Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 158

Biodiesel is a diesel-equivalent fuel derived from biological sources (such as vegetable oil) which can be used in unmodified diesel-engine vehicles. It is thus distinguished from the straight vegetable oil or waste vegetable oil used as fuels in some diesel vehicles.

Biodiesel fuel is a fuel made from the oil of certain oilseed crops such as soybean, canola, palm kernel, coconut, sunflower, safflower, corn, and a hundreds of other oil-producing crops. The oil is extracted by the use of a press and then mixed in specific proportions with other agents, which causes a chemical reaction (Figure B-1). The results of this reaction are two products, biodiesel and soap. After a final filtration, the biodiesel is ready for use. After curing, the glycerin soap which is produced as a by-product can be used as is, or can have scented oils added before use.

Figure B-1 Biodiesel Production Path.

Biodiesel is made through a chemical process (transesterification) whereby the glycerin is separated from the fat or vegetable oil. The process leaves behind two products: (i) methyl esters and (ii) glycerin (a valuable by-product usually sold to be used in soaps and other products).

The primary products of transesterification are methyl esters (biodiesel) and glycerol. 100 lbs of soybean oil are reacted with 10 lbs of methanol and 1 lbs of catalyst yielding 100 lbs of biodiesel and 10 lbs of glycerol. Glycerol is decanted and removed first as it is heavier and sinks. Methanol is recycled in the process by washing both the glycerol and biodiesel with water to remove unreacted methanol and return it to the process. Glycerol, a valuable by-product, is approximately 88% pure and can be further processed to pharmaceutical grade glycerol.

Biodiesel is a liquid which varies in color between golden and dark brown depending on the feedstock from which it is produced. In general, biodiesel compares well to crude oil-based diesel (Table B-5). Pure biodiesel fuel (100% esters of fatty acids) is called B100. When blended with diesel fuel, the designation indicates the amount of B100 in the blend, e.g., B20 is 20% B100 and 80% diesel, and B5 used in Europe is contains 5% B100 in diesel.

Table B-5 Comparison of properties of biodiesel from various sources.

Property	Source: waste cooking oil	Source: animal tallow	Commercial diesel fuel
Density (kg/l at 15oC)	0.890 -0.897	0.856 - 0.877	0.075-0.840
Flash point (oC)	196	30 to 35	67 to 85
Pour point (oC)	11	-6 to 0	-19 to -13
Cetane number	54	58.8 - 61	40-46
Ash content (% w/w)	0.004	0.022 – 0.025	0.008-0.010
Sulfur content (% w/w)	0.06	0.18	0.35-0.55
Carbon residue (% w/w)	0.33	-	0.35-0.40
Water content (% w/w)	0.04	-	0.02-0.05

The properties of biodiesel compare very favorably with the properties of crude oil -derived diesel. For example: (i) biodiesel is a safe, non-toxic, biodegradable, and renewable fuel that produces lower regulated vehicle emissions, (ii) biodiesel is oxygenated and has a higher flashpoint making it safer to handle and store. Biodiesel also has a higher cetane number providing a smoother running engine and extending engine life, and (iii) biodiesel can also be used in locomotives, as heating oil, in generators, and as a bio-remediator in oil spills.

Biodiesel (fatty acid methyl esters; FAME) is a notable alternative to the widely used crude oil-derived diesel fuel since it can be generated by domestic natural sources such as soybeans, rapeseeds, coconuts, and even recycled cooking oil, and thus reduces dependence on diminishing crude oil fuel from foreign sources. In addition, because biodiesel is largely made from vegetable oils, it reduces lifecycle greenhouse gas emissions by as much as 78%.

Vegetable oils and animal fats belong to an extensive family of chemicals called lipids. Lipids are bio-products from the metabolism of living creatures. As a result, they can be found widely distributed in nature. Their bio-functions are diverse, but they are most known for their energy storage capacity. Most lipids can easily dissolve in common organic solvents, meaning that they are hydrophobic. If a lipid is a solid at 25°C (77°F), it is classified as a fat; otherwise, it is oil. Typically, fats are produced by animals and oils by plants, but both are mainly made of triglyceride molecules, which are tri-esters of glycerol (a triol) and free fatty acids (long alkyl chain carboxylic acids). Other glyceride species, such as di-glycerides and mono-glycerides, are obtained from triglycerides by the substitution of one and two fatty acid moieties, respectively, with hydroxyl groups.

Biodiesel production is a very modern and technological area for researchers due to the relevance that it is winning every day because of the increase in the crude oil price and the environmental advantages.

Biodiesel is biodegradable and non-toxic, and typically produces approximately 60% v/v less net carbon dioxide emissions than crude oil-based diesel, as it is itself produced from atmospheric carbon dioxide via photosynthesis in plants.

It is practically immiscible with water, and has a high boiling point and low vapor pressure. Typical methyl ester biodiesel has a flash point of approximately 150°C (300°F), making it rather non-flammable. Biodiesel has a density of approximately 0.88 g/cm³, less than that of water. Biodiesel uncontaminated with starting material can be regarded as non-toxic, but it is recommended that no one drink any!

Biodiesel has a viscosity similar to diesel produced from crude oil (petrodiesel). It can be used as an additive in formulations of diesel to increase the lubricity of pure ULSD fuel, which is advantageous because it has virtually no sulfur content.

Much of the world uses a system known as the “B” factor to state the amount of biodiesel in any fuel mix. For example, fuel containing 20% biodiesel is labeled B20. Pure biodiesel is referred to as B100. 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 has approximately 5 to 8% less energy density, but better lubricity and more complete combustion can make the energy output of a diesel engine only 2% less per volume when compared to petrodiesel.

The higher lubricity index of biodiesel compared to petrodiesel is an advantage and can contribute to longer fuel injector life. However, biodiesel is a better solvent than crude oil-derived diesel, and has been known to break down deposits of residue in the fuel lines of vehicles that have previously been run on petrodiesel. As a result, fuel filters and injectors may become clogged with particulates if a quick transition to pure biodiesel is made, as biodiesel cleans the engine in the process.

The temperature at which pure (B100) biodiesel starts to gel varies significantly and depends upon the mix of esters and therefore the feedstock oil used to produce the biodiesel. For example, biodiesel produced from varieties of canola seed starts to gel at approximately -10 °C. Biodiesel produced from tallow tends to gel at around 16oC (61oF). Winter operation is possible with biodiesel blended with other fuel oils including #2 low-sulfur diesel and #1 diesel/kerosene, but the exact blend depends on the operating environment.

Biodiesel may contain small but problematic quantities of water. Although it is hydrophobic (non-miscible with water molecules), there are indications that biodiesel is said to be, at the same time, hygroscopic to the point of attracting water molecules from atmospheric moisture. In addition, there may be water that is residual to processing or resulting from storage tank condensation. The presence of water is a problem because (i) water reduces the heat of combustion of the bulk fuel which means more smoke, harder starting, and less power, (ii) water causes corrosion of vital fuel system components: fuel pumps, injector pumps, and fuel lines, (iii) water freezes to form ice crystals at 0°C (32 °F), and the crystals provide sites for nucleation and accelerate the gelling of the residual fuel, and (iv) water accelerates the growth of microbe colonies, which can plug up a fuel system and, in fact, biodiesel users who have heated fuel tanks therefore face a year-round microbe problem.

The extra lubrication provided by biodiesel fuel helps improve the longevity of the engine, as well as boosting engine performance, also helping eliminate engine knocks and noise. In addition, biodiesel fuel can be stored in any type of tank and has a much higher flash point (approximately 300°C, 570°F) compared to petrodiesel (approximately 150°C, 300°F).

See also: Biodiesel, Biodiesel – Feedstocks, Biodiesel – Production, Biodiesel – Technical Standards.