Due to growing environmental concerns and depleting fossil fuels, researchers are developing environment friendly and unlimited renewable energy sources based on technologies like solar, wind, biomass etc [1][2]. Biomass-based liquid biofuel like biodiesel has been found as a potentially viable alternative to petroleum diesel [3][4]. Biodiesel is non-aromatic in nature, less inflammable, biologically degradable and oxygenated with no or very less sulfur content [5][6]. Biodiesel can be a sustainable and environmentally friendly energy source because of its low toxicity and low quantity of exhaust pollutants [7][8]. Biodiesel is a long-chain fatty acid monoalkyl ester produced from edible and non-edible vegetable oils, animal fat, waste cooking oil, microbial oil and algal oil [9]. It can be produced using various techniques like supercritical fluid technique, emulsification, pyrolysis, and transesterification. Because of its simplicity and efficiency, transesterification is still the preferred method for synthesising biodiesel [10].
Short-chain alkyl esters of saturated and unsaturated fatty acids (FAs) present in all feedstock oils constitute the biodiesel composition that governs the resulting biofuel's fuel characteristics [11]. For example, the presence of a higher proportion of unsaturated fatty acids makes the biodiesel more susceptible to oxidation when exposed to air during storage or too high temperatures and led to the build up of a higher concentration of polymerised molecules in biodiesel [12]. Poor oxidation stability of fuel affects the fuel quality, giving diesel engines unsatisfactory performance due to fuel filter choking, injector fouling, and depositions in the engine combustion chamber [13]. Therefore, oxidation stability (OS) is a performance indicator of biodiesel quality suitability for engine applications. Double bonds and unsaturation in the fatty acid chain contribute to biodiesel's strong reactivity with O2, which causes the oxidation of biodiesel. However, it can be prevented by adding natural or synthetic antioxidants to biodiesel, using mixed oils for biodiesel production, limiting exposure to air, light, and moisture, or keeping it in air-tight, low-temperature conditions [14]. These additives slow down deterioration processes and improve the oxidative stability of biodiesel. The most important additives used are butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), tertbutylated hydroquinone (TBHQ), pyrogallol (PY), i.e. 1,2,3-trihydroxybenzene, and propyl gallate (PG), i.e. propyl 3,4,5-trihydroxybenzoate [15][16][17]. Processes like fractional crystallisation, oil mixing, hydrogenation etc have been also used to lower the unsaturated fatty acid concentration in oils to enhance biodiesel OS [18]. Biodiesel oxidation performance has improved, but adoption of these technologies is still restricted because of the complicated and expensive procedures needed in this transformation and increased fuel toxicity. The extent of OS normally depends on taking appropriate precautions during long-term storage and is an issue pertaining to FAs composition of biodiesel [19][20].
The fatty acid compositions also affect biodiesel's cold flow qualities (cloud and pour point) [20]. Since unsaturated fatty compounds have lower melting points than saturated fatty compounds, the high concentration of saturated fatty acids in biodiesel can explain its high cloud point (CP) and pour point (PP) [21]. The poor cold flow properties (CFP) due to the presence of a higher proportion of saturated fatty acids cause problems in engine operation like a higher rate of fuel consumption and low flowability, making it difficult to pump the fuel from the tank to the engine [22]. Further, the fuel filters, injectors and lines tend to be clogged, causing engine wear that reduces the output torque and raises the specific fuel consumption resulting in overall poor fuel efficiency [23]. Many innovative methods have been proposed to address the problem of poor low-temperature fluidity, such as ozone oxidation, winterisation, and the addition of MgO nanoparticles, as well as blending biodiesel with petroleum diesel and kerosene, transesterification with branched-chain alcohol, and modification of the fatty acid profile of biodiesel, all of which have significantly improved biodiesel's low-temperature performance [24]. The above method, which is based on physical or chemical processes, can change other properties of biodiesel, which can change the quality of biodiesel fuel. It also takes time and costs more to make biodiesel with a low CP and PP.
As pointed out above, biodiesel's fuel properties depend on the fatty acid composition of esters in biodiesel and hence on the fatty acids of the raw oil feedstock [25]. Both saturated and unsaturated fatty acids in biodiesel decide its CFP and OS [26]. Basically, both are contradictory to each other, thereby meaning that biodiesel with better CFPs exhibits poor OS and vice versa. An ideal or optimum balance between the saturated and unsaturated fatty acids is required to satisfy both simultaneously. This balance can be manipulated by various methods like using suitable additives, blending oil feedstocks, blending biodiesel with other biodiesel and petro-diesel and winterisation [27]. Very few researchers achieve this by blending different raw oil before biodiesel production. According to observations made by Almeida et al. [28], the blending of waste fish oil, Palm oil, and waste frying oil changes the fatty acid content of the oil mixture, which in turn increases the OS of biodiesel from 1.8 hours to 22 hours. Khan et al. [29] found that when Ceiba pentandra and Nigella sativa were mixed in equal proportion, the biodiesel made from this mixture had OS 2.87 times higher than Ceiba pentandra biodiesel. Giwa et al.[30] mixed Palm kernel oil(80.4% saturated and 19.6% unsaturated FAs) and groundnut oil (15.5% saturated and 84.4% unsaturated FAs) at 50:50 (v/v) and obtained a hybrid oil consisting of 47.8% saturated and 52.26% unsaturated FAs. With this change in FAs composition of mixed oil, OS of biodiesel was increased 2.55 times compared to groundnut biodiesel. Damanik et al. [31] observed 25.95 times improvement in OS of biodiesel produced from the mixture of Calophyllum inophyllum and Palm oil at 50:50(v/v). Sharma and Duraisamy [32] mixed Jatropha, Karanja, Cottonseed, Palm, and Coconut oils to make different ternary and quaternary oil mixtures. When they mixed different oils to make mixed oil biodiesel, they saw changes in the saturation and unsaturation levels of FAs in oil mixtures, which caused the acid value, density, and KV to go down while the cetane number and OS went up in mixed oil biodiesel. Goh et al. [2] blended inedible jatropha oil and waste cooking oil in 1 : 9 ratio to produce biodiesel and observed mixed oil biodiesel had a low acid value (0.2 mg KOH g− 1), high calorific value (38.4 MJ kg− 1), high OS (~ 11 h) and good kinematic viscosity (4.7 mm2 s− 1). Jurac and Zlatar [33] blended rapeseed and used frying oil in different proportion (90:10, 80:20, 70:30 and 50:50 (v/v)) and found that CP of mixed oil (50:50) biodiesel has a lower value compared to individual single oil biodiesel. Saavedra et al. [34] tested several Jatropha and Palm oil combinations to produce biodiesel (percentage of Jatropha oil in the mixture were 9.1, 18.2, 28.6, 37.0, and 41.7). Jatropha oil has 19.8% saturated and 80.2% unsaturated fatty acids, while palm oil has 49% saturated and 51% unsaturated fatty acids. When these two oils were mixed, the sample containing Jatropha oil 37% in the mixture had 36.9% saturated and 63.1% unsaturated fatty acids, and they observed mixed oil biodiesel offers the best quality parameters because of changes in fatty acid composition.
One of the new promising methods, as reported in the above literature, is mixing two or more oils with different fatty acid compositions is expected to improve biodiesel's OS and CFP [10][35][36]. From the literature survey, no work is explored to simultaneously improve the OS and CFP by using oils mixture, and best-mixed oils can be converted to biodiesel, satisfying all the fuel properties as per biodiesel standards. In the opinion that a mixture of oils is a relevant alternative to enhance the fuel properties of biodiesel with decreased expense, so this work aims to formulate a potential hybrid feedstock with the best oxidation and cold flow properties through a mixture design method for biodiesel production. Develop a cost-effective solution with a high potential feedstock oil like Jatropha, Karanja and Palm for real-world applications to improve biodiesel's low-temperature performance and oxidation stability. This research also gives essential information for large-scale commercial biodiesel production with superior biodiesel quality.