Butanol is traditionally produced from fossil propene (Shapovalov and Ashkinazi 2008). Butanol isomers can be processed also from biomass. Currently, most biobutanol technologies are based on fermentation, for example, Butamax Advanced Biofuels LLC, Swiss Butalco GmbH, American Gevo Inc. and ButylFuel LLC (2010). Biobutanol can be produced also by catalytic processes (Exelus 2010). Gasification and alcohol synthesis can produce alcohol mixtures, for example Ecalene (Wikipedia 2011).
Gasoline consists of hundreds of different hydrocarbon molecules, whereas butanol isomers are monomolecular compounds with narrow boiling point. Butanols are aromatic-, olefin-, and sulfur-free compounds. Thus, they may improve gasoline composition by dilution effect. Oxygen contents of butanols are 21.6 wt-%.
There are four isomers of butanol with the same chemical formula, but different structural arrangement. Isomers of butanol are n-butanol, isobutanol, tert-butanol, and sec-butanol. In past, “gasoline-grade” tert-butanol (GTBA) was commonly present in gasoline as a co-solvent for methanol, denaturant for ethanol and as impurity in MTBE, if produced from TBA. TBA solidifies at about 26°C, and thus co-solvents are needed for handling and usage. (API 2001). Today, bio-based isobutanol and n-butanol are considered as blending components for gasoline.
Figure 1. Chemical structure of butanol
In the US, gasoline-oxygenate blends are considered “substantially similar” if they contain hydrocarbons, aliphatic ethers, aliphatic alcohols other than methanol (up to 0.3 vol-% methanol or up to 2.75 vol-% with an equal volume of butanol or higher molecular weight alcohol). The fuel containing aliphatic ethers and/or alcohols (excluding methanol) must contain no more than 2.7 mass-% oxygen. This oxygen limit would represent about 11.7 vol-% (12.5 wt-%) isobutanol content in gasoline.
In Europe, Fuel Quality Directive 2009/30/EC allows maximum 15 vol-% butanol in gasoline. Isobutanol and tert-butanol are mentioned separately, other butanol isomers are covered by the group of “other oxygenates”, which means “other monoalcohols and ethers with a final boiling point no higher than stated in EN 228”. About 16 vol-% (17 wt-%) of isobutanol would represent gasoline oxygen content of 3.7 wt-%.
Automanufacturer’s recommendations for fuel gasoline qualities in the WWFC 2006 edition state that “Higher (C > 2) alcohols are limited to 0.1% maximum by volume.” Based on this statement butanol isomers are excluded as gasoline components.
There are no fuel requirements or standards for fuel butanol at the moment. Typical properties of butanol isomers are shown in Table 1.
Table 1. Selected properties of butanol isomers.
Melting point of tert-butanol (TBA) is about 26°C (Table 1). However, in past TBA was commonly present in gasoline as a co-solvent for methanol, denaturant for ethanol and as impurity in MTBE. Melting points of other butanol isomers are below -90°C.
Sufficient knocking resistance, octane rating, is essential for proper operation of spark-ignition engine. Octane numbers of methanol and ethanol are high. Blending octane numbers of butanol isomers are somewhat lower than those of ethanol. Of butanol isomers, the highest blending octane numbers are achieved with isobutanol: blending RON 114 and MON 94. For n-butanol, blending MON is as low as 78-81 (Table 1). Figure 2 shows octane numbers of various alcohols. Read more of octane numbers of ethanol.
Alcohols tend to increase research octane number (RON) more than motor octane number (MON). The sensitivity (difference between RON and MON) is typically 8-10 units for gasoline, while over 10 for butanol isomers. Blending MON is particularly low for n-butanol.
Figure 2. RON and MON octane numbers of various alcohols (Wallner 2012).
Butanol has a lower blending vapor pressure than ethanol. Basically, butanol does not significantly increase vapor pressure of gasoline at any blending ratio (Figure 3).
Figure 3. Vapor pressure of gasoline/alcohol mixtures (Andersen et al. 2010 API 2001, Furey 1985).
Boiling point of isobutanol is 108 °C and that of n-butanol is 117 °C. These butanol isomers bring higher boiling components into gasoline than ethanol (Figure 4). Mid-range distillation components increase substantially when butanol is blended with gasoline. E100 limit (volume evaporated at 100 °C) of the European gasoline standard (EN228) may need consideration, if butanol is used as gasoline component. Bruno et al. (2009) have reported the effect of all butanol isomers on distillation curves.
Figure 4. Examples of distillation curves for gasoline and tailored gasoline/butanol blends. EN228:2008 limits for E70, E100, and E150 limits are marked as red lines (Aakko-Saksa et al. 2011).
Generally, boiling points of alcohols increase with increasing carbon chain length (Figure 5, Wallner et al. 2012).
Figure 5. Boiling points of alcohols and distillation curve of gasoline (Wallner et al. 2012).
Heat of vaporization is substantially higher for ethanol than for gasoline. Higher heat of vaporization improves knock resistance and enables achieving higher engine efficiency, but also leads to problems to start up and to run the cold engine. (Larssen 2009). Heat of vaporization of longer chain alcohols, such as butanol isomers, is closer to that of gasoline compared to ethanol (Figures 6 and 7). Driveability with butanols is discussed in the later Chapter.
Figure 6. Heat of vaporization for alcohols, aromatics, olefins (O), paraffins (P), and ethers (E) (Piel 1990).
Figure 7. Latent heat of vaporization of various alcohols (Wallner et al. 2012).
Flame temperatures of some alcohols are lower than those of e.g. aromatics (Figure 8). However, if alcohols lead to leaning of air to fuel ratio, combustion temperature rises (Piel 1990). Flame temperatures for butanol are closer to those of gasoline when compared to ethanol.
Figure 8. Theoretical flame temperatures for alcohols, aromatics, olefins, paraffins (P), and ethers (E) assuming adiapatic process and stoichiometric air to fuel ratio. (Piel 1990).
Energy content of isobutanol is 33 MJ/kg (26.5 MJ/l) representing around 82% of volumetric energy content of gasoline. Densities of butanol isomers are higher than those for gasoline, which improves volumetric fuel economy to some extent. Theoretically, volumetric fuel consumption increases some 3.5% when 16 vol-% butanol is blended with non-oxygenated gasoline.
Butanols are not as polar compounds as ethanol. tert-Butanol is miscible with water, but solubilities of other butanol isomers are limited. Solubility of sec-butanol in water is relatively high, whereas solubilities of isobutanol and n-butanol are only around 8 wt-% at 20 °C. (BASF Technical leaflet). Isobutanol or n-butanol are not prone to migrate into water phase from gasoline in normal conditions.
Viscosities of butanol isomers are high when compared to gasoline. Viscosity of n-butanol is 2.7 mm2/s at 40°C, and that of isobutanol 2.3 mm2/s. These are close to viscosities of diesel fuel. For example, viscosity limits in the European diesel fuel standard (EN 590) are 2.0-4.5 mm2/s.