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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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