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) materials like paper, cardboard, food waste, grass clippings, leaves, wood, leather products, and other nonbiomass combustible materials like plastics and other synthetic materials made from petroleum.
In 1960, the average American threw away 2.7 pounds of trash per day. Today, the average American throws away about 4.4 pounds of trash every day. Of those 4.4 pounds, about 34% is recycled or composted, and about 13% is burned and converted to energy. The rest, about 53%, is discarded, mostly into landfills. About 85% of household trash is material that will burn, and about 61% of that is biogenic—material that is made from biomass (plant or animal products).
Waste-to-energy plants make steam and electricity
MSW is burned at special waste-to-energy plants that use the heat to make steam to generate electricity or to heat buildings. In 2013, there were about 80 waste-to-energy plants in the United States that generated electricity or produced steam. These plants burned about 30 million tons of MSW in 2013, and generated nearly 14 billion kilowatthours of electricity, about the same amount used by 1.3 million U.S. households in 2013. The biogenic material in MSW contributed about 52% of the energy from MSW that was burned in electricity-generating waste-to-energy facilities. Many large landfills also generate electricity by using the methane gas that is produced as biomass decomposes in the landfill.
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 that live 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, there are rules requiring landfills to collect methane gas for safety and rules to reduce greenhouse gas emissions.
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 place manure and 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 can also be used 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 can have positive and negative impacts
Using biomass for energy can have both positive and negative impacts on the environment. Using biomass for energy provides an alternative to using fossil fuels like coal, petroleum, or natural gas. Burning fossil fuels and burning biomass releases carbon dioxide (CO2), a greenhouse gas, but when the plants that are the source of biomass are grown, a nearly equivalent amount of CO2 is captured through photosynthesis.
Panicum virgatum (switchgrass) being grown
Using wood, and charcoal made from wood, for heating and cooking can replace fossil fuels and may result in lower CO2 emissions overall. Wood may be harvested from forests or woodlots that have to be thinned, or it may come from urban trees that fall down or that have to be cut down. Wood smoke contains harmful pollutants like carbon monoxide and particulate matter. Burning wood in an open fireplace for heating is an inefficient way to produce heat, and it can also produce air pollution. Modern wood burning stoves and fireplace inserts are designed to reduce the amount of particulates emitted by the appliance. Wood and charcoal are major cooking and heating fuels in poor countries, and the wood may be harvested faster than trees can grow. This results in 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
Producing energy by burning municipal solid waste (MSW, or garbage) and by burning wood waste in facilities like waste-to-energy plants means that less waste must be buried in landfills. Waste-to-energy plants produce air pollution when MSW is burned to produce steam or electricity. Burning garbage also releases the chemicals and substances in the waste. 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 it 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 these facilities by spraying a liquid into the chemical gas 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 plant's furnace.
A waste-to-energy furnace burns at high temperatures (1,800°F to 2,000°F) that make complex chemicals break down into simpler, less harmful compounds.
Disposing of 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 combustion 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 included 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 would contaminate ground water. Ash that is considered safe is used in municipal solid waste landfills as a cover layer. Ash is also used to build roads and to make cement blocks.
Collecting landfill gas or biogasBiogas is composed mainly of methane and CO2 that forms as a result of biological processes in sewage treatment plants, waste landfills, and livestock manure management systems. Many facilities that produce biogas also capture and burn the biogas for heat or electricity generation. The electricity generated from biogas is considered renewable and it is used in many states to meet state renewable portfolio standards (RPS). This electricity may replace electricity produced by burning fossil fuels and could result in a net reduction in CO2 emissions.
Liquid biofuels: ethanol and biodiesel
Ethanol and biodiesel were the fuels used in the first automobile and diesel engines, but lower-cost gasoline and diesel fuel made from crude oil became the dominant vehicle fuels. The federal government has promoted ethanol use in vehicles to help reduce oil imports since the mid-1970s. 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 considered 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 produced when biofuels are made and burned.
Growing plants for biofuels is controversial because the land, fertilizers, and energy used to grow biofuel crops could be used to grow food crops instead. Also, 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 to make 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, oils, and even algae.
Ethanol and gasoline blended with ethanol 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.