Cellulose Bioconversion Technology, Chapter 12

Utilizing cellulose to synthesize alternative renewable transoirtation fuels such as ethanol to replace gasoline is a technology that can provide a permanent solution to our energy needs. Lignocellulosic biomass, the source of cellulose, is the most abundant renewable resource on earth. Cellulose is readily available from agricultural residues, herbaceous crops, forestry by-products, pulp andpaper industry wastes, and municipal solid waste. Extensive research during the past decade has shown that simultaneous saccharification and fermentation (DDS) is a promising way to biochemically convert cellulose into ethanol. That process combines the enzymatic hydrolysis of cellulose to glucose by cellulolytic enzymes with the catabolism of glucose to ethanol by fermentative microorganisms.From an economic standpoint, SSF is at the center of the bioconversion process, as it makes the largest contribution to the production cost of ethanol and the capital cost of biomass-to-ethanol plant. Alternative cellulose bioconversion schemes, such as separate hydrolysis and fermentation (SHF) and direct microbial conversion (DMC), are also under consideration. SSF optimization has beenpursued by modeling its performance, validating the model through batch and continuous experimentation and scale-up, and developing a process design that minimizes ethanol cost and maximizes SSF's productivity. A detailed methodology to accurately determine the model parameters has been developed for the fermentative organism and the cellulytic enzymes. Batch experimental studies have helpedidentify the enzymatic hydrolysis as the limiting step in the SSF process and the need to enhance substrate accessibility by improving the effectiveness of biomass pretreatment. Continuous SSF studies, on the other hand, have provided information on crucial operating parameters, performance variables, and scale-up considerations, such as dilution rate and ethanol yield and productivity, as theprocess moves toward commercialization. This way, cellulose conversion factors of major importance, such as the effect of substrate and enzyme loading on ethanol productivity, the most efficient mode of operation, the effect of feedstock composition, and the desired pretreatment effectiveness, can be systematically evaluated to improve the overall biomass-to-ethanol technology.