Fuels derived from cellulosic biomass—the fibrous, woody, and generally inedible portions of plant matter—offer an alternative to conventional energy sources that supports national economic growth, national energy security, and environmental goals. Cellulosic biomass is an attractive energy feedstock because supplies are abundant domestically and globally. It is a renewable source of liquid transportation fuels that can be used readily by current-generation vehicles and distributed through the existing transportation- fuel infrastructure. Ethanol from corn grain is an increasingly important additive fuel source, but it has limited growth potential as a primary transportation fuel.
Achieving the ambitious goal of displacing 30% of the 2004 gasoline demand with biofuels by 2030 will require a rapid expansion of the fuel ethanol industry. Annual U.S. production will need to increase from about 4 billion gallons of corn grain ethanol to about 60 billion gallons per year from a variety of plant materials.
An annual supply of roughly a billion dry tons of biomass will be needed to support this level of ethanol production. A recent report by the U.S. Department of Agriculture [USDA] and DOE finds potential to sustainably harvest more than 1.3 billion metric tons of biomass from U.S. forest and agricultural lands by mid-21st Century (previous post).
The report found that only 6% of the 1.36 billion metric tons would come from grain, and since only about a billion tons are required, none of the feedstock need come at the expense of food producing acreage.
Research and technology development described in this roadmap will occur in three phases to enable industry to meet the 2025 and 2030 goals.
In the research phase, within the first 5 years, an understanding of existing feedstocks must be gained to devise sustainable, effective, and economical methods for their harvest, deconstruction, and conversion to ethanol. Research is centered on enzymatic breakdown of cellulosic biomass to component 5- and 6-carbon sugars and lignin, using a combination of thermochemical and biological processes, followed by co-fermentation of sugars to specified end products such as ethanol. Processes will be integrated and consolidated to reduce costs, improve efficacy, reduce generation of and sensitivity to inhibitors, and improve overall yields and viability in biorefinery environments.
The technology deployment phase, within 10 years, will include creation of a new generation of energy crops with enhanced sustainability, yield, and composition, coupled with processes for simultaneous breakdown of biomass to sugars and co-fermentation of sugars via new biological systems. These processes will have enhanced substrate range, temperature and inhibitor tolerance, and the capability to function in complex biorefining environments and over time scales that are economically viable.
The systems-integration phase, within 15 years, will incorporate concurrently engineered energy crops and biorefineries tailored for specific agroecosystems. Employing new and improved enzymes for breaking biomass down to sugars as well as robust fermentation processes jointly consolidated into plants or microbes, these highly integrated systems will accelerate and simplify the end-to-end production of fuel ethanol. In many ways, these final-phase technologies will strive to approach theoretical conversion limits. The new generation of biotechnologies will spur engineering of flexible biorefineries operable in different agricultural regions of the country and the world.
Three of the areas where much of the research will be conducted are described further in the following paragraphs.
Feedstocks - Plants intended for biomass production and downstream processes involving conversion to sugars and, ultimately, ethanol will be understood and designed as a system. New breeds of energy crops will be introduced with enhanced sugar content and optimized cell-wall structures for processing, including minimization of lignin and inhibitor precursors. Plant domestication and sustainable agroecosystems based on perennials engineered to increase yield, productivity, and tolerance to such stressors as drought and salinity will reach a mature state. Multiple crops will be developed for various regional and global agroecosystems.
Deconstruction - This phase will result in deployment of improved pretreatment procedures and saccharifying enzymes with enhanced catalytic rate and substrate specificity, a broader range of applications, and reduced inhibitor production. Improved understanding of cell-wall recalcitrance and enzyme action will provide design specifications for new energy crops. Advanced high throughput biological and chemical tools will be available to diagnose and manipulate enzyme-substrate interactions. Improved biocatalysts with desirable traits can be rationally designed for specific feedstocks and incorporated into molecular machines such as cellulosomes.
Fermentation and Recovery - New strains of industrial-processing organisms with such novel capabilities as co-fermentation of C-5 and C-6 sugars and high tolerance to inhibitors, alcohol end product, and temperature will contribute to a more energy and product efficient bioprocess. Systems biology investigations will have produced a predictive understanding of cellular metabolism and regulatory controls in key fermentation microbes. This knowledge will serve as a foundation for rational development of new strains with consolidated subsets of pretreatment, hydrolysis, and fermentation capabilities. High-throughput biological and chemical tools, including computational modeling for rapid analysis and manipulation in the laboratory and in production environments, will be available.
The report considers, as the state of the art or the base case, ethanol production based on concentrated acid pretreatment followed by hydrolysis and fermentation. The advanced process they consider as their goal incorporates advanced biocatalysts that augment or replace thermochemical (acid pretreatment and/or steam explosion) methods to reduce the severity and increase the yield of pretreatment followed by direct conversion of cellulose and hemicellulosic sugars in the fermentation step.
A few companies are still using acid pretreatment, but what I see as state of the art is practiced by Iogen, consisting of these steps: steam explosion, then multistep enzyme treatment followed by a fermentation system that converts both C6 and C5 sugars into ethanol, which is similar to the process being used by Abengoa Bioenergy in their plant which is scheduled to start up later this year. Researchers Lynd and Zhang are developing microorganisms that can simplify the process by combining the enzyme hydrolysis and fermentation into a single faster reacting step. They have licensed their technology to startup Mascoma.
I have identified 12 companies that are either building, have firm plans to build or are developing cellulosic ethanol processes. It doesn't seem to me that it will take until 2012 for the first plants to be built or 15 years for the advanced plants to be built. While DOE has, and will no doubt continue to contribute technology to celosic ethanol processes, industry has now taken control and will lead the way to commercial cellulosic ethanol production. I consider the money DOE will spend as worthwhile, it will do research industry can't afford and it will move up the date that ethanol production doesn't need subsidies. Compared to the billions spent on hydrogen R & D it is a bargain.
Breaking the Biological Barriers to Cellulosic Ethanol: A Joint Research Agenda, Biofuels Joint Roadmap, Office of Science and Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, June 2006
