08-11-2012, 11:02 AM
Biodiesel Production
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Introduction
Biodiesel [1–5] is a liquid biofuel obtained by chemical processes from vegetable
oils or animal fats and an alcohol that can be used in diesel engines, alone or
blended with diesel oil.
ASTM International (originally known as the American Society for Testing and
Materials) defines biodiesel as a mixture of long-chain monoalkylic esters from
fatty acids obtained from renewable resources, to be used in diesel engines.
Blends with diesel fuel are indicated as ‘‘Bx’’, where ‘‘x’’ is the percentage of
biodiesel in the blend. For instance, ‘‘B5’’ indicates a blend with 5% biodiesel and
95% diesel fuel; in consequence, B100 indicates pure biodiesel.
Advantages of the Use of Biodiesel
Some of the advantages of using biodiesel as a replacement for diesel fuel are
[1–4]:
• Renewable fuel, obtained from vegetable oils or animal fats.
• Low toxicity, in comparison with diesel fuel.
• Degrades more rapidly than diesel fuel, minimizing the environmental consequences
of biofuel spills.
• Lower emissions of contaminants: carbon monoxide, particulate matter, polycyclic
aromatic hydrocarbons, aldehydes.
• Lower health risk, due to reduced emissions of carcinogenic substances.
• No sulfur dioxide (SO2) emissions.
• Higher flash point (100C minimum).
Disadvantages of the Use of Biodiesel
There are certain disadvantages of using biodiesel as a replacement for diesel fuel
that must be taken into consideration:
• Slightly higher fuel consumption due to the lower calorific value of biodiesel.
• Slightly higher nitrous oxide (NOx) emissions than diesel fuel.
• Higher freezing point than diesel fuel. This may be inconvenient in cold
climates.
• It is less stable than diesel fuel, and therefore long-term storage (more than six
months) of biodiesel is not recommended.
• May degrade plastic and natural rubber gaskets and hoses when used in pure
form, in which case replacement with Teflon components is recommended.
• It dissolves the deposits of sediments and other contaminants from diesel fuel in
storage tanks and fuel lines, which then are flushed away by the biofuel into the
engine, where they can cause problems in the valves and injection systems. In
consequence, the cleaning of tanks prior to filling with biodiesel is
recommended.
Raw Materials for Biodiesel Production
The raw materials for biodiesel production are vegetable oils, animal fats and short
chain alcohols. The oils most used for worldwide biodiesel production are rapeseed
(mainly in the European Union countries), soybean (Argentina and the United
States of America), palm (Asian and Central American countries) and sunflower,
although other oils are also used, including peanut, linseed, safflower, used vegetable
oils, and also animal fats. Methanol is the most frequently used alcohol
although ethanol can also be used.
Since cost is the main concern in biodiesel production and trading (mainly due
to oil prices), the use of non-edible vegetable oils has been studied for several
years with good results.
Typical Oil Crops Useful for Biodiesel Production
The main characteristics of typical oil crops that have been found useful for
biodiesel production are summarized in the following paragraphs [6–10].
Rapeseed and Canola
Rapeseed adapts well to low fertility soils, but with high sulfur content. With a
high oil yield (40–50%), it may be grown as a winter-cover crop, allows double
cultivation and crop rotation.
It is the most important raw material for biodiesel production in the European
Community. However, there were technological limitations for sowing and harvesting
in some Central and South American countries, mainly due to the lack of
adequate information about fertilization, seed handling, and storage (the seeds are
very small and require specialized agricultural machinery). Moreover, low prices
in comparison to wheat (its main competitor for crop rotation) and low production
per unit area have limited its use.
Rapeseed flour has high nutritional value, in comparison to soybean; it is used
as a protein supplement in cattle rations.
Sometimes canola and rapeseed are considered to be synonymous; canola
(Canadian oil low acid) is the result of the genetic modification of rapeseed in the
past 40 years, in Canada, to reduce the content of erucic acid and glucosinolates in
rapeseed oil, which causes inconvenience when used in animal and human
consumption.
Soybean
It is a legume originating in East Asia. Depending on environmental conditions
and genetic varieties, the plants show wide variations in height. Leading soybean
producing countries are the United States, Brazil, Argentina, China, and India.
Biodiesel production form soybean yields other valuable sub-products in
addition to glycerin: soybean meal and pellets (used as food for livestock) and
flour (which have a high content of lecithin, a protein). Grain yield varies between
2,000 and 4,000 kg/hectare. Since the seeds are very rich in protein, oil content is
around 18%.
Oil Palm
Oil palm [11] is a tropical plant that reaches a height of 20–25 m with a life cycle
of about 25 years. Full production is reached 8 years after planting.
Two kinds of oil are obtained from the fruit: palm oil proper, from the pulp, and
palm kernel oil, from the nut of the fruit (after oil extraction, palm kernel cake is
used as livestock food). Several high oil-yield varieties have been developed.
Indonesia and Malaysia are the leading producers.
International demand for palm oil has increased steadily during the past years,
the oil being used for cooking, and as a raw material for margarine production and
as an additive for butter and bakery products.
It is important to remark that pure palm oil is semisolid at room temperature
(20–22C), and in many applications is mixed with other vegetable oils, sometimes
partially hydrogenated.
Sunflower
Sunflower ‘‘seeds’’ are really a fruit, the inedible wall (husk) surrounding the seed
that is in the kernel.
The great importance of sunflower lies in the excellent quality of the edible oil
extracted from its seeds. It is highly regarded from the point of view of nutritional
quality, taste and flavor. Moreover, after oil extraction, the remaining cake is used
as a livestock feed. It must be noted that sunflower oil has a very low content of
linoleic acid, and therefore it may be stored for long periods.
Sunflower adapts well to adverse environmental conditions and does not require
specialized agricultural equipment and can be used for crop rotation with soybean
and corn. Oil yield of current hybrids is in the range 48–52%.