Research aims at cutting costs of turning green plants into fuels and chemicals.
The ethanol sector of the U.S. energy market has taken a wild ride from cottage industry to a significant source of renewable fuels for automobiles. Because today's ethanol is produced mostly by fermentation of cornstarch, the emergence of more than 100 American ethanol distilleries is viewed by many as a success story for U.S. agriculture. Despite this expansion of corn-fed ethanol production, some advocates of a long-term solution to alternative fuel needs take a more somber perspective. A valuable source of human food and animal feed, corn ultimately cannot be grown in large enough quantities to supply America's ethanol needs. Moreover, considerable doubt exists as to whether corn-based ethanol represents the most cost-efficient bioenergy solution. Partly as a result of these questions, the U.S. government is showing increasing interest in an alternative to corn. The new plan calls for developing a sustainable industry that would produce ethanol fuel and biologically based chemicals from cellulose-containing trees and grasses using highly efficient, second-generation, fermentation facilities.
Benefits of biofuels
In the Department of Energy's vision, these biofuels and bio-based co-products would be produced in regional "biorefineries" to replace petroleum-derived gasoline for cars and petrochemicals used to make consumer products. Ideally, such a strategy would allay American concerns about soaring gasoline prices, the long-term availability of foreign oil, air pollution and climate change while creating jobs in rural communities and improving the U.S. balance of trade.
Today 4.8 billion gallons of ethanol—about 3 percent of the American fuel supply for cars—are used annually to make E-85 and E-10, reformulated blends of ethanol and gasoline. A clean-burning renewable fuel, ethanol helps gasoline burn more completely, significantly reducing tailpipe emissions that threaten human health and the environment.
Unlike gasoline, ethanol can be produced from rapidly growing "bioenergy crops" that store carbon absorbed from the atmosphere. Burning a gallon of ethanol barely boosts the total atmospheric carbon that threatens climate change. The reason: the carbon dioxide emitted is almost equal to the amount captured by cellulosic crops used to generate the next gallon.
Studies show that almost one-third more energy is created than consumed by ethanol production, including the energy required for all manufacturing, distribution and agricultural "tractor passes"—planting, fertilizing, and harvesting the corn used as the raw material, or feedstock.
The United States now imports 60 percent of the petroleum Americans use. A government goal is to produce 60 billion gallons of ethanol by 2030 to displace 30 percent of the gasoline needed for personal transportation. "Meeting this goal would require about 500 biorefineries that each produces 120 million gallons," says Jonathan Mielenz, leader of bioconversion research in ORNL's Biosciences Division. "Corn will certainly be part of the mix, but success will also require cellulosic energy crops. The National Corn Growers Association estimates that 17 billion gallons of ethanol could be produced from corn. The remaining two-thirds of the ethanol will likely come from biomass such as switchgrass and wood chips from harvested hybrid poplar trees."
Ethanol produced from cellulosic biomass would cost about $2.26 per gallon if available on the market today. Another government initiative aims to develop a profitable biorefinery industry—based on new technologies, cellulosic energy crops and value-added co-products—that seeks to lower that cost to $1.07 a gallon by 2012.
In DOE's vision, a biorefinery is a facility that integrates biomass conversion processes to produce fuels, power and chemicals from biomass. The concept is similar to petroleum refineries, which produce multiple fuels—gasoline, jet fuel, diesel fuel and heating oil—and also provide oil to chemical companies that produce petrochemicals for consumer and industrial products. Industrial biorefineries have been identified as the most promising route to the creation of a domestic bio-based industry.
Such biorefineries' profit potential centers on their ability to make affordable ethanol and chemicals from feedstock; produce other chemicals, process heat and electrical power from lignin wastes; and reduce pollution. Maximizing the value in biomass feedstock should enable a biorefinery to produce not only biofuel but also at least one low-volume, high-value "specialty" chemical or one low-value, high-volume "bulk" chemical. The high-value products boost profitability, and the high-volume fuel helps meet national energy needs while lowering fuel costs and reducing oil imports. Added benefits would be the production of power for the biorefinery itself and for the electrical grid, thus lowering energy costs, as well as the avoidance of greenhouse gas emissions.
ORNL's corn connection
Tennesseans have long been familiar with corn, from the hills where illegal moonshine was once produced, to Oak Ridge National Laboratory, one of the nation's leading centers of government-supported, scientific expertise in ethanol production.
Brian Davison, chief scientist for systems biology and biotechnology at ORNL, has helped lay the groundwork for such research through considerable experience producing chemicals from corn. Working with bioreactors, 10-ft.-tall cylindrical columns containing liquids and beads laden with bacteria, Davison and his colleagues demonstrated the production of ethanol, acetic acid, butanol and succinic acid from corn and other feedstock.
In the 1990s Davison's group and researchers from three other DOE national laboratories demonstrated an award-winning biological method for converting corn to succinic acid at very high yields. Succinic acid is a biologically based chemical that can replace petrochemicals for use in plastics, clothing, paints, inks, food additives and automobile bumpers.
In 1996 ORNL and the three labs signed a $7 million agreement with Applied CarboChemicals, Inc., a Pennsylvania specialty chemical company. The process was licensed to the company, which along with the four national laboratories received an R&D 100 Award from R&D magazine in 1997 for being one of the year's top technologies. In 2003 Diversified Natural Products in Scottville, Tenn., was formed from the merger of Applied CarboChemicals and two other companies. DNP will partner with Agro Industrie Recherches et Développments of Pomacle, France, to build a production plant in France. "The French fermentation facility," Davison says, "will produce 'green' succinic acid, which has enormous global demand for everything from industrial solvents and biodegradable polymers to airport runway de-icers."
ORNL also stands to benefit from the neighboring Tate & Lyle facility. Headquartered in England, the company operates the state's only corn-fed ethanol distillery in conjunction with a high fructose corn syrup plant. Tate & Lyle is opening nearby a new $100 million plant in Loudon, Tenn., with partner DuPont that will produce 1,3 propanediol, or PDO, through use of the companies' jointly developed industrial microorganism that ferments glucose extremely efficiently from cornstarch. Bio-PDO™ is a fiber and key ingredient in the production of DuPont™ Sorona®, the newest DuPont polymer that could replace petroleum-derived nylon for uses such as clothing, carpeting and plastics. At least 30 percent less energy is required to produce Bio-PDO than petroleum-based PDO. The company has stated that annual production of 100 million pounds of Bio-PDO will free up enough oil to produce 10 million gallons of gasoline.
Reinhold Mann, ORNL's associate laboratory director for Biological and Environmental Sciences, says that Tate & Lyle is interested in the outcomes of ORNL's bioenergy research and might collaborate with the Laboratory in the future.
ORNL and its partners are focusing on new methods and technologies for economically converting poplar tree and switchgrass feedstock into fuels. Besides reducing the cost of producing and harvesting the feedstock, the collaborative effort also strives to improve the efficiency of breaking down the feedstock's cellulose and fermenting the resultant sugars.
The plant-based feedstock consists of cellulose (31-49 percent), hemicellulose (16-26 percent) and lignin (19-26 percent). Cellulose in plant cell walls—the most abundant biological material and source of sugar on earth—consists of linked chains of thousands of glucose molecules. This glucose polymer present in corn and cellulosic trees and grasses is the material that is broken down by microbial enzymes into sugars, which are then fermented into ethanol in distilleries and biorefineries.
Cellulose contains six-carbon sugars—glucose. Hemicellulose, a gummy mixture of polysaccharides, contains five-carbon sugars—xylose. Lignin, which makes the plant rigid and resistant to compression, must be removed by a pretreatment consisting of heat, acids and enzymes to make the cellulose accessible to enzymatic breakdown, or hydrolysis, to produce sugar. Microbes, such as yeast, produce enzymes that ferment the sugars to ethanol, which is then separated from the microbes and water by distillation.
The biorefinery's three main processes are pretreatment, cellulose hydrolysis and fermentation of sugar to ethanol and carbon dioxide. To make the biorefinery profitable, researchers are seeking efficiencies in each of the processes. One way to cut costs is to reduce the amount of microbial enzymes needed by genetically maximizing the efficiency of microorganisms known to be effective performers of pretreatment, cellulose hydrolysis and fermentation to ethanol.
One microbe of interest can carry out two of the three processes. Mielenz and other ORNL researchers are examining this versatile bacterium in collaboration with Lee Lynd, a professor at Dartmouth College with whom Mielenz worked, both at the school and in a company the two started. For at least 15 years Lynd has been studying Clostridium thermocellum, which can both degrade pure cellulose to make sugar and ferment the sugar to produce ethanol.
"We have studied how this multitalented microbe makes its own set of cellulase enzymes, grabs hold of cellulose, tears it up, accesses the glucose and ferments it to ethanol," Mielenz says. "Part of the biology of Clostridium thermocellum is the generation of its own cellulose-degrading enzymes, reducing the fermentation cost because these enzymes do not have to be introduced separately along with added yeast. The bacterium makes more than 20 different cellulases and hemicellulases and other enzymes outside its cell."
Two challenges remain. Biologists do not yet understand how well the microbe will break down pretreated biomass, which is impure cellulose. The microbe also produces impure ethanol—alcohol contaminated with acetic acid and lactic acid. Lynd has led an effort to identify and knock out the genes in Clostridium thermocellum that produce the acids. ORNL researchers have produced whole-genome microarrays of Clostridium thermocellum, allowing them to identify which of the microbe's genes are turned on during fermentation, actively sending instructions via messenger RNA to the bacterial cell to produce encoded proteins that are enzymes.
"Eventually, we will understand how Clostridium thermocellum responds when genes are knocked out in an effort to make the microbe produce pure ethanol free of acetic acid and lactic acid," Mielenz says. "We will also study the gene expression that results when genes are modified in the hope of making the bacterium efficiently digest pretreated, less pure cellulose."
From field to fermenter
ORNL has made contributions to the economics of the bioenergy process because of one group's ability to analyze resource data to determine the locations of the nation's ethanol feedstocks and the costs of bioconversion. Bob Perlack, a resource economist in ORNL's Environmental Sciences Division and lead author of the "Billion Ton" study (see "Billion Ton Study" article), continues to analyze biomass feedstock resources considered to be in the supply chain from the field to the fermenter.
An ORNL agricultural engineer, Shahab Sokhansanj, has developed the Integrated Biomass Supply Analysis and Logistics (IBSAL) model. Using Perlack's data, this powerful dynamic model simulates collection, storage and transport operations by optimizing a supply chain for just-in-time delivery of feedstocks to a biorefinery at the least possible cost while meeting quality and sustainability specifications. Recent advances in computational tools have enabled the construction of mathematical models that ORNL researchers are using to simulate supply and transportation of agricultural biomass.
From optimizing microbial fermenters of ethanol to developing models on feeding the process, ORNL and its bioenergy partners are helping lay the groundwork for an economically sustainable biorefinery industry.—Carolyn Krause
Web site provided by Oak Ridge National Laboratory's Communications and External Relations