New Processing Steps Promise Cheaper Ethanol Production
April 3, 2006
Zhang's cost-effective pretreatment process integrates three technologies - cellulose solvent pretreatment, concentrated acid saccharification and organosolv - and overcomes the limitations of existing processes. Instead of a high-pressure system that operates between 150 and 250 degrees C, Zhang's "modest reaction" operates at atmospheric pressure and 50 C (120 F) to pretreat corn residue and free the solid polymeric sugars. In a multi-step pretreatment system, Zhang uses a strong cellulose solvent instead of highly corrosive chemicals, high pressure and high temperature to break the linkages between lignin, hemicellulose and cellulose.
During Zhang's gentler process, there is no sugar degradation or inhibitor formation. Additionally, he uses a highly volatile organic solvent to precipitate dissolved cellulose, extract lignin and enable effective chemical recycling.
After pretreatment and reagent recycling, lignocellulose can be fractionated into four products:
- Lignin.
- Hemicelluose sugars.
- Amorphous cellulose.
- Acetic acid.
"Co-products can generate more income, making a biorefinery more profitable, and enable satellite biorefineries that fully utilize scattered lignocellulose resources," said Zhang. "For instance, lignin has many industrial uses, from glue to polymer substitutes and carbon fiber; and xylose can be converted to a healthy sweetening additive - xylitol or to the precursors for nylon 6."
Amorphous cellulose, which is converted from crystalline cellulose, is another co-product from Zhang's process. In this form, the cellulose material is more accessible for further hydrolysis, resulting in a higher sugar yield, higher hydrolysis rate and less enzyme use. Zhang tested amorphous cellulose hydrolysis by adding special enzymes (Trichoderma cellulases) from Genencor International. The result is that about 97% of the cellulose is digested after 24 hours of the hydrolysis process.
The paper, "Novel lignocellulose fractionation featuring modest reaction conditions and reagent recycling" (FUEL 143), by Zhang and Lee R. Lynd of the Thayer School of Engineering at Dartmouth College, was presented March 30 as part of the Advances in Fuel Science and Technology session.
Ethanol now comes from corn kernels. "But that is food," Zhang said. "If we want to produce 30 to 60 billion gallons of ethanol, which is what is needed to meet the president's goal, we have to use the entire plant, or the stover (leaves, stalks and cobs), and leave the kernels as food." The largest challenge for bioconversion from raw materials to bioethanol is high processing cost, resulting in higher prices for bioethanol than for gasoline.
Corn stover is the most abundant agricultural residue in the U.S. The challenge is separating the sugars from the lignocellulose - the combination of lignin, hemicellulose and cellulose that form plant cell walls. Many technologies have been developed to convert lignocellulose to sugars but the costs are still high and sugar yields are low. "No one wants to take the risk -- to invest $1B in a large-size biorefinery based on lignocellulose," said Zhang.
Zhang is collaborating with the National Renewable Energy Laboratory (NREL) and Oak Ridge National Laboratory (ORNL), using NREL software to analyze the economic costs of various ethanol production strategies and ORNL facilities to test different enzymes and material performance. "NREL and ORNL have spent 30 years on lignocellulose processing, biocatalysis and bioenergy research, and are glad to cooperate on new technologies which can effectively overcome the recalcitrance of lignocellulose," Zhang said. "We hope to soon establish the first pilot plant in Virginia based on this new technology with switchgrass."
Source: Virginia Tech.