Sucrose — or Starch-Based Biofuels

Sucrose (from sugar cane, sugar beet and sweet sorghum) and starch (from starch crops such as corn, grain sorghum, potato, Jerusalem artichoke, cassava, rye, barley, sago palm and wheat) can be converted into ethanol by hydrolysis and fermentation. There is also the possibility of converting sugar in whey and starch and sugar in wastes (potato peel, spoiled fruit) into ethanol by fermentation (Acharya and Young 2008). The fermentation used to produce ethanol is usually yeast based. The reaction starts with a C6 sugar and is:

C6H12O6 ^ 2CO2 + 2C2H5OH (ethanol) .

Ethanol has its disadvantages vis-a-vis fossil-based gasoline. Its lower heating value is considerably lower (see Table 1.2), and it is hygroscopic and more corrosive (cf. Table 1.3). Against this background there have been proposals to convert ethanol to hydrocarbons (‘biogasoline’) (Tsuchida et al. 2008) or H2 (Ni et al. 2007; Kon — darides et al. 2008). However, in practice, ethanol may do well as transport fuel in Otto motors, both as such and as a mixture with fossil hydrocarbons (Szklo et al. 2007). Ethanol may also be applied in a mixture with fossil diesel fuel in diesel motors — with an additive to prevent phase separation (Fernando and Hanna 2004; Antoni et al. 2007; Wang et al. 2007; Song et al. 2007). In Otto motors, ethanol is used as a gasoline extender, octane booster and oxygenate suitable for driving during winter in temperate climates (MacLean and Lave 2003). Claims have been made that admixture of ethanol improves the fuel efficiency of Otto motors, but available evidence (Roberts 2008; Kamimura and Sauer 2008) suggests that differ­ences in average fuel efficiency are not statistically different from the differences in heating value. Ethanol can also be used to produce ETBE (ethylester of t-butanol) or ethylesters of fatty acids, which can be applied in Otto and diesel motors respec­tively. Currently the production thereof is in chemical reactors. For the combined production of ethanol from sugars and ethylesters of fatty acids, a synthesis em­ploying genetically modified Escherichia coli has been demonstrated (Kalscheuer et al. 2006).

Starch and sucrose may also serve as a basis for fermentation into butanol, or to be more precise, a mixture of acetone, butanol and ethanol (ABE). After World War II, bacterial fermentation generating ABE from starch and sucrose was applied on an industrial scale in a wide variety of countries. This production process ul­timately succumbed to the price competition of petrochemical butanol (Ng et al. 1983; Reinharz 1985; Jones and Woods 1986; Gutierrez et al. 1998; Zverlov et al. 2006; Chiao and Sun 2007; Ezeji et al. 2007; Qureshi et al. 2008a). There currently is pilot-scale industrial production of butanol by bacterial fermentation processes starting with starch or sucrose and a substantial amount of research and development aimed at ‘engineering out’ the production of acetone and ethanol (Wackett 2008). Up to about 18% ABE may be mixed with fossil diesel fuel, which is then suitable for powering diesel motors (Willke and Vorlop 2004). Butanol can be mixed into gasoline for use in Otto motors, as such or after esterification with t-butanol (Scott and Bryner 2006; Antoni et al. 2007; Ezeji et al. 2007). Butanol is a biofuel that can also be used in high thrust-to-weight applications such as aircraft engines. Butanol has the added advantages that, unlike ethanol, it will not solidify at the low tem­peratures of high altitudes at which airplanes operate and that it is not hygroscopic. Disadvantages are that the concentration of butanol achievable by fermentation is currently low and that the boiling point is high, which necessitates relatively high energy inputs for butanol distillation (Fortmanet al. 2008; Hayes 2008).

It has also been shown that the production of branched chain butanols (isobu­tanol, 2-methyl-1-butanol, 3 — methyl-1-butanol) from glucose is possible using meta­bolic engineering of micro-organisms (Atsumi et al. 2008). Such branched chain butanols can also be mixed into gasoline.