Butanol may be used as a fuel in an internal combustion engine. Because its longer hydrocarbon chain causes it to be fairly non-polar, it is more similar to gasoline than it is to ethanol. Butanol has been demonstrated to work in vehicles designed for use with gasoline without modification. It has a four link hydrocarbon chain. It can be produced from biomass (as "biobutanol") as well as fossil fuels (as "petrobutanol"); but biobutanol and petrobutanol have the same chemical properties.
Production of biobutanol 
Biobutanol 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 bacterium 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, isopropanol and ethanol.
The difference from ethanol production is primarily in the fermentation of the feedstock 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, prospective non-food energy crops such as switchgrass and even guayule in North America, 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.
Additionally, butanol production from biomass and agricultural byproducts could be more efficient (i.e. unit engine motive power delivered per unit solar energy consumed) than ethanol or methanol production.
Algae butanol 
Although biofuel demand has risen to over one billion liters (about 260 million US gallons) yearly, fermentation remains a largely inefficient method of butanol production. Under normal living conditions, Clostridium bacterial communities have a low yield of butanol per gram of glucose. Obtaining higher yields of butanol involves manipulation of the metabolic networks within bacteria to prioritize the synthesis of the biofuel. Metabolic engineering and genetic engineering tools allow scientists to alter the states of reactions occurring in the organism, utilizing advanced techniques to create a bacterial strain capable of high butanol yield. Optimization can also be accomplished by the transfer of specific genetic information to other uni-cellular species, capitalizing on the traits of multiple organisms to achieve the highest rate of alcohol production.
Using Alternate Carbon Sources 
One promising development in biobutanol production technology was discovered in the late summer of 2011 - Tulane University's alternative fuel research scientists discovered a strain of Clostridium, called "TU-103", that can convert nearly any form of cellulose into butanol, and is the only known strain of Clostridium-genus bacteria that can do so in the presence of oxygen. The university's researchers have stated that the source of the "TU-103" Clostridium bacteria strain was most likely from the solid waste from one of the plains zebra at New Orleans' Audubon Zoo.
Metabolic engineering can be used to allow an organism to use a cheaper substrate such as glycerol instead of glucose. Because fermentation processes require glucose derived from foods, butanol production can negatively impact food supply (see food vs fuel debate). Glycerol is a good alternative source for butanol production. While glucose sources are valuable and limited, glycerol is abundant and has a low market price because it is a waste product of biodiesel production. Butanol production from glycerol is economically viable using metabolic pathways that exist in Clostridium pasteurianum bacterium.
A combination of succinate and ethanol can be fermented to produce butyrate (a precursor to butanol fuel) by utilizing the metabolic pathways present in a gram-positive anaerobic bacterium Clostridium kluyveri. Succinate is an intermediate of the TCA cycle, which metabolizes glucose. Anaerobic bacteria such as Clostridium acetobutylicum and Clostridium saccharobutylicum also contain these pathways. Succinate is first activated and then reduced by a two-step reaction to give 4-hydroxybutyrate, which is then metabolized further to crotonyl-coenzyme A (CoA) . Crotonyl-CoA is then converted to butyrate. The genes corresponding to these butanol production pathways from Clostridium were cloned to E. coli.
In 2012 researchers developed a method for storing electrical energy as chemical energy in higher alcohols (including butanol). These alcohols can then be used as liquid transportation fuels. The team led by James Liao genetically engineered lithoautotrophic microorganism known as Ralstonia Eutropha H16 to produce isobutanol and 3-methyl-1-butanol in an electro-bioreactor. Carbon dioxide is the sole carbon source for this process and electricity is used as the energetic component. The process they developed effectively separates the light and dark reactions that occur during photosynthesis. Solar panels are used to convert sunlight to electrical energy which is then converted using the microorganism to a chemical intermediate. The team is now in the process of scaling up the operation and believes this process will be more efficient than the biologic process.
Improving Efficiency 
In late 2012, A new discovery should make the alternative fuel butanol more attractive to the biofuel industry. Scientist Hao Feng has found a way around the bottleneck that has frustrated producers in the past and could significantly reduce the cost of the energy involved in making it as well. His team was able to isolate the butanol molecules during the fermentation process so it does not kill the organisms and produces 100% or more butanol. After the fermentation process, they used a process called cloud point separation to recover the butanol which used 4 times less energy.
Also in late 2012, utilizing systems metabolic engineering, a Korean research team at the formally the Korea Advanced Institute of Science and Technology (KAIST) has succeeded in demonstrating an optimized process to increase butanol production by generating an engineered bacterium. Professor Sang Yup Lee at the Department of Chemical and Biomolecular Engineering, KAIST, Dr. Do Young Seung at GS Caltex, a large oil refining company in Korea, and Dr. Yu-Sin Jang at BioFuelChem, a startup butanol company in Korea, applied a systems metabolic engineering approach to improve the production of butanol through enhancing the performance of Clostridium acetobutylicum, one of the best known butanol-producing bacteria. In addition, the downstream process was optimized and an in situ recovery process was integrated to achieve higher butanol titer, yield, and productivity. The combination of systems metabolic engineering and bioprocess optimization resulted in the development of a process capable of producing more than 585 g of butanol from 1.8 kg of glucose, which allows the production of this important industrial solvent and advanced biofuel to be cost competitive.
The anaerobic bacteria C. pasteurianum, C. acetobutylicum, and other Clostridium species have metabolic pathways that convert glycerol to butanol through fermentation. However, the production of butanol from glycerol by fermentation in C. Pasteurianum is low. To counter this, a group of researchers used chemical mutagenesis to create a hyper butanol producing strain. The best mutant strain in this study "MBEL_GLY2" produced 10.8 grams butanol per 80 gram glycerol fed to the bacteria. This is an improvement compared to the 7.6 g butanol produced by the native bacteria.
Many organisms have the capacity to produce butanol utilizing an acetyl-CoA dependent pathway. The main problem with this pathway is the first reaction involving the condensation of two acetyl-CoA molecules to acetoacetyl-CoA. This reaction is thermodynamically unfavorable due to the positive Gibbs free energy associated with it (dG = 6.8 kcal/mol). Some experimentation has been done that involves increasing the carbon storage through the organism by utilizing carbon dioxide flow through photosynthetic organisms. To follow in this path of research, scientists have attempted to engineer reaction pathways that can enable photosynthetic organisms (like blue-green algae) to produce butanol more efficiently.
A study done by Ethan I. Lan and James C. Liao attempted to utilize the ATP produced during photosynthesis in blue-green algae to work around the thermodynamically unfavorable acetyl-CoA condensation to acetoacetyl-CoA. The native system was re-engineered to have acetyl-CoA react with ATP and CO2 to form an intermediate, malonyl-CoA. Malonyl-CoA then reacts with another acetyl-CoA to form the desired acetoacetyl-CoA. The energy release from ATP hydrolysis (dG = -7.3 kcal/mol) makes this pathway significantly more favorable than standard condensation. Because blue-green algae generate NADPH during photosynthesis, it can be assumed that the cofactor environment is NADPH rich. Therefore, the native reaction pathway was further engineered to use NADPH rather than the standard NADH. All of these adjustments led to a 4-fold increase in butanol production, showing the importance of ATP and cofactor driving forces as a design principle in pathway engineering.
DuPont and BP plan to make biobutanol the first product of their joint effort to develop, produce, and market next-generation biofuels. In Europe the Swiss company Butalco is developing genetically modified yeasts for the production of biobutanol from cellulosic materials. Gourmet Butanol, a United States based company, is developing a process that utilizes fungi to convert organic waste into biobutanol.
The number of biobutanol producers with commercial plants coming on line continues to grow monthly. At present, there are number of bioethanol plants, which are being converted to biobutanol plants, which should increase the number of butanol producers as they come on-line.
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.
Properties of common fuels 
|Gasoline and biogasoline||32 MJ/L||14.6||2.9 MJ/kg air||0.36 MJ/kg||91–99||81–89|
|Butanol fuel||29.2 MJ/L||11.1||3.2 MJ/kg air||0.43 MJ/kg||96||78|
|Anhydrous Ethanol fuel||19.6 MJ/L||9.0||3.0 MJ/kg air||0.92 MJ/kg||107||89|
|Methanol fuel||16 MJ/L||6.4||3.1 MJ/kg air||1.2 MJ/kg||106||92|
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% 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 (with a resulting "(R+M)/2 pump octane number" of 87, as used in North America) while t-butanol has octane ratings of 105 RON and 89 MON. t-Butanol is used as an additive in gasoline but cannot be used as a fuel in its pure form because its relatively high melting point of 25.5 °C causes it to gel and freeze near room temperature.
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. Engine design to run on 87 octane will not have any additional power/ fuel economy from at higher octane fuel.
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 critically damage components. 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 are 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.
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 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.
- Butanol is one of many side products produced from current fermentation technologies; as a consequnece, current fermentation technologies allow for very low yields of pure extracted butanol. When compared to ethanol, butanol is more fuel efficient as a fuel alternative, but ethanol can be produced at a much lower cost and with much greater yields.
- Butanol is toxic at a rate of 20g per liter and may need to undergo Tier 1 and Tier 2 health effects testing before being permitted as a primary fuel by the EPA.
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 16%. "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 vaporization will vary and may further limit the percentage of butanol that can be blended with gasoline. Consumer acceptance may be limited due to the offensive smell of butanol. Plans are underway to market a fuel that is 85% Ethanol and 15% Butanol (E85B), so existing E85 internal combustion engines can run on a 100% renewable fuel that could be made without using any fossil fuels. Because its longer hydrocarbon chain causes it to be fairly non-polar, it is more similar to gasoline than it is to ethanol. Butanol has been demonstrated to work in vehicles designed for use with gasoline without modification.
Current use of butanol in vehicles 
Currently no production vehicle is known to be approved by the manufacturer for use with 100% butanol. As of early 2009, only few vehicles are approved for even using E85 fuel (i.e. 85% ethanol + 15% gasoline)in the USA. However, in Brazil all vehicle manufacturers (Fiat, Ford, VW, GM, Toyota, Honda, Peugeot, Citroen and others) produce flex fuel vehicles that can run on 100% ethanol or any mix of ethanol and gasoline. These flex fuel cars represent 90% of the sales of personal vehicles in Brazil, in 2009. BP and Dupont, engaged in a joint venture to produce and promote butanol fuel, claim that "biobutanol can be blended up to 10%v/v in European gasoline and 11.5%v/v in US gasoline". 
See also 
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- Atsumi, Shota; Hanai, Taizo; Liao, James C. (2008), "Non-fermentative pathways for synthesis of branched-chain higher alcohols as biofuels", Nature 451 (7174): 86–89, doi:10.1038/nature06450, PMID 18172501
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- Lan, E.I., Liao, J.C. (2012). "ATP drives direct photosynthetic production of 1-butanol in cyanobacteria". Proceedings of the National Academy of Sciences (in press).
- Lan, E.I., Liao, J.C., (2012). ATP drives direct photosynthetic production of 1-butanol in cyanobacteria. Proceedings of the National Academy of Sciences (in press)
- J.L. Smith; J.P. Workman (December 20, 2007). "Alcohol for Motor Fuels". Colorado State University. Retrieved 2008-01-29.
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- Calculated from the difference in energy densities listed above
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