Dependence on fossil fuels, increasing global energy demand and concerns of greenhouse gas emissions have led to an interest in alternative fuels produced from domestic renewable sources. In recent year, interest in bio-butanol has increased due to its perceived advantages over the traditional gasoline substitute, bio-ethanol. Here I will discuss the current state of bio-butanol and the challenges and possible solutions to making bio-butanol.
1-Butanol (butyl alcohol or n-butanol) is a four carbon straight chained alcohol with a molecular formula of C4H9OH (MW 74.12) and boiling point of 118 ºC. 1-Butanol is an important chemical precursor for paints, polymers and plastics Most 1-butanol produced today is synthetic and derived from a petrochemical route. Synthetic butanol production costs are linked to the propylene market and are extremely sensitive to the price of crude oil.
Renewable 1-butanol is produced from the fermentation of carbohydrates in a process often referred to as the ABE fermentation, after its major chemical products: acetone, butanol and ethanol. The ABE fermentation is a proven industrial process that uses species of bacteria called clostridia to convert sugars or starches into solvents. Biobutanol is an attractive renewable liquid transportation biofuel with superior properties to bio-ethanol. Bio-butanol is compatible with existing fuel infrastructure, has a better energy density, higher octane rating and less hydroscopic than ethanol and can be made from more sustainable feedstocks than bio-diesel. Therefore, if bio-butanol can be made cheaply and on renewable resources, it can readily replace ethanol and bio-diesel in the biofuel market estimated to be worth $247 billion by 2020.
Commercial production
The ABE fermentation process was first developed in the UK in 1912 and was quickly adapted for commercial production during World War I and II; first to produce acetone for ammunitions and second to produce butanol for use in paint lacquers. Butanol is now the preferred solvent since it attracts a higher price in the chemical market. By the 1950s, a synthetic route was developed and the renewable solvents were no longer cost effective. Today, research into the ABE fermentation process may make bio-butanol a new competitor in the bio-fuels market.
China leads efforts to re-commercialize the ABE fermentation process by investing over $200 million into six major corn starch-fed plants with plans to expand production capacity. Traditionally, most plants operate in a semicontinuous fashion with each fermentation lasting up to 21 days. The reactors consist of a cascading series of fermentation tanks that convert seed cultures and fresh feedstock into solvents. Conventional distillation is then used to recover the acetone, butanol and ethanol. Most plants are next to ethanol plants to reduce utility and operating costs. The butanol and ethanol plants can share treatment facilities to process the aqueous waste streams produced from anaerobic fermentations. Biogas produced as a bioproduct is used for heat and power.
The challenges for ABE fermentation
In general, to make the ABE route profitable, there is a need for cheaper feedstocks, improved yields, and more efficient solvent recovery and waste water recycle. Feedstocks contribute most to production costs, typically over 75% of the total. Biobutanol profitability is extremely sensitive to any price fluctuation in the price of feedstocks. Therefore, transitioning to a cheaper (non-edible) feedstock offers the greatest opportunity for cost reduction and improved sustainability.
Butanol titer and yield
The butanol titer (concentration) and yield (fraction of feedstock converted into butanol) of the ABE fermentation is largely a function of the microorganism. Performance can be improved by inducing mutations using chemical mutagens and selecting for improve traits, specific genetic manipulation or a combination of both. There are four main solvent producing strains that have been used industrially with Clostridium acetobutylicum ATCC 824 being the best studied and manipulated strain (the others being C. saccharobutylicum, C. beijerinckii and C. saccharoperbutylacetonicum). Research into manipulating these strains has been helped through the publication of their genomic sequences. Significant progress has been made in genetically manipulating C. acetobutylicum while the progress to genetically engineer the other strains has lagged.
Feedstocks
Currently, biobutanol is economical if it is sold as a chemical commodity instead as a cheaper biofuel. For it to be sold profitably as a biofuel, the cost of feedstocks must be reduced. If cheaper waste streams (such as corn cobs, corn stover, sugar cane bagasse, wheat straw and municipal solid waste) could be converted into feed stocks then biobutanol could compete on price with ethanol for the biofuel market.
Solvent recovery
Currently, distillation is used as a robust and proven process to recover butanol from fermentations but the process is energy intensive. Improvements can be made to make the conventional distillation process for energy efficient but the biggest reduction in energy use can only be achieved by development of nonconventional means. Since butanol is toxic to the ABE strains, online removal of butanol from the fermentation will result in higher yields. Methods like gas stripping, pervaporation, reverse osmosis, vacuum fermentation, and aqueous two phase separation are being researched for online butanol removal.
What’s Michigan Doing?
In April of this year, Cobalt Technologies and American Process announced an agreement to build the world’s first industrial-scale cellulosic biorefinery to produce biobutanol. A plant is currently under construction in Alpena, Michigan. By April 2012, the Alpena Biorefinery will produce 470,000 gallons of biobutanol annually, which will be pre-sold to chemical industry partners.
Conclusions
The clostridial ABE fermentation is an old, but proven, industrial fermentation process that has be re-established recently. The clostridial ABE fermentation process is relatively simple and existing ethanol plants can be retrofitted fairly easily to produce butanol. In order to penetrate the larger biofuel market, biobutanol needs to compete on cost with ethanol despite its superior fuel properties. Reduction in feedstock cost offers the best opportunity especially since clostridia are well suited for sugars derived from cellulosic material. Further advances for both 1-butanol are likely to come from the deployment of continuous culture, especially when coupled with in situ methods for solvent extraction and recovery. The application of advances in biotechnology and engineering to the clostridia ABE fermentation process will drive down the cost of 1-butanol production.
