Sunday, November 25, 2007


Global Warming Sleight of Hand

Some people are talking about ethanol produced from corn or sugar cane (and other alcohol fuels) as a "solution to global warming": Here, here, and here, ad infinitem. (Although that last site did say it seemed "too good to be true").

Here are the chemical formulas for combustion of the two main alcohols used for fuel.

Methanol combustion is: 2CH3OH + 3O2 → 2CO2 + 4H2O + heat
Ethanol combustion is: C2H5OH + 3O2 → 2CO2 + 3H2O + heat

Notice anything about the right side of the equations, such as the inclusion of carbon dioxide in the product?

Here is the chemical formula of the burning of gasoline (octane): 2C8H18 + 25O2 = 16CO2 + 18H2O + heat

I'm no chemist, but is there the slightest advantage to burning alcohol over gasoline at least as far as carbon dioxide production is concerned?

I notice that the number of produced carbon dioxide molecules is greater in the gasoline equation, but then so is the heat. Is there lower CO2 emission, per measure of generated heat, in the alcohols' combustion? Any at all? I'm asking because the answer has so far eluded my Google search. (I couldn't decipher the Wikipedia entry).


Both reactions produce carbon dioxide. However, the CO2 produced in the combustion of gasoline is "new" CO2.

The CO2 from the ethanol is not "new" to the planet.

The ethanol is made from the fermentation of sugar. Here's the simplified reaction for that:

C6H12O6 ---> 2CO2 + 2CH3CH2OH

Notice the fermentation also produces CO2, however, this CO2 and the CO2 released when the ethanol is burned was originally absorbed by the plant (corn, sugar cane) as it grew.

Plants use CO2 to create their mass. So in a sense, the CO2 is being "recycled." This is a much simplified description as it does not take into account all of the energy (and subsequent CO2 production) needed to produce ethanol in the first place.

As far as energy, without going into the meaning of equation coefficients and stuff, gallon for gallon ethanol produces LESS energy than gasoline.
Yeah, what he said.

Still pretty flawed logic as it leaves out the carbon released in the production of the ethanol.

Long story short, ethanol is not the answer. Anyone who buys corn tortillas could tell you that.
OK, so let me see if I'm getting this, Chemgeek, if that really is your name. There is 'old', which is to say, good CO2 (derived from living plants) and 'new', which is to say, bad CO2 (derived from fossil fuels). Is that right? How do we tell which is which in the atmosphere? Does the good CO2 warm the planet less than the bad CO2?
Do plants and the sea take in only 'old' (or 'good' in my parlance) CO2 or will a corn plant, for instance, use CO2 derived from fossil fuels for photosynthesis? These are actually rhetorical questions designed to make a point.
Agreed, Mike. Not a solution at all.
For the sake of the common bystander, I am going to answer your rhetorical questions.

CO2 is CO2.

"New" CO2 is CO2 released when fossil fuels are burned. The CO2 is "new" because it had been locked up for eons as oil or coal.

"Old" CO2 is CO2 released when ethanol is burned. It is "old" because it has been in the CO2 cycle for millennia.

Both types of CO2 are chemically and physically identical. There is no way to tell the difference between the two. Plants and the ocean do not differentiate between the two.

The argument is that before the use of fossil fuels there was a static amount (or nearly so) of CO2 in the world. Since the burning of fossil fuels began, the amount has been increasing. The major point of debate is what affect does this have on our global climate.

Let me echo: Ethanol is not the answer.
Very reasonable answers from you in the face of my semi-snark, Chemgeek. Thanks for completing the record.
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