How green are biofuels?

[Eletruk]

Algae still holds the most promise.
Source: http://nextbigfuture.com/2008/08/biofuels-before-draft-animals.html

[aviken]

-183? That's saying something. But if the algae is already there in a normal / natural balanced supply, if we harvest some, are we then reducing the - effect that is already in place? I understand that this diagram is meant to be for planting, growing, harvesting & producing alternative fuel. I am a little confused by the 1-2% of crop land...can you grow algae anywhere? Or does it need to be in a large body of water?

[Eletruk]

The algae processing plants I have seen pretty much can be set up anywhere, but best when set up near a fossil-fueled power plant. The exhaust gasses are piped into arrays of algae filled glass tubes, so the CO2 from the exhaust feeds the algae which then is harvested.