Science Forums 2

[CraigD]

Here’s my not-extensively researched and vetted, “executive summary” of plant-based renewable combustion fuels, with petroleum fuels and hydrogen for comparison:

• All dry plant matter is mostly (C6H10O5)n – cellulose – and lignin, a family of big, complicated molecules consisting of roughly the ratio of elements C20H16O5.
• Plant oils are triglyceride such as C55H98O6 – similar to typical diesel fuel, C12H26, but with a bit of oxygen here and there – though much less than in their plant precursors. For all intents and purposes, plant oil is chemically indistinguishable from animal oil – rendered fat.
• Ethanol is C2H6O6, making it an alcohol – something roughly following the formula CnH2n+2O. Way less carbon than any of the above, and much more oxygen.

From this, we see that:

• Plant oils require much less energy to make than Alcohols – basically just crush, press, and filter a plant, and you get usable oil. Alternately, and much less efficiently, you can feed them to animals, then render the animals for their oil. Alcohol takes a lot of chemical transformation to produce.
• Like light petroleum gasses (CH4 methane, C3H8 propane, etc.), alcohol has less carbon in it, so is cleaner burning.

All of these fuels are essentially hydrogen delivery systems. Each has advantages and disadvantages:

• Plant oils are require little energy to produce, are easy to store and handle, but produce a lot of carbon when burnt
• Alcohol requires more energy to produce, is also easy to store and handle, but produces less carbon
• Petroleum gasses require the least energy to produce (just find them, tap them, transport them, etc) but aren’t liquids, so are harder to store and handle, and, of course, aren’t renewable. They also produce little carbon when burned.
• Liquid petroleum likewise requires little energy to produce and isn’t renewable, is easy to store and handle, but produces a lot of carbon.
• Hydrogen requires all of the energy it contains, and then some, to produce, as bad as some alcohol producing processes, is about the hardest fuel in the universe to store and handle, but produces no carbon when burned.

Though many plants are good oil suppliers, as Journey to Forever’s nice tables show, their per-acre productivity varies widely, with palm seed oil more than twice as productive as the nearest competition, and over 30 time as productive as a low-scoring plant, such as corn.

So that’s the rundown. It’s surprising, to me, how similar fuels of various kinds actually are, explaining perhaps why no single fuel offers an obvious “silver bullet” solution to the world’s energy needs.