• Keep it simple with minimal unit operations and separations and minimal capital investment.

  • Target a single agricultural residue at the beginning (corn stover is the largest source of agricultural residues in the United States).

  • Maximize titer.

  • Life-cycle assessment—need to understand overall process to make it sustainable.

  • Process integration—need to have high efficiency to fit all the pieces together.

  • Risk mitigation—take small steps within the current infrastructure.

Emptage explained the steps involved in converting sugar to ethanol: milling, pretreatments, saccharification, conversion of biomass to fermentable sugars, fermentation, and separation. He said that the key technology is fermentation since it can be applied to butanol or other products. He also explained exactly how corn is harvested and that the two main sugars that can be extracted from biomass are glucan and xylene.

Emptage explained that Dupont is taking a new approach using Zymomonas mobilis, a natural ethanol producer found in the sap of agave plants in the tropics. It has a higher yield and productivity than yeast and has the potential to be a better organism than yeast. Emptage announced a collaboration between DuPont and POET, the largest dry-grind producer of ethanol in the United States with over a billion-gallon ethanol capacity. They are working together to develop a pilot plant in South Dakota using the new technology. He said, “This isn’t a revolutionary program. This is really an evolutionary program, just adding on to the current infrastructure.”

Emptage believes that the key remaining challenges are solids handling, having an infrastructure to collect, transport, and store biomass effectively and efficiently with its low-bulk density. Another challenge is the cost of enzymes. The goal is to make the handling of the Z. mobilis derived biomass cost competitive with grain ethanol.


Michael Wasielewski of Northwestern University asked Thomas Moore about the type of light fluxes being used to investigate the solar flux. He also asked, “Since we all know that photosynthesis has control mechanisms that actually modify electron flow, based on light flux, what kind of prospectus or perspective do we have for control mechanisms in such systems?” Moore explained that one of the factors that seems to limit natural photosynthesis is the diffusion of carbon dioxide into the system for fixing, so it is important in photosynthesis to throttle back the powerful oxidant when carbon dioxide is limiting. There is a control mechanism called nonphotochemical quenching that is related to the xanthophyl. Moore’s team is working on a model system that responds to high light and quenches excited states releasing the energy as heat, and then as the light intensity comes back down again, the system shuts itself off. He also explained that the system needs to respond to the membrane potential and pH gradient across membranes. This can be done with a potential sensitive sensor.

R. David Britt of the University of California, Davis, asked about the limits to purely biological approaches. Thomas Moore said he thinks natural photosynthesis needs to be reengineered to double or triple its power of conversion efficiency. He said that solar will ultimately solve the problem. Moore then called for research focused on fuels by photosynthesis created by cyanobacteria grown on nonarable land and photovoltaics for electricity.

Douglas Ray from Pacific Northwest Laboratories asked G. Tayhas Palmore whether the process of engineering enzymes needs to be improved. Palmore said that not much is known about engineering proteins but she hopes that it can be solved using a Brown University database. Mark Emptage said that DuPont has worked with Diversa (non Verenium Corp.), which has put together a set of technologies to allow high-throughput screening and enzyme evolution to be done. He said that there is still a need for more basic understanding about how the enzymes operate on the complex structures.

Charles Dismukes of Princeton asked Mark Emptage how DuPont plans on solving the two major problems that he said need to be addressed: costs for removal of the ethanol distillation and acetic acid inhibition. To solve the first problem, Emptage explained that consolidated bioprocessing will be necessary. That technology has not yet been developed, so it is important to figure out what to do in the near term. DuPont has looked into thermopiles, but they have very low ethanol yields. DuPont is now seeking an organism that maximizes yield, which is why they chose to work with Zymomonas. Emptage explained that one way to solve the acetic acid problem is to adjust fermentation conditions to the highest pH level tolerable. DuPont has developed more acetate-tolerant strains. In DuPont’s process with ammonia, acetamide is a by-product with ammonolysis competing with hydrolysis of the acetyl groups, which lowers the total concentration of acetic acid in the process.

Douglas Ray asked whether biobased approaches are going to scale. Emptage said that scaling in fermentation is straightforward, and that tanks can be scaled almost as large as sugar and fermentative organisms. Daniel Nocera of Massachusetts Institute of Technology stated that he is worried about the long-term scaling issue for energy, which is why he supports solar. However, advances in solar energy involve discovery research that is 50 years out. Nocera went on to say that the energy problem is a basic science problem, not an engineering problem, and people should stop focusing on the complex engineering to find a solution.

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement