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. These feedstocks could instead enter the animal or human food chain, and as the global population has risen their use in producing biofuels has been criticised for diverting food away from the human food chain, leading to food shortages and price rises.
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 Ethanol fuel 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. Ethanol has a smaller energy density than gasoline, which means it takes more fuel volume and mass to produce the same amount of work.
An advantage of ethanol CH3CH2OH is that is 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 bio ethanol fireplaces. As they do not require a chimney and are "flueless", bio ethanol fires  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 alcohol-from-corn 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 un-sustainable imported oil and fossil fuels required to produce the ethanol. According to a joint research agenda conducted through the U. Department of Energy,  the fossil energy ratios FER for cellulosic ethanol, corn ethanol, and gasoline are They dynamically sense exhaust oxygen content, and adjust the engine's computer systems, spark, and fuel injection accordingly.
This adds initial cost and ongoing increased vehicle maintenance. As with all vehicles, efficiency falls and pollution emissions increase when FFV system maintenance is needed regardless of the fuel mix being used , but is not performed. FFV internal combustion engines are becoming increasingly complex, as are multiple-propulsion-system FFV hybrid vehicles, which impacts cost, maintenance, reliability, and useful lifetime longevity.
With large current un-sustainable, non-scalable subsidies, ethanol fuel still costs much more per distance traveled than current high gasoline prices in the United States. Methanol is currently produced from natural gas, a non-renewable fossil fuel. It can also be produced from biomass as biomethanol.
The methanol economy is an interesting alternative to the hydrogen economy, compared to today's hydrogen produced from natural gas, but not hydrogen production directly from water and state-of-the-art clean solar thermal energy processes. Butanol 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 ,  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. 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 is not to be confused with biodiesel which is chemically quite different and processed using transesterification rather than the traditional fractional distillation used to process green diesel. Green diesel feedstock can be sourced from a variety oils including canola, algae, jatropha and salicornia in addition to tallow.
Biodiesel is the most common biofuel in Europe. Chemically it consists mostly of fatty acid methyl or ethyl esters FAME. Oils are mixed with sodium hydroxide and methanol or ethanol and the chemical reaction produces biodiesel FAME and glycerol.
One part glycerol is produced for every 10 parts biodiesel. 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 B is by far 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 B use, although Volkswagen of Germany, for example, asks drivers to check by telephone with the VW environmental services department before switching to B B may become more viscous at lower temperatures, depending on the feedstock used, requiring vehicles to have fuel line heaters. In most cases, biodiesel is compatible with diesel engines from onwards, which use 'Viton' by DuPont synthetic rubber in their mechanical injection systems.
Electronically controlled 'common rail' and 'pump duse' type systems from the late s onwards may only use biodiesel blended with conventional diesel fuel. These engines have finely metered and atomized multi-stage injection systems are very sensitive to the viscosity of the fuel. Many current generation diesel engines are made so that they can run on B without altering the engine itself, although this depends on the fuel rail design. 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. This improves the combustion of fossil diesel and reduces the particulate emissions from un-burnt carbon. Biodiesel is safe to handle and transport because it is as biodegradable as sugar, 10 times less toxic than table salt, and has a high flashpoint of about F C compared to petroleum diesel fuel, which has a flash point of F 52 C.
Edible vegetable oil is generally not used as fuel, but lower quality oil can be 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.
To ensure that the fuel injectors atomize the fuel 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. 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 Elsbett]or 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, high in cetane, low in aromatics and sulphur 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. Bio ethers also referred to as fuel ethers or fuel oxygenates are cost-effective compounds that act as octane rating enhancers.
They also enhance engine performance, whilst significantly reducing engine wear and toxic exhaust emissions.
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Greatly reducing the amount of ground-level ozone, they contribute to the quality of the air we breathe. Biogas is produced by the process of anaerobic digestion of organic material by anaerobes. The solid byproduct, digestate, can be used as a biofuel or a fertilizer. In the UK, the National Coal Board experimented with microorganisms that digested coal in situ converting it directly to gases such as methane. Biogas contains methane and can be recovered from industrial anaerobic digesters and mechanical biological treatment systems. 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 potent greenhouse gas. Farmers can produce biogas from manure from their cows by getting a anaerobic digester AD. Syngas, a mixture of carbon monoxide and hydrogen, 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.
The resulting gas mixture, syngas, is itself a fuel. Using the 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 or turbines. The wood gas generator is a wood-fueled gasification reactor mounted on an internal combustion engine.
Syngas can be used to produce methanol and hydrogen, or converted via the Fischer-Tropsch process to produce a synthetic diesel substitute, or a mixture of alcohols that can be blended into gasoline.
Lower temperature gasification is desirable when co-producing biochar but results in a Syngas polluted with tar. The resulting products are then refined to produce a fuel that cars or other vehicles can use. Much of the gasoline in the United States contains one of the most common biofuels: ethanol. Made by fermenting the sugars from plants such as corn or sugarcane, ethanol contains oxygen that helps a car's engine burn fuel more efficiently, reducing air pollution.
In the U. In Brazil—the second-largest ethanol producer behind the U. Alternatives to diesel fuel include biodiesel and renewable diesel. Biodiesel, derived from fats such as vegetable oil, animal fat, and recycled cooking grease, can be blended with petroleum-based diesel. Some buses, trucks, and military vehicles in the U. Renewable diesel, a chemically different product that can be derived from fats or plant-based waste, is considered a "drop-in" fuel that does not need to be blended with conventional diesel. Other types of plant-based fuel have been created for aviation and shipping.
More than , flights have used biofuel, but the amount of aviation biofuel produced in accounted for less than 0. In shipping, too, adoption of biofuel is at levels far below the targets set by the International Energy Agency. Renewable natural gas, or biomethane , is another fuel that potentially could be used not only for transportation but also heat and electricity generation.
Gas can be captured from landfills, livestock operations, wastewater, or other sources. This captured biogas then must be refined further to remove water, carbon dioxide, and other elements so that it meets the standard needed to fuel natural-gas-powered vehicles. A variety of materials, or feedstocks, can be used to make biofuels.
Though corn and sugarcane are well-established ethanol feedstocks, the process of growing the crops, making fertilizers and pesticides, and processing the plants into fuel consumes a lot of energy—so much energy that there is debate about whether ethanol from corn actually provides enough of an environmental benefit to be worth the investment.