Biomass Basics
Biomass—renewable energy from plants and animals
Source: Adapted from The National Energy Education Project (public domain)
Source: Adapted from The National Energy Education Project (public domain)
Biomass is organic material that comes from plants and animals, and it is a renewable source of energy.
Biomass contains stored energy from the sun. Plants absorb the sun's energy in a process called photosynthesis. When biomass is burned, the chemical energy in biomass is released as heat. Biomass can be burned directly or converted to liquid biofuels or biogas that can be burned as fuels.
Examples of biomass and their uses for energy:
- Wood and wood processing wastes—burned to heat buildings, to produce process heat in industry, and to generate electricity
- Agricultural crops and waste materials—burned as a fuel or converted to liquid biofuels
- Food, yard, and wood waste in garbage—burned to generate electricity in power plants or converted to biogas in landfills
- Animal manure and human sewage—converted to biogas, which can be burned as a fuel
Converting biomass to energy
Solid biomass, such as wood and garbage, can be burned directly to produce heat. Biomass can also be converted into a gas called biogas or into liquid biofuels such as ethanol and biodiesel. These fuels can then be burned for energy.
Biogas forms when paper, food scraps, and yard waste decompose in landfills, and it can be produced by processing sewage and animal manure in special vessels called digesters.
Ethanol is made from crops such as corn and sugar cane that are fermented to produce fuel ethanol for use in vehicles. Biodiesel is produced from vegetable oils and animal fats and can be used in vehicles and as heating oil.
How much biomass is used for fuel?
Biomass fuels provided about 5% of total primary energy use in the United States in 2017. Of that 5%, about 47% was from biofuels (mainly ethanol), 44% was from wood and wood-derived biomass, and 10% was from the biomass in municipal waste. (Sum of percentages is greater than 100% because of independent rounding) Researchers are trying to develop ways to use more biomass for fuel.
Wood & Wood Waste
Biomass—Wood and wood waste
People have used wood for cooking, for heat, and for light for thousands of years. Wood was the main source of energy for the world until the mid-1800s. Wood continues to be an important fuel in many countries, especially for cooking and heating in developing countries.
Source: National Renewable Energy Laboratory, U.S. Department of Energy (public domain)
In 2017, about 2% of total U.S. annual energy consumption was from wood and wood waste—bark, sawdust, wood chips, wood scrap, and paper mill residues.
Using wood and wood waste
Industry, electric power producers, and commercial businesses use most of the wood and wood waste fuel consumed in the United States. The wood and paper products industry uses wood waste to produce steam and electricity, which saves money because it reduces the amount of other fuels and electricity that must be purchased. Some coal-burning power plants burn wood chips to reduce sulfur dioxide emissions.
Wood is used in homes throughout the United States for heating as cord wood in fireplaces and wood-burning appliances, and as pellets in pellet stoves. In 2017, wood energy accounted for about 2% of total residential energy consumption. In 2015, about 12.5 million U.S. households, or 11% of all households, used wood as an energy source, mainly for space heating, and 3.5 million of those households used wood as the main heating fuel.
The amounts (in trillion British thermal units) of U.S. wood and wood waste energy consumption by consuming sector and their shares (percent) of total wood and wood waste energy consumption in 2017 were
- Industrial—1,480—69%
- Residential—334—16%
- Electric power—247—12%
- Commercial—84—4%
Waste-To-Energy
Energy from municipal solid waste
Municipal solid waste (MSW), often called garbage, is used to produce energy at waste-to-energy plants and at landfills in the United States. MSW contains
- biomass, or biogenic (plant or animal products), materials such as paper, cardboard, food waste, grass clippings, leaves, wood, and leather products
- nonbiomass combustible materials such as plastics and other synthetic materials made from petroleum
- noncombustible materials such as glass and metals
In 2015, about 262 million tons of MSW were generated in the United States, of which
- 52.5% was landfilled
- 25.8% was recycled
- 12.8% was burned with energy recovery
- 8.9% was composted
Waste-to-energy plants make steam and electricity
MSW is usually burned at special waste-to-energy plants that use the heat from the fire to make steam for generating electricity or to heat buildings. In 2016, 71 U.S. power plants generated about 14 billion kilowatthours of electricity from burning about 30 million tons of combustible MSW. Biomass materials accounted for about 64% of the weight of the combustible MSW and for about 51% of the electricity generated. The remainder of the combustible MSW was nonbiomass combustible material, mainly plastics. Many large landfills also generate electricity by using the methane gas that is produced from decomposing biomass in landfills.
Waste-to-energy is a waste management option
Producing electricity is only one reason to burn MSW. Burning waste also reduces the amount of material that would probably be buried in landfills. Burning MSW reduces the volume of waste by about 87%.
Biogas
Collecting and using biogas from landfills
Landfills for municipal solid waste can be a source of energy. Anaerobic bacteria—bacteria that can live without the presence of free oxygen—living in landfills decompose organic waste to produce a gas called biogas. Landfill biogas is 40%–60% methane. The rest is mostly carbon dioxide (CO2) and small amounts of other gases. Methane is the same energy-rich compound—CH4—found in natural gas, which we use for heating, cooking, and producing electricity.
Biogas with high methane content can be dangerous to people and the environment because methane is flammable. It is also a strong greenhouse gas. In the United States, regulations under the Clean Air Act require landfills of a certain size to install and operate a landfill gas collection and control system.
Some landfills reduce methane gas emissions simply by burning—or flaring—the methane gas. Burning methane produces CO2, but CO2 is not as strong a greenhouse gas as methane. Many landfills collect biogas, treat it, and then sell the methane. Some landfills use the methane gas to generate electricity.
The U.S. Energy Information Administration (EIA) estimates that in 2016, about 278 billion cubic feet of landfill gas was burned for energy at U.S. landfills, 99% of which was used to generate about 11.2 billion kilowatthours (kWh) of electricity, or about 0.3% of total U.S. electricity generation in 2016.
Using biogas from animal waste
Some farmers produce biogas in large tanks called digesters, where they put manure and used bedding material from their barns. Some farmers cover their manure ponds (also called lagoons) to capture biogas. Biogas digesters and manure ponds contain the same anaerobic bacteria found in landfills. The methane in the biogas can be burned to heat water and buildings and for fuel in diesel-engine generators to generate electricity for the farm.
Read about a field trip to a real waste-to-energy plant or learn about the history of MSW.
Biomass & the Environment
Using biomass for energy has positive and negative effects
Biomass and biofuels made from biomass are alternative energy sources to fossil fuels—coal, petroleum, and natural gas. Burning either fossil fuels or biomass releases carbon dioxide (CO2), a greenhouse gas. However, the plants that are the source of biomass capture a nearly equivalent amount of CO2 through photosynthesis while they are growing, which can make biomass a carbon-neutral energy source.
Burning wood
Using wood, wood pellets, and charcoal for heating and cooking can replace fossil fuels and may result in lower CO2 emissions overall. Wood can be harvested from forests, woodlots that have to be thinned, or from urban trees that fall down or have to be cut down.
Wood smoke contains harmful pollutants like carbon monoxide and particulate matter. Modern wood-burning stoves, pellet stoves, and fireplace inserts can reduce the amount of particulates from burning wood. Wood and charcoal are major cooking and heating fuels in poor countries, but if people harvest the wood faster than trees can grow, it causes deforestation. Planting fast-growing trees for fuel and using fuel-efficient cooking stoves can help slow deforestation and improve the environment.
Burning municipal solid waste (MSW) or wood waste
Burning municipal solid waste (MSW, or garbage) to produce energy in waste-to-energy plants means that less waste is buried in landfills. On the other hand, burning garbage produces air pollution and releases the chemicals and substances in the waste into the air. Some of these chemicals can be hazardous to people and the environment if they are not properly controlled.
The U.S. Environmental Protection Agency (EPA) applies strict environmental rules to waste-to-energy plants, and requires that waste-to-energy plants use air pollution control devices such as scrubbers, fabric filters, and electrostatic precipitators to capture air pollutants.
Scrubbers clean emissions from waste-to-energy facilities by spraying a liquid into the combustion gases to neutralize the acids present in the stream of emissions. Fabric filters and electrostatic precipitators also remove particles from the combustion gases. The particles—called fly ash—are then mixed with the ash that is removed from the bottom of the waste-to-energy furnace.
A waste-to-energy furnace burns at high temperatures (1,800°F to 2,000°F), which breaks down the chemicals in MSW into simpler, less harmful compounds.
Disposing ash from waste-to-energy plants
Ash can contain high concentrations of various metals that were present in the original waste. Textile dyes, printing inks, and ceramics, for example, may contain lead and cadmium.
Separating waste before burning can solve part of the problem. Because batteries are the largest source of lead and cadmium in municipal waste, they should not be included in regular trash. Florescent light bulbs should also not be put in regular trash because they contain small amounts of mercury.
The EPA tests ash from waste-to-energy plants to make sure that it is not hazardous. The test looks for chemicals and metals that could contaminate ground water. Some MSW landfills use ash that is considered safe as a cover layer for their landfills, and some MSW ash is used to make concrete blocks and bricks.
Collecting landfill gas or biogas
Biogas forms as a result of biological processes in sewage treatment plants, waste landfills, and livestock manure management systems. Biogas is composed mainly of methane (a greenhouse gas) and CO2. Many facilities that produce biogas capture it and burn the methane for heat or to generate electricity. This electricity is considered renewable, and in many states, contributes to meeting state renewable portfolio standards (RPS). This electricity may replace electricity generation from fossil fuels and can result in a net reduction in CO2 emissions. Burning methane produces CO2, but because methane is a stronger greenhouse gas than CO2, the overall greenhouse effect is lower.
Liquid biofuels: ethanol and biodiesel
Biofuels are transportation fuels such as ethanol and biodiesel. The federal government promotes ethanol use as a transportation fuel to help reduce oil imports and CO2 emissions. In 2007, the government set a target to use 36 billion gallons of biofuels by 2022. As a result, nearly all gasoline now sold in the United States contains some ethanol.
Biofuels may be carbon-neutral because the plants that are used to make biofuels (such as corn and sugarcane for ethanol and soy beans and palm oil trees for biodiesel) absorb CO2 as they grow and may offset the CO2 emissions when biofuels are produced and burned.
Growing plants for biofuels is controversial because the land, fertilizers, and energy for growing biofuel crops could be used to grow food crops instead. In some parts of the world, large areas of natural vegetation and forests have been cut down to grow sugar cane for ethanol and soybeans and palm oil trees for biodiesel. The U.S. government supports efforts to develop alternative sources of biomass that do not compete with food crops and that use less fertilizer and pesticides than corn and sugar cane. The U.S. government also supports methods to produce ethanol that require less energy than conventional fermentation. Ethanol can also be made from waste paper, and biodiesel can be made from waste grease and oils and even algae.
Ethanol and gasoline-ethanol blends burn cleaner and have higher octane ratings than pure gasoline, but they have higher evaporative emissions from fuel tanks and dispensing equipment. These evaporative emissions contribute to the formation of harmful, ground-level ozone and smog. Gasoline requires extra processing to reduce evaporative emissions before it is blended with ethanol. Biodiesel combustion produces fewer sulfur oxides, less particulate matter, less carbon monoxide, and fewer unburned and other hydrocarbons, but it does produce more nitrogen oxide than petroleum diesel.