Chapter 11 - Production of cellulosic butanol by clostridial fermentation: A superior alternative renewable liquid fuel

Abstract

Butanol is a superior alternative fuel than ethanol since its energy content (30 MJ/L) is higher than ethanol (22.2 MJ/L) and close to gasoline (34.2 MJ/L). Besides, it can be used as a sole fuel in gasoline engine. It is nonhygroscopic, noncorrosive, never causes cold start problem, and compatible with existing infrastructures. The conventional biochemical production of butanol involves anaerobic clostridial fermentation. Solventogenic Clostridia such as Clostridium acetobutylicum and Clostridium beijerincki produce high yield of butanol from simple sugars such as glucose and sucrose. The conversion of lignocellulosic biomass (LCB) to butanol attracted great interest due to economic and environmental benefits. But most of the solventogenic Clostridia are unable to directly utilize LCB due to their inefficient expression of saccharolytic enzymes. Hence, thermochemical pretreatment of LCB is required for subsequent enzymatic saccharification to fermentable sugars. However, a few native Clostridial strains (13-18A, 125C, and BOH3) produce saccharolytic enzymes. The clostridial acetone–butanol–ethanol (ABE) fermentation is performed after the removal of lignin/major inhibitors generated in pretreatment to improve the butanol yield. The pH, temperature, substrate concentration, carbon to nitrogen ratio, inoculum age and level, redox potential, presence of certain compounds often influence the yield and productivity of butanol. Further, sporulation during solventogenesis, acid crash of active cells, low butanol titer, butanol/solvent inhibition, and progressive degeneration were the major challenges in clostridial ABE fermentation. Approaches such as clostridial coculture, immobilization of active cells, new process/bioreactor design, genetic manipulation, metabolic engineering, and developing novel mutant strains enhance the butanol production. Further improvements in the clostridial fermentation are necessary for economic industrial production of butanol from the LCB like agricultural residues.