Management and Sustainability

To many people, the most familiar forms of renewable energy are the wind and the sun. But biomass (plant material and animal waste) supplies almost 15 times as much energy in the United States as wind and solar power combined—and has the potential to supply much more. Biomass sources provide about 3 percent of all energy consumed in the United States. In 2002, biomass supplied about 47 percent of Consumption of Renewable Energy in the United States. Electric generation from biomass (excluding municipal solid waste) represents about 11 percent of all generation from renewable sources in the United States. In fact, biomass supplied more energy to the nation in 2002 than any other form of renewable energy, including hydroelectric power. Biomass supplied almost six times the energy of geothermal, solar and wind energy sources combined. Globally, biomass meets about 14 percent of the world´s energy needs.

Consumption of Renewable Energy in the United States
                                        2002

We also get nearly four billion gallons of ethanol, about two percent of the liquid fuel used in cars and trucks. The contribution for heat is also substantial. But with better conversion technology and more attention paid to energy crops, we could produce much more.

Estimates of the ultimate potential for biomass energy vary, and depend on agricultural forecasts, waste reduction by industry, and paper recycling. The Department of Energy believes that we could produce four percent of our transportation fuels from biomass by 2010, and as much as 20 percent by 2030. For electricity, the U.S. Department of Energy (DOE) estimates that energy crops and crop residues alone could supply as much as much as 14 percent of our power needs. 

Living plant material is the source of all biomass. There are a wide variety of biomass energy resources, including tree and grass crops and forestry, agricultural, and urban wastes. Some biomass fuel resources are waste products left over after plant materials have been used for other purposes or consumed by animals. Other biomass resources are plant materials directly harvested for their energy value. Biomass fuels are readily available throughout the world.  Biomass is a renewable energy source because the energy it contains comes from the sun. Through the process of photosynthesis, chlorophyll in plants captures the sun's energy by converting carbon dioxide from the air and water from the ground into carbohydrates (sugars, starches and cellulose), complex compounds composed of carbon, hydrogen, and oxygen. When plants die, the process of decay releases the energy stored in carbohydrates and discharges carbon dioxide back into the atmosphere. When biomass carbohydrates are burned, they turn back into carbon dioxide and water and release the sun's energy they contain. In this way, biomass functions as a sort of natural battery for storing solar energy.  As long as biomass is produced sustainably—with only as much used as is grown—the battery will last indefinitely. The use of biomass for energy does not increase carbon dioxide emissions and does not contribute to the risk of global climate change.  That is to say, if the amount of new biomass growth balances the biomass used for energy, bioenergy is carbon dioxide "neutral." There is no net increase in carbon dioxide emissions to the atmosphere.  Further, using biomass to produce energy is often a way to dispose of waste materials that otherwise would create environmental risks, including carbon dioxide emissions. However, if the crops are used and are not replanted, then the biomass can emit carbon dioxide that will contribute toward global warming.  

How biomass combustion works is very simple. From the time of Prometheus to the present, the most common way to capture the energy from biomass was to burn it, to make heat, steam, and electricity. The waste wood, tree branches and other scraps are gathered together in big trucks. The trucks bring the waste from factories and from farms to a biomass power plant. Here the biomass is dumped into huge hoppers. This is then fed into a furnace where it is burned. The heat is used to boil water in the boiler, and the energy in the steam is used to turn turbines and generators.

An approach that may increase the use of biomass energy in the short term is to burn it mixed with coal in power plants—a process known as "co-firing." Biomass feedstock can substitute up to 20 percent of the coal used in a boiler. The benefits associated with biomass co-firing include lower operating costs, reductions of harmful emissions, and greater energy security. Co-firing is also one of the more economically viable ways to increase biomass power generation today. In 2000, the Chariton Valley Biomass Project, a joint effort including Alliant Energy, the U.S. Department of Energy, and local biomass groups, began testing the co-firing of switchgrass with coal at Alliant's Ottumwa Generating Station in Iowa. The project has proved so successful that in 2005, Alliant received permission to build a permanent biomass processing facility at the plant, capable of co-firing up to five percent of its energy with switchgrass.

Biomass can also be tapped right at the landfill. When garbage decomposes, it gives off methane gas. Pipelines are put into the landfills and the methane gas can be collected. It is then used in power plants to make electricity. This type of biomass is called landfill gas.

A similar thing can be done at animal feed lots. In places where lots of animals are raised, the animals - like cattle, cows and even chickens - produce manure. When manure decomposes, it also gives off methane gas similar to garbage. This gas can be burned right at the farm to make energy to run the farm. This is anaerobic digestion.

There are more efficient and cleaner ways to use biomass than old fashioned combustion. It can be converted into liquid fuels, for example, or cooked in a process called "gasification" to produce combustible gases. And certain crops such as switch grass and willow trees are especially suited as "energy crops," plants grown specifically for energy generation.

New  ways of using biomass are still being discovered. One way is to produce ethanol, a liquid alcohol fuel. Ethanol can be used in special types of cars that are made for using alcohol fuel instead of gasoline. The alcohol can also be combined with gasoline. This reduces our dependence on oil - a non-renewable fossil fuel.

NREL has a vision to develop technology for biorefineries that will convert biomass into a range of valuable fuels, chemicals, materials, and products—much like oil refineries and petrochemical plants do. (PDF 664 KB) Download Adobe Reader.

Types of Biomass

There are many types of plants in the world, and many ways they can be used for energy production. In general, there are two approaches: growing plants specifically for energy use, and using the residues from plants that are used for other things. The best approaches vary from region to region according to climate, soils, geography, population, and so on.

The main biomass feedstocks for power are paper mill residue, lumber mill scrap, and municipal waste. For biomass fuels, the feedstocks are corn (for ethanol) and soybeans (for biodiesel), both surplus crops. In the near future, agricultural residues such as corn stover (the stalks, leaves, and husks of the plant) and wheat straw will also be used. Long-term plans include growing and using dedicated energy crops, such as fast-growing trees and grasses that can grow sustainably on land that will not support intensive food crops.

Energy Crops

Energy crops, also called "power crops," could be grown on farms in potentially very large quantities, just like food crops. Trees and grasses, particularly those that are native to a region, are the best crops for energy, but other, less agriculturally sustainable crops such as corn tend to be used for energy purposes at present.

Trees. In addition to growing very fast, some trees will grow back after being cut off close to the ground, a feature called "coppicing." Coppicing allows trees to be harvested every three to eight years for 20 or 30 years before replanting. These trees, also called "short-rotation woody crops," grow as much as 40 feet high in the years between harvests. In the cooler, wetter regions of the northern United States, varieties of poplar, maple, black locust, and willow are the best choice. In the warmer Southeast, sycamore and sweetgum are best, while in the warmest parts of Florida and California, eucalyptus is likely to grow well. 

Grasses. Thin-stemmed perennial grasses used to blanket the prairies of the United States before the settlers replaced them with corn and beans. Switchgrass, big bluestem, and other native varieties grow quickly in many parts of the country, and can be harvested for up to 10 years before replanting. Thick-stemmed perennials like sugar cane and elephant grass can be grown in hot and wet climates like those of Florida and Hawaii.

Other crops. A third type of grass includes annuals commonly grown for food, such as corn and sorghum. Since these must be replanted every year, they require much closer management and greater use of fertilizers, pesticides, and energy. While corn currently provides most of the liquid fuel from biomass in the United States, there are more sustainable ways to produce energy from plants.

Oil plants. Plants such as soybeans and sunflowers produce oil, which can be used to make fuels. Like corn, though, these crops require intensive management and may not be sustainable in the longer term. A rather different type of oil crop with great promise for the future is microalgae. These tiny aquatic plants have the potential to grow extremely fast in the hot, shallow, saline water found in some lakes in the desert Southwest.  In 2004, Green Fuel Technologies, a Massachusetts-based company, harnessed the ability to capture and use carbon dioxide emissions from power plants as a means to stimulate algae growth. The algae is then converted into a various range of fuels.  This technology, known as Emissions-to-Biofuels, is demonstrating great promise and has the potential to transform the way utilities produce energy.

Exploring Ways to Use Biomass Energy
Biomass Program
Alternative Fuels Data Center
U.S. Department of Energy Bioenergy Feedstock Information Network

Oregon State Biomass Energy site