In the context of energy production, biomass 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 farming of biomass feedstocks can reduce biodiversity, degrade soils and take land out of food production. It may also consume water for irrigation and fertilisers.
Global cultivable land and water resources are adequate to produce required biomass for food and supplementing carbon neutral energy needs where renewable electricity could not be used economically. Using biomass and renewable electricity in place of fossil fuels for the total energy needs of global peak population would not increase the anthropogenic green house gas emissions. Biomass economy has potential to emerge next to renewable energy economy.
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
+ Energy content of different fuels in the order of heating value Fuel type %H + %C Higher heating value Comment Natural gas (methane) 100 55 Diesel fuel 100 46 lower than methane because lower H/C ratio Coal (bituminous) 81 31 partially oxygenated Saw dust 53 20 highly oxygenated As seen from the table above, the chief problem with biomass, such as saw dust, as an energy source is that it has far lower energy content relative to fossil fuels.
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).IRENA (2014). "Global bioenergy supply and demand projections – a working paper for REmap 2030" International Renewable Energy Agency.
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.WBA (2019) GLOBAL BIOENERGY STATISTICS 2019 World Bioenergy Association 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 maize), 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.
Biomass conversion Raw biomass can be upgraded into better and more practical fuel simply by compacting it (e.g. wood pellets), or by different conversions broadly classified as thermal, chemical, and biochemical. Biomass conversion reduces the transport costs as it is cheaper to transport high density commodities.
Thermal conversion
Thermal upgrading produces solid, liquid or gaseous fuels, with heat as the dominant conversion driver. The basic alternatives are torrefaction, pyrolysis, and gasification, these are separated principally by how far the chemical reactions involved are allowed to proceed. The advancement of the chemical reactions is mainly controlled by how much oxygen is available, and the conversion temperature.
Torrefaction is a mild form of pyrolysis where organic materials are heated to 400–600 °F (200–300 °C) in a no–to–low oxygen environment. The heating process removes (via gasification) the parts of the biomass that has the lowest energy content, while the parts with the highest energy content remain. Approximately 30% of the biomass is converted to gas during the torrefaction process, while 70% remains, usually in the form of compacted pellets or briquettes. This solid product is water resistant, easy to grind, non-corrosive, and contains approximately 85% of the original biomass energy. Basically the mass part has shrunk more than the energy part, and the consequence is that the calorific value of torrefied biomass increases significantly, to the extent that it can compete with coals used for electricity generation (steam/thermal coals). The energy density of the most common steam coals today is 22–26 GJ/t. There are other less common, more experimental or proprietary thermal processes that may offer benefits, such as hydrothermal upgrading (sometimes called "wet" torrefaction.) The hydrothermal upgrade path can be used for both low and high moisture content biomass, e.g. aqueous 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 entails heating organic materials to 800–900 °F (400–500 °C) in the near complete absence of oxygen. Biomass pyrolysis produces fuels such as bio-oil, charcoal, methane, and hydrogen. Hydrotreating is used to process bio-oil (produced by fast pyrolysis) with hydrogen under elevated temperatures and pressures in the presence of a catalyst to produce renewable diesel, renewable gasoline, and renewable jet fuel.
Gasification entails heating organic materials to 1,400–1700 °F (800–900 °C) with injections of controlled amounts of oxygen and/or steam into the vessel to produce a carbon monoxide and hydrogen rich gas called synthesis gas or syngas. Syngas can be used as a fuel for diesel engines, for heating, and for generating electricity in gas turbines. It can also be treated to separate the hydrogen from the gas, and the hydrogen can be burned or used in fuel cells. The syngas can be further processed to produce liquid fuels using the Fischer-Tropsch synthesis process.
Chemical conversion