Table of Fuel Density and Source of Greenhouse Gas The High fuel density of octa
ID: 977389 • Letter: T
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Table of Fuel Density and Source of Greenhouse Gas The High fuel density of octane is one reason why replacement of gasoline engine has been slow. Diesel and biodiesel have similar fuel densities, and diesel engines are about 30% more efficient than gasoline engines and produce more power, but higher pollution levels from diesel fuels have limited the popularity of diesel cars in the US. From the data above, what are the advantages and disadvantages of the use of ethanol instead of gasoline? How many kg of ethanol would have to be used to equal the fuel density of one kg of octane? Would this amount of ethanol produce more or less carbon dioxide than on kg of octane? Considering the sources of these fuels, what are some advantages of ethanol and biodiesel over gasoline and diesel?Explanation / Answer
The combustion of octane can be represented by
C8H18(l) +12.5O2(g) -----> 8CO2(g) + 9H2O (l)
dHf n-octane = -249.95 kJ/mol
dHf CO2(g) = -393.51 kJ/mole
dHf H2O(l) = -285.83 kJ/mole
dHf O2 = 0 kJ/mole
heat of reaction = enthalpy of products- enthalpy of reactants =
(8*-393.51 + 9*-285.83) - (-249.95 - 0 ) =-5470.6 KJ/mol
1mole of octane corresponds to 8*12+18=114
Heat of combustion = 5470.6 Kj/mol and Fuel density = 5470.6/114=47.98 Kj/g=47980 Kj/Kg
Moles of CO2= 8 and mass of CO2/ kg of fuel =8*44/114 =3.087 kg of CO2/ kg of fuel
b The combustion of ethanol can be represented as C2H5OH (l) + 3O2 (g) -----> 2CO2 (g) + 3H2O (l)
heat of combustion of ethanol = -1368 Kj/mol
enthalpy of combustion = 2(-393.509) + 3(-285.83)-(-1368 ) =-276.51 Kj/mol
molecular weight of ethanol (C2H5OH)= 24+6+16=46=0.95 KgCO2/kg fuel
Fuel density = 276.51/46=6.01 Kj/g =6011Kj/Kg
mass of CO2/ Kg of fuel = 44/46 kg/kg=0.95 KgCO2/ kg fuel
c) C12H23 + 59.5 O2------->12CO2+11.5H2O
Molecular weight of C12H23= 167
mass of CO2 generate/Kg of fuel = 12*44/167=3.161 kg of CO2/ kg fuel
d) C19H36O2+ 13O2----------> 19CO2 +18H2O
molecular weight of Bio-diesel = 19*12+36+32=296
Mass of CO2/kg of fuel = 19*44/296=2.82 kg of CO2/ kg of fuel
b) CO2 emission are less 0.95 kgCO2/ kg fuel compared to 3.087 kg/kg of octane.
thanol has a lower heat of combustion per unit mass and since produced from fermentation process, disposal of Fermentation waste posses environmental problems,
Heating density of ethanol =6011 kj/kg
6011 kj are produced by combustion of 1 kg of ethanol
47980 kj requires 47980/6011=7.98 kg of ethanol
CO2 produced = 7.98*0.95=7.581 kg of CO2 ( more amount of CO2 is produced than that of octane)
3. Ethanol and bio-diesel can be obtained from renewable sources. Ethanol is cheaper. Bio-diesel becomes inexpensive if market for glycerol ( a major bi-product of gycerol) is looked for. CO2 neutrality can be maintained.
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