbers of neutrons varies in atoms)
Example Carbon Atomic number = 6 , (it has 6 protons and it 
also has on average 6 neutrons (Atomic mass = 6 + 6 = 12)
Important Atomic Masses
Carbon C = 12 , 
Oxygen O = 16, 
Hydrogen H = 1
Balance the masses on 
each side of the 
equation above
*
KDavis CIT 2013 10
KDavis CIT 2013 11
KDavis CIT 2013 12
27/09/2013
3
KDavis CIT 2013 13
Type of Calculation problem for this section
Biomass or Natural Gas generation of 
electricity
What level of CO2 emissions are produced by 
each fuel type?
Carbon Intensity
• Units of carbon intensity (kg CO2/MWh)
• This is the level of CO2 released per MWh of 
electricity generated.
• Calculated by determining 
– The mass of fuel required to produce a MWh
– Multiplying the fuel mass by the ratio of CO2
emitted per unit mass of the fuel
KDavis CIT 2013 14
KDavis CIT 2013 15
Example
Calculate the carbon dioxide emissions per MWh of 
output from two power stations.
(i) Power Station “A” is a CCGT unit and uses natural 
gas (CH4). The operating efficiency is 52%. (use a NCV 
of 50GJ/t)
(ii) Power Station “B” is a CHP unit and uses biomass 
(cellulose C6H1005). The combined operating efficiency 
is 87% while the electrical operating efficiency is 30%. 
(Use a NCV of 17GJ/t)
Biomass
• Carbon dioxide neutral?
• Carbon fuel source �produces other emissions
– Particulates
– Nitrogen
– Sulphur
– Dioxins
• Wetter material � lower energy density
• Phosphorus, nitrogen, water required for growth
KDavis CIT 2013 16
Energy Densities
• Wood, straw, paper etc. 13-18 MJ/kg
• Coal 29MJ/kg
• Gas 55MJ/kg
Remember conversion ratios (not given on exam paper)
1kWh = 3.6MJ
1MWh = 3.6 GJ
1000kg = 1 tonne
KDavis CIT 2013 17
KDavis CIT 2013 18
CH4 + 2O2 �CO2 + 2H2O + Energy
12 +(1x4) +2(16x2) � 12+(2x 16) + 2((1x2)+16)
16 + 64 � 44 + 36 (balanced) Ratio of CO2 emissions to fuel = 44/16 = 2.75
1MWh = 3.6 GJ Only 52% efficient, so need 1.923 MWh input = 3.6 x 1.923 GJ = 6.92GJ
Tonnes of fuel required = 6.92/50 = 0.1384t = 138.4kg CH4
CO2 emissions for this amount of fuel = 2.75 x 138.4 = 380.6kg
C6H10O5 + 6O2 � 6CO2 + 5H2O + Energy
(12x6)+(1x10)+(16x5) + 6 (16 x2) � 6(12+(16x2)) + 5((1 x2)+16) 
162 + 192 � 264 + 90
Ratio of CO2 emissions to fuel = 264/162 = 1.629
1MWh = 3.6 GJ 
Only 87% efficient, so need 1.15 MWh input = 3.6 x 1.15 GJ = 4.14 GJ
Tonnes of fuel required = 4.14/17 = 0.2435t = 243.5kg C6H10O5
CO2 emissions for this amount of fuel = 1.629 x 243.5 = 396.7kg

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