LIME: Since production of ethanol from corn - rather than from cellulosic biomass - has been feasible for quite some time, what are its limitations, and why hasn't it hit the mainstream?

Aden: Last year we produced over 4 billion gallons of ethanol with the renewable fuel standard that's in place now. That's expected to reach over 7.5 billion gallons by the year 2012. So there's going to be very much a market and a demand for ethanol from corn. But the limitations are several-fold: If you really want to make a dent in petroleum reduction and look at the big picture, corn ethanol is not going to get you very far. Projections vary but most say that the upper limit is in the 15-20 billion gallon range before you really start impacting food markets. But considering we consume over 140 billion gallons of gasoline every year, that's not really where we want to be. So that's why we look to cellulosic ethanol—because the resources available in terms of cellulosic biomass are much more plentiful than corn grain. And they don't have the food versus fuel competitive issues that exist with using a food crop like corn.

LIME: Can you walk us through the process of turning corn stalks and husks into ethanol?

Aden: The first step is what we call pretreatment, and that's really breaking apart the biomass and freeing up the matrix of components. After pretreatment, the heart of the process is the use of these cellulase enzymes to break down the cellulose into their individual sugars. These cellulase enzymes are just natural proteins that are very efficient at breaking down cellulose into glucose.

Once you have this mixture of sugars (a mixture of glucose and zylose and other sugars), you can ferment that into ethanol. Once you've fermented those sugars to ethanol, you've basically got a beer product and that beer can then run through standard distillation technology to separate and purify off the ethanol for fuel purposes. The ethanol that you get out of a cellulosic plant will virtually be no different than the ethanol that you get out of corn grain. It's just the process steps that it takes to get there.

LIME: Where are the main costs in the process? What are the key obstacles to making it competitive with standard gasoline?

Aden: Backing up five years, the main cost of the process was with the enzymes themselves. They're very expensive - they were used to stonewash jeans and for detergents and other fairly high-end applications. Within the past five years, NREL has worked with two of the largest enzyme producers in the world, Genencor and Novozymes, and the focus of this research was really to reduce the cost of those cellulase enzymes significantly for a process like this. Over the past 5 years, they've significantly reduced the cost by a factor of 20.

The second area that's really high-cost is the pre-treatment area. You have to use fairly exotic metals for the reactors. There are alternate pre-treatments that may not cost as much, but the real trade-off then is getting the yields of sugar out of those kinds of steps versus the cost that you put in.

Another significant fraction of costs is the capital cost used to process the leftover residue, or the byproduct of this entire process, which is the lignin fraction. In corn ethanol, there is an animal feed byproduct called DDG (distillers' dried grains), and that can be fed to animals and everything else. The byproduct that we get from cellulosic biomass is a lignin residue, and you can't really feed that to animals. The main use we see for this byproduct is as a combustion fuel. So you could burn this lignin residue in a boiler system and create your own steam and electricity needs, not only to power the facility, but also to have an excess that you could sell as green power to the local grids as a co-product. So it makes this whole entire process much more self-efficient and self-sustainable. You don't have to buy natural gas or coal or anything else to power your facility. It really eliminates the entire energy balance controversy that you hear about for corn ethanol.

LIME: So on what scale, here in the U.S., are we currently producing ethanol from biomass?

Aden: We can go out into our pilot plant at NREL and produce ethanol from biomass on a regular basis. What we're doing right now is really focusing on trying to make it cost-effective. These processes do have an economy of scale, meaning that the larger you can build them, the more cost-effective they will be. Our pilot plant is sized to handle one dry ton of biomass per day. On the other hand the commercial designs that we're looking at are on the order of 1,000 or 2,000 dry tons of biomass per day.

LIME: Are there other environmental trade-offs that come into play for cellulosic ethanol?

Aden: The main issues that you really have to look at with regards to biomass, and we'll use corn stover as an example, is how much of that corn stover you have to leave on the field. It actually does serve a purpose, which is for erosion control and soil nutrient levels, soil carbon levels and soil health. So every year farmers plow a good chunk of that corn stover back into the field for these very reasons. The question is: how much do they have to leave on the field and consequently how much does that leave for you to take off? That's going to vary from soil type to soil type, from one field to another, and from region to region. So it's a very important thing to look at.

The real benefit will be if we can start to grow crops that have increased yields of biomass per acre. Let's use switch grass as an example: Switch grass is nice because there's a wide geographic area that you can grow it in, from North Dakota all the way down to the Southeast. I've seen estimates anywhere from 5-10 tons of switch grass off of a field per acre. And that's a lot more than corn stover, where you can get maybe two to three tons per acre off of the field. Switch grass also uses a lot less water than corn does, and you can harvest it once and maybe even twice a year. So there's a lot of potential benefits with going toward a crop like switch grass. And it's something that we refer to as an energy crop, in that it might be grown specifically for energy purposes.