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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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