Butanol fuel

Butanol may be used as a fuel in an internal combustion engine. Because its long hydrocarbon chains cause it to be fairly nonpolar, it is more similar to gasoline than ethanol. Butanol has been demonstrated to work in some vehicles designed for use with gasoline without any modification. It can be produced from biomass (biobutanol) as well as fossil fuels. Some call this biofuel biobutanol to reflect its origin, although it has the same chemical properties as butanol produced from petroleum.

Production of butanol from biomass
Butanol can be produced by fermentation of biomass by the A.B.E. process. The process uses the bacterium Clostridium acetobutylicum, also known as the Weizmann organism. It was Chaim Weizmann who first used this bacteria for the production of acetone from starch (with the main use of acetone being the making of Cordite) in 1916. The butanol was a by-product of this fermentation (twice as much butanol was produced). The process also creates a recoverable amount of H2 and a number of other by-products: acetic, lactic and propionic acids, acetone, isopropanol and ethanol.

The difference from ethanol production is primarily in the fermentation of the feedstock — producing butanol rather than ethanol like primary fermentation product and minor changes in distillation. The feedstocks are the same as for ethanol — energy crops such as sugar beets, sugar cane, corn grain, wheat and cassava as well as agricultural byproducts such as straw and corn stalks. According to DuPont, existing bioethanol plants can cost-effectively be retrofitted to biobutanol production.

Distribution
Butanol better tolerates water contamination and is less corrosive than ethanol and more suitable for distribution through existing pipelines for gasoline. In blends with diesel or gasoline, butanol is less likely to separate from this fuel than ethanol if the fuel is contaminated with water. There is also a vapor pressure co-blend synergy with butanol and gasoline containing ethanol, which facilitates ethanol blending. This facilitates storage and distribution of blended fuels.

Energy content and effects on fuel economy
Switching a gasoline engine over to butanol would in theory result in a fuel consumption penalty of about 10% (32/29.2 = 1.095..., -1 = 0.095... = about 9.5%) but butanol's effect on mileage is yet to be determined by a scientific study. While the energy density for any mixture of gasoline and butanol can be calculated, tests with other alcohol fuels have demonstrated that the effect on fuel economy is not proportional to the change in energy density.

Octane rating
The octane rating of n-butanol is similar to that of gasoline but lower than that of ethanol and methanol. n-Butanol has a RON (Research Octane number) of 96 and a MON (Motor octane number) of 78 while t-butanol has octane ratings of 105 RON and 89 MON. t-Butanol is used as an additive in gasoline but can't be used as a fuel in its pure form since the melting point is 25.5 °C - in other words, it gels when cool.

A fuel with a higher octane rating is less prone to knocking (extremely rapid and spontaneous combustion by compression) and the control system of any modern car engine can take advantage of this by adjusting the ignition timing. This will improve energy efficiency, leading to a better fuel economy than the comparisons of energy content different fuels indicate. By increasing the compression ratio, further gains in fuel economy, power and torque can be achieved. Conversely, a fuel with lower octane rating is more prone to knocking and will lower efficiency. Knocking can also cause engine damage.

Air-fuel ratio
Alcohol fuels, including butanol and ethanol, are partially oxidized and therefore need to run at richer mixtures than gasoline. Standard gasoline engines in cars can adjust the air-fuel ratio to accommodate variations in the fuel, but only within certain limits depending on model. If the limit is exceeded by running the engine on pure butanol or a gasoline blend with a high percentage of butanol, the engine will run lean, something which can damage it. Compared to ethanol, butanol can be mixed in higher ratios with gasoline for use in existing cars without the need for retrofit as the air-fuel ratio and energy content is closer to that of gasoline.

Specific energy
Alcohol fuels have less energy per unit weight and unit volume than gasoline. To make it possible to compare the net energy released per cycle a measure called the fuels specific energy is sometimes used. It is defined as the energy released per air fuel ratio. The net energy released per cycle is higher for butanol than ethanol or methanol and about 10% higher than for gasoline.

Viscosity
The viscosity of alcohols increase with longer carbon chains. For this reason, butanol is used as an alternative to shorter alcohols when a more viscous solvent is desired. The kinematic viscosity of butanol is several times higher than that of gasoline and about as viscous as high quality diesel fuel.

Heat of vaporization
The fuel in an engine has to be vaporized before it will burn. Insufficient vaporization is a known problem with alcohol fuels during cold starts in cold weather. As the latent heat of vaporization of butanol is less than half of that of ethanol, an engine running on butanol should be easier to start in cold weather than one running on ethanol or methanol.

Potential problems with the use of butanol fuel
The potential problems with the use of butanol are similar to those of ethanol:
 * To match the combustion characteristics of gasoline, the utilization of butanol fuel as a substitute for gasoline requires fuel-flow increases (though butanol has only slightly less energy than gasoline, so the fuel-flow increase required is only minimal, maybe 10%, compared to 40% for ethanol.)
 * Alcohol-based fuels are not compatible with some fuel system components.
 * Alcohol fuels may cause erroneous gas gauge readings in vehicles with capacitance fuel level gauging.
 * While ethanol and methanol have lower energy densities than butanol, their higher octane number allows for greater compression ratio and efficiency. Higher combustion engine efficiency allows for lesser greenhouse gas emissions per unit motive energy extracted.

As an advantage, butanol production from biomass could be more efficient (i.e. unit engine motive power delivered per unit solar energy consumed) than ethanol or methanol routes. Also, some bacteria that produce butanol are able to digest cellulose, not just starch and sugars.

Possible butanol fuel mixtures
Standards for the blending of ethanol and methanol in gasoline exist in many countries, including the EU, the US and Brazil. Approximate equivalent butanol blends can be calculated from the relations between the stochiometric fuel-air ratio of butanol, ethanol and gasoline. Common ethanol fuel mixtures for fuel sold as gasoline currently range from 5% to 10%. The share of butanol can be 60% greater than the equivalent ethanol share, which gives a range from 8% to 32%. "Equivalent" in this case refers only to the vehicle's ability to adjust to the fuel. Other properties such as energy density, viscosity and heat of vaporisation will vary and may further limit the percentage of butanol that can be blended with gasoline.

Current use of butanol in vehicles
Currently no production vehicle is known to be approved by the manufacturer for use with 100% butanol, though any model that is able to run 10% ethanol blends should be able to use butanol without any problems.

David Ramey drove from Blacklick, Ohio to San Diego, California using butanol in an unmodified 1992 Buick Park Avenue. Although further long term testing must be done, it is highly likely that most late model cars can run on 100% butanol safely with no modifications.

Research Challenges
The key research challenge that must be resolved is that butanol production inhibits microbial growth even at low concentrations. The result is that the product of the fermentation is less than 2% butanol. The overwhelming majority of the fermentation broth is water, so an energy-intensive distillation step is required for purification. This may be acceptable if the goal is to produce butanol for use as a solvent, but if butanol is to gain traction as a motor fuel, energy inputs into the process need to be minimized.