By Dr. Larry P. Walker
Professor, Department of Biological and Environmental Engineering, Cornell University

With a new President and Congress stepping up to the plate to address energy and climate change issues there will likely be more attention and resources directed toward developing renewable energy resources to meet a portion of future domestic energy needs.  Bioenergy will be one of the strategic renewable energy options on the table as it has been for the past 30 years.  During this time we have seen interest in and funding for bioenergy and other renewable energy options ebb and flow depending on the price of oil, geopolitical concerns, and more recently its relevance to climate change mitigation.  Unfortunately, this lack of sustained focus and funding has compromised our ability to deploy good science and engineering to resolve some of the scientific, technical and environmental challenges that must be addressed if we are to develop sustainable bioenergy solutions.   In the sequential paragraphs, I will briefly describe some of the scientific and engineering challenges to biofuels development. 

Biomass Production
For bioenergy to play a substantial role in meeting domestic energy needs, agriculture and forestry must be able to develop and deliver plant biomass on a more massive scale.  In a recent analysis conducted by the United States Department of Energy and the United States Department of Agriculture it was determined that an additional 1.3 billion tons of biomass can be produced in the USA for bioenergy production. ¹   Although traditional crops like corn and maize represent a portion of this inventory, the bulk of the inventory must consist of dedicated energy crops if we are to meet a substantial part of the nation’s energy needs.   Of particular interest is the development of lignocellulosic materials such as switch grass and other warm and cool season grasses.   Lignocellulosic crops, such as switch grass and corn stover, can theoretically yield 80 to 100 gallons of ethanol per dry ton of materials.  Thus, improving biomass yield is an important goal if bioenergy is going to be produced on a massive scale.  Unfortunately, many of the lignocellulosic crops that are under consideration have not been subjected to intense breeding activities similar to those that give us high yield corn, wheat and rice. Thus, there exists the potential for improving yields through plant genomics and breeding programs.  Also, desirable traits, such as chemical composition changes and lower water and chemical inputs, can be engineered into these dedicated energy crops to yield more sustainable plant biomass production.  There is work underway to diversify the portfolio of dedicated energy crops.  For example, there is interest in developing photocycle-insensitive and cold tolerant energy sorghum (see photo right: Cornell energy sorghum field trials, courtesy of Dr. Stephen) by exploiting the molecular diversity of sorghum and maize in natural populations found around the world.  The goal of these prospecting activities is to expand the portfolio of dedicated energy crops that can be deployed using sustainable cultivation practices such as minimum tillage and poly-cultivation to meet the increased demand for plant resources for bioenergy production.

Feedstock Logistics
Given the seasonal nature of feedstock production and the economic imperative to fully utilize invested capital in a biorefinery, very robust and highly coupled feedstock logistic subsystems must be developed to meet national bioenergy goals.  Feedstock logistic subsystems must be able to deliver biomass feedstocks to energy conversion subsystems 350 days a year.  A key limiting factor in developing feedstock logistic subsystems is the relatively low bulk density of baled grasses, which are 30 to 40% of that of shelled corn.  This places economic limitations on how far these materials can be transported.  There are efforts underway to address this limitation through the development of advanced biomass densification techniques such as briquetting, cubing and pelletizing that are yielding bulk density approaching that of shelled corn; however, additional research and development is needed if perennial grasses are to be part of the bioenergy resource base.  In addition, feedstock logistic subsystems must be “engineered” to minimize field and storage carbohydrates losses, which can be as much as 25 to 30%, and to minimize and control modification in the physical/chemical structure of the feedstock as it moves through various harvesting, processing and storage activities. Storage options range from windrowing of round bales to ensilage of chopped grasses.   Thus, a whole range of material handling, processing, storage and transportation equipment and systems must be put in place to deliver feedstock in a robust and sustainable manner.

Conversion Technology
Eventually, plant biomass must go through a set of thermochemical or biological conversion processes to deconstruct the biomass into simpler compounds and to convert these compounds into the desired fuel. Both the thermochemical and industrial biotechnology approaches are being developed as part of the national biofuels initiative. ²  Thermochemical conversion processes such as pyrolysis and gasification that have been used to convert coal into liquid fuels and other energy products continue to be adapted for plant biomass.  Advancements in chemical catalysts offer the potential for improving the efficiency of these thermochemical conversion processes.  Industrial biotechnology, the focus of my research group, seeks to exploit the major scientific revolution that has occurred in modern biology, such as genomics, proteomics, systems biology and nanobiotechnology, to develop improved enzymatic and microbial processes for deconstructed plant cell wall carbohydrates into simple sugars and the subsequent fermentation of these sugars to ethanol, butanol and other biofuels.  Systems biology is focused on understanding and modeling interconnected biological components that are the networks of life – a system engineering approach to biology.  For example, the yeast Saccharomyces cerevisiae is an old and familiar fermentative microorganism that has long been a model eukaryote for genetic and metabolic study.  In-silico reconstructions of genome-scale metabolics have been constructed and validated, ³ and are being deployed to address a number of important questions regarding bioenergetics that underlie biomass utilization, lipid metabolism, and biofuel synthesis.  Advanced imaging systems and nano-scale confinement methods are being deployed to obtain high temporal and spatial resolution of the binding and surface mobility of fluorescently-labeled cellulases and other enzymes (see photo left: Fluorescently-labeled cellulases) on plant cell-wall materials; thus, expanding our understanding of how these molecular machines depolymerize plant material. ⁴

System Engineering
Bioenergy development presents a set of major “system engineering” challenges.  Feedstock production, logistics and conversion and distribution subsystems must be integrated seamlessly into sustainable and robust energy conversion systems (see photo right).

In addition, within each of these subsystems there are a number of state-of-the-art and evolving technologies and processes that can be coupled in a variety of ways that defines the subsystems energy, material and monetary flows, and these flows are used to assess economic and environmental sustainability performance.  There are a variety of network topologies that one can imagine, and assessing the economic and environmental sustainability of these topologies is important to the development of viable bioenergy systems.  As bioenergy systems evolve so does the complexity and diversity of the interconnecting networks between feedstock production challenges, aggregation and logistics market opportunities, scale and location optimization of conversion plants for different regional feedstocks, and environmental, economic and social issues assessments. There is much complexity and diversity that will need to be managed with the evolution of bioenergy systems and the challenge for universities and colleges today is to keep pace in training today’s engineers to address these complex moving challenges in an emerging industry.

¹ Perlack, R. D. , Wright, L.L., Turhollow, A.F., Graham, R. L., Stokes, B. & Erbach, D.C.., 2005.  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).

² Biomass Research and Development Board.  2008.  National Biofuels Action Plan, http://www.brdisolutions.com.

³ Duarte, N. C., Herrgard, M. J. & Palsson, B. O. Reconstruction and validation of Saccharomyces cerevisiae iND750, a fully compartmentalized genome-scale metabolic model. Genome Research 14, 1298-1309 (2004)

⁴ Moran-Mirabal, J. M.,  Santhanam, N., Corgie, S. C., Craighead, H. G., and Walker, L. P.  2008.

Immobilization of Cellulose Fibrils on Solid Substrates for Cellulase Binding Studies through Quantitative Fluorescence Microscopy.  Biotechnol. Bioengineering, 101: 1129-1141.

Photos:

Biomass Production
Courtesy of Dr. Stephen, Cornell energy sorghum field

Conversion Technology
Courtesy of Cornell Laboratory, Fluorescently-labeled cellulases

Systems Engineering 
(a) and (b) courtesy of Matthew McArdle, President, Mesa Reduction Engineering and Processing, Inc, (c) courtesy of Gary S. Keller, Videographer & Editor, Capture It On Video,Inc, (d) courtesy of the Biofuels Research Laboratory