Biofuel and CO2: ethanol, E-85 and gasoline, comparative

[Christophe]

From a reflection received by email (see: https://www.econologie.com/forums/bioethanol ... t5814.html ) and adapted to "my sauce", it's a bit long so here is the copied / pasted conclusion. For scientists, please do a proofreading.

At equal engine efficiency, ethanol thus emits 4,56 / 3,55 = 1,28 times more CO2 than gasoline to provide the same work.

In other words: ethanol emet, 28% CO2 in addition to gasoline.

And are not included in this calculation for ethanol:

- the CO2 production costs of the cereal or sugar plant
- the costs of refining the plant in sugar
- the costs of distillation
- energy costs of fermentation (ie the CO2 emitted to heat the molasses)

Some reminders of chemistry: sugar, fermentation, alcohol and CO2 ...

We will retain the following atomic masses which serve as the basis of all the calculations: C = 12 g / mol, O = 16 g / mol, H = 1 g / mol

a) The case of alcohol resulting from the fermentation of sugar

The sugar used to make ethanol is from the family of diholosides (sucrose or maltose) of formula: C12H22O11 (see http://fr.wikipedia.org/wiki/Sucre#Chimie ) has a molar mass of 342 grams which, by hydrolyzing (it "gains" a molecule of water), will give 2 molecules of glucose or 2 * C6H12O6 with a molar mass of 180 grams for a total mass of 360 grams.

Glucose Details: http://fr.wikipedia.org/wiki/Glucose

The mole of glucose will then ferment according to the equation:

code: Select all

C6H12O6 --> 2*C2H6O + 2*CO2

Products: 2 moles of ethyl alcohol (C2H6O = 46g / mol) and two moles of carbon dioxide (CO2 = 44g / mol).

Before being burned and neglecting any other CO2 emissions, each gram of alcohol produced will therefore release, during its "creation": 44/46 = 0.95 gram of CO2.

And this BEFORE its combustion. During the combustion of alcohol:

code: Select all

C2H6O + 7/2*O2 --> 2*CO2 + 3*H2O

or for those who prefer round numbers:

code: Select all

2*C2H6O + 7*O2 --> 4*CO2 + 6*H20

Balancing error on the reagents, rectifying (which does not change the results on the CO2):

code: Select all

C2H6O + 3*O2 --> 2*CO2 + 3*H2O

While burning, this mole of 46 g of alcohol will therefore produce 88 g of CO2 (2 moles).

Thus, a total of 3 moles of CO2 emitted per mole of burned alcohol: 1 during the fermentation and 2 during the combustion is obtained. In total 3x44 = 132 g of CO2 for 46 grams of burned alcohol.

Conclusion: each gram of alcohol thus emits during its elaboration and combustion at least 132 / 46 = 2,87 grams of CO2.

I say at least because many "CO2 positions" have not been taken into consideration: sugar production and alcohol distillation are undoubtedly the most energy-hungry.
This value of 2,87 g CO2 / g of alcohol is therefore a very minor value!

b) The case of gasoline (which alcohol is supposed to replace)

I resume without detailing the calculations of this page: combustion equations and CO2

Emissions of CO2 for a hydrocarbon of formula C2H (2n + 2) = 44n

Application to gasoline (pure octane). n = 8
[C8H18] = 8 12 * + = 18 1 114 * g / mol.
The mass of CO2 released per mole of octane consumed is: 44 * 8 = 352 g.
The fuel consumption ratio on CO2 releases is 352 / 114 = 3,09

Conclusion: each gram of gasoline emits during its combustion 3,09 grams of CO2

The cost of "refining" being, at worst, 15%, one would obtain 3.55 g CO2 for the refining and the combustion of gasoline.

c) Ethanol and gasoline energy comparison

code: Select all

PCI ethanol = 26 900 kJ/kg
PCI essence = 42 700 kJ/kg


At engine speed, 42,7 / 26,9 = more ethanol mass is required to provide the same energy, ie 1,59 times.

In isoenergy, it is therefore necessary to add a coefficient of 1,59 to the CO2 emissions of ethanol: the 2,87 grams thus become 4,56 grams. And remember: these 2,87 g were already largely minus because they did not take into account the sugar culture (but skeptics will say, with reason, that for gasoline, we did not take into account the cost of extraction and transport of crude)

General conclusion

To provide the same amount of combustion energy, the CO2 emissions are respectively:
- with gasoline: 3,55 grams of CO2
- with ethanol: 4,56 grams of CO2

At equal engine efficiency, ethanol thus emits 4,56 / 3,55 = 1,28 times more CO2 than gasoline to provide the same work.

In other words: ethanol emet, 28% CO2 in addition to gasoline.

And are not included in this calculation for ethanol:

- the CO2 production costs of the cereal or sugar plant
- the costs of refining the plant in sugar
- the costs of distillation
- energy costs of fermentation (ie the CO2 emitted to heat the molasses)

There is therefore no doubt that, based on this reasoning, ethanol of agricultural origin emits much more CO2 than gasoline. So how can state agencies and universities get CO2 reviews on ethanol so positive? Do not they take into account fermentation? The omissions are they? Or where is the flaw in our reasoning?

[Christine]

In other words: ethanol emet, 28% CO2 in addition to gasoline.

Just remember that in the case of gasoline it is about CO2 which was captive "off circuit" and which is released while the CO2 emitted by ethanol is in the circuit of life on earth. That's a huge difference and I'm not sure you can compare them just quantitatively.

[Christophe]

Document from the CNAM available here:
https://www.econologie.com/valorisation- ... -2985.html



In g CO2 / MJ according to the reasoning applied above one would find:

code: Select all

PCI ethanol = 26 900 kJ/kg
PCI essence = 42 700 kJ/kg


a) for gasoline: PCI = 42,7 MJ / kg
Emissions of CO2 = 3,55 g are 83,25 g CO2 / MJ

(b) for ethanol: PCI = 26,9 MJ / kg
Emissions of CO2 = 2,87 g are 106,7 g CO2 / MJ

[Christophe]

Just remember that in the case of gasoline it is about CO2 which was captive "off circuit" and which is released while the CO2 emitted by ethanol is in the circuit of life on earth. That's a huge difference and I'm not sure you can compare them just quantitatively.

Ah yes interesting, congratulations I like it and it is a plausible hypothesis!


So we take, in the balance of ethanol QUE CO2 non-plant origin ie fossil origin? In other words, we take into account ALL that I have NOT taken into account!

A bit like human respiration which has no impact on warming because it is carbon remaining in the "plant" cycle ... only modern agricultural techniques have ... an impact.

To dig so!

[Christophe]

I just realized one thing: the CO2 may be vegetable CO2 ... it will remain 100 120 years in the atmosphere and will add up with the fossil CO2 to force the greenhouse effect.

If there is no capture of CO2 during fermentation (which I doubt), we can say thatin the short and medium term agrofuel ethanol is worse in its contribution to the greenhouse effect than gasoline.

Is my reasoning good or not?

[jonule]

yes christophe the reasoning is good, and you see: it does not appear on the CNAM report ...
for info, ETBE is diester: oil + alcohol

down ethanol and the CNAM

[Christophe]

No jonule you confuse ETBE and EMHV ...

[jonule]

ah yes exactly right,

ETBE = ethanol
EMHV = diester (alcohol + oil)

HVP = pure vegetable oil

down ethanol and the CNAM

I just realized one thing: the CO2 may be vegetable CO2 ... it will remain 100 120 years in the atmosphere and will add together with the fossil CO2 to force the greenhouse effect.

well yes it's not hard to understand ... except that it is not counted in the "official" calculation of the greenhouse effect, which has its greenhouse effects calculation office you can imagine, after all the hay that they did; no methane calculation at home, because of plant origin.

thus: this carbon problem comes only from one thing for them, the mineral, the mineral oils and the TIPP invented at the time, disappeared at the same time as the bureau of calculation of the greenhouse effects

[jsl8]

And this BEFORE its combustion. During the combustion of alcohol:

code: Select all

C2H6O + 7/2*O2 --> 2*CO2 + 3*H2O

or for those who prefer round numbers:

code: Select all

2*C2H6O + 7*O2 --> 4*CO2 + 6*H20

The equation is not balanced, the results are therefore wrong!

[Christophe]

It's true didiou! There was an "O" on the reactive side too. Thank you there is at least 1 following

Correction:

code: Select all

C2H60 + 3O2 -> 2CO2 + 3H20

It simplifies everything

But that does not change NOTHING with the results on the CO2: a mole of C2H60 always rejects 2 moles of CO2 ...