Biomass (energy)

Biomass is a way to make energy from plants, wood, and other natural materials that were once living. It can come from wood chips, crops grown just for energy like maize or switchgrass, and even waste from farms, homes, and factories. This energy can be used for heating, electricity, and more.

One common type of biomass is wood pellets, which are small pieces of compressed wood used mainly for heating. Other plants like miscanthus and bamboo can also be used to make energy.

Using biomass for energy can help reduce pollution, but it needs to be done carefully. Growing too many crops for energy can hurt wildlife, soil, and use up water and land that could grow food. When done right, like planting new trees after cutting old ones, biomass can be a clean energy source that doesn’t add extra pollution to the air.

Terminology

See also: Bioenergy § Definition and terminology

Biomass is material from plants and other living things that have recently died, used to make energy called bioenergy. Most biomass comes from plants. Bioenergy is a kind of renewable energy that can help slow down changes in the Earth's climate.

Some people use the words biomass and biofuel together, but biofuel usually means liquid or gas fuels used in cars, trucks, or planes. Solid biofuels include things like firewood, wood chips, and wood pellets.

Types and uses

Further information: Energy crop

Biomass is material from plants and animals that we can use to create energy. Different types of biomass are used for different jobs. For example, wood and wood pieces are often used to create heat or electricity, but not for vehicles. To power vehicles, we can use biomass made from crops like corn, sugar cane, and soybeans.

Biomass can be gathered directly for energy, like wood and special energy crops. It can also come from things we already have, like leftover wood, farm scraps, and waste from our homes and factories. Using these leftovers helps us save energy and reduces waste.

Biomass conversion

Raw biomass can be changed into better fuels in different ways, such as making it denser like wood pellets, or using heat, chemicals, and biological processes. These methods make it easier and cheaper to move the material around.

When heat is used, called thermal conversion, materials are heated to create solid, liquid, or gaseous fuels. One method, called torrefaction, heats biomass to around 400–600 °F (200–300 °C) with little oxygen. This removes parts with low energy and keeps the high-energy parts, making the material easier to transport and store. Another method, pyrolysis, heats biomass to about 800–900 °F (400–500 °C) without oxygen, producing fuels like bio-oil and charcoal. Gasification heats biomass even more, to 1,400–1,700 °F (800–900 °C), creating a mix of gases that can be used for fuel or electricity.

Chemical conversion uses processes to change biomass into easier-to-use fuels. For example, vegetable oils and animal fats can be turned into biodiesel through a process called transesterification.

Biochemical conversion uses natural processes, often with tiny living things called microorganisms, to break down biomass. Fermentation can turn biomass into bioethanol, a type of vehicle fuel. Anaerobic digestion changes biomass into renewable natural gas, also known as biogas, which can be used like regular natural gas for heating, electricity, and more.

Climate impacts

Short-term vs long-term climate benefits

The Intergovernmental Panel on Climate Change (IPCC) says that modern bioenergy usually has lower greenhouse gas emissions than fossil fuels. Because of this, many IPCC pathways for reducing greenhouse gases include using bioenergy.

Some researchers say that even though forests in Europe and North America are growing, it takes too long for trees to grow back after they are cut for bioenergy. They think the European Union should only count renewable energy that saves carbon quickly as sustainable. This includes wind and solar power, as well as biomass from wood leftovers and trees that would have burned anyway.

The IPCC explains that forests can either add carbon to the atmosphere or take it away, depending on what happens to all the trees together. They say the best way to count carbon is to look at both emissions and absorption from all lands we manage, like forests. Natural events like fires and insect attacks are also taken into account.

The International Energy Agency (IEA) Bioenergy says that focusing only on short-term benefits makes it harder to reduce carbon in the long term. They compare investing in new bioenergy technology to investing in other renewable energy that won’t save much carbon until after 2030.

Most IPCC pathways that include bioenergy help reduce climate change. Without bioenergy, climate change could get worse, and it would cost more to reduce greenhouse gases elsewhere.

Carbon accounting system boundaries

Carbon positive scenarios are likely to release more carbon dioxide (CO₂), while carbon negative projects take in more CO₂ than they release. Carbon neutral projects balance emissions and absorption.

When comparing different scenarios, it is common to look at what would have happened without the project. The difference between what actually happens and what would have happened shows the real climate benefit.

There are different ways to set the boundaries for counting carbon. These boundaries can be about time, place, efficiency, or money. For example, the actual carbon intensity of bioenergy changes based on how the biomass is produced and how far it is transported.

Temporal system boundaries

Temporal boundaries decide when to start and stop counting carbon. Sometimes, carbon taken in by forests before harvesting is counted. Other times, emissions from breaking down buildings or factories at the end of their life are counted. Because carbon emissions and absorption change over time, the net carbon effect can be shown as a curve over time or as a single average number for a certain period.

The curve might show high emissions at the start if counting begins when biomass is harvested. Alternatively, if counting starts when trees are planted, the curve might show negative emissions (taking in more carbon than releasing) if there is no carbon debt from changing land use. The curve then spikes upward when harvest happens. The time it takes for harvested carbon to return to the forest is called the carbon payback time.

The average number is calculated for a specific time, like the life of a building, or goals set for 2030, 2050, or 2100. In the European Union, a 20-year period is often used.

Spatial system boundaries

Spatial boundaries decide the geographical area for counting carbon. Common choices are the edges of a single forest stand, the edges of a whole forest landscape with many stands, or a method that looks at stands as they grow over time. The IPCC suggests using the whole landscape.

Researchers also decide whether to include emissions from changing land use, like cutting down a forest to start farming. Including effects from changes in how land is used around the world is more debated because it is hard to measure accurately.

Efficiency-related system boundaries

Efficiency boundaries decide how well different biomass-combustion methods work. Different supply chains and combustion facilities release different amounts of carbon for the same amount of energy. Researchers need to choose realistic efficiency ranges to calculate how much fossil carbon is replaced by biogenic carbon. Lower efficiency means more fuel is burned and more CO₂ is released.

Economic system boundaries

Economic boundaries decide which market changes to include. Changes in demand can affect forests and carbon emissions, but these changes can be hard to measure, so some researchers choose not to include them.

System boundary impacts

The chosen boundaries greatly affect the results. Shorter carbon payback times are calculated when fossil carbon intensity, forest growth rate, and biomass conversion efficiency increase, or when the initial forest carbon stock or harvest level decreases. Shorter times are also calculated when using landscape-level accounting instead of stand-level accounting.

Comparisons of GHG emissions at the point of combustion

Greenhouse gas emissions per unit of energy at the point of combustion depend on the moisture in the fuel, differences between fuels, and how well the fuel is turned into energy. Raw biomass can have more moisture than coal, so more energy is used to evaporate the water, leading to higher CO₂ emissions per unit of heat.

Many biomass-only combustion facilities are smaller and less efficient than large coal plants. Raw biomass, like wood chips, can also have more moisture than coal, leading to higher CO₂ emissions per unit of heat. Modern combustion facilities can help with this problem.

On average, forest biomass releases 10-16% more CO₂ than coal. But what matters is the net climate effect from both emissions and absorption together. The IEA Bioenergy says the extra CO₂ from biomass is not a problem if the biomass comes from forests managed in a sustainable way.

Climate impacts expressed as varying with time

The use of boreal stemwood for bioenergy only helps the climate in the long term, while using wood residues helps both in the short to medium term and long term.

Short carbon payback times happen when the alternative to bioenergy is traditional forestry, where good wood is used for lumber and residues are burned or left to decay. Collecting these residues provides material that would have released carbon back into the air anyway, either through decay or burning. Payback and parity times depend on how fast the residues decay, which depends on where they are located and how thick they are. Residues decay faster in warm, wet areas and if they are thin, while thick residues in cold, dry boreal forests decay the slowest. If residues are burned instead of left to decay, emissions happen right away, and parity times are almost zero.

Climate impacts expressed as static numbers

The EU’s Joint Research Centre studied bioenergy emission estimates and calculated greenhouse gas savings for different bioenergy pathways. They found that most bio-based products release less greenhouse gas than fossil products, but the amount of emissions varies a lot depending on things like logistics, the type of plants used, how the land is managed, and the technology.

Because the climate benefits of different biofuels can vary, governments and organizations have set up certification schemes to make sure biomass use is sustainable. In the EU, this is the Renewable Energy Directive (RED). In the US, the Renewables Fuel Standard (RFS) limits the use of traditional biofuels and sets minimum greenhouse gas emission reductions.

The EU’s RED says that using wood pellets from forest residues for heat can save between 69% and 77% on greenhouse gas emissions compared to fossil fuels, depending on how far the pellets are transported. Using stemwood can save between 70% and 77%, and using wood industry residues can save between 79% and 87%.

For energy crops, researchers calculate static climate benefits using life cycle assessment methods. An energy crop project is carbon positive if emissions during farming, processing, transport, and burning are more than what the plants absorb. It is carbon negative if absorption is more than emissions. Perennial crops, which grow for many years, usually sequester more carbon than annual crops because their roots keep building up without yearly plowing, which releases carbon.

Albedo and evapotranspiration

Further information: Transpirational cooling (biological)

Environmental impacts

See also: Indirect land use change impacts of biofuels and Bioenergy § Environmental impacts

When we use plants and plant waste for energy, we need to think about how it affects the world around us. For example, in 2022, a group of experts said that using plants for energy can help reduce pollution from fuels, but we must make sure we do it in a way that does not harm people or nature.

Sustainable forestry and forest protection

See also: Forest management § Sustainable forest management

Experts have different ideas about whether the world’s forests are getting bigger or smaller. Some say tree cover has grown since 1982. Big, old trees are very good at taking carbon from the air, but when we cut them down, we lose that benefit. Also, taking trees can hurt the soil.

Old trees absorb more carbon than young trees, but forests with young trees are growing and taking in carbon faster. Using wood from managed forests can help fight climate change if done the right way. For example, wood can replace materials that create more pollution when made.

Data shows that many wood pellets come from places like Europe and North America, where forests are managed carefully. In these areas, forests are growing, even though the rate is slowing as trees get older.

United Kingdom Emissions Trading System has rules about how biomass can be used for energy.

Biodiversity

See also: Biodiversity loss

Using plants for energy can sometimes hurt animals and plants that live in the same area. For example, some crops like oil palm and sugar cane have been linked to fewer animals and plants in the area. Changes in the number of plants and animals can also affect how soil works.

There are ways to use biomass that help both the climate and nature, like using tree parts from forests that grow back quickly. But there are also ways that can hurt nature, like planting only one type of tree on land that used to be a grassland full of different plants and animals.

Pollution

Using too many fertilizers and pesticides can pollute soil and water. Burning plant waste in open fields can also pollute the air. However, using modern ways to turn wood into energy can clean the air compared to older methods. Burning wood in factories creates less pollution than burning coal for the same amount of electricity.