Nearly any plant material can be broken down into simple sugars.
Biofuel production focuses on taking the carbon that's already present in plants and converting it into burnable carbon-based fuels. Most of the carbon in a plant comes in the form of sugars, which can be readily converted into ethanol and less readily modified into other fuels.
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| Gamma valerolactone, the secret sauce of the new biofuel process |
Sugar is relatively easy to obtain from things like fruit and seeds, but those are also the sorts of things we like to eat. Most of the sugar in the rest of a plant, however, is locked into a complex polymer called cellulose. Figuring out a way to easily break down cellulose has been one of the major hurdles to the expansion of biofuels.
Now, researchers from the University of Wisconsin–Madison have figured out a chemical treatment that, given a bit of time, can completely dissolve any plant matter including wood. The end result is a solution containing mostly sugars, along with a few other organic molecules—some of which can be shunted off to synthesize the key ingredient of the chemical treatment itself.
The key ingredient in the chemical treatment is gamma-valerolactone, a ring-shaped molecule that incorporates an oxygen in its ring. On its own, this seems to be able to loosen up the cellulose and make it more accessible for chemical reactions. But it doesn't break it down into the individual sugars it's composed of. To do that, the researchers had to add some dilute sulfuric acid along with a bit of water (20 percent of the final solution) to keep everything in solution.
Essentially, that mixture gets sent into a heated reaction chamber stuffed with any kind of plant matter—the authors tested corn stalks after the corn had been harvested, as well as poplar and maple. If the solution is allowed to flow slowly through the reaction chamber, then it will completely break the plant material down into a mixture of sugars. The only solid material that's left is a small bit of particulate debris.
When the resulting sugar-rich solution comes out of the far end of the chamber, mixing it with a solution of table salt is enough to extract almost all of the sugar into a water solution. That solution can then be fed to yeast, which will convert it to ethanol. The remaining organic compounds stay in the organic phase. The gamma-valerolactone can then be pulled out of the organic solution and reused; the rest can be used as a chemical feedstock for other reactions—including the synthesis of gamma-valerolactone.
The researchers also found that they could speed up the reaction a bit, use a somewhat lower heat, and still extract a lot of sugar. The remaining plant matter can just be left in the reaction chamber when more material is added; if it wasn't broken down the first time, it's likely to be on the second pass.
The conversion into ethanol currently requires a fairly high degree of purification, but the authors are working on evolving a strain of yeast that operates efficiently in the solution as it comes out of the reactor. If that works out, then it will raise the efficiency of the process as a whole a bit.
And that's rather important, since they estimate that the ethanol produced by this process will be competitive as a fuel at a cost of just under $5.00. That's competitive with the enzymatic breakdown of plant material, which also liberates sugar for ethanol production. And, as noted above, neither of these processes competes with crops—in fact, they can use some of the byproducts of crops.
But right now, neither of those processes is competitive with either fossil fuels or ethanol produced from the crops themselves. So either of these technologies needs one of a few things: a big jump in efficiency, increases in the price of fossil fuels, policy decisions that limit the use of fossil fuels, or policy decisions that limit the amount of crops diverted into fuel production.
Science, 2014. DOI: 10.1126/science.1246748 (About DOIs).
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