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 and biogas that are 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 other forms of energy
Burning biomass is only one way to release its energy. Biomass can be converted to other useable forms of energy like methane gas or transportation fuels like ethanol and biodiesel.
Methane gas is a component of landfill gas or biogas that forms when garbage, agricultural waste, and human waste decompose in landfills or in special containers called digesters.
Crops such as corn and sugar cane are fermented to produce fuel ethanol for use in vehicles. Biodiesel, another transportation fuel, is produced from vegetable oils and animal fats.
How much biomass is used for fuel?
Biomass fuels provided about 5% of the energy used in the United States in 2015. Of that 5%, about 43% was from wood and wood-derived biomass, 46% was from biofuels (mainly ethanol), and about 11% was from municipal waste. 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.
In 2015, 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.
About 20% of total U.S. wood energy consumption in 2015 was by the residential sector, and wood accounted for about 3% of total residential energy consumption.
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 2012, about 2.5 million U.S. households used wood as the main heating fuel. An additional 9 million households used wood as a secondary heating fuel.
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, leather products
- nonbiomass combustible materials such as plastics and other synthetic materials made from petroleum
- noncombustible materials such as glass and metals
In 2014, about 258 million tons of MSW were generated in the United States, of which
- 53% was landfilled
- 35% was recycled and composted
- 13% was burned for energy
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 2015, 71 waste-to-energy power plants and four other power plants burned MSW in the United States. These plants burned about 29 million tons of MSW in 2015 and generated nearly 14 billion kilowatthours of electricity. The biomass materials in the MSW that were burned in these power plants accounted for about 64% of the weight of the MSW and contributed about 51% of the energy. The remainder of the 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%.
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. Biogas contains methane. Methane is the same energy-rich gas found in natural gas, which is used for heating, cooking, and producing electricity.
Landfill biogas can be dangerous to people and the environment because methane is flammable, and it is 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 control the methane gas emissions simply by burning or flaring methane gas. Methane gas can also be used as an energy source. Many landfills collect biogas, treat it, and then sell the methane. Some landfills use the methane gas to generate electricity.
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 used for heating and for generating electricity on 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 as an energy source is an alternative 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.
Switchgrass growing on a test plot for biomass production
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. MSW landfills use ash that is considered safe as a cover layer for their landfills, or they add it to concrete to make cement blocks.
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
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.