Ethanol production from lignocellulosic biomass has gained much momentum due to its benefits to energy security, reduction of green house gas emission, as well as both environmental and social sustainability. The technology for lignocellulosic ethanol production, however, is not yet fully commercialized. The major issues impeding the cellulosic ethanol production in the biochemical route include the high enzyme loadings needed, long enzymatic hydrolysis time, slow xylose fermentation and low ethanol productivity, which result in a high production cost. Ammonia Fiber Expansion (AFEXTM) is a leading alkaline pretreatment. It provides a biomass substrate with high enzymatic digestibility and high fermentation potential. Previous studies on ethanol production from AFEXTM pretreated biomass focused on a separate enzymatic hydrolysis and fermentation process (SHF). However, integrated biological processes, such as simultaneous saccharification and co-fermentation (SSCF) and consolidated bioprocessing (CBP), are strongly believed to have lower cost compared to SHF. This dissertation work studies the integrated biological processes performed on AFEXTM pretreated biomass and resolves the aforementioned issues for biochemical production of cellulosic ethanol. The slow xylose fermentation issue in hydrolysate by Saccharomyces cerevisiae 424A (LNH-ST) was quantitatively studied. The xylose fermentation inhibition was not only from degradation products but also from ethanol and metabolites generated during glucose fermentation (Chapter II). Based on such understanding, a two-step SSCF process was developed, in which xylan was hydrolyzed and fermented ahead of glucan/glucose. As a result, xylose fermentation was greatly improved (Chapter III). Through the study of conventional SSCF on AFEXTM treated corn stover (CS), it was found that pre-hydrolysis to generate a certain amount of glucose was crucial for achieving a good result (Chapter IV). A high solids loading process can save much cost for ethanol production. However, ethanol yield decreased with increasing solids loading. From an economic point of view, 6% (w/w) glucan loading was the optimal solids loading during SSCF of AFEXTM CS (Chapter V). For improvement of productivity, a continuous SSCF process using multi-stage continuous stirred tank reactors (CSTRs) was developed based on the kinetic studies of the two reactions in SSCF (enzymatic hydrolysis and fermentation)(Chapter VI). Based on the fundamental understandings of the cellulosic ethanol production, a novel industrially-relevant integrated biological process was developed. This process shortened the biological processing time (including enzymatic hydrolysis and fermentation) from 11 days to around 2 days, reduced enzyme loading by more than 1/3 and enhanced ethanol productivity by 2-3 times (Chapter VII). Consolidated bioprocessing (CBP) eliminates the enzyme cost and is believed to be the ultimate low-cost industrial configuration for cellulosic ethanol production. CBP studies using Clostridium phytofermentans on AFEXTM CS at both low and high solids loadings showed promising results (Chapter VIII & IX).
Read
