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Biodiesel
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Biodiesel - in comparison to diesel from fossil resources - can be produced from vegetable oils of different origins, e.g. soybeans, peanuts and other vegetable oils, such as used cooking oil, or even from animal waste (w1). In Europe, biodiesel is already used commercially. The most common crop used there is rapeseed, and to some extent sunflowers. The products are referred to as RME (rape seed oil methyl ester) and SME (sunflower oil methyl ester), respectively, but are now more often called FAME (Fatty Acid Methyl Ester). The European norm prEN14214 (CEN) specifies the properties of FAME.

In order to produce RME or SME, the oil is extracted from the harvested seeds, leaving seed meal behind which can be used as animal fodder. The oil is refined and undergoes transesterification with methanol, yielding glycerin as a by-product (1). Biodiesel can be used in its pure form (100% biodiesel) or blended in any ratio with regular diesel for use in compression-ignition engines (w1). RME can also be used as a lubricating additive in "zero sulphur" diesel fuels.

When comparing biodiesel used in an unmodified diesel engine with petroleum diesel, similar engine performance (i.e., power and acceleration) has been reported (w1). However, biodiesel has lower energy content, leading to a higher fuel requirement of about 15% on a weight basis. Also, depending on the source there can be lubricity concerns. FAME is prone to oxidation and therefore has a "best before date" about six months after the manufacturing. This may pose a problem for farm use since some motorized farm equipment is used periodically.

A significant potential advantage of biodiesel is that the raw materials can be produced without requiring large fossil fuel reserves, and that through this process less greenhouse gases are produced: the combustion of biodiesel leads to CO2 release to the atmosphere, but under ideal conditions not more than was previously extracted from the air during the growth of the crop. (Some additional emission may be generated by associated processes however.) Thus biodiesel can have a positive net effect on global climate compared to the use of conventional diesel.

Disadvantages of using biodiesel produced from agricultural crops involve the additional land use, as land area is taken up and various agricultural inputs with their environmental effects are inevitable (see (2)). In order to assess the environmental life cycle effects of biodiesel compared to conventional diesel, a life cycle assessment (LCA) has to be carried out (see for example (2) and (3)).

Generally, vehicle engines running on biodiesel produce emissions in nearly the same range as conventional diesel vehicles. Particulate, hydrocarbons and carbon monoxide emissions tend to be lower than from a conventional diesel but NOx emissions generally tend to be higher. Particulate emissions are mainly non- or partially combusted FAME. The soot content is generally low.

Sulfur content of biodiesel is usually lower, but life cycle emissions can be significantly higher (see for example (2)). It depends on the raw material used, the processing methods and conditions of combustion.

An ASTM specification for biodiesel used for blending is now final.

Biodiesel is biodegradable to a higher degree than petroleum based diesel, and therefore its use is particularly suitable for machinery used in ecologically sensitive areas.

FAME can have a swelling effect on paint, rubber and plastic materials. Old deposits can be dissolved and can lead to clogged fuel filters. Low density polyethylene (LDPE) and silicone rubber are permeable to RME. Most vehicle manufacturer generally recommend limiting biodiesel blends to a maximum of 5 % in older diesel engines for this reason.

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FAME can be produced by for example a farm using small scale equipment. A batch can consist of severalm3.

Biodiesel can be used as a fuel additive and therefore does not necessitate the existence of different distribution and storage systems, as would be the case for example with CNG or hydrogen. Neither does it necessitate engine modifications. Thus conventional diesel and heavy-duty vehicles, such as farm equipment and buses, can operate on the fuel (w1) (normally up to 5%) although as noted earlier a lubricity additive may be needed.

Elastomer incompatibilities have been observed which new users should be aware of.

Storage for more than six months should be avoided.

However, certain infrastructural changes would, in the case of domestic production, be required with regard to production: the agricultural production of oil crops in sufficient quantities requires relatively large areas of land, which could displace for example the production of food crops. Whether this is possible or desirable depends on the particular local conditions.

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The production of biodiesel is much more expensive than conventional diesel from fossil resources. However, in parts of Europe such as Germany and Austria, its price to the consumer is slightly lower than that of conventional diesel because biodiesel is exempt from the mineral oil tax: at the time of writing, the price of biodiesel in Germany for example was about 0.76 euro/liter (2.67 US$/gallon), compared to 0.79 euro/liter (2.77 US$/gallon) for regular diesel. This economic advantage does not reflect the production costs, which are significantly higher for biodiesel than for regular diesel. Hence the market viability of biodiesel depends heavily on governmental support.

In the US, the Department of Agriculture has established a subsidy for refiners to use soy oil as a biodiesel feedstock. In addition, fleet operators subject to the Energy Policy Act of 1992 (EPAct) fuel regulations may claim alternative fuel credits for the biodiesel portion of the fuel they consume.

Some transit authorities that use biodiesel blends have reported lower maintenance costs than for those vehicles fueled solely on conventional diesel (w1).

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In Europe and the USA, biodiesel is produced and utilized in commercial quantities. In 1998, the DOE in USAdesignated neat (100%) biodiesel ("B100"), as an alternative fuel and established a credit program for biodiesel use. However, blended biodiesel, the most common of which is referred to as B20 (20% biodiesel, 80% conventional diesel), has not been designated as an alternative fuel (w1).

In the US, centrally fueled light- and medium-duty fleets in the Midwest and East are currently the primary users of biodiesel fuel. Overall market shares are low: for example, in Germany, where biodiesel is available at about 1,000 out of a total of 16,000 filling stations, the share of biodiesel is on the order of 0.3% of the diesel sold, which is equivalent to 100,000 t. This is expected to rise to perhaps 300,000 in the foreseeable future, but even optimists do not expect the share to rise above 5-10% at the most (w3).

Several transit and school bus fleets are using biodiesel in the USA.

According to (w1), the use of biodiesel as an alternative fuel (i.e. in its pure form) is not expected to be significant, but as a blend it may increase in the US and elsewhere, although perhaps mainly in captive fleets with central fuelling or niche markets in environmental sensitive areas (w2).

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Biodiesel is characterized by relatively high production costs, which necessitate government subsidies of some form or other. In order to remain on the market in the long-term, it may turn out to be helpful, if not necessary, for the biofuels to achieve economic viability and independence from such subsidies. One factor that will limit the market share of biodiesel considerably in many parts of the world is the limited production capacity. In addition, the increase in NOx emissions in many studies remains an impediment to its use.

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