What is the energy produced per tonne of CO2CO2 released for each fuel?

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1 Answer
Jan 8, 2018

CH_4 → CO_2" "CH4CO2 at -890 (kJ)/(mol)CO_2890kJmolCO2
C_3H_8 → CO_2" "C3H8CO2 at -740 (kJ)/(mol)CO_2740kJmolCO2
C_6H_12→ CO_2" "C6H12CO2 at -654 (kJ)/(mol)CO_2654kJmolCO2
CH_3OH→ CO_2" "CH3OHCO2 at -726.4 (kJ)/(mol)CO_2726.4kJmolCO2
C_2H_5OH → CO_2" "C2H5OHCO2 at -504.9 (kJ)/(mol)CO_2504.9kJmolCO2
C_3H_7OH → CO_2" "C3H7OHCO2 at -535.4 (kJ)/(mol)CO_2535.4kJmolCO2
I leave the conversion from moles to tonnes to you.

Explanation:

The question requires calculating the amount of CO_2CO2 produced by each combustion, and then normalizing them to moles (or tonnes) of CO_2CO2. Basically, it should be observable that the unit definition should mean that it will be the SAME for any source, because we are looking at one of the products, not the fuel.

Methane heat of combustion is -890 kJ/mol
CH_4 + O_2 → CO_2 + 2H_2OCH4+O2CO2+2H2O ONE mole of CH_4CH4 produces ONE mole of CO_2CO2, so the energy release is the same: -890 (kJ)/(mol)CO_2890kJmolCO2
Propane heat of combustion is -2220 kJ/mol
C_3H_8 + 5O_2 → 3CO_2 + 4H_2OC3H8+5O23CO2+4H2O ONE mole of C_3H_8 produces THREE moles of CO_2CO2, so the energy release is: -2220/3 = 740 (kJ)/(mol) CO_222203=740kJmolCO2

Let’s try a larger hydrocarbon:
C_6H_12 + 9O_2 → 6CO_2 + 6H_2O DeltaH^o = -3924 (kJ)/(mol) C_6
ONE mole of C_6H_12 produces SIX moles of CO_2, so the energy release is:
-3924/6 = 654 (kJ)/(mol) CO_2
How about alcohols?
2 CH_3OH + 3 O_2 → 4 H_2O + 2CO_2 ∆H = -1452.8 (kJ)/(mol) C_1
TWO moles of CH_3OH produces TWO moles of CO_2, so the energy release is the same:
-1452.8/2 = 726.4 (kJ)/(mol) CO_2
C_2H_5OH + 3O_2 → 3H_2O + 2CO_2 ∆H = - 1009.8 (kJ)/(mol) C_2
ONE mole of C_2H_5OH produces TWO moles of CO_2, so the energy release is: -1009.8/2 = 504.9 (kJ)/(mol) CO_2
2C_3H_7OH + 9O_2→ 8H_2O + 6CO_2 ∆H = - 1606.2 (kJ)/(mol) C_3
TWO moles of C_3H_8 produces SIX moles of CO_2, so the energy release is: -1606.2 xx 2/6 = 535.4 (kJ)/(mol) CO_2
Recapping:
CH_4 → CO_2 at -890 (kJ)/(mol)CO_2
C_3H_8 → CO_2 at -740 (kJ)/(mol)CO_2
C_6H_12→ CO_2 at -654 (kJ)/(mol)CO_2
CH_3OH→ CO_2 at -726.4 (kJ)/(mol)CO_2
C_2H_5OH → CO_2 at -504.9 (kJ)/(mol)CO_2
C_3H_7OH → CO_2 at -535.4 (kJ)/(mol)CO_2

So, yes, easily within the magnitude of any fuel, the amount of energy produced per tonne of CO_2 emission is roughly the same.

The real differences are due to the ‘extra’ elements that need to be reacted. The higher the C/H ratio, the more energy is produced per mole (or tonne) of CO_2 emission. It can be seen from an energy utilization perspective that 'clean burning'” alcohols are really less efficient at producing power for the amount of greenhouse gas emitted!