Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 156

The term biodiesel refers to any diesel fuel substitute derived from renewable biomass. More specifically, biodiesel refers to a family of products made from vegetable oils or animal fats and alcohol, such as methanol or ethanol, called alkyl esters of fatty acids. For these to be considered as viable transportation fuels, they must meet stringent quality standards. In most contexts, unless otherwise stated, biodiesel refers to the alkyl esters produced by transesterification of vegetable oils or animal fats.

*Ethanol (ethyl alcohol, C₂H₅OH) could also be used.

Biodiesel is an alternative to crude oil-based fuels. Biodiesel is made from a variety of natural oils, especially rapeseed oil (a close cousin of canola oil) and soybean oil. Rapeseed oil tends to dominate the growing biodiesel industry in Europe, whereas in the United States, biodiesel is made from soybean oil because more soybean oil is produced here than all other sources of fats and oil combined. Palm and Jatropha oil are favored in Asian countries, but there are many other feedstock candidates, including recycled cooking oils, animal fats, and other oilseed crops.

Biodiesel is a renewable fuel that can be manufactured from algae, vegetable oils, animal fats, or recycled restaurant greases; it can be produced locally in most countries. It is safe, biodegradable, and reduces air pollutants, such as particulates, carbon monoxide, and hydrocarbon derivatives. Blends of 20% biodiesel with 80% crude oil diesel (B20) can generally be used in unmodified diesel engines. Biodiesel can also be used in its pure form (B100), but may require certain engine modifications to avoid maintenance and performance problems.

Biodiesel is made by chemically reacting vegetable oil or animal fat (or combinations of oils and fats) with alcohol (usually nearly pure methanol or ethanol) and a catalyst (sodium hydroxide). The chemical reaction converts constituents of the vegetable oil into an ester (biodiesel fuel) and glycerol. Since the biodiesel is less dense than the glycerol, it floats on top of the glycerol and may be pumped off, or the glycerol can be drained off the bottom. The fuel can then be filtered and used in heating or lighting applications. It is possible to use the product in diesel engines without further processing, but caution is advised because of the potential for engine damage and it is recommended that impurities (unreacted alcohol and sodium hydroxide) are removed by a washing.

Biodiesel is currently produced entirely from vegetable oil, waste cooking oil, animal fats, and algae. It is named biodiesel because it is derived from biological product and has similar physical properties as petro diesel. The biodiesel feedstock (oil and fats) primarily consists of triglycerides and have some amount of free fatty acids depending on their origin. The high viscosity and poor combustion properties of vegetable oils (compared to petro diesel) do not allow them to use directly as diesel substitute in engines. The direct use of vegetable oils can give rise to significant carbon deposit, lubricating oil contamination, excessive engine wear, and thus reduced engine durability. The processes such as transesterification, pyrolysis, catalytic cracking, and non-catalytic cracking have evolved to transform the vegetable oils or fats into biodiesel (fatty acid methyl ester) or biofuel.

Biodiesel is composed of long-chain fatty acids with an alcohol attached, often derived from vegetable oils. It is produced through the reaction of a vegetable oil with methyl alcohol or ethyl alcohol in the presence of a catalyst. Animal fats are another potential source. Commonly used catalysts are potassium hydroxide (KOH) or sodium hydroxide (NaOH). The chemical process is called transesterification which produces biodiesel and glycerin. Chemically, biodiesel is called a methyl ester if the alcohol used is methanol. If ethanol is used, it is called an ethyl ester. They are similar, and currently, methyl ester is cheaper due to the lower cost for methanol. Biodiesel can be used in the pure form, or blended in any amount with diesel fuel for use in compression ignition engines.

Biodiesel is typically produced through the reaction of a vegetable oil or animal fat with methanol or ethanol in the presence of a catalyst to yield glycerin and mono-alkyl esters. If methanol is used as the alcohol for transesterification, the reaction is referred to as methanolysis and methyl esters are produced, whereas ethyl esters are produced when ethanol is used for transesterification (ethanolysis). However, it is important to point out that glycerin is considered a valuable co-product that can be used in pharmaceuticals, cosmetics, and many other applications, depending on its grade and purity. The process can be represented simply as:

The process involves treatment of raw vegetable oils at low temperatures of 30 to 40°C (86 to 104°F) with methanol or ethanol in the presence of a catalyst such as sodium or potassium hydroxide (transesterification). Glycerol is produced and immediately settles out because of its low solubility in the ester product, carrying most of the dissolved catalyst with it. The upper layer containing the desired product is washed thoroughly to remove any residual catalyst and other emulsion-forming contaminants, yielding a clear, dark yellow material that can be directly used as a diesel substitute after dehydration. Phase separation can only be attained in ethanolysis under complete absence of water, whereas this process is often easier when methanol is used for transesterification.

Biodiesel may be made from virtually any oil-containing or oil-producing vegetation, including the seeds from rape, soybean, sunflower, peanut, cotton, castor and palm, or even animal fats. Other tropical oilseeds are also amenable to biodiesel production, such as Ricinus communis (mamona), Jatropha curcas (Jatropha), and Cyperus esculentus (tigernut). It may also be made from waste vegetable oils and fats from fish and chip or fried chicken establishments. The manufacturing process is totally benign, having no detrimental effect on the environment.

Biodiesel has a strong ecological appeal and many reasons have been claimed to support its utilization as a diesel substitute. In general, these include: (i) biodiesel is biodegradable and harmless, (ii) biodiesel can be produced from renewable materials such as vegetable oils and ethanol derived from biomass, (iii) methyl esters usually contain little sulfur (approximately 0.001% w/w) while this undesired component is literally absent in biodiesel produced by ethanolysis, (iv) biodiesel is a diesel substitute, biodiesel decreases soot emission considerably (up to 50%), (v) biodiesel emits approximately the same amount of carbon dioxide that has been absorbed during cultivation of the oilseed, (vi biodiesel does not contain any of the carcinogenic polyaromatic components found in diesel oil, (vii) biodiesel has been recognized as a suitable outlet for the vegetable oil industry, serving as an important tool for market regulation, (viii) biodiesel can be used in blends or as a neat fuel, (ix) biodiesel is not considered as a hazardous good because it has a flash point above 110°C (230°F), (x) biodiesel has a superior lubrication capability and increases engine life, and (xi) biodiesel can be produced with a rather straight forward technology, particularly in the case of methyl esters (methanolysis).

The use of biodiesel in a conventional diesel engine also results in substantial reduction of unburned hydrocarbon derivatives, carbon monoxide, and particulate matter. Emissions of nitrogen oxides are either slightly reduced or slightly increased depending on the duty cycle and testing methods. The higher oxygen content of biodiesel enables a more complete combustion of the solid carbon fraction to carbon dioxide, while soluble hydrocarbon derivatives are unaffected or even increased.

Producers and users of biodiesel use the “B” factor system to state the amount of biodiesel in a fuel mix, similar to the “E” system used for fuel containing ethanol. For example, fuel containing 20% biodiesel is labeled B20, whereas pure biodiesel is referred to as B100.

See also: Bioalcohols, Biofuels – First Generation, Biofuels – Second Generation, Biofuels – Third Generation, Biogas, Vegetable Oil.