Tuesday, May 3, 2011
Dilute-acid pretreatment is one of the most promising process options applicable for various lignocellulosic biomass feedstocks. NREL has developed an efficient continuous pretreatment reactor that can be operated with high-solid charge and with short residence time. Using this reactor, NREL has investigated extensively on pretreatment of corn stover. Because of these unique features, the pretreatment liquors produced from this reactor contain high level of hemicellulose sugars, the concentration surpassing 10 wt. %. On the other hand, the short residence time and low-acid condition limits the hydrolysis of hemicellulose. The pretreatment liquor obtained from this reactor thus contains relatively high amount of xylose oligomers, typically in the range of 10-40% of the total sugar. In order to fully utilize the sugars in the pretreatment liquor, oligomers need to be hydrolyzed to monomers. However, the secondary hydrolysis of this liquor was found to be much more difficult than hydrolysis of xylose oligomers in clean environment. This investigation was undertaken to verify the factors inhibiting the acid-catalyzed oligomer hydrolysis. For this purpose, the secondary hydrolysis was experimentally studied using rapid-heating small-scale batch reactors. The experiments were done for pretreatment liquor and for clean xylose oligomers. The comparison of the results indicated that a significant fraction of oligomers may be bound to lignin fragments, and some of it may also exist in branched form. Formation of LCC and Maillard reaction (reaction between reducing sugars and amino acid) was also proven to be factors significantly affecting the oligomer hydrolysis and xylose degradation.