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Not Ready for Prime Time: Making Fuel Out of Invasive Plants

When it comes to making ethanol, taking a hard, second look at seemingly great ideas is smart policy.
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Kudzu growing on trees in Georgia. (PHOTO: PUBLIC DOMAIN)

Kudzu growing on trees in Georgia. (PHOTO: PUBLIC DOMAIN)

It’s not a widely known ecological principle, but the idea that when “life hands you lemons, make lemonade” has certainly been applied to invasive species. While not always the case, plants and animals that are benign in their own home sometimes run amok in new settings, often to the detriment of local economies and existing flora and fauna. (And like so much else, expect climate change to make their penetrations more extreme.)

You can despair—or you can start squeezin’. There’s a relatively well developed "if you can’t beat ‘em, eat ‘em" movement afoot, or perhaps a-fin, with legions of “invasivores” who target locally raised but unwanted invaders ranging from feral boars to nutria to iguanas to lionfish. This isn’t just a game for carnivores—that Blob of vegetation known as the kudzu vine reportedly cooks up just fine (here are some recipes from one of the many kudzu festivals dotting the South), and might even have medicinal properties.

Not to be outdone, ecologists have suggested that invasive plants be turned into feedstock for making cellulosic (as opposed to starch) ethanol. It’s pure genius: “the plan could motivate the large-scale eradication of an array of troubling invaders, avoid land use conversion, resolve the food-versus-fuel debate, result in millions of gallons of clean-burning ethanol, and finally free us from our addiction to fossil fuels.” There are even more potential benefits, albeit somewhat bureaucratic, write the three scientists at the University of Illinois’ Energy Biosciences Institute who supplied the win-win-ad infinitum list above in an article appearing in Biological Invasions.

Ethanol production is remarkably fiddly. Most biorefineries can only handle a very specific feedstock, like plants with very similar cell walls.

So why do the trio, led by Lauren D. Quinn, end their paper by saying the invasives-to-energy concept “warrants careful scrutiny” (which is the nice academic way of saying, "I don’t think so, Tim")? Because what appears like the perfect answer on first blush faces an embarrassment of obstacles on closer inspection. It’s kind of like corn (i.e. starch) ethanol—it seemed like such a good idea, until it wasn’t.

“I’m not saying that we shouldn’t continue to look at ethanol conversion processes eventually,” Quinn was quoted in a release from the University of Illinois. “I’m just saying that right now, it doesn’t seem to make a lot of economic sense.”

Two huge roadblocks immediately appear, one philosophical and the other practical.

The philosophical argument is that we don’t want to harvest invasive plants, we want them permanently gone, root and branch. That’s a fine sentiment for the first batch or two of biomass, but eradicating something suggests there will be no second harvest. And even if the plant is a hardy enemy that’s likely to resprout, what kind of business plan features the end goal of going out of business?

You could just plant regular plots of a particularly useful invasive, as researchers are doing with Arundo donax now. But that runs in the opposite direction of getting rid of invasives. (Nomenclature Alert: An invasive we suddenly like, say, wheat, is an "exotic.")

This might not matter if ethanol stills were a little more flexible, but now we come to the practical problem. Ethanol production is remarkably fiddly. Most biorefineries can only handle a very specific feedstock, like plants with very similar cell walls. “The breakdown and processing of plant tissues to ethanol requires different temperatures, enzymes, and equipment that are all highly specific,” Quinn said. “The proportion of cellulose, lignin, and other fractionation products can differ even within a single genotype.”

This is nettlesome issue even if you’re not converting nettles, and more accommodating integrated biorefineries are coming on line from Visalia, California, to Old Town, Maine. But for the time being, this remains a problem.

You could, of course, truck your pulled-up invasives to a refinery that is prepared to handle them. This, however, adds to the processes’ carbon footprint and its cost—a drive longer than about an hour pretty much sucks all the profit out of the endeavor. And critically, moving invasives around has proven a dandy way to spread them further.

There are other questions, too–how do you profitably collect enough biomass when invasives aren’t all growing in neat rows, what happens when you leave a “weed-shaped hole” in the habitat, how will regulators react to having all this noxious weed biomass flitting around the countryside?

None of these barriers is insurmountable, but they are real. And there are other energy conversion processes around—gasification and co-generation with coal, for example—that could make use of the odd weed.

So for the time being, I’d suggest spending more time in the kitchen than the garage with that freshly pulled batch of iceplant.