Conversion of Biomass to Fuel and Energy

by W.L. Harris, National Program Leader for Engineering and Energy, Agricultural Research Service, USDA, Beltsville, MD, and Howard N. Rosen, Energy Coordinator, Forest Service, USDA, Washington, DC.

In 1991, biomass provided over 3.5 quads (a quad is a measure of energy that equals 1 quadrillion British Thermal Units, or Btu's) of the energy used in the United States. This was equivalent to the energy contained in 604 million barrels of oil which equals about 22 percent of annual U.S. oil imports.

Of this biomass energy:

Half was used for direct heating applications in the industrial sector. For example, the wood and paper products industry uses residues from its operations to supply almost all heating and about half of the electricity needed in making their products.

One-quarter was used for residential heating using wood and wood residues.

Wood and biomass from agricultural processing operations and municipal solid wastes were used to generate three-quarters of a quad of electrical energy for utility companies.

One billion gallons of ethanol, approximately 0.1 quad made primarily from corn, was used in transportation fuels.

Some of the technologies for using biomass for energy are mature, for example, burning wood for heating and generating electricity, and making ethanol from sugar and starch. Other technologies are just emerging, for example, converting cellulosic material from agricultural and forestry crops and residues into ethanol, and making biodiesel from oilseed crops and animal fats.

The potential for increasing the production of liquid fuels and energy from biomass is very large. Some estimates of this energy potential are as high as 26 quads per year. A significant expansion will depend, at least partially, upon the continued improvements in mature technologies and the development of cost-effective technologies for converting cellulosic material into ethanol, and oil from oilseed crops into biodiesel. Demand for these specific fuels and the costs of competing fuels will also influence the rate of expansion.

Ethanol From Corn New Technologies

Although interest in ethanol as an automotive fuel began in the early 1900's, the modern fuel ethanol industry began with the oil shortage in the early 1970's. It has grown from virtually zero production to a billion gallons a year. More than 95 percent of the fuel ethanol produced in the United States uses corn as the feedstock. The remainder is produced from molasses; other grains such as milo, wheat, and barley; and industrial and food processing waste products such as potato culls and cheese whey.

In 1991, more than 370 million bushels of corn were used to produce ethanol. Corn is used because of its availability and high starch content. The two main methods use proven wet- and dry-milling grain processing technologies. Dry milling is based on traditional technology for the manufacture of potable alcohol, while the wet-milling process is based on the refining of corn to starch and fructose. Except for the initial separation process, the technology for the conversion of the starches to fuel ethanol is generally the same for both types of milling methods.

More than 95 percent of the fuel ethanol produced in the United States is produced from corn. The remainder is produced from molasses, milo, wheat, barley, and industrial and food processing waste products, such as potato culls and cheese whey. USDA IA-2851-19A

Dry Milling. In the dry-milling process, corn is milled and mixed with steam and enzymes to liquefy the starch component. The next step in the process converts the liquefied starch to sugars by adding additional enzymes. The sugars are fermented to ethanol using yeast. The mixture leaving the fermenter is distilled into 190-proof ethanol (95 percent ethanol and 5 percent water) and residues, which are called whole stillage. The ethanol is dehydrated to produce the 200-proof fuel grade. A process developed at Purdue University with USDA support uses corn grits to adsorb the water in the dehydration phase and reduces the cost of production by approximately 3-4 cents per gallon. Water is removed from the whole stillage by mechanical and drying processes to produce the distillers dried grains with solubles (DDGS), which is used as an animal feed.

Using current technology, dry mills can produce 2.6 gallons of fuel-grade ethanol and approximately 16.5 pounds of DDGS per bushel of corn. The carbon dioxide (CO₂) that is produced may also be collected from the fermentation tanks for use in the beverage and food processing industry. The three products are produced in approximately equal amounts, or one-third each of the initial weight of the bushel of corn.

Wet Milling. In the wet-milling process, corn is separated into the germ, fiber, gluten, and starch components. The first step in the separation process involves soaking or steeping the corn in a mixture of water and sulfur dioxide. The soft kernels are then milled to separate the germ from the starch. The germ is dried and the oil removed. The remaining slurry is screened to remove the fibers and then centrifuged to separate the gluten. The gluten is dried to produce corn gluten meal, which is used as an animal feed. The starch is liquefied and converted to sugar, which is fermented, distilled, and dehydrated to produce fuel-grade ethanol. The thin stillage from distillation is combined with water used for steeping and the solids removed by evaporation and added to the recovered fiber to produce corn gluten feed. The corn gluten feed is exported, primarily for dairy cattle feed.

Ethanol yields from wet milling are slightly lower than yields from dry-milling plants. In addition to about 2.5gallons of ethanol from a bushel of corn, there are about 1.7 pounds of corn oil, 3 pounds of corn gluten meal (60 percent protein), 13 pounds of corn gluten feed (21 percent protein), and 17 pounds of carbon dioxide.

Dry-milling plants generally require lower initial investment than comparably sized wet-milling plants. However, the higher cost of the wet-milling plant may be offset by the higher value of the coproducts produced.

Promising Newer Technologies. Improvements have been made during the past 10 years in reducing the energy consumption for processing the corn into ethanol and in increasing the efficiency in converting the starch to sugar and fermenting the sugar into ethanol. In 1981, the energy required for processing exceeded 120,000 Btu's per gallon of ethanol produced, approximately 40,000 Btu's per gallon more than the energy content of a gallon of ethanol. In 1991, the energy consumption for processing averaged only 43,000 Btu's per gallon. The reductions in energy use have resulted from efficiency improvements throughout the entire plant, ranging from the cogeneration of electric power and steam to the use of corn grits and molecular sieves to remove the last 5 percent of the water in the ethanol.

Improved enzymes for converting the starch to sugars permit more efficient conversion at lower enzyme costs. New yeasts for fermentation have resulted in shorter fermentation times, higher levels of ethanol in the fermenter, lower residual sugars, and more sugars being converted. Higher ethanol concentrations have resulted in lower processing costs, as less water needs to be removed.

One new approach developed at a land-grant university for preparing corn to improve the efficiency of ethanol production is to inject gas to reduce the steeping time; this treatment of the corn kernels facilitates the separation of starch and protein from the other components, reducing the time required from approximately 40 hours to 8 hours.

The current method for converting the grain starch to glucose sugar involves a series of processes using enzymes. Research using membrane technology has shown that the time can be reduced from approximately 48 hours to about 5 hours.

Using the new tools of biotechnology, researchers are developing new yeasts that are both more resistant to the concentration of ethanol in the fermenter and more heat-tolerant. As indicated earlier, the higher the concentration the less water will have to be removed. Less energy will also be required for distillation because the beer broth will enter at a higher temperature.

New technology is also under development that will improve the removal of both the water from the ethanol and the solids from the water that has been removed from the DDGS or corn gluten feed. An open heat pump is used to take vapor from the top of a distillation column and mix it with the incoming beer broth in order to reduce the size of the heating Plant and the overall energy requirements. The use of molecular sieves and pervaporation technology, which involves membranes operating in a vacuum, is being tested by a number of universities and private companies. These technologies could reduce the energy currently required for both distillation and drying of the DDGS and corn gluten feed.