The oldest method of butanol production is the acetone-butanol-ethanol (ABE) bacterial fermentation by Clostridium acetobutylicum, which dates back to Louis Pasteur in 1861 [13]. The bacterial microorganism, C. acetobutylicum, was first isolated by Weizmann [13]. In the ABE fermentation process, C. acetobutylicum produces acetic, butyric, and propionic acids from glucose that can be generated from various biomass sources. Potential feedstocks include corn, molasses, whey permeates, or glucose. An enzyme catalyzed reaction of acetoacetyl-CoA transfers

CoA to acetate forming acetyl-CoA. Through a series of metabolic reactions, butyryl-CoA is produced from acetyl-CoA, which is then converted to butanol in the solventogenic pathway [33]. Acetyl-CoA can also produce ethanol and acetone from acetoacetyl-CoA. A typical process produces acetone, butanol and ethanol in the ratio 3: 6:1.

The butanol yield from the ABE fermentation of glucose is relatively low, about 15-25 wt% typically [33]. This is due to the buildup of acetic, butyric, and propionic acids along with the products acetone, butanol, and ethanol, during the fermenta­tion process. The solvents are toxic to C. acetobutylicum. The butanol destabilizes the cell membrane of the microorganisms ultimately resulting in cell death. Higher yields can be achieved by continuously removing the harmful solvents, mainly butanol, and/or by genetically modifying strains of microorganisms that can tolerate higher concentrations of butanol [33].

A butanol-tolerant mutant strain of C. acetobutylicum has been developed and designated as SA-1 [34]. This strain shows a 121% improvement in butanol tol­erance over the typical strain used in ABE fermentation. The enhancement of the strain results in an overall increase in butanol production of 13.2%. Additional advantages of the mutated strain are an increase in growth rate, more pH resistance, more effective utilization of carbohydrates, and reduction in acetone concentration by 12.5-40% [34]. Other studies using genetic and metabolic engineering have modified strains, which have resulted in an increase of about 320% in the final butanol concentration [35]. The antisense RNA process helps down-regulate genes for butyrate formation by acidogenesis and increases the butanol yield through solventogenesis. The process has resulted in strains with butanol yields of 35% [36].

Tetravitae Bioscience has combined a patented mutant strain of C. beijerinckii and a continuous, integrated fermentation process that utilizes gas stripping. C. bei­jerinckii is a species of rod-shaped anaerobic bacteria that is known for the synthesis of organic solvents, and uses a broader substrate range and better pH range than C. acetobutylicum. The solvent genes of C. beijerinckii are located on the chromo­some, which is more genetically stable than on the plasmid for C. acetobutylicum. The gas stripping process prevents the butanol concentrations from reaching toxic levels by sparging oxygen-free nitrogen or fermentation gases through the fermen­tation solution and the ABE captured in the gas are condensed [13]. The exhaust gas is then recycled back to the reactor to collect more ABE for removal. Advantages of this method are the low energy requirements, the fact that it does not remove important acid intermediates, and that it allows for efficient recovery of butanol [37].

Environmental Energy Inc. (EEI) and Ohio State University (OSU) have devel­oped a two-step anaerobic fermentation process in a joint project to produce butanol from biomass. The first process converts the feedstock carbohydrates into butyric acid through acidogenesis using C. tyrobutyricum. The second step converts the butyric acid, using C. acetobutylicum, into butanol, which results in a significant improvement from conventional processes. The butanol solution requires purifi­cation from a recovery unit after the second step reactor. EEI’s process uses a purification process that takes advantage of the azeotrope formed by butanol (55%) and water (45%), which is used to minimize the energy required for dis­tillation. These processes utilize OSU’s proprietary fibrous-bed bioreactor (FBB) that has demonstrated improvements in long-term production with a scalable pack­ing design. The packing consisted of a spiral-wound, fibrous matrix that allows for a high surface area with large enough voids to allow for a high cell density. Immobilizing the cells in the FBB minimizes the energy consumption required by the cells [33].

British Petroleum (BP) has partnered with DuPont to commercialize biobu­tanol using advanced metabolic pathways for 1-butanol. They have announced plans to produce 30,000 tons per year of biobutanol at the British Sugar facility in Wissington, UK. This will help meet the United Kingdom’s Renewable Fuels Obligation set for 2010. Along with 1-butanol, they plan on developing biocatalysts to produce higher octane isomers such as 2-butanol and iso-butanol, and to increase the interest and utility as a fuels additive or substitute [38]. BP and Dupont plan on initially marketing biobutanol to the current market as an industrial solvent and then implement a larger commercialization into fuel blending by 2010 [38].

A different approach to producing butanol utilizes a thermochemical route for the gasification of biomass by a syngas catalyst. W2 Energy Inc. is working to produce biobutanol from a Gliding Arc Tornado plasma reactor (GAT) for biomass gasifica­tion. The GAT is a non-thermal plasma system, which utilizes reverse vortex flow that allows for a larger gas residence time and ensures a more uniform gas treat­ment. An advantage to the GAT system is that because of the thermal insulation, it does not require high-temperature material, thus reducing costs [39]. The gasifica­tion of biomass is accomplished by the solid biomass undergoing a thermochemical reaction under sub-stoichiometric conditions with an oxidizing fuel. The biomass’s energy is released in the form of CO, CH4, H2, and other combustible gases (syn­gas) [40]. The syngas consists of basic elementary components, which can be made into butanol using various petrochemical techniques. Other advances in gasifica­tion technology have been made by the National Renewable Energy Laboratory’s (NREL) Battelle Labs.

3.2 Summary

Biobutanol is a renewable, biodegradable, alternative fuel, which can be used neat or blended with gasoline. Properties such as energy density, octane value, and Reid vapor pressure (RVP) are similar to gasoline; hence current vehicles can use biobu­tanol without any engine modifications. Biobutanol can be produced from biomass by the fermentation of sugars and starches or by thermochemical routes using gasi­fication. The emergence of butanol as a fuel is growing with companies such as BP, DuPont, EEI, Tetravitae Bioscience, and W2 Energy Inc. investing in new technol­ogy as well as in manufacturing. Worldwide commercialization of biobutanol can replace or enhance blends of gasoline to reduce the dependence on petroleum as well as reduce greenhouse gas emissions.