What Biomass Is Used To Generate Electricity?
Biomass refers to organic material that comes from plants and animals. This includes wood, agricultural waste, municipal solid waste, landfill gas, and biogas. Biomass contains stored energy from the sun which can be extracted and converted into other useful forms of energy like heat and electricity.
Using biomass for electricity generation is a renewable energy source that relies on organic waste material. Biomass can be directly combusted to produce steam which spins a turbine connected to a generator to produce electricity. Alternatively, biomass can be converted into a gas fuel through various processes like anaerobic digestion, fermentation, and gasification. The resulting biogas can then be burned in a power plant to generate electricity.
According to McKendry, biomass power currently accounts for about 2% of U.S. energy production with about 13.4 gigawatts of installed capacity. The largest source is forest product industry waste. Germany has the most extensive data on biomass electricity with over 8500 biomass plants generating 43 TWh annually. Overall, biomass offers a renewable and more carbon-neutral alternative to fossil fuels for power generation.[1]
Types of Biomass
There are several main types of biomass that are commonly used to generate electricity:
Wood
Wood, in the form of trees, branches, sawmill scraps, and forest residues, accounts for about 43% of biomass energy in the United States [1]. Wood can be directly combusted to produce steam for electricity generation. Wood pellets and chips are also becoming popular forms of wood biomass.
Agricultural Waste
Agricultural residues like corn stover, wheat straw, rice straw, and sugarcane waste represent another significant source of biomass. These materials are byproducts of food production and can be burned or converted into liquid biofuels.
Municipal Solid Waste
Solid waste from homes and businesses like paper, plastics, food scraps, and yard trimmings can be converted into electricity via waste-to-energy plants. Combustion of municipal solid waste accounts for about 13% of biomass energy [1].
Landfill Gas
The natural decomposition of municipal solid waste in landfills produces methane gas that can be captured and used to generate electricity. Landfill gas provides about 1.5% of biomass energy [1].
Biogas
The anaerobic digestion or fermentation of animal manure, food waste, and sewage sludge produces a methane-rich biogas that can be burned for electricity generation [2].
Wood
Wood is one of the most commonly used forms of biomass for generating electricity. Sources of wood biomass include forestry residues such as tree tops and branches that are left over from logging and forest thinning operations. Unused wood materials like crates, pallets, and demolition debris can also be utilized. In some cases, fast-growing trees and shrubs known as dedicated energy crops are grown specifically to provide biomass feedstock for energy production (EIA).
These wood sources are processed into wood chips, pellets, or other forms that can be burned to heat water, producing steam to spin turbines and generate electricity. In 2020, wood and wood waste accounted for about 1.9% of total U.S. electricity generation (EIA). Using wood biomass to produce electricity provides a productive use for forestry and agricultural residues, reduces landfill waste, and can be carbon neutral when trees are replanted to reabsorb CO2.
Agricultural Waste
Agricultural waste refers to biomass residues left over from agricultural processes like crop harvesting and livestock rearing. These residues primarily include crop residues like straw, stems, stalks, leaves, and seed pods as well as animal manure. Agricultural waste accounts for a major portion of the total biomass available globally. According to one estimate, crop residues alone amount to around 3 billion metric tons per year (https://pubmed.ncbi.nlm.nih.gov/37119925/).
Crop residues are an abundant and low-cost feedstock for biomass power plants. Common crop residues used include rice husk, wheat straw, corn stover, sugarcane bagasse, and coconut shells. Burning these residues provides heat energy to run steam turbines for electricity generation. In addition to crop residues, animal manure, especially cow dung, can also be used as feedstock in biomass gasification plants to produce syngas. Overall, agricultural waste provides a renewable and carbon-neutral way of generating electricity.
Municipal Solid Waste
Municipal solid waste (MSW) refers to everyday waste items from homes and businesses, such as paper, plastic, food scraps, and yard trimmings. MSW makes up a large percentage of the total solid waste generated in the United States each year (over 260 million tons in 2017) (1).
A significant portion of MSW is organic material that contains energy stored from photosynthesis. This “biomass” can be converted to electricity through several methods. The most common is incineration or combustion. MSW is burned at high temperatures, heating water to produce steam that spins a turbine generator to make electricity. In 2017, over 25 million tons of MSW was combusted in waste-to-energy plants to generate approximately 14 billion kilowatt-hours of electricity, which is enough to power over 1 million homes (2).
Gasification and anaerobic digestion are other MSW-to-energy technologies, though not as widely used. Gasification heats MSW to high temperatures without combustion to produce syngas, while anaerobic digestion uses bacteria to break down organic waste, releasing biogas. The syngas and biogas can then fuel generators (3).
Overall, waste-to-energy is a promising source of clean, renewable electricity from the large stream of MSW generated each year. It also reduces the amount of waste sent to crowded landfills. However, challenges include high capital costs of facilities, need for extensive emission control systems, and public concerns over pollutants (3).
Sources:
(1) https://www.eia.gov/energyexplained/biomass/waste-to-energy.php
(2) https://www.energy.gov/sites/prod/files/2016/07/f33/municipal_solid_waste_factsheet_bt16.pdf
(3) https://www.eia.gov/energyexplained/biomass/waste-to-energy-in-depth.php
Landfill Gas
Landfill gas is a type of biomass energy produced from the breakdown of municipal solid waste in landfills. As organic waste like food scraps, paper, and yard trimmings decompose, methane gas is generated as a byproduct of the anaerobic digestion process. Methane makes up approximately 50% of landfill gas. The remaining portion is carbon dioxide along with trace amounts of other gases.
Methane is highly flammable and can be captured via a network of wells and pipes installed in the landfill. The methane can then be used as a fuel source to generate electricity. According to the U.S. Energy Information Administration, over 600 landfills in the U.S. produce landfill gas that is used to generate electricity. The methane is typically burned in internal combustion engines or turbines to spin generators and produce electricity. The electricity can then be used on-site or sold to utilities.
Capturing landfill methane helps prevent its release into the atmosphere. Methane is a potent greenhouse gas with more than 25 times the global warming potential of carbon dioxide. Using landfill gas for energy helps offset fossil fuel use and reduces overall greenhouse gas emissions.
Biogas
Biogas is produced from the anaerobic digestion of organic matter such as manure, sewage, municipal waste, green waste, plant material, and crops.1 In an anaerobic digester, bacteria break down the organic matter in an oxygen-free environment, producing a gas that is primarily methane (CH4) and carbon dioxide (CO2) with small amounts of other gases.2
The methane produced from biogas can be purified to remove CO2 and other contaminants. This purified biogas, known as biomethane or renewable natural gas, contains 90% or more methane by volume.1 Methane is a clean burning fuel and can be used interchangeably with conventional natural gas.
Biogas provides a renewable fuel source that also helps manage organic waste streams and reduce methane emissions from decomposition. Capturing biogas methane converts this potent greenhouse gas into usable energy.
Converting Biomass to Electricity
There are several methods used to convert biomass into electricity:
Combustion involves burning biomass directly to produce high-pressure steam that drives a steam turbine and generator. According to the U.S. Department of Energy, combustion is the most common and mature conversion technology for producing electricity from biomass. It is relatively low cost and is widely used at industrial facilities (source).
Gasification converts biomass into a combustible gas made up of carbon monoxide, hydrogen, and methane. This gas is fed into a combustion turbine to generate electricity. According to the Whole Building Design Guide, gasification systems have lower emission levels and are more efficient (20-40%) than direct combustion (source).
Anaerobic digestion uses bacteria to break down biomass in an oxygen-free environment, producing biogas containing methane and carbon dioxide. This biogas can be used to generate electricity in a combined heat and power (CHP) system. According to the U.S. Energy Information Administration, biogas from landfills, wastewater treatment plants, and anaerobic digesters can be a renewable source for electricity generation (source).
Benefits
Biomass energy offers several important benefits. First, biomass is a renewable energy source. Plants and other biomass can regrow relatively quickly compared to the time it takes for fossil fuels to form, making biomass a sustainable source of energy (source). Second, biomass energy helps reduce waste by utilizing agricultural residues, forest residues, and municipal solid waste as fuel feedstocks. This reduces the amount of waste sent to landfills (source). Third, using landfill waste as an energy source reduces methane emissions from landfills. Methane is a potent greenhouse gas, so converting landfill waste to energy through biomass reduces greenhouse gas emissions (source).
Challenges of Using Biomass for Electricity
While biomass energy has many benefits, it also comes with some challenges. One major challenge is the high cost associated with biomass power plants. Constructing a new biomass plant requires significant upfront capital investment. According to EUBIA, biomass pre-treatment technologies like torrefaction and pelletization add extra costs. Ongoing operation and maintenance costs can also be high.
Sourcing large quantities of biomass feedstock can be difficult and expensive. Biomass power plants require enormous amounts of feedstock on a regular basis. Transporting bulky biomass long distances via truck, train or ship increases costs significantly. Storage and handling of large biomass volumes also requires infrastructure and equipment.
While often cleaner than coal, biomass energy can still produce air pollution and greenhouse gases during combustion. According to the National Sustainable Energy Laboratory, careful feedstock selection, efficient conversion technologies, and effective pollution control systems are needed to minimize environmental impacts.
