Biofuel is afuel that is produced over a short time span frombiomass, rather than by the very slow natural processes involved in the formation offossil fuels such as oil. Biofuel can be produced from plants or from agricultural, domestic or industrialbio waste.[1][2][3][4] Biofuels are mostly used for transportation, but can also be used for heating and electricity.[5]: 173 [6] Biofuels (andbio energy in general) are regarded as arenewable energy source.[7]: 11 The use of biofuel has been subject to criticism regarding the "food vs fuel" debate, varied assessments of theirsustainability, and ongoingdeforestation andbiodiversity loss as a result of biofuel production.
In general, biofuels emit fewergreenhouse gas emissions when burned in an engine and are generally consideredcarbon-neutral fuels as the carbon emitted has been captured from the atmosphere by the crops used in production.[8] However,life-cycle assessments of biofuels have shown large emissions associated with the potentialland-use change required to produce additional biofuel feedstocks.[9][10] The outcomes of lifecycle assessments (LCAs) for biofuels are highly situational and dependent on many factors including the type of feedstock, production routes, data variations, and methodological choices. This could be added to emphasize the complexity and variability in assessing the environmental impacts of biofuels.[11] Estimates about the climate impact from biofuels vary widely based on the methodology and exact situation examined.[9] Therefore, theclimate change mitigation potential of biofuel varies considerably: in some scenarios emission levels are comparable to fossil fuels, and in other scenarios the biofuel emissions result innegative emissions.
Global demand for biofuels is predicted to increase by 56% over 2022–2027.[12] By 2027 worldwide biofuel production is expected to supply 5.4% of the world's fuels for transport including 1% of aviation fuel.[13] Demand foraviation biofuel is forecast to increase.[14][15] However some policy has been criticised for favoring ground transportation over aviation.[16]
The two most common types of biofuel arebioethanol andbiodiesel. Brazil is the largest producer of bioethanol, while the EU is the largest producer of biodiesel. The energy content in the global production of bioethanol and biodiesel is 2.2 and 1.8 EJ per year, respectively.[17]
Biodiesel is produced from oils or fats usingtransesterification. It can be used as a fuel for vehicles in its pure form (B100), but it is usually used as adiesel additive to reduce levels of particulates,carbon monoxide, andhydrocarbons from diesel-powered vehicles.[18]
The termbiofuel is used in different ways. One definition is "Biofuels are biobased products, in solid, liquid, or gaseous forms. They are produced from crops or natural products, such as wood, or agricultural residues, such as molasses and bagasse."[5]: 173
Other publications reserve the term biofuel forliquid orgaseous fuels, used for transportation.[6]
TheIPCC Sixth Assessment Report definesbiofuel as "A fuel, generally in liquid form, produced frombiomass. Biofuels includebioethanol from sugarcane, sugar beet or maize, andbiodiesel from canola or soybeans.".[19]: 1795 It goes on to definebiomass in this context as "organic material excluding the material that is fossilised or embedded in geological formations".[19]: 1795 This means thatcoal or otherfossil fuels is not a form of biomass in this context.
First-generation biofuels (also denoted as "conventional biofuels") are made from food crops grown on arable land.[20][21]: 447 The crop's sugar, starch, or oil content is converted intobiodiesel orethanol, usingtransesterification, or yeast fermentation.[22]
To avoid a "food versus fuel" dilemma,second-generation biofuels and third-generation biofuels (also calledadvanced biofuels orsustainable biofuels or drop-in biofuels) are made from feedstocks which do not directly compete with food or feed crop such as waste products and energy crops.[23] A wide range of renewable residue feedstocks such as those derived from agriculture and forestry activities like rice straw, rice husk, wood chips, and sawdust can be used to produce advanced biofuels through biochemical and thermochemical processes.[21]: 448 [24]
The feedstock used to make the fuels either grow onarable land but are byproducts of the main crop, or they are grown on marginal land. Second-generation feedstocks also include straw, bagasse, perennial grasses, jatropha, waste vegetable oil, municipal solid waste and so forth.[25]
Biologically producedalcohols, most commonly ethanol, and less commonlypropanol andbutanol, are produced by the action ofmicroorganisms andenzymes through the fermentation of sugars or starches (easiest to produce) or cellulose (more difficult to produce).The IEA estimates that ethanol production used 20% of sugar supplies and 13% of corn supplies in 2021.[26]
Ethanol fuel is the most common biofuel worldwide, particularlyin Brazil.Alcohol fuels are produced by fermentation of sugars derived fromwheat,corn,sugar beets,sugar cane,molasses and any sugar or starch from whichalcoholic beverages such aswhiskey, can be made (such aspotato andfruit waste, etc.). Production methods used areenzyme digestion (to release sugars from stored starches), fermentation of the sugars,distillation and drying. The distillation process requires significant energy input to generate heat. Heat is sometimes generated with unsustainablenatural gas fossil fuel, but cellulosic biomass such asbagasse is the most common fuel in Brazil, while pellets, wood chips and alsowaste heat are more common in Europe. Corn-to-ethanol and other food stocks has led to the development ofcellulosic ethanol.[27]
Methanol is currently produced fromnatural gas, anon-renewable fossil fuel. In the future it is hoped to be produced from biomass asbiomethanol. This is technically feasible, but the production is currently being postponed for concerns that the economic viability is still pending.[28] Themethanol economy is an alternative to thehydrogen economy to be contrasted with today'shydrogen production from natural gas.
Butanol (C 4H 9OH) is formed byABE fermentation (acetone, butanol, ethanol) and experimental modifications of the process show potentially highnet energy gains withbiobutanol as the only liquid product. Biobutanol is often claimed to provide a direct replacement for gasoline, because it will produce more energy than ethanol and allegedly can be burned "straight" in existing gasoline engines (without modification to the engine or car),[29] is less corrosive and less water-soluble than ethanol, and could be distributed via existing infrastructures.Escherichia coli strains have also been successfully engineered to produce butanol by modifying theiramino acid metabolism.[30] One drawback to butanol production inE. coli remains the high cost ofnutrient rich media, however, recent work has demonstratedE. coli can produce butanol with minimal nutritional supplementation.[31] Biobutanol is sometimes calledbiogasoline, which is incorrect as it is chemically different, being an alcohol and not a hydrocarbon like gasoline.
Biodiesel is the most common biofuel in Europe. It is produced from oils or fats usingtransesterification and is a liquid similar in composition to fossil/mineral diesel. Chemically, it consists mostly of fatty acid methyl (or ethyl) esters (FAMEs).[32] Feedstocks for biodiesel include animal fats, vegetable oils,soy,rapeseed,jatropha,mahua,mustard,flax,sunflower,palm oil,hemp,field pennycress,Pongamia pinnata andalgae. Pure biodiesel (B100, also known as "neat" biodiesel) currently reduces emissions with up to 60% compared to diesel Second generation B100.[33] As of 2020[update], researchers at Australia'sCSIRO have been studyingsafflower oil as an enginelubricant, and researchers atMontana State University's Advanced Fuels Center in the US have been studying the oil's performance in a largediesel engine, with results described as a "breakthrough".[34]
Biodiesel can be used in any diesel engine and modified equipment when mixed with mineral diesel. It can also be used in its pure form (B100) in diesel engines, but some maintenance and performance problems may occur during wintertime utilization, since the fuel becomes somewhat moreviscous at lower temperatures, depending on the feedstock used.[35]
Electronically controlled 'common rail' and 'Unit Injector' type systems from the late 1990s onwards can only use biodiesel blended with conventional diesel fuel. These engines have finely metered and atomized multiple-stage injection systems that are very sensitive to the viscosity of the fuel. Many current-generation diesel engines are designed to run on B100 without altering the engine itself, although this depends on thefuel rail design. Since biodiesel is an effectivesolvent and cleans residues deposited by mineral diesel,engine filters may need to be replaced more often, as the biofuel dissolves old deposits in the fuel tank and pipes. It also effectively cleans the enginecombustion chamber of carbon deposits, helping to maintain efficiency.
Biodiesel is anoxygenated fuel, meaning it contains a reduced amount of carbon and higher hydrogen and oxygen content than fossil diesel. This improves thecombustion of biodiesel and reduces the particulate emissions from unburnt carbon. However, using pure biodiesel may increase NOx-emissions[36] Biodiesel is also safe to handle and transport because it is non-toxic andbiodegradable, and has a highflash point of about 300 °F (148 °C) compared to petroleum diesel fuel, which has a flash point of 125 °F (52 °C).[37]
In many European countries, a 5% biodiesel blend is widely used and is available at thousands of gas stations.[38][39] In France, biodiesel is incorporated at a rate of 8% in the fuel used by all French diesel vehicles.[40]Avril Group produces under the brandDiester, a fifth of 11 million tons of biodiesel consumed annually by theEuropean Union.[41] It is the leading European producer of biodiesel.[40]
Green diesel can be produced from a combination of biochemical and thermochemical processes. Conventional green diesel is produced through hydroprocessing biological oil feedstocks, such as vegetable oils and animal fats.[42][43] Recently, it is produced using series of thermochemical processes such as pyrolysis and hydroprocessing. In the thermochemical route, syngas produced from gasification, bio-oil produced from pyrolysis or biocrude produced from hydrothermal liquefaction is upgraded to green diesel using hydroprocessing.[44][45][46] Hydroprocessing is the process of using hydrogen to reform a molecular structure. For example,hydrocracking which is a widely used hydroprocessing technique in refineries is used at elevated temperatures and pressure in the presence of a catalyst to break down largermolecules, such as those found invegetable oils, into shorterhydrocarbon chains used indiesel engines.[47] Green diesel may also be called renewable diesel, drop-in biodiesel, hydrotreated vegetable oil (HVO fuel)[47] or hydrogen-derived renewable diesel.[43] Unlike biodiesel, green diesel has exactly the same chemical properties as petroleum-based diesel.[47][48] It does not require new engines, pipelines or infrastructure to distribute and use, but has not been produced at a cost that is competitive withpetroleum.[43] Gasoline versions are also being developed.[49] Green diesel is being developed inLouisiana andSingapore byConocoPhillips,Neste Oil,Valero, Dynamic Fuels, andHoneywell UOP[43][50] as well as Preem in Gothenburg, Sweden, creating what is known as Evolution Diesel.[51]
Straight unmodifiededible vegetable oil is generally not used as fuel, but lower-quality oil has been used for this purpose. Used vegetable oil is increasingly being processed into biodiesel, or (more rarely) cleaned of water and particulates and then used as a fuel. The IEA estimates that biodiesel production used 17% of global vegetable oil supplies in 2021.[26]
Oils and fats reacted with 10 pounds of a short-chain alcohol (usually methanol) in the presence of a catalyst (usually sodium hydroxide [NaOH] can behydrogenated to give a diesel substitute.[53] The resulting product is a straight-chain hydrocarbon with a highcetane number, low inaromatics andsulfur and does not contain oxygen.Hydrogenated oils can be blended with diesel in all proportions. They have several advantages over biodiesel, including good performance at low temperatures, no storage stability problems and no susceptibility to microbial attack.[54]
Biogasoline can be produced biologically and thermochemically. Using biological methods, a study led by Professor Lee Sang-yup at the Korea Advanced Institute of Science and Technology (KAIST) and published in the international science journalNature used modifiedE. coli fed with glucose found in plants or other non-food crops to produce biogasoline with the produced enzymes. The enzymes converted the sugar into fatty acids and then turned these into hydrocarbons that were chemically and structurally identical to those found in commercial gasoline fuel.[55] The thermochemical approach of producing biogasoline are similar to those used to produce biodiesel.[44][45][46] Biogasoline may also be called drop-in gasoline or renewable gasoline.
Bioethers (also referred to as fuelethers or oxygenated fuels) are cost-effectivecompounds that act asoctane rating enhancers. "Bioethers are produced by the reaction of reactive iso-olefins, such as iso-butylene, with bioethanol."[56][attribution needed] Bioethers are created from wheat or sugar beets, and also be produced from the waste glycerol that results from the production of biodiesel.[57] They also enhanceengine performance, while significantly reducing engine wear andtoxicexhaust emissions. By greatly reducing the amount of ground-levelozone emissions, they contribute to improved air quality.[59][60]
The European Fuel Oxygenates Association identifies MTBE and ETBE as the most commonly used ethers in fuel to replace lead. Ethers were introduced in Europe in the 1970s to replace the highly toxic compound.[62] Although Europeans still use bioether additives, the U.S.Energy Policy Act of 2005 lifted a requirement forreformulated gasoline to include an oxygenate, leading to less MTBE being added to fuel.[63] Although bioethers are likely to replace ethers produced from petroleum in the UK, it is highly unlikely they will become a fuel in and of itself due to the low energy density.[64]
Anaviation biofuel (also known as bio-jet fuel,[65] sustainable aviation fuel (SAF) or bio-aviation fuel (BAF)[66]) is a biofuel used to poweraircraft. TheInternational Air Transport Association (IATA) considers it a key element in reducing theenvironmental impact of aviation.[67] Aviation biofuel is used todecarbonize medium and long-haul air travel. These types of travel generate the most emissions, and could extend the life of older aircraft types by lowering their carbon footprint. Synthetic paraffinic kerosene (SPK) refers to any non-petroleum-based fuel designed to replace kerosenejet fuel, which is often, but not always, made from biomass.
Biofuels arebiomass-derived fuels from plants, animals, or waste; depending on which type of biomass is used, they could lowerCO2 emissions by 20–98% compared toconventional jet fuel.[68]The first test flight using blended biofuel was in 2008, and in 2011, blended fuels with 50% biofuels were allowed on commercial flights. In 2023 SAF production was 600 million liters, representing 0.2% of global jet fuel use.[69]
SAF technology faces significant challenges due to feedstock constraints. The oils and fats known as hydrotreated esters and fatty acids (Hefa), crucial for SAF production, are in limited supply as demand increases. Although advancede-fuels technology, which combines wasteCO2 withclean hydrogen, presents a promising solution, it is still under development and comes with high costs. To overcome these issues, SAF developers are exploring more readily available feedstocks such aswoody biomass and agricultural and municipal waste, aiming to produce lower-carbon jet fuel more sustainably and efficiently.[71][72]
In Sweden, "waste-to-energy" power plants capture methane biogas from garbage and use it to power transport systems.[75] Farmers can produce biogas from cattlemanure via anaerobic digesters.[76]
Syngas, a mixture ofcarbon monoxide,hydrogen and various hydrocarbons, is produced by partial combustion of biomass (combustion with an amount ofoxygen that is not sufficient to convert the biomass completely to carbon dioxide and water).[54] Before partial combustion the biomass is dried and sometimespyrolysed. Syngas is more efficient than direct combustion of the original biofuel; more of the energy contained in the fuel is extracted.
Syngas may be burned directly in internal combustion engines,turbines or high-temperature fuel cells.[77] Thewood gas generator, a wood-fueled gasification reactor, can be connected to an internal combustion engine.
Syngas can be used to producemethanol,dimethyl ether andhydrogen, or converted via theFischer–Tropsch process to produce a diesel substitute, or a mixture of alcohols that can be blended into gasoline. Gasification normally relies on temperatures greater than 700 °C.
Lower-temperature gasification is desirable when co-producingbiochar, but results in syngas polluted withtar.
Algae can be produced in ponds or tanks on land, and out at sea.[78][79] Algal fuels have high yields,[80] a highignition point,[81] can be grown with minimal impact onfresh water resources,[82][83][84] can be produced using saline water andwastewater, and arebiodegradable and relatively harmless to the environment if spilled.[85][86] However, production requires large amounts of energy and fertilizer, the produced fuel degrades faster than other biofuels, and it does not flow well in cold temperatures.[78][87]
By 2017, due to economic considerations, most efforts to produce fuel from algae have been abandoned or changed to other applications.[88]
Third and fourth-generation biofuels also include biofuels that are produced by bioengineered organisms i.e. algae and cyanobacteria.[89] Algae and cyanobacteria will use water, carbon dioxide, and solar energy to produce biofuels.[89] This method of biofuel production is still at the research level. The biofuels that are secreted by the bioengineered organisms are expected to have higher photon-to-fuel conversion efficiency, compared to older generations of biofuels.[89] One of the advantages of this class of biofuels is that the cultivation of the organisms that produce the biofuels does not require the use of arable land.[90] The disadvantages include the cost of cultivating the biofuel-producing organisms being very high.[90]
A bio-digester is a mechanized toilet that uses decomposition and sedimentation to turn human waste into a renewable fuel called biogas. Biogas can be made from substances like agricultural waste and sewage.[92][93] The bio-digester uses a process called anaerobic digestion to produce biogas. Anaerobic digestion uses a chemical process to break down organic matter with the use of microorganisms in the absence of oxygen to produce biogas.[94] The processes involved in anaerobic respiration are hydrolysis,acidogenesis,acetogenesis, andmethanogenesis.[95]
Global biofuel production was 81Mtoe in 2017 which represented an annual increase of about 3% compared to 2010.[7]: 12 In 2017, the US was the largest biofuel producer in the world producing 37 Mtoe, followed by Brazil and South America at 23 Mtoe and Europe (mainly Germany) at 12 Mtoe.[7]: 12
An assessment from 2017 found that: "Biofuels will never be a major transport fuel as there is just not enough land in the world to grow plants to make biofuel for all vehicles. It can however, be part of an energy mix to take us into a future ofrenewable energy."[7]: 11
In 2021, worldwide biofuel production provided 4.3% of the world's fuels for transport, including a very small amount ofaviation biofuel.[13] By 2027, worldwide biofuel production is expected to supply 5.4% of the world's fuels for transport including 1% of aviation fuel.[13]
The US, Europe, Brazil and Indonesia are driving the majority of biofuel consumption growth. This demand for biodiesel, renewable diesel and biojet fuel is projected to increase by 44% (21 billion litres) over 2022-2027.[96]
Issues relating to biofuel are social, economic, environmental and technical problems that may arise from biofuel production and use. Social and economic issues include the "food vs fuel" debate and the need to develop responsible policies and economic instruments to ensuresustainable biofuel production. Farming for biofuels feedstock can be detrimental to the environment if not done sustainably. Environmental concerns includedeforestation,biodiversity loss andsoil erosion as a result of land clearing for biofuels agriculture. While biofuels can contribute to reduction in globalcarbon emissions,indirect land use change for biofuel production can have the inverse effect. Technical issues include possible modifications necessary to run the engine on biofuel, as well asenergy balance and efficiency.
TheInternational Resource Panel outlined the wider and interrelated factors that need to be considered when deciding on the relative merits of pursuing one biofuel over another.[97] The IRP concluded that not all biofuels perform equally in terms of their effect on climate,energy security and ecosystems, and suggested that environmental and social effects need to be assessed throughout the entire life-cycle.
Estimates about the climate impact from biofuels vary widely based on the methodology and exact situation examined.[9]
In general, biofuels emit fewergreenhouse gas emissions when burned in an engine and are generally consideredcarbon-neutral fuels as the carbon they emit has beencaptured from the atmosphere by the crops used in biofuel production.[8] They can have greenhouse gas emissions ranging from as low as -127.1 gCO2eq per MJ when carbon capture is incorporated into their production to those exceeding 95 gCO2eq per MJ whenland-use change is significant.[45][46] Several factors are responsible for the variation in emission numbers of biofuel, such as feedstock and its origin, fuel production technique, system boundary definitions, and energy sources.[46] However, many government policies, such as those by the European Union and the UK, require that biofuels have at least 65% greenhouse gas emissions savings (or 70% if it is renewable fuels of non-biological origins) relative to fossil fuels.[99][100]
The growing demand for biofuels has raised concerns about land use and food security. Many biofuel crops are grown on land that could otherwise be used for food production. This shift in land use can lead to several problems:
Competition with Food Crops: The cultivation of biofuels, especially in food-insecure regions, can drive up the cost of food and reduce the amount of land available for growing essential crops. This can exacerbate global food insecurity, especially in developing countries.
Deforestation and Habitat Loss: To meet the increasing demand for biofuels, large areas of forests and natural habitats are being cleared for agriculture. This deforestation leads to the loss of biodiversity, threatens wildlife species, and disrupts ecosystems.
The expansion of biofuel production, particularly through monoculture farming (growing a single crop on a large scale), poses a significant threat to biodiversity. Large-scale biofuel crop production can lead to:
Habitat Destruction: The conversion of natural ecosystems into agricultural land can result in the loss of habitats for many plant and animal species, leading to decreased biodiversity.
Soil Degradation: Monoculture farming can deplete soil nutrients, reduce soil fertility, and increase the need for chemical inputs like fertilizers and pesticides, which can further harm surrounding ecosystems
Soil Fertility: Continuous cultivation of biofuel crops without proper crop rotation or sustainable farming practices can lead to soil depletion. Over time, the soil may lose vital nutrients, making it less suitable for farming.
Life-cycle assessments of first-generation biofuels have shown large emissions associated with the potentialland-use change required to produce additional biofuel feedstocks.[9][10] If no land-use change is involved, first-generation biofuels can—on average—have lower emissions than fossil fuels.[9] However, biofuel production can compete with food crop production. Up to 40% of corn produced in the United States is used to make ethanol[101] and worldwide 10% of all grain is turned into biofuel.[102] A 50% reduction in grain used for biofuels in the US and Europe would replace all ofUkraine's grain exports.[103] Several studies have shown that reductions in emissions from biofuels are achieved at the expense of other impacts, such asacidification,eutrophication,water footprint andbiodiversity loss.[9]
Second-generation biofuels are thought to increase environmental sustainability since the non-food part of plants is being used to produce second-generation biofuels instead of being disposed of.[104] But the use of second-generation biofuels increases the competition for lignocellulosic biomass, increasing the cost of these biofuels.[105]
In theory, third-generation biofuels, produced from algae, shouldn't harm the environment more than first- or second-generation biofuels due to lower changes in land use and the fact that they do not require pesticide use for production.[106] When looking at the data however, it has been shown that the environmental cost to produce the infrastructure and energy required for third generation biofuel production, are higher than the benefits provided from the biofuels use.[107][108]
TheEuropean Commission has officially approved a measure to phase outpalm oil-based biofuels by 2030.[109][110] Unsustainable palm oil agriculture has caused significant environmental and social problems, including deforestation and pollution.
The production of biofuels can be very energy intensive, which, if generated from non-renewable sources, can heavily mitigate the benefits gained through biofuel use. A solution proposed to solve this issue is to supply biofuel production facilities with excess nuclear energy, which can supplement the power provided by fossil fuels.[111] This can provide a carbon inexpensive solution to help reduce the environmental impacts of biofuel production.
As farmers worldwide respond to higher crop prices in order to maintain the global food supply-and-demand balance, pristine lands are cleared to replace the food crops that were diverted elsewhere to biofuels' production. Because natural lands, such asrainforests andgrasslands, store carbon in their soil andbiomass as plants grow each year, clearance of wilderness for new farms translates to a net increase ingreenhouse gas emissions. Due to thisoff-site change in the carbon stock of the soil and the biomass, indirect land use change has consequences in thegreenhouse gas (GHG) balance of a biofuel.[112][113][114][115]
Other authors have also argued that indirect land use changes produce other significant social and environmental impacts, affecting biodiversity, water quality,food prices and supply,land tenure, worker migration, and community and cultural stability.[114][116][117][118]
^Cherwoo L, Gupta I, Flora G, Verma R, Kapil M, Arya SK, et al. (2023). "Biofuels an alternative to traditional fossil fuels: A comprehensive review".Sustainable Energy Technologies and Assessments.60: 103503.Bibcode:2023SETA...6003503C.doi:10.1016/j.seta.2023.103503.
^abT. M. Letcher, ed. (2020). "Chapter 9: Biofuels for transport".Future energy : improved, sustainable and clean options for our planet (3rd ed.). Amsterdam, Netherlands.ISBN978-0-08-102887-2.OCLC1137604985.{{cite book}}: CS1 maint: location missing publisher (link)
^abcdT. M. Letcher, ed. (2020). "Chapter1: Introduction With a Focus on Atmospheric Carbon Dioxide and Climate Change".Future energy : improved, sustainable and clean options for our planet (3rd ed.). Amsterdam, Netherlands.ISBN978-0-08-102887-2.OCLC1137604985.{{cite book}}: CS1 maint: location missing publisher (link)
^Houghton J, Weatherwax S, Ferrell J (7 June 2006). Breaking the biological barriers to cellulosic ethanol: a joint research agenda (Report). Washington, DC (United States): EERE Publication and Product Library.doi:10.2172/1218382.
^Börjesson P, Lundgren J, Ahlgren S, Nyström I (18 June 2013). Dagens och framtidens hållbara biodrivmedel: underlagsrapport från f3 till utredningen om fossilfri fordonstrafik [Today's and the future's sustainable biofuels: background report from f3 to the inquiry into fossil-free vehicle traffic.] (Report) (in Swedish). Vol. 13. The Swedish Knowledge Centre for Renewable Transportation Fuels. p. 170.
^Evans J (14 January 2008)."Biofuels aim higher".Biofuels, Bioproducts and Biorefining (BioFPR).Archived from the original on 10 August 2009. Retrieved3 December 2008.
^Fukuda H, Kondo A, Noda H (January 2001). "Biodiesel fuel production by transesterification of oils".Journal of Bioscience and Bioengineering.92 (5):405–416.doi:10.1016/s1389-1723(01)80288-7.PMID16233120.
^Sukla MK, Bhaskar T, Jain AK, Singal SK, Garg MO."Bio-Ethers as Transportation Fuel: A Review"(PDF). Indian Institute of Petroleum Dehradun.Archived(PDF) from the original on 14 October 2011. Retrieved15 February 2014.
^Dinh LT, Guo Y, Mannan MS (2009). "Sustainability evaluation of biodiesel production using multicriteria decision-making".Environmental Progress & Sustainable Energy.28 (1):38–46.Bibcode:2009EPSE...28...38D.doi:10.1002/ep.10335.S2CID111115884.
^Demirbas AH (2009). "Inexpensive oil and fats feedstocks for production of biodiesel".Energy Education Science and Technology Part A: Energy Science and Research.23:1–13.
^abAbdullah B, Muhammad SA, Shokravi Z, Ismail S, Kassim KA, Mahmood AN, et al. (June 2019). "Fourth generation biofuel: A review on risks and mitigation strategies".Renewable and Sustainable Energy Reviews.107:37–50.Bibcode:2019RSERv.107...37A.doi:10.1016/j.rser.2019.02.018.S2CID116245776.
^Jacob-Lopes E, Zepka LQ, Severo IA, Maroneze MM, eds. (2022).3rd generation biofuels: disruptive technologies to enable commercial production. Woodhead Publishing series in energy. Cambridge, MA Kidlington: Woodhead Publishing, an imprint of Elsevier.ISBN978-0-323-90971-6.
^Rodionova MV, Poudyal RS, Tiwari I, Voloshin RA, Zharmukhamedov SK, Nam HG, et al. (2017). "Biofuel production: Challenges and opportunities".International Journal of Hydrogen Energy.42 (12):8450–8461.Bibcode:2017IJHE...42.8450R.doi:10.1016/j.ijhydene.2016.11.125.
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