This article is about technology aspects to produce a type of renewable energy. For production of biomass for bioenergy generation, see biomass (energy).
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Bioenergy is a type of renewable energy that is derived from plants and animal waste.[1] The biomass that is used as input materials consists of recently living (but now dead) organisms, mainly plants.[2] Thus, fossil fuels are not regarded as biomass under this definition. Types of biomass commonly used for bioenergy include wood, food crops such as corn, energy crops and waste from forests, yards, or farms.[3]
Bioenergy can help with climate change mitigation but in some cases the required biomass production can increase greenhouse gas emissions or lead to local biodiversity loss. The environmental impacts of biomass production can be problematic, depending on how the biomass is produced and harvested.
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.[4] Bioenergy has a significant climate change mitigation potential if implemented correctly.[5]: 637 Most of the recommended pathways to limit global warming include substantial contributions from bioenergy in 2050 (average at 200 EJ).[6]: B 7.4
The IPCC Sixth Assessment Report defines bioenergy as "energy derived from any form of biomass or its metabolic by-products".[7]: 1795 It goes on to define biomass in this context as "organic material excluding the material that is fossilised or embedded in geological formations".[7]: 1795 This means that coal or other fossil fuels is not a form of biomass in this context.
The term traditional biomass for bioenergy means "the combustion of wood, charcoal, agricultural residues and/or animal dung for cooking or heating in open fires or in inefficient stoves as is common in low-income countries".[7]: 1796
Since biomass can also be used as a fuel directly (e.g. wood logs), the terms biomass and biofuel have sometimes been used interchangeably. However, the term biomass usually denotes the biological raw material the fuel is made of. The terms biofuel or biogas are generally reserved for liquid or gaseous fuels respectively.[8]
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.[9] 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:
Thermal conversion processes use heat as the dominant mechanism to upgrade biomass into a better and more practical fuel. The basic alternatives are torrefaction, pyrolysis, and gasification, these are separated mainly by the extent to which the chemical reactions involved are allowed to proceed (mainly controlled by the availability of oxygen and conversion temperature).[10]
Many chemical conversions are based on established coal-based processes, such as the Fischer-Tropsch synthesis.[11] Like coal, biomass can be converted into multiple commodity chemicals.[12]
Biochemical processes have developed in nature to break down the molecules of which biomass is composed, and many of these can be harnessed. In most cases, microorganisms are used to perform the conversion. The processes are called anaerobic digestion, fermentation, and composting.[13]
Based on the source of biomass, biofuels are classified broadly into two major categories, depending if food crops are used or not:[14]
First-generation (or "conventional") biofuels are made from food sources grown on arable lands, such as sugarcane and maize. Sugars present in this biomass are fermented to produce bioethanol, an alcohol fuel which serves as an additive to gasoline, or in a fuel cell to produce electricity. Bioethanol is made by fermentation, mostly from carbohydrates produced in sugar or starch crops such as corn, sugarcane, or sweet sorghum. Bioethanol is widely used in the United States and in Brazil. Biodiesel is produced from the oils in for instance rapeseed or sugar beets and is the most common biofuel in Europe.[citation needed]
Second-generation biofuels (also called "advanced biofuels") utilize non-food-based biomass sources such as perennial energy crops and agricultural residues/waste. The feedstock used to make the fuels either grow on arable land but are byproducts of the main crop, or they are grown on marginal land. Waste from industry, agriculture, forestry and households can also be used for second-generation biofuels, using e.g. anaerobic digestion to produce biogas, gasification to produce syngas or by direct combustion. Cellulosic biomass, derived from non-food sources, such as trees and grasses, is being developed as a feedstock for ethanol production, and biodiesel can be produced from left-over food products like vegetable oils and animal fats.[citation needed]
The surface power production densities of a crop will determine how much land is required for production. The average lifecycle surface power densities for biomass, wind, hydro and solar power production are 0.30 W/m2, 1 W/m2, 3 W/m2 and 5 W/m2, respectively (power in the form of heat for biomass, and electricity for wind, hydro and solar).[15] Lifecycle surface power density includes land used by all supporting infrastructure, manufacturing, mining/harvesting and decommissioning.
Another estimate puts the values at 0.08 W/m2 for biomass, 0.14 W/m2 for hydro, 1.84 W/m2 for wind, and 6.63 W/m2 for solar (median values, with none of the renewable sources exceeding 10 W/m2).[16]
Related technologies
Bioenergy with carbon capture and storage (BECCS)
Carbon capture and storage technology can be used to capture emissions from bioenergy power plants. This process is known as bioenergy with carbon capture and storage (BECCS) and can result in net carbon dioxide removal from the atmosphere. However, BECCS can also result in net positive emissions depending on how the biomass material is grown, harvested, and transported. Deployment of BECCS at scales described in some climate change mitigation pathways would require converting large amounts of cropland.[17]
Greenhouse gas emissions from bioenergy can be low because when vegetation is harvested for bioenergy, new vegetation can grow that will absorb CO2 from the air through photosynthesis.[19] After the biomass is harvested, energy ("bioenergy") is extracted in useful forms (electricity, heat, biofuels, etc.) as the biomass is utilized through combustion, fermentation, pyrolysis or other conversion methods. Using bioenergy releases CO2. In BECCS, some of the CO2 is captured before it enters the atmosphere, and stored underground using carbon capture and storage technology.[20] Under some conditions, BECCS can remove carbon dioxide from the atmosphere.[20]
The potential range of negative emissions from BECCS was estimated to be zero to 22 gigatonnes per year.[21] As of 2019[update], five facilities around the world were actively using BECCS technologies and were capturing approximately 1.5 million tonnes per year of CO2.[22] Wide deployment of BECCS is constrained by cost and availability of biomass.[23][24]: 10 Since biomass production is land-intensive, deployment of BECCS can pose major risks to food production, human rights, and biodiversity.[25]
Alternative system boundaries for assessing climate effects of forest-based bioenergy. Option 1 (black) considers only the stack emissions; Option 2 (green) considers only the forest carbon stock; Option 3 (blue) considers the bioenergy supply chain; Option 4 (red) covers the whole bioeconomy, including wood products in addition to biomass.[26]
The climate impact of bioenergy varies considerably depending on where biomass feedstocks come from and how they are grown.[27] 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 absorb carbon dioxide from the air as they grow.[28] However, the establishment and cultivation of bioenergy crops can displace natural ecosystems, degrade soils, and consume water resources and synthetic fertilisers.[29][30]
Approximately one-third of all wood used for traditional heating and cooking in tropical areas is harvested unsustainably.[31] Bioenergy feedstocks typically require significant amounts of energy to harvest, dry, and transport; the energy usage for these processes may emit greenhouse gases. In some cases, the impacts of land-use change, cultivation, and processing can result in higher overall carbon emissions for bioenergy compared to using fossil fuels.[30][32]
Use of farmland for growing biomass can result in less land being available for growing food. In the United States, around 10% of motor gasoline has been replaced by corn-based ethanol, which requires a significant proportion of the harvest.[33][34] In Malaysia and Indonesia, clearing forests to produce palm oil for biodiesel has led to serious social and environmental effects, as these forests are critical carbon sinks and habitats for diverse species.[35][36] Since photosynthesis captures only a small fraction of the energy in sunlight, producing a given amount of bioenergy requires a large amount of land compared to other renewable energy sources.[37]
Bioenergy can either mitigate (i.e. reduce) or increase greenhouse gas emissions. There is also agreement that local environmental impacts can be problematic.[citation needed] For example, increased biomass demand can create significant social and environmental pressure in the locations where the biomass is produced.[38] The impact is primarily related to the low surface power density of biomass. The low surface power density has the effect that much larger land areas are needed in order to produce the same amount of energy, compared to for instance fossil fuels.
Long-distance transport of biomass have been criticised as wasteful and unsustainable,[39] and there have been protests against forest biomass export in Sweden[40] and Canada.[41]
Scale and future trends
In 2020 bioenergy 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.[42] Most of the global bioenergy is produced from forest resources.[43]: 3 [44]: 1
Generally, bioenergy expansion fell by 50% in 2020. China and Europe are the only two regions that reported significant expansion in 2020, adding 2 GW and 1.2 GW of bioenergy capacity, respectively.[45]
Almost all available sawmill residue is already being utilized for pellet production, so there is no room for expansion. For the bioenergy sector to significantly expand in the future, more of the harvested pulpwood must go to pellet mills. However, the harvest of pulpwood (tree thinnings) removes the possibility for these trees to grow old and therefore maximize their carbon holding capacity.[46]: 19 Compared to pulpwood, sawmill residues have lower net emissions: "Some types of biomass feedstock can be carbon-neutral, at least over a period of a few years, including in particular sawmill residues. These are wastes from other forest operations that imply no additional harvesting, and if otherwise burnt as waste or left to rot would release carbon to the atmosphere in any case."[46]: 68
^Fajardy, Mathilde; Köberle, Alexandre; Mac Dowell, Niall; Fantuzzi, Andrea (2019). "BECCS deployment: a reality check"(PDF). Grantham Institute Imperial College London.
Akhtar, Ali; Krepl, Vladimir; Ivanova, Tatiana (2018-07-05). "A Combined Overview of Combustion, Pyrolysis, and Gasification of Biomass". Energy & Fuels. 32 (7). American Chemical Society (ACS): 7294–7318. doi:10.1021/acs.energyfuels.8b01678. ISSN0887-0624. S2CID105089787.
Liu, Guangjian; Larson, Eric D.; Williams, Robert H.; Kreutz, Thomas G.; Guo, Xiangbo (2011-01-20). "Making Fischer−Tropsch Fuels and Electricity from Coal and Biomass: Performance and Cost Analysis". Energy & Fuels. 25 (1). American Chemical Society (ACS): 415–437. doi:10.1021/ef101184e. ISSN0887-0624.
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