*2.1. Biodiesel*

The triglyceride transesterification technique has been used in biodiesel production for more than 50 years [46]. During transesterification, fatty acid esters are formed when the triglycerides react with alcohol, and when the reaction is sped up by a catalyst.As the chemical processes involved in the manufacture of biodiesel are rather slow, catalysts are utilised to speed them up. Biodiesel manufacturing methods can be chemical or biotechnological, depending on the kind of catalyst used in the process. Biodiesel may be made from algal biomass in a variety of ways (including oil extraction from algal biomass) via esterification and direct transesterification of microalgae [47]. Fatty acid methyl esters (FAMEs), the chemical component of biodiesel, are usually generated in algal biodiesel processes by transesterification of algal oil with the alcohol (methanol) utilizing 98% concentrated sulfuric acid as a catalyst and n–hexane as a solvent. Extraction of oil from the microalgae without breaking their cells is a novel way in using nano catalysts for biodiesel synthesis from microalgae [48]. In situ transesterification is a promising method for avoiding oil extraction and directly converting lipids within microalgae cells to biodiesel in a single step, which might simplify biodiesel manufacturing procedures while also producing more biodiesel [49].

Generally, microalgae oil contains more free fatty acids compared to the oil from terrestrial plants; therefore, during biodiesel production from microalgae oil, free fatty acids should be esterified using acid catalysts prior to usual transesterification. In a research conducted by Ashokkumar, the fatty acids in the microalgae *Botryococcus braunii* were esterified using 1.5% concentrated sulphuric acid at a temperature of 55 ◦C for about 20 min prior to transesterification. The initial fatty acid content was 9.7% in the microalgae, and it decreased to 0.55 percent after esterification [50].

Unlike terrestrial oily plants (such as palm, soybean, or canola seed), algal oil may be recovered readily from the algal cell by compressing it, followed by solvent extraction. Mechanical crushing of algal biomass is another option, although extracting algal oil with the current technology is quite challenging. The majority of extraction methods are based on Bligh and Dryers 1959 approach. [51]. Cold pressing is the least costly method of extraction of algal oil [47]. This method may extract up to 70% of the oil contained inside the algae [52]. The addition of an organic solvent can improve the extraction level to 99%, but this comes at a cost in terms of processing [53].

When assessing the potential of different microalgal strains for biodiesel generation, its crucial to keep in mind that all microalgal oils are not suitable for the production of biodiesel. Biodiesel made from microalgae is similar to biodiesel made from other sources, such as oleaginous plants, in that it has limited oxidative stability due to a high degree of unsaturation.

Blending biodiesel with fossil fuel and/or adding chemical stabilisers can help in improving it [54–56]. The temperature-related features of diesel, such as cloud point and cold filter plugging point, become critical in cold climates for optimal fuel performance. [57]. Other important factors to be considered are the energy content which provides the intrinsic value of the fuel, acidic, and water content in the fuel which determines its corrosive nature, and the viscosity of the fuel to determine the proficient operation of the engine [54].
