Improving biodiesel’s properties
Properties including cold flow have prevented biodiesel from living up to its early potential. Emerging strategies may improve matters.
CHANDRAGUPTHAN BAHUBALI and ASHWIN GOPALAN
Wood PLC India
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Following almost 15 years of research and study, substantial progress has not been achieved to enable biodiesel to deliver competitive fuel performance. The flow characteristics of biodiesel are a core point of concern. Biodiesel’s relatively high pour point and cloud point make it an impractical fuel for several applications. Beside its oxidation instability and lower calorific value, biodiesel also has a lower chemical diversity (fewer chemical components) compared with petroleum diesel.
The lack of chemical diversity in biodiesel fuels narrows the temperature gap between the cloud point and the pour point. The presence of saturates in biodiesel is another important issue which affects its cold temperature characteristics. A large increase in cloud point occurs when the total amount of saturated esters increases. Cloud point depends mostly on the saturated ester content of the fuel and the effect of unsaturated ester composition can be negligible. On the other hand, the high unsaturation of fatty acid methyl esters causes polymerisation and oxidation of the fuel. Pour point and cloud point are the properties most affected by the presence of unsaturation.
All of these properties are believed to be affected by intermolecular forces of attraction, which become greater with increasing chain length. The presence of double bonds disrupts the attractive forces operating between molecules along the hydrocarbon chain and hence causes these physical properties to decrease in value. The cis double bonds in unsaturated components hamper crystal packing, resulting in lower pour point. Hence, although the trans configuration has a more beneficial effect on cold temperature properties, the formation of trans double bonds in unsaturated components is challenging. On the other hand, saturated methyl esters have a higher cetane number and lower NOx exhaust emissions. Branched compounds and aromatics have low cetane numbers. Fatty esters such as methyl palmitate and methyl stearate have cetane numbers comparable to hexadecane and other long chain alkanes. Viscosity increases with chain length and increasing saturation. Oxygen content reduces energy density and causes corrosion. The oxygen content in biodiesel varies by up to 11%. The hydrogen to carbon ratio varies in the range 1.4 to 1.8, whereas petroleum diesel has a value of 2.0.
Several strategies can be employed in order to modify the flow characteristics of biodiesel.
Strategy 1: controlling saturates in biodiesel
Use isopropanol or 2-butanol instead of methanol
The use of secondary alcohols such as isopropanol or 2-butanol instead of methanol leads to a product of lower pour point. However, the use of branched chain alcohol increases the viscosity of the product due to the presence of a relatively higher amount of mono- and diglycerides as a result of incomplete transesterification. This is attributed to the lower nucleophilicity and greater steric hindrance of alkyl groups adjacent to the carbon atom bearing the –OH group of the secondary alcohols. Further, the economic viability of using secondary alcohols is challenging in the current industrial scenario.
Ozonation of vegetable oils
Studies show that ozonation of neat sunflower oil gives 90% ozonide with 1,24-trixolane rings and fatty acid moieties whose chain lengths are like the corresponding fatty acid methyl ester components of biodiesel with the presence of 1,2,4-trixolane polar rings in the fatty acid chains. Ozonised vegetable oils, therefore, are potential pour point depressants for biodiesel. Microscopic analysis revealed that ozonised vegetable oils prevent the agglomeration of crystals that resulted in the formation of smaller and more regular-shaped solids, thus maintaining the fluid flow properties of the biodiesel.
depression of freezing point
Blending with kerosene leads to a decrease in cloud point and pour point but also to a significant reduction in calorific value.
Jatropha biodiesel has poor oxidation stability with good low temperature properties. On the other hand, palm biodiesel has good oxidation stability but poor low temperature properties. Therefore, an optimum mixture of Jatropha biodiesel and palm biodiesel can lead to a synergistic combination with improved oxidation stability and low temperature properties. It is reported that a Jatropha-palm 60:40 blend was tested for low-temperature properties and found to exhibit a cloud point of 10°C and a cold filter plugging point of 5°C. Thus the blending of palm biodiesel in Jatropha biodiesel exhibits an additive response in the cloud and pour point properties. Fatty esters which contain more than 50% saturates, such as Babassu, coconut, palm, tallow (beef), and so on, blended with low saturate esters, such as sunflower, soya bean, safflower, peanut, olive, linseed, corn and canola, give better low temperature properties.
Cloud point and pour point analyses were conducted according to ASTM standards. Initially, the cloud point and pour point of biodiesel (mahua methyl ester) fuel was measured. The sample biodiesel had a cloud point of 23°C and a pour point of 15°C.
The cloud point and pour point of a sample diesel fuel were measured. The cloud point was 6°C and the pour point was -2°C without additive.
An experiment was conducted to test the effect on cold flow characteristics of biodiesel-diesel blending at various weight proportions. It was observed that a lowest pour point of -2°C was achieved by blending 5 wt% biodiesel and 95 wt% diesel. The results are shown in Table 1 and Figure 1.
Further, an experiment was conducted to test the effect of additive on biodiesel-diesel blends. The lowest pour point of -7°C was achieved by blending 5 wt% biodiesel and 95 wt% diesel with an additive (Methacrylonitril, C22 and C6 acrylates). The results are shown in Table 2 and Figure 2.
developing additives to control the agglomeration of crystals
MnO2 and NiO based additives
Studies showed that manganese- and nickel based additives reduced the pour point and the viscosity of biodiesel fuels. No significant difference in engine torque and power output was observed. There were specific increases in consumption because of the low calorific value of biodiesel. But NOx emissions were high. The higher NOx value is probably due to manganese additives having a higher catalytic effect on composition, leading to an increase in the maximum temperature.
Developing polymers to control the agglomeration of crystals
The molecular structures of pour point depressants for conventional diesel fuel comprise a polymeric hydrocarbon chain with protruding polar groups. Their most widely accepted mechanisms of action include adsorption, co-crystallisation, nucleation and improved wax solubility. It is very likely that vegetable oil and their corresponding fatty acid methyl esters have strong interaction at low temperatures. However, vegetable oil lacks polar groups which must be introduced into the structure to make it an effective pour point depressant.
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