3-14: Pelletization and high solids enzymatic hydrolysis of AFEX treated corn stover

Densification of herbaceous biomass prior to shipment offers tremendous logistical advantages for a lignocellulosic biorefinery, but has the downside of being a costly operation.  Ammonia fiber expansion (AFEX™) can be performed commercially at scales ranging from 5 to over 100 metric tons (dry basis) per day, and has been shown to improve pelletization  by solubilizing and redistributing lignin, which can act as a binder.  AFEX has also been shown to be an effective pretreatment for a wide variety of grasses and agricultural residues.  Therefore, a network of small, local depots that collect, pretreat using AFEX, and densify local biomass could supply a large centralized refinery with readily hydrolysable pellets. 

To demonstrate the feasibility of this approach, AFEX-treated corn stover was pelletized and hydrolyzed at high (18-36%) solid loadings and the results compared to non-pelletized AFEX-treated corn stover.  No post-treatment of the pellets was required  to effectively convert the pellets to sugars.  Pelletization decreased the rate and extent of water absorption by the solids, allowing for simple mixing during initial liquefaction.  Sugar yields on pellets were slightly higher than unmilled, unpelletized biomass and comparable to milled, unpelletized biomass.  Hydrolysis and fermentation was also demonstrated using Zymomonas mobilis to produce ethanol in a 150 L stirred tank reactor.  Given the ease of mixing and high yields obtained with pelletized corn stover, pretreatment followed by densification could have additional advantages in the biorefinery beyond improved logistical handling of biomass.