From 2007.igem.org

Butanol: Alberta’s biofuel for the world

Douglas Ridgway

Project CyberCell

Institute for Biomolecular Design

University of Alberta

April 12, 2007

The twin perils of petroleum resource exhaustion and anthropogenic climate change have given new vigor to the search for alternative energy sources. Among the possibilities, only biomass is likely to be sufficiently carbon neutral and sustainable while being plentiful and cheap enough to power a major portion of society’s needs. Derived from forestry or agricultural waste, or purpose grown energy crops, biomass takes many forms, ranging from the easily utilized oils and carbohydrates to the plentiful, but difficult to use, cellulose. The problem then becomes transforming biomass sources into a suitable inexpensive liquid fuels for transportation, society’s primary use of petroleum. While current research and commercial efforts focus on fermentation to ethanol, the four-carbon alcohol butanol is a superior replacement for gasoline, and is attracting increasing commercial interest. Due to its combination of agricultural and energy industries, combined with world-class R&D institutions, Alberta finds itself uniquely placed for the creation of a biofuels industry around butanol. Further opportunity for disruptive advances may be created through investment in systems and synthetic biology.

Transportation consumes a substantial fraction of total world energy use, and is the least-substitutable application for petroleum products: currently 97% of the energy used for transportation comes from petroleum. Unfortunately transportation, as an industry, turns over slowly. Cars are not computers; the vehicles sold new today were designed a decade ago and will be on the road in 2020 and beyond. Any imagined vehicle technology revolution, whether electric or hybrid vehicles, hydrogen fuel cells, or efficiency improvements, can thus have only modest impact in the near term. Innovations requiring infrastructure or social use changes, such as improvements in mass transit and urban design, bicycle usage, etc would take effect even more slowly. It is thus vital that any potential innovations be as compatible as possible with the existing vehicle fleet makeup and fuel infrastructure. Ideal would be a cheap, carbon neutral, liquid fuel that could substitute for gasoline in unmodified existing vehicles and infrastructure. Among the possibilities, butanol from biomass is the best choice of fuel, if it can be produced cheaply enough.

Alternative biomass based fuels

Biodiesel:

Vegetable oils are easily converted via a transesterification reaction to biodiesel, a fuel broadly compatible with petroleum diesel. Engine and infrastructure conversion issues are minor; the principal drawback is cost of feedstock. Vegetable oils function as a specialized and highly refined energy storage for the plant, and as a result, constitute only a small fraction of total produced biomass. Total yield per acre is thus low, and essentially no feedstock is available as waste from other processes. (Even waste vegetable oil from restaurants has a higher value as a soapmaking feedstock than as a fuel.) Energy and carbon balance issues for purpose grown oil crops using fossil based fertilizers become unfavorable, negating any possible environmental or economic benefit. While some algae produce substantial amounts of oils, these are for the moment laboratory curiosities, without the substantial biological and process development required for a successful large scale application. Vegetable oils are thus too valuable as a human and animal food to burn for transportation on large scales.

Sugarcane ethanol:

Humans have been fermenting sugars to ethanol for beverages for four thousand years, and distilling them for over a thousand. Thus not only are the techniques and processes mature, but the yeast organisms themselves have been selected for tolerance to intense fermentations for hundreds of thousands of generations. For fuel ethanol, the current best practice is the Brazilian sugarcane process, which burns sugarcane bagasse as a heat source to power the distillation. Brazil can now produce ethanol from sugarcane at a price of US$0.25/l, undercutting the production cost of ethanol from fossil fuels. Although ethanol has a lower energy density compared to gasoline, and its different evaporation properties mean that engine modifications are required to run pure ethanol or high ethanol-gasoline mixes, a long history of ethanol has resulted in a substantial fraction of Brazil’s vehicles being suitable for ethanol fuel. Having been produced commercially for thirty years, since the 1970’s OPEC crisis, ethanol is being produced at 16 B liters/yr, enough to reduce Brazil’s transportation petroleum usage by 40%.

Cellulosic Ethanol:

Cellulose forms the largest store of biomass energy on the planet. Composed of linked polysaccharide chains and lignins, it forms the bulk structural material for trees, woody plants and shrubs. Its utility to the plant requires that it be difficult for microorganisms to attack or break down, which creates difficulty for fermenting cellulose to ethanol. Substantial research and development effort has been invested in identifying and mass producing enzymes capable of the necessary transformations, and creating the processes required for ethanol production from cellulose, with medium scale plants nearing (subsidized) commercialization. However, feedstock preprocessing requirements, and the relatively low concentration of produced ethanol, will impact energy balance and production cost.

The largest disadvantage of ethanol compared to gasoline is its miscibility with water. Gasoline is hydrophobic; it does not absorb water, and rapidly and completely phase separates should it get mixed with water. This phase separation is built into every part of the gasoline fuel infrastructure, from its refinement during production to the design of the cylinder in which it is burned. Gasoline storage tanks can be assumed to contain no water, except potentially a small pool at the bottom. Gasoline tankers and pipelines need no special provisions to prevent rust. Ethanol, on the other hand, is highly hygroscopic, and miscible with water in any proportion. Ethanol intended for fuel must be protected from moisture and condensation at every step from production through use. Ethanol storage tanks must be protected against corrosion. Ethanol cannot be shipped through gasoline pipelines, and instead must be transported by purpose-built trucks. And engines intended to burn ethanol must be prepared to accept several percent water in the fuel stream, without undue loss of power or unacceptable wear. As shown by Brazil, these obstacles are not insurmountable, but the required infrastructural changes are large, and should not be underestimated.

Biomass based syngas followed by a gas-to-liquids process. Industrial economies lacking access to petroleum have resorted to a variety of creative means of producing liquid fuels, of which perhaps the best known is the Fisher-Tropsch process. In this process, steam runs over hot coal producing syngas, a mixture of carbon monoxide and hydrogen, followed by the use of syngas to manufacture of a liquid hydrocarbon fuel. Syngas can also be produced from biomass, by combustion in a reducing environment. While the chemistry is well understood, process development and economics indicate that such gas-to-liquids plants will likely begin with coal, obviating the carbon-neutrality and sustainability benefits of biomass.

Biobutanol:

Butanol is a superior to ethanol as a replacement for petroleum gasoline. With a low vapor pressure, high energy density, and a gasoline-like octane rating, it can be blended into existing gasoline at much higher proportions than ethanol without compromising performance, mileage, cold starting, or volatile organic pollution standards, without modifying the fuel-air ratio, and without changing the fuel system. More importantly, butanol, like gasoline, is immiscible with water, and less corrosive than ethanol, allowing the existing gasoline infrastructure of tanks and pipelines to be used unmodified. Butanol would thus be a superior liquid fuel, if only it could be produced in a cheap and sustainable manner.

Although fermentation of biomass to butanol has a short history compared to ethanol, having been discovered in the late 19th century, it has nevertheless been proven on large industrial scales. The acetone-butanol (AB) fermentation, based around solventogenic organisms such as Clostridium acetobutylicum, produces acetone, butanol and ethanol from starches, sugars, and cellulose. First developed on a large scale during World War I, when Britain had a high demand for acetone to make cordite, the AB fermentation became the primary process for world butanol production until lower-cost petroleum-derived butanol began to take over in the 1950’s. The AB fermentation has therefore had many decades of optimization on an industrial scale. During this period, a wide variety of solventogenic bacteria have been discovered and characterized, the plant processes developed, and the potential substates identified. Currently, only China still produces butanol via AB fermentation, supplying 50% of internal butanol demand, but this is changing. Increases in petroleum prices, and concerns about sustainability and carbon emissions have revived interest in biobutanol, both from a research perspective and for commercialization. Biobutanol has several advantages over ethanol as a biofuel. One is feedstock flexibility. Solventogenic Clostridia are known to metabolize a wide range of substrates, including hexose and pentose sugars, and naturally produce cellulolytic enzymes, making cellulosic butanol production a substantially simpler proposition than cellulosic ethanol. Butanol has market advantages over ethanol as well. In addition to being a superior gasoline replacement, butanol is used as an industrial solvent in glue and paint production. This non-fuel butanol market currently pays $1-2/l, substantially above the value of ethanol, and would likely be the first market displaced by biobutanol. Butanol as a fuel additive also possesses a coblend synergy with ethanol, allowing larger amounts of ethanol oxygenate in a gasoline blend without exceeding vapor pressure limits. This would eliminate the need for special “summer” and “winter” gasoline formulations, of particular importance to Alberta. Thus butanol has an economic value exceeding that of the energy content alone, easing commercialization.

The market is becoming increasingly aware of biobutanol’s advantages. As one example, a joint project of BP, DuPont, and British Sugar announced in 2006 will convert the East Anglia UK bioethanol plant to butanol production, working from sugar beet feedstock. Other biofuel efforts include those of Amyris Biotechnologies, whose non-ethanol gasoline replacement has not been disclosed, but has technical characteristics similar to butanol. Based on the considerations above and the response of the marketplace, it would appear that butanol production from biomass is commercially viable today, without additional technological improvements.

Alberta’s Advantages

The biofuels field is unusual in its reliance on the traditionally distinct industries of oil and agriculture. Here in Alberta, the top industries are energy (with the traditional strengths in oil, gas and coal now being supplanted by oil sands revenue growth), agriculture (with wheat, hay, barley, and canola leading production), forestry, and chemicals. All of these industries would contribute to, and benefit from, biobutanol technology development. Agriculture and forestry would contribute feedstock, and the chemicals and energy sector would be responsible for production, refinement, and marketing. These would all be tied together with Alberta’s first class research establishments.

Feedstock availability. Potential biomass feedstocks in Alberta include forestry waste (particularly the increasing amounts of timber killed by mountain pine beetle, a source already slated for biomass energy in B.C.), agricultural residue such as wheat straw (typical price at the farm gate of $20-40/ton), and purpose-grown energy crops such as sugar beets. Importantly, unlike the US which must place a large tariff on imported sugar to protect domestic sugar producers, Canada is able to produce sugar at world market prices. The source of this sugar is Alberta-grown sugar beets, precisely the same feedstock that BP selected for the UK biobutanol plant. Alberta is thus fortunate to have a combination of high quality, low priced feedstocks suitable for early commercial development, coupled with very large potential feedstocks suitable for sustainable, economical, large-scale development.

Oil and gas, energy and chemicals. Alberta was built on oil, thus there is an extraordinary existing personnel and knowledge base, as well as a business environment of excellence and associated with energy projects. As the ultimately fungible commodity, energy presents an unusual challenge to business management, which the existing oil and gas industry has spent decades understanding. Energy permeates Albertan culture, in the same way that therapeutics permeate Boston, computers permeate San Jose, aerospace dominates Los Angeles, and entertainment permeates Hollywood. The resulting network effects of location are large and should not be underestimated: world-class employees and support organizations can be found, partners are available, bankers, politicians and regulators understand the business, and investors, familiar with the game, can act with competence and confidence. As one example of such a network effect, Alberta has also developed a substantial chemical industry, adding value to the hydrocarbon product chain. This industry could both provide necessary expertise in producing and marketing a biobutanol product, as well as potentially act as a consumer, using butanol’s flexibility as a feedstock for glues, paints, and plastics.

Research infrastructure. Alberta has long invested in establishing world-class research institutions, from the University of Alberta and the University of Calgary, to the Alberta Research Council, the National Institute of Nanotechnology, and many other facilities. Bioenergy projects require an unusual confluence of research expertise, from biology to energy to agriculture, all of which are well represented in the Alberta research community, due to their application to Alberta’s existing industries.

Research priorities for biobutanol

It is now clear that biobutanol is essentially suitable for production today, without further development. While existing commercializers such as BP evidently agree, there are a number of clear research avenues for making both incremental and discontinuous step improvements, with the potential for large impacts.

Process and feedstock development. Considerably more effort has been expended on optimizing ethanol fermentations for various feedstocks than butanol. While C. acetobutylicum can metabolize a wide variety of substrates, including pentose sugars derived from cellulose, the questions of how efficiently, with what kinds of preprocessing, what energy inputs, and producing what side products, all have a substantial impact on plant design and commercial success. Low-risk development efforts in these areas are required for plant design and scale-up, and could yield substantial improvements in efficiency.

Online solvent extraction for yield improvement. Like ethanol fermentations, butanol fermentations are ultimately limited in intensity by the poisoning of the microbial culture with the solvent product. Recent biobutanol research has developed a variety techniques for removing the product from the broth during fermentation, such as gas stripping, pervaporation, and solvent-solvent extraction, allowing the fermentation to continue, increasing the yield, and minimizing the post-fermentation separation required. The cost and efficiency of these techniques, and optimizing their application in the context of the rest of a plant design, requires substantial further research before they can be applied on commercial scales. There is no shortage of potential additional innovations in this area as well. For example, innovations in materials made possible by nanotechnology also have the potential to aid separation. Or, if gasoline is the desired end product, a fuel blend component could be used as the solvent in online solvent-solvent extraction, and the resulting extract stream could be directly blended into gasoline, eliminating the second solvent separation step which would normally be required. While advancing separation and extraction technology is not required for biobutanol success, there are both substantial opportunities, and substantial benefits possible.

Increasing butanol tolerance and production via synthetic biology. An alternative to online solvent extraction is to simply increase the butanol tolerance of the bacterial culture. In recent years, some hyperproducing strains of solventogenic Clostridia have been identified, which continue production up to over 3% total solvents, increasing yields, reducing culture volume requirements, and minimizing the costs of final separation. Substantial further improvements are possible. Some solventogenic bacterial genomes have been fully sequenced, and the solvent generating pathways mapped, potentially allowing process development to be optimized on the level of subcellular metabolism as well as a large-scale plant level. Beyond conventional genetic engineering, the whole-scale de novo design of organisms, known as synthetic biology, will likely result in the booting of a completely engineered genome sometime this year. Such a complete command of biological processes should enable discontinuous developments, such as the transplant of a solventogenic metabolism into a solvent-tolerant extremophile. Such approaches may be applicable to ethanol fermentation too. However, for ethanol the improvements can be only marginal, since ethanol is miscible with water at any concentration. Butanol has limited miscibility, resulting in a discontinuity in the impact of yield improvement on separation difficulty. In a butanol broth, saturated with butanol above the 7-9% solubility limit, a butanol phase will spontaneously separate and sink to the bottom, dramatically lowering separation costs. One could imagine a vat with a steady state fermentation, continuously fed a cheap, carbon neutral feedstock, with a tap at the bottom dispensing a nearly pure, ready-to-burn fuel, with no distillation required. Regardless of the ease of achieving this vision, it is clear that even modest improvements butanol tolerance could provide exponential improvements in process cost and efficiency, with a disruptive, positive impact on transportation and the environment.

Synthetic and systems biology in Alberta

Alberta has most of the pieces of the puzzle in place: existing energy and chemicals industries, agriculture and forestry, and world-class nanotechnology and engineering research. Still lacking is a major research focus on systems and synthetic biology, which, as seen above, are placed to have a substantial impact on the biofuel industry as well as many others. Albertan investment in systems and synthetic biology has the potential for synergistic impact on existing Alberta strengths to address a confluence of cusps of societal wants and needs with technological innovation.

References and Further Reading

Goldemberg J. Ethanol for a sustained energy future. Science 315:808 2007. Perspective on Brazilian ethanol by the Sao Paulo Minister for Environment.

Lynd LR et al. Microbial Cellulose Utilization: Fundamentals and Biotechnology. Microbiol Mol Biol Rev 66:506-577 2002. Consolidated bioprocessing approaches to cellulose.

Jones, DT and DR Woods. Acetone-butanol fermentation revisited. Microbiological Reviews, 50:484-524, 1986. Mitchell WJ. Physiology of carbohydrate to solvent conversion by Clostridia. Adv Microb Physiol 39:33-130 1998. Zverlov VV et al. Bacterial acetone and butanol production by industrial fermentation in the Soviet Union: use of hydrolyzed agricultural waste for biorefinery. Appl Microbiol Biotechnol (2006) 71: 587–597. 2006. Technical and historical reviews of solventogenesis and the industrial AB fermentation.

Stephanopoulos G. Challenges in engineering microbes for biofuels production. Science 315:801-804 2007. Paredes CJ, KV Alsaker and ET Papoutsakis. A comparative genomic view of clostridial sporulation and physiology. Nat. Rev. Microbiology, 3:969-978 2005. Ezeji TC, N Quresh, and HP Blaschek. Butanol fermentation research: upstream and downstream manipulations. Chem Rec 4:305-14 2004. Reviews from leading groups in bu