As we track down the back side of the petroleum curve, we will see a transition from the alkane/alcohol fueled Otto engine to a greater reliance on electric conveyance. Here is some wishful thinking-  Ethanol as a direct petroleum replacement will collapse under the weight of scrutiny as better cost data becomes available. Eventually, ethanol will be prized foremost as an oxygenate additive replacement for MTBE. 

Methanol and Fischer-Tropsch hydrocarbons from coal and biomass will provide high energy density fuels for the carbon-neutral future as petroleum scarcity drives other technologies into play. The Fischer-Tropsch liquified fuels technology from 20th century pariah states (Nazi Germany and South Africa) will assume a greater role in the post-petroleum age.

Fermentation of starch-derived glucose to ethanol and CO2 is too wasteful in the end to be attactive.  Fermentation of cellulosic material to acetate is more mass efficient. Esterification and reduction of ethyl acetate affords ethanol. One company, ZeaChem, (former coworkers, actually) is already working to bring this technology on stream. It remains to be seen how it will go over. I wish them well.

Electric power for the future will come from many sources. Distant, centralized power plants will channel energy across the grid to home-charged automobiles. Electrons travel fast and quietly over the lonely wire. They do not require fleets of ponderous 18-wheelers to move them around in limited quantities.

I see a future heavily reliant on electrons supplied from nuclear plants. Uranium-235 infrastructure will continue to supply fuel to nuclear plants for a long time. But the low abundance of U-235 (o.7 %) and the ever present proliferation potential of Pu-239 from this fuel cycle raises questions as to the wisdom of building U-235 nuke plants in the third or fourth tier states.

A more obscure nuclear fuel that is more abundant than uranium will see a phase-in as demand on the present nuclear fuel infrastructure exceeds supply.  That fuel is Th-232. Thorium-232 is  generally more abundant that uranium and has the additional benefit that it’s major isotope, Th-232 , is the nuclide of interest. Th-232 is not a fissile nuclide, but is a “fertile” isotope instead. Th-232 absorbs a neutron in a reactor seeded with U-235 or Pu-239 to provide an initial neutron flux to become Th-233, which beta decays to Pa-233 which further beta decays to U-233.  It is U-233 which is the fissile nuclide.  U-233 then participates in the fission chain reaction that generates the heat.

You can’t make a nuclear weapon out of Th-232, though in principle you could make one from U-233. The downside of a U-233 bomb is the high specific activity of this isotope.  U-233 is intensely radioactive and poses extra problems in handling.

The economics of thorium energy is advantageous in many ways to that provided by uranium/plutonium infrastructure. Thorium is abundant in monazite formations- reportedly up to 16 % thorium oxide.  The present problem with the thorium cycle is handling the intensely radioactive U-233 that remains in the spent fuel elements. Separate processing infrastructure will have to be put in place to supply reactors that burn thorium before this fuel can go forward.

An HTGR  Brayton cycle reactor with a helium turbine could provide up to 50 % thermodynamic efficiency.  Combine this reactor design with the potential cost savings of the more abundant Th-232, and you have a technology that is well set to provide power to keep the lights, cable TV, and the internet going into the post-petroleum age.

Check out the blog dedicated to Energy from Thorium. I’m writing about thorium because I think it is an important fuel and it needs to find its way to mainstream thinking.