by Thomas Foust, M.S., Ph.D., P.E. and Matthew Yung, Ph.D.

With a presidential administration committed to enacting policy and investing in technology to help the U.S. secure and develop alternative sources of energy, there is a bright horizon for clean energy.  The recent influx of government funding through President Obama’s economic stimulus package and an ongoing investment of $15 billion per year will create jobs and accelerate the penetration of technology related to clean energy.  Adopting clean, alternative energy sources will allow us to reduce our dependence upon foreign oil and decrease our contribution to climate change through greenhouse gas emissions.  While electricity-producing clean technologies (e.g. wind, solar) are vital in this effort, decreasing the U.S. dependence on petroleum for both the near- and long-term will be achieved by the development of second generation biofuels technologies that do not rely on grain or other food-based feedstocks.

Horizon for Alternative Energy

The majority of the U.S.’s energy needs is supplied by fossil fuels, with electricity primarily produced from coal and transportation fuels produced mainly from petroleum.  The significant displacement of conventional, fossil fuel energy sources will only be possible using a bouquet of the available and emerging technologies from the areas of wind, solar, biomass, hydroelectric, and geothermal.  Solar and wind power have made significant inroads and have great potential to increase their capacity for electricity generation as they address various barriers (e.g. performance, reliability, cost, capacity, grid integration).

As opposed to increasing our energy supply, another road towards clean and responsible energy use involves decreasing our consumption.  While lifestyle changes could account for a significant decrease in energy consumption, many of us would be reluctant to give up our computers, TVs, and air-conditioning.  Instead, increasing the efficiency of buildings and electronics through intelligent engineering can lead to substantial energy savings.  Increasing the efficiency of buildings represents low-hanging fruit for improvements in energy use, as buildings constitute ~70% of the U.S. electricity consumption, which can be vastly reduced by integrating both renewable features (e.g. solar) and efficiency features (e.g. computerized energy optimization tools, advanced HVAC systems, insulation) into building systems.

While advances in the aforementioned technologies will help decrease our use of electricity derived from fossil fuels, they do little to reduce our dependence on petroleum.  The U.S. uses petroleum to make its transportation fuels and currently burns approximately 140 billion gallons/year (bgy) of gasoline, 43 bgy of diesel in on-road applications, and 25 bgy of jet fuel ₁.  The dwindling supply of domestic oil and U.S. dependence on foreign oil for our transportation needs is a major concern.  While continued exploration is bound to lead to discovery of new oil reserves (such as BP’s recent discovery of the Tiber well, a huge oil reserve in the Gulf of Mexico, such discoveries will become increasingly rare and are not likely to offset the increasing demand and the associated production cost for obtaining oil from increasingly hard-to-reach places.  Some encouraging trends are emerging, spurred by policy makers, consumers are moving towards higher efficiency vehicles and these trends are likely to continue as crude oil prices continue to rise. Additionally, hybrid gas-electric vehicles are penetrating the market for light duty applications and several manufacturers have announced plans to bring fully electric vehicles to the market place in the near future.  Despite this progress, gasoline needs will continue to be high for the foreseeable future due to vehicle fleet changeover dynamics, as it takes 12 years for one complete changeover. Additionally, hybrid or fully electric technologies are not well suited for heavy vehicles and especially jets, so demands for these fuels will most likely continue to rise.  Hence, there will be a societal need for liquid transportation fuels for the foreseeable future that is ideally suited with the existing fuel distribution and vehicle infrastructure.

Biofuels

Although there is considerable debate on the impact that first generation biofuels (corn ethanol) are having on food and feed prices, the overwhelming consensus among experts is that advanced biofuels will greatly lessen any effect on food and feed prices. By using non-food resources, advanced biofuels avoid any direct competition with food and feed supplies.  The only likely impact that advanced biofuels technology will have on food and feed prices will be land use competition.

Advanced biofuels vary in terms of technical maturity, as well as in ultimate volume production potential. Several advanced biofuel technologies will be discussed, primarily from the point of technology maturity, but also in terms of reasonable estimates for production capacities over the next 10 – 15 years.  All advanced biofuel technologies offset our petroleum consumption and at the same time reduce our carbon dioxide emissions.

Corn and Cellulosic Ethanol

Ethanol (C2H5OH) paved the way for integration of a mass-produced biofuel into the marketplace.  Current production of ethanol by fermentation of the starches from corn grain is a well established technology and accounts for almost all of the U.S.’s 9.4 bgy capacity.  Additional facilities to produce corn ethanol are planned for construction and, when completed, will increase the U.S. capacity to 14 bgy within several years.  The limitation to this technology is the availability of feedstock– corn grain.  Corn is an important feed and food commodity in the U.S. and studies suggest that we cannot produce more than 15 bgy of corn ethanol without significant, unacceptable, and lasting impacts on the economics of food products that depend of corn.  There is currently no other readily available starch- or sugar-based crop in the U.S. from which to ferment ethanol in large quantities, thereby limiting production potential, unless cellulosic biomass feedstocks can be utilized.

Cellulosic feedstocks are abundant and do not directly compete with food and feed needs, and thus, cellulosic ethanol has the potential to overcome the capacity hurdles that limit corn ethanol.  By developing both i) biochemical and ii) thermochemical conversion routes, cellulosic ethanol production can utilize essentially the entire biomass resource base available, which has been estimated at 1.3 billion tons/year by mid-century in the much referenced “Billion Ton Study” ₂.  If this resource base was converted to ethanol, the potential exists to displace over 50% of our current gasoline usage.

Significant technical progress has been made on increasing the economic viability of cellulosic ethanol, chiefly by reducing the costs of production. Department of Energy (DOE) performs a rigorous state-of-technology (SOT) assessment every year to estimate production costs for a commercial-scale plant based on emerging technologies being demonstrated at the national laboratories and in the industry.  The 2008 SOT results estimate a production cost for both biochemical and thermochemical pathways in the $2.20-$2.70/gallon range ₃.  Although this is currently higher than either gasoline or corn ethanol, given that cellulosic ethanol technology is still in a pre-commercial state, substantially more can be done to reduce costs.  In order to drive the initial deployment of cellulosic ethanol, several demonstration- and commercial-scale plants are planned for construction, many with DOE support.

Based on this potential, ethanol’s partial compatibility with the existing transportation fuel infrastructure, and significant achievements in both R&D and deployment, cellulosic ethanol should remain the cornerstone of near term U.S. biofuels development.  With a continued focus on cellulosic ethanol and continued progress on cost-centered research and deployment, our nation can soon realize the benefits of advanced biofuel technology.

Advanced Biofuels (“non-ethanol”)

While cellulosic ethanol has great promise for addressing our nation’s transportation needs, it does have some limitations.  Commonly cited limitations include: i) reduced energy content as compared to gasoline, resulting in consumers experiencing a mileage penalty in today’s vehicles when compared to gasoline, ii) it is not fully compatible with the existing transportation fuel infrastructure, and iii) ethanol is only suitable as a gasoline replacement and does nothing to address the need for diesel and jet fuels.  Therefore, development of advanced biofuels should be expanded into additional technologies and fuel options.  Biochemical and thermochemical technologies are under development that will allow for the production of a variety of biofuels.

Biochemical Conversion to Butanol and Hydrocarbons

In addition to the conversion of cellulosic materials into fermentable sugars (via enzymatic hydrolysis) for ethanol production, other alcohols can be produced through fermentation.  Butanol (C4H9OH), a member of the alcohol family, can be produced by a fermentation process similar to ethanol, and has certain advantages over ethanol when used as a fuel.  In particular, as compared to ethanol, butanol has a significantly higher energy density (though still lower than that of gasoline) and it is more compatible with the existing fuel infrastructure, because of its reduced tendency to absorb water and corrode pipes. However, butanol is more difficult to produce than ethanol, and the economics and technology remain well behind that of ethanol.  BP and DuPont are actively engaged in a bio-butanol development program in the United Kingdom.  Although, in the U.S., butanol is not yet out of the starting gate and will most likely be a minor contributor compared to ethanol in the near future, it does have long-term advantages over ethanol and should be pursued as part of a robust advanced fuel strategy.   

Another emerging technology is the biochemical conversion of sugars to directly produce hydrocarbon fuels that are substantially similar to current gasoline, diesel and jet fuels produced from crude oil.  These hydrocarbon fuels would essentially be completely compatible with the existing fuel infrastructure and have a higher energy density as compared to alcohols.  The engineered microorganisms used to produce hydrocarbons have a production rate similar to the rate of microbial ethanol production ₄.  Companies such as Amyris and LS9 are on the verge of commercializing this technology and plan to make a drop-in a replacement to diesel fuel using biochemical conversion of biomass-derived sugars by 2011 ₅.  Since the production of butanol and hydrocarbons from these technologies can potentially use the exact same resource base as ethanol production, the potential production volume is also quite large.

Thermochemical Conversion to Advanced Biofuels

While thermochemical conversion of biomass to ethanol is a key component of the current cellulosic ethanol effort, thermochemical conversion technologies also show considerable promise for synthesis of fuels beyond ethanol, which need to be equally supported and pursued.  If biochemical conversion is the “elegant” method of producing alcohols and hydrocarbons from certain biomass feedstocks, then thermochemical conversion can be considered the “Swiss army knife” method, attacking a wider range of feedstocks and producing a broader spectrum of fuels. At high temperatures and pressures, this method converts biomass to intermediate liquids or gases, which can then be synthesized into fuels by numerous proven (e.g. methanol-to-gasoline, Fischer-Tropsch) and emerging technologies.  Figure 1 shows the general relationship between temperature and required residence times for biological and chemical catalytic processes used for conversion of cellulosic biomass to fuels ₆.  The much larger range of operating conditions for biomass conversion processes using chemical catalysts has the potential to greatly improve the production rates, as reaction rates often increase exponentially with increasing temperature.  Emerging technologies from University of Wisconsin and Virent Energy Systems use liquid-phase chemical catalysis to produce hydrocarbons from carbohydrates and sugars _7,8, and could be envisioned for use in a hybrid bio-thermochemical conversion.  A reported partnership between Virent and Royal Dutch Shell aims to produce 100 million gallons/year of cost-competitive hydrocarbon fuels by 2016 ₅.  Since some of the thermochemical conversion approaches show considerable promise for producing hydrocarbon fuels similar to gasoline, diesel, and jet fuels, they are a means to lowering the barriers to commercialization, since they would effectively leverage the existing petroleum refining and fuel distribution infrastructure, thus not requiring the new construction of costly infrastructure.  Since thermochemical conversion technologies can essentially capture the entire feedstock resource base, their potential production volume is also quite large, and potentially larger than for biochemical conversion as thermochemical processes are generally more robust and flexible with regard to feedstock.

Figure 1: Comparison of operating temperatures and residence times typical for chemical and biological catalysts used for conversion of cellulosic biomass to fuels.  Biological catalysts (e.g. enzymatic hydrolysis, fermentation): <70 °C, 1 atm, 2-5 days. Chemical catalysts (e.g. gasification, Fischer-Tropsch): 100-1200 °C, 1-250 atm, 0.01 sec to 1 hour 6.

Summary
Advanced biofuels are a significant step in the right direction to addressing tomorrow’s transportation fuel potential and needs.  The current successful, goal-focused effort on producing cellulosic ethanol is on target towards achieving our nation’s immediate objective, to displace imported oil, reduce greenhouse gases, and minimize food and feed price impacts.  However, we need to accelerate and expand our existing advanced biofuel efforts to include other conversion options and fuels, beyond ethanol, to truly achieve the benefits that advanced biofuels offer.

About the Authors

Thomas Foust, M.S., Ph.D., P.E. is the Biomass Technology Manager at the National Renewable Energy Laboratory, the U.S. Department of Energy’s primary laboratory for R&D of renewable energy and energy efficiency technology.  Dr. Foust oversees NREL research to guide it towards accomplishing the Department of Energy’s mission in the biomass arena.

Matthew Yung, Ph.D. is a research engineer at the National Bioenergy Center, National Renewable Energy Laboratory specializing in catalysis and thermochemical conversion of biomass to fuels.

References
1. Energy Information Administration, http://eia.doe.gov/

2. R.D. Perlack et al., “Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-ton Annual Supply,” DOE/GO-102005-2135, Oak Ridge National Laboratory, Oak Ridge, TN (2005), http://feedstockreview.ornl.gov/pdf/billion_ton_vision.pdf

3. D. Humbird and A. Aden, “Biochemical Production of Ethanol from Corn Stover: 2008 State of Technology Model,” National Renewable Energy Laboratory (NREL) Technical Report, Golden, CO (August 2009) Report NREL/TP-510-46214, http://www.nrel.gov/docs/fy09osti/46214.pdf

4. S.K. Lee, H. Chou, T.S. Ham, T.S. Lee, and J.D. Keasling, “Metabolic engineering of microorganisms for biofuels production: from bugs to synthetic biology to fuels,” Current Opinion in Biotechnology 19 (2008) 556-563.

5. J.R. Regalbuto, “Cellulosic Biofuels – Got Gasoline?,” Science 325, (14 August 2009), 822-824.

6. National Science Foundation, “Breaking the Chemical and Engineering Barriers to Lignocellulosic Biofuels: Next Generation Hydrocarbon Biorefineries,” (March 2008), http://www.ecs.umass.edu/biofuels/roadmap.htm

7. E.L. Kunkes, D.A. Simonetti, R.M. West, J.C. Serrano-Ruiz, C.A. Gärtner, and J.A. Dumesic, “ Catalytic Conversion of Biomass to Monofunctional Hydrocarbons and Targeted Liquid-Fuel Classes,” Science (17 October 2008) 417-421.

8. P.G. Blommel and R.D. Cortright, “Production of Conventional Liquid Fuels from Sugars,” Virent Energy Systems white paper (25 August 2008), www.virent.com/bioforming/virent_technology_whitepaper.pdf