Renewable Energy Technology 3(2+1)

Lesson 32. Properties of biodiesel

1. Density

Density is the measure of ratio between mass and volume.  Specific gravity is the ratio of the density of a substance to the density (mass of the same unit volume) of a reference substance.  Bio-diesel is slightly heavier than conventional diesel fuel (specific gravity 0.88 compared to 0.84 for diesel fuel).

2. Cetane Number:

Cetane number (CN) is a dimensionless descriptor of the ignition quality of a diesel fuel (DF).  It is a prime indicator of DF quality.  It is a measure of the quality of a diesel fuel expressed as the percentage of cetane in a mixture of cetane and 1-methylnapthalene of the same quality as the given fuel.  This relates to the readiness of the fuel to self ignite when exposed to the high temperature and pressure in combustion chamber.  Regular diesel fuel has a cetane number of 42-45, where as that of biodiesel varies with feed stock.

3. Viscosity

Viscosity is a measure of resistance to flow of a liquid created due to the internal friction of one part of a fluid moving over another.  This affects the atomization of a fuel upon injection into the combustion chamber. 

4. Flash point

Flash point of a fuel is defined as the temperature at which it will ignite when exposed to a flame or spark. The flashpoint of bio-diesel is higher than the petroleum based diesel fuel. Flashpoint of bio-diesel blends is dependent on the flashpoint of the base diesel fuel used, and increase with percentage of bio-diesel in the blend. The flashpoint of bio-diesel is around 160°C.

5. Cold Filter Plugging Point (CFPP)

At low operating temperature fuel may thicken and not flow properly affecting the performance of fuel lines, fuel pumps and injectors. Cold filter plugging point of biodiesel reflects its cold weather performance.  This can be referred as the lowest temp at which 20 ml oil safely passes through the filter within 60 s.

6. Pour Point

The pour point of a liquid is the lowest temperature at which it becomes semi solid and loses its flow characteristics.  This is the temperature at which crystal agglomeration is extensively enough to prevent free pouring of fluid.  To assess the pour point, thermal analytical method – sub ambient differential scanning calorimetry (DSC) can be used.

7. Cloud Point

During cooling temperatures, solid wax crystal nuclei may be formed in biodiesel at submicron and invisible range.  If there is any further temperature reduction, the crystals to grow.  The temperature at which crystals become visible (d ≥ 0.5 μm) is called as cloud point.  At this point, crystals form cloudy or hazy suspension.  Biodiesel generally has higher cloud point than diesel fuel.

8. Oxidative stability

Biodiesel is susceptible to oxidation upon exposure to air. The oxidation process ultimately affects fuel quality.  Rancimat apparatus is used to test the oxidative stability of biodiesel.

9. Iodine Number

Iodine number refers to the amount of iodine required to convert unsaturated oil into saturated oil.  It refers to the presence of unsaturated fatty acids in the fuel.

10. Free and Total glycerol

The degree of conversion completeness of the vegetable oil is indicated by the amount of free and total glycerol present in the bio-diesel. If the actual number is higher than the specified values, engine fouling, filter-clogging etc can occur.

11. Sulfur content

Biodiesel generally contain less than 15ppp sulfur. ASTM D 5453 test is a suitable test for such low level of sulfur.

12. Lubricity

The lubrication of the pump is provided by lubricity property of the fuel also.  Due to lack of lubricity several parts of the pump can wear out even with sufficient viscosity.  The lubricity of the fuel depends on the oil source, transesterification process, type of additives used etc.  BOCLE ( Ball on Cylinder Lubricity Evaluator ) and HFFR(High Frequency Reciprocating Rig) are commonly used for evaluating the lubricity of the fuel.

13. Sulfated Ash

Sulfated ash is controlled to ensure that all the catalysts used in the transesterification process are removed.  Presence of ash can cause filter plugging and or injector deposits.

14. Acid number/Neutralization number

Acid number reflects the presence of free fatty acids or acids used in manufacture of biodiesel. It also reflects the degradation of biodiesel due to thermal effects.

15. Water Content

The water content in biodiesel and its blends plays a major role in growing microbes when water is present in the fuel. The solvency properties of the biodiesel can cause microbial slime to detach and clog fuel filters.

16. Methanol/ethanol content

High levels of free alcohol in biodiesel damage to natural rubber seals and gaskets present in fuel pumps and injectors. 

 

Biodiesel standards

There are more Biofuel standards by various organizations.  They are

• ASTM D6751 (United States): Standard Specification for Biodiesel (B100) Blend Stock for Distillate Fuels

• EN 14213 (Europe): Heating Fuels: Fatty Acid Methyl Esters (FAME).

• EN 14214 (Europe): Automotive Fuels: FAME for Diesel Engines.

• Provisional Australian biodiesel standard.

• Provisional Brazilian biodiesel standard ANP 255

• Provisional South African biodiesel standard.