Biofuel could be produced at reasonable cost using the following process:
1) Grow a type of grass that cows can digest, such as Pennisetum flaccidum, Dactylis glomerata, or Tripsacum dactyloides.
Most proposals for cellulosic biofuel have focused on plants (such as Miscanthus) yielding the most dry mass, but if cows can't digest something well, then microorganisms in a reactor probably can't either.
2) Harvest the grass 2 or 3 times a year, sending it to a processing facility.
The grass can also be grazed instead of doing the last harvest of a year.
3) Sterilize the grass with steam.
4) Add the grass and calcium carbonate to a bioreactor tank.
The bioreactor should contain microorganisms that:
• digest cellulose
• grow at pH 5.8
• produce butyrate, without a buildup of acetate or lactate
The microorganisms need to maintain a higher intracellular pH to grow. This would be done with Na+/H+ or K+/H+ antiporter proteins. So, the solution needs some sodium or potassium salt.
Something like M. elsdenii H6F32 might be a reasonable starting point for metabolic engineering here.
The main net process is: 2 glucose + CaCO3 -> 4 H2 + 5 CO2 + calcium butyrate
5) Collect precipitated calcium butyrate from the bioreactor.
If necessary, calcium butyrate can be purified by dissolving it in some alcohol.
Excess microorganisms from the bioreactor can be removed and used as fertilizer.
6) Heat the calcium butyrate.
Calcium butyrate can be converted to calcium carbonate and (mostly) dipropyl ketone by heating it to 400 C.
Heat transfer can be a problem here, because the calcium carbonate can form insulating films on surfaces. Perhaps a fluidized bed reactor or a molten salt (eg ZnCl2) could be used to heat the calcium butyrate.
7) Use the dipropyl ketone as fuel.
Dipropyl ketone has a high octane rating and produces relatively little pollution in piston engines. Using it in current cars may require replacing some fuel hoses, but isn't too difficult.
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