Glycerol was produced on an industrial scale by fermentation in the first quarter of the twentieth century (especially during World War I) but then declined, unable to compete with chemical synthesis from petrochemical feedstocks.106 A similar historical fate occurred with the ABE fermentation-producing “solvents,” that is, acetone, butanol, and ethanol in various proportions. Beginning (as with fuel etha­nol) with the oil crises of the 1970s, renewed interest was evinced in the technology, aided greatly by the accelerating advance of microbial physiology and genetics at that time.115116 The microbial species capable of this multiproduct biosynthesis are clostridia, which also have remarkable appetites for cellulosic and hemicellulosic polymers, able to metabolize hexose sugars and pentoses (usually, both xylose and arabinose).117,118 This again parallels the drive to produce ethanol from lignocellu — losic biomass substrates (chapter 3, section 3.3.2.5). It came as no surprise, there­fore, when the neologism “biobutanol” (for и-butanol, C4H9OH) appeared. DuPont, Wilmington, Delaware, and British Petroleum are the companies most associated with the development of butanol as an advanced biofuel and which aim to market biobutanol by the end of 2007; according to the DuPont publicity material (www2. dupont. com), biobutanol’s advantages are persuasive:

• Butanol has a higher energy content than ethanol and can be blended with gasoline at higher concentrations for use in standard vehicle engines (11.5% in the United States, with the potential to increase to 16%).

• Suitable for transport in pipelines, butanol has the potential to be intro­duced into gasoline easily and without additional supply infrastructure.

• Butanol/gasoline mixtures are less susceptible to separate in the presence of water than ethanol/gasoline blends, demanding no essential modifications to blending facilities, storage tanks, or retail station pumps.

• Butanol’s low vapor pressure (lower than gasoline) means that vapor pres­sure specifications do not need to be compromised.

• Production routes from conventional agricultural feedstocks (corn, wheat, sugarcane, beet sugar, cassava, and sorghum) are all possible, supporting global implementation.

• Lignocellulosics from fast-growing energy crops (e. g., grasses) or agricul­tural “wastes” (e. g., corn stover) are also feasible feedstocks.

The principal hurdles to process optimization were in manipulating cultures and strains to improve product specificity (figure 6.11) and yield and in reduc­ing the toxicity of butanol and O2 (the fermentation must be strictly anaerobic)

Carbon Source

FIGURE 6.11 Variation in butanol production with two strains of Clostridium acetobutylicum grown on six different carbon sources. (Data from Singh and Mishra.118)

to producing cells.118119 Notable among advances made in the last decade are the following:

• Isolation of hyperproducing strains — Clostridium beijerinckii BA101 expresses high activities of amylase when grown in starch-containing media, accumulating solvents up to 29 g/l and as high as 165 g/l when adapted to a fed-batch fermentation with product recovery by pervaporation using a silicone membrane.120122

• Gas stripping has also been developed as a cost-effective means to remove butanol and reduce any product inhibition.123

• At the molecular level, the high product yields with hyperproducing strains can be ascribed to a defective glucose transport system exhibiting poor regu­lation and a more efficient use of glucose during the solventogenic stage.124

• The demonstration that the ABE fermentation can utilize corn fiber sugars (glucose, xylose, arabinose, and galactose) and is not inhibited by major sugar degradation products of pretreated lignocellulosic substrates.125126

• Overexpression of a single clostridial gene to increase both solvent produc­tion and producer cell tolerance of product accumulation.127

• Improved understanding of the molecular events causing loss of productiv­ity in solventogenic strains spontaneously or during repeated subculturing or continuous fermentation.128129

A technoeconomic evaluation of a production facility with an annual capacity of 153,000 tonnes published in 2001 estimated production costs for butanol of $0.29/kg ($0.24/l, assuming a density of 0.8098 kg/l, or $0.89/gallon), assuming a conversion
efficiency of 0.50 g products per gram of glucose and corn as the feedstock.130 The calculations were noted to be very sensitive to the price paid for the corn, the worst — case scenario costs reaching $1.07/kg; with the best-case scenario, the production costs were probably competitive with conventional gasoline (at that time showing a high degree of price instability), allowing for a lower energy content (figure 5.1).

The downstream processing operations for the ABE fermentation are necessar­ily more complex than for fermentations with single product, for example, ethanol. Not only can the insoluble materials from the harvested fermentation be used as a source for animal feed production, but the fermentation broth must be efficiently fractionated to maximize the economic returns possible from three saleable solvent products. Detailed analysis of a conventional downstream process modeled solvent extraction (by 2-ethyl-1-hexanol), solvent stripping, and two distillation steps to recover 96% of the butanol from a butanol-dominated mix of products.131 An optimal arrangement of these downstream steps could reduce the operating costs by 22%.

Advanced bioprocess options have included the following:

• A continuous two-stage fermentation design to maintain the producing cells in the solventogenic stage132

• Packed bed biofilm reactors with C. acetobutylicum and C. beijerinckii133

• A continuous production system with a high cell density obtained by cell recycling and capable of operation for more than 200 hours without strain degeneration or loss of productivity134

• Simultaneous saccharification and fermentation processes have been inves­tigated by adding exogenous cellulase to poorly cellulolytic strains135

A novel feedstock for biobutanol production is sludge, that is, the waste product in activated sludge processes for wastewater treatments; this material is generated at 4 x 107 m3/year in Japan and most is discharged by dumping.136 Adding glucose to the sludge supported growth and butanol production and a marked reduction in the content of suspended solids within 24 hours. In the Netherlands, domestic organic waste, that is, food residues, have been tested as substrates for the clostridial ABE fermentation, using chemical and enzymic pretreatments; growth and ABE forma­tion were supported mainly by soluble sugars, and steam pretreatment produced inhibitors of either growth or solvent formation.137138

Echoing the theme of recycling is the MixAlco process, developed at Texas A&M University, College Station, Texas; this can accept sewage and industrial sludges, manure, agricultural residues, or sorted municipal waste) as a feedstock, treated with lime and mixed with acid-forming organism from a saline environment to produce a mixture of alcohols that are subsequently thermally converted to ketones and hydro­genated to alcohols, predominately propanol but including higher alcohols.139 This is another fermentation technology awaiting testing at a practical commercial scale.