After $100 Million, Exxon Backs Off Algae as Fuel

[GliderGuider]

34. It's possible, but difficult, to have a closed carbon cycle.

The carbon is an energy carrier. the important questions are where the carbon in the fuel comes from and where all energy comes from. The only totally carbon neutral process would be if all the carbon comes from either atmospheric carbon or short-term storage like soil. Plus all the energy used - to grow the biomass, process it and transport the fuel - must come from either direct or indirect solar sources like sunlight, or wind or tidal electricity.

It's possible (though difficult in practice) to do this without violating the second law, because it's permissible to lose some of the embodied solar energy to entropy. What's not permissible is to introduce any carbon from long-term stores like fossil fuels. That is very difficult and expensive in practice, so there is usually some degree of "marketing imprecision" in the carbon accounting.

One of the biggest problems, the one we saw with corn ethanol, is the question of where one draws the system boundaries for carbon accounting purposes. If they're tight enough it makes the accounting much more attractive - for instance if you don't count the source of the electricity used to run the processing plant, the fuel in the delivery trucks, or the embedded carbon in the equipment due to its manufacture. If you draw the boundary around just the growing and harvesting, and then compare that carbon to the the carbon in the finished product, a lot of inconveniences just disappear from view.

I think it's safe to say that there will be no carbon truly neutral biofuels in the near or medium term.