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05/01/2019
Liquid Bio fuels
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bio-diesel
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Another way of extracting energy from biomass is through the use of
vegetable oils as fuel. Many edible and non-edible oil crops are being
grown in some countries for use as possible petroleum fuel
substitutes. Because of problems associated with the high viscosity of
the straight vegetable oil, they are also processed into a less viscous
esterified oil known as bio-diesel. Biodiesel has fuel characteristics
comparable to diesel.
bio-diesel
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What Is Biodiesel?
It is a mono alkyl ester of long chain fatty acids derived from
renewable lipid feedstock, such as vegetable oils and animal fats, for
use in compression ignition (diesel) engines and produced by
transesterification of triglycerides or free fatty acid feedstocks by an
alcohol.
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Biodiesel Production
There are two steps in biodiesel production
• Production of vegetable oils
• Conversion of veg. oil to Biodiesel
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….the expansion of oil crop plantations for biodiesel production on a
large scale may increase deforestation in
countries like Malaysia, Indonesia and Brazil. Furthermore, the line
between food and fuel economies is blurred as both of the fields are
competing for the same oil resources. In other words, biodiesel is
competing limited land availability with food industry for plantation of
oil crops.
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Non-edible oilseeds
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Non edible oil seed crops for biodiesel
In order to overcome this devastating phenomenon, suggestions and
research have been made to produce biodiesel by using alternative or
greener oil resources like non-edible oils. The non-edible vegetable oils
such as Madhuca indica, Jatropha curcas and Pongamia pinnata are found
to be suitable for biodiesel production under the experimental conditions. It
has been found that the yield of methyl ester from karanja oil under the
optimal condition is 97–98%. Oil content in the Castor bean, Hemp and
Pongamia seed is around 50, 35 and 30-40 % respectively. Neem seed
contains 30% oil content.
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Process of extraction of vegetable oils
Generally, mechanical extraction is used to extract oil from seeds. Sometimes,
solvent extraction route is adopted, with the use of solvents such as hexane. It
can also be a combination of both the processes. For small-scale processes,
usually mechanical extraction is employed.
The first step is to remove extraneous matter from the incoming seed utilizing
sieves, etc. Depending upon the seed being processed, various types of
grinding, cracking, flaking, and rolling equipment are used to access the oil.
In order to further improve oil recovery during the process, all seeds are heated,
typically using steam.
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In the solvent extraction method, oil is extracted using an organic
solvent, generally hexane. The cake is washed with solvent under a
countercurrent flow, producing an oil/solvent mixture (termed as miscella)
and a residual meal-containing solvent.
The miscella is subjected to evaporation and steam stripping to recover
hexane from oil.
After oil extraction, the next step is the refining process, which generally
includes degumming, neutralization, bleaching, deodorization, and further
refining.
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Why vegetable oils unsuitable for direct firing in engines ?
Vegetable oils as fuel in conventional diesel engines, can lead to a number of
problems as the injection, atomization, and combustion characteristics of
vegetable oils in diesel engines are significantly different from those of diesel.
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Characterization of liquid fuels
Density and Specific gravity
Kinematic viscosity
Cetane number
Heating value
Cloud point
Pour point
Flash point
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Density and Specific gravity
Density is …….
The density of liquids in relation to water is known as
specific gravity. Water is given a value of one. Liquids
with a specific gravity less than one are lighter than water,
while those with a specific gravity greater than one are
heavier than water. It is interesting to note that most
flammable liquids have a specific gravity of less than one,
therefore they would float on top of water.
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Kinematic viscosity
is the ratio of absolute or dynamic viscosity to density - a quantity in
which no force is involved. Kinematic viscosity can be obtained by
dividing the absolute viscosity of a fluid with it's mass density
ν = μ / ρ (2)
where
ν = kinematic viscosity
μ = absolute or dynamic viscosity
ρ = density
The kinematic viscosity of the liquid fuel is measured as by the standard
method ASTM D 445 .
If v< 1 cst=> not pumpable
If v>10cst => not atomize
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Cetane number
Cetane number or CN is a measure of a fuel's ignition delay; the time
period between the start of injection and the first identifiable pressure
increase during combustion of the fuel. In a particular diesel engine,
higher cetane fuels will have shorter ignition delay periods than lower
cetane fuels. Biodiesel from vegetable oil sources have been recorded
as having a cetane number range of 46 to 52, and animal-fat based
biodiesels cetane numbers range from 56 to 60.
Cetane is a collection of un-branched open chain alkane molecule
that ignites very easily under compression, so it was assigned a cetane
number of 100, while alpha-methyl naphthalene was assigned a cetane
number of 0. All other HCs in diesel fuel are indexed to cetane as to
how well they ignite under compression. The cetane number therefore
measures how quickly the fuel starts to burn (auto-ignites) under diesel
engine conditions
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Heating value
The calorific value of a fuel is the quantity of heat produced by its
combustion - at constant pressure and under "normal" conditions (i.e. to 0oC
and under a pressure of 1 atm). Higher the heating value better is the fuel (
in relation to density)
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Cloud point
The cloud point of a fluid is the temperature at which dissolved solids are no
longer completely soluble, precipitating as a second phase giving the fluid a
cloudy appearance. cloud point refers to the temperature below which wax in
diesel or biowax in biodiesel form a cloudy appearance
Pour point The pour point of a liquid fuel is the temperature at which it becomes semi solid
and loses its flow characteristics. In general a high pour point is associated with a
high paraffin content
Flash point
The flash point of a volatile material is the temperature at which it can vaporize to
form an ignitable mixture in air. Measuring a flash point requires an ignition source.
At the flash point, the vapor may cease to burn when the source of ignition is
removed
“Flash point” refers to both flammable liquids and combustible liquids. There are
various standards for defining each term. Liquids with a flash point less than 60.5
°C or 37.8 °C are considered flammable, while liquids with a flash point above
those temperatures are considered combustible.
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It can be seen that kinematic viscosity of these oils varies between 30
and 40 cSt at 38 ºC, which is due to their large molecular mass (600–900).
The high viscosity of vegetable oils interferes with the injection process and
leads to poor fuel atomization. The inefficient mixing of oil with air
contributes to incomplete combustion, leading to heavy smoke emission,
and
the high flash point attributes to lower volatility characteristics. These
disadvantages, coupled with reactivity of unsaturated vegetable oils, do not
allow efficient engine operation for long periods of time.
Solution
These problems can be solved if the vegetable oils are chemically modified
to bio-diesel, which is similar in characteristics to diesel.
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Production of bio-diesel
Bio-diesel is produced by transesterification of triglycerides and fatty acids
present in oil
Transesterification is the process of exchanging the alkoxy
group of an ester compound by another alcohol.
where Rl, Rll, Rlll are the long chains of fatty acids
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Bio-diesel Properties
Bio-diesel is a clear amber-yellow liquid with a viscosity similar to petroleum
diesel.
• With a flash point of about 150 ºC, bio-diesel does not ignite as readily
as petroleum diesel (76 ºC). It is classified as a non-flammable liquid by OSHA
(Occupational Safety and Health Administration), US, though it will of course
burn if heated to a high enough temperature. Therefore, it is far safer to use it
as a transport fuel.
• Bio-diesel gels at lower temperatures (about 0 ºC) than petroleum diesel,
which limits its pure form use in cold climates.
• Unlike petro-diesel, bio-diesel is biodegradable and non-toxic, and it
significantly reduces toxic and other emissions when burned as fuel.
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