Butanol fuel
Genetically modified organisms
Obtaining higher yields of butanol involves manipulation of the metabolic networks using
Although unproven commercially, combining electrochemical and microbial production methods may offer a way to produce butanol from sustainable sources.[9]
Escherichia coli
- E. coli is an organism for which several tools of genetic manipulation exist, and it is an organism for which an extensive body of scientific literature exists.[10] This wealth of knowledge allows E. coli to be easily modified by scientists.
- E. coli has the capacity to use lignocellulose (waste plant matter left over from agriculture) in the synthesis of isobutanol. The use of lignocellulose prevents E. coli from using plant matter meant for human consumption, and prevents any food-fuel price relationship which would occur from the biosynthesis of isobutanol by E. coli.[10]
- Genetic modification has been used to broaden the scope of lignocellulose which can be used by E. coli. This has made E. coli a useful and diverse isobutanol bio-synthesizer.[12]
The primary drawback of E. coli is that it is susceptible to
Clostridia
n-Butanol can be produced by
A strain of Clostridium can convert nearly any form of cellulose into butanol even in the presence of oxygen.[17]
A strain of Clostridium cellulolyticum, a native cellulose-degrading microbe, affords isobutanol directly from cellulose.[18]
A combination of
Cyanobacteria
- Cyanobacteria grow faster than plants[22] and also absorb sunlight more efficiently than plants.[23] This means they can be replenished at a faster rate than the plant matter used for other biofuel biosynthesizers.
- Cyanobacteria can be grown on non-arable land (land not used for farming).[22] This prevents competition between food sources and fuel sources.[22]
- The supplements necessary for the growth of cyanobacteria are This presents two advantages:
- Because CO2 is derived from the atmosphere, cyanobacteria do not need plant matter to synthesize isobutanol (in other organisms which synthesize isobutanol, plant matter is the source of the carbon necessary to synthetically assemble isobutanol).[23] Since plant matter is not used by this method of isobutanol production, the necessity to source plant matter from food sources and create a food-fuel price relationship is avoided.[22]
- Because CO2 is absorbed from the atmosphere by cyanobacteria, the possibility of bioremediation (in the form of cyanobacteria removing excess CO2 from the atmosphere) exists.[23]
The primary drawbacks of cyanobacteria are:
- They are sensitive to environmental conditions when being grown. Cyanobacteria suffer greatly from sunlight of inappropriate wavelength and intensity, CO2 of inappropriate concentration, or H2O of inappropriate salinity, though a wealth of cyanobacteria are able to grow in brackish and marine waters. These factors are generally hard to control, and present a major obstacle in cyanobacterial production of isobutanol.[24]
- Cyanobacteria bioreactors require high energy to operate. Cultures require constant mixing, and the harvesting of biosynthetic products is energy-intensive. This reduces the efficiency of isobutanol production via cyanobacteria.[24]
Cyanobacteria can be re-engineered to increase their butanol production, showing the importance of ATP and cofactor driving forces as a design principle in pathway engineering. 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).[25][26]
Bacillus subtilis
Saccharomyces cerevisiae
Saccharomyces cerevisiae, or S. cerevisiae, is a species of yeast. It naturally produces isobutanol in small quantities via its valine biosynthetic pathway.[28] S. cerevisiae is an ideal candidate for isobutanol biofuel production for several reasons:
- S. cerevisiae can be grown at low pH levels, helping prevent contamination during growth in industrial bioreactors.[10]
- S. cerevisiae cannot be affected by bacteriophages because it is a eukaryote.[10]
- Extensive scientific knowledge about S. cerevisiae and its biology already exists.[10]
- As a eukaryote, S. cerevisiae is genetically more complex than E. coli or B. subtilis, and is harder to genetically manipulate as a result.[10]
- S. cerevisiae has the natural ability to produce ethanol. This natural ability can "overpower" and consequently inhibit isobutanol production by S. cerevisiae.[10]
- S. cerevisiae cannot use five-carbon sugars to produce isobutanol. The inability to use five-carbon sugars restricts S. cerevisiae from using lignocellulose, and means S. cerevisiae must use plant matter intended for human consumption to produce isobutanol. This results in an unfavorable food/fuel price relationship when isobutanol is produced by S. cerevisiae.[10]
Ralstonia eutropha
Cupriavidus necator (=Ralstonia eutropha) is a Gram-negative soil bacterium of the class Betaproteobacteria. It is capable of indirectly converting electrical energy into isobutanol. This conversion is completed in several steps:[31]
- Anodesare placed in a mixture of H2O and CO2.
- An electric current is run through the anodes, and through an electrochemical process H2O and CO2 are combined to synthesize formic acid.
- A culture of C. necator (composed of a straintolerant to electricity) is kept within the H2O and CO2 mixture.
- The culture of C. necator then converts formic acid from the mixture into isobutanol.
- The biosynthesized isobutanol is then separated from the mixture, and can be used as a biofuel.
Feedstocks
High cost of raw material is considered as one of the main obstacles to commercial production of butanols. Using inexpensive and abundant feedstocks, e.g., corn stover, could enhance the process economic viability.[32]
Improving efficiency
A process called cloud point separation could allow the recovery of butanol with high efficiency.[34]
Producers and distribution
DuPont and BP plan to make biobutanol the first product of their joint effort to develop, produce, and market next-generation biofuels.[35] In Europe the Swiss company Butalco[36] 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.[37][38] Celtic Renewables makes biobutanol from waste that results from the production of whisky, and low-grade potatoes.
Properties of common fuels
Isobutanol
Isobutanol is a second-generation biofuel with several qualities that resolve issues presented by ethanol.[10]
Isobutanol's properties make it an attractive biofuel:
- relatively high energy density, 98% of that of gasoline.[39]
- does not readily absorb water from air, preventing the corrosion of engines and pipelines.[10]
- can be mixed at any proportion with gasoline,[40] meaning the fuel can "drop into" the existing petroleum infrastructure as a replacement fuel or major additive.[10]
- can be produced from plant matter not connected to food supplies, preventing a fuel-price/food-price relationship.[10][11][12][27]
- assuming that it is produced from residual lignocellulosic feedstocks, blending isobutanol with gasoline may reduce GHG emissions considerably.[41]
n-Butanol
Butanol better tolerates water contamination and is less corrosive than ethanol and more suitable for distribution through existing
Fuel | Energy density |
ratio
|
Specific energy |
vaporization
|
RON | MON | AKI |
---|---|---|---|---|---|---|---|
Gasoline and biogasoline | 32 MJ/L | 14.7 | 2.9 MJ/kg air | 0.36 MJ/kg | 91–99 | 81–89 | 87-95 |
Butanol fuel | 29.2 MJ/L | 11.1 | 3.6 MJ/kg air | 0.43 MJ/kg | 96 | 78 | 87 |
Anhydrous Ethanol fuel | 19.6 MJ/L | 9.0 | 3.0 MJ/kg air | 0.92 MJ/kg | 107 | 89 | 98 |
Methanol fuel | 16 MJ/L | 6.4 | 3.1 MJ/kg air | 1.2 MJ/kg | 106 | 92 | 99 |
The
A fuel with a higher octane rating is less prone to
Butanol characteristics: air-fuel ratio, specific energy, viscosity, specific heat
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 ethanol or a gasoline blend with a high percentage of ethanol, 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.[42][43]
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.[47]
Substance | Kinematic viscosity at 20 °C |
---|---|
Butanol | 3.64 cSt |
Diesel | >3 cSt |
Ethanol | 1.52 cSt |
Water | 1.0 cSt |
Methanol | 0.64 cSt |
Gasoline | 0.4–0.8 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.[48]
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.[42]
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
Consumer acceptance may be limited due to the potentially offensive
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 a 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% Gasoline and or any mix of ethanol and gasoline up to 85% ethanol (E85). 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[15] that "biobutanol can be blended up to 10%v/v in European gasoline and 11.5%v/v in US gasoline".[50][51] In the 2009 Petit Le Mans race, the No. 16 Lola B09/86 - Mazda MZR-R of Dyson Racing ran on a mixture of biobutanol and ethanol developed by team technology partner BP.
See also
- Alcohol to jet fuel
- Air-fuel ratio
- Bioalcohol
- Biofuel
- Biodiesel
- Biohydrogen
- Bioconversion of biomass to mixed alcohol fuels
- Butanol
- Catalyst
- Dimethyl ether
- Distillation
- Emission standards
- Energy crop
- Ethanol fuel
- Formic acid: can be used as an intermediary to produce isobutanol from CO2 using microbes[52][53][54]
- Gevo Biofuels
- Industrial fermentation
- List of vegetable oils used for biofuel
References
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- ^ The Korea Advanced Institute of Science and Technology (KAIST) (October 23, 2012). "Highly Efficient Production of Advanced Biofuel by Metabolically Engineered Microorganism". ScienceDaily.
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- ^ Washington University in St. Louis (January 28, 2008). "New Techniques Create Butanol, A Superior Biofuel". ScienceDaily.
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- ^ Engineering Toolbox
- ^ "Product Safety - n-Butanol". dow.com. Dow Chemical Company. Archived from the original on 2015-04-02. Retrieved 2013-07-09.
- ^ "BP-DuPont biofuels fact sheet" (PDF). BP and DuPont. Archived from the original (PDF) on 2012-02-29. Retrieved 2013-07-25.
- ^ "Boosting Biomass-to...Butanol?". Green Car Congress. July 20, 2005. Retrieved 2008-01-29.
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External links
- Biobutanol (EERE).
- Biobutanol research news from Green Car Congress
- Butanol 3D view and pdb-file