Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 181

ASTM International (ASTM), formally known as the American Society for testing materials, is an international organization which develops and publishes information on the technical standards of various products, materials, systems, and services. It is one of the largest and most highly regarded standards development organizations in the world. The available literature on the performance of biofuels when compared with traditional fossil fuels normally uses ASTM and ISO (International Standards Organization) specifications and parameters. The specifications provide details on requirements for fuel characteristics as well as the relevant standard test methods to use for each. The common international standard for biodiesel is EN 14214, while ASTM 6751 is most referenced in the United States. In Germany, the requirements for biodiesel are fixed in the DIN EN 14214 standard.

With regards to biodiesel, most of the world uses a system known as the “B” factor to state the amount of biodiesel in any fuel mix, in contrast to the “BA” or “E” system used for bioalcohol. Pure biodiesel is referred to as B100, while fuel containing 20% biodiesel is labeled B20.

The standards ensure that the following important factors in the fuel production process are satisfied: (i) complete reaction, (ii) removal of glycerin, (iii) removal of catalyst, (iv) removal of alcohol, and (v) ensuring the absence of free fatty acids. Basic industrial tests to determine whether the products conform to the standards typically include gas chromatography, a test that verifies only the more important of the variables above. Fuel meeting the quality standards is very non-toxic, with a toxicity rating (LD50) greater than 50 mL/kg.

One of the most important fuel properties of biodiesel and conventional diesel fuel derived from crude oil is viscosity, which is also an important property of lubricants. Ranges of acceptable kinematic viscosity are specified in various biodiesel and crude oil standards. Reducing viscosity is one of the main reasons why vegetable oils or fats are transesterified to biodiesel because the high viscosity of neat vegetable oils or fats ultimately leads to operational problems such as engine deposits. The viscosity of biodiesel is slightly greater than that of petrodiesel but approximately an order of magnitude less than that of the parent vegetable oil or fat. Biodiesel and its blends with petrodiesel display temperature-dependent viscosity similar to that of neat petrodiesel. Influencing factors are chain length, position, number and nature of double bonds, as well as the nature of the oxygenated moieties.

Classic biodiesel has a higher cloud point (temperature at which a fuel becomes hazy or cloudy and starts to gel) than petrodiesel. This makes its use impractical in cooler climates and limits its potential market. Other important chemical and physical properties described in ASTM standards for biodiesel are (i) total acid number, TAN, which indicates the presence of free fatty acids and carboxylic acids present, (ii) corrosion, which is the potential for copper corrosion, (iii) low temperature performance, which is described by the pour point and the cloud point, and (iv) oxidation stability.

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 fuel’s thermal energy 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).

Approximately 11% of the weight of B100 is oxygen. The presence of oxygen in biodiesel improves combustion and therefore reduces hydrocarbon, carbon monoxide, and particulate emissions; but oxygenated fuels also tend to increase nitrogen oxide emissions. Engine tests have confirmed the expected increases and decreases of each exhaust component from engines without emission controls.

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 petrol 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, it can’t be so volatile that it vaporizes and boils in the injectors, carburetor, fuel lines, or fuel pump, which could prevent it from being metered correctly. Also, if gasoline is too volatile, moiré 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 gasoline and gasoline. 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 leaner which is advantageous for fuel economy and 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.

The factors of availability, price, and independence of manufacturer, health benefits, engine improvements, and political implications must all be carefully weighed and assessed, before educated decisions can be made by the powers that be. It is hoped however, that despite some of the obvious setbacks of biofuel production, that it is still viewed as a step in the right direction, and emerging technologies and innovative ideas will encourage improvements in what is undoubtedly, the future of fuel.