Journal of Energy Bioscience 2024, Vol.15, No.5, 289-300 http://bioscipublisher.com/index.php/jeb 294 Schizochytrium sp. could achieve a DHA yield of 38.12 g/L under optimized conditions using a cooperative two-factor ALE strategy (Sun et al., 2018). Additionally, the oleaginous yeast Rhodosporidium toruloides was able to produce 3.8 g/L of biomass with a lipid content of 34.9% when cultivated in bioethanol wastewater (Zhou et al., 2013). These examples underscore the effectiveness of genetic and process optimization strategies in enhancing lipid production for biofuel applications. 6 Transesterification of Microbial Lipids into Biodiesel 6.1 Chemical process of transesterification Transesterification is a chemical reaction where triglycerides react with alcohol (usually methanol or ethanol) in the presence of a catalyst to form fatty acid methyl esters (FAMEs), commonly known as biodiesel, and glycerol as a byproduct. This process can be carried out using various catalysts, including acid, base, and enzyme catalysts. For instance, the use of Fe2O3 nanocatalysts has shown to improve biodiesel yield significantly when compared to conventional acid (HCl) and base (NaOH) catalysts, achieving up to 86% yield under optimized conditions (Banerjee et al., 2019). Similarly, homogeneous acid catalysis using H2SO4 has been found effective, with a maximum methyl ester yield of 60% (Mathimani et al., 2015). 6.2 Role of catalysts in biodiesel conversion Catalysts play a crucial role in the transesterification process by lowering the activation energy and increasing the reaction rate. Different types of catalysts, such as homogeneous (acid and base) and heterogeneous (solid acid and base), have been explored. For example, graphene oxide (GO) as a solid acid catalyst has demonstrated high efficiency, achieving a lipids conversion efficiency into FAMEs of 95.1% in microwave-assisted transesterification reactions (Cheng et al., 2016). Additionally, enzyme catalysts like immobilized lipase have been used, providing a biodiesel yield that is seven times higher compared to alkaline-based transesterification (Teo et al., 2014). 6.3 Yield optimization strategies for biodiesel production Optimizing the yield of biodiesel involves fine-tuning various parameters such as the methanol-to-lipid ratio, catalyst concentration, reaction temperature, and time. Response Surface Methodology (RSM) has been employed to optimize these parameters, achieving a maximum biodiesel yield of 89.583% with H2SO4 catalyst under specific conditions (Chamola et al., 2019). Concurrent extraction and reaction processes have also been developed to enhance yield and reduce energy consumption, achieving yields higher than 90 wt.% (Im et al., 2014). Supercritical methanolysis is another effective method, with yields reaching up to 99.32% under optimized conditions (Shirazi et al., 2017). 6.4 Challenges in ensuring biodiesel quality Ensuring the quality of biodiesel involves addressing several challenges, including the removal of impurities, achieving the desired fatty acid profile, and meeting international standards such as ASTM and European norms. For instance, the biodiesel produced fromChlorella sp. BDUG 91771 was characterized to have a suitable Degree of Unsaturation (DU), Long Chain Saturated Factor (LCSF), and Cold Filter Plugging Point (CFPP), aligning with prescribed standards (Mathimani et al., 2015). Additionally, the supercritical transesterification process has been shown to improve the quality of biodiesel by reducing the proportion of polyunsaturated fatty acids (Jazzar et al., 2015). However, maintaining consistent quality across different batches and feedstocks remains a significant challenge. 7 Process Efficiency and Economic Feasibility 7.1 Energy balance and process efficiency in microbial conversion The energy balance and process efficiency of biohydrogen production from marine algae are critical factors in determining the viability of this biofuel. Algae, particularly macroalgae, are considered efficient sources of biomass for biohydrogen production due to their high-energy yield and sustainable nature (Kumar et al., 2021). The pretreatment of algae is essential to enhance the hydrolytic process during dark fermentation, although it can lead to the formation of inhibitory substances that need to be controlled through detoxification techniques.
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