Biomass is material from recently living organisms that are now dead and used to produce energy. Examples include wood, leftover wood pieces, plants grown specifically for energy, leftover parts of crops like straw, and waste from industries and homes. Wood and leftover wood pieces are the largest source of biomass energy today. Wood can be burned directly for energy or turned into pellet fuel or other types of fuel. Other plants, such as corn, switchgrass, miscanthus, and bamboo, can also be used as fuel. Common waste materials used for energy include wood waste, crop waste, household waste, and waste from manufacturing. Improving raw biomass into better fuels can be done using methods like heating, chemicals, or biological processes.

The effect of bioenergy on the environment depends on where the biomass comes from and how it is grown. For example, burning wood releases carbon dioxide. These emissions can be reduced if new trees are planted to replace those that were cut down, as new trees absorb carbon dioxide from the air. However, growing biomass can harm wildlife, damage soil, and use land that could be used for food. It may also require water for farming and fertilizers.

The world has enough land and water to grow enough biomass for food and to help meet energy needs using carbon-neutral methods where renewable electricity is not practical. Using biomass and renewable electricity instead of fossil fuels for all energy needs of the world’s largest population would not increase human-caused greenhouse gas emissions. A biomass-based economy could develop alongside a renewable energy economy.

Terminology

Biomass, in the context of energy, is material from recently living organisms that are now dead and used to create bioenergy. Definitions of biomass for energy may vary, such as including only plants, or plants and algae, or plants and animals. Most biomass used for bioenergy comes from plants. Bioenergy is a type of renewable energy that can help reduce the effects of climate change.

Some people use the terms biomass and biofuel as if they mean the same thing, but many governments now define biofuel as a liquid or gas used for transportation. In this way, biofuel is considered a part of biomass.

The European Union's Joint Research Centre describes solid biofuel as raw or processed organic material from living things that is used for energy, such as firewood, wood chips, and wood pellets.

Types and uses

As shown in the table above, the main issue with biomass as an energy source is that it holds much less energy than fossil fuels.

Biomass is used in different ways depending on the type:

• Primary biomass is used for heat or electricity but not for transport. Examples include wood, wood scraps, wood pellets, agricultural waste, and organic waste.
• Secondary biomass is used for transport fuels and comes from crops like corn, sugarcane, and soy.

Biomass is divided into two groups: primary biomass, which is harvested directly for energy, and secondary biomass, which includes residues and waste.

The most common primary biomass sources are wood, some food crops, and perennial energy crops. One-third of the world’s 4 billion hectares of forests are used for wood production or other purposes. Forests supply 85% of all biomass used for energy globally. In the European Union, forests provide 60% of all biomass used for energy, with wood scraps and waste being the largest source.

Woody biomass used for cooking and heating, especially in developing countries, contributes about 25 EJ of energy annually. This practice causes pollution, leading to 3.8 million deaths each year, according to the World Health Organization. The United Nations aims to stop traditional biomass use for cooking by 2030. Short-rotation coppices and forests are also used for energy and are considered sustainable. These crops could provide at least 25 EJ of energy annually by 2050.

Food crops used for energy include sugarcane, corn, and rapeseed. Sugarcane is a perennial crop, while corn and rapeseed are annual crops. Sugarcane and corn are used to make bioethanol, while rapeseed is used to make biodiesel. The United States produces the most bioethanol, and the European Union produces the most biodiesel. Bioethanol and biodiesel provide 2.2 and 1.5 EJ of energy annually. Biofuels made from food crops are called "first-generation" biofuels and have lower emission savings.

The Intergovernmental Panel on Climate Change (IPCC) estimates that 0.32 to 1.4 billion hectares of unused land could be used for bioenergy worldwide.

Residues and waste are by-products from biological materials harvested for non-energy purposes. Key examples include:

• Wood residues come from forestry or wood processing. If not used for energy, they would decay or burn, causing emissions.
• Logging residues include tree tops, branches, stumps, and small trees removed to help larger trees grow. These residues have a sustainable potential of 15 EJ annually. In the European Union, 68% of forest biomass is wood stems, and 32% is stumps, branches, and tops.
• Wood processing residues include sawdust, bark, and other scraps. These residues have an energy content of 5.5 EJ annually. Wood pellets and chips made from these residues have energy contents of 0.7 EJ and 0.8 EJ, respectively.

Agricultural residues used for energy provide about 2 EJ annually. However, their untapped potential is much higher, with global estimates ranging from 18 to 82 EJ annually. The largest share comes from straw (51 EJ). Sustainable and economically viable use of agricultural residues is expected to increase to 37–66 EJ by 2030.

Municipal waste produces 1.4 EJ of energy annually, and industrial waste produces 1.1 EJ. Wood waste from cities and industry also provides 1.1 EJ. The sustainable potential for wood waste is estimated at 2–10 EJ. The International Energy Agency (IEA) recommends increasing waste use to 45 EJ annually by 2050.

Biomass conversion

Raw biomass can be improved into more useful fuel by compacting it, such as making wood pellets, or by using different methods called thermal, chemical, and biochemical conversions. Converting biomass reduces transportation costs because it is easier and cheaper to move dense materials.

Thermal upgrading creates solid, liquid, or gaseous fuels using heat as the main energy source. The main thermal methods are torrefaction, pyrolysis, and gasification. These methods differ based on how far the chemical reactions go, which is controlled by the amount of oxygen present and the temperature.

Torrefaction is a gentle form of pyrolysis where organic materials are heated to 400–600 °F (200–300 °C) in an environment with little or no oxygen. This process removes parts of the biomass with low energy content, leaving behind parts with higher energy. About 30% of the biomass becomes gas during torrefaction, while 70% remains as solid material, often made into pellets or briquettes. This solid product is water-resistant, easy to grind, non-corrosive, and contains about 85% of the original energy. The energy density of torrefied biomass increases significantly, making it comparable to coal used for electricity generation. Common steam coal has an energy density of 22–26 GJ/t. Other methods, such as hydrothermal upgrading (sometimes called "wet" torrefaction), can process biomass with high or low moisture content, like liquid slurries. Torrefied biomass, biochar, and bio-coke can be used in blast furnaces to produce green steel, green urea/ammonia/hydrogen, and green slag cement.

Pyrolysis involves heating organic materials to 800–900 °F (400–500 °C) with almost no oxygen. This process creates fuels like bio-oil, charcoal, methane, and hydrogen. Hydrotreating is used to refine bio-oil (from fast pyrolysis) with hydrogen under high heat and pressure, using a catalyst to produce renewable diesel, gasoline, and jet fuel.

Gasification heats organic materials to 1,400–1,700 °F (800–900 °C) with controlled amounts of oxygen and/or steam to create syngas, a gas rich in carbon monoxide and hydrogen. Syngas can be used as fuel for engines, heating, or electricity generation in gas turbines. It can also be processed to separate hydrogen, which can be burned or used in fuel cells. Syngas can be further refined into liquid fuels using the Fischer-Tropsch process.

Biomass can be converted through chemical processes to make fuels easier to store, transport, and use, or to take advantage of properties from the conversion process. Many chemical methods are similar to coal-based processes, such as the Fischer-Tropsch synthesis. A chemical process called transesterification converts vegetable oils, animal fats, and greases into fatty acid methyl esters (FAME), which are used to make biodiesel.

Biochemical processes use natural methods to break down biomass molecules, often using microorganisms. These processes include anaerobic digestion, fermentation, and composting. Fermentation converts biomass into bioethanol, a fuel used in vehicles. Anaerobic digestion converts biomass into renewable natural gas (biogas), which is produced at sewage treatment plants, dairy farms, and landfills. When properly treated, biogas can be used in the same ways as fossil fuel natural gas.

Climate impacts

The climate effects of bioenergy depend on where the materials used to create energy come from and how they are grown. For example, burning wood releases carbon dioxide. These emissions can be reduced if new trees are planted in a well-managed forest, as the new trees will absorb carbon dioxide from the air as they grow. However, growing crops for bioenergy can harm natural ecosystems, damage soil, and use water and synthetic fertilizers.

About one-third of the wood used for heating and cooking in tropical areas is taken from forests in a way that harms the environment. Producing bioenergy often requires a lot of energy for harvesting, drying, and transporting materials, which can release greenhouse gases. In some cases, the changes to land use, growing crops, and processing can lead to more carbon emissions from bioenergy than using fossil fuels.

Using farmland to grow biomass can reduce the land available for growing food. In the United States, about 10% of motor gasoline is replaced by corn-based ethanol, which uses a large part of the corn harvest. In Malaysia and Indonesia, clearing forests to grow palm oil for biodiesel has caused serious environmental and social problems, as these forests are important for absorbing carbon and supporting wildlife. Since plants capture only a small part of sunlight’s energy, producing a certain amount of bioenergy needs a lot of land compared to other renewable energy sources.

The IPCC says: "Modern bioenergy usually produces fewer greenhouse gas emissions than fossil fuels." Most of the IPCC’s plans to reduce emissions include using bioenergy technologies widely.

Some research groups say that even though forests in Europe and North America are growing more trees, it takes too long for trees to regrow after being cut down. Bioenergy from sources that take a long time to balance carbon emissions may not help reduce climate change quickly. These groups suggest the EU should only consider bioenergy as sustainable if it can balance carbon emissions in less than 10 years, such as wind, solar, or biomass from wood waste or fast-growing plants.

The IPCC also states: "The total carbon balance of a forest depends on adding up the carbon changes in all its parts." The only way to fairly calculate carbon changes for managed lands, like forests, is to include both emissions and absorption. After subtracting natural events like fires or insect damage, the remaining changes show human influence.

IEA Bioenergy says focusing only on short-term benefits may make it harder to reduce carbon emissions in the long term. They compare investing in new bioenergy technologies with investing in other renewable energy, like expanding battery production or building rail systems, which only reduce emissions after 2030. Protecting forests can help reduce emissions quickly, but long-term benefits from sustainable forest management provide ongoing resources.

Most of the IPCC’s plans to reduce emissions include using bioenergy widely. If bioenergy is not used enough, climate change may worsen, or other sectors may need to take on more emission-reduction efforts, which could increase costs.

Scenarios that are "carbon positive" release more carbon than they absorb, while "carbon negative" projects absorb more carbon than they release. "Carbon neutral" projects balance emissions and absorption equally.

Alternative scenarios, such as "reference scenarios" or "counterfactuals," are used to compare results. These scenarios range from small changes to major ones, like protecting forests or avoiding bioenergy entirely. The difference between scenarios shows how much carbon emissions can be reduced.

Other choices, like "system boundaries," decide which carbon emissions or absorption are included in calculations. These boundaries include time, location, efficiency, and cost factors.

For example, the carbon intensity of bioenergy depends on how biomass is produced and how far it is transported.

Temporal boundaries decide when to start and end carbon calculations. Sometimes early events, like carbon absorption in forests before trees are cut, are included. Other times, later events, like emissions from destroying buildings, are also counted. Since carbon emissions and absorption change over time, results can be shown as time-dependent (like a curve) or as an average over a set period.

A time-dependent curve might show high emissions at the start (when trees are cut). If the starting point is moved to when trees are planted, the curve could dip below zero (carbon negative) if no carbon debt from land use exists and trees absorb more carbon. The curve then rises sharply at harvest. The harvested carbon is then moved to other areas, and the curve shows how much carbon is in those areas over time. The "carbon payback time" is how long it takes for harvested carbon to return to the forest, and "carbon parity time" is when two scenarios have the same carbon levels.

A static carbon value is an average of emissions over a specific time, like the lifespan of a building or a policy goal (e.g., 2030, 2050). In the EU, a 20-year period is used for land use changes. Static values are often preferred in laws and are called "emission factors" or "greenhouse gas savings percentages" for specific bioenergy methods. The EU’s percentages for bioenergy in its Renewable Energy Directive are based on life cycle assessments.

Spatial boundaries define the geographic limits for calculating carbon emissions or absorption.

Environmental impacts

The environmental effects of producing biomass must be considered carefully. In 2022, the International Energy Agency (IEA) stated that "bioenergy is an important part of reducing carbon emissions in the energy transition as a near-zero-emission fuel," and that "more efforts are needed to speed up modern bioenergy use to meet the Net Zero Scenario while ensuring bioenergy production does not harm people or the environment."

The Intergovernmental Panel on Climate Change (IPCC) notes that scientists disagree about whether the world's forests are shrinking or growing. Research shows that tree cover increased by 7.1% between 1982 and 2016. The IPCC explains that while carbon stored in trees is decreasing in tropical regions, it is increasing globally because temperate and boreal forests are growing more.

Old trees absorb carbon dioxide (CO₂) at a high rate, and cutting them down means losing their future ability to absorb CO₂. Harvesting trees also reduces carbon stored in soil.

Old trees absorb more CO₂ than young trees because they have larger leaf areas. However, over time, an old forest stops absorbing CO₂ because emissions from dead trees balance the absorption by living trees. Old forests are also more likely to be damaged by natural events, such as fires or insect infestations, which release CO₂. The IPCC explains that "older forests store more carbon but absorb less CO₂ than younger forests, which remove CO₂ from the atmosphere more quickly."

The IPCC states that changing forests from unmanaged to managed can have mixed effects on the climate. Managed forests store less carbon than unmanaged forests, but they grow faster and absorb more carbon. Using harvested wood efficiently can help reduce CO₂. There is a balance between keeping forests as carbon stores and using some of their carbon as renewable fuel to replace fossil fuels in hard-to-decarbonize sectors.

The IPCC says that using sustainably produced forest biomass as a replacement for carbon-heavy materials and fossil fuels can reduce atmospheric CO₂ more than simply preserving forests.

The IEA Bioenergy reports that forests managed for timber, bioenergy, and other wood products can help reduce climate change more than forests managed only for conservation. Three reasons are given:

• Mature forests have a reduced ability to absorb CO₂.
• Wood products can replace materials that produce more greenhouse gases during production.
• Carbon in forests can be lost due to natural events like wildfires or insect infestations.

Data from the Food and Agriculture Organization (FAO) shows that most wood pellets are made in regions with sustainably managed forests, such as Europe and North America. In 2019, Europe (including Russia) produced 54% of the world’s wood pellets. Between 1990 and 2020, forest carbon storage in this region increased from 158.7 to 172.4 gigatonnes. In the European Union, above-ground forest biomass grows by about 1.3% each year, but the growth rate is slowing as forests age.

The United Kingdom’s Emissions Trading System allows companies to count zero emissions for biomass used in non-energy purposes, such as manufacturing. For energy use, like electricity or heating, biomass must meet sustainability standards.

Producing biomass for energy can harm biodiversity. Crops like oil palm and sugar cane have been linked to reduced biodiversity. Changes in biodiversity also affect natural processes like decomposition and soil health.

Win-win scenarios (beneficial for both climate and biodiversity) include:
– Using whole trees from coppice forests, thin residues from boreal forests with slow decay, and all residues from temperate forests with faster decay.
– Creating multi-functional bioenergy landscapes instead of expanding monoculture plantations.
– Planting mixed or naturally regenerating forests on former farmland.

Win-lose scenarios (beneficial for the climate but harmful to biodiversity) include planting forests on ancient, biodiverse grasslands that were never forests or using monoculture plantations on former farmland.

Lose-win scenarios (harmful to the climate but beneficial for biodiversity) include natural forest growth on former farmland.

Lose-lose scenarios include using thick forest residues, like stumps from boreal forests with slow decay, and replacing natural forests with plantations.

Other issues include soil and water pollution from fertilizers and pesticides, and air pollution from burning crop residues.

Traditional wood use in cook stoves and open fires produces harmful pollutants, which can harm health and the environment. Switching to modern bioenergy improves quality of life, reduces land degradation, and protects ecosystems. The IPCC says modern bioenergy has "large positive impacts" on air quality. Traditional bioenergy is inefficient, and replacing it has major health and economic benefits. When burned in industrial facilities, most pollutants from wood decrease by 97–99% compared to open burning. Burning wood produces less particulate matter than coal for the same amount of electricity.