Bioenergy: An Overview
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Introduction
The energy stored in biomass (organic matter) is called bioenergy. Bioenergy can be used to provide heat, make fuels, and generate electricity. Wood, which people have used to cook and keep warm for thousands of years, continues to be the largest biomass resource. Today there are also many other types of biomass we can use to produce energy. These biomass resources include residues from the agriculture and forest industries, landfill gas, aquatic plants, and wastes produced by cities and factories.
Because they come from organic matter, biomass resources are renewable. For example, many biomass resources are replenished through the cultivation of fast-growing trees and grasses. As these trees and grasses grow, they remove carbon dioxide—a major greenhouse gas—from the atmosphere. This is important because bioenergy, like fossil fuels, can produce carbon dioxide. However, the net emission of carbon dioxide from bioenergy will be zero as long as plants continue to be replenished.
Today, we depend on biomass to provide about 3 to 4 percent of our energy in the United States. And we continue to expand our use of bioenergy. We're even learning more about how to produce the same high-quality materials and chemicals from biomass, such as those that presently come from petroleum.
Biopower
Hundreds of U.S. power plants use biomass resources to generate about 65 billion kilowatt-hours of electricity each year. The wood and paper products industries generate and use about two-thirds of this power. Solid wastes from cities fuel most of the remaining biopower plants, providing enough electricity to meet the needs of nearly 7 million Americans.
Biopower plants come in all sizes. Today's biopower plants have a combined capacity of about 10.3 gigawatts, which is about 1.4 percent of our nation's total electrical generating capacity. However, with better technology and expanded use of biomass resources, the nation could generate as much as four-and-a-half times more biopower by 2020.
Of all the forms of renewable energy, only hydropower produces more electricity than bioenergy does. Like hydropower, biopower is available 24 hours a day, seven days a week. Other forms of renewable energy, such as solar or wind power, have lower availability since they are produced only when the sun shines or the wind blows.
Several types of biopower systems are currently in use or under development. These systems include direct combustion, cofiring, gasification, and small modular systems.
Direct Combustion
Direct combustion involves the burning of biomass in a boiler to produce steam. The pressure of the steam then turns a turbine attached to an electrical generator, which makes electricity. Coal-fired power plants employ similar technology but use fossil fuel in their boilers. Most of today's biopower plants use a direct combustion system. Researchers are evaluating other advanced processes that are even more efficient than direct combustion.
Cofiring
Cofiring systems can burn up to 15 percent biomass when mixed with coal in some boilers. Cofiring biomass with coal reduces emissions and produces fewer of the chemicals that cause acid rain. Many existing coal plants could use a cofiring system with only a few modifications. Therefore, this system has a significant potential for growth in the near future. To make cofiring biomass more attractive to power companies, researchers are investigating improvements to the cofiring process and better technologies for minimizing emissions.
Gasification
Engineers are developing new technologies to produce biogas from biomass. Biogas consists of methane (found in natural gas) together with hydrogen, and other gases. Researchers are learning how to produce higher quality biogas by studying coal gasification systems. Some new gasification technologies make biogas by heating wood chips or other biomass in an oxygen-starved environment.
A second method for making biogas is to let landfills do the work. As paper and other biomass decay inside a landfill, they naturally produce methane. Methane can be recovered from landfills by drilling wells into the landfill and piping the gas to a central processing facility for filtering and cleaning. Clean landfill gas is then ready to fuel a biopower plant or help heat a building.
Biogas can be burned (or co-fired) in a boiler to produce steam for electricity generation. Biogas can also fuel gas turbines or combined-cycle generation systems. In a combined-cycle system, pressurized gas first turns a gas turbine to generate electricity. Then, the waste gas from the gas turbine is burned to make steam for additional power production.
Pyrolysis
Researchers are also investigating a smoky-colored, sticky liquid that forms when biomass is heated in the absence of oxygen. Called pyrolysis oil, this liquid can be burned like petroleum to generate electricity. Petroleum, however, is almost never used any more to generate electricity. There's a greater need to use petroleum as a source of gasoline, heating oil, and petrochemicals. Because pyrolysis oil can also be refined in ways similar to crude oil, it may also be more valuable as a source of biofuels and biobased products than for biopower generation. Unlike direct combustion, cofiring, and gasification, this technology is not yet in the marketplace.
Modular Systems
Researchers are particularly interested in improving small systems sized at 5 megawatts (MW) or less. These so-called modular biopower systems can use direct combustion, cofiring, or gasification for power generation. They are well suited for generating biopower from locally grown resources for small towns, rural industries, farms, and ranches.
Modular systems may be a good choice where power lines are not available. Clusters of modular biopower systems in rural areas may eradicate the need for power companies to build larger, more expensive power plants.
Biofuels for Transportation
Biomass is the only renewable source of transportation fuels. These renewable fuels, called biofuels, produce fewer emissions than petroleum fuels. Biofuels also can help us reduce our dependence on foreign sources of fossil fuels. We can open up foreign markets for U.S. products and technologies. And, we stimulate growth in industry and in rural areas, making farming and forestry more profitable.
Ethanol
Fuel ethanol is a form of the alcohol found in wine and spirits, but rendered unfit for drinking through the addition of a small amount of gasoline or other denaturant. Industry currently makes ethanol from the starch in grains—such as wheat, corn, or corn byproducts—in a process similar to brewing beer. Each year, we blend more than 1.5 billion gallons of ethanol with gasoline to improve vehicle performance and reduce air pollution.
Most gasoline blends contain about 10 percent ethanol and 90 percent gasoline. This mixture works well in cars and trucks, those you see on the road everyday, designed to run on gasoline. In addition, fuel containing 85 percent ethanol is available, primarily in the Midwest. This fuel, called E85, can be used in flexible fuel vehicles. Flexible fuel vehicles can run on either E85, straight gasoline, or any mixture of the two. Each year, automobile manufacturers produce more than 700,000 flexible fuel vehicles.
Researchers are investigating technologies for making ethanol from the cellulose (fiber) component in biomass, like municipal solid wastes and agricultural residues left in the field after harvest. This type of ethanol is called bioethanol. Bioethanol reduces exhaust emissions from carbon monoxide and hydrocarbons. In addition, by displacing gasoline components such as sulfur, bioethanol helps reduce the emissions of toxic effluents from automobiles.
Biodiesel
Biodiesel can be made from vegetable oils, animals fats, or recycled grease. Industry produces about 20 million gallons of biodiesel from recycled cooking oils and soybean oil.
Like ethanol, biodiesel is primarily used as a fuel blend. Diesel blends usually consist of 20 percent biodiesel with 80 percent petroleum diesel. This mixture runs well in a diesel engine and does not require engine modifications.
Biodiesel is not yet widely available to the general public. Some federal, state, and transit fleets, as well as tourist boats and launches, use blended biodiesel or pure biodiesel. Industry is currently looking at using biodiesel in circumstances where people are exposed to diesel exhaust, in aircraft to control pollution near airports, and in locomotives with unacceptably high emissions. Biodiesel may increase nitrogen oxide emissions but it reduces carbon monoxide, particulates, soot, hydrocarbons, and toxic emissions when compared to pure, petroleum diesel.
Biobased Products
Whatever products we can make with fossil fuels, we can make nearly identical or better ones from biomass. The difference between a chemical derived from plants and an identical chemical made from petroleum is simply their origin. This difference is important because plants are renewable and petroleum is not. Biobased products also often require less energy to produce than petroleum-based products. In addition, they can be made from "useless" wastes.
Our nation produces more than 300 billion pounds of biobased products each year, not counting food and feed. Biobased products include plastics, cleaning products, natural fibers, natural structural materials, and industrial chemicals made from biomass. Such chemicals are sometimes referred to as "green" chemicals because they are derived from a renewable resource.
Phenol is an example of a green chemical. Typically made from coal tar, phenol can also be extracted from pyrolysis oil, the liquid formed when biomass is heated with oxygen. Wood glues—which are used to make plywood, molded plastic, and foam insulation—are some of the materials that can be made with phenol.
Scientists have discovered how to release the sugars that make up starch and cellulose in plants to not only manufacture biofuels but also the following biobased products:
- Antifreeze
- Brake fluid
- Glues
- Softeners
- Artificial sweeteners
- Acids used in making cheese and soft drinks
- Biodegradable plastics
- Food thickeners such as xanthan gum
- Gels for toothpaste, medicines, and paints.
Carbon monoxide and hydrogen are also important building blocks for biobased products. When biomass is heated with a small amount of oxygen present, these two gases are produced in abundance. Scientists call this mixture biosynthesis gas. Once it is cleaned up, biosynthesis gas can be used to make:
- Electricity
- Alcohol fuels
- Antifreeze
- Acids used in making photographic films, textiles, plastics, and synthetic fabrics
- Sulfur-free gasoline
- Sulfur-free diesel fuels
- Many other products made with fossil fuels, including plastics.
Biobased products are so varied it's unlikely that industries in the future will limit themselves to making just one of them. Rather, biorefineries could become commonplace.
Biorefineries
Modeled after petroleum refineries, biorefineries would produce a diverse and flexible mix of fuels, electricity, heat, chemicals, and materials. To keep costs down, they would be built near a specific biomass resource, such as cornfields. The biorefinery itself would incorporate the best available technology for converting the local resource into the desired mix of products.
Today, our nation's food industries manage integrated systems to grow crops and produce food, animal feed, natural fibers, alcohol fuel, biodiesel, and assorted biobased products. Similarly, our forest products industries produce not only lumber, wood products, and paper products, but also electricity, heat, chemicals, and materials. In a real sense, these industries have already created our nation's first biorefineries.
Biomass Resources
Biomass resources are plentiful and varied throughout the country. They are primarily wastes, food crops, and energy crops.
In the Pacific Northwest and the Southeast, for example, the forest products industry uses its wastes and residues to make electricity and heat for its own operations. Instead of filling up a landfill, sawdust, bark, paper pulp, wood shavings, scrap lumber, wood dust, and paper provide low-cost bioenergy. In Hawaii, a plant is using bagasse (a fibrous residue from sugar cane processing) to make particleboard.
In the Midwest, farmers grow corn and soybeans for ethanol fuels and bioproducts. A South Dakota firm sells truck bed liners made from soybeans. A Minnesota firm makes shrink wrap, clothing, candy wrappers, cups, food containers, home and office furnishings, and other biodegradable products from a chemical building block derived from corn starch. A consortium of farmers, businesses, and utilities in Iowa is growing 4000 acres of switchgrass as an energy crop for cofiring with coal in utility boilers.
A similar consortium in the Northeast is growing hybrid willow trees as energy crops, also for cofiring with coal. A number of cities in the Northeast generate electricity from their biomass-rich solid wastes instead of burying them in landfills. A utility in Vermont is experimenting with a new system to make biogas from wood chips.
Every day, a fast-food corporation delivers hamburgers all over the country in clamshell containers made, in part, from starches recovered from the production of French fries and potato chips.
The use of these resources is laying the foundation for future bioenergy use. However, if we want to increase our bioenergy resources and lower the costs of producing them, we must rely more on energy crops and less on food crops. As our understanding of agricultural science grows, we'll be able to grow more and better energy crops. Potential energy crops include poplars, willows, switchgrass, alfalfa stems, and sweet sorghum.
Compared to conventional farming, energy crops require less fertilizer and fewer chemicals to control weeds and insect pests. With sustainable farming practices, we can use energy crops to prevent erosion, and to protect water supplies and quality. Researchers are developing perennial grass and tree crops with life expectancies of 7 to 10 years after planting. Research has shown that soil carbon, one indicator of soil quality, increases measurably under energy crops in as few as 3 to 5 years. These crops can potentially restore the cultivation and water-holding capacity of soil degraded by intensive crop production. In all these ways, energy crop farming helps us preserve our cropland for future generations.
What Lies Ahead
No one can predict the future, but with bioenergy, there are intriguing possibilities. Researchers believe they can significantly improve the technologies for making electricity, heat, and fuels from biomass. They are investigating advanced gasification systems, fuel cells, and combination technologies that produce heat and electricity. Advanced technologies should be able to produce bioenergy more efficiently and at lower costs than today.
Another interesting possibility researchers are investigating is meshing the development of bioenergy with fossil-fuel energy. For instance, it should be possible to process biogas to pipeline quality. Pipeline quality biogas would increase natural gas supplies for home heating and electrical power generation. Cofiring biomass directly with coal for power generation is a strong possibility for the future.
Looking ahead, some analysts have begun to talk about a "carbohydrate economy," in which plants would be a major source of electricity and fuels, as well as construction materials, clothes, inks, paints, synthetic fibers, pharmaceuticals, and industrial chemicals. According to studies by the Shell International Petroleum Company and the Intergovernmental Panel on Climate Change, biomass could satisfy between one-quarter and one-half of the world's demand for energy by the middle of the 21st century. This projection implies a world full of biorefineries, where plants provide many of the materials we now obtain from coal, oil, and natural gas.
It is too soon to know whether the future holds thousands of locally owned biorefineries producing many different products from a locally grown energy crop. What we do know is that any future increases in the use of bioenergy will benefit farmers and rural communities. Each new biorefinery will make nearby farms more profitable. Farm income will rise because farmers will be able to sell both the food and energy they grow. Biorefineries will also boost regional employment and help reduce local energy costs.
Not surprisingly, some oil companies and petrochemical industries have already begun to explore bioenergy. At the same time, bioenergy researchers are discovering their products and methods have some things in common with the petroleum industry.
Bioenergy holds great promise for the future. But to realize this promise, key challenges must be met. First, the cost of bioenergy needs to be lowered. As long as it costs less to make electricity, transportation fuels, and products from fossil fuels than it does to make them from biomass, people will be reluctant to invest in bioenergy. We also must ensure that increasing our use of bioenergy will not adversely affect our environment. Finally, we must work together to facilitate the growth of an integrated bioenergy industry that links resources with the production of a variety of energy and material products.
