Biobutanol: A Viable Biofuel with Advantages Over Ethanol as a Gasoline Additive
Intro
Biobutanol is butanol, or butyl alcohol, derived from a biomass source. Butanol can also be made from hydrocarbons and that is known as petrobutanol. As a biofuel, butanol has some significant advantages over the most common biofuel, ethanol, or ethyl alcohol. As a C4 hydrocarbon biobutanol is more similar to gasoline than to ethanol. Butanol has a lower vapor pressure and a higher energy content than ethanol. Biomass feedstocks for biobutanol are similar to those for ethanol, grains like corn, sugar beets, sugar cane, and other biomass. There is also cellulosic butanol, like cellulosic ethanol, made from plant wastes. Biobutanol can also be enhanced by bacteria, yeast, or fungi, and made from algae as a feedstock with cyanobacteria. More specifically, genetically modified bacteria offer some future possibilities for biobutanol production. One challenge for biobutanol is that more bioethanol than biobutanol can be produced from a bushel of corn. Biobutanol has been in development as a modern biofuel for many years now and has fluctuated in value. More recently, there are biobutanol gasoline blends in use for road vehicles in parts of the U.S.
Fermentation, Biosynthesis, Cyanobacteria, and Substrates
Biobutanol production relies on ABE fermentation - acetone-butanol-ethanol. One economic impediment to alcohol fuels like ethanol and butanol is that they are limited by inefficient fermentation rates. Escherichia coli (E. coli) bacteria is useful in the commercial production of biobutanol since in a genetically engineered form it produces the highest yields of isobutanol of any microorganism. Isobutanol is a second-generation biofuel that has significant advantages over ethanol. E. coli is ideal as an isobutanol bio-synthesizer for other reasons as well: it has been studied extensively, it is very manipulatable with genetic engineering, and it has the ability to use lignocellulose (from agricultural waste) to make isobutanol. The process still faces economic hurdles though. Bioreactors are also susceptible to bacteriophages that may damage fermentation. Scientists are trying to genetically engineer that susceptibility out of new strains.
Clostridia is another bacterium that can make isobutanol. It is very good at making isobutanol from cellulose. It was once used to make acetone form from starch. Acetone was made from corn starch and molasses in both World Wars in biobutanol plants through such fermentation. The acetone was used in the manufacture of smokeless gunpowder and rocket propellant, but butanol was still the main product. In the 1960’s it began to be more economic to make butanol from petroleum products. Other potential bio-synthesizers of isobutanol include the bacterium Bacillus subtilis, the yeast Saccharomyces cerevisiae, and the soil bacterium Ralstonia eutropha.
Genetically engineered cyanobacteria, a form of algae, are also a good feedstock for isobutanol. It does not require the use of plants. It grows faster than plants. It grows in water and sunlight and takes CO2 from the atmosphere, a climate plus. Drawbacks are a need for specific wavelengths of sunlight and a medium of precise salinity, two conditions which are difficult to control. Cyanobacteria bioreactors also require more energy to operate. These energy intensity, salinity, and sunlight requirements impact the economics of making isobutanol from cyanobacteria.
Metabolic engineering is used to allow an organism to use a cheaper substrate. Fermentation requires sugars as a substrate so cheaper sugars, like glycerol instead of glucose, could make the process cheaper. Glycerol is cheap and abundant as a waste-product from biodiesel production. Other processes are being explored to recover butanol with higher efficiency. Enzymes are used to catalyze reactions. Fermentation chemistry and genetic engineering are two technologies used to make fermentation more efficient and the search is ongoing to find the most economic components for catalysis and for substrates.
The Timeline of Microbial Biobutanol Production below is from the Ph.D. dissertation of Reyna Gomez-Flores at the University of Western Ontario, 2018
Projects
DuPont and BP have a joint venture to develop, produce, and market next-generation biofuels. Biobutanol is a big part of that. Swiss company Butalco is exploring biobutanol production that uses a fungi to convert organic waste into biobutanol. There are also plans for an E85B fuel mixture that is 85% ethanol and 15% butanol to be run in existing E85 engines. BP and DuPont claim that a 10% biobutanol blend with gasoline is possible with no engine modification.
Th U.S. Coast Guard began a year-long engine test of 16.1 % biobutanol blended fuel for ships in 2012-2013. At the time the price of oil was high. They chose biobutanol over natural gas, ethanol blends, and biomass liquid fuels based on maturity, performance, safety, and logistics. Their supplier used metabolic engineering to develop yeast-based isobutanol. They were also doing engine tests where they were running engines on the biobutanol regularly for months then tearing the engines apart to analyze for corrosion and other issues. I don’t know the results of this project but I can make a guess that cost is still an issue relative to current oil and gasoline prices.
Eastman Chemical Company also had a biobutanol project in the works in 2012, utilizing a genetically engineered Clostridium bacterium for biosynthesis. Bioacetone and biobutanol made by the process are used in coatings, molded plastics, and personal care products.
Biobutanol is also promising as a biofuel for use as jet fuel. However, like some other biobutanol and other biofuel applications, there is still economics relative to fossil hydrocarbons, so significant subsidization is also required for profitability. Aviation biofuel does qualify as a non-CO2 emitting fuel but there has been difficulty in applying it to the EU’s Emissions Trading Scheme (ETS) at least as of 2016.
Two companies manufacturing biobutanol in the US, Butomax and Gevo, have had some patent disputes in the past. The companies also make by-products including solvents/coatings, plastics, and fibers. This helps them diversify. Both companies registered for “on-highway vehicle sales with EPA” by June 2018. Biobutanol blends are currently sold in select parts of the U.S.
A more recent breakthrough in biobutanol production was recently announced in a new paper in the Journal of the American Chemical Society, This involves a new metal organic framework that can more efficiently separate or recover biobutanol from the fermented biomass broth. It removes a significant obstacle. Current focus is on scaling up the process.
Properties of Isobutanol that Favor it Over Ethanol
The energy density of isobutanol is 98% that of gasoline. It does not readily absorb water from air like ethanol and so prevents corrosion of engines and of pipelines. Ethanol that absorbs water can separate from the gasoline with which it is mixed. Butanols, especially n-butanol, or normal butanol, that has a slightly different chemical formula than its isomer, isobutanol, resist such separation. It can be mixed at any proportion with gasoline – ie. it can replace gasoline or be an additive to gasoline. Isobutanol has a high octane rating similar to ethanol and so, like ethanol, is suitable as an additive to boost octane rating. N-butanol has a lower octane rating and is not suitable for this purpose. It can be made from plant matter not connected to food supplies. Butanols are less damaging to engines than ethanol since they can be mixed in at higher ratios before retrofitting or modification would be required. This is because butanols have an air-fuel ratio and energy content more similar to gasoline than ethanol does.
References:
Biobutanol, in U.S. Dept of Energy, Alternative Fuels Data Center, Fall 2018
Researchers Make Key Advance Toward Production of Important Biofuel – by Oregon State University, in Journal of the American Chemical Society, accessed in Phys.org
Butanol Fuel, entry in Wikipedia
Biobutanol: The Next Big Biofuel? – by Jessica Ebert, in Biomass Magazine, May 2008
Isobutanol to the Rescue: The U.S. Coast Guard is Testing Isobutanol Gasoline Blends in its Fuel Engines – by Chris Hanson, in Biomass Magazine, Oct. 25, 2013
The Business of Biobutanol: Acquisitions, Patent Infringement Disputes Continue – by Erin Voegele, in Biomass Magazine, Jan. 9, 2012
Promising Jet Fuel Market Looms for Upgraded Bioethanol, Butanol - by Kapil Lokare, in Ethanol Producer Magazine, Feb. 5. 2016
Biobutanol Production from Cellulosic and Sugar-Based Feedstock from the Corn Plant – by Reyna Gomez-Flores. Ph.D. Dissertation, The University of Western Ontario, in Electronic Thesis and Dissertation Repository, April 24, 2018
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