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Pure Vegetable Oils

Diesel engines initially perform to much the same standard with pure vegetable oil as with diesel. In the past pure vegetable oils have been mainly used in tractors on farms. Pure vegetable oils create problems in turbocharged direct injection engines with charge air coolers, such as those used in trucks.

Table 4.1 compares some of the physical and chemical properties of diesel, canola oil and methyl esters. Vegetable oils have higher density than diesel, but lower energy content (gross calorific value). Vegetable oils have lower carbon content than diesel, which means lower

CO2 emissions per litre of fuel burnt. CO2 emissions per kilometre travelled may not be lower, however, due to the lower energy content of the vegetable oils and a higher proportion of multi bonded carbon compounds. The major difference in physical characteristics between canola and diesel is in the viscosity. Canola is more than 12 times as viscous as diesel at 20oC, and remains more than six times as viscous even after heating to 80oC.

 

These high viscosity levels create problems for the use of canola, or other pure vegetable oils, as an unmodified fuel. The flow of the fuel from tank to engine is impeded, which can result in decreased engine power. Fuel filter blockages may also occur. The multi-bonded compounds pyrolyse more readily and engines can suffer coking of the combustion chamber and injector nozzles, and gumming, and hence sticking, of the piston rings – a progressive decline in power results. If left unchecked, dilution of the crankcase oil can lead to lubrication breakdown. Long-term tests have verified that there is a build-up of carbon deposits in the injection nozzles and cylinder heads.

The viscosity problem can be mitigated by preheating the oil and using larger fuel lines, by blending diesel and vegetable oils, or by chemical modification (i.e. producing biodiesel).

Apart from the viscosity difficulties, vegetable oils may result in starting difficulties due to a high temperature being required before the oil will give off ignitable vapours. They also have a relatively slow burn rate as a result of the low cetane rating, which makes vegetable oils unsuitable for high speed engines.

 

Biodiesel is a generic name for fuels obtained by transesterification of a vegetable oil. This produces a fuel with very similar combustion properties to pure diesel, but with lower viscosity. Often biodiesel refers to rapeseed oil methylester (RME), the main European biodiesel. Esterified soybean oil is the main United States source of such fuel, called Soy diesel. Figure 4.1 depicts a flow chart of the esterification process.

 

Biodiesel can be used in a diesel engine without modification. Mittelbach (1998) quotes a cetane number of 48 for rapeseed methyl ester but notes that this can be increased to 59 if the biodiesel is made from the ethyl esters of tropical oilseeds. Mann (1998) claims a cetane number of 56 for soydiesel. The fuel consumption of biodiesel per kilometre travelled is similar to that for diesel when biodiesel is used as a diesel blend. Biodiesel has a lower energy content than diesel that leads to increased fuel consumption when pure biodiesel is used (Taberski et al., 1999).

The greenhouse gas emissions arising from the process depicted in Figure 4.1 depend on the amount of fossil fuel involved in the production of the alcohol. If methanol is used then this process is described by the equation.

C3H5(OOCR)3 + 3CH3OH -> 3RCOOH3 + C3H5(OH)3

(Triglyceride) (Methanol) (Methylester) (Glycerine)

The term “triglyceride” in the equation may be either vegetable oil or tallow. From a chemical point of view, the differences between various plant and animal derived fats are due to the structural variations of fatty acids contained in fat molecules.

In most fats, the length of the fatty acid carbon chain ranges between C16 and C18. There are also differences in the degree of saturation (number and position of double bonds) in acid molecules. Saturation is the major factor determining physical properties of fats. Highly unsaturated vegetable oils are low viscosity liquids, while fully saturated animal fats are solid at ambient temperature.

From the point of view of the transesterification process itself, these differences in molecular structure are insignificant in terms of process parameters or energy demand. The greenhouse gas emissions arising from the process depicted in Figure 4.1 depend mostly on the amount of fossil fuel involved in the production of the alcohol as given by Sheehan et al. (1998: p. 147), who estimate that 5% (by mass) of the carbon emissions are fossil-fuel carbon.

For example, if methanol is used, overall emissions will be higher because current production of methanol involves solely fossil-fuel feedstocks such as natural gas or coal. By contrast, if the use of ethanol produced from renewable resources (biomass) using bioprocesses is contemplated, greenhouse emissions will be lower. Methanol can be produced by the gasification of biomass but this is currently not done.


Jatropha

 

Research on Jatropa Curcas, (Duke 1983) makes the following observations:-

Shrub or tree to 6 m, with spreading branches and stubby twigs, with a milky or yellowish rufescent exudate. Leaves deciduous, alternate but apically crowded, ovate, acute to acuminate, basally cordate, 3 to 5-lobed in outline, 6–40 cm long, 6–35 cm broad, the petioles 2.5–7.5 cm long. Flowers several to many in greenish cymes, yellowish, bell-shaped; sepals 5, broadly deltoid. Male flowers many with 10 stamens, 5 united at the base only, 5 united into a column. Female flowers borne singly, with elliptic 3-celled, triovulate ovary with 3 spreading bifurcate stigmata. Capsules, 2.5–4 cm long, finally drying and splitting into 3 valves, all or two of which commonly have an oblong black seed, these ca 2 x 1 cm (Morton, 1977; Little et al., 1974).

 

 

Jatropha curcas grows almost anywhere – even on gravelly, sandy and saline soils. It can thrive on the poorest stony soil. It can grow even in the crevices of rocks. The leaves shed during the winter months form mulch around the base of the plant. The organic matter from shed leaves enhance earth-worm activity in the soil around the root-zone of the plants, which improves the fertility of the soil. Climatically, Jatropha curcas is found in the tropics and subtropics and likes heat, although it does well even in lower temperatures and can withstand a light frost. Its water requirement is extremely low and it can stand long periods of drought by shedding most of its leaves to reduce transpiration loss. Jatropha curcas is also suitable for preventing soil erosion and shifting of sand dunes.
Analysis of the Jatropha curcas seed shows the following chemical composition:

* Moisture 6.20 %
* Protein 18.00 %
* Fat 38.00 %
* Carbohydrates 17.00 %
* Fiber 15.50 %
* Ash 5.30 %


The oil content is 25 – 30% in the seeds and 50 – 60% in the kernel. The oil contains 21% saturated fatty acids and 79% unsaturated fatty acids.There are some chemical elements in the seed, Cursin, which are poisonous and render the oil not appropriate for human consumption.

The comparison of properties of Jatropha curcas oil and standard specifications of diesel oil :

Specification

Standard specification of Jatropha curcas oil

Standard specification of Diesel

Specific gravity

0.9186

0.82/0.84

Flash point

240/110°C

50°C

Carbon residue

0.64

0.15 or less

Cetane value

51.0

> 50.0

Distillation point

295°C

350°C

Kinematics Viscosity

50.73 cs

> 2.7 cs

Sulpher %

0.13 %

1.2 % or less

Calorific value

9,470 kcal/kg

10,170 kcal/kg

Pour point

8°C

10°C

Colour

4.0

4 or less

 

 

 

 

 

 

 

 

Physical and chemical properties of diesel fuel and Jatropha curcas oil:

Property

Jatropha curcas Oil

Diesel Oil

Viscosity (cp) (30°C)

5.51

3.60

Speciflc gravity (15°C/4°C)

0.917/ 0.923(0.881)

0.841 / 0.85

Solidfying Point (°C)

2.0

0.14

Cetane Value

51.0

47.8 to 59

Flash Point (°C)

110 / 340

80

Carbon Residue (%)

0.64

< 0.05 to < 0.15

Distillation (°C)

284 to 295

< 350 to < 370

Sulfur (%)

0.13 to 0.16

< 1.0 to 1.2

Acid Value

1.0 to 38.2

 

Saponification Value

188 to 198

 

Iodine Value

90.8 to 112.5

 

Refractive Index (30°C)

1.47