In 2010 worldwide biofuel production reached 105 billion liters (28 billion gallons US), up 17% from 2009, and biofuels provided 2.7% of the world's fuels for road transport, a contribution largely made up of ethanol and biodiesel.
[2] Global
ethanol fuel production reached 86 billion liters (23 billion gallons US) in 2010, with the United States and Brazil as the world's top producers, accounting together for 90% of global production. The world's largest biodiesel producer is the
European Union, accounting for 53% of all biodiesel production in 2010.
[2] As of 2011, mandates for blending biofuels exist in 31 countries at the national level and in 29 states/provinces.
[3] According to the
International Energy Agency, biofuels have the potential to meet more than a quarter of world demand for transportation fuels by 2050.
[4]
[edit]Liquid fuels for transportation
The fuels that are easiest to burn cleanly are typically liquids and gases. Thus liquids (and gases that can be stored in liquid form) meet the requirements of being both portable and clean burning. Also, liquids and gases can be pumped, which means handling is easily mechanized, and thus less laborious.
[edit]First generation biofuels
'First-generation' or conventional biofuels are biofuels made from sugar, starch, and vegetable oil.
[edit]Bioalcohols
Main article:
Alcohol fuel
Biologically produced
alcohols, most commonly
ethanol, and less commonly
propanol and
butanol, are produced by the action of
microorganisms and
enzymes through the fermentation of sugars or starches (easiest), or cellulose (which is more difficult).
Biobutanol (also called biogasoline) is often claimed to provide a direct replacement for gasoline, because it can be used directly in a gasoline engine (in a similar way to biodiesel in diesel engines).
Ethanol fuel is the most common biofuel worldwide, particularly
in Brazil.
Alcohol fuels are produced by fermentation of sugars derived from
wheat,
corn,
sugar beets,
sugar cane,
molasses and any sugar or starch that
alcoholic beverages can be made from (like
potato and
fruit waste, etc.). The ethanol production methods used are
enzyme digestion (to release sugars from stored starches), fermentation of the sugars,
distillation and drying. The distillation process requires significant energy input for heat (often unsustainable
natural gas fossil fuel, but cellulosic biomass such as
bagasse, the waste left after sugar cane is pressed to extract its juice, can also be used more sustainably).
Ethanol can be used in petrol engines as a replacement for gasoline; it can be mixed with gasoline to any percentage. Most existing car petrol engines can run on blends of up to 15% bioethanol with petroleum/gasoline. Ethanol has a smaller
energy density than does gasoline; this fact means that it takes more fuel (volume and mass) to produce the same amount of work. An advantage of ethanol (
CH3CH2OH) is that it has a higher
octane rating than ethanol-free gasoline available at roadside gas stations which allows an increase of an engine's
compression ratio for increased
thermal efficiency. In high altitude (thin air) locations, some states mandate a mix of gasoline and ethanol as a winter
oxidizer to reduce atmospheric pollution emissions.
Ethanol is also used to fuel bioethanol
fireplaces. As they do not require a chimney and are "flueless", bio ethanol fires
[5] are extremely useful for new build homes and apartments without a flue. The downside to these fireplaces, is that the heat output is slightly less than electric and gas fires.
In the current
corn-to-ethanol production model in the United States, considering the total energy consumed by
farm equipment, cultivation, planting,
fertilizers,
pesticides,
herbicides, and
fungicides made from petroleum,
irrigation systems, harvesting, transport of feedstock to processing plants, fermentation, distillation, drying, transport to fuel terminals and retail pumps, and lower ethanol fuel energy content, the net energy content value added and delivered to consumers is very small. And, the net benefit (all things considered) does little to reduce imported
oil and fossil fuels required to produce the ethanol.
[6]
Although corn-to-ethanol and other food stocks have implications both in terms of world food prices and limited, yet positive, energy yield (in terms of energy delivered to customer/fossil fuels used), the technology has led to the development of
cellulosic ethanol. According to a joint research agenda conducted through the U.S. Department of Energy,
[7] the fossil energy ratios (
FER) for cellulosic ethanol, corn ethanol, and gasoline are 10.3, 1.36, and 0.81, respectively.
[8][9][10]
Even dry ethanol has roughly one-third lower energy content per unit of volume compared to gasoline, so larger / heavier fuel tanks are required to travel the same distance, or more fuel stops are required. With large current unsustainable, non-
scalable subsidies, ethanol fuel still costs much more per distance traveled than current high gasoline prices in the United States.
[11]
Butanol (
C4H9OH) is formed by
ABE fermentation (acetone, butanol, ethanol) and experimental modifications of the process show potentially high
net energy gains with butanol as the only liquid product. Butanol will produce more energy and allegedly can be burned "straight" in existing gasoline engines (without modification to the engine or car),
[12] and is less corrosive and less water soluble than ethanol, and could be distributed via existing infrastructures.
DuPont and
BP are working together to help develop Butanol.
E. coli have also been successfully engineered to produce butanol by hijacking their
amino acid metabolism.
[13]
[edit]Biodiesel
In some countries biodiesel is less expensive than conventional diesel.
Biodiesel is the most common biofuel in Europe. It is produced from oils or fats using
transesterification and is a liquid similar in composition to fossil/mineral diesel. Chemically, it consists mostly of fatty acid methyl (or ethyl) esters (
FAMEs). Feedstocks for biodiesel include animal fats, vegetable oils,
soy,
rapeseed,
jatropha,
mahua,
mustard,
flax,
sunflower,
palm oil,
hemp,
field pennycress,
pongamia pinnata and
algae. Pure biodiesel (B100) is the lowest emission diesel fuel. Although
liquefied petroleum gas and hydrogen have cleaner combustion, they are used to fuel much less efficient petrol engines and are not as widely available.
Biodiesel can be used in any
diesel engine when mixed with mineral diesel. In some countries manufacturers cover their diesel engines under warranty for B100 use, although
Volkswagen of
Germany, for example, asks drivers to check by telephone with the VW environmental services department before switching to B100. B100 may become more
viscous at lower temperatures, depending on the feedstock used. In most cases, biodiesel is compatible with diesel engines from 1994 onwards, which use '
Viton' (by
DuPont) synthetic rubber in their mechanical
fuel injection systems.
Electronically controlled '
common rail' and '
unit injector' type systems from the late 1990s onwards may only use biodiesel blended with conventional diesel fuel. These engines have finely metered and atomized multi-stage injection systems that are very sensitive to the viscosity of the fuel. Many current generation diesel engines are made so that they can run on B100 without altering the engine itself, although this depends on the
fuel raildesign. Since biodiesel is an effective
solvent 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 engine
combustion chamber of carbon deposits, helping to maintain efficiency. In many European countries, a 5% biodiesel blend is widely used and is available at thousands of gas stations.
[14][15] Biodiesel is also an
oxygenated fuel, meaning that it contains a reduced amount of carbon and higher hydrogen and oxygen content than fossil diesel. This improves the
combustion of biodiesel and reduces the particulate emissions from un-burnt carbon.
Biodiesel is also safe to handle and transport because it is as
biodegradable as sugar, 10 times less toxic than table salt, and has a high
flash point of about 300 F (148 C) compared to petroleum diesel fuel, which has a flash point of 125 F (52 C).
[16]
In the USA, more than 80% of commercial trucks and city buses run on diesel. The emerging US biodiesel market is estimated to have grown 200% from 2004 to 2005. "By the end of 2006 biodiesel production was estimated to increase fourfold [from 2004] to more than" 1 billion US gallons (3,800,000 m
3).
[17]
[edit]Green diesel
Main article:
Green diesel
Green diesel, also known as
renewable diesel, is a form of diesel fuel which is derived from renewable feedstock rather than the fossil feedstock used in most
diesel fuels. Green diesel feedstock can be sourced from a variety of
oils including
canola,
algae,
jatropha and
salicornia in addition to
tallow. Green diesel uses traditional
fractional distillation to process the oils, not to be confused with biodiesel which is chemically quite different and processed using transesterification.
“Green Diesel” as commonly known in
Ireland should not be confused with dyed green diesel sold at a lower tax rate for agriculture purposes, using the dye allows custom officers to determine if a person is using the cheaper diesel in higher taxed applications such as commercial haulage or cars.
[18]
[edit]Vegetable oil
Straight unmodified
edible vegetable oil is generally not used as fuel, but lower quality oil can and has been used for this purpose. Used vegetable oil is increasingly being processed into biodiesel, or (more rarely) cleaned of water and particulates and used as a fuel.
Also here, as with 100% biodiesel (B100), to ensure that the
fuel injectors atomize the vegetable oil in the correct pattern for efficient combustion,
vegetable oil fuel must be heated to reduce its
viscosity to that of diesel, either by electric coils or heat exchangers. This is easier in warm or temperate climates. Big corporations like
MAN B&W Diesel,
Wärtsilä, and
Deutz AG as well as a number of smaller companies such as
Elsbett offer engines that are compatible with straight vegetable oil, without the need for after-market modifications.
Vegetable oil can also be used in many older diesel engines that do not use
common rail or
unit injection electronic diesel injection systems. Due to the design of the combustion chambers in
indirect injection engines, these are the best engines for use with vegetable oil. This system allows the relatively larger oil molecules more time to burn. Some older engines, especially Mercedes are driven experimentally by enthusiasts without any conversion, a handful of drivers have experienced limited success with earlier pre-"Pumpe Duse"
VW TDI engines and other similar engines with
direct injection. Several companies like
Elsbettor Wolf have developed professional conversion kits and successfully installed hundreds of them over the last decades.
Oils and fats can be
hydrogenated to give a diesel substitute. The resulting product is a straight chain hydrocarbon with a high
cetane number, low in
aromaticsand
sulfur and does not contain oxygen.
Hydrogenated oils can be blended with diesel in all proportions. Hydrogenated oils have several advantages over biodiesel, including good performance at low temperatures, no storage stability problems and no susceptibility to microbial attack.
[19]
[edit]Bioethers
- Note:Landfill gas is a less clean form of biogas which is produced in landfills through naturally occurring anaerobic digestion. If it escapes into the atmosphere it is a potential greenhouse gas.
- Farmers can produce biogas from manure from their cows by using an anaerobic digester (AD).[23]
Main article:
Gasification
Syngas, a mixture of
carbon monoxide,
hydrogen and other hydrocarbons is produced by partial combustion of biomass, that is, combustion with an amount of
oxygen that is not sufficient to convert the biomass completely to carbon dioxide and water.
[19] Before partial combustion the biomass is dried, and sometimes
pyrolysed. The resulting gas mixture, 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.[24] The wood gas generator is a wood-fueled gasification reactor that can be connected to an internal combustion engine.
- Syngas can be used to produce methanol, DME and hydrogen, or converted via the Fischer-Tropsch process to produce a diesel substitute, or a mixture of alcohols that can be blended into gasoline. Gasification normally relies on temperatures >700°C.
- Lower temperature gasification is desirable when co-producing biochar but results in a Syngas polluted with tar.
[edit]Solid biofuels
When raw biomass is already in a suitable form (such as
firewood), it can burn directly in a stove or furnace to provide heat or raise steam. When raw biomass is in an inconvenient form (such as sawdust, wood chips, grass, urban waste wood, agricultural residues), the typical process is to densify the biomass. This process includes grinding the raw biomass to an appropriate particulate size (known as hogfuel), which depending on the densification type can be from 1 to 3 cm (1 in), which is then concentrated into a fuel product. The current types of processes are
wood pellet, cube, or puck. The pellet process is most common in Europe and is typically a pure wood product. The other types of densification are larger in size compared to a pellet and are compatible with a broad range of input feedstocks. The resulting densified fuel is easier to transport and feed into thermal generation systems such as boilers.
One of the advantages of solid biomass fuel is that it is often a by-product, residue or waste-product of other processes, such as farming, animal husbandry and forestry.
[25] In theory this means there is no competition between fuel and food production, although this is not always the case.
[25]
A problem with the combustion of raw biomass is that it emits considerable amounts of
pollutants such as
particulates and PAHs (
polycyclic aromatic hydrocarbons). Even modern pellet boilers generate much more pollutants than oil or natural gas boilers. Pellets made from agricultural residues are usually worse than wood pellets, producing much larger emissions of
dioxins and
chlorophenols.
[26]
Notwithstanding the above noted study, numerous studies have shown that biomass fuels have significantly less impact on the environment than fossil based fuels. Of note is the U.S. Department of Energy Laboratory, Operated by Midwest Research Institute Biomass Power and Conventional Fossil Systems with and without CO2 Sequestration – Comparing the
Energy Balance,
Greenhouse Gas Emissions and Economics Study. Power generation emits significant amounts of greenhouse gases (GHGs), mainly
carbon dioxide (CO
2).
Sequestering CO
2 from the power plant
flue gas can significantly reduce the GHGs from the power plant itself, but this is not the total picture. CO
2 capture and sequestration consumes additional energy, thus lowering the plant's
fuel-to-electricity efficiency. To compensate for this, more fossil fuel must be procured and consumed to make up for lost capacity.
Taking this into consideration, the
global warming potential (GWP), which is a combination of CO
2, methane (CH
4), and
nitrous oxide (N
2O) emissions, and energy balance of the system need to be examined using a
life cycle assessment. This takes into account the upstream processes which remain constant after CO
2 sequestration as well as the steps required for additional power generation. Firing biomass instead of coal led to a 148% reduction in GWP.
[edit]Second generation biofuels (advanced biofuels)
Second generation biofuels are biofuels produced from sustainable feedstock. Sustainability of a feedstock is defined among others by availability of the feedstock, impact on
GHG emissions and impact on biodiversity and land use.
[28] Many second generation biofuels are under development such as
Cellulosic ethanol,
Algae fuel[29].,
biohydrogen,
biomethanol,
DMF,
BioDME,
Fischer-Tropsch diesel, biohydrogen diesel, mixed alcohols and wood diesel.
Cellulosic ethanol production uses non-food crops or inedible waste products and does not divert food away from the animal or human food chain.
Lignocellulose is the "woody" structural material of plants. This feedstock is abundant and diverse, and in some cases (like citrus peels or sawdust) it is in itself a significant disposal problem.
Producing ethanol from
cellulose is a difficult technical problem to solve. In nature,
ruminant livestock (like
cattle) eat grass and then use slow enzymatic digestive processes to break it into
glucose (sugar). In
cellulosic ethanol laboratories, various
experimental processes are being developed to do the same thing, and then the sugars released can be fermented to make ethanol fuel. In 2009 scientists reported developing, using "synthetic biology", "15 new highly stable fungal
enzyme catalysts that efficiently break down cellulose into sugars at high temperatures", adding to the 10 previously known.
[30] The use of high temperatures, has been identified as an important factor in improving the overall economic feasibility of the biofuel industry and the identification of enzymes that are stable and can operate efficiently at extreme temperatures is an area of active research.
[31] In addition, research conducted at
Delft University of Technology by Jack Pronk has shown that
elephant yeast, when slightly modified can also create ethanol from non-edible ground sources (e.g. straw).
[32][33]
The recent discovery of the fungus
Gliocladium roseum points toward the production of so-called
myco-diesel from cellulose. This organism (recently discovered in rainforests of northern
Patagonia) has the unique capability of converting cellulose into medium length hydrocarbons typically found in diesel fuel.
[34] Scientists also work on experimental
recombinant DNA genetic engineering organisms that could increase biofuel potential.
Scientists working with the New Zealand company Lanzatech have developed a technology to use industrial waste gases such as carbon monoxide from
steel mills as a feedstock for a microbial fermentation process to produce ethanol.
[35][36] In October 2011, Virgin Atlantic announced it was joining with Lanzatech to commission a demonstration plant in Shanghai that would produce an aviation fuel from waste gases from steel production.
[37]
Scientists working in Minnesota have developed co-cultures of
Shewanella and
Synechococcus that produce long chain hydrocarbons directly from water, carbon dioxide, and sunlight.
[38] The technology has received
ARPA-E funding.
[edit]Biofuels by region
There are international organizations such as IEA Bioenergy,
[39] established in 1978 by the
OECD International Energy Agency (IEA), with the aim of improving cooperation and information exchange between countries that have national programs in bioenergy research, development and deployment. The
U.N. International Biofuels Forum is formed by
Brazil,
China,
India,
South Africa, the
United States and the
European Commission.
[40] The world leaders in biofuel development and use are Brazil, United States, France, Sweden and Germany. Russia also has 22% of worlds forest
[41] and is a big biomass (solid biofuels) supplier. In 2010, Russian pulp and paper maker, Vyborgskaya Cellulose, said they would be producing pellets that can be used in heat and electricity generation from its plant in Vyborg by the end of the year.
[42] The plant will eventually produce about 900,000 tons of pellets per year, making it the largest in the world once operational.
Biofuels currently make up 3.1%
[43] of the total road transport fuel in the UK or 1,440 million litres. By 2020, 10 per cent of the energy used in UK road and rail transport must come from renewable sources – this is the equivalent of replacing 4.3 million tonnes of fossil oil each year. Conventional biofuels are likely to produce between 3.7 and 6.6 per cent of the energy needed in road and rail transport, while
advanced biofuels could meet up to 4.3 per cent of the UK’s renewable transport fuel target by 2020.
[44]
[edit]Issues with biofuel production and use
There are various social, economic, environmental and technical issues with biofuel production and use, which have been discussed in the popular media and scientific journals. These include: the effect of moderating
oil prices, the "
food vs fuel" debate,
poverty reduction potential,
carbon emissions levels,
sustainable biofuel production,
deforestation and
soil erosion, loss of
biodiversity, impact on
water resources, as well as energy balance and efficiency. The
International Resource Panel, which provides independent scientific assessments and expert advice on a variety of resource-related themes, assessed the issues relating to biofuel use in its first report
Towards sustainable production and use of resources: Assessing Biofuels.
[45] In it, it outlined the wider and interrelated factors that need to be considered when deciding on the relative merits of pursuing one biofuel over another. It concluded that not all biofuels perform equally in terms of their impact on climate, energy security and ecosystems, and suggested that environmental and social impacts need to be assessed throughout the entire life-cycle.
Although there are many current issues with biofuel production and use, the development of new biofuel crops and second generation biofuels attempts to circumvent these issues. Many scientists and researchers are working to develop biofuel crops that require less land and use fewer resources, such as water, than current biofuel crops do. According to the journal "Renewable fuels from algae: An answer to debatable land based fuels",
[46] algae is a source for biofuels that could utilize currently unprofitable land and waste water from different industries. Algae are able to grow in wastewater, which does not affect the land or freshwater needed to produce current food and fuel crops. Furthermore, algae are not part of the human food chain, and therefore, do not take away food resources from humans.
The effects of the biofuel industry on food are still being debated. According to a recent study entitled "Impact of biofuel production and other supply and demand factors on food price increases in 2008",
[47] biofuel production was accountable for 3-30% of the increase in food prices in 2008. A recent study for the
International Centre for Trade and Sustainable Development shows that market-driven expansion of
ethanol in the US increased maize prices by 21 percent in 2009, in comparison with what prices would have been had ethanol production been frozen at 2004 levels.
[48] This has prompted researchers to develop biofuel crops and technologies that will reduce the impact of the growing biofuel industry on food production and cost.
One step to overcoming these issues is developing biofuel crops best suited to each region of the world. If each region utilized a specific biofuel crop, the need to use fossil fuels to transport the fuel to other places for processing and consumption will be diminished. Furthermore, certain areas of the globe are unsuitable for producing crops that require large amounts of water and nutrient rich soil. Therefore, current biofuel crops, such as corn, are unpractical in different environments and regions of the globe.
[edit]Current research
There is ongoing research into finding more suitable biofuel crops and improving the oil yields of these crops. Using the current yields, vast amounts of land and fresh water would be needed to produce enough oil to completely replace fossil fuel usage. It would require twice the land area of the US to be devoted to soybean production, or two-thirds to be devoted to rapeseed production, to meet current US heating and transportation needs.
[citation needed]
Specially bred mustard varieties can produce reasonably high oil yields and are very useful in
crop rotation with cereals, and have the added benefit that the meal leftover after the oil has been pressed out can act as an effective and biodegradable pesticide.
[50]
The
NFESC, with
Santa Barbara-based Biodiesel Industries is working to develop biofuels technologies for the US navy and military, one of the largest diesel fuel users in the world.
[51] A group of Spanish developers working for a company called
Ecofasa announced a new biofuel made from trash. The fuel is created from general urban waste which is treated by bacteria to produce fatty acids, which can be used to make biofuels.
[52]
[edit]Ethanol biofuels
As the primary source of biofuels in North America, many organizations are conducting research in the area of
ethanol production. The National Corn-to-Ethanol Research Center (NCERC) is a research division of
Southern Illinois University Edwardsville dedicated solely to ethanol-based biofuel research projects.
[53] On the Federal level, the
USDA conducts a large amount of research regarding ethanol production in the United States. Much of this research is targeted toward the effect of ethanol production on domestic food markets.
[54] A division of the U.S.
Department of Energy, the
National Renewable Energy Laboratory (NREL), has also conducted various ethanol research projects, mainly in the area of cellulosic ethanol.
[55]
[edit]Algal biofuels
From 1978 to 1996, the
U.S. NREL experimented with using algae as a biofuels source in the "
Aquatic Species Program".
[56] A self-published article by Michael Briggs, at the
UNH Biofuels Group, offers estimates for the realistic replacement of all
vehicular fuel with biofuels by utilizing algae that have a natural oil content greater than 50%, which Briggs suggests can be grown on algae ponds at
wastewater treatment plants.
[57] This oil-rich algae can then be extracted from the system and processed into biofuels, with the dried remainder further reprocessed to create ethanol. The production of algae to harvest oil for biofuels has not yet been undertaken on a commercial scale, but
feasibility studies have been conducted to arrive at the above yield estimate. In addition to its projected high yield, algaculture — unlike
crop-based biofuels — does not entail a decrease in
food production, since it requires neither
farmland nor
fresh water. Many companies are pursuing algae bio-reactors for various purposes, including scaling up biofuels production to commercial levels.
[58][59] Prof.
Rodrigo E. Teixeira from the
University of Alabama in Huntsvilledemonstrated the extraction of biofuels lipids from wet algae using a simple and economical reaction in
ionic liquids.
[60]
[edit]Jatropha
Main articles:
Jatropha and
Jatropha Oil
Several groups in various sectors are conducting research on Jatropha curcas, a poisonous shrub-like tree that produces seeds considered by many to be a viable source of biofuels feedstock oil.
[61] Much of this research focuses on improving the overall per acre oil yield of Jatropha through advancements in genetics, soil science, and horticultural practices.
SG Biofuels, a San Diego-based Jatropha developer, has used molecular breeding and biotechnology to produce elite hybrid seeds of Jatropha that show significant yield improvements over first generation varieties.
[62] SG Biofuels also claims that additional benefits have arisen from such strains, including improved flowering synchronicity, higher resistance to pests and disease, and increased cold weather tolerance.
[63]
Plant Research International, a department of the
Wageningen University and Research Centre in the Netherlands, maintains an ongoing Jatropha Evaluation Project (JEP) that examines the feasibility of large scale Jatropha cultivation through field and laboratory experiments.
[64] The Center for Sustainable Energy Farming (CfSEF) is a Los Angeles-based non-profit research organization dedicated to Jatropha research in the areas of plant science, agronomy, and horticulture. Successful exploration of these disciplines is projected to increase Jatropha farm production yields by 200-300% in the next ten years.
[65]
A group at the
Russian Academy of Sciences in
Moscow published a paper in September 2008, stating that they had isolated large amounts of lipids from single-celled fungi and turned it into biofuels in an economically efficient manner. More research on this fungal species;
Cunninghamella japonica, and others, is likely to appear in the near future.
[66] The recent discovery of a variant of the fungus
Gliocladium roseum points toward the production of so-called
myco-diesel from cellulose. This organism was recently discovered in the rainforests of northern
Patagonia and has the unique capability of converting cellulose into medium length hydrocarbons typically found in diesel fuel.
[67]
[edit]Greenhouse gas emissions
According to Britain's
National Non-Food Crops Centre, total net savings from using first-generation biodiesel as a transport fuel range from 25-82% (depending on the feedstock used), compared to diesel derived from crude oil.
[68] Nobel Laureate
Paul Crutzen, however, finds that the emissions of nitrous oxide due to nitrate fertilisers is seriously underestimated, and tips the balance such that most biofuels produce more greenhouse gases than the fossil fuels they replace. Producing lignocellulosic biofuels offers potentially greater greenhouse gas emissions savings than those obtained by first generation biofuels. Lignocellulosic biofuels are predicted by oil industry body CONCAWE
[1] to reduce greenhouse gas emissions by around 90% when compared with fossil petroleum
[citation needed], in contrast first generation biofuels were found to offer savings of 20-70%
[69][not in citation given]
Some scientists have expressed concerns about land-use change in response to greater demand for crops to use for biofuel and the subsequent carbon emissions.
[70] The payback period, that is, the time it will take biofuels to pay back the carbon debt that they acquire due to land-use change, has been estimated to be between 100–1000 years depending on the specific instance and location of land-use change. However, no-till practices combined with cover crop practices can reduce the payback period to 3 years for grassland conversion and 14 years for forest conversion.
[71] Biofuels made from waste biomass or from biomass grown on abandoned agricultural lands incur little to no carbon debt.
[72]
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