Biomass (energy)

Biomass, in the context of energy production, is matter from recently living (but now dead) organisms which is used for bioenergy production. Examples include wood, wood residues, energy crops, agricultural residues including straw, and organic waste from industry and households. Wood and wood residues is the largest biomass energy source today. Wood can be used as a fuel directly or processed into pellet fuel or other forms of fuels. Other plants can also be used as fuel, for instance maize, switchgrass, miscanthus and bamboo. The main waste feedstocks are wood waste, agricultural waste, municipal solid waste, and manufacturing waste. Upgrading raw biomass to higher grade fuels can be achieved by different methods, broadly classified as thermal, chemical, or biochemical.

The climate impact of bioenergy varies considerably depending on where biomass feedstocks come from and how they are grown. For example, burning wood for energy releases carbon dioxide; those emissions can be significantly offset if the trees that were harvested are replaced by new trees in a well-managed forest, as the new trees will remove carbon dioxide from the air as they grow. However, the establishment and cultivation of bioenergy crops can displace natural ecosystems, degrade soils, take land out of food production and consume water resources and synthetic fertilisers.

In 2020 biomass produced 58 EJ (exajoules) of energy, compared to 172 EJ from crude oil, 157 EJ from coal, 138 EJ from natural gas, 29 EJ from nuclear, 16 EJ from hydro and 15 EJ from wind, solar and geothermal combined. Approximately 86% of modern bioenergy is used for heating applications, with 9% used for transport and 5% for electricity. Most of the global bioenergy is produced from forest resources.

Biomass Pellet

The IEA's Net Zero by 2050 scenario calls for traditional bioenergy to be phased out by 2030, with modern bioenergy's share increasing from 6.6% in 2020 to 13.1% in 2030 and 18.7% in 2050. The IPCC (Intergovernmental Panel on Climate Change) believes that bioenergy has a significant climate change mitigation potential if implemented correctly. Most of the IPCC's pathways including substantial contributions from bioenergy in 2050 (average at 200 EJ).

Terminology

Biomass (in the context of energy generation) is matter from recently living (but now dead) organisms which is used for bioenergy production. There are variations in how such biomass for energy is defined, e.g. only from plants, or from plants and algae, or from plants and animals. The vast majority of biomass used for bioenergy does come from plants. Bioenergy is a type of renewable energy with potential to assist with climate change mitigation.

Some people use the terms biomass and biofuel interchangeably, but it is now more common to consider biofuel to be a liquid or gaseous fuel used for transportation, as defined by government authorities in the US and EU. From that perspective, biofuel is a subset of biomass.

The European Union's Joint Research Centre defines solid biofuel as raw or processed organic matter of biological origin used for energy, such as firewood, wood chips, and wood pellets.

Types and uses

Further information: Energy crop

Different types of biomass are used for different purposes:

• Primary biomass sources that are appropriate for heat or electricity generation but not for transport include: wood, wood residues, wood pellets, agricultural residues, organic waste.
• Biomass that is processed into transport fuels can come from corn, sugar cane, and soy.

Biomass is categorized either as biomass harvested directly for energy (primary biomass), or as residues and waste: (secondary biomass).

Biomass harvested directly for energy

The main biomass types harvested directly for energy is wood, some food crops and all perennial energy crops. One third of the global forest area of 4 billion hectares is used for wood production or other commercial purposes, and forests provide 85% of all biomass used for energy globally. In the EU, forests provide 60% of all biomass used for energy, with wood residues and waste being the largest source.

Woody biomass used for energy often consists of trees and bushes harvested for traditional cooking and heating purposes, particularly in developing countries, with 25 EJ per year used globally for these purposes. This practice is highly polluting. The World Health Organization (WHO) estimates that cooking-related pollution causes 3.8 million annual deaths. The United Nations Sustainable Development Goal 7 aims for the traditional use of biomass for cooking to be phased out by 2030. Short-rotation coppices and short-rotation forests are also harvested directly for energy, providing 4 EJ of energy, and are considered sustainable. The potential for these crops and perennial energy crops to provide at least 25 EJ annually by 2050 is estimated.

Food crops harvested for energy include sugar-producing crops (such as sugarcane), starch-producing crops (such as corn), and oil-producing crops (such as rapeseed). Sugarcane is a perennial crop, while corn and rapeseed are annual crops. Sugar- and starch-producing crops are used to make bioethanol, and oil-producing crops are used to make biodiesel. The United States is the largest producer of bioethanol, while the European Union is the largest producer of biodiesel. The global production of bioethanol and biodiesel provides 2.2 and 1.5 EJ of energy per year, respectively. Biofuel made from food crops harvested for energy is also known as "first-generation" or "traditional" biofuel and has relatively low emission savings.

The IPCC estimates that between 0.32 and 1.4 billion hectares of marginal land are suitable for bioenergy worldwide.

Biomass in the form of residues and waste

Residues and waste are by-products from biological material harvested mainly for non-energy purposes. The most important by-products are wood residues, agricultural residues and municipal/industrial waste:

Wood residues are by-products from forestry operations or from the wood processing industry. Had the residues not been collected and used for bioenergy, they would have decayed (and therefore produced emissions) on the forest floor or in landfills, or been burnt (and produced emissions) at the side of the road in forests or outside wood processing facilities.

The by-products from forestry operations are called logging residues or forest residues, and consist of tree tops, branches, stumps, damaged or dying or dead trees, irregular or bent stem sections, thinnings (small trees that are cleared away in order to help the bigger trees grow large), and trees removed to reduce wildfire risk. The extraction level of logging residues differ from region to region, but there is an increasing interest in using this feedstock, since the sustainable potential is large (15 EJ annually). 68% of the total forest biomass in the EU consists of wood stems, and 32% consists of stumps, branches and tops.

The by-products from the wood processing industry are called wood processing residues and consist of cut offs, shavings, sawdust, bark, and black liquor. Wood processing residues have a total energy content of 5.5 EJ annually. Wood pellets are mainly made from wood processing residues, and have a total energy content of 0.7 EJ. Wood chips are made from a combination of feedstocks, and have a total energy content of 0.8 EJ.

The energy content in agricultural residues used for energy is approximately 2 EJ. However, agricultural residues has a large untapped potential. The energy content in the global production of agricultural residues has been estimated to 78 EJ annually, with the largest share from straw (51 EJ). Others have estimated between 18 and 82 EJ. The use of agricultural residues and waste that is both sustainable and economically feasible is expected to increase to between 37 and 66 EJ in 2030.

Municipal waste produced 1.4 EJ and industrial waste 1.1 EJ. Wood waste from cities and industry also produced 1.1 EJ. The sustainable potential for wood waste has been estimated to 2–10 EJ. IEA recommends a dramatic increase in waste utilization to 45 EJ annually in 2050.

Environmental impacts[edit]

The environmental impacts of biomass production need to be taken into account. For instance in 2022, IEA stated that "bioenergy is an important pillar of decarbonisation in the energy transition as a near zero-emission fuel", and that "more efforts are needed to accelerate modern bioenergy deployment to get on track with the Net Zero Scenario [....] while simultaneously ensuring that bioenergy production does not incur negative social and environmental consequences."

Sustainable forestry and forest protection

IPCC states that there is disagreement about whether the global forest is shrinking or not, and quote research indicating that tree cover has increased 7.1% between 1982 and 2016. The IPCC writes: "While above-ground biomass carbon stocks are estimated to be declining in the tropics, they are increasing globally due to increasing stocks in temperate and boreal forests [...]."

Old trees have a very high carbon absorption rate, and felling old trees means that this large potential for future carbon absorption is lost. There is also a loss of soil carbon due to the harvest operations.

Old-growth spruce forest

Old trees absorb more CO₂ than young trees, because of the larger leaf area in full grown trees. However, the old forest (as a whole) will eventually stop absorbing CO₂ because CO₂ emissions from dead trees cancel out the remaining living trees' CO₂ absorption. The old forest (or forest stands) are also vulnerable for natural disturbances that produces CO₂. The IPCC found that "[...] landscapes with older forests have accumulated more carbon but their sink strength is diminishing, while landscapes with younger forests contain less carbon but they are removing CO₂ from the atmosphere at a much higher rate [...]."

The IPCC states that the net climate effect from conversion of unmanaged to managed forest can be positive or negative, depending on circumstances. The carbon stock is reduced, but since managed forests grow faster than unmanaged forests, more carbon is absorbed. Positive climate effects are produced if the harvested biomass is used efficiently. There is a tradeoff between the benefits of having a maximized forest carbon stock, not absorbing any more carbon, and the benefits of having a portion of that carbon stock "unlocked", and instead working as a renewable fossil fuel replacement tool, for instance in sectors which are difficult or expensive to decarbonize.

The "competition" between locked-away and unlocked forest carbon might be won by the unlocked carbon: "In the long term, using sustainably produced forest biomass as a substitute for carbon-intensive products and fossil fuels provides greater permanent reductions in atmospheric CO₂ than preservation does."

IEA Bioenergy writes: "forests managed for producing sawn timber, bioenergy and other wood products can make a greater contribution to climate change mitigation than forests managed for conservation alone." Three reasons are given:

1. reducing ability to act as a carbon sink when the forest matures.
2. Wood products can replace other materials that emitted more GHGs during production.
3. "Carbon in forests is vulnerable to loss through natural events such as insect infestations or wildfires"

Data from FAO show that most wood pellets are produced in regions dominated by sustainably managed forests, such as Europe and North America. Europe (including Russia) produced 54% of the world's wood pellets in 2019, and the forest carbon stock in this area increased from 158.7 to 172.4 Gt between 1990 and 2020. In the EU, above-ground forest biomass increases with 1.3% per year on average, however the increase is slowing down because the forests are maturing.

United Kingdom Emissions Trading System allows operators of CO2 generating installations to apply zero emissions factor for the fraction used for non-energy purposes, while energy purposes (electricity generation, heating) require additional sustainability certification on the biomass used.

Biodiversity

See also: Biodiversity loss

Biomass production for bioenergy can have negative impacts on biodiversity. Oil palm and sugar cane are examples of crops that have been linked to reduced biodiversity. In addition, changes in biodiversity also impacts primary production which naturally effects decomposition and soil heterotrophic organisms.

Win-win scenarios (good for climate, good for biodiversity) include:

• Increased use of whole trees from coppice forests, increased use of thin forest residues from boreal forests with slow decay rates, and increased use of all kinds of residues from temperate forests with faster decay rates;
• Multi-functional bioenergy landscapes, instead of expansion of monoculture plantations;
• Afforestation of former agricultural land with mixed or naturally regenerating forests.

Win-lose scenarios (good for the climate, bad for biodiversity) include afforestation on ancient, biodiversity-rich grassland ecosystems which were never forests, and afforestation of former agricultural land with monoculture plantations.

Lose-win scenarios (bad for the climate, good for biodiversity) include natural forest expansion on former agricultural land.

Lose-lose scenarios include increased use of thick forest residues like stumps from some boreal forests with slow decay rates, and conversion of natural forests into forest plantations.

Pollution

Other problems are pollution of soil and water from fertiliser/pesticide use, and emission of ambient air pollutants, mainly from open field burning of residues.

The traditional use of wood in cook stoves and open fires produces pollutants, which can lead to severe health and environmental consequences. However, a shift to modern bioenergy contribute to improved livelihoods and can reduce land degradation and impacts on ecosystem services. According to the IPCC, there is strong evidence that modern bioenergy have "large positive impacts" on air quality. Traditional bioenergy is inefficient and the phasing out of this energy source has both large health benefits and large economic benefits. When combusted in industrial facilities, most of the pollutants originating from woody biomass reduce by 97-99%, compared to open burning. Combustion of woody biomass produces lower amounts of particulate matter than coal for the same amount of electricity generated.