Converting thin stillage (a major byproduct of the dry-mill corn ethanol process) to methane for use in the ethanol production process could increase the net energy balance ratio of corn ethanol at a conventional from dry mill from 1.26 to 1.70, according to a new report published online 5 August in the journal Environmental Science & Technology.
The study by a team from Washington University in St. Louis; Viable Bio-Projects LLC; and GTL Resources evaluated the conversion of thin stillage in laboratory-scale thermophilic anaerobic sequencing batch reactors for conversion to methane. They found that augmentation of cobalt as a growth factor to the thermophilic anaerobic digestion process is required.
After reaching sustainable operating performances, the methane potential in the reactors was 0.254 L CH4/g total chemical oxygen demand (TCOD) fed. Together with a reduction in the mass of solids that needs drying, methane generation translates to a 51% reduction of natural gas consumption at a conventional dry mill, which improves the net energy balance ratio from 1.26 to 1.70.
At the design hydraulic retention time of 10 days, the digesters achieved TCOD, biodegradable COD, volatile solids, and total solids removal efficiencies of 90%, 75%, 89%, and 81%, respectively. We also found that struvite precipitation occurred in the thermophilic digesters during the course of the study, resulting in possibilities for nutrient recovery.
Matthew T. Agler, Marcelo L. Garcia, Eric S. Lee, Martha Schlicher, and Largus T. Angenent (2008) Thermophilic Anaerobic Digestion to Increase the Net Energy Balance of Corn Grain Ethanol. ASAP Environ. Sci. Technol., doi: 10.1021/es800671a