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Biobutanol

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Biobutanol is a fuel from natural sources (no petroleum). Butanol may be used as a fuel in an internal combustion engine. 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.[1] It can be produced from biomass 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.

Contents

Production of butanol from biomass

Butanol can be produced by fermentation of biomass. 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.[2]

There are a dozen companies pursuing production of biobutanol via fermentation and pyrolysis. Pyrolysis is a process where biomass and organic waste can be heated to decompose matter into basic building blocks and then reformulated into fuel. The process is analogous to hydrocracking and reformulation of hydrocarbon chains. [3]

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.[4] [5][6]

Properties of common fuels

Fuel Energy
density
Air-fuel
ratio
Specific
energy
Heat of
vaporization
RON MON
Gasoline 32 MJ/L 14.6 2.9 MJ/kg air 0.36 MJ/kg   91–99   81–89
Butanol 29.2 MJ/L 11.2 3.2 MJ/kg air 0.43 MJ/kg   96   78
Ethanol 19.6 MJ/L   9.0 3.0 MJ/kg air 0.92 MJ/kg 130   96
Methanol 16 MJ/L   6.5 3.1 MJ/kg air 1.2 MJ/kg 136 104

Energy content and effects on fuel economy

Butanol is reported to yield 36 MJ/kg (15,500 BTU/lb) when burned. This can be expressed volumetrically as 29.3 MJ/l (104,800 BTU/US gal).

Switching a gasoline engine over to butanol would in theory result in a fuel consumption penalty of about 10% but butanol's effect on mileage is yet to be determined by a scientific study. Potential of higher gas mileage seems reasonable as biobutanol is a mono component fuel--only one type of molecule. In theory such fuel can be compressed to higher ratio and then achieve higher degree of burn or higher efficiency.[7] 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.[8]

Octane rating

The octane rating of n-butanol is very 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.[9] 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. [10]

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. [11][12]

Specific energy

Alcohol fuels have less energy per unit weight and unit volume than gasoline but at the same time require richer mixtures. 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

Substance Kinematic
viscosity
at 20°C
Butanol 3.64 cSt
Ethanol 1.52 cSt
Methanol 0.64 cSt
Gasoline 0.4–0.8 cSt
Diesel >3 cSt
Water 1.0 cSt

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.[13]

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.[14]

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.
  • 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.
  • The viscosity of butanol is much higher than for gasoline or ethanol, which could have negative effects on the fuel system.
  • 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, the bacterium producing butanol is 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 vehicles 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 butanol vehicles

Currently no production vehicle is known to be approved by the manufacturer for use with 100% butanol. The use of butanol in a vehicle which is not approved for this is not recommended as it may cause damage to the vehicle.

See also

External links

References

  1. butanol.com
  2. Dupont Fact Sheet on Biobutanol
  3. Biobutanol Uses, Production and News
  4. Dupont Fact Sheet Biobutanol
  5. ext.colostate.edu
  6. USAtoday
  7. Biobutanol.com
  8. ethanol.org
  9. UNEP.org-Properties of oxygenates
  10. [1]
  11. ext.colostate.edu
  12. USA today
  13. Engineering Toolbox
  14. ext.colostate.edu


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