Cellulosic ethanol from corn - a quick route to hundreds of millions of gallons

Cellulosic ethanol production in the US is far behind the ambitious targets set in the Renewable Fuel Standard, and one of the reasons for the delay is the enormous capital investment needed to build out a significant cellulosic fuels industry. Given today's challenging investment climate, first-of-a-kind risks in cellulosic technologies, and relatively large capital investment needed per gallon of production, cellulosic ethanol facilities have been slow to arrive.

But what if cellulosic technology could be deployed across the existing corn ethanol fleet with almost no capital investment required? What if cellulosic ethanol could be produced side-by-side with corn ethanol and increase the capacity of corn ethanol plants? What if hundreds of millions of gallons of cellulosic ethanol were produced in the US as soon as this year?

A recently published patent application from DuPont Industrial Biosciences suggests that near zero-capex cellulosic ethanol could become a reality. In US Patent Application 20120276593 Mian Li and Colin Mitchinson describe a process in which a standard cellulase enzyme package is added to the dry grind corn ethanol process. In the normal dry grind process, ground corn is heated with alpha-amylase enzymes to liquefy the starch component. This turns the corn into a mash, which is then digested into sugars by glucoamylase enzymes (saccharification) and fermented into ethanol by yeast. However, corn grain contains around 8-10% of cellulosic fiber which is not digested by alpha-amylases or glucoamylases. Because the enzymes aren't able to break this fiber down into sugars, it sticks around and is not made into ethanol. Typically, this fiber is recovered along with protein and oil and used in animal feed.

While the standard corn ethanol enzymes can't break down fiber, the cellulases which have been developed for breaking down biomass in a cellulosic biorefinery are quite capable of doing this. Recognizing the opportunity, the DuPont researchers added their cellulase package, Accellerase 1500, to the saccharification step along with glucoamylase. They observed as much as 3% more ethanol produced than in the normal process in a number of different trials reported in the patent application. The increase in yield may come from sugar released from the cellulose in the corn or from starch becoming more accessible due to the breakdown of fiber which entraps it. Importantly, they did not have to make major modifications to the rest of the process in order to see the yield boost.

Considering the best (optimistic) case yield increases and some of the enzyme loadings described in the application, one can compare the enzyme cost against the ethanol yield boost. One ton of dry corn produces about 118 gallons of ethanol, so this technology could contribute an extra 3.5 gallons. This might be worth about $10 or more, depending on whether it gets credit as cellulosic fuel. At the lowest useful enzyme dosage of 0.08%, 1.6 lbs of cellulase enzyme would be required per ton of corn. The most recent NREL process design costed cellulase enzymes at $1.90/lb, hence the enzyme cost would be $3 to get $10 worth of additional ethanol yield. Even if the enzyme cost tripled, the cellulase yield boost may still be attractive, especially if the ethanol gets a cellulosic credit.

If it is possible to scale these results from the lab into commercial production, then cellulases could be used to unlock cellulosic ethanol from corn in existing biorefineries, just by sprinkling in some additional enzymes. Corn ethanol production capacity in the US is over 14 billion gallons, suggesting that it would be possible to produce as much as 420 million more gallons through deployment of cellulases in the process (although some modifications would be needed to apply this technology to wet mill ethanol facilities). At least some of this increased ethanol would likely be derived from cellulose so that it could legally supply the volumes of cellulosic fuel mandated by RFS2, and it would greatly increase the supply beyond the 10 million gallons in the standard in 2012. Finally, this could be a large-scale, cost effective use of cellulase enzymes which would get them out into the market and encourage producers to drive their costs down. Lower enzyme costs would then make cellulosic ethanol from other sources such as ag wastes and energy crops much more affordable and enable the cellulosic industry to grow faster.

P.S. This simple enzyme addition concept is different from the "bolt-on" technologies being pursued by NCERC, NEAtech, and Quad County. These require new equipment to be added to the corn ethanol plant to convert the cellulose, while the DuPont concept uses cellulase enzymes to break down the corn fiber within the existing plant's equipment.