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Prickly Pear: The New Petroleum?

Biofuels are a great idea, but making them from corn is a bad idea. Corn is a water-intensive crop, and dedicating hundred of thousands of acres to growing fuel instead of food isn’t the smartest use of resources for a planet that is low on both fresh water and food.

No Corn


Wouldn’t it be great instead if we could find a way to efficiently use all the arid land being created by desertification? Maybe grow some fuel that doesn’t require water; a plant that virtually takes care of itself? Rodrigo Wayland Morales, a college student in Chile, suspected that Opuntia ficus-indica, or prickly pear, might fit the bill. He hated to see tons of discarded cactus pads going to waste each year in his country after they’d been contaminated by insects. Seemed like there was a better use for them, so he obtained funding to build the world’s first biogas cactus project in the very dry Chilean desert region known as the Elqui Valley.



Since then, Morales has created partnerships with companies in Chile, Brazil, Ecuador, and Colombia to build anaerobic biogas projects. Using anaerobic digestion, the prickly pear is turned into biogas, which can then be used for a variety of purposes. It can be used to generate electricity, it can be used in place of natural gas, and it can be used in specially adapted motor vehicles instead of gasoline. Cactus power!

This all made the news a year and a half ago, but failed to make much of a splash. As per usual, a bigger media splash came from Western researchers who conducted some research but didn’t actually build a biogas reactor. But, hey, there’s nothing like a peer-reviewed study, amiright?

These UK researchers considered prickly pear the perfect biofuel source because cacti don’t require much water. But why is that? Turns out the secret is crassulacean acid metabolism (CAM). I’d never heard of it either.

CAM is how cacti perform photosynthesis; it’ what allows them to grow on arid and semi-arid land that can’t grow corn, sugarcane, or any other of those ethanol crops that receive outlandish subsidies. CAM plants close their stomata during the day and open them at night. Nighttime is when they gather carbon dioxide from the air. They convert it into an acid called malate. Then, during the daytime, the malate is sent to the chloroplasts and converted back into carbon dioxide. Because the stomata remain closed during the day, the plant doesn’t lose water from evaporation like other plants do. This process allows plants such as the pineapple and the cactus to thrive in arid areas. Khan Academy has the science-y details.



Semi-arid and arid land comprises 12 to 18 percent of the Earth’s land. Most of it, minus Las Vegas, Dubai, Cairo, and other ill-placed cities, is pretty barren. Some of it is used as grazing land, while the rest just sits there. Mike Mason, one of the UK researchers involved in the cacti study thinks that somewhere between 4 and 12 percent of the world’s semi-arid land would need to be converted to cacti cropland to provide as much electricity around the world each year as natural gas does today. It doesn’t have to all be hot desert, either. The beauty of prickly pear is that it tolerates cold snaps well and can be used as a feed for livestock. Plus, we get to eat the fruit.



How does one go about turning prickly pear into biogas? You harvest the fin-like pads, which just happen to have the percentage of fibrous material that is the sweet spot for anaerobic digestion. This is the process by which microorganisms break down organic material in the absence of oxygen. Prickly pear pads are ideal because they degrade quickly—five to ten times faster than manure. This means the time needed to convert the plant material to usable gas is . . . I don’t know, because Google wouldn’t tell me. But it’s shorter than for many other plants.

Even more good news: The byproducts of the anaerobic digestion process that creates biogas are digestate and water. Water needs no introduction. It’s a good thing, and we can always find a use for it. Digestate is the organic material leftover afterward. It’s a great fertilizer. Not only does it help soil retain moisture and plants grow, it also inhibits some types of plant diseases. Cactus = Biogas + fertilizer + water. It’s a win-win-win.

The only sticking point is the fact that biogas is mostly methane. Methane is a greenhouse gas. When it is burned it also emits carbon dioxide. However, biogas advocates like to remind people that this is still better than burning fossil fuels because the cactus absorbed carbon from the air while it was growing. Thus, it emits the same amount of carbon that it has recently absorbed, making it carbon neutral.

So maybe the prickly pear is a win-win-win-win.



Posted on: July 13, 2015, 2:20 pm Category: Admin

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