Dave's Energy

Friday, August 11, 2006

Burning ethanol in your car releases CO2

Every day in the news I hear that consumption of ethanol produces no CO2.

This is of course not true. When you burn a gallon of ethanol in your car's engine, CO2 is released. Ethanol gets fewer miles per gallon than gasoline and so, on a per-mile basis, releases at least as much CO2 into the atmosphere as conventional gasoline. It is true that CO2 will later be absorbed by growing more corn to make more ethanol, and that full-cycle analysis presumably makes ethanol closer to being carbon-neutral (however, see DOE link below). Unfortunately, though, some in the media hear that ethanol is carbon-neutral and then mistakenly state that consumption of ethanol produces no CO2.

Ethanol production and consumption generates lots of CO2. Not a surprise to anyone who pays attention to chemistry or thermodynamics, but it bears repeating so that we can understand that the hopes of sequestering carbon may lie in making sure it is produced at centralized locations. Producing CO2 at the tailpipe of the car, whether from burning ethanol or gasoline, still adds to the carbon balance, regardless of whether the crops later absorb it. Note that we could grow other things to absorb carbon, without then combusting those things, and we would then be carbon negative - which is even better than being neutral.

Just remember these ethanol chemical aspects:

CO2 is released when ethanol is fermented from corn starches:
C6H12O6 + Enzyme = 2 C2H5OH + 2 CO2
The amount of CO2 by molecular weight is roughly equal to the amount of ethanol by molecular weight.

Then, when you burn that ethanol (C2H5OH) in conjunction with oxygen, it produces about twice it's weight in CO2, slighty less than gasoline, depending on other atmospheric conditions:
C2H5OH + 3 O2 = 2 CO2 +3H20

The U.S. Department of Energy published a report on full-cycle CO2 that suggests that even with full-cycle absorption by the relevant plants, corn-based ethanol CO2 output was less than 5% different than from gasoline. So, next time your hear that ethanol produces no CO2, be a wary consumer of that information...

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2 Comments:

  • You are of course, absolutely correct that burning ethanol produces CO2. The problem is that our government and the governments of the world have conspired to say it doesn't count: "Under international agreement, CO2 from ethanol and other biofuels are not counted at the tailpipe" - http://www.eia.gov/tools/faqs/faq.cfm?id=307&t=11
    Ludicrous, I know. They also ignore the fact that burning ethanol is even more toxic to humans than gas. http://phys.org/news172391700.html#jCp

    By Anonymous Anonymous, at 9:29 PM  

  • Yep, it not only produces CO2, there's an additional step that oil doesn't have - it needs to be planted (more CO2), and perhaps fertilized (more CO2), and perhaps sprayed for weeds and bugs (more CO2. Big "oops". Here's a "what if" for you. What if all the increase in CO2 was due to the inefficiency of ethanol and its production?

    By Blogger 4TimesAYear, at 5:41 PM  

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