Biofuel Back to the Future

A century ago, farmers relied on these big, steampunk-y contraptions called threshing machines to bring in the harvest. The machines were portable, and expensive–they were usually owned by a third party, or by a cooperative of farmers. The threshers traveled from farm to farm, region to region, separating grain from stalk and turning crops into commodities.


Pictured: This threshing machine's body lies a mouldering in a barn, but its spirit is marching on. From Flickr user exfordy, via CC.

Now, researchers from the University of Minnesota are hoping to repeat history with a portable machine that could turn prairie grasses, small trees and corn stalks into liquid biofuel. It's a nifty idea that could be great for both the environment and rural economies…provided the boys in the back room can work out a few bugs.

Portable microwave pyrolysis could be the future's answer to the threshing machine. Obviously, what's being made is different, but the basic idea is the same: Take this big machine around from farm to farm and use it to help farmers turn plants into a higher-profit product.


Pictured: A higher-profit product.

Pyrolysis is all about using heat to break down organic materials into a form better suited to usable, commercial energy. To get things cooking, the University's system relies on microwaves, stronger versions of the same technology you use to make popcorn and heat up leftover pizza. It's a handy, and somewhat outside-the-box, approach. Typically, before any material is put into a pyrolysis system, it has to be ground into tiny pieces to improve the transfer of heat through the mass. But as you may have noticed, microwaves heat up the center of a solid object just fine. If you're cooking on the stove, it saves time to break a chicken breast into smaller chunks. But microwave that breast whole, and the center cooks at about the same rate as the outside. Same principal applies here. Using a well-established technology like microwaves also means the University's pyrolysis set-up could, potentially, produce fuel for less upfront cost compared to typical pyrolysis systems, and some of the other biofuel-making methods.

The main product of the University's system is a liquid fuel. It does produce enough combustible syngas that, once started, it can power itself. But, in general, liquid is what comes out. On one hand, this is a bit limiting. Other methods of breaking down organic material focus on producing just the syngas, a veritable chemistry Christmas present of hydrogen and carbon monoxide. Syngas can be burned like natural gas. You can use it to make certain chemicals that normally have to be derived from fossil fuels, like the ones used in agricultural fertilizers. Or you can turn it into a liquid fuel. Whatever you want. If alternative fuel production were baking, syngas would be the water and flour.

The liquid fuel produced by pyrolysis, on the other hand, is more like ending up with cupcake batter. Still nice, but you'll only be making dessert. On the other hand, if you really want cake, microwave pyrolysis gets you to that endpoint in fewer steps.

But biogas also needs some cleaning up. An engine will run on fresh biogas, but over time the acidic fuel will tear it apart. Paul Chen, senior research associate in the Department of Bioproducts and Biosystems Engineering, is one of the researchers working on the University of Minnesota microwave pyrolysis system. He says his team is working with chemical catalysts that can make biogas more engine-friendly, but they're still trying to figure out the best way to tackle the problem.

Another kink that still needs to be worked out in the portable pyrolysis plan: The whole "portable" part.


Pictured: Not a portable system.

Right now, the University's machine is a Rube Goldberg-like mass of conveyors and pipes that stands almost two stories tall with a floorplate that would fill a three-car garage. It is innovative, but it's not quite ready to load on a truck. Chen and company say they're close to working out a design for a smaller pyrolysis system they could take on the road. Armed with a $500,000 grant, they hope to have the pilot version built by early next year. If it works, the system could give farmers a relatively easy way to produce fuel for use on their own farms or, if it traveled with a tanker truck, that they could sell through local farming cooperatives, which already have a license to sell and ship fuel.