Encyclopedia of Renewable Energy. James G. Speight

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СКАЧАТЬ methods], and oxidation stability (normally evaluated using Differential Scanning Calorimetry and Oxidation Stability Index). In addition, according to the EMA (Engine Manufacturers Association), a blend of crude oil diesel fuel meeting ASTM D975 and 100% (neat) biodiesel fuel meeting either ASTM 6751 or EN 14214, where the biodiesel content of the blended fuel is no more than 20% biodiesel by volume (B20), shall meet the requirements identified in at the point of delivery of the fuel to the end user.

      In terms of the effects of biodiesel on fuel filters, use of biodiesel blends which do not meet up to the designated specifications, have shown to drastically reduce filter life. Blends greater then B20 may have enough of a solvent to break down the varnish deposits on the walls of existing fuel storage tanks or fuel systems. The breakdown of these varnish deposits will contaminate the fuel with particulate, which can cause fuel filters to plug rapidly.

      Another disadvantage of biodiesel is that it tends to reduce fuel economy. Energy efficiency is the percentage of the thermal energy of the fuel that is delivered as engine output, and biodiesel has shown no significant effect on the energy efficiency of any test engine. Volumetric efficiency, a measure that is more familiar to most vehicle users, is usually expressed as miles traveled per gallon of fuel (or kilometers per liter of fuel). The energy content per gallon of biodiesel is approximately 11% lower than that of crude oil diesel. Vehicles running on B20 are therefore expected to achieve 2.2% (20 % x 11 %) fewer miles per gallon of fuel.

      Areas of concern and interest are for the biofuels industry to have in place a good quality control protocol for the measurement of bioalcohols, to avoid metal corrosion from water and acid corrosion (due to weak and strong acids and inorganic chlorides in solution). Also of importance are the limits set on phosphorous content (less than 5.0 mg/L in ethanol) to prevent engine catalyst deterioration, and copper content (less than 0.1 mg/kg), along with a sulfur content less than 10 mg/kg.

      Up to a 10% blend level, the performance of bioethanol-blended gasoline is similar to ordinary gasoline. At higher levels, however, some engines may begin to exhibit problems, for example, stumbling under slight acceleration. The fuel also has more aggressive properties at higher concentrations of bioethanol which increases the possibility of deterioration of some components. Gasoline must be volatile enough to move from the carburetor or injectors into the cylinders and to vaporize prior to combustion. However, gasoline cannot have such a volatility that allows it to vaporize and boil in the injectors, carburetor, fuel lines, or fuel pump, which could prevent it from being metered correctly. Also, if the gasoline is too volatile, more evaporates into the air adding to environmental problems. There are a number of volatility specifications to ensure suppliers get this balancing act right. Adding bioethanol to gasoline as low-level blends increases the volatility of the blended fuel.

      The Engine Fuel Specifications Regulations specify volatility measures for bioethanol-blended petrol. The limits for blends are similar to those for gasoline so as to ensure no changes in vehicles are required. Bioethanol introduces more oxygen into the fuel. In vehicles with simple fuel metering systems such as carburetors, this causes the mixture to become a little leaner. Leaning is good for fuel economy and is generally good for lowering some types of exhaust emissions. However, it may cause some engines to stumble if they are already tuned reasonably lean. If a vehicle stumbles on bioethanol-blended gasoline, re-tuning should solve the problem. A vehicle tuned correctly for use on ordinary gasoline would normally not exhibit problems when using bioethanol blends.

      See also: Biodiesel – Properties.

      Biodiesel – Transesterification

      Transesterification (alcoholysis) is the conversion of triacylglycerol lipids by alcohols to alkyl esters without first isolating the free fatty acids (May, 2004). The purpose of transesterification of vegetable oils to their methyl esters (biodiesel) process is to lower the viscosity of the oil. The transesterification reaction is affected by alcohol type, molar ratio of glycerides to alcohol, type and amount of catalyst, reaction temperature, reaction time, and free fatty acids and water content of vegetable oils or animal fats. The transesterification reaction proceeds with or without a catalyst by using primary or secondary monohydric aliphatic alcohols having 1–8 carbon atoms as follows:

Generally, the reaction temperature near the boiling point of the alcohol is recommended. The reactions take place at low temperatures (approximately 65°C, 160°F) and at modest pressures (2 atm, 1 atm = 14.7 psi). Biodiesel is further purified by washing and evaporation to remove any remaining methanol. The oil (87%), alcohol (9%), and catalyst (1%) are the inputs in the production of biodiesel (86%), the main output. Pretreatment is not required if the reaction is carried out under high pressure (9,000 kPa) and high temperature (240°C, 465°F), where simultaneous esterification and transesterification take place with maximum yield obtained at temperatures ranging from 60 to 80°C (140 to 175°F) at a molar ratio of 6:1. The alcohols employed in the transesterification are generally short chain alcohols such as methanol, ethanol, propanol, and butanol. It was reported that when transesterification of soybean oil using methanol, ethanol, and butanol was performed, 96–98% of ester could be obtained after 1 hour.

      Biodiesel – Transesterification, Catalytic

Transesterification reactions can be catalyzed by alkalis or enzymes. The catalytic transesterification of vegetable oils with methanol is an important industrial method used in biodiesel synthesis. Also known as methanolysis, this reaction is well established using acids or alkalis, such as sulfuric acid or sodium hydroxide as catalysts. However, these catalytic systems are less active or completely inactive for long chain alcohols. Usually, industries use sodium or potassium hydroxide or sodium or potassium methoxide as catalyst since they are relatively cheap and quite active for this reaction. Enzyme-catalyzed procedures, using lipase as catalyst, do not produce side reactions, but the lipases are expensive for industrial-scale production and a three-step process was required to achieve a 95% conversion. The acid-catalyzed process (Table B-7) is useful when a high amount of free acids is present in the vegetable oil, but the reaction time is on the order of 48 to 96 hours, even at the boiling point of the alcohol, and a high molar ratio of alcohol was needed (20:1 wt/wt to the oil).

Table B-7 Schematic of the catalytic transesterification process for biodiesel production.

Feedstock	Reactor	Products
Vegetable oil	Catalyst	Biodiesel
Methanol*		Glycerol
*Ethanol may also be used.

      The transesterification process is catalyzed by alkaline metal alkoxides, and hydroxides, as well as sodium or potassium carbonates. Alkali-catalyzed transesterification with short-chain alcohols, for example, generates high yields of methyl esters in short reaction times. The alkaline catalysts show high performance for obtaining vegetable oils with high quality, but a question often arises; that is, the oils contain significant amounts of free fatty acids which cannot be converted into biodiesels СКАЧАТЬ