Glycerol represents 10% by weight of typical triglycerides, and biodiesel production generates large amounts of this coproduct (figure 6.3). This obviously marginalizes any chemical routes to glycerol synthesis that are either cost-inefficient or otherwise suboptimal in a competitive manufacturing environment.82 By any manufacturing route, glycerol production provides a good input to FT conversion to liquid alkanes because glycerol can be converted over platinum-based catalysts into syngas at rela­tively low temperatures, 225-350°C.104 The gas mixture from glycerol conversion at 300-450°C has a molar excess of H2 over CO (up to 1.83:1), a high ratio (up to 90:1) between CO and CO2, and only traces of methane. With a subsequent FT step, the overall conversion of glycerol to hydrocarbons can be written as

25C3O3H8 ^ 7C8H8 + 19CO2 + 37H2O

and is a mildly exothermic process (enthalpy change -63 kJ/mol glycerol). High rates of conversion of glycerol into syngas were observed using glycerol concentra­tions of 20-30% (w/w).

Unlike ethanol, a major fermentation product of only a selected few microbial species, glycerol is ubiquitous because of its incorporation into the triglycerides that are essential components of cellular membranes, as well as being accumulated in vegetable oils (figure 6.1). Unlike methanol, which is toxic to microbial species as well as higher animals and has no biosynthetic pathway, glycerol is benign and has a well-characterized route from glucose and other sugars (figure 2.3). In fermentations for potable ethanol, the priority is to regulate glycerol accumulation as its formation is a waste of metabolic potential in fuel alcohol production — and much research effort has, therefore, been devoted to minimizing glycerol formation by yeasts by, for example, regulating the glucose feeding rate to maintain an optimal balance of CO2 production and O2 consumption.105

Conversely, maximizing glycerol production by yeasts is also straightforward, successful strategies including:106

• Adding bisulfite to trap acetaldehyde (an intermediate in the formation of ethanol), thus inhibiting ethanol production and forcing glycerol accumula­tion to restore the balance of intracellular redox cofactors

• Growing yeast cultures at much higher pH values (7 or above) than tradi­tionally used for ethanol fermentations

• Using osmotolerant yeasts — glycerol is often accumulated inside yeast cells to counteract the adverse effects of high osmotic pressures

Suitable osmotolerant strains can accumulate 13% (w/v) glycerol within four to five days. Even more productive is the osmophilic yeast Pichia farinosa that was reported to produce glycerol at up to 30% (w/v) within 192 hours in a fed-batch fermentation with glucose as the carbon source as a molar yield of glycerol from glucose of 0.90.107

Fixing a maximum theoretical yield of glycerol from glucose is difficult because it is highly dependent on the totality of biochemical routes available to the producer cells. With Saccharomyces cerevisiae genetically manipulated to overproduce glycerol, the glycerol yield was 0.50 g/g of glucose, that is, a molar yield of 0.98.108 With this genetic background, a yield of 1 mol of glycerol/mol of glucose consumed may be the maximum obtainable (figure 6.10). Although an osmotolerant Saccharomyces strain isolated from sugarcane molasses could accumulate higher levels of glycerol (up to 260 g/l), the molar yield was still 0.92.109 An osmotolerant Candida glycerinogenes was, however, reported to produce glycerol with a molar yield of 1.25.110 Breaking through the limitation imposed by redox cofactors can, therefore, be accomplished by natural biochemistry. Some yeasts harbor a well-characterized “short circuit” for NADH oxidation, bypassing an energy conservation mechanism via an alternative oxidase, a mitochondrial enzyme constitutively expressed in industrial strains of Aspergillus niger used in citric acid prorduction.111113 Similar enzymes are known in plants, fungi, and many types of yeast; overexpressing the alternative oxidase gene in another yeast of industrial relevance (Pichia pastoris) resulted in a small increase in growth rate.114 Because the functioning of the alternative oxidase system is thought to circumvent any restrictions imposed on productivity by redox cofac­tor regeneration in citric acid producers, a bioconversion of each mole of glucose to produce nearly 2 mol of glycerol might be achievable.

Even with presently known levels of glycerol production, it is technically feasible to process a harvested filtered culture broth directly for syngas formation from glyc­erol.104 Because 50% of the annual costs of FT fuels from biomass were considered to be capital-related, combined syngas/FT conversion of glycerol would be expected to entail markedly lower costs, eliminating the need for a biomass gasifier and gas­cleaning steps.89,104