Introduction about Biodiesel:
Many experts have suggested using this biodiesel as an alternate fuel for existing petrodiesel with rising concerns over climate changes because of its renewability, self-sustainability, high energy content, and absence of harmful chemicals prone to result in toxic emissions (for instance, sulphur). The overall processing and handling process of this biofuel is easier and safer than other petro products due to its enhanced fuel properties, which also make it readily compatible with already available CI engines (Srinivasan and Jambulingam, 2018).
Anatomy of Biodiesel:
As discussed earlier, biodiesel is an ester functional grouped organic chemical molecule with a long carbon chain fatty acid (FA) moiety at one end and an alcohol moiety at another end.
Biodiesel is classified into saturated and unsaturated biodiesel based on the degree of saturation in its FA moiety. Accordingly, saturated biodiesel exhibits an increased concentration of saturated FAs, while unsaturated biodiesel exhibits increased concentration of unsaturated FAs as their moieties; however, both saturated and unsaturated biodiesel tends to have a significant distribution of unsaturated FAs in it. Saturated FAs resemble a linear chain of aliphatic (-CH2) group placed in an orderly manner and give rise to the zigzag structure. On the other hand, unsaturated FAs appear non-linear and display a bend due to the double bond in its molecular structure, which results in a kink (shown in figure 2) (Gunstone and Harwood, 2007).
Furthermore, the oxygen atoms in their carboxylic/ester functional are highly negative, while the hydrogen atoms are highly positive; whereas, the carbon atoms are both electropositive and electronegative. In simple words, only the oxygen and carbon molecules of FAs react throughout its life cycle, while hydrogen remains inert. Hence, bond cleavages are noted specifically at oxygen-hydrogen (O-H) bonds during transesterification and hydrolysis; and C=C during oxidation. One must have an excellent fundamental understanding of these FAs to choose the suitable feedstock for biodiesel.
“It’s FAEs what’s inside Biodiesel that Counts”:
Looking into it, biodiesel is a mixture of multiple fatty acid alkyl esters (FAE) molecules made available in a definite proportion, depending on the nature and type of feedstock from which it is produced. Moreover, these FAEs are derived from the FAs of the oil or fat used as feedstock and always exist in the form of triglycerides. Most of the time, no two feedstocks report identical fatty acid composition or distribution in their oil/fat, making each feedstock unique from the other. One species of FA found in abundance for a feedstock may or may not be available in abundance for another feedstock; at times, it can be found only in traces or even absent. For instance, palmitic acid (C16:0) is distributed up to 44% in palm oil, while beef tallow reports only 25-27%; meanwhile, it was reported in traces for castor oil (Montoya et al., 2014; da Silva Martins et al., 2018; Srinivasan et al., 2019; Yeboah et al., 2020).
Most importantly, no new species of FAEs are formed during transesterification in abundance (except for thermally decomposed FAs), and only the esters corresponding to FAs, which are already available in the oil or fat, are reported. Above all, the overall properties and behaviour of oil/fat are contributed by these FAs, which are influenced by their feedstock’s cell metabolism. Interestingly, this metabolism is decided based on the geography, nature of the habitat, genetics and even the food is taken. Overall, any biodiesel’s fuel properties and engine characteristics are entirely dependent on its FAEs, which is again dependent on the FAs in its feedstock. Thus, one can easily predict biodiesel’s behaviour by simply understanding its feedstocks’ FA composition and can decide its application.
Feedstock for biodiesel – an Overview:
As discussed earlier, each biodiesel exhibits its uniqueness in its FAE composition and can be used accordingly to achieve maximum benefit from it. Hence, utmost care must always be given while choosing the suitable feedstock for producing the biodiesel, depending upon the nature of its end application. It is already known that molecules of saturated FAs are closely packed to each other and gets solidified easily at room temperatures. This behaviour is retained even after it is converted into its FAE. However, the temperature for its crystallization is drastically reduced. Yet, these FAEs tend to crystallize at temperatures slightly higher than 0OC. Without a doubt, it is evident that saturated biodiesel is not suitable for operating engines in cold regions or conditions; in contrast, unsaturated biodiesel can be operated at both hot and cold conditions. Thus, any feedstock with higher saturation is unfit for operating in cold temperatures.
Similarly, feedstocks offering low to medium carbon chained FAs can produce low to medium power outputs. In contrast, feedstocks with high carbon chained FAs are suitable for deriving higher power outputs. Besides that, feedstocks with highly saturated long-chain FAs can produce biodiesel, which can be used for both generator and transport applications. Contrarily, feedstocks with low viscous unsaturated FAEs can be used for burner based applications since saturated biodiesel leads to clogging and gumming at lower temperatures. Ultimately, the feedstock selection is entirely based on customers’ choice and availability in that particular region. However, technical concerns have to be accounted for for its resourceful utilization and capital savings.
The science behind the Theory:
Having seen the varieties in biodiesel for different types of application, it is indispensable for one to understand the science involved in it. First, any FA or FAE is identified based on its carbon chain length and degree of unsaturation. Eventually, these parameters enable one to decide the life cycle of these FA/FAEs and conclude their effectiveness during their application. Starting with the rendering of oil/fat, feedstocks with low to medium carbon chained FAs, and unsaturated FAs can be easily rendered using mechanical extraction. In contrast, long-chained saturated FAs require heat treatment for rendering them from the non-fatty residues of their feedstock. The overall production process of biodiesel is deeply influenced by the distribution of FAs and plays a crucial role in deciding the reaction parameters. For example, the molar ratio and the catalyst concentration are directly proportional to the molecular weight of the FAs, which, again, is a function of carbon chain length and degree of unsaturation. Likewise, the other parameters like reaction temperature and time depend on the carbon chain length and degree of unsaturation. Any long-chained, saturated FA would require higher reaction parameters. Low to medium chained, saturated FA would require slightly reduced reaction parameters; in contrast, unsaturated FAs would require mixed reaction parameters. Next up, fuel properties like density, kinematic viscosity, cetane number, calorific value and flash point, saponification and iodine value can be expressed in terms of carbon chain length and degree of unsaturation (Srinivasan and Jambulingam, 2020). Density decreases with the increased degree of saturation and chain length but increases for an increased degree of unsaturation.
Other properties are directly proportional to carbon chain length and inversely proportional to the degree of unsaturation. Hence, for good quality biodiesel, the feedstock should have slightly reduced viscosity, good cetane number and calorific value. Talking about its engine characteristics, the presence of fuel-bound oxygen in biodiesel’s molecular structure ensures the complete oxidation of fuel at its maximum. Also, biodiesel with a higher cetane number will produce high in-cylinder pressure, heat release rate and reduced ignition delay. Biodiesel with a high calorific value will likely provide high thermal efficiency under the reduced fuel consumption rate and can be mostly reported upon using saturated biodiesels with low viscosity. In emission characteristics, saturated biodiesel reported reduced CO and HC. In contrast, both saturated and unsaturated biodiesel reported high NOX value depending upon the fuel injected viscosity and its cetane number (Srinivasan et al., 2020).
Commercialized feedstocks for biodiesel production:
To summarise the above discussion, biodiesel is entirely dependent on the FA composition of its feedstock and understanding its composition can help us decide the suitable biodiesel for suitable application. Numerous scientists and researchers have identified many feedstocks; only a few have been scaled up for commercial purposes. Though these selected feedstocks might have desired properties slightly below the expectations of an ideal feedstock, they are unarguably agreed over the pretext of their availability, resourcefulness and cost-effectivity. Some globally proclaimed feedstocks include oil from soybean, palm, jatropha, cottonseed, safflower, canola, sunflower, rapeseed, and inedible beef tallow, waste grease and waste cooking oil (Sani et al., 2012). In recent times, high FFA wastes have also been identified as highly resourceful feedstocks for biodiesel production.
These raw materials are not evenly distributed on a global scale but are distributed regionally throughout the world. For instance, oil palm is widely used as feedstock in Malaysia and Indonesia, while soybean oil is used as feedstock throughout American continents. As far as India is concerned, jatropha and pongamia oil have been used as feedstock for biodiesel production for a very long period. Nowadays, waste tallows are used as an efficient feedstock because of their superior quality and properties. They are primarily used in countries that pioneer leather tanning and meat processing (especially beef meat). Usage of this feedstock encourages the concept of waste to energy conversion and serves as an effective technique for reducing waste disposal.
These tallows can also be used as raw material for direct heating applications; however, it is highly recommended to process into suitable biofuel compatible with engines and burners for deriving maximum energy from it. Figure 3 shows the global distribution of feedstock used for biodiesel production.
More importantly, biodiesel’s quality relies on the nature of feedstock used and other factors like refining of crude extracted oil/fat, optimized production process, refining of produced biodiesel, handling and storage. Yet again, even these factors are dependent on the FAs present in the feedstock. Hence, choosing a feedstock for biodiesel production is purely application based and the most suitable match must always be chosen for better end-user outputs.
Gokul Raghavendra Srinivasan
PhD scholar with over 21 publications on topics covering biodiesel and bio-kerosene.