(Partly) Renewable Ethanol


Judging from a range of recent studies, corn ethanol is about one quarter renewable. Even by the most optimistic estimate — the one the Renewable Fuels Association repeatedly cites — corn ethanol is only 40% renewable.* The US may have displaced 4% of its gasoline use with energy from ethanol in 2008, but less than 2% of that energy came from sunlight captured on those 21 million acres of corn. Most of it came from the natural gas and coal used to make fertilizer and run ethanol plants.

Ethanol energy output:fossil energy input

A cursory comparison of study results emphasizes the differences between them. Delving into the data a little further shows that there is actually a remarkable degree of consistency between the studies, despite the different figures they come up with in the end. All agree that ethanol processing is by far the most energy-intensive component of the lifecycle, consuming nearly two-thirds as much energy as we get from burning the fuel. Growing the corn requires the next biggest energy investment: Most conclude that farming requires about a third as much energy as is available from the ethanol (Pimentel and Patzek’s estimate is higher; Kim and Dale’s is lower). All agree that transporting the corn and distributing the ethanol are pretty minor components of the energy investment.

Ethanol energy output:fossil energy input

So if ethanol processing takes almost two-thirds as much energy as ethanol delivers, and farming takes almost a third, how can there be any net energy benefit to making corn ethanol?

The answer lies with the co-product. For every three truckloads of corn that go into an ethanol plant, about one truck comes out full of dried distiller’s grain, a protein-rich animal feed. Since the byproduct is useful, each of the studies assigns a “co-product” credit to the process. A certain proportion of the energy that went in to the process is subtracted because the distiller’s grain co-product is assumed to have saved energy needed to grow animal feed.

Without the co-product credit all of the studies agree that corn ethanol would be less than 15% renewable.* Among the five studies that conclude there is an energy return to making corn ethanol, the co-product credit accounts for most of the benefit reported.

Renewable Proportion of Corn Ethanol

The size of the co-product credit is what really distinguishes the six studies. Those that assign a larger proportion of the energy investment to the co-product conclude that there is a greater energy return to ethanol production.

Relationship between co-product credit and energy ouput ratio

Obviously, how the co-product credit is calculated is important. Since dried distiller’s grain is a high protein feed that can substitute for soybean meal, most of the studies use an estimate of the amount of energy needed to produce the soybean meal equivalent of the distillers grain that comes out of the ethanol plant. This seems logical, but Kim and Dale point out that a sure way to improve the energy return from corn ethanol is to reduce the efficiency of soybean production (!).

The most optimistic numbers come from Shapouri et al. (2004), who calculate a credit based on the proportion of energy used to make dried distillers grain in a typical ethanol plant. They consider the amount of energy used to dry distiller’s grain an energy credit, because it is used to make the co-product, not ethanol. The net energy calculated using this method is equivalent to the net energy from a plant in which the distiller’s grain drying phase is omitted completely and animals are fed wet distiller’s grains.

Ethanol promoters understandably wish that any discussion of net energy associated with ethanol production would simply go away. This spring an advocacy group called Ethanol Across America issued a brief headlined with a statement fashioned to end all debate:

Study after study after study confirms that ethanol production from corn produces more energy than it takes to make it, period. End of story. So why is this still an issue? When you look at the facts, it simply isn’t.

The problem is, of course, that corn ethanol is being billed as a renewable fuel when it is currently mostly non-renewable. It’s true that most studies say corn ethanol does better than break even from a net energy perspective, but not much better. Calling ethanol 25% renewable (or 75% non-renewable) just doesn’t have the same ring as calling it renewable. Honesty and good policy require that we tell it like it is, though. For ethanol and other “renewable” fuels, I think we’d be well served to abandon the convenient renewable/non-renewable dichotomy, and start qualifying renewable claims with a percentage.

If you’ve read this far, you might have me pegged as an opponent of first generation (sugar and starch-based) ethanol. I’m not. Many such opponents (e.g. zFacts.com) argue that this is a mature technology, it’s already as good as it’s likely to get, and we should spend our time and resources on other pursuits, like learning to make ethanol out of cellulose. I disagree.

I think there’s plenty of room to improve first generation ethanol production. Here are some ideas:

  1. Learn to grow feedstock crops without synthetic nitrogen fertilizer. By using biologically-fixed nitrogen, instead of synthetically fixed nitrogen we can cut the amount of energy that goes into grain production by 30-50%. That’s huge. Study after study after study after study confirms that corn yields will not fall by an equivalent amount. If we’re concerned about net energy, then low-input farming is the way to go.
  2. Learn to use other feedstock crops. American farmers are very good at growing corn, but there are plenty of other crops that can produce lots of sugar and starch for conversion to ethanol. India and China have recognized that sweet sorghum and sweet potato can probably out-perform corn, particularly in low-input systems. America’s single minded focus on corn has led an increasing proportion of corn farmers to grow corn after corn, which is a poor management practice that ultimately compromises yields and demands more agricultural inputs. Diversifying feedstock demand can help restore crop rotations to American farms.
  3. Re-integrate crop, animal, and fuel production systems. Drying distiller’s mash down into dried distiller’s grain is one of the most energy intensive operations at an ethanol plant. Distiller’s mash makes a fine animal feed, but it doesn’t ship or store well. The extra energy investment to dry it is unnecessary if the plant is linked to an animal-based farm with a local and immediate demand for feed. Organic waste products from the animals and the ethanol plant can be digested anaerobically to make methane for renewable energy, and the material left over after digestion can be recycled back onto the crop land as fertilizer. A study published earlier this year by Liska et al. concludes that a closed-loop corn ethanol plant like this reduces greenhouse gas emissions by 67% and increases the net energy ratio to 2.2, making corn ethanol that’s 55% renewable.* I suspect that closed-loop plants will work better on a small scale than a large scale: Here, in Kentucky, the move to drying distiller’s mash began when bourbon distilleries got too big for the farms that once were coupled with each distillery.
  4. Stop using nonrenewable fuels for fermentation and distillation. All of the studies that conclude that cellulosic ethanol plants will offer a better energy return than first generation ethanol plants assume that the energy the cellulosic plants use for fermentation and distillation will come from plant matter, not from natural gas and coal. This difference accounts for almost all of the anticipated energy benefit expected of cellulosic ethanol. We don’t need to wait for advanced technology to become available to start converting first generation plants to draw their heat from renewable sources, like wood or crop residues. Brazilians have known this for years: Most of the energy that drives their ethanol production comes from sugarcane residues left over after sugar extraction.

By combining tactics I suspect it’s possible to make first generation ethanol competitive with cellulosic ethanol in terms of net energy return, and to move in this direction immediately, rather than waiting for the expensive and experimental enzyme technology that seems to be holding up development of the cellulosic ethanol segment. The only way that either will ever replace a substantial portion of our gasoline consumption, though, is if we can dramatically reduce our consumption. Over the past two years Americans have done just that, reducing oil use by 9%. That’s a much bigger bite out of the energy pie than we got by turning 21 million acres of corn into ethanol

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