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### carbon dioxide output??

Posted: Sat Aug 25, 2007 6:07 am
for eg. combustion of CH4:
CH4(g) + 2O2(g) ---> CO2(g) + 2H2O(l)

enthalpy of combustion: (-393.51) + 2(-241.82) - (-74.81) = -802.34 kj/mol

energy output per unit mass:
802.34 x 1000/16(molar mass) = -50146.25 kj/kg

energy output per unit volume:
-50146.25 x 0.645(mass density) = -32344.33 kj/m3

1 mole of CO2= 44g
1 mole of CH4= 16g
At STP the volume of an ideal gas occupies 0.0224 m3...

the CO2 energy output per mass....
-802.34 kj/mol divided by 12 g/mol equals 66.86 kj/g

am i doing the right thing, and how would you calculate the CO2 output per unit volume?

Many thanks.

### Re: carbon dioxide output??

Posted: Sat Aug 25, 2007 10:03 pm
mojo wrote:for eg. combustion of CH4:
CH4(g) + 2O2(g) ---> CO2(g) + 2H2O(l)

enthalpy of combustion: (-393.51) + 2(-241.82) - (-74.81) = -802.34 kj/mol

energy output per unit mass:
802.34 x 1000/16(molar mass) = -50146.25 kj/kg

energy output per unit volume:
-50146.25 x 0.645(mass density) = -32344.33 kj/m3

1 mole of CO2= 44g
1 mole of CH4= 16g
At STP the volume of an ideal gas occupies 0.0224 m3...

the CO2 energy output per mass....
-802.34 kj/mol divided by 12 g/mol equals 66.86 kj/g

am i doing the right thing, and how would you calculate the CO2 output per unit volume?

Many thanks.

I did not verify your enthalpy calculation. Taking it as correct, the enrgy output per unit mass is correct.

I don't know where your density number came from. I would use molar volume at STP. Note that you accidentally used molar mass of 12, not 16.

-802.34 kj/mol / 0.0224 m3/mol = -35818.75 kJ/m^3

As to CO2, 1 mol CH4 --> 1 mol CO2. Of course, it won't be 22.4 L, because it won't be at STP, it will be HOT! The obvious temperature change of combustion means it would be better to work in moles or in mass.
(or, alternatively, after it cools to STP, the mole of CO2 will occupy 22.4 L)

Note that methane is a stronger greenhouse gas than CO2 (by 21X) so it is better to burn methane than allow it to escape. Still better to keep it sequestered.

Posted: Sun Aug 26, 2007 5:14 am
however i need to work out the CO2 output per unit mass and volume,
the units would be kj/kg and kj/m3,correct?

the CO2 energy output per mass....
-802.34 kj/mol divided by 12 g/mol equals 66860 kj/kg

the CO2 energy output per unit volume:
66860 x 0.645 (CH4 mass density)= 43124.7 kj/m3

Have i done these calculations properly?
Thanks.

Posted: Sun Aug 26, 2007 5:57 am
If you have to look at CO2 per unit of energy out, then you need to partiition the entholpy change of combustion into that attributable to carbon (yielding CO2) and that attributable to hydrogen (yielding water vapor). Each of those combusts alone so figures are available, but I don't know how to partition the heat of formation of methane. (ratio?)

In very rough figures, you can look at energy per mole of hydrogen (there are two moles of jydrogen in a mole of methane) and attribute the balance to carbon. About 1/3 the total energy of combustion of methane is due to the carbon, the balance to hydrogen. (On a per mole basis, methane has more hydrogen than hydrogen.)